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Using FFT in labview

Hi, I am a new user with labview and I am using a premade program called continuous measurement and logging(NI_DAQmx). I am using a force sensor and reading the outputs. There is a lot of noise and it seems like using FFT is the best way to go about it. The issue is that I don't completely understand what is happening in the diagram, so I don't know where the FFT should go. If someone could help me figure that out or has another way to do this please let me know. Thank you!!

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Message 1 of 19
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Hi emberd,

 


@emberd wrote:

The issue is that I don't completely understand what is happening in the diagram


This is the main problem here…

Why don't you talk with those who created this VI?

Why don't you read their documentation?

 


@emberd wrote:

I am using a force sensor and reading the outputs. There is a lot of noise and it seems like using FFT is the best way to go about it.


Why do you think applying the FFT will help you with a noisy signal?

Which result do you expect from the FFT?

Would a filtering function be suited (better) here?

Best regards,
GerdW


using LV2016/2019/2021 on Win10/11+cRIO, TestStand2016/2019
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Message 2 of 19
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FFT is not a tool to eliminate noise. If you have noisy data, you simply get a noisy transform.

 

Can you explain the problem instead? Tell us what you need to do (eliminate noise? find the dominant frequency? something else?). Do not tell us how you want to do it (FFT). Have you tried a flux capacitor instead?

 

(Do you know how to get a simple screenshot ? If you don't, do  Google search! Attaching a 6MB blurry photograph is not useful. You could also just attach the VI)

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Message 3 of 19
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I have tried looking for documentation and while I have found some that details certain aspects, NI has not released their own explanation which is why I am having trouble. If you know of any place I could find a good explanation please let me know.  When you say why don't you talk with the people that created the VI, who would that be?

 

I believe that using FFT would be the best solution as my force sensor is not in a 'safe' location'. I am hoping I can distinguish different frequencies and eliminate them to see what is important. My scale is very small 100 micro volts so any noise is hard to look at. 

 

I will look into filtering, thank you.

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@emberd wrote:

I believe that using FFT would be the best solution as my force sensor is not in a 'safe' location'. I am hoping I can distinguish different frequencies and eliminate them to see what is important. My scale is very small 100 micro volts so any noise is hard to look at. .


Engineering is not religion, so beliefs don't matter.

 

So you you have certain frequencies that you want to eliminate, leaving the rest of the signal the "important" (your words!) part. Do you know where these stray frequency are coming from (60Hz? Vibration of the natural frequency of the device? something else? Are they always the same?)

 

Can you show us a typical signal and point out what's important and what's not.  What is the data rate? What frequencies are noise and what are important? If the experiment is finite, do you have a model to describe the data? Maybe you could do a nonlinear fit. Maybe you could do signal averaging to see all the frequencies better?

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Message 5 of 19
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Sorry, I am on a computer that had very spotty internet connection so I had to take a picture of my screen. I am trying to get data on very small changes in voltage, but the noise is basically the size of my voltage changes. I need to find a way to minimize or figure out what my noise is. I was thinking I could figure it out using FFT to find the frequencies. 

 

I am using a PXIe-1073 to a PXIe-6363 to a SCB-68 then to my sensor, if that gives any indication of where the noise may be from. 

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Screenshot 2024-05-21 141205.png

image.png

  

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Message 7 of 19
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This is the data I'm collecting. That spike that goes down the farthest is my point of interest. I have made a model to fit my data by using heavier object to create a bigger change in voltage. I don't know what the stray frequencies are and I was hoping to use the fourier transform to figure that out. 

The sample rate is 1000 

 

image.png

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Message 8 of 19
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The noise looks quite random. How does the spike look like for heavier objects? It is always in the same spot?

 

Do you have a datafile?

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Message 9 of 19
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It records continuously, so just wherever I apply pressure and when I decided to stop it.

I attached a picture where I applied pressure using my hand and in that case the difference is much clearer. 

image.png

 

This is a file of me just letting the sensor sit there with a poke at around 20.5 or data point 8588. 

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