mecanum vs. omni

[EricH]

Originally Posted by Lil' Lavery

A mecanum is really a form of holonomic drive (as mentioned earlier), but instead of angeled wheels with straight rollers, a mecanum has angeled rollers on straight wheels. Instead of applying a large portion of the force from not a large number of motors, mecanums apply a smaller portion of force from more motors.

I understand what a mecanum is; my team worked with them between 2003 and 2005 but decided they weren't worth it. My "sketch" indicates only roller direction, and not wheel direction (the axis is along the line)

Originally Posted by Lil' Lavery

Pushing:
Assuming both sets of wheels have identical traction, it is really the same. You can push on each from angels where you will get partial resistance from all the wheels, or locations where you will ignore 2 wheels (as you will be pushing along the rollers) and fight the full resistance of 2 wheels.

With a traditional omni, those locations are much easier to hit. Also, you now have a 4v2 motor battle. 2 can't beat 4 very easily in pushing power, although it can be done. (Assuming that both robots have 4-motor drivetrains). With a mecanum, if you hit it just right (only four locations that are very small), you get less resistance, but it's easy for said mecanum to spin away.

Originally Posted by Lil' Lavery

Ease of build:
Mecanums will adapt easier to a "traditional" (kit of parts) frame than a holonomic system. AM Planetaries would be a poor choice for a drive train though (given current rules). The AM CIM motor planetary is more for manipulator use, with a 181:1 reduction.

I just threw the AM planetary in because I didn't want the BB. Any gearbox will do.

Originally Posted by Lil' Lavery

Maneuverability:
Both Mecanum and Holonomic drives can drive in arcs, spin in place, translate in any direction, and "frisbee" (rotate while translating in a straight line) given proper programming.

Then the question is, which is easier?

Originally Posted by Lil' Lavery

Traction:
In general, neither system gives a great amount of traction (because of the general principle of rollers), and each wheel has a direction it can be pushed by (in theory) only overcoming the rolling resistance of the rollers. AndyMark sells both plastic roller omni-wheels, and the "trick wheels" (which you can have rubber and neoprene rollers, with higher traction).

Traction is not very good, but most omnis that people use seem to not be AM, and of the AM's I've seen, very few are "trick". Most mecanums are a) custom built or b) AM, and both (tend to) use sticky material. The thing is, what role do vectors play? for each wheel, you have two vectors. One is in line with the wheel; one is at an angle. Those vectors govern how much traction/force is applied in any given direction. Mecanums can apply a lot of force going sideways, but a standard omni can too.

[joking, almost]I have an idea: let's settle this. 357, 1138, and 1540 (mecanum) in a series of tests against 116 (2005) and two other onmiwheeled robots (maybe 67 2005 for a three-wheeled version?). Tests include maneuverability, simplicity, and traction (old-fashioned shoving match, choose your direction of push). The better system is the one that wins the most tests.[/joking, almost]