## 2 Materials and Methods  
### Working Principle  
The ground pressure measurement system consists of two parts: hardware and software. Among them, the hardware system includes a pressure sensor, 9-axis attitude sensor, GPS module, Arduino controller, signal amplifier, wireless serial communication module, Internet of things wireless module, and computer equipped with Arduino software. In addition, the software system mainly includes technology for data acquisition, data filtering, wireless data transmission, data display, data storage, and other functions. The pressure sensor converts the force of the soil on the skateboard of the rice direct seeding machine into a resistance signal, and the signal amplifier converts the resistance signal into an analog voltage signal [18]. The Arduino controller uploads the data to the mobile phone and computer through the wireless transmission module and the Internet of things module after filtering and analyzing the voltage signal. Figure 2 shows the structure of the ground pressure measurement system. Figure 3 shows the flow chart of the ground pressure measurement system. The detailed information of the components used and measurement instrumentations has been provided in Table 1.  
Figure 2: Structure of the ground pressure measurement system. 1–4: Forward resistance sensor, 5–12: ground pressure sensor, 13–24: signal amplifier, 25: 24 V power supply, 26: 12 V power supply, 27: Arduino controller, 28: 9-axis attitude sensor, 29: GPS module, 30: wireless serial port transceiver module, 31: Internet of things wireless module, 32: Internet of things measurement system—computer terminal, 33: Internet of things measurement system—mobile terminal, 34: wireless serial communication measurement system.  
According to the experiment requirements, the measurement system can remotely set the sensor parameters, control the data acquisition frequency, initialize the sensor, etc. After the data acquisition starts, the Arduino controller will remotely upload the data to the mobile phone and computer, display the current measurement data in real time, and allow the user to view historical data and save measurement data. Figure 4 shows the software flow of the ground pressure measurement system.  
The ground pressure measurement system has 12 independent pressure data acquisition channels. The data from each acquisition channel do not interfere with each other. Nos. 1-4 sensors are forward resistance sensors with a height of 10 cm from the bottom plane of the skateboard, which can effectively measure the resistance caused by the mud at the front end of the skateboard. Nos. 5-8 are the front pressure sensors and Nos. 9-12 are the rear pressure sensors. These are mainly used to measure the front and rear ground pressure and analyze the combination with forward inclination angle and forward resistance. Figures 5 and 6 show the pressure sensor installation location.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sensor** & **Model** & **Operating Voltage** & **Output** & **Measuring Range** & **Measurement Accuracy** \\ \hline Forward resistance sensor & JHBM-H3 & 5–10 V & 350 \(\pm\) 3\(\Omega\) & 0–5 kg & \(\pm\)0.1\% \\ Ground pressure sensor & JHB8–1–5 KG & 5–10 V & 350 \(\pm\) 3\(\Omega\) & 0–5 kg & \(\pm\)0.1\% \\ Signal amplifier & BSQ3 & 24 V & 0 \(\pm\) 5 V & 0–700 \(\Omega\) & \(\pm\)0.05\% \\
9-axis attitude sensor & WTGAHRS1 & 3.0–5.5 V & Serial communication & \(\pm\)180\({}^{\circ}\) & \(\pm\)0.1\({}^{\circ}\) \\ GPS module & ATGM332D & 3.0–5.5 V & Serial communication & 0–515 m/s & \textless{0.1} m/s \\ Wireless serial & CC2530 & 3.0–5.5 V & Serial communication & 250 m & \(\pm\)0.1\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Brief information about components used and measurement instrumentations.  
Figure 3: Flow chart of ground pressure measurement system.