Stability and performance of active vibration isolation systems

Abstract

Previous experimental work on active vibration isolation systems has shown that the gain in the feedback loop is limited because of the stability of the system. However, theoretical analyses of these systems showed that they are unconditionally stable. These discrepancies are investigated in this thesis and it is demonstrated that the instrumentation plays a crucial role in the stability and performance of an active isolation system. A holistic analysis of decentralised vibration isolation control systems integrating (a) the structural dynamics (b) the signal conditioning devices and (c) the actuators, to assess the stability and performance of the systems is adopted. In addition, the effect of losing control of one of the decentralised loops is also investigated.

In the stability analysis two frequency regimes are identified; (a) a low frequency regime and (b) a high frequency regime. In the low frequency regime, the phase advance of the high-pass filters, which are inherent in the control system, causes the instability. In the high frequency regime instability is caused by time delay, or phase lag (from low-pass filters). Simple formulae are derived for simplified systems, which give the frequencies at which the systems become unstable and the maximum gains that can be applied to each system. These formulae can be used as simple rules of thumb for the analysis of stability of more complex systems.

Considering the three control strategies, it is established that the acceleration feedback control system has the lowest maximum gain, and velocity and displacement feedback control systems have good low frequency stability. With time delay in the feedback systems, the displacement feedback control strategy is the most susceptible to instability. Similarly, with low-pass filters in the system, the displacement feedback control strategy is again the most susceptible. Thus velocity feedback control is proven to be most attractive.

Uncertainty due to component failure is also investigated in a two-channel velocity feedback system and it is found that failure does not affect the stability of the system, but it does degrade the performance significantly. It is observed that in a system with failure, only a small improvement on performance is possible from the system with no control. Thus, with respect to component failure, increasing gain in a system with failure does not cause the system to become unstable but does not improve the performance significantly.

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