Compass

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Magnetic-fields.jpg

This shows the general distribution and strengths of the Earth's magnetic field on teh surface.

Magnetic.gif

Compasses react to magnetic fields, in particular pointing to magnetic North, to find True North, an offset needs to be applied, which will adjust headings to indicate the direction of North Pole.

Contents

[edit] HMC6352

HMC6352


[edit] CMPS03

CompassBreadboard.jpg

Breadboard with Compass module, ShiftBrite, Boarduino, calibration button with indicator, & FTDI serial out connected.

[edit] Components

CMPS03.jpg
DC-Boarduino.jpg
ShiftBrite.jpg
FTDI-USB.jpg

Other potential components

Future potential SMD components

Other products

[edit] Schematic

CompassSchematic.jpg

Indicates wiring for RGBpixel LEDs, as opposed to ShiftBrite

[edit] Code

The following will give a compass heading via serial monitor with the CMPS03

/*
CMPS03 with arduino I2C example

This will display a value of 0 - 359 for a full rotation of the compass.

The SDA line is on analog pin 4 of the arduino and is connected to pin 3 of the CMPS03.
The SCL line is on analog pin 5 of the arduino and is conected to pin 2 of the CMPS03.
Both SDA and SCL are also connected to the +5v via a couple of 1k8 resistors.
A switch to callibrate the CMPS03 can be connected between pin 6 of the CMPS03 and the ground.
*/

#include <Wire.h>

#define ADDRESS 0x60 //defines address of compass

void setup(){
  Wire.begin(); //conects I2C
  Serial.begin(9600);
}

void loop(){
  byte highByte;
  byte lowByte;
  
   Wire.beginTransmission(ADDRESS);      //starts communication with cmps03
   Wire.write(2);                         //Sends the register we wish to read
   Wire.endTransmission();

   Wire.requestFrom(ADDRESS, 2);        //requests high byte
   while(Wire.available() < 2);         //while there is a byte to receive
   highByte = Wire.read();           //reads the byte as an integer
   lowByte = Wire.read();
   int bearing = ((highByte<<8)+lowByte)/10; 
   
   Serial.println(bearing);
   delay(100);
}


This code will provide x/y magnetic strength and computed angle with the HM55B

/*
/////////////////////////////////
Htachi HM55B Compass
parallax (#)

AUTHOR:   kiilo kiilo@kiilo.org
License:  http://creativecommons.org/licenses/by-nc-sa/2.5/ch/

http://parallax.com/Store/Microcontrollers/BASICStampModules/tabid/134/txtSearch/hm55b/List/1/ProductID/98/Default.aspx?SortField=ProductName%2cProductName
http://sage.medienkunst.ch/tiki-index.php?page=HowTo_Arduino_Parallax_HM55B_Kompass
http://playground.arduino.cc/HM55B

/////////////////////////////////
*/
#include <math.h> // (no semicolon)
//// VARS
byte CLK_pin = 8;
byte EN_pin = 9;
byte DIO_pin = 10;

int X_Data = 0;
int Y_Data = 0;
int angle;

//// FUNCTIONS

void ShiftOut(int Value, int BitsCount) {
  for(int i = BitsCount; i >= 0; i--) {
    digitalWrite(CLK_pin, LOW);
    if ((Value & 1 << i) == ( 1 << i)) {
      digitalWrite(DIO_pin, HIGH);
      //Serial.print("1");
    }
    else {
      digitalWrite(DIO_pin, LOW);
      //Serial.print("0");
    }
    digitalWrite(CLK_pin, HIGH);
    delayMicroseconds(1);
  }
//Serial.print(" ");
}

int ShiftIn(int BitsCount) {
  int ShiftIn_result;
    ShiftIn_result = 0;
    pinMode(DIO_pin, INPUT);
    for(int i = BitsCount; i >= 0; i--) {
      digitalWrite(CLK_pin, HIGH);
      delayMicroseconds(1);
      if (digitalRead(DIO_pin) == HIGH) {
        ShiftIn_result = (ShiftIn_result << 1) + 1; 
        //Serial.print("x");
      }
      else {
        ShiftIn_result = (ShiftIn_result << 1) + 0;
        //Serial.print("_");
      }
      digitalWrite(CLK_pin, LOW);
      delayMicroseconds(1);
    }
  //Serial.print(":");

// below is difficult to understand:
// if bit 11 is Set the value is negative
// the representation of negative values you
// have to add B11111000 in the upper Byte of
// the integer.
// see: http://en.wikipedia.org/wiki/Two%27s_complement
  if ((ShiftIn_result & 1 << 11) == 1 << 11) {
    ShiftIn_result = (B11111000 << 8) | ShiftIn_result; 
  }


  return ShiftIn_result;
}

void HM55B_Reset() {
  pinMode(DIO_pin, OUTPUT);
  digitalWrite(EN_pin, LOW);
  ShiftOut(B0000, 3);
  digitalWrite(EN_pin, HIGH);
}

void HM55B_StartMeasurementCommand() {
  pinMode(DIO_pin, OUTPUT);
  digitalWrite(EN_pin, LOW);
  ShiftOut(B1000, 3);
  digitalWrite(EN_pin, HIGH);
}

int HM55B_ReadCommand() {
  int result = 0;
  pinMode(DIO_pin, OUTPUT);
  digitalWrite(EN_pin, LOW);
  ShiftOut(B1100, 3);
  result = ShiftIn(3);
  return result;
}


void setup() {
  Serial.begin(9600);
  pinMode(EN_pin, OUTPUT);
  pinMode(CLK_pin, OUTPUT);
  pinMode(DIO_pin, INPUT);

  HM55B_Reset();
}

void loop() {
  HM55B_StartMeasurementCommand(); // necessary!!
  delay(40); // the data is 40ms later ready
  Serial.print(HM55B_ReadCommand()); // read data and print Status
  Serial.print(" ");  
  X_Data = ShiftIn(11); // Field strength in X
  Y_Data = ShiftIn(11); // and Y direction
  Serial.print(X_Data); // print X strength
  Serial.print(" ");
  Serial.print(Y_Data); // print Y strength
  Serial.print(" ");
  digitalWrite(EN_pin, HIGH); // ok deselect chip
  angle = 180 * (atan2(-1 * Y_Data , X_Data) / M_PI); // angle is atan( -y/x) !!!
  Serial.print(angle); // print angle
  Serial.println("");

}


See also: Compass_Vibro_Anklet

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