Converting a Driven Base Vibration Test to a Fixed Base Modal Analysis
Qualification vibration tests are routinely performed on prototype hardware. Model validation cannot generally be done from the qualification vibration test because of multiple uncertainties, particularly the uncertainty of the boundary condition. These uncertainties can have a dramatic effect on the modal parameters extracted from the data. It would be valuable if one could extract a modal model of the test article with a known boundary condition from the qualification vibration test. This work addresses an attempt to extract fixed base modes on a 1.2 meter tall test article in a random vibration test on a 1.07 meter long slip table. The slip table was supported by an oil film on a granite block and driven by a 111,000 Newton shaker, hereinafter denoted as the big shaker. This approach requires obtaining dominant characteristic shapes of the bare table. A vibration test on the full system is performed. The characteristic table generalized coordinates are constrained to zero to obtain fixed base results. Results determined the first three fixed base bending mode frequencies excited by the shaker within four percent. A stick-slip nonlinearity in the shaker system had a negative effect on the final damping ratios producing large errors. An alternative approach to extracting the modal parameters directly from transmissibilities proved to be more accurate. Even after accounting for distortion due to the Hann window, it appears that dissipation physics in the bare shaker table provide additional damping beyond the true fixed base damping.
KeywordsMagnesium Nylon Sine Flange Cyan
Unable to display preview. Download preview PDF.
- Mayes, Randy L. and Bridgers, L. Daniel, "Extracting Fixed Base Modal Models from Vibration Tests on Flexible Tables", Proceedings of the 27th I nternational Modal Analysis Conference, Orlando, FL, February 2009, paper 67.Google Scholar
- Allen, Matthew S., Gindlin, Harrison M. and Mayes, Randall L., "Experimental Modal Substructuring to Extract Fixed-Base Modes from a Substructure Attached to a Flexible Fixture", Proceedings of the 28th International Modal Analysis Conference, Jacksonville, FL, February 2010, paper 164.Google Scholar
- Crowley, John R., Klosterman, Albert L., Rocklin, G. Thomas and Vold, Havard, "Direct Structural Modification Using Frequency Response Functions", Proceedings of the Second International Modal Analysis Conference, Orlando, Florida, February 1984, pp. 58-65.Google Scholar
- Hensley, Daniel P., and Mayes, Randall L., “Extending SMAC to Multiple References”, Proceedings of the 24th International Modal Analysis Conference, pp.220-230, February 2006.Google Scholar
- Carne, Thomas G., Martinez, David R. and Nord, Arlo R., "A Comparison of Fixed-Base and Driven Base Modal Testing of an Electronics Package", Proceedings of the Seventh International Modal Analysis Conference, Las Vegas, Nevada, February 1989, pp. 672-679.Google Scholar
- Beliveau, J.G., Vigneron, F.R. and Soucy, Y., and Draisey, S., "Modal Parameter Estimation from Base Excitation", Journal of Sound and Vibration, Vol. 107, January 1986, pp. 435-449.Google Scholar