Sensor Question

[Bob - Uni]

Hello Andrew,

Thanks for that guys, that was a top rundown for me.

Re Carters point about the frequency of input signals interfering more
than nescesary: I had considered this, and I was wondering how I would
set up all the interupt priorities etc etc. Obviously the timing of
the injectors and igntion are of crucial importance, but then again if
you miss reading a MAP sensor just as the throttle is undergoing
sudden change under big boost (or similar kind of extreme condition)
then you could also potentially be in for a bit of detination, or even
just irritating hesitation. Then I got to thinking about a modular
designed EFI computer, where there is a dedicated micro for input
signals, conditioning etc, a micro for the injecting, a micro for the
ignition, a micro for user interfacing etc etc, all working off a bus
using an agreed protocol. It seems like overkill, but you could use
markedly cheaper micros, simplifies problems like input interrupts
interfering with output interrupts etc, and also would allow one to
build an engine using a normal distributor and fuel injection, and
then later down the track as money, time, whatever was no longer an
issue, be able to just plug in another daughterboard for ignition, fit
the required external components, and save the hassle of buying
(building) a full new computer. Thoughts? Ideas?

Tuesday, December 11, 2001, 8:42:03 AM, you wrote:

AT> Carter,

AT> Thanks for sticking to the original thread :)

AT> The ring gear was brought up in efi332 a while back, and was discarded for a
AT> number of reasons including the concerns you wrote about below.  One other
AT> valid concern: It's quite possible that a tooth will break off, making a
AT> difficult situtation much worse :(

AT> -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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-- 
Best regards,
 Bob                            mailto:rcs07 at uow.edu.au

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