An Autonomous Multi-Robot System for Stigmergy-Based Construction Michael Allwright A dissertation submitted to the Department of Computer Science Paderborn University for the degree of Doktor der Naturwissenschaften (doctor rerum naturalium) September 8, 2017 Thesis supervisor: Prof. Marco Dorigo, Ph.D. Thesis co-supervisor: Dr. Navneet Bhalla, Ph.D. Swarm Intelligence Group Department of Computer Science Paderborn University Heinz Nixdorf Institute Fürstenallee 11 33102 Paderborn, Germany Phone +49 5251 60-6200 [email protected] www.hni.uni-paderborn.de Abstract Original version in English An autonomous construction system (ACS) is envisioned to be a solution to the con- struction of structures in environments that are too hazardous for humans and where remote operation is not possible. Drawing inspiration from the construction behavior of social insects, we advance the state of the art by proposing a completely decentral- ized control strategy for a multi-robot ACS. This control strategy is based on previous work by Theraulaz and Bonabeau, who demonstrated multi-agent construction using a three-dimensional lattice-based simulation. In order to study the proposed decentralized control strategy, we set about designing a multi-robot ACS that consists of two components, a building material and autonomous robots. In our multi-robot ACS, an autonomous robot is capable of locating the build- ing material in its environment, picking it up, transporting it, and attaching it to a structure. The multi-robot ACS is designed to be completely autonomous and capable of constructing three-dimensional structures. To test and evaluate our multi-robot ACS and its decentralized control strategy, we provide two implementations of our system. The primary implementation is realized using hardware and the secondary implementa- tion is realized using simulation. Through the use of both implementations, we demonstrate how our decentralized con- trol strategy can be used to coordinate the autonomous construction of three-dimensional structures. After discussing these demonstrations, we conclude this thesis by suggesting future research directions. Übersetzung ins Deutsche Ein autonomes Bausystem ist eine Lösung für den Bau von Strukturen in Umgebun- gen, die für Menschen zu gefährlich ist und wo auch das Fernbedienen nicht möglich ist. Inspiriert durch das Bauverhalten von sozialen Insekten, treiben wir den Stand der Technik voran und schlagen eine vollständig dezentralisierte Steuerungsstrategie für ein Multi-Roboter-Bausystem vor. Diese Vorgehensweise basiert auf der bisherigen Ar- beit von Theraulaz und Bonabeau, die eine dreidimensionale Gitter-basierte Simulation verwendet haben um Bauverfahren mit einem Multiagentensystem zu demonstrieren. Um die vorgeschlagene dezentralisierte Steuerungsstrategie zu untersuchen, wurde ein Multi-Roboter-Bausystem entwickelt. Dieses Bausystem besteht aus zwei Kompo- ii Abstract nenten: dem Baustof und autonomen Robotern. Ein autonomer Roboter in unserem Multi-Roboter-Bausystem ist in der lage den Baustof in seiner Umgebung zu lokalisieren, aufzuheben, zu transportieren, und an einer Struktur anzubringen. Das Multi-Roboter- Bausystem wurde mit dem Schwerpunkt auf ein völlig autonomes Bauen von dreidimen- sionalen Strukturen entwickelt. Um unser Multi-Roboter-Bausystem und seine dezen- tralisierte Steuerungsstrategie zu testen und zu bewerten, werden zwei Implementierun- gen vorgestellt: Eine Hardware-basierte Realisierung und eine Implementierung mittels Simulation. Durch den Einsatz beider Implementierungen wird demonstriert, wie unsere dezen- tralisierte Steuerungsstrategie autonome Konstruktion von dreidimensionalen Strukturen koordinieren kann. Nach der Diskussion dieser Demonstrationen schließen wir diese Ar- beit mit Ausblick auf zukünftige Forschungsmöglichkeiten ab. Acknowledgements The research presented in this thesis was completed under the supervision of Marco Dorigo and Navneet Bhalla. I am sincerely grateful to Marco and Navneet for their time and continuous support over the past years while this research was being undertaken. Their support has assisted me with keeping the research on track, formulating my ideas clearly, and preparing content for submission in highly regarded journals and conferences. The simulation work in this thesis was made possible by Carlo Pinciroli, who assisted me with his ongoing development of the ARGoS simulator. I am also very grateful to Haitham El-faham, who completed the mechanical design for a prototype of the autonomous robot’s manipulator as part of his Master’s thesis. Furthermore, I would like to thank Haitham El-faham for developing the magnetism plugin for the ARGoS simulator. I am also grateful to Anthony Antoun, with whom I collaborated to create the irst prototypes of the stigmergic block. Furthermore, I would like to thank Christoph Scheytt and Uwe von der Ahe from the System and Circuit Technology Research Group and Eric Klemp and Michael Brand from the Direct Manufacturing Research Center (DMRC) at Paderborn University for their technical support and for providing access to the facilities which were used to evaluate the irst prototypes of the hardware described in this thesis. I greatly appreciate the assistance from Juergen Maniera at Paderborn University, who supported this research by organizing our budget, tracking our purchase orders, and managing our outsourced manufacturing requests. I would also like to thank the Inter- national Graduate School of Dynamic Intelligent Systems and, in particular, Eckhard Stefen and Astrid Canisius for the organization and support with the administrative aspects of the program at Paderborn University. Finally, I am very grateful to my mother, Rosemarie Allwright, who raised me to always ask questions, nurtured my curiosity about the world, and continuously supported me from the very beginning. Contents Abstract i Acknowledgements iii Contents v 1 Introduction 1 1.1 Construction by autonomous robots . 1 1.2 Construction in nature . 5 1.3 Problem statement . 6 1.4 Contributions and publications . 9 1.5 Thesis structure . 12 2 Related Work in Multi-Robot Construction 13 2.1 Overview . 13 2.2 Construction using centralized infrastructure . 15 2.3 Construction without centralized infrastructure . 22 2.4 Summary . 33 3 System Architecture 41 3.1 Design of a multi-robot ACS . 41 3.2 Adaptation of a decentralized control strategy . 42 3.3 Architecture of our multi-robot ACS . 45 3.4 Summary . 49 4 Implementation of the Hardware 51 4.1 Stigmergic blocks . 51 4.2 Autonomous robots . 57 4.3 Veriication of the hardware . 75 5 Implementation of the Simulation 79 5.1 The ARGoS simulator . 79 5.2 Extensions to the ARGoS simulator . 80 5.3 Modeling hardware in simulation . 90 5.4 Veriication of the simulation . 94 vi Contents 6 Results and Demonstrations 97 6.1 Demonstrations using hardware . 97 6.2 Demonstrations in simulation . 106 6.3 Discussion . 109 6.4 Summary . 113 7 Conclusion 115 7.1 Overview . 115 7.2 Future work . 116 7.3 Summary . 120 A Description of a Stigmergic Block in ARGoS 123 B Description of an Autonomous Robot in ARGoS 127 Bibliography 133 CHAPTER 1 Introduction 1.1 Construction by autonomous robots 1.1.1 Motivation Robotic construction systems are envisioned to be one solution for building permanent and temporary structures in environments that are too hazardous for humans and as such, are an area of research of NASA’s Jet Propulsion Laboratory [31–33, 90–92]. Such construction systems could be autonomous or could be remotely operated by a human. While an autonomous construction system (ACS) is more complex to design and pro- gram, it is able to function without a human operator. This autonomy is necessary in environments where wireless signals can not penetrate (e.g. the ocean loor, or in un- derground mines) or where the latency of the signal prevents real-time operation (e.g. between Earth and a non-terrestrial body). 1.1.2 Classiication of ACS conigurations We classify the coniguration of an ACS using the following criteria: the number of types of robots used; the total number of robots performing construction; and the type of build- ing material. We categorize building materials as either passive, semi-active, or active. In this work, we deine passive building materials as neither containing electronics nor being capable of locomotion. Furthermore, we deine semi-active building materials as containing electronics but not being capable of locomotion, and active building materi- als as containing electronics and being capable of locomotion. There are three common ACS conigurations: Single-robot ACS: consists of a single robot that arranges passive or semi-active build- ing materials into structures. Self-assembly ACS: consists of active building materials (or robots) that rearrange themselves into structures1. Multi-robot ACS: consists of multiple robots that cooperate to arrange building ma- terials into structures. The building materials may be passive or semi-active. 1We restrict our deinition of a self-assembly ACS to only cover systems that use active building materials, i.e. building materials that are capable of locomotion. 2 1 Introduction Coniguration Advantages Disadvantages Single-robot ACS • Less complex • Low redundancy (robot is to develop and program an SPOF) • Low unit cost when scaling to larger structures Self-assembly ACS • High redundancy • High unit cost when scaling to larger structures • Highly complex to develop and program Multi-robot ACS • Moderate redundancy • Moderately complex