A Two-Degree-of-Freedom Ankle Exoskeleton With Control of Plantarflexion and Inversion–Eversion

Balancing during motion involves humans using inversion–eversion control, a task that robotic exoskeletons can help with. In this study, we developed a two-degree-of-freedom robotic ankle exoskeleton that provides active assistance in both inversion–eversion and plantarflexion using a hybrid material design. This exoskeleton is lightweight, weighing 1.2 kg fully assembled, and offers a rapid step response, demonstrated by rise times of 69 ms <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 8.4e <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>4 for plantarflexion, and 95 ms <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 5.4e<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula>2 for inversion–eversion. The device presents a gain-limited torque control bandwidth of 16 Hz for plantarflexion and 12 Hz for inversion–eversion. It can generate peak torques of 70 N<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula> m for plantarflexion and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 14 N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula> m for inversion–eversion. In trials where two healthy individuals walked using the device, tracking errors were found to be on average 1.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 1.4 N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula> m for plantarflexion and 1.0 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.7 N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula> m for inversion–eversion. A single subject was also used as a case study to determine the potential effect of inversion assistance on frontal plane balance. The subject was able to decrease the step-width variability during walking by 58.3% when compared to the no exoskeleton condition. These results indicate that this device could serve as an effective tool for developing and testing balance-centric methodologies in controlled human subject studies.