Adaptive control of active magnetic bearings to prevent rotor-bearing contact

Active Magnetic Bearing (AMB) systems offer various advantages over conventional bearings but due to their limited force capacity, with high levels of vibrations the rotor may come into contact with retainer bearings. Under conventional PID control, when a rotor comes into contact with its retainer bearings it remains in contact, until the rotor is run down and the system shut down. This may not be acceptable in some applications, such as aerospace and automotive applications. In this paper, a recursive open-loop adaptive control (ROLAC) algorithm is presented, as an extension of the existing open loop adaptive controller (OLAC), that updates the control force amplitude and phase at each sampling period for rapid response to changes in external excitations. The effectiveness of the algorithm in counteracting a sudden change of rotor unbalance is demonstrated by simulation and experimental results. The experimental system consists of a flexible 2 m long rotor with a mass of 100 kg supported by two radial active magnetic bearings. A simulation model of the system, including the contact dynamics, was used to assess the feasibility of the suggested controller before applying it to the experimental system. Depending on excitation levels, it is shown that the proposed controller is fast enough to prevent contact in most cases. If contact does occur the impact is minimized, and the method is able to recover the rotor position quickly. The proposed controller is implemented in real time and applied to the experimental system. It is shown that the controller works efficiently as predicted by the simulation studies.