Sensor for capturing a moving material web

Abstract

A sensor (1) serves for capturing a moving material web (2). It has at least one light source (4) and at least one light detector (10). At least one polarization element (15) is provided between the two, which polarization element influences polarization properties of the light transmitted by the polarization element (15) as a function of an electric field. Markings such as for example metal strips (13) in the material web (2) can thus be captured in the light detector (10) without problem.

Claims

1. A sensor for capturing a position of a moving material web having a surface with a region, comprising a marking provided on the surface of the moving material web within the surface region, the surface region also including a surface portion outside said marking, wherein said marking is situated under a coating of clear varnish on the surface of the moving material web, said marking having a position being captured by said sensor, wherein said sensor comprises at least one light source transmitting emission light to said region of said material web and said emission light being influenced by said material web producing detection light, said sensor further comprising at least one light detector, which receives said detection light, wherein in order to achieve a high contrast between said s region outside said marking of said material web and said marking, at least one polarization element is provided between said at least one light source and said at least one light detector, said at least one polarisation element being penetrated by said emission light, said sensor further comprising capacitor electrodes, producing an electric field that influences said polarization of said emission light by way of applying an electrical voltage between said capacitor electrodes, wherein said at least one polarization element is arranged spatially between said capacitor electrodes, and wherein said at least one light detector is configured as a camera.

2. The sensor according to claim 1, wherein said at least one polarization element comprising at least one liquid crystal.

3. The sensor according to claim 1, wherein said at least one polarization element comprising at least one Pockels cell.

4. The sensor according to claim 1, wherein said at least one polarization element comprising at least one Kerr cell.

5. The sensor according to claim 1, wherein at least one polarization filter being provided between said at least one light source and said at least one light detector.

6. The sensor according to claim 1, wherein said at least one light source being configured such that said emission light is linearly polarized.

7. The sensor according to claim 1, wherein said material web having a normal and said emission light strikes said material web at an acute angle with respect to said normal on said material web.

8. The sensor according to claim 7, wherein said acute angle approximately corresponds to the Brewster angle.

9. The sensor according to claim 5, wherein said material web having a normal and said emission light strikes said material web at an acute angle with respect to said normal on said material web.

10. The sensor according to claim 9, wherein said acute angle approximately corresponds to the Brewster angle.

11. The sensor according to claim 1 wherein said marking comprises a metal strip.

12. The sensor according to claim 11 wherein said metal strip is embedded in the surface of said material web.

13. The sensor according to claim 11 wherein said metal strip comprises aluminium.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWINGS

(1) Other advantages and characteristics of this invention will be explained in the detailed description below with reference to the associated figures that contain several embodiments of this invention. It should however be understood, that the figures are just used to illustrate the invention and do not limit the scope of protection of the invention.

(2) Wherein:

(3) FIG. 1 shows a schematic principle illustration of a sensor,

(4) FIG. 2 shows a first embodiment of a polarization element and

(5) FIG. 3 shows a second embodiment of a polarization element.

DETAILED DESCRIPTION OF THE INVENTION

(6) A sensor 1 according to FIG. 1 serves for capturing a moving material web 2, which moves along a direction of motion 3 which is directed towards the observer. The sensor 1 has a light source 4 which emits emission light 5. Arranged downstream of the light source 4 is a polarization filter 6, which linearly polarizes the emission light 5. The emission light 5 that is thus polarized strikes the material web 2—at an acute angle 7 measured with respect to—a normal 8. Part of the incident light is reflected by the material web 2, wherein the reflectivity depends both on the polarization state of the incident light and on the material properties of the material web 2. The reflected detection light 9 finally travels to a light detector 10, which is configured for example as a camera. This can be used to capture a detail 11 of the material web in a spatially resolved manner. An additional polarization filter 6 can optionally be arranged upstream of the light detector 10.

(7) In the exemplary embodiment according to FIG. 1, the material web 2 consists for example of a polymer carrier 12, in which a marking 13 in the form of a metal strip is embedded. This marking 13 consists for example of aluminium, but alternatively can also be made of any other metal. Provision is made for this marking 13 to be captured by the sensor 1, so that the material web 2 is guided depending on the position of the marking 13. This could be achieved in principle by a suitable choice according to a bright field or dark field illumination. In the present exemplary embodiment, the material web 2 is, however, additionally provided on the upper side with a clear varnish 14. This clear varnish 14 forms a dielectric layer on the surface of the material web 2, such that both bright field illumination and dark field illumination fail in the case of this material web type.

(8) In order to still capture the position of the marking 13 in the light detector 10, a polarization element 15 is provided in the beam path between the polarization filter 6 and the material web 2. This polarization element 15 can influence the polarization of the transmission light 16 that passes through it as a function of an applied electric field. In this manner, the polarization of the transmission light 16 can be changed such that the marking 13 is capturable in the light detector 10 with high contrast. Alternatively or additionally, the polarization element 15 can also be arranged—as indicated in dashed lines—between the material web 2 and the light detector 10, without the function of the apparatus 1 being adversely affected. The use of two polarization elements 15 on either side of the material web 2 is also conceivable.

(9) FIG. 2 shows, on the left-hand side, a first embodiment of the polarization element 15. The polarization element 15 in this case has a liquid crystal 17, which is for example in a nematic phase.

(10) The liquid crystal 17 is arranged spatially between two capacitor electrodes 18, 19. Said capacitor electrodes 18, 19 have grooves 20 which are in each case mutually parallel. The grooves 20 of the capacitor electrode 18 are rotated by 90° with respect to the grooves 20 of the capacitor electrode 19. Liquid-crystal molecules 17a are here arranged in each case parallel to the respective grooves 20 such that they—viewed in the direction of the transmission light 16—assume a helical configuration. As a result, the polarization direction of the transmission light 16 is rotated by 90°. The angle of rotation in this case corresponds to the angle between the grooves 20 of the capacitor electrodes 18, 19.

(11) The right-hand side of FIG. 2 again shows the polarization element 15. In contrast to the left-hand illustration, here a voltage source 21 is connected between the capacitor electrodes 18, 19. Owing to the applied voltage, the capacitor electrodes 18, 19 produce an electric field 22 in the liquid crystal 17. If the electric field 22 is strong enough, the liquid-crystal molecules 17a of the liquid crystal 17 align themselves parallel to the electric field 22. The effect of the liquid crystal 17 on the polarization of the transmission light 16 thus disappears. Therefore, its polarization direction remains unchanged. By varying the voltage of the voltage source 21, the proportion of the regions of the liquid crystal 17 which are aligned with the electric field 22 can be varied. In this manner, various polarization directions can be mixed in the transmission light 16.

(12) Alternatively, the liquid crystal 17 can, depending on the physical properties, also rotate the polarization of the transmitted light by applying a voltage and leave the electric field unchanged.

(13) FIG. 3 shows an alternative embodiment of the polarization element 15. The polarization element 15 here has a birefringent crystal 23, which is provided with the capacitor electrodes 18, 19. Said capacitor electrodes 18, 19 are in turn connected to the voltage source 21. The light radiates through the birefringent crystal 23. Owing to the linear electro-optical effect, the refractive index of the birefringent crystal 23 changes linearly with the electric field 22 produced by the capacitor electrodes 18, 19. Owing to the birefringence of the birefringent crystal 23, said change in refractive index results in a rotation of the polarization plane of the transmission light 16. The angle of rotation is here proportional to the electric field 22 and thus also proportional to the applied voltage produced by the voltage source 21. The birefringent crystal 23 together with the capacitor electrodes 18, 19 forms a Pockels cell 24.

(14) If alternatively or additionally a non-linear electro-optical effect is utilized in the birefringent crystal 23, the arrangement of birefringent crystal 23 and the capacitor electrodes 18, 19 produces a Kerr cell 25.

(15) Since some of the embodiments of this invention are not shown or described, it should be understood that a great number of changes and modifications of these embodiments is conceivable without departing from the rationale and scope of protection of the invention as defined by the claims.

REFERENCE SYMBOL LIST

(16) 1 sensor 2 material web 3 direction of motion 4 light source 5 emission light 6 polarization filter 7 acute angle 8 normal 9 detection light 10 light detector 11 detail 12 polymer carrier 13 marking 14 clear varnish 15 polarization element 16 transmission light 17 liquid crystal 17a liquid-crystal molecule 18, 19 capacitor electrode 20 groove 21 voltage source 22 electric field 23 birefringent crystal 24 Pockels cell 25 Kerr cell