Load Cell Voltage

[Elrod..JM]

Hi everyone!

  I am new to Labview and I am working on creating a VI that displays force values from a load cell that has been installed in an industrial tool. The signal is being carried from the load cell into a PC by a NI-9237 module and USB carrier. I created a custom linear scale and virtual channel (Custom Voltage w/ Excitation) in the VI based upon the manufacturer's calibration profile.

  I have run into a problem where the waveform graph that is produced during data acquisition is indicating that the applied force is exceeding the capacity of the load cell. To confirm this I have recorded the voltage output at the load cell and within the labview program. What I have found is that the voltage signals do not match!!! For example while under a constant load, I will measure a voltage of 0.013 Volts with a multimeter, however the Labview VI indicates that the measured voltage is 0.029 Volts.

  Does anyone have any thoughts why there would be an inaccuracy in the recorded voltage?

Thanks,

-JME

[pjr1121]

Is it configured as single or double ended?

[Lukin]

The 9237 does not output a voltage.  It outputs a mv/V signal so comparing it directly with a DMM will not work as the output is a function of your exictation voltage.  Take a look at the Bridge- Coninuous Input.vi in the example finder (Help> Find Examples>Hardware Input and Output>DAQmx>Analog Input).  You will need to have some information about your sensor.  

[Elrod..JM]

Lukin,

  I am using a Honeywell Model 31 load cell with an output of ~2 mV/V. I used the exact output from the calibration profile to create the custom scale in the VI with an excitation voltage of 10V per Honeywell.

Best,

JME

[Lukin]

I have never used that method.  I still think you should take a look at the example I mentioned.  In my very quick look at your load cell I don't see any reason it won't work.  But your method might work as well.  I have never tried that. 

Beyond that why do you have so many auto indexing inputs on your for loop?  That could be the source of your issue.  Look at this Help Link.  It should help you understand how for loops and auto indexing works.  Genearly its good practice to not have more than one auto indexing inputs on a single loop.  Really I am not sure why you even need the for loop.

[ben64]

You're probably exceeding the maximum excitation power the 9237 can provide (150 mW) so the 9237 is reducing the excitation voltage. 10V in a 350 ohms bridge is 286 mW. If you want to use a 10V excitation voltage you have to use an external supply and set the voltage excitation source to external.

Ben64

[Elrod..JM]

@Ben64, thanks for pointing out the fact that I am exceeding the max power for the 9237. I will try to reconfigure the set up using the 9174 chassis to supply the excitation voltage.

@Lukin, the VI that I developed for the project was based upon code was that was posted online.

https://decibel.ni.com/content/docs/DOC-11136

The initial code that I developed did not have the For loop and simply used the scaling factor as a multiplier after data was acquired, but I still had the same problem. My initial thoughts was that the error was a result of not establishing two custom scales at the beginning and that is why I adopted the current version of the VI.

Thanks for the advice,

-JME