Getting outside DC wiring onto a constantly rotating surface
February 18, 2013 7:17 PM   Subscribe

For a research project, I have several pieces of equipment mounted on top of a large metal rotating circular surface. At the moment, all equipment is battery powered. I would like to come up with an inexpensive way of getting DC power and signal wires to a computer off the surface to analyze data in real time. Besides that constraint, going down thru the center isn't really an option since the rotating shaft is solid metal.

Ideas I've had so far:
Conductive Ball and Socket Joints
Pass-through circuitboard mounted using ball-bearings
Whatever magic is used for wireless chargers
posted by gzimmer to Technology (8 answers total)
Best answer: Typically, i think you'd use slip rings and brushes for this kind of thing, but that takes up a lot of space, since you need one set per line.
posted by ArgentCorvid at 7:27 PM on February 18, 2013

Best answer: Unless you are wanting to transmit SCSI or something equally conductor intensive then simple slip rings and brushes are the way to go. You can stack them up with minimal space between rings and can easily get a half dozen per inch.
posted by Mitheral at 7:37 PM on February 18, 2013

Response by poster: After googling the hell out of slip rings, I've learned so much.
Looks like I'll be going with:
Great site too!
posted by gzimmer at 8:12 PM on February 18, 2013

Any wireless tech can get signal off easily. IR or RF are trivial over small distances and may require you to process on the rotating surface to convert, although I have done some direct analog transmission via FM off such things. (Most recently strain gauge signals off of a rotating shaft via FM. Very cute.)

Likewise, power requirements dictate what you'll need, but it seems to me that one set of conductors is enough, if you use DC/DC converters on the platform to generate your local power from a single main power bus. If you isolate the opposite ends of the axle shaft, you can take DC in from each side. AC can be moved over space via transformer coils, but there will be drag. You could use a magnet and rotating coil, too, but again, how much power is critical in determining if this is feasible.

Your description omits several things that would make it easier to respond. My experience with slip rings goes from super el-cheapo consumer electronics devices (antenna controllers) to $10,000 fiber optic slip rings, so the range is huge, depending on constraints.

If it's test equipment and one-off design, most of the cost is non-recurring so approach and materials are less important.

Some questions:

Size of this?
Signal types?
Weight of rotating payload?
Tolerance for contact noise?
Power requirements on platform?
Rotating speed?
Tolerance for rotating variations?
Desired lifetime?
Other environmental factors?

Test equipment is product design without regard to recurring costs. Most of it is non-recurring engineering labor. The rules are a lot different for this type of thing. I prefer it because it's simpler and more fun.
posted by FauxScot at 9:21 PM on February 18, 2013

I wouldn't trust any kind of sliding contact to stay free enough from electrical noise to make for a reliable long-term data connection. If I were building this thing using slip rings, I'd use them to supply AC mains power to a PSU mounted on the rotating platform, and do all the data transfer via WiFi or Bluetooth.

Apart from noise considerations, this approach minimizes the amount of current the slip rings are required to carry which should make them last a little longer.

But I really don't like slip rings - they strike me as noisy, wear-prone and inelegant. If this were my project, I'd probably be wasting lots of time getting near-field magnetic power transfer to work, or designing a custom switching power supply around a rotating transformer, or figuring out a way to leech power from the rotor side of the brushless inductive drive motor that I was using to spin up the platform.
posted by flabdablet at 10:56 PM on February 18, 2013 [1 favorite]

I'm not sure how cumbersome it would be but if your device is rotating inside a magnetic field you could generate your own power on-board by induction.
posted by tracer at 8:28 AM on February 19, 2013

For quick, cheap and easy, you can't beat slip rings. Depending on what you need to transfer, you can get slip rings that will handle audio, video, even gigahertz signals with fidelity. Or you can use less expensive slip rings and digitize your analog data before transfer. You can reduce the number of rings you need by packetizing your data onto one signal line.
posted by JackFlash at 9:49 AM on February 19, 2013

Response by poster: I should be able to find out how noisy the slip ring is by comparing the results from it to the data logger result. If it turns out to be an issue I'll look into WiFi data logging.

I was mostly trying to find a way to get my data into Labview since I'm comfortable working with it and having physical wires is necessary for the terminal board.
posted by gzimmer at 7:58 AM on February 20, 2013

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