A semi-active knee prosthesis based on a flexible soft bladder and an irregular four-bar linkage mechanism

This paper presents a polycentric pneumatic semi-active knee (SAK) based on a flexible soft bladder and an irregular four-bar linkage mechanism. The SAK innovatively employs the flexible soft bladder to drive the irregular four-bar linkage mechanism, providing active torque for knee extension. Additionally, an elastic recovery device is designed to provide return torque for knee flexion. In active mode, an air pump inflates the bladder to drive the four-bar linkage mechanism for active extension, while the elastic recovery device stores elastic potential energy. In passive mode, the stored elastic potential energy is released to provide recovery torque for the passive flexion. Furthermore, this paper analyzes the driving characteristics of the knee actuator using the principle of virtual work, and designs a lower limb test platform to measure its active and recovery torque with a more accurate torque measurement method that can eliminate the influence of the knee actuator's weight on the measurement results. Experimental results demonstrate that, the measured output torque closely aligns with the calculated values within a certain angular range, and the recovery torque provided by the elastic recovery device configured with a single elastic band also achieved the expected performance, with the maximum recovery torque reaching 0.56 Nm. The SAK proposed in this paper not only features an instantaneous center of rotation trajectory that aligns with the human knee joint, leading to better coordination between the knee prosthesis and the human body, but also offers advantages such as low cost, compact structure, and monotonic control.