Design and Simulation of Direct and Indirect Back EMF Sliding Mode Observer for Sensorless Control of PMSM

This article compares two sensorless control algorithms for a permanent magnet synchronous machine (PMSM) based on the back-EMF sliding mode observer (SMO). Indirect SMO (I-SMO) treats the back-EMF voltages as a disturbance. Direct SMO (D-SMO) considers back-EMF voltages as state-space variables. The same phase-locked loop (PLL) is used for both observers for extraction of the rotor position and speed values from the observed back-EMF voltages. In a sensorless control, the observed speed is used as feedback for the PI controller, and the observed position is used in the Park transformations. Both observers have been implemented and tested with standard field-oriented control. Simulation results indicate rather comparable speed and position estimation precision for both, but the D-SMO indicates slightly higher precision in steady-state. Even more, a tuning procedure of the D-SMO is more straightforward when compared to the I-SMO. Thus, the D-SMO was further verified experimentally with the OP 5600 rapid prototyping device and with a 350 W PMSM drive. Experimental results of the D-SMO are included at the end of the paper