Reading a load cell with USB-6210

[Juanfer_jf]

Hello!

I'm trying to read a load cell (500 N; 2mV/V at 10 Vcc; datasheet ) with my DAQ USB-6210. Output should go from 0 to 20 mV.

I guess I have enough resolution with the USB-6210 (16 bits AI, +/- 0.2V Input Range).

On my VI I get the output and, to get a estable value, I use a Point by Point Median Filter.

As the offset is not 0 when I run the VI, I do perform a substract operation to ajust it. Then I just multiply the voltage value to obtain the load value in Kilograms (1 V x 1000mV/1V x 500N/20mV x 1kg/9.81N = 2548.42).

The program seems to work properlly, but the values that I obtain are far away from the load that I'm aplying (the relative error is near to 30%).  I've tried the same VI with other load cells and the values that I read are very relatable (error < 2%)...

Any idea of what could be the problem with this load cell? Is it the Point by Point Median Filter? Should I use any other digital Filter (I don't have many idea on how to use filters, I used the Median Filter because the signal shows lot of noise, so I wanted to get a estable value).

Thank you in advance!

[GerdW]

Hi Juan,

there's a difference between a median filter and a mean filter (aka averaging filter). Do you know it?

Apart from that issue: we cannot discuss code that isn't attached to your message...

Btw. where's the excitation coming from?

[Juanfer_jf]

Thank you for your answer!!

I do understand. Is there anything that I can use to perform the MEAN instead of the median??

I thought I did attach the VI, sorry!!

10 Vcc coming from a power source (Tenma 72-10050 ).

Thank you!!

[BertMcMahan]

I'd switch to a Butterworth point-by-point filter, and I'd also set your system to run at more than 1 sample at a time, then you can average them together. I doubt this is the main issue though since you say it works with other load cells.

Part of your problem though is that you're using the +/- 1V input range, not 0.2V (in the "Signal Input Range" in your DAQ assistant) so you're not getting the resolution you think you are.

Your subtraction might also be wrong. Is your error linear, or does it always have a constant offset? Load cells can drift with temperature and other environmental concerns, so it's wise to tare your signal each time you run it.

Also, are you sure you've got your cables run properly? Your DAQ assistant is set to Differential (which is good), so you need to be connected to AI 0+ and AI 0-, not AI GND. If you're grounding one side of the signal, you'll ruin the measurement and could cook your load cell.

I'd check your system with a known-good multimeter to see if it's a software or a hardware problem.

[Juanfer_jf]

Thanks for your answer!!

I will try to use the Butterwroth filter, but I don't know which settings use to configure the filter, because the signal without any kind of filter shows lot of noise...

I think that the error is not lineal; the bigger gets the load to measure, the bigger is the error obtained... I thing it has nothing to do with temperature, I try to obtain values always with same conditions

The hardware is in perfect conditions and the values that I obtained with the multi-meter shows it too...

I think the problem is related to noise and signal conditioning