Arduino and Load Cells

I have been doing some experiments to get load cells working with Arduino.
This is a collection of my experiences.  Something about:

• What is a load cell
• How did I wire it up to an Arduino board
• What crazy application did I use it for?

• Light was controlled by using diode boxes and Sonny Windstrupsgreat code
• Music was controlled by using Pure Data to read the Arduino board and generate a midi-controller to use in Ableton Live.
• Dancing: Allison Lorenzen and others. Mounting: Thomas Fabrik. Rigging: Karl Gillick. Interactive music: Bo Boye.

So.. What is a load cell anyway

A digital weight.

Usually a piece of steel containing one or more Strain Gauges arranged in a Wheatstone bridge.

How to get a load cell

The load cell I have been using is a commercial one. (SKANTRONICS FH SERIES – STAINLESS-STEEL Shear Beam Load Cell)

These are quite expensive if you are heading to buy one.

Break a digital weight

There is actually a load cell sitting inside any digital weight you can find. Paper weight, Bath weight.

Buy or find a cheap one, and take it apart. Then rehook the loadcell inside.

Its a little bar of steel with 4 wires coming out of it.

The 4 wires are pretty standard color coded.
Look here for a good reference:http://www.controlweigh.com/loadcell_colors.htm

You can play around with the gain to get the accuracy you want.

Connecting a load cell to an Arduino board

You need some sort of microvolt amplifier to read the minute change in volt over the bridge.
ina125P – An IC that does it all for you.

• Very simple to use.
• Cheap. Around 7 € at Farnell
• Usable for many different instrumentation amplifications

The resistor sets the gain. Look in the datasheet of the ina125p for details.

Simple arduino example

This is a simplified example of how to convert your load cell analog readings to kilo/load.

// Arduino as load cell amplifier
// by Christian Liljedahl 
// christian.liljedahl.dk

// Load cells are linear. So once you have established two data pairs, you can interpolate the rest.

// Step 1: Upload this sketch to your arduino board

// You need two loads of well know weight. In this example A = 10 kg. B = 30 kg
// Put on load A 
// read the analog value showing (this is analogvalA)
// put on load B
// read the analog value B

// Enter you own analog values here
float loadA = 10; // kg
int analogvalA = 200; // analog reading taken with load A on the load cell

float loadB = 30; // kg 
int analogvalB = 600; // analog reading taken with load B on the load cell

// Upload the sketch again, and confirm, that the kilo-reading from the serial output now is correct, using your known loads

float analogValueAverage = 0;

// How often do we do readings?
long time = 0; // 
int timeBetweenReadings = 200; // We want a reading every 200 ms;

void setup() {
  Serial.begin(9600);
}

void loop() {
  int analogValue = analogRead(0);

  // running average - We smooth the readings a little bit
  analogValueAverage = 0.99*analogValueAverage + 0.01*analogValue;

  // Is it time to print? 
  if(millis() > time + timeBetweenReadings){
    float load = analogToLoad(analogValueAverage);

    Serial.print("analogValue: ");Serial.println(analogValueAverage);
    Serial.print("             load: ");Serial.println(load,5);
    time = millis();
  }
}

float analogToLoad(float analogval){

  // using a custom map-function, because the standard arduino map function only uses int
  float load = mapfloat(analogval, analogvalA, analogvalB, loadA, loadB);
  return load;
}

float mapfloat(float x, float in_min, float in_max, float out_min, float out_max)
{
  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

DC offset

If you are having an offset in you load cell, you need to add a resistor to compensate. This circuit should show you how:

Other considerations

• I am using the 5V of the Arduino board as exitation to the load cell, not the 5v-ref on the chip
• Using a 1760 kg load cell I was able to sense 0.1 kg changes
• You may have to swap the voltage applied to the bridge to get a decent range on the analog signal
• Measure the output voltage (input to analog 0) while you change your load, and adjust the gain until you get a good sense
• Analog input at the arduino board is used. Play around with the aref – analogreference – to get a good sense.
• analogReference in my sketch was set to INTERNAL making it 1.1 V – Wich gave me a good range