How should I convert data in 2D array to a time domain signal for performing FFT?

[vgrchada]

I have strain and time data in the 2D array and now I need to perform FFT. FFT Spectrum (real imag) or (magnitude phase).vi has the input from a time domain signal, which is a cluster of time data and Y axis data. How can I convert my data in the same form, so that I can perform FFT with hanning window?

Please open the excel sheet. To briefly explain you what's there in it, the column 1: sample points; column 2: time in seconds; column 3: strain gage values at 26in from one end of the simply supported beam; column 4,5....34: strain gage values at 66in,86in,....646in from one end of the simply supported beam, respectively. The attached vi file is used get the excel values to Labview and show the waveform chart. Open the vi, then give the file path to the excel sheet. In the start cluster control, put Column 2 and Row 620. In the end cluster column, put column 3 and Row 845. From the waveform chart, The values of each strain gage from the range of 6 to 8.5 seconds will be ploted in waveform chart. Now, I need to perform FFT with hanning window.

Note:The waveform chart X-axis is showing the sample points instead of time in seconds.

[vgrchada]

I forgot to tell you that the FFT spectrum (real imag) or (magnitude phase).vi are nothing but the FFT icons that are available in any version of LabVIEW. I attached an other vi to this message, where you will see the time domain signal is in the cluster and wired to all the cases in the FFT case structures. All I need is to convert my 2D array same as time domain signal and wire it to FFT.   

Thank you

[johnsold]

Index Array will select the strain channel you want.  The only reason you need the time channel is to determine the sampling rate. Take two consecutive elements and subtract to get dt. The sampling frequency fs = 1/dt.

The Amplitude and Phase Spectrum.vi accepts 1D arrays as inputs and produces 1D arrays as outputs.  You can use the df output to scale a graph of the spectrum.

Lynn

[vgrchada]

Hi Lynn, thanks for your suggestion. I did everything as you said, except the output of the magnitude is used to scale a magnitude spectrum instead of df output. Please open the vi and assign the path to excel sheet. Now, in the start cluster assign 3 to Column, 620 to Row, in the end cluster assign 4 to Column, 845 to Row. Now run the vi. You will see the leakage in the magnitude graph. Where can I put the windowing for amplitude and spectrum.vi so that I can avoid leakage?

[vgrchada]

Use the excel sheet attached below but not the previous file. Thank you.

[johnsold]

If you open the Amplitude and Phase Spectrum.vi block diagram you will see that the first thing it does is to take a standard FFT. So just put the Window VI in the array line at the input to the Spectrum VI.

Lynn

[vgrchada]

Thank you Lynn. I appreciate for sending me the pictorial answer too. However, it seems to have a strange problem. The windowing of the signal should reduce the leakage but not the magnitude.

The magnitude of the signal has dropped from 4.1 to 2.3 by using the window. What do you say?

After I get that problem solved, the next step is to arrange those peak magnitude values according to the beam distances (26in, 46in,....646in) and join those magnitudes with a smooth curve to form the first mode shape.

Venu

[johnsold]

Venu,

I do not have time to dig into the details now but the window process significantly modifies the time domain signal. This graph adds the third plot showing the output of the Hanning Window VI.

Lynn

[vgrchada]

Thanks for your suggestions so far. I understand that the further questions I asked are not pertinent to this discussion. Excel and many other softwares are taking long time to do anlysis. So, I choosed labVIEW to perform analysis on enormous data. Anyway, I sincerely appreciate for what you did. Let me know if somebody can give suggestions to me with my other questions.

Venu