Comparison of experimental and operational modal analysis on a laboratory test plate

Operational modal analysis (OMA) is widely used whenever the dynamic characteristics of structures that do not fit into a laboratory are desired. In addition, OMA offers a test of the structure under its real boundary conditions which may sometimes be preferable for validation of numerical models. Theoretically, the natural frequencies and damping ratios should be identically estimated by an OMA test and an experimental modal analysis (EMA) test. However it is still often reported that EMA tests are more reliable. The present paper presents a thorough comparison of EMA and OMA tests of a Plexiglas plate. The experiments were carefully designed, to ensure that the plate was tested under similar boundary conditions. Estimated modal parameters from the EMA test and OMA test are presented and compared, for the first ten modes of the plate. It is found that natural frequencies are deviating by less than 0.3%, damping ratios by less than 7%, whereas cross-MAC values between the mode shapes of the two tests are found to be above 0.99. The experimental test was conducted first by an EMA test, followed by the OMA test and finally another EMA test was conducted in order to catch any time-variance. It is concluded that no significant differences were found between modal parameters obtained by OMA and EMA.