Mecanum vs Treads

[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.

[Bob Steele]

During this discussion there seems to be some disagreement about what you meant by "treads". Did you mean regular treaded wheels or a tracked system?

This will be a game that is going to have a ton of contact. Mechanums have an inherent inefficiency because they can't use all of the force directly in one direction. They, therefore, are not by any stretch of the imagination a means to push around other robots or push through other robots.

They do have the ability to be agile and avoid some of the contact.

Because of the "pinch points" on either side of the pyramids this year... this agility might be compromised to a certain extent. They are great in the open field... because other robots can't strafe. As I said this is somewhat negated by the way the field is this year.

I think you should decide what you want to do and just be aware of the limitations AND the advantages of the mechanum (or any other drive).

You might look up a cool drive called the Octocanum drive system implemented by Team 488 in 2011. It has the advantages of both systems, albeit it is complicated... very cool though...great design work done by a great team (Go XBOT!!!)

No hate here...just opinions....why may vary.... (<understatement)

[wireties]

FIRST Team 1296 used mecanum the last couple of years. As mentioned in this thread there advantages and disadvantages. To make effective use of a mecanum setup the frame & drive train requires precision design and assembly (so all 4 wheels are on the ground at all times), the weight distribution must be close to ideal (so the same amount of weight is over each wheel), the velocity of each wheel controlled (so encoders required at each wheel) and the software well done. If you do it correctly, mecanum is nice. We rarely met a robot that could block us or push us around, we were just too fast and too manoeuvrable. Our drivers could fake out defensive bots kinda like a running back juking a linebacker, spin around them etc. But we did not make it to St Louis, it might not have gone as well against better competition.

That said 1296 is going with a 6-wheel drive this year and building a climber.

[Ether]

Originally Posted by 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.

^^Much wisdom here^^

One other factor to mention: precision mecanum wheels are expensive. The affordable ones most teams use can have significant variation in roller friction and roller axial free-play if not very carefully assembled. This can also affect the precision of the robot's motion.

[Taylor]

Originally Posted by nikeairmancurry

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

If I remember correctly, 100% of 9-sided teams won Einstein. So you should do that.

Just because something has never been done, doesn't mean it's impossible. See: everything ever.

[Peter Matteson]

Originally Posted by Taylor

If I remember correctly, 100% of 9-wheeled teams won Einstein. So you should do that.

Just because something has never been done, doesn't mean it's impossible. See: everything ever.

I believe it's 100% of 9 sided robots, it was 3 wheeled.

No 9-wheeled robot has made it.

Also I believe 3 wheeled robots are 2 for 3.
67 -2005 Champion
16 - 2008 Finalist
148 -2008 Champion

[cmrnpizzo14]

Originally Posted by StevenB

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

I don't think that this is ever a good strategy......

341, 67, and 1717 did not make it to Einstein last year, ergo shooting 5+ balls in hybrid, utility arms, and unique end game strategies are a bad choice.

If this is how we think, there would be no new ideas ever.

Out team personally is using mechanums this year because we believe that being able to maneuver around the defense is more effective than withstanding the hits or pushing bots. I don't think that a bump less than .5" tall will be any sort of noticeable problem for the mechanums. I might be wrong, but I wouldn't be too worried about it.

Also, nice job, Eric, on those treads they are sweet. (and dreadfully heavy)

[Chris C. 2410]

That may have depended on which mecanums you used.

What I would say is from a repair standpoint, mecanums will be quite a bit easier. I am a huge fan of treads, but replacing them is tricky enough that I wouldn't consider using them unless you had a really good reason. There are ways to do it quickly, for example slotting the frame, but you have to have confidence in your ability to fabricate it properly. Mecanums are nice because if you have a problem with one, it is a self contained module. You can pull one off and pop a replacement in much faster than with a tread system.

My question is if you were debating treads, why not reasses the merit of the standard 6WD system? It seems like you are debating highly mobile vs. heavy and slow.

[Nemo]

If somebody forced me to choose between mecanum and treads, I'd choose mecanum. Not because I think it would be good this year (I don't), but because treads are a bunch of extra work while mecanum can be quickly bought and assembled without any fancy design and fabrication.

It seems like an obvious 6WD kind of year to me. The climbing and frisbee pickup is so complicated and difficult that I wouldn't want to spend any precious time designing a drive system, unless it somehow tied into the climbing ability.

Strafing doesn't seem like much of a benefit this year. Like last year, you can pivot to aim your shot. I liked it in 2011 when strafing made it easier to score tubes.

[J.Warsoff]

Mecanum wheels will allow you high maneuverability options (e.g. strafing), which would indeed come in handy this year, since there are no humps or anything on the field. This can help get you around defending robots, or be a good defending robot yourself.

On the other hand, treads will give you much more traction, therefore giving you more pushing power, as you did find.

All of this is going to depend on what kind of robot you want to build. Defensive bot? Offensive bot? Maybe a little bit of both? Other things like how you shoot, acquire discs, and other factors will also be variables in this.

[Ido_Wolf]

I'd definitely leave mecanum (at least only mecanum wheels or just 4 wheels, as 6 wheels seem fine and are probably the way our team's gonna roll) out this year. While the ability to maneuver is important by all means, and throwing frisbees might be slightly more simple mechanically, compared to last year's balls (depending on the way you tackle that problem), you really don't want to be pushed way off your stance when trying to shoot. When a robot is so easy to push that every single tackle messes its shots up so bad that it doesn't even seem to be remotely close, it's a very significant issue for scouters who watch it.

Regarding your pick-up issue, it's only the 4th day so it's not too late to rethink your concept, or maybe scrap the pick-up system overall and just rely on the feeded; your chosen mechanisms should be modified so that they stand in line with your ideal strategy, not vice-versa.

[commodoredl]

Mecanums?
Treads?
Why not both?

(Not actually endorsing mecanums, treads, or mecanum-treads)

[Bryscus]

That. is. AWESOME!

- Bryce