Automotive (including motorbikes) a bad environment electrically, physicall=
y
and environment.  A starting point might be to do everything that you have
ever learned about noise reduction.

Cranking currents can create current and voltage spikes. Voltage spikes
close to the battery voltage plus the inductive kick of the starter and cur=
rent
spikes that can reach perhaps 100amps in a motor bike and 2-300 amps
in an automobile. The current spikes can introduce considerable current in
close completely separate circuits.

Ignition noise has lots of high frequency components.

I would make sure that everything on the odometer was bypassed and
probably would use a small cap across the reed switch to effectively
lower the signal impedance.

Shielding the PIC24 board will help quite a bit. The noise can be induced
on the circuit board. Metal box with good contact between parts and
solidly grounded on the bike.

Walter Banks
--
Byte Craft Limited
 http://www.bytecraft.com





Electron wrote:

> It's a motorbike, really, not a car.
>
> I'm exploring noise and EMI issues, here's a very simple example, I hope =
You
> can help me understand how to make the system more robust.
>
> So, let's talk about a specific example: the motorbike has a reed-like se=
nsor
> on the front wheel. The brake disc contains a small magnet. When the whee=
l
> make a full revolution, the magnet passes by the reed, and this is shorte=
d.
>
> Measuring the digital odometer showed a very small current (~100uA IIRC),
> and also because I have important power consumption constraints, I decide=
d
> (as I want to measure those wheel revolutions) to use a pull-up resistor
> value of 100 kohm on T1CK input of a PIC24. T1CK is a schmitt trigger inp=
ut.
> The reed will short it at each wheel revolution, and the Timer1 counter w=
ill
> increment.
>
> It works.. however, when the engine is running I get spurious additional
> counts. Even when the electric starter is cranking (but I keep the engine
> kill to ground, so the engine won't start).
>
> Looks like EMI, but I can't shield the cable, as it has to remain the ori=
ginal
> cable.
>
> I wonder, how does the original digital odometer cope with the noise? Tru=
e
> I could lower the pull up resistance and probably solve the problem this =
way
> (as long as current draw doesn't become excessive, but as it is normally
> open, and the pulse is short, it's unlikely to be a problem) but it puzzl=
es
> me that measuring the original digital odometer current I get such a low
> value. Is it really that low, or may I suspect something clever is in pla=
ce?
> (e.g. it does draw larger currents but only for a very short time?).
>
> More generally, what are Your thoughts on this matter? How would you inte=
rface
> the reed with T1CK in a noisy environment?
>
> I'm thinking about all..:
>
> 1) one single ground to my electronic circuit (to avoid ground loops) - t=
he
> reed is grounded somewhere and I get the "hot" wire.
>
> 2) shield the circuit if necessary (but this doesn't seem to be the probl=
em,
> as noise seems to come from the bike's odometer wire, which I cannot shie=
ld).
>
> 3) bigger bypass? Ferrite beads? Would they help for the EMI? Sure filter=
ing
> the power wouldn't help, as I get interference even using a stand alone b=
attery.
>
> 4) bypass/bead on the sensor wire then? It should improve things but do I
> still get a nice pulse then? T1CK is a schmitt trigger, that should help =
at
> least with the slower rising edge.
>
> Anyhow, if You experienced automotive or more in general EMI-issues peopl=
e
> can give me some hints and insights, I'd appreciate it a lot.
>
> With kind regards,
> Mario
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist

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