Differential Synchros
In some cases, it is necessary to detect and transmit error signals representative of two angular positions, and in such a manner that the receiver element of a synchro system will indicate the difference or the sum of the two angles. This is achieved by introducing a third synchro into either a torque or control system, and using it as a differential transmitter. Unlike TX or CX synchros, a differential transmitter (designated TDX or CDX) has an identically wound stator and rotor which in the application to a torque synchro system are interconnected as shown in Figure 4.3.14.
At (a) the TX rotor is shown rotated clockwise through 600 while the rotor of TDX remains at electrical zero; all the magnetic fields rotate, and the rotor of TR takes up the same angular position as the rotor of TX. If now the TX rotor remains at electrical zero, and the TDX rotor is rotated clockwise through 150 say, the fields of both synchros remain in the electrical zero position because their position is determined by the orientation of the TX rotor (diagram (b)). However, a 150 clockwise rotation of the TDX rotor without a change in the position of its field is equivalent to moving the rotor field 150 anticlockwise whilst leaving the rotor at electrical zero. This relative angular change is duplicated in the stator of TR and so its rotor will align itself with the field i.e. for a 150 clockwise rotation of the TDX rotor, the TR rotor will rotate 150 anticlockwise.
Assume now that the TX rotor is rotated through 600 clockwise, and the TDX rotor through 150 clockwise, then because the TR rotor will rotate 150 anticlockwise, its final angular movement will be equal to the difference between the two input angles i.e. it will turn through 450 (diagram (c)). The differential effect is of course reversed when the TDX rotor is rotated in the opposite direction to the TX rotor, so that the TR rotor rotates through an angle equal to the sum of the two input angles. By reversing pairs of leads either between TX and TDX, or between TDX and TR, any one of the rotors can be made to assume a position equal to the sum or difference of the angular positions of the other rotors.
In the same way that differential transmitter synchros can be used in torque synchro systems, so they can be used in systems utilizing control synchros to transmit control signal information on the sum or difference of two angles. The basic arrangement is shown in Figure 4.3.13
