Collision Safety and Post-Collision Response of Series Clutch Actuator Robots in Human-Robot Interaction

Collaborative robots, or cobots, face ongoing safety challenges when operating in direct contact with human operators. This study explores how Series Clutch Actuators (SCAs) can enhance collision safety beyond mere detection and avoidance, focusing on strategies for mitigating sustained forces during and after collisions. Three distinct post-collision response strategies were implemented to enable a human to move the robot after an impact, while the robot remains stationary against gravity: partially engaged clutches (off-power torque-limit), minimally engaged clutches (clutch torque-limit equivalent to torque from gravity), and continuously slipping clutches due to motor input. We used the collaborative robot Nicebot-7 with SCA-joints. Unlike previous iterations of Nicebot, Nicebot-7 does not require passive gravity compensation. The torque instability of the friction clutches in Nicebot-7 was experimentally evaluated and addressed with damping terms when the clutches slipped. Experimental evaluations were conducted in different motion planes (horizontal and vertical), enabling an analysis of escape forces. Results demonstrate that continuously slipping clutches minimize post-collision escape forces especially against gravity. This improvement is particularly relevant for quasi-static collisions, where users experience sustained compression (see Fig. 1). We demonstrate how cobots can become safer to operate, reducing risk to human partners.