Study On Vibration Isolation Of Fluid-type Floating Raft System

Low frequency vibration isolation is a difficult technical problem. Generally, toisolate low frequency vibrations, the natural frequency of system must be small,which leads to lower supporting stiffness and larger mass of the system. And thesemethods are very limited in practical vibration isolation system design. In addition,another important technical problem of vibration isolation is whether the workingfrequency of the isolator can be easily tuned when dealing with the multi-excitedfrequencies working conditions. In this paper, a novel fluid-type floating raft isolatorwas developed based on anti-resonance law. The advantage of this isolator is that theanti-resonance frequency of the system can be tuned via changing the added mass offlowing fluid rather than changing the supporting stiffness. By this way, any specificfrequency vibration can be completely isolated, especially at very low frequenciesdomain.The presented model of isolator in this paper mainly consists of the fluidcontainer, floating body, springs and fluid medium. The mathematical model wasderived by fluid momentum equation and vibration theory. The vibration isolationmechanism was analyzed. The relationship of optimum isolation frequency and liquiddepth of floating raft immersed fluid was obtained, and the main parameters affectingthe isolator performance were discussed. Numerical simulations of various operatingcondition were made based on the flexible basis mathematical model. It was foundthat the anti-resonance occurs over a wide band of frequencies if the liquid depthchanges in a limited range; the added mass of fluid makes natural frequency of thesystem move to the direction of low frequency. Utilizing the anti-resonance, the forcetransmissibility of the fluid-type floating raft could be reduced at very low levels.A test prototype of fluid-type floating raft has been designed and built to verifythe theoretical model. In the test, the performance of the presented model wasanalyzed and compared with the classic double layered vibration isolator, whichcorresponded to the case that no fluid in the container. The results showed that theprototype offered an excellent performance of vibration isolation at anti-resonancefrequency; the natural frequency of the system could be effectively reduced to someextent, and the isolator has a good performance at low frequency vibration isolation;The testing curve of the optimal isolation frequency with the corresponding liquid depth of floating raft immersed fluid was obtained and compared with the theoreticalone, the same changing trend of test and theory was observed. Because the dampingvalues of the system were uncertain, the curve peak of the test is smaller than thetheoretical model at the natural frequency.