The need for accuracy equipment availability testing

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Unformatted text preview: all random signals, will be altered by any impedance mismatch between the system and the exciter. The number of power spectral averages used in the pure random excitation approach is a function of the reduction of the variance error and the need to have a significant number of averages to be certain that all frequencies have been adequately excited. 10 1 Power Spectrum − Pure Random 10 0 Magnitude 10 −1 10 −2 10 −3 0 5 10 15 Spectral line (bin) 20 25 30 Figure 5-14. Signal Energy Content - Pure Random Pseudo Random - The pseudo random signal is an ergodic, stationary random signal consisting of energy content only at integer multiples of the FFT frequency increment (∆ f ). The frequency spectrum of this signal is shaped to have a constant amplitude with random phase. This characteristic is shown in Figure 5-15. If sufficient delay time is allowed in the measurement procedure for any transient response to the initiation of the signal to decay (number of delay blocks), the resultant input and output histories are periodic with respect to the sample period. The number of power spectral averages used in the pseudo random excitation approach is a function of the reduction of the variance error. In a noise free environment, only one average (per input) may be necessary. (5-49) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + 10 1 Power Spectrum − Pseudo Random 10 0 Magnitude 10 −1 10 −2 10 −3 0 5 10 15 Spectral line (bin) 20 25 30 Figure 5-15. Signal Energy Content - Pseudo Random Periodic Random - The periodic random signal is an ergodic, stationary random signal consisting only of integer multiples of the FFT frequency increment. The frequency spectrum of this signal has random amplitude and random phase distribution. This characteristic is shown in Figure 5-16. For each average, input signal(s) are created with random amplitude and random phase. The system is excited with these input(s) in a repetitive cycle until the transient response to the change in excitation signal decays (number of delay blocks). The input and response histories should then be periodic with respect to the observation time (T) and are recorded as one power spectral average in the total process. With each new average, a new history, random with respect to previous input signals, is generated so that the resulting measurement will be completely randomized. The number of power spectral averages used in the periodic random excitation approach is a function of the reduction of the variance error and the need to have a significant number of averages to be certain that all frequencies have been adequately excited. 10 1 Power Spectrum − Periodic Random 10 0 Magnitude 10 −1 10 −2 10 −3 0 5 10 15 Spectral line (bin) 20 25 30 Figure 5-16. Signal Energy Content - Periodic Random (5-50) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + Burst Random (Random Transient) - The burst random signal is neither a completely transient deterministic signal nor a com...
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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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