Development of a compact ring type MDOF piezoelectric ultrasonic motor for humanoid eyeball orientation system

A ultrasonic motor (USM) has been developed to meet the specific demands for compact actuators, which can drive humanoid eyeball to realize multi-degree-of-freedom (MDOF) motions. This USM has a ring shape composite stator, and employs combinations of axial bending mode and in-plane nonaxisymmetric mode to generate three type ellipses at four driving feet, pushing the spherical rotor to rotate around x, y and z axes respectively. FEM was used in the design process, including tuning the mode frequencies by sensitivity analysis, as well as optimizing the stator dimensions by transient simulations, where the shapes and attitude angles of the driving ellipses were retracted and analyzed with the deduced method, together with the driving effects comparison of two feasible exciting methods, achieving uniform driving behavior among the four feet. A prototype has been fabricated to verify the working principles, designs and simulations. The mechanical performance around x, y and z axis were measured under different electrical excitements and preload conditions, and typical stall torque is 17.5 mN m and the no-load speed of 82 r/min in x, y rotation, meanwhile stall torque is 12.5 mN m and the maximum speed of 43 r/min in z rotation, which meet the presented requirements.