Examination of an outer-rotor-type multidegree-of-freedom spherical ultrasonic motor

An outer-rotor-type multidegree-of-freedom (MDOF) spherical ultrasonic motor (SUSM) can be applied directly as the MDOF-joint of a robot and manipulator. The SUSM consists of a spherical shell outer rotor, a rotation shaft and two same stator vibrators. The outer rotor is surrounding the stators with a spherical surface. The stator vibrators can excite three kinds of vibration modes, and the outer rotor can rotate around the stators on three axes by combining and switching the different vibration modes. This paper describes the construction and the operating principle of this type of an MDOF SUSM, modal analysis results of the stator vibrator, and some measurement results from trial manufacturing. The outer-rotor-type MDOF SUSM is useful for realizing a simple, compact and flexible actuator system, and a large torque-to-weight ratio is expected to be realized.