Sensorless velocity control of permanent-magnet synchronous motors

A sensorless velocity controller for the permanent-magnet synchronous motor is designed using discrete-time reduced order models. A nonlinear observer estimates rotor position and velocity from stator current measurements, thus eliminating the need for shaft-mounted sensors. The observer is based on least-squares optimization, and exploits a piecewise-bilinear representation of the back-emf (speed voltage) function. By smoothly switching from an observer-based closed-loop control to an open-loop control at low velocity, observer ill-conditioning is avoided and reliable operation is achieved under all operating conditions. The new scheme delivers performance comparable to sensor-based closed-loop control, yet requires only a moderate amount of on-line computation. Experimental results provide evidence of the viability and performance of the new scheme.