FFT Integration of Accel Signals
FFT Integration of Accel Signals
(OP)
Using a 4-channel OR24 analyzer with high sensitivity accels. Application is building vibration.
My analyzer will digitally integrate - no problem. But the frequencies of interest are low (<10Hz).
When I integrate to the required (by the client) velocity, I see what looks like a ski slope. That does not show up in the raw, unintegrated signal. My accels' are supposed to be accurate to less than a Hz. And the ICP cut-off frequency on my analyzer is 0.35Hz.
Is the integrated-to-velocity spectrum accurate at the very low frequencies? And, if so, is there a simple way to explain the low frequency slope that a non-FFT user can understand?
My analyzer will digitally integrate - no problem. But the frequencies of interest are low (<10Hz).
When I integrate to the required (by the client) velocity, I see what looks like a ski slope. That does not show up in the raw, unintegrated signal. My accels' are supposed to be accurate to less than a Hz. And the ICP cut-off frequency on my analyzer is 0.35Hz.
Is the integrated-to-velocity spectrum accurate at the very low frequencies? And, if so, is there a simple way to explain the low frequency slope that a non-FFT user can understand?





RE: FFT Integration of Accel Signals
Incidentally I am a little bit suspicious of the frequency response of your accelerometers.
Bear in mind that digital integration is just a case of dividing by the frequency, so any low frequency noise will suddenly become apparent.
Cheers
Greg Locock
RE: FFT Integration of Accel Signals
http://www.wilcoxon.com/knowdesk/ACF217.pdf
Integration increases the effect of any broad-band low-frequency noise. If noise is present due to sensor overload from heavy impacting or mounting problems, ski slope will be there.
Even without these problems the normal noise can be a problem if you are particularly interested in low frequency applications. Some discussion on the subject of overcoming ski slope monitoring low-speed equipment using SKF monitoring equipment:
http://www.skfcm.com/service/support2/New%20Library/CM3051%20CMVA60%20ULS.pdf
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RE: FFT Integration of Accel Signals
On our various general purpose machinery monitoring data collectors/analyzers To make the best of the situation we have to supply power to the accelerometer for at least 30 seconds before acquiring data. This was very apparent monitoring a double integrated overall signal. Even with continuous power The overall value was HUGE for several seconds. On later collector versions we had a double-key-strike option allowing us to initiate the data collection only when we were ready, after the overall had settled out, which reduced the ski-slope nicely. Still not at the low frequencies you are attempting to monitor.
When I used an earlier collector with a neck-strap I found my normal fidgeting could induce a huge ski-slope. I was bending the cable at ~ 1 Hz.
More than one accelerometer vendor has mentioned both mechanical and thermal settling-out times are required to avoid spurious ski-slope. Maybe one of these would help?
http://www.foxunlimited.com/Fur-Girls.jpg
RE: FFT Integration of Accel Signals
This charge amp model performs integration on the analog acceleration signal in the time domain after passing the signal through an analog high-pass filter. The high-pass filter frequency has several settings. A typical setting might be 1 Hz. The filter removes the DC offset which would otherwise cause the slope effect in the velocity signal.
Tom Irvine
www.vibrationdata.com
RE: FFT Integration of Accel Signals
Cheers
Greg Locock