| 摘要 |
PURPOSE: To decide with precision a reference position of a scanning device by roughly deciding a mark position on an increment scale, and then interpolating between marks by utilizing a trigonometric function from a projection angle with which a mark is projected. CONSTITUTION: When a scanning device 10 moves on an increment scale, the number of a mark 14 is counted with a counter 16 and a calculator 18, for roughly deciding a movement distance. Then, with an angle measurement device 20, marks a, b and c are projected onto a projection surface 24 comprising a CCD, so that points a', b' and c' and anglesα' andβ' are imaged. These are, with a counter 28 and the calculator 18, converted into corresponding angle valuesαandβ. Based on the projection anglesαandβ, the coordinate value of a reference point O which agrees with a projection center part of an optical system 22 is obtained by using a trigonometric function. Thus, without depending on guiding precision of the scanning device 10, a reference point position is decided with high precision.
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