Efficient experimental modal analysis of symmetric structures

A technique is presented which demonstrates improved efficiency for conducting modal analyses on symmetric structures. This technique utilizes structural symmetry, reciprocity, and multi-channel data acquisition to limit the required measurement grid imposed on a symmetric structure to cover only one of the symmetric regions of the structure. Modal analysis results are presented on an equipment rack shelf (rectangular symmetry) and a five-bladed propeller (cyclic symmetry) using both traditional (full grid) and symmetric (partial grid) modal analysis techniques. Over the frequency range measured, the traditional and symmetric approaches yield similar results, however a difference in phasing between symmetric sections is observed for a relatively small number of modes in both the shelf and propeller. The data acquisition time required for the symmetric approach is equal to the time required for the traditional approach divided by the number of symmetric regions (25% for the shelf and 20% for the propeller).