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ISAAC's Casters
Posted by Dodd Stacy at 03/22/2001 1:14 PM EST
Engineer on team #95, Lebanon Robotics Team, from Lebanon High School and CRREL/CREARE.
In Reply to: Keep Casters Around
Posted by Matt Leese on 03/22/2001 12:13 PM EST:
: I don't think banning casters would solve the drive train problem. You can use casters very effectively. You can also use casters very badly. Our robot this year has front casters and works just fine. We never have problems getting over the bridge (note: we do have four wheel drive in the back). Getting rid of casters because some teams don't use them well would be like getting rid of the plywood because some teams use it badly (yet another thing we used successfully this year). I think the main thing rookie teams need is more mentoring by veteran teams -- not forcing them into a particular design. If you think that rookies are building bad robots, offer to help out a rookie team. Don't make their job more difficult. Rookies need all the help they can get.
: Matt who was never on a rookie team but his team sort of counts this year because they haven't existed since 1998
Here's my attempt to pass on to new rookie teams what we learned about casters and 2 wheel drive in our rookie year (97) with our first FIRST robot, ISAAC (for Asimov). To my great embarassment, we didn't figure this out till we were practising with ISAAC before the 98 season. Had we grasped the now-obvious physics before the Manchester Regional, our only competition that year, I firmly believe we would have been Regional champs.
Secure a turkey (or a concrete block, or some reasonably heavy thing) in the middle of a grocery store shopping cart, and then push it forward and let it go, casters first. Altho' it may curve a bit, it's basically stable in a directional sense. Now push it forward with the casters trailing and let it go. Once the cart begins, for the smallest of reasons, to turn, it will quickly whip around at least 180 and maybe keep on going.
If you look at the forces that the shopping cart must exert on the turkey in order to make the turkey deviate from straight line motion, forces which have to be developed by the interaction of the wheels with the ground, you will see the reason this happens.
With casters front and 2 drive wheels in the rear, the drive wheels can exert a rotational torque on the ground to maneuver the cart around curves, but it helps to have most of the bot's weight on the drive wheels. You too can try all of this on your next shopping trip, but I don't know you.
With casters rear, you're toast. Once your bot begins to spin, very soon you do not have enough traction on the drive wheels to exert enough rotational torque on the ground to fight it, and you spin out.
Understand these physics, and you can use casters to great advantage. Ignore them at your peril. The main key with drive plans that include casters, in my opinion and experience (and understanding of the physics) is to keep the drive wheels under the bot's rotational center of gravity or close. 'Course I can't shop at the Coop anymore, so keep my comments in context.
Dodd
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