A nonsingular fast terminal sliding mode controller with varying boundary layers for permanent magnet synchronous motors

A nonsingular fast terminal sliding mode (NFTSM) controller with varying boundary layers is proposed to solve the problem that speed-regulation systems of permanent magnet synchronous motors (PMSM) are susceptible to parameter uncertainties and disturbances of external loads. The sliding mode control law for speed loop is developed through building the NFTSM surface with fast convergence in a finite time and using the second-order model of rotary speed and q-axis stator reference current so that the errors caused by the first-order model of rotary speed and q-axis stator reference current are decreased and the singular problem of the terminal sliding mode surface is avoided. Furthermore, the varying boundary layer is designed to make the system states switch to a small boundary layer when the error decreases to a threshold so that the coordinated control between chattering and tracking accuracy is realized. Numerical simulation results and a comparison with the traditional PI control show that the proposed NFTSM control scheme achieves smaller overshoot and faster response, and the steady-state error is near to zero. Moreover, the NFTSM has strong robustness in parameter variations and disturbances of external loads, and ensures a higher steady-state precision of the speed with a lower chattering.