Variable Stiffness Gripper for Fast and Safe HumanRobot Object Handover

Abstract

In collaborative robotics, achieving high-speed yet safe object handover remains a key challenge. Rigid grippers pose impact risks to humans, whereas compliant grippers compromise grip stiffness and positional accuracy. This paper presents the three-finger rapid variable-stiffness gripper (RVSG), which modulates grip stiffness within one second through a simplified hardware mechanism by adjusting only the elastic cable initial tension T-0 and the finger spacing w1. Based on a simplified model, the variation in gripper stiffness was simulated by tuning T-0 and w(1), and subsequently validated with a prototype implementation. By adjusting these two parameters, the prototype attains a variable stiffness range of 12.25 to 75.33 N/m, corresponding to a relative stiffness variation efficiency of 615%. Collision safety tests measured contact forces between 0.17 and 1.01 N, representing a 92.5 to 98.7% reduction compared to commercial rigid grippers. These results demonstrate a lightweight, low-complexity solution for fast and safe human-robot handovers, with potential applications in assembly, logistics, and surgical assistance. Future work will extend the approach to three-axis stiffness control and develop higher-order models that incorporate friction and effective contact distance.