Behavior-Based Coordinated Control for Robotic Planetary Cliff Descent

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 flows. Information about water is critical in the search for past life. Highly sloped terrain is difficult, and in most cases 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 rappeller robot down the cliff using tethers. The anchors use actively controlled winches to assist the rappeller in navigation about the cliff face and then retreat to safe ground. All three robots function as a team to explore the cliff face. This paper describes a behavior-based control system that allows the robots to function as a team. Stability requirements for safe operation are identified. Behaviors are defined for the system and a command fusion method is described that determines the resulting robot actions. The control architecture has been applied to an experimental system in both laboratory and field environments.