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Unformatted text preview: 40 160 180 0 200 Figure 534. Case 10: Burst Random Excitation with Cyclic Averaging (567) Coherence 0.6 Coherence 0.6 +UCSDRLRJA CN20263663/664 Revision: June 12, 2001 + Input: 1x Output: 1x Phase (Deg) 180 0 10
0 1 0.9 10 −1 0.8 0.7 Magnitude (g/lb) 10 −2 0.5 0.4 0.3 0.2 0.1 10 −3 10 −4 0 20 40 60 80 100 120 Frequency (Hertz) 140 160 180 0 200 Figure 535. Case 11: Random Excitation with Hann Window
Input: 1x Output: 1x Phase (Deg) 180 0 10
0 1 0.9 10 −1 0.8 0.7 Magnitude (g/lb) 10 −2 0.5 0.4 0.3 0.2 0.1 10 −3 10 −4 0 20 40 60 80 100 120 Frequency (Hertz) 140 160 180 0 200 Figure 536. Case 12: Random Excitation with Hann Window and Cyclic Averaging (568) Coherence 0.6 Coherence 0.6 +UCSDRLRJA CN20263663/664 Revision: June 12, 2001 + Input: 1x Output: 1x Phase (Deg) 180 0 10
0 1 0.9 10 −1 0.8 0.7 Magnitude (g/lb) 10 −2 0.5 0.4 0.3 0.2 0.1 10 −3 10 −4 0 20 40 60 80 100 120 Frequency (Hertz) 140 160 180 0 200 Figure 537. Case 13: Pseudo Random Excitation with Cyclic Averaging
Input: 1x Output: 1x Phase (Deg) 180 0 10
0 1 0.9 10 −1 0.8 0.7 Magnitude (g/lb) 10 −2 0.5 0.4 0.3 0.2 0.1 10 −3 10 −4 0 20 40 60 80 100 120 Frequency (Hertz) 140 160 180 0 200 Figure 538. Case 14: Periodic Random Excitation with Cyclic Averaging (569) Coherence 0.6 Coherence 0.6 +UCSDRLRJA CN20263663/664 Revision: June 12, 2001 + It is clear that in many of the measurement cases, the multiple coherence can be improved dramatically using simple excitation, averaging and digital signal processing methods. Note that, as the multiple coherence improves, dramatic changes in the FRF magnitude accompany the improvement (factors of 2 to more than 10). When estimating modal parameters, the frequency and mode shape would probably be estimated reasonably in all cases. However, the damping and modal scaling would be distorted (over estimating damping and under estimating modal scaling). Using these results for model prediction or FE correction would bias the predicted results. The most important conclusion that can be drawn from the results of this measurement exercise on a lightly damped mechanical system is that accurate data is an indirect function of measurement time or number of averages but is a direct function of measurement technique. The leakage problem associated with utilizing fast Fourier transform (FFT) methodology to estimate frequency response functions on a mechanical system with light damping is a serious problem that can be managed with proper measurement techniques, like periodic and pseudo random excitation or cyclic averaging with burst random excitation. Hybrid techniques demonstrated in this paper clearly show that a number of measurement techniques are acceptable but some commonly used techniques are clearly unacceptable. It is also important to note that while ordinary/multiple coherence can indicate a variety of input/output problems, a drop in the ordinary/multiple coherence function, at the same frequency as a lightly damped peak in the frequ...
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This note was uploaded on 09/29/2013 for the course MECHANICAL ME taught by Professor Regalla during the Fall '11 term at Birla Institute of Technology & Science, Pilani  Hyderabad.
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