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Unformatted text preview: guration when an impact form of excitation is to be used. (5-35) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + Figure 5-10. Typical Test Conﬁguration: Shaker (5-36) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + Figure 5-11. Typical Test Conﬁguration: Impact Hammer Single and multiple input estimation of frequency response functions (FRFs) via shaker excitation has become the mainstay of most mechanical structure measurements, particularly in the automotive and aircraft industries. While there are appropriate occasions for the use of deterministic excitation signals (sinusoids), the majority of these measurements are made using broadband (random) excitation signals. These signals work well for moderate to heavily damped mechanical structures which exhibit linear characteristics. When the mechanical structures are very lightly damped, care must be taken to minimize the leakage error so that accurate frequency response function (FRF) data can be estimated in the vicinity of the modal frequencies of the system. Frequently, when random excitation methods are compared to deterministic methods (sinusoids), the comparisons are questionable since proper procedures for eliminating the leakage error have not been followed. Historically, a number of random excitation signals have been utilized, together with appropriate (5-37) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + digital signal processing techniques [1-5], to obtain accurate FRF data. The most common random signal that is used in this situation is the pure random signal together with a Hann window. This signal is normally generated by the data acquisition system utilizing built-in random signal generator(s) or via external random signal generator(s). While this approach does not eliminate the source of leakage and the effect of applying the Hann window must be considered, this approach is normally considered as a baseline random excitation method for estimating FRF measurements since this method is available with almost any data acquisition system. Other forms of random signals (pseudo random, periodic random, burst random, etc.) utilize more control or frequency shaping of the excitation signal(s) and generally require digital-toanalog (DAC) converter(s). For this reason, some of these alternate methods are infrequently available and therefore not used. This is unfortunate since these methods often yield a superior FRF measurement in less total testing time. When FRFs are measured on lightly damped systems, great care must be taken to eliminate the leakage error. Regardless of the type of excitation signal hardware involved (random signal generator or DAC), there are random excitation methods that can nearly eliminate the leakage error. In some cases, one approach will be superior on the basis of minimizing the total test time but on the basis of accurate, leakage-free FRFs, one of the methods will always work if test time can be sacriﬁced. Note that these altern...
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