IEEE - Aerospace and Electronic Systems - December 2021 - 26

NASA Space Robotics Challenge 2 Qualification Round: An Approach to Autonomous Lunar Rover Operations
Hauler: designed for task 2, it was designed to transport
collected resources back to the processing plant.
Excavator: designed for task 2, it was designed to
excavate resources below the lunar surface with a
four degrees-of-freedom manipulator with a bucket
end-effector.
The SRC2 lunar and robot models utilized the simulation
Figure 1.
Depictions of the provided rovers in the competition qualification
round. These rovers were expansions of a medium sized
four wheeled robot, called as Base Rover. Each rover was
designed by the competition for a specific goal in the qualification
round tasks. The Scout rover was used for resource
exploration and localization (tasks 1 and 3). The Hauler and
the Excavator rovers were collaboratively used in resource
collection and excavation, respectively (task 2). See the section
" Overview of the Challenge " for detailed explanation of
the tasks.
environment Gazebo [9], which offers an interface with the
robot operating system (ROS) [10]. ROS is a framework that
facilitates the development of robotics software through
hardware abstractions and interfaces, package management,
and interprogram communication. Central to ROS is the
approach it takes to facilitate the information flow between
programs, referred to as nodes. Any node can read/write
(publish/subscribe) to ROS topics sending messages and
allowing the information to be accessible by several nodes
simultaneously. An ROS service carries out a task and provides
information about this task to a client node, which
requests the task be carried out.
In the qualification round, the competitors were
expected to overcome several hardware constraints and
technical challenges similar to a planetary exploration
mission as follows.
between 5 m and 25 m above the surface of the virtual
lunar environment. Additionally, after reporting
the object position, the rover should find the processing
plant, approach, and align itself with a specific
marker on the station.
As mentioned before, there were three lunar rovers
provided by the competition, as shown in Figure 1.
These rovers were all expansions of a " base rover, " a
medium-sized four-wheeled robot with individual control
of the wheel steering angles and motor torques.
The torque and velocity of the wheels were constrained
so that the rover would only cruise at a maximum
speed of 1.5 m/s, and the steering angle was constrained
to 90, allowing a great range of driving
possibilities. The competition provided a tuned proportional-integral-derivative
controller for controlling
the steering angles and motor torques of each wheel.
The base rover was equipped with sensors, including
a planar LiDAR, a stereo camera, and an inertial
measurement unit (IMU) to support localization and
perception. The LiDAR and cameras were actuated and
could be tilted up and down.
Each specialized rover was adapted for its task as
described as follows.
Scout: designed for task 1 and task 3, it was
equipped with a sensor that can detect and identify
the volatiles in the environment.
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1) Having no GPS or similar satellite based lunar system
for localization.
2) Having no communication with the base station (e.g.,
beacon signal) requiring full autonomy for the rovers.
3) Using coupled and limited range sensor package
(LiDAR and stereo camera).
4) Using single stereo camera in a low feature, dark
environment with some permanently shadowed areas
which impair visual odometry (VO) performance.
5) Dealing with steep slopes in the terrain that create
significant slip and prevent the rovers from climbing
in a crater and stop on hills.
6) Dealing with limited detection of volatiles to short
range distances, which could only be performed by
a specialized rover.
7) Dealing with randomly distributed obstacles, volatile
locations, initial rover and processing facility poses,
and CubeSat (i.e., for task 3) for each simulation
seed.
8) Working with time limitation (45 minutes).
To overcome these constraints and challenges, we identified
and designed the subsystems and capabilities as detailed
in the section " Systems Design " and implemented them in
our task strategies as explained in " Task Strategies. "
IEEE A&E SYSTEMS MAGAZINE
DECEMBER 2021

IEEE - Aerospace and Electronic Systems - December 2021

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