Novel Variable Stiffness Spring Mechanism: Modulating Stiffness Independent of the Energy Stored by the Spring

Theory suggests a linear relation between stiffness and the energy stored by a linear helical spring at constant deformation. This relation implies that increasing the stiffness of a helical spring upon deformation requires more energy at larger deformations. State-of-the-art variable stiffness spring actuators, used to drive robots and human assistive and augmentation devices, are characterized by a similar relation: increasing stiffness as the spring is deformed costs more energy as more energy is stored by the spring. This feature imposes an apparently fundamental limitation on variable stiffness spring actuation in demanding tasks, such as lifting more, jumping higher, or running faster, because, in all these tasks, the variable stiffness spring should store a considerable amount of energy and provide different stiffness to accommodate different weights in lifting, heights in jumping, and speeds in running. Here, we present an innovative variable stiffness spring design, where the energy cost of changing stiffness is independent of the energy stored by the spring. The key element of the new design is a novel floating spring which changes stiffness without changing the energy stored by the spring. Springs possessing the aforementioned feature could pave the way towards variable stiffness robot actuation and human augmentation using smaller motors and smaller battery packs.