EP2791617A1

EP2791617A1 - Material thickness measuring device

Info

Publication number: EP2791617A1
Application number: EP12858673.2A
Authority: EP
Inventors: Bengt ÅKERBLOM
Original assignee: Daprox AB
Current assignee: Daprox AB
Priority date: 2011-12-14
Filing date: 2012-12-13
Publication date: 2014-10-22
Also published as: WO2013089627A1; EP2791617A4

Abstract

The present invention regards a material thickness measuring device(1)for measuring the thickness (t) of non-magnetic web (3) applied to a magnetic material (5). The device (1) comprises an optical sensor device (7, 9, 45) for a first interaction with an upper side (11) of the web (3) as an optical reference element (29). It also comprises a reluctance transducer sensor device (31) for a second interaction with the magnetic material (5) as a magnetic reference element on the opposite side of the web (3). A control unit (39) is adapted for each point of measuring (P, PC) to calculate the thickness (t)from said interactions. The reluctance transducer sensor device (31) comprises a hollow conical magnetic core (41) around which circumference at least two coils (33, 35) of different diameter are mounted. The coil (33) having the smallest diameter is situated nearest the web (3) to be measured. The optical sensor device (7,9, 45) is adapted for sensing light beams (19) transferred through the hollow conical core (41) in directions which are oblique relative the plane (29) of said upper side (11).

Description

Material thickness measuring device

TECHNICAL FIELD

The present invention relates to a material thickness measuring device according to the preamble of claim 1.

BACKGROUND

The present invention relates to the manufacture of web coated sheets, such as painted sheet metal, wherein the paint is non-magnetic and the sheet is a magnetic material.

Especially, the present invention regards the measurement of the thickness of such coatings. It is desirable that the measurement can be performed with great accuracy. It is thus essential that the material thickness measuring device per se is robust and that the sensor devices of the latter are reliable and measure a first and second distance to the web and magnetic material respectively with high accuracy and precision. There are different types of material thickness measuring devices within the technical field. For example, WO 2010/104466 discloses an apparatus for non-contact measuring thickness of non-metal coating on surface of metal matrix. This apparatus described in WO

2010/104466 is developed by the applicant of the present application and the apparatus has a reliable function and works well. However, the material thickness measuring device is now under further development, since it is desirable to achieve an even greater accuracy in measurement of the coating.

The object of the invention is thus to provide a material thickness measuring device that contactless measures the thickness of a non-magnetic web applied onto a magnetic material, which material thickness measuring device manages to measure the thickness with a very high degree of accuracy.

The object of the invention is also to provide a compact material thickness measuring device, which suitably can be mounted onto a robot arm.

A further object of the invention is to provide a stable material thickness measuring device.

And also, there is an object to develop already existing material thickness measuring devices for reaching a stable material thickness measuring device managing to measure the thickness exact, despite that the device works in a production-line having a rough work environment.

SUMMARY OF THE INVENTION This has been solved by the material thickness measuring device defined in the introduction, which material thickness measuring device is characterized by the features claimed in the characterizing part of claim 1.

In such way a high accuracy in measurement can be achieved. The spot which the reluctance transducer measures can therefore be made concentrated, due to the smaller diameter of the coil, generating flux towards the magnetic material. At the same time, the hollow conical core permits transmission of light or optical signals with an oblique angle towards the upper side of the web making it possible to measure even deep indentations in the surface of the upper side, which indentations having steep walls.

Preferably, the two coils are locked onto the hollow conical magnetic core by cone-shaped press-fit connection.

In such way the device is made stable and the coils are mounted stable onto the core which provides a further high accuracy in measurement.

Suitably, the optical sensor device is a conoscopic holography apparatus for providing a contactless three-dimensional measuring of said upper side. By the use of such an apparatus, the accuracy in measurement can be further high, since such an apparatus can also measure the depth of an indentation having steep walls. Thereby the thickness of the web can be determined very exactly, such as a paint layer having a thickness of 1 μηι to 300 μηι. The adaption of the conoscopic holography apparatus makes it possible to detect smaller variations of the texture and roughness of the web surface and therefore a very exact first interaction measurement can be achieved. The first and second interactions are used by the control unit for calculating the thickness of the non-magnetic web.

Preferably, the material of the hollow conical magnetic core is stainless steel and the material of a respective coil bobbin is titanium. Thereby the device can be made even more stable which promotes for a further high accuracy in measurement of the web thickness, or at least in measurement of the reference distance to the magnetic material of the second interaction. The hard and stable coil bobbins can thus be pressed onto the cone-shaped core, making a stable package of core and coils of the measuring device. This will further promote for a high accuracy in measurement of the web thickness. Suitably, the hollow conical magnetic core is configured to direct and concentrate the magnetic flux to an area of a common point of measuring adjacent and surrounding a point of measuring of the first interaction.

This is achieved by concentration of the magnetic flux to an area near the measuring point. This is provided due to the smaller diameter of the coil nearest the magnetic material to be measured, i.e. being nearest during the use of the material thickness measuring device.

Preferably, the optical sensor device is adapted to emit and receive light to determine a first distance to the upper side of the web, and the reluctance transducer sensor device is adapted to emit electrical signals to determine a second distance to the magnetic material.

In such way the first distance between the upper side and a reference point and the second distance between the magnetic material and the same reference point can be used by the control unit to calculate the difference between the first and second distance, thereby the thickness of the web. Stepwise measuring is provided by the material thickness measuring device continuously and the control unit calculates a mean value regarding the thickness of the web for a determined section of the web. Suitably, for the first interaction with the upper side of the web, light emanating from the point of measuring is circularly polarized by a polarizer.

Preferably, the light is generated as concentrated laser light.

Suitably, the inside of the hollow conical magnetic core is provided with a matt black surface.

In such way is provided that the laser beam will not be reflected within the hollow core. Preferably, the reluctance transducer is provided with a static field magneto resistive sensor device adapted for said calculation, and which is arranged symmetrically between the two coils.

In such way is achieved a stable material thickness measuring device. In case of eventual thermal expansion of the core material, such expansion would not affect the positions of the static field magneto resistive sensor device relative both coils. The conical core is preferably provided with the surrounding upper and lower coils, the upper coil acts as a reference for the lower coil. Between the both coils and in a symmetrical position relative the upper and lower coil is a DC-field sensor in the form of a magneto resistive element arranged as for measuring the static fields. The both coils are controlled to generate (by means of the control unit and a current supply) a respective flux flowing around each coil with the same direction. As the coils are arranged above each other, the respective flux from each coil will meet each other and will work in opposite directions. This is due to the adaption of a balanced system which is not disturbed by the magnetic field present in and caused by the magnetic material. The coils are arranged to co-operate with the DC-field sensor. The DC-field sensor and the coils are connected to the control unit via conductors. The resulting flux produced between the two coils within the area of the DC-field sensor is in such way kept equal to zero. If the magnetic material property (such as a buckle, thickness variation of the sheet metal) changes, the reluctance (i.e. the material resistance against the magnetic flux) also will change for the lower coil. Thereby a magnetic flux is started through the DC-field sensor as well, which flux will be detected by the DC-field sensor. The DC-field sensors transfer this detection to the control unit which directs, via a zero detector (not shown), the current supply to change the current to the upper coil so that the resulting flux again becomes equal to zero through the DC-field sensor. A difference between the both coils' current intensities is thus present. An output signal is obtained by measuring this difference between the currents supplied to the respective upper and lower coil. The control unit is fed with this output signal and compare it with set data for calculating the proper second interaction or second distance. Such a zero detection of the magnetic flux through the DC-field sensors involve that a simple measure of the second distance can be performed, as the reluctance transducer is not sensitive to magnetic fields and is not bulky. The conical shape of the iron core, wherein the lower coil is provided onto the smallest diameter of the iron core, i.e. the lowest point and nearest the material to be measured, thus provides a coil with relative small diameter. Such small diameter promotes that the magnetic flux from the lower coil, meeting the magnetic material, can be concentrated around the point of measuring used by the optical sensor device. This flux concentration near the point of measuring will thus provide a great accuracy.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will be more readily understood by reference to the following detailed description taken with the accompanying drawings, in which: FIG. 1 is a schematic illustration of a material thickness measuring device according to a first embodiment;

FIG. 2 is a schematic illustration of a material thickness measuring device according to a second embodiment; FIG. 3 is a schematic illustration of a point of measuring illustrating irregularities in a web to be measured;

FIG. 4 is a schematic illustration of a material thickness measuring device according to a third embodiment;

FIG. 5 is an illustration taken in cross-section of the conical core of FIG. 4; FIGs. 6a and 6b are schematic illustrations of a mounting procedure of coils onto an iron core according to a fourth embodiment; and

FIG. 7 is a schematic illustration of a conical iron core comprising three coils according to a fifth embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings, wherein the sake of clarity and understanding of the invention some details of no importance are deleted from the drawings.

FIG. 1 is a schematic illustration of a material thickness measuring device 1 for measuring the thickness t of non-magnetic web 3 applied to a magnetic material 5, such as a metal sheet, according to a first embodiment. The material thickness measuring device 1 comprises an optical sensor device (optical distance-measuring device 7) in the form of a conoscopic holography apparatus 9 for measure of a first distance d1 from a reference level RF to an upper side 11 of the web 3 to be measured.

The conoscopic holography apparatus 9 comprises a laser system 13 for generating a concentrated laser beam 15 which is being output, during the use of the material thickness measuring device 1 , from a laser beam generator 16 through an optical assembly 17 of the apparatus 9. The laser beam 15 is reflected from the surface of the upper side 1 1 of the web 3, as a return beam 19, and is transferred into a beam splitter assembly 21 , wherein the return beam 19 is deflected to a distance measuring device 23. As being schematically described, the return beam 19 is reflected as a bundle 25 of beams of a conical shape which widens in the direction from the upper side 1 1 of the web 3. The advantage of using the conoscopic holography apparatus 9 is that it manages to measure also deep depressions (reference 32 see Fig.3) or cavities having steep walls in the surface of the upper side 11 , which will be explained further with regard to FIG. 3. The conical shape of the reflected bundle 25 of beams requires a relative broad lens 27 having an extension greater than a point of measuring P to be measured by the apparatus 9. The reference level RF can be determined to correspond with the plane of the lens 27 and the first distance d1 is measured from the reference level RF to the upper side 11 of the web 3. A mean value of the first distance d1 is calculated by a control unit 39 with regard to the measured deep steep wall depressions of the web 3 surface. The conoscopic holography apparatus 9 is thus adapted for a first interaction with the upper side 1 1 of the web 3 as an optical reference element 29.

The material thickness measuring device 1 according to this embodiment furthermore comprises a reluctance transducer sensor device 31. The reluctance transducer sensor device 31 comprises a lower 33 and upper 35 coil. The upper 35 coil works as a reference for the lower coil 33. The both coils 33, 35 generate a respective flux F flowing around each coil 33, 35 with the same directions. As the coils 33, 35 are arranged above each other, the respective flux F from each coil 33, 35 will meet each other and will work in opposite directions. Thereby is achieved a balanced system which not will be disturbed by the magnetic field present in, and caused by, the magnetic material 5. The coils 33, 35 are arranged to co-operate with DC-field sensors 37. The resulting flux, produced between the two coils 33, 35 within the area of the DC-field sensors 37, is in such way kept equal to zero. If the magnetic material property (such as due to a buckle, thickness variation of the sheet metal etc.) changes, the reluctance (i.e. the material resistance against the magnetic flux) also will change for the lower coil 33. Thereby a magnetic flux is started through the DC-field sensors 37 as well, which flux will be detected by the DC-field sensors 37. The DC-field sensors 37 transfer this detection to the control unit 39 (via conductors not being shown) which changes the current to the upper coil 35 (reference coil) so that the resulting flux again becomes equal to zero through the DC-field sensor 37. A difference between the both coils' 33, 35 current intensities will thus be given. An output signal (not shown) is obtained by measuring this difference between the currents supplied to the respective lower 33 and upper 35 coil. The control unit 39 is fed with this output signal and compare it with set data for calculating the proper second interaction or second distance d2.

The reluctance transducer sensor device 31 is thus provided for a second interaction with the magnetic material 5 as a magnetic reference element on the opposite side of the web 3 (opposite the upper side 11). The control unit 39 is adapted for each common point of measuring P to continuously calculate the thickness of the non-magnetic web 3 from the first and second interactions. That is, the control unit 39 calculates the thickness t of the web 3 by subtracting the value of the first distance d1 (measured by the optical distance-measuring device 7) from the value of the second distance d2 (measured by the reluctance transducer sensor device 31).

The reluctance transducer sensor device 31 furthermore comprises a hollow conical magnetic core 41 around which circumference the two coils 33, 35 are robust mounted. The coils 33, 35 are of different diameter. The coil 33 having the smallest diameter is mounted (during the use of the material thickness measuring device 1) nearest the web 3 to be measured. In such way the point of measuring P regarding the second distance d2 can be as small as possible, which implies great accuracy. This is due to the conical shape of the iron core 41 as the conical shape having the smallest diameter is nearest the web 3 during use. In such way a robust material thickness measuring device 1 is provided, which produces a very exact measurement. The optical sensor device (optical distance-measuring device 7) in the form of a conoscopic holography apparatus 9 is adapted for sensing the beams 15, 19 transferred through the hollow conical core 41 in directions which are oblique relative a plane 43 of the non-magnetic web's 3 upper side 1 1. The laser system 13 thus generates a laser beam 15 going through the optical assembly 17 and is reflected from the surface of the upper side 11 of the web 3 as a return beam 19 in the form of the bundle 25 of beams having a conical shape, which widens in a direction seen from the upper side 1 1 of the web 3. Light emanating from the point of measuring P is in this embodiment circularly polarized by a polarizer (not shown).

In such way a high accuracy in measurement can be achieved. The common point of measuring PC, defined as the point which the optical distance-measuring device 7 and the reluctance transducer sensor device 31 have in common for momentary measurement, can due to the conical shape of the core 41 be highly concentrated. The measured area, or second interaction, measured by the reluctance transducer sensor device 31 lies adjacent and around the point of measuring P measured by the optical distance-measuring device 7.

The lower coil 33 generates a flux F towards the magnetic material 5. The hollow conical core 41 permits transmission of the laser beam 15 and reflection of return beams 19, wherein it is possible to measure even deep indentations in the surface of the upper side 11 of the web 3, which indentations having steep walls (reference 59, see Fig. 3).

FIG. 2 is a schematic illustration of a material thickness measuring device 1 according to a second embodiment. This embodiment includes an optical sensor device (optical distance- measuring device 7) which is in the form of a pivoting confocal measurement sensor device 45 comprising a multi-lens optical system 47. The pivoting action, being performed during the use of the material thickness measuring device 1 , is achieved by an electrical motor (not shown) driving the system 47 about a hinge (not shown). The confocal measurement sensor device 45 includes a halogen light source based optical controller 49 for emitting

polychromatic white light transferred via a fiber optic cable 51. The pivoting confocal measurement sensor device 45 is pivotable in a direction y for scanning the upper side 11 of the web 3. In such way the detection of even steep walls of depressions on the upper side 11 surface of the web 3 can be detected giving a more precise measurement. The

polychromatic white light is focused by means of the sweeping action of the multi-lens optical system 47 onto the upper side 1 1 of the web 3. The lenses (not shown) of the multi-lens optical system 47 are arranged so that the white light is dispersed into monochromatic light by controlled chromatic aberration. The first distance d1 to the upper side 1 1 can be assigned to each wavelength, and the wavelength which is exactly focused onto the upper side 1 1 is used for measurement. The reflected light from the upper side 1 1 is passed through a confocal aperture 53 and further to a spectrometer 55, which detects and processes the spectral changes. The measured first distance d1 from a reference level RF to the upper side of the web 3 is thus achieved with high precision and extreme spatial resolution due to the oscillating motion of the pivoting confocal sensor arrangement. By the scanning of a slot area of the upper side 1 1 of the web 3 by means of the pivoting confocal sensor arrangement 45, instead of detecting a single spot, the accuracy can be even more exact. This is due to the fact that a larger area of the upper side 11 of the web 3 can be scanned. The reflected light (reflection from the upper side's 1 1 scanned slot area in the form of different wavelengths) being detected by the pivoting confocal sensor arrangement 45 forms a wedge-formed reproduction of reflected light having an angle a essentially corresponding with the angle β of the internal conical shape of the hollow iron core 41. As the sweeping action of the confocal sensor device 45 provides a reception of angled

spectrometric reflection in the form of wedge, the conical shape of the hollow iron core 41 provides conditions suitable for a reluctance transducer sensor device 31 comprising robust mounted coils 33, 35 of different diameter.

FIG. 3 is a schematic illustration of a common point PC of measuring, illustrating irregularities 57 in a web 3 to be measured by the conoscopic holography apparatus 9 of Fig. 1. The Fig. 3 schematically illustrates an enlarged cross-section of a sheet metal 5' having a paint layer 3' applied onto one side of the sheet metal 5'. A laser beam is output, during use of the material thickness measuring device 1 , from a light source (not shown) and hits the surface of the upper side 1 1 of the web 3. In case of a deep depression 32 or cavity in the web's 3 upper side 1 1 , the laser beam 15 hits the steep walls 59 of the depression 32. The light from the laser beam 15 is reflected from the web's 3 upper side 1 1 , including the reflections from said depressions 32, as return beams 19, and through the lens 27 (see Fig. 1) onto a distance measuring device. The return beams 19 are reflected as a bundle 25 of beams having a conical shape (only partly shown). The advantage of using the conoscopic holography apparatus 9 is that e.g. it manages to measure also deep depressions 32 or cavities having steep walls 59. The sheet metal 5' moves in direction r relative the device 1 and by the high accuracy in measurement, the speed of movement can be increased, which is cost-effective in the production of painted sheet metal articles. FIG. 4 is a schematic illustration of a material thickness measuring device 11 according to a third embodiment. For making the function of the reluctance transducer sensor device 31 even more robust, the material thickness measuring device 1 comprises a metal sheet cover 61 providing that the magnetic flux in a controllable manner flows through the cover 61 instead of being directed away by a near and accidentally placed interfering electronic equipment (not shown). The hollow conical iron core 41 is made of magnetic stainless steel. The core 41 is provided with a conical outer surface 63. A lower coil 33 and an upper coil 35 with different diameter are locked via a respective bobbin 65 onto the conical outer surface 63 by a cone-shaped press-fit connection. The bobbins 65 are made of titanium and the locking of the coils 33, 35 onto the conical stainless steel core 41 is performed by a press-fit of the bobbins onto the core 41. The material of titanium provides a secure and robust mounting of the coils 33, 35. High strength titanium (and titanium alloys) has superior structural efficiency, especially as service temperatures increase. This is important for the robust design. The low modulus of elasticity of titanium also promotes the fixture of the bobbins onto the core. The titanium cannot be magnetized and is corrosion-resistant, which features are essential for the functionality of the device. Each bobbin 65 comprises an internal thread 67 matching an external thread 69 of a respective cylindrical shaped shoulder 71 protruding from the conical outer surface 63. It is now possible to tighten each bobbin 65 to contact the conical outer surface 63 of the hollow conical core 41 for a press-fit locking assembly. Such assembly is robust, which is preferred regarding non-contact measurements with as small aberrations as possible. The conical shape of the core 41 corresponds with the conical shape of the bundle of the return beams 19. The thickness of the core 41 is even, in addition to the areas of the core 41 where said shoulders 71 are provided. The coils 33, 35 are arranged to co-operate with DC-field sensor elements 37' in a way similar to that being described above. The DC-field sensor elements 37' and the coils 33, 35 are connected to the control unit 39 via conductors. The number of DC field sensor elements 37' is eight. They are placed with rotational symmetry in circumference around the core 41. Each DC field sensor element 37', also called static field magneto resistive sensor device, is adapted for the calculation of the second distance d2, which calculation is performed by the control unit 39 for each common point of measuring PC. The DC field sensor elements 37' are arranged symmetrically between the two coils 33, 35 along an imaginary straight line between the coils as shown by the broken line B in Fig. 4. In such way a stable material thickness measuring device 1 is achieved. In case of e.g. thermal expansion of the device 1 parts, the symmetrical position of the DC field sensor elements 37' provides that said balanced system still works with high accuracy during all environmental conditions, which makes it a robust device. The respective flux from each coil 33, 35 meets and works in opposite directions in said balanced system and the DC field sensor elements 37' still works with high accuracy despite eventual thermal expansion of the parts. The optical sensor 7 is adapted for sensing the return beams 19 transferred through the hollow conical core 41 in directions which are oblique relative the plane 43 of the non-magnetic web's 3 upper side 1 1.

FIG. 5 is an illustration taken in cross-section of the conical core 41 of FIG. 4. The bobbins 65 comprise internal threads 67 matching external threads 69 of the core 41. Each bobbin 65 is tightened onto the core 41 , wherein an internal conical surface 72 of the bobbin 65, which surface 72 has a conical shape that corresponds with the conical shape of the outer surface 63 of the core 41 , will come into a press-fit locking contact, thereby providing a robust assembly of the core 41 and coils (not shown). The coils are rigidly mounted in the respective bobbin.

FIGs. 6a and 6b are schematic illustrations of a mounting procedure of coils 33, 35 onto an iron core 41 according to a fourth embodiment. The bobbins 65 and magnetic flux sensing detectors 37" (magneto resistive elements) (only two are shown) arranged between the bobbins 65 are fixed in a package 73 having an internal conical shape corresponding with the shape of the core 41. A nut 75 and a washer 77 are mounted onto an external thread 69 of the core 41 after the placement of the package 73 onto the core 41 as being shown in Fig. 6b.

FIG. 7 is a schematic illustration of a conical iron core 41 comprising three coils 33, 35, 35' according to a fifth embodiment. The third top position coil 35' serves as a reference standby coil. In such way a further robust system is achieved. The inside of the hollow conical magnetic core 41 is provided with a matt black surface. In this embodiment is welding 79 sections providing the securing of the bobbins of the package in a press-fit.

Although particular embodiments have been disclosed herein in detail, this has been done for purposes of illustration only, and is not intended to be limiting with respect to the scope of the appended claims. The embodiments can also be combined. In particular, it is contemplated by the inventor that various substitutions, alterations, and modifications can be made to the invention without departing from the spirit and scope of the invention as defined by the claims. For instance, also other types of optical distance-measuring devices can be used in combination with the hollow conical iron core. The conical shape of the core may have different angles than those being shown in the figures. The core can have an even thickness or varying thickness which implies that the internal conical surface has a different angle than that of the external conical surface. The material selection for the different parts of the material thickness measuring device may vary. Preferably, hard stainless steel can be used for the core and titanium can be used for the bobbins. Also other suitable materials can be used. Different types of webs can be measured, such as films, adhesive coatings, resin layers etc.

Claims

1. Material thickness measuring device for measuring the thickness (t) of non-magnetic web (3) applied to a magnetic material (5), the measuring device (1) comprises an optical sensor device (7, 9, 45) for a first interaction with an upper side (1 1) of the web (3) as an optical reference element (29) and a reluctance transducer sensor device (31) for a second interaction with the magnetic material (5) as a magnetic reference element on the opposite side of the web (3), a control unit (39) is adapted for each point of measuring (P, PC) to calculate the thickness (t) from said

interactions, characterized by that the reluctance transducer sensor device (31) comprises a hollow conical magnetic core (41) around which circumference at least two coils (33, 35) of different diameter are mounted, the coil (33) having the smallest diameter is situated nearest the web (3) to be measured and by that the optical sensor device (7, 9, 45) is adapted for sensing light beams (19) transferred through the hollow conical core (41) in directions which are oblique relative the plane (29) of said upper side (1 1).

2. Material thickness measuring device according to claim 1 , wherein the two coils (33, 35) are locked onto the hollow conical magnetic core (41) by cone-shaped press-fit connection.

3. Material thickness measuring device according to claim 1 or 2, wherein the optical sensor device is a conoscopic holography apparatus (9) for providing a contactless three-dimensional measuring of said upper side (1 1).

4. Material thickness measuring device according to any of claim 1 to 3, wherein the material of the hollow conical magnetic core (41) is stainless steel and the material of a respective coil bobbin (65) is titanium.

5. Material thickness measuring device according to any of the preceding claims,

wherein the hollow conical magnetic core (41) is configured to direct and concentrate the magnetic flux (F) to an area of a common point of measuring (PC) adjacent and surrounding a point of measuring (P) of the first interaction.

6. Material thickness measuring device according to any of the preceding claims,

wherein the optical sensor device (7) is adapted to emit and receive light (15, 19) to determine a first distance (d1) from a reference level (RF) to said upper side (11), the reluctance transducer sensor device (31) is adapted to emit signals to determine a second distance (d2) from said reference level (RF) to the magnetic material.

7. Material thickness measuring device according to claim 6, wherein for the first

interaction with the upper side (1 1) of the web (3), light emanating from the point of measuring (P) is circularly polarized by a polarizer.

8. Material thickness measuring device according to claim 6 or 7, wherein the light is generated as concentrated laser light.

9. Material thickness measuring device according to any of the preceding claims,

wherein the inside of the hollow conical magnetic core (41) is provided with a matt black surface.

10. Material thickness measuring device according to any of the preceding claims,

wherein the reluctance transducer sensor device (31) is provided with at least one static field magneto resistive sensor device (37, 37', 37") adapted for said calculation, and which is arranged symmetrically between the two coils (33, 35).