Rotating machines may, for reasons of gyroscopic coupling and aerodynamic influences, exhibit speed dependent characteristics. For systems that incorporate active magnetic bearings (AMBs) for rotor levitation, a range of control strategies are available for implementation to achieve desired closed loop dynamic characteristics. Over a large operating speed range, it may not be possible stabilize a rotor by using only one linear time-invariant (LTI) H controller that has speed independent characteristics. This paper addresses that problem by introducing a switching controller system. The system contains a number of (LTI) H controllers, designed to cover particular speed ranges, together with an external signal to drive the switching action. When speed dependent unbalance forcing acts, each of the controllers is able to attenuate vibration of the rotor over the specific speed ranges. The controller switching points are selected according to a metric that takes account of measured rotor lateral vibration. However, a sudden switching action may induce significant transient disturbance and give rise to rotor vibration overshoot, possibly to the extent of causing rotor-stator contact. To alleviate this problem, a ramp switching signal may be introduced such that two controllers may be switched smoothly over a given time period. Further smoothing modifications are possible to avoid ‘over-control’ and obtain a better transient performance. The unstable points of each H controller are also discussed with respect to upper threshold speeds, above which controller switching is not allowable.

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