Nonlinear Proportional-Integral Disturbance Observers for Motion Control of Permanent Magnet Synchronous Motors

In this letter, we present nonlinear proportional integrator (N-PI) disturbance observers (DOBs) for enhancing the motion tracking performance of a surface-mounted permanent magnet synchronous motor in rapidly varying speed regions. By presenting an N-PI-DOB for load torque estimation with a torque modulation technique, we show that the electro-mechanical system, including the motion controller, disturbance observers, and inverter controller, can be presented in a three-cascaded form, in which the stability analysis of the closed-loop system can be easily performed. After analyzing the disturbances of the current tracking error dynamics, we designed another N-PI-DOB and a Lyapunov-based nonlinear controller for the current loop to enhance the motion tracking performance. With these N-PI-DOBs and the motion controller, we analyzed the stability of the motion tracking error dynamics and estimation error dynamics. We experimentally performed a comparative study with and without N-PI-DOB to validate the effectiveness of the proposed method under the conditions of unknown load torque and rapid speed variation.