MOOGINC.CONTROLS OIVISION,EASTAURORA,N.Y. 14052

A BRIEF HISTORY OF ELECTROHYDRAUllC SERVOMECHANISMS-

R. H. MASKREY W. J. THAYER This article appeared in the ASME Journal of Dynamic Systems Measurement and Control, June 1978

Fluid power control, that is the trans- cessed more easily in the electronic of lo4 to 106, the servovalve is a very mission and control of energy by means medium than as pure mechanical or effective forward loop “amplifier” as well of a pressurized fluid, is an old and well fluid signals, while the delivery of power as an electrical to hydraulic transducer. It recognized discipline. The growth of fluid at high speeds could be accomplished is the development of the servovalve, power has accelerated with our desires best by the hydraulic servo. This mar- then, that has paced and will continue to control ever increasing quantities of riage of electronics and into to pace the growth of electrohydraulics. power and mass with higher speeds and electrohydraulic servomechanisms cre- greater precision. More specifically, where ated both a solution to an existing class GENEALOGY precise motion control is desired and of control problems and a demand for a Electrohydraulic servomechanisms space and weight are limited, the con- whole new strain of components. The trace their roots back through a variety venience of high power-to-weight ratio evolution of these components is really of disciplines. The earliest contributor is makes hydraulic servomechanisms the a story of increasingly demanding appli- the fundamental work in hydraulics from ideal control elements. cations each of which caused the cre- the early Greek and Roman civilizations. ation of better, or more efficient, or more During the Industrial Revolution a num- The demand to achieve more accurate reliable, or faster devices. To satisfy this ber of eminently practical inventions led and faster control at high power levels, demand, new manufacturing methods especially in the areas of machine tools, to the development of a variety of hydro- had to be conceived and original testing mechanical devices which increased the primary flight controls, and automatic techniques developed. fire control produced an ideal marriage influence of hydraulics. A second branch of hydraulic servomechanisms with elec- The key element in this family of of the family tree is the field of process tronic signal processing. Information mechanisms is the electrohydraulic ser- controls, where pneumatic devices for could be transduced, generated, and pro- vovalve. With power gains on the order signal transmission were developed, re- fined and eventually used in power con- Ktesbios is credited with the invention of of the variable stroke trol applications and in hydraulics. Mean- what must have been the first hydraulic and the accumulator. while, the advances in machining tech- servomechanism. Among his many in- During the 19th century, countless ventions was the water clock, which was nology were leading to the use of close feedback devices were invented, but only used to keep time over a long period, fitted parts, tighter dimensional controls those that had applicability to prime and better structural materials which cre- typically months. movers such as hydraulic governors ated an environment for high pressure Time was recorded by measuring the found acceptance. Hence, the technology hydraulic systems. The demand for high level of water in a vessel with the rate of servomechanisms was limited to pressures was accelerated during World of flow into the vessel held constant. mechanical and hydromechanical devices War I, especially by manufacturers of Ktesbios solved this problem of constant up through the start of the 20th century. naval equipment, and ultimately led to a flow by controlling the water level (hence Then came the application of electrical whole industry manufacturing high pres- pressure) in an upstream vessel which solutions to control problems, and an sure components. The spread- fed the timing vessel through a fixed acquiescence to a more theoretical treat- ing acceptability of this medium helped orifice. The water level was controlled by ment. Because of this diversification, the to fuel the demand for more sophisti- a float which opened and closed, propor- “Age of Enlightenment” came to servo- cated electrohydraulic control. tionally, a variable orifice feeding the up- mechanisms in the .1920’s, 30’s, and stream vessel. By any rigorous definition A third branch of the family tree grows 40’s with the involvement of people like this is a servomechanism which is still from the relatively recent field of electro- Kupfmuller, Nyquist, Oldenburger, Black, in use today, although not in water mechanical devices. Large motors and and Bode. clocks. In fact, the demise of the water solenoids used as prime movers, spawned The early 1900’s saw several signifi- clock was presaged in the 14th century a field of electromechanical servomech- cant developments, including the use of by the mechanical clock, and the float anisms, where miniature versions, (AC oil as the fluid medium, the use of elec- valve for hydraulic control appears to or DC servomotors and relays) were used tric actuation on directional control have gone with it for the next four as control devices. This provided three valves, and the development of pumping centuries. benefits: first, it led to miniaturization of elements. Mechanical feedback servos the motors (better magnets and materi- Float valves were next used in England appeared as vehicle power steering units als); second, it provided a reason for around 1750 for controlling level in the late 1920’s even though they expanding the amount of knowledge in in domestic water supplies and in steam were not popularly applied to automo- the field of closed loop controls; and boilers. During this time, the 18th cen- biles until the late 1940’s. third, it led to the development of a family tury, designs for steam boilers prolif- During the period prior to World War of electromechanical transducers. The erated in England, in Russia, and eventu- II, several significant events occurred. In first development led directly to the ser- ally throughout the rest of Europe. This the process control field, pneumatics vovalve, while the latter two enhanced period, of course, was the Industrial were being used for computation, control the climate and created the demand for Revolution and it made its mark felt on and signal transmission. This work re- better closed loop control. hydraulic servos. As the boiler inventions grew, so did the control schemes. Many quired mechanical/pneumatic transduc- The latest contributor to the family is varieties of flow valves were patented, ers, and created a need for valves. Aska- the field of electronics. While the vacuum as well as a new device to control steam nia Regulator Company and Askania- tube amplifier was the device that made pressure. By 1800 these were called Werke, Germany, developed and patented the field of electrohydraulic control pos- regulators, a term that became more a valve using the jet pipe principle, in sible, it has been the advancement in common to all feedback devices, and which fluid pressure is converted into solid state electronics, allowing a signifi- some that were not. momentum of a jet. The jet is directed cant reduction in size and power con- between two receivers where the momen- The Industrial Revolution created in sumption, that has made it practical. In tum is recovered as a pressure or flow. its wake a flood of gadgets and inven- addition to the contribution to drive elec- Similarly, Foxboro developed the nozzle- tions that today are essential ingredients tronics, solid state technology has pro- flapper valve which uses the cylindrical of all electrohydraulic systems. Bramah’s vided new and innovative measuring de- curtain orifice area formed by a flat plate hydraulic press in 1796, for example, vices, novel transducers that open up moving toward a sharp edged nozzle. required the invention of a cup packing new fields of control, and expanded These two devices, Figs. 1 and 2, would by Henry Maudslay to make it work. This means of handling signal information so play important roles in the future devel- is the forerunner of our seal technology. that more complex control problems can opment of the servovalve. Those presses, consisting of a hand be solved. pump and a cylinder, gained popularity Siemens of Germany developed a dual- in the manufacturing industries of En- input valve that accepted mechanical EARLV HISTORY gland and doubtlessly led to the use of inputs through a spring and electrical The earliest contributors to fluid power steam driven pumps and the eventual inputs through a moving coil, permanent probably do not have their deeds re- transmission of power through hydraulic magnet motor. While relatively primitive corded, however the earliest meaningful lines. During the late 1800’s, many En- by today’s standards, this valve was used contributor to our subject must surely glish towns had central high pressure in a closed position loop, and became a be Ktesbios [l],’ an Alexandrian living hydraulic systems for power distribution. forerunner of valves used in aircraft during the period 285 to 247 B.C. Living Although this scheme was eclipsed by automatic flight controls. in a time and place well known for its the more efficient use of electric power ‘Numbers in brackets designate References at outstanding scientific achievements, distribution, it did lead to the invention end of paper.

2 HALF BRIDGE (SINGLE NOZZLE) FULL BRIDGE (DOUBLE NOZZLE)

FIGURE 2 - Flapper-nozzle Valve

FIGURE 1 - Jet Pipe Valve

Several attempts at closed loop elec- flow. The spool was moved by a direct rmplemented feedback from the second trohydraulic control were made, of which actrng motor, usually a DC solenoid. Pro- stage by causrng the first-stage bushing the Trebel [2] patent rllustrated in Frg. 3 portronal control was achieved by caus- to follow the second stage motton (drrect gives a typical example. Here mechanical rng the solenoid to act against a spring mechanical follow up). motion signals from a rate gyro are and varyrng the motor current to cause About the same time, the Dynamic summed a change rn position, hence valve flow. wtth motions generated by a DC Analysrs and Controls Laboratory at motor that, rn turn, electrically sums MIT [5] added two improvements to the signals from an attrtude gyro with inner In 1946 Trnsley [3] of England patented the frrst two-stage valve. It used a direct two-stage valve. The first was the use loop feedback of cylinder position. The of a true torque motor (a permanent resulting error signal moves a four-way actrng solenord to drove a spool for the magnet, variable reluctance actuator), sliding spool frrst stage, whrch then drove the second provrding correcting flow Instead of a solenord, which results in a to the cylinder. spool stage wrth differential pressures. This was an early attempt to overcome srgnrficant power savings and an rm- the low force levels in srngle-stage valves provement In Irnearrty. Second was the POST WAR and provrde better hysteresis with less use of a spool position transducer to DEVELOPMENTS envrronmental sensitivity. electrically feed back second-stage posi- tron. The use of electrrcal feedback In a Electrohydraulic servomechanisms Shortly afterwards, this was Improved hrgh gain loop helped to reduce the made an important jump as a result of effect of high fraction levels at the frrst- World War II, for several reasons: upon by Raytheon and Bell Aircraft [4] who developed two-stage valves that stage spool, and thereby Improve the The pace of hardware development (materials, fluids, electronics etc.) was accelerated by the heavy R & D efforts.

Automatic control theory was devel- oped and proven. It was documented and became universally accepted.

A whole class of knowledgeable people who found automatic control challeng- ing were turned loose in the post war commercial and aircraft environments.

Some of the post war developments electrohydraulic components are dra- matic, but most are evolutionary, involv- ing improvements over existing limita- tions. The best way to understand these developments then is chronologically. The history of electrohydraulics during this period is really a history of the servovalve, its evolution and growth. By the end of the War, a servovalve was principally a sliding spool moving inside a sleeve which served to meter hydraulic FIGURE 3 - Tiebel Patent

3 CANTILEVER BIAS SPRING RETURN PORT valve static characteristics. This friction MARKET GROWTH, effect IS known as valve threshold. I AEROSPACE In 1950 W. C. Moog, Jr. [6] developed Because of the evolution in hardware, the first two-stage servovalve using a the applrcation of electrohydraulic servos frictionless pilot stage (Fig. 4). A flapper has grown from a rather small, sophis- and nozzle variable orifice was used in ticated market to a much larger, lower conjunction with a fixed orifice to drive cost and less complex market which a second-stage spool in a three-way exists today. mode. The flapper-nozzle valve was driven by a torque motor, and spool The market origins were in the mili- position was achieved by a spring acting tary, where electrohydraulics were orig- drrectly on the spool. The advantages of inally used for radar drives, guidance thus construction were an appreciable platform droves, and controls for missile launchers. The high cost of early electro- PRESSURE reduction in valve threshold, and a high PORT dynamic response because of the lower hydraulic servos, both in initial invest- mass of the first-stage parts. Frequency ment and reliability, was seen as a ne- cessity for the performance achieved. FIGURE 4 - Single-nozzle Two-stage response on the order of 90” at 100 Hz Servovalve (1950) was possible allowing for the use of Electrohydraulrcs were the exclusive pur- servovalves in high gain positron servos. view of the military for the following reasons: The use of mechanical force feed- back in a two-stage valve was pioneered Military budgets could afford the de- T. H. Carson [7] in 1953 (Fig. 5). A velopment costs. tension spring was used to generate force (torque) on the torque motor proportional Improved performance was often the dominant goal of military equipment, to the spool position, thus closing a and performance compromises were -iE mechanical loop around the spool posi- tion. By combining the frictionless first unacceptable. stage wrth mechanical feedback from the The staff required to do the analytical second stage, he was able to improve and design work in this technology threshold, improve dynamrc response was found mostly at military prime because of lower first-stage displace- contractors, whether industrial or re- ments, and reduce temperature and pres- search oriented. sure caused spool position anomalies because of increased first stage gain The use of hydraulic power was com- allowed by feedback. mon in many military vehicles. An improvement in environmental sen- Because of the successful early ap- sitivity was achieved by Moog [8] in 1953 plications and the resulting pressure for with the introduction of a symmetrical, FIGURE 5 - Mechanical Feedback better hardware, the list grew to include double-nozzle orifice bridge. This reduced Two-stage Servovalve (1955) flight controls for missiles and radar environmentally caused null offsets. antenna positioners. In aircraft, electro- About the same time reliability was im- hydraulics were used for things like sta- proved by Wolpin [9] who introduced a bility augmentation in flight control sys- means of isolating the torque motor from tems (where limited authority automatic the fluid. control was added to the manual flight TORQUE MOTOR FLUID SUPPLY In 1957 R. Atchley [lo] devised a two- controls) and in dynamically tuning mag- TO JET I stage servovalve with a first stage based netron cavities for radar generation and on the Askania jet pipe valve. This valve anti-jamming protection. Helicopters be- (Fig. 6) provided a single oil inlet path came an early proving ground for electro- Instead of the dual path in flapper-nozzle hydraulic servos because of their critical valves, thus providing a measure of reli- need for stability augmentation. ability for a particular failure mode. The advent of the space age, especially Throughout this period a number of with the magnitude of activity in the specialty devices were developed to speak late 50’s and 60’s, brought a new field to peculiar needs. Some of these include: with conditions similar to those just servovalves with dynamically shaped mentioned, plus the need to position pressure feedback for stabilization of very large masses at high speed together resonant loads; redundant servovalves with an unprecedented demand for reli- that detect their own failures and isolate ability. The primary use of electrohydrau- them automatically; servovalves with lies was in flight control. Control was positive pressure feedback to compen- achieved by vectoring the basic thrust of \ / FILTER VALVE CONTROL PORTS sate statically for structural compliance; the vehicle, whether moving the nozzle SPOOL servo-actuators which vote between mul- of a liquid or solid rocket engine or the tiple inputs for redundancy; and three- entire motor on some solid rockets, or FIGURE 6 - Jet Pipe Servovalve (1957) stage servovalves for higher flow capacity. by directing the exhaust stream either

4 with liquid secondary injection orthog- the industrial marketplace was creating tries. While early robots required very onal to the stream or by positioning a demand for electrohydraulic servos. little unique from electrohydraulics other vanes in the stream itself. Electrohydrau- One of the first major users was the than economy, recent developments (es- Iic servoactuators have been used for machine tool Industry. Numerical control pecially in the area of precision assembly guidance and control by one of the above and computer control of mulling machines robots which have the ability to follow schemes in almost every space launch was just beginning, and electrohydraulics contours and incorporate tactile feed- vehicle. Beyond the launch vehicle itself, were used In most of the early machine back) do begin to press limits of tech- however, little need exists because of the table positioning servos. These were typ- nology. low power and low dynamic requirements ically hydraulic servomotors, used to turn A second area is in processing of of orbiting vehicles. lead screws, directly replacing the human plastic. This is a relatively young indus- operator. As NC control expanded to try, thus tends to move faster with re- The space program, In addition to Include riveting machines, punch presses, spect to new processing techniques. Be- broadening the base of the available nibblers, tube benders etc., so did the cause of the high forces needed, hydrau- market, brought a demand for more need for electrohydraulic servos. reliable hardware in all areas: electrlcal, lics were a natural for early injection mold- mechanical and hydraulic. This led to the The machlne tool market, while not Ing and blow molding machines. The development of more reliable compo- demanding In the same way as military wider use of engineering plastics, and nents as well as new techniques for and space applications, brought a whole the higher cost of raw material, have led achieving reliable performance, and new host of new and different requirements to a demand for servocontrol in both methods for inspection and quality con- to the fluld power industry. The pressures injection and blow molding in order to trol of manufacturing, all of whtch have of cost and reliability brought about new improve part qualtty with a minimum led to better hardware and greater ac- manufacturing techniques and standardi- amount of material. A related field, die ceptance of the control medium. Some zation. Hydraulics were viewed as messy casting, has also demanded precision of the most complex examples of redun~ and excessively sensitive to dirty envi- control over what was previously a rela- dant flight controls are evident in the ronments, and this led to the concept, tively brute force process, and hence space shuttle orbiter and launch vehrcle. or problem, of mass education of the turned to electrohydraulics. Injection Four channels of information and control users of hydraulic equipment. This in molding and die casting have accelerated are summed hydraulically with appro- Itself did much to spread the market- the development of higher flow servo- priate sensing and shutdown logic in ability and acceptance of electro- valves. each flight surface power actuator. hydraulics. A unique application of electrohydrau- Meanwhile, a whole new generation of More sophisticated industrial applica- lies is in the Vibrosres2 field. This is a hlg,!i performance aircraft was evolving, tions were a direct result of the military means of oil exploration where the geo- logical signature of the ground is taken accompanied by new demands and new influence, prrncipally in the testing field. by simultaneously vibrating the ground problems for fluid power control. With A.n early combination of both was the in two or three locations at various fre- the increase in size, speed, and cost of use of electrohydraulic servos in flight quencies and measuring the reflected commercial transport aircraft came the simulators. Electrohydraulics are used energy. These exciters tend to be very necessity to fly under adverse weather for motion control of six degree of free- high frequency, high power devices, and conditions (Category 3 landings), hence dom aircraft simulators and for generat- have resulted In higher flow, higher re- the need for more reliable flight control ing stick reaction forces for the pilot. The systems. The solution involved high need for large dynamtc forces in struc- sponse servovalves to meet the need. tural testing aircraft flight surfaces, mis- power, high authority flight controls in Turbine controls, both gas and steam, redundant groups of three, such that a sile components, and environmental test- are large users of electrohydraulic servos single failure could not cause failure of ing of aviontcs equipment led to the de- and have made demands on system velopment of high response, high force the system. The SST added a new dimen- components that have resulted better sion to the problem because of its chang- testing devices. Dynamic test equipment reliability. ing aerodynamic stability with speed and has since grown into an industry of its loading. The supersonic transport re- own where, today, everything from auto- A natural consequence of the indus- quired an automatic flight control system mobiles to shipping cartons are tested on trial application of fluid power servo con- (stability augmentation) that was operat- electrohydraulic shakers. trol, was the automation of mobile equip- ing full time, and resulted in develop- ment. While flurd power is relatively com- Non military industrial applications ment of four-channel “voted” servo- mon in agricultural and construction have sprung up in a wide variety of areas mechanisms. In spite of its demise, the equipment, electrohydraulic servocontrols in the past three decades. Some of these Boeing SST has an impact on the devel- are not. One of the first uses for electro- are worthy of note because of the contri. opment of flight control servos. The cur- hydraulic servocontrol was for road grad- bution to control technology; some, just rent generation of military aircraft has ing and paving and railroad bed main- because of their uniqueness. Industrial been designed to take advantage of this tenance where loops are closed around automation; that is, the mechanization thinking. By designing an aircraft with level sensors and position transducers of specialized manufacturing processes marginal stability, it can be made much following guide wires to generate motions or the control of these processes by a more maneuverable and produce less of large road grading or paving machines, stored program, was a logical outgrowth drag, thereby using less fuel. However, or curb pavers. of the NC efforts. The number and types it must then have a full time automatic of these control systems are almost limit- More recently, the use of electrohy- flight control, which means redundant less, but a few are a particular note. draulic servocontrolled hydrostatic trans- electrohydraulic servoactuators. Prominent among these are controls for missions has opened up whole arenas of industrial robots. Robots came of age possibility for automatic control. While INDUSTRIAL about 15 years ago, bringing a new di- the use of hydrostatic transmissions is About the same time, the late 1950’s, mension of automation to many indus- ‘Trademark of Continental Oil Company.

5 not new, the development of low cost level of technology. Within the major re- nate the areas of typical applications. pumps and motors specifically for trans- gion of normal electrohydraulic control, This is done in Fig. 8 for some typical mission uses, and compatible servo- several zones can be indicated where commercial applications and in Fig. 9 for mechanisms to precisely control swash- various forms of control are dominant. corresponding aerospace applications. plate position, creates a new dimension The application labels are not meant to Note that this region encompasses in fluid power control. A variable dis- be restrictive, but rather to indicate char- about three decades of frequency and placement pump/motor combination is acteristic areas. For example, there are power. The low power, high frequency highly efficient in comparison to a servo- watchmaking machine tools that use low demands are typically satisfied by single- valve (which is a throttling element), so flow servovalves with 500 Hz natural fre- stage servovalves and represent instru- servocontrol of a pump/motor provides quency, and there are large spindle drives ment type servos. The high power, low fluid power control at power levels well in the 50 horsepower category that use frequency demands are often those above that achieved with servovalves, but hydrostatic drives. Yet, the majority of where remote control is a more impor- at some sacrifice in performance. Spe- machine tool servovalves fall around 100 tant criteria than performance, and these cifically, dynamic response suffers due to Hz and 5 to 20gpm. are often satisfied with hydrostatic drives. compliance of the entrapped fluid be- Technically,echnically ypically,the requirements the requirements above 2 abovein3/sec 2 tween the pump and motor, and static AREAS FOR GROWTH (cis), or l/s gpm, are satisfied with two- positioning accuracy is poor due to the While it would appear that electro- stage servovalves, and above 50 gpm characteristically high coulomb friction hydraulic servocontrols have reached a with three-stage servovalves. The single- of rotary hydraulic motors. point where a major change seems un- stage valves are, of course, the highest likely, remember that their history has Hydrostatic servos have been used as response, while three-stage valves are been one of evolution with few major open loop control elements in heavy con- slower, and hydrostatics the slowest. changes. There are still several expected struction equipment and agricultural ve- It is interesting to plot commercially areas for growth. hicles. By closing a feedback loop around available servovalves on this graph. A the transmission, various parameters specific valve should be able to satisfy n Higher Pressures - today capability such as speed, horsepower, or torque can exists for 4000 psi and higher hy- a requirement below and to its left, how- be controlled. This allows use of hydro- ever it may be larger and more complex draulic systems, though systems are static servos in machine tool spindle commonly limited to 3000 psi. Since than necessary. As a result, there are drives, cranes and hoists, tracked vehicle many sizes and configurations of avail- higher pressures represent better steering systems and power take-offs. able servovalves that fill the shaded power and weight efficiency, we should see a trend toward higher pressure region. systems as better components are TODAY Of more interest, though, is to desig- developed. In order to appreciate where electro-

hydraulics is today, it helps to summarize ------7--‘-‘--’ the technology as it exists. Typically, .-+ h ’ there are two dominant performance ---f--+-~-t--f I parameters that can be used to classify most electrohydraulic servos. One of these is size, that is power or flow rate, and the second is dynamic response. For convenience we can represent power as the primary power required to the control device, and represent dynamic response by the dynamic performance of the con- trol element. Fig. 7 shows a graph where I I i the ordinant is power level and the ab- 1 ] I I RAriGE OFI scissa is frequency. In order to appreciate i--C-i------L---l--.! CDN”ENT’ONAL ELECTROHYORAULIC------size, the ordinant is also calibrated with the load flow available at 1000 psi. Note that these axes are logarithmic.

There are three dominant regions to this graph. The major region represents the range of power.response where elec- 2 trohydraulics are generally used. At lower t power levels and lower frequencies, other 1 means are usually more satisfactory, 07 such as direct electromechanical control, ‘gpm = ciV3.85 or electropneumatic, and in many cases :: “L non proportional control. The region of 1 5 7 10 20 30 40 50 70 loo 200 300 400 500 700 loo0 high power and high frequency repre. FIGURE 7 - FREQUENCY OF SERVOVALVE 90” PHASE LAG--Hz Range of Control for sents requirements beyond the present Electrohydraulic Servomech anisms CONTROL DYNAMICS

6 n Higher Flow - the trend to automate higher power processes (such as die casting) will continue, coupled with the desire to go faster with existing systems for improved cycle time or efficiency. These will present demands to replace existing servovalves with 1 70 those of higher flow capacities. 100 - b M n Lower Cost - as fluld power technol- g7o-h ogy moves downward into less so- m 50 - g 30 wg 40 - phisticated and more cost sensitive i 20 3 m 30 - markets, there WIII be pressure to 2 2; &- 20 - 2 supply compon.ents to today’s stan- g 10 dards but at lower cost. One cause for 6; 7 this will be choosing electrohydraulics QlO-g 5 for reasons such as safety or operator 8; 7-$4 convenience, rather than for perfor- - z 3 P 5 mance. Since many of these potential 4 - E 2 areas represent a large market, the 3- 5 chances for volume production, hence lower cost, does exist.

W Better Response-frequency response represents a technical frontier to to- day’s servovalve, and probably the greatest technological challenge. The “L need for higher response will be felt 5 7 10 20 30 40 50 70 loo 2aJ 3w 400 500 700 loo0 where servos must interface at the FIGURE 8 - FREQUENCY 0~ SERVOVALVE 900 PH ASE LAG-HZ Spectrum of power end with electronics, for exam- Industrial Applications CONTROL DYNAMICS ple in radar drives, laser pointers, and in the material test field.

W Energy Savings - as we become more energy conscious, brute force solu- tions will no longer be acceptable and control systems will be evaluated by 7w their efficiency. The challenge is to WI avoid over-designing fluid power sys- 400 300 tems, and use smarter controls to 1 average the demands or limit horse- power draw.

REFERENCES

1 Mayr, O., Origins of Feedback Control, MIT Press, 1970, p. 11.

2 U.S. Patent 2,234,326 H. W. Tiebel filed i i I A I R INLET November 1939 - issued March 1941. 3 English Patent 620,688 Tin&y applied May 1946 - accepted March 1949. 4 Boyar. R. E., Johnson, B. A., and Schmid, L., “Hydraulic Servo Control Valves,” WADC Technical Report 55-29, Wright- Patterson Air Force Base, Ohio, 1955, p. 23, 35. 5 Blackburn, J. F., Reethof, G., and Shearer, J. L.. Fluid Power Control, The MIT Press and Wiley, 1960, pp. 403.406. 6 U.S. Patent 2,625,136 W. C. Moog filed April 1950 - issued Jan. 1953. 7 U.S. Patent Z934.765 T. H. Carson filed Sept. 1955 - issued Apr. 1960. t 8 U.S. Patent 2,767,689 W. C. Moog filed May 1953 - issued Oct. 1965.

9 U.S. Patent 3,209,782 M. P. Wolpin filed VL May 1955 - issued Oct. 1965. FIGURE 9 - 5 7 10 20 30 40 50 70 100 200 300400500 700 loo0 Spectrum of FREQUENCY OF SERVOVALVE 90° PHASE LAG-Hz 10 U.S. Patent 2,884,907 R. D. Atchley filed Aerospace Aug. 1957 - issued May 1959. Applications CONTROL DYNAMICS

7