Patent Description:
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.

XP004062828 concerns a study evaluating the accuracy of different camera calibration and measurement methods used in 3D stereo vision with CCD cameras. In this context, the study discloses a non-coplanar calibration object comprising calibration positions arranged in a pattern of rows and columns, wherein the calibration positions have mutually different heights (<FIG> (right)).

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:.

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:.

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:.

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

According to an third, unclaimed 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.

<FIG>, <FIG> show a bead-apex drum <NUM> for producing a bead-apex <NUM>. In this exemplary embodiment, the bead-apex drum <NUM> is formed as a circular disc <NUM> having a central hub <NUM> and a bead-apex support surface <NUM> extending circumferentially about the central hub <NUM>. The bead-apex drum <NUM> has a reference plane P, i.e. its mounting plane or its bottom surface, and a base profile B for supporting a bead-apex <NUM> relative to the reference plane P. The bead-apex drum <NUM> is typically mounted to a drum seat or drum drive (not shown) and driven in rotation about a rotation axis S1 extending concentrically through the central hub <NUM> in a direction perpendicular to the reference plane P.

A bead-apex <NUM> is formed by first applying a bead <NUM> on the bead-apex support surface <NUM> around the central hub <NUM> of the bead-apex drum <NUM>, followed by an apex <NUM> that is applied around the bead <NUM>. The bead-apex support surface <NUM> 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.

<FIG>, <FIG> further show a measuring system <NUM> for measuring the bead-apex <NUM> on the bead-apex drum <NUM>. Said measuring system <NUM> is preferably a laser-triangulation measuring system, having a laser <NUM> for projecting a laser line L on the bead-apex <NUM> and a camera <NUM> for capturing an image of said projected laser line L. The camera <NUM> has a field of view FOV as shown in <FIG>.

<FIG> show a calibration tool <NUM> for calibrating the measuring system <NUM> as shown in <FIG>, <FIG>. The calibration tool <NUM> is arranged to be placed in the same position as the bead-apex drum <NUM>. In other words, the calibration tool <NUM> temporarily replaces the bead-apex drum <NUM> when the measuring system <NUM> is to be calibrated.

As shown in <FIG>, the calibration tool <NUM> comprises a tool body <NUM> that is rotatable relative to the measuring system <NUM> about a rotation axis S1 perpendicular to a reference plane P. Preferably, the calibration tool <NUM> replaces the bead-apex drum such that the rotation axis S1 of the calibration tool <NUM> corresponds to the rotation axis S1 of the bead-apex drum <NUM> prior to its removal. Moreover, the tool body <NUM> 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 <NUM> and the calibration tool <NUM> may be the same. Hence, the calibration tool <NUM> can be representative of at least some characteristics of the bead-apex drum <NUM>.

The rotation axis S1 extends in an axial direction and defines a radial direction R perpendicular to the rotation axis S1 and a circumferential direction C about said rotation axis S1.

In this exemplary embodiment, the tool body <NUM> extends over only a part of a full circumference about the rotation axis S1. In particular, the tool body <NUM> is formed as a circular segment. The tool body <NUM> may for example extend over less than one-hundred-and-eighty degrees of the circumference about the rotation axis S1, 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 S1. The tool body <NUM> may have an integral or Monobloc shape. Alternatively, the tool body <NUM> may comprise several interconnected parts, elements, segments or sections that form the different features of the tool body <NUM>, as described below.

As best seen in <FIG>, the tool body <NUM> is provided with a plurality of calibration surfaces <NUM> that define a pattern G of calibration positions K. In this exemplary embodiment every calibration surface <NUM> is distinct from the other calibration surfaces <NUM>, i.e. delimited from the other calibration surfaces <NUM> by a clear boundary. The calibration surfaces <NUM> may for example be formed by distinct interconnected parts of the tool body <NUM>. Hence, every calibration surface <NUM> 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 <NUM> according a predetermined pattern. A continuous surface may hold an infinite number of calibration positions K, only limited by the accuracy of the camera <NUM>.

In <FIG>, the pattern G comprises ten columns A1-A10 extending in the radial direction R away from the rotation axis S1 and five rows B1-B5 extending in the circumferential direction C about the rotation axis S1. 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 <NUM>.

As best seen in the radial cross section of <FIG>, for each column A1-A10 the tool body is provided with recesses <NUM> extending between the calibration surfaces <NUM> within the respective column A1-A10. Each recess <NUM> spaces apart two calibration surfaces <NUM> in the radial direction R. At each transition from the respective calibration surface <NUM> to an adjacent one of the recesses <NUM> each calibration surface <NUM> within the respective column A1-A10 defines a calibration edge <NUM>. Conveniently, at least one of the calibration positions K may be chosen at one of said calibration edges <NUM>.

As best seen in the radial cross section of <FIG>, 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 S1. Similarly, as best seen in the circumferential cross section of <FIG>, for each row B1-B5 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 A1-A10, the calibration surfaces <NUM> within the respective column A1-A10 extend in a common plane D, as shown in <FIG>. The common plane D extends at an oblique angle to the reference plane P. Alternatively, the calibration surfaces <NUM> may be in different planes, i.e. in stepped and/or parallel planes (not shown). When using stepped calibration surfaces <NUM> in the columns A1-A10, the recesses <NUM> are not necessary to distinguish between the calibration surfaces <NUM>. The oblique angle may be different or the same for every column A1-A10 to reflect different shapes of apexes.

As shown in <FIG>, for each row, the calibration surfaces <NUM> within the respective row are stepped in the height direction H from one of the calibration surfaces <NUM> to the next one of the calibration surfaces <NUM> in the circumferential direction C. Because of the steps between the calibration surfaces <NUM>, no recesses are necessary. If the calibration surfaces <NUM> in the respective row B1-B5 are however arranged in a common plane (not shown) similar to the calibration surfaces <NUM> in the columns A1-A10, then recesses may be provided between the calibration surfaces <NUM> 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 <NUM> 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 <NUM> to another can for example be stepped, abrupt, gradual or smooth.

As best seen in <FIG>, 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 S1. Preferably, the sequential reduction in height has a constant decrement or decrease relative to the reference plane P.

As best seen in <FIG>, 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 <FIG>, 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 <NUM> 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 <NUM> may be compared to the predetermined heights of the calibration surfaces <NUM> to calibrate the measuring system <NUM>.

A method for calibrating the measurement system <NUM>, in particular the laser-triangulation measurement system, with the use of the aforementioned calibration tool <NUM> will be elucidated below with reference to <FIG>.

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

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 <NUM>. 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 <NUM>, as shown in <FIG>, so that the measuring system <NUM> can recognize the transition at the calibration edge <NUM> as a calibration position K.

When the image is captured in step e), the measuring system <NUM> 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:.

In step f) the bead-apex drum <NUM> is provided with its reference plane P in the same position as the reference plane P of the calibration tool. Hence, the measuring system <NUM> 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:.

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

<FIG> shows a strip production line <NUM> for producing strips <NUM>, in particular for the tire manufacturing. The strip production line <NUM> comprises a conveyor <NUM>, in this example a roller conveyor, which is interrupted along a measuring line T at a measuring position to allow a measuring system <NUM> to measure of characteristics of the strip <NUM>, 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 <NUM> is measured at the measuring line T. In this example, the measuring system <NUM> comprises a back-light unit <NUM> for emitting light towards the measuring line T and a first camera <NUM> and a second camera <NUM> opposite to the back-light unit <NUM> to detect the light passing at the measuring line T along the side edges of the strip <NUM> in a manner known per se.

To calibrate the measurements of the measuring system <NUM>, a calibration tool <NUM> is provided. The calibration tool <NUM> is arranged to be mounted between the back-light unit <NUM> and the cameras <NUM>, <NUM> in the measuring position. As shown in more detail in <FIG>, the calibration tool <NUM> comprises a tool body <NUM> extending in a longitudinal direction Y3.

The tool body <NUM> comprises a calibration section <NUM> with one or more calibration elements <NUM> and a validation section <NUM> with one or more validation elements <NUM>. In <FIG>, the calibration tool <NUM> is positioned in a calibration position in which the longitudinal direction Y of the tool body <NUM> 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 <NUM> of the calibration section <NUM>.

The calibration tool <NUM> is reversible or invertible about an inverting axis V1 between the calibration position, as shown in <FIG>, and a validation position, as shown in <FIG>. In the validation position, the measuring line T extends across the one or more validation elements <NUM> of the validation section <NUM>. Hence, the calibration section <NUM> and the validation section <NUM> are effectively inverted. In other words, the calibration section <NUM> and the validation section <NUM> alternate positions at the measuring line T or switch positions when inverting about the inverting axis V1.

Preferably, the calibration section <NUM> and the validation section <NUM> are arranged adjacent to each other in a lateral direction X2 perpendicular to the longitudinal direction Y3. In this exemplary embodiment, the inverting axis V1 extends perpendicular to the longitudinal direction Y3 and the lateral direction X2 between the calibration section <NUM> and the validation section <NUM>. More in particular, in this specific embodiment, the inverting axis V1 is upright, vertical or substantially vertical. Alternatively, the inverting axis may also extend parallel to the measuring line T between the calibration section <NUM> and the validation section <NUM> or parallel to the lateral direction X2 through the center of both sections <NUM>, <NUM>.

As shown in <FIG>, the calibration tool <NUM> comprises one or more mounting elements <NUM> for mounting the calibration tool <NUM> to a support relative to the measuring system <NUM> of <FIG>. As shown by comparing <FIG>, preferably, at least one of the one or more mounting elements <NUM> is in the same position after inverting the calibration tool <NUM> about the inverting axis V1. Hence, the calibration tool <NUM> can be mounted in substantially the same way in both positions.

As shown in <FIG>, the one or more calibration elements <NUM> comprises a plurality of calibration elements <NUM> arranged in a pattern extending in the longitudinal direction Y3 of the calibration tool <NUM>. Similarly, the one or more validation elements <NUM> comprises a plurality of validation elements <NUM>. However, the validation elements <NUM> are offset in the longitudinal direction Y3 with respect to the calibration elements <NUM>.

As best seen in <FIG>, the first camera <NUM> and the second camera <NUM> are arranged for observing a first end portion <NUM> and a second end portion <NUM>, respectively, of the calibration tool <NUM>. In particular, the camera <NUM>, <NUM> observe a region of the calibration tool <NUM> where the side edges of the strip <NUM> would normally pass across the measuring line T. The one or more validation elements <NUM> comprises a first group of two or more validation elements <NUM> at the first end portion <NUM> and a second group of two or more validation elements <NUM> at the second end portion <NUM>. Preferably, each group comprises three or more validation elements <NUM>.

In this exemplary embodiment, the one or more calibration elements <NUM> and/or the one or more validation elements <NUM> are through-holes. This makes the calibration tool <NUM> 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.

<FIG> and <FIG> show an alternative calibration tool <NUM> according to a third embodiment of the invention that differs from the calibration tool <NUM> according to the second embodiment of the invention in that has a calibration section <NUM> with one or more calibration elements <NUM> which are stepped or have stepped features <NUM> 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 <NUM> again has validation elements <NUM> that are offset in the longitudinal direction Y3 with respect to the calibration elements <NUM>. Like the previously discussed calibration tool <NUM> according to the second embodiment of the invention, the alternative calibration tool <NUM> 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 <NUM> and the validation section <NUM>.

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.

Claim 1:
Calibration tool (<NUM>) for calibrating a laser-triangulation measuring system, wherein the calibration tool comprises a tool body (<NUM>) that is rotatable relative tc the measuring system about a rotation axis (S1) perpendicular tc a reference plane (P), wherein the tool body is provided with one or more calibration surfaces (<NUM>) that define a pattern (G), in particular a radial grid, of calibration positions (K), wherein the pattern comprises at least three columns (A1-A10) extending in a radial direction (R) away from the rotation axis and at least three rows (B1-B5) extending in a circumferential direction (C) 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 (H) 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.