Unformatted text preview: pletely ergodic, stationary random signal but contains properties of both signal types. The frequency spectrum of this signal has random amplitude and random phase distribution and contains energy throughout the frequency spectrum. This characteristic is shown in Figure 5-17. The difference between this signal and the random signal is that the random transient history is truncated to zero after some percentage of the observation time (T). Normally, an acceptable percentage is ﬁfty to eighty percent. The measurement procedure duplicates the random procedure but without the need to utilize a window to reduce the leakage problem as long as both the input and output decays to zero in the observation time (T ).
0 Power Spectrum − Burst Random 10 −1 Magnitude
−2 10 −3 0 5 10 15 Spectral line (bin) 20 25 30 Figure 5-17. Signal Energy Content - Burst Random The burst length (0-100%) is chosen so that the response history decays to zero within the observation time (T). For moderate to heavily damped systems, the response history will decay to zero very quickly due to the damping provided by the system being tested. These systems do not cause a leakage error in the ﬁrst place. For lightly damped cases, burst random will force the response to decay to zero in the observation time (T ) primarily due to the exciter system characteristics. Exciter systems, particularly electromagnetic, attempt to match the excitation signal to some physical characteristic of the exciter. Typically, this means that the displacement, velocity or acceleration of the armature of the shaker will attempt to match the excitation signal. (Note that this is normally an open loop control process; no attempt is made to exactly match the excitation signal.) Electromagnetic shaker systems work either in a voltage or current feedback conﬁguration in order to control the shaker according to the desired input signal. Voltage feedback refers to the type of ampliﬁer in the exciter system that attempt to match the voltage (5-51) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + supplied to the shaker to the excitation signal. This effectively means that the displacement of the armature will follow the excitation signal. Therefore, if a zero voltage signal is sent to the exciter system, the exciter will attempt to prevent the armature from moving. This damping force, provided by the exciter/ampliﬁer system, is often overlooked in the analysis of the characteristics of this signal type. Since this measured input, although not part of the generated signal, includes the variation of the input during the decay of the response history, the input and response histories are totally observable within the sample period and the system damping that will be computed from the measured FRF data is unaffected. Current feedback refers to the type of ampliﬁer in the exciter system that attempt to match the current supplied to the shaker to the excitation signal. This effectively mea...
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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.
- Fall '11