Quote:
Originally Posted by Ether
The correct configuration is a "X" configuration when viewed from the top of the bot, which is the same as an "O" configuration when viewed from the bottom of the bot. In other words, if you have a bot with the wheels mounted in an "X" configuration when viewed from the top, and you tip the bot on its side and view it from the bottom, it will appear as an "O" configuration.
If your wheel configuration looks like an "O" when viewed from the TOP, it is not the correct configuration, and the bot will not spin properly.
For those of you new to this discussion, here's what we mean by "X" and "O" configurations:
"X" configuration:
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"O" configuration:
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First off, nice ascii art. Good description of the problem as well.
I have been lurking on this thread for a while, but I kept quiet because I didn't really have anything worth adding... ...until now.
When this issue was first introduced in my mind as a potential problem, I had to think about which way I would orient the wheels if nobody told me the answer and I hadn't yet done the experiment. Sadly, I got it wrong. My first thought was to say that I wanted the wheels on the ground in the X position.
My reasoning went like this: I wanted the small wheels oriented such that it would make it easy to spin the robot about its center. An X seems to provide this feature. And it is true as far as it goes -- if you have the wheels in an X configuration and you lock the main axle of the wheels (i.e. turn the motors off), you should be able to spin the robot about its center fairly easily.
BUT... ...then I thought, "What is going to provide that spinning force?"
Well... ...it is SUPPOSED to be the motors driving the main axles of the wheels, of course.
BUT, the way Mecanums work is that they only apply force to the ground in the direction of the axles of those little wheels (the same can be said for holonomic wheels, by the way).
Now that X configuration is a disaster because all the wheel force vectors go right through the center (or very near the center) of the robot!
Torque is force times moment arm: Small moment arms => small torques.
Even if the wheels are putting out largish traction forces, that force is multiplied by as small moment arm. The net result is a robot that has issues spinning about its center.
So... when it comes to Mecanums the O is the way to go.
Cheers,
Joe J.