how does crab drive work?

Hi,

Next year I will probably have the opportunity to move on to another team other then my own (I’m graduating). It is highly possible that I’ll be starting a rookie team. Anyways, with my limited knowledge on robot drive systems, other then our own team (we had a 2-speed shift on the fly system), I would like to build a crab drive robot which would probably happen over the summer.

What I’d like to know, is from any teams, that have succesfully made a crab drive system, how does it work. If you use omni wheels, to get a strafing ability I’d also like to know how that works. I’ve been looking at Wildstang’s site, and have gather basic blueprints of their system, but what would really like to know is:

what kind of motors are used on each wheel?
how are they geared?
how are they configured?
What motors are used to turn the wheels (either 1 motor for 2 wheels, or 1 motor for 1 wheel)?
If you gear the motors up or down, what size sprockets are used?
Anything special that we show know about your system (angled gears, strength of the chains, speed, power, weight, unique programming)?
and most importantly, what kind of control sytem is used (what does each joystick do)?
Basically any information you can provide about a crab drive system would be awesome.

If possible, it be awesome to have some pictures, but whatever you can come up with I’d really appreciate. Thank you very much.

Most crab drives works with all the wheels driving independently, one motor driving one chain, which is turning all the wheels at the same time, making them all going the same direction.

There is a good picture on this site somewhere, Ill try to find it and post it here.

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Crab drive is a challenge. Wildstang has been doing it for years and makes it look really easy!

our team has tried it 2 times. only one was sucessful.

Our first time being our second year. We had 2fp and 2 drill on the drive part, and then we had the window motor turning the all the wheels at the same time. There was a big chain that went around the whole robot connecting all the wheels so they all turned together.

Last year. We had a bit of a more complicated system. All 4 wheels were independant. The front modules had the Chip motors, and the back had the drill and the fisherprice. They were turned by globe and window motors. Each ‘module’ was a small tank thread with pully stock and pully on it. There was a pneumatic on each one so that it could go down so it was flat to the ground for mega pushing power or come up so that it was in crabing mode to turn easier. Well the turning of the modules wasn’t working at the first regional so we eventualy just went to having the front wheels and the back wheels turning independantly. That didn’t work real well either so we locked it into tank drive. Eventualy we gave up post season, and took out the whole system and replaced it with tank drive and casters.

As far as controls go. We had a button the first year so that you could push the button on the joystick and go into crab mode. Where ever you pointed the right joystick it would orient the wheels in that direction and go forward. It had ‘zones’ set up so that if you pointed it from back to front it would find the shortest route to rotate the wheels there. and would also make sure it wasn’t twisting more then 360 in one direction ever so that it wouldn’t twist up the wires going to the motors and such. If you let the button up you just had a normal 2 joystick tank steering system. It was really nice.

Last years controls we insane. And never really worked. So i’m not gonna get into em.

Its not an easy system to do without alot of thinking and planning ahead of time. I think that penn made a pretty good crab system once if you want to look into that.

some answers to your questions:

what kind of motors are used on each wheel?

I believe that most teams use the drill motors and/ or chips to drive the wheels. but this all depends on the set up. If your crab drive is going to have one constant chain moving all the wheels at once then it doesn’t matter what motors you use for each wheel as long as they are geared correctly to match output speed.

how are they geared?

well besides making sure the wheel speed is the same on all the wheels, the motors are normally mounted above the wheels and the gears go down through the wheel casing.

how are they configured?

This all depends… some teams have a 2 wheel crab drive with omni wheels, some have a 4 wheels system where all the wheels are physically timed together, some teams have 4 independent wheels and are programed in sync. The configuration will depend on a few things, 1) what motors you have in the kit and which ones are most simple to match speed on, 2) which type of system you want to have, 2 wheel, 4 wheel, etc. 3) and it also depends on your teams resources, there can be alot of custom parts that have to be made for specific types of systems and what equipment your team has access to might determine the type of drive you build

What motors are used to turn the wheels (either 1 motor for 2 wheels, or 1 motor for 1 wheel)?

I have seen the Fisher Price, globes, and windows all used. the globes are usually the ones when it is each wheel turning on it’s own, the FP and window are used when there is a system moving more then one wheel

If you gear the motors up or down, what size sprockets are used?

this all depends on your speed and for the most part you will gear things down, putting over 6500 rpm (chips) or over 1400 rpm (drills ) isn’t usually a good idea and the size of the sprocket all depends on how much you want to gear them down.

Anything special that we show know about your system (angled gears, strength of the chains, speed, power, weight, unique programming)?

the only thing that I can think of is that crab drives need to be carefully planned because of how involved they can be

and most importantly, what kind of control system is used (what does each joystick do)? there are a number of things you can do., but I believe the most common is using one joystick where a hard left, turns the wheels horizontal, the same as a right turn.

   --    --
            |  |  |  |     joystick forward /back                                  
             --    --      
       ____    _____
      |       |   |         |  joystick left or right
       ------   --------

    those boxes are wheels sorry for the crudeness  
 

I hope this helps… I know that there are some good pictures of crab drives in the gallery and you can learn a lot looking at what other teams do…good luck

Here is our crab control system code as a starting point

(About half way down)

We used the drills and FP for drive and 3rd drill for steering, I would strongly recomend the Globes for independant front rear steering motors

You can PM me for info or questions.

*Originally posted by TEAM_74 *
what kind of motors are used on each wheel?
What motors are used to turn the wheels (either 1 motor for 2 wheels, or 1 motor for 1 wheel)?

2 modules use FP, 2 use drill
1 globe motor turns the two left wheels, the other globe turns the two right wheels

and most importantly, what kind of control sytem is used (what does each joystick do)?

I don’t have much knowledge about the specific mechanical details, but I can help with the controls. Ours is controlled with two joysticks. If you leave the left stick alone, the right stick is simply the IFI single stick drive system. The left stick controls the wheel angle. If you move it half way left, all 4 wheels turn at a 45 degree angle left. Move it all the way left and they turn to 90 degrees left. We expanded the crab to turn 180 degrees each direction this year by using the negative-y axis. If the left stick is in the negative-x / negative-y corner (back & to the left) the wheels turn to 135 degrees left. Continue to full back with x in the middle and the wheels turn to 180 degrees left. Same for the right side. Once you crab past a certain point, the x-axis of the right joystick is ignored and all wheels drive at the same speed.

I may have confused you (that’s what I get for being brief & writing this quickly) so ask questions if you have any. Some time over the summer I want to put together a formal description of our crab control, and hopefully it will make more sense.

If you’re going to IRI (and if we go too) stop by our pits and we’ll let you drive it.

**If possible, it be awesome to have some pictures, but whatever you can come up with I’d really appreciate. Thank you very much. **

We’re still uploading our build season pictures, but hopefully when we’re done we’ll have some good ones that will help you out. Our gallery is at http://www2.wildstang.org/gallery

btw, nice name :slight_smile:

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This year, 190’s Crab Goat was a rather simple design. We had our drill motors running into the FIRST 90 degree gear box, with our main wheels connected to the straight through shaft, and the crab wheels connected to the 90 degree shaft. I’ll post a diagram of exactly how we accomplished that later.

The crab wheels themselves were raised and lowered by the Van Door motor using a simple linkage. We just used timing on the motor – no limit switches.

The control was very simple. In regular operation, the crab wheels only drove straight, so the operator just pushed a button to drop them down, and moved one of the joysticks right or left to move the robot. We did have emergency kicker mode, which allowed us to drive with our crab wheels only (just in case something happened to our main wheels), but I’m not exactly sure how those were controlled.

I’ll get you a diagram as soon as I can get a decent drawing program up on my PC.

Watch all four wheels turn when you see one in traffic! I figure they stole it from FIRST!:stuck_out_tongue:

Check out the new Chevy Silverado QuadraSteer with Chief Delphi rear steering, Just kidding but Delphi does really make it for us. Steers up to about 13 degrees and makes a pickup turn like a saturn coupe!

Once upon a time, a long long time ago Team 47 introduced an alternative drive system to the FIRST community. I remember the first couple of years and the expressions on peoples faces … priceless! We have successfully implemented this style of steering/drive system 6 of the 8 years we have been around. The 2 years we didn’t have it were, the 1st year and this year.
We have created 6 different systems - depending on how many motors Dr. Joe could strongarm out of my tightly held grip. We have had 4 wheel drive independent 4 wheel steer, 4 wheel drive chain driven closed loop steering, 2 wheel drive closed loop steering, any way…you get the idea.
Since then it has been very fun watching where and how often this type of drive / steering has been tried (some very successfully, aka; wildstang)
I’ll request Dr. Joe create a history of swerve and a description of the systems we have used - and post it as a white paper. One other word of caution, This style of drive / steering system can get very tricky, it can require a bit of precision machining and a bit of programming to match up the mechanism with the electronics. The reason we didn’t have it this year, is because we didn’t think we could have it and still stay within the $3500 K3 Rule limit - and we knew we would be a bit short on the resources required (people, machinery and equipment) needed to do it justice. All in all - I think our drive system did just fine.

everybody that has posted has been a really big help, I appreciate everything that you have done. My only remaining question deals with how to make a housing for the drill motors and CIM’s/Fisher Price motors on top of the wheels. Are they just mounted on top or is there a special configuration that houses them?

We do hope to go to IRI, if we can afford it. If not, I’ll drive down just for the competition. (hehehe…a chance to drive wildstang’s robot, an offer nobody can pass up)

But if anyone has any more tips or ides (maybe a couple pics) that’d be awesome. Thank you very much

As for mounting of the motors, the motors are mounted the same as any other gearbox that you would create. The difference is that instead of making the gearbox in a horizontally long configuration with the motors in line flat with the wheels you stand it up on end and mount it vertically so the swing of the steering wheels in the chassis is reduced and the wheels can be mounted closer to the edge of the chassis. See the wildstang pictures for a good look

Btw if you want to do a search on this I believe the official name for this type of drive is synchro drive.

By the way if you want to do a search on this I believe the official name for this type of drive is synchro drive. Whooohooo I found it. A guide to steering robots. I was looking for this. The only steering method they do not mention is holonic (aka omni directional) steering.
http://www.cs.dartmouth.edu/~robotlab/robotlab/courses/cs54-2001s/locomotion.html