Re: Proximity sensor for RSG

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

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

I think your real problem is going to be finding a sensor that is EMI 
immune. The magnetic sensors (hall effect) are fairly low level signals (at 
least internally).
I'd think some sort of optical pickup is your best bet.  If you've got a 
bit of optics (i.e. some small convex lenses) you could make something that 
can be a bit of distance from the rotor, and furthermore, which can keep 
all the electronics in a shielded box.   A laser diode makes a nice bright 
source that's visible, for alignment. A red filter over the detector helps 
improve the SNR, and you can just use any old photodiode or transistor as a 
detector, and capacitively couple the output.  To restrict the field of 
view, you can either use a lens, or just put the detector at the end of a 
tube that's painted black on the inside, so it can't "see" anything except 
the target spot.

If you really want to get the SNR up, make your targets out of that 
retroreflective tape (ScotchBrite reflector...)

At 11:32 AM 3/20/2003 -0700, you wrote:
>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