I. Introduction
As Robot manipulators show high efficiency and accuracy in performing various tasks, they are widely used in manufacturing, processing, and many other related industries [1]. Specifically, to make the movement more flexible and avoid collision and singularity, robot manipulators with redundant degrees of freedom (DoFs) are preferentially adopted [2]. However, the redundancy brought in also tremendously increases the complexity in trajectory generation based on inverse kinematics (IK), especially in case of controllable requirements on both the position and the orientation of the robot end-effector.