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.

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.

Slab Desynn

If the voltage at the 3 tappings of the transmitter of a basic Desynn are measured as the wiper arms are rotated 360°, it will be seen that they produce a sawtooth waveform as opposed to a sinewave. 
This results in the pointer of the indicator not following the transmitter exactly.  In most instances the difference is insignificant, however their may be certain circumstances where it cannot be overlooked.

The solution is to use a modified Desynn transmitter called a 'slab Desynn'.  In a slab Desynn, the resistor is wound on a slab former and has the power supply connected to it, whilst the wiper arms now provide the output to the receiver, there being 3 wiper arms each displaced from the next by 120°.  The output from this device is a sinewave.  It can be connected to the same type of indicator and operates in the same way as the basic Desynn.

Synchronous Data Transmission

Synchronous data transmission systems are designed to indicate the position of a component or control surface that cannot be directly observed.  The systems fall into one of two categories; d.c. systems called 'Desynn Systems'

There are a variety of different types of Desynn systems available:

The Basic Desynn is generally operated by a rotary motion, however linear versions are also found.  The conversion from linear to rotary motion being achieved by a push rod and gear wheel.

The Micro Desynn was designed to magnify the small movement obtained by such items as pressure measuring devices.  They are operated by linear motion.

The Slab Desynn was designed to overcome signally errors inherent in the basic Desynn system.  In the vast majority of instances the errors in the basic Desynn 

Basic Desynn

 

In the basic Desynn system the transmitter comprises an endless resistance wound on a circular former, this arrangement being referred to as a 'Toroidal Resistance'.  Equally spaced at 120° intervals around the resistor are 3 tappings, it is to these that the signal wires are connected.  Running on the resistor are two wiper arm type contacts that are spaced apart by 180° and insulated from one another, it is to these that system power is applied.

The indicator comprises a two pole permanent magnet rotor, pivoted to rotate inside a soft iron stator, the pointer being attached to the spindle.  The stator carries three star connected windings that are connected to the three wires coming from the tappings of the transmitter.