Research On Power Flow Transmission And Active Control Of Floating Raft Isolation System

The investigation of vibration transmission characteristics and vibration control strategy of complex system from the viewpoint of power flow and control theory is one of the leading research issues in the field of vibration and noise control. As a vibration isolation measure applied to the ships, the floating raft isolation device has attracted more and more attention for the navy and shipbuilding enterprises. Based on the common floating raft isolation device in the engineering, this thesis is mainly focused on the passive and active transfer matrix modeling of the rigid and elastic floating raft isolation system.A mechanical model for a rigid floating raft isolation system when excited by combined multi-dimensional forces and moments is established, where multi-dimensional wave effects in elastic mounts are taken into account. The mobility transfer equation for every subsystem and power flow expression are derived utilizing the substructure mobility synthesis method. It is shown that the coupled rigid body modes of the machine and raft are excited in low frequency domain, while multiple peaks are excited on the power flow curve due to flexural mode resonance of foundation and standing wave effect of isolators in middle-high frequency domain. The power flow excited by the pitching moment has an important effect on total power flow transmission. The power flow induced by axial force transmitted to the foundation and reponse velocity is the most influential factor to the total power flow. The vibration isolation performance can be improved by reducing stiffness of two layers of isolators, and increasing mass of raft and elastic modulus of the foundation.A mechanical model for an elastic floating raft isolation system when excited by combined multi-dimensional forces and moments is established. Mobility transfer equation for the elastic floating raft is deduced, and power flow spectrums for the rigid and elastic raft isolation system are compared. The results show that the two power flow curves almost coincide in low frequency domain, while they have obvious differences in high frequency domain due to excitation of flexural mode resonance of the raft and coupling effect with the foundation modes.A feedforward control mechanical model for a floating raft isolation system when excited by combined multi-dimensional forces and moments is established. Mobility transfer equation for a subsystem combining elastic mounts and secondary actuators connected in parallel is derived. Control strategies for minimization of total power flow, axial power flow, axial force and axial response velocity are proposed, and expressions for optimal control forces are presented. It is shown that the power flow curve has only two peaks caused by transverse mode resonance of the machine and raft in low frequency domain, while the control effect becomes weaker in high frequency domain where flexural mode resonance of the foundation has notable effect on the power flow. The overall control performance can be optimized by choosing total power flow transmitted to the foundation as the cost function.