Hi-res crank sensor.

[einarp]

It seems I was not expressing myself very exact when I
presented this:

  *     x
  _   _   _   _   _   _
_/ \_/ \_/ \_/ \_/ \_/ \_


I should also have mentioned that I assumed the sensor signals
would be run through a zero crossing detector first. Like for
example the LM1815. A short pulse would come out of it starting
at the zero crossing of each sensor. That will again coincide
with the point where it is "on the top of" the tooth, like "*"
in the above drawing.

The pulses out of "X" will come inbetween the pulses from "*".
Set the pulse time short enough that the pulses from the two
sensors will not be active at the same time even at the highest RPM.

It should look something like this:

__/\  _____/\  ______/\  _______/\  ____    Sensor "*" output
    \/       \/        \/         \/    
______/\  _______/\  ______/\  ______/\  __ Sensor "x" output
        \/         \/        \/        \/    

___II________II________II________II_____  Zero Cross "*"
________II________II________II________II_ Zero Cross "x"
___II___II___II___II___II___II___II___    OR of the above


My point with this is that each of the two sensors are detected
at a point of their waveform that is not dependent on the shape
of the tooth, or the distance sensor/tooth. It's only depending
on one sensor being above the middle of the tooth at the same time
the other is midway between two teeth.

If we had a nice sinewave shaped signal out of the sensor, it 
would have been much simpler. We could just detect positive
and negative peaks of one sensor. But look at the sensor signal
and you will see it looks more like my ASCII drawing than a
sinewave at low RPM. It will approach a sinewave as speed
increases and the "_" part (of the sensor signal) shrinks.

I think some of the the previous postings assume a sinewave
signal.

For the DIY'er with lots of time on hand: Log the time interval
between zero crossings related to each cylinder during cranking,
and you can tell if one cylinder is low on compression relative
to the others. It should be possible assuming you have 300 pulses
per revolution as you would get from the above picking pulses off
of a 150 tooth flywheel ring.



--
einarp at ade.no  ( Maserati Biturbo Spyder )