Study on Adaptive Adjustment of Variable Joint Stiffness for a Semi-active Hip Prosthesis

The moment and impedance of the hip antagonist muscle groups can change dynamically with movement, but existing passive hip prostheses are unable to achieve this function and limit the walking ability of hip amputees. This paper presents a novel adaptive adjustment stiffness system for a semi-active hip prosthesis that can simulate the function of hip antagonist muscles according to real-time detection of gait phase information and improve the walking ability of hip amputees. A method for predicting hip prosthesis kinematic information in advance is proposed to improve real-time performance of stiffness adjustment. The kinematic information at the amputee's healthy side is acquired from the posture sensor, and the kinematic information of the prosthetic side is predicted using a nonlinear autoregressive neural network model. To realize adaptive adjustment of variable stiffness joint, we have developed a stiffness controller based on PID algorithm. The target values for stiffness controller are obtained by analyzing the walking of the healthy person, and the actual stiffness of the hip joint on the prosthetic is determined using angle sensors and torque sensors from a customized embedded system. The hip joint stiffness tracking experiments show that the stiffness controller can effectively improve the regularity of the stiffness curve of the prosthetic joint, and the joint torque is significantly increased. The gait symmetry tests show that gait symmetry indices \({\text{SI}}\) and \({\text{R}}_{{\text{II}}}\) were improved by 86.3% and 85.1%, respectively. The average difference in bilateral gait length of walking amputees was reduced from 6.6 cm to 0.7 cm. The adjustable stiffness system proposed in this study can adaptively adjust the stiffness of the prosthetic joint to the amputee's gait, effectively improving the amputee's walking ability.