# Virtual Pheromone Based Network Flow Control For Modular Robotic Systems
## 1 Introduction  
Inspired by using natural ant pheromone in path searching and selection [1], the idea of artificial pheromone has been applied for many robotic systems [2]. Pheromone inspired stigmergy has advantages of simplicity and distributivity so this approach provides a suitable solution for robotic modules with limited computing resources. Artificial pheromone was used for robot foraging, group following, and cooperation in applications of modular robotic systems [3; 4; 5; 6; 7; 8; 9; 10]. Light intensity is often used to mimic the level of pheromone concentration for robot coordination [7; 8; 9; 10]. Pheromone gradient is used as a mechanism for robot foraging and coverage [3; 4]. However, in complex network of robotic modular configuration in which data packets must be transferred through a desired path between modules, stigmergy communication is not sufficient as it does not guarantee the communication path over a designated time.  
With the limitations of energy consumption, production cost, and hardware resources, local directional communication is preferable for modular robotics [11]. Infra-red line-of-sight communication becomes a popular communication mechanism of modular robotics because it provides not only the mean for data transmission but also the directional information of physical communication channels that is important for neighbourhood discovery & awareness, link maintenance of neighbour-to-neighbour communication, and routing protocols in a network of physically connected robotic modules. Utilizing the directional information of physical connection between neighbourhood modules, network data flow control can be solely built-up by the local directional communication mechanism.  
Bi-directional data flows play the key role in developing applications of modular robotic systems. In a multi-path network of robotic configuration, sending data packets to undesired destinations could lead to unexpected results, thus a communication and routing protocol for inter-communication between robotic modules becomes essential. A number of communication protocols have been designed under the inspiration of pheromone-based communication of ant colony [12; 13; 14; 15; 16]. The main objective of these protocols is to create communication paths connecting robotic modules through a mobile ad-hoc network (MANET). However, these protocols require substantial communication hardware resources, thus it is impossibly applied for modular robotic systems using limited communication and computing resources, e.g., infra-red line-of-sight communication mechanism. Indeed, directional channel selection and correct path construction are challenges of modular robotic systems with limited resources that cannot handle complex communication mechanisms.  
In this paper, to tackle challenges of path formation and selection of bi-directional communication paths in modular robotic networks, we propose a virtual pheromone-based network flow control that only relies on line-of-sight local communication. This control mechanism is based on the integration of gradient-based and virtual pheromone-based routing algorithms on the foundation of our distributed communication protocol presented in [17]. We formulate the research problem through discussion of technical challenges of establishing communication paths in modular robotic systems in Section 2. In Section 3, we describe our developed virtual pheromone based network data flow control algorithm. Real experiments through an education and entertainment game on our newly developed modular robotic system are addressed in Section 4. Section 5 draws conclusions and future work.