Analyzing the total structural intensity in beams using a homodyne laser doppler vibrometer

The total structural intensity in beams can be considered as composed of th ree types of waves: bending, longitudinal, and torsional. In passive and ac tive control applications, it is useful to separate each of these component s in order to evaluate their contribution to the total structural power flo wing through the beam. In this paper, a twisted z-shaped beam is used in or der to allow the three types of waves to propagate. The contributions of th e structural intensity, due to these waves, are computed from measurements taken over the surface of the beam with a simple homodyne interferometric l aser vibrometer. The optical sensor incorporates some polarizing optics, ad ditional to a Michelson type interferometer, to generate two optical signal s in quadrature, which are processed to display velocities and/or displacem ents. This optical processing scheme is used to remove the directional ambi guity from the velocity measurement and allows nearly all backscattered lig ht collected from the object to be detect. This paper investigates the perf ormance of the laser vibrometer in the estimation of the different wave com ponents. The results are validated by comparing the total structural intens ity computed from the laser measurements, with the measured input power. Re sults computed from measurements using PVDF sensors are also shown, and com pared with the non-intrusive laser measurements.