gearwheel encodjng

[RABBITT_Andrew at mail.orbeng.com.au]

>My problem is this, I don't know what the min. diameter of the wheel 
>should be.  At 3deg. per tooth edge, the tooth length gets quite 
>small as the diameter goes down.

The standard HE flywheel encoder pickup will sense material some mm 
before it passes the sensor axis, and also some mm after.  These 
operate and release *distances* will limit your encoder diameter.  For 
example, an 60-tooth encoder I designed at 210mm mean diameter, had a 
~1:2 physical mark-space ratio for a 1:1 electrical mark-space ratio. 
 For a normal IC engine, the frequency response is not an issue if you 
keep your electrical mark-space ratio near 1:1.  

You'll need to test your pickup to determine the operate and release 
distances unless you have the manufacturers specifications.