DIY PCB slip ring - Page 1
Forum section: electronics / beginners

author: FrankT
author_rank: Regular Contributor
timestamp: March 09, 2021, 04:21:37 am
content: There's a few post on the forum already that are discussing slip rings. I want to try and build a pcb type, like in the picture...The first version will just be a pcb with no special coating (expensive).I'm wondering what I can use for contacts?I've seen "Grounding Contacts", but they seem a bit big, and probably not suitable for a continuous sliding contact.The other thing I was thinking of was repurposing motor brushes, the small ones on leaf spring like this (although they have curved faces to match the commutator)Any thoughts? These are really expensive - the cheapest I've seen is $100+ on aliexpress, which is too much for a $20 robot arm joint.Thanks,Frank

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2021, 05:43:10 am
content: Yeah, you're on the right track. You can get tin and gold plated RFI/EMI spring contacts, and board-to-board and other types of spring, pogo-pin, etc. contacts. They come pretty small, I mean they're used in cell phones for example. Take your pick.Tin plate might actually be modestly reliable. Can get that on the PCB, or Pb-free/RoHS HASL, usually standard.Gold plate should be good enough on the springs as is, but the hard part is the PCB: ENIG is extremely thin, only good for a few cycles really. You need hard gold 30µin+ or something like that, which requires a few provisions from the fab (they need to make electrical contact to all the plated nets, then those connections need to be broken apart for use -- in edge connectors, the connection is made by extending the edge, then milling off the excess, beveling it in the process), and is a custom spec run ($$$).FYI, I wouldn't expect too much current capacity through it. Under an ampere should be fine, give or take? More than that, I'd want to design it carefully, maybe use multiple contacts, consider thicker copper, maybe stitching vias to hold it down better or something, test it under load and in motion, see how warm it gets -- that sort of thing.Tim

author: Gyro
author_rank: Super Contributor
timestamp: March 09, 2021, 10:18:22 am
content: I don't think you mentioned whether the motion would be single direction or bidirectional. Looking at your first photo, it looks as there would be a significantly higher contact wear (dig-in tendency and wandering) in one direction than the other. you might need to consider contacts where the suspension is more symmetrical.I have a powerful old Printed Motors (PML) pancake brushed servomotor in the garage. This has an FR4 PCB rotor (many, many poles) and uses standard graphite motor brushes in standard rigid vertical brush holders, bearing on the commutator area of the PCB. This area is gold plated, but after what looks like considerable use, part of this has worn down to the underlying copper. It does show that graphite brushes can work on PCB track though (and at very high speed).https://www.printedmotorworks.com/brushed-pancake-motors/

author: jpanhalt
author_rank: Super Contributor
timestamp: March 09, 2021, 10:30:59 am
content: Have you considered a "clockspring" like is used to connect electrical devices on the steering wheel to system electronics in many/most cars.They can easily go ±360° or 720° but not continuous rotation. New ones are about $100, but ones from a salvage yard will be cheaper. Winding your own wouldn't be that difficult ether. I think they are fairly low noise, which would be an advantage.Here's a new example:https://www.summitracing.com/parts/smp-csp-223?seid=srese1&gclid=EAIaIQobChMIx5-E6f6i7wIVavbjBx2G8guTEAQYBCABEgJmFPD_BwE

author: fordem
author_rank: Regular Contributor
timestamp: March 09, 2021, 02:59:54 pm
content: Quote from: jpanhalt on March 09, 2021, 10:30:59 amI was about to ask how far does this robotic arm need to rotate - most don't rotate continuously making slip rings unnecessary - laptop hinges (and the LCD cables that pass through them) get quite a work out - my laptop lid get opened & closed numerous times a day, every day, and those cables last in some cases, decades - the "secret" is that they twist axially rather than flex.

author: FrankT
author_rank: Regular Contributor
timestamp: March 09, 2021, 08:12:56 pm
content: Thanks for the replies. Some more details...My current robot arm has the wires running on the outside (wiring wasn't planned). Some joints which are capable of full rotation, will get tangled, especially after homing when they are far away from the home position. The other limitation is the movement range - some joints can only move +/-100 degrees. The plan is to make a modular joint, like is used in the UR5 robot, or similar to this open source one -https://youtu.be/4o3d7_WZ_DQbut using a hypocycloid gearbox instead of a harmonic drive or planetary gearbox.So motion is bi-directional. Limited to about 60 rpm. I need 2 conductors for RS422 signal, plus gnd and 24v. I was thinking of using 6 conductors, so power can use 2 pairs.The clockspring method is interesting - if I can limit the motion to a couple of revolutions. The action of tightening the spring could probably be detected. But it does end up being quite thick, getting thicker for each conductor - compared to 2 pcbs face to face.The description of the pancake motor is promising - for a first try, with brushes and a thick copper board. I had a quick look on pcbway, and hard gold plating is +$300 for set up, regardless of the number of boards - so maybe something for the future.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 09, 2021, 08:46:16 pm
content: The clocksprings I have disassembled (grand total of 2) use flat ribbon cable (like FPC/FFC) wound on the natural axis. It could get get a little tight in either direction because it is in an outer shell.With just ±100°, I might consider ordinary spaghetti wire or a coiled microphone cord. Spaghetti wire is high stranded wire with a soft insulation. McMaster lists cable for continuous machine flexing.For your power, what is the max current you need? Electrical noise with a slip ring for data may create a lot of problems. You might use something like a clockspring for data and slip rings/carbon brushes for power, but I suspect you don't need a lot of amps (>5A to 10A).

author: FrankT
author_rank: Regular Contributor
timestamp: March 09, 2021, 08:49:00 pm
content: I'm not sure what the power will be. I'm currently using small stepper motors which use less than 1 amp each, but I want to use compact brushless motors, which can peak a lot higher.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2021, 09:58:38 pm
content: If you are going to flex a flat cable (not for continuous rotation), there are "continuous flex-rated" cables that can handle many more cycles than a standard ribbon cable. One of many sources:https://www.cicoil.com/unshielded-cable/flexare rated for 107cycles.