Level meter

Abstract

To reduce a reflection wave which causes a stray signal wave in a level meter.SOLUTION: A level meter 10 includes a sensor substrate 42, a radio wave shaping member 59, a dielectric lens 48, and a radio wave absorption member 11. At the sensor substrate 42, a transmission part 43T and a reception part 43R are mounted. The radio wave shaping member 59 includes: an element case 49 for covering an internal space 51 containing the transmission part 43T and the reception part 43R mounted on the sensor substrate 42; a wave guide pipe 45 in which one end is communicated electromagnetically with the element case 49; and a horn 46 in which a radio wave path gradually enlarges toward the travelling direction of transmission radio waves from the other end of the wave guide pipe 45. The dielectric lens 48 is provided at the tip of the horn 46, and deflects the radio wave which has gone through the radio wave shaping member 59 from the transmission part 43T with respect to an object. The radio wave absorption member 11 is provided along an internal wall of the element case 49, and absorbs the radio waver.SELECTED DRAWING: Figure 9

Description

[Technical field]

[0001] The present invention relates to a level meter that measures the level of an object. [Background technology]

[0002] In containers that store fluid substances such as liquids, powders, and granules, level meters are sometimes used to measure the height of the interface of the substance, that is, the level (liquid surface level, powder top surface level, etc.) (Patent Document 1). As such a level meter, Patent Document 2 describes one in which a radio wave transmission signal is sent into the container during measurement operation, and an echo signal reflected by the surface of the fluid substance is received after a transit time that depends on the level. Millimeter waves are generally used as the radio wave transmission signal.

[0003] Level meters that use electromagnetic transmission signals are usually configured with a millimeter wave radar. Millimeter wave radars mainly use a patch antenna that is made up of a pattern formed on a circuit board to transmit and receive radio waves. In order to improve the directionality of the radar, it is common for the radio wave signal output from the patch antenna to be passed through a horn antenna and a lens antenna to narrow the spread of the radio waves before being emitted.

[0004] Instead of a patch antenna, it is possible to use a radar IC with an antenna-on-package structure, which is more versatile and less expensive than a patch antenna. Radar ICs often have separate transmission and reception waves, but to configure a level meter, it is necessary to unify the transmission and reception paths as a radar. By having an element case, a waveguide, a horn, and a lens as elements for this, the transmission and reception paths are unified and the directivity is improved. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2014-002091 A [Patent Document 2] DE 102015113955 A1 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in known level meters using radar ICs, the signal wave is reflected by the element case, lens, etc. in no small way. Therefore, when the surface of a fluid material is located close to the level meter, the reflected wave is received as a stray signal wave, and the stray signal generated by this is superimposed on the detection signal as noise. Therefore, when detecting the level at a short distance, there is a problem that the margin to the detection limit is reduced.

[0007] Furthermore, for example, if the fluid substance to be detected for level has a lower dielectric constant than water, the level of the reflected wave will be lower than that of water depending on the dielectric constant, and so detection stability will be reduced when detecting the level of a substance other than water with a low dielectric constant at close range.

[0008] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to reduce reflected waves that cause stray signal waves in a level meter. [Means for solving the problem]

[0009] In order to solve the above problems, a level meter as an example of an embodiment of the present invention includes: A substrate on which a transmitter and a receiver are mounted;

[0010] a radio wave shaping member having an element case that covers a space including the transmitter and receiver mounted on the substrate, a waveguide having one end electromagnetically connected to the element case, and a horn whose radio wave path gradually expands from the other end of the waveguide toward the traveling direction of the transmitted radio wave; a dielectric lens provided at a tip of the horn for deflecting the radio wave passing through the radio wave shaping member from the transmitter toward an object; The element case further includes a radio wave absorbing member disposed along an inner wall thereof for absorbing radio waves.

[0011] In a level meter as another embodiment of the present invention, a radio wave absorbing member is provided along an inner wall of the horn.

[0012] As yet another embodiment of the present invention, a level meter includes: a metal casing that houses the substrate, the radio wave shaping member, and the dielectric lens; and a mounting member made of a dielectric material for contacting the dielectric lens and mounting the dielectric lens to the metal casing.

[0013] As yet another example of an embodiment of the present invention, a level meter has a dielectric lens formed from a lens material including a low dielectric constant material having a relative dielectric constant of 2 to 3 and a high dielectric tangent material having a dielectric tangent higher than that of the low dielectric constant material.

[0014] As yet another example of the embodiment of the present invention, in the level meter, the high dielectric tangent material is polychlorotrifluoroethylene resin.

[0015] In a level meter as yet another example of an embodiment of the present invention, the waveguide is formed of a waveguide, and makes a transmitted wave from the transmitter and a received wave to the receiver coaxial with each other.

[0016] As yet another example of the embodiment of the present invention, in the level meter, the dielectric lens has a protruding cross-sectional shape that protrudes toward the waveguide.

[0017] As yet another embodiment of the present invention, a level meter includes: A casing is provided to house a substrate, a radio wave shaping member, and a dielectric lens; The casing is capable of being attached to a container that stores a flowable substance; an adapter that is attached to the container and to which the casing can be attached; The adapter is configured such that, when the casing is attached, a first distance from a surface of the flowable material to the substrate is longer than a second distance from the surface of the flowable material to the substrate when the casing is attached to a container. Effect of the Invention

[0018] According to the level meter of the present invention, the radio wave absorbing member is provided along the inner wall of the element case that covers the space including the transmitter and receiver mounted on the board, and absorbs radio waves. Therefore, it is possible to absorb unnecessary reflected waves other than those from the object. Therefore, it is possible to reduce reflected waves that cause stray signals in the level meter. As a result, the detection performance of the level of the surface of the object close to the level meter is improved. [Brief description of the drawings]

[0019] [Figure 1] Three-dimensional diagram of a level gauge. [Diagram 2] Vertical cross-section of a level meter. [Diagram 3] FIG. 1 is a diagram showing a state in which a level gauge is attached to a tank that contains an object. [Figure 4] FIG. 2 is a diagram showing the travel path of a transmitted wave. [Diagram 5] FIG. 4 is a diagram showing the travel path of a reflected wave. [Figure 6] FIG. 2 is a block diagram showing an example of the relationship between components of a level meter. [Figure 7] FIG. 2 is a diagram showing an example of the configuration of a radar control unit and a transmitting / receiving unit. [Figure 8] FIG. 4 is a diagram showing the relationship between a transmitted wave and a reflected wave. [Figure 9] FIG. 2 is a longitudinal sectional view of a main part of a first example of a level meter. [Figure 10] 13 is a diagram showing the intensity of a reflected wave that is a stray signal and the intensity of a reflected wave from an object. [Figure 11] 13 is a diagram showing an improvement in detection performance of the level of an interface of an object at a short distance from the level meter. [Figure 12] FIG. 4 is a vertical cross-sectional view of a main part of a second example of a level meter. [Figure 13]13 is a diagram showing a collimated state of a transmission wave from the level meter of FIG. 12. [Figure 14] 13A and 13B are diagrams showing the state of a wave transmitted from a level meter when a dielectric lens does not have a convex portion that protrudes toward a waveguide. [Figure 15] 3D diagram showing the level gauge attached to the adapter. [Figure 16] FIG. 4 is a cross-sectional view showing the state in which the level meter is attached to the adapter. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] A level meter 10 as an example of an embodiment of the present invention will be described below with reference to the drawings. The three-dimensional view of Fig. 1 shows the level meter 10 of this embodiment. The level meter 10 is a device that measures the level of an object to be measured (e.g., liquid, powder, granules, etc.). The level to be measured is the height of the interface of the object. A specific example of the level is the height of the liquid surface contained in a container.

[0021] The level meter 10 has a longitudinal direction A. In FIG. 1, the level meter 10 has a generally cylindrical shape extending in the longitudinal direction A. The level meter 10 has a casing 15 and a sensor unit 16. The casing 15 and the sensor unit 16 are aligned along the longitudinal direction A. The sensor unit 16 is disposed at one end side (lower side in FIG. 1) of the level meter 10 in the longitudinal direction A, and the casing 15 is disposed at the other end side (upper side in FIG. 1) in the longitudinal direction A. One end of the sensor unit 16 in the longitudinal direction A is a measurement end portion 40. In FIG. 1, the casing 15 and the sensor unit 16 each include a portion having a cylindrical shape extending in the longitudinal direction A.

[0022] In the following, one end side of the longitudinal direction A of the level meter 10 may be referred to as the lower side, and the other end side of the longitudinal direction A may be referred to as the upper side. The sensor unit 16 measures the level of an object with the lower side of the longitudinal direction A facing the object. A measurement axis is set in the sensor unit 16 in FIG. 1. The sensor unit 16 measures the level along this measurement axis. The sensor unit 16 in FIG. 1 measures the level of the object with the measurement end 40 facing the object. When the level meter 10 measures the level of an object, it is preferable that the longitudinal direction A is the same as the direction of change in the level of the object to be measured (for example, a liquid such as water). In other words, it is preferable that the direction of the measurement axis of the sensor unit 16 is the same as the longitudinal direction A. For example, when the level meter 10 measures the height (level) of a liquid, it is preferable that the longitudinal direction A is the same as the direction of change in the height of the liquid level, that is, the vertical direction (height direction, gravity direction).

[0023] In the level meter 10 in Fig. 1, radio waves serving as transmission waves are transmitted from the measurement end 40 toward an object, and the reflected waves reflected by the object are received by the measurement end 40. The level meter 10 calculates the level of the object based on the transmitted waves and the reflected waves.

[0024] The sensor section 16 has a mounting screw section 18 with a thread engraved on the surface of a cylinder, provided above the measurement end 40. Furthermore, a mounting section 17 with a diameter larger than that of the mounting screw section 18 is provided above the mounting screw section 18. The mounting section 17 in FIG. 1 has a nut-like shape. The mounting section 17 may have any structure that allows the level gauge 10 to be mounted to an object (such as a tank containing liquid), and is not limited to a nut-like shape. For example, the mounting section 17 may have a cylindrical shape with a non-slip protrusion. The non-slip protrusion of the mounting section 17 prevents the level gauge 10 from slipping when the level gauge 10 is mounted or removed (rotational attachment / detachment) while rotating around the longitudinal direction A relative to the object. Instead of the mounting section 17 and the mounting screw section 18, a mounting flange for mounting the level gauge 10 to the object may be formed. Even when a mounting flange is formed, the mounting section 17 and the mounting screw section 18 may be provided in addition to the mounting flange.

[0025] A display unit 20 is provided on the casing 15, which is disposed above the sensor unit 16. The display unit 20 is disposed on the outer surface of the casing 15, which extends along the longitudinal direction A. In Fig. 1, a flat portion along the longitudinal direction A is provided on part of the outer circumferential surface of the casing 15, and the display unit 20 is disposed on this flat portion.

[0026] An operation switch 30 is also arranged on the outer surface of the casing 15 where the display unit 20 is arranged. The operation switch 30 in FIG. 1 is arranged adjacent to a rectangular active matrix display that is long along the longitudinal direction A, and includes setting keys 32 and direction keys 33 that are aligned along the longitudinal direction A. The direction keys 33 also have an up key 34 and a down key 35 that are aligned along the longitudinal direction A. A user of the level gauge 10 can change the operating parameters of the level gauge 10 by operating the operation switch 30. In particular, the user can change the settings of the display content and level range of the display unit 20 by operating the operation switch 30.

[0027] A connection part 12 is provided on the upper side of the casing 15. The connection part 12 in Fig. 1 has a cylindrical shape extending along an axis parallel to the longitudinal direction A. The outer peripheral surface of the cylindrical connection part 12 forms a connection thread part 14 having threads.

[0028] A status light 52 is provided between the connection unit 12 and the display unit 20 so as to surround the lower side (base) of the connection unit 12. In FIG. 1, the connection unit 12 is cylindrical and extends along an axis parallel to the longitudinal direction A, so the status light 52 is disposed so as to surround the axis parallel to the longitudinal direction A. The lighting state of the status light 52 changes according to the measured level of the object. The user can roughly know the status of the object by visually checking the lighting state of the status light 52.

[0029] The internal structure of the level meter 10 will be described with reference to Fig. 2. Fig. 2 is a cross-sectional view showing a cross section of the level meter 10 in Fig. 1 taken along a plane intersecting the display unit 20 and parallel to the longitudinal direction A.

[0030] A sensor IC 41 and a sensor substrate 42 supporting the sensor IC 41 are disposed inside the sensor unit 16. The sensor IC 41 transmits and receives radio waves and processes signals to measure the level of an object. The signal processed by the sensor IC 41 is transmitted to a display substrate 60 inside the casing 15.

[0031] For example, a monolithic microwave integrated circuit (MMIC) is used as the sensor IC 41. The MMIC is an IC in which a plurality of semiconductor components that transmit radio waves, receive radio waves, and perform signal processing based on the transmitted and received radio waves are integrated into a single semiconductor device (one chip). In the sensor IC 41 in FIG. 2, an antenna is integrated into the MMIC using antenna in package (AiP) technology or antenna on package (AoP) technology. That is, in FIG. 2, the sensor IC 41 is an antenna-integrated package (antenna on package) in which a transmitting section that transmits radio waves and a receiving section that receives radio waves are integrated into a single semiconductor device. By using an antenna-integrated MMIC as the sensor IC 41, the volume occupied by the configuration for transmitting and receiving radio waves is reduced, and the overall dimensions of the level meter 10 are made compact. The sensor IC 41 is not limited to an antenna-integrated MMIC alone, and may include a plurality of ICs. The sensor IC 41 may include, for example, an antenna-integrated MMIC and a microcomputer. The antenna-integrated MMIC may include a radio wave transmitting antenna, a radio wave receiving antenna, and a circuit that controls radio wave transmission and reception, and the microcomputer may include a circuit that performs signal processing and arithmetic processing based on a signal received from the antenna-integrated MMIC.

[0032] The sensor IC 41 is mounted on the lower surface of the sensor substrate 42. The sensor substrate 42 is an electronic circuit substrate in which various electronic circuit elements are arranged on a plate made of an insulating material such as glass or resin. In FIG. 2, the sensor substrate 42 is arranged so as to be oriented perpendicular to the longitudinal direction A (horizontal direction in FIG. 2). When the sensor IC 41 is an antenna-integrated MMIC, the sensor IC 41 includes a transmitter 43T that transmits radio waves and a receiver 43R that receives radio waves. The sensor IC 41 includes the transmitter 43T and the receiver 43R, so that the sensor unit 16 has the transmitter 43T and the receiver 43R. The sensor IC 41 is mounted on the lower surface of the sensor substrate 42, so that the transmitter 43T and the receiver 43R are arranged on the lower surface of the sensor substrate 42 (one end side in the longitudinal direction A).

[0033] The measurement end 40 is located below the sensor substrate 42. The measurement end 40 in FIG. 2 includes, as an internal structure, a waveguide 45, a horn 46, and a dielectric lens 48 as a waveguide. When the sensor IC 41 is an antenna-integrated MMIC, an electric wave that becomes a transmission wave is transmitted from the sensor IC 41. The transmission wave transmitted from the sensor IC 41 is guided toward the object through the waveguide 45, the horn 46, and the dielectric lens 48 in this order. The reflected wave reflected by the object and incident on the measurement end 40 is guided toward the sensor IC 41 through the dielectric lens 48, the horn 46, and the waveguide 45 in this order. The waveguide 45 makes the transmission wave from the transmitter 43T and the reflected wave to the receiver 43R, i.e., the received wave, coaxial. When the portion where the mounting screw portion 18 is provided on the lower side of the sensor portion 16 is cylindrical, its cross section is circular. In this case, the circular dielectric lens 48 can be disposed below the cylindrical portion (one end side in the longitudinal direction A). In other words, if the sensor unit 16 has a cylindrical portion on the lower side, the cylindrical portion can serve both as a portion to be attached to the installation target (such as a tank) of the level meter 10 and as a location for disposing the dielectric lens 48, so that the overall dimensions of the level meter 10 can be made compact.

[0034] On the other hand, a rotation mechanism 19 is provided inside the casing 15 disposed above the sensor unit 16. The rotation mechanism 19 in FIG. 2 is disposed at the lower end (one end in the longitudinal direction A) of the casing 15, that is, between the casing 15 and the sensor unit 16. The rotation mechanism 19 can rotate the casing 15 relatively to the sensor unit 16 around the rotation axis 19A. For example, the rotation mechanism 19 is a mechanism having a shaft extending along the rotation axis 19A between the casing 15 and the sensor board 42. The shaft is connected to the casing 15 via a bearing or the like disposed around the shaft, and is configured so that the rotation of the casing 15 around the rotation axis 19A is not transmitted to the sensor unit 16. It is preferable that the rotation mechanism 19 can hold the casing 15 and the sensor board 42 in a state where the relative rotation angle between the casing 15 and the sensor board 42 is a desired angle. For example, the rotation mechanism 19 may include components such as a damper element that provides rotation resistance and a rotation lock that prevents rotation so that the rotation mechanism 19 can be held at a specific rotation angle. In addition to the above configuration, the rotation mechanism 19 may include an engagement protrusion formed on the sensor unit 16 and a plurality of hooks provided on the casing 15. The engagement protrusion is disposed on the outer periphery of a cylindrical portion at the upper part of the sensor unit 16, and is formed long along the circumferential direction. The engagement protrusion of the sensor unit 16 may be a continuous shape that goes around in the circumferential direction. The plurality of hooks of the casing 15 are disposed in an annular shape at intervals corresponding to the engagement protrusions of the sensor unit 16. Each hook engages with a lower surface of the engagement protrusion. Each hook rotates while sliding in the circumferential direction when it receives an external force. When the rotation mechanism 19 includes the engagement protrusion and the hook, the casing 15 may be provided with an elastic body that generates a pressing force that pushes the gap between the sensor unit 16 and the casing 15 apart. By providing the elastic body, the sliding resistance between the engagement protrusion and each hook is adjusted. The rotation mechanism 19 in FIG. 2 is disposed so that the rotation axis 19A is parallel to the longitudinal direction A (vertical direction in FIG. 2). The display unit 20 is disposed on a flat surface formed at a position offset toward the rotation axis 19A in the casing 15. In other words, the flat surface of the casing 15 is located away from a plane including the rotation axis 19A.By providing the rotation mechanism 19, the user can orient the display unit 20 in a direction that is easy to view by rotating the casing 15. In particular, when the level meter 10 is attached to a container (such as a tank) that contains a measurement target by the mounting screw portion 18, the user can orient the display unit 20 in a direction that is easy to view without moving the entire level meter 10.

[0035] A display board 60 is disposed inside the casing 15 above the rotation mechanism 19. The display board 60 is an electronic circuit board in which various electronic circuit elements are disposed on a plate made of an insulating material such as glass or resin. In FIG. 2, the display board 60 is disposed so as to be parallel to the longitudinal direction A (vertical direction in FIG. 2). A signal processing circuit for controlling the display unit 20 is mounted on the display board 60. The display board 60 receives a signal from the sensor IC 41 and converts it into a signal for displaying the level of the object measured by the sensor unit 16 on the display unit 20. The display board 60 in FIG. 2 is disposed on the back side of the display unit 20 (inside the casing 15 in FIG. 2).

[0036] The display unit 20 in FIG. 2 includes a display device 28 and a transparent display window 29. The display device 28 is, for example, an LCD, particularly an LCD capable of color display. The display window 29 is a plate made of an optically transparent material such as glass, acrylic, polyarylate, or polycarbonate, and optically transmits the display content of the display device 28 to the outside of the casing 15. The display device 28 is oriented with the display surface 20A aligned along the longitudinal direction A. Also, in FIG. 2, the display substrate 60 is oriented along the display surface 20A.

[0037] Since the display device 28 and the display substrate 60 are oriented along the longitudinal direction A, the components for operating the display device 28 of the display unit 20 are arranged along the longitudinal direction A (height direction) inside the level gauge 10. Therefore, the area occupied by the components for operating the display unit 20 in the direction perpendicular to the longitudinal direction A (radial direction) is reduced, and the overall dimensions of the level gauge 10 are compact. In Figs. 1 and 2, the diameter of the cylindrical casing 15 is smaller than the diameter of the sensor unit 16. Note that the diameter of the casing 15 does not necessarily have to be smaller than the diameter of the sensor unit 16, and the diameters of the casing 15 and the sensor unit 16 may be the same, or the diameter of the sensor unit 16 may be smaller.

[0038] The status light 52, which is provided between the display unit 20 and the connection unit 12 above the display unit 20 (the other end side in the longitudinal direction A), has a plurality of status LEDs 50 serving as light sources and a transmission window 53 that diffuses the light emitted by the status LEDs 50 in a direction intersecting the longitudinal direction A. A pillar for supporting the connection unit 12 may be provided between the status LEDs 50 and the transmission window 53. The plurality of status LEDs 50 are arranged above the display board 60 and the display unit 20. The lighting state of the status LEDs 50 changes according to the level of the object measured by the sensor unit 16. The transmission window 53 including a member for diffusing light (such as a light diffusion film) is arranged above the status LEDs 50. The light emitted from the status LEDs 50 is diffused through the transmission window 53 in a direction intersecting the longitudinal direction A and is guided to the outside of the casing 15. Therefore, as the lighting state of the status LEDs 50 changes according to the level of the object, the lighting state of the status light 52 changes. The transparent window 53 includes a member for diffusing light, and the status lamp 52 (particularly the transparent window 53) is arranged to surround an axis parallel to the longitudinal direction A, so that the light emitted from the status LED 50 is diffused uniformly in all directions around the connection portion 12. Each of the status LEDs 50 may emit light in a single color (e.g., red, yellow, green, etc.), or may emit light by switching between multiple colors (e.g., red, yellow, green, etc.). When the status LED 50 emits light by switching between multiple colors, multiple light-emitting elements that emit light in different colors may be included in one LED, or a combination of multiple LEDs that emit light in different colors may be arranged as the status LED 50. The status LED 50 may emit light by mixing multiple colors. When the status LED 50 emits light by switching between multiple colors, the status LED 50 may emit light in a color corresponding to the color of the section of the gauge 24 (FIG. 1) that corresponds to the level range to which the measured level belongs. It is preferable that each of the status LEDs 50 can be switched between a non-emitting state (light-out state), an emitting state, and a blinking state. The light source of the status lamp 52 may be any light source capable of controlling the lighting state, and may be, for example, a light-emitting element using an organic electroluminescence (EL).

[0039] The connection part 12 is disposed on the other end side (upper side) of the display part 20 in the longitudinal direction A and on the extension of the rotation shaft 19A. In FIG. 1, the connection part 12 is provided at the upper end (the other end in the longitudinal direction A) of the casing 15. The connection part 12 includes an external input terminal 12C and an external output terminal 12D that are connection terminals with an external device. The external input terminal 12C is a terminal for inputting a signal or power, or both, from the outside to the level meter 10. The external output terminal 12D is a terminal for outputting a signal from the level meter 10 to the outside. The external output terminal 12D may include a plurality of signal lines or a plurality of output terminals. For example, a signal may be output to the outside from different signal lines or different output terminals depending on the level range to which the measured level belongs. Since the connection part 12 is disposed on the extension of the rotation shaft 19A, the position of the connection part 12 does not change even when the casing 15 is rotated relative to the sensor part 16 by the rotation mechanism 19. Therefore, even if the casing 15 is rotated by the rotation mechanism 19, there is no problem in the connection with the outside by the connection part 12. The position where the connection part 12 is provided is not limited to the upper end of the casing 15. For example, the connection part 12 may be provided at a position on the surface of the casing 15 where the user can access (approach) from the direction in which the display part 20 is viewed by the user. In this case, it is expected that there will be no obstacles within the direction in which the user views the display part 20 so that the user can see the display part 20. Therefore, when the connection part 12 is provided at a position where the user can access from the direction in which the display part 20 is viewed by the user, it is easy to secure a space for routing the cable connected to the connection part 12, and the level meter 10 can be easily installed. The connection part 12 may be provided on the back side of the display part 20. In this case, the connection part 12 can be provided in a dead space that is not being used, so that the level meter 10 can be made smaller. The external output terminal 12D may include a plurality of signal lines or a plurality of output terminals. For example, signals may be output to the outside from different signal lines or different output terminals depending on the level range to which the measured level belongs.

[0040] An example of how the level gauge 10 is used will be described with reference to Fig. 3. Fig. 3 is a diagram showing a state in which the level gauge 10 is attached to a tank 70 that contains an object 72. The object 72 is a liquid such as water, and the level Y of the object 72 is the height from the bottom of the tank 70 to an interface 74 (liquid level) of the object 72.

[0041] The tank 70 contains water that becomes the object 72 in, for example, a water treatment facility. For example, when the object 72 in the tank 70 is supplied to a water treatment process or the like, the level Y of the object 72 in the tank 70 decreases. When the object 72 is replenished in the tank 70, the level Y of the object 72 in the tank 70 increases. For example, a water inlet 75 is provided on the outer wall (upper wall in FIG. 3 ) of the tank 70. A water inlet pipe 76 is fluidly connected to the tank 70 via the water inlet 75. The water inlet pipe 76 is connected to a water inlet device 78 (e.g., a device including a pump, a valve, etc.) provided outside the tank 70. The water inlet device 78 is a device that supplies (injects) the object 72 into the tank 70 from outside the tank 70. The water inlet device 78 adjusts the amount of water injected into the tank 70 depending on the level Y of the object 72 in the tank 70. The water inlet device 78 stops injecting water depending on the level Y of the object 72 in the tank 70. The water injection device 78 controls the replenishment of the object 72 to the tank 70 in response to the level Y of the object 72 in the tank 70, which decreases as the object 72 is consumed, for example by a water treatment process, so that the level Y of the object 72 in the tank 70 is within a predetermined range.

[0042] The level gauge 10 in FIG. 3 is attached to the upper side of the tank 70. An attachment hole 71 is provided on the upper side of the tank 70. The attachment hole 71 is a screw hole, and the level gauge 10 is attached to the tank 70 by screwing the attachment screw portion 18 of the level gauge 10 into the attachment hole 71. For example, a user of the level gauge 10 can screw the attachment screw portion 18 into the attachment hole 71 by rotating the nut-shaped attachment portion 17 with the tip of the attachment screw portion 18 aligned with the attachment hole 71. The structure for attaching the level gauge 10 to the tank 70 is not limited to this. For example, the attachment hole 71 may not have a thread, and a nut may be separately screwed into the attachment screw portion 18 exposed inside the tank 70, so that the level gauge 10 is attached to the tank 70. Alternatively, the top surface of the tank 70 may be open, and the level gauge 10 may be attached to a mounting bracket provided above the tank 70 using a nut and the attachment screw portion 18. Alternatively, the method of attaching the level gauge 10 to the tank 70 is not limited to screwing using the mounting thread portion 18, and threads do not have to be formed on the outer circumferential surface of the sensor portion 16. For example, the level gauge 10 may be attached to the tank 70 by providing a flange on either or both of the level gauge 10 and the tank 70 and fastening the flange to the level gauge 10 or the tank 70 with a bolt.

[0043] In FIG. 3, the longitudinal direction A of the level gauge 10 attached to the tank 70 is the same as the direction in which the level Y of the object 72 changes.

[0044] The level meter 10 transmits radio waves, which become transmission waves Tx, from the measurement end 40 toward the object 72. Then, the transmission waves Tx are reflected by the interface 74 of the object 72, and the reflected waves Rx are received by the measurement end 40. The level meter 10 calculates the level Y of the object 72 based on the transmission waves Tx and the reflected waves Rx. For example, when performing measurement using a Time of Flight (ToF) method, the level meter 10 calculates the distance YA from the measurement end 40 to the interface 74 based on the difference between the transmission waves Tx and the reflected waves Rx, and calculates the level Y based on the distance YA. For example, when performing measurement using a radar method using a Frequency Modulated Continuous Wave (FMCW), the level meter 10 calculates the distance YA from the measurement end 40 to the interface 74 based on the frequency of a waveform obtained by mixing the transmission waves Tx and the reflected waves Rx, and calculates the level Y based on the distance YA.

[0045] A connection cable 92 is connected to the connection part 12 of the level gauge 10. The connection cable 92 connects the level gauge 10 to a control device 90 (such as a programmable controller) provided outside the tank 70. An analog signal indicating the level Y of the object 72 measured by the level gauge 10 is transmitted to the control device 90 via the connection cable 92. The control device 90 controls the operation of the water injection device 78 according to the measured level Y.

[0046] Fig. 4 is a diagram showing the travel path of the transmitted wave Tx. Fig. 5 is a diagram showing the travel path of the reflected wave Rx. With reference to Figs. 4 and 5, the following will explain how the transmitted wave Tx is guided to the object 72 through the horn 46 and the dielectric lens 48, and how the reflected wave Rx is guided to the receiving unit 43R.

[0047] The sensor unit 16 has a transmitter 43T that transmits a transmission wave Tx and a receiver 43R that receives a reflected wave Rx. Specifically, the transmitter 43T and receiver 43R are included in a sensor IC 41 disposed inside the sensor unit 16. The transmitter 43T and receiver 43R are a semiconductor electromagnetic wave generator and a semiconductor electromagnetic wave receiver, respectively, mounted on a chip of the sensor IC 41.

[0048] Between the transmitter 43T and the object 72, an enclosure wall 47, a waveguide 45, a horn 46, and a dielectric lens 48 are provided in the order of proximity to the transmitter 43T. The enclosure wall 47 surrounds a space including the transmitter 43T and the receiver 43R on the sensor board 42, and is connected to the waveguide 45. The enclosure wall 47 is conductive. The waveguide 45 is a hollow pipe formed of a conductor. The horn 46 is surrounded by a tapered wall that surrounds a space connected to the waveguide 45. The tapered wall of the horn 46 is conductive. These are arranged so that the direction of the directivity of the waveguide 45 and the horn 46 coincides with the direction of the optical axis 40A of the dielectric lens 48. In FIG. 4 and FIG. 5, the measurement end 40 is arranged so that the optical axis 40A is parallel to the longitudinal direction A (the length direction of the level meter 10). Hereinafter, the direction of the directivity of waveguide 45 and horn 46 may also be referred to as optical axis 40A.

[0049] The transmission wave Tx transmitted from the transmitter 43T passes through the space in the surrounding wall 47 and the waveguide 45, and then enters the dielectric lens 48 through the horn 46. In Figs. 4 and 5, the equiphase surface of the radio wave is shown by a broken line in the surrounding wall 47, the horn 46, and below the measurement end 40. In the waveguide 45, the equiphase surface should be shown as a straight line, but in order to make the traveling direction of the radio wave easier to understand, a curved broken line that bulges toward the traveling direction (the lower side in Fig. 4 and the upper side in Fig. 5) is shown. The waveguide 45 converts the transmission wave Tx into a spherical wave that travels spherically from the exit end of the waveguide 45 along the optical axis 40A, regardless of the relative positional relationship between the transmitter 43T and the waveguide 45. As a result, the exit end of the waveguide 45 can be regarded as a point-like transmission source. The diameter of the waveguide 45 is preferably set to a diameter that allows only fundamental mode radio waves to pass through the waveguide 45 so that the output end of the waveguide 45 becomes an ideal point-like transmission source. On the other hand, if the diameter of the waveguide 45 is smaller than half the wavelength of the radio waves, the radio waves will not pass through the waveguide 45. For example, when the frequency of the radio waves is 50 GHz-70 GHz, the wavelength is about 4.3 mm-6.0 mm, and the half wavelength is about 2.15 mm-3.0 mm. In this case, if a diameter larger than 3 mm and smaller than 4.3 mm is selected as the diameter of the waveguide 45, only fundamental mode radio waves can pass through the waveguide 45. The diameter of the waveguide 45 that satisfies the above condition is, for example, 4 mm. If the dimension of the waveguide 45 in the direction along the optical axis 40A is short, radio waves other than fundamental mode will also pass through the waveguide 45 even if the diameter of the waveguide 45 satisfies the above condition. For example, the dimension of the waveguide 45 in the direction along the optical axis 40A may be one wavelength of radio waves or longer. The longer the dimension of the waveguide 45 in the direction along the optical axis 40A, the longer the dimension of the level meter 10 in the longitudinal direction A. After passing through the waveguide 45, the transmitted wave Tx travels as a spherical wave in the horn 46 and is converted into a plane wave by the dielectric lens 48.

[0050] As shown in FIG. 4, the transmission wave Tx travels as a spherical wave inside the horn 46, and then the transmission wave Tx that is incident on the dielectric lens 48 is refracted between the horn 46 and the dielectric lens 48 and between the dielectric lens 48 and the surrounding air (atmosphere), and is guided in a direction parallel to the optical axis 40A. Since the optical axis 40A is parallel to the longitudinal direction A, the transmission wave Tx is guided in the direction in which one end (lower side) of the sensor unit 16 in the longitudinal direction A is facing, that is, in the direction of the object 72. The transmission wave Tx travels as a plane wave corresponding to the effective diameter of the dielectric lens 48 and a diffracted wave around it. The transmission wave Tx forms a radio wave beam with high directionality. Therefore, the area of ​​the detection target surface (the area of ​​the interface 74 of the object 72 where the transmission wave Tx hits) is narrowed, and unnecessary reflections due to surrounding obstacles and the like are reduced.

[0051] The transmission wave Tx guided to the object 72 is reflected at the interface 74 of the object 72, becoming a reflected wave Rx. As shown in FIG. 5, the reflected wave Rx traveling as a plane wave is received at the measurement end 40. The reflected wave Rx received at the measurement end 40 is first incident on the dielectric lens 48. The reflected wave Rx incident on the dielectric lens 48 is refracted between the surrounding air and the dielectric lens 48, and between the dielectric lens 48 and the horn 46, and travels through the horn 46 as a spherical wave approaching the optical axis 40A. As a result, the reflected wave Rx passes through the waveguide 45 and is guided to the receiving section 43R at the end on the enclosure wall 47 side as a radio wave emitted from a point-like transmission source.

[0052] The waveguide 45, the horn 46, and the dielectric lens 48 arranged inside the sensor unit 16 guide the traveling direction of the radio wave in the direction of the optical axis 40A by their respective directivities for the radio wave, and therefore, when combined, the sensor unit 16 shows strong directivity for the radio wave as a whole. Therefore, even if the lengthwise dimension (length along the longitudinal direction A) of the sensor unit 16 is small, the transmission wave Tx and the reflected wave Rx can be appropriately guided by coaxializing the transmission wave Tx and the reflected wave Rx by the waveguide 45. Furthermore, since the transmission wave Tx and the reflected wave Rx are appropriately guided, the transmission of the transmission wave Tx and the reception of the reflected wave Rx can be performed by the common measurement end 40, even though the position of the transmitter 43T and the position of the receiver 43R are different in the sensor IC 41. Therefore, by using the waveguide 45, the horn 46, and the dielectric lens 48, the designer of the level meter 10 can reduce the lengthwise dimension of the level meter 10 including the sensor unit 16, and the overall dimension of the level meter 10 can be made compact.

[0053] Next, the relationship between the components of the level meter 10 will be described with reference to FIG. 6. FIG. 6 is a block diagram that shows an example of the relationship between the components of the level meter 10. As shown in FIG. 6, the sensor IC 41 of the sensor unit 16 includes a transmitting / receiving unit 43, a radar control unit 44, a storage unit 63, and a calculation unit 64. The transmitting / receiving unit 43 has a transmitting unit 43T that transmits a transmission wave Tx and a receiving unit 43R that receives a reflected wave Rx. The sensor IC 41 may include multiple ICs. For example, the sensor IC 41 may include an antenna-integrated MMIC and a microcomputer.

[0054] The radar control unit 44 has a transmission control unit 80 that determines the waveform of the transmission wave Tx, a radar transmission and reception circuit 81 that performs mutual conversion between digital signals and radio waves, and a signal processing unit 89 that performs signal processing based on the transmission wave Tx and the reflected wave Rx. When the sensor IC 41 includes an antenna-integrated MMIC and a microcomputer, the antenna-integrated MMIC may have the parts of the radar control unit 44 excluding the signal processing unit 89 (the transmission control unit 80 and the radar transmission and reception circuit 81) and the transmission and reception unit 43, and the microcomputer may be configured to have the signal processing unit 89, the storage unit 63, and the calculation unit 64.

[0055] The storage unit 63 stores various set values ​​(data) related to the operation of the level meter 10. The calculation unit 64 performs various calculations related to the operation of the level meter 10 based on the set values ​​stored in the storage unit 63 and the signal processing results of the signal processing unit 89. The storage unit 63 includes a storage device such as a RAM or a ROM. The calculation unit 64 includes a processor such as a CPU. The storage unit 63 and the calculation unit 64 may be provided on the display board 60 in the casing 15. Alternatively, the sensor unit 16 and the casing 15 may be provided with separate storage units 63 and calculation units 64, respectively, and the stored data and the calculation processing in charge may be shared between the sensor unit 16 and the casing 15.

[0056] On the other hand, the display board 60 of the casing 15 has an input section 65 and an output section 66. The input section 65 is an interface circuit that inputs an input provided from outside the level meter 10 as a signal to the level meter 10. The input provided from outside the level meter 10 is, for example, a user's operation of the operation switch 30, and a control signal provided from an external device (such as the control device 90) via the external input terminal 12C. The input section 65 stores in the memory section 63, for example, flag information indicating that the operation switch 30 has been operated, and data such as a setting value provided via the external input terminal 12C.

[0057] The output unit 66 is an interface circuit that outputs to the outside a signal generated inside the level meter 10. For example, the output unit 66 changes the display content of the display unit 20 and the lighting state of the status lamp 52 according to the calculation result (such as the value of the level Y) by the calculation unit 64. The output unit 66 also transmits the calculation result by the calculation unit 64 to an external device via the external output terminal 12D.

[0058] Measurement of the level Y by the level meter 10 will be described in more detail with reference to Fig. 7 and Fig. 8. Fig. 7 is a diagram showing an example of the configuration of the radar transmission and reception circuit 81 and the transmission and reception unit 43 included in the sensor IC 41. Fig. 8 is a diagram showing the relationship between the transmitted wave Tx and the reflected wave Rx.

[0059] As shown in FIG. 7, the radar transmission / reception circuit 81 includes a ramp generator 82, a power amplifier 83, a low-noise amplifier 84, a mixer 85, a low-pass filter 86, and an analog-to-digital converter 87.

[0060] The level meter 10 of this embodiment measures the level Y by a radar method using FMCW. The ramp generator 82 is connected to the transmission control unit 80. When the ramp generator 82 receives data indicating the waveform of the transmission wave Tx determined by the transmission control unit 80, it generates a transmission signal having the waveform of the transmission wave Tx according to the data. Here, a waveform that repeatedly increases and decreases in frequency is used as the waveform of the transmission wave Tx.

[0061] FIG. 8 shows a graph showing the change in frequency of the transmission wave Tx (and the reflected wave Rx) over time. In FIG. 8, the frequency of the transmission wave Tx increases linearly from the minimum value (Min) as time passes, and when it reaches the maximum value (Max), it returns to the minimum value again. In this way, the frequency of the transmission wave Tx repeatedly increases and decreases. The frequency increase and decrease pattern is not limited to this, and for example, a pattern in which the frequency decreases linearly from the maximum value as time passes, and when it reaches the minimum, it returns to the maximum value again may be used. Alternatively, a pattern in which the frequency increases and decreases linearly between the maximum value and the minimum value may be used. The transmission wave Tx used in this embodiment is a radio wave in the 60 GHz band, and the minimum value of the frequency is, for example, 58 GHz, and the maximum value is, for example, 69 GHz. The frequency band used is not limited to this, and for example, a frequency of 77 GHz to 81 GHz may be used.

[0062] The transmission signal generated by the ramp wave generator 82 in Fig. 7 is amplified by the power amplifier 83 and sent to the transmitting unit 43T. The transmitting unit 43T generates radio waves having a waveform corresponding to the transmitted transmission signal and transmits the radio waves as a transmission wave Tx to the object 72. The transmission wave Tx is reflected by the interface 74 of the object 72 to become a reflected wave Rx, which is received as a received signal by the receiving unit 43R. The reflected wave Rx becomes a wave out of phase with respect to the transmission wave Tx.

[0063] As shown by the dashed line in Fig. 8, the reflected wave Rx is a wave that is shifted from the transmitted wave Tx by a time difference Δt. This time difference Δt is a value that corresponds to the distance YA (Fig. 3) from the measurement end 40 to the interface 74 of the object 72. Since the reflected wave Rx travels back and forth between the measurement end 40 and the object 72, there is a relationship of Δt = 2 × YA / c, where c is the speed of light.

[0064] Then, a frequency difference ΔF occurs between the transmission wave Tx and the reflected wave Rx according to the magnitude of the time difference Δt. There is a certain relationship between the frequency difference ΔF and the time difference Δt that depends on the waveform of the transmission wave Tx. The waveform of the transmission wave Tx is a waveform in which the frequency changes linearly with the passage of time. In other words, there is a certain relationship between the frequency difference ΔF and the time difference Δt that depends on the frequency change per unit time in the waveform of the transmission wave Tx. For example, the frequency of the transmission wave Tx increases linearly from the minimum value (Min) as time passes, and reaches the maximum value (Max). In this case, the relationship between the frequency difference ΔF and the time difference Δt is uniquely determined by the difference between the maximum and minimum values ​​of the frequency of the transmission wave Tx, which is the bandwidth of the frequency modulation, and the relationship of the frequency change with the passage of time. Therefore, the level meter 10 can calculate the time difference Δt based on the frequency difference ΔF, which is the difference between the transmission wave Tx and the reflected wave Rx. Then, the level meter 10 can calculate the distance YA (Δt×c / 2) from the time difference Δt. Furthermore, the level meter 10 can calculate the value of the level Y based on the distance YA. Specifically, the difference between the depth of the tank 70 (the distance from the bottom of the tank 70 to the measurement end 40) and the distance YA is the value of the level Y. Note that since there is a certain relationship between the frequency difference ΔF and the time difference Δt that depends on the waveform of the transmission wave Tx, it is possible to obtain the correspondence relationship between the frequency difference ΔF and the distance YA in advance. The correspondence relationship between the frequency difference ΔF and the distance YA may be stored in advance in the storage unit 63 of FIG. 6.

[0065] 7, a received signal corresponding to the reflected wave Rx received by the receiving unit 43R is input to a mixer 85 via a low-noise amplifier 84. A transmission signal corresponding to the waveform of the transmitted wave Tx output from the ramp wave generator 82 is also input to the mixer 85, and the mixer 85 generates a signal corresponding to a mixed wave Mx obtained by mixing the waveforms of the transmitted wave Tx and the reflected wave Rx. Specifically, the mixer 85 mixes the transmitted signal and the received signal to generate an IF signal (IF: Intermediate Frequency) corresponding to the mixed wave Mx.

[0066] The IF signal corresponding to the mixed wave Mx has a waveform including high frequency components originating from the 60 GHz frequency band of the transmitted wave Tx and the reflected wave Rx, and low frequency components corresponding to the frequency difference ΔF between the transmitted wave Tx and the reflected wave Rx. The IF signal corresponding to the mixed wave Mx is input to a low pass filter 86, which extracts a low frequency waveform corresponding to the frequency difference ΔF. The extracted low frequency waveform is input to an analog-to-digital converter 87. The analog-to-digital converter 87 converts the low frequency waveform into a digital value and outputs it to a signal processing unit 89.

[0067] The signal processing unit 89 converts the low-frequency waveform output from the analog-digital converter 87 into a frequency signal Px by fast Fourier transform processing or the like. The frequency signal Px is a signal that indicates the strength of the wave for each frequency, and the frequency corresponding to the maximum peak PS of this frequency signal Px is the frequency difference ΔF between the transmitted wave Tx and the reflected wave Rx. The signal processing unit 89 transmits the frequency signal Px to the calculation unit 64 in FIG. 6.

[0068] The calculation unit 64 calculates the values ​​of the distance YA and the level Y based on the frequency signal Px. When calculating the values ​​of the distance YA and the level Y, the calculation unit 64 refers to set values ​​stored in the storage unit 63. For example, the storage unit 63 stores the correspondence between the frequency difference ΔF and the distance YA, a value for calculating the level Y from the distance YA (the depth of the tank 70), and the like.

[0069] Depending on the measurement environment, a peak other than the maximum peak PS may appear in the frequency signal Px due to an element other than the interface 74 of the object 72 (for example, equipment such as a stirrer provided in the tank 70). Even if multiple peaks exist in the frequency signal Px, the calculation unit 64 can identify only the maximum peak PS originating from the object 72 by performing appropriate calculations. For example, data on the frequency signal Px obtained in a state in which the object 72 is not present (a state in which the tank 70 is empty) may be stored in advance in the storage unit 63. The calculation unit 64 can identify the maximum peak PS originating from the object 72 by examining the difference between the frequency signal Px obtained in a state in which the object 72 is not present and the frequency signal Px obtained in a state in which the object 72 is present.

[0070] After calculating the frequency difference ΔF corresponding to the maximum peak PS of the frequency signal Px, the calculation unit 64 calculates the distance YA and the value of the level Y based on the frequency difference ΔF and the set value stored in the storage unit 63. The calculation unit 64 transmits the calculated value of the level Y to the output unit 66. The output unit 66 changes the display content of the display unit 20 and the lighting state of the status lamp 52 according to the value of the level Y. The value of the level Y is sent to the control device 90 (FIG. 3) through the external output terminal 12D. The output unit 66 may output a binary or multi-value control signal based on the result of comparing the calculated value of the level Y with a threshold value to the control device 90 through the external output terminal 12D. Alternatively, instead of the value of the level Y itself, a signal indicating that a specific control should be performed according to the state of the level Y may be sent to the control device 90. For example, when the level Y exceeds a specific threshold value or falls below a specific threshold value, a signal indicating that the operation of a pump, a valve, or the like should be changed may be sent to the control device 90.

[0071] Fig. 9 shows a detailed internal structure of the sensor unit 16 of the level gauge 10 according to the first embodiment of the present invention. As shown in the figure, an element case 49 is formed by a surrounding wall 47. Although the shape of the internal space 51 of the element case 49 is arbitrary, the element case 49 shown in Fig. 9 is formed in a rectangular parallelepiped shape to match the shape of the sensor IC 41 to be accommodated in the internal space 51. Therefore, the surrounding wall 47 as the inner wall of the element case 49 has four planar faces on the inner surface. The element case 49 has a bottom wall 31 as an inner wall in a portion to which the waveguide 45 as a waveguide is connected.

[0072] The dielectric lens 48 is made of a dielectric material such as synthetic resin or glass. The element case 49, the waveguide 45, and the horn 46 are made of members having formability and radio wave reflectivity. Examples of such members include aluminum die-cast members plated with nickel, machined aluminum members anodized with alumite, zinc die-cast members plated with nickel, stainless steel members, and resins such as ABS and liquid crystal polymers plated with nickel.

[0073] Inside the casing 54 of the sensor unit 16, there is provided an element case 49 covering the internal space 51 including the sensor IC 41 mounted on the sensor board 42 and equipped with a transmitter 43T and a receiver 43R, a waveguide 45, and a radio wave shaping member 59 having a horn 46.

[0074] The radio waves transmitted from transmitter 43T of sensor IC 41 are reflected by element case 49 inside sensor unit 16 and surface 48A of dielectric lens 48 facing the inside of sensor unit 16. In horn 46, the radio waves reflected by surface 48A of dielectric lens 48 are further reflected. The reflected waves become stray signal waves and exist inside sensor unit 16. Among them, the strength of the radio waves reflected by the inner surface of element case 49 is significantly higher.

[0075] A radio wave absorbing member 11 that absorbs reflected waves that become stray signal waves is provided on the inner surface of the bottom wall 31 and the inner surfaces of the four surrounding walls 47 that constitute the element case 49, i.e., at locations along the bottom wall 31 and the surrounding walls 47. Any material can be used as the radio wave absorbing member 11 as long as it can absorb reflected waves that are reflected from parts other than the interface 74 of the target 72 shown in FIG. 3 out of the radio waves transmitted from the transmitter 43T of the sensor IC 41.

[0076] Specifically, examples of the radio wave absorbing material include a radio wave absorbing material that uses the principle of dielectric loss, a radio wave absorbing material that uses the principle of magnetic loss, and a radio wave absorbing material that uses the principle of reflection loss. Examples of the radio wave absorber used in the radio wave absorbing material include a magnetic radio wave absorber, a dielectric radio wave absorber, and a conductive radio wave absorber.

[0077] A radio wave absorbing sheet can be suitably used as the illustrated radio wave absorbing member 11. In the radio wave absorbing member 11 in the form of a radio wave absorbing sheet, radio waves entering the sheet from the front surface are attenuated as they pass through the sheet in the thickness direction, are reflected by the back surface of the sheet, and are attenuated again as they pass through the sheet in the thickness direction from the back surface to the front surface, and part of the radio waves pass through the front surface of the sheet while the remainder is reflected by the front surface, and are attenuated as they pass through the sheet in the thickness direction, repeating this process.

[0078] It is necessary to take measures on the back side of the sheet so that radio waves can be reliably reflected. As described above, when the element case 49 is made of a conductor such as aluminum, reflection can be achieved without any problems by simply attaching the radio wave absorbing sheet directly to the element case 49. On the other hand, when the element case 49 is made of resin, it is necessary to reliably reflect radio waves by attaching a sheet-like conductor, i.e., a shielding material, to the back side of the radio wave absorbing sheet. A radio wave absorbing sheet integrated with a sheet-like conductor can also be attached to the element case 49 made of a conductor together with the sheet-like conductor.

[0079] Examples of materials for the radio wave absorbing member 11 include a silicone base material with a silicon carbide filler, a ceramic material with silicon carbide and sintered, a silicone grease with silicon carbide to form a paste, and a silicone adhesive with silicon carbide to form a coating agent.

[0080] In detail, it is desirable that the radio wave absorbing member 11 has a low relative dielectric constant. A low relative dielectric constant can effectively prevent radio waves that are about to enter the radio wave absorbing member 11 from being reflected on the surface of the radio wave absorbing member 11. For this purpose, it is preferable that the radio wave absorbing member 11 is actually made of a low dielectric constant material whose relative dielectric constant is 2 to 3. In addition, it is desirable that the radio wave absorbing member 11 has a high dielectric tangent. A high dielectric tangent can effectively absorb and reduce radio waves that pass through the radio wave absorbing member 11, i.e., the radio wave absorbing sheet, i.e., reflected waves that become stray signal waves.

[0081] From this perspective, the above-mentioned silicone-based materials and ceramics are materials and substrates with low relative dielectric constants, while silicon carbide is a material with a high dielectric tangent.

[0082] Examples of materials with low relative dielectric constant include polypropylene, silicone, epoxy, urethane, chloroprene, and ceramics. Examples of materials with high dielectric tangent include ferrite, silicon carbide, and titanium oxide. As in the above example, a preferred embodiment is one in which a filler made of a material with a higher dielectric tangent than the material with low relative dielectric constant is mixed into a base material made of a material with low relative dielectric constant.

[0083] The sheet-like conductor attached to the back surface of the radio wave absorbing sheet is preferably made of aluminum foil, copper foil, carbon fiber, etc. By being made of aluminum foil, copper foil, carbon fiber, etc., it can flexibly conform to the shape of the element case 49.

[0084] FIG. 10 is a graph showing the intensity of the reflected wave that became a stray signal wave and the intensity of the reflected wave from the interface of the object. In FIG. 10, the horizontal axis represents the distance from the level meter 10, and the vertical axis represents the intensity of the signal detected by the level meter 10. In the figure, the solid line shows an example of a detection signal when the sensor unit 16 includes the horn 46, the waveguide 45, and the element case 49, as in the level meter 10 configured as shown in FIG. 9, but does not include the radio wave absorbing member 11. In the figure, the dashed line shows an example of a detection signal when the sensor unit 16 does not include any of the horn 46, the waveguide 45, the element case 49, and the radio wave absorbing member 11. As shown in the figure, the sensor unit 16 includes the horn 46, the waveguide 45, and the element case 49, and thus the signal intensity is higher than when these are not included.

[0085] In FIG. 10, the pulse-like detection signal 21 on the right side represents the detection signal for the surface of the detection target, i.e., the interface. The portion in FIG. 10 where the signal intensity is high represents the detection signal 22 resulting from detection of a reflected wave from other than the interface of the target, i.e., a reflected wave that has become a stray signal wave. As shown in the figure, the signal intensity of the detection signal 22 of the reflected wave that has become a stray signal wave increases as the distance to the vertical axis, i.e., the level meter 10, approaches. The signal intensity of the detection signal 22 of the reflected wave that has become a stray signal wave is higher than the signal intensity of the detection signal 21 of the interface at a distance position that is somewhat closer to the level meter 10. In the region where the signal intensity of the detection signal 22 of the reflected wave that has become a stray signal wave is higher than that of the detection signal 21 of the interface, the detection signal 21 of the interface is buried in the detection signal 22 of the reflected wave that has become a stray signal and therefore cannot be detected any more. In other words, when the interface of the target approaches the level meter 10 to a certain degree, it is no longer possible to detect the level of the interface.

[0086] The signal strength of detection signal 21 for the interface increases as the interface approaches level meter 10. This is because the closer the interface is to level meter 10, the higher the strength of the reflected wave from the interface when received by level meter 10. Furthermore, the lower the relative dielectric constant of the object to be detected, the lower the signal strength of detection signal 21 for the interface. The relative dielectric constant of water is approximately 80, while the relative dielectric constant of cotton seed oil is 3.1. For this reason, the intensity of detection signal 21 of the reflected wave from the interface is lower when detecting the level of cotton seed oil in level meter 10 than when detecting the level of water.

[0087] FIG. 11 is a graph showing that the detection performance of the level of the interface of the detection target object at a short distance from the level meter 10 is improved by attaching the radio wave absorbing member 11 to the element case 49 as shown in FIG. 9. As in FIG. 10, the horizontal axis represents the distance from the level meter 10, and the vertical axis represents the strength of the signal detected by the level meter 10. In the figure, the signal strength represented by the dashed line is the same as the signal strength represented by the solid line in FIG. 10. That is, the dashed line in FIG. 11 shows an example of a detection signal in the case where the sensor unit 16 includes the horn 46, the waveguide 45, and the element case 49, but does not include the radio wave absorbing member 11, as in the case of the level meter 10 configured as shown in FIG. 9. In the figure, the solid line shows an example of a detection signal for the level meter 10 configured as shown in FIG. 9. That is, an example of a detection signal in the case where the sensor unit 16 includes all of the horn 46, the waveguide 45, the element case 49, and the radio wave absorbing member 11 is shown.

[0088] 11, i.e., the detection signal when the radio wave absorbing member 11 is provided, is compared with the detection signal when the radio wave absorbing member 11 is not provided, and in a position distant from the level meter 10, i.e., a position where an interface of an object is present, there is not much difference between the intensity of the detection signal 23 of the solid line when the radio wave absorbing member 11 is provided and the intensity of the detection signal 21 of the dashed line when the radio wave absorbing member 11 is not provided. In detail, the difference between the intensity of the detection signal 23 of the solid line interface when the radio wave absorbing member 11 is provided and the intensity of the detection signal 21 of the dashed line interface when the radio wave absorbing member 11 is not provided is about 10 dB. In contrast, in a position close to the level meter 10, the intensity of the detection signal 13 of the solid line for the reflected wave that became a stray signal wave when the radio wave absorbing member 11 is provided is significantly lower than the intensity of the detection signal 22 of the dashed line for the reflected wave that became a stray signal wave when the radio wave absorbing member 11 is not provided. The difference in intensity reaches about 30 dB.

[0089] An arrow 25A in Fig. 11 indicates a change in the peak value of the detection signal 21 of the interface when the interface of the object approaches the level meter 10 in the case where the radio wave absorbing member 11 is not provided. An arrow 25 in Fig. 11 indicates a change in the peak value of the detection signal 21 of the interface when the interface of the object approaches the level meter 10 in the case where the radio wave absorbing member 11 is provided. The arrows 25A and 25 are inclined upward to the left in Fig. 11, which indicates that the intensity of the detection signal increases as the interface approaches the level meter 10 as described above. The distance from the level meter 10 corresponding to the point where the dashed detection signal 22 in the case where the radio wave absorbing member 11 is not provided and the arrow 25A intersect represents the detection limit short distance 36 of the level meter 10 in the case where the radio wave absorbing member 11 is not provided. The distance from the level meter 10 corresponding to the point where the solid detection signal 13 in the case where the radio wave absorbing member 11 is provided and the arrow 25 intersect represents the detection limit short distance 37 of the level meter 10 in the case where the radio wave absorbing member 11 is provided.

[0090] It is clearly shown that the detection limit short distance 37 of the level meter 10 when the radio wave absorbing member 11 is provided is significantly closer to the level meter 10 compared to the detection limit short distance 36 of the level meter 10 when the radio wave absorbing member 11 is not provided. In other words, when the radio wave absorbing member 11 is provided, the interface of the object can be detected at a distance closer to the level meter 10 compared to when the radio wave absorbing member 11 is not provided.

[0091] 11 also shows a detection signal 26 at the interface of another object having a lower relative dielectric constant than the object generating the detection signal 23 at the interface when the radio wave absorbing member 11 is provided. For simplicity's sake, the detection signal 26 represents both the detection signal when the radio wave absorbing member 11 is not provided and the detection signal when the radio wave absorbing member 11 is provided. However, since the peak value of the detection signal when the radio wave absorbing member 11 is not provided and the peak value of the detection signal when the radio wave absorbing member 11 is provided differ by about 10 dB as described above, only the state of this difference, i.e., the difference between the two peak values, is shown in FIG.

[0092] An arrow 27A in Fig. 11 indicates a change in the peak value of a detection signal 26 of the interface when the interface of the object approaches the level meter 10 in the case where the radio wave absorbing member 11 is not provided. The distance from the level meter 10 corresponding to the point where the detection signal 22 resulting from detection of a reflected wave that has become a stray signal wave intersects with the arrow 27A in the case where the radio wave absorbing member 11 is not provided represents the close detection limit distance 38 of the level meter 10 in the case where the radio wave absorbing member 11 is not provided. An arrow 27 in Fig. 11 indicates a change in the peak value of a detection signal 26 of the interface when the interface of the object approaches the level meter 10 in the case where the radio wave absorbing member 11 is provided. The distance from the level meter 10 corresponding to the point where the detection signal 13 in the solid line resulting from detection of a reflected wave that has become a stray signal wave intersects with the arrow 27 in the case where the radio wave absorbing member 11 is provided represents the close detection limit distance 39 of the level meter 10 in the case where the radio wave absorbing member 11 is provided.

[0093] The change in the peak value of the detection signal 26 when detecting the interface of another object having a low relative dielectric constant as described above has a lower signal strength than the change in the peak value of the detection signal 23 when detecting the interface of an object having a not low relative dielectric constant. However, even in this case, it is clearly shown that the close detection limit distance 39 of the level meter 10 when the radio wave absorbing member 11 is provided is significantly closer to the level meter 10 than the close detection limit distance 38 of the level meter 10 when the radio wave absorbing member 11 is not provided. In other words, when the radio wave absorbing member 11 is provided, even when detecting the interface of another object having a low relative dielectric constant, the interface of the object can be detected at a distance closer to the level meter 10 than when the radio wave absorbing member 11 is not provided. In other words, the detection stability at a distance closer to the level meter 10 is improved.

[0094] 11, based on the characteristics of the solid line detection signal 13 for the reflected wave that became a stray signal wave when the radio wave absorbing member 11 is provided and the dashed line detection signal 22 for the reflected wave that became a stray signal when the radio wave absorbing member 11 is not provided, at a position close to the level meter 10, the difference between the close detection limit distance 39 by the level meter 10 when the radio wave absorbing member 11 is provided and the close detection limit distance 38 by the level meter 10 when the radio wave absorbing member 11 is not provided is larger when detecting the interface of another object having a low relative dielectric constant as described above, compared to when detecting the interface of an object having a not low relative dielectric constant. In other words, when detecting the interface of another object having a low relative dielectric constant, the detection stability at distances close to the level meter 10, in other words the detection performance at short distances, is further improved compared to when detecting the interface of an object having a not low relative dielectric constant.

[0095] This is because, as shown in FIG. 11, the detection signals 13, 22 for the reflected waves that have become stray signal waves at a position close to the level meter 10 exist at a greater distance from the level meter 10 as the signal strength decreases.

[0096] Therefore, when detecting the level of oil, pellet-like resin, or the like stored in tank 70 as a container shown in FIG. 3, the level up to the vicinity of the top plate of tank 70 can be detected reliably.

[0097] The mounting of radio wave absorbing member 11 on horn 46 will now be described.

[0098] As described above, the intensity of the reflected wave from dielectric lens 48 is lower than the intensity of the reflected wave from element case 49. For this reason, even if level meter 10 is small and the reflected wave from dielectric lens 48 becomes a stray signal wave, so long as this does not substantially adversely affect detection of the reflected wave from the interface of the target by sensor IC 41, horn 46 may have an exposed metallic inner surface.

[0099] However, if the level meter 10 is not small, there is a high possibility that the reflected wave from the dielectric lens 48 will have a substantial adverse effect on the detection of the reflected wave from the interface of the target by the sensor IC 41. In such a case, as shown in Fig. 9, it is preferable to also attach the radio wave absorbing member 11 to the inner circumference of the horn 46 in a portion close to the surface 48A of the dielectric lens 48. For example, when the aperture of the dielectric lens 48 is 37 mm, the preferable range for attaching the radio wave absorbing member 11 is a range of 5 to 15 mm in the vertical direction from the dielectric lens 48. The sheet-like radio wave absorbing member 11 can be attached by sticking it to the inner circumference of the horn 46 with, for example, an acrylic double-sided adhesive tape.

[0100] If the radio wave absorbing member 11 is installed over too wide an area, the radio wave absorbing member 11 will absorb a large amount of the reflected waves from the interface of the object, making it impossible to accurately detect the level of the interface. From this point of view, it is desirable to consider the installation range of the radio wave absorbing member 11 depending on, for example, the magnitude of the S / N ratio.

[0101] 12 shows a detailed internal structure of the sensor unit 16 of the level meter 10 according to a second embodiment of the present invention. Here, a low dielectric constant material is used instead of a metal material for the mounting structure of the dielectric lens 48 to the metal casing 54 in the sensor unit 16, thereby preventing the generation of reflected waves from the mounting structure. Furthermore, the dielectric lens 48 is made of a specific material, which absorbs and reduces the reflected waves that pass through the dielectric lens 48.

[0102] The mounting structure of the dielectric lens 48 to the metal casing 54 will be described. In the level meter 10 of the first example shown in FIG. 9, an inner flange 55 is integrally formed at the lower end, i.e., the opening end, of the metal casing 54 of the sensor unit 16. In contrast, a pair of outer flanges 56, 56 are integrally formed along the direction of the optical axis 40A at the edge of the dielectric lens 48. The dielectric lens 48 is inserted downward into the casing 54 from the opening at the upper end of the casing 54, and the lower outer flange 56 abuts on the inner flange 55 of the casing 54, so that the dielectric lens 48 is positioned and fixed inside the casing 54. A packing 57 such as an O-ring is provided between the pair of outer flanges 56, 56 to seal between the dielectric lens 48 and the casing 54. However, in this configuration, the metal inner flange 55 abuts on the outer flange 56 of the dielectric lens 48, and there is a concern that a reflected wave that becomes a stray signal wave may be generated from the inner flange 55 of the metal casing 54.

[0103] 12, a second example of a level meter 10 is provided with a mounting member 58 made of a low dielectric constant material. The mounting member 58 hooks onto one outer flange 56 of the dielectric lens 48 and also hooks onto an appropriate location on the casing 54, thereby fixing the dielectric lens 48 in a positioned state relative to the casing 54.

[0104] With this configuration, the mounting member 58 made of a low dielectric constant material is in contact with the outer flange 56 of the dielectric lens 48, so that the reflected waves that become stray signals at the contact surface between the outer flange 56 of the dielectric lens 48 and the mounting member 58 can be reduced compared to the reflected waves that become stray signal waves at the contact surface between the outer flange 56 of the dielectric lens 48 and the metallic inner flange 55.

[0105] In order to reduce the generation of reflected waves from the dielectric lens 48 that become stray signal waves, it is preferable that the dielectric lens 48 is formed of a high dielectric loss tangent material. As the high dielectric loss tangent material for this purpose, polychlorotrifluoroethylene (PCTFE) resin is particularly preferably used. By forming the dielectric lens 48 from a high dielectric loss tangent material, radio waves passing through the dielectric lens 48 are effectively attenuated. In particular, polychlorotrifluoroethylene (PCTFE) resin is a high dielectric loss tangent material as well as a low dielectric constant material, and therefore it is possible to obtain the effect of reducing reflected waves that become stray signal waves by reducing the reflectance of radio waves.

[0106] By forming the dielectric lens 48 from a high dielectric tangent material, the original transmission wave directed toward the interface of the object and the original reflected detection wave reflected at the interface of the object and incident on the sensor unit 16 are slightly attenuated. However, compared to the disadvantage of attenuation, reducing the occurrence of reflected waves that become stray signal waves brings about better results in detection by the sensor unit 16.

[0107] In the sensor unit 16 of the level gauge 10 of the second embodiment of the present invention shown in FIG. 12, the cross-sectional shape of the dielectric lens 48 is suitable for reducing side lobes. In detail, the transmission wave incident on the dielectric lens 48 from the transmitter 43T of the sensor IC 41 and transmitted through the dielectric lens 48 includes a main lobe 61 passing through the central part of the dielectric lens 48 toward the interface of the object, and a side lobe 62 passing through the peripheral part of the dielectric lens 48 and diverging to the periphery of the level gauge 10. The side lobe 62 is a noise component with a low intensity caused by leakage or diffraction of the transmission wave, but it always occurs. The generated side lobe 62 becomes a reflected wave from a surrounding structure in the tank 70 to which the level gauge 10 is attached, as shown in FIG. 3. In other words, it causes a reflected wave that becomes a stray signal wave from an area close to the level gauge 10.

[0108] In order to reduce side lobes 62, dielectric lens 48 has surface 48A facing the inside of horn 46, which has a convex cross-sectional shape that protrudes toward waveguide 45. Furthermore, dielectric lens 48 has surface 48B facing the outside of level gauge 10 formed to be relatively flat compared to surface 48A facing the inside of horn 46. This is different from dielectric lens 48 of level gauge 10 of the first example shown in Fig. 9, in which surface 48A on the waveguide 45 side is a flat end and surface 48B facing the outside of level gauge 10 has a convex cross-sectional shape.

[0109] Fig. 13 shows the state of a wavefront 67 of a transmission wave Tx when the dielectric lens 48 shown in Fig. 12 is used. Fig. 14 shows the state of a wavefront 68 of a transmission wave Tx when the dielectric lens 48 shown in Fig. 9 is used. In Fig. 14, a side lobe 62 is clearly observed. In contrast, the occurrence of a side lobe is not observed in Fig. 13.

[0110] 12, it is possible to reduce the intensity of signals originating from other than the main lobe 61. This makes it possible to reduce the generation of reflected waves from areas close to the level meter 10, and to prevent reflected waves that cause stray signal waves from areas close to the level meter 10 from entering the level meter 10.

[0111] Although not shown in the figure, the surfaces 48A, 48B of the dielectric lens 48 are not smooth surfaces but, for example, have fine irregularities, which can reduce the generation of reflected waves from areas close to the level meter 10, which also cause stray signals.

[0112] By adopting the configuration shown in FIG. 12, particularly in a small level meter 10, it is possible to reduce the generation of reflected waves that cause stray signal waves without attaching radio wave absorbing material 11 to the inner circumference of horn 46 as described above.

[0113] FIG. 12 illustrates a compact level gauge 10. The compact level gauge 10 illustrated in FIG. 12 differs from the non-compact level gauge 10 illustrated in FIG. 1 and the like in that it does not include a display unit 20 or a rotation mechanism 19 for the display unit 20. Instead, in the compact level gauge 10 illustrated in FIG. 12, the display unit is separate from the level gauge 10 and is installed at a distance from the level gauge 10. FIG. 12 illustrates the compact level gauge 10 and a cable 69 for transmitting signals to the display unit (not illustrated). In the compact level gauge 10 illustrated in FIG. 12, only a status light 52 is provided.

[0114] In Fig. 11, the position on the vertical axis is the installation position of the level gauge 10. When the level gauge 10 is installed at the position of the top plate of the tank 70 as shown in Fig. 3, the position of the top plate of the tank 70 is the position on the vertical axis in Fig. 11. In contrast, a case where the level gauge 10 is moved and installed above the top plate of the tank 70 will be described below.

[0115] In this case, the top plate of the tank 70 is located at a distance 91 from the vertical axis, i.e., from the level gauge 10. At this distance 91, as shown in the figure, the detection signal 13 of the reflected wave, which causes a stray signal, is at a much lower level than the detection signals 23 and 26 of the interface.

[0116] That is, by changing the position of the level meter 10 to a position above the top plate of the tank 70, the detection limit close distance can be brought closer to the top plate of the tank 70 shown in Fig. 3. That is, in the tank 70, the level of the interface 74 up to a position near the top plate can be detected accurately without any hindrance. Therefore, for example, the change in the level of the interface 74 in a small tank 70 can be detected accurately over a wide range.

[0117] 15 and 16 show an example of an adapter 93 suitable for relocating or installing the level gauge 10 at a position above the top plate of the tank 70. The adapter 93 is formed in a cylindrical shape, and has a flange 94 at the bottom end for attaching the adapter 93 to the top plate of the tank 70. An internal thread 95 is formed in a penetrating state at the top end of the adapter 93 for fixing the level gauge 10 to the adapter 93 by screwing in the connection threaded portion 14 of the level gauge 10.

[0118] By providing the adapter 93, the sensor board 42, on which the sensor IC 41 with the receiving unit 43T is mounted, in the level gauge 10 is moved from the top plate of the tank 70, i.e., from the solid vertical axis drawn in Fig. 11 to the left in Fig. 11. The amount of movement corresponds to the height 96 of the adapter 93.

[0119] In each of the examples shown in Figs. 9 to 16, technical features of the other examples can be incorporated to the extent possible. [Explanation of symbols]

[0120] 10 Level gauge 11 16 Sensor section 17 Mounting part 18 Mounting screw part 40 Measurement end 41 Sensor IC 42 Sensor board 45 Waveguide 46 Horn 47 Enclosure Wall 48 Dielectric Lens 48A surface 48B surface 52 Status Light 54 Casing 58 Mounting material 59 Radio wave shaping materials 70 Tank 72 Object 74 Interface 93 Adapter Tx transmitter Rx Receiver

Claims

1. A substrate on which a transmitter and a receiver are mounted; a radio wave shaping member having an element case that covers a space including the transmitter and receiver mounted on the substrate, a waveguide having one end electromagnetically connected to the element case, and a horn whose radio wave path gradually expands from the other end of the waveguide toward the traveling direction of the transmitted radio wave; a dielectric lens provided at a tip of the horn for deflecting the radio wave passing through the radio wave shaping member from the transmitter toward an object; The level meter further comprises an electric wave absorbing member provided along an inner wall of the element case for absorbing electric waves.

2. 2. The level meter according to claim 1, wherein the radio wave absorbing member is provided along an inner wall of the horn.

3. a metal casing that houses the substrate, the radio wave shaping member, and the dielectric lens; 2. The level meter according to claim 1, further comprising a mounting member made of a dielectric material for mounting said dielectric lens to said metal casing in contact with said dielectric lens.

4. 2. The level meter according to claim 1, wherein the dielectric lens is formed of a lens material containing a low dielectric constant material having a relative dielectric constant of 2 to 3 and a high dielectric loss tangent material having a dielectric loss tangent higher than that of the low dielectric constant material.

5. 5. The level meter according to claim 4, wherein the high dielectric tangent material is polychlorotrifluoroethylene resin.

6. 2. The level meter according to claim 1, wherein the waveguide is formed of a waveguide tube and coaxially couples a transmission wave from the transmitter and a reception wave to the receiver.

7. The level meter according to claim 1 , wherein the dielectric lens has a convex cross-sectional shape that protrudes toward the waveguide.

8. A casing is provided to house a substrate, a radio wave shaping member, and a dielectric lens; The casing is capable of being attached to a container that stores a flowable substance; an adapter that is attached to the container and to which the casing can be attached; 2. The level gauge of claim 1, wherein when the casing is attached to the adapter, a first distance from a surface of the flowable material to the substrate is longer than a second distance from the surface of the flowable material to the substrate when the casing is attached to a container.

Images