Experiments in the Control of Unbalance Response Using Magnetic Bearings

Unbalance response is a common vibration problem associated with rotating machinery. For several years, researchers have demonstrated that this vibration could be greatly alleviated for machines using active magnetic bearings through active control. Many of the control strategies employed fall into a class which the authors have termed adaptive open loop control. In this paper, three algorithms in this class are presented and their performances are examined experimentally. These algorithms are (1) a non-recursive control law with simultaneous estimation, (2) a recursive control law with simultaneous estimation, and (3) a recursive control law with gain scheduling according to operating speed. Each algorithms was coded in C and executed on a high-speed, multi-tasking digital controller. The advantages and disadvantages of each algorithm are illustrated by examining experimental results from a laboratory magnetic bearing rotor rig. These results clearly demonstrate the high degree of synchronous vibration attenuation (over 30 dB) which can be achieved with adaptive open loop methods. The response of these algorithms to a sudden change in 'simulated imbalance' is used to evaluate their relative transient performances. These results indicate the benefits of recursive control laws in adapting the synchronous open loop control currents to cancel the vibration. The ability of each of the algorithms to adapt the open loop control during changes in rotor speed is also examined. On this test, the recursive gain scheduled algorithm shows superior performance: rotor midspan vibration is almost completely eliminated over the operating speed range. However, surprisingly, the non-recursive control law shows better performance than the recursive law with simultaneous estimation. This result is explained in terms of the stability of the adaptation process.