Fill-level measuring device

Abstract

The invention relates to a waveguide (14) for a multi-frequency radar fill-level measuring device (1), which is used to determine at least one fill-level value (L1, 2, 3) of a filling material (2) and can operate in at least two frequency bands (f1, 2, 3) that differ significantly from each other. The waveguide (14) runs in a housing neck (15) which is used to distance the high-frequency units (10, 11, 12) of the fill-level measuring device (1) from the passive transmitting / receiving structure (13) for high-temperature applications. According to the invention, the waveguide (14) has a cross-section along the conductor axis (x) over which the dielectric value changes. This ensures that the radar signals (S, RHF1, 2, 3) of all frequency bands (f1, 2, 3) are efficiently transmitted between the high-frequency units (10, 11, 12) and the transmitting / receiving structure (13).

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, as well as to a dielectric waveguide for such a level measuring device.

[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 been established for level measurement of filling materials in containers because they are robust and low-maintenance. A further advantage of non-contact measuring methods is the ability to measure the fill level almost continuously. In the field of continuous level measurement, radar-based measuring methods are therefore predominantly used. The term "radar" in the context of this invention 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 the frequency, the higher the measurement resolution can be achieved. The pulse transit time method and FMCW ("Frequency Modulated Continuous Wave ,r ). Radar-based level measurement is described in more detail 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 they achieve greater beamforming for a given antenna size and generally provide more bandwidth, which can be used for greater range resolution. One such application area is high-precision level measurement in refinery tanks.

[0006] However, there are also various disadvantages associated with radar signals at higher frequencies or in higher frequency bands, which can lead to impairments or even failure of the level measurement in certain applications. These disadvantages are largely due to interactions between the radar measurement and the filling materials to be measured, the atmosphere above the filling materials and, to some extent, container shapes, environmental and installation conditions as well as official regulations. Level measurement in grain silos, among others, is an area of ​​application in which a wide beam cone or a low frequency band is advantageous: Due to the granular nature of the filling material in this area, this can lead to 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 is missed by the antenna orThe transmit-receive structure of the level measuring device is not received.

[0007] In order to be able to use the advantages of different frequencies or in different frequency bands, the publication WO 2023099269 A1 describes a level measuring device that can determine the fill level in several, clearly defined frequency bands depending on the situation or application. The problem here, however, is that the radiation and reception of the radar signals must be ensured in all frequency bands. This is particularly relevant for high-temperature applications, since in these cases the antenna or, as a passive transmit / receive structure, is spaced from the components of the high-frequency components by a housing neck in order to protect them from the temperature influences from the interior of the container. Since the waveguide arranged in the housing neck for signal bridging can only be designed for a single frequency band, this reduces the transmission efficiency for all other frequency bands.The invention is therefore based on the object of overcoming this problem.

[0008] The invention solves this problem by means of a dielectric waveguide by means of which radar signals can be guided along a particularly straight conductor axis, comprising: a first end region into which the radar signals can be coupled in or out, and an opposite, second end region for coupling in and out of the radar signals.

[0009] The waveguide according to the invention is characterized by a cross-section relative to the guide axis, across which the dielectric value varies. Due to the resulting dielectric value gradient in the plane of the cross-section, the waveguide's transmission capability is not limited to a single frequency band.

[0010] Within the scope of the invention, it is generally not predetermined how the changing dielectric value is realized. Key influencing factors here are the position and number of frequency bands implemented in the level gauge. For example, in the case of two frequency bands, the cross-section can be divided into at least a first segment with a first dielectric value and a second segment with a second dielectric value.

[0011] In this case, it is advantageous to form the second segment, particularly coaxially around the first segment. The segments can, for example, have a rectangular, elliptical, or circular shape. With such a central design of the segments around one another, it is essential that the dielectric value decreases from the inside to the outside relative to the conductor axis. This means that the first dielectric value of the inner, first segment is higher than the second dielectric value in the outer, second segment. This design ensures that the inner segment also contributes to the transmission of the low frequency band, since the HE11 fundamental mode of all frequency bands is capable of propagation here.

[0012] Accordingly, it is advantageous to design the cross-sectional dimensions (radius r or the longest edge length r) of the segments in particular according to the general physical relationship between cut-off frequency and dimension:

[0013] 1.84 * c '^ 1 ' 2 ~ 2nr 1 segment

[0014] (circular cross-sectional shape). In case of a rectangular cross-sectional shape, its dimensions shall be as per to the corresponding frequency band (fi,2). Where c is the material- or dielectric-dependent signal propagation velocity.

[0015] In order to achieve different dielectric values ​​in the segments, the segments can be made from different materials, each with different dielectric values. Suitable materials for this purpose include PEEK, PTFE, HDPE, PEI, or PFA. If the waveguide is made from different materials, it can be manufactured, for example, by segment-by-segment casting, lamination, or extrusion. However, it is also conceivable to manufacture the waveguide from a monolithic material and, if necessary, subsequently process it so that the dielectric value changes across the cross-section. One possible implementation in this context is for the material to have a correspondingly different porosity and / or density across the cross-section. This can be achieved in particular using 3D printing. Laser drilling or etching are also conceivable.

[0016] Based on the waveguide according to the invention, a radar-based level gauge can be realized that can access at least two different frequency bands for various applications for determining the level of filling materials. For this purpose, the level gauge comprises the following components:

[0017] A first high-frequency unit which is designed to generate a first radar signal within the first frequency band and to receive a corresponding first received signal, a second high-frequency unit which is designed to generate a second radar signal within the second frequency band and to receive a corresponding second received signal, wherein the second frequency band is significantly lower than the first frequency band, a passive transmit / receive structure, such as a focusing radar lens or a horn antenna, via which the radar signals from both high-frequency units can be transmitted towards the filling material and, after reflection at the filling material surface, can be received as corresponding received signals, and an evaluation unit which is designed to determine a fill level value based on at least one of the received signals.

[0018] The waveguide according to the invention serves to guide radar signals of both frequency bands. For this purpose, the radio-frequency units are connected to the waveguide via its first end region. The transmit / receive structure is coupled to the second, opposite end region.

[0019] This inventive design of the level gauge is particularly advantageous in high-temperature applications, where the passive transmit / receive structure is separated from the active and correspondingly temperature-sensitive radio-frequency units by a housing neck. In this case, the waveguide runs through the housing neck, so that the transmit / receive structure is connected to the radio-frequency units at the radio frequency level despite the separation.

[0020] For the purposes of the invention, the frequency bands are always clearly demarcated from one another if their center frequencies are at least a factor of two apart and their bandwidth is each narrower than a fifth of their center frequency. A clear demarcation also exists for the purposes of the invention if the center frequencies of the frequency bands are at least a factor of four apart and their bandwidth is 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 within 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.

[0021] In principle, the inventive design of the waveguide or level gauge is applicable not only to two different frequency bands, but theoretically to any number. In the case of three frequency bands, the waveguide accordingly comprises, for example, a third segment with a third dielectric value, in which case a third radio-frequency unit is also required for generating / processing radar signals in the third frequency band.

[0022] The advantage of multi-frequency level measuring devices is that, depending on the application, the most suitable frequency band can be selected or set on the level measuring device. The level measuring device can be designed to automatically set the appropriate frequency band, for example, depending on which frequency band the level value can be determined most reliably or accurately.

[0023] In relation to the level measuring device according to the invention, the term "unit" is understood to mean any circuit group that is 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 the unit 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. They show:

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

[0026] Fig. 2: a detailed view of the level measuring device according to the invention, and Fig. 3: a cross-sectional view of the waveguide according to the invention.

[0027] 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 f-1,2,3 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 f-1,2,3 at, for example, 6 GHz tends to be suitable. Low frequency bands are also more 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 the container s, a frequency band f-1,2,3 as high as possible is advantageous due to the flat fill surface, since this essentially allows for a potentially higher distance resolution.

[0028] As a rule, the level measuring device 1 is connected via a separate interface unit, in which for example “4-20 mA”, “PROFIBUS”, “HART 1 , or “Ethernet 1is 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 fill level measuring device 1 can also be communicated. To determine the fill level L, the fill level measuring device 1 is mounted above the filling material 2 at a known installation height h above the brine of the container 3. The fill level measuring device 1 is attached and aligned in such a pressure- and media-tight manner to a corresponding opening in the container 3 that only an electrically passive transmitting / receiving structure 13 of the fill level measuring device 1 is directed vertically downwards into the container s in the direction of the fill material 2.The transmit / receive structure 13 can, for example, be based on a focusing radar lens, as indicated in Fig. 1. In principle, it is also possible to use an antenna, such as a horn antenna, as the transmit / receive structure. The active components 10, 11, 12 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.

[0029] Radar signals SHFI,2,3 are transmitted within predefined frequency bands towards the surface of the filling material 2 via the transmit / receive structure 13. After reflection of the radar signals SHFI,2,3 at the filling material surface, the level measuring device 1 receives the reflected reception signals RH I ,2,3 again via the transmit / receive structure 13. The signal propagation time t between transmission and reception of the respective radar signal S, RHFI,2,3 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,2,3. 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 fzpi,2,3 represents the intermediate frequency signal ZFI,2,3, which is obtained after receiving and mixing the radar signal S, RHFI,2,3, according to the signal propagation time t between transmission and reception. f'1 ,2,3 is the preset and therefore known frequency change rate of the transmitted radar signal SHFI,2,3. The frequency fz i, 2, 3 of the intermediate frequency signal ZFI,2,3 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 an appropriate calibration. The level measuring device 1 can then use this to determine the fill level L according to d = h - L, provided the installation height h is stored in the level measuring device 1 or in the higher-level unit 4.

[0030] To determine the signal propagation time t or the corresponding fill level value L based on the low-frequency intermediate frequency signal ZFI,2,3, the fill level measuring device 1 comprises a correspondingly designed evaluation unit in which the FMCW or pulse propagation time measuring principle is implemented. A high-frequency unit 10, 11, 12 is used in the fill level measuring device 1 to generate the radar signal SHFI,2,3 to be transmitted and to create the corresponding intermediate frequency signal ZFI,2,3. In this case, the corresponding high-frequency unit 10, 11, 12 can, for example, each comprise a correspondingly designed phase-locked loop (PLL) on the transmit side if the FMCW method is implemented.On the receiving side, a mixer and a subsequent Fourier transformation logic are used in the evaluation unit in this case to detect the frequency fzpi,2,3 of the intermediate frequency signal ZFI,2,3 corresponding to the distance d.

[0031] As shown in Fig. 1, the transmit / receive structure 13 is arranged inside the container 3, while the high-frequency units 10, 11, 12 are arranged in a separate housing outside the container 3. In order to protect the high-frequency units 10, 11, 12 from any thermal stresses from the container interior, or to separate the container interior from the high-frequency units 10, 11, 12 in accordance with explosion protection regulations, the housing is spaced from the transmit / receive structure 13 by a housing neck 15. For this purpose, the housing neck 15 is designed to be correspondingly long.

[0032] The center frequency or frequency band fi,2,3 of the radar signal SHFI,2,3 is to be selected primarily depending on the area of ​​application and, in particular, depending on the type of filling material 2: For highly accurate level measurement, such as in oil storage tanks, a frequency band fi,2,3 as high as possible is advantageous, whereas for an uneven or wavy filling material surface, a radiation angle of the transmitting / receiving structure 13 that is as wide as possible and thus a comparatively low frequency band fi,2,3 is advantageous. In the context of the present invention, the term "radiation angle" refers to the solid angle at which the transmitting / receiving structure 13 has a defined, equal transmission intensity or reception sensitivity of, for example, -3 dB.

[0033] In order to be able to be used under these different application conditions, the level measuring device 1 shown in Fig. 1 is accordingly capable of emitting radar signals SH1, SH2, SH3 in three different frequency bands fi,2,3, whereby the frequency bands fi,2,3 do not overlap but are clearly separated from one another. The selection of the frequency band fi,2,3 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 fi,2,3 itself. In the second case, the level measuring device 1 can be designed such that it independently selects the underlying frequency band fi,2,3 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.

[0034] As shown in the detailed view of Fig. 2, the level measuring device 1 comprises three separate high-frequency units 10, 11, 12 which are designed to generate corresponding radar signals SHFI,2,3 ZU within the respective frequency band f1,2,3 and to receive or process the corresponding received signals RHFI,2,3 ZU after reflection. In the illustrated embodiment, the first high-frequency unit 10 operates, for example, at a center frequency of 180 GHz or in a corresponding first frequency band f1. The second frequency band f2, in which the second radar signal SHF,2 is generated by the second high-frequency unit 11, has, for example, a center frequency of 80 GHz. The third high-frequency unit 12 generates the corresponding third radar signal SHF3 with a center frequency of 26 GHz in the lowest of the three frequency bands f1,2,3.

[0035] All three frequency bands fi, 2, 3 or the underlying radar signals S, RHFI, 2, 3 are transmitted and received via the same transmit / receive structure 13. For this purpose, in the variant of the level measuring device 1 shown in Fig. 2, the three high-frequency units 10, 11, 12 are arranged on a common printed circuit board substrate 16. In order to transmit the radar signals S, RHFI, 2, 3 between the printed circuit board substrate 16 and the transmit / receive structure 13, the length of the housing neck 15 must be overcome. For this purpose, the level measuring device 1 comprises a dielectric waveguide 14, which is arranged within the housing neck 1 between the printed circuit board substrate 16 and the transmit / receive structure 13. For the transmission of the radar signals S, RHFI, 2, 3 therebetween, a first end region 140 of the waveguide 14 is connected in a signal-technical manner to the circuit board substrate 16 or the corresponding high-frequency paths.The second end region 141 of the dielectric waveguide 14, opposite the guide axis x, is coupled to the transmit / receive structure 13. In the illustrated embodiment, this is designed as a focusing radar lens.

[0036] In order to be able to guide the radar signals S, RHFI,2,3 of all three frequency bands T,2,3 with the lowest possible losses, the waveguide 14 has a structure according to the invention, which is shown in more detail in Fig. 3: Accordingly, the cross section of the waveguide 14 is divided into three segments 142, 143, 144. The cross section runs orthogonally to the guide axis x between the end regions 140, 141.

[0037] In the embodiment shown, all three segments 142, 143, 144 of the cross-section have a rectangular shape, with the first segment 142 located centrally within the second segment 143. The second segment 143 is in turn coaxially enclosed by the third segment 144. The first segment 142 is made of a material with a first dielectric value that is higher than the second dielectric value of the material from which the second segment 143 is made. The third segment 144 is in turn made of a material that has a third dielectric value that is lower than the first and second dielectric values. This results in a stair-step-like gradient of the dielectric value outwards in relation to the cross-section of the waveguide 14.

[0038] As can be seen from Fig. 3, the individual segments 142, 143, 144 adjoin one another without a gap. This can be achieved in manufacturing technology, for example, by casting the respective materials segment by segment. Due to the step-like dielectric value gradient toward the outside and the gap-free transition between the segments 142, 143, 144, the radar signal R, SHF,3 of the lowest frequency band fs propagates over the entire cross-section of the waveguide 14, while the radar signal R, S^ of the middle frequency band f a propagates only through the inner, first segment 142 and the middle, second segment 143. The radar signal R, SHF.I of the first, highest frequency band fi is limited in this respect to the inner, first segment 142. In order to achieve this inventive effect, the edge lengths of the respective longest edge r of the individual segments 142, 143, 144 are according to to be tuned to the respective frequency band fi,2,3 in order to obtain maximum transmission in the waveguide 14 across all frequency bands fi, 2, 3. In contrast to the embodiment shown in Fig. 1 to Fig. 3, it is of course also conceivable that the waveguide 14 does not have a straight, but rather a correspondingly curved conductor axis x, as required.

[0039] List of reference symbols

[0040] 1 level gauge

[0041] 2 Filling material

[0042] 3 containers

[0043] 4 Superior unit

[0044] 10 First high-frequency unit

[0045] 11 Second high-frequency unit

[0046] 12 Third high-frequency unit

[0047] 13 Send / receive structure

[0048] 14 Dielectric waveguide

[0049] 15 Case neck

[0050] 16 circuit board

[0051] 140 First end region of the waveguide

[0052] 141 Second end region of the waveguide

[0053] 142 First segment of the waveguide cross-section

[0054] 143 Second segment of the waveguide cross-section

[0055] 144 Third segment of the waveguide cross-section d Distance fi,2,3 Frequency bands h Installation height

[0056] L Fill level

[0057] RHFI,2,3 Receive signals r Edge length or radius of the waveguide cross-section

[0058] SHFI,2,3 radar signals x conductor axis

Claims

Patent claims 1. Dielectric waveguide (14) by means of which radar signals (S,RHFI,2,3) can be guided along a guide axis (x), comprising: A first end region (140) into which the radar signals (S,RHFI,2,3) can be coupled in or out, a second end region (141) for coupling in and out of the radar signals (S,RHFI,2,3), which second end region is opposite the first end region in relation to the conductor axis (x), characterized in that the waveguide (14) has a cross-section along the conductor axis (x) over which the dielectric value changes.

2. Waveguide (14) according to claim 1, characterized in that the cross section is divided into at least a first segment (142) with a first dielectric value and a second segment (143) with a second dielectric value.

3. Waveguide (14) according to claim 2, characterized in that the second segment (143) is formed in particular coaxially around the first segment (141).

4. Waveguide (14) according to claim 3, characterized in that the first dielectric value is higher than the second dielectric value.

5. Waveguide (14) according to claim 2 to 4, characterized in that the segments (142, 143, 144) have a rectangular, elliptical or round shape.

6. Waveguide according to one of claims 2 to 5, characterized in that the segments (142, 143, 144) are each made of different materials with different dielectric values.

7. Waveguide according to at least one of claims 1-5, which is made of a monolithic material, characterized in that the material has such a different porosity over the cross-section that the dielectric value changes over the cross-section.

8. Radar-based level measuring device for determining at least one level value (L-1,2,3) of a filling material (2), comprising the following components: A first high-frequency unit (10) designed to generate a first radar signal (SHFI) within a first frequency band (f1) and to receive a corresponding first received signal (RHFI), a second high-frequency unit (11) designed to generate a second radar signal (SH2) within a second frequency band (f2) and to receive a corresponding second received signal (RH2), wherein the second frequency band (f2) is lower than the first frequency band (f1), a transmitting / receiving structure (13) via which the radar signals (SHFI,2,S) of the high-frequency units (11, 12) can be transmitted towards the filling material (2) and, after reflection at the filling material surface, can be received as corresponding received signals (RHFI,2,S), a waveguide (14) according to one of the preceding claims, which is designed to guide the radar signals (S,RHFI,2,3) via the first end region (140) with the high-frequency units (11,12) and is connected to the transmitting / receiving structure (13) via the second end region, and an evaluation unit which is designed to determine a fill level value (Li) based on at least one of the received signals (RHFI,2,3).

9. Level measuring device according to claim 8, comprising: A housing neck (15) in which the waveguide (14) runs, so that the transmitting / receiving structure (13) is spaced from the radio frequency units (11, 12).

10. Level measuring device according to claim 8 or 9, wherein the transmitting / receiving structure (13) is designed as a focusing radar lens.

11. Level measuring device according to one of claims 8 to 10, wherein the transmitting / receiving structure (13) is designed according to one of claims 5-7, and wherein the dimensions of the first segment (142) in the case of a circular cross-sectional shape are in particular according to 1.84 * c ^• nr 142 or in the case of a rectangular cross-sectional shape according to is designed for the high, first frequency band (fi), and / or wherein the dimensions of the second segment (143) in the case of a circular cross-sectional shape are in particular according to 1.84 * c fz — ö 2TT * r 143 or in the case of a rectangular cross-sectional shape according to designed for the second, lower frequency band (f2).

Images