[archiver]

Posted by Patrick Dingle at 03/21/2001 10:27 PM EST


Coach on team #639, Red B^2, from Ithaca High School and Cornell University.


In Reply to: wheelchair wheel just aren't going to cut it
Posted by Ken Leung on 03/20/2001 3:10 PM EST:



I agree with you completely... but have a couple comments to add. Torque is important, but torque doesn't mean anything if your robot can't grip the carpet! What we're dealing with here is static friction. Static friction tells you the maximum amount of sideways (perpendicular) the wheels of your robot can exert on the field before they slip. This maximum force is equal to the coefficient of friction multiplied by the "normal force" (the force of the robot on the ground). So there are two ways to increase the maximum amount of force your robot can exert without slipping:
1) a heavier robot
2) increasing coefficient of friction

There are many ways to increase the coefficient of friction... grooves, "wonder wheels", etc... However, there is a maximum grip that you can possibly get on carpet, and you can't do much better than grooved wheels. Therefore, if you've already grooved the wheels and stuff, and still have trouble, then you most likely have a light robot, and you can't do too much better with increasing coefficient of friction... unless you go to treads.

Another point: power all wheels. Unpowered wheels is just a waste of traction & torque.

Anyway, enough physics from me!

Patrick

: Good call about the traction...

: Usually when people are thinking about a drive train, they usually only think about how much gear ratio the motors have, or how much speed/torque do the motors transfer onto the wheels.

: However, that's only part of the problem. The over all goal of a drive train is to transfer one form of energy into another, and one form of an motion to another. Although motors take care of turning electrical energy into mechanical energy, you still have to think about how to transfer that rotational mechanical energy into linear motion.

: The most common way of solving that problem is using wheels or thread. But you have to understand that those things work because of a major thing: Friction.

: And from that standpoint, you have to start worry about how to get maximum friction allowed on the wheels/threads in order to get most from the rotational energy. Then you have analyze how the wheels/threads react under different kind of stress: the weight of the robot pushing onto the wheels, the carpet pushing against the wheels with the same amount of force the wheels are pushing against the carpet, side friction when spinning in circle... etc.

: Finally you have to look and see if the material on the wheels are capable of taking so many stress, then think about the trade off between the life-time of the wheels, the energy you are losing, the cost of manufacturing, and the need for the competition...

: After all these work, you should be able to conclude that normal wheelchair wheels are just not going to cut it, that is without any improvements anyway... But of course most people just need to try them out and immediately see the lack of traction.

: Ha. Who need all the analysis when you can just test the stuff out?