Variable structure control of a magnetic suspension system

This paper presents a variable structure control (VSC) design procedure for robust stabilization and disturbance rejection of a magnetic suspension system. The procedure is based on a method called reaching law method complemented by a sliding mode equivalence technique [1]. First, the basics of variable structure control system design are reviewed with emphasis on the reaching law method. Second, the dynamics of the magnetic suspension system are described in terms of airgap deviation, airgap flux and the electromagnet voltage. Third, in order to use the VSC to control the airgap deviation using the electromagnet voltage, new system dynamics are developed to make the electromagnet input voltage as the forced term of the dynamic equation of the airgap deviation. Then the new system is described in state variable form and a VS controller is designed for this system using the reaching law method. Finally several simulation results are presented. The results showed that robust stability against parameter variations and disturbance rejection (for any class of disturbance) are achieved using the proposed VS controller.