Publications

2021
Dyke, Shirley J., Karen Marais, Ilias Bilionis, Justin Werfel, and Ramesh Malla. 2021. “Strategies for the design and operation of resilient extraterrestrial habitats.” SPIE Smart Structures + Nondestructive Evaluation Conference. Abstract
An Earth-independent permanent extraterrestrial habitat system must function as intended under continuous disruptive conditions, and with significantly limited Earth support and extended uncrewed periods. Designing for the demands that extreme environments such as wild temperature fluctuations, galactic cosmic rays, destructive dust, meteoroid impacts (direct or indirect), vibrations, and solar particle events, will place on long-term deep space habitats represents one of the greatest challenges in this endeavor. This context necessitates that we establish the know-how and technologies to build habitat systems that are resilient. Resilience is not simply robustness or redundancy: it is a system property that accounts for both anticipated and unanticipated disruptions via the design choices and maintenance processes, and adapts to them in operation. We currently lack the frameworks and technologies needed to achieve a high level of resilience in a habitat system. The Resilient ExtraTerrestrial Habitats Institute (RETHi) has the mission of leveraging existing novel technologies to provide situational awareness and autonomy to enable the design of habitats that are able to adapt, absorb and rapidly recover from expected and unexpected disruptions. We are establishing both fully virtual and coupled physical-virtual simulation capabilities that will enable us to explore a wide range of potential deep space SmartHab configurations and operating modes.
Carey, Nicole E., Paul Bardunias, Radhika Nagpal, and Justin Werfel. 2021. “Validating a termite-inspired construction coordination mechanism using an autonomous robot.” Frontiers in Robotics and AI. Abstract
Many species of termites build large, structurally complex mounds, and the mechanisms behind this coordinated construction have been a longstanding topic of investigation. Recent work has suggested that humidity may play a key role in the mound expansion of savannah-dwelling Macrotermes species: termites preferentially deposit soil on the mound surface at the boundary of the high-humidity region characteristic of the mound interior, implying a coordination mechanism through environmental feedback where addition of wet soil influences the humidity profile and vice versa. Here we test this potential mechanism physically using a robotic system. Local humidity measurements provide a cue for material deposition. As the analogue of the termite's deposition of wet soil and corresponding local increase in humidity, the robot drips water onto an absorbent substrate as it moves. Results show that the robot extends a semi-enclosed area outward when air is undisturbed, but closes it off when air is disturbed by an external fan, consistent with termite building activity in still vs. windy conditions. This result demonstrates an example of adaptive construction patterns arising from the proposed coordination mechanism, and supports the hypothesis that such a mechanism operates in termites.
Melenbrink, Nathan, Ariel Wang, and Justin Werfel. 2021. “An Autonomous Vault-Building Robot System for Creating Spanning Structures.” IEEE International Conference on Robotics and Automation. Xi'an, China. Abstract
Research in autonomous robots for construction has largely focused on ground-based robots whose reach constrains the size of what they can build, or on climbing or aerial robots that build solid or unroofed structures. Autonomous construction of larger, multistory buildings, or bridges spanning unsupported distances, would require robots that build sturdy structures supporting their own weight. In this paper, we present VaultBot, a system of autonomous robots that build a load-bearing spanning vault using identical modular blocks. The custom blocks employ mechanical and other features to facilitate robotic manipulation and locomotion, and can be removed from and replaced in an assembled structure as a way of repairing damage. We characterize the system's performance and failure modes, and demonstrate reliable autonomous assembly for a structure composed of 46 blocks. Blocks can be made collapsible and deployable as a way of reducing mass and volume that must be transported to a construction site. Such a system could be used to help enable construction of protective shelters in challenging environments, such as disaster relief scenarios, arctic settings, or extraterrestrial habitats.
Carey, Nicole E., and Justin Werfel. 2021. “Collective Transport of Unconstrained Objects via Implicit Coordination and Adaptive Compliance.” IEEE International Conference on Robotics and Automation. Xi'an, China. Abstract
We present a decentralized control algorithm for robots to aid in carrying an unknown load. Coordination occurs solely through sensing of the forces on or movement of the shared load. Robots prevent undesired motion of the load while permitting movement in the task-relevant subspace, and stabilize against unexpected events by a transient decrease in compliance. The algorithm requires no direct communication between agents, and minimal knowledge of the system or task. We demonstrate the approach in simulation using a commercially available compliant robotic platform.
2020
Melenbrink, Nathan, Katja Rinderspacher, Achim Menges, and Justin Werfel. 2020. “Autonomous anchoring for robotic construction.” Automation in Construction 120: 103391. Abstract
Advances in construction automation have tended to focus on either automating conventional earthmoving equipment or on the discrete assembly of superstructure elements. Neither paradigm has addressed anchoring introduced material into soil, a critical task for virtually all useful structures. Simple anchoring can be achieved by driving posts (discrete linear elements) or sheet piles (interlocking profiles) into the ground, serving as a foundation for a later superstructure. In this paper we present Romu, a wheeled robot that uses a combination of a vibratory hammer and its own body mass to drive both posts and piles into the ground. We report on the effects of hardware parameters on pile driving performance, and demonstrate operation in both controlled and natural environments. Romu is first configured to drive interlocking sheet piles. In addition to their utility as foundations, such walls could be useful directly as check dams, interventions used to prevent erosion and promote groundwater recharge in arid regions. We use simulations based on real-world terrains to explore the potential impact of a fleet of such robots deployed over a large watershed region, using a simple reactive approach to dynamically determine dam placement. Romu is then configured to drive a range of readily available building materials that commonly serve as posts. These include wooden slats that can be used for sand fencing, an intervention used to collect wind-blown sand to build barrier dunes. Post driving performance is characterized for a range of materials, and finally the use case of sand fencing is evaluated using physical tests at 1:10 scale in order to predict its potential impact. To broaden the utility of such robots in field settings, directions for future work include refinement of the hardware for improved operation in more terrains, increased capabilities for fuller autonomy, and integration with other construction tasks for more complex projects.
Bardunias, Paul, Daniel S. Calovi, Nicole Carey, Rupert Soar, J. Scott Turner, Radhika Nagpal, and Justin Werfel. 2020. “The extension of internal humidity levels beyond the soil surface facilitates mound expansion in Macrotermes.” Proceedings of the Royal Society B 287: 20200894. Abstract
Termites in the genus Macrotermes construct large-scale soil mounds above their nests. The classic explanation for how termites coordinate their labour to build the mound, based on a putative cement pheromone, has recently been called into question. Here we present evidence for an alternate interpretation based on sensing humidity. The high humidity characteristic of the mound internal environment extends a short distance into the low-humidity external world, in a "bubble" that can be disrupted by external factors like wind. Termites transport more soil mass into on-mound reservoirs when shielded from water loss through evaporation, and into experimental arenas when relative humidity is held at a high value. These results suggest that the interface between internal and external conditions may serve as a template for mound expansion, with workers moving freely within a zone of high humidity and depositing soil at its edge. Such deposition of additional moist soil will increase local humidity, in a feedback loop allowing the "interior" zone to progress further outward and lead to mound expansion.
Gershenson, Carlos, Vito Trianni, Justin Werfel, and Hiroki Sayama. 2020. “Self-Organization and Artificial Life.” Artificial Life 26 (3): 391-408. Abstract
Self-organization can be broadly defined as the ability of a system to display ordered spatio-temporal patterns solely as the result of the interactions among the system components. Processes of this kind characterize both living and artificial systems, making self-organization a concept that is at the basis of several disciplines, from physics to biology and engineering. Placed at the frontiers between disciplines, Artificial Life (ALife) has heavily borrowed concepts and tools from the study of self-organization, providing mechanistic interpretations of life-like phenomena as well as useful constructivist approaches to artificial system design. Despite its broad usage within ALife, the concept of self-organization has been often excessively stretched or misinterpreted, calling for a clarification that could help with tracing the borders between what can and cannot be considered self-organization. In this review, we discuss the fundamental aspects of self-organization and list the main usages within three primary ALife domains, namely "soft" (mathematical/computational modeling), "hard" (physical robots), and "wet" (chemical/biological systems) ALife. We also provide a classification to locate this research. Finally, we discuss the usefulness of self-organization and related concepts within ALife studies, point to perspectives and challenges for future research, and list open questions. We hope that this work will motivate discussions related to self-organization in ALife and related fields.
Melenbrink, Nathan, Justin Werfel, and Achim Menges. 2020. “On-site autonomous construction robots: Towards unsupervised building.” Automation in Construction 119: 103312. Abstract

Real-world construction projects typically require three groups of tasks: site preparation (earthmoving, leveling), substructure (anchoring, foundations), and superstructure (load-bearing elements, facade, plumbing, wiring, etc.). Advances in construction automation have revealed a gap between industry and academic research, where industry efforts have been focused on automating conventional earthmoving equipment and embracing prefabrication in order to reduce the amount of work that needs to be done on site, while academic efforts have largely concentrated on proposals for on-site additive manufacturing or discrete assembly, which may be of limited applicability to industry. This review presents a broad range of advancements in construction automation research, and finds that achieving fully autonomous construction in unstructured environments will require considerably more development in all three groups of construction tasks, as well as a particular emphasis on coordinating myriad construction tasks between different task-specific robots. Consideration is given to both mature technologies (conventional equipment widely used in industry) and emerging technologies (novel machines designed for autonomy). Key findings from the survey suggest that achieving the goal of fully autonomous construction will require more attention to be paid to site preparation and substructure tasks, material-robot systems (co-designed robots and building materials), embedded sensing, auxiliary construction tasks, and coordinating operations between robot systems. More general lessons from the literature indicate that making incremental improvements to mature technologies may benefit the industry in the short term, but there are considerable limitations to adding autonomy to equipment designed for human operators. Instead, we perceive a demand for novel hardware to be developed for specific tasks, in each case based on fundamental principles and at the appropriate scale, as well as for an increase in interdisciplinary research. We suggest that the reported shortage of skilled labor in the industry can be met with an increased emphasis on training for leveraging advances in automation.

2019
Melenbrink, Nathan, and Justin Werfel. 2019. “A Swarm Robot Ecosystem For Autonomous Construction, 2017.” Robotic Building: Architecture in the Age of Automation, edited by Gilles Retsin, Manuel Jimenez, Mollie Claypool, and Vicente Soler, 88-90. München: DETAIL.
Carey, Nicole E., Daniel S. Calovi, Paul M. Bardunias, J. Scott Turner, Radhika Nagpal, and Justin Werfel. 2019. “Differential construction response to humidity by related species of mound-building termites.” Journal of Experimental Biology 222: jeb212274. Abstract
Macrotermes michaelseni and M. natalensis are two morphologically similar species occupying the same habitat across southern Africa. Both build large mounds and tend mutualistic fungal symbionts for nutrients, but despite these behavioural and physiological similarities, the mound superstructures they create differ markedly. The behavioural differences behind this discrepancy remain elusive, and are the subject of ongoing investigations. Here we show that the two species demonstrate distinctive building activity in a lab-controlled environment consisting of still air with low ambient humidity. In these conditions, M. michaelseni transports less soil from a central reservoir, deposits this soil over a smaller area, and creates structures with a smaller volumetric envelope than M. natalensis. In high humidity, no such systematic difference is observed. This result suggests a differential behavioural threshold or sensitivity to airborne moisture that may relate to the distinct macro-scale structures observed in the African bushland.
Amir, Yaniv, Almogit Abu-Horowitz, Justin Werfel, and Ido Bachelet. 2019. “Nanoscale Robots Exhibiting Quorum Sensing.” Artificial Life 25 (3): 227-231. Abstract
Multi-agent systems demonstrate the ability to collectively perform complex tasks (e.g., construction, search, and locomotion) with greater speed, efficiency, or effectiveness than could a single agent alone. Direct and indirect coordination methods allow agents to collaborate to share information and adapt their activity to fit dynamic situations. A well-studied example is quorum sensing (QS), a mechanism allowing bacterial communities to coordinate and optimize various phenotypes in response to population density. Here we implement, for the first time, bio-inspired QS in robots fabricated from DNA origami, which communicate by transmitting and receiving diffusing signals. The mechanism we describe includes features such as programmable response thresholds and quorum quenching, and is capable of being triggered by proximity of a specific target cell. Nanoscale robots with swarm intelligence could carry out tasks that have been so far unachievable in diverse fields such as industry, manufacturing, and medicine.
Melenbrink, Nathan, and Justin Werfel. 2019. “Autonomous Sheet Pile Driving Robots for Soil Stabilization.” 2019 International Conference on Robotics and Automation (ICRA). Montreal, Canada: IEEE. Abstract
Soil stabilization is a fundamental component of nearly all construction projects, ranging from commercial construction to environmental restoration projects. Previous work in autonomous construction has generally not considered these essential stabilization and anchoring tasks. In this work we present Romu, an autonomous robot capable of building continuous linear structures by using a vibratory hammer to drive interlocking sheet piles into soil. We report on hardware parameters and their effects on pile driving performance, and demonstrate autonomous operation in both controlled and natural environments. Finally, we present simulations in which a small swarm of robots build with sheet piles in example terrains, or apply an alternate spray-based stabilizing agent, and quantify the ability of each intervention to mitigate hydraulic erosion.
Calovi, Daniel S., Paul Bardunias, Nicole Carey, J. Scott Turner, Radhika Nagpal, and Justin Werfel. 2019. “Surface curvature guides early construction activity in mound-building termites.” Philosophical Transactions of the Royal Society B 374 (1774): 20180374. Abstract
Termite colonies construct towering, complex mounds, in a classic example of distributed agents coordinating their activity via interaction with a shared environment. The traditional explanation for how this coordination occurs focuses on the idea of a "cement pheromone", a chemical signal left with deposited soil that triggers further deposition. Recent research has called this idea into question, pointing to a more complicated behavioral response to cues perceived with multiple senses. In this work, we explored the role of topological cues in affecting early construction activity in Macrotermes. We created artificial surfaces with a known range of curvatures, coated them with nest soil, placed groups of major workers on them, and evaluated soil displacement as a function of location at the end of one hour. Each point on the surface has a given curvature, inclination, and absolute height; to disambiguate these factors, we conducted experiments with the surface in different orientations. Soil displacement activity is consistently correlated with surface curvature, and not with inclination nor height. Early exploration activity is also correlated with curvature, to a lesser degree. Topographical cues provide a long-term physical memory of building activity in a manner that ephemeral pheromone labeling cannot. Elucidating the roles of these and other cues for group coordination may help provide organizing principles for swarm robotics and other artificial systems.
2018
Gershenson, Carlos, Vito Trianni, Justin Werfel, and Hiroki Sayama. 2018. “Self-Organization and Artificial Life: A Review.” The 2018 International Conference on Artificial Life (ALIFE 2018). Abstract
Self-organization has been an important concept within a number of disciplines, which Artificial Life (ALife) also has heavily utilized since its inception.  The term and its implications, however, are often confusing or misinterpreted.  In this work, we provide a mini-review of self-organization and its relationship with ALife, aiming at initiating discussions on this important topic with the interested audience. We first articulate some fundamental aspects of self-organization, outline its usage, and review its applications to ALife within its soft, hard, and wet domains. We also provide perspectives for further research.
Melenbrink, Nathan, and Justin Werfel. 2018. “Local force cues for strength and stability in a distributed robotic construction system.” Swarm Intelligence 12 (2): 129-153. Abstract
Construction of spatially extended, self-supporting structures requires a consideration of structural stability throughout the building sequence. For collective construction systems, where independent agents act with variable order and timing under decentralized control, ensuring stability is a particularly pronounced challenge. Previous research in this area has largely neglected considering stability during the building process. Physical forces present throughout a structure may be usable as a cue to inform agent actions as well as an indirect communication mechanism (stigmergy) to coordinate their behavior, as adding material leads to redistribution of forces which then informs the addition of further material. Here we consider in simulation a system of decentralized climbing robots capable of traversing and extending a two-dimensional truss structure, and explore the use of feedback based on force sensing as a way for the swarm to anticipate and prevent structural failures. We consider a scenario in which robots are tasked with building an unsupported cantilever across a gap, as for a bridge, where the goal is for the swarm to build any stable spanning structure rather than to construct a specific predetermined blueprint. We show that access to local force measurements enables robots to build cantilevers that span significantly farther than those built by robots without access to such information. This improvement is achieved by taking measures to maintain both strength and stability, where strength is ensured by paying attention to forces during locomotion to prevent joints from breaking, and stability is maintained by looking at how loads transfer to the ground to ensure against toppling. We show that swarms that take both kinds of forces into account have improved building performance, in both structured settings with flat ground and unpredictable environments with rough terrain.
2017
Melenbrink, Nathan, Paul Kassabian, Achim Menges, and Justin Werfel. 2017. “Towards Force-aware Robot Collectives for On-site Construction.” Association for Computer Aided Design in Architecture (ACADIA), 382-391. Abstract
Due to the irregular and variable environments in which most construction projects take place, the topic of on-site automation has previously been largely neglected in favor of off-site prefabrication. While prefabrication has certain obvious economic and schedule benefits, a number of potential applications would benefit from a fully autonomous robotic construction system capable of building without human supervision or intervention -- for example, building in remote environments, or building structures whose form changes over time. Previous work using a swarm approach to robotic assembly generally neglected to consider forces acting on the structure, which is necessary to guarantee against failure during construction. In this paper we report on key findings for how distributed climbing robots can use local force measurements to assess aspects of global structural state. We then chart out a broader trajectory for the affordances of distributed on-site construction in the built environment and position our contributions within this research agenda. The principles explored in simulation are demonstrated in hardware, including solutions for force-sensing as well as a climbing robot.
Melenbrink, Nathan, Panagiotis Michalatos, Paul Kassabian, and Justin Werfel. 2017. “Using Local Force Measurements to Guide Construction by Distributed Climbing Robots.” IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS). Abstract
Construction automation has historically been driven by top-down implementations of specific tasks, which are neither responsive nor resilient to dynamic situations, and often require centralized control or human supervision. Previous work on robotic assembly has generally neglected to consider forces acting on the structure, whether in the completed structure alone or throughout the building process. In this paper, we investigate the utility of local force measurements in guiding construction by a distributed team of strut-climbing robots, focusing on a scenario involving building an unsupported span out across a gap in a two-dimensional vertical plane, as a step towards building a bridge. We show that such measurements enable robots to build structures that cantilever significantly further than those built by robots without access to such information, while maintaining stability throughout the building sequence. We consider both structures securely anchored to the ground and those resting unanchored atop it, using a counterbalancing approach in the latter case to permit cantilevering. The principles explored in simulation are also demonstrated in hardware, including a prototype strut-climbing robot and truss components, incorporating a cost-effective sensor implementation that reports the requisite force information.
Green, Ben, Paul Bardunias, J. Scott Turner, Radhika Nagpal, and Justin Werfel. 2017. “Excavation and aggregation as organizing factors in de novo construction by mound-building termites.” Proceedings of the Royal Society B 284 (1856): 20162730. Abstract

Termites construct complex mounds that are orders of magnitude larger than any individual and fulfill a variety of functional roles. Yet the processes through which these mounds are built, and by which the insects organize their efforts, remain poorly understood. The traditional understanding focuses on stigmergy, a form of indirect communication in which actions that change the environment provide cues that influence future work. Termite construction has long been thought to be organized via a putative "cement pheromone": a chemical added to deposited soil that stimulates further deposition in the same area, thus creating a positive feedback loop whereby coherent structures are built up. To investigate the detailed mechanisms and behaviors through which termites self-organize the early stages of mound construction, we tracked the motion and behavior of major workers from two Macrotermes species in experimental arenas. Rather than a construction process focused on accumulation of depositions, as models based on cement pheromone would suggest, our results indicated that the primary organizing mechanisms were based on excavation. Digging activity was focused on a small number of excavation sites, which in turn provided templates for soil deposition. This behavior was mediated by a mechanism of aggregation, with termites being more likely to join in the work at an excavation site as the number of termites presently working at that site increased. Statistical analyses showed that this aggregation mechanism was a response to active digging, distinct from and unrelated to putative chemical cues that stimulate deposition. Agent-based simulations quantitatively supported the interpretation that the early stage of de novo construction is primarily organized by excavation and aggregation activity rather than by stigmergic deposition.

Werfel, Justin, Donald E. Ingber, and Yaneer Bar-Yam. 2017. “Theory and associated phenomenology for intrinsic mortality arising from natural selection.” PLOS ONE 12 (3): e0173677. Publisher's Version Abstract

Standard evolutionary theories of aging and mortality, implicitly based on assumptions of spatial averaging, hold that natural selection cannot favor shorter lifespan without direct compensating benefit to individual reproductive success. However, a number of empirical observations appear as exceptions to or are difficult to reconcile with this view, suggesting explicit lifespan control or programmed death mechanisms inconsistent with the classic understanding. Moreover, evolutionary models that take into account the spatial distributions of populations have been shown to exhibit a variety of self-limiting behaviors, maintained through environmental feedback. Here we extend recent work on spatial modeling of lifespan evolution, showing that both theory and phenomenology are consistent with programmed death. Spatial models show that self-limited lifespan robustly results in long-term benefit to a lineage; longer-lived variants may have a reproductive advantage for many generations, but shorter lifespan ultimately confers long-term reproductive advantage through environmental feedback acting on much longer time scales. Numerous model variations produce the same qualitative result, demonstrating insensitivity to detailed assumptions; the key conditions under which self-limited lifespan is favored are spatial extent and locally exhaustible resources. Factors including lower resource availability, higher consumption, and lower dispersal range are associated with evolution of shorter lifespan. A variety of empirical observations can parsimoniously be explained in terms of long-term selective advantage for intrinsic mortality. Classically anomalous empirical data on natural lifespans and intrinsic mortality, including observations of longer lifespan associated with increased predation, and evidence of programmed death in both unicellular and multicellular organisms, are consistent with specific model predictions. The generic nature of the spatial model conditions under which intrinsic mortality is favored suggests a firm theoretical basis for the idea that evolution can quite generally select for shorter lifespan directly.

Pages