hall sensor simple questions

[Bernd Felsche]

nacelp writes:

>> >As I understand it, a hall sensor can detect a metal mass approaching the
>> >sensor i.e. a distributor cam rotating and a hall sensor sensing the
>lobes.

>> The Hall sensor picks up a chnage in the magnetic field. It can be
>> used to sense the position of a camshaft drive gear tooth.

>?. What signifigance is a camshaft drive tooth.
>Camshaft posistion sensor, yes, the two thou are entirely different, the
>posistion sensor is to find a relative TDC, or sync for a specific cylinder.
>The only thing constant about camshaft gear teeth is that there is an even
>number.

The camshaft position is indicative of the piston "position" and the
"stroke" (i.e. if it's induction or power for example). As it
rotates at half the crank speed, you can identify for example which
injector and plug to fire with a solid-state ignition.

>> "Reluctance" sensore are also used commonly. A reference signal to
>> identify a particular tooth on the gear wheel is used so that the
>> absolute position of the camshaft can be determined. If you're using
>> an encoder wheel, the most accurate (but also more costly) method,
>> then a "missing tooth" is quite a common method of identifying one
>> particular position.

>Actually a missing notch is best.  Take a steel wheel, and cut several
>notches into it.  Those are easiest to find.  Specially at cranking speeds.

>> You *need* none at all. Basic engine timing can be determined from
>> the camshaft position, with a TDC and BDC signal required as a
>> minimum. The ECU can interpolate intermediate positions given the
>> elapsed time between TDC and BCD.

>If your implying just two distributor reference pulses good luck.  I'd
>imagine real hard starting in cold weather when the cranking speed is
>erratic.
>   I'd use at least one notch for every two cylinders if crank, and 1 per
>cylinder distributor, as mins.

There is one "mark" for every TDC in a four-cylinder engine - which
if it's a conventional layout. So you get two pulses per crankshaft
revolutions, just like in a Kettering ignition.

>> >Crank shaft sensors seem hard to implement on an old engine with no stock
>> >electronics what so ever. I was thinking about using the distributor's
>cam
>> >with a hall sensor in order to get spark timing and as well as Fuel
>> >Injection timing and thereby doing away with crank sensors completely. Is
>> >this possible? are the hall sensors accurate enough to not only detect a
>> >lobe peak but the lobe's approach and exit?

>> The common method is to use a magnet with an "interruptor" wheel.
>> Edges of the openings are treated (nominally) as the TCD position
>> for each cylinder - the Hall sensor is accurate if the distributor
>> does not have any advance mechanism.

>???, the sensor in it's operation don't care if there is an advance
>mechanism.  If there is a timing problem in the advance mechanisms, then
>timing will be erratic.  Just the same as if there were points, IR detector
>or anything else triggering off of the distributor shaft.

The sensor may not car, but your ECU is going to have a big problem
trying to determine precise times for ignition and injection. The
ECU would have to reverse-map the mechanical advance and overlay the
vacuum advance/retard as well. The variable-advance distributors
also introduce an amount of jitter if you have to rely solely on the
distributor signal for timing.

>> Such a method has in the past been employed successfully by VW's
>> Digifant, which integrates fuel injection and ignition.  More
>> accuracy is possible with camshaft and/or crankshaft sensing to
>> detect transitions in engine speed more easily and reliably; as well
>> as allowing sequential injection and other goodies.

>??. Accuracy (at least in the mechanical sense), is a function of the number
>of references, not number of sensors.  You can do equally well with crank or

I think I said that. The Digifant architecture (similar to
L-Jetronic) has only the pulses from the Hall sensor in the
distributor as a timing reference.

Digifant's "cousin", Motronic, has crank-angle (and optionally
camshaft) position sensing.

>cam, the only time you need both is in SEFI (meaning timing or fuel).  If
>you want to measure crankshaft accleration rates accurately then you need a
>crank sensor, since there is always "gearing" for the distributor, and that
>alllows for slow, and erratic signaling for that level of resolution,
>needed.  You can get by with dissy, but the resolution of input, and
>filtering gets more complex, and hence more room for error.

I think I said that. I apologise for not explaining it in sufficient
detail.


-- 
Real Name: Bernd Felsche
    Email: nospam.bernie at perth.DIALix.com.au
     http://www.perth.dialix.com.au/~bernie - Private HP
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