## 3 Results and Discussion
### Test on the Displacement Measurement of Fertilizer Discharge  
In order to achieve the purpose of precise fertilization, it was necessary to determine the discharge capacity of different fertilizer dispensers [25; 26]. The test materials were commonly used granular fertilizers: urea (N), diammonium phosphate (P), and potassium sulfate (K). Before the test, the fertilizer was put into the fertilizer tank, and the servo motor was controlled through the PLC controller. It drove the trough wheel type fertilizer discharging device to rotate at a certain speed and put the material receiving container at the fertilization outlet. The fertilizer amount could be obtained by weighing the weight of the fertilizer in the receiving vessel. Then, a stopwatch was used to time the fertilizer discharge. The test was repeated 3 times for each fertilizer device at each speed. The test results are shown in Table 2 and fertilizer discharge measurement test is shown in Figure 8.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Category**} & \multirow{2}{*}{**Loading/Unloading**} & \multicolumn{5}{c}{**Weight/kg**} \\ \cline{3-7}  & & **50** & **100** & **150** & **200** & **250** \\ \hline \multirow{2}{*}{Before calibration} & Loading & 49.85 & 100.05 & 150.01 & 200.02 & 249.99 \\ \cline{2-7}  & unloading & 49.88 & 99.96 & 149.97 & 200.03 & 249.98 \\ \hline \multirow{2}{*}{Maximum error/\%} & 0.24\% & 0.05\% & 0.02\% & 0.015\% & 0.008\% \\ \cline{2-7}  & Loading & 50.01 & 99.97 & 149.98 & 199.99 & 250.00 \\ \cline{2-7}  & unloading & 49.99 & 100.02 & 149.99 & 200.00 & 250.01 \\ \hline \multirow{2}{*}{Maximum error/\%} & 0.02\% & 0.03\% & 0.013\% & 0.005\% & 0.004\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Pressure sensor calibration test measurement results.  
Figure 8: Fertilizer discharge measurement test.  
According to the test results in Table 2, in the controller, three gears of low, medium, and high were set up for the motor speed, corresponding to the motor speed of 10, 30, and 50 r/min, respectively. Then, the corresponding calibration data nitrogen (N) average fertilizer outputs were 52.4, 46.9, and 41.8 g/r, and the standard deviations were 1.3, 2.8, and 1.2, respectively; phosphorus (P) average fertilizer outputs were 64.3, 60.3, and 55.8 g/r, and the standard deviations were 2.9, 3.1, and 0.8, respectively; potassium (K) average fertilizer outputs were 86.0, 82.7, and 79.4 g/r, and the standard deviations were 1.0, 4.3, and 1.1, respectively.  
It could be seen that potassium fertilizer discharged the largest amount of fertilizer per turn, followed by phosphate fertilizer, and nitrogen fertilizer was the smallest, because the bulk density of the three fertilizers of nitrogen, phosphorus, and potassium was different. The amount of fertilizer per revolution of the grooved wheel type fertilizer discharging device is larger at low speed and smaller at high speed, which is mainly related to the weight of the fertilizer carried between the outer side of the groove wheel and the outer shell. The standard deviation of the fertilizer output per revolution was different for the three kinds of fertilizers of nitrogen, phosphorus, and potassium at different speeds. Nitrogen fertilizer was the largest, followed by phosphate fertilizer, and potassium fertilizer was the smallest, because the fluidity of the three fertilizers of nitrogen, phosphorus, and potassium is different.