## 5 Experimental Validation
### Experiments in the Real Environment
#### 5.2.2 Landing Line Experiments  
To verify the effectiveness of the landing line algorithm, an experimental field was designed according to the specifications of the experimental platform in Figure 15. The following parameters were used in these experiments: (1) the distance between the FPLIR and the power line was set to \(\mathrm{L}=2\) m; (2) the flight height of the FPLIR in the fast landing phase was set to 2 m. Once the position was reached, the FPLIR entered the precise landing line phase; (3) the flight speed was set to 1 m/s, which helped to ensure the accuracy and repeatability of the test. The experimental setup is shown in Figure 16.  
Figure 15: The autonomous landing line system settings used in our experiments. During the fast landing phase, the FPLIR takes off at the given speed and continuously estimates its position using VIO until it reaches a set height. During the entire process, the FPLIR only moves in the \(z\)-axis direction, but due to the drift of VIO, there may be slight movements in the \(x\)- and \(y\)-axis directions. In addition, the actual flight altitude may have some errors with respect to the predetermined altitude due to the limitations of the control accuracy. Figure 17 shows the motion process of the FPLIR during the fast landing phase, where the asterisk indicates its current position. Once the FPLIR has reached the predetermined height, it enters the precise landing line phase. In this phase, the FPLIR performs the following steps: (1) use visual sensors to obtain images of power lines and segment the images through a semantic segmentation algorithm to obtain the power lines in the image; (2) fit the segmented power lines and calculate the slope of each point on the power line, in combination with an SLA generation model, in order to calculate the SLA; (3) choose the closest SLA to minimize energy loss during the landing process; (4) take the optimal landing position as the expected trajectory planning location and plan a landing trajectory; (5) use the trajectory tracking controller to carry out the landing trajectory, and perform FPLIR landing on the power line. Figure 18 shows the process of the precise landing line phase, which includes three parts in each state: the spatial position of the FPLIR, the semantic segmentation result of the power line, and the calculated power line slope. The spatial position of the FPLIR is marked with a red box. Figure 16: Schematic diagram of the experimental setup. Figure 17: The fast landing stage of the FPLIR. As seen in Figure 18, the FPLIR can accurately segment the power line in the image during the landing process while simultaneously calculating the slope of the power line. In addition, the SLA is calculated to be less than or equal to 8\({}^{\circ}\) by the coupling model of the FPLIR and the power line of the test platform. The red box indicates the current position of the FPLIR. Figure 19 shows the results of the FPLIR landing line experiment, including the overall trajectory of the FPLIR during the landing line and the tracking error during the precise landing line. The FPLIR first moves along the blue trajectory to reach the endpoint p\({}_{1}\) of the fast landing line phase at a height of 2.05 m. Then, it moves along the yellow trajectory to reach the endpoint p\({}_{2}\) of the precise landing line phase and successfully lands on the power line. Figure 19b shows the tracking errors of the FPLIR in the x-, y- and z-directions during the precise landing line phase. The FPLIR mainly moves along the x-axis direction; therefore, it will accelerate and then decelerate during the landing process, resulting in the error in the \(x\)-axis direction first increasing and then decreasing. It can be seen from Figure 18 that the slope of the power line in the field of view of the FPLIR is always less than 2\({}^{\circ}\), which meets the requirement of an SLA. Therefore, the robot only needs to move a  
Figure 18: Precise landing stage for the FPLIR. (**a**) Take off. (**b**) fly to appropriate height. (**c**) adjust the pose.