Nonlinear Disturbance Rejection for Magnetic Levitation Systems 1

Abstract: In this paper, a position regulation control strat-egy is developed for a magnetic levitation system operating in the presence of a bounded, nonlinear, periodic disturbance. The proposed controller utilizes a saturated control force input in con-junction with a learning based disturbance stimator to asymp-totically regulate the target mass to a desired set point position despite the actuator's unidirectional limitation of exerting only an attractive force on the target mass (i.e., the control input can only generate an attractive force while the earth's gravitational field is utilized to produce the repulsive action). In addition, the control development only requires the disturbance input to be bounded and periodic in nature. Simulation results are included to illustrate the performance of the control strategy. 1 In t roduct ion With ever increasing demands placed on precision and re-liability within manufacturing and research environments, magnetic levitation systems are finding increased utiliza-tion within such applications as machine tooling due to their non-contact (i.e., low friction) force exertion and their capability for active attenuation of mechanical vibrations. However, magnetic levitation systems suffer from various control complexities uch as: i.) open-loop instability, ii.) inherent nonlinearities within the system model, iii.) a uni-directional force input, and iv.) continuous biasing. In re-sponse to these difficulties, researchers have employed var-ious modelling and control development techniques to ad-dress the aspects of i.) through iii.). For example, Charara et al. [2] constructed a nonlinear model of an inertial wheel supported by active magnetic bearings, for which a sliding mode controller was then designed to stabilize the system. In [7]