Method for checking the position of elements in a tire in an X-ray inspection system, X-ray inspection system for carrying out such a method, and use of such an X-ray inspection system for carrying out such a method

Abstract

Method for checking the position of elements (99) in a tire (90) of a tire type in an X-ray inspection system (1), wherein the X-ray inspection system (1) comprises an X-ray tube (10), a line-type X-ray detector (15) and a manipulator, and the method comprises the following steps: - Using a three-dimensional model of the tire (90) of the tire type in which the potential positions of the elements (99) in the tire (90) are described; - Recording of two-dimensional X-ray line images of the elements (99) of the tire (90) consisting of image points (33) which are described by rays from the X-ray tube (10) through the element (99) to the X-ray detector (15); - Assigning the image points (33) of an element (99) from a two-dimensional X-ray line image to the three-dimensional model of the tire (90) by assigning the intersection of a straight line through the X-ray tube (10) to the image point (33) from the two-dimensional X-ray line image to the potential position of the element (99) of the three-dimensional model as a point in space for the image point (33), whereby individual lines are recorded while the tire (90) is rotated and assembled to form the two-dimensional X-ray line images; - in which a measurement of the distance between two of the aforementioned points in the space of the element (99) in the three-dimensional space of the model of the tire (90) is subsequently carried out along an arbitrary measuring distance; - and in which the potential position of the elements (99) in the three-dimensional model of the tire (90) extrapolates the position of the elements (99) even in the case of potential displacements of the elements (99) in both directions.

Description

[0001] The present invention relates to a method for inspecting the position of tire elements, in particular cords or belts in a tire of a specific tire type, in an X-ray inspection system. It also relates to such an X-ray inspection system and the use of such an X-ray inspection system for carrying out such a method.

[0002] Tires are reinforced with metal belts or cords, especially in the tread area. To verify that these belts or cords are correctly inserted, the tires are X-rayed to visualize the belts or cords within the tires. The resulting images are evaluated to identify deviations from a desired standard.

[0003] In the X-ray inspection of radial and bias-ply tires, X-ray line scan images are generated using a wide-angle X-ray tube and a U-shaped X-ray detector array. Typically, the tire is viewed from bead to bead in one line. While the tire rotates, individual lines are recorded. These are referred to in this application as X-ray line scans. These are then combined to form a complete image depicting the entire tire.

[0004] This type of image acquisition involves two fundamental problems: By projecting a three-dimensional object onto a two-dimensional X-ray line image, position and depth information in the beam direction is lost on the one hand, and on the other hand, due to the complex imaging geometry, position and distance information cannot be directly determined as physical quantities in the X-ray line image.

[0005] A key inspection criterion in tire X-ray testing is the evaluation of the relative position of the various tire components, especially the steel belts or the cords that comprise the steel belts. For example, the distance between two belt edges must not be outside a specified tolerance range. Tolerance ranges of this type are generally specified as physical quantities and usually include a precise definition of the measurement points to be selected. Due to the problems mentioned above, neither a free selection of the measurement points nor a direct assessment of the relative position is possible.

[0006] DE 10 2012 213 244 A1 discloses an X-ray testing method for tires. To create an overall X-ray image of the tire, the tire is rotated and, with the aid of an X-ray tube arranged inside the tire and an X-ray detector arranged outside the tire, a plurality of strip-shaped individual images of the tire sidewalls and the tread lying between them are recorded, extending transversely to the circumferential direction of the tire. To measure structures in the acquired overall X-ray image, the number of recorded pixels along a distance to be measured is counted. The position of the distance to be measured on the tire surface is calculated based on the known size of the pixels, an underlying geometry of the tire, and the geometric arrangement of the tire, X-ray tube, and X-ray detector relative to one another.

[0007] US 2010 / 0 220 910 A1 discloses an X-ray inspection method and an X-ray inspection system that compares a scanned object with an idealized object. A 3D model of the idealized object is generated, and a 2D reference image created from it is compared with a 2D image created from the scanned object. Only the quality of the scanned object is checked—that is, whether a defect exists at all—but not the quantity of any detected defect.

[0008] US Pat. No. 5,737,383 A discloses a method and device for examining tires, which is essentially identical in design to that of DE 10 2012 213 244 A1 described above. A 2D X-ray image of the tire's cords is created and compared with a 2D reference image. This allows it to be determined whether the cords are incorrectly arranged.

[0009] In a valuable contribution to the state of the art, DE 10 2013 001 456 B3 describes a method for calibrating an X-ray inspection system for a specific tire type. The position information of tire components is first determined automatically or manually in the form of image coordinates in the X-ray line scan image and then converted into physical quantities using a tire-type-specific geometric calibration. A geometric calibration assigns each detector pixel a physical height and width in the X-rayed object. The distance between two measurement points defined in the X-ray line scan image results from the combination of the pixel heights and widths along a path between the measurement points. The measurement points cannot be freely selected but are implicitly located in the calibration plane. This makes it impossible, for example, to determine the horizontal distance between two tire components relative to a radial section, or is subject to systematic deviation.

[0010] Therefore, the object of the present invention is to avoid the aforementioned disadvantages of the prior art.

[0011] This object is achieved according to the invention by a method having the features of patent claim 1. Advantageous embodiments are specified in the subclaims.

[0012] The object is then achieved by a method for testing the position of elements in a tire of a tire type in an X-ray inspection system, wherein the X-ray inspection system has an X-ray tube, a line-shaped X-ray detector and a manipulator and comprises the following steps: - Using a three-dimensional model of the tire of the tire type in which the potential positions of the elements in the tire are described - Recording of two-dimensional X-ray line images of the elements of the tire consisting of pixels described by a vector from the X-ray tube through the element to the X-ray detector - Assignment of the image points of an element from the two-dimensional X-ray line image to the three-dimensional model of the tire by assigning the intersection point of a straight line through the X-ray tube with the vector of the image point from the two-dimensional X-ray line image with the potential position of the element of the three-dimensional model as a point in space for the image point.

[0013] According to the invention, it is also provided that the distance between two of the aforementioned points in the space of the element is subsequently measured in the three-dimensional space of the tire model along any desired measuring distance. This also allows measuring distances outside of a calibration plane to be realized, for example, the vertical distance between the end of the ply turnup and the end of the chafer. The potential position of the elements in the three-dimensional model of the tire extrapolates the position of the elements, even in the case of potential displacements of the elements in both directions.

[0014] Preferably, the measuring section runs along the cross-section of a tire belt layer or horizontally within the tire's cross-section. The first alternative allows the design-related different curvature of the individual belt layers to be incorporated into the determination of the belt width, thus improving measurement accuracy by reducing systematic measurement errors. The second alternative allows the horizontal distance between the ends of two consecutive belt layers to be measured particularly precisely, relative to the tire cross-section.

[0015] A further advantageous development of the invention provides that the elements are cords of a belt. This results in an improved accuracy for determining the pairwise distances between the belt cords.

[0016] A further advantageous development of the invention provides that the recording of the two-dimensional X-ray line images of the elements of the tire takes place in several positions of the tire into which the tire is brought with the manipulator.

[0017] The object is also achieved by an X-ray inspection system having the features of patent claim 5. This is an X-ray inspection system with an X-ray tube, a line-shaped X-ray detector and a manipulator, further comprising a control device configured to carry out a method for checking the position of elements in a tire according to one of the preceding claims.

[0018] The object is also achieved by using an aforementioned X-ray inspection system for carrying out a method according to the invention, as described above together with its further developments.

[0019] Further details and advantages of the invention will now be explained in more detail with reference to an embodiment shown in the drawings.

[0020] They show: Fig. 1 a schematic imaging geometry in tire testing, Fig. 2 a schematic representation of a measuring procedure for evaluating the position of two belts or measuring the distance between the ends of a ply turn-up and a chafer, Fig. 3 a schematic projection of various measurement objects onto a detector row and the calibration plane, Fig. 4 a schematic representation of the systematic measurement error, Fig. 5 a parameterization of a model component, Fig. 6 a cross-section through a tire model and an extended tire model and Fig. 7 a tire model and an imaging model.

[0021] In Fig. Figure 1 schematically shows an X-ray inspection system 1 used to perform X-ray inspections of tires 90, particularly radial and cross-ply tires. X-ray line images are generated using an X-ray tube 10, particularly a wide-angle X-ray tube, and an X-ray detector 15, particularly a U-shaped X-ray detector line. While the tire 90 rotates, individual lines are recorded. These are then combined to form an overall image depicting the entire tire 90. Typically, one line of the overall image shows the tire 90 from bead core 92 to bead core 92, including the carcass 91.

[0022] This type of image acquisition involves two fundamental problems: By projecting a three-dimensional object onto a two-dimensional X-ray line image, position and depth information are transmitted in the beam direction, i.e. along the rays referred to in this application as vectors 35 (see Fig. 3 and Fig. 4), and on the other hand, due to the complex imaging geometry, position and distance information cannot be directly determined as physical quantities in the X-ray line image.

[0023] In Fig. 2 is a cross-section, comparable to Fig. 1, represented by a tire 90 (without the X-ray system 1). A key test criterion in the X-ray inspection of tires 90 is the evaluation of the position of the various tire components, here in the form of layers of cords 99 (generally referred to as elements 99 in this application), relative to one another; these are, in particular, steel belts consisting of cords 99. Of interest here is the true value of the horizontal distance A between the ends of the belt edges 95. Other test criteria include, for example, the width of the individual steel belts or the vertical distance between the layer turn-up end 93 and the chafer end 94. Generally, such test criteria consist of a definition of the measuring points to be selected and a tolerance range for the measured quantity.

[0024] Due to the problems mentioned, the methods currently in use do not allow for a free choice of measuring points or a direct assessment of the situation, which is Fig. 3 and Fig. 4 is illustrated.

[0025] In Fig. Figure 3 schematically illustrates how the position information of tire components in the X-ray inspection system 1 is initially determined automatically or manually in the form of image coordinates in the X-ray line scan image and then converted into physical quantities using a tire-type-specific geometric calibration. A geometric calibration assigns each image point 33 (detector pixel) in the X-ray detector 15 a physical height and width in the X-rayed object, here the tire 90, along a vector 35 (beam of the X-ray tube 10). The distance between two measurement points defined in the X-ray line scan image results from the combination of the pixel heights and widths along a path between the measurement points. The measuring points cannot be freely selected, but are implicitly located in the calibration plane 40 (see Fig. 4).

[0026] In Fig. Figure 4 illustrates how, for example, determining the horizontal distance between two tire components relative to a radial section is not possible or is subject to a systematic error. The systematic measurement error depends on the shape of the calibration plane 40 and the imaging geometry. On the one hand, there is the horizontal distance B obtained in the calibration plane 40 by calibration, and on the other hand, the true value of the horizontal distance A.

[0027] In the following, an inventive method is presented that solves the problems listed above. This method is based on a model of a tire type that contains the idealized position of the tire components in space. The model of a tire component k represents a two-dimensional surface in three-dimensional space that is rotationally symmetrical with respect to the tire's axis of rotation 90; it can therefore be defined as a function fk:(α,t)∈[0°,360°)×[tk0,tk1]→(x,r,α)∈(−∞,∞)×[0,∞)×[0°,360°)with 0≤tk0≤tk1≤1 In doing so, as is the case in Fig. 5, a representation in cylindrical coordinates is used. Due to the rotational symmetry of a tire 90, it is sufficient to consider a two-dimensional model representing the cross-section of the tire 90. The parameter α in the above representation can therefore be omitted for the time being.

[0028] The model is extended using a suitable method to take into account any production-related positional shifts of the individual components, see Fig. 6. This results in an extended model function f˜k:t∈[0,1]→(x,r)∈(−∞,∞)×[0,∞) with fk(α,t)=(f˜k(t),α) for∈[tk0,tk1] and α∈[0°,360°)

[0029] The extended model is embedded into the imaging model according to the actual imaging geometry, which represents the position T ∈ [0,∞) 2the X-ray tube 10 and the positions of the detector pixels L i ∈ [0,∞) 2 , i = 1...number of detector pixels., contains, as in Fig. 7. For each pixel and for each component of the tire model, the preimage f̃ k (t k,i ) under the X-ray projection, that is, the parameter t k,i ∈ [0,1], such that f̃ k (t k,i ) = T + s · (L i - T) for a suitable s ∈ [0,∞).

[0030] The preimage of a pixel with respect to a model component is the point at which a ray emanating from the X-ray source 10 to this pixel intersects the model component.

[0031] For a point P = (m, n) in the X-ray line image with row index m and column index n, the corresponding tire component k is first identified. Then, the preimage f̃ k (t k,n ) =(x k,n , r k,n), which contains the x-position of the point in space and the radial distance to the x-axis (the x-axis corresponds to the rotation axis of the tire 90). The row index m is equivalent to the angular position α m in the tire 90, so that the reconstructed three-dimensional position of P is the point (x k,n , r k,n , α m ) in cylindrical coordinate representation. The representation in Cartesian coordinates is obtained using the coordinate transformation (x,r,α) → (x,r · cos(α),r · sin(α)).

[0032] This method can be used to approximately determine the position of individual tire components identifiable in the X-ray line scan by reconstructing the positions of all pixels representing the component in space. To assess the relative position of the tire components, the position of the component edges is reconstructed and subsequently measured according to the test specification. Fig. 2 and Fig. The true values ​​A of the horizontal distances of interest shown in Figure 4 are generally in a range between 5 mm and 50 mm.

[0033] In summary, it can be stated that the described method according to the invention makes it possible to reconstruct the original three-dimensional position in the tire 90 for a point in the X-ray line image that can be assigned to a component, in particular a cord 99 or a steel belt to which such a cord 99 belongs, of the tire 90. This results in the possibility of measuring and evaluating the position of the tire components in the three-dimensional reconstruction of the tire 90. For example, it can be determined whether there is a sufficiently large distance in the horizontal direction relative to the cross-section of the tire 90, i.e. parallel to the tread of the tire 90 or orthogonal to the acting weight of the vehicle, for two belt edges 95 that are projected onto exactly the same position in the X-ray image.Compared to the approaches currently in use, the described method according to the invention thus enables the realization of a significantly larger number of different measurement tasks and at the same time a reduction of the systematic measurement deviation.

[0034] Typical measuring tasks include a) the measurement of the horizontal distance between two belt edges 95, b) measuring the vertical distance between the ends of the turn-up and the chafer, c) the determination of the belt width, ie the actual extension of a belt layer along its cross-section, d) measuring the offset at the splice of the belt layers, e) the determination of the pairwise spacing of the cords of a belt layer.

[0035] Measurement tasks a) and b) can be realized using the described method. Currently used state-of-the-art approaches generally do not allow this. For measurement tasks c) to e), the described method leads to an increase in measurement accuracy by reducing the systematic measurement error. List of reference symbols 1 X-ray inspection system 10 X-ray tubes 15 X-ray detector 33 pixels 35 Vector 40 Calibration level 90 tires 91 Carcass 92 Bead core 93 Layer-turning 94 chafer end 95 belt edge 99 Element; Cord A true value of the horizontal distance B value of the horizontal distance related to the calibration plane

Claims

[1] Method for testing the position of elements (99) in a tire (90) of a tire type in an X-ray inspection system (1), wherein the X-ray inspection system (1) has an X-ray tube (10), a line-shaped X-ray detector (15) and a manipulator, and the method comprises the following steps: - using a three-dimensional model of the tire (90) of the tire type in which the potential positions of the elements (99) in the tire (90) are described; - recording two-dimensional X-ray line images of the elements (99) of the tire (90) consisting of pixels (33) which are described by rays from the X-ray tube (10) through the element (99) to the X-ray detector (15); - Assigning the image points (33) of an element (99) from a two-dimensional X-ray line image to the three-dimensional model of the tire (90) by assigning the intersection point of a straight line through the X-ray tube (10) to the image point (33) from the two-dimensional X-ray line image to the potential position of the element (99) of the three-dimensional model as a point in space for the image point (33), wherein individual lines are recorded while the tire (90) is rotated and are combined to form the two-dimensional X-ray line images; - in which the distance between two of said points in the space of the element (99) is subsequently measured in the three-dimensional space of the model of the tyre (90) along any desired measuring distance; - and in which the potential position of the elements (99) in the three-dimensional model of the tire (90) extrapolates the position of the elements (99) even in the case of potential displacements of the elements (99) in both directions. [2] Method according to claim 1, wherein the measuring section runs along the cross section of a belt layer of the tire (90) or horizontally in the cross section of the tire (90). [3] Method according to one of the preceding claims, in which the elements (99) are cords of a belt. [4] Method according to one of the preceding claims, in which the two-dimensional X-ray line images of the elements (99) of the tire (90) are recorded in several positions of the tire (90) into which the tire is brought by the manipulator. [5] X-ray inspection system (1) with an X-ray tube (10), a line-shaped X-ray detector (15) and a manipulator, further comprising a control device which is designed to carry out a method for checking the position of elements (99) in a tire (90) according to one of the preceding claims. [6] Use of an X-ray inspection system according to claim 5 for carrying out a method according to one of claims 1 to 4.

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