Quote:
Originally Posted by Andrew Schreiber
Actually, I would make the opposite argument. Mechanically a Nonadrive (one that does not need to cross bumps) is only slightly more complicated than a standard 6wd robot. Remember, the perpendicular wheel does not need to raise or lower if it is a flat field. From a programming standpoint it is much much simpler. The hardest part about mechanum drive machines is controlling them. It involves either lookup tables and interpolating results or using lots of sins and cosines or matrices (Thank you Ether). A nonadrive is simple in that for translation all you need to do put the Y component of your translation stick to the left and right drive motors and the X component to the cross drive motor(s) Rotation is a little trickier but you could just scale the left and right based on your rotation input. Either way, much simpler than mechanum.
It should also come out near the weight of a mechanum drive train. Remember mechanum needs 4 gearboxes, Nonadrive needs 3. Yes Nonadrive requires 4 cylinders but if you were planning on using air anyway that is not a ton of weight.
I will admit that the Nonadrive has, at least after a cursory glance, more failure points.
TLDR, there are benefits and drawbacks to both systems (like any systems) but neither one is inherently more complicated.
EDIT: Appended note about Ether's solution.
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One item to consider with the nonadrive is that even on flat ground you will VERY likely want your center wheel to still be actuated by a pneumatic cylinder.
There is a reason stools usually have 3 legs. Having a 4 legged stool is more difficult. From experience (we attempted a solid 6 wheel rectangular drivetrain in '08 with all omnis), having your corner wheels and the middle wheel all touch if they are fixed is nearly impossible. This is why you must have pneumatics - if you rock on the center sideways drive wheel you will end up going in circles. It needs to push against the floor, but not with enough force to actually support the robot. That means some sort of suspension is required.
We used a laser to get our frame and 6 wheels straight, then realized that the floor many of the arenas was imperfect as well. We didn't much want to go back and engineer suspension, so we dropped the system and went standard 6 wheel.
It looked like this:
.......Wheel.........
........................
W....................W
h.....................h
e.....................e
e.....................e
l......................l
........................
W....................W
h.....................h
e.....................e
e.....................e
l......................l
........................
.......Wheel.........
We saw several teams using this same drive train this year, and they all seemed to experience the same problems we did during our prototyping before we threw it out - with no suspension the robot reacts differently depending on which wheels are touching the ground (Obvious when looking at it, perhaps not so obvious when designing it).