HX711 and 10kg Load Cell reading issue

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barcesat
Posts: 1
Joined: Tue Nov 14, 2017 3:07 pm

HX711 and 10kg Load Cell reading issue

Post by barcesat » Tue Nov 14, 2017 3:51 pm

Hi Sparkfun people,

I'm having an issue for more than a week now.
I want to read 6 load cells individually and display the result on an LCD with other abilities (such as autocalibration, save the calibration factors in memory, etc.)

I'm using these products:
1) Load Cell - 10kg, Straight Bar (TAL220) SEN-13329 https://www.sparkfun.com/products/13329
2) SparkFun Load Cell Amplifier - HX711 SEN-13879 https://www.sparkfun.com/products/13879
3) Teensy 3.2 DEV-13736 https://www.sparkfun.com/products/13736

I'm having a big difficulty in a place that I once succeeded*, that is, calibrating and reading the load cell.
At first, I wanted to use the teensy 3.2 for the task, and it seemed to work.
However, later I've found out that the original library* has timing issues* and therefore will give funny results working with the HX711.
I tried to make a copy of the library with the changes to match the teensy and got pretty much the same problems.
That's when I reverted to use an arduino uno.
I tried to use the original library and another alternative method* that worked for me in the past but what I realized is this problem (described by steps):
1) I tare the scale (taking a long average zero weight measurement)
2) I take a long average measurement of a known weight (100 grams) that I tested on a different scale and save that value
3) I take a free measurement of the scale and calculating the weight using this formula:
weight [in grams] = ((measured_value - zero_value) / 100grams_value * 100)

I used to get either a ~0 grams or a ~100grams (depends if I put the weight or not of course) but I'm getting ~80 grams for the 100g weight and no zero (sometimes it's a negative number, sometimes it's positive).

This problem happens whether I'm using one HX711+ load cell pair or all the 6 pairs.

Wiring:
The pin definition is as follows (arduino UNO)
HX711 cell(DATA, CLOCK);

HX711 scale1(2, 3);
HX711 scale2(4, 5);
HX711 scale3(6, 7);
HX711 scale4(8, 9);
HX711 scale5(10, 11);
HX711 scale6(12, 13);

The GND lines are shared between all of the HX711's and the arduino,
The VCC lines are shared between all of the HX711's and the arduino,
The VCC lines are shared between all of the HX711's and I'm using a voltage regulator (78L33) to run them on 3.3V and giving it a steady 5v@2A power supply.

What am I doing wrong?
Are there any further investigations that I can do?

Thank you in advance for every bit of information,
Ziv

*former successful project of mine with a load cell
https://github.com/barcesat/Arduino-Sca ... ht_lcd.ino

*original library
https://github.com/bogde/HX711

*timing issues with teensy:
https://github.com/bogde/HX711/issues/84
https://forum.sparkfun.com/viewtopic.ph ... 20#p196520

*alternative calibration method and version of the library
https://www.youtube.com/watch?v=nGUpzwEa4vg
http://www.iforce2d.net/sketches/LoadCellDemo.zip

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