Book review: Self-reconfigurable Robots — An Introduction [Stoy, Brandt and Christensen] by Anders Lyhne Christensen

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Self-Reconfigurable Robots - an Introduction by Kasper Stoy, David Brandt and David J. Christiansen

Self-Reconfigurable Robots - an Introduction by Kasper Stoy, David Brandt and David J. Christensen

Research on self-reconfigurable robots started in the late 1980s when
the idea of cellular robots emerged. The vision was to develop a
system of autonomous cells that could jointly form different shapes to
accomplish tasks. Over the past two decades, significant progress has
been made and several self-reconfigurable robots have been built. The
book “Self-reconfigurable Robots — An Introduction” by Stoy,
Brandt, and Christensen is the first book on self-reconfigurable
robots to appear. The authors have divided the material into 10
chapters covering topics ranging from the history of
self-reconfigurable robotics and hardware to different approaches to
self-reconfiguration and future research challenges. The book is
aimed at graduate students and researchers. The material is, however,
presented in a language and at a level of technical detail that allow
anyone with an interest in self-reconfigurable robots to enjoy the
book.

Chapter 1 introduces the reader to the field of self-reconfigurable
robotics by providing two possible scenarios in which
self-reconfigurable robots could be particularly useful: i) a
planetary exploration scenario, in which a self-reconfigurable robot’s
morphological flexibility enables it to navigate in different
environments, and ii) a morphing production line scenario in which
reconfigurable robots autonomously assemble furniture. The authors
then present examples of two self-reconfigurable robots, namely the
CONRO and the ATRON. A brief history of self-reconfigurable robots is
then provided. The history starts at Fukuda’s CEBOT concept and Yim’s
PolyPod, while some of the more recent systems covered include Shen et
al.’s SuperBot and Zykov et al.’s Molecubes. The authors propose an
interesting classification for self-reconfigurable robotic systems
based on the number of modules in the system: pack robots consisting
of tens of modules, herd robots consisting of hundreds of modules, and
swarm robots consisting of very large numbers of modules. The
classification is justified by two observations, namely that (i) systems
of different sizes tend to be suitable for different tasks, and (ii)
different types of control are appropriate for systems of different
sizes.

Chapter 2 is titled “Designing Self-Reconfigurable Robots”. In the
chapter, the authors stress the interdependences between hardware,
control, morphology, task(s), and environment(s). Some of the
tradeoffs that have to be made in the design of self-reconfigurable
robotic systems are also discussed. A distinction is made between
design goals and characteristics of systems. Versatility,
adaptability, robustness, and cheapness are listed as desirable design
goals for most robotic systems, whereas systems can be characterized
according to their degree of reconfigurability, scalability,
responsiveness, and how well they meet the functional requirements in
their domain of application.

Chapter 3 focuses on the mechanical design of self-reconfigurable
robotic systems. The purpose of the chapter, as stated by the authors,
is to “enlighten software designers about the cost of implementing a
solution at the hardware level … [and] to provide the hardware
designer with a starting point for designing self-reconfigurable
robots”. The chapter starts off with a discussion of the different
types of self-reconfigurable robots, namely chain-type, lattice-type
and hybrids. The authors introduce different aspects of mechanical
design: reconfiguration in two dimensions and in three dimensions,
module geometry, module autonomy, the use of sub-modules,
heterogeneous modules, bipartite designs, and actuator strength.
Different types of connectors, that is, magnetic connectors,
mechanical connectors, and electrostatic connectors, are also
discussed.

Chapter 4 is titled “Electrical Design of Self-Reconfigurable
Robots”. The majority of the chapter is devoted to a discussion of
communication and to discussions of the advantages and disadvantages
of different types of communication, namely local communication,
global communication, and multimode communication. The authors briefly
debate the use of external power supplies vs. on-board batteries. A
table summarizing the computational hardware, type of communication
technology, and on-board sensors for 20 different self-reconfigurable
robots is also provided.

Chapter 5 is an introduction to the problem of
self-reconfiguration. Self-reconfiguration is difficult for several
reasons: (i) modules are often subject to motion constraints,
(ii) modules can get trapped inside or outside hollow substructures,
(iii) a robot usually has to remain in one piece during reconfiguration,
and (iv) when multiple modules move at the same time, congestion can
arise. An overview of approaches to simplify the problem of
self-reconfiguration is then provided, namely (i) the use of
meta-modules, that is larger modules that are composed of a number of
smaller modules, (ii) reducing the size of the space of global
configurations by only allowing for a limited set of local
configurations, (iii) the use of scaffolds of modules on which other
modules can move, and (iv) the use of a few, predefined intermediate
configurations during reconfiguration. For each of the simplification
approaches, the authors discuss why the approach simplifies the
reconfiguration problem and at what cost.

In Chapter 6, the problem of how to find a sequence of module moves
that will reconfigure the robot from an initial configuration to a
goal configuration is discussed. Configurations of a robot can be
represented as nodes in a graph while edges between nodes represent
the move (or moves) necessary to go from one configuration to
another. The self-reconfiguration problem as seen from a search
perspective essentially boils down to finding a path from the node
presenting the initial configuration to the node representing the goal
configuration. However, as the authors demonstrate, the exponential
growth in the size of the search space makes brute-force approaches
infeasible — even for systems composed of relatively few
modules. Various heuristics and distance metrics are discussed. The
authors give examples of how simplifications, such as meta-modules and
scaffolds, have been used in order to make search a feasible approach
to the problem of self-reconfiguration. As noted at the end of Chapter
6, the use of search often necessitates centralized control and a
global view of the system.

Self-reconfiguration viewed as a distributed control problem is the
topic of Chapter 7. In the discussion, the authors assume that modules
know their current location and their goal location with respect to
the other modules. The challenge for each module is to determine how
and where to move next in order to close the distance to their goal
location. Different movement strategies are discussed, namely random
movements, local rules, coordinate attractors, gradient attractors,
and recruitment. The authors also discuss different representations of
the goal locations and how a target shape can be grown using
transition rules.

Chapter 8 is on the topic of self-reconfiguration as a side effect of
task-execution. The authors discuss several simulation-based works on
cluster-flow. In cluster-flow, a robot as a whole moves forward by
continuously letting modules from the back of the robot wander to the
front. The authors then present Bojinov et al.’s simulation-based work
on task-driven self-reconfiguration through growth and Ishiguro et
al.’s work on reconfiguration as a side effect of module
oscillations. Toward the end of Chapter 8, the authors discuss some
unaddressed challenges in self-reconfigurable robotics, namely for
robots to maintain balance during self-reconfiguration in environments
with gravity and reconfiguration in robots composed of heterogeneous
modules.

Chapter 9 discusses control of self-reconfigurable robots in fixed
configurations. The focus is mainly on locomotion, but one page is
devoted to a discussion of issues associated with manipulation. The
authors discuss some of the approaches that have been proposed and
studied, namely gate control tables, hormone-based control, role-based
control, and the challenges related to distributed control. The
PolyPod and the CONRO are used as examples.

Chapter 10 is titled “Research Challenges” and the authors discuss
some of the main challenges that need to be overcome before
self-reconfigurable robots can be used in real world applications. It
is argued that while we may soon see systems of up to tens of modules
(pack robots) outside of labs, a number of fundamental research
questions, such as how to program and control large systems, still
have to be addressed before self-reconfigurable swarm systems can take
on real world tasks. The focus of the chapter is therefore on
challenges for systems of tens of modules (pack robots) and for system
of hundreds of modules (herd robots). After a discussion of the
complexities associated with carrying out real world tasks, the
authors advocate a transition toward a common framework for the
control of self-reconfigurable robots. Under such a framework, it
should be possible to discuss how to move from isolated low-level
behaviors such as a gait, to controllers capable of solving more
advanced tasks. The authors outline what such a framework could be: a
behavior-based-like framework that takes the distributed and modular
nature self-reconfigurable robots into account. The authors discuss
the role of basic behaviors, behavior adaption, behavior selection,
and a novel concept called behavior mode. A behavior mode
corresponds to a set of basic behaviors and a robot shape. A switch
from one behavior mode to another causes the robot to execute a new
set of basic behaviors and triggers self-reconfiguration. In this
way, a complex task may be divided into a number of simpler subtasks
each for which a dedicated behavior mode can be designed. The authors
end the book stating that this is an exciting time for
self-reconfigurable robots because several fundamental problems have
been solved and because self-reconfigurable robots may soon be used in
real world scenarios.

The book, “Self-reconfigurable Robots — An Introduction” is a
well-structured and easily comprehensible introduction to the field of
self-reconfigurable robots. The book is the first of its kind, and it
brings together much of the research related to self-reconfigurable
robots that has been conducted over the past two decades in a single,
coherent, introductory text. The book covers a lot of ground — from
module geometry to distributed control of complex gaits. Each chapter
contains a “Further reading” section with categorized references to
publications on the topics and systems discussed in the chapter. Both
beginners and experienced researchers will therefore find the book a
valuable addition to their personal library.

Anders-Lyhne-Christensen is a current an Assistant Professor
at Lisbon University Institute (ISCTE-IUL),Portugal and a
researcher at Institute of Telecommunications.

Anders Lyhne Christensen

His research focuses on autonomous robotics, multirobot systems,
communication in large scale systems, swarm intelligence,
fault tolerance, self-assembly, evolutionary computation,
and high performance computing. He received a Ph.D. in
2008 from IRIDIA, CoDE, Université Libre de Bruxelles /
Belgium. He also received a Master’s degree in bio-informatics
at Aalborg University / Denmark in collaboration with deCode
Genetics / Iceland. After his Master’s studies, He spent two
years at Critical Software SA. / Portugal on R&D focused on
high performance computing and mission critical systems.

During his professional career, he have worked on IT projects
in industries ranging from space and defense to multimedia,
computer games, and web development. He have worked with
customers such as IBM, Microsoft, ESA, Sun, Landmark Graphics,
MSC Software, and Century Dynamics.

Editor’s note: Anders Lyhne Christensen is in no way
related to David J. Christensen.

References:

H. Bojinov, A. Casal, and T. Hogg. Emergent structures in modular self-

reconfigurable robots. In Proceedings of the IEEE International Conference
on Robotics and Automation (ICRA’00), volume 2, pages 1734-1741. IEEE
Press, Piscataway, NJ, 2000.

A. Castano, W.-M. Shen, and P. Will. Conro: Towards deployable robots
with inter-robots metamorphic capabilities. Autonomous Robots, 8(3):309-
324, 2000.

T. Fukuda and S. Nakagawa. Approach to the dynamically reconfigurable
robotic system. Intelligent & Robotic Systems, 1(1):55-72, 1988.

A. Ishiguro, M. Shimizu, and T. Kawakatsu. Don’t try to control everything:
An emergent morphology control of a modular robot. In Proceedings of
the IEEE/RSJ International Conference on Intelligent Robots and Systems,
pages 981-985. IEEE Press, Piscataway, NJ, 2004.

W.-M. Shen, M. Krivokon, H. Chiu, J. Everist, M. Rubenstein, and
J. Venkatesh. Multimode locomotion via SuperBot reconfigurable robots.
Autonomous Robots, 20(2):165-177, 2006.

M. Yim. Locomotion with a unit-modular reconfigurable robot. PhD thesis,
Department of Mechanical Engineering, Stanford University, Stanford, CA,
1994.

V. Zykov, E. Mytilinaios, M. Desnoyer, and H. Lipson. Evolved and designed
self-reproducing modular robotics. IEEE Transactions on Robotics, 23:308-
319, 2007.

E. H. Østergaard, K. Kassow, R. Beck, and H. H. Lund. Design of the atron
lattice-based self-reconfigurable robot. Autonomous Robots, 21(2):165-183,
2006.

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