## 4 Experiment
### Experiments Outdoors  
#### 4.3.1 Experimental Scheme  
The stability and sensitivity of variable spray system were tested by outdoor spray deposition experiment, the experiment was carried out in a paddy field of the Zengcheng Research and Teaching Base (113 deg38'15'' E, 23 deg14'37'' N) of South China Agricultural University, Guangzhou. The experimental  
\begin{table}
\begin{tabular}{c c c} \hline
**Duty Cycle (\%)** & **Volume Diameter (\(\upmu\)m)** & **Spray Angle (\({}^{\circ}\))** \\ \hline
40 & 99.81 & 98 \\
45 & 98.72 & 99 \\
50 & 97.85 & 97 \\
55 & 97.46 & 99 \\
60 & 97.32 & 98 \\
65 & 97.07 & 99 \\
70 & 96.73 & 100 \\
75 & 96.77 & 98 \\
80 & 96.49 & 101 \\
85 & 96.26 & 102 \\
90 & 95.79 & 104 \\
95 & 95.65 & 105 \\
100 & 94.59 & 105 \\ \hline \end{tabular}
\end{table}
Table 1: Diameter of droplet volume and spray angle at different duty ratios of PWM square waves.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Duty Cycle (\%)** &
\begin{tabular}{c} **Target Flow Rate** \\ **(L-min\({}^{-1}\))** \\ \end{tabular} & **Actual Flow Rate** \\ \hline
40 & 0.1550 & 0.1508 & 2.71 \\
45 & 0.1705 & 0.1682 & 1.35 \\
50 & 0.1860 & 0.1805 & 1.34 \\
55 & 0.2072 & 0.1950 & 5.89 \\
60 & 0.2324 & 0.2290 & 1.46 \\
65 & 0.2945 & 0.2  
site is the same as the prescription map. The outdoor experiment site photo of the plant protection drone variable spray system is shown in Figure 15.  
According to the prescription diagram, the prescription values of adjacent units are different. In order to explore the uniformity of droplet deposition in each unit and the droplet deposition at the boundary of adjacent units, the sampling bands such as S1, S2 were shown in Figure 16, in which the operational units each were set to 10 m \(\times\) 10 m. Figure 16 shows only the layout of the sampling points in the adjacent operational units, and the other units were arranged in the same way. In UAV flying one sortie, the total number of sampling bands was 124. A sampling band was set up in the center of each operation unit, such as S4 and S10. In order to study the change of spray volume at the boundary of the operation unit, the sensitivity of the variable spray system was evaluated. The sampling bands were set on the boundary of the adjacent operation unit and to be 1 m and 2 m away from the boundary line on its both sides, as shown in Figure 16. S1, S7 and S13 were the sampling bands on the boundary line; S5, S6, S8 and S9 were the sampling bands on the left and right sides of the dividing line, respectively. The amount of droplet deposition, the sediment density and the sedimentation uniformity of aviation plant protection operations are important parameters reflecting the quality of spraying [26]. In order to study the law of droplet deposition between various regions, a Rhodamine B (soluble fluorescent tracer) solution with a concentration of 5 g/L was used instead of the pesticide solution, and a Mylar card with a size of 50 mm \(\times\) 80 mm and water-sensitive paper with a size of 28 mm \(\times\) 75 mm were used to collect the sprayed droplets to analyze the deposition density and deposition uniformity. Nine sampling points were evenly arranged at an interval of 1 m on each collection belt, and both the Mylar card and water-sensitive paper were fixed at a height of about 50 cm from each sampling point. The layout of Mylar card and water-sensitive paper is shown in Figure 17.  
Figure 15: Spray test site.  
The experiment was conducted on 5 November 2018, and the mature rice was selected as the tested crop. A total of four sorties were tested, the flight speed of the plant protection drone was stable at 5 m/s and the flight altitude was 2 m. At the same time, a portable ultrasonic micro-automatic weather station (Hberw6-3, Shenzhen Hongyuan Technology Co, Ltd., China) was used to measure the environmental information, and the height of the weather station was set at 2 m above the ground. The test was carried out four times in total. The environment parameters are shown in Table 3.