Instantaneous Frequencies Identification Of Time-varying Bridge Structures Based On New Signal Processing Techniques

The response signals of bridge structures during service are not only time-varying and non-stationary,but also closely-spaced modes.Until now,most parameter identification methods on time-varying structures are based on the concept of "frozen time",which ignores the fact that the structural parameters changs with time.Moreover,it is difficult for traditional signal processing methods to identify responses with closely-spaced modes.As such,several modal parameter identification methods are proposed by combining and improving advanced signal processing methods,aiming to deal with the existing time-varying,non-asymptotic and closely spaced modes signals in bridge structures.The validity and accuracy of the proposed method are verified by numerical examples and time-varying structural tests.The main research work and innovations of this dissertion are as follows:(1)The variational mode decomposition method is introduced into the filed of civil engineering to overcome the shortcome of synchrosqueezing wavelet transform,that is,it can not identify closely-spaced modes paprameters accurately.After that,variational mode decomposition method and synchrosqueezing wavelet transform are combined to present a new instantaneous frequency identification method.In the new method the signal is first decomposed by variational mode decomposition and then the instantaneous frequencies of component signals are extracted by synchrosqueezing wavelet transform.Several numerical examples and a time-varying cable test are employed to verify the effectiveness and the accuracy of the method.(2)Since the standard synchrosqueezing wavelet transform can not suppresse the diffusion along the time axis,a new parameter identification method called improved synchrosqueezing wavelet transform is proposed to achieve good resolutions in both time and frequency domain.In this method,the analytic mode decomposition theorem and recursive Hilbert transform are combined to cope with non-asymptotic and closely-spaced response signals.After that,the zoom synchrosqueezing wavelet transform is introduced to squeeze the wavele coefficents in a specific region but not the whole time-frequency plane,leading to better time-frequency resolution in a concerned region.Finally,a cantilever beam test with abrupt mass reduction is designed to verify the effectiveness of the method.(3)An improved arccosine function algorithm is proposed.The improved method gets rid of previous methods which use Hilbert transform to construct analytical signal to obtain instantaneous frequency.Instead,the instantaneous phase of the structural response signal is obtained by segmenting the time axis and using the inverse cosine function.Then,instantaneous frequency is solved by its definition.