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Posted by Gordon McComb on April 27, 2006, 12:49 pm
cnichols@greenvalleyschool.com wrote:
>
> Here's a picture of a 3-wheel robot I built:
> http://www.greenvalleyschool.com/Curtis/3Wheel.jpg
> A motor drives the front wheel and steers the front wheel via a servo.
> There is a shaft encoder on the front wheel. I'm thinking that I
> should be able to keep track of wheel angle and distance from a
> starting (x,y) coordinate (and starting robot angle) and be able to
> know the present (x,y) position and heading.
>
> Any ideas about practicality of this approach? Potential problems? Any
> good code? (I am pretty good with math but think there may be some
> better approaches than the brute force trigonometry I have in mind.
Take a look at this robot to see how a similar three-wheel robot uses a
wheel encoder:
http://www.robotics.com/arobot/
Yours is a bit different in that the steering wheel is also the driven
wheel, but the basic concepts are similar.
The problem with any solution where there is just single encoder on the
front wheel of the tricycle arrangement is you can calculate distance,
but not bearing. You'll need to infer bearing by the position of the
steering motor, and this is inherently inaccurate. Well, wheel odometry
is inherently inaccurate to begin with, but there are ways to minimize
it. You might consider putting encoders on the rear wheels instead, and
then you'd use ordinary odometry techniques you can commonly find with a
Y or G search. For starters, there's a Sourgeforge project that
discusses this issue:
http://rossum.sourceforge.net/papers/
and you can find a variety of different implementations, some with
posted code, on various robotics retail and hobby pages. Here's one, for
example, based on a commercially available quadrature encoder board
designed for small bots:
http://www.nubotics.com/support/ww01/example_code.html
Sorry...don't see any examples for Vex.
-- Gordon
PS: I'm sure you are aware Radio Shack is discontinuing Vex, and they
have everything half off. The sale used to be "double secret" but the
sale prices are now listed on the radioshack.com Web page.
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