JP4130459B2 - Bottom reflector for radar level gauge - Google Patents

Description

  The present invention relates generally to liquid level measurement, and more particularly to a bottom reflector for a radar liquid level gauge.

  Radar methods have been widely used for several years to measure liquid levels in various tanks. In such liquid level measurement, microwaves are transmitted toward the liquid surface, microwaves reflected from the liquid surface are received, and the liquid level in the tank is determined from the propagation time of the transmitted and reflected microwaves. By calculating, the distance from the top of the tank to the liquid surface stored in the tank is measured.

  The inherent limitations of radar level measurement are that most liquids are at least partially permeable to the microwaves used, and in low level measurements, the microwaves that are probably reflected from the bottom of the tank are It interferes with the microwave reflected from. That is, the distance resolution may be insufficient to distinguish the microwave reflected from the tank bottom from the microwave reflected from the liquid surface. Therefore, if the liquid level is low, the microwave reflected from the bottom of the tank is likely to interfere with the measurement. This problem can occur at liquid levels lower than a few tenths of a meter, depending on the type of radar level gauge. This is particularly a problem with tanks having a flat bottom, which contains liquids such as petroleum products that are at least slightly permeable.

  In some tanks, the bottom can be tilted. In such tanks, there is generally no radar-echo interference problem, but when the tank is empty, the radar beam deviates from the level gauge receiver so that the microwave reflected from the bottom of the tank Not received. If the tank is empty, it is desirable to have a relatively strong radar echo from the bottom to confirm that it is empty. In other cases, there is an unknown layer of sludge at the bottom of the tank, and even a flat bottom may emit ambiguous radar echoes, or no radar echoes at all.

  The problem of an empty tank not receiving radar echo from the bottom can be solved by welding a simple reflective structure to the bottom of the tank. However, such a reflective structure can cause interference when the liquid level is slightly above the reflective structure if the liquid in the tank is at least partially permeable to microwaves. become.

  Therefore, the main object of the present invention is a bottom reflector for a radar level gauge that measures the liquid level in a tank, when the instrument is used for low level measurement. It is to provide a bottom reflector that reduces microwave interference reflected from the bottom of the tank but still provides sufficient microwave reflection when the tank is empty.

  In this regard, it is a specific object of the present invention to provide a bottom reflector that is suitable for its use regardless of whether the liquid level is measured through a tube.

  It is another object of the present invention whether the radar level gauge is a frequency modulated continuous wave (FMCW) radar device, a pulse radar, or any other type of distance measuring radar. It is to provide a bottom reflector suitable for their use.

  Another object of the present invention is to provide a bottom reflector that is simple, reliable, efficient, accurate, precise, easy to manufacture and install, and low cost. .

  These objects are achieved, inter alia, by the bottom reflector as claimed in the appended claims.

  The inventor found that at a given height in the tank, preferably at the bottom of the tank, to reflect the microwave signal when there is no liquid in the tank or only very low liquid level. A bottom reflector that can be placed in close proximity and includes a reflecting structure, wherein the reflecting structure is a first for microwave signals when the liquid level is above the reflecting structure. Having a reflection coefficient, and having a second reflection coefficient for the microwave signal when the liquid level is below the reflecting structure, the first reflection coefficient being substantially smaller than the second reflection coefficient It has been found that the above objective can be achieved by providing a bottom reflector.

  Preferably, the liquid is a petroleum product such as, for example, crude oil, liquefied petroleum gas (LPG), liquefied natural gas (LNG), other liquefied hydrocarbons, or at least partially generic to microwaves. It is liquid. Such products generally have a dielectric constant in the range of 1.6-3, while the atmosphere above the liquid level is generally in the range of 1-1.03, depending on the contents and its pressure. Has a dielectric constant of

  Furthermore, preferably, when the liquid level of the liquid is above the reflecting structure, the microwave signal reflected from the reflecting structure is weaker than the microwave signal reflected from the liquid level, advantageously Is much weaker. If the liquid level is below the reflective structure, the microwave signal reflected from the reflective structure can be compared to the microwave signal reflected from the liquid level, or Slightly stronger is preferred.

  Various reflective structures can be used to obtain the desired function based on the difference in dielectric constant between the liquid and the atmosphere above it. These reflective structures are described in detail in the present description, and various groups of preferred embodiments, ie, resonant structures such as cut-off gratings, dipoles, dielectric reflectors, polarization rotating structures, etc. Grouping into microwave guide structures that are bendable over small distances, to guide transmitted and reflected microwave signals, and structures that are particularly applicable when microwave free space propagation is used Can do.

  Regarding the group of polarization rotation structures, the microwave signal used for liquid level measurement has a specific polarization state, and the first and second reflection coefficients for the specific polarization state are Please understand that they are given. Therefore, the action of rotating the polarization of a microwave signal that is linearly polarized during reflection in the liquid is equivalent to reducing the reflection coefficient for the microwave signal in that particular polarization state.

  The main advantage of the present invention is that the liquid level can be measured without interference from microwaves reflected from the bottom of the tank. A clear reflection from the bottom reflector is obtained when the tank is empty or nearly empty, i.e. when the reflecting structure of the bottom reflector is raised above the level of the liquid to be measured. This indicates that the tank is empty or the liquid level is low.

  A radar level meter as used herein includes not only large vessels but also processing equipment such as, for example, reactors, centrifuges, mixers, hoppers, sorting machines (graders), or heat treatment furnaces and similar devices, Used to measure liquid levels in tanks used in food chemistry, medicinal chemistry, biochemistry, genetic chemistry, and petrochemistry.

  Other features of the present invention and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments of the present invention and the accompanying FIGS. These are provided for illustration only and are not intended to limit the invention.

  A preferred embodiment of the present invention will be described with reference to FIG. 1, which schematically shows in side view an apparatus for radar level measurement. The device may be a frequency modulated continuous wave (FMCW) radar device, a pulse radar device, or any other type of distance measuring radar.

  Many frequencies can be used for radar liquid level measurement, but bands close to 5.8, 10 and 25 GHz are widely used. In a tank where air bubbles and contamination occur frequently, the one with the lowest frequency is the most common. This is because the microwave signal at this frequency is hardly affected by such deterioration.

  The radar liquid level meter indicated by reference numeral 11 in FIG. 1 is placed above the opening of the tank or the roof of the container 12 filled with the liquid whose liquid level is to be measured. Preferably, the liquid is a petroleum product such as crude oil, liquefied petroleum gas (LPG), or liquefied natural gas (LNG). Such products generally have a dielectric constant in the range of 1.6-3, while the atmosphere in the tank 12 above the liquid level is generally 1 depending on the gas content and its pressure. Has a dielectric constant in the range of ~ 1.03.

  The radar level gauge transmits a microwave signal toward the liquid surface in the tank 12 and receives the microwave signal reflected from the liquid surface in the tank 12. Furthermore, the radar level gauge comprises or is connected to a signal processing device (not explicitly shown) for calculating the liquid level of the liquid in the tank from the propagation time of the transmitted and reflected microwave signals. Is done. Typically, a substantially vertical tube 13 is provided to guide the transmitted and reflected microwave signals. The tube 13 is attached to the tank by a support member 14 and is perforated to obtain the same liquid level inside and outside, although density stratification may occur. However, the level gauge can operate in free space propagation mode and therefore the tube 13 can be omitted.

  A bottom reflector 15 is provided near the bottom of the tank 12 to reflect the microwave signal when no liquid is present in the tank. When a tube is used for waveguiding, such as tube 13 of FIG. 1, bottom reflector 15 is typically placed at the lower end of the tube. Alternatively, the bottom reflector is mounted on the bottom of the tank.

  According to the present invention, the bottom reflector 15 includes a reflecting structure (not explicitly shown in FIG. 1), which has a liquid level higher than that of the reflecting structure. The first reflection coefficient for the microwave signal when it is above, and the second reflection coefficient for the microwave signal when the liquid level is below the reflecting structure. The coefficient is substantially smaller than the second reflection coefficient.

  With such equipment, correct liquid level measurement without interference by microwaves reflected from the bottom can be ensured. This is particularly important when measuring the low liquid level in the tank 12. When the liquid level drops below the reflecting structure of the bottom reflector 15, not a little more strong reflection is obtained, indicating that the liquid level is low.

  It should be noted that the first and second reflection coefficients are advantageously so-called “in-polarization” reflection coefficients. The in-polarization reflection coefficient is the ratio of the microwave signal in a specific polarization state (ie, its amplitude) reflected from the surface of the bottom reflector to the incident microwave signal (ie, its amplitude). And the specific polarization state is given by the polarization of the microwave signal reflected by the surface of the liquid. Therefore, if the incident microwave signal is linearly polarized, the specific polarization is in the linearly polarized state. This is because a linearly polarized microwave signal does not change its polarization state even if it is reflected by the surface of the liquid. In the case of a left circularly polarized incident microwave signal, the specific polarization is a right circular polarization. This is because the rotation of the electric field is reversed when the microwave signal is reflected from the surface of the liquid.

  If the liquid level is above the reflecting structure, it is advantageous that the microwave signal reflected from the reflecting structure is weaker than the microwave signal reflected from the liquid level, much more It is further advantageous to be weak. If the liquid level is below the reflecting structure, the microwave signal reflected from the reflecting structure is comparable to or more than the microwave signal reflected from the liquid level. Slightly strong is preferred. Reflections that are too strong should be avoided.

  Preferably, the first reflection coefficient is less than 0.2, more preferably less than 0.1, and even more preferably less than 0.05. The second reflection coefficient is preferably greater than 0.1, more preferably greater than 0.2. Furthermore, the second reflection coefficient is less than 0.5, more preferably less than 0.4, and most preferably less than 0.3.

  In most cases, it is preferable to deflect or absorb the reflected microwave signal reflected from the bottom of the tank 12 by the structure 16. The structure 16 may be an inclined plate, a conical surface, a piece of radar absorbing material, or the like.

  In the following sections, various specific embodiments of the bottom reflector of the present invention are described in detail.

Certain preferred embodiments
Figures 2a-2e show various bottom reflectors based on the principle of waveguide cut-off frequency. Each bottom reflector includes a grating structure with a grating separation slightly less than λ / 2, where λ is the vacuum wavelength of the microwave signal, and the grating structure is above the liquid level. At some point, the microwave signal is prevented from being transmitted, and instead the microwave signal is reflected. As the liquid level rises above the lattice structure, the wavelength of the microwave signal decreases due to the dielectric constant of the larger liquid. Thus, the grating appears wider to the microwave signal, and the microwave signal can pass through the grating structure.

In FIG. 2 a, a bottom reflector is shown that includes a lattice structure configured within the tube 13. In this case, the tube 13 is configured to assist microwaves propagating in the H ₁₁ mode having the direction of the electric field substantially indicated by the arrow 22. The lattice structure consists of a plurality of conductive, preferably metal ribbons 23 with a separation slightly less than λ / 2 so that propagation is cut off when the liquid level is below the lattice structure. To.

For example, at 10 GHz, the spacing can be 14 mm and the ribbon 23 can have a height of 28 mm, which corresponds to a distance of λ _m / 2 in the waveguide formed by the ribbon. Here, λ _m is the wavelength of the microwave signal in the liquid in the tube 13 (that is, when the liquid level is below the lattice structure). With a ribbon height of λ _m / 2, the propagation does not reflect within a certain frequency band, like a standard radar radome. Later, since lambda _m will be used as a local wavelength in the pipe 13, lambda _m is to be understood that the different in different circumstances of the present description.

A support member 24 made of metal or some dielectric material is used to support the lattice structure. Figure 2a shows a grid structure with straight ribbons, it may be bent to fit the electrical field pattern of the waveguide mode H _11. Furthermore, the grating structure can be modified to match other waveguide propagation modes.

  In one embodiment, the bottom reflector of the present invention comprises a mesh grid structure. As another modification, a portion of the ribbon can be made of an attenuating material that can both reduce reflection band transmission and increase the usable bandwidth.

In another embodiment, as shown in FIG. 2b, a grid structure including a plurality of circular conductive strips concentrically configured in the tube 13 is used to support the H ₀₁ mode of radio waves. Get the action to do. The field lines 22 in the H ₀₁ waveguide mode are purely annular, and therefore several concentric, short cylindrical tubes or rings 25 are configured parallel to the field lines. Referring to this 10 GHz embodiment, the distance between the rings is about 14 mm, but modified according to the H01 mode cylindrical waveguide pattern, the height is about 28 mm. Support elements (not shown) of dielectric material or metal material are included in actual use. Similar to H ₁₁ and H ₀₁ , structures for other waveguide modes are possible.

For example, in the case of an _E01 mode microwave signal, the ribbon should be configured radially.

  In FIG. 2c, a portion of the bottom reflector is shown in cross-sectional side view. This embodiment is similar to the embodiment of FIG. 2a, and a straight ribbon 23 is shown. However, this embodiment further comprises an antireflective structure that includes pins 26 arranged horizontally, parallel to the ribbon 23. Pins 26 are located above and below the ribbon and are close together at a distance of λ / 4. Similar to the corresponding details of the radar radome, there are several alternatives for obtaining similar functions in order to increase the usable range of frequency and liquid permittivity, which will be obvious to those skilled in the art. Easy to understand.

  When immersed in liquid, the bottom reflector is also transparent in the opposite direction, so if the bottom of the tank is flat or nearly flat, the reflection from the bottom of the tank will combine back into the tube 13 and return from the liquid surface Measures must be taken to avoid the possibility of interfering with the reflections. Various alternatives are possible. Microwaves transmitted through the bottom reflector can be deflected or absorbed.

  FIG. 2 d shows a 45 ° metal reflector 27 mounted below the bottom reflector 15 in the tube 13. The reflector is attached to the tube 13 or the bottom of the tank (not shown) by a support structure 28.

  Depending on the shape of the tank, a similar deflecting reflector (referred to herein as a deflector) can be used with any bottom reflector embodiment of the present invention. The angle can be very different from 45 °, and very different shaped obstacles can be used to spread the radar wave to prevent the radar wave from entering the tube 13 again. it can.

  The bottom, or cone attached to the tube, is another usable deflector embodiment. Such a portion is indicated in FIG.

  FIG. 2e shows a suitably shaped damping material member 29, for example filled with carbon, placed at the bottom of the tank below the bottom reflector 15 in the tube 13 in order to reduce the reflection at the bottom. PTFE and the like are shown. The member 29 is shown in broken lines to show that the actual shape can be very different from the box shape as shown. For example, the member can be formed as a standard anechoic absorber.

  FIG. 2 e shows the bottom reflector 15 mounted in the tube 13, but the bottom reflector can alternatively be integrated with the deflecting or absorbing member 29. Such a bottom reflector design will provide a true reflection from the bottom of the tank rather than from the bottom of the tube 13. Some of the bottom reflectors described herein are very small (one or two λ / 2 dipoles), which clearly makes such integration simple and easy .

  FIG. 2f shows the reflecting structure of the bottom reflector for different frequencies and for the different dielectric constants of the liquid (1.7 and 2.5) in the absence of water and immersion conditions, ie the liquid level of the liquid respectively. It is a reflection figure showing reflection when it is above and below the body. Also, permeation is shown in the absence of water conditions.

  The solid line shows that when the grating is in the air, it is nearly totally reflected by the cut-off condition, and the dashed / dotted line shows the corresponding transmission through the λ / 2 grating. The dotted line indicates a small reflection when the dielectric constant is the lowest, and the broken line indicates the corresponding reflection when the dielectric constant is the highest within the range of use of the dielectric constant value. For the middle dielectric constant in this range, the reflection is smaller. Furthermore, the reflection is fairly constant over a fairly large frequency range, ie 9.5 to 10.5 GHz.

  Figures 3a to 3e show various bottom reflectors based on the principle of resonant structure. Each bottom reflector includes a resonant structure that includes a number of dipoles that are tuned to resonate in a waterless condition, for example, to provide a clear reflection when the tank is empty. When the resonant structure is immersed in the liquid, the dipole is adjusted so as not to resonate, in which case the reflection is considerably reduced.

In FIG. 3a, a bottom reflector is shown that includes two stacked dipoles 32. FIG. The two dipoles 32 are tuned to provide maximum reflection when the dielectric constant of the surrounding medium is very close to 1 (ie, air or gas), while the vertical distance between them is It is selected close to a quarter wavelength (λ _m / 4) in a typical liquid that may have a dielectric constant of 2.1. These design constraints will provide the desired reflection conditions for the radar wave 31 as shown in FIG. The dipole 32 is attached to a metal or dielectric vertical support pin 33. If the vertical support pins 33 are metal, the complete structures 32, 33 can be molded from the same plate piece (as shown in FIG. 3a). The structures 32, 33 are attached to some support 34 on or above the bottom. The dipole length can typically be slightly less than 14-15 mm at 10 GHz.

  FIG. 3b shows a bottom reflector that includes an element having two sets of crossed dipoles 32, 35 to achieve the polarization independent function. In addition to the dipole 32 shown in FIG. 3a, two intersecting dipoles 35 of the same length are provided. The crossed dipoles are preferably not connected and FIG. 2 c shows mounting on the dielectric pin 33.

In FIG. 3 c, a reflective structure such as the embodiment of FIG. 3 a or 3 b is placed in the center of the tube 13. For H ₁₁ mode, it is possible to place the resonant reflector in the center of the waveguide 13. For well-known polarization, a single polarization model 32 can be used. Alternatively, an optional dipole 35 can be added to make the reflector independent of polarization.

In many waveguide modes, the transverse electric field is low in the center. This case can be compensated by using several resonant structures, typically as shown in FIG. 3a. In FIG. 3d, such an embodiment in the tube 13 is shown. Mode-specific reflections can be obtained using from two to four resonant structures located according to the electric field pattern of the mode of the microwave signal. The FIG. 3d embodiment is particularly suitable for use with microwave signals in the _H01 propagation mode.

Another way to obtain a resonant structure for the H ₀₁ mode is to place a ring 37 in the center of the tube 13 as in the configuration of FIG. The ring 37 resonates when the periphery is one wavelength (or an integral multiple of the wavelength) under waterless conditions. This resonance changes under conditions with water. Two or more rings can be stacked like the dipole of FIGS. 3a-3b. Since the electric field is small in the middle of the tube 13 propagating in the H ₀₁ mode, the reflection is much smaller than that obtained by the embodiment shown in FIG. 3d, which is a reflection of some liquefied gas, for example. May be advantageous for less liquid.

This embodiment is also applicable to a microwave propagating in mode other than H _01. In the case of H ₁₁ mode microwaves, this ring shape will, for example, make the reflection polarization independent and can be used as an alternative to the crossed dipole embodiment.

FIG. 3f shows a dipole structure consisting of two dipoles stacked vertically at a distance corresponding to λ _m / 4 when immersed in a common liquid (in this case ε = 2.1). Shows reflection. The length of the dipole corresponds approximately to maximum reflection when the dipole is dry (ie, when the dipole is slightly shorter than λ / 2). For typical bandwidths, reflections under immersion conditions are at least 20 dB weaker than in air (or the surrounding gas). This figure should be compared to the strongest reflection (under waterless conditions) which is generally 10-20 dB below the reflection on the metal surface. The decrease in reflection under immersion conditions is a result of dipole detuning and vertical spacing that cause the two dipoles to cancel each other. Thus, this dipole combination results in reflections that resemble oil surfaces in the absence of water conditions and considerably lower reflections under immersion conditions. Three or more dipoles can be stacked to suppress the radar echo under immersion conditions in a wider band, and a resonance structure other than the dipole can be used.

  Figures 4a-4b show various bottom reflectors based on the principle for dielectric radar radomes.

In FIG. 4a, a dielectric plate 41 is shown, which has a plurality of holes 42, preferably through holes (unlike a common radome). The thickness of the plate 41, and the volume of the holes compared to the volume of the plate, to minimize reflection, is an effective thickness of λ _m / 2 when the plate 41 is below the liquid surface (however, , Λ _m is selected here to refer to the wavelength of the microwave signal in the dielectric in which the holes are filled with liquid. In the absence of water condition, the effective dielectric constant of the plate 41 changes and thus its thickness that the microwave signal undergoes changes. Thus, reflection is increased because the half-wave condition is no longer met.

  Another suitable geometry for achieving similar unequal behavior is a plate composed of horizontally disposed dielectric bars or dielectric pins.

  Another reflective structure with a fairly thin plate 43, which is flat but fitted with a small dielectric member 44 on its bottom side, is shown in FIG. 4b. The flat upper side facilitates accurate reference measurement by conventional mechanical means.

FIG. 4c is a reflection diagram for a partially filled dielectric plate over the general frequency range, ie the plate shown in FIG. 4a. In this example, the dielectric plate is a PTFE plate with 50% perforations. That is, half of the volume is PTFE and half is empty. Under waterless conditions, the average dielectric constant is the average of the dielectric constants of PTFE and air (gas), and under immersion conditions, the average dielectric constant is the average of the dielectric constants of PTFE and liquid. In immersion conditions, the thickness is very close to λ _m / 2 in the middle of the band, reducing the reflections considerably, but under waterless conditions some reflections are desired, but this is an effective electrical This is achieved when the thickness deviates significantly from λ _m / 2.

  In all of these cases discussed with reference to FIGS. 4a-4c, the exact shape of the liquid-filled void is less important. As will be readily appreciated by those skilled in the art, a portion of the dielectric material can be made of an attenuation material that reduces both reflection and transmission (such as carbon-filled PTFE).

  Figures 5a to 5c show various bottom reflectors based on the principle of polarization rotation structures.

  In FIGS. 5a-5b, a reflector known as a twist reflector in antenna engineering is shown. W. Rudge et al., “The handbook of antenna design (Volume of Antenna Design Handbook), Volumes 1 and 2, Peter Peregrinus Ltd., 1986, pp. 184-185, and author R. C. Johnson“ Antenna Enhingen Hingen Tenhing. edition (Antenna Engineering Handbook (Third Edition)) "McGraw-Hill, Inc. 1993, 17-28 and 17-29. A section of these texts is incorporated herein by reference.

The twist structure 51 includes a plurality of straight and parallel ridges (ridges) 52 and is made of a conductive material. It is preferably a metal plate and can be cast to obtain multiple ridges. The height of the ridge is close to lambda _m / 4, when the liquid is above the ridges 52 of the reflector 51 has a spacing of λ _m / 4~λ _m / 2. The reflector 51 can be located at the lower end of the tube 13 and the thin ridge 52 is oriented 45 ° from the average electric field 53 of the incoming microwave signal propagating in the H ₁₁ waveguide mode in this configuration. .

In order to understand the twist function, the incoming electric field can be thought of as a superposition of two electric fields directed at −45 ° and + 45 ° with respect to the electric field line 53. One of the electric fields is parallel to the ridge 52 and will be reflected from the top of the ridge 52, and the other polarization is hardly affected by the ridge 52, but is a reflector exposed between the ridges 52. It will be reflected from the structure. The height of the ridge 52 of λ _m / 4 introduces a relative phase shift of 2 × 90 ° = 180 °, which reflects the reflected microwave electric field 54 into the incoming microwave electric field. It will twist 90 ° to 53. This behavior is similar to the function of a standard twist reflector in antenna engineering. However, the striking difference is that the dielectric constant of the material filling the space between the ridges depends on whether it is liquid or air / gas. After the tank is emptied, the twist function is only partial, and reflection from the reflector 51 can be obtained.

  An important feature of this embodiment is that the reflector is very thin and can therefore be configured very close to the bottom of the tank and the liquid level measurement can be made closer to the bottom.

  One skilled in the art will appreciate that a more complex structure allows for a wider tolerance for different dielectric constants of the liquid and for different frequencies.

  Furthermore, the microwave signal used in the liquid level measurement in the embodiment identified above has a specific polarization state, and the first one found above and in the appended claims. And that the second reflection coefficient is given for that particular polarization state. Therefore, the action of rotating the polarization of a microwave signal that is linearly polarized during reflection in the liquid is equivalent to reducing the reflection coefficient for the microwave signal in that particular polarization state.

The principle of polarization rotation is also applicable to change the reflected microwave propagation mode to another mode, as shown by FIG. 5c. The twist structure can be implemented by a spiral structure, which changes the propagation mode of the reflected microwave from H ₀₁ to E ₀₁ . The contents of which are incorporated herein by reference. O. Edvardson, U.S. Pat. No. 4,641,139. A similar function can be obtained by a number of dipoles 55 appropriately arranged on a plate 56 made of conductive material or damping material. This configuration has similarities to the configuration shown in FIG. 3d, but here there is a plate 56 and there are more dipoles 55 oriented in different directions. Furthermore, the dipoles 55 of FIG. 5c are single and are not stacked.

The dipole forms a frequency sensitive surface that provides a reflection that is comparable to the reflection of the bottom plate 55, but out of phase with the polarization. When the incident microwave propagates in the tube 13 in the H ₀₁ mode, the reflected microwave will propagate mainly in the E ₀₁ mode when the reflector is immersed in a liquid and properly designed. . Unlike known frequency sensitive surfaces, this surface will be detuned when the reflector is in a waterless condition and a large amount of reflected microwave will propagate in the _H01 mode.

  Thus, with the proper design of the reflector of FIG. 5c, if the reflector is immersed in liquid, there will be very little reflection of the incoming waveguide mode, and if the reflector is in air or gas. A substantially stronger reflection of the incoming waveguide mode is realized.

  Depending on the material of the bottom plate, the reflection in the incoming waveguide mode can be very strong (when the plate 55 is made of metal), i.e. almost total reflection, or (the plate 55 May be comparable to the reflection from the oil surface (in the case of a suitable damping material). As will be readily appreciated by those skilled in the art, attenuating material can be included in any one of the polarization rotation embodiments to tune the reflections in waterless conditions.

  FIG. 6 shows a bottom reflector based on the principle of local reduction of the cross-sectional area of the tube. To reduce or eliminate radar echo from the bottom of the tank below the tube 13, the tube diameter is converted to a smaller diameter and the radar wave is bent in a small space to guide the tube 13 away from the tube 13. You can use a device that can. One embodiment using only materials suitable for placement close to the tank bottom 60 (ie, stainless steel only) is based on a funnel 61 that reduces the diameter to a diameter close to the single mode waveguide diameter. Is. Such a waveguide 62 can easily be provided with a 90 ° bend 63 proximately above the bottom 60 in order to guide the microwave in another direction through the waveguide portion 64.

  The waveguide portion 64 can be easily designed to provide low reflection as long as the waveguide is filled with a liquid of fairly well known properties (including a range of hydrocarbons). The distance from the tank bottom 60 and the end of the tube 13 can be very short. The funnel 61 and the waveguide can be attached to the tank bottom 60 or the tube 13.

  If the waveguide 62 is narrow so as to limit the propagation to one mode when the waveguide 62 is immersed in a liquid, the propagation will be cut off in a waterless condition, and the empty tank will Will have a reflection at the bottom.

  An alternative to the funnel 61 is that at the bottom of the tank, the conical conductive or resistor structure is oriented at least substantially from the bottom end of the tube (not shown), with its top facing substantially upward in the tube. It is to place them at intervals corresponding to the diameter of the tube. A space between the bottom end of the tube and the envelope of the conical structure provides a smooth transition from the tube above the conical structure to the coaxial waveguide formed by the tube with the conical structure. Needed to get. As the microwave is propagating from the bottom end of the tube, the microwave will be deflected away from the tube. An attenuating material can be provided in the coaxial waveguide that is formed. Appropriate resonant structures (eg, half-wave slots in coaxial waveguides) can provide a bottom reflector that has very low reflection when immersed in liquid and much higher reflection in waterless conditions. .

  Finally, FIG. 7 shows a bottom reflector applicable when free-space propagation of microwaves is used, i.e. when no tube is present to guide the microwaves. The radar level gauge 11 is placed on top of the tank 12 with a main direction 77 of the antenna beam 76 that is near vertical so that it can receive reflections from the liquid surface 71. A dotted line 76 indicates the width of the antenna beam. When the liquid surface 71 approaches the bottom portion 75, a general interference problem occurs, and the same problem occurs when there is water 74 that only covers the bottom portion 75. The position of the water level is not well known at first and can result in a strong radar echo from the bottom 75.

（ここで、ｈは液位計１１から底部反射器７２までの距離である）の面積が反射に作用する。そのため、底部反射器は、少なくとも有効な反射面積の実質的な部分を占める面積を有するべきである。さらに、底部反射器７２が図２ａの反射器のような構造体を有するとき、どんな水やスラッジをも蓄積しないことになり、有利である。 When the present invention is applied in this case, the bottom reflector 72 is placed on the support member 73 close to the bottom. The bottom reflector 72 can be similar to FIG. 2a, but must be much larger. about

The area (where h is the distance from the level gauge 11 to the bottom reflector 72) acts on the reflection. Therefore, the bottom reflector should have an area that occupies at least a substantial portion of the effective reflective area. Furthermore, it is advantageous when the bottom reflector 72 has a structure like the reflector of FIG. 2a, since it will not accumulate any water or sludge.

  In certain embodiments of the present invention, the bottom reflector 72 can have a smaller size. If the phase shift of transmission through the bottom reflector 72 is controlled, a portion of the reflection from the bottom or water that passes through the bottom reflector 72 will cancel that portion of the reflection that travels outside the bottom reflector 72. Become. Such an arrangement reduces not only the reflection from the bottom reflector 72, but also the reflection from the bottom or from the water surface. The support pin 73 should be as low as possible relative to the expected water level.

  It will be appreciated that the bottom reflector principle described with reference to FIGS. 2-5 is also applicable to a level gauge that is configured to transmit and receive microwaves and is not guided by any tube. If a contractor wants to understand.

1 is a schematic side view of a radar level gauge including a bottom reflector according to the general principles of the present invention. FIG. FIG. 3 is a schematic top view of a bottom reflector according to a first preferred specific group of embodiments of the present invention. FIG. 3 is a schematic top view of a bottom reflector according to a first preferred specific group of embodiments of the present invention. Figure 2 is a schematic cross-sectional side view of a bottom reflector according to a first preferred specific group of embodiments of the present invention. Figure 2 is a schematic cross-sectional side view of a bottom reflector according to a first preferred specific group of embodiments of the present invention. Figure 2 is a schematic cross-sectional side view of a bottom reflector according to a first preferred specific group of embodiments of the present invention. FIG. 3 is a schematic diagram of the amplitude of the microwave signal reflected and transmitted respectively by the bottom reflector according to the first preferred specific group of embodiments as a function of frequency. Figure 3 is a schematic side view of a bottom reflector according to a second preferred specific group of embodiments of the present invention. Figure 3 is a schematic perspective view of a bottom reflector according to a second preferred specific group of embodiments of the present invention. FIG. 4 is a schematic top view of a bottom reflector according to a second preferred specific group of embodiments of the present invention. FIG. 4 is a schematic top view of a bottom reflector according to a second preferred specific group of embodiments of the present invention. FIG. 4 is a schematic top view of a bottom reflector according to a second preferred specific group of embodiments of the present invention. FIG. 4 is a schematic diagram of the amplitude of the microwave signal reflected and transmitted respectively by the bottom reflector according to the second preferred specific group of embodiments as a function of frequency. FIG. 5 is a schematic perspective view of a bottom reflector according to a third preferred specific group of embodiments of the present invention. Figure 4 is a schematic side view of a bottom reflector according to a third preferred specific group of embodiments of the present invention. Schematic of the amplitude of the microwave signal reflected and transmitted respectively by the bottom reflector according to the third preferred specific group of embodiments as a function of frequency. FIG. 6 is a schematic top view of a bottom reflector according to a fourth preferred specific group of embodiments of the present invention. FIG. 5 is a schematic cross-sectional side view of a bottom reflector according to a fourth preferred specific group of embodiments of the present invention. FIG. 6 is a schematic top view of a bottom reflector according to a fourth preferred specific group of embodiments of the present invention. Figure 6 is a schematic cross-sectional side view of a bottom reflector according to a fifth preferred specific group of embodiments of the present invention. FIG. 9 is a schematic perspective view of a bottom reflector according to a sixth preferred specific group of embodiments of the present invention.

Claims (45)

A bottom reflector (15) for a radar liquid level meter (11), wherein the liquid level gauge is configured to transfer a liquid level (71) stored in a tank (12) from the top of the tank. Transmitting a microwave signal toward the surface, receiving the microwave signal reflected from the surface of the liquid, and storing the liquid of the liquid stored in the tank from the propagation time of the transmitted and reflected microwave signal When the bottom reflector can be placed at a predetermined height in the tank and there is no liquid at the predetermined height in the tank. In a bottom reflector (15) adapted to reflect the microwave signal,
The bottom reflector (15) has a first reflection coefficient for the microwave signal when the liquid level of the liquid is above the reflecting structure, and the liquid level of the liquid is When located below the reflecting structure, the reflecting structure (23-24; 25; 23, 26; 27; 29; 32, 33; 32) having a second reflection coefficient for the microwave signal. 35; 36; 37; 41,42; 43,44; 51-52; 55-56; 61; 72)
Bottom reflector said first reflection coefficient and said second small Sai Ri by the reflection coefficient (15). Wherein said first reflection coefficient of said reflecting structure for microwave signals, bottom reflectors described in I請 Motomeko 1 smaller than the reflection coefficient of the liquid surface with respect to the microwave signal.   The second reflection coefficient of the reflecting structure with respect to the microwave signal is substantially the same as or slightly larger than the reflection coefficient of the liquid surface with respect to the microwave signal. The bottom reflector described.   4. A bottom reflector as claimed in any one of claims 1 to 3, wherein the liquid has a dielectric constant in the range of 1.6-3.   The liquid is a group of liquids comprising crude oil, liquefied petroleum gas (LPG), liquefied natural gas (LNG), other hydrocarbon liquids, and liquids that are at least partially permeable to the microwave signal. 5. A bottom reflector as claimed in any one of claims 1 to 4.   The microwave signal has a specific polarization state, and the first and second reflection coefficients are given to the microwave signal in the specific polarization state. A bottom reflector as described in any one of the above.   The bottom reflector according to any one of claims 1 to 6, wherein the microwave signal is in a specific frequency range. Bottom reflector according to any one of claims 1 wherein the first reflection coefficient have smaller than 0.2 to claim 5. 9. A bottom reflector as claimed in claim 8, wherein the first reflection coefficient is less than 0.1. 10. A bottom reflector as claimed in claim 9, wherein the first reflection coefficient is less than 0.05. Bottom reflector according to any one of from the second reflection coefficient is 0.1 from the large hearing claims 1 to 10. 12. A bottom reflector as claimed in claim 11, wherein the second reflection coefficient is greater than 0.2 . Said second reflection coefficient, the bottom reflector as claimed in any one of the small old claim 1 from 0.5 to claim 7. The bottom reflector as recited in claim 13, wherein the second reflection coefficient is less than 0.4. The bottom reflector as recited in claim 14, wherein the second reflection coefficient is less than 0.3. The liquid level meter, a bottom reflector according to any one of claims 1 to 15 including the tube of the transmitted and reflected microwave signal lead straight direction that will be guided (13). It said bottom reflector (15) is, before Kinamari bottom reflector of claim 16 in the lower end portion can be placed in the perpendicular direction of the tube (13). 18. The microwave signal according to claim 16 or 17 , wherein the microwave signal has a specific propagation mode, and the first and second reflection coefficients are given to the microwave signal of the specific propagation mode. Bottom reflector. 19. A bottom reflector as claimed in any one of claims 16 to 18, wherein the reflective structure is a cut-off grating (23-24; 25; 23, 26; 27; 29). It said microwave signal propagates in H ₁₁ mode, the cut-off grid, according to claim 19 wherein including a plurality of flat rows of strips arranged in the electric field and the flat row of the microwave signal (23) Bottom reflector. Said microwave signal propagates in H ₀₁ mode, claim wherein a cut-off grid, including the microwave signal a number of concentrically arranged circular strips arranged in the electric field and flat line of (25) A bottom reflector as described in item 19 . The bottom reflector according to any one of claims 1 to 21 , further comprising a deflecting structure (27) disposed below the reflecting structure. The bottom reflector according to any one of claims 1 to 21 , further comprising an absorption structure (29) disposed below the reflecting structure. The reflecting structure, a bottom reflector according to any one of claims 16 to claim 18 which is a co-sounding structure. 25. A bottom reflector as claimed in claim 24, wherein the resonant structure comprises a dipole (32, 33; 32, 35; 36; 37). 26. A bottom reflector as claimed in claim 24 or claim 25 , wherein the resonant structure comprises stacked dipoles (32). 27. A bottom reflector as claimed in any one of claims 24 to 26 , wherein the reflective structure comprises two intersecting dipoles (32, 35). Said microwave signal propagates in H ₁₁ mode, the resonant structure, before Kinamari claim comprising a single dipole elements arranged in the central axis (32, 35) in the perpendicular direction of the tube (13) 24 bottom reflector according to any one of up to claim 27. Said microwave signal propagates in H ₀₁ mode, the resonant structure, before Kinamari straight direction of the tube (13) in a plurality of dipole elements arranged in the periphery which is spaced from the central axis (36) 28. A bottom reflector as claimed in any one of claims 24 to 27 , comprising: Said microwave signal propagates in H ₀₁ mode, the resonant structure, according to claim claims 24 before containing ring (37) having an axis of symmetry coincides with the central axis of Kinamari straight direction of the pipe (13) 27. A bottom reflector as described in any one of the items up to 27 . 19. A bottom reflector as claimed in any one of claims 16 to 18, wherein the reflecting structure is a dielectric reflector (41, 42; 43, 44). 32. A bottom reflector according to claim 31 , wherein the dielectric reflector is a perforated plate (41, 42). It said dielectric reflector, bottom reflector of claim 31 including the Ru with the down small dielectric member (44) Water earnestly arranged plates (43). The bottom reflector according to any one of claims 16 to 18, wherein the reflecting structure is a polarization rotating structure (51 to 52; 55 to 56). 35. A bottom reflector as claimed in claim 34 , wherein the polarization rotation structure is a twist reflector structure (51) comprising a ridge (52). 35. A bottom reflector as claimed in claim 34 , wherein the polarization rotation structure comprises a reflector (55) and a plurality of differently oriented dipoles (56). The reflecting structure is a microwave guiding structure, the microwave guide structure is arranged in front Kinamari straight direction of the pipe (13), via the microwave guide structure, the transmission and reflection bottom reflector according to item 1 claim 16 to claim 18, microwave signal is guided away from the interior of the front Kinamari straight direction of the pipe (13). 38. A bottom reflector as claimed in claim 37 , wherein the microwave guiding structure is a funnel (61). The said bottom reflector is provided in order to transmit the said microwave signal toward a liquid level, without using the pipe | tube of the vertical direction which guides the said microwave, The any one of Claim 1-15 A bottom reflector (15) according to paragraph 1. The reflecting structure, the cut-off grid, co-sounding structure, a dielectric reflector, or the bottom reflector of claim 39 which is polarization rotation structure. 41. A bottom reflector as recited in claim 40, wherein the resonant structure includes a dipole. The reflecting structure (72) is below the radar level gauge,

Claim A bottom reflector occupying at least a part amount of the area, the above equation, h is the distance to the bottom reflector from the radar liquid level meter, lambda is the wavelength of the microwave signal 42. A bottom reflector as claimed in any one of claims 39 to 41 . When the area of the bottom reflector (72) is smaller than the area of the microwave signal at the height of the bottom reflector and the liquid level of the liquid is above the bottom reflector, The phase shift of the microwave signal transmitted through the reflector (72) is controlled so that the portion of the microwave signal reflected from the bottom of the tank outside the bottom reflector (72) and 42. The part of any one of claims 39 to 41 , wherein the microwave signal portions reflected from the bottom of the tank that have passed through the bottom reflector (72) cancel each other. Bottom reflector. 43. The bottom reflector according to any one of claims 1 to 42 , wherein the bottom reflector can be placed close to the bottom of the tank. A radar level gauge (11) system comprising a bottom reflector (15) according to any one of claims 1 to 44 .

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