Optical shaft encoder question

Jitter with single-channel encoders is pretty common. The jitter occurs
because even with a Schmitt  trigger there can be noise from the
photocoupling.

You didn't actually mention that it's single-channel but the description
sounds like it. If that's the case, how about adding another transistor
and LED, and buy either the LS7083 or LS7084 chip from USDigital.com
(http://www.usdigital.com/products/ls7083-ls7084/ ). They're about $3
each, and have circuitry in them to clean up the signal. And since
you're now working with a quadrature encoder, you'll have 6X4 (or 24)
transistions, which is a little better resolution.

-- Gordon


Mark Haase wrote:

You don't say how you are processing the signals. The problem, as you have
spotted, is that transitions may not be 'clean' - consider what happens if
you stop moving exactly on a transition point, and then random noise will
cause a train of signals.

IMHO the very best way to deal with this is to build a state machine for the
edge transitions in hardware - then these unwanted transitions merely result
in dithering between two states. If you attempt this in software, then your
'reaction' time (worst case interrupt response or sampling loop period) must
be less than the smallest possible glitch, such that you respond to all
transitions. This can be a severe constraint on robots with a simple single
'brain'. You can slow the response of the phototransistor, but if you slug
it too much then you will lose counts at speed.

These days the simplest way of doing this sort of thing is to use a
dedicated 8 pin micro (avr, pic - $2) which runs the state machine and
maintains a fairly long counter for position. With an internal clock of a
few MHz it will cope with any glitch. This approach removes a time critical
operation from your main 'brain', which can simply 'ask' for the current
'position' when it needs it.

Have fun

Dave

The Schmitt trigger on the hex buffer sounds like it's doing the
conditioning, which it should be able to haver a fair crack at
provided the noise from the sensor is less than the hysteresis. The
hysteresis is there to deal with exactly the situation you describe -
stopping exactly at the transition point.

To the OP - how much hysteresis are you using on the Schmitt triggers?
If it's not enough and the IC doesn't let you tweak it, you could make
a Schmitt trigger from an op-amp and tweak the resistors to get the
desired result. This is all /much/ easier with an oscilloscope to see
what's going on.


Tim
--
You are being watched. This gives you power.