Topic 13 Servomechanism

Malaysian Institute of Aviation Technology

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Topic 5:

Servomechanism


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Objectives

After studying the material in this chapter, you should be able to:

1. Explain on the classification of the open and closed loop control system.

2. Explain on the feedback/follow-up system of a closed loop controller.

3. Explain on the meaning of the term : null, hunting,dead band and damping

4. Describe on the degree and method of damping


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Definition

A servomechanism is a force amplifier mechanism where the output accurately follows the input but with greater power.

Control system can be divided into two basic types:

a. Open Loop Control System

b. Close Loop Control System


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a. Open Loop

A system whereby external action is required to

control the loop manually.

Output controlled by the input only


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b. Close Loop

The control of loop is automatic within the system. Output controlled by the input with some form of

feedback or follower.


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Basic system of Servomechanism consist of following components:


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The unit that control ailerons, horizontal stabilizer and rudder are called servo motors.

They are class of devices that include synchros and resolvers.

Servos for the flight control systems (FCS) have important characteristics; they will not jam or cause other parts to become

entangled in the motor.

If an FCS fails, the aircraft returns to manual control.

Servo mechanisms, also called SERVO SYSTEMS or SERVOS for short, have countless applications in the operation of

electrical and electronic equipment.

Introduction to Servo Systems


8 Main Flight Control Surface


9 Synchros


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It is often necessary to operate a mechanical load that is remote from its source of control in working with:

Radar and antennas Aircraft control surface Flight Directors Computing devices Many other equipments

Introduction to Servo Systems (contd)

Servo motor


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To obtain smooth, continuous, and accurate operation, the mechanical loads are normally best controlled by synchros.

As you already know, the big problem here is that synchros are not powerful enough to do any great amount of work.

This is where servos come into use.

A servo system uses a weak control signal to move large loads to a desired position with great accuracy.

The key words in this definition are move and great accuracy.



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Elevator and rudder control cables


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In many large aircraft, control surfaces are moved with motors.

A pilot does not have the strength to move control surfaces in some aircraft and, like power steering in an automobile

The controls are servo assisted.

In the most advanced aircraft;

Control surfaces are manipulated by a simple wrist action controller called a side stick

Surface are moved only by servo, with no mechanical connection to the controller

This is fly by wire, where signals from the side stick are transmitted to a computer which adjusts the control surfaces



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Fly by wire - A321 Cockpit


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The difference between fly by wire and servo-assist is that fly by wire uses digital data and control surface positions are determined by a computer.

Conventional servo assist typically uses analog signals and does not involve a computer.

Servo assist, even in the largest aircraft, is a simple feedback control system.

The important point in servo-assisted control system is that there are motors already in place.



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In general, synchros are excellent for sensing angular position, but are not effective as torque-producing motors.

To overcome this problem,

An AC or DC motor, possibly even with a gear reduction drive, to provide ample torque

The synchro to measure the position of the indicator

The position of the indicator mechanism is compared to the desired position, and the error is amplified and applied to the driving motor.

This technique applies to a broad class of electrical or electrical-mechanical systems called servo systems.



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Synchro Controlled Servomechanism

The CT rotor is at 90 to CX rotor and so the system is in the null condition


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The CX rotor has been rotated so that an error signal has been produced This will drive the motor (clockwise) until the null position reached


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The CX rotor has been rotated so that an error signal has been produced This will drive the motor (counterclockwise) until the null position is reached


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In figure above:

A synchro-controlled servo system is used as an input

The output is made up of an indicator and other associated rotating components (load)

The mass or friction would not allow a simple synchro to be used

The input synchro signal is connected to a synchro that is coupled to the indicator shaft.

When used in this application, special synchros, called control transformers (CTs), are used.

The rotor and stator impedances of the control transformer are higher than those of a simple synchro, which allows them to interface more

easily with electrical circuits.



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If the input synchro and the control transformer in the indicator were set to exactly the same angle, the rotor output from the control transformer

would be zero.

When theres an angular difference between the two synchros, there will be rotor voltage,

The amplitude is proportional to the sine of the error angle and the phase is indicative of the algebraic sign of the error angle.

If the error angle is positive, the output from the synchro rotor is in phase with the driving voltage;

If it is negative, the two are out of phase.



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If the AC voltage from the rotor of the synchro were amplified and applied to a two-phase AC motor, the motor would operate so as to drive the servo system toward a zero error angle.

At this point, the output from the synchro would be zero and the motor would cease to run.

There are two positions at which the error voltage is zero; one at the desired angle and one 180o from the desired angle.



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When the servo system is near its desired position, the motor is still running at a relatively high speed.

Because the motor and the other components of the servo system have an inertia, the high motor speed near the desired servo position will cause the servo to overshoot the position and end up on the opposite side of the desired null.

The error voltage will cause the motor to change direction and approach the null point again.

Depending on the amount of inertia, the system could overshoot again and the process be repeated.

A servo system that oscillates continuously is said to be unstable.

Servo Behavior


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Continuation of the overshooting about the null position is called hunting. Damping of the system can reduce the oscillations and prevent hunting.

A servo system in which oscillations exist, but cease after a few passes of the null point, is said to be underdamped.

If the system approaches the null point without any oscillations in the minimum time, the system is said to be critically damped.

If the system approaches the null point without any oscillations but requires an excessive amount of time, the system is said to be overdamped.

The damping of a servo system can be controlled either electrically or mechanically.

Usually it is easier to control damping electrically, and most systems set the characteristics of the servo by means of electrical components.


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Degree of Damping


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Method of Damping

a. Viscous Frictional Damping Consist of a thin disc metal (copper or aluminium) on the

output shaft rotating between the pole of a permanent magnet.

Rarely used because: i. Consume power

ii. Causes or widens dead band which is the amount of

error that can exist without correction.


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b. Velocity Feedback Damping

By a tacho-generator attached to the output shaft which provide a small AC to produce voltage proportional to the

angular velocity of the shaft (motor).

Cosume less power.


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Summary

1. Define of open and closed loop control

system.

2. Explain on the meaning of the term : null,

hunting,dead band and damping?

3. Describe on the degree and method of

damping?


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Topic 5: Servomechanism ii


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Objectives


1. Explain the principle and operation of a synchro

transmission system namely:

i. Control synchro

ii. Torque synchro

iii. Differential synchro

iv. Resolver synchro


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Synchro Transmission

System


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Control Synchro

Used as error detectors in servo mechanism. Comprises of two synchro units, a control transmitter (CX) and

a control transformer (CT).


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Example of synchro transmitter

Control Synchro


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Operation

Reference signal applied to CX rotor and created a magnetic field link to CX stator coils.

CX stator EMFs produce currents through the coils of CT stator coils and set up magnetic fields in the CT.

EMFs induced in CT rotor depend on angle between rotor and stator field.

R1 aligned to S1 of the CX and the rotor of CT at 90 degrees to S1 is the Null Position. (No error)


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Torque Synchro

Used in instrument repeater systems (data indicating).

No amplification of torque takes place.

Movement of an input shaft is converted into an electrical signal and transmitted to move a pointer on a meter.


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Operation

AC applied to TX rotor and created a magnetic field in TX rotor which is the primary of a transformer.

This primary field effect the three coils S1,S2 and S3 (secondary transformer).

If the rotors misaligned, the currents will set up a resultant magnetic field in TR stator to which the TR rotor will align.

When the rotors are aligned, the stator EMFs on the TX and TR are the same. No current flows. (Null Position)

TR rotor follows TX shaft.


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Principle of torque synchro measurement


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Resolver Synchro

Commonly used in older analog computers, flight director and remote indicating compasses system on the aircraft

They are used in dealing with problems that are often occur in navigation: fixing the relative position of the two points.

Has two sets of stator coils arranged at 90 degrees and two set of rotor coils also at 90 degrees


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Positions are fixed in two way:

1. Measuring range and bearing (Polar coordinates)

2. Measuring X and Y coordinates (Cartesian coordinates)


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Polar to Cartesian Conversion Operation

= x

= y


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Cartesian to Polar Operation


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Differential Synchro

Used when the output required to be the difference between two input shaft angles.

The rotor outputs are connected to the three stator coils of TR or CT

May be used in either torque,control or resolver synchro system.


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Operation

TDX acts as a three winding transformer.

The induces EMFs of TX stator applied to stator coils of TDX which produces a resultant magnetic field.

Induces EMFs TDX rotor coils applied to TR stator coils.

The resultant field produced in TR stator is combination of both TX and TDX rotor position.

TR rotor moves to align with the resultant field (difference between two shaft angles).


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Example of combination of input and the resultant output


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Summary

1. Difference of Torque synchro & Control synchro?

2. How differential synchro operate?

3. Resolver synchro used for what application?


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Objectives


1. Explain the meaning of transducer.

2. Explain the operation and state the use of E and I Transformers.

3. Explain the operation and state the use of Inductive Transmitter.

4. Explain the operation and state the use of Capacitive Transmitter


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Transducer Transducer

A device, component machine, system or combination of

these that is used to convert one form of energy into

another.

Example types of transducers:

1. Temperature transducer convert temperature changes into electrical voltages (or mechanical switching)

2. Pressure transducer change barometric pressure into electrical voltage

3. Motor convert electrical signal into mechanical (rotary/linear) .


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E and I Transformers

Construction E core the centre limb is the primary coil and

the 2 outer are the secondary coils

Secondary windings are connected in series opposition, therefore the voltage induces in

them will oppose each other.

I bar pivoted in the center and attached to whatever we are trying to measure the movement

of, e.g. in servo altimeter, the I bar is connected

to the capsules.

Changes in the position of the I bar changes the reluctance at the upper of lower arm.


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OPERATIONS

Figure a: I Bar Neutral

The flux in the top and bottom limbs will be the same

The emf induced into two coils B and C will be the same but of opposite phase.

The output will be zero


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Figure (b) : I Bar Position 1

When the I bar is moved by the sensing element, more flux cutting coil B (less air gap) and less flux cutting coil C (larger air gap).

The emf induced in coil B is greater than in coil C. The output is the difference between these two giving an output that is in phase with input.


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Figure (c): I Bar Position 2

When the I bar moved in opposite direction, the emf induced into coil C is greater than coil B

The output will be anti-phase to the input. The amplitude of the output will depend on the amount of movement of the I bar.


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Linear Voltage Differential Transformer (LVDT)


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Applications

These type of sensor is used in:

Servo altimeters Acceleration sensors Servo instruments Air data computers Cabin pressure transducer


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ADC Electro Mechanical


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Inductive Transmitter

Construction

Has a coil supplied with alternating current set against two vanes. Voltage will be induced into the secondary coil depending on the inductance of the vane next to the coils.

Amount of inductance depends on the type of vane material used (permeability value,)

(a)


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Operation

A common inductive transmitter is made of aluminium and ferrite material. The null position is when the inductive coil is positioned on the join between ferrite and the aluminium vanes, figure (a).

If the vane is moved so more of the aluminium vane is beside the coils, less inductance results, figure (b).

Conversely when the vane move more of the ferrite vane is beside the coils, greater inductance results, figure (c).

One of the main application of inductance transmitter is position indication.


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The capacitance of a capacitor depends on:

1. The distance between the two plates

2. The area of the plates

3. The dielectric constant of the material between the plates

Capacitance Transmitter


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Capacitance Transmitter

Construction

Capacitive transmitter are simply variable capacitor (air dielectric)

Movement of the input shaft effectively alters the area of the

plates facing each other.

Has a rotor and stator of intermeshing plates.


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Operation

The amount of stator and rotor intermeshing is controlled by the rotation of a shaft by mechanical output.

As illustrated in figure above, when the stator and rotor plates are fully intermeshed, the capacitance is high.

Conversely, when the stator and rotor plates are partly meshed, the capacitance is low.


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Applications

A common use of capacitance transmitters is a fuel quantity indicator system. As the fuel rises in the tank, air is displaced by fuel and the dielectric changes to increase the capacitance of the unit.

While the fuel level goes down, so the capacitance goes down. Air has a dielectric constant of 1 and aircraft fuel has a dielectric constant of approximately 2.

Simple capacitive tank unit


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Tank unit installation (B777)


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Example of Fuel Tank Units


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Summary

1. What is E & I bar? The construction?

2. How E & I bar operate?

3. The use if E & I bar?

4. Construction of inductive transmitter?

5. Amount of inductance depends on?

6. Give 3 factors that effect capacitance of

a capacitor?

7. Application of capacitive transmitter?

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Topic 13 Servomechanism