Task-Space Control for Powered Knee–Ankle Prostheses

Abstract

As the capabilities of powered lower-limb prostheses continue to improve, there is an opportunity to shift the focus of prosthetic control beyond purely joint-level considerations and incorporate whole-body task-based information to better coordinate user and device interaction. This paper presents a control framework, termed task-space control (TSC), that determines the desired torque response of a knee-ankle prosthesis during level-ground walking and stair ascent based on task-level objectives, specifically ground reaction forces (GRFs) and center-of-mass (CoM) kinematics. TSC is evaluated with two individuals with knee disarticulation. While results were subject-specific, relative to a phase-based impedance controller, TSC with CoM feedback reduced stair-ascent gait cycle duration for one individual by 17.6%, and reduced stair-ascent CoM tracking error for the other by 27.6% vertically and 32.4% horizontally. More broadly, while exclusively considering task-level objectives to compute actuator torques, TSC still produced key joint-level kinematic outcomes for multi-joint and multi-modal control, like plantarflexion push-off in late stance for level-ground walking and powered knee extension for step-over-step stair ascent. However, subjects did not prefer certain normative characteristics, like early stance knee flexion during level-ground walking. These preferences raise questions about whether fully replicating normative joint characteristics should be the primary target for prosthetic control, or if system-level approaches like TSC may be better suited for adapting prosthesis behavior in conjunction with the user.