Patent US 6,965,211

A robot apparatus 1 is to be electrically charged autonomously. An electrical charging device 100 is provided with two markers, namely a main marker 118 and a sub-marker 119, and the heights of the markers are pre-stored in the robot apparatus. When the robot apparatus 1 is to find the direction and the distance to the electrical charging device 100, a CCD camera 20 finds the direction vector of the marker from the photographed image. This direction vector is transformed into a position vector of a camera coordinate system {c} and further into a position vector of the robot coordinate system {b}. The coordinate in the height-wise direction in the robot coordinate system {b} is compared to the pre-stored height to find the distance between the markers and the robot apparatus and the direction of the robot apparatus.