# Research Progress on Synergistic Technologies of Agricultural Multi-Robots
## 3 Research Progress on Synergistic Technologies of Agricultural Multi-Robots
### Environment Perception
#### 3.2.1 Co-Location Technology  
The concept of co-location was originally proposed in 1994 by [PERSON] and [PERSON] in Japan [33] The concept refers to a robot \"sharing\" its positioning results with other mobile robots and to other robots using this shared information to integrate their calculation results to improve the accuracy of positioning themselves and, in turn, sharing their positioning results with other mobile robots, repeatedly achieving the precise positioning of mobile robots [34]. According to the collaborative positioning method, this approach can be divided into active positioning, passive positioning, and interactive positioning [35]. Figure 3: The distributed architecture of an agricultural mobile multi-robot system (**a**) Agricultural multi-robot agricultural condition monitoring in different areas of the vineyard [28]. And the robots exchange their information for autonomous work. (**b**) Distributed structure diagram of a multi-robot. Each robot can exchange information with other robots through communication and make decisions autonomously [26]. However, no research on interactive localization has been reported in the literature of agricultural multi-robots. Active positioning  
In the absence of information exchange, the robot relies on its sensors to obtain relative distances and angles by observing the neighboring robots for self-positioning [35; 36]. As shown in Figure 4, the leading robot guides the follower robot to steer along the leading robot reference route [37]. For example, the black-and-white checkerboard feature board was fixed to the leader robot as the following feature, and the 3D information of each corner point on the black-and-white checkerboard feature board was obtained by the binocular vision camera fixed to the following robot, and the information was analyzed to finally obtain the longitudinal spacing, lateral offset and heading declination of the following robot relative to the leader robot. Using this navigation information to realize the automatic following of the following vehicle, the following system of master-slave orchard operation vehicle is established [38]. Passive positioning  
In an environment where information exchange exists, the robot indirectly obtains the relative distance and angle through the \"observed\" data provided by the friendly neighboring robot to perform its positioning [35]. As shown in Figure 5, the follower robot dynamically creates a reference heading for itself from the position point of the leading robot [39]. Passive positioning mode is usually used in combination with kinematic control models and is one of the most used in master-slave robots. For example, GPS positioning was used for multiple robots, and the travel trajectory of the pilot robot was the main one. Under the premise of communicable, the pilot robot estimated the motion trajectory of  
Figure 4: Active positioning model. Figure 5: Passive positioning model [39]. other robots using the kinematic model, and the difference between the sensor positioning and the estimated positioning value was calculated by the other robots or the pilot robot, and the lateral and longitudinal displacement of each robot was adjusted according to the difference [40]. However, the above positioning methods are highly dependent on the positioning accuracy of the leader robot, which requires good stability and robustness. When the leader robot fails, it is easy to cause the localization accuracy of the whole team to drop, or even the localization fails. In contrast, the interactive localization approach has an environment of information exchange, where robots achieve joint localization through these steps of mutual observation, data exchange, and information fusion [41]. In this case, the robots are in the same position in the team and there is no master-slave distinction, namely, they do not depend on the positioning accuracy of a fixed robot, and this type of phenomenon is reduced.