A Novel Direct-Drive Rotary–Linear Machine and Ripple Reduction by Harmonic Current Injection Approach

This article presents a direct-drive multistage rotary–linear machine (RLM) with a dual permanent magnetic (PM) structure. The RLM has prominent direct-drive characteristics, such as high torque/force density, and a deeply integrated mechanical structure, as well as a significant degree of decoupling between the two dimensions of motion. A harmonic current injection method is proposed to reduce torque and force ripples, which are mainly caused by machining tolerances and manufacturing processes. The basic structure, winding design, and design deduction of the proposed RLM are introduced first. Then, the electromagnetic performance of the proposed dual-PM RLM is evaluated by finite element analysis and compared with the data of an existing RLM and the model with mechanical tolerance. Furthermore, the harmonic current injection (HI) method is designed. HI is adopted in motor control to effectively reduce the torque and force ripples of the machine and enhance the controllability of the RLM motor. Finally, the feasibility of the design scheme and HI is confirmed by testing the prototype.