In many industries, particularly the steel, plastics, rubber and lumber industries, material is produced by high speed extrusion, rolling, drawing, casting, sawing or other similar techniques wherein the output is carried along a conveyor-type system. Such techniques preclude the use of any measuring system requiring contact with the object, and, in the case of materials such as hot rolled steel, the temperature and heat of the material is too great for a contact-based measurement system. In order to maintain the required high output rate, non-contact techniques have been proposed to calculate the dimensions of the material.
U.S. Pat. No. 3,187,185 to Milnes discloses an apparatus for determining the surface contour and thickness of an article by non-contact means. The apparatus consists of one or more light beam sources and a camera which is preferably connected to a receiver. In using this apparatus, the beam source rotates such that the beam of light travels from one side of the specimen being measured to the other. The camera records the image of the line that the beam makes as it travels across the specimen, and the distance between this line and a reference line is proportional to the thickness of the specimen.
U.S. Pat. No. 4,271,477 to Williams discloses a method and apparatus for the measurement of the width and thickness of relatively thick metal workpieces. This apparatus uses two cameras, displaced vertically and longitudinally from each other, above the workpiece to measure the apparent width of the slab. The thickness of the slab may then be calculated, knowing the apparent width of the images from the two cameras and the vertical distance of the images from the surface upon which the workpiece is supported.
Also known are devices for contact-free thickness measurement of an object in which a beam is periodically deflected onto the object and the time at which the beam strikes each of a plurality of detectors is monitored. From the differential in time at which the beam strikes the various detectors, the thickness of the object may be calculated (see U.S. Pat. Nos. 4,068,955 and 4,192,612 to Bodlaj).
In U.S. Pat. No. 4,710,808 to Hoogenboom et al., there is disclosed a system for measuring the thickness of an object in which light from two sources, which are located on opposite sides of the object, is reflected from the object and the image thus formed is monitored by a video camera and the thickness of the object is determined from the light pattern of the images.
The prior art described above has several short-comings: the beam generator is constantly moving and thus susceptible to mechanical breakdown; the system measures differential thickness in relation to a calibration standard and thus cannot easily be used to accurately measure the thickness of objects with varying thicknesses; and others.