Robust encoderless speed control of a synchronous machine by direct evaluation of the back-EMF angle without observer

This paper proposes an encoderless field-oriented control scheme for a permanent-magnet synchronous machine (PMSM) based on the back-EMF voltage. A polar stator coordinate representation is used to design a simple and robust position and speed estimator. The angle of the rotor is directly calculated based on stator current and voltage measurements as well as on knowledge of stator resistance and stator inductance. The estimated angle is filtered and used directly for current and torque control. Furthermore, an algebraic equation to estimate the speed is proposed. The direct estimation scheme is shown to be robust to parametric uncertainties and easily extendable with filters to cope with measurement noise. The estimator outputs are driftless, even under parametric offsets and without any feedback compensation. As advantages, there is no need to design an asymptotic observer, the parametric robustness is improved, there is no open-loop integration and no complicated tuning of feedback mechanisms, and the dynamics are very good. The scheme is validated experimentally.