A Parametric Wave-Structured 3-DoF Compliant Joint with Tunable Stiffness
Anjum Saeed, Mihai Dragusanu, Monica Malvezzi, Domenico Prattichizzo, Gionata Salvietti
Abstract
Soft–rigid tendon-driven robotic hands are widely adopted due to their simple fabrication and effective compliance, enabling robust and adaptive grasping. However, achieving dexterity, such as in-hand manipulation, remains challenging because actuation systems typically constrain finger trajectories. This paper presents a novel parametric wave-structured 3-DoF compliant joint with tunable stiffness, designed to enhance dexterity while maintaining a compact form factor. The joint combines a compliant structure and a particular geometry with a Twisted String Actuation (TSA) system, allowing simultaneous modulation of joint stiffness and the mobility of a universal joint that can be used to resemble flexion/extension and abduction/adduction motion of the human hand fingers. Two tendons, independently actuated, control asymmetric bending and stiffness regulation, while a third tendon drives flexion/extension. Analytical modeling and numerical simulations are provided to characterize the kinematics, statics, and stiffness modulation properties of the joint. A functional prototype demonstrates significant improvements in workspace and dexterity when integrated as the base joint of a wearable robotics supernumerary finger. Experimental evaluations validate the proposed design and confirm its potential as a versatile building block for dexterous, lightweight, and adaptive robotic hands.