Adjusting the Quasi-Stiffness of an Ankle-Foot Prosthesis Improves Walking Stability during Locomotion over Compliant Terrain

Despite significant advances in the design of robotic lower-limb prostheses for individuals with impaired mobility, there is a need for further progress in improving the robustness, safety, and stability of these devices in a wide range of activities of daily living. Although powered prostheses have been able to adapt to different speeds, conditions, and rigid terrains, no control strategies have been proposed for addressing walking over compliant surfaces. This work proposes a continuous admittance controller that adjusts the ankle quasi-stiffness of a powered ankle-foot prosthesis and improves gait stability during locomotion over compliant terrain. The proposed controller is evaluated with walking experiments on an instrumented treadmill that can accurately change the walking surface stiffness. In these experiments, the proposed controller accurately changes the prosthesis ankle quasi-stiffness across a wide range of 10 - 20 Nm/deg, while improving local dynamic stability compared to a standard phase-variable controller. The proposed controller can significantly improve the performance of lower-limb prostheses in dynamic and compliant environments frequently encountered in daily activities, resulting in improved quality of life for people with lower-limb amputation.