Simultaneous Reduction of Normal- and Tangential-Direction Force Ripples in Permanent-Magnet Linear Synchronous Motors

This article presents the force ripple reduction method for iron-cored permanent-magnet linear synchronous motors (PMLSMs) in both the tangential and normal directions. Iron-cored PMLSMs yield significant geometry-driven and current-driven ripples in the moving and normal directions due to the magnetic saliency in the airgap, deteriorating the servo control performance and causing unwanted vibration and noise. In order to resolve such issues, a control method is proposed in this article to simultaneously reduce the motor force ripples in both directions, using position-dependent force constants. Considering the effects of the D- and Q-axis current on the motor force directions in PMLSMs, the identification methods are presented for the force constants and the geometry-driven ripples in the tangential and normal directions to be used in the force ripple reduction method. An experimental testbed of PMLSM-driven stage is constructed to validate the motor force parameter identification methods and ripple reduction method. Using the proposed methods, the significant force ripple reductions are experimentally achieved as 91.2 % and 94.5 % in peak-to-peak and RMS (root-mean-square) values in the tangential direction, and 82.4 % and 86.6 % in the normal direction, without sacrificing the motor thrust performance.