To achieve the highest performance, the stepper motor can be treated as a 2 phase brushless servo motor. Current to the motor will then be controlled as a function of the error signal just as with standard servo motors.
Robolink: Articulated joint modules for robots. igus® now offers a lightweight, maintenance- and corrosion-free range of robot joints for humanoid systems or other automated applications. The carbon-fiber plastic joints can rotate and oscillate freely via four wire ropes. No more expensive milling, cutting or DIY systems. Moving mass is reduced to a minimum. The actuators (engines, pneumatics and hydraulics) and the control module (DP, PC) are kept separate. Contact igus® for more information and free samples.