The VariLeg is an exoskeleton allowing a paraplegic to walk. It was used for competing on an obstacle course at the first Cybathlon. It integrates an adjustable stiffness in the knee joint to improve the walking performance. However, the adjustable stiffness mechanism (ASM) of the VariLeg is bulky and heavy, which hampers the handling of the exoskeleton. Hence, the choice of an ASM concept that only needs small springs is essential. This study benchmarks six state-of-the-art ASMs regarding their needed energy storage capacity, thus their potential for a high compactness. The benchmark is performed with the requirements of the VariLeg and a second requirements set, which can be fulfilled by all six ASMs. The benchmark can be transferred to other requirements as well. It is based on models of the ASMs with their design parameters optimized for the given requirements set. The benchmark reveals large differences between the performances of the investigated ASM concepts of up to a factor of five in the energy storage capacity. This compactness benchmark is a useful design tool to choose a suitable mechanism to realize a compact implementation. More compact ASMs will improve the handling of assistive robots with a physically adjustable stiffness, such as the VariLeg, to support handicapped people in everyday life.

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