Criterion for the Design of Low-Power Variable Stiffness Mechanisms

Designing robotic systems capable of low-power operation, inherent to their compliant actuation, has been elusive in practical application. In this paper, we propose a physical measure to mathematically define mechanical designs that are suitable to realize stiffness modulation with low power cost. Using this measure, we present a mathematical formulation of an ideal variable stiffness mechanism unaffected by the external load during its operation. We then analyze several existing mechanisms from the literature to relate design features with analytical conditions inherent to low power stiffness modulation in practical designs. Through this analysis, we identify an approximate practical realization of an ideal actuator capable of stiffness modulation with inherently low power cost. Similar to a number of existing efficient variable stiffness mechanisms, this mechanism is able to hold a given stiffness setting with zero input force under no external load. However, unlike many other previously designed mechanisms, it enables infinite range stiffness modulation using finite control forces. A practical variable stiffness mechanism that is capable of infinite range stiffness modulation using finite control forces leads to lower power cost and reduced energy consumption.