This paper presents the design and preliminary evaluation of an elas-tic lower-body exoskeleton. Human legs behave in a spring-like fashion while running. We selected a design that relied solely on material elasticity to store and release energy during the stance phase of running. The exoskeleton included a novel knee joint with a cam and a Bowden cable transferring energy to and from a waist-mounted extension spring. We used a friction-lock clutch controlled by hip angle via a pneumatic cylinder to release the cable during swing phase for free movement of the leg. The design also incorpora-ted a composite leaf spring to store and release energy in the distal portion of the exoskeleton about the foot and ankle. Preliminary test data for our target subject showed that his typical leg deflection was 0.11 m with leg stiffness of 16 kN/m while running at 3.0 m/s. We used these values to set the desired stiffness (60%15% of the normal leg stiffness, or 9.62.4 kN/m) and deflection (0.11 m) of the exoskeleton. We created simplified multi-body and full finite element quasi-static models to achieve the desired system stiffness and validate our results, respectively. The final design model had an overall stiffness of 7.3 kN/m, which was within the desired range. We fabricated a single-leg prototype of the exoskeleton that weighed 7.1 kg. We tested the exoskeleton stiffness quasi-statically and found a stiffness of 3.6 kN/m. While running, the exoskeleton provided 30% of the total leg stiffness for two subjects. Although the stiffness was lower than desired, the fabricated prototype demonstrated the ability of a quasi-passive exoskeleton to provide a significant portion of an individual’s leg stiffness while running.

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