The function of the spine and its morphological effect in quadruped robot locomotion

In quadruped animals, spinal movements contribute to locomotion in terms of controlling body posture, providing the foundation to generate leg movement, and integrating limb and trunk actions. Inspired by this biological findings, we develop two quadruped models featuring different numbers of spinal joints to demonstrate the spine-driven locomotion behaviors. To gain a deep understanding of how the locomotion is achieved by axial driven propulsion and how the spinal morphology affects locomotion, we exclusively employ actuated spinal joint(s) to the model with a minimalistic control strategy. We choose three individuals from these two models and analyze their behaviors in terms of gait properties, i.e., angle of attack, ground clearance, and movement of the center of mass. The results show that employing the spinal morphology with two joints can greatly enhance the stability and speed of locomotion. Among several advantageous properties of the two spinal joint model we identify the following. First, it allows the robot to adjust the movement of the center of mass to stabilize itself. Second, by providing more freedom to bend the spine, the robot can pull the rear legs forward, thus increasing the stride length. Finally, locomotion with this model exhibits two flight phases and greater flight proportion during each stride, similar to what it is observed from running cheetahs, which make significant difference in the speed and the gait.