Multi-Robot Cooperation in Space: A Survey Jurgen¨ Leitner Department of Automation and Systems Technology Helsinki University of Technology, PL 5500, 02015 TKK, Finland Email: [email protected].fi Abstract—This paper reviews the literature related to multi-robot topic in political science and other human sciences, e.g. Axelrod research with a focus on space applications. It starts by examining & Hamilton in 1981 [6] published a work on the famous prisoner’s definitions of, and some of the fields of research, in multi-robot systems. dilemma. An overview of space applications with multiple robots and cooperating multiple robots is presented. The multi-robot cooperation techniques Cooperating behaviours are a subset of collective behaviours, in used in theoretical research as well as experiments are reviewed, and which the cooperation can be manifold and usually is not clearly the applicability for space applications is investigated. defined. Examples of cooperating in nature (e.g. bees and ants) Keywords-Cooperative systems, Satellite applications, Space Vehicle show possibilities for ”simple” robots to work together to solve Control, Intelligent robots, Intelligent systems, Mobile robots a very complex task. The mechanism of ‘cooperation’ may be incorporated into the system in various ways, by dynamics, by I. INTRODUCTION design or by accident. This survey will present multi-robot systems and especially In general there exist two groups of cooperation: how those systems incorporate cooperation. It includes a short Passive Cooperation: The robots do not use explicit communi- discussion of taxonomy in multi-robot systems; a more thorough cation, the cooperation appears only when the whole system is overview and background information can be found in [1], [2]. observed (sometimes named emergent cooperation or behaviour). The main focus of this review are multi-robot systems and their One example are robots that sense each other only as obstacles applications in areas where cooperation and collaboration between and plan their way around these. The decision making and action robots is found. A special focus is placed on space applications. planning is local only and not communicated to the other agents. Section II will give a short introduction to multi-robot systems, Active Cooperation: A communication link is used for cooper- a definition of cooperation and an overview of the taxonomy used ation, where agents may be actively coordinating their decision- in the literature. making and actions. This does not necessarily mean radio or Section III focuses on literature describing multi-robot systems (wired) electronic communication, including also other sorts of in (mainly planned) space applications. Examples of the few current communication and communication via the environment. applications of multiple spacecrafts working together will be listed. A special case of active cooperation is the case of tight coop- Section IV gives a short round-up and conclusion. eration, in which the robots need to coordinate in very detail the action they are going to perform, e.g. cooperative construction and II. MULTI-ROBOT COOPERATION transportation [3], [7], [8]. A. Introduction Though there has been a lot of theoretical research in this field, Multi-robot systems have been of interest to researchers for a experimental and real world implementations have only recently long time and the topic has become more and more interesting started to emerge. There are various reasons for this, including over recent years and an increasing amount of research is done communication costs and problems, unreliability and sensor noise today in the field of robot cooperation. in the real world [9]. A good summary with good reasoning why to chose multi-robot Multi-robot systems have the potential to perform better than systems over a single robot can be found in [3], the main reasons single robots in a variety of fields, but it has been seen that only for choosing a multi-robot system over a single-robot are also clearly designed multi-robot systems achieve a good performance. presented in [1]–[4]. More research is needed to make those systems use cooperation as ubiquitously as it appears in nature. B. What is Cooperation? Relevant fields of research are: Distributed Artificial Intelligence The Oxford English Dictionary (ODE) defines “to cooperate” as (DAI), multi-robot systems, which in turn relies heavily on the “to work together, act in conjunction (with another person or thing, research done in Multi-Agent Systems (MAS), high-level control to an end or purpose, or in a work)”. In robotics cooperation is and theoretical computer science. Similarities to problems in those not very often explicitly defined and the few definitions tend to be fields suggest that techniques and solutions found there can be very broad, some include communication, some progressive results applied in the area of multi-robot cooperation. (e.g. increasing performance). The few exceptions are listed in [2]. Cooperative and collaborative robotics started with the introduction C. Taxonomy of behaviour-based control into robotics. This paradigm is bio- There are various terms, most of them not clearly and uniquely logically inspired and encouraged researchers to find cooperating defined, that describe multi-robot systems. systems in nature, which then were used for multi-robot systems As defined by Dudek et al. [1], multi-robot systems can be [5]. Cooperation is also a very long and much discussed research classified with the following taxonomy: Problem-based Classification: Depending on the task multi- In publications of multi-robot systems for space applications very robot systems might be a better choice than a single robot [1]. The often humans are included as members of the team, working closely groups are defined by Tasks that: (a) require multiple agents, (b) together with the robots to complete the explorative tasks. Areas are traditionally multi-agent, (c) are inherently single agent, and of interest in research regarding this are human robot interaction (d) may benefit from multiple agents. [14] and sliding autonomy [3], [15], [16]. Size of the Collective: single robot (SIZE-ALONE), a minimal- ist multi-robot system (SIZE-PAIR), a limited amount of multiple A. Planned Missions and Visions robots (SIZE-LIM) and an infinite (very large compared to the Several space missions, where multiple mobile robots play a problem) amount of robots (SIZE-INF) (used for huge wireless central role, are currently proposed. The research and funding of sensor networks or robot swarms). those areas has increased in the last years, mainly due to the Communication: Interaction via environment (no direct com- above mentioned exploration visions announced by various space munication) (COM-NONE), interaction via sensing (local only) agencies [17]–[19]. Some of these missions are presented here. (COM-NEAR), interaction via a communication link (wide area) 1) In-Orbit Operation and Satellite Formations: Many multi- (COM-INF). A detailed description can be found in [2]. satellite applications, especially in cooperation, are envisaged but Reconfigurability: systems without reconfiguration abilities very few are planned or even partly funded. The main focus in (ARR-STATIC), coordinated rearrangement (e.g. change of for- research is currently on optimizing formation flying (with respect mation),(ARR-COOR) and dynamic arrangement (ARR-DYN). to fuel usage) and on-orbit servicing, which might also help the Composition: homogeneous (CMP-HOM) and heterogeneous development of in-orbit construction for larger structures. (CMP-HET) systems. Another possibility are marsupial systems In the field of simulation [20] proposed a trajectory/path planning that have a homogenous group of small robots, that can be technique based on dynamic networks, with simulation in 3D transported by a ”mothership” [10], [11]; therefore CMP-MAR is for use with satellites. These systems are only in the very early introduced. development stage and do not provide optimizations of, e.g. fuel Control: Centralized (CTL-CEN), decentralized (CTL-DEC) consumption. For simulation and testing purposes the MIT has and hybrid (CTL-HYB) architectures exist. created a satellite testbed, the Free-Flying Robot Testbed (FFRT), These classifications and terms described here will be used in to verify planning and control algorithms experimentally. It allows the rest of this review when describing the other publications, the for a 2D simulation of micro-gravity satellite control using air- research and the projects presented. bearings [21]. On-Orbit Servicing (OOS): On-orbit servicing is an increas- III. SPACE APPLICATIONS ingly interesting field in space applications. Some tests of servicing Using multiple, modular and reconfigurable robots has a few systems have already been performed, but those spacecraft usually possible advantages in space, where the systems have very strict have only passive cooperation. No direct communication between requirements. These advantages range from saving weight (used the servicing and the to-be-serviced craft are used. as multiple tools), compressed form (saving space) to increasing A European consortium of space companies proposed the HER- robustness (increasing redundancy). Being light-weight is important MES OOS system. It is planned to use fuel from damaged, since the weight is directly proportional to the cost of launching overloaded satellites as well as their fail-safe fuel at EOL and and deploying the system into space, hence smaller size is better