Laser-based broad-band spatially dense automated data acquisition system

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Virginia Tech

The scanning laser Doppler vibrometer (SLDV) provides the capability necessary for collecting a broad-band spatially dense data set. A laser-based broad-band spatially dense automated data acquisition system is presented and verified. The motivation for such a system is to provide vibration data at many frequencies and spatial locations to allow a more complete dynamic characterization of a structure.

The acquisition system interfaces an SLDV to a frequency analyzer and a Silicon Graphics Crimson workstation. The system allows the user to specify the frequency range, scanning area, and spatial density. The system moves the laser beam, collects and downloads data for the specified area and spatial density. The system post-processes the data compensating the measurements for calibrations and then computes the complex-valued Frequency Response Function (FRF), and coherence plots. The data acquisition system is used to collect FRF data for a beam hanging with simulated free-free boundary conditions. Examples of the FRFs are presented and the data verified by extracting the natural frequencies, damping, and mode shapes of the system.

The experimental natural frequencies were compared with those predicted by Euler-Bernoulli beam theory. The comparison yielded less than an 8% difference for the first seven transverse bending modes extracted. The mode shapes were compared by the Modal Assurance Criterion to show the orthogonality of the modes and a comparison with theory. In each of the cases above, the comparisons yielded results that indicated the modes were both orthogonal and close to theory. The overall conclusion based on the above verification was that the system can be used to collect spatially dense broad-band vibration data that can be used to characterize structures.