6 wheel holonomic drive feasibility

[T^2]

Let's break down your response.

Thanks for your reply. The reason we're trying to push with a mecanum drivetrain is that "not having enough traction and power" is the only argument anyone on my team has made against getting mecanum wheels (and they're saying it a lot).

The "traction and power" issue has largely been addressed already in this thread. To sum up, traction in the forward and reverse direction is not significantly weaker on mecanums compared to standard wheels, and forward/reverse power is reliant only on the motors and gearboxes. In the sideways direction, of course, mecanums lose out in both. (There are many threads explaining why this is.)

If there's a way to have both in addition to holonomic steering, there would be no reason not to get the wheels (besides price and assembly and possibility that it won't help at all and whatnot)

You're completely wrong. Not only are there significant mechanical downsides (weight, size, complexity, number of gearboxes required) to mecanum, but they also come with a huge opportunity cost. The time and money you waste on those things could be much better spent training your students to make a drivetrain that works, and what's left over can be spent on the rest of your robot.

[Ether]

Originally Posted by T^2

The "traction and power" issue has largely been addressed already in this thread. To sum up, traction in the forward and reverse direction is not significantly weaker on mecanums compared to standard wheels

For properly operating mecanums (e.g. rollers can spin freely etc), and for the same tread material, mecanums have less traction in the fwd/rev direction than a standard wheel.

[ekapalka]

Originally Posted by T^2

Let's break down your response.

Originally Posted by T^2

You're completely wrong.

Loving your enthusiasm. I'm pretty sure I covered all my bases in my previous response in the parenthesis

(besides [...] the possibility that it won't help at all and whatnot)

Whatnot means I'm not sure of what will happen. Also, we already have the wheels, it's just whether or not we want to use them that's an issue.

Originally Posted by T^2

The time and money you waste on those things could be much better spent training your students to make a drivetrain that works, and what's left over can be spent on the rest of your robot.

I'm getting the feeling that you have something against mecanum wheels... well, to each his own :) We have plenty of drivetrains that work, and even more people who know how to use them. Teams that decide to use mecanum wheels have to start somewhere, right?

[Pault]

Originally Posted by ekapalka

I'm getting the feeling that you have something against mecanum wheels... well, to each his own We have plenty of drivetrains that work, and even more people who know how to use them. Teams that decide to use mecanum wheels have to start somewhere, right?

My opinion (and I think his opinion, and the opinion of many people on this forum) is that just because you can build a mechanum drive doesn't mean you should. Maybe your team thinks differently, and thats perfectly fine. But the point was that the resources you put into a mecanum drive would be better spent on improving either your tank drives (no tank drive is perfect, there is always room for improvement) or the other mechanisms on your robot. Again, thats an opinion, and like you said, to each his own.

If you have want your robot to have pushing power, mecanum is definitely a terrible choice (this is an opinion backed by a lot of evidence). The are drivetrains out there that can give you omnidirectional movement and pushing power (swerve, octocanum, and probably some other rare ones), but only the best teams (think IRI caliber) are able to make one without devoting nearly all their resources to it. Generally, unless your strategy to play a certain year's game revolves completely around omnidirectional movement with pushing power, you should just stick with tank drive (or mecanum if you really insist). (I'm addicted to parenthesis (like, really addicted)).

[Nate Laverdure]

Originally Posted by Pault

My opinion (and I think his opinion, and the opinion of many people on this forum) is that just because you can build a mechanum drive doesn't mean you should.

Now is the perfect time to build a mecanum drivetrain. Your experience may differ, but I couldn't understand mecanum's bad reputation until I built my own version and saw firsthand how the drivetrain's actual behavior differed from the way I'd imagined the system would behave. If you get it done in the next 4 months, you'll be able to carry all of your "lessons learned" into the 2014 build season.

[IndySam]

Originally Posted by ekapalka

I'm getting the feeling that you have something against mecanum wheels... well, to each his own We have plenty of drivetrains that work, and even more people who know how to use them. Teams that decide to use mecanum wheels have to start somewhere, right?

Mecanum wheels are a wonderful exercise and learning experience for a team. If you guys want to experiment with them I say go for it.

When it comes to winning matches however, remember that the game should decide what type of drive you use, a mecanum drive might not match the game. We have done it once with great results and I could see them being very helpful in one other season I have participated in, that's 2 out of 9 seasons for me.

[Nemo]

Originally Posted by IndySam

Mecanum wheels are a wonderful exercise and learning experience for a team. If you guys want to experiment with them I say go for it.

When it comes to winning matches however, remember that the game should decide what type of drive you use, a mecanum drive might not match the game. We have done it once with great results and I could see them being very helpful in one other season I have participated in, that's 2 out of 9 seasons for me.

Which two games? I assume 2011 is one of them.

[evanperryg]

Originally Posted by ekapalka

How about this: a 6-8 CIM drivetrain (not necessarily FRC legal) made for pushing (i.e just throw on some extra motors for forward pushing). That HAS to add at least some additional pushing/pulling power, regardless of how much traction mecanum wheels have (for all intensive purposes, the mecanum wheels could be plaction wheels).

The thing about mecanums and omnis is that they are designed to slide. That, in fact, is the defining aspect that makes them different from any other wheel. That extra omni might help you push forward and backward(emphasis on 'might'), but it will be absolutely useless pushing sideways because the rollers are passive. If you want to have omnidirectional drive that can push, you have 2 options: swerve or octocanum. And, in order for either of these options to be effective at pushing, you have to execute the design flawlessly. You are right, more motors adds more pushing power(if you do it right) but most of that power will be negated by the fact that the wheels are designed to slide around.

Originally Posted by T^2

You're completely wrong. Not only are there significant mechanical downsides (weight, size, complexity, number of gearboxes required) to mecanum, but they also come with a huge opportunity cost. The time and money you waste on those things could be much better spent training your students to make a drivetrain that works, and what's left over can be spent on the rest of your robot.

I feel like I remember seeing Karthik posting something that was very similar to this... Anyway, I agree fullheartedly. In addition to what you mentioned here, they also can have some severe mechanical issues. We did mecanum once, back in 2010 (first and last time doing holonomic). Somehow, something got bent so we could only strafe. That was also the heaviest, the least reliable , and the most complicated drivetrain we have ever built.

In my opinion, if you want to do a holonomic drive, mecanum is possibly the worst option. The wheels themselves are expensive, it offers no potential for pushing, and it is a nightmare to program. A well-executed tank drive and a decent driver will maneuver just as well as a typical mecanum drivetrain. If you really, desperately want to do holonomic drive for some reason, I would suggest either swerve or octocanum. Remember, both of these are extremely complicated, weigh more than tank, and without a good driver they will not make your robot any better.

How many bots that won champs/IRI used mecanum? None.
How many bots were even on Einstein that used mecanum? None.
How many mecanum bots were invited to IRI? One, and it was probably the best executed mecanum drive this year.

[Ether]

Originally Posted by evanperryg

[mecanum] is a nightmare to program

Could you elaborate a bit on that? What did you have in mind?

[ekapalka]

Originally Posted by evanperryg

[...] and it is a nightmare to program.

The only thing that makes me feel any better about programming mecanum wheels is this information (courtesy of Ether; thanks, by the way) and the mecanum classes in WPI (we're using C++), which I have been reading over and over since I found out we had the mecanum wheels (that doesn't mean I should rely on it, but it gives us hope that we can program it). We already have four wheels, four gearboxes, four motors, and four Talon SRs, all with no dedication at the moment. We're currently designing the chassis with room for a central gearbox (see original post), not necessarily for omni wheels, but for the possibility that we go back to skid steering. Hopefully it'll be water or plasma cut from sheet metal, so adding in additional holes for such things won't be that much more difficult (if we're getting it water-jet. If it's plasma cut it won't quite be as easy). Thanks for your input, by the way :)

[Oblarg]

Mecanum is not a nightmare to program. It has been done many times before; the knowledge is there, and so is the code itself if you wish to download it. It becomes somewhat harder if you want field-oriented control, but the difficulty there is with the gyro, not with the actual drive.

Mecanum is a fairly good, cheap solution to the omnidirectional problem. It is not a 100% solution; it has notable shortcomings (effectively lower coefficient of friction than traction wheels with the same material, lots of frictional losses when moving sideways, losses due to imperfections in wheel design/construction), but also notable benefits (ease of construction, ease of maintenance, relatively low cost). It is not going to compete with a swerve drive or with a hybrid drive, but it will give you an agile robot (acceleration in FRC, with the exception of 2009's silly teflon wheels/driving surface, has never been traction-limited) and that can strafe. For some games, this is all that is needed; 449's use of mecanum in 2008 was a huge asset for our team, as the anti-roadblock rule in that game made the lack of pushing power a near non-issue. For others, it is not.