Test data and acceleration signal
Test data and acceleration signal
(OP)
Hi,
We are going to measure accelerations from piece of machinery. Goal really is displacement by double integration.
Before attempting this, I have done some preliminary work with simple cantilever beam. In short, I glued a strain gage to fixed end and mounted single axis accelerometer to the free end. I set up beam into decaying vibratory motion.
I calibrated system such that I know displacement at the end given strain (beam theory holds...checked.) We stay linear the whole time.
First few amplitudes are about 2 gs at 10Hz. I cannot get free end velocity integrated from acceleration signal to match even close. Not to even mention displacement.
I am not filtering acceleration signal because it is well-defined.
Does anyone see what is being done wrong here? Does decaying signal introduce a problem here, maybe?
We are going to measure accelerations from piece of machinery. Goal really is displacement by double integration.
Before attempting this, I have done some preliminary work with simple cantilever beam. In short, I glued a strain gage to fixed end and mounted single axis accelerometer to the free end. I set up beam into decaying vibratory motion.
I calibrated system such that I know displacement at the end given strain (beam theory holds...checked.) We stay linear the whole time.
First few amplitudes are about 2 gs at 10Hz. I cannot get free end velocity integrated from acceleration signal to match even close. Not to even mention displacement.
I am not filtering acceleration signal because it is well-defined.
Does anyone see what is being done wrong here? Does decaying signal introduce a problem here, maybe?





RE: Test data and acceleration signal
Also, integrating accel. twice to get disp. is not really a good idea. It will be difficult to get anything worth while.
Fe
RE: Test data and acceleration signal
RE: Test data and acceleration signal
FeX32 - Yes, sampling frequency checks. What would you recommend as better approach?
BobM3 - Acceleration is decaying sine wave. Yes, velocity is sine wave as well. Also, velocity is at zero at max. acceleration points...as it should be. Amplitude is way off. Integrating this velocity signal produces nearly linear ramp.
This is quite interesting little experiment, because it appears so simple. School teaches double integration to get displacement...real life is not so simple. Thanks.
RE: Test data and acceleration signal
Can you use a proximity sensor? I've used these for measuring the displacements of gears on a timing drive before.
- Steve
RE: Test data and acceleration signal
RE: Test data and acceleration signal
If your signal is naturally windowed like this then Fourier transform the signal (with no other windowing), high pass filter to remove DC and as many lower specral lines as you dare. Do the integration in the frequency domain (divide by minus omega squared) then inverse Fourier transform back to time domain.
You might also want to filter out some of the very high frequnency crud as this will probably just be noise.
I have done this quite successfully a number of times before. It works best if you design your test with this type of processing in mind. i.e have plenty of pre-trigger on the signal and leave plenty of time for the decay. Then you can apply a 1/2 cosine taper window to the start and end of the signal to make sure you don't get leakage.
M
--
Dr Michael F Platten
RE: Test data and acceleration signal
RE: Test data and acceleration signal
MikeyP - Yes, signal is exactly as you describe. Thanks a lot.
RE: Test data and acceleration signal
Are you integrating in the time domain or the frequency domain?
Cheers
Greg Locock
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RE: Test data and acceleration signal
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RE: Test data and acceleration signal
Fe
RE: Test data and acceleration signal
M
--
Dr Michael F Platten
RE: Test data and acceleration signal
In the real problem this needs to be fixed, so frequency domain integration may be the key like MikeyP said. I am having problem processing signal in frequency domain as I am quite new to this. I get back.