Swerve vs. Mech?
 

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 :rolleyes: What does everyone else think? Mechs or swerve?

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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?

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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?

Re: Swerve vs. Mech?
 

Be very wary of doing anything with mecanum or any type of omni-wheels this year. Two big reasons....

The Ramp...

and Traffic ...

Omni wheels are easy for a team to push around...

As someone said before... all drive trains are trade-offs and they take lots of resources... this year the question will be does a team dare to use omniwheels even if they don't go on the ramps.?..

I think there is going to be huge amount of pushing around on the field this year... especially in the midfield...

if you are using Omnis in a 4 wheel drive put them on the back and then you might be able to go up the ramp... just be really careful when you are coming down the other side... its just so easy to slide sideways with omni wheels...

You will have to be aligned pretty square to the ramp to try it... and hope that no other robot comes along to push you sideways...

My suggestion is you need to power all the wheels... you are going to need the pushing ability... but of course the trade off of using sticky wheels all around is going to be loss of turning control...

so perhaps wheels with a medium amount of traction all around say a coefficient of friction of .75-.8 ... you could turn easier...

Get a variety of wheels and see what you can do...

You can use really sticky wheels if you do a swerve... mechanically they are not extremely difficult...the programming is the hard part...

Don't be set back about the design... we designed our Skunk Swerve last year AFTER the kickoff...

if you need some help .... we are not far away from you guys!!
We can help!!

Good luck

Re: Swerve vs. Mech?
 

Swerves are fairly heavy, and to lighten them without ever actually doing one before means a team risks quality. Mecanums are the 'easy' way to upgrade during the actual season, because if all else fails with the code you can easily just tank steer like usual. Additionally there's aren't any tricks to mounting a Mecanum wheel since it goes in the same spot as a normal wheel.

The BEST is definitely crab this year*. Yet just because that's a probable reality doesn't mean your team should do it. It's complicated in both hardware and software.

*AFAIK. YMMV. Only applies to certain strategies. See the other robots at your local competition arena for details.

Re: Swerve vs. Mech?
 

Swerve versus Mech/Holonomic is a discussion that seems to come up every year.

My team did a crab drive last year, and swerve drive the 2 years before that. I can say that they're great for somethings (maneuverability), but at a huge cost.

In 2007/2008, we made the full 8-motor 4-wheel independent swerve drive. All 4 CIMs, 2 FP, and 2 Globes went to this application. What did that leave us elsewhere? Not all that much. The 6 most powerful motors in the it all went to the drive. Worse yet, in terms of pushing power, many (most?) teams w/ a simple 4/6WD with 4 CIMs were stronger. Why? Using high traction wheels (IFI/Pneumatic) would mean that it would be necessary to gear down the steering motors further. If they steer too slow, there's a noticeable delay between going straight and translating. You get one or the other, power of maneuverability.

Another thing that is commonly overlooked is HOW you control the robot. If you have "full swerve", you can control it rotate/translate (the same way you play a FPS game, for example). Difference is, in a FPS (and most other games where you use this control system), the system responds instantly and repeatably. In a robot system, it does not. The end result? It becomes difficult for the driver to fully utilize this system. Now you have a heavy, resource expensive (as far as time, (maybe money), and motors) system that's difficult to program and more difficult to control.

Last year, we downgraded a bit to a left/right side linked crab drive system. With the slick surface, it was unnecessary to put 4 CIMs to drive, since 2 would (in our assessment) always be sufficient to break the wheels loose. This system was really no less complicated than the independent swerve system. When you link sides, you need some way of passing power around. The benefit, though, is that you now can use less motors (minimum of 4 for left/right linked, I suppose you could get away with 2 for all linked). I suppose another advantage is controlability/programming. The control scheme we used last year was simple, left side of drive points in the direction of your left joystick, and the right side of the drive follows the right joystick. In my opinion, it was a little more controlable, but still very difficult to fully utilize (most of the time it was enough to just drive like a tank-drive robot, forward w/ minor adjustments to get balls).

Now, I have never built a mech/holonomic drive system before. I am also quite afraid to go in such a direction. It seems, to me, as if a mechanum drive would, at least, have trouble going over the ramp. I don't think a holonomic drive can do it at all, but that's just my gut feeling. Now, consider the alternative to the ramp, the tunnel. First of all, its narrow. Second, a mechanum/holonomic system would, almost undoubtedly, be unable to (or have great difficulty to) push a conventional 4/6/8/10/12... wheel drive system. If you are really the biggest scoring threat on your alliance, just about any robot would be capable of largely eliminating your ability to maneuver across the field. How big of a deal is that? I can't say for sure. Maybe crossing zones won't be necessary at all, but even then, your inability to stand your ground would prove to be quite detrimental, I suspect.

Just my 2 cents.

EDIT: Oh, as far as the comments about swerve drives being less robust goes, I wholeheartedly disagree. The only year we had issues with the swerve drive was in 2007, mostly due to calibration and chains. If you build a swerve drive, avoid chains if possible, and make a system that, once calibrated, will not lose calibration. If I recall correctly, we recalibrated 0 times during the season and never lost functionality in either side of the drive during competitions. A properly engineered swerve drive is, in my opinion, as robust as a well engineered conventional drive.

REEDIT:
While that is true, you can also do the same with a swerve/crab system. I would imagine that swerve/crab is better than mechanums in tank mode because the wheels would have more traction.

Re: Swerve vs. Mech?
 

Wait a second ... iirc 540 did Mecanum drive in 2007 as part of the Sparky Triumverate (and won VCU right?) -- thus it should be very easy for your team. This simplicity is probably why they chose it. It'll be practically just as good for your team as swerve since your team will probably have more time to practice with it than they would swerve.

Re: Swerve vs. Mech?
 

Well you could try a four wheel drive with a wide wheelbase orientation which should help with turning. Six wheel with dropped center can also be nice.

Removing mecanum, swerve, and killough ("holonomic" really isn't specific), you're not left with really any omni-directional drive (those which possess three degrees of freedom). At this point, you need to optimize your ability to turn, but at the same time, to help against defense you need the ability to push.

With this in mind, if you can manage a shifting drive train to allow you to have both speed and power available, that could be the best for you (then again, it may not).

Re: Swerve vs. Mech?
 

Yeah, that's another reason why we chose it. I'm just worried that it's not going to be as advantageous this year with less room to maneuver. I'm trying to design a 6wd just because I think we really need the traction. Thoughts?

Re: Swerve vs. Mech?
 

If the team has decided to go with a Mecanum drive train, focus on it. You can tighten the rollers to get more forward traction, yet you will lose strafing torque. Mecanum drive trains are good a pushing if they square up on the object being pushed before pushing or strafing into it (look at 384 in 2008 on Galileo). Evaluate those tradeoffs and stick with the Mecanum decision!

Re: Swerve vs. Mech?
 

We have looked at mecanums as well as a 6WD drop center arrangement. Two things come to mind that push me a little in favor of the 6wd.
1. Mecanums come with a price penalty we may not be able to afford.
2. They also come with a fairly significant weight penalty. I think there is going to be a strong concentration of pneumatics this year which tend to be heavy. I'm afraid to develop a drive train that uses up my weight budget. We've built the 6WD drop-center before and if it's done right it can turn on a dime AND climb ramps. We will probably need to make a shorter wheelbase because of the steepness of the bumps but I think it can work. We'll be doing the calculations and scale modeling this week.
We'd love to get your thoughts.

Re: Swerve vs. Mech?
 

Price aside, the weight will vary depending on your 6WD. If you use say AM Supershifters and 35 chain, you might even end up weighing as much as if you did mecanum with Toughbox nanos (I haven't checked this yet).

Honestly, any drive system can work, the question is really which one is best suited to your goals (and your driver's style).

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

Where did you order them from?

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

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.

Re: Swerve vs. Mech?
 

Every year is a physical year. High-speed maneuvering is critical in every game, so a swerve drive almost always presents some compelling advantages. You'll notice however, that most of the winning teams still use a simpler drivetrain.

What maneuver can you do with swerve that you can't with mecanum? A mecanum base can move in any direction with any amount of spin, meaning that it has perfect control of all three degrees of freedom.