Andrew Kunz wrote:
>
> Hi All,
>
> Been a while since I've been around, but I need help again.
>
> I need to gain a little understanding on Hall sensors, I think.
>
> I need to "see" a steel setscrew (grubscrew) in a fiber wheel as the wheel
> rotates at varying RPM from 1000 RPM to 60000 RPM.  I'm only interested in
> determining instantaneous RPM, not in position data.
>
> Does anybody know of products which work in this situation?


Hi Andrew, yep the allegro sensors Jinx posted
about will do the high speed stuff, these are
incorporated in high speed motors and sense the
multi poles so I know 1kHz will be ok. The ones in
some motorcycle speedo sensors work at 4kHz+ ok.

A trick with sensing a tiny metal grubscrew
is to use an external magnetic field, like a
small (stationary!) strong magnet next to your
hall sensor, and as the screw goes past it affects
the flux near the sensor. You might even have to
get some steel "poles" and make a small airgap,
but with fiddling i'm sure you could get it working
ok.

So the big question is, why not use optical? Metal
grit will build up on the hall magnet, could be
nasty. But optical has it's own problems. Optical
will never have a mechanical effect or load the
shaft though...
-Roman

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