Investigation On Modal Identification At Ambient Excitation

Ambient Excitation is one kinds of natural excitation method ,it doesn't needs valuable excitation equipment and doesn't break off the normal service of the system ,convenient and time efficient, has been prodigious developed in the resent years. The output-only signals is generally and all has the characteristics of have very small amplitude, the randomness ,the noise influence and the number of data are all very big, and the input is unknown, so it is difficult to identify the modal parameter. Wavelet analysis method can project the signal to the time-frequency field, composite the time-frequency spectrum, it is a new subject which has rapidly developed in the recent 10 years. In this paper, we use the Morlet wavelet to analyze the time-frequency signal. Use the property of comparison between the wavelet and the signal, to identify the modal parameter. Its main contents comprise the following aspects:The traditional identification method of modal parameter and the development of identification method of modal parameter under the ambient excitation of were investigated, made a summary of the time field and frequency field methods. There is a particular depiction on the wavelet transform which belongs to the time-frequency filed, mainly analyzed the continuous wavelet transform of the Morlet wavelet and its identification of the modal parameter.According to the situation of the output-only signals, we need to identify the modal parameter of the system which is under ambient excitation, use the random decrement technique to obtain the free response. We modify the random decrement technique, so we can obtain the improved free response use the same number of data. Use the modified Morlet wavelet to analyze the free response, decompose the multi-degree-of freedom system into single-degree-of-system, based on the wavelet coefficient and the connection of the nature frequency and the damping, we can obtain the modal parameter of the system which is under the ambient excitation. The modal of GARTEUR is an applying example of this method, its result indicates the method is efficient, the identification have a good precision.Then we investigate the modal of truss-mass system, make modal analysis of experiment and then use the finite element analysis all obtained the modal parameters, the both results may be confirmed. At the last, we use the wavelet transform to analyze the response signal which is obtained from the modal analysis of experiment, get the modal frequency and the damping ratio of the mass-pole modal, and validate the efficiency of the wavelet analysis.