Variable Stiffness Spring Leg With Human-Driven Stiffness Adaptation

Tiange Zhang; David J. Braun

RA-L 20260 citations

Variable Stiffness Spring Leg With Human-Driven Stiffness Adaptation

Abstract

The bicycle transmission enables effortless gear shifting to amplify the force produced by human limbs, but it cannot amplify limb power. Conventional variable stiffness springs, placed in series or parallel with human limbs, can amplify both force and power by increasing stiffness. However, amplifying power requires energy proportional to the amount of energy stored in the spring. Here, we present a human-driven robot leg that functions as an energetically conservative variable stiffness spring. Using a pedaling mechanism, the leg stores energy supplied by the human and allows stiffness to be increased before the stored energy is released. This approach amplifies both force and power, analogous to shifting gears while pedaling a bicycle. The prototype confirms force amplification, power amplification via stiffness modulation, and low-cost stiffness adjustment, demonstrating the potential for human-driven robot exoskeletons that employ a pedaling mechanism to enable human energy input and exceed biological force and power limits during energy release.

BibTeX

@inproceedings{ral2026_variablestiffnes,
  title = {Variable Stiffness Spring Leg With Human-Driven Stiffness Adaptation},
  author = {Tiange Zhang and David J. Braun},
  booktitle = {RA-L 2026},
  year = {2026}
}