Optimal and robust digital current controller synthesis for vector-controlled induction motor drive systems

The vector-controlled induction-motor drive is a multi-input multi-output system. By using modern control techniques, all feedback loops can be closed simultaneously, in addition to obtaining optimal gains for the controllers. A systematic approach to the design of the digital current controllers is proposed. A vectorcontrolled induction-motor-drive system is usually viewed as a full-state feedback problem, where the non-measurable states are estimated using suitable reduced-order observers. Here, the induction-motor-drive system is viewed as a linear quadratic (LQ) tracker problem with output feedback, as this approach uses only the measurable states of the system and provides flexibility in choosing the control structure. The current controllers are designed for the torque dynamics of the induction motor. The controller should also be so designed that the whole drive system is stable for a class of induction motors, i.e. the whole drive system should be stable in the face of uncertainties in the parameters of the induction motor. Therefore, it becomes essential to build stability robustness into the controller design. Here, the proposed systematic approach determines the optimal controller gains for a given performance specification under the constraint that the whole system has stability robustness for uncertainties in the specified parameters of the induction motor. The concepts are illustrated by simulation.