Camshaft/crankshaft sense

[John Dammeyer]

At 07:47 PM 22/08/1996 -0700, you wrote:
>In the designs I've read for an electronic ignition system, there is
>usually a missing notch in the sense wheel to sense one full rotation of
>the sense wheel.  My question is, to do the initial calibration for the
>electronic ignition controller, does the controller take (up to) one
>rotation to make sure that its assumptions about TDC are correct before
>starting to fire the plugs?
Actually it should take only a couple of teeth if the tooth and the gap are
symetrical.  try it this way:

Wait for opposite polarity of tooth sensor.  ie: look for edge. and then
start a counter accumulating some frequency until opposite edge occurs.

Save counter and start again till first edge happens again.  

The two counted values should be roughly equivilent.  If the one interval is
three times the other - two gaps and a missing tooth - you've already found TDC.

Otherwise treat this like a UART looks at bits in the bit window.  Use half
the previous interval time to count from an edge into the middle of the next
tooth time and you should find another tooth. (Logic high?) otherwise you've
found TDC 

if ( ToothSensor != TRUE) {
    TDC = TRUE; 
    }
else {
    TDC = FALSE;
    // keep counting till edge and use half of that count to check for
presence of the next tooth
};


>
>A second question is, does anyone sense crankshaft and camshaft positioning
>separately?  This sounds silly, but it would let one sense timing
>belt/chain failure almost instantly, so one could inhibit plug firing which
>might save some bent valves.  In theory, you could sense belt/chain wear,
>but I don't think you'd want to spend the money on that level of precision.

Engine would do at least enough revs through momentum of flywheel or manual
transmission coupling through clutch that bent valves are a fact of life.


John



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