1 SERVOMECHANISMS A servomechanism (servo) is a type of control system whose output is the position of a shaft. They may be controlled remotely when used in conjunction with synchro devices. Synchros themselves transmit position information but cannot amplify torque to move heavy loads. Used with servomechanisms, an output to control such a load can be obtained to give a desired result in relation to an input. 1.1 OPEN LOOP SYSTEM In this system, an input is applied and an output obtained. Figure 1 shows an example; assume an aircraft rudder controlled by an open loop system.

INPUTTRANSDUCER

MOTOR LOAD

DEMAND

RESPONSE

DEMANDSIGNAL

AMP

Open Loop System Figure 1

The demand, made by the pilot on the rudder bar, is picked up by the transducer which converts it to an electrical signal; i.e. the demand signal. This signal is amplified and fed to the motor, which responds by moving the load; i.e. the rudder. There is no positional feedback and the pilot does not know if the rudder has adopted the position requested.

1.2 CLOSED LOOP SYSTEM In the closed loop system, the demand is made in the same way. In a basic system, positional feedback would be given to the pilot who would make adjustments accordingly but this is not practical with systems such as aircraft flying controls. Figure 2 shows a closed loop automatic system.

INPUTTRANSDUCER

SERVOMOTOR LOAD

OUTPUTPOSITION

TRANSDUCER

ERRORDETECTOR

POSITIONFEEDBACK

ERRORSIGNAL

AMP

Closed Loop System Figure 2

An output position transducer has been added to the servomotor and this feeds back any difference between input demand and output to an error detector. The error detector outputs an error signal to the amplifier to make any positional corrections necessary at the servo motor and thus the load (or rudder) is positioned as demanded. If for example the pilot wanted to move the rudder 5°, a demand is made at the rudder bar and this is converted to a voltage at the transducer, say +5 volts. The error detector immediately gives an output signal corresponding to +5 volts input and this is amplified to drive the motor, moving the rudder. The output position transducer converts the output position to an electrical signal, which corresponds to the new position of the rudder. As this happens, this signal, (feedback), is fed back to the error detector until the demanded position is achieved and the input is negated. Now, there is no error signal and no output. The feedback has reached -5 volts.

1.3 FOLLOW UP If in our example the rudder were to be displaced from its demanded position, or from the optimum speed at which the demanded position may be achieved, an error signal occurs. In the way described, there is a feedback signal and the system returns to its demanded position or speed. This process is called 'follow up'. 1.4 TYPES OF SERVO There are two main classes of servomechanism - remote position control (RPC) servos and velocity control servos (velodynes). a) RPC servos. These are used to control the angular, or linear position

of a load. A typical example of the use of a RPC servo is the control of the direction in which a radar scanner is pointing.

b) Velodynes. These are used to control the speed of a load. In this

case, the speed of the driving motor is made proportional to the input demand (usually a voltage). A typical example of the use of a velodyne is the control of a radar scanner, which is required to rotate with a constant angular velocity. It may be necessary to change the velocity of rotation from time to time and the velodyne must be capable of doing this and maintaining the new velocity set by the input demand.

1.5 FEEDBACK 1.5.1 POSITIONAL FEEDBACK Positional feedback is obtained from transducers positioned at the output. The feedback element, or transducer, converts the output shaft angle into a signal suitable for operating the error detector. In this case a voltage signal. The simplest form of element is a R-pot, or a helical potentiometer similar to that used as a control element. In practice, helical potentiometers are used since they give 360° coverage, which a R-pot cannot provide. Figure 3 shows positional feedback in a dc system.

ERRORDETECTOR SERVO

MOTOR

LOAD

TACHOGEN

FEEDBACKELEMENT

POSITIONALFEEDBACK

VELOCITYFEEDBACK

CONTROLELEMENT

Positional Feedback Figure 3

Figure 4 shows a R-Pot & Helical Potentiometer

E

θ iPROPORTIONAL

TO θ i

Ei

R-POT

HELICAL POTENTIOMETER

E

θ i

Ei

PROPORTIONAL

TO θ i

R-Pot & Helical Potentiometer Figure 4

In ac systems, other components are used to provide positional feedback. Synchros are employed in some servomechanisms. These will be discussed later.

1.6 ROTARY VARIABLE DIFFERENTIAL TRANSDUCER (RVDT) The RVDT is an inductance transmitter having a primary stator coil, an iron rotor coil and two secondary stator coils. Figure 5 shows the operation of a RVDT.

R TS

L3

L1 L2

IRON CORECONNECTED TO

MECHANICALINPUT

PRIMARYCOIL

R TS

R TS

1. ZERO POSITION 2. ROTATED CLOCKWISE

3. ROTATED COUNTER CLOCKWISE

RVDT Operation Figure 5

The mechanical input changes the position of the iron core. The position of the core changes the magnetic coupling between the primary and the secondary stator coils. When the input rotates, one of the secondary coils receives more magnetic flux and this induces a higher voltage in that coil. The other secondary coil receives less magnetic flux, so a lower voltage is

induced. The difference between voltages induced in the secondary stator coils is proportional to the rotated angle. This is an AC Ratio Signal. Figure 5.1: The position of the iron core is zero. The magnetic field induced

by primary coil L3 is equally divided between L1 and L2. Therefore the voltage R-T is zero.

Figure 5.2: The iron core is turned clockwise. Now there is more coupling

between L3 and L2, and less coupling between L3 and L1. The voltage between T and S increases and the voltage between R and S decreases.

Figure 5.3: The iron core turned counter-clockwise. Now there is more

coupling between L3 and L1, and less coupling between L3 and L2. The voltage between T and S decreases, while the voltage between R and S increases.

The difference between figure 5.2 and 5.3 is that the output-voltage between R and T is of opposite phase. The output measured between R and T is an AC RATIO signal. The Linear Variable Differential Transducer (LVDT) is also an inductance transmitter with similar components and similar in operation but of course, the movement detected is linear and not rotary.

1.7 CAPACITANCE TRANSMITTER An example of a capacitance transmitter can be seen in a simple fuel gauging system as in Figure 6.

TANK UNIT

FULL

EMPTY

AMPLIFIERSTAGE

REF C

2 - PHASEMOTOR

AMPLIFIER UNITREF

PHASE

INDICATOR

LOOPA

LOOPB

IS

IB

DISCRIMINATIONSTAGE

Capacitance Transmitter

Figure 6 This system depends upon the comparison of two capacitance values. One in Loop A, which is the variable capacitance of a tank unit and the other in Loop B, which is fixed. A current is developed in each loop; IS in loop A; IB in loop B. The two loops form a bridge with resistor R across it. If the tank is full, then current IS is the greater. With the tank empty, IS falls so that IB is the greater. Note: The currents act in opposite directions so that a potential is developed across resistor R of a polarity dependent on the direction of current flow and of a magnitude dependent on the size of the current. This signal is transmitted to an amplifier, which powers a 2-phase motor to drive an indicator and a balance potentiometer.

When the balance potentiometer moves as a result of change in fuel level, it adjusts IB, rebalancing the bridge formed by loop A and loop B. Now, no current flows through resistor R, no signal is developed across R and the new fuel level is displayed at the indicator. 1.8 VELOCITY FEEDBACK The inherent friction of a basic servomechanism is very small and so the device may be able to oscillate fairly freely. This means that the load may oscillate about its final required position, an effect known as 'hunting'. The time taken for the load to come to rest at the required new position is called the 'response time' and ideally will be as short as possible. A process known as 'damping' achieves the desired response time. Figure 7 shows graphs of the results of different degrees of damping.

INPUTDEMAND θi

&OUTPUT θo

TIME

SUDDEN CHANGEIN INPUT DEMAND θi

OUTPUTUNDERDAMPEDRESPONSE θo

θo OVERDAMPEDRESPONSE

θo REQUIREDRESPONSE

Velocity Feedback

Figure 7 It can be seen from this that excessive oscillation takes place if the device is underdamped, while overdamping results in too long a response time. Viscous friction damping by using a mechanical brake or eddy current damping are possible answers to the damping problem but are rarely used due to their inefficiencies. Velocity feedback damping is a more effective method and uses a signal proportional to the velocity, or rate of movement of the output shaft, as a feedback signal to compensate for oscillation of the load. Tacho-generators are used to obtain this feedback signal.

1.9 DC TACHO-GENERATOR The dc tacho-generator is mounted on the output shaft of the servomechanism so that it is rotating at the same speed as the load. The dc tacho-generator is a normal small dc generator with a separately excited field. It will, therefore, produce a dc voltage, which is directly proportional to the speed at which it is driven and whose polarity depends upon the direction of rotation. 1.10 AC TACHO-GENERATOR The ac tacho-generator, used to provide velocity feedback damping in ac servo systems, is mounted on the output shaft so that it rotates at the same speed as the load. The ac tacho-generator is usually a drag-cup generator, which produces an alternating voltage of the same frequency as the ac supply. However, the amplitude of the voltage depends upon the speed of rotation and the phase of the voltage leads or lags the ac supply, depending upon the direction of rotation. In an automatic RPC servo, there is no operator and the braking required is produced by attaching a tacho-generator to the output shaft as shown in Figure 8 below.

θO θO

CONTROLLER&

AMPLIFIER

SERVOMOTOR LOAD TACHO

GENERATOR

θOθO

VELOCITY FEEDBACK

POSITIONAL FEEDBACK

VOLTAGE PROPORTIONALTO SPEED OF OUTPUT

SHAFT

ERROR = θi - θO

θi

CONTROLS THEAMOUNT OF

VELOCITY FEEDBACK

NEGATIVE TOERROR SIGNAL

A.C. Tacho-Generator

Figure 8 The tacho-generator produces a voltage proportional to the angular velocity of the output shaft. A suitable fraction of this voltage is fed back to the input of

the controller and amplifier in opposition to the error signal, which is produced in the usual way. This is negative feedback, also known as velocity feedback. 1.11 SYNCHROS

1.11.1 INTRODUCTION AC transmission systems are generally known as synchros because of their synchronous action in reproducing the angular movement of a shaft. As mentioned previously, they cannot transmit torque to any appreciable degree but can be used in conjunction with servomechanisms. 1.12 TORQUE SYNCHRO 1.12.1 PRINCIPLE OF OPERATION The principle of a synchro is that of the transformer, where the primary winding is wound onto a rotor and is rotated with respect to a fixed stator winding. The size and phase of the output voltage is dependent on the direction and angular displacement between the primary and secondary windings. The torque synchro comprises two electrically similar units: the transmitter (TX) and the receiver (TR) which are interconnected by transmission lines. The TX and TR have very similar construction. Each has a rotor carrying a single winding concentrically mounted in a stator of three windings, the axes of which are 120° apart. It should be noted that the TX and TR torque synchros are not identical. The difference is that the TR synchro has an oscillation damper added, so that when its rotor rotates to a given position, it does not oscillate as it comes to rest. The rotors of both TX and TR synchros are energized from the ac supply and produce an alternating flux which links with their corresponding stators S1, S2 and S3. This process is the normal transformer action, with the rotors corresponding to the transformer primary winding and the stators to the secondary windings. Consider the case when the two rotors are not aligned. The three voltages induced in each of the two sets of stator windings are different. Currents therefore flow between the two stators and a torque is produced in each synchro which is directed in such a way that the two rotors must align themselves. Normally, the TX rotor position is controlled by the input shaft, while the TR rotor is free to turn, so it is the one which aligns itself with the TX rotor. In this way, any movement of the TX rotor due to movement of the input shaft is repeated synchronously by movement of the receiver rotor. Torque synchros are used for the transmission of angular position information and flight instrument systems is a typical application. Figure 9 shows a Torque Synchro and circuit symbol.

ROTORFIELD

CURRENTFLOW

STATORFIELD

S1

S2

S3

R1

R2

INPUTSHAFT

OUTPUTSHAFT

S1 S1

S3 S3S2 S2

CIRCUIT SYMBOL

Torque Synchro Figure 9

Figure 10 shows the construction of a torque synchro.

STATOR ROTOR COMPLETEASEMBLY

STATORWINDINGS

SHELL

LOWER ENDCAP

SHAFTBEARING

COILS

CORE

LEADS TOSLIP RINGS

SLIPRINGS

STATORLEADS

ROTORLEADS

Torque Synchro Construction Figure 10

1.13 CONTROL SYNCHRO The basic control synchro system has two units; a synchro control transmitter (CX) and a synchro control transformer (CT) connected as shown in Figure

11.

INPUTSHAFT

S1 S1

S3 S3S2 S2

A.C.SUPPLY

M

SERVOMOTOR

A.C.SUPPLY

CX CT

Control Synchro

Figure 11 1.13.1 PRINCIPLE OF OPERATION The CX synchro is similar to that used in the torque synchro system. The control transformer has a stator, which in design and appearance resemble the synchro units already discussed but with high impedance coils to limit the alternating currents through the coils. Further differences in the CT are that the rotor winding has its coils wound so that no torque is produced between it and the stator magnetic fields and the rotor is not energized by the supply voltage applied to the rotor of the control synchro. The CT rotor acts as an inductive winding for determining the phase and magnitude of error signal voltages. The signals, after amplification, are fed to a two-phase motor, which is mechanically coupled to the CT rotor. A control synchro system is at electrical zero when the rotor of the CT is at 90° with respect to the CX rotor. This is the situation as shown in Figure 10 above.

If the input shaft is rotated and the CX rotor is disturbed, voltages are induced in the CX stator and currents flow down the transmission lines to the stator windings S1, S2 and S3 of the CT. A magnetic flux is produced, depending on the amount of displacement of the CX rotor and the orientation of its displacement. This flux links with the rotor of CT, inducing a voltage into it, again depending on the amount, or rate of displacement, and its orientation. The voltage, or error voltage, representing the electrical difference between the rotors of CX and CT, is then amplified and passed to the control phase of a two-phase motor. The ac reference phase supply is fixed. The motor now rotates. Its direction depends on the phase of the error signal, as can be seen from Figure 12.

APPLIED VOLTAGE

CLOCKWISE ROTATIONVOLTAGE IN-PHASE

ANTI-CLOCKWISE ROTATIONVOLTAGE OUT-OF-PHASE

Phase Error Signal

Figure 12 As it rotates, the motor drives the rotor of CT in such a direction as to reduce the error voltage to zero and the new position is reached. By using the error signal amplified by a servo amplifier, a servomotor can be driven to move a control surface as in Figure 11.

1.14 DIFFERENTIAL SYNCHRO There are two types of differential synchro system:

♦ Torque.

♦ Control. In each, a special type of synchro is inserted between the synchros of the basic torque or control systems. It is called a ‘differential synchro’ and differs from the basic synchros in that it has a three-phase stator and rotor. In a torque differential system it is abbreviated to TDX and in a control differential system, CDX. The inclusion of this synchro between a torque transmitter and receiver or control transmitter and transformer permits an additional input to be algebraically added to, or subtracted from, the system. The layout of a differential synchro and its circuit symbol are shown at Figure 13.

S2

S1 S3

R2

R1 R3

ROTOR

STATOR

CIRCUIT SYMBOL

S1

S2

S3

R1

R2

R3

Differential Synchro

Figure 13

Figure 14 shows the construction of a differential synchro

STATORASSEMBLY

ROTORASSEMBLY

SKEW CUT TOENABLE SMOOTHER

RUNNING

STATORCONNECTIONS

STATORWINDINGS

ROTORCOILS

Differential Synchro Construction

Figure 14

1.15 TORQUE DIFFERENTIAL SYNCHRO Figure 15 shows a differential synchro system set up for the SUBTRACTION of two inputs.

INPUTSHAFT 60º

INPUTSHAFT 15º

OUTPUTSHAFTθ1 – θ2

TX

TDXTR

60º 45º15º

60º

45º

Torque Differential Synchro

Figure 15 Note that the rotors of the normal transmitter TX and receiver TR are supplied in parallel with the single-phase ac supply. The stator windings of the TX are connected to the stator windings of the TDX and its three rotor windings are connected to the three-stator windings of the TR. The rotor of the TDX is not energized by the ac supply. The circuit is such that one input shaft turns the TX rotor and the second input shaft drives the TDX rotor. The TDX receives an electrical signal corresponding to a particular angular position of the TX rotor, which it modifies by an amount corresponding to the angular position of its own rotor. This modified signal appears at the TDX output and is transmitted to the receiver, where it produces an angular flux, which is the difference of the rotor angles of the two transmitters TX and TDX. If the TDX rotor is locked in one position, the TX/TR chain acts as a normal torque synchro system with a transformer placed between TX and TR.

1.16 CONTROL DIFFERENTIAL SYNCHRO Figure 16 illustrates a control differential synchro system.

INPUTSHAFTθ1

INPUTSHAFTθ2

OUTPUTSHAFTθ1 – θ2

CX CDX CT

ERRORSIGNAL

Control Differential Synchro

Figure 16 As with the straight control synchro system, the ac supply is only applied to the transmitter rotor. The transformer rotor produces an error signal, which after amplification is applied to a motor, causing the CT rotor to move. Apart from these differences the action of the control differential transmitter is the same as for the torque differential synchro system. Torque differential synchros have been used to combine a direction finding loop reading and a compass reading, in navigation systems, to give a true bearing. Control differential synchros, combined with servomotors, are used for moving much heavier loads such as radar scanners where the subtraction or addition of two inputs may be necessary.

1.17 RESOLVER SYNCHRO This type of synchro is used to convert voltages, which represent the CARTESIAN co-ordinates of a point, into POLAR co-ordinates and vice versa. 1.17.1 POLAR AND CARTESIAN CO-ORDINATES A vector, representing an alternating voltage, can be defined in terms of ‘r’ and the angle it makes with the X-axis: angle (θ). These are the polar co-ordinates of the vector written as r/θ. Figure 17 shows the vector diagram for Polar and Cartesian co-ordinates.

θ

r

X

Y

POLAR CO-ORDINATES = r/θ

CARTESIAN CO-ORDINATES X = r COS θ

CARTESIAN CO-ORDINATES Y = r SIN θ

Polar & Cartesian Co-ordinates

Figure 17

1.17.2 RESOLVER SYNCHRO OPERATION The resolver synchro consists of a stator and rotor, each having two windings arranged in phase quadrature as shown in Figure 18.

INPUT SHAFT S2

S1

S3

S4

R2

R1

R4R3

ROTOR STATORR1

R2

R3 R4

S1

S4S3

S2

a

b

Resolver Synchro Figure 18

Figure 16b represents the resolver differently for ease of explanation. The resolver has two coils, R1 R2 and R3 R4 at right angles to each other and attached to an input shaft. The stator consists of two coils S1 S2 and S3 S4, also placed at right angles to each other.

1.17.3 CONVERSION FROM POLAR TO CARTESIAN CO-ORDINATES For this purpose, one of the resolver coils is short-circuited, say R3 R4, and the other, R1 R2, has an alternating voltage applied to it. The magnitude of this voltage (r) and the angle (θ) through which both rotor coils are turned, represent the polar co-ordinates r/θ. Figure 19 shows a resolver synchro to carry out this function.

θ

ROTOR FLUXR1R1

R2R2

R3R3 R4R4

S1S1

S4S4S3S3

S2S2

R SIN θ

90º 180º 360º270ºθ

R

R COS θ

MAXVOLTS

NOVOLTS

STATOR

Polar to Cartesian Co-ordinates Figure 19

Consider firstly that the rotor shaft position is such that the R1 R2 coil magnetic field links completely with the stator winding S1 S2, i.e. the coils are aligned. The maximum voltage will therefore be induced in coil S1 S2. Since the stator coil S3 S4 is at right angle to stator coil S1 S2, there will be no voltage developed across it due to R1 R2 coil's magnetic field. When the shaft is rotated at constant speed through 90°, the rotor coil R1 R2 is now in phase quadrature to stator S1 S2, which has zero volts induced in it. However, R1 R2 rotor coil is now aligned with stator coil S3 S4 and this now has maximum voltage induced in it. As the shaft continues to rotate, a cosine voltage wave

is developed across S1 S2 stator and a sine voltage wave across S3 S4 stator

cos’ and ‘r sin’ summed together result from the input voltage at R1 R2 and

ge lue of the alternating flux. Its value may be

Cartesian to Polar Co-ordinates Figure 20

coil. ‘rrotor rotation r/. The result represents the cartesian co-ordinates. 1.17.4 CONVERSION FROM CARTESIAN TO POLAR CO-ORDINATES In this arrangement, there are two voltage inputs and these represent the cartesian co-ordinates. They are VX = r cos and VY = r sin θ (Refer Figure 15). VX is input to S1 S2; VY is input to S3 S4. The two together develop an alternating magnetic flux representing the cartesian co-ordinates in the stator. R1 R2 is connected to an amplifier, which drives the output load and the rotor in such a direction as to null the rotor and stop the motor. R3 R4 has a voltainduced in it dependent on the vacalculated using Pythagoras' Theorum √VY² + VX² . Figure 20 shows the layout for performing the above.

R4R3

S1S1

S2S2

θ θS3 S4

R1

R2 SM

TO LOADVX = r COS θ

VY = r SIN θ VY 2 + VX2

R4 R2

CIRCUIT SYMBOL

R1

R3

S1

S3

S4 S2

1.17.5 USE OF RESOLVER SYNCHROS The ability to develop receiver signals at 90° is used, for example, in VOR systems, ADF systems using a non-rotating loop, in autopilots and in flight directors. 1.18 E AND I BAR TRANSMITTER Figure 21a shows an E and I bar transmitter. These devices convert mechanical movements into electrical signals (transducer) and are used in various systems as required. Figure 19a shows an E and I bar as applied to a servo-altimeter.

A.C. EXCITATION

SUPPLY

RESULTANTWAVEFORM

a b

E & I Bar Transmitter

Figure 21 The ‘E’-bar has a coil wound round the centre limb. This coil is supplied by an ac excitation supply. A magnetic flux is set up within the ‘E’-bar and when the ‘I’-bar is equidistant from the outer limbs of the ‘E’-bar, the waveforms transmitted are equal and opposite (Figure 21b). No output results. If the ‘I’-bar is moved (in this case by capsules) one end of the ‘I’-bar is brought in

closer proximity to the opposite limb of the ‘E’-bar. The air gap here is reduced, the magnetic field strengthens and the signal from the upper limb oil is increased. (Figure 21b).

r

‘E’ -bar back to the position nulls the signal so that no signal is produced.

c The opposite end of the ‘I’-Bar moves further away from its associated ‘E’-balimb, and the resultant signal is weaker. In the case of the servo-altimeter, moving the