Method and apparatus for observing, detecting and correcting periodic structures in a moving web

Abstract

Abstract The diffraction pattern, produced by the periodic structure of a moving web is stationary. Characteristics of the periodic structure may be observed in the diffraction pattern. Deviations from a norm of a characteristic of the periodic structure are detected by imaging the diffraction pattern on a mask which is a matched filter for the characteristic. In a specific application the periodic structure is the weft of a moving cloth web, the characteristic is weft alignment, and the matched filter mask is a slit. In order to obtain an alternating current output signal, the cloth is illuminated by an oscillating slit of monochromatic light. Misalignments of the weft produce rotations in the diffraction image which may be detected by means of a pair of photodetectors located behind a slit in the plane of the diffraction image. As in the prior art a plurality of such weft misalignment detectors may be located across the moving cloth web and the angular deviations of the weft at each detector compared to produce a control signal for controlling existing automatic devices for making weft misalignment corrections. In an alternative embodiment, relative rotation between the diffraction image and a slit produces a peak output from a photodetector when the slit is aligned with a linear pattern in the diffraction image. The output of a pulse generator driven in synchronism with the slit or image is counted from an initial reference position until the peak is attained to provide a digital output of the angular orientation of the pattern. In another embodiment of the invention, the detector takes the form of a photochromic sheet illuminated by an ultraviolet diffraction pattern and collecting optics focusing the ultraviolet radiation passing through the photochromic sheet upon a photodetector. A perfect moving web will produce a stationary diffraction pattern forming an opaque mask in the photochromic sheet and substantially no ultraviolet light will fall on the photodetector. Short term deviations from normal will cause diffracted light to pass through the transparent areas of the photochromic material producing a signal at the photodetector. The detector may also be illuminated with infrared radiation which tends to bleach it. The frequency of the periodicity of the moving web may be detected by combining diffracted light with a reference beam of light from the same source of illumination or a source of illumination coherent therewith to thereby produce a beat amplitude at a photodetector in synchronism with the passage of the periodic structure.

Inventors

• 👤 Peter H Langenbeck

Bibliographic Details