Observer-based variable structure control in microstepping for permanent magnet stepper motors

In this paper, we propose observer-based variable structure control (VSC) in microstepping for permanent magnet stepper motors (PMSMs) without position feedback. The back-emfs of the currents dynamics are defined as the disturbance terms. The observers for the back-emf estimation are designed. The proposed observers are in the form of the high pass filter. Observer-based VSC is developed to guarantee local exponential stability of the desired currents for microstepping by only currents feedback. The proposed controller is robust to back-emf. The proposed controller does not require any transformation so that it could reduce the commutation delay. Experimental results show that the proposed method results in improvement in position tracking over the microstepping with proportional and integral current feedback. Since proposed method guarantees local exponential stability of the desired currents required for microstepping, overall more than 60% improvement in tracking error was obtained in experimental results. And the tracking error ripple is also reduced.