# Research Progress on Synergistic Technologies of Agricultural Multi-Robots
## 3 Research Progress on Synergistic Technologies of Agricultural Multi-Robots
### Environment Perception
#### 3.2.3 Mapping  
Once the multi-robot has determined its position, it also needs to determine information about the multiple robots' surrounding environment, for instance, the presence of obstacles. Mapping is the task of accurately describing the spatial position of the robot working environment and various objects (such as obstacles and road signs) in the environment, that is, to establish a spatial model (two-dimensional or three-dimensional) or map [26]. The purpose of creating this map is to provide path planning for the robot, so the map must be easy for the robot to understand and computationally manipulate and accommodate revision when new environmental information is detected [64]. At present, many methods have been developed for constructing environmental models for multiple robots, which are mainly summarized in the three types of grid-based model, geometric mode, and topological mode [22]. Probably due to the low environmental information of the topological model, no literature was found on the use of multiple robots in agriculture. On the contrary, the grid model and the geometric model provide abundant information about the agricultural environment, the purpose of multi-robot operations is clear, and more literature is applied to agricultural operations. And the detailed environment model facilitates the task allocation to multiple robots, real-time observation of multiple robots' motion, effective coordination mechanisms, and detection of robot motion faults. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Fusion Level** & **Data Level** & **Feature Level** & **Decision Level** \\ \hline Processing level & Bottom layer & Middle layer & High level \\ \hline Sensor types & Homogeneous & Heterogeneous & Homogeneous/Heterogeneous \\ \hline Degree of dependence on the sensor & Maximum & Medium & Minimum \\ \hline Advantages & Less information loss; & Takes into account the advantages of data and decision; & Less calculation; \\  & High precision; & Fusion is more convenient; & A small amount of communication data; \\  & & Wide range of applications; & Good real-time performance; \\ \hline Disadvantages & A large amount of calculation; & & \\  & Poor fault tolerance; & Global decision; & A large amount of information loss; \\  & The algorithm is complex; & Strict requirements for sensor & Low precision; \\  & A large amount of communication data; & pretreatment; & Need to balance constraints \\  & Poor real-time performance; & & (speed, reliability, cost, etc.); \\ \hline Fusion algorithm & Wavelet transform; & & \\  & Artificial neural networks; & Kalman filter; & Bayesian theory (Particle filter); \\  & Weighted average; & & \\  & Production rules, etc. & Extended Kalman filter, etc; & The statistical decision, etc.; \\ \hline Applications & None & Drove in a greenhouse [56] & Seeded fields [58] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of multi-sensor information fusion processing levels [62; 63]. Grid model  
The uniformly distributed grid represents the working environment of the robot, and each grid corresponds to the position of whether an obstacle exists or not [22]. For example, if a grid has an obstacle, it is marked as 1; otherwise, it is 0. As shown in Figure 6, [PERSON] et al. [28] obtained georeferenced pictures through UAVs and then created a full map of a vineyard. The processing entails first dividing the UAV workspace into multiple regular grids and defined borders (line segments of workspace in the grid-based environment) using [PERSON]'s linear algorithm (BLA). Then, the procedure to distinguish the subareas in the grid-based environment is a recursive flood-fill algorithm that picks an empty cell (not marked as occupied) and floods in four directions while there are empty cells, and each flooded cell is marked as occupied. This mapping procedure is repeated until all nodes of the grid are occupied by georeferenced pictures.