jimmyjimbob
Military
- Jan 13, 2004
- 5
Carried out ( to required spec) a low frequency vibe evaluation and test on a unit. Then carried out shock pulses and left the monitor accelerometers on to measure shock response. Customer asked why there was a marked response on the shock measurements on the monitor channels whereas there was no response on the sine sweeps.
My explanation was that the shock tests being short duration have excited higher frequencies than the vibration test was allowed to check for. ( remember I am dictated to keep within the limits of a tight customer produced specification) I suggested that we could sine sweep to a higher frequency just to check.
My question is coming up now. I was able to use cursors on my vibe controller to measure the ''time'' between the response peaks and 1/t=f therfore tell him the frequency of the prominent sinusoidal response. I suggested that the frequencies that are generated by the shock pulse as I had been told were up to 1/duration, so for a 6ms duration half sine pulse we would excite frequencies up to about 167Hz. However other engineers in my company say that the 6ms is only half the 'sine wave' and therfore the total cycle is of 12 ms duration for calculation and therefore the frequencies excited are up to 83 Hz.
Can someone please help put my mind at ease with the definitive truth here. I seem to remember FFT ing a half sine pulse and generating a lot of 'lobes' in the frequency domain dipping to zero at 1/t, 2/t, 3/t etc but each lobe diminishing in amplitude from its predecessor and there was something about -3db points too.
Anyone help?????
My explanation was that the shock tests being short duration have excited higher frequencies than the vibration test was allowed to check for. ( remember I am dictated to keep within the limits of a tight customer produced specification) I suggested that we could sine sweep to a higher frequency just to check.
My question is coming up now. I was able to use cursors on my vibe controller to measure the ''time'' between the response peaks and 1/t=f therfore tell him the frequency of the prominent sinusoidal response. I suggested that the frequencies that are generated by the shock pulse as I had been told were up to 1/duration, so for a 6ms duration half sine pulse we would excite frequencies up to about 167Hz. However other engineers in my company say that the 6ms is only half the 'sine wave' and therfore the total cycle is of 12 ms duration for calculation and therefore the frequencies excited are up to 83 Hz.
Can someone please help put my mind at ease with the definitive truth here. I seem to remember FFT ing a half sine pulse and generating a lot of 'lobes' in the frequency domain dipping to zero at 1/t, 2/t, 3/t etc but each lobe diminishing in amplitude from its predecessor and there was something about -3db points too.
Anyone help?????
lease see FAQ731-376 for tips on how to make the best use of Eng-Tips.