This paper describes a theoretical and experimental investigation into an active four-mount vibration isolation system, in which electromagnetic actuators are installed in parallel with each of the four passive mounts placed between a three-dimensional piece of equipment and a vibrating base structure. Decentralized velocity feedback control is applied, where each actuator is operated independently by feeding back the corresponding equipment vibration velocity at the same location. Although one end of the actuator acts at the sensor position on the equipment, the system is not collocated because of the reactive force at the other end acting on the flexible base structure, whose dynamics may be strongly coupled with the mounted equipment. The investigation of this actuator installation and its practical implementation are the motivation of this research. Isolation of low-frequency vibration is considered where the equipment can be modeled as a rigid body and the mounts as lumped-parameter springs and dampers. A general theoretical formulation for analysing multiple-mount vibration isolation systems using the impedance method is presented and is used to investigate the control mechanisms involved. Experimental results show that up to 14 dB reduction in the kinetic energy of the equipment can be achieved in practice. If very high gains are used in the experiments, however, instability occurs at low frequencies due to phase shifts in the transducer conditioning electronics
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