## 2 Proposed Screw Unfastening Strategy  
As introduced above, this study presents an effective screw unfastening system for a six-DOFs robot equipped with an end effector composed of a special screw-tightening tool and a camera, as shown in Figure 4. In this system, a servo unfastening tool consists of a motor, a reducer, inner displacement and torque sensors, and an output rotation shaft. There is a regular hexagonal groove at the end of the rotation shaft, having a slightly larger size than the target screw head and an elastic mechanism, as shown in Figure 5. To realize the disassembly operation, the tool is mounted at the end of the robot so that it can guide it to the target screw. It has its control system, which communicates with the robot through the Beckhoff module and exchanges messages with a PC through the TCP/IP protocol. During the operation, a signal for performing the unfastening operation is sent to the unfastening tool when a robot arrives at the desired location. Namely, the screw-unfastening operation is performed through the cooperated motion of the tool and robot. The signals for stopping motions are sent to the tool and robot at the same time. Finally, the robot finishes the disassembly operation and prepares for the next unfastening task. In the proposed strategy, the unfastening operation of a hexagonal screw is divided into four steps, as shown in Figure 6. The first step is to adjust the pose of an end-effector camera to obtain high-quality images of a hexagonal screw, which is necessary for the accurate calculation of its pose. In this step, the robotic pose relative to the workpiece is first measured through a characteristic circle to determine the start and target poses of the robotic end effector. Then, based on the measurement result, the robotic motion is planned for the subsequent process of camera pose adjustment. In this study, the robot's target pose is calculated to ensure that the end effector is parallel and stays at a certain distance above the screw, which is helpful to improve the screw image quality for the second step. Figure 4: Structure of the robotic end effector. Figure 5: Servo unfastening tool with an elastic mechanism. Figure 6: The flowchart of the screw unfastening process. The second step is to calculate the screw pose and then move the robotic end effector forward and lock the screw, which is a pre-action for the third step. In this step, a hexagon screw's pose in the robotic base frame is calculated based on the geometry analysis results of a hexagonal shape. The calculated pose is used to plan the robotic motion in the locking action of a screw by the end-effector tool. This step makes the proposed strategy adaptable to the disassembly operation of screws with different loose states by providing the correct feedback on the target screw's pose for the robotic motion. The third step is to unfasten a screw through the cooperated motion of the robot and unfastening tool. In this step, the disassembly quality can be improved by selecting proper values of the motion parameters, which can be obtained by the analysis of the torque variation over time during the unfastening action. The fourth step is to move a screw away from the workpiece. In this step, the robot continues its motion along a planned trajectory; the tool stops its rotating motion but maintains the normal force so that the screw does not fall. Finally, the robot puts the screw into a container and proceeds to the next unfastening operation. The proposed strategy aims to improve the efficiency and quality of screw disassembly, as well as the adaptability of a robotic system at a reduced cost. Particularly, the screw pose calculation method based on a vision system is proposed to provide accurate information for the one-time engagement action of a tool and a hexagonal screw. The engagement duration has a major impact on the bolt disassembly efficiency.