Flexi-SEA: Flexible-Shaft-Driven Series Elastic Actuator for Wearable Robots

Series elastic actuators (SEAs) are widely used in wearable robots due to their precise torque control, backdrivability, and low mechanical impedance. However, SEAs that assist distal joints like the ankle often have a large moment of inertia due to components such as springs and frames, negatively affecting biomechanics, and energy efficiency during walking. Therefore, cable-driven SEAs have been introduced to distribute mass, but they face inherent limitations, including human discomfort from joint stress, shear forces against the skin, and force transmission delays due to cable slack. This article proposes the Flexible-shaft-driven SEA (Flexi-SEA), an innovative SEA designed to reduce its moment of inertia without the drawbacks of cable-driven SEAs. The Flexi-SEA features a heavy motor at the proximal part, connected via a flexible shaft to a spring-loaded end-effector at the distal joint. While the flexible shaft can transmit bidirectional torque with low inertia, its nonlinear, asymmetric, and spatially varying torsional stiffness poses control challenges. To ensure high torque precision and robustness, a controller based on system linearization and a disturbance observer is implemented. The performance of the Flexi-SEA was evaluated through a series of tests, including human-involve experiments.