Modal Parameter Identification Using The Wavelet Transform Based On Ambient Excitation

Nowadays, more and more attention is focused on structural damage detection and healthy assessment with nondustructive test. It is required to understand their dyamic properties during the inspection, assessment and health monitoring of civil engineering structures. Structural modal parameters reflect the dynamic properties that can be obtained by either traditional or ambient vibration identification technique. Due to the huge volume of civil engineering structures, the vibration excitation is difficult; the modal identification based on environmental excitation is available. The system identification of engineering structures is now a very active and challenging research topic.The state of the art studies are carried out in this thesis on the modal parameter identification of civil engineering structures under operational conditions.The research is focused on the algorithm, implementation and real case applications of modal parameter identification using the wavelet transform based on ambient excitation.Firstly, the theory of sampling and frequency mixing is introduced in this thesis. The advantages and disadvantages of various methods varying from Fourier analysis to short -time Fourier analysis and then to wavelet analysis which is focused on are studied. The description of spectrum analysis of stationary signal provides the later actual engineering calculation of modal parameter a theoretical basis.Then, the theory of modal parameter identification using the wavelet transform is introduced. The discussion of the selection of wavelet function and wavelet function parameters is presented for the low-frequency intensive modal of civile engineering structures. The free vibration response can be acquired by using random reductions technology from the vibration response. The frequency domain methods of structural modal parameter identification under ambient excitations are comprehensively discussed in the thesis. A Matlab based modal analysis software for large civil engineering structures is developed. In addition, two simulation examples are presented.Finally, the methods mentioned above are applied to two actual projects. From the results, it is concluded that, STFT and Wavelet methods can achieve a better identification precision in practical application, and the wavelet method can realize a desirable precision in both time and frequency domain because of its variable windows. However, the selection of the wavelet function is significant, which influences the analysis effect and efficiency. The appropriate wavelet function can be chosen by theoretical analysis and repeated attempt. It is convenient, effective to gain the instantaneous frequency by extricating the wavelet-ridge from the continous cmor wavelet transform of free vibration signal. The wavelet method can denoise the original signals effectively, restrain the high-frequency components, and extrude the low-frequency components, which provide more useful information. The method based on level threshold value can attain a far better denoise outcome than on glabal threshold value, but losing some information of original signals.