Mecanum vs Treads

[StevenB]

I suggest you count the number of Einstein robots which used treads and mecanums.

Don't count on mecanums to compensate for a poor harvester design, and don't count on treads to make you immune to defense. Both will disappoint. In this game, where the field is wide open, the maneuverability of mecanums is not as beneficial as it has been in some years. You'll probably need to make some full-field sprints, and both mecanums and treads sacrifice power that you'll desparately want.

[nikeairmancurry]

Originally Posted by StevenB

I suggest you count the number of Einstein robots which used treads and mecanums.

If I remember correctly that number is still 0 for mecanums.

[Kevin Sevcik]

Also important, mecanums need all four wheels solidly on the ground to function correctly. See that drawing of that bump around the entire pyramid? That bump means mecanums are going to hate driving near the pyramids. Any precision driving you try to do near the pyramids is guaranteed to take twice as long on mecanums as it would on a 6 wheel drive.

[MARS_James]

Originally Posted by Kevin Sevcik

Also important, mecanums need all four wheels solidly on the ground to function correctly. See that drawing of that bump around the entire pyramid? That bump means mecanums are going to hate driving near the pyramids. Any precision driving you try to do near the pyramids is guaranteed to take twice as long on mecanums as it would on a 6 wheel drive.

I don't understand this at all, how does the bump make mecanums not work? We used them last year and got double or single bridge balances very easily

[Anupam Goli]

Originally Posted by MARS_James

I don't understand this at all, how does the bump make mecanums not work? We used them last year and got double or single bridge balances very easily

It has to deal with weight. Meccanum drive relies on an even weight distribution on each wheel for accurate strafing and turning. if you don't have that, the robot will strafe in a slightly offset angle. Introduce a different height for 2 wheels, and suddenly your back wheels have more weight than your front. It only affects strafing though, if I remember correctly.

Also, some non selling points for meccanum:

Actually learning to take advantage of strafe is harder than it looks. Have you ever played Halo and strafed at all? I don't think i've strafed at all when fighting in Halo 4. I'll bet when you play halo, the majority of the time you move the player's orientation and have him walking in that direction instead of strafing. The only times I strafe is to snipe and to avoid shots, both of which probably won't be done in this year's FRC game.

Meccanum also uses the vectors of motion on the wheel to produce strafing motion. Unless your PID is tuned to godly precision, you're always going to have some cancelled force, reducing your strafing accuracy and your power.

Also, the coefficient of friction on those wheels is low. Unless your control system has taken account for your driver
s tendencies, its going to be a lot easier to slip and waste even more drivetrain power.

See if these outweigh any advantage you think you may be getting. I don't know, they might for you.

[JamesCH95]

I would questions two things in your thought process.

First do you really think picking up frisbees will be worth compromising your drivetrain decision? It may be an okay strategy for qualifications, but if you're picking up non-trivial amounts of discs in eliminations it means either 1) your alliance partners are missing lots of shots (very bad, you probably can't make up for them) or 2) your opponents are missing lots of shots (in which case they aren't a very serious opponent anyway) or 3) it's the last 30s of the match, human players are throwing discs in, and you can reliably score more points by hanging than a last-minute grab and shoot of a few discs.

Second, if your robot is touching your pyramid you're more or less protected from being interfered with while shooting. Additionally, your robot can be aligned against the pyramid to help your aim.

In my mind you don't have a really strong argument for either mecanum wheels or treads. To pick between the two I would say treads would make your more effective at breaking past defensive robots, provided they could be fast enough to effectively traverse the field end-to-end (10-12ft/s I'd guess). However, I think a 6WD drivetrain would be at least as effective as treads.

[philso]

Originally Posted by Kevin Sevcik

See that drawing of that bump around the entire pyramid? That bump means mecanums are going to hate driving near the pyramids.

Kevin is wrong! The mechanums are innanimate objects so they are not capable of hating. It will be the pit crew who have to fix the mechanums after the drivers bump into the lip around the pyramids who will be doing the hating.

At Alamo last year, we learned that we had to check that all the rollers on all four mechanums were turning freely after each and every match. If a roller was binding, we removed it and bent the aluminum spider back out. Bumping into the edge of the bridge (lowered) was just one way to bend the aluminum spiders. Just a few (2-3) binding rollers caused the robot to deviate from the expected path in unpredictable ways. We could see the driving performance degrade throughout a match. I am sure this messes up your force vector diagrams.

We also found that the rollers on our mechanums would bind when new because the urethane had coated the inside of the brass bushings. It took a lot of work to disassemble them and clean them out so that they would spin freely.

www.youtube.com/watch?v=1mrNH0Czq0Il

[MamaSpoldi]

Originally Posted by Wing

It has to deal with weight. Meccanum drive relies on an even weight distribution on each wheel for accurate strafing and turning. if you don't have that, the robot will strafe in a slightly offset angle. Introduce a different height for 2 wheels, and suddenly your back wheels have more weight than your front. It only affects strafing though, if I remember correctly.

In our experience, a properly implemented gyro-stabilization in the control loop for the mecanums make them very controllable and maneuverable. In addition you can certainly take advantage of the protection of touching the pyramid for shooting without fear of being pushed/bumped.

[Kevin Sevcik]

Originally Posted by MARS_James

I don't understand this at all, how does the bump make mecanums not work? We used them last year and got double or single bridge balances very easily

Take a look at the force vector diagram for the mecanum drivetrain. The wheels always apply force at 45 degrees from the direction of the wheel rim. The only reason you can drive straight with them is because opposite pairs of wheels push against each other, cancelling the sideways force and resulting in straight motion. When you take one of those wheels out of the equation or start shifting weights unevenly between wheels, things start getting wonky pretty quickly. The fact that the rollers aren't perfectly frictionless means driving straight isn't affected quite as much as strafing, but the effect is still there.

[Ryan Dognaux]

I'll be the voice from the other side since so many seem to immediately dismiss mecanum wheels.

Originally Posted by Kevin Sevcik

Any precision driving you try to do near the pyramids is guaranteed to take twice as long on mecanums as it would on a 6 wheel drive.

I don't think this is true, at all. Guaranteed? Would love to know why it's guaranteed.

One set of wheels, either front or back, should have some kind of suspension or adjust-ability built in to compensate for the variations in the floor around the pyramid in order to maintain driving as usual. However, I'd argue that even without it you would probably still be able to strafe with two front wheels on the raised part & two wheels off the raised part. Even weight distribution would be important but it would still work, perhaps with a little driver compensation involved.

If your team is considering mecanum wheels, it needs to be because your team thinks it gives you a competitive advantage and that it plays a part in your overall design strategy. Maybe your frisbee intake from the slot is on the side of your robot, so you can strafe into the wall and pick up frisbees but have your shooter on the front or back of your robot. That's just one example. A mecanum drive also means you're choosing to play pure offense, which isn't a bad thing. Give your driver enough time to learn how to really use the drive effectively.

My opinion is that you have a wide open field this year, fewer obstacles than last year to navigate around and a quick mecanum drive could be very effective if the driver is well-versed in using it.

I don't get this notion of dismissing mecanum drives just because they haven't 'been to Einstein.' The majority of teams in FIRST haven't been to Einstein, that doesn't mean the choices they made or will make for their robots are wrong.

[buchanan]

Mecanum drivetrains, like every other, have advantages and disadvantages, but some of the disadvantages being discussed here are overstated and/or misinformation.

Even weight distribution is optimal, but not an absolute requirement for straight driving. So long as each wheel has sufficient loading for sufficient traction, it will go the way it's supposed to. The problem only arises when you're at the limit, trying to get more acceleration/deceleration/pushing force than the wheels can transmit. This is true for every drivetrain, though holonomic ones are generally to be more sensitive to it and have more interesting behavior when it happens. The better the weight distribution, the higher the limit, but until there's slippage behavior stays correct.

Concerns about non-flat surfaces tend to be exaggerated for two reasons. One, it's not quite true that loss of traction on one (or even two) wheels guarantees incorrect behavior. The effect of traction loss varies between nothing and complete failure to move, depending on which wheels lose traction and what motion is being attempted; in practice it's usually something in between. Two, this criticism assumes an infinitely rigid chassis. Even if you could make such a thing, it's a basic of holonomic drivetrain design with >3 wheels that you don't try. In practice, even without intentional flexibility/suspension, a basic kitbot type frame has all the flex necessary to keep things like tower base plates from being problems.

[Anupam Goli]

Originally Posted by buchanan

Mecanum drivetrains, like every other, have advantages and disadvantages, but some of the disadvantages being discussed here are overstated and/or misinformation.

I'm not sure if experience is misinformation though. In 2012, my old team (1002) used meccanum wheels, and the center of gravity was somewhere close to the back. This affected driving in more than one way. The robot strafed at offset angles, and sometimes, especially on the key, the robot wouldn't even go straight properly! It was not at all an optimal experience. I'm not trying to say don't do meccanums, I'm just trying to say that you should keep weight distribution in mind.

[buchanan]

Experience is important, and often trumps theory. But it is possible to go wrong tracing back from practical experience to theoretical root causes.

There are challenges in getting a holonomic drive to go straight, but they probably have more to do with how computed wheel speeds map to actual wheel speeds than with traction under low/moderate force. In particular, motors tend to be non-linear, and since holonomic drives regularly run different motors at different speeds/loads, the final wheel speeds relative to one another end up different than the ratios the drive calculations ask for.

Closed loop motor controls can prevent this problem; gyro feedback can compensate for it. Without some such mechanism, it's hard to get these things to drive straight, no matter how good the weight distribution.

Weight distribution matters (for other reasons), and driving straight is a challenge (for other reasons), but they're not linked quite so directly as it might appear.

[Dumper FTW 435]

I didn't know there was so much hate for mecanums

I would like to say that my team used mecanums quite effectively in 2011 and we were able to get it to work near perfectly without dealing with weight issues or most of the other issues people claim mecanums chronically suffer from. Of course, I think 2011 was a better year for mecanums for this year, but after seeing both treads and mecanums work well, I don't think we should just dissmiss one or the other without seriously considering them. I'm more curious about specific things this year that would suggest treads over mecanums or vice versa, I think the general pros and cons of treads vs mecanums are already well established.

[JamesCH95]

Originally Posted by Dumper FTW 435

I didn't know there was so much hate for mecanums

I would like to say that my team used mecanums quite effectively in 2011 and we were able to get it to work near perfectly without dealing with weight issues or most of the other issues people claim mecanums chronically suffer from. Of course, I think 2011 was a better year for mecanums for this year, but after seeing both treads and mecanums work well, I don't think we should just dissmiss one or the other without seriously considering them. I'm more curious about specific things this year that would suggest treads over mecanums or vice versa, I think the general pros and cons of treads vs mecanums are already well established.

In my opinion yhey ought to be compared to the pluses and minuses of a wheeled tank drive and various wheeled tank drive configurations with omni and traction wheels. Treads and mecanum drives aren't the only drives out there.

In 2010 we used mecanum wheels with reasonable success without using any suspension other than a flexible chassis. Driving was reasonably consistent even getting onto and over the bumps. The point many posters are trying to make is that you should be a little careful when designing a mecanum drive.

Edit: there are some really good posts out there discussing the theory, implementation, and control of a mecanum drive by Ether (among others) a thread search might yield some great information.