stepper-servo system

To get to grips with the programming of the parallax propeller chip
I'm going to upgrade my RC based sinker EDM. As the chip has 8
processors it should be possible to have one creating pulses for the
EDM, one for controlling the stepper on the z-axis and others doing
the GUI etc.

I will have a DC voltage comming from the spark gap, this will be a
low pass filtered version of the gap voltage, essentially proportional
to the gap voltage. It will be digitized and available to the
processor as feedback for the stepper motor.

I was initially considering just setting the stepper off at a constant
speed, then using an algorithm to adjust the speed and direction based
on the amount of time spent in the "machining zone". However some
form of true servo type system might be preferable.

I hoping someone might know a bit about using a stepper motor within a
servo loop. Of course the feedback is to be implemented digitally
which makes things even more tricky for me, I can remember the term z
transform but not much else.

Graham