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Strange perodic fluctuations in voltage measurement

Hello there,

 

I had set up a very simple VI for measuring the pressue via a USB-6009 DAQ unit. I had no problems reading voltage from several pressure transducers until just about a week ago. Some strange periodic fluctuations came from nowhere (see attachment). I tried new power supply, new DAQ unit, rewriting the VI. None of this worked. Could someone help me solve this problem?

 

Thanks,

yanxiang

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Message 1 of 18
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it seems to be some noise. try to take the measuremnt without connecting the sensor and then analyse what you get. if these fluctuations gone then it seems to be problem from your sensor side. one more thing, can you apply on DAQ card some defined analog signal (sine wave etc) and then analyse.

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Hi Tirmizi,

 

Thanks for the reply. I have two DAQ cards installed on my computer, one with 3 sensors connected and one with 2 sensors. Every one of them has the same fluctuations. The sine wave singal is a good idea. Will try that out and post the results. That said, I have connected the card directly with two AA batteries, whose voltage should be stable. But that still showed the same fluctuations. Do you think that can be considered as a defined signal?

 

yanxiang

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Message 3 of 18
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What type of process are the pressure transducers measuring? Could it be a real fluctuation in pressure?

 

EDIT: Just saw your post with the battery test- how long of a run is the wiring for the transducers and for the battery test?

 

-AK2DM

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"It’s the questions that drive us.”
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Hi AK2DM,

 

Sorry if I understood your question wrong, but the lengths of the wires for different sensors vary from ~15" to ~60". For the battery test, I used the 15" wire, perhaps even shorter.

 

yanxiang

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what you want to say is that you applied a DC volatage (using AA battries) on DAQ card and measure it and have the same fluctualtions. do you process your data means FFT some thing and then display or you display the raw data (time domain).

 

what happened if you simply take measuremnt from your DAQ without connecting any thing on input and display it. is there still be these flucations present

 

if you donot find any fluctuations when traking measurement without connecting any signal then the problem lies on sensor/cable or connector side. you have to check for them by changing cable first and then go to sensor

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Message 6 of 18
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This seems very odd. I have been thinking about it for a while and cant think what it could be. Could you use an oscilloscope and check the 5v and the vref pins on the 6009? The device is not isolated so I am wondering if the spikes are coming from the pc via the usb. Try a different pc, port and cable perhaps?
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Dont try and measure an open circuit daq channel, it will pick up noise. Less so on the 6009 because of the input resistors but still true. Instead, link out he channel and you should get 0v. I think the battery test is more representative though.
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Hi Michael78 and Tirmizi,

 

Thank you both for suggestions. I have been trying things for the past few days and still experiencing this issue. So I am open to any suggestions. I just restarted the computer but the problem still persists. I have two identical cards running in different VI's and both have this noise. I will try the DAQ card on a labmate's computer and see whether or not I will see the fluctuations.

 

yanxiang

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I would check around the lab to see if you have any kind of equipment that works at that period.  Like maybe a chamber with a solenoid.

Bill
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(Mid-Level minion.)
My support system ensures that I don't look totally incompetent.
Proud to say that I've progressed beyond knowing just enough to be dangerous. I now know enough to know that I have no clue about anything at all.
Humble author of the CLAD Nugget.
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