Robust two degree of freedom regulators for velocity ripple elimination of AC permanent magnet motors

A general AC PM (Permanent Magnet) motor control system consists of a motion controller, a current tracking amplifier, a feedback encoder and the motor itself. The motion controller generates two analog commands to the current tracking amplifier and the three phase currents are reproduced at the motor terminals. However, DC offsets are always present at the motor terminals due to the DAC (Digital to Analog Converter) offsets of the motion controller and the current sensor offsets of the current tracking amplifier. These current offsets generate sinusoidal torque disturbance and hence produce velocity ripples. Such a disturbance cannot be rejected by using a simple PI (Proportional plus Integral) control. Furthermore, the current offsets drift with time and temperature so that an off-line compensation does not work satisfactorily. In this paper, a robust 2DOF (Two Degree of Freedom) regulator containing the internal model of the sinusoidal disturbance is proposed to accomplish disturbance rejection and constant speed tracking