Subject:
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Re: Newbie needs Help
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 7 Jun 2006 04:59:21 GMT
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Viewed:
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4516 times
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In lugnet.robotics, steve <sjbaker1@airmail.net> wrote:
> Why waste our limited weight budget weighing down wheels that aren't
> generating any traction? (Presuming we aren't concerned about steering
> by turning the idle wheels).
Thought provoking question. It led me to the conclusion that when the bot is
generating it's maximum possible drawbar pull (as limited by the motor and drive
train), the weight on the undriven front wheels (I'm assuming a rear wheel drive
configuration) should be exactly zero. More than zero is weight not being used
to get traction from the rear drive wheels. Less than zero is the front end
lifting case. Exactly zero is optimal. And I think that holds regardless of
drawbar height. Thoughts?
If you buy that, then given that there is a maximum possible weight limit, it
follows that it is possible to get the drawbar too high. The "too high" case is
where we get negative weight on the front wheels at less than drive-train
limited drawbar pull.
Also, consider an opponent with a drawbar higher than ours. He will generate a
lifting force on our drawbar. Assuming our cable attachment point is to the
rear of the tire/ground contact point, the lifting force on our drawbar creates
a torque tending to force our front end down, giving us >0 weight on the front
axle. In that case, we want to move weight backward until we have zero weight on
the front end again. So, now we can compare drawbar heights analytically. Two
bots: Bot A and bot B have identical drawbar pull. Bot A and bot B have
idenitical weight W. Bot A has a higher drawbar than bot B. Therefore, bot B
will optimally place its weight further back. When both bot's A and B have
placed W optimally, who wins? And furthermore, does it depend on cable length,
since that determines the important angles?
-dave
P.S. In my farm tractor analogy, I'm picturing a plow or subsoiler, where the
down force on the drawbar is negligable compared to the force of pulling the
tool through the soil.
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Message has 1 Reply:
 | | Re: Newbie needs Help
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| (...) That makes sense. (...) Yeah - I agree. So we have to sum some force vectors here. If our robot is pulling to the left, we have a horizontal 'drive' force to the left and a force in the tow rope going off to the right - plus a force due to (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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Message is in Reply To:
| | Re: Newbie needs Help
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| (...) Yeah - bad for a tractor with front-wheel steering - and a need to steer - not so bad for a robot with either no steering or rear-wheel skid steering. This robot really doesn't care if it's front end gets light - but for a tractor it's a major (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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