Rectangular Thrust Control Methods of PMLSM for Stroke and Dead Centers Tracking of Free-Piston Linear Generator

In the presence of frequent and significant combustion variations, the motion control of the piston is a critical and challenging task for the stable operation of the free-piston linear generator (FPLG). In this respect, this paper presents a rectangular thrust control method to shape the motion characteristics of the piston by actively controlling the electromagnetic thrust of the permanent-magnet linear synchronous motor (PMLSM). The thrust magnitude of the PMLSM during the compression stroke and expansion stroke in one motion cycle is constant, with the direction changing with the piston velocity. The thrust is adjusted cycle-by-cycle based on the tracking errors of the previous cycles. The control algorithm is easy to implement with double closed-loop cascade control structure. Under the proposed method, the motion stroke, top dead center (TDC) and bottom dead center (BDC) of the piston can be precisely regulated with fast transient response and high robustness. The control effect of the proposed methods are examined through experiments under different conditions on a PMLSM-based motion platform.