Online identification of serious faults, such as cracks and unbalance, is significantly important for the safe, smooth and efficient performance of rotating machines such as microturbines, fuel cell blowers, turbo compressors and air cycle machines. These high-speed machines use foil bearings as supports due to their oil-free, wide temperature range, low cost, high adaptability, high stability and environmental friendliness nature. Therefore, this paper proposes a mathematical model-based identification technique for the simultaneous estimation of unbalance and crack fault parameters in a rotor-foil bearing system. For the execution of this methodology, the mathematical model of a four-degrees-of-freedom transverse cracked and unbalanced rigid rotor with foil bearings supports is presented, and its dynamic behaviour is studied. The rotor is composed of a rigid shaft with two offset discs and one middle disc. The offset discs result in a gyroscopic effect at high rotor speeds. To model the transverse crack, a switching crack function is considered, which contains multi-harmonic frequency components. Hence, a full spectrum analysis of frequency domain responses has been performed to show the nature of rotor responses both at positive and negative frequencies. The prime motive of this spectrum analysis is to illustrate a model-based identification algorithm for estimating system and fault parameters, such as the foil bearing stiffness and damping coefficients, additive stiffness related to crack faults, as well as the discs unbalance magnitude and phase. Testing of the algorithm has also been carried out at multiple spin speeds against measurement signal noise in rotor responses and bias errors in rotor system parameters to check its effectiveness and robustness. The algorithm is found to exhibit an outstanding outcome in the estimation of parameters.
