Sensorless PBC of induction motors: a separation principle from ISS properties

In this paper the design of sensorless controllers for induction motors is approached from the perspective of establishing structural properties of the control law that allow for dealing with the observer design in a systematic way. In particular it is shown that sensorless passivity-based control (PBC) of this kind of machines leads to some input-to-state stability properties of the control error dynamics that establish a separation principle for the closed-loop composed by the motor, the PBC and any sensorless observer. The importance of the result concerns the fact that it is proved that no matter how the observer is designed, boundedness (convergence to zero) of the observation error guarantees that the control error also will be bounded (tend to zero). Evidently, the well-known lack of global observability properties of the motor model is still present in the system operation. The usefulness of the contribution is illustrated via some numerical simulations.