Swerve vs. Mech?

[kirtar]

Originally Posted by 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?

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).

[yoshibrock]

Originally Posted by JesseK

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.

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?

[JesseK]

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!

[rcflyer620]

Originally Posted by yoshibrock

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?

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.

[kirtar]

Originally Posted by rcflyer620

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.

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).

[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.

[StevenB]

Originally Posted by 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.

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.

Originally Posted by yoshibrock

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.

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.