JP7088532B2 - Level detector - Google Patents

Description

The present invention relates to a level detection device, and more particularly to a level detection device using microwaves.

Surface level detection of liquids and powders is required in various fields such as industrial production, agricultural production, wastewater treatment and water resource management. In particular, when the liquid or powder is stored in the tank, various level detection devices are used to detect the surface level of the liquid or powder in the tank.

For example, in Patent Document 1, a capacitance is detected by an electrode mounted in a storage container and protruding into the storage container, oscillated at a frequency inversely proportional to the capacitance, and based on the oscillation cycle. A capacitive level detection device that outputs a detection signal of the level position of the stored object is described.

Further, in Patent Document 2, a blade that swings around the center of swing is swung by a drive machine, and it is detected that the swing angle of the blade is equal to or less than a predetermined value, and the object to be measured that comes into contact with the blade. A pendulum type level detector equipped with a level detection circuit for detecting the level of the above is described.

Patent Document 3 describes an ultrasonic level sensor that emits ultrasonic waves from a probe fixed to the outer wall of the container to the inner surface of the container and detects the presence or absence of liquid by the difference in decay time between the emitted wave and the reflected wave. The device is described.

Japanese Unexamined Patent Publication No. 2006-322770 Japanese Unexamined Patent Publication No. 2006-145265 Japanese Unexamined Patent Publication No. 2004-28765

In the level detectors used in the past, in many cases, a part of the measuring instrument is installed in the tank. For example, in Patent Document 1, the electrode is arranged in the tank, and in Patent Document 2, the blade is installed in the tank. However, in order to maintain the strength and airtightness of the tank and to simplify the installation and maintenance of the device, it is preferable that the measurement can be performed from outside the tank. Especially for tanks that store corrosive substances such as acids such as sulfuric acid, hydrochloric acid, and fluoroacid and alkaline solutions, tanks that store toxic substances, and applications where contamination avoidance is strictly required in fields such as the food industry and pharmaceuticals. In the tanks used, level detection devices that can measure from outside the tank are desired from the viewpoint of protecting the measuring device, avoiding leakage of contents, and preventing foreign substances (outside air, dust, germs, etc.) from entering the tank. ing. The ultrasonic level sensor device of Patent Document 3 can determine the presence or absence of liquid from the outside of the tank, but when an outer wall (ceramic, etc.) or air, which is difficult for ultrasonic waves to propagate, intervenes, measurement by ultrasonic waves is performed. Has the problem of being difficult to apply.

An object of the present invention is to provide a level meter capable of measuring the surface level of a fluid or powder such as a liquid or a slurry inside the tank from the outside while maintaining the airtightness of the tank.

The level detection device of the present invention is a level detection device provided with a level sensor for detecting the level of the stored items by using microwaves on the outer wall of a tank for storing the stored items. In this level detector, the level sensor can be mounted exposed to the outside of the tank. According to this level detection device, by using microwaves, it is possible to detect the level of fluid (liquid, slurry) or powder inside the tank while maintaining the airtightness of the tank. Moreover, since the level sensor is exposed to the outside of the tank, maintenance and inspection of the level sensor is easy.

The level detection device receives the first signal level of the microwave reflected from the contents in the tank and received by the level sensor, and reflected from the inner surface of the tank or the reflecting plate provided in the tank to be received by the level sensor. It may be a level detection device configured to detect the presence / absence of the contained object at the measurement position based on the difference from the second signal level of the microwave. For example, the level detection device detects the output DC voltage of the microwave received by the level sensor by being reflected from the inner surface of the tank or the reflector when the tank is empty or when there is no stored object at the detection position. By comparing the value with the value detected at the time of measurement, the presence or absence of stored items at the measurement position may be detected.

The level detection device includes an oscillator in which the level sensor oscillates microwaves, a first antenna that transmits the microwaves to the outside of the main body, and a second antenna that receives the reflected microwaves. A level detection in which a gap is provided between the antenna surface including the front surface of the first and second antennas and the outer surface of the tank, and the size of the gap is set so as to maximize the signal level difference. It may be a device. For example, the level sensor includes a sensor positioning member installed on the outer wall of the tank and a sensor body installed on the positioning member, and the distance between the antenna surface of the sensor body and the tank outer wall is determined by the positioning member. It may be adjustable. According to the level detection device having such a configuration, the sensitivity of level detection can be optimized according to the type of stored items. Since the presence or absence of stored items at a predetermined level can be detected by a single sensor, the configuration for detection can be simplified.

The level detection device may be a level detection device having a reflector arranged inside the tank facing the level sensor and reflecting the microwave. This makes the tank compact because the reflector does not exist outside the tank. However, it may be a level detection device having a reflector arranged on the outer wall surface of the tank facing the level sensor and reflecting the microwave.

In the level detection device, a microwave Doppler sensor can be used as the level sensor.

According to the level detection device of the present invention, the level of liquid, slurry, powder, etc. can be detected while maintaining the airtightness of the tank. Since the level detection device of the present invention uses microwaves for detection, it is possible to detect the presence or absence of stored items even when air is present between the sensor and the stored items.

It is a schematic diagram which shows the structure of the level detection apparatus which concerns on one Embodiment of this invention. When calculated under the conditions shown in Table 1, it corresponds to the sensor displacement when the tank is empty (solid line) and when there is an item at the measurement position (broken line) and the output DC voltage and the difference (dotted line). It is a figure which shows the fluctuation. In Example 2, when the water in the acrylic tank is measured through an acrylic plate having a thickness of 20 mm, the measured values are when the tank is empty (solid line) and when there is a stored object at the measurement position (dashed line). It is a figure which shows the fluctuation corresponding to the sensor displacement of an output DC voltage and the difference (dotted line). It is sectional drawing (the side view about the sensor main body) which shows the arrangement state of the sensor of the level detection apparatus which concerns on one Embodiment of this invention. It is sectional drawing when the reflector is installed in the tank in the same structure as FIG. FIG. 3 is a cross-sectional view showing a case where a window made of a substance that transmits electromagnetic waves is provided on the wall of a tank that does not transmit electromagnetic waves, and a sensor similar to that in FIG. 4 is installed to measure a powdery stored object. It is a schematic diagram which shows one configuration example when the level detection apparatus of this invention is used for the level switch of a liquid tank. It is a figure which shows the time-dependent change of the sensor output when the level of the stored matter (ash) is changed in Example 4. FIG.

The level detection device of the present invention is a level detection device provided with a level sensor for detecting the level of the stored items by using microwaves on the outer wall of a tank for storing the stored items. Hereinafter, the level detection device according to the embodiment of the present invention will be described with reference to the drawings. In each drawing, the same reference numerals are given to the corresponding configurations, and the description thereof will be omitted.

FIG. 1 is a conceptual diagram showing a configuration of a level detection device according to an embodiment of the present invention. The level detection device 1 of the present embodiment is installed on the outer wall surface 20a of the wall 20 of the tank 2, sandwiching the level sensor 10 exposed to the outside of the tank wall 20 and the storage portion of the tank, and the above-mentioned level. A reflector (reflector) 13 installed on the outer wall surface of the tank is provided at a position facing the sensor 10.

The level sensor 10 includes a sensor positioning member (sensor guide) 12 installed on the tank outer wall 3 and a sensor main body 11 held by the sensor positioning member 12. A predetermined gap (air layer) G is provided between the antenna surface 11c on the front surface of the sensor main body 11, that is, the exit surface, and the outer wall surface 20a of the tank, and the sensor positioning member 12 sets the gap z between the gaps G. It is configured to be adjustable. The sensor main body 11 includes an oscillator 11d that generates high frequencies, an antenna 11a for transmission, an antenna 11b for reception, and a mixer 11f. The AC input A is supplied to the oscillator 11d, and the sensor output signal S is output from the mixer 11f.

In the configuration shown in FIG. 1, the microwaves transmitted from the transmitting antenna 11a are the tank outer wall surface 20a (or the sensor side wall surface of the sensor cover described later) and the tank wall 20 as shown by the solid line, the broken line, and the alternate long and short dash line in the figure. At the interface between the container 3 (or air) (tank wall inner surface 20b), the tank inner wall surface 20c facing the storage unit 21, and the reflector (reflector plate) 13 attached to the tank outer wall surface outside the tank. Each is reflected and received by the receiving antenna 11b. The output S from the sensor is a DC component of the product of the transmission voltage E _e (t) and the reception voltage _Er (t), and the frequency 2f component appearing in this product is removed by a filter (not shown). .. The output DC voltage _{Em when there is an item in the tank and the output DC voltage E 0 when} the tank is empty are expressed by the following equations (1) and (2), respectively.

In the above equation, f is the carrier frequency, E _e is the transmission voltage, E _ro is the reception voltage from the tank wall (or sensor cover), E _rc , E ^* _rc is the reception voltage from the stored items, and E ^* _rm is. Received voltage from the reflector, 2D is the antenna spacing, c ₀ is the speed of light, c _c is the speed at which microwaves pass through the tank wall, cm is the microwave speed in the container, lc is the thickness of the tank wall, l _T Is the thickness of the stored item (inner diameter of the tank).

As an example, when each parameter is set as shown in Table 1 below, the values of _Em (broken line) and E _o (solid line), and the difference between the two ΔE (dotted line) = Em _{−E 0} are displaced z. Is shown in FIG. 2 as a plot with the horizontal axis.

As seen in FIG. 2, the magnitude of ΔE can be adjusted by adjusting the gap z of the gap G, and the presence or absence of stored items is detected by the output signal S (DC voltage) from the sensor. Can be done.

The above formula describes the case where the reflector is installed on the outer wall surface of the tank, but when the reflector is not used, E ^* _rm may be set to 0. In the actual use of the device, the gap may be adjusted so that ΔE becomes maximum according to the stored items, and the device may be used as a level detection device. As an example, FIG. 3 shows an example of E ₀ (solid line), Em (broken line), and ΔE (dotted line) actually measured in Example 2 described later.

FIG. 4 shows a cross-sectional view of a portion where the level sensor 10 according to the present invention is installed in the tank (side view of the sensor body). In this embodiment, the tank wall 20 is made of a resin such as acrylic. The sensor positioning member 12 is fixed to the outer wall surface 20a of the tank. Fastening means 12a such as a bolt may be used for fixing. For example, a male screw 11e may be formed on a part of the outer surface of the sensor main body 11 and screwed into the female screw 12e formed on the positioning member 12. At that time, the distance z of the gap G between the antenna surface 11c and the outer wall surface 20a is adjusted according to the screwing depth so as to maximize the signal strength when detecting the signal strength of the stored object as the subject. ..

When the tank is made of a material such as resin or ceramic that allows electromagnetic waves (microwaves in the present invention) to pass through, a sensor may be installed outside the tank as in the above embodiment, but the tank emits electromagnetic waves. If it is made of a material that does not allow electromagnetic waves to pass through, an opening may be provided in the wall of the tank and the opening may be closed with a window made of a material that allows electromagnetic waves to pass through, or a sensor with a cover made of a material that allows electromagnetic waves to pass through may protrude into the tank. You may let me. That is, the level sensor 10 may be attached to the tank wall so as to be exposed to the outside from the tank wall 20. Since the level sensor 10 is exposed to the outside of the tank, maintenance and inspection of the level sensor can be facilitated, and the gap G can be adjusted according to the contents of the tank. However, if necessary, the antenna surface of the sensor may be arranged inside the tank. Further, although FIG. 4 shows a case where the level sensor and the reflector are installed on the outer wall surface of the tank, the reflector may be arranged inside the tank if necessary.

FIG. 5 shows a state in which the reflector 13 is installed in the tank. Further, as described above, the reflector is not an indispensable configuration, and in some cases, the reflector may not be used. If the relative permittivity of the stored items in the tank is relatively high, the stored items can be detected without a reflector. Further, when the tank wall is made of a material that does not allow electromagnetic waves to pass through, the reflection intensity when microwaves from the level sensor are reflected by the tank wall on the opposite side of the sensor is high, and a reflector is unnecessary.

FIG. 6 shows a state in which a window 21 made of a material (for example, resin) that allows electromagnetic waves to pass through is provided in an opening of a tank wall 20 made of a material that does not allow electromagnetic waves to pass through, and a level sensor 10 is attached. Here, the level sensor 10 has the same configuration as the level sensor shown in FIG. The electromagnetic wave (broken line) transmitted through the window 21 is reflected by the stored object 3 (powder) and received by the receiving antenna of the level sensor 10. When there is no storage object 3, the microwave reflected from the inner wall surface 20c on the opposite side of the level sensor 10 is received by the receiving antenna of the level sensor 10.

In each of the configurations shown in FIGS. 4 to 6, the output signal S from the level sensor 10 differs (except for a specific z value) between the case where there is a stored object and the case where there is no stored object, and the measurement is performed based on the difference. It is possible to determine the presence or absence of stored items at the site. That is, in each embodiment, the output voltage from the level sensor 10 with and without the stored items is measured, and the position of the sensor main body 11 is adjusted so that the voltage difference is maximized by the positioning member 12. , It may be used as a level detection device.

As the level sensor 10 used in the level detection device 1 of the present invention, a microwave Doppler sensor can be used. As a sensor equipped with a microwave transmitting antenna and a receiving antenna, a Doppler sensor has been developed for motion detection of automobiles and people.
The frequency of the microwave used may be, for example, 10.5 GHz to 10.55 GHz or 24.05 GHz to 24.25 GHz. Although not limited in terms of the configuration of the invention, in practice, it is necessary to use a frequency permitted by law, and in Japan, the above frequency is permitted for sensor applications. In countries where other frequency bands are allowed, different frequencies may be used.

The level detection device can also be used as a level switch for stopping or starting loading of stored items when the stored items in the tank reach a predetermined level.
FIG. 7 is a schematic cross-sectional view showing an example when the level detection device of the present invention is installed in the liquid tank 2. The upper part of the tank 2 is connected to the inflow path 5A via the first valve 4A, and the lower part is connected to the discharge path 5B via the second valve 4B. Two level detection devices 1A and 1B (in this case, sensors 10A and 10B) of the present invention are installed at predetermined positions on the tank wall 20, and the detection device and the valve are connected to the controller 6. In the detection devices 1A and 1B, the installation intervals 2D of the level sensors 10A and 10B are adjusted so as to maximize the sensitivity. For example, when the second valve 4B is opened to discharge the liquid 3 in the tank and the liquid level drops below the measurement level of the second detection device 1B, the controller 6 detects a change in the output voltage and the second Issue a command to close the valve 4B of 2. Next, the first valve 4A is opened, and the liquid 3 is replenished in the tank 2. When the liquid level exceeds the measurement level of the first detection device 4A, the controller 6 detects a change in the output voltage and closes the first valve 4A. In this way, the level of the stored object 3 in the tank 2 can be held so as to be between the sensors 10A and 10B.

[Example 1]
As the subject 1, foamed styrene (thickness 150 mm) having a foaming rate of 99% and a relative permittivity of 1.006, and as the subject 2, water (relative permittivity 80) is placed in an acrylic tank having a volume of 144 mm × 144 mm × 144 mm (width 150 mm). I prepared what I had stored. Further, as a reflector, a copper plate having an area of 365 mm × 365 mm and a thickness of 0.3 mm was installed at positions of 0.15 m, 0.30 m, 0.60 m, and 3.00 m from the outer surface of the expanded polystyrene on the sensor side or the outer wall surface of the acrylic tank. The experiment was conducted under three conditions: when the sensor was directly installed on the outer surface of Styrofoam or the outer wall surface of the acrylic tank, when a polytetrafluoroethylene cover having a thickness of 1 mm was provided, and when an acrylic cover having a thickness of 3 mm was provided. A microwave Doppler sensor (manufactured by Kansai Automation) with a frequency of 24.15 GHz is used as the sensor, and it depends on the output voltage when the antenna surface is moved to a position of 15 mm every 0.5 mm from the sensor tip surface and the presence or absence of stored items. The voltage difference was calculated. The results are shown in Table 2.

In Table 2, T1 uses 1 mm thick polytetrafluoroethylene as a sensor cover, A3 uses 3 mm thick acrylic, and T1A3 uses 1 mm thick polytetrafluoroethylene in front of the 3 mm thick acrylic sensor cover. When installed, A3a1A3 indicates a case where acrylic having a thickness of 3 mm is installed with a space of 1 mm (air layer) from 3 mm acrylic (tank wall).

In the case of water having a relatively high dielectric constant, the average value of the maximum value and the average value of the minimum value of the voltage difference do not change much depending on the position of the cover, the material of the tank wall surface, and the reflector. In addition, although not shown in the table, the presence of the substance could be confirmed even when the reflector was not used. It is considered that this is because most of the electromagnetic waves are reflected by the water as the subject. On the other hand, in the case of expanded polystyrene having a low dielectric constant, improvement in sensitivity is clearly recognized by installing a reflector at a position of 0.15 m near the sensor. The spatial period in which the maximum and minimum values of the voltage difference appear is 6.5 to 7 mm, which is close to the half wavelength of 6.2 mm at 24.15 (GHz). In this example, it can be seen that the search for the maximum sensitivity by adjusting the position of the gap G (FIG. 1) may be performed within an adjustment range of up to about 7 mm. Since the experiment using foamed styrol can be seen as a simulated experiment in the case of measuring a powder having a low relative permittivity, it can be seen that the level detection device of the present invention is also useful for measuring the powder.

[Example 2]
Water is stored in an acrylic tank (A3) having a thickness of 3 mm, and an acrylic having a thickness of 20 mm and / or a natural rubber (R20) having a thickness of 20 mm is laminated thereto, and the same sensor as in Example 1 is installed therein. A detection experiment was conducted from outside the tank. No reflector was used. The results are shown in Table 3. Further, as an example, FIG. 3 shows a graph of the case where measurement is performed through an acrylic plate having a thickness of 20 mm and a tank wall (A23). In FIG. 3, the solid line shows the output DC voltage when the tank is empty, the broken line shows the output DC voltage when there is water at the level detection position, and the dotted line shows the difference ΔE (the scale is different) between the two.

表３に見るように、タンク壁面の厚さと素材により、出力最大値は変化する。他方、ｚ＝５～１００ｍｍの範囲で、出力が最大値となるときに、空タンクとの出力差も最大になっていた。したがって、レベル計としての使用にあたっては、タンクに水があるときに出力最大となる条件に変位（ｚ値）を設定すればよい。

As shown in Table 3, the maximum output value changes depending on the thickness and material of the tank wall surface. On the other hand, in the range of z = 5 to 100 mm, when the output reached the maximum value, the output difference from the empty tank also became the maximum. Therefore, when using it as a level meter, the displacement (z value) may be set under the condition that the output is maximized when there is water in the tank.

[Example 3]
When a polyethylene resin tank specified in JISZ1710 was prepared and a microwave Doppler sensor was installed directly on the tank wall, measurement was performed via 20 mm acrylic resin (A20), and measurement was performed via natural rubber having a thickness of 20 mm. Case (R20), three layers (A3R20A20) of acrylic resin with a thickness of 3 mm, natural rubber with a thickness of 20 mm, and acrylic resin with a thickness of 20 mm are laminated on the tank wall, and a microwave Doppler sensor is installed on the outermost layer of each. , An experiment was conducted to detect kerosene in the tank. No reflector was used. The results are shown in Table 4. As shown in Table 4, a voltage difference was detected by adjusting the displacement z, although it depends on the conditions of the tank wall.

[Example 4]
As an example of the level sensor, an experiment was conducted to detect ash in a pottery brazier from outside the brazier. No reflector was used. The ash in the brazier was piled up above the sensor level 10 to 17 seconds after the start of the output measurement by the sensor, and the ash was scraped out to be lower than the sensor level within 34 to 44 seconds. The output level of the Doppler sensor during this period is shown in FIG. In this experiment, the measurement was done through a thick ceramic (brazier wall), and the ash level was not uniform in front of the sensor, so air was also present, but as seen in FIG. 8, the wall surface. Changes in the level of nearby ash were detected as changes in sensor output.

[Example 5]
In order to examine the effect of the reflector when detecting a subject with a low dielectric constant with a sensor, Sample 1 with a foaming rate of 99% and a relative permittivity of 1.007 and Sample 1 with a foaming rate of 97% and a relative permittivity of 1.015. Three types of copper reflectors of 30 mm x 27 mm, 30 mm x 11 mm, and 30 mm x 6 mm are installed with foamed styrol sandwiched between them at a distance of 25 mm from the antenna surface of the Doppler sensor, and when there is air between the antenna surface and the reflector. The displacement z of the sensor was adjusted so that the output became 0V, and the change in the output power from 0V when the sample was sandwiched between the antenna surface and the reflector was read. The results are shown in Table 5.

As shown in Table 5, it can be seen that by using the reflector, even a sample having a low relative permittivity can be used as a level detection device. From this result, it can be expected that even for a sample having a low relative permittivity such as powder, the level can be easily measured by using the reflector 13 shown in FIGS. 1 and 6, for example.

According to the level detection device of the present invention, the level of the fluid or powder in the closed tank can be suitably measured from outside the tank. As the sensor of the level detection device, a Doppler sensor having a high penetration rate can be used, and a simple and inexpensive configuration can be used.

1 Level detection device 10 Sensor 11 Sensor body 12 Positioning member 13 Reflector (reflector)
11a Output antenna 11b Receiving antenna 11c Antenna surface 11d High frequency oscillator 11e Thread 2 Tank 20 Tank wall 20a Tank wall outer surface 20b, 20c Tank wall inner surface 21 Window (cover)

Claims (4)

A level sensor that detects the level of the stored items by oscillating and receiving microwaves is provided on the outer wall of the tank that stores the stored items.
The level sensor is exposed and attached to the outside of the outer wall of the tank.
The first signal level of the microwave reflected from the contents in the tank and received by the level sensor, and the second signal of the microwave received by the level sensor reflected from the inner surface of the tank or the reflector provided on the tank. Based on the difference from the level
In a level detection device configured to detect the presence or absence of the stored object at the measurement position ,
The level sensor includes an oscillator that oscillates microwaves, a first antenna that emits the microwaves to the outside of the main body, and a second antenna that receives the reflected microwaves, and the first and second antennas. A level detection device in which a gap is provided between the antenna surface including the front surface of the tank and the outer surface of the tank, and the size of the gap is set so as to maximize the signal level difference . The level detection device according to claim 1 , wherein the level detection device includes a reflector arranged inside the tank so as to face the level sensor and reflect the microwave. The level detection device according to claim 1 , wherein the level detection device includes a reflector arranged on the outer wall surface of the tank so as to face the level sensor and reflect the microwave. The level detection device according to any one of claims 1 to 3 , wherein the level sensor is a microwave Doppler sensor.

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