Superior low-speed control of a permanent magnet synchronous motor with digital encoder

Low-speed control is indispensable to several servo applications. Generally, motor speed acquired from the so-called M method is averaged by counting the number of pulses from the encoder at a fixed time period. As a result, a long detection delay, due to speeds lower than the minimum measurable value determined by the sampling frequency and the resolution of the encoder, may degrade the performance or even make systems unstable. In this paper, instead of using disturbance torque observers, a reference model derived from a full-order observer to denote the permanent magnet synchronous motor (PMSM) model is employed to approach instantaneous speed and improve the low-speed control performance. Two structures of model reference control are proposed. A proportional - integral - differential (PID) controller is located on the feedback path for one structure and on the forward path for the other. Additionally, in the latter structure, a composite signal obtained by adding motor speed and the estimated speed timed respective weighting factors is fed back to the input terminal to compare the speed command. This will further improve the performance. A digital signal processor-based drive provides experimental results, which show the improved speed resolution from 7.5 r/min to 1 r/min and the effectiveness of the system with loading. © 2011 Institution of Mechanical Engineers.