3-Dimensional Position Measurement Technology

Many systems that measure the position and orientation of an object in any space, and utilize those measurements have been developed for such applications as the positional control of robots and the analysis of worker movement in a field of production. Generally, the triangulation method shown in Fig. 1 is widely known for measuring the position of an object in 3D space. However, systems that use the triangulation method require two or more cameras that must be specifically positioned relative to one another and installed with an automatic focus system, making it difficult to simplify their structures. To resolve this issue, Fuji Xerox has developed a 3D position measurement technology that only requires a single camera without the need for focusing.

Methods of calculating the position and orientation of an object in 3D space using images captured with a single camera have been actively researched. When the configuration of target points (markers) on an object is known, the position and orientation of the markers can be calculated from 2D image patterns consisting of more than six markers captured with a camera. With the markers on the same plane (and not on the same straight line), both position and orientation can be calculated from just four markers.

The measurement technology developed by Fuji Xerox uses five LEDs as markers, and captures them with a single camera. The camera transforms the point light source into a ring pattern without using automatic focus as shown in Fig. 2, but by using the spherical aberration of the lens. The entire ring pattern is used to detect the center of the ring, thus allowing this camera to calculate the position of the image more precisely than a normal camera that captures the point light source as a point pattern. Also, with a normal camera, the field of view is narrowed when enlarging the point for better position accuracy, while the camera used for this technology can capture ring patterns without narrowing the field of view.

When the object (card with markers) is small in size as shown in Fig. 3, measuring the tilting direction of a plane consisting of four LEDs is sometimes difficult. By adding a fifth LED on a plane different from the one with four LEDs, however, the tilting direction of the plane can be easily detected.

When a 51-mm square card with five LEDs was set four meters away from the camera (distance L in Fig. 4), the position accuracy in the z-axis direction (optical axis of the lens) was within 0.7% of distance L, that in the x-axis and y-axis directions was within 0.1% of distance L, and the angle accuracy around the y-axis was 1.5 degrees. These results proved that the position and orientation of an object can be measured with high accuracy using equipment as simple as a camera (1.2 megapixels) and a small card.