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 02:16:58 GMT
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Viewed:
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4025 times
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In lugnet.robotics, Brian Davis wrote:
> In lugnet.robotics, Dave Curtis wrote:
>
> > > something that raises your end of the tow rope above your
> > > oponent's end. Then the act of pulling exerts a downward
> > > force on your robot and an upward force on your opponent's.
> >
> > Ummm.... that's not the way I remember the mechanics. There
> > is a torque created that is force on the drawbar times the
> > drawbar height from ground.
>
> I don't think Dean was talking about torque from the rope tipping the robot;
> I think he was talking about the fact that a raised attachment point for the
> rope can result in a downward component of the tension, increasing the force
> down on the robot and thus potentially increasing the force of friction.
Sure, I understand the argument. But this old farm boy has both a few hours in
the driver's seat of a farm tractor, as well as a couple of engineering degrees,
both of which prompted my cautionary statements. Let me start by saying that I
agree whole-heartedly with you that experimental results trump paper analysis
every time. That said, here is some paper analysis :-)
+-+
|L+----Drawbar <--force L---
+-+
_______Ground____ --force T-->
(wheel and axle not shown, because my ASCII art isn't that good. sorry.)
There is a force L in the -X direction that is the pull of the load on the
drawbar.
There is a force T in the +X direction that is the traction of the wheel against
the ground.
To the extent that T > L, your tractor+load system accelerates to the right.
(F=MA and all that jazz...)
There is a torque of L times the distance from drawbar to ground that wants to
lift the front end. All I'm saying is that raising the drawbar increases that
torque. Now, if the load connector from drawbar to load is at an angle, there
is a Y component to the force, yup, that's true. How does it impact your own
traction versus the other guy's traction? Hmmmm... ??? Time to head for the
workbench.
I'm not looking for an argument, I'm just cautioning that in a tractor+load
situation the above mentioned torque is a significant design challenge, and
anything that makes it worse needs to be well tested. In a farm tractor,
getting a "light front end" means loss of control... I can attest that this
causes the operator some amount of mental discomfort. A robot, OTOH, may not
care if the front end comes off the ground, so long as you can drag your
opponent across the finish line.
>
> > That torque will lift your front end.
>
> The torque from the rope tension can certainly reduce the downward force at
> one end of the vehicle, but it can increase it at the other - you want the
> driven wheels where the downward force is increased.
The farm tractor solution is: 1) low drawbar, 2) long chassis, 3) as much weight
forward as possible. Without going through the analysis, my guess is that
getting the weight forward will be a bigger contributor to traction than you
could get by using your opponent's weight to pull your own back end down.
By now, the original poster is probably sorry he asked :-) I'm certainly not
sorry he asked, this has been fun to think about.
-dave
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Message has 2 Replies:
 | | 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)
| | | Re: Newbie needs Help
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| (...) Dave, Actually I am very happy and grateful. I am getting a crash course. Couldn't have asked for more. Raj. (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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| (...) I don't think Dean was talking about torque from the rope tipping the robot; I think he was talking about the fact that a raised attachment point for the rope can result in a downward component of the tension, increasing the force down on the (...) (18 years ago, 6-Jun-06, to lugnet.robotics)
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