Device and method for detecting defects in a strand-like product

Abstract

The invention relates to a device for detecting defects in a strand-like product positioned in a measurement region of the device, wherein the device comprises a first light source which extends in a first direction of extent and which diffusely emits light, preferably visible light or infrared light, onto the measurement region from a plurality of locations along the first direction of extent, wherein the device also comprises a first sensor which detects the quantity of light emitted by the first light source after transmission through the measurement region, and wherein an evaluation unit is provided which is designed to identify a change in the quantity of light detected by the first sensor as indicating a defect in the product. The invention also relates to a corresponding method.

Description

[0001] Device and method for detecting defects in a strand-shaped product

[0002] The invention relates to a device for detecting defects in a strand-shaped product arranged in a measuring area of ​​the device. The invention also relates to a method for detecting defects in a strand-shaped product.

[0003] Stranded products with, for example, a round cross-section, such as cables or pipes, are typically inspected for defects immediately after production, for example, in an extrusion line. Apart from any defects, the product is homogeneous in the conveying direction. These defects can include, for example, deposits, local widenings, or constrictions. Defect detection should be non-contact and performed while the product is conveyed through a measuring area.

[0004] Currently, such products are examined by measuring the shadow cast by the product on a detector, such as a photodetector, using collimated or divergent light sources. The shadow is projected onto the detector, and changes in the shadow image are used to infer the presence of a defect. The problem here is that locally occurring defects across the product's circumference only affect the shadow cast if they are positioned close to the light source, depending on their size. As the defect size decreases relative to the product's cross-section, the detection range decreases. To solve this problem, it has been proposed to combine multiple such detection systems.However, up to twelve such detection systems may be required to reliably detect a local defect with a height of one-hundredth of the product radius, regardless of its position around the product's circumference. This involves considerable effort and corresponding costs. Therefore, conventional detection devices typically only include up to three such systems and can thus only reliably detect local defects with a height of approximately one-fifth of the product radius or greater, independent of their position. Camera-based systems would be another option. However, these are even more complex and consequently involve even higher costs.

[0005] Based on the prior art described above, the invention aims to provide a device and a method of the type mentioned at the outset, with which even small defects in a product can be detected in a simple and reliable manner, as independent of position as possible.

[0006] The invention solves the problem through independent claims 1 and 15. Advantageous embodiments can be found in the dependent claims, the description and the figures.

[0007] For a device of the type mentioned at the outset, the invention solves the problem in that the device has a first light source extended in a first direction of extension, which diffusely emits light onto the measuring area from a plurality of locations along the first direction of extension, that the device further has a first sensor which detects the quantity of light emitted by the first light source after passing through the measuring area, and that an evaluation device is provided which is configured to recognize a change in the quantity of light detected by the first sensor as a defect in the product.

[0008] The strand-shaped product can, for example, be a product with a partially or fully convex cross-section, in particular a partially or fully circular cross-section. As also explained, the product can, for example, be a cable or a pipe. The product can consist partially or entirely of a plastic. The product is homogeneous in its longitudinal direction, which can, for example, correspond to a conveying direction, and is free of defects. It can have been manufactured in an extrusion apparatus and, with the device or method according to the invention, can be measured, in particular directly after manufacture in the extrusion apparatus, and for this purpose, as explained below, conveyed through the measuring area of ​​the device. As also explained, the defects to be detected can, for example, be deposits, local widenings, or constrictions.The detection according to the invention is contactless.

[0009] The inventive approach also utilizes, in principle, a change in the shadow cast by the product located in the measuring area onto the first sensor, caused by a defect in the product. However, it avoids or reduces the problem described above of the decreasing detection ranges as the defects become smaller. For this purpose, instead of a collimated light source that produces a sharp image on the first sensor, a first light source with diffuse emission extending over a large area is used. The first light source emits light from a multitude of locations along a first direction of extension over a wide angular range with the most constant intensity possible.While diffuse light radiation does cause a loss of contrast compared to the prior art when a defect is optimally positioned relative to the detection system consisting of the first light source and the first sensor, thus increasing the requirements for the signal changes to be detected in order to continue detecting the same defect size as in the prior art, the inventive approach allows the detection range for defects to be significantly extended across the entire circumference of the product. In contrast to the prior art described above, a sharp image of a shadow is not produced on the first sensor.

[0010] The first sensor does not generate light; rather, it measures the total amount of light detected by the sensor after the product has passed through the measuring area. A defect leads to a change in the amount of light detected by the first sensor, for example, due to a change in the product's cross-section in the case of local widening or constriction. Based on this, the evaluation unit recognizes a change in the amount of light detected by the first sensor as a defect in the product. A threshold value for a minimum change in the detected light quantity can be predefined, above which such a change is reported as a defect. This prevents false detections due to external interference or signal noise.

[0011] The first light source extends along a first direction of extension. It can be extended in a flat or curved plane that includes the first direction of extension. The extension of the first light source, and for example also of the first sensor, in a direction transverse to the measuring area or transverse to the longitudinal direction of the product to be measured is preferably greater than the measuring area or the cross-section of the product to be measured. In this way, light radiation passing by the product can also be reliably detected. The first sensor is, for example, arranged on one side of the measuring area opposite the first light source. The first direction of extension can be a straight or a curved direction.Combinations of straight and curved sections of the extension direction, combinations of straight sections facing different directions, or combinations of curved sections with different radii of curvature are also possible. If the extension direction is straight, it can, for example, be oriented at an angle between 70° and 110°, particularly between 80° and 100°, preferably 90°, to the longitudinal direction of the strand-shaped product to be measured. The beam angle of the first light source, or the arrangement of the first light source relative to the first sensor, can be such that the light radiation emitted by the first light source completely covers the first sensor when the product is not within the measuring range. The diffuse emission of light from the first light source occurs with the most uniform intensity possible.According to the invention, it is therefore possible to detect even small local defects with a height of less than one tenth, preferably less than one thirtieth, further preferably less than one hundredth of the cross-section of the product or the radius of the product over an extended angular range on the circumference of the product of more than 90°, preferably more than 120°, further preferably more than 180°, and to recognize them as defects by the evaluation device.

[0012] Although the inventive approach requires light sources or sensors to be extended over a larger area compared to the prior art, it allows for more comprehensive detection of even smaller defects with fewer light sources or sensors, and thus in a simpler and more cost-effective manner compared to the prior art.

[0013] The problem of reduced contrast resulting from the inventive evaluation of the amount of light instead of a sharp image of the shadow can be addressed in various ways, as explained in more detail below. For example, simply increasing the radiant power of the first light source can ensure that a relatively weaker detection signal becomes absolutely stronger. Increased radiant or light power is advantageous for amplifying the useful signal against noise. The light sources used according to the invention can, for example, have several watts of optical power.

[0014] The light sources usable according to the invention can include light-emitting diodes (LEDs) or laser diodes. Suitable sensors include, for example, photodetectors.

[0015] ...16CCD sensors or extended photodiodes. The sensors can be line sensors or, for example, two-dimensional sensor arrays. The light sources can also be arranged in a cell-like structure in the respective direction of expansion or spread out over a flat or curved plane.

[0016] Depending on the design, the first light source can emit visible or infrared light. Infrared light has the advantage that current artificial light sources emit essentially only visible light. Therefore, there is minimal interference from ambient light in the infrared radiation range.

[0017] The first light source can comprise a planar light source extending in the first direction of extension and / or a plurality of individual light sources arranged side by side in the first direction of extension. A planar light source means that diffuse light is emitted from essentially every point of the light source along the direction of extension. If the light source is composed of several individual light sources, each of these individual light sources emits diffuse light accordingly.

[0018] In a further embodiment, the first sensor can extend in a first sensor dimension direction, for example, on one side of the measuring area opposite the first light source. The first sensor dimension direction can be parallel to the first dimension direction. The sensor can also extend in a flat or curved plane that contains the first sensor dimension direction. With the aforementioned embodiment, particularly reliable detection of light after it has passed through the measuring area containing the product. The first sensor dimension direction can also be straight or curved. Combinations of straight and curved dimensions are also possible.

[0019] ,.. / 7or curved sections of the first sensor extension direction or combinations of straight sections of different directions or combinations of curved sections with different radii of curvature are possible.

[0020] In a further embodiment, the device may also include a second light source extending in a second direction of extension, which diffusely emits light onto the measuring area from a multitude of locations along the second direction of extension, the device may further include a second sensor that detects the quantity of light emitted by the second light source after passing through the measuring area, and the evaluation device may be configured to recognize a change in the quantity of light detected by the second sensor as a defect in the product.

[0021] In this configuration, two light source sensor pairs are provided, illuminating the product located within the measuring area from different directions and detecting the corresponding light radiation. The second sensor can be positioned on the side of the measuring area opposite the second light source. This second sensor can extend in a second sensor dimension. This second sensor dimension can be parallel to the first. The same principles apply to the second dimension and the second sensor dimension as described for the first dimension and the first sensor dimension. By using two light source sensor pairs, the detection range for defects can be extended across the entire circumference of the product.With a suitable arrangement of the light source sensor pairs, reliable detection of defects across the entire circumference of the product is possible with just two such pairs. The light source sensor pairs can be arranged on different planes.

[0022] ... / 8, in particular offset in the longitudinal direction or conveying direction of the product to be measured. The second light source can also emit visible light or infrared light. Furthermore, the second light source can also comprise a planar light source extending in the second dimension and / or a plurality of individual light sources arranged side by side in the second dimension. More than two light source-sensor pairs can also be provided, for example, three light source-sensor pairs. The light source-sensor pairs can all be arranged in different planes, in particular offset in the longitudinal direction or conveying direction of the product to be measured.

[0023] In a further embodiment, the first and second light sources can diffusely illuminate a product arranged in the measuring area with light over its entire circumference, within an angular range of at least 270°, preferably 360°. The first and second sensors receive the light emitted by the light sources and incident upon them. By having the first and second light sources extend transversely to the measuring area or the longitudinal direction of the product, for example, over a larger area than the cross-section of the product to be measured or the measuring area itself, and by having the first and second light sources each emit diffuse light within a large angular range, illumination of the product located in the measuring area can be achieved over its entire circumference, thus enabling the detection of defects over the entire product circumference.As previously explained, the first and second directions of extension can be straight directions. They can then be aligned at an angle between 70° and 110°, for example, approximately 90°. Similarly, the first and second sensor directions of extension can also be straight directions, as previously explained, and can then be aligned at an angle between 70° and 110°, for example, approximately 90°. The first and second light sources, together with the first and second sensors, can form a complete square around the measuring area or the product to be measured. For complete illumination of spatially extended products over a circumference of 360°, there can be a spatial overlap between the directions of extension of the light sources and the sensor directions.For this purpose, due to space constraints, the first light source-sensor pair (comprising the first light source and first sensor) and the second light source-sensor pair (comprising the second light source and second sensor) can be arranged in different planes, offset in the longitudinal direction or conveying direction of the product to be measured. The first pair (comprising the first light source and first sensor) and the second pair (comprising the second light source and second sensor) can be designed and arranged in the same way, only rotated relative to each other by a specific angle, for example, between 70° and 110°, preferably between 80° and 100°, and more preferably by 90°.

[0024] The device can further include a conveying unit that transports the strand-shaped product through the measuring area along a conveying direction during the measurement. The conveying direction can run in the longitudinal direction of the strand-shaped product. As already mentioned, the product can have been produced in an extrusion device. It can then be measured directly downstream of the extrusion device using the device or method according to the invention. The extrusion device can be part of the device according to the invention.

[0025] The device can further comprise an additional sensor arranged downstream of the first sensor in the conveying direction of the strand-shaped product, which also detects the quantity of light emitted by the first light source after passing through the measuring area. The evaluation unit can then be configured to...

[0026] ... / 10 to evaluate the difference signal of the first sensor and the second sensor. By arranging two sensors one behind the other in the conveying direction of the product and evaluating the difference signal between the measurement signals of the sensors, interference from extraneous light, i.e., light not emitted by the light sources according to the invention, is reduced, since such extraneous light affects both sensors at least similarly, whereas an error signal generated by a defect only shades one sensor at a time in the case of short defects and, in the case of extended defects, initially affects one of the sensors after entering the measuring range and then, as it exits the measuring range, also the other sensor.Additionally, this design reduces disruptive effects that can be caused by changes in the product's position, such as oscillation, in conjunction with inhomogeneous illumination, provided that such inhomogeneity affects both sensors at least similarly. The second sensor is separate from the first and positioned at a distance from it. The second sensor can be designed and positioned like the first sensor, except that it is positioned at a distance along the longitudinal or conveying direction of the product. Of course, a second sensor could also be paired with a second sensor positioned downstream of the first, in the conveying direction of the stranded product. This second sensor would also measure the quantity of diffuse light emitted by the second light source after passing through the measuring area.The evaluation unit could then be designed to evaluate a differential signal from the second sensor and the additional sensor associated with it.

[0027] In a further embodiment, it can be provided that the first light source extends over the entire circumference of the measuring range, and that the first sensor extends over the entire circumference of the measuring range, wherein the first light source and the first sensor extend in different planes.

[0028]

[0029] In particular, the sensors are offset along the product's length. As explained above, completely measuring the product around its circumference using two light source / sensor pairs can require spatial overlap of the light sources or sensors. This, in turn, necessitates relatively long or large light sources or sensors. An alternative that allows the use of shorter or smaller light sources and sensors would be to use three light source / sensor pairs, which, if appropriately arranged around the product, can measure it around its entire circumference. However, due to space constraints, this may require arranging the three light source / sensor pairs in three different planes, thus undesirably increasing the overall installation space of the device.The aforementioned configuration, in which the first light source and the first sensor extend completely around the measuring area, enables a complete measurement of the product over its entire circumference in just two planes, which can be oriented perpendicular to the longitudinal direction or conveying direction of the product. The first light source is aligned with the plane of the first sensor, with the illumination lighting the measuring field from all directions.

[0030] The first light source can extend circularly around the measuring area, and the first sensor can extend triangularly around the measuring area, for example, in an equilateral triangle. A symmetrical arrangement of light source and sensor could be disadvantageous with regard to shadowing. Furthermore, the first sensor can be constructed in a structurally simple way from three sensor segments, each extending along a straight line, forming a triangle. The first light source can be composed of individual light sources. These individual light sources can be arranged in a circular shape in a particularly simple manner to achieve the circular shape.

[0031] In a further embodiment, an aperture can be provided that blocks out part of the light emitted by the first and / or second light source.

[0032] ... / 12 so that it does not reach the first and / or second sensor. As explained in more detail below, differently designed and arranged apertures can be used. The underlying principle for all apertures is that only light passing close to or directly onto the product contributes to a relevant signal change caused by a defect, since only this light is blocked by corresponding changes on the product or, in the case of a missing cross-section, is not blocked. Light paths that run outside this desired measurement field do not contribute to the useful signal, i.e., changes caused by defects, but they do contribute to the overall signal. They thus reduce the contrast and make it more difficult to detect defects. Accordingly, the overall signal can be reduced by using apertures that only block those light paths that do not contribute to the useful signal, without affecting the useful signal itself.If additional sensors are planned, corresponding apertures can also be assigned to them. To counteract the reduced total amount of light caused by apertures, more powerful light sources can be used if necessary. Alternatively, other optical elements that restrict the emission of diffuse light to the measurement area, such as lenses, are also conceivable.

[0033] In one embodiment, the aperture can directly limit the measuring range. The aperture can extend in the first or second direction of expansion. It can also limit the measuring range laterally. For this purpose, the aperture can be arranged in a measuring plane of the measuring range, particularly in a plane passing through a center of the measuring range. It can, for example, comprise a slit-shaped aperture opening. An aperture in the measuring plane or the product plane represents a particularly simple way of blocking unwanted radiation. The distance of the aperture walls or edges from the measuring range must be...

[0034]

[0035] The measuring range should be large enough so that only the desired light paths are obscured by the aperture.

[0036] Another possibility is to assign multiple apertures to individual light sources of the first and / or second light source. Each aperture blocks a portion of the light emitted by the first and / or second light source, preventing it from reaching the first and / or second sensor. This more complex method of blocking light is suitable for multiple point light sources. Such individual apertures ensure that the respective light sources emit light only into a specific area, particularly the measurement area. While this design is more complex compared to a single aperture in the measurement plane, it allows for the use of an individually optimized aperture for each light source, instead of a single aperture for the entire illumination.

[0037] Another possibility is to position an aperture in the longitudinal direction, or in the conveying direction, of the strand-shaped product. This approach can increase the relative signal strength of short defects in the longitudinal or conveying direction of the product. This reduces the length of the product under consideration, thereby increasing the proportion of the defective area within the sensor's detectable range and consequently generating a relatively higher signal change.

[0038] In a further embodiment, it can be provided that the first and / or second sensor is assigned a spectral filter for filtering out ambient light and / or that the first and / or second sensor is assigned a frequency filter for filtering out a frequency range of the received light. The inventive approach is inherently sensitive to changes in ambient light, since these changes do not necessarily correspond to changes in the ambient light due to the evaluation of the light quantity rather than a sharp image of the shadow cast.

[0039] ... / 14 a change in shading. To reduce this effect, spectral filters can be used that allow light of the desired illumination to pass through the light sources while filtering out as much ambient light as possible. Since room lighting is now largely limited to the visible spectrum of light, this approach is particularly effective when the light sources emit light in the non-visible spectral range. Furthermore, if the conveying speed of the product is known through the measuring range, an expected frequency range for the error signal generated by a defect can be determined. Signals outside this expected frequency range can be ignored accordingly. Frequency filters can be used for this purpose. This reduces the sensitivity to changes in ambient light and to changes in the product's position.If further sensors are assigned to the first and / or second sensor, these may also have spectral filters and / or frequency filters assigned to them.

[0040] In a further embodiment, the light emitted by the first and / or second light source can be modulated light, in particular frequency-modulated light, and the measurement signal received by the first and / or second sensor can be demodulated, in particular frequency-demodulated, or be demodulated, in particular frequency-demodulated. This embodiment represents another way to reduce the undesirable influence of ambient light by modulating the illumination. This effectively transforms, for example, a frequency range of the unwanted ambient light from the frequency range of the desired illumination to be evaluated.

[0041] In a further embodiment, it can be provided that the first light source comprises a plurality of individual light sources arranged side by side in the first direction of extension and / or that the second light source comprises a plurality of individual light sources arranged side by side in the second direction of extension.

[0042] ,.. / 15 includes the fact that the light from different individual light sources is modulated differently, in particular frequency-modulated, and that the measurement signal received by the first and / or second sensor is differently demodultable, in particular frequency-demodulated, or is demodultable differently, in particular frequency-demodulated. When using multiple individual light sources, different modulation frequencies can be used for different light sources. For example, each individual light source can be modulated differently. This makes it possible to separate signals generated by any individual light source from other signals by appropriate demodulation of the associated sensor and thus evaluate them separately. Accordingly, a defect signature can be searched for in each of these signal components.Depending on the location of the defect, the fault signatures contribute differently to individual signals. Therefore, the relative signal strength of one or more of these sub-signals is greater than that of the overall signal, making it easier to identify a fault signature in at least one sub-signal. If additional sensors are installed, the aforementioned configuration can also apply to them.

[0043] In a further embodiment, the first sensor and / or the second sensor can have a mirror that reflects light to the first sensor and / or the second sensor. If further sensors are provided, they can also have such mirrors. The evaluation according to the invention requires relatively large sensors. To nevertheless limit the absolute size of the sensors, mirrors can be used that increase the effective sensor size. This is possible according to the invention because it is irrelevant for the invention at which exact location of the sensor light falls after passing through the measuring area. Rather, the total amount of light detected by the sensor is evaluated. Any mirrors can be designed such that essentially all light passing through the sensor is reflected.

[0044]

[0045] The measurement area is reached and does not reach the detector directly, but is directed onto the detector by the mirrors.

[0046] The invention also solves the problem by providing a method for detecting defects in a strand-shaped product using a device according to the invention. The device can be configured accordingly to carry out the method according to the invention.

[0047] Exemplary embodiments of the invention are explained in more detail below. They are shown schematically:

[0048] Figure 1 shows a conventional device for detecting defects in a strand-shaped product.

[0049] Figure 2 is a diagram illustrating measurement results of the device according to Figure 1.

[0050] Figure 3 shows a device according to the invention for detecting defects in a strand-shaped product,

[0051] Figure 4 is a diagram illustrating measurement results of the device according to Figure 3.

[0052] Figure 5 shows another diagram with measurement results of the device shown in Figure 3,

[0053] Figure 6 shows a further embodiment of the device according to the invention.

[0054] Figure 7 shows a further embodiment of the device according to the invention,

[0055]

[0056] Figure 8 shows a further embodiment of the device according to the invention,

[0057] Figure 9 shows a further embodiment of the device according to the invention, and

[0058] Figure 10 shows a further embodiment of the device according to the invention.

[0059] Unless otherwise stated, the same reference symbols in the figures denote the same objects.

[0060] The conventional device shown in Figure 1 for detecting a defect in a strand-like product 100 comprises a light source 102 that emits divergent light 104 onto the product 100. Opposite the light source 102, a detector 106 is arranged, which receives the light emitted by the light source 102. In the setup shown in Figure 1, two defects 108 and 110 on the circumference of the product 100 are shown as examples. As can be seen in Figure 1, the defect 108, due to its position, changes the shadow cast on the sensor 106, while the defect 110, due to its position behind the horizon of the light source 102, does not.

[0061] Figure 2 illustrates this in a diagram, showing the relative signal change generated by a defect on sensor 106 as a function of the defect's angular position in degrees for various defect radii. The generated relative signal change decreases as the defect size decreases. It can also be seen that the detectable angular range of the defect's position also decreases with decreasing defect size.

[0062] Figure 3 shows a device according to a first embodiment, wherein in a central measuring area of ​​the device a [feature] is shown in Figure 3.

[0063]

[0064] The drawing plane is shown as a strand-like product 10 with a circular cross-section in this example. Product 10 could, for example, be a cable or a pipe. During measurement, product 10 can be conveyed along its longitudinal axis through the measuring area of ​​the device by means of a conveying device (not shown in detail), in Figure 3 into the drawing plane.

[0065] The device according to the invention comprises a first light source 12, which in the illustrated example comprises a plurality of individual light sources 14 extending along a first direction of extension, in Figure 3 in a vertical direction. On the side of the measuring area or of the product 10 opposite the first light source 12, there is a first sensor 16 extending in a first sensor direction of extension. In the illustrated example, the first sensor direction of extension is parallel to the first direction of extension of the first light source 12. The device further comprises a second light source 18, which comprises a plurality of individual light sources 20 extending along a second direction of extension, in Figure 3 in a horizontal direction. On the side of the measuring area or of the product 10 opposite the second light source 20, there is a first sensor 16 extending in a first sensor direction.A second sensor 22, extending along a second sensor extension direction, is located on the product 10. The second sensor extension direction is arranged parallel to the second extension direction of the second light source 18. Furthermore, it can be seen in Figure 3 that the first extension direction and the first sensor extension direction are arranged perpendicular to the second extension direction and the second sensor extension direction, so that the first and second light sources 12, 18, together with the first and second sensors 16, 22, form a square frame around the measuring area and the product 10 to be measured. The first and second light sources 12, 18, as well as the first and second sensors 16, 22, each extend transversely to the longitudinal direction of the product 10.

[0066]

[0067] The product 10 is measured over a considerably larger area than its measuring range or cross-section. Furthermore, Figure 3 shows that the first light source 12 and the second light source 18 overlap spatially, intersecting in the upper right area of ​​the square formed. The first sensor 16 and the second sensor 22 also overlap, in the lower left area of ​​the square. For space reasons, the first light source-sensor pair, consisting of the first light source 12 and the first sensor 16, is therefore arranged in a different plane than the second light source-sensor pair, consisting of the second light source 18 and the second sensor 22. The planes of the light source-sensor pairs are offset along the longitudinal direction of the product 10, thus extending into the plane of the drawing in Figure 3.

[0068] In the illustrated example, the individual light sources 14, 20 of the first and second light sources 12, 18 each emit diffuse light over a wide angular range onto the measuring area and the product 10 located therein, respectively. The individual light sources 14, 20 can emit light constantly and with consistent, and in particular, uniform intensity during a measurement. For example, the individual light sources 14, 20 can emit light in the infrared frequency range. The individual light sources 14, 20 can be, for example, LEDs or laser diodes. The first and second sensors 16, 22 can be photodetectors, such as CCD sensors or extended photodiodes. They detect the quantity of light emitted by the first and second light sources 18 after passing through the measuring area in which the product 10 is located. The sensors 16, 22 thus only detect the total amount of incident light from the light sources 12 and 18, respectively.In particular, no focused image of the shadow cast by product 10 is displayed and evaluated on sensors 16, 22.

[0069] It is possible that the first and / or second sensor 16, 22 is assigned a spectral filter for filtering out ambient light and / or a frequency filter for filtering out a frequency range of the received light. In this way,

[0070] ,.. / 20 it must be ensured that, for example, the first sensor 16 only detects light from the first light source 12 and the second sensor 20 only detects light from the second light source 18.

[0071] It is also possible that the light emitted by the first and / or second light source 12, 18 is modulated light, and that the measurement signal received by the first and / or second sensor 16, 22 is demodulated accordingly. It is also possible that the light from the individual light sources 14, 20 of the first and second light sources 12, 18 is modulated differently, and that the measurement signal received by the first and / or second sensor can be demodulated differently.

[0072] The measurement signals from the first and second sensors 16, 22 are also connected to an evaluation unit 24, which detects a change in the amount of light detected by the first or second sensor 16, 22 during the conveying of the product 10 through the measuring area as a defect in the product 10. In contrast to the prior art shown in Figure 1, according to the invention, the measuring area or the product 10 arranged therein is diffusely illuminated with light over the entire circumference of the product 10, i.e., over an angular range of 360°, by means of the light sources 12, 18. Thus, in contrast to the prior art described above, defects can be easily detected regardless of their position on the circumference of the product 10.

[0073] This will be explained in more detail with reference to Figures 4 and 5. In Figure 4, the relative signal change 26 as a function of the position of the defect in degrees across the circumference of the product 10 is shown as a dashed line, as it occurs with a device according to Figure 1. The corresponding relative signal change 28 as a function of the angular position of the defect in degrees, as obtained with the approach according to the invention, is shown as a solid line in Figure 4. The relative signal change 26 was plotted for an idealized collimated light source, and the relative

[0074] ,.. / 21 Signal change 28 for an idealized diffuse light source, which therefore illuminates every point of the sensor equally from every point of the light source. The height of the defect is one hundredth of the radius of the product 10. It can be seen that with the approach according to the invention, an evaluable relative signal change due to a defect is possible over a considerably larger angular range across the circumference of the product.

[0075] Figure 4 illustrates this relationship for a single diffuse light source and sensor pair, which, incidentally, has a smaller size than the embodiment shown in Figure 3. With larger light sources and sensors, or with a smaller distance between them, the detectable angular range according to the invention can be further increased. As explained, the configuration shown in Figure 3 with two sensor-light source pairs allows coverage over the entire circumference of the product 10.

[0076] This will be illustrated by the diagram in Figure 5, which shows experimental data for the device depicted in Figure 3. In this case, the maximum signal difference (normalized) generated by a defect is plotted against the angular position of the defect over the circumference of the product 10 in degrees for the two pairs of first light source 12 and first sensor 16, and second light source 18 and second sensor 22. The signal difference 30, shown as a dashed line in Figure 5, represents the quantity of light radiation received from the first light source 12 as detected by the first sensor 16. The maximum signal difference 32, shown as a solid line in Figure 5, represents the quantity of light radiation received from the second light source 18 as detected by the second sensor 22, in each case after the measurement area has been irradiated with the product 10 attached to it.For the experiment, product 10 was prepared with a defect, and the change in the amount of light received by sensors 16 and 22 was measured as product 10 with the defect was conveyed through the measuring range as a function of the.

[0077] .. / 22 The orientation of the product, in particular the angular position of the defect, is measured. Not shown in Figure 5 is a background signal received by sensors 16 and 22, which is caused by noise or irregularities on the product 10. In the example of Figure 5, this signal ranges between 0.1 and 0.2 with a maximum signal difference (normalized). The diagram in Figure 5 shows that for every angular position of the defect around the circumference of the product 10, at least one of sensors 16 or 22 detects a significant, evaluable change in the amount of light received, which can be reliably identified as a defect on the product 10 by the evaluation unit 24.

[0078] Further possible embodiments of the device according to the invention will be explained with reference to Figures 6 to 9. For illustrative purposes, only one pair consisting of a first light source 12 and a first sensor 16 is shown. It is understood that the embodiments can also be used with several sensor-light source pairs, as shown in Figure 3.

[0079] As can be seen in the diagram of Figure 4, the evaluation of the light quantity according to the invention, in contrast to a sharp imaging of the shadow cast with collimated light as in the prior art, reduces the contrast when detecting defects. To counteract this problem, an aperture 34, which can be, for example, slit-shaped, is arranged in the measuring plane in Figure 6 and limits the measuring area with the product 10 arranged therein. The aperture 34 blocks light paths running outside a desired measuring area, so that the first sensor 16 receives only the desired useful signal, in particular changes caused by a defect. In this way, the contrast for detecting a defect can be increased.

[0080] In the embodiment according to Figure 7, instead of the aperture 34 arranged in the measuring plane of the measuring range, the individual light sources 14 are assigned

[0081] ,.. / 23 individual apertures 36 are provided, each ensuring that the individual light sources 14 emit light into the measuring area containing the product 10 only within a limited range, as illustrated in Figure 7 for one of the individual light sources 14 at reference numeral 38. This aperture arrangement also allows the useful signal to be increased relative to the overall signal, and thus the contrast. For example, aperture 34 can also extend in the conveying direction of the product 10.

[0082] In the embodiment according to Figure 8, the first sensor 14 has a smaller extent in the first sensor extent direction compared to the previously discussed embodiments. Furthermore, mirrors 40 are provided at the ends of the extent of the first sensor 14 in the illustrated example, arranged perpendicular to the first sensor extent direction, which reflect light to the first sensor 14 and thereby effectively enlarge it, since according to the invention only the detected amount of light is evaluated.

[0083] Figure 9 shows a top view of the strand-shaped product 10, which is conveyed from bottom to top. It can be seen that, downstream of the first sensor 16 in the conveying direction of the product 10, a further sensor 42 is provided. This second sensor 42 also detects the quantity of light emitted by the first light source 12 after passing through the measuring area containing the product 10. The second sensor 42 can be configured like the first sensor 16. If, in this case, a defect 44 in the product 10 passes through the measuring area, the first sensor 16 and, at a slightly later time, the second sensor 42 will detect a corresponding change in the detected amount of light. The evaluation unit 24 can then evaluate a differential signal from the first sensor 16 and the second sensor 42 to, for example, eliminate unwanted effects caused by ambient light.

[0084]

[0085] In the embodiment shown in Figure 10, the first light source 12 is composed of individual light sources extending along a circular shape around the product 10. The first sensor 16 has three sensor sections, each extending along a straight line, forming an equilateral triangle. The longitudinal axis of the product 10 passes through the center of the circle formed by the first light source 12 and the triangle formed by the first sensor 16. The first light source 12 is arranged in a first plane oriented perpendicular to the longitudinal direction of the product 10. The first sensor 16 is arranged in a second plane, also oriented perpendicular to the longitudinal direction of the product 10 and offset from the first plane along the longitudinal direction of the product 10. Thus, in Figure 10, the planes are offset into the plane of the drawing.As explained above, this design allows for a complete measurement of product 10 across its entire circumference in a space-saving manner.

[0086]

[0087] Reference symbol list

[0088] 10 products

[0089] 12 First light source

[0090] 14 individual light sources

[0091] 16 First sensor

[0092] 18 Second light source

[0093] 20 individual light sources

[0094] 22 Second sensor

[0095] 24 Evaluation unit

[0096] 26 Signal change

[0097] 28 Signal change

[0098] 30 Signal difference

[0099] 32 Signal difference

[0100] 34 aperture

[0101] 36 aperture

[0102] 38 area

[0103] 40 mirrors

[0104] 42 Additional Sensor

[0105] 44 Missing item

[0106] 100 product

[0107] 102 light sources

[0108] 104 lights

[0109] 106 Detector

[0110] 108 Missing Item

[0111] 110 missing item

[0112]

Claims

Claims 1. Device for detecting defects (44) in a strand-shaped product (10) arranged in a measuring area of ​​the device, characterized in that the device has a first light source (12) extending in a first direction of extension, which diffusely emits light, preferably visible light or infrared light, onto the measuring area from a plurality of locations along the first direction of extension, that the device further has a first sensor (16) which detects the quantity of light emitted by the first light source (12) after passing through the measuring area, and that an evaluation device (24) is provided which is configured to recognize a change in the quantity of light detected by the first sensor (16) as a defect (44) of the product (10).

2. Device according to claim 1, characterized in that the first light source (12) comprises a planar light source (12) extending in the first direction of extension and / or a plurality of individual light sources (14) arranged side by side in the first direction of extension.

3. Device according to one of the preceding claims, characterized in that the first sensor (16) extends in a first sensor extension direction, preferably on one side of the measuring area opposite the first light source (12).

4. Device according to one of the preceding claims, characterized in that the device further comprises a second The device comprises a second light source (18) extending in the direction of extension, which diffusely emits light, preferably visible light or infrared light, onto the measuring area from a plurality of locations along the second direction of extension, the device further comprising a second sensor (22) which detects the quantity of light emitted by the second light source (18) after passing through the measuring area, and the evaluation device (24) is configured to detect a change in the quantity of light detected by the second sensor (22) as a defect (44) of the product (10).

5. Device according to claim 4, characterized in that the first and second light sources (12, 18) diffusely illuminate a product arranged in the measuring area with light over an angular range of at least 270°, preferably 360°, over the circumference of the product.

6. Device according to one of the preceding claims, characterized in that it further comprises a conveying device which conveys the strand-shaped product (10) through the measuring area along a conveying direction during the measurement.

7. Device according to claim 6, characterized in that the device further comprises a further sensor (42) arranged downstream of the first sensor (16) in the conveying direction of the strand-shaped product (10), which also detects the quantity of light emitted by the first light source (12) after passing through the measuring area, and that the evaluation device (24) is configured to evaluate a differential signal of the first sensor (16) and the further sensor (42).

8. Device according to one of the preceding claims, characterized in that the first light source (12) extends over the entire circumference of the measuring area, preferably circularly around the measuring area, and that the first sensor (16) extends over the entire circumference of the measuring area, preferably triangularly around the measuring area, wherein the first light source (12) and the first sensor (16) extend in different planes, in particular offset in the longitudinal direction of the product (10).

9. Device according to one of the preceding claims, characterized in that an aperture (34, 36) is provided which blocks a portion of the light emitted by the first and / or second light source (12, 18) so that it does not reach the first and / or second sensor (16, 22).

10. Device according to claim 9, characterized in that the aperture (34) limits the measuring range and / or that a plurality of individual light sources are provided for apertures (36) assigned to the first and / or second light source (12, 18), which block out part of the light emitted by the first and / or second light source (12, 18) so that it does not reach the first and / or second sensor (16, 22) and / or that the aperture (34) is arranged in the longitudinal direction of the strand-shaped product (10).

11. Device according to one of the preceding claims, characterized in that a spectral filter for filtering out ambient light is assigned to the first and / or second sensor (16, 22) and / or that a frequency filter for filtering out a frequency range of the received light is assigned to the first and / or second sensor (16, 22).

12. Device according to one of the preceding claims, characterized in that the light emitted by the first and / or second light source (12, 18) is modulated light, and that the measurement signal received by the first and / or second sensor (16, 22) is correspondingly demodultable.

13. Device according to one of the preceding claims, characterized in that the first light source (12) comprises a plurality of individual light sources (14) arranged side by side in the first direction of extension and / or that the second light source (18) comprises a plurality of individual light sources (20) arranged side by side in the second direction of extension, that the light from different individual light sources (14, 20) is modulated differently, and that the measurement signal received by the first and / or second sensor (16, 22) can be demodulated differently.

14. Device according to one of the preceding claims, characterized in that the first sensor (16) and / or the second sensor (22) has a mirror (40) which reflects light to the first sensor (16) and / or the second sensor (22).

15. Method for detecting defects (44) in a strand-shaped product (10) using a device according to one of the preceding claims.

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