| Summary: | 碩士 === 長庚大學 === 機械工程學系 === 100 === Modal analysis is an essential tool for deriving reliable models to study the dynamics of machine structures. This is a study of structural dynamics under vibrational excitation.
Experimental modal analysis (EMA) is a process of determining modal parameters (such as frequencies, damping ratios, and mode shapes) of a linear, time invariant system experimentally. EMA is performed using excitation by an impulse hammer on a structure and considering a limited number of measurement points. The impulse generated by the hammer is recored along with the responses. On the other hand, the operational modal analysis (OMA) is a technique that characterizes a structure on the basis of output responses only. The responses of the structure are the vibrational signals resulted in operation. The OMA is an emerging technique for studying structural dynamics, which has been applied to complex structures that are often difficult to analyze using EMA.
The computer aided engineering software, which combines the finite element method and numerical analysis, has become ae mainstream engineering tool. Although a FEM package, such as ANSYS, can be used to produce a simulated result, the results obtained require validation. The validation can be carried out with experimental data by the technique of model updating. Since the purpose of model updating to study how modifications of the design variables will influence the system's response and how an error function that describes the deviation between analytical and experimental data, can be minimized.
In this study, the modal analysis is based on the OMA. In this these, the principles of the OMA are described first, the method of PolyMAX with frequency domain identification is then applied. In the experiments conducted by OMA both methods of single run and multi-run are used to measure the structural dynamic characteristic. The results are compared with the EMA and FEM. From the results obtained in this study the OMA is recommended over the other methods for modal testing of mechanical structures.
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