Servo control use current control for rapid adjustment of motor torque. Voltage control will not be good for servo applications due to inherent delays before the control passes to adjust current. In PWM it is a delay in the motors electrical time constant L/R; in square wave control it is a sequence of delays at the capacitor of DC-link, electric time constant L/R of motor etc.
To obtain current control we use, so called, "current controlled PWM". There too, we have two options;
(a). Hysteresis current control mode
(b). Fixed frequency current control mode
(a). Hysteresis current control mode
This PWM acts to constrain the motor current I to a specified shape and amplitude, as suggested by the outer loops (e.g. Speed loop) of the closed loop control system. This requires motor current feedback as an input to the PWM modulator. Desired current is the other input.Switching principle is,
