A major problem for isolating vibration from large marine machinery rafts is how best to deal with excited resonances. These generate large forces on the hull that create a significant vibration problem. The passive design of such mounts typically represents a compromise between providing good vibration isolation and good machinery alignment under seaway motion. The ‘Smart Spring’ isolation system, a new hybrid passive–active approach to solving this problem that is being developed by BAE Systems, has been described in a series of earlier papers. The fundamental concept utilizes digitally controlled actuators to apply forces that are independent of local displacement while controlling the response of the structure's rigid body modes. The current paper describes recent work to extend the functionality of the isolation system to deal specifically with discrete-frequency vibration sources. Two novel harmonic control strategies are introduced and evaluated using a six-degrees-of-freedom experimental active mount. It is shown that a strategy that employs recursive least-squares estimation provides both exceptionally high isolation performance and rapid convergence
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