In this paper, a robust model-free nonsingular terminal sliding-mode control (MFNTSMC) algorithm based on the ultra-local model is proposed to reduce the influence of permanent magnet (PM) demagnetization for PM synchronous motors (PMSMs). First, the PMSM mathematical model in normal and demagnetization is described, and the ultra-local model of speed loop and current loop is constructed based on the input and the output of the PMSM vector control system. Then, the MFNTSMC method is proposed and adopted to design the speed controller and d-q-axis current controller, and the sliding-mode observer is designed to estimate the unknown terms of the ultra-local model. Finally, compared with the PI control method and model-free control method, the results of simulations and experimentations show that the MFNTSMC method can improve the dynamic response while maintaining robustness of PMSM driven system, reduce the dependence of the design of controller on the precise PMSM model, and has fault-tolerant control function for PM demagnetization fault. INDEX TERMS Ultra-local model, permanent magnet synchronous motor (PMSM), demagnetization, model-free nonsingular terminal sliding mode control (MFNTSMC), sliding mode observer (SMO).

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