A Parametric Wave-Structured 3-DoF Compliant Joint with Tunable Stiffness
Anjum Saeed, Mihai Dragusanu, Monica Malvezzi, Domenico Prattichizzo, Gionata Salvietti
AI summary
Problem
Tendon-driven soft-rigid robotic hands struggle with dexterous in-hand manipulation because fixed tendon routing constrains finger trajectories to predefined paths, while existing variable stiffness designs are often overly complex or lack multi-directional motion.
Approach
The authors designed a parametric wave-shaped compliant joint actuated by three twisted string tendons, allowing independent control of asymmetric bending, flexion/extension, and structural compression to modulate stiffness on demand.
Key results
- Analytical and numerical kinematic and static models for the 3-DoF joint and twisted string actuators
- Experimental characterization confirming stiffness tunability through geometric parameters like joint length and thickness
- Functional prototype integration into a wearable supernumerary robotic finger
- Demonstrated significant workspace and dexterity improvements over conventional designs
Why it matters
Offers a compact, adaptable joint architecture that advances the development of lightweight, dexterous soft robotic hands and wearable assistive devices for complex manipulation tasks.
Abstract
Soft–rigid tendon-driven robotic hands are widely adopted due to their simple fabrication and effective com- pliance, 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 ten- don 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 vali- date the proposed design and confirm its potential as a versatile building block for dexterous, lightweight, and adaptive robotic hands.