At 12:29 PM 2/8/2012, Bob Blick wrote:
>I need to design a device that interfaces to the speed sensor of heavy
>trucks. These are variable reluctance sensors and will remain connected
>to the vehicle's ECU, my circuit will piggyback. These are two-wire,
>differential signals full of common-mode noise and vary in amplitude
>greatly with frequency and also position as the tailshaft bearing wears
>in the vehicle.

I have designed a circuit for this kind of thing, used commerically so I ca=
n't share it,=20
but can give some tips.

At the final step it's just a comparator with hysteresis to give square wav=
e output for=20
AC input. You need to filter the signal. You want it AC coupled so it handl=
es +ve and=20
-ve swings and cases there is a DC offset, but consider the lowest frequenc=
y=20
(speed) you require it to work at. Sensitivity/gain must be appropriate for=
 lowest=20
required speed. Too much gain and/or not enough hysteresis and you'll pick =
up=20
noise.

You need to limit the voltage at some point so it doesn't damage your circu=
it, but not=20
load the reluctance sensor enough so that your vehicle's ECU thinks there i=
s a=20
problem (and fails to operate). For this reason alone you want to stay away=
 from=20
using the ABS signal so as not to compromise safety of the vehicle.

I used a single ended circuit design. Not all sensors are differential (man=
y are just 1=20
wire to circuit, other side of sensor is GND), even with a differential sen=
sor when=20
connected to the ECU it may become GND refferenced in which case AC coupled=
=20
single ended can work fine. If not, refference one side to GND with a resis=
tor, but=20
check carefully this doesn't affect ECU. Can't really generalise as there a=
re many=20
variations on sensor topology.

Some cases where it is difficult or impractical to gain access to an existi=
ng speed=20
sensor, a good solution is to add one - use a commercial off the shelf indu=
ctive=20
proximity sensor, 3 wire (power, GND, square wave signal out). Pick up spee=
d pulse=20
from a driveshaft bolt or similar, or add a toothed wheel.


--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.