Proximity sensors extending out from both sides of this robot ensure there is ground to roll on. If one sensor detects a drop-off the robot turns int the other direction. If both sides detect a cliff then the robot gets very anxious. The result is a perimeter following behavior that emerges from the simple rules of turning away from edges and going striaght if no edges are detected. Like many of our bots, this cliff avoider makes ample use of the binary proximity sensor from pololu.