METHOD AND DEVICE FOR DETECTING DEFECTS OF A STRAND-LIKE PRODUCT

Abstract

A method is provided for detecting a defect of a strand-like product conveyed in a conveying direction at a conveying speed of more than 50 m/min. The method includes determining a frequency of a lateral movement of the strand-like product. The lateral movement occurs in a direction that is transverse to the conveying direction. Terahertz radiation is emitted at a wavelength by at least one transmitter onto the strand-like product conveyed in the conveying direction. The terahertz radiation is reflected by the strand-like product and received by at least one receiver. A temporary change in the terahertz radiation received by the at least one receiver is detected. A defect in the strand-like product is inferred when the temporary change in the terahertz radiation received by the at least one receiver is higher than the frequency of the lateral movement of the strand-like product.

Claims

1-21. (canceled)

22. A method for detecting a defect of a strand-like product conveyed in a conveying direction at a conveying speed of more than 50 m/min, the method comprising: determining a frequency of a lateral movement of the strand-like product, wherein the lateral movement occurs on a direction that is transverse to the conveying direction; emitting terahertz radiation at a wavelength by at least one transmitter onto the strand-like product conveyed in the conveying direction; reflecting the terahertz radiation by the strand-like product; receiving the reflected terahertz radiation by at least one receiver; detecting a temporary change in the terahertz radiation received by the at least one receiver; and inferring a defect in the strand-like product when the temporary change in the terahertz radiation received by the at least one receiver is higher than the frequency of the lateral movement of the strand-like product.

23. The method according to claim 22, wherein the strand-like product comprises a pipe extruded in an extrusion device, and wherein the defect comprises extrusion residue inside the pipe.

24. The method according to claim 22, further comprising conveying the strand-like product at a conveying speed of more than 75 m/min in the conveying direction.

25. The method according to claim 22, further comprising forming a wall thicknesses of the strand-like product that is smaller than a wavelength of the THz radiation used.

26. The method according to claim 22, further comprising: forming a first derivative of the terahertz radiation received by the at least one receiver; and inferring the defect when the derived terahertz radiation exceeds a defined threshold value.

27. The method according to claim 26, further comprising forming a n-th derivative of the terahertz radiation received by the at least one receiver, wherein n2, and inferring the defect when the n-fold derived terahertz radiation exceeds a defined threshold value.

28. The method according to claim 26, further comprising defining the threshold value based on the conveying speed of the strand-like product in the conveying direction.

29. The method according to claim 22, further comprising filtering the terahertz radiation received by the at least one receiver using a band-pass filter.

30. The method according to claim 22, further comprising emitting a terahertz signal by the least one transmitter, wherein the terahertz signal comprises a bandwidth that is less than a frequency that corresponds to a spatial resolvability of a diameter of the strand-like product.

31. The method according to claim 22, wherein the at least one transmitter emits a terahertz signal with a bandwidth of less than 5% of an average frequency of the terahertz signal.

32. The method according to claim 22, further comprising structuring the at least one transmitter to emit a terahertz continuous wave signal at a frequency with a substantially constant amplitude.

33. The method according to claim 22, further comprising configuring multiple transmitters to emit the terahertz radiation from different directions onto the strand-like product conveyed in the conveying direction, and configuring multiple receivers to receive the terahertz radiation reflected by the strand-like product.

34. The method according to claim 33, further comprising configuring characterized the multiple transmitters to emit terahertz radiation of different frequencies.

35. The method according to claim 34, further comprising configuring the multiple receivers to evaluate the received terahertz radiation using demodulation of the different frequencies of the emitted terahertz radiation.

36. The method according to claim 32, further comprising summing squares of the received radiation signals before the defect is inferred.

37. The method according to claim 22, further comprising focusing the terahertz radiation emitted by the at least one transmitter such that a region of the strand-like product irradiated by the terahertz radiation in the conveying direction is smaller than in a direction transverse to the conveying direction.

38. The method according to claim 22, further comprising orienting at least one of (i) the at least one transmitter and (ii) the at least one receiver with respect to the strand-like product such that a main beam direction of the terahertz radiation extends obliquely to the conveying direction of the strand-like product.

39. A device for detecting a defect of a strand-like product conveyed in a conveying direction at a conveying speed of more than 50 m/min, wherein the strand-like product performs a lateral movement transversely to the conveying direction when being conveyed in the conveying direction, the device comprising: at least one transmitter configured to emit terahertz radiation onto the strand-like product conveyed in the conveying direction; at least one receiver configured to receive terahertz radiation reflected by the strand-like product; and an evaluation apparatus configured to infer the defect of the strand-like product from a temporary change in a frequency of the terahertz radiation received by the at least one receiver that is higher than a frequency of the lateral movement of the strand-like product.

40. The device according to claim 39, further comprising an extrusion device, wherein the strand-like product comprises a pipe extruded in the extrusion device, and wherein that the defect to be detected comprises extrusion residue inside the pipe.

41. The device according to claim 39, further comprising: multiple transmitters configured to emit terahertz radiation from different directions onto the strand-like product conveyed in the conveying direction, and multiple receivers configured to receive the terahertz radiation emitted by the multiple transmitters and reflected by the strand-like product.

Description

DESCRIPTION OF THE DRAWINGS

[0037] Exemplary embodiments of the invention are explained below in greater detail using schematic drawings, in which:

[0038] FIG. 1 schematically illustrates an embodiment of a device for detecting defects of a strand-like product.

[0039] FIG. 2 schematically illustrates a part of the device shown in FIG. 1 in a first operating state.

[0040] FIG. 3 schematically illustrates the embodiment of FIG. 2 in a second operating state.

[0041] FIG. 4 schematically illustrates a part of the device shown in FIG. 1.

[0042] FIG. 5 shows a terahertz radiation signal received using the device of FIG. 1 and/or an embodiment of the disclosed method for detecting defects of a strand-like product.

[0043] FIG. 6 illustrates an embodiment of received terahertz radiation signal from FIG. 5 in a processed state.

[0044] The same reference signs refer to the same objects in the figures unless indicated otherwise.

DETAILED DESCRIPTION

[0045] The device according to the invention represented in FIG. 1 comprises an extrusion device 10 for extruding a strand-like product 12, in the present case a thin plastic pipe 12. The strand-like product 12 may have a small outer diameter of, for example, less than 10 mm, preferably less than 5 mm After exiting the extrusion device 10, the strand-like product 12 is conveyed in a conveying direction 14 that corresponds, at the same time, to the longitudinal axis of the strand-like product 12. The conveying speed in the conveying direction 14 may comprise, for example, more than 50 m/min, preferably more than 75 m/min During the movement of the strand-like product 12 in the conveying direction, said product can also perform a lateral movement transversely to the conveying direction, as illustrated by the arrow 16 in FIG. 1. This lateral movement has a maximum speed that is significantly less than the maximum speed in the conveying direction, for example at least 10 times less. The lateral movement illustrated by the arrow 16 may be a substantially periodic lateral oscillation. The lateral oscillation may have a comparatively low frequency of less than 10 Hz, for example less than 2 Hz, for example approximately 1 Hz.

[0046] After exiting the extrusion device 10, the strand-like product 12 generally travels through one or more cooling sections 18, in which the strand-like product 12 is cooled down for the purpose of cooling, for example by spraying on a coolant. A rolling-up device 20, in which the strand-like product 12 can be rolled up into a roll, is arranged at the end of the device represented in FIG. 1. In the example represented in FIG. 1, a transceiver 22 is arranged downstream of the cooling section 18, which transceiver comprises a transmitter for emitting terahertz radiation onto the strand-like product 12 and a receiver for receiving terahertz radiation reflected on boundary surfaces of the strand-like product 12. The terahertz radiation emitted by the transceiver 22, reflected on the strand-like product 12, and received again by the transceiver 22 is illustrated in FIG. 1 by means of the arrow 24. Measurement signals of the transceiver 22 or rather receiver are supplied to an evaluation apparatus 28 via a line 26. The evaluation apparatus 28 is structured to infer a defect of the strand-like product 12 from a temporary change in the terahertz radiation signal received by the at least one receiver. This is explained in greater detail below with reference to FIGS. 2 and 3. If the evaluation apparatus 28 detects a corresponding defect, it can for example emit a corresponding error signal, as illustrated in FIG. 1 by the arrow 30. It can also influence the extrusion device 10, as illustrated by the arrow 32, for example change production parameters of the extrusion device 10 or stop the extrusion device 10.

[0047] In FIG. 2, the strand-like product 12 is represented in cross-section. Here, an internal space 36 enclosed by a cross-sectionally circular wall 34 of the strand-like product 12 in the form of a pipe can be seen. In FIG. 2, there is no defect, in particular no extrusion residue, in the internal space 36 of the strand-like product 12. The transceiver 22 or rather the receiver of the transceiver 22 accordingly receives an accordingly regular or rather uniform signal, except for a uniform signal oscillation caused by any lateral movement of the strand-like product 12. The terahertz radiation is reflected on the different boundary surfaces of the strand-like product 12, in particular on the outer face facing the transceiver 22 as well as on the inner face of the wall 34 facing the transceiver 22. Multiple reflections may also occur.

[0048] FIG. 3 shows the representation from FIG. 2 in another operating state. As a result of moving the strand-like product 12 in the conveying direction, in the state represented in FIG. 3, a defect 38 remaining in the internal space 36 of the strand-like product 12 in the form of extrusion residue is in the field of view of the terahertz radiation. Due to the additional boundary surfaces as well as the change in boundary surfaces, this leads to a rapid temporary change in the received terahertz radiation signal.

[0049] This will be explained in more detail based on FIGS. 5 and 6. In FIG. 5, the reflected terahertz radiation signal received by the transceiver 22 is plotted as a raw signal in arbitrary units against the time in milliseconds [ms]. A rapid change in the signal with a frequency of, for example, approximately 300 Hz can be seen between about 20 ms and 25 ms. This rapid change in the signal is caused by the defect 38. As can already be seen in the raw signal of FIG. 5, said defect can be easily discriminated from a subsequent uniform oscillation of the received terahertz radiation signal, which may be caused, for example, by a lateral oscillation of the strand-like product 12. FIG. 5 also shows that, due to the lateral oscillation of the strand-like product 12, the oscillation of the terahertz signal has a much lower frequency than the change in the signal caused by the defect 38, in the present case only about 30 Hz, for example.

[0050] FIG. 6 shows a mathematically processed version of the raw signal shown in FIG. 5. Here, the received and mathematically processed terahertz radiation signal is again plotted in arbitrary units as a function of the time in milliseconds [ms]. In order to arrive at the processed signal shown in FIG. 6, squares of the second mathematical derivatives of the raw signal shown in FIG. 5, which derivatives are filtered, for example, by means of a band-pass filter, are used. In particular, if, for example, multiple transceivers are used, as is explained in greater detail below based on FIG. 4, it is possible to add up the received terahertz radiation signals of the various transceivers in order to arrive at the evaluation signal represented in FIG. 6. In FIG. 6, the defect 38, which is already fundamentally discernible in the raw signal in FIG. 5, stands out very clearly from the remaining signal curve. The oscillation of the strand-like product 12 in the lateral direction no longer has a noticeable effect. As can be seen in FIG. 6, a suitable threshold value of the mathematically processed terahertz radiation signal can be established in a simple manner for the output of a defect.

[0051] Another exemplary embodiment will be explained below with reference to FIG. 4. In the exemplary embodiment according to FIG. 4, in the example shown, three transceivers 22 are arranged so as to be distributed over the circumference of the strand-like product 12, which transceivers each emit terahertz radiation onto the strand-like product 12 and receive terahertz radiation reflected by boundary surfaces of the strand-like product 12, as illustrated again by the arrows 24. It should be noted that, in FIG. 4, only the main radiation direction in which the strongest reflections are produced is represented. It should be understood that the transceivers 22 each emit terahertz radiation which completely covers the strand-like product 12 in cross-section.

[0052] As already explained, the received terahertz radiation signals of the transceivers 22 in FIG. 4 can optionally be added up after mathematical processing in order to obtain the processed radiation signal shown in FIG. 6. It is also possible that the different transceivers 22 emit terahertz radiation of different frequencies, such that the received terahertz radiation can be assigned to the individual transceivers 22. In this way, it can for example be ensured that, in each case, only one receiver receives terahertz radiation from one transmitter in each case. It would also be possible, for example, for terahertz radiation from various transmitters to be received by all receivers and for said terahertz radiation to be accordingly demodulated, for example, in the case of a rigidly defined differential frequency between the transmission frequencies.

[0053] Of course, further transceivers 22 may also be arranged so as to be distributed over the circumference of the strand-like product 12, for example at regular intervals over the entire circumference. By providing multiple transceivers 22, any dependence of the detection of defects on direction can be counteracted.

LIST OF REFERENCE SIGNS

[0054] 10 Extrusion device [0055] 12 Strand-like product [0056] 14 Conveying direction [0057] 16 Arrow [0058] 18 Cooling section [0059] 20 Rolling-up device [0060] 22 Transceiver [0061] 24 Arrow [0062] 26 Line [0063] 28 Evaluation apparatus [0064] 30 Arrow [0065] 32 Arrow [0066] 34 Circular wall [0067] 36 Internal space [0068] 38 Defect