I. Introduction
In recent years, research on robot manipulators has been rapidly growing in various fields, such as medical apparatus [1], manufacturing industry [2] and humanoid robots [3], [4]. Redundant manipulators possess higher flexibility compared with non-redundant manipulators due to their redundancy on degrees-of-freedoms (DOFs), which guarantees that they can finish primary tasks of the end-effector as well as subtasks.