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 robustness, safety, and torque transparency. In particular, the proposed method reduces the residual torque to 0.62 N<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula>m at 1 Hz, demonstrating improved performance compared to conventional proportional–derivative control. In addition, the actuator achieves accurate impedance rendering during dynamic interaction tasks, highlighting its potential for wearable robotic assistance and rehabilitation applications.