Zero-position Offset Calibration of PMSM Based on I/f Control Strategy and Slide Mode Observer for Electric Power Steering System

In this paper, I/f control strategy combined with slide mode observer (SMO) are applied to calibrate the zero-position offset between permanent magnet synchronous motor (PMSM) and its rotor position sensor (RPS) for electric power steering (EPS) system. The I/f control strategy can ensure the motor operates in a medium speed smoothly with full torque capacity; while the SMO offers a reliable estimation of the rotor position of the motor itself. As a result, the offset between motor and its RPS can be acquired. In addition, the phase delay angle caused by zero-order holder (ZOH) in discretization of the digital controller and the steady state error of the SMO have been analyzed in detail, and effectively compensated with the bi-directional calibration. According to relative experiment results, it can be figured out that the proposed method can accurately align the zero-position offset angle whether the motor operates on-load or off-load. And moreover, the impacts of the calibration current, the dead time effect caused by the power inverter, and the variation of the motor parameters in SMO have also been discussed. According to comparisons between other existing calibration methods, it can be confirmed that the proposed method is more suitable for zero-position offset calibration of the PMSM in EPS production line.