We used a "home made" slip ring system for transferring data from hydro 
turbine and other slow speed shafts.

The last project we did this on was interesting.  The turbine turned at 
87 RPM, but the only place we could install a slip ring was a coupling 
that was 7.5 FEET in diameter.  The slip ring was just a shade under 24 
feet in circumference.

To make the slip ring, we used standard copper-clad PCB material and 
mechanically removed copper to leave 4 tracks of about 1/2" width with 
about 1/2" space between them.  Carbon brushes rode on each track.  We 
transmitted power (low voltage DC) and a conditioned, multiplexed 
signal.  We were measuring temperature, so we scanned each sensor for a 
few seconds, with one reading set to zero to determine the starting 
point of the sequence.  Worked really well despite our nightmares of the 
slip ring assembly flying apart.

Jon
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