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.
