Spectral measurements

[MaddieM]

Hello,

Our group is looking for help with collecting and processing data using the Spectral Measurements Express VI. We are collecting acoustic data through a DAQ assistant, averaging 30 samples of collected data and calculating the FFT (RMS) using the Spectral Measurements Express VI as shown in the file, Time Averaging FFT - 30_samples.vi. This VI seems to run efficiently and we have been collecting reliable data using this VI. When we expanded this same concept to where the FFT data is further manipulated, we ran into problems. I have attached the second file, Contour_DAQ_data.vi. This is being used as a Sub-VI. We are noticing that once the acoustic DAQ data is collected and the FFT processed, that data is not being cleared in between function calls. It shows the same data every time. It seems to be collecting new acoustic data, but somehow spits out the FFT data that was processed during the first function call every time. How do we rectify this error?

Thank you for your time. 

Maddie

[Bob_Schor]

I'm assuming that "our group" refers to your Senior Engineering Design (a.k.a Capstone) Team, right?

I regret having to tell you this, but if you want to use LabVIEW in your Project "because it is so easy to do, it has all these Express VIs that do all the work for you", you've been sold a Bill of Goods.  LabVIEW is a reasonable platform for Senior Design, but you need to put a little bit of effort, at least working with an Instructor or other LabVIEW Guru to learn the basics of Data Acquisition, DAQmx, MAX, the reasons to avoid many (most?) Express VIs, and the reasons to avoid the Diabolical Dynamic Data Wire.

Get your team together, decide whether or not you are all going to "learn together" (I recommend this), do all the Basic Tutorials, and find yourself a LabVIEW Guru to get you started on the right path (hint -- what you posted isn't it ...).

Bob Schor

[lucaso3 · Accepted solution]

Hello MaddieM,

Based on what I see on the VIs you attached, it makes sense the behavior that you are having. The DAQ Assistant is being configured to be a finite acquisition and it is being placed outside a loop. For this reason, it will be only executed once.

Now, the date being pulled out of the DAQ Assistant is being passed to the different loops in a way that every time it executes, it will obtain the initial data that the DAQ Assistant pulled (again, it only pulled it once). It might be a good idea to verify if we can use an architecture to help you build your system. If the FFT takes too long and you need to have quick readings, I would recommend using a producer consumer architecture.

Also, in order to have a more efficient code, I would suggest you to see the lower level VIs. You should be able to have the same functionality of the Espress VIs if you use them correctly. For guidance, you can use the shipping examples (LabVIEW > Help > Find Examples), the community examples, tutorials, and white papers.

I hope this answer might help you getting in track to what you are seeking for.

Regards,

Luis Calderon

Applications Engineer / CLD / CTD

[MaddieM]

Thank you for taking the time to respond! You gave me a lot of things to consider.