Active vibration isolation with a distributed parameter isolator

Abstract

Conventional vibration isolators are usually assumed to be massless for the purpose of modelling. This simplification tends to overestimate the isolator performance because of neglecting the internal resonances (IRs) due to the distributed mass effects in the isolator, which is especially important for lightly damped metallic isolators. Previous research on the problem of IRs is not particularly comprehensive, because it does not clarify the characteristics of the distributed parameter isolator. Furthermore, with the development of active vibration isolation, there is a need to investigate the effects of isolator IRs on the control performance and stability for commonly used control strategies. Effective ways to attenuate these effects are also required. This thesis concerns the active vibration isolation of a piece of delicate equipment mounted on a distributed parameter isolator, which is modelled as different idealised configurations under various types of deformation. The model is first developed to determine the effects of IRs on a single-degree-of-freedom system with a distributed parameter isolator. This analysis is then extended to include the resonance behaviour of the supporting structure. Simple expressions are derived which describe the behaviour of various types of distributed parameter isolator. The parameters which control the isolator performance at various frequencies are clarified theoretically and experimentally. The effects of IRs on control performance and stability of several control strategies are determined and compared. Absolute Velocity Feedback (AVF) control is shown to be the optimal solution to minimise the mean square velocity of the equipment mass supported by a distributed parameter isolator. A stability condition for an AVF control system containing a distributed parameter isolator is proposed. Based on this condition, different approaches to stabilize such a control system are presented. Experimental work is carried out to validate the theoretical results. Based on the improved knowledge of the characteristics of IRs in the distributed parameter isolator, different approaches which can suppress the IRs are proposed. AVF control with more damping in the isolator is demonstrated to be effective in attenuating the IRs theoretically and experimentally. Absolute velocity plus acceleration feedback control and AVF control on a fraction of the isolator length are also shown theoretically to be effective ways to attenuate the IRs and improve the isolation performance over a broad range of frequencies.

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