Model based identification of crack and bearing dynamic parameters in flexible rotor systems supported with an auxiliary active magnetic bearing

Abstract

In this paper, possibility of identification of a crack appearing in flexible rotor systems supported with active magnetic bearing has been investigated. A cracked rotor with active magnetic bearing (AMB) support in auxiliary bearing configuration has been analysed for identification of crack and end support bearing stiffness. Presence of AMB in an auxiliary bearing configuration provides additional damping to the rotor system, thus attenuates the vibrations induced by rotor flaws. Identification strategies based on vibration responses of such a rotor may not produce correct results. However, loss of diagnostic information due to attenuation of the vibration signal could be compensated for, by supplementing the vibration signal with the AMB control current history. With inclusion of the control current history, the crack force and other parameters could be identified. A cracked rotor with multiple discs has been modelled by standard finite elements method, with consideration of gyroscopic effect due to rigid discs. Breathing behaviour of the crack has been modelled with a switching crack excitation function, containing multiple harmonics of both forward and backward nature. Full spectrum of the frequency domain of the response is utilised to develop the identification algorithms. This algorithm identifies the crack force in form of additive crack stiffness and simultaneously estimates the disc unbalances, end support bearing stiffness and active magnetic bearing dynamic parameters as well. The algorithm has been tested in a simple rotor system for the measurement noise and bias errors in system parameters, and found robust.