H_∞ Control Approach to a Magnetic Levitation System by 4 Points Attraction with Nonlinear Characteristics

Abstract

An H_∞ control system design for a magnetic levitation system by 4 points attraction is presented. In the levitation system, a vehicle which runs as the secondary in a reluctance-type linear motor is levitated by four pairs of attraction forces and guided by two pairs of attraction forces. Because it has contactlessness, in the field of semiconductor products, its application is favorable for ultra-clean environments in microscopic processing. In the control system design, we consider the influence of both disturbances and uncertainties in the model. The main disturbances stem from the position sensors. The uncertainties are divided into electromagnetic and mechanical ones: the former are due to the gain change in the current amplifier, the influence of leakage flux and linearization error in the magnetic circuit and the latter are due to the changes of the mass and the monment of inertia of the vehicle. Therefore, the designed controller is indispensable to guarantee robustness of this system for both stability and performance. The controller design is based on the standard H∞-optimal control problem. As the novel features in this paper: We have obtained the low sensitivity and the robust stability for this system design. Further, (1) there are two-poles on j ω-axis in the controlled model; (2) an integrator is included in the controller so that equivalently there are three poles on j ω-axis in the model. Finally, several experiments, comparisons and simulations are carried out to verify the low sensitivity and robustness of the designed control system.