Robust Controller Design for AMB Levitation Recovery

A control method is proposed for relevitating an active magnetic bearing (AMB) system which has lost levitation due to an acute external fault. The proposed method involves switching to a robust recovery AMB controller once a delevitating event has been detected. The novel recovery controller is uniquely designed for levitation recovery using the robust control strategy μ-synthesis with performance weights which bound the disturbance force of the touchdown (TD) bearing on the rotor. Also, in contrast to typical AMB controller design, the recovery controller is designed to allow potential deflection up to the distance of the TD bearings so as to mitigate amplifier saturation during recovery. The proposed method is demonstrated on an AMB test rig. Details on recovery controller design for the test rig are presented. Rotor drop tests are first used to tune a simple TD bearing model. Then, a numerical simulation is performed to find the frequency response of the TD bearing force on the rotor. The simulated frequency response is used to craft a force bounding performance weight which is then used for robust controller synthesis. The experimental rotor is run at 2000 RPM and a fault is induced by stopping current flow to the AMB coils. After approximately 2 s running on the TD bearings, the power is restored. When no recovery scheme is used, the rotor responds violently, hitting the top of the TD bearing before recovering levitation. With the proposed recovery scheme, the rotor relevitates quickly without unwanted dynamics.