Stepper motor not following encoder exactly

I'm using an A-B quadrature encoder with open collector (2000 pulses / rev) to supply pulses to a Automation-Direct STP-DRV-4850. The drive is set to "quadrature following" mode and the parameters have been set so that the motor shaft turns 360 degrees every time the encoder turns 360 degrees (the motor is 200 pulses / rev)

The shaft driving the encoder turns 90 degrees in 0.35 seconds, then stops for about 0.35 seconds before turning another 90 degrees. I would expect the motor to follow accordingly, but it sometimes over or under-shoots 90 degrees by several (perhaps as much as ten) degrees.

Looking for a possible solution...

It sounds like the stepmotor is missing torque for the load inertia, and/or accelerates/decelerates too fast.
As a result, it looses or overruns unpredictable number of steps, towards the load inertia direction.

The ratio of pulses is 10 to 1 could it be a rounding or math error in the conversion? Seems to me possible since the amount of error is about the same ratio to the distance traveled

what is stopping and starting acceleration. It can not bang run and bang stop.

Besides everything already mentioned, check the following:

* Make sure the motor running current isn't set too high or too low.
* Check your coupling
* Do you have a scope? Check the input signal to the drive. Over / undershooting can easily result from a noisy signal.

If all else fails and you have a good signal, you can always switch to a closed-loop stepper. Telemecanique (Lexium MDrive hybrid models), Oriental (Alphastep) and others make a number of different models that work perfectly well.

Solved - bad encoder, right out of the box!

Thanks for the input.

I have seen that one time one an IMS stepper motor. Encoder mounting hole wasn't tapped deep enough, so screw head was rubbing optical disk. Took a while to find that.