A nonlinear state observer for the sensorless control of a permanent-magnet AC machine

This paper presents a sensorless speed regulation scheme for a permanent-ma gnet synchronous motor (PMSM) based solely on the motor line currents measu rements. The proposed scheme combines an exact linearization-based controll er with a nonlinear state observer which estimates the rotor position and s peed. Moreover, the stability of the closed-loop system, including the obse rver, is demonstrated through Lyapunov stability theory. The proposed obser ver has the advantage of being insensitive to rotation direction. It is sho wn how a singularity at zero velocity appears in the scheme and how it can be avoided by switching smoothly from the observer-based closed-loop contro l to an open-loop control at low velocity. The system performance is tested with an experimental setup consisting of a PMSM servo drive and a digital- signal-processor-based controller for both unidirectional and bidirectional speed regulation.