Load cell calibration and coding

Projects General Guidance
1

Hi,

I am trying to understand the best way to calibrate a tensile load cell using the Arduino. The load cell is 4 wired and goes from 20-1000 lbs. I used an INA129P to amplify the output signal. I will upload a schematic as soon as I have it ready, but here is a description of the setup:

The load cell excitation is connected to 3V from a power supply and and the V+ and V- go to the INA129P inamp. The inamp is getting +/- 15V from another power supply. All the grounds are connected to the virtual ground made from the +/- 15V power supply. A 680 ohm resistor is used for the gain giving me a gain of about 73.

Initially the output was connected to a high precision multi-meter, so that I could observe the voltage reading of the output. For some reason when I put a load on the cell the voltage increased opposite to the offset. I initially had a negative offset of about 200mV and adding load pushed it towards the positive. Reversing V+ and V- connections just reversed the signs on the multi-meter; positive offset going towards negative when load is applied, so I stuck with the negative offset.

I connected the output to an LM324AN opamp to use as a summing circuit. I added a small voltage of 0.3V to the summing circuit to get a positive offset of around 30mV. Now as I added weight the output increased in the positive direction from the positive offset.

I am just confused right now on how to calibrate it. I hooked up an Arduino Uno, connected the ground pin and took the output from the LM324 to an analog input and printed the analogRead*(5.0/1023.0) to see the the readings coming in.

I was planning to take two readings of different weights and using them to create a scale for the arduino to compare the voltage with. I am just confused whether this approach will give me good accuracy. I've tried searching other solutions and everything used different hardare. And those were also for lower capacity load cells.

Can anyone give me suggestions on how to approach the Arduino code? Also, I'd like to know if I'm doing something inadvisable in my hardware approach. Some of you may not like the INA129P, but that's the one I'm limited to right now.

2

I would start with just getting the Arduino to output the raw analog value. There's an example sketch in the examples folder which does this.

Then add different weights (1kg, 2kg, 3kg...) and write down the numbers. See if you get a straight line when you plot it in Excel or your favourite spreadsheet/chart program.

3

Dump the whole INA lot, and get a HX711 breakout board.
Leo..