In context
By 2023, lower-limb exoskeletons for rehabilitation were increasingly common, but most relied on rigid actuation, which limited compliance and raised safety concerns in human-robot interaction. Series elastic actuators (SEAs) offered some compliance but had fixed stiffness, restricting control bandwidth. Variable stiffness actuators (VSAs) promised a solution, yet existing designs often struggled with limited stiffness adjustment ranges or added complexity and weight.
What was reported
Researchers at Nanjing University of Aeronautics and Astronautics and the Affiliated Jiangning Hospital of Nanjing Medical University proposed a reconfigurable variable-stiffness principle for a lower-limb exoskeleton. The design adjusts joint stiffness by changing the preload of a spring via a pulley-block system, and the stiffness range can be further expanded by reconfiguring the number of pulley groups used—without disassembling the actuator.
The exoskeleton features an active knee joint driven by the variable-stiffness actuator, while hip and ankle joints are passive. The actuator uses a servo motor with a Bowden cable to reduce leg inertia, and a dedicated stiffness-adjustment motor with a self-locking worm gear to control spring preload efficiently. A theoretical stiffness model was developed and validated through simulations and prototype experiments, showing close agreement between theoretical and measured stiffness curves.
Based on the torque-deflection characteristics of the variable-stiffness drive, the team implemented a sensorless walking-following control method. Tests at different speeds showed a maximum interaction torque of 0.8243 N·m between the exoskeleton and the human body, demonstrating effective gait following without force sensors.
Why it mattered
This work addressed a key limitation of variable-preload VSAs—their narrow stiffness range—by introducing a reconfigurable mechanism that multiplies stiffness adjustability. The design enhances safety and comfort in rehabilitation exoskeletons, potentially accelerating their adoption in clinical and home settings.
“The test results show that the maximum interaction torque between the exoskeleton and the human body is 0.8243 N·m, which proves that the exoskeleton can follow the movement of the human body.”
Source: 《机器人》期刊 (robot.sia.cn) · Published 2023-05-09 · “基于变刚度驱动的下肢外骨骼”