Method and apparatus for detecting defects in strand-shaped products

Abstract

The invention relates to a method for detecting defects (38) in a strand of product (12) conveyed in a conveying direction (14), comprising emitting terahertz radiation (24) by at least one transmitter (22) onto the strand of product (12) conveyed in said conveying direction (14) and receiving the terahertz radiation (24) reflected by the strand of product (12) by at least one receiver (22), and wherein the defects (38) in the strand of product (12) are inferred from a temporal change in the signal of the terahertz radiation received by the at least one receiver (22). The invention also relates to a corresponding device.

Description

Technical Field

[0001] The present invention relates to a method for detecting defects in a strand-shaped product conveyed in a conveying direction. The present invention also relates to an apparatus for detecting defects in a strand-shaped product conveyed in a conveying direction.

Background Art

[0002] For example, a thin plastic tube, a so-called microduct, into which a data cable made of an optical fiber is pushed at a later time, can be manufactured by an extrusion device. This type of plastic tube has a small outer diameter and inner diameter. Thin tubes, such as medical infusion lines and other medical tubes, can also be manufactured in this type of plant. In the case of this type of medical tube, it may be a single or multi-lumen tube. During the extrusion process, there is a possibility that residues of the material extruded from the extruder leak irregularly without being controlled. This type of extrusion residue falls, for example, at a rhythm of one day or several days, adheres to the inside of the extrusion tube on the inner wall of the tube, and may at least partially reduce the free cross-sectional area of the tube. This type of residue may remain and move inside the tube even when a liquid is guided during later use, be carried, and as a result, may lead to blockage of the tube or leak in an uncontrollable manner. Any of these scenarios must be absolutely avoided in medical applications. This type of residue generates corresponding waste.

[0003] Generally, due to the high conveying speed of extruded strands, especially thin ones, it has hitherto been impossible to detect defects, such as extrusion residues remaining inside the tube, to a satisfactory extent online. In the prior art, currently, two methods for detecting defects have been established. According to the first method, a mandrel of about 80% of the inner diameter of the tube is magnetically held inside the tube. Thereafter, when the mandrel moves away from its holding position, specifically, when it is moved away from its holding position by extrusion residues located inside the conveyed tube, a corresponding error signal is output. According to the second method, after extrusion is completed, a ball of about 80% of the inner diameter of the tube is blown over the entire roll of the extrusion tube by compressed air. If there is no extrusion residue inside, the ball will come out after passing through the entire tube. However, if the ball does not come out, this indicates the presence of extrusion residues. For example, in order to pass through properly and reliably, a wire is often passed through a medical tube during final production.

[0004] Known detection methods are complex and cannot be detected non - contact. Furthermore, at least in the latter method, detection can only be performed after the completion of the production process, and accordingly, a large amount of waste will be generated. Accurate localization of defects is difficult or impossible. Furthermore, known detection methods are only suitable for tubular products.

SUMMARY OF THE INVENTION

PROBLEM TO BE SOLVED BY THE INVENTION

[0005] Therefore, proceeding from the prior art described above, the object of the present invention is to provide a method and an apparatus of the type described at the beginning that can reliably and accurately detect defects in a conveyed strand - shaped product, specifically extrusion residues that narrow the cross - section inside the extruded strand - shaped product, online, without labor and non - contact.

MEANS FOR SOLVING THE PROBLEM

[0006] The present invention achieves this object according to independent claims 1 and 19. Advantageous embodiments can be found in the dependent claims, the description, and the drawings.

[0007] Regarding the method of the type described at the beginning, the present invention aims to radiate terahertz radiation to a strand-like product conveyed in the conveying direction by at least one transmitter, receive the terahertz radiation reflected by the strand-like product by at least one receiver, and infer a defect of the strand-like product from a temporary change in the signal of the terahertz radiation received by at least one receiver.

[0008] Regarding the device of the type described at the beginning, the present invention provides at least one transmitter that radiates terahertz radiation to a strand-like product conveyed in the conveying direction, and at least one receiver that receives the terahertz radiation reflected by the strand-like product, and further provides an evaluation device designed to infer a defect of the strand-like product from a temporary change in the signal of the terahertz radiation received by at least one receiver, thereby achieving the object.

[0009] Stranded products are, in particular, uniform products. A defect-free stranded product has a substantially constant profile, particularly with respect to shape and material. Specifically, it may be an elongated product. The stranded product may be a tube. The stranded product may have a small outer diameter and thus also a small inner diameter. The stranded product may be a product extruded by an extrusion device. The transport direction in which the stranded product is transported may specifically extend along the longitudinal axis of the stranded product. During the detection measurement according to the present invention, the stranded product may be transported at a high transport speed. The stranded product may be composed of, for example, a plastic material, glass, or paper. The stranded product may be a thin tube, a so-called microduct, into which, for example, a data cable made of an optical fiber material is subsequently pushed. The stranded product may include a conductive stranded component, such as an electrical cable. Further, the stranded product may be, for example, a thin tube for later medical use, such as an infusion line. In the case of this type of medical tube, these may be single or multi-lumen tubes. Further, the stranded product may be a profile, for example, a plastic profile. Plastic profiles are often very complex and there are substantially no suitable options for measuring dimensions online during manufacturing. The present invention is also suitable for this type of product and can identify defects occurring in the profile, such as extrusion residues.

[0010] The measurement of geometric parameters by terahertz radiation, such as the diameter and / or wall thickness of a strand conveyed in its longitudinal direction, for example a relatively large-diameter plastic tube conveyed relatively slowly, is known, for example, from International Application Publication No. WO 2016 / 139155 A1. The present invention is based on the discovery that even in the case of very small-diameter strand-shaped products conveyed very rapidly, defects such as extrusion residues located inside the tube can be reliably detected by terahertz radiation based on an appropriate evaluation of the signal of the received terahertz radiation. For this purpose, according to the present invention, terahertz radiation is emitted by at least one transmitter towards a strand-shaped product conveyed in the conveying direction, and the terahertz radiation reflected by the strand-shaped product is received by at least one receiver. The strand-shaped product may at least partially transmit the emitted terahertz radiation, and specifically may transmit it completely. Thus, at least part of the terahertz radiation is emitted through the product and is thus reflected at the interfaces on the outside and inside of the strand-shaped product. In this way, it is possible to detect defects, specifically extrusion residues located inside the strand-shaped product. However, the strand-shaped product may also be composed of a material that does not transmit the emitted terahertz radiation, such as metal. In this case, only defects on the outside of the product can originally be detected.

[0011] The terahertz radiation emitted by at least one transmitter impinges on the strand-shaped product, is reflected outside the strand-shaped product, and, if there is sufficient transparency, is reflected at the inner boundary surface. The reflected terahertz radiation reaches at least one receiver, and the receiver receives the reflected terahertz radiation as a signal of the terahertz radiation. At least one receiver records the temporal progression of the received terahertz radiation. The strand-shaped product moves in the transport direction through the measuring structure during the measurement according to the invention. In the best scenario, the strand-shaped product moves solely in the transport direction, for example along its longitudinal axis. In this case, if there are no defects, the signal of the terahertz radiation received by at least one receiver is substantially constant. Next, the occurrence of a defect, for example the occurrence of an extrusion residue located inside the strand-shaped product, is particularly clearly evidenced in the temporal progression of the signal of the terahertz radiation recorded by at least one receiver, or more precisely in its temporal variation.

[0012] According to the present invention, even when there are no defects and the received terahertz radiation signal is not constant, it is possible to detect defects from the terahertz radiation recorded by at least one receiver. In practice, especially small-diameter strand-shaped products that are conveyed particularly quickly move simultaneously in the transverse direction, specifically also in the direction transverse to the conveying direction, when being conveyed in the conveying direction. This transverse movement may specifically be a transverse vibration. The transverse movement may be, for example, a periodic movement. Next, at least one receiver receives a signal of amplitude-modulated and phase-modulated terahertz radiation even when there are no defects. This has not been possible to detect defects online in the prior art so far. For example, at least one receiver receives a periodic radiation signal even when there are no defects. The present invention is based on the discovery that even in such cases, defects bring about characteristic changes in the received terahertz radiation signal. For example, the transverse vibration of a strand-shaped product conveyed in the conveying direction generally has a low vibration frequency of less than 50 Hz, specifically less than 10 Hz, for example about 1 Hz. Furthermore, the strand-shaped product generally moves much faster in the conveying direction than in the transverse direction orthogonal to the conveying direction. The (maximum) moving speed of the strand-shaped product in the conveying direction, or more precisely the moving frequency, may be, for example, at least 10 times the (maximum) moving speed of the strand-shaped product in the direction transverse to the conveying direction, or more precisely the moving frequency. The inventors of the present application have discovered that in the case of a small-diameter strand-shaped product conveyed quickly in the conveying direction, due to defects such as extrusion residues remaining inside, temporary, specifically short-term, or transient, or rapid, or high-frequency changes appear in the received terahertz radiation signal. This applies especially when there are no defects, despite the possibility that the strand-shaped product may move in the transverse direction because the strand-shaped product has a substantially constant profile with respect to shape and material.

[0013] Defects cause anomalies that result in changes corresponding to the reflection behavior of the strand-shaped product due to their shape and, in some cases, material deviations originating from the material of the strand-shaped product. For example, defects of generally irregular shape, such as extrusion residues remaining inside the product, can cause diffuse reflection or irregular reflection of terahertz radiation compared to the inner and outer boundary surfaces of a defect-free product. As a result, the signal of the radiation received by at least one receiver will change accordingly. Since strand-shaped products are generally conveyed rapidly in the conveying direction, especially compared to lateral movement, the changes in the reflection behavior caused by defects are distinguishable and thus evaluable as rapid, temporary changes, specifically changes more rapid than lateral changes, or more precisely, high-frequency changes, in the temporal progression of the signal of the reflected terahertz radiation recorded according to the present invention.

[0014] Therefore, according to the present invention, defects of a strand-shaped product conveyed in the conveying direction can be inspected non-contact, online, from the outside, simply and reliably. Waste can be minimized. Detection according to the present invention can be performed in real time. Further measures can be taken based on the detected defects. For example, a warning signal can be output and / or the detected defects can be displayed, specifically in real time, and / or the production parameters of the extrusion device can be affected, and it is even possible to stop the extrusion device.

[0015] As a contrast to other possible measurement methods for detecting defects non - contact, such as the use of visible light or laser light, the measurement according to the invention using terahertz radiation has the advantage that terahertz radiation is hardly affected by disturbances that may occur in difficult measurement environments such as extrusion plants. For example, in this type of extrusion plant, as is well known, the extruded product is cooled by spraying cooling water, such as water, onto the surface of the product. In that process, a significant amount of steam is generated, which hinders optical measurement methods. As a possibility for detecting defects, it is also conceivable to use, for example, X - ray radiation. However, on the one hand, the cost becomes quite high, and on the other hand, strict safety requirements and the corresponding effort are required.

[0016] In the case of the evaluation according to the invention, the terahertz transmission signal radiated by at least one transmitter can be mixed with the terahertz reception signal received by at least one receiver. Thereafter, band - pass filtering, for example, low - pass filtering can be performed. The signal generated by the reflection of terahertz radiation results, in principle, from the wavelength L of the terahertz radiation, the distance d between at least one transmitter and / or at least one receiver and the interface surface causing the reflection, and the intensity s of the reflection.

[0017]

Number

[0018] Here, in the above formula, it is assumed that the transmitter and the receiver are in the same place, specifically in the form of a transceiver. The total received signal can be understood as the sum of all reflections.

[0019]

Number

[0020] At least one transmitter and at least one receiver may be arranged in substantially the same location. These may be integrated, for example, into a transceiver. At least one transmitter may emit single-frequency terahertz radiation. The frequency of the terahertz radiation may be, for example, in the frequency range from 10 GHz to 3 THz. The frequency of the terahertz radiation may be so-called millimeter waves. As already explained, using terahertz radiation can obtain reliable measurement results, especially in difficult process environments where optical systems such as laser systems have difficulties. At least one receiver and / or the corresponding transceiver preferably have I and Q channels. Therefore, quadrature amplitude modulation, which is well known to those skilled in the art, is basically possible. The amplitude modulation of two orthogonal carriers can be understood as the amplitude modulation and phase modulation of one carrier. Two baseband signals are called the in-phase component the I channel and the quadrature component the Q channel.

[0021] According to the present invention, an evaluation device of the device according to the present invention can specify a threshold value indicating a defect when exceeded, for a recognized temporary change in the signal of the terahertz radiation received by at least one receiver. For example, when a defect is detected, as already explained, a corresponding signal can be output, for example, an alarm, or an error counter, or other means can be implemented.

[0022] Similarly, as already explained, the strand-shaped product may be a tube extruded by an extrusion device. The defect detected according to the present invention may be, as explained, extrusion residue inside the tube. The device according to the present invention may comprise an extrusion device.

[0023] The strand-shaped product can be conveyed in the conveying direction at a conveying speed exceeding 50 m / min, preferably exceeding 75 m / min. Furthermore, the strand-shaped product may have an outer diameter of less than 10 mm, preferably less than 5 mm. When the strand-shaped product is a tube, the tube may correspondingly have an inner diameter of less than 10 mm, preferably less than 5 mm.

[0024] According to another embodiment, the (detected) structure or wall thickness of the strand-like product may be smaller than the wavelength of the terahertz radiation used. By evaluating the change in the signal of the received radiation, the present invention does not rely on elucidating the structure or, for example, the wall thickness. In conventional measuring devices, FMCW radiation is used. For this reason, for example, in order to measure the wall thickness of a tube, a correspondingly large bandwidth is required. As a result, generally, only a minimum wall thickness of about the wavelength can be measured. By using the method according to the present invention, it is possible to measure a structure or, more precisely, a wall thickness that does not require a large bandwidth and is particularly smaller than the wavelength of the terahertz radiation.

[0025] According to another embodiment, the present invention provides that when the strand-like product is conveyed in the conveying direction, it also performs a lateral movement in a direction transverse to the conveying direction, and the defects of the strand-like product are inferred from a temporary change in the signal of the terahertz radiation received by the receiver only when the temporarily changed frequency is higher than the frequency of the lateral movement of the strand-like product. The lateral change in the position of the product caused by the lateral movement leads to a change in the distance between at least one transmitter and / or receiver and the strand-like product, or more precisely, the boundary surface that reflects the terahertz radiation. Such a change in distance causes a phase change in the received terahertz radiation signal and further weakly changes the signal intensity. As already explained, such a change in the position of the product usually occurs mainly in the low-frequency range of less than 50 Hz, specifically less than 10 Hz, for example less than 2 Hz. Also, the defects of the strand-like product are related to the change in the signal. For example, if there are foreign substances such as extrusion residues inside the product, the reflection behavior changes. For example, additional reflections occur and / or the shape of the boundary surface changes locally, resulting in changes in the phase and / or reflection intensity of the terahertz radiation signal. This type of change usually has significant high-frequency components above 10 Hz, specifically above 50 Hz and less than 1 kHz. The frequency value can vary depending on the conveying speed. This makes it possible to reliably distinguish the lateral change in the position of the product due to the lateral movement from the defects. Even if there are lateral vibrations of the product within the frequency range of the defects, as long as they are small enough (amplitude x frequency) to be distinguishable from the defects, there is basically no problem. In the case of stronger vibrations, it is necessary to identify such vibrations in order to be able to filter them. However, such strong vibrations may reduce the measurement sensitivity and complicate the evaluation. Therefore, it should be the aim to prevent strong vibrations of the strand-like product in the above high-frequency range. For example, the defects of the strand-like product can be inferred only when the frequency of the temporary change is at least 5 times, preferably at least 10 times, higher than the frequency of the lateral movement of the strand-like product.According to the present invention, in order to prevent problems during the detection of defects caused by lateral movement, means can be provided to reduce the oscillation frequency of a strand-shaped product in a direction transverse to the transport direction, in a sufficient manner such that in particular all defects are reliably detected. Such means may comprise, for example, guide means corresponding to the strand-shaped product when the strand-shaped product is transported in the transport direction.

[0026] According to another particularly practical embodiment, a first derivative of the signal of the terahertz radiation received by at least one receiver can be formed. Next, if the simply derived signal of the terahertz radiation exceeds a defined threshold value, a defect can be inferred. By means of this derivative, in particular a temporary change in the received signal of the terahertz radiation can be determined by evaluating it in a particularly reliable and simple manner. As already explained at the beginning, the total signal received by at least one receiver can be understood as the sum of all individual reflections.

[0027]

Number

[0028] Here, the reflection from the product S P and the reflection from the interface surface in the environment of the product S U can basically be distinguished. Since the environment does not change during the measurement, the following equation applies.

[0029]

Number

[0030] Therefore, the derivative of the received terahertz radiation signal is caused only by changes in the interface of the strand-like product. When a defect-free product moves through the measurement structure according to the present invention without lateral movement, the reflection does not change and the derivative becomes zero. However, for example, if the strand-like product has low-frequency, periodic, lateral vibrations, a regular derivative signal will be generated accordingly. In contrast, a defect causes a temporary and relatively rapid change in the derivative signal and can thus be measured and identified in a reliable manner. The degree of the defect is essentially correlated with the intensity of the derivative of signal S. Therefore, this can be used to define a threshold value, and the change in the terahertz radiation signal derived therefrom can be defined as a significant defect and detected and / or displayed. The derived terahertz radiation signal can also be further processed to reduce the influence of disturbances and lower the detection threshold. For example, a receiving filter for minimizing the noise of the derived signal is conceivable and useful.

[0031] According to another embodiment, the nth derivative of the terahertz radiation signal received by at least one receiver can be formed, where n >= 2. If the n-fold derived terahertz radiation signal exceeds a defined threshold value, a defect can be inferred. The threshold values of the first derivative and the nth derivative may be different threshold values. Also, for example, by forming the second derivative, a rapid change in the received terahertz radiation signal can be investigated. In particular, this type of change can be defined more precisely. For example, the direction of the change can be investigated, and further information regarding the defect can be determined therefrom. Other functions may also be suitable for evaluating a rapid change in the received terahertz radiation signal.

[0032] According to another embodiment, the threshold value may be defined according to the conveying speed of the strand-like product in the conveying direction. The derivative of the received terahertz radiation signal is essentially proportional to the conveying speed of the product. If the conveying speed of the product is known, the derivative can be corrected accordingly when defining the threshold value.

[0033] The signal of the terahertz radiation received by at least one receiver can be filtered by a band-pass filter. The band-pass filter may be, for example, a band-pass filter that filters low-frequency signal components generated by the lateral movement of a strand-like product rather than defects. This simplifies the evaluation further.

[0034] According to another embodiment, at least one transmitter can emit a terahertz signal having a bandwidth less than the frequency corresponding to the spatial resolution of the diameter of the strand-like product. Accordingly, at least one receiver may have a reception bandwidth less than the frequency corresponding to the spatial resolution of the diameter of the strand-like product. In the prior art, a bandwidth of the speed of light / (2 * Refractive index * feature size) is applied. For example, when the feature size to be analyzed is 1 mm and the typical refractive index is 1.5, a bandwidth of 100 GHz is required. However, terahertz transceivers having this type of bandwidth are expensive and complex, and in particular, this type of terahertz transmitter is subject to special approval procedures. However, according to the present invention, it has been discovered that the method according to the present invention eliminates the need for spatial resolution as in the prior art, and thus the need for a large bandwidth, and instead can operate with a bandwidth smaller than the frequency required for the spatial resolution, specifically, the diameter of the strand-like product. As a result, according to the present invention, a simpler and more cost-effective transceiver can be used.

[0035] According to another related embodiment, at least one transmitter can emit a terahertz signal having a bandwidth of less than 5% of the average frequency of the terahertz signal, preferably less than 3% of the average frequency of the terahertz signal, more preferably less than 2% of the average frequency of the terahertz signal. As a result, the conventional ISM (Industrial, Scientific, and Medical) band can be used, and special approval procedures can be avoided. For example, the bandwidth is 1% or less of the average frequency. For example, the ISM band of 122 to 123 GHz can be used.

[0036] According to another embodiment, at least one transmitter can emit a terahertz continuous wave signal at a frequency having a substantially constant amplitude. Thereby, particularly good evaluation can be obtained. Therefore, at least one transmitter of the device according to the present invention may be a transmitter that emits a terahertz continuous wave signal at a frequency having a substantially constant amplitude.

[0037] According to another embodiment, the present invention provides that terahertz radiation is emitted to a strand-like product in which the terahertz radiation is conveyed from various directions to the conveying direction by a plurality of transmitters, the terahertz radiation emitted by the plurality of transmitters and reflected by the strand-like product is received by a plurality of receivers. The orientation of the boundary surface that reflects the terahertz radiation has a decisive influence on the strength of the reflection. Therefore, a defect boundary surface that is not preferably directed may have a much smaller change in the temporal progression of the signal of the received terahertz radiation than the same defect with different directions. In order to cancel such an undesired direction dependence, in the above-described embodiment, terahertz radiation is emitted to the strand-like product from different directions, for example, dispersed on the circumference of the strand-like product, and a plurality of transmitters and a plurality of receivers for receiving the terahertz radiation reflected by the product are provided. When the transmitter and receiver are integrated as a transceiver, accordingly, a plurality of transceivers can be provided, for example, dispersed on the circumference of the strand-like product.

[0038] Multiple transmitters can emit terahertz radiation of different frequencies. Therefore, the multiple transmitters of the device according to the present invention may be designed to emit terahertz radiation of different frequencies. When there are multiple transmitters, each transmitter preferably emits a terahertz continuous wave signal at a single frequency, for example, at a frequency having a substantially constant amplitude. In the above-described embodiment, the frequencies of the terahertz radiation emitted by different transmitters are different from each other. As a result, different signals emitted by different transmitters and received by the receiver are distinguishable from each other in evaluation. For example, by using an appropriate frequency filter, each receiver can reliably receive only the terahertz radiation emitted by a specific transmitter.

[0039] According to another embodiment, there is a defined differential frequency between the frequencies of the terahertz radiation emitted by the multiple transmitters, and the terahertz radiation emitted by the multiple transmitters is received by all receivers in any case, and the signals of the terahertz radiation received by the receivers can also be evaluated by demodulation of the respective differential frequencies. By using synchronized transmitters or transceivers having fixed differential frequencies, there is no need to perform filtering such that a specific receiver receives only the terahertz radiation of a specific transmitter. Instead, the signal modulated at the differential frequency of the transmission frequencies of the relevant transmitters is demodulated by the receiver accordingly. As a result, the sensitivity of the measurement according to the present invention is further improved, and the direction dependence is further canceled.

[0040] According to another embodiment, the received radiation signal can be added, in some cases, after a mathematical process, such as a first or second derivative, and in particular the square of the received radiation signal can be added, before the defect of the strand-shaped product is inferred. Thereby, a particularly simple evaluation method can be obtained. Furthermore, the error signals of all transmitters and / or receivers are reliably evaluated. For example, adding the squares of the absolute values of all the derived radiation signals is carried out as a particularly simple possibility in signal fusion. This is also possible when using a plurality of transmitters and / or receivers, and / or transceivers without using the Q channel.

[0041] According to another embodiment, the terahertz radiation emitted by at least one transmitter can be focused and / or shielded such that the range of the region of the strand-shaped product irradiated by the terahertz radiation in the transport direction of the strand-shaped product is smaller than when it is transverse to the transport direction. Thus, when viewed from the transport direction, the size of the measurement spot is smaller than the size of the measurement spot transverse to the transport direction. As a result, the discrimination and identification of defects can be further improved. For this purpose, the device according to the invention may comprise a corresponding focusing device and / or a screening device.

[0042] According to another embodiment, at least one transmitter and / or at least one receiver may be directed at the strand-shaped product such that the main beam direction of the terahertz radiation extends obliquely with respect to the transport direction of the strand-shaped product. The main beam direction may extend opposite to or obliquely to the transport direction of the strand-shaped product. For example, the main beam direction may extend at an angle of less than 80°, preferably 70° or less, with respect to the transport direction. The inclination position of the transmitter and / or receiver, particularly the transceiver, makes it possible to change the signal signature of the defect. Specifically, as a result, the frequency of the signal change increases, and the vibration frequency of the transported product and the frequency of the signal change can be better distinguished. For example, due to the change in the distance between the defect and the transmitter and / or receiver caused by the transport speed v, the signal change is approximately at a frequency f = 2 * v / λ * cos(α), where λ is the wavelength of the terahertz signal and α represents the angle between the transport direction and the connecting line between the defect and the transmitter and / or receiver. Since the transmitter and receiver transmit and receive directionally, respectively, the defect is measured only within a limited angular range depending on the orientation. Therefore, the direction perpendicular to the product (α is approximately equal to 90°, and thus cos(α) is close to 0) results in a much lower frequency than the inclined position. For example, when the transport speed of the product is 60 m / min, the wavelength is 2.5 mm (corresponding to 120 GHz), and the angle is α = 70°, a frequency of approximately 270 Hz is expected. Empirically, this results in a large gap with respect to the vibration frequency of the product, and as a result, even if the vibration frequency is within the range where an error signal exists in the case of the perpendicular direction, the vibration and the defect can be clearly distinguished. For a more pronounced inclination position, the frequency can be further increased if necessary to also handle higher frequency vibrations.

[0043] The device according to the invention, specifically at least one transmitter and / or at least one receiver, and / or the evaluation device may be designed to carry out the method according to the invention. Thus, the method according to the invention can be carried out by the device according to the invention.

[0044] Exemplary embodiments of the present invention will be described in more detail below with reference to schematic diagrams.

Brief Description of the Drawings

[0045]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

[0046] The same reference numerals refer to the same objects in the figures unless otherwise indicated.

Modes for Carrying Out the Invention

[0047] The device according to the invention represented in FIG. 1 comprises a strand-shaped product 12, in this case an extrusion device 10 for extruding a thin plastic tube 12. The strand-shaped product 12 may have a small outer diameter of less than 10 mm, preferably less than 5 mm, for example. After exiting the extrusion device 10, the strand-shaped product 12 is simultaneously conveyed in a conveying direction 14 corresponding to the longitudinal axis of the strand-shaped product 12. The conveying speed in the conveying direction 14 may be, for example, more than 50 m / min, preferably more than 75 m / min. During the movement of the strand-shaped product 12 in the conveying direction, the product may also perform a lateral movement in a direction transverse to the conveying direction, as indicated by arrow 16 in FIG. 1. This lateral movement is considerably smaller than the maximum speed in the conveying direction and has a maximum speed that is, for example, at least 10 times smaller. The lateral movement indicated by arrow 16 may be a substantially periodic lateral vibration. The lateral vibration may have a relatively low frequency of less than 10 Hz, for example less than 2 Hz, for example about 1 Hz.

[0048] After exiting the extrusion device 10, the strand-like product 12 generally moves through one or more cooling sections 18, where the strand-like product 12 is cooled for cooling purposes, for example, by spraying cooling water. A winding device 20 capable of winding the strand-like product 12 into a roll is arranged at the end of the device shown in FIG. 1. In the embodiment shown in FIG. 1, a transceiver 22 is arranged downstream of the cooling section 18. This transceiver comprises a transmitter that emits terahertz radiation to the strand-like product 12 and a receiver that receives the terahertz radiation reflected at the interface of the strand-like product 12. The terahertz radiation emitted by the transceiver 22, reflected by the strand-like product 12, and received again by the transceiver 22 is indicated by arrow 24 in FIG. 1. The measurement signal of the transceiver 22, or more precisely the receiver, is supplied to the evaluation device 28 via line 26. The evaluation device 28 is designed to infer defects in the strand-like product 12 from the temporary changes in the signals of the terahertz radiation received by at least one receiver. This will be explained in more detail below with reference to FIGS. 2 and 3. When the evaluation device 28 detects a corresponding defect, it can issue a corresponding error signal, for example, as indicated by arrow 30 in FIG. 1. Also, as indicated by arrow 32, the evaluation device 28 can influence the extrusion device 10, for example, by changing the production parameters of the extrusion device 10 or stopping the extrusion device 10.

[0049] In FIG. 2, the strand-like product 12 is represented in cross-section. Here, an internal space 36 surrounded by the circular wall 34 of the cross-section of the tubular strand-like product 12 can be seen. In FIG. 2, there are no defects, specifically extrusion residues, in the internal space 36 of the strand-like product 12. Accordingly, the transceiver 22, or more precisely the receiver of the transceiver 22, receives a regular, or more precisely a uniform signal, excluding the uniform signal oscillation caused by the lateral movement of the strand-like product 12. The terahertz radiation is reflected at different interfaces of the strand-like product 12, specifically the outer surface facing the transceiver 22 and the inner surface of the wall 34 facing the transceiver 22. Multiple reflections can also occur.

[0050] Figure 3 shows the depiction of Figure 2 in another operating state. As a result of moving the strand-shaped product 12 in the conveying direction, in the state represented in Figure 3, a defect 38 remaining in the form of extrusion residue in the internal space 36 of the strand-shaped product 12 enters the field of view of the terahertz radiation. Due to not only the addition of the boundary surface but also the change of the boundary surface, the defect leads to a rapid temporary change in the signal of the received terahertz radiation.

[0051] This will be described in detail based on Figures 5 and 6. In Figure 5, the signal of the reflected terahertz radiation received by the transceiver 22 is plotted as a raw signal in arbitrary units against time in milliseconds [ms]. Between about 20 ms and 25 ms, for example, a rapid change in a signal having a frequency of about 300 Hz can be seen. This rapid change in the signal is caused by the defect 38. As can already be seen from the raw signal in Figure 5, the defect is easily distinguishable from the subsequent uniform oscillation of the received terahertz radiation signal, which can be caused, for example, by the lateral vibration of the strand-shaped product 12. Figure 5 also shows that the oscillation of the terahertz signal caused by the lateral vibration of the strand-shaped product 12 has a much lower frequency than the change in the signal caused by the defect 38, which is, for example, only about 30 Hz in this case.

[0052] Figure 6 shows a mathematically processed version of the raw signal shown in Figure 5. Here, the received and mathematically processed terahertz radiation signal is plotted again in arbitrary units as a function of time in milliseconds [ms]. To arrive at the processed signal shown in Figure 6, the square of the second mathematical derivative of the raw signal shown in Figure 5 (this derivative is filtered, for example, by a band-pass filter) is used. Specifically, as will be explained in more detail below based on Figure 4, for example, when multiple transceivers are used, it is possible to add the terahertz radiation signals received by the various transceivers to arrive at the evaluation signal represented in Figure 6. In Figure 6, the defect 38, which was already basically distinguishable in the raw signal of Figure 5, stands out very clearly from the remaining signal curve. The lateral vibration of the strand-like product 12 no longer has a significant effect. As can be seen in Figure 6, an appropriate threshold for the mathematically processed terahertz radiation signal can be established in a simple way for defect output.

[0053] Another exemplary embodiment will be described below with reference to Figure 4. In the exemplary embodiment according to Figure 4, in the illustrated example, three transceivers 22 are arranged to be distributed on the circumference of the strand-like product 12, and each of these transceivers emits terahertz radiation to the strand-like product 12 and receives the terahertz radiation reflected by the boundary surface of the strand-like product 12, as indicated again by the arrow 24. Note that in Figure 4, only the main radiation direction where the strongest reflection occurs is shown. It should be understood that each of the transceivers 22 emits terahertz radiation that completely covers the strand-like product 12 in cross-section.

[0054] As already explained, the signal of the terahertz radiation received by the transceiver 22 in FIG. 4 can be optionally added after mathematical processing in order to obtain the processed radiation signal shown in FIG. 6. Also, different transceivers 22 can emit terahertz radiation of different frequencies, and it is also possible to assign the received terahertz radiation to individual transceivers 22. In this way, for example, in any case, only one receiver can surely receive terahertz radiation from one transmitter. Also, for example, terahertz radiation from various transmitters can be received by all receivers, and if the terahertz radiation is, for example, a differential frequency strictly defined between transmission frequencies accordingly, it will also be possible to demodulate it.

[0055] Of course, additional transceivers 22 may be arranged, for example, at regular intervals over the entire circumference so as to be distributed on the circumference of the strand-like product 12. By providing a plurality of transceivers 22, the direction dependence of defect detection can be canceled.

Description of Signs

[0056] [[ID=***]] 10 Extrusion device 12 Strand-like product 14 Conveying direction 16 Arrow 18 Cooling section 20 Take-up device 22 Transceiver 24 Arrow 26 Line 28 Evaluation device 30 Arrow 32 Arrow 34 Circular wall 36 Internal space 38 Defect It should be noted that some of the tags from ID 8 to ID 10 seem to be missing in the original text, so I've left them as they are in the translation with "***" to indicate the potential omission. If there's any specific correction or additional information for these, please let me know.

Claims

Claim 1 A method for detecting a defect (38) in a strand-shaped product (12) conveyed in a conveying direction (14), comprising: when the strand-shaped product (12) is conveyed in the conveying direction (14), it also performs a lateral movement in a direction transverse to the conveying direction (14); terahertz radiation (24) is radiated by at least one transmitter (22) onto the strand-shaped product (12) conveyed in the conveying direction (14), and the terahertz radiation (24) reflected by the strand-shaped product (12) is received by at least one receiver (22); each receiver (22) corresponding to each transmitter (22) is arranged at substantially the same location as the respective transmitter (22); a defect (38) in the strand-shaped product (12) is inferred from a temporary change in the signal of the terahertz radiation received by the at least one receiver (22); the defect (38) in the strand-shaped product (12) is inferred from the temporary change in the signal of the terahertz radiation received by the receiver (22) only when the frequency of the temporarily changed terahertz radiation signal is higher than the frequency of the lateral movement of the strand-shaped product (12). Claim 2 The strand-shaped product (12) is a tube extruded by an extrusion device (10), wherein the detected defect (38) is an extrusion residue inside the tube. The method according to claim 1 Claim 3 The structure or wall thickness of the strand-shaped product (12) is smaller than the wavelength of the terahertz radiation used. The method according to any one of claims 1 or 2 Claim 4 forming a first derivative of the signal of the terahertz radiation received by the at least one receiver (22); when the first derivative of the signal of the terahertz radiation exceeds a defined threshold, a defect (38) is inferred. The method according to any one of claims 1 to 3 Claim 5 forming an nth derivative of the signal of the terahertz radiation received by the at least one receiver (22), where n >= 2; a defect (38) is inferred when the nth derivative of the signal of the terahertz radiation exceeds a defined threshold. The method according to any one of claims 1 to 4 Claim 6 The method according to any one of claims 4 or 5, characterized in that the threshold value is defined according to the conveying speed of the strand-like product (12) in the conveying direction (14).

7. The method according to any one of claims 1 to 6, characterized in that the signal of the terahertz radiation received by the at least one receiver (22) is filtered by a band-pass filter.

8. The method according to any one of claims 1 to 7, characterized in that the at least one transmitter (22) emits a terahertz signal having a bandwidth less than the frequency corresponding to the spatial resolution of the diameter of the strand-like product (14).

9. The method according to any one of claims 1 to 8, characterized in that the at least one transmitter (22) emits a terahertz continuous wave signal at a frequency having a substantially constant amplitude.

10. The terahertz radiation (24) is emitted by a plurality of transmitters (22) to the strand-like product (12) conveyed in the conveying direction (14) from various directions, The method according to any one of claims 1 to 9, characterized in that the terahertz radiation (24) emitted by the plurality of transmitters (22) and reflected by the strand-like product (12) is received by a plurality of receivers (22).

11. The method according to claim 10, characterized in that the plurality of transmitters (22) emit terahertz radiation (24) of different frequencies.

12. There is a differential frequency defined in any case between the frequencies of the terahertz radiation (24) emitted by the plurality of transmitters (22), The terahertz radiation emitted by the plurality of transmitters (22) is received by all receivers (22) in any case, The method according to claim 11, characterized in that the signal of the terahertz radiation received by the receiver (22) is evaluated by demodulation of each of the differential frequencies.

13. The signal of the received radiation is added before a defect (38) of the strand-like product (12) is inferred, and in particular, the square of the signal of the received radiation is added when reliably evaluating the error signals of all transmitters (22) and / or all receivers (22). The method according to any one of claims 1 to 12, characterized in that.

14. The terahertz radiation (24) emitted by the at least one transmitter (22) is focused and / or shielded such that the extent of the region of the strand-shaped product (12) irradiated by the terahertz radiation (24) in the transport direction (14) of the strand-shaped product (12) is smaller than in the case where it is transverse to the transport direction (14), according to any one of claims 1 to 13.

15. The at least one transmitter (22) and / or the at least one receiver (22) are directed towards the strand-shaped product (12) such that the main beam direction of the terahertz radiation (24) extends obliquely with respect to the transport direction of the strand-shaped product (12), according to any one of claims 1 to 14.

16. A device for detecting a defect (38) in a strand-shaped product (12) transported in a transport direction (14), wherein the strand-shaped product (12) also performs a lateral movement transverse to the transport direction (14) when being transported in the transport direction (14), the device comprising: at least one transmitter (22) that emits terahertz radiation (24) towards the strand-shaped product (12) transported in the transport direction (14); at least one receiver (22) that receives the terahertz radiation (24) reflected by the strand-shaped product (12); and each receiver (22) corresponding to each transmitter (22) is arranged at substantially the same location as each respective transmitter (22), the device further comprising an evaluation device (28) designed to infer the defect (38) of the strand-shaped product (12) from a temporary change in the signal of the terahertz radiation received by the at least one receiver (22), the device being further designed such that the defect is inferred from the temporary change in the signal of the terahertz radiation only when the temporarily changed frequency of the signal of the terahertz radiation is higher than the frequency of the lateral movement of the strand-shaped product (12).

17. The device further comprises an extrusion device (10), wherein the strand-shaped product (12) is a tube extruded by the extrusion device (10). The device according to claim 16, wherein the detected defect (38) is an extrusion residue inside the tube.

18. The device comprises a plurality of transmitters (22) that emit terahertz radiation (24) from various directions to the strand-shaped product (12) conveyed in the conveying direction (14), The device according to any one of claims 16 or 17, characterized in that the device comprises a plurality of receivers (22) that receive the terahertz radiation (24) emitted by the plurality of transmitters and reflected by the strand-shaped product (12).

19. The device according to any one of claims 16 to 18, characterized in that specifically the at least one transmitter (22) and / or the at least one receiver (22), and / or the evaluation device (28) are designed to implement the method according to any one of claims 1 to 15.

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