Robust Piston Control for PMLSM-Driven Free-Piston Linear Generator by Progressively Updating Thrust-Position Trajectory

Free-piston linear generator (FPLG) is considered as a potential technology in the field of hybrid vehicles due to its unique operation characteristics. However, the special structure of no crank-link also leads to the difficulty of piston control. In the presence of frequent combustion variations, the stable operation of FPLG is the biggest challenge it currently faces. In this respect, this article presents a robust piston control method by progressively updating thrust-position trajectory (PUT-PT) of permanent-magnet linear synchronous machine (PMLSM) in FPLG. According to the target points and desired speed, the required motor thrust is obtained based on the energy balance relationship. The calculation of thrust is repeated with a certain position interval in the whole target stroke, so as to obtain the updating thrust-position trajectory. A high-order fast terminal sliding-mode load force observer (HOFTSM-LFO) is designed to estimate the combustion work of an engine based on thermodynamics. Finally, a thrust compensation mechanism is added to the control scheme to counteract modeling errors to maintain high-precision dead centers positions. A sufficient theoretical analysis is provided with an emphasis on thrust calculation and observer design. The performance of the PUT-PT is successfully verified on an 18 kW PMLSM motion platform and a simulated single-cylinder FPLG.