Automatic head positioning system using PSD-equipped camera-based photostereometry and a 5-degree-of-freedom robotized chair : calibration and accuracy verification

: In measurements of jaw movements employing an X-ray television, the orientation of the patient's head must be normalized relative to the measuring device. To avoid interferring with natural jaw movements caused by using a positioning jig, we have developed an automatic head positioning system consisting of a head position sensor and a robotized chair. The position and orientation of the head is monitored by detecting the three-dimensional positions of four non-coplanar LED markers mounted on the head, by means of a set of two photoelectronic cameras. If they are incorrect, the head is moved into the required position, using a 5-degree-of-freedom robotized chair. To accomplish this accurately, the positional relations of system components must be determined precisely. This paper proposes a least-square technique of calibrating the relationship between the position sensor and the chair. Consequently, accurate positioning is achieved. Accuracy verification studies revealed that overall positioning errors lay within 2.42 mm and 1.28 degrees with respect to the position and orientation, respectively, if the markers are secured to the head support of the chair.