Velocity Control Servomechanisms

In some applications it is the rotational speed of a shaft and not its position that must be controlled.  A Rate Servo is shown in the diagram .  The input demand signal is used to control the angular velocity of the output shaft and not its position.  To make the speed of the driving motor exactly proportional to the input demand a servomechanism is essential.  If a servomechanism were not used the speed of the output motor would vary with changes in the supply voltage or any changes of the friction in the motor or its load.

Note that there is no position feedback.

Movement of the speed control potentiometer produces a voltage proportional to the demanded speed.  The tacho-generator provides a voltage proportional to the angular velocity of the output shaft.  If there is a difference between these two signals an error voltage will be fed to the amplifier.  The output of the amplifier will accelerate or decelerate the motor until the output of the tacho-generator produces a voltage exactly equal to the input demand voltage and the motor will run at the demanded speed.

Coulomb Friction Damping

In a servomechanism with no frictional forces, all the torque developed, which is proportional to the error is used to accelerate the load.  However, when friction damping is introduced, the torque developed must first overcome the frictional force before the system can be set in motion. 

  If d represents the error that must exist before the system can develop sufficient torque to overcome the frictional torque, then if the error is less than d when the system is not in motion, the system will remain at rest with that amount of error.  The response to a step function input of a servomechanism with coulomb friction damping is shown below; the response curve of an undamped system is also given so that a direct comparison can be made. 

From Figure it can be seen that each successive oscillation of the output shaft is reduced in amplitude by the retarding effect of the friction damping until a point is reached when, with the output shaft velocity at zero, the error is less than d; at this point the output shaft will come to rest.  However, as shown in Fig 8, the output shaft may not be at its correct position and a permanent error may exist; this error is known as positional error.   

The response curve given in Figure indicates two overshoots before the system finally comes to rest, but the number of overshoots for a given degree of damping will depend upon the size of the initial error in relation to the amount of error required to overcome the friction torque; an initial error of 2d would produce no overshoot, whilst a large initial error compared with 2d would produce several overshoots.  In practice, coulomb friction damping is not used and, although always present, it is kept to a minimum.

Synchro Types

Synchro types may be classified as follows:

• Torque transmitter
• Torque receiver
• Torque differential receiver
• Torque differential transmitter
• Control transmitter
• Control transformer
• Control differential transmitter
•   Resolver

Torque Transmitter  -  TX

Used to generate an electrical signal corresponding to the angular position of a mechanical component.  The rotor is connected to the component and the stator kept stationary.  The electrical signal is derived from the position of the rotor relative to the stator.  The TX  is generally used as the transmitting element in a remote position indicating system.

 Torque Receiver  -  TR

The rotor of a torque receiver, which is free to turn, moves to a position dependent on the electrical angular information received from its connected torque transmitter or torque differential transmitter.  The TR is generally used as the receiving element (indicator) in a remote position indicating system.

Torque Differential Receiver  -  TDR

The torque differential receiver is electrically connected to two torque transmitters.  The rotor of the TDR, which is free to move, aligns with the stator field.  The position of the stator field depends on the inputs from the two transmitters, and the way in which they are interconnected.  By suitable connection, the TDR can be made to indicate the sum of the transmitter inputs, or the difference between them.

Torque Differential Transmitter  -  TDX

The torque differential transmitter has a stator that receives electrical positional information from a torque transmitter, and a rotor which is mechanically positioned.  This enables it to transmit electrical information corresponding to the sum, or difference, between the electrical input and its own rotor angle.

Control Transmitter  -  CX

Used to generate an electrical signal corresponding to the angular position of a mechanical component.  The rotor is connected to the component and the stator kept stationary.  The electrical signal is derived from the position of the rotor relative to the stator.  The TX  is generally used as the position transmitting element in a remote position control system.

 Control Transformer  -  CT

A CT is electrically connected to a CX and is used to produce an electrical signal for driving a servo system.  The electrical signal produced, is an a.c. voltage with an amplitude and phase dependent on the position of the rotor relative to the stator.

Control Differential Transmitter  -  CTX

A CTX receives electrical information from a CX and has a rotor which can be mechanically moved.  This enables it to transmit an electrical signal proportional to the sum or difference in angle between the electrical input and its own rotor position.

Resolver

A resolver has two mutually perpendicular windings on the rotor and another two on the stator (4 windings in total).  It can resolve an input signal into its sine and cosine components, perform the operations of vector addition and subtraction or convert polar to cartesian co-ordinates and vice versa.

Aircraft Servomechanisms

Servomechanisms can be classified according to two main categories:

• Open loop systems.

• Closed loop systems.

OPEN LOOP

In an open loop system, the input demand generates an electrical equivalent of the demand position. This signal is amplified to the required power level and applied to a motor to position the load. The speed of response and the final position of the load depend on the following factors:

• Any variations in load conditions.

• Frictional forces within the motor and its load, and any mechanical interconnections.

• Variations in power supplies.

• The value of the demand voltage.

• Variations in amplifier gain.

As the open loop system suffers from the variable factors shown above, the output is unlikely to follow the input precisely and cannot provide the close tolerance required.

CLOSED LOOP

If the errors in the output of a system are detected and fed back to the input so that the necessary corrections can be made to eliminate the error, the system is said to be a closed loop system. A Closed loop system is shown below.

The essential features of a closed loop system are:

• Information concerning the behaviour of the load is fed back to the input. This is called feedback.

• The position of the output (feedback) is compared to that demanded by the input. Typically in a summing amplifier.

• The production of an error signal proportional to the difference between the demand and feedback signals.

• Power amplification of the error signal to control the load.

• Movement of the load in such a direction as to reduce the error signal to zero, at which point the output is the same as that demanded by the input.