Subject:
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Re: Back wheel construction advice
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Newsgroups:
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lugnet.robotics
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Date:
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Sun, 16 Mar 2003 13:53:32 GMT
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Original-From:
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Steve Baker <sjbaker1@airmail.net^Spamless^>
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Viewed:
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848 times
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Teoman Naskali wrote:
> Does anybody have any good construction ideas for the back wheel of a 3
> wheeled robot, the wheel is mainly for balance, and it will have to rotate
> freely, the front wheels have separate motors so the back wheel is dragged,
> the robot can rotate whithout moving forward.
Firstly, do you really need a wheel?
Certainly on carpet - and on other smooth surfaces like wood flooring
and linoleum, I've had the best success with those 2x2 stud black
domes ('gliders') mounted underneath the robot.
If you engineer the beast so that the center of gravity is over
the center of the wheels and the robot more or less balances on
them - then you can put one of those on each end of the robot and
since it's more or less balanced on it's wheels, there is hardly
any weight on the two gliders and they slide easily.
It's a good idea to balance the robot's weight like this and
having the wheels in the middle makes it turn in place - which
makes for easier programming when you want it to go somewhere.
If you are really dead set on a third wheel then you need it to
be built like the wheels on a shopping cart. It has to 'caster'
so the axle that the wheel is on is not directly below the axle
that the entire caster rotates around. The distance that the
vertical turning axle is from the horizontal rolling axle is
important.
If you make that distance too short, the wheel won't turn in
the direction the robot is going very easily and your robot
may skid all over the place.
If you make the distance too large then as the wheel straightens
out (which it'll now do easily), the whole body of the robot
will be shifted sideways quite a bit. Rotating in place without
moving forwards will be just about impossible with the distance
set too large.
Use the thinnest wheels with smooth treads for casters - you don't
want them to grip the ground because that makes them harder to rotate.
I like those thin grey pulley wheels with the thin rubber tyre on
them...or no tyre at all if the surface you are driving on is
very smooth.
Here is a classic caster design - although you can make it much
more simply using more modern Lego parts:
(look at figure 26)
http://fredm.www.media.mit.edu/people/fredm/papers/artoflego/node18.html
Even with casters, it may be better to put your main drive wheels
in the center of the robot and have one caster at each end of
the robot just to balance it.
A third option is to have the vertical axle be precisely over
the horizontal axle (just what I told you not to do!) and to
drive the rotation of that third wheel using a motor. This
is very difficult to get right and needs a rotation sensor or
something so your software knows which way the wheel is pointing.
Since the rotation sensor isn't all that precise and loses data
at very low rotation speeds, this turns out to be an especially
hard design to get right.
My experience is that casters are a pain in the neck - and unless
you are on a really rough surface like an outdoor concrete driveway,
or you need lots of ground clearance or something, the domed gliders
are by far the best answer.
---------------------------- Steve Baker -------------------------
HomeEmail: <sjbaker1@airmail.net> WorkEmail: <sjbaker@link.com>
HomePage : http://www.sjbaker.org
Projects : http://plib.sf.net http://tuxaqfh.sf.net
http://tuxkart.sf.net http://prettypoly.sf.net
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Message is in Reply To:
 | | Back wheel construction advice
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| Does anybody have any good construction ideas for the back wheel of a 3 wheeled robot, the wheel is mainly for balance, and it will have to rotate freely, the front wheels have separate motors so the back wheel is dragged, the robot can rotate (...) (22 years ago, 14-Mar-03, to lugnet.robotics)
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