[chronojump] Added the force sensor tare and calibration process

[Xavier Padullés <xpadulles src gnome org>]

commit 8079ead9991e092ba055ce0395198a3c6fdcd8e8
Author: Xavier Padullés <x padulles gmail com>
Date:   Wed Aug 9 15:34:15 2017 +0200

    Added the force sensor tare and calibration process

 arduino/ForceSensor/tare_calibration.ino |   96 ++++++++++++++++++++++++++++++
 1 files changed, 96 insertions(+), 0 deletions(-)
---
diff --git a/arduino/ForceSensor/tare_calibration.ino b/arduino/ForceSensor/tare_calibration.ino
new file mode 100644
index 0000000..55366a4
--- /dev/null
+++ b/arduino/ForceSensor/tare_calibration.ino
@@ -0,0 +1,96 @@
+/*
+  Example using the SparkFun HX711 breakout board with a scale
+  By: Nathan Seidle
+  SparkFun Electronics
+  Date: November 19th, 2014
+  License: This code is public domain but you buy me a beer if you use this and we meet someday (Beerware 
license).
+
+  This is the calibration sketch. Use it to determine the calibration_factor that the main example uses. It 
also
+  outputs the zero_factor useful for projects that have a permanent mass on the scale in between power 
cycles.
+
+  Setup your scale and start the sketch WITHOUT a weight on the scale
+  Once readings are displayed place the weight on the scale
+  Press +/- or a/z to adjust the calibration_factor until the output readings match the known weight
+  Use this calibration_factor on the example sketch
+
+  This example assumes pounds (lbs). If you prefer kilograms, change the Serial.print(" lbs"); line to kg. 
The
+  calibration factor will be significantly different but it will be linearly related to lbs (1 lbs = 
0.453592 kg).
+
+  Your calibration factor may be very positive or very negative. It all depends on the setup of your scale 
system
+  and the direction the sensors deflect from zero state
+  This example code uses bogde's excellent library: https://github.com/bogde/HX711
+  bogde's library is released under a GNU GENERAL PUBLIC LICENSE
+  Arduino pin
+  2 -> HX711 CLK
+  3 -> DOUT
+  5V -> VCC
+  GND -> GND
+
+  Most any pin on the Arduino Uno will be compatible with DOUT/CLK.
+
+  The HX711 board can be powered from 2.7V to 5V so the Arduino 5V power should be fine.
+
+*/
+
+#include "HX711.h"
+
+#define DOUT  3
+#define CLK  2
+
+HX711 scale(DOUT, CLK);
+//The factor to convert the units coming from the cell to the units used in the calibration
+float calibration_factor = 915; // Usual value With Chronojump strength gauge
+//float raw_data = 0;
+
+//Data comming from the cell after resting the offset weight
+float offsetted_data = 0;
+
+//Data resulting of appying the calibration_factor to the offsetted_data
+float scaled_data = 0;
+
+//The weight used to calibrate the cell
+float weight = 0.0;
+
+void setup() {
+  Serial.begin(115200);
+  Serial.println("HX711 calibration sketch");
+  Serial.println("Remove all weight from scale and press Enter");
+  while(Serial.available() == 0){}
+  Serial.read();
+  Serial.println("Adjusting tare...");
+  scale.tare(255); //Reset the scale to 0
+  
+  Serial.println("Enter the weight you will use");
+  while(!Serial.available()){}
+  weight = Serial.parseFloat();
+  
+  Serial.println("Put the weight on the scale and press Enter");
+
+  Serial.flush();
+  Serial.read();
+  while(!Serial.available()){}
+  Serial.println("Calibrating...");
+
+  //mean of 255 values comming from the cell after resting the offset.
+  offsetted_data = scale.get_value(255);
+
+  //offsetted_data / calibration_factor
+  calibration_factor = offsetted_data / weight / 9.81; //We want to return Newtons.
+  scale.set_scale(calibration_factor);
+
+  //mean of 255 values after being scaled to the desired units
+  scaled_data = scale.get_units(255);
+  Serial.print("Calibration factor:");
+  Serial.print("\t");
+  Serial.println(calibration_factor);
+}
+
+void loop() {
+
+//  //Raw data
+//  raw_data = scale.read_average(100);
+//  Serial.print(raw_data);
+//  Serial.print("\t");
+//
+
+}