In conventional modal testing, accelerometers are used to sense structural response data which is processed to obtain the natural frequencies, damping and mode shapes of the structure under test. In the case of light-weight structures like composites where mass loading and other local effects of these transducers are not negligible, optical instruments like the laser doppler vibrometer (LDV) are used. The availability of real-time scanning LDV's has introduced many interesting measurement possibilities. By applying a time-domain sorting algorithm, we have recently demonstrated the use of such a scanning LDV to simulate multiple discrete sensors distributed over the test structure. In the method developed in this paper, we process the scanning LDV velocity output signal in the frequency domain to obtain directly the deflection shape of the vibrating structure in a functional (series) form. The technique is illustrated by measuring the second mode shape of a light-weight cantilever beam. A discussion of the limitations of the method and comparisons with theoretical predictions are also included.

by ASL