A Modal Parameter Extraction Method Of Structure Based On The Signal Processing

Damage will appear in the civil engineering structures during their lifetime with the effect of loads and other harmful factors. As a result, it sometimes will bring about significant economic losses and safety incidents. So it is necessary to make efficient diagnosis, evaluation and prognosis for the health condition of serving structures.The structure modal parameters, such as nature frequency, stiffness, damping parameter can be changed when damage occurs. So the modal parameter variety may be regarded as the index that the structure damage occurs, we may use the structure modal parameter variety to detect the different structure damage. In this paper the damage feature extraction techniques for engineering structure based on signal processing method are studied to provide a basis for structural health monitoring.In order to extract modal parameters, a structure damage detection method based on the Laplace wavelet correlation filtering is studied. The effect of the various parameters on the correlation filtering process is also studied through the simulation. By making correlation filtering to the response signal of the ASE structure's impulse vibration, conclusions were as follows: Wavelet analysis is a process of correlation analysis, larger wavelet coefficients can be obtained only when the Wavelet function matches with characteristics of the feature to be extracted. Therefore, the choice of wavelet function is crucial to the feature extraction. The Laplace wavelet which has a unilateral attenuation characteristic resembles the impulse response signal of the structure. When using the Laplace wavelet correlation filtering, it treats the target as a single degree of freedom system, so the result will be one modal parameter, not all modals. It will have bad effect on modal parameter extraction when all modals responses mixed together. In this case, the extracted modal parameter is obtained depends on their amplitude. Because of the vibration response signal of the actual structure is as the situation mentioned above, it is difficult to use the Laplace wavelet correlation filtering to extract modal parameter correctly.The principle, algorithm and problems of the Empirical Mode Decomposition (EMD) method are studied. In order to fix the Mode Mixing Problem of the EMD, the Ensemble Empirical Mode Decomposition method (EEMD) and its characteristics are introduced. Some issues in using the EMD and EEMD method to decompose signals by simulation are investigated. Due to all modals responds mixed together in vibration response signal of the actual structure, a modal parameter extraction method based on EEMD and the Laplace wavelet correlation filtering are developed. firstly, use the EEMD method to decompose the vibration response signal to get a modal response signal from the mixed responds of all modals, which can eliminate the bad effect of the mixed modals responds. Then the Laplace wavelet correlation filtering is adapted to the IMF which related to the modal response signal to get the right modal parameter. By using this method to different cases of the ASCE structure, the 3^rd order nature frequency is extracted accurately. It's shown by experiments that: The structure modal parameters will change when damage occurs, nature frequency of the structure have significant difference between different damage situations. So the changes of the structure modal parameters can reflect the situation of structure health condition. Through using this method to extract modal parameters of structure online, the result can give the evidence for the assessment of structure healthy condition.