techniques or by dragging a cursor
over the 3D terrain using the mouse.
A basic route can be created by
dragging a 3D rover model to each successive waypoint in the terrain model.
The operator’s experience is necessary
to select reasonable routes and to plan
drives with the tight operational communication windows. The operators
must select the appropriate traverse
and localization methodologies.
Once a sequence of desired locations relative to the terrain is decided,
motion commands can be generated
for path planning using a grid-based
estimation of surface traversability
applied to local terrain. Motion
commands are then stored and packed
to be sent to Mars.
• Rover perception (Mars). Reliable
perception is mandatory when the
robot is navigating, because dead reckoning information is clearly unreliable,
especially in rocky terrain. Cameras are
used because they provide a lot of information if compared to other sensors.
In particular, Visual Odometry is
used when navigating. This algorithm
computes the rover displacement
between two successive camera images.
Specifically, an image is taken before a
rover’s movement. After the movement,
another image is taken mostly of the
same terrain area. Features present in
both images are then associated, and
the corresponding displacement is
computed. As a result, an estimate of
the current rover motion is computed.
agreement in the robotics community
about the morphology and the design
principles of rovers for space exploration. These are necessarily application-driven. Rovers must adhere to the concept of “Robotic Field Geologist,” i.e., a
semi-autonomous rover is considered as
a replacement for a team on Earth, able
“to do in one day what a field geologist
can do in about 45 seconds.”
In other words, the goal is to
create a cyber astrobiologist, deploying
scientific instruments (i.e., the scientific
payload) over wide areas. The
mechanical design and the cognitive
architecture of planetary rovers are
arranged to guarantee the traversal of
uneven and rocky terrain according to
From the mechanical point of view,
typical key requirements are: (1) the
rover must traverse over obstacles of
25 cm in maximum dimension; ( 2) the
rover must traverse over slopes of a
nominal tilt of 16 degrees; ( 3)
moreover, it should cope with hard,
high traction terrains and soft
deformable soils; ( 4) and with a temperature between -100 and 20 Celsius,
in order to carry the scientific payload;
and ( 5) each vehicle is approximately
1.4 m long and 1.2 m wide.
Furthermore, lithium-ion batteries
are used for saving mass and volume.
From a mechanical perspective, the
current state-of-the-art locomotion techniques assume that rovers are six-wheel
drive, four-wheel steered vehicles with a
specifically designed suspension system.
In particular, the rover suspension
system is a mechanical assembly called
rocker-bogie that connects the six wheels
to the body of the rover itself (see Figure
4). In order to increase robustness to
possible hardware faults, all six wheels
are independently driven by DC motors.
Moreover, in order to increase the
mobility capabilities, front and rear
wheels are independently steered,
allowing the rover to turn in place, as
well as to execute more complex turns.
The two front and rear wheels are
steered by identical DC motors. A
differential connects the two rocker-bogie systems to the main rover body.
The rocker-bogie design is characterized by a number of interesting
properties: (1) safe traversal of obstacles whose dimensions are in the same
order of magnitude of the diameter of
the wheels (about 30 cm); ( 2)
withstanding a tilt of 45 degrees in any
direction without overturning; ( 3)
absorbing the most of the impact load
during terrain traversal, which is particularly important for the scientific
payload carried around during mission
execution; and ( 4) passively keeping all
• Rover locomotion (Mars). Given the
rough nature of the Martian surface,
cutting-edge state estimation techniques must be used to precisely allow
a rover to reach a given target position.
In general, reaching a target is a three-step process: terrain traverse, homing,
and fine positioning. Once a given area
is reached with a sufficient precision,
scientific operations can be arranged,
and the results sent back to Earth.
Nowadays, there is a widespread FIGURE 4
SERVO 01.2008 51