Variable Stiffness Soft Robotic Fingers Using Snap-Fit Kinematic Reconfiguration

Versatile and secure grasping in robotic systems remains a difficult challenge to address when objects possess a wide range of different properties (size, weight, friction coefficient, etc.). The human hand is often the primary source of inspiration for many technologies addressing this challenge, and a notable feature of our hands is that they can vary their stiffness to match the requirements of the task, e.g., become stiffer or more compliant depending on specific requirements. Many robotic devices have been proposed in the literature mirroring this capability, either using an adjustable internal tension mechanism similar to what happens with human tendons or another physical phenomenon yielding the same effect. This article proposes a new type of soft robotic fingers using a novel method to produce a variable stiffness achieved by modifying the kinematic structure of the fingers using snap-fit joints, a very simple alternative to most variable stiffness mechanisms. The resulting modification of the geometry and kinematics of the fingers, including their number of degrees of freedom, allows to greatly alter the intrinsic stiffness of the grasp produced by these fingers. A notable feature of the proposed new design is that one pair of fingers can be used to switch the stiffness of another pair if a dual arm robot is used.