Spectrum anaysis of an accelerometer

[mehdi33]

HI,

I have a signal from an accelerometer which is mounted on a rotating part  ( 2000 rev/ min = 33.33 Hz). After filtering the signal and deducing the displacement by double integration , I tried to do a FFT spectrum analyzer , but the result is wrong. Normally I should have the first peak at 33.33 Hz (1 X) , but the result shows a noise between 5 and 40 Hz!

Here is my program VI and the TDMS file.

Could someone help me please!

[mikeporter]

The first thing is to look at the accelerometer data in the time domain to check noise levels. Accelerometers are notorious for noise problems.

Mike...

[mehdi33]

Hi Mike,

Thanks for your answer. I am a beginner in labview and I don't know how to check noise level of my signal and how to remove it . Could you help me please 🙂

[billko]

It's not a LabVIEW thing; rather it's understanding how to filter an accelerometer signal - something best done in hardware.

[mikeporter]

You are acquiring the data in the time domain. Graph the data coming from the DAQ.

Mike...

[mehdi33]

Hi Bill

Is that mean that my VI program is correct and that the problem should be in the hardware ?

[mehdi33]

Hi Mike

Here is my graph in the time domain!

[Bob_Schor]

I'm a little confused.  I plotted the data from your TDMS file, which includes time stamps, and could see a prominent oscillation at 3KHz.  This is pretty high for something that you think has a fundamental at 33.3 Hz (two orders of magnitude higher, in fact).

So now it might be important to ask "Just what are you measuring"?  You talk about an accelerometer, but say nothing about whether it is a linear or angular accelerometer, and if linear, how it is mounted on the motor, whether it is on the "stationary" part of the motor (and how it is oriented with respect to other accelerations, such as the more-or-less constant acceleration due to gravity) or on the rotating part (where its orientation with respect to the axis of rotation and the orientation of that axis with respect to gravity are important factors).

Sometimes when you are dealing with data and trying to figure it out, it helps to look at the data (which is what I just did).  Plot the data.  See what happens when you take the frequency response of the data in its "raw" state (it would be very interesting to see if you get most of your power at 3KHz -- please report back to us).

As I'm sure you know, integrating a signal emphasizes the low frequency components while attenuating those at higher frequency -- indeed, a first-order low-pass filter is sometimes called a "leaky integrator".  Before turning an integrator loose on your data, you should see what is there first.

Bob Schor

[mikeporter]

Ewwwwwww... No wonder you aren't seeing anything. Your first job is to clean up all that noise. As Bob indicated you need to bandlimit your input. A simple lowpass RC filter will work for that. You also need to look at how you are connecting to the accelerometer.

How are you connecting to it?
What is your sample rate?
What DAQ device are you using?
How far is your computer from the accelerometer?
What is your engineering background?

Mike...