Swerve Drivetrain.

[Mk.32]

Hello all, Team 3647 here.
We are done with our 2011 season and moving to prepare for our next season.
One thing we saw that really neat was swerve drive systems.
So we want to build one on a test bot to experiment and test.

After looking around we found the andymark modules:
http://www.andymark.com/ProductDetai...ctCode=am-0760
We also have a basic idea of how rig everything up.

We are planning on having the left side wheels and right side wheels hooked to separate power trains. Then the top right and bottom left wheels linked together for turning and then the top left and bottom right linked together for turning. So we can have all the wheels at 45 degree angles so we can spin in place and then strafe with turning the wheels 90 degrees.

So my question what do you guys think? How do you run your swerve drives? Also how do you have your controllers setup? What gearing ratio do you use? Also any documentation would be greatly appreciated.

Thanks for all the help.

[big1boom]

We have done Right/Left Power split, Front/Back turning. It allows translation in any direction, good turning ability, on the spot rotation like a tank drive, and leaves the middle open for a manipulator.

We have also done all 4 powered together, Front/Back turning. Simple to control, difficult to chain up.

I would recommend Right/Left Power, Front/Back turning.

[EricH]

There are quite a few ways to do a swerve drive. There's fully independent steering (each module has its own turning motor). There's all turning at once. (This one really needs some other way to turn--or a turret, noting 118's 2007 robot.) There's front/back and left/right module linkages.

I don't think your turning solution is optimal--it'll work for turning in place, but it could be really nasty for anything else, programming-wise, especially if you get a wheel off from the rest by mistake. And the chain runs are going to be especially interesting--you're forming an X with the chains, and hopefully you find a good way to do that without risking fouling up. I've seen that done before, with 4 runs, and it worked OK, but I think it was the propulsion power runs from the center to the edge and the rotation runs on the outside.

What I might do, as an experiment (and I don't know if anybody's fully run this on coaxial), is to turn all the pods together, but have left/right or front/back motor units. Think of it like tank drive where the wheels can pivot for sliding sideways. It's somewhat easier to do with the Wild Swerve module (also available from AM/221 Robotics Systems), but could be really interesting to try a coaxial version.

Incidentally, I've never been on a team that used swerve drive. The closest was 2009, two years after I left, and if there hadn't been a trailer towing behind the robot it would have been a turreted robot instead of a pivoting drivetrain.

[Mk.32]

At the SD regional Team 1717 had a really nice swerve drive and a others i don't remember atm.
What is the difference between the Wild Swerve module and the ones i linked to in the OP?
And sliding sideways, we were thinking about that, but would it work?

[EricH]

Originally Posted by Mk.32

At the SD regional Team 1717 had a really nice swerve drive and a others i don't remember atm.
What is the difference between the Wild Swerve module and the ones i linked to in the OP?
And sliding sideways, we were thinking about that, but would it work?

The difference is that the Revolution modules are coaxial, and the Wild Swerve have their motor in the module. (Oh, and they're based off two different teams' designs--Revolution is from 118 Robonauts, and Wild Swerve comes from 111 WildStang.)

Coaxial swerve means that the shaft located at the pivot axis (which the module tends to rotate around) also transmits power to the drive wheel. The motor is completely off the module, other than the chain run. Non-coaxial means that you have to be careful not to twist the motor power wires as the module turns (unless the GDC allows slip rings again), as the motor is in the module. Less chain, but more wire.

Sliding sideways=strafing. Sorry I didn't make that clear. And if you're not going to do that with a swerve drive, then you may as well not build a swerve drive...

[apalrd]

Originally Posted by EricH

...(unless the GDC allows slip rings again)...

Not true.

Originally Posted by FRC Game Manual, Section 4 (The Robot), Rev K

<R40> All active PD Board branch circuits shall be wired with appropriately sized wire:
Application
Minimum wire size...<snipped table>
The branch circuit may include intermediate elements such as COTS connectors, splices, COTS flexible/rolling/sliding contacts, and COTS slip rings, as long as the entire electrical pathway is via appropriately gauged conductors.

If they are COTS, and appropriately gauged, they're fine.

If you want to build a swerve drive, build one in the fall. You probably won't be able to do it in 6 weeks without prior experience in swerve-building.

[EricH]

Originally Posted by apalrd

Not true.
If they are COTS, and appropriately gauged, they're fine.

Read the entire post. Note that I said "GDC allows" and "again". And know your FRC rules history. Slip rings are an on-again, off-again legal part. (190 had custom ones in 2008 that were legal--there wasn't a prohibition on custom ones then. COTS ones were not legal at that time, IIRC.) COTS ones even more so. This is one of the few times that I remember COTS slip rings being legal for competition use.

And note that the rules change every year.

So, IF the GDC chooses to allow slip rings again, then you don't have to worry about motor wires twisting. Otherwise, you do.

[Mk.32]

That team 1640 doc was exactly what i was looking for, information wise.
Thank you very much.
If we do go this route we are definitely building a test robot off-season first.
And what rule exactly is it that dis-allows slip rings?

[GGCO]

We're using the AM swerve modules, and they're fantastic! The hardest part though is the code base. Make sure you have a TON of time and good programmers to do this.

[Ether]

Originally Posted by GGCO

The hardest part though is the code base. Make sure you have a TON of time and good programmers to do this.

There are a couple of short papers here which explain the kinematics of, and give example code for, 4-wheel-independent steering and speed control.

[EricH]

Originally Posted by Mk.32

And what rule exactly is it that dis-allows slip rings?

This year, there is not one. In fact, they are explicitly allowed.

In past years, rules varied on legality. In future years, the rules may or may not allow them. Hopefully, they will be allowed. But, we can't be certain.

[Mk.32]

k that makes sense.
Thanks for the clear up.

[Mk.32]

Also another question,
with the wild swerve drive modules
http://www.andymark.com/ProductDetai...ctCode=am-0496
do we need to blot both the top and bottom plates down?
or would just blotting the top plate be enough?

[cire]

Originally Posted by Mk.32

Also another question,
with the wild swerve drive modules
http://www.andymark.com/ProductDetai...ctCode=am-0496
do we need to blot both the top and bottom plates down?
or would just blotting the top plate be enough?

You need to bolt both the top and bottom plate, otherwise it will just bend the top bracket.

[Clem1640]

2011 is our 2nd year of doing Pivot Drive. 1640 is very satisfied with the performance of this drive-train. Our drivers love it! We've designed and built our own. Info is available at:
http://wiki.team1640.com/index.php?t...II_Drive_Train

1640 has taken the approach of independently driving and steering each wheel. This puts a lot of investment into the drive-train, but also provides a fly-by-wire flexibility. Drive modes are programmable. The robot has mecanum agility (or better) with tank traction. A hell of a combination.

This is a great drive-train, but not easy to build or program. Many teams which start a Pivot program fail and abandon it. The AndyMark modules are probably a good starting point.