Fractional Order Sliding Mode Control for Permanent Magnet Synchronous Motor Speed Servo System via an Improved Disturbance Observer

A fractional order sliding mode control (FOSMC) method is developed in this paper to deal with the control problem of permanent magnet synchronous motor (PMSM) speed servo system subject to multiple disturbances including model uncertainties, unknown constant disturbances and harmonic disturbances. The lumped exogenous disturbances and uncertainties of the PMSM speed servo are estimated by an improved disturbance observer (DO) and an extended state observer (ESO), respectively. Then, a novel FOSMC law is developed by incorporating the feedforward compensation and a fractional order switching law. The stability of the closed-loop system is established based on Lyapunov stability approach. Under the FOSMC scheme, the tracking performance and robustness of the PMSM servo system are improved simultaneously in the presence of mismatched disturbance torques and measurement noise. The effectiveness and advantages of the proposed method are demonstrated by the PMSM speed regulation experiments and the comparisons with some existing methods.