A nonlinear approach to the control of magnetic bearings

In this paper, nonlinear aspects of the control of magnetic bearings are studied. The authors design a nonlinear feedback law for the positioning of a shaft, and study the possibility of avoiding premagnetization currents, usually required when applying tangent linearization techniques. Using the flatness property, they propose simple solutions to the motion planning and stabilization problems according to the current complementarity condition or the current almost complementarity condition, that ensures that only one electromagnet in each actuator works at a time, or that one approaches arbitrarily close to this situation. The feedback synthesis is presented in both current and voltage control cases. In the latter case, a hierarchical control scheme, based on time-scale separation, is proposed, to avoid unbounded voltages at switchings that might result from the voltage linearizing feedback. Some implementation aspects are described and some experiments presented.