A Safe Robot Arm with Safe Joints and Gravity Compensator

Abstract

This study presents a robot arm equipped with safe joints and multi DOFs gravity compensators. The safe joint, also referred to as "Spring-clutch", is simple passive mechanism that consist of a spring, a cam, and a joint torque sensor. When the torque applied is lower than a pre-set threshold, the Spring-clutch serves as a rigid joint between the input and output. When the applied torque exceeds the threshold, the Spring-clutch is released and is free to rotate like a revolute joint, which significantly reduces the collision force to avoid damage to the robot, as well as to humans. In addition, a compact joint torque sensor is installed in the Spring-clutch to measure the torque at the joint. Also, the analysis of energy and torque shows that the proposed mechanism can function as a gravity compensator capable of static balancing. Since joint torques vary in accordance with the pose of a manipulator (i.e., rotation angles), a Spring-clutch with a constant threshold torque cannot always guarantee the maximum collision torque in some poses of a manipulator. To overcome this limitation, a gravity compensator is adopted to eliminate the gravitational torque. In this research a bevel gravity compensator is applied which can perform static balancing completely. This paper describes the design principles and fabrication of the safety mechanisms and the robot arm.