Stiffness direction stabilization and inertia matrix diagonalization of robot manipulator by biarticular muscle

This paper suggests two advantages of the biarticular muscle which has been said to play an important role in human´s motion control; one is stiffness direction stabilization of an end effector which results in stabilization of the posture, and the other is inertia matrix diagonalization in the dynamics of a robot manipulator. In order to analyze these characteristics, we adopt three-pair six-muscle model of human arm including biarticular muscles and derive simple relationship between an endpoint force/position and three muscle torques based on the absolute angle of two joints. Based on the derived equation, the relationship between the stiffness in the workspace and in the muscle torque space is calculated and the direction of stiffness that can be enhanced by the biarticular muscle is revealed. Finally, we suggest a new dynamics of the planar robot manipulator with three muscle torque input where the inertia matrix is diagonalized.