I have found that steppers dont have enough torque and are prone to
positioning errors due to many reasons ( overload, power spikes etc )    I
have had great success with 2 photo-interupters and a model train loco
motor - position reflective marks at known points and add a home position.
if you miss the mark due to inertia then PWM slowly back, the multipole
motors are great for slow speed running...

Rob Bakker
Research Technician
Psych Dept
University of Waikato