Encoder-Motor Misalignment Compensation for Closed-Loop Hybrid Stepper Motor Control

Field-Oriented Control (FOC) of hybrid stepper motors allows high performance in motor movements. In FOC, the shaft position is tracked through an encoder (or a similar device), and the stator magnetic field orientation is continuously adjusted to maintain the desired load angle, i.e. the phase between the magnetic fields produced by the stator windings and the rotor magnet. Unfortunately, measuring the load angle with high accuracy is not trivial. For example, in a typical 200 step/turn motor, the electro-mechanical configuration repeats every 7.8°, and a 5% load angle accuracy requires a 5/50 = 0.1% absolute accuracy in the alignment among motor windings and encoder and/or their coupling. When this is not achieved, torque and velocity are affected by oscillations. In this paper a simple solution is proposed where the misalignment errors are mapped with an open-loop motor run, and then compensated during the normal FOC employment. Experiments with a 1.1 Nm, 2-phase, 200-step/turn hybrid stepper motor show how the proposed method reduces the velocity oscillations in a constant torque condition.