Mecanum or 6WD

[Ether]

Originally Posted by Josh Goodman

speed is irrelevant because whether you have two CIMs or 4 CIMs running your drivetrain, you will go the same speed. Obviously.

It's not obvious. If you have 4 CIMs, you can gear it down less and go much faster.

[indubitably]

From a builder's standpoint, 6WD can be built from the KOP, but macanum is also very easy to build as the mounts mimic that of a 4WD.

From a driver's standpoint, I would have to say that both drivetrains present an equal amount of challenge to drive. The most important thing to remember is to tailor the controls to your liking and don't be afraid to practice with multiple layouts.

From a programming standpoint, 6WD is as simple as it gets. While mecanum can be challenging at first, there are many example programs available that basically brings the need to understand the physics behind mecanum down significantly.

From a strategist's standpoint, I feel traversing the feild quickly will benifit your team much more than being able to strafe in the scoring zone. No one ever said that the tubes needed to be perpendicular with the racks before they are placed. Placing on a slight angle should not be a big deal for a good arm. Also, simply rotating to place may be faster than strafing to place in some situations.

Our team used mecanum last year and I definately felt like it was a strong drivetrain, but seriously lacked in the area of bullying robots. Since i feel like crossing the feild this year is going to be rigged with other robots getting in your way, I feel speed and power are going to be an advantage over omnidirectional movement.

6WD for us this year.

[pfreivald]

Is anybody else anticipating robots getting stuck and/or having a hard time in the lanes (including leaving the lanes around the towers)?

If you have a defense bot preventing you from zipping right out of the lane and past the tower, you want to be able to go around the tower as fast as possible to get by that robot -- or be able to just manhandle it out of the way.

I'm seeing defensive bottlenecks as very possible in this area of the field, but it still doesn't inform the decision between 6WD or mecanum, because each has a 'solution' to the problem.

[Madison]

Originally Posted by Paul Copioli

We had a pretty decent debate between traditional swerve (think 111 or 71), mecanum, Nonadrive (148 & 217), Octocanum (I know someone will do it this year), and 6WD skid.

In this context, what is octocanum? Four mecanum wheels along with four other wheels?

[Chris is me]

I'm pretty sure Octocanum is a Nonadrive-style setup using mecanum wheels instead of omni wheels.

[BrendanB]

Originally Posted by Madison

In this context, what is octocanum? Four mecanum wheels along with four other wheels?

It sounds like 8 omnis since some team did a 6 omni drive last year.

[Josh Goodman]

Originally Posted by pfreivald

Is anybody else anticipating robots getting stuck and/or having a hard time in the lanes (including leaving the lanes around the towers)?

If you have a defense bot preventing you from zipping right out of the lane and past the tower, you want to be able to go around the tower as fast as possible to get by that robot -- or be able to just manhandle it out of the way.

I'm seeing defensive bottlenecks as very possible in this area of the field, but it still doesn't inform the decision between 6WD or mecanum, because each has a 'solution' to the problem.

Yeah, I was thinking this too. One strategy I was thinking about is the human player popping a tube through the feeder so it lands by the safe zone line (still within the boundaries). This would allow for the robot not going all the way in the safezone, therefore harder to get blocked (Can back up or pull out around the tower) and the opposition alliance still can't get the tube.

Just one thought.

[EStokely]

Main reason (as of day 2 of the build) for not doing a Mecanum drive, the work best when each wheel has an equal force on it (weight) the fact that we will be moving arms (or something) around to score will be shifting that weight, admittedly that will be in a "safe" zone. But while on the field these shifts can lead to chaotic motions from a Mecanum drive.

But the drive isn't *completely* off the list yet , but 6 wheel is easy, light, inexpensive, easy to program and drive, predictable, robust, able to run while damaged and we tend to have better years when we use 6WD

[Siri]

Originally Posted by Steve_Alaniz

...It IS possible to make your own Urethane Mechanums and come closer to the traction belt co-efficients. For that matter, replace the rollers on the AM mechanums with a urethane roller... but I'm too lazy...I also have a plan for locking the wheels on a mechanum but have never bothered to build it. No one can have their cake and eat it too.

Valid point. My use of the AM numbers wasn't supposed to be an end-all-be-all commentary on mecanum but rather, on the mecanum that teams asking this question on the Sunday after kickoff will more than likely use. Locking rollers are a beast in and of themselves, no doubt, but they're not a beast you first debate during build season.

That said, I wasn't aiming to dismiss the trade-off either. It's a team choice. Personally, I've never seen the appeal of mecanum, but that's my team/drive team and the games. Simulate it, watch '07 videos*, work with your drive team and builders, weigh your options. Either may be right. What I find odd is the view that this is the year of mecanum simply because of the scoring challenges. I've never seen a game that necessitated a certain drivetrain, and certainly not mecanum. It may be true, but it's a heck of a statement make without testing, especially considering the results in 2007.

*PatrickS has got this down, by the way. Consider that you [general pronoun] may be over-stating the alignment necessary. I dropped many a misaligned tube on the rack in 2007 with no issue. Make some pegs and test it out.

[Ether]

Originally Posted by EStokely

Main reason (as of day 2 of the build) for not doing a Mecanum drive, the work best when each wheel has an equal force on it (weight) the fact that we will be moving arms (or something) around to score will be shifting that weight, admittedly that will be in a "safe" zone. But while on the field these shifts can lead to chaotic motions from a Mecanum drive.

Just to be clear: the force changes themselves do not affect the behavior of the mecanum, unless the force on a wheel (or wheels) becomes so small that the wheel(s) loses traction and begins to slip. Then you'll get the odd behavior mentioned.

In other words, as long as the wheels don't slip, the kinematics remain unchanged.

[Steve_Alaniz]

Originally Posted by Ether

The above statement is not correct.

http://www.chiefdelphi.com/media/papers/download/2739

In the forward direction for example, mecanum is the equivalent of 4 motors driving 4 wheels with the rollers locked, if the vehicle is not traction-limited.

If the vehicle is traction-limited, then mecanum is 41% more force than two motors driving two wheels with the rollers locked.

Not to start an argument but the white paper actually confirms what I was saying. In fact, it states in the body that "Therefore, assuming no roller bearing friction, the “pushing force” (and speed) of the vehicle is the same in the fore/aft and sideways directions." So I don't see how you got the 41% unless you are including friction from the bolt/axle but I would be happy to hear how you reached these conclusions.


Steve

[Charmander]

If deciding on 6WD, what type of 6WD is recommended? Omni Wheels in the middle, all omni, Middle wheel lowered, etc. I am currently leaning toward all omni 6WD because of the increased maneuverability. Any thoughts?

[Madison]

Originally Posted by Charmander

If deciding on 6WD, what type of 6WD is recommended? Omni Wheels in the middle, all omni, Middle wheel lowered, etc. I am currently leaning toward all omni 6WD because of the increased maneuverability. Any thoughts?

Don't do it. Omniwheels in all 6 wheel locations will allow you to be pushed sideways with practically no resistance.

I'd look at 6 traction wheels with a dropped center, 4 traction wheels with 2 omniwheels on one end, or 2 traction wheels in the center with omniwheels on the four corners.

[Paul Copioli]

Madison,

Octocanum is nonadrive with mechanums replacing omni wheels and the kicker drive.

Paul

[Madison]

Originally Posted by Paul Copioli

Madison,

Octocanum is nonadrive with mechanums replacing omni wheels and the kicker drive.

Paul

Got it. Thanks, Paul.

Your prediction that at least one team will build it will come true.