Control of Powered Ankle-Foot Prostheses on Compliant Terrain: A Quantitative Approach to Stability Enhancement

TL;DR

This study introduces an admittance-based control strategy for powered ankle-foot prostheses that dynamically adjusts stiffness to improve gait stability on compliant terrains, potentially reducing fall risk for amputees.

Contribution

The paper presents a novel adaptive control approach specifically designed for soft terrains, validated through experiments showing improved stability over standard controllers.

Findings

Enhanced gait stability on compliant surfaces

Consistent performance across different ground stiffness levels

Potential reduction in fall risk for prosthesis users

Abstract

Walking on compliant terrain presents a substantial challenge for individuals with lower-limb amputation, further elevating their already high risk of falling. While powered ankle-foot prostheses have demonstrated adaptability across speeds and rigid terrains, control strategies optimized for soft or compliant surfaces remain underexplored. This work experimentally validates an admittance-based control strategy that dynamically adjusts the quasi-stiffness of powered prostheses to enhance gait stability on compliant ground. Human subject experiments were conducted with three healthy individuals walking on two bilaterally compliant surfaces with ground stiffness values of 63 and 25 kN/m, representative of real-world soft environments. Controller performance was quantified using phase portraits and two walking stability metrics, offering a direct assessment of fall risk. Compared to a standard phase-variable controller developed for rigid terrain, the proposed admittance controller consistently improved gait stability across all compliant conditions. These results demonstrate the potential of adaptive, stability-aware prosthesis control to reduce fall risk in real-world environments and advance the robustness of human-prosthesis interaction in rehabilitation robotics.