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Unformatted text preview: 15) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + be reduced by the use of weighting functions or by cyclic averaging. The second situation occurs when a signiﬁcant propagation time delay occurs between the input and output as may be the case with acoustic measurements. If a propagation delay of length t is compared to a sample function length of T , a low estimate of coherence will be estimated as a function of the ratio t/T . This propagation delay causes a bias error in the frequency response and should be removed prior to computation if possible. 5.2.3 Multiple Input FRF Estimation
Multiple input estimation of frequency response functions is desirable for several reasons. The principal advantage is the increase in the accuracy of estimates of the frequency response functions. During single input excitation of a system, there may exist large differences in the amplitudes of vibratory motion at various locations because of the dissipation of the excitation power within the structure. This is especially true when the structure has heavy damping. Small nonlinearities in the structure will consequently cause errors in the measurement of the response. With multiple input excitation, the vibratory amplitudes across the structure typically will be more uniform, with a consequent decrease in the effect of nonlinearities. A second reason for improved accuracy is the increase in consistency of the frequency response functions compared to the single input method. When a number of exciter systems are used, the elements from columns of the frequency response function matrix corresponding to those exciter locations are being determined simultaneously. With the single input method, each column is determined independently, and it is possible for small errors of measurement due to nonlinearities and time dependent system characteristics to cause a change in resonance frequencies, damping, or mode shapes among the measurements in the several columns. This is particularly important for the polyreference modal parameter estimation algorithms that use frequency response functions from multiple columns or rows of the frequency response function matrix simultaneously. An additional, signiﬁcant advantage of the multiple input excitation is a reduction of the test time. In general, using multiple input estimation of frequency response functions, frequency response functions are obtained for all input locations in approximately the same time as required for acquiring a set of frequency response functions for one of the input locations, using a single (5-16) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + input estimation method. Another potential advantage of the simultaneous measurement of a number of columns or rows of the frequency response function matrix is the ability to use a linear combination of frequency response functions in the same row of the matrix in order to enhance speciﬁc modes of the system. This technique is analogous to the...
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- Fall '11