## 3 Results
### Experiment on Indoor Positioning Accuracy of Binocular Stereo Vision  
To accurately evaluate the 3D positioning accuracy of _C. oleifera_ fruits with the binocular stereo vision system, an indoor experiment platform was built using an industrial robot. The indoor experiment platform mainly consisted of an industrial robot with a six-axis mechanical arm as the core, a teach pendant, the binocular stereo vision system and a mobile bracket, as shown in Figure 6. The model of the industrial robot was Borunte 0707A, and it had been calibrated before delivery. The motion accuracy was \(\pm\)0.03 mm, the arm span was 700 mm, and the probe was fixed at the end of the industrial robot. The _C. oleifera_ fruit was fixed on the top of the mobile bracket. The probe was accurately moved to touch the mark point of the _C. oleifera_ fruit vertically by operating the teach pendant. The three-dimensional space coordinate of the current tail end of the probe under the coordinate system of the mechanical arm was displayed on the teach pendant at that moment. The coordinates (data compensation had been performed in combination with the radius of the _C. oleifera_ fruit) were recorded as the real values. The three-dimensional coordinates of the center point of the _C. oleifera_ fruit were measured using the binocular vision system with the _C. oleifera_ fruit detection and positioning algorithm and were taken as the measured values and converted to the robot coordinate system through coordinate transformation. A plurality of groups of experimental data were obtained by changing the three-dimensional position of the _C. oleifera_ fruit by moving the mobile bracket. The errors between the real values and the measured values in the _X-_, _Y-_ and _Z-_axis directions were \(\Delta X\), \(\Delta Y\) and \(\Delta Z\), respectively, and \(\Delta E\) was used as the 3D positioning comprehensive error of binocular stereo vision and the standard for measuring the positioning accuracy [31]:  
\[\Delta E= \sqrt{\Delta X^{2}+\Delta Y^{2}+\Delta Z^{2}} \tag{6}\]  
The pixel coordinate system of a binocular camera can be expressed through coordinate transformation because of the linear relationship between it and the 3D space coordinate system of the mechanical arm [32]. In this study, the origin of the binocular camera coordinate system was set at the optical center of the left camera, and the origin of the robot coordinate system was the projection point of the first axis rotation center on the bottom surface of the base. The coordinates conversion equation was expressed using a homogeneous matrix, as shown in Equation (7):  
\[ \begin{bmatrix}X_{C}\\ Y_{C}\\ Z_{C}\\ 1\end{bmatrix} = \begin{bmatrix}R&t\\ 0^{T}&1\end{bmatrix}\begin{bmatrix}X_{W}\\ Y_{W}\\ Z_{W}\\ 1\end{bmatrix}  \tag{7}\]  
Figure 6: Indoor experiment platform. where \((X_{C},\,Y_{C},Z_{C})\) refers to the three-dimensional coordinates of the marked points of the _C. oleifera_ fruits in the camera coordinate system; \((X_{W},\,Y_{W},\,Z_{W})\) denotes the three-dimensional coordinates of the marked points of the _C. oleifera_ fruit in the robot coordinate system; and \(R\) and \(t\) represent the rotation matrix and the translation matrix of the transformed coordinate system, respectively. The three-dimensional space coordinates of the center point of the _C. oleifera_ fruit in the robot coordinate system were obtained by calculating the coordinates in the camera coordinate system according to Equations (2)-(4) and inputting the coordinates into Equation (7). In the camera coordinate system, the 3D positioning accuracy of the spatial points within the field of view of the binocular camera was tested in the depth planes of 400 mm, 450 mm, 500 mm, and 550 mm, respectively. In the experiment, the test points were uniformly set along the \(X\)-axis and \(Y\)-axis in each depth plane, and the distance between adjacent points was 50 mm.