Fill-level measuring device

Abstract

The invention relates to a transmitting / receiving assembly (11) for a temperature-resistant radar-based fill-level measuring device (1), the transmitting / receiving assembly being used to efficiently emit and receive radar signals (SHF1,2, RHF1,2) in different frequency bands. For this purpose, the transmitting / receiving assembly (11) is based on a wave guide (111), by means of which the temperature-sensitive high-frequency units of the fill-level measuring device (1) are spaced apart from the possibly hot contents (2). The radar signals (SHF1,2, RHF1,2) are emitted and received via the end region of the wave guide (111). According to the invention, the wave guide (111) has an inner cross-section (D) through which only the radar or receive signals (SHF1, RHF1) of the lowest frequency band propagate exclusively in the fundamental mode, while the radar or receive signals (SHF2, RHF2) of the higher frequency band must propagate predominantly in a higher-order mode. In order that all frequency bands are nevertheless emitted with a common phase center and thus efficiently at the end region, according to the invention an adjustment element (112) is disposed there which converts the radar signals (SHF2) of the high frequency band into lower modes in the emission direction. As a result of this design, the transmitting / receiving assembly (11) can be compactly and, at the same time, efficiently designed.

Description

[0001] Level measuring device

[0002] The invention relates to a radar-based level measuring device that can be adapted to a wide variety of applications.

[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 corresponding 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.

[0004] Non-contact measuring methods have become established for level measurement of filling materials in containers because they are robust and low-maintenance. Another advantage of non-contact measuring methods is their ability to measure the fill level virtually continuously. Therefore, radar-based measuring methods are predominantly used in the field of continuous level measurement. In the context of this invention, the term "radar" refers to signals or electromagnetic waves with frequencies between 0.1 GHz and 300 GHz. Due to the principle, the higher the frequency, the higher the measurement resolution can be achieved. The pulse transit time method and FMCW ("Frequency Modulated Continuous Wave") have become established measuring methods. Radar-based level measurement is described in more detail, for example, in "Radar Level Detection, Peter Devine, 2000".

[0005] Typical frequency bands approved for radar-based level measurement are 26 GHz, 60 GHz, 80 GHz, and 120 GHz, and increasingly also 180 GHz and 240 GHz. Higher frequency bands are advantageous for many applications because, for given antenna dimensions, greater beam focusing is achieved and generally more bandwidth is available, which can be used for greater distance resolution. One such application area is, for example, high-precision level measurement in refinery tanks.

[0006] However, there are also various disadvantages associated with radar signals with higher frequencies or in higher frequency bands, which can lead to impairments or even failure of the level measurement value in certain application areas.

[0007] These disadvantages are largely due to the interactions between radar measurement and the materials being measured, the atmospheres above the materials, and, to some extent, container shapes, environmental and installation conditions, and regulatory requirements. Level measurement in grain silos, among others, is an application where a wide beam cone or a low frequency band is advantageous: Due to the granular nature of the material in these areas, this can lead to a very diffuse reflection of the radar signal. With a narrow beam cone or a high frequency band, the reflected received signal can be deflected so strongly from the vertical that it cannot be received by the antenna array of the level measuring device.

[0008] To utilize the advantages of frequencies or different frequency bands, the publication WO 2023099269 A1 describes a level measuring device that can determine the level in several, clearly defined frequency bands depending on the situation or application. However, transmitting and receiving the respective radar signals in their different frequency bands via one and the same antenna is problematic, as each antenna is optimized for a specific frequency. With a broadband antenna design, corresponding compromises must be made for all frequency bands. The use of multiple antennas in a transmit / receive arrangement, in turn, requires more space on the level measuring device.The situation is further complicated when the fill level has to be measured in containers with elevated process temperatures, as the high-frequency units are very sensitive to temperature, especially at high frequencies.

[0009] The invention is therefore based on the object of providing a radar-based level measuring device for several, clearly separated frequency bands, which overcomes these disadvantages.

[0010] The invention solves this problem by means of a transmit / receive arrangement for a radar-based level measuring device, by means of which radar signals can be transmitted in at least two clearly different frequency bands and, after reflection, can be received as corresponding receive signals. For this purpose, the transmit / receive arrangement comprises: a waveguide, with o at least one coupling structure, via which the radar signals can be coupled into the waveguide or the receive signals can be coupled out of the waveguide, o an internal cross-section which is dimensioned such that the radar or receive signals of the lowest frequency band can propagate exclusively in one of their fundamental modes, and o a first end region, via which the radar signals can be transmitted orthe received signals are receivable, a matching element arranged at the first end region, which is designed to convert the radar signals of the high frequency band in the radiation direction into at least one lower mode, in particular into the fundamental mode. In a scientific context, the underlying operating principle is disclosed, for example, in "Flexible Radar Front End with Multimodal Transition at 300 GHz," M. Geiger et al.; 2020 IEEE / MTT-S International Microwave Symposium (IMS).

[0011] The advantage of this transmitting / receiving arrangement according to the invention is that the radar signals can be transmitted or received in all frequency bands with a common phase center, provided that the internal cross section is dimensioned such that the radar or reception signals of the high frequency band according to

[0012] D > 2 * A2 are capable of propagation in a higher-order mode. From an optical perspective, this makes it easier to arrange a focusing radar lens in front of the first end region of the waveguide in order to focus the radar signals of all frequency bands more strongly in the radiation direction. This allows the transmit / receive arrangement to be designed more compactly overall. In this context, it is particularly advantageous to arrange the radar lens in front of the first end region of the waveguide in such a way that the radiation angle of the radar signal in the lowest frequency band completely covers the radar lens in order to achieve the highest possible signal intensity.

[0013] To convert high-frequency radar signals into lower modes, the matching element can be made of a dielectric material and / or can be designed to be conical or tapered in the direction of radiation. The matching element can be attached to the waveguide, for example, by providing a corresponding sub-segment on the matching element that protrudes into the first end region, thus securing the matching element there.

[0014] The cross-sectional shape of the waveguide is not fixed within the scope of the invention. The shape can, for example, be square. However, it is particularly advantageous if the waveguide has a round internal cross-section, since in this case the adapter element can also be designed to be rotationally symmetrical, which simplifies production and attachment to the waveguide.

[0015] The transmitter / receiver arrangement according to the invention is particularly suitable for radar-based level measurement of filling materials. A corresponding level measuring device accordingly comprises the following components:

[0016] A high-frequency unit designed to generate radar signals or process corresponding received signals in at least two clearly separated frequency bands, in particular according to the FMCW method, and an evaluation unit designed to determine the fill level based on the received signal from at least one of the frequency bands.

[0017] In this case, the transmitting / receiving arrangement according to the invention serves to transmit the radar signals towards the filling material and to receive corresponding reception signals after reflection at the filling material surface.

[0018] This enables the level gauge to measure efficiently in each of the clearly separated frequency bands, depending on the application. Within the meaning of the invention, the frequency bands are clearly separated from one another if their center frequencies are at least a factor of two apart and their bandwidths are each narrower than a fifth of their center frequency. Within the meaning of the invention, a clear separation also exists if the center frequencies of the frequency bands are at least a factor of four apart and their bandwidths are each narrower than half of their center frequency. The center frequency of a frequency band is defined as the frequency that is exactly in the middle of the frequency band. According to this definition, for example, a frequency band with a center frequency of 26 GHz and a bandwidth of 2 GHz extends from 25 GHz to 27 GHz.

[0019] Furthermore, the term "unit" within the scope of the invention 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 unit can therefore comprise corresponding analog circuits for generating or processing corresponding analog signals. However, the respective unit 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 unit 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 a unit are arranged on a common circuit board or on several connected circuit boards.

[0020] The invention is explained in more detail with reference to the following figures. They show:

[0021] Fig. 1: A level measuring device according to the invention on a container, Fig. 2: a transmitting / receiving arrangement according to the invention at low frequency band, and

[0022] Fig. 3: the transmit / receive arrangement at high frequency band.

[0023] For a basic understanding of the invention 1, 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 in which the fill level measuring device 1 determines the fill level L: In the case of a coarse-grained filling material 2 and correspondingly diffuse reflection, a comparatively low frequency band, for example, 6 GHz, tends to be suitable. Low frequency bands are also more suitable for foaming filling materials, since in this case the foam does not have a reflective effect. In the case of a refinery tank as the container 3, the highest possible frequency band is advantageous due to the flat fill material surface, since this essentially allows for a potentially higher distance resolution.

[0024] As a rule, the level measuring device 1 is connected via a separate interface unit, such as “4-20 mA”, “PROFIBUS”, “HART 1 , or "Ethernet" to a higher-level unit 4, such as a local process control system or a decentralized server system. The measured fill level value L can be transmitted via this interface, for example, to control inflows or outflows of the container 3 if necessary. However, other information about the general operating status or for parameterizing the level measuring device 1 can also be communicated via the interface.

[0025] 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 a pressure- and media-tight manner to a corresponding opening of the container 3 such that a transmitter / receiver arrangement 11 of the level gauge 1 is directed vertically downwards into the container s in the direction of the filling material 2.

[0026] By means of the transmitting-receiving arrangement 11, radar signals SHFI,2 are transmitted within predefined frequency bands in the direction of the surface of the filling material 2. After reflection of the radar signals SHFI,2 at the filling material surface, the level measuring device 1 receives the reflected reception signals RHF-I,2 again via the transmitting-receiving arrangement 11. The signal propagation time t between transmission and reception of the respective radar signal S, RHFI,2 is according to proportional to the distance d between the level measuring device 1 and the filling material 2, where c represents the media-dependent and usually at least roughly known propagation speed of the respective radar signal S, RHFI,2. The signal propagation time t can be determined by the level measuring device 1, for example, using the FMCW or the pulse propagation time method. In the case of FMCW, the frequency fzFi,2 of the intermediate frequency signal ZFI,2, which is obtained after receiving and mixing the radar signal S, RHFI,2, according to

[0027] _ fzFl,2 ~ f'1,2 is the signal propagation time t between transmission and reception. f'1,2 is the preset and therefore known frequency change rate of the transmitted radar signal SHFI,2 within the respective frequency band. The frequency fzrv of the intermediate frequency signal ZFI ,2 can be determined, for example, by its Fourier transformation. This allows the level measuring device 1 to assign the measured propagation time t to the respective distance d, for example based on a corresponding calibration. The level measuring device 1 can then

[0028] L = h — d in turn determines the fill level L, provided that the installation height h is stored as a parameter in the level measuring device 1.

[0029] To determine the signal propagation time t or the corresponding fill level value L based on the low-frequency intermediate frequency signal ZFI ,2, the fill level measuring device 1 comprises a suitably designed evaluation unit in which, for example, the FMCW or pulse propagation time measuring principle is implemented. A radio-frequency unit connected to the transmit / receive arrangement 11 is used to generate the radar signal SHFI,2 to be transmitted and to create the corresponding intermediate frequency signal ZFI ,2. If the FMCW method is implemented, the radio-frequency unit can, for example, comprise a suitably designed phase-locked loop (PLL) on the transmit side.In this case, a mixer and subsequent Fourier transformation logic are used on the receiving side of the high-frequency unit in order to detect the frequency fzFi,2 of the intermediate frequency signal ZFI ,2 which corresponds to the distance d. The center frequency or the frequency band of the radar signal SHFI,2 is to be selected depending on the area of ​​application and in particular on the type of filling material 2: For highly accurate level measurement, such as in oil storage tanks, the highest possible frequency band is advantageous, whereas with an uneven or wavy filling material surface, the widest possible radiation angle of the transmitting / receiving arrangement 11 or a comparatively low frequency band is advantageous. The term “radiation angle” in the context of the present invention refers to the solid angle at which the transmitting / receiving arrangement 11 has a defined, equal transmission intensity orReception sensitivity of, for example, -3 dB.

[0030] In order to be used under these application conditions, the level measuring device 1 shown in Fig. 1 is capable of emitting radar signals SHFI, SH2 in two different frequency bands, whereby the frequency bands do not overlap but are clearly separated from one another. The selection of the frequency band on the basis of which the level measuring device 1 determines the level value L can either be specified manually, or the level measuring device 1 selects the most suitable frequency band itself. In the second case, the level measuring device 1 can be designed so that it independently selects the underlying frequency band depending on certain parameters, such as a possible rate of change of the level value L. This is also described in the publication DE 10 2021 131 690 A1.

[0031] To generate signals in the different frequency bands, the level gauge 1 accordingly comprises two separate high-frequency units designed to generate corresponding radar signals SHFI,2 ZU within the respective frequency band and, after reflection, to receive and process the corresponding received signals RHFI,2,3 ZU. The first high-frequency unit operates, for example, at a center frequency of 180 GHz or in a corresponding first frequency band. The second high-frequency unit generates the corresponding second radar signal SH 2 in the lowest frequency band with a center frequency of 26 GHz.

[0032] With regard to possible temperature stress from the interior of the container 3, it is important to design the level measuring device 1 such that the high-frequency units in particular are thermally protected against this. Fig. 2 and Fig. 3 therefore show an inventive design of the transmit / receive arrangement 11, which offers protection against thermal stress in this regard and simultaneously enables efficient transmission and reception in all frequency bands: Accordingly, the transmit / receive arrangement 11 is based on a waveguide 111 for the radar signals SHFI,2 to be transmitted and the receive signals RHFI,2, which are coupled into and out of the waveguide 111 by the high-frequency units via a coupling structure 1111. The length I of the waveguide 111 must be dimensioned such that sufficient thermal insulation is provided.

[0033] In the embodiment shown in Fig. 2, the coupling structure 1111 is arranged in the end region of the waveguide 111 that is spaced from the interior of the container 3. In contrast to this embodiment shown, it is also possible for the coupling structure 1111 to couple laterally into the waveguide 111, and / or for a separate coupling structure to be provided for each frequency band. Since the coupling structure 1111 is connected to the respective radio-frequency unit and is therefore arranged at the end region of the waveguide 111 that is remote from the process, the radio-frequency units are correspondingly protected by the waveguide 111 from heat from the interior of the container 3.

[0034] The radar signals SHFI,2 are transmitted or the received signals RHFI,2 are received via the opposite, first end region of the waveguide 111, i.e. the end region which, in the installed state of the level measuring device 1, faces the filling material 2. According to the invention, the waveguide 111 has an internal cross-section D through which the radar or received signals SHFI,2, RHFI,2 of the lowest frequency band can propagate exclusively in one of the fundamental modes, i.e., TEn in the case of a round cross-sectional shape or TE in the case of a rectangular cross-sectional shape. For this purpose, the internal cross-section D (i.e., the diameter or the edge length in the case of a rectangular cross-sectional shape) must be determined according to the formula for the cutoff frequency where Ai is the wavelength of the radar signal SHFI, RH I in the low frequency band. On the other hand, the dimensioning according to This ensures that the radar signal SH I in the low frequency band is propagated or radiated exclusively in the fundamental mode. This can be seen in Fig. 2, which shows the E-field distribution of the low frequency band within or in the radiation direction in front of the waveguide 111. Fig. 3 illustrates the corresponding E-field distribution of the radar signal SnF2 in the high frequency band: As can be seen, the radar signal SHF2 in the high frequency band within the waveguide 111 does not propagate exclusively in the fundamental mode due to the previously described dimensioning of the internal cross-section D, since the above formulas require the following relationship:

[0035] D > 2 * A2

[0036] As a result, the high frequency band is transmitted predominantly in higher-order modes, since the internal cross-section D for the high frequency band is "too wide" for purely mono-mode transmission. According to the invention, a dielectric matching element 112 is therefore arranged at the first end region, by means of which the radar signals SHF2 of the high frequency band are converted in the radiation direction into lower modes, including a predominant proportion of the fundamental mode. The radar signal SHF2 generated in the lower frequency band is not influenced by the matching element 112 with regard to its mode, since the structural size of the matching element 112 is significantly smaller than the wavelength of the low frequency band, at at least 1:16.

[0037] The function of the matching element 112 is based on its material properties on the one hand and its shape on the other: The embodiment of the matching element 112 shown in Fig. 2 and Fig. 3 tapers in a step-like manner in the radiation direction. The number of steps depends on the number of different frequency bands in which the radar signals S, RHFI, 2 are transmitted and received. Accordingly, the embodiment of the matching element 112 shown in Fig. 2 and Fig. 3 has two steps. The dimensioning of the step height or taper depends on the material from which the matching element 112 is made: The higher the dielectric value of the material, the smaller the step height or taper can be. Suitable materials for this purpose are particularly PTFE, PFA or PEEK.

[0038] In the embodiment of the matching element 112 shown in Fig. 2 and Fig. 3, its attachment to the first end region of the waveguide 111 is effected by means of two partial segments 1121 which are designed as a monolithic component of the matching element 112 and protrude into the first end region of the waveguide 111 in a form-fitting manner with respect to the inner cross section D.

[0039] Overall, according to the invention, the radar signals SHFI,2 of both frequency bands are emitted with a common phase center by the matching element 112. This makes it possible to use a common radar lens 113 for the radar signals S, RHFI,2 of both frequency bands, which is connected upstream of the waveguide 11 in the transmission direction. As can be seen in Fig. 2 and Fig. 3, the transmitting / receiving arrangement 11 is designed such that the first end region of the waveguide 11 facing the filling material 2, or the matching element 112, is located at the focal point of the radar lens 113. The radar lens 113, in turn, makes it possible to achieve a greater concentration of the radar signals S, RHFI,2, which in turn makes it possible to detect a greater distance d.

[0040] List of reference symbols

[0041] 1 level gauge

[0042] 2 Filling material

[0043] 3 containers

[0044] 4 Superior unit

[0045] II Transmit / receive arrangement

[0046] III Waveguide

[0047] 112 Adjustment element

[0048] 113 Radar lens

[0049] 1111 Coupling structure

[0050] 1121 Partial segment of the matching element c Propagation speed of the radar signals

[0051] D Internal cross-section d Distance fzFi,2,3 Frequency of the intermediate frequency signals f'1 ,2,3 Frequency change rate of the emitted radar signal h Installation height

[0052] L Fill level

[0053] RHFI,2 receive signals

[0054] SHFI,2 radar signals

[0055] AI ,2 wavelengths of the radar signals

Claims

Patent claims 1 . Transmitting / receiving arrangement (11) for a radar-based level measuring device (1), by means of which radar signals (SHFI,2) can be transmitted in at least two different frequency bands and, after reflection, can be received as corresponding received signals (RHFI,2), comprising: a waveguide (111), with o at least one coupling structure (1111) via which the radar signals (SHFI,2) can be coupled into the waveguide (111) or the received signals (RHFI,2) can be coupled out of the waveguide (111), o an internal cross-section (D) which is dimensioned such that the radar or received signals (SHFI, RHFI) of the lowest frequency band can propagate exclusively in one of their fundamental modes, and o a first end region via which the radar signals (SHFI,2,S) can be transmitted orthe reception signals (RHFI,2) are receivable, a matching element (112) arranged at the first end region, which is designed to convert the radar signals (SH 2) of the high frequency band in the radiation direction into at least one lower mode, in particular one of their fundamental modes.

2. Transmitting / receiving arrangement according to claim 1, wherein the internal cross-section (D) is dimensioned such that the radar or receiving signals (SHFI, RHFI) of the high frequency band are capable of propagation in a higher-order mode.

3. Transmitting / receiving arrangement according to claim 1 or 2, comprising: A converging radar lens (113) which is arranged in front of the first end region of the waveguide (111), in particular such that the matching element (112) is located at the focal point of the radar lens (113).

4. Transmitting / receiving arrangement according to claim 1, 2 or 3, wherein the matching element (112) is made of a dielectric material.

5. Transmitting / receiving arrangement according to claim 4, wherein the matching element (112) is designed to be conically tapered or stepped in the radiation direction.

6. Transmitting / receiving arrangement according to claim 4 or 5, wherein the matching element (112) comprises at least one partial segment (1121) which projects into the first end region of the waveguide (111) in such a way that the matching element (112) is fixed to the waveguide (111).

7. Transmitting / receiving arrangement according to one of the preceding claims, wherein the waveguide (111) has a rotationally symmetrical internal cross-section (D).

8. Level measuring device for determining a level (L) of a filling material (2), comprising the following components: A high-frequency unit which is designed to generate radar signals (SHFI,2) in both frequency bands, in particular according to the FMCW method, or to process corresponding received signals (RHFI,2), a transmitting / receiving arrangement (11) according to at least one of the preceding claims, by means of which the radar signals (SHFI,2,3) are Filling material (2) can be emitted and, after reflection at the filling material surface, corresponding reception signals (RHFI,2) can be received, and an evaluation unit which is designed to determine the filling level (L) on the basis of the reception signal (RHFI,2) from at least one of the frequency bands.

9. Level measuring device according to claim 8, which is designed to transmit or receive the radar signals (SHFI,2,3) simultaneously or cyclically alternately in the at least two different frequency bands.

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