Identification and compensation of torque ripples of a PMSM in a haptic context

A haptic system is an articulated mechanical structure with motors, sensors, embedded electronics as well computer software allowing force feedback. It enables the user to interact with virtual reality through the sense of touch and sight. It has two main operating modes. In the blocked mode the haptic system should be able to reproduce virtual shocks. In the transparent mode, the haptic system should be as transparent as possible. The transparency is defined as the quality of the free movement while displacing outside the blocked regions. A Permanent Magnet Synchronous Motor (PMSM) is used to drive the mechanical structure. Nowadays PMSM connectors use ultra-powerful magnet in order to reduce the motor volume with respect to the delivered electromagnetic torque ratio. Furthermore the use of those magnets, torque ripples phenomena (cogging torque) can be observed and it can deteriorate the haptic interface performances. In this paper a torque ripples identification and compensation method is proposed and evaluated experimentally. The developed least square identification procedure uses the measured currents, while the PMSM is driven at constant speed, in order to reconstruct the cogging signal. The main advantage of the proposed method is that it doesn´t use the motor mechanical structure characteristics in order to calculate its parameters.