Homogeneous Dynamic Control for Stair Ascent in a Swing-Assist Knee Prosthesis

Stair ascent is a challenging task for people with transfemoral amputation. It can be made substantially easier with a swing-assist prosthesis, actively supplementing the prosthesis nominally passive behavior to help the user place their foot on the next stair tread. A significant control challenge is providing power without competing with the user's agency during the swing-phase movement. This paper presents a new control approach for stair ascent swing-phase assistance in swing-assist prostheses. The approach is designed to supplement swing-phase movement with power without introducing an additional exogenous control input, leaving the user as the sole source of prosthesis movement. Namely, this is achieved by adding power to modify the homogeneous dynamics of the prosthesis's passive behavior. This control approach is developed in the paper, implemented on an experimental prosthesis, and assessed in stair ascent trials with three unilateral transfemoral amputees, comparing it with their daily-use device. Experimental results demonstrate, for step-over stair ascent aggregated across participants, the proposed approach: 1) increased peak knee angle by a factor of 2.5; 2) improved symmetry from 41% to 84% (where 100% is perfectly symmetric); and 3) required 2 times less hip effort to achieve a given knee motion, all relative to daily-use prostheses.