Method and apparatus for determining refractive index in surface region of object

Abstract

The present invention provides a method for determining the refractive index of a surface region of a heated strand or plate-like object (10) coming from a production system that has not yet fully cooled to ambient temperature, comprising: a) irradiating a surface (16) of the object (10) with terahertz radiation at an angle of incidence; b) receiving the terahertz radiation reflected from the surface (16) of the object (10); c) determining the refractive index of the surface region of the object (10) from the ratio of the emitted terahertz radiation to the reflected terahertz radiation; d) taking into account the influence of the surface properties of the object (10) on the reflected portion of the terahertz radiation when determining the refractive index; 1. A method comprising: Furthermore, the invention relates to a corresponding device.

Description

Technical Field

[0001] The present invention relates to a method and an apparatus for determining the refractive index in a surface region of a heated strand-shaped or plate-shaped object sent from a production system that has not been completely cooled to the ambient temperature.

Background Art

[0002] For example, in an extrusion system, strand-shaped or plate-shaped plastic objects are manufactured and conveyed along the conveying direction, for example, through a cooling line, until they are completely cooled to the ambient temperature and concomitantly until they are completely cured. Immediately after leaving the extrusion system, in the region on the extension line of the conveyor line, the objects are not yet completely cooled and thus not cured.

[0003] From WO2016 / 139155A1 (Patent Document 1) and DE102018128248A1 (Patent Document 2), methods and devices are known for determining the refractive index of a strand-shaped or plate-shaped plastic object by irradiating the object with terahertz radiation and receiving the terahertz radiation reflected by the object. This is the average refractive index across the cross-section of the object or the irradiated surface of the object. Based on this, geometric parameters of the object, such as the wall thickness of a pipe, can be reliably determined even if the refractive index is initially unknown.

[0004] In particular, when measuring an object immediately after it exits the extrusion device, the geometric parameters confirmed by this method may deviate from the actual geometric parameters of the object in a completely cooled state. Obtaining the refractive index and geometric parameters after the object is completely cooled and cured would likely yield more reliable results. On the other hand, in order to be able to intervene in the production process as early as possible when the geometric parameters are incorrect and thereby minimize scrap, it is desirable to determine the geometric parameters as early as possible after the object exits the extrusion system, for example.

[0005] Therefore, even if the object is not yet completely cooled and is measured in a state where it has not hardened, it is necessary to be able to accurately determine the refractive index.

[0006] The methods for determining the refractive index described at the beginning and the methods known from the prior art are also not optimally suitable for reliably determining the refractive index, for example, with a portable measuring device, a so-called handheld unit. Therefore, it is further necessary to be able to reliably determine the refractive index even in such cases.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, based on the prior art described, the object of the present invention is to be able to reliably determine the refractive index even when the object is not yet completely cooled.

Means for Solving the Problems

[0009] The present invention achieves the object through independent claims 1 and 17. Advantageous embodiments are disclosed in the dependent claims, the description, and the figures.

[0010] Regarding the method of the first-mentioned type, the present invention a) irradiating terahertz radiation at an incident angle to the surface of the object, a step; b) receiving the terahertz radiation reflected from the surface of the object, a step; c) a step of determining the refractive index in the surface region of the object from the ratio of the radiated terahertz radiation to the reflected terahertz radiation; d) a step of considering the influence of the surface characteristics of the object on the reflected portion of the terahertz radiation when determining the refractive index; The object is achieved through the above steps.

[0011] The above-described type of apparatus for achieving the object comprises: - a transmitter for transmitting terahertz radiation at an incident angle with respect to the surface of the object; - a receiver for receiving the terahertz radiation reflected from the surface of the object; - an evaluation device designed to determine the refractive index in the surface region of the object from the ratio of the radiated terahertz radiation to the reflected terahertz radiation; and - the evaluation device is designed to also consider the influence of the surface characteristics of the object on the reflected portion of the terahertz radiation when determining the refractive index.

[0012] The measurement object according to the present invention sent from the production system is in a heated state and not completely cooled to the ambient temperature. Therefore, there may be a viscous component inside it that has not yet completely cured and will only cure upon further cooling. On the other hand, the surface region of the object is already cured. During the curing process by additional cooling, the material shrinks. The production system may be, for example, an extrusion system. Accordingly, the object can be an object extruded by an extrusion system. The object may be, for example, an object made of plastic. The object may be, for example, a pipe. Furthermore, the object is conveyed along the conveying direction during the terahertz irradiation according to the present invention and may be, for example, a strand-like object along its longitudinal axis. For this purpose, the apparatus according to the present invention may be provided with a conveying device. Furthermore, the apparatus according to the present invention can also constitute the object itself.

[0013] In the present invention, terahertz radiation is radiated onto the surface of an object at an incident angle that is assumed to be known. The surface of the object reflects at least part of the incident terahertz radiation according to its reflectivity. The reflected terahertz radiation is received by a receiver. The terahertz radiation can exist, for example, within a frequency range of 10 GHz to 3 THz. It may be so-called millimeter waves. A transmitter that emits terahertz radiation and a receiver that receives the reflected terahertz radiation can be arranged substantially at the same location. For example, they may be integrated together in a transceiver.

Advantages of the Invention

[0014] The present invention is first based on the concept that the refractive index can be reliably measured in a surface region of an object that has already been largely cooled and hardened. This is based on the insight that an object that is not yet completely cold has a viscous part, and there is a narrow transition region, a so-called recrystallization region, between this viscous part and the already hardened components. Therefore, overall, there are three types of layers with significantly different properties regarding density, strength, and thus refractive index. When the average refractive index is measured over the entire cross-section of an object that still has a viscous part by the method described at the beginning with respect to the prior art, a refractive index value that does not correspond to the refractive index of the material in a fully hardened state may be obtained, especially with respect to the still viscous components. This can have a distorting effect on the determination of geometric parameters such as the wall thickness of a pipe.

[0015] According to the investigation of the applicant, in the process of further cooling and hardening the pipe with a viscous part, it has been found that the length and diameter of the pipe only undergo slight changes, that is, less than 1%, even when further cooled to room temperature. However, during the period until it is completely cooled and hardened, a shrinkage of about 10% of the pipe material can be confirmed. It can be concluded that this shrinkage is substantially entirely due to the still viscous part of the material of the object. Since the refractive index of the material changes with the density of the material, there may be a measurement error when obtaining the average refractive index of an object with a still viscous part. This can be avoided by measuring the refractive index in the surface region of an object that has already been completely cooled and thus hardened, as provided according to the present invention.

[0016] To measure the refractive index of the surface region, the well-known Fresnel equation is used. This equation can calculate the refractive index of the surface region, particularly at the surface of the object, from the ratio of the terahertz radiation reflected from the surface of the object to the terahertz radiation radiated onto the surface from a known incident angle. The reflectivity of the object to be determined, that is, the ratio of the reflected radiation intensity to the irradiated radiation intensity, can be confirmed from the received reflected radiation portion if the irradiated radiation intensity is known. The reflectivity shows a mathematical relationship between the refractive index in the surface region of the object and the environment of the object. The refractive index of the environment of the object is known and can be assumed to be 1 for air. The incident angle can be 0° in the simplest case, that is, the radiation can be perpendicularly incident. The Fresnel equation can be simplified as follows.

[0017] [Number]

[0018] Here, R: Reflectivity n 1 : Refractive index of the environment of the object (1 for air) n 2 : Refractive index of the surface region of the object is shown.

[0019] However, additional investigations by the inventors have revealed that the use of this method for determining the refractive index of a heated strand or plate-shaped object sent from a production system that is not completely cooled to ambient temperature using terahertz radiation is very complex. This is particularly true when determining the portion of the radiation reflected in relation to the incident radiation. It has been revealed that the intensity level of the incident terahertz radiation varies greatly depending on the distance from the object, and the portion of the terahertz radiation reflected from the surface varies depending on the positional relationship, particularly the surface of the object.

[0020] The distance and curvature from the object can be effectively set and considered by performing a careful measurement setup. However, the situation is different because the surface of the object can hardly have an impact within the scope of the measurement setup. In order to overcome this problem that particularly occurs when using terahertz radiation, in the determination of the refractive index according to the present invention, it is provided to consider the influence of the surface characteristics of the object on the reflected portion of the terahertz radiation. Thereby, the influence of the surface that changes the reflected portion is considered, and the corresponding distortion of the determination result of the refractive index can be avoided. For example, extremely perpendicular irradiation, and thus reflection, of terahertz radiation, that is, reflection at an incident angle of 0°, can thereby always be achieved. The incident angle that can be surely set is very important as an input variable when obtaining the refractive index with the Fresnel equation, even if the surface state of the object is difficult or unknown.

[0021] It should be noted that the refractive index is also affected by the frequency of the terahertz radiation used. When referring to the refractive index here, it means the refractive index with respect to the frequency of the terahertz radiation used.

[0022] According to one embodiment, the influence of the surface shape of the object can be considered in step d). In this embodiment, in particular, the concept is based on the fact that irregular surface shapes such as dents, protrusions, and grooves deviate the assumed incident angle, such as 0°, from the actual incident angle, and thus the reflection angle. This can be avoided by considering the surface shape.

[0023] According to another embodiment, in step d), calibration can be performed on an object having a known reflectivity, preferably a reflectivity of substantially 1. For calibration, the surface of the object can be provided with a coating having a known reflectivity, preferably a reflectivity of substantially 1. The object used for calibration can be the object determined with respect to its surface refractive index, that is, the object itself sent from the production system. Next, it is assumed that, for example, the surface shape of the object conveyed along the conveyance direction during terahertz irradiation does not change significantly in the conveyance direction, for example, in its longitudinal direction. This can be well approximated for an object sent from, for example, an extrusion system. For example, for calibration, the surface of the measurement object can be treated as if it has a defined reflectivity, preferably as high a reflectivity as possible, for example, a reflectivity of 1. For example, the coating can be a foil coating or a spray coating. The foil can be, for example, a thin metal foil. The spray coating can be, for example, a water spray coating or a coating with another liquid having a high reflectivity.

[0024] When the measurement according to the present invention is carried out with the transmitter (and receiver) for terahertz radiation under a certain arrangement, the (actual) incident angle of the terahertz radiation can be inferred from the reflected part of the radiation, because the reflected part of the radiation in normal incidence must correspond to, for example, a known reflectivity of 1. If there is a deviation here, a deviation from the assumed incident angle of 0° will be inferred. Also, by using the surface thus provided and searching for the arrangement of the transmitter (and receiver) where the reflected part of the radiation most corresponds to a known reflectivity, for example, about 1, it is conceivable to reposition the transmitter (and receiver) accordingly and find the normal incidence. Calibration with the actual object to be measured can obtain the best results because calibration is performed using the actual surface shape of the object. However, when the shape of the object including the surface shape can be approximately assumed to correspond to another object having a known reflectivity, such as a metal strand, especially a metal pipe or a metal plate, it is also possible to use something different from the object to be measured for calibration. The positional relationship (distance, etc.) between the transmitter and the receiver with respect to the object used for calibration must match very accurately with the object actually measured. The above calibration can be carried out both before and after determining the refractive index of the untreated object according to the present invention. In order to avoid the influence of surface treatment on the determination result of the refractive index, it may be preferable to carry out calibration after determining the refractive index of the untreated object according to the present invention. In this case, of course, readjustment of the transceiver is not necessary.

[0025] According to another embodiment, in step d), the terahertz radiation can be radiated at different positions on the surface of the object and / or at different angles of incidence with respect to the surface of the object, the reflected terahertz radiation is received, and the refractive index is determined from the portion of the reflected terahertz radiation having the maximum intensity. Thus, the arrangement of the transmitter (receiver) of the terahertz radiation can be changed to obtain the maximum reflected radiation power, that is, normal incidence can be obtained. And around the maximum reflected radiation intensity, it is assumed that the angle of incidence substantially corresponds to 0°. According to another embodiment regarding this point, the transmitter (and receiver) for the terahertz radiation can rotate while transmitting the terahertz radiation around at least one rotation axis and / or can rotate around the object. In principle, it is considered possible to circularly orbit around a tubular object, to change the angle of incidence one-dimensionally or two-dimensionally, for example, to make the transmitter and the receiver perform a pendulum motion. In each case, the maximum intensity can be grasped and used as a basis for determining the refractive index. For this purpose, for example, it is conceivable to tilt (rapidly) the transmitter and the receiver electrically or mechanically or to make them spiral. In this regard, for example, a so-called galvanometer scanner can be considered. The maximum intensity can, in principle, be determined, for example, by fitting to an envelope curve. This embodiment can also be considered for a handheld unit, especially a handheld unit in which the support of the handheld unit is omitted by using a device for rapidly scanning the surface of the object, especially compared to the inevitable wobbling of the hand having the handheld unit. Of course, it would also be conceivable in principle to provide a plurality of transmitters and receivers that irradiate the surface of the object with terahertz radiation and receive the reflected radiation, for example, at different locations or distributed on the circumference of the object. And the reflected radiation with the highest intensity can be used for subsequent evaluation. In addition to or instead of grasping the maximum value, it is also possible to determine the average refractive index from the average value of the received reflected terahertz radiation or from the quadratic average value of the received reflected terahertz radiation.

[0026] According to another embodiment, the average refractive index can further be determined across the cross-section of the object. This average refractive index can also be determined from terahertz radiation that is radiated onto the object and reflected from the object, in which case the radiation that shines through the object at least partially and the radiation portion reflected from the internal interfaces of the object are received and evaluated. The refractive index in this regard can be determined as described, for example, in WO2016 / 139155A1 (Patent Document 1) or DE102018128248A1 (Patent Document 2). Accordingly, reference is made to these documents. An advantage is that the same measuring and evaluation device, in particular the same transmitter and receiver, as for determining the refractive index in the surface region of the object can be used. It is understood that the reflected signals from different surfaces can be identified by time or frequency so that they can be individually evaluated for measurement. The terahertz radiation can be emitted, in particular, in FMCW mode (frequency-modulated continuous wave) or in pulse mode.

[0027] According to another embodiment, the comparison between the average refractive index and the refractive index of the surface region of the object can be used to infer the shrinkage during the process of completely cooling the object to ambient temperature. The refractive index of a material depends, inter alia, on temperature and in particular on the density of the material. Since the still viscous parts are less dense than the completely cooled regions, the refractive index of such still viscous parts is also lower. Thus, from the difference between the average refractive index taking into account the viscous regions and the surface refractive index not taking into account the viscous regions, the presence and the portions of the viscous regions can be inferred, and thus the expected shrinkage rate can be reliably inferred. This also applies when the measurement is carried out substantially immediately after leaving a production system such as an extrusion system.

[0028] According to another embodiment, in step d), the predicted intensity of the terahertz radiation reflected by the object can be determined using the finite element method, taking into account the radiation characteristics of the transmitter for terahertz radiation and the distance from the transmitter to the object. This portion of the predicted reflected terahertz wave corresponds in particular to the portion when the angle of incidence = angle of reflection = 0°, i.e., normal incidence. If the intensity of the actually measured reflected radiation deviates from the expected value, the deviation from the assumed angle of incidence of 0° can be inferred, and this can be taken into account mathematically or corrected in the measurement setup.

[0029] According to another embodiment, the temperature in the surface region of the object can be determined from the determined refractive index in the surface region of the object. As explained, the refractive index is temperature-dependent. Therefore, the temperature at the position or location where the refractive index is determined can be inferred from the determined refractive index.

[0030] According to another embodiment, the object may be a pipe, and the terahertz radiation reflected from the inner surface of the pipe wall is also received, and the refractive index on the inner surface of the wall is determined from the ratio of the irradiated terahertz radiation to the terahertz radiation reflected from the inner surface of the wall. Of course, the attenuation of the terahertz radiation along the path through the material also needs to be considered.

[0031] According to another embodiment in this regard, the refractive index on the inner surface of the wall portion can be used to determine the temperature on the inner surface of the wall portion and / or the temperature difference and / or temperature gradient between the temperature on the inner surface of the wall portion and the temperature in the (outer) surface region of the object. By taking such temperature into account, for example, material (batch) changes can be detected and calculated and removed, so that the determination of the refractive index according to the present invention can be carried out independently of such changes. Furthermore, taking into account the temperature difference or temperature gradient is particularly interesting for materials with varying composition and refractive index in cross-section, such as pipe walls. This is the case, for example, with foamed materials having strongly reflecting interfaces. In principle, the present invention can also be used to determine the refractive indices of a plurality of different material layers in a material, especially when the object has a layered structure. For this purpose, the terahertz radiation reflected at the boundary layer between the layers can be evaluated to determine the refractive index.

[0032] It is also conceivable to determine the average temperature of the object from the determined temperature in the surface region of the object and the determined temperature on the inner surface of the wall portion. It may also be possible to determine the average temperature of the object from the determined average refractive index across the cross-section of the object. And the average temperatures of the two determined objects can be compared. From this comparison, it is possible to infer changes in the material properties of the object, such as material changes, component mixing, batch changes, etc. It is also conceivable to measure the temperature in the surface region of the object using a temperature measuring device independent of the material, such as a pyrometer. The temperature on the inner surface of the wall portion can be determined from the measured temperature and the temperature difference determined between the temperature on the inner surface of the wall portion and the temperature in the surface region of the object. Therefore, the absolute internal temperature independent of the material can be determined without providing a corresponding temperature measuring device inside, but this is often actually impossible.

[0033] By comparing the average temperature obtained from the average refractive index determined as described above with the absolute surface temperatures of, for example, the outer and inner surfaces of the wall of a pipe measured in this way, if a corresponding difference occurs, it is possible to infer the mixing of other materials or a material change such as a new lot.

[0034] In principle, in order to consider the surface characteristics of the object, particularly the surface shape, it would also be possible to optically additionally measure the surface, for example, by means of laser radiation. And when determining the surface refractive index according to the present invention, any shape of the surface can be considered. Laser triangulation can be used for the measurement, for example.

[0035] Also, it is advantageous to provide a reflector on the side opposite to the transceiver of the object to prevent the measurement result from being affected by fluctuations in the intensity of the transmitter or fluctuations in the sensitivity of the receiver.

[0036] According to another embodiment, the device according to the present invention can be a handheld unit, that is, a unit that can be manually carried. In this way, the refractive index can be reliably determined from the outside in a very simple manner. Also, according to the present invention, reliable measurement is possible even when using a portable unit.

[0037] The device according to the present invention, particularly its evaluation device, can be designed to implement the method according to the present invention. Therefore, the method according to the present invention can be implemented by the device according to the present invention.

[0038] Hereinafter, exemplary embodiments of the present invention will be described in more detail with reference to the drawings.

Brief Description of the Drawings

[0039]

Figure 1

Figure 2

Figure 3

Best Mode for Carrying Out the Invention

[0040] In the drawings, unless otherwise specified, the same reference numerals refer to the same components.

[0041] FIG. 1 shows a vertical cross-sectional view of the apparatus according to the present invention. FIG. 2 shows a horizontal cross-sectional view of the apparatus from FIG. 1. In the exemplary embodiment shown in FIGS. 1 and 2, a heated object 10 that has been sent from a production system, such as an extrusion system, and is not yet completely cooled to the ambient temperature, and in which most of the surface region has already been cooled and completely cured, but still has a viscous portion inside, is conveyed through the apparatus by a suitable conveying device (not shown in detail). In FIG. 1, it extends along its central longitudinal axis (cylinder axis) perpendicular to the plane of the drawing, and in FIG. 2, it extends from bottom to top. The object 10 in this embodiment is a cylindrical plastic pipe 10. That the cross-section of the pipe 10 is circular can be easily determined from the cross-sectional explanatory view shown in FIG. 1. In particular, the wall 12 of the pipe 10 has an outer surface 16 with a circular cross-section that defines a hollow space 14 and an inner surface 18 also having a circular cross-section.

[0042] The device according to the invention shown in FIGS. 1 and 2 also comprises a transmitter for irradiating terahertz radiation and a receiver for receiving the terahertz radiation irradiated by the transmitter and reflected by the interface of the pipe 10. In the example shown, the transmitter and the receiver are formed by the terahertz transceiver 20. Of course, spatially separated transmitters and receivers, for example facing each other, may also be provided. The reference numeral 22 schematically represents the radiation optical system, which in this embodiment comprises a biconvex lens 21 and a cylindrical lens 23 for terahertz radiation. Of course, other radiation optical systems are also conceivable. For example, it is also possible to use a combination of a biconvex lens and a cylindrical lens. Furthermore, it can be identified in FIGS. 1 and 2 that the optical axis 25 of the optical system formed by the transceiver 20 and the radiation optical system 22 is perpendicular to the longitudinal axis of the pipe 10. The transceiver, i.e., the transceiver 20, is further connected to the evaluation device 30 by line 28. It should be noted that the radiation does not have to be focused at the center of the pipe 10 and can have any other beam characteristics. In particular, parallel radiation can also be emitted. Furthermore, the direction of the irradiated radiation can be controllably changed by means of a suitable adjustment device.

[0043] The reference numeral 26 denotes a cylindrically curved reflector for terahertz radiation having a longitudinal axis running in the longitudinal axis direction of the pipe 10 guided inside the device. The curvature midpoint of the reflector 26 coincides with the curvature midpoint of the measured pipe 10 such that the focal line of the cylindrical reflector 26 coincides with the longitudinal axis of the pipe 10. The reflector 26 makes it possible to amplify the measurement signal and improve the discrimination of different measurement signals received by the receiver. Furthermore, it helps to compensate for fluctuations in the transmission power or reception sensitivity of the transmitter and the receiver. It should be noted that the reflector 26 can also have different geometric shapes. Of course, it is also possible to completely omit the reflector 26.

[0044] By means of the device according to the invention shown in FIGS. 1 and 2, in particular the transceiver 20, terahertz radiation is radiated onto the outer surface 16 of the pipe 10 at an angle of incidence of 0° if possible. The terahertz radiation reflected from the outer surface 16 of the object is then received by the transceiver 20. By means of the evaluation device 30, the refractive index in the surface area, in particular on the outer surface 16 of the pipe 10, is determined from the ratio of the irradiated terahertz radiation to the reflected terahertz radiation. The refractive index can be calculated in the manner described above, in particular by using the Fresnel equations.

[0045] The figure shown in FIG. 3 illustrates the basic problem of this procedure. For this purpose, as a test of a completely cooled and hardened circular cylindrical plastic pipe 10, the device shown in FIGS. 1 and 2 was rotated 360° around the plastic pipe 10, and in each case terahertz radiation was radiated onto the outer surface 16 of the plastic pipe 10, and the terahertz radiation reflected from this outer surface 16 was received again by the transceiver 20. Assuming that the plastic pipe 10 has a substantially homogeneous refractive index, the measured intensity amplitude of the reflected terahertz radiation should only be slightly affected by the rotation.

[0046] FIG. 3 shows the intensity amplitude of the reflected terahertz radiation actually received by the transceiver 20 during the rotation. The intensity of the reflected terahertz radiation is plotted on the Y-axis in arbitrary units, and the rotation angle of the device, in particular the transceiver 20, is plotted on the X-axis. As a result, it became clear that intensity fluctuations of more than 20% occur, which cannot be explained by the irregularities of the refractive index of the material of the plastic pipe. This variation is due to the surface characteristics of the plastic pipe, in particular the irregular surface shape. Thereby, the angle of incidence of the terahertz radiation, and thus also the angle of reflection, changes according to the shape of the surface during the rotation of the transceiver 20, and as a result, corresponding variations in the received intensity of the reflected terahertz radiation occur.

[0047] To solve this problem, for example, the maximum value of the radiation intensity received over a full rotation can be determined, and based on this, the refractive index assuming an incident angle of 0° can be calculated. This evaluation assumes that the incident angle when the maximum intensity occurs is substantially 0°.

[0048] To consider the influence of the surface characteristics of the pipe 10, particularly the surface shape of the pipe 10, other procedures are possible with the device according to the present invention. Thus, the transceiver 20 can be pivoted or spiraled, for example, so that terahertz radiation is emitted at different positions on the outer surface 16 of the pipe 10 and / or at different incident angles with respect to the outer surface 16 of the pipe 10. As a result, the maximum value can be determined from the received reflected radiation portion and used as a basis for further evaluation to determine the refractive index. The transceiver 20 can have adjustment means incorporated therein for movement.

[0049] It is also possible to calibrate the measured pipe 10 having a known reflectivity, preferably a reflectivity of 1. To do this, after determining the refractive index according to the present invention on the surface of the pipe 10, a coating having a known reflectivity, preferably a reflectivity substantially equal to 1, such as a foil coating or a spray coating, is applied, and the measurement of the reflected terahertz radiation according to the present invention can be carried out again. If there is a deviation from the reflected radiation intensity expected based on the known reflectivity, this can be taken into account by subsequent calculation to correct the previously determined refractive index.

[0050] As shown in FIGS. 1 and 2, the pipe 10 is at least partially transmissive to the irradiated terahertz radiation, and the irradiated terahertz radiation is thus additionally reflected at the interface of the pipe 10, and these additional reflected radiation portions are also received by the transceiver 20. Therefore, the device shown in FIGS. 1 and 2 can also be used to determine the average refractive index across the cross-section of the pipe 10, as described in, for example, WO2016 / 139155A1 (Patent Document 1) or DE102018128248A1 (Patent Document 2). In this regard, reference is made to the cited documents.

[0051] The temperature can also be determined based on, for example, the average temperature at the outer surface 16 and / or the inner surface 18 of the pipe 10, or across the cross-section of the pipe 10, as described above. For example, within the range where the temperature of the outer surface 16 is measured by a temperature measuring device independent of the material, the absolute value of the temperature at the inner surface 18 of the pipe 10 that is independent of the material can also be confirmed by the method described above.

[0052] Furthermore, the shrinkage of the pipe 10 during its complete cooling and thus complete hardening can be inferred from the comparison between the determined average refractive index and the determined refractive index in the region of the outer surface 16 of the pipe 10.

[0053] Based on the refractive index thus obtained, additional geometric parameters of the pipe 10 can be known, for example, by using the measurement of the propagation time of the radiation portions reflected at different boundary surfaces of the pipe 10. This is related to, for example, the outer diameter shown in the figure by reference numeral 32, or the wall thickness of the pipe 10 shown by reference numerals 34 and 36. The values determined using the measurement values of the propagation time for the optical wall thickness or the optical diameter can be converted into the geometric wall thickness or diameter using the values obtained for the refractive index by a method known per se.

[0054] The device shown in the figure can be a permanent installation designed to rotate, for example, around the pipe 10, or can also be a portable hand-held device.

Description of Reference Numerals

[0055] 10 Object / Pipe 12 Wall 14 Hollow Space 16 Outer Surface 18 Inner Surface 20 Transceiver 21 Biconvex Lens 22 Radiation Optics 23 Cylindrical Lens 25 Optical Axis 26 Reflector 28 wires 30 evaluation device 32 outer diameter 34 wall thickness 36 wall thickness

Claims

1. A method for determining the refractive index in the surface region of a heated strand-shaped or plate-shaped object (10) sent from a production system that has not yet been completely cooled to the ambient temperature, comprising: a) irradiating the surface (16) of the object (10) with terahertz radiation at an incident angle; b) receiving the terahertz radiation reflected from the surface (16) of the object (10); c) determining the refractive index in the surface region of the object (10) from the ratio of the emitted terahertz radiation to the reflected terahertz radiation; d) considering the influence on the surface characteristics of the object (10) in the reflected portion of the terahertz radiation when determining the refractive index. A method, characterized by comprising the above steps.

2. The method according to claim 1, characterized in that, in step d), the influence due to the surface shape of the object (10) is considered.

3. The method according to claim 1 or claim 2, characterized in that, in step d), calibration is performed on the object (10) with a known reflectivity, preferably the object (10) with a reflectivity substantially equal to 1.

4. The method according to claim 3, characterized in that, for the calibration, a coating with a known reflectivity, preferably a coating with a reflectivity substantially equal to 1, is applied to the surface of the object (10).

5. The method according to claim 4, characterized in that the coating is a foil coating or a spray coating.

6. The method according to any one of claims 1 to 5, characterized in that, in step d), the terahertz radiation is irradiated at different positions on the surface (16) of the object (10) and / or at different incident angles with respect to the surface (16) of the object (10), the reflected terahertz radiation is received, and the refractive index is determined from the portion of the reflected terahertz radiation having the maximum intensity.

7. The method according to claim 6, characterized in that the transmitter (20) for the terahertz radiation rotates while transmitting the terahertz radiation around at least one rotation axis and / or rotates around the object (10).

8. The method according to any one of claims 1 to 7, characterized in that the average refractive index across the cross-section of the object (10) is also determined.

9. The method according to claim 8, wherein the average refractive index is also determined from the terahertz radiation irradiated on the object (10) and the terahertz radiation reflected from the object (10).

10. The method according to claim 8 or claim 9, wherein the shrinkage of the object (10) during the process of being completely cooled to the ambient temperature is estimated by using a comparison between the average refractive index and the refractive index in the surface region of the object (10).

11. In step d), the predicted intensity of the terahertz radiation reflected by the object (10) is determined by using the finite element method, considering the radiation characteristics of the transmitter (20) with respect to the terahertz radiation, and considering the distance of the transmitter (20) from the object (10), according to the method of claim 7.

12. The method according to any one of claims 1 to 11, wherein the temperature in the surface region of the object (10) is determined from the refractive index determined in the surface region of the object (10).

13. The object (10) is a pipe (10), and the terahertz radiation reflected from the inner surface (18) of the wall of the pipe (10) is also received, and the refractive index at the inner surface (18) of the wall is determined from the ratio between the irradiated terahertz radiation and the terahertz radiation reflected from the inner surface (18) of the wall, according to the method of any one of claims 1 to 12.

14. The method according to claim 13, wherein the refractive index at the inner surface (18) of the wall is used to determine the temperature at the inner surface (18) of the wall, and / or the temperature difference between the temperature at the inner surface (18) of the wall and the temperature in the surface region of the object (10), and / or the temperature gradient between the temperature at the inner surface (18) of the wall and the temperature in the surface region of the object (10).

15. The method according to any one of claims 1 to 14, wherein the object (10) is a strand-shaped or plate-shaped plastic object (10) sent from an extrusion system.

16. The method according to any one of claims 1 to 15, wherein the terahertz radiation is pulsed terahertz radiation or FMCW terahertz radiation.

17. An apparatus for determining the refractive index in the surface region of a heated strand-shaped or plate-shaped object (10) sent from a production system that has not yet been completely cooled to the ambient temperature, a transmitter (20) for transmitting terahertz radiation at an incident angle with respect to the surface (16) of the object (10); a receiver (20) for receiving the terahertz radiation reflected from the surface (16) of the object (10); an evaluation device (30) designed to determine the refractive index in the surface region of the object (10) from the ratio of the irradiated terahertz radiation and the reflected terahertz radiation; comprising: the evaluation device (30) is designed to also consider the influence of the surface characteristics of the object (10) on the portion of the reflected terahertz radiation when determining the refractive index, characterized by the apparatus.

18. The apparatus according to claim 17, further comprising an adjustment device for adjusting the transmitter (20) for irradiating the terahertz radiation to different positions on the surface of the object and / or at different incident angles with respect to the surface of the object, the receiver being designed to receive the reflected terahertz radiation, and the evaluation device (30) being designed to determine the refractive index from the portion of the reflected terahertz radiation having the maximum intensity.

19. The apparatus according to claim 18, wherein the adjustment device comprises a device for rotating the transmitter (20) around at least one rotation axis and / or a device for rotating the transmitter (20) around the object (10) while transmitting the terahertz radiation.

20. The object (10) is a pipe (10), the receiver (20) is designed to also receive the terahertz radiation reflected by the inner surface (18) of the wall of the pipe (10), and the evaluation device (30) is designed to determine the refractive index on the inner surface (18) of the wall using the portion of the reflected terahertz radiation and a known incident angle, characterized by the apparatus according to any one of claims 17 to 19.

21. To measure the temperature in the surface area of the object (10), a temperature measuring device independent of the material, preferably a pyrometer, is also provided, and the evaluation device (30) is designed to determine the temperature of the inner surface (18) of the wall from the measured temperature and the temperature difference determined between the temperature of the inner surface (18) of the wall and the temperature in the surface area of the object (10). The device according to any one of claims 17 to 20, characterized in that.

22. The device according to any one of claims 17 to 21, characterized in that the device is a hand-held device.

23. The device according to any one of claims 17 to 22, characterized in that the terahertz radiation is pulsed terahertz radiation or FMCW terahertz radiation.

24. The device according to any one of claims 17 to 23, characterized in that it is designed to execute the method according to any one of claims 1 to 16.

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