OBSERVATIONS AND CONCLUSIONS Stereophotogrammetry techniques provide additional measurement capabilities that compliment the current array of traditional measurements by providing an alternative that favors high-displacement and low-frequency vibrations typically difficult to measure with accelerometers and laser vibrometers. The techniques presented here generated results that showed a very high level of correlation to the reference finite element model of the Base-Upright. Therefore, the application of the image-based systems for vibrations and modal analysis is a viable technique for the development of structural dynamic models. Previous studies showed that when low-speed cameras are used in conjunction with forced-normal-mode-testing, both digital image correlation and 3D point-tracking can accurately capture mode shapes as long as measurable displacements are present. The biggest difference between the traditional and the FNMT-based techniques is the approach taken to measure multiple modes. Accelerometers and scanning laser Doppler vibrometers measure multiple modes over a broad frequency range point by point. Conversely, the two image-based approaches measure all points simultaneously. If FNMT is used, all approaches measure the response of one mode at a time. DIC can provide an incredibly fine spatial resolution that would be unobtainable with accelerometers or would take a significant amount of time with a scanning LDV. Had the entire surface of the upright been patterned and the same facet settings used, roughly 40,000 effective data points could have been measured. Combining DIC and 3DPT with high-speed cameras enables simultaneous measurement of multiple modes over a wide frequency range. The high-speed 3DPT results were the best obtained in these studies through the first 5 modes of the BaseUpright. Nearly all MAC values were higher than 99%. Though not presented here, time traces from DIC could theoretically be exported and used to calculate mode shapes just as was done with the 3D point-tracking. A higher pointdensity may be obtained with DIC, but larger amounts of data recording needs to be considered. For a given measurement, unless local strain data is desired, the use of 3DPT is recommended. Another possible use for DIC or 3D point-tracking is the measurement of rigid body motion of a test article suspended in a free-free condition. Measuring structures that exhibit large displacements or rigid body motion is difficult for LDVs, because the specific position on the structure at which the laser is pointing will change. Accelerometers provide an inertial reference frame, but their useful ranges usually begin at frequencies above those where rigid body motion occurs. The use of the optically-based displacement measurements presented in this work (DIC and 3DPT) provides an alternative measurement technique that increases the envelope over which engineers can now make vibration measurements. ACKNOWLEDGEMENTS The authors gratefully appreciate the financial support for this work provided by the U.S. Army Research Office Nanomanufacturing of Multifunctional Sensors Ref. Award Number: W911NF-07-2-0081 and the National Science Foundation under Grant No. 0900534, entitled “Dynamic Stress-Strain Prediction of Vibrating Structures in Operation”. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or Army Research Office. The authors would also like to thank Tim Schmidt of Trilion Quality Systems for providing insight and equipment necessary to conduct the measurements. REFERENCES [1] Butland, Adam, “A Reduced Order, Test Verified Component Mode Synthesis Approach for System Modeling Applications,” Master’s Thesis, University of Massachusetts Lowell, January 2008. [2] Nicgorski, Dana, “Investigation on Experimental Issues Related to Frequency Response Function Measurements for Frequency Based Substructuring,” Master’s Thesis, University of Massachusetts Lowell, January 2008. [3] Wirkkala, N.A., “Development of Impedance Based Reduced Order Models for Multi-Body Dynamic Simulations of Helicopter Wing Missile Configurations,” Master’s Thesis, University of Massachusetts Lowell, April 2007. [4] Pingle, P., J. Sailhamer, & P. Avitabile, “Comparison of 3D Laser Vibrometer and Accelerometer Frequency Measurements,” Proceedings of the IMAC-XXVII, February 9-12, 2009 Orlando, Florida USA. [5] Warren, Chris, Pawan Pingle, Chris Niezrecki, & Peter Avitabile, “Comparison of Image Based, Laser, and Accelerometer Measurements,” Proceedings of the IMAC-XXVIII, February 1-4, 2010, Jacksonville, Florida USA. [6] PONTOS – GOM mbH, Mittelweg 7-8, 38106 Braunschweig, Germany. 251
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