PMSM sliding-mode control based on novel reaching law and disturbance observer

To improve the performance of the speed servo system in the permanent magnet synchronous motor (PMSM) and to reduce the influence of disturbance in the control system, a sliding mode control method was proposed based on a novel reaching law and disturbance observer. A novel reaching law was presented to solve the contradiction between sliding mode surface reaching time and the system chattering in the regular reaching law, and which can simultaneously improve the system response speed. A sliding mode disturbance observer (SMDO) was employed to estimate the system's lumped disturbances, such as parameter variations and external disturbances. The estimated value was utilized as a feed-forward to compensate for the speed controller and to further increase the anti-disturbance ability of the system. The switching gain of SMDO was designed as a function of the observed error of disturbance to suppress chattering of the sliding mode estimated value. The simulation results demonstrate that the novel reaching law has an improved dynamic system response speed compared to the regular reaching law and that it can accurately and rapidly track the step speed signal. The SMDO accurately observes the varying system disturbance. When load disturbance is added to the system, the maximum speed fluctuation under PI control is 75 r·min^-1, whereas the sliding mode control, which is based on the novel reaching law and disturbance observer, records a maximum speed fluctuation of 30 r·min^-1 and guarantees better and more robust system performance. The experimental results demonstrate that the system that relied on the proposed sliding mode control method can rapidly track a speed command of 400 r·min^-1 without overshoot. The regulation time is 0.12 s and the steady-state accuracy is ±4 r·min^-1. The SMDO can accurately estimate the system disturbance without overshoot while also improving the system's anti-disturbance ability. When the motor is operating at a steady speed of 400 r·min^-1 and then is added a 0.6 N·m load torque disturbance, the method based on the novel reaching law and disturbance observer gives a maximum speed fluctuation of 23 r·min^-1. This speed fluctuation represents a reduction of 8% compared to that with PI control. The simulation and experimental results are in good agreement. These results indicate that the proposed control method can improve the dynamic and robust performance of the speed servo system and effectively alleviate the chattering of the sliding mode control system.