A satellite undergoing a mode survey test. The results are used to update the satellite's loads analysis model.
Mode survey tests establish the structural dynamic properties of spacecraft and complex launch vehicle components such as the upper stage and fairing. These measurements are used to adjust the analytical finite-element models that are, in turn, used to develop the loads analysis structural dynamic models.
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A satellite undergoing a mode survey test. The results are used to update the satellite's loads analysis model. |
Mode survey tests are typically conducted with multiple shakers that impart low-level random forces through tube-like stingers. Two to four shakers are usually positioned to impart forces at various locations on the test article. The imparted forces are measured with small force gages attached between the test article and the shaker stingers. System vibration is measured with sensitive accelerometers, and 300 to 500 accelerometers are not unusual for a mode survey test of a complex satellite structure. Time-series data analysis techniques are then used to compute transfer functions (which describe the mathematical relationship between system input and output) relative to each shaker as though the shaker forces were applied one at a time.
The locations of the applied forces and accelerometers must be chosen carefully to ensure that the test will generate adequate data. Aerospace has developed procedures for determining appropriate measurement locations. By automatically calculating factors such as mass-weighted effective independence and iterative residual kinetic energy, these methods have helped reduce the time required to conduct reliable mode survey tests.
Once the transfer functions have been computed from the measured data, optimization procedures are used to extract the normal or natural modes of vibration and their associated natural frequencies and damping values. Mode-X, a tool for extracting modal parameters from transfer function data, was developed at Aerospace to support mode survey tests on Air Force programs. Aerospace-derived parameters are often used to supplement contractor-derived parameters that eventually get used to adjust the analytical loads analysis models.
—J. A. Lollock