Research On Modal Parameters Identification Based On Local Mean Decomposition Method

The identification of modal parameter of vibrating structures is widely usedin mechanical engineering, aerospace, civil, defense industry and other fields.In-accordance with whether the vibration system’s stiffness, damping, and qualityparameters will change with time, the vibration system can be distinguished astime-varying and time-invariant system.There are many researches about modalparameter identification method for time-invariant system, which are also moremature. However, how to effectively identify the modal parameters of multi-degr-ee-of-freedom time-invariant vibration system still needs further study. Time-var-yingvibration systems canbe divided into linear time-varying system and nonlineartime-varying system. This paper is going to mainly discuss the linear time-varying system, of which the vibration response signal is an amplitude-modulatedandfrequency-modulated non-stationary signal. And its modal parameter identification is also more difficult, while the researches focused on this aspect at homeandabroad are skinny.The Local Mean Decomposition (refers to LMD) can self-adaptively dividea multi-component signal into several PF components with physical significance (Product Function, refers to PF). Each PF component is the product of a pure FM signal and an envelope signal, and the instantaneous frequency of the compon-ent is directly educed from pure FM signal, while the envelope signal is the am-plitude. The combination of the instantaneous frequency and amplitude of all c-omponents can obtain the original signal and its time and frequency distribu-tion. Essentially, the LMD method is dividing the multi-component signal self-adaptively into amplitude-modulated and frequency-modulated signals. Thus it canbe applied into the processing of non-stationary signal with multi components.By introducing The local mean decomposition method into the field of mo-dal parameter identification,this paper thoroughly and systematically studied themodal parameter identification method of time-varying and time-invariant vibra-tion system based on The local mean decomposition.Main works:Theoretical study of Local Mean Decomposition (LMD) modal parameter id-entification method. Through theoretical analysis, the paper proposed the process of the method based on LMD decomposition of modal parameter identification，and identified the instantaneous amplitude and instantaneous frequency of FMand AM signal using LMD method and EMD method. The comparison and an-alysis of the results of the two methods show that LMD decomposition method,in the treatment of linear time-varying modal parameter identification, is superiorthan the EMD method.The modal parameter identification of time-invariant system. According tothe response characteristics of the time-invariant vibration system and combin-ed with a linear fitting， the paper theoretically brings out the process of themodal parameter identification of time-invariant system based on LMD，and val-idates the effectiveness of this method by analyzing the data from the simulation of single-degree-of-freedom andmulti-degree-of-freedom time-invariant systemand the experience of the transient response of a cracked beam.Modal parameter identification of linear time-varying system. The vibrationresponse of linear time-varying system is non-stationary signal. According to thecharacteristics that using the LMD decomposition method can effectively analyzeand process non-stationary signal analysis, the paper researches the modal par-ameter identification of time-varying system based on LMD decomposition meth-od.Structural parameter identification of time-varying SDOF system. Based onlinear time-varying modal parameter identification and further research in thestructural damage, imitated by the structure stiffness changes of a time-varyingSDOF system,the LMD method proposed in this paper accurately identified the s-tiffness changes of SDOF system. And it provides effective method for structuraldamage identification under ambient excitation.The Local Mean Decomposition method is applied in the identification ofmodal parameter of time-invariant and linear time-varying system and the struc-tural parameter of SDOF system in this article. The analysis of simulation exa-mples and experimental data shows that the method in this paper can effectivelydecompose the response of multi-degree-of-freedom vibration system self-adapt-ively, in accordance with its modal fremquency, into several sigle-order modalresponse signals，and then, it can effectively identify the modal parameter ofvibration system. This method is of significant value in engineering application.