Reduction of periodic speed ripples in PM synchronous motors using iterative learning control

PM synchronous motor drives are widely used for high-performance servo applications where torque smoothness is an essential requirement However, parasitic torque pulsations do exist in PMSM drive due to nonsinusoidal flux distribution, cogging and errors in current measurement. A consequence of these torque ripples is speed oscillations that deteriorate the drive performance particularly at low speeds. The outer loop PI speed controller does suppress these periodic speed ripples to a certain extent, but is not adequate enough for many high performance applications. In this paper, the authors propose a simple iterative learning control (ILC) scheme for generating a compensation reference current to augment the PI speed controller output to further reduce the speed ripples. The proposed ILC controller is applied in conjunction with the outer loop PI speed controller and therefore can be easily integrated to any of the existing PMSM drive system. Experimental verification of the proposed scheme is carried out, and test results obtained demonstrate the effectiveness of the proposed control scheme in reducing speed ripples.