Measuring Force with a NI9237 and Omega LC304 Loadcell

[KurtzC1]

Hi Everyone!

I am using a NI9174 USB Chassis with a NI9237 for measuring force with an Omega LC304 loadcell, and I am having trouble getting the loadcell to output correct measurements.

I am using one loadcell and am powering it internally with the NI9237 with 10V. The problem I am having is the values coming out the Read VI do not correlate with the expected voltage or force. The expected voltage with a 25lb weight is ~4.5mv. I don’t think the NI9237 is broken, because I powered the loadcell internally with the NI9237 and using a multimeter it yielded 4.5mv with a 25lb weight on it. I checked this value by powering it with a benchtop power supply and it still yielded 4.5mv with a 25lb weight. The loadcell is also brand new so I don’t think it is broken.

Therefore the only reason I can think of it not working is the LabVIEW Code. I am using the AI Force Two Point Linear, and I am inputting the calibration from Omega as “Scaling Information.”

Two be honest, it could be my bridge completion. I don’t exactly know how to determine the best one for my application. I have used full bridge completion in the past and it worked fine.

Any help would be appreciated!

Attached is the code and images.  

0 lbs 

25 lbs

[RavensFan · Accepted solution]

I think you are effectively scaling things twice.  When you put in a scale, it converts the electrical units (mV/V)  to a load.  But then you proceed to do math on that to try to convert it to load again.  You shouldn't need any of the math in the loop.

Also, with your scaling factor, your scale factor is 1.853 mV/V.  But you are proceeding to multiply that by 10 which is the 10V excitation your device is suppling and entering 18.531.   18.531 is the mV with 10V of excitation.  It is NOT 18.531 mV/V.

So put that in with the 100 lbs of full scale torque, I think you've effectively divided your scale factor by 10.  That is why you see ~2.5 in your Y1 array.  That is 2.5 lbs, but is really 25 lbs once you fix the scale factor.

Fix the scaling factors, get rid of the math on the Y array, and I think you'll see your value in the waveform chart show your load in lbs.

If you change your DAQmx read to 1 Channel N samples, you can get rid of the inner For Loop.

[KurtzC1]

You hit the nail on the head. The issue was that I was inputting mV into the task and not mV/V.