System and method for measuring the thickness of a sheet-like material

Abstract

System (1) and method for measuring the thickness of a sheet-like material (2). The following are provided: a measuring station (3); a conveyor device (4) configured to feed the sheet-like material (2) through the measuring station (3); at least one measuring device (6) arranged in the measuring station (3) and equipped with a lower probe (11) and an upper probe (12) between which the sheet-like material (2) advances; a control unit (13); and a calibration master (14) that is configured to calibrate the measuring device (6) and that has two calibration elements (16, 17) that are arranged facing each other at a certain distance from each other so that the sheet-like material (2) can pass between the two calibration elements (16, 17).

Description

[0001] DESCRIPTION

[0002] “SYSTEM AND METHOD FOR MEASURING THE THICKNESS OF A SHEET-LIKE MATERIAL”

[0003] TECHNICAL SECTOR

[0004] The present invention relates to a system and a method for measuring the thickness of a sheet-like material.

[0005] The present invention is advantageously used to measure the thickness of a sheet-like material, for example metal, which is used to produce the electrodes of an electric battery. The description below makes explicit reference to this, while remaining generally applicable.

[0006] PRIOR ART

[0007] The thickness of a metal sheet-like material (typically copper for the anode and aluminium for the cathode) used to produce electric batteries must be equal to the nominal value (with limited tolerances) and uniform along the entire length of the material. T o ensure that a sheet-like material meets specifications, the sheet-like material is measured by a thickness measuring system which generally comprises a conveyor configured to advance the sheet-like material through a measuring station containing several measuring devices arranged at a certain distance from each other to measure the thickness of the sheet-like material in different zones simultaneously. Each measuring device comprises a lower probe and an upper probe that are aligned with each other in a measuring direction perpendicular to the surface of the sheet-like material and between which the sheet-like material is passed. In this way, the lower probe measures the distance between the lower probe and a lower wall of the sheet-like material and the upper probe measures the distance between the upper probe and an upper wall of the sheet-like material. The thickness of the sheetlike material can be determined simply using these measurements and the distance (known with a high degree of precision) between the lower probe and the upper probe.

[0008] To calibrate each measuring device and thereby reduce measurement errors, it is known to place a calibration master of known thickness (with a very high degree of precision) in the vicinity of the measuring device. When the measuring device needs to be calibrated, the measuring device takes a measurement of the calibration master (either by moving the calibration master to the measuring device, which remains stationary, or by moving the measuring device to the calibration master, which remains stationary).

[0009] For example, patent application No. DE10060144A1 discloses a system for measuring the thickness of a web- or plate- shaped object by means of a measuring device placed at a certain distance from the object to be measured. The system further comprises a calibration device to recalibrate periodically the distance between the measuring device and the object to be measured. The measuring device and the calibration distance are mounted on the same supporting frame. When the measuring device needs to be calibrated, the object to be measured has to be removed.

[0010] DESCRIPTION OF THE INVENTION

[0011] The purpose of this invention is to provide a system and a method for measuring the thickness of a sheet-like material that allows the measuring device to be calibrated with a high degree of precision and without requiring the removal of the sheet-like material during calibration (since removing the sheet-like material is a rather lengthy and complex task and should therefore be avoided if possible).

[0012] The present invention provides a system and a method for measuring the thickness of a sheet-like material, according to the attached claims.

[0013] The claims describe embodiments of the present invention and are an integral part of the present description.

[0014] SHORT DESCRIPTION OF THE DRAWINGS

[0015] The present invention is described below with reference to the attached drawings, which show a non-limiting example embodiment of the invention, in which:

[0016] • Figures 1 and 2 are two different perspective views of a system for measuring the thickness of a sheet-like material, with parts removed for clarity;

[0017] • Figure 3 is a perspective view of a measuring station of the measuring system in Figures 1 and 2;

[0018] • Figure 4 is a perspective view of a measuring device and a corresponding calibration master of the measuring station in Figure 3;

[0019] • Figure 5 is a perspective magnified view of the calibration master in Figure 4; and

[0020] • Figure 6 is a schematic view of a measuring device paired with a corresponding calibration master.

[0021] PREFERRED EMBODIMENTS OF THE INVENTION

[0022] In Figures 1 and 2, reference number 1 denotes, as a whole, a system for measuring the thickness of a sheet-like material 2 (schematically shown in Figure 2) which is used for the production of electrodes in an electric battery. The sheet-like material 2 can be entirely or partially metal.

[0023] The measuring system 1 comprises a measuring station 3 in which the thickness of the sheet-like material 2 is measured and a conveyor device 4 (only partially illustrated) configured to feed sheet-like material 2 through the measuring station 3 in an advancement direction D1. According to a preferred embodiment and as shown in the figures, the advancement direction D1 is horizontal, i.e. parallel to the ground. Among other things, the conveyor device 4 comprises a pair of idle rollers 5, i.e. which are mounted to rotate freely around their respective rotation axes perpendicular to the advancement direction D1. According to the preferred embodiment and as shown in the figures, the rotation axes of the rollers 5 are horizontal, i.e. parallel to the ground. As it passes through the measuring station 3, the sheet-like material 2 moves between and in contact with the two rollers 5, i.e. bearing against them.

[0024] Alternatively, the conveyor device 4 may include a pair of pneumatic rollers 5. In this case, the rollers 5 are fixed (do not rotate) and the sheet-like material 2 moves between them without contact.

[0025] The measuring system 1 comprises three measuring devices 6, which are usually but not necessarily twinned with each other (i.e. entirely identical), each of which is configured to measure the thickness of the sheet-like material 2 in a respective zone. The three measuring devices 6 are arranged in different positions to simultaneously measure the thickness of the sheet-like material 2 in different zones of the sheet-like material 2. According to other embodiments (not illustrated), a different number of measuring devices 6 is provided. A single measuring device 6 can be provided, or two, three, four, five six, seven ... measuring devices 6 (the number of measuring devices 6 depends on the width of the sheet-like material 2 and the number of zones where the thickness of the sheet-like material 2 is to be checked).

[0026] According to a preferred embodiment, the measuring devices 6 are arranged to measure the thickness of the sheet-like material 2 between the two rollers 5, i.e. in a space delimited upstream and downstream by the two rollers 5 in the advancement direction D1.

[0027] The measuring system 1 comprises a base 7 provided with a set of ground support feet and possibly also wheels and a support body 8 with a central opening (best illustrated in Figure 3 and in jargon called “O-mount” due to the similarity to the letter “O”) on which the measuring devices 6 are mounted. The position of the measuring devices 6 on the support body 8 is adjustable according to the measurement needs and / or the dimensions of the sheet-like material 2. Once assembled, the measuring devices 6 are rigidly connected to the support body 8. The sheet-like material 2 is passed through the central opening in the support body 8. In particular, a beam 9, which is arranged horizontally and bears the support body 8, is rigidly fastened to the base 7. According to a preferred embodiment, the support body 8 is mounted on the beam 9 to move in a translation direction D2 perpendicular to the advancement direction D1. According to the preferred embodiment and as shown in the figures, the translation direction D2 is horizontal, i.e. parallel to the ground. The measuring system 1 comprises an actuating device 10 (for example an electric motor, which is shown schematically in Figures 1 and 2) configured to move the support body 8 in the translation direction D2.

[0028] As more clearly shown in Figure 4, each measuring device 6 is equipped with a lower probe 11 and an upper probe 12 that are aligned (coaxial) with each other and perpendicular to the respective upper and lower surfaces of the sheet-like material 2 in an alignment direction D3. According to the preferred embodiment and as shown in the figures, the alignment direction D3 is vertical and is perpendicular to the advancement direction D1 of the sheet-like material 2 and to the translation direction D2. The conveyor device 4 feeds the sheet-like material 2 between the two probes 11 and 12, i.e. in the space between the two probes 11 and 12. The lower probe 11 thus sees (captures) a lower surface of the sheet-like material 2 and the upper probe 12 thus sees (captures) an upper surface of the sheet-like material 2. The lower probe 11 is configured to detect (measure) a lower distance (present) between the lower probe 11 and the lower surface of the sheet-like material 2. Similarly, the upper probe 12 is configured to detect (measure) an upper distance (present) between the upper probe 11 and the upper surface of the sheet-like material 2. Both probes 11 and 12 emit a signal representative of the measured distances. According to a preferred embodiment, the probes 11 and 12 are optical probes and are in particular confocal chromatic probes.

[0029] The measuring system 1 includes a control unit 13 (shown schematically in Figure 1) configured to determine (calculate) the thickness of the sheet-like material 2 as a function of the lower distance and the upper distance detected by the probes. More specifically, the control unit 13 is configured to determine (calculate) the thickness of the sheet-like material 2 as a function of the signals from the two probes 11 and 12 and an offset determined during calibration. More precisely, the thickness of the sheet-like material 2 is calculated by adding the sum of the signals from the probes 11 and 12 to the offset calculated during calibration.

[0030] As shown in Figure 3, the measuring system 1 comprises, for each measuring device 6, a corresponding calibration master 14 configured to calibrate the measuring device 6 to enable thickness measurements. The calibration masters 14 are supported by the base 7 in a fixed position (in particular they are fastened to the beam 9) and are arranged near the corresponding measuring devices 6 so that the translational movement of the measuring devices 6 (due to the translational movement of the support body 8) in the translation direction D2 can pair each measuring device 6 with the corresponding calibration master 14.

[0031] As shown in the figures, the calibration master 14 comprises a bearing frame 19, which is preferably C-shaped and provided with two arms substantially parallel to each other. The overall dimensions of the bearing frame 19 are known a priori (i.e. in advance) with a high degree of precision as they are measured beforehand in the metrology room.

[0032] To the arms of the bearing frame 19 are fastened at least one opposing pair of calibration elements 16 and 17, which are flat, parallel to each other, and spaced apart. More specifically, each calibration master 14 comprises at least one lower calibration element 16 and at least one upper calibration element 17 parallel to the lower calibration element 16 and spaced apart from the lower calibration element 16 to form an empty space between the two calibration elements 16 and 17. The distance between the calibration elements 16 and 17 of each calibration master 14 is sufficient to allow the sheet-like material 2 to pass (safely) between the two calibration elements 16 and 17, thereby enabling the calibration masters 14 to remain in the measuring station 3 even when the sheet-like material 2 is present. The sheet-like material is present and continues to pass between the calibration elements 16 and 17 of each calibration master 14 even during calibration.

[0033] As the calibration elements 16 and 17 of each calibration master 14 are fastened to the arms of the bearing frame 19, the position, and thus the distance, at which each pair of calibration elements 16, 17 are arranged facing each other is known in advance. There is no relative movement between the calibration elements 16 and 17 of each calibration master 14.

[0034] It should be noted that the bearing frame 19 to which the calibration elements 16 and 17 are fastened is distinct from the support body 8 on which the measuring devices 6 are mounted. In other words, the calibration elements 16 and 17 and the measuring devices 6 are supported from two distinct supporting elements, that is the bearing frame 19 for the calibration elements 16 and 17 and the support body 8 for the measuring devices 6.

[0035] According to the preferred embodiment and as shown most clearly in Figure 5, each pair of calibration elements 16 and 17 is fastened to a frame 15 which is in turn fastened to the bearing frame 19.

[0036] The dimensions of the bearing frame 19 of the calibration master 14 and in particular the length of the arms of the bearing frame 19 are dependent on the position and the number of the corresponding measuring devices 6 to be paired with the calibration master 14. The bearing frame 19 is designed accordingly.

[0037] A calibration master 14 may also have a structure enabling pairing with a plurality of measuring devices 6 (i.e. more than one measuring device 6) and may have a plurality of pairs of calibration elements 16 and 17 spaced apart from each other.

[0038] For example, according to a preferred embodiment shown in the figures, in particular in Figure 3, the measuring system 1 comprises three measuring devices 6 and two calibration masters 14, the bearing frames 19 of which have different dimensions.

[0039] A first calibration master 14, shown on the left in Figure 3, comprises a bearing frame 19 provided with a lower arm and an upper arm, which are long and to which two pairs of calibration elements 16 and 17 are fastened in distinct positions spaced apart from each other. The position of the pairs of calibration elements 16 and 17 along the arms of the bearing frame 19 depends on the position of the measuring devices 6 to which this calibration master 14 is to be paired. For example, in the embodiment shown in Figure 3, the distance between the two pairs of calibration elements 16 and 17 corresponds substantially to the distance between the measuring device 6 positioned centrally in relation to the support body 8 and the measuring device 6 on the left. The first calibration master 14 allows a single bearing frame 19 to be used to calibrate the two measuring devices 6. A second calibration master 14, shown on the right in Figure 3, comprises a lower arm and an upper arm that are shorter than the arms of the first calibration master 14, and comprises a single pair of calibration elements 16 and 17. The second calibration master 14 allows only one measuring device 6 to be calibrated, in particular the measuring device 6 shown on the right in Figure 3.

[0040] According to an alternative embodiment not shown in the figures, a single calibration master 14 may be provided, the dimensions of which, in particular the length of the arms, are such as to comprise as many pairs of calibration elements 16 and 17 as there are measuring devices 6 mounted on the support body 8.

[0041] As explained above, the distance at which the two calibration elements 16, 17 are arranged facing each other is known in advance. More specifically, as shown in Figure 6 the external distance M1 and / or the internal distance M2 between the two calibration elements 16 and 17 are known a priori with a high degree of precision as they are measured beforehand in the metrology room. In particular, the external distance M1 is measured between the outer surfaces of the calibration elements 16 and 17 (i.e. between the surfaces of the calibration elements 16 and 17 which do not face each other, i.e. are arranged opposite each other and face the respective probe 11 or 12), while the internal distance M2 is measured between the inner surfaces of the calibration elements 16 and 17 (i.e. between the surfaces of the calibration elements 16 and 17 that face each other directly).

[0042] If the calibration elements 16 and 17 are made of transparent material, the thickness of each calibration element 16 and 17 is also measured beforehand in the metrology room and therefore known a priori with a high degree of precision.

[0043] According to a preferred embodiment referred to in Figure 6, the calibration elements 16 and 17 are made of (at least partially) transparent material. More specifically, they (at least partly) comprise transparent optical glass. Optical glass is a very good surface for the calibration of chromatic confocal probes as it minimizes the chance of probe saturation and reduces the effect of surface roughness, for example in a ground metal surface. According to an alternative embodiment, the calibration elements 16 and 17 are metal plates (for example steel) which preferably have a mirror polish to minimize surface roughness.

[0044] It is important to note that the calibration master 14 can also be used to perform the “dark" procedure on the probes 11 and 12, during which the probes 11 and 12 are brought to respective black plastic blocks 18 mounted on the frame 15 near the calibration elements 16 and 17. Under these conditions, the amount of light returned to the optical fibre under “dark" conditions is determined.

[0045] According to a preferred embodiment, the calibration masters 14 can be removed from the base 7 for periodic verification in the metrology room and the fastening thereof to the base 7 (in particular to the beam 9) enables the adjustments required to align them in the translation direction D2 of the support body. The calibration masters 14 are relatively large, substantial structures with a relatively high operating radius compared to the mounting surfaces on the base 7. This makes it necessary to certify the calibration masters 14 in the metrology room using a procedure that takes into account the deformation of the calibration masters 14 stressed by both the weight force and the pressure from the touch probe of the measuring machine used in the metrology room. In practice, the calibration masters 14 must be mounted on the measuring machine used in the metrology room so that the force of gravity acts thereon in the same direction and the same sense as in real usage conditions, i.e. when the calibration masters 14 are mounted on the measuring system 1.

[0046] During normal operation of the measuring system 1 , the sheet-like material 2 is advanced by the conveyor device 4 in the advancement direction D1 and passes through the measuring station 3 guided and supported by the two rollers 5 while the measuring devices 6 detect the thickness of the sheet-like material 2. The actuating device 10 can move the support body 8 alternately right and left (i.e. swing it in the translation direction D2) so as to cyclically vary the reading point of the thickness of the sheet-like material 2 of each measuring device 6.

[0047] During normal operation of the measuring system 1 , the measuring devices 6 can be calibrated at the same time or at different times. For example, it is possible to calibrate just one measuring device 6 at a time (while the other measuring devices 6 measure the thickness of the sheet-like material 2), to calibrate some of the measuring devices 6, for example two, at the same time (while the remaining device 6 measures the thickness of the sheet-like material 2), or to calibrate all the measuring devices 6 at the same time. This depends on the positioning of the calibration masters 14 with respect to the respective measuring devices 6 and / or the structure of the calibration masters 14, in particular the bearing frame 19 and the position of the pairs of calibration elements 16 and 17 fastened thereto. The above-mentioned calibration is carried out without interrupting the feed of the sheet-like material 2 and removing it from the measuring station 3. During calibration the sheet-like material 2 continues to advance through the space between the two calibration elements 16 and 17 of each calibration master 14.

[0048] According to a different embodiment (not illustrated), the calibration masters 14 are mounted movably on the base 7 to move in the translation direction D2 and / or in the advancement direction D1 so that it is the calibration masters 14 that are moved to pair with the corresponding measuring devices 6 and not vice versa. In this embodiment, i.e. when the calibration masters 14 are mounted movably on the base 7, the support body 8 bearing the measuring devices 6 can also be mounted fixed on the base 7. In other words, the pairing between the measuring devices 6 and the corresponding calibration masters 14 can be achieved by moving the calibration masters 14 to the measuring devices 6, which remain stationary, or by moving the measuring devices 6 to the calibration masters 14, which remain stationary (as in the embodiment shown in the attached figures).

[0049] According to a different embodiment (not illustrated), each measuring device 6 is always paired with the corresponding calibration master 14. For example, the calibration master 14 can be mounted on the support body 8 together with the measuring device 6. Alternatively, the calibration master 14 may be mounted on the beam 9 in a fixed position and comprise a bearing frame 19 and calibration elements 16 and 17 long enough to ensure that the calibration elements 16 and 17, made of transparent material, are always interposed between the probes 11 and 12 and the sheet-like material 2 during the translational movement of the measuring devices 6 (under the effect of the translational movement of the support body 8). In both embodiments, the probes 11 and 12 of each measuring device 6 always see (capture) the sheet-like material 2 through the calibration elements 16 and 17, which are transparent. In addition, in these embodiments, each measuring device 6 can be calibrated continuously, or with each thickness measurement, or periodically, or after a predetermined number of measurements.

[0050] To calibrate a measuring device 6, the measuring device 6 is paired with the corresponding calibration master 14 (as shown in Figure 6), i.e. it is arranged so that the probes

[0051] 11 and 12 of the measuring device 6 see (capture) the calibration elements 16 and 17 of the calibration master 14.

[0052] The measuring device 6 can be calibrated in different ways depending on the type of material (transparent or not) used to make the calibration elements 16 and 17 of the calibration master 14 and / or how the measurement is taken (i.e. if the measurement is taken in air and each probe 11 and 12 is directly facing the sheet-like material 2, or if the measurement is taken through the transparent material used to make the calibration elements 16 and 17 of the calibration master 14).

[0053] The different cases are described below with reference to Figure 6, in which:

[0054] - T 1 and T2 correspond respectively to the distances between the probes 12 and 11 and the outer surfaces of the calibration elements 17 and 16;

[0055] - T3 and T4 correspond respectively to the distances between the probes 12 and 11 and the inner surfaces of the calibration elements 17 and 16.

[0056] If the thickness is measured in air, i.e. the measurement is taken when each probe 11 and

[0057] 12 is directly facing the sheet-like material 2, the measuring device 6 can be calibrated using the distances T1 and T2, regardless of the material used to make the calibration elements 16 and 17 of the master 14, without requiring any additional actions. Alternatively, if the calibration elements 16 and 17 of the master 14 are made of (partially) transparent material, calibration can be performed using the distances T3 and T4. In this case, a correction factor has to be applied to the thickness measurement of the sheet-like material 2, taking into account the distortion of the measurement field of the probes caused by the light passing through the transparent material. It is also necessary to periodically verify the correctness and accuracy of the calibration and the correction factor by repeating the calibration of a sample object of known dimensions certified in the metrology room. However, this operation requires the feed of sheet-like material 2 to be interrupted and temporarily removed, thus partially reducing the advantages related to the calibration according to the invention.

[0058] Similarly, if the calibration elements 16 and 17 of the calibration master 14 are made of (partially) transparent material and are placed between the probes 11 and 12 and the sheet-like material 2 during the measurement so that the measurement is taken through that transparent material, the measuring device 6 can be calibrated using the distances T3 and T4 without requiring any additional actions. Alternatively, calibration can be performed using the distances T 1 and T2. In this case, a correction factor has to be applied to the thickness measurement of the sheet-like material 2, taking into account the distortion of the measurement field of the probes caused by the light passing through the transparent material. It is also necessary to periodically verify the correctness and accuracy of the calibration and the correction factor by repeating the calibration of a sample object of known dimensions certified in the metrology room. However, this operation requires the feed of sheet-like material 2 to be interrupted and temporarily removed, thus partially reducing the advantages related to the calibration according to the invention.

[0059] It is important to note that the distances T3 and T4 to the inner surfaces of the calibration elements 16 and 17 can only be used when the calibration elements 16 and 17 are (at least partially) transparent and therefore allow light to pass through. The correction factor mentioned above takes into account the refractive index of the calibration elements 16 and 17 which can be determined by experimentation using the measurement of the thickness of the calibration elements 16 and 17 taken in the metrology room. If the calibration elements 16 and 17 are not (at least partially) transparent, it is obvious that the distances T3 and T4 cannot be used.

[0060] According to other embodiments (not illustrated), the measuring system 1 described above is used to measure the thickness of a sheet-like material, including a non-metal material, used in applications other than electric batteries.

[0061] The embodiments described here may be combined with each other.

[0062] The measuring system 1 described above has several advantages.

[0063] First of all, the measuring system 1 described above allows calibration of the measuring devices 6 without requiring removal of the sheet-like material 2, which can continue to advance through the space between the two calibration elements 16 and 17 of each calibration master 14. In fact, in this type of process, removing the sheet-like material 2 from the measuring system 1 is very expensive and it is therefore very advantageous to have a calibration system for the measuring devices 6 that allows the calibration procedure to be carried out regularly even in the presence of the sheet-like material 2 without having to stop the production line in operation.

[0064] The measuring system 1 according to the invention is particularly advantageous if the support body 8 of the measuring system 1 has a central opening (“O-mount”) as described above and illustrated in the figures. Indeed, in this case, the measuring devices 6 can be calibrated not only without removing the sheet-like material 2 but also minimizing the footprint of the measuring system 1 in the direction orthogonal to the advancement direction D1.

[0065] It is known to use support bodies with a C-shaped structure, i.e. with an open side, which allow the measuring devices to be calibrated without removing the sheet-like material by moving the support body in a direction orthogonal to the direction of movement of the sheet-like material and moving the measuring devices away from the footprint of the rollers to be paired with calibration devices outside the measuring system. This significantly increases the footprint of the measuring system. In addition, the C-shaped structure of the support body is known to be less rigid than the “O-mount”, which negatively affects the measurement.

[0066] In summary, the measuring system 1 according to the invention allows use of the support body 8 structure, which better guarantees high performance in terms of measurement (i.e. the “O-mount”) while simultaneously calibrating the measuring devices in the presence of the sheetlike material and without increasing the footprint beyond the measuring system itself.

[0067] Despite the limitations in terms of rigidity detailed above, the measuring system 1 according to the invention may alternatively comprise a C-shaped support body 8, while retaining the advantages mentioned.

[0068] In addition, the measuring system 1 described above enables particularly precise and accurate calibration of the measuring devices 6.

[0069] Finally, the measuring system 1 is simple and relatively inexpensive.

Claims

CLAIMS1. Measuring system (1) for measuring the thickness of a sheet-like material (2) and comprising: a measuring station (3); a conveyor device (4) configured for feeding the sheet-like material (2) through the measuring station (3); at least one measuring device (6) that is arranged in the measuring station (3) and is equipped with a lower probe (11) and an upper probe (12) between which the sheet-like material (2) advances; a control unit (13); and at least one calibration master (14) configured for carrying out the calibration of said at least one measuring device (6); wherein the lower probe (11) is configured for detecting a lower distance existing between the lower probe (11) and a lower surface of the sheet-like material (2); wherein the upper probe (12) is configured for detecting an upper distance existing between the upper probe (12) and an upper surface of the sheet-like material (2); wherein the control unit (13) is configured for determining the thickness of the sheet-like material (2) as a function of the lower distance and of the upper distance; the measuring system (1) is characterized in that the calibration master (14) comprises two calibration elements (16, 17) that are arranged facing each other at a certain distance from each other so that the sheet-like material (2) can advance between the two calibration elements (16, 17) during the calibration of said at least one measuring device (6).

2. Measuring system (1) according to claim 1 , wherein, in order to carry out the calibration, the measuring device (6) is paired with the corresponding calibration master (14) so arranged that the probes (11 , 12) of the measuring device (6) frame the calibration elements (16, 17).

3. Measuring system (1) according to claim 1 or claim 2, wherein an external distance (M1) measured between external surfaces of the calibration elements (16, 17) laying oppositely to each other is known in advance, and / or an internal distance (M2) measured between internal surfaces of the calibration elements (16, 17) laying facing each other are / is known in advance.

4. Measuring system (1) according to any one of claims 1 to 3, wherein said at least one calibration master (14) includes a C-shaped bearing frame (19) having two arms that extend substantially parallel to each other and to which the two calibration elements (16, 17) are fixed.

5. Measuring system (1) according to claim 4, wherein the overall dimensions of the C- shaped bearing frame (19) are known in advance.

6. Measuring system (1) according to claim 4 or claim 5, comprising a support body (8) on which said at least one measuring device (6) is mounted, the C-shaped bearing frame (19) to which the two calibration elements (16, 17) are fixed being distinct from said support body (8).

7. Measuring system (1) according to any one of the preceding claims , wherein, in order to carry out the calibration, the control unit (13) is configured for employing the probes (11 , 12) for measuring distances (T1 , T2) with respect to external surfaces of the calibration elements (16, 17) laying oppositely to each other.

8. Measuring system (1) according to any one of the preceding claims, wherein the calibration elements (16, 17) are at least partially transparent optical glasses.

9. Measuring system (1) according to claim 8, wherein, in order to carry out the calibration, the control unit (13) is configured for employing the probes (11 , 12) for measuring distances (T3, T4) with respect to internal surfaces of the calibration elements (16, 17) laying facing each other.

10. Measuring system (1) according to claim 8 or claim 9, wherein the measuring device (6) is permanently coupled to the corresponding calibration master (14) so that the probes (11 , 12) frame the sheet-like material (2) only ever through the calibration elements (16, 17).

11. Measuring system (1) according to any one of the preceding claims, wherein: the measuring device (6) is movably mounted so as to translate along a translation direction (D2) that is perpendicular to a feeding direction (D1) of the sheet-like material (2); and an actuator device (1) is provided and is configured for moving the measuring device (6) along the translation direction (D2).

12. Measuring system (1) according to any one of the preceding claims and comprising: a base (7); and a support body (8) that supports the measuring device (6) and features a central opening through which the sheet-like material (2) advances.

13. Measuring system (1) according to any one of the preceding claims, wherein the conveyor device (4) includes a pair of rollers (5) on which the sheet-like material (2) runs while passing through the measuring station (3).

14. Measuring system (1) according to claim 13, wherein the measuring device (6) is arranged so as to perform the thickness measurement of the sheet-like material (2) between the rollers (5).

15. Measuring system (1) according to any one of the preceding claims, wherein the calibration master (14) includes a plurality of pairs of calibration elements (16, 17) spaced apart from each other.

16. Measuring method for measuring the thickness of a sheet-like material (2) and including the steps of: feeding the sheet-like material (2) through a measuring station (3) in which a measuring device (6) is arranged and is equipped with a lower probe (11) and an upper probe (12) between which the sheet-like material (2) advances; detecting a lower distance between the lower probe (11) and a lower surface of the sheetlike material (2); detecting an upper distance between the upper probe (11) and an upper surface of the sheet-like material (2); determining the thickness of the sheet-like material (2) as a function of the lower distance and of the upper distance; and carrying out the calibration of the measuring device (6) by means of a calibration master (14); the measuring method is characterized in that the calibration master (14) includes two calibration elements (16, 17) that are arranged facing each other at a certain distance from each other so that the sheet-like material (2) can advance between the two calibration elements (16, 17) during the calibration of the measuring device (6).

17. Measuring method according to claim 16, wherein the distance at which the two calibration elements (16, 17) are arranged facing each other is known in advance.

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