The present paper address the analysis of Ultrasonic motors, UM, as driving technology in impedance controlled robotic manipulators. A focus is given on rehabilitation robotics as an emerging field in which impedance controlled approaches are more suited. The article gives some general remarks on impedance control, briefly introduces a UM driven miniature SCARA robot and describes a model of the manipulator and UM that serves the goal of gaining insight in the expected behavior of the controlled system. The experimental results and evaluation of UM as driving technology make use of a set of figures of merit previously defined. Eventually, some concluding remarks are given.

The application of a repetitive controller with zero phase error tracking compensation (ZPETC) to the speed control problem of travelling wave ultrasonic motors is investigated. The proposed control strategy is aimed at reducing the typical speed ripples of ultrasonic motors as well as compensating the load and temperature changes. Repetitive controllers are capable of improving the response of dynamical systems under periodic disturbances by learning from the stored error valves of preceding periods. Since it is desirable for the stabilizing compensator of repetitive controllers to be zero phase and perfect error tracker, the ZPET controller is a natural choice when the servo's closed-loop transfer function has non-minimum zeros. The performance of the proposed controller is compared against that of a PI-repetitive controller and its effectiveness experimentally demonstrated.