This paper presents an analytical framework for designing variable stiffness mechanisms based on a general model of potential energy and physical constraints. The approach defines classes of mechanisms from first principles and identifies those capable of infinite stiffness modulation using bounded motor forces. A prototype consistent with the theoretical model was built and experimentally validated, showing constant motor force regardless of output deflection or stiffness under external load.
Why it matters: Variable stiffness mechanisms are critical for energy-efficient robotics, yet their design is often guided by heuristics. This work establishes analytical conditions that enable systematic design and experimental realization of efficient mechanisms, advancing the development of next-generation actuators for robotics and human augmentation.