Speed closed-loop drive system of permanent magnet synchronous motor using low-count QEP

A speed closed-loop drive system of permanent magnet synchronous motor (PMSM) typically requires the rotor position feedback from the high-count quadrature encoder pulse (QEP) sensor equipped with the motor. Unlike the brushless dc motor, the high resolution of rotor position is necessary in PMSM drive because the instantaneous rotor position is required for its field orientation on the synchronously rotating dq-axis coordinate. In addition, the bandwidth of the calculated speed depends on the resolution of measured rotor position as well. Unfortunately, the resolution of the QEP (e.g., 1000-2000 pulse/rev) heavily affects the overall cost. In low cost applications, the PMSM drive using the high-count QEP is therefore impossible. This paper proposes the rotor position extrapolation using the polynomial estimator from the low-count QEP and implement in the complete speed closed-loop drive system of PMSM. Simulation results using MATLAB/SIMULINK are presented to initially validate the proposed algorithm in the speed closed-loop drive system. According to these results, the improved speed control in a high speed range can be observed when using low-count QEP of 3 and 6 pulse/rev. However, in the low speed range the speed control still requires the higher count QEP. Finally, the experimental results are included and discussed to fully validate the overall system.