It is interesting that you posted this. I've been giving this design some thought recently as a way of proving that force applied through a roller can be effectively lost with no useful conversion.
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Originally Posted by Mr. Lim
In my experience, I've noticed that Mecanum wheels often spin faster than the robot is moving, even when driving straight forward and backwards, much like the McCannots would, except not nearly as badly. Many say that it's because the Mecanum wheels are losing traction with the carpet, but I didn't see that being 100% the case. My theory is that the Mecanum rollers do roll, even when going straight forward or backward.
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I believe in a perfect setup the mecanum rollers wouldn't roll in straight forward/backwards driving. However I suspect there are artifacts of reality that probably make them spin in most cases. There are so many ideal design constraints. If the wheel centers aren't at the corners of a square you will get some slipping that will result in roller movement. If the weight isn't evenly balanced it could cause the same.
Quote:
Originally Posted by Mr. Lim
I think it's fairly obvious that these "McCannot" wheel rollers will definitely roll when driven straight forwards or backwards, resulting in less than ideal robot movement.
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This is clearly true for a robot with only McCannot wheels. Let's take a moment and consider the case where you have 2 McCannots in front and the other wheels are standard traction wheels. Let's take the example further and say all the axles are coupled together so the wheel movement is exactly the same. Let's further assume that this is driven in such a way that there is no loss of traction or slipping. In that case, I wouldn't expect the McCannots' rollers to spin at all.
I believe this is similar to what happens in a mecanum drive going straight forward. If you only look at a diagonal pair of wheels they contribute motion in a 45 degree angle. If you watch a robot doing that, you will see the rollers spinning. When you run all the wheels, the opposite diagonal set is pushing the wheels along the axis transverse to the rollers. The net effect should be (in a perfect setup) that the rollers don't spin because the opposite diagonal sets are countering the movement lost to the rollers.
Quote:
Originally Posted by Mr. Lim
My question is for those who believe that mecanum rollers WON'T spin when driving straight forward or backwards. How many degrees do you need to start angling the McCannot rollers until they stop rolling completely when driving straight forward or backward? 90? 45? 1? 0.00000001?
I don't know the answer to this, but I'd be curious to see what others think...
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My answer to this question, I believe that any roller angle between an Omni wheel and a Mecanum wheel can create this effect because the wheels on the opposite side have enough movement to counteract the losses accross the rollers. Once you go past that from Mecanum to McCannot you are losing more than half your potential movement to the rollers, if the rollers are frictionless. There isn't enough movement left to counteract the opposite diagonal's losses. Friction changes that though. With enough friction in the rollers, even the McCannot could have non-spinning rollers.
Andy