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Abstract

A gauge for measuring the thickness of an unsupported web. One triangulation sensor is located above and the other is located below the web. Each triangulation sensor includes a laser whose beam is bounced off the web to a position sensitive detector in the sensor, which determines the distance of the surface of the web from the sensor. The sensors travel in synchronization along tracks, across the direction of web travel. To detect and compensate in the measurements for any sag or deflection in the track, there is a further laser either on one of the sensors or on the frame of the gauge and a respective further detector on the other one of the sensors and/or the frame of the gauge. The further detector detects the shifting of the sensor toward or away from the web and generates a signal which is combined with the triangulation signals from the sensors to calculate the thickness of the web. In one embodiment, a further laser is on one sensor and the further detector is on the sensor at the opposite side of the web. In another embodiment, a further laser is on each sensor while the further detectors are in the frame of the gauge. In another embodiment, the further laser is on the frame while the further detector is on the sensor. In the latter embodiment, a beam splitter splits the beam between the further detector and a photosensitive diode for zeroing the further detector.

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First Claim