Anyone tried a slip ring?

[LondonBoy29]

Hi,

we have built some swerve drive modules, but the motors are turning as well in this design, so we thought that since the rest of the build team doesn't want to make the motors stationary, we needed another solution.

I welcome other solutions, but I think a slip ring might work best.

Any suggestions / experience?

[EricH]

Slip rings are tricky--not necessarily because they're tough to work with, but because the GDC may or may not allow them. Last year, they were legal, and 973 used them on an offseason swerve, IIRC. Before that, the last time I remember them being legal was 2008 (190 used a custom one at the base of their mast).

[ratdude747]

Originally Posted by EricH

Slip rings are tricky--not necessarily because they're tough to work with, but because the GDC may or may not allow them. Last year, they were legal, and 973 used them on an offseason swerve, IIRC. Before that, the last time I remember them being legal was 2008 (190 used a custom one at the base of their mast).

ditto.

the other concern i would have is wear, which would depend more on your implementation and your driver's driving habits. and by wear, i mean loss of good contact, causing unusually high resistance, a loss in motor power, and a noticeable amount of heat.

[Al Skierkiewicz]

If your design is not going to be greater than 360 degrees of rotation, you should be able to get along with just wire going down the center of your pivot. Slips rings add weight and size to the module and provide one more place where series resistance creeps up on your efficiency.
From R40 2011

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.

Emphasis mine, you may need to prove the slip rings you use are capable of the stall current in the motor you use to satisfy that last part. Assuming the rules remain the same.

[Al Skierkiewicz]

I should also have included a caveat to not use mercury filled electrical contacts. This fits under the 'hazardous materials' sections of the robot rules.

[AdamHeard]

Originally Posted by EricH

Slip rings are tricky--not necessarily because they're tough to work with, but because the GDC may or may not allow them. Last year, they were legal, and 973 used them on an offseason swerve, IIRC. Before that, the last time I remember them being legal was 2008 (190 used a custom one at the base of their mast).

No slip rings there; just bevel gears.

[1986titans]

We had planned on using slip rings in our 2010 robot before they were deemed illegal.

They weren't all that necessary anyways; our coding was set up to automatically untwist the wires if they got wrapped up too much from turning (three revolutions, I think). Here's a video of it in action. I think you can see it "correcting" itself at 2:12.

[Ninja_Bait]

I think the potential for power loss is too great. I agree with Al and Paul that you can get away with having enough slack to allow the module to rotate, and just code the robot to avoid entanglement. Maybe you could use a reel of some sort if you don't like loose wires.

[anthonyttu]

If you weren't worried about the power losses. They can be very noisy. So I would rule out using one in any application involving sensors or analog components.

[Big Ideas]

Sliprings are one of those devices that are a great solution if nothing else will work. It's exciting to see a design working well, but, the design can be very time consuming to do right.

It doesn't help that First is very conservative with wire sizing. If COTS slip rings are required they may be expensive and heave for the current ratings needed.

One possible solution is to hack the commutator and bell housing to an adequately sized DC motor. "Commutators" are segmented slip rings.

We did a slip ring design last year, but it was a short term exercise on the way to a better solution.

[Dillon Carey]

Originally Posted by Al Skierkiewicz

Slips rings add weight and size to the module and provide one more place where series resistance creeps up on your efficiency.

If your worried about size and weight, then you should probably look into a co-axial module anyway.

[Al Skierkiewicz]

Originally Posted by Big Ideas

It doesn't help that First is very conservative with wire sizing.

The wire size comes right from the NEC for open frame wiring.

[DonRotolo]

As an alternative to slip rings, take a cue from the auto industry, where they need to wire an airbag on the steering wheel.

Clock-spring wiring.

Basically, you wrap some wire in a controlled space, loosely, allowing perhaps 2 or 3 turns in each direction easily. Use software to ensure you don't exceed that.

[Retired Starman]

After having spent 26 years running a planetarium which contained dozens of solid silver slip-rings, I would rather gnaw off my right arm than try to use slip-rings on a FIRST robot. Those things are nothing but trouble!

You mostly likely will trade the solution to one problem for about a dozen other problems.

If you insist on going the slip-ring route, consider solutions to the following:
1) Dirt. It gets ground into your rings with unpredictable performance results.
2) Design rings with multiple brushes, to keep contact if one hits a clot of dirt.
3) Shield against nuts, screws, washers, bolts, shavings, etc. etc. bouncing up from the carpet. Getting metallic junk in your high-current slip-rings can have SPECTACULAR results.
4) Allow for wear. With the pressure of the brushes, some materials which are good conductors won't last long under use.
5) A damaged slip-ring may be very difficult to replace between game matches.

Sounds like a good engineering project!

Dr. Bob

[Chexposito]

Originally Posted by 1986titans

We had planned on using slip rings in our 2010 robot before they were deemed illegal.

They weren't all that necessary anyways; our coding was set up to automatically untwist the wires if they got wrapped up too much from turning (three revolutions, I think). Here's a video of it in action. I think you can see it "correcting" itself at 2:12.

ditto