Electronic Instrumentation Conference

ELECTRONIC INSTRUMENTATION CONFERENCE

HOBART 1972

These papers have not been edited by the respective Institutions and no responsibility is taken for the views expressed. - CONTENTS -

Computer Based Telemetry Systems for Remote Monitoring and Control R.A. Smythers, M.A., Senior Applications Engineer, Cutler-Hammer Australia Pty Ltd 1 Radio-Frequency Modulation and Demodulation of Lasers for Communication D.J. Cole, B.E.E.(Melb.), M.I.E.(Aust.), F.I.R.E.E., Principal Research Scientist. Division of Radiophysics, C.S.I.R.O., Sydney 9 An Instrumentation System for a Natural Gas Pipeline B.E. Richards, Fellowship Diploma of Electronic Engineering, Development Engineer, Relays Pty Ltd.... 16 Data Acquisition System for Astronomical Photoelectric Photometry D.G. Thomas, M.f.E.Aust, A.M.I.R.E.E.Aust, Engineer II, Mount Stromlo and Siding Spring Observatories, The Australian National University 22 Application of Solid State Logic in a Zinc Stacking Line Thomas Michael Fulton, B.E. (Elect) Uni. of NSW, Graduate I.E.Aust, Graduate I.E.E. London, Electrical Engineer, E.Z. Co. of A/asia Ltd, Risdon, Tasmania 29 Very Low Noise Amplifiers for Semiconductor X—Ray Detectors J.E. Eberhardt, B.E.(UQLD), M.E.(UNSW), Instrumentation and Control Division, Australian Atomic Energy Commission 38 A Telemetry System for Electrical Distribution Networks F. Zillhardt, Ing. (grad.). Senior Systems Engineer, Cutler-Hammer Australia Pty Ltd 45 Digital Companding Techniques C.J. Kikkert, B.E.(Hons.), Dept of Electrical Engineering, University of Adelaide, Adelaide, S.A 52 Vibration Technique for Rot Detection in Wood Poles A.D. Shaw, Dip.E.E., M.I.E.Aust., Development Engineer, Retail Supply Branch, Hydro-Electric Commission of Tasmania 57 Fault Detecting in a Processor-Controlled Telephone Switching System N.W. McLeod, B.Sc, M.I.E.(Aust), A/g Engineer Class 3, Australian Post Office Research Laboratories ... 65 Digital Phase Meter M. Imber, B.Eng.(Elec), University of Melbourne 74 A Computer Controlled Circuit Tester N.J. Gale, B.Eng. (Honours), Australian Post Office Research Laboratories, Melbourne 80 Technical Control Facilities for Common User Data Network Centres K.V. Sharp, A.R.M.I.T., Engineer Class 4, Postmaster-General's Department 80 An Accelerometer for Engines and Rotating Machines W.S. Leung, B.Sc., Ph.D., M.I.E.E., Sem.M.I.E.E., Senior Lecturer, University of Hong Kong; W.F. Ma, B.Sc. (Eng.), M.Sc.(Eng.), Lecturer, University of Hong Kong; and C.C. Lau, B.Sc., University of Hong Kong 90 Development of Telephone Traffic Measurement Equipment and it« Application in the Australian Telephone Network C.W. Pratt, Ph.D., Australian Post Office; and LA.Tyrrell, B.E., M.I.E.IAust), Australian Post Office 95 Metering Control and Alarm Indication for Power Rectifiers used in a large Electro-Chemical Zinc Plant M.J. Healy, B.E. (Uni. of Tas.), Grad.I.E.Aust., Electrical Engineer at The Electrolytic Zinc Company of Australasia Limited 104 A Direct Reading Digital Instrument for the Measurement of Speed of Road Vehicles G. Ganky, B.E., Grad.I.E.Aust., ').C.A., formerly Research Student, University of Melbourne: and A.E. Ferguson, M.E.E., M.I.E.Aust, F.I.R.E.E.Aust, Reader, University of Melbourne 112 Security of Supervisory Control Systems — Two Case Studies B.M. Lewis, B.Tech., M.I.E.Aust., Electrical Contracts Engineer, Control Systems. Electricity Trust of S.A. . . 117 A Presence and Passage Vehicle Detector G.F. Shannon, B.E., Ph.D., M.I.E.Aust, Senior Lecturer, Elec. Eng. Dept., University of Queensland; and N.V. Chuong, B.E., Research Assistant, Elec. Eng. Dept, University of Queensland 125 Digital Frequency, Time and Time Deviation Equipment J.W. Tamke, B.E.(Hons), Grad.!.E.Aust, M.I.I.C.A., Assistant Test Engineer, Power Electricity Trust of South Australia 129 Manipulators: An Instrumentation and Control Survey N. Newman, B.E., M.Eng.Sc, M. I.E.Aust; and K.E. Tait, B.E.(Hons). B.Sc., Ph.D., M.I.E.Aust Postgraduate Student and Senior Lecturer, Department of Control Engineering, School of Electrical Engineering, University of N.S.W 136 cc The Precise Measurement of Weak Magnetic Fields Ronald Green, B.Sc., Ph.D., A.M.I. R.E.E.; and John M.Stanley, B.Sc 142 Force-Feedback: A Review N. Newman, B.E., M.Eng.Sc, M.I.E.Aust, a post graduate student in the Dept of Control Engineering within the School of Electrical Engineering at the University of New South Wales 150 The Accuracy of Electrical Measurements made by Electronic Techniques J.M. Warner, B.Sc., Non-Member, Australian Post Office Research Laboratories, Melbourne 156 A Polar Co-ordinate Multiparameter Display J.A. Coekin, B.Sc., Ph.D., M.I.E.Aust, Senior Lecturer in Charge of Electrical Engineering, James Cook University of North Queensland 161 Low Cost Computer Compatible Data Logger A. Ceresa, M.I.E.(Aust), M.I.R.E.E.(Aust), Experimental Officer, Division of Irrigation Research, CS.I.R.O. . 168 The Generation of Pseudo-Random Binary Sequences- for Testing Very Wide Band Systems J.A. Coekin and J.R. Wicking, Electrical Engineering Division, James Cook University of North Queensland . . 173 Development in Microwave Instrumentation for Industrial Process Control D.W. Griffin, B.E., B.A., Ph.D., C.Eng., Reader in Electrical Engineering, University of Adelaide 180

Precise Function Generation under Digital Control Sl D.A. Pucknell, B.Sc., B.E., M.I.E.E., Senior Lecturer in Electrical Engineering, University of Adelaide, : c Adelaide, South Australia 186 New Solid State Microwave Generators for Industrial Applications c C.J. McRae, B.Tech., B.E., M.I.E.Aust. (Student); and D.W. Griffin, B.E., B.A., Ph.D., E.Eng. (Reader), : c Department of Electrical Engineering, The University of Adelaide 193 ; P Digital Diffusion Analogue „ M.N. Svilans, B.Sc., B.E.(Hons), University of Adelaide 201 Electronic Techniques in Telemetering Systems CR. Farrell, B.E., Electronics Engineer, M.W.S. & D.B., Sydney 206 The Detection and Location of a Noise Source in a Dispersive Medium G.J. Cybula, B.Sc., (Tech.) Grad. I.E. Aust, Electrical Engineer, Operations Division, Australian Atomic ; i Energy Commission; and T.J. Ledwidge, B.Sc., Ph.D., C.Eng., M.I.E.E., M.lnst P. Head Engineering Physics Section, Engineering Research Division, Australian Atomic Energy Commission 215 , The Acquisition of Wind Tunnel Data using a PDP15/20 Computer < RoyE. Kane, B.E..M.I.E. Aust 222 A Theoretical Study of the Effects of Dispersion on Cross-Correlograms ' G.J. Cybula, B.Sc.(Tech), Grad. I.E.Aust., Elect. Eng., Operations Div., A.A.E.C.; and ' R.W.Harris. B.Sc., M.Sc, Ph.D., A.M.I.R.E.E,(Aust), Research Scientist, Eng. Research Div., A.A.E.C. ... 229 AC-to-DC Conversion for Rapid Precise Digital Measurements E.L. Harris, B.Sc., M.Sc, M.I.R.E.E., Senior Lecturer in Electrica! Engineering, N.S.W. Institute of Technology 234 COMPUTER BASED TELEMETRY SYSTEMS FOR REMOTE MONITORING AND CONTROL

R.A. Smythers, M.A., Senior Applications Engineer, Cutler-Hammer Australia Pty Limited

SIMIABXt Major operating objective» for computer based reaote monitoring and control ayeteae are presented from which emerge the prime requirements of flexibility and ease of expandability. A typical system configuration containing a number of alternatives is then examined with major features of the system receiving detailed attention. The major objective ot tbia examination is to consider soae of the alternatives, and their consequences, that become possible once a computer forms the heart of auch a system. This article is not concerned with basic technology involved but rather with system design concepts and philosophies that ensure compatible and efficient utilisation of components through the system. Finally, consideration of possible future trends and developments endorse the primary requirements for flexibility and where the possibility of future needs are considered during the design of the initial system.

XNTBODUCTION« For many decades conventional IMJDR OBJECTIVES FOR MONITORING AND CONTROL telemetry systems have been used to provide remote SYSTEUS: The following simnariae the major object- monitoring and control facilities of plant and dis- ives that must be satisfied by present day, large tribution networks. However, many of the industries scale remote supervisory systems. These objectives that employ such Systeme hare undergone significant are presented in two stages, firstly to provide - and radical changes during the last few years. This is particularly true of the public supply utilities, 1. Meaningful presentation of selected operating electricity, gas and water. These utilities have been data on a periodic aoä operator request basis. subjected to an ever increasing demand pattern along 2. Automatic presentation of alarm conditions, in a with increasingly more complex and stringent distrib- meaningful manner so as to bring the operator's ution problems. Associated with this change has been attention to areas of the system that need his an increasing awareness of the necessity to operate attention. with respect to tight economic objectives and constraints. These requirements have generated the need 3. The ability to calculate and present to the for more powerful and flexible data gathering and operator derived parameters such as efficiency. data reduction facilities that provide vital assist- 4. Easy to use control facilities. Ideally the ance to the human operator in his system assessment security aspects of making control actions and control activity. To meet this need the variable should be looked after automatically. program computer has «merged as a potentially powerful and flexible tool. This subject is currently Attract- 5. Ability to perform regular periodic control ing considerable attention (Bef.1-4) and a large functions automatically. amount of operating experience is now available (Bef. 5-9). Unfortunately not all this experience has been 6. The ability to build up an historical record, on complementary or enthusiastic. It is not the intent- a regular and on demand basis, in an efficient ion ot this article to dwell on this aspect, however, manner. it is worth noting that in many applications the 7. The ability to readily «ccosmodate network ex- variable program computer has simply formed a direct pansion and modification. substitute for the conventional, fixed store program unit. In such cases, hardware problems have received These are the areas where a computer based system early attention, whereas development of suitable soft- will make an immediate contribution, however, from ware has only received attention at as advanced stage the use of the above facilities and analysis of of the project. rfhis approach has resulted in in- historical data a number of things will emerge. efficient and untidy systems where the power and Typical operating patterns will become apparent, sophistication of the computer has not been fully models can be formulated to assess network response exploited due to limitations imposed by the presence and security etc. In this fashion the system may of restrictive hardware with untidy and inaccessible eventually be extended to provide additional operat- software. This experience would often have bsen ional aids, the major being as followsj- avoided if the system hafi been designed and conceived 8. To estimate both short and long t«rm consumer an a total entity, whsr« the compatibility and inter- requirements. action of all system elements, both hardware and software, had bten considered from the commencement of the 9. To generate optimal solutions to meet the demaads project. of (8) within the constraints of economic and security. 10. To perform economic and security assessment on of alarm conditions, is brought to an Outstation. At any configuration that the operator defines. each Outstation information is conditioned, as necessary, ready for transmission to the Master Station. In 11. To provide comprehensive alarm analysis with some general a number of Outstations will share a comon automatic response. communication link to the Master Station. This link 12. To perform data reduction and present operating will be terminated, according to the appropriate and efficiency summaries at points in a manage- regulations, via a termination unit. A Telemetry ment structure where they are required. Interface Unit (THJ) acts as a buffer between the incoming data and the Computer Data Highway. 13. To estimate system response and subsequent behaviour to proposed system re-configuration. At the Master Station, Pata files, either in core store or on backing store, contain relevant data As the above tools become available to an operator his describing every point in the system. The computer role will change. He «rill be relieved of attending to program breaks down into three major groups. Firstly many hitherto mundane and periodic tasks and thus will there are the main executive routines that are re- be able to give more time to unusual and often- potent- sponsible for the required priority and time sharing ially critical situations. In short his role will between the various system programs. Secondly there change from network control to network management. are the individual system programs that perform the The main concern of the remainder of this article is calculations and logical functions as defined by the to define a system that will allow the initial set of system requirements. Finally there are the peripheral objectives to be achieved. However, it is essential service routines responsible for the input/output to keep in mind the future possibilities in order to requirements of all peripherals. accommodate the result of increased system understand- The operators control desk contains audio and visual ing as it becomes available. It ie easily understood alarm annunciators, fixed and selective display that a prime design objective, to allow for the above facilities and network control facilities. Provision possibilities, must be to provide system flexibility is also aade to allow the operator to modify and up- and ease of expandability. date point date stored in computer memory. Type- THE COMPUTES BASED SYSTfli: Fig. 1 shows a writers and loggers provide fnr the permanent record- typical large scale computer based telemetry system. ing of events and system summaries. A number of alternatives features, for such as stand- Two standby philosophies are shown. One shows full by and backing store, are shown in this schematic. duplication of a major component while the other shows Briefly the system works as follows. Suitable trans- a simple standby facility that provides only essential ducers convert physical quantities into analogue or features. digital type signals. These signals are conveyed via Data exchange within the system is controlled by the direct wire links to a local Outstation, generally system and executive programs. For large networks using D.C. or Frequency Division Multiplexing (FDM) communication between Master Station and Outstation techniques. In a similar fashion status information, will use Time Division Multiplex (TDM) techniques. describing the condition of devices or the existence

OFF LIME ANDi STANOBY SYSTEM C. P. STAND1Y (REDUCED) OPERATORS CONSOLE SHALL MIMIC I

SMALL STANDBY CONSOLE

CENTRAL CENTRAL PROCESSOR PROCESSOR Nβ a NU (STANDBY)

FIG 1. TYPICAL COMPUTER BASED SYSTEM Tin- above will now be discussed in detail, emphasis requires each Outetation to have a unique link with will be placed on achieving the correct level of the Master Station along with an associated register sophistication throughout so that powerful system at the Master Station. The request-reply system features are not degraded due to the presence of non works by the Master Station broadcasting, on a shared compatible or less advanced philosophy elsewhere. communication network, a message that has two main components. The first component will be an Out- PRIMARY TRANSDUCERS: The selection of a trans- station address with the second part being either a ducer will depend upon the physical quantity to be request for a certain data word or an instruction to measured, the location, required accuracy and servie- the Outstation to perform some control action. All ability, safety constraints etc. Experience will Outstations receive and interrogate this message, and play a major role in this exercise. Care must be thus the appropriate Outstation will respond accord- taken to ensure that transducer accuracy is not de- ingly. This approach may be extended whereby the graded due to the scanning or transmission process. second part of the message could call for the This is particularly relevant with a pulse counting sequential transmission of a nuaber of data points. type instriasent. Analogue transducers with standard However, this feature must be used with care as it 0-10 mA, or 4-20 mA output allow standardisation of tends to reduce the flexibility of the scanning Outstation input networks which in turn allows a arrangements possible. It is readily understood modular approach to Outetation design to be undertaken. that the request-reply principle provides the degree The concept of telemetry type output from transducers of flexibility required by the" systems being consider- is currently being considered. However, this approach ed. In this fashion the Master Station may manipu- will probably cause considerable problems unless a late the transmission network so that priority is standardised form of output is adhered to. A more given to those areas of the system currently of important concept is to transmit the raw digitized interest to the operator or where some crisis exist*. output to the Master Station and to perform all liner- Scanning arrangements that exploit this flexibility isation, compensation etc. by computer program. In a will be discussed later. similar fashion the computation of derived quantities from a number of measurements nay be assigned to the The efficiency of the communication system could be conputer. The advantages of this approach are ob- expressed as a function of the amount of valid data vious and represent a good example of where the power handled by the network in a given period of time with of the computer should be exploited. However, the some penalty imposed that was related to the amount one great objection raised to this approach is the of undetected erroneous data handled during the same tine period. Such an efficiency factor will be re- loss of the stand alone capability of the instrument. lated to:- It ia interesting to note that this approach is now emerging in the process control industry and the op- 1. Speed of Transmission. erating experience of one company that pioneered this field has amply justified this change in concept 2. Message format. (Ref.11). 3. Degree of error detection built into the format. 4. Quantity of communication media. THE OUTSTATION: A prime requirement for an Outetation associated with a computer based Master It is accepted practice for message formats to be Station is that it should possess a high level of designed so that undetected message corruption is flexibility and ease of expandability. If this is exceptionally low. The techniques use range from not the case, then a major advantage of having a the inclusion of a parity bit to a full repeat of the computer at the Master Station is diminished. A message. A typical system might require an Out- major step in this direction is achieved by designing station to firstly re-transmit the message received the Qutstation on a modular basis with a range of from the Master Station and then to transmit the re- plug in type boards to handle standard inputs and quired data followed by its inverse. Generally the output«. TIU includes all necessary memory and checking capability in order to confirm the validity of these Another desirable feature that an Outetation should exchanges, and to initiate a repeat if necessary. possess, to be fully compatible with a computer This philosophy possesses one advantage, only good based Master Station, is some form of self reporting information is passed to the next system level, name- facility. With this feature if any one of a uumber ly the Master Station computer. Thus a form of. of selected status bits at the Outstation undergoes hierarchy structure emerges where the lower level ia a change of state, the Outetation will immediately responsible for checking its data before onward trans- alert the Master Station. The Master Station can mission. This may be ideal philosophically but it then give this situation its immediate attention. results in a rather low transmission efficiency and a This uelt reporting ability will significantly in- fair amount of hardware at the telemetry data inter- crease «canning efficiency, particularly in a system face. Other approaches are possible once a digital with many hundreds of status bite» coeputer is present at the Master Station. Consider- able hardware saving may be achieved if the TIU were DATA TRANSMISSION METHODS AND COOES: The de- to became a simple register just clocking it« pulaaa tailed design of the communication networks, from in and alerting the computer when it had received a primary transducer through Outetation.. to Master complete word. All validity checking would then be Station, involves the consideration of many factors, carried out by software modules. Transmission effic- and ba« received auch attention elsewhere (fief.12,13). iency will alao be increased by checking i received However, for large scale system« of the type being measurement against physical constraints, in contrast considered a TDM system will, in general, represent to calling for a repeat. These checke may be as the optimum choice for transmission between Out- stringent as desired. Normal out of limit check« may station and Master Station. Basically .there are two be applied along with rate of chaftge checks. Know- categories of TDM, free-running and address-reply. ledge of the physical limits and dynamic behaviour of The free-running TDM has certain disadvantages; it the plant involved «rill allow these limits to be set. The first example in Fig. 2 is used when a consider- This aethod of checking nas an additional advantage able degree of parity exists between the data of in that the performance of the primary transducer and each Outstation, while tbe second example account* Outstation input networks are also being checked. for the case where the majority of neasurentente in a Clearly the combination and possibilities are endless, system are different. The principle in the first the nain point of the above discussion is to show that example is to store the data, that characterises the with a computer based Master Station hardware and measurement type, once. A simple bit list is attach- system complexity may be reduced by placing greater ed that indicates at what Outstation this measurement reliance upon software. This, in turn, will enhance exists. In the second case a complete data liat i* system reliability. stored for every measurement in the system. For a typical example of 200 measurements with 20 measure- THE COMPUTER: The central processor jf any ment types the total data storage using example one coEj-uter baaed telemetry system will have been select- is 140 twelve bit words, whereas 1400 words are ed from a range of standard production processors. required using example two layout. This demonstrates This has many advantages. Firstly, the processor the need to develop system software that will accept will have been developed to meet the many stringent different data file configurations. In this fashion requirements of a wide range of applications and tbe system designer will he able to select the most operating environments. Secondly, large numbers of efficient storage techniques for a given application. processors will have been produced, hence production This is just another example of tbe need to maintain problems will have been overcome and operating exper- flexibility. However, experiene shows that eyatem* ience will be wide resulting in lower coat and in- requiring the flexibility of the variable program creased reliability. computer invariably lack the constraints required to implement the layout of the first example. For this The latest trends in the computer manufacturing reason the use of backing store, generally * drum, or industries are for smeller, lower cost but except- disc, is becoming increasingly popular. .Different ionally powerful processors. This will lead to forms of backing storage will be considered later« consideration of forming computer based data gathering points between Outstatione and central Master The importance of system flexibility and ease of Station. Data checking and some data reduction could expandability has been continually emphasised. This then be performed at this point, results being onward is achieved by the software storage of data files, as transmitted to the Master Station along with similar the information in these files can be easily updated data fro« many other points. If the present trend by overwriting. In a similar fashion they can be continues swell computers, with fairly extensive easily extended, provided suitable provision was made input/output features, sight replace the larger Out- at the conception of the system. However, it is this station. area that is generally completely overlooked or given very little attention during initial design consider- The choice of a suitable computer depends upon aany ation. It is of paramount importance that an easily factors. For proceusors enjoying the ease service operated but very secure facility is provided that backup and reliability, the sensible solution is allows the operator to update basic data. This invariably tbe cheapest computer that is fast enough aspect is fully discussed later, when operator to do the job and has sufficient storage capacity. facilities are considered. However, when assessing computer cost the availability of suitable software must be considered. Aβ systems THE COMPUTER PROGRAMS: The computer program become more complex, requiring large backing stores, has already been described as providing three major computer to computer links,, and more demanding per- types of routines, the executive, tbe system and ipheral« auch as cathode ray tubes, ihe most attract- tbe peripheral service routine. It is impractical to ive overall economic solution will be the computer describe how these routines interact as this depends for which a good software operating system has been so much on the processor being used, particularly its developed and is well tried. hardware priority and interrupt structure. However, there are some important concepts that must be ad- THE DATA FILES: The data files, held /ithin hered to for successful program development. Firstly the computer core or on backing,store, contain all - the concept of modularity. This requires the sys- the necessary details of every point in the telemetry tem programs to be built up of a number of individual system. Fig. 2 shows two examples of possible modules each responsible for a unique task. For measurement file layout for » 12 bit word machine example, the limit checking of a flow rate will be Such layout* can follow a variety of patterns. pet formed by one module while its integration will 4 BITS 4 BITS 4 BITS 4 SITS 4 BITS A JL...... ,_ J TYRE Ni | POINT h• 1 O. S. ADDRESS SCAN PRIORITY PRINT FORMAT POINT MS | SCALE FACTOR TAPE FORMAT SCAN PRIORITY PRINT FORMAT X CRT CO-ORQ | Y CRT CO-ORD. SCALE FACTOR TAPE FORMAT HI6H LIMIT X CRT CO-ORD. | Y CRT C0-0RO. LOW LIMIT HIGH LIMIT O/S MARKER LOW LIMIT 2CLJ* TYPE 2. FiS. 2 tWA FILE LAYOUTS be tbe responsibility of another Module. In a similar A number of alternatives have to be considered once fashion tbe subsequent display of the integral will the decision to include a backing store has been be organised by a peripheral output nodule. A taken. second- important concept is the use of data and work buffers for cooaunication between modules. This 1. Type of backing store. concept is particularly relevant to tbe telemetry 2. Will program as well as data be stored on type system where prime data ia stored in list form. backing store. This technique results in an a»synchronous node of working which, combined with the nodular structure, 3. The need for autonomous data transfer between allows program modification and expansion to be core and backing store. undertaken in a controlled and predictable fashion. 4. Reliability required and security of data. The other advantage of a nodular approach is the increased efficiency of store usage as duplication 5. The need for hardware nnd data formst capabil- is avoided. However, development of this fora of ity with other machines (for off-line analysis). software requires a highly disciplined approach with Space does not allow a comprehensive coverage of all considerable system design and specification long these aspects and their inter-relationships. The before any software coding is attempted. random access of the drum and disc store make them ideal for storage of on-line program or data, with SCANNING AHRANGflffiNTS: The simplest, and mo et the magnetic tape unit being suitable for storage of often used, scanning arrangements is a straight- historical data prior to analysis on an off-line forward sequential scan of all data points, with machine. Before deciding if autonomous transfer is control messages 'interrupting as necessary. The use necessary, some careful attention should be given to of a digital computer at the Uaster Station allows a system timing. Without the AT unit program execution considerably more ambitious and flexible scanning is suspended while data exchange takes place, and system to be operated. The assignment of an updating hence system response may be reduced. However, in a priority-, for each piece of data, provides such a telemetry type system, system response is almost system. When deciding the appropriate priority for certainly limited by the message exchange rate over a given measurement the following points should be the communications network, with tbe main system consideredt- design criteria being to keep this network running at its maximum speed. In many computer systems this can 1. Normal rate of change of the variable being be achieved without the additional hardware expense measured. and software compexity of an AT unit. However, this 2. The need to monitor this variable for alarm consideration might be outweighed by the use of a software package based on minimum core residence of conditions. data and program and hence reliance of an AT unit. 3. Its contribution to derived quantities and hence the effect of their accuracy. OPERATOR/SYSTEM INTEHFACT: The computer 4. Required update rate when being monitored by provides for data to be stored in many forms and to the operator. be rapidly presented to the operator as the need arises. This allows the design of compact and effic- The first three points above will result in a base ient operators consoles where the operator can ex- priority being assigned to every measurement which amine his system in general terms and then zoom in, will be stored in the data file area for each as it were, on some detailed area. Fig. 3 shows the measurement. Point 4 above will be handled by rajor aspects of such a console. This design also assigning a higher priority, on a temporary basis, satisfies the criteria for easy expandability, the while the variable is selected for display. Similar point selection mechanism easily provides for con- arrangements will be made if it is required to log siderable system expansion without the requirement or record a certain variable. Alarm/status words for additional hardware. Some major aspects of the may be fitted into this structure, but this will Operator/System interface are considered below. depend upon the degree of self reporting of the alarm words. OPEBATOH DISPLAY FACILITIES.• Fig. 3 show* the This system outlined above represents a very efficient overall system condition being continuously displayed technique of providing variable priority scanning. on a «mall mimic diagram, where the total system is This arises due to a very simple software module split into a number of areas. For each area the sequentially interrogating the data files and arrang- mimic diagram has or-> general alarm indication and a ing the system scanning sequence according to the few major variables on display such as total »re* priority it finds associated with each particular power consumption. A study of this overall display- data point, log and display, routines.etc. will will indicate to tbe operator' any area that might simply manipulate this priority and in no way inter- warrant more detailed examination. This can then be fere or interrupt the software module responsible for achieved in a variety of ways. A complete display of maintaining the scanning cycle. the area may be selected on one of the Cathode Hay Tubes (CRT). A very meaningful technique of present- BACKING STOfcE« The need for backing store ing the network state on a CRT screen is to display «rises fromt- dynaaic data against a background of the network schematic. The CRT can also, of course, be used to 1. Increasing system size. . present various system summaries and breakdowns in 2. The inclusion of complex diaplay or logging tabulated or graphic form. An important aspect of systems (CRT for example). using a CRT for data presentation is not only the •peed with which data becoues available but it does 3. The requirements to store large quantities so without the production of vast quantities of paper of historical data. ' containing summaries that cease to be of interest operator to select the required control point, Bet o O up the required position or set point on rotary switches and input this requirement to the computer. o I i in —1— llll| A program module will then arrange to monitor the Q3S3 valve position and output the inching commands as necessary. This approach can be further developed whereby the inching commands are output at interval» I1I1I4I2IVI BiamiiAl slightly leas than the time taken for each step to be HI fully achieved. This baa the effect of achieving the new position in a ramp fashion in contrast to a series of ramps and plateaus and thus overcomes all the pro- """"Mill blems associated with the frequent stop-start of ALARM ICRTSEL drive machinery. Thus the advantage of continuous ODoa control is achieved but with the security of the inching control. However, the most efficient exploit- CONTROL LOG SEL. ation of tbe computer is to allow it to exercise feedback control in order to maintain a desired value of some dependent variable such as flow. This re- once examined. An excellent example of the use at quires the definition of a control algorithm that this approach is the system of National Distribution pays respect to the system dynamics and required Centre of the Central Electrical Generating Board of response. the United Kingdom (Ref.9). The above could then be extended to automatically The CRT display system can be backed up by selective maintain daily profiles, such as gas holder flows. display facilities, two of which are shown in Fig.3. To display a Measurement on one of these facilities, OPERATOR AMETOUENT OF STORED DATA: The the operator will set up th« point address on the flexibility and ease of expandability provided by display selector switches and operate the display the use of a digital computer are significantly button. Thereafter the display will be updated at a reduced unless the operator is provided with an easy frequency dependent upon the priority assigned to to use, but sufficiently secure, mecheaism to update this measurement. It is useful tc note that the and modify stored data. Provision of this facility point address does not have to correspond to the is essential to allow for change in network configur- Outstation and point hardware address. It can be ation or plant being taken out of service etc. defined on a «ore convenient basis, such as area and Lack of attention to this area in some recent systems type of neasurement, the conversion to the true hard- has caused severe disappointment. One proposal to ware address being handled by a program module after overcome this problem postulates a form of soft- interrogation of the measurement data files. hurdware for data storage (Ref.14). While these th proposals are sound and effective in principle, they j* OPERATOR CONTROL FACILITIES: The control represent an unfortunate retrograde step as the inat requirements that may be provided by tbe computer corporation of more hardware and hence computer inter- tw based telemetry system fall into three categories. face must result in lower reliability, apart from the requirement to carry more spares and servicing c« 1. ON/OFF Control. capability. Operator facilities for amending stored al data must possess the following features:- po 2. Position or Set Point Control. in 3. Periodic Control. 1. Be independent of formate and techniques used ex be For security reasons a two or three stage sequence to actually store the data in computer memory. of events is normally required to effect a single 2. Be independent of location of storage in 1. control action, with both operator and automatic computer memory. 2. checks being made at each stage. The presence of th- digital computer allows the scope and nature of such 3. Possess a high level of automatic validity checks to' be easily extended to meet additional checking. 3. requirement«. This may often allow overall system 4. Maintain a complete record of all modifications. 4. cost to be reduced by replacing low level hardware interlocks by program modules. 5. Make provision for inclusion if basic data tapes are reloaded after system shutdown. The making of an ON/OFF type control action is There are a variety ol ways all the above may be pr straightforward whereas the positioning of a valve fa regulator set point presents some alternatives. achieved. The first two iteas are achieved by using the normal operator references to all points within CO However, security consideration« dictate that an pr inching type of control ia used. This is achieved the system, plus plain englisb codes to describe the required modification. Consider the use of an input S« by th« output of a aeries of discreet commands, each is command causing the valve position to change by a typewriter to describe the temporary removal from service of pump mm&er 04 at pumping station number ad small step. Conventional practice has always relied in on the operator to generate tbe series of necessary 12. The modification command could be input on the control typewriter by typing TS CP. 12/04. The TS Tb commands i.e. one per step. This is very time of conaiaiicg and also very restricting as the control indicates temporary suspension, CP defines Control Point and 12/04 describes the actual point, at this at feature on the console is then committed for a au lengthy period. In addition the operator must point the operator may inspect what he has typed, and if satisfied operation of carriage return, line im monitor tbe valve position. However, with a computer CO at the Master Station +,hi» activity can be made •OΓ.' feed, could cause the appropriate software module to to efficient. The facilities shown in Fig.3 allow the scan the control point data files, check that a point 12/04 exists, check that it is currently in an operational state and if all check* are satisfied, The second example shows a fixed store processor establish a non-operational Barker bit in its data associated with a simple operator console. This word. This modification could then be output on a approach has some advantages, in that it provides logging typewriter along with current time to form a standby for the complete system, from the TIU inter- record and allow a final check. Clearly the check» face hut its great disadvantage is that it only that nay be Mde are endless, for example a P could provides simple operational facilities and hence IΒ have been included in the above message, indicating (be places greater reliance and burden on the operator. a pump. This could have been checked against an entry Not all failures necessarily require standby capabil- the in the control data files confirming that 12/04 was ity, for example, failure of backing storage can indeed a prop. An alternative approach, if a CRT often be tolerated by operating with a reduced system. existed in the system, would have been to call for a Under these circumstances minionmi system data and list on the CRT of all control points at station 12. only essential system programs will all reside in An arrow could have then been manipulated to point to computer core. CRT presentation and complex checking Pomp 04 and a •Temporary Suspend1 button operated. routines will be forfeited. Alternatively, in a Alternatively the point could have been selected as large system, where there is considerable backing normal on the control console and a 'Temporary store, many other possibilities emerge. Two drums Suspend' pushbutton operated. may be provided, one for system data and another for data collection. If the system data drun was to fail The above example is fairly simple, but should serve a dump of the other drum could be made onto tape and to demonstrate that data modification may be effect- it could then take over the failed drums role. Data ively carried out by the operator using only the collation would, of course, be suspended. The normal system description codes. The last item in intention in the above discussion has been solely to the above list, the automatic inclusion of temporary demonstrate that the system can be configured to data after re-load, invariably causes most difficul- prevent total shutdown in the event of a major ties and often only compromise solutions are reached. component failure. The details of standby arrange- Obviously this item is heavily dependent upon general ments depend a great deal upon the nature of the arrangements made for system standby discussed in actual system being considered. The previously the next section. One solution, although rather mentioned system of the UK CEGB (Ref.9) has a very untidy, is for each modification to cause the auto- high level of built-in standby. matic generation of a piece of data tape, on which the modification is recorded. These could then be input following the basic data tape. FUTURE TBEHDS AND DEVELOPMENTS: The growth in both size and operating complexity of the energy producing and distribution industries will undoubted- SYSTEM STANDBY! An essential feature of any ly continue. In addition mankind is becoming in- monitoring and supervisory system is that it must creasingly aware that traditional energy resources possess an extremely high degree of reliability and are finite and hence the concern for optimal exploit- that system failure must not place the network in ation will usiM greater significance. Indeed, it jeopardy. Reliability can be increased by incorpor- is already being pointed out that the independent ating redundancy in the system. There are essentially production of different forms of energy do not two ways in which redundancy Bay be included. Major necessarily represent the optimal approach to meet a components may be either completely duplicated or country's energy needs. Another activity that will alternatively major areas may be replaced by less probably increase in popularity is the exchange of powerful devices that only provide essential operat- electrical energy between countries. ing features. When deciding upon the nature and extent of such provisions the following points must All these considerations will dictate the need for be considered. continuing development and cost reduction in both hardware and software, but will also require increase 1. Expected component failure rate. in scope and power of the supervisory control system. 2. Extent to which a major component failure The change will take place chiefly in developnent will disrupt system operation. and use of Mathematical models describing both the network and the constmwr. These models will not only 3. Typical time to diagnose and repair. be used for network control but also in long range 4. Possibility and cost of providing component system planning. Such tools are in limited use redundancy standby or reduced capability. today but chiefly in an off-line mode. The natural development is for SUCK models to be available for Fig. 1 shows two possibilities, firstly the central on-line use, with suitable adaptive ability. Under processor is duplicated. In this configuration these circumstances, it will be essential to separate failure o? the on-line prccessors will result in. the low. level data gathering and control activities complete loss of system capability while the off-line and the higher level modelling and management control processor is being switched to the on-line state. activities. For this reason a. hierarchy structure Some systems have been designed where this switching of processing capability will emerge. This approach is automatic, however, care must be exercised in has been well exploited within the constraints of adopting this rather ambitious approach ax» it can the process control unit (lief.15) but it clearly has involve considerable hardware and software complexity. more greater and far reaching implications when This arrangement has a useful feature that can often considered in the context of a complete country's offset the additional processor cost, in that the energy supply problem. Such structure is very standby processor may be used for off-line work, dependent upon computer/computer data links. Indeed, such as system modelling or demand forecasting. An the possibility is that such links will expand on a immediate complexity arises in such a case with the horizontal as well as vertical basis, if only in the consideration of the standby machine having access interests of system standby. It is evident from to the on-line data for use in its off-line role. these considerations that future-possibilities oust Questions of system security naturally arise. be considered when designing the lower level of the

7 r

system. For thia reason there i« a clear requirement 13. CAMERON, A.H. and BREWSTER, P.J. - Communic- for urgent attention to be given to the standardizat- ation and Data Acquisition in a Gas Supergrid. ion of computer to coaputer data links; embracing Electronics and Power, December '69. p.427. not so-auch the nature of the media but rather the code and general operation rules (Ref.16). «.•DICKINSON, G.C. - "Soft-Hardware". The Key to Accessibility, Flexibility in Supervisory CONCLUSIONS: The main objective of this System Master Stations. article was not only to demonstrate the potential 15. ROM, J.F. - The Application of the Hierarchy advantages to be gained by use of the digital com- System to On-line Process Control. The Journal puter in teleaetry type systems, but also to high- of British Institution of Radio Eng. Vol.24 light soae of the alternative approaches and their No. 2. August '62. consequences. The dominant factor is tbat a system must be designed as a total systea where the capabil- 16. DAVIES, D.W. - Teleprocessing and Data ities of each component or« so integrated and f*n—i»nil ill inn of the Future. Electronics and exploited tbat maximum system efficiency is achieved. Power, December '71. p.464. The importance of this consideration is endorsed when the future potential is considered.

ACKNOWLEDGEMENTS: The author wishes to express his thanks to the Management of Cutler-Haamer Australia Pty. Limited for permission to publish this article.

REFERENCES: Note: All references marked * were presented at the conference "Centralised Control Systene",: The I. Institution Electrical Engineers, London, September 1971 and are published in the IEE Conference Publication No. 81. 1.* RU1PEL, D. - Equipment for Grid Automation. 2.* SMETHBi, P.S. and HARDY, R.H.D. - Computer« - Their place in Modern Industrial Teleaetry. 3,* FIELDEN, C.J. and TURTIE, D.P. - Minicomputers in Centralised Control Systems. 4.« SWOBODA, G. - General Deliberations on Inter- operation Between Telecontrol Systems and Process Computers. 5.-* WEISS, Th, - The Central and Regional Super- visory Control Systea of the Bernese Power Company Network. 6.* JENNIONS, U.S. and THOMAS, J.R. and BUTCH, R.H. and MARLOf, K.C. - Computer Based Telemetry and Control of a Water System. 7.« SAMINADEN, Y. and ANCLAUX, J.F. and NICOL, M. - Electric Power Systen Control. 8.* GUTSMANN, A. - Centralised Operation oi the Zeam Power Station* of the Tauernkraftwerke, A.G. 9.* CROOK, D.W.E. and HARRIS, J.H. - The Use of conum On-line Computers to Provide Cathode Ray Tube firs Display and Computational Assistance *t the (Ref CBGB'i national Council Centre. expe 10. CAMBtON, A.H. - Metering and Control Problems by Involved in the Supply and Distribution of dist; Natural Gcs. Measurement and Control Vol.2 neon March '68, shor thel< 11. OAHXIN, E.B. - Computational Methods of a whici Dedicated Computer System for Measurement and info Control of Paper Machines. TAPPI 24th Engin- beam eering Conference, San Francisco, Sept.'69. prob 12.« EDWARDS, R.D. - Mixed Teleaetry Systems for vapo Industrial Use. sein lenc more quen ampl RADIO-FREQUENCY MODULATION AND DEMODULATION OF LASERS FOR COMMUNICATION DJ. Cole, B.E.E.IMelb.), M.I.E.(Aust); F.i.R.E.E. Principal Research Scientist, Division of Radiophysics, C.S.I.R.O., Sydney

SUMMARY. This paper gives a brief historical survey of R.F. modulation of light beams mentioning some of the most recent experiments. Properties of lasers and their suitability for communication purposes are considered in connection with the type and method of modulation. The receiver sensitivity required is indicated by the effect of the atmosphere on propagation and the detector sensitivity. An outline is given of experimental work, currently being conducted by the author, for transmission of information over a short base- line at a radio astronomy site.

I. INTRODUCTION

The history of radio-frequency modulation as well as variable attenuation in the path of light beams extends back several decades. of the beam. The use of the potential One of the most notable early applications information-carrying-capacity of a laser was the Scophony large screen television receiver developed about 1935 in which light beam of 10 bits per second has only re- modulated by diffraction caused by R.F. cently been shown to be possible by Denton ultrasonic waves travelling in a transparent and Kinsel in their proposed 24-channel, liquid was used to reproduce the picture. 224-megabit-per-channel pulse code modulation More recently a number of surveying instru- system, the basic circuits of which have been shown to be able to be made using currently ments using modulated light beams for dis- available techniques (Ref. 5). tance measurement have been developed. Notable amongst these is the Mekometer, which employs a xenon flash tube as a light source Radio-frequency modulated laser beams and a Pockels cell modulator (Ref. 1,2). With have shown slow acceptance in competition the advent of lasers the more accurate sur- with conventional centimetre and millimetre veying instruments usually employ a radio links; nevertheless apart from the continuous-wave (CW) laser with CW radio- potentially large bandwidth of the laser frequency amplitude modulation. Distance is other advantages exist, such as the small then measured by phase comparison between transmitting radiator size, the small beam the outgoing signal and the signal returned divergence (0.1 milliradian with collimation) by a retro-reflector placed at the distant giving rise to freedom from interception and point. avoidance of ground reflections which are present in most radio links, and the fact Interest in the use of laser beams for that sidelobes are practically non-existent. communication purposes arose immediately the first laser was made by Maiman in 1960 The choice of the laser is governed by (Ref. 3) and there were some notable early the method of modulation. High power Q- experiments, including voice communication switched lasers are not useful for communica- by an R.F. amplitude modulated laser, over a tion by pulse modulation, since the repetition distance of 118 miles in 1963 using a helium- frequency is low (<100 kHz), making the duty neon laser (Ref. 4) as well as more than one cycle and information-carrying capacity low. short-distance laser television link. Never- Continuous-wave lasers have been used for theless these were only experimental systems pulse modulation with a suitable light valve, which in no way approached the potential but it is apparent that there are more ef- information-carrying-capacity of the laser ficient methods, such as mode locking. Fre- beam and which did not attempt to deal with quency modulation may take place at the laser problems such as attenuation caused by water line frequency, but it is more usual to ampli- vapour, clouds etc., and the effects of tude modulate a CW laser with a frequency scintillation caused by atmospheric'turbu- modulated R.F, carrier. The gallium- lence. Laser communication systems developed arsenide laser diode is well suited for pulse more recently have employed pulse and fre- modulation and it can also be modulated with quency modulation, both being independent of CW radio-frequency at cryogenic temperatures. amplitude fluctuations due to scintillation, It has the disadvantage of having a bean : divergence of about one radian which neces- 0.11. The helium-neon laser radiates at a sitates the use of expensive short-focal- wavelength of 0.633 um and also at 3.39 urn, length optics. where its output and efficiency are a quarter of those for the visible radiation. Demodulation of the received signal may be The red radiation is convenient for experi- by the use of a photo-multiplier or a photo- mental purposes, since it is readily trans- diode or may take the form of an optical mitted through glass lenses and its heterodyne with a laser local oscillator, attenuation in the atmosphere is only slight- using a photo diode or photo-multiplier as ly greater than that of the near infra-red. the mixer. Much more power is available from the CO2 II. PROPERTIES OF THE LASER (Ref. 6) laser with a wavelength of 10.6 um and efficiency up to 201. For sealed tubes power The laser is the optical analogue of the up to 100 W is available with a life greater maser. The medium used (e.g. a gas) is one in which three or more atomic or molecular than 103 h. Being well in the infra-Ted, energy levels exist. In the rest-state the lenses and mirrors present some difficulty, population of the lower levels exceeds that glass and quartz having to be replaced by of the higher levels, but in the pumped con- germanium, gallium arsenide etc. Both gas dition, energy put into the medium causes lasers are pumped by a D.C. electrical dis- population inversion. By having the medium charge between an anode and a cold cathode within an optically-resonant (Fabry-Perot) or a tungsten filament. cavity, radiation emitted by some of the population returning to lower levels, trig- The most promising CW laser for communica- gers the restoration of the normal population uses neodymium doped yttrium aluminium tion distribution. Under Q-switching garnet (Nd:YAG). Originally used for quite conditions, the Q of the cavity is kept low high powers of 750 W when excited from a during pumping and then is suddenly raised xenon or krypton lamp, satisfactory operation so that there is a large transfer of popula- at 100 W has been obtained with tungsten- tion to low levels and simultaneously a large iodine lamps for pumping, with life limited pulse of coherent light is produced. Under to 1000 h by the lamps. More recently it CW conditions the pumping is continuous, and has been shown possible to pump the Nd:YAG with cavity Q constant there is a continuous laser with light from a matrix of light- transfer of population from low to high and emitting diodes operating at room temperature from high to low levels, the result being a continuous light output which is not normal- with a life expectancy of 10 h. Here the ly a single frequency but a band of fre- power output is only a few milliwatts quencies occupying the frequency range over (Ref. 9). The efficiency of this laser is which the laser gain is greater than the between 0.5$ and 3* and it radiates at wave- cavity losses. This band has a spectrum of lengths of 0.532 urn and 1.064 urn. frequencies (called axial or longitudinal modes) differing from each other by C/2L, Semi-conductor junction diode lasers are where C is the velocity of light and L the capable of peak pulse power of a few tens of length of the Fabry-Perot Tesonator. The watts at room temperature, but at low tem- bandwidth of the modes is in the region of peratures (77 K) CW outputs of greater than 100 mW are obtained, while there is one case 10 Hz. The excitation of the axial modes of a pulsed gallium arsenide laser diode tends to be random and the laser output is giving 30 W average power at cryogenic tem- more noisy than necessary unless mode-locking peratures (Ref. 10). The wavelength is is used (Ref. 7). Under locked conditions dependent on temperature and doping with the modes are stabilized by introducing a elements such as phosphorus and is in the frequency C/2L into the cavity, for example region of 0.9 ym. The efficiency is be- by mechanically driving one of the resonator tween It and 40*. mirrors with a piezo transducer. Lateral modes also exist, but can be reduced to the The light power radiated from the gallium dominant mode by reducing the cavity Q for arsenide junction laser i* proportional to unwanted modes with a diaphragm, or by tilt- the junction current once- the lasing ing one of the cavity mirrors. Single threshold has been exceeded. Below the longitudinal mode lasers have been success- threshold it behaves as a light-exitting fully developed for communication purposes diode with a much broader spectral line and (Ref. 8), but also mode-locking has been a somewhat lower efficiency. Unlike the used to arrive at a high repetition rate gas and YAG lasers the beam divergence is pulse output. about a radian, making it less convenient for many applications. Amplitude modulation Continuous-wave lasers suitable for com- is obtained by adding a small time-varying munication include the helium-neon and CO. current to a steady O.C. pumping current. However, the gallium arsenide laser is much gas lasers. The former, one of the earliest more suited to pulse modulation. developed, and with a life expectancy of 10 h, was an obvious choice despite its low III. MODULATORS power 100 mW maximum and low efficiency of Some methods of pulse modulation within

10 the laser cavity have been mentioned above. IV. ELECTRO-OPTIC MODULATORS Other intra-cavity modulating techniques include R.F. pumping of a gas laser (leading to The Pockels Cell employs a suitable crys- a short tube life), pulse modulation by mode talline material which has a high coefficient locking with a light modulator within the - of bi-refringence, and is transparent at the cavity excited by a sine wave at the fre- laser wavelength. Potassium di-hydrogen quency C/2L (Ref. 11), and cavity dumping in phosphate (KDP) and potassium di-deuterium which the light energy stored in the cavity phosphate (DKDP) are amongst materials em- by virtue of its high Q is tapped at regular ployed in commercially available modulators. intervals to give pulses several times the Single crystals of lithium tantalate and CW output. barium sodium niobate which have much higher electro-optic coefficients have become avail- Apart from indirect methods of modulation able recently. Cases are recorded where such as the piezo-electric effect, and modula- LiTaO3 and LiNbO3 have been optically damaged tion by diffraction caused by ultra-sonic by the laser beam even when the power density waves in a transparent liquid, modulation has been relatively low (Ref. 12); however, outside the laser cavity is obtained by l^NNbOj,. seems to be immune to damage. electro-optic effects in liquids (Kerr Cell) and in crystals (Pockels Cell) and by Some properties of modulator crystals are magneto-optic modulators. Since the Kerr given in Table I. Cell is relatively insensitive as well as being inconvenient, the Pockels Cell will be Amplitude modulation in the Pockels Cell considered in some detail as the more prac- results from interference between the ordi- tical. Gigahertz modulation has been ob- nary ray and the extraordinary ray in a bi- tained with considerable bandwidths by both refringent crystal. If plane polarized lumped circuit and travelling wave versions light is passed through the crystal, the of the Pockels Cell (Ref. 12,13,14',15). emerging beam will, in general, be ellipti- Driving power is between 1 and 5 mW per mega- cally polarized due to the different velo- hertz of bandwidth. A tunable single side- cities of the 0 and e rays. If the crystal band electro-optic ring modulator (Ref. 16) is subjected to a varying electric field, the has been used to demonstrate frequency modula- e-ray velocity may be varied, with the result tion of a CW laser about its optical fre- that the polarization may be altered. Re- quency . moval of the plane polarized light with a

TABLE I PROPERTIES OF CRYSTALS SUITABLE FOR ELECTRO-OPTIC MODULATORS (Ref. 17,18)

f \3 v Material Symmetry* Tc £3 nl n3 r63 (K) : 33 ^n3y 13 (kV)

KH2PO4 42 m 123 21 1.51 1.47 9.7 14

KD2PO4 42 m 111 48 1.51 1.47 18 7.5 LiNbO3 3 m 1470 28 2.286 2.200 20 3.4 LiTaOj 3 m 890 43 2.176 2.180 24 2.8

Ba2NaNbs015 im 2 833 33 2.32 2.22 1.57 4 mm 0.48 Sr0.2SBa0.75Nb2°6 470 160 2.32 2.26

Symbols: Tc is the Curie temperature in kelvins £j is the dielectric constant along the optic axis n, is the refractive index along the x, axis

n3 is the refractive index along the x3 (optic) axis T63' rc are ^or *^e crvstal claaped, i.e. above all piezo resonances, -12 and are in metres x 10 per volt. V^ for axes equal. *Hernann-Mauguin symbols; see (Ref. 30). i 11

[ crossed polarizing prism results in an half-wave voltage is optical shutter. For a light ray of intensity l^, directed (5) along the optic or x, axis, and linearly polarized in the plane containing the x^ or where d is the electrode spacing and I is the length of the light path. In this case sen- x. axis,* and subjected to an electric field sitivity can be increased by making d/i along the x, axis, it may be shown for the" small. KDP group of crystals (Ref. 17,18) that the For lithium tantalate and lithium niobate emergent light intensity polarized parallel intensity modulation may be obtained with the to the incident polarization is field along the Xj axis and light directed along the x axis. Here 1/2 CD 2 ,, _ A d (6) If a quarter-wave retarder is inserted so as to delay the e-ray by n/2, then the n3 r33 " nl r13 intensity becomes where r33 and r13 are the appropriate electTO- 1/2 (2) optic tensors and have the same sign and where A is the free space wavelength of the n, is the refractive index for the path incident illumination, of the e-ray. I is the length of the optical path The modulation index is related to Vff by through the crystal, An is the change in refractive index, and m * 2J,(Ä (7) IT 3 where V is the peak applied voltage An >• -i- r63 Er (3) J, is a Bessel function of the first kind In (3) n. is the refractive index for the and the first order. O-ray, For m - 1, V - 0.383 V^. rfi, is the appropriate electro-optic tensor, and Modulators employing LiTaOj and LiNbOj E3 is the field strength along the x3 are constructed to make i/l as small as pos- or optic axis. sible. The laser beam is optically reduced Equation (2) shows a linear relationship for to pass through the crystal parallel to the small changes in An and is most commonly *2 axis, which is made long (about 10 urn), used in modulators. The voltage for a shift in phase of ir radians while the field is applied parallel to the is Xj axis, which is made small (about G.2S am), thus making Vff small. (4) Various other crystal cuts have been used 2n0 r63 in experimental modulators, but when tested at low frequencies (below piezo resonance) For the electric field parallel to the light there is some doubt as to the contribution of path V^ is not affected by the crystal dimen- the piezo-electric effect to the electro- sions so that sensitivity can only be in- optic effect (Ref. 20,21). creased by passing the light through more than one crystal with corresponding elec- In electro-optic modulators it is not un- trodes in parallel. common to arrange for the light to traverse the crystal twice (by reflection from an end face) or even more times. Some modulators A transverse electro-optic effect in KDP use a zig-zag path in the crystal and some can also be obtained with the light directed include the crystal in a resonant cavity. along A path perpendicular to the x- axis These methods of increasing sensitivity re- and at 4S* to the x2 axis with the electric sult in a reduction in bandwidth, but this may be tolerable. Where the electrode field parallel to the Xj axis. Here the length is comparable with a wavelength a travelling-wave structure may be necessary. One of the simplest ways of doing this is to *For convenience of notation x,, x, and x, include the crystal as part of the dielectric are used for the optical a, b and c axes re- of a terminated stripline. spectively. See for example (Ref. 4).

12 V. MAGNETO-OPTIC MODULATORS In a photo detector, the photo current is Comparatively little work has been done eP, in this field, and yet it appears to be an I. (8) important one. Yttrium iron garnet (YIG) has "-si an absorption of 0.3 dB/cm at wavelengths between 1.15 and 4.5 um at room temperature. where n is the ratio of free charge carriers Saturation optical rotation is 172° per to photons and is called the quantum centimetre at 1.5 urn. Having low losses efficiency (and varies between It this material is attractive for a magneto- and 701), optic modulator. Gallium doped YIG has been e is the magnitude of the charge on the used in a modulator with a bandwidth of 200 electron, MHz and a driving power in the region of one h is Planck's constant, milliwatt per megahertz (Ref. 22). Because v is the optical frequency, of the drive requiring low voltages at .fair- PL is the optical power. ly large currents the magneto-optic modulator lends itself much more conveniently to In an avalanche photo-diode or a photo transistor circuitry. multiplier this current is multiplied by the current amplification, M^ But the dark cur- VI. PHOTO-DETECTORS (Ref. 23) rent and current due to ambient light, are also amplified by the same factor. Since for The ideal detection system makes use of wide-band applications thermal noise sets a the coherent properties of the laser to limit to diode sensitivity, an optimum value eliminate noise due to incoherent light and (M t) for M can be found when the amplified to give pre-detection amplification. Such a system could employ a laser as a pre- shot noise is equal to the thermal noise. The detection amplifier or as the local oscil- optical input which will give a signal-to- lator for a heterodyne detector. Heterodyne noise ratio of unity (known as the noise detection is essential for a laser frequency- equivalent power) (Ref. 25) is then given by modulated about its line frequency (Ref. 24). Less ideal detectors are the photo-multiplier and the photo-diode. The former is suitable N.E.P. * C9) where high sensitivity up to gigahertz fre- M R quencies is required, but interference from opt eq' ambient light must be removed. Cross-field photomultipliers may be used successfully at where B is the bandwidth in hertz, several gigahertz. Semi-conductor diodes k is Boltzmann's constant, though low in sensitivity at gigahertz fre- T is the effective temperature of the quencies are useful for short-distance com- detector load, which includes the munication. If higher sensitivity is required equivalent noise input temperature of silicon and germanium avalanche pnoto-diodes subsequent amplifiers and can be in give signal-to-noise ratios nearly comparable the region of 1000 K R is the equivalent load resistance on with those of photo-multipliers over a much q wider range of wavelengths from ultra-violet the diode, taking into account the in- to infra-red. Indium antimonide photo put resistance of the following ampli- diodes are usable to 5.6 um and mercury cad- fier. . mium telluride to 15 ym; at still longer Since low N.E.P. is desirable, r\ M„„» R_„ wavelengths photo-resistive detectors may be Opt C\\ employed. may be used as a figure of merit for comparing some photo-detectors (see Table II). TABLE,II PROPERTIES OF SEMI-CONDUCTOR PHOTO DETECTORS (Ref. 23)

Photo detector Wavelength range Req Mopt Mopt Req (urn) («) Ge PIN 0.6 to 1 .6 120 1 11 Si point contact operating in avalanche mode 0.4 to 1 .0 250 10 160 Si surface barrier 0.4 to 1 .0 600 1 25 Si avalanche 0.4 to 1 .0 250 100 1600 Ge avalanche 0.6 to 1 .6 250 25 400

13 In the case of the photodiode without ava- movable telescope to the central receiver. lanche amplification the value of R is These cables have been shown to have a very high and variable attenuation as well as a limited by the bandwidth and the depletion large coefficient of electrical length with layer capacitance, so that temperature. It is also expected to use the laser link for accurate measurement of tele- 1 (10) scope spacing. The experimental arrangement 4BC employs a S mW He-Ne laser adjusted for the TM - lateral mode, but without longitudinal Since the diode sensitivity is thermal noise 00 limited, mode locking. Modulation is obtained with a commercial DKDP electro-optic modulator (V -.2.2 kV), together with a ir/2 retarding (11) ff »I« plate and crossed polarizing prism, followed These equations serve as a guide to the re- by a collimator reducing beam divergence to ceived signal power at 100t modulation re- less than 0.2 milliradian. For the receiver quired to give an acceptable signal-to-noise a 3" lens is used to focus the laser light on margin at the detector output. a silicon PIN photo diode (HPA420S)• With the beam collimated, but without the modula- VII. ATMOSPHERIC EFFECTS ON TRANSMISSION tor in place, it was found that with a separation of 900' between the transmitter and The major effect for short-distance links receiver a signal power of 1 mW was received is scintillation caused by turbulence in the by the photo-diode. With the same separa- air due to heat or wind. If a small photo- tion and the beam modulated 101 by the EOM cell is placed so that only a small part of at a frequency of 16 MHz, the signal-to-noise the light from the laser falls on it, varia- ratio with a receiver bandwidth of 0.4 MHz tions in intensity of 10 dB under average was 26 dB. A LiTaO, modulator is being de- conditions over a distance of 1000 feet may veloped and it is expected to be about 10 be observed. If the receiver optical system times more sensitive than the DKDP modulator is such that all the received light is col- and to be useful at several hundred MHz. lected by it, observations over 1000 feet have shown that the photo-detector current IX. CONCLUSIONS varies by less than 1 dB. Observations made over a distance of four miles have shown that While a fully utilized modulated laser the entire laser beam can be displaced by up communication system may.carry 10 bits per to a quarter of a milliradian (Ref. 26). The second, relatively simple systems may be made receiving optical system must therefore allow with bandwidths of tens of megahertz. The for such effects, by having sufficient col- Nd:YAG laser operating in the near infra-red lecting area. appears most suitable as a very reliable transmitter. Low voltage electro-optic Attenuation due to rain and mist depends modulators are well developed, but magneto- on prevailing weather conditions and even a optic modulators may be expected to find short-distance link would be useless in an increasing use in the future. Ninety-eight are» where heavy fogs are frequent. For per cent reliable communication over dis- Australian conditions the figures of Carroll tances of less than half a mile may be ex- and Poronnik are a valuable guide (Ref. 27). pected in many parts of Australia where fogs Their light-emitting diode link operating at are not encountered. 0.9 um over half a mile near Sydney led to the conclusion that attenuation of 6 dB or '.ess can be expected for 98* of the year, and REFERENCES 20 dB or less for 99t of the year. Attenua- tion due to rain is generally slight, but a 1. Froome, K.D. and Bradsell, R.H. - Distance heavy fog can eliminate the signal completely. Measurement by Means of a Light Ray Modu- lated at a Microwave Frequency. Jour. In the choice of operating wavelength, Sei. InstTum., Vol. 38, Dec. 1961, absorption bands due to water vapour should pp. 458-462. be considered. The visible spectrum is particularly free, but absorption occurs in the 2. Froome, K.D. and Bradsell, R.H. - A New ultra-violet and in the infra-red below 1 urn. Method for the Measurement of Distances Between 1 urn and 20 urn there are several ab- up to 5000 Ft by Means of a Modulated sorption bands, most of which are narrow. At Light Beam. Jour. Sei. Instrum., still longer wavelengths, absorption is Vol. 43, March 1966, pp. i29-133. almost continuous (Ref. 28). 3. Maiman, T.H. - Stimulated Optical Radia- VIII. EXPERIMENTS BEING CARRIED OUT tion in Ruby Masers. Nature, Vol. 187, Aug. 1960, pp. 493-494": Experiments are being conducted on a low information capacity link between the two 4. Goodwin, F.E. - A Review of Operational radio telescopes of the Parkes interferometer Laser Communication Systems. Proc. (Ref. 29), with the object of eliminating the I.E.E.E.. Vol. 58, Oct. 1970, trailing coaxial cables connecting the pp. 1746-1752. 14 5. Denton, R.T. and Kinsel, T.S. - Terminals 18. Kaminow, I.P. and Turner, E.H. - Electro- for a High Speed Optical Pulse Code Modu- Optic Light Modulators. Applied Optics. lation Communication System: Vol. 5, Oct. 1966, pp. 1612-1628. I. 224 Mbit/s single channel II. Optical multiplexing and demulti- 19. Nye, J.F. - Physical Properties of Crys- plexing tals. Oxford Univ. Press, 1960. Proc. I.E.E.E., Vol. 56, Feb. 1968, pp. 140-154. 20. Ley, J.M. - Low Voltage Light-Amplitude -Modulation. Electronics Letters. Vol. 2, 6. Geusic, J.E., Bridges, W.B. and Pankove, Jan, 1966, pp. 12-13. J.I. - Coherent Optical Sources for Com- munications. Proc. I.E.E.E., Vol. 58, 21. Hulme, K.F. - Oblique-Cut Longitudinal Oct. 1970, pp. 1419-1437. Electro-Optic Modulators. I.E.E.E. Jour, of Quantum Electronics. Vol. QE-7, 7. Smith, P.W. - Mode Locking of Lasers. June 1971, pp. 236-239. Proc. I.E.E.E., Vol. 58, Sept. 1970, pp. 1342-1357. 22. LeCraw, R.C. - Wideband Infrared Magneto- Optic Modulation. I.E.E.E. Trans. Mam., 8. Draegert, D.A. - Single-Longitudinal-Mode Vol. MAG-2, Sept. 1966, p. 394. Nd:YAG Laser. I.E.E.E. Jour, of Quantum Electronics, Vol. QE-S, June 1969, 23. Anderson, L.K. and McMurty, B.J. • High- p. 300. , Speed Photo Detectors. Proc. I.E.E.E., Vol. 54, Oct. 1966, pp. 1335-1349. 9. Allen, R.B., Dierschke, E.G., Gilbert, B.C. and Kaisty, R.W. - Room Temperature 24. De Lange, O. - Optical Heterodyne Detec- Operation of a Diode-Pumped YAG:Nd Laser. tion. I.E.E.E. Spectrum, Vol. 5, I.E.E.E. Jour, of Quantum Electronics, Oct. 1968, pp. 77-85. Vol. QE-5, June 1969, pp. 300-301. 25. Jones, R. Clark - Phenomenological 10. Eros, S. - High Average Power Ga As Description of the Response and Detecting Pulsed Laser Illuminator. I.E.E.E. Jour, Ability of Radiation Detectors. Proc. of Quantum Electronics, Vol. QE-7, I.R.E., Vol. 47, Sept. 19S9, pp. UTT" June 1971, p. 295. 1SÖ2. 11. DiDomenico, M., Geusic, J.E., Marcos, 26. Bucher, E.A., Lerner, R.M. and Niessen, H.M. and Smith, R.G. - Generation of C.W. - Some Experiments on Propagation Ultra-Short Optical Pulses by Mode- of Light Pulses Through Clouds. Proc. Locking in the Y Al G:Nd Laser. Appl. I.E.E.E., Vol. 58, Oct. 1970, pp. "156*4- Phys. Letters, Vol. 8, April 19657 1567. pp. 180-183. 27. Carroll, W, and Poronnik, K. - An Infra- 12. Kaminow, I.P. and Sharpless, W.M. - Per- red Solid State Optical Communication formance of LiTaOj and LiNbOLiNbOjj Light Modu- System. Proc. I.R.E.E. (Aust.). lators at 4 GHz. Applied Optics. Vol. 6, Vol. 31, July 1970, pp. 212-219. pp. 351-352. Feb. 1967, 28. Kopeika, N.S. and Bordogna, J. - Back- ground Noise in Optical Communication 13. Riez, R.P. and Biazzo, M.R. - Gigahertz Systems. Proc. I.E.E.E., Vol. 58, Optical Modulation. Applied Optics, Vol. 8, July 1969, pp. 1393-1396. Oct. 1970, pp. 1571-1577. 14. Chow, K.L., Comstock, R.L. and Leonard, 29. Batchelor, R.A., Cooper, B.F., Cole, D.J. and Shimmins, A.J. - The Parkes Inter- W.B. - 1.5 GHz Bandwidth Light Modulator. ferometer. Proc. I.R.E.E. (Aust.), I.E.E.E. Jour, of Quantum Electronics, Vol. QE-5, Dec. 1969, pp. 618-620. Vol. 30, Oct. 1969, pp. 305-313. 15. White, Gerard - A One Gigabit Per Second 30. Wood, Elizabeth A. - Crystals and Light. Optical PCM Communications System. Van Nostrand Momentum Book No. 5. Proc. I.E.E.E.. Vol. 58, Oct. 1970, D. Van Nostrand Inc., 1964. pp. 1779-1780. 16. Page, P. and Pursey, H. - Tunable Single Sideband Electro-Optic Ring Modulator. Opto-Electronics, Vol. 2, Jan. 1970, PP. 1-4. : 17. Fang-Shang Chen - Modulators for Optical Communications. Proc. I.E.E.E.i Vol. 58, Oct. 1970, pp. 1440-1457. [ I 15 AN INSTRUMENTATION SYSTEM FOR A NATURAL GAS PIPELINE

B.E. Richards, Fellowship Diploma of Electronic Engineering, Development Engineer, Relays Pty Ltd

I. INTRODUCTION INPUT VARIABLE FROM PIPE LINE Natural gas obtained from Bass Straight enters the pipeline system of the Gas and Fuel Corporation of Victoria at the Longford Metering Station where measurements of gas pressure, teaperature, flovrate and quality are taken. From Longford gas is trans- ported to the Dsndenong City Gate via a 108 mile 30 TRANSDUCERS inch diaaeter pipeline operating up to a maximum pressure of 1000 psig. At Dandenong the gas is metered and reduced in pressure before delivery to the Melbourne area. A lU inch pipeline from the vest of the City transports gas to Geelong consumers. The instrumentation system provides supervision over A-0 CONVERTERS Figu 300 miles of natural gas transmission pipelines. The Gas and Fuel Corporation required continuous monitoring and logging of data variables obtained from metering stations located on the pipeline at Longford, Dandenong, Ringwood, Footscray, Brooklyn and Corio. The data variables included gas pressure, tempera- TELEMETRY ture, flovrate and quality parameters. All data was TRANSMITTER to be made available at the control desk situated in the Dandenong Control Centre. An alarm system was required to advise the dispatcher of abnormal system conditions and record any abnormalities for future analysis. This paper describes aspects of the instrumentation and telemetering system supplied for V.R SIGNAL TO A.RO. LINE the continuous supervision of the pipelines and meter stations. Figure 1. Remote metering station equipment Figu II. TOE EQUIPMENT accept an input of the actual physical quantity being (ii) measured (i.e. pressure, temperature, flow, etc.) and in The installation at each remote metering 3tation provide an output of a proportional electrical quant- the consists of primary measurement transducers, analog ity (usually current or voltage) which can be con- to digital converters, the telemetry transmitter and veniently measured using electronic techniques. a power supply as shewn in figure 1. Power failure (iii dete is sensed at each station. The number of data (a) Pressure Measurement variables measured at each station is as follows: Longford (11), Dandenong (8), Ringwood (5), Brooklyn Pressure data is measured using "force-balance" (6) and Corio (6). "zer types of transducers mounted directly on the pipe- for line. These transducers provide analog direct comp The control centre equipment consists of the current outputs proportional to input pressure with teleeetry receivers, the data logger, the control a zero-displacement constant. Figure 3 shows the (b) desk display ,• local transducers and analog to digital input-output characteristic of a typical pressure converters, the alarm system and a power supply as transducer used in the system. shown in figure 2. Data is transmitted from the reaote metering stations to the control centre over chan This type of transducer offers certain advant- pipe leued A.F.O. voice frequency channels. ages: depei III. PRIMARY MEASUREMENT TRANSDUCERS curr (i) Two wires, only connect the transducer to the temp« current measuring equipment, i.e. a simple current The transducers are used as the interface betwee. press loop. tion' the pipeline and the instrumentation system. They

16 (c) Flowrate Measurement V fr SIGNALS FROM A.P 0. LINES Flowrate data is obtained from existing rotary meters whose speed is directly proportional to the gas flow through the measuring point. Tacho- 1 1 1 generators are coupled to the meters, providing TELEMETRY analog output voltages proportional to the flowrate«. RECEIVERS (d) -Measurement of Gas Quality Variables

Gas quality variables included such parameters u specific gravity, water content, hydrogen sulphide content, sulphur content and calorific value. To ALARM SYSTEM DIGITAL CLOCK provide an input to the telemetering equipment an analog voltage for each quality variable is obtained by passing a constant current through a precision slidewire whose wiper is coupled to the pen of a chart recorder. The voltage- developed across the slidewire resistance is therefore proportioual to the DATA LOGGER TYPEWRITER position of the pen. The chart recorders are driven by associated gas analysis equipment which formed H part of the initial plant at Longford Metering Station and were in service prior to the installation of the telemetering system. DISPLAY CONTROL DESK EQUIPMENT IV. ANALOG TO DIGITAL CONVERTERS Electrical analog signals (either voltages or currents) obtained from the transducers are fed to Figure 2. Control centre equipment the inputs of the a-d converters. The output of each a-d converter is in the form of a parallel, three-digit, binary coded decimal (BCD) code. The a-d converters are scaled so that the digital number OUTPUT represented by the output code corresponds directly CURRENT to the value of the input variable. This enables the codes received at the control centre to be simply decoded and displayed to represent the physical input 'ZERO ELEVATION' quantity directly without the need for further data processing. CURRENT (a) Type Used

The "Dual slope" integrating type of a-d con- INPUT PRESSURE verter operating at a rate of approximately five conversions per second is used throughout the system. Figure 3. Pressure transducer characteristic This type was selected as it offered the following advantages. ing (ii) The loop resistance is not critical and changes and in the loop resistant ? do not significantly affect (i) Good noise rejection characteristics; the output nt- the accuracy. code is proportional to the average value of the input signal occuring during the sampling interval (iii) Open circuit loop conditions are easily (approximately 100 milliseconds), effectively filter- detected. ing out most noise components of the input signal. The main disadvantage is the presence of the (ii) The principle of operation is inherently very "zero elevation" constant, i.e. the current flowing lineer when operating at comparatively slow speeds. for zero input pressure. Ibis constant must be compensated for in the monitoring equipment. (iii) Good temperature stability was obtained using straight-forward circuit design, the predominant (b) Temperature Measurement causes of temperature drift being the reference voltage source and the scaling resistors. Components Gas temperature data is obtained by detecting having low temperature coefficients suitable for changes in a resistive sensing element mounted in the these functions are readily available. pipeline. A transducer converts the temperature dependent resistance changes to an output analog The comparatively slow conversion rate is not a current. The input-output characteristic of the disadvantage as the gas variables being measured take temperature' transducer is similar to that of the a number of seconds to change value by a significant pressure transducers and also requires "zero eleva- amount. tion" compensation. (b) Zero Elevation Suppression When the code word is at logic level "1" a mo*»- lating signal gates a tone burst of the higher frequ- Removal of the zero elevation component of the ency (F.), and when the logic level is "0" a tone analog- current derived from the current loop type of burst or the lower frequency (FQ) occurs. These transducer is accomplished using a simple analog tone bursts have a duration of 12.5 milliseconds and technique. A two-input summing amplifier is used the sequence of events is illustrated in figure U. as a preamplifier to the a-d converter. One input So during the scanning cycle a string of "l" and "0" is the voltage developed across a shunt located in tones are generated corresponding to the logic levels the transducer current loop. This voltage is there- of the a-d converter output bits being scanned. Ihe fore proportional to the transducer output current. synchronizing condition is generated by allowing both The other input is a stable reference voltage of tones to be switched on simultaneously. opposite polarity set so that the preamplifier gives zero output when the transducer loop current is at (d) Transmission of V.F. Tonee the "zero elevation" value. The tones representing the "1" and "0" states are (c) Digital Code Scaling individually filtered by band-pass filters to remove unwanted harmonics that result from the modulation The full scale value of the output code is equal process. The two signals are then summed at a line to the maximum count of the BCD counter located with- transformer whose secondary is connected to a 600 oh« in the a-d converter. By resetting the converter at A.P.O. voice frequency telegraph channel. This is a number determined by the socket wiring of each shown in figure 5. plug-in a-d converter unit the output code range can be selected. Ibis also allows identical a-d converter units to be used in locations where different 0 110 output ranges are required without the need for SCANNER »calibration • OUTPUT

V. THE DATA TRAKSMISSIOH SYSTEM To convey the pipeline data from each remote metering station to the control centre a time-divi- JLJUUUUL «-V sion multiplex data transmission system is used. (a) The Scanner nn n n "»•<• A free running pulse generator (clock) operating at a frequency of 1*0 Hz provides the timing pulses to operate the scanner. These pulses are counted by a binary counter whose outputs drive an 80 - way binary to decimal decoder. The decoder output lines are used to scan each a-d converter output bit sequentially, thus generating a serial code word contain- TONE fi ing the a-d converter data. The binary counter is reset when it counts the 80th clocic pulse and the scanner then recommences the cycle. This occurs Figure k. Modulation of tones every two seconds. (e) Detection of V.F. Tones (b) Code Word Format At the control centre the tones are separated by Each serial word is framed by a synchronizing a pair of bandpass filters as shown ia figure 6. pulse which la added aa the 80th bit. The synchron- Each tone is fed to a receiver containing an auto- izing pulse is the simultaneous transmission of a "1" matic gain control (a.g.c.) stage and an amplitude and a "0" bit and marks the beginning and end of modulation (a.m.) detector to identify the presence every word. or absence of a tone. The output waveform from each receiver is identical to the corresponding A bistable multivibrator performs a parity check modulating signal at the transmitter and from these on the information bits and this is transmitted as waveforms the transmitted data is reconstructed. the 79th bit. This ensures that an even number of "1" bits are always transmitted between adjacent (f) Reconstruction of Data synchroniilng pulsti. This leaves 78 bits for transmission of information, and the transmission A binary counter is reset when the synchronizing sequence is as follows: syne, 78 information bits, condition exists, i.e. when simultaneous detection of parity, sync, etc. F1 and F. occurs. The counter then counts the number of bite received since the last synchronization (c) Modulation. pulse. An 80 - way binary to decimal decoder connected to the counter output lines gives a discrete Two oscillators with a frequency spacing of 120 output to each of a series of temporary memories, Hz are used as carriers. Their frequencies are set consisting of a pair of capacitors, in turn. On thr at the centres of a pair of standard 120 Hz voice count corresponding to a particular code word bit one frequency (v.f.) telegraph channels. of the two capacitors, which represent the "1" and "0" states respectively, charges. The charged 18 pulse. This pulse causes each bist .ble to be set to the "1" or "0" state depending on which capacitor in the associated temporary memory is charged. The OSC. fo OSC. fi discharge time constant of the capacitors is large enough to allow the first temporary memory capacitor to hold enough charge until the transfer pulse occurs at the end of the current word.

The transfer pulse occurs at the end of each received code word provided that three code checks are satisfied. These checks are made to ensure that only a valid data transfer occurs and are as follows:

(i) Total Count

The binary counter must count up to the total number of bits in the code word, i.e. 80. This checks that no bits have been lost or gained during transmission. LINE

(iii) Synchronization TO A.PO. LINE The synchronizing pulse must occur at the sane Figure 5. Basic transmitter instant as checks (i) and (ii) are satisfied. If all three checks are satisfied simultaneously, FROM A.PO. LINE the transfer pulse updates the output meaory bistables so that their states represent the valid received code that has just been received. If one (or more) of these checks is not satisfied, then the transfer pulse is inhibited. This prevents updating of -he output memories whenever codes become distor- LINE ted due to excessive mise on the line. If a valid TRANSFORMER code is not received within ten seconds, a "data fail" alarm is given to warn the dispatcher of abnormal line conditions. During periods of line outage the "last known" data is held indefinitely in the output memories until the next valid code is FMER BANOmSS FILTER received. The data transmission system described above offered certain advantages.

(i) Good noise immunity A.6.C. A.6.C. By using a dual tone technique positive detection of both "ones" and "zeroes" occurs. The system does not rely on the absence of a tone to detect a "zero". This gives the system good noise immunity.

DETECTOR "1" DETECTOR "0" (ii) Rejection of distorted codes * Hoise transients large enough to distort the cod* are of little concern as these codes are rejected. Figure 6. Basic receiver They do not result in a transfer of false information. capacitor now represents the tit being received. The next received bit will step the counter to the (iii) Good stability next traporary memory, and one of its capacitors will charge to represent the state of this bit. This No critical adjustments have to be made. process repeat» until all temporary memories are set This simplify» setting up procedures and allow» the up to states representing the received code. system to be unaffected by minor changes in operating speed, pulse lengths, frequencies, etc. The logic states represented by charges on the "1" and "0" capacitors in the temporary memories are transferred to output memory bistables by a transfer 19 (iv) Bearer sharing The binary count is decoded by a series of gates whose-outputs become the scanner output lines. Each A number of systems can be operated simultaneous- line corresponds to a particular typewriter carriage ly and independently over a common bearer. This is position and is connected to the four wire* from the done for the three Western data links (Pootscray, telemetry receivers representing the BCD value of Brooklyn, Corio). Each of these systems use a the incoming data digit that is required to fill different pair of frequencies. this particular carriage position. Each data link occupies 2U0 Hz of bandwidth which The allocation of carriage positions to data gives freedom from interference when operating at a digits is done at a patching matrix located at the transmission rate of Uo bits per second. This in- rear of the main equipment cubicle.' The matrix formation rate is suited to pipeline applications, also provides spacing and decimal points, and may be e.g. oil, gas, water, etc. varied if required to provide a different log sheet format. The two second cycle time gives a maximum data transfer delay of four seconds. This is quite As each digit, space or decimal point is scanned, acceptable for the pipeline application due to the its code appears on a common U-line data bus which slowly changing nature of the gas data variables. operates the appropriate typewriter solenoid to generate the log. VI. THE DATA LOGGER da" VII. CONTROL DESK DISPLAY (a) Format Six groups of 3-digit readout modules are mounted The telemetered data is logged by a 23 inch elec- on the control desk. Each group contains 3 "Nixie" tric typewriter located in the control room at tubes and associated decoder/driver circuits. Data Dandenong. Each data variable is typed in a partic- from each metering station is displayed on each read- ular column of the log sheet whenever the' printout out module. The dispatcher selects a data variable sequence is initiated. Column headings are pre- of interest by pressing a labelled pushbutton mounted printed on the log sheet and groups of columns are adjacent to the readout module allocated to that arranged under main headings indicating the metering variable. This generates an address code that gates station from where the data is derived. The time of the required data variable through to the readout day is entered in the first column of the log sheet. module, and illuminates the pushbutton to indicate When the printout sequence is initiated, one line of the variable currently being displaced. With this data shoving the status of the pipeline is generated. type of readout system the dispatcher can only observe one data variable from each metering station (b) Initiation at a time, but greatly reduces the control desk area used for readout. The printout sequence may be initiated in one of these ways. VIII. THE ALARM SYSTEM (i) Automatic initiation When the values of critical data variables exist outside preset specification limits an alarm is Printout occurs at a pre-dete-mined time interval. given, to advise the dispatcher of abnormal system A selector switch is provided adjacent to the type- conditions. When an alarm occurs an audible device writer to enable automatic printout intervals of 15, sounds in the control room and the data logger types 30 or 60 minutes to be selected as required. one record, showing.the alarm value in red. (ii) Manual initiation Alarms are detected using a digital comparison technique. Specification limits are preset at a A pushbutton is provided at the typewriter to patching panel located at the rear of the main equip- initiate the printout sequence whenever required by ment cubicle. The digital method of detection has the dispatcher. the advantage that no error is introduced, and the alarm settings require no calibration. (iii) Alarm initiation IX. THE DIGITAL CLOCK Printout occurs whenever an abnormal system condition exists. The alarm system continuously moni- A digital clock is. used to provide time of day tors incoming data and initiates the printout seq- information at the control desk, where hours and uence if any out-of-specification data value is det- minutes are displayed on a 4-digit readout module. ected. If an alarm condition occurs, the data The time data is also used to initiate automatic variable existing outside the preset specification printouts of the data logger, and is printed as the limits is printed in red. Alarm values Continus to first entry on each log sheet record. be printed red on subsequent printouts for as long «B the alarm condition exists. The.clock operates by frequency division of pulses derived from a crystal-locked oscillator using (c) Printout Sequence Control a series of BCD counters. Hours,' minutes and seconds are displayed on six "Nixie" tubes which are ' At the commencement of the printout sequence a driven via BCD decoders connected to the counter out- binary counter starts counting pulses derived from a put lines. The crystal locked oscillator was used pulse generator running at about 20 pulses per second. in preference to the 50 Hz mains'frequency to enable

2P operation during periods of mains failure. X. CONCLUSIONS A digital type of system is preferred to alternative methods for the pipeline instrumentation system as no error is introduced into this data variables after the analogue to digital conversion stage. The use of a secure code word format for data transmission allowed good data reception under adverse line conditions. The Method of displaying and recording the data variables is compact and efficient. The modular construction of the system simplifies future expansion. Spare wired sockets are provided to allov approximately 20? increase in the number of da4"- variables telemetered and logged. Spare rack space is provided to extend the system particularly with a view to future remote control of the pipeline values etc. Due to the use of solid state components throughout the system it is anticipated that maintenance will be minimised. ACKNOWLEDGEMENT The author extends his thanks to the Officers of the Gas and Fuel Corporation of Victoria and to the Management of Relays Pty. Ltd. for their co-operation and permission to present this paper. DATA ACQUISITION SYSTEM FOR ASTRONOMICAL PHOTOELECTRIC PHOTOMETRY

D.G. Thomas, M.I.E.Aust., A.M.I.R.E.E.Aust., Engineer II, Mount Stromlo and SidingSpring Observatories, The Australian National University

I. SOlOILRr: In the course o£ reviewing the next step in the Observatories1 Photoelectric requirements, a need arose for a system to record, by means of a Plexowriter, manually entered digital data and two analog» inputs viz. a two channel integrator. Other requirements led to the development of a system which would handle a maximum of ten words each consisting of eight characters. The system as it now stands can be set up to handle any combination of digital and analogue data inputs up to a maximum of eighty characters including "»pace" "pause" "Carriage Return" and "Line Feed" functions. The interfaces provided «ill drive a Plexowriter, an IBM Output writer, or a Facit Tape Punch. The system described, using T.T.L. Medium Scale Integration, is sufficiently flexible to meet possible future requirements. At the same time the opportunity has been taken to put forward a proposal to uptdate the design of the present standard low current integrator which is described in the latter part of this paper. II. INTRODUCTION III. MAIN FEATURES FIGl Astronomical Photoelectric Photometry is usually This system has been designed to satisfy what performed by placing one or more Photomultipliers could be considered flexible but basic requirements behind a suitable beam splitting arrangement together and these may be summarized as follows:- with suitably placed apertures and filters or other means such as reflective gratings to facilitate the 1. The system should bs simple and self meat treuent of the light intensity of a star at contained. various wavelengths or bands of wavelengths. 2. The system should be useable with all The output current(s) of the photomultiplier(s) digital or all analogue data sources or any may be measured by one of three following methods viz: combination up to a maximum of ten words of eight characters. (a) direct measurement by means of a suitable amplifier and a chart recorder; 3. A programming arrangement to control system functions viz:, operation of the cloolc and (b) a direct current integrator rogether with a or the output device, chart recorder or a analogue to digital converter and digital data recording system; 4. The system should be able to drive a number of output devices. If extra sockets are (c) a pulse counting system and a digital data provided in mounting frame, then a number of recording system. the devices may be used simultaneously e.g. tape punch and electric typewriter, the There are many arguments for or against the minimum system cycle time automatically aethod» of (b) and (c) above, which are the ones adjusting itself to the slower device. mostly used now. Harold Johnson (Ref. 1) and Andrew Young (Hef. 2) discuss this question in depth. 5. The integrators should be as simple as possible for the Astronomer to use. In general the time available for Data System to acquire digital portion of the data such as manually IV. DATA MULTIPLEXER entered data, aidereal tine, star position etc., is of tht order of ten seconds i.e. the time taken to The Data Multiplexer shown in the lower part of integrate the current or count the pulse outputs of figure 1., consists of up to ten multiplexer boards the photOBultipliers. In the case of the analogue (see Fig. 2). outputs this is determined by number of analogue outputs and the cycle time of the Digital Voltmeter in use. FIGt

22 UNES

_ WORD COUNTER RESET , <- — — — '

FIGURE 1 Showing system as set up for photoelectric photometry

SERIALIZED DATA TO DATA BUS

DIGITAL DATA INPUTS ^

8 TUNES 8'2'UNES • VUNES ^^el'8*UNESi v YIIIIIIY Yi i in lY YllllllY Tin niT DECODER AS 8 INPUT -- DEMULTIPLEXER MULTIPLEXER

CHARACTER CONTROL UNES WORD QONTROL LME

TO PRWTOR PUNCH AS RKXMCD - INSTRUCT UNE CHARACTER PROGRAMME AJ SOCKET

-*.UNE FEED -»CARRIAGE RETURN 1 -»SPACE

FIGURE 2 Digital multiplexer Bach contains an eight input four line multiplexer appropriate holes punched in channels 1 - 4 as well consisting of four eight input multiplexer packages as 5 and 6. However sign is represented as 101011 with bare collector outputs. (Ref. 3, 4). Also for positive and 101101 for the negative sign. located on each board is a four line to ten line decoder which is used to generate coranands associated - The signal on the LF bus is also used to stop with each of the eight inputs, which are routed to the clock in the Word and Character Control board. the Output Device "Punch" or "Print", "Space", "Carriage Return" (CR) and Line Feed" (LP) control VI. WORD «TO CHARACTER CONTROL lines, or to the "skip" or "pause" lines to the "Word and Character Control" board. The system relies on the Word and Character Control board for the signals to step the multiplexers The outputs of each of the multiplexers which across each of the data inputs (see Fig. 4). The scan the data inputs under the control of signals Character Control lines are comon to all of the from the "Word and Character Control" Board are multiplexers whilst the word control lines remove the connected to a data bus consisting of four lines, inhibit signal from each board, i.e. each group of which feed the 4 bit serialized data to the "Output eight inputs, in turn. Device Interface" board. The clock is formed from a aonostable multi- V. OUT HJT DEVICE INTERFACE vibrator (Ref. 5) (I/C1) and is triggered initially by the R/S Flip Plop (I/C2) being "set" by the Four different interface boards have been designed closure of the start switch. The clock pulses are to couple the various output devices may be used to maintained by signals received back from the Output the Data and Control Lines. The one shown in Figure device oa the PR Line, but if no signal is received 3 is designed to drive a Facit Punch in ASCII code. back from the Output Device the skip "line" nay be It should be noted that only the codes for the, prograoMtd, by means of the diodes on the multiplexer numerals 0-9, polarity, space, and Carriage Return board, to enable a second monpetab'le (l/C5) on the and Line Feed are used. Word and Character Control Board to maintain clock operation. This skip mode can be maintained as long One feature of. this particular board is the use as is required by system requirements. of the bare collector decoder to detect the BCD digits greater than 1001 in order to inhibit channel Provision is made for the insertion of a pause 5 of the punch control. Reference to ASCII code will comaand by aeans of a diode on the multiplexer board show that digits 0-9 are represented by the *hich inhibits the clock mono3tab1« (l/0i), and so holds the prograaae until a signal such as a "Read" Command is received i*roo thP timer controlling a current integrator or sealer.

OOCK>

PUNCH INSTRUCT

PUNCH flEADT

PIOiJRB 5 Facit ?umh Driver (A3

24 RAUSE

Smc SKIP MONOSTABLE rs < OUTPUT DEVICE READY

< RESET WORD CTR TO 0

5m« OCTAL DECADE 4-10 UNE WORD MONOSTABLt DECODER SELECT COUNTER 124 COUNTER »LINES STROBE PRESET WORD COUNTER CHARACTER SELECT -*- CLOCK SIGNAL M0N06TA8LE < SYSTEM RESET

SYSTEM READY «*-

FIGURE 4 Word and Character Control

PWNTOR PUNCH PWNT OR PUNCH UNE •*- COMMAND FROM DV.M. WORD SELECT UNE ON LAST DIGITAL RD BOARD. JL 4 LINES TO -—_•# «L 0 Q ^>0.V.M. READ WORD COUNTER« • *D"TYP£ FUP FLOP [PRESET] C

-e CLOCK

firm iiiii it. __..,_ zzz^^ ANALOGUE

JUMPER REQURED UNES MOS ANALOGUE SWITCH V ]|- TTTTTTTfTT WORD UNES 0-9 nriTirnf/i zzzzzzzzzz z z z z z! ANALOGUE INPUTS

FIGURE 5_ Analogue Multiplexer

25 VII. ANALOGUE DATA MULTIPLEXER An alternative application of the system is recording one word of manually entered data and the Four functions are performed by the Analogue second word, in this case the output of a digital Data Multiplexer Board (see Pig. 5) vizi frequency meter, which is cycled one or more timee. The frequency, meter in this application is driven by (a) analogue multiplexer which can acan up to a voltage to frequency converter. ten analogue inputs under the control of one or more of the word control lines and feed IX. CONSTRUCTION each to a Digital Voltmeter; The frame is arranged so that the circuit boards (b) a ten input gate to sombine any of the word plug into the sockets situated behind the front panel control signals and feed it resulting from the back of the unit. These sockets carry the signal to the last Digital Multiplexer l/C power supply bus, the control lines and the four board. This Digital Multiplexer scans the line serialized data bus. Mounted at the other end outputs of the Digital Voltmeter or Counter; of each circuit board are sockets for the cabling to equipment external to the unit. This arrangement (c) a "D" type flip flop which produces a does not unduly complicate the circuit layout but clocked Read Command pulse for the Digital does reduce the amount of internal wiring. For a Voltmeter (D.V.1I.) and is Reset by the particular task only che required number of boards "Print or Punch" Instruct from vhe D.V.M. need to be inserted in the frame. which is also fed to the. Output device; Space has been allocated in the frame for boards (d) a "Preset to H" pulse to the (ford Counter on to perform level changing and logic inversion the Control Board. functions in order to utilise the older but still serviceable counters and Digital Voltmeters which VIII. OPERATION seem to become obsolete in ever decreasing periods of time. The system as described is capable of being used in «any different modes. One such mode would be to X. AUTOMATIC RANGS CHANGING INTEGRATOR record say, with a Flexowriter, the following blocks of digital data viz: The general form of the Integrator used at the Observatory is shown in Figure 6 and is based on (i) star serial number entered by means of a circuits described at the I.H.E.E. Aust. Radio and thumb wheel switch; Electronic Engineering Convention in Uay 1963 (Ref. 6) see also Refs. 7-10. (ii) sidereal time, It consists of a transistorized d.c. amplifier (iii) telescope right ascension; with an input stage consisting of push pull electrometer tubes. A digital timer using integrated (iv) telescope declination; circuits provides the signals for operating the relays "A" and "B" in the correct mode. (v) photometer filter settings and aperture settings; Leakage of the charge from the capacitor C is due to: and say four integrator outputs. (a) leakage due to moisture affecting the The system would operate in the following manner ceramic capacitor range changing switch; see Fig. 1:- On receipt of a start command, the integrator timer coaaeneee its timing period. (b) leakage across the insulation of the B Curing this interval which nay be between 10 and 100 relay contact; seconds, the clock starts and the data multiplexers scan the digital data, at the epeeds to suit the (c) leakage across the A relay contact; Flexowriter»this will take about 5 seconds. A , "Pauae" signal stops the system clock which . , (d) leakages from the amplifier input to ground. recommences on receipt of a"Read Command"signal fron the Integrator Timer. —9 10 If C » 10 fds. then,we can measure 10 amps. in 100 sec. Suppose we wish to keep the leakage to The Digital Multiplexer which is connected to 1 the D.V.ll. then scan the outputs of the D.V.M. and at one thousandth of this value'say 10" 5amps., then the aaae time the D.V.ll. input is connected to each the relay contact leakage resistance would need to be 1014ohme. cf the integrator outputs by means of the analogue switch. A "read"' cn—ind being given to the D.V.M, j The leakage requirements across the "B" relay at correct timee in the cycle. At the end of the (Fig. 6) can be relaxed by usi-ig the circuit of D.V.M. read cycle the "Print or Punch" line is Figure 7. . ,._ driven to ground and thus activates the Output Device, When the "B" relay is closed the resistor R is When digital data its being recorded the system connected between the amplifier output and ground in cycle time is the clock pulse width (5 milliseconds) the Reset mode and tbu3 the situation is the same as plus ihe cycle tiM of the Output device for the in Figure 6. However when the relay is open and Flexowriter (106 milliseconds) i.e. 115 milliseconds. integration of the input current 'i? in progress, then In the case of Analogue data, the cycle time of the contact B1 is connected between the amplifier output Digital Voltmeter is added to the above figure. 26 FIGURE 6 SiapUfied Integrator FIGURE 7 Showing method of reducing relay contact leakage

C3 LOGIC FOR AtB CONTROL AtBMOS SIGNALS FROM II-VTV TO RANGE -* ••• SWITCHES TIMER MOS SWITCHES 1 I C2 TTL-MOS DRIVERS

M SHIFT < ——i—r*n—r—i REGISTER

AI B1 82 PRESET INPUT ' INTERGRATOR "HI OUTPUT

t RANGE OECADE AMPUFKR DATA •IOV- COUNTER WITH MOSFET REF (BCD) INPUT COMPARATOR

FIGURE 8 Showing nain features of Automatic Range Changing Integrator

27 (10 volts) and ground,R being about 10 ohms and XIII. REFERENCES does not contribute to the leakage, but contact B2 is AP now connected between the amplifier.input and ground. 1. JOHNSON H.L, 1964 Stars and Stellar Systems 2. If the-amplifier has a gain of 1Cr then the. voltage Ed. Hiltner W.A. (Chicago, Univ.-of Chioago Press,) across the relay contact B2 is 1 millivolt. So that P 174. the relay leakage resistance need only be 10"^ times the value required in Figure 6. 2. YOUNG A.T. 1969 ''Photometric Analysis: IX Optimum Ose of Photomultipliers", Applied Optics If we use a number of MOS analogue switches Vol.-8, No. 12, pp 2431 - 2447. (Ref. 11) which have leakage resistances of 1011 ohms and employ the principles outlined above, we can 3. "Designing with tu.S.I." 1969 Signetics eliminate the range changing- switch and the B relay Corporation Sunnyvale California. leakage thus reducing the two main causes of leakage. 4. "T.T.L. Multiplexers and Demultiplexers", .. In our integrators, leakages at the amplifier Application Note AH-37, 1970, National Semiconductor input are very small. In the updated version, the Corporation Santa Clara California. amplifier would consist of a Linear integrated circuit operational amplifier with push pull insul- 5. »T.T.L. One Shot SN74121" Texas Instruments ated gate Field Effect Transistors these having a ' Application Report Bulletin C.A. 128 Texas typical leakages of 10~14 amps. (Ref. 12). Instruments Inc. Dallas Texas. SUN An integrator circuit using1 these principles is 6. THOMAS D.G. »Transistor Circuitry for wei shown in Figure 8. The voltage comparator gives a Astronomical Photoelectric Photometry", Proc. pulse output when the amplifier output exceeds say I.R.E.E. Aust. Vol. 27 Ho. 1 Jan. 1966 pp 19-21. Thi 10V and switches in the next capacitor until the CO« integration process is terminated by the timer. This 7. LITTAEUR R. "Ion Current Integrator", Rev. Sei. is done by means of a serial in parallel oat shift Insts., Vol. 25, 1954, pp 148-152. INT register and some logic gates. The counter BCD output shows which capacitance ranges, are in use. 8. GARDINER A.J. and JOHNSON H.L. "Integrator 1» < Type D.C. Amplifier. Rev, Sei. Insts. Vol. 26 The: A3 the right terminal of each capacitor is pp 1145 - 1147. returned to the amplifier the worst case for the for leakage would be eight analog switches in parallel 9. WARREN J.B. and ARGYLE P.E. "The Integrating therefore maximum leakage due to the equivalent of Exposure of the Dominion Astrophysical Observatory" the "B" relay of Figure 5» and the range changing Publication of the Dominion Astrophysical Observatory and switches is Vol.10 No. 4 March 1956 pp 323 - 330. and Input voltage of amplifier x 8 .11 10. WEITBRECHT R.H. "Current Integrator for 10 and Astronomical Photoelectric Photometry" Rev. Sei. waa Irists. 1957 Vol. 28 pp 883 - 888 x 8 11. "MOS Analog Switches" Application Note AN-38 -14 May 1970 National Semiconductor Corporation Santa ate = 8 x 10 amps Clara California. - , :.•• fac vat which is small, compared to the 1C.-10 amps we wish to measure. 12,. Data Sheets for HDIG 1886 and HDIG 8551 MOS Division, Hughes Aircraft Company, Newport Beach, California. the XI. CONCLUSION- .. ' •ua pro 13. Product Guide, R.C.A.' MOB Field Effect : The system as described is simple, low in cost Transistors, MOS-160., Radio Corporation of America, and satisfies the requirements for' which it was Harrison, N.J. originally designed. The possible variations in the aeq mode of operation and the ease with which it can 14. "PET Differential Amplifier" Application Note to i drive various output devices make it well suited for du AN-251, Motorola Semiconductor Products Inc., the foreseeable data acquisition requirements of tbel the Observatories. Phoenix, Arizona...... -.,.,..•-*.-... . 15. "FIELD EFFECT TRANSISTORS IN CHOPPER AND XII. ACKNOWLEDGEMENTS ANALOG SWITCHING CIRCUITS« APPLICATION NOTE- AN-220, Motorola Semiconductor Products Inc., Phoenix, I wish to thank Mr, J.S. Cocmbs of the John Arizona. Curtin School of Medical Research, for stimulating discussions over many years on low current and voltage measurements, also Mr. M, Gamlin of the Research School of Physical Sciences,

The helpful comments of Mr. W.M. Ruting are gratefully appreciated.

baa| £0 APPLICATION OF SOLID STATE LOGIC IN A ZINC STACKING LINE

Thomas Michael Fulton, B.E.(Elect.) Uni. of N.S.W., Graduate I.E.Aust, Graduate I.E.E. London Electrical Engineer, E.Z. Co. of A/asia Ltd, Risdon, Tasmania

;or

SUMMARY Zinc slabs emerging from a casting machine are mechanically stacked in readiness for automatic weighing and strapping.

| This paper describes how a solid state logic system controls the stacking process and examines the solid state components used.

INTRODUCTIOK

More than half of all zinc handled in the plant is cast in rectangular slabs of 60 pounds each. These are assembled In stacks of 40 in preparation for weighing and strapping.

The prime factors in selection of stack size and method-of-assembly were customer requirements 3.5.7.9 ft 11« LAYERS and transportability.

Considerable time was spent on investigation and development before the final stack configuration was decided on.

The configuration finally arrived at incorporates many advantages such as strength, compactness, 4.6.6ft 10* LAYERS facility of strapping, ease of handling and effective water run off.

Counterbalancing these advantages however, was the complex arrangement of slabs in the stack. This must have tested the ingenuity of designers when the proposal for mechanical stacking was first-advanced.

The stacking machine (Stacker^,which they subsequently built, and which is described in the pages to follow, is an equipment, the operations of which, duplicate those of the men who formerly assembled the stack manually. Figure 1. Composition of the Stack

I THE STACK . Alternate layers above the base layers contain slabs The arrangement of slabs in the stack can be which have been rotated through 90 degrees to enhance seen by reference to Figure V below. the Strength of the stack.

(a) Factors in Stack Design ' .; Slabs in the first and third.layers are inverted to prevent sharp edges from bearing on the steel The stack is composed of 40 slabs made up d£ strappings. two base layers or two slabs *ach, and nine top layers of four slabs each. (b) Stack Formation

The placement of the two pallet layers at the A trolley unit is driven by three pneumatic base allows room for the insertion of the prongs of a cylinders designated "A". "B" and."C". It moves fork-lift truck. from its de-energised position to any of three dump positions labeled "a", "b" and "c".

29 To go to position "a" cylinder "A" must be energised. after which the trolley traverses, still rotated ,to position "a", where the second slab Is released. To go to position "b" cylinders "A" and "B" The trolley then returns to the fully-back position must be energised. at the same time restoring to its unrotated state.

To go to position "c" cylinders "A" and "B" and "C" All slabs of the stack are inverted except the muse be energised. six of layers 1 and 3. Hence these slabs are assembled a different-way-up from the other 34 slabs The trolley grips are designed to lift two slabs of the stack. at a time and dump them either together in one position or separately In two positions. Each pair of II STACKER EQUIPMENT (Refer to Figure 3) slabs placed on the stack by the grips is termed a "dump". All Stacker items shown in Figure 2 can be classified in two groups, namely "control equipment" and "controlled equipment". Aα implied the'fcontroi As the first and second layers contain two slabs 1 each only one dump is necessary to fill each of these equipment! is equipment designed to direct and co- layers. ordinate the Stacker, whereas the "controlled equipment" is the mechanical or electrical equipment For layers 3, 5, 7, 9 and 11 two duaps per which carries out the control instructions. layer are necessary. As all slabs in three layers lie in the same direction no rotation of the grips Stacker "control equipment" consists of « logic is necessary. cubicle, a swltchgear cub'cle and a Console for indication and manual operation. Layers 4 , 6,8 and 10 also require two dumps per layer. However, before making the first dump Stacker "controlled equipment!'is listed below:- of each of these layers, the trolley rotates through 90 degrees. At position "c" one slab is deposited,

A+S+C

A+S

bee

-Figure Z Arrangement of Stacker Equipment

30 Turnover Unit (a) Turnover Unit Trolley Unit Stack Aligning Frame This is a pneumatically-driven machine which Magazine Conveyor Table, and inverts all slabs for the stack except those of Magazine Conveyor layers 1 and 3. Refer to Figure 4 for a description of its control. It is appropriate to refer to the function of the Feed Conveyor (FC) here. Although not controlled by the Stacker logic this fixed-speed belt provides a transport link for slabs from the Cooling Conveyor, through the Turnover Unit, to the Trolley Unit.

Before proceeding with descriptions of individ- — MEM ual Stacker controls an introduction should be given to the circuit which is common to nearly all these controls, namely, the memory circuit. T/o] Ill THE MEMORY CIRCUIT CTR-J This simple circuit, drawn in Figure 3 consists of two logic switching units called NOR units, each with its output fed back to the input of the r. RSTE other. Hi

INPUT T/0 COMPLETE DET.

OUTPUT Figure 4. Operation of the Turnover Unit Control A slab detector (DET) senses each slab before it reaches the turnover ur.it, registering the slab count on the TURNOVER COUNTER (T/0 CTR).

The decoded output of the COUNTER will not affect the SLAB DETECTOR signal at the MIXER (MIX) Figure 3. The Memory Circuit except for slabs 1, 2, 5, 6, 7 and 8 (i.e. the slabs of layers 1 and 3).

When the a.c. supply is switched on an "OFF" For slabs other than Nos.l, 2, 5, 6, 7, or 8 pulse, generated in logic d.c. supply, operates NOR the MIXER output seta the MEMORY and, after a time 2, ensuring that the OUTPUT of the memory is delay introduced by TIMER (T) (to allow the slab to initially set to zero (logic "0"). travel between the SLAB DETECTOR and the T/0 machine) the T/0 solenoid (T/0 SOL) is energised, and the slab A logic "1" INPUT to the memory changes the inverted. output of N0R1 to "0". All inputs to N0R2 are then at "0" causing the circuit OUTPUT to switch to "1". When the inverting discs of the machine have nearly conipieted their rotation the T/0 COMPLETE This OUTPUT remains at "1" indefinitely, even DETECTOR (SET) produces a signal to reset the MEMORY. if the INPUT changes (as feedback from the OUTPUT This immediately de-energiiei the T/0 solenoid, re- holds MORI output at "0"). Only when the memory turning the discs to their normal position. RESET signal is applied does it revert to "0", restoring the circuit to its original state. For slabs 1, 2, 5, 6, 7 and 8 the decoded output of the T/0 counter cancels the SLAB DETECTOR IV INDIVIDUAL STACKER CONTROLS (Ref.l) signal at the MIX2R, preventing the MEMORY from being set, and so inhibiting the operation of the T/0 As the arrangement of Stacker equipment has SOLENOID. been looked"at, and the basic operation of the memory circuit described, individual Stacker After the 40th slab has been counted a decoded controls will now be examined in the following signal RST II resets the T/0 COUNTER, preparing it order :- for the next count. Turnover Unit Trolley Unit . (b) The Trolley Unit Stack Aligning Frame Magazine Conveyor Table The function of thi« pneumatically-driven unit and Magazine Conveyor is to grip the slabs as they arrive from the FEED CONVEYOR and place them in their correct stack position. 31 The three basic motions of the Trolley Unit are In the more involved operation of grips open for the "grip close and open" motion, the "grip rotate rotated dumps, the trolley reaches poslon "c" and and return" notion and the "trolley traverse" motion. a combination of signals from the TROLLEY POSITION DETECTOR and the LAYER AND DUMP COUNTERS (L & D CTRS) The "grip close and open" motion and the produce a MIXER output which RISETS MEMORY 1 only.. "trolley traverse" motion take place in the dumps of GRIP 1 SOLENOID de-energises, and the first slab Is all layers, whereas the "grip rotate and return" deposited at "c". The trolley traverses to position notion occurs only in the first dumps of layers 4, 6» "a", where TROLLEY POSITION DETECTOR "a" transmits a 8 and 10. signal to RESET MEMORY 2. GRIP 2 SOLENOID de- GC energises, and the second slab Is depaoaited at "a". Refer to Figure 5 for details of the "grip close and open" motion. .Up to this point in the description of the trolley unit operation the grip rotate and return motion has only been referred to in passing. It is now described in more detail by reference to Fig.6. CRIP I SOL GRII SLAB rH LS3 OKI GRIP XT L&D TRS]

Figure GOA

Figure 6. Trolley Grips Rotate and Return Control

To achieve the desired stack configuration th« trolley grips must rotate on the first dumps of layers 4, 6, 8 and 10. For these dumps the LAYER AND DUMP COUNTERS (L&D CTRS) produce a signal to GC Figure 5. Operation of the Grip Close & Open Control set the MEMORY as soon as the trolley commences to move out (i.e., as soon as the L§3 inhibit signal removed). The MEMORY (MEM) energises the TIMER (7 The grip close operation is the same for all which, after a delay (inserted to allow the trolle L1C slabs, that is, it is independent of the position to traverse clear of obstacles) will operate the of the (lab in the stack. This operation is as GRIP ROTATE SOLENOID (GRIP ROT SOL) to rotate the follow» :• grip« through 90 degrees.

After the SLAB DETECTOR (SLAB DET) registers a The return rotate operation is triggered by second sUb in the SLAB COUNTER (CTR), and after a signal, GOA, generated only after both grips have short delay introduced by the TIMER, T (to allow it opened. This signal RESETS the MEMORY,, de- to COM alongside the first slab), MEMORIES 1 and 2 energising the GRIP ROTATE SOLENOID andreturning Figure f (MEM 1 & 2) are set by the TIMER signal. Both GRIP grip* to theii unrotated state. SOLENOIDS (GRIP 1 SOL AND GRIP 2 SOL), are energised and the slabs are lifted clear of the FEED CONVEYOR. The "grip close and open" and the "grip rotate and return" motions have MO far been des« Whereas the grip close circuit operation is ed. Now consider the trolley traverse motions bj DETEC independent of vhlch pair of slabs are handled the reference to the three traverse operating circuit (MIX 21 grips open circuit is not. Two distinctly different of Figures 7, 8 and 9 below. MEMORlj grips opening routines take place, namely that for the rotated firstdumps of layers 4, 6, 8 and 10, The generation of two basic signals in the and that for the unrotated dumps of these layers and verse motion, namely.the 52 signal and the GOA i . traverj all other layer*. will first be examined with reference to Figure* folly and 9, respectively. Take firstly the simpler case of the grips open A| operation for unrotated dtatps, and again refer to The closure of both grips actuates both GRIP CO Figure 5. The trolley having completed its traverse CLOSED DETECTORS (DET) of Figure 9. Outputs froe descrif will arrive at position "a" or "b" or "c". The these DETECTORS result in a signal, 55 from MI} relevant TROLLEY POSITION DETECTOR (TROLLEY POS DET) (MIX 1). This signal is connected to the trcve- will signal to RESET MEMORIES 1 and 2, and both solenoid circuits of all three solenoids A, B a' energil grips will release. solenol DUMP 32 or DUMP has been reached In Che stack construction :-

SOL A for traverse out to position "a" S) SOLS A and B for traverse out to position "b" SOLS A and B and C for traverse out to position "c"

TRAV

LID CTRSJ-

GAIP I» OPEN DET 2

LS* GOA

Figure 7. Operation of Traverse Solenoid A

Figure 9. Operation of Traverse Solenoid C

The grips open operation was not as simple as the grips close operation. Similarly, the traverse return operation is not as simple as the traverse-out operation. As with the grips open operation two distinctly different traverse return routines take GC place, namely, that for the rotated first dumps of layers 4, 6, 8 and 10, and that for the unrotated dumps of all other layers.

Take firstly, the simpler case of traverse return operation for unrotated dumps, Here both grips have opened simultaneously to deposit the slabs In position "a" or "b" or "c". The GOA signal resulting from the two grips opening RESETS the MEMORIES of all three TRAVERSE SOLENOID circuits. The three SOLENOIDS are thus de-energised and the trolley returns to its fully-back position.

Figure 8. Operation of Traverse Solenoid B ta the more involved case of traverse return for the rotated first dumps of layers 4, 6, 8 and 10 all the TRAVERSE SOLENOIDS would have been energised by Opening of both grips actuates both GRIP OPEN the 58 signal. On reaching position "c" only GRIP 1 DETECTORS (DET) of Figure 7. Outputs fron these opens ^ previously described). The opening of GRIP DETECTORS result in a signal, GOA, fro« MIXER 2 1 produces a signal, E5$, (Figure 7) which RESETS Che (MIX 2). GOA acts as a RESETTING signal for the MEMORIES of TRAVERSE SOLENOIDS B and C, but not A. MEMORIES of all three traverse circuits. The trolley is driven pneumatically to position "a", where GRIP 2 opens. Both grips are then open and the Now, to commence descriptions of the trolley GOA signal which results RESETS the MEMORY of Figure traverse operations, start with the trolley unit 7. de-energising TRAVERSE SOLENOID A. Th« trolley fully-back, and the trolley grips open. returns to its fully-back position.

After the counting of a second slab by the SLAB This section on the trolley unit operation would COUNTER of Figure 5, both grips close (a> previously not be complete without some reference to the described). functions of the LAYER and DUMP COUNTERS.

The <5S signal generated by the grips closing can The LAYER COUNTER is a 4-blt binary counter energise the following combinations of traverse which counts the number of layers as the stack is solenoids, depending on inhibits from the LAYER AND being built. The counter counts one on every DUMP COUNTERS, depending in turn, on which LAYER AMD descending increment of the magazine conveyor table. 33 Decoded output signals from the LAYER COUNTER, slabs for the next stack. in conjunction with signals from the DUMP COUNTER, determine the destination of the dumps by controlling The table lower operation is described with the trolley traverse solenoids A, B and C and the reference to Figure 11. grips release operation. Decoded output signals are also used for indication and various inhibits in the logic circuitry. C The DUMP COUNTER is a 1-bit binary counter used DE to count the number of dumps in any layer. In addition to controlling the traverse solenoids with the LAYER COUNTER the DUMP COUNTER provides inhibiting signals for various parts of the logic circuitry. (c) Stack Aligning Frame This frame corrects irregularities in the stack formation ensuring that the stack will be sufficiently compact for strapping. FDLll CM C START) Stack layers 1 to 8 are aligned in the progressive lowering of the magazine conveyor table (which Figur« supports the partly-built stack) through the frame.

Stack layers 9 to 11 are aligned by the pneu- 1 matic lifting of this frame after completion of the FRAME stack. DOWN MIX now d< 2 (TU INHIBIT) OET 1 The operating sequence for lifting the frame is _». FFO clear described by reference to Figure 10. CLEAR] (MCMAN.INHIBIT) MEM0R1 A signal, Lll (indicating that the last layer ACTÜA1 of the stack has been completed) triggers off the lift operation by setting the MEMORY (MEM) and energising the FRAME LIFT SOLENOID (SOL). UP LB Figure 10. Frame Lift Control Circuit ACTUA The MEMORY output also produces a signal, FDLll rece: from MIXER 1 to start the MAGAZINE CONVEYOR, providing that 1, the FRAME UP DETECTOR signals that the (e) frame is fully-up, and 2, the FD-input signals that the MAGAZINE CONVEYOR TABLE is fully-down. TD ACTUATOR When the MAGAZINE CONVEYOR has removed the com- CONTACTOR Its pleted stack a signal CT resets the MEMORY returning LC the the frame to its fully-down position. The FTB interlock signal from the FRAME DOWN DETECTOR i< uaed to ensure that the trolley will not posit traverse (to start a new stack) until the frame it dell; fully down.

•Hi'! The FFD interlock signal, also from the FRAME is gi DOWN DETECTOR, is used to ensure that the MAGAZINE CONVEYOR will not be manually started when the frame is fully-down. (d) Magazine Conveyor Table The function of this electrically-powered table Figure 11. Magazine Conveyor Table Lower Control is to provide a base of controlled height for the stack during its construction. Layer complete signal, LC, aeta the MEMORY to The table commences in the fully-up position and generate an output signal, TD. TD energises the lowers automatically with the formation of each TABLE DOWN ACTUATOR CONTACTOR and simultaneously layer. •tarts the TIMER (T). After timing out the TIMER signal RESETS the MEMORY to stop the ACTUATOR. Finally it deposits the stack on the MAGAZINE CONVEYOR. The TABLE DOWN LIMIT SWITCH (TDLS) maintains the MEMORY in a RESET condition,- so inhibiting the When the completed stack has been removed by ACTUATOR operation after the table has bottomed. the MAGAZINE CONVEYOR the table rises to accept Figui 34 Signal FDL1X, received on completion of a stack, sets the MEMORY (MEM) to energise the MAGAZINE CON- VEYOR SOLENOID (MC SOL). TU ACTUATOR Near the end of the magazine conveyor stroke MC POSITION DETECTOR 1, and then 2, operate, RESETTING CONTACT» the MEMORY and stopping the conveyor. CTn L 1 h MEM DET — Interlock signal, PFD froe the stack aligning frame circuit, prevents manual starting of the conveyor when the frame is down, by maintaining the MEMORY In a RESET state.

V TYPICAL STACKING SEQUENCES TFU RESET Two stacking sequences are now described, namely those for slabs 9 and 10 and for slabs 39 and 40.

(a) Stacking Sequence for Slab's 9 and 10

1. both slabs inverted by the turnover unit. Figure 12. Magazine Conveyor Table Raise Control 2. 9 counted by the slab counter before reaching the ingot stop below the trolley grips. The Magazine Conveyor table raise operation is now described with reference to Figure 12. 3. 10 also counted, and moves alongside 9.

When the completed atack has moved sufficiently 4. 9 and 10 gripped and lifted clear of the clear of Che unloaded magazine conveyor table the Feed Conveyor. CLEARED TABLE DETECTOR (CT DET) pulses to set the MEMORY, which in turn, energises the TABLE UP (TU) 5. three traverse cylinders and the rotate ACTUATOR CONTACTOR. cylinder operate to simultaneously traverse and rotate the grips. On reaching the required height the TABLE FULLY UP LIMIT SWITCH (TFULS) opens to de-energise the 6. rotated grips arrive at position "c"; grip 1 ACTUATOR, leaving the table raised and ready to opens to deposit 9. receive the first slabs of a new stack. 7. two traverse cylinders de-energise; the (e) Magazine Conveyor trolley, still rotated, returns to position "a".

This is the output unit of the Stacker equip- 8. grips open to deposit 10. ment. It 1* a pneumatically-powered conveyor and its function, in relation to «tacking, i* to convey 9. trcjley return-rotates, traversing to the fully- the completed stack «way leaving rooa for the back position as it does so. comaencement of a new atack. (b) Stacking Sequence for Slabs 39 and 40. Further along the process line it operate« to position the stack for weighing, «trapping and Operations 1 to 4 are the same as for slabs 9 and 10. delivery to a fork-lift truck. 5. three traverse cylinders operate, and the grip« A description of the Magazine Conveyor control creverse to position "c". i« given below by reference to Figure 13. 6. both grips open depositing 39 and 40 at "c".

7. trolley traver»es to the fully-back position, the stack having been completed. FDLII MC FFD 8. frame lifts to align the' top three layers of the stack.

9. magazine conveyor operates to remove the stack.

10. when stack is clear magazine conveyor table ascends and frame descends. MC POSITION 11. when frame is fully down trolley can traverse DET with slabs 1 and 2 for the next stack.

Figure 13, The Magazine Conveyor Control Circuit 35 Vi COMPONENTS OF CONTROL EQUIPMENT a b (a) General s

All stacker control equipment may be classified T in three groups, namely, input equipment, output VANE + 20V OC r equipment and logic equipment. This classification a is made below. I I Input equipment n d Proximity switches, limit switches, pushbuttons, KNsmvm ADJ. filter paks and signal converters. 4

Output equipment. T p O.C. amplifiers, a.c. amplifiers, solenoid valves, b a.c. contactors and indicating lights. a CD w*re* c Logic equipment. (MH WWW CCMTHOL OMU) V T NOR units, OR units, time delay units, transfer units couuft cooo m/m (KM UWSONLV) r and decade counters. T d Figure 14 shows typical units belonging to each of a these groups. d t All equipment In the three groups with the ex- r ception of the solenoid valves belongs to the Square 0 Norpak range. F c Components of this range make use of transistor, u thyristor and diode switching, and are rated for ii operation In ambient temperatures of 0 C to 60 C, without any correction factor.

Logic units of the range are encapsulated in moulded cases filled with resin, rendering them imune from normal environmental damage.

Connection between units is accomplished by 23/.0076 wire with special crimped tapered pins at each end. An Insertion tool is used to ensure that the pins a?e fitted with the proper force. The resulting connection is capable of withstanding vibration and other forces for an indefinite time. Removal of the connection is accomplished by rotating the insertion tool, thus turning and loosening the pin.

Standard logic voltages in the Norpak range are -20/0/+20 volts d.c., - 5Z, obtained from a power unit designed for the range. The -20 volts Is the standard logic "1" signal, while the +20 volts is Figure 14. Typical Input, Logic and Output Devices used mainly for biaasing.

With involved controls of many different opera- The Standard NOR Unit - Refer to Figure 14 for an in- tions, such as the one described herein, space side view of this unit. It makes use of a transistor becomes a consideration. Here the Norpak range switch and accommodates three normal and one special provides dual advantages of compactness of wafers Input. The Pak is available in standard sizes of within blocks and compactness of the blocks them- either 6 or 20 wafers, each wafer providing acomplete selves. The consequent reduction of system slse 4-input switch with a single output. A single volt- 1 carries with it the advantage of shorter- Inter- age wafer, -20/0/+20 volts d.c., supplies all NOR » connecting wiring. of the block. (For higher switching loads "Power" and "Universal" NOR blocks are used. These operate (b) Individual Components (Kef. 2). on the same principle as the Standard NOR).

The main features of the Norpak solid state The OR Unit - This unit consists of seven wafers each switching components used in the Stacker control are wafer containing a one-Input OR 'and a two-input OR. described below. Wafers of the OR block can be coupled up to provide

36 a multiple-input OR. For example, this block could Auxiliary d.c. Power Supply Unit - A 110 volt, 50 Hs be connected to supply a 21-input OR.- No power a.c. supply is rectified to produce the unfiltered, supply is required for the OR unit. full-wave output of -24 volts d.c. available at the terminals of the unit. The Time Delay Pak - This Pak uses transistors, resistors, diodes and a capacitor to provide an This auxiliary supply is used in the Stacker control adjustable time delay commencing from the applica- to operate solenoid valves connected In switching tion of a logic "1" input signal. It has two timing amplifier output circuits. ranges, namely 0.1-10 seconds and 3-300 seconds. Re- moval of the input at any time will instantaneously CONCLUSION - A Stacker fault, however minor, drive the output to zero. The Pak requires a -20/0/ could render the Stacker, Weigher and Strapper +20 volts d.c. logic supply. equipments ineffective.

The Transfer Pak - This Pak has two outputs, one in'- Under these circumstances either the whole line put and two control gates. An output signal can only would shut down, or manual operation would be intro- be obtained when the input changes from "I" to "0", duced to replace the ineffective equipment. and for this change output No.1 will switsh only if control No.l is held at 0 voles, and output No.2 In both cases considerable inconvenience and will switch only if control No.2 is held at 0 volts. loss of output could result. The main applications of the unit are in the resetting of memory circuits and in binary counters. Application of well proven solid state components in an equally well designed Stacker logic The BCD Decade Counter - This binary coded control should go a long way towards reducing such decimal counter employs the 12 4 2 code, resetting faults, and so towards improving the overall after the tenth count. Its eight outputs can be reliability of the line. decoded for any count up to 10. With two counters this range can be extended up to 100. The unit ACKNOWLEDGMENTS requires a -20/0/+20 volt d.c. logic supply. I wish to express my appreciation to Square D Filter Pak - This Pak contains 12 identical resistor Limited, Swindon and to E & J Engineering Ltd., capacitor networks in 6 wafers, 2 per wafer. It is Swansea, for allowing me to print this paper used to provide a buffer against stray signals in describing their design. input leads carrying normal logic-level voltages. My gratitude is also due to my colleagues at D.C. Amplifier - Application of a logic "1" signal to Risdon who assisted with compiling and typing, to the input terminal of this amplifier will switch a 24 the Design Office who so capably handled the produc- volt d.c. auxiliary supply voltage across a load tion of all figures and to the E.Z. Co. management having a minimum resistance of 20 ohms. who granted me permission to publish this paper.

A.C. Amplifier - A thyristor in this amplifier RFFERENCES circuit fires in response to a logic "I'1 input signal switching a 120 volt, 50 Hz supply across a load 1. E & J ENGINEERING LTD. - connected to the amplifier output. The amplifier Static Control for Ingot Stacker can carry currents of up to one amp with loads such Trade Literature, 1971. as starters, contactors and solenoids. 2. SQUARE D LIMITED - Proximity Switch - This matchbox-sized, stainless- Norpak Static Control steel-housed unit consists of an inductive sensing Trade Literature, 1964-68. circuit feeding Into a detector circuit. Two complementary outputs change logic states on the passing of a metallic vane across the sensitive face of the switch. Adjustment of sensitivity is possible by means of a screw at the back of the unit. Supply connections are +20/0 volts d.c.

Logic D.C. Power Supply Unit - A 110 volt, 50 Hz a.c. supply is rectified, smoothed and regulated to provide the following d.c. outputs which are available at the terminals of this unit :-

-20/0/+20 voles d.c., -5% (for logic signal and bias purposes), and a -130 voles d.c. (for logic Input devices such M* push buttons and limit switches).

This power unit also incorporates an "OFF" supply circuit which generates a pulse to reset memories whenever power is first applied to the system.

37 VERY LOW NOISE AMPLIFIERS FOR SEMICONDUCTOR X-RAY DETECTORS

J.E. Eberhardt, B.E.(UQLD), M.E.(UNSW), Instrumentation and Control Division, Australian Atomic Energy Commission

SUMMARY Semiconductor diodes may be used for the energy-intensity spectral analysis of X-rays which produce pulses of photo-Ionisation current in the diode. Typical diodes are of silicon 50 ram2 in area, 4 «am in depletion layer thickness, have a capacitance of about 1 pF and have a leakage current of less than 10" 14 A when operated at 80 K with 1000 V reverse bias. The diodes output impedance is greater than 10*5 Q. Typical signals are current pulses of less than 3 x 10"16 coulomb ( » 2000 electron-hole pairs) in 10"8 s. To best match the characteristics of the detector diode, the preamplifier uses a junction field effect transistor (JFET) at its input. The JFET is cooled to 130 K to improve its noise performance. Conventional preamplifiers use commercial JFETs and glass enclosed multi kilomegohm feedback resistors both of which mist be carefully selected for low noise generation. The noisy feedback resistor may be eliminated by the use of the recent techniques of charge pumping or pulsed optical feedback. At A.A.E.C. R.E. the latter technique has been used.

The JFET dice is removed from its commercial packaging and remounted on boron nitride, a low loss dielectric for reproducible low noise performance. The preamplifier integrates the input signals and leakage current and when the accumulated charge is about 2 x 10"" coulomb it is removed in 4 μs by optical enhancement of the JPET's drain-gate leakage current. Careful mechanical design reduces the microphonic noise. Without very rigid mounts and electrostatic shielding of the preamplifier input connection, vibrations of only one micron would significantly increase the system noise. When suitable band pass filtering amplifiers are used with pulsed optical feedback preamplifiers the system noise is typically 22 rms electrons (3.6 x 10-18 coulombs rms) and may be as low as 9 rms electrons.

I I INTRODUCTION consider the signal production process in the X-ray

'• :!' detector. . An X-ray detector diode (Fig. 2) is a + ; : •' When a beam of energetic X-rays or electrons is p-y-n device which is reverse biased so that the V 'I directed at a surface, the elements in the surface region (high resistivity n-type) is totally depleted. Th« IK; *bsorb the energy and emit some of it as monoenergetic An X-ray enters the depletion layer through the thin 1 optimum , i'f,! fluorescent X-rays. The energies and relative surface barrier contact where it nuty. transfer its pile the input stage. This is certainly true of up). preamplifiers for X-ray detectors where there is little to choose between the many circuits published For somewhat simpler amplifiers with simple RC (4,5,6) at least on the basis of potential noise high and low pass filtering, analytical expressions performances. However there may be important are available which describe the equivalent noise differences in the secondary characteristics such as contributions (ENC) (in electron-hole pairs, rms) of closed loop bandwidth, gain stability and performance noise sources at the preamplifier input. at high energy input (count) rates. Fig. 4 is a simplified schematic of the most popular conventional For a detector or preamplifier input leakage front end circuit. The detector is dii-ect coupled to current (II) the mean square input current fluctuation the gate of a JFET which is in a folded cascode circuit, the cascode transistor having a bootstrapped (in) • 2It,qaf where af is the frequency interval in which the noise is measured and q is the electronic collector load resistor. A resistor of several kilo- charge. Any resistors or lossy dielectric in parallel megohms in parallel with the feedback capacitor sinks with the input may be represented by a noise the detector leakage current and defines the JFET'a operating point. The open loop gain (> -4000) has equivalent input leakage current Ieq - 2kT/qRp (amps) where Rp is the total parallel input resistance in one dominant pole (the cascode load) and circuit is ohms. For an amplifier with equal time constant (to) stable under all input load connections. high and low pass filters it can be shown (2) that the ENC due to parallel noise sources can be expressed as The detector operates in a vacuum cryostat at 77 K to 130 K, has a leakage current of < 10"l2 A and (1.15 x 197 a capacitance of Rf 1 pF. To keep the stray input capacitance down, the JFET, feedback capacitor and (rms electron-hole pairs) resistor are mounted in the cryostat near the where In is the total equivalent input current. detector. The JFET is conveniently cooled to its optimum temperature of about 130 K which not only The shot noise of the input JFET is a series reduces its gate leakage current but also increases noise voltage source characterised by an equivalent its transconductance by more than half. The input noise resistance Rn a/ 0.7/gm (3) where gm is the FET is conventionally an n channel silicon JFET, transconductance of ths JFET. When a charge almost always type 2N3823 or 2N4416, possibly as a sensitive preamplifier is used, the input impedance selected low noise type with an 'in-house' number. is capacitive and added input capacitance increases MOSFETs are not used since they have never proven as the noise figure by attenuating the signal. The ENC quiet as JFETs, it is thought because they are surface due to series noise sources is given (2) by effect devices.

15 2 (ENC)N » (1.92 x 10 TReqCin /«"o)7 III. COMPONENT YIELD PROBLEMS (rms electron-hole pairs) Preamplifiers produced as described above have, where Req is the equivalent input noise resistance and on occasion, been capable of amazingly low noise Cin is the total input capacitance. figures but only after rigorous selection of the input JFET, the feedback resistor and the JFET The above equations demonstrate that there is an operating temperature. The yields of low noise JFETs optimum filtering time constant (ENOjaZj,^'^ and in particular, were so poor that published results (ENC)N0c(l/t'o) « They also show as would be expected (7) often bore annotations such as 'FET No. 36'I that In and the (ReqCin2) product must be minimised Selection was expensive and time consuming. and that reducing the temperature of the JFET is Commercial JFETs in metal rather than plastic advantageous. Since the input capacitance- encapsulation exhibited better yields but little transconductance ratio is relatively constant for correlation could be found between measurable silicon JFETs, there is obviously little.point in characteristics and their noise performance Improve- using a JFET which has an input capacitance much ments in the manufacturing processes eliminated the greater than the rest of the input load capacitance. trapping levels which had been present in the bandgaps Practical limits to f are set at the low end by the of earlier JFETs and had been responsible for many o peaks and valleys in the JFETs noise as its variable width of the detector current pulses and at temperature was varied (7). This is Shockley-Read- the high end by 'pile-up' or overlap of the pulses Hall (8) noise and-is maximum when the Fermi level which are randomly distributed in time. To Is usually approaches one of the trapping levels. Cooling the between 0.1 and 10 μs. JFET much below 130 K leads to increased noise. 10 Impact ionization in the channel produces increased As an example consider a preamplifier whose 10 gate leakage current (9). The gate-drain threshold ß feedback resistor is at 130 K with' amplifier voltage for this anomalous leakage current is a filtering time constants of 6 μs. Ieq will then be sensitive function of temperature being about 11 V about 2.3 pA leading to an (ENC)^ cf 12.6 rms for a 2N4416 (Texas Instr.) JFET at 130 K and electrons if other leakage currents are negligible. decreasing for lower temperatures. Assuming that the JFET has a gm of 4 mA/V at 130 K and that the total input capacitance is 6 pF leads to ah (ENC)N of 5.1 rms electrons. Of course in It was realised (10) that the brown glass practice Req will be larger than 0.7/gm. filling in the JFET's header was a lossy dielectric and a prime cause of the variable noise performance of the JFETs. A study of JFETs in commercial and 39 Special low loss beryllia packages (11) has confirmed channel JFET is to be used at the preamplifier input this view. In that study it was found that dice (the best JFETs are n channel) and produces bigger specially mounted on beryllia and alumina headers reset pulses. The opposite polarity of detector were virtually always lower in noise than commercially connection is not generally as suitable for low mounted devices and that the statistical spread of capacitance mountings. The highest quality preamplif- the noise performance from device to device was ten iers presently available use the pulsed optical times lower (Fig. 5). These specially mounted JFETs feedback method. are not commercially available but high yields are available simply by removing the dice and gate wire Recently there have been reports (17) of MOSFETs assembly from its commercial packaging and remounting with especially thin gate insulation which may be it on a low loss dielectric using an epoxy resin. induced to leak by ultra-violet light. The MOSFETs could be used in room temperature pulsed optical reset The several kilomegohm feedback resistor is electrometer circuits if a conveniently controllable another yield problem. Resistors in this range are ultra-violet light source were available. Normal usually prepared costmercially by pyrolizing praprietry MOSFETs usually have low leakage protective diodes organic compounds on low loss ceramic substrates. The connected to their gates and these could be used for resulting carbon film may have a spiral cut into it optical coupling. to increase the resistance. The resistance element is then encapsulated in an inert-gas filled glass V. A.A.E.C. R.E. PREAMPLIFIER CONSTRUCTION tube about one to two inches long (Fig. 6). AND PERFORMANCE Unfortunately the equivalent circuit of such a device is not simply a resistance but rather a distributed The cooled preamplifier-detector front end RC network. This causes the resistors parallel loss assembly is shown in cross section in Fig. 11. Most impedance to decrease rapidly as frequency increases of the assembly is of free machining aluminium alloy (Fig. 7). For example a resistor with a d.c. with pure aluminium X-ray filter rings in appropriate resistance of two kilomegohms may have an a.c. places. The X-ray detector is mounted in a black resistance of only one kilomegohm at 16 kHz leading acetyl plastic (Delrin) holder to prevent light to an ENC of 50 rms electron hole pairs instead of 35 coupling from the GaAs LED which would lead to rms electron hole pairs. The rate of change of positive feedback. The detector mounting plate is impedance with frequency varies from resistor to supported 5 mm from the beryllium window on a boron- resistor and yields of 50 kilomegohm resistors for nitride insulating block. Hot pressed moisture systems with 22 rms electron hole pair noise figures resistant grade boron nitride is a good thermal are typically less than 107.. conductor, an excellent electrical insulator and is easily machined, having the mechanical properties of IV. IMPROVED FEEDBACK METHODS a quality chalk crayon. Under vacuum this block's leakage current is less than 10*1-2 A with 3 kV across The noisy and unreliable resistor can be it even after extensive handling without gloves. replaced by an optically coupled light emitting diode (LED) and photodiode (Fig. 8) (12,13) and it was The p-V-n'*' diode has an n* guard ring on its n realized by Goulding et al. (14) that for the usual rear contact which is connected to ground, diverting detector connection the drain-gate junction of the the 10*12 A diode surface leakage current from the JFET was a suitable photodiode. The JFET would have JFET's input and also electrostatically shielding the been remounted to remove che lossy header material wire connecting the detector to the JFET mounted and so would be open to light. This system produced about 3 mm below it. Without the electrostatic low noise preamplifiers but the overall d.c. feedback shielding, the gate wire, which is near ground was non linear, possibly due to non linearity in the potential, would be 5 mm at the most from the detector current input- light output characteristic of the LED. mounting plate which is biased at about 1000 V. The capacitance between the wire and the plate would not 1 1 This non linearity can be avoided if pulsed be less than lO" ' F and if the wire to plate spacing optical feedback is used (15). Fig. 9 is a block changed by 0.5 um (0.017.) then the change in the input capacitance would be 10"^ F and the change in input diagram of such a preamplifier. In this system the 3 net input leakage current is integrated along with charge (10-18 p x 10 V) would be 10-15 coulombs. signal pulses on the feedback capacitor and when a This is three times as large as a typical signal pulse. preset output voltage level is reached the LED is The mechanical resonant frequency is usually below the turned on. The light induced JFET leakage current pass band of the filter amplifier reducing the effects rapidly resets the JFET's gate-source bias and when a of such noise contributions. Mechanical resonators lower preset voltage level is reached the LEO is are excited by the boiling of the liquid nitrogen turned off. The feedback loop output voltage is a coolant. saw tooth waveform with superimposed signal voltage steps. The resets can produce very great overload The remounted JFET assembly (Fig. 12) is screwed pulses in the filter amplifier but these, can be dealt to an aluminium block incorporating the reset LED and with by signs! gating methods. Pulsed optical; feed- a zener diode heater (not shown). A thin walled back ha* the added advantage over resistor feedback stainless steel tube rigidly supports the block and ox ordinary optical feedback that the shot noise of provides, a thermal impedance of 300 K/watt between the current removal device is not added to the other the cold clamp at e» 77 K and the JFET at «* 130 K. noise sources. The detector's leakage current is adjusted so that the preamplifier resets at least once every twenty Another method, charge pumping (16), uses the seconds and each approximately two. volt reset interval 6. detector as the capacitor and the JFET's gate-source takes about 4 μs. This reset interval indicates.a junction «s the diode of a d.c. restorer (Fig. 10). light induced leakage current of about 0.2 MA when the Unfortunately the charge-pump system;requires the LED drive current is 0.6 A. opposite polarity of detector connection if an n 40 Some selection of remounted JFETs is still 7. BLALOCK, T. V. - Wide band low noise Charge required but all have noise figures of less than 25 Sensitive Preamplifier, IEEE Trans. Nucl. Sei.. rms electrons with half less than 20 rras electrons Vol. NS-13, No. 3, June, 1966, pp 457-67. wfvh a 2 pF load capacitance. A logic output pulse which proceeds the reset by 1/2 ps allows signal path 8. SPAULDING, R.A. - Field Effect Transistor Noise gates to be closed before the reset transient occurs. at Low Temperatures, Proc. IEEE. Vol. 56, No. 5, Reset transients are about 2 V, signal step pulses May, 1968, pp 886-7. about 3 mV (for a 6 keV X-ray) at the output of the charge sensitive loop. 9. RYAN, R.D. - The Gate Currents of Junction Field- Effect Transistors at Low Temperatures, Proc. VI. CONCLUSIONS IEEE. Vol. 57, No. 6, June, 1969, pp 1225-6.

It has been demonstrated that the performance of 10. RADEKA, V. - State of the Art of Low Noise conventional preamplifiers for high impedance trans- Amplifiers for Semiconductor Radiation Detectors, ducers is dominated by the non-ideal characteristics Proc. Int. Symp. Nucl. Elect.. Versailles. France of the feedback resistor and the JPET packaging Sept., 1968, Vol. 1, pp 46-1 to 46-28. rather than the JFET itself or the rest of the feedback loop. JFETs are readily removed from their 11. KERN, H. E. and McKENZIE, 3> M. - Noise Studies commercial packaging and when placed on low loss of Ceramic Encapsulated Junction Field Effect ceramic mounts have better, more uniform noise Transistors (JFETs). IEEE Trans. Nucl. Sei.. performances. When used in a dry nitrogen atmosphere Vol. NS-17, No. 3, June, 1970, pp 425-32. with a selected feedback resistor such JFETs have had consistently low noise figures (< 25 rms electrons at 12. KANDIAH, K., STIRLING, A., TROTMAN, D. L. and 25 C) (11) and gate leakage currents less than 5 x WHITE, G. - A Fast High Resolution Spectrometer 10-13 amps. for Use with Nuclear Radiation Detectors, United Kingdom Atomic Energy Authority Report, The techniques of optical feedback, pulsed AERE-R-5852. August, 1968. optical feedback and charge pumping have all been applied to cooled JFETs giving consistent low noise 13. KANDIAH, K. and STIRLING, A. - A Direct-Coupled results and should be applicable at room temperature Pulse Amplifying and Analyzing System for Nuclear where high impedance transducers are being used. Particle Spectrometry, Proc. Second Conf. Seml- conductor Nuclear-Particle Detectors and Circuits, ACKNOWLEDGEMENTS Gatlinburg, 1967, pp 495-505.

We thank A. McC. Beech and P. Weir for their 14. GOULDING, F. S., WALTON, J. T. and MALONE, D. F. work in designing the detectors and fabricating the An Optoelectronic Feedback Preamplifier for High- electronics described in this paper, V. L. Gravitis resolution Nuclear Spectroscopy, Nucl. Instr. for supplying the ore sample spectra, Dr. J. K. and Meth.. Vol. 71, 1969, pp 273-9. Parry, Dr. A. J. Tavendale, Dr. E. M. Lawson and Mr. R. D. Ryan for their interest, support and critical 15. LANDIS, D.A., GOULDING, F. S. and JAKLEVIC, J. M. reading of the manuscript. Performance of a Pulsed Light Feedback Preamplifier for Semiconductor Detector X-ray REFERENCES Spectrometers, Nucl. Instr. and Meth.. Vol. 87, 1970, pp 211-3. 1. BERTOLINI, G. and COCHE, A. (Eds), Semiconductor Detectors. Amsterdam, North Holland, 1968, 518pp 16. RADEKA, V. - Charge Amplification without Charge Leak Resistor, IEEE Trans. Nucl. Sei.. Vol. NS-17, 2. GOULDINC, F. S. - Semiconductor Detectors tor No. 3, June, 1970, pp 433-9. Nuclear Spectroietry, Nucl. Instr. and Weth., Vol. 43, 1966, pp 1-54. 17. FROHMAN-BENTCHXOWSKY, D. F. - ROM can be Electrically Programmed and Reprogrammed and 3. VAN DER ZIEL, A. - Gate Noise in Field Effect Reprogrammed , Electronics. Vol. 44, Transistors at Moderately High-Frequencies, No. 10, May, 1971, pp 91-5. Proc. IEEE. Vol. 51, No. 3, March, 1963, pp 461-7. FIGURE CAPTIONS

4. MEYER, 0. - Ein Rauscharmer Ladungsempfindlicher Figure 1 X-ray fluorescence spectra of Broken Hill Vorverstärker mit Feld-sffekt-transistoren, ore at various stages of processing (supplied Nucl. Instr. and Meth.. Vol. 33, March, 1965, by V. L. Gravitis, Isotope Applications Research pp 164-6. Section, A.A.E.C. R.E.).

5. ELAD, E. and NAKAMURA, M. - High Resolution Figure 2 A guarded n+ contact X-ray detector diode. Beta- and Gamma-r«y.Spectrometer, IEEE Trans. Nucl. Sei.. Vol. NS-14, No. I, Feb., L967, Figure 3 Block diagram of semiconductor X-ray pp 523-31. spectrometry system.

6. SMITH, K. F. and CLINE, J. E. - A Low-noise Figure 4 Schematic of a conventional preai.iplifier Charge Sensitive Preamplifier for Semiconductor for a semiconductor X-ray detector. Detectors using Parallel Field-effect transistors, IEEE Trans. Nucl. Sei.. Vol. NS-13, No. Figure 5 Noise performance of commercially packaged 3, June, 1966, pp 468-76. JFETs and specially mounted JFETs at 298 K (11).

41 Figure 6 Construction of a kilomegohm resistor and its equivalent circuit.

Figure 7 Resistor loss impedance versus frequency for some high value resistors (10), curves 1, 2, Pyrofiltn HR1250, 5 kilomegohm, curve 3, Allen Bradley, BB 1/8 W, 2 kilomegonm.

Figure 8 Block diagram of a preamplifier with optically coupled feedback.

Figure 9 A pulsed optical feedback preamplifier.

Figure 10 A charge pumping preamplifier.

Figure 11 A cross section cf the front end assembly of an A.A.E.C. R.E. pulsed optical feedback preamplifier.

Figure 12 An A.A.E.C. R.E. remounted JFET.

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ex FIGURE 12 equ tec •th 44 A TELEMETRY SYSTEM FOR ELECTRICAL DISTRIBUTION NETWORKS

F. Zillhardt, Ing.(grad) Senior Systems Engineer, Cutler-Hammer Australia Pty Limited

SUMMARY: The Remote Supervisory Equipment for the Metropolitan Transmission System, Perth, Is described. The equipment is of the conventional, solid-state logic and relay Interface type. Details are given of the specification for the system. The technical layout of the Power System Control Centre with the control room and the high tension mimic board is presented and the Information flow from the Control Centre in East Perth to the six outstations 1s shown in some detail. From the transducers at the outstations to the display and recording In the control centre the telemetry is shown in detail. Each outstation has 64 measurands, 56 of which are selectible. The selection 1s made on push-button banks close to the associate indicating meter fitted on the mimic board. Sixteen measurands of each outstation are continuously displayed and any of these can be selected for recording and fed Into a graphical recorder. The A-D Converter used 1s a four channel device. A scanner makes it possible to have sixteen inputs continuously available. The gating signals for the scanner and the A-D Converter are derived from the transmitter logic. At the Control Centre precision D-A Converters supply a constant current to the Indication meters and recorders. 1.0 INTRODUCTION fully integrated). The advantage of this is that all Telemetry and telecontrol for electrical distri- 'design bugs' have been eliminated. The manufacture bution networks are not new. It is a long estab- of the hardware 1s high quality based on past lished fact that it is economical and efficient to experience and in general the equipment is of high use data communication for measuring and controlling standard. plants from a distance. The basic concepts for these comnunication systems have been written down long ago. The layout of the Metropolitan Transmission System, There are many possible conditions for a data-trans- Perth, is shown in figure 1. mission system I.e. different transmission systems, modulation methods, coding methods and data storage WT Western Terminal means. It 1s not easy to select the most suitable NT Northern Terminal possible combination. There has to be a close co- operation between designer and user to specify a ST Southern Terminal particular system. Public utilities, such as KU Kwinana Gen. St. electricity commissions, provide a round the clock service therefore the basic requirement for the SF Sth. Frm. Gen. St. design is reliability. The final design for the EP East Perth Gen. St. Remote Supervisory Equipment for the Metropolitan Transmission System, Perth has been determined, as U Communication Link in so many other cases, by the cost factor. All communication systems consist of three basic elements; a source, a sink and a transmission channel. The designers task Is to match efficiently the source and sink within the limitations of the transmission channel. The transmission channel is a source of a wide variety of disturbances; impulse and random noise, cross talk, distortion, fading, time delays or line drop-outs. All these disturbances can Introduce errors into the message during transmission. There are many methods of Improvement on the source, the sink and the channel as well as error control techniques. The design of the system described Is based on experience of other systems already operating. The equipment is, to speak In the language of computer technology, a 'second generation' as opposed to 'third generation* equipment (solid- state viz:- FigU Metropolitan Transmission Systan, Perth 45 TABLE 1 SPECIFICATION OF FUNCTIONS QUANTITIES OF FUNCTIONS FUNCTION DESCRIPTION uantities eauiooec Full Otv. eauiDoed Full MT NT ST CT Total SF KM Total CONTROLS (PSCC TO OS) Circuit Breaker Trip/Close 50 42 40 38 50 6 6 20 Transformer Mas ter/Tra 11 er 4 4 2 5 5 2 2 2 Transformer Parallel/Separate 4 4 2 5 5 2 2 2 Transformer Raise/Lower 4 4 2 5 5 2 2 2 Select Measurand (8 groups of 8) 56 56 56 56 64 32 32 64 Telephone Call 1 1 1 1 1 1 1 1 Remote Alarm Relay Reset 1 1 1 1 1 1 1 1 RETURN IND. FROM CONTROLS (OS TO PSCC} Circuit Breakers open 50 42 40 38 50 6 6 20 Transformer Master 4 4 2 5 5 2 2 2 Transformer Parallel 4 4 2 5 5 2 2 2 Transformer Tap Change In Progress 4 4 2 5 5 2 2 2 Pre-set Alarms for Line currents 8 8 8 8 8 8 8 8 Telephone Call 1 1 1 1 1 1 1 1 System Alarms 3 3 3 3 3 3 3 3 Command Code Checks 5 5 5 5 5 4 4 4 ALARMS A INDICATIONS (OS TO PSCC) Circuit Breakers open(Indication only) SI 116 76 in 120 88 120 120 Alarms 80 62 76 64 80 6 6 6 Measurand Selected 56 56 56 56 64 32 32 64 TELEMETERS (OS TO PSCC) Continuous B1-D1rect1onal (10 Bit) 8 8 8 8 9 7 7 10 Selective Un1-Directional (9 Bit) 7 7 7 7 7 2 2 2 Transformer Tap Position (6 Bit) 5 5 5 5 5 4 4 4 System Alarms 1 1 1 1 1 1 1 1 Judging on the present trend this system will be 1s switching the ladder network of a precision 0-A the last with solid-state logic equipment by Cutler- Converter. Tue alarms and Indications are received Hammer, Australia« not because of technical reasons and linked to the High Tension Mimic Board for display. but economical reasons, the future will show fully Flg. Integrated systems. Control functions Initiated at the Control Centre are being transmitted by the P.S.C.C. equipment. From the original tender specifications for the transmission system by the user to the final specifi- 2. THE CONTROL ROOM cation by the designer a great amount of co-operation The layout of the control room in the East Perth between the two parties took place. The final Generating Station Control Centre Is shown 1n figure function specifications for the six sub-systems for 2. the sub-stations known as Western Terminal, Northern Terminal, Southern Terminal, Cannington Terminal and The Control Room 1s located on the first floor of the generating stctions at South Fremantle and the Power System Control Centre building. The floor Kwinana »rt shown 1n shortform 1n Table 1. Is of hardwood construction and the cables are fed through cable trays which lead to the room below the At the six Outstations (0/S) measuring transducers Control Room. In this room the supervisory equip- convert the telemetered quantities of voltages, ment cubicles are placed. The walls of the Control currents, megawatts and megavars Into suitable Room »re made up by the Control Board for the high electrical signals which are fed Into the A-0 tension metropolitan transmission system, Perth. flow, Converter Input Interface equipment, scaled and There are also three free-standing consoles located directi linked to the A-D Converter via a scanner. The In the centre of the Control-Room. To digital equivalent of the analogue Input to be Control transmitted to the Power System Control Centre 2.1 The Control Board permanJ (P.S.C.C.) or Base Station (B/S), located at East button I Perth Generating Station, 1s an eight bit pure binary The Control Board is formed Into the shape of an 14/iOOl number plus polarity bit and the full scale deflec- octagonal room with eleven bays. An aluminium frame or the I tion (FSD) bit. The 0/S also sends two-state is supporting the laminex faced chip-board panels rail. Indication such as *CB Closed' and alarms to the which make up the board. The overall height of the It.:, P.S.C.C. The 0/S receives from the P.S.C.C. control frame Is 9 feet 5 Inches. The panels contain the be stri commands such as 'CB Trip' which have to be executed. mimic diagram for the system and hold discrepancy and The communication channels or links used are rented control switches, annunciators, push-button banks, Thif telephone lines from the P.M.G. or private pilot meters, recorders and Instruments. install cables« done The mimic diagram shows busbars, switching equip- descril At the P.S.C.C. the eight bit pure binary number ment (circuit breakers, Isolators, switches) and a 46 load flow diagram. The acrylic escutcheons for the testing. Since all the panels have been despatched Control 4 Discrepancy and Discrepancy only switches with the looms, terminal rails and cables attached, are: square for Oil Circuit Breakers (OCB), octagonal packing was somewhat difficult. for motorised Isolators, and circular for Manual Isolators or Earth Switches. Ninety-one panels, 2.2 General Facilities equipped and blank ones, make up the wall. This amounts to about 885 square feet panel area. There The three consoles standing inside the Control are two main types of panels, the bottom panels about Room are: the Tap Changer Console for adjusting the 73 inches high and 20 or 40 inches wide, and the top. outputs from the power transformers, the Generated panels about 37 inches high and 20 or 40 Inches wide. Quantities Console, for indicating the amount of power generated at each power station, and the Trend Recorder Selection Console, for connecting various graphic recorders to Indicated quantities. The Tap Changer Console is housing up to 32 Tap Changer Modules and the voltmeters for the busbar voltages 66 kV, 132 kV and 330 kV (1n future). Belcw each meter is a bank of push-buttons to select the required sub-system. In addition there is a bank of push-buttons to accept the alarms of the particular system. Each Tap Changer Module is fitted with switches and numerical indicators. The indicator, a nixie display, is showing the tap position of the particular transformer. Also mounted in the module is a Tap Raise/Lower Switch, a Master/Trailer Switch and a Parallel/Separate Switch. The front of the module is covered with a Traffolyte identity label. The connecting cable attached to the module can be placed into any socket position within the console. The Generated Quantities Console is fitted with six (1n future eight) spinning reserve meters, mega- watt meters and megavar meters. Three instruments show the total spinning reserve, power and reactive power. Three Landis & Gyr simulators are mounted inside the Console. The Trend Recorder Selection Console is housing the aural alarm unit. A group of 76 and a group of 88 post office pattern jacks are mounted, with the associated alarm lamps (for F.S.O.) adjacent, on the Fig. 2 Control Room Layout front panel. All jacks are labelled with the signal source they represent. A group of 8 plugs, assoc- The bottom panels have the mimic diagram attached iated with the 76 jacks for uni-directional measurands and are fitted with the control and discrepancy keys. and a group of 4 plugs, associated with the 88 jacks The top panels are fitted with the voltage, current for bi-directional measurands are located on the top and temperature indication meters. Bay 4 is the of the front panel of the Console. time and frequency area, the panels have big cutouts to fit the continuous recorders for: 66 kV, 132 kV, Each plug with cord 1s labelled with the location 330 kV (future), frequency, total load and spinning of the Trend Recorder It 1s connected to. reserve. There is also a barograph, frequency standard Instrument, standard time clock, time 3. THE REMOTE SUPERVISORY EQUIPMENT difference clock and time difference Indicator mounted on the panels in this bay. The transmission equipment is of the time-division- multiplexing (t.d.m.) type and phase-reversal modul- Bay S is allocated for the Indication of the load ation Is used. Since serial transmission is used the flow. One panel In Bay 8 is used for 8 uni- two basic parameters, namely the 'Data Pulse Rate' and directional and 4 bi-directional Trend Recorders. the number of 'Pulses per Message 31ock* must be To ease Installation and commissioning of the specified. The Audio frequency carrier 1s chosen Control Board each panel has a wiring loom or looms around the usual practical limit of B (Bandwidth) permanently connected to all its switches, push- pulses per second. The transmission link is a button banks or annunciators. The wire used 1s telephonic channel therefore B 1s about 3 kHz 14/.0076 P.V.C. rated 240 V.A.C. The outgoing end An Important constraint In industrial telemetry of the loom 1s terminated with a 48 way terminal and telecontrol systems is time. In most cases a rail. Each terminal rail has a cable connected to maximum limit is imposed on the time Interval between it. The cable has a 50 way plug on Its end and can initiation of a control and Its operation. This has be straight plugged Into the equipment cubicle. a first order effect on the modulation rate. It This arrangement has great advantages during appears, therefore, that the pulse-rate.needs to be as installation, because very little wiring has to be high as the channel bandwidth allows to achieve the done on site. .. The termination of the panels as fastest overall system response. However, there 1s described above is also of advantage in 'in-house' a minimum time limit imposed on the length of each 47 block of data by the Input/output devices of the selector lines. The address bits are pre-wired 1n ! system. the first five bit positions. : A baud rate of 150 bauds and carrier frequencies All receivers and transmitters work from DC at of I860 Hz, 2220 Hz, 2580 Hz and 2940 Hz had been 50 volts. selected. The speech is nominal in the channel from 300 Hz to 1600 Hz. 3.2 General Systems Approach The equipment is continuously scanning the Infor- Having decided to use continuous scanning TELSCAN, mation and on a directional basis it is duplex a modulation rate of 150 bauds and carrier frequen- operated. The P.S.C.C. and 0/S Is simultaneously cies from 1860 Hz to 2940 Hz, only the following transmitting and receiving. points had to be answered: the number of transmitters and receivers to be used per sub-systan and the , 3.1 Solid-State System TELSCAN number of addresses required to cover all functions shown In the 'full total1 columns of Table 1. The TELSCAN Is a standardised system by Cutler-Hammer answers to these points are given in 3.2.1 (P.S.C.C. • Australia for remote supervisory control and Cubicle) and 3.2.2 (0/S Cubicle). ; telemetry systems. The total of the transmission system is made up Information Is transferred In complete messages, by six 0/S Cubicles at their respective outstations each message consists of 64 coded pulses of voice and six corresponding P.S.C.C. Cubicles at the frequency tone. Eight of these pulses are used for Control Centre at East Perth Generating Station. * address coding, error checks, and synchronisation and f1fty-s1x for information. The system basically As an arrangement for future data logging, every comprises a transmitter and a receiver. indication and alarm is providing a clean change- = over contact from their respective interface relays. The information to be transmitted Is scanned Telemeters and tap position Indications have only sequentially, thus converting It from parallel to make contacts wired out. serial form, and at the receiver, the incoming pulses are stored until the end of the message, when they Both Cubicles are three rack cubicles with a rack are reconverted to a parallel form for read-out. size of 19 in. The approximate overall cubicle size is 66 in. Wide x 24 in. Deep x 102 in. High. Up to eight messages can be transmitted on one The cable entries are for data at the top and power link, each message having a particular address, and V.F. at the bottom. The terminations of the and the message being transmitted sequentially. In data cables are 50 way sockets. each message, the 64 pulses follow each other in time, and occupy the same frequency band. 3.2.1 P.S.C.C. Cubicle 7 The Audio signal is modulated by the message pulses so that successive pulses have a phase Figure 3 shows the layout of the P.S.C.C. Cubicle, f difference of 180°. This technique 1s called The Controls Tx and Rx are located in rack 1. :. phase reversal modulation and enables a high signal- to-noise ratio to be achieved. Rack 1 Rack 2 Rack 3 At the receiver» the original pulse waveform is re-constructed. The polarity of the output wave- Tx form is unimportant, as pulses are identified by FUSE & ALARM measuring the time between successive pulse edges, CONTROLS PANEL D-Α CONVERTER not their polarity. CHANNELS High noise immunity Is obtained by means of a band-pass filter and DC integrating circuitry. Initial protection against line surges 1s by 4 Address LINE CHANGE- 16 OFF standard line protection units and Isolating trans- 1860 Hz OVER UNIT i 4. formers, where fitted. Final protection Is given by zener diodes at the transmitter output and diodes 1n the receiver Input. If a system requires more than one 64 bit message con '/! 1 block to cover all the control, supervisory and Rx Rx Rx sa> telemetry functions then a multiple address system que may be used. RETURN IND. ALARMS & TELEMETERS. bas In a multiple address system the transmitter Is FROM CONTROLS INDICATIONS nfOT required to scan the Inputs from each address In turn. If the system, for Instance, requires three The 64 bit message trains, each message will be given a rev separate address. The message associated with 2 Address 5 Address 4 Address sue address 1 will be transmitted first, followed 2220 Hz 2580 Hz 2940 Hz tra Immediately by address 2, followed by address 3 and back to address 1. F1g.3 P.S.C.C. Cubicle Layout The I The particular address amplifier being operated agai for any1 message applies a 0 volt output to the set In rack 2 the Rx for alarms and Indications is sigri of 64 Information gates used for that address. All housed plus the line change-over unit and the fuse modJ ti;e other sets of Information gates have their and alarm panel. The two communication links, comd diodes held non-conducting by 12V on their address main line and stand-by line are connected via reva 48 screw terminals to the line change-over unit. Fuses 5. COMMUNICATION LINKS for all frames in the three racks are fitted In the fuse and alarm panel, alarm relays are plugged into In paragraph 3 all the transmitting frequencies sockets on the front of the panel. used have been mentioned. A bandwidth of 300 Hz to Rack 3 is equipped with the telemeters Rx and 3100 Hz for the data transmission 1s required, there- sixteen 0-A Converter channels plus the D-A Converter fore rented P.M.G. lines and private pilot cables are Power Supply. quite sufficient. 3.2.Z Outstation Cubicle Each link between the P.S.C.C. and the six outstations is in form of two quads. One quad Is the The layout of the Outstation Cubicle is shown in main link and the other quad is the standby link. figure 4. One can see It is arranged in a way to The two pairs for each link are used as 'Go' and match the P.S.C.C. functionally In layout as much as 'Return' pairs. possible. Again rack 1 1s reserved for Controls, rack 2 for alarms & Indications and rack 3 for 5.1 Lines telemeters, this gives it a logical division which helps to service the equipment and Isolate faults The rented P.M.G. lines are 10 lbs per single wire easily. Whole functional sections can be dis- mile, paper insulated, quads, lead sheathed, and may connected without Interference with the other parts be either V.F. loaded or unloaded. These cables of the system. have a typical attenuation characteristic of 0.7 to 0.8 dB per loop mile. Rack 1 Rack 2 Rack 3 The multicore pilots have a greater attenuation and high cost but are very reliable. Since both DC-AC INVERTER links are routed differently the link security is Tx FUSE & ALARM high and the probability of complete lin« drop-out PANEL 50VDC - 240VAC is very low. RETURN IND. The distance between the Control Centre in East FROM CONTROLS Perth and the Outstations or better the length of communication links is from 6.7 miles (WT) to 28 A-D Converter miles (KW). LINE CHANGE- SCALER 2 Address OVER UNIT SCANNER AND 5.2 Line Protection 2220 Hz TAP POS. ENC. Special insulated cables are used within the limits of the dangerous area around the power or switching Rx Tx station. Line isolating shorting plugs for each A-0 CONVERTER wire are used. Surge Arrestors from each wire to station earth are connected as well as fuses in each CONTROLS ALARM & wire. The equipment is isolated from the lines by INDICATIONS Tx line isolating transformers. The line side insulation of the transformer is 15 kV and the equip- TELEMETERS ment side 2 kV. In longer lines, line section transformers are used. Both sides are Insulated for 20kV. 4 Address 5 Address 4 Address 1860 Hz 2580 Hz 2940 Hz 6. LINE CHANGE-OVER UNIT The line change-over unit has two plug-in printed F1g.4 Outstation Cubicle Layout circuit cards, the actual Line Change-Over & Monitor Card and an Audio Frequency Filter Card. 4. PHASE REVERSAL MODULATION Both lines, the main line and the stand-by line Phase reversal modulation has, like other PM the are connected to the change-over card. In the event best tolerance to noise but at somewhat increased of failure of the main line, the circuit automatically complexity. This is true with low modulation rates, switches to stand-by line. If the stand-by line say 5D bauds to 400 bauds. At higher rates fre- fails during operation of the equipment on main line quency modulation seems to have the edge, this is an alarm indicates this event and the operator can based on practical experience of many manufacturers take precautionary measures.. If both lines fail the of telemetry equipment. circuit connects the output signal path to OV. Isolation transformers are provided for all the input The phase reversing is done with a ring modulator. pairs and a hybrid transformer for the telephone The output of the modulator consists of a phase input connection. Telscan Inputs and outputs are reversed V.F. carrier In which the time between directly coupled via appropriate resistor networks - successive reversals corresponds to the Information unbalanced relative to signal OV. Free relay transmitted. contacts are available for signal Indication and data logger, etc. The relative signal levels can be The demodulator works on a frequency doub.er basis. adjusted by means of pre-set potentiometers. The doubler output is amplified and divided by two again to eliminate the modulation. The original Four Telscan receivers and transmitters can be signal and the unmodulated one are then ring de- accommodated simultaneously together with one speech modulated and applied to a comparator circuit. The channel (an external low pass filter is required to comparator will show a d.c, shift each time a phase separate the speech frequencies from the Tel scan reversal occurs. ' tones). All signal input and outputs are 600 ohms 49 impedance. Voltage insulation between the windings of 2940 Hz. Ivery address has four blocks of binary of the line transformers is 550V rms. bits allocated, representing the amplitude of four analogue measurands. The firit 5 bits of each 7. TELEMETERING message are coded with the address,the sixth bit Is used as a check bit to indicate a possible scanner Each of the six sub-systems can have up to 64 failure. measurands. Nine or ten are continuous and the The digital equivalent of the analogue input to remainder is selected to make up sixteen measurands, be transmitted to the P.S.C.C.is an eight bit pure the number transmitted. binary number, plus the full scale deflection bit The selection of the selectable measurands takes and, if required, the polarity bit. place on push-button banks located below their The transmitter is continuously scanning the four associated indicating meters on the Control Board. addresses and with them ti.e message content. At the The overall accuracy between the analogue inputs at same time the transmitter is gating the A-0 Converter the outstations and output at the P.S.C.C. is better and the Scanner. The gating pulses are derived from than - IS of FSD. the 1-J matrix of the transmitter scanner and the output of the Address Amplifiers. 7.1 Measurand Selection On the receiving s'dea four address receiver with The measurand selection buttons are banks of eight four output stores is storing the measurand datas and and mechanically Interlocked. They have two change- continuously updating the content. over contacts. One contact is used for the actual selection of the measurand by marking one bit in the The general measurand scanning arrangement is message to be transmitted. The second contact is shown in Figure 5. connected to the corresponding relay in the output store for the return indications of controls and 7.3 Measurand Scaling and Scanning feeds the lamp which is located inside the pushbutton. As soon as the measurand is selected and All measurands are scaled at all times from an the return indication relay changes state the lamp input of a 10mA or 5-0-5mA analogue current to an changes from light to dark. Since all the buttons output voltage of lOOmV. There are also eight are mechanically interlocked only one measurand preset-current alarms, all eight alarms are trans- within its block can be selected. At the out- mitted continuously but only one analogue value is stations measurand selection relays, interlocked or selected and transmitted at a time. The current non-interlocked, connect the scaled analogues to the is first channeled through a potentiometer across A-D Converter Scanner Cards. which a corresponding voltage is picked up and fed into a comparator circuit for alarm initiation. All eight comparator circuits have separate power 7.2 Telemeter Transmission supplies to eliminate any influence on each other. Sixteen measurands are continuously transmitted by a four address TELSCAN transmitter at a frequency Out of the total of measurands sixteen are A-0 CONV. A-0 TELSCAN SCANNER CONVERTER TRANSMITTER Gating Pulses ~-1 r

CHANNEL AOMESS 1 1

CHANNEL ADDRESS 2 2

CHANNEL lAOURESS 3 3

CHANNEL ADORESS 4 4 I I I J I | Fig.5 General Measurand Scanning Arrangement 50 connected to the Scanner sixteen are switched throl the A-0 Converter. The Scanner Cards are and have four analogue ir number of analogues scann transmitter addresses, expanded. The scanner a «ay that the A-D Conve and the transmitter Is sc A-D Converter at the r1gh * In 7.4. «r m 7.4 A-0 Converter The Cutler-Hammer anal operates on the voltage principle. It 1s a medi| Instrument. In the analogue sectic quency is generated pr This frequency 1s then circuit on a lOOmS tin stored In a buffer store | is accumulated In the Besides the magnitude | also contains the polarit This information is then | circuitry, where it is av TELSCAN data transmission Inputs »re provided be held 1n the buffer st to prevent a change of re period. When using scanning te have enough time between (fourth measurand) to swi take another analogue ret mission at the beginning measurand) In this addre« By applying the 'stop simultaneously commuting address just before tran time can be saved. In switched to the next add nation of the previous b buffer store. The 'sta applied just after compi previous block. Each channel of the / scanner cards. The so switches and an SCR is i capacitor in each link ' pulse to the anodes of off. Because of that < can be on at any one tii The ADC gets Its pow The DC-AC Inverter is c supply and supplies 240 reason for this arrange independent from mains 7.S 0-A Convtrsic In all six P.S.C.C. Converter channels ir* consists of a precisioi connected to the Scanner Cards and four out of those switched by the relay contacts of the output store sixteen are switched through to the four channels of relays for the measurands, and an amplifier to give the A-D Converter. 10mA (FSD) constant current Into a load of up to The Scanner Cards are based on an address basis 1200 ohms. A plug-in power supply provides AC for and have four analogue inputs each, therefore, the each channel. The power supply Is a 3K Hz square number of analogues scanned depends on the number of wave DC-AC Inverter. The input voltage to the transmitter addresses. The present system can be power supply Is 50V D.C. expanded. The scanner cards are switched in such a way that the A-D Converter can read the analogue Two D-A Converter channels are mounted on one plug and the transmitter is scanning the output of the -1n card. Each channel has a transformer for the A-D Converter at the right time, this is described power input and therefore all the channels are 1n 7.4. completely Isolated from each other. This makes It possible to summate the output currents of the 7.4 A-D Converter different channels without any common mode problems. The constant current outputs of the D-A Converters The Cutler-Hammer analogue-to-digital converter are linked to the trend recorder selection console In operates on the voltage to frequency conversion the Control Room. From this console the analogues principle. It is a medium accuracy, solid-state are fed through indication meters on the mimic boards Instrument. and possibly through additional meters on the generated quantities console. Since constant current Is In the analogue section of the converter, a fre- supplied, all these connections are series quency is generated proportional to the input voltage. connections. This frequency 1s then measured in the counter circuit on a lOOmS time scale and the resultant count If there 1s a need to record a particular measur- and a recorder can be plugged Into the current pass stored In a buffer store while the following reading and this is done on the trend recorder selection is accumulated In the counter. console. Besides the magnitude information, the buffer store also contains the polarity and over-range indication. 8. CONCLUSION This information 1s then fed to the interface The equipment described Is of the conventional circuitry, where 1t 1s available for scanning by the solid-state type and the system has built in much TELSCAN data transmission system. redundancy. Nevertheless, these factors give added Inputs are provided to allow the Information to security to the system although on the economical be held In the buffer store during scanning, so as side there is no advantage. to prevent a change of reading during the scanning The fact that the equipment Is not of the newest period. design has the advantage that the system is fully de-bugged and manufactured with experience. It is When using scanning techniques, the problem 1s to redundant to have one P.S.C.C. cubicle for each of have enough time between the end of the last block the six sub-systems, however, in the case of trouble (fourth measurand) to switch to the next address and in one sub-system on the control centre side there is take another analogue reading In time for trans- no Interference with the other five sub-systems. mission at the beginning of the first block (first The division of the equipment on the basis of measurand) 1n this address. functions to be handled and separate provision of By applying the 'stop* pulse to the ADC and transmitters and receivers for controls, alarms and simultaneously commuting the scanner to the next telemeters has the great advantage of eace of servic- address just before transmission of the stored value, ing and fault location and therefore cuts on down- time can be saved. In other words the ADC is time for the equipment. switched to the next address immediately the Infor- mation of the previous block has been stored In the The use of a main-line and stand-by line as buffer store. The 'start' signal to the ADC Is bearers and their automatic change-over in the case applied just after completion of transmission of the of line drop-out adds extra safety. The power previous block. supplies for the A-D Converter, the preset-current alarms and the D-A Converters are all obtaining their Each channel of the ADC 1s connected to four input voltages from the 50 V D.C. power supply for scanner cards. The scanner circuits form a loop of the system, as a result no dependency on the 240 V switches and an SCR Is used as main switch. A loop A.C. mains exists. The 50 V D.C. power supply Is capacitor In each link transfers a negative voltage equipped with stand-by facilities and hence power pulse to the anodes of the other SCRs, turning them failure will have no Influence on the equipment off. Because of that only one out of four switches except of course when a breakdown In one of the can be on at any one time. Inverters occurs. The ADC gets Its power from a DC-AC Inverter. 9. ACKNOWLEDGEMENT The DC-AC Inverter 1s connected to the 50V power supply and supplies 240 V A.C. to the ADC. The The author wishes to thank the Management of reason for this arrangement 1s to make the telemetry Cutler-Hamer Australia for the encouragement to Independent from mains power failure. present the paper. 7.5 D-A Conversion and Measurand Display In all six P.S.C.C. cubicles sixteen 0-A Converter channels are plugged in. Each channel consists of a precision ladder network, which is 51 DIGITAL COMPANDING TECHNIQUES

C.J. Kikkert, B.E.(Hons.) Department of Electrical Engineering, University of Adelaide, Adelaide, South Australia

same as unity bit APCM. A more detailed description SUMMARY: of pulse code modulation and delta modulation can This paper deals with the requirements for the be found in modern communication texts (Ref.1,2). design of digital companding techniques in either delta For delta modulation, syllabic companding using or pulse code modulation. analogue techniques has been proposed by several Both delta and pulse code modulation convert workers (Ref. 3*7) but their systems do not have the analogue signals into binary signals and in both these advantages that digital techniques can offer. systems the dynamic range is normally small. By the Figure 1 shows a comparison between the binary use of companding, the dynamic range can be extended transmitted signals obtained from PCM, APCM and AM for Since both delta and pulse code modulation are digital a certain input signal. In Figure 1 the input voltage methods, they are well suited to the use of digital is expressed in terms of a constant A, called the step companding techniques. size. The binary transmitted signal normally contains a measure of the system performance. By observing certain patterns in this binary signal and using the occurrence or non-occurrence of these patterns to change the gain of the modulator and demodulator, companding can be obtained. The selection of the binary pattern and the rate of change of gain of the modulat- or and demodulator, determines both the point at which the companding operates and the attack and decay times. The ratio of the largest to the smallest value of the gain determines the dynamic range. By the use of digital circuitry, the gain can be controlled with sufficient accuracy over a large dynamic range.

The paper deals with the principles involved in APCM selecting the binary patterns to control the gain of the modulator and as examples a delta modulation system and a pulse code modulation system with companding AM ratios of 60dB are discussed. Comparison of tht binary transmitted signal and dtmodulattd output of PCM.APCM and AM 1. Introduction. Figure 1. In many applications, analogue data are collected at a transducer which is remote from the processor and The input power can be normalised by dividing it often th« transmission of these data is through a by the squats of the step size. The resulting normal- noisy Mdiun, making conventional transmission in prac- ised input power Is a very usefull parameter for the tical. In order to overcome the adverse transmission design of the companding since it is a measure of over- conditions modulation systems such as FM, pulse width load. The corresponding term in amplitude modulation modulation or code modulation isust be used. is modulation depth. Feir a code modulator without companding the step size la fixed and the normalised input Code modulation is a group of modulation methods power is thus directly proportional to the input power. where the analogue input is sampled and where at each sampling instant a code word representing the input is Since code modulation approximates the input signal, n generated. Pulse code modulation (PCM), differential distortion, called quantisation distortion, is introdu- pulse code modulation (APCM), delta modulation (AM) ced. A typical plot of the resulting signal to quanti- and their variants are examples of code modulation sation noise ratio (SNR) versus input signal is shown systems. in Figure 2(a). PCM is widely used in computer interfaces v/hore It can be seen that for a code modulator without it is known as analogue to digital conversion. In companding, a high SNR is only obtained over a narrow APCM, the difference between the analogue inpu<: at range of input signals. Companding however varies the the present sampling interval and its value at the step size such that the normalised input power remains previous sampling interval is transmitted. SK is the at the value corresponding to the optimum SKR for a 52 Region of Region of overload constant 'companding quantisation 2X

-overload |

Relative occurrence of a typical control word 20 «0 60 80 I Relative input in dB. Comparison between the performance of companded 20 10 60 and uncompanded code modulators Normalised input power in dB. Figure 2. Figure t. wide range of, input powers. The SNR will thus be the must be increased thereby reducing the normalised input maximum value as is shoVm in Figure 2(b). The compand- power. Similarly if the relative occurrence is less ing virtually stretches a point on the SNR curve in than X, the step size must be reduced. Figure 2(a) horizontally to give Figure 2(b). There are two practical methods by which digital 2. Digital companding principles. companding can be controlled (a) linear control, and Figure 3 shows a block diagram of a code modulator (b) semilogarithmic control. Logarithmic control will with companding. lead to unnecessarily complex circuitry without any significant improvement in the performance over semilogarithmic control. analogue */• code binary transmitted (a) Linear control input ^ generator signal For linear control the step size is changed linearly, the rate of increase or decrease is thus fixed. Consider the control word depicted in Figure t. If the local occurrence of a control word is used to increase the k demodulator step size by K(l-X) units and the step size is decreas- dr ed by KX units if the control word does not occur, the average step size will adjust itself until the system operates at the desired optimum normalised input power. step step size normalised input K is a positive constant. If the relative occurrence of size store power detection the control word is Y the net increase in the step size = (increase in step size due to the control word Block diagram of a companded code modulator occurring) . x (the relative occurrence of the control word). - (decrease in'step size due to the control word not occurring), x (the relative.occurrence of the control word not occurring.) Figure 3. = K(1-X)Y - KX(l-Y) = K(Y-X) The code generator generates different control If Y> X the step size is increased as required, words until the correct one is obtained, which is then until the optimum step size occurs when Y=X. If Y < X transmitted. A measure of the normalised input power the step size is decreased as required. The step size is detected and this is used to control the step size is thus automatically adjusted to obtain the optimun store which in turn controls the multiplier. Provided SNR. no transmission errors have occurred, the step size at the transmitter and receiver will be the same. The attack and decay time constants are dependent on the relative occurrence of the control words, the In order to derive a measure of the normalised amount by which the gain is changed and the sampling input power, one must select one or more binary patt- period. The ratio of the attack to decay time constants erns or control words, the relative occurrence of which is X to 1-X and this can thus be selected by the proper ; varies with the normalised input power. The best con- choice of control words. trol can be achieved if the function of relative occurrence of the control word versus normalised input (b) Semilogarithmic control - power is a monotonic increasing or decreasing function. Logarithmic control can be achieved by using a Figure 4 shows a typical graph of the relative occurr- nercentage change of gain instead of an absolute change ^ ence of a suitable control word versus normalised input of gain. If A is the Fractional increase of step size power. The optimum SNP is obtained when the relative when a control word occurs and B is the fractional de- : occurrence of the control word is X. crease of steo size when no control word occurs, the If the relative occurrence is more than X, the average step size will remain constant when the relative normalised input power is loo high and the ster size occurrence of tiie control word is X if A and B are selected according to Piuation (1). ~~> 53 (1 • A)X (1 - (i) It is however difficult to achieve true logarithmic control if digital circuitry is to be used, since the step size is changed by A.N or B.N where N is the input/7\demod contents of the digital counter storing the step size. ' \ output The counter can only count in integers but A.N and B.N need not be integers, so that true logarithmic control can not be achieved. One can however approximate logarithmic control by approximating A.N and B.N. Two possibilities for this truncation will be discussed; (1) Approximate A.N and B.N to the nearest integer. This will give a good approximation of logarithmic control but complex circuitry will result. (2) Change the step size by A.M or B.H where M is N truncated to the nearest binary integer. For example when N is between 8192 and 16383 (i.e. between 213 and (2*1»-!)), M is 8192 (i.e. 213). Linear counting is thus used over a nearly 2:1 range of the contents of _ the counter. For large changes in the contents of the counter, the counting is approximately logarithmic binary transmitted signal since M is directly related to N. The companding ratio, which is defined as the ratio of the smallest to the largest step size is _n uininruin determined by the number of bits in the forward and control words reverse counter and can thus be chosen at will, provided the analogue circuitry of the modem is capable of handling the dynamic range. The same principles apply if the relative occurrence of the control word is a decreasing function of normalised input power. The step size should now decrease if a control word occurs and increase if no control word occurs. 3. Examples of designs using digital companding. (a) The first example uses linear companding and will step size help to explain the principles explained above. Figure 5. Consider a single integration delta modulation system with a sampling frequency 31 times the input frequency. The control words choson are 00 and 11. The relative occurrence of the control word increases with normalised input power as shown in Figure 4. If a control word occurs the step size is increased by 1 unit end if no control word occurs it is decreased by 1 unit. (c) Figure 5 shows the transient response of this digital companded delta modulation system. It can be seen that as soon as the transient is applied, slope overload occurs, generating control words which increase the step size. The step size is thus automatically adjusted to obtain the required normalised input power. curves for (b) Digital syllabic companding delta modulation patterns indi- (DSCDM). cated or their This delta modulator was designed for speech applications, a 40KHz sampling frequency and a companding complement ratio of 60d£. The development and the design of DSCDM are discussed elsewhere in more detail (Kef. 8, 20 40 9, 10). This design illustrates the approach that should be followed for the design of digital companding Relative input in dB. for code modulators. Possible control words The following parameters must be selected: Figure 6. (i) Tht sampling frequency. For delta modulation and speech transmission a sampling frequency of uoKHz gives These results were obtained by.computer simulation of A t an adequate performance,this value- was thus used. delta modulation as discussed in Appendix 1. For DSCDM the control words 11 and 00 were chosen. (a) it (ii) The control word. Figure 6 shows the relative occ- put wave: urrence of several control words that can be used. lator. 54 (iii) The type of companding and the companding ratio Semilogarithmic companding and a companding ratio of 60dB were used. (iv) The increment and the decrement of the step size due to control words i.e. the constants A and B. the constants were optimised by computer simulation to yield i = ^ and B = TT . The step size store consists thus of a forward and reverse binary counter, where the clock pulses are inserted 6 bits from the most significant bit and one or two clockpulses are used, depending on whether a control word has occurred or not. Figure 7 shows a comparison between the performance of uncompanded delta modulation and DSCDH. It also shows the performance obtained from the DSCDH hardware. The measuring technique is discussed in Ref. 11.

compandtd d«!lta module- :40hHz.. \ rtsutti for I ardwart. 3.UV (R.M.S.) r ! 20 60 80 Figure 8. Rttativt nput in dB. Figure 7. Figure 8 shows two photographs which illustrate the companding. It can be seen that the normalised input remains the same over a wide range of input powers. (c) Digital syllabic companded PCM. Digital companding can be applied to PCM as well. The design presented here is applicable to 7 bit PCM with a sampling frequency of BKHz and speech as input. Similarly to DSCDN, semilogaritheic companding with a 60dB companding ratio was chosen. The control words chosen were whether the two most significant bits, excluding the sign bit, were non zero. One will thus have a control word if the amplitude is greater than a quarter of the maximum amplitude. The relative occurrence of the control word again varies with a shape similar to Figure 1. The constants A and B to give the optimum performance were obtained by computer simulation as A = - i Figure 9. The resulting performance, as obtained by computer simulation is compared with the performance of (b) It will cause an error in the step size. uncompandtd PCM in Figure 9. The first effect will normally disappear with tiee, but the error in the step size may persist under certain *•• Effect» of transmission errors. conditions. The step size stores in the transmitter A transmission error will cause two effects: and receiver will synchronise if the upper or lower limits of the step size are obtained at both location«. (a) It will generate an error in the demodulated out- In many situations this will periodically occur. With put waveshape, similar to the uncompanded code modu- speech for instance, pauses occur between words which lator. are sufficient to produce the smallest step size,caucing 58 the step size stores to synchronise. HOSOKAWA, S. and YAMASHITA, K., "Companded delta modulation coders of the h power and If the step size does not normally reach the limit-, 2/3 power types". Electron Comm. Japan. synchronisation can be achieved artificially by short- Vol.51A, No.11, November 1968, pp.18-26. ing the analogue input of the modulator periodically during the periods that no measurements are made at 6. HÄUSER,K. and ZARDA, S., "The design of digitally the receiver. A second method of obtaining synchron- controlled delta modulation CODECS." isation is to transmit the contents of the transmitter Proceedings I.R.E.E.. Vol.32, No.7, July 1971, | step size store and use these to set the receiver pp.286-295. | ;tep size store to the required value. This will however lead to more complex circuitry. 7. SHINDLER, H.R., "Delta Modulation". I.E.E.E. 5. Conclusions. Spectrum. October 1970, pp.69-78. A method for designing digital companding has 8. KIKKERT, C.J., "Digital techniques in delta been presented. This method is applicable to PCM, modulation". I.E. (Aust.). Data Transnission I i APCH, AM and their variants. Since the companding Conference. Brisbane. June 1970 and I.E.E.E. uses digital techniques it has the advantages that Trans. Communication Technology. Vol.Com.19, digital techniques can offer. The principles outlined No.4, August 1971, pp.570-574. are however applicable to analogue techniques as well. The resulting code modulators will have a wide 9. KIKKERT, C.J. and The University of Adelaide, dynamic range, making them suitable for signal trans- "A method for improving modulation efficiency. mission, analogue delay lines (the delay is obtained Aust. Patent No. C34963/71, by delaying the binary transmitted signal in a digital U.S. Patent No. 193,110. delay line), analogue to digital conversion and many other applications. 10. KIKKERT, C.J., "Digital techniques in delta Appendix 1. modulation." Thesis (Ph.D.), University of Adelaide, 1972. COMPUTER SIMULATION. Delta and Pulse Code Modulation was simulated on 11. KIKKERT, C.J., "Measurements of quantisation the CDC6400 computer. Two designs presented in this distortion for random inputs." Proc. I.R.E.E. paper were oriented towards speech transmission. Aust., Vol.32, No.12, December 1971, Speech was simulated by filtering a pseudo random pp. 459-464. sequence through a digital filter with a frequency response similar to speech. This waveform was then amplitude modulated with a low frequency wave, simulating syllabic variations of power. Any other input signal can be obtained in either a similar fashion or by direct generation. The input signal then passes through a simulated code modulator and at each sampling interval, signal and quantisation noise contributions are calculated, enabling the SNR to be evaluated. ACKNOWLEDGEMENT. The author wishes to thank the staff of the Electrical Engineering Department at the University of Adelaide, STH. AUST., and in particular Mr. D.C. Pawsey and Dr. B.R. Davis for their encouragement, helpfull comments and suggestions.

REFERENCES. 1. PANTER, F.F., "Modulation, noise and snectral McGraw Hill, New York, 1965. 2. CARSON, A.B., "Communication Systems." McGraw Hill, New York, 1968. 3. BROLIN, S.J. and BROWN, J.M., "Companded delta modulation for telephony." I.E.E.E. Trans. C.9, muni cat ion Technology. Vol. Com. 16, No.l, February 1968. GREEFKES, J.A. and DE JAGER, F., "Continuous delta modulation." Philips Research Report 23, No.2, April 1968, pp.233-246.

56 VIBRATION TECHNIQUE FOR ROT DETECTION IN WOOD POLES A.D. Shaw, Dip.E.E., M.I.E.Aust. Development Engineer, Retail Supply Branch, Hydro-Electric Commission of Tasmania

SUMMARY A new method Is proposed for rot detection in wood poles used for electricity distribution purposes. The test method has been developed for the detection of concealed ground-line rot in natural round hardwood poles used in Hobart, Tasnanla, but it holds promise of being adapted by any ucer of buried wood poles, whether treated to minimixe the effects of rot, or in the natural condition, and say have wider applications such as for narine piles and detecting dangerous trees in public parks. A resonant vibration technique is employed which Is capable of detecting seriously defective poles not at present accessible to inspection. It is essentially non-destructive in nature and provides a degree of precision and security not formerly available. Present pole testing methods are shown to be economically wasteful and deficient, safety wise, where poles have concealed rot. The equipment used is readily available, portable, inexpensive and can be operated by one man of the calibre of workers now employed in pole testing.

I. INTRODUCTION Because of these deficiencies, an investigation The present state of the art of wood pole sound- was commenced several years ago in an effort to ness testing in the electricity supply industry car. approach the problem In a quantitative way. scarcely be regarded as satisfactory. In fact, this activity would probably be unique in having remained The following methods which seemed to offer a virtually unchanged, since it was first found neces- possibility of a solution were investigated in turn:- sary to institute regular pole inspection and replacement programmes. 1. Radiation propagation from a radio-active source. The deficiencies in present: testing methods need little elaboration, have developed by 'rule-of-thumb', 2. X-ray photographs. and are essentially vague. 3. Seismic. Present methods commonly applied throughout the 4. Ultra-sonic vibrations. world, and with particular reference to the Common- 5. Measured Impact. wealth of Australia, are of two general types:- 6. Sonic vibrations. (1) Aural interpretation of sound waves. The only practicable method proved to be some form (2) Direct investigation with auger, pick and of sonic vibration, and the following discourse axe, or other special device designed to presents a method of rot detection of wood poles at penetrate the inner fibres of the pole, to the ground-line, based on a sonic vibration, which detect areas of rot near the ground-line. satisfies the requirements of Improved precision in assessment and also Is, for all practical purposes, The use of sound waves depends on the interpret- non-destructive in nature whilst at the same time ation placed on the sound of impact made by a hammer- being operable by present pole inspectors with some blow when the pole is struck. training. Direct investigation involves cutting into the The principle employed is that of a measurement pole and further accelerates decay by permitting of emergent energy resulting from a vibration force freer access of bacteria. Simultaneously, the applied through a diameter of sound wood at an arbit- strength of the pole is further reduced by the rarily selected, but constant height from the ground, removal of good wood. and an observation oi the energy variation curve through the pole at points between the emitted signal Because of the uncertainty of the methods and the region of the ground-line. employed, pole inspectors tend to be conservative and poles are frequently condemned when they have The energy variation curve is found to be quite yesrs of life remaining. This is no reflection on different from that of a good pole, if the pole under the ability of pole Inspectors in general, but rather test has serious rot at or around the ground-line. indicative of the need for a precise testing procedure to augment and largely replace existing methods. By assessment of tests conducted on a large sammle 57 of poles having serious rot in this region It has If the exciting force and velocity components of been possible by empirical means to arrive at a mechanical impedance are brought into phase with etch characteristic curve which always indicates a bad other, 4 becomes zero, the Imaginary component of pole. equation (1) disappears and the general expression simplifies to: Each pole is used as Its own standard. This approach eliminates the problems posed by non- 2. TT" R uniformity between poles. The assumption made is that the condition of a pole several feet above where R Is the dynamic resistance or damping effect, ground is, for all practical purposes, satisfactory. Ref. (3). The condition of the vibrating system at resonance is therefore such that the mechanical II. THEORETICAL CONSIDERATIONS impedance Is a minimum. When a sinusoidal force Is applied to a solid III. APPLICATION OF THEORETICAL CONSIDERATIONS homogeneous object, a resulting motion of the same frequency of vibration occurs at the point of applic- The problem of determining the patterns of energy ation of the force and also throughout the object flow through homogeneous cylinders proved on examin- being vibrated. ation, to be relatively Inaccessible to mathematical treatment. This prompted the author to perform an The object possesses properties of elasticity and investleatlon of vibration phenomena through cylinders damping, which can be represented dlagrammatically of uniform density and moderate dimensions. These for the steady-state condition In accordance with cylinders were subjected to a controlled sinusoidal Fig. 1, Ref. (1). vibration, transverse to their major axes.

FIG. 1 The purpose of the investigation was to arrive at an experimental datum, using cylinders as a comparison< with good and defective polet- and good pole samples of ' similar dimensions. The cylinders were tightly wound paper rolls of commercially available paper, run off to the correct dimensions. A 14 inch diameter cylinder weighing 62.2 lbs. per cubic foot and an 11 inch diameter cylinder weighing 44.6 lbs. per cubic foot were thus obtained. Do 00 The timber species under review were Eucalyptus SPRING Globulus (Blue Gum) and Eucalyptus Obllqua (Stringy Bark), having air dry densities ranging from 48 to (ELASTICITY) 67 lbs. per cubic foot and the particular samples weighed 62.8 lbs. per cubic foot and 66.8 lbs. per cubic foot for the 14 inch and 11 inch diameter Diagram of Vibration Components specimens respectively. The selection of specimen Appropriate to a Homogeneous Object. size was based on the average dimensions of electric power distribution poles which usually have a diameter at the ground-line of 14 inches grading to 12 inches, The differential equations which describe the with an occasional pole as small as 11 inches In : motion of the system can be written In ten» of the diameter at this point. applied force and velocity and there Is a fixed phase relationship between the sinusoidal force and the The paper rolls were wound on small diameter ; notion of all parts of the object. cardboard formers, and the central pipes were filled i with a tightly rammed sawdust/cement grout of similar Mechanical Impedance, Z, has been defined as the dry density to the paper. The rolls therefore, { ratio of a force-like quantity to a velocity-like could not, in the strictest sense, be regarded as quantity, when the arguments of the real (or Imagin- quite homogeneous. Also the layers of paper con- ary) parts of the quantities Increase linearly with stituted a variation from the uniform. Nevertheless, tlae - Ref. (2). the rolls were « convenient practicable solution and a reasonable datum on which to gauge wood poles and Mechanical Impedance is proportional to the pole samples. Paper rolls and wood pole samples applied force and to the inverse of the resulting were 4 feet in length. velocity in the same system and under steady-state conditions of force and velocity and, hence, Fig. 2 Illustrates the situation observed between i frequency. the 14 Inch diameter paper roll and wood pole samples. Similar results were obtained for the 11 Inch diameter The complex algebraic expression connecting paper roll. mechanical impedance, force applied and resulting velocity can be written thus: The phase angle for a signal of given frequency, will not be the same for different wood pole samples 1. Z - £ (cos 4 + J sin 4) of the sane species and dimensions, since no poles are Identical. Timber density within a given pole where 4 Is the phase angle between the rotating is not altogether uniform, due to the effects on vectors of applied force F and velocity V. 58 growth rate of yearly seasonal variations and also through-put of energy which can be obtained and »till long term weather cycles• However, the main problem ensure the portability of test equlpaent. is the infinite variety in the magnitude of radial cracks and their severity. In hardwood poles, these FIG. 3 are numerous and give rise to anomalies in cross- section. FIG. 2 0* 3" 6' 0* r> 9* 3' 12" 6" 15' 9 Iβ' 12" 21' 15'- 2«: • TYPICAL KSONAMt-LIM Iβ*. CURVE 27' • - TVMCM. NON RESOMaNT-lMf 21" CUM« 30' 2*" 14 IN PIPER ROIL U IN WOOD POLE SPECIMEN 3* IN SOUND CONDITION 27" 36 30! 0 WO 200 PERCENTAGE RMS VELOCITY AT TRANSDUCER 33". 361 0 fOO 200 Energy Variation Curves of Cracked New Poles. PERCENTAGE RMS VELOCITY AT TRANSOUCfR

FIG. 4 Energy Variation Curves - Resonant-Like and Non-Resonant Like Frequencies.

In the instance of cracked wood poles, it is observed experinentally, that there can be large divergencies in velocities between signals transmitted at closely adjoining frequencies, where one is resonant like, and the other non-resonant like. A radial crack in a pole is a partial discontin- HI SONANT CUM uity and can be represented as a large localised NOH «SONANT CUM increase in mechanical impedance, when the force and velocity waves are not in phase. A pole may be regarded as a stack of thin sections, each varying in greater or lesser degree from Its fellows. Sections havlv.g no dlscontin- utiles will have lower mechanical impedances than those having discontinuities. Such varying nechan- lcal impedances will result in velocities of emergent waves having random values. It follows that as Z Iβ a minimum at resonance, the effect of radial cracks at resonance is also a nlnimua. This is observed experimentally in Figs. 3 and 4. It is, therefore, necessary to select a frequency of 0 100 200 excitation which exhibits a pronounced resonant like KKENTAGC "MS VCLOCITV AT TRANSOUCE» effect, both from the point of view of obtaining a standard situation for all poles for the purpose of securing a datum, and also to obtain the best Energv Variation Curves of Cracked A«ed Good Poles.

59 Fig. 5 shows Chat the through-put of energy is FIG. 6 unaffected to any significant extent by the point of excitation beinR away from a precise diameter, so long as the departure from a diameter is small. However, the lateral energy distribution curve is exceedingly distorted by significant departures from the true diametral situation. VARIABLE FREQUENCY FIG. 5 OSCILLATOR

VACUUM NAMPUFKRX TUBE 100 VOLTMETER SPECMEN ELECTRO- L MAGNETIC «0 \ HAMMER

CATHODE «0 \ I R.MS RW / fVEIOCITV PER \ V / OUTPUT CENT \ / 40 \

i' I" t" mm ir 6 Schematic Arrangement. ORCUMFERENTUl DISTANCE PROM TRUE CENTRE

V. TEST METHOD AND RESULTS - PAPER CYLINDERS AND SIMILAR HOOD POLE SAMPLES Energy Distribution About True Diameter of 14 Inch Diameter Pole. The electro-magnetic hammer was firmly supported against the sample which was resting on end on the fl ground and allowed to vibrate against a clout driven IV. TEST EQUIPMENT into the specimen near the top. On the opposite side of the specimen, on a The test equipment consisted of a commercially diameter through the clout, a nail was driven available electro-magnetic shaker with an output of horizontally and the transducer solidly attached about 10 volt amps converted to operate as a small to it. hammer of very low mass, a variable frequency oscillator and amplifier to drive the hammer, a read out device and a 12 volt automotive battery. Other nails had previously been placed at inter- The oscillator circuit was arranged to provide a vals down the specimen in a vertical line through the square wave form. top nail. Values of output in terms of velocity were observed by progressively moving the transducer down the specimen from nail to nail, the hammer Initially a cathode ray oscilloscope was used in tandem with a vacuum tube voltmeter, both being remaining fixed and with unvarying output in terms driven by an electro-magnetic transducer, but as the of frequency and power, at an arbitrarily selected vibration patterns became well known, it was found value. In effect, the hammer delivered a steady simpler to use the voltmeter alone. Later the stream of tiny impulses of identical magnitude into electro-magnetic transducer and vacuum tube voltmeter the pole or paper specimen. were replaced by a vibration meter and plezo-electric transducer, which produced similar, but somewhat more It was,found that by selecting various frequen- refined results. Both instruments registered R.M.S. cies, it was possible to reproduce within close values of velocity. limits, the energy variation curve for paper and wood pole specimens, which were without cracks, so The equipment used, and its arrangement, is set long as frequencies were not as low as about 50 Hz, out diagrammatlcally in Fig. 6. and that curves were Independent of resonance conditions. This was not always so with pole specimens which had prominent cracks. These findinRS, being basic, have already been discussed in Section III, APPLICATION OF THEORETICAL CONSIDER- ATIONS.

60 VI. TEST METHOD AND RESULTS - WOOD POLES Table 11. Vibration Test. Results of Application of 150Z Criterion. This method varied little fron testa on the samples except that, resonant like conditions were first obtained, observations were only taken at four Condition as Assessed after equidistant points fron 2 feet 6 Inches above ground sectioning Total to 6 inches below ground, and these were taken through Dangerous Not Dangerous two diameters, 90 degrees apart. Condemned 20 10 30 Tests were conducted on good poles, recently installed, both pressure-impregnated and natural Not Condemned * 17 53 round, which showed a good measure of agreement, and 70 also natural round poles, which were known to be in poor condition near the ground-line. Such poles were Total 37 63 100 subsequently extracted whole for evaluation, quite large increaaes in signal being evident, when compared with readings obtained for corresponding positions of * These poles had decayed exteriors. Such poles good poles. fall within the category of poles for which normal methods are acceptable, and for which the application This preliminary work culminated In a similar test of some sophisticated device is unnecessary. programme of 100 natural round poles of various sices and pole top construction, which were selected by an Table Hi. Normal Method in Combination with experienced foreman, after the pole inspector had seen Vibration Method. them. Each of these poles was bored in the suspect region and found to be decayed in some degree, although not necessarily condemned by the pole Condition as Assessed after Inspector, or regarded as being excessively decayed sectioning and at the end of Its useful life. Total Dangerous Not Dangerous Defects sought were those of a concealed nature Condemned 37 10 47 and not visible, although some poles also had visible defects. Not Condemned 0 53 53

Following tests with the equipment, poles were extracted whole, and sectioned at each test point. Total 37 63 100 The percentage reduction of area of good wood due to rot was measured, and an individual assessment of each pole was made in tern of whether it was considered Fig. 7 depicts a series of typical graphs of dangerous or not. It was considered that a pole poles having dangerous defects near the ground-line, would be dangerous, allowing a reasonable margin of and similar graphs of good poles, illustrating time between condemning and renewal, if its strength wide differences in characteristics which will enable were reduced below SOZ of its original strength. By the detection of defective poles. Typical sections selecting arbitrarily a minimum value of 150Z of the of a bad pole are seen In Fig. 8, sections 1 to 4 orlglcal observed velocity output at any of the six grading from 2 feet 6 Inches above ground to 6 Inches positions of test below the two used for the datum, below ground respectively. It is of Interest to as the criterion for condemning a pole, the following note the wide variations in the geometry of rot evaluation resulted:- between sections and also the extent of some creeks.

Table 1. Field Inspection of 100 polet by Fig. 2 demonstrates the divergence which a good Pole Inspector using Normal Methods. wood pole shows from the more Ideal case of the paper roll. It will be noted that whilet the curve for the good pole shows some distortion, this is not such Condition as Assessed after as will confuse the issue between the characteristics sectioning of good and bad poles. Total Dangerous Not Dangerous The Increase in value of the resonant like signal, where rot is encountered, is explained in terms of vibration theory, in that light materials have a Condemned 29 36 65 lower mechanical impedance than heavier ones. Rot manifests Itself as a material of lower density than Not Condemned 8 27 35 surrounding good wood. The less dense material reduces the mechanical imped«nee, and for a given Total 37 63 100 resonant like input signal, emergent velocities «re higher than would be the case through a section of uniformly good wood.

61 FIG. 7 VIII. EFFECTS OF INACCURACIES IN SETTING UP THE EQUIPMENT

MO KLB It is desirable in setting up for testing, that (New) nails he accurately located, since it is obvious that r.^ MOD PW.U (MIO) significant errors «ill affect the characteristic

However, small vertical displacements are not very important, neither is the effect of not being exactly on a diameter when setting up for the top read inc. FIR. 5 shows that the effect of serious decent ring results in a severe drop in signal, which mav prevent a satisfactory test fron being obtained, consequently it Is desirable to be careful to obtain a near-centre position.

2« Fie. 5 also shows, however, that minor departures up to 1 inch from the true diameter are acceptable. 24 // The dramatic differences shown between resonant ri like and non-resonant like curves, indicate that great care must be taken to ensure that a resonant 36 like point is obtained before testing, where cracks ri are present. It will be noted that with new poles the divergencies between resonant and non-resonant curves are not extreme, but that these increase 0 100 200 significantly with aged good poles. A pole that PfBCENMOE SMS VElOCirr a TMNSOUCER has dried out often has more cracks than a new pole and this is verv evident when coaparing Figs. 3 and 4. Such cracks will tend to channel vibratlonal energy into preferred longitudinal paths, and give rise to increases in observed velocities in the lower regions Energy Variation Curves of Good and Had of poles under test. Poles Under Resonant Like Conditions. As it Is not possible to assess whether a pole Is cracked, merely from inspection, this requirement Is Fin. « fundamental to a meaningful test.

Resonant characteristics are more or less easily obtained, depending on the point on the spectrum being worked. A particularly good point was obtainable in the range, 10ft - 120 Hz which produced a large increase in signal above the basic energy curve. This had the advantage of being exceedingly easy to detect and select.

Nails should be driven in a uniform distance which, in Itself, is unimportant for natural round poles. In all cases it is imperative, however, that nails he driven so as to make the best possible contact with the pole and not into defective wood.

A depth of 4 Inch into solid material is recom- mended. This depth would also be suitable when extending Investigations to pressure-impregnated poles. Little effect is observed if nails are not driven strictly normal to the major axes, however, this should be a desirable objective.

IX. EFFECTS OF FlHLn CONDITIONS

The main problems in the field arise from weather. Poles which have been rain-soaked or water-logged, require further Investigation. However, it is known that there is little effect so long as the pole has not been subjected to heavy rain over a prolonged period of several days.

Tvplcal Cross Sections of a Rad Pole. A pole which has been thoroughly saturated around ehe ground-line presents a problem In that the observed values of velocity at the ground-line are 62 higher Chan the well established characteristic curve By more precise evaluation than is afforded using for poles in normal conditions. normal methods, the number of Incorrectly assessed, but dangerous poles can be significantly reduced and Wind la the main problem and gusts above 10 Biles an error made during operating is likely to create a per hour tend to make testing difficult as they prod- 'fail safe' situation, because higher random readings uce minute novements of the poles, which interfere tend to occur at frequencies away fron resonant like with signal Intensity. For thie reason it is desir- peaks, the only consequence of which is that a pole able to obtain a strong basic input signal, which might- be condemned prematurely. minimizes the effects of wind. At its present stage of development the equipment It has been found that it is essential to penet- has limitations, in that water-logged poles do not rate to six inches below ground level for tests to be conform to the normal pattern and cannot be assessed effective. Working down Co ground level only, correctly. Also, windy conditions make testing produced results in the 100 pole sample statistically difficult to the point of abandonment during some no better than are obtained by present methods. days. The problem can be mitigated to some extent, but is Inherent in the system and tends to restrict This presents a difficulty where poles are set in its use to reasonably calm weather. It appears that solid pavements, however it has been possible to keep results are not significantly affected by superficial excavation to a very minor level and the problem is dampness of poles caused by rain so long as this is serious only where poles are entirely surrounded by not prolonged. concrete. It is reiterated that tests up to date have been X. TESTING STAFF, TIME INVOLVEMENT, COST OF EQUIPMENT restricted to defective natural round poles. Pilot routine tests on such poles are encouraging and have Ideally, testing staff would be at the tradesman confirmed in large measure the conclusion that the level, as the equipment is essentially complex, test is very effective. however it has been operated successfully by a pole inspector. Once the technique of setting up is It may, therefore, be anticipated that an exten- mastered, the routine to obtain an assessment is very sion of teating to pressure-impregnated poles, using simple as the read-out device is calibrated in the similar techniques of test and subsequent evaluation, form of "good" and "bad" zones. should result in the introduction of the system for these poles also. The time to test a pole during previous tests averaged 20 minutes. This time Is considered excess- In Hobart, only about half the total poles ive and was affected by the requirement to move the inspected have central concealed defects, many poles transducer progressively from point to point, and being subject to external deterioration, and hence also to check that there had been no change in input may be assessed by present methods. The statistics during the protracted period whilst readings at refer to poles where an attempt was made to obtain a different points were obtained. larger proportion of poles with hidden defects than occurs during normal pole Inspections. As the input level can change if left for long periods, this procedure adds significantly to the time By employing similar evaluation techniques it may for testing. be possible to extend the uses of the device to such things as marine piles and trees in public parks or The problem can be circumvented by using a trans- any construction involving timber, which is likely ducer for each nail and a switching unit, which will to suffer deterioration with time. reduce the tine of testing to about 12 minutes. This time is considered acceptable and a system of this ACKNOWLEDGEMENTS type is to be introduced shortly. This paper would be Incomplete without mention of The cost of providing the equipment described and the enthusiastic help given by colleagues, partic- modified for field use by the addition of transducers ularly in the preparation of testing equipment, and and switching unit has been $1200. It could be that in the arduous and repetitive task of obtaining basic • unified approach to the problem of cost could reduce data. this somewhat, a* the experimental data revealed a progressively broadening view, which led to modific- Similarly, the staff of the University of Tas- ations from time to time. mania greeted the task of basic investigation with great enthusiasm. Particular thank* are due Prof- XI. CONCLUSIONS essor G.R.A. Ellis and Dr. B.I.H. Scott of the Physics Department and Profesaor C.H. Miller and The system can be operated after a short training Messrs. J. Beresford and R. Wherrett of the Elect- period, by personnel of similar background to that of rical Engineering Department. presitnt pole inspectors, and determination of pole condition is v*ry simple. The equipment is easily Finally, thanks are due to the Commissioner of portable and can be operated by one man. The cost the Hydro-Electric Commission of Tasmania, Sir Allan of labour per pole is about one and a half times that W. Knight, for his kind permission to publish this for present methods. However, this is not very paper. significant when considering the large economies available from extension of pole-life in many inntances.

63 Reference» (I) VAN SANTEH, G.U. - Mechanical Vibration. 3rd «d. Eindhoven, Philip» Technical Library - 1961, 45p. (2) HARRIS, CM. and CREDE, C.E. - Shock and VihratIon Handbook. New York, McGraw Kill Book Co. 1961, Section 1, 20p. (3) KELLER, A.C. - Fundamental» far Mechanical Impedance Analyals. Spectral Dynaalca Corporation of San Diego. Technical Publication Ho. M-2, 6-67.

64 FAULT DETECTING IN A PROCESSOR-CONTROLLED TELEPHONE SWITCHING SYSTEM

N.W. McLeod, B.Sc, M.I.E.(Aust), A/g Engineer Class 3, Australian Post Office Research Laboratories

SUMARX. The Switching and Signalling Section of the Research Laboratories of the Australian Post Off let Is developing a processor-controlled telephone trunk exchange, as part of a national field trial of a signalling system (the C.C.I.T.T. No.6 Signalling System). This paper describes the test procedures for this switching system, and the test equipment developed for their implementation. A system such as this involves th« marriage of Hardware (the Switch Block and associated control circuitry) and Software (the operational progress in to* control processor). It is not practicable to take the Exchange and "plug it in" to the Processor, hoping for the best. Rather, extensive testing is required involving many phases of sub-system testing and gradually bringing these parts together in various configurations of simulation.

I INTRODUCTION have been kept (Ref.6). Semiconductor control and interface circuitry replace many other electro- The Switching System discussed here is a Stored- mechanical functions. Digit analysis, signalling and program controlled telephone exchange developed part- overall control and supervision functions have been icularly for the National Field Trial of the CCITT allocated to software in the processor. The APO Bo.6 No.6 Signalling System (Ref.l). The No.6 System is a Switch is a mixture of a number of different techno- common channel scheme, specified by the CCITT of logies, electro-mechanical, electronic (integrated which the Australian Post Office (APO) is a partici- circuits), and processor control. pating member (Refs 2,3,it). It concentrates all the signalling information for a group of speech circuits The control of the No.6 Switch is based on the between processor-controlled exchanges on to one sig- principles shown in Fig.2 (Ref.7). The system is nalling link. made up of a number of groups of controlled equipment, each group consisting of a number of units. Tht The place of this exchange, the No.6 Switch, in groups are closely related to interface equipment the trial network is shown in Fig.l. It works with which provides access to the processor. Software a. processor-controlled exchange at the Overseas Tele- operations in the processor generate instruction» cooninications Consiission (Australia) (OTC) in and addresses which are passed through it* output to Sydney. the Instruction Highway, which is connected to all interface equipment. Depending on the group address The switch-block of the No.6 Switch uses 100 on the highway, the appropriate group interface will lines of a standard electro-mechanical crossbar trunk respond, and pass the instruction to the vAt (or exchange to switch the speech paths. (A typical units) of that group, dependent on the unit address crossbar trunk exchange in the Australian network is included with the instruction. equipped for 2000 or UOOO lines) (Ref.5). There are generally two types of instructions The standard equipment has been extensively modi- sent on the highway, drive or scan. The former will fied for electronic control. The crossbar switches cause some function, such as a relay, to operate in and the associated electro-mechanical control cir- the unit. A scan Instruction will cause the unit to cuits that select a path through the crossbar switch send data back through the Interface via the Data

MELBOURNE SV0NEY NETWORK (34) MIO NO. S SWITCH INTERNATIONAL SPEECH PATHS PKTMMK

PROCESSOR PROCESSOR

A.P.O. NO. S TRIAL EXCHANGE L.M.E. A.K.E EXCHANGE »WINDSOR, MELBOURNE O.T.C. NATIONAL AND INTERNATIONAL TRIALS PADOINQTON SYDNEY

Fig. 1 - C.C.I.T.T. lfo.6 Signalling Syttm Field Trial Hetuoek 65 GMX» WO. 1 OMXff MO. 2 OBOU. NO. 3 OHOUP NO. 4 to the Ho. 6 Swltdh are aade up of a three part word: G - Group "Address - to Indicate which group of ! UNFI» OF , circuits is being addressed. i U - Unit AAireas •- to Indicate which unit in this j group is 'being addressed. ; T - Function Code - to indicate the function to be performed by this unit. This instruction has the following fonsat. Bits:-.1,2,3,1» 5,6, ... 11 12,13.1V.15.16 Group Add. Unit Add. Function Cede This f oraat allows a total of 16 groups, 128 units per group, and 32 functions, which is aeple for the ' Ho.6 Switch. ; II INSTRUCTION DATA HWIWWV The control of the Ko.6 Switch aaplegrs siaplwt : HfGHWHV operation, that is, there la only one control -pro- ; cessor. Koreally, for security reaaaas, two (or i «ore) control processors would be u*ed - duplex ope- | W^. 2 - Pzvoeesor Controlled Equipment ration. Tbe original aysteai design was 'baaed oc dup- ! lex operation, using two processors, two processor : access higbwey», and two sets of interTace eqaifaent. ; Highway to toe input of the processor. The duplication would have *een taken rlgat down to ! tbe individual controlled unit, sucn that a single ; Other connecting 'highways are included. One .pro- Tlip-^flop could be set or revet independently froai • vides the processor with simple direct monitoring of either tbe interface equipment. Another interconnect» the Ject «as to assess the Ho. 6 Signalling Sys'tem, not an groups of interface equipment,, to allow the transfer exercise in duplex control. Hence, for acre effici- ', of infonMtion between the groups without involving ent use of rcisources, lapleaentation of the *t>,6 ; tbe processor and, thus,, saving some of the latter's Switch control was limited to simplex operation. • time. Information for the processor is collected frost ; In the Ho.6 Switch, these highways «re collect- the to.6 Switch by .scanning every circuit on a re- >• ively grouped under the title "Processor Accos High- petitive basis. It is important thtt the processor ; way'".. becosKfa aware of new data within a particular time : delay. Hence,, some circuits are scanned 'every 50m*, ; She Processor used in the Wo.6 Switch operates on even though the circuit condition may -only change a sixteen-bit word. Hence the Instruction and Data every three -minutes. This type of scanning, in gene- , Highways are based on 16 bits. For optimum equipment ral, makes poor use 'of processor time. However, It layout» and simplifieation and minimisation of gat- does represent some simplifications in system design, ing* each unit has one unique address. Instructions and the Ho.6 Switch is small enough to not overload

Ml. 5 - Tt* field Trial tmhmg* %ritdii% tquipmtnt Fig. 4 - The Control Prooeetor and Peripheral Equipment t trord: the processor's capacity. In a larger system, a more oup of sophisticated scheme would probably be employed using interrupts. t in thit The telephone exchange switching equipment is shown in Fig.3. The first two racks of equipment contain the crossbar speech-path switches. The remaining racks contain the equipment associated with the individual incoming and outgoing speech circuits and the processor access interface equipment. The control processor is shown in Fig.U.

II THE PHASES OF TESTING

Processor time is at a premium before the cutoyer of a stored program controlled exchange due to the extensive testing of operational programs (software). At the same time, testing of the switching equipment (hardware) must proceed. A worthwhile saving of processor time is possible by simulating some of the processor control functions so that equipment can be developed without excessive demands on processor time.

In the No.6 project, the testing of equipment is divided into a number of different categories. Fig. S - The Electronic Circuit Teeter (a) "Hardware Testing Hardware", (HW-HW) where test equipment is used to run the hardware through a series of tests. In the Research Laboratories, an instrument called the "Electronic Circuit Tester" (Fig.5) has (b) "Software Testing Hardware", (SW-HW) where spec- been specially developed for both electrical and ial programs are written by which the processor functional testing. It is used in all stages of de- drives the hardware. These programs are not the sign and development of equipment, including the operational programs to be used in the completed testing of new devices, the prototyping and testing exchange. of printed circuit boards, modules, and the testing of sub-systems. (c) "Hardware Testing Software", (HW-SW) where portions of the operational programs are run with por- The Electronic Circuit Tester has twenty-five tions of the switch-block. Simple functions are per- lines, that is, modules with up to 25 pins may be formed in the hardware, and the response of the soft- controlled and monitored. Each of these lines may ware is checked. have one of ten independent functions connected to it via ten position "thumb-wheel" switches. Functions (d) "Software Testing Software", (SW-SW) where por- available include power supplies, input keys associ- tions of the operational programs are run with spec- ated with circuits to provide bounce-free drive for ial simulation programs, and a sequence of tests att- the manual simulation of pulse trains, output level empted. GO/NOGO indicators, a pulse generator, and metering.

(e) "Operation". When significant portions of the The primary application of the tester 13 a manual operational software and hardware are tested. The instrument. For the prototype and fault finding sta- rystem can be exercised with special test calls and ges, the user needs the full flexibility of manual extensive monitoring. testing. This is not always available with automated testing because of the constraints imposed by a rigo- As described above, the processor and hardware rous sequence of tests. are connected by a single processor-access highway. • Information is distributed on this highway on Twist- The use of this tester has assisted greatly in ed-pair cables. Special Integrated circuits have the completion of a number of major project« within been designed in the Research Laboratories for Driv- the short tin« available. It has also allowed the ing and Terminating the Twisted-pair lines, and completion of these projects with no subsequent de- Custom-built by A.W.A. in Sydney (Ref.6). This high- bugging or modification being necessary. The ex- way is a very convenient point to separate the pro- perience gained with this tester is being used in the cessor and switch block, and insert various test development of a Mark II version. The new teeter equipment. will have general improvement in characteristics taking advantage of state-of-the-art components. Also, Ill PRELIMINARY TESTING the number of test lines will be increased from 25 to 70 to cope with the increased size and complexity of In the development of digital equipment, it is modern printed circuit cards and complex Integrated essential that testing be performed from the beginn- Circuits. ing, including basic components. Experience has shown that all Integrated Circuits should be tested During the production phase of the No.6 Switch, on delivery. automatic test equipment was used for the hundreds of printed circuit cards used, as described in a com- 67 panion paper (Ref.9). This equipment used a specially produced peripheral associated with a control processor. (The same processor that is used for the No.6 Switch.)

IV THE PROCESSOR SIMULATOR

During the development of the switch block, the processor was being employed on the testing of the operational programs, and was therefore not available for hardware testing. In view of this, a device called the processor simulator was developed. It was used for "Hardware Testing Hardware", by disconnect- ing the processor access highvay from the processor, and connecting the highway to the simulator instead, as shown in Fig.6. The peripheral register shown there is a simple buffer system to interface the electrical characteristics of the processor to those of the processor access highway in the switch-block. It also performs some minor logic functions.

OATA KWV. ——4 -X PROCESSOR PERIPHERAL REGISTER INSTRUCTION -X H'WAV Pig. 7 - The Prooeeeor Simulator

level. The Instruction word and clock are sent to MINIPROGRAMMER SIMULATOR the exchange via the instruction highway.

The logic diagram of the simulator is shown in Fig. 6 - Testing the Switch Block with the Fig.8. (Symbols are in accordance with Ref.10.) Prooeeeor Simulator The 16 keys used to set the required instruction code are shown on the lefthand sue. These are grouped according to the G, U, and V pattern defined above» The prime function of the simulator (Fig.7) is to A seventeenth key is provided which controls the generate a 16 bit instruction word together with a clock line. This pulse is used in the Ho.6 Switch clock bit. Each bit is manually controlled by its to clock the flip-flop relating to the unit and individual key which sets it to e logical high or low function addressed by the instruction.

TO MINIPROORAMMER OATA HWIjM

r NORMAL INT. DATA HHKAY t PR

GROW 1 - 4 4 PR

UNIT 1 - 7 7 PR

FUNCTION 2 - f 4 PR

FUNCTKM 1

CLOCK t PR

It is »ith loads struc proce Fig. 8 - Prooeeeor Simtlator, Logic Diagrcn the n 68 Timing circuitry included in the simulator may be employed to chop or cycle the instructions set on •ATA HWY the keys. These pulses assist the operator when -X PROCESSOR PERIPHERAL cr ^D using an Oscilloscope to monitor the response of ths REGISTER r NO.6 exchange to the instruction words. The timing may INSTRUCTION X 1- SWITCH be derived from the internal or external oscillator, H-WAY 1 or under manual control. This allows the equipment U to be tested at various speeds - high speeds to check propagation delays, etc. and low speeds to allow ~1 visual monitoring of some operations. MINIPROGRAMMER ' SIMULATOR

Gating is provided to chop or cycle the instruction words under the control of the timing circuitry. Fig. 9 - Testing the Switch Blook with the If the cycle generator (a binary counter) is em- Prooeaeov Simulator and Nini-Programer ployed, the same function code is sent to all addresses, one at a time. This allows the operator, for example, to "Reset" every unit in the Exchange. PROCESSOR The "Fl Alternate" function causes the Fl Bit of SIMULATOR the instruction word, the sixteenth bit, to adopt 1 cr 0 values on alternate cycles of the timing cir- •r'S INSTRUCTION cuitry. Generally, the Fl Bit of the five function WORD ~ TO code bits F5 to Fl indicates whether the function is CODING^™ "true or false". For example, if the F5 to F2 bits KEYS were set to a code meaning "seize" and Fl were set to 1, the unit addressed would become seized. Alter- nately, if Fl were set to 0, the unit would be released. This facility may be used, for example, to cause a relay to turn on and off continuously.

The resulting instructions are buffered through complementary drivers to the instruction highway of the processor access highway.

A major feature of the simulator is its display panel. The instruction word generated in the simulator is decoded and used to drive a panel employing some 250 lamps, which are organised in a manner similar to the system coding format sheet. This assists the operator greatly in setting the 16 keys to the pattern required for a particular instruction word. The alternative of calculating the binary pattern from the coding sheet and setting the keys accordingly would be much more time consuming, tiring and prone to error. The panel also includes a set of lamps to indicate the information being returned from the exchange on the data highway.

This instrument is of great assistance in the testing of the hardware systems. Commencing with the interface equipment, and adding one unit at a time, individual operation is tested. Then the units and groups are tested collectively for any erroneous interactions• The simulator on its own is also a valuable training aid, using the 16 keys and the display panel to demonstrate the coding format.

V THE MIHI-PROGRAMMER CONTROL FUNCTION 0« «ATOM The mini-progranmer, a controllable circulating ITIME DELAYS ETC.) store, is an add-on accessory for the simulator that is used for "Hardware Testing Hardware", as shown in fig.9. It is a storage device using MOS shift-registers with a capacity of 32xl6-bit words. The operator SWITCH loads a sequence of up to 32 exchange control instructions that are typical of those generated by a IACTIVEI processor routine. Loading is achieved by setting the required instruction on the 16 control keys on Fig. 10 - Mini-Programer, Logics Diagrm 69 the simulator and pressing the "load" button on the mini-programmer. The display panel of the simulator assists the operator in setting the required codes. Once the 32 instructions are loaded, the mini-programmer is changed to the output mode where it repeatedly sends this series of instructions to the exchange via the simulator.

Each instruction word has a three bit word stored with it. These additional bits are used to provide control functions for the mini-programmer in the readout mode, such as synchronising pulse output, wait on switch open, and time delays.

The logic diagram of the mini-programmer is shown in Fig.10. It connects to the simulator at the" control keys as shown in Figs 8 and 10. When loading, the signal flow is from the simulator control keys to the input of the mini-programmer shift registers. At the same time, the outputs of the shift registers are inhibited from feeding back to the inputs. While coding the instruction word on the simulator, the required mini-programmer control function is also set by the operator. Fig.ll - The Hini-Progvamer

In the readout mode, the signal flow is from the output of the mini-programmer shift registers through the gates with the inhibit inactive and out to the these instructions vhile incrementing the address on simulator to the Instruction Highway. The output of each cycle is of great value. At the same time as the inhibit gates is also fed back to the input of providing drive instructions, scan instructions can the shift registers. It is essential in this mode be used. The data collected can be used in the test that the control keye of the simulator are in the program to "test the success of each cycle. These open state. Interlocks are included to ensure this. tests would be very tedious for an operator to per- The mini-programmer control function is operated on form with the simulator and mini-programmer. as appropriate for each instruction. For example, if the function Oil, meaning wait on "Switch A" open, is An example of a complex "Software Testing Hard- present, the shift pulse will be inhibited until ware" test run on the No.6 Switch is the test on the "Switch A" is closed. This facility allows the auto- Switch Block. A program was used to connect every matic synchronising of the mini-programmer with elec- incoming circuit to every outgoing circuit, some tromechanical equipment. 3000 combinations. Each interconnection involved some six drive and six scan instructions, and timing In all, the mini-programmer (Fig.ll) has five to compensate for the slower speed of operation of modes: the electromechanical equipment. The final results 1 - clear (feedback gates inhibited, and high frequ- are displayed as a print-out in the form of a matrix ency internal oscillator connected to shift in- of incoming versus outgoing circuits, showing the puts ); results of each attempt. Any consistent faults are 2 - manual load; readily detected on the matrix. 3 - manual readout (the instructions stored in the mini-programmer may be confirmed on the simula- This test took about one hour to run. An operator's display panel); tor would tqke days, even if he had the concentration It - automatic readout (synchronised with the timing and persistence. circuitry of the simulator, and responding to the stored control functions); A complicating factor in the case of the No.6 5 - Program Dump (described in section VIII below). Switching is the arrangement of the switch block employed. To ensure efficient traffic handling capab- By using the mini-programmer, blocks of exchange ilities and high reliability, there is a lot of re- equipment may be put through a variety of functional dundancy in the switch block. Any one of dozens of tests with the opportunity of monitoring performance paths can be used to connect two circuits. The in detail. The equipment has sufficient flexibility choice lies with effectively random call distributors to allow detailed inspection of any switching faults in the switch control circuitry. It nay be necessary discovered. to run the interconnect program a few times to gain reasonable confidence in switch operation. VI TEST PROGRAMS It is essential to perform extensive "Hardware" Special programs are written for the "Software Testing Hardware" tests on the individual units be- Testing Hardware" phase. Once basic hardware opera- fore attempting the above test program. Only a few tion hae been tested, advantage can be taken of the faulty circuits will fill the result matrix vitb so flexibility and speed of the processor for further many failure« that the operator will not be able to more rigorous tests. assess the cause. Besides, it is generally easier to diagnose a hardware fault while doing simple unit The software facility of loading a series of in- testing with HW-HW equipment. structions with variable addresses and repeating 70 VII PROCESSOR MEMORY DISPLAY 16 inches. It has above 8OOO plated-through holts, and mounts some 300 Integrated circuits, and 512 As the development of the operational program ad- light-emitting diodes. The printed circuit layout vances-, the "Hardware Testing Software" tests are for this board was produced on an Automatic Drafting performed. Portions of the operational program are Machine installed at the Munitions Factory, Maribyr- used in the processor, with special linkages inserted, nong, Victoria. It took the machine seven hours to for the portions that are omitted. The processor is draw the full sized master for one side of the board. connected to the tested portions of the exchange This would probably have taken a technician »ore like hardware where simple operations are performed such seven- weeks. as pushing-up a relay, or operating a blocking key. Scanning sections of the software should detect these The display is invaluable in aystee testing and changes and modify data words cr status buffers in debugging and will also provide a useful display for the processor accordingly. Some means is needed to demonstrating the operation of the exchange. examine these stores to confirm correct operation. This can be done via the console teletypewriter, but VIII INSTRUCTION DUMP this is relatively time consuming and, in many cases, too slow to catch rapidly occurring events. Also, Another use of the Bdni-prograaawr is as an in- the operator has "miles" of paper to examine. struction "duap", as mentioned in Section V above. In this mode, it is connected to the instruction The memory display was developed to overcome this highway between the processor and the exchange at the difficulty. It consists of a matrix of 512 light- peripheral register, as shown in Fig.13. emitting diodes (semiconductor light sources) with associated storage. The matrix is organised with 16 bits in a horizontal rev representing one word. DATA KWY. A total of 32 rows provides a capacity of 3£sl6 bit PROCESSOR PROCESSOR PERIPHERAL words. REGISTER HIGHÄAY MO.« j SWITCH INSTRUCTIO A small program within the processor contains a HWAY list of the memory locations for display. At regular 1 X—] intervals of approximately 15ms, the display is 1 driven with the contents of these locations. Alter- natively, the program could be organised to drive the MINIPROGRAMMER' J SIMULATOR display whenever the locations are addressed.

Fig.13 - The Instruction Dump - the Mini-Programer Mode S

It then stores the last 32 instructions in its shift register memory. That is, when a new word is stored, the word stored 32 time periods earlier is lost. The mini-programmer is then disconnected fron the highway and plugged into the processor simulator, as shown in Fig.lU. The words stored can then be read out in turn via the display on the simulator panel. Using this facility, an operator may check on the instructions being generated by the processor operational programs.

OATA HWY.

PROCESSOR PERIPHERAL REGISTER NO.« SWITCH

MINIPROGRAMMER < SIMULATOR Hg.12 - The Pvoeneor Memory Display

Physically, the display (Fig.12) consists of a Fig. 14 - Beading the Dumped Irwtruotiona narrow profile case that may be aounted on a cabinet or wait, like a chart. A 'perspex front panel is provided with space for indicating which words are dis- In the Ho.6 Switch, a lot of time is spent scann- played and what each bit in the word represents. The ing circuits to observe line conditions, and keep the perspex panel is side lit by fluorescent tubes for software status buffers up to date. Consequently, pre-prepared photographic aasks to be used with the there are many more scan instructions than drive in- •ore frequently required groups of aeaory locations. structions. A control is included in the •ini-prc- graaier that cause» it to ignore any «can instruc- All display and storage components are aounted on tions in this mode, storing only the «ore relevant one double-sided printed circuit board measuring 23 x drive instructions. 71 This instrument is a powerful tool in the "Hard- Other tests are based on simple test-phone calls. ware Testing Software" phase. It may also be used These include seixure (when the handset is lifted), for demonstration purposes. At any time during ex- effect of dialling, operation of switch block on change •operation, the mini-prograemer may be dis- completion of dialling, called party's phone ringing, connected from the highway and the last 32 instruc- and release. tions examined. XI TESTIHO ACCESSORIES IX TESSAR As well as the major pieces of test equipment For the "Software Testing Software" phase, a described above, numbers of minor pieces of test special system, called TESSAK (Test Event Sequencing, equipment were also developed. simulating And Recording), has been developed by Mr. A.W. Thies. TESSAR employe a general control Amongst these it the monitoring extension card, program with a special table of conditions relating with buffered light-emitting diodes, to indicate the to the particular test being performed. The portions logic states at the inputs and outputs of the ex- of the operational program being tested, TESSAR and change logic cards. the TESSAR table are all loaded into the processor core. No hardware is connected to the processor (except for printers, etc.)«

The operational program comnences operation and will generate an instruction word to be sent to the hardware. When it attempts to output this word, but detects that there is no hardware connected, it attempts to go into the close-down routine, as it would under normal operation. However, with the TESSAR program in core, control is transferred to TESSAR instead.

When TESSAR takes control, the real-time clocks used in the operational program are frozen, so that TESSAR can perform all the operations requested by the operator. It determines what the last instruction word was and outputs it on the printer, together with any other relevant information. It also uses this information in conjunction with the TESSAR table to determine the appropriate simulation and returns control to the operational program.

For example, it may have been a scan instruction that the program attempted to output. TESSAR will examine its simulation table to determine what the Fig.16 - The Monitoring Sxttnaicn Card scan result should have been for that instruction. It inserts this result in the location which the data would have reached under normal operation, and re- The extension card plugs into the printed cir- turns control to the operational program. The latter cuit card socket in the chassis, where the card under then continues as if nothing had happened and pro- test is normally inserted, as shown in Fig.15. The ceeds to analyse the data it thinks came from the ex- signal lines on the socket are extended across the change . extension card to a similar socket on the other end j of It, wh«re the circuit under test is inserted, lea- Simulation testing of this form is used for the vine it completely clear of the other cards in the testing of the operational programs of most processor chassis. ; controlled systems. It involves a lot of vork, but Is essential to prove software operation. For example, TESSAR testing on the No.6 Switch represents some three man-years work.

X OPERATIONAL PROGRAMS

Once significant portions of the operational software and hardware are tested, test of the complete or partly complete system may be attempted.

The first of these is the idle condition test. (ONC CIRCUIT EUEMENT SHOWN.) When the sy«t«m is idle, the processor is still continuously scanning all circuits. While in this state, all software tables should be inactive. Thia Fig.16 - The Monitoring Extension Circuit Diagram can be monitored vith the Processor Memory Display. Also, no drive Instructions should be sent to the exchange - as monitored by the mini-prograismer. Any The extension card used here also has a secondary such instruction would be caused by a stray noise board attached to it, where the monitoring circuit is ! pulse or failure of the data highway, etc. mounted, one element of which la shown in Fig.l6. 72 The circuit cards in the No.6 Switch used 50 pin References connectors. Therefore, there were 50 such circuits on the extension card. When the input of a TTL Inte- 1 Crew, G.L., "C.C.I.T.T. Signalling System Ho.6 - grated circuit is left floating, the input adopts the Field Trial Design". P.M.G. Research Laborator- high logic condition. Most circuit cards have some ies Report 6U00, December 1968. unused pins, and it is preferable for the monitor lamps to stay off in these cases. Hence, a non- 2 Crew, G.L., "C.C.I.T.T. System N0.6 - A Common inverting buffer (71(17) was used and the lamp is Channel Signalling Scheme". Telecomminications activated when a low appears on the line. Journal of Australia, Vol.18, No.3, October 1966, page 251. An isolation switch was included on the monitor circuit to allow for cases where excessive voltages, 3 C.C.I.T.T. White Book, Vol. VI, Part XIV - which would damage the buffer, were present. "Specification of Signalling System No. 6". This was a very valuable tool in the testing of k. Crew, G.L., "C.C.I.T.T. Signalling System Ho.6 - the hardware, particularly, when testing the response Field Trial-Project Review". P.M.G. Research of the equipment to instructions from the processor Laboratories Report 6567, December 1970. simulator. 5 The Australian Post Office, "Introduction to Crossbar Switching - Part 1" CF6OO - 1969. XII CONCLUSION 6 McLeod, B.W., "C.C.I.T.T. Signalling Ho.6 Field The testing phases discussed above are required Trial - Processor-Controlled ARM Exchange Equip- for the complete development of a system. No one ment". P.M.G. Research Laboratories Report 6661, test is sufficient and no test can be omitted without 1972. making the others more complicated, or incomplete. Stored-program-controlled systems present some diffi- 7 McLeod, N.W., "C.C.I.T.T. Signalling System Ho.6 culties resulting from the marriage of hardware and Field Trial - Digital Addressing ead Control of software. The differences of approach and technology Switching Equipment". P.M.G. Research Labora- of hardware and software system designers must not be tories Report 6662, 1972. allowed to introduce conflicts, or cause inconsisten- cies. Rather, the best of both worlds must be adop- 8 McLeod, N.W., "Complementary Data Line Receivers ted to produce the optimum system. Means of testing and Driver Circuits". Proe. IREE Australia, Vol. must be considered from the very start and included 32, No.6, June 1971, page 21k. in the system design. 9 Gale, N.J., "A Computer Controlled Circuit Tester". Electronic Instrumentation Conference, ACKNOWLEDGEMENT I.E. Australia, May 1972. The permission of the Senior Assistant Director- 10 Standards Association of Australia "Graphical General, Australian Post Office Research Labora- Symbols for Electrotechnology, Logic Symbols" tories, to publish this paper is acknowledged. AS1102 9 - 1971.

73 DIGITAL PHASE METER

M. Imber, B.Eng.(Elec) University of Melbourne

I INTRODUCTION II GENERAL DESCRIPTION OF THE SYSTEM The measurement of phase difference between The block diagram of the complete instrument two periodic signals of the aatne frequency and is shown in Figure 1. Both channels are identical waveshape ha» in the past been achieved in a up to the set-reset flip flop (SRFF). The first variety of ways. The majority of these are stage in the phase meter is to compare correspond- entirely analogue but recent designs tend to in- ing points on the waveforms. The only restriction« corporate some digital methods because of the on the waveforms are that they are of the same form sec improved accuracy, stability, consistency of results and frequency - that is, no appreciable distortion oir the and low cost. noise is introduced by the device causing the phase cry shift - and that the corresponding points occur The first operation is to detect corresponding only once every cycle. There are two logical rec points on the incoming waveforms. In analogue choices for these points: re« instruments, the time difference between these points is converted to a voltage. This is com- (i) the maximum or minimum points on pha pared to a reference and subsequently displayed on the waveform, and a meter (1), (5). The meter scale will often be compressed to give, say, 3 degrees full scale (iij the zero crossing points. for deflection giving greater relative accuracy. In diff this method the accuracy of the instrument is The first type is difficult to define accurately and is a ri dependent on the accuracy of the reference voltage, amplitude dependent. On the other hand, zero fig« the components in variable phase shift networks, crossing points are well defined, can be detected just the resistors in the meter circuit and the meter independently of amplitude and are unique every itself. Absolute accuracies are typically £4 c'egrees, cycle (one positive going and one negative going). though relative accuracies as high as +_ . fdegrees Thus this method was used in preference to the have been attained. first. The More recent designs have used a voltage to To determine the zero crossing points frequency conversion with a resultant display on a accurately, a detector with fast and large output digital frequency counter. Ehret, et al., (2), used changes for small changes of input about zero was a digital divider section in their design to extend required. This function was provided by an in- the frequency range of the instrument. A method tegrated circuit high speed differential comparator; which is used for phase measurement of very high the |iA7lOC. The input-output characteristics of frequency signals uses a sampling process to lock this device are shows in Figure 2. the input frequency with a standard intermediate frequency of about 1 KHz. The phase oi this low At the output of the zero crossing detectors «re intermediate frequency is then measured by an have two square waves in constant phase relationship analogue method using a meter display (4). with the incoming waveforms. These square waves are differentiated and trigger high speed monostable To obtain greater accuracy it is necessary to multivibrators with pulse widths of 15 nS. The use digital techniques in measuring the phase differentiator ensures that the pulses will only be difference of two signals. In addition, digital produced on the positive going elopes of the square methods are more easily implemented, simpler in waves. The channel A pulse seta the SRFF (that is, their hardware and free from any zero setting it causes the "1" output to go high) and the channel adjustments. B pulse resets it (causes the "1" output to return to the low state). This set-reset process produces a The approach chosen in the design described pulse train from the SRFF with a mark-space ratio below emphasizes simplicity while endeavouring to proportional to the phase difference between the two maintain accuracy and uses digital techniques input signals. The output of the SRFF is connected throughout. to one input of a logic NAND gate. The other input is connected to a high frequency square wave crystal oscillator and the output fed to a digital frequency counter, with gate times available in decade multiples up to 10 seconds. A gate time of one 74 Zero crossing Differentiator Monostable detector A A set 1 — channel A Set-Re set flip flop Zero crossing detector Differentiator Monostable reset 0 channel B B B

Digital frequency Fixed divider NAND — counter with one ratio of 1000 gate second gate 3. 6 MHz square wave oscillator nt il Figure 1. Block diagram of digital phase meter nd- »ni orm second was used for convenience. Depending on integrated circuits as these are cheap and con- n or the phase difference of the input signals and the venient to use. Thus the crystal oscillator •e crystal oscillator frequency, a number of square frequency must be a decade multiple of 36. A high waves will be transferred by the NAND gate to be oscillator frequency is desirable as it increases the recorded on the frequency counter. The counter resolution of the instrument. However, it also reading will be given by brings about an increase in cost because of the very high speed dividers and logic required. Hence phase difference crystal oscillator freq. an oscillator frequency of 3. 6 MHz was chosen 360 fixed divider ratio which meant a fixed divider of 1000. Figure 3 shows a typical sequence of waveforms for a phase for a gate time of one second. To obtain phase shift of 90 degrees. difference directly on the frequency counter with a resolution of . 1 degree (as only four significant The instrument as described has an error of figures of the phase - XXX. X degrees - were less than +_ 1 degree over the range of input justified) it follows that frequencies from 20 Hz to 20 KHz. However the accuracy improves to +^ . 2 degrees in the centre crystal oscillator frequency in of this range. 360 x fixed divider ratio " in DESIGN REQUIREMENTS The fixed divider consists of divide-by-ten (a) Zero Crossing Detector The differential comparator integrated circuit is a standard high gain amplifier with »r; internal feedback and the output levels compatible with TTL logic. An 8 mV input swing about zero 3 r was required to change the output logic level from one state to the other. This meant that for a we 2 / 10 V peak to peak sine wave input |+ 5 V about •hip / zero), an absolute phase of .09 degrees could be res resolved because ble 1 5 sin .09° = .008. / 0 re If the input level dropped to 1 V peak to peak, the is, resolution would be reduced to . 92 degrees. The el -1 input« of the differential comparators were DC to -6-4-2024 coupled to eliminate the inherent phase shifts in a Input voltage (mV) AC coupling networks. Thus any DC component on io the incoming signal would cause a non unity mark- :wo space ratio with a subsequent error in the phase ed reading. The effects of nonlinearities on the inptil at Figure 2. Input-output characteristic» •ignal are discus ted in Appendix I. •tal of the zero crossing detector. The output of the zero crossing detector responded to a change in input after a delay of 20 nS 75 (a) Input to channel A zero crossing detector.

(b) Input to channel B zero crossing detector.

(d) Output of channel B zero crossing detector.

(e) Differentiator output from r r channel A.

(f) Differentiator output from r r channel B. (g) Output of Set-Reset flip flop. it T (h) Output of gated 3.6 MHz square fi wave. «M oi Ul n< Figure 3. Waveforms through the circuit for a phase shift of 90 degrees.

and had a minimum rise time of 20 nS. Thus there for the resistor and capacitor. These values was a 40 nS delay through the device. This delay were matched to +_ 5 percent between channels will Introduce a phase error depending on the which was sufficiently close for no extra error si frequency of the input signal as given by the to be introduced. Oβ following expression: and „a (c) Monostable Multivibrator phase error = 40 x 10 x frequency x 360. A very narrow pulse was required from the At 20 KHc, the error introduced from this source monostable because the SRFF would not allow two is about . 3 degrees whereas the maximum high pulses (one from each channel) to occur frequency error is 2 degrees as will be explained simultaneously at its two inputs. To allow an in section (g) below. absolute phase resolution of . 1 degrees, the pulse width would need to be less than 1/3600 of one (b) Differentiator cycle of the input signal . Thus at 1 KHz, a maximum pulse width of eli The differentiator must operate over a wide inj frequency range and provide pulses for the 1 mS or 280 nS ph, monostable multivibrator only once per cycle of 3&W thv the input signal. This was accomplished with in a simple RC differentiator coupled into the mono- would be required. A minimum pulse width of gw stable via the base of its input transistor. Both 14 nS would be required at 20 KHz - the upper Ho channels used the same preferred component values frequency limit imposed on the instrument. de, 76 However, the accuracy at this frequency is limited and not the relative voltages between channels, the bv Uu- crystal oscillator frequency rather than the voltage input to channel A was kept at its maximum monoslable pulse width as will be explained below. allowable value of 10 V peak to peak. High The shortest stable pulse width that the monostable stability resistors and styroseal capacitors were could produce was 15 nS. This is sufficiently used in the phase shift networks and their values short to eliminate the mono stable as an error were measured on an impedance bridge at 1 KHz source over the instrument's frequency range. with an accuracy of +_ . 1 percent. Three networks were constructed covering three overlapping (d) Set-Reset Flip Flop frequency ranges from 20 Hz to 120 KHz. Table 1 give8 the preferred values of the components in The SRFF was constructed from two high each network and the frequency range covered for speed TTL NAND gates with propogation delays which the amplitude ratio between channels A and of about 8 nS. The delay from the channel A B was less than 10:1. The variation of each of input to output was 8 nS whereas that from channel B to the output was 16 nS. This aesymetry would produce a maximum error of . 05 degrees at the Table 1. upper frequency and so can be ignored. Network R, Frequency range (e) Frequency Counter and Gate Kit KO & Hz The frequency counter used was external to 1 1 l 10 1 20 - 1. 5K the instrument and was a General Radio model 2 1 . i 1 . 1 200 - 15K 1)92 counter. The upper frequency limit was 32 MHz and it contained its own 10 MHz crystal 3 1 .01 1 .01 1.4K - 120K oscillator with a short term stability of better than 2 parts in 10 . This accuracy is at least an order of magnitude greater than the rest of the instrument. the component's values with frequency from 20 Hz The gate time of one second was derived from the to 20 KHz (the range of the impedance bridge) was frequency counter's crystal oscillator. The 3.6 checked and found to be negligible. The phase MHz oscillator of the instrument was calibrated shift of the networks was calculated on a against this crystal oscillator. Because the least computer along with the sensitivity of the phase significant digit displayed on the counter was one for a . 1 percent variation in R., C., R,. C and tenth of a degree, and there is an in herent +_ 1 in ? the last digit of any digital counter, an error of frequency. This was found to be less than ±..11 +• . 1 degrees would exist because of the display. degrees in all cases.

(f) Test Phase Shift Network (g) High Frequency Errors With an instrument which has a potential The phase meter has an inherent high accuracy of . 1 degree, it becomes a task in frequency limitation since the SRFF output must itself to check this accuracy with known standards. gate a finite number of square waves from the There were no available phasemeters in the 3.6 MHz oscillator. At 20 KHz, the maximum frequency range being considered which remotely number of oscillator square waves that can be approached this resolution and so the phase shift gated in one period of the SRFF output is given by of a simple passive RC network was calculated and 3. 6 MHz used as a check on the instrument's accuracy. The = 180 pulses network used is shown in Figure 4. 20 KHz and will occur for a phase shift of 360 degrees. Therefore, the smallest phase difference that can be measured is 360 degrees/180 puises or vw—ll 2 degrees as at least one pulse must be counted. sine wave However, at this frequency we are averaging the oscillator B phase reading over 20, 000 periods, because of the and A input input one second gate time of the counter. Thus there is a statistical probability that for a phase shift of less than 2 degrees a pulse will sometimes be counted and other times not because the signal source is not locked to the 3. 6 MHz oscillator. Figure 4. The RC phase shift network All of this means that the effective accuracy at used for testing the instrument. this frequency will in fact be better than 2 degrees. (h) Low Frequency Errors Because the network was passive (to eliminate the possibility of DC offsets on the As the frequency decreases the theoretical inputs to the differential comparators with active accuracy improves until at 1 KHz we have . 1 degree phase shift networks) it had some attenuation and resolution per cycle of the incoming signal. Our thus the eero crossing detector in channel B did not actual resolution will be somewhat better since in some cases have sufficient input amplitude to averaging takes place over 1000 periods at tins give the zero crossings within +_ . 1 degrees. frequency. If the input frequency were an However, as the measurement "process is only integral number of cycles per second then the low dependent on the absolute voltage into each channel frequency limit with a gate time of one second is 77 1 Hz with no loss of accuracy (and for a gate time of 10 seconds is . J Hz, etc.). However, if the input frequency is not an integral number of hertz, then the gate could omit different parts of the (a) first and last cycles and give an error depending on the actual frequency and phase shift of the input signals. Consider the simple case of a 1. 5 Hz signal with a phase difference between chennels A and B of 180 degrees. Figure 5(a) shows the input signal to the frequency counter. The counter only counts for one of the one second interval s shown on the diagram. These intervale can occur in several relative positions as shown in figures 5(b). (c) & (d). In 5(a) it is only counting pulses for one-third of a second and will therefore read 120 degrees. This is an error of -33-1/3 percent. Figure 5. Demonstration of low frequency In 5(c) it is counting for two-thirds of a second errors which can occur. and will give a + 33-1/3 percent error. In 5(b) it is counting for exactly half a second and no (a) 1.5 Hz signal into frequency counter. 5. error will be observed. If the frequency is closer to 1 Hz, then the counter will miss more of the (b), (c) & (d) Three possible positions of second cycle giving a lower maximum resultant the gate time of the frequency counter. error. For a frequency closer to 2 Hz, more of 6. the second cycle will be counted and a lower maximum error will again be obtained. For a V FURTHER DEVELOPMENT given frequency the error will approach zero as the phase shift approaches 360 degrees. For a Because of the simplicity of operation and frequency of 20. 5 Hz, the maximum error will be potential accuracy of this design, another 7. about 2.4 percent. phasemeter is at present under construction in which more emphasis is placed on the zero IV RESULTS crossing detector to overcome the shortcomings of the prototype design. The new design will have The experimental and theoretical results are five cascaded differential amplifier - limiter shown in Figure 6 for the three networks used. stages with DC feed back to compensate for any The ordinate is the phase error while the abscisaa DC level shift of the input signal and any even- Appc shows the frequency, actual phase difference and harmonic distortion. The number of stages will peak to peak amplitude of the channel B input also provide sufficient gain \o allow a 60 db signal. The channel A input was held constant at dynamic input range. The differentiator and insti 10 V peak to peak for all measurements. The over- monostable multivibrator have been eliminated bein; all theoretical error is £ . 2 degrees in the middle and fast (Schottky) TTL gates using both zero of the first two ranges and ±,1.1 degrees at the crossings of the input signals have been used. Of Wi limits of these ranges. The* third range has a Thus the resultant display will be 0 - 180 degrees oftei different theoretical error because the upper lead or lag. This makes the circuitry simpler relai frequency limit of the instrument is being approached. and less prone to errors which could occur due to mistriggering of the monostable from noise on The fixed bias error of about 1.1 degrees in the input signals at the zero crossing points. A to m the first network and .36 degrees in the second frequency counter has been built into the unit and thes« is caused by two main factors: has a six digit readout (XXX. XXX degrees) to disto eliminate any +_ 1 count error in the one-hundredth sigm (i) the finite input impedance of the zero degree position. The local crystal oscillator has outpi crossing detector, and been increased in frequency to 72 MHz and this be el will improve the high frequency range of the not is (ii) the input offset voltage of the pA710C. instrument. The gate time for the counter is also harrr derived from this oscillator and so any frequency at ea The error is greater in the first network as the drift in the oscillator will be automatically com- for output impedance is greater than that in the pensated for by the drift in gate time to give a is thi second. This was a necessary shortcoming as correct phase reading. using there were no larger stable capacitors available sourc for Cj and C^. The input of channel A was not VI CONCLUSIONS does circu affected by impedance variations because the From the results obtained from the prototype desig oscillator used at the signal source had a 5011 •econ output impedance. phase meter, the potential accuracy and simplicity of the instrument can readily be seen. wavei The new design under construction should increase detect Resultant Specification for the Instrument the accuracy and frequency range by a factor of areas about 20. trigge Input impedance 250 Kβ erron Frequency Range 20 Hx - 20 KHz VII REFERENCES over r Signal Input Amplitude Range 10V - IV as the Error (maximum over range) +_ 1 degree 1. Frater, R.H. - A Precision Phase Meter. will t« IEEE Trans, on Instr. and Measurement. Vol. IM-15, No. 1-2, March-June, 1966, PP 9-1" 78 2. Ehret, R.L., Wood, L. E., Thompson Jr., Acknowledgements M.C. - Linear Integrated Circuit Phase Meter. IEEE Trans, on Instr. and Measumt. The author wishes to thank Messrs. A.E. Vol. IM-18, No. 3, September, 1969, pp. Ferguson and R.L.G. Kirsner for their valuable 157-160. assistance in the preparation of this paper. 3. Maxwell, D.E. - A 5 to 50MHz. Direct Reading Phase Meter with Hundredth-Degree Precision. IEEE Trans, on Instr. and Measumt. Vol. IM-15, No. 4, December, t> 1966, pp. 304-310. i> u 4. Yen, Chu-Sun. - Phase Locked Sampling y Instruments. IEEE Trans, on Instr. and Measumt. Vol. IM-14, No. 1-2, March- (a) June, 1965. pp. 64-68.

5. Johnson, G.D. - A Linear Phase Comparator - • for 10KHz. Proc. IREE, Aust., Vol. 30, - - February, 1969, pp. 54-56. 6. Epstein, W.S. - Digitized Phasemeter. 20 50 100 200 500 IK 2K F US Bur, of Stds. - Journal of Research Eng, 20 0 -10 -30 -60 -80 P and Instr., Vol. 68C, No. 4, October-December, 4.5 4. 8 4.5 3.3 2.0 1.0 A 1964, pp. 223-226. 7. McKinney, J.E. - Digitized Low -Frequency / Phasemeter Assembled from logic Modules. s US Bur, of Stds. . Journal of Research. s / Eng, and Instr., Vol. 7IC, No. 3, July- o September, 1967. u s u V (b)

Appendix 1 The measurement technique used in this t instrument only relies on zero crossing points 200 500 IK 2K 5K 10K 20K F. being unique with no DC offset of the signal. This 70 50 30 10 -10 -40 -60 -75 P. allows the m easurement of the phase of many types 1.0 2.5 3.1 3.3 2.5 1.5 A. of waveforms. However, as the sine wave is most often encountered in practice, only the problems relating to this type of waveform axe discussed. t There are two main factors which contribute 1 to make a sine wave non-perfect. The first of these is harmonic distortion. Even-harmonic I distortion may shift the effective DC level of the signal to give a non-unity mark-space ratio at the f 7 output of the zero crossing detector. This would o 1 / (c) be eliminated by a form of DC feedback but was t not included in this design for simplicity. Odd j harmonics give equal and opposite contributions y at each zero crossing, giving an error of . 1 degree for . 18 percent total odd-harmonic distortion. It I \ • is thus necessary to minimize the distortion by using a high quality oscillator as the signal •> source and ensuring that the circuit under test 2K 5K 10K 20K 50K 100K F does not approach overload. In a passive linear 1.2 2.4 3.3 3.2 2.4 1.3 A circuit, such as that used for checking this 65 45 20 0 -30 -50 -65 P design, there are no overload problems. The second factor causing non-uniformity of sine waves is noise. Because we are accurately Measured results detecting zero crossing points, any noise in these Theoretical result» areas will cause either prematur» or late F. Frequency (Hz) triggering of the monostable and thus give P. Actual phase difference (degrees) erroneous results. However, as we are averaging A. Channel B amplitude (peak to peak V) over many periods of the incoming signal, and as the noise present is usually random, the errors will tend to cancel. Figure 6. Results. (a) Network 1, (b) Network 2, (c) Network 3. pp 9-1°

79 A COMPUTER CONTROLLED CIRCUIT TESTER

N.J. Gale, B.Eng.(Honours), Australian Post Office Research Laboratories, Melbourne

SUMMARY. The tester described in this paper is well suited to the static testing of printed circuit boards containing digital integrated circuit packages. The tester is computer-driven, which allows it to be very flexible and able to perform thorough testing at high speed. The tester is at present in use in the A.P.O. Research Laboratories. Experience has shown that testing rates of between UO and 100 boards per hour or l60 IC packages per hour can readily be achieved.

Tolerance and dynamic tests may also be required on each card, but often may be done as part of the test of the system of which a particular card is part.

I INTRODUCTION and paper tape reader, an operational program with associated sub-routines for printout and data selec- The growing use of Medium and Large Scale Integration tion, a buffer between processor and circuit board has led to increasingly larger slices of systems adaptor which houses decoding and interface circuit- being accomnodated on individual circuit boards, ry, and an adaptor to interface with the circuit increasing the need for powerful and flexible systems board under test an« supply power to the board. to provide comprehensive testing of manufactured cir- Fig. 1 portrays the system seen from the software cuit boards. point of view.

Testing of digital logic circuitry can be quite a The ccatputer outputs a number of words to the buffer straight forward matter. The major types of tests which, in turn, applies a set of logic states to the are: input pins of the board under test. The board may be either a normal printed circuit card or a card con- (i) Static tests taining integrated circuit package sockets into which (ii) Tolerance tests are plugged the integrated circuits to be tested. (iii) Dynamic tests In fact, of these types the static tests detect the The conditions on the input pins as well as those on majority of faults leaving only a small percentage the output pins of the card are fed back via the of marginal faults (both voltage- and timingwise) buffer to the computer. A comparison is made between undetected. the expected state previously recorded and the observed state of each pin. If the two states are not This paper describes a tester which has been devel- the same, an error message is printed on the tele- oped in the Research Laboratories of the Australian typewriter. The message gives details of the test Post Office to perform static test sequences on number and the pin number of the faulty pin. After printed circuit board3 or on individual integrated detecting a fault, the program pauses. The operator circuit packages. The tester is computer-driven, may choose to abort the test and start again on an- vMch means that all decisionmaking, comparison, and other board or continue testing in order to gain more test sequencing is under program control. The hard- detail of the extent of the fault. For simple cir- ware for the tester is therefore fairly simple, being cuits, the fault may be deduced from the printout. mainly decoding, storage and printed circuit card Alternatively, the board may be tested manually in access circuitry. greater detail at a later stage. Each sequence of passing a set of logic states out to the pins of the II NOTATION board under test and comparing the observed and ex- pectsd conditions is termed a test. Up to one hund- Word; used in the sense of a computer word, In this red tests may be performed in a given test run under the present dimensioning of the operational program. case, the computer uses 16 bit words.

Operational Program: the drive program in the com- The buffer is at present wired to test circuit boards puter which carries out the decision-making, con- of up to sixty-six pins. Since the processor may par is on and test-sequencing functions. only output one sixteen bit word at a time, a total of five words is required to set up one test. The Ill SYSTEM DESCRIPTION words are stored in sixty-six stores in the buffer, having been directed to the correct locations by the This system consists of a computer with teleprinter decoding logic. Readout circuitry in the buffer

80 gates the pin condition« of the board under teat to (b) Repeat Test. the information bus which, in turn, transaite the information to the computer. The time taken to per- It is possible to repeat a specified sequence of two fora each teat is generally of the order of 700 tests a specified nuaber of times before continuing microseconds. This puts the total time to perfora with further tests, this is useful for applying a one hundred tests at around 70 milliseconds. clock pulse to a counter, for exaaple, provided that the output does not change during the clocking sequ- The program in the computer which drives the buffer ence. Aβ in (a) above, the request to repeat tests uses data which have been prepared for the type of and the number of repeats required are set In the board under test. These data consist of a set of fifth word of the test. words vhich define which pins on the board are input pine and which are output pins. These are followed (c) Use of a Serviceable Card as Data Source. followed by the words which define the individual tests one onwards. (Up to 100.) Each test word con- Provision has been made to enable a known good cir- tains both the required conditions for the input pins cuit board to be used as a reference to define the and the expected conditions for the output pins', output data for the testing. A data tape for the having the advantage that the seme words can be used board type must still be prepared. However, only both to set up the test and to compare the results. the conditions to be set up on the input pins of the board under test need be defined, since the output The arrangement of setting the input pin states and conditions are effectively matched with the refer- then reading the eet-up conditions along with the ence. The test system is set up in the usual manner. output pin states has the advantage that the buffer However, before a test is run on the non-t«eted card, nay be "looped back on itself" by marking all pine as the reference board is inserted in the test socket. input pins. The Computer may then test the terminal The computer outputs data to the buffer which sets input set-up operation without reference to external up states on the input pins of the reference board. test equipment. When the output states of the reference board are received by the computer, they are stored In the Under normal circumstances, a "eelf-test" always pre- data table as expected states of the twin card re- cedes a test session to check that the terminal is quiring test. Once the reference card run is com- operating correctly. plete, the system is automatically reset to normal mode so that the twin cards may be tested in the After loading the programs« up to ten different data normal manner. For future reference, the acquired tapes may be loaded in our present system. The part- data table may be printed out and dumped on paper icular data to be used for a test may then be select- tape. ed by typing the tape number on the teletypewriter. Bequest for a different data table may be made from (d) Pause in Testing. the buffer control panel and the data defined by type-in from the typewriter. The operator may instruct the program to pause after any given test has been performed. This facility may The adaptor forme a flexible link between the buffer be used to aid on-line debugging. Often a faulty and the board under test. It is used to apply power output signal is due to some internal circuitry (e.g. to the board under test and can also be used for a flip-flop) not being set correctly several tests logic level conversion, or to apply loading to spe- before the fault appeared on the output pins. A cific pins. pause may therefore be requested several tests before the known failure point and an independent logic IV FACILITIES probe used to check signals not connected to the I/O pins of the circuit board. Alternatively, the teat- The use of processor control allows the tester a ing may be allowed to proceed, one test at a time, large degree of flexibility. The. buffer is largely by requesting a pause after each test. interface and decoding circuitry and therefore does not contribute in a major way to the way in which (e) Alteration. testing is carried out. Additional facilities may readily be added to the tester by changing the opera- The information in the data tables may be altered, tional program, or adding suitable sub-routines. The on-line, from the teletypewriter. This facility may following section describes some of the extra facili- also be used to write a simple data table on-line. ties which have been added. This list is not ex- Information is typed in the following format: haustive by any means, but it gives an example of the sort of flexibility which is inherent in the test ALTER DATA FROM TEST 1, TO TEST 10, system. PIES HIGH - - --"" PIHS LOW 2A (a) Delay Facility. DOHZ! A variable delay may be inserted between setting-up (The sections underlined are typed by the operator. the input conditions on the test board and reading The other information is the program response.) Each the output conditions. This may be necessary to operation is performed after a comma is typed. This allow the output conditions to stabilise, especially means in the example that pin 39 will be set low, If there ax« relays or delay element« in the tested even though it was initially set high. circuit. The maximum delay available is 2Uo milliseconds, programmable to within 100 microseconds. (f) Test Data Checking Facility. The delay is contained in the fifth data word for each test. The delay only applies to the test in This facility allows the operator to check the data vhich it was requested and defined. tables. Situations can occasionally arise in which 81 the beard under test is good but a fault message is printed due to an error in the data tape. By typing the word to be checked, its value in the data store will be printed out by the processor so that a visual comparison can be made.

ACKNOWLEDGEMENT

The permission of the Senior Assistant Director- General, Australian Post Office Research Labora- tories, to present this paper is acknowledged.

CARD UNDER TEST

TESTER BUFFER OPERATIONAL AND ADAPTER SUBROUTINES

FIGURE 1. TEST SYSTEM.

82 TECHNICAL CONTROL FACILITfES FOR COMMON USER DATA NETWORK CENTRES

K.V.Sharp, A.R.M.I.T., Engineer Class 4, Postmaster-General's Department

1. INTRODUCTION as accessing facilities, for testing and monitoring on the various lines. (Figure 1.) The Australian Post Office is currently establishing the Common User Data Network (CUDN), a stored program- This area has been .called the Technical Service controlled computer data-switching complex with Area (T.S.A.) and, while it is under the control of switching centres located in each of the mainland the Officer-in-Charge of the CUDN centre, it does not capital cities. provide facilities for testing the actual computer equipment apart from the input/output interfacing The system has been designed to cater for the needs terminals. of subscribers whose interests are spread throughout Australia, with consequent data accumulation being 3. TESTING EQUIPMENT diversified. It is the capability of the CUDN to transfer this data from the collection points to a The provision of test equipment can be subdivided conmon processing point, or to any other desired roughly into three main groups :- delivery point. (i) Telegraph equipment for lines operating The network is based on Univac 418-III processors in direct current in polar mode up to ISO bits/ a fully duplicated installation pattern and operates in second. (There is a possibility of a real-time environment. A major factor in the design extension to 200 bits/second at a later date) . has been "data protection", i.e. once data has been (ii) Data equipment for lines operating via modems input to the system it must not be lost. in the range 200 bits/second to 4,800 bits/ second, The network provides transmission facilities at (iii) Line testing equipment. speeds ranging from low speed telegraph (50 bauds) Standardised equipment for performing through the full range of data speeds to 4,800 bits/ resistance, continuity, etc. tests on cable second. Consequently, terminal equipment which will be circuits. used by the organisations operating through the system will vary to meet the particular requiressnts of the In «Mition to the above, special facilities for organisation. Teleprinters will be both 5 and 8-level the convenience of the testing technician have b.en and will operate at speeds of up to 150 bauds. Connec- Provided and these include :- tion may be either on a direct current (D.C.) basis or via modems- Other data terminal equipment will be (i) Loud-speaking, pushbutton dialling telephone connected via modems operating up to 4,800 bits/second facilities. as will C.R.T. visual display units and their multi- (ii) Pushbutton accessing panel for connection of plexing consoles. test equipment to the selected line, (iii) Console mounting of test equipment. Each user has the option of providing his own Network (i) Bias Distortion terminated and provision is made to allow f (ii) Peak Telegraph Distortion testing of either 2 or 4 wire circuits. (iii) Error Count I, The following tests can be performed - j The tester is intechangeable with the terminal [ equipment and input and output interchange (i) Foreign Battery (A or B side of pair) | circuits are compatible with CCITT V24, E.I.A. (ii) Earth (A or B side of pair) j RS-232 and ISO recommendations. (iii) Loop Resistance | (iv) Insulation Resistance ! Transmission speeds up to 4,800 bits/second are provided with stability of better than 1 part in (b) Portable Cathode Ray Oscilloscope - Tektronix! 453. 104. Output signals may be :- (c) Telephone Facilities. To permit the Binary 0, Binary 1, 1 : 1, 3 : 1, 1 : 3, 7 : 1, operating technician complete freedom of 1 : 7 and 511 bit pseudo random sequence. movement, telephone facilities have been provided with pushbutton dialling and loud- Receive speeds in the range 30 bits/second to speaking output. A normal telephone handset may be provided if required for privacy but 9,600 bits/second. would not be a normal provision. Four Bias distortion is indicated on a meter while telephone-circuits can be accommodated althow' telegraph distortion is displayed as a two-number the general design of the console caters for I digital readout. simultaneous use by two technicians only. (e) T, Although the actual instuaentation used is of Error count is displayed as a binary coded lamp T! interest and has been described in SOB« detail display on the front panel with a reading to a t>: this paper is to describe the design, layout . maximum of 2047. An additional lamp is lit if this ti and operation of the accessing and instrument number is exceeded. A six-digit resettable (i selection techniques used and to detail the electromagnetic counter can extend the range of the Cc considerations taken into account in the error count to 2048 x 10 Pa mechanical design of the console itself. th Removal of the outer cover of the tester permits ir it to be rack mounted on the console. As has been mentioned, the Technical Service ac Area is an adjunct to the main CUDN equipment pa room and is not intended to perform testing th on the computer equipment, other than the line pr interfaces. Consequently, the activities in the area will be generated as the result of - , 84 (i) Computer Alarms; and By the use of the "Terminate" button a (ii) user Complaints, decision is made as to whether the test and will be directed to the Technical Service Area equipment is connected in a terminated or because of suspected fault conditions in input/output monitor mode. interfaces, modems, outstation terminal equipment or lines. Normally two access circuits are provided between the TAI and the Accessing Panel, the Because of this method of generation of work, and the maximum number of circuits being usable on a inherent idea that the particular service is at the Test Console being six. time of report "Unservicable", there does not on the surface appear to be a necessity for "no-break" accessing Interlock arrangements on the pushbuttons prevent of the particular faulty service. However, due to the the same piece of test equipment being connected possible requirement of performing monitoring activities, to two lines simultaneously, although connection either after repair action has been completed or as a of more than one piece of test equipment to the means of "trapping" an intermittent condition,, it was one line is allowed. decided that all line accessing should be on a "no-break" basis. Monitoring could then be effected on the data The layout of the Accessing Panel is such that stream in a non-interfacing condition and could be visual separation of the pushbuttons for the removed without introducing errors. three groups of test equipment is effected. (Figure 3). It became necessary then to design a system of carrying all lines to CUDN through the Technical Service When testing a medium or high-speed data line using Are* where testing could b<.; effected, and from there to the Trend Data Set, it is necessary to select the computer room. the correct test modem to match the particular circuit under test. Operation of the button (d) Test Access Interface. The unit designed to enable connected to the Trend tester before selection lines to be diverted to the test console was of a modem has been made initiates a flashing designated the Test Access Interface and was light alarm in the Trend tester pushbutton. designed around a patching block manufactured by This alarm is reset only when a modem is the German firm - KRONE. For standardisation, selected. four wire blocks were provided for all circuits even though some D.C. Telegraph circuits utilise (f) Test Console - Mechanical Design. The various only two wires. locations of the Technical Service areas necessitated a console design which could be Basically the KRONE block consists of input and located in a free-standing location, or output tags for each wire, with a pair of contacts alternatively placed against a wall or in between. The insertion of the plug into partition. Consequently, access to the inside the block simply diverts the input to the cord and of the console had to be provided from either permits it to be returned to the block on the underneath, or from in front. Ultimately pair) / other side of the plug. (Figure 2) If the cord both entries were provided but it was decided is simply a closed loop the plug may be inserted that all cabling and power wiring would be and withdrawn from the block without causing a ducted in from the bottom and distributed from "break" in the circuit. the central column.

The KROHE blocks which cater for 25 circuits/ Power outlets for the various pieces of test block are mounted in cabinets which cater for equipment were mounted along a plinth running 750 lines. Cabling is terminated directly on die lenth of the console. the tag blocks and the eabinet was designed to permit the blocks to be withdrawn to the fromt The structural design of the console was decided to facilitate termination. upon after considering several factors :-

Since the Technical Service Area is built on (i) Appearance. This had to be in keeping a floor plenum, construction cabinet is with the general appearance of computer intended to be run under the cabinets and installations while, at the same time, entered from the bottom. providing access for cabling, fault clearance and replacement of panels and (e) Test Console - Line and Equipment Accessing. instruments. A good balanced The diversion of a circuit to the Test Console appearance was essential. by insertion of a plug into the KRONE block on (ii) operational ergonomics the Test Access Interface (TAI) extends 8 wires (iii) Acceptable size. This was important, (both sides of a 4 wire circuit) to the Test not only for siting, but also for Console, and in particular to the Accessing manufacture and handling. Panel. This panel provides the technician with (iv) East of installation and future relocation the capability of then connecting various test if required. instruments to the line for fault clearance (v) Suitable mounting arrangements for the activity. No equipment is connected to the test instruments. particular line until the technician selects that equipment appropriate to his test by The final design met all these criteria and, pressing the zelated pushbutton. after colour-matched painting in the computer equipment colourscheme was completed and all panels were fitted, together with the t«tt 85 instrumentation, a good working unit was completed. (Figure 5.)

The accessing panel, measuring approximately IS" x 18" and fitted with 82 illuminated ' pushbuttons, presented a problem with mounting and cabling if made simply as a "lift-in" panel. The design was modified to allow the panel to be mounted on slide tracks of a robust design. Any attention required on the panel can now be performed without the necessity of special j handling techniques. (Figure 6.)

The console is designed for two technicians to be seated at once, and all instruments and pushbuttons can be reached from the seated position. If only one technician was considered necessary for the amount of work involved, a mobile chair allows access to the full console with minimum effort.

Ample writing area of laminated surface material permits records to be consulted, recorded and amended as necessary.

The first CUDN installation in Brisbane will be followed by the Melbourn Centre, and consoles have been manufactured and delivered to these centres.

4. ACKNOWLEDGEMENTS

The majority of the design and much of the manufacture of the prototype console was carried out in the Telegraph Laboratory, Postmaster-General's Department, Melbourne. The Officer-in-Charge, Mr. S. Sanders, controlled the design activities in the Laboratory, with considerable assistance from Mr. A. MacCaskill (mechanical design and manufacture), Mr. L. Merlo (control and access design), Mr. W. Tait (Data facilities), and Mr. B. Anderson (telephone facilities). All those concerned are Technical Officers in the Postmaster-General's Department, Headquarters, Telegraphs and Data Branch, Melbourne.

86 COMPUTB) AREA Vs3 IUI CONSO

SERVICE MODEM AREA m iMI i i CA—KT« rrrrn irr

FIG. I BASIC CENTRE LAYOUT (TYPICAL)

LINE (TOR>

EQUIPMENT FIC2 .KRONE BLOCK ASSEMBLY (TO BE MOUNTED WITH POLARIZING SLOTS IN JACKS TO THE UH.$

87 COUNTO ACCIMMNO. TKtr acr

nUNSMMHOM •IT •CT

DC LMC TMT FMCMNCV COUNT» wr MML'A* PANKL

FIG.3 CONSOtE FACE LAYOUT

mr TMt < vou me

•tui. »hi um,

T|AMl

1 H i

i i i i I I I I I I I

3 I • I I I I I

FI6.4 ACCESS ftkNtL (LAYOUT)

88 FIG. 5 TEST CONSOLE

FI&. 6 ACCESS PAKJEL MOUNTING

89 AN ACCELEROMETER FOR ENGINES AND ROTATING MACHINES

W.S. Leung, B.Sc, Ph.D., M.I.E.E., Sem.M.I.E.E. Senior Lecturer, University of Hong Kong W.F. Ma, B.Sc.(Eng.), M.Sc.(Eng.) Lecturer, University of Hong Kong C.C. Lau, B.Sc. University of Hong Kong

SUMMARY.- The paper describes an electronic system for measuring and displaying acceleration and retardation of engines and rotating machines. The special features of the system are: (1) a transducer, which converts the instantaneous speed into an instantaneous d.c. e.m.f., and (2) an electronic network, which differentiate! the speed and produces a d.c. voltage proportional to the acceleration or retardation. Acceleration-tine and acceleration-speed curves can be produced on a C.R.O. The transducer is in the form of a special horoopolar d.c. generator which is capable of generating a d.c. ripple-free voltage. The electronic differentiator, which consists of a sampling network and a comparator, is so designed as to produce an output voltage that is relatively free from the adversed effects of any harmonics in the input voltage cr drift in the d.c. amplifiers. As the size of the homopolar generator is comparable to that of a tachometer, the accelerometer can conveniently be used to measure and record the acceleration of a variety of engines and rotating machines. The accelerometer is used to measure the running-up torque of an Induction motor whose characteristics are known. The accuracy of the measurements is confirmed. I.- INTRODUCTION by any appreciable drift in the d.c. amplifiers used in the differentiator. This paper also When an engine or a motor is started from rest, presents a special electronic differentiatial it is often desirable to obtain the acceleration- network using sampling techniques similar to that time and acceleration-speed relationships by described by Broomal I and Riebman^ in their analogue direct measurement. The minimum torque generated by computer. Instantaneous values of the d.c. voltage the engine during the running-up period may well proportional to speed are sampled at a fast rate. determine the critical value of the load on which Adjacent sampled voltages are grouped into pairs. the engine is started. Electromagnetic torque The two voltages in each pair are being compared meters are available commercially, operating on the and their difference is an incremental change of principles of the Weidmann effect. However, they voltage during the interval by which the sampling suffer from the disadvantages that the transducers of the pair of voltages is separated. If the are not easily detachable and transferrable from interval is kept constant, the incremental change machine to machine and that they require elaborate, of voltage may be taken as a measure of the and hence expensive, »ccesaorle» Including instantaneous rate of change of speed (i.e. the oscillators etc. The popular device for measuring instantaneous acceleration) of the engine or »peed is the common tachometer, which is a very small rotating machine. The sampling differential network conventional a.c. or d.c. generator, producing a removes the common objections to the use of voltage that is directly proprtional to its speed. differentiation as a technique in solving problems The corresponding acceleration would readily be with electronic circuits. deduced if the tachometer's voltage could be differentiated to obtain the rate of change of the II.- ACCELEROMETER magnitude of the voltage. This condition rules out the use of an a.c. voltage for the measurement of Fig. 1 shows the block diagram of the the speed. Even a d.c. voltage generated from a accelerometer which consists of a hoaopolar d.c. conventional d.c. tachometer is also not generator H mechanically coupled to the engine E differentiable since the d.c. voltage picked up whose acceleration and speed characteristics tct across a rotating commutator is inherently saddled to be measured and displayed. The output voltage with ripples. At the output of the differentiator, of H in millivolts is amplified by a linear d.c. the component of the voltage corresponding to amplifier A. The output of A is connected to the accelera.ion is overwhelmed by the component of the sampling differential network S through a filter F. voltage obtained by the differentiation of the The output voltage of A, which is proportional to ripples in the input voltage. To overcome this speed, and the output voltage of S, which is difficulty, a special small monopolar d.c. proportional to acceleration, are connected to the generator!>2 is used as the transducer. As the C.R.O. for measurement and display. As the physical output d.c. voltage of the monopolar generator is size of the homopolar d.c. generator H is ripple-free, it is differentiable by a simple RC comparable to that of an ordinary tachometer, it circuit. However, the accuracy of such a circuit3 can readily be coupled to the engine by contact j is adversely affected by its circuit constants and friction. Both the homopolar generator H and the

90 stationary. The magnetic circuit concitts of: (1) r - -\ an inner ferromagnetic solid cylinder I with the > E i shaft extending at both ends, (2) a circular permanent magnet F, (3) an outer ferromagnetic hollow cylinder 0, which is closed at one end and (4) a small radial air gap between the inner solid cylinder I and the outer hollow cylinder 0. Located at the air gap is a thin hollow brass cylinder T, which' is open at both ends. The permanent magnet P will send a homopolar flux through the thin S hollow brass cylinder T. The stationary inner 1 core E rests on bearings B at the two ends. The thin hollow brass cylinder T is fixed mechanically _J to the driving shaft D, which in turn nay be coupled to the rotating machine with the unknown ir speed and acceleration. When the thin hollow brass cylinder T is rotated, an e.m.f. proportional C.R.O. to the speed is generated on Its surface along the axial direction. This will be the output voltage of the homopolar generator and the voltage can be picked up across a pair of carbon brushes Fig. 1 Accelerometer Block Diagram in contact with the thin hollow bras« cylinder T at the two ends. The instantaneous generated e.m.f. is given by the following equation: sampling differential network S will be described B*v (I) separately in some details. The amplification factor of the linear d.c. amplifier is of the order i.e. the triple product of flux density B, axial of a hundred for an output voltage of 0-50 mV from length of flux penetrating and the linear the homopolar generator H. The function of the velocity v of the flux. For the experimental filter F is to prevent interference from any homopolar generator, B * 0.50 W/M2 and / » 5 cm. irregularities in the input voltage to the sampling An average velocity of 2 H/sec. will generate differential network S. However, the presence of a d.c. e.m.f. of 50 mV. The homopolar distribution the filter F will inevitably distort the results of flux at the air gap together with the to some extent. For a well-designed homopolar commitatorless method for collecting the generated generator H and an interference-free environment, voltage enables the d.c. output voltage to be the filter F may be left out. The C.R.O., which continuous and ripple-free. In order to avoid is an ordinary low-frequency, two-beam oscilloscope any disturbance of the homopolar circumferentlally- with facilities for an external time base, is used uniform flux distribution in the air gap, the to display and record speed-time, acceleration-time outer hollow cylinder core 0 is split into two and acceleration-speed characteristics of the parts separated by a thin non-magnetic ring N engine E. through which connection leads from one of the brushes and from the possible excitation coil may III.- HOMOPOLAR GENERATOR be brought out. A photograph of the experimental monopolar generator is shown in Fig. 3. Fig. 2 shows a simplified sectional view of the homopolar generator which is capable of generating a ripple-free d.c. voltage signal whose magnitude 1« directly proportional to the speed of the generator. The core of the generator is

Fig. 3 Monopolar Generator Fig. 2 Section of Monopolar Generatcc

91 IV.- DIFFERENTIATOR The common objection to differentiating a transient d.c. voltage is that any minute distortion in the original voltage due to harmonics and external interference will be considerably magnified in the differentiated voltage. The noise component often overwhelms the signal component in the output voltage of the differentiator. This difficulty is largely overcome by the sampling differential network which works on the following principles: Fig. U shows any given voltage function. The slope

Fig. 5 Block Diagram of Differentiator Circuit

clock pulses with frequency f . The monostables give very narrow sampling pulSea which are used to open the gate G^ and close the gate G„, and vice versa. Alternate magnitudes of the input voltage with frequency f is sampled and stored in memories Ml and M2, These values are fed to the comparator C for comparison via gates G. and G, which are opened and closed simultaneously by a Fig. 4 Voltage Characteristic chain of pulses with the frequency f . A partial F circuit of the sampling network is given in Fig. 6 of the function at any time t, is given by in which the transistors are of the commonly-used general-purpose types. Vl •V 2 TT ' (2) The gain A of the sampling differential At network is given Aβ at approaches zero, the expression will be the derivative of the function at t^, i.e.

dv lim Av (3) V.- dt at -» o 4t A v may be obtained by sampling and temporarily storing a pair of Instantaneous voltages v. and v,. TO ml The difference between the two voltages can be obtained by means of a comparator. For a constant TOC rate of sampling, the function of iv will be directly proportional to the derivative of the given voltage. The accuracy of differentiation will be increased with an increase in the"sampling rate f . In the absence of any input voltage, the c comparator is kept in the quiescent state. The odd-number pulset and the even-number pulses are memorised separately, each for, a duration of 2 At, i.e. until a pulse of a new magnitude appears. By means of • scries of gating pulse«, appropriate Fig. 6 Partial Circuit for Memory and Gates parts of the odd-number and even-number pulseit -are . simultaneously fed to the comparator whose output by the ratio of At to 4- , i.e. is the derivative of the sampled voltage. A' block . diagram of the circuit is shown in Fig. S. The 2f two monostables ml or m 2 are steered by a flip- (4) flop FF and the latter is operated by a chain of 1/f.

92 Kor unity gain in the entire process, an

amplification factor of fc/2fs is to be provided by the C.R.O. The phaseshift of the differential network is not constant and is dependent on whether the number of sampling pulses is an even multiple of the input frequency and on the phase relationship between the sampled voltage and the clock pulses at t = o. An example of the worst case is given in Fig. 7, where f /2f is an integer

Function Even PIIIs«.' Odd Pulse

... «. «•• lolcagf Derivative

(A) Abcve Speed-time Curve Below Acceleration-time Curve

Fig. 7 Triangular Voltage Function

and a voltage is sampled by an even-number pulse at t=0. Maximum phaseshift is given by 2f s 2 * = A « f /£ (5) emax. s c

In practice, since f >> f , the phaseshift is negligibly small. c As the output of the comparator is the difference between two adjacent sampled voltages, any noise or distortion of a (B) Acceleration-speed Curve frequency^) f will be practically eliminated in the output voltage of the comparator. Fig. 8 Induction Motor Starting Characteristics V.- EXPERIMENTAL RESULTS (Without External Rotor Resistance)

The accelerometer described above is used to measure the acceleration of a three-phase slip- ring induction motor whose starting characteristics are known. Photographs of speed-time, acceleration- time and acceleration-speed characteristics of the motor without and with external rotor resistances are taken and shown in Fig. 8 and Fig. 9 respectively. The sampling effect is indicated by the dotted appearance of the acceleration curves. Acceleration is proportional to the net starting torque of the motor. Since the motor is started on no load, the acceleration is approximately equal to the gross starting torque generated in the motor. A comparison of Fig. 8B end Fig. 9B shows the effect of the external rotor resistance on the starting torque of the motor. The acceleration-speed curve of the motor without external rotor resistance was reproduced in Fig. 10 for comparison with its corresponding theoretical curve. It can be seen that the two curves are in close agreement, thus verifying the ,(A) Above : Speed-time Curve accuracy of the accelerometer. Below ; Acceleration-time Curve

93 distortion particularly «hen the frequency of the latter is much less than the sanpling frequency. An accelerometer with either the honopolar d.c. generator or the sampling differential network will function satisfactorily and the use of both will produce remarkably good results. The measurement of acceleration by an accelerometer with both the hpmopolar generator and the sampling differential network is shown to be highly accurate. The accelerometer has a number of advantages over the commercial torqueaetert including its portability and considerably lower cost.

RIFEMHCES

1. LEUNG, W.S. and MA, W.F. - Machine Torque Measurement with Tacho-generator. Electrical Times. October, 1969.

2. LEUHG, W.S. and MA, W.F. - Measuring and (B) Acceleration-speed Curve Recording System for Speed and Acceleration using a Special Honopolar DC Tacho-generator. British Patent No. 1228070, April, 1969. Fig. 9 Induction Motor Starting Characteristics (With External Rotor Resistance) 3. SIMMONDS, R.K. and BHAGNAT, t.G. - Visual Display of Motor Torque-Speed Characteristics. International Journal of Electrical Engineering Education. Vol. 2, 1965.

~ ~ — Theoretical Curve 4. MOOKkU., J. »nd RIEBMAH, L. - A Sampling Analogue Computer. Proceedings of I.R.E.. Measurement by Aeceleromcter Vol. 40, No. 5, May, 1952. I.

Fig. 10 Acceleration-speed Characteristics of an Induction Motor

VI.- CONCLUSIONS

The accelerometer is a simple means for measuring the acceleration (or retardation) of any rotating engine or machine. The transducer of the accelerometer can be mechanically coupled to the •«gin« by contact friction In the saae way at * tachometer and Che •carting or braking characteristics of the engine or machine can i conveniently be displayed or recorded on the screen of a C.R.O. Acceleration i» obtained by the sllmple principle of differentiating speed. The paper '!•,, presents two key components for the accelerometer, namely (1) the homopolar d.c. generator which can produce a differentlable voltage proportional to speed and (2) the sampling differential network which Is capable of effectively differentiating a voltage with a considerable amount of noise and

94 DEVELOPMENT OF TELEPHONE TRAFFIC MEASUREMENT EQUIPMENT AND ITS APPLICATION IN THE AUSTRALIAN TELEPHONE NETWORK

C.W. Pratt, Ph.D., Australian Post Office and LA. Tyrrell, B.E., M.I.E.(Aust.) Australian Post Office

SUMMARY. The paper discusses the data requirements for planning and supervision of the telephone network. Data of two basic categories are required: occupancy (traffic load) and dispersion (origin-destination). Traffic measuring equipment has been developed within the Australian Post Office to provide comprehensive occupancy and dispersion data throughout the network. It is planned to use the equipment at key exchanges in the trunk network and the larger terminal exchanges ''exceeding 2000 subscribers). This represents some 600 exchanges throughout the Commonwealth. The data will be computer legible since computer aid is needed for editing and processing.

The data to be acquired are presented in a number of different forms and dispersed over various devices In the switching path of the telephone exchange. The fundamental design problem was to translate the data as presented by the switching equipment into suitable binary patterns, to assemble these patterns into ordered formats and write then on punched paper or magnetic tape. The philosophy of the system design and the operation of the equipment are explained. Several devices within the system are described in some detail. I. TRAFFIC AND NETWORK PLANNING. The calls in progress in the telephone network are required for extending buildings, and ordering collectively known as telephone traffic; these calls switching equipment and large transmission systems. are of course the reason for existence of the The planning process is aimed at ensuring that network, the prime operational objective being to circuits and switching equipment are provided at switch calls from calling subscriber to called the right places and in the right quantities. subscriber with a very low probability of failure Very large capital expenditures depend on these due to technical faults or insufficiency of circuits. plans and it is therefore necessary to base them Telephone traffic represents the revenue-earning on the best possible data. These data are obtained life blood of the network and since this revenue by traffic measurements, but before going into currently runs at a rate of over $300M per annum, details, it is necessary to give a general picture considerable attention must be paid to its smooth of the Australian network. disposal. (a) The Australian Telephone Network. The number of calls in progvese at any point in the telephone network varies from moment to moment as From its inception in 1912 up to 1960, the automatic calls are established and disconnected, and also telephone network was based on the step-by-step varies throughout the day in response to business system which uses bl-motlonal Strowger selectors. and social activity in the conmunity. The momentary Except for final selectors which deal with the last variations are of course quite unpredictable, but two digits of the called number, in general one rank the daily pattern of traffic follows a more or less of switches Is required to deal with each dialled regular shape. In providing the network it is digit, and there is a close relationship between the necessary to reach a compromise between cost on the dialled digits and the path by which a call is routed one hand and service to subscribers on the other: through the network. Discriminating selector to achieve an economical network It is necessary to repeaters permit a strictly limited amount of insist that subscribers tolerate a small probability alternative routing: up to nine direct routes each of encountering congestion during the busiest with only one alternative route. Over the past 12 period of the day. years, network growth has been implemented using the Swedish Ericsson crossbar system which Is a so called register controlled coonon control syatsm in which There are two fundamental requirements for ensuring dialled digits are stored in a memory devlc* that the network caters for traffic demands at all (register), and analysed to the extent necessary to times. Firstly, the network must be supervised at decide the appropriate path (outgoing route) to regular intervals and where Inadequacies appear in which to connect the call. This analysis of digits switching equipment or numbers of circuits, short allows routing to be separated from numbering to tern action must be taken. Secondly, It Iβ some extent, and allows economies to be achieved by essential to plan the orderly growth of the network comprehensive use of alternative routing techniques. some years ahead since quite long lead times are 95 With alternative routing, there «ay be two or more occupancy measurement the number of simultaneous .ilttTnatlve paths fro« origin to destination occupations in each group of devices is measured and ••xchange, and these are searched In a fixed order of recorded, and this is the more usual type of preference when attempting to establish a call. If measurement especially for lsrger exchange. ill circuits on the first choice route ar« busy» Circuit occupancy measurements are taken in ARK'a then the cell Is offered to the second choice route, or P.A.B.X.'s, or for special studies such as the .ind no on. analysis of gradings, when it is convenient and useful to observe each device individually, and Vhere are three generic types of crossbar exchange: record Its status, i.e. busy or idle. ARM exchanges for the 4-wire switched trunk network, ARF exchange« for 2-wlre tsndea switching centres The discriminating selector repeaters (DSR'e) of .ind teralnal exchangee In larger cities, and ARK the step-by-step system with their limited exchanges for small rural applications. Within each alternative routing capability, and the crossbar £onerlc type there ere aome variations which In turn switching system with its extensive alternative have a bearing on the traffic data which nay be routing facilities have created a need for derived fro« the«. dispersion Information, i.e. the percentagea of traffic from an origin exchange to the various (b) Traffic Measurements. possible destination exchanges In the network. Dispersion information can be of two kinds: call As Just described, the Australian network consists dispersion which consists simply of percentages of at the present tiae of a Mixture of step-by-step calls to the various possible destinations, and and crossbar equipment. The traffic measurements traffic dispersion which consists of the percentage! required for supervision and planning purposes are of traffic to destinations. As a generalisation, of two kinds: occupancy measurements and dispersion call dispersion is easier and cheaper to obtain as measurements. it requirea only a count of calls to the various possible destinations, irrespective of their average Occupancy measurements are vital for exchangee of all durations. If calls to a particular destination types and involve observation of the numbers of are very short, then although they aay be numerous, simultaneous occupations In the various groups of the average number in progress simultaneously will devices In an exchange. Here "devices" means all be quite low. Hence call dispersion can be a those items of equipment which participate in the misleading indicator of origin-destination traffic disposal of traffic. The term includes the circuits and hence of the circuit requirements. By contrest which carry the conversations themselves but also traffic dispersion takes account of differences in items of common equipment such as registers, markers, call holding times in different traffic streams, and code senders, etc., which are occupied for shorter is therefore ouch to be preferred for network periods during the establishment of calls but must planning purposes. be provided in edequate numbers to cater for all likely demands. To plan the network the percentages of traffic to various destinations obtained from dispersion It would of course be possible to take occupancy measurements must be used in conjunction with an measurements on a continuous basis but this is not absolute measure of the buay hour traffic as necessary alnce adequate accuracy is obtained by derived from occupancy measurements. Thus a observing each group of devices at regular intervale comprehensive traffic measurement system must permit of, aay three minutes, and averaging the number both occupancy and dispersion measurements to be found busy over an appropriate period of time. taken. Current occupancy measurement practice therefore employs scanning techniques, and by use of suitably (c) Measurement Practice. designed automatic equipment a large number of. device groups can be observed within each scanning Traffic measurements are of course not new, but cycle. have been taken for many years using & variety of equipment of steadily increasing sophistication. Traffic conditions of course vary throughout the The earliest measurement techniques involved a day, but for design purposes attention la fecussed visual count of occupied switches, but this wes on the time consistent busy hour, which la the clock subsequently displaced by electrical scanning of hour (commencing on the hour or half-hour) for which device groups. Dispersion recorders were also the traffic, averaged over the five business days of built for the step-by-step system. the week. Is the highest. The measurement sessions during the day must of course be extensive enough to Traffic measurement field staffs in each State ensure that the buay hour is Included. Time administration take measurement» In exchanges consistent busy hour traffic figures derived from periodically. The frequency of measurement varies the scanned observations yield the magnitudes of with the importance of the exchange. Trunk the traffic flowa in the network in absolute terms, switching centres are measured once or twice a year and theae are used for network supervision, i.e. to as are also tandem exchangee. Less important demonstrate deficiencies and aurplussee of circuits exchanges are measured at up to two year intervals, in the existing network. They are alao used, although it is hoped to reduce the Interval to suitably adjusted for seasonal effects, as base data about one year. Measurements are taken throughout for traffic forecasts to plan the extension of the the year although as many as possible are network in detail. concentrated in the busy season to reduce seasonal corrections. It Is necessary to distinguish between two types of occupancy measurement, known respectively as route Most measurements are made using portable measuring occupancy and circuit occupancy. In a route and recording equipment. The measurement procedure 96 requires the portable equipment to be set up and the initial tape will indicate whether errors have thoroughly tested. Readings are then taken been made in setting up the measurement. It is then typically over the five business days of the week, a matter of judgment whether the measurement can with usually two (morning and afternoon) reading continue or must recommence after clearing all faults. sessions per day, but sometimes three (an evening session as well) in residential areas where social The output from processing includes the following: activity can produce an evening peak. The sessions must of course be sufficiently long to ensure that Occupancy: the time consistent busy hour is included. Data are recorded on punched paper tape for subsequent - Traffic averages (half-hour by half-hour) for every computer processing, although the use of magnetic device group. tapes is now commencing. When the measurement is complete, the portable equipment is dismantled and - The time consistent busy hour and its average moved to the next exchange. traffic over a five day measurement period for each device group. II. THE TRAFFIC DATA EQUIPMENT PROJECT. - Similar data for combinations of device groups. Over the past five years the Australian Post Office has been developing traffic data equipment suitable - A summary of the time consistent busy hour, and for extracting data from crossbar exchanges. The the busy hour traffic, both measured and seasonally equipment will be installed in the more important corrected. exchanges first, and it is planned to equip all switching centres, and terminal exchanges exceeding Dispersion: 2000 lines in capacity. This will require installations in approximately 600 of the 3200 - Percentages of traffic, and average call holding automatic exchanges in the Australian network, at a times for all identified destinations, and for cost exceeding S2M. certain groups of destinations.

The equipment will consist of a mixture of III. GENERAL SYSTEM DESCRIPTION. permanently installed and portable units. A rack will be installed in each equipped exchange, and The traffic measuring equipment consists access wiring to the various measurement points essentially of three main parts: throughout the exchange will also be permanent. Certain basic units will be placed permanently in . equipment to collect and assemble the data the rack so that exchange maintenance staff are able from the various sources within the exchange. to keep selected device groups under surveillance and observe any abnormal traffic behaviour. The . equipment which controls the recording of resc of the equipment required for full scale the data which has been collected. measurements »ill be portable. . the data recording equipment. The equipment permits occupancy and dispersion measurements to be made simultaneously. Occupancy Figure No.l illustrates the inter-relationship of data are gathered and recorded at scan intervals the different equipments for ARF crossbar exchanges. which can vary between wide limits: 15 seconds up to 60 minutes. These scans are initiated by a time (a) Collecting And Assembling The Data. clock. On the other hand dispersion data are recorded in real tine as calls are set up and For each type of switching equipment there is a disconnected. The output tape therefore contains a complete traffic measuring system which collects mixture of the two types of data which must be both dispersion and occupancy data from the segregated and processed by the computer. telephone exchange. The occunancy equipment is the same for all types of exchange, but many differences (a) Data Processing. occur beween the dispersion equipment for ARM and the various types of ARF exchanges. As may be seen The traffic data equipment project incorporates a from Figure No.l tne data to be acquired is presented centralised data processing system, physically in different forms and is dispersed over various located in Melbourne, to which data tapes will be devices in the switching path of the telephone exchange. Collection of the dispersion data is submitted for editing, validation, and processing. initiated by circuit conditions within the switching The validation and processing phases require equipment as calls are set up and disconnected; reference to data about the exchange itself: details whereas the traffic measuring equipment.through of switching equipment, number of devices in each the agency of its time clock, controls the collection group and so on. For this purpose a special master of occupancy data. The data collection equipment file is to be created which will be updated as part translates the data as presented by the switching of the overall measurements programme. equipment into binary signals, assembles the binary information into a suitable pattern-and extends a The use of magnetic tape as a recording medium signal indicating that data is ready for recording. creates certain problems, since it is not possible to detect or read the data on the tape visually. Hence after setting up an exchange for measurement, The main functions of the data collection equipment data will be gathered for about half a day, and a may therefore be summarised as: short tape sent for editing and validation. In the meantime, measurement «ill continue using a second . detection of conditions which indicate that tape. Reports from the editing and validation of data is ready for collection. 97 TELEPHONE EXCHMKE EQUIPMENT

DATA COLLECTION EQUIPMENT

MIOftlTV ANO SEQUENCING MODULE

RECORDING EQUIPMENT Block Diagram of Traffic Measuring System for ARF Exchanges. Figure No.l

. generation of circuit conditions to enable the device. The equipment can record the condition of [' data to be collected. more than 40 devices per second. f< . assembly of the data in its correct format. (ill) The data collection equipment for dispersion j' measurements records the following information about i . extension of a signal to indicate the each call monitored: . ;• availability of data for recording. . the Inlet Identity, (1) The data collection equipment for route occupancy Measurements enables the number of devices . the dialled digits, occupitd withir a group of up to 200 to be recorded. A total of 400 groups, each containing up to 200 the outgoing route, devices, can bt measured in less than one minute. . the call starting time, (li) The data collection equipment for circuit occupancy measurements Identifies each device within . the call finishing time. a group and records its condition. Whereas the previous measurement merely sums the number of The methods of obtaining dispersion data are devices within a group which are occupied, this dictated by the type of exchange equipment on which measurement records the condition of each individual the measurements are taken. As previously described 98 the Australian Post Office operates a number of finishes with an end of block character. Between different switching systems so that it has been the header character and the end of block character necfssary to design data collection equipment to 'alwavs binarv 62) are recorded the relevant data. suit each case. There is however a common approach to each system of obtaining traffic dispersion data It mav be observed that each frame comprises six bits in that a number of inlets to a switching stage are of data plus a bit in the seventh track to provide observed, and the information mentioned above is odd parity. The parity bit provides a simple check obtained for all traffic passing through these of the correctness of each frame of data. It is inlets. For ARF exchanges up to 320 inlets are subsequently checked bv the tape recorder before observed to obtain •'he traffic sample, while for recording and again by the data processing equipment ARM exchanges 400 inlets are observed, yielding a to ensure that the data has not been disturbed by sample of up to 5,000 calls per hour. noise, faulty tape or any other agency.

(iv) All the data collection eauipments assemble

CALL RELEASE BLOCK CIRCUIT OCCUPANCY BLOCK TIME BLOCK

ABCOEFGH A 8 C D A B C 0 E F G

5 IO IS 2O

2 7 12 17

1 6 II 16

A 4O A. 2 A. 52 B 1-16 CIRCUIT BLOCK Ne a -I BINARY DISPLAY OF B O->-S95 \ EXCHANGE IDENTITY C C J OCCUPANCY OF IO CIRCUITS C. O>-59.- J l D END OF BLOCK 62 0 O-3O DAYS AND WEEKS 0 I BINARY DISPLAY OF E O-23 HOURS £ [CIRCUIT OCCUPANCY F O-S9 MINUTES F J i G END OF BLOCK 62 G O-59 SECONDS H END OF BLOCK 62

Typical Block Formats.

Figure No.2

99 devices may he available for recording at the same oilled a parity error character, is written after UIIU-, .1 system is needed to determine the order in th« corrected frame of data. The presence of the whli'h data from the various sources is to be parity error character is used later In the data recorded. In addition to selection of a particular processing phase to identify characters of doubtful .source, provision must be made for the data from validity. British Standard 3968 : 1968 also requires that source to be extracted frame by frame in the that the data be assembled Into blocks of up to 2048 correct sequence. characters, each such block of data to be terminated by a longitudinal check row of even parity and U) The data from those sources involved in the separated from adjacent blocks by an Inter-record setting up of calls is available for relatively gap of 19 millimetres. The tape recorder incorporate« short durations, in some cases as little as 200mS. the logic needed to satisfy these further requirements In addition, up to four calls may be switched of the format specification. simultaneously through the inlets being observed. The magnetic tape recorder can record a character (ii) The recording density of 200 characters per every 3.3mS. It is therefore necessary to employ a inch on the aagnetic tape is quite low. The system of priorities for recording the various' selection of a format incorporating such a low blocks to minimise loss of data. A different system character density was mad* considering that the of priorities has been devised for each exchange recorders would often operate in environment« where system. The typical blocks Illustrated in Figure there was only limited control over temperature, No.2 have recorded against them the priorities that humidity, or dust levels. In these circumstances a apply within the ARF traffic measuring system. low character density offers a certain protection Dispersion data which are generated by the switching against the intrusion of errors. equipment itself are given the higher priorities. Route occupancy and circuit occupancy data are IV. DESCRIPTION OF THE EQUIPMENT. given the lower priorities since these measurements are generated by the traffic equipment Itself and The traffic measuring equipment haa been designed in can therefore be controlled to allow the dispersion modules, or building blocks, so that the measuring data first use of the recording equipment. systems which are required to perform the occupancy and dispersion measurements in the different types (11) The extraction of data, frame by frame, from of exchanges may be built from different combination« the data collection equipment is performed under the of standard nodules. A total of 25 modules was control of a stepping chain. There is, howevar, a required to cover all circumstances. Brief complication that arises from the non-uniformity of descriptions of several modules are set out below. the lengths of the blocks of data. Provision Iβ made in the circuitry to determine the length of (a) The Data Bus. the block appropriate to each data source and to control the extraction process accordingly. Before examining the system in any detail it must first be understood that all devices which collect (c) Recording The Data. and assemble data are permanently connected to a common bus comprising six wires. The six wires Punched paper machines or magnetic tape recorders (or data bus) are the medium by which the data is record the data. Both types of recorder are transferred from the collection points to the purchased from commercial sources. The paper tape recorder. Buffer amplifiers to adjust voltage machines, depending on their type, are capable of levels and filter out noise disturbances are recording 20 te 100 characters per second, while introduced Into each of the six wires at the the incremental magnetic tape recorders can record interface to the recording equipment. 300 characters per second. The relatively higher speed of the magnetic tape recorders is essential (b) Digital Ohmmeter. for the complete traffic measuring systems. In the past, paper tape has been used extensively since The measurement of route occupancy, that Is the traffic dispersion equipment has not been available number of occupied devices within a group, is and the speed of paper tape machines is adequate essentially a measurement of resistance. Each for occupancy measurements. The various timing device within a group, while it is occupied, applies constraints enumerated in Section III (b) (i) earth via a 100 K ohm resistor to a coamoning point. I '• Indicate the need for magnetic tape recorders in Consequently the resistance between the common point conjunction with traffic dispersion measurements. and ground is an indication of the number of circuit« occupied. A simple block diagram demonstrating the (1) As Mentioned earlier both the paper tape operation of digital ohmmeter is sketched on machines and magnetic tape recorders write the data Figure No.3. The digital ohameter basically In computer legible format. The magnetic tape consists of a Wheatstone bridge which has a recorder warrants a more detailed description in differential amplifier across the balance arms. To this regard. It is a seven track machine which uses balance the bridge the differential amplifier causes an incremental movenent to record 200 characters a series of resistors to be switched in and out of per inch on 's inch magnetic tape. The data is the balance arm until a null is achieved. There are written onto the tape in a format which accords with in fact eight resistors and the sequence in which the specifications of British Standard 3968 : 1968, they are applied to the balance arm is such that when which are in effect a definition of the conditions a null has been obtained the setting of the resistor* that must be observed for the data on the tape to be indicates in binary the number of device* occupied. legible to a computer equipped with a suitable Digital ohaaeter is capable of measuring the reader. To this end the tape recorder checks the occupancy of 1 to 200 devices. parity of each frame of data which it receives. Incorrect parity is corrected and a unique character, A number of protection circuits are incorporated to 100 1 ret IOOKA «8 cd COMMON POINT DEVICES BEING MEASURED atei «nti 1 set

VARIABLE RESISTANCE

DIGITAL -sov OHMNETER

Block Diagram; Operation of Digital Ohmmeter.

Figure No.3. es it. guard the bridge against the effects of foreign detectors. To identify the register connected to a nt electrical potentials. particular inlet, a 15 Khz tone is fed out through lite the inlet over the switched connection to the (c) Register Identifier. associated register. The tone is then fed from the register to the register identifier where it Is In AST exchanges the dialled digits associated with detected by the detectors corresponding to the each call are stored in what is known as a register. register's x and y co-ordinates within the matrix. o For a group of 320 ARF inlets up to 100 registers es may be required to perform this function. Since an The identity of the register is used by other Inlet through which a call is to be switched may be equipment to access the digit store of the register, connected to any of the 100 registers it Is necessary encode the digits In binary coded decimal and insert then to be able to identify the register which is parity In preparation for the data being recorded on :or» connected and has stored the digits for the call, so tape. I. that these digits may be extracted and stored on the magnetic tape. (d) Standard Priority And Sequencing Module. The register identifier consist* of a 10 x 10 The Standard Priority and Sequencing Module (SPASM) performs the following functions: :o transformer «atrix corresponding to the x and y co-ordinates of the 100 registers, and 20 tone 101 Identifies the various data collection possible discrete components were used to obtain the equipments with data available for recording. required speed of operation, but in those cases where the circuit operations were complex, . Selects the source of data to be recorded in integrated circuitry was exploited for its accordance with a standard set of priorities. compactness and cheapness relative to discrete components. Scant. each frame of data from the data collection equipment, commencing with the (a) Choice Of Components. header character and through the data proper in the correct sequence. The various considerations mentioned above have led to a system which Incorporates a wide range of Inserts an end of block character after the devices: last frame of data from each recording source. . Electromechanical equipment to access and (d) store the data in those circumstances . Controls the operation of the recorder by where speed of operation is not a signalling forward to the recorder whenever significant constraint. data is available for recording. . Discrete semi-conductor coaponent» to (i) The sequence of operation by which SPASM controls access and store data which are only the recording of a block of data is described below: present for short periods (less than 200mS), and also to perform certain . Of those data collection equipments which analogue functions. have signalled that they have data ready for recording SPASM selects the one with the . Low voltage integrated circuitry (the highest priority. RTL family) to handle relatively coaplex logical functions which must be performed SPASM then applies an enabling condition to at high speeds. extract the six data bits of the first frame of data. The electromechanical logic of the switching equipment is characterised by large inductive . At the same tine a signal is sent to the voltage surges and contact bounce. This behaviour recorder commanding it to record the frame does not affect other electromechanical logic but of data presented to it. The frame semi-conductor components performing logical VI. comprises the six bits of data which appear functions will not function satisfactorily in such on the six wire data bus plus one bit of an environment unless special attention is paid to parity generated from the data proper. The the suppression of noise and,the effects of contact first frame of data will as previously bounce. Under these conditions electromechanical described be the header character for the solutions to circuit problems tend to be relatively block. cheap and simple. Therefore wherever speed of operation was not the prime factor, electro- . The recorder sends a revertive signal back to mechanical equipment was used. In all other SPASM as soon as the data has been recorded. circumstances discrete semi-conductor components or integrated circuits were used. Discrete semi- . On receipt of the revertive signal SPASM conductor components operating directly off the repeats step Nos. 2 to 4 above until all 50 volt supply were preferred to integrated VII. the frames of data for the block have been circuitry for simple functions in spite of their recorded. inefficient utilization of space. Low voltage integrated circuits were the choice for complex . After the last frame of data has been written logical processes although their use was SPASM causes an end of block character to be complicated by their low noise immunity and the added to indicate the llmi* of the block of need to provide D.C. to D.C. converters for the low data. voltage.supply.

. SPASM then selects the next source of- data as (b) Compelled Sequence Operation. described in step No.l. In view of the large differences in the speed of V. CIRCUIT PRACTICES. operation of the different systems of logic it was decided that any interworking equipment should As an introduction to the circuit practices within operate in compelled sequence. That is, the the traffic measuring equipment it is instructive to circuitry was arranged so that where two devices first consider the circuitry of the exchange interwork, one must check the condition of the switching equlpaent. The crossbar switching other before being enabled to move to its next equipment designed by L.M. Ericsson and onerated hv circuit condition. This method of operation avoids the Aiisfr.-ilfan Poet Office consists mainly of relays the need for timing conditions to be exploited or operating from a 50 volt D.C. supply. To gather introduced to ensure that circuit operations proceed the requisite data fron this electro-mechanical in their correct order. equipment the measuring equipment must in general operate an order more quickly than the switching (c) Noise Protection. i-(|iilpment to avoid loss of data. This in turn Implies che use of semi-conductor devices for many Extensive precautions were taken to protect the low lunrtians where high speed is necessary. Wherever voltage logic from the effects of noise introduced 102 from high voltage logic by either capacltlve or inductive coupling. The digital ofammeter and SPASM already described are two of the devices which «•ploy low voltage logic. The low voltage circuitry was physically removed as far as possible fro« wires or components energised directly from the normal 50 volt exchange supply. The wiring forms within the system were carefully designed so that wires carrying low voltage logic and those carrying high voltage logic either ran at right angles to each other or were maintained at a separation of at least five centimetres.

(d) Contact Bounce. The contacts of relays can bounce for varying periods depending on a number of factors: the type of relay, the number of contacts, the state of adjustment and so forth. Under some conditions the contacts may bounce for periods of up to 20mS» High speed seal-conductor logic on the other hand generally requires that any input signal should be characterised by a single unambiguous change of state. Accordingly wherever relay contacts were used to drive high speed logic various interfaces were introduced after the relay contacts to monitor their electrical condition. A typical Interface ignores all transitions until a period of 4mS has elapsed since the last transition. Then a single change of state Iβ generated on the output of Che interface with the rise time or fall time required by the following logic.

VI. PRESENT STATE OF DEVELOPMENT. The circuit and route occupancy equipment has been manufactured in reasonably large quantities. Equipment from early production runs has been giving satisfactory service for some two years. The first production run of equipment for traffic dispersion measurements in ARF exchanges has just been conpleted. The testing of prototype equipment for traffic dispersion measurements in ABM exchanges is currently in progress.

VII. ACKNOWLEDGEMENTS. A large and complex project such as the one described, executed over a period of years, inevitably involves many people at both professional and subprofessional level in all phases of its development - design, prototype testing, documentation and field trials. The authors wish to acknowledge the contributions of all those persons with a past or present Involvement in the project, especially N.M.H. Smith who was the original prime mover, and the numerous technical officers whose efforts made the project possible. The authors thank Che Director-General of the Australian Post Office for permission to present the paper.

103 METERING CONTROL AND ALARM INDICATION FOR POWER RECTIFIERS USED IN A LARGE ELECTRO-CHEMICAL ZINC PLANT

M.J. Healy, B.E.(University of Tasmania), Grad.I.E.Aust. Electrical Engineer at The Electrolytic Zinc Company of Australasia Limited

SUMMARY This paper outlines Che metering, control and alarm indication of power rectifiers at the Electro- lytic Zinc Company of Australasia Ltd., Risdon Works. Although the individual components discussed in this paper are not a recent breakthrough in instrument technology, the paper covers -he application of instrumentation to a large industrial complex. Particular mention is made of the components of two recently installed rectifiers purchased from overseas and gives an insight to the type of instrumentation and control which is being offered by overseas manufacturers.

I INTRODUCTION II METERING

Power rectifiers with ratings totalling more Metering has become an important factor in the than 110,000 kilowatts have been installed at the E.Z. Company's Risdon Works operation and accounting. Electrolytic Zinc Company of Australasia Ltd., Risdon As new plants are commissioned considerable interest Works in Hobart, Tasmania, and normally operate con- is placed on the efficiency and general operating be- tinuously to supply d.c. power to six cell units for haviour of new sophisticated equipment. The expanding the production of zinc and two hydrogen batteries industry at the Risdon Works requires more than for the production of ammonia. For the purpose of 100,000 kW of power at 11,000 volts purchased as bulk accurate production control, cost analysis and ful- supply from the Hydro-Electric Commission which meters filling power consumption agreements, it is the total supply to the Risdon Works. For the essential that relevant metering and control of the purpose of cost analysis, each section of the operat- rectifiers be of a high standard in accuracy and ing plant is accurately metered on the high voltage reliability. It is also essential that the down input with conventional induction type kilowatt-hour time of a rectifier due to a fault be reduced to meters. Overall load summation of 13 incoming feed- a minimum by installing elaborate fault finding ers to the Risdon Works is obtained by the electron- alarm systems. ic summation of the energy consumption of groups of feeders as metered by induction type kilowatt-hour Energy consumption on the high voltage a.c. in- meters fitted with electronic transmitter units. It put to each rectifier is mettred by means of induc- is essential that the maximum demand of power to tion type kilowatt-hour meters. The summation of Risdon Works does not exceed that which if contracted several of the a.c. energy consumptions as metered for in any metering period. Metering of high voltage by each kilowatt-hour meter is achieved with a high energy consumption of individual loads and Riedon degree of accuracy by means of an electronic summa- Works total load must therefore have a high degree of tor. D.C. currents of the order of 30,000 amps are accuracy and reliability. For close production con- measured with high accuracy clamp-on d.c. current trol in the electrolytic process, where zinc is transformers. extracted from a zinc sulphate solution, accurate measurement of the large d.c. currents supplied by Under normal operations, the rectifiers are the rectifiers to the various cell units is required. automatically controlled by means of constant current High accuracy clamp-on d.c. current transformers are controllers. Coarse control is obtained by conven- used for this purpose. tional on-load tap changers. Fine control on recently installed rectifier equipment is obtained (a) A.C. Metering . by saturable reactor control, which, depending on the degree of saturation, raises or lowers the mean Each rectifier equipment is supplied by 11,000 d.c. voltage and consequently the mean d.c. current volt power cables terminated in high voltage «witch- supplied by the rectifiers. ing stations located within the Risdon Work* boundaries. Current transformers with ratios of Many aspects of each rectifier equipment such the order of 800/1 A and voltage transformers with as a.c. and d.c. currents, temperatures, cooling ratios of 11,000/110 V, transform current and volt- media flow, diode fuses, surge protection circuits ages to signals which can be safely handled by con- and so on, are monitored continuously by a solid ventional induction type kilowatt-hour meters. The at.-itf alarm system. Serious faults initiate an three phase kilowatt-hour meters are connected in a .•il.irm signal and simultaneously initiate a trip similar manner to the two wattmeter method for signal tc Isolate the faulty rectifier equipment from measuring three phase power and have only two current tin- !iiuli voltage supply. Slight faults initiate an elements and two voltage elements. From time to time •ij.irm .siKn.il only. 104 the kilowatt-hour meters are checked against a. : The waveform diagram of Fig.2 is a graphical rotating standard and kept to within 0.2 per cent of description of a two-channel summator receiving two that standard. pulses simultaneously. At time tO we can consider that channel stores SI and S2 have received no pulses Incoming feeders from the Hydro-Electric Conn- from the transmitters Tl and T2. At time tl, the ission substation to the various high voltage switch- channel stores receive impulses simultaneously and ing stations are oetered in.groups by using simulation, . these 'are. immediately accepted by the channel stores current transformers, the secondary currents- of which changing etace. At time t3, a scanning pulse to are fed into similar kilowatt-hour meters mentioned - channel-1 AND gate transfers the contents of channel above. These kilowatt-hour meters are fitted with store'SI to channel register Rl. Similarly, at time electronic transmitter units-which send pulse signals t2 the contents of S2 are transferred to R2. into a remote electronic•.•sunmator.J. '; . ; ••:•.. • •• Immediately the channel registers change state, that ' is at times. c2, t3, t4, t5 and so on, an impulse is The following paragraphs' describe the basic • transmitted to the total register which operates in principle of operation of the electronic •uonatpr a similar manner. system. For simplicity of argument only' a two - channel system is dealt with, whereas the actval system at Risdon Works has four channels operating in a system capable of handling eight channels: The block diagram of Fig.l shows the connection of CHANNEL I SCANNER a two-channel system. • • : n n n n ft CHANNEL 2 SCANNER LJL4JjL_JlijL CHANNEL I INPUT n! ! h! 11 CHANNEL 2 INPUT Jt_L L I I CHANNEL STORE S,

i i

CHANNEL STORE S2 JT L

CHANNEL REGISTER Rl JÜLJ

CHANNEL REGISTER R2 L) I 11 i i i TOTAL REGISTER TR JLJ 1 '

Figure 2.. A Waveform Diagram of a Two-channel System Receiving Two Impulses Figure 1. A Block Diagram of a Two-channel Simultaneously Electronic Summator Metering Equipment (1) Electronic Transmitter Unit Conventional kilowatt-hour meters Ml and M2 are fitted with electronic transmitter units Tl and T2 The electronic transmitter unit has been which transmit electric pulses to the remote designed to give a series of output pulses corres- «umeator. The movement* of the' two meters are quite ponding to the kilowatt-hours metared by the conven- independent so that It la possible for Tl and T2 to tional kilowatt-hour meter. This unit ties certain transmit pulses simultaneously. To ensure that both advantages over mechanical pulse transmitters in pulses «re sumaated correctly, the input pulses to that it eliminates contact bounce and reduces the each channel are stored on transistorised binary mechanical load on the meter. The transmitter unit circuit stores Si and S2; respectively. T-s outputs consists of a printed circuit board upon which la of the stores SI and S2 set AND gates 61 and G2, mounted a framework supporting two sets of feedback depending on the state of the »tores, to permit coil* and a shaft carrying two slotted aluminium •canning pulses PI add P2 to pass sequentially to the discs. Two oscillator circuits are included in the channel registers Rl. and R2 and art the same time to printed circuit. The rotor shaft of the meter the total register Tit;.: Each register circuit drives drives the slotted aluminium discs and gives a a stepping motor which advances step by step and variable electromagnetic coupling between the feed- registers the pulse on acyclometer dial. back coils of the oscillator circuit*. Each time a 10.5 slot In the disc passes between the two coils, the These pulses are amplified and fed to the coils of oscillator is driven into a high frequency oscilla- the totalising register stepping motor. The waveform tion, which is formed into an electrical impulse by diagram oi: Fig.3 displays graphically the operation the selected values of the circuit components and of the divide-by-four unit. transmitted to the electronic summa tor. When a solid part of the aluminium disc is between the coils the eddy current loss in the disc is sufficient to prevent feedback and therefore prevents oscillation. The two oscillators function in turn as the discs are driven by the meter and the unit transmits pulses at DIVIDER INPUT JULJULJUUULJL the rate of one pulse per 12.5 kilowatt-hours of metered energy. FIRST -J-2 BINARY JULTLTL-Γ (11) Channel Units SECOND -!> 2 BINARY A channel unit is contained on a printed circuit board, one board comprising two complete MONOSTABLE OUTPUT n n channel units. Each channel unit consists of the input channel store, the AND gates and the register drive circuits. Figure 3. A Waveform Diagram of a Divide-by-four Input pulses from a transmitter unit are fed Unit onto load resistors and pass through filter circuits to trigger alternatively the transistor bases of the channel input store binary circuit which is (iv) Scanner Unit essentially a bistable multivibrator designed to respond to only the positive going edge of the A scanner arrangement for a four-channel simula- pulses. tor is illustrated in Fig.4 and consists of a clock multivibrator, a clock monostable, two binary The output of the channel store is connected counters and four AND gates. through emmitter follower stages to the AND gates where scanning pulses from the scanner unit control the action of the gates. BINARY COUNTERS The register drive circuit is also a bistable multivibrator which responds to only positive going pulses. The register drive is triggered by the output of the AND gates and the collector loads of the multivibrator transistors are the two coils of the stepping motor. A change in state of the binary circuit of the register corresponds to a change in polarity of the poles on the stepping motor stator. The number of poles on the rotor and stator are the seme, so that a change of state in the binary circuit causes the rotor to move one pole pitch. Uni- directional motion is assured by means of a magnetic bias in the motor. Each channel register circuit also has a differentiated output to drive a divider unit. At the Risdon Works a divide-by-four unit is employed to match the impulse rate of the sum of four Risdon Works meters, impulsing at 12.5 kWh per pulse, with the Hydro-Electric Commission meters which impulse at SO kWh per pulse. AND (lli)Divider Unit GATES As each of the four channel registers change P4 state the differential circuits generate pulses which (M+A+B) CMtÄ+B) CM+A+B) CM4Ä+B) are added and fed into the divide-by-four unit. Division by four is obtained by connecting two bistable multivibrators in cascade. The output Figure 4. A Block Diagram of a Four-channel pulses of the four channel register drives trigger Scanner Circuit the first multivibrator and the output of tnis stage triggers the second multivibrator. The clock multivibrator is an astable circuit The output from the divide-by-four unit is con- producing a square wave at each.collector output. verted Into pulses by means of a monostable circuit Components are selected so that the frequency of the which gives a pulse output for every positive going clock multivibrator output is 30 hertz. One output edge of the output waveform from the divider unit. c of the clock multivibrator is coupled to the clock 106 nonostable to trigger th pulses at 30 hertz. The co prevent simultaneous adjacent scanner outputs Ing time of the binaries fed to all the AND gates output can only occur in stable pulse output. Th multivibrator is coupled which only responds to j The outputs of the first square waves with half t multivibrator and are cc ers to the second binarj stage only s. „spends to f and generates square wa\ frequency one quarter tt The outputs of M, A, four diode AMD gates to \ form PI, P2, P3 and P4 s

CLOCK MULTIVIBRATOR

MONOSTABLE ji

FIRST BINARY

SECOND BINARY U

SCANNERS

Figure 5. A Waveform DJ Scanner Cir«

(v) Voltage Supplies The voltage supplie« equipment are obtained i A 50 volt d.c. input to to a zener diode chain v voltage supplies to the (b) D.C. Metering Several rectifier* c Work* have a d.c. output and 11,000 amp«. monostable to trigger this stage with positive going Two recently installed rectifier equipments, pulses at 30 hertz. The purpose of the monostable is referred to as R261 and RZ62, each have a d.c. output to prevent simultaneous impulses appearing on of approximately 700 volts and 16,500 amps. These adjacent scanner outputs due to the inherent switch- two equipments have their output paralleled so that ing time of the binaries. The monostable output M-ds they can supply a total of 33,000 amps d.c. to one fed to all the AND gates so that a positive going zinc cell unit. RZ61 and RZ62 are each 12 pulse output can only occur in the duration of the mono- rectifier equipments and are separated in phase by stable pulse output. The other output of the clock 15 degrees to give an overall 24 pulse system for the multivibrator is coupled to the first binary counter two equipments. The rectifier transformers have two which only responds to positive going input pulses. secondary windings, one connected in star and the The outputs of the first binary counter A and A are other in delta. Each secondary winding supplies a square waves with half the frequency of the clock three phase bridge circuit. To ensure that a d.c. multivibrator and are coupled through emitter follow- fault from positive to negative cannot occur within ers to the second binary counter. Again, this latter the rectifier equipment, each rectifier is divided ao stage only z spoads to positive going Input pulses that the positive rectifier cubicle is mounted on one and generates square wave outputs 6 and t with a side of the 13,750 kVA rectifier transformer and the frequency one quarter that of the clock multivibrator. negative cubicle on the other side.

The outputs of M, A, A, B and B are combined in A d.c. current transformer with ratio 10,000 A four diode AND gates to produce the scanning wave- d.c/5A a.c. is mounted on the negative d.c. bar of form Pi, P2, P3 and P4 as shown in Fig.5. the delta bridge. Likewise, an identical d.c. current transformer is mounted on the positive d.c. bar of the star bridge. The d.c. current transformers have a common 200 volt, 50 herts supply and generate a square wave output with amplitude dependent on the magnitude of d.c. current passing through CLOCK the d.c. bars. The outputs from the two d.c. current MULTIVIBRATOR transformers are summed by means of a summation XLTLTLTUTJ" c current transformer with ratio (5A + 5A)/5A and the output current of this latter unit is fed to a trans- MONOSTABLE JLJLJLJLJLJLM mitter unit. The transmitter unit transforms the current to a F!RST A small milliamp signal. This current is rectified, BINARY filtered and transmitted to remote indicating meters Ä through screened cable for a distance of more than 1,000 feet. The current to Che indicating meter is B such that 2 milliamps d.c. at the meter is equivalent SECOND to 20,000 amps d.c. output from the power rectifier BINARY equipment, a metering ratio of 10,000,000 to 1. B Indication of output current of each rectifier is of interest to the operators of Risdon Works Power Control Centre in that they can control each rectifier output from zero to full load current.

For the production of zinc, greater interest is placed on the total d.c. current to any zinc cell SCANNERS unit, rather than on the d.c. output of each rectifier.

The total d.c. current to each of four cell units, including the cell unit supplied by RZ61 and RZ62 rectifiers, is measured by high accuracy clamp- on d.c. current transformers. Each d.c. current transformer is rated at 35,000 amps d.c. and has an Figure 5. A Waveform Diagram of a Four-channel error guaranteed to be less in magnitude than 0.2 per Scanner Circuit cent when the load current is between 50 and 110 per cent of rated current. These-d.c. current transformers are supplied with 415 volt 50 herts supply (v) Voltage Supplies and have & high quality square wave output which is transformed by an auxiliary current transformer, The voltage supplies for the electronic summator rectified and transmitted to remote metering equip- equipment are obtained from a DC/DC converter unit. ment which may have a d.c. burden of up to 30 watts. A 50 volt d.c. input to the unit Is filtered and fed The current to the metering equipment it such that 1 to a zener diode chain which provides several d.c. amp d.c. at the metering equipment is equivalent to voltage supplies to the summator equipment. 35,000 amps d.c. input to a zinc cell unit.

(b) D.C. Metering The d.c. teetering supplied by the high accuracy clamp-on d.c. current transformer consists of three Several rectifiers on the Zinc Plant at Risdon units. An indicating ammeter, supplied by this unit, Works have a d.c. output of approximately 600 volts is mounted beside the indicating ammeters of each and 11,000 aaps. rectifier supplying that cell unit. Thus the Power 107 Control Centre operators can observe the d.c. current equipment. supplied by each rectifier and also the total d.c. current supplied to each cell unit. The second unit supplied by the d.c. current transformer is a high accuracy commutator type ampere hour meter fitted A.C. INPUT with a cyclometer type dial. This meter has an adjustable shunt to tune the meter to a high degree of accuracy. The current flows to the armature through two brushes of gold wire and through a three sector commutator made out of noble metal. To reduce friction and to centralise the armature, the lower bearing is magnetically discharged. The power Intake to this meter is only 0.7 watts. The Electro- lytic Section of the Zinc Department require the ampere-hour readings to each cell unit to a high degree of accuracy. The final unit supplied by the d.c. current transformer is an accurately calibrated shunt. Leads from the shunt are fed to a Cambridge slide wire potentiometer and as a matter of routine testing, the ampere-hour meter is kept to within the required accuracy.

The d.c. voltage of each cell unit is also closely observed by the Power Control Centre operators. Voltmeters, directly connected to the d.c. busbars, are located in the Power Control Centre adjacent to the ammeter indicating the total current to that cell unit. With the rectifiers controlled by constant current controllers, any change in cell unit solution resistance causes corresponding changes in the d.c. voltage and therefore the d.c. power of that cell unit. Power Control Centre operators ensure that the voltage does not rise to levels which might damage the rectifier equipment and which might cause the cell unit to draw excessive power upsetting the overall load of Risdon Works. The d.c. voltmeters are also regularly checked for accuracy. AMPLIFIER

Ill CONTROL

The control of the two recently installed rectifier equipments, RZ61 and RZ62, will be described in this section. Current control of each rectifier is an operational requirement so that any current from zero amps up to approximately 33,000 amps d.c. can be supplied to the zinc cell unit when the two recti- ac. OUTPUT fiers are operating in parallel.

Each rectifier is supplied through a 3-phase 11,000 volt, 500 MVA oil circuit breaker which is Figure 6. A Block Diagram of the Overall Control capable of interrupting and isolating the high voltage Circuit of a Rectifier Equipment supply from the rectifier equipment.Each rectifier is also fitted with off-load d.c. isolators in the negative and the positive d.c. bars. These isolators (a) On-Load Tip Changer are capable of carrying almost 20,000 amps d.c. and serve to isolate each rectifier from the cell unit The characteristic voltage/current curve of a

bus. bars. -. — „:.. •• zinc cell unit is relatively flat due to the back e.m.f. of the cells. For this reason, a small change Coarse control of each rectifier is obtained by In d.c. volts gives a large change in d.c. current. a conventional on-load tap changer. Fine control is For reasonable current control using the on-load tap obtained by saturable reactors and can extend for changer it is necessary for each tap step to cause a about 3 tap steps to give, in conjunction with the small change in d.c. voltage of the order of 10 volts. tap changer, a smooth stepless control from zero amps The tap changer is part of the regulating transformer. to full load current of the rectifier equipment. The The tapping sequence includes a reversing switch so control circuit has facilities for operating the that a series transformer supplied by the tap changer coarse and fine controls automatically or manually. can buck or boost the voltage supply to the rectifier A seldom used feature is the ability to drop the d.c. transformer primary winding in 33 tap steps. output voltage range to a very low value by changing the rectifier transformer primary winding from a Initiation of a tap change is made manually by a delta connection to a star connection by means of an push button or automatically by the operation of a off-load switch. The -$iock diagram of Fig.6 shows relay. The signal to change a tap is fed to a tap the overall control circuit for each rectifier change initiation circuit where it energises a tap 108 raise or a tap lower contactor. The contactor closes to supply the tap changer motor to drive the mechanism in one direction or the other corresponding to the input signal. Cam operated contacts energise various relays to give indication that the tap cha«g- er is in progress, the tap changer has not completed a cycle, a high or low limit of the tap change travel has been reached and so on. Included in the tap change control circuit is a step-by-step circuit to ensure that in the manual operation, only one tap can be initiated in one operation of the push button. The automatic operation of the tap changer has time delayed relay contacts to allow the saturable reactor control circuit to resat before the next cap change is initiated.

(b) Saturable Reactors Fine control of the rectifier equipment is obtained by means of saturable reactors. Each Figure 7. The Characteristic B-H Curve of a rectifier has a total of twelve saturable reactors, Saturable Reactor Showing Relevant one in each' arm of the two sets of 3 phase bridge Operating Points rectifiers. Six reactors are mounted in the rectifier negative cubicle and six in the positive cubicle, each group of six reactors being separately considered to be a time delayed switch in which the controlled. impedance of the reactor is infinite during the time delay, but zero after the time delay. The effect of The core of She reactors is made up of several the saturable reactor is to delay the firing of the annul! of magnetic steel cores mounted one above the incoming rectifier arm and thus subtract a notch from other and mechanically held together. The primary the rectifier output voltage. The mean d.c. voltage winding is the a.c. bar input to one arm of the rect- decreases due to this notching effect and consequent- ifier, similar to the bar primary of a current trans- ly the mean d.c. current is reduced. Fig.8 shows the former. The control winding consists of several effect of the saturable reactors on the d.c. output turns of a conductor suitable for approximately 10 voltage when no control current is applied and also amp current, which is wound on the magnetic core when some control current is applied. Notching the similar to the secondary of a current transformer. d.c. voltage in the manner described can generate The control winding is terminated on a readily severe harmonics. It is for this reason that such accessible terminal board. Such saturable reactors fine controllers only cover a small range of the are simple robust and effective components for the order of about 2 per cent of the total output volt- control of large d.c. currents. age. The greater part of the range is covered by the tap changer with the fine control operating between The operation of a saturable reactor is govern- taps. ed by the B-H characteristics of the iron core of the reactor. Ideally the B-H curves are almost vertical when the iron is unsaturated and almost horizontal when the iron is saturated with a sharp knee at the point of saturation. D.C VOLTAGE When the rectifier arm is conducting the reactor is driven hard into saturation to the point K of the B-H curve of the reactor shown in Fig.7. This is due to the very high ampere turn, Id, of the primary winding of the reactor. When the rectifier arm ceases to conduct, the operating point of the reactor moves back to point L where the reactor can CO 120 IK 24t be considered to be fully saturated. Now if a small to 120 1» 340 number of control ampere turns, Icl, are fed through DEGREES 0E6REES the reactor in the opposite direction to the recti- Cb) fier arm current, then the reactor will unsaturate and the operating point will move to point M. An increase in the control ampere turns to Ic2 would Figure 8. D.C. Output Voltage Wave nove the operating point to point N. A further in- (a) With No Saturable Reactor Control and crease in control ampere turns to Ic3 Is not justi- (b) With Saturable Reactor Control fied since the operating point then moves to point P which saturates the reactor in the opposite direction. Thus with s few ampere turns of the control (i) Manual Control wlnalng the reactor can be brought from the fully saturated to the fully unaaturated state. The amount Under emergency conditions, when the automatic by which the flux density B, drops from the fully control circuit fails, the manual control circuit for saturated state, point L>corresponds to the voltage the tap changer and the saturable reactors would be which is absorbed by the reactor. The reactor can be used. A 200 volt supply originating from the recti- 109 fier equipment auxiliary transformer Iβ fed to a pulses to each pair of conducting thyrlstors are syn- mall hand operated auto transformer, the output of chronised with the voltage supply to those thyrlstors which is stepped down by a 500VA transformer. The but may be shifted in phase depending on the control output of the latter transformer is split to give signal. Thus the thyrlstors can be delayed in firing two identical supplies. Each supply is rectified so that the mean d.c. current output from the thyris- with a single phase bridge rectifier and passed tor stack can be varied. This current is fed to one through a heavy choke and a low impedance variable group of saturable reactors through the same choke resistance to the saturable reactors. One supply is and variable resistor as mentioned above for the connected to the reactors in the rectifier negative manual control circuit. cubicle, the other supply Is connected to reactors in the rectifier positive cubicle. The supplies are The operation of the automatic circuit Iβ brief- balanced. Variation of the control on the hand ly summarised in the following paragraphs. operated auto transformer increases or decreases the d.c current through the reactor control windings If the actual current as detected by the d.c. and thus controls the rectifier output. current transformer rises to a higher level than the set current then the difference will be generated by The purpose of the heavy choke and the variable the mixer circuit. The difference will be amplified resistance in the reactor control circuit is to and fed to the pulse phase shifter which will advance smooth the control current, to provide voltage the firing pulses of the thyristors. The thyristor« signals for tap change initiation relays and to will thus be switched on for a greater portion of the dampen out the reflected spiky voltages transformed cycle to feed a greater current to the control wind- through the reactors from the rectifier conduction. ing of the saturable reactors. The reactors will absorb more rectifier output voltage which will de- (ii) Automatic Control crease the mean d.c. current to the set value. The entire operation has a relatively high speed of Noroally the rectifiers are operated auto- response. matically. A 200 volt supply from the rectifier equipment auxiliary transformer is fed to a stabil- If the difference between the actual current and ised power supply. One output from the power supply the set current had been greater, then a larger provides stabilised voltage rails for the autonatic control current would have been generaced by the control circuit amplifier. A second output Iβ fed to thyristor stack. At a preset value of control a high quality rheostat. This rheostat is mounted on current the tap change lower initiation relay opera- the control desk of the Power Control Centre and is tes to reduce the rectifier output current by means graduated for use as the d.c. current setting con- of the tap changer in conjunction with the saturable troller. The voltage output signal from Che rheontat reactor control. Aβ the actual rectifier output is split into two supplies, one for each group of current approaches the set current, the difference is saturable reactors, and each «upply is fed to a .six- reduced, the saturable reactor control current, ing circuit. reduces to de-energise the tap change lower relay so that the final adjustment is made by Che eaturable The d.c. current transformer for the positive reactor controller. output of the delta bridge has an auxiliary current transformer of ratio 5A/0.1A in its secondary A similar procedure applies to the condition circuit. The output of this small current transform- where the actual rectifier output current is lower er is a square wave which is rectified and fed to a than the set current. loading resistance. The voltage signal output from this resistance represents the d.c. current output IV ALARM INDICATION from the rectifier equipment and is fed to the ease mixing circuit as the current control signal. The central element of semiconductor rectifier equipment is the Individual diode. The silicon diode An identical circuit arrangement is made for used in modern power rectifiers is a robust unit cap- the current signal from the negative output of the able of carrying very high currents to ths order of star bridge thus making two separate control circuits several thousand amps for a few milliseconds and for each group of saturable reactors. blocking peak transient reverse voltages of the order of 3,500 volts. Diodes are particularly sensitive Any difference between the current setting to heat and to very high voltages and every effort if signal and Che actual current as detected by Che d.c. made to ensure that the diodes are not subject to current transformer generates a email difference out- extremes of heat and voltage. Excessive heat can be put froa the mixing circuit which is amplified and generated by excessive currents through the diode« fed into a.pulse phase shifting circuit. The varia- for relatively long periods of time or by failure of tion of the phase angle of the pulse is determined by the designed coolin3 equipment for the diodes. Power Che magnitude of the control signal input to the rectifier equipment is built up of a large number of pulse phase shifting circuit. The output pulse has diodes in parallel In each arm of the rectifiers. an amplitude of approximately 10 volts and a pulse For some applications rectifiers may have diodes also width of 30 microseconds and is fed between the connected in series to give a high voltage output. cathode and gate of a thyristor. If a serious fault occurs within the rectifier The thyristor stack Is a single phase full wave equipment it is essential that the equipment be bridge supplied from the rectifier equipment auxil- rapidly isolated from the high voltage supply before iary transformer. The voltage is stepped down by a the fault develops into a major crisis. On the other 500 VA transformer before being fed to the thyris- hand, if a fault is not so serious, isolating the tors. The thyrlstors are unable to conduct until equipment from its supply cannot be justified and the they receive a pulse between cathode and gate. The equipment is allowed to continue operating. 110 Surrounding the rectifier is a considerable When an alarm condition occurs, the sequence amount of sophisticated protection equipment, many card energises the visual output circuit, together aspects of which are continuously monitored by a with the flasher bus to produce a flashing visual solid state alarm system. indication of the alarm. At the same time the audible output bus is energised to sound a continuous The cooling media for the diodes, each dissipat- alarm. This condition will persist until the ing some 400 watts, is usually vast and expensive. "accept" pushbutton is operated when the visual out- Several rectifiers installed at the Risdon Works are put will change to a steady bright light and the outdoor oil cooled equipments. The oil is circula- audlb-le output will be silenced. When the alarm ted through the diode and fuse bars and pumped condition returns to normal the visual indicator My through pipes to large banks of radiators cooled by be restored to its normal dim condition by operating several electric air fans mounted on the radiator the "reset" pushbutton. Operation of the "reset" framework. The radiator fan motors, the oil pump pushbutton before the alarm condition return« to motors, the oil pressure and temperature, are all normal,returns the logic to the original alarm monitored by the solid state alarm system. condition where the flashing visual indication and the continuous audible output Is repeated. If a diode fails, experience has shown that it usually fails short circuited. If the faulty diode Operation of the "test" pushbutton activates is not isolated, it acts as a short circuit on the the input logic of all the sequence cards in the transformer secondary and could cause extensive system. All visual indicator« and the audible output damage to the transformer. High speed fuses are will respond as if the alarm conditions had appeared connected 'in series with each diode and cut off the on all inputs. The "test" pushbutton, therefore, current before it reaches its prospective maximum. fully tests the entire logic circuitry of the alarm When the diode fuse operates it causes a special system. auxiliary fuse fitted with a spring loaded plunger to operate. The plunger ejects so that it can be The recently installed rectifier equipments at visually observed and at the same time it closes a the Risdon Works each have 37 components monitored contact to initiate a signal to the solid state by the solid state alarm system. Several of the alarm system. With one diode isolated from an arm alarms are paralleled reducing the total number of carrying several diodes in parallel, the other diodes alarm points at the solid state alarm system to 28. are stressed having to carry extra current. The This elaborate system enables Power Control Centre circuit arrangement is such that a diode fuse opera- operators to diagnose and locate the fault so that tion trips the high voltage circuit breaker to they can take appropriate action to ensure that isolate the equipment from the supply until the the rectifier equipment is operated safely and to faulty diode is replaced. ensure that the down time of the equipment is kept to a minimum. Many other aspects of the equipment, such as air temperature of the rectifier cubicles, a.c. in- V CONCLUSION put current, d.c. output current, surge circuits and the conventional protection devices of the recti- Modern rectifier equipment Iβ generally robust, fier transformer are continuously monitored by the reliable and efficient equipment with a relatively solid state alarm system. When the protective long life in excess of 10 years. The technology of device operates following a serious or a slight rectifier equipment advances through the years, but fault it initiates a signal, which may pass through not nearly as fast as that of electronic instrumenta- an interposing relay to a set of alarm relays. tion. Industrial engineers are continually faced Serious fault signals pass through the alarm relays with the problem of purchasing new equipment to re- to the high voltage circuit breaker tripping coll to place inefficient, obsolete equipment or as a new open the breaker, A signal to an a'arm relay ener- installation in an expending industrial complex. gises that relay to close a contact in the input These engineers, working under economy minded mana- circuit of the solid state alarm system. The con- gers, seek out good quality equipment at reasonable tact closes instantaneously but is time delayed on costs. opening to ensure that a positive alarm signal Is indicated on the solid state alarm system. The metering, control and alarm indication for the large power rectifiers at the Risdon Works have The alarm system is solid state, all the logic proved to be reliable and satisfactory at this functions being carried out in integrated circuit stage. With moves to high accuracy and greater pro- blocks. These integrated circuits are soldered to duction output»engineers work to improve the gold-plated printed circuit cards which plug in to existing equipment which often entails the purchase compact multi-socket blocks mounted in the compon- of new and more sophisticated components. ents housing. Each sequence card contains logic circuitry for two alarm points and the alarm relay VI ACKNOWLEDGMENTS normally open contacts are wired directly to these cards. The author wishes to express his appreciation to the management of the Electrolytic Zinc Company of The normal logic level is between +3 and +6 Australasia Ltd. for granting permission for this volts, the alarm logic level is at 0 volts. Each paper to be printed based on material contained in sequence card is connected to a number of common the Company's files, and to the Company staff for supplies which provide the controls for the alarm their assistance in compiling this paper. accept function and for the built-in system test function. The common supplies also provide power for the audible output circuit and lamp flasher circuit. in I A DIRECT READING DIGITAL INSTRUMENT FOR THE MEASUREMENT OF SPEED OF ROAD VEHICLES

G. Banky, B.E., Grad.I.E.Aust. D.C.A., formerly Research Student, University of Melbourne A.E. Ferguson, M.E.E., M.I.E.Aüst., F.I.R.E.E.Aust., Reader, University of Melbourne

SUMMARY. The paper discusses the design of a new instrument* which measures the 3peed of cars passing two fixed detectors on the roadway. The speed is indicated directly in miles per hour on a digital display.

Digital measuring techniques are used throughout and timing is controlled by a crystal oscillator, TTL circuits are used almost exclusively.

This application requires an instrument which will measure the reciprocal of time to give a direct reading of speed. The principle adopted is to divide the clock frequency so that the resultant generated signal has a i frequency which is inversely proportional to the time interval. This frequency is then proportional to the car's speed and is measured by counting the number of cycles in a preset interval after the car has passed. The number of cycles counted can be made proportional to the speed, hence making the instrument direct reading.

* Provisional patent PA5171 dated 10th June, 1971.

I. INTRODUCTION The meter is scaled to read directly in m.p.h. from 90 m.p.h. to 30 m.p.h. (a) The Amphometer Amphometers have been used widely over the last In 196S the Victorian Police introduced the Amph- six years, but in spite of their apparent simplicity ometer to measure the speed of motor vehicles at they suffer from some problems associated with the selected points in the State's road system. moving coil meter and the environment in which they are used. In service, the operators must carefully Regulations under the Motor Car Act in 1966 note that the instrument's zero error is negligible legalized this instrument for measuring road traffic and that the initial capacitor voltage is correctly speeds and made the Electrical Engineering Department set. Component stability and temperature coeffic- in the University of Melbourne the Statutory Author- ients are short-comings of the method which limit the ity for certifying each instrument for a period of long term accuracy. Before re-certification about not more than six months. In the certification pro- 30% of the instruments need some form of maintenance. cess a lead seal is placed on each instrument and a written certificate issued, stating that the speed (b) Other Instruments For Measuring Vehicle Speed indicated is accurate to within one part in thirty. Other vehicle speed measuring instruments have The instrument requires two pneumatic detecting been described in literature over the years. Many tubes to be temporarily nailed to the road surface a such analogue types (Ref. 1,2,3), using either the distance of 88 feet apart. As a vehicle crosses a charge or discharge of a capacitor, seem generally to detector the pressure wave generated by the tyre suffer from faults similar to those found In the rolling across the rubber tube closes a diaphragm amphometer. Indeed, the amphometer is perhaps the switch. The start and stop signals so generated are most satisfactory. used to tine the vehicle's passage over the pre- scribed distance. Digital timing circuits (Ref. 4,5) counting the number o£ cycles of a stable oscillator, offer a much The timing instrument is a capacitance discharge more accurate method of measuring the timed interval circuit, the capacitance voltage being monitored by a and the digital display will be more reliably booked moving coil meter. The initial capacitance voltage by an operator. However, time is inversely propor- is selected by adjusting the full scale deflection of tional to speed and hence a conversion table oust be the meter in the ready position. used. reg« A transistor controlled relay starts the dis- Speed measuring instruments using the Doppler at charge upon the actuation of the first pneumatic shift to obtain velocity (Ref. 6), often called the switch. The second pneumatic switch operates another Radar type, have been used. Except under relatively relay which mechanically clamps the movement of the clear highway conditions with light traffic, some moving coil meter using a mechanism like a small disc ambiguity may exist about which vehicle is resolved brake. (Ref. 7). This type of instrument has been chall-

112 eiiged many times in the courts, In Australia they which has been subject to dramatic change« tn rectnl have not been widely adopted. years (Ref. 10).

Laser beam instruments (Ref. 8) and efforts to, III. SYSTEM DESCRIPTION reduce the Radar ambiguity are currently under development, but either will be expensive. (a) The Reciprocal Time Problem

(c) Direct Reading Digital Instrument When speed is measured by observing the transit time of a vehicle over a fixed base, then the speed Recent developments in digital microelectronics will be proportional to the reciprocal of the transit have made possible the realisation of compact, In- time. expensive and highly reliable counting circuits for time measurement. In addition, similar digital The measurement of time by counting the number of circuits can be used to display reciprocal time, cycles of an oscillator Is simple to instrument, but which, with appropriate data processing can make the in this application it can only be used provided the Instrument direct reading in miles per hour or any frequency is inversely proportional to the tine other unit of speed, interval measured. The above principle has been adopted after considering several alternatives, II. DESIGN CONSIDERATIONS including the usual computer method using an accumulator to store the dividend and multiple subtraction (a) Design Objectives of the divisor until the sign changes, and the use of "Read Only" memories. From the experience gained In the operation and calibration of the Amphometer, it was decided to Since the range of time intervals to be dealt design an Instrument which met the following objec- with in an Instrument designed for speeds from 90 tives: m.p.h. to 25 m.p.h. is less than 1/4, the variation in frequency required is not large. (i) A total error less than 1 in 30 in the range 25 miles per hour to 90 miles per hour; It is unnecessary to continuously control the frequency to be measured since sufficiently small (ii) The Instrument should directly display in errors can be obtained by changing a clock frequency digital form vehicular speed to eliminate any in discrete steps over the timed Interval. errors which may result from incorrect decoding by the operator using tables.

For use by the Mobile Traffic Police, the instrument: must also be: (iii) Portable, light enough to carr> and require no OUT more power than can be supplied from a small rechargeable battery with 8 amp-hour capacity;

(iv) Compatible with the presently used analogue device in that it should be triggered by two pneumatic tubes placed eighty-eight feet apart across the roadway; IN I TTT (v) Economically viable and able to compete with Data Inputs the analogue instrument;

(vi) Reliable under all predictable environmental Fig.1 - Variable Frequency Generator Circuit. ccndltlons, Including rough handling and trans- portation in the boot of cars, and should not Figure 1 shows the schematic circuit of the require calibration or zero setting in normal variable frequency generator (Ref. 11). Two four bit use; counters are operated in cascade with the all "1" condition detected in a multiple input MAND gate. (vli) Maintenance free for periods of at least six The output of this gate Iβ used to generate the out- months (except for battery recharging). put signal from the variable frequency generator and to reset the two counters to the data Input count After a careful survey of the logic circuit pattern. Ihe division ratio is therefore controlled families available in 1970, it was decided that by the binary code pattern applied to the data Input Transistor Transistor Logic (TTL) would be used terminals of the counters. The binary code pattern (Ref. 9). A wide range of digital sub-eyeteae were is obtained from a "divisor code" generator consisting of a set of "down counters" which have an input available though such a design decision cannot be timing pulse period which determines the rate of regarded as other than one of several possibilities change of the frequency division. at the time.

The TTL family contained a range of special pur- (b) System Block Diagram pose sub-system«, all designed for operation from 5V with non-critical Interconnection, and their design The essential features of the system block dia- appeared to have some degree of stability in an area gram are shown Iα Figure 2. timing pulses control

113 the divisor code generator. This generates a code nT -(5) which controls the division ratio of the variable frequency generator so that the input frequency is dx nT -(6) divided in discrete steps determined by the number of dv V timing pulses which have entered the divisor code generator. This will be a minimum at the maximum speed to be measured and is -7.4 msec./m.p.h. at 90 m.p.h. INPUT f Hence nT 667 at 90 m.p.h. VARIABLE "tr COUNTER AND FREQUENCY DISPLAY The choice of T (or n) determines the maximum GENERATOR error Introduced by controlling the variable frequ- • »A * * * ency generator in discrete "lepe. The larger the divisor n the greater will be the number of DIVISOR CODE divider circuits to be used. GENERATOR In this case T has bees chosen at 10 msec, and A n at 67 resulting in a maximum error of the order TIMING PULSE GENERATOR of \ In 67 at 90 m.p.h. CONTROLLING D.C.G. At the minimum speed of 25 m.p.h. the required value of n is 240 which is within the capacity of a four stage binary counter.

Fig.2 - Variable Frequency Divider. The clock frequency input to the divider can now be found once the sampling interval t of the dis- When the timing pulse generator output ceases, at play counter has been determined. This Introduces a the end of the interval to be measured, the variable delay in displaying the reading. It has been chosen frequency generator output is counted for a preset at 0.25 sec. time and displayed in digital form. = ^Γ-vn - 24.0 kHz. To be direct reading the output of the variable t frequency generator must be inversely proportional to time and the number of cycles counted must equal the However, in order to obtain a three digit display vehicle speed. a clock frequency of ten times this value Is used.

The timing circuit design depends on the follow- Since the divisor must be a whole number there ing relationships between parameters. will be some error in the instrument reading.

Let T » the time of traverse of the vehicle For an interval T sec. to be timed, the divisor over the 88 ft. detecting base n required is ——- where n should be a whole

and v • velocity of vehicle in m.p.h. number. In practice n is made the nearest whole T number the fraction —- being rounded off 60 T then T (1) to the value in the range: 60 (n - 0.5) < T < (n + 0.5). (2) T Although the clock pulses and the start signal are not synchronous, negligible error occurs from Let £c input clock frequency to the variable frequency divider this source since the timing pulse generator is controlled from the start signal and its 100 Hz output and n divisor ratio for any time interval x will always be synchronous with the start signal to within one period of the clock.

then f -j°- « output frequency of the Table I shows the instrument reading for various variable frequency divider representative speeds iu intervals of 0.5 m.p.h. It -(3) is seen that the errors introduced by using a minimum divisor of 67 at the maximum speed are well within The output frequency f is sampled for an inter- the design objective. val t sec. during which the number of cycles counted must equal the speed of the vehicle in m.p.h. In addition, the counter operating the display may also Introduce an ambiguity of 1 count, i.e. an i.e. error of 0.1 m.p.h. in the reading. This is not included in Table I.

If the variable frequency divider is controlled In steps by timing pulses of period T sec. then:

114 The divisor code generators can similarly be TABLE I obtained from two four bit binary down counters. Error The output from the variable frequency generator Vehicle Traverse Instrumental Speed Time Divisor in- is counted by a three decade counter, the output of Reading m.p.h. t m sec. m.p.h. which is decoded and displayed on low voltaga digital tubes. 90.0 667 67 89.6 - 0.4 89.6 + 0.1 89.5 670 67 Resetting circuits must be provided to clear the 674 67 89.5 + 0.6 89.0 display and reset the divisor code generator. The 88.2 - 0.3 88.5 678 68 start and stop circuits must operate from dosing 88.2 + 0.2 88.0 682 68 contacts and must be interconnected so that sub- 87.0 - 0.5 87.5 686 69 sequent contact bounce does not affect the system. 60.0 0 60.0 1000 100 The following sequence cakes place in operation. 59.4 - 0.1 59.5 1009 101 On turning on the power supply the DCG "fills" with - 0.2 59.0 10P 102 58.8 timing pulses after which a "stend-by" pilot light 58.5 1026 103 58.3 - 0.2 turns on. The "reset" button resets the display - 0.3 58.0 1035 104 57.7 counter to zero, resets the timing pulse generator to zero output and primes the "start" circuit. A 0 40.0 1500 150 40.0 signal from the "start" circuit switch resets the 39.5 1519 152 39.5 0 DCG to zero, resets the sampling timer, turns on 39.0 ' 1539 154 39.0 0 pulses from the TFG and primes the "stop" circuit. 38.5 1558 156 38.5 0 This "stop" signal inhibits the entry of further 38.0 1579 158 38.0 0 timing pulses to the DCG and opens the seapling gate for the required C.25 sec. At the end of the samp- 26.0 2308 231 26.0 0 ling period the timing pulses re-enter the DCG until 25.5 2353 235 25.5 0 it fills and again turns on the "stand-by" pilot. 25.0 2400 240 25.0 0 (e) Description Of The System IV. TESTING OF THE PROTOTYPE

A block diagram of the system Iβ shown in Figure (a) Variable Frequency Generator Test 3. A crystal oscillator ensures that the clock frequency will be stable in service without the need The divisor code generator and variable frequency for adjustment. It supplies the input frequency for generator were tested with pulses supplied by a pulse the variable frequency generator and controls the generator and the output frequency measured by a timing pulse generator period of 10 mS and the digital frequency meter. All divisors were correct. sampling timer gate period. (Ref. 12). (b) Beat Test Since two four bit counters can divide the input frequency by up to 255, this is sufficient to cover There were two heat tests carried out in the the range of speeds. laboratory, in accordance with B.S. 2011 :Pt. 2B :-

240 240 kHz kHz CRYSTAL VARIABLE SAMPLING COUNTER AND FREQUENCY DIGITAL DISPLAY OSCILLATOR GENERATOR GATE

4 Hz

DIVISOR CODE SAMPLING GENERATOR TIMER

100 Hi 100 Hz

TIMING PULSE 6ENERAT0R ...... b-.J

i ; r START RESET P.B. STOP CIRCUIT CIRCUI

TO ROAD CONTACTS TO ROAD CONTACTS MAIN TIMING CIRCUITS

• CONTROL CIRCUITS

Fig.3 - System Block Diagram.

115 1

(1) The instrument was put into an oven whose temper- sacrifice in accuracy) or by increasing the oscilla- ature was slowly increasing from 25°C. to 57°C. tor frequency in the ratio of 1.609/1. Checks on the instrument's readings were carried out at 5°C. intervals. It was triggered in con- The digital instrument described has an initial junction with the University's Amphometer cali- cost somewhat greater than the present amphometer. brator*. The temperature was then held at 57°C. However, since so much of the circuit is realized in for about thirty minutes at the end of which one integrated electronics and all mechanical relays and of the display tubes failed. Later it was found the moving coil meter have been eliminated from the that this failure was due to the insulation design, it is felt that the higher initial cost will aelting on some of the interconnecting wires. be more than offset by lower maintenance.

(ii)The instrument was left Iα direct sunlight (temp- In service, the elimination of any setting up erature approximately 50°C.) for two hours. Once adjustment and the digital display should reduce again the readings were monitored. most of the human error which can contribute to the errors which are present in the amphometer using a In both tests the readings were unaffected by the moving coil instrument to read speed. environmental temperatures, and no other failures have occurred. REFERENCES

(c) "On-the-Road" Assessment 1. REICH, H.J., T00MM, H., "A Ballistic Meter for Measuring Time and Speed", Review of Scientific Two pairs of cables were set up in Park Street, Instruments, Vol. 12, No. 2, February 1941, Carlton, in order to compare the operation of the pp. 96-98. prototype with an "Amphometer" (SM 20) from the Victoria Police. 2. "The Accurate Measurement of Short Time Inter- vals", Electronic Engineering. Vol. 16, No. 194, Firstly, the readings for "typical traffic" April 1944, p. 470. crossing the test area was obtained. Then the readings of the two instruments were compared with a 3. LAKER, I.B., WHITING, P.D., "A Speed-meter for police car, whose driver attempted to maintain a use at High Traffic Flows", Electronic Engineer- constant velocity, as indicated by his dashboard ing, Vol. 27, No. 329, July 1955, pp. 284-286. 'speedo'. 4. SCHWARZ, H., "Speed Survey Instruments, Traffic The results demonstrated that the design object- Engineering and Control, Vol. 5, No. 10, February ives had been realized. 1964, pp. 604-606.

(d) Battery Load 5. "Vehicle-speed Monitor gives Digital Readout", Electronics and Power. Vol. 16, No. 11, November It is imperative that the instrument is operated 1970, p. 405. from its own internal battery supply. The requirements are nominally 5 V at 0.8 A although operation 6. BARKER, J., "Radar Measures Vehicle Speed", down to 3.8 V is satisfactory, the only change being Traffic Quarterly, Vol. 2, No. 3, July 1949, that the indicator brightness is reduced. pp. 239-251.

Five rechargeable alkaline cells having a nominal 7. ISHI, T.K., "Analysis of Target Speed Determin- 6 Amp-Hour capacity have been used. The instrument ation with Doppler Radar", IEEE Trans, on operates satisfactorily for up to 7 hours with the Instrument and Measurement. Vol. IM-19, No. 2, terminal voltage dropping from 6.5 V to 5.4 V during May 1970, pp. 86-91. this period. 8. WATSON, Jr., R.C. , LEWIS, R.D., WATSON, H.J., V. CONCLUSION "Instruments for Motion Measurement using Laser Doppler Heterodyne Techniques", ISA Transactions, Increasing the number of discrete frequencies Vol. 8, No. 1, January 1969, pp. 20-28. from the VFG by increasing the number of counter stages can increase the ratio of times which can be 9. GARRETT, L.S., "Integrated-circuit Digital Logic reciprocated, and/or decrease the maximum error. Families; Part I, Part II and Part III", IEEE Spectra», Vol. 7, No. 10, 11, 12, October, By reducing the sampling period the display can November and December 19.70. be made to approximate more closely an instantaneous display but in this application the 0.25 sec. samp- 10. "Fall 1969 Condensed Catalogue". Signetics Cor- ling has proved quite satisfactory. poration, California, 1969.

Since S.I. units will b« adopted in the future, 11. BROCK, L., "Designing with M.S.I.", Vol. 1, methods of conversion need consideration. It can be Counters and Shift Registers. Signetics Corpora- done either by changing the spacing between the tion, 1970. detection points (reducing this to 16.65 metres, i.e. to 54.6 ft., gives a reading In km/hour but with some 12. CHAPMAN, R.A., "A Simple Oscillator with Option- al Crystal Control", Applications Brio! . So. * Depending on the setting of a selector switch, this D.002, Fair child Austral ta. Ply. Ltd., Melbourne, crystal controlled device will provide the "contro- May 1968. lling" pulses required to simulate one of seven different vehicular speeds. 116 SECURITY OF SUPERVISORY CONTROL SYSTEMS - TWO CASE STUDIES

B.M. Lewis, B.Tech., M.I.E.Aust. Electrical Contracts Engineer, Control Systems, Electricity Trust of South Australia

9MUSZ, A comparison ia made of the security of two auperviaoiy control systems purchased by the Electricity Trust of South Australia. Each system ia analysed froa the viewpoint of internal defects only and coopered on that basis. The techniques used in each system to achieve a certain level of security ace described. One system ia found to be significantly aore secure than the other.

I imODXTIOH II SXSTSI A The supervisory control syatens eaployed by the System A is based on a variation of the Kleotricity Trust of South Australia are engaged in "address check before operate* scheme. The the »note superviaion and control of 132 k7 and addreaa code of the selected point is encoded, 275 kV transmission terminal st&tious> 33 kV and transmitted to the remote station, and then 66 kT substations and gas turbine power stations. returned to the control station. The returned code The penalty for insecure design of such systems oust agree with the stored form of the original could be the destruction of major plant or possibly code before any command can be issued to the remote the loss of human life. station. This paper considers the security of two of The scheme is described diagraamatleaUy the Trust*s supervisory control systems, each of in figure 1. which are based on electronic technology but each «•ploying quite different techniques. The emphasis The security of the system is described below here is on the abLMty of a system to remain secure by reference to each of the elements of the system In the event of a defect within the system itself. and the possible outcome of one or aore defects However, external influences can also affect system within each eleaent at any given time. security and therefore those influences considered to be critical to the security of a particular (a) Point Selection system should be included in a security analysis. Protection against the simultaneous selection of more than one point is achieved by tnt addition of a simple monitoring circuit which deteots if

POINT »LECTICN "~l ADDEESS _J ADDRESS ADDRESS ADORESS —^ —* •^ ENCODING GENERATION RECEPTION STORAGE

COMMAND MB SELECTION * COMMAND ADDRESS ADDRESS ADORESS ADORESS ALLOWANCE COMPARISON DECEPTION n m tE6ENERATim OE COOING • 1 l OUTPUT 1 CONTROL e r COMMAND tOMMAtHH.,,, COMMAND COMMAND ENCODING GENERATION] RECEPTION DECODING J

CONTROL STATION REMOTE STATION

Fig, 1 System A. Address check before operate.

117 sore than one point has been selected (either by (d) Address Reception; remote station. operator error, equipment defect or a combination An interesting feature of System A ie that regeneration of the address code is carried out before it is completely stored, the regenerated cod« being derived from the second stage of the receiver shift register. If a defect in the address recepticj FROM unit (e.g. a defective clock) causes incorrect POINT registration of the address code, then the method of SELECTION regeneration ensures that the erroneous code that li UNIT stored will be identical to that which is trans- ALARM mitted back to the control station. Subsequently, STOKE the address comparison unit will inhibit the generation of a command and the remote station address store will be automatically reset upon receipt of a Fig. 2 Point Selection Monitoring Circuit reset pulse from the control station. (e) Address Storage of both). The circuit is shown above in figure 2. Protection against defects in the address Beaiatora Rl and R2 are chosen so that trans- storage unit is achieved by testing each bistable istor TS1 is non-conducting »hen one point is select- in the store before the address code is completely ed but is conducting when two or more points are stored. Failure of any one bistable will Inhibit tb« selected» address storage process. This testing process is simplified by the fact that each address code trans- If a defect causes a single point selection mitted from the control station is preceded by an under quiescent conditions, then the 2-step pro- initial 1 and 0 (or mark and space) which is passed cedure (i.e. point selection; conmand selection) through the address storage unit ahead of the inherent to the system will prevent a naX-operation. address code. Receipt of the initial 1 and 0 at tb» However the system would be in a critical state as output of the address storage unit implies that each any subsequent command selection would be immediately bistable has functioned correctly and therefore that directed to the remote station. This potential the address code is stored correctly. However the hazard can be avoided by strict adherence to the possibility remains that a fault can develop daring specified operating procedure. Provided a point the latter part of the storage process and so cause selection ia always made before a command selection a false selection. then the action of the point selection monitoring circuit will ensure that a fault is detected. (f) Address Decoding If two sljniltaneous defects occur in the point Protection against defects in the address de- selection unit such that a valid point selection is coding unit is achieved in a similar manner to that inhibited whilst an invalid point selection is described for the point selection unit in (a) abov«. generated, then a false operation will occur if a Protection against the simultaneous selection of command is issued. more than one point is achieved by the addition of s simple monitor to detect when more than one output Therefore the point selection process is con- of the address decoder has been energised. The sidered to be secure with one defect but not circuit is identical in principle to that shown in necessarily secure with sore than one defect. figure 2. (b) Address Encoding If a defect causes a single point selection under otherwise quiescent conditions, the tvo-atep An address check encoder is used (not shown in procedure inherent to the system will prevent a mal- figure l) to provide protection against defects in operation. However a false selection can occur if the address encoder itself. The address check more than one defect exists in the address decoding encoder uses the output of the point selection unit unit such that a single, but incorrect, output is to encode an address and this address ia compared produced. with the received address code. Thus a false selection cannot occur as a result of either one or (g) Address Comparison more defects in the address encoder unless an identical defect or set of defects also occurs in If a defect occurs in the address comparison the address check encoder. unit the outcome will be secure provided that the encoded «nidress does in fact correepond exactly with (c) Address Generation and Regeneration the rece.red address code. However, if a further defect elsewhere in the system has censed a disparity Protection against defects In the address gen- between the two codes then the possibility exists eration and regeneration units is achieved by means that a simultaneous defect in the address comparison of the "address check before operate" principle unit will allow a command to proceed. inherent to the system. A false selection cannot occur as a result of either one or more defects in (h) Command allowance the address generation or regeneration processes unless an equal number of compensating defects The sane reasoning applies in the case of a occur in each. defect in the command allowance unit as given abore for the address comparison process. A defect in the 118 command allowance unit together with a further defect a point selection. The separations between the cod* lii the system such that the received address code for telemeter and the codes for trip and close are differs from the original address code will possibly therefore an important feature of the design, A min- allow a command to proceed. imum of 3 defects are required to transpose a telemeter command into a close command whereas a minimum (i) Command Selection of 5 defects are required to transpose & telemeter command into a trip command. If a defect occurs in the command selection unit before any point selection has been made the encoding (k) Output control of a command cannot occur unless the address comparison unit indicates that a satisfactory address The output control unit provides the final con- transmission has taken place. Such an indication tact closures that determine the action carried out without a point selection could only occur if a de- on the plant. The two-step control procedure adopted fect also exists in the address comparison unit. with this system allows a single low-power relay to Under these circumstances the command selection be used for each point selection together with heavy- process can be regarded as single fail-safe. duty relays for each of the trip and dose commands. The relay configuration is shown in figure 3 below However, if a defect occurs in the command together with a timing circuit to ensure that the selection unit after a point selection operation final circuit closure and subsequent opening i6 per- but before a command selection then the possibility formed by the command relay contacts and not by the exists that some command will be issued. Subject to lower rated contacts of the point selection relays. these conditions the command selection unit should be regarded as insecure. If a defect occurs in either a point selection relay or a command selection relay than the (j) Command Encoding, Generation, Reception and possibility exists that an incorrect operation will Decoding. be carried out on tho plant following receipt of a valid command. In particular, if a telemetry comand Protection against defects in the command en- is received whilst either the trip or close command coding, generation, reception and decoding units is relay contacts are closed, then an attempt will be achieved by appropriate coding of the system commands. made to operate an item of plant. Furthermore the The coding structure employed by System A is based on point selection relay contact nay become defective a purely binary representation, and the command codes as a result. Thus the output control unit shown are as follows: in figure 3 is insecure. Command to trip 10011101 Command to close 11001011 Command to telemeter 11010010 Command to rescan 11100011 Command to reset 11100001 The commands to trip, close and telemeter refer to each point and each command must be preceded by

CLOSE reoM i ADDHESS DECODING j UNIT KOMMAN0 GATING

Fig. U Output Control Failure Protection

The output control process can be made single fail-safe by the addition of a simple monitoring circvit to detect when either of the following conditions occur: (i) that a point selection relay contact is closed at the time of the command gating TO signal: (ii) that a command relay contact is closed AOOO.ESS & ADDRESS fcCOMMANO GATING. COMMAND at the time of the command gating signal. DECODING UNITS STOP In the event of either (i) or (ii) the address gating signal must ba inhibited. A simple circuit to achieve this aim is shown in figure U-

Fig. 3 Output control circuit - System A

119 F

(1) Power supply these circuits at the remote station is critical to the security of the system as it can be shown that a System A employs 3 DC/fiC/bC converters together reduction in the bias supply by 60% or more can cans« with voltage regulators to provide 2 regulated a mal-operaticn of the output control circuit. supplies for the logic circuits and one regulated Detection of a supply abnormality should be used to: supply for the voice-frequency signalling (VFS) (1) disconnect the primary power to each circuits. In the event of a defect in any of the of the other supplies; power supplies at the control station it is necess - (ii) initiate transmission of a supply ary to protect against: failure alarm to the control station; (i) the spurious generation of an address and or command code; (lii) energise a local supply failure alarm (ii) the simulation of an address or coamand at the remote station. code by communication channel noise (in the absence of transmitter output power). Ill SISTEM B Similarly, in the event of a defect in any of System B employs a single transmission, single the remote station power supplies it is necessary operation scheme together with a coding arrangement to ensure that in which two comctands are allocated to e ach point; (1) a false operation cannot occur; and that one command to trip and one command to close. The (ii) a "supply failure" elan is raised at concepts of address and coamand are thus combined the control station to al-zrt operators Into a single control code. Bach coamand is preceded and maintenance staff. by a station address code such that up to 8 remote stations may share the same control station» Bach Each of the logic circuit power supplies of command code consists of thrse 8-bit characters System A are interlocked so that failure of either corresponding to the units, tena and hundreds of the one will lead to the disconnection of primary power to each of them. However the power supply to the VFS circuits is not interlocked with the other TO STATION ADDRESS supplies. Protection against spurious generation and transmission of address or connand codas la ENCODER therefore not completely ensured in the event of absolute failure of either of the logic circuit TO HUNDfcCDS COMMAND power supplies. Zn the event of a marginal failure of any supply, voltage monitoring circuits will ENCODES. ensure that the supply is switched off. TO TENS COMMAND Protection against code simulation in the A_ TO UNITENCODES COMMANS D absence of transmitter output power from the control ENCODE* station (or intermediate repeater station, if applicable) la ensured by the provision of a trans- FKOM mission monitor circuit at the remote station. OTHER During the normally quiescent condition character- CONTROL istic of this system the control station generates SWITCHES a continuous constant frequency signal. Any inter- 11 ruption of this "normally marking condition for a Fig. 6 Command Selection period In excess of 5 seconds will be detected by the transmission monitor circuit, the output of which may be used to: decimal number allocated to that command. A simpli- (l) inhibit the operation of all output fied diagram of the scheine is shown in figure 5. control circuits; and (ii) energise a local alarm at the remote (a) Command Selection station. Two contacts of each control switch are wired At the remote station, protection against mar- to the command selection unit as shown in figure 6, ginal failure of any supply Iβ achieved by voltage and used to energise a itation address relay and monitoring circuits. The correct functioning of three command relays.

COMMAND 51 AI ION ADDRESS CODE 1 SELECTION EHCOOINÖ 6EHEBATI0q

COMMAND CODE 1 I-ENCODING 1CHECK, f CONTROL. STATION REMOTE STATION

5 System B. Single Transmission, Single Operation.

120 Protection against any single defect in the control switch is obtained by means of the two contact arrangement shown. However, protection against a defective relay is achieved by means of the code generation unit which will not operate . unless both the station address encoder and the units RECEIVED command encoder have been energised. COOE -—C (o)- (b) Station Address and Command Encoding The station address and command encoding units translate the contact inputs from the command selection unit into 8 bit codes in accordance with :=€^^i@ figure 7.

A B D K K c l 2 ei 0 0 0 0 0 1 1 0 0 s 1 0 0 0 1 1 0 1 1 2a 2 0 0 1 0 0 0 0 1 3 0 0 1 1 0 1 1 0 U D 1- 0 0 0 0 1 0 u 5 3 1 0 1 0 1 0 1 o o 6 0 1 1 0 1 1 1 1 Iβ Fig. 8 Remote Station Code Check Circuit 7 D 1 1 1 1 0 0 0 a 8 L 0 0 0 0 1 1 1 9 1 0 0 1 0 0 0 0 (d) Code Reception and Code Check Each incoming code is received and checked Fig. 7 Station Address & Command Codes simultaneously. The following conditions muet be satisfied i A, B, C, K, = odd Transposition of one valid code into another SBB, C, D, KKi = odd cannot occur unless either 4 or 8 defects exist. The ZO, D, K,,Kf odd 2 A, B, C, VT, K validity of each code is confirmed by the code check K2, K33 = even. unit prior to code generation. The station address and command encoding units are therefore secure with As the code8 are available in sequential fora up to 7 defects but not including U defects. simple counting circuits can be used to check these requirements. For example, if a counter is available (c) Code Generation and Code Check that will allow discrimination of the incoming bite, then a circuit such as that shown in figure 8 con The outputs of the encoders are sequentially be used. transferred to the shift register of the code generation unit, commencing with the station address, Protection against incorrect registration of am then the hundreds command code, the tens code and incoming code is provided by the station address finally the units command code. Prior to each recognition circuit. If the station address cod« It transfer all shift register stages are reset. This incorrectly registered because of a defect within condition is confirmed by the code check unit and the code reception unit (e.g. faulty shift register) if satisfactory the transfer is allowed. If the then the station address code is not .«cognised and shift register is found to be not completely reeet so subsequent codes are rejected until a station then a system elan is initiated. After each trans- address code hjg been recognised. fer the contents of the shift register are checked by the code check unit. Each code must satisfy the The code reception process is not completely following conditions: secure aa an intermittent defect could conceivably cause incorrect registration of a valid command cod*. Z A, B, C, K, «odd A parallel code check performed on the contents of 2 B, C, D, Kp = odd the shift register would overcome this deficiency. 2 A, B, D, K, = even S A, C, 0, K^ - even (e) Command Storage These conditions are checked by the logic The operation of each bistable element in the circuits derived in Appendix I. If the conditions command storage unit is checked before any consod are satisfied the code is released and transmitted code is transferred to it. This is done in two to the remote station . If not satisfied, the steps as follows-: encoders are reset and a system alarm is initiated. (i) Each bistable is reset and the condition "all outputs equal 0" is checked by mease Protection against defective operation of the of a 12 input OR gate. shift register during code generation is achieved by (ii) The contents of the code reception unit are the remote station cods check described in paragraph then transferred to each of the three (iv) below. The code generation process is there- sections of the command store. Aβ the code fore secure with up to 7 defects, but excluding reception unit should contain no data at 4 defects. the time of the transfer the effect i« to 121 J aet each bistable. The condition "all IV EXTERNAL INFLUENCES outputs equal 1" is then checked by means of a 12 input NAND gate. The security analysis of any supervisory control c: system that is based on electronic techniques mat Subsequently, the command code is transferred to include consideration of the following factors: the command store provided that the code checks have proved to be correct. The command storage process is (a) Characteristics of the communication therefore secure provided we can disregard the poss- (i) Amplitude, frequency and phase stability E: ibility of a defect occurring after the above checks of carrier. P> have been carried out. (ii) Amplitude - frequency response, (iii) Phase-frequency response, (iv) Level of electrical noise. (v) Inte rruptions. OUTPUT RELAYS (vi) Channel bandwidth and loss. UNITS These factors must be assessed with duo regard to the characteristics of the baseband signal, the coding techniques employed, and the characteristics of the transmitter and receiver units. Such an UNITS 9 assessment may indicate the need to monitor the signal/noise ratio at the receiver input, the duration of any interruption, and the level of distortion of the baseband signal.

(b) Station environment, (i) Temperature, (ii) Electrical noise on power supply leads and input-output leads, (iii) Over-voltages on input and output leada. «SET COMMON These factors mist be assessed with due regard STORM to the types of components used and the design and construction techniques employed. (c) Failure of prerequisite services. (a Absolute or partial failure of the primary power supply may imply the need for power supply inter- 2 sec locking and sequence control on shut-down and restoration. Fig. 9 Output Control Circuit - System B (d) Human factors. Errors by operators and technicians nuat bo (f) Command Decoding assumed and reasonable precautions taken to eocure the system against them. At least two defects must occur within the command decoding unit before a false command can be V CONCLUDING REMARKS issued. The analyses of the two systems A and B are auen- (g) Output Control arized in Appendix H in terms of the number of da- defects that can be tolerated by each element of the The output control circuit of System B is shown system without loss of security. System B Iβ aeen to in figure 9 and it can be seen that operation of an be more secure then system A in all reepecte with ttu output relay requires that: exception of command decoding and power supply. Where- (a) RLA pot be energised; as system B will reiaain secure with any single defect, (b) RLB be energised; and system A could become insecure in the event of a de- (c) RLC be energised. fect in either the command selection eleeent, the address or command storage elements or the output RLA is adjusted such that it will not operate control element. With the exception of the output unless two or more output relays have been energised. control element, the possibility of insecure operetta Therefore say single defect cannot cause a false can probably be tolerated as the defect oust occur control action. Protection against faults within within a relatively short tine interval. In the case the final output relay is obtained by the multi-con- of the command selection decent, this tiae interval tact arrangement shown. can be minimized by appropriate operating procedures. The output control element can be made single fail- (h) Power Supply safe by the addition of a failure detection circuit such as that shown in figure 4. Each power supply of System B is monitored by a 'no-volt' relay that detects an absolute failure of Finally, the reader is reminded that the influ- the supply. The logic circuit power supply is also ence on security of defects within the data acquisi- monitored for % under-voltage and over-voltage con- tion and telemetering circuits of a supervisory conditions in the event of which the system is either trol system also requires consideration even though reset (-5%) or switched off {*%). there may be no direct feedback between acquisition 122 and control. The security of such circuits becomes Extracting C and C critical with the introduction of direct feedback. [(A*3 > AB')C •(A^' • AB)c' J Kx VI ACKNOWLEDGEMENTS [(A*3 • AB')C' * (A'31 • The permission of ths Chief Engineer of the Electricity Trust of South Australia to present thie Using De Morgan's theorem, (A'B t Aβ') * , r paper is acknowledged. (A3 * AB) Therefore, we require:

APPENDIX I 1 1 [(AV • AB)' 0 • (A'B • AB) C ] Kx •

DERIVATION OF 2ODE, p CIRGITTT - SYSTEM B 1 [(AV «• AB)^' • (A'B • AB) C ] K^ =1 Snmmarv of possible codes: Again using Oβ Morgan's theorem; A B Kl (ÄV • AB)'c' • (A'B' • AB)C ]' K • 0 0 0 0 1 1 0 0 x 0 0 0 1 1 0 1 1 (A'B'* AB)V • (A'B' • AB)C ] K^ « 1 0 0 1 0 0 0 0 1 0 0 1 1 0 1 1 0 Again using Dβ Morgan's theorem: 0 1 0 0 0 0 1 0 0 1 0 1 0 1 0 1 [(A'B' • AB)V • (A'B1 • AB)C I« K* • 0 1 1 0 1 1 1 1 0 1 1 1 1 0 0 0 [(A3 • AB) C • (A 3 • AB)C 1 K. » 0 1 ^^N. J X 1 0 0 0 0 1 1 1 1 0 0 1 0 0 0 0 Gopditiona to be checked: 2 A, B, C, K, is odd Z B, C, D, K, is odd 2 A, B, D, K, is even 2 A, C, D, Z. is even

(a) Consider A. B. C. A B c h t I I 0 A 3 C X-, The above equation can be simply realized 0 0 x l I I l with 2-input AND and NOR gates as shown above. Essential 0 0 1 o A 3 CK. i i + (b) Similarly for Z B, C, D, Kg* 0 1 0 0 A 3C K. » -I- terms 0 1 1 1 A 3CK, (c) Consider A. B. D. K-, i I-Μ 1 0 0 AB C t^ 0 A B D X 1 0 1 1 non- I I I I 0 0 0 0 A 3 D K3 1 1 0 1 ABC'K, essential t i 0 0 1 1 A 3 DK 1 1 1 0 ABCK, terms 3 t t Essential For £ A, B, C, K^ to be odd, we require either 0 1 0 1 A 3D K3 of the terms listed in the colunn marked x to be t i terms true. The three non-essential terms are included 0 1 1 0 A 3DK- so that a general purpose solution will result. i i

1 0 0 1 AB D K3 Algebraically, we require:

I I I 1 0 1 0 AB'DK3 A'B'C'^ • AVCK^ + A*33*1 • AB C K, • Non- I i •aeential 1 1 0 0 ABD K. terms

ABDK, Re-arranging terns: For 2. A, 3, D, K, to be even, we require (A*33^ • AB'C^) • either of the terms listed in the column marked • to be true. = 1 i Algebraically, we require;

Extracting K^ and K,: • A'3DK • A'H'D'X^ • AVDK3 • A'3D'K 3

(A 3C • AB C) • (A 3 C • ABC )J = 1 ABVK3 • AB'OKJ • ABD'K3 • ABDKj (A'BC' • ABV) • (A'a'c • ABC)J

123 Re-arranging terms: APPENDIX II

(AVDK., • ABDKj) *-(A'BDK( 3 • AB NUMBER OF DEEECTS POSSIBIÄ WITHOUT LOSS OF + (A'BJ • AB DK^ ) = 1 SYSTEM A Extracting D, D*, and K^: Selection V • AB)D • ( B • AB')D' ] (i) Point 1 (ii) Command 0 or 1 V • AB)D' • 'B • A')DB ] KJ

Jsing De Morgan's theorem: Encoding

1 1 [(A'B* + AB)D • (A'B • AB)V ] K + (i) Address 3 (ii) Command <3 or<5 or [ (A'B' • AB)D' • (A'B' • ABJ'D] K3' = l Generation

(AB • AB) D • (A 3 * AB)D J^* (i) Address 1 (ii) Command <3 or<5 or [ (A'B' • AB)D' • (A'B • AB)'D] K3 = I Reception [ (A'B1 • AB)'D + (A'3* • AB)D'J' K^ •

1 (i) Address 1 [(A'3 + AB)D" • (A'B* + AB)'D] K3 = o (ii) Command <3 or<5 The above equation can be realized by- connecting the previously derived circuit as Storage shown below: (i) Address 0,N (ii) Command 0,N Decoding

(i) Address 1 (ii) Command <3 or<5 Output control 0 1

Power supply 1 1

totally defective

(d) Similarly for Z A, C, D,

124 A PRESENCE AND PASSAGE VEHICLE DETECTOR

G.F. Shannon, B.E., Ph.D., M.I.E.Aust. Senior Lecturer, Elec. Eng. Dept., University of Queensland N.V. Chuong, B.E., Research Assistant, Elec. Eng. Dept, University of Queensland

SUMMARY: In attempting to control the flow of traffic across an intersection in a manner that recognises changes in traffic demand at that Intersection, information about the demand must be used. Typically this information relates to a count of vehicles or to the presence of a vehicle at a given location. For the traffic control to be efficient this base information must be reliable and accurate. Commercially available detectors do not combine the functions of presence and passage detection and are subject to counting errors. This paper describes the design of a presence and passage detector whose performance is reliable and very accurate.

I INTRODUCTION

A variety of methods have been used to detect the presence of vehicles on roads but for one reason or no anothert2) the inductive loop of wire buried in the road surface offers the best detector. This loop of wire is excited by an A.C. voltage and the resulting current flow establishes a magnetic fisld. When 108 metal objects enter this field, induced eddy currents act to decrease the magnetic field. The field change may be detected as 106 (i) A change In inductance of the loop, (ii) A change in resonant frequency of the circuit of which the loop is part, 104 or (ill) A change in phase between the current flowing in the loop and a reference signal.

This paper is concerned with the design of a 102 circuit to detect the field change by one of the above methods and use this information to indicate -LOOP- the passing or continued presence of a vehicle. The FMt oassage detector provides an output suitable for 100 α-iving a counter from which rates of flow m?y be -2 0 6 8 X> tt 14 16 1 Court detrained or the output may be used with another olloop FIG.1 .. VARIATIONS IN LOOP INDUCTANCE detector to measure vehicle speed. The presence ASAINST MSTAWC.g OF VEHICLE detector provides an output which indicates when a PROM LOOP 1 Holdgn ) vehicle is in the vicinity of the loop.

Environmental changes such as temperature or Figure 2 shows that a stationary vehicle parked humidity variation should not interfere with the over portion of the loop will' yield a steady contrib- operation of these detectors and the vehicle count ution to the inductance and the effect of a passing should not be subject to errors. The choice of circuit elements is very such influenced by the need vehicle is added to this. to satisfy these requirements. Ill EXTREME CASES FOR TRAFFIC DETECTION II CHARACTERISTICS OF THE FIELD CHANGE The most onerous demands for traffic detection result from certain extreme situations. These are Figure 1 shows a graphical representation of the namely - loop inductance as a function of position of a vehicle over the loop. It is noted that the percentage change is only about 6X at the best and (1) to detect a car that passes over only a fraction as low as IX in some instances. It is also noted of the width of the loop. that the change may not be monotonically .increasing or decreasing. (2) to detect the passage of a vehicle even when 125 possible to use a crystal controlled oscillator to remove some of these problems but now the stability of 110 the components in the bridge circuit becomes significant. The phase detector makes use of logic circuitry to compare the two signals, one from the voltage applied to the loop and the other from the current flowing in the loop. Each of these is half wave rectified and fed as inputs to a logic gate. This may be either an AND or an OR gate. The width of the IOC output pulse indicates how much the signals are in or out of phase. This detector operating from a crystal controlled oscillator removes the performance objections associated with drift and environmental 104 change: it also provides a greater percentage level change than the direct frequency or Inductance change

Mini The output of the logic gate is averaged to yield 102 a voltage level. This level change may be used directly to give an indication of the presence of a vehicle. Similarly the rising or falling edge of the • IOOP- Feet level change could be used to provide a count of the 100 passage of vehicles. This implies that for passage -2 6 B 10 12 14 18 count the output of the logic gate should be Centre differentiated. All of the above circuit otLoop FIG. 2 VARIATIONS IN LOOP INDUCTANCE ACAIKST DISTANCE OF VEHICLE possibilities have been used in one or other of the commercially available detectors. However there still PSOM LOOP C Effects from parked exists ample opportunity for erroneous counting. If vehicle ). a vehicle in crossing the detecting loop stopped and started several times there would be a pulse for each stop and start and hence an erroneous passage count. another one Is parked over portion of the loop. A vehicle parking over the loop and subsequently moving off could easily be miscounted. (3) to detect the passage of two cars which are travelling in very close proximity. These malfunctions can be eliminated only by giving careful attention to the design of the output The first case means that the inductance will circuits from which passage and presence are indicatsc.| change by about 12 only and consequently the detecting circuitry needs to be very sensitive; the It was therefore determined that the output of a second means that the level at which the change takes phase detector used in conjunction with a loop fed by pul place may vary; the third requires a rapid response a crystal controlled oscillator would provide the 3(f to changes of level. basic information from which successful passage and two presence indicators must operate. Most of the IV METHOD OF DETECTION problems of accuracy and reliability of performance occur with these output circuits. The three methods of detecting the field change mentioned above may, by using suitable circuitry, V DESIGN PHILOSOPHY result in a voltage level change. This change in level could be used to operate a switch which would Indicate passage or presence. However such a system To eliminate the possibility of a double count would not yield a passage count under condition (2) for one vehicle, a second pulse caused by that vehicloj above and would also be subject to erroneous must be ignored. A device that can be switched on behaviour if environmental changes caused any drift once and then ignore all subsequent attempts to switch | in circuit component values. it on until it has been switched off will achieve this. Such a device is a bistable multivibrator. When the state of the bistable changes, pulses can Thus while it is convenient to use level change again be generated and these used for counting to indicate presence of a vehicle it will be more purposes. convenient to differentiate the change to obtain pulses and use these for passage detection. A presence detector may be arranged to indicate The choice of method of detecting the field vehicle presence for as long as the d.c. level change is made largely by considering the stability output from the phase detector exceeds a given reliability and convenience of the circuits to threshold level. realise each method. For purpose of illustration assume that car 1 A detector built to detect a change in frequency enters the loop and before it leaves car 2 aj.so has very simple circuitry. The loop of wire in the enters the loop. Then car 1 leaves followed by car roadway naturally forme part of the free running 2. The output from the phase detector wouid typicall; | oscillator. Because of the use of such an oscillator be of the form shown in Figure 3(a). The output of spurious output may be obtained as a result of the presence detector would be as in Figure 3(b). Fo: | component values being effected by environmental passage detection the output of the phase detector is change. With amplitude change detection it is differentiated to give curve (c) Figure 3. These pulses are fed to the bistable multivibrator which

126 variation in circuit component values may cause some variation in the quiescent output of the phase detector. To prevent this causing false presence detection the switching level of the output must be set well above the level of these stray variations. 2nd vWiicic This precaution will also eliminate lock in. leaves loop Output from A Schmitt trigger having two distinct switching Phase Detector levels will satisfy this requirement and will therefore be used in the new design. Presence Detection The voltage difference between the ON and OFF Differentiator states of the trigger was designed to be 7V. Through Output a differentiator located at the trigger output, pulses will be generated as the trigger is 3witched ON and Output frem OFF. These pulses are then used to operate the Passage Bist.Mu*. presence relay driver which is a bistable multivibrator . (Figure 4).

Postage Detection To Pass, o/pgatt FIG.3 .-POSSIBLE WAVEFORMS FOB FIG.4 -BLOCK 01AftWAM FOW THE VEHICLE DETECTION. PRESENCE OETECTOB OUTPUT WE, then has an output Figure 3(d). This output is again differentiated to give Figure 3(e). If the negative pulse is used for vehicle counting the input to the The use of pulses reduces the effect of drift passage counter would be as in Figure 3(f). This and also provides a pulse when presence detection would indicate a count of one for two vehicles terminates. This pulse is also used to give passage passing. However if the output of the presence count. detector is also differentiated another negative pulse wculd be obtained which when added to Figure A capacitor in the relay driver circuit ensures 3{f) yields Figure 3(g) which then gives a count of that initial operating conditions are established two for two vehicles passing. ready for correct functioning of the detector when the power is turned on. Thus it is seen that passage and presence detection can be achieved simultaneously from the (b) Design of the Passage Detector output of the same phase detector. Coalescing the two detections in this way offers several To overcome drift problems, pulses corresponding conveniences. There is a saving in number of loops to the changes In d.c. output of the phase detector and phase detectors; further, with this detector it will be used for passage detection: in particular the is possible to have a loop cover two lanes of traffic pulse occurring when a vehicle leaves the loop will be and yield an accurate count. used to drive the counter. By using a bistable multivibrator it is possible to eliminate the overcount (a) Design of the Presence Detector problems of other detectors, Tor once the bistable has been switched by a positive pulse, further positive The presence detector is required to have the pulses will have no effect until a negative pulse has following features:- caused the bistable to switch back.

(1) absolute presence detection A block diagram of the passage detector output gate is shown in Figure 5. (2) sensitivity high enough to detect high-bed trucks The d.c. output of the phase detector is differentiated and the resulting pulses are amplified (3) absence of spurious output to ensure efficient switching. This amplifier has three stages: a current amplifier, a voltage ampli- (4) may not be locked in fier and an output buffer stage. Feedback provides some automatic gain control. The amplifier has an (5) provide additional passage pulses in overall gain of about 200. special cases. At the output of the amplifier, positive and To obtain absolute detection of vehicle presence, negative pulses must be distinguished from each the output gate or at least part of it must be. d.c. other. Simple diode gating achieves this. coupled to the phase detector. Drift in settings and

127 With these new designs the paralysed or locked on condition experienced with other designs is absent: hence there is no undercounting in the tailgating FromPns. o/p gate situation (i.e. closely following vehicles). From Phos« Extreme testing also failed to show any overcount Oct problems. Axle count on semi-trailers is however OIFF. possible. This is not seen as an adverse character- 4 istic however as such a vehicle occupies at least as much road as two or three cars.

FIG. 5 - BLOCK DIAGRAM FDR THE No problem of latch on occurred because the use PASSAGE OETECTO» of level switching prohibits this. OUTPUT GATE. VII ACKNOWLEDGEMENTS

This work was carried out under a research Positive going pulses are buffered through an contract with the Queensland Department of Main Roads emitter follower to switch one state of the bistable and the authors wish to express their appreciation to multivibrator: negative pulses are inverted before that Department for the assistance given to the being used to switch the other state. No succession project and for permission to publish this paper. of like pulses can cause two switchings. REFERENCES The change of state of the multivibrator resulting from a vehicle leaving the loop is then used to switch 1. Chuong, N.V., "Inductive loop vehicle detectors", a relay driver which is a monostable multivibrator. M.Eng.Sei. Thesis, University of Queensland.

Under normal non-detection conditions the relay 2. Anderson, R.L., "Electromagnetic loop vehicle is energised. When a vehicle which has entered the detectors", IEEE Trans. Vehicular Technology, loop leaves the loop the monostable is momentarily Vol. VT 19, No. 1, Feb. 1970. switched to its unstable state and the relay is released for a short period. The duration of this transition period depends on the resetting time of the monostable and may be adjusted by a variable resistor in the circuit.

The monostable multivibrator also accepts pulses from the presence detector. As discussed above these are sometimes needed to correct the vehicle aount. The presence detector provides a pulse to the passage detector when the last vehicle to leave the loop does so. If only one vehicle had been involved there would also be a pulse available from the passage detector itself. This must not yield a double count. If the pulses arrive simultaneously there will not be a double count: once the monostable has been switched to the unstable state b" one pulse a second pulse arriving will have no effect. Hence, provided the two pulses are not time spaced by more than the transition period, erroneous counting will not occur.

If vehicles are travelling slowly across the loop the passage pulses may not be sufficiently big to yield a count. However the phase change will cause presence detection and when the vehicle leaves the loop passage count will be obtained from the presence circuit.

The monostable also serves as a power-failure alarm. It is energised while the power is on: if power fails it is de-energised and will give «y continuous presence detection which can be arranged to operate an alarm.

VI PERFORMANCE OF THE NEW DESIGNS

The new designs were tested under both field and simulated conditions 2nd found to perform accurately and reliably.

128 DIGITAL FREQUENCY, TIME AND TIME DEVIATION EQUIPMENT

J.W. Tamke, B.E.(Hons), Grad.I.E.Aust, M.I.I.C.A., Assistant Test Engineer Power Electricity Trust of South Australia

SWOtfLM Orer the last few years a steadily Increasing variety of general purpose logic Modules have been ia- troduoed by semi oonduotor manufacturers. Because of the improved control of the technological methods uatd la the production of integrated oircuits, more and more devices can now be manufactured in a single operation. Basic logio elements have been combined to form store complex circuits; for example counters and registers.

Because theM increasingly oomplex Mediua Scale Integration (MSI) logic blocks can be manufactured in a single prooess, they are much cheaper than the sans oirouits formed from discreet components. Design engineers are now able to construct digital instrumentation more economically than was possible in the past.

Thia paper describes time and frequency equipment designed and built of MSI logic bloolcs and employing only a quick periodic calibration check. The equipment has been in use in the control room of the Torrens Island Power Station for the last three years without a failure. Torrens Island is the most modern Power Station to be commissioned by the Electricity Trust of South Australia.

1 INTRODUCTION 11 SYSTEM BLOCK DIASBAH

In British Countries the frequency of power gene- The overall block diagram is shown in Figure 1. ration is 50 Hs. The turbines are designed for 3000 rpm operation and drive a two pole alternator. The The standard oscillator shown in Fig. 1 supplies resultant frequency of generation is 50 Hs. a standard 50 Hz to all three units. The standard tine unit oounts the standard 50 Hs and displays time Because of sow industrial requirements it is ne- in twenty four hour notation. ossssry to maintain the frequency of generation as dose to 50 Hz as possible. If the demand for power The time deviation unit continually integrates rises appreciably the speed of all machines connected the error between the two frequencies (i.e. mains and to the power grid will fall and hence the frequency standard) and displays the result as time deviation. will fall. Thia is an indication to the operators to increase power to the turbine and so increase the speed and bring the frequency back to 50 HΒ. STANDARO Although generating authorities attempt to keep OSCILLATOR the frequency at 50 He all the time it ia impossible to do so because AC power must be made when it is de- nanded and the deomd at any time is not precisely known. Because of this uncertainty in demand the frequenoy change» slightly over a given period »suiting STANDARO TIME SYSTEM in an error between the time shown by mains frequency TIME DEVIATION FREQUENCY clocks and standard clocks. It is necessary for the operators to be able to see this error, or time deviation, so that it may be kept to a minimum.

In order to achieve satisfactory time and ftsquen- oy control three display* are necessary: MAINS FREQUENCY (1) Standard Tie» (2) Frequency FIG. 1 System Block Diagraa (3) Time Deviation As a basis for time keeping an accurate stable 50 H* The mains frequency unit displays average »ins signal is necessary as a substandard. frequency to - .01 Hi over 50 nains periods. Fresh information is displayed once every 1.1 seconds. The frequency unit uses the standard 50 Hs as a check signal only.

129 !

111 STAMDAHD OSCILLATOR The standard oscillator consists of a 102.4 kHz crystal oscillator enclosed in a double copper block which is maintained at a constant temperature using an oven controller. The oscillator circuit uses sil- KH3— icon transistors adequately underated and high stability components for long term accuracy. G. 2 Standard Oscillator The 102.4 kHz is progressively divided by 2 using basic integrated circuit logic blocks as shown in Figure 2. The result after 11 successive divisions is a square wave of 50 Hz. This square wave is amplified in an isolation amplifier and distributed to the other three units as well as driving an anologue clock.

1 HP «• T.W.S T.W.S 1 ose. T.W.S RB. EP 1 1 CONTROL ENABLE r—• LOG!C COMPA«Mr l UK L COMPAKAIUK L COMPAKAIUK

1 50 Hz + (K> •=• SO + 2« STO. COUNTER COUNTER COUNTER

SECONDS MINUTES HOURS DISPLAY

FIG. 3 Standard Digital Clock

IV STANDARD TIKE DISPLAY Standard time is displayed in hours, minutes and less reset push button. The desired time is set into seconds using the twenty four hour clock notation. A the TOS, the ley lock enabled and the yaah button de- block diagram is shown in Figure 3. pressed at the desired time. Under normal operation the standard 50 Hz supply When the key lock is energised a high frequency is first divided by 50 to give 1 second pulses. These oscillator transmits a signal to the control logic. are then progressively divided by 60 (to give display At the instant the push button Is depressed the nor- in seconds) 60 again (for minutes) and 24 for hours. mal operation of the clock is disabled. Instead of The dividers are constructed using integrated circuit all the hours, minutes and seconds decades counting binary coded decimal decade counters which may be the standard 50 Hz serially each deoade i» stepped in- programed to divide by 10, 2 or 5 and binary coun- dependently by the high frequency oscillator. When ters which may be programmed to divide by 2, 6 or 12. the BCD numbers in each decade equals the maber in A suitable combination of these integrated circuits the corresponding TWS that particular decade does not will result in networks capable of dividing by 50, 60 receive any more pulses from the high frequency oscil- and 24. lator and the number it contains is frozen at the number contained in the corresponding TWS. When each of The Binary Coded Decimal (BCD) output from the the hours, minutes and seconds decades are inhibited dividers is decoded into decimal using integrated from counting high frequency oscillator pulses the circuit decoders. These decoders are capable of dri- oontrol logic automatically opens the gate and the ving the cold cathode (nixie) tubes which are used as clock returns to normal operation by counting the 50 the final display. Hz standard frequency. To set the time six contactless Thumbwheel Switch- The error in aettin/j the dock by this method 1« es (TVS) are provided on the front panel of the in- the reaction time of thb person engaged in resetting strument together with an enable key and a contact- the clock minus a maxima of 59 high frequency oscil- 130 ,.i lator periods. The error introduced is mainly the Logic circuits are necessary to detect when the I reaction time of the operator and so a very high fre- tine deviation passes through zero. How this is a- I quency oscillator is not required. The oscillator is chieved is explained below. ' shown in Figure 4. A block diagram of the instrument is shown in Figure 5. The mains and standard 50 Hz supplies ar» filtered to remove unwanted noise which is always pre- I5V sent around power stations. The filtered sine waves 39K are then squared, divided by 5 and passed through pulse generators to produce 5 mioroseoond pulses for »very 5th input oycls of F and F . These are then directed to either the up or down count lines of a three deeade Up/Down counter via the direction of count logic.

If the up and down pulses arrive at the counters at the same tin» erratic operation of the counter results. A Comparator is used to separate coincident and overlapping pulses and its operation is explained later. A zero detecting circuit automatically ohanges the direction of count of f and f pulses whenever the count passes through cero. ThS number oontained in the counter may be set to any value by dialling the 11 required tine deviation in the TVS provided on the front panel enabling the set oount logic and depress- ing the pushbutton provided. The number dialled in the TWS is asynchronously loaded iato the Up/Down FIG. 4 High Frequency Oscillator counters.

The frequency of oscillation is 800kHz resulting in a

59 seconds or -.074 maecs. maximum error of - OvXJ X

V INTEGRAL FREQUENCY ERROR TRANSDUCER or TIME DEVIA- TION METER COUNT The basis for the design of this instrument is PULSE »m UP the versatility of an integrated circuit Up/Down SHAPES & counter. This is used to integrate the error between DIVIDER DIRECTION two frequency inputs namely standard 50 Hz frequency PULSE OF COUNT (F ) and Bains 50 Hz (F ). The standard frequency SEPARATOR LOGIC COUNT count« in one direction and the mains frequency in fs DOWN the other direction. If the two frequencies are different the counters gradually count up or down. For example if P is 50 Hz and F is 49.95 Hs for a period of 100 seconds the resulting count is 5 which can * be made to represent a time deviation of gOOO - 4995 50 50 or 0.1 seconds.

In the transducer constructed both frequency signals are first divided by 5 to obtain frequencies of f (10 Hz) and f (approximately 10 Hz). Thus in the SIGN 13 DECADE UP/DOWN COUNTER |_ aoove example over a period of 100 seconds the Up/ Down counter would receive 1000 counts in one direc- ZERO tion fron f and 999 in the otiter direction from fm« DETECTION The resultant accumulated oount of 1 can be displayed & SIGN as a time deviation of 0.1 seconds. CHANGE Because the counter accumulates the modulus of the difference in counts on the up and down lines the direction of count of f and f must change as the sign of the time deviation changes. If the time deviation is positive f counts down and f counts up. Then if fB> f the positive deviation continues to increase; " f_< f. the deviation decreases. If the tine deviation is negative f counts up and f counts down. Then if f< f the negative deviation Become» FIG. 5 Digital Time Deviation Heter aore negative; If f*>f the deviation approaches tero. 8

131 (a) Direction of Count Control between the pulses a four bit binary comparator is used. The comparator compares the absolute value of The direction of count is controlled by logic cir- two digital numbers A and B. Depending on the values cuits which detect a zero count and control the sign of A and B, pulses are given out on two different line of the display. The circuits also reverse the direc- X and Y as shown by the truth table in Figure 6. tion of count of f and f in accordance with Table 1. m 3

TABLE 1

tm INPUT OUTPUT Deviation positive Deviation negative -X s X V f count up count down COMPARATOR A>8 0 1 0 m A< B 0 0 t f count down count up Α-B 0 1 1 , 1 ITT A4B 1 0 0

Bβ B< B2 B, However; a distinction oust be made between two 8 oases PK. 6 Digital Comparator (1) the deviation passing through zero (2) the deviation approaching zero but not passing through zero. Thus by connecting the comparator as shown in Vigor« 7 overlapping Up/Down pulaes nay be separated as the out- (i) Deviation passing through zero puts on the X and Y lines are both 'high' when the Up and Down pulses overlap. This corresponds to the tine In this ca3e the sign and direction of count of when the corresponding digital numbers formed by the f and f oust be changed. Sons indication of when Up and Down pulses at the comparator are equal. the count passes through zero is required. This is aohieved by the Up/Down counters used. If the count- (d) Display ers are reading +00.0 and a count down occurs the counter will read +99«9 and will give out a "borrow The numbers contained in the Up/Down counters pulse". (See 7(a) later.) The count of +99.9 is not vary at a frequency of 10 H*. Hence it is necessary displayed because of the display system used. (See to use quad latches as described in section 7(d) with 5(dJ.) This "borrow pulse" cannot be used to change • 1 Hz latch pulse derived from F before decoding the the sign and direction of count since the counter value of the time deviation in decoder/driver integra- would still be reading +99.9» When the next pulse on ted circuits and displaying the result. A nonflicklng the up line occurs the counter returns to +00.0 and display results. gives out a "carry pulse" (See 7(a).) This is used to change the sign and direction of count of f and f . The counter thus reads -00.0. A similar argu- UP ft» 1 ment holds if the counter reads -00.0 and a count down occurs first. It must be remenbered that the counter accumulates the modulus of f and f pulses and that the sign is external to theTJp/Down counters. ove tio (ii) Deviation approaching aero qui abl If the counter reads +00.0 or -00.0 and a count up occurs first, no "carry pulse" is generated and so no sign change or direction of count change is initia- OOWN(O) fre. ted. This corresponds to the case of counting down lem to zero and then counting up again with the same sign. Dom Iw (b) Zero Detecting Circuit the eee< The zero detecting circuit detects when the coun- ue ter holds 00,0 irrespective of sign and when a "carry PK. 7 Pulse Sepaxator thei pulse" occurs a sign and direction of count change is lope initiated. V1 DIGITAL FREQUENCY METER

(c) Simultaneous Up/Down Count Pulses The frequency meter is required to have * resolu- the tion of .01 Hz with an update time of approximately COM For correct operation, the Up/Down counters re- one second to be of any use for frequency control of • is quire that the up and down count pulses be separated power grid. A conventional frequency meter could not in time. If there is any overlap between the pulses fulfil both requirements simultaneously and so a new random counting results. Hence it is necessary to approach was necessary» accurately develop f and f to be the same length; nominally 5 mioroseconäs. To separate any overlap

132 Since the frequency of power generation in Austra- lia is nominally 50 Hz small inaccuracies of the frequency meter outside the working range of the power frequency is ad mis sable. LATCH A down count frequency meter was designed for a centre frequency of 50.00 Hz. Mains frequency F is RESET fed to the meter and reduced by a divide by 50 circuit to give a pulse of 50 mains periods; that is a pulse of length ^- seconds. This is nominally 1 seo- DECADE DOWN COUNTER ''m ond long.

A ' count down1 electronic counter resetting to LATCH (1)0000 at the beginning of each pulse period counts down the number of accurate 5 Uli pulses during the

time -SΓ seconds. The 5 kHz oscillator is an internal OISPLAV

crystal controlled oscillator At the end of the 5000 x 50 pulse period the counter reads 10000- If PIS. 8 Digital Frequency Meter the mains frequency is 50.00 Hz the counter will read 5OOO. If the frequency is 49.95 Hz the counter will read 4995» A calibration table is shown in Table 11. V11 LOGIC BLOCKS USED The logic blocks used in the construction of the TABLE 11 equipment are Transistor Transistor Logic of Medium Scale Integration. Some of the more complex circuits are described below: INDICATED FREQUENCY CORRECTION (a) Up/Down Decade Counter 5O.7I - 49.30 0 50.72 - 51.00 & 49.29 - 49.01 + .01 51.01 - 51.21 & 49.00 - 48.80 + .02 This is the basic logic block used for the Time 51.22 - 51.40 & 43.79 - 48.61 + .03 Deviation Meter and the Frequency Meter. The device used is capable of being preset to any number from 0 51.41 - 51.56 & 48.60 - 43.45 + .04 51.57 - 51.71 & 48.44 - 48.30 to 9« A load input controls the asynchronous entry of + .05 these numbers and sets all outputs to the appropriate 51.72 - 51.84 4 48.29 - 48.17 + .06 state. Counting is performed through two clock lines 51.85 - 51.97 & 48.16 - 48.04 + .07 one controlling the count in the up direction and the 51.98 - 52.03 & 48.03 - 47.93 + .03 other in the down direction. Two outputs Borrow and 52.09 - 51.19 * 47.92 - 47.82 + .09 59.20 - 59.30 & 47.31 - 47.71 Carry are connected to the clock Inputs of subsequent + .10 counters to provide for counting numbers greater than 9. A connection diagram and pulse diagrams showing the operations of clearing, loading and counting up and The frequency displayed is the average frequency down with the resultant carry and borrow pulses is over the 50 mains periods. To obtain similar resolu- shown in Figure 9. Note that when counting either up tion using a conventional frequency meter would re- or down the other clock line must be "high' quire a sampling tine of 100 seconds which is unsuit- able for power grid control. (b) Four Bit Binary Comparator A block diagram is shown in Figure 3. The input frequency is divided by 50 to generate a pulse of The use of the 4 Bit Binary Comparator has al- length 50 periods. During this tine the 4 Decade ready been described as a pulse separator. It is a Down Counter is stepped down using the 5 kHz pulses. circuit which is used to compare the numsrloal values Immediately after the 50 period pulse the count in of two 4 bit binary numbers A 4 B. Outputs indicate the counters, which is the frequency during the last whether second, is gated to the storage latches where the value is decoded and displayed. The down counters are (1) A greater than B then reset to (1)OOOO and a new 50 period pulse deve- (2) A less than B loped to begin a new cycle. (3) A equal to B

! The accuracy of the instrument is dependent upon A strobe input overrides all other inputs and places , the accuracy of the internal 5 kHz oscillator and the the outputs X A Y in a definite state. Figure 6 shows correct operation of the divide by 50 circuit. This a connection diagram and logic table. is easily checked by substituting the standard 50 Hz for the mains 50 Hz. The reading should be 50.00 (c) Quad Latch - .01 Hz. Where non flicking displays are required a four bit storage element utilising latch connected gates is used to perform the asmory function. The information

133 bits to be stored are supplied to ons 3et of inputs. Upon strobing the circuits with a latch pulse the inputs are transferred am stored on the outputs. The value stored on the outputs is independent of the val- B.C.O. IN ue on the inputs during the absence of the latch pulse. 1 i i i QUAD LATCH LATCH "PULSE • i « i

v DECODER cc 'IN CLEAR BORROW CARRY LOAD 4IN BIN ORIVER

1 i i ii n UP/ DOWN COUNTER 1 2 3 5 6 7 8 9

'2'lN 20UT 'OUT DOWN UP A'oUT 8OUT GROUND

UP CARRY UP CARRY UP FIG. 10 Display System

DOWN SORROW DOWN BORROW DOWN (d) Binary Coded Decimal to Decimal Decoder/Driver CASCADING COUNTERS This is a monolithic BCD to Decimal decoder capable of driving cold cathode indicating tubes direct. The BCD number '•'•<> be decoded is applied to the four CLEAR Jl input lines; and the unique output corresponding to the decimal equivalent of the input number falls to a LOAD logic low level causing that particular cathode to TU glow. Figure 10 shows the use of the circuit with a "1'lN ILL. quad latch. ILL. (e) Power Supplies For all units except the standard oscillator power is supplied from the AC mains. The power supplies U3e a regulated and short circuit protected integrates '8'lN circuit fabricated on a single silicon chip. It is capable of supplying up to 1 aap at voltages from 5? UP upwards.

OOWN Typical power supplies of +5V and are shown in Figure 11.

'OUT

"2ÖUT

CARRY I I I I U I I BORROW ITT I i N* IN 0 ' 8 19 10 1(1 2 0 19 1 COUNTER

FIG. 9 Up/Down Counter FIG. 11 Typical Power Supply

Figure 10 shows the use of the latch in conjunc- The power suprly for the standard oscillator Iβ tion with a Decoder/Driver and cold cathode display taken from the Power Station DC battery and reduced to tube. the required voltages using a DC/fcC converter.

134 A backup supply from the AC mains has only been used on very rare occasions.

VIII CONCLUSIONS

With the advent of Mediun Scale Integration it is economically possible to construct time frequency and time deviation equip cent for power station use using solely digital techniques. Even in the noisy environment of a power station the equipment is highly reliable. The hardware is maintenance free thus saving instrument personnel time and the complete ays ten needs only a monthly calibration check.

IX ACKNOWLED&EMENT The author wishes to acknowledge The Electricity Trust of South Australia for permission to present this paper.

pa. t. sr to to a > i a

po- jliei rated Lβ 5? ion

C —o

•5V

is •a t«

135 MANIPULATORS: AIM INSTRUMENTATION AND CONTROL SURVEY

N. Newman, B.E., M.Eng.Sc, M.I.E.Aust and K.E.Tait, B.E.(Hons), B.Sc, Ph.D., M.I.E.Aust. Postgraduate Student and Senior Lecturer, Department of Control Engineering, School of Electrical Engineering, University of N.S.W.

SDUÜRT: Current application of remote manipulators are outlined in this paper and a general comparison «lth the haw arm and hand nade and aspects of the latter which are likely to be of significance in the development of •ophistioated remote manipulators are discussed. A survey of instrumentation used to detect sensory infor- •ation for manipulators is presented, interspersed with some general comments on associated probleea. Sea« of the controls applied to manipulators, and possible control strategies are outlined. It is suggested that development of sophisticated remote manipulators will result in the extension of some areas of the supportiT* technologies. I. IHTRODUCTIOH. Considerable effort has been expended in extending Although correspondence between walking and ourrent man's capabilities into dangerous environments where methods of locomotion is not close, the function- he does not naturally belong, such as in space, the mobility is more readily achieved by the applied seas and in radiation contaminated areas. (Ref. 1) methods. For other senses and functions, auch as The methods of human extension can be broadly cate- touch, feel, temperature, taste and smell, the gorised into two groups, namely, those of environmen- development of the respective technologies does not tal support to Kan, such as diving and space suits; yet permit transfer of human senses, although many and those utilising remote control devices, euch as devices exist whereby transfer of isolated sensor space probes. The concept of deployment of a remote information in symbolic form occurs. controlled device of human form has been receiving considerable attention, as it is a compromise between Accepting that the senses of sight and hearing these two major categories. Such a device has ideally can be competently transfered in space the next most the versatility of the human but not the attendant important factor in the transference of a hand with logistic liabilities, it places the senses of a man its attendant arm. A hand, as well as providing at a remote site without physically moving him there. passive sensory information of touch, feel, tempera- It should be noted that such mechanisms are ones for ture and heat flow, also is capable of interaction huMn extension and are not programmed automata which with the environment so permitting appreciation of have similar external physical appearance and suppor- other characteristics of items in the environment, tive technology. such as mechanical compliance and mass. The full significance of the hand in the elementary learning Remote controlled devices of human form can behaviour of children was not widely appreciated currently be supplied with comparitively good means until the thalidomide mishap necessitated effort of video and audio sensing which approach the into the support of these youngsters. The hand is limitations of sensitivity and range of human the human's prime nethod of reaction with his ability. Given the sensible siting of receptors, environment. suoh as head mounted television receivers and head phones suitably supported and controlled, it is The above brief introduction to remote manipulator! possible for a human operator to project his senses is but one aspect of their use. They have existed of vision and hearing to the remote site of the since the metalworkers first used elementary tongs to television camera and microphones. The anecdote of assist them with hot metal forging. The application one researcher, who had been operating a head mounted of manipulators is as broad as is the usage of the television receiver with a 2 to 1 correspondence with ten» is widespread, the camera (that is 20 degrees of camera rotation for 10 degrees receiver and head rotation) for several i.'ani ulators assist, extend or rehabilitate human hours and short y after hie r-version to normal f'motions and may be categorised into five areas, functioning ricked his neck while attempting to look namely: over hie shoulder, (Ref. 2) gives some tenuous support to the reasonable state of technology in (a) Augmentative, those which have greater limitation! remote viewing. Various methods have been employed than the human, for exannle, forging manipulators by man to provide locomotion to vehicles such as and micromanipulators. wheels, tracks, propellers, and jets, and quite sophisticated methods of control have been devised (b) Eacilitative, those which relieve humans of and the use of remote control is by no means new. tedious tasks, for example an industrial robot.

136 ;e) Hostile environment, those which 30 where rumens muscles to achieve apparently the same action. It Is may not, for example radioactive research master- of note that the human Infant requires a considerable slave manipulators. time to achieve competence with the control of hie hands. The muscles are composed of small individual (d) Orthotic, those which assist an impaired human muscle spindles of comparitively small size, each function, for example an arm brace, apparently subject to individual control.

(e) Prosthetic, those which replace a lost human The senses used in a hand and arm have not been function, for example, an artificial arm. fully defined and the sensors employed in the Iramua frame have not been comprehensively investigated. Manipulators are essentially mechanical devices, Proximity sensing is possibly achieved by way of however, the advent of electronics and microminatur- hairs, contact sensing by means of pressure sensitive isation has caused the field to be improved by sensors receptors, and position sensing may be by yet unde- providing information for both knowledge and control fined receptors in the joints or by signals generated feedback, inbuilt control ca;>abilities, sophisticated in the muscles. Temperature sensing may be achieved signal processing, etc. Electronics has enabled some by boiling minute quantities of liquids or expansion semblance of a nervous system to be extended into of fluids. The receptors feed into a discrete system, mechanical arms. so presumably they have a threshold if used for a direct consequence/action relationship, or utilise II, THE HDMAN P10TOW7E:. pulse frequency modulation. Other derived senses such as surface roughness (tactile?), shape (feel?), Manipulators are extensions of human arms, and as meohanieal compliance, electrostatic, wind movement such an appreciation of the functions and limitations and heat flow are presumably the result of integrated ->f the hand forme a significant background to develop- processing of many and/or different individual sensors. :ental requirements. The human hand and arm are an The packaging ratio of individual sensors must be high, extremely complex arrangement of framework, actuators, for if one considers that the hairs on one arm, whioh sensors, control systems and signal channels which are comparitively insensitive devices, are capable of ar» capable of physical growth and regeneration. When interpreting contract, direotion and force, then the the hand is connected with the senses of sight and entire hand/ana complex must contain myriads of hearing and with the spinal column and brain, the sensors. overall device with its sensing, information processing and control is an extraordinarily complex The number of signal channels required to support sophisticated mechanism whioh is, in all probability, the many individual sensors and muscle spindles is several orders of magnitude more complex than the enormous. The rate of transmission of information is largest engineered eyetern brought under control by comparitively slow which may be responsible for the man. stability of the overall system. It appears that if feedback to the master control is enhanced by artif- Man has arrived at his present state by a process icial means it is possible to control an individual of evolution over an extensive period and is biologi- nerve amonjst the many millions. (Ref. 4) cally unique in several ways, one of which is the levelopment of a thumb with more degrees of freedom The control of the hand Is a complex problem which than that of any other mammal. The reason for this is still open to debate. There are apparently levels development has been assessed as due to the inter- of hierarchial control as the nervous system ascends action between Man's predecessors and the tools and the arm, through the spinal column and to the various weapons which he discovered. (Ref. 3) The rapid levels of the brain. The exact functioning of these increase of technology has tended to separate man levels of control has not been resolved. There are froa his tools, and in any case, the rate of evolution also apparently "short circuit" paths which permit could not keep pace with possible biological improve- rapid responses to dangerous situations, for example, ments dictated by available technology. reaction to excessive temperatures appears to be much faster than the speed that the nerve transmission The framework of an arm consists of bones which would permit. There is apparently no master reference are in most oases held together when in tension by system within the human 1'rane, and supply of data may ligaments and where bones move differentially they are be such that a each particular instance it is convert- often separated by synovial cavities which contain a ed to task oriented information. Conflicting informa- fluid whioh provides lubrication and cushioning. Con- tion is apparently resolved with respect to accuracy tacting surfaces of articulating bones are mostly of and the particular task. The ability to learn actions a complicated shape, the exact reason for which has and relate them to subsequent problems indicates some not been fully established. form of sub-programmed development and adaptability, and it is interesting to speculate the comparison between actions of a competent squash player and the The actuators of the arm and hand are muscles electronic hardware and software that would be needed which grow directly onto a bone, or like in some to duplicate his accuracy, speed and adaptability. cases in the hand, use essentially non-active fibres, (tendons) to overcome transmission problems round articulating joints. Since muscles can only act in The above are a fen of the major points of engin- tension, normally on« muscle opposes an other to give eering interest in the human ara and hand. At first control. Muscles mostly exert tension in such a si^-ht one is surprised at the lack of definitive data manner that several degrees of freedom are effected on the hunan bein-;, however, as one ap ireclates the and several muscles appear to achieve similar function» complexity of the body, and the considerable differ- The analysis of functioning of a human hand is diffi- ences between individuals, the reason for such a lack cult due to the number of muscles involved, and this of data becomes evident. It is of note that the ie also compounded by different people us ing different application of modelling techni-ues have apparently 137 'ssiated in the understanding of nany human Subsysteme movement about an axis as pressure is supplied to the and the use of varied electronic sensors have aided bellows. Research is being done on ooncepts of mag- the development of medical science (Ref. 5). The netic muscles and electric step motors are being used increasing involvement of engineers with medical on one artificial arm (Ref. 10). problems nay result in improvement in the rate of " understanding the natural human system. IV. SEK30RS.

III.FRAUWORK AND ACTUATOKS. The knowledge of position is of prime significant;» when controlling a remote object. The majority of If one considers the motions of the index finger manipulator systems rely on visual feedback, whether one finds four degrees of controlled freedom, namely direct or with television, and audio cues when the flexion and extension round the distal-middle, middle- manipulator strikes an object. Artificial arms have proximal phalangeal joints and between the proximal employed several devices to assist the user in deter- ph&langeal and the metacarpal and adduction and abdu- mining position of the prosthesis, and it is of inteet ction round the proximal phalangeal and the oetacar- that a certain amount of information had been gathered oal; and three degrees of limited passive freedom indirectly by the user in older types of mechanically with axial rotation about those three joints. The operated appliances, «here the applied force and move- complete arm hand corn-lex can be assessed as having ment of the operatiiig muscles was mentally converted some 60 odd degrees of freedom. To build a duplicate to an a:precistion of appliance position. Thla point of an arm/hand with respect only to ths degrees of was emphasised by the first use of externally powered freedom, would tax the ingenuity of the framework and prostheses when degraded sensing by the intact portion actuator designers equally as well as that of the of the anatomy occured. The mechanical moments of th» control system designer. As consideration of frame- prostheses as exerted on the intact human body are alio work and actuators dictate approaches from the control of relevance in establishing position information. area it is thought necessary to briefly review aspects Vibrating electrical reeds have been used to indicate of framework and actuators. elbow position to the arm stump, apparently with good improvement for the "ulind" control of arm prostheses Two general approaches to design occur in frame- (Ref. 11). One prosthesis uses movement of the scapula works, namely function and appearance. In both cases to direct prosthesis movement and close feedback is ohese often overriding requirements may dictate poor used to pass position information back to the control- performance from other attributes. A heavy manipula- ling scapula movement. (Ref. 12) Master-slave manip- tor may have little correspondence with the operator's ulators do have a position correspondence, however, as natural capabilities and he nay require extensive the prime function is force reflection position cor- training to use it. In achieving satisfactory cosme- respondence can be upset under dynamic conditions. tic appeal in an artifical arm it may become funct- Measurement of relative positions of arm and hand seg- ionally difficult to use. ments to satisfactory standards of accuracy ie, in itself, not a difficult instrumentation task. However, Fin or journal joints appear to ne used exclusive- the penally of sensor cost, weight and complexity ly, possibly because of their low cost and ease of indicates that the ability to sense position by use of oonstruction, especially -.»hen rotary actuation ie used, the actuators ecrl yed is of engineering concern. Step •"he linear relationship of sear reduction is of motors are being used in one prosthetic arm, and may rarariount significance. prove of benefit in the position sensing regard, however one of the requirements of a manipulator is con- Special arm geometries have been developed to sidered to be the ability to stall and even back drive ease the control problens, an exariple is a prosthetic without damage of components or loss of information. arm whose inherent geometry ensures thit a held glass of water remains upright for norial ranges of move- Although there are many proximity sensors available, ment. (Ref. 6) As may be expected, the achievement such as capacitive, inductive and pneumatic, no manipu- of one functi -n »ill tend to -leßrade the general lators have apparently yet used any.. Considering the purpose use of the «ihole system. state of development and recourse to visual sensing as the primary control, this is not surprising. Limitations of space to contain sensors, actuators and power supplies can im; ose considerable restraint Contact switching, which does not, in essence, onthe framework design. Hon-human forms of framework exist in the human prototype, have been In several are hydrosfcatic cylinders which are used to extend applications to initiate actions of the manipulator. and contract a power manipulator framework as a »hole One a,.-plication is in the provision of a sensing and an "elephant trunk like" prehensile am (Ref. 7). indication to en anaesthetic hand, that is impaired sensing, whereby a mercury filled thin tube is coiled Host types of actuators have been useä in oae form to form a glove, when the hand touches an object the c# another, hydrostatic transmission is favoured in mercury path is broken, consequently an indication is large power manipulators while electro-nechanical is jiven to the wearer to exercise caution so as not to favoured in smaller master-slave manipulators. damage his hand. (Ref. 13), Automata manipulators Although electric Biotors are used in some prosthetic use contact switching to direct movement of jaw, or of hands these seems to be a -eneral use of pnuematics the manipulator as a whole, especially for obstacle for some of the more r-eeirt complex ams. Several avoidance, llaster-slave manipulators will sense coat" ttnique pneumatic actuators have been developed for act with an object. orthotic and prosthetic use, for exanple the I.cKibben Muscle, (Ref. 8) this is essentially a tube which If indication of surface roughness is synonymous "if* balloons when gas is admitted and causes tension the tactile sense, then there are a variety of displays forces to be develoned alon;; the long axis, and the mechanical, pr.euumatic and electrostatic, employed in Israeli 3ellows, (=.ef. 9} which jiveo rotational the assistance to blind persons, which may be of use is

138 the recovery of tactile information (Ref. 14). However^ electrodes with either aotive or passiv« radio oontaot most of the current devices appear to be discrete, with external devices. (Ref. 18) although the packing of receivers can be quite high. No reference can be found to any sensor device used V. CONTROL. to detect tactile information for use in manipulators Of note is the use of a gramanhone cartridge to sense Ideally the control systems desired for a reset* slippage of a held object in one prosthesis, as the manipulation system would be such that the operator object slips through the fingers it generates noise would not be aaare of their existence, his hand would »hich is picked up by the cartridge, amplified and be transposed to the reraote location. However, prac- used to hold the force of the fingers at a sufficient ticalities and economics dictate what can be done« level. (Ref. 15) As surface roughness is apparently (Ref. 19) In the long term information is required sensed by finger movement over the surface when a to be either passed in an analogue fora fron the re- degree of accuracy is required, then perhaps develop- mote site to the operator, for example temperature ment of the cartridge idea may sufice for this sense. sensors to temperature recievers, or the information needs to be transposed into biologically equivalent Although the sensing of force in a human can be signals to be processed by the operator by bypassing perceived on the small scale, auch as in the tactile his sensors. The variety of visual display* for snese of the hand and with hairs, its detection on informing operators of the state of portion of a the large scale is not clear. It has been suggested system is considerable; however, if the operator that it might be due to sensors within the joints, requires his full vision for transfer to the remote by strain measurement within the bones, or by infor- site, then current display iectaology ceases to be mation processing of the many small skin sensors. As appropriate. It must be anticipated that manipulation implied with the preceeding paragraph there are requirements will initiate specific investigations apparently no small scale sensors in use. There are into suitable information displays. several large scale devices used, one of the most simple is that of BODSSO who used a small sheet of polarised material to detect the stress in a hook The large number of sensors required in an ideal prostheses, this gives a visual feedback to the wearer system are unlikely to be duplicated for a consider- ss to the forces being used on the object held in the able time, and the problem of transmitting and pro- hook. (Ref. 9) TOMOVTC with hie artificial hand cessing a very lar^e amount of information soon initially used pads of a carbon mixture to regulate becomes apparent as the number of sensors increases. the automatic grip on the prosthesis, he later used The number of channels for information transmission pressure sensitive paint. (Ref. 16) Other artificial rai>idly becomes exceseive. The human hand is subject hands have used devices based on the piezoelectric to a complex arrangement of hierarchical control a« principle, Force reflecting master-slave manipulators the information passes from the sensors to the nerves, connect the corresponding motions so that forces are to the spinal column and to the brain. It may be reflected between the motions of the operator and tV.a possible to identify factors and transfer informa- master to the slave and the object. tion in symbolic form to other sense receptors on the operator, for axanple with very few sensors it may be If the sense of touch is equated to sensing of possible to identify the size and shapes of objects shape, then it could be artificially detected by an and transmit the information to a sensor array on the array of position sensors. The developing art of operators back which informs him separately of eise pattern recognition may prove of significance. Dev- and sharie. elopment of stress patterns on a sheet of suitably polarised material is an interesting visual display Certain reflex actions, which in the human body nay device. be controlled from the spine, or even earlier in the nervous chaia, have been designed for some prosthetic Ho known use is made of the various temperature appliances. Investigations into prehensible actions sensing devices to indicate the temperature of a man- of the thumb gripping action has revealed that the ipulator. Temperature sensitive paint, thermocouples human system adjusts the ^rip to some 110 per cent of etc could possibly be used without the need fox sepa- that needed to prevent slipping, and this is presumably rate development. carried out by a process of trial and error. (Ref. 12) It would seer,: appropriate that a remote manipulator It is expected that heat flow, when in contact system should have relex actions incorporated; however, with an object, would be established to sufficient this >>oint is debatable and to a certain extent re- acouracy by processing temperature information with volves around the time delay between the manipulator respect to time. and the operator.

Sensing of the direction signals of the brain in Assuming that the mechanics of an operators brace the form of eleotromyography has achieved considerable and a remote nanipulatcr arm would be of significant importance in both research and diagnostic techniques. mass and inertia there is a rejuirement to remove the Slectromyograohy is also used to control prostheses, mechanical interactions of these sechanisms from the orthotic devices and anti-G manipulators. (Ref. 17) operator. This is seen as the major disadvantage with Electrodes used for these purposes have developed from current master-slave manipulator systems especially implantable electrodes inserted into the muscles and under dynacic conditions. Reduction of operator electrically attached to the apparatus, increase in fatigue by reflecting only portion of the force of the amplifier technology permitted surface electrodes to system statics and dynamics also reduces sensitivity. be used. The problems associated with surface elec- One early heavy hydraulic master-slave manipulator trodes, such as cross signal sensitivity and movement reduced some of the static forces by suitable compu- of Bkin with respect to the muscle may be overcome with tation. (Ref. 20). current developmental work on surgical i

139 One orthotic research aid has used a computer to ACKNOWLEDGaCENT. store set piece programs for an assistive manipulator to carry out specific tasks. (Ref. 21) A parallel is made to the assistance of •sabers of the with elementary learning theory is of note. The Royal Australian N&vy and the University of lev ideal would be programs which were adapted by exper- South Wales in the preparation of the paper. ience as refinement of operation occurred and flexibility to apply these programs to slightly different REFERENCES. situations. The device in question suffered from a time lag between action by operator and commencement 1. JOHNSEN, E.G. and CORLISS, V.R. - of selected operation. Applications in Teleoperator wlnn ^ Hew York, Wiley-Xnteracienco, 1971, 252p, If scaling is required for micromanipulation or amplification then scaling of some factors may 2. NASA - lanrove the sysoej? for its particular use rather than NASA SP-5O81, 1970, 243P. i 1! direct scaling of all dimensions. This point leads on to correct analysis of the task of the manipulator. 3, NAPIER, J. - The Evolution of **"» HVtf - 3CIHB- Manipulators are at present more adapted to special IITC AHERICAH, Vol. 2077 *>• 6, SsoSber, 1962, purpose tasks rather than general purpose ones, pp. 56-62. whenoe specification of attributes for the tasks in 2( question results in devices which are not facsimiles 4. of the hunan band. he Isolati Circuits. AUERICAH J. PHTSIC » Toi. 4} Coneious control of many degrees of freedom of Ho. 6, Decssibsr, 1970, pp. 348-361. f 21 both prosthetic and orthotio devices has not proved entirely successful and later work seems to be dir- CASS, N. et. al. ede. - RU—t of the Math eoted towards task oriented control such as holding ate an object steady with respect to a reference and ioal Engineering. Melbourne, 1971, 279p. •oviag the terminal device using a simple co-ordinate system. However, it could quite readily be argued 6. ENGER, S. - The Baals for a Prosthetic Shoulder that the problem is essentially one of correctly Analogue **nd a View of 7pp»r—^»fr F""ctlon. interfacing the device with the operator. UEDICAL ABB BIOLOGICAL ENGIHEERIHG, Vol. 5, 1967, PP. 455-462. The use of interlocks and overriding controls may bs expected in a sophisticated manipulator. The use 7. ANDERSOK, V.C. and HORN, R.C. - Tensor Ar» of these devices is not in itself difficult; however, Manipulator Design. ASME Paper 67-BE-57, "1967. the correct definition of the problem, especially if it is complex, is likely to prove far from easy. GEDDES, L.A. and HOFF, B.E. - Artificial Ifasoles. SLIDE HÜLS, Vol. 22, No. 10, February, 1962, The ability of a human to open a door handle and pp. 10-11. door is often quoted 83 an example of a task which is almost impossible to achieve with a manipulator, BODSSO, D. - A Six as manipulators have not the ability to adapt them- Lieb for Thalidomide Children. BIO-JSEDI selves to exerting forcee against the best line of ENGINEERING, Vol. 4, No. 7, July, 1969» PP. 31> mechanical resistance and often, in the case in point, tear the door bodily from its hinges. The ability of the hand to adapt to tasks may not be 10. STEIN, A. - tfefoelectrlc Control Systest for Ara- entirely capable of definition, and one may need to Hand Prosthesis. ELECTRONICS LETTERS, Vol. 7, investigate the operation of a facsimile in order to No. 10, Kay 20, 1971, P. 334. define in engineering terms the various operations. 11. KANN, R...'. and REBÜERS, S.D. - Kinesthet VI.COHCLUSION. in/r for the EMG Controlled 'Boston Am'< TRAKS. 1IAN-H&CHTNE SYSTEMS, Vol. 1IUS-11, Ho. 1, Extending mans sphere of activity by remote man- llarch, 1970, pp. 110-115. ipulation is at an early stage of development in comparison to the prototype of the human arm. The teoh- 12. Inst. Ueohanical Engineers. - Symposia« on the nology of remote manipulation can be regarded as one Basic Problems of Prehension. Movement and Con- of application which derives its background from many trol of Artificial Limbs. PROC. I. HECH. E., more specialised technologies and it is consequently Vol. 183, It. 3J. 1968-69. limited by the limits of the component technologies, ipplioation of instrumentation and control are poss- 13. PFEIFFER, E.A. et. al. - An Experimental Device ibly the two areas receiving most concentrated to Provide Substitute Tactile Sensation fro* the efforts at the present time. The limitation« of Anaesthetic Hand. 1EDICAL AND BIOLOGICAL ENGIN- applied technologies relevant to remote manipulation E2RSJG, Vol. 7, 1969, pp. 191-199. will be extended, in some cases by specific investigation. Although it may be some tine before remote 14. iX£, P.if. and BLISS, J.C. - Sensory Aids for the manipulation technology becomes of significance, Blind: A Challenging Problem with Lessons for perhaps one of its most beneficial results will be the Future. PROC. I2EE, Vol. 58, No. 12, the cross fertilization it will engender between December, I97O, pp. 1878-1898. distinct technologies and, one hopes, help in the overall advancement of technology. 15. IlAAS, li.A. - Phonograph Cartridge Senses of Artificialländ. Ho. 23. 1966. pp. 12-H. 140 16. TOMOVIC, R. - Artifioial Hand Responds to Touch. ELECTRONICS, Vol. 35, No. 20, Kay 18, 1 pp. 76-82.

17, SCOTT, R.H. - Myoalectri«yoale c Control of I- ARCHIV3S PHYSICAL :.ü2DICTE AIJD REHABILITATION, Vol. 47i No. 3, March, 1966, pp. 174-181. 1Q. HERBERTS, P. et. al. - Implantation of Micro- Circuita. for liroalectric Control of Frostheeas. J. BORE ADS JOINT SORGST, Vol. 5O-B, Wo. 4, Noveober, 1966, pp. 780-791.

19. GAVRILOVIC, U.V.. and BEHHETT WII£0K, A. «da. -

itiea. Belgrad«, Togoalav Coonittee for Bleo- tronios and Automation, I970, 6O8p,

20. UOSHER, R.S. - Aa Bleetrohvdraullo Bilateral ator. THANS. JUZERIGAN NUCtSAH , Vol. 3, Ho. 2, I960, pp. 482-483. 4) 21. BAHNITOC, E. and V-1JTBCHEK, Jw.J. - aatars for an Orthotic Arm Aid. iper 63-WA-282, 1963, 12p.

141 THE PRECISE MEASUREMENT OF of the induction principle which enabled statle mag- WEAK MAGNETIC FIELDS netic fields to be measured. The method was vised for the detection of submarines during World War II, and since then has been fully developed as a most Ronald Green, B.Sc, Ph.D., A.M.I.R.E.E. useful tool in exploration geophysics. Again only John M. Stanley, B.Sc. a relative measure of the field was obtained. The device was directional, measuring only the component of the field resolved along the orientation of the sensor. Resolution was typically of the order of 50 micro Gauss. A bibliography of fluxgate magnetometers has been published. (Primdahl, 1970).

Various less well known principles were investigated but it was not until the discovery of nuclear magnetic resonance by Purcell and Bloch in 1946 that a new era was opened in the domain of accurate magnetic field measurement. This phenomenon linked accurately and linearly the value of the magnetic INTRODUCTION field to a circular precession frequency which was a characteristic of an atomic nucleus. The measurement of magnetic fields is a matter of considerable Importance. The extremely widely differ- The "Proton Precession" magnetometer was the ing magnetic properties of materials k—oduce a varied first successful adaption of this principle. The degree of perturbation of the Earth's natural magne- magnetic moment of the protons contained in a fluid tic field. Measurement of this perturbation provides such as water or benzine were aligned approximately a means of remotely sensing the presence of a foreign East-West by the application of a strong magnetic body having magnetic properties different from its field in that direction. On the removal of the environment. This has immensely varied application, polarising field, the aligned magnetic dipoles pre- ranging from the mapping of mineral orebodies to the cessed in unison about the Earth's field and could location of buried archaeological artifacts; from be used to induce a voltage at the precession fre- detecting the inclusion of foreign matter in food quency in a coil wound around the container of the containers during processing, to the tracking of fluid. The frequency was directly proportional to enemy weapon carriers. In addition! much of our the field intensity, linked by a physical constant knowledge of the Earth itself and the Sun and Solar known as the "gyromagnetic ratio" of the proton. System Is based on magnetic field measurements both from the Earth's surface and from satellites in space. For many years this instrument was the standard reference for magnetic measurements in both the It is thus not surprising that much effort has field and the laboratory. It measured the absolute been directed toward the improvement of magnetic value of the total field vector to an accuracy of field measuring devices. The magnetic fields of con- the order of 5 micro Gauss. Its principle limita- cern are mostly less than one Gauss in intensity and tion however, was that before each measurement are thus termed "weak" with respect to fields pro- could be made, the protons had to be realigned, a duced in the laboratory, or contained within atomic process taking 2 or 3 seconds. Thus time variations nuclei. The Earth's magnetic field, for example, is of period less than about 5 seconds could not be typically 0.5 Gauss. recorded, a factor greatly limiting the instruments versatility, particularly In the domain of aerial One of the first magnetometers to be developed magnetic surveying. for geophysical work was the "Schmidt Balance". In principle it consisted of a delicately suspended In 1957, Dehmelt (Dehmelt, 1957) suggested a magnetic needle. The maximum torque tending to align means of optically detecting "electron nuclear this needle with the Earth's magnetic field gave a resonance" in an alkali vapour. The method was measure of the magnetic intensity. Such an instru- adopted by Bell and Bloom (1957) at Varlan Associ- ment was limited to a sensitivity of 10 micro Gauss ates who produced the first "optically pumped and was prone to drift. The device was only capable alkali vapour magnetometer". Since that time con- of giving relative field values. siderable development has taken place principally by Varian in the United States and at Thompson Induction coils consisting of many thousands of C.S.F. in France. Both these companies market com- windings on a ferrite core have been used where mag- mercially available instruments for ground and netic fluctuations of frequency ranging between aerial application. The instrument typically has a about 1 and 50 Hz were to be recorded. The threshold sensitivity of 0.1 micro Gauss over a dynamic range of sensitivity may be typically 5 micro Gauss. In- of from a few hundred micro Gauss to nearly one duction coils have been used to measure time varying Gauss and will respond to magnetic field fluctuation» "magnetotelluric" currents within the Earth's crust as rapid as 200 Hz. (Usher, et al., 1964). and to measure the magnetic field from space satellites. In the latter case the spin of the satellite Because it is the most versatile of the now com- provided the tine variation of the field necessary mercially available magnetometers, the alkali vapour for the instrument to operate. Integration then made type has been selected as the prime example In this the value of the field directly available. (Stefant review. However, magnetometer development is not 1963, Grlvet et al., 1961). yet static. In 1964 the first 'practical SQUID (Super Conducting Quantum Interference Device) was The "fluxgate" magnetometer was a modification operational. (Lambe et al 1964). The prototype had a short term seneltivity of 0.1 micro Gauss over a magnetic dipole radiation with the selection rule dynamic range of 0.01 Gauss to 1 mill! Gauss, but It that AF • 0,+1-only. Zeeman transitions (between was hoped that eventually a sensitivity of 10~9Gauss different mp levels) are similarly constrained by might be reached. Miniature helium refrigerators the rule Amp « 0,±lonly. Let us now consider what have been developed for use in satellites and it happens to such an electron when illuminated with seems that the SQUID might find its greatest applic- a beam of circularly polarised Dl light. ation in space research. The circular polarisation of a light beam THE PRINCIPLE OF OPTICAL PUMPING implies that all photon angular momentum is either parallel or antiparallel to the direction of pro- The expression "optical pumping" was first used pagation. If the light Iβ "left" circularly polar- by Kastler (1951) to describe the assembling of ised for example, then the photon angular momentum atomic spin orientation.* into a non-equilibrium dis- is parallel to the direction of propagation and its tribution in which the majority of spins were absorption by an alkali electron must lead to a net orientated in a particular direction from out of an gain of one unit of angular momentum provided the equilibrium distribution. To describe the pumping external magnetic field Is also parallel to the procedure, the notation of optical spectroscopy has light beam. In this case we have restricted still been adopted. Vector quantities are represented A further the selection rule constraining the Zeeman and the corresponding quantum number, A. transitions. When left circularly polarised lighl. interacts with an electron, mp may change by +1 Alkali atoms have a single valence electron, only. (Similarly, for right circular polarisation, located in the,2 S^ ground state. Transitions between Amp - -1). Hence, if only left circular polarised this and the first excited level involve energies In Dl radiation is present, its absorption can only 2 the region of optical wavelengths. Since the first lead to a P£ excited substate with Increased mp. excited level is a F level with orbital angular The Doppler effect will, in general, allow trans- momentum TL - 1, and the total electron spin j>. can itions to either of the J_ states provided the only be ±%, there are in this level two possible selection condition on mp remains satisfied. When values of the total electronic angular momentum the electron relaxes to the ground state, the Is. (1 = k + i> hence J = 3/2 and 1/2). These two normal selection rule Amp - 0,tl applies, as the states of the P level differ slightly in energy and - relaxation is independant of the means of excitation. electron transitions from them to the ground state As these three transitions are equally likely, there give rise to the familiar doublet observed in the is a probability of 2/3 that the electron will spectra of the alkali metals. The two components of return to the ground state with a greater value of mp. i.e., a greater component of spin along the this doublet are designated D^ and D2 corresponding reference axis than it had previously. to J = 1/2 and 3/2. (Figure 1). The existence of nuclear spin J_ further divides this doublet Into a number of substates known as the "hyperfine" states. The atom may absorb and reradiate repeatedly (Figure 1, 2nd column). Each of these hyperfine with the result of a net migration of valence states has slightly different energy arising from electrons to the ground state having greatest value the interactions of the electron with the nuclear of mF. In the case of rubidium 87 this is the magnetic dipole moment and the electric quadrupole £•2, mp«2 substate. It can be noted from Figure 1 moment. The different energies of this hyperfine that in the 2P£ level, there does not exist a sub- structure arise from the discrete number of quantum state having mp»3. Hence, if only left circular mechanically allowed orientations of the nuclear polarised Dl radiation is present to energise these angular momentum !_ relative to the total electronic optical transitions, then electrons in the mp»2 angular momentum J_j_ As dicated by the rules of level of the ground state cannot be excited further quantisation, the allowed orientations are such that by absorbing the Dl photon. They have been "pumped" the total angular momentum F_ is restricted to only into an unnatural condition of maximum spin align- the values 1? » J+J» I+i-1» • • • Irl- °* particular ment. Provided there is no mechanism enabling relevance to the optically pumped magnetometer is a relaxation from mF"+2 to a lower value, photons still further splitting of these hyperfine states from the "pumping light beam can no longer be when a weak external field is applied to the system. absorbed, and the alkali vapour has become trans- (Figure 1, 3rd column). This phenomenon is the well parent. known Zeeman Effect and is valid for external fields of up to about one Gauss. The splitting arises from Depumping from the ordered substate can be the different energies associated with allowed brought about by several means. For example, from orientations of a given £ with the external field H.. the previous discussion it is apparent that If the The only allowed orientations of £ are those which direction of the external field is reversed, the give projections mF along the Ü direction, where sign of mp (the component of F_ along the field) mp « F,F-1,...-F. Conveniently, the energy differ- must also be reversed. Hence the ordered electrons ence between the Zeeman levels is directly propor- are now in a level of negative angular momentum tional to the magnitude of H in the weak field limit. from which they may again absorb photons from the Figure 1. depicts the ground and first excited state pumping beam. (Dehmelt, 1957). electron energy levels of an alkali atom with nuclear angular momentum l_ » 3/2, and their subdivision Alternatively, should there be presenr quanta in the presence of a weak magnetic field. Rubidium of radiation having energy equal to the transition 87 for example, has a nuclear angular momentum of between Zeeman levels, then under certain conditions 3/2. these may interact with the aligned electrons, returning them to a depumped state. The frequency Hyperfine transitions (between substates of of this radiation is known as the "Larmor Precession different F_) involve the absorption or emission of Frequency" of the ground state electron. It Iβ

143 directly related to the separation energy of the detected with a photocell, and the signal suffic- Zeeman levels and is hence also directly proportional iently amplified and suitably phase shifted before to the external magnetic field. It is thus the quan- being fed back to the depumping coll, then a closed tity to be measured if the magnetic field is to be loop self oscillating circuit would result. The determined. resonance must necessarily be at the Larmor frequency if there was to be optical coupling to close the If we monitor the intensity of the pumping light loop. The Experimental Plot of Figure 4 shows the beam passing through the absorption cell as an orientation dependence of the signal amplitude from electromagnetic field is swept through the Larmor a Caesium self oscillator having the depumping r.f. frequency, (Figure 2a) a minimum will be observed. field parallel to the pumping light beam. This corresponds to the reabsorption of photons from the beam after the cell has been depumped on every The Sensor. cycle of the electromagnetic field at the Larmor frequency. (Figure 2b). In principle this is how Figure 3 illustrates a single cell sensor we determine the magnetic field. assembly. The light emitted from a spectral lamp was focused Into a parallel beam before being In this discussion it has been assumed that passed through a narrow band dielectric interference there is no mechanism enabling the spontaneous spin filter at the 01 wavelength of 894.4 run. The relaxation of the aligned population. In practise filter was designed to effectively block the D2 this is not the case and collisional processes bet- wavelength of 852.0 nm. The Dl light was then ween different atoms of the vapour, and with the circularly polarised before passing into a cylind- walls of the container in which the atoms are held, rical, temperature stabilised absorption cell may cause substential mixing of the different mp containing partial pressures of caesium vapour at 6 leveis. This phenomenon has been the centre of 5xlO" torr (35*C), a krypton buffer at 12 torr and attention of many researchers and there has been a nitrogen quenching agent at 8 torr. (Clack and much publication of theoretical and experimental Stanley, 1971). Transmitted light was focused onto findings corresponding to varied alkali vapour en- a silicon photovoltaic diode operated as a current vironments. Pottier (1968) and Violino (1968) are source. The r.f. coil providing the HI (depumping) recent examples. The particular requirement of an field was wound around the absorption cell. alkali vapour magnetometer is that a high degree of orientation be quickly obtained and that spontan- A bifiler heating coil was wound onto an alumin- eous relaxation be slow when considered relative to ium sleeve which fitted over the HI coil and the Larmor frequency. A simple and reliable tech- absorption cell. The aluminium served to eliminate nique for the manufacture of absorption cells capacitive coupling between the heater and the specifically for alkali vapour magnetometers has windings of the HI coll, and also to uniformly be»n described by Clack and Stanley (1971) and distribute the heating. A thermistor located on Stanley (1971). They employed a heavy noble gas the cell surface provided an error function to (krypton) to act as a "buffer" gas to reduce Che control a proportional a.c. heater circuit. The number of alkali/alkali (caesium) and alkali/glass effects of thermal changes in the vapour cell collisions and combined this with nitrogen having principally arose from Doppler broadening of the the property of "quenching" the relaxation radia- spectral line. The observed variation in the phase tion which might otherwise interfere with the of the feedback with temperature, suggested that pumping process. Collision of the aligned alkali the sensor had a limiting thermal coefficient of with either of these gases will not usually result about 0.5 micro Gauss per degree C. (Usher and in disorientation. Stuart, 1970). A thermal stability of 0.2'C was thus required to achieve an instrument sensitivity THE SELF OSCILLATING TYPE CAESIUM VAPOUR of Q.I micro Gauss. MAGNETOMETER. The components of the sensing unit were insul- Two approaches may be taken in the utilisation ated with polystyrene foam, and encased in a rosin of optical pumping for a magnetometer system. Des- bonded cylinder of 3 inch diameter and 7 inch cribed below in detail is the simpler of the two. length. It is this type which has been built in the Depart- ment of Geophysics at the New England University, The Spectral Lamp. Armidale, N.S.W. for mineral exploration purposes and for installation in that Department's "Cooney The spectral lansp is probably the most critical The Underground Geophysical Observatory". (Green and component in systems utilising optical pumping. Sydenham, 1971). Figure 3 represents the components The output intensity must be stable from d.c. to of the completed instrument. at least an order of magnitude above the maximum Larmor frequency. The most satisfactory lamps at Principle of Self Oscillation. present in use consist of a small glass ampoule containing approximately 0.1 mg of natural caesium It has been shown that when an alternating in a krypton buffer gas at 1 torr pressure. (Clack electromagnetic field of the Larmor precession and Stanley, 1971). This lamp was energised into frequency was applied to an optically pumped vapour discharge by an r.f. excitation radiated from a small | cell (as in Figure 2), Che absorption character- coll about the ampoule. This coll formed part of let ice of the vapour behaved very much like a the tuning circuit of a single transistor power partial shutter, blinking open and nhut at the oscillator. The firing of the lamp was aided by Lannor frequency in phase with tin- periodic pumping strong electric fields, but the discharge was main- and depumping of the vapour. It then followed that tained more stably by high frequency magnetic If tlil» modulation of the transmitted light beam WAS excitation. An oscillator circuit capable of deliv-

144 I

er lim approximately 3 watts at 130 MHz is In present The Counting and Recording Circuits. use. The impedance of a gas discbarge tubo Is highly Jependant upon the bulb temperature and the contained The design of counting logic for the magnetometer vapour pressure. Forming part of the tuned circuit was determined largely by the intended use of the as the bulb does, its temperature must be stabilised instrument. For field use where a digital display if uniform impedance and discharge is to be maintain- in magnetic field units, and a hold time of a few ed. Ideally, the bulb temperature would be independ- seconds was desirable, frequency counting was em- ently stabilised, but heat resulting from the r.f. ployed. But for maximum resolution with a rapid discharge can conveniently be used to provide the sample rate suitable for analogue recording only, a necessary thermal control. In practice, satisfactory period counting circuit was simplest. Both Systeme results were obtained by limiting the power deliver- are described. ed by the oscillator, with a thermistor located on the bulb surface. Lamp ignition was assisted by the (a) The Frequency Counter: In this system the application of maximum power when the thermistor was magnetometer signal cold. was counted for a precisely known time determined by an internal reference oscillator. Indeterminacy of Another design consideration of importance was the counting procedure allows for an error of ± 1 Hz the arrangement of thermal gradients across the bulb. of the magnetometer signal. In a 1 second counting Excess alkali metal must be inhibited from condensing period this corresponds to a resolution of ± 2.8 on the front surface of the bulb, from where it can micro Gauss. To make full use of the resolution of mask the light output. To minimise self reversal of the sensor would therefore require a counting time the pumping radiation, it is important that Dl phot- of about 28 seconds. In practice, this time can be ons should not have to pass through cool regions of reduced by multiplying the magnetometer signal fre- gas where reabsorption may take place. The suitable quency. design of the bulb shape, and provision of a thermal sink at the rear of the lamp have overcome these The principle advantage of the frequency counting problems. system is that the Larmor frequency may readily be converted into magnetic field units by the suitable The Loop Electronics. choice of counting period. In a 1 Gauss field, the Larmor frequency for caesium is 349,869 Hz. Hence , The amplifier for a self oscillating system was a gating time of 2.85821 seconds corresponding to 10 required to have sufficiently high gain to compensate cycles of a standard oscillator tuned to this fre- for the very low level of modulation of the optical quency, will pass that number of cycles from the beam. Experience has shown that the most efficient magnetometer corresponding to the magnetic field in application of the silicon photovoltaic diodes under units of 1 micro Gauss (i.e. in a 1 Gauss field, the conditions of low light intensity, is their use as magnetometer will count 10^ also, but in a 0.5 Gauss a current source. For caesium in the Earth's field, field, the Larmor frequency will be reduced by one the Larmor frequency extends over the red end of the half and the cycles counted in 2.8 seconds will only commercial broadcast band, and the removal of radio be 500,000). interference was a vital design consideration. Where an accuracy of 0.1 micro Gauss was desired A low input impedance band pass amplifier was with a reasonably short counting time, it became built consisting of four cascaded common emitter necessary to multiply the magnetometer signal by a stages with feedback. The transfer Impedance of the factor of 16 using four frequency doubling stages. circuit was nominally 105 volt/amp and constant phase The signal then corresponded to 5,597,904 Hz per was maintained over the working frequency range. A Gauss. A standard oscillator at this frequency will preset variable phase circuit followed enabling the gate 107 Hz in 1.78638 seconds, and each count will r.f. field in the feedback coil to be set 90* out of represent 0.1 micro Gauss. Automatic recycling of phase with the optical modul tion. Self oscillation the counting was used and each value digitally dis- will normally occur within 30* of optimum but a shift played for nearly 1.8 seconds while the next count in the resonance frequency will result. A monitor was taking place. output was obtained via a buffer stage. During self oscillation a signal to noise ratio of 20:1 was In this instrument the Binary Coded Decimal from typical with a 1 volt peak to peak signal. the buffer stores of 3 consecutive digits (selectable) was transformed to analogue with a monolithic The Thermal Control Unit. digital to analogue converter. With this arrangement the highest recordable frequency on maximum The absorption cell in the sensor unit was therm- sensitivity was that of 5 second period. ally stabilised to 35° ± 0.1°C by a proportional heater. To eliminate perturbing magnetic fi-lds A disadvantage in the use of frequency counting from the heater windings, a 50 foot bifiler element is a spurious modulation of the recording caused by was heated with alternating current at 500 Hz. The 50 Hz magnetic fields which were derived from un- circuit consisted of a twin Tee oscillator, with a balanced mains electricity. (Stuart, et al., 1967). thermistor controlling the current gain in a buffer stage. Output from the oscillator was fed into a (b) The Period Counter: Where analogue record- conventional quasi complementary, class B audio ing only was required, amplifier capable of delivering 10 Watts through an simpler logic was obtainable by counting the stand- 8 ohm load. ard oscillator signal during the time taken for a precisely determined number of magnetometer signal cycles to occur. The error in counting time then depended upon the signal to noise ratio (R) of the

145 magnetometer signal, amounts to T/2uR where T is the period of the signal. Considering the worst value f - (10S) drifts The battery supplies were zener stabilised to the of sensors has presented a more complex problem. operating voltages of +24 volts for the lamp and Allen (1968) assessed the results of 17 months of heater, ± 15 volts for the loop electronics, digital comparison between a proton precession magnetometer to analogue stage, crystal oscillator and oven and and a series of self oscillating single cell rubid- + 5 volts for the counting logic. Cell capacities ium magnetometers. He concluded that an undisturbed were designed to provide 8 hours continuous operat- sensor may drift over a range of 20 micro Gauss and ion. that a range of ± 2 micro Gauss could be expected over a one week period. Absolute Accuracy and Stability. There are a number of reasons for long period The absolute accuracy and stability of an a.v. drift of an a.v. magnetometer. From the electronic (alkali vapour) magnetometer were best considered components we can expect phase changes as reactive separately. The absolute accuracy was dependant upon components alter value. Photocell capacitance two principle factors. The first of these concerns being a function of current and hence lamp intensity, an asymmetry of the resonance line related to the may introduce phase drift in the loop. Phase effects "Back-Goudsmit" effect which disturbs the linearity can be of the order of 1 micro Gauss per degree. of the Zeeman energy levels with respect to the Behaviour of the alkali vapour is prone to change. magnetic field. One of the consequences of this is Linewidth is dependant upon the pumping light that the Larmor frequencies of the different Zeeman intensity, vapour pressure of alkali, buffer press- components are not coincidental, and for caesium in ure, cell dimensions and vapour temperature, all of a 0.5 Gauss field there is a variation of 6.7 Hz. which are liable to change. While endeavour has This difference is less than each line width so that been made to control these factors, the findings of the spectrum is not resolved. The optical pumping Allen may be attributed to their presently remain- procedure produces an Inequality between the ing summed effect. different components and an asymmetry in the overall line results. The sense of the asymmetry depends upon the sense of the polarised light, which in turn, is equivalent in the pumping operation to the ALTERNATIVE ALKALI VAPOUR MAGNETOMETER SYSTEMS sense of the magnetic field. Hence the rotation of the sensor about the magnetic field direction pro- The simple single cell, self oscillating mag- duced a change of 6.7 Hz to the Larmor frequency in netometer has been shown co have failings due a 0.5 Gauss field. That corresponds to 19 micro principally to either phase loop problems or asym- Gauss. The second factor influencing the absolute metry of the spectral line. The French have made accuracy of the self oscillating sensor was phase greatest progress in overcoming these difficulties lag around the loop. It has been found that self by their development of the following magnetometer oscillation will occur provided the depumping field systems: is within about 30* either side of optimum. But the resonance frequency was effected by approximate- The Phase Locked Magnetometer. ly 20 Hz corresponding to 70 micro Gauss over this range. Consquently, each sensor must be calibrated Figure 5 represents the components of the phase by adjusting the variable phase circuit provided, if locked magnetometer system. An rf oscillator was absolute accuracy is required. This problem lias frequency modulated at an audio frequency ft so as also been overcome by use of the alternative sensor to scan about the Larmor frequency. The trans- systems. mitted light beam through the absorption cell was consequently modulated at ft. After amplification, the phase of the detected signal was compared with The maximum sensitivity of the instrument was the initial modulating signal to determine the Halted by the short term stability of the sensor, position of the mean generator frequency Vg with which was in turn, determined by Internal electronic respect to the niddle of the resonance line. The noise. Usher and Stuart (1970) gave an express ion synchronous detector generated an error signal for the rms fluctuation 6f in terms of the Zeemar. which was used in a control loop to maintain the llnewldth <5v, the measuring bandwidth W, the ampli- generator frequency Vg in coincidence with the fier bandwidth w and the signal to noise ratio of Larmor frequency. This system overcame the phase the electronics R, within the bandwidth. They problems encountered with the self oscillating loop. (Grivet and Malnar, 1967).

146 Hultibeam Sensor. APPLICATIONS It has previously been shorn that asymmetry of The proposal of new applications for precise |the Zeeman splitting was responsible for the orlent- magnetic field measurements has become a common Eition dependence of the sensor. It was shown that event since the development of a.v. magnetometers. (the sense of the asymmetry was dependant upon the An Indication only of the scope of these can be jiign of polarisation of the pumping beam relative to obtained from the examples briefly considered I the sense of the magnetic field. It was proposed by below. bloom (1961) that a twin cell arrangement be used {such that electrons in one cell be pumped into the Perhaps one of the most striking successes of [highest M_ state while those in the other be pumped the a.v. magnetometer has been in the field of {into the lowest M_ state. Thus the effective absorp- archaeological mapping. Where the a.v. instrument tion line became the sum of the previous asymmetric has excelled, was in the detection at items of I line with its mirror Image. Symmetry oust result. pottery, stone walls, middens and humus deposits § Iα practice, the two cells were on either aid« of buried to depths of many feet In sediment. ! the lamp as in Figure 6, so that the direction of {the beam was opposite in either cell, and thus In The most common application of magnetometers [counter alignment with the magnetic field. The has been In the field of geological prospecting. I line sense of polariser when viewed along the dir- Total field, vertical field and gradient measure- ection of light propagation, was then used In each ments made from the air or from the ground can be I fide. interpreted in terms of size, shape, orientation, depth and magnetic susceptibility of mineral bearing The dual cell arrangement has other very import- formations. The high quality of surface magnetic [ut advantages. When the single cell sensor was data obtainable with a.v. magnetometers has bean 1 rotated about 180*, the phase of the feedback field particularly suited to digital analysis with com- I had to be shifted through 90* to allow for the light puters and many advances have recently been made Iβ I beam reversing direction. The phase change became this direction. (Giret, 1965., Taiwan!, 1965). I unnecessary witn the dual cell arrangement and hence [the solid augle of orientation through which the In Zoological field research, some small mammals f sensor would operate without adjustment was doubled. with restricted home ranges have had magnets attach- [Experimentally, this was nearly 3ir steradians. ed to them and they have been relocated with magnetometers. A further merit of the dual cell configuration |vas its noise cancelling characteristics. Fluctu- The mapping of remnant magnetism occuring in ations in lamp intensity, sensor temperature and stripes across the ocean floors has provided positive I related instabilities, had equal and opposite effects evidence of "sea floor spreading" and continental Ion either component and thus did not alter the peak drift. I resonance frequency. Solar produced micropulsations in the Earth's The French team at Thompson C.S.F. have combined magnetosphere are monitored from ground station three orthogonal dual cell magnetometers in a magni- geomagnetic observatories around the world. ficently engineered system for airborne use. Their instrument operated in any orientation with align- This same solar activity Is responsible for ment error of less than G.04 micro Gauss per degree. inducing large "magnetotellurlc" currents within the Each twin beam component derived pumping light from Earth's crust. Measurement of these provides a means the one lamp, and depumping was achieved using the of calculating a resistivity profile of the crust to phase locked method. With the experimental perform- a depth of seveei. hundred kilometers. ance figures of Table 1, this instrument must represent the ultimate in engineering of this type The cause of planetary magnetic fields is a of magnetometer. The instrument was described by topic about which little is known. Magnetometers tfellleroux (1969, 1970). have been orbiting the Earth for several years now and one has been sent in the Mariner spacecraft to The Magnetic Gradiometer. Mars, to investigate this phenomenon.

For exploration type applications a resolution The scope for industrial use of magnetometers of 0.1 uG Iβ frequently a disadvantage, being too as metal detectors Is varied. Magnetometers also •usceptible to "noise" anomalies. The excess sensi- have military uses like locating submarines or tivity can, however, be put to great advantage by detecting munition dumps either hidden underground Maturing the field difference between two sensors or in dense jungle. The authors recently recorded tty 1 H apart. The mathematical Implications of the build up of charge for five hours before an obtaining gradient data instead of, or *a well as, electrical storm broke out. Where blasting In total field Information are extensive and varied. dangerous situations must be carried out, It Iβ of Soae of the applications have been listed In Section great safety importance to be aware of the likeli- 5 and their mathematical details discussed in the hood of lightning, as this discharge has been references. (Aitken t Tite, 1962., Hood and McClure, known to predetonate expletives. 1965., Hood, 1965).

147 SUMMARY Kastler, A., 1951. "Optical Pumping", Phvsica. 17_, 191. A brief resume of weak magnetic field magnetometer systems was given, and a'1 comparison made Laebe, J., Silver, A.H., Mercereau, J.E. & Jaklea- between conventional "relative^'field" instruments vic, R.C., 1964. and those using nuclear magnetic resonance to pre- Phys. Letters. 11, 16. cisely determine absolute magnetic field values. 12, 159. The recently developed alkali vapour instrument was considered In detail as an example of a magnetic Meilleroux, J.L., 1969. "Locked Type Caesium resonance magnetometer. The mechanism of "optical Vapour Magnetometer". pumping" and the principle' of the alkali vapour Thompson - CSF Pub. magnetometer was given. A single cell, self oscillating instrument was described in detail and its Meilleroux, J.L., 1970. "Progres Hecenta Sur Le performance and limitations assessed. Consideration Magnetometre a Vapeur de Caesium Type "Aeservi". was made of the refinements now. Incorporated in a Rev, de Physique Appliquee, J5., 121-130. highly developed, commercially available caesium vapour magnetometer and a selection of its applic- Pettier, L., 1968. These 3. me cycle, ations given. •'•. ''•'••• Universite de Paris.

REFERENCES Prindahl, F., 1970. "Bibliography of Fluxgate Magnetometers". Allen, J.H. 1968. "Long Term Stability of Self Pub. Earth Physics Branch. Dept. Energy nines & Oscillating Rb Magnetometers". Resources, Ottawa, Canada. J. Geomag & Geoelect. .20, 3, 197-204. Vol. 41, 1,1970.

Aitken, M.J. & Tite, M.S. 1962. "A Gradient Magneto- Stanley, J.M., 1972. "Reply by Author to P. meter, Using Proton Free Precession". Violino's Comments on "The Manufacture of J. Sei. Inst. .39, 625-629. Alkali Vapour Cells for Optical Pumping Experiments". Bell, W.E. & Bloom, A.L., 1957. "Optical Detection J. Phys E: Sei. Inst. of Magnetic Resonance in Alkali Metal Vapour" Phys. Rev. 107. 6, 1559-1564. Stefant, R., 1963. "Induction Magnetometers in Space". Bloom, A.L. 1962. "Double Beam Sensor". Ann. Geophvs. 19. 250. Applied Optics. 1, 61. Stuart, W.F., Ciarrocca, S. & Usher, M.J., 1967. Clack, D.J. & Stanley, J.M. 1971. "The Manufacture "Stray Field Modulation Effects in the Rb of Alkali Vapour Cells for Optical Pumping Magnetometer". Experiments" J.Sci. Inst. .44, 618-620. J. Phvs. E. Sei. Inst. 4., 758-760. Talwani, M., 1965. "Computation with the help of a Dehmelt, H.G. 1957. "Modulation of a Light Beam by digital computer of magnetic anomalies caused Precessing Absorbing Atoms". by bodies of arbitrary shape". Physical Rev. 105. 6, pp 1924. Geophysics 30. 5, 797.

Giret, R.I. 1965. " Some Results of Aeromagnetic Usher, M.J. & Stuart, W.F., 1970. "Stability and Surveying with a Digital Caesium Vapour Magneto- Resolution of a Rb Magnetometer". meter". J. of Phvs. E.: Sei. Inst. 2, 203-205. Geophysics 30, 5, 883-890. Usher, M.J., Stuart, W.F. & Hall, S.H., 1964. "A Green, R. and Sydenham, P.H., 1971. "The Cooney self oscillating Rb Magnetometer for Geomagnetic Geophysical Observatory". Measurements". Aust. Phvs. 8., 11, 1971. J. Sei. Inst. 41, 544-547. Grivet, P.A. & Malnar, L., 1967. "Measurement of Violino, P., 1968. "Optical Pumping Rates in an Weak Magnetic Fields by Magnetic Resonance". Alkali Vapour In the Presence of a buffer Cas". Advances in Electron Phvslcs. 23. 39-151. Nuovo Clmento 54B, N.I. Mar, 1968. ********************* Grivet, P., Sauzade, M. & Stefant, R., 1961. "Induction Magnetometers In Space" SENSITIVITY: 0.1 uGause Rev. Gen. Elect. 70. a317. DYNAMIC RANGE: 0.2 - 0.7 Gauss ORIENTATION EFFECT: < 0.04 uGauss/deg. Hood, P. 1965. "Gradient Measurements In Aeromag- TEMPERATÜRE RANGE: -40*C to +50*C. netic Surveying". THERMAL DRIFT: < 0.1 uGauss/deg. C Geophysics. 30. 5. 891-902. POWER CONSUMPTION: < 80 watts WEIGHT: "Bird" - 15 lb. Hood. P. & McClure, 1965. "Gradient Measurements in Electronics - 45 lb. Ground Magnetic Prospecting". Table 1. Specification figures of the six beam, Geophysics. 30, 403. P551-05. Thompson C.S.F. "Locked Type Caesium Vapour Magnetometer" for aerJjl survey.

148 •*. ii --..

FlNt -äT SJttUCTl,« FIGURE 3. Component diagram of the self- &<*i- Hurt* oscillating magnetometer.

FIGURE 1. Diagrammatic splitting of energy levels as relevant Interactions are added to the Bohr model. The quantum numbers are those of an alkali atom having a nuclear spin I - 3/2, for example Rb87. Energy level spacing is not to scale and for simplicity, the Zeeman splitting of the 2P state is 3/2 not shown.

FIGURE 4. Dependence of signal amplitude upon sensor orientation (Caesium, self oscillator).

FIGURE 5. The Phase Locked Magnetometer Syatem.

FIGURE 2. (a) Schematic diagram of the Apparatus, (b) The absorption of photons from the beam when the cell is being depumped by the electromagnetic field at the Larmor frequency. The transmitted intensity Bay vary by up to 20%. (Bloom, 1960).

FIGURE 6. Dual cell, self-oscillating magnetometer. 149 FORCE-FEEDBACK: A REVIEW

IM. Newman, B.E., M.Eng.Sc, M.I.E.Aust, a post graduate student in the Department of Control Engineering within the School of Electrical Engineering at the University of New South Wales

Y. The reliance of nan on force information fron his hand and arm ie outlined. Advances in remote control have generally been such that force sensing has been supressed. Reports of applications and investigations into the uee of generated force feedback in a simple servomechanism control loop are briefly discussed, as are reports on research and deployment of force-reflection in dual servomechanisms used in master-sieve manipulators. Possible current areas of research are surmised and a orojiosis on the possible Ions tern use of force-reflecting computer controlled manipulators is nade« 1. Man, when usin^ his hand as a rranir-ulator, usec reliance on the sense of vision as the prime method many sources of sensory information to aid him in the of remote control, with assistance from the sense of control of any particular operation. Vi3ion, sound, hearing to provide sone alarm indications. temperature, tauch and feel are some of the senses which may be used. The relative importance of these In recent years «an has turned towards environ- sources of sensory information is hard to rank due to ments other than his own in quest of discovery and the adaptability of the human control system in pro- exploitation. His own limitations have created con- cessing a very large amount of sensory information, siderable terriers, nost of which have been overcome and also the task of manipulation is general, v/hence in some form or another, examples are space and information of paranount significance for one task diving suits. There is a growing awareness that there may be completely inconsequential in the next. is a requirement to produce an image of man to go to these hostile environments for detailed exploration Man's hand i3 unique, as his thumb has five dej- and exploitation, which has the abilities of loan, but rees of active freedom whence he has uore ability to not some of his biological and psychological limita- use his hands for manipulation than any other compe- tions. An intergral part of a human inage is a mani- ting species (Hef. 1). The reason for this unique- nulatim- ana which can react with the environment. ness has been assessed as due to evolutionary interaction of Man's predecessors with tools and weapons. The increased abilities of the medical profession The ability of early Man to extend his attributes by in sustaining the lives of naimed and impaired acci- the use of hand held tools and treasons gave hin dis- dent victims, caused in essence by the increased tinct advantages. Apart from the effect of the tools technology of üan's tools and weapons, has led to an on the external aspect being processed, the tool is increasing demand for prosthetic and orthotic appli- also able to reflect some information bad: to the ances and caused research to be directed towards supporting hand, thus tools can be seen as sensory fundamental understanding of human manipulative extensions of man. The significance of the passive activity, ./hile there are other requirements for senses of vision and hearing on the learninj pattern prosthetic and orthotic arms, a similarity of require- of animals has been given its due accord, as also has ments with those for remade manipulation exists, the ability of Man to communicate by s eech v:th his especially for the need to react with the environment. kind. However, the ability of Van to act and react with his environment by means of his manipulating Assuring that a manipulator arm and hand has avai- hands has perhaps not been given it due in the -able sufficent and necessary information on position, learning development of the human individual and the whether by visual means or by sensors on the arm, the species as • whole. next nost significant sensory indication which enable« one to react with the environment, is force. Force The Industrial revolution changed i'an from a ver- information when coupled with that of position permits satile tool user into a specialised rcaehine user. one to establish det&ils on the mechanical compliance Automation has reiaoved "an to the Position of a of an object under investigation, this in turn permits nachine supervisor. There are exceptions to such optimal decisions on work tactics, for example the sweeping generalisations, such as the specialised establishment of the plane of action of the wrist is machine maker; however, the general trend of Ian in opening a door knob oust be resolved by using com- industry to become a watcher instead of doer is pliance information. Force information when coupled evident. As Van has become removed from his tools in vith that of iiosition, visual signs of deformation and apace, whether it be Tor reasons of augmentation or a reservoir of experience, permits man to undertake • facilitation, there has been an almost universal wide range of manipulative tasks such as surgical

150 dissection and boilennaking, and to do these generally of the overall tracking loop is of interest. in a manner which uses an optimal amount of energy. ni.K)RC2-F&amCK IN A SIliPIJä CONTROL LOOP. There are several mechanical senses which the human arm and hand employ, such as the tactile sense in establishing shape, contact sensing and proximity sensing1 of hairs. It is unresolved whether the human sensing of force is due to its own sensors or the processing of other sensory information. However, the current state of the art in sensor packaging and processinj abilities dictates that application of force feedback to manipulative devices will initially follow the macro approach,

II. TXLLKRS, .THESIS AKD JOYSTICKS. Pig. 1. DITOSD RESET The Human control of ships and aeroplanes is one The first use of a control signal which can be area where the sensory use of force reflection, from attributed to force was possibly divided reset to the actuating device on its environment to the human assist in the remote position control of gun turrets controller, has recieved sone cognisance. The amount in the Hoyal Navy. (Ref. 7) The advent of fast of force reflected to the operator is dependent on moving aircraft targets dictated the change over fro« several factors relating to the control and controll- manual follow to remote position control: however, ability of the vehicle and its medium. >'foen force the existence of Dany gun turrets necessitated modi- reflection is coupled with other sensory information fication rather than replacement. The mechanical and experience the operator has more "feel" for his slackness of modified manual turrets give rise to task. unacceptable set-back on firing «hen the control chain used normal methods of position, velocity and accele- Steering control of ships developed from sweep oars ration feedbacks with associated compensation. By to a rudder and tiller, and then to «heels coupled using two position measuring devioee, one coupled to mechanically to the rudder head. Although mechanical the driving motor and one to the driven element, the losses decreased the sensitivity of the steering sys- problem of torque disturbance was overcome. (Ref. 8 tem for reflecting force the increased sizes of Pig. 1). The development of gun turrets designed for vessels ensured that more force v

Pilot control of aircraft intially had direct iiec- hanical oouyilirr; between the joystick and rudder pedals with the control surfaces. Heflection of the forces acting on the control surfaces assisted the pilot in his task. The increase in size and speed of aircraft overtook the physical abilities of the hwan, nith pure mechanical assistance, to control them, and servo controls were developed. (Ref. 3) The speed of response of the vehicle together with the direct Jig. 2. ,.£AS0ITi3 TORQUE APTER AHZ34ECHER chain of control is of note. Generally power assisted controls of aircraft reflect forces to the ,'>ilot, In developing th

151 •ut. Tm . «m I«

(b) Simplified Systems.

Fis. 4. 1EASURED ARUATOTE TORQJJE ASTER TAN. Sie possibility of replacing the elements of jso| : computed torque feedback were investigated by 'IAN, at the*university of Sydney (Ref. 13, Tig. 4)* By emp- (a) Diagrsmetic System. loying a d.c. motor with separate field excitation, a feedback signal was obtained by sensing the arnatuxe current which is proportional to torque within the linear region of the amplifier. By using this sat- •ut. Tm . »mm -He ••«»- *e (*•'*') •*»<">> urating non-linearity to operate for large displacements in an optimal switching mode, and for final alignment as a linear system with force-feedback, a system with good step response can be constructed« Conclusions were that the system would be insensitive to variation of load, provided motor torque to inertia was large and that the load was small compared with the available driving power. Care needs to be taken in the comparison of the (b) Simplified System. four sub-systems illustrated, as they form only sec- Pig. 3, COMPUTED TORQÜE AFT3H SA'./AIIO tions of a complete system. Investigations by SAWABO and TAi: into the effects ot mechanical resilience were The possibility of using a derived torque signal not reported on. Removal of the resilience from the instead of a torque transducer was investigated by two system simplifies the problem, but may not nec- SA'ffANO. (Ref. 11,12, Pig. 3) A confuted torque essarily represent the particular system. The system was established from the motor input variable and the using armature current feedback, which reduces the output speed. Various non-linearities, such as requirement for a tachogenerator and ir.proves perform- amplifier saturation, motor dead zone and ripple of ance, may prove of significance in the mainstream of tachoganerator output, were given their due accord. servomechanism developaent. The study disclosed a resemblance -to a relay servomechanism with non-linear velocity feedback and that 17. FORCE-RSFLECTIOH BETiVSiSH TYÄ) SERVO LOOPS. the system with computed force-feedback could be designed to have an optimum response for a fixed initial mechanical master-slave manipulators inertial load, with altering inertia the response consisted of a single control loop for each motion becomes oscillatory or overdamped as in the case of (Ref. 14, 15). The master and slave were mechanically a simple optimum non-linear control system. coupled by rods, tapes and ropes. Physical limitatloni of these mechanical manipulators gave impetus to the development of electro-mechanical manipulators* The Conounication link became an electric cable, thereby giving the device more flexibility of movement and resulted in the creation of two separate coatrol loops suitably interconnected for each motion. Arganne National laboratory, in the D.S.A., led development in the master-slave manipulator field. (Ref. 16 and 17)«

(a) Biagramatic System.

(a) Basic System.

152 Tm

*m - A, - * Pig. 6. BILATERAL SERVO; PORCB- (b) Reduction of Amplifier«. POSITIONAL PORCE-BEFLECTOG.

(o) System with flyetereeis and Load Inertia. Fig. 7. BILATERAL SERVO; F0RCE-POSITI0HAL Complete System would have Motor FORCE-REFLECTING WITH FORCE-FEEIBACK. Inertia, Friction and Backlash. A force-positional foroe-refleoting bilateral Fig. 5. BILATERAL SERVO; POSITIONAL - servo was proposed (Ref. 16 and Fig. 6), and analysed FORCE-REFLECTING. (Ref. 10). A variation of this, with foroe-feedbaok used to reset the torque developed in the aaster nie first conoept of master-slave connection was system (Fig. 7), has had the same early work with as a positional-positional force-reflecting servo- further development (Ref. 22) and analysis with ea- mechanism (Fig. 5). and it was soon appreciated that phasis on the detrimental effects of friction (Ref.23). two separate amplifiers was unneccessary. Synthesis This type of system in its basic configuration has of such a mechanism was limited to considerations of reduced the effect of systemdynamics on the operator, a simple system without the attendant mechanical hys- the master system being the only section effecting teresis and inertia loading present at both the slave the sensitivity of operators "feel". Improvement In and master. (Ref. 18) An analogue computer analysis system stability, when considering the complete oon- of this type of device supported practical conclusions plez system of practical mechanical components, w*h on the abilities of such master slave systems. (Ref. also reported. The main dissadvantage is the torque 19) The system is capable of excellent torque refle- transducers, from both the practical and cost view- ction in the steady state, but under dynamic condit- points. Torque transducers introduce extra resilience ions the inertia, friction and resilience of both the into the system. However, the saving of tachogenera- master and slave assume significance which in most tors assists from both the weight and cost aspects. oases swamps the load torque developed and reflected by the slave. This basic system, associated with The problen of deployment of force-reflecting conventional methods of velocity feedback and conpen- systems over considerable distances gave rise to lation, appears to be the only one used in master- investigation into suitable control strategies where slave manipulators which are commercially available the time delay nas large (Ref. 24) and to stability (Ref. 20). criteria where the time delay was comparatively small (Hef. 25). General Electric developed an electrohydraulic taster-slave manipulator the "Handyman". (Ref. 22) The external load on a force-reflecting servo- This used hydraulic pressure feedback to achieve force mechanism may vary from nothing, free movement in a reflection and due to «eight considerations of the vacuum, to infinitity, collision with an effective complete device computed the torque necessary to hold ianoveable object. The load of the operator on the the slave at a given position as earlier attempts at i..astur end is also variable as he changes his effect- mechanically balancing the motions had apparently been ive inertia, danping and spring constant to meet futile. The concept of using a computed torque nece- chani-ins circumstances (Ref. 26). It is this broad ssary to hold the device in a static position was used ranje of external loads which makes design and the in the Italian "Mascot" master-slave manipulator selection of appropriate tests difficult. Torque or (Ref. 20). force is the variable of prime interest, though position is a äijnificant consequence. v. A::D The investigation of active devices used in traok- inj controls is oeen as a logical development of

153 investigation into passive ones. The Tiae of a torque logical development of iian'a tool and weapon techno- in modifying the action of a human controller in logy. Force-reflecting servomechanisma appear to addition to inertia, friction and spring constant offer the best manner of transfer of force intona- could either provide the controller with additional tion from a remote image to the operator. Development Information, or act as an aid to improve his tracking of force-reflection devices must be expected. Should performance. a considerable refinement of force-reflection mechanisms and manipulators occur then the effects on soc- The possible economics of reduced components and iety may be significant. increased performance with torque feedback instead of tachometric feedback would appear to assure further investigation and development work on torque-feedback in a single control loop. is made to the assistance of members of the Royal Australian Navy and the University of Hew South Vale* The mainstream of manipulator control development in the preparation of this paper. has been towards the digital computer control of a manipulator in either the manner of an automaton or REFERENCES. by limited supervisory control using switch inputs. The interest of the Electro-technical Laboratory, 1. UAPIER, J. - The Evolution of the Hand. SCIENTI- Tokyo, in using force information as sensory input to FIC AJ-ERICAN, Vol. 207, No. 6, December, 1962, the control of a manipulator (Ref. 27), nay be the pp. 56-62. start of renewed investigation into force in manipu- tore. ifcieter-Blave manipulators have been analogic 2. BUCHAKAH, G. - Steering Gears, in INSTITUTE devices with cooperatively little external computa- IIARINE ENGINEERS - "The Running and Maintenance tion. Digital computer control of a master-slave of Karine Ikchinery" 4th. ed., 1955, pp. 216-228. manipulator, with the attendant advantages of calculated compensations, must be anticipated within a 3. JOHNSON, R.i;. - Artificial Peel For Sarvo-Booated few years. Haeter-slava manipulators have not used Manual Controls. CONTROL ENGINEERING, Vol. 12, eeryo-Asaistance to drive the master arms, although No. 2, IJarch, 1965, pp. 67-71. this general area has been investigated in conjunction with mem-amplification projects (Ref. 20). 4. BÜIttOiJS, A.A. - Control Feel and the Dependent General Electric have indicated that significant Variable. HUUAN FACTORS, Vol. 7, Ho. 5, October, theoretical performance gains occur when the controls 1965, PP. 413-422. of materials handling devices, like bulldozers, axe fitted with force-reflection between the actuator and 5. a.'0!VL3j, '.V.B. and SHERIDAN, T.B. - The 'Peel' of control levers, and it may be presumed that this has Rotary Controls: Friction and Inertia. HuTUH had further consideration (Ref. 29). FACTORS, Vol. 8, No. 3, June, 1966, pp. 209-215. Although prosthetic research has produced several 6. -/EISSEKBERGER, S. and SHERIDAN, T.B. - Pjnfy^cs devices to adjust automatically the gripping force of Human Operator Control Systems Usiz|g_Tactlle of a hand, priorities in other directions would appear Feedback. THAIS. AMERICAN SOCIETY MECHANICAL to mitigate against short term advancements using EK&IHi&RS, Vol. 84, Series D, Ho. 2, June, 1962, force as a significant variable. Work on orthotic PP. 297-301. devices may perhaps prove relevant (Ref. 30). 7. LaKP-BT, .7.E.C, - Naval Applications of Electrical When the engineering problems of force-reflection Remote - Positional Controllers. J. INSTITUTION have been overcome and accurate and detailed master- SLUCTRICAL ENGINEERS, Vol. 94, Part IIA, No. 2, olave manipulator hands are developed, then the start 1947, PP. 236-251, also remarks pp. 253-254, and of almost a new era may occur. Knowledge was passed reply p. 255. on initially by the spoken work, then by longhand writing, by printing, and lately by computer. It is 8. TAYLOR, ?.Ii. - Servomechanisma• London, Longman*, suggested that skill of hand is essentially an indi- I960, pp. 229-233. vidually acquired attribute and that technique and knowledge form only secondary supportive roles to 9. CLAUSEN, H. - Ba that of experience. Although practical guidance and and Structures. ENGINEER, Vol. 199, Ho. 5172, observation aid the development of skill of hand, March 11, 1955, 3P« tbexe exists ao way of accurately passing on experience. The ability to record the dynamic actions 10. AEZBAECffiiR, H.C. - Servomechanisms with Force end forces exerted by the hand of a doyen o£ some Feedback. ANL-6157, 1960, 119p. skilled art, like that of a surgeon, and the subsequent playback onto the hands of a trainee may reduce 11. SA'.YAIIO, 3. - Servomechaniama with Computed Torque significantly the time needed to bring successive Feedback. BULLETIN JAPAN SOCIETY KECKAHICAL generations to the competence of their predecessors. iSi'GINEERS, Vol. 8, Ho. 29, February, 1965, 28 Acceleration of the development of skill of hand must pp. 48-55. follow. 12. SA./Ai'iO,,S. - An Adaptive Control System to load 29 VI.CONCLUSION. Inertia Variations. BULLETIN JAPAN SOCIETY HECH- AKICA1 EXGINiiERS, Vol. 9, No. 33, February, I966, Force information from the hand is of prime sign- ??. 77-85. ificance in the reaction of Man with his environment. 30. Deployment of Man-like devices which transpose the 13. HAS, J.S.C. - P.C. Servo-System with Torque Feed- s.eneee of man to a distance from the operator are a back. CONTROL, Vol. 12, Ho. 126, December, 1968,

154 pp. 1037-1040, and Vol. 13, No. 127, January, 1969, pp. 43-47. 14. GOERTZ, R.C. - Master-Slave Manipulators. ANL- 311, 1949, 6p. 15. CURTIS, VT.K. - The Development of the Master- Slave Manipulator. NUCLEAR KNERGY, Vol. 4, November, 1963. PP. 4-16. 16. GOERTZ, R.C. and BEVILACQUA, F. - Servos for Remote Manipulation. IRil CONVENTION RHCORD, 1953, Pt. 9, PP. 103-109. 17. BURNETT, J.R. - Force Reflecting Servos Add 'Feel' to Remote Control». CONTROL EJTGIHITERISG, Vol. 4, No. 7, July, 1957, PP. 82-87. 18. CHALMERS, 77.J. - Servomeohaniam Synthesis Through Betwork Parameters. Ph.D. Thesis, Purdue Univer- sity, 1955, 87p. 19. SPOONER, K.G. and WEAVER, C.H. - An Analysis and Analogue — Computer Stndy of a Force-Reflecting Positional Servoaechanism. TRANS. AIEE, Vol. 74, Pt. 2, January, 1956, pp. 384-387. 20. GALBIATI, L. et. al. - A Compact and Flexible Servoevetem for Masttw-Sla PROC. 12th. CORF. REMOTE SYSTEMS TECHNOLOGY, 1

21. MOSHER, R.S. and WENDEL, B. - Force-Beflecting Electrohydraulic Servomanipulator. ELECTRO- TECHNOLOGY, Vol. 66, December, 1960, pp. I38-I4L 22. FLATAU, C. - Compact Servo Master-Slave Manipula- tor with Optimized Communication Links. BIIL 13810, 1969, 18p. 23. KOVARICK, V.J. - Notes on Force-Reflecting Systems BNL, AGSCD Technical Note No. 80, 1967, 11p. 24. FERRELL, V/.R. - Delayed Force-Feedback. HUT.'AN FACTORS, Vol. 8, October, i960, pp. 449-455. 25. THOMPSON, U.K., VACROUX, A.G. and HOFFJAN, C.J. - Application of Pontryagin's Time Lag Stability Criterion to Force-Reflecting Servomechanisns. 9th. JACC, 1968, pp. 432-443. 26. AGARWAL, G.C., BKHKAN, B.I', and STARK, L. - Studies la Postural Control Systems: Torque Disturbance Input. IEEE TRANS. SYSTH.S SCIiflCS & CYBHRIJETICS, Vol. SSC-6, No. 2, April, 1970, pp. 116-121. 27. IN0D3, H. - Computer Controlled Bilateral Mani- pulator. BULLETIN JAPAN oOCIOTT MECHANICAL ENG- INEERS, Vol. 14, No. 69, larch, 1971. PP. 199-207. 28. KIZEN, N.J. - Machines with Strength. SCIENCE J., Vol. 4, No. 10, October, 1968, pp. 50-55. 29. KOSHER, R.S. - New Concept Joins I.ian to Kachine to Perform Gigantic Tasks. SAE J., Vol. 75, No. 9, September, 1967, pp. 72-75. 30. ALU3I, J.R., KAHCHAK, A. and OTCIflX, V.L. - Orthotic Manipulators. ARTIFICIAL LE.S3, 1969, pp. 261-270.

155 THE ACCURACY OF ELECTRICAL MEASUREMENTS MADE BY ELECTRONIC TECHNIQUES

J.M. Warner, B.Sc, Non-Member, Australian Post Office Research Laboratories, Melbourne

SUMMARY. The use of electronic instruments is essential in some types ot measurement. The published specifications of the more complex electronic instruments, such as digital voltmeters, are capable of misinterpre- tation, even by technical personnel. This paper highlights those parts of such specifications which, in the author's experience, are not unequivocal and makes particular reference to the problems of alternating current measurement.

I INTRODUCTION Modern DYMs are among the most sophisticated measuring instruments made and have passed the stage About kO years ago, the requirements for electrical where the accuracy is inferior to that of a good measurement reached the stage where they could not be portable indicating instrument. Today, the accuracy met by the indicating instruments of that time, al- claimed by some makers suggests that they could be though these were probably within one order of the used to replace the traditional potentiometer and limiting sensitivity and certainly the same order of standard cell in the standards laboratory. Instru- accuracy as today's instruments. The measurement of ments with a digital display are susceptible to a alternating current, particularly at the higher frequ- phenomenon that could be called the "credibility encies, was becoming increasingly important and there effect". Because there is no obvious reading error, were requirements of high impedance, extended frequ- all the digits are to be believed. When the first ency range and high sensitivity that the instruments three digit instruments were made, the accuracy was of the time could not provide. These needs brought usually limited by the finite resolution and an about the development of the first electronic instru- error of one digit was believable. Today, six and ments, vacuum tube voltmeters and, while the desir- seven digit instruments are made and it should be able features listed above were attained, the accu- apparent that the same limitations on accuracy do racy of the measurements declined. In the last 20 not apply. years, there has been a great increase in both the quantity and complexity of electronic instruments for When we wish to examine the accuracy of digital making measurements of electrical quantities as well voltmeters and, particularly, to compare the pub- as those for measurement of other quantities, which lished specifications of different manufacturers, we are not the concern of this discussion. find some difficulty due to differing methods of writing specifications, (in what is a very competi- It is known that we can often obtain an improvement in tive business there is also some advantage in skilful our product quality or output or our operational specification writing.) The method used differs from efficiency (depending on the type of industry con- that almost standardized form that is used for indi- cerned) by closer tolerance specification somewhere cating instruments, and the chances of misinterpre- in our respective processes. Closer tolerance speci- ting are many. Errors are usually quoted in terms fication almost always requires higher accuracy mea- of the reading and range in combination, thus: surement and with the introduction of automation into +_ X% of reading +. Y55 of range. At full scale, this industry there arose the need for many of these mea- becomes + (X+Y)2 ot reading but at ;he range change surements to be made automatically as well. This was point, usually 1/1C of range, it becomes +, (X+10Y)*. for various reasons; so that information could be fed A typical 0.01)5 + 0.01JJ instrument has an error of back into a process controller, so that the measure- 0.02!f of reading at full scale and O.lljt of reading ment could be made without operator error, for the at 1/10 full scale. Figure 1 shows the effect of IV results to be printed out or punched onto a tape for various typical two-part accuracy specifications of digestion by a computer for further calculation or error expressed as a percentage of the reading, which other statistical purpose. As well as all this, the is what the user wants to know. demand was for increasingly higher accuracy. Ill OVEB-RANGING AND THE "HALF-DIGIT" II THE ACCURACY OF DIGITAL VOLTMETERS Some makers provide overranging by putting a "1" The most common autoEatic measuring instrument in front of the most significant digit, so that a is the self balancing digital voltmeter which, with three digit instrument can read up to 1999 instead the appropriate accessories, is the basis for most of 999. Some people call this a 3 1/2 digit instru- data logging systems as well as a most convenient ment. There have been various overranges used, from tool for investigations and general measurements. 10% to 300%. Most manufacturers do not permit over- 0. 156 1V ± (0.00U* of input + 0.002* of rang«) 10,100,1000V • (0.003* of input • 0.001* of rang«)

For 90 days (l8°C to 28°C)

0.1V range + (0.005* of input + 0.005* of ran«*) 1V i (0.005* of input + 0.002* of range) 10,100,1000V £ (O.OOU* of input «• 0.001* of irange)

For 1 year (18°C to 28°C)

t .IX of * .1% 0.1V range +, (0.02* of input + 0.005* of ranee) of full la IV + (0.02* of input + 0.002* of range) 10,100,1000V + (0.01* of input + 0.001* of rang*)"

Comparison with another maker's product ii not simplified by the change in temperature range for the -± .1% o< full sal various entries. The more interesting question is -+ .1« of nacKna that of the event occurring at zero time - the calibration of the DVN against an external standard, the uncertainty of which should be added to the figures above. There are very few laboratories in Aust- V ralia that have the facilities for dc voltage measurement up to 1000V with an uncertainty lees than 0.01*. For ac measurement, this would be even worse and, as will be shown later in this paper, there are difficulties with waveform. The calibra- ± .OIK of nrtina tion, every 90 days of even a moderately accurate ± .01% of full «cat* .02 DVM, is obviously outside the capabilities of the ordinary meter calibration laboratory and requires the very best in the way of standards and they will ± .01% of rwcftnc of necessity be the traditional potentiometer, volt ± .008% of full «at* box and standard cell. .01 A fairly common expression found in specifications IΒ "typical" as typical accuracy, typical stability and \± .005% of raiding, + .002% of full tori« 30 on. Typical accuracy is higher than "guaranteed" accuracy by anything up to one order and is generally ,.006 10 20 SO 100 regarded with mistrust by engineers became of the Indication (% of full lack of definition of the expressioc, and the fact that in their past experience they have never been fortunate enough to have bought a "typical" instru- Fig. 1 - Error expressed as % of reading. ment.

On the subject of stability, one maker (J. Fluke) has ranging on the maximum range (1000 Volt). There is given the following definition of "typical" stability consequently a certain aaount of doubt as to what ie for his 0.00b* instrument: the "range" for the purpose of calculating errors. On a 3 1/2 digit instrument with 100$ over-ranging "Thorough error analysis studies into total is full scale 999 or 1999? Iβ the error 0.1* or instrument stability, taking into account the docu- 0.05$ of range? There are marginal advantages for mented stabilities of individual components and uti- the specification vriter (the maker's, not the pur- lizing probability end statistical methods, indicate chaser's); four digits with 20* over-range has a one that typical instrument stability (defined as a spe- count error of 0.0082 compared with 0.01* for a meter cification met by 80* to 90* of all instruments) is without over-range. Very few makers quote worst case !+0 ppm (O.OOU*) peak to peak per year. An instru- accuracy (at the range change point) and certainly ment so categorized need be calibrated only once per none headlined it. year to meet all specifications". Other makers might well follow this example. IV STABILITY V OTHER ERRORS The constancy of accuracy of digital voltmeters is usually given in the same two part form together The overall error in a DVM is the total of all with a time limit. Below is an extract from the ca- or some of the following errors due to: talogue of one maker. (a) Finite resolution "Accuracy (b) non-linearity of basic range (c) range multipliers or attenuators (to 120* of range or 1100V maximum input) (d) pre-amplifiers (e) internal reference For Sk hours (23°C +. 1°C) (f) supply mains changes (g) effects of temperature 0.1V range £ (O.OOU* of input + 0.0055t of range) (h) drift in zero setting 157 {i) frequency of input users would be happy to receive a list of corrections (j) waveform. to be applied - the normal result of a "calibration" operation. There are alf.o other effects that are not properly

classed as errors but are capable of causing errors VI .t TEMPERATUBE EFFECTS when used in a particular manner, such as current fed out from the input terminals. Several components in a DVM will be temperature dependent and the overall temperature dependence will If the specification is given for an overall error, not necessarily be linear. The designer will make it it should list those causes that are included, but small over a small range about a normal ambient but it is more common for the user to have to do his own outside that range it majr be non-linear. He may arithmetic, particularly, if the exercise is the com- quote an overall figure which may be quite large, or parison of allegedly similar instruments from diffe- give the coefficient of each part, leaving the user rent makers (as in the examination of public tenders). to do his own statistics. Most makers have adopted Accuracy statements such as the one quoted below make the practice of quoting the accuracy first over a such a comparison very tedious as all makers do not very narrow temperature range (23+l°C for most instru- use the same form. ments of American origin) and give additional errors for other temperatures. If the temperature coeffici- "AC Accuracy ents differ for various input ranges, then a two part specification is often used: At 23°C i 1°C (nominal calibration temperature), relative humidity less than 70?; "Temperature Coefficients (0° to l8°C -and 28° to 50°C)

30 Hz to 5 kHz + (0.05? of input + 0.0025)5 of range) 0.1V Range from 0.001 to 500 VAC £ 0.1? of input from 500 to 1100 VAC. i (0.0007? of input + 0.0005? of range)/°C

5 kHz to 10 kHz +_ (0.07J& of input + 0.005)5 of range) IV Range from 0.001 to 500 VAC +_ 0.1? of input from 500 to 1100 VAC. + (0.0007? of input + 0.0003? of range)/°C

10 kHz to 20 kHz +_ (0.15? of input + 0.01? of range) 10, 100, 1000V Ranges from 0.001 to 1100 VAC. £ (0.0005? of input + 0.0002? of range)/°C" Over the temperature range 13°C to 35°C (55°F to 95°F), relative humidity less than 70°?; In this example, the effect of temperature over the range l8-28°C is included in the accuracy statement 20 Hz to 5 kHz +_ (0.1? of input + 25 uv) from 0.001 given in section IV for accuracy and stability. to 1100 VAC. VII REJECTION OF UNWANTED SIGNALS 5 kHz to 10 kHz +. (0.15? of input + 25 uv) from 0.001 to 1100 VAC. Interference from ac signals has a greater effect on de DVM operation than it has on VTVMs or any other 10 kHz to 20 kHz +0.3? of input from 0.1 to 1100 VAC. indicating type instruments. The effect of series 20 kHz to 50 kHz £ 0.5? of input from 0.1 to 110 VAC. mode unwanted signals. which originate mainly from 50 kHz to 100 kHz £ 1? of input from 0-1 to 110 VAC. the supply mains, can be reduced by either a low pasB 10 Hz to 20 Hz +_ (0.3? of input + 100 uv) from 0.001 filter or inherently by the integrating system used to 1100 VAC. in scene types of DVM. A low pass filter will reduce 5 Hz to 10 Hz + (1? of input + 250 uv) frin 0.001 to the reading speed to the extent that it may take 1/2 1100 VAC. to 1 second to settle within 0.01? of the final reading, although the time without filter may be few Outside the 13°C to 35°C temperature range, the above milliseconds. The written specification may headline specifications may be derated at 0.003?/°C (below the time of the unfiltered performance and the re- 5 kHz) or 0.005?/°C (above 5 kHz) to the extremes of jection appropriate to filtered performance, which 0°C and 50°C (32°F and 122°F). " can be misleading. Slow balancing may not be important for sane type of usage in a laboratory, for There is a tendency to make input resistances high instance, but for data logging applications the high to reduce internal heating as well as reduce circuit rejection of the integrating instrument is prefer- loading. It is well known that high valued resist- able. ors are less stable than lower valued and the makers provide adjustable trimmers that are adjusted at the It should be noted that, although it is possible to 30, 60, or 90 day "calibration". Also, many instru- apply 100V ac on the 100V dc range without obtaining ments have a built-in reference, a zener diode or a a change in reading greater than 0.01?, it is standard cell against which the instrument is "cali- not reasonable to expect the same performance on lov- brated" at the user's demand by a front panel switch er ranges. In fact, low ranges which use pre-ampli- or button or even automatically in some cases. There fiers frequently have clamping diodes for overload is also a screw driver adjustment inside so that the protection and these produce spurious signals when change in this reference with time can be compensa- overloaded. ted. In practice, the DVM is not "calibrated" in the usual sense when sent to the standards laboratory, VIII ERRORS 13 A.C. MEASURIHG INSTRUMENTS but the internal trimmers are adjusted so that ity reads correctly within a specified tolerance. Few So far, the matters discussed in this paper may 158 be taken to apply equally to instruments for the mea- amount of the harmonic. The effect of several har- surement of either direct or alternating quantities. monics will be additive and in the worst case the There is an important source of error that affects error will be the arithmetic sum of the harmonic amo- the measurement of alternating quantities only: the litudes. relationship between the type of instrument and the waveform of the quantity being measured. Hie effect (b) Average Responding Instruments. applies to all ac measuring instruments, ordinary indicating meters, all types of electronic Volt- The effects of even and odd order harmonics are meter as well as differential and digital voltmeters. quite different. The following aumnary is derived from an examination of the effects of harmonic in- The amplitude of an alternating wave may be specified fluence given in references 1 and 2 for the case by a single value provided the waveform is fully where the amount of distortion is less than 5%. defined. The three well known parameters are the peak, average and root mean square (ras) values and (i) Even-ordered harmonics can produce errors up to they are interrelated as follows: rms amplitude form factor = average amplitude where H is the amplitude of the harmonic expressed u ... peak amplitude a percentage of the fundamental. The "•»•»ITTI error crest factor = ^ ,gltude occurs when the harmonic is in phase with the fundamental. The error due to several even harmonics can and for a sine wave these have fixed values of be up to 1.1107 and l.Ullfe, respectively. ,H2 h 2 The significance of the difference between the three parameters is often overlooked. As the majority of The effect is small, h% of 2nd harmonic producing measurements is concerned with energy and power, less than 0.1/t error. the rms value is the most important. For measuring the breakdown of insulating materials or calibrating (ii) Odd-ordered harmonics can produce the greatest aa oscillograph, the peak value is more important, errors up to and for measurements of electrolysis, the average H, Hc value is more appropriate. However, the rms value is always assumed to be specified unless otherwise stated. The error produced by the lower order odd harmonics is the worst and 3% of 3rd harmonic can produce an Unfortunately, the rms value is the most difficult error in an rms measurement made with an average re- to measure in a practical manner. Dynamometer, sponding instrument between +1% and -lit depending on thermocouple and electrostatic instruments all the phase relationship. measure rms but suffer from limitations of sensitivity, frequency range, robustness and price as well When the distortion in a supply is given only as as resolution. Peak rectifying diode voltmeters for a per cent harmonic content and the order of the har- the measurement of radio frequency voltage and aver- monics is unspecified, then the worst case assumption age rectifying meters for lower frequencies are more is also that it is entirely 3rd harmonic. practical. Although they do net measure rms values, but peak or average the scales of these practical in- The accuracy claims for ac measuring instruments struments are always scaled so that they indicate also require a term to be added for the accuracy the rms value for a sine wave by applying either the of the local standard and it should be noted that crest factor or form factor and frequently declare the Australian national Standards Laboratory cali- on the scale "RM3 of a Sine Wave". brates true rms measuring devices for the frequency range up to 10 kHz with an uncertainty at the pre- While the sinusoid may be simple to describe sent time of O.OlJf. Consequently, a figure of at mathematically, it is rarely encountered in reality least that magnitude must be added to any maker's in the electrical field. The accuracy with which a claims. peak or average responding/rms scaled voltmeter measures the rms value of a distorted sine wave voltage While the majority of ac DVKb is average responding/ depends on the amplitude of the harmonics, their or- rms reading, there are some true rms instrument* der, and phase relationship to the fundamental. It available as well as accessory type ac/dc conver- is not practical to calculate the correction for a ters which are true ras operated. The true rme in- particular amount of distortion because of the diffi- struments use thermocouples and are much slower to culty in measuring the phase relations. However, reach final balance than the average responding types the errors can be calculated for the most unfavour- and, in general, are claimed to have worse accuracy able phase conditions and this "worst case error" than the average responding types. This will, of used as a qualifying uncertainty for the measured course, only be realized on sine waveforms of low value. The effects of harmonics differ with the type distortion. of instrument and are considered separately.

(a) Peak Responding Instruments. for a sine wave with a single harmonic added the maximum error occurs when a peak of the harmonic co- incides with that of the fundamental. The error in rms measurement will in this case be equal to the 159 References

1. Martin, F.C. "RMS Measurement of AC Voltages" Instruments and Control Systeme, Vol. 35» January 1962, pp 65-71.

2. Oliver, B.H. "Some Effects of Wavefora on VTVM Readings". Hewlett-Packard Journal, April, Nay, 0.2 Ptak mponding June 1955- (T.H.D.)

.06

.02

O.OfK , Avtraot r«panding (3rd Htrmonlcl .005 7 Avtrag* rMponding < (2nd Harmonic) /

.002

0.001% 0.001% 002 .005 0.01« .02 .06 0.1% 0.2 0.5 1.0% Distortion

Fig. 2 - Error in r.m.s. voltage indication of peak and average responding instruments.

IX CALIBRATION

The calibration of a.c. measuring DVMs requires a signal source of very low distortion and high amplitude stability (because of the slow response of true rms instruments of the thermocouple type). The range multipliers (attenuators or voltage dividers) are usually provided with adjustable capacitors to compensate for unequal stray capacity distributions. The resistive trimmers for the multipliers are adjusted at a low frequency and the capacity trinmers at a high frequency - usually 100 kHz. The supply of 1000V at this frequency with low distortion is probably the most stringent requirement for the signal source and its matching transformer as the combined capacity of the DVM, circuit wiring and the calibration standard constitute an surprisingly high loading, even if it is reactive power.

X CONCLUSION

Electronic measuring instruments are becoming increasingly used and they can posses particularly high short term accuracy. Although traditional calibration laboratories have no great confidence in digital voltmeters, they are acknowledged to be a very convenient tool.

-I« ACKNOWLEDGEMENT

The permission of the Senior Assistant Director- General of the Australian Post Office Research Labora- tories to publish this paper is acknowledged.

160 A POLAR CO-ORDINATE MULT I PARAMETER DISPLAY

J.A. Coekin, B.Sc, Ph.D., M.I.E.Aust. Senior Lecturer in Charge of Electrical Engineering, James Cook University of North Queensland

SUMMARY. After considering the need for a multiparameter display, and e*nnHTiiT»e •ome of the innate capabilities of the human operator, the paper explains the principles of the polar co-ordinate diagraa and describes an electronic system to generate the diagram on a CRT. The paper goes- on to discuss some subjective testing and closes with a consideration of some potential areas of application of the display. INTRODUCTION expose this information in a meaningful wey to the first-class pattern-recogniser we all Advances in automation, computing and carry in our heads. electronic techniques - especially for instrumentation, are making it increasily II THE TASK OF INTERPRETATIOH. possible to monitor and control complex processes and systems of many different types. To determine the desirable features of a !hese systems may consist of heavy industr- new form of display, it is worthwhile to ial plant such as generators or chemical consider the bases on which the human processors, precision equipment such as in observer is able to recognise and interpret high-performance aircraft, navigational aids patterns of visual information. and weapon guidance, or - in an entirely different field - the human body in major Interesting experiments have been surgery and intensive care. However, no carried out (Ref.1) to test the ability of matter what the system, there is a very untrained observers to monitor multichannel important common feature which will concern alphanumeric information for trends or us here, namely the interface between the specific critical combinations. Four system and the human supervisor. Even if factors were incorporated; (1) the number control is largely carried out automatically, of channels to be monitored,(2; the number somewhere there is a vital link upon which of combinations or signals to be detected, the responsible human operator must depend (3) the range of levels within each channel, for his understanding of the state of the (4) the rate of information change. To system. summarise the results; subjects detected nearly every combination while watching 8 Aβ techniques of transducing and instru- channels of information which changed every mentation improve it also becomes possible 10 seconds, even though there were several to provide more and more information about combinations to watch for. They managed the system under observation. It may not 90 per cent detection with 16 channels all be useful Information of course, it may changing every 10 seconds and 80 per cent at even confuse the pattern-recognition task 5 seconds. At this latter speed they could the operator has to perform. Information achieve 90 per cent with 12 channels. With can be presented on meters, chart recorders, up to 16 channels the number of levels made coloured lamps, numeral indicator tubes, little difference. These remarkable results CRTs, and via computer print-out, plotters show how good the human observer is at and graphic displays. spotting specific patterns of information. It Iβ no exaggeration to say that one of There is ample evidence that the human the most important things needed by modern being is very adept at recognising and inter- technology is a new breed of information preting large amounts of visual information display. For example, it is relatively easy - we do it all the time. Ye can Quickly using modern techniques to derive a great "take-in" a scene and detect its main deal of information about the activity of features; we can recognise a complex shape, various parts of the human cortex during a face say, after a very long period of time, certain mental disorders. The problem is In other words we have good perception and knowing how best to examine that information. memory for shape. It is also true that we Various computations can be performed but it can much more quickly interpret a graph than would be a major advance to be able to two columns of figures. Undoubtedly our

161 Visual system much prefers to receive information in parallel - it can handle it. It is strange then that we more often than not provide a human operator with his visual information in serial form. Control panels and consoles contain rows of meters and indicators to be read individually. Obviously control quantities must be known separately, but for a quick appraisal of the total state of a system, an operator may as well step-back and scan the whole panel for the over-all pattern it indicates - neglect- ing the numerical information. So unless the supervision is to be undertaken by computer, a display that indicates the total state of a system would be very useful especially if it facilitated the detection of trends in system behaviour. There is a small but important category of operator which needs to be borne in mind in this matter of displays - the non-expert supervisor or tester. There are many instances where items from a production line or process are examined by testers who have little or no understanding of the technical significance of the variables they are asked to measure. They are required to select and categorise acceptable items. Any display provided for these operators must be complete and stand in its own right. There will be Fig.1. The basic eight-variable polar further comment on this aspect later. diagram.

So in view of the inherent capabilities The amplitudes of the input variables of the human operator it is possible to can be scaled so that under normal conditional approach the design of a new form of display the plotted points produce a regular octagon with several particular points in mind. (shown dotted), with which all deviations (1) System technology needs a method of can be compared. In any particular displaying many parameters at once, not application attention would have to be necessarily in numerical form. given to the disposition of the parameters (2) We need not be afraid of presenting around the diagram. For instance in record- the human operator with a lot of information ings from some industrial processor, it might all at once, he can take it provided it is be decided to plot a certain group of, say, truly in parallel. pressures or flow-rates in a particular part (3) To make the best of the human of the diagram. But whatever the arrangement; perception and memory cf shape, the display with scaling or without it,normality becomes should have a form which is easily recog- associated with a particular shape which an nised and remembered. operator soon knows instinctively. It has (4) The display should be particularly been suggested (Ref.2) .that the diagram useful for detecting trends in the state of could be used as the basis for a chart, the system. subsequent sets of readings being plotted on transparent sheets overlaying the original. Ill THE POLAR CO-ORDINATE DISPLAY. A particular use, in the recording of the One form of display which seeks to meet physiological parameters of a hospital the requirements of the previous section, patient was cited, but the polar diagram and which is thought to have much to commend does not appear to offer anything over the it (Ref.2,3,4) is the polar co-ordinate type. normal set of graphs in this sort of In principle it is very simple. The amplit- application. On the other hand, it is udes of the system variables are plotted difficult to relate a number of variables outwards along radiale from a central common plotted on different graphs. The polar point and then the adjacent points are diagram could be used as a summary, indicat- joined up to produce a closed shape. This ing the total state at a particular moment. is illustrated by the eight-variable diagram shown in Fig.1. There is no doubt that the most fitting context for the polar co-ordinate display is where the variables are changing fairly rapidly, or even very rapidly with time. A suitable medium for a time'varying display is therefore necessary, the CRT being the most reasonable choice.

162 Vf THE OSCI JfOSt tOOAiti contains a CR! nay be easily be more or lee system deacrib must hare X an known relative identical aapl: assumed in the Provided the made as to the input variable > electronic plot- difficult, it 1 of the variables between 0 and +1 assigned to the . 2,4,6,8) can aia; oscilloscope X ai 4 and 6 mist be i diagonals are obt appropriate input is the sine or co the plotted point; 1. x « r « +o. 2. X = 7g 3. X » +0.7173 4. X = 0 5. X * r * -0.1 ma 6. X = m 7. X a :lf7nj 8. X * 0

,'ht

•t at, B

INPUT

Flg. Z. Block so display IT THE OSCILLOSCOPE DISPLAY. Figure 2 shows the block diagram of a simple electronic system that has been built Host monitoring or control equipment to generate the eight-variable display. A contains a CRT or an oscilloscope, or one number of standard electronic techniques and may be easily incorporated, so it seems to" circuits have been used. Obviously with a be more or less ideal,for the task. For the larger number of variables, different system described below, the oscilloscope methods might be considered. The input must have X and Y amplifiers with at least timing pulsea are applied at 7kHz to give a known relative amplifications, or preferably uniformly-bright display. The cyclic control identical amplifiers. The latter will be which consists of bistable circuits and a assumed in the subsequent discussion. diode encoder, produces a sequenoe of 8 sample pulses which are used to opes the Provided the decision has already been analogue gates at the input and output at the made as to the association between each apt opriate moment. A phas« splitter input variable and a particular radial, the provides positive and negative versions of electronic plotting of the variables is not each signal and these are applied to the difficult. It is assumed that the amplitudes corresponding output gates, as shown. Pig.3 of the variables are presented as voltages shows a photograph of the actual display between 0 and +10 volts. The variables obtained. Tektronix and Solartron oscillo- assigned to the rectangular radiale (Inputs scopes have been used. 2,4,6,8) can simply be applied to the oscilloscope X and Y inputs except that Memory may be added to the facilities; 4 and 6 must be involved. The points on the using long time-conetant circuits for short- diagonals are obtained by multiplying the term memory, and magnetic tape recording for appropriate input amplitudes by 0.71, which long-term storage. In either case the is the sine or cosine of 45 degrees. Thus procedure is to store the amplitudes at the the plotted points are defined by: moment of choice and then later to sample 1. X = Y m +0.71V-I the stored information and the active values alternately, thus producing two diagrams on 2. X = 72 Y=O the display to demonstrate the changed conditions. The use of a storage tube will 3. X = +O.7IV3 Y = -O.7IV3 also provide continuous information on 4. X = 0 Y = -V4 changing conditions. 5. X = Y = -O.7IV5 6. X = -7, Y=0 7. X = -O.7IV7 8. X = 0 +V,8

CYCLIC TIMING CONTROL 'PULSES

, 2 SAMPLE PULSES

7 *

PHASE- POS. 7 1 PLITTER NEC

4 3

SAMPLE PULSES

Fig.2. Block schematic of the electronic display generator.

163 Fig.3. The oscilloscope eight-parameter display.

An alternative method of generation has been examined in which two sine waves, shifted iti phase by 90 degrees, are used to produce a circular time-base. The sine waves are then modulated appropriately and the shape depicted in Fig.4 is obtained. Fig.4. Type of display derived from a nodulated circular time-base. As will be explained in a later section on subjective tests, it is very easy to detect changes in the shape of this sort of display, but to aid judgement a circular graticule can be used or a special transparent overlay prepared which indicates salient features to be detected. (a) Displays With More Parameters. Naturally, as the number of parameters i i increases the electronic circuits become more complex. However, the same approach I - can be used. For instance, the circuit for a 12-parameter display, shown in essence in Fig.5, would need to provide Vcos 60 and Vain 60 instead of just the sine or cosine of 45 degrees. This would lead to slightly more complication in the switching of the output gates, and an extra stage in the cyclic control with associated encoder and input gates.

Fig.5. The principles of a twelve- parameter display. 164 V SUBJECTIVE TESTS. VI ABEAS OP APPLICATION. Using the oscilloscope display of Pig.3 (a) Industrial Supervision,. preliminary subjective tests were carried out on five untrained observers to see It has already been suggested that the whether they could (a) detect small polar co-ordinate type of display could play simultaneous changes in two or more paramet- an important role in any situation where it ers, (b) ascertain the category of the is necessary for a human operator to control complete diagram, and (c) detect particular multiparameter systems. The display could features. Although only five observers were be provided either as part of the original used the pattern of results soon became console of meters etc., as a summary as it obvious. After a few minutes of instruction were, or at a remote position at which the each observer was given four tests in which operator performs other functions or super- the "normal octagon" shape was always shown vises other equipment. Facilities could be first. Each test was composed of ten dia- provided to switch-In parameters from grams and it must be emphasised that the various positions. normal shape was not shown between diagrams. Even computer control does not remove In test 1 a series of ten diagrams was the need for display. There may be shown in which there was either no change at occasions when the inherent properties of all from the normal octagon or there was an the display provide a better communication increase of .10 per cent in one parameter and interface than displayed alphanumeric a decrease of 10 per cent in another. In characters, in which case the polar diagram this series only, the observers were told could be generated by the computer. when they were correct. The subjects detected 85 per cent of the changes correctly, Usually the parameters derived from a although this was only the first trial, and real system will vary according to different only took 7 to 10 seconds for each diagram. laws; it is unlikely they will all vary With another series the same accuracy was linearly. At first thought, it would seem achieved at 7 or 8 seconds. Eetter that that with training an operator would be able 80 per cent was obtained at 5 to 7 seconds. to cope with such a situation. However, One observer found every change at 4.5 this aspect is under investigation. If a seconds per diagram; another two found computer is involved linearization would be 88 per cent at 5 seconds. possible. In the second test, the subjects were It is a common feature of engineering asked to perform a much more difficult task. systems, that control parameters are not They were asked to detect whenever both independent of one another. Rather there is parameters 1 and 2 changed (increase or a common frustration that variation of one decrease) or, both 4 and 6 changed. Note parameter affects at least one other and that one pair are adjacent to one another, control becomes difficult. It could well be the others are not. The remarkable result that the polar display can help in this was that all the observers got all the situation simply because it gathers all the answers correct at only 3.5 seconds per variables together thus facilitating the diagram. achievement of a delicate compromise. The third test, again of 10 diagrams, (b) Intensive Care. involved the description of the display as a whole, the subjects being asked to say which The success of intensive care depends quadrants were accentuated and which depress- upon constant nursing and medical vigilance. ed. This classification task was no problem The polar display could be used to advantage at all. either at the bedside or at some central console, depending upon the layout of the In the fourth test, the observers were rest of the system. At a central console asked to look for one or more of a set of there could be a display for each patient or features shown on a card next to the display. automatic cycle switching on the one CRT. The features included, for example, a Numerical information would always be horizontal line between points 4 and 5 or 3 available for more detailed analysis. and 4; and a right-angle formed by the lines joining 6,7 and 8. They found 97 per cent (c) Product Inspection. of the features at 8 seconds per diagram. The inspection of industrial products The untrained observers performed very usually involves some measurements using well indeed and it may be supposed that special equipment and an overall judgement trained operators would have no difficulty by either an inspector fully conversant with at all in detecting changes and trends in the technical significance, or an unskilled the display. operator trained only to read and record various numerical information. If the necessary measurements can be automated then the polar display could be used to Indicate

165 the total category of the product. A trans- Curing the search for a method of parent overlay could be used on the CRT displaying spoken words, it was decided to which would define the boundaries of accept- attempt to plot the values of component ability of each variable, as shown in Pig.6. frequencies on a polar diagram. Data was available for twenty-four component frequencies of a series of words spoken by the same person. These were plotted on the twenty- four radii of a polar diagram and are shown in Pig.7. Since characteristic shapes were obtained it was decided to attempt to use the oscilloscope display. This proved to be successful, especially using a storage tube. Unfortunately even the same word spoken repeatedly by the same person varies so there needs to be some further investigation on the feasibility of this method. It must be added that component frequencies are not necessarily the best criteria on which to characterise speech, but in this case the information was available. VII CONCLUSIONS. The polar co-ordinate display is strange and unfamiliar because it departs from what we normally expect in a display. But it has several features which recommend it for use in the supervision and description of situations which involve a large number of Category 1 Category 2 variables. The CRT version greatly enhancea the possibilities for application. The important aspect as far as users are concerned is tc take a long serious look at Pig.6. A display with overlay for display requirements and to be prepared to product inspection. try the unconventional, VIII REFERENCES. (d) The Examination of Multiple Sets of Data. 1. GOU1D, J.D. and SCHAFFER, A. - Automatic pattern-recognition processes Visual monitoring of multi-channel depend for their success on being able to displays. Human Factors. 7, 1966, work with those parameters which contain pp. 69-76. most information. But because the amount of information is so large it is not always obvious which parameters to choose for the 2. WOLFF, H.S. Signal received but not decision-making. It has therefore been understood. IEE Colloquium on the suggested that if many sets of this infor- Interpretation of Biological sumlla. mation were presented rapidly to an 1967. observer, rather like a movie film, he 3. COEKIN, J.A. A versatile presentat- would be able to pick out those parameters ion of parameters for the rapid which changed most or least. recognition of total state. Proc. In a preliminary test, an observer was BE Int.Sym. on Man-Machine SVatema. shown four series of ten diagrams each and Sept.19b9. asked to say which parameters varied most in each series. One showing of the series was 4. HANSEL, C.E.H. (Professor of insufficient, 3 or 4 being necessary. The Psychology, Swansea) Private observer successfully picked out the param- Communication. eters which changed most. From an analysis of the known parameter values it became clear that the recognition was not on the basis of the number of changes or of deviations about the "normal" value (5 volts), but of the total variation in volts of the parameter added up from set to set in the series. This process is under further investigation. (e) Speech Analysis and Display.

166 f6 f.

•12

Fig.7. Polar diagrams of the component frequencies of the spoken words REED, HOD and HEAD, (to same scale).

167 !

LOW COST COMPUTER COMPATIBLE DATA LOGGER

A. Ceresa, M.I.E.(Aust), M.I.R.E.E.(Aust) Experimental Officer, Division of Irrigation Research, CS.I.R.O.

I INTRODUCTION the DPM is converting normally, in this case at five conversions per second, and the multiplexer has its A requirement to automatically scan and record 50th coil energised. (Normal position at end of a up to 50 points with a predetermined time interval scan.) When the elapsed timer, T, operates, the provided the motive for a system using established next conversion complete pulse fron the DPM starts items as much as possible and therefore requiring the following sequence. It pulses the bistable unit minimum design effort. of the multiplexer therefore energising coil one, inhibits further conversion complete pulses being The Schematic of the system is illustrated in received and applies a hold signal to the DPM for Fig. 1 and consists of a 50 way 4 pole reed uni- one second, allowing the reed switches and any selector, M, which connects the transducers in turn amplifiers time to settle. When the hold signal is to a Digital panel Meter, DPM, the binary coded removed, the next conversion complete pulse gates the decimal output at the DPM together with other informa- required information into the input buffer of the tion is interfaced and fed to a paper tape punch, P. interface, I, and characters are assembled and punched until a record or frame is completed. During To allow meaningful information to be recorded this time the multiplexer bistable unit is pulsed and a control, C, an interface, I, and an elapsed timer, coil two is energised repeating the cycle. T, were constructed using standard TTL components. Consequently the bistable unit of the multiplexer is actuated until the 50th coil is again energised. II THE OPERATION OF THE SYSTEM This closes a reed which operates a schmitt trigger, the resulting pulse is delayed and then used to reset The initial conditions for operation are that the control flip flop, thereby finishing the scan.

transducer input fig. 1 BASIC SCHEMATIC 168 I Referring to figures 2a & b showing the waveforms multiplexer and simplified logic for the scanning cycle, the operation is as follows -

The conversion complete pulse from the DPM is gated through the first AND gate when the elapsed timer control flip flop, C, goes true producing the delayed waveforms C, D, and E.

Waveform, C, inhibits further pulses being received whilst D, provides a hold signal to the DPM and also the multiplexer change signal. The final delayed pulse, D4, produces the waveform, E, which gates the next conversion pulse through to the reading and punching section. This pulse also restarts the scanning cycle. When the multiplexer has its final coil (50th) energised a further delayed pulse, D5, resets the control flip flop, C, completing the scan. Vβ

lit fig. 2 (a) SCANNING LOGIC

the ing and ! 3 sec. is n it n sset n n JL n St. —*J2nd reading 50 D

E n n fl fig.2 (b) SCANNING WAVEFORMS punch The reading and punching section diagram is shown in Figure 3. P buffer j output l

On the receipt of a pulse from the scanning • i circuit (WAVEFORM, F), th@ required input information is gated in parallel into a register which holds 9 characters. A clock is then started which produces COUNT flip a train of pulses followed by a punch command. The — ro MM train of pulses serially shifts one character from • flop the register to the output buffer and the punch command signal is fed to t*ie paper tape punch to < • • produce the character. After 9' sets of pulses the clock i J clock is inhibited and the circuit awaits a further

conversion waveform, F. »registe r Jl j from scanningI circuit 169 fig.3 RECORDING LOGIC Ill MULTIPLEXER

TTie multiplexer used was a 4 pole 50 way reed type with a self-contained pulsing circuit. Briefly, each channel has one coil, and one reed in that coil holds the coil energised while another prepares the circuit for the next coil. ro-irJ | ^Integrator Even channels connect to the X rail while odd channels connect to the Y rail and a bistable unit switches the supply to X and Y rails alternately. An T" Sj S2 " zero detector auto restart circuit is incorporated to energise a coil when the supply is switched on. 2 A2 Considering the circuit diagram figure 4 with external power connected to rail X and RLO energised contact RLO/l provides a hold for RLO and RLO/2 connects RL1 control to rail Y. At the next pulse the bistable unit switches the supply to rail Y and RL1 is energised. In this way each coil is progressively energised and scanning can be accomplished if contact RLN/2 is connected to RLO as shown.

When the power is applied to the circuit the relay coil RLS has current flowing for a short while sufficient to operate contact RLS. Similarly, the bistable unit is biased to the, X, rail. Therefore a path is established to energise ceil RLO, and hence ensure consistent starting.

Ihe purpose of capacitors 00, Cl, C2 etc. is to delay the release of RLO/2, RLl/2, BLz/z etc. so that the succeeding coil can be primed ready for the next pulse.

Advantages - IV DIGITAL PANEL METER (DPM) fig. 5 Low power requirements Low contact resistance Several digital panel meters were examined and Independent contacts instead of common they could all have performed satisfactorily with of wiper circuit course suitable interface changes. Most used the High reliability same technique for the analog-to-digital conversion, Sealed contacts easily replaceable reed i.e. dual slope integration. capsules In the dual slope integration or the double ramp technique it is required to integrate both the

unknown input (VA) and the known reference (Vref) and • Vβ compare their slopes. Figure 5 illustrates a typical

system. Initially, both Sj and S2 are off, and the output of the integrator is at some negative voltage with the counter precleared, i.e. at zero. When the output of the integrator (a ramp with a slope +VA/RC) crosses zero going positive, the counter is gated on. This applies the clock to the counter which counts until it reaches maximum (2n), and as it returns to zero the resulting overflow signal switches Sj off

and S2 on applying the known reference voltage to the

integrator, giving a ramp with a slope of ~\re^/fC. The counter will continue to count until the output of the integrator again crosses zero, going negative. This is detected by the comparator (zero detector) and the counter is inhibited, thus the contents of the counter is the digital representation of the input V^.

The DPM finally selected was from DATA TECHNOLOGY having a range of 0 - 2 volts represented fig.A REEO MULTIPLEXER CIRCUIT by four digits and where accuracy was better than 0.1%. This meter performed beech tests reliably and DIAGRAM. sufficient software was supplied with it to understand and use it effectively.

170 PAPER TAPE PUNCH

A paper tape punch was selected which could be directly interfaced to standard transistor transistor logic (TTL) and be actuated asynchrously while having few moving parts thereby reducing maintenance to a minimum.

An incremental motor drives the paper tape via a capstan and the punching mechanism uses solenoid actuated punching pins. Standard 8'1 reels of paper tape can be accommodated and a take-up spool is i incorporated. fig. 7 A complete tape can be punched in just over Z\ days so that site visits could be restricted to 3 per week. is constant e^ will decrease linearly with increasing temperature until a tail off occurs at higher temperatures.

o o oooo Over the linear range, the junction voltage i*

ooooooo 6j - ej0 + d(T - To) ooooooo oo where ejo - initial junction voltage o o o To = initial temperature O 00 O T - final temperature o ooo o d = diode temperature coefficient in volts per degree and of course is negative fig.6 EXAMPLE OF PAPER TAPE FRAME In practice the diode used for soil temperature (1217340 CR LF) was a IN2326 which provides good linearity over the required range say -5 to 45°C with a sensitivity of approximately 2mV per °C, the constant current supply being 1mA from a 5V source.

PAPER TAPE FORMAT The IN2326 is hermetically sealed in a transistor can with a diameter of 6mm and a length of The punched paper tape is coded in even parity 10mm. The leads enter the base of the package and ASCII format which can be processed by standard the diode is thermally coupled to the surroundings Fortran programmes. Consequently, the user of the principally through the can rather than the leads. equipment can write and modify them to suit his The time constant of the device varied from several requirements* seconds, when immersed iu water, to approximately a minute in still air. As a protection against Referring to Fig. 6, the frame or the paper tape moisture and subsequent corrosion a screen insulated representation per channel consists of 9 characters. pair of wires was soldered to the diode leads and the The first two characters indicate the multiplexer can with the bare portion of the leads was potted in channel that is currently connected to the DPM araldite, this probably resulted in a doubling of the (Example 12), the next four characters record the time constant but as the frequency of reading was one DPM"s output (Example 1734), whilst the seventh every 15 minutes it was not considered it would character denotes the polarity of the DPM output adversely affect measurements. (Example 0 - positive number) and finally the last two characters, Carriage Return and line feed, are The remaining transducers consisted of several necessary for the computing process. solar radiation measuring instruments and a nuaber of thermocouples. TRANSDUCERS

Germanium diode thermometers were mainly used as temperature sensors because of their high sensitivity, cheapness, linearity and ease of use.

The principle of operation of the diode for temperature measurement can be basically illustrated by referring to figure 7. If the battery voltage E is large compared to the diode junction voltage ej, the diode forward current, If, is essentially constant and controlled by adjusting Rj. Thus if If

171 CONSTRUCTION

The microcircuits were assembled onto double circuit boards 8 inches by 5 inches which plugged into a standard 19 inch rack. This also contained the DPM and the logic power supply. The rack was installed into a weather proof heat insulated metal container which housed the Multiplexer, an input patch board and the paper tape punch.

Access to the equipment was from the front and top.

CONCLUSIONS

The equipment was field tested during the summer and preliminary results have been encouraging. Further evaluation is in progress to determine the system reliability.

To cater for the demand for further logging facilities coupled with the availability of digital transducers a benefit in cost and reliability could be achieved if a purely digital system was considered.

Work is in progress on this using data selectors and counters in place of the reed multiplexer and DPM respectively.

ACKNOWLEDGEMENTS

The permission of the Chief of the Division of Irrigation Research to present this paper is gratefully acknowledged.

172 THE GENERATION OF PSEUDO-RANDOM BINARY SEQUENCES FOR TESTING VERY WIDE BAND SYSTEMS J.A. Coekin and J.R. Wicking Electrical Engineering Division, James Cook University of North Queensland

SUMMARY. After explaining the baeia of testing systems with noise, the paper describes the advantages to be gained in using pseudo-random binary noise. The generation of pseudo- randoa binary sequences at very high bit rates is discussed, and then the general theory is given for a method using parallel shift registers by which sequences are produced at a bit rate which is a multiple ot the highest obtainable with a single register. The paper closes with a consideration of the practical implementation and experimental work to date. I TBSTJXG WITH NOISE AND IMPULSES. the increasing use of pseudo-random noise which has a number of desirable properties as Since all electrical systems are subject will become apparent later. to the random disturbances of some form of noise, noise itself has assumed an important A linear system may be characterised by role as a test 'signal'. Moreover, the wide determining its response to an impulse input, spectrum of broadband or "white" noise is since the output is equivalent to the trans- often a more exacting and therefore satis- fer function of the system. Unfortunately factory test of a system than simply a rep- impulses are difficult to generate and lica of the likely real-life signal the dangerous to use. Generation is a problem system will carry. An example of this is the because clearly the output can at best be use of white noise to test a multichannel only an approximation to the ideal. Impulses telephone system which will normally only are dangerous because when small enough not carry speech. Different types of systems to threaten parts of the system, they may not are being increasingly studied by simulation, have enough energy to disturb it decisively either in the form of scale models or on an in the presence of noise of comparable analogue or hybrid computer. This facilit- amplitude. If large enough to be disting- ates the determination of noise response, be uished from the noise, the system may be It of vibrations on a building or random driven into non-linear operation. variations in a control system. However, it is possible to achieve an Unfortunately, as a test signal white Impulse response without actually using an noise is not entirely satisfactory for sever- impulse test signal. Assuming some form of al reasons. For instance it is preferable noise input the impulse response can be that the power density spectrum is flat over obtained mathematically as follows; provided the frequency-band of interest, but it is not that the autocorrelation function of the always possible to achieve this particularly noise input is an impulse (i.e. the noise at low frequencies with the commonly-used bandwidth is larger than the system band- noise generators (Ref.1). Furthermore, sig- width), then the cross-correlation of th# nals generated by these sources are small and system response with the test signal will be •ust be amplified, thus introducing further the impulse response of the system (Ref.2). low-frequency components such as drift which The cross-correlation is achieved by are indistinguishable from the noise signal. multiplying the output and a delayed version Another disadvantage of white noise is its of the input, and then averaging. Ideally atatistlcal variance. While it may be corr- the averaging time should be infinite, which ect to subject a system to a perfectly random is impracticable. However, the statistical variance introduced by using a finite tie* input, there may be certain desirable prop- is acceptable provided the time is reason- erties which the input should have. To ensure ably large. White noise has an ideal auto- that these properties are included in the correlation function but also has all the noise the system must be tested for a long disadvantages discussed above. The statist- time, eertainly longer than if the properties ical variance problem is overcome by could be deliberately built-in to the signal. pseudo-random noise which,, aa its naa* Shis disadvantage is particularly important implies, is a signal which behaves like in testing systems in which the operation- noise but is in fact periodic and may be time of interest is very short. These short- designed to have certain propert- comings of "natural white noise have promoted 173 iee (Ref.3). A number of binary codes exist Problem by using integrated circuits in which will produce a periodic two-level which layout is optimised, although there ie signal which is random but s;nthesised and here the problem of having to take what Iβ which has an autocorrelation function that available. The final solution will probably is approximately impulsive. (Ref.4). This be a judicious mixture of both methods. For is referred to as Pseudo-Random Binary Noise instance the system to be described later (PRBN). These signals have spectra and will employ a tunnel-diode flip-flop which power-density functions similar to white will toggle at 1.5Obits/see., but it is not noise but have controlled properties. As the suitable for use in a shift-register because number of bits in a sequence increases, the there is no master-slave facility. The fast- autocorrelation becomes an increasingly est IC flip-flop with this facility, avail- better approximation. In the cross-correlat- able commercially, is the UBCI III which hai ion process, the averaging time can be made a quoted maximum toggling speed of 33OMbite/ equal to the period, in which case the out- sec. but a confirmed maximum in the shift- put is repeatable - no statistical variance. register context of about 160Kbite/sec. The Since the signals consist of a digital wave- new method of generation presented here was form, the multiplication in the cross-correl- developed in order to find a way of ueing ation is achieved by a simple gating function. these circuits in a configuration that would The PRBN will not correlate with the actual permit a substantially-higher effective bit noise on the system so the input does not rate. The method basically involves the have to be large enough to swamp the natural modulo-2 addition of an »-sequence and a noise. Thus a damaging large-amplitude test delayed version of the same sequence, signal is not required, and the test can producing the same sequence at the output, even be performed while the system is operat- but at twice the speed. The general case will ing normally. now be considered. II GENERATING PSEUDO-RANDOM BINARY Ill GENERAL THEORT OF NEW METHOD. SEQUENCES (PRBS). When an m-sequence Si is sampled at One easily-generated and therefore often every (n+1)/2x bit, where n is the period of used form is the nm" or maximal-length sequence. the sequence in bits and x is an integer Such a sequence is readily generated (Ref.5) 0,1,2..., then according to Oolomb (Ref.9) using a shift register and modulo-2 the output sequence obtained is a delayed (exclusive-or) feedback from the last stage replica of the original sequence Si, of the register and at least one other provided (n+1)/2z is an integer. selected stage. The choice of the feedback stages and the implementation are well known Now suppose that Si is modulo-2 added (Ref.5). to another sequence which is a replica of Si but delayed in time by J bits and a half-bit The highest possible bit-rate of the (i.e. it is staggered). It is now necessary sequence is of coarse limited by the maximum to represent the output sequence by two modu- operating speed of the switching circuits lo-2 equations since a transition may occur which make up the register and the modulo-2 in the second input sequence in the middle adder. Therefore if we wish to test very of a bit in the original sequence. This ie wide band systems by this method, the fastest illustrated in Fig.1. The two equations are circuits and configurations must be applied. therefore:- High speed logic families are available using tunnel diodes (Ref.6) and have been Si ® Si-J e Si-K .(1) successfully implemented in commercial equipment. However, the outstanding problem where K is a constant delay. in the use of tunnel diode logic is that the diode has to be reset before each logic and Si @ Si-tf-(n+1 )/2x = Si-L ... (2) operation can be performed. This disadvantage has been overcome by a method (Ref.7) where L is a constant delay. in thich each logic element is duplicated so that one is being reset while the other is Now let L-K - P(n^1)/2x functioning. This arrangement requires a complex clocking waveform. A similar method where P is a constant. (Ref.8) has been used to produce a sequence at 200Ün>it8/sec. using pumped diode circuits Eqn.2 now becomes and delay lines. There are in fact two main lines of approach to the design of high speed Si® Si-J-(n+1)/2x a Si-K-P(n+1)/2x ...(3) PRBS generators. On the one hand there are especially conceived and designed circuits The next term in the output, due to eqn.1 ie; using high speed discrete devices in unique configurations, much like the two systems Si-(n+1)/2x@ Si-J-(n+1)/2x = Si-£-(n+1 )/2x just mentioned. The physical construction of these circuits may amount to a consider- (4) able problem bearing in mind the need to provide subnanosecond switching speeds. The other approach seeks to avoid this sort of

174 ORIGINAL

SEQUENCE Si

the STAGGERED SEQUENCE Si -J Iβ lid It

MODULO- 2 Will ADDITION

original Of bit rate Sj © Si-J=Si-K 'Si © Si-J-(n+1)/2x sSi-L

Pig.1. Modulo-2 Addition of Original and Staggered Sequences.

The next term due to eqn.2 is:- Hence the final output sequence can be e written. re @ SW-2(n+1)/2x

x Si-K, Si-g-P(B+1). Sl-g-(n-t-i). = Si-K-(P+1)(n+1)/2 (5) 2X 2X

Si-K-fP+1)(n-f1) x 2 .(6)

is: x

175 For this sequence to be a shifted replica of Si, it must have equal increments which must eq: also be powers of 2. i. Thus P=4 FASTER Therefore the output sequence is a shifted replica of Si if and only if I-K = (n+1)/2.2x CLOCK Hot« that the resultant sequence may be described as a shifted version of the parent sequence(s), sampled at a different increment. The sampling increment in the resultant is (n+1)/2.2x or (n+1)/2x+1 which indicates that 2-STAGE the powvr of 2 in the denominator may be considered to have increased by one due to RfNG- the modulo-2 addition. (The value of z COUNTER being appropriate to the generating sequences), The output sequence (eqn.6) nay therefore be written as Si-Z, Si-K-n+1 , Si-£-2(n+1). Si-K-3(n+1) pt+1 2x+i gX+J

It now only remains to solve for J. Subtracting (n+1)/2.2x from each term in Fig.2. Arrangement of Two-Register eqn.1 gives Generator. Si~(n+1)/2.2X @ Si-J-(n+1)/2.2x Since the value of z appropriate to the • Sl-K-(n+1)/2.2x generating sequences is zero, then the resultant sequence can be written (from but according to eqn.3 eqn.7); x x Si @ Si-J-(n+1)/2 s Si-Z-(n+1)/2.2 Si-Z, Si-K-J x x 3 a .'. J + (n+1)/2 = (n+1)/2.2 loa x Si-X-3.(n+1), .(9) orj = -(n+1)/2.2 (8) 17 This is a delayed version of Si sampled at (a) Practical Realization. every (n+1)/2 bits; the output bit-rate thus being twice that of the generating eac Fig.2 shows two four-stage shift sequences. ace registers in parallel with their outputs out modulo-2 added to give the final sequence. The method is readily expanded to more mati The registers are driven by alternate clock than two registers. Fig.3 illustrates the eig] pulsea thus giving the effect of staggering four-register case. The final output is 12 one register by a half-bit. In order to now generated at four times the bit rate in determine the initial loading of the second any single register. The loading rules can register it is first necessary to establish be derived on the basis of eqn.8. is the appropriate value of z. epe« For the first two registers in Fig.3, fad The original sequence i.e. that produced z « 0 and the equation for the generation of the by the first register, has not undergone the first bit of the output is the any actual sampling process but is funda- pula mentally equivalent to the sequence produced Si® Si + (n+1)/2 « Si-Z .(10) by sampling at every (n+1) bit. Since the resp sampling increment is defined in the fore- chan The result of the modulo-2 addition is a prop going analysis as (n+1)/2*, then clearly at sequence given by expression 9. this stage z>0. Thus the initial loading one of the second register is to be advanced by It (n+1)/2 » 8 or, because of periodicity, This (sequence is to be modulo-2 added addi with another, and eqn.8 with z « 1, indicate« are delayed by 7. (In eqn.8 the negative sign that this other sequence must be advanced by indicates advancement since J is a delay). (n+1)/4. The first term will therefore be a co Si-Z+(n+1)/4. The equation that gives the 0 generation of this bit is obtained froa bina 176 eqn.10 by adding (n+1)/4 to all indices, (Ref.11). This circuit may be clocked by positive or negative pulses which make it i.e. Si + (n+1) @ Si + 3 (n+1) = Si-K+n+1 ideal for the present purpose. Narrow pulses are required to operate the counter so the outputs from the registers would need to be differentiated. The passive R-C

STER |CLLOC( K I 4-STAGE RING I I I t XXJNTER

he 1 r T—i Pig.3. fcur-^ti^e

The generating sequences (from registers differentiator is not satisfactory because 3 and 4 in Pig.3) must therefore have initial of the long exponential delay which effect- loadings advanced by (n+1)/4 and 3(n+1)/4. ively increases the pulse width. An interesting solution (Ref.12) is shown in 17 IMPLEMENTATION. Fig.4. If a step is applied, the output t pulse has a duration of 2L/c where L is the The four register case described, with length of the 25 ohm stub. The pulse also each register capable of 160Mbits/sec. would, has equal rise and fall times, and with a according to the theory above, produce an negative-going step a negative pulse results. output sequence at 640Mbits/sec. The same The problem now remains to terminate the e mathematical argument can be applied to the output line without degrading the pulse. e eight-register case which would produce a It is envisaged that this will be possible 1.2Crbit/sec. output bit rate. using an ultra-high-speed transistor in the in common-base mode, the output of which may drive the flip-flop. an The first major problem of implementation is to achieve modulo-2 addition at such high speeds. However, there is an important The remaining problem is the rise-time factor which emerges from an examination of improvement of the outputs of the shift of the different times at which the outputs of registers. One solution is to use tunnel- the shift-registers change. Since the clock diode monostable circuits and a particular pulse to each register is staggered with design is available (Ref.13) with a cycle respect to the previous one, no two registers time of 3OOps and pulse rise time of 7Ope. change at the same time. Now the basic There may be some difficulty with combining property of a modulo-2 adder is that if only the monostable outputs to trigger the one input changes, the output must change. flip-flop. It is therefore possible to perform the addition on the transitions, provided they are clearly separated in time. Fortunately a counter fast enough for this does exist.

Ortel (Ref.10) describes a one gigahertz binary counter of the same design as Chow 177 50A

[25 ohm) shorted stub

output stage of register 1.

Pig.4. Circuit Arrangement to Produce Narrow Symmetrical Pulses.

T EXPERIMENTATION. VII ACKNOWLEDGEMENTS. The basis of the method presented here The authors wish to thank the Research has been confirmed by a computer investig- Laboratories of the Australian Post Office ation and by the construction of a low-speed for their support under contract CO.40861. model using TTL components. A single flip- flop and two pulse-forming circuits were VIII REFERENCES. used for the ring-counter which clocked two 4-stage registers. The feedback was obtained 1. ANDERSON G.C. et al, Pseudo-Random and from stages 3 and 4. The register outputs Random Test Signals, Hewlett-Packard were modulo-2 added by an exclusive - OR Journal Vol.19, No.1, sept.1967, gate and by the transition-counting method pp. z-16. using another flip-flop. Synchronising the system was no problem since with this logic 2. STAPLETON C.A. Identification using family the clocking is inhibited during set Artifical Noise. Research Report, and reset opertaions. The initial setting of University of N.S.W., Sept.1970. the output flip-flop was derived from the output of the exclusive - OR gate. The same 3. ZORN G.A. Random Process Simulation and property for the set and reset inputs holds Measurements. N.T. McGraw-Hill. for the MECL III D-type flip-flops so this feature will be included again. A tunnel- 4. CORRAN E. and CUMMINS J. Binary Codes diode flip-flop has been constructed using with Impulsive Auto-Correlation 1mA, diodes and it has been triggered from Functions for Dynamic Experiments. both a monostable and a common-base stage. Atomic Energy Eat, ffinfrith. U.K., teport No.zio ^AAEW-zio; sept, l962. VI CONCLUSIONS. 5. YOUNG R.G. P.R.B. Sequence Correlator It is possible to generate pseudo-random Using ICs. Electronic E"^ veering T sequences at very high bit-rates, of the Vol.41, No.499, Sept.19bi7 pp. 41-43 order of IGblt/sec, by using the configuration of shift registers described. The higher 6. CHO T. and ZIMBEL N. Univer - a last, speed achieved thus facilitates the testing Versatile Multi-Logic Digital Amplifier of wide-band systems. There are still some for Micro-Logic Circuits, Proc. IEEE problems with the implementation of the Vol. 52, No.12, Dec.1964, p.1591. modulo-2 addition but several tunnel diode circuits are available and other techniques, some original, may be employed.

178 7. COOPERMAN M. Gigahertz Tunnel Diode logic RCA Review Sept.1967 pp.424-459. 6. MAROLF R. 2OQMbit/sec. Pseudo-Random Sequence Generators for Very Wide Baud Secure Communications Systems, Proc. N.E.C. Vol.14, P.183-187, 1963. 9. GOIOMB, S.W. Shift Register Sequences. Holden-Day, San Francisco, 1967. 10. ORTEL W.A One Gigacycle Binary Counter, Proc. IEEE, Vol.52, No.12, Dec.1964, p.1746. 11. CHOW W.F. Tunnel Diode Digital Circuitry 1KB Trans, on Electronic Comp Vol.EC-9, ITo.3, Sept.1900, pp.295-301. 12. ROSS G.P. Transient Analysis of Certain TEM-Mode Four-Part Networks. Trans. -on Microwave Theory. No.10, Nov.1966, p.528. 13. ORTEL VT. The Uonostable Tunnel Diode Trigger Circuit, Proc.IEEE Vol.54, No.7, July 1966, P.936.

179 DEVELOPMENT IN MICROWAVE INSTRUMENTATION FOR INDUSTRIAL PROCESS CONTROL

D.W. Griffin, B.E., B.A., Ph.D., C.Eng. Reader in Electrical Engineering, The University of Adelaide

SUMMARY. A comprehensive range of transducers exists for relating non-electrical quantities to electrical quantities at microwave frequencies. Until recently, industrial application was hampered by the relatively expensive microwave source and power supply needed for operating any particular measuring system. The development of more reliable and efficient solid-state microwave sources requiring relatively simple low voltage power supplies now gives microwave instrumentation an all-solid-state status and has stimulated renewed interest in transducer design and industrial application. Microwave transducer developments are discussed in detail under two classification headings; transducers for mechanical quantities and those for determining the physical properties of materials. The principles underlying basic types of transducer and microwave signal processing assemblies are explained and various specific applications are discussed. A new type of transducer that exploits the properties,of solid state microwave negative resistance devices (TED and IMPATT devices in particular) is proposed and simple means of compensating against temperature effects explained.

I. INTRODUCTION and a data display unit. In a process control application the rest of the units forming the control loop In general, industrial process control involves would follow the microwave detector. A block diagram the measurement of non-electrical variable:, such as of the basic features of most industrial microwave amounts of material, flow rates, geometrical dimen- instruments is shown in Figure 1. The variable sions, physical properties, etc., and use of the quantity to be measured and possibly controlled, measurements to effect control so that a particular provides an input signal to the microwave transducer objective such as product uniformity or programmed in the assembly. The microwave parameter that is re- changes, can be achieved. The development, in recent lated to the required variable can be any one of a years, of more reliable and efficient solid-state wide range that includes reflected wave amplitude, sources of microwave power requiring relatively phase or frequency, transmitted wave amplitude or simple low voltage power supplie:; has overcome the phase, cavity resonant frequency, wave polarisation, main disadvantages that stood in the way of the etc. Other variables will provide inputs to the introduction of microwave instrumentation in indus- transducer and it is necessary to minimise their try . As a result, a new impetus has been evident in influence on the related microwave parameter. the development of microwave transducers for use in such harsh environments as steel rolling mills, In general the transducer is not a microwave margarine production plants, etc., (Ref. 1, 2). The generator. A microwave signal processing assembly is challenge to all types of transducer is mainly that therefore necessary. It is energised by a microwave they should accurately measure the required variable source and part of this power is used to produce a and at the same time not respond significantly to microwave output signal which is a measure of the others associated with the process or the environ- microwave parameter that is related to the input ment. As a result of this the design of a measure- variable to be measured. Some or all of the variables ment system for an industrial process usually other than the one to be measured may influence the involves not only the prime variable but also other microwave signal processing assembly so here again the variables to which response must be reduced below designer must minimise the effects of unwanted inputs. certain levels. It is because of the existence of The microwave output signal is detected and the vol- difficult environments with many associated variables tage or current output is used for display and that the search for alternative transducers for measurement or operation of the next unit in the con- measuring specific variables continues and evolution- trol system. ary improvements to microwave components are of great III. relevance. For instruments that provide continuous monitoring in a control application some means of calibration without significant interruption to the process is In II. GENERAL PRINCIPLES OF INDUSTRIAL MICROWAVE usually desirable. Advantage can be taken of the INSTRUMENTS. rapid response time of most microwave assemblies simple compared with that of other parts of the closed loop The microwave measuring assembly generally conso that calibration and adjustment is effected in a sists of a microwave source, a transducer, .some time interval short enough to cause negligible dis- microwave signal processing components, a cetector turbance. Depending upon the detailed nature of the motion Figure 180 rROL assemblies for measuring mechanical quantities. The assembly of Figure 2(a) can be used to measure surft contour detail and mechanical vibration associated with, for example, the commutator on an electrical machine (Ref. 3, 4). Input variable Other variables to be measured Hi Components associated with production of primary microwave signal Calibration j Transducer Transducer.

ical Related microwave Other variables iy parameterameter III velop- power Signal processing t in 1 111 (a) under 1 sing Microwave. Microwave signal source ~™^ processing assembly

Microwave Interference response factors I 1 Signal processing loop quantity (b) agram ill /e

Detection of microwave icer quantity and display of » re- magnitude. a

.on, Output for operation of control loop (c) 1 Transducer •y is rave a Figure 1 General operational features of microwave instruments for industrial applications. .ables :he .n the transducer and signal processing assembly on-line Source tputs. calibration nay be undertaken at various points - not • K3 IT* J signal -ol- 1 necessarily the input port of the transducer. processing (d) con- III. MEASUREMENT OF KINEMATIC QUANTITIES AND PHYSICAL DIMENSIONS. :or- Figure 2 Schematics of microwave assemblies for •ation In the case of metal objects relatively simple measuring kinematic quantities and s microwave assemblies can be used to 'determine the physical dimensions simple kinematic quantities and the physical (a) Surface contour and vibration measurement. dimensions. Almost all of the microwave energy inci- .oop dent on a metal surface will be reflected and carry (b) Measurement of the presence of objects or i a precise information about the nature, position and their velocity (Doppler). s- notion of the surface. To illustrate these points (c) Measurement of the thickness of aetal sheet. the Figure 2 shows schematically a series of microwave (d) Measurement of the diameter of metal rod.

181 The transducer is the open end of a waveguide which constant and will be set at a position on the has been reduced to the smallest practicable cross- cavity resonance characteristic that results in rapid sectional dimensions to give maximum resolution. The change in detector signal with increase or decrease end is placed as close as practicable to the object in tape thickness. under test. The microwave source is connected via an isolator to the E port of a magic tee type microwave Where complete reflection from the object is not hybrid so that one half of the available power pro- practicable, as in monitoring the diameter of ceeds to the transducer and one half to an adjustable extruded rod or tube, the assembly of Figure 2(d) can short circuit termination. The microwave signal that be applied (Ref. 1, 7). Transmission between two reaches the detector will be the phasor resultant of antenna-type transducers will depend upon the diaae- components reflected by the object under examination ter of an interposed rod. The transmitted signal and the face of the short circuit. With a short obtained with rod of nominal size can be balanced circuit across the transducer aperture the detector against a fixed reference signal by using a bridge signal can be set to a minimum, near zero, by adjust- assembly that incorporates two magic tees. The tees ing the position of the instrument short circuit. in this ease are used simply as a power divider (left With the rotating object present the detector will hand side component) and a signal adder (right hand indicate the difference between the reflection co- side). Variation in rod size free the nominal value efficient of the object and the calibrating short yields a resultant signal in the detector proportion- circuit as sensed by the transducer. Clearance al to the change from balance signal level through between the transducer aperture and the reflecting the transducer path. surface has a big effect on the signal and can be calibrated by making measured changes to this distance. By displaying the signal on an oscilloscope IV. MEASUREMENT OF CERTAIN PHYSICAL PROPERTIES OF a record of the detailed surface deviations from a MATERIALS. perfect circle under dynamic operating conditions can be obtained by synchronising the sweep with the Assemblies similar in principle to those of rotation. Random non-synchronous variations in the Figure 2 can be used for measuring various physical displayed signal would then be indicative of vibra- properties of materials. It is usually necessary for tion arising from worn bearings, etc. the microwave energy to pass through the material which behaves as a lossy dielectric. Water has a To detect the presence or absence of an object or pronounced effect on the attenuation coefficient of to determine its velocity and in turn its length as materials at microwave frequencies and so many of the it moves, the assembly of Figure 2(b) is simple and applications are to situations where moisture content effective (Ref. 1). All of the reflected power is of importance in a process (Ref. 8). Figure 3 gathered by the transducer passes through the circu- shows assemblies similar in principle to those of lator to the detector. There will be some direct Figure 2 but adapted for measuring the moisture con- leakage from the source to the detector. In detect- tent of various materials. The main adaptation lies ing the presence of an object there should be an in the detail associated with the transducer. increase in reflected signal that is large compared with the leakage level. Thus leakage acts as a The moisture content of textiles and paper can be threshold level and limits the range at which the monitored using the assembly of Figure 3(a). The instrument will operate satisfactorily. In measuring material passes belt-like between an antenna and a object velocity the frequency of the reflected signal relatively large metal plate. If the material is a is shifted by Doppler effect and the leakage signal quarter wavelength from the plate the electric field is required so that the Doppler shift can be obtained associated with the microwave standing wave pattern through mixer action as the frequency difference and the absorption due to the moisture will be a between transmitted and reflected signals (Ref. 5). maximum. The instrument can either be calibrated to The design of the transducer is basically a problem indicate moisture content accurate to one or two in antenna design. The length of a moving object, percent or arranged to indicate changes from a desir- such as a steel billet, can be measured as the time ed content. The arrangement of Figure 3(b) is integral of the velocity throughout the period that superior to that of 3(a) where the moisture content the object is passing a second transducer that is large because all of the microwave power is detects its presence by reflection from its side. incident on the specimen and all of the reflected power passes to the detector. Examples of more complicated transducer and microwave signal processing assemblies suitable for Where the moisture content is small the cavity precision measurement of specific object dimensions assembly of Figure 3(c) may be more suitable (Ref. 9). are shown in Figures 2(c) and (d). The thickness of Provided there is little reflection at the transducer metal tape can be measured as it emerges from the horns the microwave power passes through the specimen rolls by assembling a microwave cavity whose resonant and returns to the circulator via the left haad side length is dependent on tape thickness (Ref. 1, 2, 6). port. Waves propagate within the cavity from the iris via the circulator to the right hand face of the tape, In the case of materials such as margarine that back via the circulator to the left hand face of the can be arranged to flow uniformly along a metal pipe tape and finally via the remaining part of the circu- the filled pipe can be used as a waveguide over a lator to the iris. If the tape thickness increases, certain length between input and output directional then the length of the resonator decreases. Fcr sig- couplers as shown schematically in Figure 3(d) nal processing a magic tee can be used as in Figure (Ref. 10). 2(a). A variable impedance is adjusted to give a null in the detector when tape of desired thickness Apart from the measurement and control of mois- is present. The source frequency must remain ture content, there are various physical changes in

182 materials that can be detected by microwaves all of which are relevant to industrial production. Thee« changes include the solidification of concrete, gypsum, plastics and epoxy resins (Ref. 1), the • Q- ingredient ratio of mixtures, vapour content in gases including the humidity of air, progress of baking and heat treatment processes (Ref. 11) and species- Transducer specific chemical reactions if electron spin reso- L I nance effects are exploited.

It will be evident from the diversity of materials mentioned above that a certain amount of custom engineering is necessary, especially in connection with the transducer, if an accurate con- Source I tinuous measurement for operating a control loop is Signal processing to be achieved. The literature on this subject i« characterised by the following features, (a) (a) a large number of patents are referred to going back to 1945 in which particular methods of measuring specific properties of materials or .sjjort circuit for objects are disclosed, —I calibration (b) operating principles and performance figures for some systems are given

(c) the physical composition and structure of (b) particular materials, e.g. margarine, and the influence that these features hav>^ on microwave propagation is explained

(d) methods of eliminating the effects of unwanted variables in particular assemblies are described.

(e) since most of the assemblies developed in the past used klystrons as microwave sources invariably the primary source in the assembly is of fixed frequency or varied in a predetermined fs- way independent of the quantity being measured.

V. A NEW MICROWAVE TRANSDUCER PRINCIPLE metal -pipe The new solid state microwave oscillators, by flow their nature, offer properties that allow several improvements and at least one basic departure from the instrumentation previously developed for use with klystrons as sources. The improvements will be dealt with in another paper at this conference (Ref. 12). The basic departure is the following. Transferred electron devices (TED) and impact avalanche and V* i transit time (IMPATT) devices exhibit negative I I (d) resistance over a wide range of frequencies and therefore can be made to oscillate at a frequency that is determined by the dimensions and nature of the mounting structure. It can be anticipated then that transducers for a wide range of physical Figure 3 Schematics of microwave assemblies quantities will be developed which effect either the frequency or the amplitude of oscillation or both. for measuring certain physical properties of materials. The following assembly, which the author is (a) Moisture content of textiles, paper developing, illustrates what will be called the TED etc. microwave oscillator transducer. Referring to Figure (b) Moisture content of .thick sheet 4 a drawn wire whose diameter is to be measured materials. passes through the cavity of a TED oscillator. The (c) Moisture content of relatively dry wire passes through a region of high electric field materials. and parallel to the field lines if its diameter is (d) Moisture content of fluid materials small or through a region of high magnetic field and such as margarine. normal to the field lines if its diameter is large. The frequency of oscillation is shifted by an amount

183 frequency fxt. by approximately the transmission cavity bandwidth. Variations in the oscillator small frequency relative to the cavity resonance will cause wire coupling amplitude variations at the detector. Such varia- factor tuning tions can only be brought about by factors that affect the oscillator cavity and not the transmission cavity. Wire diameter is such a factor but not temperature because with proper design both cavities can be affected equally by temperature changes. Thus the transmission cavity acts as a simple discrimina- tor and results in an assembly that is relatively insensitive to temperature change. Relevant microwave- solid state device performance features are dealt with in another paper (Ref. 12). (a) Oscillator Transmission Detector transducer cavity The design problems presented by the oscillator type transducer are unusual in so far as they involve a knowledge of the methods of designing with the nonlinear negative resistances and reactances associated with TED and IHPATT devices. It is evident from the discussion of measuring assemblies in the previous sections of this paper that the wide range of variables to be measured lead to a wide range of transducer and signal processing combinations. It is yet to be established how wide a range of measuring task.the oscillator transducer can be designed to handle, but it can be noted that the internal impedance of solid state devices presently (b) available range over at least two orders of magni- ose res tude and their operation in conjunction with a diverse range of cavity and electromagnetic mode con- Af figurations has been demonstrated already. A variety ose of ways of compensating for the effects of unwanted variables appears possible of which the simple dis- criminator described above is only one. Extreme sensitivity of measurement tends to be associated with cavity assemblies. This is.because very large unloaded quality (Q) factors (up to several thousand) can be achieved and small perturbations of tuning or Q measured with conventional assemblies energised by highly stable microwave signal sources. TED (c) wire diam. oscillators can be designed to serve as such sources but in addition they can be operated most efficiently in resonant structures with loaded Q factors of ID or Detected less. Hence it does not necessarily follow that only signal extremely sensitive transducers can arise from exploitation of the oscillating transducer idea.

VI. CONCLUDING REMARKS

From the discussion of established microwave methods of measuring a wide range of industrial (d) wire diam. variables the following advantages appear to be realisable if proper design for the intended application is undertaken.

Figure Simple microwave oscillating trans- (a) the measurement can be contactless and neither ducer for monitoring the diameter of damages nor affects the object drawn wire. (b) the entire quantity or volume of the material can usually be measured; on-line operation can usually be arranged; sampling can usually be that depends on the wire diameter as shown in Figure avoided i+(c). For control about a nominal diameter do, the frequency perturbation due to diameter changes will (c) the system response can usually be made be approximately linear and the amplitude of oscilla- sufficiently fast for all practical purposes. tion will be constant. The output of the oscillator is loosely coupled to a detector through a trans- (d) the transducer can usually be made sufficiently mission cavity which is tuned to resonate at a fre- rugged to operate reliably in the most adverse quency f greater or less than the oscillator of environments.

184 (e) control loop calibration of a process can usual- 10. KRASZEWSKI A., "Determination of water content ly be effected without significant interruption in bi-phase amorphous mixtures by microwave to the process because of the relatively rapid method", Proc. 1971 European Microwave Confer- response of the microwave part of the loop. ence, Stockholm, Aug. 1971, pp 08/2:1-4.

The evolution of devices and techniques for 11. SVENKEBRINK J. "Microwave equipment for microwave engineering lead to the expectation that detecting small length changes, as for existing schemes for microwave measurement of indus- metallography and seismology", Proc. 1971 trial variables will be improved and completely new European Microwave Conference, Stockholm, Aug. schemes will be developed. 1971, pp C9/4-.1-4.

12. McRAE C.J. and GRIFFIN D.W., "New solid state VII ACKNOWLEDGEMENT microwave generators for industrial applications", Proc. Electronic Instrum. Conf., Hobart, This work forms part of a study of transferred 1972. electron devices being supported by the Australian Research Grants Committee, the Radio Research Board and the Rome Air Development Center Post-Doctoral Programme. As well as this work on applications, projects relevant to the design, production and evaluation of this type of device are being supported.

VIII REFERENCES

1. STUCHLY S.S., KRASZEWSKI A. and RZEPECKA M., "Microwaves for continuous control of the industrial process", Microwave Jl., vol. 12, no. 8, Aug. 1969, pp 51-7.

2. STUCHLY S.S., "State of art in microwave methods and techniques for measuring non electrical quantities", Proc. 1971 European Microwave Conference, Stockholm, Aug. 1971, pp. C8/S:l-ll.

3. RYAN A.H. and SUMMERS S.D., "Microwaves used to observe commutator and slip ring surfaces during operation", Elec. Eng., vol. 73, no. 3, March 1954, pp 251-7.

COHN G.I. and EBSTEIN B., "A microwave non- contacting tracing technique for automatic contour-following machines", Proc. NEC, vol. 12, Nov. 1956, pp 982-995.

5. HERLO A.L., "Automotive radar for the prevention of collisions", Trans. IEEE, vol. IECI-11, no. 1, Feb. 1964, pp 1-6.

6. BEYER J., van BLADEL J. and PETERSON A., "Micro- wave thickness detector", Rev. Sei. Instrum., vol. 31, no. 3, March 1960, pp 313-6.

7. BERGLING C. and HENOCH B., "On-line measurement of the diameter of bare or isolated metallic wires", Proc. 1971 European Microwave Confer- ence, Stockholm, Aug. 1971, pp C9/2:l-4.

SUMMERHILL S., "Microwaves in the measurement of moisture", Instrument Review, no. 10, Oct. 1967, pp H19-422.

9. LINDBERG T., "Microwave moisture measurements in the industry by cavity resonatpr technique", Proc. 1971 European Microwave Conference, Stockholme, Aug. 1971, pp C8/l:l-4.

185 PRECISE FUNCTION GENERATION UNDER DIGITAL CONTROL

D.A. Pucknell, B.Sc, B.E., M.I.E.E. Senior Lecturer in Electrical Engineering, University of Adelaide, Adelaide, South Australia

1. Summary and Introduction. In the study, instrumentation and control of engineering systems it is often necessary to generate D.c.v.s. arrangement voltage or current waveforms of precisely known form. For example, sinusoidal, exponential and triangular forms are frequently required. Several basic approaches to function generation are possible including, (a) the use of sinusoidal oscillators and shaping circuits, (b) the use of arrays of linear integrated circuits^) and (c) the use of a programmed digital computer with suitable digital to analogue (d/a) convertor outputs.

Approaches (a) and (b) generally give limited flexibility and restricted accuracy whilst approach where m (c) involves the exclusive or time shared use of the number digital computer during the whole period of function erence generation. is ang» (2) An interesting alternative is to use a number com. If of interconnected d/a convertors driven by a binary weighte counter to generate terms to form polynomial approx- outputs imations to the desired functions. The computer is Input c the lea then only involved to the extent o setting initial maximum parameters and starting the process of generation which will then continue until stopped or modified by the computer. This paper briefly outlines this method of gen- Figure 1. FΓ erating functions and then goes on to show that good realise precision and accuracy are readilv achieved in theory = R.(G.D) V. (2) given b and in practice. ab ref 2. Method of Generation. where (GD) is a conductance term contributed by the digitally controlled conductance. where K The method proposed''"'' requires an array of interconnected d/a converters of the multiplying type, The interconnected arrangement of the d/a •Wax convertors for function generation is shown in that is to say, each convertor must have an output It Figure 2 and it may be seen that the arrangement voltage V such that age (V QUt generates a series of terms each containing a o = K.D.V (1) power of (R.G.D.). out ref. out The arrangement of Figure 2 assumes that the where K is a scaling constant, D is a digital input resistors "R" and conductance elements "Gn" are and Vpgf is the input reference voltage against which OR similar in all convertors and that all convertors digitisation is taking place. are switched from the same digital input "D". V =V The arrangement used by the author is based on out F the digitally controlled voltage source (d.c.v.s.) If digital input D is a regular function of which is basically an operational amplifier interconn- time, as will be the case when "D" is derived from the outputs of a binary counter having an | where C ected with scaling resistor "R" and digitally contx-oll- ite ser input at clodk frequency "fc", then each of the ed conductance elements "Gn" as shown in Figure 1. Referring to Figure 1, the output voltage V^ between generated terms has the form A and B can be shown to be 1 Th< VO(R.G.D) = Vfa.t)" (3) K ent the terms ir 186 In practice, a large number of terms means a large number of d/a convertors. The number to be used is limited by economic considerations and, therefore, these studies are confined to series truncated at (at) thus requiring four convertors in the generator. Function Generator The accuracy now deteriorates as (at) increases arrangement but to construct "continuous" cosine waves for example the maximum value (at) may be limited so that

(at) = R.G (K +1) = 1 IK +1] o max 2 (5a) max K = 2N - Where max

Figure 2. where m is an integer = 0.1,2 mmax> "Vnax is tne number of d/a convertors used, t is time, VR is a reference voltage input to the first d/a stage and "a" is angular velocity. If the switched conductance elements are binary weighted and driven in order of significance from the caw • (»•»> outputs of a binary counter of "N" bit, and if Go is Gtntttlcd COS <»U fö IO "' the least significant conductance element, then the maximum conductance G is given by

= (2N - 3) G (U) Figure 3. Prom consideration of expression (3) it will be When the conditions set out in expression (5a) are realised that the value of (a.t) at any count "K" is !1 given by met then a quarter cycle is generated as "K is advanced from zero to K as shown in Figure 3. (a.t). = R.G .K (5) max o The quarter cycles generated may then be used to where K is an integer which advances from zero to construct a continuous sine wave as shown in Figure "*. ^max (s 2" - 1) in units at clock pulse frequency "f ". The range of functions which can be generated is quite wide, particularly so when it is noted that It is now easy to scale and sum the output volt- m age (V in Figure 2) such as individual terms in VR(at) are available which, for out the first stage for example, gives a ramp voltage V (at) which can be used to generate triangular wave- V = V Cos(at R out R > forms. Amplitude in all cases is determined by a d.c. OR reference voltage Vp and frequency (or time constant etc.) is basically controlled by clock frequency f V =v =V c out RV "R L* into the counter. Both can be given a high degree 1 i. a of accuracy and stability. where C., C etc «re coefficients which for an infinite series would nave the values 3. Factors Affecting Frequency, Time Constant or Period. Cj - 1., C2 - 2., Cn - n. Further consideration of expression (5) reveals The accuracy with which these expressions repres- that generated frequency etc. depends primarily on ent the desired function depend on the number of the clock frequency f and also on the precision, terms in the series and the maximum value of (at). 187 f

.- (R.G K)' (S.G K)1* -i Y V 1 K " R L " -CT- I (7)

Z * ZerOICounter M Max"»J settinQ "Continuous*Cosine generatIon

Figure t. Figure 5. stability and accuracy of scaling resistor R and the conductance elements "G ". At any count "K1 the value of Yj< remains constant until n The actual generated frequency "f_" in the case the next clock pulse advances the count to (K+l). This of sine waves may be obtained by considering Figure 4 is clearly demonstrated by Figure 5 which also shows and expression (5a) noting that each quarter cycle that the value of (at) advances in A(at) increments requires one sweep of the counter between zero and as shown. However, the true cosine function is analog- maximum. Therefore for a fixed input frequency of ue in nature and changes from a value "ZK" to a value Z fc the period generated "tg" is dependent on "N" such (K+1) between counts K and (K+l). If &(at) is small, that then from Figure 5, it may be seen that the average +2 amplitude error "Ex" between K and (K+l) is given by t = 2<« > T ..... (6) K K ™ 2* K (K+l) —— IBJ ! OR (N+2) Expression (8) is valid for all generated functions -f = 2 (6a) where Z is the true value. The average error Ej< may V therefore be computed at intervals over a range of where T = 1/f c c values of "K" and used as a measure of the goodness A further and most significant, benefit arises of the generated waveform. from the fact that initial presetting of the state of The factors which effect the error, Eg, may be the count K gives full control of initial 'phase angle summarised as follows: and/or initial value of the generated function. Pre- set facilities are readily available in many M.S.I, (i) The range of the value of (at). For sine circuit packages. and cosine waves for example the range is such that 0) of terms generated to implement In all cases it will be realised that the gener- the polynomial approximation. For these studies ated function is a stepped approximation to the desir- m=U. ed function. This is quite apparent in Figure 3. The size of step io determined by the value allocated to (iii) The stability and accuracy of reference voltage the least significant conductance Go which, in turn, VR- for a given value of R and for a particular range in (iv) The number (N) of stages in the control counter. the value of (at), is determined by the number of bits "N" in the control counter. This directly affects increment A(at). (v) The choice of coefficients "C " (e.g. C and Ci» It is possible to express the value of the gener- n 2 in expression (7)). ated function in terms of count "K" and by considering expression (5), the truncated cosine function for In considering these factors it will be realised example may be expressed as that the last two can be varied at will and used to 188 I minimise the average amplitude error E . i D 27.0 K 1 2- 2 0 i .634826 499273 -0.25% -0.16% A further measure of the goodness of the generated ' E ! -° ! 28.0 .633224 498494 -0.53% -0.32% (uaveform is to compute Mean and Mean Sq. values. Mean Ideal values : MEAN = .636622 MEAh SQUARE =.500000 Mean Square values are readily computed as follows:

N K=2 -1 It will be seen that an optimum value for C4 is to be found in the proximity of C =26.0 and at this point (9) 4 MEAN (TK) the Mean and Mean Square values are close to ideal and

2 Luv.r the error EK is minimised. The next plot is of the

optimum values of C4 for various values of "N" and this is presented in Figure 7. K MEAN' Sr,UAP£ = L (10) K=0 . A Nova digital computer with an ARDS graphics I facility was used to calculate these values and plot I' U|< on a logarithmic scale for various values of N and various values of coefficients C .

until* This ws s alog- O1LG4 f«r CO. (witilM lue all, e Figure 7. by The relevant calculated values are set out in Table II. Table II. Cosine function, N=6 to 11, C =2.0, Opt. C. tions «••, nti C4 ay vr«r fg vere» CvtM K. Coefficnts Calculated %Error iFrom Ideal C Curve N S I 4 MEAN MEAN SQ MEAN MEAN SQ Figure 6. The first example, Figure 6, is a plot of EK for the A 11 2.0 1 25.64 .636667 .499979 +.006% -.004% cosine function generated as in expression (7). The 6 '10 2.0 ,' 25.68.636697 .500002 +.011% +.0004% example chosen indicates the effects of varying coeff- C 9 2.0 • 26.08 .636552 .500128 -.01% +.025% icient Cn alone for a fixed size of counter (N=9) and ! D 8 2.0 i 26.80 .636159 .500413 -.07% + .08% i E 7 2.0 . 28.4 .635611 .501130 -.16% + .23% ch with coefficient C2=2.0. The range of values plotted correspond to a quarter cycle i.e. (at) varies from 0 F 6 2.0 30.8 .636357 .503473 -.043% + .69% to n/2 along the horizontal axis to a linear scale. ! Ideal values: MEAN = .636622 MEAN SQ = .500000 nt ies The corresponding calculations for Mean and Mean It will be seen reasonably accurate sine waves are Square for reference voltage VR = 1 volt are summaris- generated for all value of NA8. With coefficients Cμ ed in Table I. age chosen as in Table II, the average error E« has a max- Table I. Cosine function, N=9, 03=2.0, vary C^. imum value of 0.33% for all values of N from 8 to 11 and Table II shows that the mean and mean square values are Cer. within less than ±0.1% of the ideal in ail cases. Coefficients Calculated I % Error/Ideal Curve MEAN MEAN SQ MEAN |MEAN SQ Better results may be achieved if both coefficients c» c C2 and Cj, are "optimised" for best Mean and Mean Square A 2.0 . 40435 .502112 +0.6% values. The relevant plots of E'K are given as Figure 8 id B 2.0 25.0 .638415 .501070 +0.29* +0.2% and the calculated Mean and Mean Square values are 2.0 5 C 26.0 .636552 .500128 -0.01? +0.024? set out in Table III.

189 r

i.—'—• 0-1 f ** :> vV V\ \ \\ t \ 4 III V'a 1 11!"* 11

r f

Opt. C,«"« C4. (CM)

figure 8. Figure 9. Table III. Cosine function: N=6 to 10, Opt. and Table IV. Sine function, N=6 to 10, C =1, Opt. C .

Coefficnts.! Calculated %Error from Ideal Coefficnts Calculated %Error from Ideal c Curve N C MEAN MEAN' SO MEAN MEAN SO. :urve l MEAN MEAH SQ. MEAN IMEAN 30. j 2 a i S A 10 1.0 6.55 .636784 .500034 + .016%! +. ooel' A 10 2.01 ' 26.48- .636236 .500047 -.06% + .01% B 9 1.0 6.56 .636676 .500157 + .008%; +. 03% B 9 2.00 26.08! .636481 .5C0092 -.022% +.018% C 8 1.0 6.60 .636658 .500860 + .004%! +. 161 C 8 1.18 : 24.6 .63.7048 .499942 +.065% -.012% 0 7 1.0 6.65 .635817 .500972 -.13% ! +. 13% D 7 | 25.2 . .636213 .50)039 -.064% +.008% \.n E 1.0 6.75 .634230 .501401 -.38% | +. 281 E 6 1.95 J25.8 j .635780 .50)865 -.12% +.17% i 6 ! MEAN : >, MEAN SQ [deal values: MEAN = .636622 MEAN SQ = .500000 j Ideal values: .63662: = .500000 From Figure 9 it will be seen that the error E It will be seen that the Mean and Mean Square val- K has an initial value at K=0 which depends on the value ues are within +.17% to -.12% limits in all cases even allocated to "N". Further consideration reveals that for a simple system using a six bit counter and 6 bit this initial error is, in fact, approximately half t.-.e d/a convertors. increment A(at). The initial error can to eliminated It is also possible to generate sine waves by lim- therefore by modifying expression 11 as follows

iting the number of d/a convertors to 3 (m=3) and using 3 a polynomial approximation to the sine function as in i~(F.G K) P.G (R.G X) — expression (11) , = V, f IIa) -(R.G .K) (R.G .K) 3 _ Y.- = V, (11) where = h A(at).

f Coefficient C^ may now be "re-optimised" and the Sine and Cosine functions are intchangeable with this system of generation since the initial value in all results obtained are set out in Figure 10 and Table V. cases can be set by suitably presetting the state of It will be noted from Figure 10 and Table V that the the control counter. error curves and the mean and mean square values generated are quite similar over the range of values of "N". Results plotted and calculated as in previous This means fiat coarse control of frequency can be obt- cases are set out in Figure 9 and Table IV. ained using a 10 bit counter and three 10 bit d/a convertor bv orogressively discarding the less significant bits. For example it may be seen from expression (Pa) that when 4 bits are discarded and the clock frequency fc kept constant then the generated frequency will be increased bv 2U i.e. by a factor of 16. The arrangement 190 E, 1-0

•0-1» \ f

•• / •

..«I ,

-r—"I •ft

l-o !

\- I.MCMKK.

Figure 10. Figure 11. Table V: Sine Function, N=6 to 10, Opt. C^ no initial -(at) Table VI. Exponential function e N=10, C1=l,

C2=2, C3=6, C^ Opt. Coefficnts. Calculated j%Error from Ideal dea:i MEAH . MEAN SQ. MEAH MEAN SQI Coe ffic Lents Calc. %Error from Curve K MEAN Ideal _sci 1.0.6.52 • .636875 .499765 +.04% Cl C2 C3 ^4 361' 1.0 ! 6.50 , .636854 .499623, +.036% -.076% .632164 n t, 1.0, 6.17 I .636799 .499389.! +.027% -.12% A 10 1.0 2.0 6.0 29.0 +.006% 1.0 ! 6.43 j .637046 i .499752 +.065% -.05% 1.0 I 6.34 ! .637301' .50C032 +.11% +.006% Ideal Mean value = .632121 Ideal values: MEAN = .636622, MEAV SQ = .500000 case when all coefficients are adjusted for optimum. using m=3 does not give as precise generation as in the 4. Conclusions. case when m=4 but the results achieved are nevertheless .ue quite impressive. The preceeding sections of this text indicate quite clearly that a wide range of repetitive or non-repetit- it The discussion so far has been concerned mainly ive functions may be generated at will using this method. with sine wave generation but it will be apparent that Discontinuities provide no difficulties to generate and the same optimisation techniques can be applied to any it ^has already been pointed out that the initial value generated function. or phase angle of the generated function can be preset to any desired point. The frequency, or time constant, la)! For example, Figure 11 and Table 6 give the results at I is determined by a clock frequency which can be set most obtained when generating an exponential function (e~( * precisely. Coarse control of the frequency or generat- using a 10 bit system and covering a range of values of ed time constant can be effected by varying the nunber (at) such that O^ a ^1.0 and using the approximation "N" of stages in the control counter this also affects (R.G .K) (R.G .K)' (R.G K)" the coarseness of the stepped functions generated. o o o V. I v- The use of a control counter to drive the generator E- leaves the computer free to do other work and the comp- (R.Go .K) uter is only involved where parameters setting or adj- — (12) ustment is required. iv- The amplitude of the generated function is primarily it The results in this case are given in Figure 11 and determined by a d.c. reference voltage and this can Table VI. impart a high degree of absolute accuracy to the a.c. It will be seen that the process has been optimis- waveforms generated as it is possible to standardise ed using C^ coefficient only. This leads to hardware them against a standard cell. simplicity and gives almost as good results as in the The accuracy with which the generated function 191 r

tracks the ideal may be kept within close limits by (a) The use of a.c. or time varying reference inputs suitable adjustment of coefficients. Coefficient in place of the d.c. reference voltage used in the NEV adjustment in practice is effected by precision resis- studies. tor or resistor/potentiometer networks. (b) Variation of the interval between clock pulses at Hardware simplicity is achieved throughout part- the input to the system in place of the clock frequ- icularly when it is realised that the d/a converters ency fc. used are frequently already available as part of small digital computer systems in engineering situations. (c) In the arrangement used by the author, scaling re- The studies reported in this paper were confined to sistor R could be replaced by a device having resistan- systems comprising four d/a convertors and it will ce controlled by a suitable input voltage or current. be seen that very good precision is possible. This This gives in effect, a third input to the d/a convert- has been demonstrated in this case of continuous sine ors. wave generation where it is possible to achieve close (d) The suggested system allows for the generation of amplitude tracking and also Mean and Mean Square repetitive and non-repetititve functions at will, in values within much less than 0.1% of ideal using a ten the case of repetitive functions, the method used to bit system (N=10). construct the function gives great flexibility allowing Greater precision was not attempted in these for example the generation of full or half wave rect- studies since ±0.1% limits are more than adequately ified sine waves of precise amplitude and term. Some precise for the vast majority of engineering applic- other possibilities are dealt with in reference (2). ations. However, improved precision and accuracy will result if (a) the number of terms in the series is in- The arrangement then is extremely flexible, capab- creased i.e. more d/a convertors and switched conduct- le of very high accuracy and precision and is yet ances are used and/or (b) if the range of values over economical in hardware. which (at) is varied is decreased. 5. Acknowledgements. For example, in the case of sine waves it would The author wishes to acknowledge the invaluable be possible to restrict the generated range of (at) assistance of colleague Mr. R.C. Hash in building to 0 to ir/i». Repetitive sinewaves can then be con- and testing the system and recording results. structed using the approach outlined in Figure 12 giving extremely precise generation. The Australian Research Grants Committee is also to be thanked for financing work in this and related areas. 6. References.

(1) BOTOS, R., "A Low Cost Solid State Function Co unter Generator," Application note AN 510, Motorola Semiconductor Products Inc.

(2) PUCKNELL, D.A., "Interface Conversions and Function Generation Using Digitally Controlled Voltage Sources," Proc. I.E.E., Vol.117, No.5, Hay 1970, pp.912-916.

(3) ROBERTS, L.G., "Conic Display Generation Using Multiplying d/a Convertors," I.E.E.E. Trans., 1967 EC16, p.369.

Z*Ztro; M«Ma*m: • »C/O cos/si nt ,(N+3) -Tfl =2' 'X Hiqh Precislon Sine generation

Figure 12. At the other end of the scale tolerably good sine waves and exponential function generation (better than ±1%) can be achieved using only three d/a convertors and a si/ pie 6 bit counter with 6 switched con- ductances in tich d/a converter. Finally, the method can be applied to the generation of special complex functions and in this respect t- e following possibilities are open:

192 NEW SOLID STATE MICROWAVE GENERATORS FOR INDUSTRIAL APPLICATIONS at C.J. McRae, B.Tech., B.E., M.I.E.Aust. (Student) and D.W. Griffin, B.E., B.A., Ph.D,. C.Eng. (Reader) Department of Electrical Engineering. The University of Adelaide

SUMMART. The introduction of solid state microwave generators now makes industrial application of microwave techniques «ore attractive. Close examination of the available generators is essential to ensure that these are suitable for all the applications envisaged. The properties of the tvo generators of interest, IT"1J. the impact-avalancba-transit-time, and transferred electron devices need to be investigated closely to determine their respective roles in industrial equipment. A study of the performance of each device with respect to power output, frequency range and stability is essential for effective design. This paper coapares the operating characteristics of the generators and presents the relative merits of each in Meeting the -i-winit of a variety of applications. For instance such demands could range fron simply the need for microwave power to those application« «hen low noise levels and even high frequency stability are essential. Initially, the basic operating aeehanisss are discussed and the different nodes of operation that result are explained. The performance features associated with the various Modes are listed and used as a basis for optimal device and operating aode selection. The essential features include output power, frequency range and stability, and complexity at the associated microwave nounting structure. Finally, a summary of the current stage of development is included together with indications of future extensions in industrial usage, pending successful advancements in technology based on the apparent advantages predicted by theory.

I INTRODUCTION. since solid state devices are strictly limited in site by thermal properties alone the w4«^ available out- Since the application of silicon diodes in World War put RF power must be limited accordingly. II microwave radar installations there has been steady development in semi-conductor technology over Whilst acknowledging the superiority of tubes in the the past three decades. Prom the advent of the regions of moderate to high microwave power, transistor in 19U7 solid state device research and replacement of tubes by solid state devices at lower development teams have continually aimed for higher power levels can be expected, and, of greater power, higher frequency performance. Up to this importance, the range of applications of low power last decade semi-conductor development had come to an microwave generators invariably will be extended. abrupt halt Just short of the microwave range and, Past experience should teach us that the transition apart from microwave transistors and frequency multi- from tube to semi-conductor technology ',n not merely a plier chains each with their inherent limitations, a fashionable trend to achieve that 'all solid state real breakthrough did not occur until the avalanche appearance* in systems, but rather a move towards and transferred electron devices were demonstrated as lower cost, smaller and, in particular, more reliable possible useful sources of microwave energy. equipment. In the light of the advantages of the new Subsequent research and development has confirmed microwave generators to be presented the use of micro- these Initial predictions and the major part of this waves in many applications previously reserved for paper outlines the properties of these devices and other transducing mediums should now be considered. the expected applications in industrial instrumentation. n SOLID STATE KtCROWAVS SODBCKS. In the meantime, whilst solid state devices were There are today five important classes of solid state being developed and largely replacing thermionic devices for generating microwave power, namely, tubes in most applications, the various special microwave transistors, transistor driven varaotor microwave tubes have advanced considerably in both harmonic-generator chains, tunnel diodes, avalanche output power and frequency rang« since their initial transit-time devices and transferred electron use in World War II radar. It should be emphasised devices. (Kef. 1, 2, 3, it.) Transistors, varactor at this point that these new solid state generators diodes and tunnel diodes are pure junction devices are not expected to replace, in the foreseeable and show 9tnvn restrictions in their output power • future, microwave tabes in those applications frequency characteristics. The operating principles demanding high powers (tons of kilowatts, cw). This of these devices will not be discuued and the is not unexpected since we still find tubes holding a limitations are shown graphically in figure 1. firm footing In the high power field at lower radio Avalanche transit-time devices contain junctions, but frequencies such as in the output stages of tele- some of the processes essential to their operation vision and broadcast radio transmitters. It is occur in the bulk of the material whilst transferred basically a question of power density - the smaller electron devices are pure bulk devices devoid ot any the device, the lesser the available power - and junctions except at the contacts. 193 Applying a bias step to this circuit will result in ; "ringing*, the signal frequency dependent upon l, c ! and Q, and the amplitude decaying exponentially with time owing to the duping of 0. If, therefore, a negative conductance, -O, is coaneeted to the circuit the net conductance is sere and hence the oscillation 10* will continue with constant amplitude. The equivalent circuits will be shown to include an affectiv« negative conductance as in figure 2(b), (e) and so \\ varactor multiplier with correct biaeaing and connection to a resonant 103 circuit the oscillation will build up to an amplitude i * \ \ when the total conductance is sero.

2 \ 10 \ (a) Avalanche Transit-Time Devices, \ transistor (i) DffATT device. 10 Following Head1« proposed multilayer n^-p'-i-p** structure in 1958 tor generating microwave power, 1 tunnel diode based on a combination of impact avalanche breakdom [ 1 i and electron transit-tiee effects, the principle was j experimentally verified in 1965 when Johnston, De 1 10 100 Loach and Cohen achieved pulsed power outputs of 80mV at 12 Olli. The so called MPATT (imoaet avalueh» TB2WESCT (GHs) and transit-time) diodes have been developed consTderably"eince then and from figure 3 now FIO. 1. Power versus frequency characteristics for represent important solid state power sources. Th» typical Junction-type microwave generators.

Since the emphasis is on bulk devices we can Halt oar ^•Vv. X/t thermal limit study to elementary resistance oscillators based on the Mgatlre differential resistivity displayed by the 100 devices to be described in some detail below. A -Os/ baale resonant circuit consists simply of a parallel connection of an inductance, I, capacitance» C, and a 10 conductance, 0, as depicted in figure 2(a). i >.

1 a i/f* v "~ x o.w. THAPATT eleotroni't + pulsed TRAPA3T limit .01 _ • o.w. HCPATT o pulsed DtPATT 001 1 100 L JREQDBrCioT (OEs)

FIG. 3. Power versus frequency characteristics for avalanche devices, including thermal (for cw only) and electronic limits. Duty cycle 10-3 or less for pulsed performance.

device consists of two regions, one being a relatively narrow avalanche region and the other the drift region A steady reverse bias is applied Just below impact FIO. 8. Simplified equivalent eircult diagraM + Resonator circuit (i) and dasped oscil- Ionisation level at the n**-p Junction. Super- lations for net positive conductance (11). imposed on this bias is an HF voltage each that avalanche breakdown occurs daring the positive half (b) P-α junction In avalanche breakdown. Avalanche region (i)j negative resiitanee cycle, but the field is within breakdown level for -Ra, depletion layer capacitance, Ca and the negative half cycle. As a result the holes inductance, la, due to avalanche build up. generated in the avalanche region are injected into Drift region (11)) negative resistance the drift region and the device current reaches a -Rd, depletion layer capacitance, Cd. maximum at the end of the positive half cycle. (O Transferred electron device. Hence the current lags the RP voltage by Tf/2 radians. 194 During the remainder of the RF cycle the injected boles travel with constant velocity to the n*+ region, (a) 2 the width of the drift region being such that an additional lag of TT/2 radians is introduced with the result that the current is n radians out of phase with the applied voltage and so the device exhibits negative resistance at the fundamental frequency. Variations in the Read structure have been developed offering wider band width performance, but in basic principle the mechanics are similar and so do not warrant further discussion. In general IMPACT 10 devices can provide cw power outputs up to hundreds ELECTRIC FIELD (kv/om) of milliwatts in the frequency range U to 20 GHs. (Hefe. 5, 6, 8, 13.) (b) I (ii) TRAPATT device. mriiii ofamio oontaot active layer An important mode of operation of avalanche devices (n-type OaAs) is found in the trapped plasma avalanche triggered ofamio oontaot transit (TRAPATTToivice (Refe.~l, 2, 9, 13). If The amplitade of the microwave component of the voltage generated in certain types of IMPACT structures exceeds a critical value a different carrier response results. When avalanching occurs, the charge FIQ. U. (a) Drift velocity of condaetion electrons density growth far exceeds the background doping in OaAs as a function of electric field density and as the generated boles and electrons (b) Bulk transferred electron device. separat« the field between falls close to sero. Hence a high device current exists tor a low terminal voltage. The voltage rises to a high value again as If a non-resonant and resistive load is connected to the plasma clears and the cycle is repeated. The the device, the current in the load wiU rise when TRAPATT oscillation has a somewhat lower frequency the dipole domain is collected at the anode and fall than that of the IMPACT device, but, being a Urge when a new domain nucleates at the cathode. The signal instability, higher peak output powers can be repetition frequency will be close to the inverse ef expected. The oscillation is self sustaining once the domain transit time and so the mode of oscil- Initiated, but requires a major perturbation to lation is referred to as the transit time mode. start. The frequency is Inversely proportional to the width of the depletion layer and so unlike the Repetition frequencies less than that for the transit LSA device, to be discussed below, the output power time mode can be achieved if the load is resonant and cannot be increased merely by increasing the bulk of the RF voltage superimposed on the dc bias results in the material for a given frequency. The TRAPATT the net device voltage swinging below the sustaining oscillator can be summarised as a device having high level Juat as the domain la arriving at tne anode. efficiency, high peak microwave output power, but The formation of a new domain is delayed until the restricted to the lower microwave frequency range voltage again rises above the threshold value, hence (figure 3). lengthening the period between pulses. This mod* is known as the delayed domain mode. If, however, the device voltage swings below the sustaining level (b) Transferred Electron Devices, before the domain reaches the anode, quenching will occur and a higher than transit time frequency results (i) Transit time devices. This is called the quenched domain mode. Bulk negative resistance in certain semi-conductors resulting from the transfer of electrons from a high Regardless of the mode of operation the frequency of mobility to a low mobility state was predicted in oscillation is limited by the transit time of the 1961 by Ridley and Watkins. Experimental veri- domain which in turn is determined by the thickness fication was achieved in 1963 by Qunn. Figure l»(a) of the active layer. Hence when operated into a shows the drift velocity of conduction electrons as a resonant circuit the transferred electron device will function of electric field for n-type Gals. For oscillate at its transit tiae frequency with up to fields greater than 3Kv/cm the drift velocity $0 per cent decrease as the circuit is toned for decreases as the electric field is increased. In a delayed domain mode performance. Alternatively, the typical device depicted schematically in figure li(b) circuit may be tuned to provide higher frequencies on applying a bias voltage electrons accumulate at with the quenched domain mode which, under suitable the cathode with an equal depletion at the anode. operating conditions, may be extended into the limitei Increasing the electric field above threshold results space charge accumulation (USA) mode. in the cathode accumulation layer being launched into the bulk of the material and owing to the negative Treating the LSA mode separately, the transit time differential velocity -field characteristic this devices are frequency Halted and since their accumulation layer develops into a dipole domain as operation is dependent upon the formation of high it moves towards the anode. This α-aaain is a field domains there must be an output power limit, relatively high resistance region wit» a correspond- the domain field necessarily being kept within ing high electric field whilst the field on either avalanche breakdown level. Furthermore, there is, side of the domain is below threshold. The same ideally, no difference in cw and peak pulsed power type of dipole may nucleate also at notches in this levels, but in practice higher peak powers are doping profile between cathode and anode. obtainable owing to thermal limitations. Future 195 r

domain mode outlined above. Here the characteristiei of the device and associated resonant circuitry an 1000 - such that any accumulation layer is quenched before maturing into a high field domain. The field throughout the bulk of the active layer is therefore virtually uniform and above threshold for most of th« 100 RF cycle. The portion of the cycle when the field is below threshold (figure 6) is Just sufficient to allow the quenching of the charge accumulation whilst 10 when above threshold the accumulation process is retarded by the lower carrier mobility. The frequency of oscillation is not restricted by transit ties effects and can be several times transit time 1 "^\ frequency as determined by the resonant circuit. la addition the »«•»<»»- output power obtainable is not set by domain field limitations. In practice the power is limited by thermal af Zmtfm ealy sad no high .1 peak poised performance la coaslbl» «ith laerasee la • TED (incl. LSA) cw duty cycle as more efficient —tmods of beat slaking o LSA pulsed 10-3 duty are developed. Frequency limits have mot been con- .01 or less , \ clusively established, but the intervalley transfer 10 100 time of the electrons would certainly influence the FREQUENCY (GHz) upper limit. A better understanding of the physic* of the transferred electron effect will allow a mere accurate prediction of the frequency range, but at FIG. 5. Power versus frequency characteristics for this time useful microwave powers can be expected up transferred electron devices, including to $0 OHs as indicated in figure 5. thermal (for cw only) and electronic limits. Ill APFIICATIORS.

development will improve the heat dissipation and so In considering industrial applications of solid state lengthen the duty cycles allowable. For microwave microwave generators in instrumentation, systems power of the order of teas of milliwatts in X-band demanding high powers such as dielectric and transit time devices offer low cost, low noise, induction heaters can be excluded. Furthermore, tbt reliable sources with a minimum complexity in assoc- current state of development would indie e that such iated circuitry. (Refs. 1, 2, k, 7, 13.) applications call for the use of microwave high power tabes and it would not be realistic to envisage (It) Limited space charge accumulation (LSA) these being replaced by arrays of, say, ISA devices devices. in the foreseeable future. In general, apart from heating, there appears to be minimal requirements for The LSI aode is an extreme case of the quenched moderate to high power (cw or pulsed) devices in instrumentation. Since both the ISA and TBAPATT devices are essentially relatively high power pulstd generators they can be excluded from farther consideration at this time. Likely present day appli- L derlo* length cations for solid state microwave generators would n oarrler deaeity dictate a choice between the IMPATT avalanche and e electronic charge transit time transferred electron devices, and possibly the microwave transistor for cw performance in the lower microwave frequency range (X-band).

high frequency stability is not an important specification in industry apart from frequency ELECTSIC FIELD, S (T-EL) drifts that may result in interference with neigh- bouring systems. Whereas such frequency stability is essential in communications thus Justifying the use of crystal oscillator driven varactor chains as local oscillators, for example, their usage in instrumentation is generally not warranted. Essentially the need is for a cheap, reliable sour« of microwave power requiring a minimum of associated bircuitry and relatively simple bias controls. In its simplest form such a source need be little more sophisticated than the common flashlight as a light source. HO. 6. LSI mode of oscillation (a) Telocity (current) - electric field (a> Transferred Electron Transit Time Oscillator. (voltage) characteristic (b) device voltage - time wave shape (high Transit time oscillators are now available commer- Q-factor circuit) cially to provide tens of milliwatts microwave power in X-band. The output tm normally ew although 196 J n

palaed performance is usually possible if desired. In presenting the described new solid state generators Such oscillator« lend themselres particularly to in industrial instrumentation three •»•<»> points eaa be those Applications in which the state of the system stated. Firstly, devices are now available to is measured including sise, velocity, acceleration replace the equivalent microwave tubes for all and position. These new instruments offer ragged existing applications thus making the use of micre- design and perforaance in environments often found to waves as a transducing medium auch more attractive be too hostile for alternative Methods of measurement and practicable than possible alternative methods. Although the cost of a single device is already in— Optimal directivities with a minimum of scatter can what lower than for a tube, further decreases la eest be expected, allowing «ore accurate siie and position can be expected as applications expand allowing neasureasnta. Telocity measurement, anu hence economical mass production including integrated acceleration Beasurement by differentiation, are well circuit techniques. suited tor transferred electron oscillator applic- In ation, the frequency difference between the incident Microwave tube installations have always beea balky, and reflected signals providing a direct numerical employing coaxial lines or waveguidnfor traasaisalMi indication of the speed of the moving object. A and it is improbable that aaas production will be block diagram of a simple Doppler radar asseably is possible on a oeapetitive soale with solid state shown in figure 7. It is obvious that the equipment. Apart from this cost factor other application for this type of equipment extends beyond advantages include site, reliability and sljEplleity of associated circuitry and operation. Secondly, in the iamediate future high (pulsed) power solid state oscillators will be available coaaar- cially la the fere of ISA and TBAPATT device equipment possibly allowing new developments in instra- TED res. —» coupler aentatien doas.wH.ng this type of source. eavity Finally, solid state oscillators have properties that allow departures from classical microwave instrumentation techniques and so sew types of transducers d.c. bias appear practicable (Bef. Iβ). In these new fields circulator the apparent applications may not be realised in practice unless the influence of external parameters the such as teaperature and bias voltage on the device performance is understood and methods of compensation mixer adopted where necessary. Per example, the frequency de- filter of oscillation of a transferred electron device is tector dependent upon both the bias voltage and teaperature, typical departures from centre frequency being shown in figure 8. Hence in transducers, where changes in audio output oscillator frequency represent changes in the measured quantity, influence from fluctuations in bias voltage and teaperature must be minimal. As a first step in design an adequately stablised bias supply 710. 7. Block diagram of a TED Soppier systea. bO {centre temp. 3O°C industrial instrumentation allowing the production of 30 :\ •centre frequency large numbers with considerable decrease in unit cost 10 GHz Hence applications that used to be economically out of reach new become potentially practicable. 20 Typical nen-induetrial systems would include collision avoidance for aircraft, boats and road ~ 10 motor vehicles, police radar, anti-eldd systeas for M rail and road vehicles and personnel detection systeas such as in burglar alarms and safety equip- 1. aent. -10 (b) Avalanche IMPACT Device. For operation at higher frequencies the IMPATT device offers an acceptable replacement for the -30 transit time transferred electron device or in those application« where a higher ev output power is desired. In the past Si technology has provided -UO relatively noisy IMPATT oscillators,' but current -16 6 io ?b A research and development favours QeAs as a substitute AT (°C) for Si with the aim to decrease the noise level, levertheleas although high noise levels would not be tolerable in coaaunication systems any degradation in FIG. 6(a) Variation of frequency of transit time Industrial applications as a result of such noise is transferred electron oscillator with doubtful. temperature. P' 197 open to the designer and so the possibility of frequency drifts in the device must be accepted. IV Where such drifts arelikely to cause intolerable errors a control loop can be incorporated to mlnimii (a) these adverse effects of temperature as, for indicated in figure 9. (i)

d.c. me«sored Source valiable Bias 10 volts centre frequency 6.I4 GHz o\\> T 1 Y! ^^~^ ^J Regulator H i K/) 1

f temperature Controll« looitor fluctuations

FIG. 8(b) Variation of frequency of transit tie» transferred electron oscillator with bias voltage, should be used and to avoid the effect of teeperature FIG. 9. Temperature compensated TED microwave on the device itself a high RP energy storage cavity transducer assembly. is necessary. (Ref. 17.) The change in frequency due to the thermal expansion of the evrity can be compensated by using a double cavity configuration (Ref. 11*). In practice, however, the choice of a sufficiently high energy storage cavity say not be

TABLE 1. Distinctive features of solid state bulk microwave generators and potential applications.

DEVICE ADVANTAGES OTHER FEATURES APPLICATIONS Nature domain Low AM aad FM noise Hide frequency range General purpose low tnzn Low effioieaejr power miorowave source Doppler radar Looal osoillator Measurement meohanioal quantities Materials analysis Miorowave osoillator transducer

LSA TED Highest pulsed power o/p Speoial microwave Moderate pulsed power Hot transit time limited structure required radar IMPATT Highest CW power o/p Relatively noisy Radar High freouenoy (mm wave) Measurement mechanioal performance quantities Materials analysis Miorowave osoillator

TRAPATT High pulsed power o/p Limited frequenoy range Pulsed power radar Highest effioienojr Speoial miorovav* Series operation structure required feasible

798 r

17 FUTURE DEVELOPMENTS. double drift region (Ref. 13). (a) Transferred Electron Derices. (ii) TRAPATT device. (i) Transit time devices. The main limitation in the TRAPATT oscillator is the restricted range of frequencies. Current research The transit tine device, including the quenched is aimed at extending the range of frequencies of domain and delayed modes, currently offers near oscillation by using alternative device structures, optimum performance allowed by the present materials and improving the bias control and associated technology. Theoretically, there is no difference circuitry for ease of starting. between the maximum continuous and pulsed power levels since the output power is limited by the V CONCLUSIONS. aaxiiiium domain field that must remain within avalanche breakdown level. This level can be raised The present state-of-the-art of the four main types ' with better quality material, but any improvement of semi-conductor negative resistance oscillators has cannot be expected to exceed one order of magnitude. been outlined and their respective application* The basic limitation is that due to temperature tabulated, based on their characteristics inherent effects and so the average output power will tend to from the basic modes of operation. Whilst the , increase with improved heat sinking. Apart from transferred electron transit time devices and the these moderate advancements expected with the avail- TRAPATT avalanche devices are nearing their theor- ability of better quality material and more efficient etical Units the frontiers for LSA and IMPATT methods of heat dissipation transit tine devices devices are rapidly progressing with higher quality would appear to have reached their optimum perform- factors (pf2) being reported regularly. In the case ance and are already being exploited commercially as of LSA oscillators, although theoretical frequency low cost, reliable sources where tens of milliwatts limits appear to be set essentially by the inter- of low noise microwave power is required. Such valley transfer time some extension may well b« applications include pumps for paramps, local possible with the availability of improved material. oscillators, low power Doppler oscillators and For both LSA and IMPATT devices a considerable general purpose RF generators. Increase in cw power is expected with improvements in fabrication including better materials and optimised (ii) LSI devices. heat sinking. Although a single semi-conductor oscillator cannot be expected to reach the same power Considerable development can be expected in this levels as a microwave tube, merely on the basis of field both in available power output and frequency power density considerations, arrays of devices may range. Again the limitation in power is essentially present a challenge in the higher power fields, a problem of heat dissipation, but since there can be particularly poised power, in the future. Sech no domain formation the second problem of avalanche development, however, is several years away and so a breakdown does not exist. Peak power outputs of the realistic present day sunmary would place microwave order of kilowatts have been achieved and the average tubes and semi-conductor devices in the high power power level will be raised as longer duty cycles are and lower power fields respectively. On the pf* permitted with improved device and heat sinking scale, Junction devices must inevitably hold the efficiency. The satisfactory operation of arrays of position in the lower microwave frequency range (say, many (thousands) of LSA devices is feasible without < U Ghs), transit tiae devices from h Oh» up to, say, sacrificing benefits of reliability, cost and system 12 Ghs for cw and TRAPATT devices for pulsed complexity, thus providing power outputs comparable operation, LSA devices up to approximately 50 Ohz for to those of the higher power microwave tubes. pulsed operation whilst IMPATT devices are the choice Although such high power fields present an attractive for high cw power up to 50 Ghz and beyond. goal optimism should not replace the reality that tubes are already providing these relatively high The availability of relatively cheap, reliable micro- output powers and they too surely will be improved in wave sources of energy will allow the widespread us« the future. Conservatively, at this stage, there- of microwaves in many applications previously not fore, LSA oscillators will be available for lower cw «•ploying this medium. performance and pulsed outputs up to several kilowatts. ACKNOWLEDGEMENTS. (b) Avalanche Transit Time Devices, This study forms part of a project on the development of transferred electron devices »upported by th» (i) IMPATT device. Radio Research Board, the Australian Research Grants CoBmittee and the RAM Post-Doctoral Program. Aβ in the case of all four devices discussed IMPATT devices are characterised by the maximum available REFERENCES. output power, d.c. to microwave conversion efficiency noise, frequency range and reliability. Prom the 1. GIBBONS G. and MINDEN H.T. - Avalanche and Ounn- characteristics listed previously the IMPATT oscil- Effect Microwave Oscillators. Solid State lator can provide the highest cw output power with Technology, Vol. 13, No. 2, Feb. l?70, pp. 37-ii8. the widest frequency range - a most useful generator of mm waves% efficiency and reliability rate with 2. STERZER F. - Microwave Solid-State Power Source«. those of the other devices, but unfortunately the Hicrowave Journal, Vol. II, No. 2, Feb. 1968, noise levels are relatively high. Lower noise, pp. 67-7U. higher efficiency and higher power IMPATT devices have been produced at research level by using GaAs as 3. SZE S.M. and RTOER R.M. - Microwave Avalanche a replacement for Si and reconstructing to include a Diodes, PROC. IEEE, Vol. 59, No. 8, Aug. 1971, I 199 pp. h. CARROLL J.E. - Mechanisms in Ounn Effect Micro- wave Oscillators, The Radio and Electronic Eng.. Vol. 3k, July 1967, pp. 17-30. 5". DELOACH B.C. Jr. - Modes of Avalanche Diodes and their Associated Circuits, IEEE Jl. Solid State Circuits, Vol. 50-lj, No. 6, Dec. l?oy, pp. 37b-

6. AIlAMS J.T. - Capability of a Projected 1975 Airborne Solid-State Phased-array Radar, Micro- wave Journal, Vol. 1U, No. 9, Sept. 1971, pp. Z3-UÖ. 7. BERSON B.E. - Transferred Electron Devices, Proc. 1971 European Microwave Conference, 1, Al/Sl-9, 1971. 8. EDWARDS R. - Recent Advances in Avalanche Diode Microwave Sources, Proc. 1971 European Mi Conference, 1, A5/S1-10, 1971. 9. WILSON W.E. - Pulsed LSA and TRAPATT Sources for Microwave Systems, Microwave Journal, Vol. lit, Aug. 1971, pp. 33-iiH 10. EASTMAN L.F. - The Capabilities and State of the Art of Ounn and LSA Devices, Int. Microwave Symp. Digest, 1969, pp. 163-9. 11. BRAVMAN J.S. and EASTMAN L.F. - Thermal Effects of the Operation of High Average Power Gunn Devices. Trans. IEEE. Vol. ED-17, Sept. 1970, pp. 7Ui-5O. 12. McRAE C.J. and GRIFFIN D.W. - Studies of the Operational Problems of LSA Oscillators, Proc. Radio Research Board Symposium on Microwave flShinlcation, Feb. 1972, pp. 16/1-19. 13. GRIFFIN D.W. - Survey of Developments in Trans- ferred Electron and Avalanche Microwave Oscillators, Proc. Radio Research Board Symposium on Microwave CouKunlcation, Feb. 1972, pp. 13/1-

Hi. GRIFFIN D.W. - Developments in Microwave Instru- mentation for Industrial Process Control. Proc. The Inat. of Engineers. Anat. Electronic Instru- mentation Conference, May 197Z - this Tciume. 15. STUCHLY S., KRASZEWSKI A. and RZEPECKA M. - Microwaves for Continuous Control of Industrial 2. Processes, Microwave Journal. Vol. 12, No. 8, Aug. 1969, pp. 51-57. 16. SAAD T. (Editor-in-Chief), The Microwave Engineers' Technical and Buyers' Guide, '1970. 17. HOBSOW G.S. " Measurement of the Variation of Frequency with Ambient Temperature of Microwave Semiconductor Oscillators, ^. Physics E, Sei. Instr., Vol. 3, Oct. 1970, ppp. Ö01-5. 16. BARANOWSKI J.J. and HIGGIK'S V.J. - Gallium Arsenide IMPATT Diode3, Microwave Journal, Vol. 12, No. 7, July 1969, pp. 71-76.

200 DIGITAL DIFFUSION ANALOGUE

M.N. Svilans, B.Sc., B.E.(Hons), University of Adelaide

1. Introduction. 1 half thickness of slab. The design here presented was evolved to fill a From practical experience, g is known to have a need for a diffusion analogue in the field of human definite upper bound for its time-rate of change. Thia physiology, connected-with the study of decompression is important in the estimation of error later on. sickness. (b) Numerical Analysis. Analogues have been built and used successfully to study transient heat flow and mass diffusion (e.g. By the application of standard numerical techni- Refs. 1 6 2). However, the particular restrictions ques, (1) can be reduced to a 2nd order finite differ- imposed in this case, namely, 12 hours continuous ence equation. In effect, both tine and space are operation and small diffusion rate (~10~5 cm^/sec.) divided into finite intervals of length k and h respec- are too severe for these methods to be practical. For tively. Only the values of the function u at the points the same reason, use of analogue computation techniques separated by these intervals are used. Thus, must be discarded due to problems of drift in amplifiers t = sk s o,1,2 • • • The basic requirement is the solution of the x = rh r = o, ±1, ±2, diffusion equation in one dimension over a finite int- For brevity, let erval of space, with time-varying boundary conditions. = uCrh sk) = u(x ,t) <<•) This can be conceived as an infinite slab of Vβ « diffusible material, lying in the Y-Z plane with Then, for the normalised case (D=l), the solut- boundaries at x = ±1. As the surface gas concentration may be written (Fef.3) ion is changed, the question may be asked at any time: u a( 2e) u at what position within the slab does the peak gas r,s+l Vl,s * r,s concentration occur, and what is its value? (5) Since the solution is required in real time, re- where —- (6) cording of the boundary conditions for later processing and calculations must be carried out as the boundary conditions (not known in advance) become available. Thus, the process of calculating u at the subsequent time interval is a simple matter, involving The digital method employed does just this. Whilst only 3 of the previous values. Equation (5) is then it eliminates the error due to leakage of capacitors, the basis of all the calculations. drift and other sources, it introduces truncation and rounding errors. However, the magnitude of these can 3. Error and Stability. be predicted, and with suitable choice of parameters, In approximating the exact partial differential reduced tc within specified tolerances. equation, discretization or truncation error, €f, is introduced. As can be expected, this diminishes as 2. Mathematical Model. the chosen intervals are made smaller, at the cost of (a) Exact Formulation. having to use more values in calculations. The problem expressed mathematically would A fair estimate of truncation error is given by appear thus: . (Ref. 3) h2 32u h" 33u *•»$ • ----<» with boundary conditions This suggests that the error is very dependent on the u(-l,t) = u(l,t) = g(t) (2) choice of a . This is indeed so, as can be seen free Figure 1, which was obtained by direct comparison of the and initial condition exact and numerical solutions for a typical boundary u(x,0) = 0 (3) condition. The expected optimal value of a is 1/6. The dependence of error on h is not critical. For thia where u is the gas concentration D the diffusion coefficient purpose, h = j^r (giving 130 intervals) is sufficiently t the time small. g gas concentration near the surface

201 r

tOO 2OO 3OO 4OO SOO

i—v^ (a) basic process.

requir of the may as (b) use of registers. ands a occur Fig.1. Dependence of truncation error on a.

Stability is also critically dependent on the choice of a. A numerically stable process is achieved if a<,\ (Ref.3.). The above considerations place a well within this range. When a The value a = T? was finally chosen to facilitate (c) calculation of "centre" value. to ano multiplication by this factor using simple s,hift and cycle. add operations. Fig.2. Optimizing storage space. The remaining inherent source of error is due to rounding. The effect can be minimised by using more bits to represent numbers. A wore1 length of 16 bits For the adopted h = 66 locations are is quite sufficient to reduce this error to small proportions. necessary, with no increase in storage efficiency possible. The bulk of these are provided by a MOS 3. Implementation of Numerical Method. (metal-oxide-semiconductor) 1024 (= 64 x 16) bit dynamic shift register, the remainder by TTL MSI (a) Storage requirements. (medium scale integration) shift registers. As suggested by (5), a matrix is necessary for (b) Arithmetic. storing each of the values ur>s with columns corresponding to time intervals and rows to distance inter- The algorithm requires three fixed point arith- vals. metic operations to be performed in serial logic, this It is neither practical nor necessary to record being cheaper and requiring less lower than the para- past values of Uj, once new ones have been calculated. llel process. These are (i) addition (ii) multiplic- s ation by a constant and (iii) comparison of 2 This allows the matrix to be reudced to 2 columns, used binary numbers starting with the least significant alternately (Figure 2(a)). digits. Only (ii) and (iii) will be discussed here, The problem is so formulated as to make u an (i) being easily implemented with standard techniques. even function of x. As a result, Uj, will be equal to s As before mentioned, a was chosen to expedite u-r,s» enabling the number of rows to be halved by discarding redundant information. multiplication. Noting that a = 3J = g" * ~Z2 * Optimum reduction in storage is achieved by multiplication by it consists of dividing the multip- using 3 temporary locations (registers) for the oper- licand by 8 and 32 and adding. This corresponds to a ands, reinserting the newly calculated value back into right shift of „ and 5 binary digits respectively be- the same column (see Figure 2(b)). fore addition. Similarly, multiplication by The question may arise as to how the end values (1 - 2a) =TS- = =• * — + — is the sum of the multi- of the column are calculated. The first value is the lo £. o lb plicand shifted 1, 3 and 4 places respectively. This input boundary condition. The last or "centre" value is calculated by using the space symmetry of u. This is illustrated in Figure 3. will be explained later in more detail (Figure 2(c)). 202 (c) Execution of algorithm. Equation (5) can be implemented by using a logic circuit, essentials of which are shown in Figure 4(b). The.main components are the memory (102U bit shift register), and 4 16 bit shift registers X,Y,Z, S. Figure H(a) explains the notation of gates G1-G5. scrioHr. '^V To understand the operation, consider the mem- poroll«l-out shift ory and registers at time t=s.k. At this point, X nt will contain u , while memory - u, etc. * »s 3 ,s A cycle of calculation is initiated by setting binary adder* gates Gj = G2 = G3 = 1. During the next 16 clock cycles, referred to as one word cycle, the contents ofS are shifted into Z and memory . S contains the boundary value, uljS+1, provided by the input transducer. The contents of y will now be U2,s» that of x»u3,s* These are the values necessary to calculate U2,s+1- Fig. 3. Method of serial multiplication. Now the gates are set so that Gj=l, 63=63=0. Consequently, during the following word cycle u2,s+i is transferred to memory, uj s to Z, U3 a to Y and Comparison of two numbers, A and B, essentially U4jS to X. The logic is set'to calculate u3 s+1- requires a flip-flop whose output, Q, indicates which So the cycles continue until the last value, : of the two operands is the greater. At the outset, Q u^, jS is in X. Here the symmetry of the original mat- may assume either 0 or 1. As the digits of both oper- rix is used, as mentioned earlier. Gate Gj is set to ands appear in sequence at the input, changes of state zero, keeping the others unchanged. The result: on the occur as shown in the table below: next word cycle, u1)S+1 is overwritten (of no consequ- An Bn Qn ence), so that u„.i's appears in both X and Z, while "n,s in V. 0 0 On-1 0 1 0 Calculations for this time interval are complet- 1 1 On-1, ed with one more word cycle, during which % s+i goes 1 0 1 to memory, and Uj sf.j to X. Clocking can now stop until the next time interval arrives and begins the When all 16 bits have been checked, Q is transferred process again. to another flip-flop to control gating for the next cycle. (A>B, Q will be 1.) Normally, gate G^ is kept at a logical '1'. To reset the memory and registers, clocking is performed as for calculations, exception being that G^O.

G C-GA+G.B (a) gate notation. lis i- .ic-

(b) ma:r. logic. (c) maximum value extractor. L-

Fig.

203 (d) Extraction of maximum value. ectly into the Y input of the C.R.O. The time base can be triggered from the clock circuitry. The particular item of interest in the solution of the diffusion equation is the peak or maximum value. 5. Applications. Extraction of this is slightly complicated by the use Due to the great versatility of the design, this of a serial process. analogue has possibilities in other fields besides The method adopted uses 2 shift registers M,N the physiological, for which it was developed. (Figure 4(c)). The input to this circuit is the same A valuable feature is the ease of varying the as that to the memory. On the first word cycle, the diffusion coefficient by varying the rate of sampling boundary value is shifted from S to N. On the next of the boundary value. Also, the accuracy can be imp- word cycle, various events occur. The new value, roved with little extra cost in complexity and calc- U2,s+1 *s shi^ted int0 N- Simultaneously, the contents ulating time. I of *H is recirculated and compared bit by bit with the new value (as explained in the foregoing section). With suitable adjustments, the analogue can simp- ' The result of this comparison controls the new setting lify semiconductor manufacture by providing the doping! of gate Gj. This in turn determines into which reg- profile in process. Changes in diffusion rates caused! ister the following calculated value is shifted. Nat- by temperature variations can be compensated for. 1 urally, it is arranged to always overwrite the smaller of the two. At the end of calculations the setting Excessive gradients of temperature, the cause of | of G5 will indicate which of M or N contains the cracking and breakages in heat treatment of metals and; maximum. This is then passed to the output transducer. glass can be obviated, whilst maintaining optimum \ cooling rates, with the aid of the analogue. (e) Synchronization. Any application where the 1 dimensional diffusion It is anticipated that the most likely error equation is relevant holds promise for the analogue to occur will be the loss of clock synchronization. in both monitoring and control capacity. This can be overcome by using the location occupied by the boundary value for a special code. As this 6. Conclusion. code word emerges from the mehiory, suitable logic can The digital approach to the diffusion problem be provided to resynchroniae the clock, should an error appears to be superior to other methods in most resp- be detected. ects, accuracy, potentially small size, versatility, long operating time to name but a few. t. Input and Output. However, practical problems may arise when the (a) Input. design is built. Digital systems are immune to noise Input to the analogue is in the form of pre- below a certain threshold. Once noise interference ssure varying from 0 to 300 feet of water (0-9 atmos- exceeds this, the induced errors are serious and, pheres). At the time of writing, no transducer with more often than not, non-recoverable. good linearity over this range was available. Another area of concern is the input transducer. Therefore it is proposed to use a Gray coded If a pressure transducer is used in conjunction with disc on the spindle of a standard pressure gauge. The an A/D converter, scaling is facilitated at the pressure can be sampled by an array of photodiodes. expense of temperature compensation and drift prob- The scaling involved is totally determined by the lems, which can be substantial over a long period. coded disc. If the Gray-coded disc method is used, scaling the input to a suitable digital range entails the use Although the diffusion analogue works on mass of a separate disc for every situation. This may diffusion and concentrations of gas within a solid, prove to be an obstacle. Only practical tests will there is a direct relationship between the partial show how serious this will be. pressure of the gas at the face of the solid and the concentration just within the solid. This permits the The building can be attempted with some reassur- pressure to be used as the basis of all calculations. ance as to the soundness of logic design since the whole device was simulated on a computer, thereby (b) Output. eradicating errors of logic. The analogue provides more information than is 7. Acknowledgements. usually necessary. Therefore the output procedure will differ with the information required. Two main The motivation for this project was mai/ily due types of output are envisaged. to the research conducted by Dr. D.H. LeMeeisurier a* the Adelaide Aeromedical Research Laboratory. In the case of the analogue being used by a diver, an indication of the peak gas tension (concentration) Many of the problems inherent in the design would is required. This can be done by simply placing LED's not have been solved if it were not for the assist- (light emissive diodes) along the periphery of the ance and supervision of Dr. B.R. Davis of the Elect- depth gauge scale. The value of the peak (in pressure rical Engineering Department at the diversity of terms) will be indicated by the illumination of the Adelaide. appropriate LED. This has the advantages of presenting an immediate and simple indication of the peak 8. . References. supersaturation and the same meter can be used as the 1. PASCHKIS, V. and HEISLER, M.P.: "The accuracy of input transducer. measurements in lumped R-C cable circuits as used in the study of transient heat flow," The other output method is the provision of a Trans. Elec.Eng. , April, 19f4, Vol.63, graphic display of the concentration - distance prof- p.165. ile on a standard C.R.O. The memory is recirculated, whilst its contents are presented to a D/A converter, word by word. The output of the converter feeds dir-

204 2. HILLS, B. A.: "A thermodvnamic and kinetic approach to decompression sickness", Libraries Board of South Australia, 1966.

3. FROBERG, C.E.: "Introduction to numerical analysis", Addision Wesley, 1966, pp.268-269.

205 ELECTRONIC TECHNIQUES IN TELEMETERING SYSTEMS

CR. Farrell, B.E., Electronics Engineer, M.W.S. & D.B. Sydney

Introduction: The Sydney Water Board uses a variety of telemetering systems. These systems will be briefly described and the electronic technique for each phase stated, enabling the reader to evaluate the technique to his own circumstances. Supplementary comments appear where appropriate e.g. on reliability. Two of the systems use radio links, the others use physical lines. 1. SEWAGE PUMPING STATION (SPS) TELEMETERING SYSTEM: connected to the station "satellite". The switches are connected by a pipe, finished flush with the (£ Some 200 SPS's lift sewage from low level areas to well wall to prevent blockage by caught rags. An \" nain sewer carriers. Operation is intermittent and interposed kerosene buffer prevents sewage corroding p automatic and the early warning telemetering system the pressure switch bellows. A microswitch on the notifies a central Service Centre of station defects final contactor of the motor starter senses pump in any of the 200 stations, within 15 minutes of the operation. A Float switch detects flooding of the malfunction onset. machinery well. The station power supply is The system is interrogation type, each station monitored and by using AND gates, the failure of one being interrogated in turn. Time for interrogation or more phases is detected. ! and reply is 2 seconds. The reply signal, (b) Remote Station Operation: | representing multifunction status of the station in the form of conditions (tones) 1 to 8, is compared The transducers are on/off switches and are "3 to the previous reply for that station, and print- gated to give one of eight outputs, each representing out, read out and alarm are automatically triggered combined conditions from various transducers. The n a change of status. An optimum network of PMG gate selects a transistor switch coupling the and-lines is used, the stations being connected in appropriate high stability capacitor in the reply roups of up to ten to a single PMG pair which conveys oscillator. .11 signals to and from all stations on that group. When interrogation occurs, a frequency conscious a) Remote Station Transducers: reed operates a multivibrator timer which connects power co the gating and oscillator modules for half a 0-15 p.s.i. pressure switches (15 inch operat- second. The input amplifier has an Automatic Gain ional differential, 0.25 inch reproducibility) which Control circuit which allows a wide range of input pick up the level of sewage in the wet well are

BATTERY BATTERY CHARGER PH3NE BELL

MAO MULTI- POWER i/AE CONDITION LINE VIBRATOR FAILURE TERMINATION ,, mJFIER B TIMER GATING DETECTOR FREQUENCY \ SELECTIVE I REED I

REPLY STATUS 3 PHASE TELEPHONE OSCILLATOR DETECTION SWITCHES.

Fig.1 Remote Station Satellite - Block Diagram

206 --_-- I

levels. The unit contains a rechargeable standby and gated to the appropriate memory. The comparitor ' iittery with 24 hour capacity protected against full is an AND-OR gate combination, which sets up one of discharge by a solid state low voltage sensor. two outputs for either SÄHE or DIFJBREKT oondition. A telephone system is also incorporated using After 6 pulses the outputs are unclasped, the the same PMG lines, again with reed selection. To appropriate one acting-SAME condition output initiate • avoid loading the line a high impedance input is reset of the station, old and new condition memories; " required to the satellite (10 K ohms) which is by- DIFFERENT condition output triggers the BR-3KAECH passed during interrogation by voltage reversal on flip flop memory, which cuts off drive to th« pinoh the line. Difficulties with this unit are primarily wheel, stopping the tape, (in approximately 50 faults on PMS private lines. Also, the use of milliseconds), resets the NEW COHDITIOH meeory and polarised voltages on the PMG lines causes faults, restarts the timing chain. If the different condition interruption to conversation during interrogation is is confirmed, the RESEARCH module turns on the FRZRT sometimes inconvenient and currently there is a flip flop, gating a lOHz Oscillator into a sealer of shortage of reeds which are the least reliable 12 driving a sequence detersinator with 12 outputs- ' component. Satellite cost is about $250 each. one for each memory module. The deteiminator gatea the appropriate figure from each memory in turn into (c) Central Datalogger: the serial input printer, providing priat out of th* A prerecorded program on one of two tracks on an old and new conditions, SPS number and tin». Th« : 3 head reversible tape deck initiates interrogation. eleventh position of the sealer rescans the HIV . Nine tones are used in pairs, providing 36 (PCg ) CONDITION memory to choose the black or red print ; command signals through filters, DC amplifiers and a solenoid and simultaneously, the read out and audible diode matrix. Currently 19 command signals are group alarm are triggered. The twelfth position resets the ; selection, 10 are station selection and 2 are tape PRINT flip flop, which in turn resets the RESEARCH, reversal (allowing 36 groups, 360 stations at 1.8 reenergiaing the pinch wheel and resetting station second intervals). The command is stored in a and old condition memories. It also turns on the transistor or flip-flop memory. The group memory RECORD flip flop to erase the old condition and forward biases diodes connecting the group line to the record the new. The recording phase is timed to datalogger, the station memory selects one of ten coincide with the reading of the next station comsand tuning capacitors in the interrogation oscillator. tone and the erase and record circuits are de-energised by the earlier mentioned trailing edge of the Using a diode matrix the station command signals are shaped command signal for the next station, which fed into a common wave shaper. The trailing edge of resets the RECORD and NEW CONDITION memories the shaped wave then triggers a critical timing chain simultaneously with the triggering of the next timing for the interrogate, reply,read old condition, compare, chain. stop tape, reinterrogate, recompare, start tape, erase, record sequence. Timing is by counting from The SFS number is independent of the group and a continuously running digital clock divided down station number. An 12 x 10 Cross-Bar having 9 from a 10 kHz crystal into 200mS pulses. Until the contact pairs per crosspoint is used to decode from advent of daylight saving, the clock was designed group and station to SPS number. The Group memory for reset every leap year. selects a solenoid for a horizontal bar (3 groups As the timing chain starts, the interrogation to each solenoid); Station memory selects a solenoid oscillator is switched to the waiting group line. for the vertical. Thus for 3 different SFS'a, the 9 After 2 pulses the oscillator drops out allowing contacts for a particular crosspoint are closed and the reply tone to feed the NEW CONDITION filters and a 9 transistor matrix separates these 9 contacts their associated memory circuit. The old condition into 3 groups of 3 i.e. 3 stations,each with a 3 is then read from the tape, passed through filters digit number (units, tens, hundreds). To do this,

010 NEW ( TAI CONDITION LINE — \OEC — COMFHRin» SYSTEM TEST CONDITION FILTERS » FILTERS 1 TERMINATION MEMORY MEMORY

\ 1 i f coyMANQ PRINT-OUT STATION FHTERS, CROSS-BAR SELECTOR, DSU1DER, DISPLAY OESEMCH TTHER DECODER INTERROGATE HEMORr

Fig.2 Central Datalogger - Block Diagram

207 each transistor switch is operated by 12 group memories, and connected to 120 contact pairs (only one is closed) each of which is connected to a digit rail. The transistor switches are scanned in turn by the print sequence determinator, to determine which digit rail is energised. Conversley in telephone dialling, the dialled SPS number is converted by uniselectors and a diode ffiitrix to select a group line and station tone. Typical print out font is 17-134O6OO-meaning condition 1 has now replaced condition 7 at SPS 134 occurring at 6.00 a.m. Manually triggered SYSTEM TEST and ALARM LOG are provided and these are achieved by manipulating the comparator. Cost .to design, manufacture, install and commission the Datalogger was approximately $50,000.

Since commissioning, the main problems have been associated with the tape carriage mechanism, which has recently been redesigned and is now expected to be satisfactory. Power supply frequency stability of l£ is satisfactory, but long term frequency drift occurs in the emergency power supply which is located in an air conditioned room. These aside, the system has been extremely trouble free, particularly the crossbar unit.

>.. WATER PUMPING STATION (WPS) TELEMETERING SYSTEM With approximately 200 Service Reservoirs, each separated from its associated pump, the Board requires telemetry both for automatic control end supervisory purposes. The majority of reservoirs have only a few functions requiring telemetering and time is not a critical factor, making analogue tone telemetering an economic proposition. Maximum economy is achieved by use of both time and frequency ME» over multiplexing. Op to the order of 12 functions »out (either simple amplitude modulation for on-off functions or frequency shift key tor 3 state functions or high security 2 state) can be telemetered by multichannel voice frequency transmission economically; digital transmission becomes more economic for a larger number. .3 Typical multiplexing arrangement for one From each reservoir, it is necessary at least module at the Central Location to telemeter water level for two purposes - i.e. for local pump control and for supervisory functions hammer drives up scale from zero during mark-time, at a central location. For the first, detection is the other downscale from full scale during space- at specific levels by an on-off pressure switch time, each hammer having a spring return to its rest which achieves high accuracy, and for supervisory ' position. There is a fixed time for the full «ark- purposes, continuous indication is obtained by space signal (l5 seconds). Thus for any water pressure bellows in a commercial Impulse Duration height , since the driving time upscale and downscale Transmitter. The bellows mechanically alter the totals 15 seconds, both hammers touch together e.i proportion of time for which a motor driven cam the appropriate position, and one hammer will drive doses a low friction switch, the length of closure the ivcoxäLag pointer from its last resting position. being linearly related to the water level height. If the total tine is less than 15 eeconds, the haaan Both transducers switch the emitter follower of one will not meet, and the pointer can not be driven in of twelve (maximum) oscillators to a mixer and the wrong direction. thence to line for the duration the switch contacts are closed. At many pumping stations, reservoir heights and pumping station functions are tine and frequency At the pumping station the tone receiver multiplexed and retransmitted. A commercial time consists of 12 parallel filters each operating a multiplexing transmitter and receiver are utilised relay. With the exception of the impulse duration having 15 information time slots of 40 seconds, and signal, each channel operates an on-off control or a final time slot for synchronisation which is longer display function. A commercial Impulse Duration The timing is from synchronous motor driven cams, the Receiver follows the relay action,plotting it as a 40 seconds ensures that at least one complete 15 water level height. The transformation is electrical second signal is intercepted. to mechanical with two solenoid driven hammers moving in opposite directions across a scale. One The Board manufactures its own amplitude

208 TABLE 1 SAMPLE JUNCTIONS TRANSMITTED OVER tfIDEHBRE PIPEHEAD HONE LINK

Function Description type Actuating Device(s) Locatio Intermediate Devices Terminal Device(s) Pump 1 Vane Position T Pulley attached to pump Widemere Pulley in VDI TSL Receiver Display casing Transmitter Pump 1 Running T Auxiliary Run contact it N.A. Lamp indicator Reflux Valve open T Limit Switch, on aim » N.A. Lamp indicator operated by valve actio Butterfly valves open T Limit Switch built into it N.A. Lamp indicator valve actuators 84" main flow T Venturi flow meter ii Pressure bellows is TOI Receiver Display VJ)I transmitter Butterfly valves opening T Auxiliary contact on it Intermediate Relay Lamp indicator or closing relay operating solenoi Croup 'A' 'B1 & 'C1 alarm T Refer Table II it Group relay Uro up alarm indicator Pump 1 set up for 84"main T Bank on Pump Function tt Intermediate Relay Lamp indicator switch; valve switch Control taken at Widemere T Bank on Remote-Local it N.A. Lamp indicator Control Switch Isolating valve set ready T&C Auxiliary contact on tt Intermediate Relay; Lamp indicator; Ready- to close setting relayj"Open" Refer below to-trip Relay limit switch Pump 1 overloaded T Current Transformer it Overload Relay Lamp indicator Start pump on 72" main C Push Button Pipehead Interlocking Relays; Hotor starter Function Routing swit ches Open Vane " " M C H 11 a Relay Motor on Actuator Emergency Stop Pump C Push Button tt it Motor Valve Solenoid Valve 8O# open (Hydraulic T iercury switch on n n Lamp indicator leak) Hydraulic arm Test and Set Isolating Setting Relay; Reply Valve 72" main C J*ush Button il N.A. Relay Trip Isolating Valve 72" c 'ush Button; ready- n N.A. Valve actuator main to-trip relay Control accepted at T ?ush Button n N-.S. Lamp indicator Fipehead

VDI - Variable Duration Impulse factor, 5 x 12 channel standard amplitude modulated T - Telemetering tone units were used, although this application C - Control would be more suitably carried out using digital techniques and frequency shift tone equipment« modulated voice frequency tone units, at an approximate cost.of $700 for a 6 channel Transmitter - Details of Widemere are as follows* Receiver set, flOOO for 12 channel. The tone equipment and the impulse duration equipment are The station has 2 pumps and a number of valves. noteworthy for their simplicity, providing remarkable It will be an unmanned station on remote manual reliability in an extremely large system. The main control from the Supervising Control Centre at Pipe weakness are line failures, and damage from lightning Head. strikes, which are serious weaknesses on pump control lines» Located in the Supervisory Control Centre at Pipe Head will be the push button controls to stop/ The tone equipment is used for a 'large variety of start each pumping unit, position the control •-ignal and control functions, which are best illustr- vanes on each pump and individually open/close the ated by referring to a major water pumping station, valves V85 and V86. Adjacent to these push buttons Widecere WPS No.147. For telemetering and control will be recorded the individual flow rates in the of this station remotely at Pipe Head, some 90 72" and 84" diameter delivery mains plus recordings functions are involved. Because time was a critical of the positions of the control vanes on each pump.

209 r

TABLB II a-ister .. ( mechani 'fcen th SAMPLES OF GROUPED FUNCTIONS FROM TABLE 1 serial tone, height Function Description Actuating Device(s) a furth which t Pump 1 Unscheduled Shutdown Start Solenoid Contactor, timer, Aux. Bun Contactor interrc " " Power Failure Relay de-energising " " Motor Overload Thermal Overload; Circuit Breaker; Overcurrent Relay Es " " Vanes Faulty Aux. Hun Contactor, timer, limit switch evapors " " Suction Pressure low Pressure switch switch« Thermostatically controlled switch " " Pump Oil Temp. High wiper E Full Load Current relay " " Approaching Overload contact Various valves faulty Limit Switches for Fully Open, Fully Closed, plus timer any fu: Reflux Valve will not open Pump Stop-relay, User, limit switch oscillc Reflux Valve Oil Pressure Low Pressure switch Valve Pit Water Level High Bouyancy switch the sei Ambient Temperature High Thermostatically controlled relay Forced Entry Switches Ri frequei As the and soi Indicating lights will be provided to show the upper reaches of 4 rivers feeding the extensive depend« availability of each pump, the running of each pump Varragamba Catchment area. The data from these 4 interfi and a warning that a pump is approaching the condition remote stations is transmitted by radio link to a of overload» central control room at Varragamba. The system is interrogation type, and employs digital technique. B< Adjacent to the controls will be three alarm the wa; Information supplied is the accumulated rainfall a 12 V. annunciators each of which will represent a group of iiave ai alarms as follower- and evaporation, both with a resolution of 0.01 inches and river height with a resolution of 0.01 ft. The Group A - Unscheduled shutdown or tripping of any state of the remote station batteries is monitored, pumping unit in V.P.S. 147. (This alarm activating an alarm should any battery fall to an requires urgent adjustment of other unacceptably low voltage. equipment in Headworks system). The primary sensor for the rainfall measurements Group B - W.P.S. No.147 is approaching conditions consists of a tilting bucket gauge the contact of of unscheduled shutdown unless attended which updates a local raingauge and a 4 digital to urgently. electromagnetic register with each point of rainfall; the movement of the float wheel on the river height Group C - Non-urgent alarm which could be recorder is measured by a servo-motor-driven 4 digit investigated on next periodic visit to register which is updated every 5 minutes. The V.P.S. 147. differential between the register and the float wheel is detected by a set of contacts which cause the Located at Widemere W.P.S. 147 will be a register to follow the float wheel movement until duplicate set of push button controls, indicators the differential between it and the register is and recorders except that all alarms will have cancelled. individual annunciators in lieu of grouped alarms. The evaporimeter consists of a 3 ft. diameter Also located in Widemere V.P.S. 147 will be a inner tank and a 4 ft. diameter outer tank, the rim mode selector switch so that control of the pumping of which has been set flush with the surrounding units and the corresponding valves will be either ground level. The inner tank is connected by an from Pipe Head or Videmere (but not simultaneously). underground pipe to a service pit several feet from Details of some functions transmitted over the tone the evaporimeter tank. Inside the service pit are links are shown in Table 1. two float wells, in which the water level is hydrostatically balanced with the water level in the Another notable application for the tone equip- inner tank. The larger float drives a local recorder ment involves transmission of water flow readings which plots continuously the variations in over a telemetering line. The flow measured by a evaporimeter water level. Vonturi meter, is converted with a differential pressure cell to a current output (4 to 20 mA) and by Coupled to the same float is a servo-eotor- using comnercial equipment the current electronically driven system which updates a four digit register converted to a variable duration impulse, for every 5 minutes, using the contact detected transmission to line. differential follower technique described above. The function of the smaller flott will be described 3. HYDR0L0GICA1 DATA STATION (H.D.S.) TELEMETERING later« SYSTEM When the station is interrogated a tiaer switches Fig. 4 This system provide early warning data from the the radio transmitter on, the audio circuits and

210 ! Tister scanner are energised. The scanner,which is ; mechanical,first monitors the low voltage alarm and *;hen the station identification number producing a serial train of pulses of the audio frequency reply FILAMENTS t P.T.T. . tone. The scanner then switches to rainfall, river ] height and evaporation registers in turn producing I a further three bursts of tone pulses, following NTEfDOCAJlON ; which the station shuts down and awaits the next 1 interrogation. FILTER TfWSCENER Each of the rainfall, river heights and evaporation registers consist of four 10 position switches, one switch for each digit, each with a CMHIFCL CCNTKOL wiper arm earthing the appropriate contact. Each COMMAND 0MM contact is scanned in sequence, pulsing the reply FILTER oscillator until the earthed contact, which prevents any further pulses in that digit from reaching the oscillator. There is a homing position which steps the search to the next digit. AUDIO REPLY Radio equipment is Frequency Modulated,at a OSCtLATOR frequency of 44.85 MHz;into Yagi directional antennae. As the remote stations are near the bottom of gullies and some miles from Warragamba, there is some dependence on reflected signals, with resultant interference to the radio signals in some conditions. BATTERY Because the remote stations are so far out of the way, power is from windmill generators charging DRAIN SCANNER a 12 Volt 200 Amp Hour battery bank. The windmills I FILL have automatic feathering, and the generators are CONTROL REGULATOR t CHARGER

I WIND DRIVEN RAINFALL EWkPORAWh RECORDER RECORDER

BATTERY mwasvas BATTERY OURGER AUTO Fig« 5 Typical Remote Station - Block Diagram CHANGE-OVER equipped with automatic voltage regulators.

At the master station, these pulse trains are interpreted and printed out in digital form on two electric typewriters which also record the date and WARRAGAMBA CONTROL ROOM time derived from a digital clock supplied with the equipment. The letter also contains a matrix program board which enables the operator to "plug-up" any one NJERKXHTOR POWER »PROGRAM of three basic programmes for the interrogation of SUPPLES UNIT SELECTION the out-stations. A switch is provided to select the appropriate programme for the prevailing weather conditions. A typical choice of programmes would be:

Programme 'A* for dry weather periods - three transmissions per day - at 8 a.m., 9 a.m. and 8 p.m.

PRUT-OUT DECODER 1 UNITS Programme 'B for wet weather - seven transmissions per day - at 4 a.m., 8 a.m. 9 a.m. 12 noon, 4 p.m.( 8 p.m. and 12 midnight.

Programme "C for storm conditions likely to cause flooding - twenty four transmissions for each ALARM station per day - hourly on the hour« UNIT A manual control is also provided to interrogate any station at other tines when necessary.

Pig. 4 Warragamba Tranceiver Hut - Block Diapram Irrespective of which programme is selected,

211 TABLE 3

TYPICAL PRINTOUT HDS SYSTEM

Date Time Batt. Station Rainfall River Evaporation Volts No. Heii-htn 69.12.16 12.22 H 1 2.13 in 11.63 ft 0.99 in 69.12.16 12.24 H 3 1.90 in 3*04 ft 0.93 in 69.12.16 12.27 LV 2 1.76 in 5.35 ft 0.90 in

as soon as all 8 a.m. readings have been receivedt the and relays* master station will transmit a special command signal to all the out-stations to activate the automatic A typical section of printout is reproduced in refill/drain system on the evaporimeter. On the table 3. receipt of this special command signal, the smaller float in the service pit will operate micro switches 4. WATERMAIN PRESSURE RECORDING to energise solenoid valves which will either fill or drain the inner tank to a point nominally 3 inches In order to take steps to prevent damage being below the rim of the tank. After allowing an hour caused by "water hammer" in Watermains, a system has for the tank surface to settle, the master station been developed to transmit watermain pressure from will interrogate each station in turn at 9 a.m. to a number cf points, to a local point via a radio determine an exact reading of the tank level which link where comparisons with local pressure and the will become the reference level for evaporation operation of motors and valves can be made. measurements over the next 23 hours terminating at 8 a.m. the following day. This method can actually be used for monitoring a number of remote points, where information is A ball valve is used to maintain the water level available in the fonn of a DC voltage. The main within 1 inch of the rim of the outer tank of the advantage of the system is that it is transportable, evaporimeter, both tanks of which are replenished able to be set up in a few minutes where a need from a 200 gallon supply tank mounted on a stand arises. inside the equipment house. Rainwater from the roof of this house is piped to the supply tank. The pressure transducer is connected directly to the watermain. The pressure transducers DC The meteorological stations are sited some output voltage varies between 0-40mA. This signal distance from the corresponding river gauging is then amplified to a level 0-4V, converted in the station in order to be above maximum flood levels. voltage-to-frequency convertor to an AC signal in Vith one exception the transmitter houses are located the frequency range 400Hz - 3KHz. on higher ground than the corresponding meteorological stations so as to obtain more wind for the generators The output from the convertor is fed into a and improve the radio communication. All three sites monostable multivibrator which shapes it to a square- are interconnected with undergro'ind cables to wave and thence into a standard frequency modulated constitute a complete hydrological data station, all mobile radio transmitter with a carrier frequency equipment of which is powered by the one battery in the 70MHz band. bank. At the receiving end is a standard mobile At the receiving end, in addition to the actual receiver. The audio signal from the receiver is printer, the Warragamba Centre contains the Interr- amplified and then fed into a Schmitt trigger to ogation Programmer run from a mains driven timer, increase the stability of the signal* From the manual interrogation selectors, tirac and dete units, Schmitt trigger the signal is fed into a monostable decoder, printer,programmer and alarms. This consists multivibrator to give a square wave with uniform basically of electro-magnetic and mechanical logic, pulse width and from the multivibrator into an mainly synchronous clocks, step-by-step uniselectors lf.tegrator, which averages the outward signal-as the

-i VOLTAGE 70 FREQUENCY CONVERTER MODULE [ VOLTAGE I PRESSURE DC. 0-4V 400Hz M0N0STA3LE I A TO TRANSMITTER TRANSDUCER AMPLIFIER FREQUENCY Whiz MULTIVIBRATOR CONVERTER I Fig. 6 Transmitting Point - Block Diagram

212 r i FREQUENCY TO VOLTAGE CONVERTER MODULE i HXhte SCHMITT .TL RECEIVER MONOSTABLE JL DC. CHART AMPLIFIER INTEGRATOR i 3KHz TRIGGER MULTIVIBRATOR RECORDER

i L

Fig. 7 Receiving Point - Block Diagram

frequency changes the output level changes. From Each unit will be mounted inside a pillar type the integrator comes a DC signal which varies with housing, similar to that used by the P.M.G. for frequency and hence reproduces the DC input at the cable junctions, which is 12 inches in diameter and transmitting end and drives the chart recorder. stands 37 inches above footpath level. A £" diameter copper pipe will connect the watermain to The Pressure Transducer is a general purpose the unit, the only other connection being an type which converts fluid pressure on the diaphragm underground cable to an adjacent P.M.G. cable pit, to a controlled strain in an excited wheatstone all operating power being fed through the P.M.G. bridge of strain gauge windings in which all four line so that an external 240 Volt alternating current anas are active. The displacement is transmitted supply will not be necessary. through a rod from the diaphragm to a spring element. The resulting flexure of this spring tilts the The binary coded pulses will be conveyed over an sapphire posts on which the strain gauge windings existing network of telemetry lines connecting the are mounted increasing strain and hence resistance reservoirs to Head Office where the data will be fed in the windings at the other end. The thermal into a register to verify the validity of each and mechanical symmetry of the unit ensure that the message before passing into a process computer work- output is a function of the pressure applied to the ing in real time. Prior to the installation of such diaphragm. The full range output is nominally 40mV a computer, the coded pulse information could be with a source impedance of 350 Ohm. stored on magnetic tape to facilitate batch processing by our existing commercial type computers. The DC Amplifier is a linear I.C. Amplifier. Its output is connected to the voltage-to-frequency If the development of the pressure sensor is converter which features a differential amplifier successful, the system will be extended to the and an unijunction oscillator whose frequency of measurement of differential pressure. This would oscillation depends on the input voltage, followed then revolutionise the field of flow measurement, by monostable multivibrator for i niform pulse width. eliminating entirely the need for flow meter gauge houses, chart changing, clock winding, inking and In the receiver it is necessary after the the present inevitable manual task of processing the Schmitt trigger to use a monostable multivibrator charted information. Such a system would provide to provide uniform pulse width to feed the integrator. positive identification synchronised data (as The DC output voltage from the integrator is adjusted opposed to slow running charts) and any malfunction to zero output voltage at an input frequency of would be detected within lrj- minutes of its 4O0Hz. This DC signal is fed into the chart recorder occurrence thereby giving an early warning of loss where zero DC signal represents the 400Hz frequency, of valuable records. and this corresponds to zero DC and hence zero pressure at the input to the transmitter end. The system envisaged would involve a transmission speed of approximately 70 Band. 5. DIGITAL TELEMETRY SYSTEMS (b) In Conjunction with Frequency Shift Keying (a) Hydraulic Gradient Surveys (fl.G.S.) The Board is currently calling tenders for a At the present time we are engaged on a research Digital Telemetry and Control System to receive and development contract with one of the major status, alarm and analogue information from four Australian instrument companies to manufacture remote stations interconnected by land line, and to prototype digital sensors for measuring the gauge transmit command signals to these stations. It is pressure at selected points on the distribution anticipated that Frequency Division Multiplex network. These devices must be low cost (approxim- equipment will be necessary. Continuous updating ately $400 each), robust enough for permanent without manual reset is required, as are control field mounting and suitable for connection to sequences of the Select-Check-Operate type. Analogues '•Private Lines" rented from the P.M.G.'s Department. include rates of flow. Keying speeds near 100 baud

213 r

are anticipated. Tenderers were advised that a three level PSK using Mark, Space and Centre Rest levels i.e. keying with Return to Zero is preferred.

6. OTHER APPLICATIONS

The Board has used other telemetry techniques from time to time and is finding more frequent use for various types of datalogger both for telemetry applications and otherwise. It has just developed an expandable 200 input datalogger scanning and printing information at 10 stations per second. This equipment utilises a commercial printer and Digital Voltmeter for Analogue to Digital Conversion, the remainder being made by Board's forces.

An unusual application of telemetry involved continuous scanning of a digital clock, a digital pressure gauge and a crack in a cement pipe under 400 p.s.i. pressure,using a number of cameras and a video mixer, the results being viewed and video taped at a remote point. ACKNOWLEDGEMENTS

The author wishes to thank various colleagues in the Board for their assistance in preparation of this papea The Sydney Water Board has granted me permission to publish this paper and the information contained herein.

214 THE DETECTION AND LOCATION OF A NOISE SOURCE IN A DISPERSIVE MEDIUM

G.J. Cybula, B.Sc, (Tech.) Grad. I.E. Aust. Electrical Engineer, Operations Division, Australian Atomic Energy Commission T.J. Ledwidge, B.Sc., Ph.D., C.Eng., M.I.E.E., M. Inst P. Head Engineering Physics Section, Engineering Research Division, Australian Atomic Energy Commission

SUMMARY

The transit time of a signal in a medium can be measured by many techniques. When the signal has random- like properties then the cross correlation between spaced detectors can, with certain restrictions, yield the transit time between the detectors. In this paper a distinction is made between phenomena which are convected by the medium and those which are propagated as a wave motion through the medium. The latter signals may experience a significant dispersive effect and in these cases, the cross correlation of the raw signals yields a result which indicates that between spaced detectors the degree of correlation is zero. A simple theory is postulated to explain these results and meaningful correlations are obtained using only ths envelope of the original filtered signals.

The results of an experimental programme, designed to investigate the acoustic frequency spectra generated when a liquid boils in an annular passage, are used to test the method outlined above. Freon was pumped vertically over a heated cylindrical rod mounted inside a circular tube. The signals emanating from the boiling process were detected by piezo-electric accelerometers fastened at either end of the containment tube. Amplitude spectra (plotted with the power supplied to the heated rod as a parameter) clearly indicated the power at which boiling cotrmenced.

The position along the tube at which the boiling was active was determined by time delay correlation between the envelopes of the signals from the two accelerometers.

I. INTRODUCTION attenuation of y rays do not require penetration of the pipe walls and thus can easily traverse a The problem of detecting and locating the position length; however since this method depends on changes of an unwanted noise source is well known to many in density between the source and detector, its people. Although numerous methods are already in sensitivity is greatly affected by the ratio of pipe common usage on such problems, it is felt that the wall thickness to the fluid diameter. technique described in this paper may have potential in certain circumstances. Such a circumstance could The method described here is illustrated by be, for example, the leaking of a gas or a liquid using the acoustic noise generated by boiling. It from a pipe line buried underground. In this case the offers significant advantages over the other methods existence of the leak is undesirable and the passage since it can detect the onset of boiling as well as of the fluid through the leak could constitute a pin-point its location and is not affected by the source of acoustic energy which would travel through dimensions of the pipe. the walls of the pipe as an elastic wave. This method was developed as part of an investiga- Another example could be the problem of determin- tion of heat transfer phenomena in annular nuclear ing whether boiling is occurring in the liquid, and fuel elements. The work was carried out in the exactly at what point the boiling takes place. laboratories of the A.A.E.C's Engineering Research Division at Lucas Heights. Whether the detection of boiling is simple or difficult, usually depends on the type of liquid, the II. METHOD FOR LOCATING THE POSITION OF NOISE SOURCE temperature and pressure at the point of interest, and the physical structure of the duct. So far only The case considered here is the simple one in a few methods exist which can successfully detect which the source and the detectors are co-linear. In boiling in a pipe. If the liquid is electrically a practical situation co-linear can be taken to mean conductive an integrating conductivity Void gauge or the case in which the disturbance from the noise a conductivity- probe can be useful. If the fluid is source is propagated along a pipe, bar or wire (whose an insulator a capacitance type void gauge may be radius of curvature which is much greater than the employed» Both methods involve penetration of the wave length of the disturbance). It is further pipe wails and consequently do not facilitate traver- assumed that two detectors are placed in such a sing of the pipe length in order to locate the point manner that the source lies at an unknown location of onset of boiling. Void detectors based on the N between them. If the known distance between the 215 detectors is k and the unknown distances from the Auto-Correlation Function, RXX(T), defined as follows; detectors are k. and k respectively, then letting X. and x. be the times taken for the disturbance to Sxx<(o) Lim i travel tne distances k and k at a common velocity V we have T_»co 1 TT f • v RXX(T) - Lim ± j x(t) x

where X_,(ai) is the Fourier transform of a sample of x(t) taken over a period T and * denotes the complex from which conjugate. Xj - 1/2 &t + v<^ - X2)] 4. Sxx(ü)) and Rxx( t) describe the same information j>. In order to determine I, then it is necessary to know in different ways; the former in the frequency >, the velocity of propagation in the medium, V, and the domain, the latter in the time domain. These j difference in the transit times AX. functions form a Fourier transform pair. [, j V can be measured in any practical situation by Similar expressions exist for y(t) and also for [ either simulating a noise source at a known location the information common to x(t) and y(t). The most | or positioning a third detector in the system. The important of these in this work is the cross \ latter method uses an equation like 2 or 3 in which correlation function Rxyfr) defined as follows: i= the distance k is known and X is measured.

It is clear from the above argument that two Rxy - Lim ± x(t)y

A brief summary of the theory of noise analysis is A fuller theoretical treatment of the effect of given in the next section. dispersion on the cross correlation is given in [2]. A simplified heuristic argument is used here to III. RELEVANT NOISE THEORY demonstrate the important finding that the cross correlation of broad band signals propagated Is is not the intention here to give a full through a dispersive medium yields a zero resultant. treatment of noise theory but rather to highlight the parts relevant to the measurements reported in this Suppose that the signal generated at the source paper. Many texts are available to the interested is given by: reader, for example Bendat and Piersol [l]. s In a situation when a disturbance is propagated through a medium a signal designated x EAi

216 love of propagation is given by: of the test section, on the outside of the connecting flanges as shown in Figure 1. The output of these V = Vo [1 - nrn (aAi)2]

where

Vo Fi is the velocity of propagation

in an unbounded medium. Converting the above Top Acceleremcter equations into the time domain results in the following: .To Tap« fWcordtr

X, 2: X. mit (a/a) Assumed location The cross correlation function Rxy(t) can now be of onsit of Nucleate written as: boiling

2 Rxy(i) = 2Ai (T) cos tu.f T-(X1-X2)(l-imt(a/M)))

In a non-dispersive medium the term involving the wavelength of the disturbance is zero and in this Heated Rod case Rxy(t) reaches a clearly defined maximum when T = AX. In the present case, however, the peak is Outer Tube spread over the whole of the frequency range with a consequent loss of the definition of the location of the peak. In all the practical cases investigated involving sound waves in steel structures the cross correlation was found to be zero.

The above argument is based on a lossless medium Bottom Accel «rometer and on the use of sensors with a flat frequency response over the whole of the spectrum of interest. To Tape In real systems neither of these assumptions are true and in consequence an even greater loss of resolution Recorder in the correlation peak, than that predicted above, is to he expected.

V. THE USE OF THE ENVELOPE OF THE SIGNAL

The previous section discussed the processing of the raw signal. This is equivalent to attempting a Freon inlet transit time estimate using phase velocity measurements. An alternative and later shown to be a successful method is to use the envelope of the Figure 1. Test Section Simulating a Fuel Rod signals from the two detectors. This is the same as considering x(t) = SAHt-^) and y(t) = £Ai(t-X2) in which the effects of dispersion have been reduced noise transducers was recorded on magnetic tape whicj by moving to a very much lower frequency signal. was later processed on Noise Analysis equipment. Presumably it would be possible to take the envelope Figure 2 shows the instrumentation set-up. of the envelope and so on until meaningful correlations were obtained. In this work with The onset of nucleate boiling was deduced by signals mainly below 40 kHz then the first envelope comparing amplitude spectra taken when no boiling was found to be sufficient. took place (no power applied to test section), and when boiling was definitely established. Since all The next section discusses one typical background noises appear on both spectra they are application of the technique applied to the case automatically accounted for, and any peaks that when the noise source was the boiling of liquid Freon appear on the boiling spectrum only must be due to in an annular passage. - . noise .created by the growth and collapse of vapour bubbles. VI. EXPERIMENT AND RESULTS The location of the point of onset of nucleate The experiment used an electrically heated tube boiling requires a cross correlation of the two cooled on the outside by liquid Predn 12. The heated signals. The first and fourth traces in Figure 3 1 tube simulated a nuclear fuel rod, and the Freon show the raw signals teceived from the two «ccelero- simulated cooling water. The purpose of such meters at the instant when nucleate boiling starts. experiments is to investigate the boiling processes The high frequency bursts which can be seen in the and critical heat fluxes which may occur during the traces are each due to a vapour bubble growing or operation of water cooled reactors. collapsing. For each burst in the top trace there is a corresponding burst in the fourth trace, with Two accelerometers were mounted one at either end lt.. 217 r

Ensemble Envelope Averager petector (Spect. Dgn. Corp.) From Top (CRO) • t \ j Accelero meter Real Time Analyzer i _ X-γ (Spectral) Plotter (Dynamics Corp.) Bond Pass Filter Mognetk Tope Recorder (Ampex) Band Poss C.R.O. Filter ) Correlator X-Y (HP) T Plotter From Bottom Envelope Acceierometer Detector

Figure 2. Instrumentation set up and signal flow paths >

RAW SIGNAL

EXTRACTEOI 17-4 kHi

RAW SIGNAL

EXTRACTED 174kHi

| I I I I .V I" ,,'» 'J

Figure 3. Signals from top and bottom accelerometers at various stages of processing. Note presence of visual cross correlation between respective traces 218 an apparent delay betwe<

Attempts at cross c< consistently failed due dispersive medium. Howi previously by Ledwidge ' cross correlation on thi growth or collapse of a frequency content of th< fruitful. For this pur] of the bursts in the fi'. be extracted by passing band-pass filters, then and smoothing before api The second and fifth tri extracted frequencies ai show the corresponding i envelopes of the events seen more clearly in th: which frequency to extrj the boiling and non-boi 4 and 5.

Figure 4 shows a 3-< spectra against power a] the onset of nucleate b< 0.30 kW. Figure S show: just before the onset o: boiling spectrum taken <

o li

Figure 4a. Three dimene amplitude spectrum with section. Onset of nucle approx. 0* ^ kW. "The powe in 44 seconds. Top acce 18 db.

Figure 4b. Same as Figu accelerometer. Input at an apparent delay between the two signals. It should be noted that both Figures 4 and 5 serve a dual purpose. Firstly, the onset of nucleate Attempts at cross correlating the two signals boiling can be determined from the spectra by either consistently failed due to the effect of the the general rise in amplitude or by the appearance dispersive medium. However, from work carried out of new frequencies. Secondly, the frequency to be previously by Ledwidge [3] it seemed possible that a extracted for the purpose of cross correlation can cross correlation on the events - an event being the be found. The cross correlation in this experiment growth or collapse of a bubble - rather than in the has been performed on the frequency of 17.4 kHz and frequency content of the events, might be more Figure 6 shows the cross correlogram. fruitful. For this purpose the frequency content of the bursts in the first and fourth traces had to The information depicted in this Figure 6 is be extracted by passing both signals through narrow crucial in the location of the noise source since band-pass filters, then demodulated by rectification it gives AX the time delay between the arrival of and smoothing before application to the correlator. the signal at the top and bottom accelerometers. The second and fifth traces in Figure 3 show the Figure 6 gives AX» 0.81 ms. V must be determined extracted frequencies and the third and last traces before d can be found. This is quite easily show the corresponding demodulated signals, or the accomplished by holding the test section conditions envelopes of the events. The apparent time shift is just below the onset of boiling and striking the seen more clearly in this case. The decision on pipe at a point below the bottom accelerometer. which frequency to extract was made with the help of The noise thus created appears at both accelerometers the boiling and non-boiling spectra shown in Figures at different times. If the signals from the 4 and 5. accelerometers are applied to two band-pass filters set at the predetermined centre frequency and then, Figure 4 shows a 3-dimensional development of via envelope detectors, are displayed on a CRO screen spectra against power applied to the test section, or applied to a pulse controlled time counter, the the onset of nucleate boiling takes place at about required velocity of propagation is easily measured. 0.30 kW. Figure 5 shows a non-boiling spectrum taken In the work described here it was found to be just before the onset of boiling, and a superimposed 1,496 ms (probably the propagation was as a boiling spectrum taken during the onset. flexural wave). Since i. equalled 2.44 m, we find that:

= i (2.44 - 1496 x 0.81 x io-3)

= 0.61 m .

That is, the boiling was most active from an acoustic sense at about 61 cm below the top accelerometer.

VII. CONCLUSIONS AND RECOMMENDATIONS

The acoustical method for detection and location of nucleate boiling is fairly new. Although it has often been discussed and suggested in the past, its practical application posed considerable problems » FMOUfNCV (KH>) due to the extensive Noise Analysis equipment required. This equipment is now readily available. Figure 4a. Three dimensional view of growth of It is not suggested that the prospective user of amplitude spectrum with increasing power on test this method should invest in such equipment, however section. Onset of nucleate boiling occurs at he should at least have access to the equipment approx. 0.3 kW. The power was raised from 0 to 2.2 kW during the preliminary investigations of his in 44 seconds. Top accelerometer input attenuated particular problem. Once the amplitudes and 18 db. frequencies of interest have been determined, .it should be relatively simple to construct and test an instrument operating on a fixed frequency and capable of displaying the onset of boiling and its location.

Although not the easiest to Implement, this method offers some significant advantages over others discussed earlier. Firstly, its application does not disturb the system under observation. No penetrations of the pipes or vessels are required. Secondly, traversing mechanisms are not needed in order to locate the position of the noise source. Thirdly, the nature of the liquid in the system is not important.

The method outlined in this paper is easily adopted to the general problem of detecting and Figure 4b. Same as Figure 4a but from bottom locating the position of a noise source in a variety accelerometer. Input attenuated 3 db. of environments. 219 17-4

Figure 5. Superimposed boiling and non-boiling amplitude spectra from top accelerometer

(mV)

(t)dt

6500

Figure 6. Cross correlation on the envelopes. Speed reduction « 512, AT 33.3 ms, N « 8 x 1024. Taken during onset of nucleate boiling.

220 VIII. ACKNOWLEDGEMENTS

Many colleagues in the Engineering Research Division of the A.A.E.C. helped considerably in the preparation of this paper by offering comments and., suggestions on an early draft. The authors are particularly indebted to Mr. F.A. Rocke for his critical conments concerning the nature of the relationship between the degree of correlation and the amount of dispersion in the medium.

Members of the Heat Transfer Section were responsible for operating the heat transfer loop from which the accelerometer signals were obtained. The signals were processed by Mr. W. Carr. Their help and assistance in these matters is gratefully acknowledged.

IX. REFERENCES

1. Bendat, J. S. and Piersol, A. G. - Measurement and analysis of random data.. John Wiley, New York. (1966).

2. Cybula, G. J. and Harris, R. W. - A theoretical study of the effects of dispersion on cross correlograais. Presented at this Conference.

3. Ledwidge, T. J. - Detection of local overheating in liquid metal cooled reactors. Thesis (Ph.D.) Univ. of Aston, Birmingham. (1969).

4. Saxe, R. F., Sides, W. H. and Foster, R. G. - The detection of boiling in nuclear reactors. Journal of Nuclear Energy, Vol. 25, 1971.

221 THE ACQUISITION OF WIND TUNNEL DATA USING A PDP15/20 COMPUTER

Roy E. Kane, B.E., M.I.E.Aust.

SUMMARY - This paper deals with a system, based o« a PDP15/20 computer, for the acquisition of data from the Weapons Research Establishment wind tunnels. A broad outline of the types of experiments performed in the tunnels and the types of transducers used to obtai. the data is given. The large quantity of analogue and digital data generated during an experiment must be rapidly and accurately acquired and recorded and the signal conditioning and interfacing to the computer are discussed. The system is designed for the acquisition of data, on-line or off-line data reduction and display, and some computer control of model attitude and probe position.

I. INTRODUCTION (I. THE PDP15/20 COMPUTER AND ASSOCIATED COMMERCIAL EQUIPMENT The Aerodynamic Research Group at the Weapons Research Establishment operates several wind tunnels, the two main ones Some brief details of the commercial equipment obtained for being a continuous flow tunnel (S-l) capable of a speed range from this system are given below. about 300 feet per second to Mach 2.8, and a blow down tunnel (S-3) for use in the Mach number range 2.8 to 5. Models of aircraft or PDP 15/20 Advanced monitor system incorporating: other research shapes are mounted in the air stream on a support (commonly called a sting) attached to an attitude control gear which PDP15 Central Processor sets the angles of incidence and roll. The two most common types (8192 word memory, 18 bit word size, 800 nano- of experiment require the measurement of aerodynamic forces on the second cycle time). model or the pressure distribution on the model surface. Other types of experiment call for probing of the temperature, pressure or I/O Processor velocity distribution in the flow field around the model or for flow 1 investigation using a variety of flow visualisation techniques such as (Data chann? controller - 8 bidirectional channels the schlieren optical method. and addressable I/O (input/output) bus). KSR35 Teletype Aerodynamic forces on a model are typically measured with a six component strain gauge balance attached to the sting; pressures PC 15 High Speed Paper Tape Reader and Punch are measured using strain gauge pressure transducers and temperatures (300 Hz reader, 500 Hz punch). are monitored using thermocouple or thermistor probes. Flow velocity and temperature may be measured with a hot-wire anemo- KE15 Extended arithmetic element meter. Where the pressures at a large number of points need to be (18-bit ADD, MULTIPLY and DIVIDE; measured a Scanivalve is used, which allows the pressures on up to 36-bit SHIFT and NORMALIZE). 48 ports to be measured sequentially using the one pressure transducer. TCO2D DEC Tape controller (capable of controlling up to 8 DEC tape transports. During an experiment the test conditions are monitored by Data transfer rate — one 18 bit word every measurements of air stream pressure (using servo-manometers fitted 200 microseconds). with digital shaft encoders), air stream temperatures (using thermistors or thermocouples), and model pitch and roll position (using shaft TU55 DEC tape transports encoders). (each DEC tape unit provides storage for 150000 18-bit words). To meet the experimenters' requirements for very accurate and rapid acquisition of this large quantity of data, a data acquisition TU25A Incremental Tape transport system based on a PDP 15/20 computer is being installed. The system (7 channels I.B.M. compatible, 700 steps per described in this paper has been designed for acquisition of data from second incremental write, 25 inches per second tunnel S-l, on-line or off-line data reduction and display, and some synchronous read). computer control of model attitude and probe position. The system will later be expanded to acquire data from other tunnels and could TR03D Controller for TU25A Incremental Tape Unit. be extended to monitoring and control of tunnel conditions. ADC24-14B-B Raytheon Multiverter (16 channel multiplexed analogue to digital converter with sample and hold. Input nngr. ± 10 volt; output - Binary - 2's complement, 13 bits plus sign).

222 VPISA Storage Display It is obvious that the operator provides inputs of some kind to most of the sub-systems and in addition he receives information (Provides a 1024 point X i 024 point display on by means of monitors and indicators of various types from every screen 854 inches x 6-3/8 inches). sub-system. HI. THE DATA ACQUISITION SYSTEM The experimenter is likely to be a person whose primary interest is in performing an aerodynamic experiment; he will not understand, The Data Acquisition System is shown schematically in or even be interested in the electronic details of the equipment; figure I.

POP IS COMPUTE* H I

MUX. A/D 4 i , i > i h of 4 mmm COMPUTE« r ANAIMUC MODEL RUHP. TRAV. SCHUCRCN SCAHI. OATA CONTROL 1 PAHAM. SUR-5VST. SUB-SYST. SUt-SVST. SUB-SVKT. SUB-SVST. SUB-SVST. SUb-SYST. H > i i < t I f-t • 1 1 ATTITUDE TRAV. CAMERAS. SCAMi- ANAIOCUE i . i CONTROL i •CAS 4 i i I i UMTS VALVC INPUTS •EAR COKTROi.

i I TELETYPE «NO EXPERIMENTER DISPLAY IS CONSOLE

POWER TO ALI OTHER SUBSYSTEMS. supplies

Figure I. System schematic

For convenience the system has been broken down into the foUowing he will expect the equipment to perform the tasks of data acquisition sub-systems:- and control reliably and quickly, to present him with sufficient real time information for him to assess the progress of his experiment and Analogue Data Sub-System modify his attack if necessary, and to record all of his data accurately for later off-line manipulation. The task of both the hardware Computer Control Sub-System designer and the software developer is to meet this requirement as nearly as possible and the degree to which it is met provides the Run Parameter Sub-System measure of the success of the system. The early pha« of the design Model Paramc *.er Sub-System therefore, must involve the experimenter/s and both the hardware and software designers in an attempt to define as we!! as possible, the real Traverse Gear Sub-System requirements. It would be unrealistic to believe that this would lead to a perfect system, particularly since many of the capabilities of Schlieren Sub-System such a system will not become apparent until it is in use, i.e. there Scanivalve Sub-System will be a learning period. Power Supply Sub-System The philosophy that has been adopted with this system is to meet the known requirement as fully as possible but to retain as These sub-systems will be discussed in some detail in later sections. much flexibility as possible to allow for later moditication both c< hardware and software. The experimenter has been included in the schematic because of the importance of the interface between him and the equipment.

223 I 'V. ANALOGUE DATA SUB-SYSTEM High level analogue data are taken directly to the multiverter patching panel. The analogue data sub-system is designed for the signal conditioning and acquisition of low level (typically less than (a) Sub-System Noise Level ± 50 millivolts) and high level (typically ± 10 volts) data. A schematic of the sub-system is shown as figure 2. The sub-system has been designed to provide a basic system noise level in the order of 1 microvolt peak-to-peak for low

HIGH LEVEL DATA SOURCES

BALANCE CALIBRATION NETWORKS NETWORK

LOW SIGNAL POMS LEVEL MONITOR COMPUTER 1OX DATA AMPS DATA AND SOURCES DISTRIBUTION

PRECISION SUPPLIES

Figure 2. Analogue data sub-system schematic

The low level data typically arises from strain gauge bridges (force level data with the data amplifier bandwidth set to less than balances and pressure transducers) and thermocouple probes. The lOHz. To achieve this figure considerabu emphasis has been high level data may arise from bridge excitation voltages, position placed on the following techniques:- potentiometers and thermistor probes. (i) Careful shielding and eart,.;ng systems. The strain gauge bridges are d.c. excited (S volts) from Hewlett Packard 6111A power supplies; both the outputs from the bridges (ii) The use of low thermal solder (70% Cd, 30% Sn) where soldered and the excitation voltages are measured and outputs are taken as a connections were unavoidable. The thermal e.m.f. between fraction of the excitation voltage. this solder and tinned copper wire was found experimentally to be about an order less than that between normal solder (60% Pb, The low level data are amplified by a bank of ten data 40% Sn) and tinned copper wire. Similar results were obtained amplifiers (Astro Data Model 889). The outputs of the amplifiers between the solders and gold plated connector contacts. x.i taken to a signal distribution and monitoring panel where a bank of msters provides approximate signal level monitoring to aid in (iii) Minimising the use of connectors in the data lines and, where optimum gain setting, and thence to the multiverter patching panel connectors were necessary, specifying gold plated contacts and and a switching panel which allows for selection of a data channel to crimped'connections. be monitored by a d.c. X-Y recorder (Hewlett Packard Model 2000AR). A highly accurate and stable calibration source has been The noise threshold is ultimately established by the data amplifiers designed for periodically calibrating the data amplifiers for zero which have a low frequency noise specification of 1 microvolt peak- offset, gain, and deviation from linearity. A balance unit provides a to-peak at the input PLUS 1 millivolt peak-to-peak at the output i.e. bank of balancing networks for backing cff zero offset in strain for gains less than 1000 the output noise predominates and for gains gauge bridges where necessary. greater than 1000 the input noise predominates. In fact the data amplifiers exceed their specification and some experimentally Low level data may be acquired from a number of sources e.g. measured noise levels measured at the multiverter patching panel are ti.e wind tunnel, a balance calibration room (where force calibration shown as a function of data amplifier bandwidth with a gain of 1000 of strain gauge balances is carried out) or, later, from other tunnels. in figure 3. So ihat the signal conditioning may be performed as close as practical to the data source, some of the units of the low level analogue data (b) Sub-System Accuracy sub-system (viz. precision power supplies, data amplifiers, balance networks, calibration unit) are built as plug-in modular units and may The system accuracy limitation is determined by the multiverter. be physically moved from tunnel racks to calibration room racks as The ± I bit accuracy threshold for a 14 bit conversion is 1 part required. in 16 384 (1:2' *) of full scale (±10 volts), i.e. the basic accuracy threshold at the input to the multiverter is approximately 224 Her i 1.22 millivolts. Hence, with a gain of 1000. the accuracy the peripherals and hence the high rate data are kept on short trans- threshold at the input to the data amplifiers is approximately mission paths. This approach also has a practical advantage that, for ±1.2 microvolts. Therefore the output of a strain gauge servicing purposes, the bulk of the logic is in the one physical location bridge with a range of ± 10 millivolts may be expected to be The logic design uses ii xgrated circuits (mostly TTL logic) and measured with an error of approximately ± 0.012'/>. an medium scale integration (MSI multiplexers, counters, comparators) w with some discreet components (mainly open collector transistors for There are. of course many other factors which will contribute line driving). to the ultimate error of an experiment (one of the major factors being temperature affects within the strain gauge Interfacing to the PDP15 is relatively straight forward and full balances) but the aim has been to produce a system in which details may be found in reference I Briefly the computer has an the system error will not be a large contributing factor to the I/O (input/output) bus system, the bus cable is "daisy-chained" from experiment error. one peripheral to the next and up to 256 separately addressable peripheral devices may be connected to it. Provision is made for The system software has been designed to consider strain single cycle and multi-cycle data transfers, transfers using Direct gauge outputs as fractions of the excitation source, to calculate Memory Access (D.M.A.), connection to the Automatic Priority gain, linearity ;nd deviation from linearity from the amplifier Interrupt (A.P.I.) system, if fitted, and transfers of data to and from calibration information, and to correct the acquired data for the accumulator. For the peripherals designed for this data any variations in these parameters. acquisition system, transfers to and from the accumulator are used.

The IOT (input output transfer) instruction contains the peripheral device select code (which must be decoded by the peripheral) and issues any OT all of three separate execute pulses (IOPI, IOP2, IOP4) at different times in the machine cycle. These three pulses may be used for a variety of functions (with some restrictions) and the most common usage is:

IOP! . Skip status (see below). (OP2 : Read data into accumulator. (The peripheral's registers are strobed onto the bus). IOP4 : Write data out of accumulator. (The accumulator word is strobed onto the bus). 10 tOO IK 10 K W/B DATA AMPLIFIER BANDWIDTH (Hi) The peripherals are designed to use the interrupt system. When a peripheral posts an interrupt (low level on the PROG INT line), the programme being executed is interrupted, the current instruction is stored and the machine enters a skip routine. This routine issues a Figure 3. Analogue data sub-system data amplifier skip instruction (containing the IOP1 pulse) to each peripheral in output noise with a gain of I 000 tum until a reply is obtained on the SKIP RQ line, thus identifying the peripheral seeking the interrupt. The machine is then routed to Jhe servicing routine for that peripheral The interrupt line is used in V. DIGITAL DATA HANDLING the data acquisition system for posting READ requirements for the iered various peripherals and for signalling operations "DONE" to the Digital data are generated within the tunnel system from a computer. l variety of sources. Some examples of thtse include: - yto VI. COMPUTER CONTROL SUB-SYSTEM foVb, Shaft encoders - used for monitoring pitch and roll position :ned (ICbit cyclic progression €.?. code). The computer control sub-system provides the primary means of B.C.D. thumbwheel switches - used for entering various para- communication between the experimenter and the computer once e meters and demands. the appropriate programmes have been loaded. Due to the difficulty tnd of predicting requirements here before gaining operational experience, B.C.D. logic codes - from the various digital controllers (e.g. a simple panel and interface have been installed v.ith ample scope for the Traverse Gear Controller). later sophistication. s Binary code words - used for entering command information The present panel incorporates four data tag buttons: ''ERROR". k- from the control panels (e.g. commands to read in data, cease "INITIAL ZERO". "FINAL ZERO", and "MEASURED DATA". ,e. reading etc.). ins These are used to identify the data point being taken. When the "ERROR" button is depressed and a data point is 'aken, this In addition some of the control sub-systems require digital indicates to the computer that the previous data point was in error data from the computer (e.g. position demands to the traverse gear, and that the present one should be substituted for it. The ire commands to the roll control gear). XX) "INITIAL ZERO" and "FINAL ZERO" tags are normally used at the beginning and end of a run respectively and the "MEASURED The digital data requirements are grouped under the various DATA" tag is used for taking data during a run. sub-systems and for interfacing purposes each sub-system is dealt with as being one or more peripherals to the computer. 'The basic- The other three switches on the panel are "SINGLE READ". rter. design philosophy has been to place as much of the logic circuitry as "REPETITIVE READ" and "CEASE READING". The "SINGLE irt possible in the one rack in close proximity to the computer, and to °.EAD" switch is used to signal the computer to take a point of data racy assemble the digital data to that point. This approach was.chosen once only from those data sources requited for this experiment. because, in general, the rates at which the digital data change are The "REPETITIVE READ" signals the computer to take data slow compared with the data transfer rate between the computer and 225 F I

i-epetitively. At present it is necessary to programme the data rate; Since the acquisition of these parameters is vet. .«». irwt ' later a real time clock and facility for selecting the data rate will be described above for the run parameter sub-system it ES sufficient here '< included. The "CEASE READING" switch is used to terminate a to list the parameters. These are:- r series of repetitive readings. Tunnel S-l pitch position (1PP) - 13 bit C.P. shaft encoder, j Operation of the "SINGLE READ", "REPETITIVE READ" Tunnel S-1 roll position (1RP) - a 13 bit C.P. shaft encoder. j or "CEASE READING" switches causes an interrupt, and the skip routine identifies the Computer control sub-system as the peripheral Tunnel S-l required incidence (UN) - the incidence to the i seeking an interrupt. The computer then reads in the "COMP" flow at which the model is required - this is entered into J. data word which is a binary coded word which allows identitication B.C.D. thumbwheel switches on the local pane» at the | of the read switch being used and the data tag required for the experimenters' console. particular experiment. The software required to perform the data Tunnel S-l required sideslip (1SS) - entered in the same manner acquisition will be referred to briefly in a later section. as required incidence. Tunnel S-l total pressure (1TP) — this is monitored by a 13 bit As mentioned previously, it is anticipated that this system will shaft encoder fitted to a servo manometer system monitoring be updated as soon as sufficient operational experience is gained. tunnel total pressure. It is hoped that, having loaded the programmes, the experimenter should be able to communicate with the computer entirely by means Tunnel S-3 roll position and tunnel S-3 pitch position - these [ of this sub-system. This means that a variety of indicators (e.g. have been included to allow for connections when the other "data outside limits", "amplifiers saturated", "data not correctly high speed tunnel is brought into the system. acquired" etc.) will be required and a number of additional commands (e.g. "plot data" etc.) may be required. The interface provides the normal interrupt and skip facilities and the multiplexers for reading the parameters onto the I/O bus. VII. RUN PARAMETER SUB-SYSTEM Again facilities are provided for reading in only these parameters (as opposed to taking a full point of data via the computer control sub- system) and two read switches "READ S-l" and "READ S-3" are The run parameter sub-system provides the means of entering provided. These instruct the computer to read in these parameters the run identification parameters into the machine. Four run para- only, code convert them and type out the parameters, and will meters are used, Run Number (RN), Data Point Number (DP), normally be used during setting up and checking the system. Mach number (MN), and configuration number (CN). The sub- system comprises two panels, a local panel on the experimenter's console and a remote panel in the tunnel vicinity, and a logic and The pitch position is controlled by a three phase motor and the interface module. panels provide for two speed control of the pitch motor via relays. The roll position is driven by a permanent magnet stepper motor and The run number, Mach number and configuration number can the sub-system contains the logic, decoders and driver to provide be set into B.C.D. thumbwheel switches on the local panel. The three speed control of roll position. Visual readout of pitch and rol' data point number is incremented by one by a machine instruction positions are provided by synchro systems and the shaft encoders each time a data point is taken and is reset to 001 either by a manual provide the digital readout for the computer. reset or each time the run number is altered. Only the run number and data point number are displayed at the remote panel. The The requirement to set and hold a model at a required incident logic module contains the number control and display logic and the or required sideslip provides an interesting exercise in machine conti I interface logic. The numbers are multiplexed onto the I/O bus data The sequence of events required is:- line (via a set of MSI four bit multiplexers) by the last two of the device select lines. (Normally the first six of the device select code The experimenter manually sets the model pitch angle.. lines are known as device select lines and the last two are known as He sets the required incidence or required sideslip into the sub-device lines). demand switches, using the unwanted set to indicate which of the two possible solutions he requires. A "RKAD" switch is included on the RUN PARAMETER e.g. If required sideslip is desired and not required incidence, local panel and this will normally be used during setting up for an the required incidence switches are set to +90° if the experiment when it is required to read in only the RUN PARA- correct sideslip with positive incidence is required, and tc METERS. (As explained in the previous section the READ -90° if the correct sideslip with negative incidence is switches on the computer control panel will be used when a full required. data point is to be taken). The READ switch on ihe run parameter panel causes a programme interrupt and the skip routine identifies He then operates the "adjust roll position" switch. the sub-system requiring the interrupt. In this case a small routine will be used to read in the run parameters only and type them out The "adjust roll position" switch posts an interrupt which on the teletype. initiates the following sequence.

VIII. MODEL PARAMETER SUB-SYSTEM The computer reads in pitch position, roll position, required incidence and required sideslip. The model parameter sub-system provides both a data The machine computes the roll position which would be acquisition function and a control function. It contains equipment required to satisfy the demand. Software will be provided t correct for anticipated model support (sting) bending in this calculation. Provide manual control of model pitch and roll position. It then compares the actual roll position with the required r> Provide for acquisition of the model parameters. position and issues an instruction which* steps the roll gear o step (approximately 0.012 degrees) in the appropriate direct Provide computer control of roll position to enable a This instruction aiso initiates a delay sufficient for the mote model to be r'aced and maintained at a required incidence step and settle. or sideslip to the tunnel axis. At the end of the settling delay another interrupt is posted ( that during this delay the machine has returned to theexen of other programmes). 226 The process is then repeated until the actual roll position required position, the dr-tum numbers are set into the position equals the calculated required roll position. demand switches, and the "SET DATUM" switches are operated. The machine then extinguishes a "Model Rolling" lamp to The counter sets to the number in the demand switches and there- indicate that the demand has been satisfied. after all positions are measured relative to that position.

IX. THE TRAVERSE GEAR SUB-SYSTEM The reason for building a separate controller rather than placing the entire traverse gear under computer control is that it is i>2 traverse gear is a mechanical device for positioning and anticipated that a considerable amount of setting up may be required moving a probe throughout the air stream in the tunnel working during preparation for an experiment and it was preferable to leave section. It has four axis control with a traverse capability of the computer free for off-line data computation during this time. 14 inches along the X axis, 6 inches along the Y axis, 9 inches along Full computer control is available through the interface register and the Z axis and ± 240° in roll. It is capable of being positioned to the same degree of control is also available from either of the panels. within 0.001 inch in the linear directions and 0.1 degree in roll. The axial movements are controlled by permanent magnet stepper X. SCHLIEREN SUB-SYSTEM motors. The schlieren optical facilities available include the capability The traverse gear sub-system includes an entire digital for taking still and movie photographs of the schlieren patterns, using controller with the computer providing for data acquisition and for continuous or flash illumination, and closed circuit television one of the sources of position demands and initiation of traverses. facilities. Facilities are also available for colour schlieren and A schematic of the controller for one axis is shown in figure 4. quantitative schlieren work.

POSITION READOUT TO INTERFACE -• TO DI6ITAL POSH. DISPLAY

SET DATUM

INTERFACE CONTROL •.CD. •.CO. U/D MOTOR STEPPER RE6ISTER MUX COMPARATOR COUNTER OECODCR MOTOR P05N. IΓ DEMAND

MOTOR LOC. PANEL DIRECTION X DEMAND

COMP. TRAV. INIT. CONTROL LOCAL TRAV. INIT. LIMIT SWITCHES REM. PANEL X DEMAND REM. TRAV. INIT. STEP COMPLETE

TO OTHER I; AXES SPEE0 SELECTION CLOCK

Figure 4. Traverse gear controller schematic (one axis)

Position demands may be entered from three sources, the local The schlieren sub-system provides the camera and lighting operating panel, the remote operating panel or the register in the control and switching, and introduces a display of run number and computer interface. One of the demand sources is selected 'jy the data point number (see Section VII above) into the photographic control multiplexer and the B.C.D. comparator compares the frame for film identification. The data point number can be incre- demanded position with the actual position (contained in the B.C.D. mented each time a still photograph is taken if it is required to be up/down counter). On receipt of a traverse initiate command from used as a frame numbering system. the appropriate source the clock is gated to the up-down counter and sends a pulse to the motor decoder. The clock is then gated out XI. SCANIVALVE SUB-SYSTEM until the step complete pulse comes back from the stepper motor. This means that a pulse cannot be sent to the motor unless it has The Scanivalve (Scanivalve Corporation) is a device which properly responded to the previous pulse. allows the pressure applied to up to 48 ports to be sequentially sampled and measured by one transducer. The Scanivalve sub- A "Set Datum" facility allows the up-down counters to be system provides comprehensive control and interface circuitry and reset to the number in the demand lines, thus making it possible to control panels required to scan the ports in a variety of ways and to choose any required probe position as the datum from which all communicate with the computer during acquisition of the data. The other positions are measured. This may be zero or any other system is designed to control the Scanivalve in either of two tunnels. number required as a datum. The probe is positioned at the The output of the pressure transducer is handled through the analogue 227 r

data sub-system in a similar manner to any other low level signal The aim is to set up the software so that the computer will except for the software coordination of scanning and acquisition. interrogate the experimenter via the teletype. He will be asked to type in such information as an experiment log, detailing which sub- A choice of nine scanning rates from 0.1 second/port to systems are being used for the experiment and which data are to be SO seconds/port is available so that an appropriate rate for the taken, multiverter channel allocation, and data output requirements. plumbing dimensions may be selected. An installation where the The computer will then check that it has all handlers and operating Scanivalve can be placed close to the point of measurement (and routines required to perform the experiment, ask for those it does not hence the length/diameter ratio of the tubing, and pipe volume/ have to be loaded, and signify when it is ready to proceed with the transducer volume ratio are small) has a short settling time and high experiment. sampling rates are feasible. In order to achieve maximum settling time the data acquisition is performed at the end of the sampling - The degree of sophistication that can be achieved and the limits time on any given port. The clock initiates the data acquisition and within which on-line data reduction can be performed are yet to be a "multiverter done" signal from the computer steps the Scanivalve determined and will no doubt be controlled by memory capacity. to the next port. It then holds on that port until the next clock pulse initiates the data acquisition again. Facilities are available tor XV. EXPANSION TO INCORPORATE ADDITIONAL TUNNELS acquiring data from each port or from alternate ports only, for stepping a single port, for setting in the number of ports to be scanned The proposed incorporation of the other wind tunnels into the (after that port, the unit "homes" rapidly to port number one) and system has not yet been studied in depth. A significant factor is for holding on a port and taking data from that port repetitively. that these tunnels are in a separate building some 600 feet away from The latter facility is necessary for leak testing an installation. Three the computer and this introduces some new design problems. A sets of B.C.D. switches are available for entering the values of refer- design study will be carried out to determine the most efficient means ence pressures which may be connected to some ports for calibration of achieving the data transfer. Depending upon the quantity of data purposes. this will probably show that a serial data path is necessary for the digital data and hence a parallel-serial-parallel communication link In addition there is a "trap pressure" facility which may be will probably be required. used for trapping the pressures on all ports at some instant. This is achieved through a Scanivalve cut-off valve which samples the.inlet In the previous data acquisition system, the strain gauge bridges pressures and holds them in small reservoirs for subsequent sampling. were a.c. excited (60 Hz) and the low level analogue data were trans- This facility is useful when a profile of the pressure on ports at the ferred over the 600 feet of cable. This will not be satisfactory with one instant is required when rapidly varying pressure histories are d.c. excitation and therefore the data amplifiers and associated being measured. equipment will have to be placed in the tunnel vicinity. Whether the I. analogue data should be transferred at the high level (±10 volts) or There are two control panels, one on the local console and one whether it will be necessary to digitise before transmission is not in the tunnel vicinity. The interface provides the facilities for quite so obvious. reading in the port number, reference pressures etc. and for the machine control of the Scanivalve and coordination of scanning and Only when data requirements, data bandwidth, transfer rates, data acquisition. noise and accuracy and control requirements have been specified can a study be undertaken to define the optimum approach. XII. POWER SUPPLY SUB-SYSTEM XVI. CONCLUDING COMMENT A number of types of power supply are provided and most are housed in a single rack. The supplies are regulated using LM309 A system has been built for the acquisition of analogue and integrated circuit regulators and fold-back current limiting. For digital data from a wind tunnel complex. The system also provides improved noise immunity the logic Vcc supplies are regulated again for manual and computer control of model and probe position and for at the logic modules. All supplies are distributed in shielded cables. the control and operation of ancillary equipment such as schlieren equipment and Scanivalve pressure sampling equipment. The system XIII. CONSTRUCTIONAL TECHNIQUES noise level for low frequency analogue data is in the order of ± 1.5 microvolts peak-to-peak with a maximum system error for strain All logic circuitry is built on printed circuit cards using up to gauge bridge outputs in the order of ± 0.02%. The system has been 80 connections on 0.050 inches centres. The modules in the logic designed to handle experimenters' known requirements and is rack are plug-in modules for ease of servicing and in general each sufficiently flexible to allow for future modifications which will module contains a separate sub-system or logical part of a sub-system. undoubtedly be required as experience is gained. Inter rack wiring from one main distribution frame to the other uses 20 pair and 6 pair twisted and shielded wiring. Each active logic line XVII. ACKNOWLEDGEMENTS is twisted with its own grounw wire. Terminations to the main frames are by wire wrapping. The author expresses his acknowledgement of the detailed work of Mr. B. Humski in developing and maintaining the system wiring Analogue data lines are separately screened pairs and where details and in carrying out the system testing and acceptance. The co- possible are routed via different ducts from the logic and control operation of the software developer, Mr. I.C. Heron, and of wiring. Mr. E.R.A. Landers who helped establish the experimenters' requirements is alFj acknowledged. Mr. H.R. Naumann, previously of XIV. SYSTEM SOFTWARE Weapon«; Research Establishment and now of the South Australian Institute of Technology, did some initial design work on the traverse This paper has been written primarily from the point of view of gear controller. The permission of the Weapons Research the hardware designer. As mentioned previously the design of Establishment, Department of Supply, to present this paper is system hardware and software must be very closely coordinated at all acknowledged. stages of design. Since the system will be used by experimenters not familiar with computer hardware, the aim has been to produce a XVIII. REFERENCES system wherein programmes may be loaded prior to an experiment and thereafter, the experimenter's communication with the computer 1 Digital Equipment Corporation, "Interface Manual- is limited to his actual requirements for acquisition and control. PDP-15 Systems".

228 J 7

A THEORETICAL STUDY OF THE EFFECTS OF DISPERSION ON CROSS-CORRELOGRAMS

G.J. Cybula, B.Sc(Tech), Grad.I.E.Aust., Elect. Eng., Operations Div., A.A.E.C. •• R.W. Harris, B.Sc, M.Sc, Ph.D., A.lvi.l.R.E.E.(Aust), Research Scientist, Eng. Research Div., A.A.E.C.

SUMMARY. A meaningful cross-correlation cannot be obtained between two raw signals derived from acoustic disturbances which have travelled through a dispersive medium. However, this does not mean that cross- correlation techniques are not applicable in work with dispersive media since, with suitable signal processing, satisfactory results can be obtained. The work described here is a theoretical study of the effects of dispersion on the cross-correlation function. The relevant noise theory is examined and with the help of a suitable computer programme, frequency deviations and transit time delays are introduced into the calculation of cross-correlation coefficients to observe their influence. A general approach to the use of cross-correlation techniques in dispersive media is described.

I. INTRODUCTION II. BASIC THEORETICAL CONSIDERATIONS Cross-correlation is an important measurement The cross-correlation coefficient, R(T), technique, and is usually employed in the determina- between the two signals, x(t) and y(t), is defined tion of time delay between two signals emanating from as: the same source. Such a determination is required for transit time measurements (Refs. 16 2). Many media exhibit some form of dispersion associated with R V(T) - (l/T)/ x(t-r)y(t)dt. (1) the propagation of a disturbance. This dispersion usually manifests itself in the fact that the velocity of propagation of a disturbance is a where T is the averaging time. function of the frequencies contained within it. If the sensor employed to convert the disturbance to an If x(t) is a. sine wave (A.sinaJt) and y(t) is electrical signal responds to spatial characteristics simply x(t) with a delay x» (A.sinm(t-X)), then (related to the wavelengths being propagated), then equation (1) becomes: in a dispersive medium, a frequency deviation whose 1 magnitude depends on the original values of the r frequencies present, will occur. If the sensor K„(T) = (l/T) / A.siita)(t-T).A.sinu3(t-X)dt. responds to the temporal characteristics (related to the frequencies being propagated), then in a dispersive medium the delay time will be a function (A2/2) of the frequencies present. The effects of frequency deviations are considered - (A2/2T)sin[2aJT-ayr-a>x] (2) for three cases: The situations under consideration have oT > 100, so (a) single sine waves, that the dominant term on the RHS of equation (2) is (b) a group of five sine waves with the same the first term, and this has a naxiaua when: frequency deviation for each component, and O)(X-T) (3) (c) a group of sine waves with no frequency deviation for the centre component, but with Thus there are many maxima apart from the important a progressive frequency shift for all other one when x * T. This situation arises only when the components away from the centre. artificial case of a single sine wave is being considered. The effect of transit time dispersion is considered for a group of five frequencies with a Now, if a frequency deviation occurs. delay which is a simple linear function of frequency.

229 T

equation (1) becomes: normalised values of R and T (normalised by dividing by the values of R ana T with no frequency j A.sin[ (t-K}]dt deviation) are plotted for various frequency 2 deviations and averaging times. o A2 JsJsini [ (ujj- 2 '*'- te

- sin

C4)

where

In considering a group of sine waves, it is simpler to perform the integration of equation (1) numerically (this is the approach of commercially available correlators) and thus the expression for R becomes: m x(i.At-n.A-r)y(i.At), (5) (n.A-r) = (1/m) ) / [ 0-01 i=1 09 01 0 7 0 6 OS 0-4 0-3 0-r 0-1 where At is the tine between samples, AT is the time NORMALISED »..«;„> ANO t If» delay step, and m is the number of samples. Figure 1. Effect of frequency deviation and III. RESULTS AND DISCUSSION FOR SINGLE FREQUENCY averaging time on cross-correlograms. WITH ASSOCIATED FREQUENCY DEVIATION

A frequency of 5,000 Hz was chosen for this analysis with a delay of 0.82S ms. The values of The effects are symmetrical in that the magnitude R were calculated at steps of 0.033 ms initially, of the effect is the same for say +10 Hz and -10 Hz and finally smaller steps were used to determine more deviation, except that the time delay shifts in precisely the maximum value of R and the delay CO opposite directions. which gave this value. The averaging time (T) varied from 0.01 s to 1,000 s. The frequency and time IV. RESULTS AND DISCUSSION FOR A GROUP OF FIVE factors were chosen to parallel an experimental FREQUENCIES WITH FREQUENCY DEVIATION investigation (Ref. 1), and to prevent the possible appearance of any unusual effects due to too The five frequencies were chosen to represent a artificial a choice of numbers. The calculations simple wave packet. The values and weighting factors were carried out on the A.A.E.C. IBM 360/50 computer. are given in Table I. The frequency deviation varied from 0.001 Hz to 100 Hz. Two main facts arise from an examination of the results: Table I. Frequency breakdown of wave packet

(a) The time delay yielding a maximum in R Number Frequency Weighting shifts with increasing frequency deviation (N) (Hz) factor and increasing aversging time. The maximum change in delay time was about 10% before 1 3,400 0.5 the value of R became too small to be 2 4,200 0.8 meaningful. ' 3 5,000 1.0 4 5,800 0.8 thi 5 6,600 (b) The larger the value of frequency deviation, 0.5 CO] the smaller the value of averaging time (T) gr« before a meaningless value of R was in computed. A meaningless value of R was chosen to be one where the value oFTt 0.1% Calculations were carried out using equation (5) for of the value with no frequency deviation. two cases: crc (a) The same frequency deviation for all sin These general trends are apparent from an examination (al of the family of curves in Figure 1, where the frequencies (Af). for 230 J A frequency deviation linearly changing as a deviation law) for an increase in the averaging tiae. function of frequency (Af is frequency However, an increase in averaging time produced less dependent). The frequency deviation is given error in the estimation of the value of the delay for by: a maximum in R for the situations involving five frequencies. r deviation = (N-3)Af (6) V. RESULTS AND DISCUSSION FOR GROUP OF FIVE where N is the number of the frequency in the group (see Table I) and Af is the frequency FREQUENCIES WITH TRANSIT TIME DEVIATION deviation factor. The group of frequencies and weighting factors given in Table I wore chosen to investigate the effect Thus for case (b) the first frequency deviates by of transit time deviation. The relationship between -2,Af, the fifth by +2.Af, and the centre frequency transit time and frequency was linear and similar to remains unaltered. There is a large nuaber of Equation (6) - relationships that could be used. Equation (6) was chosen because it is of a similar from to that Delay time = 8.25xlO"4 [1 - (N - 3)A] . (7) occurring in time delay deviation. Since the magnitude of delay time deviation is The computer programme was arranged to sample the usually less than 1%, values of A greater than 0.01 signals at 1,000 points in time. Since the tiae were not considered in detail. A value of A - 0.1 between samples was 0.033 as tue total saaple length produced a completely nonsensical correlogram. The was 0.033 sec. The choice of 0.033 ms between samples results are summarised in table 3 where it is precluded the possibility of aliasing errors for the apparent that a transit tine deviation factor of 1% frequencies chosen. produces less than 0.5* error in a transit time determination. The computed cross-correlogram for no dispersion is shown in Figure 2. Table III. Results for calculations on Table II lists both the normalised time for a effects of transit time deviation naxiaum in Ri:') and the normalised value of R(R i) foPboth cases (a) and (b) as detailed above for AT* 1 and 10. The table also gives the results for a single frequency of 5,000 Hz, where Af was the Normalised R Normalised frequency deviation. One calculation was carried out (R p at max?y (T0 for max. for a 10 times longer averaging time (0.33 s) and the x results are also included in Table II. 0.100 0.66 1.480 0.010 0.97(0.95) 1.004(1.003) Table II. Results for effects of various 0.008 0.98 1.003 forms of frequency deviation 0.006 0.99 1.002 0.004 1.00 1.002 0.002 1.00 1.001

T1 Single f 1.000 1.004 Figures in brackets refer to ten times Af « 1 longer averaging time. Af/f « 0.0002 Group of (i) 1.000 1.004 five CÜ) 0.990 1.000 Table IV compares the effects of transit time and frequency deviations of the same magnitude for Single f 0.830 1.040 A = 0.002 and Af/f = 0.002 (case (ii), linear Af » 10 (0.078) (1.035) frequency deviation). Frequency deviation produces a larger effect. Group of (i) 0.810 1.040 Af/f * 0.002 five (0.076) (1.036) Table IV. Comparison of time delay and (ii) 0.810 1.244 frequency deviation (0.369) (1.030) Numbers in brackets refer to calculations for larger averaging time. 1 Deviation V T If values for ££ greater than 10 are employed, the correlogram degrades quickly. Figure 3 shows the Time delay 1.001/ 1.001 correlogram for case(ii) when Af = 20. The correlo- gran> for Af * 10 is compared with that for Af = 0 Frequency 0.810 1.244 in figure 4. Table 2 shows that the maximum value of the cross-correlation degrades more rapidly for the single frequency and the group of five, case (i), (all frequencies deviating by the same amount) than for the group of give, case (ii), (linear frequency 231 Figure 2. Cross-correlogram for group of five frequencies

-0-5

-10

S. Cross-corrclogram for linear frequency deviation where Af = 20.

232 -10

Figure 4. Comparison of cross-correlograms for frequency deviation (linear law, Af = 10) and for no deviation (Af = 0).

VI. CONCLUSIONS Thus, in dealing with random signals, some form of signal pre-processing must be employed (such as There are significant errors in the determination filtering and envelope detection) together with the of both the value of the delay time for a maximum in smallest possible averaging time. If the pre- the cross correlation and also the value of the processing employs some form of filtering, then the maximum for both frequency and transit time deviations. filters must have identical characteristics as the As a general rule, the error in the magnitude of the effect of any frequency deviation for large averaging cross-correlation is larger than the error in the times can completely destroy the correlation. delay determination, and the magnitude of the cross- correlation becomes very small before a significant VII. ACKNOWLEDGEMENTS error in delay occurs. An increase in averaging time produced a lowering of the magnitude for the cross- The encouragement of Or. T.J. Ledwidge is correlation but often a lessening of the error in the gratefully acknowledged. delay determination. VIII. REFERENCES The effect of a frequency deviation was more pronounced than a transit time deviation. 1. Cybula, G.J. and Ledw;id<;e, T.J. - The Detection and Location of a Noise Source in a The analysis has only considered coherent Random Dispersive Medium. Presented signals, whereas the experimental situation is often at this Conference. concerned with the analysis of random signals where the phase relationships cannot be determined. Long 2. Garrard, G. and Ledwidge, T.J. - Measurement averaging times are required to obtain significant of Slip Distribution and Average results for random signals (often referred to as Void Fraction in an Air Kater Mixture. ensemble averaging). However, the analysis has shown International Symposium on Two-phase that longer averaging times produce smaller Systems, Haifa, Israel, 1971. correlations if any form of dispersion is present.

233 r

AC-to-DC CONVERSION FOR RAPID PRECISE DIGITAL MEASUREMENTS

E.L Harris, B.Sc, M.Sc, M.I.R.E.E., Senior Lecturer in Electrical Engineering, N.S.W. Institute of Technology

SUMÄRY - This paper deals with the conditioning of sinusoidal ac signals for measurement using dc-responding digital instruments. To allow analogue-to-digital conversion, the average value of the ac signal is obtained by precision full-wave rectification followed by filtering. Rapid digital measurement demands a filter with the minimum least-significant-digit settling time and compatible ripple attentuation. Design equations are developed which relate these criteria to the optinun pole locations for a two-pole filter. The development uses approximations based on the dc &tep response and the second-harmonic frequency response of the filter. The results can be applied to automatic measurement systems, multiplexed systems, and digital voltmeters.

Many instrumentation applications require the rapid digital representation of an ac phenomenon. Vft.1 Precisian irfrt r n\ T« The stepi involved are illustrated in figure 1. A Z^ _ j ^ 1 ^S J £ [_=U Äue-io- digital converter

phenomt Figure 2. AC-to-dc interface. rectification could be used, but full-wave rectification reduces the amount of filtering required. This approach gives a dc output proportional to the mean Figure 1. Digital representation of an ac phenomenon. of the half-cycle average values of the ac input. If the shape of the input waveform is known, calibration problem arises because most analogue-to-digital con- can be expressed in other terms — rms, for example. verters either require a constant input during conversion or respond to the average value of the input The amount of filtering required depends upon 1) voltage during the conversion period. Either charac- the resolution and 2) the inherent filtering — often teristic will give a falst representation of an ac called the normal mode rejection — of the analogue- input, even if a sample-and-hold device precedes the to-digital converter. When speed of measurement is analogue-to-digital converter. important, as is often the case in systems applications, the settling time of the filter must also be For ac measurements, an interface is required specified. which will accept an ac voltage input and produce a dc voltage output with the following characteristics: This paper deals with the design of the filter 1. shown in Figure 2, for the case when the ac input The average value is proportional to the voltage is sinusoidal. Calibration is based on the desired property of the ac voltage. average value of the full-wave-rectified waveform. The design criteria are developed for a two-pole 2. The ac content (ripple) is small enough not filter which has the minimum settling time for a to affect the least significant digit of the digital output. given attentuation of ripple components. Such an interface might be required for extending II. THE FILTERING PROBLEM the capabilities of an existing dc digital voltmeter, or the interface could be designed as an integral Consider the input to the filter, shown in part of a digital multimeter (Ref..I). Figure 3. This signal, v,(t),is a full-wave- rectified replica of v(t),1 a sinusoidal signal which One approach to this interface problem is to is zero before some arbiträr/ time reference. We can process the ac voltage through a precision rectifier write and filter, Figure 2. Either naif-wave or full-wave v(t) = 0 for t < 0, (1) v(t) = Vmsim)ot for t * 0. (2) 234 III. THE FILTER TRANSFER FUICTION The general form for the transfer function of a second orcL-r all pole low pass filter is

F(s) = C(s (5)

where C (s) and V,(s) are, respectively, the IT 3TP Laplace transforms of the output c(t) and the input vj(t); and k is the dc gain. The poles may be either TIME in SECONDS complex or real, as shown in Figures 4 and 5.

Figure 3. Input to the filter. Then v^(t) can be expressed (Ref.2) by the Fourier series = v cos im>ot 1 S- plone V« . 17 * m/0 (m m even \ cos 2u)ot -j| cos4wot 1

subject to the constraint that v,(t) = 0 for t < 0. (4) X A low pass filter is required to reduce the alternating components of Equation 3 to an acceptable level. Two common limits on these alternating components are 1) the accuracy and 2) the resolution of the analogue-to-digital converter. The resolution has Figure 4. Complex poles of equation 5. been chosen here for two reasons. Firstly, this is usually the more stringent of the two criteria. Secondly, if the alternating components are constrained to be less than the resolution of the 1(0 analogue-to-digital converter, this will minimize periodic jitter in the least significant digit. Having determined the required high frequency e-plane attenuation, the filter must then be optimised so that the output comes within and stays within a specified deviation from the final dc value in the shortest possible time. Given a limit of deviation, the previously mentioned time is called the settling time, Ts. Again the resolution of the analogue-to- digital converter is chosen as the limit of -O». deviation. A first order (one pole) filter can be used where only a small attenuation of the ripple components is required, but the settling time will be longer than for a properly designed second order filter. This restriction may be prohihibitive where large attenuations are required. In addition, a second order (two pole) filter can use smaller component values titan the corresponding first order filter; this is particularly important for low frequency operation. Figure S. Real poles of equation S.

235 r

If 0 s c < 1, the poles are complex and Reference to Equation 3 shows that the magnitude of the ac components fall off rapidly with increasing c, = cos ö (6) order. For example, the 4o>o term has 1/5 the magnitude of the 2o, term, and the 6w0 term has 1/7 in Figure 4. The distance fron the origin of the the magnitude of the 4^ term. Furthermore, from s-plane to either of the complex poles is u> . If Equation 9, the attentuation of the ripple components n 2 z 1, the poles are real and ü>n = /uiai2, Figure 5. falls off inversely with a . Hence, at the output of the filter, the contribution due to the 4ai0 term has IV. RIPPLÜ ATTENUATION only 1/20 the magnitude of that due to the 2

Equation 7 is plotted in Figure 6 (Ref. 3) for k=1 and various values of c. (10)

Equation 10 reduces to the criterion

o>n < too/ 3L . (11)

In Equation 11, Ü>0 is the lowest input frequency for which, with a full scale input, the filter will reduce the ripple magnitude below the least- significant-digit level. The derivation of Equation 11 assuned no filtering in the analogue-to- digital converter. If there is filtering of a factor H < 1 at frequency 2u>0, then the design criterion becomes

u>n /3HL . (12)

Most digital applications require better than Figure 6. Sinusoidal frequency response for 0.15 resolution or L = 0.001. For this value of L, Equation 7. Equation 11 yields u>« < 18.3o> , which certainly satisfies the original assumption that 2oj0»u>n, Note from Figure 6 that z affects the maTiitude of Equation 8. the peaking near o> = o> . However, the lugh frequency attenuation is relatively independent of x, when this V. MINIMUM SETTLING TIME attenuation is large, say more than an order of magnitude (20dB). (a) The Transient Response This can be also seen from Equation 7 when The settling time of the output of the filter is determined from the transient response of u> = im>0» o>n (8) the output c(t). Let £ signify the Laplace transform and i the inverse Laplace transform. Then the first term in the denominator of Equation 7 dominates, and Then from Equation 3,

kuj_ =l[ for (9)

— — •*' • (b) Ripple Reduction Criterion.

Thus we have the first design criterion: the attenu- -_2 ation of the high frequency ripple components is (13) independent of x, and depends only on o> . IS

236 From Equations 5 and 12, (b) Settling-Time Criterion for a dc Step Input C(s) =JUc(t)] = F(s) V (s) 1 The development in Section VI shows that the 2v k terms due to the ac components of the input—those m other than the first and second on the right-hatri side in Equation 15 —make a negligible contribution to the transient response. The settling tijne is therefore 2 determined primarily by the second term on the right- hand side of Equation 15. This represents the output "3 " due to a dc step input to the filter, and the magnitude-normalized response is 2 s TF" ( c(t) = 1 - sinfü^/l-c't + 6). (18)

(14) Equation 18 is plotted in Figure 7 (Ref.6) versus normalized time for various values of c,.

Then 1 c(t) =X" [F(s)V1(s)]

2V k -C + 8)

3 2 cos (2u) t- r(u*-4«*) + (4cVo) ;

+ 6-] )

cos (4(J in u n o Figure 7. Normalized transient response for a dc step input to the filter.

Assume that «n has been fixed by the ripple re- (15) auction requirement, Equation 11, and consider the transient waveforms of Figure 7. There is overshoot Where 6 = cos ? as given by Equation 6 and followed by an oscillatory transient for 0 < c <1• Figure 4, and The envelope of the transient decays exponentially, such that each excursion from the steady state value, c(t) =1, is less than the preceding excursion. The 6. = tan value of the first and largest excursion above the 2 [ui*+(2i«o) ] steady state value is the peak overshoot

i = 1,2,3,... (16) M_ (19) 4Uo)n(u0 tan and the time taken to reach Nt, is the peak time rt i = 1,2,3,... (17) (20) Equations 13 and 15-17 are derived using Laplace transform tables (Ref.4) and trigonometric formulae Equations 19 and 20 are derived by differentiating for the sums of angles (Ref.5). Equation 18 with respect to t, and then solving for the smallest value of t < 0 for which this derivative

237 J is zero. - In 6 Observe in Figure 7 that Mp decreases with ?but Tp (22) increases. These results follow directly from Equat,- ions 19 and 20. Observe, further, that the time Equation 22 is plotted in Figure 9, which shows that required to reach c(t) = 1 for the first time de- 0.9 < ? < 1 for 6 < 10 . (This value of 6 corres- creases as c is decreased, and that the same is true ponds to 0.1% of full scale on the analogue-to- for any value of 0 < c(t) < 1. digital converter.) Consider C = ti such that c(t) = M_ , which occurs at Tp , is the largest output excursion touch exceeds the steady state value, c(t) = 1. Because the magni- 10 tude of the transient oscillatory waveform decreases exponentially for t>0, all succeeding excursions , above or below c(t) =1 are smaller in magnitude than V From Equation 19, < 1 for all t, > 0. There- fore the largest deviation below c(t) =1 occurs for Pi. In particular, the deviation of magni- Q. tude Mp where c(t) = 1 - NL, occurs at t = T . ti ti . 1 This is illustrated in Figure 8. From the definition f> 1 1 1 ICf I0"5 Kf* Itf1 DEVIATION 6 »•M.

Figure 9. Plot of Equation 22 which gives c, the damping ratio (Figure 4), for minimum settling time as a function of 6, the I-M, maximum acceptable deviation from a steady K -state value of 1. The actual settling time is found from Equations 18 and 19. T is the smallest value of t < 0 for which

O T« Tp t, seconds 1 - c(t) 7TT (23)

Using Equation 18 with Equation 23, and simplifying Figure 8. Transient response for a dc step input to the result, Equation 24 the filter with c=5,. + e> - /TT2" e"c(wnTs " ein(o>n of settling time, T is the lOOMp % settling time (24) for c, = ^. That is, T is the tlfce it takes for c(t) to come within and stay within +Mp of the relates T and ?. The solution of Equation 24 for steady state value c{t) =1. normalizes o^Tg as a function of either E; or 6 is most easily accomplished with the aid of a digital The development which follows in Section (cj computer. shows that T si Ts, the minimun lOOMp % settling (c) Ju,tification of mnimm dc.Step ^^^ Time time for a dc step input. The implication for design is that once 6, the maximum acceptable devia- Assume that, as is shown in Figure 8, 6 = NU is tion from the steady state value, has been determin- t, the maximum acceptable deviation from the steady- ed from the analogue-to-digital converter, c is state value c(t) = 1. Consider Figure 7 and v. = C > chosen so that M wil1 first reacn c 2 n From Equation 19, e1. The output c(t) for C = C2 W = 1-14, at time T ,>T ,. Hence the output for ? = Pti s<: sx 6 * e' (21) GjWill settle to c(t) = 1 +Mp more quickly than for which can be solved for c, to yield 238 •

any other u *t^ And, in general, for the 2i th harmonic,

Next consider Figure 7 and c = c, < £.,. Although A 6 L i = 1,2,3,... (32) the output c(t) for c = S, will first reach c(t) = i 1-Mp at time Tg3 < Tgl, the overshoot, Mp > ti t 3 The phase angles of the transient term are found from Mp = 6, exceeds the maximum allowable deviation from Equations 8 and 17. the steady state value. The output for z; = c, cannot possibly settle to 1+6 until t > Tp . Tp exceeds i * tan 1,2,3,... (33) T , so the minimum settling time occurs tor C * 5,. This minimum settling time T and the corresponding From Figure 4 and Equations 6 and 33. damping ratio £ are related By Equation 24, and this e damping ratio is related to the acceptable deviation 9i i - 1,2,3,... (34) 6 by Equation 22. Hence the magnitude-normalized transient compc.Tent of VI. EFFECT OF TRANSIENT ac TERMS Equation 15 can be written

Return now to Equation IS and the coefficient ct(t) = e + e) •y ..2 (25) On" - 6L Zj(2i sin((e, which multiplies both of the terms i = 1 (35) (t) = cos (2uot + (26) The infinite series in Equation 35 can be expressed as a partial fraction expansion, Equation 36. and (t) sin (27)

The term f1(t) in Equation 26 is, after multiplication by 2 VjikA,, the steady state component of the i = 1

output due to the second-harmonic frequency, 2u . (36) Similarly, the term g1(t) in Equation 27 is, after 2 multiplication by 2 Vzk/L, the transient component of L, 4i i = 1 1 = 1 the output due to the^second harmonic. The two series on the right hand nide cf Equation 36 converge (Ref.7) to 1 and - IT2 , respectively. Note that he coefficient A, in Equation 25 is 1 2T the same as that given in Equation 7 for the magni- Hence, the coefficient of the second transient term tude frequency response of the filter with u= 2u. ? The factors k and - 2 are contained in Equations 3 in Equation 35 is -6L (! - TT? ) • - 0.533L. The 1 TX and 15. Due to the ripple reduction criterion, sun of the two terms within the brackets in Equation Equations 8, 9, 11 and 25, 35 is calculated using the phasor diagram in Figure 10 and the relation between the sides and angles of Ax < jl (28)

where L is the least significant digit, expressed as a decimal fraction of the full scale of the analogue- to-digital converter. Similarly, from Equation 15, coefficient

A2 = '- V (29) 2 2 - 16o)0 ) + (8Cü)nu)( which multiplies the transient term

2 g (t) = -_§l5 __ sin (d)n/l -£ 1-. * 62) (30)

•• can be simplified to . L A2< ?0' (31) Figure 10. Phasor diagram for Equation 36.

239 ny plane triangle (Ref.8). The magnitude of the sum The overall conversion time is most affected by is the filter when T is much longer than the analogue- to-digital conversion time. When the converse is R * [l2 + (0.53310* - 2(0.533L) cos (37) true, the filter has little effect on the overall conversion time. If the analogue-to-digital con- and the incranental angle is version time is comparable to T , the overall conversion time can be reduced by almost a factor of two 0.S33L sin 26 sin-l . (38) by starting at t • 0. However, then the optimum filter requires a different design approach, and Equations 11, Zi and 39 no longer give valid miniinm- For high resolution (L«l) and 6 £ L (See Section settling designs. These design equations are most VII), the second term on the right-hand side of useful for the first of the three cases. Equation 38 car be neglected. For O.lt resolution and 6 * L, Equations 6 and 22 give 6 = 23.5°. Then, Then o> and C are chosen as described above, from Equation 37, R - 1.00073; and from Equation 38, Equation 2? gives an upper bound on the »»iri«^ a = 0.022°. Thus the difference between the result- settling time for the iifut of Figure 3. The longest ant transient and the dc-step transient is negligible; settling time occurs for a dc value just above a similar results apply for other values of L« 1. quantizing level of the analogue-to-digital converter. Even here, the settling time can be less than that Therefore, the calculations of Section V. — given in Equation 24 by as much as one full period of based on a dc-step input only — produce settling- the ac ripple. The settling time decreases as the dc time results which also apply when the in^it is a value rises toward the next quantizing level. full-wave rectified .sine wave. VIII. AREAS FOR FURTHER RESEARCH. VII. COMBINED RIPPLE ATTENUATION AND MINIMUM SETTL- ING TIME. The design criterion given is based on a peak-to- peak ripple of L and a maximum transient overshoot (a) Final Design Criterion. of L/2. Different relative weightings may produce a shorter settling time. T increases as UL is de- A criterion has been developed for a filter with creased to reduce the ripple. However, larger over- the minimum settling time, using the following con- shoots can be tolerated for this case and the straints: effective settling time may be reduced. 1. The peak-to-peak value of the second- The design criterion given assumes that u and ; harmonic component of the ripple is less than can be realized exactly. Exact initial realization the least significant digit of the analogue-to- is not possible; variation of w and c with time and digital converter. This determines u from temperature must also be considered in any practical Equation 11, which is repeated below. design. The effects of an increase in £ can be easily calculated, but a decrease in c produces a discontinuous change in the settling time because of n o the periodic nature of the transient. A similar 2. The peak overshoot for a dc-step input is statement applies for a decrease or increase in ii>n. less than one-half the least significant digit of the analogue-to-digital converter. This The results of these sensitivity calculations determines c from Equation 22 with 6 * L/2. should be combined with revised relative weightings. New iesign criteria can include the effects of - ln(L/2) (39) expected component tolerances. 2 [IT + (In IX. CONCLUSIONS. (b) Discussion of Design Criterion. A design criterion has been developed for the pole locations for a two-pole filter. The filter Equations 11 and 39 are the required design gives •°i"™« settling time with compatible ripple equations for minimum settling time, provided the attentuation when the input is a full-wave rectified inequalities are taken to the limit of equality. The sine wave. Areas of application include rapid effect of inherent filtering (normal-mode rejection) analogue-to-digital conversion of simsoidal signals. in the analogue-to-digital converter can be included by using Equation 12. These equations define a filter with a settling time no greater than that given by Equation 24, assuming a reading is acceptable when it is within ± 1 digit (±L) of the correct value. Analogue-to- digital conversion can commence any time t > T ,and the inaccuracy due to the filter response wiUrbe less than ±L. Equations 11 and 39 can easily be modified to accommodate other limits, for example ± >sL or ± 2L.

240 X. REFERENCES. 4. Nixon, F.E. - Handbook of Laplace Trans- ionnation, 2nd ett. Prent ice- 1. Harris, E.L. - Design Innovations for an Hall, 1965, pp. 192 5 251. Integrated Circuit Multimet- er. Proc. IOCST, Sydney, S. - Standard Mathematical Tables. 1970, pp. 5Ö-51." lutn ed. chemical Kubber Fublish- ing Co., 1954, pp. 344. 2. Millnan, J. & - Electronic Devices and Halkias, C.C. Circuits. 6. Dorf, op. cit., pp. 88. McGraw-Hill, 1967, pp. 602. 7. Jolley,L.B.W. Summation of Series. 2nd ed. 3. Dorf, R.C. - Modern Control Systems. Dover Publications, 1961, No.397 Addison Wesley, 19t>7, pp. 187. pp. 74 § No. 336 pp. 62. 8. op. cit., pp. 346.

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