COF help?

[FourPenguins]

Originally Posted by jgraber

Lovely math.
Re COF measurements: Is there a difference between static friction with a rolling wheel, vs static friction on a fixed wheel? It sounds like most people are measuring with a drag sled fixed wheel.

I have observed, but not measured, significantly lower COF when static friction is lost.

[Jared Russell]

Rolling friction (where the surface of the wheel at its bottom point doesn't slip relative to the floor) should be equivalent to static friction for non-deformative surfaces.

[jgraber]

Originally Posted by jgraber

Re COF measurements: Is there a difference between static friction with a rolling wheel, vs static friction on a fixed wheel? It sounds like most people are measuring with a drag sled fixed wheel.

Originally Posted by Abwehr

Rolling friction (where the surface of the wheel at its bottom point doesn't slip relative to the floor) should be equivalent to static friction for non-deformative surfaces.

Yes, so I have been taught as well. I'll rephrase my comment/question as
"When reporting local COF measurements, please include details of your methodology, such as using a fixed wheel drag sled, vs using a weighted robot with KOP wheels pulling a scale pulling a dragsled.

[Gary Dillard]

Originally Posted by Abwehr

Rolling friction (where the surface of the wheel at its bottom point doesn't slip relative to the floor) should be equivalent to static friction for non-deformative surfaces.

So I assume you have an algorithm in your drive code that determines when your wheel is slipping and backs off the power? Certainly possible - compare omega R of the wheel to integrated speed at the wheel from the yaw rate and accelerometer. Or just calculate it from static friction and never apply more than that amount of power, but if you do start to slip it's not self correcting since the power requred decreases. Otherwise if you're applying full power to the wheels you can expect them to slip.