Joint Stiffness Analysis and Regulation for Underactuated Soft Grippers Based on Monolithic Structure

Underactuated tendon-driven fingers are a simple and effective solutions for realizing robotic grippers and hands whose range of applications is very wide and could contribute to accomplish Sustainable Development Goals (SDG), as for instance SDG8 (Decent work and economic growth) and 9 (Industry, Innovation and Infrastructure). One of the main drawback of these robotic fingers is that, due to the small number of actuators, they lacks in dexterity and are poorly adaptable to different tasks. In this paper, we introduce a passive elastic joint to be implemented in monolithic robotic fingers in which the stiffness can be actively regulated by applying a pre-compression to the structure, controlled by a twisted-string actuator (TSA). The paper describes the working principle of the joint and investigates on the relationship between pre-compression and flexural stiffness both with numerical simulations and with experimental measures.