Figure 1: (a) Photo of the Base-Upright (BU) with dimensions. (b) First 8 analytical frequencies and mode shapes of the BU. Results of Previous Studies Prior work using slow-speed cameras with phase-stepped sampling and forced normal-mode testing (FNMT) yielded highlyaccurate measurements of the BU mode shapes at many points [5]. The results of the study showed that DIC and 3DPT can be used to measure mode shapes that correlate very well to those obtained using accelerometers and a scanning laser Doppler vibrometer. When the accelerometer and laser data was acquired, shaker excitation was provided at an angle 45 degrees relative to all three principal axes so that all modes would be excited. Figure 2a depicts shaker mounted to the BU with the laser and accelerometer measurement points indicated by red dots and their corresponding numbers. At these measurement locations optical targets were placed on the structure that are measured and tracked by the dynamic photogrammetry system PontosTM [6]. An overlay of FRFs in the z-direction measured by an accelerometer and the laser Doppler vibrometer at point 3 is shown in Figure 2b. A frequency domain, polynomial curvefitter was then used to extract the modal parameters and mode shapes. Figure 2: (a) Shaker orientation and measurement points. Red dots correspond to accelerometer measurement points. Green squares represent SLDV points from a previous study. (b) Sample FRF from previous study [4]. All four measurement approaches were used to acquire the first and third modes of the structure studied and then were correlated to each other as well as a highly accurate finite element model. Excellent correlation between the measurements and the FEM was obtained; each MAC value was above 97.8. When the experimental results were compared, all MAC values were above 95.6. Figure 3 presents a comparison between the FEM analytical mode shape and the accelerometer, laser Doppler vibrometer (LDV), DIC, and 3D point-tracking based mode shapes for modes 1 and 3 of the BU. ¾” 24” 30” 24” 244
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