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Position & Orientation measurement help!

Discussion in 'Electronic Design' started by [email protected], Jan 20, 2006.

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  1. Guest

    I've started my senior project and I'm running into some questions..
    What the project is, is a modular device that will need to record the
    x,y,z displacement from its original location and its orientation
    (tilt, yaw and roll degrees). I've been searching around and it looks
    like the orientation can be done with a mixture of gyro sensors and
    analog output compasses (i think it's called a dinsmore compass). But
    anyways, what's boggling me is a solution to calcluate the current
    position. I've read about the accelerometers and it sounds like
    there's a lot of error involved. And our spec is to have the device
    measuring for 30mins, at a sample rate of ~30Hz, so I am assuming there
    can be a lot of error introduced and propagated through time. And I
    thought of GPS before we got some specs, but due to their slow reading
    rate that is out of the question. Does anyone know of a good way to
    measure position other than using accelerometers? or is there a way to
    perhaps use a three axis accelerometer without much errors, or
    filtering out errors? Thanks!
     
  2. Guest

    Search on "inertial navigation". Some of the predecessors to GPS - like
    Loran - may still be around, and mobile phones have a pretty good idea
    where they are with respect to the local transmitters, not that I've
    got any idea how you could exploit that.
     
  3. MK

    MK Guest

    From basic equations of motion:

    s = (at^2)/2

    so a = 2s/t^2

    If you want 1m error after 30 minutes the offset error you can tolerate in
    an accelerometer is

    2/3.24E6 m/s/s or roughly 0.1uG.

    You can buy inertial guidance packages off the shelf and one this good will
    cost > $100k.

    If you can think of a way of resetting your position every few seconds in
    some way then the accelerometers get a lot more reasonable.

    Of course there are other error sources and it gets worse if its moving
    fast, tilting etc.

    Look at NAV420 on www.xbow.com - this lloks like a GPS/sensor combination -
    no idea how much it costs but it may give you ideas.

    Have fun.

    Michael Kellett

    www.mkesc.co.uk
     
  4. artie

    artie Guest

    What's the volume you need to cover? If the volumes are relatively
    small (cubic meters), you might look at polhemus technologies
    (www.polhemus.com) -- they make motion and orientation tracking
    systems.
     
  5. Guest

    I'm not exactly sure on the range of volume that will be tracked. We
    are still waiting for some detailed specs.. Assuming from the range of
    recording of 30 minutes, it can have a great amount of displacement.
    But ultimately this device is to be put on like a videocamera, so I
    guess the MAX would be a person walking in one direction for 30 minuts
    (1~2 miles?). I'll dig into the inertial guidance and polhemus tech
    and see what I can dig up.. Thanks so far!
     
  6. You can have simple, cheap or accurate. Choose one of these attributes
    at the cost of the other two.

    --
    Regards,

    Adrian Jansen adrianjansen at internode dot on dot net
    Design Engineer J & K Micro Systems
    Microcomputer solutions for industrial control
    Note reply address is invalid, convert address above to machine form.
     
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