Nonlinear sliding mode controller using disturbance observer for permanent magnet synchronous motors under disturbance

We propose a nonlinear sliding mode controller (NSMC) using disturbance observer (DOB) for permanent magnet synchronous motors (PMSMs) under disturbance. A nonlinear gain, which is a function of a sliding surface, is incorporated to improve the performance of the sliding mode controller (SMC). The chattering in the proposed controller is mathematically proven to be lower than that in a conventional sliding mode controller (CSMC). In the NSMC, the sliding surface exponentially converges to zero when the sliding surface is far from zero, and then reaches zero within a finite time, similar to the behavior of the CSMC near zero. The exponential convergence property ensures stability without disturbance information. The chattering performance under the unmodeled dynamics of the PMSM is mathematically analyzed using the describing function method. A DOB is used to compensate for disturbances, such as parameter uncertainties and load torque. The closed-loop stability under the perturbation term, which is caused by the disturbance estimation error, is guaranteed without disturbance information.