Method for contactlessly checking a surface quality of a polished surface

Abstract

The invention relates to a method for contactlessly checking a surface quality of a polished surface, in particular a highly polished surface (1), of a metal strip (2), in particular a cast film strip, in an open state or in a state closed to form an endless strip, wherein a laser beam generated by a laser (3) is irradiated onto the surface (1) in a main irradiation direction (4) and is reflected by the surface (1) in a main reflection direction (5), wherein to assess the surface quality, intensities of portions of the laser beam deflected at surface anomalies (7) of the surface (1) in the direction of a camera (6) are detected by means of said camera (6), which is arranged in a dark field outside a region of the main reflection direction (5) of the laser beam.

Description

[0001] METHOD FOR NON-CONTACT VERIFICATION OF THE SURFACE QUALITY OF A POLISHED SURFACE

[0002] The invention relates to a method for non-contact testing of the surface quality of a polished surface, in particular a high-gloss polished surface, of a metal strip, in particular a film casting strip, in an open state or in a closed state to form an endless strip.

[0003] Belts, especially endless belts, which require a very high surface quality, are typically ground and / or polished during their manufacture, repair, or maintenance. Such belts are frequently used in film casting processes, for example, in the production of films or foils used in the manufacture of screens.

[0004] When polishing or grinding a belt, especially an endless belt, structures can form on the belt's surface that are not normally visible to the naked eye due to their size. That is, these structures are below the resolution limit of the human eye. With conventional polishing processes, a certain regularity can develop in the arrangement of these structures on the belt surface, whereby groups of regularly arranged structures can become visible in reflection at certain angles of incidence. Since the structures are transferred into the product, the product manufactured on the belt, especially an endless belt, can also be affected. Depending on the viewing angle, the imprints of the groups of structures transferred into the product can become visible in reflection, for example, in the form of colored, especially blue, lines, so-called "blue lines."The blue color arises from roughness in the blue wavelength range (Rayleigh-Rice scattering model). Currently, only visual inspection of the bands for surface anomalies of the aforementioned type is performed, which compromises the quality of the inspection.

[0005] It is therefore an object of the present invention to overcome the disadvantages of the prior art and to provide a method by which a simple and reliable verification of surface quality is made possible.

[0006] This problem is solved according to the invention by a method of the type mentioned at the outset in that a laser beam generated by a laser is directed onto the surface in a principal direction of incidence and is reflected from the surface in a principal direction of reflection, wherein, in order to assess the surface quality, intensities of components of the laser beam deflected from the principal direction of reflection towards the camera at surface anomalies are detected by means of a camera arranged outside a region of the principal direction of reflection of the laser beam in a dark field.

[0007] The solution according to the invention makes it possible to reliably detect surface anomalies of the endless belt using measurement technology and to assess the quality of the surface.

[0008] According to an advantageous embodiment, it can be provided that the main direction of incidence of the laser beam and an optical axis of the camera run at an angle of 90° + / - 5% to each other.

[0009] Advantageously, the camera and the laser are arranged in a common housing, which is closed except for one side facing the surface to be inspected.

[0010] The housing can have an aperture on its surface to be inspected, wherein the aperture has at least a first elongated opening for the laser beam generated by the laser and at least a second elongated opening for the at least one camera, wherein the at least one first elongated opening and the at least one second elongated opening are perpendicular to each other and / or together form a T-shaped opening, and the at least one camera is shielded from background light by means of the aperture.

[0011] The distance between the aperture and the surface to be inspected is 1 - 20 mm, in particular 5 - 10 mm.

[0012] To improve the signal-to-noise ratio, it may be necessary to shield at least one camera from backlight by means of an aperture.

[0013] Preferably, a line scan camera is used.

[0014] To enable a quantitative assessment, at least one characteristic value for surface quality can be calculated from the intensities recorded by the camera. It has proven particularly advantageous that this at least one characteristic value is a ratio of at least one maximum measured intensity to at least one average measured intensities.

[0015] According to a preferred embodiment of the invention, it is provided that the camera and the laser are moved together over the surface of the metal strip to be inspected and / or that the metal strip is moved relative to the camera and the laser.

[0016] Advantageously, the relative position of the camera and laser to each other remains unchanged during the measurement.

[0017] To better understand the invention, it is explained in more detail with reference to the following figures.

[0018] They each show, in a highly simplified, schematic representation:

[0019] Fig. 1 shows a measurement setup for carrying out the method according to the invention;

[0020] Fig. 2 shows an aperture to protect a camera of the measuring setup from Fig. 1;

[0021] Fig. 3 shows a curve of a characteristic value for surface quality as a function of a distance to an edge of a metal strip.

[0022] It should be noted at the outset that in the differently described embodiments, identical parts are provided with the same reference numerals or component designations, and the disclosures contained in the entire description can be applied analogously to identical parts with the same reference numerals or component designations. Furthermore, the positional designations chosen in the description, such as top, bottom, side, etc., refer to the figure directly described and illustrated, and these positional designations must be applied analogously to the new position if the position changes.

[0023] According to Fig. 1, the inventive method is used to check the surface quality of a polished surface, in particular a high-gloss polished surface 1 of a metal strip 2, for example, a film casting strip. The metal strip 2 can be in an open state or in a closed state forming an endless strip. A laser beam generated by a laser 3 is directed onto the surface 1 in a principal direction 4 and reflected by the surface 1 in a principal reflection direction 5. To assess the surface quality, the intensities of components of the laser beam deflected by surface anomalies 7 of the surface 1 in the direction of the camera 6 are detected by means of a camera 6 arranged outside the region of the principal reflection direction 5 of the laser beam in a dark field.

[0024] The laser beam generated by laser 3 can, for example, have a wavelength between 300 and 1100 nm. The shorter the wavelength of the light emitted by the laser, the stronger the scattering at the surface anomalies 7.

[0025] The main direction of incidence 4 of the laser beam and an optical axis 12 of the camera 6 preferably run at an angle of 90° ± 5% to each other. In this context, "camera" refers to any device that can record light intensity on a digital storage medium or transmit it to such a medium via an interface. The camera 6 does not necessarily have to include optical components such as a lens. The camera 6 can also be simply an optoelectronic sensor that can be connected to a corresponding processor for signal evaluation, for example, a suitably programmed microprocessor or an external computer. The camera 6 is particularly preferably a line scan camera.

[0026] If the camera 6 has only one sensor or sensor array but no upstream optical components, then a surface normal to a sensor surface corresponds to the direction of the optical axis of the camera 6.

[0027] As can be further seen from Fig. 1, the camera 6 and the laser 3 can be arranged in a common housing 8, which is closed except for one side facing the surface 1 to be checked.

[0028] The housing 8 can have an aperture 9 on its side facing the surface 1 to be inspected, which shields the camera 6 from background light. The aperture 9 can be made, for example, of laser felt or laser protective paper.

[0029] According to Fig. 2, the aperture 9 can have a first elongated opening 10 for the laser beam generated by the laser 3 and at least a second elongated opening 11 for the camera 6. The opening 10 and the opening 11 are perpendicular to each other. As can be seen from Fig. 2, the two openings 10 and 11 can form a common T-shaped opening.

[0030] A characteristic value K for the surface quality can be calculated from the intensities recorded by camera 6. This characteristic value K preferably represents a ratio of a maximum measured intensity to at least one average measured intensities. The characteristic value K can be calculated as follows:

[0031] In the formula above, ai represents the light intensity measured by the camera. If the characteristic value K in the formula above exceeds a target value S (Fig. 3) that is greater than 1, for example, the value 2, then the presence of a surface anomaly, in particular in the form of a "B incline", can be inferred.

[0032] The camera 6 and the laser 3 can be moved together across the surface 1 of the metal strip 2 to be inspected during the measurement. Alternatively or additionally, the metal strip 2 can also be moved relative to the camera 6 and the laser 3. However, the relative position of the camera 6 and the laser 3 to each other remains unchanged during the measurement. The surface 1 is preferably scanned in paths perpendicular to a longitudinal direction of the strip.

[0033] In Fig. 3, the characteristic value K is plotted on the ordinate and a distance A to a band edge of the metal strip 2, which serves as a reference position, is plotted on the abscissa. Thus, the position of the surface anomaly or the "B incline" can also be located.

[0034] Reference numeral list

[0035] 1 surface

[0036] 2 metal bands

[0037] 3 lasers

[0038] 4 Main direction of incidence

[0039] 5 Main reflection direction

[0040] 6 cameras

[0041] 7 Surface anomaly

[0042] 8 cases

[0043] 9 aperture

[0044] 10 Opening

[0045] 11 Opening

[0046] 12 optical axis K characteristic value S target value

Claims

Patent claims 1. Method for non-contact inspection of the surface quality of a polished surface, in particular a high-gloss polished surface (1), of a metal strip (2), in particular a film casting strip, in an open state or in a closed state to form an endless strip, characterized in that a laser beam generated by a laser (3) is directed onto the surface (1) in a principal direction of incidence (4) and is reflected by the surface (1) in a principal direction of reflection (5), wherein, for the purpose of assessing the surface quality, intensities of components of the laser beam deflected by surface anomalies (7) of the surface (1) in the direction of the camera (6) are detected by means of a camera (6) arranged outside a region of the principal direction of reflection (5) of the laser beam in a dark field.

2. Method according to claim 1, characterized in that the main direction of incidence (4) of the laser beam and an optical axis (12) of the camera (6) are at an angle of 90° + / - 5% to each other.

3. Method according to claim 1 or 2, characterized in that the camera (6) and the laser (3) are arranged in a common housing (8) which is closed except for one side facing the surface (1) to be inspected.

4. Method according to claim 3, characterized in that the housing (8) has an aperture (9) on its side facing the surface (1) to be inspected, wherein the aperture (9) has at least a first elongated opening (10) for the laser beam generated by the laser (3) and at least a second elongated opening (11) for the at least one camera (6), wherein the at least one first elongated opening (10) and the at least one second elongated opening (11) extend transversely to each other and / or together form a T-shaped opening, and the at least one camera (6) is shielded from background light by means of the aperture (9).

5. Method according to claim 4, characterized in that the distance between the aperture and the surface to be tested is 1 - 20 mm, in particular 5 - 10 mm.

6. Method according to any one of claims 1 to 5, characterized in that as Camera (6) a line scan camera is used.

7. Method according to one of claims 1 to 6, characterized in that at least one characteristic value (K) for the surface quality is calculated from the intensities recorded with the camera (6).

8. Method according to claim 7, characterized in that the at least one characteristic value (K) is a ratio of at least one maximum measured intensity to at least one mean value of measured intensities.

9. Method according to one of claims 1 to 8, characterized in that the camera (6) and the laser (3) are moved together over the surface (1) of the metal strip (2) to be tested and / or that the metal strip (2) is moved relative to the camera (6) and the laser (3).

10. Method according to one of claims 1 to 9, characterized in that the relative position of the camera (6) and the laser (3) to each other remains unchanged during the measurement.

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