Synchronous Sensor Runout and Unbalance Compensation in Active Magnetic Bearings Using Bias Current Excitation

Sensor runout and unbalance are the dominant sources of periodic disturbances in magnetic bearings. Many algorithms have been investigated for individual runout and unbalance compensation but the problem of simultaneous compensation of the two disturbances remains essentially unsolved. The problem stems fundamentally from a lack of observability of disturbances with the same frequency content and is critical for applications where the rotor needs to be stabilized about its geometric center. A credible way to distinguish between the synchronous disturbances is to vary rotor speed but speed variation is not acceptable for many applications. We present in this paper a new method for simultaneous identification and compensation of synchronous runout and unbalance at constant rotor speed. Based on traditional adaptive control design, our method guarantees geometric center stabilization of the rotor through persistency of excitation generated by bearing stiffness variation. The variation in magnetic stiffness is achieved through perturbation of the bias currents in opposing electromagnetic coils in a manner that does not alter the equilibrium condition of the rotor. Our theoretical results are first validated through numerical simulations and then experiments on a laboratory test-rig. The experimental results adequately demonstrate efficacy of our approach and provide clues for future research directions.