A Dual-Mode Hydraulic Actuator for a Quasi-Passive Load-Carrying Exoskeleton in Multiple Conditions

Lower limb exoskeleton robots have been widely researched for load-carrying assistance. Recently, quasi-passive exoskeletons using low-power elements to modulate mechanical characteristics have emerged. However, achieving effective damping and stiffness across varying tasks and loads remains challenging. This letter proposes a dual-mode actuator (DMA) inspired by knee joint impedances, achieving controllable damping force and high spring stiffness for assistance, with good backdrivability for human-robot transparency. Based on the DMA, a quasi-passive lower limb exoskeleton (QLLE) is proposed and evaluated during loaded walking and squatting. Experi-mental results demonstrated that the DMA achieved a sinusoidal damping force tracking error of 5.9% at 1 Hz, a spring stiffness of 21.7 N/mm, and an unassisted backdrive force of 14.3 N for compression. In addition, QLLE assistance reduced the maximum net metabolic cost by 8.5% during walking and 15.6% during squatting, with the load effectively transferred to the ground. These findings highlight the potential of QLLEs in real-world applications, such as manual material transportation.