02-29-2012 07:46 AM - edited 02-29-2012 07:49 AM
Hello, this is the first time I have used the forum so please be gentle.
I have adapted a Vi to interpret a data packet from an Xbee wireless module. I have successfully interpreted the 'analogue' portion of the incoming packet and displayed it in the form of a chart depicting the time domain signal.
However, as hard as I try, I have had absolutely no success when attempting to display that exact same signal by means of frequency domain analysis.
I have attached a picture of the Vi in the hope that someone may be able to help. I appreciate any comments, thank-you.
03-05-2012 02:18 PM
Hi, thankyou for your reply. The Xbee module transmits a packet of data containing 22 bytes. The value of '7E' signifies the start byte of the data packet which I am using to index the other bytes.
At this point I am only interested in the 20th and 21st byte which denotes the Most Significant Byte & Least Significant Byte of a analogue sample. I have added these two bytes together and have successfully plotted them by means of the chart depicted in the Vi.
I desperately wish to use a chartto display the same value by means of spectral analysis so I can see what frequency components exist at certain amplitudes.
My analogue sample represents a vibration signal.
I hope this makes sence and appreciate your interest!
03-06-2012 04:11 PM
I have tried many things. the closest i can get is to get a FFT graph where the amplitude along the side varies but no plot / line shows on the graph. I only need to identify frequencies below 20 Hz. The time domain graph works fine (maybe slow).
Please, if anyone can help, it would be so appreciated. I have been working on this for so long and cannot make anymore progress.
03-07-2012 12:31 AM
I'm no expert and I can't look at your latest vi as I only have access to LabVIEW 2010, but judging from your diagram I see no timing information being collected. In your diagram the waveform chart looks like it is just using a default timing. How fast are you collecting these samples? You need to create (recreate) the waveform using your samples,dT and the build waveform primitive.
Hope this helps.
03-07-2012 10:55 AM
Thanks for the reply. Im very new to labview.
I have constructed a waveform as shown in my diagram. "waveform chart 2" shows the signal in the time domain.
Is this what you meant? The Xbee samples analogue values @ 1k samples per second, but this may be not 100% acurate after transmission of the data. I think I will be able to tweak the accuracy of the system using a known waveform at the input and adjusting the sampling until the output is satisfactory.
Now. It can be seen that I have attempted to get a graph to depict the frequenct components of the newly created waveform but the "result" graph returns no results??? all that i see is a change in the amplitude numbers @ the side of the graph but no plot??? Im guessing this is not the correct method to achieve frequency analysis.
03-07-2012 03:29 PM - edited 03-07-2012 03:33 PM
Sorry, doesnt seem like my picture has shown up.
I have further attempted to display the frequency data, but no matter the signal. the frequency plot remains @ zero??