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Unformatted text preview: This signal is normally generated by the application of a static force through a cable. The cable is then cut or allowed to release through a shear pin arrangement [39] . Table 5-4 summarizes the advantages and disadvantages for the most commonly used excitation signals. Excitation Signal Characteristics Steady Pure Pseudo Periodic Fast Impact Burst Burst State Random Random Random Sine Sine Random Sine Minimize Leakage Signal-to-Noise Ratio RMS-to-Peak Ratio Test Measurement Time No Very High High Very Long No Fair Fair Good Yes * No Yes No Yes Fair Fair Very Short Yes * Yes * No No Yes Fair Fair Fair Yes * No Yes No Yes Yes Yes Yes Fair Fair High Low High High Low High Fair Very Very Very Short Short Short Yes * Yes * No Yes No No No No Yes * Yes * No Yes Yes * No Yes No Controlled Frequency Content Yes Controlled Amplitude Content Yes Removes Distortion Characterize Nonlinearity * Special Hardware Required No Yes TABLE 5-4. Summary of Excitation Signals (5-59) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + 5.3.6 Excitation Example - H-Frame The following example presents a single FRF measurement on an H-frame test structure in a test lab environment as a representative example. The configuration of the test involved two shaker locations (inputs) and eight response accelerometers (outputs). The test results are representative of all data taken on the H-frame structure. This H-frame test structure is very lightly damped and has been the subject of many previous studies. For all FRF measurement cases, the same test configuration was used. Sensors were installed and left in place; no additions or changes were made to the test configuration other than altering the excitation, averaging and digital signal processing parameters. Therefore, any changes in the FRF measurements are assumed to be due to the change in measurement technique and not due to a test set-up variation. The test results were repeated to be certain that the results are representative. All FRF measurements are estimated using the H 1 estimation algorithm using 1024 spectral (frequency) lines of information. The frequency bandwidth is from 0 to 250 Hertz for the 1024 spectral lines; only the first 80 % of the spectral lines (0 to 200 Hertz) are displayed in order to exclude the data affected by the anti-aliasing filters. The FRF data is plotted with phase above log magnitude. The log magnitude portion of the plot also contains the relevant multiple coherence plotted on a linear scale in the background. The log magnitude scaling is annotated on the left side of the plot and the multiple coherence scaling is annotated on the right side of the plot. Fourteen representative cases were measured on this structure. The relevant excitation and digital signal processing characteristics of each case are shown in Table 5-5. (5-60) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + Case Signal Type Frequency Shaping No No No Pseudo No Pseudo No Pseudo No No No No Pseudo No Per...
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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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