Research On Vibration Isolation Characteristic And Active Control Of Floating Raft

The application of floating raft isolation system can be a very effective way to depress thestructure-borne sound and mechanical vibratory energy transmission from vibration source to the shiphull, so it is widely used in large-scale rotating machinery of the vessel. The floating raft isolationsystem is a complicated and coupled system because it contains several sub-systems which aremulti-body vibration system with elastic coupling. But when the ship is voyaging in low speed, thefrequencies of excitation forces on the machines are often within the low-frequency band, it will causethe floating raft isolation system fail to work properly. Besides, the system will have a poor performanceif the vibration source is changed to variable-frequency excitation for it is a kind of the passiveisolation.New types of composite material have been widely used with the development of material scienceand one of them is the sandwich structure. The sandwich structure is made up of a upper skin layer, alower skin layer and a core layer. The upper skin and lower skin layer are often made of metal and hardplastic while the core layer is often made of foam and honeycomb structure. The sandwich structureoffers the advantage that the whole system is much lighter in addition to the high stiffness and strength.The application of viscoelastic material which replaces steel in engineering can reduce vibration andnoise and the significance for improving the survivability of submarines is obviously there.This thesis focuses on the study of the vibration control approach, taking advantage of the floatingraft isolation system. Above all, the dynamic model of the floating raft isolation system without thedynamic vibration absorbers (DVAs) is derived by assembling the mobility matrices of subsystemswhen a combined excitation acts on the source. According to the relationship between the force and thevelocity on each subsystem’s interfaces, the power flow expressions of the systems are derived, and theimpact of the structural parameters on the power flow transmitted into the subsystems of the isolationdevices are also analyzed. In order to improve the performance of the vibration reduction in lowfrequency domain, dynamic vibration absorbers (DVAs) are mounted on the subsystems of floating raftrespectively and the dynamic models of the floating raft isolation system with the dynamic vibrationabsorbers (DVAs), adaptive vibration absorbers (AVAs), active-adaptive vibration absorbers (AAVAs)are derived respectively. The performance of the vibration reduction under different types andconditions is evaluated by comparing the power flow transmitted into the foundation before installing DVAs with that after installing ones. A floating sandwich raft isolation system is presented and itsdynamic model is derived in the same way proposed in the last paragraph. The structural dynamiccharacteristics of the sandwich raft are studied. Subsequently, the dynamic model of the floatingsandwich raft isolation system is derived by assembling the mobility matrices of subsystems. Finally,the control behavior of the AAVAs is verified by employing the adaptive active vibration absorbers(AAVAs) for the active vibration control of the floating raft isolation system with different controlstrategies. Numerical simulation results show that the control strategy based on the total power flowminimization is much better than the other ones.