Switched control of a magnetic levitation system with stick-slip

Magnetic levitation systems can be used in many applications such as precise positioning. Repulsive configurations are open-loop stable and offer other interesting characteristics. However, these applications may present stick-slip effects due to the friction forces. The combination of the highly non-linear magnetic forces and the stick-slip effects result in a complex control problem. This article presents the identification, model analysis and control system design for an experimental repulsive magnetic levitation system. The design is based on the principles of state feedback linearization. In previous reports it was shown that the performance of feedback linearization control of similar devices is degraded by the parameter uncertainty introduced by the friction. In this work, the performance of the feedback linearization control is improved by adding an outer-loop linear controller with integral action. This controller was designed according to classical frequency analysis. Experimental results show better transient responses and low steady state errors. Nevertheless, the integral action and the friction force increase the stick-slip oscillations. Stick-slip motion is eliminated through a switching control strategy based on the experimental characterization of the stick-slip sliding surface. The resulting control scheme allows preserving the low steady state error of the integral control law and eliminates the stick-slip motion. This is accomplished through a relatively simple controller when compared with previous reports. Experimental results show the effectiveness of the proposed scheme.