Apparatus and method for determining wall thickness of an object

Abstract

The present invention relates in particular to an apparatus for determining the wall thickness of a flat or elongated object. The apparatus comprises a first measuring device having a first transmitter for terahertz radiation and a first receiver for terahertz radiation, and a second measuring device having a second transmitter for terahertz radiation and a second receiver for terahertz radiation. The first measuring device is arranged such that the first transmitter radiates terahertz radiation onto the object and the first receiver receives the terahertz radiation reflected from the object's boundary and emitted by the first transmitter. The second measuring device is arranged such that the second transmitter radiates terahertz radiation onto the object and the second receiver receives the terahertz radiation reflected from the object's boundary and emitted by the second transmitter. The apparatus comprises an evaluation device that applies the measurements of the first and second receivers to determine the wall thickness of the object. The first transmitter emits terahertz radiation within a first frequency range having a first bandwidth around a first center frequency, and the second transmitter emits terahertz radiation within a second frequency range having a second bandwidth around a second center frequency different from the first center frequency. The evaluation device is configured to determine a plurality of periodic reference profiles that approximate a signal profile of the measurements of the first receiver, compare the determined periodic reference profiles with the signal profile of the measurements of the second receiver, and identify the periodic reference profile that best corresponds to the signal profile of the measurements of the second receiver. The evaluation device is also configured to determine a wall thickness of the object based on the identified reference profiles. The present invention also relates to a corresponding method.

Description

[Technical Field]

[0001] The present invention relates in particular to an apparatus for determining the wall thickness of a flat or elongated object. The apparatus comprises a first measuring device having a first transmitter for terahertz radiation and a first receiver for terahertz radiation, and a second measuring device having a second transmitter for terahertz radiation and a second receiver for terahertz radiation. The first measuring device is arranged such that the first transmitter radiates terahertz radiation toward the object and the first receiver receives the terahertz radiation reflected from the object's boundary and emitted by the first transmitter. The second measuring device is arranged such that the second transmitter radiates terahertz radiation toward the object and the second receiver receives the terahertz radiation reflected from the object's boundary and emitted by the second transmitter. The first transmitter radiates terahertz radiation within a first frequency range having a first bandwidth around a first center frequency, and the second transmitter radiates terahertz radiation within a second frequency range having a second bandwidth around a second center frequency different from the first center frequency. The apparatus comprises an evaluation device that applies the measurements of the first and second receivers to determine the wall thickness of the object.

[0002] The invention also relates to a method for determining the wall thickness of a flat or elongated object, in particular, in which a first transmitter radiates terahertz radiation onto the object, a first receiver receives the terahertz radiation reflected from an interface of the object and emitted by the first transmitter, a second transmitter radiates terahertz radiation onto the object, and a second receiver receives the terahertz radiation reflected from an interface of the object and emitted by the second transmitter, the first transmitter radiates the terahertz radiation within a first frequency range having a first bandwidth around a first center frequency, and the second transmitter radiates the terahertz radiation within a second frequency range having a second bandwidth around a second center frequency different from the first center frequency. [Background technology]

[0003] Such devices or methods are used, for example, to measure the wall thickness of plastic pipes produced in extrusion equipment. Plate-shaped objects can also be measured using such devices and methods. The object may also be made of glass. To unambiguously determine the thin-walled optical thickness of such objects using terahertz radiation, the frequency and / or phase difference of radiation components resulting from reflection at a boundary layer, which typically defines the wall of the object, is determined. This can be achieved, particularly in the case of thin walls, by mixing two signal components of the radiation reflected at the boundary. For easier evaluation, the low-frequency component of the mixed signal can be isolated. This isolated signal is modulated at a frequency proportional to the difference frequency between the signal components and thus the wall thickness.

[0004] Determining this frequency is difficult in situations of signal interference due to unknown physical phenomena, such as noise, further reflections, or unknown dispersion of the object. This is especially true when the signal available for evaluation consists of only one or only a portion of the oscillation period of the frequency to be determined due to the limited bandwidth of the terahertz radiation used for the measurement. For a given object material and wall thickness, the bandwidth of the terahertz radiation used for the measurement determines the signal range available for determining the frequency of the signal to be evaluated, specifically the signal period(s) available for evaluation. Therefore, to unambiguously determine the frequency of the signal to be evaluated, whose frequency is proportional to the wall thickness, it is desirable to be able to evaluate at least one complete period, preferably multiple periods, of the signal to be evaluated. Against this background, this can be achieved by increasing the bandwidth of the measurement device. However, this is technically difficult. In any case, this significantly increases the cost of the measurement device. Furthermore, a higher bandwidth requires the use of a higher center frequency, since various components of the measurement device, such as lenses, high-frequency conductors, or frequency multipliers, are limited within their relative bandwidths. However, at higher frequencies, it becomes more difficult to unambiguously assign frequencies to the measured signal profile because the potentially matching frequencies in the signal profile are closer together. Furthermore, the higher frequencies of terahertz radiation increase the absorption of the radiation signal in many materials, making reliable measurement and evaluation even more difficult or even impossible.

[0005] German Patent No. 102017207648B4 discloses a method and apparatus for measuring the layer thickness of an object. In the method, electromagnetic radiation is radiated onto the object by one of a plurality of transmitters in at least two measurement steps. The frequency bands of the individual measurement steps are different subranges of the bandwidth. In each measurement step, secondary radiation reflected from the boundary layer of the object is detected, and a measurement signal is determined for each measurement step. The measurement signals of the individual measurement steps are combined according to their respective frequency bands to form an evaluation signal. Specifically, this is done by plotting the measurement signals of the individual measurement steps on a common frequency axis according to their respective frequency ranges. To determine the layer thickness of the object, the fundamental frequency of the evaluation signal is determined by adapting the measurement signals plotted on the common frequency axis. In this way, measurements over the entire bandwidth are approximated by individual measurement steps within a subrange of the bandwidth. It is particularly envisioned that this method enables precise measurements using multiple cost-effective narrowband electromagnetic wave transmitters and receivers.

[0006] However, the proposed integration of measurement signals in German Patent No. 102017207648B4, in which each signal covers a partial range of the total bandwidth and the measurement signals of the individual measurement steps are adapted on a common frequency axis, requires that each transceiver used for the individual measurement steps have high signal quality to eliminate ambiguity during the adaptation process. As a result, the required transceivers are still relatively expensive. Furthermore, the integration of individual measurement signals proposed in German Patent No. 102017207648B4 requires knowledge of the relative amplitudes of the individual signal portions, which ideally requires at least one calibration, but this may vary depending on the situation due to differences in absorption in the layer under consideration or differences in the orientation of the object relative to the individual transceivers. Not taking the relative amplitudes into account, or taking them incorrectly, can result in modulation in the combined signal, which can distort the determination of the fundamental frequency and, therefore, the layer thickness. This is particularly difficult when the partial range of the bandwidth covered by the individual measurement signals is narrow. As a result, relatively wideband and therefore expensive transceivers are required. Particular problems arise with moving objects, such as those produced in extrusion machines and conveyed through the measurement area in the conveying direction. The method proposed in DE 10 2017 207 648 B4 combines the individual measurement signals via a common frequency axis and adapts the measurement signals, making it susceptible to deviations in the individual measurements, especially if the measurements are not performed simultaneously, at the same position on the object, or from the same direction. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] German Patent No. 102017207648B4 Summary of the Invention [Problem to be solved by the invention]

[0008] Based on the above-mentioned prior art, the present invention is therefore based on the problem of providing an apparatus and a method of the type mentioned at the beginning, which allows unambiguous metrological determination in a simple and cost-effective manner of the wall thickness, in particular of flat or elongated objects, even of thin walls, and in particular of objects that are moving during the measurement. [Means for solving the problem]

[0009] The present invention solves the problem with the independent claims 1 and 17. Advantageous embodiments can be found in the dependent claims, the description and the drawings.

[0010] The present invention solves the problem of an apparatus of the type mentioned at the beginning in that the evaluation device is configured to determine a plurality of periodic reference profiles that approximate the signal profile of the measurement values ​​of the first receiver, to compare the determined periodic reference profiles with the signal profile of the measurement values ​​of the second receiver, and to identify from the determined periodic reference profiles the periodic reference profile that best corresponds to the signal profile of the measurement values ​​of the second receiver, and further in that the evaluation device is configured to determine the wall thickness of the object based on the identified reference profiles.

[0011] With respect to the method of the type mentioned at the beginning, the present invention solves the problem of determining a plurality of periodic reference profiles that approximate the signal profile of the measurements of a first receiver, comparing the determined periodic reference profiles with the signal profile of the measurements of a second receiver, identifying from the determined periodic reference profiles the periodic reference profile that best corresponds to the signal profile of the measurements of the second receiver, and determining the wall thickness of the object based on the identified reference profile.

[0012] As explained at the beginning, the object to be measured may be made of, for example, plastic or glass. Since the object to be measured is at least partially transparent to terahertz radiation, the terahertz radiation radiated onto the object is reflected by the boundary surfaces that define the walls of the object to be measured. If the object is flat, the object may be, for example, plate-shaped. If the object is elongated, the object may be, for example, strand-shaped. In particular, the elongated object may be cylindrical, in particular hollow cylindrical. The object may also be tubular, in particular a plastic or glass tube. The object may be manufactured in an extrusion device. At the time of the measurement of the present invention, the object may not be completely cooled, in particular the object may still have flowable portions. The object may be transported through the measurement range of the device by a transport device. In particular, elongated objects may be transported along their longitudinal axis. As explained, if the object is moving in this way during the measurement, it is particularly difficult to determine the layer thickness.

[0013] The first transmitter and the first receiver for terahertz radiation may be substantially co-located. The first transmitter and the first receiver may be combined to form a first transceiver. The second transmitter and the second receiver for terahertz radiation may also be substantially co-located. The second transmitter and the second receiver may be combined to form a second transceiver. As explained above, the terahertz radiation emitted by the first or second transmitter penetrates at least partially into the object and is reflected by the object's boundary surface, specifically the surface defining the wall of the object being measured. In this specification, when terahertz radiation is mentioned, it particularly refers to electromagnetic radiation within the frequency range of 1 GHz to 6 THz, and particularly within the frequency range of 50 GHz to 1 THz. These are therefore so-called millimeter waves or submillimeter waves. Specifically, the terahertz radiation is FMCW radiation (frequency modulated continuous wave radiation).

[0014] The first and second transmitters emit terahertz radiation within frequency ranges around different center frequencies. As described in more detail below, the present invention uses evaluation of terahertz radiation emitted at different frequencies to unambiguously determine wall thickness. This invention is significantly simplified compared to the prior art. The evaluation device determines multiple periodic reference profiles that correspond as closely as possible to the signal profile recorded by the first receiver. For example, multiple periodic reference profiles that correspond as closely as possible to the signal profile of the first receiver measurement can be determined, preferably using a numerical curve fitting method. For example, at least two, preferably at least three, and / or no more than 10, preferably no more than five, periodic reference profiles that best correspond to the signal profile of the first receiver measurement can be determined. For example, exactly three or exactly four periodic reference profiles can be determined. In principle, as explained at the beginning, particularly in the case of FMCW terahertz radiation, it can be assumed that the frequency of the signal profile of the first receiver measurement is proportional to the wall thickness of the object being measured. As explained above, the frequency and / or phase difference of the signal components resulting from reflection at the boundary surfaces that limit the thickness of the wall to be measured can be determined to form a signal profile of the measurement value of the first receiver. Also, as explained above, the radiation components reflected at the boundary surfaces that define the wall can be mixed. The signal profile can be filtered before evaluation, for example, using a corresponding frequency filter, in particular a low-pass filter, and thus its low-frequency components can be attenuated. As explained above, this signal profile is modulated with the difference frequency of the signal components reflected at the two boundary surfaces of the wall, and thus with a frequency that is proportional to the wall thickness.

[0015] To solve the problem of unambiguously determining the frequency of the signal profile, as explained above, a second measuring device is first used, operating with terahertz radiation of another frequency, as in German Patent No. 10207207648B4. However, in contrast to German Patent No. 10207207648B4, the measurement signals of the first and second measuring devices are not combined in the present invention. Rather, the present invention simplifies the evaluation by first determining, in a first step, multiple periodic reference profiles that correspond as closely as possible to the signal profile measured by the first receiver. Due to at least interference in the signal profiles and the limited bandwidth of the terahertz radiation of the first measuring device, it is difficult to identify the periodic reference profile from the group of determined periodic reference profiles that best corresponds to the signal profile. Therefore, in the second step, instead of integrating the measurement signals, the determined periodic reference profile (only) is compared with the signal profile measured by the second receiver. This signal profile can also be generated by determining the frequency and phase difference of signal components resulting from reflection of the terahertz radiation emitted by the second transmitter at the boundary layer of the wall. Similarly, the signal components can be mixed as described above. The measurements of the second receiver can be filtered by a frequency filter, in particular a low-pass filter, before being evaluated by the evaluation device.

[0016] Next, based on a comparison of the signal profile of the measurement value of the second receiver with the already reduced number of possible periodic reference profiles determined in the first step, a periodic reference profile that best corresponds to the signal profile of the measurement value of the second receiver can be easily identified from the group of periodic reference profiles determined in the first step. Specifically, the phase position of the periodic reference profile is evaluated. Because the frequencies of the first and second measurement devices are different, the phase of the periodic reference profile determined relative to the signal profile of the first receiver differs significantly within the frequency range of the second receiver. This allows, in particular, based on a comparison with the phase of the signal profile of the second receiver, to easily and clearly identify from the periodic reference profiles a periodic reference profile that ultimately best matches the signal profiles of both the first and second receivers. The periodic reference signals are still relatively close to each other in terms of their phase position relative to the signal profile of the first receiver. As a result, as described above, the (fundamental) frequency of the periodic reference profile identified as suitable allows the wall thickness of the object to be determined in a simple manner.

[0017] The comparison of the reference profile with the signal profile of the second receiver is significantly simplified because the number of possible reference profiles has already been significantly reduced in the first step. Because the first step does not yet require the selection of a single specific periodic reference profile, the first transmitter and first receiver do not require structurally complex and expensive wideband transmission and reception. Furthermore, because the number of periodic reference profiles that need to be compared with the signal profile of the second receiver in the second step is very limited, the second measurement device can operate at a bandwidth low enough that it cannot identify the frequency of the signal profile by itself. Note that if there is a frequency difference between the first and second measurement devices, the phase of the periodic reference profile within the frequency range of the second receiver will be significantly different. By evaluating the phase position, an appropriate reference profile can be unambiguously selected from a set of predetermined reference profiles, even if the second measurement device has a particularly narrow bandwidth. Therefore, in contrast to the prior art, the second measurement device can be designed to be particularly cost-effective or narrow-band, and in particular more cost-effective or narrow-band than the first measurement device. Furthermore, the method according to the invention is substantially insensitive to deviations between the measurements of the first and second measuring devices, especially if the first and second measuring devices do not measure simultaneously, at the same position on the object, or from the same direction. Therefore, according to the invention, it is not even a problem to measure an object that is transported through the measuring range while performing the measurements.

[0018] Overall, by combining measurements with two measuring devices operating at different frequencies and selecting an appropriate reference profile with the second measuring device from a set of reference profiles determined by the first measuring device, the wall thickness of an object can be unambiguously determined using a relatively simple measuring device, even for thin walls. For example, the wall thickness of the object to be measured may be less than 10 cm, even less than 5 cm, or even less than 1 cm.

[0019] Of course, multiple second measurement devices are also conceivable, in which case the periodic reference profile determined in the first step can be compared with signal profiles from multiple second receivers, which can further improve the evaluation, especially if the multiple second measurement devices transmit and receive within frequency ranges around different center frequencies.

[0020] According to one embodiment, the signal profile and the periodic reference profile can be phase profiles or frequency profiles. As explained above, radiation components reflected by the boundary surfaces defining the walls of the measurement object can be mixed in the signal profile of the first or second receiver. Due to physical laws, there is a fixed relationship between the phase and frequency of a signal profile. Therefore, for example, in evaluating the phase profile, the phase can be used to determine the frequency of the signal profile and therefore the wall thickness. This can improve accuracy. The phase profile can be a profile of the phase of the terahertz radiation plotted against the frequency of the terahertz radiation of the first or second receiver. The frequency profile can be a curve of the frequency of the terahertz radiation of the first or second receiver plotted against time.

[0021] According to a further embodiment, the periodic reference profile may be a harmonic reference profile. Harmonic reference profiles are specifically known as cosine or sine waves. The signal profile of the first or second receiver from the mixed radiation components reflected at the two boundary surfaces of the wall ideally corresponds to such a harmonic profile. This further simplifies the evaluation according to the present invention.

[0022] As already explained, the periodic reference profile that best corresponds to the signal profile of the measurement value of the second receiver can be identified by comparing the phase of the determined periodic reference profile with the phase of the signal profile of the measurement value of the second receiver. The evaluation device of the device according to the invention is appropriately configured for this purpose. Such evaluation of the phase position significantly simplifies the selection of an appropriate reference profile, even for a narrow-band second measurement device. In particular, different amplitudes of the signal profiles of the first and second measurement devices are of little importance when evaluating the phase position.

[0023] According to a further embodiment, the second center frequency may be smaller than the first center frequency. For example, the second center frequency may be equal to the first center frequency multiplied by a factor in the range of 0.3 to 0.7, preferably in the range of 0.4 to 0.6. Furthermore, the first and second frequency ranges may not overlap. For a particularly simple and unambiguous evaluation, it is desirable to initially select a low center frequency for the second transceiver to keep the density of reference profiles matching the signal profile of the second receiver low. In particular, this prevents the possibility that adjacent periodic reference profiles determined for the signal profile of the first receiver both match the signal profile of the second receiver. On the other hand, this avoids high absorption of terahertz radiation. Only frequencies close to zero need to be avoided, since the phases of all solutions converge there, making differentiation difficult. Furthermore, since the phase difference in the signal profile of the second receiver is approximately proportional to the difference between the first and second center frequencies, the frequency of the terahertz radiation of the second measurement device must be sufficiently separated from the frequency of the first measurement device. This further simplifies the identification of a periodic reference profile that optimally matches the signal profile of the second receiver. The above-mentioned frequency differences have proven particularly advantageous, since the phase difference between adjacent solutions is greatest within these ranges. Furthermore, based on the above considerations, the approach according to the present invention is clearly superior to direct evaluation using only a single transceiver and with a correspondingly higher bandwidth.

[0024] For example, the first center frequency may be in the range of 100 to 200 GHz, preferably in the range of 120 to 180 GHz. Correspondingly, the second center frequency may 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 examples. Depending on the particular application, the center frequencies may have other values.

[0025] According to a further embodiment, the second bandwidth can be smaller than the first bandwidth. As mentioned above, the present invention still allows the wall thickness of an object to be determined unambiguously. At the same time, a measuring device with a low bandwidth can be realized particularly easily and cost-effectively.

[0026] According to a further embodiment, the first bandwidth may correspond to less than one period of the determined periodic reference profile. For example, the first bandwidth may be less than 50 GHz, preferably less than 40 GHz. As explained in the introduction, the bandwidth of the terahertz radiation of the first (and second) measurement device determines which portion of the period of the signal profile and, accordingly, which portion of the periodic reference profile is available for evaluation. The inventive combination of a first measurement device and a second measurement device operating at different frequencies does not require the bandwidth of the first measurement device to be selected large enough to unambiguously identify the periodic reference profile. This means that the first measurement device can be realized more simply and cost-effectively, while still being able to unambiguously determine the wall thickness. For example, the first bandwidth may correspond to less than three-quarters of a period, preferably less than half a period, of the determined periodic reference profile. This also avoids the disadvantages of measurement devices with wide bandwidths explained in the introduction.

[0027] In view of the above background, it is further possible to correspond to less than a half period of a periodic reference profile with a determined second bandwidth. For example, the second bandwidth may be less than 30 GHz, preferably less than 20 GHz. For example, the second bandwidth may correspond to less than a quarter period, preferably less than an eighth period, of the determined periodic reference profile. Since the second measuring device is only used to confirm the validity of the measurement results of the first measuring device, it does not need to have a bandwidth close to the bandwidth required to clearly identify the periodic reference profile. The design and cost of the device according to the present invention are simplified.

[0028] In this specification, the following is applied. Period length = speed of light / (2 * optical thickness of the layer), where optical thickness = thickness * refractive index. Therefore, for the bandwidth, the following is correspondingly applied. Bandwidth < N * speed of light / (2 * optical thickness of the layer), where N = 3 / 4, 1 / 2, 1 / 4, 1 / 8 of the respective limit values of the mentioned period ratio.

[0029] The device may further include a transport device for transporting an object during measurement by the device. For example, an elongated object may be transported along its longitudinal axis. As described at the beginning, the object can be manufactured, for example, in an extrusion device. Therefore, after being discharged from the extrusion device, the object can be transported through the device by the transport device during measurement. Also, the device may include an extrusion device. Also, the device may include an object.

[0030] According to a further embodiment, the first and second transmitters may be arranged to radiate terahertz radiation onto the object while being offset from one another in the direction of transport of the object. In particular, the first and second transmitters can be arranged to radiate terahertz radiation at substantially the same angle and at substantially the same location on the surface of the object. To achieve optimal measurement and evaluation results, it is ideal for the first measuring device to accurately measure the same cross-section of the object from the same direction as the second measuring device. This can be achieved, for example, by arranging the first and second transceivers offset to account for the transport speed when the object is transported through the device during measurement. The offset depends on the transport direction, and the first and second transceivers are operated in a clocked manner. However, the present invention does not rely on the first and second measuring devices to accurately measure the same wall thickness. In particular, if the second transceiver operates at a relatively low center frequency, some deviation in the measurement position is acceptable. This is the case when the object's shape changes sufficiently slowly with respect to position and / or time, which is typical for the extrusion of plastic or glass objects. A corresponding arrangement with the first transceiver offset relative to the second transceiver simplifies the measurement setup and allows each setup to be optimized for each frequency range.

[0031] According to a further embodiment, the first transmitter and the second transmitter are constituted by a common transmitting device that emits both terahertz radiation in a first frequency range having a first bandwidth around a first center frequency and terahertz radiation in a second frequency range having a second bandwidth around a second center frequency different from the first center frequency, and / or the first receiver and the second receiver are constituted by a common receiving device that receives both terahertz radiation (40) in a first frequency range having a first bandwidth around the first center frequency and terahertz radiation (42) in a second frequency range having a second bandwidth around a second center frequency different from the first center frequency. Thus, in this embodiment, a single transmitter or only a single transmitter is provided that can transmit or receive terahertz radiation in different bandwidths and thus form both the first transmitter or receiver of the first measuring device and the second transmitter or receiver of the second measuring device. Simultaneous transmission and reception of terahertz radiation in different frequency bands is also possible in the sense of a frequency diversity scheme. This allows for a particularly compact measurement device, since at least some components can be used for both frequencies and a common device can therefore be smaller than two dedicated devices. This can be advantageous, for example, for portable devices and can also reduce costs. A further advantage is that measurements can be performed at the same location and from the same direction substantially simultaneously.

[0032] The method according to the invention can be carried out by means of the device according to the invention. The device according to the invention can therefore be configured to carry out the method according to the invention. [Brief explanation of the drawings]

[0033] An embodiment of the invention is explained in more detail below on the basis of the drawings.

[0034] [Figure 1] 1 is a schematic side view of an apparatus according to the present invention; [Figure 2] FIG. 2 is a cross-sectional view of the device shown in FIG. 1. [Figure 3]FIG. 2 shows the first step of the method according to the invention. [Figure 4] FIG. 3 illustrates a second step of the method according to the invention. [Figure 5] 4A and 4B are further diagrams illustrating the method according to the invention;

[0035] Unless otherwise specified, the same reference numerals in the figures refer to the same items. DETAILED DESCRIPTION OF THE INVENTION

[0036] 1 and 2 show a tubular strand 10, in this case a tube 10, specifically a plastic tube 10. The tube 10 has a wall 12, a cavity 14 enclosed by the tube 10, an outer surface 16 having a circular cross section, and an inner surface 18, also having a circular cross section and bounding the cavity 14. In this example, the tube 10 is extruded in an extrusion device 20 and transported along its longitudinal axis by a suitable transport device (from left to right in FIG. 1). After leaving the extrusion device 20, the tube 10 first passes through a first cooling section 22, where it is cooled. The tube 10 is still strongly heated and has not completely solidified, i.e., it still contains a fluid component (melt) that can be recrystallized. The first cooling section 22 can include a calibration device, specifically a calibration sleeve. The tube 10 is forced into the calibration device, for example, by vacuum and atmospheric pressure inside the tube 10. As a result, the outer diameter of the tube 10 preformed by the extrusion device 20 is finally set. In a further step, the tube 10 passes through at least one further cooling section 24, in which further cooling takes place. The dashed lines in Fig. 1 below indicate that further cooling sections can be provided. After the tube 10 has completely solidified, it is cut to length in a cutting device 26, for example a flying saw.

[0037] Between the first cooling section 22 and the further cooling section 24 there is a first measuring device 28 and a second measuring device 30 arranged downstream of the first measuring device 28 in the conveying direction of the tube 10. The first measuring device 28 comprises a first transceiver 32 comprising a first transmitter for terahertz radiation and a first receiver for terahertz radiation. The second measuring device 30 comprises a second transceiver 34 comprising a second transmitter for terahertz radiation and a second receiver for terahertz radiation. Furthermore, the first measuring device 28 comprises a first reflector 36 on a side of the tube 10 opposite the first transceiver 32. The second measuring device 30 also comprises a second reflector 38 on a side of the tube 10 opposite the second transceiver 34. Terahertz radiation 40 emitted perpendicular to the direction of propagation into the tube 10 by a first transmitter of the first transceiver 32 is accordingly partially reflected by the boundary surface of the tube 10, partially reflected by the reflector 36, and returns to the first receiver of the first transceiver 32. Correspondingly, terahertz radiation 42 emitted perpendicular to the direction of propagation into the tube 10 by a second transmitter of the second transceiver 34 is reflected by the boundary surface of the tube 10 and by the reflector 38 before returning to the second receiver of the second transceiver 34. FIG. 2 shows an example of this for the first measuring device 28. The reflectors 36, 38 can be curved, as shown by way of example for the reflector 36 in FIG. 2. The measured values ​​of the first and second receivers are available to an evaluation device 44 of the device. In the method described below, the thickness of the front wall 46 and / or rear wall 48 of the tube 10 can be determined by an evaluation device 44 based on the terahertz radiation 40, 42 received by the first and second receivers of the first and second measurement devices 28, 30.

[0038] The first transmitter of the first measuring 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 measuring 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 may be lower than the first center frequency. For example, the second center frequency may correspond to the first center frequency multiplied by a factor in the range of 0.3 to 0.7, preferably in the range of 0.4 to 0.6. Furthermore, the first frequency range and the second frequency range may not overlap. Furthermore, the second bandwidth may be smaller than the first bandwidth. For example, the first bandwidth may be less than 50 GHz, preferably less than 40 GHz. The second bandwidth may be less than 30 GHz, preferably less than 20 GHz. For example, the first center frequency of the first transmitter of the first measuring device 28 may be in the range of 130 to 160 GHz. The second center frequency of the second transmitter of the second measurement device 30 may be, for example, in the range of 80 to 90 GHz. The first transmitter and the second transmitter may emit FMCW radiation. The arrangement of the first and second measurement devices 28, 30 and their activation timing may be selected so that, at a given transport speed of the tube 10, the first transmitter and the second transmitter emit terahertz radiation 40, 42 at substantially the same angle and at substantially the same location on the surface of the tube 10. The terahertz radiation 40, 42 is then received by the first and second receivers, respectively, accordingly.

[0039] FIG. 3 shows a signal profile 50 of the measurement of the first receiver of the first measuring device 28, shown as a solid line. The signal profile 50 is obtained, for example, by mixing signal components reflected at the boundary surface of the pipe 10, which borders the front wall 46. This mixed signal is then filtered by a low-pass filter. The signal profile 50 is modulated at a frequency proportional to the wall thickness of the front wall 46 of the pipe 10. The thickness of the measured wall 46 may be, for example, less than 1 cm, less than 2 mm, or approximately 1.8 mm. As can be seen from FIG. 3, the bandwidth of the first measuring device 28 does not cover the entire period. Rather, only a portion of the period is visualized. FIG. 3 also shows deviations from the typically assumed ideal harmonic curve due to various components resulting from interference.

[0040] The evaluation unit 44 identifies four reference profiles 52, 54, 56, and 58 from a plurality of periodic reference profiles, for example, using a numerical curve fitting method. The periodic reference profiles are specifically harmonic reference profiles, and the identified periodic reference profiles are those from the plurality of reference profiles that best approximate the signal profile 50. In FIG. 3, the determined periodic reference profiles 52, 54, 56, and 58 are shown with dotted, dashed, and dash-dot lines. As can be seen from FIG. 3, it is impossible to unambiguously determine the reference profiles 52, 54, 56, and 58 that best correspond to the signal profile 50 based on the signal profile 50 of the first receiver alone.

[0041] The periodic reference profiles 52, 54, 56, 58 are compared with a signal profile 60 of a second receiver measurement of the second measurement device 30 to identify the reference profile that best corresponds to the signal profile 50. The reference profile is identified from the harmonic reference profiles 52, 54, 56, 58, which were determined based on the first receiver signal profile 50. This signal profile 60 is shown in FIG. 4 as a solid line. Again, this is the result of mixing signal components reflected at the two interfaces that bound the front wall 46, and the signal profile 60 is again low-pass filtered. FIG. 4 shows the signal profile of the second receiver measurement of the second measurement device 30. The signal profile is plotted across the second frequency range of the second measurement device 30. FIG. 4 further shows that the signal profile 60 of the second measurement device 30 corresponds significantly less than one period of the signal profile 60. This ideally corresponds to a harmonic curve, and is due to the interference signal and the extremely low bandwidth from the second measurement device 30. In the illustrated embodiment, the bandwidth of the second measurement device 30 is only about 12.5 GHz, compared to about 37.5 GHz for the first measurement device 28. It is not possible to unambiguously identify the frequency based on the signal profile 60 of the second receiver-only measurements.

[0042] Next, FIG. 4 shows the periodic reference profiles 52, 54, 56, and 58 determined according to FIG. 3. While the reference profiles 52, 54, 56, and 58 are still relatively close in FIG. 3, it can be clearly seen that they are significantly farther apart in FIG. 4. This is due to the different frequency range of the second measurement device 30. Based on FIG. 4, the reference profile 56, indicated by the dashed line, can be clearly identified as the reference profile 56 that best corresponds to the signal profiles 50 and 60. Therefore, the frequency of this harmonic reference profile 56 can be used as a reference for determining the thickness of the anterior wall 46.

[0043] For illustrative purposes only, FIG. 5 shows the frequency ranges of the first and second measurement devices 28, 30, along with their respective signal profiles 50, 60 and periodic reference profiles 52, 54, 56, 58. The frequency ranges of the first and second measurement devices 28, 30 are shown across a wide frequency range from 0 to 180 GHz. In fact, in accordance with the present invention, and in contrast to the prior art discussed above, no such signal profile integration is performed. However, FIG. 5 clearly shows the phase divergence of the reference profiles 52, 54, 56, 58. In accordance with the present invention, the divergence is exploited to ensure that the phases of the reference profiles 52, 54, 56, 58 remain closely spaced within the frequency range of the signal profile 50 of the first measurement device 28. [Explanation of symbols]

[0044] 10 tubes 12 Wall 14 Cavity 16 Exterior 18 Inner 20 Extrusion equipment 22 Cooling Section 24 Cooling Section 26 Cutting device 28 First Measuring Device 30 Second measuring device 32 First Transceiver 34 Second Transceiver 36 First Reflector 38 Second Reflector 40 Terahertz radiation 42 Terahertz radiation 44 Evaluation equipment 46 Front wall 48 Back wall 50 signal profiles 52 Reference Profiles 54 Reference Profiles 56 Reference Profiles 58 Reference Profiles 60 signal profiles

Claims

1. In particular, a device for determining the wall thickness of a flat or elongated object (10), said device comprising: a first measuring device (28) comprising a first transmitter for terahertz radiation (40) and a first receiver for terahertz radiation (40); a second measuring device (30) comprising a second transmitter (42) for terahertz radiation and a second receiver (42) for terahertz radiation; Equipped with the first measuring device (28) is arranged such that the first transmitter emits terahertz radiation (40) at the object (10) and the first receiver receives the terahertz radiation (40) reflected from an interface of the object (10) and emitted by the first transmitter; the second measuring device (30) is arranged such that the second transmitter emits terahertz radiation (42) at the object (10) and the second receiver receives the terahertz radiation (42) reflected from an interface of the object (10) and emitted by the second transmitter; The first transmitter emits terahertz radiation (40) within a first frequency range having a first bandwidth about a first center frequency, and the second transmitter emits terahertz radiation (42) within a second frequency range having a second bandwidth about a second center frequency different from the first center frequency; the apparatus comprises an evaluation device (44) that applies the measurements of the first and second receivers to determine the wall thickness of the object (10); the evaluation device (44) is configured to determine a plurality of periodic reference profiles (52, 54, 56, 58) that approximate a signal profile (50) of the measurement of the first receiver, compare the determined periodic reference profiles (52, 54, 56, 58) with a signal profile (60) of the measurement of the second receiver, and identify the periodic reference profile (52, 54, 56, 58) from the determined periodic reference profiles (52, 54, 56, 58) that best corresponds to the signal profile (60) of the measurement of the second receiver; The evaluation device (44) is also configured to determine a wall thickness of the object (10) based on the identified reference profile (52, 54, 56, 58).

2. 2. The apparatus of claim 1, wherein the signal profile and the periodic reference profile (52, 54, 56, 58) are phase profiles or frequency profiles.

3. 3. The device according to claim 1, wherein the periodic reference profile (52, 54, 56, 58) is a harmonic reference profile (52, 54, 56, 58).

4. 4. The apparatus according to claim 1, wherein the evaluation device is configured to identify the periodic reference profile (52, 54, 56, 58) that best corresponds to the signal profile (60) of the second receiver measurement based on a comparison of the determined phase of the periodic reference profile (52, 54, 56, 58) with the phase of the signal profile (60) of the second receiver measurement.

5. 5. The device according to claim 1, wherein the second center frequency is lower than the first center frequency.

6. 6. The device according to claim 5, wherein the second center frequency is equal to the first center frequency multiplied by a factor ranging from 0.3 to 0.7, preferably from 0.4 to 0.

6.

7. 7. The device of claim 1, wherein the first frequency range and the second frequency range do not overlap.

8. 8. The device according to claim 1, wherein the second bandwidth is smaller than the first bandwidth.

9. 9. The device according to claim 1, wherein the first bandwidth corresponds to less than one period of the determined periodic reference profile (52, 54, 56, 58), preferably less than or equal to three-quarters of a period, more preferably less than or equal to half a period.

10. 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. 11. The device according to claim 1, wherein the second bandwidth corresponds to less than half a period, preferably less than a quarter period, more preferably less than an eighth period of the determined periodic reference profile.

12. 12. The device according to any one of claims 1 to 11, characterized in that the second bandwidth is less than 30 GHz, preferably less than 20 GHz.

13. 13. Apparatus according to any one of claims 1 to 12, characterized in that the apparatus also comprises a transport device for transporting the object (10) during measurement by the apparatus.

14. 14. The device according to claim 1, wherein the first transmitter and the second transmitter are arranged to radiate terahertz radiation (40, 42) onto the object (10) while being offset from each other in a transport direction of the object (10).

15. 15. The apparatus of claim 1, wherein the first transmitter and the second transmitter are arranged to emit terahertz radiation (40, 42) at substantially the same angle and at substantially the same position on the surface of the object (10).

16. the first transmitter and the second transmitter are configured by a common transmitting device; the common transmitter emits both terahertz radiation (40) within the first frequency range having the first bandwidth around the first center frequency and terahertz radiation (42) within the second frequency range having the second bandwidth around the second center frequency different from the first center frequency; and / or the first receiver and the second receiver are configured by a common receiving device; 16. The apparatus of claim 1, wherein the common receiving device receives both terahertz radiation (40) within the first frequency range having the first bandwidth around the first center frequency and terahertz radiation (42) within the second frequency range having the second bandwidth around the second center frequency different from the first center frequency.

17. In particular, a method for determining the wall thickness of a flat or elongated object (10), comprising: a first transmitter emitting terahertz radiation (40) to the object (10), and a first receiver receiving the terahertz radiation (40) reflected from an interface of the object (10) and emitted by the first transmitter; a second transmitter emitting terahertz radiation (42) to the object (10), and a second receiver receiving the terahertz radiation (42) reflected from an interface of the object (10) and emitted by the second transmitter; 1. A method of transmitting terahertz radiation (40) within a first frequency range having a first bandwidth about a first center frequency, and a second transmitter of transmitting terahertz radiation (42) within a second frequency range having a second bandwidth about a second center frequency different from the first center frequency, comprising: determining a plurality of periodic reference profiles (52, 54, 56, 58) that approximate the signal profile (50) of the first receiver measurement, comparing the determined periodic reference profiles (52, 54, 56, 58) with the signal profile (60) of the second receiver measurement, and identifying the periodic reference profile (52, 54, 56, 58) from the determined periodic reference profiles (52, 54, 56, 58) that best corresponds to the signal profile (60) of the second receiver measurement; determining a wall thickness of the object (10) based on the identified reference profile (52, 54, 56, 58).

18. 18. Method according to claim 17, characterized in that the object (10) is transported through the measuring range of the device by a transport device during the measurement.

19. 19. Method according to any one of claims 17 or 18, characterized in that the method is carried out by a device according to any one of claims 1 to 16.

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