6 wheel holonomic drive feasibility

[ekapalka]

My team has been tentatively considering designing a 6 wheel drive using two sets of mecanum wheels with a centrally located set of omni wheels (a little like the one on this thread) to give the robot pushing power. When I say tentatively, I mean like we may-or-may-not be designing our next-gen chassis with holes for two central gearboxes (which could also be used in the event that we give up on holonomic and resort to skid steering). What I was wondering is how to control the center wheels so as to not conflict with the mecanum wheels while still providing pushing power. I've considered having each side sync with the speed of the mecanum wheels if and only if that particular side is turning in unison, but that would cause them to drag when strafing at an angle or when rotating while strafing. I've also considered setting them to match the averaged RPM of all four wheels, but I don't think that would work either. One other thing that came up in discussion was to have a non-powered wheel to measure the absolute speed forward of backwards and sync the middle wheels with that, but that wouldn't work if you're in a pushing match and not moving at all. What do you think? Is the only way to do this really to use a ball differential?

[AdamHeard]

You will need some sort of differential to make that work.

This drive won't give you more pushing power (per physics, actual results may slightly vary) in any meaningful way.

Your efforts are probably better focused on better solutions.

If you're dead set on holonomic with good pushing power, octonum (488 2011, 525 a few years?) is probably what you want to look at.

[IndySam]

It is a big misnomer that mecanum wheels lack pushing power, this is really not their weak point. Their big problem is the lack of resistance pushing sideways and omni wheels will not help you there.

[ekapalka]

Originally Posted by AdamHeard

You will need some sort of differential to make that work.

Hmm... I can't believe that there's no way to do this by using encoders and math... I understand why you say it wouldn't help much to use a differential

Originally Posted by IndySam

It is a big misnomer that mecanum wheels lack pushing power, this is really not their weak point. Their big problem is the lack of resistance pushing sideways and omni wheels will not help you there.

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).

Thanks for your replies

[Ether]

Originally Posted by ekapalka

for all intensive purposes, the mecanum wheels could be plaction wheels

What do you mean by "intensive purposes" in this context?

[ekapalka]

I just meant that no matter how much power or traction the robot has, adding two more wheels and 2-4 more motors will add at least some additional traction and/or power (at least in our application). I'm fairly certain that that's what I meant :) We have no plans to go back to plaction (and I don't think it would work too well).

If it's the messed up figure of speech you were asking about, I would like to accredit that to auto correct (it's so intrusive on Android phones). For all intents and purposes.

[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.

[T^2]

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.

It doesn't HAVE to have additional pushing power, at all. The two main failure modes of traction are motor stall, and loss of static friction with the ground. If the second item happens, adding more motors will not do anything.

That's completely beside the point, though. The real question is, why in the world are you trying to push with a mecanum drivetrain?

[ekapalka]

Originally Posted by T^2

The real question is, why in the world are you trying to push with a mecanum drivetrain?

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). 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) :)

[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?