## 2 Proposed Screw Unfastening Strategy
### Robotic Motion-Guided Camera Pose Adjustment Method  
This work aims to develop an effective vision-based disassembly strategy for screws located in 3D complex structures, including different working planes. The measurement accuracy of a vision system has a major impact on the effectiveness and efficiency of the proposed strategy. This section proposes a camera pose-adjustment method guided by robotic motion, aiming to improve the image quality of a screw. To solve this problem, a characteristic circle needs to be placed on the structure to be disassembled. The image processing of the characteristic circle aims to realize the camera pose adjustment to obtain high-quality images of the target screw, which has a significant impact on the calculation accuracy of a screw pose. In the proposed strategy, there is one characteristic circle on each working plane, and screws on the same plane can be disassembled. The camera pose adjustment is completed by three steps: the initial camera view adjustment, the orientation adjustment and the position adjustment. These are presented as follows in detail. _(1) Initial camera view field adjustment_: The camera view is firstly adjusted by regular searching of the characteristic circle through the robotic motion based on the global recognition and positioning, aiming to make a screw and the characteristic circle located in the same image. According to the imaging principle, the object distance is positively related to the camera view field but negatively to the area covered by the target in an image, which means that the object distance adjustment should consider both the camera view field and the image quality. In this study, the general adjustment is performed by the robotic motion based on the above principle, which makes the characteristic circle and the target screw be located in the same image and their image area as large as possible. Generally, when the distance between the characteristic circle and a screw is very long, the image quality will be greatly affected, which will reduce the location accuracy of a screw and can result in a failure of the engagement of the screw and unfastening tool. To analyze the allowable distance range of the proposed robotic system, experimental tests were performed. In these tests, the distance between the hole and a screw was set to be in a  
Figure 8: Examples of a complex disassembly environment. (**a**) Obstacles between target screws. (**b**) Target screws with different loose states. range from 20 mm to 300 mm. Testing results are given in Table 2, from which it can be seen that the accuracy was greatly affected by the distance \(L_{hts}\) and area percentage \(Ratio_{A}\). Therefore, the strategy is only applicable to cases when the distance is less than a certain value, which is defined by a specific requirement for the measurement accuracy. For realizing the unfastening action successfully, the sizes of the screw and tool end should satisfy the condition of \(d_{1}<d_{2}\), as shown in Figure 9. Accordingly, the minimal gap \(g\) is limited by the following condition,  
\[g<(e-s)/2 \tag{1}\]  
where values of two parameters \(e\) and \(s\) are defined by the nut size specification standard. In this study, the target screws' parameters are as follows: \(e=17.77\) mm and \(s=16\) mm. The minimal gap satisfies the condition of \(g<0.885\) mm according to the standard GB/T5783-2000. In the proposed robotic system, \(g=0.88\) mm, which indicates that the location error cannot exceed the minimal gap of 0.88 mm. Therefore, aiming to improve the reliability of the proposed strategy, based on the test results in Table 2, the distance between the hole and a screw cannot be larger than 200 mm. For most industrial robots, such as the KUKA robot used in this work, their positioning accuracy is always high enough considering structural flexibility and control resolution, and can reach \(\pm 0.02\) mm to \(\pm 0.05\) mm. The accuracy of the positioning of the robot is far less than the minimal gap when performing screw unfastening.