Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> It is suggested that a system of chemical substances, called morphogens, reacting together and diffusing through a tissue, is adequate to account for the main phenomena of morphogenesis. Such a system, although it may originally be quite homogeneous, may later develop a pattern or structure due to an instability of the homogeneous equilibrium, which is triggered off by random disturbances. Such reaction-diffusion systems are considered in some detail in the case of an isolated ring of cells, a mathematically convenient, though biologically unusual system. The investigation is chiefly concerned with the onset of instability. It is found that there are six essentially different forms which this may take. In the most interesting form stationary waves appear on the ring. It is suggested that this might account, for instance, for the tentacle patterns onHydra and for whorled leaves. A system of reactions and diffusion on a sphere is also considered. Such a system appears to account for gastrulation. Another reaction system in two dimensions gives rise to patterns reminiscent of dappling. It is also suggested that stationary waves in two dimensions could account for the phenomena of phyllotaxis. ::: ::: The purpose of this paper is to discuss a possible mechanism by which the genes of a zygote may determine the anatomical structure of the resulting organism. The theory does not make any new hypotheses; it merely suggests that certain well-known physical laws are sufficient to account for many of the facts. The full understanding of the paper requires a good knowledge of mathematics, some biology, and some elementary chemistry. Since readers cannot be expected to be experts in all of these subjects, a number of elementary facts are explained, which can be found in text-books, but whose omission would make the paper difficult reading. <s> BIB001 </s> Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> In insect societies, a number of very striking collective structures are formed by individuals linking themselves to one another. One such example is an army ant bivouac. These structures are termed self-assemblages and are part of a more general and important aspect of insect societies - intermediate-level parts - in which functional group-level adaptive structures are formed. These parts are, in a sense, the tissues and organs of complex insect societies. Here we review the natural history of self-assemblages in insect societies. We find that at least 18 different types of structure exist: bivouacs, bridges, curtains, droplets, escape droplets, festoons, fills, flanges, ladders, ovens, plugs, pulling chains, queen clusters, rafts, swarms, thermoregulatory clusters, tunnels, and walls. These self-assemblages are found in a variety of species of ants, bees, and wasps, but (as far as we are aware) not in termites. The function of these self-assemblages can be grouped under five broad categories which are not mutually exclusive: 1) defence, 2) pulling structures, 3) thermoregulation, 4) colony survival under inclement conditions, and 5) ease of passage when crossing an obstacle. The paucity of our knowledge concerning the factors that favour self-assemblage formation and the likely proximate mechanisms are highlighted. <s> BIB002 </s> Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> A swarm-bot is a robotic entity built of several autonomous mobile robots (called s-bots) physically connected together. This form of collective robotics exploits robot interactions both at the behavioral and physical levels. The goal of this paper is to analyze the physical performance of a swarm-bot as function of its size (number n of s-bots composing it). We present three tasks and the corresponding swarm-bot performances. In all three tasks we show superlinear performances in a range of n where the physical forces applied in the structure fit to the robot design. This superlinear performance range helps in understanding which swarm-bot size is optimal for a given task and gives interesting hints for the design of new application-oriented swarm-bots. <s> BIB003 </s> Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> In this paper, we review half a century of research on the design of systems displaying (physical) self-assembly of macroscopic components. We report on the experience gained in the design of 21 such systems, exhibiting components ranging from passive mechanical parts to mobile robots. We present a taxonomy of the systems and discuss design principles and functions. Finally, we summarize the main achievements and indicate potential directions for future research. <s> BIB004 </s> Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> In certain multi-robot systems, the physical limitations of the individual robots can be overcome using self-assembly—the autonomous creation of physical connections between individual robots to form a larger composite robotic entity. However, existing robotic systems capable of self-assembly have little or no control over the morphology of the self-assembled entities. This restricts the adaptability of such systems, since robots can carry out certain tasks more efficiently if their morphology is specialized to the task. In this paper, we extend the distributed mechanism presented in (Christensen et al. in IEEE Robot. Autom. Mag. 14(4):18–25, 2007) that allows autonomous mobile robots to self-assemble into specific morphologies. We present a simple language, SWARMORPH-script, that allows for concise descriptions of the rules that govern the distributed morphology growth process. Local visual communication allows physically connected robots to send and receive strings. A string can be a rule identifier that triggers execution of predefined logic for extending a morphology. Alternatively, whole scripts can be communicated and subsequently executed on the receiving robot. On real self-propelled robots capable of self-assembly, we demonstrate how specific morphologies can be constructed, how the size of a morphology can be regulated, and how multiple morphologies can be assembled. We also show how the transmission of entire scripts gives the robots the capacity to participate in the formation of morphologies of which they had no a priori knowledge. <s> BIB005 </s> Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> We examine the ability of a swarm robotic system to transport cooperatively objects of different shapes and sizes. We simulate a group of autonomous mobile robots that can physically connect to each other and to the transported object. Controllers – artificial neural networks – are synthesised by an evolutionary algorithm. They are trained to let the robots self-assemble, that is, organise into collective physical structures and transport the object towards a target location. We quantify the performance and the behaviour of the group. We show that the group can cope fairly well with objects of different geometries as well as with sudden changes in the target location. Moreover, we show that larger groups, which are made of up to 16 robots, make possible the transport of heavier objects. Finally, we discuss the limitations of the system in terms of task complexity, scalability and fault tolerance and point out potential directions for future research. <s> BIB006 </s> Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> This research work illustrates an approach to the design of controllers for self-assembling robots in which the self-assembly is initiated and regulated by perceptual cues that are brought forth by the physical robots through their dynamical interactions. More specifically, we present a homogeneous control system that can achieve assembly between two modules (two fully autonomous robots) of a mobile self-reconfigurable system without a priori introduced behavioral or morphological heterogeneities. The controllers are dynamic neural networks evolved in simulation that directly control all the actuators of the two robots. The neurocontrollers cause the dynamic specialization of the robots by allocating roles between them based solely on their interaction. We show that the best evolved controller proves to be successful when tested on a real hardware platform, the swarm-bot. The performance achieved is similar to the one achieved by existing modular or behavior-based approaches, also due to the effect of an emergent recovery mechanism that was neither explicitly rewarded by the fitness function, nor observed during the evolutionary simulation. Our results suggest that direct access to the orientations or intentions of the other agents is not a necessary condition for robot coordination: Our robots coordinate without direct or explicit communication, contrary to what is assumed by most research works in collective robotics. This work also contributes to strengthening the evidence that evolutionary robotics is a design methodology that can tackle real-world tasks demanding fine sensory-motor coordination. <s> BIB007 </s> Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> Robots are said to be capable of self-assembly when they can autonomously form physical connections with each other. By examining different ways in which a system can use self-assembly (i.e., different strategies), we demonstrate and quantify the performance costs and benefits of (i) acting as a physically larger self-assembled entity, (ii) letting the system choose when and if to self-assemble, (iii) coordinating the sensing and actuation of the connected robots so that they respond to the environment as a single collective entity. Our analysis is primarily based on real world experiments in a hill crossing task. The configuration of the hill is not known by the robots in advance--the hill can be present or absent, and can vary in steepness and orientation. In some configurations, the robots can overcome the hill more quickly by navigating individually, while other configurations require the robots to self-assemble to overcome the hill. We demonstrate the applicability of our self-assembly strategies to two other tasks--hole crossing and robot rescue--for which we present further proof-of-concept experiments with real robots. <s> BIB008 </s> Swarm robotics: a review from the swarm engineering perspective <s> Self-assembly and morphogenesis <s> We introduce spatially targeted communication - a communication method for multirobot systems. This method allows an individual message sending robot to isolate selected message recipient robots based on their spatial location. The recipient robots can then be sent information targeted solely at them, even if the sending robot uses a broadcast communication modality. We demonstrate spatially targeted communication using a heterogeneous multirobot system composed of flying robots and ground-based self-assembling robots. Flying robots use their privileged view of the environment to determine and communicate information to groups of ground-based robots on what morphologies to form to carry out upcoming tasks. <s> BIB009
Description In robotics, self-assembly is the process by which robots physically connect to each other. Self-assembly can be used for different purposes. For example, to increase stability when navigating on rough terrains or to increase the pulling power of the robots. Morphogenesis is the process that leads a swarm of robots to self-assemble following a particular pattern, and can be used by the swarm to self-assemble into a structure that is particularly appropriate for a given task. For example, self-assembling into a line can allow to pass on a narrow bridge, while a blob-like shape will make moving on rough terrain more stable. Source of inspiration Self-assembly can be observed in several species of ants. Ants are able to physically connect in order to perform different tasks. Some examples of structures Fig. 7 Examples of the self-assembly collective behavior created by ants are bridges, rafts, walls and bivouacs BIB002 . Self-assembly and morphogenesis are studied also by developmental biology: scientists study how cells develop and self-organize to form tissues and organs BIB001 . Approaches From the swarm robotics perspective, there are two main challenges: how to self-assemble into a desired target structure (i.e., morphogenesis), and how to control the obtained structure to tackle specific tasks. Works focusing on the first issue are usually based on probabilistic finite state machines and rely on communication for coordination. Works focusing on the second issue, make use either of artificial evolution or of probabilistic finite state machines. Results A review of the literature on self-assembly and morphogenesis has already been presented by BIB004 . For this reason, in this paper, we discuss only some examples of recent works. O'Grady et al. (2009a) presented a morphogenesis behavior for self-assembling robots that are able to signal docking points on their body to other robots using LEDs. Different structures, such as lines, stars and circles, can be obtained by having the robots signal docking points in different positions (see Fig. 7(a) ). A related scripting language for the morphogenesis process has been presented by BIB005 . Both works were realized in the context of the Swarm-Bots project . Results on the control aspect of self-assembly depend strongly on the goal of the system considered. BIB008 demonstrated that physically connected robots can navigate through difficult terrains better than robots that are not connected. In O'Grady et al.'s work, robots randomly explore an environment with slopes. Each robot is able to measure the steepness of these slopes and when a slope is steeper than a certain threshold, it can initiate a self-assembling procedure. Once connected into a structure, the robots can navigate in hazardous terrains thanks to the high mechanical stability given by the new morphology. BIB003 showed that physically connected robots are able to cross a ditch that is too large for a single robot to overcome (see Fig. 7(b) ). Finally BIB006 showed that physically connected robots are able to obtain better results, in terms of speed and distance, in the transportation of heavy objects compared to non-connected robots. The Symbrion and Replicator projects tackled both the morphogenesis and the control aspects of self-assembly  . In these projects, swarms of selfassembling robots capable of creating 3D structures are studied. Such robots are able, when connected, to share energy and computational resources with their neighbors. Another aspect of self-assembly is how to make the swarm decide who should assemble with whom. In the work of BIB007 , a duo of robots has to assemble to each other without prior knowledge of who will grip and who will be gripped. The authors proposed a solution based on artificial evolution and recurrent neural networks which can make time-dependent decisions. BIB009 used a heterogeneous approach: a flying robot is used to recognize the task to tackle and guide ground-based robots. The flying robot communicates to the ground-based robots which robots should self-assemble and what kind of structure to create to tackle the task.