This paper investigates on developing a novel model-based identification technique for the simultaneous identification of severe faults such as the unbalance in the rotor and transverse crack in the shaft supported on foil bearings. With plenty of advantages over rolling element bearings or fluid film bearings, foil bearings have been used as the supported bearings in rotating machines such as fuel cell-electric air compressors, blowers, expanders, air cycle machines, etc. In the present article, a rotor model consisting of a cracked and unbalanced rotor with a disc in the middle supported by foil bearings has been considered for easier understanding of online identification of faults in high-speed rotating machines. Dynamic equations of motion of the rotor-foil bearing system have been derived based on the equivalent stiffness concept of shaft-foil bearing, inertia force, unbalance force, and crack force relying on the switching crack concept. The solutions of the equations, i.e., time domain displacement responses, orbit plots, etc. have been obtained numerically using the Simulink inbuilt Runge-Kutta method for different values of spin speed of the rotor and ramp-up speeds. The shaft centreline orbit is found to have eight shaped and asymmetric about the axes due to presence of crack and unbalance faults. The force due to unbalance fault gets dominated over the crack force at the higher speeds. Moreover, the orbit line is also observed to be thicker at higher level of noise addition in the responses. As the switching crack force contains multiple harmonics, a full spectrum analysis has been done to investigate both the forward and backward rotor whirls. The frequency-based rotor displacement is utilized to illustrate an identification algorithm for the estimation of the dynamic coefficients of foil bearings, additive crack stiffness, and magnitude as well as phase of disc unbalance. The identification algorithm is found to be quite suitable for the estimation of system and faults parameters even with addition of different levels of noise signal and modelling errors.

High-speed rotors on gas foil bearings (GFBs) are applications of increasing interest due to their potential to increase the power-toweight ratio in machines and also formulate oil-free design solutions. The gas lubrication principles render lower (compared to oil) power loss and increase the threshold speed of instability in rotating systems. However, self-excited oscillations may still occur at circumferential speeds similar to those in oil-lubricated journal bearings. These oscillations are usually triggered through Hopf bifurcation of a fixed-point equilibrium (balanced rotor) or secondary Hopf bifurcation of periodic limit cycles (unbalanced rotor). In this work, an active gas foil bearing (AGFB) is presented as a novel configuration including several piezoelectric actuators that shape the foil through feedback control. A finite element model for the thin foil mounted in some piezoelectric actuators (PZTs), is developed. Second, the gas-structure interaction is modelled through the Reynolds equation for compressible flow. A simple physical model of a rotating system consisting of a rigid rotor and two identical gas foil bearings is then defined, and the dynamic system is composed with its unique source of nonlinearity to be the impedance forces from the gas to the rotor and the foil. The third milestone includes a linear feedback control scheme to stabilize (pole placement) the dynamic system, linearized around a speed-dependent equilibrium (balanced rotor). Further to that, linear feedback control is applied in the dynamic system utilizing polynomial feedback functions in order to overcome the problem of instability.