Direct velocity feedback control of equipment vibration

Abstract

This study investigates the performance of the active strategy of Direct Velocity Feedback control (DVFB) in reducing vibration transmission from a flexible vibrating base to a mounted equipment structure. The first objective of coupling an active controller to existing passive mounts is to attenuate the low frequency amplification effect generated by the passive isolation system. Passive damping can be added in the mounts to limit this adverse effect but this decreases the high frequency performance provided by the passive isolation. In a previous study, a large vibration reduction was achieved using DVFB control on a rigid piece of equipment, whose rigid body modes were shown to be strongly reduced despite the flexibility of the base and without any effect on the high frequency isolation. The main advantage of absolute velocity feedback control is the extreme simplicity of the technique, which provides a decentralised control with a minimum of signal processing. Motivated by the good results obtained for the isolation of rigid equipment structures, this study extends the control strategy to the isolation of large flexible structures coupled to a flexible vibrating base structure by a set of passive isolators. Both the equipment and base structure flexibility has to be accounted for as well as the multi-transmission paths created by the use of several passive mounts. The final objective is not only to estimate the local reduction of the control at the mount junctions on the equipment structure but to assess the global effect of the control on the equipment structure dynamics A simple rigid equipment mounted on a single mount is first considered to assess two practical methods of implementing the control, using either reactive or inertial actuation. An inertial actuator is shown to have stability limits at low frequencies, which are inherent to the actuator dynamics, whereas a reactive control force can provide an unconditionally stable plant to control. Inertial and reactive DVFB controls are then implemented on a mounted flexible composite panel at the mount junctions. Both single and multichannel controllers are considered in simulations and experiments. Both inertial and reactive controls exhibit low frequency instability that limits the maximum feedback gain. Because of the actuator dynamics, an inertial implementation appears to be more gain-limited than a reactive implementation, which provides strong local isolation above 100 Hz as each control channel then generates a skyhook damping effect. A large attenuation is thus obtained in the frequency range of passive isolation amplification for a reactive control, which is not destabilised by the longitudinal resonances occurring in the mounts. The experimental implementation of the reactive control also encounters stability problems at high frequencies which appear to be due to the actuator dynamics. The isolation effect of DVFB control is rather local but an implementation of three control channels, one at each mount junctions, provides global control over the equipment.

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