Sliding mode control of PMSM drives subject to torsion oscillations in the mechanical load

A control system for permanent magnet synchronous motor electric drives with a significant torsion vibration mode is presented based entirely on sliding mode principles to achieve robustness against external load torques and parametric modelling uncertainties in the motor and/or the driven mechanical load. The sliding mode control law respects the vector control condition by keeping the direct axis current component approximately zero as well as controlling either the rotor or load position to follow the demanded position with prescribed closed loop dynamics. When controlling the load position, the torsional oscillations are completely eliminated. To avoid control chatter, a boundary layer is introduced by replacing the relay control switching transfer characteristic (signum function) by a high gain with the same control saturation limits. The user is only required to provide the demanded position and specify the settling time of the step response, no controller tuning being necessary. The simulations predict that the desired robustness will be achieved.