Design of the passive joints of underactuated modular soft hands for fingertip trajectory tracking

In this letter, we propose a method to design tendon-driven underactuated hands whose fingertips can track a predefined trajectory, when actuated. We focus on passively compliant hands composed of deformable joints and rigid links. We first introduce a procedure to determine suitable joints stiffness and tendon routing, then a possible realization of a robotic underactuated finger is shown. The kinematic and kinetostatic analysis of a tendon-driven robotic finger is necessary to define the overall stiffness values of the finger joints. A structural analysis of the element constituting each passive joint allowed to define a relation between the stiffness and joint's main dimensional and material properties. We validated the proposed framework both in simulation and with experiments using the robotic Soft-SixthFinger as a case study. The Soft-SixthFinger is a wearable robot for grasping compensation in patients with a paretic hand. We demonstrated that different fingertip trajectories can be achieved when joint stiffness and tendon routing are properly designed. Moreover, we demonstrated that the device is able to grasp a wider set of objects when a specific finger flexion trajectory is designed. The proposed framework is general and can be applied to robotic hands with an arbitrary number of fingers and joints per finger. The modular approach furthermore allows the user to easily customize the hand according to specific tasks or trajectories.