GB1360992A - Papermaking - Google Patents

Abstract

1360992 Automatic control of position REED INTERNATIONAL Ltd 21 Oct 1971 [21 Oct 1970] 50028/70 Heading G3R [Also in Division G1] In a Fourdrinier type papermaking machine the position of the "dry-line", i.e. the boundary between wet and dry portions of the paper web, is detected by illuminating the appropriate area with visible or infrared light and monitoring the intensity of the resulting reflections. The intensity varies with the position of the boundary because the reflectance is specular from the wet surface and diffuse from the dry. The monitored signal may be used to control the position of the line. In an embodiment this is done firstly by adjusting the amount of suction in drying boxes, then, if this approaches limits, the dilution of the raw material is altered, and if this also approaches limits the refining process is modified Fig. 1 (not shown). Tube rolls and foils may also be adjusted to regulate the drying. A single photo-multiplier (2), Figs. 2, 3 (not shown) is arranged behind a lens system and shutter (3) which rotates at 1 r.p.s. to admit reflected and incident light from the source alternately, with a brief blanking period between each. A secondary shutter and associated light source and detectors (15, 16, 17) provides a discriminating signal, and the photo-multiplier can be slidably replaced by a mirror system (19, 20) for alignment purposes. The whole detector system is enclosed in a box 4 to which pressurized air is supplied and exhausted through vents which direct it across the lens surface. The signal from the photo-multiplier is connected through a buffer amplifier 21, Fig. 5, to a switch 22 which samples the 1 Hz signal at 10 KHz. This is operated by a multi-vibrator 23 operating at constant frequency but whose mark/space ratio is varied according to the difference between a voltage proportional to the signal corresponding to the light source and a reference voltage. By this means the signals emerging from the switch 22 are compensated for changes in lamp intensity and photo-multiplier sensitivity. These signals are integrated at 24 and separated into reflected and incident light signals respectively by a second switch 25 operated by the secondary shutter. The output circuit 26 and buffer amplifier 28 for the respective signals have long time constants to sustain the voltage during the respective "off" periods, and in the output circuit 26 during longer periods of accidental interruption of the light beam. This occurrence is detected by a separate threshold circuit, not shown, which stops switch 25 in the "off" (to 26) position. The circuits are fully described and component values given Figs. 6A, B (not shown). The switches comprise field effect transistors, which act as gates, filters are used to block noise, and signals are limited by Zener diodes for protection purposes.