Experimental determination of modal parameters for lightweight structures

Abstract eng:
Despite powerful simulation tools, experimental modal analysis still remains indispensable method for reliable investigation of the structural behavior. Numerical structural models like the finite ele­ment based ones can in many cases capture the structural behavior to a satisfactory extent of reliability. Nev­ertheless some information might still be missing or cannot be accurately represented in such models. This is especially valid for material damping properties, which can still not be captured accurately enough merely by a numeric simulation, where they can be implemented only as assumed values. Modal analysis can con­tribute to FE model improvement in such cases and it brings a number of other advantages like in machine diagnosis, trouble-shooting problems, health monitoring or other fields. In this work a methodology for exper­imental identification of modal parameters is presented based on measurement of the frequency responses of structures with complex geometries. The overall investigation of structural behavior is performed on a funnel shaped inlet of magnetic resonance tomograph. Several identification methods are implemented and compared: complex exponential, least-squares complex exponential and polyreference least-squares complex exponential. We have implemented the modal parameters identification methodology within our own graphical user inter­face supported by MATLAB, to create an independent tool for modal analysis. The estimation methods are compared and the comparison results are summarized showing based on tabular representation and stabilization diagrams significant advantage of the proposed methodology for determining eigenfrequencies, damping coef­ficients, mode shapes and residues for complex structures investigated in broad band of frequencies. Runtime for the execution of algorithms can vary depending on the applied method, assumed order of the model used for estimation, as well as on the number of measurements, i.e. inputs and outputs.

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