Recent developments in dynamic legged locomotion have focused on encoding a substantial component of leg intelligence into passive compliant mechanisms. One of the limitations of this approach is reduced adaptability: the final leg mechanism usually performs optimally for a small range of conditions (i.e. a certain robot weight, terrain, speed, gait, and so forth). For many situations in which a small locomotion system experiences a change in any of these conditions, it is desirable to have a variable stiffness leg to tune the natural frequency of the system for effective gait control. In this paper, we present an overview of variable stiffness leg spring designs, and introduce a new approach specifically for autonomous dynamic legged locomotion. We introduce a simple leg model that captures the spatial compliance of the tunable leg in three dimensions. Lastly, we present the design and manufacture of the multi-directional variable stiffness legs, and experimentally validate their correspondence to the proposed model.
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