Is there a way to make a grippy mecanum wheel? I know they are usually low traction rollers, and use those rollers to obtain omni-directional movement, but is there any way to add more traction to the rollers, while keeping the fluid, omni-directional movement?
This is because most people who say this are merely miss-speaking. What they are actually referring to is the ability of non-powered wheels to roll. For example if an macanum drive bot were to be pushed from the side (straffing) it would “roll” as though it had no traction.
In a pure pushing match they perform well. They can suffer from gearing problems because they generally have to be geared high for good speed and are rarely used with shifting gearboxes so they can suffer when it comes to power.
If you look at a mechanum wheel or an omni wheel, they have rollers. They achieve omni-directional movement through the manipulation of force vectors (which Ether will be able to explain far more eloquently than I will). The reason they are considered low traction is because the rollers on the wheels are not powered, and so the robot is resting, essentially, on tiny un-powered wheels, which are far more push-able that powered wheels.
The reason mecanum/omni robots can sometimes win in a pushing match is that if the pushing force is parallel with the axis of rotation, then the rollers aren’t spinning (and so as the angle of the pushing force approaches parallel to the axis of rotation, the less the robot can be pushed).
I hope that helps
EDIT: It’s already been answered, but I hope that adds.
In all seriousness though the real issues with the mecanum wheels in terms of pushing are this:
-They aren’t 100% effective at transferring power, a lot of it is lost. This forces you to have to gear higher to be able to go at a reasonable speed which makes you less capable of pushing. It is difficult to do a shifting gearbox with mecanum wheels because each wheel has it’s own gearbox and shifting gearboxes are quite heavy.
-They are easily pushed sideways. They are designed to do this so that you can strafe, but most of the time when pushing is done it is not done front of pushing bot to front of bot being pushed but front of pushing bot to side of bot being pushed.
Hope this helps. As a team that used mecanum wheels last year I would advise against them because when the eliminations roll around and you are facing defense, being able to be pushed sideways easily really hurts your chances and it makes winning very difficult.
Trust me, I’m not going to use those things ever in my 4 years as an FRC student. I’m just curious about them, why they are considered “low traction”, and how one could manipulate them to make better traction.
You are right. They are not Omni’s. We’ve used both. Though they don’t roll easily, they do roll. Try pushing a macanum bot sideways when it is not powered. What I was trying to explain to SuperNerd256 is that a macanum bot in most orientations would roll off the tilted bridge with no power applied to prevent it. And, while we are at it, given identical gearboxes and friction coefficients of wheels, macanums would lose in a head to head with conventional wheels. This is possibly what SuperNerd256 was referencing.
You could implement some type of brake, electronic or physical. Either way, traction is only half the problem most teams have with mechanums. When strafing, you have less than 100% power and speed off a traction drive(I believe close to 60%). Therefor, in many situations a traction drive can get to the same place as a mechanum in equal or less time, even if there is a turn involved.
Actually watched 357 shed half of a mecanum this season at Philly. As another team that uses mecanums, 1370 had tremendous success as a defensive bot, pushing other bots around the field. While mecanums do have their limitations, they are nowhere as bad as their reputation will lead you to believe.
Just a few points, as this debate is never-ending every year.
We geared our mecanums 17.5:1 (CIM shaft to wheel), and found this to be perfect. at 12:1 we were TOO fast. The same speed on last year’s bot with kit wheels gave us a perfectly capable and quick base.
The weight of shifting 4 gearboxes isn’t extreme. We use dewalt gearboxes, with a cim and mount it’s under 5lbs each. For comparison, the AM super shifter is 4lbs without CIM’s. The problem becomes shifting all 4 at the same moment.
This year is unique, where you’ve got a large protected area. An efficient mecanum can out maneuver a defensive robot, as we’ve done many times this year.
Except Kevin is wrong. I’ve tried pushing unpowered mecanums sideways (software testing for 2337 in 2010). I was physically unable to push the robot sideways. Along the diagonals was a different story though. Still not easy but most definitely easier. None of this involved any sort of fancy software which would enable the wheels to hold position.
As for grippy mecanums, if you ever get a chance talk to 357 about their process. Their wheels have quite a bit of grip and they are one of the few teams I’ve seen use mecanums well.
However, I am a firm believer in the theory that if you are pushing with a mecanum wheeled robot you are doing it wrong.