This paper on "FE Model Correlation & Mode Shape Updating Using Qualification Test Data…A Case Study on the Olympus Satellite" was presented at the NAFEMS World Congress on The Evolution of Product Simulation From Established Methods to Virtual Testing & Prototyping - 24-28 April 2001, The Grand Hotel, Lake Como, Italy.

Abstract

Satellite equipment is subjected to shaker tests with various shaker levels to ensure their structural integrity and launch survivability. Traditionally 2 types of excitation signals are applied to the shaker table: random and swept sine. For large structures, this is sometimes complemented with a high-level acoustic qualification test. A fundamental problem in a shaker test is that it is difficult to control the load in the tested structure since the number of notching channels is hardware limited. A possible solution to this is to use a global measure of the shaker load, which can be easily obtained by the use of a multi-axial Force Measurement Device (FMD) on the control channels in combination with a Signal Processing Unit (SPU) [1]. To have an idea of the maximum excitation a test specimen can accept during a shaker test, a mathematical model is used to generate transfer functions that show the maximal stress/displacement at critical locations in the model as function of the forces and moments that excite the structure at the base. These transfer functions can then be combined with the highest allowed stress/displacement levels to estimate the largest force load the satellite can accept during the test. As a consequence, the mathematical model needs to be of high quality, and thus be test-verified. This is only a first reason why it is of vital importance to dynamically correlate the FE model with experimental data and eventually to further fine-tune and update the model. A more important reason is that finite element models are used for the estimation of loads during the launcher (rocket) flight. By combining the FE model with forces and moments, measured in-flight at the launcher/satellite interface, one can estimate the loads on the satellite at various locations. These calculated loads then determine which load levels need to be achieved during the vibration test as a minimum. The FMD is also used to verify that the desired loads are in fact achieved.
This paper illustrates the process of correlation and updating on the Olympus satellite. Instead of building another expensive and time-consuming classic modal survey test set-up, the measured data from the low-level, medium level and high level qualification test is used, processed and fed into a conventional modal analysis package in order to estimate the natural frequencies, the damping ratios and the mode shapes. Using this data as reference, the initial finite element model is correlated and fine-tuned towards resonance frequencies and MAC values.
The work related to the Force Measurement Device has been carried out under Contract 12014/96/NL/FG of the European Space Agency. The results and the pictures are provided as a courtesy from Ingemansson [2].