Subject:
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Re: shaft encoders for a track drive
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Mon, 15 Feb 1999 18:09:28 GMT
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Original-From:
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mr4@andrew.cmu.+StopSpammers+edu
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Viewed:
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1044 times
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Hi. We built encoders for a skid steered wheeled robot. There is a picture
of it at:
http://www.andrew.cmu.edu/user/mr4/minesweeper.html
We used an IR Led and reciever pair, and a white disk with 24 radial black
lines, and had pretty good luck with the encoder, but bad luck with the info
gathered. The encoder counted alright (after we built a comparitor circuit
to convert the analog sensor signal to digital), however, skid steering is
very hard to model, making the odometry info obtained not very useful for
accurate turning. In otherwords, a tracked (or wheeled) skid steering
vehicle, by its nature, slips when it turns. Because of this, it is hard to
correlate wheel rotations to degrees turned. Even if you can make a
determine a correlation through experimentation, this relationship will not
be consistant for different surfaces.
Encoders are not useless though, as they will still give a good "ballpark
figure" on distances traveled going straight.
Anyway, a quick easy way to build one without using optics or worrying about
A/D conversion is to get a magnetic switch (like on a bicycle speedometer),
and mount little magnets on a fixture that turns with the tracks / wheels.
You will never get as good resolution as with optics, because there is a
limit to how close together the magnets can be, but for a tracked vehicle on
rough terrain, we have found that the error accumulated from slippage
prevents obtaining very accurate info anyway.
A really good resource for this stuff is the book:
Navigating Mobile Robots : Systems and Techniques
by J. Borenstein, H. R. Everett, Liqiang Feng
good luck, if I can help further, let me know,
--Michael
/*
Michael N. Rosenblatt, sophomore
Carnegie Mellon University
(412) 683-5333
mr4@andrew.cmu.edu
http://www.andrew.cmu.edu/user/mr4
*/
--On Mon, Feb 15, 1999 12:26 PM -0500 "Steve Wall" <SWALL@flemingc.on.ca>
wrote:
> Has anybody out there constructed shaft encoders for a track driven
> MOBOT. If any body has I would appreciate any help you could give me.
> I wasn't going to use shaft encoders but I guess they are needed
> for many things correct? Is there anyway to build encoders without
> using the led's and the colored disks? or is that the easiest way?
>
> ___________________________________________________
> Steve Wall
> Sir Sandford Fleming college
> Swall@flemingc.on.ca
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