Condition monitoring of cracked shaft using active magnetic bearings

The concept that changes in the dynamic behaviour of a rotor could be used for general fault detection and monitoring is well established. Current methods rely on the response of the machine to unbalance excitation during run-up, run-down or normal operation, and are mainly based on pattern recognition approaches. Of all machine faults, probably cracks in the rotor pose the greatest danger and research in crack detection has been ongoing for the past 30 years. For large unbalance forces the crack will remain permanently open and the rotor is then asymmetric, which can lead to stability problems. If the static deflection of the rotor due to gravity is large then the crack opens and closes due to the rotation of the shaft (a breathing crack), producing a parametrically excited dynamical system. Although monitoring the unbalance response of rotors is able to detect the presence of a crack, often the method is relatively insensitive, and the crack must be large before it can be robustly detected. Active magnetic bearings (AMB) have been used in high speed applications or where oil contamination must be prevented, although their low load capacity restricts the scope of applications. Recently a number of authors considered the use of AMBs as an actuator that is able to apply force to the shaft of a machine. If the applied force is sinusoidal, then the presence of the crack generates responses containing frequencies at combinations of the rotor spin speed and applied force. This paper discusses some of the issues to be addressed to enable this approach to become a robust condition monitoring technique for cracked shafts. In particular the following are considered and demonstrated using simulated results: the character and frequency of the force, the dynamics of the AMB, the choice of signal features for the detection, the effect of higher frequency resonances, the sensitivity to small crack depths, and the risk of reducing machine life by increased forcing, the response of other different system parameters. In addition the prospects of using AMBs to extend the life of the machine are discussed briefly.