Arduino sketch to turn step / direction signals into DC servo motor position control - 'stepper simulator'
Requires libraries:
FlexiTimer2.h http://playground.arduino.cc/Main/FlexiTimer2
PID_v1.h http://playground.arduino.cc/Code/PIDLibrary
AFMotor.h https://github.com/adafruit/Adafruit-Motor-Shield-library PLUS my own custom version hacked to work with ardumoto from sparkfun: https://www.sparkfun.com/products/9815
Encoder.h http://playground.arduino.cc/Main/RotaryEncoders
For only one motor, the basic arduino Uno will work, but for more than one motor something like a mega2560 is needed so the interrupts and motor pwm don't conflict.