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What's the 3rd axis on a compass — Parallax Forums

What's the 3rd axis on a compass

photomankcphotomankc Posts: 943
edited 2011-12-02 10:39 in Accessories
Trying to understand what the 3rd axis on a 3 axis compass module is? In mobile device parlance it means that it's tilt compensated so it doesn't need to be level to get an accurate reading. However that does not seem to be what it means for the world of IC compass so I I'm kind of at a loss as to what that 3rd axis relates to? Is it just that 3 axis is required just to get the best accuracy overall but the tilt compensation must all still be done with accelerometers and software in all of them?

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2011-12-02 08:13
    A digital compass module measures the magnetic field strength. A 2 axis compass measures this on an X-Y plane and assumes that the compass is level. A 3 axis compass measures this on an X-Y-Z space. Given the 3 values, you can compute the direction in space of the magnetic field axis and compensate for any tilt of the sensor.
  • photomankcphotomankc Posts: 943
    edited 2011-12-02 08:50
    Hmmm, ok the Parallax document when browsing through it suggested that to compensate you would need to have have accelerometers which I found confusing with it being labeled a 3 axis compass:

    "Without tilt compensation the device will need to be level for accurate headings. For tilt compensation, you will need an accelerometer."
  • Mike GreenMike Green Posts: 23,101
    edited 2011-12-02 09:12
    Any compass, including this one, measures the direction of the local magnetic field axis. Assuming you're got nothing significant to interfere with the earth's magnetic field, the direction of the axis still depends on where you are on the earth's surface. Unless you either know where you are on the earth's surface or you can tell where the center of the earth lies (or are already known to be level), you can't do the tilt compensation.
  • Dave HeinDave Hein Posts: 6,347
    edited 2011-12-02 10:39
    Read the Wikipedia article on the Earth's magnetic field http://en.wikipedia.org/wiki/Earth%27s_magnetic_field . This should help explain what's going on. If you can ensure that the sensor is held flat, and not tilted you don't need the third axis. However, if the sensor is tilted you will need the third axis and a vertical reference, such as an accelerometer.
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