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?
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.
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.
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
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 
What do you mean by “intensive purposes” in this context?
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**.You could do it with independent drive of all 6 wheels (i.e. 6 drive motors, one for each wheel). Control the mec wheel speeds as you normally would (i.e. based on standard mec inverse kinematics), and control the omni wheel speeds based on inverse kinematics of the FWD and ROT command components.
I doubt whether this will provide much (or even any) benefit compared to standard 4-wheel mec though.
That’s exactly what I was trying to say So you don’t think encoders will be necessary? Why would it not add any pushing power?
I purposely worded it the way I did:
Control the mec wheel speeds as you normally would (i.e. based on standard mec inverse kinematics), and control the omni wheel speeds based on…
To control the wheel speeds properly requires an encoder on each wheel. But you could probably make it work somewhat using open-loop voltage control.
Why would it not add any pushing power?
If you custom-made the center omni wheels using high-traction treaded rollers there might be a small difference. Seems like an awful lot of time, expense, and effort when there are better ways to achieve increased pushing force.
1114 has published a fantastic paper on the economics of drive-base selection for different teams.
Drivetrain Selection (or: Why every team should build a tank.)
A good resource. The comments about programming mec and omni seem somewhat dated.
thanks
Just to be clear:
All caps is considered shouting.
Bold is not considered shouting. I bolded it for context only.
No offense was taken, and no apology required.
(I went back and edited the prior post to remove the bold, lest my intent be misunderstood)
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?
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)
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.
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.
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.
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)).
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.