Compliant Finger Joint with Controlled Variable Stiffness based on Twisted Strings Actuation.

Underactuated tendon-driven fingers are a simple, yet effective solution, for realizing robotic grippers and hands. The lack of controllable degrees of actuation and precise sensing is compensated by the deformable structure of the finger, which is able to adapt to the objects to be grasped and manipulated, and also to implement grasping strategies based on environmental constraint exploitation. One of the main drawbacks of these robotic fingers is that, due to the limited number of actuators, they can only realize a limited number of movements. Finger closure motion realized by activating the tendon depends on finger mechanical properties, and in particular on elastic joint stiffness. In this paper, we introduce a passive elastic joint to be implemented in monolithic 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, investigates the relationship between pre-compression and flexural stiffness, and finally shows its application to a robotic finger composed of three phalanges.