Design of a Semi-Active Ankle Foot Prosthesis Using a Pneumatic and Hydraulic Hybrid for Stiffness and Energy Timing Control.

Lower-limb amputees require ankle-foot prostheses with adjustable stiffness and energy return timing to adapt to varying walking speeds, as well as adequate ankle push-off power to propel the body forward. Most passive prostheses utilize energy storage and return with carbon fiber blades (CFBs), but their single stiffness and early energy return timing limit their effectiveness for propulsion. Quasi-active or powered prostheses with CFBs also fail to fully utilize the energy storage and return capabilities of the CFB. As a result, many quasiactive prostheses lack precise energy return timing, while powered prostheses rely on large motors or bulky hydraulic cylinders. In this paper, we present the Pneumatic and Hydraulic Hybrid Prosthesis (PHHP), designed to adjust stiffness and energy return timing. The system leverages the compressible and incompressible properties of pneumatic and hydraulic systems to enable both stiffness adjustment and stored energy delivery timing. The PHHP includes three pneumatic chambers of varying sizes that adjust the resistance of a hydraulic cylinder by turning valves on and off, enabling variable stiffness. The hydraulic cylinder stores energy from the carbon fiber foot's deformation and releases it for push-off assistance via a hydraulic valve. Theoretical and experimental results show the PHHP's potential for push-off assistance and variable stiffness (24.3-54 N/mm), making it adaptable to different walking speeds for lower-limb amputees.