Modal Analysis: A Comparison between Finite Element Analysis (FEA) and Practical Laser Doppler Vibrometer (LDV) Testing

Natural frequencies and modal shapes simulated in FE (Finite Element) environments are not simple to validate with physical testing or calculous, especially when relating to complex shaped components. In this paper a Laser Doppler Vibrometer (LDV) system is utilized as an aid in validating the natural frequencies, modes and mode shapes computed from an FEA model of a motorcycle swing arm. The experimental errors observed help to illustrate the difficulty of replicating the correct boundary conditions when dealing with this sort of analysis, due to the fact that the FEA environment is usually ideal. In this case the analyzed component was supported with a very basic set-up; however, slight changes in test configurations and repeat multiple tests assisted in reducing experimental errors ensuring continuity in measurements and the exclusion of non-coherent values which were not relevant. A constant reference surface of the component was used in order to visualize the LDV computed modal shapes, even though the excitation point of the component was varied. This last combination was fundamental for validation.