Servomechanism

• A servomechanism is a particular type of

automatic control system.

• It is the action of the control system causing

the output (the position or velocity of a shaft)

to slavishly follow an input demand, that has

given the device its name of servomechanism

(from the Latin word servus – a slave).

• This section will be confined to the basic

principles of servomechanism operation.

SERVOMECHANISMS

A transducer can be described as a device which converts one type of energy into another.

The defining characteristics of a servomechanism is that the controlled output of a mechanism is automatically compared with the controlling input.

The difference between the settings or positions of the output and the input is called an error signal, which acts to bring the output to its desired value.

Servomechanisms may be mechanical, electrical, hydraulic or optical.

This module deals with mechanical/electrical devices.

Magnetic transducers include synchro systems

SERVOMECHANISM

• A device used to provide control of a desired operation through the used of feedback.

• A servo is a small device that has an output shaft.

• This shaft can be positioned to specific angular position by sending the servo a coded signal.

• As long as the coded signal exist in the input the servo will maintain it’s angular position of the shaft.

• Coded signal changes – The angular position of the servo shaft changes.

• Servo is used in Auto-pilot in air planes by using the Radio controlled to position control surface like – Elevator and Rudder.

• Rudder – Flat, broad piece of wood/metal hinged on the stern of a boat, ship and Airplane for steering.

Servo

• A self correcting close loop control system.

• Usually containing some mechanism, such as a Motor

• The motor runs, thus controlling some other device.

• When the device moves in wrong direction or wrong angle, an Error Voltage or Error Angular is produced.

(The error signal is some way supplied by being matched between reference signal).

• The error signal is matched and compared with reference signal and supplied to activate the servo

CONTROL SYSTEMS

• Automatic control systems are devised to regulate or govern a flow

of energy and, therefore, can include electronic, electro-mechanical, pneumatic, hydraulic and mechanical devices.

• Their arrangement and complexity varies with the function they have been designed to perform, together with the required speed and accuracy.

• The output to be controlled can take many forms; it could be for example the automatic piloting or stabilising of an aircraft, the precise positioning of a radar aerial, or maintaining the position of an inertial platform, regardless of any manoeuvres the vehicle to which it is mounted performs.

• Disregarding the nature of the tasks involved, the basic components and their arrangements have a strong family likeness and they behave in very similar ways.

• A common theory can, therefore, be applied to all forms of automatic control.

TRANSDUCERS

• A transducer can be described as a device which

converts one type of energy into another.

• The defining characteristics of a servomechanism is that the controlled output of a mechanism is automatically compared with the controlling input.

• The difference between the settings or positions of the output and the input is called an error signal, which acts to bring the output to its desired value.

• Servomechanisms may be mechanical, electrical, hydraulic or optical.

• This module deals with mechanical/electrical devices.

• Magnetic transducers include synchro systems.

TRANSDUCERS

• A device which converts one quantity into another

quantity, specifically when one of the quantities is electrical.

• Thus, a loudspeaker converts electrical impulses into to sound.

• A microphone converts sound into electrical impulses,

• A photocell ( Solar-cell) converts light energy into electrical energy.

• A thermocouple converts heat into electricity.

• An Antenna converts electromagnetic waves into electrical current.

• CRT converts electrical signals into visual form.

3 Kind of Transducer 1.Sensor:- • Used to detect a parameter in one form and report it un another

form of energy.

• Usually electrical or digital signal: such as Tachometer.

2.An Actuator:- Used for transformation of energy or in other words, an actuator

is the one which get actuated or stands responsible for the output

action.

Actuator uses the electrical energy to perform mechanical

movements.

3.Ultra-Sonic Transducer:- Used to switch back and forth many times a second between acting

as an Actuator or to produce ultra-sonic waves.

Acting as a sensor to detect the ultra-sonic waves

SYNCHRO SYSTEMS

• Synchro systems are used as an integral part of many avionic systems in aircraft,

such as

a) instrument systems

b) auto-pilots.

• The synchro systems can be:

a) Torque systems

b) Control systems

c) Resolvers

• Servo mechanisms may also be classified according to 2 main categories:

a) Open Loop

b) Closed Loop

CLOSED LOOP SYSTEM

OPEN LOOP SYSTEM

The input transducer converts the angular movement into a demand

voltage which is then amplified.

This amplified voltage drives the motor which turns the output shaft

connected to a load

A problem with this configuration is that there is no way to ensure

that the output is following changes of the input.

Possible problems that may occur are:

a) The output voltage will vary as the gain of the amplifier alters with time

and temperature

b) the frictional forces within the motor and load will change with velocity,

temperature and load.

c) Variations of supply and frequency will cause variations of speed of the

motor and ultimately the load speed and position, even if the load torque

remains constant.

As these factors cannot be precisely controlled, the open loop system is

not good enough for close tolerance control.

More control of the system is achieved by using a closed loop system

OPEN LOOP SYSTEM

CLOSED LOOP SYSTEM

A closed loop system feeds output positional information to an

error detector which compares it with the demand. Fig shows a simplified closed loop ystem

The input transducer converts an angular movement into a demand voltage.

The demand voltage is input to the error detector.

If the input shaft and output shaft are lined up, there will be no difference

between the demand and the feedback voltage so no error voltage is produced.

There is no drive to the motor which is connected to the load and a feedback

transducer.

The feedback transducer produces a feedback voltage proportional to the

angular movement of the output shaft.

CLOSED LOOP SYSTEM

• If the input shaft rotates 60 degrees clockwise:

• The input transducer produces a voltage equivalent to 60 degrees clockwise.

• The error detector has 60 degrees right and zero degrees fed into it.

• The output is a 60 degrees right voltage.

• The error voltage is amplified

• The motor turns 60 degrees right

• The output shaft turns the load and feedback transducer.

• The feedback transducer outputs a feedback voltage equivalent to 60 degrees right.

• The error detector has 60 degrees right (demand) and 60 degrees right (from feedback).

• The output is at the required position and the error is zero.

• The sequence is repeated whenever there is a rotation of the input shaft.

CLOSED LOOP SYSTEM:-

A self-correcting close-loop control system, usually containing

some mechanism, such as a motor that is made to run, thus

controlling some other device, until an error signal is some way

supplied by what is being controlled matches the reference signal

activating the servo.

Servo Motor:- A motor operated by the output signal of a servo amplifier.

Depending upon the end application of servo system, the motor

signal may or may not be corrected.

Servo Amplifier:-

A highly stable amplifier designed to expressed for use in a

servo mechanism

Servo Loops:-

In a control system( particularly in servo amplifier), the output

to input feedback loop through which automatic control is effected.

TORQUE SYNCHROS

INTRODUCTION

• Torque synchros are mainly used in

position indicator systems where the

indicating mechanisms (pointers,

gauges), do not need a great deal of

torque to operate them.

Torque Synchro Symbol

TORQUE SYNCHRO

CONSTRUCTION

The basic construction of a synchro is a stator

body into which are set 3 sets of recessed

windings set 120 degrees apart.

A rotor is positioned between the stator

windings and supplied with an alternating current

supply.

:

In some civil airline diagrams, the synchro may be labelled as follows

S1 = x R1 = cold

S2 = z R2 = hot

S3 = y

When using S1, S2 and S3 designations, they are counted clockwise with

S2 at the top.

When using x, y and z designations, they are counted anti-clockwise with

z at the top.

This module will use S1, S2, S3, R1 and R2.

Stator

Rotor

I/P AC

TORQUE SYNCHRO OPERATION

• In the synchro units’ windings, the

voltages which are induced by the

TRANSFORMER principle from the

ROTOR are not of equal magnitude and

their amplitude will be dependent on

the RELATIVE ANGLE between the

rotor and each stator winding

TORQUE SYNCHRO SYSTEM

INTRODUCTION

• In a torque synchro system one of the

synchros is called the torque transmitter

(TX) for measuring input angular

movement, eg. flight control position.

• The other synchro is called the torque

receiver (TR) which moves a pointer, eg.

flight position indicator.

TORQUE SYNCHRO SYSTEM DIAGRAM

Diagram layout that may be used in circuit diagrams.

TORQUE SYNCHRO SYSTEM OPERATION

• When the rotors are at the same angular position to each other, as in Fig, the system is in correspondence and because the induced voltages (transformer action) in the stators of the TX and TR are equal, no current flows between the stators.

• If the rotor of the TX is now moved 20 degrees clockwise (+20 degrees), the amplitude of the secondary voltages in S1, S2 and S3 change.

• This causes an unbalance in the stator voltages between the TX and TR resulting in a current flowing between the stators.

• The alternating currents in S1, S2 and S3 of the TR cause a resultant magnetic

field in the stator which is displaced by 20 degrees clockwise.

• The rotor of the TR is also producing an alternating magnetic field which is in

phase with the TX.

• Both the alternating fields produced by the stator and rotor react to each other.

• As the rotor is free to move, it will rotate 20 degrees clockwise until

correspondence is achieved.

• No current flows and the TX and TR are at the same angle

SYNCHRO ERRORS

The system has some problems: HUNTING –

This error is most pronounced when there is a large angular input to the rotor of the TX over a short period of time.

This causes the rotor of the TR to FOLLOW UP quickly to correspondence.

The momentum generated causes the TR rotor to overshoot the required position.

The feedback is now producing an error in the other direction which causes the TR rotor to take up the required position.

To reduce hunting, the TR in most torque systems is fitted with an eddy current damping disc.

It is not necessary to fit it to the TX, however, the synchros are manufactured so that the TX and TR are interchangeable.

DEADBAND –

• This error occurs because of imperfections and friction in the system.

• As the TR rotor is reaching its position of correspondence, the torque on the

rotor is reducing.

• It eventually becomes so small that it cannot overcome the imperfections

and friction, leaving the TR rotor out of correspondence with the TX.

• The angular difference between the Tx and TR is known as the DEADBAND.

• The main advantage in TORQUE SYNCHRO system is, it uses very

little power consumption when at rest as it only draws power when the

TX and TR are displaced (not in correspondence).