Rules Regarding Custom Slip Rings

Technical Electrical
1

Hi, our team is considering building a custom slip ring, but we have some questions regarding the rules:

I couldn’t find anything regarding the rules for exposed live wire. Will our slip ring have to be fully enclosed?

Additionally, would we be allowed to attach a motor to the slip ring? R56 states:

CUSTOM CIRCUITS shall not directly alter the power pathways between the ROBOT battery, PDP, motor controllers, relays (per R29-B), motors and actuators (per R27), pneumatic solenoid valves, or other elements of the ROBOT control system (items explicitly mentioned in R66)
I am not sure what “directly alter the power pathways” means. Does this mean that we can use a slip ring to power a motor as long as the power goes directly through the slip ring as if it was a normal wire? What counts as altering the power pathways?

2

oh boy.

3 Likes
3

R54 is what you are looking for

R54. Branch circuits 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/rated elements.

The slip rings need to be COTs, not custom. and rated for the current. If the circuit powers a motor, then it is a motor circuit not a custom circuit. There are a few listed exceptions.

4

as long as you manufacture your system with COTS contacts, it is legal. If you were to make custom brushes it would not be.

1 Like
5

This is a very spicy take I have not considered before

2 Likes
6

He is one who communes with the Spirit of the Rules.

4 Likes
7

as long as you manufacture your system with COTS contacts, it is legal. If you were to make custom brushes it would not be.

Can we CNC a brass circle and then use COTS brushes or is that not allowed

8

honestly, no idea. A COTS brass circle might be legal.

9

Ultimately it would be settled by your LRI. (Q&A typically punts these sort of questions) But, I would expect if it was not rated as system, you are going to run into issues at inspection.

10

If you did that, it would be a FABRICATED ITEM, not COTS. Appropriate definitions from the glossary are below:

COTS: Commercial off the Shelf, a standard (i.e. not custom order) part commonly available from a VENDOR for all teams for purchase

FABRICATED ITEM: any COMPONENT or MECHANISM that has been altered, built, cast, constructed, concocted, created, cut, heat treated, machined, manufactured, modified, painted, produced, surface coated, or conjured partially or completely into the final form in which it will be used on the ROBOT

Additionally, note the last portion of R54:

as long as the entire electrical pathway is via appropriately gauged/rated elements.

So, to sum up… if you want to create a custom slip ring, you would need to make it out of properly rated COTS parts. For example, a COTS copper ring, electrically rated for the current (or more) of the associated breaker, and COTS brushes, also properly rated.

In theory, you could make a very rough (and poorly performing, I’m sure) slip ring by stripping a length of wire, forming a loop with it, and using the end of another piece of wire as a brush. Performance would suck (in terms of disconnects, high impedance likely tripping the breaker repeatedly, etc). Consider all that, and then consider R8:

ROBOT parts shall not be made from hazardous materials, be unsafe, cause an unsafe condition, or interfere with the operation of other ROBOTS

Any attempt at a custom electrical component like this would invite heavy scrutiny for safety, even if it passed the other rules.

So, instead I would ask: What’s the intended purpose of such a device? Why do existing COTS supplies not meet your needs? What requirements do you have in your design that could be slightly modified to allow for a different solution?

11

We have a shooter that has unrestricted rotation. There’s a motor on it for the flywheel though, so when it’s wired it wont be able to rotate freely without a slip ring. We’re probably just going to opt for a long wire since using a slip ring seems too difficult

12

Yeah, that’s why most teams have a restricted motion on their shooters. With the challenges this year, I don’t see much need for more than 180 degrees of motion, but since you won’t have other robots on the field, it would be relatively easy to get 360 degrees - you don’t have to worry about a robot coming by and catching on the wire loop. Such a loop typically runs up near the bearing, then loops out to go around the bearing. However, if you have room in your design (height wise), you can run the wire up over the shooter, and drop it straight down to the center of rotation. This means the wires twist when rotating, instead of pulling taught around the outside of the shooter. This can let you get more rotation out of it, but there will still be a limit!

It’s fun to consider the possible design options to get around such an issue. In reality, though, I would revisit your requirements and ask “how far does the shooter really need to rotate?” With the challenges this year, it’s quite possible the answer will be “not very much” - limiting yourself to 180 degrees might be enough. I know my team ran with a 90 degree limit last year.

Finally, i strongly encourage hard stops if you have a rotation limit. You want it to be impossible to rotate more than your wires will allow, otherwise something will go wrong at some point and rip the wires out!

1 Like
13

@Ryan_Dognaux do we need a Thrifty Slip-ring for Integrating Multi-Functional Drives ?

2 Likes
14

One should always ask questions like this. Many of the top teams do. The lead mentor for 2056 talks about this aspect of their design process in the “2056 Ways to Maximize Your on Field Performance” YouTube video. I know I have heard the 118 mentors talk about this but it might not have been on a video.

15

Team THRUST did this with their turret in 2016. I believe they had a standard COTS slip ring to allow continuous rotation.

Similarly, you could easily ask whether you need a turret at all. A fixed shooter where you rotate the whole chassis can work (even more so if there are no defenders). You simplify your robot a lot at the cost of potentially slower cycle times. so, ultimately it is about optimization rather than requirements.

Another way to approach this is to put full continuous rotation as a “desirement” rather than a requirement. It is easy to see that if you had full continuous rotation that it frees up the drivers to be able to collect balls wherever they are on the field and the turret can stay locked on the target such that the drivers can shoot without needing to re-orient their chassis first. It is easy to see that if you had a way to do that, theat your design would potentially be more optimized. With 180 degrees of rotation, the drivers would need to be able to adapt to some limits on the direction the drivetrain could be pointed in in order to be able to shoot. but there is a cost associated with that level of function.

By making it a desirement rather than a requirement, it is easier to trade-off against other design options and see the pros and cons of each.

16

There are COTS slip rings out there that can do 40A, but those will only let you have a single shooter motor, and no CAN (if it’s a Falcon). I would like to see something a little higher current, maybe an 80A ring with 2 or 4 extra wires for CAN, but it’s something of a niche item.

17

Relevant to this thread:

18

This is an image of a ‘clock spring’ contact used for the connections between a vehicle steering wheel and the rest of the car. Modern cars have all sorts of steering wheel-mounted switches and stuff, so reliable connections (i.e., NOT slip rings) are needed.

Look closely and you can see four tape-like flat conductors that alternate between coiling onto the center or the outer hub; this has the flat conductor bending and rolling, which allows for a very long lifetime. The thickness of the conductors determines their current capacity.

This is likely not available as a COTS, but being completely passive, I speculate that it is not a custom circuit but a type of wire. If you were to do something like this, it could accommodate several revolutions of the turret. A car steering wheel generally turns about 4 times lock-to-lock; you can design for more if desired.

image
image671×713 179 KB

4 Likes
19

The “S” could be interpreted as “Scrapyard” :wink:

If one were to use high-strand count 12 AWG wire, it might be possible to make a version of this mechanism that would handle the currents required by the large motors or motor controllers. Otherwise, standard ribbon cable could be used for the signal level connections.

20

Robonauts did this on their 2017 turret. They have a wire snake for the wires. If you study their reveal video from 2017 you can spot it. There is a nice shot of the turret rotating at 1:07 - 1:09 where you can see the wire snake feeding the wires. There are a couple of decent views from above showing the snake at 0:22 and 0:12 and several other places. The shots go by pretty quickly, so you need to be quick to hit the pause button. They had a shorter version of the same horizontal wire snake concept on their 2020 bot.

1 Like