TERAHERTZ MEASURING DEVICE AND METHOD FOR CALIBRATING A TERAHERTZ MEASURING DEVICE

Description

[0001] The invention relates to a terahertz measuring device for measuring the wall thickness of an object, wherein the terahertz measuring device comprises a terahertz transmitter rotatable by means of or along a rotating ring about a measuring area which receives the object during a measurement, and a terahertz receiver rotatable by means of or along the rotating ring about the measuring area, which receives terahertz radiation emitted by the terahertz transmitter onto the object after reflection by an object arranged in the measuring area, and wherein the terahertz measuring device further comprises an evaluation device configured to determine a wall thickness of the object from the measurement data of the terahertz receiver.

[0002] The invention further relates to a method for calibrating a terahertz measuring device for measuring the wall thickness of an object, wherein the terahertz measuring device comprises a terahertz transmitter rotatable by means of or along a rotating ring about a measuring area which receives the object during a measurement, and a terahertz receiver rotatable by means of or along the rotating ring about the measuring area, which receives terahertz radiation emitted by the terahertz transmitter onto the object after reflection by an object arranged in the measuring area, and wherein the terahertz measuring device further comprises an evaluation device configured to determine a wall thickness of the object from the measurement data of the terahertz receiver.

[0003] Terahertz measuring devices enable highly accurate measurement of geometric parameters, such as wall thickness, of objects that are at least partially transparent to terahertz radiation, for example, plastic pipes, even in challenging measurement environments. For this purpose, terahertz radiation is emitted from the terahertz transmitter onto the object, where it is reflected by the object's interfaces. The reflected terahertz radiation is then received by the terahertz receiver. An evaluation unit determines, for example, the object's wall thickness by analyzing the travel times of the different radiation components.

[0004] Terahertz measuring devices are used to measure objects extruded in extrusion dies, such as plastic pipes. Measurement of the object's geometric parameters is possible shortly after it exits the extrusion die. It is also known from WO 2016 / 139155 A1 to determine the refractive index of the object's material from the received terahertz radiation. For a comprehensive measurement of the object, for example, a pipe around its circumference, the terahertz transmitter and receiver can be rotated synchronously with a rotating ring or along a rotating ring of the terahertz measuring device around the object, such as the pipe. This allows for continuous or intermittent measurements of parameters such as the wall thickness, outer diameter, and / or inner diameter of the pipe around its circumference.This allows for the detection of ovality or deviations, for example in wall thickness around the circumference, caused by sagging (i.e., the gravity-induced sinking of still-meltable portions of the pipe during the solidification process). If necessary, adjustments can be made to the extrusion equipment based on this information.

[0005] Terahertz measuring devices of this type must be calibrated before being put into operation, and usually also at regular intervals during operation, for example according to the ISO 9001 standard. Currently, cylindrical or tubular calibration objects are manufactured for this purpose and measured by a certification body prior to a calibration procedure, for example, with regard to wall thickness. Such a calibration object, for example a tubular one, is inserted as centrally as possible into the measuring range of the terahertz measuring device, which is defined by the rotating ring. The terahertz measuring device then measures the wall thickness of the calibration object as described above. The measurement results can be compared with the previously certified values ​​for the wall thickness, and the terahertz measuring device can be calibrated accordingly.

[0006] This type of calibration presents several problems. Firstly, suitable calibration tubes, for example made of a dimensionally stable glass material, cannot be manufactured with a sufficiently precise and uniform wall thickness, either circumferentially or longitudinally. It should be noted that the terahertz measuring devices described above achieve a measurement accuracy with a tolerance in the range of a few micrometers, specifically less than 5 µm. Since manufacturing calibration tubes with consistently high accuracy within this tolerance range is practically impossible, even slight positioning deviations of the calibration tube in the terahertz measuring device—be it a minor rotation or displacement in the longitudinal direction—lead to corresponding deviations in the wall thickness measured by the terahertz device. This makes the calibration process extremely difficult and time-consuming.For example, for customers who use such terahertz measuring devices during operation to measure geometric parameters of, for example, extruded pipes, it is practically impossible to perform the calibration themselves. This increases the workload for customers who typically have to submit calibration certificates at regular intervals, for example, according to the ISO 9001 standard. In this case, they often have to contact the manufacturer of the respective terahertz measuring device.

[0007] Furthermore, due to the large manufacturing tolerances of such calibration tubes, certification bodies only certify measurement results with tolerances in the range of + / - 50 µm. Given the at least ten times better measurement accuracy of the terahertz measuring device, such tolerances are unsuitable for the calibration process. Certified measurement data with tolerances in the range of + / - 1 µm would be required or even desirable.

[0008] From DE 10 2020 116 810 A1, a terahertz measurement method for measuring a pipe made of plastic or rubber is known, the refractive index of which is not assumed to be known. To determine the geometric wall thickness of the pipe, a gap (i.e., the distance between two inserted terahertz sensors) and the gap time (the travel time of the terahertz radiation in the gap) are determined in a calibration step preceding the measurement. For this purpose, a double-sided calibration reflector is positioned in the measurement area so that its two side faces are perpendicular to the optical axis of the terahertz sensors. The calibration reflector can, for example, be a precision sheet with a very precisely known thickness.Subsequently, the first and the opposite second terahertz sensors each emit terahertz radiation onto the calibration reflector, and the side surface of the calibration reflector facing each terahertz sensor reflects the terahertz radiation back to that sensor. In this way, two idle transmission times are determined from the terahertz sensors to their respective sides of the calibration reflector. The idle transmission time between the terahertz sensors is determined by summing the two idle transmission times and the known thickness of the calibration reflector. Taking this determined idle transmission time into account, the wall thickness of the pipe can be determined during a subsequent measurement, even if the refractive index is unknown.

[0009] SCHREINER N ET AL: "Mobile Handheld FMCW Terahertz Multilayer Thickness Inspection", 2019 44TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ), IEEE, September 1, 2019 (2019-09-01), pages 1-2, XP033636756, DOI: 10.1109 / IRMMW-THZ.2019.8873942 describes a mobile handheld device for terahertz measurement of multilayer structures, comprising an FMCW terahertz transceiver. As part of a verification of the handheld device, wall thicknesses of different objects with known wall thicknesses were measured using both measuring devices, and the measurement results of the devices were compared.

[0010] Based on the prior art described above, the invention is therefore based on the objective of providing a terahertz measuring device and a method of the type mentioned at the outset, with which the calibration of a terahertz measuring device is possible in a more precise and simpler manner, in particular also for customers who use the terahertz measuring device in ongoing operation.

[0011] The invention solves the problem through independent claims 1 and 18. Advantageous embodiments can be found in the dependent claims, the description and the figures.

[0012] For a terahertz measuring device, the invention solves the problem by... that a plate-shaped calibration object with a known wall thickness, at least partially transparent to the terahertz radiation emitted by the terahertz transmitter, is arranged within the rotating ring; that the terahertz transmitter can be brought into a rotating position so that the terahertz transmitter emits terahertz radiation along a vertical principal beam direction onto the calibration object; that the terahertz receiver can be brought into a rotating position so that the terahertz receiver receives terahertz radiation reflected from interfaces of the calibration object and / or radiating through the calibration object; that the evaluation device is designed to determine the wall thickness of the calibration object from the measurement data of the terahertz receiver and to calibrate the terahertz measuring device by comparing the determined wall thickness with the known wall thickness of the calibration object.

[0013] For a method of the type mentioned above, the invention solves the problem by the following steps: A plate-shaped calibration object with a known wall thickness, at least partially transparent to the terahertz radiation emitted by the terahertz transmitter, is arranged within the rotating ring. The terahertz transmitter is moved into a rotating position so that it emits terahertz radiation along a vertical main beam direction onto the calibration object. The terahertz receiver is moved into a rotating position so that it receives terahertz radiation reflected from and / or passing through the calibration object. The wall thickness of the calibration object is determined from the measurement data of the terahertz receiver by the evaluation unit, and the terahertz measuring device is calibrated by comparing the wall thickness determined by the evaluation unit with the known wall thickness of the calibration object.

[0014] The terahertz measuring device according to the invention operates as described above in relation to the prior art. Terahertz radiation is emitted from the terahertz transmitter, which is rotatable with or along the rotating ring, onto an object located in the measuring area, for example, a plastic tube extruded in an extrusion device. The terahertz radiation penetrates the object, which is at least partially transparent to the radiation, and is reflected at its interfaces. It can also pass through the object. The reflected and / or transmitted radiation components are received by the terahertz receiver, which rotates synchronously with the terahertz transmitter, and the measurement data is supplied to the evaluation unit. Based on this, the evaluation unit determines the wall thickness and, if applicable, other geometric parameters, such as the outer and / or inner diameter or ovality of the object, for example, the tube.The terahertz transmitter and receiver can be rotated around the object during measurement, particularly around the circumference of a pipe being measured. The terahertz transmitter can continuously or successively emit terahertz radiation onto the object. This radiation is received by the terahertz receiver after reflection at the object's interfaces and / or after passing through the object, and then evaluated by the evaluation unit as described. In this way, the geometric parameters can be determined around the circumference of the object, for example, the pipe. The terahertz radiation emitted by the transmitter can, for example, have a frequency in the range of 1 GHz to 10 THz. This is the fundamental frequency around which the terahertz radiation typically exhibits a certain bandwidth.

[0015] According to the invention, a plate-shaped calibration object, at least partially transparent to terahertz radiation, is arranged within the rotating ring. As explained in more detail below, the calibration object can be arranged in the rotating ring for a calibration process. It can be detachably arranged in the rotating ring. However, it is also possible for it to be permanently or fixedly arranged in the rotating ring. The calibration object can, for example, be arranged on the inside of the rotating ring or on the inside of or within a housing of the rotating ring. The calibration object can, for example, have a round shape, in particular a circular shape, or, for example, a rectangular shape, in particular a square shape. The wall thickness of the calibration object is known. It can, for example, have been determined in advance by a certification body.The terahertz transmitter is brought into a rotational position, in particular by rotating the rotating ring or the terahertz transmitter along the rotating ring, in which it emits terahertz radiation along a vertical principal beam direction onto the calibration object, for example, vertically from top to bottom or vice versa. If the terahertz radiation emitted by the terahertz transmitter has an aperture angle, the principal beam direction corresponds to the central beam or the center direction of the emitted radiation angle or radiation cone. The terahertz receiver is brought into a rotational position in which it receives the terahertz radiation emitted by the terahertz transmitter as measurement data after its reflection at the interfaces of the calibration object and / or after passing through the calibration object.The terahertz transmitter and terahertz receiver can, for example, be arranged opposite each other on the rotating ring and be rotatable synchronously by rotating the ring, so that the terahertz receiver is opposite the terahertz transmitter in every rotational position. The terahertz receiver receives the terahertz radiation emitted by the terahertz transmitter onto the object after it has passed through the calibration object. It is also conceivable that the terahertz transmitter and the terahertz receiver are arranged essentially in the same location, as explained in more detail below. In this case, the terahertz receiver receives the terahertz radiation emitted by the terahertz transmitter after reflection at the interfaces of the calibration object.Furthermore, as will be explained in more detail below, a reflector for the terahertz radiation emitted by the terahertz transmitter can be arranged at a location on the rotating ring opposite the terahertz transmitter and the terahertz receiver located at the same location. This reflector reflects the radiation back to the terahertz receiver located at the same location as the terahertz transmitter, possibly after multiple reflections at the interfaces of the object.

[0016] The evaluation unit determines the wall thickness of the calibration object from the measurement data of the terahertz receiver, as described above. In particular, the evaluation unit can analyze the transit times of the terahertz radiation reflected at the object's interfaces. If the terahertz transmitter is located on the rotating ring opposite the terahertz receiver, the evaluation unit can analyze any change in transit time caused by the calibration object being positioned in the beam path compared to when it is not. If the refractive index is known or determined, the wall thickness can also be calculated from this. The evaluation unit compares the wall thickness determined in this way with the known wall thickness of the calibration object, for example, one determined and provided by a certification body. Based on this comparison, the evaluation unit calibrates the terahertz measuring device.

[0017] According to the invention, the evaluation unit thus forms an evaluation and calibration unit. It can be constructed as a single unit or in multiple parts. For example, the calibration of the terahertz measuring device can be performed by a different part of the evaluation unit than the wall thickness determination. The evaluation unit can be located at the same location as the terahertz measuring device or at a location separate from it. If it is constructed in multiple parts, one part, for example, the part that performs the wall thickness determination, can be located at the same location as the terahertz measuring device, and another part, for example, the part that performs the calibration, can be located at a location separate from the terahertz measuring device.

[0018] The calibration object used according to the invention is plate-shaped, in particular planar and flat. The interfaces of the calibration object that bound the wall to be measured are in particular planar and parallel. As explained, the wall thickness of the calibration object can be known from an external measurement, for example, carried out by a certification body in accordance with ISO 9001. The calibration according to the invention also meets the requirements of ISO 9001. A corresponding calibration certificate in accordance with ISO 9001 can be issued.

[0019] The plate-shaped calibration object can be inserted into the rotating ring, for example, in the lower region, particularly at its lowest point, as determined by the rotating ring's position. The rotating ring can have a fixed housing into or onto which the calibration object can be placed. This allows for simple, gravity-based arrangement and storage. As already explained, the terahertz transmitter and, if applicable, the terahertz receiver located at the same point, can emit terahertz radiation vertically downwards from the upper region of the rotating ring, particularly at its highest point, as determined by the rotating ring's position.A reverse arrangement is also possible, in which the terahertz transmitter and, if applicable, the terahertz receiver located at the same position emit terahertz radiation vertically upwards from a lower region of the rotating ring, particularly from the lowest point of the rotating ring, onto the calibration object. As will be explained in more detail later, a reflector for the terahertz radiation can be located at the opposite point of the rotating ring. An advantage of this arrangement is that the radiation angle in the area of ​​the terahertz transmitter or transceiver is essentially limited to its near field. The calibration object(s) can therefore have significantly smaller dimensions than in the reverse arrangement. For example, calibration objects with a diameter of 100 mm are conceivable for circular designs, or 100 mm x 100 mm for square designs.

[0020] The use of a plate-shaped calibration object according to the invention offers several advantages. A plate-shaped calibration object can be manufactured with significantly higher accuracy and uniformity of wall thickness compared to cylindrical or tubular calibration objects. The wall thickness tolerance of + / - 1 µm described above can be reliably achieved with a plate-shaped calibration object, even one made of glass, unlike a cylindrical or tubular one. This allows a certification body to provide certification with sufficiently high accuracy for the known wall thickness used in the calibration comparison according to the invention.Any positional deviations, for example in the longitudinal direction within the measuring range of the calibration object, do not significantly affect the measurement result and thus the calibration, unlike with cylindrical or tubular calibration objects. The alignment of the terahertz measuring device, in particular the terahertz transmitter and the terahertz receiver, with respect to the calibration object inserted in the rotating ring can be reliably and easily adjusted by the rotatability of the terahertz transmitter and terahertz receiver according to the invention, so that the terahertz radiation strikes the object along a vertical main beam direction. As explained in more detail below, the positioning accuracy of the calibration object within the rotating ring can be precisely defined, for example, by markings or mechanical supports or holding devices.

[0021] In combination, these features of the terahertz measuring device according to the invention enable simple and precise calibration, particularly for customers who use the terahertz measuring device during operation to measure the geometric parameters of objects. This also makes obtaining regular calibration certificates in accordance with ISO 9001 straightforward.

[0022] This is especially true in contrast to terahertz measuring devices, which have a plurality of stationary terahertz transmitters and terahertz receivers arranged across the entire measuring range. Calibrating these devices, particularly those terahertz transmitters that do not radiate along the vertical axis, is then subject to the problems described earlier.

[0023] The process steps claimed in the independent method claim do not necessarily have to be carried out in the sequence stated therein. For example, the terahertz transmitter and / or the terahertz receiver can be brought into their respective rotational positions for the calibration measurement before or simultaneously with the arrangement of the calibration item in the rotating ring.

[0024] The calibration object can be made of glass, depending on the design. A calibration object made of ceramic would also be possible. The use of glass or ceramic is preferred over plastic because plastics are generally not sufficiently dimensionally stable. While glass or ceramic behaves differently with respect to its refractive index than conventional plastics, which, for example, have an average refractive index of approximately 1.5, whereas glass can have a refractive index of around 2.1, the measurement accuracy and repeatability of the terahertz measuring device can still be reliably derived using a calibration object made of glass or ceramic. Plastic would also be a conceivable material, provided it exhibits sufficient dimension stability. Pertinax is one example.

[0025] According to a particularly practical embodiment, the terahertz transmitter and receiver can be combined into a single terahertz transceiver. A terahertz transceiver integrates a terahertz transmitter and receiver into one component, so that the terahertz transmitter and receiver are located in the same position. The terahertz transceiver is then rotatable with or along the rotating ring.

[0026] Furthermore, a reflector for terahertz radiation can be provided opposite the terahertz transmitter, reflecting the terahertz radiation emitted by the terahertz transmitter back to the terahertz receiver. The reflector can be rotatable around the measuring range with or along the rotating ring. In particular, the reflector can be arranged opposite a terahertz transceiver on the rotating ring. The reflector reflects the terahertz radiation emitted by the terahertz transmitter, especially the terahertz radiation that may pass through the object after multiple reflections at the object's interfaces, at least partially back to the object and, after passing through the object again, possibly after further multiple reflections at the interfaces, at least partially back to the terahertz receiver. The reflector can be synchronized with the terahertz transmitter and the terahertz receiver or the terahertz transceiver.Rotate the ring so that in every rotational position of the terahertz transmitter and terahertz receiver or terahertz transceiver it is opposite them and reflects radiation emitted by the terahertz transmitter back to the receiver.

[0027] With the aforementioned configuration, it is also possible to determine the refractive index of an object located within the measuring range or of the calibration object itself. In this way, a reliable correlation can be established between the material of the calibration object and the regularly varying material of an object subsequently measured with the terahertz measuring device, for example, an extruded plastic object such as a pipe. For instance, the refractive index can be determined by comparing the transit time of the terahertz radiation emitted by the terahertz transmitter through the measuring range with an object or calibration object located within the measuring range, to the transit time of the terahertz radiation through the measuring range without an object or calibration object located within it, as described, for example, in WO 2016 / 139155 A1.

[0028] With further refinement, the reflector can serve as the calibration object. The reflector is then at least partially transparent to terahertz radiation. Its wall thickness is also known or can be measured by a certification body if necessary. Simultaneously, the plate-shaped reflector offers sufficient reflective properties for terahertz radiation, enabling it to fulfill the reflection function described above. The reflector can, for example, be made of a glass material, as explained for the calibration object. This design is particularly advantageous because a calibrated wall thickness is always available for measurement. Even when the terahertz transmitter and terahertz receiver or terahertz transceiver are rotating, this wall thickness value ensures that all acquired measurements are of the highest precision.Of course, it is also conceivable that the reflector is combined with another calibration object, as explained in more detail below. The reflector can be either permanently installed in the terahertz measuring device or detachably mounted within it. A permanently installed reflector as a calibration object has the advantage of ensuring continuous measurement accuracy. Alternatively, the reflector can be detachably mounted and thus be interchangeable, allowing the operator of the terahertz measuring device to have its dimensions certified by a certification body at desired intervals. This would allow compliance with the ISO 9001 standard.

[0029] In a further embodiment, a marking for receiving the calibration item can be provided on the rotating ring. This marking can be located on a fixed part of the rotating ring's housing. It indicates how an operator must insert the calibration item into the terahertz measuring device so that it is correctly positioned and aligned for the calibration measurement. This simplifies use, even for less experienced operators.

[0030] In a further embodiment, a holder, in particular a mechanical holder, for receiving the calibration item can be provided on the rotating ring. This holder can, in turn, be arranged on a housing of the rotating ring. The holder further simplifies the positioning of the calibration item. For example, the holder can have one or more projections that define a specific position for the calibration item, allowing it to be easily inserted between the projection(s). For instance, the projection can have lateral limits for the calibration item to be inserted. This ensures the correct positioning of the calibration item reliably and permanently.

[0031] Another possible mounting method involves inserting a retaining tube or cylinder, for example made of plastic, into the rotating ring and placing the calibration object onto this tube or cylinder. The retaining tube or cylinder can be aligned vertically along its longitudinal axis. It is particularly advantageous if the inner diameter of the retaining tube corresponds approximately to the width of a window or reflector located on the underside of the rotating ring. The retaining tube can then be slid onto the window or reflector. This aligns and centers the retaining tube, ensuring that any calibration object placed on it is precisely aligned at a right angle to the main beam direction of the terahertz radiation.

[0032] The bracket can also have a base for receiving the calibration object, so that the bracket simultaneously forms a spacer towards the terahertz transmitter and / or terahertz receiver or the terahertz transceiver or the reflector.

[0033] In a particularly practical design, the holder can be made of foam. Such a foam can be easily inserted without risk of damaging components of the terahertz measuring device and does not affect the measurement, as it is transparent to terahertz radiation.

[0034] In a further embodiment, a second, plate-shaped calibration object with a known wall thickness, at least partially transparent to the terahertz radiation emitted by the terahertz transmitter, can be arranged within the rotating ring parallel to and at a distance from the (first) calibration object. This allows the measurement of the two wall thicknesses of a pipe, as typically performed by the terahertz measuring device, to be replicated. The evaluation software of the evaluation unit then does not need to be adapted for the calibration measurement. The second calibration object can be identical to the (first) calibration object. However, a different design is also conceivable, for example, different wall thicknesses for the (first) and second calibration objects.In this way, a distance measurement can also be carried out using terahertz radiation on, for example, parallel interfaces of the two calibration objects, for example, on outer interfaces facing away from each other, corresponding to a diameter measurement of a pipe.

[0035] The holder can include a spacer between the first and second calibration items. The spacer can, for example, be frame-shaped. It can leave a gap between the calibration items, allowing the terahertz radiation to pass through this gap without the risk of the spacer distorting the measurement result. This distance ensures a clearly separate measurement of the wall thicknesses of the two calibration items. The holder can also accommodate the second calibration item in a particularly practical manner.

[0036] It is also possible to provide a holding device located outside the rotating ring and / or on a non-rotating part of the terahertz measuring device, which holds the calibration object within the rotating ring. This holding device can also be combined with the holder, so that the holding device supports the holder for the calibration object. The holding device can, for example, include a stand that can be placed on the floor next to the terahertz measuring device. This design eliminates the need to modify the rotating ring itself. It is also particularly well-suited for retrofitting existing terahertz measuring devices.

[0037] According to a further embodiment, the calibration object can be arranged within the rotating ring such that the interfaces of the wall of the calibration object to be measured are oriented perpendicular to the vertical. This makes measuring the wall thickness and evaluating the received measurement data particularly easy. The marking, holder, or holding device according to the aforementioned embodiments can be designed, in particular, to ensure the aforementioned arrangement of the calibration object with the interfaces of the wall oriented perpendicular to the vertical.

[0038] In a further embodiment, the terahertz transmitter and / or the terahertz receiver can be arranged outside the rotating ring, the rotating ring having a window that is at least partially transparent to terahertz radiation, through which the terahertz radiation enters and / or exits the measurement range. Such a window serves to protect the sensor components, in particular the terahertz transmitter and terahertz receiver or the terahertz transceiver. It is designed to generate as few reflection signals from the terahertz radiation as possible, which could interfere with the measurement evaluation.The window can, for example, contain a thin film, such as a plastic film like PTFE, where reflection echoes on the front and back of the film cancel each other out. This occurs, for instance, because the terahertz radiation reflects without phase shift upon entry and exhibits a 180° phase shift upon exiting the denser medium. The film can be housed in a holder, such as a flange. The terahertz transmitter and receiver can be positioned slightly outside the rotating ring. They can be mounted to the rotating ring via a connecting bracket, allowing them to rotate with the ring. The reflector can also be positioned (slightly) outside the rotating ring and mounted to it via a connecting bracket.The rotating ring can also have a window in the reflector area that is at least partially transparent to terahertz radiation and can be designed as described above. Naturally, the terahertz transmitter and / or the terahertz receiver and / or the reflector can also be mounted directly on the rotating ring and rotate with it. In this case, for example, a ring window that is at least partially transparent to terahertz radiation can be arranged inside the rotating ring to protect the terahertz transmitter and terahertz receiver or reflector. The ring window can also be designed so that it generates as little reflection signal from the terahertz radiation as possible, which could interfere with the measurement evaluation. In particular, it can be constructed as described above regarding the window. The ring window can also serve as a support for the calibration object in a particularly practical way. The terahertz transmitter and terahertz receiver or...The terahertz transceiver and, if applicable, the rotating ring supporting the reflector can have a U-shaped profile, so that it rests on the ring window with the free ends of the legs of the U-shaped profile and / or holds the ring window.

[0039] The terahertz measuring device can have a preferably annular housing in which the rotating ring and the other components are arranged. The housing serves to protect the components arranged inside it, in particular the terahertz transmitter and terahertz receiver or the terahertz transceiver, and optionally the reflector.

[0040] As explained, the calibration object can, for example, be made of glass. It can be made of quartz glass or borosilicate glass, in particular. Such glass materials can be manufactured with exceptional dimensional precision, exhibit particularly high dimensional stability, and can therefore be measured with very high accuracy and certification. For example, a borosilicate glass of type MPAbor made of Borofloat SG is conceivable. The calibration object has the greatest possible surface flatness. The dimensions of the calibration object can generally be selected flexibly according to the dimensions of the terahertz measuring device, especially the rotating ring. By way of example, dimensions of up to 250 mm in diameter for a circular design, up to 250 mm in width for a rectangular design, or up to 250 × 250 mm² for a square design are mentioned.Depending on the arrangement of the terahertz transmitter, smaller dimensions are also possible, with a diameter or width of, for example, 100 mm, or, in the case of a square design, 100 x 100 mm. The thickness of the calibration object can be selected depending on the specific application.

[0041] The results of the calibration according to the invention can be recorded and documented. In this way, a history of the calibration measurements can be created and evaluated over a longer period. This allows, for example, the detection of long-term deviations in measuring components.

[0042] The method according to the invention can be carried out with a terahertz measuring device according to the invention. Accordingly, the terahertz measuring device according to the invention can be configured to carry out the method according to the invention.

[0043] Exemplary embodiments of the invention are explained in more detail below with reference to the figures. They schematically show: Fig. 1 a terahertz measuring device for illustrating a calibration process according to the prior art in a vertical sectional view, Fig. 2 a terahertz measuring device according to the invention for illustrating a calibration method according to the invention in a vertical sectional view according to a first embodiment, Fig. 3 a terahertz measuring device according to the invention for illustrating a calibration method according to the invention in a vertical sectional view according to a second embodiment, Fig. 4 a terahertz measuring device according to the invention for illustrating a calibration method according to the invention in a vertical sectional view according to a third embodiment, Fig. 5 a terahertz measuring device according to the invention for illustrating a calibration method according to the invention in a vertical sectional view according to a fourth embodiment, Fig.6a terahertz measuring device according to the invention for illustrating a calibration method according to the invention in a vertical sectional view according to a fifth embodiment, and Fig. 7the terahertz measuring device according to . Figure 6 in a opposite Figure 6 Sectional view rotated by 90°.

[0044] Unless otherwise stated, the same reference symbols in the figures denote the same objects.

[0045] The in Figure 1The terahertz measuring device shown comprises a terahertz transceiver 10, which is arranged via a connection (not shown) on a rotating ring 12 (circular in this example) and is rotatably mounted to it through 360°. The terahertz transceiver 10 comprises a terahertz transmitter and a terahertz receiver located at the same position. In the area of ​​the terahertz transceiver 10, a window 14, at least partially transparent to the terahertz radiation emitted by the terahertz transmitter, is arranged in the rotating ring 12. The window 14 can be configured as described above. The terahertz radiation emitted by the terahertz transmitter of the terahertz transceiver 10 can pass through the window 14 into the interior of the rotating ring 12, which forms a measuring area 16 of the terahertz measuring device, and back out again.

[0046] On a side of the rotating ring 12 opposite the terahertz transceiver 10, a reflector 18 for terahertz radiation is arranged, which reflects the terahertz radiation emitted by the terahertz transmitter back to the terahertz transceiver 10 and thus to the terahertz receiver, as shown in Fig. 1 This is illustrated by arrow 20. The measurement data from the terahertz receiver are available at an evaluation unit 22 of the terahertz measuring device. During operation, an object to be measured, for example a plastic pipe, is arranged in the measuring area 16, with the longitudinal axis of the plastic pipe in Figure 1the path extends into the plane of the drawing. Terahertz radiation emitted by the terahertz transmitter of the terahertz transceiver 10 is reflected multiple times by interfaces of the plastic tube. After passing through the plastic tube, the repeatedly reflected radiation components are reflected by the reflector 18 and, after passing through the object again and undergoing further multiple reflections at the interfaces of the plastic tube, return to the terahertz transceiver 10 through the window 14. Based, for example, on an evaluation of the travel times of the terahertz radiation reflected at the different interfaces, the evaluation unit 22 determines geometric parameters, in particular the wall thickness of the plastic tube.

[0047] Figure 1Figure 1 illustrates a conventional calibration method for such a terahertz measuring device. A tubular calibration object 24 made of glass is positioned in the measuring area 16. The wall thicknesses of the tubular calibration object 24 are determined beforehand, for example, by a certification body. Based on the operating principle described above, the terahertz measuring device determines the wall thicknesses of the tubular calibration object 24. The terahertz measuring device is then calibrated by comparing the determined wall thicknesses with those previously determined, for example, by the certification body.

[0048] One disadvantage of this known method is that the tubular calibration object 24 cannot be manufactured with a wall thickness accuracy that corresponds to that of the terahertz measuring device. Consequently, the wall thicknesses cannot be certified with the required accuracy by a certification body. For this reason alone, precise calibration with the tubular calibration object 24 is not possible. Furthermore, the tubular calibration object 24 must be positioned very precisely and centrally within the measuring area 16 in a complex manner. Even small deviations in its position distort the measurement result, as lateral displacement of the tubular calibration object 24 within the measuring area 16 can lead to inaccurate readings. Figure 1the wall thicknesses measured by the terahertz measuring device are altered, and on the other hand, as explained, the tubular calibration object 24 cannot be manufactured with a sufficiently constant wall thickness, neither around its circumference nor in the longitudinal direction. Also, for example, a change in position in the longitudinal direction, in Fig. 1 This also leads to a distortion of the measurement result, as it extends into the drawing plane.

[0049] In Figure 2 A terahertz measuring device according to the invention for carrying out the calibration method according to the invention is shown in a first embodiment. It differs from the one described in Fig. 1 The terahertz measuring device shown here has respect to the calibration object. Thus, in the embodiment according to... Fig. 2A plate-shaped calibration object 26 made of glass is arranged within the rotating ring 12, which has plane-parallel interfaces with high flatness that bound the wall to be measured. In the Fig. 2 In the illustrated embodiment, a holder 28, comprising a circumferential projection 27 and a base 29, is arranged on the rotating ring 12, precisely defining the position and orientation of the calibration object 26 on the underside of the rotating ring 12. The calibration object 26 is thus arranged precisely and securely, and the arrangement is simple and can be easily performed even by less experienced operators.

[0050] As in Figure 2As can be seen, the calibration object 26 is arranged within the rotating ring 12 such that the interfaces 30, 32 of the wall to be measured are oriented perpendicular to the vertical. Furthermore, it can be seen that the terahertz transmitter of the terahertz transceiver 10 is in a rotating position, so that the emitted terahertz radiation is directed downwards along a vertical main beam direction onto the calibration object 26. While the terahertz transceiver 10 is thus located at the highest point of the rotating ring 12, the reflector 18 is correspondingly located at the lowest point of the rotating ring 12. In this way, it is ensured that the terahertz radiation falls perpendicularly onto the interfaces 30, 32 of the calibration object 26. This simplifies the measurement and evaluation.

[0051] The calibration object 26 can, for example, be made of quartz glass or borosilicate glass, as explained above. It is also conceivable that the calibration object 26 is made of ceramic. The holder can, for example, be made of foam. Instead of the holder 28, (optical) markings could also be provided on the inside of the rotating ring 12, which specify a position for the calibration object 26.

[0052] Figure 3 shows a further embodiment of a terahertz measuring device according to the invention, which is largely based on the one described in Fig. 2 The terahertz measuring device shown corresponds to this. It differs from the one in Fig. 2 The terahertz measuring device shown here is described with regard to the mounting of the plate-shaped calibration object 26 within the rotating ring 12. In the initial example according to Fig. 3For this purpose, a holding device 34 is provided outside the rotating ring 12, which holds the calibration object 26 inside the rotating ring 12. The holding device 34 is designed like a stand and can rest on a surface within the space of the terahertz measuring device by means of a stand base 36. It is also conceivable that the holding device 34 is attached to a non-rotating part of the terahertz measuring device.

[0053] Figure 4 shows another embodiment that differs from the one in Figure 2The illustrated embodiment differs, firstly, in that the terahertz transceiver 10 is located on the underside of the rotating ring 12 and the reflector 18 is arranged opposite it on the top side of the rotating ring 12. The window 14 is also located on the underside of the rotating ring 12, so that the terahertz radiation emitted by the terahertz transceiver 10 is emitted vertically from bottom to top. Furthermore, the embodiment differs according to Figure 4 from the in Figure 2 The illustrated embodiment with regard to the mounting of the calibration item 26. Thus, in Figure 4A retaining tube 38, for example made of plastic and oriented vertically with its longitudinal axis, is inserted into the lowest position of the rotating ring 12 as a support. The retaining tube 38 forms a support for the calibration object 26. The inner diameter of the retaining tube 38 corresponds approximately to the width of the window 14 or a flange accommodating the window 14, so that the retaining tube 38 can be placed vertically onto the window 14 or the flange with its longitudinal axis, whereby the window 14 or the flange simultaneously centers and aligns the retaining tube 38. Furthermore, this advantageously ensures that the calibration object 26 is reliably and precisely aligned at a right angle to the terahertz radiation. The retaining tube 38 also ensures the distance required for the measurement between the calibration object 26 and the window 14.

[0054] The in Figure 5The illustrated embodiment has the same measuring setup with terahertz transceiver 10, window 14 and reflector 18 as the embodiment according to Figure 4 . In the Figure 5In the illustrated embodiment, in addition to the first calibration object 26, a second plate-shaped calibration object 40 is provided, which is at least partially transparent to the terahertz radiation of the terahertz transmitter and is arranged parallel to and spaced apart from the first calibration object 26. The second calibration object 40 can be identical to the first calibration object 26. However, the calibration objects 26 and 40 could, for example, also have different thicknesses. A frame-shaped spacer 42, which can be made of foam, for example, is arranged between the calibration objects 26 and 40. It leaves a gap centrally between the calibration objects 26 and 40, so that the terahertz radiation can pass through the gap without the risk of the spacer 42 distorting the measurement result. In this way, the measurement of two adjacent wall thicknesses can be simulated during calibration.For example, based on an evaluation of the terahertz radiation, a distance measurement to the outer interfaces of the calibration objects 26 and 40 facing away from each other can be carried out, corresponding to a diameter measurement of a pipe.

[0055] Another advantage of the arrangement of terahertz transceiver 10, window 14 and reflector 18 according to the Figures 4 and 5 The advantage is that in the area of ​​the terahertz transceiver 10, the antenna's radiation angle is essentially limited to its near field. The calibration object(s) can therefore have significantly smaller dimensions than in the arrangement according to the Figures 1 to 3 For example, calibration objects with a width of 100 mm or, in the case of a square design, with dimensions of 100 mm x 100 mm are conceivable.

[0056] The Figure 6 and 7Figure 1 illustrates a further embodiment, which differs from the embodiments described above in that the rotating ring 12 and the other components are arranged in a housing 46 of the terahertz measuring device. Such a housing 46 can also be used in the embodiments according to the above. Figures 1 to 5 be provided for. Furthermore, in the exemplary embodiment, the transceiver 10 is according to the Figure 6 and 7 directly adjacent to the rotating ring 12. Instead of the window 14, a ring window 44, at least partially transparent to terahertz radiation, is provided within the rotating ring 12, protecting the transceiver 10 and the reflector 18. The calibration object 26 is inserted into the underside of the ring window 44, which thus simultaneously forms a support for the calibration object 26. Figure 7It can be seen that the rotating ring 12 has a U-shaped profile, with the annular window 44 being supported at the free ends of the legs 48 of the U-shaped profile and thus being held in a defined position within the rotating ring 12. The design with the annular window 44 could also be used in the embodiments according to the Figures 3 to 5 be planned.

[0057] The arrangement of the transceiver 10 and the reflector can be chosen arbitrarily in the different embodiments. For example, by appropriately rotating the rotating ring 12, it would also be possible in the embodiment according to the Figures 1 to 3 and 6, 7 The terahertz transceiver 10 is arranged at the lowest point of the rotating ring 12, and the reflector 18 at the highest point. Accordingly, in the embodiment according to the Figures 4 and 5by appropriate rotation of the rotating ring 12 the terahertz transceiver 10 can be positioned at the highest point of the rotating ring 12 and the reflector 18 at the lowest point.

[0058] As explained above, the reflector 18 can also serve as the calibration object. In this case, the reflector 18 can replace the calibration object 26 shown in the figures and / or the one shown in Figure 5 The reflector 18 may be provided for in the calibration object 40 shown, or in addition to it. The reflector 18 can then, for example, be made of a glass material, such as the glass material described for the calibration objects 26 and 40. The reflector 18 can be fixedly or detachably arranged in the terahertz measuring device. Reference symbol list

[0059] 10 Terahertz transceiver 12 Rotating ring 14 Window 16 Measuring range 18 Reflector 20 Arrow 22 Evaluation unit 24 Tubular calibration item 26 Plate-shaped calibration item 27 Projection 28 Holder 29 Base 30, 32 Interfaces 34 Holding device 36 Stand foot 38 Holding tube 40 Second calibration item 42 Spacer 44 Ring window 46 Housing 48 Leg

Claims

1. A terahertz measuring device for measuring the wall thickness of an object, wherein the terahertz measuring device has a terahertz transmitter (10) that can be rotated with a rotary ring (12) or along a rotary ring (12) about a measurement region (16) that encompasses the object during a measurement, and a terahertz receiver (10) that can be rotated with the rotary ring (12) or along the rotary ring (12) about the measurement region (16) and that receives terahertz radiation emitted by the terahertz transmitter (10) onto the object after being reflected by an object arranged in the measurement region (16), and wherein the terahertz measuring device further comprises an evaluation apparatus (22) that is designed to determine a wall thickness of the object from the measurement data of the terahertz receiver (10), wherein • a planar calibration object (26) that is at least partially transparent to the terahertz radiation emitted by the terahertz transmitter (10) and that has a known wall thickness is arranged inside the rotary ring (12), • the terahertz transmitter (10) can be brought into a rotary position, such that the terahertz transmitter (10) emits terahertz radiation along a vertical main beam direction onto the calibration object (26), • the terahertz receiver (10) can be brought into a rotary position, such that the terahertz receiver (10) receives terahertz radiation that is reflected by interfaces (30, 32) of the calibration object (26) and / or that passes through the calibration object (26), characterized in that • the evaluation apparatus (22) is designed to determine the wall thickness of the calibration object (26) from the measurement data of the terahertz receiver (10), and to calibrate the terahertz measuring device based on a comparison of the determined wall thickness with the known wall thickness of the calibration object (26).

2. The terahertz measuring device according to claim 1, characterized in that the terahertz transmitter (10) and the terahertz receiver (10) are formed by a terahertz transceiver (10).

3. The terahertz measuring device according to one of the preceding claims, characterized in that a reflector (18) for terahertz radiation opposite the terahertz transmitter (10) is provided, which reflector reflects terahertz radiation emitted by the terahertz transmitter (10) back to the terahertz receiver (10).

4. The terahertz measuring device according to claim 3, characterized in that the reflector (18) can be rotated with the rotary ring (12) or along the rotary ring (12) about the measurement region (16).

5. The terahertz measuring device according to one of claims 3 or 4, characterized in that the reflector (18) forms the calibration object.

6. The terahertz measuring device according to one of the preceding claims, characterized in that a marking for receiving the calibration object (26) is provided on the rotary ring (12).

7. The terahertz measuring device according to one of the preceding claims, characterized in that a holder (28, 38) for receiving the calibration object (26) is provided on the rotary ring (12).

8. The terahertz measuring device according to claim 7, characterized in that the holder (28) has a base (29) for receiving the calibration object (26).

9. The terahertz measuring device according to one of claims 7 or 8, characterized in that the holder (28) consists of a foam material.

10. The terahertz measuring device according to one of the preceding claims, characterized in that a second planar calibration object (40) that is at least partially transparent to the terahertz radiation emitted by the terahertz transmitter (10) and that has a known wall thickness is arranged inside the rotary ring (12) in parallel with and at a distance from the calibration object (26).

11. The terahertz measuring device according to claim 10 and one of claims 7 to 9, characterized in that the holder comprises a spacer (42) between the calibration object (26) and the second calibration object (40).

12. The terahertz measuring device according to claim 11, characterized in that the holder also receives the second calibration object (40).

13. The terahertz measuring device according to one of the preceding claims, characterized in that a holding apparatus (34) arranged outside the rotary ring (12) and / or on a nonrotating part of the terahertz measuring device is provided, which holding apparatus holds the calibration object (26) inside the rotary ring (12).

14. The terahertz measuring device according to one of the preceding claims, characterized in that the calibration object (26) is arranged inside the rotary ring (12) in such a way that the interfaces (30, 32) of the wall of the calibration object (26) to be measured are oriented perpendicularly to the vertical.

15. The terahertz measuring device according to one of the preceding claims, characterized in that the terahertz transmitter (10) and / or the terahertz receiver (10) are arranged outside the rotary ring (12), wherein the rotary ring (12) has a window (14) which is at least partially transparent to the terahertz radiation and through which the terahertz radiation enters the measurement region (16) and / or exits the measurement region (16).

16. The terahertz measuring device according to one of the preceding claims, characterized in that the calibration object (26) consists of a glass or a ceramic.

17. The terahertz measuring device according to claim 16, characterized in that the calibration object (26) consists of a quartz glass or a borosilicate glass.

18. A method for calibrating a terahertz measuring device for measuring the wall thickness of an object, wherein the terahertz measuring device has a terahertz transmitter (10) that can be rotated with a rotary ring (12) or along a rotary ring (12) about a measurement region (16) that encompasses the object during a measurement, and a terahertz receiver (10) that can be rotated with the rotary ring (12) or along the rotary ring (12) about the measurement region (16) and that receives terahertz radiation emitted by the terahertz transmitter (10) onto the object after being reflected by an object arranged in the measurement region (16), and wherein the terahertz measuring device further comprises an evaluation apparatus (22) that is designed to determine a wall thickness of the object from the measurement data of the terahertz receiver (10), characterized in that the method comprises the following steps: • a planar calibration object (26) that is at least partially transparent to the terahertz radiation emitted by the terahertz transmitter (10) and that has a known wall thickness is arranged inside the rotary ring (12), • the terahertz transmitter (10) is brought into a rotary position, such that the terahertz transmitter (10) emits terahertz radiation along a vertical main beam direction onto the calibration object (26), • the terahertz receiver (10) is brought into a rotary position, such that the terahertz receiver (10) receives terahertz radiation that is reflected by interfaces (30, 32) of the calibration object (26) and / or that passes through the calibration object (26), • by means of the evaluation apparatus (22), the wall thickness of the calibration object (26) is determined from the measurement data of the terahertz receiver (10) and the terahertz measuring device is calibrated based on a comparison of the wall thickness determined by means of the evaluation apparatus (22) with the known wall thickness of the calibration object (26).

19. The method according to claim 18, characterized in that it is carried out using a terahertz measuring device according to one of claims 1 to 17.