MATERIALS
FOR THE
AEROSPACE INDUSTRY
Flexible Delivery Student Learning Materials for
Apprentice Aircraft Engineers / Mechanics
AVIONICS
MODULE NAA09
Synchros and servomechanisms
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.
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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
Module introduction
vii
Before you start
vii
How this module is organised
vii
References
ix
Glossary
x
Section 1
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
Section 2
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
Check your progress 2
2–17
Section 3
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
Section 4
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
Check your progress 4
4–33
Module review
R–1
Learning outcomes checklist
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
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Module introduction
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.
• 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
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
Avia-tion AdministraAvia-tion PublicaAvia-tions, 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.
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Glossary
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.
direction the servo motor will drive.
Remote position control These are used to control angular or linear position of a
servos 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
1 Error detection
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devices
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.
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.
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 degreesout of phase with the input
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.
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.
Error output Movable core
Secondary Secondary
Primary
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.
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.
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.
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.
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.
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.
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.
(C) Movable plate Fixed plate
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.
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 degreesof sensing. Figure 1.14 shows the theoretical
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 degreesof 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.
2 2 2 1 1 3 3 3 B
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.
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.
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.
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.
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.
_________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________ _________________________________________________________________
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.
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.
_________________________________________________________________ _________________________________________________________________
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.
2 DC synchronous
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systems
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
• inspect and test a DC synchronous system and troubleshoot as required • identify which indications DC synchronous systems are used to indicate:
• position • pressure.
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.
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 degreesapart and 120 degreesfrom 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.
Airgap Open circuit in winding here Movable wiper Damping ring Pointer 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.
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.
+
-Rotatable contact arm
Resistor with taps equally spaced
Unlimited rotation transmitter Indicating element Rotatable
magnetised core
Laminated iron core with 3 coils placed 120° apart
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.
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.
S N Open circuit in winding Transmitter Indicator Movable wiper arm Power supply
Rotatable magnetised core
C1 C2 Damping ring Laminated iron core Air gap
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.
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
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.
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.
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 N S S + -24v DC supply Transmitter Indicator Slip rings N S
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.
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.
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 2 2 1 1 3 3 3 B + -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.
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.
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.
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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.
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?
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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
degreesof movement.
Give reasons for your answer.
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5 Explain what will happen to the indicator of a three coil Selsyn when power is removed.
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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.
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8 Briefly describe the construction of the micro Desynn transmitter. Use a diagram to help you.
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9 Describe the types of mechanical devices best suited to operate the:
slab Desynn
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micro Desynn
_________________________________________________________________ 10 Briefly describe the slab Desynn transmitter.
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3 AC synchronous
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systems
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
• 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.
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.
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.
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.
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.
Cam - shaped discs Bellows Armature Stator 26 V AC 400 Hz Transmitter Indicator Indicator Transmitter 26 V 400 Hz Lamination & bobbin assy Damping magnet & disk N S
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 Rotor field Torque receiver 115 V AC R1 R2 S1 S2 S2 S1 S3 S3
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 S3.
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.
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
3 Draw a circuit diagram of the ratiometer system, showing the circuit arrangement.
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.
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.
S1 S2 S2 S2 S1 S3 S3 R1 R2 R1 R2 TX DT TR R2 S3 S1 R3 R1 S S R R R S
