In context
By 2030, China is projected to have over 30 million stroke patients and more than 80 million people with disabilities, driving demand for lower-limb rehabilitation exoskeletons. While many research groups have developed such devices, rigid exoskeletons face challenges in structural alignment and human-robot interaction, limiting their practical feasibility. This work, published in the Chinese journal Robot in 2023, addresses these issues through a modular, compliant series elastic actuator (SEA) design.
What was reported
Researchers from the Chinese Academy of Sciences and Yanshan University designed a series elastic joint featuring a novel multi-branch planar scroll torsion spring. This elastic element enables linear stiffness within a rated deformation range (up to 0.22 rad) and nonlinear stiffness beyond it, allowing overload protection and passive compliance. The joint is split into motor and reducer modules, achieving a compact profile (65 mm thick, 85 mm wide) with a rated torque of 51.8 N·m and a total mass of 1,870 g.
A rigid-flexible coupling dynamic model was established and identified step-by-step. Under a model predictive control framework, an iterative linearization method was developed to solve the nonlinear, non-convex optimization problem, incorporating control constraints to prevent torque spikes and jitter. Experiments demonstrated accurate trajectory tracking, reduced control energy consumption, and effective disturbance rejection under constraints.
The elastic element's stiffness was validated via finite element analysis and calibration experiments, showing a linear stiffness of 57 N·m/rad and a maximum nonlinear stiffness of 2,500 N·m/rad. The design allows stiffness switching based on external load, enhancing safety and comfort during human-robot interaction.
Why it mattered
This work contributes a compact, lightweight SEA design with variable stiffness and a constraint-aware control strategy, addressing key barriers to practical rehabilitation exoskeletons. The approach offers a pathway to safer, more comfortable assistive devices, with implications for broader applications in compliant industrial robots and human-robot collaboration.
"The proposed control method can track different reference trajectories, effectively reduce control energy consumption and suppress external disturbances under the condition of control constraints."
Source: 《机器人》期刊 (robot.sia.cn) · Published 2023-09-16 · “面向康复外骨骼的串联弹性关节设计与控制”