Chainless Mecanum Drive

[Alan Anderson]

Direct drive mecanum is trivially easy. You just need the appropriate reduction in your gearbox for the size of wheels you're using. Like this.

[Clem1640]

We designed a chainless mechanum chassis as a mental exercise during the summer of 2008. Included suspension. We never built this design.

Here's the link:
http://wiki.team1640.com/index.php?title=Mecanum

Cheers,

[spiffyspleen]

Thanks! How heavy is a standard nonadrive? Five motors, 9 wheels, and four pneumatic cylinders, that must add up...

[EricH]

Originally Posted by spiffyspleen

Thanks! How heavy is a standard nonadrive? Five motors, 9 wheels, and four pneumatic cylinders, that must add up...

On a sheet-metal chassis, which can be lighter than a more standard box-tubing chassis.

For further questions on the Nonadrive, you might want to take a look at the following two threads (and yes, there are two, one for each team that used it): 217 and 148--and definitely watch the videos that open the threads. (Warning: watching said videos may cause a sore jaw, due to said jaw hitting computer, desk, or floor.) Unfortunately, I can't seem to find tech specs...

But I do know where CAD files can be found!

To echo what Chris said, you need to know what you're doing to try a nanodrive. They'd have had a hanger on their robots in competition if they hadn't decided to reinforce their drive and make other improvements--and that's another scary part, they improved it from the videos.

[kgzak]

Originally Posted by Alan Anderson

Direct drive mecanum is trivially easy. You just need the appropriate reduction in your gearbox for the size of wheels you're using. Like this.

We learned this the hard way. During Overdrive we tried a mecanum drive and our robot would not move. At first it was thought to maybe be a code issue but turned out to be that our gear boxes (stackerboxes) were not geared down enough to move our robot.

[Chris is me]

Not to take away from innovation and technical achievement, but I don't really see the Nonadrive as some kind of new Holy Grail in FRC drivetrains, and it is absolutely not something a team without the engineering resources to design a direct drive mecanum drive should do. It's not exactly a cheap, light, or simple drivetrain.

[spiffyspleen]

So if we're using the 6 in mecanums, what is the appropriate gear box reduction?

[Ether]

Originally Posted by spiffyspleen

So if we're using the 6 in mecanums, what is the appropriate gear box reduction?

12.75:1 seems to be a reasonable compromise between speed and torque (more or less depending on the game, of course)

Just curious, can you share a bit with us about your team's decision process that led you to your decision to explore mecanum wheels?

[spiffyspleen]

Our main reason for trying them now is so that we have experience with them if we decide to use them for the FRC season. We bought them last year but decided not to use them for FRC because we didn't have any experience with them and we didn't think they would work well on the ramp. We want to learn how to use them because they are one of the most manueverable drive trains. From what I saw at the Portland regional last year the teams that used them were able to position themselves very well with the ball and goal and ended up doing pretty good. Basically, we think that they could be advantagous in this years competition and we want to be able to use them if they are.

[EricH]

Originally Posted by spiffyspleen

Basically, we think that they could be advantagous in this years competition and we want to be able to use them if they are.

The key words in this statement are: "could be" and "if".

Right now is a really good time to do a mecanum drive as an offseason project. You get a chance for rookies to build a robot, you test out new technology, and you gain knowledge and experience.

But the problem is that when all you have is a hammer, every problem looks like a nail. When Kickoff shows up, you will be tempted to use mecanum, even if the game is reminiscent of 2002 (where high traction, great pushing and low maneuverability were great benefits). Trust me, you will be tempted.

So, what you do is you take your offseason robot and a competing drivetrain (say, 6WD drop-center, or a 4WD built from the Kitbot) and run them through a series of tests, either individually or against each other with an offense vs defense. Then you evaluate which is better.

I'll let you in on a little secret: 330 has had the capability to use mecanum drive since 2005; development/small-scale prototyping started around 2003, with a full-scale base during the 2005 build season (thanks to the Kitbot frame). The closest they've ever come was 2009 with a turning 6WD system in the middle of their robot. The only time they've used wheels with rollers in competition was omni wheels in 2010, for added agility on a 4WD. They just haven't found it advantageous to use them...yet.

[Chris is me]

Actually, Eric, I'd say they're doing exactly the right thing by trying it now. There's really no better of a time to see if you can get it working and learn about the drawbacks of such a system. I mean, one could say the very same thing about a six wheel drop prototype (ignore for a minute that 6 wheel drop has been consistently very successful).

If I'd say anything it's to make sure you guys are very, very critical of your final project and to make sure the advantages are actually called for in your design process, and that the disadvantages are not absolute. I would say a lot of teams that build mecanum drives get a little distracted by the whole coolness factor and build it without regard to the realities of pushing and whether or not strafing is required.

[EricH]

Originally Posted by Chris is me

Actually, Eric, I'd say they're doing exactly the right thing by trying it now. There's really no better of a time to see if you can get it working and learn about the drawbacks of such a system. I mean, one could say the very same thing about a six wheel drop prototype (ignore for a minute that 6 wheel drop has been consistently very successful).

Which is exactly what I said in the second paragraph (right after pointing out the key words).

I also pointed out that just because you have an uber-widget doesn't mean that you should use it, but having it is nice because if you should use it, you can do it very quickly.

OK, so I also suggested a comparison test that should be easy to whip up come build season...

[Ether]

Originally Posted by spiffyspleen

Our main reason for trying them now is so that we have experience with them if we decide to use them for the FRC season. We bought them last year but decided not to use them for FRC because we didn't have any experience with them and we didn't think they would work well on the ramp. We want to learn how to use them because they are one of the most manueverable drive trains. From what I saw at the Portland regional last year the teams that used them were able to position themselves very well with the ball and goal and ended up doing pretty good. Basically, we think that they could be advantagous in this years competition and we want to be able to use them if they are.

Well, there's a lot of information here on the CD forums about mecanum wheels, both pro and con. If you haven't yet searched the forums and read the links, you might want to consider doing that.

[JesseK]

Spot on about the direct-drive Mecanum setups. It's very efficient, though be sure to clean the rollers every couple of matches.

An interesting thing about nonadrive is the penta drive configuration. 5 omni wheels can be driven by 3 motors for a team with limited resources that wants agility.

- Less code complexity than most non-standard (skid) drive systems
- Less expensive than Mecanum or Killough (traditional 4-wheel Omni)
- Arguably more traction than "out-of-the-box" Mecanum, and definitely more traction than Killough
- In recent years, it would leave at least 1 CIM available for other things
- Can be used in 4WD or 6WD skid configurations, though that is highly coupled with need-based strategy

I've toyed with a concept that uses 5 wheels in nonadrive's pentadrive configuration with the middle wheel being a traction crab module that pivots via pneumatic linkage (pneumatic to keep the code simple). This concept gives a mid-grade complexity while also potentially providing some of the agile+tractive advantages of nonadrive without the weight. Of course it doesn't have the natural anti-turn capability of nonadrive, but that's the trade off. The concept was inspired by nonadrive and the limited pictures I've seen of 330's 2009 bot.

Unfortunately I've been overruled for our offseason prototype due to team survivability, so maybe another team will prototype it?

[Chris is me]

I would suggest that a slide drive config that wants at least 1 CIM for something else use 1 CIM + 1 FP in the drivetrain instead of just 1 CIM. Acceleration is helped a pretty good amount.