An Ankle-Foot Prosthesis Emulator with Control of Plantarflexion and Inversion-Eversion Torque.


Ankle inversion-eversion compliance is an important feature of conventional prosthetic feet, and control of inversion, or roll, in robotic prostheses could improve balance for people with amputation. We designed a tethered ankle-foot prosthesis with two independently-actuated toes that are coordinated to provide plantarflexion and inversion-eversion torques. This configuration allows a simple lightweight structure with a total mass of 0.72 kg. Strain gages on the toes measure torque with less than 2.7% RMS error, while compliance in the Bowden cable tether provides series elasticity. Benchtop tests demonstrated a 90% rise time of less than 33 ms and peak torques of 180 Nm in plantarflexion and 30 Nm in inversion-eversion. The phase-limited closed-loop torque bandwidth is 20 Hz with a 90 Nm amplitude chirp in plantarflexion, and 24 Hz with a 20 Nm amplitude chirp in inversion-eversion.

The system has low sensitivity to toe position disturbances at frequencies of up to 18 Hz. Walking trials with five values of constant inversion-eversion torque demonstrated RMS torque tracking errors of less than 3.7% in plantarflexion and less than 5.9% in inversion-eversion.These properties make the platform suitable for haptic rendering of virtual devices in experiments with humans, which may reveal strategies for improving balance or allow controlled comparisons of conventional prosthesis features.



Figure: Two degrees of freedom ankle-foot prosthesis 

(Left) Plantarflexion occurs when both toes rotate together and inversion-eversion occurs when the medial and lateral toes move in opposite directions.

(Right) A tethered ankle-foot prosthesis was developed.


Video: A tethered ankle-foot prosthesis


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         S.H. Collins*, M. Kim*, T. Chen, T. An ankle-foot prosthesis emulator with control of plantarflexion and inversion-eversion torque. In Proceedings IEEE International Conference on Robotics and Automation, 1-6, 2015 (* equal contributor). pdf