Slip Control during Slope Descent for a Rover with Plowing Capability

Recent efforts in planetary robotic exploration aim toward craters, skylights, and other depressions with challenging terrain conditions. The access to such places requires traversing on extreme slopes where high levels of slip greatly hamper rover mobility and control. To successfully reach valuable targets such as water ice and mineral outcrops in these locations, slip must be promptly arrested. The work presented here develops an automatic system for a plowing-capable rover that controls slip during descent on steep unconsolidated slopes. The slip control system is implemented around the robot's plow, and has two main components: a slip estimation subsystem and the slip controller. Slip estimation is performed through a visual odometry algorithm based on monocular optical flow. Two approaches were explored for the slip controller: PID and fuzzy logic control. The design of the controllers was aided by a model of the rover-terrain system formulated specifically for this purpose. Field testing was carried out on conditions relevant to lunar crater exploration. The experimental results showed that the control system is able to keep slip to a minimum for different commanded vehicle speeds and slopes as steep as 31°. As a consequence, this work expands current rover mobility and control capabilities by enabling precise descent on steep slopes of unconsolidated material. ii Dedication To my mother, whose tireless efforts and perseverance have opened invaluable opportunities in my life. To my father, whose professional excellence inspires all my endeavors. iii Acknowledgements I would like to thank my advisor, David Wettergreen, for his experienced guidance and constant support throughout the development of this work and, in general, during my time in the Robotics Institute. I am taking with me lessons that go beyond the academic setting. I would also like to thank my former professor, Aarón Sariñana, for being amazingly generous with his time and for his insightful advice on controls. I am grateful that he has kept sharing with me his knowledge and expertise after I graduated from college. was always available to answer my never ending questions on terramechanics. Chuck's assistance during field testing was incredible, and he also gave Icebreaker another breath of life to complete the final experiments. David Kohanbash was very helpful in troubleshooting several quirks of Icebreaker. Finally, I would also like to thank Colin Creager and Kyle Johnson from NASA Glenn Research Center for their wide support and ample time flexibility during field testing.