a mobile robot includes a robot chassis having a forward end , a rearward end and a center of gravity . the robot includes a driven support surface to propel the robot and first articulated arm rotatable about an axis located rearward of the center of gravity of the robot chassis . the arm is pivotable to trail the robot , rotate in a first direction to raise the rearward end of the robot chassis while the driven support surface propels the chassis forward in surmounting an obstacle , and to rotate in a second opposite direction to extend forward beyond the center of gravity of the robot chassis to raise the forward end of the robot chassis and invert the robot endwise .