Fill-level measuring device

Abstract

The invention relates to a fill-level measuring device (1) for determining the fill level (L) of a filling material (2), comprises the following components: a high-frequency unit (10) for generating and emitting radar signals (SHF) along a beam axis (a) towards the filling material (2) and for receiving corresponding received signals (RHF) after the signals are reflected on the surface of the filling material, an evaluation unit for determining the fill level (L) on the basis of the received signal (RHF), and two radar lenses (11, 12) positioned one behind the other in the beam axis (a). According to the invention, the apertures of the radar lenses (11, 12) are selected in such a way that the mutual spacing (b11 + f12) thereof is the sum of the image distance (b11) of the first radar lens (11) and the focal length (f12) of the second radar lens (12). As a result, the radar signals (SHF, RHF) can be guided through a thermally insulating housing neck (131). Furthermore, by virtue of the design of the apertures within the housing neck (131) according to the invention, a radar window (14) used for explosion protection can be positioned in such a way that the beam guidance is only minimally limited.

Description

[0001] Level measuring device

[0002] The invention relates to a radar-based 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 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. A further advantage of non-contact measuring methods is their ability to measure the fill level virtually continuously. Therefore, radar-based measuring methods are predominantly used for continuous level measurement (in the context of this patent application, the term "radar" refers to signals or electromagnetic waves with frequencies between 0.03 GHz and 300 GHz). In principle, the higher the frequency, the higher the measurement resolution that can be achieved. The pulse transit time method and FMCW ("Frequency Modulated Continuous Wave") have become established measurement 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, as well as 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. For higher frequencies above 100 GHz, the device's internal radio-frequency unit, which generates and processes the radar signals, is implemented in the form of integrated circuits (“ / C”). The radar signals are transmitted or received directly via the IC package (better known as “AiP = Antenna in Package”) or even directly on the semiconductor chip (“AoC = Antenna on Chip”).The structures for such integrated antennas must be kept small due to the limited chip area, resulting in a correspondingly wide radiation angle. In the context of the present invention, the term "radiation angle" generally refers to the solid angle at which the respective antenna exhibits a defined, equal transmission intensity or reception sensitivity of, for example, -3 dB relative to a corresponding beam axis. The term "beam axis," in turn, refers to the vector along which the radar signal is transmitted with maximum intensity.

[0006] In order to transmit the radar signals with sufficient focus towards the filling material despite IC-based radiation, a focusing radar lens must be installed upstream of the radio-frequency unit at such high frequencies, which, however, leads to increased design and manufacturing complexity. A further design conflict arises in this context from the fact that the radio-frequency unit must be encapsulated in accordance with explosion protection regulations. An alternative approach, in which the radar signals from the radio-frequency unit are not freely emitted towards the radar lens, is described in the publication WO 2023 / 285300 A1. According to this approach, the radar signals from the radio-frequency unit are coupled into an antenna via a dielectric waveguide. The problem with this, however, is the small waveguide cross-section at high frequencies, which leads to a correspondingly delicate design and, consequently, difficult manufacturing.

[0007] The invention is therefore based on the object of providing a radar-based level measuring device that overcomes these disadvantages. The invention solves this problem by providing a level measuring device for determining the level of a product, comprising the following components:

[0008] - A high-frequency unit designed o to generate radar signals, particularly in a frequency band above 100 GHz, and to radiate them along a beam axis towards the filling material and o to receive corresponding reception signals after their reflection at the filling material surface,

[0009] - an evaluation unit designed to determine the fill level based at least on the received signal,

[0010] - a first radar lens arranged in the beam axis, and

[0011] - a second radar lens arranged behind the first radar lens in the beam axis with respect to the radio frequency unit.

[0012] The first radar lens and / or the second radar lens can be made of PTFE, PEEK, PEI, PFA or PEE.

[0013] According to the invention, the level gauge is characterized by the fact that the apertures of the radar lenses are specially coordinated with one another: According to the invention, the apertures of the two radar lenses are selected such that their distance from one another along the beam axis corresponds to the sum of the image distance of the first radar lens and the focal length of the second radar lens. This means that the first radar lens is positioned at a distance along the beam axis from the focal point of the second radar lens by the amount of its image distance.

[0014] This inventive design is particularly advantageous if the housing of the level gauge includes a housing neck for thermally insulating the temperature-sensitive units from the container interior. In this case, the housing neck can serve, on the one hand, to structurally space the second radar lens from the first radar lens. The inventive design of the apertures is advantageous in that the resulting beam path of the radar signals within the housing neck is relatively narrow and thus not disrupted by any components that protrude into the housing neck due to the design.

[0015] Secondly, a radar-permeable, yet gas- and pressure-tight window can be arranged within the housing neck in the beam path of the radar signals in order to hermetically separate the housing neck and thus make the level measuring device explosion-proof, in particular in accordance with the ATEX Directive 2014 / 34 / EU. For this purpose, the radar-permeable window can preferably be made of glass or ceramic. The inventive design of the apertures in turn enables the window to be arranged at the focal point of the second radar lens. According to the invention, this is equivalent to the window being arranged away from the first radar lens by the distance of its image width. This has a particularly advantageous effect on the pressure resistance of the level measuring device: Due to the resulting radar beam guidance, the window can be made smaller and thus thinner while maintaining the same pressure resistance.By making the window thinner, the transmission of radar signals is less affected.

[0016] Efficiency, especially in transmitting radar signals, can be further increased by designing the first radar lens to minimize reflection of the radar signal emitted by the high-frequency unit. This can be achieved in several ways:

[0017] - The first radar lens can be designed in such a way that it has a slight tilt relative to the beam axis, i.e. between 0.2° and a maximum of 15°.

[0018] - The first radar lens can have a coating or a particularly groove-shaped structure with a depth of particularly one quarter of the wavelength of the radar signal on a surface facing the high-frequency unit.

[0019] - The first radar lens can be structurally divided into two halves with respect to the beam axis, which in turn are offset from each other by a quarter of the wavelength of the radar signal with respect to the beam axis. The term "unit" in the context of the invention generally refers to any design or encapsulation of the electronic circuits required for the specific intended use, e.g. for high-frequency signal processing or as an interface. Depending on the intended use, the corresponding unit can therefore comprise corresponding analog circuits for generating or processing corresponding analog signals. However, the 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 process 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 irrelevant whether various electronic circuits within the unit are distributed 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 radar-based level gauge on a container, and

[0022] Fig 2. A sectional view of the level measuring device according to the invention.

[0023] To provide a basic understanding of the level measuring device 1 according to the invention, Fig. 1 shows a container 3 with a filling material 2 whose fill level L is to be determined. Depending on the type of filling material 2 and the area of ​​application, the container 3 can be up to more than 100 m high. The conditions in the container 3 also depend on the type of filling material 2 and the area of ​​application. For example, exothermic reactions can lead to high temperature and pressure loads. In the case of dust-containing or flammable substances, appropriate explosion protection conditions must be observed inside the container. As a rule, the level measuring device 1 is connected to a higher-level unit 4, such as a local process control system or a decentralized server system, via a separate interface unit in which a protocol such as "4-20 mA", "PROFIBUS", "HÄRT" or "Ethernet" is implemented.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 of the level measuring device 1 can also be communicated.

[0024] In order to be able to determine the fill level L independently of the prevailing conditions, 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 to a corresponding opening of the container 3 in such a pressure- and media-tight manner that only a second radar lens 12 of the fill level measuring device 1 is directed vertically downwards into the container 3 towards the fill material 2, while the other components of the fill level measuring device 1 are arranged outside the container 3 in a separate housing 13.

[0025] Radar signals SHF are transmitted within a predefined frequency band via the second radar lens 12 in the direction of the surface of the filling material 2. After reflection at the filling material surface, the level measuring device 1 receives the reflected reception signals RHF again via the second radar lens 12. The signal propagation time t between transmission and reception of the respective radar signal SHF, RHF is according to proportional to the distance d between the level measuring device 1 and the filling material 2, whereby the variable “c” represents the at least roughly known radar propagation speed. The signal propagation time t can be determined by the level measuring device 1, for example, using the FMCW or pulse propagation time method. This allows the level measuring device 1 to assign the measured propagation time t to the respective distance d, for example on the basis of an appropriate calibration. The level measuring device 1 can then determine the filling level L according to d = h — L, provided that the installation height “h” is stored in the level measuring device 1. To determine the signal propagation time t or the corresponding filling level value L based on the incoming receive signal RHF, the level measuring device 1 comprises an appropriately designed evaluation unit in which, for example, the FMCW or pulse propagation time measuring principle is implemented.A corresponding high-frequency unit 10 in the level gauge 1 is used to generate the transmitted radar signal SHF and to preprocess the received signal RHF. If the FMCW method is implemented, the high-frequency unit 10 can be based on a PLL ("Phase Locked Loop") on the transmit side, for example. On the receive side, the high-frequency unit 10 can in this case include a Fourier transformation logic. This serves to identify the frequency maximum in the intermediate frequency signal corresponding to the signal propagation time t, which is characteristic of FMCW and is obtained after mixing the received signal RHF with the transmitted radar signal SHF.

[0026] The frequency or frequency band in which the high-frequency unit 10 of the level measuring device 1 generates the radar signal SHF is selected primarily depending on the nature of the filling material 2: In the case of highly accurate level measurement, a comparatively high frequency band, for example, 180 GHz, is implemented in principle. An inventive design of the level measuring device 1, which is easy to manufacture even at such a high frequency band, has explosion protection compliance, and can be designed to be temperature-resistant, is explained in more detail with reference to Fig. 2: Basically, the IC-based high-frequency unit 10 comprises a printed circuit board substrate 100, which is arranged in the housing 13. As a result, the beam axis a, under which the radar signals S, RHF are emitted or received by the IC, is aligned orthogonally to the printed circuit board substrate 100.The printed circuit board substrate 100, together with the high-frequency unit 10, is secured within the housing 13 in a potting encapsulation 15. This encapsulation is filled with a potting compound (not explicitly shown) during the manufacturing process to encapsulate the high-frequency unit 10 in accordance with explosion protection regulations. The beam axis a is thus defined by appropriate attachment points for the printed circuit board substrate 100 within the potting encapsulation 15.

[0027] As can be seen from Fig. 2, a focusing first radar lens 11 is arranged in front of the high-frequency unit 10 in alignment with the beam axis a. Furthermore, the housing 13 of the level measuring device 1 in the illustrated embodiment comprises a housing neck 131, so that the high-frequency unit 10 is appropriately spaced from the container interior for its thermal protection. The end region of the housing neck 131 which, in the assembled state, faces away from the container interior adjoins the first radar lens 11 in alignment with the beam axis a. As shown in Fig. 2, the housing neck 131 is aligned in the direction of the beam axis a. The opposite end region of the housing neck 131, which in the assembled state faces the filling material 2, is closed off by a second radar lens 12, wherein the second radar lens 12 is again arranged and aligned in the same beam axis a as the first radar lens 11.

[0028] Overall, the bundling of the two radar lenses 11, 12 is independent of whether it is the transmitted radar signal SHF or the received radar signal RHF due to the generally reciprocal radar properties. Since plastics such as PTFE, PEEK, PEI, PFA or PEE cause a corresponding refraction of the radar signals S, RHF due to their dielectric value, the radar lenses 11, 12 can be made of one of these materials, e.g. by injection molding or hot stamping. As illustrated in Fig. 2, the distance fn + bi2 of the two radar lenses 11, 12 from one another along the beam axis a and their apertures are coordinated such that this distance results in the sum of the image length bi 1 of the first radar lens 11 and the focal length fi2 of the second radar lens 12. In other words: The first radar lens 11 is arranged at a distance bn from the focal point of the second radar lens 12.The definitions of the image length “b” and the focal length “f” are based on the generally known imaging equation.

[0029] 1 1 _ 1 b + g 7 with “g” as object distance.

[0030] Fundamentally, this inventive design ensures that the housing neck 131 can be designed to be comparatively narrow with respect to the beam axis a, thus providing good thermal insulation from the container interior. Furthermore, a particular advantage of this inventive aperture design is that the extent of the radar beam path at the focal point of the first radar lens 11 is logically minimal, as outlined in Fig. 2. This makes it possible to constructively position a radar-permeable window 14 at this point, which separates the housing neck in a pressure- and gas-tight manner, thus complying with explosion protection requirements. Due to its location at the focal point, the window 14 only minimally disrupts the propagation of the radar signals S and RHF, since it only needs to have a small diameter due to the beam guidance.This allows the window thickness to be reduced without compromising the certification of the level gauge 1 with regard to relevant directives, specifications, and standards, such as the ATEX Directive 2014 / 34 / EU. A reduced window thickness, in turn, reduces the absorption of the radar signals S, RHF, which increases the performance efficiency of the level gauge 1. List of reference symbols.

[0031] 1 level gauge

[0032] 2 Filling material

[0033] 3 containers

[0034] 4 Superior unit

[0035] 10 High frequency unit

[0036] 11 First radar lens

[0037] 12 Second radar lens

[0038] 13 housings

[0039] 14 Radar-permeable window

[0040] 15 Encapsulation

[0041] 100 PCB substrate

[0042] 131 Housing neck a Beam axis bi2 Image distance of the second radar lens d Distance fu Focal length of the first radar lens h Installation height

[0043] L Fill level

[0044] RHF reception signals

[0045] SHF radar signals

Claims

Patent claims 1. Level measuring device for determining a level (L) of a filling material (2), comprising the following components: - A high-frequency unit (10) which is designed to o generate radar signals (SHF) and to radiate them along a beam axis (a) towards the filling material (2) and o receive corresponding reception signals (RHF) after their reflection at the filling material surface, - an evaluation unit designed to determine the fill level (L) based at least on the received signal (HF), - a first radar lens (11) arranged in the beam axis (a), and - a second radar lens (12) arranged in relation to the high frequency unit (10) is arranged behind the first radar lens (11) in the beam axis (a), characterized in that Apertures of the radar lenses (11, 12) are selected such that their distance (bi 1 + fi2) along the beam axis (a) to each other is the sum of - the image distance (bn) of the first radar lens (11 ), and - corresponds to the focal length (fi2) of the second radar lens (12).

2. Level measuring device according to claim 1, comprising: - A housing (13) with a housing neck (131 ) through which the second radar lens (12) is spaced from the first radar lens (11 ).

3. Level measuring device according to claim 2, comprising: - A radar-permeable window (14), which is made in particular of a glass or a ceramic, which is arranged at the focal point of the second radar lens (12) or at the image distance (bn) of the first radar lens (11 ) is arranged in a hermetically separated manner in the housing neck (131 ).

4. Level measuring device according to claim 3, wherein the window (1) is designed to be gas and pressure tight, in particular according to ATEX Directive 2014 / 34 / EU.

5. Level measuring device according to one of the preceding claims, wherein the first radar lens (11) and / or the second radar lens (12) are / is made of PTFE, PEEK, PEI, PFA or PEE.

6. Level measuring device according to one of the preceding claims, wherein the high-frequency unit (10) is designed to generate or process the signals (S, RHF) in a frequency band at least 100 GHz, in particular at 180 GHz.

7. Level measuring device according to one of the preceding claims, wherein the first radar lens (111) has a defined tilt of in particular between 0.2° and 15° relative to the beam axis (a), and / or wherein the first radar lens (111) has a coating or a particularly groove-shaped structure with a depth of in particular a quarter of the wavelength of the radar signal (SHF, RHF) on a surface facing the high-frequency unit (10).

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