Design and Control of a PRS-Based Series Elastic Actuator for the Knee Exoskeleton

This article presents a novel knee exoskeleton actuator based on a planar rotary spring, aimed at improving torque transparency and reducing structural complexity in wearable robotic applications. To achieve a compact and lightweight design, the spiral geometry of the neutral surface and arm thickness is optimized using a physics-based model under multiple mechanical constraints. To enable compliant control, a cascaded impedance controller was implemented, with feedforward and friction compensation integrated into the inner loop torque control. To improve torque transparency, the effect of a disturbance observer with a leveraging coefficient was analyzed and compared under different damping ratios. Passivity is maintained through adaptive gain regulation and a velocity-threshold-based time domain passivity approach. The resulting prototype achieves a high torque-to-mass ratio compared to existing series elastic actuator designs. Experimental results further confirm the system’s robustn...