Passive control of bipedal robots via tail morphology
Raphael, Jonathan S.T.
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Legged robots offer significant advantages over their wheeled and treaded counterparts, enabling access to huge sectors of otherwise non-navigable terrain. To develop these walkers many engineers have looked to nature for inspiration, but the field of bipedal research has been focused almost exclusively on human locomotion. Other than Homo sapiens, the only regularly bipedal walkers are members of Theropoda, a clade that includes modern day birds as well as all the carnivorous dinosaurs. Whereas birds evolved extensively for flying, their ancestors, e.g. Velociraptor and Tyrannosaurus Rex, were much more specialized for dynamic terrestrial motion. We submit that there is good reason to look to theropod body geometry for an efficient alternative walking model. In this thesis, a novel model was developed in order to examine the mechanics of such specialized bipedal motion. Instead of a traditional anthropomorphic model maintaining a vertically balanced torso, this research synthesized a dinomorphic model that consists of a horizontal spine with counterbalanced torso and tail masses pivoting around the hip joint. The system model developed herein was an extension of the simple rimless wheel representation and aimed to capture critical events in the cycle of bipedal motion while avoiding chaotic regimes. Mathematical models and computer simulations were designed iteratively and in parallel. Once the system dynamics and the energy losses from inelastic impact were derived, then all the equations were nondimensionalized. Theoretical bounds on efficiency were found, and an attempt was made to experimentally quantify the effects of each geometric system parameter. A region of improved performance was identified, indicating non-negligible benefits to tailed morphologies over tail-less ones. It is suggested that further research might adapt and apply this model to the more complex bipedal compass gait. Ideally these findings will enable and encourage the design of legged robots with a horizontal load-bearing frame, demonstrating advantage over anthropomorphic walkers.
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