1 Active Exploitation of Redundancies in Reconfigurable Multi-Robot Systems Thomas M. Roehr1 Abstract—While traditional robotic systems come with a existing hardware is a significant advantage, even more when monolithic system design, reconfigurable multi-robot systems a team of robots can perform this process autonomously. can share and shift physical resources in an on-demand fash- The autonomous exploitation of features of a reconfigurable ion. Multi-robot operations can benefit from this flexibility by actively managing system redundancies depending on cur- multi-robot system is, however, a significant challenge in- rent tasks and having more options to respond to failure cluding practical issues regarding distributed communication events. To support this active exploitation of redundancies in and planning approaches. Additionally, an application in a robotic systems, this paper details an organization model as space context requires risk mitigation strategies and high safety basis for planning with reconfigurable multi-robot systems. standards. Therefore, enabling planning approaches that permit The model allows to exploit redundancies when optimizing a multi-robot system’s probability of survival with respect to a to exploit redundancy and sharing of resources between robotic desired mission. The resulting planning approach trades safety systems will be one step forward towards safe long-term against efficiency in robotic operations and thereby offers a operations of autonomous multi-robot systems. By introducing new perspective and tool to design and improve multi-robot a Model for Reconfigurable Multi-Robot Organisations (More- missions. We use a simulated multi-robot planetary exploration Org) in this paper, we offer a modelling approach with focus mission to evaluate this approach and highlight an exemplary performance landscape. Our implementation of the organiza- on (physically) reconfigurable multi-robot systems, although tion model is open-source available (https://github.com/rock- the model can embed classical non-reconfigurable robots. This knowledge-reasoning/knowledge-reasoning-moreorg). enables a planning approach which exploits reconfigurability Index Terms—Multi-Robot Systems; Planning, Scheduling and as described in Section IV. The focus of this paper is, however, Coordination; Space Robotics and Automation; Reconfigurable on the design and role of the organization model in this Robots context. The organization modeling and planning approach for re- I. INTRODUCTION configurable multi-robot systems finds its initial motivation in space applications. In this paper we also refer to this ECONFIGURABLE multi-robot systems introduce a context to provide an exemplary use of our suggested mod- R new dimension to the design of future robot missions elling approach. Meanwhile, the organization model builds on since they permit robots to exchange physical subsystems. ontologies and is therefore extensible , so that users can add This flexibility to shift subsystems can be exploited to actively new subsystems, functionalities, robots, custom properties and manage the level of redundancy of individual robots. This inference rules by extending the ontological description. is especially interesting costly planetary space operations, which require highly redundant robots. The state of the art in planetary space missions are, however, single robotic systems. A. History and Related Work Despite the fact that international space agencies operate Implementations of reconfigurable multi-robot systems exist with multiple rovers on the same planet, cooperation between within a spectrum ranging from industrial robots which allow arXiv:2106.12079v1 [cs.RO] 22 Jun 2021 these system has not been targeted. With the consideration of an end effector exchange to fully self-reconfigurable multi- building up infrastructure, creating habitats to prepare human robots systems [5, 16, 8]. Research in the area of recon- presence and supporting safe operations, this paradigm will figurable multi-robot systems has initially been driven by have to shift. the latter, i.e., the concept of so-called self-reconfigurable Current planetary space operations have to rely on ground systems. Their main design characteristic is a high-level of operators for adaptations and repair, which leads to a very redundancy of mostly homogeneous modules, which can au- slow and costly process. The dependency on earth-based tomatically restructure to establish a target structure; a broad maintenance, or even hardware deliveries should be minimized for future long-term space mission to achieve a An incremental mission design offers an alternative and the concept is depicted in Figure 1. Here, not only software, but also hardware subsystems can evolve with the experience made in previ- ous missions and incrementally improve already operating multi-agent systems. The possibility to extend or refurbish 1DFKI GmbH Robotics Innovation Center Bremen, Robert-Hooke-Str. 1, Fig. 1. Schematic description of an incremental design of planetary space 28359 Bremen, Germany, correspondence: [email protected] missions using reconfigurable multi-robot systems. 2 review of self-reconfigurable multi-robot systems is provided can endure impacts up to a certain degree without breaking, by Chennareddy, Agrawal, and Karuppiah [6] and Liu, Zhang, resilience results from the ability to recover from error and and Hao [17]. Self-reconfiguration aims a providing highly return into a functional state. resilient systems, i.e., being able to recover from disrup- Especially resilience is a key to survival, and not only in tive (structural) changes and failures. These highly-modular natural systems, but for technical and social systems alike systems typically suffer, however, from limited capabilities shown by examples collected from Zolli and Healy [36]. and thus lack broad applicability. Planning approaches in Resilient systems, however, rely on their capability to adapt. this context focus on the transition between two organization Therefore, reconfigurability can contribute to an increased states, e.g., Baca et al. [1] apply coalition game theory to resilience of robotic systems. Evans’ conceptual framework optimize the organizational state. They characterize coalitions, is general enough to be applied to reconfigurable multi-robot however, based on strong assumptions with a utility function systems, and his categorization of maneuvers can be similarly which (a) has a static utility for each agent independent of the applied for a characterization of robotic activities: protective coalition it will be embedded into, and (b) cannot account for and corrective activities count as defensive maneuvers. interface compatibility issues leading to constrained coalition The design of a space robots is typically focusing on formation. The swarm-bot system developed by Mondada et al. defensive measures by adding redundancies and preparing fail- [18] initially takes a middle ground and uses simple structured, ure handling strategies. What the flexibility of reconfigurable yet reconfigurable swarm-based system in combination with a multi-robot systems offers, however, is the possibility for an behavior-based control approach to exploit reconfigurability. active management of these redundancies, for instance, to They are able to illustrate team capabilities that arise from adapt the organization to respond to functional requirements or superaddition such as gap and obstacle traversal as well as to optimize the redundancy level across all available systems. (heavy) object transport. An active management with a global optimization policy will Similarly to the swarm-bot system, our work targets re- treat all resources equally. This means, that the controller configurable multi-robot systems which consist of individual of a robotic mission can influence the level of resources agents that can already be considered capable robots. Wilcox redundancies. As a side effect active management might even et al. [33], for instance, developed the reconfigurable six- result in an overall cost-optimized system design, by reducing legged robot ATHLETE to support infrastructure build up on the mean redundancy level of the multi-robot system. planetary surfaces. Although Wilcox et al. [33] did approach Continuous optimization of an organization structure can automation of reconfiguration procedures, they did not develop also be found in Model of Organisation for multI-agent high-level planning approaches to fully exploit reconfigurabil- SystEms (MOISE+). Hubner,¨ Sichman, and Boissier focus ity. For similar capable robots reconfigurability focuses on the on a design pattern to control the reconfiguration process adaption of internal subsystems, e.g., the Scarab rover [2] is and identify key components. They outline an architecture able to adapt its locomotion platform. Reid et al. [22] show to continuously optimize an organization structure to achieve how to exploit this kind of reconfigurability with a dedicated main organization’s objectives. Objectives for a (sub)team can sampling-based (motion) planning approach after modeling the be defined as Hierarchical Task Network (HTN) in a so- reconfiguration space of the motion planning system. called scheme, which leads to the definition of a behavioural In the context of organization research outside of the pattern, e.g., they use playing soccer as primary example. robotics domain Dignum [12] looks at reconfiguration of orga- A reconfiguration process or rather transition from one team nizational processes involving