FFT Vibration Data 2

[Croco76]

thread384-257416
Am new to vibration and any help is greatly appreciated. I have around 1.4 million points of data taken over every 0.002,0.003 secs. and g's (Acceleration) in both X,Y&Z. I need to develop a vibration profile.
How do I transform these (g's vs Time) to Amplitude vs. Freq and eventually to get a PSD vs Freq?

Am familiar with matlab but don't know much about FFT and vibration profiles.

 

[Strong]

Your sample rate was only 500 samples/sec.. The F-max would be 250 Hertz or less. Was there a good anti-alias filter ahead of the A/D converter? If not, then don't waste you time converting to the frequency domain.

Walt

 

[Croco76]

Thanks Walt. I'm a Thermal Analysis/CFD guy and very naive in mechanical/vibration stuff. Our vibration guy is away and a client needs this data asap. I have no clue as to what anti-alias means..... so if I dont have to go thru. a freq. domain, how do i get this data (Time, g's_x, g's_y & g's_z) into a vibration profile?
1. Do I just plot g^2/freq vs. freq?
2. Is freq. just 1/time?
3. With over 1million points of data, how do i deal with the frequencies.
5. How did you determine that F-max < 250 Hz?

 

[Strong]

Croco,

You don't have any easy task. Here is a link to FFT references:

http://www.fftw.org/links.html

Q1: Yes after FFT
Q2: Yes, but you can't simply take reciprocal of waveform to get spectrum
Q3: Typically time data is divided into data blacks like 1024 points
Q4: Nyquist, another term you need to know

I use DasyLab for general signal analysis. Perhaps you could do your job with a download demo of that or similar software. You must be ready for an education, either way.

Walt

 

[Strong]

Croco,

Another good reference, see related posting about PSD:

http://cp.literature.agilent.com/litweb/pdf/5952-8898E.pdf

Walt

 

[izax1]

vibrationdata.com will have all the SW (Standalone) you need, but Walt is right, you need to know what you're doing.

 

[Tmoose]

"How did you determine that F-max < 250 Hz? "
page 29 in the HP/Agilent Application note 243 is a start.

For sample rate I think about the aliens that are monitoring my behaviour. If they take a sample once every 24 hours, and the first sample happens to be at about 2 in the morning, in their report they will conclude that I sleep all the time. They need to sample at a much faster rate to capture the details of my exciting, fast paced existence.