Date: Mon, 8 Apr 2013 18:54:08 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: What's a Hall sender do?
In-Reply-To: <24E93AC6-5CC6-49F0-BE1F-E2E4B189D7C5@gmail.com>
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At 03:41 PM 4/8/2013, James Felder wrote:
>What's the difference between the Hall Effect sensor in a computer
>keyboard and the one in a Vanagon distributor?
I don't think keyboards typically use Hall-effect devices as that
would be a very expensive way to do it, but in principle, no difference.
In the distributor the Hall-effect sensor and a magnet are mounted on
a common base with a small gap between them. The distributor shaft
carries a steel wheel like a jar lid with equally spaced tabs and
openings in the vertical portion, one tab and opening per cylinder
(so four each for us). These pass through the gap in the Hall
generator assembly. When the gap is open, the sensor "sees" the
magnet and generates a tiny voltage which closes a transistor switch
path to ground. When the gap is closed by a tab, the sensor no
longer generates a voltage and the switch opens. Because the spacing
is equal, the result is a square wave. On the Digijet system the
distributor has centrifugal and vacuum advance/retard mechanisms, and
the Hall generator output goes first to the idle stabilizer module
which advances timing if the idle drops too low. Otherwise it passes
the pulse straight through to the ignition module on the firewall
which generates the ignition primary pulse that drives the coil. The
distributor output also feeds rpm and rough timing info to the ECU so
it can compute and time the injection pulses.
On the Digifant system the distributor simply reports rpm and TDC to
the ECU. From these and other inputs the ECU computes ignition
advance or retard and outputs a primary ignition impulse at the
correct time, as well as controlling the injection timing and
quantity. Also on the Digifant system idle stabilization is
accomplished by a much fancier system that operates a tiny auxiliary
throttle (the Idle Stabilizer Valve) instead of fiddling with
timing. Because this system depends heavily on the throttle position
switch, the Digifant engines will not idle down properly if the
switch does not close.
A Hall-effect keyboard would probably have tiny magnets mounted on
the keys, with the sensors and amplifier circuitry beneath them, one
set per key. It could be used to make a keyboard where the
electronics were totally sealed up, but it would be sensitive to
nearby magnets (and expensive).
Yours,
David
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