To improve the velocity tracking of permanent magnet stepper motors (PMSMs), the cogging torque must be rejected. Most controllers for PMSMs use direct-quadrature (DQ) transformation to force the mechanical dynamics to become linear. However, feedback control using DQ transformation may be negatively affected by the fact that the commutation is imprecise due to time delay of the current measurements and position encoder feedback. In this paper, an internal model principle (IMP) based velocity tracking controller for PMSMs is proposed to track the desired velocity and reject the cogging torque without DQ transformation. The controller consists of a velocity tracking controller and IMP to reject the cogging torque. The controllers for mechanical dynamics and electrical dynamics are independently designed. The velocity tracking controller generates the desired torque to track the desired velocity. A commutation scheme is proposed to determine the desired current for the desired torque. The current tracking controller guarantees the desired current. We propose field oriented control (FOC) for this scheme. The stability of the closed-loop system is proven using passivity. Since the cogging torque is equal to the sinusoidal disturbance, an add-on type IMP is designed to cancel the cogging torque while preserving the merits and the performance of the predesigned controller and its closed-loop stability. Simulation results are used to demonstrate the performance of the proposed method.
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