Device and method for determining wall thickness of object

Abstract

The invention relates to a device for determining the wall thickness of an object, in particular a flat or elongated object, comprising a first measuring device comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation, and comprising a second measuring device comprising a second transmitter for terahertz radiation and a second receiver for terahertz radiation, the second measuring device comprises a second emitter for terahertz radiation and a second receiver for terahertz radiation, the first measuring device being arranged in such a way that the first emitter emits terahertz radiation onto the object and the first receiver receives terahertz radiation emitted by the first emitter and reflected by a boundary surface of the object, the second measuring device is arranged in such a way that the second emitter emits terahertz radiation onto the object and the second receiver receives the terahertz radiation reflected by the boundary surface of the object and emitted by the second emitter. The first emitter emits terahertz radiation in a first frequency range having a first bandwidth around a first center frequency, and the second emitter emits terahertz radiation in a second frequency range having a second bandwidth around a second center frequency different from the first center frequency, and wherein the evaluation device is designed such that the first emitter emits terahertz radiation in the first frequency range, and the second emitter emits terahertz radiation in the second frequency range, a plurality of periodic reference curves approximate to the signal curve of the measured values of the first receiver are determined, and the determined periodic reference curves are compared with the signal curve of the measured values of the second receiver. The evaluation device is further designed to determine the wall thickness of the object on the basis of the determined periodic reference curves and to identify a periodic reference curve that optimally conforms to the signal curve of the measured values of the second receiver from the determined periodic reference curves. The invention further relates to a corresponding method.

Description

Technical Field

[0001] The invention relates to a device for determining the wall thickness of an object, in particular a flat or elongated object, the device comprising a first measuring device comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation, and a second measuring device comprising a second transmitter for terahertz radiation and a second receiver for terahertz radiation, the first measuring device being arranged such that the first transmitter emits terahertz radiation onto the object and the first receiver receives the terahertz radiation emitted by the first transmitter and reflected by a boundary surface of the object, and the second measuring device being arranged such that the second transmitter emits terahertz radiation onto the object and the second receiver receives the terahertz radiation emitted by the second transmitter and reflected by a boundary surface of the object, the first transmitter emitting terahertz radiation in a first frequency range having a first bandwidth around a first center frequency, and the second transmitter emitting terahertz radiation in a second frequency range having a second bandwidth around a second center frequency different from the first center frequency, and the device comprising an evaluation device, at which the measured values ​​of the first and second receivers are provided in order to determine the wall thickness of the object.

[0002] The present invention further relates to a method for determining the wall thickness of an object, in particular a flat or elongated object, wherein a first transmitter emits terahertz radiation onto the object, and the terahertz radiation emitted by the first transmitter and reflected by a boundary surface of the object is received by a first receiver, and a second transmitter emits terahertz radiation onto the object, and the terahertz radiation emitted by the second transmitter and reflected by the boundary surface of the object is received by a second receiver, the first transmitter emitting terahertz radiation in a first frequency range having a first bandwidth around a first center frequency, and the second transmitter emitting terahertz radiation in a second frequency range having a second bandwidth around a second center frequency different from the first center frequency. Background Art

[0003] This device or method is used, for example, to measure the wall thickness of plastic pipes produced in extrusion systems. Plate-shaped objects can also be measured using this device and method. These objects can also be made of glass. To unambiguously determine the optical thickness of such thin-walled objects using terahertz radiation, the frequency and / or phase differences of the radiation components, resulting from reflections at the boundary layer defining the wall to be measured, are typically determined. Especially for thin walls, this can be achieved by mixing the two signal components of the radiation reflected at the boundary surface. For simpler evaluation, the low-frequency component of the mixed signal can be separated. This separated signal is modulated with a frequency that is proportional to the difference frequency between the signal components and, therefore, to the wall thickness.

[0004] Determining the frequency is challenging in the face of signal interference, for example due to noise, other reflections, or unknown physical phenomena such as unknown dispersion / scattering of the object. This applies particularly when the signal available for evaluation consists of only one oscillation period of the frequency to be determined, or even only a fraction of one oscillation period, due to the limited bandwidth of the measuring terahertz radiation. Given the material and wall thickness of the object, the bandwidth of the measuring terahertz radiation determines the signal range available for determining the frequency of the signal to be evaluated, and in particular, the signal period available for evaluation. In other words, to unambiguously determine the frequency of the signal to be evaluated, which is proportional to the wall thickness, it is desirable to be able to evaluate at least one complete period of the signal to be evaluated, or even better, multiple periods. Against this backdrop, this can be achieved by increasing the bandwidth of the measuring device. However, this is technically difficult and significantly increases the cost of the measuring device. Furthermore, a larger bandwidth requires the use of a higher center frequency, as various measuring device components, such as lenses, high-frequency conductors, or frequency multipliers, are limited in their relative bandwidths. However, higher frequencies make it more difficult to uniquely assign a frequency to the measured signal curve. This is because the frequencies matching the signal curve may be relatively close to one another. Furthermore, for many materials, the higher frequencies of terahertz radiation lead to increased absorption of the radiation signal, which can additionally make reliable measurement and evaluation difficult or even impossible.

[0005] DE 10 2017 207 648 B4 discloses a method and apparatus for measuring the layer thickness of an object, wherein electromagnetic radiation is irradiated onto the object in at least two measuring steps, each of which is performed by one of a plurality of transmitters. The frequency bands of the individual measuring steps are different sub-regions of a bandwidth. In each measuring step, secondary radiation reflected by the boundary layer of the object is detected and a measurement signal is determined. The measurement signals of the individual measuring steps are combined into an evaluation signal according to the respective frequency bands, in particular by plotting the measurement signals of the individual measuring steps according to the respective frequency ranges onto a common frequency axis. To determine the layer thickness of the object, the fundamental frequency of the evaluation signal is determined, in particular by fitting the measurement signals plotted onto the common frequency axis. In this way, individual measurement steps within sub-regions of the bandwidth should be able to approximate measurements performed over the entire bandwidth. In particular, this should enable accurate measurements using multiple, relatively economical, narrowband transmitters and receivers for electromagnetic radiation.

[0006] However, by combining the measurement signals, each covering a sub-region of the entire bandwidth, as proposed in DE 10 2017 207 648 B4, and fitting the measurement signals of the individual measurement steps plotted on a common frequency axis, each transmitter and receiver used in each measurement step must have high signal quality to eliminate ambiguities during the fitting process. Consequently, the required transmitters and receivers remain relatively expensive. The combination of the individual measurement signals proposed in DE 10 2017 207 648 B4 also requires knowledge of at least the relative amplitudes of the individual signal components. This ideally requires at least one calibration, but can vary in different situations due to varying absorption rates in the layer being examined or due to different orientations of the object relative to the individual transmitters and receivers. If the relative amplitudes are not taken into account or are incorrectly taken into account, modulations may result in the combined signal, which can distort the determination of the fundamental frequency and, therefore, the layer thickness. This presents a particular challenge when the bandwidth sub-regions covered by the individual measurement signals are small. Consequently, transmitters and receivers with larger bandwidths, and thus higher costs, are also required. Particular problems also arise with moving objects, such as those produced in extrusion plants and conveyed through the measurement region in the conveying direction. Due to the merging of the individual measurement signals onto a common frequency axis and the fitting of the measurement signals, the method proposed in DE 10 2017 207 648 B4 is sensitive to deviations between the individual measurements, especially when the measurements are not taken at the same time, at the same position on the object, or from the same direction. Summary of the Invention

[0007] Therefore, starting from this prior art, the object of the present invention is to provide a device and a method of the type described above, with which the wall thickness of, in particular, flat or elongated objects can be clearly determined in a metrologically simple and economical manner even in the case of thin walls, in particular for objects that move during the measurement.

[0008] The invention achieves this object by means of independent claims 1 and 17. Advantageous embodiments are given in the dependent claims, the description and the drawings.

[0009] For the device of the type described above, the present invention achieves the purpose in such a way that the evaluation device is constructed to determine a plurality of periodic reference curves that are close to the signal curve of the measurement value of the first receiver, compare the determined periodic reference curves with the signal curve of the measurement value of the second receiver, and identify the periodic reference curve that best matches the signal curve of the measurement value of the second receiver from the determined periodic reference curves, and the evaluation device is also constructed to determine the wall thickness of the object based on the identified reference curve.

[0010] For the method of the type described above, the present invention achieves the object in such a way that a plurality of periodic reference curves that are approximate to the signal curve of the measurement values ​​of the first receiver are determined, the determined periodic reference curves are compared with the signal curve of the measurement values ​​of the second receiver, and a periodic reference curve that best matches the signal curve of the measurement values ​​of the second receiver is identified from the determined periodic reference curves, and the wall thickness of the object is determined based on the identified reference curve.

[0011] As previously explained, the object to be measured can be made of plastic or glass, for example. The object is at least partially transparent to terahertz radiation, so that the terahertz radiation emitted to the object is reflected at the boundary surfaces of the wall to be measured that defines the object. If it is a flat object, the object can be, for example, plate-shaped. If it is an elongated object, the object can be, for example, strip-shaped. The elongated object can be, in particular, cylindrical, in particular hollow cylindrical. It can be a tubular object, in particular a plastic or glass tube. The object can be produced in an extruder. At the time of the measurement according to the invention, the object may not have completely cooled down, that is, in particular, it may still have parts that are still flowable. The object can be transported through the measuring area of ​​the device by means of a conveying device, in particular, the elongated object can be transported along its longitudinal axis. As already explained, determining the layer thickness is difficult, especially when the object undergoes such movement during the measurement.

[0012] The first transmitter and first receiver for terahertz radiation can be located essentially at the same location. The first transmitter and first receiver can be combined to form a first transceiver. The second transmitter and second receiver for terahertz radiation can also be located essentially at the same location. The second transmitter and second receiver can be combined to form a second transceiver. As already explained, the terahertz radiation emitted by the first or second transmitter at least partially penetrates the object and is reflected off the object's boundary surfaces, in particular, off the surfaces of the walls that define the object to be measured. When referring to terahertz radiation in this document, this refers in particular to electromagnetic radiation in the frequency range of 1 GHz to 6 THz, and in particular to electromagnetic radiation in the frequency range of 50 GHz to 1 THz. This therefore refers to so-called millimeter waves or submillimeter waves. Terahertz radiation is in particular FMCW radiation (Frequency Modulated Continuous Wave radiation).

[0013] The first and second transmitters emit terahertz radiation within frequency ranges surrounding different center frequencies. As will be explained in detail below, the present invention utilizes an evaluation of terahertz radiation emitted at different frequencies to achieve a significantly simplified and unambiguous determination of the wall thickness compared to the prior art. The evaluation device determines a plurality of periodic reference curves that approximate the signal curve received by the first receiver as closely as possible. For example, a plurality of periodic reference curves can be determined using a preferred numerical curve fitting method, each of which corresponds as closely as possible to the signal curve of the measured values ​​of the first receiver. For example, at least two, preferably at least three, and / or no more than ten, preferably no more than five, periodic reference curves can be determined that best match the signal curve of the measured values ​​of the first receiver. For example, exactly three or exactly four periodic reference curves can be determined. As previously explained, in principle, particularly with FMCW terahertz radiation, it can be assumed that the frequency of the signal curve of the measured values ​​of the first receiver is proportional to the wall thickness to be measured. As also explained above, to form the signal curve of the measured values ​​of the first receiver, the frequency differences and / or phase differences of the individual signal components formed by reflection at the boundary surfaces defining the wall whose thickness is to be measured can be determined. As also explained above, the radiation components reflected at the boundary surfaces defining the wall can be mixed. Prior to evaluation, the signal curve can be filtered, for example, using a suitable frequency filter, in particular a low-pass filter, thereby reducing it to low-frequency components. Furthermore, as explained above, the signal curve can be modulated with a frequency that is proportional to the difference frequency of the signal components reflected at the two boundary surfaces of the wall, and thus proportional to the wall thickness.

[0014] To resolve the aforementioned problem of unambiguously determining the frequency of the signal curve, a second measuring device is first used, as in DE 10 207 207 648 B4, which operates with terahertz radiation at a different frequency. However, unlike the proposal in DE 10 207 207 648 B4, the present invention does not combine the measurement signals of the first and second measuring devices. Instead, the present invention simplifies the evaluation by first determining a plurality of periodic reference curves that best match the signal curve of the measured values ​​of the first receiver in a first step. Because identifying the periodic reference curve that best matches the signal curve from the set of determined periodic reference curves is difficult, particularly due to interference in the signal curve, but also due to the limited bandwidth of the terahertz radiation of the first measuring device, in a second step, the measurement signals are not combined, but (only) the determined periodic reference curve is compared with the signal curve of the measured values ​​of the second receiver. This signal curve can also be generated by determining the frequency and phase differences of signal components formed by reflection of the terahertz radiation emitted by the second transmitter on the boundary layer of the wall. The signal components can also be mixed, as explained above. The measured values ​​of the second receiver can also be filtered by a frequency filter, in particular a low-pass filter, and then evaluated by the evaluation device.

[0015] By comparing the possible periodic reference curves determined in the first step and reduced to a smaller number with the signal curve of the measured values ​​of the second receiver, it is now possible to easily identify the periodic reference curve that best matches the signal curve of the measured values ​​of the second receiver from the set of periodic reference curves determined in the first step. In this case, the phase positions of the periodic reference curves are evaluated in particular. Due to the different frequencies of the first and second measuring devices, the phases of the periodic reference curves determined for the signal curve of the first receiver are significantly separated from each other within the frequency range of the second receiver. Thus, based on a comparison, in particular with the phases of the signal curve of the second receiver, it is possible to easily and unambiguously identify the periodic reference curve that ultimately best matches both the first and second receiver signal curves from the periodic reference curves whose phase positions are still relatively close to each other for the signal curve of the first receiver. As already explained, the (fundamental) frequency of the periodic reference curve identified as matching now allows for a simple determination of the wall thickness of the object.

[0016] The comparison of the reference curves with the signal curve of the second receiver is significantly simplified because the number of possible reference curves is already significantly reduced in the first step. Since it is not necessary to select a single reference curve from the determined periodic reference curves in the first step, the first transmitter and the first receiver do not need to transmit or receive with a complex and expensive high bandwidth. Furthermore, in the second step, only a very limited number of periodic reference curves need to be compared with the signal curve of the second receiver. Furthermore, given a corresponding frequency difference between the first and second measuring devices, the phases of these periodic reference curves are significantly separated from each other within the frequency range of the second receiver. This allows the second measuring device to operate with a low bandwidth that would not allow the frequency of the signal curve to be discerned alone. By evaluating the phase position, even when the bandwidth of the second measuring device is particularly narrow, the matching reference curve can be unambiguously / uniquely selected from the previously determined set of reference curves. Consequently, in contrast to the prior art, the second measuring device can be designed to be particularly economical or have a narrow bandwidth, and in particular, more economical or have a narrower bandwidth than the first measuring device. The method according to the invention is also significantly less sensitive to deviations between the measurements of the first and second measuring devices, in particular when the first and second measuring devices do not measure at the same time, at the same position on the object, or from the same direction. Consequently, according to the invention, there are no measurement problems even when measuring objects that are transported through the measurement area.

[0017] Overall, by combining measurements using two measuring devices operating at different frequencies and selecting a matching reference curve from the set of reference curves determined by the first measuring device using the second measuring device, the wall thickness of the object can be unambiguously determined even in the case of thin walls using a relatively simply designed measuring device. The wall to be measured can, for example, have a thickness of less than 10 cm, further less than 5 cm, or further less than 1 cm.

[0018] Of course, it is also conceivable to use multiple second measuring devices. In this case, the periodic reference curve determined in the first step can be compared with the signal curves of multiple second receivers. This can further improve the evaluation, especially if the multiple second measuring devices transmit and receive in frequency ranges around different center frequencies.

[0019] According to one embodiment, the signal curve and the periodic reference curve can be phase curves or frequency curves. As described, radiation components reflected by the boundary surface defining the wall to be measured can be mixed in the signal curve of the first or second receiver. Due to the laws of physics, there is a fixed relationship between the phase and frequency of the signal curve. Therefore, for example, the phase in the phase curve can be used for evaluation to determine the frequency of the signal curve and, ultimately, the wall thickness. This can improve accuracy. The phase curve can be a plot of the phase of the terahertz radiation versus the frequency of the terahertz radiation of the first or second receiver. The frequency curve can be a plot of the frequency of the terahertz radiation of the first or second receiver versus time.

[0020] According to another embodiment, the periodic reference curve can be a harmonic reference curve. Harmonic reference curves are known to be, in particular, cosine-shaped or sine-shaped. Ideally, the signal curve of the first or second receiver, consisting of mixed radiation components reflected at both boundary surfaces of the wall, corresponds to such a harmonic curve. This further simplifies the evaluation according to the present invention.

[0021] As already explained, the periodic reference curve that best matches the signal curve of the second receiver's measured values ​​can be identified by comparing the phase of the determined periodic reference curve with the phase of the signal curve of the second receiver's measured values. The evaluation device of the device according to the present invention is designed accordingly for this purpose. This evaluation of the phase position significantly simplifies the selection of a matching reference curve even for a narrow-bandwidth second measuring device. In particular, different amplitudes of the signal curves of the first and second measuring devices are largely irrelevant when evaluating the phase position.

[0022] According to another embodiment, the second center frequency can be lower than the first center frequency. For example, the second center frequency can be equal to the first center frequency multiplied by a factor in the range of 0.3 to 0.7, preferably by a factor in the range of 0.4 to 0.6. Furthermore, the first and second frequency ranges may not overlap. To achieve particularly simple and unambiguous evaluation, it is primarily desirable to select low center frequencies for the second transmitter and the second receiver so as to keep the density of reference curves matching the signal curve of the second receiver low. In particular, this prevents two adjacent periodic reference curves determined for the signal curve of the first receiver from both matching the signal curve of the second receiver. Furthermore, this avoids high absorption of terahertz radiation. Frequencies close to zero must be avoided, as the phases of all solutions converge at these frequencies and are therefore difficult to distinguish. Furthermore, the terahertz radiation frequency of the second measuring device should be sufficiently far from the terahertz radiation frequency of the first measuring device, as the phase difference in the signal curve of the second receiver is approximately proportional to the distance between the first and second center frequencies. This further simplifies the identification of the periodic reference curve that best matches the signal curve of the second receiver. The frequency differences described above have proven to be particularly advantageous, since the phase differences between adjacent solutions are greatest in these ranges. Based on the considerations described, the solution according to the invention is also significantly superior to direct evaluation using only one transmitter and receiver with a correspondingly higher bandwidth.

[0023] The first center frequency can be, for example, in the range of 100 GHz to 200 GHz, preferably in the range of 120 GHz to 180 GHz. The second center frequency can correspondingly be in the frequency range of approximately 60 GHz to 100 GHz, preferably in the range of 70 GHz to 90 GHz. However, these values ​​are merely exemplary. Depending on the specific application, the center frequency can also have other values.

[0024] According to another embodiment, the second bandwidth can be smaller than the first bandwidth. However, according to the present invention, as described above, the wall thickness of the object can still be determined unambiguously. At the same time, a measuring device with a narrower bandwidth can be implemented particularly simply and economically.

[0025] According to another design, the first bandwidth can be less than one period of the determined periodic reference curve. For example, the first bandwidth can be less than 50 GHz, preferably less than 40 GHz. As described above, the bandwidth of the terahertz radiation of the first (and second) measuring device determines the range available for evaluation of the period of the signal curve and accordingly the range available for evaluation of the period of the periodic reference curve. By the combination according to the invention of the first measuring device with the second measuring device operating at different frequencies, it is not necessary to select the bandwidth of the first measuring device to be so large that the periodic reference curve can already be clearly identified thereby. Thereby, the first measuring device can be implemented more simply and economically, but the wall thickness can still be clearly determined. For example, the first bandwidth can not exceed three quarters of the period of the determined periodic reference curve, preferably not exceed half a period. Thereby, the above-mentioned disadvantages of measuring devices with large bandwidths are also avoided.

[0026] Furthermore, in this context, the second bandwidth can be less than half a period of the determined periodic reference curve. For example, the second bandwidth can be less than 30 GHz, preferably less than 20 GHz. For example, the second bandwidth can be not greater than a quarter of the period of the determined periodic reference curve, preferably not greater than an eighth of a period. Since only the reasonableness of the measurement result of the first measuring device is checked using the second measuring device, the second measuring device does not have to be close to the bandwidth required for clearly identifying the periodic reference curve. The structure of the device according to the invention is simplified and the cost is reduced.

[0027] Here, the length of one period = speed of light / (2 × optical thickness of the layer), where the optical thickness = thickness × refractive index. That is to say, for the bandwidth, correspondingly: bandwidth < N × speed of light / (2 × optical thickness of the layer), where N = 3 / 4, 1 / 2, 1 / 4, 1 / 8 of the corresponding extreme value of the periodic component.

[0028] The device can furthermore include a conveying device, by means of which the object is conveyed through the device during the measurement. For example, an elongate object can be conveyed along its longitudinal axis. As described above, the object can be manufactured, for example, in an extrusion device. During the measurement, the object can accordingly be conveyed through the device by means of the conveying device after the object has left the extrusion device. The device can also include an extrusion device. The device can also include the object.

[0029] According to another embodiment, the first and second transmitters can be arranged such that they emit terahertz radiation toward the object while being offset relative to one another in the conveying direction of the object. In particular, the first and second transmitters can be arranged such that they emit terahertz radiation at substantially the same angle and at substantially the same location on the surface of the object. For optimal measurement and evaluation results, it is ideal for the first measuring device to measure exactly the same section of the object from the same direction as the second measuring device. If, for example, the object is conveyed through the device during measurement, this can be achieved by staggering the first transmitter and receiver, and the second transmitter and receiver, in the conveying direction and activating them in a clocked manner, taking into account the conveying speed. However, the present invention does not rely on the first and second measuring devices measuring exactly the same wall thickness. In particular, when the second transmitter and receiver operate at a lower center frequency, the measurement positions can deviate to a certain extent. This is particularly true when the geometry of the object changes slowly enough in space and / or time, as is often the case with extruded plastic or glass products. A corresponding offset arrangement of the first transmitter and receiver relative to the second transmitter and receiver simplifies the measurement structure and allows an optimization of the corresponding structure for the corresponding frequency range.

[0030] According to another embodiment, the first transmitter and the second transmitter can be formed by a common transmitting device, which transmits terahertz radiation in a first frequency range with a first bandwidth around a first center frequency and in a second frequency range with a second bandwidth around a second center frequency different from the first center frequency, and / or the first receiver and the second receiver can be formed by a common receiving device, which receives terahertz radiation (40) in a first frequency range with a first bandwidth around the first center frequency and in a second frequency range with a second bandwidth around a second center frequency different from the first center frequency and in a second frequency range with a second bandwidth. Thus, in this embodiment, only one transmitter or only one receiver is provided, which can transmit or receive terahertz radiation in different bandwidths and thus constitutes both the first transmitter or receiver of the first measuring device and the second transmitter or receiver of the second measuring device. Here, terahertz radiation can also be transmitted and received simultaneously in different frequency bands in the form of a frequency diversity method. This allows for a particularly compact measurement system, as at least some components can be used for both frequencies, and the common device can be smaller than two dedicated devices. This can also be advantageous for handheld devices, for example. This can also reduce costs if necessary. Another advantage is that measurements can be performed at the same location, from the same direction, and virtually simultaneously.

[0031] The method according to the invention can be implemented using the device according to the invention. Accordingly, the device according to the invention can also be configured to implement the method according to the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] An embodiment of the present invention is described in detail below with reference to the accompanying drawings.

[0033] Figure 1 The device according to the invention is shown in a schematic side view,

[0034] Figure 2 Show Figure 1 A cutaway diagram of a device in FIG.

[0035] Figure 3 A diagram is shown for explaining the first step of the method according to the invention,

[0036] Figure 4 shows a diagram for explaining the second step of the method according to the invention, and

[0037] Figure 5 A further diagram is shown for explaining the method according to the invention. DETAILED DESCRIPTION

[0038] Unless otherwise indicated, like reference numerals in the drawings refer to like objects.

[0039] exist Figure 1 and 2 1 shows a tubular strip 10, in the present case a tube 10, in particular a plastic tube 10, which has a wall 12, a cavity 14 defined by the tube 10, an outer surface 16 which is circular in cross section and an inner surface 18 which is also circular in cross section and defines the cavity 14. In the present example, the tube 10 is extruded in an extrusion device 20 and conveyed by suitable conveying means along the longitudinal axis of the tube, in Figure 1 The tube 10 is conveyed from left to right in the extrusion device 20. After leaving the extrusion device 20, the tube 10 first passes through a first cooling section 22, in which the tube 10, which is strongly heated and not yet completely solidified, i.e. has a flowable component (melt) that is still to be recrystallized, is cooled. The first cooling section 22 can include a calibration device, in particular a calibration sleeve, onto which the tube 10 can be pressed, for example, by means of a vacuum and the normal pressure inside the tube 10. As a result, the outer diameter of the tube 10, which was preformed by the extrusion device 20, is finally fixed. In the further course of the process, the tube 10 passes through at least one further cooling section 24, in which further cooling takes place. By Figure 1 The dotted line in the back shows that other cooling sections can also be set. After pipe 10 is fully solidified, in a cutting device 26 such as with a suspension saw, said pipe is cut into predetermined sections.

[0040] A first measuring device 28 and a second measuring device 30 are provided between the first cooling section 22 and the further cooling section 24. The second measuring device 30 is located downstream of the first measuring device 28 in the conveying direction of the tube 10. The first measuring device 28 includes a first transceiver 32, which contains a first transmitter and a first receiver for terahertz radiation. The second measuring device 30 includes a second transceiver 34, which contains a second transmitter and a second receiver for terahertz radiation. The first measuring device 28 also includes a first reflector 36, which is located on the side of the tube 10 opposite the first transceiver 32. The second measuring device 30 includes a second reflector 38, which is also located on the side of the tube 10 opposite the second transceiver 34. Terahertz radiation 40 emitted perpendicularly to the conveying direction by the first transmitter of the first transceiver 32 onto the tube 10 is partially reflected by the boundary surfaces of the tube 10 and partially reflected by the reflector 36, and then returns to the first receiver of the first transceiver 32. Accordingly, the terahertz radiation 42 emitted by the second transmitter of the second transceiver 34 perpendicular to the transport direction of the tube 10 returns to the second receiver of the second transceiver 34 after being reflected on the boundary surface of the tube 10 and on the reflector 38. Figure 2 The reflectors 36, 38 can be curved, as in Figure 2 30 for reflector 36. The measured values ​​of the first and second receivers are supplied to an evaluation device 44 of the device. The evaluation device 44 can determine the thickness of the front wall 46 and / or rear wall 48 of the tube 10 in the manner described below based on the terahertz radiation 40, 42 received by the first and second receivers of the first and second measuring devices 28, 30.

[0041] The first transmitter of the first measurement device 28 emits terahertz radiation 40 within a first frequency range having a first bandwidth around a first center frequency. The second transmitter of the second measurement device 30 emits terahertz radiation 42 within a second frequency range having a second bandwidth around a second center frequency different from the first center frequency. The second center frequency can be lower than the first center frequency. For example, the second center frequency can be equal to the first center frequency multiplied by a factor in the range of 0.3 to 0.7, preferably 0.4 to 0.6. Furthermore, the first frequency range and the second frequency range can have no overlap. Furthermore, the second bandwidth can be lower than the first bandwidth. For example, the first bandwidth can be less than 50 GHz, preferably less than 40 GHz. The second bandwidth can be less than 30 GHz, preferably less than 20 GHz. The first center frequency of the first transmitter of the first measurement device 28 can be, for example, in the range of 130 GHz to 160 GHz. The second center frequency of the second transmitter of the second measurement device 30 can be, for example, in the range of 80 GHz to 90 GHz. Both the first and second transmitters can emit FMCW radiation. The arrangement of the first and second measuring devices 28, 30 and the time of their activation can be selected so that the first and second measuring devices emit terahertz radiation 40, 42 at substantially the same angle and to substantially the same position on the surface of the tube 10 at a predetermined conveying speed of the tube 10, and the terahertz radiation is then received by the first or second receiver respectively.

[0042] Figure 3 The signal curve 50 of the measured values ​​of the first receiver of the first measuring device 28 is shown as a solid line. The signal curve 50 is formed by a mixture of individual signal components reflected, for example, at the boundary surface defining the front wall 46 of the tube 10, with this mixed signal additionally filtered by a low-pass filter. The signal curve 50 is modulated with a frequency that is proportional to the wall thickness of the front wall 46 of the tube 10 in the present case. The wall 46 to be measured can have a thickness that can be, for example, less than 1 cm, for example less than 2 mm, for example approximately 1.8 mm. As in Figure 3 As can be seen further in FIG, the bandwidth of the first measuring device 28 does not cover a complete cycle. Instead, only a portion of the cycle is visible. Figure 3 It can also be seen that due to the different interference components, the signal deviates from the ideal harmonic curve that is expected in principle.

[0043] The evaluation device 44 determines four reference curves 52, 54, 56, 58 from a plurality of periodic reference curves, in particular harmonic reference curves, for example by means of a numerical curve fitting method, which best approximate the signal curve 50 among the plurality of reference curves. Figure 3In FIG, the determined periodic reference curves 52, 54, 56, 58 are shown by dot-dashed lines, dashed lines and dot-dashed lines. Figure 3 As can be seen in FIG, based solely on the signal curve 50 of the first receiver, it is not possible to uniquely determine the reference curves 52 , 54 , 56 , 58 that best correspond to the signal curve 50 .

[0044] In order to identify the reference curve that best corresponds to the signal curve 50 from the harmonic reference curves 52, 54, 56, 58 determined from the signal curve 50 of the first receiver, the periodic reference curves 52, 54, 56, 58 are compared with the signal curve 60 of the measured values ​​of the second receiver of the second measuring device 30. The signal curve 60 is Figure 4 This signal curve is again obtained by mixing the signal components reflected at the two boundary surfaces defining the front wall 46 , and the signal curve 60 is also filtered by a low-pass filter. Figure 4 The signal curve of the measured value of the second receiver of the second measuring device 30 is shown, which is plotted over the second frequency range of the second measuring device 30. Figure 4 It can also be seen that, due to interfering signals and due to the significantly smaller bandwidth of second measuring device 30, which in the example shown is only approximately 12.5 GHz compared to approximately 37.5 GHz of first measuring device 28, signal curve 60 of second measuring device 30 is significantly shorter than one period of signal curve 60, which ideally corresponds to a harmonic curve. Unambiguous frequency determination is not possible based solely on signal curve 60 of the measured values ​​of the second receiver.

[0045] Now, in Figure 4 Again, according to Figure 3 Determined periodic reference curves 52, 54, 56, 58. Here it can be clearly seen that due to the different frequency ranges of the second measuring device 30, Figure 3 The reference curves 52, 54, 56, and 58 are still relatively close to each other. Figure 4 can be clearly distinguished from each other. Figure 4 The reference curve 56 shown in dashed lines can be clearly identified as the reference curve 56 that best corresponds to the signal curves 50 and 60. The frequency of this harmonic reference curve 56 can accordingly be used as a basis for determining the thickness of the front wall 46.

[0046] exist Figure 5, the frequency ranges of the first and second measuring devices 28, 30 are shown in FIG. 1 over a large frequency range from 0 to 180 GHz, together with the corresponding signal curves 50, 60 and the periodic reference curves 52, 54, 56, 58, for illustrative purposes only. In fact, in contrast to the prior art described above, according to the present invention, no such merging of the signal curves takes place. Figure 5 It can be clearly seen that the phases of the reference curves 52, 54, 56, 58, which are still close to each other in the frequency range of the signal curve 50 of the first measuring device 28, are separated from each other in the frequency range of the signal curve 60 of the second measuring device 30. This situation is utilized according to the present invention.

[0047] Reference Signs List

[0048] 10 tubes

[0049] 12 walls

[0050] 14 Cavity

[0051] 16 outer surface

[0052] 18 inner surface

[0053] 20 Extrusion device

[0054] 22 cooling section

[0055] 24 cooling sections

[0056] 26 cutting device

[0057] 28 First measuring device

[0058] 30 Second measuring device

[0059] 32 first transceiver

[0060] 34 Second transceiver

[0061] 36 First Reflector

[0062] 38 Second reflector

[0063] 40 terahertz radiation

[0064] 42 terahertz radiation

[0065] 44 Evaluation Device

[0066] 46 anterior wall

[0067] 48 back wall

[0068] 50 signal curve

[0069] 52 reference curves

[0070] 54 reference curves

[0071] 56 reference curves

[0072] 58 reference curves

[0073] 60 signal curves.

Claims

1. A device for determining the wall thickness of an object (10), in particular a flat or elongated object, comprising a first measuring device (28), the first measuring device comprising a first transmitter for terahertz radiation (40) and a first receiver for terahertz radiation (40), and a second measuring device (30), the second measuring device comprising a second transmitter for terahertz radiation (42) and a second receiver for terahertz radiation (42), the first measuring device (28) being arranged such that the first transmitter transmits terahertz radiation (40) onto the object (10), the first receiver receives the terahertz radiation (40) emitted by the first transmitter and reflected by a boundary surface of the object (10), and The second measuring device (30) is configured such that a second transmitter transmits terahertz radiation (42) toward the object (10), and a second receiver receives the terahertz radiation (42) emitted by the second transmitter and reflected by a boundary surface of the object (10), the first transmitter transmits terahertz radiation (40) in a first frequency range having a first bandwidth around a first center frequency, and the second transmitter transmits terahertz radiation (42) in a second frequency range having a second bandwidth around a second center frequency different from the first center frequency, and the device comprises an evaluation device (44), to which the measurement values ​​of the first receiver and the second receiver are supplied in order to determine the wall thickness of the object (10), It is characterized in that The evaluation device (44) is configured to determine a plurality of periodic reference curves (52, 54, 56, 58) that are similar to the signal curve (50) of the measured values ​​of the first receiver, compare the determined periodic reference curves (52, 54, 56, 58) with the signal curve (60) of the measured values ​​of the second receiver, and identify a periodic reference curve (52, 54, 56, 58) that best matches the signal curve (60) of the measured values ​​of the second receiver from the determined periodic reference curves (52, 54, 56, 58), and the evaluation device (44) is further configured to determine the wall thickness of the object (10) based on the identified reference curves (52, 54, 56, 58).

2. The device according to claim 1, characterized in that The signal curve and the periodic reference curve (52, 54, 56, 58) are phase curves or frequency curves.

3. The device according to any one of the preceding claims, characterized in that The periodic reference curve (52, 54, 56, 58) is a harmonic reference curve (52, 54, 56, 58).

4. The device according to any one of the preceding claims, characterized in that The evaluation device is configured to identify a periodic reference curve (52, 54, 56, 58) that best corresponds to the signal curve (60) of the measured values ​​of the second receiver based on a comparison of the phase of the determined periodic reference curve (52, 54, 56, 58) with the phase of the signal curve (60) of the measured values ​​of the second receiver.

5. The device according to any one of the preceding claims, characterized in that The second center frequency is lower than the first center frequency.

6. The device according to claim 5, characterized in that The second center frequency is equal to the first center frequency multiplied by a coefficient in the range of 0.3 to 0.7, preferably in the range of 0.4 to 0.

6.

7. The device according to any one of the preceding claims, characterized in that The first frequency range and the second frequency range do not overlap.

8. The device according to any one of the preceding claims, characterized in that The second bandwidth is smaller than the first bandwidth.

9. The device according to any one of the preceding claims, characterized in that The first bandwidth is smaller than one period of the determined periodic reference curve (52, 54, 56, 58), preferably not larger than three quarters of a period, and further preferably not larger than half a period.

10. The device according to any one of the preceding claims, characterized in that The first bandwidth is less than 50 GHz, preferably less than 40 GHz.

11. The device according to any one of the preceding claims, characterized in that The second bandwidth is smaller than half a period of the determined periodic reference curve (52, 54, 56, 58), preferably not larger than a quarter of a period, and further preferably not larger than an eighth of a period.

12. The device according to any one of the preceding claims, characterized in that The second bandwidth is less than 30 GHz, preferably less than 20 GHz.

13. The device according to any one of the preceding claims, characterized in that The device further comprises a transport device, by means of which the object (10) is transported through the device during the measurement.

14. The device according to any one of the preceding claims, characterized in that The first emitter and the second emitter are arranged so that they emit terahertz radiation (40, 42) toward the object (10) while being offset from one another in a transport direction of the object (10).

15. The device according to any one of the preceding claims, characterized in that The first emitter and the second emitter are arranged such that the first emitter and the second emitter emit terahertz radiation (40, 42) at substantially the same angle and towards substantially the same location on the surface of the object (10).

16. Apparatus according to any one of the preceding claims, characterised in that The first transmitter and the second transmitter are formed by a common transmitting device, which transmits terahertz radiation (40) in a first frequency range with a first bandwidth around a first center frequency and also transmits terahertz radiation (42) in a second frequency range with a second bandwidth around a second center frequency different from the first center frequency, and / or the first receiver and the second receiver are formed by a common receiving device, which receives terahertz radiation (40) in a first frequency range with a first bandwidth around the first center frequency and also receives terahertz radiation (42) in a second frequency range with a second bandwidth around a second center frequency different from the first center frequency.

17. Method for determining the wall thickness of an especially flat or elongated object (10), wherein: A first transmitter emits terahertz radiation (40) toward the object (10), and a first receiver receives the terahertz radiation (40) emitted by the first transmitter and reflected by a boundary surface of the object (10), and a second transmitter emits terahertz radiation (42) toward the object (10), and a second receiver receives the terahertz radiation (42) emitted by the second transmitter and reflected by the boundary surface of the object (10), the first transmitter emitting terahertz radiation (40) in a first frequency range having a first bandwidth around a first center frequency, and the second transmitter emitting terahertz radiation (42) in a second frequency range having a second bandwidth around a second center frequency different from the first center frequency, The invention is characterized in that a plurality of periodic reference curves (52, 54, 56, 58) that are similar to the signal curve (5) of the measurement value of the first receiver are determined, the determined periodic reference curves (52, 54, 56, 58) are compared with the signal curve (60) of the measurement value of the second receiver, and a periodic reference curve (52, 54, 56, 58) that best matches the signal curve (60) of the measurement value of the second receiver is identified from the determined periodic reference curves (52, 54, 56, 58), and the wall thickness of the object (10) is determined based on the identified reference curve (52, 54, 56, 58).

18. The method according to claim 17, characterized in that During the measurement, the object (10) is transported through the measuring region of the device by means of a transport device.

19. The method according to any one of claims 17 or 18, characterized in that The method is implemented by the apparatus according to any one of claims 1 to 16.

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