RADAR CIRCUIT ANALYSIS 13-14
THE DIFFERENTIAL SElSYN
In a mechanical system, a differential con- 52 nects three shafts together in such a way that the amount that one shaft turns is equal to the dif- zsv ference between the amounts that the other two turn. The differential selsyn is named according to the two functions it performs. If the shaft of the differential selsyn serves to indicate the differ- ence in shafts positions of two other selsyns, the other two selsyns are generators and the differ- ential selsyn is a motor. If the differential selsyn shaft position is to be subtracted from that of a 53 selsyn generator (or vice versa) and the differ- ence to be indicated by the rotor position of a i----OV---o-i motor, the differential selsyn is then a generator. ~---OV--~~_-4 ~ The stator of the differential generator or motor is very similar to the stator of the ordi- nary selsyn. It consists of three sets of coils 0 wound in slots and spaced 120 apart around the Differential Transformer Action at 0° Position inside of the field structure. The rotor, however, differs considerably from that of an ordinary The selsyn generator just above is connected selsyn. It is cylindrical in shape and has three sets to a differential selsyn in which the rotor leads of coils wound in slots and equally spaced around are open. Both shafts are in the 0° position. Since the circumference. Connections to the external the stator coils of the differential selsyn connect circuits are made through three brushes riding on to the stator coils of the generator, the voltages three slip rings on the rotor shaft. AE in the are equal in magnitude and are in phase. Since ordinary selsyn, the motor has an oscillation the differential is a 1:1 transformer, the induced damper and the generator does not. There is no voltages are equal in magnitude and are in phase connection to the llOv AC as in the case of the with the stator voltages. When the generator ordinary selsyn. The three leads to the rotor are shaft is turned to some other position, the volt- designated Rl, R2, and R3. Electrical zero is the ages in the stator coils of the differential and, position in which rotor coil R2 is aligned with consequently, those induced in the rotor coils stator coil 82, R1 with 81, and R3 with 83. will be the same as those in the stator coils of the Electrically, the differential selsyn acts as a 1:1 generator. Thus the rotor voltages of the differ-- transformer with the stator coilsthe primary and ential are as shown in the rotor position diagram the rotor coils the secondary. Due to the air gap, on page 13-10, as the generator shaft is rotated there must be a few more turns on the stator with the differential shaft remaining in the 0° coils than on the rotor; hence, the transformation position. A motor with its stator windings con- is not a 1:1 ratio when the rotor coils are con- nected to the rotor windings of the differential nected as the primary. will have the correct voltages applied causing it
BALL 8EA~lNG
Rofor of Differenfial Selsyn RADAR CIRCUIT ANALYSIS 13·15
generator rotor is in the 345° position, the field axis is 15° clockwise from the axis of the S2 52 secondary coil, the voltage induced in it equals 52 cos (-15°). Likewise the voltages induced in Sl and S3 equal 52 cos (-255°) and 52 cos (-135°). These voltages set up a field in the motor which equals the vector sum of the three fields of the stator coils, resulting in a field with an axis at the 345° position. The rotor of a motor accordingly moves to the 345° position. Now in the differential selsyn, the composite primary field has reached the 0° position while the secondary (rotor) coils have rotated 15°. This
53 51 makes the field axis 15° clockwise from the axis of the R2 coil. The induced voltages then have the values previously stated and shown in the OV ----4'"1 transformer action illustration to the left. If the motor had its stator coils connected to the terminals R1, R2, R3 of the differential, the Transformer Action at 15° Position voltages applied to it would cause the rotor of the motor to move to the 345° position. to assume the same position as the generator Conversely, if the rotor of the differential were rotor. This means, that with the differential turned to the 345° position, the motor would rotor in the 0° position the system functions as turn to the 15° position. If the generator shaft it would if the differential were out of the circuit position is at 0°, the voltages induced in the and connections were directly from generator to rotor of the differential selsyn are the same motor. as if the rotor of the differential were at 0° When the generator rotor is in the 0° position, and the rotor of the generator were rotated in and the rotor of the differential is turned to 15° the opposite direction. as shown directly above, the voltages induced Thus, whenever the shafts of both generator in the rotor coilswill be as indicated. The voltages and differential rotate, the resultant field and between terminals R1-R2, R2-R3 and R3-R1 are induced voltages vary with the difference of the respectively 63v, 87v, and 24v and the R2-R3 angular position of the shafts. A motor connected voltage is in phase with the R1-R2 voltage across to the rotor coils of the differential will assume a the rotor of the generator. The other voltages are shaft position equal to the difference of the angu- 180° out of phase. These voltages correspond to lar positions of the other two shafts. The mathe- the voltages induced in the stator windings of a matical proof, a quite long one, is beyond the generator in which the rotor is at the 345° posi- scope of this manual. Briefly, it involves setting tion. To understand this action, consider the up equations for the voltages induced in each relative angle of the inducing magnetic field axis of the rotor coils of the differential by each of and the axis of the secondary coils. When a the sta tor coils and adding the induced voltages
DIFFERENTIAL GENERATOR 60·
GENERATOR 180· MOTOR 1200
Selsyn System with Differential Generator RAQAR CIRCUIT ANALYSIS 13-16
in pairs. A qualitative explanation of what causes the motor shaft to turn to the indicated position is fairly simple to understand. For example, take the case illustrated at the bottom of page 13-15. The generator rotor is at the 180° position.
Hence the voltages induced in 81, 82, 83 are 115V
26v, 52v, 26v, respectively in the phases in- DIRECT CONNECTIONS dicated. All the voltages 81, 82, 83 of the dif- ferential are identical. These voltages set up a magnetic field in the differential which has the same position, 180°, for its axis as the field in the generator. If the axis of the rotor of the differ- ential generator were aligned with the 0° posi-
tion, the induced voltages would be identical to il5V
the stator voltages. But since the rotor coil axis DIFFERENTIAL REVERSED is shifted to the 60° position, the axis is only 120° from that of the magnetic field. Therefore the voltages induced in R1, R2, R3 are 52 cos 120°, 52 cos 240°, 52 cos 0° volts respectively. The vol.tagesfrom R1 to R2, from R2 to R3 and from R3 to R1 are therefore Ov, 78v and 78v with ___ R~l~rR~' ~R' R, R2-R3 being 180°out of phase with R1-R2 of the 1i5V generator. When these voltages are applied to "A" REVERSED the motor, it will cause its rotor to turn to the position such that the voltages induced in its stator coils by the rotor just balance these voltages. This happens when the rotor is at the 1200 position, the difference between 1800 and 60°. R, R, By reversing pairs of leads either between the 115V
generator and the differential or between the "S"R~RSED differential and the motor, you can make any Various Connec.ron for Selsyn System one of the shafts assume a position equal to the sum or difference of the angular positions of the required, the use of a differential is avoided if at other shafts. The proper connections for various all possible. types of operation are shown in the illustration to the right. The Control Transformer A differential selsyn cannot be a 1:1 trans- The uses of selsyn are to cause two shafts to former in both directions. Therefore the perfect rotate in synchronization and to produce an balance of voltages indicated is not strictly error voltage which indicates the difference in accurate. The differential selsyn is designed such position of two shafts. The systems discussed to that the voltages induced in the rotor coils are this point are for synchronization purposes. For equal to the voltages applied to the stator wind- the purpose of producing error voltages, there is ings provided the shaft is at 0°. When a differen- a type of selsyn known as the control transformer tial is connected between a generator and a which is used in conjunction with a generator. motor, some current flows in the generator's Both the generator and the transformer have stator circuit, reducing the output voltages due their shafts -connected to loads. The voltages to the IR drop. These lower than normal volt- induced in the stator coils of the generator are ages are induced in the rotor windings of the applied to the stator of the control transformer. differential. They are too small to buck out the These voltages induce in the rotor of the control motor voltages; and some current flows in the transformer (not connected to the 115v AC) a motor stator circuits. The generator currents, voltage, the magnitude and polarity of which usually higher, reduce the accuracy of the system. depends upon the relative positions of the two Therefore, when a very high degree of accuracy is shafts. RADAR CIRCUIT ANALYSIS 13-17
The stator of a control transformer is very similar to that of a generator or motor except that 52 52 the coils are wound with more turns and finer wire. The purpose of this is to make the im- COMMON pedance high enough to prevent high currents COMMON from flowing. Remember that no voltage is induced in the stator coils of the control trans- former to buck the applied voltage and keep the R1 current within safe limits. The stator current is R2 determined by the impedance of the stator wind- ings and is practically independent of the rotor's R2 position. R1
The rotor of a control transformer looks some- 5ElSYN MOTOR OR CONTROL TRANSFORMER what like the rotor of a differential selsyn. Yet GENERATOR IN 00 POSITION IN THE 00 POSITION there are some important differences. Instead of three groups of windings spaced 120° apart, all Comparing Electrical Zero Position the windings are in series and have their external connections through two slip rings and brushes. the 90° position. The axis of the coil will then be The shaft always connects to a load making an aligned with the composite magnetic field and oscillation damper unnecessary. The coils con- there will be a maximum (55v) voltage induced tain many turns of fine wire. The turns ratio in it. The phase polarity of the R1-R2 voltage is such that the maximum output voltage, with will be identical to the R1-R2 voltage of the normal stator voltages, is 55 volts. There is no generator rotor. At other positions you can appreciable current in the rotor windings. The determine the magnitude of the induced voltage, rotor is not free to turn to any particular posi- by resolving the magnetic field into two com- tion when voltages are applied to the stator ponents one of which is perpendicular to the windings. The electrical zero position of the axis of the coil and induces no voltage, and the rotor is such that no voltage is induced in the other which is aligned with the coil and induces rotor windings. This means that the coil axis is at the right angle position as shown in the illus- tration at the-upper right. Note that the termi- nals are labeled exactly as are those of a motor or generator. R1 This is how the control transformer operates. 115V t ov. In the illustration just to the right, the generator R2 shaft is at 0° and the control transformer shaft is at 0° position. The axis of the magnetic field set si up by the stator voltages is in the 0° direction- C. T. at right angles to the rotor windings. It induces GENERATOR no voltage in the rotor windings. Now assume that the control transformer rotor is turned to No Voltage Induced with Rotor at 0° Position
CROSS SECTION
Typical Control Transformer Rotor RADAR CIRCUIT ANALYSIS 13-18
55V
~~ I0- ~~ ;;; wOw '«"> wZ ":r-,,,N o ~~ 0 '"w ~~ 0 ~~2< ,,0 wW >"'>- -c < ~N "':rl''- ~ 0'" ~O"-~ "" ~ ~~ ~~ ~ u .., 6~ , I 01- 0 ~~
1800 2700 3600
POSITION OF CONTROL TRANSFORMER SHAFT
Induced Voltage vs Shaft Position a voltage. The component of the field which is that if the control transformer (CT) shaft parallel to the coil axis is proportional to the sine leads the generator shaft by any number of of the angle to which the control transformer degrees up to 180, the output is in phase with rotor is turned. Note in the induced voltage vs the generator rotor voltage but, that if the shaft position graph directly above that voltages CT shaft lags the generator shaft by any num- above the zero axis are in phase with the gener- ber of degrees up to 180, the output is opposite ator rotor voltage, R1-R2, and the voltages below in phase. the axis are 180°out of phase. Obviously then the voltages from the 180° position to the 360° SERVOMECHANISMS position are 1800 out of phase. You have already seen how selsyns transmit When the rotor of the generator is turned to data from one place to another. When the shaft some position other than 0°, the magnetic field of the control transformer will followand assume to be driven at the remote position is connected the same position, and when the rotor of the to an indicating device or some light load, the control transformer is turned the same position, selsyn motor is capable of developing the neces- the output will be zero volts, since the axis of the sary torque. But if the load is greater and more coil is perpendicular to the field. The rotor, torque is necessary, then torque amplification is starting from this position, on rotating in a required. In this case the load is driven by a counterclockwise direction will cause the volt- reversible adjustable speed motor which is ages induced in the control transformer to assume called a servo motor. The complete system, in- the values indicated in the above graph, where eluding the motor and its control devices, is the abscissa is the difference in angular posi- called a servomechanism. The speed and direction tion of the two rotors. This action indicates of rotation of the motor are controlled bv the
CONTROl TURNING MECHANISM LOAD DEVICE (SERVO MOTOR)
___ MECHANICAL CONNECTION
AC INPUT S2 VOLTAGE
AMPLIFYING DEVICE
Simple Servomechanism RADAR CIRCUIT ANALYSIS 13-19
magnitude and phase of an "error voltage" which hence the control in the servo system is always is produced by any misalignment of the two accomplished by varying the armature voltage. shafts (the load shaft and the control shaft). Placing a variable resistor in series with the Obviously, the direction of rotation must be such armature to control the armature voltage is as to reduce the misalignment causing the error wasteful since the resistor itself dissipates con- voltage. siderable power. Usually armature voltage is The principles of the servo system in its controlled by a DC generator driven by an AC simplest form are shown in conjunction with the motor. The Ward Leonard drive, discussed later, illustration on the preceding page. Assume that is an example of this system. the control device is a crank or a hand wheel Servo motors operate with constant fields. The and the load is the antenna. To the handwheel field is supplied either by permanent magnets or there is connected a selsyn generator, and to by DC applied to the field coils. The method of the antenna a control transformer. The stator applying DC voltage to the field coils is wasteful windings of the generator and control trans- of power, since power is consumed even when former are connected electrically as shown. the motor is not turning. In addition the design The output of the control transformer is an of the equipment is much more complicated than AC voltage that depends upon the relative the permanent magnet arrangement. Principally, shaft positions for magnitude and phase. There because of this reason, the permanent magnet is is no output when the shafts are in the same the more widely used. One problem in the use of position, and accordingly no output from the permanent magnets is the' danger of demagnetiz- amplifying device. Hence the servo motor ation of the magnet by the field set up by cur- does not turn and the antenna remains sta- rent flow in the armature. This is overcome by tionary. To change the position of antenna, the inclusion of a special winding which is con- you merely turn the handwheel to the desired nected in series with the armature and wound in position. This produces an error voltage in opposite direction. In this winding, called the the control transformer. This voltage is rec- compensating winding, there is set up a com- tified and increased in magnitude by the am- pensating field which results from current flow plifying device and then applied to the servo through it. Since the current in the compensating motor, causing it to turn the antenna to the wind varies with the armature current, the com- desired position. pensating field balances out the armature current The selsyn generator and control transformer and protects the permanent magnets. may be interchanged in position. However, such The Ward Leonard Drive an interchange necessitates five electrical leads In the Ward Leonard drive shown on the from one position to the other and gives no ad- next page, AC motor turns at a constant speed vantages. If the control device rotates contin- always in the same direction. It drives the arma- uously in one direction, the servo motor will tures of the large and small DC generators. The rotate in the same direction at the same rate small generator provides excitation voltages for but with a small angle of lag. Remember that an both the large DC generator and the motor. The error voltage is necessary before torque is de- DC motor field is constant, being supplied by veloped. The greater the torque required, the field coils. The generator field is not constant. greater will be the angle of lag. Both its polarity and magnitude are controllable The Servo Motor by a field rheostat. When the variable arm is at An AC motor, inherently a constant speed the point M there is no voltage applied to the device, is not suitable for the requirements of a generator field coils and consequently there is no servomechanism and therefore a DC motor is output voltage, no voltage applied to the arma- used. There are two methods of controlling the ture of the DC motor, and no motor operation. speed and direction of rotation-controlling the When the variable arm is between M and N, a voltage applied to the field coils, and controlling voltage of a certain polarity is applied to the the armature voltage. Controlling the field volt- field coils of the generator, the generator output age permits control of speed only from a certain has a certain polarity and causes the motor to minimum speed upward, since speed varies turn in a certain direction. When the variable inversely as the flux set up by the field. However, tap is located between M and P the generator reduction of speed to zero at times is necessary; field voltage is reversed in polarity, the generator RADAR CIRCUIT ANALYSIS 13-20
~:-.- ~ N
LOAD
Word Leonard Drive output is reversed in polarity and the motor duced by the control transformer (CT) is turns in the opposite direction. In either case the fed into the control amplifier the output of motor speed depends upon the voltage applied which is a DC voltage having a polarity and to the field coils and is controllable by the rheo- magnitude that is determined by the phase stat. Thus, you can control both the speed and and magnitude of the CT voltage. The out- put of the control amplifier is applied to the direction of rotation of the motor. It will there- field of the DC generator of the Ward Leonard fore function very weU as a servo mechanism drive starting the action described in the last providing that the generator field is controlled paragraph. The circuits of the control amplifier by means of an error voltage instead of the field are explained later. The purpose of selsyn rheostat. generator G2 and selsyn motor Ml is to furnish information to the operator as to the position of A Servo System Using the Ward Leonard Drive the antenna. In some cases this could be dis- When the Ward Leonard drive is used for a pensed with since the scope pattern furnishes the turning mechanism the servo system operates as same information when the scan is B type or PPI shown directly below. The error voltage pro- type.
CONTROL SELSYN GENERATOR TRANSFORMER G2 C.T.
CONTROl AN,PLlFIE.'
230Y 30 1I0V AC
Servo System Using Ward Leonard Drive RADAR CIRCUIT ANALYSIS 13-21
The Amplidyne The amplidyne is a special type DC generator commonly used for the DC generator in a system
similar to the one described. Such a system is OUTPUT TO LOAD known as an amplidyne servomechanism. The word amplidyne means dynamo amplifier.
The DC generator may be considered an CONTROl FIELD COMPENSATING FielD amplifier since a small amount of power applied to the field coils controls 10 to 100 times as much power in the output of the generator. Of course, the additional power comes from the mechanical '"::J I I power which turns the armature. This mechanical QUADRATURE AXIS power is comparable to the B voltage of a vacuum tube amplifier. If greater amplification is The Amplidyne desired than possible with a single DC generator, then two are used in cascade, that is, the output same as it is in an ordinary generator. The rota- ofthe first (the exciter)is the input to the second. tion of the armature through this quadrature field sets up a voltage at right angles to it, that is, along the direct (horizontal) axis. Brushes located on the direct axis have the load connected 1 ST. STAGE AMPLIFICATION to them. When the load draws current, a magne- tic field is set up along the direct axis due to the OUTPUT armature reaction of this second stage. Since the TO LOAD field due to armature reaction in each stage is ninety degrees from the field producing it, the EXCITER 0 DC GENERATOR reaction field of the second stage is 180 from the control field and tends to cancel it out, thereby reducing the amplification of the amplidyne. This is comparable to degeneration in a vacuum DC Generators as Cascade Amplifiers tube amplifier. In order to prevent this cancella- While an exciter was used in the Ward Leonard tion of the control field, compensating winding drive system, it may not be considered two on the direct axis and in series with the brushes stages of amplification, since the input to the on the direct axis sets up a field which opposes exciter was uncontrolled. When two generators and cancels out the armature reaction of the second stage. are used in cascade as is shown above, the ampli- fication is the product of the two amplifica- As shown helow the various magnetic fields of tions. For example, if each amplifies the power an amplidyne occupy certain positions and have 30 times, the total amplification is 900. certain directions. The symbol ct>1is the control The amplidyne performs like a two-stage DC field, ct>the2 field due to armature reaction of the generator-amplifier with the two stages combined first stage or the quadrature field, ct>ais the into one armature. The input goes to the control armature reaction of the second stage and ct>bis field which is on the direct axis of the machine. the field due to the compensating windings. Rotation of the armature in the control field sets up a voltage at right angles to the field as shown at the top of the page. Brushes are placed on this axis. If a load were connected to these brushes, the machine would become an ordinary DC generator. However, in the ampli- dyne these brushes are short circuited and quite high currents flow in the armature producing a magnetic field along the quadrature (vertical) axis. This field is much stronger than the control ACTUAL EFFECTIVE field. This is known as armature reaction the Magnetic Fields in Amplidyne R A D A R CI R CUI TAN A LY 5 I 5 1 3 - 22
Since, by design,
~ COlLI BIAS RESISTOR
\COt'\TROt ERROR VOLTAGE FROM C. T. FIELDCOILS OF AMPLIDYNE ,;{, V,
110VA.C. '--- 1
Bosic Control Amplifier Circuit RADAR CIRCUIT ANALYSIS 13-23
ERROR VOLTAGE IN PHASE WITH 110V ERROR VOlTAGE 180' OUT OF PHASE WITH 110V.
EPI EJ>2 1'---+----\---+----'1-----1 OV. 1'---+----\---+----'1----1 OV.
1----+---+---~+-~-+-----1OV.
E,. r--t------=::.'k::--+-----::;~=------IBIAS E,.
I---t----+----+---+----I OV.
E" E" f---t-----".,..--+-----:..fL---J
NO INPUT ,.-
Control Amplifier Currents and Voltages
If a greater voltage is applied to the input load impedance and use this practically constant transformer TJ, it will result in more cur- voltage to feed into a DC amplifier. rent passing through coil 1 and less through coil 2 Hunting is quite likely to take place in a servo- which causes the servo motor to turn faster. mechanism unless some method is devised to 0 When the error voltage is 180 out of phase prevent it. Hunting means that the servo motor with the 110 volts, the grid of V~will go positive drives the load past the desired position causing and the grid of VI negative at the time the plates an error voltage which in turn reverses its direc- go positive. As a result, the current flow through tion and causes the motor to swing back and coil 2 will increase and that through coil 1 will forth past the final position several times before decrease. This causes the flux in the amplidyne stopping. Under some conditions continued os- to increase but in the opposite direction so cillation may take place. The usual anti-hunt that the output voltage is opposite in polarity. The servo motor will turn in the opposite circuit consists of feeding back a portion of the direction. amplidyne output into the input of the control amplifier. This voltage is proportional to the In actual practice some sort of filter circuit is speed of the motor and is applied so that it employed to smooth out the pulsations of the control amplifier output. This makes the output makes the grid of the conducting tube more more steady. One way of accomplishing this is negative (in other words, it decreases the gain of to connect resistors in parallel with the amplidyne the amplifier) and decreases the amplifier output control field windings. During the negative and the motor speed. In this way the motor, in alternation of the plate voltage, the self-induced effect, acts as a brake on its own speed. The mag- voltage of the inductance of the control field nitude of the feedback decreases as the speed maintains the field current which finds a com- decreases and allows the motor to rotate the load plete circuit through the shunting resistor. to the desired position without unnecessary Another way of making the output more nearly hunting. The amount of feedback is adjustable pure DC is to put an RC circuit in to act as the by a potentiometer. RADAR CIRCUIT ANALYSIS 13-24
SElSYN GENERATOR
115V. A. C.
FlUX GATE COMPASS AZJM. MOTOR
ANTENNA
ANTENNA AND SWEEP COIL IN DEAD ASTERN POSITION ZERO SET
~~} TORQUE AMPURER 53 MAGNESYN I SWEEP COIL
SWITCH IDENTICAL TO THAT IN 3-3 LEAD
115V. A. C.
SELSYN' MOTOR
Typical Selsyn System Employing Azimuth Stabilization
USE OF SERVOMECHANISMS FOR AZIMUTH tained by shifting the sweep coils of the CRT STABILIZA TJON whenever the plane changes its heading. The The heading of an airplane is quite changeable. antenna is driven by the azimuth motor at a For this reason, it is desirable sometimes for the rate of approximately 24 RPM. A selsyn gener- presentation of data on the indicator scope to re- ator is geared to the antenna through a 10:1 main in the same position regardless of changes gear. The generator is connected to a differential in heading of the plane. For example if the 0° selsyn which has its output connected to a selsyn position of the PPI scan is always true north it is motor. This motor is geared, through another much easier to compare the scope picture with 10:1 gear to the sweep coils and causes them to the map of the region. In order to accomplish rotate about the neck of the CRT. If the differ- this, it is necessary to change the relative posi- ential generator is in the electrical zero position, tions of the antenna and sweep on the scope by the rotation of the sweep coils is in synchroniza- an amount equal to the change in heading. This tion with the rotation of the antenna. The pur- can be effected by applying azimuth stabiliza- pose of the 10:1 gears is to reduce to 1/10 of its tion to the antenna drive assembly. value the lag inherent in a selsyn system. In this system, if the selsyn motor lags the generator by Selsyn System Employing Azimuth Stabilization 100, it produces only 10 lag between the antenna The circuits shown above are those of a typical and sweep coils. The actual error is a small frac- 0 set employing stabilization. Stabilization is ob- tion of 1 • The increased accuracy is gained by RADAR CIRCUIT ANALYSIS 13-25
the possibility of the antenna and sweep coils sweep coils. The illustration below shows the being 36°, or some multiple thereof, out of syn- cams and switches at successive steps in the rota- chronization. Of course the selsyns are always tion. This out-of-step rotation will continue until in step but since 36° rotation of the antenna (or both switches are open at the same time (see C), sweep coils) will mean one complete rotation of then the sweep coils will stop and remain in that the selsyn motor, there is the possibility that position (180°) until the antenna reaches the this error could occur. To guard against such an position where it will closethe switch. This occurs occurrence, cams are attached to the shafts of the at 23° before the dead astern position. The sweep antenna and the sweep coils. These are called coil rotor will then electrically interlock and phasing cams and operate switches in the Rl-Sl cause the antenna and sweep coils to be in step and R3-S3leads from the differential generator to once more. the selsyn motor. The cam attached to the anten- The description of the operation thus far na shaft closes the two switches during 46° of assumes that the differential generator was in the rotation centered about the dead astern position. electrical zero position. This is true if the AZIM. The cam attached to the sweep coil shaft opens STAB switch is in the OFF position. In this type switches in the same leads during 36° of rotation, operation, the control transformer in the phasing also centered about the astern position. If the unit has its stator windings connected to a zero two shafts are in synchronization the antenna set magnesyn. The magnesyn is a device similar cam closes the switches 5° ahead of the position in performance to a selsyn generator. The control where the sweep coil cam opens the switches and transformer in this particular set up is known as holds them closed for 5° past the position where an autosyn. The output of the control trans- the sweep coil cam again closes its switches. former, after being amplified by the torque Thus the circuits are complete at all times and amplifier, causes the torque motor to turn the the selsyn motor drives the sweep coils in step CT rotor and differential selsyn rotor to the with the antenna. desired position which is the electrical zero posi- To illustrate the action assume that when the tion of the differential generator. system is turned on and the antenna is point- When using azimuth stabilization, turn the ing somewhere in the aft sector, the sweep switch AZIM. STAB to the ON position. This dis- coils are at a position corresponding to dead connects the control transformer from the zero ahead. Both sets of phasing switches are closed, set magnesyn and connects it to the flux gate and the antenna and sweep coils start rotating. compass magnesyn. The switch also shorts out There is large error between the antenna and the phasing switches and connects the differential
Action of Phasing Switches R A D ARC IRe U I TAN A I. Y SI S 1 3 - 2 6
generator directly to the selsyn motor. The sweep coil motor to lead or lag the antenna magnesyn in the flux gate compass produces generator by the same angle. By proper connec- voltages to set up a field in the control transform- tion and adjustment the sweep coils move in er which is aligned with the position of the rotor such a way that the OC position of the scope is in the rnagnesyn. This position is dependent true north. In order to determine the plane's upon the position of the compass. The error heading from the scope pattern a switch con- voltage produced by the control transformer, nected to the antenna causes the bias of the CRT after amplification, turns the control transformer to decrease when the antenna points dead ahead. rotor and that of the differential selsyn to the position corresponding to the compass position. This results in a brighter trace which indicates Since the rotor of the differential generator is the planes heading. The purpose in shorting out shifted by an angle equal to the difference be- the phasing switches is to permit the antenna tween the plane's heading and true north, its and sweep coils to be out of step, as is the case output voltages will be such as to cause the when the plane's heading is other than north.
r