D.J. Braun, V. Chalvet, Chong T.-H., S.S. Apte and N. Hogan, Variable Stiffness Spring Actuators for Low Energy Cost Stiffness Modulation, vol. 35, no. 6, pp. 1435-1449, IEEE Transactions on Robotics, 2019.
This paper introduces a variable-length leaf-spring actuator designed to modulate stiffness with very low energy cost. Unlike conventional variable stiffness actuators that require two motors to control both stiffness and equilibrium position, this design uses a single motor solely for stiffness modulation and does not perform net mechanical work unless externally loaded. Experiments show that the actuator can effectively support human joints by providing stiffness augmentation during tasks such as postural stabilization and weight bearing.
Why it matters: Human joints regulate stiffness through coactivation of muscles, a process that is metabolically expensive. By providing low-energy stiffness modulation, this actuator concept can restore or extend human capabilities in rehabilitation, assistive devices, and human–machine collaboration, without the high energy demands of traditional actuators.