Level sensor for detecting the fill level of a medium in a container

Description

[0001] The invention relates to a fill level sensor for detecting a fill level of a filling medium in a container.

[0002] The level sensor comprises a generator with a feed line, an antenna, a power supply, and a controller. The generator, antenna, power supply, and controller are essential components of the level sensor.

[0003] The generator is designed both to generate electromagnetic waves at a resonant frequency and to output the electromagnetic waves via the feed line. The feed line has a line impedance with a line impedance value.

[0004] The feeder is located between the feed line and the antenna and is designed to transmit the electromagnetic waves from the feed line to the antenna. For example, the feeder is another feed line.

[0005] The antenna is designed to radiate the electromagnetic waves into a container with a filling medium.

[0006] Electromagnetic waves are transmitted in a single direction from the feed line through the feeder to the antenna. The electromagnetic waves emitted by the antenna have a propagation direction that essentially coincides with the transmission direction.

[0007] The controller is designed to detect a change in the near field around the antenna of electromagnetic waves emitted by the antenna due to a change in the fill level of a filling medium in a container. Furthermore, the controller is designed to detect a fill level of the filling medium in the container by evaluating the detected change in the near field.

[0008] The near field is formed by the electromagnetic waves emitted by the antenna, and the change in the near field occurs due to the change in the fill level of the filling medium in the container. The change in the fill level displaces another medium, such as air, in the vicinity of the antenna, or vice versa. A change in the fill level of the fill medium in the vicinity of the antenna changes the near field of the antenna if the fill medium and the other medium have different permittivities. This is assumed below.

[0009] A change in the antenna's near field also causes a change in the antenna's tuning, which also changes the reflected power versus frequency. To detect the fill level of a filling medium in a container, either the absolute reflected power at a specific frequency or a curve of the reflected power versus frequency can be used. However, the use of the absolute reflected power at a specific frequency is susceptible to tolerances and environmental conditions. This is not the case with a curve of the reflected power versus frequency, and it can also distinguish between different filling media if they have different permittivities.

[0010] For example, level sensors with planar patch antennas in combination with surface acoustic wave resonators are known from the state of the art. Patch antennas are half-wave resonators. Level sensors with quarter-wave resonators are also known. A disadvantage of these level sensors is their sensitivity to changes in the near field around the antenna, which only ensures the detection of levels in media with a permittivity greater than a certain permittivity value, making these level sensors unsuitable for media with lower permittivity. Furthermore, patch antennas generate parasitic resonances, especially in small containers, which complicates the detection of the level of a medium in the container. Furthermore, the implementation of highly sensitive patch antennas that are separated from a medium in a container is difficult.

[0011] The documents US 5 675 259 A and GB 2 569 540 A show state-of-the-art level sensors.

[0012] An object of the present invention is therefore to provide a level sensor with increased sensitivity so that it is also suitable for media with lower permittivity.

[0013] The problem is solved by a level sensor having the features of patent claim 1.

[0014] In the fill level sensor according to the invention, the supply comprises a resonant circuit. The resonant circuit and the antenna together have a resonant input impedance in the transmission direction. The resonant circuit transforms the resonant input impedance at the resonant frequency into a real impedance with a predetermined resonant impedance value. The antenna has an antenna resonant frequency that differs from the resonant frequency. The near field of the antenna is free from influence by a fill medium and is instead surrounded by another medium or a vacuum. The other medium is preferably air. It should be noted that the other medium and the fill medium have different permittivities.A change in the near field around the antenna due to a change in the fill level of a filling medium, for example in the direction of the antenna, causes a reduction in the resonance frequency if the previously performed transformation was carried out with air as an additional medium and the filling medium has a higher permittivity than air.

[0015] Accordingly, the resonant frequency is not a fixed frequency, but lies within a resonant frequency range, and the generator is designed to generate electromagnetic waves within this resonant frequency range. The respective resonant frequency depends on the antenna's near field.

[0016] The design of the control system to detect a change in the near field around the antenna includes, for example, an evaluation of the resonance input impedance.

[0017] The level sensor according to the invention has the advantage over the described level sensor known from the prior art of increased sensitivity for changes in the near field around the antenna, so that it also ensures the detection of levels of filling media with lower permittivity.

[0018] In one embodiment of the level sensor, the feed additionally has a terminating circuit. The terminating circuit, the resonant circuit, and the antenna together then have a terminating input impedance in the transmission direction. The terminating circuit sets the terminating input impedance to the line impedance value. If the line impedance value of the feed line is, for example, 50 ohms, then the terminating input impedance value is also 50 ohms. Thus, no reflections occur at a transition between the feed and the feed line. The design of the control for detecting a change in the near field around the antenna includes, for example, an evaluation of a shift in the resonant frequency. The reduction in the resonant frequency described above is such a shift.

[0019] In a further development of the above embodiment, the terminating circuit comprises a quarter-wave line. The quarter-wave line is preferably implemented using an air gap. In a particularly preferred embodiment, the terminating circuit consists of a quarter-wave line.

[0020] In a further embodiment, the antenna resonance frequency is higher than the resonant frequency. This has the advantage that the antenna is smaller than if the antenna resonance frequency were lower than the resonant frequency. A further advantage is that the resonant frequency range is larger. Otherwise, the resonant frequency range would be limited to a frequency range between the antenna resonance frequency and the first harmonic.

[0021] In principle, the generator is designed to generate electromagnetic waves with any resonant frequency. However, for detecting the fill level of a medium in a container, resonant frequencies within the microwave frequency range have proven particularly effective. A microwave frequency range, for example, is the frequency range from 1 GHz to 300 GHz. Therefore, one embodiment of the fill level sensor provides for the generator to generate electromagnetic waves with a resonant frequency within the microwave frequency range.

[0022] In a further embodiment, the antenna is designed for direct contact with the filling medium. An alternative to direct contact is indirect contact between the antenna and the filling medium. Direct contact has the advantage over indirect contact that smaller changes in the near field around the antenna can be detected, further increasing sensitivity. This ensures the detection of fill levels in filling media with even lower permittivity.

[0023] In a further embodiment, the controller is additionally designed to signal the detected fill level of a filling medium. For example, the controller has a bus interface, and the signaling is performed by transmitting a corresponding bus message via the bus interface and a bus to a control room, where it is then displayed to a user.

[0024] In a further embodiment, the feed comprises a cylindrical inner conductor and a hollow cylindrical outer conductor for transmitting the electromagnetic waves. Furthermore, the outer conductor and the inner conductor are arranged coaxially with each other. Consequently, the outer conductor is arranged around the inner conductor. Such a coaxial feed has advantageous properties for the transmission of electromagnetic waves.

[0025] Preferably, a section of the inner conductor also forms the antenna. State-of-the-art level sensors feature planar patch antennas as antennas. Compared to a planar patch antenna, the section of the inner conductor serving as the antenna is smaller, which also makes the level sensor smaller. Due to its smaller size, the level sensor is also suitable for applications that offer limited installation space. Furthermore, unlike a patch antenna, the section of the inner conductor serving as the antenna only excites a near field, which is why the antenna is also suitable for correspondingly smaller containers.

[0026] In a further embodiment of the level sensor, the resonant circuit and / or the termination circuit are arranged between the inner conductor and the outer conductor. This arrangement of the resonant circuit and / or the termination circuit results in a further reduction in the size of the level sensor and provides advantageous electrical properties for the electromagnetic waves.

[0027] Fill levels of filling media in containers often need to be determined, whereby the filling media have temperatures, especially high temperatures, which represent thermal stress for the generator and the controller, for example, and impair their functions. Since the terminating input impedance is set to the line impedance value, no reflections occur at the transition between the supply and the feed line, which is why the generator and the controller can be arranged at a distance from the supply, whereby they are thermally decoupled from the filling media and their thermal load is reduced. To also reduce the thermal load on the supply, one embodiment of the level sensor provides that the outer conductor has cooling fins. These cooling fins are designed to dissipate heat transferred from a filling medium to the supply to an area surrounding the level sensor.

[0028] In a further embodiment of the level sensor, the resonant circuit comprises a dielectric and a high-frequency absorber. The dielectric is preferably a glass, in particular a metal-fused glass. If the level sensor has the previously described coaxial feed, then in a particularly preferred embodiment, the resonant circuit consists of the dielectric, the high-frequency absorber, and the inner conductor.

[0029] A container with an interior space for a filling medium, in which the fill level of the filling medium is to be detected, has a wall on which the fill level sensor is arranged. If the antenna of the fill level sensor is to be in direct contact with the fill medium at a specific fill level, an opening must be provided in the container wall. Therefore, in a further embodiment, the fill level sensor has a process window that closes the opening in the container wall and arranges the antenna in the interior of the container.

[0030] In a further development of the above embodiment, in conjunction with a resonant circuit comprising a dielectric, the dielectric also serves as the process window. The dielectric ensures effective, robust, and chemically resistant separation of the rest of the level sensor from the medium.

[0031] In a further embodiment, the resonance impedance value is in the interval from 150 ohms to 300 ohms.

[0032] In state-of-the-art level sensors arranged on a container, it often happens that the near field around the antenna is impaired by the excitation of higher modes of the electromagnetic waves in the container, which are excited by the radiated electromagnetic waves, particularly in the region of a transition from the feed and / or antenna into the container. The excitation occurs due to discontinuities in the electromagnetic field of the electromagnetic waves at the transition. Higher modes are modes above the fundamental mode of an electromagnetic wave. The container itself is also often a waveguide for the transmission of electromagnetic waves. If waveguide modes are excited by the electromagnetic waves in the container, standing waves can form in the container, which lead to ambiguities when detecting a change in the near field.If the container is a hollow cylinder, the fundamental mode is calculated using the following formula: . f = 1,8412 ⋅ c 0 2 π ⋅ r ⋅ ε r

[0033] In order to avoid waveguide modes in a container, a further embodiment provides that the fill level sensor has a mode filter on the antenna and that the mode filter is designed to avoid the excitation of higher modes of the electromagnetic waves in a container.

[0034] In a further embodiment, the mode filter comprises a horn with a horn interior, the antenna extends into the horn interior, and the horn interior expands continuously in the transmission direction. The electromagnetic waves emitted by the antenna are guided by the horn within the horn interior, thereby reducing or even eliminating discontinuities in the electromagnetic waves at the transition from the antenna to the container.

[0035] Preferably, a cross-sectional area of ​​the horn interior is circular with respect to the transmission direction, and a radius of this cross-sectional area is determined, taking into account the geometry of the interior of a container and the highest assumed permittivity of a filling medium, such that only excitation of higher modes of the electromagnetic waves in the container, which are propagable above the fundamental mode and whose frequencies are above the resonant frequency, occurs. Preferably, the radius is a minimum radius of the horn interior. Since the horn interior continuously widens in the transmission direction, the minimum radius of the horn interior is at the end of the horn into which the antenna extends.

[0036] In a further embodiment, the horn is provided with recesses for the penetration of a filling medium into the horn interior. This ensures that if the fill level of a filling medium changes in the area of ​​the horn, no gas pockets form in the horn interior, but rather the horn interior is filled with the filling medium according to the fill level.

[0037] In detail, there are numerous possibilities for designing and developing the fill level sensor. Reference is made to the claims subordinate to claim 1 as well as to the following description of a preferred embodiment in conjunction with the drawing. The drawing shows Fig. 1 shows an embodiment of a fill level sensor which is arranged in a wall of a container, Fig. 2 shows the embodiment of the fill level sensor in a sectional view, Fig. 3a shows an embodiment of a mode filter in a sectional view, and Fig. 3b shows the embodiment of the mode filter in a perspective view.

[0038] Fig. 1shows an abstract embodiment of a fill level sensor 1 for detecting the fill level of a filling medium in a container. The fill level sensor 1 is arranged on a wall 2 of a container 3. The container 3 has an interior space 4. The interior space 4 has a fill level 5 of a fill medium 6. Above the fill medium 6 is air as another medium. The fill medium 6 and the air have different permittivities. An opening 7 is formed in the wall 2 of the container 3, through which the fill level sensor 1 has access to the interior space 4 of the container 3.

[0039] Fig. 2 shows an abstract longitudinal section of the level sensor 1 from Fig. 1The level sensor 1 comprises a generator 8 with a feed line 9, an antenna 10, a feed 11, a controller 12, and a mode filter 13. The generator 8 is designed to generate electromagnetic waves with a resonant frequency and to output them via the feed line 9. The resonant frequency is 6 GHz and thus in the frequency range of microwaves. The feed line 9 has a line impedance with a line impedance value of 50 ohms.

[0040] The feed 11 is arranged between the feed line 9 and the antenna 10 and is designed to transmit the electromagnetic waves from the feed line 9 to the antenna 10.

[0041] The feed 11 has a cylindrical inner conductor 14 and a hollow cylindrical outer conductor 15 for transmitting the electromagnetic waves. The outer conductor 15 and the inner conductor 14 are arranged coaxially to one another and have circular cross-sectional contours. The outer conductor 15 also has cooling fins 16. The cooling fins 16 are designed to dissipate heat transferred from the filling medium 6 to the feed 11 to the environment. The feed 11 also has a dielectric 17 in the form of a metal-fused glass and a high-frequency absorber 18. The dielectric 17 also serves as a process window. The dielectric 17, together with the outer conductor 15, closes the opening 7 in the wall 2 of the container 3. Accordingly, the dielectric 17 is also designed to be resistant to the filling medium 6 and to the pressure and temperature in the container 3.Both the dielectric 17 and the high-frequency absorber 18 are ring-shaped and arranged between the cylindrical inner conductor 14 and the hollow cylindrical outer conductor 15.

[0042] The feed 11 has a resonant circuit 19 and a termination circuit 20. The resonant circuit 19 has the dielectric 17 and the high-frequency absorber 18. The termination circuit 20 has a quarter-wave line, which is implemented using an air gap. Thus, both the resonant circuit 19 and the termination circuit 20 are arranged between the inner conductor 14 and the outer conductor 15. In the present exemplary embodiment, the feed line 9 has the same inner conductor 14 and the same outer conductor 15 as the feed 11 and a feed dielectric 21 between the inner conductor 14 and the outer conductor 15. The feed line 9 is also shown as a single line in the area of ​​the generator 8 and the controller 12.

[0043] The electromagnetic waves are transmitted in a transmission direction 22 from the feed line 9 via the feed 11 to the antenna 10. The antenna 10 is designed to radiate the electromagnetic waves into the interior 4 of the container 3 containing the filling medium 6. A section 23 of the inner conductor 14 forms the antenna 10, in that the section 23 protrudes beyond the dielectric 17 in the transmission direction 22. The antenna 10 is designed for direct contact with the filling medium 6. When the fill level 5 of the medium 6 is sufficiently high, the antenna 10 is in direct contact with the filling medium 6.

[0044] In the transmission direction 22, the resonant circuit 19 and the antenna 10 together have a resonant input impedance ZR. The antenna, in particular, provides a capacitive contribution to the resonant input impedance ZR. The resonant circuit 19 transforms the resonant input impedance ZR at the resonant frequency and without any influence on the near field of the antenna 10 by the filling medium 6 into a real impedance with a predetermined resonant impedance value ZR = 200 ohms. The transformation therefore occurs before a fill level 5 of the filling medium 6 is detected, i.e., before the productive use of the fill level sensor 1.

[0045] In the transmission direction 22, the termination circuit 20, the resonant circuit 19, and the antenna 10 together have a termination input impedance ZE. The termination circuit 20 sets the termination input impedance ZE to the line impedance value of 50 ohms. Since the line impedance of the feed line 9 and the termination input impedance ZE are equal, the electromagnetic waves do not generate any reflections at the transition from the feed line 9 to the feed 11.

[0046] The feed 11 with its essential components of cylindrical inner conductor 14, hollow cylindrical outer conductor 15, annular dielectric 17 and annular high-frequency absorber are therefore coaxially designed.

[0047] The controller 12 is designed to detect a change in the near field around the antenna 10 of electromagnetic waves emitted by the antenna 10 due to a change in the fill level 5 of the fill medium 6 in the interior 4 of the container 3. A change in the near field is caused by a change in the fill level 5 of the fill medium 6 in the interior 4 of the container 3 in the vicinity of the antenna 10, since the fill medium 6 and air have different permittivities. A change in the near field causes a change in the adaptation of the antenna 10, which also changes the reflected power over frequency. A change in the near field around the antenna 10 due to a change in the fill level 5 of the medium 6, for example, in the direction of the antenna 10, causes a reduction in the resonant frequency. To detect a change in the reflected power, the controller 12 is connected to the feed line 9 in the present embodiment.

[0048] Furthermore, the controller 12 is configured to detect the fill level of the filling medium 6 in the interior space 4 by evaluating the detected change in the near field. The controller 12 is also configured to signal the detected fill level of the filling medium 6.

[0049] Fig. 3a is a longitudinal section through an embodiment of a mode filter 13 and Fig. 3bis a perspective view of this mode filter 13. The mode filter 13 is designed to prevent the propagation of higher modes of the electromagnetic waves in the interior 4 of the container 3. The mode filter 13 has a collar 24 and a horn 25 with a horn interior 26. The horn interior 26 guides the electromagnetic waves emitted by the antenna 10. In this exemplary embodiment, the collar 24 serves to arrange the mode filter 13 on the feed 11. In other exemplary embodiments, it is arranged on the wall 2 of the container 3. When the mode filter 13 is arranged, the antenna 10 projects into the horn interior 26. The horn interior 26 expands continuously in the transmission direction 22.

[0050] In this exemplary embodiment, the horn interior 26 is rotationally symmetrical with respect to the propagation direction 22. Thus, all cross-sectional areas are circular with respect to the transmission direction 22 of the horn interior 26. In particular, a cross-sectional area 27 is circular with respect to the transmission direction 22 of the horn interior 26, and a radius 28 of the cross-sectional area 27 is determined taking into account a geometry of the interior 4 of the container 3 and a maximum assumed permittivity of the filling medium 6, so that propagation of higher modes of the electromagnetic waves in the interior 4 of the container 3 is avoided. The radius 28 is a minimum radius of the horn interior 26. The antenna 10 passes through the cross-sectional area 27. It is therefore located in the horn interior 26 where this has the minimum radius.

[0051] The horn 13 has three recesses 29 for the penetration of the filling medium 6 into the horn interior 26. This ensures that changes in the fill level 5 of the filling medium 6 in the area of ​​the horn 25 do not result in air inclusions in the horn interior 26, but that the horn interior 26 is filled with the filling medium 6 according to the fill level 5 of the filling medium 6. Reference symbol

[0052] 1Level sensor 2Wall 3Container 4Interior 5Level 6Filling medium 7Opening 8Generator 9Feed line 10Antenna 11Feed 12Control 13Mode filter 14Inner conductor 15Outer conductor 16Cooling fins 17Dielectric 18High-frequency absorber 19Resonant circuit 20Termination circuit 21Feed dielectric 22Transmission direction 23Section 24Collar 25Horn 26Horn interior 27Cross-sectional area 28Radius 29Recess ZR Resonance input impedance ZE Termination input impedance

Claims

1. Fill level sensor (1) for detecting a level of a fill medium in a container, - wherein the fill level sensor (1) comprises a generator (8) having a feed line (9), an antenna (10), a supply (11) and a controller (12), - wherein the generator (8) is designed to generate electromagnetic waves having a resonant frequency and output the electromagnetic waves via the feed line (9) having a line impedance with a line impedance value, - wherein the supply (11) is arranged between the feed line (9) and the antenna (10) and is designed to transmit the electromagnetic waves from the feed line (9) to the antenna (10), characterized in - that the antenna (10) is designed to radiate the electromagnetic waves into a container with a fill medium, - that a transmission of the electromagnetic waves occurs in a transmission direction (22) from the feed line (9) via the supply (11) to the antenna (10), - that the controller (12) is designed to detect a change of electromagnetic waves radiated by the antenna (10) in a near field around the antenna (10) by a fill level change of a fill medium in a container and to detect a fill level of the fill medium in the container by evaluating the detected change in the near field, - that the supply (11) has a resonant circuit (19), - that the resonant circuit (19) and the antenna (10) together have a resonant input impedance (ZR) in the transmission direction (22), and the resonant circuit (19) transforms the resonant input impedance (ZR) at the resonant frequency into a real impedance having a predetermined resonant impedance value, and - that the antenna (10) has an antenna resonant frequency different from the resonant frequency.

2. Fill level sensor (1) according to claim 1, characterized in that the supply (11) comprises a terminating circuit (20) and that the terminating circuit (20), the resonant circuit (19) and the antenna (10) together have a terminating input impedance (ZE) in the transmission direction (22) and the terminating circuit (20) sets the terminating input impedance (ZE) to the line impedance value.

3. Fill level sensor (1) according to claim 2, characterized in that the terminating circuit (20) has a quarter wavelength line, wherein the quarter wavelength line is preferably implemented using an air gap.

4. Fill level sensor (1) according to any one of claims 1 to 3, characterized in that the antenna resonant frequency is greater than the resonant frequency.

5. Fill level sensor (1) according to any one of claims 1 to 4, characterized in that the resonant frequency is in a frequency range of microwaves.

6. Fill level sensor (1) according to any one of claims 1 to 5, characterized in that the antenna (10) is designed for direct contact with the fill medium.

7. Fill level sensor (1) according to any one of claims 1 to 6, characterized in that the controller (12) is designed to signal the detected fill level of the fill medium.

8. Fill level sensor (1) according to any one of claims 1 to 7, characterized in that the supply (11) has a cylindrical inner conductor (14) and a hollow cylindrical outer conductor (15) for transmitting the electromagnetic waves, and the outer conductor (15) and the inner conductor (14) are arranged coaxially with respect to one another.

9. Fill level sensor (1) according to claim 8, characterized in that a section (23) of the inner conductor (14) forms the antenna (10).

10. Fill level sensor according to claim 8 or 9, characterized in that the resonant circuit (19) and / or the terminating circuit (20) are / is arranged between the inner conductor (14) and the outer conductor (15).

11. Fill level sensor (1) according to any one of claims 8 to 10, characterized in that the outer conductor (15) has cooling fins (16).

12. Fill level sensor (1) according to any one of claims 1 to 11, characterized in that the resonant circuit (19) has a dielectric (17) and a high-frequency absorber (18), wherein the dielectric (17) is preferably a glass, in particular a metal-fused glass.

13. Fill level sensor (1) according to claim 12, characterized in that the dielectric (17) is also a process window.

14. Fill level sensor (1) according to any one of claims 1 to 13, characterized in that the resonance impedance value is in the interval from 150 ohms to 300 ohms.

15. Fill level sensor (1) according to any one of claims 1 to 14, characterized in that the fill level sensor (1) has a mode filter (13) at the antenna (10) and that the mode filter (13) is designed to avoid waveguide modes in a container.

16. Fill level sensor (1) according to claim 15, characterized in that the mode filter (13) has a horn (25) with a horn interior (26), that the antenna (10) extends into the horn interior (26), and that the horn interior (26) widens continuously in the transmission direction (22).

17. Fill level sensor (1) according to claim 16, characterized in that a cross-sectional area (27) is circular with respect to the transmission direction (22) of the horn interior (26), and that a radius (28) of the cross-sectional area (27) is determined taking into account a geometry of an interior of a container and a highest permittivity to be assumed of a fill medium such that excitation of waveguide modes in the container is avoided.

18. Fill level sensor (1) according to claim 17, characterized in that the radius (28) is a minimum radius of the horn interior (26).

19. Fill level sensor (1) according to any one of claims 16 to 18, characterized in that the horn (25) has recesses (29) for penetration of a fill medium into the horn interior (26).