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This paper describes the implementation of an independent two-channel controller based on absolute velocity feedback and its performance in improving the isolation from base vibration of a mounted rigid equipment structure characterized by two-degrees of freedom. A single-channel controller is also investigated. If the base structure were rigid, a collocated control strategy based on feedback of the equipment absolute velocity reduces the vibration transmission by skyhook damping. In this study, the vibrating base is flexible so that no rigid ground is available to react the secondary forces off. The direct velocity feedback (DVFB) control implemented here is shown to be very stable, however, so that high control gains could be applied. Effective damping ratios of up to 600% in the modes of the suspended system could be introduced by the two control channels. The passive isolation performance is thus dramatically improved by the two-channel controller: the heave mode is reduced by up to 40 dB, whereas the amplitude of the pitching mode is attenuated up to 26 dB. The experimental results also show a global improvement in the vibration caused by the resonances of the base plate over the frequency range of control [0–200 Hz]. The control effect decreases with frequency as a consequence of the increasing efficiency of the passive isolation. It is also shown that if the feedback gains are equal for the two control channels, the control effect is the same as adding equal damping terms to the two modal responses of the mounted equipment. Finally, the control is shown to be robust to changes in the plate support dynamics, since adding masses at various positions on the base plate did not destabilize the system. |
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