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Best way to determine the speed relative to the ground?

Posted by Jonas Arvidsson on March 16, 2010 at 3:34pm
Hello! I'm in a group at Chalmers University of Technology that are working on a self stabilized quadcopter with position hold, see picture.

We've got a little problem: We need to know the speed relative to the ground in x, y and z. What's the best way to handle this?

THANKS!


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Tags: airspeed, chalmers, quadcopter, speed, stabilized
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Replies

  • Jonas Arvidsson March 17, 2010 at 3:48pm
    Thanks everyone for your input!

    We have decided to control our quadcopter with position (not velocity) using GPS, an absolute pressure sensor and a magnetometer.

    More info and pics about the quadcopter will be posted soon! =)
  • Diarmuid Wrenne March 17, 2010 at 3:42am
    While the absolute error for GPS chips can be quite big, their drift in the short term (minutes) is very small. So for distances relative to the start point, they are pretty good. You need to be outdoors and have very good sat readings.

    That said, you're reqs are probably pretty tight and for testing you will need a system that will work indoors (for testing at least). Then it is pretty much image flow that you need to use, like the parrot quad copter (at CES). They use a downward facing camera and process the image into, I think, 16 sectors. They then find edges or other distinctive features in the image and compare XY positions of those from frame to frame to determine orientation and movement.

    Works indoors (using horrible carpet with nasty designs) and outdoors. Needs a fair bit of processing power, but small arm boards should do it. http://designsomething.org/leopardboard/default.aspx

    Look up optical flow in google

    http://www.google.ie/search?hl=en&safe=off&client=firefox-a...

    Great looking platform you have. Any info on specs, etc

    Cheers

    Diarmuid
  • Jonas Arvidsson March 16, 2010 at 3:50pm
    That's a pretty good idea! But how good will the measurements of the speed be, calculated by GPS?
  • anaximander March 16, 2010 at 3:45pm
    There's a couple of ways to go about this. The simplest is just to differentiate the latitude and longitude given by GPS, and that'll give you your x and y. GPS altitude isn't really accurate enough to give a z, so for that you need an airspeed, from something like the pressure sensor sold on this site. Then just compare the airspeed, which is sqrt(x2 + y2 + z2) to the groundspeed, which is just sqrt(x2 + y2), and from the difference between the two you can get your z velocity.
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