Quasi passive bounding of a quadruped model with articulated spine

The motion of spine in passive quadrupedal bounding and its effect on leg behavior were presented in this paper. First, a simplified sagittal plane model of quadruped mammals was introduced. The model's body contained one articulated spine joint. Next, a control method was developed to obtain symmetrical bounding of the model. In this method, the spine joint was locked when the spine reached its maximum bend. It was held until vertical velocity became opposite in value. These actions resulted in a specific type of bounding, which was defined as quasi passive bounding. Despite its simplicity, this system captures properties of real animals. Plenty of bounding conditions were considered in the analysis of system kinetics. Passive bounding of the model with stiff spine was also analyzed. A comparison of the maximum leg compression between the two models suggests the use of spine joint for legged systems at a high speed. Moreover, to obtain realistic motion of quadruped mammals, evolution of the spine joint angle shows that trunk stiffness in extension is greater than that in flexion.

► We model quadruped mammal's trunk as two rigid beams linked by elastic joint.
► We lock the joint at maximum extension or flexion to obtain symmetrical running.
► The running is continuous and periodical.
► The constant spine stiffness leads to bigger joint angle at maximum extension.
► A stiffer spine in extension than flexion is important in running of real animal.