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4 SYNCHRONOUS DATA TRANSMISSION
4.2 SYNCHRO SYSTEMS
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4.1.2 SLAB DESYNN
If the voltage at the 3 tappings of the transmitter of a basic Desynn are measured as the wiper arms are rotated 360°, it will be seen that they produce a sawtooth
waveform as opposed to a sinewave. This results in the pointer of the indicator not following the transmitter exactly. In most instances the difference is insignificant, however their may be certain circumstances where it cannot be overlooked.
The solution is to use a modified Desynn transmitter called a 'slab Desynn'. In a slab Desynn, the resistor is wound on a slab former and has the power supply connected to it, whilst the wiper arms now provide the output to the receiver, there being 3 wiper arms each displaced from the next by 120°. The output from this device is a sinewave. It can be connected to the same type of indicator and operates in the same way as the basic Desynn.
4.2 SYNCHRO SYSTEMS
Synchro's are electromagnetic devices used to transmit positional data electrically from one location to another. They have an advantage over Desynn's in that they can also be used to compute the sum of two rotations, or the difference in angle between them. They are used in applications requiring low output torques.
Servo systems, which will be examined in the next section, employ synchros in conjunction with an amplifier and a controlling motor to provide an automatic control mechanism. They are used in applications requiring output torque's greater than those which can be produced by a synchro.
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4.2.1 SYNCHRO TYPES
Synchro types may be classified as follows:
• Torque transmitter
• Torque receiver
• Torque differential receiver
• Torque differential transmitter
• Control transmitter
• Control transformer
• Control differential transmitter
• Resolver
4.2.1.1 Torque Transmitter - TX
Used to generate an electrical signal corresponding to the angular position of a mechanical component. The rotor is connected to the component and the stator kept stationary. The electrical signal is derived from the position of the rotor relative to the stator. The TX is generally used as the transmitting element in a remote position indicating system.
4.2.1.2 Torque Receiver - TR
The rotor of a torque receiver, which is free to turn, moves to a position dependent on the electrical angular information received from its connected torque transmitter or torque differential transmitter. The TR is generally used as the receiving element (indicator) in a remote position indicating system.
4.2.1.3 Torque Differential Receiver - TDR
The torque differential receiver is electrically connected to two torque transmitters.
The rotor of the TDR, which is free to move, aligns with the stator field. The position of the stator field depends on the inputs from the two transmitters, and the way in which they are interconnected. By suitable connection, the TDR can be made to indicate the sum of the transmitter inputs, or the difference between them.
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4.2.1.4 Torque Differential Transmitter - TDX
The torque differential transmitter has a stator that receives electrical positional information from a torque transmitter, and a rotor which is mechanically positioned.
This enables it to transmit electrical information corresponding to the sum, or difference, between the electrical input and its own rotor angle.
4.2.1.5 Control Transmitter - CX
Used to generate an electrical signal corresponding to the angular position of a mechanical component. The rotor is connected to the component and the stator kept stationary. The electrical signal is derived from the position of the rotor relative to the stator. The TX is generally used as the position transmitting element in a remote position control system.
4.2.1.6 Control Transformer - CT
A CT is electrically connected to a CX and is used to produce an electrical signal for driving a servo system. The electrical signal produced, is an a.c. voltage with an amplitude and phase dependent on the position of the rotor relative to the stator.
4.2.1.7 Control Differential Transmitter - CTX
A CTX receives electrical information from a CX and has a rotor which can be mechanically moved. This enables it to transmit an electrical signal proportional to the sum or difference in angle between the electrical input and its own rotor position.
4.2.1.8 Resolver
A resolver has two mutually perpendicular windings on the rotor and another two on the stator (4 windings in total). It can resolve an input signal into its sine and cosine components, perform the operations of vector addition and subtraction or convert polar to cartesian co-ordinates and vice versa.
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4.2.2 SYNCHRO SCHEMATICS
All synchros are represented by the same basic schematic symbol which may be drawn in any one of three different ways:
This is the simplest and possibly the most commonly used representation in
maintenance manuals. The code letters are inserted in the centre circle to identify the type and function of the synchro.
Used when an explanation is given of the operation of a synchro. The schematic shows the rotor in the zero degree position.
This is now commonly used when an explanation is given of the operation of a synchro.
Note: By convention, the vertical winding in the last 2 schematics is identified as S2, the lower right as S1 and the lower left as S3.
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Differential transmitter and receiver synchros can be represented schematically by any one of the following symbols.
The resolver synchro can be represented schematically by any one of the following symbols.
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4.2.3 XYZ SYNCHRO SYSTEM
Synchros often appear in aircraft wiring and schematic diagrams with the letters X, Y, Z indicating the free end of each stator winding and the letters H and C indicating the ends of the rotor.
S1 --- X S2 --- Z S3 --- Y R1 --- H R2 --- C
When connections to earth are required, the stator wire designated S2 or Z is earthed and the C end of the rotor winding is earthed.
4.2.4 SYNCHRO SUPPLIES
Synchros used in aircraft data transmission systems are operated from either 115V 400Hz or 26V 400Hz alternating current supplies. Radio systems commonly employ 26V 400Hz.
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4.2.5 TORQUE SYNCHRO SYSTEM
Torque synchro systems are used where the turning force or torque required is very small. The system only produces sufficient torque to move a pointer over a scale, or to operate a micro switch, because of this they are limited to indicating systems.
4.2.5.1 Construction
The torque synchro system comprises a Torque transmitter (TX) and a Torque Receiver (TR) interconnected as shown below.
In practice:
• R2 and S2 will be connected to earth.
• The rotor of the TX will be mechanically rotated by suitable means appropriate to the system whose positional information has to be transmitted.
• A pointer, which will indicate the transmitted data, is normally attached to the rotor of the TR.
The ac power supply is connected to both rotors, the rotors being connected in parallel.
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With supply current flowing, voltages are induced is the stator winding of both the TX and TR by transformer action. With the rotors in the same angular position, as shown in the diagram, the voltages in the TX and TR will be equal and opposite, hence no current will flow in the stator coils and interconnecting wires. The system is said to be balanced or nulled.
The voltage induced in the stator coils will depend on:
• The ratio of the number of turns on the rotor to the number of turns on the stator and,
• the angular position of the rotor with respect to the stators.
For the position of the rotors shown in the diagram, the voltages induced in the stators of both transmitter and receiver would be:
• S1 half maximum voltage
• S2 maximum voltage
• S3 half maximum voltage
If the transmitter rotor is rotated through any angle, the voltages induced in the stator coils of the TX will change. The voltages induced in the stator coils of the TR will remain unchanged. This creates potential differences across the
interconnecting wires, and current flow in them. The current flows produce magnetic fields around the stator windings which combine to form a resultant field across the stator of both the TX and TR.
A torque reaction will now exist between the resultant stator field and the field that exists around the rotor. This torque reaction will exist at both the TX and TR.
The rotor of the TX is held by the system whose positional information has to be transmitted and cannot move. The rotor of the TR is however free to rotate and moves around in response to the torque.
Once the TR rotor is in the same angular position as the transmitter rotor, the voltages induced in the stators will again be equal and opposite, current will cease to flow and the system will once again be balanced.
To ensure accuracy of the system there must be sufficient current flow to produce a torque even for small changes in transmitter position. This requires the impedance of the windings to be very small. Under normal operating conditions this is of no concern, however, should the receiver pointer jam then a large potential difference would exist between the TX and TR with resulting high currents. This can easily lead to one or both of the synchros burning out.
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A loss of supply to the TR rotor will result in Low Torque operation with possible 180° error.
A loss of supply to the TX rotor will result in no operation of the synchro.
An open circuit on one stator line will result in the receiver oscillating between 2 points approximately 75° apart.
A short circuit between 2 stator lines will result in the receiver being displaced by 0°, 60°, 120°, 180°, 240° or 300° and movement in 180° steps.
The table below shows the results or effects of a number of cross connections.
Cross Connections Fault Symptoms rotates in same direction as the transmitter.
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4.2.6 ELECTRICAL ZERO
The electrical zero setting of a synchros provides a standard means of aligning synchros units so they will all have the same position at the same instant. This setting provides a common reference point at which all synchros are set before being installed.
Electrical zero is defined as the position of the rotor with respect to its stator when the voltage between S1 and S3 is zero and the voltage at S2 with respect to S1 or S3 is in phase with that of R1 with respect to R2. It simply means that the rotor is parallel to S2 and that R1 is at the top. By connecting the voltmeters as shown electrical zero can be determined. V1 should indicate zero and V2 should indicate a value less than the supply voltage. Remember that if R2 were at the top V1 would still indicate zero but, if the voltage between R1 and S2 would be antiphase and V2 would indicate a value greater than the supply voltage.
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4.2.7 DIFFERENTIAL TORQUE SYNCHRO SYSTEM
A differential synchro system consists of a differential synchro used in conjunction with a synchro transmitter and receiver. It is electrically connected as shown in the diagram below.
The first thing to notice is that the rotor of the differential synchro has three equally spaced windings and is connected to the transmitter and receiver stators. When connected as shown it will provide an output which is the difference between the two inputs from the mechanical drives. It can also be wired to produce an addition of the two inputs. There is no connection between the differential synchros and the
supply.
4.2.7.1 Operation
Consider the differential synchro to be three 1:1 transformers between the three stator windings of the transmitter and the three stator windings of the receiver.
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When the system is set as shown in the diagram above, (the interconnecting wires have been removed for clarity), the induced voltages in the stators and across the transformers will be equal and no current will flow in any of the interconnecting wires.
If the transmitter (on the left) is turned by 60º, the TX stator voltages will change and current will flow around the stator windings. Resultant fields will be set up and the TR rotor will feel torque, so the rotor will turn until, again, the voltages are equal and current stops flowing.
An important thing to remember is that all three components feel the torque reaction created by the interaction of rotor and stator fields, but because the transmitter rotors are mechanically connected to other systems they will not be free to move.
Only the receiver rotor (on the right) is free to respond.
If the TX is left stationary and the TDX is rotated by 15º the voltages will be different and current will flow around the stator windings. A torque reaction will occur and the rotor on the receiver will turn until the voltages are equal and current stops flowing.
It should be noted that when the TDX is wired as shown, clockwise rotation of the TDX results in anticlockwise rotation of the TR.
If both the TX and the TDX were rotated then the TR would show the difference between the two movements.
The differential synchro need not always be a transmitting device. The system could be arranged with two transmitting synchros and a TDR with a pointer attached. Under these conditions, the torque differential receiver (TDR) is the receiving element, but the system will respond as previously described to show the
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4.2.7.2 Sum of Two Inputs
If the S1 and S3 connections between the TX and TDX are crossed and the S1 and S3 connections between the TDX and TR are also crossed, the system will
algebraically add the two mechanical inputs.
i.e. TX 30° clockwise TDX 15° clockwise moves the TR 45° clockwise.
TX 30° anticlockwise TDX 15° clockwise moves the TR 15° anticlockwise.
A similar arrangement of cross connections could be used on a TDR type system to make the TDR show the sum of two inputs.
4.2.8 CONTROL SYNCHRO SYSTEM
Control synchros are used in electromechanical servo and shaft positioning systems. They only produce a signal representative of the position of the transmitter. This signal can then be amplified many times to power very large motors that can move very large loads to a desired position.
4.2.8.1 Construction
In construction, control synchros are similar to torque synchros but because they do not have to handle any motive power for driving a load they may be of lighter
construction. Also, because the signal from the receiver is going to be amplified to drive an output, the impedance of the windings can be made much higher and there is no danger of the system burning out. The control synchro system is the most common of all synchros and has extensive use in aircraft instrument and
navigational systems.
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4.2.8.2 Operation
In a control synchro system the ac power supply is only connected to the rotor of the transmitter, the CX. The signal representing the position of the transmitter is
obtained from the rotor of the receiving element, the CT. Note that in the balanced or nulled position, the rotors of the CX and CT are at 90° to each other.
When the rotor of the CX is in the position shown, maximum voltage is induced in stator S2 and half maximum voltage is induced in stator windings S1 and S3. No emf's are induced in the stator windings of the CT, therefore a potential difference exists between each stator winding of the CX and CT and currents flow in the transmission wires.
The current flowing in the CT stator windings produce magnetic fields that combine to form a magnetic field across the stator. This alternating field cut's the rotor winding. The emf induced in the CT rotor winding depends on the position of the rotor relative to the resultant field. When the rotor winding is parallel to the resultant field, maximum voltage is induced in it, when the rotor is at 90° to the resultant field, zero emf is induced in it.
The amplitude of the induced emf is proportional to the sine of the angle between the rotor and resultant field. The phase of the induced emf depends on whether the rotor is clockwise or anticlockwise of the balanced or nulled position. The control transformer can therefore be considered as a null detector and is most often used in servo systems.
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CT Rotor Position EMF Induced in CT Rotor
90° clockwise
90° anticlockwise
5° clockwise
5° anticlockwise
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4.2.8.3 Control Synchro Servo System
The diagram below shows a control synchro system employed in its most common role as part of a servo system.
As shown, the system is balanced, zero emf is induced in the CT rotor, there is no output to the servo motor and the motor and pointer are stationary.
If the rotor of the CX is now moved clockwise, the resultant field across the stator of the CT will also move around clockwise. The rotor of the CT is now no longer at 90°
to the resultant field and therefore has an emf induced in it.
The emf is applied to a discriminator amplifier to sense its phase relationship to the excitation supply, to obtain direction information, and then applied to the motor. The motor turns, driving the pointer and at the same time driving the rotor of the CT towards the balanced position (90° to the resultant field).
When the rotor is at 90° to the resultant field, the induced emf falls to zero and the motor stops, the pointer having moved to indicate the new position.
If the rotor of the CX had been moved anticlockwise, the error signal in the CT rotor would have been of opposite phase and the motor would have turned in the
opposite direction to once again null or balanced the system.
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4.2.9 DIFFERENTIAL CONTROL SYNCHROS
These are in common use. Their operation is the same as for Torque differential
These are in common use. Their operation is the same as for Torque differential