OBD-II toothed wheels for crankshaft position sensing

[Lamari, Matthew]

I participated in the construction of an ECU that fully controlled the 
ignition (didn't get to fuel) of a car.  We drove the thing around and all 
was cool.  It comprised an advance table.  The crank sensor wheel had 2 
sensors on it, that is, an event was generated every half rev.

When to actually spark was done relative to this.  Knowing in time base the 
advance to use in a situation corresponded to delays from this point, which 
use of interrupts could catch in time (the thing ran as well as with its 
archaic stock ignition.)

We were also going to implement a backup mode whereby the system could run 
with only one functional.  But what we had was effectively a 2 tooth wheel. 
 Position in the cycle can be guessed from the time since the last signal 
and the wheel's effective speed, or time to reach a position predicted.  Of 
course, acceleration and deceleration make the previously measured speed 
inaccurate; but the wheel is spinning so fast and accelerating so 
comparitively slowly that the difference from one sample to the next isn't 
that great.

Okay, we had output compare functionality on the microprocessor to spark 
right on time.

I don't know where my colleague got his hands on this sensor (magnetic) but 
are they available?

And what is the benefit of the many (50+) toothed sensor?  Is it durability 
without maintenance, simplification of hardware, or is there some benefit 
which I am missing?

Thanks.
Matthew Lamari,
mlamari at origin.ea.com