Résumé
Achieving stable contact on uneven terrain remains a key challenge in humanoid robotics, as most feet rely on rigid or passively compliant structures with fixed stiffness. This work presents the design, fabrication, and analytical modeling of a compact soft pneumatic submodule capable of tunable longitudinal stiffness, developed as a proof-of-concept unit for adaptive humanoid feet. The submodule features a tri-layer architecture with two antagonistic pneumatic chambers separated by an inextensible layer and reinforced by rigid inserts. A single-step wax-core casting process integrates all materials into a monolithic soft-rigid structure, ensuring precise geometry and repeatable performance. An analytical model relating internal pressure to equivalent stiffness was derived and experimentally validated, showing a linear stiffness-pressure relation with mean error below 10% across 0-30 kPa. Static and dynamic tests confirmed tunable stiffness between 0.18 and 0.43 N<middle dot>m/rad, a rapid symmetric response (2.9-3.4 ms), and stable stiffness under cyclic loading at gait-relevant frequencies. These results demonstrate the submodule's suitability as a scalable building block for distributed, real-time stiffness modulation in next-generation humanoid feet.