Stabilizing Sensorless Control Down to Zero Speed by Using the High-Frequency Current Amplitude

Sensorless control of permanent magnet synchronous machines (PMSM) down to zero speed using high-frequency (HF) signal injection is an established method. Stable operation during transient states still presents a challenge for this method, especially for motors with very small anisotropies. This paper introduces a new method for stabilizing the sensorless control scheme. It is shown that a misalignment between the estimated dq coordinate system and the actual rotor position causes a variation of the high-frequency current amplitude. Calculated values based on theoretical observation are compared to actual measurements of a hybrid stepper motor used as a PMSM. Several issues that can be used to determine if a motor is suitable for this approach are addressed. Finally, the effectiveness of the new control scheme is validated by implementation in the existing sensorless control of a hybrid stepper motor.