Modeling, identification, and compensation of pulsating torque in permanent magnet AC motors

Permanent magnet ac motors generate parasitic torque pulsations, owing to several electromagnetic phenomena, such as imperfections in the motor and in the associated power inverter. Excitation of the mechanical resonances on the load side is generally the worst consequence of such disturbances; speed oscillations arise which may dramatically limit the performance in high-precision applications. A compact model of the pulsating torque in sinusoidal permanent magnet motors is presented in this paper, based on a combination of theoretical analysis and experimental identification. On-line identification algorithms and a compensation scheme are outlined, as well. The control scheme autonomously identifies the parameters of the disturbance model through simple closed-loop motion experiments, The compensation is then applied as a modification of the output of the position controller. Experiments carried out on an industrial robot demonstrate the effectiveness of the control scheme in suppressing oscillations, both on the motor and on the load side.