# Wheel Slip/Relation to Center of Gravity

Hello,
We have a differentially steered robot with a free-rotating front wheel. We initially had our robot on blocks and visually measured the number of ticks per revolution for the left and right wheel that our encoder was producing. We got a number of ~350 for both wheels. However, when we attempt to measure it on the ground we get a number of ~300 for the left wheel and ~350 for the right wheel. Is it likely/plausible that this is caused by the center of gravity being to the right of center and that is causing the left wheel to slip more than the left? Thanks in advance!

Hello.

I think you’re missing some information about how you’re measuring this. Where is any notion of slip coming from? There’s some number of encoder counts you get per wheel revolution, and if you’re not getting them, it sounds like you’re not keeping track of the counts right. If you find that the robot (or a side of it) is not traveling as far per wheel revolution as you expect, then you’re probably dealing with a wheel slip issue. That issue will likely crop up to some extent; one way to deal with it is to have gradual acceleration of your wheels.

- Jan

Thanks for the quick reply! I think I might be using the term slip incorrectly–I don’t mean to say that it is not travelling as far per revolution, I actually mean the opposite of that. The robot is travelling farther per measured wheel revolution than it should. I think this is caused by one wheel sliding across the ground rather than spinning. My thought was that an unequal weight on the two wheels could be causing one of the wheels to be farther off the ground and be sliding for some of the revolution rather than spinning.

What you’re describing is slipping, too; I just think it’s less likely. I’m coming from the assumption that your wheel is trying to drive the robot, so any slip leads to more turns on the wheel than corresponding distance traveled. If you’re trying to slow the left side down, with the robot’s forward movement causing the wheel to rotate, then any slip will cause your wheel to travel less than what you would expect. If that’s going on, you could help confirm that by trying the first case (driving the left wheel) and seeing that you get more counts than you expected. If you’re getting fewer counts either way, you’re probably losing them somewhere else.

- Jan