Fill-level measuring device

Abstract

The invention relates to a calibration device (5) for calibrating radar-based fill-level measuring devices (1) or the high-frequency module thereof. For this purpose, the device (5) is based on a main part (51) in which a waveguide (52) extends within a flat plane (A). The waveguide (52) is characterized by a coupling region (521), an end region (522) that reflects radar signals (SHF, RHF), and a calibration path (dkal) extending therebetween. According to the invention, the waveguide (52) or the calibration path (dkal) is spirally nested. As a result, a very compact design is achieved with respect to the length of the calibration path (dkal) or of the waveguide (52) so that the calibration device (5) can also be used for recalibrations at the site of use of the fill-level measuring device (1).

Description

[0001] Level measuring device

[0002] The invention relates to the calibration of radar-based level measuring devices or the calibration of their high-frequency module.

[0003] In process automation technology, appropriate field devices are used to record relevant process parameters. To record the respective process parameters, suitable measurement principles are implemented in the field devices to record process parameters such as level, flow, pressure, temperature, pH value, redox potential, or conductivity. The Endress+Hauser Group manufactures and distributes a wide variety of field device types and variants.

[0004] Non-contact measuring methods have become established for level measurement of filling materials in containers because they are robust and low-maintenance. Radar-based level measurement is described in more detail, for example, in "Radar Level Detection, Peter Devine, 2000." A key advantage of radar-based level measurement is its ability to measure the level virtually continuously. Therefore, radar-based measuring methods are predominantly used for continuous level measurement. In the context of this invention, the term "RadaP" refers to signals or electromagnetic waves with frequencies between 0.03 GHz and 300 GHz. Due to the principle, the higher the absolute bandwidth or frequency, the higher the measurement resolution can be achieved. The pulse transit time method and FMCW ("Frequency Modulated Continuous Wave") have become established measurement methods.

[0005] To ensure the potentially high resolution of the radar measurement, it is also essential to calibrate the level measuring device or its high-frequency module at least during production.

[0006] In the case of legal-for-trade level measurement, it is also mandatory to recalibrate the level gauge at prescribed intervals during operation. The problem with this is that the level gauge must be removed from the site of use and brought to the appropriate calibration line. The calibration line requires a certain length depending on the measuring range.

[0007] The invention is therefore based on the object of achieving the most efficient calibration of radar-based level measuring devices under these conditions.

[0008] The invention solves this problem by providing a calibration device that can be used to separately calibrate radar-based level gauges or their high-frequency modules. The calibration device comprises:

[0009] - A basic body that encloses a section of a planar plane,

[0010] - a waveguide which runs in the plane within the cutout, with o a coupling region, o an end region which reflects radar signals and is opposite the coupling region, and o a defined calibration section in between.

[0011] The device is characterized by the fact that the waveguide or calibration section is designed with a spiral-shaped nesting in the plane. This can be achieved either by dividing the waveguide into semicircular base segments, so that the waveguide forms a circular spiral shape. Or the waveguide is divided into straight base segments, which are arranged at a defined angle to the next base segment and connected by correspondingly circular-segment-shaped connecting segments, so that the waveguide forms a particularly square, hexagonal, or octagonal spiral shape. In these cases, all connecting segments of the waveguide can have the same radius. The advantage of the spiral design is that the calibration section can, in principle, be designed to be very compact for its length.A further doubling of the calibration length with the same size base body section is optionally possible by dividing the waveguide along the spiral into two parallel waveguide paths, whereby the two parallel paths are to be connected by a connecting path in the center of the spiral.

[0012] Overall, the calibration device according to the invention exhibits low radar attenuation due to the kink-free waveguide guide. This, in turn, enables a high signal-to-noise ratio and thus high-resolution calibration, which can be achieved based on the following process steps:

[0013] - Coupling a radar signal into the waveguide via the coupling area by the level measuring device or its high-frequency module,

[0014] - Extracting a corresponding reception signal from the waveguide after reflection of the radar signal at the end area, and

[0015] - Calibration based on the received signal and the known calibration length of the waveguide.

[0016] Due to the compact design of the calibration device, this procedure can be carried out not only during the production of the level measuring device, but also during measuring operation, for example to recalibrate the level measuring device.

[0017] In principle, the level measuring device to be calibrated can be most easily connected to the calibration device if the coupling area is located on the outside of the base body, i.e. opposite the center of the spiral.

[0018] For efficient coupling, especially in the case of a rectangular cross-section of the waveguide, a polarization filter can be arranged in front of the coupling area and in particular can be designed in such a way that the radar signal is coupled into the waveguide via a dielectric waveguide or a dielectric lens.

[0019] Due to its design principle, the waveguide must have an electrically conductive inner surface. This can be achieved, for example, by manufacturing the base body from an electrically conductive material such as aluminum, copper, or stainless steel, or by applying an electrically conductive coating to the inner surface of the waveguide, for example, with a silver coating.

[0020] The planar arrangement of the waveguide enables simple fabrication of the calibration device, as the base body can be designed in two parts, so that the two corresponding parts of the base body are separated by the plane in which the waveguide runs when assembled. In this case, the waveguide can be manufactured, for example, by milling along the surface of the base body parts. The cross-sectional diameter of the waveguide and its cross-sectional shape must be dimensioned depending on the frequency of the radar signal or the desired mode.

[0021] The calibration device can be further improved in terms of signal-to-noise ratio by placing a hermetically sealed window at the coupling area, allowing the waveguide to be subjected to a vacuum or filled with an inert gas. This is also advantageous if the inner wall of the waveguide has a corrosion-prone coating, such as silver.

[0022] To avoid having to maintain a single calibration section, the waveguide can also be modified to incorporate radar interference at one or more defined positions, particularly in the form of a cross-sectional narrowing or widening. In terms of radio frequency technology, these radar interference points thus perform the same function as the reflective end region of the waveguide.

[0023] In relation to the invention, the term "module" is understood to mean any circuit group intended for a specific application, e.g., as an interface or for high-frequency signal processing. Depending on the application, the respective module can therefore comprise corresponding analog circuits for generating or processing corresponding analog signals. However, the respective module can also comprise digital circuits, such as FPGAs, microcontrollers, or storage media, in conjunction with corresponding programs. The program is designed to carry out the required method steps or apply the necessary computing operations. In this context, various electronic circuits of the module within the meaning of the invention can potentially also access a common physical memory or be operated using the same physical digital circuit.It is not relevant whether different electronic circuits within the module are arranged on a common circuit board or on several connected circuit boards.

[0024] The invention is explained in more detail with reference to the following figures. Shown are:

[0025] Fig. 1 : A radar-based level gauge on a container,

[0026] Fig. 2: an exploded view of a first embodiment of the calibration device according to the invention, and

[0027] Fig. 3: a plan view of a second embodiment of the calibration device according to the invention.

[0028] To provide a basic understanding of the invention, Fig. 1 shows a container 3 with a filling material 2, the fill level L of which is to be determined by a radar-based fill level measuring device 1. Depending on the type of filling material 2 and the area of ​​application, the container 3 can be up to 100 m high. The type of filling material 2 and the area of ​​application also determines the optimal frequency band fi ,2,3 in which the fill level L is determined by the fill level measuring device 1: In the case of a coarse-grained filling material 2 and correspondingly diffuse reflection, a comparatively low frequency band fi ,2,3 at, for example, 6 GHz is generally suitable. Low frequency bands are also suitable for foaming filling materials 2, since in this case the foam does not have a reflective effect.In the case of a refinery tank as container 3, a frequency band as high as possible (fi,2,3), for example, 80 GHz or 180 GHz, is advantageous due to the flat filling material surface, as this inherently allows for a potentially higher distance resolution. Level measuring device 1 is usually connected via a separate interface module, which can accommodate signals such as "4-20 mA," "PROFIBUS," or "HART." 1 , or

[0029] "Ethernet is implemented, connected to a higher-level unit 4, such as a local process control system or a decentralized server system. The measured fill level value L or the pure distance value d can be transmitted via this, for example, to control inflows or outflows of the container 3. However, other information about the general operating status of the level measuring device 1 can also be communicated.

[0030] To determine the fill level L, the level gauge 1 is mounted above the filling material 2 at a known installation height h above the brine of the container 3. The level gauge 1 is attached and aligned in such a pressure- and media-tight manner to a corresponding opening of the container 3 that only an electrically passive antenna is directed vertically downwards into the container 3 toward the filling material 2. The active components of the level gauge 1 are located outside the container 3 in a separate housing, which is attached to the opening of the container 3, for example, via a flange.

[0031] The level gauge 1 transmits radar signals SHF via the antenna within a defined frequency band, for example, at 6 GHz or 180 GHz, towards the surface of the filling material 2. After the radar signals SHF are reflected at the filling material surface, the level gauge 1 receives the reflected received signals RHF via the antenna. The signal propagation time t between transmission and reception of the respective radar signal S, RHF is determined according to proportional to the distance d between the level gauge 1 and the medium 2, where c represents the media-dependent and generally at least roughly known propagation speed of the respective radar signal S, RHF. The signal propagation time t can be determined by the level gauge 1, for example, using the FMCW or pulse propagation time method. This allows the level gauge 1 to assign the measured propagation time t to the respective distance d based on an appropriate calibration.

[0032] The level measuring device 1 can then determine the fill level L according to d = h — L, provided that the installation height h is stored in the level measuring device 1 or in the higher-level unit 4.

[0033] Since the propagation speed c of the respective radar signal S, HF is only roughly known and is also influenced by the internal signal routing, the above relationship between signal propagation time t and distance d at the level measuring device 1 must be determined by calibration.

[0034] Therefore, at least during the production of the level measuring device 1, a calibration is performed on a defined and known calibration section dkai. The calibration can be performed either after completion of the entire level measuring device 1 or beforehand on only its high-frequency module. This allows the same calibration section dkai to be used for different variants of level measuring devices 1 that use the same type of high-frequency module. Particularly in applications subject to legal metrology, cyclic recalibration of the level measuring device 1 during measuring operation is also required.

[0035] In principle, calibration is performed by transmitting the radar signal SHF along the calibration section dkai and receiving the corresponding received signal RHF after reflection at a suitable end region 522 of the calibration section dkai. As a result of the calibration, the transit time t measured by the level gauge 1 is assigned to the calibration section dkai according to t_2eal.

[0036] The length of the required calibration section dkai depends significantly on the desired measuring range, which must cover up to 100 m. Therefore, providing a sufficiently long calibration section dkai can be problematic, particularly during recalibration during measurement operation. A calibration device 5 according to the invention, which can be designed compactly considering the desired calibration section dkai, is explained in more detail in the exploded view of Fig. 2: Accordingly, the calibration device 5 is based on a metallic base body 51, which is designed in two parts. The base body 51, or each of the parts 51a, b in the illustrated embodiment, has a square basic shape.

[0037] As can be seen from the exploded view in Fig. 2, the two parts 51a, b each have a planar, square surface. At these surfaces, the two parts 51a, b border one another in a planar manner in the assembled state and are congruent with respect to their basic square shape. This defines a square section A of a planar plane. Within the plane section A, i.e., along the adjoining surfaces, a waveguide 52 is formed as a calibration section dkai. Due to the planar arrangement of the waveguide 52, it can be manufactured by milling the surface of one of the parts 51a, b or by congruent milling in the surfaces of both parts 51a, b.

[0038] In the embodiment shown in Fig. 2, the two parts 51a, b of the base body 51 are connected by a screw connection 525, with the individual screw holes for this being located next to the waveguide 52. In contrast to the embodiment shown, the two parts of the base body 51 do not necessarily have to be made of an electrically conductive material, as long as the inner wall of the waveguide 52 is electrically conductive. In the case of non-metallic parts 51a, b, this can be achieved, for example, by sputtering or chemical vapor deposition ("CVD") of an electrically conductive coating, such as silver, on the inner wall of the waveguide 52. According to the invention, the waveguide 52 is designed in a spiral shape, with its spiral shape in the embodiment shown in Fig. 2 being composed of rectilinear base segments 523 that are arranged at right angles to the respective next base segment 523.The base segments are each connected by quarter-circular connecting segments 524, resulting in a square-spiral nesting. At the center of the spiral is the end region 522 of the waveguide 52, which reflects the radar signals SHF, HF. In relation to the waveguide 52, opposite the end region 522, i.e., at the edge of the square base body 51, there is a coupling region 521 to which the level measuring device 1 or its high-frequency module can be coupled. Thus, the calibration section dkai is defined by the waveguide 52 between the coupling region 521 and the end region 522. For this purpose, the radar signals SHF, RHF can be coupled into the waveguide 52, for example, via a dielectric waveguide that can be inserted into the coupling region 521. Not explicitly shown in Fig.2, that the waveguide at the coupling region 521 can also be structurally closed by a radar-permeable window, so that the waveguide 52 can be subjected to a vacuum or an inert gas for less interference radar transmission.

[0039] Fig. 3 illustrates a further embodiment of the calibration device 5 according to the invention. There, too, the coupling region 521 is arranged at the edge of the square base body 51, i.e., opposite the center of the spiral. However, in contrast to Fig. 2, the waveguide 52 is divided into semicircular base segments 523', whereby the waveguide 52 or the calibration section dkai forms a circular spiral shape. Furthermore, the waveguide 52 is divided along the spiral into two parallel waveguide paths 52a, b. These are connected to each other for high frequency purposes in the center of the spiral by a connecting path 52c. Accordingly, in this embodiment, in addition to the coupling region 521, the reflective end region 522 of the waveguide 52 is also located at the edge of the square base body 51.Due to the parallel guidance of the waveguide paths 52a, b within the spiral, the calibration distance dkai is doubled without the base area of ​​the base body 1 increasing.

[0040] List of reference symbols

[0041] 1 level gauge

[0042] 2 Filling material

[0043] 3 containers

[0044] 4 Superior unit

[0045] 5 Calibration device 51 Base body

[0046] 52 Waveguide 521 Coupling area 522 End area

[0047] 523 basic segment

[0048] 524 connection segment

[0049] 525 screw connection

[0050] A Level cutout d Distance h Installation height

[0051] L Fill level

[0052] RHF reception signal

[0053] SHF radar signals

Claims

Patent claims 1. Calibration device (5) by means of which a radar-based level measuring device (1) or a high-frequency module thereof can be calibrated, comprising: - A base body (51 ) which encloses a section of a planar plane (A), - a waveguide (52) which runs within the cutout in the plane (A), with o a coupling region (521), o an end region (522) reflecting radar signals (SHF, RHF), and o a defined calibration section (dkai) therebetween, characterized in that the waveguide (52) and the calibration section (dkai) are formed in a spiral-shaped manner.

2. Device according to claim 1, wherein the waveguide (52) is divided into semicircular basic segments (523') so that the waveguide (52) forms a circular spiral shape.

3. Device according to claim 1, wherein the waveguide (52) is divided into rectilinear basic segments (523) which are arranged at a defined angle to the respective next basic segment (523) and are connected by corresponding circular segment-shaped connecting segments (524) in such a way that the waveguide forms a particularly square, hexagonal or octagonal spiral shape.

4. Device according to claim 3, wherein all connecting segments (524) of the waveguide (52) have the same radius.

5. Device according to one of the preceding claims, wherein the coupling region (521) is located opposite the center of the spiral on the waveguide (52).

6. Device according to claim 5, wherein the waveguide (52) is divided along the spiral into two parallel waveguide paths (52a, b) which are connected in the center of the spiral by a connecting path (52c).

7. Device according to one of the preceding claims, wherein the base body (51) is made of an electrically conductive material, in particular aluminum, copper or stainless steel, and / or wherein the waveguide (52) has an electrically conductive coating, in particular a silver coating.

8. Device according to one of the preceding claims, wherein the base body (51) is designed in two parts, so that the two corresponding parts (51a, b) of the base body (51) are separated by the plane (A) in the assembled state.

9. Device according to one of the preceding claims, comprising: - A hermetically sealed window arranged at the coupling region (521), wherein the waveguide (52) is subjected to a vacuum or filled with an inert gas.

10. Device according to one of the preceding claims, comprising: - A polarization filter which is arranged in front of the coupling region (521) and is designed in particular to couple the radar signal (SHF) into the waveguide (52) via a dielectric waveguide or a dielectric lens, wherein the waveguide (52) has a rectangular cross-section.

11. Device according to one of the preceding claims, wherein the waveguide (52) has a radar interference point, in particular in the form of a waveguide cross-sectional change, at at least one defined position.

12. Method for calibrating a radar-based level measuring device (1) or a high-frequency module thereof by means of the device (5) according to one of the preceding claims, comprising the following method steps: - coupling a radar signal (SHF) into the waveguide (52) via the coupling area (521), - coupling out a corresponding reception signal (RHF) after Reflection of the radar signal (SHF) at the end area (522), and - Calibration based on the received signal (RHF) and the known calibration distance (dkai) of the waveguide (52).

13. Method according to claim 12, which is carried out during the manufacture of the level measuring device (1 ) or as part of the measuring operation.

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