Different Drivetrains

[Paul Richardson]

Originally Posted by emeraldstorm

Did your bot turn easily with the grippy wheels in the center? Did it move quickly? Did you have to worry about the omnis taking the force of hitting the obstacles?

I feel like I'm missing an obvious design cue with this. My first thought was "they should've put the pneumatic wheels on the ends and the omni in the center so it could absorb the hits while still being able to turn (with the raising and lowering of the front and rear)", then I wondered if rotation would still be feasible with the friction.

When you think about turning scrub (the resistance to turning from friction), it helps to look at the path each wheel will take when the robot pivots, because this determines the friction force you'll see.

To start simple, imagine a Segway rotating in place about its center. If you traced the path of the wheels, they would draw a circle, right? If you looked at the wheels at any point along that circle, they would be pointing tangent to the curve. In other words, in order to follow the curve, the wheel just needs to roll forwards as the Segway rotates, and there is no sideways movement (which would cause friction). So you can see that the center wheels of a robot have almost no contribution to turning friction.

However, if you switch to a 4/6/8 wheel robot and imagine the circle traced by the corner wheels, you see that the wheels are not tangent to that curve. The corner wheels can't just roll along, they have to slide sideways, too. This is where nearly all turning resistance comes from.