Omni Wheels Vs. Mecanum Wheels

[RunZach]

Hi,
My team just finished the North Star Regional (#4687). This year we used standard KOP wheels, but we want to start Alpha testing new ones. What are the advantages of Mecanum Wheels versus Omni Wheels? Also is there another wheel type we should consider?

Thanks!
-Zach

[Jon Stratis]

Well, that's a pretty wide open question.

Lets start with mecanum wheels. You need 4, one in each corner of a "normal" rectangular robot. Power each one separately (4 motors, 4 gearboxes), and set them up so the rollers form an "X" when looking from the top of the robot. With this setup, you can drive and turn like you do with a "normal" tank drive, but you can also strafe - move sideways or at an angle without rotating the robot.

With omni wheels, there are a number ways to set them up. Assuming you can keep them all level, you can set them up to get similar control as mecanum drive, by having a pair parallel to each other, and another pair perpendicular to the first (either one in each corner, turned 45 degrees, or one on each side: left, right, front, and back). You can also use them to make turning easier. For example, if you find it difficult to turn, or see that your robot bounces around when turning, having a "normal" traction wheel in the middle and an omni in each corner on each side will make the robot turn a lot smoother. Or if you have 4-wheel drive, having one pair of omni's and one pair of traction (omni's probably in the front) makes turning easy, but it puts your center of rotation towards the traction wheels.

There are other drive train configurations available, like swerve, octocanum, and apparently something my students were talking about today called "butterfly"... I'm still not entirely clear on how that one works, but apparently they got it from another team at the regional. However, those are drastically more complex and difficult to do right.

[cgmv123]

This post goes into excellent detail about non-omni-directional wheels: http://www.chiefdelphi.com/forums/sh...05&postcount=9 Though what form of traction wheels you use is less important than the kind of drivetrain, since most traction wheels are reasonably interchangable. (All AndyMark wheels use the same mount pattern.)

Mecanum wheels are only used in mecanum drivetrains (at least for driving).

Simple omni-wheels are used only to reduce scrub forces when turning tank-style drivetrains that also use traction wheels or in holonomic drivetrains where (typically) 4 omniwheels are set up in a square pattern.

[Trent B]

To elaborate on butterfly drive as Team Neutrino has used it in the past two years:

Is this drive train each corner has a small traction wheel and a larger omni or traction wheel. By using pneumatic cylinders, a module can be moved to allow the robot to drive on either all the small traction wheels (slower, high pushing force, low maneuverability) or on the larger wheels (faster, lower force, high maneuverability).

Here is an older blog post on the modules used last year: http://www.teamneutrino.org/seasons/...bot/butterfly/

[Ginger Power]

Originally Posted by Jon Stratis

There are other drive train configurations available, like swerve, octocanum, and apparently something my students were talking about today called "butterfly"... I'm still not entirely clear on how that one works, but apparently they got it from another team at the regional. However, those are drastically more complex and difficult to do right.

A butterfly drive is an articulating drivetrain with some sort of traction wheels paired with omni wheels (very similar to octocanum). In our configuration (which we kept off our robot for weight purposes and lack of necessity for our strategy) we have a 4" omni wheel paired with a custom made 2.5" traction wheel with rough top tread. Each module is actuated by a 1 1/16" bore 1" stroke pneumatic cylinder. We have found that butterfly drives are not overly complex but I wouldn't recommend using one without experimenting with it in the off season. The team that has really made this design famous is 3928 Team Neutrino. They had a butterfly drive for Ultimate Ascent and it worked extremely well for them. They have an even better version of it on their Aerial Assist bot. I believe Aren Hill a former mentor for Neutrino current mentor for 148 Robowranglers conceived the idea and could explain it much better than I can.

Edit: Trent B beat me to it

[Electronica1]

Originally Posted by Ginger Power

They had a butterfly drive for Ultimate Ascent and it worked extremely well for them. They have an even better version of it on their Aerial Assist bot. I believe Aren Hill a former mentor for Neutrino current mentor for 148 Robowranglers conceived the idea and could explain it much better than I can.

Didn't 148 conceive and run the first butterfly drive in 2011?

[EricH]

Originally Posted by Electronica1

Didn't 148 conceive and run the first butterfly drive in 2011?

Sort of... but you're off by a year.

148 and 217 built a "nonadrive" in 2010--in fact, they collaborated on it. The butterfly was omnis and traction, and a 5th omni was mounted perpendicularly to the rest of the drivetrain (for sideways motion).


I'd actually thought about (and tried to convince my team to do) one back in the mid-2000s I want to say '06 or so, and I know a team that did a traction/traction butterfly-type setup back in '03, so they've been around a while, in concept at any rate. These days, more teams have the resources to pursue 'em, and are more willing to share designs, so they've been getting more common.

[RunZach]

Originally Posted by Jon Stratis

Well, that's a pretty wide open question.

Lets start with mecanum wheels. You need 4, one in each corner of a "normal" rectangular robot. Power each one separately (4 motors, 4 gearboxes), and set them up so the rollers form an "X" when looking from the top of the robot. With this setup, you can drive and turn like you do with a "normal" tank drive, but you can also strafe - move sideways or at an angle without rotating the robot.

With omni wheels, there are a number ways to set them up. Assuming you can keep them all level, you can set them up to get similar control as mecanum drive, by having a pair parallel to each other, and another pair perpendicular to the first (either one in each corner, turned 45 degrees, or one on each side: left, right, front, and back). You can also use them to make turning easier. For example, if you find it difficult to turn, or see that your robot bounces around when turning, having a "normal" traction wheel in the middle and an omni in each corner on each side will make the robot turn a lot smoother. Or if you have 4-wheel drive, having one pair of omni's and one pair of traction (omni's probably in the front) makes turning easy, but it puts your center of rotation towards the traction wheels.

There are other drive train configurations available, like swerve, octocanum, and apparently something my students were talking about today called "butterfly"... I'm still not entirely clear on how that one works, but apparently they got it from another team at the regional. However, those are drastically more complex and difficult to do right.

Do mecanum wheels have more traction then omni wheels, or KOP wheels for that matter?

[audietron]

A favorite of ours is using traction wheels on a six wheel drop center drive. The tractions that you choose kind of varys on what you want to be doing more of. We have found that the 4 inch versawheel spike tread is really nice for good grip and still being low to the ground. The 3.25 inch diamond tread wheels are also good but wear down a bit quicker. so these are bit better for more offensive purposes.

[Abhishek R]

Originally Posted by RunZach

Do mecanum wheels have more traction then omni wheels, or KOP wheels for that matter?

Mecanum wheels have the least traction out of any wheel.

[Electronica1]

Originally Posted by EricH

Sort of... but you're off by a year.

148 and 217 built a "nonadrive" in 2010--in fact, they collaborated on it. The butterfly was omnis and traction, and a 5th omni was mounted perpendicularly to the rest of the drivetrain (for sideways motion).

I was considering nonadrive as a different drive because of the 5th omni, they did not run the drive that everyone now calls butterfly drive until 2011.

[EricH]

Originally Posted by RunZach

Do mecanum wheels have more traction then omni wheels, or KOP wheels for that matter?

For omnis, in general, yes, mecanums have more traction. General caveat that there are omni setups that would that statement would not apply to.

As far as KOP wheels, most mecanums won't have quite as much traction. That tends to be due to the fact that mecanum traction surfaces happen to be rollers... However, a mecanum system can give a fair amount of pushing power--not that you EVER want to have to USE that pushing power! In the individual wheel, the effect is more pronounced than in the whole mecanum system.


By the way, one thing that I should make clear: The above statements are for the whole drive system, not just one wheel. That's something that you need to take into consideration.


Some other things to consider: Plaction wheels (or other wheels with conveyor-belt tread attached) are very good in a pushing match. Blue nitrile tread is favored, but wedgetop and roughtop tread are common. Colson wheels are reputed to give good traction--I've never dealt with 'em in FRC applications, so I can't comment on that.


(Side note: you're asking about mecanums having more traction than KOP wheels--you do know what a mecanum wheel is, right?)

[EricH]

Originally Posted by Abhishek R

Mecanum wheels have the least traction out of any wheel.

Sorry, that's omnis you're talking about. I'd take a mecanum drive over an omni drive any day.

[Abhishek R]

Originally Posted by EricH

Sorry, that's omnis you're talking about. I'd take a mecanum drive over an omni drive any day.

Our main drive is omni, and we've been able to push around mecanum bots all day, in both first and second gear (Orlando seems to be heavy on mecanum compared to Alamo).

On a side note: I have yet to see any bot that has ever made me want to use mecanum, even when we used it pretty effectively in 2010.

[JVN]

Originally Posted by Electronica1

Didn't 148 conceive and run the first butterfly drive in 2011?

It is difficult to track the origins of an idea...

In the pre-season of 2010 148 build a drivetrain which combined an H-drive (5 omni wheels, 4 in "normal" configuration and 1 sideways) with 4 drop down traction wheels. We called this a Nonadrive, since it has 9 wheels and we really like the number 9.

In 2010 we ran this same configuration on our competition robot Armadillo.

In pre-season 2011 we prototyped an Octocanum drivetrain (an articulating drivetrain which switches between 4 mecanum wheels and 4 traction wheels). This is something which I believe several other teams had done in the past.

We didn't like the performance of the mecanum wheels we had at the time, and switched them out for omni-wheels as part of a new experiment. This configuration could not move sideways, but had some cool features we liked. We jokingly called it "Butterfly" drive since you could swat it aside like a butterfly when it was in omni-mode. I believe we were the first to experiment with this configuration.

In 2011 we ran this same configuration on our competition robot Raptor.

We ran an evolved configuration in 2013 on our robot Viper.

This year, we're running another cool evolution of the Nonadrive which involves 2 center Omni-Wheels, which we're calling the "Decadrive." This is on our robot Vader.

We like articulating drivetrains. We've been playing with them since pre-season 2010, and we will probably continue to play with them. We're actually starting to work on a "History of the Robowrangler Drive" which will hopefully turn into something cool for our website.