Sensor Question

[Carter Shore]

One problem with using a distributor to generate
timing is the jitter due to mechanical backlash in the
drive. Many use some sort of helical gear arrangement
off of the camshaft, often in common with the oil pump
drive. Any wear in the system will result in timing
innacuracies. To compensate, you would have to run a
more conservative advance map.

Using the starter ring gearteeth has some issues. The
environment is not friendly; clutch dust, road dirt,
water, bits of metal from the gearteeth, oil leaks
(engine and transmission), heat, vibration, etc. The
frequency of a 150 tooth gear at 9,000 RPM is 2250 Hz.
So pulsewidth is around 440 microseconds. Since the
ECU is already pretty busy, it may not appreciate
handling an interrupt so often. That means some kind
of frequency divider circuitry (and intermediary
signal conditioning if you use a mag pickup). Well
within the state of the art, just more bother. 
As for pickups, you have 3 main choices, opto,
inductive, or hall.
Both opto detectors and hall sensors must be able to
handle the frequency (not all can). The opto emitter
and detector, and the hall sensor are semiconductors,
so you must specify parts capable withstanding the
environment. An inductive sensor can handle the
frequency, but the air gap may be critcal and require
fine adjustment. The sensing edge of the ring gear may
also require machining to insure that the runout is
small enough to yield reliable operation.

With an opto sensor, whether you choose a reflective
or transmissive scheme, the obvious issue is
obscurement of the beam by debris.

An inductive or hall pickup can be affected by a
buildup of ferrous dust or particles, since the sensor
must be biased with a magnet to detect the teeth.

Detection of TDC (or equivalent) can be accomplished
by altering one tooth somehow, to give a detectably
different waveform.

If I was forced to make a choice, I'd probably go with
inductive. The detector itself is more robust. To
combat crud buidup, I would probably fashion a 'wiper'
that would sweep the pickup each revolution. A small
piece of teflon or whatnot on the tip of one geartooth
(maybe 0.02-0.05" thick). Maybe spring load it so the
root of the starter pinion gear could compress it
during starts.

Lucky for me, I'm not faced with such a choice.

Carter Shore

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