Mecanum Wheels

[Hawiian Cadder]

http://www.fingertechrobotics.com/pr...mecanum-set-hg

what if instead of using 4 6 inch mechs, you used 8 or even 12 of those. that would greatly decrease the amount of rotational inertia in the wheels, those little wheels are also higher quality.

[Ether]

Originally Posted by Marc S.

When was the last time you saw a mecanum robot going 18ft/s(?)...

I suppose it would get pretty heavy and costly to have four shifting gearboxes, but it would be possible I guess.

...and then stop in less than 5ft?

Doing a little back-of-the envelope math...

assuming constant deceleration

a = (V*V)/(2*x) = (18*18)/(2*5) ~ 32 f/s/s ~ 1g

... so you'd need an effective CoF of 1.

To get that with mec, you'd need about 1.4 CoF between the tread material and the carpet. So you'd have to make your own custom rollers with high-traction tread.

[Ether]

Originally Posted by Hawiian Cadder

what if instead of using 4 6 inch mechs, you used 8 or even 12 of those

The software would get interesting, unless you plan to mount 3 coaxially at each of 4 locations and drive all three 3 wheels within each coaxial set at the same speed (which would be approximately correct but not quite, resulting in friction).

[Hawiian Cadder]

i was actualy thinking of mounting all 3 right next to each other on a standard frame setup. i would probably use #25 chain directly driven from a cim ( no gearbox)

[Ether]

Originally Posted by Hawiian Cadder

i was actualy thinking of mounting all 3 right next to each other on a standard frame setup. i would probably use #25 chain directly driven from a cim ( no gearbox)

By "right next to each other" I assume you mean "in the same plane" and not "coaxially".

Such being the case, a correct inverse kinematic translation would require that each of the 3 wheels in a set be driven at slightly different speeds.

If you just chain them together and drive all 3 at the same speed, you'll get some scrubbing of the contact patch which means additional loss of power and traction.

[Hawiian Cadder]

i think that the same thing would happen if the were mounted co-axially. while it is true that while using all 3 degrees of freedom there would be some scrub, during normal translation there would be none, conceptually it is 3 entirely separate mechanum bases, driving in the same space, at the same speed.

[Ether]

Originally Posted by Hawiian Cadder

i think that the same thing would happen if the were mounted co-axially. while it is true that while using all 3 degrees of freedom there would be some scrub, during normal translation there would be none, conceptually it is 3 entirely separate mechanum bases, driving in the same space, at the same speed.

I believe that is correct.

[craigboez]

Looking through the links on this thread, I see that the number of rollers per wheel seems to vary based on who is designing them. Is there an optimum number?

[Ether]

Originally Posted by craigboez

Looking through the links on this thread, I see that the number of rollers per wheel seems to vary based on who is designing them. Is there an optimum number?

It depends on what you are trying to optimize.

[EricH]

Ether is right. If you want to optimize a smooth ride, lots of rollers are better (less "flat" distance between crowns of rollers). If you want to optimize manufacturability, fewer rollers are better (to a point--4 rollers to a wheel will most likely really, really make life miserable for the drivers). And it will depend on your design.

The center-hub designs typically carry about 6-8 rollers per wheel from what I've seen; the AM outer-hub design carries something like 32. I've seen an outer-hub design carry 8, and a combination of outer and inner carry something like 20. It varies depending on design and wheel designer.

[Kevin Sevcik]

Fewer rollers also means fatter/wider wheels, assuming diameter and angle stay constant. The rollers have to be longer to cover a sector of the wheel, and since they're at a 45 deg angle, that means they'll stick out farther to the sides as well.

Also, can everyone in the thread agree that the Mecanum vs. 6WD discussions were already done this season in numerous other threads? If someone thinks that horse isn't dead enough, I implore them to revive one of those threads or at least give us a break until next season.