I have recently had the opportunity to play with some AC synchronous stepper motors. They feel a little different than a normal stepper as they do not cog when you rotate the shaft "no permanent magnet". From reading the documentation on the drives they just sync to the line frequency. The drives I am playing with say that they have a set RPM for 60HZ and another for 50HZ. This instantly sparked an idea in my head! What if you ran it from a variable frequency? Would it produce a proportional speed to frequency ratio? And if so I would think a small variable frequency drive closed with a PID loop would be pretty inexpensive compared to servos. And it would be a lot more precise than a normal open loop stepper.