Operational modal analysis of a rectangular plate using non-contact excitation and measurement

This work presents a non-contact experimental technique for measuring modal parameters of a rectangular aluminum plate using only the output data. Operational modal analysis (OMA), or output-only modal analysis, has been widely used especially when the input is unknown and difficult to measure. OMA is used to analyze both the out-of-plane and in-plane vibrations of the plate in the frequency range of up to 15000 Hz under white noise acoustic excitation in a direction of interest. It is shown that OMA can be performed when the types of measurement at the measurement and reference points are different. A single-point laser vibrometer and a free-field microphone are used to simultaneously measure the responses of the plate in a non-contact manner, with the microphone measurement serving as the reference. The pressure measured by the microphone near the reference point is shown to be proportional to the normal surface acceleration at the reference point, and the cross power spectral densities obtained by the current test method can be used for modal parameter estimation. A method for measuring the in-plane modes of the plate by shining the laser beam on the plate surface with an incident angle is developed. Experimental modal analysis (EMA) is also performed on the plate using an impact hammer and the laser vibrometer. The measured natural frequencies and mode shapes of the out-of-plane and in-plane modes of the plate using OMA and EMA are compared well with those calculated using commercial finite element software.