## 3 Control Methods Used in Agricultural Vehicle Path Tracking Control
### Dry Field Agricultural Machinery  
The essence of path tracking is that under the developed control algorithm, the target vehicle can track the planned path within a certain accuracy range so that the vehicle can work along the planned path. Tractors can form different operating units with other agricultural machinery to complete tasks such as plowing, sowing, fertilization, and cultivation. The operating environment of tractors is complex and changeable, and the driving skills of drivers vary from person to person, resulting in problems such as low efficiency, decreased operation quality, and waste of land resources in field operation. In order to address these issues, many experts have conducted research on the path-tracking control technology for autonomous tractors. [PERSON] et al. (2018) developed a 4 WS tractor path tracking algorithm, which uses the tractor's kinematic and dynamic model and the linear control law to control the steering of the tractor. Winter wheat sowing test results showed that the lateral tracking deviation of straight sowing is less than 0.05 m, and the heading angle tracking deviation is less than 1 degree. To improve the tractor's ability to track variable curvature paths, [PERSON] et al. (2012) designed a variable curvature path tracking method based on the front wheel \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Vehicle Models** & **Pros** & **Cons** & **Comments** \\ \hline \multirow{4}{*}{Kinematic model} & \multirow{4}{*}{Easy to build, small computational burden} & Neglecting vehicle dynamics & Suitable for agricultural vehicles under low-speed conditions, \\  & & parameters, introducing errors & four-wheel vehicles are often \\  & & under assumed conditions, & simplified as bicycle kinematic \\  & & poorer robustness & models, which are still popular in \\  & & & existing research. \\ \hline \multirow{4}{*}{Dynamic model} & Analyzing the force during & \multirow{4}{*}{Dynamic parameters are difficult to obtain online} & The accuracy of the model is \\  & the vehicle movement process, & & difficult to obtain online \\ \cline{1-1}  & good robustness to external & & further research is needed. \\ \cline{1-1}  & parameter changing & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of agricultural vehicle models in path tracking control. Figure 6: Block diagram of the path tracking control system. angle feedforward compensation strategy, which comprehensively considers the driving speed and target path curvature to adjust the look-ahead distance dynamically. The agricultural land surface usually fluctuates greatly, and tractors will be affected by interference, such as the interaction between the supporting agricultural machinery and the uncertain ground. When wheel slip occurs, the tracking error of tractors will increase and even affect subsequent agricultural production activities. [PERSON] et al. [39] developed a sliding mode controller with a nonlinear perturbation observer, and the new controller performed better when slip occurred compared with backstepping controllers and model predictive controllers. [PERSON] et al. [40] constructed a second-order sliding mode (SOSM) controller for a tractor path tracking system, fully considering longitudinal and lateral wheel slips. [PERSON] et al. [41] proposed a novel adaptive second-order sliding mode controller (ASOSM), which alleviates the chattering problem in traditional sliding mode control and improves the convergence speed and anti-interference ability of the system. [PERSON] et al. [42] proposed a novel adaptive sliding mode control (ASMC) method for path tracking of unmanned agricultural vehicles. This method is robust in the presence of parameter uncertainty, disturbances, and changing road conditions. During the straight-line operation of the tractor, stable high-speed driving will improve its work efficiency, but when turning on the ground, the driving speed should be reduced to improve the stability of the steering and reduce the turning distance.