It is not yet clear of what type, and how much, intelligence is needed for a planetary rover to function semi-autonomously on a planetary surface. Current designs assume an advanced AI system that maintains a detailed map of its journeys and the surroundings, and that carefully calculates and tests every move in advance. To achieve these abilities, and because of the limitations of space-qualified electronics, the supporting rover is quite sizable, massing a large fraction of a ton, and requiring technology advances in everything from power to ground operations. An alternative approach is to use a behavior driven control scheme. Recent research has shown that many complex tasks may be achieved by programming a robot with a set of behaviors and activation or deactivating a subset of those behaviors as required by the specific situation in which the robot finds itself. Behavior control requires much less computation than is required by tradition AI planning techniques. The reduced computation requirements allows the entire rover to be scaled down as appropriate (only down-link communications and payload do not scale under these circumstances). The missions that can be handled by the real-time control and operation of a set of small, semi-autonomous, interacting, behavior-controlled planetary rovers are discussed

Miller, David P.

English

NASA Technical Reports Server

N91-20686
The Real.Time Control of Planetary Rovers Through Behavior Modification
David P. Miller
Jet Propulsion Laboratory/California Institute of Technology
M.S. 301-440, 4800 Oak Grove Drive
Pasadena, CA 91.109
(8 ! 8) 354-9390
dmiller@ ai.nasa.jpl.nasa.gov
Abstract
It is not yet clear of what type, and how
much, "intelligence" is needed for a planetary rover
to function semi-autonomously on a planetary
surface. Current designs assume an advanced AI
system that maintains a detailed map of its
journeys and the surroundings, and that carefully
calculates and tests every move in advance. To
achieve these abilities, and because of the
limitations of space-qualified electronics, the
supporting rover is quite sizable, massing a large
fraction of a ton, and requiring technology advances
in everything from power to ground operations.
An alternative approach is to use a behavior
driven control scheme. Recent research has shown
that many complex tasks may be achieved by
programming a robot with a set of behaviors and
activating or deactivating a subset of those
behaviors as required by the specific situation in
which the robot finds itself. Behavior control
requires much less computation than is required by
traditional AI planning techniques. The reduced
computation requirements allows the entire rover to
be scaled down as appropriate (only down-link
communications and payload do not scale under
these circumstances). This paper discusses the
missions that can be handled by the real-time
control and operation of a set of small, semi-
autonomous, interacting, behavior-controlled
planetary rovers.
1. Introduction:
There are many possible uses for unmanned
planetary rovers. Rovers with a high degree of
autonomy can carry out missions that serve
science, operations, and space exploitation goals.
For example, rovers can be used on the Moon to
perform site certification for possible manned
outposts and science instrument sites. On Mars,
science instruments need to be placed and soil and
rock samples need to be gathered from a wide
variety of terrains. To reduce light-time delays and
the need for communications (and its inherent
infrastructure of relay satellites etc), rovers with at
least semi-autonomous capabilities, are highly
desired.
1.1 Plan Control for Rovers
The autonomous system control that has
been proposed for a rover, to accomplish the tasks
mentioned above, is shown in Figure 1. The rover
senses its environment, combines that with
previous knowledge (from earlier and orbital views)
and then builds a map of its surroundings. A path
planner finds a trajectory through the map. The
trajectory is simulated, producing run-time
expectations, and these expectations are monitored
during the actual rover traverse. If an expectation is
violated, the rover performs a reflex stop, and starts
the cycle over again. Under normal circumstances
the cycle is repeated every five to fifteen meters.
Such a system has been successfully implemented,
and tested under realistic conditions [Miller89,
Gat90].
The implemented system required just under
one billion machine instructions per meter of
travel. While the code used in this experimental
system was by no means optimal, by the time all
the functionality and reliability improvements are
made, it is believed that the real number will be
within a factor of three. This means that a rover
that needed to travel at a speed of one kilometer per
hour would need a (space qualified) computational
capability of between 80 and 250 MIPS. Some of
this could be offloaded onto special purpose
computation systems, but none the less,
computation becomes a major driver for a planetary
rover, both in power and mass.
The system studies that have been undertaken
[Pivirotto90] have confirmed this, indicating that a
rover with some onboard autonomy would need to
mass between 600 and 842kg (the test vehicle we
used massed over 1100kg). In large part, this mass
is due to the system control algorithms.
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https://ntrs.nasa.gov/search.jsp?R=19910011373 2020-03-19T19:05:14+00:00Z
1.2 Low.Mass Rovers are Needed
For almost all imagined uses of rovers, the
job is best done ff it can be done several times, in
many places, under differing conditions. Traditional
rovers weigh several hundred kilograms, and under
the best circumstances will probably be able to
move a few kilometers a day. It will not be
economically feasible to place more than a few
such rovers on a planetary surface. Nor will it be
logistically possible to have those few rovers visit
all the desired sites. Additionally, the risk of losing
one of these scarce and expensive resources will
make it difficult to put a rover in the places where
it could do the most good: in the previously
unmapped or geologically unknown areas of a
planetary_.
S_mf_ _ skp_
W_ta Su_ Aa
Figure 1. Planning Cycle
All of the problems mentioned above go
away ff a low-computation method for
autonomously controlling rovers could be
developed. Low computation greatly lowers the
power needs of the rover, which can pearly reduce
its mass. Once the power mass cycle is broken, it
is possible to scale the entire rover. The strength to
weight ratio of mechanical systems improves as a
system is scaled smaller. This in turn makes the
system stronger and stiffer, which will reduce the
accuracy of the control needs, further reducing the
power and payload needs of the rover, allowing it to
be scaled even smaller. The rover cannot be made
arbitrarily small, because it is necessary for it to
carry some sort of payload, but improvements in
instrument miniaturization [Waltman89] indicate
that that may not be much of a limitation. A rover
must also travel through its environment, but
remember that an ant can travel anywhere a person
can, and many more places in addition.
To keep this all within todays technology, if
one could build mini-rovers that massed a few
kilograms, and could operate autonomously, then
many useful things could be done with them. Mini-
rovers could be sent in far greater numbers, and to
potentially far riskier sites. It now appears feasible
to design a small rover that can accomplish most of
the science and operations objectives that
previously would have been handled by a large
rover. Key in building a mini-rover is behavior
programming.
2. Behavior Control for Rovers
Behaviorprogramming isa methodology
thatallowsa setofinteractingreactionstobe
programmed into a robot so that they work
together. For example, if a robot has a behavior
that causes the robot to turn so as to maximize the
value being returned by its fight side range finder,
and has a behavior to cause the robot to turn away
when its left side proximity sensor is activated,
then that robot will exhibit the behavior of
waveling around the perimeter of a room in a
clockwise direction.
Some behavior languages have been created
[Brooks86, Gatg0], and several robots have been
programmed with these languages to perform some
interesting tasks [Brooks89, Gatg0]. Central to
having a robot perform something of interest is the
ability to have the robot's behavior modified by
cues in the environment, or by the robot's own
actions.
Figure2 shows thesensorsand actuatorsofa
small robot named "Tooth". Tooth is a robot
massing just under two kilograms, and containing
two eight-bit micro-processors and four kilobytes
of memory. Its memory is filled with the behavior
program shown in Figure 3. Tooth's mission is to
go around a room, picking up "toys" that have been
left near the walls of the room, and to bring the
toystoabeaconlocatedsomewhere nearthecenter
of the room. The robot carries out this task using
nine interacting behaviors. The look for toy
behavior just has the vehicle move along in a
straight line. The follow light behavior uses the
photocells to decide on the direction of the beacon.
If the beacon can be seen, this behavior sends a
steering command away from the beacon. This
forces the robot towards the wails. The dead-end,
obstacle avoidance, unthrash, and stall behaviors all
keep the robot from crashing into anything, or
getting stuck. When the object beam sensor detects
something, then the robot uses the pickup toy
behavior to grasp the object. If it is successful,
then that behavior causes the steering signal from
the follow-light behavior to be inverted. This
causes the robot to head towards the beacon. When
363
the photocells report a light above a certain
threshold brightness, then the drop toy behavior
turns on. This overrides the look for toys behavior,
causes the robot to stop, drop the toy and back
away a few centimeters. Since the robot is no
longer holding the toy, the follow-light signal is
no longer inverted, and the robot turns away from
the light and goes in search of more toys. [Gat90]
gives more details on these exveriments.
m
Figure 2: The Tooth Robot
The experiment outlined above shows that
simple behaviors can be linked together to exhibit
complicated actions. The experiment above had the
robot perform all the major parts of a sample return
mission. The robot moved following certain
parameters. When it came across something that
matched its sample criteria, it acquired it. It then
brought the sample back to a marked beacon
(simulating a return vehicle dock). All the while,
the robot avoided obstacles in its path. This
technology is sufficient for a variety of missions.
3. Target missions:
There are many missions that can be
performed by behavior-controlled robots. The
advantages of these small, autonomous rovers are
many. Because of their small size, low mass, and
high strength, these rovers could be placed on a
planetary surface without the expense and mass of a
traditional soft lander. [Miller90] outlines how
many mini-rovers could be landed on Mars using
parachutes and areoshells, and how communications
can be maintained through ground relays. On the
Moon, mini-rovers could be landed using an
updated version of the Ranger seismometer capsule
[Ranger63]. Navigation can be handled by
referencing the robot to a coded radio signal, such
as that used in VOR aviation radios.
Figure 3: The Behavior Program for
Tooth
3.1 A Lunar Mission
On the Moon, one of the major activities of
rovers would be deploying science instruments
(e.g., VLFA). The VLFA is large antenna
consisting of several hundred elements laid out over
a sixty kilometer long spiral arc. The exact
placement is not crucial, but the elements should
be distributed evenly along the arc. The elements
themselves are self contained and mass
approximately one kilogram. A previous study
undertaken by the Battelle Corporation [Easter88]
chose to use 1060kg rover that could carry the
approximately 300kg of antenna elements. The
rover contained large robotic arms for implanting
the antenna elements. Since the VLFA is to be
placed on the Lunar Farside, a series of relay
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satellites would be placed in lunar orbit to allow
near continuous communications with the rover.
There are several ways the VLFA could be
set up using behavior controlled mini-rovers. The
simplest method would be to build one antenna
element into each mini-rover, and land
approximately 300 on the lunar farside. It would be
easy to have the rovers distribute themselves
relative to one another, in the desired pattern. This,
most inefficient use of mini-rovers, would still
result in a considerable mass savings over the
Battelle proposal.
One could also send up, say fifty mini-
rovers, that could each retrieve antenna elements
from a central cache. It would then be trivial to
have the rovers place the antenna elements in the
proper arrangement about a VOR transmitter
located at the cache. Each rover would have
prestored the coordinates (radial and range) of the
six antenna elements it was to place. Simple
behaviors would cause the rovers to home in on the
transmitter 611 they could spot an antenna element.
Once they picked that up, they would circle the
transmitter 611 they located the proper radial. They
would then hem outward along the radial the proper
distance and deposit the antenna element. No longer
holding anything, they would head back towards the
cacheand repeat the process with the next set of
position coordinates. This scheme would require
only about 500kg landed on the Moon, much lower
lander masses, and onlythe relaysatellites required
by the VLFA.
Any activity that needs to take place in a
pattern can easily be done with behavior-controlled
mini-rovers. A simple non-directional radio-beacon
along with a range encoder can be used to have a
rover go in circles, spiral in or out, or rendezvous
at the beacon. By adding the radial information that
comes from a VOR, a mini-rover could have its
behaviors direct it to a specific point relative to the
beacon. All the while, other behaviors can be used
to keep the rover from hanging up on obstacles, or
gettingstuckindead-ends.
3.2 A Mars Mission
On Mars, there are several unanswew.d
questionswhich require roverstowork atmany
diverselocations.Inparticular,the searchfor
carbonates, and the emplacement of science stations
(seismic and meteorological) are very suitable for
behavior-controlled mini-rovers. These tasks require
rovers in many different terrains in areas that are
many thousands of kilometers apart, and are most
useful in areas where the rover may never be able to
get out (eg., inside extinct volcano craters). Here,
the basic philosophy should be put down a lot of
rovers, all over the place, and have them report
back when they find something interesting.
More narrow scope missions on Mars are
also applicable. A question, which could be key
for the design of a sample return mission, is the
thickness of the weathering rind on Mars rocks.
Scientific opinion ranges from a millimeter to
several centimetea_. The extent of the rind will
determine the type of coring that will be necessary
for a sample return mission. A small rover, armed
with a small circular saw, would be able to answer
this question (or at least determine if the rind is
more than a centimeter). Such a rover would have
behaviors to get it close up to rocks of various
sizes. It would have a list of criteria, as soon as it
found a rock matching some of those, it would
come up to the rock, and using the control
behaviors, cut off a slice. It would then take
several images and relay them to Earth, then go off
to find its next sample. With the proper cameras,
such a rover could gather information about the
weathering rind, and about the makeup of many
Mars' rocks.
The weathering rind mission should be done
very soon. Such a mission could be done very
economically. Two or three mini-rovers would be
dropped to the planet's surface viaparachutes. A
small relay orbiter would be left in orbit. Each
rover would find an appropriate sample, take the
images, and send a signal to the orbiter. When the
orbiter was in range, it would broadcast a ready to
receive signal, and the rover could dump the images
directly from the camera CCDs to the orbiter. This
way, the rovers need no mass storage, and can get
by with very simple electronics.
The information from these images is crucial
in designing a proper rover for a detailed science
sample return mission. If the weathering rind is
several centimeters then a rover large enough to do
rock coring is necessary for many experiments. If
the rind is on the order of a millimeter, then a
sample retarn mission using behavior-controlled
mini-rovers (see [Miller90]) would be more than
adequate, and much more cost-effective.
4. Conclusions
Controlling a rover through behavior-control
can allow such a rover to be greatly reduced in size
365
and complexity with little or no loss in capability.
Planetary rover missions, in most cases, are rover
missions because the mission objectives require the
sensors to be in many different locations. All
planetary missions are mass constrained. Behavior
controlled mini-rovers are a way of getting beyond
the mass constraints and increasing the effective
mobility of the system. Mobility is increased by
making the myers more autonomous (detailed
direction from Earth does not fit well with the
behavior-control paradigm) and by being able to
provide more rovers, dropped at more locations, for
a given mass allolment, then is possible using
traditional control or planning techniques.
Behavior-control, when combined with nano-
technolgy, also holds the promise of being able to
undertake wholly new types of missions
[Brooks89].
Acknowledgements: The research described in
this paper was carried out by the Jet Propulsion
Laboratory -- California Institute of Technology
under a contract with the National Aeronautics and
Space Administration.
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The real-time control of planetary rovers through behavior modification

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