Aviation Servomechanisms (4.3) PDF

Servomechanisms (4.3) Learning Objectives 4.3.1.1 Recall the following terms: open and closed loop, follow-up, servomechanisms, analogue, transducers and feedback (Level 1). 4.3.1.2 Describe the principle of operation and use of the following synchro system components and features:...

Servomechanisms (4.3) Learning Objectives 4.3.1.1 Recall the following terms: open and closed loop, follow-up, servomechanisms, analogue, transducers and feedback (Level 1). 4.3.1.2 Describe the principle of operation and use of the following synchro system components and features: resolvers, differential, control and torque, transformers, inductance transmitters, and capacitance transmitters (Level 1). Summary Servomechanism theory is an introduction to control systems. Servomechanisms are automatic devices used alongside electronic, hydraulic, or mechanical systems. They use error-sensing negative feedback to correct the action of a mechanism and keep it running at the desired operating conditions. In aircraft, during normal flight, a digital signal communicates the pilot's instructions electrically to control servomechanisms that position the plane's control surfaces as needed. If the servomechanism was not there, the pilot would have to control every aspect of the flight. However, in modern systems servomechanisms are monitoring sensor feedback including accelerometers to keep the aircraft stable. Without servomechanism technology flight-control systems would be nearly impossible and the large safe aircraft taken for granted today would be impractical. 2022-11-10 B1-04 Electronic Fundamentals Page 127 of 163 CASA Part 66 - Training Materials Only  Servomechanism Terminology Servomechanisms In control engineering a servomechanism, sometimes shortened to servo, is a device that uses an external power source to move a load with forces at a higher level of energy than the input level. A servomechanism is a type of control system. Servomechanism systems are used for measurement and control in a range of aircraft systems such as pressurisation, brakes, flight control, engine control, etc. The term applies to systems where the feedback or error-correction signals help control mechanical position, speed, attitude or any other measurable variables, as well as those systems with no feedback. For example, an automotive power window control is a servomechanism, as there is a larger output force than operators force - the operator does this by observation. By contrast a car's cruise control uses closed-loop feedback, which automates the system. Typically, a servomechanism is a control system that provides the following: A command device to control the program or final process. Amplification to strengthen and modify the command signal. Work instrumentation to manipulate the controlled process. Feedback provision to initiate corrective action when needed. In aviation, there are two main classes of servomechanism: position control and speed control. Flight control servo system 2022-11-10 B1-04 Electronic Fundamentals Page 128 of 163 CASA Part 66 - Training Materials Only  Servomechanism Terminology The following diagram is accompanied by some description outlining key elements of a basic servomechanism. © Aviation Australia Basic servomechanism Controller The controller is the part of the process that manages or controls the entire system. In the basic servomechanism diagram above, it is a computer processor. Disturbance Disturbance of a system is something that forces the output of a system to change without a demanded change at the input. For example, turbulent winds can cause changes in heading, bank angle or height. Feedback Feedback can be mechanical such as a shaft or gear system, some physical movement (in autopilot it may be called aerodynamic), or electrical (voltage or current). Mechanical feedback is usually detected by a transducer and converted to an electrical signal as modern systems are computer controlled. Feedback is derived from a comparison of an actual response to a desired response (servomechanism input), this could be a position of velocity to help obtain proper system operation. Systems employing feedback are called closed-loop systems; the feedback closes the loop. 2022-11-10 B1-04 Electronic Fundamentals Page 129 of 163 CASA Part 66 - Training Materials Only  Null Point In physics, the null point means the point in a field where the field quantity is zero. The field quantity is zero because the two or more opposing forces at the null point cancel each other out. In transducer terminology, the null point is the point where the output voltages are neither positive or negative therefore zero. In synchro terminology, the null point is the point where the output position matches the input desired position, therefore no demanded movement. Summing Point A summing point is the part of a system where error signals are modified by other factors such as output rate or position. In modern systems, summing of these factors is a software function and therefore may only be seen in a schematic diagram as a method to show virtual operation. Transducers A transducer is a device, component machine, system or combination of these that is used to convert one form of energy into another. There are many different types of transducers: Temperature transducers, which convert temperature changes into electrical voltages (or mechanical movement, for example a bimetallic strip) Electric motor converting electrical energy into kinetic energy Pressure transducers, which change barometric pressure into electrical voltage. The transducers described in this module are only those which convert to electrical signals. Transducers that produce other energy outputs from electrical inputs we refer to as actuators. 2022-11-10 B1-04 Electronic Fundamentals Page 130 of 163 CASA Part 66 - Training Materials Only  Open-Loop Control Open-loop control is where the control action is independent of the output. The open-loop control does not self-correct when disturbances causes output drift and this may result in large deviations from the required value. The system may use either full manual control or be connected to a timer. For example, a plane experiences a strong crosswind, if the system was open-loop it would require the pilot to manually compensate via controls. A timer may be used in open-loop systems that just run via a timing sequence. An example of this is a clothes dryer, it does not check if the clothes are dry (feedback) - it simply runs through a standard timing sequence. Aviation Australia Open-loop control system block diagram As explained, the open-loop control system does not use a comparison of the actual result and the desired result to determine the control action. The primary advantage of open-loop control is that it is simpler to implement and less expensive than closed-loop control. The disadvantage of open-loop control is that errors caused by unexpected disturbances are not corrected (automatically). Typically the servomechanisms used on aircraft are closed-loop, however servomechanisms do exist in open-loop format. A simple motor used to rotate a television-antenna is an example of an open- loop servomechanism. 2022-11-10 B1-04 Electronic Fundamentals Page 131 of 163 CASA Part 66 - Training Materials Only  Control Systems and Feedback A control system is the general term for any system that manages, commands, directs, or regulates the behaviour of other devices or systems using control loops. It can range from smaller systems such as a home heating controller to large industrial systems for control processes and machines. Control systems are of course also used extensively in aviation. A servomechanism is an electronic control system. For continuous control, a feedback loop is used to control a process or operation. Closed-loop control system diagram The control system compares the measured output with the desired set point and adjusts the system accordingly. The control system shown is known as a closed-loop control system. Closed-Loop Control Closed-loop control differs from open-loop control in that feedback is added to the system. Feedback consists of measuring the difference between an actual output parameter and the demanded result. By using the difference, the closed-loop control system will drive the actual result towards the desired result. The advantage of a closed-loop control system is that it gives more accurate control over the process. 2022-11-10 B1-04 Electronic Fundamentals Page 132 of 163 CASA Part 66 - Training Materials Only  Follow-Up In the simplest terms “follow-up” is the act of cancelling out the control that destabilised a system to create a controlled change. Using the example of a driver turning a corner in a car. The steering inputs they use to navigate the turn are described in the following sequence. In position A, on approach to the corner, the car is in a stable condition traveling at a constant speed in a straight line with the front wheels pointed straight ahead. At position B, the driver turns the steering wheel to turn the corner. The direction of the front wheels has been changed, destabilising the car and initiating the change in direction. From positions C to D the position of the front wheels is constant so the car is re-established in a stable condition during the turn. At position E, the “follow-up” takes place as the driver once again changes the stability of the car by turning the steering to the left, this time to get the car to go in a straight line again. Aviation Australia Follow-up example using a car turning a corner Follow-up in an aircraft goes even further, the pilot rolls the aircraft to initiate a turn to change heading. Once the turn is established the ailerons must be centred to stop the rolling motion (not follow-up). The aircraft remains in a banked condition as the aircraft turns. 2022-11-10 B1-04 Electronic Fundamentals Page 133 of 163 CASA Part 66 - Training Materials Only  As the aircraft approaches the required heading the “follow-up” can now happen. The pilot performs the follow-up action by moving the control column in the opposite direction rolling the aircraft out of the bank. The pilot centres the control column to complete the follow-up action once the aircraft is on the correct heading and the aircraft is straight and level. The term ‘follow-up’, when applied to an automatic flight control servomechanism, is used to describe an error feedback signal that is generated by the error detection circuits. The purpose of a follow-up signal is to return the aircraft to stable flight at the selected altitude or heading. To achieve a new altitude or heading, an automatic flight control system servomechanism must move a control surface to initiate a change in aircraft pitch or roll. The control surface will return to the neutral position with the aircraft pitch or roll adjusted to enable the aircraft to fly toward the new altitude or heading. As the aircraft is approaching the required altitude or heading a follow-up signal is sent to the servomechanism to adjust the control surface once more. The follow-up signal prevents the aircraft from flying past (or overshooting) the selected altitude or heading by adjusting the flight control surfaces in the opposite direction. Aviation Australia Follow-up signal of an automatic flight control system The servomechanism uses the follow-up signal to adjust the control surface so that the aircraft will change it’s pitch or roll angle to return to stable flight at the selected altitude or heading. As the aircraft approaches the selected altitude or heading, the follow-up signal is larger than the error signal and drives the controls back to a neutral position. It then decreases the follow up signal until the error and follow-up signals are both at zero, and the aircraft is on the new heading or altitude. 2022-11-10 B1-04 Electronic Fundamentals Page 134 of 163 CASA Part 66 - Training Materials Only  Analogue Transducers The information regarding the position or rate of a controlled device is often accomplished with an analogue transducer in place of a potentiometer. As previously introduced, a transducer converts a mechanical input to an electrical output. In aircraft, the analogue transducer converts the differing position or rate of movement of the physical flight control surface into a variable electrical output signal that can be processed by the controller. Aviation Australia Analogue transducer converts positional data into electrical data for the controller Closed-loop control systems must include the means to measure the variables being monitored or controlled in the system. These variables are many in number and might include one or more of the following; position, displacement, force, fluid level, pressure, flow rate, temperature and velocity. 2022-11-10 B1-04 Electronic Fundamentals Page 135 of 163 CASA Part 66 - Training Materials Only  Hydraulic pressure transducer The devices that measure variables are analogue devices in that their outputs in terms such as voltage, current, frequency, pressure, etc., are varying indications of the conditions they represent such as mechanical displacement, volume, weight, speed, flow, temperature, pressure, etc. Devices capable of translating physical variables into an equivalent electrical variable are called electrical transducers, or, for ease, just transducers. Most transducers are of the passive variety, requiring an external source of electrical excitation for their operation. Active transducers develop an output without external excitation. The thermocouple is such a device and is used for temperature measurement, another is the tachogenerator that can be used to produce a voltage or frequency, based on speed of rotation and can even power the circuits of the control elements. A strain gauge is used as a transducer to convert applied force, pressure, weight or tension into electrical signals. 2022-11-10 B1-04 Electronic Fundamentals Page 136 of 163 CASA Part 66 - Training Materials Only  Strain gauge / load cell Hall Sensors Hall sensor theory is based on the phenomenon that the electromotive force appears in the direction perpendicular to both the current and the magnetic field when applying a magnetic field perpendicular to the current to the object through which current is flowing. When a current is applied to a semiconductor plate with no magnetic field, the current is spread out across the whole plate. Whereas with a magnetic field present the current flow is pushed to one side of the plate and a voltage corresponding to the magnetic flux density and its direction is output by the hall effect sensor. © Aviation Australia Hall effect sensor 2022-11-10 B1-04 Electronic Fundamentals Page 137 of 163 CASA Part 66 - Training Materials Only  Digital Transducers Modern systems use more digital transducers. There are two basic position sensing systems in general use. Incremental Encoder The first is the incremental encoder which outputs a number of square wave pulses from one end of the movement to the other end. The system can then add or subtract the number of pulses moved and the remainder is the numerical position. Aviation Australia Incremental encoder Absolute Encoder An absolute encoder requires many more light sources and sensors (one for each digit required) but the output of the detectors gives the exact position as a binary number to an accuracy dependent on the number of bits. Shown is a 10-bit wheel giving a resolution of 0.35 degrees. © Aviation Australia Absolute encoder 2022-11-10 B1-04 Electronic Fundamentals Page 138 of 163 CASA Part 66 - Training Materials Only  Synchro Systems I Linear Variable Differential Transformer A Linear Variable Differential Transformer (LVDT) is an electromechanical device which translates straight line motion into a linear alternating current (AC) signal (proportional to the amount of movement). The LVDT has one primary and two secondary windings. The two secondary windings are connected so that the voltages induced in each coil are in opposition. The LVDT has a magnetic core (also called an armature) that is positioned by a linear motion. When the armature is in the central position, the secondary windings have an equal voltage induced in each coil. The LVDT output is the sum of both coils. As the coils are wired in opposition, one coil will be in phase with the primary voltage and the other coil will be 180° out of phase, with the voltage in each coil being equal. The result is that they cancel each other out, giving an output of zero. Aviation Australia Linear Variable Differential Transformer (LVDT) The LVDT displacement transducer operates as below. 2022-11-10 B1-04 Electronic Fundamentals Page 139 of 163 CASA Part 66 - Training Materials Only  RDP Group LVDT operation Where Ext/Exc is the primary excitation, Sec 1 is the secondary 1, Sec 2 is secondary 2. With the two coils in opposition, the smaller voltage is cancelled out, leaving the residual of the higher voltage. The phase would be that of the higher voltage. If the armature is moved the same amount in the opposite direction, the output voltage would be the same magnitude, however the phase of the output would be opposite. Therefore, the direction of the armature movement determines the phase of the output and the amount of movement determines the magnitude of the output. 2022-11-10 B1-04 Electronic Fundamentals Page 140 of 163 CASA Part 66 - Training Materials Only  RDP Group When the two coils are in opposition the smaller voltage is cancelled out The main advantage of the LVDT transducers over other types of displacement transducers is their high degree of robustness. This is derived from their very principle, in which there is no physical contact across the sensing element and so there is zero wear in the sensing element. This also means that LVDTs can be made waterproof and in a format suitable for the most arduous applications. The LVDT principle of measurement is based on magnetic transfer, which also means that the resolution of LVDT transducers is infinite. The smallest fraction of movement can be detected by suitable signal-conditioning electronics. An LVDT comprises a coil former or bobbin onto which three coils are wound. The first coil, the primary, is excited with an AC current. The other two coils, the secondary coils, are wound such that when a ferritic core is in the central linear position, an equal voltage is induced into each coil. However, the secondary coils, are connected in opposition so that in the central position the outputs of the two secondary coils, cancel each other out. 2022-11-10 B1-04 Electronic Fundamentals Page 141 of 163 CASA Part 66 - Training Materials Only  Capacitance Transmitters A capacitive sensor can be connected to 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. Aviation Australia Capacitance Bridge Vary the plate area, varying the capacitance 2022-11-10 B1-04 Electronic Fundamentals Page 142 of 163 CASA Part 66 - Training Materials Only  A capacitance transmitter has a rotor and stator of intermeshing plates which is shown in the image below. The relative position of the rotor and the stator plates determines the capacitance value. Therefore, if you vary the plate area, you are varying the capacitance. The anode and cathode of a capacitance transmitter is the rotor and stator. The amount of stator and rotor intermeshing is controlled by the rotation of a shaft by a mechanical input. When the stator and rotor plates are fully intermeshed, the capacitance is high. When the stator and rotor plates are partly meshed, the capacitance is low. Capacitance transmitter 2022-11-10 B1-04 Electronic Fundamentals Page 143 of 163 CASA Part 66 - Training Materials Only  Inductance Transmitters An inductive transmitter or more correctly, a mutual inductance transmitter has two coils, one coil supplied with alternating current, surrounded by a secondary output coil. The transmitter is set against a vane with two areas of different permeability. The portion of the secondary voltage induced into the output coil is determined by the level of permeability of the metal next to the coils. The amount of inductance varies depending on the position of the sensor in relation to the ferrous or non-ferrous component of the vane. An example of this inductive transmitter is shown in the image below. Aviation Australia Inductance transmitters A common inductive transmitter is made of aluminium and a ferrite material. The null position is when the inductive coil is positioned on the join between the ferrite and the aluminium vanes giving half the maximum output. A displacement of the inductor from the join either increases or decreases the mutual inductance. In the centre position, you can see that the coil is centred over the join between the aluminium and the ferrite vanes. If the vane is moved so that more of the aluminium vane is beside the coils, less inductance results due to the vane's properties. 2022-11-10 B1-04 Electronic Fundamentals Page 144 of 163 CASA Part 66 - Training Materials Only  Inductance transmitter 2022-11-10 B1-04 Electronic Fundamentals Page 145 of 163 CASA Part 66 - Training Materials Only  Pendulous Transmitters The pendulous transmitter illustration shows the operation of another type of transmitter that has a primary (input) and secondary (output) winding. A pendulous transmitter is used to detect shifts from the vertical (or horizontal). If the core is secured vertically to the aircraft structure (as illustrated) and the aircraft is level, the core and frame will be in a central position. If the aircraft tilts from level the pendulous frame will still hang vertically but it will be displaced relative to the fixed core. In the left-hand diagram, with the core centred in the frame, the flux of the primary winding does not intersect with the secondary, which therefore has no EMF induced in it. Aviation Australia Pendulous monitors When the frame displaced from around the core there is a low reluctance path for some of the primary flux that intersects with the secondary winding. The direction of the displacement also determines the direction of the magnetic flux through the core. When the frame is displaced to the left of the core the input and output signals are inphase and when the frame is displaced to the right of the core the two signals have an opposite phase relationship. 2022-11-10 B1-04 Electronic Fundamentals Page 146 of 163 CASA Part 66 - Training Materials Only  Synchro Principles Synchros are an electromagnetic device, that electrically transmits the rotational position data from one aircraft component to another. Synchros are low current devices transmitting rotational angle information using 3 or 4 wires and an AC input signal. Aircraft synchros are used as a rotary position sensor for aircraft control surfaces, remote position indicators for radar systems. They are also used in autopilot systems as signal input devices. Synchros will typically contain both primary and secondary windings, comparable to that of a motor. Synchros will often have a primary winding that rotates, as well as secondary windings that have either 2 windings at 90 degrees or 3 windings that are 120 degrees apart and connected in star to provide varying voltage ratios. Resolver Synchro One type of synchro is the resolver synchro. One of its functions is to convert alternating voltages representing the cartesian (rectangular or XY) coordinates of a point into polar coordinates (rho- theta) represented by an alternating voltage and the angular position of a shaft. The resolver can also convert polar coordinates to cartesian ones, which is called resolving. The left diagram below shows the polar coordinates of the point P represented by the vector r and the angle θ it makes with the baseline X. The right diagram shows the same point whose location is now identified by the cartesian coordinates X and Y. Polar and cartesian coordinates The stator and rotor in a common type of synchro resolver have two windings each with their axes perpendicular to each other. The illustration below shows alternative schematics and the symbol for synchro resolvers. The schematics shown are in the electrical zero position. In various applications resolvers may have additional windings, or only one stator or rotor winding. 2022-11-10 B1-04 Electronic Fundamentals Page 147 of 163 CASA Part 66 - Training Materials Only  Schematics and symbols for synchro resolvers 2022-11-10 B1-04 Electronic Fundamentals Page 148 of 163 CASA Part 66 - Training Materials Only  Resolver Synchro Operation As with the voltages in the output winding of a transformer, the voltage out changes with the sine of the angle between them. If the coils are parallel, maximum output is the output and if they are perpendicular the output would be zero. By having the two stator windings at 90 degrees apart one will give the sine of the rotor angle and the other will give cosine of the angle. Aviation Australia Resolver angles 2022-11-10 B1-04 Electronic Fundamentals Page 149 of 163 CASA Part 66 - Training Materials Only  Resolver Components Connection to the rotor is made by brushes and slip rings, or inductive coupling. Resolvers using the inductive method are referred to as brushless types. The inductive (brushless) resolvers offer up to 10 times the life of brush types and are insensitive to vibration. Brushless resolver rotor and armature 2022-11-10 B1-04 Electronic Fundamentals Page 150 of 163 CASA Part 66 - Training Materials Only  Synchro Systems II Synchro Categories There are two general categories of synchro systems: Torque synchro - Torque synchros use current in the rotors to generate torque to move the load. Torque synchros contain components called torque transmitters, torque receivers, and torque differentials. Control synchro - Control synchros, on the other hand, are voltage control devices using very small currents. Control synchro system components are called control transmitters, control differentials and control transformers. Torque synchro system 2022-11-10 B1-04 Electronic Fundamentals Page 151 of 163 CASA Part 66 - Training Materials Only  Synchros Purpose The use of a synchro system in position sensing and data transmission is very common in aircraft, especially in automatic pilot systems. It is a fast and accurate method of transmission and control and it provides an accuracy of approximately 0.5% for torque synchro systems and much higher for control synchro systems. The synchro is essentially a rotary transformer whose secondary output voltages depends upon the primary input voltage and 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. Torque Synchro System To convert a mechanical movement into electrical signals and then transmit the signals to another location, a system of torque synchros 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 of the rotor. The markings on the terminals are the same for both; S for stator and R for rotor. The symbols used in electrical drawings are also the same for both. Sometimes the word "indicator" may be used instead of "torque receiver". Aviation Australia Torque synchro system 2022-11-10 B1-04 Electronic Fundamentals Page 152 of 163 CASA Part 66 - Training Materials Only  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. 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° clockwise, the stator field of the TR will follow it and move 30° 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. Synchro Symbols Synchro transmitters and receivers are virtually the same, and so are their schematic symbols. The image below shows three examples of the way in which synchros are drawn. Top left is the most commonly used, top right is usually used when the operation is explained, and bottom middle is usually on wiring diagrams. Synchro transmitters schematic symbols (different ways they are drawn) 2022-11-10 B1-04 Electronic Fundamentals Page 153 of 163 CASA Part 66 - Training Materials Only  Torque System Accuracy Due to the fact that torque is derived from the current in the stators and that this current is zero when the transmitter and receiver are aligned, any small misalignment will generate a small current to re-align them, the torque will also be very small. Torque synchro systems can only drive light loads such as indicators with an accuracy requirement of 0.5% of ±1.8°. Any friction in the system will reduce the accuracy as it will take more error to generate enough current to provide torque to overcome the friction. Differential Transmitter (TDX) Torque Differential Synchro System A torque differential synchro can be used to transmit the following: An electrical signal that is the sum or the difference of two inputs - one mechanical, the other electrical (TDX) A mechanical signal that is either the sum or the difference of electrical inputs from two synchro transmitters (TDR) 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° apart. In this case the stator acts as the primary of the transformer and the rotor acts as the secondary. Aviation Australia Differential Transmitter (TDX/TDR) 2022-11-10 B1-04 Electronic Fundamentals Page 154 of 163 CASA Part 66 - Training Materials Only  The illustration below shows a three-component synchro system, consisting 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. Aviation Australia The differential synchro is not directly connected to the AC supply 2022-11-10 B1-04 Electronic Fundamentals Page 155 of 163 CASA Part 66 - Training Materials Only  Differential Synchros The torque differential transmitter (TDX) and the torque differential receiver (TDR) are electrically identical. The only difference in their construction is that the TDR has a damper, which serves the same purposes as the damper in the TR. It prevents the rotor from oscillating. Aviation Australia The Torque Differential Transmitter (TDX) and the Torque Differential Receiver (TDR) are electrically identical 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° apart. 2022-11-10 B1-04 Electronic Fundamentals Page 156 of 163 CASA Part 66 - Training Materials Only  Differential Subtraction When using the differential synchro as a transmitter, it can be used to add or subtract information to/from the remote torque receiver. The system 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. Aviation Australia Differential transmitter operation The TX rotor is held at 45° and the TDX rotor is turned to 15°. The TR rotor will turn back from 45° to 30°. The 15° signal from the TDX has modified the signal from the TX. Imagine if TDX were at 45° as well. Subtracting one input from the other with TR indicating 0°. 2022-11-10 B1-04 Electronic Fundamentals Page 157 of 163 CASA Part 66 - Training Materials Only  Aviation Australia Difference between the electrical signal A and the mechanical signal B Differential Addition The illustration shows the same set-up as used for subtraction, except that both the input and output leads of the TDX are changed. This means 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. The rotor of the TR will turn to a position of 85° clockwise. Aviation Australia Input and output leads of the TDX are changed 2022-11-10 B1-04 Electronic Fundamentals Page 158 of 163 CASA Part 66 - Training Materials Only  Differential Transmitter (TDX) Symbols Aviation Australia Differential Transmitter (TDX) symbols 2022-11-10 B1-04 Electronic Fundamentals Page 159 of 163 CASA Part 66 - Training Materials Only  Control Synchro Systems As stated previously torque synchro systems produce a relatively small mechanical output, they are suitable only for very light loads. Even when the torque system is moderately loaded, it is never entirely accurate because the receiver rotor requires a slight amount of torque to overcome its static friction. This leads into the high accuracy, high torque system, the Control Synchro system. The system uses a Control Transmitter (CX), a Control Transformer (CT), and Control Differential Transmitter (CDX) the system also requires an amplifier and a motor to produce torque. The fact that an amplifier provides the power supply for the motor to provide torque means that it is a very flexible system driving indicators with low torque requirements to very large torque systems for positioning many tonnes of load very accurately. The CX, CDX and CT which are used for the small indicator system are the same units used to position the large systems. All that has changed is the amplifier and the motor. Aviation Australia Control synchro system The power supply is connected to the CX rotor only. The CX, CDX and the CT have the same wiring from stator to stator as the TX,TR and the TDX discussed previously, except for higher Inductive reactance windings in the CX, CT and CDX. The increased reactance is due to a much large number of turns in each winding, this gives a much larger voltage for a small movement which is then amplified to drive the motor. The motor is connected to the rotor shaft as well as the load, as the load is repositioned the difference between the CT an CX is reduced. the smallest movement of the CX gives an output from the CT which is then amplified and the motor drives, this makes the system very accurate. 2022-11-10 B1-04 Electronic Fundamentals Page 160 of 163 CASA Part 66 - Training Materials Only  Aviation Australia Control synchro system Since we discussed the theory and operation of the TX and the TDX earlier, the CX and CDX work the same way. However, the CT operation will be described as follows. The CT rotor winding is connected to the amplifier. The rotor voltage is dependent on the angle between the stator magnetic field and the rotor coil. When they are aligned, the voltage would be maximum, and when they are perpendicular, the voltage will be zero. The zero voltage when the coil is at a right angle to the magnetic field means this is the null position. The CT will always be driven by the motor until the magnetic field and CT rotor coil are at 90 degrees. The voltage from the CT rotor can be called the error voltage as it shows the difference between CT rotor and the null point. The CT rotor signal is then passed to an amplifier whose output can control a motor capable of producing the correct amount of torque. © Aviation Australia Magnetic field angle to the coil induction 2022-11-10 B1-04 Electronic Fundamentals Page 161 of 163 CASA Part 66 - Training Materials Only  Control Synchro System Operation Let us see how this system works with a servo system to move heavy equipment. The illustration below shows a block diagram of a typical servo system that uses a control synchro system. Assume the shaft of the CX in this system is turned by some mechanical input. This causes an error signal to be generated by the CT because the CX and the CT rotors are now out of correspondence (null point has moved, leaving the CT with an error voltage). The error signal is amplified by the servo amplifier and applied to the servomotor. The servomotor turns the load, and through a mechanical linkage called “response”, it also turns the rotor of the CT. The servomotor then turns the rotor of the CT so that it is once again in correspondence (at null point) with the rotor of the CX, the error signal drops to zero volts, and the system comes to a stop. Control synchro system operation 2022-11-10 B1-04 Electronic Fundamentals Page 162 of 163 CASA Part 66 - Training Materials Only  Control Transformer Symbols Note: the only difference between TX/TR and CX/CT Wiring diagram symbols is that the CX/CT have arrows showing the Null Point 90° away from Magnetic field. Control Transformer symbols 2022-11-10 B1-04 Electronic Fundamentals Page 163 of 163 CASA Part 66 - Training Materials Only