Unformatted text preview: ns that the acceleration of the armature will follow the excitation signal. Therefore, if a zero voltage signal is sent to the exciter system, the exciter will allow the armature to move, preventing any force to be applied by the exciter system. The characteristic of a voltage feedback exciter system for a burst random exciation is shown in the following ﬁgures. Note the difference between the desired burst random signal and the actual force measured. 1.5 1 0.5 0 -0.5 -1 -1.5 500 1000 1500 2000 Figure 5-18. Burst Random - Signal to Shaker (5-52) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + 1.5 1 0.5 0 -0.5 -1 -1.5 500 1000 1500 2000 Figure 5-19. Burst Random - Signal from Load Cell (Voltage Feedback)
-3 6 4 2 0 -2 -4 -6 500 1000 1500 2000 Figure 5-20. Burst Random - Signal from Accelerometer For very lightly damped systems, the burst length may have to be shortened below 20 percent. This may yield an unacceptable signal to noise ratio (SNR). The number of power spectral averages used in the burst 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. plus the exciter/ampliﬁer system trying to maintain the input at zero (voltage feedback ampliﬁer in the excitation system). Slow Random - The slow random signal is an ergodic, stationary random signal consisting only of integer multiples of the FFT frequency increment. This signal behaves just like the pseudo random signal but without the frequency shaping of the amplitude. The slow random signal is generated by cyclic averaging a random signal in order to produce digitally comb ﬁltered excitation signal(s) with the proper characteristics. (5-53) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + MOOZ Random - The MOOZ random signal is an ergodic, stationary random signal consisting only of integer multiples of the FFT frequency increment frequency band limited to the frequency band of a ZOOM fast Fourier transform (FFT)(F min to F max ). The MOOZ (ZOOM spelled backwards) random signal requires synchronization between the data acquisition and the digital-to-analog converter (DAC). The MOOZ random signal is essentially a slow random excitation signal adjusted to accommodate the frequencies of a ZOOM FFT. The relationship between delay blocks and averages for some of the most commonly used random excitation methods are summarized in Table 5-3. TABLE 5-3. Excitation Characteristics (5-54) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + Hybrid Random Excitation Methods
Several random excitation methods have recently been demonstrated that are hybrid methods involving combinations of burst random and pseudo random, burst random and periodic random together with cyclic averaging. Burst Pseudo Random - Figure 5-21 shows the energy content of a hybrid excitation method that combines pseudo random with burst random. This excitat...
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