Design of safe joint with variable threshold torque

Abstract

This study presents a passively variable threshold torque Spring-Clutch (PVSC). When human beings and robots work together as in the service robot example, physical safety of the former should be guaranteed. A passive mechanism is usually preferred because of its high reliability and fast response to collision. The PVSC comprises the Spring-Clutch mechanism (in that a spring and cam are utilized for generating threshold torque), and an adjuster (an additional cam and lever mechanism to adjust the threshold torque). It functions as a rigid joint between the input and output. However, when the applied torque exceeds the threshold value, the PVSC joint is fully released and becomes freely rotatable to guarantee safety during contact. The threshold torque is passively adjusted according to the gravitational torque and, therefore, a constant releasing torque is maintained for all joint rotation angles. The threshold torque of the PVSC increases (or decreases) in response to the variation in the deflection of the spring (achieved by using the adjuster) according to the rotation angle (or gravitational torque). Experiments were performed and the PVSC was implemented in the second joint of the SS-Arm IV.