As we started machining a turret we have came upon the question of could we make it a continuously rotating turret with 360 degree + of rotation by wire tracks or such. We found it to be completely do-able. The question is; is it doable AND legal? If so what are the limitations on wire to wire connections (finger like brushes?). Any insight would be great! Thanks
But depending on how many wires you had going up into your turret…you’d need quite a few wire tracks.
We’re using some brushes in a setup on our robot too. Not for a turret, but to achieve the same effect. As far as we could telll after reading all the rules and going through the flow-chart, there was no reason it should be illegal. Tho we have a backup plan just in case.
They make just about the only FIRST-legal slip ring that I know of. If you use your own brushings, expect to come under a LOT of scrutiny during inspection. The nice part about using COTS items is that the manufacturer spends a lot of time and effort guaranteeing certain performance metrics (in this case, voltage and current capacity). It is much harder to convince an inspector that your homemade brushes act in a safe and reliable manner.
FIRST doesn’t fool around when it comes to the transmission of electricity throughout the robot…
I’m not sure if this qualifies as a legit part, but here’s a link to a Make Magazine article on how to make your own slip ring assembly out of plastic and ball bearings, that can handle decent power levels:
Any comments about the legality of using this design on a FIRST robot?
As much as I’d like to use slip rings (commercial or otherwise) one year - they could be very handy for swerve drives - I’d be a bit worried:
<R58> All wiring and electrical devices must be electrically isolated from the ROBOT frame; the ROBOT frame must not be used to carry electrical current (this isolated ground arrangement is necessary due to polarity reversals that occur under certain operating conditions such as during motor direction reversals).
<R85> Electrical devices must be wired in accordance with Section 8.3 Robot Rules, using commercially available copper wire for all electrical connections. Aluminum (or other non-copper material) wire is prohibited. For examples of acceptable circuit designs, please see the 2007 Robot Power Distribution Diagram.
I think you could argue that a commutator involves using the robot frame to carry electrical current, illegal according to <R58>; unfortunately, ‘frame’ isn’t defined. Even if you argue that the commutator isn’t part of the actual robot frame, but a separate, isolated piece, I certainly think a commutator is an example of an electrical connection using something other than copper wire, in contravention of <R85>.
I don’t have the FIRST experience to know for sure, but I agree with others that a conventional “slip ring” might not be in compliance with R58 / R85.
However, we are also trying to build a continuous 360-degree turret this year. (The turret we had last year for our shooter only turned about 100 degrees.) Our approach this year is to use a homemade “slew ring bearing” out of a couple of lazy susan bearings, and then pass the wires up through the center of the bearing. The wires will take a 360-degree twist each time the turrent goes around, but we intend to put a fair bit of slack into the cabling and “count” the rotations (in software) for the turret. We anticipate that few matches will involve more than 2 or 3 complete rotations, so we don’t need to be able to handle an infinite amount of rotation. We plan to have software enforce a maximum number of rotations in either direction, as well as to use an indicator on the OI to let the driver know which way to go to “unwind.” In between matches, we intend to “unwind” the wires manually.
Can’t wait to see your 360-degree turret! (For that matter, we can’t wait to see if our own will actually work!)
BTW… OOC, before I forget, how much is the Mercotac unit, anyway? Their 830 (8-conductor unit) is definitely cool. You could tie several 30A motors to it, with data lines to spare, but I didn’t see prices on their site. (I worry whenever a site makes you RFQ an item. That normally means it’s outrageously EXPENSIVE!)
Anyway… First off, to answer one question "Frame" is commonly defined as the metallic chassis of the robot, the cases of the motors, etc. They don’t want the chassis used as a power ground return, for a number of safety related reasons (“floating ground wiring” only). This IS a good rule.
(<R58>) - IMHO R58 isn’t a problem here at all. As long as you don’t use the frame itself for ANY of the connections, it doesn’t apply. (IOW, simply add another ring!)
(<R85>) This however, is a different story…
But remember - A slip ring is a CONNECTOR, not a “wire”, and you ARE allowed to use “connectors” to your heart’s content! Therefore, I would argue that as long as it’s either a COTS slip ring assembly, made completely from COTS items (copper pipe, PVC, graphite brushes, etc…), or SCAVENGED from a COTS item (e.g. take the ring set out of an old alternator), AND it is sized to handle the current needed, AND it’s insulated properly, it therefore isn’t a “wire”, and R85 doesn’t apply.
But, IANAFRL (I am not a FIRST Rules Lawyer)… so I definitely think once you have decided on a way you want to do the slip rings, a Q&A question posing it is in order to get a final ruling on the subject.
Other slip ring ideas to explore (and/or comment on):
A “hardware store” homebrew version may be simply made using a pair of nested PVC pipes, separated by flanged bearings on the ends. Slice up some rings from the next size larger copper pipe for wrapping around the inner PVC tubing. The outer pipe has a series of “T” connectors along it to hold motor brushes (an opposing pair for each ring, connected together). Now use hardware store replacement spring loaded graphite motor brushes within the “side lobes” to contact the copper rings, and screws on their endcaps to give you a connection point. You’ll need a grooved or X-shaped stick glued into the middle to hold the center still, yet allow wiring into the core. (I’ve seen this on the net before, but I couldn’t find a good photo for it. Sorry…)
Yea, that’s a lot of fab at this stage of the game, but it’s all hardware store items!
For those interested, here are a few more links for homebrewed & COTS slip rings:
A COPPER version of the above homebrew slip ring set:
United Equipment Accessories (no clue on price & availability)
Michigan Scientific Corp (again, no clue on price & availability)
…and lastly, Global Spec search engine results (52 slip ring manufacturers found - but you must register to use the engine’s output)
I hope this helps!
Last season we posted on the FIRST Q&A forum about whether we could use the aforementioned Mercotac slip ring.
The answer was (surprisingly) YES.
It was under $200. Surprisingly.
Slip rings are allowed this year. I would consider a COTS slip ring to be a connector. But a “home made” slip ring would invite more investigation by the inspection team.