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Unread 23-06-2002, 23:37
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Join Date: May 2001
Location: Pontiac, MI
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You the man Dodd!!!! You the Maaaan!!!

Posted by Ernie P at 1/26/2001 10:21 AM EST


Engineer on team #548, RoboStangs, from Northville High School and Robert Bosch.


In Reply to: Re: F = u x N, At Most - "myewww"
Posted by Dodd Stacy on 1/25/2001 11:20 PM EST:



Hey Dodd,
Wow .... thank you for your response. The weak link being "in the carpet" was the hole in my understanding of the traction system. By distributing the weight to multiple wheels we don't change the propulsion force much ... But, we do increase our ability to transfer that force without damaging the surface. This makes complete sense.
Thanx, Ernie P

p.s. Good luck on your white paper!!!!!

: OK, Ernie. Wake up - here comes the second installment. Now we know why it's important to maximize N on the DRIVEN wheels (or "drive interface surfaces&quot to have the greatest PROPULSION force. Now, how do we maximize u (mu), and why DO those guys use tank treads?

: Think of mu, the friction coefficient, as relating to the tendency of one surface to mechanically interlock with another surface on a microscopic scale. Teeth in teeth. We all know how to get great robot traction on a FIRST carpet. Cover your wheels/treads with those nifty little stiff wire brushes in the SPI catalog they use to clean gradoo out of files. The little wire needles sink down into the carpet, make a great mechanical interlock, and produce a marvelous mu.

: There are lots of other ways to do it, but they all involve carrying the robot's weight onto the carpet over a small surface area of actual contact (needles, wires, spikes, etc) so that they sink in and mechanically interlock with the carpet. Terrific. We have a very high mu and all of the robot's weight on the drive wheels and we can push anything that's in the way. Look out!

: Until the carpet says uncle. It has a certain shear strength (ability to withstand horizontal force, per square inch of surface), and then it lets go. In the end, it is the weak link, the limiting factor. How do we deliver more horizontal PROPULSION force if the carpet can only take so much force per square inch before it shreds? Apply the force over more square inches. If you want the ultimate in straight line traction without damaging the carpet, make a 36" long by 30" wide tank tread drive with each cleat of the track being a wire brush. (Just don't try to turn.)

: By way of closure, the game is all about balancing the factors that limit performance. There is the torque/force that the drive system can deliver to the traction interface (motor and gearing). There is the friction coefficient that can be developed at the traction interface (surface micro geometry). There is the normal force that can be applied to the traction interface when the weight shifts as the robot goes about ALL of its operations (chassis layout and drive train). And, finally, there is what the carpet can sustain before it fails.

: It makes no sense to develop enough traction, without damaging the carpet, to win an ox pull and then gear your motor so tall as to stall when climbing the bridge. Conversely, it's a lost cause to gear way down for pushing power and then run smooth hard drive wheels with no weight on them. I apologize for belaboring what is known and obvious to many. I'm hoping to dispel some of the confusion I've read here from both the question and the answer side.

: Dodd


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