Predicting Redundancy of a 7 DOF Upper Limb Exoskeleton Toward Improved Transparency between Human and Robot

Abstract

For a wearable robotic system which includes the same redundancy as the human arm, configuring the joint angles of the robotic arm in accordance with those of the operators arm is one of the crucial control mechanisms to minimize the energy exchange between human and robot. Thus it is important to understand the redundancy resolution mechanism of the human arm such that the inverse kinematics solution of these two coupled systems becomes identical. In this paper, the redundancy resolution of the human arm based on the wrist position and orientation is provided as a closed form solution for the practical robot control algorithm, which enables the robot to form the natural human arm configuration as the operator changes the position and orientation of the end effector. For this, the redundancy of the arm is expressed mathematically by defining the swivel angle. Then the swivel angle is expressed as a superposition of two components, which are reference swivel angle and the swivel angle offset, respectively. The reference swivel angle based on the wrist position is defined by the kinematic criterion that maximizes the manipulability along the vector connecting the wrist and the virtual target point on the head region as a cluster of important sensory organs. Then the wrist orientation change is mapped into a joint angle availablility function output and translated to the swivel angle offset with respect to the reference swivel angle. Based on the inverse kinematic formula the controller can transform the position and orientation of the end-effector into the joint torque which enables the robot to follow up the operator's current joint configuration. The estimation performance was evaluated by utilizing a motion capture system and the results show that there is a high correlation between the estimated and calculated swivel angles.