Mecanum Drive control

[AriMindell]

Originally Posted by Ether

Tyler,
Some of the things you wrote are still a bit ambiguous to me. I think it would be enlightening to discuss it further. Would you be interested in doing that? I don't want to pester you if you don't want to do that.

I, for one, would love to see that discussion

[Ether]

Originally Posted by AriMindell

I, for one, would love to see that discussion

OK, I'll kick it off with this.

If you make certain simplifying assumptions (which under certain adverse conditions may not be too realistic; more on this if there is interest), mecanum wheels do not "slip" unless they are torqued so hard that they lose traction. And yes, they lose traction more easily than normal wheels because of the nature of the reaction forces between the floor and the rollers.

Under these assumptions, the rollers are rolling, not slipping. The rolling of the rollers is what makes mecanum do what it does.

The computation to turn a driver command of a simultaneous combination of forward/reverse plus strafe_right/left plus rotate_CW/CCW into four wheel speeds is called inverse kinematics. The four wheel speeds resulting from this computation will cause the desired robot motion, if the resulting reaction forces between the roller and floor are less than the available traction so that the rollers roll without slipping.

On the other hand, taking 4 instantaneous measured wheel speeds (one for each wheel) and turning them into the corresponding instantaneous robot motion (forward/reverse plus strafe_right/left plus rotate_CW/CCW) is called forward kinematics.

If the rollers are not slipping, it is possible to do the forward kinematic computation. And in theory, you could integrate the resulting instantaneous robot motion over time to find the robot position and heading. I do not know if any teams have done this with a fruitful degree of success.

[Oblarg]

I could be wrong, but I believe he's suggesting that mecanum rollers are prone to "sticking" and not rolling freely (this would be consistent with my experience of certain brands/models of mecanum wheel). Under these conditions, the wheels of a strafing mecanum robot could be said to "slip," and throw off the correspondence between encoder readings and movement.