Module 4.1

Module 4Electronic Fundamentals

4.3 ServomechanismsElectronic FundamentalsSemiconductorsPart 1 - SynchrosElectronic FundamentalsSemiconductorsObjectivesDefine the word 'synchronous

State the function of a synchronous data transmission system

List the categories of synchronous data transmission systems

Give a simple description of operation of the following desynn systems;

BasicMicroSlab

State the difference between synchro and servomechanismsElectronic FundamentalsSemiconductorsObjectives cont...State the use, letter designation and identify the three variations of circuit symbols and terminal names for the following synchros;

Torque transmitterTorque receiverControl transmitterControl transformerTorque differential receiverTorque differential transmitterControl differential transmitterResolver

List the power supplies used for aircraft synchronous data transmission systemsElectronic FundamentalsSemiconductorsObjectives cont...Recognize and give a simple description of operation and use of the following synchro systems;

TorqueDifferential TorqueControlDifferential Control

Recognize and define the balanced/null position of torque and control synchro systemsElectronic FundamentalsSemiconductorsSynchro SystemsSynchronous data transmission systems are designed to indicate the position of a component or control surface that cannot be directly observed.

These systems fall into one of two categories;

Desynn Systems (D.C.)Synchro Systems (A.C.)

These systems have a transmitting element and a receiving element connected by wires

Synchronous means happening at the same time

When the transmitter is moved, the receiving element will follow that movement instantlyElectronic FundamentalsSemiconductorsDesynn SystemThere are a variety of different types of Desynn systems available;

Basic Desynn - operated by a rotary motion, however linear versions are also found

Micro Desynn - designed to magnify the small movement obtained by such items as pressure measuring devices. Operated by linear motion

Slab Desynn - designed to overcome signally errors inherent in the basic desynn system. Operated by a rotary motionElectronic FundamentalsSemiconductorsBasic Desynn ConstructionTransmitter (toroidal resistor) - an endless resistance wound on a circular former

Signal wires are attached to 3 tapings, equally spaced at 120 intervalsSystem power is applied to two wiper arms, spaced 180 apart, running on the resistor

Receiver - a two-pole permanent magnet rotor, with attached pointer, pivoted to rotate inside a soft iron stator

The stator carries three star connected windings that are connected to the signal wires coming from the transmitter

Electronic FundamentalsSemiconductorsBasic Desynn OperationD.C. power is applied to the wiper arms of the transmitter; current divides equally and flows in both directions around the resistor

Voltage at tapping 1 will be 28 volts. Voltage at tapping 2 and 3 will be 9.3 volts The potential differences at the three tapings cause currents to flow in the wires that connect to the receiver

The current creates magnetic fields around the 3-stator windings in the receiver, which combine to produce a resultant field across the stator

The permanent magnet aligns with the resultant magnetic field of the stator

Electronic FundamentalsSemiconductorsSlab Desynn OperationIn basic desynn systems, the receiver pointer may slightly lag behind the transmitter. This problem can be solved by using the slab desynn system.

Transmitter power is supplied to a resistor wound on a slab former. 3 wiper arms, 120 apart, provide the output to the receiver

Receiver - the slab desynn transmitter can be connected to the same receiver as the basic desynn system

Operation the slab desynn system operates in the same way as the basic desynn system

Electronic FundamentalsSemiconductorsFail Safe DevicesIn a desynn system, if the D.C. power to the system fails, the pointer will remain in its last position.

If the instrument fails, it should respond in such a way that the fault will be identified.

Fail safe is achieved by fitting a small permanent magnet in the indicator

Under normal operation, the field of the permanent magnet is weak in comparison to the fields produced by the coils and therefore has no effect

When power is removed, the small permanent magnet attracts the permanent magnet rotor, moving the pointer off scale.Electronic FundamentalsSemiconductorsSynchros and ServosSynchros are electromagnetic devices used to transmit positional data electrically from one position to another

They can also be used to compute the sum of two rotations or the difference in angle between them

Synchros used in aircraft data transmission systems are operated from either 115V 400Hz or 26V 400Hz A.C. supplies

Servo systems use 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 synchroElectronic FundamentalsSemiconductorsSynchro ClassificationSynchro system components may be classified as follows;

Torque transmitter (TX) - the rotor is mechanically positioned. Used for transmitting angular position of the rotor relative to the stator

Torque receiver (TR) - the rotor is free to turn. Develops a torque dependent on the difference between its rotor position and the angular information received from a torque transmitter or torque differential transmitter

Control transmitter (CX) - the rotor is mechanically positioned. Used for transmitting angular position of the rotor relative to the stator

Control transformer (CT) - supplied with electrical angular information and which supplies a voltage proportional to the sine of the difference between the electrical input angle and its own rotor position angleElectronic FundamentalsSemiconductorsSynchro Classification cont...Torque differential transmitter (TDX) - the rotor is mechanically positioned. Modifies angular information received from a torque transmitter and transmits the sum of or difference between the electrical input and its own rotor position angle

Torque differential receiver (TDR) - the rotor is free to turn. Develops a torque dependent on the difference between its own rotor position and the sum of or difference between the two sets of angular information received from two connected torque transmitters

Control differential transmitter (CDX) - the rotor is mechanically positioned. Modifies angular information received from a control transmitter and transmits the sum of or difference between the electrical input angle and its own rotor position angle.

Resolver - has two perpendicular winding on the rotor and two on the stator. 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. Electronic FundamentalsSemiconductorsSynchro Symbols

CONTROLTRANSFORMERS

TRANSMITTERSand RECEIVERSNOTE:When the symbol is used in a diagram, a letter designation TX, TR or CX will appear next to, or inside, the symbol to clarify which type it is

TRANSMITTERS, RECEIVERSAnd CONTROL TRANSFORMERS

TRANSMITTERS, RECEIVERSAnd CONTROL TRANSFORMERSElectronic FundamentalsSemiconductorsSynchro Symbols

DIFFERENTIALSNOTE:When the symbol is used in a diagram, a letter designation TDX, CDX, TDR will appear next to, or inside, the symbol to clarify which type it is DIFFERENTIALS

DIFFERENTIALSElectronic FundamentalsSemiconductorsSynchro Symbols cont...RESOLVER

RESOLVER

RESOLVERElectronic FundamentalsSemiconductorsTorque Synchro System - ConstructionTorque synchro systems are used where the turning force or torque required is very small.

The torque synchro system comprises an interconnected Torque transmitter (TX) and Torque Receiver (TR)

A.C. power is supplied to both rotors, which are in parallel

R2 and S2 will be connected to earth

The transmitter rotor will be mechanically rotated

A pointer is attached to the receiver rotor, which is free to rotateElectronic FundamentalsSemiconductorsTorque Synchro System - OperationBalanced or Null

When supply current flows in the rotor windings, voltages are induced is the stator winding of the TX and TR

If the rotors are in the same angular position, the voltages in the TX and TR will be equal and opposite; no current will flow in the stator coils; the system is said to be balanced or nulled

For the position of the rotors in the diagram, the voltages would beS1 half maximum voltageS2 maximum voltageS3 half maximum voltageElectronic FundamentalsSemiconductorsTorque Synchro System - OperationTransmitter rotated

When the TX rotor is rotated, the voltages in the TX stator coils will change, while the voltages in the TR stator coils remain the same

Current flows in the wires, creating magnetic fields around the stator windings, that creates a resultant field across the TX and TR stators

A torque reaction will exist at both the TX and TR. The TR rotor will moves around in response to the torque

Once the TR rotor is in the same angular position as the TX rotor, the voltages in the stators will be equal and opposite, current stops and the system will be balanced

Electronic FundamentalsSemiconductorsElectrical ZeroElectrical 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 is in phase with that of R1 with respect to R2.

The electrical zero setting of synchros provides a standard means of aligning synchro 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

It simply means that the rotor is parallel to S2 and that R1 is at the top

Electronic FundamentalsSemiconductorsDifferential Torque Synchro System - ConstructionA differential synchro system will provide an output that is the difference, or sum, between the two inputs from the mechanical drives.A differential synchro system consists of a differential synchro (TDX) used in conjunction with a synchro transmitter (TX) and receiver (TR)

A.C. power is supplied to the TX and TR rotors, which are in parallel

There is no connection between the TDX and the A.C. supply

The transmitter rotor will be mechanically rotated

A pointer is attached to the receiver rotor, which is free to rotate

Electronic FundamentalsSemiconductorsDifferential Torque Synchro System - OperationBalanced or Null

When the system is set as shown in the diagram below, the induced voltages in the stators and across the transformers will be equal; no current will flow the interconnecting wires

Electronic FundamentalsSemiconductorsDifferential Torque Synchro System - OperationTX rotated

If the TX is turned by 60, the TX stator voltages will change and current will flow across the stator windings

Resultant fields will be set up, and all three components feel a torque reaction

Only the TR rotor is free to respond, so it will turn until the voltages are equal and current stops flowing

Electronic FundamentalsSemiconductorsDifferential Torque Synchro System - OperationTDX rotated

If the TX is left stationary and the TDX is rotated by 15 the voltages will be different and current will flow around the stator windings

A torque reaction will occur and the TR rotor will turn until the voltages are equal and current stops flowing

If both the TX and the TDX were rotated then the TR would show the difference between the two movements

Electronic FundamentalsSemiconductorsDifferential Torque Synchro System - OperationThe differential synchro system could be arranged with two TX and one TDR

Under this condition, the TDR is the receiving element, but the system will respond as previously described to show the difference in the two inputs

If the interconnecting wires between S1 and S3 of the TX and TDX, and between S1 and S3 of the TDX and TR, are crossed, the system will algebraically add the two mechanical inputs

Electronic FundamentalsSemiconductorsControl Synchro System - ConstructionControl synchros are used in electromechanical servo and shaft positioning systems. They only produce a signal representative of the position of the transmitter. The control synchro system comprises an interconnected Control transmitter (CX) and Control Transformer (CT)

A.C. power is only supplied to the CX rotor

The CX rotor will be mechanically rotated, but the CT rotor is fixed

The balanced or null position occurs when the CX and CT rotors are 90 apart

Electronic FundamentalsSemiconductorsControl Synchro System - OperationWhen the CX rotor is parallel to stator S2, maximum voltage is induced in stator S2 and half maximum voltage is induced in stators S1 and S3

No EMFs 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 wires

The current flowing in the CT stator windings produces a resultant magnetic field

This resulting field cuts the CT rotor winding producing an induced EMF in the CT rotor

Electronic FundamentalsSemiconductorsCT Rotor Induced EMFThe amplitude of the EMF induced in the CT rotor 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 systemsElectronic FundamentalsSemiconductorsControl Synchro Servo System - OperationBalanced or Null

When the system is balanced, zero EMF is induced in the CT rotor, there is no output to the servomotor and the motor and pointer are stationary

Electronic FundamentalsSemiconductorsControl Synchro Servo System - OperationTransmitter rotated

When the CX rotor is rotated, the resultant field across the CT stator will move. The CT rotor will no longer be at 90 to the resultant field and an EMF will be induced in it

The EMF is applied to a amplifier to sense its phase relationship to the supply voltage

Direction information from the amplifier is applied to the motor, which will turn, driving the pointer and at the same time driving the CT rotor

When the CT rotor is at 90 to the resultant field, the induced EMF falls to zero and the motor stops turning. The pointer indicates the new position

Electronic FundamentalsSemiconductorsDifferential Control Synchro System - ConstructionDifferential control synchro systems operate the same way as torque differential synchro systems.

They can also be wired to produce an electrical signal proportional to the sum or difference between two inputs.

Electronic FundamentalsSemiconductorsQ & Ahttp://tutmo.net/lisans_modul/

http://www.rfcafe.com/references/electrical/NEETS%20Modules/NEETS-Module-15-1-1-1-10.htm

Electronic FundamentalsSemiconductorsPart 2 - ServosElectronic FundamentalsSemiconductorsObjectivesState the function of servomechanisms

State the properties of servomechanisms

List the categories of servomechanisms

Identify the following servomechanisms;

Open LoopClosed Loop

State the problem associated with open loop servomechanisms

List the factors that affect the operation of open loop servomechanisms

List the essential features of ServomechanismsElectronic FundamentalsSemiconductorsObjectives cont...State the function of the following parts of a remote position control servomechanisms;

TransducerAmplifierMotor

Define the following forms of servomechanism feedback;

PositionVelocity

Identify the symbol for and state the function of a summing device

Recognize closed loop servomechanisms with the following feedback;

PositionVelocityElectronic FundamentalsSemiconductorsObjectives cont...Give a simple description of operation of the following servomechanism components;

E & I BarTachogenerator

Give a simple description of operation of the following servo systems;

DCAC

Give a simple description of operation of the following transducers;

Linear Variable Differential Transform (LVDT)Rotary Variable Differential Transformer (RVDT)Induced EMF speed sensorInduction proximity sensorCapacitance transmitterElectronic FundamentalsSemiconductorsServomechanismsServomechanisms (Servos) are control systems whose outputs are slaved to follow the demands of the input.

Servos provide the precise control and power required to operate complex machines that humans are unable to provide

Servos possess the following properties;

Error activatedHave power amplificationContain moving partsAutomatic in operation

Servomechanisms can be classified according to two main categories;

Open loopClosed loopElectronic FundamentalsSemiconductorsOpen Loop

In an open loop system;

The input demand generates an electrical signal equivalent to the demand position

The signal is then amplified to the required power level and applied to a motor

The motor turns to position a loadElectronic FundamentalsSemiconductorsOpen Loop Problems The speed of response and the final position of the load depend on the following factors;

Variations in load conditionsFrictional forces in the motor and its loadMechanical interconnectionsVariations in power suppliesValue of the demand voltageVariations in amplifier gain

PROBLEM Because of the variations mentioned above, the output of an open loop system is unlikely to follow the input precisely and cannot provide the close tolerance required by many complex machines.Electronic FundamentalsSemiconductorsClosed LoopIn a closed loop system, any error in the output is detected and fed back to the input so that the necessary corrections can be made to eliminate the error.

Feedback - information concerning the behavior of the load is fed back to the input

Summing amplifier compares the position of the output (feedback) to that demanded by the input

Error signal - proportional to the difference between the demand and feedback signals

Amplifier - amplifies the error signal to control the load

Motor - moves the load in such a direction as to reduce the error signal to zeroElectronic FundamentalsSemiconductorsRemote Position Control Servo SystemsRemote Position Control (RPC) servo systems are a form of closed loop servo system used to control the position of a remotely located device in response to a change in the demanded input.

The essential parts of an RPC servo are as follows;

Transducers - a device for converting one form of energy into another, for example, electrical to mechanical, heat to electrical or light to electrical. In servo systems these are generally used to convert a mechanical input to an electrical signal for the servo

Amplifier - The amplifier increases the power of the input signal to a level suitable to drive the device being positioned. Large mechanical work outputs are therefore possible for very small work inputs

Motors - Motors are used to move the device being controlled. They are usually coupled to a gearbox and produce either a linear or rotary motionElectronic FundamentalsSemiconductorsPositional FeedbackPositional feedback is used to make the output shaft of a servo system exactly follow the input shaft position.

Electronic FundamentalsSemiconductorsPositional Feedback - OperationBalanced or Null

When the angular positions of the output shaft and input shaft are the same, the demand and feedback voltages from the potentiometers will be equal

The summing amplifier will subtract the feedback signal from the demand signal resulting in no error signal (0 volts)

With no error signal applied to the amplifier, the motor will be stationary

Electronic FundamentalsSemiconductorsPositional Feedback - OperationError Signal

When the input shaft is first rotated, there will be a change in the demand voltage. The the feedback voltage will remain unchanged because the output shaft has not yet rotated

The summing amplifier will subtract the feedback signal from the new demand signal resulting in an error voltage (polarity depends on the direction of input shaft rotation)

The motor now runs in the direction determined by the polarity of the error voltage

The load is repositioned and the potentiometer wiper moves, changing feedback voltage

When the output shaft is realigned with the input shaft, the feedback and demand voltages will be equal again. The error signal will be zero, and the motor will stop

Electronic FundamentalsSemiconductorsVelocity Control Servo SystemVelocity Control Servo Systems are a form of closed loop servo system used to control the rotational speed of a shaft and not its position.

Electronic FundamentalsSemiconductorsVelocity Control Servo SystemBalanced or Null

The speed control potentiometer produces a voltage proportional to the demanded speed

Feedback is provided by the tachogenerator which produces a voltage proportional to the angular velocity of the output shaft

A summing amplifier will subtract the feedback signal from the demand signal. When the feedback and input demand signals are equal, no error signal is produced

The motor continues to run at the current speed

Electronic FundamentalsSemiconductorsVelocity Control Servo SystemError Signal

At the instant the speed control potentiometer is first moved, the input demand voltage changes, but the tachogenerator produces the same voltage as before

The summing amplifier subtracts the feedback signal from the demand signal and an error signal will be produced that is proportional to the difference between the two signals

The error signal is fed to the amplifier, which accelerates or decelerates the motor until the output of the tachogenerator is equal to the input demand voltage, at which time the motor will run at the new demanded speed

Electronic FundamentalsSemiconductorsA.C. Servo TransducersWhile D.C. servo systems use simple resistive transducers, different methods are required for A.C. servo systems, such as;

TachogeneratorE & I BarElectronic FundamentalsSemiconductorsE & I Bar TransducerBalanced or Null

A winding on the centre limb of the E bar carries an A.C. excitation supply

Secondary coils are connected in series opposition

With the I bar in the centre position equal flux will flow in the outer limbs of the E bar, the voltages induced in the two secondary coils will be equal and opposite and will therefore cancel out and there will be no output signal

Electronic FundamentalsSemiconductorsE & I Bar TransducerError Signal

If the I bar is displaced from the central position, more flux will flow in the limb of the E bar with the smaller air gap and less flux will flow in the limb with the larger air gap

The induced voltages in the two windings will no longer cancel out and an output voltage will be produced

The phase of the output voltage is determined by the direction of movement of the I bar

The magnitude is determined by how far the bar moves. In a servo system the amount of movement will be kept small due to the follow-up action.

Electronic FundamentalsSemiconductorsE & I Bar TransducerThe E & I Bar may also be used to convert linear movement to an electrical signal.

When the I bar is moved linearly, by an evacuated capsule, the induced voltages in the two windings will no longer cancel out and an output voltage will be produced

Electronic FundamentalsSemiconductorsAC TachogeneratorTachogenerators provide the velocity feedback for servo systems.

Tachogenerators normally produce a voltage with the same frequency as the supply voltage

With the drag cup stationary no voltage is induced in the secondary winding as it is placed at right angles to the primary winding and the output is zero

As the output shaft drives the rotor, the cup rotates and rotating eddy currents are induced in the cup

The eddy currents in the cup will induce a voltage across the output winding

The amplitude of the voltage will be proportional to the speed of rotation and the phase will be dependent on the direction of rotationElectronic FundamentalsSemiconductorsLinear Servo Systemhttp://www.youtube.com/watch?v=zQuRQHHw8DIElectronic FundamentalsSemiconductorsDC Servo SystemBalanced or Null

The voltage at the input and output potentiometer wipers depend on the input and output shaft positions (i and o)

When the wiper arms are in the same position, the voltages will be the same and there will be no error signal from the amplifier; the servo motor will not turn (balanced)

Electronic FundamentalsSemiconductorsDC Servo SystemError Signal

Any difference in the position of the two wipers results in a voltage difference, which is applied to the amplifier to create an error signal

The motor drives the load in a direction corresponding to the polarity of the error signal. When alignment is reached the error signal falls to zero, the motor field disappears and the motor stops.

Electronic FundamentalsSemiconductorsAC Servo System

Balanced or Null

When the rotor of the CT is at 90 to the rotor of the CX, no EMFs are induced in the rotor of the CT and no error signal is sent to the amplifier

With no error signal applied to the amplifier, the motor will not turn and the load will remain stationaryElectronic FundamentalsSemiconductorsAC Servo System

Error Signal

With a misalignment in the system, an EMF is induced in the rotor of the control transformer and an error signal is applied to the amplifier

The error signal is amplified and passed to the motor, which drives the load in one direction or another (according to the phase of the induced EMF)

As the load is repositioned, the CT rotor is realigned until there is no output from the CT, resulting in no input to the amplifier, and the motor stopsElectronic FundamentalsSemiconductorsTransducersThere are several types of transducers that can be used with servo systems.

Linear Variable Differential Transform (LVDT)

Rotary Variable Differential Transformer (RVDT)

Induced EMF speed sensor

Induction proximity sensor

Capacitance transmitterElectronic FundamentalsSemiconductorsLinear Variable Differential TransformerLinear variable differential transforms (LVDT's) are used to produce an electrical signal proportional to a linear movement.

LVDT's consist of a moveable iron core that is mounted inside three windings wound on a coil former

The centre winding is the excitation winding and is connected to an A.C. reference voltage

The two outer windings are connected in series opposition and provide the output

Electronic FundamentalsSemiconductorsRotary Variable Differential TransformerRotary Variable Transformers (RVDTs) produce an electrical signal proportional to a rotational movement.

RVDT's consist of one fixed iron core mounted inside a coil, and one moveable iron core that is mounted inside two output coils

The centre winding is the excitation winding and is connected to an A.C. reference voltage

The two outer windings are connected in series opposition and provide the output

Electronic FundamentalsSemiconductorsInduced EMF Speed SensorInduced EMF speed sensors are used to measure rotational speed of items such as engine shafts.

This type of transducer comprises a coil and a permanent magnet and requires a steel target for its operation

If the target is moved continually back and forward past the transducer, the flux density continually increases and decreases

This changing flux induces an EMF in the transducer

Electronic FundamentalsSemiconductorsInduction Proximity SensorInductive proximity sensors are used in sensing systems such as those used to sense the position of the landing gear.

An inductive proximity sensor has four main components; oscillator, coil, detection circuit and output circuit

Placing a piece of steel near the coil increases its inductance, which in turn increases the inductive reactance of the coil

Increasing the inductive reactance reduces the A.C. current flow in the coil, which is detected and used to provide a signal to indicate when the steel is in close proximity to the sensor

Electronic FundamentalsSemiconductorsCapacitance TransmittersCapacitance transmitters consist of a variable capacitor located inside a liquid, such as an aircrafts fuel tank or hydraulic reservoir.

The capacitor mounted inside the fuel tank is called a tank unit, and is normally constructed from two aluminum alloy tubes, one mounted inside the other and separated by insulating supports

The capacitance of the tank unit will vary depending on the quantity of fuel in the tank

The dielectric constant of fuel is higher than that of air, so when the fuel level rises between the two aluminum tubes, the capacitance of the tank unit will rise

Electronic FundamentalsSemiconductorsQ & Ahttp://tutmo.net/lisans_modul/

http://www.rfcafe.com/references/electrical/NEETS%20Modules/NEETS-Module-15-1-1-1-10.htm

Electronic FundamentalsSemiconductors