Performance Assessment of a Multi-Frequency Controller Applied to a Flexible Rotor Magnetic Bearing System - Contact Dynamics

Vibrations associated with external disturbances and rotor faults, such as cracks, usually exhibit themselves as harmonic components of the synchronous frequency. The essence of the multi-frequency form of the synchronous recursive open loop adaptive controller (ROLAC) is that it can minimise a number of vibration components simultaneously, for example, synchronous rotor vibration at frequency Ω, its harmonics and sub-harmonics within any defined range. This requires on-line identification of the speed dependent partial receptance matrix by using a multifrequency test signal incorporating all of the pertinent harmonic components. The question arises: What is the degradation in performance if the rotor comes into contact with its retainer bearing? This may arise when a magnetically levitated rotor is installed on a movable base frame, for example on board a ship or an aircraft. A simulation study is described to examine this question. It comprises a flexible rotor supported by two active magnetic bearings. Eight displacement transducers are positioned along the rotor. A local PID controller is provided for each axis of the magnetic bearings to ensure stability and alignment of the rotor at a central position. An outer ROLAC loop is incorporated at each bearing to control the vibrations at discrete frequencies of 0.5Ω, Ω, 2Ω and 3Ω. In addition, a multi-frequency disturbance was applied to the rotor causing contact with its retainer bearing. The performance of the multi-frequency ROLAC is assessed in preventing contact, or recovering the rotor position if contact occurs.