SYNCHROS AND RESOLVERS

Jenny

Synchros

It is the name given to a variety of rotary, electromechanical, position-sensing devices.

a variable coupling transformer. Functions –

supplying data by positioning dials and pointers control devices in servo systems.

Types Torque synchro

to move light loads such as dials, pointers, or similar indicators. Control synchro

designed to move heavy loads such as gun directors, radar antennas, and missile launchers,machine tools.

Torque synchros include transmitters (CG), differentials (CD) and receivers (CR).

Control synchros include transmitters (CG), differentials (CD) control transformers (CT), resolvers (CS), linear transformers (LT) and

the two hybrid units: transolvers (CSD) and differential resolvers (CDS).

Construction of synchro transmitter-receiver Each synchro contains a rotor, similar in appearance to the

armature in a motor, and a stator, which corresponds to the field in a motor.

The synchro stator is composed of three Y-connected  windings (S1, S2, and S3).

The rotor is composed of one single winding (R1 and R2).

TRANSMITTER The magnitude of the induced voltage depends on the angle

between the rotor fields and the resultant axis of the coils forming that stator phase.

VS1-3 = KVR2-1 sin θVS3-2 = KVR2-1 sin (θ+ 120°)VS2-1 = KVR2-1 sin (θ + 240°)

where K is the maximum coupling transformation ratio (TR), which is defined as TR=Vout(max.)/Vin

θ is the rotor position angle. VS1-3 is the voltage from the S1 terminal to the S3 terminal. The transmitter, therefore, supplies information about the rotor

position angle as a set of three output voltages.RECEIVER An instrument which will measure the magnitude of these voltages,

examine their time-phase relationships, and return them to their original form: a shaft position.

Such a device is the synchro receiver (CR). These two units form the most basic synchro system.

Torque in receiver The torque developed in a synchro receiver results from the tendency of two

electromagnets to align themselves The strength of the magnetic field produced by the stator determines the torque. The field strength depends on the current through the stator coils. As the current through the stator is increased, the field strength increases and more

torque is developed Synchro torque receivers, commonly called synchro receivers, are electrically

identical to torque transmitters of the same size except for the addition of some form of damping

The rotor is connected to the same ac source as the transmitter and assumes a position determined by the interaction of its magnetic field with the magnetic field of the stator.

BASIC SYNCHRO SYSTEM OPERATION

TORQUE DIFFERENTIAL SYNCHRO SYSTEMS

TX-TDX-TR

TX-TDR-TX

Control synchros

Control synchros by themselves cannot move heavy loads.

These devices are used for providing and dealing with control signals in servo systems where accurate angular transmission to a mechanical load is required.

There are three types of control synchros: the CONTROL TRANSMITTER (CX), the CONTROL TRANSFORMER (CT), and the CONTROL DIFFERENTIAL TRANSMITTER (CDX).

Control transmitter (cx) The Control Transmitter is really a high

impedance version of the Torque Transmitter and is possibly the most common of all synchros.

A Control Transformer is a high impedance version of the Torque Receiver with the rotor winding aligned at 90 degrees from that of a TR.

The output voltages from the CX are normally fed to a Control transformer (CT) to form a Control Chain.

Synchros in machine tools

Working

Servo system using control chain

Synchros are more commonly used in machine tools than resolvers A single synchro can represent an angular rotation unambiguously in very

narrow range(4-6mm) A number of synchros are coupled with gears having reduction ratio 10:1 As the table moves,the switching system switches from one to the other till

only fine control works

Digital synchro

Resolver

The Resolver is a form of synchro (Resolvers are very often called Synchro Resolvers) in which thewindings on the stator and rotor are displaced mechanically at 90º

The Resolver therefore exploits the sinusoidal relationship between the shaft angle and the output voltage.

The simplest Resolver would have a rotor with a single winding and a stator with 2 windings at 90º to each other.

If we assume that the rotor is excited by an AC reference voltage:

A Sin ω t

Then the voltages appearing on the stator terminals will be:

S1 to S3 = V Sin ω t Sin θ and S4 to S2 = V Sin ω t Cos θ, θ is resolver shaft angle

These voltages are known as Resolver format voltages

Resolver with two rotor windings Either one of rotor windings or one of stator windings shorted. Known as Data Transmission Resolvers, Control chain

a Resolver Transmitter (Symbol TX) and a Resolver Control Transformer (Symbol RC)

Resolver control chain

Application in machine tools

If rotor is coupled to the lead screw of machine tool, the voltages in stator indicate angle of rotation and hence displacement of table.

Selsyn transducer-3 stator windings

Difference between synchro and resolver

3 stator winding Stators 120 deg apart Output taken from

stator

2 stator winding Stators 90 deg apart Output taken from

stator or rotor

Comparison with other angular transducers

Cost ,in descending order Absolute optical Encoders Synchros and Resolvers Potentiometers Contact Encoders

Resolution and Accuracy High accuracy

Static and Dynamic Mechanical Loading Less in the case of the optical encoders and the

Synchros and Resolvers than in the case of the potentiometer and the contact encoders

Environmental Considerations The combination of synchro and its associated

electronics meet more stringent temperature, humidity, shock and vibration specifications than any other shaft angle transducer.

This is one of the main reasons why Synchros and Resolvers are preferred for Military and Aerospace applications.

Noise Immunity High degree of noise immunityIf true transformer

isolation is used at the conversion end No 3 or 4wire long distance problem