Friday, August 30, 2013

Arduino: simple compass with HMC5883L + Library

Introduction

One of the most popular I2C-compatible magnetometer is the Honeywell HMC5883L. These sensors’ solid-state construction with very low cross-axis sensitivity is designed to measure both the direction and 
the magnitude of Earth’s magnetic fields, from milli-gauss to 8  gauss. 
In this tutorial I'll try to: 
  1. Introduce how a magnetometer works
  2. Explain how to retrieve the heading from the magnetometer data
  3. Provide the little library I wrote for Arduino IDE

How does a magnetometer work?

An electronic magnetometer like the HMC5883L is based on the Anisotropic Magnetoresistance phenomenon. Mastering the physics that descibe the phenomenon is not an easy task, since this is a huge field whose depths we cannot hope to begin to plumb in these few words. 
Basically, the a magnetic field interacts with the path of the current flowing through a ferrous material, according to the Lorentz Law hence the resistance of the material seems to change to the observer. You can imagine as if the bar of ferrous material (e.g InSb) grows longer, raising its electric resistance. Therefore measuring the change in the resistance we can estimate the magnetical field! The Equation that rules the phenomenon is in the image below. For a further investigation of the matter, especially on the electronics a magnetometer is based upon, you could read this.





From the raw data to the north! 



Supercomputer models of Earth's magnetic field from nasa.gov


In a compass, the magnetic field you measure is the earth's one. It is tangential to the surface of the planet and it flows from north to south. The HMC5883L has three different axis to calculate the headings, as you may not know the tilt of your device (i.e. our quadcopter) when you need the data! Anyway for this example we will assume that the sensor is flat on a table, so we don't have to worry about its tilt. Therefore we'll use only X and Y axes data.

We'll assume Hz =0


Hence the angle between the Y axis and the magnetic north will be, according to the quandrant:

Direction (y>0) = 90 - [arctan(x/y)] * 180 / π
Direction (y<0) = 270 - [arctan(x/y)] * 180 / π
Direction (y=0, x<0) = 180.0
Direction (y=0, x>0) = 0.0

First of all we have to scale the raw data according to the scale we chose.
The valid gauss values are: 0.88, 1.3, 1.9, 2.5, 4.0, 4.7, 5.6, 8.1. Of course for a geo-compass we just need 1.3 Ga, that leads us to a 0.92 [mG/LSb] of resolution and a gain of 1090 [LSb/Gauss]. The code I provide with this post is based on the code found here, but at the time this post is written, the original code won't work. There are some huge bugs as floating point number comparison that will not allow you to change the scale factor of the sensor, and some queer bugs on error handling (basically that code doesn't check for error at all, as you can easily prove executing it: it will always display an error setting the scale, and setting the measurement mode. More oddly this latter error display the same message because the error variable is not reset after its use). Of course even my library will have some bugs too, and it's not complete at all, but it's a good start to familiarize with the sensor itself.

Here is the code with a lots of comments:

On the image below you can see how the heading measured with an iPhone 4 is quite close to the one we read from Arduino. There are many margin of improvement. First, we ought compensate the potential tilt of the device using the accelerometer data from the ADXL345, for example using the info on my previous post! Moreover, my breadboard has an aluminium ground plane at its bottom, which can obviously make harder for the structure to sense the magnetic field and/or could drift it and create an offset.


39 comments:

  1. Thank you very much for the good info and library!!!!

    ReplyDelete
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  3. Hi, thanks for the article and the fix for the SetScale error. Did you notice if the opposite direction of 0 ° match with 180°? My sensor keeps returning about 205° in that position. Any ideas about whats going on?

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  4. how to calculate the angle of inclination, for my city is:
    Magnetic declination: + 0 ° 10 'EAST
    Declination is POSITIVE

    ReplyDelete
  5. I refer to this value
    declinationAngle float = 0.0404;
    your article is very good

    ReplyDelete
  6. Declination is not always added to get the true bearing. If your declination is E, the magnetic bearing will be larger than true, so subtract declination ("-=") to get true bearing.. This does make a difference.

    ReplyDelete
  7. Using two of these can you get a heading and declination?

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  8. hi the code doesn't work
    Error compiling
    Arduino: 1.6.6 Hourly Build 2015/10/14 10:42 (Windows 7), Board: "Arduino Mega ADK"

    Build options changed, rebuilding all
    fatal error: WProgram.h: No such file or directory

    #include

    ^

    compilation terminated.

    exit status 1
    Error compiling.

    This report would have more information with
    "Show verbose output during compilation"
    enabled in File > Preferences.

    ReplyDelete
    Replies
    1. I got same problem.. then I downloaded the same code from here http://bildr.org/2012/02/hmc5883l_arduino/ it works perfectly.. I did not know why these kind of things happen because these all are same code.. no idea

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    2. This comment has been removed by the author.

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    3. This comment has been removed by the author.

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    4. I received the same error when compiling - From another forum tried a suggestion to change the include statement Wprogram.h in the cpp file to Arduino.h. Used a text editor to modify the cpp file in the arduino libraries folder and compiled and that fixed the compile error.

      Delete
  9. i got a problem, my degree values only 221.something and 220.something, when I rotate the compass 360 degrees, the value is still the same, 221.something and 220.something, can you help me?

    ReplyDelete
  10. "For" cycle I would like to read the data multiple times in a row.
    Unfortunately, the cycle read datas only once, but several times. Can you help me fix it?
    correct: 10, 12, 9, 11
    incorrect: 10, 10, 10, 10

    ReplyDelete
    Replies
    1. :) CODE:
      MagnetometerRaw raw = compass.ReadRawAxis();
      MagnetometerScaled scaled = compass.ReadScaledAxis();
      for(int i=0;i<5;i++){
      switch(i) {
      case 0:
      sXTomb[0] = scaled.XAxis;
      break;
      case 1:
      sXTomb[1] = scaled.XAxis;
      break;
      case 2:
      sXTomb[2] = scaled.XAxis;
      break;
      case 3:
      sXTomb[3] = scaled.XAxis;
      break;
      case 4:
      sXTomb[4] = scaled.XAxis;
      break;
      }
      delay(100);
      }

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  11. I am using HMC5883L for my project. I have interfaced it with the arduino and executed the program given in the website. But the problem is it is showing the same angle (66.68) when I rotate & if I do it in other places I am facing the same problem. I have also added declination angle of that place. So please help me how to make it work properly.

    ReplyDelete
  12. I am using HMC5883L for my project. I have interfaced it with the arduino and executed the program given in the website. But the problem is it is showing the same angle (66.68) when I rotate & if I do it in other places I am facing the same problem. I have also added declination angle of that place. So please help me how to make it work properly.

    ReplyDelete
  13. You have really shared a informative and interesting blog post with people..
    used measurement equipment

    ReplyDelete
  14. What is the reason of using scaled.XAxis and not the raw values?

    I mean, for the calculation of the headline, the operation is arc_tan and this the same for atan(y*scale / x*scale) and atan(y/x).

    I believe the scale is important when you really want to measure the magnitude of the magnetic vector components.

    Any thoughts?

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  15. Wonderfull !! thank you so much !!

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  16. i can't open the link for the library, can someone give me the correct link?

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  17. Can you please explain what is x-axis,y-axis referenced to?

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  18. having problem with wire.h library

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  19. Hi. I'm using GY-273 module (HMC5883L based shield). I can't read information from this module.
    I ran the sketch but the information I receive is:
    0 0 0. If I unplug any SDA or SCL pin, the information printed is -1 -1 -1. This means I have communication.
    I tried many libraries and arduinos and shields.
    Can someone help me, please: Thanks a lot.

    ReplyDelete
    Replies
    1. i'm experiencing the same thing. did u find any way out?
      please revert!

      Delete
  20. I am using LSM303 sensor. It says that internally it uses HMC5883 sensor. I compiled it for Arduino nano and compiler refused. What is the reason, the library is not working with nano? original 303 library code is running well on sensor so wiring is correct.

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  21. hi. perfect. but i have problems reading angles from 270 to 360. rest of quadrnts ok. Any help?

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  22. homemade geomagnetic magnetometer
    https://github.com/IvanKor/geomagnetic-magnetometer

    ReplyDelete
  23. Hi, I cannot install library correctly, can you tell me how to install correctly and it is not ZIP , it is RAR file.

    ReplyDelete
  24. I really like your guide. Maybe you can take a look at this store selling the HMC5883L : https://voltatek.ca/sensors/150-hmc5883l-triple-axis-compass-magnetometer-sensor-module.html
    Thanks!

    ReplyDelete