Eric R Snow wrote:
Ned,
I went ahead and supplied 5 volts to the encoder and followed your
instructions for scope use. I could watch the transitions on the
screen. Thanks. If the carriage is moved back and forth .0002 the
transition will show. Still, the mysterious backlash seems to be
present. Small movements seem OK but not large ones. It's hard to
count transitions on the screen and watch the indicator too. I got
someone to count the transitions on the screen while I watched the
indicator. BTW, the indicator is OK, it doesn't have bachlash. Still,
I can't see if the shaft is moving or not. So the laser is probably
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

Hi Eric,
I have designed the counters and state machines used to decode
quadrature encoders like you are using and there is a good possibility
that the problem you are seeing is in the readout head. By putting the
scope on the encoder outputs and confirming that they transition every
0.0002 inches including the first, you have confirmed that the encoder
is working correctly. The only other test would be to verify that the
transitions are on different 0.0002" boundaries for the two channels.
One should change on 1,3,5... and the other should change on 2,4,6...

Assuming that the encoder is wired correctly to the encoder, if you
don't see the backlash in the scope transitions, it is not in the
encoder/mechanical system. Look through the manual for the readout head
for configuration stuff about filtering the input data stream. Many of
the low cost quadrature encoders are pretty noisy and may generate
multiple transitions. It is common to build some kind of filtering into
the state machine that turns quadrature transitions into counts. If
there is an option to disable filtering on the head, try it.

Another possible source of the problem would be an "index" sensor input.
Since quadrature sensors are relative position sensors, a third sensor
that triggers once per revolution or at one end of a mechanism can give
you a calibrated "home" start point to give an absolute position. If
there is any mention of tying the index input to +5V or ground, make
sure you have done it. There may be exception states in the quadrature
decoding state machine to deal with the index sensor. Or if there is a
configuration option to disable it, make sure it is disabled.

Good Luck,
Bob