Originally Posted by BenX02 (Post 854539)

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

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

Originally Posted by rc_cola1323 (Post 854540)

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.

Originally Posted by Doug G (Post 854544)

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.

Originally Posted by BenX02 (Post 854539)

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.

Originally Posted by hillale (Post 854549)

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

Originally Posted by BenX02 (Post 854539)

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.

Originally Posted by BenX02 (Post 854539)

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.

Originally Posted by BenX02 (Post 854539)

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?

Originally Posted by BenX02 (Post 854539)

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

Originally Posted by big1boom (Post 854582)

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

Originally Posted by big1boom (Post 854602)

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.

Originally Posted by EricH (Post 854623)

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.

Originally Posted by BenX02 (Post 854773)

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.