Syn & Servo

NATIONAL TRAINING MATERIALS FOR THE AEROSPACE INDUSTRY

AVIONICS

MODULE NAA09 Synchros and servomechanisms

SECTIONS 1- 4 Flexible Delivery Student Learning Materials for Apprentice Aircraft Engineers / Mechanics

Managing Agent: Department Training and Education Coordination, NSW. (DTEC)

ACKNOWLEDGMENTS Written by Brian Camp Produced by The Learning Design Centre Kangan Institute of TAFE PO Box 299 Dallas Vic 3047

© Australian National Training Authority (ANTA) 1997 Published by: Australian Training Products Ltd (formerly ACTRAC Products Ltd) All rights reserved. This work has been produced initially with the assistance of funding provided by the Commonwealth Government through ANTA. This work is copyright, but permission is given to trainers and teachers to make copies by photocopying or other duplicating processes for use within their own training organisation or in a workplace where the training is being conducted. This permission does not extend to the making of copies for use outside the immediate training environment for which they are made, nor the making of copies for hire or resale to third parties. For permission outside these guidelines, apply in writing to Australian Training Products Ltd.(formerly ACTRAC Products Ltd). The views expressed in this version of the work do not necessarily represent the views of ANTA. ANTA does not give warranty nor accept any liability in relation to the content of this work. GPO Box 5347BB, MELBOURNE, Victoria 3001, Australia Telephone +61 03 9630 9836 or 9630 9837; Facsimile +61 03 9639 4684 First Published October 1997 STOCKCODE : DP 5010 A09 WBK Printed for Australian Training Products Ltd (formerly ACTRAC Products Ltd) by Document Printing Australia

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Contents ○

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Module introduction

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vii

Before you start

vii

How this module is organised

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References

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Glossary

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Section 1

Section 2

Error detection devices Learning outcome 1

1–1

Introduction

1–2

Error detector systems

1–2

Activity 1

1–21

Review

1–22

Check your progress 1

1–23

DC synchronous systems Learning outcome 2

2–1

Introduction

2–3

DC synchronous systems

2–3

Selsyn system operation

2–7

Desynn system operation

2–9

Testing and inspection of DC synchronous systems

2–13

Applications of DC synchronous systems

2–14

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Section 3

Section 4

iv

Activity 1

2–15

Review

2–16

Check your progress 2

2–17

AC synchronous systems Learning outcome 3

3–1

Introduction

3–3

AC synchronous systems

3–3

Torque synchro system

3–9

Activity 1

3–11

Inspection, testing and fault finding

3–25

Control synchro system

3–25

Inspection and testing

3–31

Synchrotel

3–31

Resolver synchro

3–35

Conversion from polar to cartesian coordinates

3–36

Activity 2

3–45

Review

3–47

Check your progress 3

3–48

Servomechanism systems Learning outcome 4

4–1

Introduction

4–3

Servomechanism

4–3

Terms associated with servomechanisms

4–3

Open loop and closed loop systems

4–8

Types of servomechanisms

4–9

Activity 1

4–19

Servomechanism systems

4–20

Causes of hunting

4–24

Activity 2

4–25

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Inspection and test

4–26

Review

4–32

Check your progress 4

4–33

Module review Learning outcomes checklist

R–1 R–1

Answers to activities Section 3

A–1

Section 4

A–2

Answers to check your progress questions Check your progress 1

C–1

Check your progress 2

C–2

Check your progress 3

C–4

Check your progress 4

C–5

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Module introduction ○

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The purpose of this module is to introduce you to the principles and applications of synchronous indicating devices and the servomechanisms they control. This module provides the background knowledge required so that a student can, with further knowledge, meet the National Aerospace Competency Standard listed: A11, A21 and A25. Modern day aircraft have numerous systems operating over which pilots need to maintain some form of control and monitoring. To try and have each of these systems displayed in the cockpit would require a very large amount of flexible cabling and mechanical indicating devices. A synchro system is a remote indicating system, in which the needle of an indicator moves in synchronism with the device being monitored. Because the needle and sending device are electrically connected, the indicating system is much lighter, more efficient and more reliable. These synchro systems can also be used to control servo systems, which will drive various aircraft systems.

Before you start Before you begin this module you should have completed: • NAA07 Electrical Principles 2: AC • NAA08 AC Machines and Polyphase Systems.

How this module is organised This module should take you about 40 hours to complete if you study full time in a classroom. If you are completing this module in your workplace or at home, you can work at your own pace.

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Each section includes: • reading materials • activities • check your progress questions. These activities and questions will assist your learning. Answers to activities and check your progress questions are at the end of the module. Because you may be working on different aircraft or systems, you may need to discuss some answers with your trainer. This module is divided into sections each covering one learning outcome: • error detection devices • DC synchronous systems • AC synchronous systems • servomechanisms.

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References ○

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The information provided in these module notes should provide you with enough knowledge to meet the assessment criteria. If you would like to do some more reading on these topics or other related topics, here are some suggested learning resources. Airframe and Powerplant Mechanics, Airframe Handbook, EA-AC65-15A, Federal Aviation Administration Publications, Washington DC, USA. Aviation Technician Integrated Training Program, Avionics Fundamentals, EA-AV, Aviation Maintenance Publishers Technical Publications, USA, 1987. Aviation Technician Integrated Training Program, Airframe Textbook, EA-ITP-A2, Aviation Maintenance Publishers Technical Publications, USA, 1992. Civil Aviation Safety Authority, Civil Aviation Safety Authority - C.A.O. 108.56 and 108.6 Peters, D., Aircraft Maintenance Text 4: Basic Functional Devices and Systems. Australian Government Publishing Service, Canberra, 1989. Pallett, E.H.J., Aircraft Instruments, Third edition, Pitman Publishing, London, 1987. Pallett, E.H.J., Automatic Flight Control, Second edition, Granada Publishing, 1983. These books may be available in your local library or TAFE library. Your training supervisor will be able to assist you to find the appropriate books.

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

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Glossary ○

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Here is a list of terms that you may come across for the first time in your study. This list is not a complete list, so we have left you some space at the end to add in any words that you come across which you do not understand. Accelerometer

A device which detects acceleration of the aircraft in its plane of sensing.

Damping

A force applied to a system to control oscillations around a required position.

Desynn

A DC self synchronous system using a set of coils wound on an iron core and connected up in a delta formation.

Differential

The difference in two readings.

Differential transformer

A transformer with two windings, a primary and secondary. The secondary is wound in two sections opposing each other.

Error signal

This signal is produced by the error detector, it is the difference between the required output position, and the position that the output is actually in.

Feedback

The actual position of the output is feedback so that it may be compared with the required output.

Mechanism

A system of mutually adapted parts working together.

Micro sensor

A resistive sensing device capable of reacting to very small movements.

Null

The term null is used to describe the condition where there is no error signal being produced by the error detection device.

Pendulous monitor

A position sensing device which uses a freely suspended pendulum as its sensor.

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Reference signal

This signal is supplied to the amplifier to enable it to determine in which phase the error signal is in. It is the phase relationship which will determine in which direction the servo motor will drive.

Remote position control servos

These are used to control angular or linear position of a load and can be used to rotate a load such as a control surface.

Selsyn

A DC self synchronous system using a set of coils wound on an iron core and connected up in a star formation.

Servo device

A power driving device usually electric or hydraulic which can produce motion or forces at a higher level of energy than the input level and be used to move a heavy part of the aircraft structure.

Slab sensor

A resistive sensor with the resistor wound on a curved former in order for it to produce a sinusoidal output.

Summing point

A point into which signals may come from as many as three directions but go out only in one direction.

Synchronous

A function which occurs at the same time as some other function.

Synchronous system

An indicating system where the transmitter and indicator are connected together electrically in such a way that the position of the indicator will always be a copy of the transmitter position.

System alignment

For the system to work correctly the controlling transmitter, the load, and the feedback transmitter must be aligned at zero. In this way, when the controller calls for, shall we say 2 degrees of movement, when the load reaches 2 degrees the output and input signals will be null.

Thermistor

A resistor whose resistance changes with a change of temperature. It can have either a positive or negative characteristic.

Torque

A force tending to cause rotation.

Toroidal resistor

A resistor wound on a circular former.

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.

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

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1 Error detection devices ○

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Learning outcome 1 At the end of this section, you should be able to describe the construction and operation of error detection devices.

Assessment criteria You will have achieved the learning outcome when you can: • identify the following error sensing devices: • differential transformers: – LVDT – E and I bar – C and Y • pendulous monitors (accelerometers) • inductive • capacitive • resistive • describe the construction of the error sensing devices listed above • describe the operation of the error sensing devices listed above.

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1–1

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Introduction In the operation of synchro-servo device systems, it is necessary to be able to plot the changes in positions and forces that take place in the various components making up the system. To do this, devices which are capable of detecting these changes must be built into the system, one device being used to detect each item monitored. In this section you will learn about the construction and operation of the error detecting devices which make up these systems.

Error detector systems Servo mechanism systems are used for the measurement and/or control of such aircraft systems as cabin temperatures, fluid pressures, fuel flow rates, radar antenna positions and many more. In its simplest form, the detector monitors two positions or two voltages, one of which is usually a control and the other variable. The output is either the sum or difference of the two measured positions and becomes the error signal which can be applied to an amplifier, creating an output signal to drive a servo of some type. The error detecting devices can take many forms, among them being: • differential transformers • pendulous monitors • inductive transducers • capacitive transducers • resistive networks.

Differential transformers The differential transformer is similar in design to a standard transformer, in that it has a primary and secondary winding. However the secondary winding will always be in two sections, which are connected in series, opposing each other. The position of a moveable iron core determines the phasing and magnitude of the secondary output. Figure 1.1 below shows the basic format of a differential transformer.

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Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Figure 1.1: Differential transformer with the core centred

Operation With the core in the central position, the magnetic field created by the primary will link evenly with both windings. The EMF induced in each secondary will be of equal value, but opposite in phase. Figure 1.1 shows the induced EMF of the secondaries. The nett result will be a zero output from the secondaries. If the iron core is moved up so that it is linking more closely with secondary A, the induced EMF in secondary A will exceed the induced EMF in secondary B. The output will be in the phase relationship of secondary A, with a value equal to the difference in the two induced EMF and 180 degrees out of phase with the input voltage. Figure 1.2 shows the output with the iron core linking secondary A.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

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NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Figure 1.2: Differential transformer with the core linking secondary A

If the iron core is moved down so that it is linking more closely with secondary B as shown in Figure 1.3, the induced EMF in secondary B will exceed the induced EMF in secondary A. The output will be in the phase relationship of secondary B, with a value equal to the difference in the two induced EMF and in phase with the input voltage.

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Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Figure 1.3: Differential transformer with the core linking secondary B

In all the examples above, the value of the output will be governed by the degree of linking of the iron core between the primary and each secondary winding. The basic differential transformer we have just looked at, can be made in many different forms, depending on the method by which the magnetic linkages are arranged.

The linear variable differential transformer (LVDT) In this type of transformer the primary and secondary coils are wound on hollow cores to allow a moveable iron core to pass through them. The core is generally connected to the device being measured, which gives a linear movement of the core. With the core centred, the magnetic field will link evenly with both coils, giving approximately equal output EMF from both sets of coils. The nett error signal will be almost zero. Minor differences in the characteristics of the coils will always give some small output, because of phase differences, but this can be reduced to negligible values by careful positioning of the core. Figure 1.4 shows the construction of the linear variable differential transformer.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

1–5

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Movable core Secondary

Secondary

Primary

Error output Figure 1.4: The linear variable differential transformer

As the core moves to one side, the magnetic linkage will transfer to that side increasing the induced EMF on that side and decreasing it on the other side. The nett output signal will have a linear relationship to the amount of movement of the iron core.

The E and I bar transformer This device is so called by the shapes of the components. The transformer coils are wound on the legs of the E core with the primary on the centre core and the secondaries on the outer cores. The I bar may be pivoted at the centre. It is generally actuated by linear devices, although it can be adapted to limited circular movement. When it is moved toward one end, the reduced air gap will create a stronger magnetic linkage with that end, giving an output signal relative to the end in contact. The amplitude of the error signal will depend on the amount of rotation of the I bar. Figure 1.5 shows the form of the E and I bar transformer.

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Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Figure 1.5: The E and I bar transformer

An application of the E and I bar is in an acceleration and side slip sensor. When an aircraft maintains an attitude change which is less than one which can be sensed by the gyros, an acceleration sensor can provide an output in a direct relationship to the attitude change. An I bar, suspended on springs in the sensing axes, is able to sense acceleration in that plane. Under constant velocity, the I bar will maintain its position giving a zero output from the secondary. If acceleration or deceleration forces are detected, the I bar will be displaced as a function of the acceleration forces acting upon it. This will induce an EMF in the secondary in the way we have already described. This EMF will be a signal, which will carry details of the displacement. After application to an amplifier, it will provide power to the relevant servomotor to correct for the change of attitude. Figure 1.6 shows the E and I bar acceleration sensor.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

1–7

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Figure 1.6: E and I bar acceleration sensor

The C and Y bar transformer This transformer, like the E and I bar transformer, is operated by the magnetic coupling between the primary and secondary windings. In this transformer the core takes the shape of a Y, much like the traditional core used for a star transformer. The primary is wound on one leg and the opposing secondaries are wound onto the other two legs.

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Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

The linking iron core takes the form of a C, which is moveable around the outside of the windings and is centred on the primary winding. It has the advantage of being able to be actuated by circular motion. Figure 1.7 below shows the basic form of the C and Y transformer.

Figure 1.7: The C and Y transformer

Operation With the iron core in the central position, the magnetic coupling between the primary winding and the two opposed secondary windings, will be exactly equal, hence inducing equal but opposite EMF. The nett output signal from the secondary will be zero. Rotation of the iron core in one direction will cause the linkage on the exposed leg to be reduced and the linkage on the more enclosed leg to be increased. The output signal will be in phase with the exposed leg and equal to the difference in the values of the induced EMF’s. Rotation of the iron core in the opposite direction will give exact opposite output to the one described above. Figure 1.8 shows the outputs obtained from the C and Y transformer.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

1–9

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Figure 1.8: The output from a C and Y transformer

Pendulous monitors (accelerometers) A pendulum monitor is a device designed to monitor the attitude or acceleration of an aircraft. It can be used to detect long term attitude changes by reacting to the actual movement of the aircraft rather than the rate of movement. It is an extremely sensitive device capable of reacting to attitude changes too small to be detected by the primary reference devices. It can also react to a side slipping motion of the aircraft. The heart of the monitor is a pivoted pendulum frame, which moves outside of a core which is wound with a primary and secondary winding. In the vertical or neutral position, the magnetic flux links through both ends of the core equally, resulting in a zero induced EMF in the secondary winding. As the aircraft tips to one side the pendulum is displaced. The magnetic field is now deflected through one side of the core inducing an EMF in the secondary. The value of the induced EMF is proportional to the angle of deflection of the pendulum. If the aircraft is deflected to the opposite side, the polarity of the induced EMF will be reversed. Figure 1.9 shows the actions of the pendulum monitor.

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Figure 1.9: Actions of the pendulum monitor

Inductive sensors You will remember from NAC06 and NAA07 that if we add an iron core to an inductance coil, the inductance increases. We can use this characteristic to make a very effective inductive sensor for linear position sensing applications. The iron core is connected to the sensed item and the coil connected either into a bridge circuit or as part of an oscillator circuit. An E and I bar type can be used in this type of detector. The output polarity and value of the bridge will then be a measure of the position of the core. If the coil is used in an oscillator circuit, the mechanical position of the core determines the frequency of oscillation. This will be sensed and converted into a mechanical position signal which can be applied to another circuit or system. Figure 1.10 shows the variable inductance error sensor.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1–11 © Australian National Training Authority (ANTA) 1997

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Figure 1.10: Inductive sensors

Capacitive sensors In a capacitor, the distance between the plates directly varies the capacitance. In the capacitive sensor, one plate of the sensor is connected to the function being sensed. Like the inductive sensor, the capacitive sensor can be connected into a bridge circuit or an oscillator circuit. Movement of the plate will vary the capacitive reactance and give either an output from the bridge or a change in frequency of oscillation, which is then converted into a measure of the mechanical position of the capacitor plate. Figure 1.11 shows the capacitive sensor.

1–12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 1: Error detection devices © Australian National Training Authority (ANTA) 1997

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Movable plate

Fixed plate

(C)

Capacitance varies with position of movable plate Figure 1.11: The capacitive sensor

Resistive sensors Variable resistors A variable resistor can be used to detect the position of a particular device. For example, a flap actuator has a linear movement to extend the flaps. Coupled to that is a lever which moves the wiper arm of a variable resistor. The output from the wiper arm is connected to a voltmeter calibrated to read in degrees of flap extension. Figure 1.12 shows a simple resistive detector circuit.

Figure 1.12: Simple resistive sensor

Circular resistors To enable a rotary position to be transmitted to a device such as a Desynn system, a circular resistance network is established. The outputs from the points 1, 2 and 3 when applied to a three coil receiver, will create a magnetic field which will always be in the same position as the sender wiper arm. Figure 1.13 shows a simple circular resistive network.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1–13 © Australian National Training Authority (ANTA) 1997

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Figure 1.13: A simple circular resistive network

Micro sensors When the movement available to actuate the sensor is very small, the basic resistive network can be modified into a device called a micro sensor, which will still give maximum electrical output with a minimum of mechanical movement of the wiper arms. Imagine that two circular resistances, called toroidal resistors are joined up in parallel, with the wiper arms insulated from each other but linked together, one wiper arm on each resistor. They will operate as one resistor, with movement of the wiper arms providing full 360 degrees of sensing. Figure 1.14 shows the theoretical construction of the micro sensor.

1–14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 1: Error detection devices © Australian National Training Authority (ANTA) 1997

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Figure 1.14: A theoretical micro sensor

This circular arrangement of resistors is altered to give it a linear movement by cutting the outer resistor at point 3 and the inner resistor between points 1 and 2 and opening them out into a straight line. By interconnecting the pickoffs as shown in Figure 1.15 we will have the three tappings arranged so that 45 degrees of movement of the sensing device, will move the wiper arms over the length of the resistors, equalling 360 degrees of electrical movement. This arrangement is used to reduce friction errors. The wire is wound on a square or circular former, the square being the most efficient.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1–15 © Australian National Training Authority (ANTA) 1997

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2

1

3

2 3 B

1

3

2

Figure 1.15: The linear micro sensor

Linear micro sensors Standard square shaped coils create errors called cyclic errors, which are accentuated by other errors due to the friction of the wiper arms moving over the windings large area of resistance wire. None of these can be completely removed. By modifying the shape of the winding former, the sawtooth characteristics of the output can be converted into a sinewave. This transmitter/sensor is called a slab sensor, from the shape of the resistance winding. The wiper arms become the moveable contact arms, with the power supply connected to the resistance winding. Figure 1.16 shows a slab winding. As the wiper arms rotate, the output taken from the pickoffs will produce waveshapes similar to a three phase wave, which will be a function of the angle of rotation of the wiper arms.

1–16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 1: Error detection devices © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Figure 1.16: The slab winding

Temperature controlled resistive devices A thermistor is a temperature sensitive resistive device, which can have either a positive or negative temperature coefficient of resistance. Because of their high sensitivity they can be used in a bridge circuit, whose output then becomes a function of the sensed temperature. Figure 1.17 shows a temperature controlled resistive sensor.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1–17 © Australian National Training Authority (ANTA) 1997

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Figure 1.17: A temperature controlled resistive sensor

A change of temperature will give an output from the bridge whose polarity and magnitude is a direct function of the sensed temperature. This output can be then applied to a differential amplifier for use in driving a servo in another part of the circuit.

Linearity of resistive sensors No two applications of resistive sensors are exactly the same in their requirements. The resistors required for one system may need to give a purely linear output, for another a non-linear output, for a third, a sinusoidal output may be required. For example, in the slab Desynn, the output required is sinusoidal.

1–18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 1: Error detection devices © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

To achieve this, the type and layout of the winding can be designed to achieve the required output. This will include the use of trimmer resistors connected in different positions in the circuit and will also be determined by whether the actuation is linear or circular. Ways of achieving the required characteristics can be: • winding the resistance wire with uneven spacing over its length • changing the wire size over the length of the resistor • substituting different wire types at intervals along the resistor • designing the shape of the card to match the resistance required • using stepped cards, having a different size or type of wire on each step • in film type resistors, a non uniform film gives the required characteristics • in a square card potentiometer, rotation of the slider gives a sinewave output. Any of these techniques can be combined to give the required resistance values. Typical examples of different resistor types are shown in Figure 1.18.

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1–19 © Australian National Training Authority (ANTA) 1997

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Figure 1.18: Examples of non-linear resistance elements 1–20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 1: Error detection devices © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Activity 1

Examine an aircraft available to you for error detectors used for devices such as flaps, cowl flaps, heater control valves, altitude hold detectors etc. Briefly describe the type of detectors. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1–21 © Australian National Training Authority (ANTA) 1997

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Review Before you move on to Section 2, work through the Check your progress questions to see how well you understood Section 1. If there is anything you are not sure of, revise the relevant work before you begin the next section. If you would like additional information to help your understanding of any part of this section, use the reference books listed at the beginning of this module. When you are satisfied with your progress, move on to Section 2, which covers DC synchronous systems.

1–22 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 1: Error detection devices © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Check your progress 1

1

Describe where the output signal from an error detection device can be used (for example: a/p, flight director). _________________________________________________________________ _________________________________________________________________

2

The secondary winding of the differential transformer has an unusual characteristic. Describe this winding. _________________________________________________________________ _________________________________________________________________

3

Briefly describe the construction of the LVDT. _________________________________________________________________ _________________________________________________________________

4

An output can be obtained from an E and I bar error detector by _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

5

When used as an acceleration sensor, the I bar is actuated by _________________________________________________________________

6

Describe the methods of actuation available for the C and Y transformer. _________________________________________________________________ _________________________________________________________________

Section 1: Error detection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1–23 © Australian National Training Authority (ANTA) 1997

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7

The main advantage of the pendulous monitor is that it can _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

8

True or false? The position of a moveable iron core can be sensed by a frequency sensing device in an inductive sensor. Explain how this is done. _________________________________________________________________ _________________________________________________________________

9

True or false? The mechanical position of the sensed item can be determined by sensing the distance between the plates of a capacitor. Explain how this can be done. _________________________________________________________________ _________________________________________________________________

10

The easiest way to display the position of a wiper arm from a simple variable resistive sensor is to apply it to a ____________________________

11

A circular resistive network can be used when it is required to _________________________________________________________________

12

When the movement available to actuate a sensor is only very small, the resistive sensor is modified into a device called a _____________________ _________________________________________________________________

13

The cabin air temperature can be controlled by using a _______________ as part of a bridge network.

1–24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 1: Error detection devices © Australian National Training Authority (ANTA) 1997

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2 DC synchronous systems ○

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Learning outcome 2 At the end of this section, you should be able to describe the operation of DC synchronous systems and test them for serviceability.

Assessment criteria You will have achieved the learning outcome when you can • identify the different DC synchronous systems: • Selsyn:

– two coil – three coil • Desynn:

– slab – micro • describe the operation of the Selsyn synchronous system: • two coil • three coil • limitations • describe the operation of the Desynn synchronous system: • slab • micro • limitations

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2–1

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• inspect and test a DC synchronous system and troubleshoot as required • identify which indications DC synchronous systems are used to indicate: • position • pressure.

2–2

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Introduction The pilot of an aircraft needs to know what is happening in the services of the aircraft during flight. Unfortunately it is often difficult to get that information because the items are located away from the cockpit. For example, flap positions are critical to safe flight and automatic flight controls interact with their controlling computers. This type of information and numerous other physical quantities have to be continually monitored for the systems to work correctly. A direct mechanical linkage, such as flexible drive, between the component and its indicator or computer have been used in the past, but in today’s modern aircraft, long runs of flexible drives are no longer used because they are: • inaccurate • inefficient • costly to install and maintain • cause a weight penalty. This can be done much more efficiently by electrical remote indicating systems usually called data transmission systems. In the electrical remote indicating systems, the movements of an input shaft are converted into a suitable electrical signal by one of the error detectors described in Section 1. There are two main methods of transmitting data, the first by using an DC powered system, the second by using an AC powered system. In this section we will be looking at the DC powered system.

DC synchronous systems In any synchronous system, the alignment of the input shaft is converted into an electrical signal, which can be transmitted to a receiver, where an indicator device will be moved to directly mirror the position of the input shaft. DC systems have largely been superseded by AC systems, however many light aircraft and older aircraft still use them. A study of them is included here because an understanding of the simple DC systems will make understanding the AC systems much simpler.

General The term Selsyn is a contraction of self synchronous, and relates to many of the remote indicating systems which are used to transmit system information from remote areas of the aircraft to the cockpit instruments.

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2–3

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Trade names such as Selsyn and Desynn are commonly used to identify DC self synchronous systems. We will look at these in the following texts.

Selsyn systems Two coil Selsyn This system like most remote indicating systems consists of three main parts. These are: • the indicator • transmitter • interconnecting wiring. An example is shown in Figure 2.1 below. Indicator The indicator consists of a laminated iron core which has a small air gap cut through its circumference to establish a high reluctance path to the magnetic field. Two field coils are mounted on the core 120 degrees apart and 120 degrees from the air gap. The moving element is a small permanent magnet rotor to which the pointer is fitted. The rotor is surrounded by a non ferrous damping ring that assists in providing a smooth operation. Transmitter The transmitter is a circular resistance strip over which a wiper moves under the control of the medium being measured. This motion varies the voltage that is applied to the junction of the two coils of the indicator to control the level of current that flows in each coil, that is, establishes a ratio of currents between the coils that is dependant upon the wiper arm position. As the applied system voltage is fed to both the transmitter resistance and the indicator coils which are connected in series, minor variations of supply voltage will have little or no effect.

2–4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2: DC synchronous systems

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Open circuit in winding here Airgap

Damping ring Pointer

Movable wiper

-

+

Transmitter

Indicator

Figure 2.1: The two coil Selsyn system schematic

Three coil Selsyn The three coil Selsyn system like the two coil system consists of: • indicator • transmitter • interconnecting wiring as shown in Figure 2.2 below. Indicator The indicator consists of a circular laminated iron core on which are mounted three separate coils spaced 120 degrees apart. The coils are electrically connected into a delta type wiring arrangement, with three connecting leads being taken away to corresponding points on a toroidal wound transmitter resistance unit. The indicator rotor is a circular permanent magnet mounted on the pointer shaft. The rotor is free to rotate within a damping ring and the laminated coil frame. A weak magnet is mounted below the coil assembly to move the indicator rotor assembly and pointer off scale when the power is removed. Transmitter The transmitter is a toroidal wound resistance having tappings 120 degrees apart. Two wiper brushes, insulated from each other move around the resistance unit 180 degrees displaced from each other carrying direct current into the windings.

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2–5

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The wiper brush unit is moved by the medium being measured and the power supply from the aircraft bus-bar is positioned around the resistance as it moves. The positive is connected to one brush, and negative to the other brush. +

Laminated iron core with 3 coils placed 120° apart

Rotatable contact arm

Rotatable magnetised core Resistor with taps equally spaced Unlimited rotation transmitter

Indicating element

Figure 2.2: The three coil Selsyn schematic

The Desynn indicating system This system is similar to the three coil Selsyn with the exception that this system connects the stator coils in a star or wye arrangement. The system is used for the indication of position and pressure. The circuit arrangements are such that there are several types of transmitter units available for special applications. These may be classified as follows: • a basic rotary motion, or toroidal resistance transmitter for position indication • slab Desynn rotary motion transmitter system which is a variation of the basic system and was also designed for pressure measurement • a micro Desynn or linear motion transmitter system, which is employed when the available mechanical movement is small, as is the case for pressure measurement. These were introduced to you in section under resistive error detectors, however we will look at them in more detail here. The principle of operation of all three types of transmitters remains the same, as each one was developed from the rotary motion, toroidal transmitter type.

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Selsyn system operation Two coil system Looking at Figure 2.3 below, you can see that as the wiper arm is moved in the transmitter towards the positive end of the resistance strip, the current in coil, C2 decreases while that in coil C1 will increase. Rotatable magnetised core

C

C

1

2

S

Open circuit in winding

Movable wiper arm

Damping ring

N

Laminated iron core Air gap

Power supply

Transmitter

Indicator

Figure 2.3: The two coil Selsyn operation

As the wiper arm moves across the transmitter resistance, the resultant magnetic field of the indicator, which mirrors the transmitter position, shifts from a point 60 degrees on one side of the air gap to 60 degrees on the other side of the gap. The field movement pulls the rotor magnet with it to change the pointer position. Wherever the wiper arm moves to, the magnetic field will follow. The function of the air gap in the laminated iron core is to prevent the magnetic field going around the core when only one coil is carrying current. When only one coil is magnetised the high reluctance path causes some of the field flux to travel across the ring.

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Factors affecting the accuracy of the Selsyn type indicating system are: • spacing of the turns on the transmitter resistance element • resistance matching of the indicator coils • positioning of the coils on the laminated core.

Three coil operation Looking at Figure 2.4 below, you can see that as the transmitter brushes are moved over the resistance by the medium being measured, power will be applied to two points around the windings. The voltages at the three transmitter tappings will be varied.

Figure 2.4: The three coil Selsyn operation

This in turn varies the value of current that flows in the indicator coils. The magnetic field that is created in each coil, establishes a resultant magnetic field that exactly mirrors the transmitter wiper arm position and attracts the indicator rotor to that position, moving the pointer across the indicator scale. Therefore the pointer position is dependant upon brush position which is in turn controlled by the variation in the measured medium.

Limitations With all Selsyn systems the pointer is capable of following the transmitter wiper brush position through 360 degrees, although in practice the indicator scale is limited to 180 degrees or 300 degrees depending upon the application. 2–8

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2: DC synchronous systems

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When measuring pressures, the pressure sensing device will be chosen to suit the pressure range required. Because both transmitter resistors and indicator coils are powered from a single source, minor voltage variations do not cause any significant errors.

Desynn system operation The basic Desynn system All of the Desynn systems in use are a development of the basic system. We will look at that first, so that the other system will be easier to understand. The standard Desynn indicator consists of a circular laminated stator on which is mounted the star wound, three phase winding. A two pole permanent magnet rotor is supported within a brass tube. The pointer is connected to the spindle and is capable of 360 degree of rotation. In general practice, the scales cover either 180 or 300 degrees. This type of indicator has a small weak permanent magnet called a pull off magnet, fitted to the end plate to act to move the pointer off scale should the power fail. The indicator is connected by three leads to the transmitter tappings on the toroidal resistance. The transmitter consists of a toroidal wound resistance having tappings 120 degrees apart. Two diametrically opposed wiper contacts form an arm that is rotated across the resistance by means of gearing and a lever connected to the medium being measured. The contact arm has the positive supply connected to one side and the negative to the other. As the arm is moved over the resistance the voltage is varied to the coils of the indicator.

Operation The movement of the transmitter arm causes the wiper contacts, which are insulated from each other, to create different potentials at the transmitter tappings. This in turn will cause current to flow to the indicator coils, setting up magnetic fields. These fields are dependant upon transmitter arm position and will attract the indicator rotor resulting in a change in pointer position to correspond to the new transmitter arm position. Supply voltage to this system is not critical because position of the magnetic field in the indicator is controlled by the relative strengths of the three line currents in the stator coils.

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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A 24 V system will work satisfactorily within the voltage range of 20 to 29 volts.

Figure 2.5: The basic Desynn system

The slab Desynn system As we saw in Section 1, this system was produced in an effort to remove the saw toothed output voltage characteristics of the basic system, which produces errors in the indications. The slab type construction produces an output which is a function of the sine of the angular displacement of the wiper arm from a fixed reference point. The three point contact increases the friction as compared to the wiper arm method of the basic system, however the use of good contact materials and burnishing of the resistance wire surface, has further reduced the overall frictional errors of the system.

Construction The transmitter resistance element consists of a slab former over which the wire is wound. One side of the slab is convex and it is over this surface that the contacts are positioned and moved by the medium being measured. The contacts are mounted upon a spindle and are spaced 120 degrees apart. Electrical connections are made via slip-rings to the indicator stator coil windings.

2–10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2: DC synchronous systems

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Operation The operation of the slab system is such that the output taken from the pickoffs can produce waveshapes similar to a three phase wave, which can be stopped at any point. The voltages picked up from the slab are transmitted to the indicator stator coils. The resultant current that flows in the stator coils, sets up a magnetic field which controls the position of the permanent magnet rotor of the indicator. The pointer, which is positioned on magnetic rotor shaft, provides an indication of the pressure being sensed.

N S +

-

N

24v DC supply

S

Slip rings

N S Indicator

Transmitter

Figure 2.6: The slab Desynn

In the application of pressure measurement, it is common for a Bourdon tube sensing element to be used to provide the motion of the wiper or contact arm spindle.

Micro Desynn This type of transmitter was briefly described in Section 1. It is different to the normal position type transmitter, in that it must be able to measure small linear movements and because of this its construction is quite different. However the principle of operation is the same and the indicator used is the same type as the basic Desynn uses. The transmitter is constructed from two cylindrical bobbins of resistance wire that are parallel to each other. The bobbins are tapped and electrically connected together to produce an output that is the same as the toroid. The arrangement is developed from the basic circular Desynn system shown in Figure 2.5 above.

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–11

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Figure 2.7: Concentric toroidal resistors

For the micro sensor, imagine that two concentric toroidal resistances have been cut at a point and laid out with the ends joined and three tappings made as before. It will be seen from Figure 2.8, that movement of the brushes is limited or one or the other brush would run off the resistance wire. The second resistance, provided with corresponding tapping points for each brush, will also have the movement limited to half the length of the resistance.

Figure 2.8: Micro sensor windings 2–12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2: DC synchronous systems

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

However if the tappings of the second resistance are repositioned so as to allow the brushes to be linked together, but still 180 degrees apart electrically, the arrangement as shown Figure 2.9 will be obtained.

2

1

3

2 3 + B -

1

3

2

Linear actuation

Figure 2.9: The micro Desynn

The brushes can now be moved together over the whole range of the resistance and such movement will correspond to one revolution of the contacts of a toroidal transmitter, resulting in 360 degrees of indicator pointer movement. The mechanical part of the transmitter consists of a bellows mounted in a housing which is connected to the source of pressure. Movement of the bellows under the influence of the pressure, is transmitted to the brushes by means of a push rod.

Limitations The limitations which applied to Selsyn systems a few pages back will also apply to Desynn systems

Testing and inspection of DC synchronous systems Any test or inspection of avionics equipment should be commenced with a thorough visual inspection for signs of: • overheating • cable abrasion • contamination • mechanical damage.

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–13

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The equipment manufacturers documentation will give you all the technical data needed to carry out voltage and resistance checks on any synchro device. This must always be referred to when testing or trouble shooting, do not rely on your memory or that little black book. When a fault occurs, test for output voltages and coil resistances as per the handbook. Once the faulty component has been identified, replace it with a serviceable item and re-test.

Applications of DC synchronous systems Selsyn systems are able to be used for either position or pressure measurement, depending on the type of sensing device employed. Desynn systems can also be used for either position or pressure measurement, however the slab and micro Desynn systems are particularly suited to pressure measurement because of their ability to react with small movements.

2–14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2: DC synchronous systems

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Activity 1

Referring to the detectors found in the Section 1 Activity 1, locate one on an aircraft, draw and describe the systems it is used in.

_________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–15

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Review Before you move on to Section 3, work through the Check your progress questions to see how well you understood Section 2. If there is anything you are not sure of, revise the relevant work before you begin the next section. If you would like additional information to help your understanding of any part of this section, use the reference books listed at the beginning of this module. When you are satisfied with your progress, move on to Section 3, which covers AC synchronous systems.

2–16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2: DC synchronous systems

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Check your progress 2

1

If the supply voltage to a 28 volt two coil Selsyn system drops to 24 volt when the aircraft is operating on batteries, what effect will this have on the operating characteristics of the system? _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

2

True or false? The field coils of a two coil Selsyn are mounted 60 degrees either side of the air gap in the iron core. Give reasons for your answer. _________________________________________________________________ _________________________________________________________________

3

True or false? A transmitter with closely packed fine resistance wire will be more accurate than one with larger more loosely packed turns. Give reasons for your answer. _________________________________________________________________ _________________________________________________________________

4

True or false? The indicator of the three coil Selsyn is restricted to 180 degrees of movement. Give reasons for your answer. _________________________________________________________________ _________________________________________________________________

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–17

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

5

Explain what will happen to the indicator of a three coil Selsyn when power is removed. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

6

Describe the differences between the basic Selsyn and Desynn systems. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

7

Draw and describe the rotor and pointer used for the slab Desynn indicator.

_________________________________________________________________ _________________________________________________________________ _________________________________________________________________

2–18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2: DC synchronous systems

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

8

Briefly describe the construction of the micro Desynn transmitter. Use a diagram to help you.

_________________________________________________________________ _________________________________________________________________ 9

Describe the types of mechanical devices best suited to operate the: slab Desynn _________________________________________________________________ micro Desynn _________________________________________________________________

10 Briefly describe the slab Desynn transmitter. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

Section 2: DC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2–19

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2–20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 2: DC synchronous systems

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3 AC synchronous systems ○

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Learning outcome 3 At the end of this section, you should be able to describe the operation of AC synchronous systems and test them for serviceability.

Assessment criteria You will have achieved this learning outcome when you can: • identify the basic AC synchronous systems: • autosyn • magnesyn • ratiometer • describe the operation of a torque synchro system: • synchro transmitter (TX) • synchro receiver (TR) • differential transmitter (TDX) • symbols • purpose • null point

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3–1

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• inspect and test a torque synchro system and troubleshoot as required: • wiring connections: – crossover – open circuit • rotor relationships • describe the operation of a control synchro system: • synchro transmitter (CX) • synchro receiver/control transformer (CT) • differential transmitter (CDX) • symbols • purpose • null point • inspection and test a control synchro system: • wiring connections • rotor relationships • describe the operation of a synchrotel system: • stator • rotor • purpose • null point • define the following terms: • cartesian coordinates • polar coordinates • sine signals • cosine signals • describe the operation of a resolver synchro system: • resolver (RS) • symbol • purpose.

3–2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Introduction With the advent of high speed aircraft, it was necessary to introduce systems of data transmission which are fast and accurate. These systems are known as AC synchro data transmission systems, and consist of AC synchronous systems, torque transmission systems and control transformer systems.

AC synchronous systems Autosyn This system uses 26 V AC 400 HZ power from the aircraft’s instrument power supplies. The term Selsyn is often used to describe this self synchronous system, which can be used to measure and indicate such things as fuel flow, oil pressure, and flap position. The name autosyn is derived from automatic synchronism. The units are of similar construction, the transmitter and indicator being variable transformers, the rotors being the primaries and the stators being secondaries. They are connected in parallel. Figure 3.1 shows the connections of an autosyn.

Operation In this system the transmitter has its rotor physically positioned by the medium to be measured, whilst the rotor of the indicator moves because of magnetic action. When power is applied, the current in the rotors sets up an alternating flux that induces a voltage into the stators. The position of the rotors determines the value of voltage induced into each segment of the stators. Whenever two rotors have the same physical position, both stators will have the same voltages induced into their corresponding segments, and since they are connected in parallel, no potential difference exists, and no current will flow between the units. This position is called the in correspondence condition and no pointer movement will take place. When the two rotors do not have the same physical position, the voltages induced into the stators will not be the same. This will now cause a potential difference to exist and current will flow through the connecting wiring. The current flow will create a motor action that moves the indicator rotor until both rotors are again aligned. Whenever the two rotors are out of alignment, their voltages differ, and they are said to be out of correspondence.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

3–3

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Figure 3.1: Autosyn system

Summary From the above, we find that as the transmitter rotor is moved due to a change in the value of the medium being measured, the stator voltages would not be the same in both the transmitter and indicator. This causes the out of correspondence condition to occur, and the current that flows sets up a magnetic action within the indicator stator. This interacts with the rotor field causing the indicator to turn until the in correspondence condition occurs. The main disadvantage of this system is that the pointer will remain on scale when the power fails, which could give the crew misleading information about the system being monitored. Many of these systems incorporate a power off flag to alert the crew to a power failure situation.

Magnesyn This system makes use of 26 V AC 400 HZ single phase AC power from the aircraft’s supply. It can be used wherever a mechanical movement is available. The system consists of a transmitter and indicator connected electrically and is more compact, lighter, and simpler than an autosyn. The transmitter consists of the mechanical actuating mechanism and the transmitter, which can be either a rotary type or a linear type. The theory of operation is the same for both, and the rotary type is described here.

3–4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

The rotor is a permanent magnet attached to, and positioned by, the actuating shaft. The stator consists of a circular laminated core, upon which is wound the excitation coil, and a tapping is made at each 120 degrees on the coil away from the input. Outer laminations within the housing encircle the outside of the stator, and provide a return path for the magnetic flux. The indicator is of the same construction, except that the rotor is attached to a pointer which indicates the medium which is being measured.

Operation When the permanent magnet rotor is placed inside the ring or stator of soft iron, the flux lines will establish a flux within the ring. If a coil is wound around the ring and connected to an AC supply, the ring will become magnetically saturated twice each cycle when the current reaches its peak. The rotor flux is forced out of the ring because the ring now has a higher reluctance than the air surrounding the rotor. When the excitation current is at zero, the rotor flux cuts across the excitation coil inducing an EMF, this is generated in all three sections of the stator windings. The amount and phase of the EMF in each section is dependent on the position of the permanent magnet rotor. When the indicator rotor corresponds with the transmitter rotor, identical changes to the EMF’s will take place to their respective stators. There will be no difference in potential at each tapping, and therefore no current flow. Figure 3.2 shows the layout of a magnesyn system. When the mechanical mechanism moves the transmitter rotor, the EMF will differ in the stator windings creating a difference in potential, and current will flow in the interconnecting wires. As the transmitter is mechanically held, the receiver rotor will turn to align itself with the transmitter rotor thus moving the pointer around the scale.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

3–5

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Figure 3.2: Magnesyn system

3–6

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Ratiometer In this system, the measurement of pressure is obtained by measuring the ratio of two alternating values of current. A pressure sensitive element (bellows) causes the linear movement of two armatures, positioned inside two stator coils in such a way, that an increase of current is produced in one stator, and a decrease of current in the other. A small change of inductance at the stators, results in a relatively large ratio of current between the stator coils which is measured on an AC ratiometer.

Operation The circuit is arranged in such a way that when the transmitter and indicator are connected together they form an AC bridge. Any change in pressure will cause the bellows to move the armature cores within their stators, which will result in a change of inductance in the stator coils. This, in turn produces a differential change of current in the coils of the electro magnetic elements of the indicator. The current flowing in these coils produce an alternating flux in the cores on which they are wound. The shading ring on each core causes the flux in that section to lag behind the main field flux thus producing a sweeping flux action across the pole faces. This will produce a torque into the cam shaped discs due to the interaction of the sweeping flux and the eddy currents induced into the discs. The turning motion is such that the disc moves to reduce the effective radius in the air gap. When the effective radius is reduced, the disc impedance increases, thus reducing the torque. Conversely, an increase in radius creates an increase in torque because of a decrease in impedance through the disc. When a change in position of the transmitter armature causes an increase in current at one arm of the bridge and a decrease in the other, the moving element rotates in a direction determined by the coil having the increased current. The movement of the indicating element is designed so that the torques produced in both coils are in opposition, and therefore as the element rotates, the torque that produces rotation is decreasing while the opposing torque is increasing. This will mean that rotation will stop when the torques are balanced. Figure 3.3 is a schematic of ratio system. Two capacitors are included in the circuit to reduce the effect of changes in phase displacement of the induced currents, brought about by an increase in frequency, and the impedence of the discs. These changes will cause an increase in the ratio of the currents in the bridge. Errors resulting from changes in temperature in the laminated iron cores are reduced by means of a high temperature coefficient resistance connected across the bridge.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

3–7

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26 V AC 400 Hz Lamination & bobbin assy

Cam - shaped discs

N

Stator

Damping magnet & disk

S

Armature

Bellows

Transmitter

Indicator

Indicator

26 V 400 Hz

Transmitter

Figure 3.3: Ratiometer schematic

3–8

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Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Torque synchro system To convert a mechanical movement into electrical signals and then transmit the signals to another location, a system of torque syncros is used. The system consists of two items, a torque transmitter (TX) and a torque receiver (TR). Both items are similar except that the receiver will contain some form of damping to prevent oscillations in the rotor. The markings on the terminals are the same for both, S for stator R for rotor and the symbols used in electrical drawings are the same for both. Sometimes the word indicator is used instead of torque receiver.

Torque synchro transmitter operation A circuit will be created if the three stator windings of a TX synchro are connected to the same connections of a TR synchro. When a voltage is applied to the TX rotor, the magnetic field generated by the current in the rotor, will induce a voltage in each of the stator windings by transformer action. The current flowing in the three windings will create three magnetic fields which will combine to produce one field. Lenz’s law states that whenever a magnetic field cuts through a coil inducing a voltage in that coil and causing a current to flow, that current will generate its own magnetic field. This field will oppose the original field as shown in Figure 3.4. The resultant magnetic field in the stator is in the opposite direction to the magnetic field in the rotor. If the TX rotor is turned to any angle the magnetic field of the stator will still oppose the field of the rotor.

Resultant stator field

S2

S2

Torque receiver

R1 115 V AC R2 Rotor field

S3

S1

S3

S1

Figure 3.4: Torque synchro

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

3–9

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Torque synchro receiver operation The current that flows in stator S1 of the TX will also flow in the TR stator S1, it will flow up the TX windings and down the TR winding. Both coils are wound in the same direction but their magnetic fields will lie in opposite directions. The same applies to S2 and S 3. Because the individual fields lie in opposite directions the resultant fields in the TX and TR stators will also be in opposite directions. The TX rotor field and the TR stator field are lined up in the same direction as shown in Figure 3.5.

Figure 3.5: Torque synchro

3–10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Activity 1

1

Draw a diagram showing an autosyn system in correspondence and out of correspondence.

2

Draw a diagram to show the power failure indication of an autosyn system.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–11 © Australian National Training Authority (ANTA) 1997

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3

Draw a circuit diagram of the ratiometer system, showing the circuit arrangement.

4

Draw the basic parts of a torque synchro system.

3–12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

If we turn the TX rotor 30 degrees clockwise, its magnetic field will be at an angle of 30 degrees with a line through the axis of S2. The stator magnetic field must oppose the rotor magnetic field and therefore the stator field will rotate 30 degrees until it opposes the rotor field. The currents flowing in the TR stator are equal but opposite and will oppose the TX stator field and line up with the TX rotor field as shown in Figure 3.6. It can be said that whatever angle the TX rotor takes up, the TR stator field will align itself in the same direction.

Figure 3.6: Torque synchro

Up until now we have only looked at the system without the TR rotor, if we now include the TR rotor we can see what the results will be. The TR rotor and the TX rotor are now connected in parallel, creating magnetic fields in both rotors which are in phase, therefore their fields will always be in the same direction. If the TX rotor is turned 30 degrees clockwise, the stator field of the TR will follow it and move 30 degrees away from its rotor field. The two magnetic fields in the TR will be out of line, and an attraction will exist between the two. This will cause the TR rotor to turn and bring the two fields into line Figure 3.7 shows the two rotors now in line.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–13 © Australian National Training Authority (ANTA) 1997

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Figure 3.7: Torque synchro

3–14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Differential transmitter (TDX) • A torque differential synchro can be used to transmit either: • an electrical signal which is the sum or the difference of two inputs, one mechanical, the other electrical • a mechanical signal which is either the sum or the difference of electrical inputs from two synchro transmitters • a corrective signal to compensate for errors in various parts of a system. This means that they can be either a transmitter TDX or a torque receiver TDR. The principle of operation is the same as the torque transmitter, however the rotor is designed with three separate windings which are placed electrically 120 degrees apart. In this case the stator acts as the primary of the transformer, and the rotor the secondary.

Differential synchro operation Figure 3.8 shows a three component synchro system, it consists of a torque transmitter, a differential synchro, and a torque receiver. The stator leads of the torque transmitter are connected to the stator leads of the differential synchro. The rotor leads of the differential synchro are connected to the stator leads of the torque receiver. The differential synchro is not connected to the AC supply. TX

DT

S

S2

S2

S

R1

TR

R

S

S2 R1

R2

R

R

R2

R2

S3

S1

S3 R3

R1

S1

S3

S1

Figure 3.8: Torque synchro

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–15 © Australian National Training Authority (ANTA) 1997

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If both the torque transmitter and the differential synchro rotors are at zero degrees, the rotor of the torque receiver will also be at zero degrees. The magnetic field in the rotor of the DT is parallel to the R2 winding, the magnetic field in the stator of the TR will be parallel to the S2 winding. The rotor of TR will be attracted to the magnetic field of its stator and will line up with it. The rotor of TR is on 0 degrees, its rotor field is also at 0 degrees. The magnetic field of the TR stator will line up with the rotor field, but will oppose it. As the stators of TR and DT are connected in series, the resulting magnetic field in the DT stator will be parallel to the magnetic field in the TR stator, but will be in the opposite direction. The magnetic field in the DT stator will induce a voltage in the DT rotor. The resulting rotor field will line up with the stator field but will be in the opposite direction. We will now determine the angular position of the DT rotor field relative to the R2 winding. This field as shown in Figure 3.9 will be positioned 180 degrees in relation to the R 2 winding. We can now tell the position of the TR stator field in relation to its S2 winding. This is because the DT rotor and the TR stator are connected in series, so that the two fields are parallel but have opposite directions. When the DT rotor field is positioned at 180 degrees, the TR stator field will be positioned at 0 degrees. The TR rotor will position itself at 0 degrees as shown in Figure 3.9.

3–16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Figure 3.9: Torque synchro

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–17 © Australian National Training Authority (ANTA) 1997

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Differential transmitter operation When using the differential synchro as a transmitter, it can be used to add or subtract information to the remote torque receiver. Subtraction The system shown in Figure 3.10 is used to produce a difference output from the two inputs to the differential transmitter. The two inputs come from the movement of the shaft of the TDX and an electrical input from the TR stator. The signal transmitted to the TR is the difference between the electrical signal A and the mechanical signal B. The shaft of the TR will position itself at an angle equal to A to B.

Figure 3.10: Differential subtraction

In Figure 3.11 the TDX rotor is held at 45 degrees and the TX rotor is turned to 45 degrees the TR rotor will turn back from -45 degrees to 0 degrees. The 45 degrees signal from the TX has cancelled the signal from the TDX.

3–18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Figure 3.11: Differential addition

In Figure 3.12 the TDX stator field is at a 45 degrees angle with respect to the S2 axis due to the 45 degrees clockwise shift of the TX rotor. The TDX rotor field is parallel to the R2 winding. The field of the TR stator must be parallel to its S2 winding. The TR rotor will line up with its stator field turning back from -45 degrees to 0 degrees.

Figure 3.12: Differential addition

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–19 © Australian National Training Authority (ANTA) 1997

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Addition Figure 3.13 shows the same set up as used for subtraction in Figure 3.10, except that both input and output the leads of the TDX are changed. This will mean that the shaft of the TR will revolve to a position whose angle will be equal to the angles travelled by the shafts of the TX and TDX. If the rotor of the TDX is held at 40 degrees clockwise, and the rotor of the TX is turned from -45 degrees through an angle of 90 degrees to a final position of 45 degrees clockwise, the rotor of the TR will turn from -5 degrees to a position of 85 degrees clockwise as shown in Figure 3.14.

Figure 3.13: Differential addition

3–20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Figure 3.14: Differential addition

When the TX rotor is turned through 90 degrees clockwise, the stator field of the TDX moves anti-clockwise, through the same angle. As the TDX rotor remains at 40 degrees clockwise from S2 the TDX stator and rotor fields act along a line 85 degrees to the left of the R 2 winding of the rotor as shown in Figure 3.15.

Figure 3.15: Differential addition

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–21 © Australian National Training Authority (ANTA) 1997

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The stator field of the torque receiver, now acts along a line 85 degrees to the right of the S2 winding and the TR rotor turns 85 degrees clockwise to align itself with this field.

Symbols Synchro transmitters and receivers are virtually the same, and so the schematic symbols for them are the same. Figure 3.16 shows three examples of the way in which synchros are drawn, (a) is the most commonly used, (b) is usually used when the operation is explained, and (c) is usually on wiring diagrams.

Figure 3.16: Differential addition

Unfortunately there are two standards used - one British and the other American.

3–22 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

S1

R1 R2

S3

S2

Figure 3.17: The British configuration symbols

S2

R1 R2

S3

S1

Figure 3.18: The American configuration symbols

The differential synchro is shown in a similar fashion, and Figure 3.19 shows the three ways in which it is drawn.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–23 © Australian National Training Authority (ANTA) 1997

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Figure 3.19: Symbol

Purpose The use of synchros in position sensing and data transmission, is very common in aircraft, especially in automatic pilot systems. It is a fast accurate method of transmission and control, and provides an accuracy of approximately 0.5%. The synchro is essentially a rotary transformer whose secondary output voltage depends upon the primary input voltage, and upon the position of the rotor. The simplest system consists of two synchros connected together electrically, one is called a transmitter, the other a receiver. The purpose of the receiver is to take up the same position as the transmitter.

Null point If the transmitter and receiver rotor position are identical the rotor fields produced are identical both in magnitude and direction. The voltages induced in the corresponding stator coils will be equal in both magnitude and phase, and zero current will flow through the stator coils. No current flow, no stator field produced, no torque developed, null point.

3–24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Inspection, testing and fault finding Synchros are delicate items and should be handled carefully. When inspecting a synchro system you should be on the lookout for the following conditions. • corrosion • loose or broken connecting wires • discolouration due to overheating • physical damage • any other abnormalities. Inspection is simply a case of using your eyes and common sense. When testing a system you must carry out the requirements as laid down in the appropriate manuals. Make sure that the equipment used is the type that is called up in the manuals, and that it is serviceable and within its calibration date. Ensure that the test is carried out in the sequence stated in the manual, not in any adhoc fashion. The test results should be written on the test sheet and must be within the called for parameters. Fault finding is usually a case of approaching the problem in a logical way, and using a lot of common sense. The most common fault in any synchro system is wiring. Such things as crossed connections, short circuits, and open circuits will cause malfunctions within the system. If you check for a supply voltage at the input, and then check for an output, the results will then allow you to discover the culprit.

Control synchro system In the previously described torque transmission system, the output element exerts a torque which tends to align its rotor with the angular position of the input shaft. When positioning heavy loads, for example a radar antenna, the torque synchro is inadequate, and a system which provides an output in the form of an electrical signal is used. This signal is then passed to an amplifier whose output can control a motor capable of producing the correct amount of torque. The syncro system is still used, but the normal synchro receiver is replaced by a unit called a control transformer, which takes the signal from the transmitter and turns it into a control voltage.

Synchro control transmitter (CX) The synchro control transmitter, like the torque transmitter is wound with a three phase output winding in the stator and a moveable rotor winding. When an AC voltage is applied to the rotor winding a voltage will be induced into the stator winding, the phase and value being dependant on the rotor position.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–25 © Australian National Training Authority (ANTA) 1997

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Synchro control transformer (CT) The stator of the control transformer is similar in design to a torque unit, but is fitted with high impedance coils to limit the current flowing through the windings. Unlike the rotor of synchro receiver, the winding of the transformer rotor is housed in slots in a cylindrical rotor. This ensures that the rotor is not subjected to any applied torque when the magnetic field of the stator is displaced. Figure 3.20 shows the circuit of a synchro control transmitter (CX) connected to a synchro control transformer (CT). The rotor of both units are at electrical zero. The axis of the transmitter rotor winding is aligned with the axis of S2 winding of its stator, the control transformer rotor winding is at right angles to the axis of the S2 stator winding. Control transmitter

Control transformer

S2

S2 R1

115v 400 Hz

R1

R2

R2

Voltage output

S1 S3

S3

S1

Figure 3.20: Control synchro

In this situation the magnetic field created by the current in the transmitter rotor winding gives rise to magnetic fields in the two stators, the axis of these fields being in line with the axis of the S2 winding of each stator. Rotating the transmitter rotor in either direction from the electrical zero position will produce a corresponding angular movement of the axis of the magnetic field of both stators. The axis of the transformer rotor winding and the axis of the transformer stator magnetic field are no longer at right angles to each other. The flux of the stator field begins to induce an EMF within the turns of the rotor winding. The magnetic field of the stator induces an EMF in the rotor winding, the amplitude of which will increase as the transmitter rotor is moved further away from the electrical zero position. When the transmitter rotor has travelled through 90 degrees from electrical zero, the axis of the transformer rotor winding and the axis of the transformer stator field are parallel. At this point maximum flux transfer is achieved and so maximum EMF is induced in the rotor winding.

3–26 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

The effect of transmitter rotor displacement is shown in Figure 3.21. Control transmitter

Control transformer

Transmitter rotated 90°

S2

S2 Reference supply

R1 R2

S3

R1 R2

90° S1

Rotor field

Maximum output volts

S3

Resultant stator field

S1

Resultant stator field

Figure 3.21: Control synchro

The amplitude of the control transformer output voltage is relative to the angular displacement of the CX rotor from the datum position. The output amplitude is zero when the CX rotor is rotated from the datum position and increases to a maximum as the CX rotor is moved through 90 degrees in either direction. It is either in phase or in complete anti phase with the reference voltage applied to the CX rotor winding. This is according to whether the displacement of the CX rotor from the datum position is clockwise or anti clockwise. The CT output senses the CX rotor displacement, and as the output is a simple voltage, it can be applied to a power amplifier whose output can be used to drive a heavy mechanism. Figure 3.22 shows the rotor in the clockwise and anticlockwise position.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–27 © Australian National Training Authority (ANTA) 1997

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Figure 3.22: Control synchro

3–28 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Differential synchro transmitter The differential synchro transmitter (CDX) is the same as the torque differential transmitter mentioned earlier. However the rotor is always mechanically driven. Figure 3.23 shows a synchro transmitter connected to the stator of a differential synchro transmitter and the rotor connected to a synchro receiver. the voltages appearing at the receiver are modified by the angular position of the CDX. The resultant magnetic field in the stator of the CR is also dependant upon the connections between all three items. The resultant voltages can represent either the sum or the difference depending on the connections.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–29 © Australian National Training Authority (ANTA) 1997

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Differential synchro transmitter

Synchro transmitter

(a)

R1 R2

Synchro receiver

S1

S1

R1

S1

S2

S2

R2

S2

S3

S3

R3

S3

45° clockwise

R1 R2 30° clockwise

15° clockwise

Difference clockwise (b)

R1 R2

S1

S1

R1

S1

S2

S2

R2

S2

S3

S3

R3

S3

45° clockwise

R1 R2 30° clockwise

15° clockwise

Sum clockwise (c)

R1 R2

S1

S1

R1

S1

S2

S2

R2

S2

S3

S3

R3

S3

45° clockwise

R1 R2 60° anticlockwise

15° clockwise

Sum anticlockwise (d)

R1 R2

S1

S1

R1

S1

S2

S2

R2

S2

S3

S3

R3

S3

45° clockwise

R1 R2 30° anticlockwise

15° clockwise

Difference anticlockwise Figure 3.23: Control synchro wiring

3–30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Symbols Figure 3.24 shows the usual symbols for representing a differential synchro. S2 R2

R1

S3

S1

R1

S2

R2

S3

R3

S1 R3 Figure 3.24: Differential synchro symbols

Purpose These units can be used to compensate for errors in various parts of a system, or to add information, and are found in navigation and radar systems.

Null point For any synchro to be accurate, it is important for the unit to be electrically zeroed. To be in the zero position, the voltage between S1 and S3 must be zero, and the phase of the voltage at S2 must be the same as the phase at R1.

Inspection and testing The components of the control synchro system are similar to the torque synchro system, and therefore the inspection and testing procedures are the same.

Synchrotel Stator A normal 3 phase synchro stator is used, but the rotor is in 3 separate parts.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–31 © Australian National Training Authority (ANTA) 1997

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Rotor The rotor consists of a hollow aluminium cylinder, a fixed single phase rotor winding, and a cylindrical core which the rotor revolves around. The rotor shaft is supported in bearings and is connected to pressure sensing bellows, as shown in Figure 3.25. The rotor of the sychro transmitter is energised by a 26 V 400 Hz single phase supply which will induce voltages into the transmitter stator. As this stator is connected to the synchrotel stator then a resultant radial alternating flux is established across it. When a pressure is applied to the sensing bellows, the synchrotel rotor will turn and due to its oblique shape, parts of it will be cut by the radial flux of the stator current. This will produce currents in the rotor winding, which is electrically connected to a synchro control transmitter, whose rotor will then follow the synchrotel rotor. As the synchrotel rotor is pivoted around the core cylinder, a flux axis will be created in the core. Because the rotor winding is also fixed around the core, the core flux will induce an alternating voltage in the winding. The positions of the rotor and stator flux will determine the amplitude and phase of the output voltage. The synchro control transmitter output is supplied to an amplifier and then to the control phase of a two phase servo motor, which will drive the synchro transmitter rotor around until there is no voltage induced in the rotor winding. In other words to the null position, which corresponds to the measured pressure.

3–32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Figure 3.25: Synchrotel system

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–33 © Australian National Training Authority (ANTA) 1997

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Purpose The synchrotel acts as a servo loop system and is usually used as a low torque control transformer or transmitter.

Null point The null point is achieved when there is no output from the rotor winding.

Cartesian coordinates A set of numbers which locate a point in space with respect to a collection of mutually perpendicular axes. In Figure 3.26 the cartesian coordinates of point P are X = 8, Y = 6 and are written 8, 6. These figures are obtained by drawing lines at right angles to the X and Y axes to meet at point P. The intersection of the lines on the axes are known as the cartesian coordinates.

Y 7 P

6 5 4 r

3 2 1

θ

X

0 1

2

3

4

5

6

7

8

9

Figure 3.26: Cartesian coordinates

3–34 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Polar coordinates A point in a plane may be represented by coordinates rθ where θ is the angle between the positive X axis and a line from the origin to the point and r the length of that line. In Figure 3.27 the polar coordinates of point P are determined by drawing a straight line from zero of the X and Y axes to point P, the length of this line (r) is one coordinate, and the angle between line r and the X axis is the other. The polar coordinates of P are r, angle q. With the use of trigonometry we can convert polar coordinates to cartesian coordinates the value of the Y axis can be determined by the equation: sin q = Y = r sin q. The value of Y for any point P will vary as the sine of the angle q. By using the same method to determine the value of the X axis: cos q = X = r cos q. The value of X will vary as the cosine of the angle q.

Sine angles When the polar coordinates of a point and the slant range are known, the vertical component or Y cartesian coordinate can be found from the formula Y = r Sin q.

Cosine angles When the polar coordinates of a point and the slant range are known, the horizontal component or X cartesian coordinate can be found from the formula Y = r Cos q.

Resolver synchro The resolver syncro from the outside looks like any other synchro, but internally it is very different. It has two rotor windings mounted on the one rotor, and has two stator windings.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–35 © Australian National Training Authority (ANTA) 1997

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The two rotor windings are wound 90 degrees apart, as are the stator windings. The synchro resolver is a rotating transformer, where either the stator or the rotor is acting as the primary. This is dependent on which one is receiving the input signal. The output from the secondary winding of a rotating transformer is proportional to the angular displacement between primary and secondary windings, provided they have equal amount of turns. The ratio of the windings is 1:1 Figure 3.27 shows the relationship of the rotor and stator windings.

Figure 3.27: Resolver synchro

Conversion from polar to cartesian coordinates When converting polar to cartesian coordinates, the known facts are the polar coordinates which is the known distance r and the angle q, which is the angle between the induced voltage and the mechanical axis of the rotor field to which the voltage is applied. The required facts are cartesian coordinates X and Y, which give the location of point P. A voltage proportional to the known distance r is applied to one rotor winding, which will produce a magnetic field in the rotor with its axis in line with the mechanical axis of the winding. This field will induce voltages into the stator windings. In Figure 3.28 the winding S1/S2 will have a maximum induced voltage because it is parallel to the rotor field, but S3/S4 will have zero voltage induced because it is 90 degrees to the rotor field.

3–36 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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If the rotor is turned at a constant speed, the voltage across S1/S2 will fall to zero after the rotor has turned 90 degrees. The voltage is in phase with the voltage applied to R 1 to R2 during the first 90 degrees of displacement, and anti-phase from 90 degrees to 270 degrees, and then back to in-phase from 270 degrees to 360 degrees. It is the measure of the cosine of the displacement. At electrical zero, the windings S 3/S4 will have zero voltage induced, but at 90 degrees displacement of rotor winding R1/R 2 maximum in phase voltage will be induced and will vary sinusoidally throughout 360 degrees. The S3/S4 voltage is directly proportional to the sine of the rotor displacement.

Figure 3.28: Resolver synchro application

The phase depends on the angle of displacement of the rotor, the angle being identified by the amplitude and phase of the voltages in the stator winding S3/S4. The sum of the outputs from both stators gives the input voltage and rotor movement in cartesian coordinates. r cos q + r sin q. In Figure 3.29, the voltmeters are converted to give an instantaneous reading of cartesian coordinates X and Y, in miles. This reading will be: • the known distance r X cosine of angle θ = VX • the known distance r X sine of angle θ = VY.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–37 © Australian National Training Authority (ANTA) 1997

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Applied AC Applied AC voltage voltage R1 proportional propor tional totoknown known distance r distance r R2

S1

Angle turned

θØ n

Resultant magnetic Resultant field magnetic field

V V

S2 Vx = distance r × cos angle Vx = Distance rr x cos. angle

R3

R4 S3

V V

S4 Vy Distance rrr×xsine sineangle angle V = distance y

Figure 3.29: Resolver synchro - basic distance reading

In Figure 3.30, A is a radar station on an airfield and B is an approaching aircraft. The radar scanner has picked up an unidentified aircraft P and the trace shows it at distance r and elevation q or 10 miles at an angle of 36 degrees 52 minutes. As the horizontal position of aircraft B with respect to the radar station is known, it would be useless to relay this information to aircraft B. It would be more useful to identify P by: • its horizontal position from A • its vertical height from A. This now pin points P in terms which are useful to aircraft B. The known facts are the polar coordinates 10 miles at 36 degrees 52 minutes The required facts are the cartesian coordinates X horizontal position and Y vertical position. It is now necessary to convert from polar to cartesian coordinates.

3–38 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

(Y)6

P

4 Height (miles)

r

2 θ

B

0 2

4

6

8 (X)

10

12

Distance (miles)

Figure 3.30: Coordinates

Conversion from polar to cartesian In Figure 3.31, a resolver for converting polar to cartesian coordinates is shown. If 1 volt is equal to 1 mile then the AC voltage representing r in Figure 3.30, is 10 volts, and is applied to one rotor. As the ratio of the windings is 1:1 then there will be 10 volts across stator 1 and zero volts across stator 2 will increase sinusoidally from zero to maximum, the EMF change is the sine of angular displacement.

Figure 3.31: Resolver synchro - angular readings

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–39 © Australian National Training Authority (ANTA) 1997

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r cos θ = X measured across stator 1 r sin θ = Y measured across stator 2 The voltmeters give an instantaneous read out of X and Y. As the radar scanner determines slant range and the angle of elevation as shown in Figure 3.32. Applied AC voltage R1 applied proporVtional r to known distance 10v = r R2 V induced = r cos θ = X = 10 × .8000 = 8v = 8 miles

S1

θ = 36°Angle 52' turned

n θ Ø

Resultant magnetic Resultant field magnetic field

V V

S2 Vx = distance r × cos angle Vx = Distance r x cos. angle

R3

V induced = r Sin θ = Y = 10 × .6000 = 6v = 6 miles

R4 S3

V V

S4 Vy = Distance rr x sine angle

Figure 3.32: Resolver synchro - slant range reading

The terminals R3 and R4 of the unused rotor windings are shorted out to improve accuracy. The flux of R1 and R2. This flux will always be at 90 degrees to the unused rotor winding R 3 and R4 and will not affect it. Any unwanted flux will create a large flux in R3 and R4 which will oppose the unwanted flux. This limits the effects of unwanted flux and improves the accuracy of the resolver.

3–40 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Conversion from cartesian to polar When converting cartesian coordinates to polar coordinates, the known values are the cartesian coordinates X and Y, and the unknown coordinates are the polar values distance and an angle. Voltages proportional to X and Y are applied to the stator windings which causes stator magnetic fields to give a resultant field with its direction and strength dependant on the applied voltages. The magnetic field set up by the stator voltages will induce voltages into the rotor windings as shown in Figure 3.33.

Figure 3.33: Resolver resolution - induced voltages

A nullmeter is connected across one of the rotor fields and a voltmeter is connected across the other rotor field as shown in Figure 3.34. When the rotor is turned so that the winding containing the nullmeter is 90 degrees from the stator field, as no voltage is induced and the nullmeter will read zero. The windings of R3/R4 will be

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–41 © Australian National Training Authority (ANTA) 1997

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parallel to the stator field and will have maximum voltage induced into them. This voltage will be proportional to r, and if the voltmeter is made to read miles, it will give a direct read out of this distance. The angle through which the rotor is turned is noted, and so both polar coordinates are supplied. R1 Shaft

N R2

S1 Resultant magnetic field

Vx S2

R3 V R4

S3

Vy

S4

Figure 3.34: Resolver synchro - range reading

If we use our previous example of the radar set on an airfield and also Figure 3.36, we can workout the conversion from cartesian to polar coordinates. VX = 8 volts and VY= 6 volts and represent the cartesian coordinates. The fluxes of the two stators will combine to give a resultant magnetic field as in Figure 3.35 The resolver rotor is turned until a zero voltage is shown on the nullmeter. The angle through which the rotor turns is the polar angle coordinate q, which in our example is 36 degrees 52 minutes. If R1/R2 is at null, R 3/R4 must have maximum voltage induced due to being parallel to the resultant field. This voltage represents the polar coordinate r which is 10 volts = 10 miles. Figure 3.36 shows the mathematical equation.

3–42 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Figure 3.35: Polar coordinates

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–43 © Australian National Training Authority (ANTA) 1997

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Symbols Figure 3.36 shows the most common ways in which a synchro resolver is shown in schematic drawings.

Figure 3.36: Resolver synchro - schematic

Figure 3.37 shows how it is represented on a block diagram.

RS Figure 3.37: Resolver synchro - circuit symbols

Purpose The purpose of the synchro resolver is to change position data from one form of coordinate to another by converting alternating voltages. The voltages represent the cartesian coordinates of a point, and by changing them to a shaft position and a voltage, will now represent the polar coordinates of that point. The synchro resolver may also be used in the reverse manner.

3–44 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Activity 2

1

Draw diagrams to show the difference between the British configuration and the American configuration for synchro transmitters and receivers.

2

Describe the construction of the rotor of a synchrotel. Use a diagram to assist you.

_________________________________________________________________ _________________________________________________________________

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–45 © Australian National Training Authority (ANTA) 1997

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3

Describe the construction of the windings in a resolver synchro. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

4

Draw a block diagram symbol for a synchro resolver.

3–46 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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Review Before you move on to Section 4 work through the Check your progress questions to see how well you understood Section 3. If there is anything you are not sure of, revise the relevant work before you begin the next section. If you would like additional information to help your understanding of any part of this section, use the reference books listed at the beginning of this module. When you are satisfied with your progress, move on to Section 4, which covers servomechanism systems.

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–47 © Australian National Training Authority (ANTA) 1997

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Check your progress 3

1

True or false? The rotor of a Magnesyn is energised from a 26 V AC 400 Hz supply. Explain your answer. _________________________________________________________________ _________________________________________________________________

2

Which of the following gives a mechanical output: TR CT TDR CX. _________________________________________________________________

3

Which of the following gives an electrical output: TX TR. _________________________________________________________________

4

The purpose of a synchro system is to: • transmit a voltage • transmit a position • transmit a torque. _________________________________________________________________

5

Briefly describe the construction of the sensing element of the ratiometer system. _________________________________________________________________ _________________________________________________________________

3–48 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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6

A basic torque synchro system converts _ signals into ____ signals.

7

A TDX does what to the information sent to the TR? _________________________________________________________________ _________________________________________________________________

8

True or false? The voltages of all the windings in a CX-CT circuit are either in-phase or 180 degrees anti-phase to reference. Explain your answer. _________________________________________________________________

9

True or false? The rotor of a CT is excited by an external source. Explain your answer.

10

What turns the Synchrotel rotor? _________________________________________________________________

11

In a resolver synchro the rotor and stator windings are wound how many degrees apart? _________________________________________________________________

12

What is the purpose of the synchro resolver? _________________________________________________________________

Section 3: AC synchronous systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–49 © Australian National Training Authority (ANTA) 1997

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3–50 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 3: AC synchronous systems © Australian National Training Authority (ANTA) 1997

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4 Servomechanism systems ○

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Learning outcome 4 On completion of this module you should be able to describe and test servomechanism systems.

Assessment criteria You will have achieved competency in this learning outcome when you can: • define the terms associated with servomechanisms: • null • error signal • feedback • reference signal • system alignment • hunting • overshooting • deadband • response time • time lag • damping:

– under – over – critical

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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• define the difference between an open loop and a closed loop system • describe the operation of a servomechanism system to block diagram level identifying the components: • command transmitter • error detector • amplifier • summing point/network • servo • position feedback • rate feedback • acceleration feedback • modulator/demodulator • identify differences in operation between types of servomechanism systems: • AC • DC • hybrid • advantages • limitations • identify the causes of hunting • inspect, test, and align a servomechanism system and trouble shoot as required.

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Introduction A synchro-servo system is no use to us unless the information gathered by the detectors can be put to useful work. This is done by servomechanisms which provide the muscle power to move control surfaces, close valves or move other parts of the aircraft’s mechanical system. In the glossary you would have read that: A servo device is a power driving device usually electric or hydraulic which can produce motion or forces at a higher level of energy than the input level and be used to move a heavy part of the aircraft structure. A mechanism is a system of mutually adapted parts working together.

This section combines the two and introduces you to servomechanisms.

Servomechanism An automatic feedback control system for mechanical motion. It is used to provide remote control of heavy loads such as control surfaces, and has an error correcting system. The servomechanism is used extensively in the automatic control of an aircraft’s flight, mainly to position the control surfaces to provide the correct flight attitude. It is capable of detecting and correcting errors at its output, which are due to external forces acting upon the load it is controlling. This is achieved by the use of a closed loop system, and to be classed as a servomechanism it must conform to the following. • detect the difference between input and output (error detection) • amplify the error signals (power amplification) • provide feedback (closed loop) • capable of continual operating (continuous operation).

Terms associated with servomechanisms All the terms you see listed here are mentioned in the glossary, but we have repeated them here for more direct reference.

Null The term null is used to describe the condition where there is no error signal being produced by the error detection device.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

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Error signal This signal is produced by the error detector, it is the difference between the required output position, and the position that the output is actually in.

Feedback The actual position of the output is feedback so that it may be compared with the required output.

Reference signal This signal is supplied to the amplifier to enable it to determine in which phase the error signal is in. It is the phase relationship which will determine in which direction the servo motor will drive.

System alignment For the system to work correctly the controlling transmitter, the load, and the feedback transmitter must be aligned at zero. In this way, when the controller calls for, shall we say 2 degrees of movement, the load will move until it reaches 2 degrees. The output from the controller (error signal) and feedback signals will be equal and therefore null out.

Hunting This form of instability is when the output shaft swings back and forth through the required position, even though the input shaft is stationary. The main causes of this condition are: • a large time lag in the system • the lack of velocity feedback. The servomechanism cannot respond in zero time to a change of input, and so corrective action will lag the change of input. The corrective action is still in progress when the error has reduced to zero, and the system will overcorrect, causing an opposite correction. The output shaft now oscillates around the desired position as shown in Figure 4.1.

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Figure 4.1: Hunting characteristics

Damping To prevent oscillations some form of damping is introduced. There are four levels of damping and are listed as follows: • underdamped, the load oscillates several times before coming to rest • overdamped, there is no chance of oscillations and the load is positioned slowly • critical damping which is between oscillations and no oscillations • optimum damping, there is slight overshoot, but it takes the least amount of time to reach the correct position.

Mechanical damping To ensure fast positioning of the load, the system should be damped at slightly less than critical damping. The damping should be accurately adjustable, and the easiest way of creating more friction would be to apply some form of brake. This could be done in one of three ways. • a disc could be made to rotate in thick oil • a hysteresis type braking system • a coulomb damper, where a disc rotates against some type of friction material.

Figure 4.2: Friction damping

Like all things, mechanical or friction damping as shown in Figure 4.2 has its problems: Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

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• it reduces efficiency through the dissipation of energy • cooling will be required for large friction devices • motor power would need to be increased due to the added load • extremely difficult to maintain the correct level of damping.

Electrical damping To overcome the problems associated with mechanical damping, a system called velocity or electrical feedback damping can be used instead. This type of damping is achieved by fitting a tacho generator to the output shaft as shown in Figure. 4.3. The tacho generator will produce a voltage which will be proportional to the angular velocity of the output shaft. Part of this voltage is fed back into the amplifier in opposition to the error signal as a negative feedback, or velocity feedback as it is known. The idea of velocity feedback, is to reduce the net input to the amplifier to zero and then reverse it before the output shaft reaches its final position. By adjusting the amount of feedback correctly, the momentum of the load, which is acting against the reversed torque, will bring the load to rest as it reaches its required position.

Figure 4.3: Electrical feedback

Response and overshoot If the input demand changes rapidly from one value to another in a servo system, it can be seen in a graph as being a step input which is shown in Figure 4.4.

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Figure 4.4: Response and overshoot graphs

Initially the system is at rest with θ1 = θ0. In Figure 4.3 θ1 suddenly changes to a new value, but θ0 cannot follow immediately and the error will increase from zero to θ1 and a large torque is applied to the load. As the load accelerates and θ0 decreases, the error and torque are reduced until θ0 reaches the desired value and the error and torque become zero.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

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The servomechanism will readily oscillate because by the time θ0 reaches the required value, the load has acquired momentum and overshoots the error now increases in the opposite sense and a reverse torque is applied which brings the load to rest, but then accelerates it back again and causes another overshoot.

Deadband In a continuous control system, the error must be large enough to overcome starting friction, and will only respond to errors above a certain amount. The amount of the error either side of the correct position is called the deadband. If the system will not react unless the error is 0.01 inch on either side of the correct position, then the system has a deadband of 0.02 inch.

Response time As we saw in the overshooting paragraph, there is a definite time taken for the servo system to react. The larger the load being driven the longer will be the response time.

Time lag This function is closely tied with response time. If the system is a purely mechanical system, the time lag for a response will be longer than for one which is electronically controlled.

Open loop and closed loop systems Open loop system In this type of system, the controlling operator will make a comparison between the command input and the controlled output, and then make adjustments manually to obtain the required output. Whilst this system works, it is entirely dependent on the operator being there and having continual control.

Closed loop In this type of system the controlled output is compared with the commanded input and is automatically adjusted so that the output equals the input. This is achieved through the use of feedback loops. The error or difference is continuously providing a signal to change the output and cancel the input when the two match. As an example, the thermostat will automatically stop the compressor when the temperature inside a refrigerator has reached the desired amount of chilling.

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The difference between the two systems may be summarised as follows: • In the open loop system, the command is cancelled by some visual reference and manual adjustment to the output. • In the closed loop system, the command will be cancelled automatically when the output attains the amount ordered by the input. This is the feature of negative feedback which applies an opposite signal to remove the original signal and so cancel the input signal.

Types of servomechanisms These can be placed into two main types: • remote position control (RPC) • rate control servos (velodynes).

RPC servos These are used to control angular or linear position of a load, and can be used to rotate a load such as a control surface.

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.

Servomechanism A basic servomechanism may have the following elements: • command input transmitter • error measuring device • error voltage • amplifier • servomotor • output • position feedback • rate feedback • acceleration feedback. Figure 4.5 shows the relationship of the above elements.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

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Position feedback

Summing point

A

EMD Command transmitter

Servo motor Output shaft

Damping

Load

Figure 4.5: Servomechanism

Basic servo operation Figure 4.6 shows a simple RPC servomechanism where the output shaft is moving a load such as a control surface.

Figure 4.6: Basic servo operation

We will assume that the shaft has taken up a position that agrees with the position demanded by the input shaft and the motor is stationary in a steady state. • θ1

=

quantity equal to input shaft position

• θ0

=

quantity equal to output shaft position

• e

=

error voltage which equals θ1 - θ0

• EMD is the error measuring device.

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If we now say that the control surface has to be changed to another position, the input demand q, is suddenly changed. We now have a difference between the actual load position and the desired input signal, and the resulting error signal (e) is amplified and will cause the motor to drive to bring the control surface to the new position. As the control surface nears the required position, the error signal and the motor drive are progressively reduced, until the condition is reached where the error signal and the motor drive is zero. The motor will stop and the control surface is at its new position. The period during which the output is changing in response to the change in demand, is called the transient time. When this period has been completed, the system is in a steady state. The time taken to reach the steady state after a change in demand is the response time.

Summing point Most modern systems will have a summing point. Here the output from the EMD, damping and other feedback signals are summed, and the output is then supplied to the amplifier

Servo device A power driving device usually electric, hydraulic or pneumatic, which can produce motion or forces at a higher level of energy than the input level and be used to move a heavy part of the aircraft structure.

Position feedback A signal sent from the output of the servo or the moveable component, back to the amplifier which will oppose the error signal to halt the movement of the servo at the required position.

Rate feedback A signal proportional to the speed of the servo motor, which is summed at the summing point and modifies the command error signal to control the rate at which the servo drives.

Acceleration feedback A signal proportional to the rate of acceleration of the servo, which is summed at the summing point and reduces the command error signal to control the rate at which the servo can accelerate the load, reducing stress of mechanical components and the tendency to overshoot.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–11 © Australian National Training Authority (ANTA) 1997

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Modulation - demodulation Stable amplification of DC signals is difficult to achieve because the amplifiers tend to suffer from drift which causes variations in the output. To overcome this problem and still provide high gain, the DC signal is used to modulate a suitable AC carrier. The AC signal is amplified and then demodulated to extract the amplified DC signal. Figure 4.7 is a block diagram of this kind of system. EMD Mechanical input

Error signal

AC MOD

DC

AC A

DC MOD

DC position feedback q DC DC velocity feedback

Tacho gen

Load

Shaft

Servo motor

Figure 4.7: Modulation

Ring bridge modulator One such circuit used to carry out the modulation function is called the ring bridge modulator and is shown in Figure 4.8, with no DC input signal. The AC reference voltage will cause current to flow through D3 and D 4 on one half cycle and D 1 and D2 on the other half cycle. All the current flow is through the rectifier diodes and the secondary windings of T 1. There will be no output from T2.

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AC REF

A

DC I/P

+

_ T1

_

+

D1 R1

D2

D3 D4 B

AC O/P

T2 Figure 4.8: Ring bridge modulation - no input signal

If a DC signal, which is positive at A, is applied, it will bias the rectifier bridge and D1 and D4 will not conduct. During the positive half cycle of the reference voltage, D2 will conduct causing current to flow through the top half of the primary windings of T2. This is indicated by the dotted arrows in Figure 4.9. During the negative half cycle of the reference voltage, D 3 will conduct and the current will flow in the lower half of T2 as shown by solid arrows. The AC output from the secondary windings of T2, is in the same phase as the reference voltage. For a positive DC input the AC output is in phase with the reference voltage.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–13 © Australian National Training Authority (ANTA) 1997

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Figure 4.9: Ring bridge modulation with a positive input signal at point A

For a DC signal which is negative at A, diodes D2 and D3 will not conduct. During the positive half cycle of the reference voltage, D1 will conduct causing current to flow through the upper half of the primary windings of T2. This is indicated by the dashed arrows in Figure 4.10. During the negative half cycle of the reference voltage, D 4 will conduct and the current will flow through the lower half of the primary winding T2, as shown by solid arrows. The AC output is out of phase (anti-phase) with the reference voltage.

4–14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

AC REF

T1 _

+ _

A _

+

D1

DC I/P R1

D2

D3 D4

C

B+

T2

AC O/P D

Figure 4.10: Ring bridge modulation with a negative input signal at point A

Ring bridge demodulator The same circuit is used to de-modulate the AC signal. Figure 4.11 shows a ring bridge demodulator with no AC error signal input, the AC reference voltage causes current to flow in D 3 and D4 for one half cycle, and D1 and D2 during the second half cycle. There is no current flow through resistor R1, no AC input and no DC output.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–15 © Australian National Training Authority (ANTA) 1997

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AC REF

T2 A

T1

D1

D2

D3

R1 DC O/P

D4 AC I/P B

Figure 4.11: Ring bridge demodulator - no input

In phase error signal When the AC reference signal is in a half cycle making D3 and D4 forward biased (point E positive), an AC error input that makes point D on the secondary of T 1 negative, will reverse bias D4. The current path as shown by solid arrows in Figure 4.12, from D through D3, to E then via the centre tap to R1 will make point A positive with respect to point B.

4–16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

AC REF

_

+

+

_ E

T2 _

F

A

T1

D1 + C

AC I/P

D2 +

DC O/P _

D3 D4

R1

D +B Figure 4.12: Ring bridge demodulator - in phase error signal

On the second half of the cycle, the negative reference voltage (point E negative in Figure 4.12) will forward bias D1 and D2. The AC error input (point C negative), will reverse bias D1 so that the current flow is through D2. The current flow, shown by dashed arrows is from C through D 2 , to F and via the centre tap to R1 in such a way that point A is still more positive than point B. Out of phase error signal Figure 4.13 shows the AC input 180 degrees out of phase with the reference signal, the current flow through R1 is reversed making point A more negative to point B. During the first half cycle, the positive reference voltage will again forward bias D4. The error signal will make point D positive. The current will flow from F through D4 to D on T1 then from the centre tap of T1, through R1 back to T2 as shown by the solid arrows. During the second half cycle the reference voltage forward biases D 1 and D2. Current flows from E through D 1, T1 and R1 as shown by the dashed arrows.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–17 © Australian National Training Authority (ANTA) 1997

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Figure 4.13: Ring bridge demodulator − out of phase error signal

It can be seen that the ring bridge demodulator will detect both amplitude and phase of the incoming AC signal.

4–18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Activity 1

1

List four of the main elements of an RPC mechanism. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

2

Define the term response time. _________________________________________________________________

3

State the three methods of mechanical damping. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–19 © Australian National Training Authority (ANTA) 1997

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Servomechanism systems In Sections 1, 2 and 3 we have looked at the different components which form part of a servomechanism system. They may operate on a DC supply, an AC supply or a combination of both. Lets have a brief look at how these systems work.

AC servomechanism operation When the load is in the required position, there will be zero voltage induced into the windings of the control transformer CT. The position of the rotor of the control transmitter CX is at 90 degrees to the rotor of CT by moving the mechanical input to a new setting, the rotor of CX will also move inducing a voltage into the rotor of CT. This signal will now be fed to the amplifier, where it will be amplified and then sent to the servomotor to not only turn the load but also turn the rotor of CT until it is again at 90 degrees to the rotor of CX. Now the rotor of CT will have no voltage induced into its windings, and the drive to the servo motor will be zero and the load will be in the new position. The output from the rate generator is used to achieve velocity feedback and is antiphase to the error signal from the CT rotor windings. The voltage required from the rate generator to achieve the required damping is set by RI and sent to the amplifier. Figure 4.14 shows the circuit of an AC RPC which uses a control transformer as the error detecting device.

4–20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

115 v 400 Hz

AMP

R1

26v 400 Hz

Rate gen Cx

Servo motor

Load

Ct

115v 400Hz

Mech input

Figure 4.14: AC servomechanism

DC servomechanism operation A stationary DC RPC servomechanism is shown in Figure 4.15 the load remains stationary until there is a reason to move it again. If the operator wishes to move the load, the required position will be selected by moving the wiper of RI. RI and R2 form a balanced potentiometer error detector. Which assumes that their wipers are in the mid position. A 28 V DC floating supply is developed across each resistor. As the centre of R2 is at zero, the top of both resistors is at +14 V and the bottom of the resistors are at -14 V with respect to earth. If the operator moves the wiper of R1 to the top, the input to the servo AMP will be +14 V which will then be amplified and used to drive the servo motor to turn the load. At the same time the servo motor will drive the wiper of R2 towards the top. On reaching the top of the resistor the voltage difference between the two wipers is zero and therefore the voltage to the amplifier becomes zero. The servomotor will now stop and the load has taken up its new position.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–21 © Australian National Training Authority (ANTA) 1997

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Fitted to the system is a DC generator or tacho generator whose output polarity will depend upon its direction of rotation. The voltage will always oppose the error signal from the balanced potentiometer. The voltage generated by the tacho generator is proportional to the output shaft speed, and R3 is adjusted to provide the correct amount of output to produce the correct amount of velocity feedback for damping. The input to the amplifier will be the sum of the error signal and the output of the tacho generator. When the load approaches the required position, the error voltage will decrease the small amount of tacho generator output from R3, which is opposite in polarity to the error signal, will eventually cancel the error signal, and there will be zero volts to the amplifier. The servomotor will stop driving, but momentum will continue to drive the load to the required position. The tacho generator will still produce an output which will now be above the error signal and of opposite polarity. This small voltage will be amplified and cause the servomotor to run in the reverse direction and slowing the load. If R3 is adjusted correctly, the load will stop in the required position without oscillation.

Figure 4.15: DC servomechanism

4–22 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Hybrid (AC/DC) servomechanism operation In Figure 4.16 the servomechanism shown uses synchros in an AC error detection system, where the AC error is amplified but then converted by a phase sensitive rectifier to DC to drive the servomotor. The operation of the error detection and velocity feedback parts of the system work the same as for an AC servomechanism.

Figure 4.16: Hybrid servomechanism

Advantages of servomechanisms • remote control of systems • small command input signals used • multiple input commands possible • can be fully automated or integrated into computerised control systems • can be used in explosive liquid or gas environments • can be used underwater.

Limitations of servo systems • electrical, hydraulic or pneumatic power required at all times to provide operation • component failure can disable system • difficult to override in the event of a system failure.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–23 © Australian National Training Authority (ANTA) 1997

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Causes of hunting Hunting was described earlier as A form of instability which occurs when the output shaft swings back and forth through the required position, even though the input shaft is stationary.

The main causes of this condition are: • a large time lag in the system • the lack of velocity feedback • wear in moving parts • unusual external forces • variations in power supply inputs • restriction in moving parts caused by such things as: • lack of lubrication • damage to bearing surfaces.

4–24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Activity 2

1

State the main difference between open and closed loop. Have a look around your workplace or aircraft and try and find some examples of both systems. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

2

Why is modulation/demodulation required? _________________________________________________________________ _________________________________________________________________

3

Describe deadband. _________________________________________________________________ _________________________________________________________________

4

What is used in an AC servomechanism as the error detecting device? _________________________________________________________________ _________________________________________________________________

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–25 © Australian National Training Authority (ANTA) 1997

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Inspection and test Any aircraft system can suffer a malfunction, therefore there will be an occasion when you will have to make adjustments, do alignments or trouble shoot a synchro system. This section will help you with these basic procedures. The most obvious inspection is a visual one, so look for any external damage, such as broken wires, pushed back pins in plugs, etc. check part numbers and serial numbers, MOD status, and serviceability labels. When carrying out tests, either on the aircraft or in the workshop ensure that you have the correct publication. Carry out the test with the test equipment as called up in the maintenance manual, following the instructions precisely. Ensure that all test equipment has been calibrated and is within the calibration date.

System alignment To obtain the correct performance from a servomechanism the synchro devices must be zeroed. In order to identify the positions of the input device, a reference must be established, which is a position defined as zero and all alignments are made with respect to this position. The procedure for servicing and aligning servomechanisms will be laid down in manufacturers instructions, or the appropriate aircraft publications. These instructions should be followed, for only a general description is given here. Before any electrical zeroing can take place, the load and mechanical unit must be placed in its zero position and the shaft locked into position.

Transmitter (CX) The CX synchro must be electrically zeroed so that the voltages induced into the stator coils will correspond exactly with the mechanical zero of the system. Figure 4.17 shows the connections. 1

Disconnect the stator connections to the synchro, and connect a jumper wire between S3 and R2.

2

Connect a voltmeter between S2 and R1 and apply 115 V between R1 and R 2.

3

Rotate the barrel of the synchro until 37 V is indicated on the AC voltmeter.

4

Remove jumper wire and AC voltmeter.

This will give a coarse zero position, and to adjust for a fine position.

4–26 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

1

Connect the AC voltmeter between S1 and S3.

2

Rotate the barrel until the minimum voltage is indicated on the AC voltmeter.

3

Remove the AC voltmeter and reconnect the stator connections.

Figure 4.17: CT synchro adjustment

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–27 © Australian National Training Authority (ANTA) 1997

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Control transformer (CT) The CT synchro must be electrically zeroed to accept the input signals to the system and produce an error output that is representative of the input. The procedure is similar to that used for the transmitter. Figure 4.18 shows the connections. 1

Disconnect the stator connections to the CT synchro and connect a jumper wire between S1 and R1.

2

Connect the secondary of a variac to S1 and S3 and adjust the output to 78 V.

3

Connect a AC voltmeter between S3 and R2 and rotate the barrel of the CT synchro until the AC voltmeter reads null. Remove the variac, jumper wire and AC voltmeter.

Once again, the above is for coarse adjustment, and to adjust for fine: 1

Connect the jumper lead between S1 and S 3, the AC voltmeter between R1 and R2, and the variac to S2 and S 3.

2

Adjust the variac output to 78 V.

Rotate the barrel of the CT synchro until the AC voltmeter reads null. Remove the variac, jumper lead and AC voltmeter, reconnect the stator connections. It should be remembered that the electrical zero position of the CT syncro, is 90 degrees from that of the CX sychro.

4–28 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Figure 4.18: CT synchro adjustment

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–29 © Australian National Training Authority (ANTA) 1997

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Troubleshooting The following conditions can cause total or partial failure in a servomechanism.

Power failure A suspected power supply failure can be checked quite easily by measuring the AC volts across the rotor winding of the transmitter check the fuse.

Open circuit An open circuit in either rotor winding will cause the receiver synchro to stick in one position. If a stator winding had an open circuit the operation would be sluggish.

Short circuits Will cause the fuse to blow or, at the worst, component and wiring burn out. Whichever, the servomechanism will have ceased operation.

Incorrect wiring This condition usually results in reversed direction. The following is a table of common synchro system faults.

4–30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Symptom

Cause

CT synchro follows CX synchro sluggishly

Open circuit between R1 and R2

360 degrees rotation of CX rotor causes CT rotor to oscillate between 60 degrees and 180 degrees or 240 degrees and 300 degrees

Open circuit at S1

360 degrees rotation of CX rotor causes CT rotor to oscillate between 120 degrees and 240 degrees

Open circuit a S2

CT rotor follows CX rotor but 180 degrees difference

R1 and R2 reversed

CT rotor turns in opposite direction from CX rotor

S1 and S3 reversed

Reference voltage fuse blows

R1 shorted to R 2

CT rotor locks at 0 degrees or 180 degrees CX rotor locks at 0 degrees or 180 degrees

S1 shorted to S3

The above table gives a few examples of faults and symptoms in synchros, they are generally logical and clear cut. The servomechanism system is a closed loop, so there is no correct or incorrect place to start, it is really a matter of gathering the facts, and then following the loop until you find the faulty item.

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–31 © Australian National Training Authority (ANTA) 1997

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Review Now work through the Check your progress questions to see how well you understood Section 4. If there is anything you are not sure of, revise the relevant work. If you would like additional information to help your understanding of any part of this section, use the reference books listed at the beginning of this module.

4–32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Check your progress 4

1

What is meant by null? _________________________________________________________________ _________________________________________________________________

2

What is the reference signal? _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

3

Explain hunting. _________________________________________________________________ _________________________________________________________________ _________________________________________________________________

4

Where will you find the procedure for aligning a servomechanism? _________________________________________________________________ _________________________________________________________________

5

What is the result of an open circuit in a rotor winding? _________________________________________________________________ _________________________________________________________________

6

If the reference voltage fuse blows, what is the cause? _________________________________________________________________ _________________________________________________________________

Section 4: Servomechanism systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4–33 © Australian National Training Authority (ANTA) 1997

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

4–34 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section 4: Servomechanism systems © Australian National Training Authority (ANTA) 1997

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Well done! You can now say that you have finished this module, and now know all about error devices, synchronous systems and servomechanisms. Remember that aircraft can be dangerous things, especially when electrical power and hydraulics are applied. With engines running, there is the added danger of spinning props, jet intakes and exhausts. If you are in an instrument workshop, cleanliness is a must, and remember instruments are very delicate. Please observe all safety regulations. IF IN DOUBT, ASK! Now review what you have learnt by following the learning outcome checklist.

Learning outcomes checklist Learning outcome 1:

Describe the construction and operation of error detection devices.

Assessment criteria

I can do it

I can’t do it

I’m unsure

Identify the following error sensing devices: •

differential transformers: •

LVDT

•

E and I bar

•

C and Y

•

pendulous monitors (accelerometers)

•

inductive

•

capacitive

•

resistive.

Describe the construction of the error sensing devices listed above. Describe the operation of the error sensing devices listed above.

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

R–1

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Learning outcome 2:

Describe the operation of DC synchronous systems and test them for serviceability.

Assessment criteria

I can do it

I can’t do it

I’m unsure

Identify the different DC synchronous systems: • Selsyn:

•

•

two coil

•

three coil

Desynn: •

slab

•

micro.

Describe the operation of the Selsyn synchronous system: • two coil •

three coil

•

limitations.

Describe the operation of the Desynn synchronous system: • slab •

micro

•

limitations.

Inspect and test a DC synchronous system and troubleshoot as required. Identify which indications DC synchronous systems are used to indicate: • position •

R–2

pressure.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Module review © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Learning outcome 3:

Describe the operation of AC synchronous systems and test them for serviceability.

Assessment criteria

I can do it

I can’t do it

I’m unsure

Identify the basic AC synchronous systems: • autosyn •

magnesyn

•

ratiometer.

Describe the operation of a torque synchro system: • synchro transmitter (TX) •

synchro receiver (TR)

•

differential transmitter (TDX)

•

symbols

•

purpose

•

null point.

Inspect and test a torque synchro system and troubleshoot as required: • wiring connections:

•

•

crossover

•

open circuit

rotor relationships.

Describe the operation of a control synchro system: • synchro transmitter (CX) •

synchro receiver/control transformer (CT)

•

differential transmitter (CDX)

•

symbols

•

purpose

•

null point.

Inspect and test a control synchro system: • wiring connections •

rotor relationships.

Module review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

R–3

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Describe the operation of a Synchrotel system: • stator •

rotor

•

purpose

•

null point.

Define the following terms: • polar coordinates •

cartesian coordinates

•

sine signals

•

cosine signals.

Describe the operation of a resolver synchro system: • resolver (RS) •

symbol

•

purpose.

R–4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Module review © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Learning outcome 4:

Describe and test servomechanism systems.

Assessment criteria Define the terms associated with servomechanisms: • null •

error signal

•

feedback

•

reference signal

•

system alignment

•

hunting

•

damping: •

under

•

over

•

critical

•

overshooting

•

deadband

•

response time

•

time lag.

I can do it

I can’t do it

I’m unsure

Define the difference between an open loop and a closed loop system. Describe the operation of a servomechanism system to block diagram level identifying the components: • command transmitter •

error detector

•

amplifier

•

summing point/network

•

servo

•

position feedback

•

rate feedback

•

acceleration feedback

•

modulator/demodulator.

Module review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

R–5

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Identify differences in operation between types of servomechanism systems: • AC •

DC

•

Hybrid

•

advantages

•

limitations.

Identify the causes of hunting. Inspect, test, and align a servomechanism system and troubleshoot as required.

R–6

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Module review © Australian National Training Authority (ANTA) 1997

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Section 3 Activity 1 1

Your diagram should show that no potential difference exists and no current flow between the units.

2

Your diagram should show that the indicator pointer will remain on scale when the power fails.

3

Your diagram should show an AC bridge.

4

Your diagram should show a torque transmitter and torque receiver.

Activity 2 1

British = S 1, S2, S 3 American = S2, S1, S3

2

Refer to figure 3.25

3

Refer to figure 3.27

4

RS

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A–1

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

Section 4 Activity 1 1

Input Error measuring device Error voltage Amplifier Servomotor Output Position feedback Damping

2

The time taken by the system to reach a new steady state after a change of demand.

3

Viscous damping Hysteresis damping Coulomb damping

Activity 2 1

Open loop requires the command to be cancelled by a manual adjustment. Closed loop the command is cancelled automatically when the output equals the input.

2

DC signals do not provide stable amplification and cause variations in DC output.

3

The error either side of the correct position of the system.

4

A control transformer.

A–2

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Answers to activities © Australian National Training Authority (ANTA) 1997

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Answers to check your progress questions ○

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Check your progress 1 1

To an amplifier to drive a servo.

2

The winding is divided into two parts and wound to oppose each other.

3

A hollow cored differential transformer with a straight iron core which can move in either direction.

4

Outputs are generated in both coils but moving the iron bar towards one end of the core will create a greater output from one coil. Moving the core to the other end will create an inverse output. The phase relationship will be determined by the position of the core and the amplitude will be determined by the degree of coupling

5

Inertia as the frame is carried forward.

6

The C and Y transformer can be operated by linear or rotary motion.

7

The pendulous sensor can detect long term changes or changes too small to be detected by other devices.

8

True. The inductor forms part of an oscillator circuit. Movement of the core changes the inductive reactance and hence the frequency of oscillation.

9

True. The capacitor forms part of an oscillator circuit. Movement of the plates changes the capacitive reactance and hence the frequency of oscillation.

10 A simple voltmeter graduated to read the quantity being measured.

© Australian National Training Authority (ANTA) 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C–1

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

11

Indicate a rotary position.

12

Micro sensor.

13

Thermistor.

Check your progress 2 1

As the transmitter and receiver are connected in parallel this change will have no effect.

2

False. The gap forms the third pole of the core as the coils are mounted 120 degrees away from the gap.

3

True. The closely packed windings can give closer variations of resistance, hence greater accuracy.

4

False. The indicator is capable of giving 360 degrees of rotation.

5

The pull off magnet will cause the needle to move off scale.

6

The major difference is the method of connection of the coils. The Selsyn is connected in delta and the Desynn in star.

7

N S +

-

N

24v DC supply

S

Slip rings

N S

Transmitter

C–2

Indicator

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Answers to check your progress questions © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

8 3

3

Outer resistors 2 2

1

3

1

3

Resistors opened out

3 1

2

3

Inner resistors 2

1

2

1

3

2 3 + B -

1

3

2

Linear actuation

Answers to check your progress questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

C–3

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

9

Slab Desynn. Bourdon tube or any circular motion transmitter. Micro Desynn. Pressure operated diaphragm or bellows.

10

The transmitter resistance element consists of a slab former over which the wire is wound. One side of the slab is convex and it is over this surface that the contacts are positioned and moved by the medium being measured. The contacts are mounted upon a spindle and are spaced 120 degrees apart. Electrical connections are made via slip-rings to the indicator stator coil windings.

Check your progress 3 1

False. The rotor is a permanent magnet, the 26 V AC 400Hz supply is connected to the stator.

2

TR and DR

3

TX

4

Transmit a position.

5

A pressure sensitive device which will cause a linear movement of two armatures.

6

Mechanical signals into electrical signals.

7

Adds or subtracts.

8

True. The output can only ever be in phase or 180 degrees out of phase. The amplitude will vary with the degree of rotation.

9

False. The rotor is connected to the output of one CX.

10

Sensing bellows.

11

90 degrees.

12

To change position data from one form or co-ordinate to another by converting alternating voltages.

C–4

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Answers to check your progress questions © Australian National Training Authority (ANTA) 1997

Synchros and servomechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NAA09

Check your progress 4 1

No error signal is being produced by the error detection device.

2

A signal supplied to the amplifier so that it may determine which phase the error signal is in.

3

Hunting: The time lag in the system where corrective action lags the change of input.

4

Manufacturers’ instructions or aircraft publications.

5

The receiver synchro will stick in one position.

6

R1 and R2 are shorted together.

Answers to check your progress questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . © Australian National Training Authority (ANTA) 1997

C–5

NAA09 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synchros and servomechanisms

C–6

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Answers to check your progress questions © Australian National Training Authority (ANTA) 1997