Sliding Control of Magnetic Bearing Systems

This paper proposes a sliding control scheme to deal with the nonlinear, uncertain dynamics of magnetic bearing systems. The controller is designed based on the Thevenin equivalent of a network model that characterizes both the main electromechanical interaction and the secondary electromagnetic effects such as flux leakage, fringing fluxes, and finite core permeance. The controller consists of two parts: the nominal control part that linearizes the nonlinear dynamics, and the robust control part that provides robust performance against the uncertainties. Because the unidirectionality of magnetic forces may render the control law ill-defined in some scenarios, a lemma is proved and can be used to select proper control parameters to guarantee the performance. Moreover, a control modification is also suggested in order to reduce the intensive control pulsation in the absence of gravity. The proposed control scheme is applied to the thrust bearing of a magnetically levitated rotor. Simulations and experiments indicate that the control system is capable of maintaining stability and consistent performance.