Method and device for controlling a production plant for sheet-shaped or strand-shaped bodies

Description

[0001] The invention relates to a method for controlling a production plant for plate-shaped or strand-shaped bodies, in which the body is conveyed along a conveying direction through a measuring area, in which the body is irradiated in the measuring area by means of measuring radiation in the gigahertz or terahertz frequency range, wherein the measuring radiation penetrates at least partially into the body, and in which measuring radiation reflected by the body is detected and the refractive index of the body is determined on the basis of the detected measuring radiation.

[0002] The invention further relates to a device for controlling a production plant for plate-shaped or strand-shaped bodies, comprising a conveyor device for conveying the body along a conveying direction through a measuring region of the device, a transmitter device for irradiating the body in the measuring region with measuring radiation in the gigahertz or terahertz frequency range, wherein the measuring radiation penetrates at least partially into the body, a detection device for detecting the measuring radiation reflected by the body, an evaluation device which is designed to determine the refractive index of the body on the basis of the measuring radiation detected by the detection device.

[0003] For example, DE 10 2016 103 298 A1 discloses a terahertz measuring device and a terahertz measuring method for determining at least one layer thickness of a test object using time-of-flight measurement. WO 2016 / 139155 A1 further discloses a device and a method for measuring the diameter and / or wall thickness of a strand, even when the refractive index is unknown. This allows for a precise determination of the diameter and wall thickness of pipes, for example, even when the refractive index is unknown or not reliably known.

[0004] DE 20 2018 006 144 U1 discloses a device for measuring a tubular strand emerging from an extrusion device, in which the diameter and / or wall thickness and / or shape deviations of the tubular strand can be determined, particularly using terahertz radiation. Based on the determined values for the diameter and / or wall thickness and / or shape deviations, the extrusion device can be controlled and / or regulated. The refractive index of the strand material can also be determined to determine the diameter or wall thickness or shape deviations.

[0005] Furthermore, DE 10 2015 110 600 B3 discloses a method and a device for determining a layer property of an extruded product produced in an extrusion process by irradiating it with terahertz radiation and measuring at least one feed rate or feed quantity of the feed material fed into an extruder. DE 10 2015 110 600 B3 addresses the problem that, in foamed layers of the extruded product, the refractive index, which depends on the material density, is unknown. For non-foamed layers, however, the refractive index is assumed to be known according to DE 10 2015 110 600 B3. To determine the refractive index of foamed layers, DE 10 2015 110 600 B3 proposes, in addition to measuring the extruded product with terahertz radiation, using data or measurement signals relating to the quantity of material fed into the extrusion process. This amount of material is obtained gravimetrically or volumetrically according to DE 10 2015 110 600 B3.The refractive index of the foamed layer determined in this way is intended to provide information about the degree of foaming of the foamed layer. Based on the determined refractive index, the extruder feed rate can be regulated in order to achieve a desired degree of foaming. According to DE 10 2015 110 600 B3, the weight of the material fed to the extruder as granules is measured using a weighing device. However, the volume can only be determined if the specific gravity of the granules is known and constant. Both of these are often not the case in practice. Furthermore, according to DE 10 2015 110 600 B3, the material input to the extruder is measured and, on this basis, a prediction is made about the refractive index of a section of the extruded product irradiated by terahertz radiation.However, making a statement about the refractive index of a specific section of the extruded product from the extruder input requires that neither the rotational speed nor the haul-off speed of the extruder, nor the degree of shrinkage change due to a change in temperature during extrusion. These conditions are not reliably met in practice, so the known method is subject to a corresponding inaccuracy.

[0006] While according to DE 10 2015 110 600 B3 the refractive index of a foamed layer can be determined and used to regulate the feed rate to achieve a desired degree of foaming, DE 10 2015 110 600 B3 assumes a known refractive index for non-foamed layers. In practice, however, the refractive index of non-foamed layers also changes for various reasons. Such changes are not detected according to DE 10 2015 110 600 B3. Furthermore, since the refractive index according to DE 10 2015 110 600 B3 is determined indirectly volumetrically or gravimetrically based on the material fed to the extruder for extrusion, assignment to a specific section of the body extruded from the material is difficult, as explained. Accordingly, the control or regulation of the extrusion system provided for in DE 10 2015 110 600 B3 is also inaccurate.

[0007] In addition, there is a need to obtain more information about the production process in order to achieve more targeted and precise control of the production plant.

[0008] US 2009 / 0045536 A1 discloses a device and method for measuring the thickness distribution of an extruded strip stretched transversely to a conveying direction using, for example, infrared radiation. The thickness of the strip can be determined based on the absorption of the infrared radiation. In the event of undesirable thickness deviations transversely to the conveying direction of the strip, an extruder can be controlled for correction.

[0009] WO 2013 / 007250 A1 discloses a method and a device for producing an extruded extruded profile from multiple blend components. In this process, the coordinates of the boundary layers of at least individual blend components of the extruded profile are recorded using a measuring method that penetrates the extruded profile. Electromagnetic radiation can be used. Based on the recorded measured values, for example, the extruder speed can be controlled or the tool contour can be adjusted, for example, to change the contour of the extrusion tools.

[0010] From WIETZKE S ET AL: "Terahertz spectroscopy: A powerful tool for the characterization of plastic materials," terahertz spectroscopy is a technique known for the non-destructive and non-contact analysis of macromolecules for monitoring plastic processing. Terahertz radiation can be used to measure, for example, filler content or water content.

[0011] J. HAUCK ET AL: "Terahertz inline wall thickness monitoring system for plastic pipe extrusion" also describes a method for inline measurement of the wall thickness of extruded plastic pipes using terahertz radiation. This should also enable the measurement of multi-layer systems or foamed pipes.

[0012] Based on the explained prior art, the invention is based on the object of improving the control of a production plant for plate-shaped or strand-shaped bodies.

[0013] The invention solves the problem by the features of independent claims 1 and 10. Advantageous embodiments can be found in the dependent claims, the description and the figures.

[0014] For a method of the type mentioned at the outset, the invention achieves the object in that at least one production parameter of the production plant is controlled on the basis of the refractive index determination, wherein the refractive index is determined at several points in time during the conveyance of the body through the measuring area and the at least one production parameter is controlled on the basis of a temporal change in the refractive index and / or wherein measuring radiation is radiated onto different locations of the body, wherein the refractive index is determined at the different locations of the body and the at least one production parameter is controlled on the basis of a spatial change in the refractive index.

[0015] For a device of the type mentioned at the outset, the invention achieves the object in that a control device is provided which is designed to control at least one production parameter of the production plant on the basis of the refractive index determination, wherein the evaluation device is designed to determine the refractive index at several points in time during the conveyance of the body through the measuring area, wherein the control device is designed to control the at least one production parameter on the basis of a temporal change in the refractive index and / or wherein the transmitting device is designed to radiate measuring radiation onto different locations on the body, wherein the evaluation device is designed to determine the refractive index at the different locations on the body, and wherein the control device is designed to control the at least one production parameter on the basis of a spatial change in the refractive index.

[0016] The strand-shaped or plate-shaped bodies produced in the production plant can, for example, be bodies made of plastic or glass. The body can, in particular, be a non-foamed body, i.e. one that does not have a foamed portion, for example a foamed layer. The strand-shaped body can, for example, be a tubular body, for example a plastic or glass tube. The plate-shaped body can, for example, be a plastic or glass plate. The body produced in the production plant can already have (essentially) completely received its final shape at the time of the measurement according to the invention. However, the shaping can also not yet be completed at the time of the measurement. The body can still have very high temperatures of, for example, over 2000°C at the time of the measurement, especially if it is a glass body.The body is conveyed, particularly longitudinally, through the measuring area and irradiated there with gigahertz or terahertz measuring radiation. Difficult measuring conditions prevail in production plants of the type concerned here. This is particularly true when measuring the body early during its final shaping or when its shaping has just been completed. This is generally desirable in order to be able to react early to any unacceptable deviations in the production plant and to avoid unnecessary scrap. However, in this measuring area there is a high risk of contamination from the production process. In addition, cooling fluid such as cooling water is often applied to the body or components of the production plant to cool it. This leads to splashing water and the development of steam.Optical measurement methods, such as laser light, generally have problems in such measurement environments. Such problems can be avoided by the inventive use of gigahertz or terahertz measurement radiation, since such measurement radiation is generally largely insensitive to difficult measurement environments of the type described.

[0017] The measuring radiation is emitted by a transmitting device and directed onto the body to be measured. The measuring radiation penetrates the body at least partially, preferably completely. In particular, the measuring radiation can radiate completely through the body. The measuring radiation is reflected at boundary surfaces of the body, and the reflected measuring radiation is received by a receiving device. The transmitting device and the receiving device can be combined in a particularly practical manner into a transceiver. Of course, multiple transmitting devices and multiple receiving devices can also be provided, which irradiate the body, for example, from different directions and receive the reflected measuring radiation. If multiple transmitting devices and receiving devices are provided, these can be combined in pairs to form a transceiver, in a particularly practical manner.

[0018] Based on the detection of the reflected measuring radiation, the refractive index of the body material and the absorption of the measuring radiation by the body can be determined. While DE 10 2015 110 600 B3 assumes a known refractive index for non-foamed layers, the invention takes into account that the refractive index, especially of non-foamed materials, can change in practice for various reasons. For example, additives are added to the extrusion material for extrusion products, such as plastic pipes, for various reasons, for example to reduce the conductivity of the material, as sun protection, or similar. Users of extrusion systems sometimes use premixed material mixtures in which the additives are already added by the manufacturer. In some cases, however, users also produce the material mixtures themselves by adding additives to the base material.Especially in the latter case, undesirable variations in the amount of additive added can occur. If the proportion of an additive added to the extrusion material changes, this can be quickly and reliably detected using the refractive index measurement and remedied by appropriate control intervention in the production system.

[0019] According to the invention, the refractive index or absorption is determined in particular directly based on the measurement radiation reflected by the body, in particular exclusively based on the measurement radiation reflected by the body. According to the invention, gravimetric or volumetric measurements are not required to determine the refractive index or absorption, for example, of a material extruded in an extrusion device, as is still provided for in DE 10 2015 110 600 B3. Thus, according to the invention, the determined refractive index is assigned to a specific section of the body more reliably, thus enabling more precise control.

[0020] According to the invention, at least one production parameter of the production plant is controlled based on the refractive index determination. The invention is based on the surprising discovery that the refractive index and / or the absorption coefficient of the measured body, in particular a temporal or spatial change in these values, provide information about the production process, on the basis of which the production process can be controlled. In practice, irradiation with gigahertz or terahertz radiation is often carried out anyway in order to determine geometric parameters of the body, such as surface contour, diameter, thickness, or wall thickness(es). Accordingly, according to the invention (by the evaluation device) at least one geometric parameter of the body can also be determined, such as its surface contour, diameter, thickness, or wall thickness(es).As explained, the refractive index can also be determined anyway for a precise determination of the geometric parameters. According to the invention, the gigahertz or terahertz radiation and, if necessary, the determination of the refractive index and / or absorption are now further used to draw conclusions about the process and control the process accordingly, whereby this control can, in particular, be carried out automatically. In this way, the invention enables a simple and reliable improvement of the production process.

[0021] The invention is based in particular on the finding that a temporal or spatial change in the refractive index and / or absorption is an important parameter for the control or regulation of the production plant. For this purpose, the refractive index is determined at several points in time and / or for several locations on the body, in particular distributed over the circumference of the body. From a change in the determined temporally or spatially distributed data for the refractive index, undesirable changes in the production process are inferred. The production plant is controlled on this basis.

[0022] The refractive index or absorption can be determined, for example, at regular intervals while the object is conveyed through the measuring area. This allows a trend in this data to be identified. This can be used to derive necessary control interventions in the production facility. For example, a decreasing or increasing value of the refractive index or absorption over time indicates undesirable changes in the production process.

[0023] In the case of a strand-shaped body, the measuring radiation can in particular be radiated onto different locations distributed over the circumference of the body. In the aforementioned embodiment, a plurality of transmitting devices and receiving devices, for example a plurality of transceivers, can be provided which are arranged such that they direct the measuring radiation to different locations on the body and receive the respective reflected measuring radiation. For example, a plurality of transmitting devices and receiving devices, for example a plurality of transceivers, can be arranged distributed over the circumference of a strand-shaped body. However, it would also be conceivable for at least one transmitting device and at least one receiving device, for example at least one transceiver, to be arranged in a spatially variable manner, for example so as to be rotatable around a strand-shaped body. The aforementioned embodiment enables the detection of a spatial distribution of the refractive index orof absorption. This, in turn, can be used to determine the necessary control interventions in the production facility. For example, a systematic change in the values of refractive index or absorption with location indicates a faulty production process. For example, if extruded plastic material flows undesirably, the refractive index or absorption may be different on the underside of the strand-like body than on its top. This can be detected and taken into account when controlling the production process.

[0024] The absorption can be determined, for example, by comparing the intensity of the measuring radiation emitted by the transmitting device with the intensity of the measuring radiation received after reflection, for example at a rear interface of the body facing away from the transmitting and receiving devices. The refractive index can be determined, for example, as explained in WO 2016 / 139155 A1. In this case, for example, the travel time of the measuring radiation emitted by the transmitting device through the measuring area with the body arranged in the measuring area can be compared with the travel time of the measuring radiation through the measuring area without the body arranged therein. The refractive index of the material can then be determined mathematically from the change in travel time, as will be explained in more detail below. For this purpose, a transmitting device and a receiving device can be arranged, for example, on opposite sides of the measuring area.However, it would also be possible, for example, to arrange a transmitting device and a receiving device on one side of the measuring area and to arrange a reflector on an opposite side of the measuring area.

[0025] As already mentioned, the refractive index can be determined by comparing the travel time of the measuring radiation emitted by the transmitting device through the measuring area with a body arranged in the measuring area with the travel time of the measuring radiation through the measuring area without a body arranged therein. Particularly if the body is tubular, the travel time of the measuring radiation emitted by the transmitting device through a first wall section facing the transmitting device and through a second wall section facing away from the transmitting device can also be taken into account for determining the refractive index.

[0026] As explained in WO 2016 / 139155 A1, for example, in tubular bodies the wall thickness W d1 one of the at least one transmitting device facing wall sections of the body or the wall thickness W d2 of the wall section of the body facing away from the at least one transmitting device can be determined according to the following formulas: W d 1 = 1 2 Δ T wd 1 − Δ T R ⋅ Δ T wd 1 Δ T wd 1 + Δ T wd 2 c W d 2 = 1 2 Δ T wd 2 − Δ T R ⋅ Δ T wd 2 Δ T wd 2 + Δ T wd 1 c with: Δ T wd 1 Time difference between the measuring radiation reflected at the outer boundary surface facing the at least one transmitting device and at the inner boundary surface of the wall section of the body facing the at least one transmitting device, Δ T wd2 Time difference between the measuring radiation reflected at the inner boundary surface facing the at least one transmitting device and at the outer boundary surface of the wall section of the body facing away from the at least one transmitting device, Δ TR Change in the transit time of the measuring radiation emitted by the at least one transmitting device and received by the at least one receiving device after passing through the body, caused by the material of the body guided through the device, cPropagation speed of the measuring radiation in air

[0027] For example, the above equation for W d1 can be changed to: W d 1 = 1 2 Δ T wd 1 ⋅ c 1 − Δ T R Δ T wd 1 + Δ T wd 2

[0028] Furthermore, the following applies: W d 1 = 1 2 Δ T wd 1 ⋅ c K with: c K Propagation speed of the measuring radiation in the body

[0029] Therefore, the following still applies: c K = c 1 − Δ T R Δ T wd 1 + Δ T wd 2

[0030] This means that the refractive index n of the tubular body is: n = c c K = 1 1 − Δ T R Δ T wd 1 + Δ T wd 2

[0031] The refractive index of the tubular body can thus be determined mathematically, taking into account the change in propagation time caused by the body as well as the propagation times of the measuring radiation through the first and second wall sections. Volumetric or gravimetric measurements are not required.

[0032] According to a further embodiment, a data trend can be created based on the refractive index and / or absorption values determined at several points in time during the conveyance of the body through the measuring area. The production system can then be controlled based on a detected temporal change in the data trend, for example, a decrease or increase in the data trend over a certain period of time. For this purpose, the data trend can be derived over time. If the calculated value of the derivative exceeds or falls below a specified target value, control intervention can then take place in the production system.

[0033] According to a further embodiment, a spatial value distribution can be created based on the values for refractive index and / or absorption determined at various locations on the body. The production system can then be controlled based on a detected spatial change in the value distribution. As already explained, the refractive index and / or absorption can be determined, in particular at multiple locations distributed over the circumference of a tubular body, for example. In this way, a spatial distribution of the determined values of refractive index and / or absorption over the circumference of the tubular body can be determined. If a particularly systematic change occurs here, for example significantly higher values of refractive index and / or absorption on the underside of the body compared to the top, it can be concluded that there is undesirable downward flow of the material, i.e. so-called sagging.This can then be counteracted by appropriate control interventions in the production process. For example, a spatial derivation of the value distribution can be performed. If the calculated value of the derivation exceeds or falls below a specified target value, control intervention in the production system can then be initiated.

[0034] According to a further embodiment, the body can consist of a plastic material, wherein the production plant comprises an extrusion device for extruding the plastic material, wherein at least one production parameter of the extrusion device is controlled based on the refractive index determination and / or the absorption determination. According to a further related embodiment, an output performance of the extrusion device can be controlled as a production parameter. Alternatively or additionally, it is also possible for a mixing ratio of at least two materials to be extruded fed to the extrusion device to be controlled as a production parameter. In extrusion devices, two materials can be mixed to form a mixture to be extruded. This often involves admixtures in a main plastic material.For example, graphite or glass fibers can be mixed into a carrier plastic such as polypropylene (PP) or polyethylene (PE). Such admixtures are often present in only small amounts, for example less than 1 wt.%. This makes adjusting the mixing ratio difficult and reliably detecting an incorrect mixing ratio is often not possible. Ready-made mixtures obtained from different manufacturers, for example, also vary considerably in terms of their mixing ratio. Depending on the material, such admixtures have a significant influence, for example, on the refractive index. This is particularly true for materials whose refractive index deviates significantly from the refractive index of the carrier material. This is the case, for example, with graphite or glass fiber admixtures in carrier plastics such as PP or PE. The aforementioned design takes advantage of this.It was surprisingly found that, based on the determination of the refractive index or absorption according to the invention, an inadmissible deviation of the mixing ratio from a predetermined mixing ratio can be reliably detected even with small admixture proportions and that the mixing process in the extrusion device can be intervened accordingly.

[0035] As already explained, additives added to a base material, particularly in an extrusion line, can have a significant impact on the refractive index or absorption of the material. According to a further embodiment, the determined refractive index and / or absorption values can be used to determine the proportion of an additive added to the material used to produce the body, and the production line can be controlled based on the determined proportion of the additive. This ensures that the desired mixing ratio between base material and additive is maintained at all times.

[0036] According to a further embodiment, the at least one production parameter of the production plant can be controlled in a closed control loop based on the refractive index determination and / or the absorption determination. This is then a so-called closed-loop control. The control device therefore forms a control device. In particular, fully automatic control can take place in which no manual intervention is required. The control device can, for example, receive a determined value for the refractive index and / or the absorption as a controlled variable. This is compared with a target value for the refractive index and / or the absorption as a reference variable. If the comparison reveals a control deviation, the control device can control production parameters, for example a mixing ratio of an extrusion device, until the control deviation is back within a permissible range.All of the configurations for controlling the production plant mentioned in this context can also be used to regulate the production plant.

[0037] The method according to the invention can be carried out using a device according to the invention. Accordingly, the device according to the invention can be designed to carry out the method according to the invention.

[0038] The invention also relates to a production system for plate-shaped or strand-shaped bodies, comprising a device according to the invention for controlling the production system, and comprising a conveyor device for conveying the body along a conveying direction through the measuring area of the device according to the invention. The device according to the invention or the production system according to the invention can also comprise the plate-shaped or strand-shaped body.

[0039] An embodiment of the invention is explained in more detail below with reference to the figures. They show schematically: Figure 1 shows a diagram in which a refractive index of a tubular body determined using a device or method according to the invention is plotted over time, Figure 2 shows a diagram in which a refractive index determined using a device or method according to the invention is plotted over an angle of rotation around the tubular body, Figure 3 shows a representation of a device according to the invention with the tubular body shown in cross section.

[0040] Unless otherwise stated, the same reference symbols in the figures refer to the same objects.

[0041] In the diagram of the Figure 1A temporal progression of the refractive index determined according to the invention is shown for a body measured in a production facility using a device according to the invention or the method according to the invention, for example, a tubular body. In the diagram, the refractive index n is plotted against time t. In the example shown, the refractive index n decreases over time.

[0042] In Figure 2 The diagram shows the spatial distribution of the refractive index, determined using a device according to the invention or the method according to the invention in a production plant, in particular for a tubular body. In particular, the diagram shows the Figure 2The refractive index was determined at various locations around the circumference of the tubular body. For this purpose, a combined transmitting and receiving device, acting as a transceiver, was rotated around the circumference of the tubular body, with measuring radiation being emitted onto the tubular body and the measuring radiation reflected by it being measured by the receiving device. In the diagram of the Figure 2 The refractive index n is plotted against the rotation angle ω of the transmitting and receiving device. It can be seen that the refractive index initially passes through a minimum in an angular range between 0° and 180° and then approaches its original value again.

[0043] In Figure 3 An example of a device according to the invention is shown, with which the values according to the diagrams according to the Figures 1 and 2can be determined. In the example shown, the device comprises a transceiver 10, comprising a transmitting device and a receiving device for gigahertz or terahertz radiation. The measuring radiation in the gigahertz or terahertz frequency range is emitted by the transceiver 10 onto a tubular body 12 conveyed along its longitudinal direction through a measuring region of the device, as indicated by the arrow 14 in Figure 3 The measuring radiation penetrates the tubular body 12 and is reflected at various interfaces of the tubular body 12, as illustrated by arrows 14, 16, 18 and 20. A certain portion of the radiation exits the tubular body 12, as indicated by arrow 22 in Figure 3This radiation component is reflected in the example shown by a reflector 34, so that this radiation component returns to the transceiver 10. The measurement radiation reflected at the interfaces is also received by the transceiver 10. The measurement data from the transceiver 10 are transferred to an evaluation device 24, as shown in Figure 3 by the dashed arrow 26. The evaluation device 24 can, for example, determine the refractive index of the material of the tubular body 12 in the manner explained above. This refractive index determination can be repeated at regular intervals during the conveyance of the tubular body 12 through the measuring area of the device, for example over a predetermined period of time, from which a diagram as in Figure 1It would also be conceivable, for example, to rotate the transceiver 10 (and the reflector 34) around the tubular body 12, to transmit measuring radiation distributed over the circumference of the tubular body 12 to different locations during the rotation and to receive the reflected measuring radiation in each case and to determine therefrom a spatial distribution of the refractive index, as shown in the diagram of the Figure 2 is shown. In particular, if the refractive index is determined as described above, the measured values are repeated with an angular period of 180°.

[0044] The values for the refractive index determined by the evaluation device 24 can be fed to a control device 28 in the example shown, as in Figure 3 illustrated by the dashed arrow 30. The control device 28 can control at least one production parameter of the Figure 3extremely schematically shown at the reference numeral 32 production plant, as in Figure 3 illustrated by the dashed arrow 34. The at least one production parameter can be, for example, a mixing ratio of two materials fed to an extrusion device of the production plant. List of reference symbols

[0045] nRefractive index tTime ωAngle of rotation 10Transceiver 12Tubular body 14Arrow 16Arrow 18Arrow 20Arrow 22Arrow 24Evaluation device 26Dashed arrow 28Control device 30Dashed arrow 32Production plant 34Dashed arrow 36Reflector

Claims

1. Method for controlling a production plant (32) for plate-shaped or strand-shaped bodies (12), in which the body (12) is conveyed along a direction of flow through a measurement area, in which the body (12) is exposed in the measurement area to measurement radiation in the gigahertz or terahertz frequency range, wherein the measurement radiation penetrates at least partially into the body (12), and in which measurement radiation reflected by the body (12) is detected, characterized in that the refractive index (n) of the body (12) is determined based on the detected measurement radiation without gravimetric or volumetric measurements, in that at least one production parameter of the production plant (32) is controlled based on the refractive index determination, wherein the refractive index (n) is determined at several points in time while the body (12) is conveyed through the measurement area and the at least one production parameter is controlled based on a change of the refractive index (n) over time and / or wherein different locations of the body (12) are exposed to measurement radiation, wherein the refractive index (n) is determined at the different locations of the body (12) and the at least one production parameter is controlled based on a spatial change of the refractive index (n).

2. Method according to claim 1, characterized in that the refractive index is determined from a comparison of the runtime of the measurement radiation emitted by the transmitting device through the measurement area with a body arranged in the measurement area with the runtime of the measurement radiation through the measurement area without a body arranged therein.

3. Method according to claim 2, characterized in that the body (12) is a tubular body (12), and in that, for determining the refractive index, the runtime of the measurement radiation emitted by the transmitting device through a first wall section facing the transmitting device and through a second wall section facing away from the transmitting device is furthermore taken into account.

4. Method according to one of the preceding claims, characterized in that the absorption of the measurement radiation by the body (12) is also determined based on the detected measurement radiation, characterized in that at least one production parameter of the production plant (32) is controlled based on the absorption determination, wherein the absorption is determined at several points in time while the body (12) is conveyed through the measurement area and the at least one production parameter is controlled based on a temporal change of the absorption and / or wherein different locations of the body (12) are exposed to measurement radiation, wherein the absorption is determined at the different locations of the body (12) and the at least one production parameter is controlled based on a spatial change of the absorption.

5. Method according to one of the preceding claims, characterized in that a data trend is created based on the values of the refractive index and / or absorption determined at several points in time while conveying the body (12) through the measurement area, and in that the production plant is controlled based on a detected change over time in the data trend, and / or in that a spatial value distribution is created based on the values for the refractive index and / or absorption determined at the different locations of the body (12), and in that the production plant is controlled based on a detected spatial change of the value distribution.

6. Method according to one of the preceding claims, characterized in that the body (12) is made of a plastic material, wherein the production plant (32) comprises an extrusion device for extruding the plastic material, and in that at least one production parameter of the extrusion device is controlled based on the refractive index determination and / or the absorption determination.

7. Method according to claim 6, characterized in that a delivery rate of the extrusion device is controlled as a production parameter and / or in that a mixing ratio of at least two materials fed to the extrusion device and to be extruded is controlled as a production parameter.

8. Method according to one of the preceding claims, characterized in that a proportion of an additive added to the material used for the production of the body (12) is determined based on the determined values of the refractive index and / or absorption, and in that the production plant is controlled based on the determined proportion of the additive.

9. Method according to one of the preceding claims, characterized in that the at least one production parameter of the production plant (32) is controlled in a closed-loop control system based on the refractive index determination and / or the absorption determination.

10. Device for controlling a production plant (32) for plate-shaped or strand-shaped bodies (12), comprising a conveyor for conveying the body (12) along a direction of flow through a measurement area of the device, a transmitting device for emitting measurement radiation in the gigahertz or terahertz frequency range onto the body (12) in the measurement area, wherein the measurement radiation penetrates at least partially into the body (12), a detection device for detecting the measurement radiation reflected by the body (12) and an evaluation device (24), characterized in that the evaluation device (24) is adapted to determine the refractive index (n) of the body (12) based on the measurement radiation detected by the detection device without gravimetric or volumetric measurements, in that a control device is provided which is adapted to control at least one production parameter of the production plant (32) based on the refractive index determination, wherein the evaluation device (24) is adapted to determine the refractive index (n) at several points in time while the body (12) is conveyed through the measurement area, wherein the control device is adapted to control the at least one production parameter based on a change of the refractive index (n) over time and / or wherein the transmitting device is adapted to emit measurement radiation onto different locations of the body (12), wherein the evaluation device (24) is adapted to determine the refractive index (n) at the different locations of the body (12), and wherein the control device is adapted to control the at least one production parameter based on a spatial change in the refractive index (n).

11. Device according to claim 10, characterized in that the evaluation device (24) is adapted to determine the refractive index from a comparison of the runtime of the measurement radiation emitted by the transmitting device through the measurement area with a body arranged in the measurement area with the runtime of the measurement radiation through the measurement area without a body arranged therein.

12. Device according to claim 11, characterized in that the body (12) is a tubular body (12), and in that, for determining the refractive index, the evaluation device (24) is adapted to furthermore take into account the runtime of the measurement radiation emitted by the transmitting device through a first wall section facing the transmitting device and through a second wall section facing away from the transmitting device.

13. Device according to one of claims 10 to 12, characterized in that the evaluation device (24) is adapted to further determine the absorption of the measurement radiation by the body (12) based on the measurement radiation detected by the detection device, wherein the control device is adapted to control at least one production parameter of the production plant (32) based on the absorption determination, wherein the evaluation device (24) is adapted to determine the absorption at several points in time while the body (12) is conveyed through the measurement area, wherein the control device is adapted to control the at least one production parameter based on a temporal change of the absorption and / or wherein the transmitting device is adapted to emit measurement radiation onto different locations of the body (12), wherein the evaluation device (24) is adapted to determine the absorption at the different locations of the body (12), and wherein the control device is adapted to control the at least one production parameter based on a spatial change of the absorption14. Device according to one of claims 10 to 13, characterized in that the evaluation device (24) is adapted to create a data trend based on the values for the refractive index and / or absorption determined at several of points in time while the body (12) is conveyed through the measurement area, and in that the control device is adapted to control the production plant based on a detected change over time of the data trend and / or that the evaluation device (24) is adapted to create a spatial value distribution based on the values for the refractive index and / or absorption determined at the different locations of the body (12), and in that the control device is adapted to control the production plant based on a detected spatial change of the value distribution.

15. The device according to any one of claims 10 to 14, characterized in that the body (12) is made of a plastic material, wherein the production plant (32) comprises an extrusion device for extruding the plastic material, and in that the control device is adapted to control at least one production parameter of the extrusion device based on the refractive index determination and / or the absorption determination.

16. Device according to claim 15, characterized in that at least one production parameter is a delivery rate of the extrusion device and / or in that at least one production parameter is a mixing ratio of at least two materials fed to the extrusion device and to be extruded.

17. Device according to any one of claims 10 to 16, characterized in that the control device forms a control device (28) which is adapted to control the at least one production parameter of the production plant (32) based on the refractive index determination and / or the absorption determination in a closed-loop control.