Another chapter to the 6WD vs. Mecanum debate

[Oblarg]

Originally Posted by Ether

If by "power" you mean "maximum force" this is not correct. Even with the same material on the mec and the 6WD, the mec will have less maximum pushing force, as explained in many posts here on CD.

By "omni holonomic" I was referring to the drive system of four omni wheels placed in square or diamond configuration. If that's not what the term means, then I'm sorry, that's what it's always referred to in our team.

No, I don't mean maximum force. I know mecanum wheels do start skidding before 6wd drive, but honestly I *rarely* see a pushing match actually reach that point in competition, and with the pinning rules in place it's even more of a nonissue now than it has been in previous years.

And yeah, the rollers turn while rotating, but it's not as if someone hitting you from the side will cause you to start spinning.

Originally Posted by Chris is me

This is not true.



Also not entirely true - it's worth noting a mecanum drive more or less necessitates a lower traction roller.



It can also be pushed from the front more easily due to those spinning rollers.

Keep in mind just because there's no "at the goal" defense doesn't mean there is no defense.

I've had to push against 4wd and 6wd drive robots with mecanum before, and not once have I seen the rollers spin - try it yourself, you'll notice that the difference is not nearly as big as most people make it out to be. Yes, there's a lower cap before mecanum wheels start skidding, but how often have you seen wheels skid in a FIRST pushing match? It's not exactly a common occurrence - in my time in FIRST I really can't recall ever seeing our mecanum wheels skidding on the carpet. In my experience, the idea that "mecanum drive gets pushed around a lot" is almost entirely a myth.

People put way too much store in "If we end up exactly head to head and are pushing each other, my robot will win out." That's not a realistic situation. In the context of an actual competition, in which your goal is not to push head to head with other robots, mecanum is not at a significant disadvantage in terms of being pushed. Take into account the fact that defense isn't allowed to push you while you're scoring, and the best way to get into the scoring zone will probably be dodging the defensive robot rather than pushing it (in fact, are you even allowed to push another alliance's robot into your scoring zone?), and I really don't see what the benefit of a 6wd is.

[JamesBrown]

Originally Posted by Oblarg

And yeah, the rollers turn while rotating, but it's not as if someone hitting you from the side will cause you to start spinning.



I've had to push against 4wd and 6wd drive robots with mecanum before, and not once have I seen the rollers spin - try it yourself, you'll notice that the difference is not nearly as big as most people make it out to be. Yes, there's a lower cap before mecanum wheels start skidding, but how often have you seen wheels skid in a FIRST pushing match? It's not exactly a common occurrence - in my time in FIRST I really can't recall ever seeing our mecanum wheels skidding on the carpet. In my experience, the idea that "mecanum drive gets pushed around a lot" is almost entirely a myth.

People put way too much store in "If we end up exactly head to head and are pushing each other, my robot will win out." That's not a realistic situation. In the context of an actual competition, in which your goal is not to push head to head with other robots, mecanum is not at a significant disadvantage in terms of being pushed. Take into account the fact that defense isn't allowed to push you while you're scoring, and the best way to get into the scoring zone will probably be dodging the defensive robot rather than pushing it (in fact, are you even allowed to push another alliance's robot into your scoring zone?), .

2006 Boston Regional Finals Matches 1 and 2 come immediatly to mind for me. Every thing you say you have never seen occur happens in those matches and the drive train on the defensive bot was not that good, it was a poorly executed 4wd design.

I have seen Mecanum drives pushed directly backwards by a 4 or 6 wheel drive despite trying to push forwards. There is definitely a disadvantage to mecanums whenever there is pushing.

Originally Posted by Oblarg

and I really don't see what the benefit of a 6wd is.

I think the advantage to 6wd is pretty obvious and pretty much common knowledge, it is simple, reliable and easy to manuever. There is a reason that the majority of teams on Einstein every year seem to be 6 or 8wd

[Oblarg]

Originally Posted by JamesBrown

I think the advantage to 6wd is pretty obvious and pretty much common knowledge, it is simple, reliable and easy to manuever. There is a reason that the majority of teams on Einstein every year seem to be 6 or 8wd

I think that's probably because the majority of teams in FIRST don't use mecanum. Walk around the pits at regionals if you don't believe me.

Mecanum is unequivocally more maneuverable than 6wd, and mechanically it's no more complex (and you can get working drive code online, I believe our team has published ours), and it's perfectly reliable - our team has never had a major drive failure (one which made our drive unusable or even less maneuverable than a 4wd) due to the mecanum wheels.

I encourage you to build a mecanum bot and push against a 6wd with it. I think you'll be surprised at how hard it is to actually get the mecanum to skid.

[Chris is me]

Originally Posted by Oblarg

I think that's probably because the majority of teams in FIRST don't use mecanum. Walk around the pits at regionals if you don't believe me.

Maybe that's because it's not as good - the VAST majority of the best teams in FIRST don't use mecanum, and it's not because they're stupid.

Mecanum is unequivocally more maneuverable than 6wd,

Is it? Honestly? I think 6wd maneuverability is quite underrated - there's a reason there's literally never been a mecanum on Einstein.

I encourage you to build a mecanum bot and push against a 6wd with it. I think you'll be surprised at how hard it is to actually get the mecanum to skid.

We've pushed plenty of mecanum bots no problem (and yes, the rollers spin)

[Oblarg]

Originally Posted by Chris is me

Maybe that's because it's not as good - the VAST majority of the best teams in FIRST don't use mecanum, and it's not because they're stupid.



Is it? Honestly? I think 6wd maneuverability is quite underrated - there's a reason there's literally never been a mecanum on Einstein.



We've pushed plenty of mecanum bots no problem (and yes, the rollers spin)

No, I think there are plenty of teams who have never tried it - I know that 449 had never used mecanum until '07, and since then we've used it in both games which allowed it. There's a lot of stigma around mecanum based on this myth that it gets pushed all over the place - anyone who has had experience driving a mecanum bot will tell you that this is nowhere near as big an issue as you think it is.

No, the rollers do not spin unless your wheels are already skidding - the physics of this has been explained multiple times earlier in the thread. As has been mentioned earlier in the thread, the only power difference mecanum suffers is the fact that the wheels skid sooner than traction wheels. You will never start pushing a mecanum bot and see the rollers immediately start spinning - it's the same reason that acceleration of mecanum drive is identical to that of a traction drive.

Yes, it is unequivocally more maneuverable. You turn just as well, you accelerate just as well, and you have an extra axis of movement. If that's not unequivocally more maneuverable, then nothing is.

[EricH]

I'm going to throw a wrench into the works.

Back in 2005, 330 had this debate a bit ourselves. We decided to work a compromise: We'd build our frame to run both (quite easy to do) and rig it with 6WD and rig our kit frame with mecanum (6" fully nylon wheels, KOP trannies from that time frame--single speed, single CIM). At that time, our 2003 robot was still operational, so we decided to test against that (4WD, 2-speed, drill motor transmissions).

We could not beat the 4WD in speed. We could not get around it for shooting a gap between two goals. I distinctly remember the mecanum losing a push test, and after the first 5-10 seconds, it wasn't a contest! If we'd been hit trying to cap, we'd have lost position, every time we took a hit. We had trouble strafing with an off-balance load (tetra on a pole). If I had video, I'd post it. Just on motors, we should have been able to push the 2003 robot, 4 CIMs versus 2 drill motors, roughly equal weights.

The 6WD stayed on the competition robot. To this day, 330 has not done a competition mecanum. Not because we're scared to, but because the GDC has yet to give us a game where it's an advantage given our play style. The only competition omni-directional drive we've ever done was a 6WD rotational drivebase on our 2009 robot, because it fit a key part of our game plan (avoid getting pinned if possible).

Oh, and one other thing: At that point, we had two years of experience with small-scale mecanum, both programming and hardware.

[Kevin Sevcik]

I think it should probably be noted that declarations about how rare mecanums were on Einstein don't really mean much till after 2007. At least.

2007 was the first year andymark offered mecanum wheels. So pre-2007, the only teams attempting this would be teams with the time and patience to make their own wheels and try them out.

Even then, I think it's a little arguable that they've had much of a chance at shining since 2007. Look at the run down:
2007: Debut of AM mecanums, with all the first-run, low volume problems.
2008: Mecanums arguably weren't much of an advantage.
2009: Lunacy, slicks only
2010: Bumps and plywood under the field reduced the effectiveness of mecanums.

So I don't think there have been all that many years where mecanums were both commonly available and obviously useful for that year's game. Now that may be because they just aren't ever going to be useful to the game, but I'd want a few more games to actually make that call.

[sgreco]

I compiled this information on a thread a while ago....It's not entirely complete, but if it means anything to this conversation I have the drivetrains of the winning alliance members of the championships.

2010
67 – 8 WD
177 – 8 WD (articulated is sets of two, front and back)
294 – 6WD

2009
67 - 6WD wide
111 - 4 wheel crab (non-coaxial), wide
971 - 6WD wide

2008
1114 - 6WD long
217 - 6WD long
148 - three-wheeled crab (coaxial), nonagon-shaped robot

2007
177 - 6WD long
987 - 6WD long
190 - 6WD long

2006
217 - 6WD long
522 - Treads, long
296 - 2WD long, Omnis in front

2005
67 - three-wheeled crab (non-coaxial). Flop bot.
330 - 6WD long
503 – 4WD long, omniwheels in rear


2004
71 - 4WD long
494 - 4WD long
435 - 2WD long, with casters in front

2003
111 - Four-wheeled non-coaxial crab (with dropdown skid for turning)
469 – 4WD Long
65 - 4WD Wide

2002
71 - 4WD flop bot with casters in front
173 - 4WD long
66 – 4WD long

2001
71 - ?
294 - ?
125 - ?
365 - ?
279 - ?

2000
255 - ?
232 - ?
25 - ?

1999
176 - 4WD long w/ Omnis in front
1 - tank treads, long
48 – 4WD, long

1998
45 – 4WD long with Omnis in front.

1997
71 - ?

1996
73 - ?

1995
100 - ?

1994
144 - ?

1993
148 - ?

1992
126 - ?

Important to note that mecanum wheels have not been available to first teams for this entire time, but the numbers are stacked in favor of skid steers. From 04 to 05 the dimensions changed from 30x36 to the current size, 28x38, so there are fewer 6 wheel bots as you go back before 2005.

I've had people argue that the reason skid steers win more is baecause they are more common, but I prefer to view this as "the best teams win, and by no coinsidence, they are skid steer bots." So basically they are winning because they are good, not because they are greater in numbers. They are simple, intuitive to drive, and comparatively pretty light, which leaves weight for an affective manipulator.

[IndySam]

We are building a mecanum drive. We feel it's best for our strategy.


BUT!!!!

We are also building into our drive the ability to quickly change to 6 wheel drop center. The holes are already cut into our frame. Add a nano box and a little chain and off we go.

Our first regional is week three, if we were week one I doubt we would attempt the mecanum setup. If we didn't have the ability or funds to build a second robot for practice and testing we wouldn't try mecanum.

[klmx30302]

Our team decided on mecanum drive day 2 of build season, we had resurrected our 2007 chassis into a driving mecanum proto-chassis by the end of week one. We have 1/2 our chassis done now (waiting on more brackets from 80/20) and because we are making most of our robot from 80/20 we will probably make a practice chassis. I think that because 2007 was the introduction of andymark mecanum wheels that it seems they havnt had a decent chance to prove them selves since then, this seems to be that year.

[pfreivald]

As a team that does not have the space to create a full field for practice purposes, I can say that the biggest advantage mecanum gave us last year is one reason the team was enthusiastic about using it this year:

It controls exactly like a first person shooter, so kids good at these types of videogames have an inherent knack for mecanum controls that they do not for skid steering.

That, and nostalgia. 2010 was a great year for us, and that mecanum drive had a lot to do with it.

[Oblarg]

Originally Posted by klmx30302

Our team decided on mecanum drive day 2 of build season, we had resurrected our 2007 chassis into a driving mecanum proto-chassis by the end of week one. We have 1/2 our chassis done now (waiting on more brackets from 80/20) and because we are making most of our robot from 80/20 we will probably make a practice chassis. I think that because 2007 was the introduction of andymark mecanum wheels that it seems they havnt had a decent chance to prove them selves since then, this seems to be that year.

AndyMark's mecanum wheels have certainly improved over the years.

The '08 6'' mecanum wheels were pretty bad - the plate was far too thin and would bend in on the rollers. Still, even with this problem our '08 bot was one of the most maneuverable at ever regional we attended; it was a nightmare to maintain, though (especially as I was the one fixing them in the pit between matches...ah, the memories).

No mecanum in '09, sadly.

The '10 6'' mecanum wheel were a vast improvement - thicker plate, better rollers. They still had occasional problems with sticky rollers, so you'd have to play with the lock nuts between every match to free them. Much less of a pain than '08, though. They also, like the '08 wheels, were manufactured to allow the rollers to be reversed, which while good in theory did get in the way of maintaining them a bit. Overall I'd give them a B - they did their job, and were certainly an improvement.

A quick glance at this year's generation of 6'' mecanum wheel seems to reveal that they've fixed most of my problems with the previous generations - they have removed the generic plate in favor of "left" and "right" wheels, which has allowed them to fix the access problems. The bolts seem to protrude a lot further out, as well, which makes it much more unlikely for you to unwittingly loosen them enough during maintenance to lose a roller in a match (this happened to us once or twice last year). The plate is nice and thick, too - they're claiming it supports a 400 pound load per wheel, which is great.

Unfortunately, having graduated and now being in Colorado means I won't get to see them in person, but I'm hoping they've finally worked all the kinks out of the design this year. They're certainly heading in the right direction, if nothing else.

[TroyCDH]

Yes 6WD can push harder than Mecanums. So? I did not catch Dean saying this year's main goal was to push around other robots. In fact I see there is a rule against pinning and contact during certain match times.

Next year, when the goal is simply to push each other I will be on my way to northern Minnesota for these:

http://www.mattracks.com/

Good luck to all, have a safe build season!

[klmx30302]

I feel like mecanum bots will do pretty well this year. We also used mecanum in 2007. We resurrected our 2007 chassis and put the electronics from last years robot to do prototyping/let software have more then 6 hours with the robot. I just don't see a well designed mecanum bot having problems out-maneuvering a 6wd bot.
Here are our 2007 drive tests:
http://www.youtube.com/watch?v=v_vUYyJAkCc
http://www.youtube.com/watch?v=YbluSr2Mf5Q
http://www.youtube.com/watch?v=W0XN6iSvCXo
Here are a few more:
http://www.youtube.com/watch?v=L01Ok-6AMhM
http://www.youtube.com/watch?v=RXGbo3tQapE
http://www.youtube.com/watch?v=ZnoMhVEqx1c
http://www.youtube.com/watch?v=oamC4esbGfI
http://www.youtube.com/watch?v=KfOLERgWQrU
http://www.youtube.com/watch?v=fdS8BmQom58&NR=1

[Ian Curtis]

Originally Posted by Oblarg

I encourage you to build a mecanum bot and push against a 6wd with it. I think you'll be surprised at how hard it is to actually get the mecanum to skid.

When the Blue Alliance comes back up, watch matches with team 40 in 2006. They had a mecanum drive with a drop down "traction plate" to stay in place when they wanted to. When they were on the mecanum wheels, they got pushed all over the field, when the traction plate was down, teams rammed them until their angle was off and with the mecanum wheels they couldn't fight back for their initial position.

Mecanum wheels have strengths that traditional skid steer drives do not. Pushing is not one of them.