Swarm-Bot: A New Distributed Robotic Concept

Swarm-Bot: A New Distributed Robotic Concept
Autonomous Robots 17, 193–221, 2004
c 2004 Kluwer Academic Publishers. Manufactured in The Netherlands.
Swarm-Bot: A New Distributed Robotic Concept
FRANCESCO MONDADA
Autonomous Systems Lab - EPFL, Lausanne, Switzerland
[email protected]
GIOVANNI C. PETTINARO
IDSIA - USI/SUPSI, Manno-Lugano, Switzerland
[email protected]
ANDRE GUIGNARD
Autonomous Systems Lab - EPFL, Lausanne, Switzerland
[email protected]
IVO W. KWEE
IDSIA - USI/SUPSI, Manno-Lugano, Switzerland
[email protected]
DARIO FLOREANO
Autonomous Systems Lab - EPFL, Lausanne, Switzerland
[email protected]
JEAN-LOUIS DENEUBOURG
CENOLI—Université Libre de Bruxelles, Belgium
[email protected]
STEFANO NOLFI
Institute of Cognitive Sciences and Technologies—CNR, Roma, Italy
[email protected]
LUCA MARIA GAMBARDELLA
IDSIA - USI/SUPSI, Manno-Lugano, Switzerland
[email protected]
MARCO DORIGO
IRIDIA—Université Libre de Bruxelles, Belgium
[email protected]
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Abstract. The swarm intelligence paradigm has proven to have very interesting properties such as robustness,
flexibility and ability to solve complex problems exploiting parallelism and self-organization. Several robotics
implementations of this paradigm confirm that these properties can be exploited for the control of a population of
physically independent mobile robots.
The work presented here introduces a new robotic concept called swarm-bot in which the collective interaction
exploited by the swarm intelligence mechanism goes beyond the control layer and is extended to the physical level.
This implies the addition of new mechanical functionalities on the single robot, together with new electronics and
software to manage it. These new functionalities, even if not directly related to mobility and navigation, allow to
address complex mobile robotics problems, such as extreme all-terrain exploration.
The work shows also how this new concept is investigated using a simulation tool (swarmbot3d) specifically
developed for quickly designing and evaluating new control algorithms. Experimental work shows how the simulated
detailed representation of one s-bot has been calibrated to match the behaviour of the real robot.
Keywords: swarm intelligence, swarm robotics, distributed robotics, reconfigurable robotics, collective robotics,
physics-based simulation
1.
Introduction
Applications like semi-automatic space exploration
(Visentin et al., 2001), rescue (Casper et al., 2000),
or underwater exploration (Ayers et al., 1998) need robust and flexible robotic systems. Most of these applications require systems combining the following three
basic characteristics:
Robustness. Unstable, very complex or extreme environments require robustness to severe hardware failures.
Versatility. The complexity of the task needs versatility
in hardware shape and functionality. The robot has
to perform well in very different terrains and in very
different tasks such as displacement, exploration or
object transportation.
All terrain navigation. Complex unstructured environments such as distant planets or catastrophic environments need a very flexible and efficient all-terrain
navigation.
The SWARM-BOTS project1 aims at combining
swarm intelligence (Bonabeau et al., 1999) and physical self-assembling features to provide the above mentioned characteristics to a group of 35 robots.
The swarm-bot robot concept, as well as the hardware implementation, have been developed in parallel
to a simulator. This last is intended to provide the following supporting functionalities:
– the accurate prediction of both kinematics and dynamics of a swarm-bot in 3D;
– the evaluation of hardware design options for different components;
– the design of swarm-bot experiments in 3D worlds;
and
– the efficient investigation of different distributed
control algorithms.
The work presented here reports the hardware and
software development carried out from October 2001.
It is expected that a group of 35 real robots are going to be available by the Fall of 2004. Currently, two
single robot prototypes are available. Preliminary control tests on a group of more than two robots (see the
companion paper in this issue (Dorigo et al., 2004))
are for the moment possible only within the simulation
environment (swarmbot3d).
The next section presents the swarm-bot concept
in more details and places it into its research context
(Section 2). The physical hardware implementation of a
swarm-bot component (s-bot) is illustrated in Section 3.
The swarmbot3d simulation environment is discussed
in Section 4. Section 5 is dedicated to the experimental
comparison between simulated and real s-bots while
final conclusions are drawn in Section 6.
2.
Concept and Related Work
The objective of the SWARM-BOTS project is to study
a novel approach to the design, hardware implementation, test, and use of a self-assembling, self-organizing,
metamorphic robotic system called swarm-bot. This
approach finds its theoretical roots in recent studies
Swarm-Bot
Figure 1. A graphic visualization of the s-bot concept. The main
body (turret) is equipped with passive and active gripping facilities,
sensors and electronics. The lower body (traction system) is equipped
with tracks and it hosts the batteries. The diameter of the main body
is 116 mm.
in the field of swarm intelligence, that is, in studies
exploiting the self-organizing and self-assembling capabilities shown by social insects and by some other
animal societies (Bonabeau et al., 1999).
An important part of the project consists in the physical construction of at least one swarm-bot, that is, a selfassembling and self-organizing robot colony composed
of a number (30–35) of smaller devices, called s-bots
(Fig. 1). Each s-bot is a fully autonomous mobile robot
capable of performing basic tasks such as autonomous
navigation, perception of the environment and grasping of objects. In addition to these features, one s-bot is
able to communicate with other s-bots and physically
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connect to them in flexible ways, thus forming a socalled swarm-bot. Such a robotic entity is able to perform tasks in which a single s-bot has major problems,
such as exploration, navigation, and transportation of
heavy objects on very rough terrain (see Fig. 2 for an
example). This hardware structure is combined with
a distributed adaptive control architecture loosely inspired upon ant colony behaviors (Dorigo et al., 2004).
The final goal of the SWARM-BOTS project is illustrated by a scenario describing the type of operation
that this novel robotic concept aims at achieving. This
scenario consists in transporting a very heavy object
from its initial location to a target defined by a light.
The light cannot be seen from the area where the object
is initially placed and there are several possible paths
for the transport. These paths have different lengths and
include large obstacles and holes. The scenario itself
is split in four stages. During the first stage a group of
s-bots searches for a heavy object, grasps it and starts
moving it in a collective way. In parallel, another group
of s-bots disperses in the environment to search for the
goal. In the second phase the s-bots create a path linking
initial and goal positions for the object. In the third and
fourth phases, the s-bots transporting the object have
to move over a hole and through a narrow passageway
by reconfiguring their position around the object.
This scenario emphasizes several key features of the
swarm-bot concept, and particularly the three aspects
mentioned in the introduction (Section 1), that is, robustness, versatility, and rough terrain navigation. This
concept would therefore be well adapted for exploration in extreme rough terrain. The collective robustness and the self-assembling versatility can be used to
climb obstacles and transport objects also in situations
Figure 2. Graphic visualization of how the rigid gripper can be used to connect in a secure way s-bots among themselves so to form chains for
overcoming large obstacles or holes.
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where a single robot acting alone could not succeed.
This gives a swarm-bot a clear advantage over existing
collective robotic systems in rough terrain conditions.
Additionally, distributed hardware and control provide
strong robustness to failures, which is an advantage
over both classic rovers and self-reconfigurable robots,
that often have a centralized control (Kamimura et al.,
2001; Yim et al., 2002). Even if a swarm-bot concept
cannot be used to form complex 3D structures, it fits
well rough terrain situations and can perform well in
search tasks where dynamic assembly and disbanding
are required.
In the next sections the details of the swarm-bot
concept are presented within the research context of
exploration in rough terrain conditions. The description is structured following the three main features
needed by this type of robots: robustness (Section 2.1),
versatility (Section 2.2), and rough terrain navigation
(Section 2.3). In each section, a comparison is drawn
between the swarm-bot concept and the related stateof-the-art robots.
2.1.
Robustness to Hardware Failures
The problem of robustness to physical damages plays
a crucial role in unstructured and unstable environments, such as those found in post-catastrophic situations or space exploration. Large obstacles, holes in
the ground, unstable hindrance, fire, explosions, water,
chemicals or other dangerous agents can cause damages to a robotic system. In order to ensure the most
efficient task execution, a system has to be fault tolerant and ensure operation even if a large part of it, such
as half of the hardware, is lost.
2.1.1. State of the Art. A widely used technique to
overcome hardware failures is redundancy. Most of the
literature on fault tolerant systems deals with minor
failures that can be corrected with a robust control or
with systems which have intrinsic redundancy, like distributed communication networks. A typical example
exploiting intrinsic redundancy is the failure of a node
in a communication network. In this case the system,
if well controlled, can continue to operate using the
remaining working parts. To face this type of partial
failure, which is the most common in engineering systems, the main design effort has to be placed in the
control part of the system (Stengel, 1991). An efficient
fault tolerant control is based on failure detection and
correction. Both of them need a major design effort
and an accurate model of the system. To correct major
failures, additional and specific hardware redundancy
becomes necessary.
In case of exploration in extreme environments,
hardware failures can be frequent and major. Here robust control is not anymore sufficient and redundancy
has to be introduced also at the hardware level, building
in this way multi-robot systems.
Most of the research done in this direction is known
under the name of collective robotics and represents
a very active field. An overview can be found for instance in a survey by Parker et al. (2000). A major
focus of this research community is distributed control, and there is little research on exploiting the collective self-organization at the hardware level. The main
motivation of collective robotics research is the coordination of several systems (Gerkey and Matarić, 2002;
Agassounon et al., 2001; Melhuish, 1999; Flocchini
et al., 2000) and the robustness that can be achieved
by the redundancy of the whole system (Parker, 1998;
Goldberg and Matarić, 2002; Fukuda et al., 1999).
An increasing number of applications, such as space
robotic missions (Chien et al., 2000; Earon et al., 2001)
where there is a strong advantage in obtaining a more
robust system, plan to exploit this type of information
processing.
Hardware modularity and redundancy can also be
found in the field of self-reconfigurable robots. There
the research activity is complementary to that of collective robotics and is mainly focussed on hardware modularity with relatively little research on autonomous
perception and action in the environment.
Pioneering examples of self-reconfigurable robots
are MTRAN (Kamimura et al., 2001) and PolyBot
(Duff et al., 2001). An overview of existing systems and
characteristics can be found in the work of Kamimura
et al. (2001), or in the work of Yim et al. (2002).
MTRAN and PolyBot use both a large number of simple modules, they both have been physically implemented, and they both can self-reconfigure. Despite
their very good hardware flexibility, both MTRAN and
PolyBot have been designed with a centralized control perspective, which, in comparison with the decentralized ones, shows reduced robustness to failures. The latest articles on these two research works
show that MTRAN is keeping the centralized control approach (Kurokawa et al., 2003) while the PolyBot team is working on new decentralized approaches
under the name of Phase Automata (Zhang et al.,
2003).
Swarm-Bot
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Figure 3. Bottom view of the s-bot robot. A differential treelsc drive ensure the displacement of the s-bot. The motor base with the treelsc
can be oriented independently of the main body.
The first 3D self-reconfigurable robot with decentralized control has been the CONRO hardware
(Castano et al., 2000) which runs the decentralized
control developed by Støy et al. (2002) or the one developed by Salemi et al. (2001). These controllers allow the robot’s hardware modules to change their relative position while the system is running. During this
dynamic change, each involved module re-adapts autonomously its behavioral role in the system. Although
this demonstrates software robustness towards structure modifications and failures, automatic hardware
failure correction is not yet implemented and would
require a major redesign effort. Hardware failure detection and correction are in fact known to be hard to
implement in a reliable way (Blanke et al., 1997).
2.1.2. Swarm-Bot Robustness. Robustness is ensured within the SWARM-BOTS project by distributed
hardware and control. Each s-bot is a simple but fully
autonomous unit capable of displacement, sensing and
acting based on local information and decisions. This
is a clear distinction from self-reconfigurable robots,
where each unit has no mobility, very limited sensing
capabilities and acts often under the control of a central unit. The self-assembling ability of the swarm-bot
is added on top of the s-bots, enabling a swarm behavior at the level of the physically connected swarmbot system. The global task execution is obtained by
the exploitation of robot-robot and robot-environment
properties, without centralized planning and control.
Both the swarm-bot control strategy and the distributed
hardware ensure good robustness to hardware failures.
As mentioned above, the robustness of the concept
is based on the distribution of the task over a group of
s-bots, each of them able of autonomous displacement,
sensing, acting and control.
The mobility of the system is ensured by a combination of two tracks and two wheels, as illustrated
in Fig. 3. Each track is connected to the wheel of the
same side and it is controlled by an independent motor.
Wheel and track on the same side are driven by the
same motor, building a differential drive system controlled by two motors. This combination of tracks and
wheels was labelled Differential Treelsc Drive.2 Such
a combination has two advantages. First, it allows a
more efficient rotation on the spot due to the larger diameter and position of the wheels. Second, it gives to
the traction system a shape close to the cylindrical one
of the main body (turret), avoiding in this way the typical rectangular shape of simple tracks and thus making
navigation simpler.
The differential treels drive allows each s-bot to
move in moderately rough terrains,3 while more
complex situations are handled by swarm-bot
configurations.
The motor base with the treelsc can rotate with
respect to the main body by means of a motorized
axis, as illustrated in Fig. 3. This ensures an independent movement of the upper part where the sensors and the physical connections to other robots are
located.
Each s-bot is equipped with sensors necessary for
navigation, such as infrared proximity sensors, light
and humidity sensors, accelerometers and incremental encoders on each degree of freedom. In addition,
each robot is equipped with sensors and communication devices to detect and communicate with other sbots, such as an omni-directional camera, color LEDs
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all around the robot, local color detectors and sound
emitters and receivers. In addition to a large number of
sensors for detection of the environment, several sensors provide each s-bot with information about physical
contacts, efforts, and reactions at the interconnection
joints with other s-bots. These include torque sensors
on most joints as well as traction sensors on the connection belt.
Research on social insects (Camazine et al., 2001)
suggests that collective robotics could benefit from
multi-range and multi-modal sensing in order to perceive and exchange signals at multiple levels and in
several circumstances. Because of this, as well as for
more practical reasons of interference, infrared proximity (active) sensors mainly have a very short range.
Sound has instead a much longer range span. The camera, which is a passive sensor, is thought to be used
both for long and short range sensing, depending on
the features extracted from the image.
The control architecture of a swarm-bot consists
of distributed algorithms based on local information
and simple self-organization rules inspired upon ant
colony behaviors (Şahin et al., 2002; Dorigo et al.,
2004). Although this type of control algorithm does
not need much computational power, the large number
of sensors and degrees of freedom requires fast preprocessing and efficient control. Therefore s-bots are
equipped with a network of several processors, each of
them responsible for a particular sub-task in the system.
The main processor is in charge of the management
of the entire system and of the communication with a
base station for monitoring purposes. This processor
runs a standard Linux OS allowing in this way the use
of standard development tools, such as compilers and
debuggers, as well as an easy porting of custom made
robotic development tools, such as specific control libraries or monitoring tools. S-bots are also equipped
with a radio link to a base station just for monitoring
purposes (not for control).
2.2.
Versatility
The environment where a robot has to move about in
applications of extreme exploration includes a large
number of obstacles anywhere and of any kind: from
fissures to deep vertical holes, from small pebbles to
large rocks, from wires to walls, from long tubes to
compact blocks, etc. (e.g., Fig. 4). It may happen, for
instance, that robots need to be introduced into small
holes, and once inside they need to overcome large
Figure 4. The environment in rescue operation is composed of obstacles of very different size and shape, including wires, walls, tubes
and gaps.
gaps, to descend a vertical duct ending in a large void,
and finally to pass in other narrow passageways. Robots
designed to cope with only one or two of these features
are surely challenged by the other ones. It may also happen that a mission starts with a goal and ends up with
another one. For instance, a mission can start with an
exploratory phase and finish with a transportation task.
To be successful, a robot has therefore to be very versatile, that is, capable of dynamically changing shape
and control functionality depending on the situation it
faces.
2.2.1. State of the Art. Modularity is a widely used
technique to ensure versatility. At the control level,
modularity is often implemented by distributed approaches in the structure of the control system (Callen,
1998; Zhang et al., 2001) or in the control process itself, as in collective robotics. At the hardware level,
modularity and versatility are clearly represented by
the field of self-reconfigurable robots.
Modularity provides versatility at several levels in
collective robotics. A physically distributed system allows distributed sensing, acting and processing. Simultaneous distributed sensing delivers high flexibility in
placing the sensors according to the configuration of
the search space, thus improving search efficiency. A
good example of this type of situation is given by Hayes
et al. (2001) where a very difficult search task, plume
tracing, is performed using a swarm of robots equipped
with odor sensors.
Distributed acting allows versatility in transport
tasks (Kube and Bonabeau, 2000; Groß and Dorigo,
2004), exploiting the possibility to change the number
Swarm-Bot
of agents involved depending on the effort needed.
Sorting is another example where multiple agents can
improve versatility of the system (Martinoli et al.,
1999; Wilson et al., 2004). Transport (Detrain and
Deneubourg 1997), sorting (Deneubourg et al., 1991),
or structure building (Camazine et al., 2001) are typical tasks where collective robotics can take inspiration
from the behavior of social insects (Bonabeau et al.,
1999) providing efficient and versatile solutions.
At the hardware level, advanced modularity and versatility are shown by self-reconfigurable robots. These
systems are built with a large number of physical modules acting together within a unique body. Each module provides few degrees of freedom. These modules,
when assembled together, give the body an extraordinary physical versatility. An additional feature is given
by the possibility of the system to connect or disconnect
modules autonomously, enabling self-reconfiguration.
Based on such a characteristic, a robot can change
shape depending on the environment, as shown by
PolyBot (Yim et al., 2000a) and by other robots such
as MTRAN (Kamimura et al., 2001). The structure of
the modules and of the possible configurations change
very much across existing systems. The most advanced
ones show 3D configurations like snakes, tracks, spiders, and quadruped legged systems. Both PolyBot and
MTRAN have displayed transition between shapes in
hardware.
Experiments of mobile robots equipped with connection capabilities showed that it is possible to create
larger structures. This is the case of the Millibot robot
units, which have been modified to enable the creation
of a Millibot train (Khosla et al., 2002). Such a structure
is equipped with one degree of freedom between each
robot, enabling a rotation around a horizontal axis. This
allows each robot placed inside the structure to lift vertically the next robot connected to it, bending the train
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vertically. Although the whole structure seems to bring
some additional mobility when facing large obstacles,
it offers a very limited flexibility and lateral mobility.
Moreover, the version of Millibot able to create trains
has very limited sensor capabilities, due to mechanical
constraints and small size.
2.2.2. Swarm-Bot Versatility. Versatility is given in
a swarm-bot by the presence of many entities that can
self-assemble in a unique body and disband when the
union is no longer necessary. This feature combines
the properties of control versatility found in collective
robotics with some hardware versatility found in
self-reconfiguring robots. Since each s-bot is a fully
autonomous mobile robot, a swarm-bot can not only
self-reconfigure, but it can also self-assemble and
disassemble efficiently. S-bots can leave a swarm-bot
configuration, move around it and join it again when
necessary. This is a major additional feature with respect to existing self-reconfigurable robots, which form
a unique and monolithic structure. Compared with a
Millibot train, a swarm-bot can form more complex and
flexible configurations, due to better mechanical and
electronics capabilities. Each s-bot has, in fact, about
50 sensors and 9 actuators, as opposed to a Millibot unit
which has just about 10 sensors and 3 actuators. The
large number of sensors and actuators allows s-bots to
ensure more efficient connections and operations.
S-bots have two types of possible physical interconnections for self-assembling into a swarm-bot configuration: rigid and semi-flexible.
Rigid connections between two s-bots are established by a rigid gripper mounted on a horizontal
active axis (Fig. 5). Such a gripper has a very large
acceptance area allowing it to realize a secure grasp at
any angle and, if necessary, allowing it to lift another
s-bot. Similar connections are made by ants when
Figure 5. The s-bot rigid gripper rotates around a horizontal axis. It can connect either ensuring a rigid grip or leaving some freedom of
movement.
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Figure 6.
Mondada et al.
Two s-bots connected using semi-flexible connections.
they build bridges or other rigid structures (Lioni
et al., 2001). The large acceptance area is a very
significant aspect for connections taking place among
independent autonomous robots on rough terrains.
Building a self-assembling swarm-bot by means of
interconnecting robots is a very different task than
interconnecting modules in a self-reconfigurable robot.
This last can in fact compute the exact position of each
module in order to ensure precise positioning during
interconnection (Agrawal et al., 2001). This is not the
case in a swarm-bot where there is freedom of connecting at several angles and with less accuracy. This is a
very crucial difference with respect to a Millibot train,
whose units must align very accurately in order to
interconnect.
The s-bot rigid gripper can grasp other s-bots on
a T-shaped ring around the main s-bot body (turret).
If it is not completely closed, such a grasp lets the
two joined robots free to move with respect to each
Figure 7.
other while navigating on a rough terrain. If the grasp is
firm, the gripper ensures a very rigid connection which
can even sustain the lifting up of another s-bot. However, lifting with the rigid gripper more than one s-bot
is not possible. This is a major difference between a
swarm-bot and other self-reconfigurable robots, which
can instead form quite complex 3D shapes while moving and overcoming obstacles. Nevertheless, a swarmbot does not require complex 3D shapes, since its
mobility is guaranteed by the combined effort of
each s-bot.
Semi-flexible connections (Fig. 6) are implemented
by a gripper positioned at the end of a flexible arm actuated by three servo-motors positioned at the point of
attachment on the main body. The three degrees of freedom allow to extend and move laterally and vertically
the arm (Figs. 7 and 8, respectively). This structure is
a modified version of the DELTA robot (Clavel, 1988).
The gripper at the end of the arm (called in the following
“flexible gripper”) is similar to the rigid gripper mentioned above. The orientation of this gripper is kept
in a default position by the cables flexibility, but can
rotate around a vertical axis. Rigid and semi-flexible
connections have complementary roles in a swarmbot. A rigid connection is mainly used to form solid
chains for passing large gaps or obstacles (Fig. 2). A
semi-flexible connection is instead used for configurations where s-bots need to stay close to each other
but at the same time they still retain relative freedom
of movement with respect to each other (Fig. 9). A
swarm-bot can also have mixed configurations, which
include both rigid and semi-flexible connections. A
third type of connection among s-bots can take place
The semi-flexible connection can be extended, retracted, and moved laterally.
Swarm-Bot
Figure 8.
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The semi-flexible connection can be moved also vertically.
configurations envisioned are close to the examples
shown in Figs. 2 and 9. In any case, a rigid connection allows the creation of simple 3D structures, for
instance where peripheral s-bots are placed vertically
to help a swarm-bot to overcome obstacles. This type
of 3D flexibility is exploited mainly for climbing obstacles too steep for the tracks of a single s-bot.
2.3.
Figure 9. Graphic visualization of how lateral semi-flexible connections are going to be used to keep relative mobility between s-bots
while they are in a swarm-bot configuration. This flexible structure
can help for instance to pass local small obstacles.
Figure 10. Most swarm-bot configurations will include both rigid
and semi-flexible connections.
through an external object in case of a transporting task
(Fig. 10).
Rigid and semi-flexible connections are not designed to create complex 3D structures. Most of the
Rough Terrain Navigation
A robot capable of navigating in unstructured environments should be able to get across rough terrains as well
as through cavities and narrow passages. A swarm-bot
offers within this context an innovative solution in improving mobility by exploiting physical collaboration
of a collective system.
2.3.1. State of the Art. Navigation in rough terrain
conditions is mainly addressed by articulated rovers
and reconfigurable robots. Examples of rovers include
the shrimp robot (Estier et al., 2000), the family of space
exploration robots by ESA4 (Visentin et al., 2001), the
pathfinder rover used on Mars (Stone, 1996), as well
as other specific rovers for missions like volcano explorations (Bares and Wettergreen, 1999). This type of
research is mainly focussed on mechanical structures
of articulated wheels and tracks and their ability to pass
obstacles. Although most of these rovers are remotely
controlled, research aims also at developing sensors for
autonomous operation (Vandapel et al., 1999) or to help
the remote operator (Matthies et al., 2002).
Some researchers consider multiple rovers for allterrain exploration (Chien et al., 2000; Earon et al.,
2001) exploiting distributed hardware and, in some
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cases, distributed control to obtain a more robust system and better exploration performances.5 To the best
of the authors’ knowledge, nobody has yet tried to take
advantage of the collective aspect for obstacle climbing, except for some preliminary experiments using the
modified version of Millibot mentioned earlier.
Research in self-reconfigurable robots addresses the
same problem in a totally different way, building modular systems that are flexible and can walk, creep, and roll
in rough environment conditions. Simulations of PolyBot have been based on an all-terrain scenario (Yim
et al., 2000b) and the typical goal of the CONRO system
is earthquake search-and-rescue and battlefield surveillance and scouting (Castano et al., 2000). Despite these
goals, the sensors included in these developments are
mainly used for perception of the internal state of the
system and there is practically no perception of the environment. This is motivated in some cases by pure
tele-operation. Pure tele-operation, however, may not
be sufficient for efficient operation. The remote perception of the environment is in fact limited by time
delays, communication bandwidth, and representation
of the environment to the remote operator (Murphy
et al., 2000; Matthies et al., 2002). Semi-autonomous
tele-operation, using local information and performing local control, can improve operability and allow to
achieve the task in an efficient way. There is therefore
a need for including sensors on this type of robots, as
shown by recent work on the CONRO system (Støy
et al., 2002).
Another problem in rough terrain operation of selfreconfigurable robots is the contact of the robot with
the ground. Although snake-like structures can be quite
efficient, performance is less convincing in legged configurations. The contact with the ground is guaranteed in this latter case by the module at the end of
the chain. Such a module is in this way forced to
have its inter-modules connector, that is its most sensitive part, in contact with the ground. A future reconfiguration of the system might therefore fail due
to possible severe damage of the modules previously
used as feet. Solutions to this problem are yet to be
found.
The approach taken by a Millibot train is much closer
to that of the swarm-bot concept. However, despite
its ability to self-assemble and form chains that can
climb large obstacles, the very limited capacities of
each module and the limited lateral mobility of the
one-dimensional trains show strong limitations of the
entire concept.
2.3.2. Swarm-Bot Ability to Deal with Rough Terrain.
The overall mobility of a swarm-bot is guaranteed by
the mobility of each single s-bot composing it. S-bots
are not designed to be used as modules of a leg, as it
happens for instance in the case of self-reconfigurable
robots. The gripper used for the interconnection between two robots does not have sufficient torque to
support this type of structure. The configurations displayed by a swarm-bot are mainly bi-dimensional with
the possibility of lifting up lateral s-bots in order to
overcome large obstacles. The tracks of one s-bot are
therefore always the point of contact to the ground for
a swarm-bot. The possibility of rotating the tracks with
respect to the turret (Fig. 3) ensures suitable mobility of the entire structure even when s-bots are rigidly
connected.
The control of a swarm-bot structure in rough terrain conditions is strongly inspired on insect behavior
(Lioni et al., 2001; Şahin et al., 2002). Most structures
are built of chains of s-bots combined with lateral connections for overall stability. The process of passing
an obstacle is based on local push-pull operations. The
self-assembling feature is strongly exploited: a swarmbot structure is assembled, if necessary, and disbanded
as soon as possible, using in this way the robots as
much as possible as independent units.
As a final remark, the limited size of one s-bot fits
very well the constraints of a catastrophic search operation which requires introduction of a robotic unit into
very narrow entry points. This characteristic gives to
the swarm-bot entity the possibility of accessing internal voids. Once inside, a swarm-bot navigates adapting to the environment conditions which may demand
self-assembling of the swarm and disaggregation of the
same when the union is no longer needed.
3.
Hardware Implementation
This section illustrates the feasibility of the swarmbot concept, showing how it has been implemented
and briefly summarizing some preliminary results. The
discussion presents an overview of the mechanical
(Section 3.1), electronic (Section 3.2), and software
(Section 3.3) implementations of the first prototype.
3.1.
Mechanics
The design described in Fig. 1 was done so as to include
all necessary details to build a real robot. All parts were
Swarm-Bot
Figure 12.
Figure 11.
Exploded view of most of the components.
designed to be feasible and most mechanisms tested
during the design. Figure 11 shows all major parts
included in this design. Each of them was translated
into a technical manufacturing drawing (blueprint) and
then produced. Figure 12 shows the corresponding real
parts.
The production methods employed were very different depending on the type of part. Standard machining
was used for very simple components, such as some
bars of the flexible arm, or parts that needed to be metallic, such as some gears or axes. Chemical machining
203
Exploded view of most of the real parts.
was instead employed for flat parts like the electric contacts molded inside the gripper (Fig. 17). Most of the
parts were molded, which implied first to manufacture
a mold and then to create the part itself. This manufacturing approach asks for a bigger initial effort but it
allows to reproduce parts very easily. Due to the number of s-bots that we plan to produce (thirty-five), this
method allows a cheap and fast production.
At the time of writing, two s-bots have been fully
assembled and tested (Fig. 13). By March 2004, the
plan is to have a swarm-bot of 10 fully operational
s-bots.
The treelsc mechanism has shown very good
performance during tests in rough terrain conditions
(Fig. 14). The association between tracks and wheels
performs very well both in straight motion, where
tracks ensure a powerful displacement, and in sharp
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Mondada et al.
Figure 13.
Swarm-bot prototype using the rigid connection to pass a gap.
3.2.
Figure 14.
Treelsc mechanism during tests.
turns, where the wheels, which are bigger than the
tracks and placed on a bigger radius, play a key role
and ensure a very good rotation.
The rigid gripper (Fig. 15) is another important part
of an s-bot, and it has been crafted so that it can lift
another s-bot. This feature requires a good torque and
a good rigid connection between the gripper and the
s-bot to be lifted. The gripper is able to grasp rigidly
another s-bot using a lockable gripper, which can be
locked mechanically to keep its position.
Comparison results between simulated and real
robots both for a single s-bot and for a swarm-bot configuration are presented in Section 5.
Electronics
An overview of the electronic structure controlling the
robot is given in Fig. 16. The CPU is an Intel XScale
processor running Linux OS and controlling directly the
sound and camera interfaces. The camera is a standard
color web cam with a resolution of 640 × 480 pixels connected to the main processor using a USB bus.
A spherical mirror facing the vertical optics allows to
have a 360◦ panoramic view. All other devices on the
robot are controlled by local PICTM micro-controllers6
communicating with the main processor using an I2 C
bus.
The electronics is mainly included in the central sbot body (turret), but several printed circuits are located in places where they support sensors or local
control electronics. In some cases the printed circuit
is molded inside the mechanical parts, as seen for instance in Fig. 17.
The most important integration effort in size, power
consumption, and computational power has been made
at the level of the main XScale Linux board. Developed to fit in a very small size (just about a credit
card), this board has been integrated successfully inside the s-bot after a long process of prototyping and
software development. The main characteristics of this
board are: 64 MB RAM memory, 32 MB Flash memory, two slots for compact-flash cards (able to support
radio-ethernet or bluetooth), USB master and slave
Swarm-Bot
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Figure 15. CAD and real view of the gripper mechanism partially assembled to show the internal mechanics. On the real view the black gear
ensures the elevation of the gripper. Inside the gripper support a white gear ensures the symmetrical configurations of the jaws.
Figure 16.
Overview of the electronics controlling the s-bot.
Figure 17.
Gripper teeth: 3D model showing the internal printed circuit and electrical contact (left) and real part (right).
interfaces, I2 C bus and serial port. The XScale processor runs at 400 MHz. Tests of Linux running on the
board have shown a power consumption of 750 mW.
Computational tests have shown that this type of pro-
cessor can process simple algorithms on full color images (640 × 480) in 100–200 ms.
Each s-bot is equipped with two Lithium-ION accumulators placed between the tracks. The capacity
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Mondada et al.
of these accumulators is 10 Wh. Preliminary measurements show a power consumption of one s-bot between
3 and 5 W, which ensure continuous operation for at
least two hours.
3.3.
Software
The low level software is distributed among the 14
processors controlling all the functionalities. One of
them is the ARM main processor running Linux and
described above. The ten other processors are PICTM
micro-controllers, each programmed for a very specific
task. Five of them perform motor control, while the
other five perform sensor processing. The five PICTM
motor controls have been programmed in assembler,
and the remaining other eight in C. All of these processors have a part of the code managing the communication through the I2 C bus, whereas the rest manages the
custom functionalities they are responsible for. This
last part of code can include some preprocessing or
some local control loop which can be supervised by
the main CPU with commands sent over the I2 C bus.
As an example, it can be mentioned the control loop
using the torque sensors on the motors or the IR sensors
data preprocessing.
4.
The Simulation Tool
This section presents the simulation environment
(swarmbot3d) complementing the hardware part of the
swarm-bot concept described earlier.
The simulator was planned in order to cover the
current lack of commercial products or research prototypes allowing to tackle all the aforementioned aspects of the SWARM-BOTS project at the same time.
Most of the tools available on the market concentrate,
in fact, on specific aspects of the distributed intelligence paradigm and they generally deal with 2D worlds
only.
Swarmbot3d was developed to work as an aiding tool
for accurately predicting 3D kinematics and dynamics
of a single s-bot in a swarm-bot, for evaluating possible new options for hardware parts, for designing new
experimental set-ups in 3D, and for quickly evaluating
new distributed control ideas before porting them to the
real hardware (Pettinaro et al., 2002).
The main characteristics of this simulation environment can be summarized as follows.
3D dynamics. It is a 3D dynamics simulator of a multiagent system (swarm-bot) of cooperating robots (sbots).
Hardware s-bot compatibility. It provides s-bot models
with the functionalities available on the real s-bots.
It can simulate different sensor devices such as IR
proximity sensors, an omni-directional camera, an
inclinometer, sound, and light sensors.
Software s-bot compatibility. Controllers that are developed using swarmbot3d can be ported directly
to the hardware s-bot due to a common application
programming interface.
Interactive control. It provides online interactive control during simulation, useful for rapid prototyping
of new control algorithms. Users can try, debug and
inspect simulation objects while the simulation is
running.
Multi-level models. It provides most robot simulation
modules at different levels of detail. It also provides
a hierarchy of four s-bot reference models with increasing level of detail. Dynamic model switching
is an included feature which allows to change the
robot representation model in real-time. This allows
a user to switch between a coarse and a detailed level
of simulation model to improve simulation performance at any time.
Swarm handling. It allows to handle a group of robots
either as independent units or in a swarm-bot configuration, which can be thought of as an entity made of
s-bots connected to each other. The connections are
created dynamically at simulation time and can be
eliminated when the components disband. Connections may be of a rigid nature giving to the resulting
structure the solidity of a whole entity. This feature
is unique with respect to other existing robot simulators.
This section is dedicated to present several aspects
of the swarmbot3d simulation tool: from its internal
structure to how robots have been modeled.
4.1.
State of the Art
Simulation of multiple robot systems has been addressed mainly in the fields of multi-agent systems,
artificial life, distributed AI and autonomous mobile
robotics. The simulation tools developed for these areas, depending on the abstraction level, have ranged
from simple cellular automata to highly distributed realistic environments such as, for instance, MissionLab
Swarm-Bot
(MacKenzie et al., 1997), which supports execution of
multiple robots both in 2D-simulation and on actual
robotics platforms.
Another simulation package for large multi-agent
systems is Swarm (Minar et al., 1996) developed at the
Santa-Fe Institute. The modeling formalism adopted by
Swarm is a collection of independent agents interacting via discrete events. Each entity can generate events
that affect the entity itself and other agents. A simulation consists of scheduling the interactions among
agents. Although Swarm simulates multiple agents,
this discrete-event simulator is not appropriate for simulation of mobile robots.
There are three further multiple robot simulators
worth being mentioned: Player/Stage, TeamBots, and
MuRoS. They are all designed to deal with 2D
worlds, and, because of this, they do not comply
with the basic requirement of simulating s-bots in
3D. Player/Stage (Gerkey et al., 2001) is a public domain simulator developed at the Robotics Lab of the
University of South California (USC). Its characteristic is that of being a scalable multiple mobile robot
simulator with each robot moving about and sensing a two-dimensional bit-mapped environment. TeamBots (Balch, 1998) is a Java-based 2D simulator for
multi-agent mobile robotics research. Its distribution is
written entirely in Java and its release is open source.
Last, MuRoS (Chaimowicz et al., 2001) is a simulator
developed at the University of Pennsylvania. Such a
simulator allows 2D simulation of several multi-robot
applications such as cooperative manipulation, formation control, foraging, etc. It is interesting to point out
that in MuRoS tasks both loosely and tightly coupled
can be simulated. With the exception of the restriction
to 2D environments, MuRoS seems to address the same
goals of the SWARM-BOTS project, including the formation of flexible and rigid cooperating structures.
It should be mentioned for the sake of completeness
that there exists a variety of robotic soccer simulators
such as the official RoboCup simulator (Noda, 1995),
JavaSoccer, SoccerBots (part of the TeamBots package). However, all of them are used to simulate only
disconnected robots and they do not consider the possibility of dynamically creating inter-robot connections.
Finally, we mention Webots, a simulator which
was originally developed for the KheperaTM robot but
which is now able to support any type of autonomous
vehicle, including wheeled, legged and flying robots
(Michel, 1998). Earlier versions of this simulator were
purely kinematics based. However, its latest release has
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been extended to handle dynamics using a physics engine based on the Open Dynamics Engine7 libraries.
The software includes a complete library of actuators
and sensors for building customized robots. Webots
would have been an interesting candidate for simulating a swarm-bot; unfortunately, its version using dynamics became available too late to be considered in
the project.
4.2.
Structure
Swarmbot3d is a 3D dynamics simulator. This means
that it is able to take into account physical laws related
to properties such as mass, friction, or acceleration in
the usual Euclidean space. The simulator is built on
top of Vortex,8 a commercial physics engine used in
many applications, including the pioneering work of
Karl Sims on evolved creatures (Sims, 1994).
Simulation models of environments and robots, as
well as world properties such as friction, gravity, and
so on, are all defined in an external text file written in
XML format. Robot control programs are expressed in
terms of the same application programming interface
(API) available in the real robot hardware. This guarantees full portability of any control developed using
swarmbot3d.
The simulator provides a simple graphic user interface (GUI) developed in Python, a high level interpreter
language, for controlling the simulator’s parameters
(such as gravity, simulator speed, time step, and so on)
and for controlling directly each s-bot. This Pythonbased GUI provides also a command line interface for
directly interacting with the simulated robots. This feature has shown to be very useful for online debugging,
scripting, and rapid prototyping of control strategies.
4.3.
S-Bot Modeling
Swarmbot3d has been designed with three main fea-
tures in mind: (i) modularity, (ii) multi-level modeling, and (iii) dynamic model switching. Such a multifeatured design philosophy allows to build a flexible
and efficient simulator.
4.3.1. Modularity. This characteristic allows users to
have a large freedom in customizing their own swarm
according to their specific research goals. This revealed
to be very useful during the early prototype stages of the
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Table 1. Modularity of the simulated s-bot in subsystem components with different levels of abstraction.
Subsystem
Abstraction
Treelsc
⇒ 2 spherical wheels
⇒ 6 spherical wheels
⇒ 6 detailed teethed wheels
Turret
⇒ cylinder
⇒ detailed description
Rigid gripper
⇒ box with on-off connection capability
⇒ detailed toothed jaws
Flexible gripper
⇒ detailed scissor-like arm
robot hardware when its specifications often changed.
Since the simulation models were developed in parallel
with the hardware prototype, the use of modularity allowed not only to re-model a specific s-bot geometry,
but also to extend swarmbot3d with new mechanical
parts or new sensor devices which became available
throughout the hardware development. To implement
the modular design philosophy, one s-bot model was
divided in 4 subsystems: the treelsc , the turret, the rigid
gripper, and the flexible gripper. Some of these subsystems have been implemented at different abstraction
levels, as explained in the next subsection (see also
Table 1).
4.3.2. Multi-Level Modeling. This characteristic provides different models for the same part, so that an enduser is given the possibility to load the most efficient
and functionally equivalent abstraction model among
those available to represent the real s-bot.
For example, one s-bot may be loaded as a detailed
model when an accurate simulation is needed; or it
may be loaded as a crude abstraction for the evaluation of a big swarm, in which case the accuracy
of a single robot might not be a crucial aspect. People working with learning or swarm intelligence techniques might in fact be more interested in simple coarse
s-bot models. Conversely, those experimenting with
the interaction of relatively small groups of s-bots (between 5 and 10) might prefer to use a more refined
s-bot model.
Viewed in terms of subsystems, defining an abstraction for one s-bot consists in defining an opportune
combination of some of the subsystems at the desired
level of detail (Table 1). Since the use of a particular
model refinement influences considerably the time required to simulate it on a given computing hardware,
the level of abstraction has to be chosen as an opportune trade-off between simulation efficiency (speed)
and accurate reproduction of reality.
As an aid to the end-user, 4 reference s-bot descriptions differing in their level of abstraction have been defined and prepackaged: detailed, medium, simple, and
fast. Users can anyhow still select among the s-bot subsystems the combination of approximations which best
suits the specific research goals they intend to pursue.
A brief description of each reference model is outlined
in the following.
Detailed s-bot. This robot model is a quite faithful
replica of the real s-bot: all its mechanical parts are
reproduced with all the degrees of freedom required
(Fig. 18 on the left). This model replicates in details
the geometry of the real hardware (Fig. 19) as well as
the masses, centre of masses, torques, accelerations,
and speeds.
The detailed model comprises four mechanical
modules: treels, turret, rigid gripper, and flexible
gripper. Its main characteristics are reported below.
–A detailed chassis description comprehensive of
4 ground IR sensors (2 at the bottom, 1 in front
and 1 on the back).
–Six teethed wheels, three on each side, with the
two middle ones slightly larger and located outward as in the hardware s-bot.
–A detailed turret representation.
–A rigid gripper hinged on the front of the turret
and endowed with two teethed jaws.
–A flexible gripper attached through three hinges
to the s-bot’s body and endowed with two
teethed jaws.
Although the detailed model closely matches the
s-bot hardware, it lacks the caterpillar-like rubber
teethed band joining the inner wheels of each track.
The simulation of this track was computationally
very expensive and provided only a negligible gain
in the realism of simulating the real hardware.
Medium s-bot. This s-bot model is shaped with a detailed description of the treels system, although the
6 wheels are in this case defined as simple spheres
(Fig. 18, second from the left). The turret is defined
in full detail but without the presence of the flexible
gripper. A coarse representation of the rigid gripper (a hinged box) gives this model the possibility
of realizing limited on/off connections to other sbots. The turret model implements all main hardware
Swarm-Bot
209
Figure 18. Detailed (first from left), medium (second from left), simple (third from left), and fast (fourth from left) s-bot models. Fast and
simple models have both 2 wheels and a very simplified rotational turret. The medium model differs from the detailed model only by having 6
simplified spherical wheels, a simplified rigid gripper and lacking the flexible side arm.
Figure 19.
Mechanical diagram of a real s-bot.
sensors such as infrared, sound, camera, and light
sensors.
This model is still reasonably efficient and therefore it allows to develop distributed controllers over
quite large groups of units on smooth planes as well
as on uneven terrains. The simplified rigid gripper
and the absence of the flexible gripper, however, limit
its use just to single s-bot missions or to collective
tasks in groups of s-bots with simple rigid connections.
Simple s-bot.This model is a minimalistic abstraction
of the most salient characteristics identifying one
s-bot (Fig. 18, third from the left). It has a traction system made of one sphere with two spherically
shaped wheels hinged to its sides, and a turret made
of a bare cylinder. Caster wheels are added to give
mechanical stability to the model. Mass and size are
roughly the same as those of the real s-bot.
This is a model designed to test and validate control algorithms, developed using the fast model, in
environment with real gravity pull.
The simple model, benefitting from its minimal
structure and therefore from its high simulation
speed, is very useful for investigating computation
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intensive distributed control policies spread over a
large number of units (e.g. genetic algorithms) in
environments using the real gravity pull. However,
because this model implies real masses and forces,
it is not possible to run simulations using large time
steps without incurring in problems of instability.
Furthermore, its applicability is strongly limited to
environments with very limited roughness: when this
constraint ceases to hold, a more refined level of detail is required.
Fast s-bot. This model is a scaled down version of
the simple model with the same modelling structure
(Fig. 18, first from the right). However, its linear
dimensions are halved and its mass is 1/20th with
respect to the simple model.
It is meant for simple tests in environments with
1/50th of the real gravity pull. The reduced masses
and unrealistic gravity enables to use a large time
step without getting unstable simulations, at least in
flat environments. This possibility allows to increase
the simulation speed up to 10 times, although the
use of this abstraction on terrains modeled as mesh
surfaces may lead to unstable simulations if the time
step is too big (>10 ms).
To establish how computationally heavy each of the
4 models described above is, a performance evaluation
experiment was set. This consisted simply in loading
and using one s-bot at each different abstraction level
on a horizontal plane. The performance result for each
type of model is presented here in terms of its Real
Time Multiplier (RTM) value which refers to how fast
real time can be simulated. Table 2 summarizes each
model characteristics and gives a typical RTM value
for each of them. RTM values were obtained running
the tests on a dual 3.06 GHz Xeon PC with one nVidia
QuadroFx 1000 graphics card.
4.3.3. Dynamic Model Switching. Using the hierarchical abstraction levels introduced above, it has been
implemented in the simulator a way to change the used
s-bot representation during simulation (Pettinaro et al.,
2003). The availability of such a feature allows a user,
for example, to start a simulation with the simplest abstraction level for one s-bot when the terrain onto which
it moves is flat and to switch to a more refined model
representation when the environment or the interaction
among s-bots require a more detailed treatment. Dynamic model changing allows swarmbot3d to increase
Table 2. Comparison of features among detailed, medium, simple,
and fast s-bot simulation models. Yes and no respectively indicate
the presence or absence of a particular feature in the model.
Model
Driving wheels #
Detailed
Medium
Simple
Fasta
6
6
2
2
Yesb
No
IR proximity sensors
Yes
Yes
Yesb
Ground sensors
Yes
Yes
No
Camera
Yes
Yes
No
No
Sound module
Yes
Yes
Yesb
Yesb
Rigid gripper
Yes
Yesd
Noc
Noc
Flexible gripper
Yes
No
No
No
Dynamical bodies #
23
8
6
6
Typical RTM
3
31.8
94.7
960a
Time step
10
10
10
100a
a Measurement
taken with 1/50th of gravity and 1/20th of mass.
sample-based look-up table.
c No physical gripper model: connections are possible using virtual
links.
d Coarse version, i.e. sticking box.
b Using
simulation speed by introducing complexity only when
it is needed.
This feature, however, assumes that all representation models are compatible with each other, that is,
all models show the same behaviour when a particular robot command is issued. It is therefore important
to adjust each model so that speed, mass, and geometry are calibrated to ensure compatibility, even if they
differ in representation detail. For this reason, it was developed an extension of the simple model named basic
model possessing a simple on/off connecting device,
which was previously available just for the medium
model, and a ray traced model of the IR sensors (see
Section 4.4).
Thanks to this model changing mechanism, users
can use the basic model when the terrain is flat, change
to the medium model when the terrain gets rough or
change to the detailed model when the flexible arm is
needed. Currently, such a model switching has to be
carried out manually, however work is in progress for
investigating ways for automating this feature, so to
leave to the simulator the decision on when changing
the abstraction level.
4.4.
Sensor Modeling
Real s-bots are equipped with several types of sensors which are read by the control program running on
Swarm-Bot
Table 3. Sensors types and their implementations in the simulation environment. Each sensor may have multiple implementations
which can be chosen by the user.
S-Bot sensors
weighted by their inverse squared distance and some
scaling factor. At present, light shadowing is not implemented. Sound sensors are implemented in a similar
way, disregarding also in this case shadowing.
Implementations
Proximity sensor
Sample table based
Ray tracing
Ground sensor
Ray tracing
Sound
Instant on-off with spatial decaying intensity
Sound wave propagation with spatial decaying
intensity
Speed sensor
Standard library function
Torque sensor
Standard library function
4.5.
Inclinometer
Comparison with the absolute XZ plane
Light sensor
Ray tracing based with shadowing
Camera
Abstract camera using high level objects
Low-level fish-eye view
Performance Evaluations of the Different
Abstraction Models
A crucial point concerning swarmbot3d is how it
performs with respect to an increasing number of
units populating its world and for different abstraction
models.
To evaluate the computational load, the 4 model abstractions introduced in Section 4.3.2 were examined.
The experimental test was carried out by loading into
the simulator an increasing number of disconnected sbots of the same kind. The simulator performance was
quantified by checking, as done with the single s-bot,
the real time multiplier (RTM) value. The hardware
employed in this performance evaluation was a dual
3.06 GHz Xeon PC with one nVidia QuadroFx 1000
graphics card.
The readings obtained for a smooth plane terrain are
reported in Table 4, where, for comparison purposes,
data is shown also for 5 connected s-bots on a plane
and for 5 disconnected s-bots on a rough terrain.
All measurements were taken using a time-step of
10 ms, except for the fast model, that, because of the
large step used with it, showed instability when used
on rough terrain.
each s-bot. Swarmbot3d implements 8 types of sensors
matching those available on the real units. Some sensors (such as the speed and torque sensors) have been
implemented using standard library functions of the
underlying physics engine, while others needed implementation and calibration with the real sensors. Table 3
summarizes the virtual sensors and their implementation currently available in swarmbot3d.
The infrared sensors used by the proximity sensor
and the ground sensors are simulated using ray-tracing
by probing the sensor vicinity with 5 rays (1 central
and 4 peripheral) within the sensing cone of each virtual sensor. The 5 rays detect any intersection of nearby
objects and compute an average distance value which is
subsequently converted to an integer sensor response.
The mapping function has been obtained by linear regression of the experimental values.
The light sensors are modelled in the simulator by
summing up contributions of all known light sources
5.
Comparisons Between Simulated
and Real S-Bot
This section presents a number of experiments conducted to compare the mechanical behaviour of the
Table 4. Simulator performance for different abstraction models. The numbers represent real time multipliers, i.e., the
ratio between simulated real time and simulation time (the higher, the better).
Disconnected S-Bots (smooth plane)
Model
Fasta
Conn. S-Bots
Disconn. S-Bots
5 on rough terrain
1
2
5
10
20
40
5 on smooth plane
960
490
200
96
43.3
17.6
88.1
20
Simple
94.7
49
Medium
31.8
15
Detailed
3
a Measurements
1.4
211
147
9.6
4.3
1.8
8.5
14.9
4.7
2
0.7
0.1
2.2
2.4
0.5
0.2
0.09
0.04
0.2
0.4
taken with a time step of 100 ms and 1/50th of the real gravity pull.
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Mondada et al.
Figure 20. Comparison of linear motion errors of the three s-bot simulation models with respect to different terrain roughness. The HR values
correspond to descriptive values: 0 for flat, 1 for almost flat, 2 for minimally rough, 4 for little rough, 8 for mildly rough, 16 for rough, and 32
for very rough.
various simulated s-bot models with respect to the real
s-bot. A good correspondence was found between the
detailed model and the real s-bot in all cases. The
medium model was able to approximate acceptably
well the detailed one in many situations, whereas the
fast and simple models were sufficient only in certain
simple environments.
5.1.
Motion Comparison
This experiment compares how different models differ
during forward linear motion on terrains with different
levels of roughness. To do so, each simulation model
was placed randomly on a terrain and then assigned a
random heading. The terrain roughness was controlled
within the simulator by specifying a height range (HR)
parameter. Given the reduced size of the fast model, the
terrain used for that s-bot abstraction was also scaled
down by half its original size.
Figure 20 shows the motion errors which are obtained by letting each s-bot model run at medium speed
(15.24 cm/s) for 10 seconds. The vertical axis plots
the projection of the travelled distance onto the randomly selected initial direction. Depending on the terrain roughness this distance decreases because the sbot is not able to retain a straight course. The small
differences in distance on flat terrain are caused by cal-
ibration errors of the velocity due to differences in the
wheel diameters among the various s-bot.
Figure 20 shows that the rough terrain motion of the
medium model closely follows the behaviour of the detailed one: the constant offset is due to differences in
wheel size (see above). Both simple and fast models
quickly fail to retain linear motion even on minimally
rough terrains. Since the detailed model replicates quite
closely the behaviour observed on the real robot, this
suggests that also the medium one can reasonably approximate it, at least as far as pure locomotion on rough
terrain is concerned. The fast and simple models therefore are not suitable for experiments involving very
rough terrain.
5.2.
Passing a Gap
In rough terrain situations, it may be the case that one
s-bot or a group of s-bots have to pass gaps or holes.
We ran therefore an experiment to study how s-bots
behave in these situations. To quantify the behaviour of
the different simulation models with respect to the real
s-bot, two planes were placed close to each other with
a variable gap (Fig. 21). We observed how each model
reacted to changes in the size of the gap and compared
the results with what observed using one real s-bot.
This experiment was carried out for one s-bot and then
repeated for connected s-bots.
Swarm-Bot
213
Table 5. Gap experiment for a single s-bot. The s-bot had to maneuver across a gap of different width.
Each row corresponds to a certain gap width and reports the results.
Width (mm)
Fasta, b
Simple
Medium
Detailed
Real
40
Overcome
Overcome
Overcome
Overcome
Overcome
45
Overcome
Overcome (70%)
Stuck (40%)
Overcome (90%)
Stuck (10%)
Overcome
Overcome
50
Overcome
Stuck
Stuck
Overcome
Overcome
57
Overcome
Stuck
Stuck
Overcome (50%)
Stuck (50%)
Stuck
60
Overcome
Stuck
Stuck
Stuck
Stuck
65
Stuck
Stuck
Stuck
Stuck
Stuck
a Observations
taken separately with 1/50th of gravity and 1/20th of mass.
the fast model is half the size of the simple model, the gaps’ widths should be divided by 2 for
having a correct comparison.
b Since
Figure 21.
From left to right: The 3 s-bot models while traversing a gap of 40 and 60 mm, respectively.
5.2.1. Single S-Bot. The results of the experiment described above are reported in Table 5. Each table entry
corresponds to the modal value of 12 observations. All
tests were carried out by using a low speed of 2.8 cm/s.
By observing the table, we see that the simple model
can cope with gaps up to 40 mm, starts having trouble
with gaps of 45 mm, and gets stuck with gaps of 50 mm
or wider. The medium and detailed models, instead, do
not have problems with gaps up to 40 mm. Beyond this
size, the presence of the teeth on the wheels of the
detailed s-bot makes a difference. This feature, in fact,
by acting as a surrogate of caterpillar tracks, mimics
remarkably well the behaviour observed on the real
s-bot which gets also stuck with gaps of 55–57 mm.
The fast model was tested in a separate environment
with low gravity (1/50th of the normal one) and with a
high time step value (100 ms). This model overcomes
gaps of 60 mm and behaves therefore similarly to the
detailed model, although it moves in an unreal environment. Thus, it can be used as a rough approximation of
the real s-bot functional behaviour.
5.2.2. Connected S-Bots. While for a single s-bot
the maximum traversable gap width is around 60 mm,
a connected structure is expected to pass wider gaps,
even larger than the diameter of a single s-bot (about
116 mm). We ran an experiment using two connected
s-bots both in simulation and in the real world (Fig. 22).
The gap passing experiment was repeated, only in simulation, using chains of three and four robots. Table 6
summarizes the observed maximum gap widths for
successful traversals. Each table entry for the simulated s-bot corresponds to the modal value of 12
observations.
5.3.
Climbing a Step
When navigating on rough terrain one s-bot has to confront itself with all sorts of hindrances. Some may be
overcome by getting around them, some others can instead be overcome by climbing over. Climbing a step
is one of these latter and it is important to evaluate how
214
Mondada et al.
Figure 22. Gap traversal sequence using two s-bots. The same sequence is reported in the left column for the simulated s-bots and in the
right column for the real s-bots. From top to bottom: (1) the first s-bot in front of the gap calls for help, (2) the second s-bot connects with
the rigid gripper, (3) both s-bots pass the 70 mm gap, (4) the second s-bot releases its gripper. A movie of the experiment is available at
www.swarm-bots.org .
s-bots behave when they have to face such an obstacle.
In order to do so, two experiments were carried out:
one using a single s-bot and one using two s-bots in
a connected configuration. The capability of overcoming a step was in both cases quantified by progressively
varying its height with respect to the ground.
5.3.1. Single S-Bot. In this experiment one s-bot,
detailed model, placed on the ground started to
move towards a step. The experiment was performed
with the s-bot moving both forward and backward.
The height of the step was varied in intervals of
1 mm.
Swarm-Bot
Table 6. Gap traversal experiment using swarmbot3d simulation. The values represent observed maximum gap width for successful traversal for different
numbers of connected s-bots.
(see next section), one single s-bot going backward
topples over both in the simulated world and in reality.
Gap width (mm)
S-bots #
Detailed
Real
1
57
57
2
100
100
3
190
Not available
4
240
Not available
Table 7. Maximum step climbing height for one s-bot moving
backward (bw) and forward (fw).
Detailed S-bot
Real S-bot
Step (mm)
bw
fw
bw
fw
≤15
Pass
Pass
Pass
Pass
16
Fail
Pass
Fail
Pass
23
Fail
Pass
Fail
Pass
24
Fail
Fail
Fail
Fail
It was observed that the maximum step height which
a single detailed s-bot was able to cope with was 15 mm
when moving backward and 23 mm when moving forward (Table 7). Each table entry for the simulated s-bot
is the outcome of 12 observations. The difference between the forward and backward behaviours both in the
simulated s-bot and in the real one is due to the center of mass of the turret which is 40 mm off centered
towards the gripper.
Figure 23 shows that with a step of 32 mm, which
is the height at which two connected robot can pass
Figure 23.
215
5.3.2. Connected S-Bots. This experiment was set
by letting a pair of simulated s-bots, detailed model,
first connect and then navigate backward towards a
step. By varying the height of the step, it was observed that the two robots were able to pass steps up to
32 mm, in accordance with what experienced with the
real s-bot.
Figure 24 shows four stages in passing the limit step
of 32 mm. First, the two s-bots approach the step in
backward formation. Second, as soon as the first robot
senses the step with its rear ground sensor, it starts
lifting itself using its connected rigid gripper. During
traversal, the robot bends its rigid gripper in the opposite direction (downward) pushing itself up. Finally,
the first robot continues its backward motion and pulls
in this way the second one over the step to complete
the procedure.
6.
Conclusions
The work reported in this paper presented a new robot
concept, called swarm-bot. Such a concept shows to
possess the three major characteristics needed for rough
terrain exploration: robustness, versatility, and all terrain navigation. These characteristics were used to discuss how the swarm-bot concept compares with similar
existing systems.
A swarm-bot, with its self-assembling capability
added on top of fully autonomous robots, opens up a
new research field situated between self-reconfigurable
and collective robotics. The concept combines hardware versatility found in self-reconfigurable robots
Equivalent behaviour of simulated and real s-bot with respect to a step size of 32 mm.
216
Mondada et al.
Figure 24. From left to right: the different phases of the step passing for the limit step of 32 mm. The first four pictures refer to the simulated
s-bot and the last four refer to the actual s-bots. A movie of the experiment is available at www.swarm-bots.org .
with control versatility found in distributed control
for collective robotics. This fundamental property
of a swarm-bot plays a key role in robotic operations to be performed on rough terrains and it allows to carry out different tasks while facing complex
and harsh environments usually found in exploration
missions.
A second and fundamental property of a swarm-bot
is robustness, provided by distributed hardware and
control. This feature is also essential for exploration
operation where the unknown and unstable environment can cause loss of robotic units.
The feasibility of the concept has been shown by
presenting the construction of the first physical prototype as well as of a 3D dynamics simulation package (swarmbot3d) complementing it. The usefulness of
this software package has been shown for accurately
simulating both the kinematics and the dynamics of
a single s-bot as well as of an entire swarm-bot, for
evaluating hardware design options for different robot
Swarm-Bot
components, for designing swarm-bot control experiments in 3D worlds, and for investigating distributed
control algorithms.
The simulation environment features modularity,
multi-level modeling, and dynamic model switching.
S-bots are defined in terms of modules with each modules expressed at different levels of detail. These characteristics make simulated s-bots fully customizable.
Dynamic model switching is unique in its kind. Such
a feature gives to the simulator the power of reducing the computational cost while keeping the accuracy
of predicting a swarm-bot behaviour within acceptable
limits.
Acknowledgments
Many thanks to Michael Bonani, Daniel Baer, Pierre
Bureau, Michel Lauria, and Vito Trianni for their help,
comments, and ideas.
This work was supported by the SWARM-BOTS
project, funded by the Future and Emerging Technologies programme (IST-FET) of the European Commission, under grant IST-2000-31010. The information
provided is the sole responsibility of the authors and
does not reflect the Community’s opinion. The Community is not responsible for any use that might be made
of data appearing in this publication. The Swiss participants to the project are supported under grant 01.0012
by the Swiss Government and the Swiss National Science Foundation. Marco Dorigo acknowledges support
from the Belgian FNRS, of which he is a Senior Research Associate, through the grant “Virtual Swarmbots”, contract no. 9.4515.03, and from the “ANTS”
project, an “Action de Recherche Concertée” funded
by the Scientific Research Directorate of the French
Community of Belgium.
Notes
1. Swarm-Bots is a European IST-FET (Future and Emerging Technologies) project, grant IST-2000-31010, for more details see
http://www.swarm-bots.org.
2. Treels is a contraction of TRacks and whEELS.
3. Obstacles no more than 3–4 cm high, 30% slope, and gaps not
larger than 3–4 cm.
4. European Space Agency.
5. http://www.sandia.gov/isrc/Swarm.html
6. PICTM micro-controllers are products of Microchip Corp. See
http://www.microchip.com for more details.
7. Open Dynamics Engine (ODE) is an open source project.
8. http://www.cm-labs.com.
217
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Francesco Mondada has an M.Sc. in Microengineering and a Ph.D.
from the Swiss Federal Institute of Technology (EPFL), Lausanne
(Switzerland). He is a member of the group who developed the
Khepera mobile robot. He is co-founder of two companies: K-Team
(robotics) and Calerga (scientific software). He has been president
and director of K-Team for 5 years. He is currently a senior researcher
at the Autonomous System Laboratory of the Swiss Federal Institute of Technology (EPFL), Lausanne. His interests include miniature robotic design, mechatronics, bio-inspired robotic research, the
development of tools to perform this research and the transfer of
robotics technology to industry.
Giovanni C. Pettinaro graduated in Computer Science at the Università di Milano, Italy, in 1989. He received the M.Sc. in Information
Technologies and the Ph.D. in Intelligent Robotics from the University of Edinburgh in 1992 and 1996, respectively. He joined in
1997 ABB Corporation in Västerås, Sweden, where he worked as a
consulting researcher in mechanical design for ABB Robotics AB,
Sweden. From 1998 till 2001, he worked as a researcher in mobile
robotics and artificial intelligence within the group of Mechatronics
and Software Systems at the Department of Industrial IT of ABB
Corporate Research. In 2001, he joined the Istituto Dalle Molle di
Studi sull’Intelligenza Artificiale (IDSIA), a multi-disciplinary research institute within the Università della Svizzera Italiana (USI)
and the Scuola Universitaria Professionale della Svizzera Italiana
(SUPSI). Since then he has worked as a researcher in robotics
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control and 3D simulation within the SWARM-BOTS project. He
is a member of IEEE, ACM, and AAAS. His research interests are
in mobile robotics, distributed robot control, sensor fusion, robot
architectures, robot learning, and swarm robotics.
Andrè Guignard got a Swiss federal certificate of proficiency as
watchmaker and afterwards an engineering degree in electronics in
Lausanne, Switzerland. He is a member of the group who developed
the Khepera mobile robot. He is co-founder of the two companies
K-Team (robotics) and CFG (electronics). He is currently senior
engineer at the Autonomous System Laboratory of the Swiss Federal Institute of Technology (EPFL), Lausanne. His interests include
miniature mechatronics applied to autonomous robotics, manufacturing techniques for miniature mechanics, and microsystems.
Ivo W. Kwee was born in Surabaya, Indonesia, in 1969. He received
his Engineering degree in Technical Physics from the Delft University of Technology, The Netherlands, his M.Sc. in Applied Physics
from the Hokkaido University, Japan, and his Ph.D. in Biomedical
Physics from the University College London, UK, in 1991, 1994 and
2000 respectively. Currently he is researcher at IDSIA, Switzerland,
working on the SWARM-BOTS project, in particular on developing
the 3D simulator and on developing learning algorithms for swarmbot control.
Dario Floreano (M.Sc., 1992; Ph.D., 1995) is a professor of the
Swiss National Science Foundation at the Swiss Federal Institute
of Technology in Lausanne (EPFL) where he is the director of the
Institute of Systems Engineering. His research activities include artificial neural networks, evolutionary robotics, swarm intelligence,
bio-mimetic electronics and robotics, and artificial life. He held senior research positions at the National Research Council (CNR) in
Rome, at the University of Stirling, at the Swiss Federal Institute
of Technology in Lausanne, and at Sony Computer Science Laboratory in Tokyo. He published more than 100 peer-reviewed papers,
authored 2 books, and edited 3 other books. His book with Stefano
Nolfi, Evolutionary Robotics, has been reprinted by MIT Press three
times since 2000. He co-organized three international conferences
and joined the program committee of more than 60 other conferences. He delivered more than 100 invited talks all over the world
to academic, industrial, and public audiences. He is on the editorial
board of the journals Neural Networks, Genetic Programming and
Evolvable Machines, Adaptive Behavior, Artificial Life, Connection
Science, and IEEE Transactions on Evolutionary Computation. He
is also co-founder and co-director of the International Society for
Artificial Life (Inc., USA), member of the Board of Governors of
the International Society of Artificial Neural Networks (Inc., USA),
and member of various international societies. He frequently serves
as advisor to the Research Division of the European Commission,
to the U.S. National Science Foundation, and to other governmental
and private institutions. He aims at building autonomous machines
that have life-like properties, that is, machines which are able to
reproduce, adapt, and evolve without human intervention.
Jean-Louis Deneubourg received his doctoral degree in Sciences
(Chemistry) from the Université Libre de Bruxelles in 1979. From
1980 to 1989 he was a research fellow at the Service de Chimie
Physique and at CENOLI, Université Libre de Bruxelles. From 1989,
he has been a researcher of the FNRS, at CENOLI. From 2003, he
is, with C. Detrain, co-director of the Department of Social Ecology. He is the author or co-author of around 180 papers, the coeditor of two books (From individual to collective behavior in social
insects with J.M. Pasteels Birkhäuser; Information Processing in Social Insects with C. Detrain, & J.M. Pasteels, Birkhäuser) and the
co-author of one book (Self-Organization in biological systems with
S. Camazine, N. Franks, J. Sneyd, E. Bonabeau & G. Theraulaz,
Princeton, in press). He is (was) member of the editorial board of numerous international journals and was involved in the organization
of many international conferences. His research concerns the collective intelligence in animal societies and their application to artificial
and human systems. He has developed integrated experimental and
theoretical tools for the study of complexity and self-organisation
in biological systems. Current research projects deal with decisionmaking, information flow, building behavior and pattern formation in insect societies and in group-living organisms. Offshoots in
Swarm-Bot
applied research include collective robotics and transportation systems. He was awarded two prizes of the Belgian Academy and, in
2004, the french Prize Goeffroy Saint-Hilaire for his work on collective intelligence.
Stefano Nolfi is a senior researcher at the Institute of Cognitive Sciences and Technologies of the National Research Council (CNR) in
Rome, where he leads the Laboratory of Artificial Life and Robotics.
He is also associate professor at Lumsa University in Rome. His research interests are in the field of neuro-ethological studies of adaptive behavior in natural and artificial agents and include: evolutionary robotics, artificial life, complex systems, neural networks, and
genetic algorithms. The main tenets underlying his work are that
behavioural strategies and neural mechanisms are understood better when an organism (living or artificial) is caught in the act, that
is, when one considers situated and embodied agents in their interaction with the environment; and that to understand how natural
agents behave and to build useful artificial agents one should study
how living organisms change, phylogenetically and ontogenetically,
as they adapt to their environment. He has published more than 70
peer-reviewed articles and a book on Evolutionary Robotics.
Luca Maria Gambardella is Research Director at IDSIA. His major research interests are in the area of optimization, simulation,
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robotics learning and adaptation, applied to both academic and realworld problems. In particular he has studied and developed several
ant colony optimisation algorithms to solve scheduling and routing problems. In these domains, the best-known solutions for many
benchmark instances have been computed. He is responsible for IDSIA robotics projects. He has led several research and industrial
projects both at national (Swiss) and European level.
Marco Dorigo received the Laurea (Master of Technology) degree in industrial technologies engineering in 1986 and the doctoral
degree in information and systems electronic engineering in 1992
from Politecnico di Milano, Milan, Italy, and the title of Agrégé de
l’Enseignement Supérieur, from the Université Libre de Bruxelles,
Belgium, in 1995. From 1992 to 1993 he was a research fellow at the
International Computer Science Institute of Berkeley, CA. In 1993
he was a NATO-CNR fellow, and from 1994 to 1996 a Marie Curie
fellow. Since 1996 he has been a tenured researcher of the FNRS, the
Belgian National Fund for Scientific Research, and a research director of IRIDIA, the artificial intelligence laboratory of the Université
Libre de Bruxelles. He is the inventor of the ant colony optimization metaheuristic and one of the founders of the swarm intelligence
research field. Its current research interests include metaheuristics
for discrete optimization, swarm intelligence and swarm robotics.
Dr. Dorigo is an Associate Editor for the journals: Cognitive Systems Research, IEEE Transactions on Evolutionary Computation,
IEEE Transactions on Systems, Man, and Cybernetics, and Journal of Heuristics. He is a member of the Editorial Board of numerous international journals, including: Adaptive Behavior, AI Communications, Artificial Life, Evolutionary Computation, Information
Sciences, and Journal of Genetic Programming and Evolvable Machines. He is the author of three books: Robot Shaping, MIT Press,
1998; Swarm Intelligence, Oxford University Press, 1999; and Ant
Colony Optimization, MIT Press, 2004. In 1996 he was awarded the
Italian Prize for Artificial Intelligence and in 2003 the Marie Curie
Excellence Award for his work on ant colony optimization and ant
algorithms.
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