Exploring Mars Using Intelligent Robots
The use of robotic rovers is an attractive and necessary option if
exploration of Mars is to go forward.Having decided on this route,further
problems come to surface.The delay for radio signals between Mars and Earth vary
between 6 to 41 minutes while the long distance imposes a low communication
bandwidth.This precludes the use of teleoperation for controlling the vehicle.(A
teleoperated vehicle is one which every individual movement would be controlled
by a human being). Therefore,some autonomy of the vehicle is needed.However,a
totally autonomous vehicle that could travel for extended periods carrying out
its assigned tasks is simply beyond the present state of the art of artificial
intelligence. This report considers the technical issues involved in the
operation of a Mars Exploration Rover. In particular,the various navigation
techniques and related technologies are discussed,while up to date robots and
their performance are used as examples.
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About to land on Mars ,source
http://nssdc.gsfc.nasa.gov/planetary/mesur.html
As mentioned earlier,the Mars
rover must possess a certain degree of autonomy.There are various degrees of
autonomy and various approaches in the way this autonomy is granted to the
rover.These factors determine the navigation technique used,but all fall under
two broad categories:
- Path-Planning navigation and
- Reactive navigation (or Real-time obstacle
avoidance navigation)
In Path-planning,some form of terrain
analysis is performed and a safe route is decided before the vehicle is
commanded to start moving.In reactive navigation,the rover moves towards a goal
location and avoids obstacles or untraversable territory as it encounters
them,without previous knowledge of their existence.
Two important path-planning techniques developed at the Jet Propulsion
Laboratory (JPL) are Computer-Aided Remote Driving (CARD) and
Semi-Autonomous Navigation (SAN) .
2.2 Semi-Autonomous Navigation (SAN)
In Semi-Autonomous Navigation
[5,6,7],the rover is given approximate routes from Earth,but plans its local
routes autonomously.Thus,some operations are performed on Earth while others
onboard the vehicle.
In this scenario,a satellite orbiting Mars sends stereo images of the areas
of interest to the ground-station on Earth.These images may have a resolution of
about a metre and enable operators to plan a safe route for the vehicle,possibly
a few kilometres in length.In addition to path planning,an elevation map
is produced by computers.Both the elevation map and the planned path
are sent to the rover.
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A rover collects samples on the surface of Mars, in this depiction by
artist Ken Hodges.(" A Mars Rover for the 1990's " [6] )
Primary task of path planning is to find an appropriate path for the rover
to follow in order to reach a goal location ,designated from Earth, avoiding any
obstacles and untraversable terrain ( e.g. elevation higher than what the rover
can compensate). The path planning proposed[8] by JPL for a SAN takes place in
two phases.
The rover is given the global terrain
map(section 3.2) from the orbiter with the goal site for the rover. The path
planner generates a global path gradient [9] using a spreading activation
algorithm[10], this algorithm takes into account all the kinematic constraints
of the rover and the traversability of the terrain to produce a global gradient.
Phase II is a refined version of phase
I, using the global gradient previously computed, the planner searches through
the local terrain maps to find a safe route that will bring it closer to the
goal location. It is assummed that the planner has three local terrain maps for
the same region with diffent resolutions ( low, medium and high). At first the
planner tries to find a set of locations (exit zones) on the low resolution map
that will bring the rover closer to the goal location using the computed local
map gradient ( as done in phase I). Then possible paths are computed to the exit
zones using the local gradient map. The paths are passed into a simulator that
determines which paths can be executed safely and the path to the top rated exit
zone is preferably chosen. If the simulator cannot find an acceptable path from
the low resolution map then the higher resolution maps are used in turn until
one is found. In the higher resolution maps exit zones will be found by using as
goal sites the exit zones of the immediately lower resolution maps. If no path
is accepted by the simulator ( quite unlikely) the rover must back up and try to
reach the goal site from a different route.
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