Mapping Encoder Values to Ankle Angles in a 3 Dof Parallel Ankle Exoskeleton

Accurate estimation of ankle joint angles is vital for advancing assistive technologies such as ankle exoskeletons. This study investigates whether three encoders within the parallel kinematics of a three degree of freedom (3-DoF) ankle exoskeleton can effectively estimate the three rotational movements of the ankle: plantar-/dorsiflexion (PF/DF), in-/eversion (IN/EV), and internal/external rotation (IR/ER). Performance was tested during walking, curve walking, and stair climbing with two healthy participants. Ankle motion was independently captured using a motion capture system, while encoder values from the exoskeleton were concurrently recorded. The root-mean-squared error (RMSE) for angle estimations ranged from 1° and 3°, indicating good accuracy, particularly for IN/EV and IR/ER, though challenges were noted in capturing peak dorsiflexion. The findings suggest that misalignment between the exoskeleton and the biological joint axis may have contributed to the observed inaccuracies, particularly for PF/DF, as evidenced by differing range of motion (RoM). Additionally, variability between the two participants may reflect the potential impact of individual anthropometry and alignment quality, which could inform future considerations for design and alignment protocols. Future model enhancements could incorporate dynamic effects and machine learning to generalize across diverse populations.