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Modal Data Extraction. EMA : obtain H( w ) w n , z , mode shape. Review :. z : SDOF method. Modal Parameters by circle fitting. Nyquist plot : Plot of Re[H( w )] versus Im[H( w )] mobility FRF. receptance (compliance) TF. mobility TF. accelerance (inertance) TF.
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Modal Data Extraction EMA : obtain H(w) wn ,z , mode shape Review : z : SDOF method
Modal Parameters by circle fitting Nyquist plot : Plot of Re[H(w)] versus Im[H(w)] mobility FRF
receptance (compliance) TF mobility TF accelerance (inertance) TF In resonance, Also
Example 7.4.1 Consider the experimentally determined compliance transfer function plotted in Figure 7.9 and calculate the number of degrees of freedom, modal damping ratios, and natural frequencies. Solution Since the magnitude plot indicates two distinct peaks, the test system is assumed to have two degrees of freedom. This is confirmed by examining the phase plot at the peaks. Since the phase is at each peak, each of the peaks corresponds to a natural frequency. Reading the vertical axis yields Next since the 3-dB down points are those two values of w where
Example 7.4.1 From the plot, these values yield and Using equation (7.14) yields Repeating this procedure for the second peak yields
Mode Shape Measurement : transfer function between response at s and input force at r For
Example 7.6.1 ,similar results can be obtained from
