Hi all,

I'm having a weird encoder problem and I'd like your advice. I'm using some
512 count incremental encoders from US Digital and trying to track the
exact angular position of a shaft attached to a motor.

First of all, I've noticed that the B output of the encoder has a slow
risetime (few microseconds, perhaps 3 or 4) compared to it's fall time, and
the A output is much more symmetrical (both rise and fall are fast). I
tried replacing the electronics part of the encoder and it made no change
to this. Both outputs are simply going to a couple of digital gates on my
board so I don't think it is caused by my electronics. I have not yet tried
replacing the encoder wheel but I don't see how that could be causing
something different to happen to chB as opposed to ch A, although replacing
that is probably the next thing I will try.

Ok, so now to the real problem: when running the shaft one direction, my
code tracks the position fine. The other way, it will randomly occasionally
loose track by a count or so perhaps every minute (shaft running at about
3000 rpm). This may seem minor but I'm doing brushless motor control and
the encoder is being used as the sensor for the commutation and it is
connected to the motor via a timing belt with a significant reduction
ratio, so there are actually only about 6 counts of the encoder per
commutation step.

This happens when running at a constant speed in one direction.

I am tracking the encoder position by interrupting on both rising and
falling edges of the XOR of A and B. In my interrupt routine, I compare the
current A and B to the previous A and B to determine which has experienced
a change and whether it is rising or falling. Based on this as well as
whether the other (unchanged) channel is high or low, I either increment or
decrement the count. I also take the gear ratio into account and compute
the correct motor shaft position.

Any ideas?

Sean

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