Lots of Wheels and F = u x N

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Posted by Ernie P at 1/25/2001 5:18 PM EST


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


In Reply to: Re: Lots of Wheels and F = u x N
Posted by Michael on 1/25/2001 3:24 PM EST:



Michael,
Thank you for the info. I'm with ya up to the tank tread part, then I lose the theoretical interpretation.

Why isn't a tank tread, which has more contact points, just distributing the same robot wieght over more contact points, and resulting in no increase in "propulsion force"?? Is it because the individual points each sink into the carpet slighly, which provides a small force vector component which is no longer normal to the flat surface of the carpet??

Once again my lowly grey matter would be infinitely expanded by your words of superior erudition.
Thanx, Ernie P

: >

: What you've written above is the formula for the force of friction alone. The total propulsion force, as you call it, is actually the sum of the forces in the horizontal direction. That's made of several components.

: Force of Wheels On Carpet = a vague definition for the forces at work, but it can and will get ridiculously complicated.
: Force of friction = m * g * u, where m is the mass of the robot, g is (9.8 m/s^2), and u is mu, the coefficient of friction.

: Treating the entire robot as a point object (which just makes life easier, even though it's not entirely accurate), we see that:

: the force of the wheels on the carpet - the force of friction = propulsion force

: >

: The normal force is affected by the addition of wheels only in that you're increasing the mass of the robot, and consequently, increasing friction. You're also adding additional friction to the system along the new bearing surfaces that additional wheels provide.

: However, the additional surface area provided by additional wheels can be advantageous in that in increases tractive effort, can sometimes just look really cool.

: It's just a matter of weighing the pros and cons. Additional wheels can mean more traction, in more places. That's good.

: But, it can also make a robot a lot harder to turn, increase friction a bit.

: That's why a tank tread design seems to be so popular. It allows for a large surface area to be in contact with the floor, with increased tractive benefits that far outweigh the added friction (in most cases . . . ask me about that some other time). But, with a curved or pointed bottom surface, it also facilitates fast rotation at a single point, rather than along the 3' length of the robot.

: Anyway, I hope my ramblings are a bit useful. Really, I do, because I really dislike physics and wouldn't want my efforts to be for naught.

: ~ Michael ~