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Unformatted text preview: ency response function, is often a direct indicator of a leakage problem. Frequently, comparisons are made between results obtained with narrowband (sinusoid) excitation and broadband (random) excitation when the ordinary/multiple coherence function clearly indicates a potential leakage problem. It is important that good measurement technique be an integral part of such comparisons. 5.3.7 Impact Excitation
Impact testing is an attempt to match the input and output data to the requirement of the discrete or fast Fourier transform that the data be a totally observed transient in the observation time (T ). While the impact is almost always totally observable, the response for lightly damped systems may not be. Special windows are often used for impact testing that accommodate the characteristics of the transient input and the response of the system to a transient input. (5-70) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + Impact excitation is widely used due to the minimal equipment required, portability and low cost of the impact devices and broad applicability to both small, medium and large size structures. However, impact testing also suffers from limitations imposed by the human control of the impact. Repeatability and consistency of the impact (force and direction) cannot be guaranteed, particularly as the test becomes long and repetitious. Care must be taken to ensure that the impact and response is not too small, not too large (overload) and that there is only one impact per observation period. When impact testing is used, windows are generally required on both the force and response data in order to minimize different errors. The force window is used to eliminate the signal coming from the impact device after the short duration impact is over. This eliminates electrical noise and spurious output from the hammer during data acquisition that is caused by motion of the impact device that does not put force into the system. The response (exponential) window is used to force the response closer to zero by the end of the observation period (T ) and should be used carefully. If the response is already near zero at time T , no response window should be added. To be theoretically correct and to allow for the effects of this response window to be accounted for, the decay rate of the exponential must be recorded and the same window should also be applied to the input data, in addition to the force window. Force Window Force windows are used to improve the signal-to-noise problem caused by the noise on the input channel measured after the impact is completed. Note that the exponential window used on the response should also be applied to the input in addition to the force window. (5-71) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + Force Window 1 0.8 Amplitude 0.6 0.4 0.2 0 −0.2 0 0.2 0.4 0.6 Time: (0−T) 0.8 1 Force Window * Exponential Window (10%) 1 0.8 Amplitude 0.6 0.4 0.2 0 −0.2 0 0.2 0.4 0.6 Time: (0−T) 0.8 1 Figure 5-39....
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