A minimum-time control strategy for torque tracking in permanent magnet AC motor drives

A minimum-time torque control strategy for permanent-magnet ac motor drives is presented. The proposed technique neither requires the solution of an HJB-type equation, which would be practically unfeasible, nor uses Pontryagin's maximum principle. In fact, the solution is obtained by an ad-hoc procedure based on the computation of reachability and controllability sets. In principle, the optimal control strategy is implemented by iteratively solving a fourth-degree polynomial equation. For its efficient implementation an algorithm based on Sturm sequences is proposed. The paper provides the detailed sequence of on-line operations required by the algorithm, for a given tolerance value on the optimal time. The proposed algorithm has been implemented on a laboratory prototype. Experimental results, showing the effectiveness of the proposed technique as well as the computational load needed for its implementation, are included in the paper.