-30 -20 -10 0 10 20 30 40 50 dB (m/s2)/N HBcc - Reference HBcc - Mobility/Impedance HBcc - Inverse FBS 0 128 256 384 512 640 768 896 1024 -180 -90 0 90 180 Phase o Hz Figure 6. System AB-TT Estimation of Beam B Translational FRF at Node 6 using Synthesized Perturbed FRF. To improve the results, additional internal DOF can be included in both the Mobility and Impedance approaches. Many cases were studied to determine an optimal procedure for choosing internal DOF. Techniques typically employed in pretest analysis (kinetic energy, modal displacements) were investigated; however, no technique produced a clear determination of best internal DOF to choose. While each DOF was used to improve the results of decoupling, only two DOF will be presented for brevity. Figure 7 displays the location of the two DOF used, translational DOF at node 1 and translational DOF at node 16. Figure 7. Internal DOF on AB-TT Presented in Mobility and Impedance Approaches. Figure 8 and Figure 9 presents the resulting translational FRF at Node 6 of Beam B using internal translational DOF located at node 1 and node 16, respectively. Translation DOF at node 1 removes the spurious peaks produced in the resulting FRF when only connection DOF are considered. Mobility approach produces the most accurate results with node 1, as a spurious peak is still present when using the translational DOF at node 16. The Impedance approach suffers at lower frequencies with either internal DOF, and is less accurate than results using only connection DOF. 179
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