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FYI, a year ago I asked here about using slip rings and rotary
transformers - specifically video head drum assemblies - to power a
rotating project. I've returned to this again after a year of
essentially-continuous project rotation.

The summary is pretty simple:
- Slip rings are a poor choice unless the project can support sealed
mercury-wetted contacts - which are expensive; otherwise, physical
wear was high and electrical noise was eventually unforgiving.

- The rotary transformers in video head drums cannot convert
sufficient energy to make them useful for significant project power,
but read on.

Using paralleled high-drive 74AC14s in push-pull as an H-Bridge to
switch (at 24MHz!) two paralleled fixed coils, and stacking the
corresponding rotating secondary coils then rectifying and smoothing,
the best I was able to achieve was 5vDC @10mA, unregulated. That
might be sufficient for some low-current functions like analog
preamplification or some processing on a small PIC board, but it is
not enough to run a Basic-X or similar processor with any significant
peripherals.

However, I learned good news. Getting digital data on and off
a rotating platform via the coils is very easy, and it is very fast -
much faster than any radio solution.

Driving one side requires only a fast buffer or inverter (I'm using
74AC14s but a 74HC14 is fine) through 200 ohms to a transformer coil
to 0v. On the receiving side, the hysteresis of a 74HC14
Schmitt-trigger inverter reconstructs the logic levels of the driven
side after adding the transformer output to a mid-logic bias from a
pair of series 10k resistors from Vcc to 0v. The transformer
secondary is connected between Vcc/2 and the Schmitt input; if the
resulting sense is wrong, reverse the transformer secondary coil.
That's all there is to it; depending on the video drum assembly,
you'll have two, four, six or eight bidirectional channels - probably
in pairs because each pair might share a common on one side.

I was able to move TTL digital data (I tried 19200 TTL serial, a 10MHz
modulator stream and a 30MHz clock simultaneously) through this link
with only a ~30nS delay. Few things are as simple and work so well.

Now then, power remains a problem...
Tom



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I said:
> ... the best I was able to achieve was 5vDC @10mA, unregulated.

Eating my words, I am now able to get _regulated_ 5v @50mA using a
pair of video head drum coils at ~2MHz, precluding using other coil
pairs for data alone since the data edges are buried by the adjacent
power flux. I now believe that I'll achieve my target of 100mA using
two paralleled pairs of coils. My challenge is now reversed; earlier
I could pass data but not power, and now I can pass power but not
simultaneous data - at least as I was attempting it.

I now see the ~2MHz power signal as a carrier for a Manchester-coded
self-clocked data stream. Encoding a Manchester stream is trivial;
decoding is more difficult, typically involving a PLL or monostables.
Has anyone encountered a simple Manchester data separator?
Tom



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FWIW, a follow up:

I am now providing ~120mA of regulated 5vDC to a rotating platform
through four video-head-drum coil pairs (similar:
http://en.wikipedia.org/wiki/Rotary_transformer) while simultaneously
carrying a flawless 750kHz serial data channel to the platform.

I've exceeded my goal but the coils are still cold - so more power and
faster data, is possible.
Tom



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That's really neat. I would have never thought of that.
Keep us informed.

Tom Becker wrote:
FWIW, a follow up:

I am now providing ~120mA of regulated 5vDC to a rotating platform
through four video-head-drum coil pairs (similar:
http://en.wikipedia.org/wiki/Rotary_transformer) while simultaneously
carrying a flawless 750kHz serial data channel to the platform.

I've exceeded my goal but the coils are still cold - so more power and
faster data, is possible.

Tom

---------------------------------
Ahhh...imagining that irresistible "new car" smell?
Check outnew cars at Yahoo! Autos.

[Non-text portions of this message have been removed]


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Would it be possible to provide further details? Info such as the model of
VCR that was butchered, rotation rate of the platform, circuits, etc would
be great.

> Tom Becker wrote:
> FWIW, a follow up:
>
> I am now providing ~120mA of regulated 5vDC to a rotating platform
> through four video-head-drum coil pairs (similar:
> http://en.wikipedia.org/wiki/Rotary_transformer) while simultaneously
> carrying a flawless 750kHz serial data channel to the platform.
>
> I've exceeded my goal but the coils are still cold - so more power and
> faster data, is possible.
>
> Tom
> ---------------------------------
> Ahhh...imagining that irresistible "new car" smell?
> Check outnew cars at Yahoo! Autos.
>
> [Non-text portions of this message have been removed]
> Yahoo! Groups Links
>


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> ... further details?

I am able to transfer useful power through the coils by switching ~450mA
through the primaries at 1.5MHz with H-bridges made of paralleled
74AC14s (three inverters per leg of two phases), at 6vDC. I'll probably
try complementary MOSFET switches at 5v in a future version, but the
AC14s at 6v can supply sufficient fast current for the moment; no
motor-specific H-bridge I could find is fast enough.

The drum has four sets of coils, all identical 2:1 _stepdown_
transformers (six-turn fixed primaries and three-turn rotating
secondaries) in VCR service; mounted upside-down, it becomes a step-up
transformer. The six-turn (secondary, as I use them) coils share a
common (which I do not use) so, with two sets of paralleled primaries,
the four coil pairs become two independent phases.

I use a 12MHz oscillator and a 22V10 GAL to implement fast logic that
provides push-pull quadrature to drive the AC14s, then full-wave rectify
each phase with 1N914s (@3MHz) on the secondary and feed a small common
smoothing cap before regulating with the rotating BX-24p's 2951. I
implemented the fast data channel by toggling the primary quadrature
phase in the GAL, decoding the secondary with a pair of fast comparators
and two flops. While this might sound complex, it uses only four ICs on
the primary and three on the secondary. I get data off the platform via
Bluetooth.

The drum I'm using came from a Samsung VCR, but I've found many suitable
units for $5 each at a Goodwill store (charity clothing and home goods
in the US), where VCRs are moderately plentiful; Saturday-morning garage
sales should be fruitful, too. Some drums are more suitable than
others; some have no exposed shaft that you can drive, some have more
coils, some fewer, some mount squarely, some have the helical scan angle
cast into the mount - and don't mount squarely. You'll probably need to
rip a few machines apart.

If there is further interest, I'll post a schematic and photos.
Tom


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> I said:
> ... the best I was able to achieve was 5vDC @10mA, unregulated.

FYI, I've come a long way with rotating platform power. Using the
same video head drum rotating transformer I started with after
abandoning slip rings, I now have 1.2Amp of regulated 5volts _and_
125mA of regulated 12volts on the platform, about seven Watts combined.

This is probably close to the practical limit since the coils run a
little warm (~40C in 25C ambient) and I've had two identical failures,
each after about a week of full load, when paths on a small circuit
board that hold connectors for the coils fused open.

I still use two pairs of coils in quadrature to also pass encoded data
on the power, although single coil pairs can use missing pulse
detection for simpler signaling like processor reset, too.
Tom



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