| 摘要 |
PURPOSE:To prevent a mis-detection even in the case of a photodetection amt. varying by a steam, etc., by providing two photodetectors for one projector and providing a slit so that the variation sensibility in the photodetection amt. due to the positional change of the body to be measured may vary on each photodetector. CONSTITUTION:The light projected from a projector 10 is photodetected by a 1st photodetector 11 and a 2nd photodetector 13 respectively through slits 12, 14 to output signals E1 and E2. The light shielding area and photodetection area at the time when a strip material 4 is located at the center position are equal and E1=E2=E0, and the variation in the photodetection area due to the positional change of the strip material 4 appears as the variation of output signals E1 and E2. At the time when a photodetection amt. is reduced with the effect of a steam, etc., the positional change of the strip 4 calculated from E1, E2, E'1, E'2, etc., becomes no relation to the reduction in the photodetection factor due to a steam, etc., provided E1, E2 become E'1, E'2. As a result the position of the strip 4 can correctly be detected even if the photodetection amt. varies by the steam coming out of a rolling mill, the variation of the external light and the oxidized scale of the strip 4.
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