A curved ultrasonic actuator optimized for spherical motors: Design and experiments

•A new actuator for spherical motor design is proposed.•The actuator contact surface is adapted to the rotor diameter.•Modal finite element analysis is used to optimize the geometry of the actuator.•The sensitivity of the actuator to tolerances and perturbations is studied.•The performance of an ultrasonic motor is reported and validates the design. Multi-degree-of-freedom angular actuators are commonly used in numerous mechatronic areas such as omnidirectional robots, robot articulations or inertially stabilized platforms. The conventional method to design these devices consists in placing multiple actuators in parallel or series using gimbals which are bulky and difficult to miniaturize. Motors using a spherical rotor are interesting for miniature multidegree-of-freedom actuators. In this paper, a new actuator is proposed. It is based on a curved piezoelectric element which has its inner contact surface adapted to the diameter of the rotor. This adaptation allows to build spherical motors with a fully constrained rotor and without a need for additional guiding system. The work presents a design methodology based on modal finite element analysis. A methodology for mode selection is proposed and a sensitivity analysis of the final geometry to uncertainties and added masses is discussed. Finally, experimental results that validate the actuator concept on a single degree-of-freedom ultrasonic motor set-up are presented.