hall sensor simple questions

[Bernd Felsche]

Toby Atwater 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.
"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.

>Crank shaft sensors can determine the position of the piston and therefore
>give the ECU a timing reference inorder to inject fuel at the right time.

The sensors are used to sense the position of the crankshaft,
not the piston. The crankshaft detrmines the relative locations of
the pistons.

>How many sensors do you need around the crank inorder to get a resolution
>that is useful? Is this multiplied by each cylinder as well? or can the ECU
>calculate where one piston is compared to another's location?

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.

>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.

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.

-- 
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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