Quote:
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Originally Posted by JVN
$.02
-JV
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Having said all that, I'll get off the soapbox and describe some of the actual physical phenomena that are being loosely described in this thread.
Robot Myth Busters - Linear Pushing Force:
Myth: Tank Treads have a significantly greater pushing force than Omni drives.
Math:
In actuality, pushing force is dependant on 2 things:
1. Traction
2. Torque
If you have lots of traction, but not enough torque, you stall your motors and/or blow your breakers.
If you have lots of torque, but no traction, you spin your wheels and can't push anyone.
The trick to designing a super-powerful pushing robot is do get as much traction as possible, then gearing your robot so that when it is under load (in a pushing match), the motors only draw 40 amps each.
Traction is dependant on:
a. Robot Weight
b. Wheel Coefficient of Friction
So your first step, is to maximize the robot weight. This is relatively easy in a game where the robots have a maximum allowed weight, and there are no heavy goals to lift up on (Refer to 2002, where teams increased pushing force by lifting 190 lb goals).
You then need to find a grippy tread. Most teams have comparable wheel-treads. The 'best' stuff is arguably the brecoflex stuff that many teams use.
Using wider wheels/tracks
won't get you more pushing power, but it will distribute the load over a greater area; this means you won't burn holes in the carpet as easily, and it will take longer for your treads to wear down.
If you've *maxed* out your pushing power, now you need to figure out your gearing (to ensure you have enough torque). As I said briefly above, the main requirement for low-gear is: "Can push against a wall, without popping your breaker?" (We use pushing against the wall, as the worst-case load scenario).
What does this mean?
Two Robots both weigh the same. One robot has tank-treads, one robot has omni-wheels. Both are geared such that their wheels slip, and their motors only draw 40 amps during "wall push". Their CG is in relatively the same place. Who will win in a pushing contest?
Neither! They will stalemate.
Reality:
It is extremely difficult to get omni-wheels with a coefficient of friction equal to the brecoflex treads. So this scenario doesn't usually happen. However, it very easily could given the right design.
Followup:
Are you sure that the surface area of the tank-treads doesn't result in a greater coefficient of friction?
No. This is true for "ideal surfaces" only. In actuality, the surfaces aren't "perfectly flat, and smooth". This results in an interlocking matrix of materials (think velcro, on a smaller scale). In this case, the surface area does play a small effect. (This is measurable.)
However-- You can calculate an "overall coefficient of friction" for a given robot configuration. This is done with a simple pull test. Based on this, it would be possible to find an omni-wheel drive which has a comprable coefficient of friction to a tank-tread drive. So the above theory of a stalemate is still valid. Again-- the difficulty comes in finding such a system.
Overall:
Robots with traction wheels, will in
most cases, win pushing matches against omni-drives. When someone invents a better omni-wheel; traction wheeled robots beware!