# Research Progress on Synergistic Technologies of Agricultural Multi-Robots
## 2 Problems with Multi-Robot Applied to Agricultural Environments  
Crops are fixed in cultivation season and time, which determines the labor demand pattern of agricultural production in a year, and agricultural operators need to make flexible responses and treatments according to the growth pattern of crops, such as plowing, planting, management, and harvesting [16]. At the same time, to adapt to the development of agricultural intensification, scale, and industrialization, and to reduce the economic losses caused by untimely processing, a collaboration of multiple farm machinery operated by people has widely appeared in agricultural production (as shown in Figure 1). In Figure 1a multiple rotary fillers are being used to plow rice fields on sloping land to safeguard food production and mitigate the impact of the phenomena of lack of labor resources, which is brought about by the New Crown epidemic [17]. In Figure 1b, multiple corn planners are employed to sow seeds in a large field, which saves labor and ensures the quality of seeding, and directly improves the yield and quality of corn planting [18]. In Figure 1c, multiple drones are used to spray pesticides in cotton fields, which could be targeted according to the types of pests and diseases, and also prevented their rapid spread in the early stages of infestation [19]. In Figure 1d, it has become a trend to manually operate multiple combines simultaneously during the wheat harvesting season to avoid the effects of rainfall on wheat quality and yield [20]. By manually operating the machines, the quality of operation of the implements relies heavily on human experience, while the use of robots instead of manual operation can free up manpower and ensure the quality of operation. However, the topography, soil, light, and climate conditions of the crop growing environment are different from those of indoor and urban transportation environments, and these conditions pose a challenge to the application of multiple robots in agriculture. Take the example of multiple harvesters harvesting grain in farmland. First, farmland is an unstructured environment, which means that the road conditions are undulating, there are various types of obstacles, and there are missing or blurred lane lines on the ground, and agricultural machines both share the same resource and interact with each other to become dynamic obstacles to each other. Secondly, agricultural operation tasks have strong requirements for operation time, such as harvesting grains in a fixed short time frame. Furthermore, the amount of grain output varies from plot to plot, and the number of agricultural machines needed and the number of operational tasks assigned to them is dynamically changing (such as harvesters with large loading capacity should match the plots with large grain output). Finally, even if the same type of farm machines work together, the characteristics of the machines are not the same (for example, the harvester with the same loading capacity, the fuel consumption is different, the harvester with high fuel consumption should be assigned the operation task more than its work cost, and its operation path should be as short as possible but the harvesting volume should be as large as possible). To ensure that multiple machines can cooperate, a multi-robot system is required to be able to organize multiple robots flexibly, quickly, and efficiently according to the changes in the environment and tasks, and to fully utilize the capabilities of each robot to finally complete the given task with high quality [21]. At the technical level, in addition to being accurately informed of the positioning information of the swarm and the environmental information of the operation, and solving collision and obstacle avoidance,  
Figure 1: Man-operated farm machinery worked in a large field. (**a**) Multiple rotary fillers plowing rice fields in spring. (**b**) Multiple seeders sowing corn in spring. (**c**) Multiple drones spraying cotton fields in the summer.