Design optimization and control of a rotary series elastic actuator for lower-limb exoskeletons

The worldwide rise in aging populations and mobility-impairing diseases has created an urgent demand for lower-limb exoskeletons that can safely provide effective assistance in daily environments. Physical safety is therefore the foremost issue in human–robot collaboration. Exoskeleton actuators must be able to absorb impact, filter unwanted vibrations, provide transparent force feedback, and yet remain as small and light as possible. Series elastic actuators (SEAs) are the preferred hardware solution to ensure safe human–robot interaction. They can effectively reduce the vibration of external force and provide force feedback during the interaction process. However, there is little research on the design optimization of SEAs for exoskeletons, leaving their intrinsic safety, energy density, and manufacturability largely unexploited. To address this problem, a systematic topology-optimization framework specifically tailored for the rotary SEA in a lower-limb exoskeleton is proposed in this paper. The topology and parameters of the SEA’s elastic element are concurrently optimized for specific energy, mass, and manufacturing cost so as to meet the compact and lightweight requirements for exoskeletons. The optimized elastic element is then fabricated and used to build a hip-joint SEA for our lower-limb exoskeleton prototype. A fuzzy singular perturbation controller is derived to realize the position tracking of the hip-joint SEA. Experiments verify the effectiveness of the proposed method. By shifting SEA design from heuristic tuning to multi-objective topology optimization, this work provides a repeatable recipe for the design of compact, lightweight, and intrinsically safe actuators.