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pxi-6251 Having Trouble Reading Load Cell Signal

 

I have connected a load cell to my DAQ card but it can not read the load cell's signal. According to the manual the DAQ card should be able to read signals from -100mV to 100mV and the load cell's signal is about 20mV. However, when I apply a force to the load cell the DAQ card doesn't seem to be able to read it. I have tried measuring the change in voltage manually using a multi-meter as a force is applied to the load cell and it appears to work.

 

I don't know what is wrong. Are my setting incorrect? I will attach a few pictures showing my settings. Also, whenever I set my max/min input range  to to +/- 10 V, I get picture #1. But when I change the max/min input range to anything else I get picture #2(on the right).

 

Any help would be greatly appreciated. Thanks.

 

PXI.jpgPXI 2.jpg

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What DAQ card are you using?  Load cells usually required some form of excitation current to function, does your load cell require external excitation from the DAQ card, or does it provide its own?  You said that you are measuring the coltage difference when a load is applied with a multi-meter, what voltages are being returned by the load cell?

 

I would also glance through the following developer zone article and see if it is relevant to what you are doing.

 

http://zone.ni.com/devzone/cda/tut/p/id/7138

Doug Farrell
Solutions Marketing - Automotive
National Instruments

National Instruments Automotive Solutions
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I guess I should have read the subject line.  PXI-6251.  It looks from the images you attached, like your voltage input is floating on the card up around 9.63 volts.  The 6251 also does not provide an excitation voltage so if that is something that your load cell requires you may need a different card.
Doug Farrell
Solutions Marketing - Automotive
National Instruments

National Instruments Automotive Solutions
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Hi Doug,

 

I don't know why the voltage input is floating around 9.63 volts...at the moment I only have the load cell connected to the card and nothing else. If the 6251 doesn't provide an excitation voltage, shouldn't the voltage be zero? The card is connected to a NI PXI 1042Q, could this be what is supplying the voltage? Sorry, I'm not very knowledgeable in this area.

When I test it using the multi-meter I get a very small value not even close to 9.63 volts. Could I have possibly set up the card wrong?

 

 

Thanks for your help.

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 To clarify, the positive and negative excitiation wires of the load cell are connected to a PXI 6733 card...so this is what provides the voltage I think. The positive and negative output wires of the load cell are connected to the PXI 6251. 

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You are correct, the 6733 is probably providing the excitation voltage to the load cell.  However it does not just automagically source, you need to tell it to output some amount of voltage or current to power the load cell.  What I would recommend is that you check out the Field Wiring Diagram to remedy any floating signals you are seeing.

 

http://zone.ni.com/devzone/cda/tut/p/id/3344

 

Doug Farrell
Solutions Marketing - Automotive
National Instruments

National Instruments Automotive Solutions
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