Displacement from accelerometer signal
Displacement from accelerometer signal
(OP)
Hello.
I'd like to have your advice about the integration of the acceleration signal to obtain the displacement.
I tried to integrate in the frequency domain by taking the fft of the acceleration signal, dividing it by -omega^2 and the displacement should be the inverse fft of the result. However the results are not good.
What do you think?
Thank you very much!
I'd like to have your advice about the integration of the acceleration signal to obtain the displacement.
I tried to integrate in the frequency domain by taking the fft of the acceleration signal, dividing it by -omega^2 and the displacement should be the inverse fft of the result. However the results are not good.
What do you think?
Thank you very much!





RE: Displacement from accelerometer signal
RE: Displacement from accelerometer signal
The accelerometers are mounted near the bearings of a rotor and I need to have the orbits.
I've already done the numerical integration but the result is drifted. If I filter the signal before performing the numerical integration, do you think it would help?
I thought about frequency domain integration because it is the procedure used on the signal analyzers, isn't it?
Thank you!
RE: Displacement from accelerometer signal
RE: Displacement from accelerometer signal
RE: Displacement from accelerometer signal
RE: Displacement from accelerometer signal
I've just finished to integrate my signal by using frequence domain integration and a time domain integration.
To validade the results I installed a proximity probe near the accelerometer and I took one set of measures.
The best results of integration was given by the frequency domain method, although there's an error of 10% on the amplitude.
The time domain integration was carried out with a trapeze integration and there was a lot of drift. Filtering the low frequencis did help but it was not enough.
I'm not writing this to go against your advice, please don't think so.
Thank you!
RE: Displacement from accelerometer signal
RE: Displacement from accelerometer signal
http://www.endevco.com/main/news/2775b.php
Filter Integrator Module 35818 provides a capability for single or double integration of the input vibration signal.
Also contact: http://www.wilcoxon.com
Be careful of very low frequency noise - see: http://www.wilcoxon.com/technotes/lowfreq.pdf
There once was a consultant telling one of our customers that our Elliott Plant air compressor had high vibration with very low frequency for some reason. When I went to the plant, converting the signal to displacement (that really was just electrical noise) would have meant the compressor was jumping up and down about one foot. Proper isolation from ground of our own accelerometer eliminated the noise.
Can also doublecheck results by hand after obtaining acceleration frequency spectrum, using this calculator:
http://www.wilcoxon.com/knowdesk_calculator.cfm
RE: Displacement from accelerometer signal
For my particular applications, I tend to use frequency domain integration wherever possible. However, I ensure that the signal I am integrating is in some sense "periodic" (e.g. starting and ending at zero). This means that you don't have to window the signal prior to the Fourier Transform and hence you avoid the corruption of the signal through convolution with the window function.
So in an experimental situation I would start acquiring data while the structutre is a rest, set the structure vibrating with my test signal, then switch off the test signal and allow the vibrations to decay away. The response would then be "leakage-free" as far as the Fourier Transform was concerned.
Of course, this may not be practicable in your situation and you may have no control over the excitation of the structure.
Remember that accelerometers (except for some specialist ones) have very poor response at very low frequencies, so you can filter out the first few (probably meaningless) spectral lines in the frequency domain to elimiate the problems of large DC terms.
M
--
Dr Michael F Platten
RE: Displacement from accelerometer signal
Thread384-70681
I think there are others
RE: Displacement from accelerometer signal
http://twiki.psas.pdx.edu/pub/PSAS/ImuCalcs/numerical.p...
As I said before, I believe it is possible to eliminate most of the drift after the first integration, since you are dealing with a cyclic situation of known frequency. In my view, this is a more accurate technique than using filters, which will inevitably destroy some good information.
RE: Displacement from accelerometer signal
I can't use the prox probes at all places that I have accelerometers because I have only one prox probe available.
RE: Displacement from accelerometer signal
RE: Displacement from accelerometer signal
http://mathworld.wolfram.com/BoolesRule.html
It's a very good site.
I've just found another way to integrate the signal, this one is giving the best results so far: I fit a sin function to my data and then I integrate it analitically.
I will try what you told me about removing the drift after the first integration :)
Thank you!
RE: Displacement from accelerometer signal
First of all, you must make sure that you don't have aliasing frequencies in your numeric acceleration signals. Otherwise you are bound to look at very high frequencies as very low ones that will in turn give rise to huge displacements. Modern delta-sigma ADC are very appropriate for this job unless you have high-quality antialiasing analog filters. No LP digital filters will help you here.
Second, you have to pass your numeric signal through a high-pass digital filter
Finally you integrate the output of this HP digital filter. One could place the HP filter after the integration in case your simple integration scheme produces LF compoents, which I doubt if your digital HP filters was good in the first place. If your transients are short, the integration scheme ought not be very sophisticated. Sophicitcated integration schemes also curtail the frequency band (I suspect).
Drift won't be an issue.
Never dream of getting displacement from dc on with seismic sensors like accelerometers. This violates the basic law of physics. There is no way to know how you move in the stellar space with an accelerometer. You need a frame of reference to do that like eddy probes do (their anchor point)
Hope this helps.
Gérard.