Optical "Slip ring"

I can't quite visualize your problem, but a 62u fiber-optic cable can have a reasonable gap and still transmit most of the light. Especially if both fibers, the fixed and rotating ones, are in a ceramic ferrule. That would of course have to be along the axis of rotation; or maybe not.

There are even bigger glass fibers, and positively huge plastic ones.

Or do you need to put the ring source on the outside of a big shaft, firing out radially? Like a shaft torque pickup or something?

John

You could use a diode laser shining along the axis of an axicon, which is a cone-shaped element (transmissive or reflective).

Cheers,

Phil Hobbs

Don't have access to the center, unfortunately.

Consider two big dinner plates an inch or so apart, one fixed and one rotating. Consider only the space between radius 6" and radius 8" (say there are thin opaque dividers at those positions). Is there some clever optical way of coupling one or more LEDs or laser(s) attached to the top rotating plate to a single fixed pickup attached to the bottom plate? No physical contact whatsoever is permissible and it must be optical, not magnetic, RF, etc.

Obviously I could put 100 LEDs or 200 LEDs around the circumference and diffuse it with a plastic ring (like a circular fluorescent tube), and a fixed pickup would see fairly constant light intensity (unlike a circular fluorescent which has a joint), but it would probably use too much power. High light intensity is useful to keep the speed up, of course.

Circular mirror? Am I missing anything obvious here?

Best regards, Spehro Pefhany

A leaky optical pipe would do it -- something like a plexiglass ring, with a glowing edge. I'm not sure how you'd couple into it so that you'd get reasonably even intensity, nor if you could control the amount of light that leaked out of the edge, but it may be a starting point.

If you get it working, let me know -- I have a need for something like this myself, just not bad enough to do the work!!!

Are you transmitting data, or FM, or something that can tolerate some amplitude wobble?

Too bad about the center not being accessable.

If the "single fixed pickup" can actually be several receivers spread over an inch or two, you wouldn't need so many transmitters on the rotating thingie. Save power by only driving only the led's that are in the ballpark of the receivers, one of several ways.

Umm, why not magnetic or RF?

John

I don't know how they are doing it but Moog advertises a Fiber Optic Rotraty Joint:

Spehro Pefhany Inscribed thus:

How are you going to get a power source onto the rotating part ?

You could use a ring of recievers and one transmitter to solve the power issue, or compromise and use several transmitters and one more (or less) recievers to minimize the worst case instantaneous misalignment between the closest reciever and transmitter pair. For example, with ~7" radius, 44" circ, 10 LEDs @ 4.4" spacing and 11 PIN diodes evenly spaced gives perfect alignment every .44" (think vernier) or a worst case misalignment of .22".

In 3-D, a shiny sphere has the nice property of scattering a plane wave equally into 4 pi steradians--as you go away from the polar axis, the gain in circumference cancels the loss due to obliquity. Unfortunately that isn't true of a cylinder in 2-D, so just shining a wide-angle beam at a silvered ring around the axis won't quite cut it.

So in general it's a hard problem, especially since there are probably things you haven't told us, e.g. that there's machine oil and dust flying around everywhere. I'd move the goal posts if I were you.

Cheers,

Phil Hobbs

Oh, sure, within reason (not 10:1)-- we hope to avoid AGC type circuitry.

Yes.

Yes, that's one possibility that I'm considering, but the angular position is not known everywhere it has to be for that to be straightforward.

Ruled out, at least for the present, due to possible interactions with other stuff. As well as "not contact" even tiny forces must be avoided with the possible exception of small precisely radial forces.

Best regards, Spehro Pefhany

If you really have do to this, the usual approach is to use a fibre bundle--you illuminate all the fibres at one end, and fan out the other ends around the circle. Then use one or a few receivers (diversity reception is always comforting).

If power is a bigger issue than complexity, you can probably use serveral transmitters and turn off the ones you aren't using at any given time.

Cheers,

Phil Hobbs

The Journey is the reward"

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Consider a spinning transmitter (LED/Laser) aiming obliquely into a fixed waveguide made of glass or plastic rod bent into a circle, with a detector at one end. Once the light gets into the rod it's trapped until it reaches the detector. Overlap the rod little at the joint/ detector to mitigate the discontinuity. Adjust the beam width for continuous coverage.

-- Joe

=A0"The Journey is the reward"

/

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One more point: you will need to roughen the entry surface to disperse some of light along the waveguide's axis.

-- Joe

IIRC that one doesn't have a hole in the middle, but it's worth a look.

Looks on-axis to me.

...Jim Thompson

The Journey is the reward"

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Since there is no direct machanical connection across the slip ring, it is possible to do a single transmitter (or reciver) fixed in orientation as the slip ring rotates around it, but which spins about its own central axis to accomodate the roatation from the equipment rotaing with the ring.

Can't see how it is possible to do 2, though, without winding them up together.

Joe already mentioned a similar setup but I'd go a step further and try something low-tech. Get a piece of "milky" Perspex or whatever, cut a disk and mount it on the shaft. Shine the LED or laser diode onto it and see if the dispersed light is going to be enough. If you modulate the source you should be able to achieve a nice ambient rejection as well even if the optical energy coming out the sides of the disc isn't stellar. Bidirectional with two modulation frequencies might also work.

Unfortunately if the beam can make it in, it'll make it out again on the next bounce. By the time it makes it all the way round, it'll have decayed to zilch. You could do this if you can drip a little water over the tube at the (hermetically sealed) detector. That way the light would bounce around by total internal reflection until it got to the detector, then a big chunk of it would couple out via the water and be absorbed in the detector. The OP would have to manage the multiple paths, though. (This isn't horrible--ordinary flat black Krylon spray paint makes an amazing index match to acrylic or fused quartz, so you can get rid of the light on one bounce.)

Cheers,

Phil Hobbs

It will certainly is lossy, but I just tried it and it works. I took a piece of clear plastic rod, 3mm dia. x 50 cm long, and shined a small flashlite at the side near one end while viewing the other end directly. A perpendicular beam gives nothing as we would expect. At a grazing angle of 10 to 20 degrees there is a definite light visible at the other end. If I sand the input end to make it frosty I can get a similar effect at around 45 degrees.

-- Joe