# Virtual Pheromone Based Network Flow Control For Modular Robotic Systems
## 1 Introduction
### Related Work  
Both global and local communication protocols have its own limitation in these existing modular robotic systems. Some types of hybrid communication was exploited but these approaches bring complexity to the hardware infrastructure. As a result, broadcasting becomes the most popular method for data transmission between robotic modules [18]. To illustrate this fact, we provide a comparison of the key properties of communication methods of the existing modular robotic systems including our system as seen in Table 1. We see that application-oriented design is the main research of such systems while communication is only considered as a simple data transmission protocol. Controlling data flow in a given configuration was only studied in a few systems but these routing protocols are relatively simple. We consider these approaches as the top-down design approach, focusing on application rather than system level. Meanwhile, we believe that, in order to enable scalability and flexibility of a modular robotic system for numerous desired applications, we must pay particular attention on a robust communication protocol that is capable of dealing with data flow in a given configuration, which can be also considered as the bottom-up design approach of modular robotic systems.  
The idea of virtual pheromone based communication path selection with low computational complexity has been becoming more popular in mobile ad-hoc networks. However, this research direction has been not mature, especially in modular robotics. One typical example can be found with using virtual ants for path construction in a robotic network [19]. The communication path is used for transmitting information between food sources and a nest by mimicking the ant foraging phenomenon. However, virtual ants are sent between the nest and food sources with a fixed rate and a specific packet is required to update the network topology during operation, leading to conflict between virtual ants at the intersection and overused communication resources.  
In [20], we have implemented a simplified virtual pheromone technique for information foraging to reducing the excessive virtual ants and updating signal in modular robotics. We developed the virtual pheromone inspired algorithm in which the nest sends out virtual ants, so-called _forward ants_, and wait for return ants from the food source. All robotic modules along the communication path randomly select communication direction without neighbourhood information. Once virtual ants reach food sources, they return to the nest and accumulate the virtual pheromone concentration on the modules along the path. If a _forward ant_ is lost on the way back, the virtual pheromone concentration is reduced by the effect of danger warning parameter. A communication path can be established with high time-consuming cost. In addition, neighbourhood awareness and link maintenance are neglected in [19; 20]. Therefore, although they demonstrated the advantages of the virtual pheromone techniques in robotic networks, it is not realistically practical for modular robotic systems.