Reference model based low-speed control of PMSM

Low speed control is indispensable to several servo applications. Generally, motor speed obtained from the so-called M method is averaged by counting the number of pulses from the coupled encoder at a fixed time period. As a result, for speeds lower than the minimum measurable value determined by the sampling frequency and the resolution of the encoder, a long detection delay time may degrade the performance or even make system unstable. In the paper, instead of using a disturbance torque observer, a nominal permanent magnet synchronous motor (PMSM) model is employed to approach the instantaneous motor speed and improve the low-speed control performance. The composite signal by adding respective weighting factor timed motor speed value and the estimated speed value is fed back input terminal to compare the speed command. The “real” motor speed will be the output from reference model instead of the one averaging the pulses from a low-resolution encoder. A digital signal processor (DSP)-based drive system will present the experimental results, which show the improved speed resolution from 7.5 rpm to 1 rpm and the effectiveness of system with loading.