Re: Tachometer Pickup

[Richard Bebbington <richard@rbebb.demon.co.uk>]

Hello everyone

On 31-Mar-99, jklein wrote:
> Actually most of the sensors that I have seen are hall effect
> sensors. The sensor picks up the magnetic field of the magnet and
> creates a pulse. This pulse is then sent to the processor and is
> counted as a single revolution. My Corvair has an aftermareket
> sensor with three wires. MY cruise control has a sensor with two
> wires. My bicycle has a 2 wire sensor. The three wire system is
> "excited" by low voltage, then the passing feild is the lever that
> closes the switch, and sends a pulse to the processor. A three wire
> system is more stable around other electrical fields (less
> electrical noise in the circuit), but for a Tach a two wire system
> is fine.
>     WARNING : A three wire system MAY NOT OPERATE ON 12 VOLTS. If
> the connector is smaller then the standard 1/4 inch SAE blade, it is
> possible that this is a low voltage lead (4,6, or 8 volts). You can
> blow out your sensor AND tach if you put a full 12 volts through the
> sensor. If the connectors are smaller then the standard blade
> terminal (usually round, but can be 1/8 blade type) check the donor
> system to see what voltage it needs to operate.
>   A properly prepared surface AND a good epoxy will hold the magnet
> (make sure you get the epoxy that will not be harmed by water and
> temperature).

Some of these sensors also work by inductance - the flywheel sensor
on my Mini has a single metal spike, with a coil of some sort mounted
on the back of it. I think it may have a magnet as well, since bits
of metal will stick to the end.
The flywheel has a deep groove in it, with a ring carrying about 20
teeth
sitting in it. The teeth sweep past the spike as the flywheel rotates.
The signals generated must be small or sensitive, since the cable
going
to the ECU is shielded.

We've used an industrial inductive proximity sensor as an engine rpm
sensor on a diesel powered hydraulic vehicle we built in work. Two of
the
bolts that held the hydraulic pump's drive adaptor onto the flywheel
are
extra long, and they stick up. The bolt heads sweep past the sensor,
which
makes the required pulses. Works up to 4 k rpm at least.....but you
need to choose a sensor with a fast enough response time. Ours even
had a built-in LED and a socket for the cable! 

Regards
-- 
Richard Bebbington ( Captain Cleaver )