5 Modal Testing of a Nose Cone Using Three-Dimensional Scanning Laser Doppler Vibrometry 45 Fig. 5.3 Positioning the laser spots at features on the test article for alignment Fig. 5.4 Baseline test setup 5.2.1 Baseline Test The baseline test was set up using the 1D laser system as shown in Fig. 5.4. The laser head was positioned approximately 1.35 m (53.1 in) from the test article, which was suspended on soft bungee cords. A modal shaker and force transducer were attached to the exterior of the part at the same axial position as the ring of holes, and a drive point accelerometer was mounted on the interior of the test article directly aligned with the force transducer. Since the laser requires line-of-sight to a point to make a measurement, multiple scans were needed to measure all sides of the test article. For each scan, the laser head was aligned with the test article’s coordinate system using the features described in Sect. 5.2, generally using four or five holes and the tip of the test article for alignment. Figure 5.5 shows an example of the alignment points used for one scan. The error in the alignment calculated by the laser system was on average 1.3 mm; compared to the grid spacing of 50–60 mm, it was thought that this error was acceptable. With the lasers aligned, the geometry could be imported and reduced to the points visible from that scan direction. Figure 5.6 shows an example of the measurement points. The baseline system struggled to measure more than four circumferential stations in one scan due to
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