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 16:33:42 GMT
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Original-From:
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steve <SJBAKER1@AIRMAILihatespam.NET>
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Viewed:
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4152 times
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Dave Curtis wrote:
> 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.
That makes sense.
> And I think that holds regardless of drawbar height. Thoughts?
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 gravity due
to the weight of the robot pointing down and a 'reaction force' from the
ground pushing upwards to equalise the total downward force.
All of these sum to produce a net force using the parallelogram rule.
If the tow rope is exactly horizontal - then the force from the drive
wheels has to be larger than the force in rope for you to win. The
weight of the robot and the reaction force are also equal so the amount
of traction we get (which depends on that reaction force) is
proportional to the weight of the robot.
OK - so what happens if the rope isn't horizontal. Suppose our tow
hitch is higher than the opposition and the rope slopes downwards to the
right.
The force due to the rope is now split into two parts. The horizontal
component of the force still has to be counteracted by the pull of the
wheels - and there is a vertical component which ADDS to the weight of
the robot and therefore INCREASES the ground reaction force - thereby
INCREASING the traction you get...which is good!
If the rope slopes upwards, the vertical component of the force is
now pointing upwards - it's reducing the reaction force and reducing
our traction.
If the rope points DOWN for us - it must point UP for our opponent - so
a downward pointing rope is a win/win thing!
However, if you get the rope to slope down by simply attaching it to
the top of the robot instead of the bottom - there are some other
consequences.
Because the force applied by our wheels is acting at the very bottom
of the robot (at ground level) and the horizontal component of the
force due to the rope is pointing in the opposite direction and acting
higher up, we have a 'moment' - a tendancy to turn the robot in a
clockwise direction (remember it's driving to our left).
If the driven wheels are the ones at the back, that moment will
tend to lift the front wheels and there is a risk of our robot
flipping onto it's back.
The trick here is to mount the tow rope on the FRONT of the robot
fairly high up.
If the robot is pulled so hard that the front wheels come off the
ground - then the rope mounting point will go UP in height. That
in turn will increase the downward slope on the rope - and we know
that having a downward slope produces a DOWNWARD force,,,,which
will tend to push the front of the robot back down again.
So a L-O-N-G nose - with the rope attached high at the front
has some interesting possibilities.
> 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.
Ah - OK. Tractor design has to be a compromise between different
kinds of load...the robot can be mindlessly optimised for just
one answer.
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Message has 1 Reply:
 | | Re: Newbie needs Help
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| (...) I'm with you until we get here. Attaching at the front of a long nose feels like a bad idea. Any side force on the tow rope will tend to pull the robot off course, and once slightly off course the force only gets worse. In any case, a (...) (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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| (...) 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 (...) (18 years ago, 7-Jun-06, to lugnet.robotics)
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