Proximity sensor for RSG

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Marco.Denicolai-at-tellabs-dot-com>

Hello all,

I would like to add to Thor's rotating spark gap a position sensor to
know when the rotating electrodes face the fixed ones. I don't need to
know the absolute position (degrees), just an digital signal is enough.
I'll use that to synchronize the primary capacitor charge and to count
the bang amount.

The RSG features a 30 cm diameter rotor, with 12 rotating electrodes (2%
thorium doped tungsten, 0.5" in diameter). The motor is a 2850 rpm,
achieving a maximum bang-rate of 570 breaks/s. I calculated:

- min. face time = 1.27/(PI*30*2850/60) = 0.28 ms
- min. period = 1/570 = 1.75 ms

I was thinking to detect the rotating electrode proximity or painted
black spots on the rotor side. I don't want to drill holes or whatever
to the rotor.

I need help in choosing the right proximity sensor: capacitive,
inductive or magnetic? What will work? Anybody with experience on this?

Regards