Questions about crab/swerve drive.
 

Hey, our team is wanting to have crab drive next year, so I have been coming up with designs. I have a few questions.

1) The design I came up with has six CIMs, one for each wheel module, and one for each side to turn the modules. My only concern with this design is that it will be heavy. Can someone show me some designs or CADs (or just explain your design) for your designs. Don't worry, I won't rip them off, I just want to get insperation.

2) This is going to sound like a dumb question, but I have heard that a water jet/computerized mill is necessary for building a crab drive system, may I ask why? The machine shop our team meets at has one, but in the past they have not let us use it.

3) As a driver, I have been wondering about the different ways you can drive a crab/swerve. I have seen teams do a tank style with two joy sticks, where it acts like tank drive but if you move the sticks sideways your robot moves sideways ect. I have also seen with one stick and a throttle, and a stick with a Z axis. But which would you say is the best?

4) I know you need lots of sensors, but what kind and where might I get them?
Thanks in advance.

Re: Questions about crab/swerve drive.
 

Ok,

1) First of all you can't use 6 CIMS. You are only allowed to use 4. To turn the modules, teams use globes or another type of motor. There are some designs on CD, I have some already drawn up and will post them up when I get back home. **school is not fun**

2)CNC machining or a waterjet makes life easier. You can simply draw up the parts, CAM it and run it. Automated machining just makes life easier. Ask them if you can use or if they can mill parts for you.

3)It is up to you on the joysticks. I prefer using tank drive.

Re: Questions about crab/swerve drive.
 

You need to use pots on the steering, or possibly encoders (startup position can be tricky). You should also consider encoders on each drive wheel. It has been said many times on these forums and is worth repeating... Building the crab modules is just one part of the system, controlling it is often toughest part.

Re: Questions about crab/swerve drive.
 

Dang! I forgot about that rule. :( I'm stupid.

I think they will probably let us use it, since we actually did good this year.

That's what I would prefer also. But I have a question. Say next years game is one rug, in previous years you usually had to use omni-wheels for tank drive because of friction, how might you get passed that on a crab drive system?

Sorry if I sound like a noob, I am with crab drive.

Okay, and all the sensors available on AM or other websites?

Re: Questions about crab/swerve drive.
 

Precision is key when building a bulletproof swerve drive. This cuts down on everything from chain play to drift. Our 2007 swerve (handmade) was down for some reason or another (thrown chains, etc.) every few matches. Our 2008 swerve (waterjetted) made it through 3 regionals and champs without the need for a fix (until IRI when one of the axles wore far enough to lose it's snap ring which was a less than 1 minute fix with our design).

If you want more info, pm Aren_Hill

Re: Questions about crab/swerve drive.
 

I see. Did you machine your wheel hums out of one piece of metal? I'm guessing not, but it's an idea.

Re: Questions about crab/swerve drive.
 

You definitely don't need a CNC to make a swerve drive, if it's going to be coaxial and you're going to use bevel gears, you probably want to find a shop to help you out. My team made a full swerve on a manual Bridgeport mill in 08.
We used a 4-axis CNC this year, but you certainly don't need that to make it good.

For sensors, my team finds hall effect sensors to be the best because they are absolute sensors which saves time centering the modules(the reading is usually more accurate for us than we found with pots and encoders).

Although you are only allowed 4 CIMs, you probably wouldn't want it for steering anyway. They are too fast. I would use the globe motors. My team used the Denso motors and they worked fine as well.

If you have any questions feel free to PM and I can answer any questions you want about mechanical aspects of swerve.

Re: Questions about crab/swerve drive.
 

Here are some pics of some modules and dt's:

http://www.chiefdelphi.com/media/photos/33368
http://www.chiefdelphi.com/media/photos/33336
http://www.chiefdelphi.com/media/photos/33335
http://www.chiefdelphi.com/media/photos/33334
http://www.chiefdelphi.com/media/photos/32573


CAD:
http://www.chiefdelphi.com/media/photos/32520
http://www.chiefdelphi.com/media/photos/32444
http://www.chiefdelphi.com/media/photos/31626
http://www.chiefdelphi.com/media/photos/31078
http://www.chiefdelphi.com/media/photos/29415
http://www.chiefdelphi.com/media/photos/29412
http://www.chiefdelphi.com/media/photos/28568
http://www.chiefdelphi.com/media/photos/25328

These are just some images and stuff on CD. If you contact the creator, they will for sure help you out. There are a ton of iterations. You just have to find one that fits your needs. On sensors, since MadTown will never do Crab (I mean NEVER). I don't know. But someone will post a link soon hopefully ;)

Re: Questions about crab/swerve drive.
 

Might as wel preface this saying that I only have 1 year (this year) experience with crab/swerve
http://www.chiefdelphi.com/media/photos/33181
This is our swerve. My brother posted images of his CAD's later in the thread.


This year we went with 1 CIM total, but for a normal year I would recommend going 1 CIM for each side (r/l) The main problem with this setup is that you have to be exact for the bevel gears, otherwise the will not mesh correctly.
We used globe motors to turn our wheel modules. I have seen other teams use FP motors, or window motors.


It is not necessary to have a computerized mill, although it does make everything easier.

We had our boxes cut out by an old CNC, then we drill pressed all the holes, then had them welded.

This year we went with 1 Joystick controlling throttle, one joystick with twist controlling translation (strafing) and rotation.

I didn't have involvement in sensors other than mounting them, but I know that we mounted a magnet onto a sensor sprocket shaft, then put the sensor right above it, no contact. This sensor then outputs a voltage that corresponds to an angle. According to our team's programmer, he has to round the values otherwise it will crash/slow down the cRIO.

If you have any more questions, please post here, or PM someone. Most people are more than willing to help

Re: Questions about crab/swerve drive.
 

This a is a swerve I drew up during the build season with the specific intention of not needing a CNC machine. However, you do need a rotary table for your mill with this design.



The sprockets for powering are not shown, but they could simply be attached with a keyed connection. The steering sprocket is shown, but the motors for steering aren't because we didin't know what method of actuation we wanted or which motors we would need elsewhere. I would recommend steering the front wheels together and the back wheels together for a little bit of flexibility in steering modes.

The CIMs can also be down into the frame. The only reason they a sticking up is because of a few specific wiring issues that pertained to my team's 2009 bot.

Re: Questions about crab/swerve drive.
 

Thanks for the help.

You said the CADs later on in the thread were your brothers right? I see those rotating things on the bottom and top of your modules, can you explain to be what those are? I see them on a lot of different designs.

Sorry about all the questions, I have a lot to learn about crab drive.

Re: Questions about crab/swerve drive.
 

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.

Re: Questions about crab/swerve drive.
 

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.

Re: Questions about crab/swerve drive.
 

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.

Well of course that's what I meant. :rolleyes:

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.

Re: Questions about crab/swerve drive.
 

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 :)

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

Re: Questions about crab/swerve drive.
 

I'm actually referring to the loads the module itself has to deal with, the sideloads on the module can be massive if you turn while moving, or are pushed, or just about any situation really.

Many modules I saw this year made me think, "wow, if that was on carpet, it would just collapse when you turn".

Working isn't good enough, it has to "Work" while turning at 20fps and getting it from another robot, or being pushed sideways by two robots. If your modules can't survive those loads, it'll be a rough season.

Re: Questions about crab/swerve drive.
 

Why not have a wheel encoder tied to a non-driving wheel. Use two at ninety degrees to each other using omni wheels and then you don't have to worry about running wires down inside the modules.
Please keep in mind that bevel gears (those normally used on coaxial drives) are relatively inefficient in use for driving and require some pretty meaty bearings to keep them aligned under load and stress.
Slip rings can be made by using circular, machined conductive surfaces, insulated from the shaft by PVC, polyeurathane or delrin. Brushes can be bought anywhere, even at Ace or TruValue. All you need to do is keep them aligned in the assembly and have some form of attachment. Using brass allows you to solder wires directly to the rings.

Re: Questions about crab/swerve drive.
 

Good point! So I guess you mean the turning bearing on the module is a critical spot. I'd have to agree with you. (A Lazy Susan probably wouldn't cut it. )
Thanks for clearing that up.

Steve

Re: Questions about crab/swerve drive.
 

Hey Al! Nice to see you... Really? You'd use slip rings? Ok but I would be concerned about the electrical noise they would make... One is bad enough but four? I guess that's why I never tried it. Still Id like to hear your thoughts.

Steve

Re: Questions about crab/swerve drive.
 

Steve,
You asked an engineering question, so I gave you an answer. I didn't say it was a good one. There is no possible way to not have some noise in a slip ring, particularly one that is carrying a lot of current. However, with large brushes or multiple brushes, you should be able to transfer enough current to be useful. Slip rings for sensor use should be fine since the current is fairly low, and the noise should also be low. Please be sure that power supply for sensors are filtered near the sensor.
There are some mercury filled rotary electric connections out there. I am against using them but I do believe the GDC answered (In a Q&A) that they could be used this year. In the hard hits, and misalignments I see on robots during inspection, I believe the chance for a mercury spill is pretty high. For that reason, I advise against their use.

Re: Questions about crab/swerve drive.
 

Steve, I don't know if what Adam saw was similar, but I saw at least two crabs this year that would not sustain the axial loads put on them by rapid change in wheel direction or by being pushed across the carpet perpendicular to wheel direction.

I am far from a mechanical expert (electrical and software are more my thing), but I am near positive that the two crabs I saw would fail in an FRC game on carpet. One team was definitely aware of this and the module was designed this way because of the game. I didn't get a chance to talk to the other team at all to see if they knew what issues they could face if we move back to carpet next year.

Re: Questions about crab/swerve drive.
 

I agree with what you are saying about crabs failing. For an example, my team used made a swerve drive that worked very well, but only on Regolith. If we put higher friction wheels on and drove it on a rug, it probably wouldn't be as successful. We were aware when we built it that the system would only work on regolith with lunar wheels, but that's the only place it had to work. You just have to keep in mind what the challenge is and what the objectives are when you are building it.

For those of you asking questions about swerve for the future, I would really look into 111 and 118. That's where my team got a lot of inspiration from.

If you are interested in looking at my team's swerve, here's a link to a brief description and some photos. http://alarmrobotics.wikispaces.com/Swerve+Design+09

Re: Questions about crab/swerve drive.
 

Just wait... next year's field will be the same Glassliner surface but with a sticky tar coating. It's purpose is to simulate the gravity on the surface of ... Jupiter! :ahh:

Re: Questions about crab/swerve drive.
 

Also, not to knock swerve drive systems but there is something to be said about simplicity. I remember several years ago our buddies from the north, 1114, did swerve. Interesting that they havent done it since then. (Not saying they aren't capable of doing it or that it is bad) Perhaps they concluded that a 6 wheel drop center drive offered more bang for their buck. Remember a swerve drive is usually a significantly harder drive train to fabricate.

Always have a contingency plan, if your fancy swerve driving system won't work you better have something you can replace it with because moving should be step one for any robot.

Adam also brings up a good point, just because teams could get away with being sloppy this year with the robustness of their drive trains does NOT mean that they should. I look at several of these robots and shudder knowing how many pieces they would have been in a couple years ago.

Re: Questions about crab/swerve drive.
 

Regarding mercury-filled electrical connectors, my team has some sitting on our shelf (in 2006 we toyed with the idea of a continuously rotating turret). We didn't end up using them in competition, but you should read the product documentation CAREFULLY if you want to. Currently, no Mercotac electrical contact (Mercotac was the only manufacturer of these as of a couple years ago) is rated for more than 30A of continuous current. This all but rules out these devices for a high-traction swerve drive.

One other warning: slip rings (especially home made ones) always add a bit of noise to a signal. Be careful when using analog sensors with slip rings! (I know from experience!)

Re: Questions about crab/swerve drive.
 

190 built a custom high-power slip ring for their 2008 robot, along with a commercial slip ring inside of it for all of the sensors. Here's a photo of their [nearly completed] slip ring during the build season:

Re: Questions about crab/swerve drive.
 

That's 30 amps per contact, right? The one we used on the 2008 TechnoKats robot's continuous-rotation arm has eight contacts. Use four of them for a pair of 30A supply and return lines, and you still have four left over for an encoder if you want.

(The list price of such things might be higher than the single-component cost limit, but many suppliers have an educational discount that they offer to any team who asks for it.)

Re: Questions about crab/swerve drive.
 

To determine the position of the wheels on a swerve drive, would one want an absolute rotary encoder or a relative rotary encoder?

Re: Questions about crab/swerve drive.
 

We've always used the Ma3 abolute analog encoder which kept its orientation so we didnt have to line up the wheels before we turned the bot on

Re: Questions about crab/swerve drive.
 

Aren,

A link would be very handy, we kinda wanted to buy some.

Thanks,

-RC

Re: Questions about crab/swerve drive.
 

http://usdigital.com/products/ma3/ ? It was the first hit on Google for "Ma3 abolute analog encoder"

Re: Questions about crab/swerve drive.
 

Thanks, multitasking off my ipod touch and internet is super slow.

Thanks,

-RC

Re: Questions about crab/swerve drive.
 

Since this thread has taken a lot of tangents about crab drive, let me throw this out.. In a coaxial system, where the power is transferred to all wheels via chain, would it be better to use a belt instead of a chain in case you get into a pushing match on a friction surface so you don't slip or lose the drive chain? This creates more side load since the belt only works with tension but if preserving your drive is the main point, would a belt drive be a better solution?
( I ask because I noticed a belt "upgrade" for my milling machine. It is currently geared and the belt preserves the system in a stall situation presumably by slipping. )


Steve

Re: Questions about crab/swerve drive.
 

I'm not going to claim expertise on the subject, but I have seen teams to use chain for this purpose and as far as I know it works very well. I have never seen a team use strictly belt to power a coax, but somebody has probably done it. 1625 and 118 would be great to teams to ask about this.

If you want some intersting info about belts vs. chains check out team 234's white paper about chains and belts

Re: Questions about crab/swerve drive.
 

Re: Questions about crab/swerve drive.
 

The difference is that in using a belt to control the turning a timing belt would have the same effect as a chain essentially, but if there was slippage, then the entire steering system would be uncalibrated, the wheels would be in different places then the program thinks, a logistical nightmare. That is assuming you monitor the turning from the motor. If you do not measure from the motor then you are complicating the modular designing of your wheels.

And if you are running with multiple modules and wheels tied together then you complicate things if your wheels become out of sink.

Re: Questions about crab/swerve drive.
 

I didn't mean for the steering, the steering should be chained. I meant for the drive system that supplies power to the wheels. Even if you settle on a system that has a motor dedicated to each wheel, should you belt drive the wheels so the belt acts like a virtual clutch to avoid over stressing the transmission on a friction surface (like carpet)?

Re: Questions about crab/swerve drive.
 

I didn't mean for the steering, the steering should be chained or electronically synced. I meant for the drive system that supplies power to the wheels. Even if you settle on a system that has a motor dedicated to each wheel, should you belt drive the wheels so the belt acts like a virtual clutch to avoid over stressing the transmission on a friction surface (like carpet)?

Re: Questions about crab/swerve drive.
 

Thats a good point. Just wondering, how do teams like 118 keep the modules alligned? Is it just tensioning out the chains correctly, or more complicated than that?

Re: Questions about crab/swerve drive.
 

Chain tensioning but their wheels are generally within 2-5 degrees off from each other. I don't think I've ever seen them perfectly parallel.

Re: Questions about crab/swerve drive.
 

This year we bound our wheels F/B. The way we did this was with chain tensioning, and physically rotating the rotation sprocket

We didn't line up our wheelboxes completely, but I would estimate that we were between 1 and 5 degrees toed-out for each wheel

Re: Questions about crab/swerve drive.
 

Hmm, here's a way to eliminate the misaligned wheels. If teams limit the total rotation to, say, 720 degrees, a couple of in-line turn-buckle style chain tensioners could be added to a coaxial crab system. 720 degrees on a 3" sprocket radius is a total ~12" of chain run needed to rotate the modules. Hence, there should be plenty of room to add two tensioners to each chain run, one on either side of the sprocket, centered between the sprockets when the wheels are approximately in the center of their rotation range. With some adjustments, the wheels could become perfectly parallel to each other.

This might not be worth the added complexity, but if toe-in is really a concern for a team it may be a start.

Re: Questions about crab/swerve drive.
 

Steve, the issue of overworking a gearbox is not an issue unique to crab drive. Every FRC robot has to take this into account during design regardless of drivetrain to make sure that they don't stall the CIMs. Our systems are a bit different than your mill in that our systems are current limited by breakers well below the stall current of the motors typically used in the drivetrain. A stall condition will trip the breakers and your robot will just stop moving for a little bit. This is still an undesirable situation so most teams try to avoid it.

The most common way to do this is to gear your system so that the motors have enough torque to make the wheels slip. Having belts that slip somewhere in your power transmission path would be another potential way to attack the problem. As a mechanical ignoramus I will avoid commenting on the pros/cons of using belt slip to address the stall condition.

Re: Questions about crab/swerve drive.
 

I had an in depth conversation with people from 118 the gears on the modules are actually a friction fit, so they just turn the modules to realign, instead of turning the motors and chains that position them.

Re: Questions about crab/swerve drive.
 

Steering with chain works great, but I wouldn't say it should be chained. I've seen teams cable. I believe Wilstang did in 2008, I didn't get to see their robot this year up close, but they may have used it again. My team used cog belt this year and it worked just as well as the chain we used the year before, the only thing about belt is that it needs more wrap around its pulleys/sprockets than chain does. I have also seen teams use a linkage, if you look at 141's robot this year it is a good example of this.

Re: Questions about crab/swerve drive.
 

[quote=Sgreco27;855714]Well OK I see what you mean. I just meant to say they needed some sure way that they would stay in sync. Additionally, having said that, we didn't use a physical link to sync our drives. We used an absolute encoder on the steering motors on both sides to "sync" them together. I just didn't want to throw that in because I was asking about a comparison between chains and belts for the dive system and that was a steering system.

Steve

Re: Questions about crab/swerve drive.
 

We've used 1 long loop of #25 chain going around the modules to steer them, the sprocket mounting system actually has slots built into it so you can loosen screws, align module and tighten them down again works out okay.

Re: Questions about crab/swerve drive.
 

Thanks for all the help guys.