Variable Structure Control Of Active Magnetic Bearings

This dissertation is concerned with the applications of variable structure control(VSC) of active magnetic bearing(AMB) to high speed milling spindles and flexible rotor systems. The accomplished creations are summarized as follows:The state space model of current-controlled AMB milling spindle is formulated by taking into account the influence of dynamics of cutting process. Based on this model, a discrete time variable structure controller is designed. The simulations show that the VSC strategy is robust against external disturbances and model perturbation as long as the control parameters are selected properly. In comparison with the PID controllers, the VSC also enables a satisfactory dynamic performance to be achieved in terms of variation in the depth of cut.As far as the regenerative chatter in milling process is concerned, the AMB alone is incapable of maintaining the system stability. To tackle this problem, a novel methodology is proposed to suppress the vibration of the spindle by means of chatter frequency identification and on-line regulation of spindle speed. Feasibility of this method is verified by simulation.The VSC by use of the sensorless magnetic bearing is investigated for the first time. The state space model of sensorless AMB milling spindle is formulated and a discrete time VSC strategy is presented to stabilize the spindle system. Simulations show that the VS controlled sensorless magnetic bearings offer better dynamic performances over the PID controllers and the cost in hardware would be much reduced by saving the high performance non-contact sensors.Based on a reduced-order model, the VSC of flexible rotor-magnetic bearing system is also investigated. A master-slave controller composed of a PC586 and a TMS320C50 DSP board is build and a flexible rotor-magnetic bearing test rig is developed. Simulation and experimental results show that the VSC is insensitive to the spillover problem due to the model truncation and parameter uncertainty.The achievements of this dissertation would most probably be useful for developing the high speed spindles and improving the active control of flexible rotor systems as well.