Examination of Hemispherical Shell Stator for Lightweight Spherical Ultrasonic Motor

A multi-degree-of-freedom, spherical ultrasonic motor (MDOF-SUSM) has been proposed to reduce the size and weight of actuator systems capable of MDOF motion. The MDOF-SUSM is expected to be used in manipulators and spacecraft because of its noiseless, self-holding, and non-magnetic properties. Previous studies have shown the feasibility of the MDOF-SUSM using a spherical stator, but the weight of the device was problematic. This paper describes a hemispherical shell stator devised to reduce the weight of the MDOF-SUSM. For the prototype stator, a suitable vibration mode search and an excitation method were investigated using finite element analysis (FEA). A combination of l2m1-mode and l1m0-mode was selected and found to be excitable by an arched piezoelectric ceramic plate bonded to the opening. A stator support mechanism using four tuning forks was also devised. The vibration characteristics of the prototype stator were experimentally investigated. The results of FEA and displacement measurement confirmed the formation of multiple natural vibration modes that can be used for rotation. Rotation with three degrees of freedom was confirmed to be possible by combining the vibration modes. In short, a hemispherical shell stator for the MDOF-SUSM was created, and it is lighter in weight than the MDOF-SUSM that used a spherical stator.