Two-degree-of-freedom control of a two-link manipulator in the rotating coordinate system

As applications and tasks of robotic manipulators become more diverse and complicated, the desired motions of the robots also become more sophisticated and complicated. In spite of this diversity of tasks, the coordinate system to describe the tasks has not been changed much; the conventional Cartesian coordinate system is still the most widely used coordinate system. It is found in this paper that the rotating coordinate system significantly simplifies the kinematics of a two-link robotic manipulator with the biarticular actuation coordination, which is inspired from human muscles that can generate torques at adjoining two joints simultaneously. Taking the advantage of this simple kinematic relationship by the rotating coordinate system and the biarticular actuator coordination, the dynamics of the two-link manipulator is analyzed, and a disturbance observer (DOB) is designed based on the derived dynamics to nominalize the actual dynamics and to reject undesired disturbances. The proposed DOB-based control algorithm can achieve better control performance in the rotating coordinate system, and comparative experiments verify the effectiveness of the proposed coordinate system and control methods.