Different Swerve Drives

[Siri]

Originally Posted by CalTran

If we had to make the decision right now, we would chain each side's module together. Our reasoning behind this is because we believe this iteration is the "simpelest" method of swerve while maintaining some amount of maneuverability. As far as we have been able to tell, the added complexity of 4 independent modules for the added driving tricks added are not worth the time or research.
Is this the right decision? If you could provide personal examples or logical reasoning as to why independent modules are a necessity vs. keeping sides tied together than that would be appreciated.

We use fully independent modules primarily because it provides more turning options, including spinning and snaking in both orientations. I don't think snaking in both orientations in most games, though we did use it this year: long for barrier crossing and some defense and wide for ball and bridge manipulation. Spinning can be valuable if you don't have a turret, though. As always, you should look at the stationary turning radius of the drivetrain you choose.

/..\
\../ ... Spin

/../
\..\ ... Snake

Originally Posted by CalTran

Thirdly, wiring. The obvious implication of swerve modules are that they are not infinitely rotating, due to the twisting of the power wires. What we would like to know is if it is possible to make one that would be infintely rotating, and how one would design for this. If not then how does one ensure that the position and rotation of the wheels are correct?

As Alan said, you don't need slip rings or wires rotating at all. All you need are power take-offs to transmit drive and steering rotation co-axially. We do it with one belt each, plus a set of bevel gears and a chain for each drive, 8.6lb each. (FRC 1640 Pivot Modules 2012)

[Ether]

We use fully independent modules primarily because it provides more turning options, including spinning and snaking in both orientations.

/..\
\../ ... Spin

/../
\..\ ... Snake

I suppose you could do the above two motions with the left and right sides chained together for steering if you added an extra gear on each side to change the front/rear steering directions. But you'd sacrifice crab motion.

[Cal578]

You could have one motor steer the left side, and another to steer the right side. When you turn both of them 90 degrees, you get crab drive.
|..|
|..| = Forward

/..\
\../ = Spin

/../
\..\ = Snake

-..-
-..- = Crab

Crab requires independent drive motors on the four wheels, since the rear wheels are aimed the opposite direction of the front wheels.

[CalTran]

Originally Posted by Cal578

Crab requires independent drive motors on the four wheels, since the rear wheels are aimed the opposite direction of the front wheels.

We intend on each wheel having independent drive motors. We're just a little shaky on the rotation motors and whether to chain rotation together or one motor each, or some weird combination (ie 1717's opposite corners). Whichever CD deems the best will be what we pursue.

[Secretspy97]

Thirdly, wiring. The obvious implication of swerve modules are that they are not infinitely rotating, due to the twisting of the power wires. What we would like to know is if it is possible to make one that would be infintely rotating, and how one would design for this. If not then how does one ensure that the position and rotation of the wheels are correct?

Yes it is possible for a wheel to spin limitlessly.

This is a CAD picture of a swerve module:


And this is a CAD picture of our entire robot's drive frame:


We mounted the CIMS next to the module and then connected a chain from the CIM to the smaller shaft on the module. The CIM controls the motors speed. We then attached two window motor to two sets of swerve modules, (one window motor for each side) by chain, in order to turn.

If you would like to just move around the field and have the robot facing the same wall for the entire match then 2 motors controlling steering (one for each side) works great. But if you want to be able to turn the robot, then you might want to think about changing to 4 motors steering. (one controlling the rotation of each wheel).

[Nate Laverdure]

Resurrecting this thread to mention that I just uploaded a whitepaper on this subject.

[Foster]

Thanks for the update of the info.

As an FYI, team 1640 is in their 4th season of "Unicorn" drive, 2010,2011,2012 and this year, 2013. Their fabrication info is on their website.

I've only seen them in practice driving it. This years incantation is quick and agile. There also seems to be some mind-meld of the driver and control system from the demos of Dewbot IX swirling around the base of the tower.

Check them out at the MAR districts.

Slip rings are reasonable for this application. Consider that the current that is commutated (sp?) in DC brushed motors goes through an interrupted slip ring.

You could make your own with copper tube, some insulators, and carbon brush blocks. To cut losses, use more blocks.

What's difficult is getting a clean signal across a slip ring.

[Grim Tuesday]

Originally Posted by three_d_dave

You could make your own with copper tube, some insulators, and carbon brush blocks. To cut losses, use more blocks.

Not on a competition robot you couldn't, unless the rules changed. R49 says any slip rings must be COTS.

[Jon Stratis]

Oh, and to ensure position/rotation of the wheels, you'll want to have feedback on them. I would suggest a potentiometer - this can tell your code exactly which direction each wheel is pointing, and would be a bit simpler than an encoder.

[Alpha Beta]

In 2010 we loved using a mercury slip ring from Mercotac. I believe we had some donated. They were later outlawed that year and we switched to a twisting loop of wire and used software to keep track of how many twists we had undergone. I think recent rules have allowed for them again.

There were 3 swerve drive systems that I know of at CowTown. 16, 3284, and 3928. Aren Hill said they wouldn't be doing swerve again. 3284 is willing to, and thought they learned alot this year. They used purchased modules from AndyMark. This weekend they had one of the modules lose a physical connection to its encoder. After adding a floating sprocket to tension the steering chain the felt like they were ready to roll again. I can't think of the last time 16 didn't do swerve.

In 2011 1717 steered the opposite corners together, allowing them to create a diamond wheel pattern and spin in place to reorient. That would have been our next iteration had we continued with swerve.

[Alan Anderson]

You can completely avoid the problem of getting electricity to a motor that swerves with the wheel. Using coaxial shafts and bevel gears, you can keep the motors stationary and transmit mechanical power through the rotating parts.