Sensor Question

[Bruce]

While possible, it's not a common occurance on many cars.  I can't recall
having ever seen a tooth break off.   Driving a car with a fubar'd starter
can ruin some teeth, but again it's not a common occurance.
  I've seen many times more reluctor coils in a distributor break then
flywheels fail.
Some times you just have to play the odds.
Bruce



From: "Andrew Theurer" <habanero at us.ibm.com>
Subject: Re: Sensor Question
> Thanks for sticking to the original thread :)
> The ring gear was brought up in efi332 a while back, and was discarded for
a
> number of reasons including the concerns you wrote about below.  One other
> valid concern: It's quite possible that a tooth will break off, making a
> difficult situtation much worse :(
> -Andrew

> > 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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