Robot is Happy creating our robot overlords one day at a time

9Aug/103

Part 1) Visualizing 3-Axis Accelerometer Readings in Processing by Christopher Hazlett

It's been a while since I've had the chance to do anything vaguely electronic. Sure, I've painted rooms in my house, installed ceiling fans, added insulation to my attic, but that's a far cry from programming in Wiring or Processing. So, thankfully, after getting my new workspace all put together, I got the chance to play with some of the parts I've had waiting in a few SparkFun boxes.

So I started playing around with a 3-Axis Accelerometer in the hopes of dreaming up some project or other. So I hooked it up to my Arduino and my Arduino to my computer and wrote a little Processing code to graph it all into pretty colors. As with all of my projects, the first step for me is understanding and since I didn't have much experience with Accelerometers a little crash course was in order. As it turns out, it's a fairly simple sensor to use (or collection of 3 sensors: x, y, z, I should say). Simply plug the VCC connector into the Arduino 3V pin (not the 5V pin. The ADXL3305 chip is only rated to 3.3V), the ground into ground and the x, y, and z pins into 0,1,2 analog pins. The code for the Arduino is simple:

The Arduino Code

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#define X_AXIS 0
#define Y_AXIS 1
#define Z_AXIS 2
 
void setup() {
  Serial.begin(9600); 
}
 
void loop() {
  int x = analogRead(X_AXIS);
  int y = analogRead(Y_AXIS);
  int z = analogRead(Z_AXIS);
 
  Serial.print(x);
  Serial.print('|');
  Serial.print(y);
  Serial.print(':');
  Serial.println(z);
}

It takes the readings in and outputs them into a formatted string '[x]|[y]:[z]'. That's it. This is just for outputting data right now, so nothing special. It gets more interesting when we look at the processing.

The Processing Code in Action

The code that makes the sweet, sweet video above isn't necessarily complicated, but there may be a few things you haven't used in Processing before.

  • map(value, low1, high1, low2, high2) - converts a value from one range into a corresponding value into another range.
  • norm(value, low, high) - converts a value into a value from 0.0 to 1.0 based on the supplied range.
  • pushMatrix() / popMatrix() - the pushMatrix() and popMatrix() methods allow you to apply rotation, translation, and other methods to a specific style. By issuing the pushMatrix() then calling the translate(), and rotateX, rotateY methods, you can then call popMatrix so those methods don't affect other elements being rendered by Processing.

The Processing Code

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import processing.serial.*;
import processing.opengl.*;
Serial myPort;
int baudRate = 9600;
int lf = 10;
 
PFont font;
int[] xAxis;
int[] yAxis;
int[] zAxis;
 
int currentX = 0;
int currentY = 0;
int currentZ = 0;
//these value were determined by taking readings from a resting position
int oneGSensorValue = 400;
float oneGMillivolt = oneGSensorValue * 4.9;
 
int totalReadings = 400;
int readingPos = 0; // the reading position in the array
 
void setup(){
  smooth();
  size(600, 300, OPENGL); 
 
  font = createFont(PFont.list()[270], 24);
  smallFont();
 
  xAxis = new int[totalReadings];
  yAxis = new int[totalReadings];
  zAxis = new int[totalReadings];
 
  for (int i=0; i < totalReadings; i++){
    xAxis[i] = oneGSensorValue;
    yAxis[i] = oneGSensorValue;
    zAxis[i] = oneGSensorValue;
  }
 
  myPort = new Serial(this, Serial.list()[0], baudRate);
  myPort.bufferUntil(lf);
 
  noLoop();
}
 
void serialEvent(Serial p){
  String inString;
 
  try{
    inString = (myPort.readString());
    currentX = xValue(inString);
    currentY = yValue(inString);
    currentZ = zValue(inString);
    xAxis = insertValueIntoArray(xAxis, currentX, readingPos, totalReadings);
    yAxis = insertValueIntoArray(yAxis, currentY, readingPos, totalReadings);
    zAxis = insertValueIntoArray(zAxis, currentZ, readingPos, totalReadings);
    readingPos = readingPos + 1; // increment the array position
  }catch(Exception e){
   println(e);
  }
  redraw();
}
 
void draw()
{
  background(#FEFFFC);
  drawGraph(xAxis, 100, color(#519050), "X - Axis");  
  drawGraph(yAxis, 200, color(#708CDE), "Y - Axis");
  drawGraph(zAxis, 300, color(#D38031), "Z - Axis");
  draw3d(currentX, currentY, currentZ);
}
 
void drawGraph(int[] arrToDraw, int yPos, color graphColor, String name){
  int arrLength = arrToDraw.length;
  stroke(graphColor);
  for (int x=0; x<arrLength - 1; x++) {
    float normalizedLine = norm(arrToDraw[x], 0.0, 700.0);
    float lineHeight = map(normalizedLine, 0.0, 1.0, 0.00, 85.0);
    line(x, yPos, x, yPos - int(lineHeight));
 
  }
  pushStyle();
  smallFont();
  stroke(#FFFFFF);
  fill(#FFFFFF);
  String gString = nfc(gFromSensorValue(arrToDraw[arrLength - 2]), 2);
  text(name + " : " + gString + " Gs", 10, yPos - 10);
  popStyle();
}
 
void draw3d(int currentX, int currentY, int currentZ){
  float normalizedX = norm(currentX, 0.0, 700.0);
  float normalizedY = norm(currentY, 0.0, 700.0);
  float normalizedZ = norm(currentZ, 0.0, 700.0);
  float finalZ = map(normalizedZ, 0.0, 1.0, 300.00, 0.0);
  float finalY = map(normalizedY, 0.0, 1.0, -3.5, 3.5);
  float finalX = map(normalizedX, 0.0, 1.0, -3.5, 3.5);
 
  pushMatrix();
  ambientLight(102, 102, 102);
  lightSpecular(204, 204, 204);
  directionalLight(102, 102, 102, -1, -1, -1);
  shininess(1.0);
  translate(500, finalZ);
  rotateY(finalY + 1.0);
  rotateZ(finalX);
  fill(#E2E8D5);
  noStroke();
  fill(#B76F6F);
  float heightWidth = finalX * 1.8;
  box(65, 65, 50);
  popMatrix();
}
 
int xValue(String inString){
  int pipeIndex = inString.indexOf('|');
  return int(inString.substring(0,pipeIndex));
}
 
int yValue(String inString){
  int pipeIndex = inString.indexOf('|');
  int colonIndex = inString.indexOf(':');
  return int(inString.substring(pipeIndex+1, colonIndex)); 
 
}
 
int zValue(String inString){
  int colonIndex = inString.indexOf(':');
  return int(inString.substring(colonIndex + 1, inString.length() - 2));
}
 
/*
This little method creates a running tally of all the incoming sensor readings
and then, when it reaches the end of the array, it pops the first one of the beginning
and inserts a new value in at the end...thus keeping a running tally of the last 400
readings (it can be for any length array, that's just what it's set to for this project).
This works a lot like an RRD graph where my inspiration came from.
*/
int[] insertValueIntoArray(int[] targetArray, int val, int pos, int maxLength){
   if(pos > (maxLength-1)){
     // if the pos == maxSize, shift the array to retain the original value
     int[] returnArray = subset(targetArray, 1, maxLength-1);
     returnArray = expand(returnArray, maxLength);
     returnArray[maxLength-2] = val;
     return returnArray;
   }else{
     targetArray[pos] = val;
     return targetArray;
   }
}
 
/*
This conversion will vary from project to project
and if you're project is relying on battery power
the reading may need to be adjusted to give you true 
one G as your battery power decreases.  All of this is due to
the output of the X,Y, and Z sensors and their coorelation to the incoming voltage at VCC
Check out the specs for the ADXL335 (part of the break out board from Sparkfun.com) here: http://www.analog.com/en/sensors/inertial-sensors/adxl335/products/product.html
*/
float gFromSensorValue(int sensorValue){
  //convert analog value into millivolts
  float mvValue = sensorValue * 4.9;
  return mvValue/oneGMillivolt;
}
 
void smallFont(){  textFont(font, 24); }
void mediumFont(){ textFont(font, 30); }
void largeFont(){  textFont(font, 40); }

This is just the first step of a larger project to create a DIY radio control using an xBee and this 3-axis accelerometer.

Happy Coding.
- Chris

About Christopher Hazlett

Chris has been programming for almost a decade, and thinks programming is even more fun when it makes a robot do stuff...even if that stuff is just a bunch of blinking lights. Chris's GIT repos: http://github.com/chazlett.
Comments (3) Trackbacks (0)
  1. Thanks for the code and video – got it working!
    Cheers!

  2. Hmm can’t seem to get the code to work.
    Is the code specific for running on a MAC or will it work for Windows XP as well?

  3. Got it working! I finally realized I need to tell it what COM port to use inside the [].
    Anyway thanks for the code. This is a great learning resource!


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