Questions about crab/swerve drive.

[big1boom]

The rotating things are Square Lazy Susans from McMaster

We have one on top, and one on bottom of each box, this defines the vertical plane of the wheel with 2 points, making it much stronger. Most swerve modules are supported from top and bottom to increase strength and reliability.

Keep asking questions, I know that I don't mind sharing information that might help someone.

[EricH]

You're all forgetting one thing.

We don't know that a swerve drive will be good for next year's game.

So, by "next year", you really mean "next fall, as a prototype", riiight? You see, crab/swerve drives are pretty complex, especially compared to tank drive. So you want to do at least one before it counts, just to help you do the one that counts (if you do it). Move up your schedule and do it in the fall, then evaluate it at the start of build to see if it'll be good for the game.

Now, for some help.

1 CIM per side is still probably too weak. It'll work, yes, but you really want 2, or 1 per wheel module.

There are a number of setups, with the 2 most common being a coaxial crab and a more standard swerve. 118 uses coax, 148 used coax last year, and some other teams do as well, I think. In this setup, all the drive motors power all the wheels. A second rotation shaft is used to rotate the modules. You could set this up to act as either all 4 steer together or as a 2x2.

The other main option is to put the motor in the drive module. That's a little tricky on the wiring side, but it's a bit simpler mechanically.

[BenX02]

Originally Posted by big1boom

The rotating things are Square Lazy Susans from McMaster

We have one on top, and one on bottom of each box, this defines the vertical plane of the wheel with 2 points, making it much stronger. Most swerve modules are supported from top and bottom to increase strength and reliability.

Keep asking questions, I know that I don't mind sharing information that might help someone.

Ah, thanks. I will look into those.

Thanks, you've been a lot of help. I think most of my questions have been answered, but thanks again.

Originally Posted by EricH

You're all forgetting one thing.

We don't know that a swerve drive will be good for next year's game.

So, by "next year", you really mean "next fall, as a prototype", riiight? You see, crab/swerve drives are pretty complex, especially compared to tank drive. So you want to do at least one before it counts, just to help you do the one that counts (if you do it). Move up your schedule and do it in the fall, then evaluate it at the start of build to see if it'll be good for the game.

Now, for some help.

1 CIM per side is still probably too weak. It'll work, yes, but you really want 2, or 1 per wheel module.

There are a number of setups, with the 2 most common being a coaxial crab and a more standard swerve. 118 uses coax, 148 used coax last year, and some other teams do as well, I think. In this setup, all the drive motors power all the wheels. A second rotation shaft is used to rotate the modules. You could set this up to act as either all 4 steer together or as a 2x2.

The other main option is to put the motor in the drive module. That's a little tricky on the wiring side, but it's a bit simpler mechanically.

Well of course that's what I meant.

And yeah, I was aware that one would be to weak. I was thinking of using six CIMs (before someone reminded me of the rule that you can only use four), one for each module, and two to turn them.

I have been thinking of the wiring, how might you go about doing the wiring for that? I'm trying to figure out a way so that they don't get wrapped around.

Thanks for the help.

[RMS11]

Originally Posted by BenX02

I have been thinking of the wiring, how might you go about doing the wiring for that? I'm trying to figure out a way so that they don't get wrapped around.

Thanks for the help.

If you have the motor on the module then you would probably need to have a mechanical stop so the module could not spin freely and some kind of wire holder that allows for all the wires to stay organized, but im forgettin the name right now. Or if you do coaxle then its not a problem

[EricH]

Originally Posted by BenX02

I have been thinking of the wiring, how might you go about doing the wiring for that? I'm trying to figure out a way so that they don't get wrapped around.

Thanks for the help.

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.

[Al Skierkiewicz]

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.

[IceStorm]

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.

[RMS11]

Originally Posted by IceStorm

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.

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

[EricH]

Originally Posted by IceStorm

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

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.

[Steve_Alaniz]

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

[Enigma's puzzle]

Originally Posted by Steve_Alaniz

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.

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.

[IceStorm]

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.

[IceStorm]

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.

[Vikesrock]

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.

[sgreco]

Originally Posted by IceStorm

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.

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).