Design, Development, and Validation of a non-backdrivable active Ankle-Foot orthosis for the TWIN lower-limb exoskeleton

Raffaele Giannattasio^1,2*Nicolò Boccardo^1,3Riccardo Vaccaro¹Heeral Bhatt¹Stefano Maludrottu¹Elena De Momi²Matteo Laffranchi¹

• ¹Rehab Technologies Lab, Italian Institute of Technology (IIT), Genova, Italy
• ²Politecnico di Milano Dipartimento di Elettronica Informazione e Bioingegneria, Milan, Italy
• ³Open University Affiliated Research Centre, Istituto Italiano di Tecnologia (ARC@IIT), Genova, Italy

This study's primary objective was to develop an Active Ankle-Foot Orthosis (AAFO) specifically designed for integration into lower-limb exoskeletons. An analysis of human ankle motion is conducted to inform the development process, guiding the creation of an AAFO that aligns with specifics extrapolated by real data. The AAFO incorporates an electric motor with a non-backdrivable transmission system, engineered to reduce distal mass, minimize power consumption, and enable high-precision position control. Capable of generating up to 50 Nm of peak torque, the AAFO is designed to provide support throughout the walking cycle, targeting pathological conditions such as foot drop and toe drag. Performance was first validated through benchtop experiments under unloaded conditions. The AAFO was then integrated into the TWIN lower-limb exoskeleton, employing an optimal trajectory planning method to generate compatible reference trajectories. These trajectories are designed to help the user maintain ground contact during the support phase while ensuring safe toe clearance and minimizing jerk during the swing phase. Finally, the AAFO's performance was assessed in real-world application conditions, with four healthy participants walking with the TWIN lower limb exoskeleton. The results suggest that the proposed AAFO efficiently reduces toe clearance, ensures stable control, and maintains low power consumption, highlighting its suitability for clinical applications.