Automated Modal Identification Of Structures With Closely Spaced Modes Based On Frequency Domain Method

With the development of society,more and more engineering structure was built.The dynamic characteristics of the structure are determined by the modal parameters of the structure,which are usually identified by some modal parameter identification methods based on the measured structure's vibration signal data,and then we can analyze the health status of the structure which are in service.Considering the advantages of The frequency domain decomposition method and Operational Modal Analysis in modal parameter identification of actual vibration structures,this paper choose the frequency domain decomposition method to identify structural modal parameters.The main research work and results are as follows:(1)This paper summarized the advantages and disadvantages of the frequency domain identification method,and the frequency domain decomposition method suitable for environmental excitation is selected as the modal parameter identification method.The modal frequencies and mode shapes identified by the frequency domain decomposition method has high modal mode accuracy,but the accuracy is low because of the truncation error in identifying damping ratio.The damping ratio identification method was improved based on the high correlation of the singular vector in the natural frequency of the structure.Eventually programming to realize the method,and the method is used to identify the modal parameters of a four degree of freedom spring mass system structure numerical model,the results show that the algorithm accuracy is higher and the damping ratio identification accuracy is satisfactory.However,the frequency domain identification method can't solve the problem of structures with closely spaced modes well.(2)This paper analyzed the reason why closely spaced modes appeared and the type of structures with closely spaced modes.The characteristics of the structure with closely spaced modes and the reasons for the low recognition accuracy in frequency domain are also analyzed.And closely spaced modes are divided into two categories according to the mixing degree of two peaks in frequency domain.In this paper,a method based on frequency response function is proposed for determining the modal order of structures with closely spaced modes.This method is applicable to the frequency domain identification method which needs to be determined in advance.(3)On the basis of the technique of frequency domain decomposition,a method to effectively identify the closely spaced modes is proposed.In this method,we need to make SVD of the cross power spectral density function matrix of the acceleration response data in the vibration structure,and then analyze the first singular value of each frequency point.By calculating and comparing the angle between the singular vector of its peak and the neighboring frequency points,the number of modes in a peak is finally determined.A three degree of freedom spring mass system with closely spaced modes is used to verify and analyze the structure.The numerical example shows that the proposed method can effectively solve the two kinds of closely spaced modes.(4)In order to reduce human participation,and automatically identify the modal parameters of the structure,thus providing further possibility for the real-time monitoring of the engineering structure,thus we chose the scale-space peak detection method to automatically detect the peak value of the singular value curve,and the parameter enclosure is set up in the improved frequency domain decomposition method.Finally,the measured data of a practical bridge are analyzed.The results of modal parameter identification show that this method can automatically identify the actual engineering structure with closely spaced modes.And the characteristics of the structure with closely spaced modes are verified by comparing the mode shapes of three periods on the same day.