Flywheel/Encoder patterns

[RABBITT_Andrew at mv8.orbeng.com.au]

>can you explain your thinking on the 60 tooth wheel.  I know 
>Electromotive uses both the 60 tooth design and the 120 tooth design.

We're looking at a 60 tooth wheel (over a 36 tooth) to improve our 
timing accuracy when we use missing tooth reference markers.  

We use the industry standard practice of measuring time interval 
between edges and using this time to extrapolate the position over the 
next interval.  Usually clocks with a 250ns to 1us time resolution are 
used to drive the counters.

There are a couple of other issues driving this too, which are 
specific to our design and not applicable to MPI systems.

Andrew Rabbitt