I bought one of these kflop motion controllers to control steppers on a small router. In the end I sold the router before fitting the card but they are worth researching as an option. Motion controller boards have been springing up everywhere over the last couple of years, so things may have moved on fromt the kflop.

You can also close the loop on the Kflop and it works with glass scales.

http://dynomotion.com/KFLOP.html