Joint variable stiffness of musculoskeletal leg mechanism for quadruped robot

When the quadruped robot is in locomotion such as jumping and running with higher speed, there is non-continuous contact force between the foot and the environment inevitably. In order to achieve the flexible force interaction of the bionic legs with the environment, it is necessary to analyze the joint angular stiffness of the bionic leg. In this article, based on the designing principles of the bionics, light-weighted, and flexible, a kind of musculoskeletal bionic leg mechanism driven by pneumatic artificial muscles is presented by inspiring from the biological cheetah anatomy and physiology muscle distribution. The kinematics of the bionic leg is analyzed to obtain the Jacobian matrix. In order to achieve high-speed jumping and soft landing of the bionic leg, a kind of foot stiffness model is presented by analyzing the foot elastic potential energy caused by the contact force. The mapping relationship between the joint stiffness matrix and the foot stiffness matrix is obtained by the Jacobian matrix. Then, the dynamics of the bionic leg is analyzed to determine the relationship between the joint angular stiffness and the pneumatic artificial muscle gas pressure. Finally, the experiment on controlling the pneumatic artificial muscles gas pressure for tracking the joint angular stiffness of the bionic leg is conducted. By regulating the pneumatic artificial muscle gas pressure, the needed pneumatic artificial muscle gas pressure that could meet the desired foot stiffness ellipse model can be determined. The study will pay a theoretical foundation for controlling the pneumatic artificial muscles to achieve the high-speed locomotion of the bionic leg.