The present invention relates to a method of, and an arrangement for measuring the contour of a cross-section of an object, such as an elongated article in general, and more particularly of an article produced by an extrusion.
The present invention is concerned with the measurement of the cross-sectional contour of an elongated article or object which is either stationary, or is advanced in its longitudinal direction, the article having an arbitrary length, wherein one or more laser beams scan the surface of, and are focused onto the surface of the elongated article, the rays reflected from the surface being formed into a light spot. Any changes in the image of the light spot, which occur as a result of any deviation of the surface from an ideal surface during the scanning process, are measured, registered and, if required, displayed after having been converted into corresponding analog and/or digital signals.
There is already known an arrangement which is capable of focusing a laser beam on the surface of an object, by utilizing an optical system, which also registers the variations in the intensity of the image of the light spot by means of a photo-detector, when changes take place in the distance between the laser and the surface of the object. Background information, with respect to known systems, is available in "Laser in Industrie und Technik" (Laser in Industry and Technology) Publisher Lexicon, "Technische Anwendung des Lasers" (Technical Application of the Laser) Publisher Springer, particularly pages 59 to 61, and Naray, "Laser und Ihre Anwendungen" (Lasers and their Applications), Publisher Akademische Verlagsgesellschaft.
The previously mentioned conventional arrangement operates in such a manner that the laser beam emanating from the laser is collimated by a collimating lens system, and the parallel light beam is then focused onto the surface of the object. A ray-dividing mirror is arranged in the path of the laser beam at the collector lens system, and is disposed at an angle of 45.degree. with the optical axis of the system. This mirror is permeable to the light impinging thereon from the direction of the collecting lens system, but is impermeable to any light arriving from the opposite direction. Accordingly, the light rays, which are reflected from the surface of the object, are reflected from the rear side of the ray-dividing mirror, and then proceed towards another mirror, which further reflects the light rays onto a collecting lens. From there, the light rays pass towards a photo-detector. A perforated diaphragm is arranged between the photo-detector and the collecting lens, the focal point of which is arranged upstream or frontwardly of the photo-detector. Advantageously, the perforated diaphragm is arranged approximately at the focal point of the lens. This perforated diaphragm is connected to an oscillator, which drives it, and thus oscillates at a certain frequency along the axial direction of the optical system. As a result of this oscillation of the perforated diaphragm, a fluctuating quantity of light reaches the photo-detector, depending on how far the perforated diaphragm is spaced from the focal point, or image.
When the perforated diaphragm moves through the focal image, then the photo-detector will indicate a maximum of light intensity at the moment of passage of the perforated diaphragm through the focal image. Now, considering the case where the distance between the surface of the object, upon which the laser beam impinges, and the laser remains constant, the variations of intensity indicated by the photo-detector of the image of the laser light spot, which are the result of the oscillation of the perforated diaphragm, will then fluctuate between two constant values. If it is assumed that the oscillation center of the perforated diaphragm is located exactly at the focal point of the collector lens, then the intensity variation indicated by the photo-detector will be a symmetrical Gaussian curve, the maximum of which corresponds to the position of the perforated diaphragm at the focal point.
When the difference between the measurement of the photo-detector in two successive positions of the perforated diaphragm, corresponding to successive maxima of light intensity, respectively, is utilized for evaluation of the measured distance, then a zero difference between these values is obtained if the distance from the focused light spot on the surface of the object to the focal image of the light spot remains the same. In every other case, that is, when the center of oscillation of the perforated diaphragm is not located at the focal image, compared to its previous position in the oscillation cycle, there is obtained either a positive or a negative value in the difference indication of the photo-detector, compared to the previous two successive positions of the perforated diaphragm.
Now, should the distance of the surface of the object from the laser vary during the scanning motion of the laser beam, then the intensity of the focused light spot on the surface of the object will also change accordingly. As a result, the intensity of the light spot obtained from the object-reflected beam, and its intensity as a function of time, as registered by the photo-detector, will also vary in the course of the oscillation of the perforated diaphragm. The intensity differences at the output of the photo-detector, which corresponds to intensity differences in the axial positions of the perforated diaphragm in the light axis of the system, are registered by the photo-detector as a potential difference.
Now, assuming that an object is moved with respect to the laser beam or, conversely, that a laser beam is moved with respect to a stationary object, then a change in the structure of the exposed surface of the object can be determined in one dimension by means of a known arrangement, provided that the surface of the object is properly positioned and adjusted; furthermore, this change may, if required, be registered and displayed. This known arrangement is not suited for a two- or three-dimensional determination of the exposed surface of a body, which is to be surveyed. This problem is encountered, for instance, when rubber or synthetic plastic materials are being extruded, and it is desired to determine their 3-dimensional shapes. Despite all technical advances, it is often not possible to maintain the cross-sectional dimensions of an extruded article with the needed precision. This problem is encountered especially in connection with articles of rubber, inasmuch as the composition of the rubber material can never be obtained in a precisely uniform manner under all circumstances; thus, a differential swelling can occur following extrusion, as a result of which the vulcanized article may exceed the range of any predetermined tolerances required of its cross-section. This undesired situation cannot be corrected in a satisfactory manner, by resorting to any currently known techniques.
It would be theoretically possible to resort, for instance, to mechanical scanning methods, which, however, have other disadvantages, as they require a certain measuring pressure to be exerted on the object which, in turn, results in a deformation of objects, especially made of rubber, and thus yield an erroneous indication. A further, at least theoretical alternative, is the utilization of an isotope-measuring method, in which the thickness of the article being extruded is determined by adsorption measurement of radioactive rays. In addition to the general disadvantages due to radioactive measurements, a specific disadvantage of such radioactive measurements is the fact that such a measuring method does not actually measure the cross-section of the object investigated, but only its surface density. A further disadvantage of the measuring method of this type is due to the fact that the sensitivity range of the measuring apparatus must be very accurately matched with the range to be measured, so that when more substantial thickness differences occur in the article being measured, it is no longer sufficient to use only a single measuring sensor.
A relatively primitive method of determining surface irregularities is the determination of the weight of a corresponding section of the elongated article. It is self-evident that this procedure will yield only very imprecise data.