How do the sampling during run up of machines?
How do the sampling during run up of machines?
(OP)
Hello everyone!
I am a student, and do some research on time-frequency analysis of vibration.So, the speed-depend vibration data during run up of machines is important to me.Recently, I did a vibration test about run up of machinetools.What I wanted to attain was the sampling data of non-stationary speed-depend vibration data.I used the BK 2635 charge amplifier and BK 4370 accelerometer which mounted on the shell of the gearbox in test.But the test was failed, the signal related to run up was not find in time-frequency plane.Please tell me how I can get a good vibration data relate to run up.Thanks!
Your Sincerely!
Gary
I am a student, and do some research on time-frequency analysis of vibration.So, the speed-depend vibration data during run up of machines is important to me.Recently, I did a vibration test about run up of machinetools.What I wanted to attain was the sampling data of non-stationary speed-depend vibration data.I used the BK 2635 charge amplifier and BK 4370 accelerometer which mounted on the shell of the gearbox in test.But the test was failed, the signal related to run up was not find in time-frequency plane.Please tell me how I can get a good vibration data relate to run up.Thanks!
Your Sincerely!
Gary





RE: How do the sampling during run up of machines?
Cheers
Greg Locock
RE: How do the sampling during run up of machines?
Your Sincerely!
gary
RE: How do the sampling during run up of machines?
First to know is the total run-up time.
Then you divide it in +- 20 to 50 pieces
In each part you take a mesurement, attention there is a relation between number of lines, max freq and sampling time.
The problem is the FFT, this algoritme thinks that the speed is constant so it works badly with a variabel speed.
Good luck
DePauw
RE: How do the sampling during run up of machines?
Get round the FFT smearing problem by making a lot of short frames of data, overlapped by 50-75%, then calculate the spectrum of each. Then take the spectrum of the magnitude of the spectrum (this is called cepstrum analysis). This will give you a time domain function with some peaks in. Hopefully the tallest peak will be at the inverse of the frequency of your instantaneous rotational speed. This works because it detects the regular harmonic structure of your signal.
This will allow you to build a map of rotational speed against time for your entire sweep. Obviously it may need to be smoothed or massaged.
You can then re-analyse the original data in any manner you see fit, you might like to heavily oversample the original data and then synchronously sample it using your speed map, for instance.
A google search on cepstrum may be educational (or it may be a complete blank)
Cheers
Greg Locock
RE: How do the sampling during run up of machines?
I guess the reasons lead to my test failed were as following:
1.The vibration during run up is too weak.I do the test in a factory,the machinetool would do a cutting work when run-up is end,the vibration was get the max when cutting.And I set the gain of amplifier according the max vibration.
2.The mounting position of the accelerometer is bad.
Thanks again! Any new suggestion are welcome!
Your Sincerely!
Gary
RE: How do the sampling during run up of machines?
Cheers
Greg Locock
RE: How do the sampling during run up of machines?
Sampling rate is 50 kHz, thus this mean fmax=20kHz?
If fmax=20kHz and you have 3200lines the resolution is 6Hz, this is 180/min, so your first line is 180/min, the second 360/min , you will have only 15 significant lines for an run-up 0-3000/min, this looks no good.
Good luck.
DePauw
RE: How do the sampling during run up of machines?
Dear GregLocock
Do you means I can sent the run-up sampling data to you for a help? The data file not very big about 1.2Mb.If you like, I can do it and send you some plots of my analysis result at the same time.But tell me how I send the data to you. Thanks!
Your Sincerely!
Gary
E-mail address:km_gy@yahoo.com.cn
Dear DePauw
In my test, fmax<1KHz,and I do a downsampling for a better frequency resolution. For example, I can set the down sampling rate is 128,you can calculate the resolution, I think it can stand for instantanous speed.Of couuse, a digital anti-aliasing filtering need be done when down sampling. The hardware anti-aliasing filtering is simplyfied, just as GregLocock mentioned.
Your Sincerely!
Gary
E-mail address:km_gy@yahoo.com.cn