(A) continuously variable transmission for physically interacting robot

Abstract

Electric motors typically used in robot actuators are used to produce low-speed, high torque performance in combination with a gear reducer to overcome the characteristics of high-speed, and low-torque, but are used in a limited speed-torque operation range due to the thermal and mechanical limitations of the system. To overcome this issue, multiple variable transmission systems have been developed. However, because of the complexity of the proposed design and the limitations in weight reduction, miniaturization, and durability of the mechanical components, the existing variable transmission mechanisms have difficulty applying to small and medium-sized robots. Moreover, due to the time delay in gear shifting process, a limited range of variable transmission, and the low durability limits to its application in the field of physically interacting robots. This study proposes an active continuously variable transmission design to overcome the limitations of the existing transmission system, which is hard to apply to robots. The proposed transmission is a type of continuously variable transmission capable of continuously shifting by a friction-based power transmission between toroidal input and output disks and spherical rollers. It has the function of preventing malfunction through the occurrence of slip between the disk and roller in the event of an external shock or overload by utilizing the intrinsic characteristic of friction-based power transmission. In addition, we enabled a wide range of gear shifting at high speed through the design of a gear ratio control module that combines a low-output motor with a planetary gear and a worm gear module. Based on these concepts, a compact design of transmission possessing a wide operation range, high energy efficiency, and overload protection functions was implemented suitable for physically interacting robotic applications. In order to maximize the performance of the proposed transmission, the hardware was manufactured through the process of setting and optimizing design parameters. Through the kinematic and dynamic modeling and experimental validations of the frictional power transmission, system output, energy efficiency of the system, gear shifting performance (range, speed), and the speed-torque curve of the output are presented.