At 10:48 PM 8/11/98 -0400, Trevor Boicey wrote:
>....
> Hall effect devices put spinning magnets in the rotor assembly in the
distributor, and a coil-type detector nearby. ....
>
> The current induced into the coil abruptly changes polarity at the
instant the magnet passes the coil. This change is detected by a module
which fires the spark.
>....
> Then main disadvantage I can think of for Hall effect units is that the
effect isn't noticeable except when the magnets are moving. So you cannot
static time a car with a hall effect sensor. ....
This is finally getting more interresting. Even though it's going just a
little astray of the original topic, it's a good answer to a good question.
Since the question was specifically about the sensor, I'd like to
embellish on the answer a bit.
I have a rally computer that uses a hall effect sensor with a pair of
magnets attached to the hub of a road wheel. This is used to count the
turns of the wheel to accurately calculate distance. The magnet is
oriented with the + and - ends leading and trailing in the motion path, so
that as the magnet passes the sensor the magnetic polarization sensed by
the hall effect unit changes very suddenly from + to -, or vise versa.
This effect can and does work at a very slow speed indeed. I can sneak up
to a reference marker on the road at a very slow pace, like 1/2 foot per
second or less. At the wheel hub that would translate into a speed of less
than one inch per second. As the center of the magnet passes the center of
the hall effect sensor the number trips over on the computer.
It is not the speed of this magnetic transition that counts, but the travel
distance required to make the polarity change in the sensor, and that
distance is particularly small. I think (it is my impression) that if you
had a the hall effect sensor in your distributor powered up and a volt
meter attached to it, that you could see that sudden change in polarity,
and that it would happen within perhaps 1/2 degree rotaion of the
distributor as you turned it back and forth by hand. The effect would be
nearly identical to having a test light attached to the contact points, and
you should be able to static time it with the hall effect sensor. You
should also be able to get the coil to spark in the same manner.
Disclaimer: You all know I'm just a Mechanical Engineer, so I could be
just blowing wind over this here funny electrical issue. Never could get a
good grip on those silly little electrons.
$.02,
Barney Gaylord
1958 MGA with an attitude
PS
Definition of a Nuclear Physicist: If you can hit it with a hammer, they
don't understand it.
BG
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