Planetry Cliff Descent using cooperative robots

Sponsors: NASA - Jet Propulsion Laboratory and NASA Nebraska Space Grant

Collaborators: NASA - Jet Propultion Laboratory

Future robotic planetary exploration will need to traverse geographically diverse and challenging terrain. Cliffs, ravines, and fissures are of great scientific interest because they may contain important data regarding past water flow and past life.
Highly sloped terrain is difficult and often impossible to safely navigate using a single robot. This paper describes a control system for a team of three robots that access cliff walls at inclines up to 70°. Two robot assistants, or anchors, lower a third robot, called the rappeller, down the cliff using tethers. The anchors use actively controlled winches to first assist the rappeller in navigation about the cliff face and then retreat to safe ground.

This project studies the coordination of these three robots so they function as a team to explore the cliff face. Stability requirements for safe operation are identified and a behavior-based control scheme is presented. Behaviors are defined for the system and command fusion methods are described. Controller stability and sensitivity are examined. System performance is evaluated with simulation, a laboratory system, and testing in field environments.

	A behavior-based control scheme was developed to navigate a steep slope while maintaining system safety. The controller will identify dangerous situations and take appropriate steps to eliminate the danger while attempting desired waypoint navigation.
Simulations have been created to analyze controller performance. They are used to study obstacle avoidance techniques, stability issues, communication bandwidth, and system sensitivity.
	Controller testing has been performed on a test ramp (right) inside this lab and also in field trials performed at JPL (below). Experiments verified the sensitivity and stability analysis. Tests also verified the ability of the controller to identify dangerous situations and make intelligent decisions to maintain safety and navigate the cliff face.
This research is done in cooperation with The Planetary Robotics Lab at NASA/JPL - Pasadena, Calif. The Planetary Robotics Lab is directed by Dr. Paul Schenker.