SCANNING LASER DOPPLER TECHNIQUE FOR MODAL TESTING OF DISTRIBUTED-PARAMETER SYSTEMS

IN conventional modal testing, accelerometers are used to sense structural response data that are processed to obtain the natural frequencies, damping, and mode shapes of the structure under test. In the case of lightweight structures like composites where mass loading and other local effects of these transducers are not eligible, optical instruments like the laser Doppler vibrometer (LDV) are used. The availability of real-time scanning LDVs has introduced many interesting measurement possibilities. In the method developed in this paper, we process the scanning LDV velocity output signal in the frequency domain to directly obtain the deflection shape of the vibrating structure in a functional (series) form. The technique is demonstrated through application to the problem of modal identification of a cantilever beam. The identified dynamic characteristics of the cantilever beam have been compared with analytical solutions. The method employs a noninvasive, noncontact procedure that avoids the problem of transducer mass loading on lightweight structures and makes possible measurements on hard-to-reach and hazardous surfaces.