Mode enhancement techniques using the MMIF and/or CMIF were used to extract modes that were difficult to extract using AFPoly™and other algorithms. These included bands of modes where there were high levels of modal coupling as defined by high off-diagonal terms in the test self-orthogonality matrix, and frequencies where the synthesized FRFs from modes extracted by AFPoly™did not match the test data well. In the following example, the 36–38 Hz region contained a high modal density and required mode enhancement. Mode enhancement techniques often work better for sets of test data with a large number of modes over a very small frequency range, such as the modes associated with local deformations of the primary mirror mass simulators. The eigenvectors associated with the minimum value of the MMIF for each mode are defined as the force patterns necessary to drive the structure into a normal mode. Figure 10.13 shows the MMIFs from the random five-shaker run for the stowed configuration. The FRF matrix was post-multiplied by these force patterns to generate a set of mode-enhanced FRFs. The imaginary part of the resulting FRF matrix at resonance is assumed to be the mode shape as it would be for a mode extracted using a normal mode tuning technique. The FRF matrix is then pre-multiplied by this shape to create a single enhanced FRF. The measured drive point FRF and the enhanced FRF are compared in Figs. 10.14 and 10.15, showing that the modes in the 36–38 Hz range are much more prevalent after the enhancement. Damping and natural frequency values were extracted from the enhanced FRF. Figure 10.16 shows the enhanced FRF and the curve-fit data for the first two modes in this frequency range. Fig. 10.12 Primary mirror mass simulator impacts—example photos of two locations 0 50 100 150 200 250 10-4 10-3 10-2 Frequency (Hz) Acceleration/Excitation Force ((m/s2)/N) -360 -270 -180 -90 0 Phase Frequency Response Function [2] (11625Y-,100Y+) [5] (11635Y-,101Y+) [8] (11630Y+,102Y-) [11] (11620Y+,103Y-) One mount location is stiffer than the others Fig. 10.11 Drive point inertance comparison 10 Modal Testing of James Webb Space Telescope (JWST) Optical Telescope Element (OTE) 111
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