Biological research has concluded that the actuation of the spine contributes significantly to the performance of quadrupeds in terms of controlling body posture, and integrating limbs and trunk actions. Inspired by this biological findings, we develop a pneumatically-driven quadruped robot called Renny with configurable spine morphology to study how the spine contributes to cheetah-like running. Three spine morphologies: rigid spine, passive spine, and actuated spine, are introduced and tested in Renny robot. In addition, we investigate the effect of the stiffness distribution of the spine muscles in the passive case. The experimental results show that the passive one where the dorsal stiffness is higher than the ventral stiffness can run faster, even faster than the rigid case. Moreover, the coordination between the legs and the actuated spine is studied in actuated spine morphology. We found that when the spinal movements are synchronized with the legs movements, the speed is much faster. In the actuated case, both flexion and extension benefit the increase of the speed by advancing limbs rapidly and increasing the limb swing.
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