TORQUE DIFFERENTIAL SYNCHRO SYSTEMS

The demands on a synchro system are not always as simple as positioning an indicating device in response to information received from a single source (transmitter). For example, an error detector used in checking weapons equipment uses a synchro system to determine the error in a gun's position with respect to the positioning order. To do this, the synchro system must accept two signals, one containing the positioning order and the other corresponding to the actual position of the gun. The system must then compare the two signals and position an indicating dial to show the difference between them, which is the error.

Obviously, the simple synchro transmitter-receiver system discussed so far could not handle a job of this sort. A different type of synchro is needed, one which can accept two signals simultaneously, add or subtract the signals, and furnish an output proportional to their sum or difference. This is where the SYNCHRO DIFFERENTIAL enters the picture. A differential can perform all of these functions.

There are two types of differential units - differential transmitters and differential receivers. The differential transmitter (TDX) accepts one electrical input and one mechanical input and produces one electrical output. The differential receiver (TDR) accepts two electrical inputs and produces one mechanical output. A comparison of the TDX and TDR is shown in figure 1-18. The torque differential transmitter and the torque differential receiver can be used to form a DIFFERENTIAL SYNCHRO SYSTEM. The system can consist either of a torque transmitter (TX), a torque differential transmitter (TDX), and a torque receiver (TR), (TX-TDX-TR); or two torque transmitters (TXs) and one torque differential receiver (TDR), (TX-TDR-TX). Before beginning a discussion of the systems using differentials, we need to provide a brief explanation on the newly introduced synchros, the TDX and the TDR.

Figure 1-18. - Torque differentials.

Torque Differential Transmitter

In the torque differential transmitter, BOTH the rotor and stator windings consist of three Y-connected coils, as illustrated in view A of figure 1-19. The stator is normally the primary, and receives its input signal from a synchro transmitter. The voltages appearing across the differential's rotor terminals (R1, R2, and R3) are determined by the magnetic field produced by the stator currents, the physical positioning of the rotor, and the step-up turns ratio between the stator and the rotor. The magnetic field, created by the stator currents, assumes an angle corresponding to that of the magnetic field in the transmitter supplying the signal. The position of the rotor controls the amount of magnetic coupling that takes place between the stator magnetic field and the rotor, and therefore, the amount of voltage induced into the rotor windings. If the rotor position changes in response to a mechanical input, then the voltages induced into its windings also change. Therefore, the output voltage of the TDX varies as a result of either a change in the input stator voltage or a change in the mechanical input to the rotor. This electrical output of the TDX may be either the SUM or the DIFFERENCE of the two inputs depending upon how the three units (the TX, the TDX, and the TR) are connected.

Figure 1-19A. - Torque differential transmitter.

Figure 1-19B. - Torque differential receiver.

Torque Differential Receiver

The torque differential transmitter (TDX) and the torque differential receiver (TDR) are ELECTRICALLY IDENTICAL. The only difference in their construction is that the receiver (TDR) has a damper, which serves the same purposes as the damper in the TR - it prevents the rotor from oscillating. The real difference in the receiver lies in its application. It provides the mechanical output for a differential synchro system usually as the sum or difference of two electrical inputs from synchro transmitters. As in the case with the TDX, the TDR addition or subtraction function depends upon how the units in the system are connected.

Basically, the torque differential receiver operates like the electromagnets we discussed earlier in this chapter. In view B, the rotor and stator of the torque differential receiver receive energizing currents from two torque transmitters. These currents produce two resultant magnetic fields, one in the rotor and the other in the stator. Each magnetic field assumes an angle corresponding to that of the magnetic field in the transmitter supplying the signal. It is the interaction of these two resultant magnetic fields that causes the rotor in the TDR to turn.

Q.30 What is the purpose of using differential synchros instead of regular synchros?
Q.31 What are the two types of differential synchros?
Q.32 Other than their physical differences, what is the major difference between a TDX and a TDR?
Q.33 What determines whether a differential synchro adds or subtracts?