# A Bacterial Chemotaxis-Inspired Coordination Strategy for Coverage and Aggregation of Swarm Robots
## 1 Introduction  
The adaption behavior of biological creatures provides a source of inspiration for the coordination of robots. Chemotaxis behavior in response to the presence of chemical concentration gradients occurs during foraging and has inspired a number of research and applications in robotic systems. Some examples include target search [1, 2], exploration of unknown environments [3], mine detection [4], as well as environmental monitoring [5, 6, 7]. The simplest microorganism with chemotaxis behavior is a bacterium. In this paper, we focus on a simple bacterium called _Escherichia coli_ (_E. coli_) and _E. coli_-inspired coordination strategies for robots. In foraging, _E. coli_ performs a biased random walk, which is called bacterial chemotaxis [8]. Indeed, each bacterium moves to the highest (lowest) concentration location of the stimulus source based on the comparison between previous memory concentration and the current ones. Actually, it is likely that most of the time, the bacteria exhibit swarming behavior through interactions. Chemotaxis behavior of _E. coli_ can serve as an inspiration source for coordination strategies of swarm robots. One of the potential advantages is that _E. coli_-inspired coordination strategy does not require the robot to have a complex physical structure to reduce the production cost, and it also contributes to the scalability of the swarm robotic system. The other advantage is that _E. coli_ does not need the long-term memory function of a particle in PSO [9]. This means that the physical robot does not need a large storage unit. Another great potential advantage is the application in the medical field. Various microrobots have been developed in micro-assembly, micromanipulation, and drug delivery applications [10, 11, 12]. In general, the main control strategy for robots depends on the motion response of remotely controlled magnetic fields. Magnetotactic bacteria, which are controlled by magnetic fields, are usedto manipulate microparticles to assemble large structures [10]. Swarming microrobots have been applied in micromanipulation inspired behaviors of bacteria [11]. The collective behavior of micro-robotic swarms is obtained by magnetic interactions (attractive and repulsive) among individual agents [12]. Hence, applying bacteria-inspired coordination strategies to the macro and micro swarm robotic systems is of great significance. In recent decades, existing chemotaxis-inspired control works have focused on a single robot mimicking chemotaxis behavior and applied it to source search tasks [13; 14; 15; 16; 17; 18], where the chemotaxis-inspired control strategies are used to steer a single robot to the extrema of an unknown gradient field. Meanwhile, for applications of swarm robotic systems, some tasks, such as path planning, task allocation, are generally converted to optimization problems. The bacteria-inspired methods are served as optimization algorithms [19; 20; 21; 22]. However, there is no direct mapping between individual robot and bacterium in bacteria-inspired optimization algorithms, because the algorithms involve operations, such as reproduction and elimination, which would be impractical to implement in a real robot. Furthermore, there is a lack of research on coordination mechanisms between robots inspired by bacteria. Thus, we aim to propose a bacteria-inspired coordination strategy for a swarm robotic system inspired by the chemotaxis of _E. coli_, in which individual robot maps a bacterium. We consider two swarm robotic tasks: Coverage and aggregation. Coverage is to uniformly deploy robots on a given bounded space without central coordination [23; 24], as is illustrated in Figure 1a. The potential practical applications include mapping of unknown environments, environment exploration, and autonomous surveillance systems. Aggregation is to guide a number of autonomous robots to rendezvous in a common place [24], as shown in Figure 1b, which is a basic behavior amongst biological creatures, such as cockroaches [25], bacteria [26].