Some explanation of the electrical circuit required for a servo valve is presented here to help readers understand how the servo valve in the servo pump operates when it is used in a closed-loop transmission. The key features of the electrical circuit for a closed-circuit, closed-loop hydrostatic transmission are shown above the dotted line in Fig. 6.28. The motor output shaft drives a transducer, typically a tachometer generator. Voltage output from the tachometer generator is directly proportional to shaft speed. As shaft speed increases, voltage increases, and vice versa.
The comparator compares the tachometer generator voltage with the command voltage. The difference between the two is the error voltage. If the tachometer generator voltage is equal to the command voltage, meaning that the motor is turning at the desired speed, then the error voltage is zero.
The error voltage is fed to a servo amplifier. The servo amplifier produces an output current proportional to the input voltage. This current is fed directly to the coil of the torque motor. (Coil is another name for the windings on the armature.) The armature rotates, initiating the sequence of events described in the previous section.