Re: Questions about crab/swerve drive.
 

Slip rings. They're these little devices that allow electronic stuff to turn in full circles without twisting wires. Or coaxial-style--then there won't be anything electronic other than maybe an encoder or so on the modules.

Re: Questions about crab/swerve drive.
 

It is good to investigate different drive types but as Eric has pointed out, don't dedicate yourselves to a system that will be less than adequate for next year's game. We do not decide on a drive system until well into the first or second week. It is strictly determined by the game and the playing surface(s). Our modules have the motor and gearbox built right into the module and wiring is run down through a hollowed out top (vertical) axle. Rotation is limited to 360 degrees or less so wrapping up the wiring is not a problem. Automated machining makes things easier to assemble but is not totally needed. Don't forget to add a bottom bearing surface to prevent the module from binding when it is hit from the side or in turning at full speed. Globe motors have more than enough torque to turn two wheel modules at a time when used properly.

Re: Questions about crab/swerve drive.
 

The slip ring idea sounds great but aren't the extremely expensive?

Were also looking into building a crab/swerve frame in the offseason here to try and figure out how to build one should we need to for next year.

One of my biggest questions was how do teams that do coaxial crab run the wires down to there encoders in the actually swerve module? without the slip ring idea above i would think they could pretty easily get wound up around themselves and one of the advantages to the coax crab in my mind would be to not have to limit your self to a range of motion.

Re: Questions about crab/swerve drive.
 

You put the encoder in the gearbox, so they never spin. you dont need them on the wheel module.

Re: Questions about crab/swerve drive.
 

The ones McMaster sells are at least $550 each. They're also huge--that particular one is 10 1/4" diameter.

I'm fairly certain that there are cheaper ones out there (and smaller); try an electronics specialty store.

Re: Questions about crab/swerve drive.
 

We wanted a crab drive this year and due to weight constraints we just used front crabdrive, which worked very well once we got the rear wheel programming worked out.
The slip ring idea is very cool but a bit exotic. We built longer shafts for our cims with banebot 16:1 transmissions and mounted our motors directly on top of the drive. The motors do not turn (other than the shafts) so no wire twisting. We only built our shafts because Banebot was out of their longer shafts and we had the ability, so that worked out for us. We made everything modular so we could switch a drive in under 5 minutes.
To be honest, the "go to team" on crab drives is 118, the robonauts, and they are really happy to share what they know and how they do it. Their system allows them to use 1 to 4 cims and depending on the game they change the number every year. They had a display at the championship of their many evolutions ( very interesting stuff ). I'd look them up and drop them an e-mail.
The ONE and only drawback is that it is a bit complicated, as you would expect. They only lost the Houston regional last year (2008) because a drive failed at the last match of the finals and they couldn't replace it in time. (the reason we made our drive modular.)
Their mentor, Lucien, is a really great guy so if you really are interested, he's always willing to help out.
Ours is an "economy" drive but if you're interested in our ideas I'll be happy to post the system... I just need to get back to the robot... AFTER I crash for awhile... (it's been a LONG season!)

Good Luck!

Steve

Re: Questions about crab/swerve drive.
 

I seen the mcmaster one's but didn't even look at the size after i seen they were over the 1 item can be no more than cost factor.

Idealy you would want a through bore type i think so that you could do the coaxial setup with it. And If I'm right about the US Digital encoders they are a 4 conductor.

So anyone know where to get a 1/2" ID through bore 4 conductor slip ring and keep the cost down on it. Most of the ones i've found searching show them being able to handle RPM's in the hundreds which would never happen.

Re: Questions about crab/swerve drive.
 

Chad, if you are going with coaxial swerve there is really no reason to put the encoder on the rotating part of the module. Put the encoder further upstream and toss out the slip ring altogether.

Re: Questions about crab/swerve drive.
 

It depends what kind of crab drive you are looking to do. If you are thinking Coaxial then 118 is definitely the team. However if you are doing a pod design with the motors in the modules then teams like 111 or 71 are better ways to go. Another exotic design is the Martians, 494 and 70 had them only anchored on wheels that were similar to what you would find in a drawer slide and turned by cabling that you would find in a bike, a very different and reliable solution that they innovated.

Re: Questions about crab/swerve drive.
 

I would think that for this years game at least you would have wanted to have the ability to put a sensor down on the wheel directly so you could see any slippage but now that I think about it more with the coaxial i guess you could really read it anywheres and only really need one reader per power house. I was just trying to cover all of our base's really before we start trying to build one.

Re: Questions about crab/swerve drive.
 

If you can get 360 degrees, then I would say that is more than enough. My team's swerve can do approx. 720, but we found it useless and limted it to about 200ish. Once you turn them more than 180 and go forward, it's the same as turning the the other way as much and you would have passed 180 then powering backwards.The other thing is that you don't necessarily need encoders. This year it was pretty much a necesity because of traction control, but there are games where you can get by fine without them. I would design it assuming you need them, then maybe evaluate whether you need them when the game comes out. If you have coax anyway, you could put the encoders outside of the modules where your motors are, then figure out the speed based on gear ratios.(this year was a little different than most because of slipping).

Re: Questions about crab/swerve drive.
 

Anyone know of a good supplier of pots and what type would be needed? Since pots can only go a fixed distance, what would a setup be like?

Re: Questions about crab/swerve drive.
 

I'm seeing an awful lot of people jump on the swerve guru bandwagon recently, and I'd to say something about that.

Before you hand out advice that someone may take as 100% perfect, think for a moment, do I REALLY know what I'm talking about? Can someone take my words, act on them, and be satisfied with the results?

Successfully using a swerve this year was a challenge, but also one much less so than on carpet. Most swerve designs I have seen this year would probably result in severe damage to the modules/turning shafts if they had been used on carpet.

Since I doubt we'll be getting anything other than carpet, that's what a new crab should probably be designed around.

Re: Questions about crab/swerve drive.
 

Could you give some examples please? Any drive that might stall will have problems but that is pretty much ...any drive. I think you are referring to a transmission problem that might occur under stress but I wouldn't want to put words in your mouth. You are correct that the slick surface allowed for easy turning but our drive actually worked better on carpet, so it was actually a challenge for the drivers to adjust to the regolith.
Our shafts were 1/2 in steel so they were not going to break but we did worry a bit about the transmission so we used the banebot 16:1. I am under the illusion we were pretty bullet proof. At least we plan to build on what we learned this year.

Re: Questions about crab/swerve drive.
 

What application do you mean? If you mean speed encoders, the banebot encoders that fit on the transmissions is a very easy solution (http://banebots.com/pc/ELECTRONICS/EN-G0561-KT).
If you mean a position sensor, USDigital has a great absolute encoder that we used to link our left and right side drives together so we didn't have a chain across the robot. (http://www.usdigital.com/products/en...ary/shaft/ma3/)
USDigital also has rotary encoders that you can use with a wheel for floor speed if you are looking to do some traction control.
Ok USDigital IS pricey but I can't argue with how easy they were to use (a simple analog signal) and reliability. I think they are worth a look.

Steve