Design, modeling, and experimental study of variable stiffness pneumatic bio-inspired soft actuators

Inspired by the actuation mechanism of octopus tentacles, this study proposes a pneumatic bio-inspired soft actuator with variable stiffness to improve load-bearing capability and manipulation performance while enabling flexible stiffness control. The designed actuator exhibits multi-modal deformation capabilities, such as elongation, bending, and circumferential deflection. To establish a theoretical framework for structural optimization, numerical simulations were carried out to investigate the influence of chamber geometry, wall thickness, and length on the actuator's behavior. A deformation analysis model was developed utilizing the Yeoh hyperelastic constitutive model and the moment equilibrium principle to characterize the correlation between input pressure and the resulting bending angle and elongation. Furthermore, a variable stiffness model was formulated using the pseudo-rigid-body model approach rooted in energy equivalence. By synthesizing material properties with discrete kinematic mechanisms, the mapping between system stiffness and actuation pressure was identified. Finally, prototypes were manufactured via rapid prototyping, and a custom experimental platform was built for validation. Experimental data confirmed the validity of both the static and variable-stiffness models. The proposed method achieved a 40% increase in stiffness and a 23.59% enhancement in horizontal contact force, thereby validating the practicality and efficacy of the pneumatic soft actuator. The strategies and findings detailed herein offer significant insights for the development of pneumatic and hydraulic soft robotics.