Functional design and experimental characterization of a soft bio-inspired robotic wrist

This paper presents TriHSA, a soft bio-inspired robotic wrist module based on three Handed Shearing Auxetic (HSA) cylinders. The proposed architecture is designed to generate coordinated three-degree-of-freedom (3-DoF) wrist motion, including flexion/extension (F/E), radial/ulnar deviation (R/U), and pronation/supination (P/S), while providing adjustable stiffness and passive compliance. By tailoring the HSA geometry for bidirectional actuation, the wrist enables both single-DoF and coupled wrist-like motions. The proposed design provides coordinated motion in all three DoFs through a differential/clutch-inspired coordination law. In the current prototype, this law is implemented through three independently controlled servo inputs to experimentally characterize the HSA-based motion-synthesis principle. A modular testing platform was used to evaluate the wrist’s range of motion, trajectory response, dynamic load behavior, and passive stiffness. The prototype achieved approximately ±33.5° RoM in both F/E and R/U, covering commonly used human-wrist-inspired functional RoM benchmarks, while the P/S range reached approximately ±23.6° and remains the main kinematic limitation of the current prototype. Under trajectory-referenced dynamic loading, the wrist exhibited torque responses of approximately 5.0 N·m in F/E and 4.9 N·m in P/S. Passive stiffness tests yielded equivalent rotational stiffness values of approximately 100 N·m/rad in F/E and 115 N·m/rad in P/S. These results support the feasibility of HSA-based multi-DoF motion synthesis for soft robotic wrist applications.