BACKGROUND

The invention relates to a calibration tool and a method for calibrating a measuring system, in particular a laser-triangulation measuring system.

Measuring systems are used in the field of tire building during various stages of production to measure the quality and/or characteristics of one or more tire components. One of said stages is the production of a bead- apex. The bead-apex is formed by subsequently applying a bead and an apex around the circumference of a bead-apex drum. The bead-apex drum may receive beads and apexes in a wide variety of shapes and sizes. Moreover, the bead-apex drum also comes in various shapes and sizes and may be replaced by another bead-apex drum when appropriate. Finally, the part of the bead-apex drum that supports the apex is typically conical to support the apex at an oblique angle to the radial direction.

SUMMARY OF THE INVENTION

A disadvantage of the known laser-triangulation measuring system is that it may become inaccurate over time. It is known to calibrate a laser-triangulation measuring system by measuring a stationary object with predetermined dimensions and by comparing the measurements with the predetermined dimensions. However, this process of calibration only provides a limited amount of feedback based on the dimensions of the stationary object. Although the measuring system may be properly calibrated for the dimensions of the stationary object, measurements in other dimension ranges are still uncalibrated and may be inaccurate .

It is an object of the present invention to provide a calibration tool and a method for calibrating a measuring system, in particular a laser-triangulation measuring system, wherein the calibration can be improved.

According to a first aspect, the invention provides a calibration tool for calibrating a laser- triangulation measuring system, wherein the calibration tool comprises a tool body that is rotatable relative to the measuring system about a rotation axis perpendicular to a reference plane, wherein the tool body is provided with one or more calibration surfaces that define a pattern, in particular a radial grid, of calibration positions, wherein the pattern comprises at least three columns extending in a radial direction away from the rotation axis and at least three rows extending in a circumferential direction about the rotation axis, wherein for each column the calibration positions within said respective column vary in height relative to the reference plane in a height direction perpendicular to said reference plane and wherein for each row the calibration positions within the respective row vary in height in the height direction relative to the reference plane.

The tool body can conveniently be rotated relative to the measuring system in the same way as the bead-apex drum. By rotating the tool body, the columns can be positioned, one-by-one, in a measuring position for measuring of the calibration positions within each column by the measuring system. In particular, the measuring system may project a laser line onto the tool body in or parallel to the radial direction so that all calibration positions in a respective one of the columns can be measured simultaneously along the same projected laser line. Each column of calibration positions represents or forms a specific height profile that can serve as a calibration for the measuring system. As the calibration positions are varied in height in both the columns as well as the rows, the measurements can be calibrated for a considerable number of calibration positions, thus providing the measuring system with a relatively large amount of feedback for various height positions.

Preferably, for each column at least half of the calibration positions and preferably all calibration positions within the respective column have different heights in the height direction relative to the reference plane. Hence, at least half of the calibration positions within the respective column generates unique calibration information for the calibration of the measuring system.

In one embodiment, for each column the calibration positions within the respective column are sequentially reduced in height relative to the reference plane in the radial direction away from the rotation axis. The sequential reduction in height can be similar to or representative of the declining height of a bead-apex supported on the bead-apex drum and can therefore provide useful calibration information for the calibration of the measuring system.

Preferably, the sequential reduction in height has a constant decrement relative to the reference plane. The calibration information generated by the calibration positions within the respective column can thus be used to determine a scale of the measuring system, in particular a scale for converting pixels to real-world units, i.e. millimeters. Alternatively, the sequential reduction in height follows a curvature. Said curvature can for example be chosen to match or correct for a certain lens distortion effect as a result of the camera used in the measuring system .

Additionally or alternatively, for each row at least half of the calibration positions and preferably all calibration positions within the respective row have different heights in the height direction relative to the reference plane. Hence, at least half of the calibration positions within the respective row generates unique calibration information for the calibration of the measuring system.

In one embodiment, for each row the calibration positions within the respective row are sequentially increased in height relative to the reference plane in the circumferential direction. The calibration positions within the respective row can thus be representative of the various heights of different bead-apexes that are supported on the bead-apex drum at the radial position of the respective row. When combined with the sequential reduction of height in the radial direction within the columns, a pattern can be formed with columns of radially declining calibration positions that per column collectively increase in height in the circumferential direction with each row.

Preferably, the sequential increase in height has a constant increment relative to the reference plane. The calibration information generated by the calibration positions within the respective row can thus be used to determine a scale of the measuring system, in particular a scale for converting pixels to real-world units, i.e. millimeters .

In one embodiment, each calibration position within the pattern has a height in the height direction relative to the reference plane that is different from the heights of the other calibration positions relative to the reference plane in the same column and the same row. Hence, each calibration position within the pattern generates unique calibration information for the calibration of the measuring system.

The skilled person will appreciate that the calibration tool according to the invention may comprises only a single calibration surface in each column, in each row or for the pattern as a whole. Such a single calibration surface could for example have a gradually declining height in the radial direction and a gradually inclining height in the circumferential direction. The measuring system would then be configured to measure at certain locations on the single calibration surface, said locations corresponding to the calibration positions. The single calibration surface could hold an infinite number of calibration positions.

In contrast, in the embodiment as shown in the drawings, for each column the one or more calibration surfaces comprises an individual calibration surface for each calibration position within the respective column. By having distinct, individual calibration surfaces, the calibration positions are not easily confused and can be easily recognized by the measuring system, i.e. by detecting transitions from one calibration surface to another .

Preferably, for each column the tool body is provided with recesses extending between the calibration surfaces within the respective column to space apart said calibration surfaces in the radial direction. By spacing the columns apart, the calibration positions are even less likely to be confused. Moreover, the presence of the recess between the calibration surfaces allows for a distinct edge and/or a base level or zero level measurement in the recess .

More preferably, each calibration surface within the respective column defines a calibration edge at each transition from the respective calibration surface to an adjacent one of the recesses, wherein at least one of the calibration positions is located at one of said calibration edges. The calibration edges are easily detectable and/or measurable and can therefore serve as an excellent calibration position.

In a further embodiment, for each column the calibration surfaces within the respective column extend in a common plane, wherein said common plane extends at an oblique angle to the reference plane. The obliquely angled common plane is similar to or representative of the obliquely declining or conical surface of the bead-apex supported on the bead-apex drum. The common plane has the additional advantage that all calibration positions are also positioned in the same common plane.

Additionally or alternatively, for each row the one or more calibration surfaces comprises an individual calibration surface for each calibration position within the respective row. By having distinct, individual calibration surfaces, the calibration positions are not easily confused and can be easily recognized by the measuring system, i.e. by detecting transitions from one calibration surface to another.

Preferably, for each row the calibration surfaces within the respective row are stepped in the height direction from one of the calibration surfaces to the next one of the calibration surfaces in the circumferential direction. The stepped height from one calibration surface to the next means that - with each subsequent column - the calibration surfaces can be easily distinguished from the calibration surfaces of the previous column in the circumferential direction of the respective row. Moreover, the height of each calibration surface may be constant in the circumferential direction between the steps, so that representative measurements for the respective calibration position can be taken at any position in the circumferential direction between the steps. Therefore, the accuracy of the rotational positioning of the calibration tool relative to the measuring system is less critical.

In another embodiment, the pattern comprises at least five columns, preferably at least eight columns. Additionally or alternatively, the pattern comprises at least four rows, preferably at least five rows. The amount of columns determines the amount of height profiles that can be calibrated. The number of rows determines the amount of calibration positions within each column, i.e. within each height profile.

In a further embodiment the tool body extends over only a part of a full circumference about the rotation axis. Preferably, the tool body is formed as a circular segment. When the tool body is not a full ring or annulus, the tool body can be relatively compact, i.e. compared to the bead-apex drum.

According to a second aspect, the invention provides a method for calibrating a laser-triangulation measuring system with the use of the calibration tool according to any one of the aforementioned embodiments, wherein the laser-triangulation measuring system comprises a laser and a camera with a field of view, wherein the method comprises the steps of:

a) providing the calibration tool at least partially within the field of view of the camera;

b) projecting a laser line onto the calibration tool with the laser-triangulation measuring system;

c) rotating the calibration tool about the rotation axis such that the laser line is projected on all calibration positions of a respective one of the columns; and

d) capturing an image of the laser line projected on all calibration positions of the respective column with the camera.

The method relates to the practical implementation of the calibration tool according to the first aspect of the invention and thus has the same technical advantages, which will not be repeated hereafter.

In a preferred embodiment of the method, step d) comprises the step of repeating steps c) and d) for another one or all of the other columns. Hence, more or all calibration positions can be measured to have a maximum amount of calibration data.

In a further embodiment of the method the heights of the calibration positions of each column relative to the reference plane are predetermined, wherein the method further comprises the step of calibrating the laser- triangulation measuring system by correlating pixels in each captured image corresponding to the calibration positions of a respective column to the predetermined heights of said calibration positions within said respective column. The correlation can result in a scale for each calibration position that converts the pixels to real-world units, i.e. micrometers, millimeters or centimeters .

In a further embodiment the method further comprises the step of:

f) providing an empty bead-apex drum relative to the laser-triangulation measuring system prior to or after steps a) to e) , wherein the bead-apex drum has a reference plane and a base profile for supporting a bead-apex relative to the reference plane, wherein the empty bead- apex drum is provided with its reference plane in the same position as the reference plane of the calibration tool;

g) projecting a laser line onto the empty bead- apex drum with the laser-triangulation measuring system;

h) capturing an image of the laser line projected on the empty bead-apex drum; and

i) determining the base profile of the empty bead-apex drum relative to the reference plane of the empty bead-apex drum.

During production of the bead-apexes, the base profile of the bead-apex drum is covered by the bead-apex currently supported on the bead-apex drum. Although the height of the bead-apex can be measured relative to reference plane can be measured, this measurement is not indicative of the actual height of the bead-apex relative to the bead-apex drum. Hence, when the base profile is determined prior to the production, i.e. when the bead-apex drum is still empty, the measuring system has more information from which the actual height of the bead-apex relative to the bead-apex drum can be determined. Preferably, the method further comprises the steps of :

j) providing a bead-apex on the bead-apex drum; k) measuring the bead-apex using the measuring system; and

l) subtracting the base profile of the empty bead-apex drum as determined in step i) from the measurements .

The result of the subtraction can be representative of the actual height of the bead-apex relative to the bead-apex drum.

According to a third aspect, the invention provides a laser-triangulation measuring system comprising a laser, a camera and a support for supporting the laser and the camera, wherein the measuring system further comprises a drum rotatable about a rotation axis for guiding a strip through the measuring system and a calibration tool for calibrating said measuring system, wherein the support is pivotable about a pivot axis between an operational position in which the camera and the laser are directed at the drum to measure the strip on said drum and a calibration position in which the laser and the camera are directed at the calibration tool.

As the laser and the camera are supported on the same or a common support, they can be pivoted together between the respective positions easily and quickly, i.e. during a short interruption in the production process of the strip or even during the production process. As the calibration position is different from the operational position, the calibration is done out-of-line.

Preferably, the pivot axis is parallel to said rotation axis.

In a further embodiment the support is arranged to pivot about the pivot axis between the operational position and the calibration position over at least forty- five degrees, and preferably at least sixty degrees. Hence, the two positions can be sufficiently spaced apart to prevent interference from the calibration tool with the production process at the operational position.

In another embodiment the camera has an optical axis, wherein the calibration tool defines a reference plane perpendicular to the optical axis of the camera when the support is in the calibration position. Hence, the camera can be positioned directly overhead the calibration tool to capture an optimal image of the laser line projected on the calibration tool.

In another embodiment the drum comprises a circumferential surface that supports the strip at a zero level relative to the camera, wherein the calibration tool comprises one or more first calibration surfaces at a predetermined height relative to the reference plane in a height direction perpendicular to said reference plane for calibrating said zero level. The calibrated zero level can be used to determine the height of the measured strip above said calibrated zero level.

In another embodiment the laser is arranged for projecting a laser line onto the calibration tool in a lateral direction parallel to the rotation axis of the drum, wherein the calibration tool comprises one or more second calibration surfaces which in the lateral direction vary in height relative to the reference plane in a height direction perpendicular to said reference plane. Hence, along a single laser line, several heights can be detected corresponding to the different second calibration surfaces. Measurement data about the height of the second calibration surfaces can be used to determine a scale of the measuring system, in particular a scale for converting and/or correlating between pixels in the image and real-world units, i.e. micrometers, millimeters or centimeters.

Preferably, the height of the one or more second calibration surfaces varies in the lateral direction according to a pattern that is repeated in said lateral direction at least twice, and preferably at least three times. By repeating the pattern, the camera can be calibrated with respect to more positions in the lateral direction, i.e. across a substantial part of the field of view of the camera in the lateral direction.

According to a fourth aspect, the invention provides a clip bar for clamping a tire component to a drum, wherein the clip bar is provided with a verification element for verifying a measuring system.

The clip bar can be mounted in a predetermined position on an empty drum, i.e. without clamping a tire component to the drum. In said predetermined position, a measuring system associated with the drum can measure the clip bar, including the verification element, to verify if the measuring system, as previously calibrated, still meets the requirements. In particular, the verification element can have a known or predetermined pattern, shape or dimension, to check whether the measurements still correspond to said known or predetermined pattern, shape or dimension. Because the clip bar can be mounted on the drum in a position that substantially corresponds to its operational position on the drum during the clamping of a tire component. Hence, the verification can be performed in-line, i.e. at the same position where the clip bar is normally mounted.

In a preferred embodiment, the clip bar has a clamping side that faces the tire component during the clamping and non-clamping side opposite to the clamping side, wherein the verification element is provided at the non-clamping side. Hence, the verification element can be easily observed from the outside while the clip bar is clamped to the drum.

In a further embodiment the clip bar has a longitudinal direction, wherein the verification element is arranged at or near one end of the clip bar in said longitudinal direction. By arranging the verification element at or near one end of the clip bar, said verification element is easily visible, even if the clip bar is covered in the center area for some reason. In a further embodiment, the verification element is a slot. The characteristics of such a slot can be easily captured with a camera and laser triangulation.

According to a fifth aspect, the invention provides a first cover plate covering an intermediate space between two drum segments of a tire building drum, wherein the first cover plate comprises one or more verification elements for verifying the measurements of a measuring system .

The verification elements on the first cover plate can be used to verify the measuring system after the initial calibration, i.e. when the measuring system has already been calibrated using one or more calibration elements and/or tools. The verification elements are arranged on the drum and can be detected when said verification elements are not covered by the tire components. Because the verification elements are provided on a part of the drum that is also present during operation of said drum, the verification can be performed on the first cover plate in-line, i.e. in the same position in which the cover plate is located during production.

According to a sixth aspect, the invention provides a tire building drum comprising one or more first cover plates according to the fifth aspect of the invention, wherein the tire building drum is rotatable about a rotation axis extending in an axial direction, wherein the tire building drum further comprises one or more second cover plates, wherein each second cover plate comprises a plurality of calibration elements arranged in a calibration pattern, wherein the verification elements of the one or more first cover plates are in different positions in the axial direction with respect to the plurality of calibration elements in the one or more second cover plates. By having the verification elements and the calibration elements in different positions, the measuring system can be verified using different values to determine if the scale determined during calibration correctly interpolates to the value that is expected at the verification elements.

The tire building drum has the same advantages as the first cover plate of the fifth aspect of the invention. In particular, because the verification elements are in different positions than the calibration elements, they can be used to verify the calibration.

Preferably, the first cover plate has a longitudinal direction, wherein the measuring system comprises one or more cameras arranged side-by-side for observing a first end portion, a second end portion and a center portion, respectively, of the first cover plate which are arranged side-by-side in said longitudinal direction, wherein the one or more verification elements comprise a one or more verification elements at the first end portion, one or more verification elements at the second end portion and one or more verification elements at the center portion. Consequently, the one or more cameras can be calibrated by capturing an image of the verification elements in the respective portion. Preferably, the one or more verification elements comprise a group of two or more verification elements per portion, more preferably three or more verification elements per portion. Most preferably, the three or more verification elements in each group form a pattern that is the same for each group. Hence, each camera can be calibrated with substantially the same pattern .

According to a seventh aspect, the invention provides a validation tool for validating measurements of a measuring system for measuring one or more tire components applied around a tire building drum, wherein the validation tool comprises an annular body extending circumferentially about a central axis and one or more reference elements which are provided on said annular body and which are representative of characteristics of said one or more tire components, wherein the validation tool is provided with a center reference for determining a center of the validation tool in an axial direction parallel to the central axis.

The validation tool may substantially correspond to the validation tool disclosed in WO 2016/122311 Al, incorporated herein by reference. The position of the validation tool on or along the tire building drum can be slightly inaccurate. This can be compensated as long as the center of the validation tool is known. Once the center has been determined, the tire building drum, with the validation tool around or alongside it, can be moved in the axial direction until the validation tool is in a position relative to the measuring system where the tire building drum would normally be during operation. Alternatively, the measuring system can offset the measurement slightly to compensate for any misalignment of the center of the validation tool relative to the measuring system.

Preferably, the validation tool is provided with one or more side references for determining one or more sides of the validation tool in the axial direction. The sides can be representative of the sides of the tire building drum.

More preferably, the center reference and the one or more side references are arranged in-line in the axial direction. Hence, the center reference and the one or more side references can be measured simultaneously, i.e. by projecting a laser line across all of the references.

According to an eighth aspect, the invention provides a calibration tool for calibrating a measuring system, wherein the calibration tool comprises a calibration section with one or more calibration elements and a validation section with one or more validation elements, wherein the calibration tool is invertible about an inverting axis between a calibration position and a validation position, wherein the calibration section and the validation section switch positions when inverting about the inverting axis.

The calibration tool can therefore also function as a validation tool, simply be changing its orientation, i.e. by flipping, reversing or inverting about the inverting axis. Consequently, no separate tooling is required to validate the measuring system after the initial calibration .

Preferably, the calibration tool has a longitudinal direction, wherein the calibration section and the validation section are arranged adjacent to each other in a lateral direction perpendicular to the longitudinal direction, wherein the inverting axis extends perpendicular to the longitudinal direction and the lateral direction between the calibration section and the validation section.

In a further embodiment the calibration tool comprises one or more mounting elements for mounting the calibration tool to a support relative to the measuring system, wherein the at least one of the one or more mounting elements is in the same position after inverting the calibration tool about the inverting axis. Hence, the same one or more mounting elements can be used to mount the calibration tool in either one of the positions.

In another embodiment the one or more calibration elements comprises a plurality of calibration elements arranged in a pattern extending in a longitudinal direction of the calibration tool, wherein the one or more validation elements comprises a plurality of validation elements which are in different positions in the longitudinal direction with respect to the plurality of calibration elements. By having the validation elements and the calibration elements in different positions, the measuring system can be validated using different values to determine if the scale determined during calibration correctly interpolates to the value that is expected at the validation elements.

In another embodiment the measuring system comprises a first camera and a second camera for observing a first end portion and a second end portion, respectively, of the calibration tool, wherein the one or more validation elements comprises at least one validation element at the first end portion and at least one validation element at the second end portion. Consequently, each camera can be calibrated by capturing an image of the validation elements in the respective portion. Preferably, the one or more validation elements comprises a first group of two or more validation elements at the first end portion and a second group of two or more validation elements at the second end portion. More preferably, each group comprises three or more validation elements.

In another embodiment the one or more calibration elements and/or the one or more validation elements are through-holes. Hence, the calibration tool can be used in a back-light system where a light bar is provided at one side of the calibration tool and the camera is provided at an opposite side of the calibration tool to capture the light that passes through the through holes.

In another embodiment the one or more calibration elements comprise stepped features that enable the measuring system to be calibrated in the height direction as well.

According to a ninth aspect, the invention provides a method for verifying the measurements of a measuring system for measuring one or more tire components, wherein the measuring system comprises at least one camera with a field of view, wherein the method comprises the steps of :

m) providing a verification element with predetermined dimensions in the field of view of the at least one camera;

n) measuring the one or more tire components and measuring the verification element simultaneously with the measuring of the one or more tire components or at predetermined intervals;

o) verifying the measurements of the verification element with the predetermined dimensions of the verification element; and

p) repeating steps n) and o) over time.

When the verification is performed in-line, it can be repeated any time during production, during certain intervals or even continuously. Consequently, when the measuring system is no longer properly calibrated, i.e. because of wear or tolerances, immediate action can be taken.

The various aspects and features described and shown in the specification can be applied, individually, wherever possible. These individual aspects, in particular the aspects and features described in the attached dependent claims, can be made subject of divisional patent applications .

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be elucidated on the basis of an exemplary embodiment shown in the attached schematic drawings, in which:

figure 1 shows a top view of a bead-apex drum with a bead-apex applied to it and a measuring system for measuring the bead-apex;

figure 2 shows a cross section view of the bead- apex drum and the bead-apex according to line II-II in figure 1;

figure 3 shows a cross section view of the bead- apex drum and the bead-apex according to line III-III in figure 1;

figure 4 shows a view of a calibration tool according to a first embodiment of the invention to replace the bead-apex drum during a calibration method;

figure 5 shows a top view of the calibration tool according to figure 4;

figure 6 shows a cross section view of the calibration tool according to line VI-VI in figure 5;

figure 7 shows a cross section view of the calibration tool according to line VII-VII in figure 5;

figure 8 shows a side view of a laser- triangulation measuring system according to a second embodiment of the invention in an operating position;

figure 9 shows a side view of the measuring system according to figure 8 in a calibration position;

figure 10 shows view from above of the measuring system in the calibration position of figure 8;

figure 11 shows a view of a tire building drum with a clip bar according to a third embodiment of the invention;

figure 12 shows the clip bar of figure 11 in isolation;

figure 13 shows a detail of the clip bar according to circle XIII in figure 12;

figure 14 shows a view of a tire building drum with a first cover plate and a second cover plate according to a fourth embodiment of the invention;

figure 15 shows the first cover plate of figure 14 in isolation;

figure 16 shows a detail of the first cover plate according to circle XVI in figure 15;

figure 17 shows a view of a tire building drum and a validation tool according to a fifth embodiment of the invention;

figure 18 shows a front view of the validation tool according to figure 17 in isolation;

figure 19 shows an isometric view of a production line for strips or sheets, a measuring system and a calibration tool according to a sixth embodiment of the invention;

figure 20 shows a top view of the calibration tool of figure 19 in isolation in a calibration position;

figure 21 shows a top view of the calibration tool of figure 20 in a validation position;

figure 22 shows a top view of a calibration tool according to a seventh embodiment of the invention; and

figure 23 shows an isometric view of the calibration tool according to figure 22. DETAILED DESCRIPTION OF THE INVENTION

Figures 1, 2 and 3 show a bead-apex drum 7 for producing a bead-apex 8. In this exemplary embodiment, the bead-apex drum 7 is formed as a circular disc 70 having a central hub 71 and a bead-apex support surface 72 extending circumferentially about the central hub 71. The bead-apex drum 7 has a reference plane P, i.e. its mounting plane or its bottom surface, and a base profile B for supporting a bead-apex 8 relative to the reference plane P. The bead- apex drum 7 is typically mounted to a drum seat or drum drive (not shown) and driven in rotation about a rotation axis SI extending concentrically through the central hub 71 in a direction perpendicular to the reference plane P.

A bead-apex 8 is formed by first applying a bead 80 on the bead-apex support surface 72 around the central hub 71 of the bead-apex drum 7, followed by an apex 81 that is applied around the bead 80. The bead-apex support surface 72 may be slightly angled to assume a conical orientation, i.e. at an oblique angle to the reference plane P. Different bead-apex drums may be provided for different bead-apexes, depending on their respective dimensions, i.e. diameter, thickness and conicity.

Figures 1, 2 and 3 further show a measuring system 9 for measuring the bead-apex 8 on the bead-apex drum 7. Said measuring system 9 is preferably a laser- triangulation measuring system, having a laser 90 for projecting a laser line L on the bead-apex 8 and a camera 91 for capturing an image of said projected laser line L. The camera 91 has a field of view FOV as shown in figure 2.

Figures 4-7 show a calibration tool 1 for calibrating the measuring system 9 as shown in figures 1, 2 and 3. The calibration tool 1 is arranged to be placed in the same position as the bead-apex drum 7. In other words, the calibration tool 1 temporarily replaces the bead-apex drum 7 when the measuring system 9 is to be calibrated.

As shown in figure 4, the calibration tool 1 comprises a tool body 10 that is rotatable relative to the measuring system 9 about a rotation axis Si perpendicular to a reference plane P. Preferably, the calibration tool 1 replaces the bead-apex drum such that the rotation axis Si of the calibration tool 1 corresponds to the rotation axis SI of the bead-apex drum 7 prior to its removal. Moreover, the tool body 10 may have similar mounting features, i.e. a mounting plane that extends in the same plane as the mounting plane of the bead-apex drum prior to its removal. More in particular, the reference planes P for measuring height on the bead-apex drum 7 and the calibration tool 1 may be the same. Hence, the calibration tool 1 can be representative of at least some characteristics of the bead-apex drum 7.

The rotation axis SI extends in an axial direction and defines a radial direction R perpendicular to the rotation axis SI and a circumferential direction C about said rotation axis SI.

In this exemplary embodiment, the tool body 10 extends over only a part of a full circumference about the rotation axis SI. In particular, the tool body 10 is formed as a circular segment. The tool body 10 may for example extend over less than one-hundred-and-eighty degrees of the circumference about the rotation axis SI, preferably over less than one-hundred-and-twenty degrees. Alternatively, the tool body may extend over a full circumference, i.e. to form a disc-like tool body similar to the disc-like bead- apex drum. The tool body may also be shaped as an annulus or ring, provided that it can still be rotatably mounted about the rotation axis SI. The tool body 10 may have an integral or Monobloc shape. Alternatively, the tool body 10 may comprise several interconnected parts, elements, segments or sections that form the different features of the tool body 10, as described below.

As best seen in figures 4 and 5, the tool body 10 is provided with a plurality of calibration surfaces 11 that define a pattern G of calibration positions K. In this exemplary embodiment every calibration surface 11 is distinct from the other calibration surfaces 11, i.e. delimited from the other calibration surfaces 11 by a clear boundary. The calibration surfaces 11 may for example be formed by distinct interconnected parts of the tool body 10. Hence, every calibration surface 11 can be measured as an individual surface. Alternatively, the pattern G may be formed by a single, continuous calibration surface (not shown) , in which case the calibration positions K are merely virtual or imaginary, i.e. the calibration positions K are chosen by the measuring system 9 according a predetermined pattern. A continuous surface may hold an infinite number of calibration positions K, only limited by the accuracy of the camera 91.

In figure 5, the pattern G comprises ten columns

A1-A10 extending in the radial direction R away from the rotation axis Si and five rows B1-B5 extending in the circumferential direction C about the rotation axis Si. As such, a radial grid of calibration positions K can be formed. The number of columns A1-A10 and rows B1-B5 may be chosen differently when a higher or lower amount of calibration positions K is required. A minimum of three columns and three rows seems necessary to provide at least some useful amount of feedback to the measuring system 9.

As best seen in the radial cross section of figure 6, for each column A1-A10 the tool body is provided with recesses 12 extending between the calibration surfaces

11 within the respective column A1-A10. Each recess 12 spaces apart two calibration surfaces 11 in the radial direction R. At each transition from the respective calibration surface 11 to an adjacent one of the recesses

12 each calibration surface 11 within the respective column

A1-A10 defines a calibration edge 13. Conveniently, at least one of the calibration positions K may be chosen at one of said calibration edges 13.

As best seen in the radial cross section of figure 6, for each column A1-A10 the calibration positions K within said respective column A1-A10 vary in height relative to the reference plane P in a height direction H perpendicular to said reference plane P and/or parallel to the rotation axis SI. Similarly, as best seen in the circumferential cross section of figure 7, for each row BI BS the calibration positions K within the respective row also vary in height in the height direction H relative to the reference plane P.

In this exemplary embodiment, for each column Al- A10, the calibration surfaces 11 within the respective column A1-A10 extend in a common plane D, as shown in figure 6. The common plane D extends at an oblique angle to the reference plane P. Alternatively, the calibration surfaces 11 may be in different planes, i.e. in stepped and/or parallel planes (not shown) . When using stepped calibration surfaces 11 in the columns A1-A10, the recesses 12 are not necessary to distinguish between the calibration surfaces 11. The oblique angle may be different or the same for every column A1-A10 to reflect different shapes of apexes .

As shown in figure 7, for each row, the calibration surfaces 11 within the respective row are stepped in the height direction H from one of the calibration surfaces 11 to the next one of the calibration surfaces 11 in the circumferential direction C. Because of the steps between the calibration surfaces 11, no recesses are necessary. If the calibration surfaces 11 in the respective row B1-B5 are however arranged in a common plane (not shown) similar to the calibration surfaces 11 in the columns A1-A10, then recesses may be provided between the calibration surfaces 11 in the respective row B1-B5 as well .

The skilled person will appreciate from the above paragraphs that the shape and relative orientation of the calibration surfaces 11 is open to variation and that the scope of the present invention is not necessarily limited to any particular shape, as long as the technical effect of providing a plurality of calibration positions K in a pattern G is obtained. The transition from one calibration surface 11 to another can for example be stepped, abrupt, gradual or smooth.

As best seen in figure 6, for each column A1-A10, all calibration positions K within the respective column A1-A10 have different heights in the height direction H relative to the reference plane P. More in particular, the calibration positions K within the respective column A1-A10 are sequentially or progressively reduced in height relative to the reference plane P in the radial direction R away from the rotation axis SI. Preferably, the sequential reduction in height has a constant decrement or decrease relative to the reference plane P.

As best seen in figure 7, for each row B1-B5 all calibration positions K within the respective row B1-B5 have different heights in the height direction H relative to the reference plane P. More in particular, the calibration positions K within the respective row B1-B5 are sequentially or progressively increased in height relative to the reference plane P in the circumferential direction C. Preferably, the sequential increase in height has a constant increment relative to the reference plane P.

Consequently, as shown in figure 4, each calibration position K within the pattern G has a height in the height direction H relative to the reference plane P that is different from the heights of the other calibration positions K relative to the reference plane P in the same column A1-A10 and the same row B1-B5. In other words, each column A1-A10 of calibration positions K forms a height profile with a different height at each calibration position K, while each calibration position K in the respective column A1-A10 also has a different height compared to the other calibration positions K in the same row B1-B5. Preferably, the decrement in the columns A1-A10 is the same for each column A1-A10 and/or the increment in each row B1-B5 is the same for each row B1-B5. In that case, the height profiles are all equally offset from one column A1-A10 to the next.

The varying heights of the calibration surfaces 11 relative to the reference plane P are predetermined, i.e. measured and verified prior to the calibration, so that the measurements of the measuring system 9 may be compared to the predetermined heights of the calibration surfaces 11 to calibrate the measuring system 9.

A method for calibrating the measurement system 9, in particular the laser-triangulation measurement system, with the use of the aforementioned calibration tool 1 will be elucidated below with reference to figures 1-7.

The method comprises the steps of:

a) providing the calibration tool 1 at least partially within the field of view FOV of the camera 91, as shown in figure 6;

b) projecting a laser line L onto the calibration tool 1 with the laser-triangulation measuring system 9, as shown in figure 5;

c) rotating the calibration tool 1 about the rotation axis SI such that the laser line L is projected on all calibration positions K of a respective one of the columns A1-A10;

d) capturing an image of the laser line L projected on all calibration positions K of the respective column A1-A10 with the camera 91; and

e) repeating the steps c) and d) for another one of the columns A1-A10.

In step a) the calibration tool 1 may be provided with its reference plane P in the same position as the reference plane P of the bead-apex drum 7 during the bead- apex production. Hence, the measuring system 9 does not have to be adjusted to capture images of the calibration tool 1.

By capturing the image of the laser line L in step d) , calibration data can be collected regarding the height profile of the respective column A1-A10. In particular, any transitions, edges or changes in height can be captured and processed by a suitable processor in the measuring system 9. Preferably, step e) involves repeating steps c) and d) for all of the other columns A1-A10. Hence, the maximum amount of calibration data can be collected.

For each column A1-A10, the calibration positions K may be located on the calibration edges 13, as shown in figure 6, so that the measuring system 9 can recognize the transition at the calibration edge 13 as a calibration position K.

When the image is captured in step e) , the measuring system 9 can be calibrated by correlating pixels in each captured image corresponding to the calibration positions K of a respective column A1-A10 to the predetermined heights of said calibration positions K within said respective column A1-A10. In particular, the captured heights of the calibration positions K within the respective column A1-A10 can be used to determine a scale for a pixel to real-world units conversion, i.e. from pixels to micrometers, millimeters or centimeters.

Optionally, the method may further comprises the step of :

f) providing an empty bead-apex drum 7 relative to the laser-triangulation measuring system 9 prior to or after steps a) to e) , as shown in figures 1-3 but without the bead-apex 8;

g) projecting a laser line L onto the empty bead-apex drum 7 with the laser-triangulation measuring system 9;

h) capturing an image of the laser line L projected on the empty bead-apex drum 7; and

i) determining the base profile B of the empty bead-apex drum 7 relative to the reference plane P of the empty bead-apex drum 7.

In step f) the bead-apex drum 7 is provided with its reference plane P in the same position as the reference plane P of the calibration tool. Hence, the measuring system 9 does not have to be adjusted. Moreover, the height of the determined base profile B can be easily compared to the heights of the calibration positions K as they are measured relative to the same reference plane P.

Finally, the method may comprise the steps of: j) providing a bead-apex 8 on the bead-apex drum 7, as shown in figures 1-3;

k) measuring the bead-apex 8 using the measuring system 9, as shown in figures 1-3; and

l) subtracting the base profile B of the empty bead-apex drum 7 as determined in step i) from the measurements .

The result of the subtraction can be representative of the actual height of the bead-apex 8 relative to the bead-apex drum 7 in the height direction H.

Figures 8 and 9 show an alternative laser- triangulation measuring system 109 according to a second embodiment of the invention. The measuring system 109 comprises a laser 190, a camera 191 and a guide roller, pulley or drum 106 that is rotatable about a rotation axis S2 for guiding a strip 108 through the measuring system 109. The camera 191 has an optical axis M and a field of view FOV. The laser 190 is placed at an oblique angle to said optical axis M. The laser 190 is arranged for projecting a laser line L onto the calibration tool 101 in a lateral direction XI parallel to the rotation axis S2 of the drum 106. The drum 106 has a circumferential surface 160 that supports the strip 108 at a zero level relative to the camera 191 of the measuring system 109. The laser 190 is arranged for projecting a laser line L onto the circumferential surface 160 of the drum 106 and across the strip 108 to measure the height profile of said strip 108 relative to the zero level. The measuring system 109 further comprises a support 192 for supporting the laser 190 and the camera 191 relative to the drum 106. The measuring system 109 is also provided with a calibration tool 101 for calibrating said measuring system 109.

As shown by comparing figure 8 and figure 9 the support 192 is pivotable about a pivot axis U between an operational position (figure 8) in which the camera 191 and the laser 190 are directed at the drum 106 to measure the strip 108 on said drum and a calibration position (figure 9) in which the laser 190 and the camera 191 are directed at the calibration tool 101. Preferably, the pivot axis U is parallel to said rotation axis S2. Preferably, the support 192 is arranged to pivot about the pivot axis U between the operational position and the calibration position over at least forty-five degrees, and preferably at least sixty degrees.

As the laser 190 and the camera 191 share the same support 192, they can be pivoted while maintaining the same relative orientation. Moreover, the laser 190 and the camera 191 can be pivoted between the respective positions easily and quickly, i.e. during a short interruption in the production process of the strip 108 or even during the production process. As the calibration position is different from the operational position, the calibration can be performed out-of-line.

As best seen in figures 8 and 9, the calibration tool 101 comprises a tool body 110 that defines a reference plane Q. The reference plane Q is preferably perpendicular to the optical axis M of the camera 191 when the support 192 is in the calibration position of figure 9.

As shown in figure 9, the calibration tool 101 comprises one or more first calibration surfaces 111 at a predetermined height relative to the reference plane Q in a height direction H2 perpendicular to said reference plane Q for calibrating the zero level. Said one or more first calibration surfaces 111 are preferably at the same distance from the camera 191 in the calibration position as the distance between the circumferential surface 160 and the camera 191 in the operational position of figure 8.

As shown in figure 10, the calibration tool 101 comprises one or more second calibration surfaces 112 which in the lateral direction XI vary in height relative to the reference plane Q in the height direction H2. In particular, the height of the one or more second calibration surfaces 112 varies in the lateral direction XI according to a pattern that is repeated in said lateral direction XI at least twice, and preferably at least three times. Hence, along a single laser line L, several heights can be detected corresponding to the different second calibration surfaces 112. Measurement data about the height of the second calibration surfaces 112 can be used to determine a scale of the measuring system 109, in particular a scale for converting and/or correlating between pixels in the image and real-world units, i.e. micrometers, millimeters or centimeters. By repeating the pattern, the camera can be calibrated with respect to more positions in the lateral direction XI, i.e. across a substantial part of the field of view FOV of the camera 191 in the lateral direction XI .

Figure 11 show a tire building drum 206 and a clip bar 201 for clamping a tire component to said drum 206. Typically, a measuring system (not shown) is arranged in the proximity of the drum 206 to measure the shape, size and/or height of the tire components on the drum 206. The drum 206 has a rotation axis S3. The clip bar has a longitudinal direction Y1 and is arranged to be placed on the drum 206 with its longitudinal direction Y1 parallel or substantially parallel to the rotation axis S3 of the drum 206. The clip bar is arranged to be retained to the drum 206 by magnetic, vacuum or mechanical retaining means.

Such clip bars are known per se. However, the clip bar according to the present invention is provided with a verification element 211, as shown in figures 12 and 13, for verifying a measuring system. In particular, the clip bar 201 has a clamping side 202 that faces the tire component during the clamping and non-clamping side 203 opposite to the clamping side 202. The verification element 211 is conveniently provided at the non-clamping side 203 so that it is easily visible from the outside. More in particular, the verification element 211 is arranged at or near one end of the clip bar 201 in the longitudinal direction Y1 thereof so that it is less likely to be covered by a tire component during production.

In this exemplary embodiment, as shown in detail in figure 13, the verification element 211 is a slot. The skilled person will however appreciate that many variations to the verification element 211 fall within the scope of this invention, as long as it can be used to verify the measurements, accuracy, calibration and/or repeatability of the measuring system.

Figure 14 shows a tire building drum 306 that is rotatable about a rotation axis S4 extending in an axial direction W to receive one or more tire components (not shown) . Typically, a measuring system (not shown) is arranged in the proximity of the drum 306 to measure the shape, size and/or height of the tire components on the drum 306. The drum 306 comprises a plurality of segments that are radially expandable and contractible. When the segments are expanded radially, the diameter and/or circumference of the drum 306 are increased and intermediate spaces are formed in the circumferential direction between the segments. The drum 306 is provided with a plurality of cover plates 301, 302 for covering the intermediate spaces between said segments.

The plurality of cover plates 301, 302 comprises one or more first cover plates 301 with one or more verification elements 311 for verifying the measurements, accuracy, calibration and/or repeatability of the measuring system. The plurality of cover plates 301, 302 further comprises one or more second cover plates 302 with a plurality of calibration elements 312 for calibrating the measuring system. The calibration elements 312 are arranged in a regular pattern across the one or more second cover plates 302 in the axial direction of the drum 306. The verification elements 311 of the one or more first cover plates 301 are offset in the axial direction W with respect to the plurality of calibration elements 312 in the one or more second cover plates 302.

Figures 15 and 16 show one of the first cover plates 311 in more detail. The first cover plate 301 has a longitudinal direction Y2 and a first end portion 321, a second end portion 322 and a center portion 323 in said longitudinal direction Y2. In this exemplary embodiment, the measuring system comprises a first camera, a second camera and a third camera arranged side-by-side for observing the first end portion 321, the second end portion 322 and the center portion 323, respectively. The one or more verification elements 311 comprises a first group of two or more verification elements 311 at the first end portion 321, a second group of two or more verification elements 311 at the second end portion 322 and a third group of two or more verification elements 311 at the center portion 323. Preferably, each group comprises three or more verification elements 311. More preferably, the three or more verification elements 311 in each group form a pattern that is the same for each group.

Figure 17 shows a tire building drum 406 for receiving one or more tire components. Typically, a measuring system (not shown) is arranged in proximity to said tire building drum 406 for measuring one or more of said tire components applied around the tire building drum 406. Figure 17 further shows a validation tool 401 for validating measurements of the measuring system. The validation tool 401 comprises an annular body 410 extending circumferentially about a central axis Z and one or more reference elements 420 representative of characteristics of said one or more tire components provided on said annular body 410. The annular body 410 is arranged to be fitted concentrically around or alongside the tire building drum 406. The validation tool 401 may substantially correspond to the validation tool disclosed in WO 2016/122311 Al, incorporated herein by reference.

The position of the validation tool 401 on or along the tire building drum can be slightly inaccurate, in particular in an axial direction W2 along or parallel to the central axis Z. This can be compensated as long as the center of the validation tool 401 is known. For this purpose, the validation tool 401 is provided with a center reference 411 for determining a center of the validation tool 401 in the axial direction W2.

Once the center has been determined, the tire building drum 406, with the validation tool 401 around or alongside it, can be moved in the axial direction W2 until the validation tool 401 is in a position relative to the measuring system where the tire building drum 401 would normally be during operation. Alternatively, the measuring system can offset the measurement slightly to compensate for any misalignment of the center of the validation tool 401 relative to the measuring system.

Optionally, the validation tool 401 is provided with one or more end or side references 412 for determining one or more sides of the validation tool 401 in the axial direction W2. Preferably, the center reference 411 and the one or more side references 412 are arranged in-line in the axial direction W2.

Figure 19 shows a strip production line 507 for producing strips 508, in particular for the tire manufacturing. The strip production line 507 comprises a conveyor 570, in this example a roller conveyor, which is interrupted along a measuring line T at a measuring position to allow a measuring system 509 to measure of characteristics of the strip 508, i.e. a tread, a carcass or a breaker ply, or the folding of a gum strip around the edge of a breaker, as it passes across the interruption. In particular, the width of the strip 508 is measured at the measuring line T. In this example, the measuring system 509 comprises a back-light unit 590 for emitting light towards the measuring line T and a first camera 591 and a second camera 592 opposite to the back-light unit 590 to detect the light passing at the measuring line T along the side edges of the strip 508 in a manner known per se.

To calibrate the measurements of the measuring system 509, a calibration tool 501 is provided. The calibration tool 501 is arranged to be mounted between the back-light unit 590 and the cameras 591, 592 in the measuring position. As shown in more detail in figure 20, the calibration tool 501 comprises a tool body 510 extending in a longitudinal direction Y3.

The tool body 510 comprises a calibration section 502 with one or more calibration elements 521 and a validation section 503 with one or more validation elements 531. In figure 20, the calibration tool 501 is positioned in a calibration position in which the longitudinal direction Y of the tool body 510 extends parallel or substantially parallel to the measuring line T. In the calibration position the measuring line T extends across the one or more calibration elements 521 of the calibration section 502.

The calibration tool 501 is reversible or invertible about an inverting axis VI between the calibration position, as shown in figure 20, and a validation position, as shown in figure 21. In the validation position, the measuring line T extends across the one or more validation elements 531 of the validation section 503. Hence, the calibration section 502 and the validation section 503 are effectively inverted. In other words, the calibration section 502 and the validation section 503 alternate positions at the measuring line T or switch positions when inverting about the inverting axis VI .

Preferably, the calibration section 502 and the validation section 503 are arranged adjacent to each other in a lateral direction X2 perpendicular to the longitudinal direction Y3. In this exemplary embodiment, the inverting axis VI extends perpendicular to the longitudinal direction Y3 and the lateral direction X2 between the calibration section 502 and the validation section 503. More in particular, in this specific embodiment, the inverting axis VI is upright, vertical or substantially vertical. Alternatively, the inverting axis may also extend parallel to the measuring line T between the calibration section 502 and the validation section 503 or parallel to the lateral direction X2 through the center of both sections 502, 503.

As shown in figure 20, the calibration tool 501 comprises one or more mounting elements 505 for mounting the calibration tool 501 to a support relative to the measuring system 509 of figure 19. As shown by comparing figure 20 and figure 21, preferably, at least one of the one or more mounting elements 505 is in the same position after inverting the calibration tool 501 about the inverting axis VI. Hence, the calibration tool 501 can be mounted in substantially the same way in both positions.

As shown in figures 20 and 21, the one or more calibration elements 521 comprises a plurality of calibration elements 521 arranged in a pattern extending in the longitudinal direction Y3 of the calibration tool 501. Similarly, the one or more validation elements 531 comprises a plurality of validation elements 531. However, the validation elements 531 are offset in the longitudinal direction Y3 with respect to the calibration elements 521.

As best seen in figure 19, the first camera 591 and the second camera 592 are arranged for observing a first end portion 511 and a second end portion 512, respectively, of the calibration tool 501. In particular, the camera 591, 592 observe a region of the calibration tool 501 where the side edges of the strip 508 would normally pass across the measuring line T. The one or more validation elements 531 comprises a first group of two or more validation elements 531 at the first end portion 511 and a second group of two or more validation elements 531 at the second end portion 512. Preferably, each group comprises three or more validation elements 531.

In this exemplary embodiment, the one or more calibration elements 521 and/or the one or more validation elements 531 are through-holes. This makes the calibration tool 501 suitable for use in a back-light measuring system. Alternatively, the calibration elements and validation elements may be provided as slits or protrusions, for example when calibrating and validating a laser- triangulation measuring system.

Figures 22 and 23 show an alternative calibration tool 601 according to a seventh embodiment of the invention that differs from the calibration tool 501 according to the sixth embodiment of the invention in that has a calibration section 602 with one or more calibration elements 621 which are stepped or have stepped features 622 in a height direction H3 perpendicular to the longitudinal direction Y3 to allow for a more accurate calibration of the height measurements of the cameras. The validation section 603 again has validation elements 631 that are offset in the longitudinal direction Y3 with respect to the calibration elements 621. Like the previously discussed calibration tool 501 according to the sixth embodiment of the invention, the alternative calibration tool 601 is reversible or invertible about an inverting axis V2 between the calibration position and a validation position. The inverting axis V2 in this case extends in the longitudinal direction Y3 between the calibration section 602 and the validation section 603.

In some of the embodiments described above, the verification of the measurements of the measuring system can be performed in-line, meaning that the tire components can be measured while simultaneously measuring one or more of the verification elements. In such embodiments, the verification element is provided within the field of view of at least one of the cameras of the measuring system. The verification step can then be repeated over time, during regular intervals or even continuously.

It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the scope of the present invention.

LIST OF REFERENCE NUMERALS

1 calibration tool

10 tool body

11 calibration surface

12 recess

13 calibration edge

7 bead-apex drum

70 circular disc

71 central hub

72 bead-apex support surface

8 bead-apex

80 bead

81 apex

9 measuring system

90 laser

91 camera

101 calibration tool

ill first calibration surface 112 second calibration surface 106 drum

160 circumferential surface 108 strip

109 measuring system

190 laser

191 camera

192 support

201 clip bar

211 verification element 202 clamping side

203 non-clamping side

206 drum

301 first cover plate

302 second cover plate

306 tire building drum

311 verification element

312 calibration element 321 first end portion

322 second end portion

323 center portion

401 validation tool

406 tire building drum

410 tool body

411 center reference

412 side reference

420 reference elements

501 calibration tool

502 calibration section 521 calibration element

503 validation section

531 validation element

510 tool body

511 first end portion

512 second end portion

505 mounting elements

507 strip production line 570 conveyor

508 strip

509 measuring system

590 back-light unit

591 first camera

592 second camera

601 calibration tool

602 calibration section 621 calibration element 622 steps

603 validation section

631 validation element

A1-A10 columns

B1-B5 rows

C circumferential direction D common plane

FOV field of view

G pattern or radial grid HI height direction

H2 height direction

H3 height direction

K calibration position L laser line

M optical axis

P reference plane

Q reference plane

R radial direction SI rotation axis

52 rotation axis

53 rotation axis

54 rotation axis

T measuring line

U pivot axis

VI inverting axis

V2 inverting axis

W1 axial direction

W2 axial direction

XI lateral direction X2 lateral direction Y1 longitudinal direction Y2 longitudinal direction Y3 longitudinal direction Z central axis