Design, modeling, and performance analysis of a 2-DOF pneumatic flexible wrist with controllable braking torque

To address the structural complexity and limited compliance of traditional rigid robotic wrists, as well as the absence of adjustable stiffness in soft wrists designed primarily for flexibility, a two-degree-of-freedom (2-DOF) pneumatic flexible wrist with controllable braking torque was developed. The wrist comprises three circumferentially arranged pneumatic actuators connected in parallel around a central pneumatic cross-hinge brake, enabling multidirectional pivoting, while the braking torque can be adjusted according to task requirements for pose stabilization. This configuration combines the high stiffness required for posture maintenance during braking with compliant actuation during motion. Theoretical models of the wrist pivot angle, pivoting direction, and braking torque were established. The pivot-angle model was empirically calibrated using the measured pivot-angle data, while the pivoting-direction and braking-torque models were validated experimentally. Mechanical performance was evaluated under constant-friction-torque and constant-pivot-angle conditions, and empirical relations were obtained by fitting the experimental data. A three-dimensional motion-capture system was also used to characterize the dynamic response under different braking conditions and excitation signals. Under the present experimental conditions, the results show that the wrist exhibits high compliance and enables continuous omnidirectional pivoting with a maximum single-axis pivot angle of 45.2°. The pneumatic cross‑hinge brake achieves multidirectional braking with continuously adjustable torque, reaching a maximum braking torque of 0.842 N·m under brake-only actuation at 0.35 MPa. During coordinated actuation–braking operation, the wrist’s overall resistance to external loads increased by a factor of 4.98 compared with its unpressurized initial state. This composite load-resisting capacity originates from the combined effects of the pneumatic actuators and the central brake.