## 2 Structural Design of Photovoltaic Panel Cleaning Robot
### Chassis Structure  
When designing the chassis structure of the photovoltaic panel cleaning robot, two different chassis layout methods of three wheels and four wheels were proposed from the perspective of road complexity, high drive and stability. The three-wheel type was driven by the wheel-track composite drive, and the four-wheel type was completely driven by the track. The layout is presented in Figure 3.  
Through the comparison of three-wheel and four-wheel structures in different performance aspects in Table 2, it can be seen that the three-wheel structure reduces the weight of the whole machine by 30% and improves the lightweight of the whole machine. In terms of the turning radius, the three-wheel type uses rear-wheel steering, resulting in a small turning radius. The four-wheel type can only use differential steering, with a large turning radius and the possibility that the track may fall off during the steering process. In terms of leveling control strategy, the three-point leveling control strategy is simple and has fast response speed. The four-point leveling control strategy is relatively complex and the system response speed is slow. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number** & **Name** & **Numerical Value** & **Number** & **Name** & **Numerical Value** \\ \hline
1 & drive power & 8 kW & 7 & turning radius & \(\geq\)2 m \\
2 & system & 20 Mpa & 8 & upper rotation & \(\pm\)180\({}^{\circ}\) \\  & pressure & & & & \\
3 & machine & 1600 kg & 9 & chassis & three-level \\  & weight & & & & \\
4 & end load & 200 kg & 10 & gradeability & 10\% \\
5 & cleaning radius & \(\geq\)2.3 m & 11 & sweeping & 1 km/h \\  & & & & & \\
6 & traveling speed & 2.5 km/h & 12 & cleaning & \\  & & & & method & dry cleaning \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Photovoltaic cleaning robot design requirement parameters. Figure 2: Robot cleaning operation. In summary, the chassis of the photovoltaic panel cleaning robot adopts a three-wheel wheel-track composite drive form using rear-wheel steering to improve the adaptability of the vehicle in a complex environment. The chassis structure was designed using Solidworks 2016 to adapt to complex road environments. The chassis structure design comprised three parts, i.e., track-driven walking device, front- and rear-wheel suspension device, and rear-wheel tire steering device. Each part was connected through the pin shaft structure. Using track wheels as the driving walking device can increase the contact area with the ground and improve traction force. The front- and rear-wheel suspension devices use three plunger cylinders to achieve adaptive leveling of the chassis. The rear tire steers independently, which can reduce the turning radius and improve the flexibility of the whole machine. The encoder was installed at its rotary center to achieve closed-loop control and improve the accuracy of control. As shown in Figure 4.  
The three-point active suspension exhibits the advantages of simple structure and no over static problems. The leveling control strategy was simplified so that the response of \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Number** & **Name** & **Three Wheel** & **Four Wheel** \\ \hline
1 & Weight (kg) & 1060 & 1530 \\
2 & turning radius size & small & big \\
3 & difficulty of leveling & simple control & complex control \\  & strategy & no & yes \\
4 & hyperstatic & rear-wheel steering & differential steering \\
5 & turning method & small & big \\
6 & driving power & small & big \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Performance Comparison between three-wheel and four-wheel models. Figure 4: Wheel–track chassis structure. Figure 3: Different chassis structure layout. the suspension system could be fast and stable. This ensured the capability of the robot to automatically level the road surface no matter how complex it is during walking so that the fast response of the active suspension and the stability of the upper arm during the cleaning process could be guaranteed.