A new control system for permanent magnet synchronous motor electric drives with a
significant torsion vibration mode in the mechanical coupling is presented based entirely on sliding mode
principles to achieve robustness against external load torques and parametric modelling uncertainties. The
user is only required to provide the demanded position and specify the settling time, no controller tuning
being necessary. The vector control condition of keeping the direct axis current component approximately
zero is satisfied as well as controlling either the rotor or load position to follow the demanded position with
prescribed closed loop dynamics. To avoid control chatter, a boundary layer is introduced by replacing the
relay control switching transfer characteristic (signum function) by a high gain having the same control
saturation limits. Any steady state errors with a constant demanded rotor angle due to the finite value of the
aforementioned gain are eliminated by means of an outer integral control loop. The simulations predict that
the desired robustness will be achieved
