Subject:
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Re: Thoughts on Turning
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Newsgroups:
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lugnet.robotics
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Date:
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Tue, 26 Jan 1999 20:59:35 GMT
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Original-From:
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alex wetmore <{alex@phred}AvoidSpam{.org}>
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Viewed:
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1137 times
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On Tue, 26 Jan 1999, Reno Pittner wrote:
> 1) I'm not sure I understand the advantages of the fifth wheel
> design being discussed. I mean, why four wheels for forward
> motion. It seems a two center wheel with front and back coasters
> would accomplish the same thing on a smooth surface and is less
> complicated for turning.
You don't even ned two caster wheels, just one works fine. This is
what I've been doing on my robots. You just need to make sure that
your main wheels aren't at the center of gravity (or close enough to
it for the robot to flip over).
> 2) The adder/subtractor differential. In a human controlled tracked
> based system, operation is controlled by switching the individual
> motors on and off. Motor speed is consistent to both through a petal.
> Actually this is a single motor with gearing for each track. In a
> a two motor system the adder/subtractor makes perfect sense.
>
> This method does not allow for smooth turning such as an arch. If
> the motors are controlled by speed, equal = straight, on motor
> greater than other = arch turn, one fwd one rev = turn in place
> I don't see the need for the differential as you are controlling the
> tracks. I don't think it will hurt but don't see the benefit. Please
> someone enlighten me. I just know this has to be a good thing.
It is rare to find two lego motors which are matched for speed.
Normally if you run a motor to each wheel you'll end up going in
curves when you mean to go straight, because one wheel will go
faster then the other. To solve this you'd need to use two inputs
which keep track of the speed of each wheel and try and equalize
them using the 7 output levels to each motor. That would probably
utilize most of the RCX's power right there. The two differential
system handles it elegantly and mechanically.
Also, the two differential system can allow for arcs. If you run
both motors at full speed you'll turn around a point located under
one of the wheels. If you run one at full and one below full then
you'll either turn around a point inside your robot or at a point
to the side of the robot. If you only run one motor at a time
you end up turning around the center of the robot, or around nothing.
There are no possibilites that the one motor to each wheel system
gives you that you don't get with the adder/subtractor, except that
there is a bit of efficiency loss with the adder/subtractor due to
going through extra gears. The better designs of it (such as the
one that my dad came up with and which is on the bottom of my
page at http://www.phred.org/~alex/lego) minimize this loss too
and work really well.
alex
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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Message has 1 Reply:
 | | Re: Thoughts on Turning
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| Thanks to all who replied. I am now clear to benefits of the adder/subtractor and will incorporate it into my bot. Cheers, Reno (...) -- Did you check the web site first?: (URL) (26 years ago, 26-Jan-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: Thoughts on Turning
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| Hi All, I'm fairly new to this list and take comfort in knowing I am not alone as the mechanically impaired. I have been involve in robotics for quite a few years, have a little electronic knowledge, and program for a living. Despite a ripe old age (...) (26 years ago, 26-Jan-99, to lugnet.robotics)
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