Nonlinear Friction Compensation for PMLSM Using an Enhanced Adaptive Friction Observer

Nonlinear friction forces are the predominant force disturbance in permanent magnet linear synchronous motors (PMLSMs) with guide rails. Moreover, nonlinear friction exacerbates the precision of position control, especially in the velocity zero-crossing area. In this article, we find that the stiffness of friction varies dramatically between the pre-sliding region and the sliding region. On this basis, the friction effects on the position errors during two regimes are analyzed in detail, and the conclusion that there will be peak position errors during the motion startup or reversal phase are achieved. Then, to compensate for friction force, we propose an enhanced adaptive friction observer (EAFO) consisting of bristle deflection prediction (BDP) and an adaptive extended state observer (AESO). The proposed BDP is used to predict high-stiffness friction in presliding regimes. Its intermediate variable is ingeniously used to design the adaptive bandwidth for the AESO, which improves the observer accuracy and robustness of the EAFO. Moreover, the design and stability analysis related to the EAFO are presented in detail. Finally, comprehensive experiments are conducted to demonstrate that the EAFO can manage complicated frictions in different motion scenarios and significantly improve the position control precision in the PMLSM system.