Torque ripple reduction in a.c. permanent magnet servo motor drives

Servo systems play an important role in industrial automation. A servo system denotes a closed loop controlled system capable of tracking required demands. One way of achieving high performance servo drive systems is to apply the closed loop control of an a.c. permanent magnet synchronous machine (PMSM). PMSM is a type of machine which rotates once three-phase a.c. voltages are supplied. The usage of permanent magnet materials contributes to the high efficiency of PMSM, and makes it a popular type of machine in industrial applications. However, the interaction between the permanent magnets and the machine stator would generate torque ripple and consequently unsmooth speed. Therefore, torque ripple of PMSM need to be considered carefully in the control of such servo systems. An innovative control scheme combining an enhanced high bandwidth deadbeat current controller and a fractional delay variable frequency angle-based repetitive controller, is developed in this work in order to minimize torque ripple. For the purpose of accurately modelling the cogging torque and flux harmonics in PMSM, a lookup table embedded PMSM model is also proposed. It has been validated by both simulative and experimental tests that the proposed control scheme is able to reduce torque ripple in a PMSM drive system effectively for a wide range of frequencies, and even during transients, which has never been achieved according to the author's knowledge. The proposed method is not only adaptive to variable frequencies, but also adaptive to the variations of electrical and mechanical parameters in normal operating conditions