In a conventional helical spring, stiffness and stored energy are linked: making the spring stiffer while it is deformed costs more energy the more energy the spring already stores. State-of-the-art variable stiffness spring actuators share this limitation. This paper presents a variable stiffness spring in which the energy cost of changing stiffness is independent of the energy stored by the spring. The key element is a floating spring that changes stiffness without changing the energy it stores.
Why it matters: Tasks such as lifting heavier loads, jumping higher or running faster need a spring that stores a large amount of energy while its stiffness is adjusted. Separating the cost of changing stiffness from the stored energy could allow variable stiffness actuators for robots and human augmentation devices to use smaller motors and smaller battery packs.