One of the frontiers in intelligent machine research is the understanding of how constructive cooperation among multiple autonomous agents can be effected. The effort at the Center for Engineering Systems Advanced Research (CESAR) at the Oak Ridge National Laboratory (ORNL) focuses on two problem areas: (1) cooperation by multiple mobile robots in dynamic, incompletely known environments; and (2) cooperating robotic manipulators. Particular emphasis is placed on experimental evaluation of research and developments using the CESAR robot system testbeds, including three mobile robots, and a seven-axis, kinematically redundant mobile manipulator. This paper summarizes initial results of research addressing the decoupling of position and force control for two manipulators holding a common object, and the path planning for multiple robots in a common workspace

Fujimura, K.

Mann, R. C.

Unseren, M. A.

English

One of the frontiers in intelligent machine research is the understanding of how constructive cooperation among multiple autonomous agents can be effected. The effort at the Center for Engineering Systems Advanced Research (CESAR)at the Oak Ridge National Laboratory (ORNL) focuses on two problem areas: (1) cooperation by multiple mobile robots in dynamic, incompletely known environments; and (2) cooperating robotic manipulators. Particular emphasis is placed on experimental evaluation of research and developments using the CESAR robot system testbeds, including three mobile robots, and a seven-axis, kinematically redundant mobile manipulator. This paper summarizes initial results of research addressing the decoupling of position and force control for two manipulators holding a common object, and the path planning for multiple robots in a common workspace. 15 refs., 3 figs

Mann, R.C.

Unseren, M.A.

UNT Digital Library

Research and development at ORNL/CESAR towards cooperating robotic systems for hazardous environments

NASA Technical Reports Server

N93-1i 9'79
RESEARCH AND DEVELOPMENT AT ORNL/CESAR TOWARDS
COOPERATING ROBOTIC SYSTEMS FOR HAZARDOUS ENVIRONMENTS
R. C. Mann, K. Fujimura, M. A. Unseren
Center for Engineering Systems Advanced Research
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831-6364
mannrc@ornl.gov
ABSTRACT
One of the frontiers in intelligent machine research is the
understanding of how constructive cooperation among
multiple autonomous agents can be effected. The effort at
the Center for Engineering Systems Advanced Research
(CESAR) at the Oak Ridge National Laboratory (ORNL)
focuses on two problem areas: (1) cooperation by multiple
mobile robots in dynamic, incompletely known
environments; and (2) cooperating robotic manipulators.
Particular emphasis is placed on experimental evaluation of
research and developments using the CESAR robot system
testbeds, including three mobile robots, and a seven-axis,
kinematically redundant mobile manipulator. This paper
summarizes initial results of research addressing the
decoupling of position and force control for two
manipulators holding a common object, and the path
planning for multiple robots in a common workspace.
INTRODUCTION
Interest in issues related to achieving effective coordination
of multiple robotic devices has been growing over the past
few years. There are two main areas of active research
related to cooperative robots: (1) path planning and
navigation for multiple mobile robots sharing a common
workspace, and (2) coordinated or cooperative use of two
or more robot manipulators to perform a set of tasks. A
useful compilation of representative publications on the
topic can be found in [1]. Proceedings of the IEEE
International Conferences on Robotics and Automation
during the past four years also represent a convenient
source of information about recent research efforts.
References [2] and [3] contain useful bibliographies
covering coordinated robot manipulators and path planning
for multiple mobile robots, respectively.
There is a broad spectrum of fundamental problems
associated with cooperating multiple robots, ranging from
high-level planning to low-level control, and to important
architectural and systems issues. Solutions to these
problems will have great impact on the safety and
productivity of operations in a number of applications
including flexible manufacturing, nuclear energy facilities,
environmental restoration and waste management, future
intelligent transportation systems, as well as space and
underwater applications.
CESAR was established during FY-1984 at ORNL for the
purpose of addressing fundamental problems and issues
arising in the development of intelligent machines. The
402
approach at CESAR involves concentrating part of the
resources on the development of an evolving ser_es of
mobile robot prototypes HERMIES (Hostile Environment
Robotic Machine Intelligence Experiment Series). These
machines serve as testbeds for new methods, and
hardware and software developments. They are used in
experimental scenarios that provide the necessary
quantitative data for testing and validation of new
approaches, as well as for performance evaluation of
different robot system components in integrated systems.
HERMIES-IIB and -III (see Figure 1) are currently
operational mobile robots [4, 5]. HERMIES-III is
equipped with the 7 degree-of-freedom CESARm research
manipulator. A second redundant manipulator will be
available in the near future. A third mobile platform with
simultaneous translational and rotational motion capability
as well as on-board VLSI fuzzy logic processors [6] is
also part of the unique experimental facilities. Among the
most recent experiment scenarios for proof-of-principle
demonstrations with the HERMIES robots were the
autonomous clean-up of simulated chemical spills [7], and
autonomous mapping of areas and objects with beta-
radiation contamination (manuscript in preparation).
This paper summarizes recent work at CESAR in the areas
of decentralized path planning for multiple mobile robots,
and control of coordinated manipulators. The current focus
of these activities is on multiple agents with heterogeneous
capabilities, with respect to sensing, manipulation,
mobility, reasoning. Application focus is derived from a
number of applied programs that can benefit from this
research in several problem areas, including site
characterization, construction, remediation.of contaminated
sites, and decontamination and decommissioning of
facilities.
COORDINATION OF MULTIPLE MOBILE ROBOTS
We have been addressing problems associated with global
and local motion planning for multiple mobile robots in a
common workspace. Previous and on-going work in this
area can be broadly characterized into approaches based on
centralized or on distributed planning and control. Early
references concerning these approaches are [8] and [9].
Most centralized approaches assume complete knowledge
of the workspace and capabilities of all the robots, and
proceed to develop a plan which is then followed by all
robots. It was shown that the amount of computation
involved is exponential in the number of robots [I0].
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Central planning for all robots in a prioritized order has
been employed [11] to avoid this complexity, at the
expense of sub-optimal solutions.
In distributed approaches, each robot develops a plan
based on information available about motions of the other
robots in the workspace. This information can be rather
limited, and usually depends on the robot's sensory
capabilities and/or on the communication bandwidth
among the different robots. Solutions can generally not be
guaranteed to be globally optimal, and deadlock situations
may occur. Recent reports on distributed approaches
include [9, 12, 13] . Among the simplifying assumptions
made in many of these contributions is the homogeneity of
the robots with respect to mobility, sensing and reasoning
capabilities.
Motivated by many potential application areas for multiple
mobile robots, e.g., nuclear environments, space,
environmental restoration and waste management, we have
been working on the problem of decentralized motion
planning for multiple heterogeneous mobile robots in a
common workspace.
A computer simulation system has been developed in
which the model for each robot or agent consists of three
modules: a planning algorithm, knowledge about the
environment, and an action interval. These modules
determine how well the agent can navigate to a destination
point based on knowledge about the environment, how
much the agent knows about the current status of its
workspace, and how quickly the agent can react to changes
in the workspace. Each robot maintains a local map of its
environment. The scope of a robot's sensor(s) is reflected
in this map. The planning module generates a path based
on the information in the map. The agents are treated as
completely independent without direct communication
among them. Motion planning is based either on visibility
[9] or on accessibility [14]. A robot computes estimated
future locations of other robots and moves in one of the
directions along which a future collision can be avoided.
Collisions are avoided by changing the direction of
motion, while moving at a constant speed. Each robot
repeats the process of planning, acting (moving), and
updating its map at a frequency determined by the action
interval.
_0
VIS, Sensor scope=2)
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Agent 1 (,_L_C,4 Sensor scope-2)
G1 Gc
2
0
,i for Agent 1
G2 Goal for Agent 2
Figure 2. Example of paths followed by two robots with
different planning algorithms.
404
The simulation system was recently described in detail [3].
Among others, our results show that distributed mobile
robots can coordinate their motions without deadlocking
when appropriately different obstacle avoidance strategies
are used by different mobile robots. The simulation system
is a useful tool for the investigation of many issues
concerning sensing and planning under uncertainty,
including reactive and high-level planning for multiple
mobile robots.
COOPERATING ROBOT MANIPULATORS
Advancements of our basic understanding of how to
accomplish efficient multi-arm manipulation will represent
benefits for many application areas for advanced robots in
unstructured environments. The effort at CESAR has
focused so far on issues arising when two manipulators
hold a common object.
Two cooperating manipulators can lift and transport an
object whose mass and/or geometry, (e.g., a long, rigid
beam) is beyond the carrying capacity of a single
manipulator. Two manipulators can directly assemble two
parts into a rigid end-product where each part is held by a
manipulator. This typically allows for the reduction or
even elimination of the expensive, custom-made fixturing
often required for a single manipulator to accomplish the
same task. Unfortunately, when two manipulators
mutually hold an object, the three form a single closed-
chain mechanism and a loss of degrees of freedom(DOF)
occurs. Strong kinematic and dynamic interactions
between the manipulators due to the shared payload result
in a constrained system motion. Clearly the manipulators
cannot function independently in this closed-chain
configuration.
In order to better understand the problems associated with
two manipulators lifting and transporting a common
object, an adequate process model describing the dynamic
behavior of the constrained system is required. In [2], a
rigid body dynamical model has been developed for two
structurally dissimilar manipulators holding a rigid object
in a three-dimensional workspace. The configuration of
the system is shown in Figure 3. The final model consists
of two sets of equations. One set constitutes the reduced
order model which governs the motion of the closed-chain
system. The generalized contact forces imparted to the
common load by the manipulators are eliminated from the
reduced order model but are calculated separately by the
other set of model equations. A nonlinear control
architecture consisting of the sum of the outputs of two
controllers is suggested, which according to the model,
leads to the exact decoupling of the position- and force-
controlled DOF during motion of the system. The
composite controller enables the designer to develop
independent, non-interacting control laws for the
simultaneous position- and force-control of the closed-
chain system.
The approach given in [2] has been generalized to the case
of two manipulators holding two types of complex
payloads: (i) a spherically jointed object and (ii) a part
containing a revolute joint. In [15], the problems of
dynamically distributing the jointed loads and of
quantifying and controlling the internal stress, strain, and
torsion component of the generalized contact forces which
were not addressed in [2] are discussed. The problem of
solving the rigid body system model for the forward
dynamics(i.e., to determine the output response to given
applied inputs) is demonstrated to be well-specified,
whereas the solution of the model for the inverse dynamics
(i.e., to determine the required inputs when the desired
output response of the system is given) is underspecified.
It is shown that the number of configuration DOF lost due
to the imposition of the kinematic constraints is the same as
the number of DOF gained for controlling the internal
stress contact forces which do not induce motion in the
shared, jointed payload. The composite control
architecture proposed in [15] completely decouples the
position- and internal stress force-controlled DOF.
CONCLUSIONS
This report provides a brief synopsis of research at
ORNL/CESAR in the area of cooperating multiple robots.
Initial efforts have been focusing on decentralized path
planning for multiple mobile robots, and on controlling
two robot manipulators holding a common object. A
simulation system was developed that allows for
investigations of different planning strategies for multiple
mobile robots that differ with respect to their sensing,
mobility, and reasoning capabilities. The system supports
the study of different reactive behaviors as well as high-
level control approaches. A rigid body dynamical model
was developed for two structurally dissimilar manipulators
holding a rigid object in a three-dimensional workspace. A
nonlinear control architecture was suggested, which
according to the model, leads to the exact decoupling of the
position- and force-controlled DOF during motion of the
system. This approach has been generalized to the case of
two manipulators holding two types of complex payloads:
(i) a spherically jointed object and (ii) a part containing a
revolute joint.
Results and lessons learned form these initial studies are
now being transferred into efforts that are more
experimentally oriented and are making use of the CESAR
mobile robots and manipulator(s) for proof-of-principle
demonstrations that are relevant to a number of applied
programs.
REFERENCES
1. "Multirobot Systems", Ed. by R. Mehrotra, M. R.
Varanasi, IEEE Computer Society Press, 1990.
2. Unseren, M. A., "Rigid body dynamics and decoupled
control architecture for two strongly interacting
manipulators," Robotica (accepted for publication).
3. Fujimura, K., "A model of reactive planning for
multiple mobile agents, IEEE International
Conference on Robotics and Automation, pp.
1503-1509, Sacramento, CA, April 1991.
4. Burks, B. L., deSaussure, G. L., Weisbin, C.R.,
Jones, J. P., and Hamel, W. R.,
"Autonomous navigation, exploration and
recognition," IEEE Expert, Winter 1987, pp. 18-
27.
5. Weisbin, C. R., Burks, B. R., Einstein, J. R.,
Feezell, R. R., Manges, W. W., and
Thompson, D. H., "HERMIES-IIII: a step
towards autonomous mobility, manipulation
and perception," Robotics, 8, 1990, pp. 7-12.
6. Killough, S. M. and Pin, F. G., "A fully
omnidirectional wheeled assembly for robotic
vehicles," ANSI Vol. 61, pp. 425-426, American
Nuclear Society 1990 Annual Meeting
Proceedings, 1990.
7. Reister, D. B., Jones, J. P., Butler, P. L.,
Beckerman, M., and Sweeney, F. J., "Demo89 -
the initial experiment with the HERMIES-II robot,"
IEEE International Conference on Robotics and
Automation, pp. 2562-2567, Sacramento, CA,
April 1991.
T 2
N-_ ^", "2 I II I II
T w z,2, u 'o
(N 2 -1)
(i) /x° X
W
Figure 3. System configuration and coordinate system assignment.
405
8. Erdmann, M. and Lozano-Perez, T., "On multiple
moving objects," Algorithmica 2, 4, pp.477-522,
1987.
9. Tournassoud, P., "A strategy for obsacle avoidance and
its application to multi-robot systems,"
Proceedings of WEE International Conference
on Robotics and Automation, pp. 1224-
1229, San Francisco, CA, April 1986.
10. Spirakis, P. and Yap, C., "Strong NP-hardness of
moving many discs," Information Processing
Letters, 19, pp. 55-59, 1984.
11. Buckley, S. J., "Fast motion planning for multiple
moving robots," Proceedings of the IEEE
International Conference on Robotics and
Automation, pp. 322-326, Scottsdale, AZ, May
1989.
12. Saito, M. and Tsumura, T., "Collision avoidance
between mobile robots," Proceedings of IEEE/RS]"
International Workshop on Intelligent Robots and
Systems, pp. 473-478, Tsukuba, Japan,
September 1989.
13. Lee, Land Bien, Z., "Collision-free trajectory
control for multiple robots based on neural
optimization network," Robotica 8, 3, pp. 185-
194, July-September 1990.
14. Fujimura, K. and Samet, H., "Time-minimal paths
among moving obstacles," Proceedings of the
IEEE International Conference on Robotics and
Automation, pp. 1110-1115, Scottsdale, AZ,
May 1989.
15. Unseren, M. A., "A rigid body model and decoupled
control architecture for two manipulators holding
a complex object," Robotics and Autonomous
Systems (accepted for publication).
406


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