Melinda Malley, Michael Rubenstein, and Radhika Nagpal. 2017. “Flippy: A Soft, Autonomous Climber with Simple Sensing and Control.” In IEEE/RSJ Intl Conference on Intelligent Robots and Systems (IROS).Abstract

Climbing robots have many potential applications including maintenance, monitoring, search and rescue, and self-assembly. While numerous climbing designs have been investigated, most are limited to stiff components. Flippy (Fig. 1) is a small, flipping biped robot with a soft, flexible body and on-board power and control. Due to its built-in compliance, flipping gait, and corkscrew gripper, it can autonomously climb up and down surfaces held at any angle relative to gravity and transition from one surface to another, without complex sensing or control. In this paper, we demonstrate the robot’s ability to flip consistently over a flat Velcro surface and 2D Velcro track, where it reliably climbs vertically, upside down and back to a flat surface, completing all the interior transitions in-between. 

Florian Berlinger, Jeff Dusek, Melvin Gauci, and Radhika Nagpal. 2017. “Robust Maneuverability of a Miniature Low-Cost Underwater Robot using Multiple Fin Actuation.” IEEE Robotics and Automation Letters (RA-L), PP, 99.Abstract

In this paper we present the design of a miniature (100 mm) autonomous underwater robot that is low-cost ($ 100), easy to manufacture, and highly maneuverable. A key aspect of the robot design that makes this possible is the use of low-cost magnet-in-coil actuators, which have a small profile and minimal sealing requirements. This allows us to create a robot with multiple flapping fin propulsors that independently control robot motions in surge, heave, and yaw. We present several results on the robot, including: (i) quantified open-loop swimming characteristics; (ii) autonomous behaviors using a pressure sensor and an IMU to achieve controlled swimming of prescribed trajectories; (iii) feedback from an optic sensor to enable homing to a light source. The robot is designed to form the basis for underwater swarm robotics testbeds, where low cost and ease of manufacture are critical, and 3D maneuverability allows testing complex coordination inspired by natural fish schools. Individually, miniature and low-cost underwater robots can also provide a platform for the study of 3D autonomy and marine vehicle dynamics in educational and resource-constrained settings. 

Kirstin Petersen and Radhika Nagpal. 2017. “Complex Design by Simple Robots: A Collective Embodied Intelligence Approach to Construction.” Architectural Design, Special Issue: Autonomous Assembly: Designing for a New Era of Collective Construction, 87, 4, Pp. 44-49.Abstract
Nature's builders – from termites to beavers – offer a model of collective intelligence that can inspire robotic construction. Kirstin Petersen, Assistant Professor in Electrical and Computer Engineering at Cornell University, Ithaca, New York, and Radhika Nagpal, Professor in Computer Science at the Harvard School of Engineering and Applied Sciences, Cambridge, Massachusetts, describe several recent projects in this field that they have been involved in, both separately and collaboratively.
Tsvetomira (Mira) Radeva, Anna Dornhaus, Nancy Lynch, Radhika Nagpal, and Hsin-Hao Su. 2017. “Costs of task allocation with local feedback: Effects of colony size and extra workers in social insects and other multi-agent systems.” PLoS computational biology, 13, 12. Publisher's Version (open-access)Abstract
(Author summary) 

Many complex systems have to allocate their units to different functions: cells in an embryo develop into different tissues, servers in a computer cluster perform different cal- culations, and insect workers choose particular tasks, such as brood care or foraging. Here we demonstrate that this process does not automatically become easier or harder with sys- tem size. If more workers are present than needed to complete the work available, some workers will always be idle; despite this, having surplus workers makes redistributing them across the tasks that need work much faster. Thus, unexpectedly, such surplus, idle workers may potentially significantly improve system performance. Our work suggests that interdisciplinary studies between biology and distributed computing can yield novel insights for both fields. 

Melvin Gauci, Monica Ortiz, Michael Rubenstein, and Radhika Nagpal. 2017. “Error Cascades in Collective Behavior: A Case Study of the Gradient Algorithm on 1000 Physical Agents.” In 16th International Conference on Autonomous Agents and Multiagent Systems (AAMAS).Abstract

The gradient, or hop count, algorithm is inspired by nat- ural phenomena such as the morphogen gradients present in multi-cellular development. It has several applications in multi-agent systems and sensor networks, serving as a basis for self-organized coordinate system formation, and finding shortest paths for message passing. It is a simple and well- understood algorithm in theory. However, we show here that in practice, it is highly sensitive to specific rare errors that emerge at larger scales. We implement it on a system of 1000 physical agents (Kilobot robots) that communicate asynchronously via a noisy wireless channel. We observe that spontaneous, short-lasting rare errors made by a sin- gle agent (e.g. due to message corruption) propagate spa- tially and temporally, causing cascades that severely hinder the algorithm’s functionality. We develop a mathematical model for temporal error propagation and validate it with experiments on 100 agents. This work shows how multi- agent algorithms that are believed to be simple and robust from theoretical insight may be highly challenging to im- plement on physical systems. Systematically understanding and quantifying their current limitations is a first step in the direction of improving their robustness for implementation.

Ben Green, 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. Publisher's VersionAbstract
Termites construct complex mounds that are orders of magnitude larger than any individual and fulfil 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 behaviours through which termites self-organize the early stages of mound construction, we tracked the motion and behaviour of major workers from two Macrotermes species in experimental arenas. Rather than a construction process focused on accumulation of depositions, as modelsbased 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 behaviour 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.
Nicole Carey, Radhika Nagpal, and Justin Werfel. 2017. “Fast, accurate, small-scale 3D scene capture using a low-cost depth sensor.” In IEEE Winter Conference on Applications of Computer Vision (WACV).Abstract

Commercially available depth sensing devices are pri- marily designed for domains that are either macroscopic, or static. We develop a solution for fast microscale 3D re- construction, using off-the-shelf components. By the addi- tion of lenses, precise calibration of camera internals and positioning, and development of bespoke software, we turn an infrared depth sensor designed for human-scale motion and object detection into a device with mm-level accuracy capable of recording at up to 30Hz. 

Elizabeth E. Esterley, Helen McCreery, and Radhika Nagpal. 2017. “Models of Adaptive Navigation, Inspired by Ant Cooperative Transport in the Presence of Obstacles.” In IEEE Artificial Life Conference (ALIFE).Abstract

Cooperative transport is an impressive example of collective behavior in ants, where groups of ants work together to move heavy food objects back to their nest over heterogeneous terrain. This behavior also serves as a model for bio-inspired robotics. While many studies have considered the mechanisms by which ants transport objects in simple settings, few have looked at how they deal with obstacles and heterogeneous terrain. A recent study on Paratrechina longicornis (crazy ants) demonstrated that groups of these ants implement a stochastic, adaptive, and robust cooperative transport strategy that allows them to succeed at navigating challenging obstacles that require moving away from their goal. In this paper, we use group-level computational models to investigate the significance and implications of this biological strategy. We develop an algorithm that reproduces important elements of the strategy, and compare it to several benchmark algorithms for a range of obstacle sizes and shapes. Our results show that, for smaller obstacles, the ant-inspired adaptive stochastic strategy is adept at efficient navigation, due to its ability to match the level of randomness it uses to unknown object size and shape. We also find some unexpected differences between our results and the original ant transport behavior, suggesting new biological experiments. 

(Best Student Paper, special commendation)
Serena Booth, James Tompkins, Hanspeter Pfister, Jim Waldo, Krzysztof Gajos, and Radhika Nagpal. 2017. “Piggybacking Robots: Human-Robot Overtrust in University Dormitory Security.” In ACM/IEEE International Conference on Human-Robot Interaction (HRI). (pdf)Abstract

Can overtrust in robots compromise physical security? We posi- tioned a robot outside a secure-access student dormitory and made it ask passersby for access. Individual participants were as likely to assist the robot in exiting the dormitory (40% assistance rate, 4/10 individuals) as in entering (19%, 3/16 individuals). Groups of people were more likely than individuals to assist the robot in entering (71%, 10/14 groups). When the robot was disguised as a food delivery agent for the ctional start-up Robot Grub, individ- uals were more likely to assist the robot in entering (76%, 16/21 individuals). Lastly, participants who identied the robot as a bomb threat demonstrated a trend toward assisting the robot (87%, 7/8 individuals, 6/7 groups). us, overtrust—the unfounded belief that the robot does not intend to deceive or carry risk—can represent a signicant threat to physical security at a university dormitory. 

(based on senior thesis, awarded Harvard Hoopes Prize)
Helen F. McCreery, Zachary A. Dix, Michael D. Breed, and Radhika Nagpal. 2016. “Collective strategy for obstacle navigation during cooperative transport by ants.” Journal of Experimental Biology, 219, 21, Pp. 3366-3375. Publisher's VersionAbstract

Group cohesion and consensus have primarily been studied in the context of discrete decisions, but some group tasks require making serial decisions that build on one another. We examine such collective problem solving by studying obstacle navigation during cooperative transport in ants. In cooperative transport, ants work together to move a large object back to their nest. We blocked cooperative transport groups of Paratrechina longicornis with obstacles of varying complexity, analyzing groups' trajectories to infer what kind of strategy the ants employed. Simple strategies require little information, but more challenging, robust strategies succeed with a wider range of obstacles. We found that transport groups use a stochastic strategy that leads to efficient navigation around simple obstacles, and still succeeds at difficult obstacles. While groups navigating obstacles preferentially move directly toward the nest, they change their behavior over time; the longer the ants are obstructed, the more likely they are to move away from the nest. This increases the chance of finding a path around the obstacle. Groups rapidly changed directions and rarely stalled during navigation, indicating that these ants maintain consensus even when the nest direction is blocked. Although some decisions were aided by the arrival of new ants, at many key points, direction changes were initiated within the group, with no apparent external cause. This ant species is highly effective at navigating complex environments, and implements a flexible strategy that works for both simple and more complex obstacles.

Melvin Gauci, Radhika Nagpal, and Michael Rubenstein. 2016. “Programmable Self-Disassembly for Shape Formation in Large-Scale Robot Collectives.” In 13th International Symposium on Distributed Autonomous Robotic Systems (DARS).Abstract

We present a method for a large-scale robot collective to autonomously form a wide range of user-specified shapes. In contrast to most existing work, our method uses a subtractive approach rather than an additive one, and is the first such method to be demonstrated on robots that operate in continuous space. An initial dense, stationary configuration of robots distributively forms a coordinate system, and each robot decides if it is part of the desired shape. Non-shape robots then re- move themselves from the configuration using a single external light source as a motion guide. The subtractive approach allows for a higher degree of motion paral- lelism than additive approaches; it is also tolerant of much lower-precision motion. Experiments with 725 Kilobot robots allow us to compare our method against an additive one that was previously evaluated on the same platform. The subtractive method leads to higher reliability and an order-of-magnitude improvement in shape formation speed. 


Lucian Cucu, Michael Rubenstein, and Radhika Nagpal. 2015. “Towards Self-assembled Structures with Mobile Climbing Robots.” In Intl. Conference on Robotics and Automation (ICRA). (pdf)
Michael Rubenstein, Bo Cimino, Radhika Nagpal, and Justin Werfel. 2015. “AERobot: An Affordable One-Robot-Per-Student System for Early Robotics Education.” In Intl. Conference on Robotics and Automation (ICRA). (pdf)
Kirstin Petersen, Paul Bardunias, Nils Napp, Justin Werfel, Radhika Nagpal, and J. Scott Turner. 2015. “Arrestant property of recently manipulated soil on Macrotermes michaelseni as determined through visual tracking and automatic labeling of individual termite behaviors.” Behavioural Processes, 116, Pp. 8-11. Publisher's Version
Johnathan Budd. 2015. “Seaturtle: A low-cost underwater Robot.” ES100 Senior Capstone Project. (pdf)
Justin Werfel, Kirstin Petersen, and Radhika Nagpal. 2014. “Designing Collective Behavior in a Termite-Inspired Robot Construction Team.” Science, 343, 6172.Abstract
Complex systems are characterized by many independent components whose low-level actions produce collective high-level results. Predicting high-level results given low-level rules is a key open challenge; the inverse problem, finding low-level rules that give specific outcomes, is in general still less understood. We present a multi-agent construction system inspired by mound-building termites, solving such an inverse problem. A user specifies a desired structure, and the system automatically generates low-level rules for independent climbing robots that guarantee production of that structure. Robots use only local sensing and coordinate their activity via the shared environment. We demonstrate the approach via a physical realization with three autonomous climbing robots limited to onboard sensing. This work advances the aim of engineering complex systems that achieve specific human-designed goals.
Michael Rubenstein, Alejandro Cornejo, and Radhika Nagpal. 2014. “Programmable self-assembly in a thousand-robot swarm.” Science, 345, 6198.Abstract
Self-assembly enables nature to build complex forms, from multicellular organisms to complex animal structures such as flocks of birds, through the interaction of vast numbers of limited and unreliable individuals. Creating this ability in engineered systems poses challenges in the design of both algorithms and physical systems that can operate at such scales. We report a system that demonstrates programmable self-assembly of complex two-dimensional shapes with a thousand-robot swarm. This was enabled by creating autonomous robots designed to operate in large groups and to cooperate through local interactions and by developing a collective algorithm for shape formation that is highly robust to the variability and error characteristic of large-scale decentralized systems. This work advances the aim of creating artificial swarms with the capabilities of natural ones.
Yong-Lae Park, Bor-Rong Chen, Nestor Presez-Arancibia, Diana Young, Leia Stirling, Robert Wood, Eugene Goldfield, and Radhika Nagpal. 2014. “Design and control of a bio-inspired soft wearable robotic device for ankle-foot rehabilitation.” Bioinspiration & Biomimetics, 9, 1. Publisher's Version
Richard Moore, Karthik Dantu, Geoffery Barrows, and Radhika Nagpal. 2014. “Autonomous MAV guidance with a lightweight omnidirectional vision sensor.” In IEEE Intl. Conf on Robotics and Automation (ICRA). (pdf)
Kirstin Petersen. 2014. “Collective Construction by Termite-Inspired Robots.” Doctoral Thesis, Harvard University. (pdf)