get minimum frequency from FFT of three signals

[Norbert_B]

fesmi.,

please step back a bit and tell us what you have to do.

All i know is that you are trying to accomplish a closed control loop whose input is a set of frequencies. This by itself is not easy and the code in your docx does not perform something like this.

Attached you can find the screenshot from my post as png file, so i hope you can now take a look into it.

As already mentioned, the screenshot does not refer to your original question, it refers to the complexity of performing FFT for a control loop....

Regarding your current issue: The result of FFT is represented in complex numbers. So you simply cannot display them in a simple graph, you have to either use a complex graph (e.g. polar plot) or you have to select the information which can be presented in real numbers, just like the magnitude.

hope this helps,

Norbert

[ruser]

Dear Nobert,

Thankyou for your help,

I could also open and view the image; thanks again...

My project in detail is :

I have a suspended whirling disc. A sensor is placed on the disc and senses and logs the linear acceleration data about the x, y and z axis. It logs these data as an excel file with the time column  and accleration columns; so I have an excel file with 4 columns.

I require to find the FFT of these accelration data in approximately, every 20 seconds ; then find the frequency at which the spectrum magnitude is minimum for all the three FFTs. This frequency is to be used by the developed controller.

In the block diagram , i had send you, i was trying offline to find the fft and find the frequency for the logged acceleration data.

The problem was i could not view the FFts; and i tried to build an array of the FFT block output with the 'build array' function and tried to sum up the elements using the 'compound arithmetic' block to add the elements of the FFT arrays element wise...

This just wasnt working... i could not find another add array element wise block either... so i was lost .. completely...

I shall try the " FFT magnitude and phase block " ; ..

Thanks,

Regards,

fesmi.

[ruser]

Hi Nobert,

I have also attached a sample acceleration data file; the original txt file ...just so you get a clearer picture of the data too..

regards,

fesmi

[Norbert_B · Accepted solution]

fesmi,

ah, i see. So you are not actually controlling something, because writing/reading files and doing lots of computations on this on a Windows system is not a deterministic procedure. So it is more or less an offline analysis and representation....

Looking into the data you are using, i have serious doubt that you are able to achieve what you are looking for. The reason for this is that the signals you have are non-periodic and change only over a "fragment" of the whole dataset. So the result of the FFT will return that the main frequency is close to 0 Hz (DC).

Please discuss the whole approach with your supervisor since i am confident that you will not be able to extract the information you are looking for from that kind of data.

hope this helps,

Norbert

[ruser]

Dear Nobert,

Thanks for the advice,

I discussed your advice with my advisor; he also remarks similarly...

But on second thoughts;...the data i send you was after the closed loop control was activated; however if it was before applying the controller; the data would be as in the attached file; would it still be a problem to find the FFT? The second time column is the correct one to be used...

thanks,

regards,

fesmi

[Norbert_B]

fesmi,

the FFT does "virtually" add the same signal at the front and the back again and again. Since your signal does include a part where the disk was nearly "at rest", you will add this part again and again. So it will modify the result of the FFT in a negative way.

Norbert 

[ruser]

Ohh.. okay, thanks for the clarification....

regards,

Fesmi