Lattice Membrane for Improved Mechanical Characteristics of Monolithic Flexure-Hinge Based Anthropomorphic Hands

Flexure-based mechanisms offer a promising alternative to traditional rigid-link joints in robotic hands. They enable simplified construction, reduced weight, and adaptability through their intrinsic compliance. This paper presents the design and mechanical characterization of a compound flexure-hinge joint with a lattice membrane for use in a monolithic anthropomorphic hand. A series of experimental tests were conducted to evaluate the joint's stiffness in flexion and torsion, which were compared to joints from existing literature. Additionally, its durability under cyclic loading was examined, together with the functional grasping capabilities of a full anthropomorphic hand utilizing this joint. Results demonstrate that the joint exhibits increased torsional stiffness of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{5. 5 1} \boldsymbol{\pm} \mathbf{0. 4 7 ~ N m m} /$</tex> degree compared to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{0. 5 2} \pm 0.08 \text{Nmm} /$</tex> degree for a simple flexure-hinge joint. Additionally, it has a smooth response under flexion at a stiffness of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.50 \pm 0.07 ~\mathrm{N} / \text{mm}$</tex> compared to a full membrane which has a variable stiffness between <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.54 \pm 0.10 ~\mathrm{N} / \text{mm}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.30 \pm 0.17 ~\mathrm{N} / \text{mm}$</tex>, caused by a buckling response. Preliminary results also showed that the joint can withstand cyclic loading without failure, but with some deformation, for over <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{3 0 0, 0 0 0}$</tex> cycles, and the ability of a hand utilizing these joints to perform essential grips for successful operation. Clear benefits to incorporating the lattice structure are presented, which include, added torsional stiffness compared to a simple flexurehinge joint, a smooth force profile compared to a full membrane. The lattice design also prevents object interference with the joint cavity during grasping. These findings support the viability of the proposed design approach for use in adaptive, lightweight and compact robotic hands, with applications in humanoid robots and upper-limb prosthetics.