116 J. P. Winkel et al. Fig. 12.21 Stability Diagrams for the CT Speed 1 (left) and CT Speed 5 (right), 1% Modal Damping Level, No Sensor/Ambient Background Noise Fig. 12.22 Cross Orthogonality Matrices of the Lowest Frequency Modes for CT Speed 1 (left) and CT Speed 5 (right), 1% Modal Damping And No Sensor/Ambient Background Noise is almost no difference in the quality of the CMIF or race track pattern of the pole estimates, except for the first three target modes. CT speed 5 with no sensor/ambient background noise was the only CT speed where the first three modes were cleanly extracted from the simulation. This can be more clearly observed in the cross-orthogonality matrices that were generated for each CT speed and shown in Fig. 12.22. Damping Parameter Sensitivity This study showed that, with no sensor/ambient background noise present, increasing levels of modal damping made identification of closely spaced target modes more difficult. Modal damping levels exceeding 3% make it extremely difficult to identify the target modes and if this is the case during actual testing, the test engineers would probably not be able to identify more than the first few target modes. In Fig. 12.23, the gold stars at the top of each stability matrix/CMIF overlay indicate what pole estimates were able to be automatically selected by the AFPOLY software. In this study, these automatically selected pole estimates were used as an indicator of how strong a mode was excited and how easy it would be to identify it. Pole estimates not automatically selected in this study indicate modes the test engineers most likely will have a very challenging time trying to identify. In an attempt to keep the study as “realistic” as possible, no manually selected poles were used even if they could be accurately selected from the stability diagram. This precluded the analyst from selecting poles based upon knowledge of the FEM modes. Figure 12.24 shows the cross-orthogonality matrices of these automatically selected modes for each level of modal damping considered at a CT speed 2 (the speed most likely to provide clean modes). When the modal damping level increased above 3%, the software’s ability to select pole estimates was cut nearly in half and only 13 of the 28 target modes were able to be extracted.
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