Swerve vs. Mech?

[yoshibrock]

I know there's a thread about effective drive systems, but I specifically want to discuss swerve drive vs. mech drive. I personally think that swerve drive would work better this year because it gives more traction to push and hold your ground, and it seems like it's going to be a physical game this year. My team voted to use mechs though, so that's too bad for me What does everyone else think? Mechs or swerve?

[Jeffy]

Today we voted for mechs, not because we thought they were "better" but because there is no way we could do swerve with our woodshop.
Swerve does everything mech can do and with better traction. The only tradeoff I can see is swerve is more complex. Too complex? You decide.

[yoshibrock]

Originally Posted by Jeffy

Today we voted for mechs, not because we thought they were "better" but because there is no way we could do swerve with our woodshop.
Swerve does everything mech can do and with better traction. The only tradeoff I can see is swerve is more complex. Too complex? You decide.

Yeah, that's basically the same thing that happened to us. Personally, I don't think it's that much more of a complex system. I also think it's more manueverable because you can do more, programming wise, with it. But it also takes more practice (arguably) to learn to drive it. I agree that it's a tradeoff.

[lexxasaurus]

Our team has been debating this too. The team voted mech but before we finalize that the drive team has decided to prototype both. At the moment I'm kinda torn in between the two because both have strong points. I'll let you guys know how prototyping goes.

[Chris is me]

There's a billion threads on this before but I"ll add my personal inexperienced cents.

All drive trains are tradeoffs, and neither of these are exceptions. Mecanum drives do not respond well to defense, and can have high power draw. Swerve eliminates the defense problems at the expense of a heavier, tougher to build, less durable drivetrain and tricky programming. Given an infinitely good shop / mentors / resources / machining / programming, I'd pick swerve over mecanum always (but a 6 wheel drive is sometimes better than both), but that's obviously not the case.

If you've never built a swerve before this year, don't start now. It's too late.

[kirtar]

Originally Posted by Chris is me

There's a billion threads on this before but I"ll add my personal inexperienced cents.

All drive trains are tradeoffs, and neither of these are exceptions. Mecanum drives do not respond well to defense, and can have high power draw. Swerve eliminates the defense problems at the expense of a heavier, tougher to build, less durable drivetrain and tricky programming. Given an infinitely good shop / mentors / resources / machining / programming, I'd pick swerve over mecanum always (but a 6 wheel drive is sometimes better than both), but that's obviously not the case.

If you've never built a swerve before this year, don't start now. It's too late.

I'll add on to that. Swerve drive also takes a lot of resources. Unless you do coaxial swerve (drive all of the wheels off of a central gearbox), you'll use probably three or four CIMs + whatever you're using to turn the wheels. In addition, it takes up a lot of space in comparison to mecanum drive. This year, where almost all interaction with the ball is below the bumper zone, you might not want to take that space under your bumper zone to put all the components of a swerve drive (then again, you also might decide that it's worth it).

In terms of complexity, as noted above, swerve is pretty ridiculous compared to mecanum. In addition, as a backup plan on mecanum, you can switch out your wheels for traction wheels.

[Alexis Howell]

Yeah. Plus mecanum wheels are really expensive. A set of four from the andy marks site is 700+ dollars.

This year our team is really considering a holonomic drive, we've got most of the resources collected from the past couple of years. Downside's lack of traction. Also worried about how difficult this would be to drive...

Anyone has experience with this drive base?

[Chris is me]

Originally Posted by Alexis Howell

Yeah. Plus mecanum wheels are really expensive. A set of four from the andy marks site is 700+ dollars.

This year our team is really considering a holonomic drive, we've got most of the resources collected from the past couple of years. Downside's lack of traction. Also worried about how difficult this would be to drive...

Anyone has experience with this drive base?

1714 did it in 2008 and it was the worst drive base we ever had. Though part of it was code issues, it was way too easy to push around and extremely vulnerable to defense. The robot would over spin almost every turn. I've heard mecanum is a bit better, but I doubt the team will ever do a holonomic drive ever again for anything.

[Alexis Howell]

Ooh yikes. Our programmers said that they already have a VI for holonomic drive... and we're trying to decide whether it's worth it to build it or not. We don't have the experience or resources for crab.

Worst ever? hmm.

[XaulZan11]

Originally Posted by Alexis Howell

Ooh yikes. Our programmers said that they already have a VI for holonomic drive... and we're trying to decide whether it's worth it to build it or not. We don't have the experience or resources for crab.

Even if you got it programmed 100% perfect, I don't think it is the answer for this year's game. A team in 2007 had a 3 wheeled holonomic drive that they seemed to be able to control very well. On the downside, it was unable to climb ramps and was very easy to shut down with defense (just sit in front of the rack and push them accross the field once they got close). If you don't face defense or don't plan to climb the bumps, then it may be a good choice.

[Alexis Howell]

Thanks for bringing that up. We were planning on just staying in one area, but defense is going to be a definite issue. Driving straight is a problem I hear too.

We want something really maneuverable, but crab and mecanum are pretty expensive... any ideas?

[Bongle]

Originally Posted by Alexis Howell

Yeah. Plus mecanum wheels are really expensive. A set of four from the andy marks site is 700+ dollars.

This year our team is really considering a holonomic drive, we've got most of the resources collected from the past couple of years. Downside's lack of traction. Also worried about how difficult this would be to drive...

Anyone has experience with this drive base?

I should probably mention that a single 6" mecanum wheel is $57, not $175 on AndyMark. We just ordered a set of them plus mounting hardware for $340.

[MikeWherley]

Originally Posted by Bongle

I should probably mention that a single 6" mecanum wheel is $57, not $175 on AndyMark. We just ordered a set of them plus mounting hardware for $340.

Where did you order them from?

[JesseK]

Originally Posted by MikeWherley

Where did you order them from?

AndyMark's site:
http://www.andymark.biz/mecanumwheels.html

We got some too; $222 for 4 wheels, plus $60 in hubs if you choose to use their hubs. The difference in weight between these mecanums and other comparable AM traction wheels on a 6WD is only 2lbs total (6WD = 6 wheels * 0.5lbs, Mec. = 4 wheels * 1.25lbs).

The real cost/weight savings for 6WD comes with the transmissions, not the wheels; yet there are other factors that 6WD must take into account such as chain, tensioners, and sprockets. Mecanum wheels can pretty much be direct drive and maintain a decent speed/torque balance with the available COTS ratios. Keep that in mind.

[alicen]

my knowledge of mecanums:

Rack n Roll-
we used the 8" andymark mecanums, and our drivetrain/chassis only weighed about 60 pounds. we ended up with the compressor and a fairly heavy lift system as well, our overall weight tht year? 110 lbs. In competition, we didn't really get pushed around at all, and one team (with 4 or 6 WD- i don't remember) actually wanted us penalized for pinning them. what worked out was the manueverability of the mecanums, and we were also able to easily get to and climb other people's ramps.

Overdrive-
we used the 6" andymark mecanums, our drivetrain was lighter, of course, but the cost that year was the way the 6" mecanums are deisgned. We had the problem of, at championships, the metal holding the wheels together DUG into the carpet, adn kept us from turning at all. that year we also went with some gearboxes which ended up not being able to pull the robot sideways, but that's another story.

so, in my experience, mecanums have never really been a problem, as long as you've got your numbers right. this year, we've already gotten some 8" andymark mecanums (about $300 for all 4) and some 12:1 banebots gearboxes. our plan is to drop the mecanums down so that they hang below the chassis about the same distance that the kitbot would.

experience with swerve: i have absolutely none. i hear it's difficult to program, etc. my only thoughts are how well it would go up the ramp, but i really don't know about that.