Identification, Based On The Parameters Of The Connection Structure Of The Fluctuation Method

The parameter identification of junctions belongs to the category of the boundary condition identification of structural dynamics, and it is also one of the most challenging fields. When studied the vibration of structures, sometime the properties of the junctions have to been identified in order to continue the analysis. In general, the exact finite element model of the individual can be set up without much difficulty. But for the properties at the junctions, such as the stiffness and damping, the model and measuring are both complex. So the results from the analysis about the whole structure cannot meet the need of the project. Then it has been paid more and more attention to the identification method using the measuring data for the junction or the boundary conditions. And the parameter identification for the junction is bringing more interest to many researchers. The conventional identification methods are mostly based on the theory of FEM (finite element method), and developed by the vibration analysis. As to lots of structures in the field of engineering, such as truss, frame structures, piping systems and so on, the models of the individual member are simple and can be established exactly though the whole structure may be very complex. So in order to make use of this merit, a method based on the wave propagation is developed in the domain of vibration analysis and control. This method analyses the structural vibration in the view of wave theory. Because of the local characteristic of wave propagating along the wave-guide, the wave method has the advantage of brevity and systematism in structural dynamics. Based on the wave method, this paper presents the wave amplitude and wave spectra method respectively from the measured data of FRFs (frequency response functions) and auto- and cross-power spectra. In addition, , the method using the data of natural frequencies to solve inversely the characteristic equations has been described in brief. The technique of wave amplitude method is to extract the characteristics of propagating wave, and then the wave mode in the wave-guide can be obtained. Therefore, the relation between the wave mode and the dynamic stiffness at the junction is derived. As to wave spectra method, it is possibleto obtain the expressions of the wave spectra in terms of the measured auto- and cross-power spectra of an array of closely spaced accelerometers. With these expressions, the junction stiffness and damping can be obtained according to the conditions of force equilibrium and displacement continuum. For the method inversely solving characteristic equations, the basic concept is to set up the characteristic equations applying wave method and solve them inversely under each of tested natural frequencies. Because of its global property, the application of this method is limited. The validity and feasibility of these approaches are using numerical examples and experiments. For the numerical examples, the junction parameter identification of a beam structure is applied to verify the wave amplitude method, and the conjunction parameter identification of a piping system is for the verification of wave spectra method. In the end, a cantilever supported by different springs is excited with stochastic signals to demonstrate these three new identification methods.