Encoder 101 was Re: Ecoder resolution terms

Jon,

That's what I thought. Just to be clear, we get 2 COUNTS (edge
transitions, really) from the rise and fall of the A pulse, and two
more from the rise and fall of the B pulse. Nowadays, that's 4 counts
per QUADRATURE CYCLE. Number of CYCLEs is equal to the number of
lines on the encoder disk. PULSES per REV can be EITHER the same as
the lines/cycles, OR twice as many. Depends upon whether you're
counting BOTH the A AND B channels, or just one.

For those who are wondering where the pulses come from, the lines of
the encoder disk block the passage of light between an LED and a
phototransistor. That's why they are called optical encoders. There
are two sets of LED/phototransistor, offset by half the distance
between the lines. This offset is what allows the encoder to "know"
the direction, because going one way the A channel will come
first;going the other way the B channel will come first... Since
there are 4 possible states, A hi, B hi, A lo, B lo; the term
quadrature is used.

In the old days, only one count per channel per line was used, EITHER
the change from low to high (rising edge), or high to low (falling
edge). So we only had 2 counts per CYCLE. But some smart guy (or
gal?) figured out that if we use BOTH transitions, we double
resolution using the same hardware. This was such a good idea that
practically every mfr. jumped on the band wagon... All aboard!

Ballendo

P.S. For copmpleteness, one other term you will hear is INDEX pulse.
This is a SINGLE line (with an additional LED/phototransistor pair)
which gives one PULSE per revolution-one rise and fall per rev.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> ballendo wrote:
>
> >Jon,
> >
> >I thought pulses meant the "whole" square wave. Which means either
200
> >(one channel) or 400(both channels)PPR for a 200 line encoder?
> >
> >
> A CYCLE is a definitive term, and when you look at it in quadrature,
> it would be from the rising edge on the A channel to the next
rising edge
> on the same channel. That would give 4 COUNTS.
>
> Most makers who use the horrible "Pulses/Rev" form mean the same as
> CYCLES/Rev. So, a 200 line encoder would also mean 200 CYCLES/Rev
> or 200 pulses/rev, as it will give 200 pulses on the A channel for
a
> complete
> turn. (It also gives 200 pulses on B per turn.)
>
> That is the standard way these terms are used, but some makers have
used
> these terms different ways. Many encoder manufacturers have slowly
> moved to Cycles/rev as the definitive term, as it is pretty hard to
> misunderstand this one. (Except when it is abbreviated to CPR,
whoer
> it could mean cycles or counts! UGHHHH!)
>
> Jon