Hexagonal Chassis + 6 CIM swerve drive
 

Hello.

As you probably know, swerve drive is cool. I've been fascinated with it ever since I discovered other teams using it (namely 1640 at MAR), and took it upon myself to try to create some CAD files for our team. We've recently been discussing using 6 CIMs on the drivetrain; as an added challenge, I wanted to see how I could use all 6 in a swerve drive. My answer was to use 2 cims on each of 3 swerve modules, instead of using 1 cim on each of 4 and having 2 left over. However, rectangles don't sit/balance very well on triangles; thus, I decided that a hexagonal chassis would be better as well. Here's my current (basic) progress. The gear ratios aren't correct, but I've placed gears everywhere necessary and the appropriate sizes can be determined and bought.

It is a coaxial design; the sprocket at the top of the drive shaft above the wheel is attached to the drive shaft, while the sprocket below that is bolted directly to the pivot case and is attached to the pivot module. Between this lower gear and the plate which has the toughbox and P60 gearboxes attached (the lower of the 2 large horizontal plates) resides a thrust bearing which is inserted into the sprocket attached to the top of the pivot module.

Please let me know if you have any questions/comments/concerns about the design, attached concepts, or anything else. I'd like to know what I should improve on this design, and (perhaps more importantly) if it will work. I do know that I need to add some bolts between the top and the bottom horizontal plates, add some more support horizontally between the two vertical supports for the wheels, make an actual mounting system (which will probably just be bolts through the bellypan of the chassis in this sort of setup), and then I need to swiss cheese everything. I also need to put in the shafts of proper lengths and add keys/retaining rings where necessary. However, I'm sure there are things that I'm forgetting; if you find anything, please let me know.

Also, I would actually appreciate some assistance figuring out the gear ratio needed from the RS550 motor we will be using; I have no idea how to figure out the torque required to turn the wheel under load. I also need to correctly gear the cim motors, but that shouldn't be too much of an issue.

Thanks,
Hades

Module:



HexBot:

Re: Hexagonal Chassis + 6 CIM swerve drive
 

This is a really neat idea. Did you consider having 6 different swerve modules? I suppose the fact that non-CIM motors are limited to 4 would pose some additional difficulty.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

This looks quite interesting, and I like it.

My only concern with the 3 swerve drives on a hexagonal chassis is that this might be a bit unstable and prone to tipping under heavy defense or with a tall robot design. The design looks like it already has plenty of power to drive the robot, so maybe an unpowered omniwheel in each of the corners without a wheel could help with stability without impacting maneuverability.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

That makes sense, thanks. I was thinking either casters or omni wheels; omni wheels would probably be easier and more effective.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

What if you mounted the motors where the wheels are now, and had a set of wheels on either side of motors? It would give you stability by having a wheel in each of the 6 corners, but the simplicity of only needing 3 modules.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

Are you saying to put 2 wheels on each module?

Edit: I did it the way I have it so that when the bot drives forward, there will be a flat side instead of a corner.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

Your design looks pretty good. However, as others have said, you might face tipping problems even with what looks like a pretty sweet setup.
You can still use 4 modules on a hexagonal chassis I think.

A note about the swerve modules you have setup: I highly recommend using the swerve design from team 221 if you go coaxial. Basically, they sell a really nice looking coaxial that is already optimized for weight. Download the CAD and use those ideas in making your own coaxial swerve.
Mainly I suggest this because it's already super light and they have very nice looking plates that you can emulate. Do you have access to a CNC?

Re: Hexagonal Chassis + 6 CIM swerve drive
 

This seems pretty good.
Basically cbale says to have one gearbox for each two independent coaxial modules. That way you only need 3 gearboxes.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

Thanks. I'll add the omni wheels shortly and post a revised drawing.

As for 4 modules, the reason I'm not using 4 is so that I can have 2 cims powering each wheel setup.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

If you've got 6 independent drive modules, you'd need to have 6 steering motors; for consistency (and performance) sake I want to use the same motors on each setup. You can't use 6 of anything except CIMs, which means you'd have to change motors for the last 2 steering motors, or do a 3/3 setup; I fear this may cause more problems than it solves, and that the issue will be much more easily remedied by adding non-driven wheels.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

If you want a 3DoF swerve, you have to drive and steer each wheel independently.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

One more thing to think about is the pure amount of power a swerve drive uses in the first place. We have had a lot of problems with that alone. Adding in more cims may make that worse. I would honestly focus more on how to make it more efficient.

If you are running a 3 wheeled 6 cim triangles are in my opinion a great drive base. With a corner you could penetrate a defense pretty fast. 1425 had a great kiwi drive, i can not imagine them if they would have had a swerve drive. plus 3 corners makes the rolling maneuver swerves are famous for easier.

For the gearing for the steering. we have found that a 132/1 for final reduction is pretty good. We messed around with faster/slower and found that any faster we might have problems with turning the wheels in a pushing match where we receive a lot of thrust.

On a side note make sure the module is mounted well if it becomes cantilever it makes it almost impossible to turn when the robot is not moving.

For weight saving you can look in to what we did where we put the steering motor in the belt from the drive belt. We managed to make the module super strong and insanely light doing this.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

You're correct, I had to think about it a while to realize that issue. You could potentially do it with half of the wheels as omnis (alternating every other), though that's not the most practical.

I'm still of the opinion though that using omnis on a swerve drive defeats the purpose of a swerve drive (Why not just build a kiwi drive if you're going to use all omnis?).

Re: Hexagonal Chassis + 6 CIM swerve drive
 

Well, for one swerve will have a lot more traction. Divorcing orientation of the chassis from the wheelbase is also a feature not found in omnis. In this case, the omnis will only be used to support the robot weight to prevent it from tipping over, instead of as steering. Many teams use this approach in tank drive to allow for easier turning (though they tend to power the omnis ...)

Re: Hexagonal Chassis + 6 CIM swerve drive
 

To the extent that the unpowered omnis are supporting weight, they are robbing traction from the driven wheels.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

If you are using coaxial swerves, I can't see a reason why not to simply use the drive sprockets to power two wheels at once, then use separate turning motors for each wheel. You can even technically use a single 6 cim gearbox to power any number of modules.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

If you do this you cant have a true 3D swerve drive (X,Y,rotation). Changing the power to each wheel is how you preform holonomic maneuvers.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

Oh really? That's annoying. Thank you for the information.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

I think at that point it would be a crab drive or something close to it i might have some hidden benefits but a true 4 wheel independent should out perform it. (or 3 wheel independent)

Re: Hexagonal Chassis + 6 CIM swerve drive
 

So random question
What happens if you have say a 8 wheel swerve and you powered the wheels in sets of 2. Could you code them to act like any 4 wheel swerve?

Re: Hexagonal Chassis + 6 CIM swerve drive
 

If you have to change speeds for holonomic motion, I don't think you could do that even if it's a multiple of 4.
Not to mention 8 coaxial swerve modules plus the associated turning motors/ gearboxes would be massive. That would weight at least 18bs for the coaxials, 2lb per shifter for 8lbs more, and 8 turning motors at 0.5lbs each plus all the chain and sprockets. So 30 lbs+ just for the swerve modules.
Livable, but I'm more worried about space even with coaxials.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

I don't think so. It is an interesting combination, but if I'm not mistaken swerve works on having the ability to control rotation about a point and not a line. You need to power each different point in the rotation at a different speed, and without some sort of differential that wouldn't be possible in swerve.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

Why not raise the omni's axel center up an 1/8th (maybe less? 3/32?). This would allow the 3 powered wheels to have optimal traction, while still having good stability when being pushed. At the expense of traction.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

You could code them like a 4 wheel swerve, be they wouldn't act like a 4 wheel swerve because it wouldn't be kinematically correct. When turning, the wheels would scrub.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

To anyone having a hard time understanding why you need to power and turn wheels individually for a true swerve:

Take a rectangle. Move it in some manner that involves both a translation and a rotation. Pick some set of arbitrary points (say the corners), and imagine those are your wheels. It should be easy to see that for any generalized movement, those points are all moving in different directions and at different speeds.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

Correct.

For anyone interested in the math, here are the equations:

http://www.chiefdelphi.com/media/papers/download/3236

Re: Hexagonal Chassis + 6 CIM swerve drive
 

You could try using 4 cims and 4 mini cims for 4 modules.

Re: Hexagonal Chassis + 6 CIM swerve drive
 

Here are 10 pictures of a "semi-swerve" drive I built at about 1/3 FRC scale, using servos to steer and independent motors to drive. The last 8 pictures show how you have to orient the wheels to move various directions, including pivoting around selected points. (These are views from below).

Re: Hexagonal Chassis + 6 CIM swerve drive
 

swerve inverse kinematics math quiz:

http://www.chiefdelphi.com/forums/sh...d.php?t=129185