Condition monitoring of rotating machinery 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 and are mainly based on pattern recognition approaches. However these methods are 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 AMBs have been proposed as an actuator to apply force to the shaft of a machine. 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. The approach is illustrated with a number of simulated examples.