Design and characterization of a compact rotary Series Elastic Actuator for knee assistance during overground walking

In wearable robotics applications, actuators are required to satisfy strict constraints in terms of safety and controllability. The introduction of intrinsic compliance can help to meet both these requirements. However, the high torque and power necessary for robotic systems for gait assistance requires the use of custom elements, able to guarantee high performances with a compact and lightweight design. This paper presents a rotary Series Elastic Actuator (SEA), suitable to be used in an active orthosis for knee assistance during overground walking. The system includes a commercial flat brushless DC motor, a Harmonic Drive gear and a custom-designed torsion spring. Spring design has been optimized by means of an iterative FEM simulations-based process and can be directly connected to the output shaft, thus guaranteeing high torque fidelity. With a total weight of 1.8 kg, it is possible to directly include the actuator in the frame of a wearable orthosis for knee flexion/extension assistance. The presented design allows to obtain a large-force bandwidth of 5 Hz and to regulate output impedance in a range compatible to locomotion assistance of elderly subjects with an age-related decay of motor performances.