a multilegged spider, able to stride over rocks and bumpy terrain. This robot, dubbed .... the terrain, it can flow over obstacles and has even climbed stairs.
ROBOTICS
When a task or terrain varies, reconfigurable robots can change their shape to get the job done
Modular Robots BY MARK YIM, YING ZHANG & DAVID DUFF Xerox Palo Alto Research Center (PARC)
IEEE SPECTRUM
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assorted abilities at Stanford University, also in California. Systems of this kind would be useful for remote, autonomous operations, particularly in hostile environs such as under the sea, at the scene of a natural disaster, or on other planets. Researchers not only at PARC but also at Johns Hopkins University, in Baltimore, Md., the National Institute of Advanced Industrial Science and Technology, in Tsukuba, Japan, the Information Sciences Institute at the University of Southern California, and elsewhere have been experimenting along these lines. The systems they have built can reconfigure themselves automatically, with no help from outside, to tackle whatever tasks and terrain they encounter.
Three promises Modular reconfigurable robots are built up from tens to hundreds, and potentially millions, of modules. These, like cells in a
PHOTOGRAPHS: ROBERT SCHLATTER
• February 2002
R
obots out on the factory floor pretty much know what’s coming. Constrained as they are by programming and geometry, their world is just an assembly line. But for robots operating outdoors, away from civilization, both mission and geography are unpredictable. Here, robots with the ability to change their shape could be of great value, since they could adapt to constantly varying tasks and environments. Modular reconfigurable robots—experimental systems made by interconnecting multiple, simple, similar units—can perform such shape shifting. Imagine a robot made up of a chain of simple hinge joints [see photos, from upper left]. It could shape itself into a loop and move by rolling like a self-propelled tank tread; then break open the loop to form a serpentine configuration and slither under or over obstacles; and then rearrange its modules to “morph” into a multilegged spider, able to stride over rocks and bumpy terrain. This robot, dubbed PolyBot, is being built and experimented with at Xerox Palo Alto Research Center (PARC), in California. In fact, as far back as 1994, a simulation of it was displaying its
human body, are few in type but many in number. Such robots are called n-modular systems (where n is the number of module types). An n-modular system holds out three promises: versatility, robustness, and low cost. Its versatility stems from the many ways in which modules can be connected, much like a child’s Lego bricks. The same set of modules could connect to form a robot with a few long thin arms and a long reach or one with many shorter arms that could lift heavy objects. For a typical system with hundreds of modules, there are usually millions of possible configurations, which can be applied to many diverse tasks. Obviously, turning a bunch of uniform modules into a versatile robot is not child’s play. To put together a useful system, solutions must be found to the complexities of programming a great many coupled, but independent, robotic units. Worse, as more modules are added, many of the programming issues get exponentially harder. These include controlling and coordinating modules to work together effectively and not collide or otherwise interfere with each other. Robustness is born of the redundancy and small number of module types. The units diagnose themselves and each other and compensate for, replace, or reconfigure themselves around any that are malfunctioning. But the overall number of modules is a factor: the more of them there are, the more likely it is that some may fail. Clearly, if just a few modules fail, others may be able to compensate for them. The main advantage of redundancy is that when one or more modules malfunction, overall function degrades gracefully, instead of failing catastrophically. Naturally, such a robotic system must have a control strategy for
dealing with partial failures. Ultimately, the system should be able to repair itself by shedding crippled units. The promise of low cost may be the most difficult to realize. Being few in type, the modules can be mass produced, and as economies of scale come into play, the cost of each one can be reduced. But how cheap can they get? That may really depend on how small they can get. At their current scale of 5 cm on a side, our modules consist of many parts and fasteners that must be assembled, some by hand, but as their size diminishes, batch fabrication becomes practical, even necessary. However, even if the cost of each module is reduced to just US $1, a complete system might require one million modules. Still, even that $1 million price tag might be worth it, especially if one modular robot can adapt to a variety of difficult tasks. Modular robots should have the most impact on those tasks that need versatility. Exploration of a distant planet should be just the thing. Its inherently unknown aspects demand that a system adapt to unexpected situations. The need for self-repair and reliability also looms large since it is difficult, if not impossible, to do on-the-spot repairs, and the cost of transporting the robot to the site is high. Urban search and rescue in buildings badly damaged by an earthquake or a bomb is another promising application.
Dissecting PolyBot PolyBot, the modular robot being developed at Xerox PARC, is a chain reconfiguration robot. As such, it belongs to one of three classes of reconfigurable robots [see “The Three Types of Reconfigurable Robot,” p. 34]. PolyBot, which has been made of as many as 100 modules, has demonstrated several abilities,
Xerox PARC’s multimodule robot, PolyBot, can move in several ways, starting as a rolling tread [first frame]. With the connectors between two modules released, it moves in a snake-like manner [second frame]. With its end modules docked to its center module, it forms a figure-8 [not shown]. Breaking the intermodule connections at the ends of the figure-8 [third frame] leads to a fourlegged spider configuration that walks like a person on crutches [last frame].
IEEE SPECTRUM
• February 2002 31
ROBOTICS Hinge axis
Infrared emitter Infrared detector
Electrical connectors
Pin Shape memory alloy latch
Connection plate
Motor housing
IEEE SPECTRUM
• February 2002
The 25-cm2 connection plate shown on this PolyBot G2 segment mates with an adjacent module. Infrared sensors align the modules for docking, and a latch made of shapememory alloy holds them together. Holes and pins add stability to the connection, with power and data transmitted via electrical connectors. “Under the hood” where they can’t be seen are the microprocessor and memory.
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including locomotion, manipulating simple objects, and reconfiguring itself. Different versions vary the on-board computational power; the on-board battery power; the ability of modules to automatically dock, attach, and detach themselves; and the power of the modules’ motors. The two used most at PARC are known as G2 and G1v4. The more powerful one, G2, is made of just two types of cube-shaped modules: a segment that has a hinge-joint between two hermaphroditic connection plates, and a node, which doesn’t move but has six connection plates [see photo, left]. Most of the functions depend on the hinged segment, which produces the robot’s movement, whereas the node’s job is to provide branches to other chains of segments. In theory, with enough nodes and segments, PolyBot could approximate any shape. Structurally, each segment is roughly the size of a cube about 5 cm on a side and has one motor that rotates the hinge. The two connection plates on either side of the hinge join it to other modules, electrically as well as physically. On every connection plate there are four electrical connectors, each with four contacts; and through the connectors electric power and communications pass from module to module. The communications network uses the CAN protocol (for controller area network), which is a popular automotive serial network standard. For physically docking and undocking, every connection plate also houses a latch. At its heart the latch is a wire made of a shape memory alloy, a nickel-titanium combination that alternates between two shapes when alternated between two temperatures. In this case, resistive heating is used. When current is run through the wire, the latch opens and releases its hold on a neighboring module. Stopping the current allows the latch to close by a return spring. Embedded in each PolyBot segment and node is a 32-bit Motorola PowerPC 555 processor (MPC555) along with 1MB of external RAM. Granted, the MPC555 is a rather powerful processor to have on every module, and its full processing power is not yet utilized. However, the goal of this research is a large, multipurpose, fully autonomous robot, which may require the complete use of these processors and memory. The G2 has two kinds of sensors: position sensors, to determine the angle between the two connection plates, and proximity sensors.
Walk This Way
module G1v4 version of PolyBot needed only
Using a snake configuration and up to
one charge to travel half a kilometer in
32 modules, PolyBot has overcome a variety
45 minutes. By using sensors to conform to
of obstacles, including crawling through
he PolyBot series of robots built at
T
the terrain, it can flow over obstacles and has
10-cm-diameter aluminum pipes, up
Xerox Palo Alto Research Center
even climbed stairs. The rolling tread has lim-
30-degree ramps, over chicken wire, and
(PARC) in California have demonstrat-
its, though. It makes wide turns—it’s tricky to
over loose debris and wooden pallets.
ed a variety of capabilities, configurations,
get a rolling tread to bend to the left or right.
A four-legged spider configuration
and ways of moving [see photos, pp. 30–31].
Snake-like gaits excel at traveling over or
walks somewhat like a person on crutches,
Obviously, for locomotion over roads and
through obstacles: a linear set of modules
two legs moving at a time [photo, right,
smooth terrain, it is hard to beat wheels. But
[next photo] can stretch longer distances
p. 31]. A realistic spider-like crawl will
although PolyBot does not have wheels, it
than can a nonlinear form, so as to cross
require a more finely tuned gait control
can turn itself into something very similar.
small ditches or go down steps.
algorithm and perhaps another pair of legs.
A chain of modules in a loop forms a
Crossing wide gaps is less to these gaits’
In a lizard configuration [not shown], four
self-propelled tank tread, which proves to be
liking: it demands more torque of the motors
modules functioning as feet connect in the
the gait that makes the most efficient use of
because the torque required at the base of
middle to a fifth module as a waist. The entity
power [see far left photo, p. 30]. Using off-
the cantilevered modules increases with the
walks like a lizard, with a large oscillating lat-
the-shelf rechargeable AAA batteries, a 10-
square of the number of those modules.
eral motion from the waist. By changing the
The first are Hall effect sensors, which measure voltage induced by magnetic flux to determine the motor’s angle with a resolution of 0.45 degrees. These also serve for commutation and are built into the segment’s 30-W brushless DC motors, which can generate 4.5 newton-meters of torque. The proximity sensors are infrared detectors and emitters mounted on the connection plates. They serve primarily to aid in docking two modules but can also be used to help the robot maneuver in tight spaces. In the next version of PolyBot, G3, the plan is to equip the segments and nodes with other proximity, tactile, and force/torque sensors, plus possibly a low-resolution CMOS camera. These sensors and the camera will help the robot with manipulating objects and interacting with the environment. A less elaborate PolyBot, the G1v4, was built mainly to prototype and evaluate a variety of configurations and gaits [see “Walk This Way,” below]. Its modules are of one type only, a hinge, and because they cannot dock and undock automatically, they must be plugged together by hand. It is powered by on-board batteries and controlled by an 8-bit microcontroller, the PIC16F877 from Microchip Technology Inc., Chandler, Ariz. PolyBot is by no means the only chain-reconfiguration robot. In the Conro system, built at the Information Sciences Institute at the University of Southern California, in Marina del Rey, every module is like every other, with two small hobby servomotors that actuate right-angled hinge joints controlled by an 8-bit microcontroller. The modules communicate with their neighbors through an infrared interface. Rather than hermaphroditic connection plates, Conro’s modules have three male connectors at one end and one female connector at the other. A system like this will easily form tree-like structures (the same structure as limbed animals) as well as structures with single loops but none with more than one loop—no figure-8s.
Programming perplexities Programming the movements of n-modular systems is a struggle. As the number of modules grows, the complexity of many of the computational tasks explodes. At the same time, though, because each module has its own computer, the com-
Gait Control How the robot moves is determined by the angle between the connection plates that each module’s motor makes. The angles, downloaded from a gait control table, result in a sequence [top to bottom] that propels the robot. -20°
Connection plate
...four steps later
putational resources increase, but only linearly. Further complications accrue from increases in the number of module types, the distributed nature of the resources, constraints posed by torque limits of the motors, failing modules, and limited communication bandwidth. To keep confusion at bay, three control techniques are being tried: gait control tables, an unusual messaging method, and a hierarchical organization. A gait control table stores precomputed motions for reference [see illustration, above]. Simple open-loop control instruc-
pattern in which its four feet lift up, the
The robot lifts the rearmost spike and
robot will walk forward or backward or
inserts it at a higher point than the
turn in either direction.
next-rearmost spike, and so on, in a wave traveling from its tail to its head.
form with up to 12 legs, one-, two-, and
A similar method is used to climb ver-
three-module legs are attached to the
tical chain-link fences, but with hooks
bottom of a rectangular plate. When the
instead of spikes attached to modules. For a robot to be truly versatile, it
the correlation between locomotion
must be able to operate human tools.
and manipulation is revealed: the legs
As a first step toward that goal,
become fingers that can manipulate
the robot was shaped into a
balls, boxes, paper, and other objects.
pair of legs and mounted on a tricycle. Each leg moved
Polybot G1v4 has climbed nearly verti-
the pedals successfully, in a,
cal porous materials (like a tree or a
well, cyclic fashion [see
ceiling tile) by using a short spike at-
photo, right].
tached to the bottom of some modules.
—M.Y., Y.Z., & D.D.
• February 2002
walking platform is turned upside down,
IEEE SPECTRUM
To build a multilegged walking plat-
Employing an inchworm-like gait,
+20°
Motor
33
ROBOTICS
The Three Types of Reconfigurable Robot
R
obots that can change shape can be
rolling like a tank tread, climbing stairs, slith-
With its less demanding programming,
classified in terms of how they do so.
ering like a snake, climbing like a caterpillar,
this class currently has the most research
They are built for chain, lattice, or
and walking like a spider.
groups working on it, including ones led by
IEEE SPECTRUM
• February 2002
mobile reconfiguration.
34
Lattice robots change shape by moving
Daniela Rus at Dartmouth University, in
The chain kind make themselves over by
into positions on a virtual grid, or lattice.
Hanover, N.H.; Cem Unsal at Carnegie Mel-
attaching and detaching chains of modules to
They are like pawns moving on a chessboard,
lon University, in Pittsburgh; and Greg
and from themselves, with each chain always
except this board has three dimensions. As
Chirikjian at Johns Hopkins University, in
attached to the rest of the modules at one or
with chain robots, all the modules remain
Baltimore, Md.
more points. Nothing ever moves off on its
attached to the robot. Planning and control
Mobile self-reconfiguring robots change
own. The chains may be used as arms for
issues become less complex when the modules
shape by having modules detach themselves
manipulating objects, legs for locomoting, or
may move only to neighboring positions with-
from the main body and move independently.
short tentacles for both manipulation and
in a lattice instead of to any arbitrary position.
They then link up at new locations to form new
locomotion. Xerox Palo Alto Research Center
The robot need only deal with what is occu-
configurations. This type of reconfiguration is
(PARC) is focusing on this class, which it has
pying the limited number of neighboring posi-
less explored than the other two because the
found to be the most versatile. A chain robot
tions in the lattice: for example, four positions
difficulty of reconfiguration tends to outweigh
has already demonstrated locomotion by
for a module that moves on a flat square grid.
the gain in functionality.
tions coupled with the mechanics of the configuration suffice for many of the capabilities demonstrated so far, including the snake, loop, and spider gaits [see again, the photos on pp. 30–31]. Most often, one module contains the set of gait control tables, which are downloaded as needed to the other modules. Still, beyond a very minimal point, no table could hold all of a robot’s possible gaits and configurations, because that number soars exponentially with the number of its modules. A PolyBot with 10 segments and two nodes, say, could form hundreds of distinct configurations, while another with 100 segments and 10 nodes could make well over a million. For any given application, though, a robot relies on a fixed and relatively small set of configurations, determined by analyzing the task to be performed. The sequence of motions by which the robot changes configuration is then planned and stored in a table. This approach does not fully exploit the versatility of the system, both for self-repair and for adaptation to the task in hand. For that, the robot would have to be able to reconfigure itself into arbitrary shapes. This is something researchers at PARC are working on. An alternative to gait control tables is a message-passing method developed by the University of Southern California’s Information Sciences Institute. The novel technique is modeled on the way a single hormone may produce a variety of responses throughout the human body. Rather than specific instructions being sent to each module, a single message flows from module to module. It is modified by some of them as it passes through and therefore sends dissimilar messages to and produces different effects on other modules. The state of the module to which the message is passed—the joint angle, for instance—dictates whether and how the message is altered. The same message, then, could change the motor angle in one segment, not change it in the next, and delete itself in the third. Another way of simplifying programming, which is well suited to chain-type robots, is to divide the robot into hierarchical portions, rather as a finger, hand, and arm form a hierarchy within
—M.Y., Y.Z., & D.D.
the body. For example, if an elbow is moved, the hand attached to it and the fingers on that hand also move. Several modules can be grouped into larger virtual modules, which can then also be grouped and a hierarchy formed. Such an organization simplifies the programming, because the motions of the modules within the smaller virtual modules matter less. A hand need not know how it got over the keyboard, just that it got there; and a shoulder couldn’t care less whether a finger typed “y” or “m,” just that the shoulder pointed the upper arm toward the desktop. In planning reconfiguration, one must first consider how the modules need to be connected, and then plan the motions needed to get there. Much as a human can be drawn as a stick figure, the connectivity of a configuration can be drawn as a graph, with lines for the segment chains and intersections of lines where nodes need to be. Graph theory provides the tools for manipulating such graphs.
The future is modular The first two generations of PolyBot have shown some of the versatility possible with these systems, most notably the use of self-reconfiguration to adapt to changes in the environment or the task. New versions of PolyBot and other modular robots are being constructed to explore other issues. For example, the next PolyBot generation, G3, will grapple with robustness and self-repair as aspects of reliability, for G3 will have over 100 modules, an order of magnitude more than any other modular robot so far. G3’s goal is to move autonomously, shift lightweight objects blocking its path, and reshape itself while moving through a pile of rubble as part of a search-and-rescue task. To cope with some of the issues that will arise with G3 and more sophisticated robots, PARC engineers plan to look to biology to see how nature solves the same problems of complex control, self-repair, and efficiency. Then we plan to adopt similar solutions in a modular robot.
•
Samuel K. Moore, Editor
