Mechanical Design of a Soft Palm and a Tendon-Driven Thumb for an Anthropomorphic Robotic Hand

Abstract

Anthropomorphic hands can be useful in many robotic applications, e. g. limb replacement, humanoid service robots, or terrain exploration. Current robotic hands achieve impressive performance in power grasping, but fall short of replicating human dexterity, for which the thumb’s mobility and the palm flexion are essential aspects. This work investigates how a robotic hand can benefit from a third degree of freedom in the trapeziometacarpal joint (TMC) and the use of soft material in the palm. In a kinematic model, three variations of a biaxial TMC did not sufficiently approximate human performance in two common benchmarks, the Total Opposition Test and the GRASP taxonomy, when a newly proposed criterion for fingertip orientation was applied. FEM simulations of the soft material indicated that the palm’s structural stiffness can be modulated by notching the soft matrix between the metacarpal bones and by increasing the tendon’s lever arm. A 3D-printed, hybrid soft-rigid prototype was built, integrating a custom ball joint for the TMC and a compliant palm with embedded bones. A starting point for the tendondriven actuation was included in the design but needs elaboration. Further evaluation and iteration of the concept would greatly benefit from a control system for the hand. The prototype developed here is the first anthropomorphic hand featuring an active flexible palm with embedded bones.