POSITION MEASURING INSTRUMENT AND METHOD FOR OPERATING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a position measuring instrument especially suitable for catching an absolute position, and a method for operating the same. SOLUTION: The position measuring instrument comprises a measuring scale and a scanning device movable in a measuring direction relatively with respect to the measuring scale. At least one first track and at least one second track are arranged on the measuring scale and scanned using the scanning device to form the first and second scanning signals corresponding to a position. The scanning signals formed from the mutually different tracks show a behavior due to the shift in mutually different longitudinal directions, so that the first scanning signal shows an erroneous position while the second scanning signal shows an accurate position, when there is the shift in the longitudinal direction of the scanning device or the measuring scale around the axis in the direction parallel to the surface of the measuring scale. The scanning device is constituted so as to form a third scanning signal showing the behavior due to the shift in the longitudinal direction different from the other scanning signal of one track among two tracks. By comparing the third scanning signal with the scanning signal of the same track, the presence of shift in the longitudinal direction generated according to circumstances is detected.