JP2011002326A - Ultrasonic liquid level meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrasonic liquid level meter in which errors due to the plate thickness of a container bottom part hardly occur, when a liquid level is detected by an ultrasonic transducer mounted to the container bottom part.SOLUTION: When the ultrasonic transducer 2 is driven into a burst as changing its frequency to emit ultrasonic waves toward a liquid level via a bottom part of a container 30, extremum values periodically appear in detection information at each frequency generated in the ultrasonic transducer 2, with the driving corresponding to the appearance of plate-thickness natural frequencies of different orders. By taking advantage of this, it is possible to specify a plurality of plate-thickness natural frequencies of different orders of the bottom part of the container 30. When the height of a liquid level in the container 30 is computed, on the basis of received signals of liquid-level reflected waves RW at a driving frequency for liquid-level detection, by performing correction on the basis of a value of the plate thickness (t), it is possible to correctly specify the height of the liquid level at all times; even when the plate thickness of the container 30 has varied and reliability, for example, when a meter is read on the remaining quantity of gas in the container 30 is improved on the basis of the height of the liquid level.

Description

  The present invention relates to an ultrasonic liquid level gauge.

  Conventionally, an ultrasonic transducer is attached to the lower surface of the bottom of the container, a measurement ultrasonic wave is sent out toward the liquid level through the bottom of the container, and the propagation round-trip time until the ultrasonic wave for measurement is reflected back at the liquid surface position There is known an ultrasonic liquid level gauge that can know the height of the liquid level by measuring (Patent Documents 1 and 2).

Japanese Patent No. 3378231 Japanese Patent No. 4083038

  In the ultrasonic liquid level meter, in order to convert the round-trip propagation time obtained by measurement into actual liquid level information, the sound speed of the liquid needs to be specified accurately. However, since the speed of sound changes depending on the type of liquid (for example, the composition of the gas when the stored liquid is city gas or LPG) and the temperature, it becomes an error factor in liquid level measurement. In conventional ultrasonic level gauges, the liquid level is calculated and output from the propagation time under the condition that the sound speed is constant, and corrections for eliminating such measurement errors are performed. The surface height has been grasped by manual calculation on the user side, taking temperature and composition into consideration. However, such a thing has to be bothered and corrected to grasp the remaining amount of liquid, which is inconvenient and cumbersome, and the liquid level height (that is, the remaining amount of liquid in the container) is manually calculated. Even if it is grasped, it is a condition that the user knows the correction coefficient corresponding to the temperature, gas composition, etc., so it goes without saying that if the correction coefficient itself is unknown, it can not be handled in the first place. .

  The problem of the present invention is that when the liquid level is detected by an ultrasonic transducer attached to the bottom of the container, the measurement error due to various factors can be corrected spontaneously on the liquid level meter side by a simple method, and thus the measurement accuracy It is to provide an ultrasonic liquid level gauge capable of improving the above.

Means for Solving the Problems and Effects of the Invention

In order to solve the above problems, the ultrasonic liquid level meter of the present invention is
An ultrasonic transducer that is attached to the bottom of a container in a system to be measured that includes a container containing liquid, sends ultrasonic waves for measurement into the liquid via the bottom of the container, and receives reflected waves of the ultrasonic waves for measurement;
In the received waveform by the ultrasonic transducer, a reflected waveform specifying means for separating and specifying the main reflected waveform derived from the main lobe of the measurement ultrasonic wave and the side reflected waveform derived from the side lobe,
Liquid level position information generating means for generating information on the liquid level position in the container based on both the main reflection waveform and the side reflection waveform;
Liquid level position information output means for outputting the generated liquid level position information.

  The ultrasonic beam emitted from the ultrasonic transducer has a main lobe (main pole) with a high sound pressure generated in the normal direction of the transmission surface (that is, the central axis direction) and an obliquely expanding direction away from the normal direction. There is a side lobe (secondary pole) with a low sound pressure to be transmitted. In the conventional ultrasonic liquid level meter, only the main lobe is used, whereas in the present invention, the side lobe is also used, and the main reflection waveform derived from the main lobe and the side reflection waveform derived from the side lobe Since the information on the liquid level position in the container is generated based on both of the above, the accuracy of the liquid level detection can be improved as compared with the case where only the main lobe is used.

  The ultrasonic transducer can be attached to a position where it can directly receive the liquid surface reflected wave of the main lobe with respect to the back surface of the bottom of the plate-like container. By receiving the direct reflection waveform of the main lobe by the liquid level, the information of the direct reflection waveform can be used as the basic information of the liquid level position as in the conventional liquid level system. On the other hand, when the mounting form of the ultrasonic transducer is adopted, the side lobe is sent out so as to spread away from the main lobe toward the inner surface of the side wall, so that the following configuration can be adopted in consideration of this. .

  That is, a reflecting member that reflects the side lobe so that a reflected wave is incident on the inner surface of the side wall of the container toward the liquid surface while the reflected wave is incident on the bottom of the container away from the receiving surface of the ultrasonic transducer. Install. The reflected waveform specifying means uses the liquid surface reflected waveform of the main lobe as the main reflected waveform, and reflects the side lobe waveform that enters the bottom of the container and multi-reflects within the bottom of the container while reaching the receiving surface of the ultrasonic transducer. As specified. The main lobe transmitted from the ultrasonic transducer is reflected at a right angle on the liquid level and returns to the ultrasonic transducer without reflection, so the propagation path matches the normal direction of the transmission surface regardless of the liquid level. The propagation length varies in proportion to the liquid level. However, because the side lobe is tilted with respect to the normal direction of the transmission surface, if the liquid surface is used for reflection, the liquid surface and the inner surface of the container side wall and the bottom surface are repeatedly reflected, and the propagation path is It changes greatly according to the liquid level position. Therefore, in the above configuration, a reflecting member that gives a stationary reflecting surface to the side lobe regardless of the liquid surface position is arranged inside the container, and the reflecting position of the side lobe is fixed by the reflecting member. The reflection propagation path of the side lobe is not affected by the liquid surface position, and the liquid surface position calculation algorithm considering the side reflection waveform can be greatly simplified.

  Specifically, the liquid surface position information generating means includes a main lobe liquid surface propagation time measuring means for measuring the liquid surface reciprocating propagation time of the main lobe from the time when the measurement ultrasonic wave is transmitted until the main reflection waveform is detected. Further, it can be configured to have side lobe propagation time measuring means for measuring the propagation time of the side lobe from when the measurement ultrasonic wave is transmitted until the side reflection waveform is detected. Further, the liquid surface position information generating means can be configured to calculate the liquid surface height in the container as information on the liquid surface position based on both the liquid surface reciprocation propagation time of the main lobe and the propagation time of the side lobe. . Regardless of the liquid surface position, the reflection propagation path of the side lobe is made constant so that the propagation time of the side lobe is constant if there is no error factor in the measurement system. In other words, if a change occurs in the propagation time of the side lobe, the change reflects an error factor generated in the measurement system. According to the above configuration, it is possible to easily realize the calculation of the liquid level considering the error factor of the measurement system based on both the liquid surface reciprocation propagation time of the main lobe and the propagation time of the side lobe.

  More specifically, the liquid surface position information generating means includes a liquid surface height calculating means for calculating the liquid surface height in the container based on the liquid surface reciprocating propagation time of the main lobe, and the calculated liquid surface height. And a liquid level correction means for correcting the value based on the propagation time of the side lobe. By correcting the liquid surface height calculated from the liquid surface reciprocation propagation time of the main lobe based on the propagation time of the side lobe, the specific accuracy of the liquid surface height can be greatly improved by a simple method. .

  For example, the liquid surface position information generating means is calculated based on the sound speed calculating means for calculating the sound speed in the liquid based on the propagation time of the side lobe, the liquid surface reciprocation propagation time of the main lobe, and the propagation time of the side lobe. And a liquid level calculating means for calculating the liquid level in the container based on the sound speed. If the type of liquid is different or the liquid level detection temperature changes, the speed of sound in the liquid changes. If the propagation path of the side lobe is constant, the propagation time changes due to the change of the sound speed in the liquid, so that the sound speed in the liquid can be accurately identified from the measurement result of the propagation time of the side lobe. Then, using the identified sound velocity and the liquid surface reciprocation propagation time of the main lobe, the liquid surface height in the container can be accurately calculated. The speed of sound in the liquid can be easily specified by, for example, storing or holding in advance the relationship between the propagation time of the side lobe and the sound speed as propagation time / sound speed information.

  In this case, it is possible to provide a liquid type / sound speed correlation information acquisition unit for acquiring a liquid type / sound speed correlation information for specifying a correlation between the type of liquid that can be stored in the container and the sound speed of each liquid. The liquid type / sound velocity correlation information acquisition unit may be configured as, for example, a storage unit (memory) of the liquid type / sound velocity correlation information, or may be configured as a communication unit that acquires the liquid type / sound velocity correlation information by wireless or wired communication. May be. For example, when the liquid contained in the container is a mixture of a plurality of types of hydrocarbons such as city gas and LPG, the speed of sound in the liquid changes according to its composition. By acquiring and grasping it, it becomes possible to specify the composition of the liquid from the sound speed specified from the measurement result of the propagation time of the side lobe. Therefore, a liquid type specifying means for specifying the liquid type corresponding to the sound speed calculated by the sound speed calculating means with reference to the liquid type / sound speed correlation information, and a liquid type output means for outputting the specified liquid type are provided. Thus, when the type of liquid in the container is unknown, it can be reversely specified from the speed of sound obtained by calculation, and the specified result can be output.

  Next, in the ultrasonic liquid level meter of the present invention, the temperature measuring means for measuring the temperature of the liquid in the container and the sound speed calculated by the sound speed calculating means or the liquid level calculated by the liquid level calculating means are calculated. And a temperature correction means for correcting at the measured temperature. The sound speed of the liquid (and hence the liquid level calculated based on the speed of sound) varies depending on the temperature of the liquid. If the temperature of the liquid in the container can be determined by measurement, the sound speed and therefore the liquid level height can be more accurately determined. Can be corrected. In particular, even when the change in sound speed due to the type of liquid competes with the change in sound speed due to the temperature of the liquid, the temperature of the liquid is separately specified by measurement, so that only the change in sound speed due to the type of liquid is determined from the propagation time of the side lobe By specifying, the liquid level can be accurately obtained. In this case, the relationship between the side lobe propagation time and the sound speed (propagation time / sound speed information) is prepared for each of various temperatures, and the liquid level height is determined using the propagation time / sound speed information corresponding to the measured temperature. It is possible to accurately specify the speed of sound for calculation.

  The reflecting member can then be attached at the lowest liquid level position detectable by the main reflected waveform or at a lower position. The liquid surface position information generating means includes a minimum liquid surface determination means that determines that the liquid surface height is less than the minimum liquid surface position on condition that both the main reflection waveform and the side reflection waveform are not detected. Can be configured. When the liquid level position in the container is lowered, the propagation time of the main reflection waveform is shortened, the main reflection waveform is easily buried in the reverberation after the main lobe is cut off, and the accuracy of specifying the liquid level is lowered. However, if the reflecting member is provided at the lowest liquid level position or lower than that, even if the main reflected waveform cannot be detected, the liquid level is determined based on whether or not the side reflected waveform by the reflecting member is detected. It is possible to easily identify whether or not is below the lowest liquid level position.

  The ultrasonic liquid level meter of the present invention specifies the reception intensity of the side reflection waveform at each frequency by driving the ultrasonic transducer while changing the frequency within a predetermined band. It is possible to provide a drive frequency adjusting / setting means for finding a frequency at which the frequency is maximized and setting it as the drive frequency of the ultrasonic transducer. The transmission efficiency of the ultrasonic container bottom is the best when the ultrasonic frequency matches the plate thickness natural frequency of the container bottom, and conversely, the different plate thickness natural frequencies (adjacent plate thicknesses) adjacent to each other. When the frequency of the ultrasonic wave is located in the middle of the natural frequency), the influence of boundary reflection becomes the largest, and the transmission efficiency is minimized. This situation holds true for both the main lobe and the side lobe. Since the transmission intensity of the side lobe is smaller than that of the main lobe, when detecting the side reflection waveform, it is possible to increase the accuracy of specifying the side reflection waveform by driving the ultrasonic transducer by selecting the frequency at which the reception intensity is maximized. .

If the incident angle θ is defined with reference to the normal direction at the ultrasonic incident point on the inner surface of the bottom of the container, the wavelength of the longitudinal sound wave incident on the bottom of the container is λ, and the plate thickness of the bottom of the container is t. ,
t / cos θ = m · λ / 2 (1)
Or
λ = 2t / (m · cos θ) (1) ′
It is. Therefore, when the longitudinal wave sound velocity is C, thickness natural frequency f _m in the ultrasonic incident angle direction,
f _m = C / λ (2)
So from (1) '(2)
f _m = (m · cos θ) · C / 2t (3)
Here, m is an integer indicating the order of the natural vibration. If the incident angle of the main lobe to the container bottom is θ _m , and the incident angle of the side lobe to the container bottom is θ _s , the frequency f _mm at which the received intensity of the main reflected waveform is maximized is
f _mm = (m · cos θ _m ) · C / 2t (4)
The frequency f _ms at which the reception intensity of the side reflection waveform is maximized is
f _ms = (m · cos θ _s ) · C / 2t (5)
It is. In general, θ _m ≠ θ _s , and when detecting the main reflection waveform, the ultrasonic transducer is driven at f _mm , and when detecting the side reflection waveform, the ultrasonic transducer is driven at different f _ms . This is advantageous in improving the S / N ratio of each reflected waveform. For example, if θ _m = 90 ° and θ _s <90 °,
f _mm = m · C / 2t (4) ′
f _ms = (m · cos θ _s ) · C / 2t (5) ′
It becomes.

Specifically, with the natural frequency relating to the longitudinal wave vibration in the thickness direction of the container bottom as the plate thickness natural frequency, the ultrasonic transducer is different from the first plate thickness natural frequency (for example, f _mm above) and the second plate. Ultrasonic driving means for sequentially driving with the thickness natural frequency (for example, the above f _ms ) can be provided. The reflection waveform specifying means specifies only the main reflection waveform in the reception waveform when the ultrasonic transducer is driven at the first plate thickness natural frequency, and also in the reception waveform when the ultrasonic transducer is driven at the second plate thickness natural frequency. May be configured to specify only the side reflection waveform.

  On the other hand, the natural frequency related to the longitudinal wave vibration in the thickness direction of the bottom of the container is defined as the natural thickness of the plate, and the first natural thickness and the second natural frequency are different from each other. Ultrasonic drive means for driving the ultrasonic transducer with a composite drive waveform obtained by synthesizing the drive waveform and the drive waveform based on the second plate thickness natural frequency may be provided. The reflected waveform specifying means specifies the main reflected waveform from the waveform component of the first plate thickness natural frequency among the received waveforms obtained when the ultrasonic transducer is driven by the composite drive waveform, and the waveform of the second plate thickness natural frequency. The side reflection waveform is specified from the formation. In this way, it is possible to detect both the main reflection waveform and the side reflection waveform at the same time with the reception intensity optimized by driving the ultrasonic transducer once with the above composite drive waveform. It is possible to efficiently improve the S / N ratio of the waveform.

The schematic diagram which illustrates the attachment form of the ultrasonic level gauge of this invention. The block diagram which shows the electrical structure of the ultrasonic liquid level meter of FIG. The figure explaining the effect | action of the reflecting plate with respect to a side lobe, and the propagation path of the reflected wave. The figure explaining the relationship between the attachment position of a reflecting plate, and a liquid level position. The graph which shows an example of the temperature dependence of the sound speed of LPG. Explanatory drawing which shows an example of the measurement waveform of the ultrasonic liquid level meter of FIG.

  An embodiment of an ultrasonic liquid level gauge according to the present invention will be described with reference to the drawings. In FIG. 1, a container 30 contains a liquid such as liquefied gas such as LPG or LNG, or kerosene (in this embodiment, LPG), and the ultrasonic liquid level gauge 1 is a liquid level of the contained liquid. This is for measuring the height of 31 from the bottom of the container (and hence the amount of liquid stored). The ultrasonic liquid level gauge 1 has a control circuit 10 and an ultrasonic transducer 2 as main parts, and the ultrasonic transducer 2 is installed on the bottom lower surface of the container 30 by a magnet (not shown) and connected to the control circuit 10 via a cable C. Has been. The ultrasonic transducer 2 sends a measurement ultrasonic wave PW (main lobe) toward the liquid surface 31 via the bottom of the container, and receives a liquid surface reflection wave RW (main reflection wave) of the measurement ultrasonic wave.

  The container 30 is a steel tank, and as shown in FIG. 2, an ultrasonic transducer 2 composed of a piezoelectric element on the bottom surface thereof is interposed through an acoustic matching layer 3 and a resonance coupling suppression layer 4 made of a silicone sheet. It is attached. The ultrasonic liquid level meter 1 is configured so that the height of the liquid level 31 in the container 30 can be measured from outside the container by the ultrasonic transducer 2. FIG. 2 shows the electrical configuration of the control circuit 10 in the form of a block diagram. The ultrasonic transducer 2 includes a plate-like piezoelectric ceramic element 2P polarized in the thickness direction, and an electrode pair formed so as to face each other across the piezoelectric ceramic element 2P so as to cover each main surface of the piezoelectric ceramic element 2P. 2e, 2e. The electrode pairs 2e and 2e serve as driving electrodes to which a driving voltage for ultrasonically vibrating the piezoelectric ceramic element 2P is applied when transmitting the ultrasonic beam for measurement, and when the reflected ultrasonic waves are received, It becomes an output electrode which outputs an electric signal accompanying vibration.

The acoustic matching layer 3 is made of a material having an acoustic impedance density higher than that of a piezoelectric ceramic element (perovskite oxide: for example, barium titanate, lead titanate, lead zirconate titanate, lanthanum lead zirconate titanate). It is desirable to do. The acoustic impedance is defined as a value S · ρ · C obtained by multiplying the product of the material density ρ and the sound velocity C by the sound wave cross-sectional area S, and in particular, the product ρ · C of the density ρ and the sound velocity C is the specific acoustic impedance or sound. This is called impedance density. Examples of the material for the acoustic matching layer include aluminum or an alloy thereof (for example, an Al—Mg alloy, an Al—Mg—Cu alloy, an Al—Zn—Mg alloy, an Al—Li alloy, etc.), magnesium or an alloy thereof. Alloys (for example, Mg—Zn alloys, Mg—Al—Zn alloys, Mg—Zr alloys, etc.), titanium or alloys thereof, beryllium, stainless steel, and other metal materials, and harder than resonance coupling suppression layers It is particularly preferable to employ a resin material such as ABS resin. Among these, it is particularly desirable to employ a material (specifically, a material in which the value of C / (ρ · C) ^1/2 exceeds 1) in which the acoustic velocity C has a large contribution in the value of the acoustic impedance density. The value of C / (ρ · C) ^1/2 is more preferably 1.3 or more, and the corresponding materials are aluminum (1.54), magnesium (1.58), ABS resin (1.46). ), Glass (1.46), and beryllium (2.62).

  The ultrasonic beam emitted from the ultrasonic transducer 2 is inclined obliquely in a direction away from the normal direction and a main lobe ML (main pole) having a high sound pressure generated in the normal direction (that is, the central axis direction) of the transmission surface. There is a side lobe SL (secondary pole) with low sound pressure that is transmitted while spreading. In the present embodiment, the main lobe ML and the side lobe SL are used in combination, and information on the liquid level position in the container based on both the main reflection waveform derived from the main lobe ML and the side reflection waveform derived from the side lobe SL. Is generated.

  The ultrasonic transducer 2 is attached to a position at which the liquid surface reflected wave of the main lobe ML can be directly received with respect to the back surface of the plate-like container bottom 30B. While the main body 30M of the container 30 is formed in a cylindrical shape, the bottom 30B is formed in a semi-rotary ellipsoid shape that protrudes downward, and is welded and joined so as to close the lower opening of the main body 30M. The ultrasonic transducer 2 is attached to the center of the back surface of the container bottom 30B so that the normal passing through the center of the transmission surface (that is, the beam center line of the main lobe ML coincides with the center axis of the main body 30M. The main lobe ML sent from the ultrasonic transducer 2 passes through the container bottom 30B in the plate thickness direction and penetrates into the liquid, and is reflected at a right angle by the liquid surface 31, and the reflected wave is reflected on the transmission surface of the ultrasonic transducer 2. It comes back with no reflection.

  On the other hand, the side lobe SL is sent out so as to spread away from the main lobe ML toward the inner surface of the side wall. On the other hand, the main lobe ML toward the liquid surface 31 is not reflected on the inner surface of the side wall of the main body 30M of the container 30 (that is, the passage toward the liquid surface 31 is allowed), and the side lobe SL is reflected. (Reflection member) 30K is attached. The radiation angle of the side lobe SL and the size of the radiation surface of the ultrasonic transducer 2 and the length of the reflector 30K in the radial direction of the container are such that the reflected wave is incident on the container bottom 30B at a position off the receiving surface of the ultrasonic transducer 2. It is stipulated to do.

  The side lobe SL reflected by the reflecting plate 30K and incident on the bottom surface of the container bottom 30B having a spheroid curved surface is reflected on the bottom surface, and then follows the same propagation locus as that at the time of incidence, and again the ultrasonic transducer 2 And the sound wave received by the ultrasonic transducer 2 again while being reflected in the container bottom 30B, and these sound waves are reflected by the main lobe ML as shown in FIG. In addition to the main reflection waveform (A), the side reflection waveform (B) is detected.

As shown in FIG. 3, the total reflection critical angle θs of the side lobe SL incident on the container bottom 30B from the liquid is calculated from the ratio between the sound speed V1 of the liquid and the sound speed V2 in the metal (steel) forming the container bottom 30B. Can be prescribed. For example, when the incident angle of the sidelobe reflected wave by the reflector 30K to the container bottom 30B is θ1, and the refraction angle of the sound wave after the incidence is θ2, according to Snell's law,
sin θ2 / sin θ1 = V2 / V1 (6)
Or
θ2 = Sin ⁻¹ {sin θ1 · (V2 / V1)} (6) ′
It becomes. The critical angle θs is
sin θs = V1 / V2 (7)
For example, when V1 = 760 m / sec and V2 = 5900 m / s,
θs = 7.401 ° (8)
It becomes. Therefore, if the angle of the side lobe SL, the size of the radiation surface, and the length of the reflecting plate 30K in the radial direction of the container are defined so that the incident angle θ1 is smaller than the angle θs, a part of the side lobe SL is the container. A side reflection waveform that can be incident on the bottom 30B and propagates through the container bottom 30B through multiple reflections and reaches the ultrasonic transducer 2 can be generated. For example, when θ1 = 5 °, the refraction angle θ2 of the side lobe entering the container bottom 30B is calculated as 42.58 ° from (6).

  FIG. 2 shows the electrical configuration of the control circuit 10. The control circuit 10 includes a microcomputer 100, and the electrode pairs 2 e and 2 e of the ultrasonic transducer 2 are connected to the input / output unit 99 of the microcomputer 100 via the changeover switch 104 and the reflected ultrasonic measurement unit 103. The microcomputer 100 includes the input / output unit 99, a CPU 98 as a processing body, a RAM 96 as a work area, a control program (including drive control of the ultrasonic transducer 2 (including mode switching processing by the changeover switch 104), and a liquid level height. And a ROM 97 which stores the calculation processing control of Connected to the input / output unit 99 are a monitor 94 that displays and outputs the measured liquid level height, and a keyboard 95 (input unit) used for inputting the thickness value of the bottom of the container.

  In addition, a drive circuit 105 and a reception circuit 103 are connected to the ultrasonic transducer 2 via a change-over switch 104 that operates according to a command from the microcomputer 100. The drive circuit 105 receives a control pulse signal from the microcomputer 100, generates a burst pulse drive signal, and outputs the burst pulse drive signal to the ultrasonic transducer 2 via the changeover switch 104. The drive frequency can be changed according to the frequency setting value of the control pulse signal by the microcomputer 100.

  The reception circuit 103 includes a filter 103a for filtering unnecessary reverberation waves and the like from the reception signal, an amplifier 103b for amplifying the reception signal after filtering, a detection circuit 103c for detecting an envelope of the reception vibration waveform, and a reception signal after detection And the reverberation signal level and the liquid surface reflection signal level are specified, the trigger signal output from the microcomputer 100 is received in response to the output timing of the burst rectangular wave to the ultrasonic transducer 2, and the liquid level is activated. A signal processing unit 103d that measures the propagation time until the reflected signal is received is included.

  In measurement, the changeover switch 104 connects the ultrasonic transducer 2 to the burst rectangular wave drive circuit 105 in response to a switch control signal from the microcomputer 100, and the measurement ultrasonic waves (main lobe and side lobe) are set at a predetermined drive frequency. ) Is transmitted as a burst rectangular wave. Since the details of the driving operation are well known from Patent Documents 1 and 2, detailed description thereof is omitted. After the measurement ultrasonic wave is transmitted, the changeover switch 104 switches the connection state of the ultrasonic transducer 2 to the receiving circuit 103 side in accordance with a switch control signal from the microcomputer 100. Thereby, the reflected wave at the liquid level of the measurement ultrasonic wave PW is received by the ultrasonic transducer 2.

  The reception circuit 103 includes a filter 103a that filters unnecessary reverberation waves and the like from the received signal, an amplifier 103b that amplifies the filtered received signal, a detection circuit 103c that detects an envelope of the received vibration waveform, and analyzes the received signal after detection The reverberation signal level and the liquid surface reflection signal level are specified, and the trigger signal output from the microcomputer 100 is activated in response to the output timing of the burst rectangular wave to the ultrasonic transducer 2, and the liquid surface reflection signal is activated. Signal processing section 103d that measures the round-trip propagation time (in FIG. 1, the total propagation time of incident wave PW and reflected wave RW).

  As shown in FIG. 6, when the ultrasonic transducer 2 is burst-driven with a drive pulse having a preset liquid level detection drive frequency fa and the drive pulse is cut off, the ultrasonic transducer 2 is detected. A reverberation signal from the container bottom 30B appears in the output (this can be excluded from the measurement by setting the mask time tmx on the microcomputer 100 side).

As shown in FIG. 3, by providing the reflector 30K inside the container 30, the received waveform of the ultrasonic transducer 2 is reflected in the echo signal (main echo transmitted by the main lobe ML transmitted through the container bottom 30 and reflected by the liquid surface 31). In addition to the reflected waveform (A)), a reflected echo signal (side reflected waveform (B)) of the side lobe SL by the reflector 30K appears. The time t1 from the start of driving the ultrasonic transducer 2 until the main reflected waveform (A) is detected is the reciprocating propagation time to the liquid surface of the main lobe ML, and basically this reciprocating propagation time. 1/2 of the value obtained by dividing the speed of sound in the liquid is calculated as information reflecting the distance from the ultrasonic emission surface of the ultrasonic transducer 2 to the liquid surface (that is, the liquid surface height). That is, if the round-trip propagation time is t1, and the sound speed in the liquid is C, the liquid level height h is
h = (t1 / 2) · C
As calculated by the microcomputer 100.

On the other hand, the reciprocating propagation time of the main lobe ML becomes shorter as the height of the liquid surface 31 becomes lower (that is, as the remaining amount of liquid in the container 30 decreases), and is less than a certain minimum liquid surface height (FIG. 4: h _min ). Then, the main reflected waveform (A) is buried in the reverberation signal and cannot be detected. The position of the reflection plate 30K that forms the side reflection waveform (B) is a height corresponding to the minimum liquid level height h _min , specifically, the reflected wave of the side lobe SL by the reflection plate 30K is reverberant. It is set at a height that produces a minimum propagation time that is not buried in the signal.

By providing the reflecting plate 30K as described above, the appearance forms of the main reflection waveform (A) and the side reflection waveform (B) are as follows according to the position of the liquid surface 31.
(1) When the liquid level 31 is higher than the reflector 30K, both the side reflection waveform (B) and the main reflection waveform (A) are detected, and the side reflection waveform (B) is the main reflection. It always appears before the waveform (A). Further, since the intensity of the side lobe SL is lower than that of the main lobe ML, the detected intensity of the side reflection waveform (B) is lower than that of the main reflection waveform (A).
(2) When the liquid level 31 is at a lower position than the reflecting plate 30K, the side reflection waveform (B) and the main reflection waveform (A) are both buried in the reverberation signal and cannot be detected. In other words, when both the side reflection waveform (B) and the main reflection waveform (A) cannot be detected, it can be determined that the liquid surface 31 is at a lower position than the reflecting plate 30K.

Here, the ultrasonic transmittance with respect to the bottom of the container is maximized when the driving frequency fa coincides with the resonance point of the bottom of the container, and is minimized when coincident with the antiresonance point. Naturally, the detection level of the reflected echo is maximal. In the present invention, both the main lobe ML and the side lobe SL having different transmission angles are used. As described above, the incident angle of the main lobe ML to the container bottom 30B is 90 ° and the side lobe SL to the container bottom 30B. When the incident angle is θ _s , the frequency f _mm (first plate thickness natural frequency) at which the received intensity of the main reflected waveform is maximized is
f _mm = m · C / 2t (4) ′
The frequency f _ms (second plate thickness natural frequency) at which the reception intensity of the side reflection waveform is maximized is
f _ms = (m · cos θ _s ) · C / 2t (5) ′
Thus, the two are basically inconsistent. For example, considering that the intensity of the side lobe SL is lower than that of the main lobe ML, if the drive frequency fa is matched with the frequency f _{ms at} which the reception intensity of the side reflection waveform is maximized, the S of the side reflection waveform / N ratio can be increased. In this case, prior to the measurement of the liquid level, the ultrasonic transducer 2 is burst driven while variously changing the frequency of the control pulse signal to the control circuit 105 output from the microcomputer 100 of FIG. Accordingly, the detection intensity of the side reflection waveform at each frequency generated in the ultrasonic transducer 2 is specified. Then, the natural frequency at which the detected intensity is maximized is set as the drive frequency fa (see FIG. 6).

  As shown in FIG. 4, by providing the reflecting plate 30 </ b> K in the container 30, the reflection propagation path of the side lobe SL is fixed regardless of the position of the liquid surface 31. Therefore, if there is no error factor in the measurement system, the propagation time of the side lobe SL is constant. Conversely, if a change occurs in the propagation time of the side lobe SL, the change reflects an error factor generated in the measurement system. The error factors are mainly the temperature and composition of the liquid. As for the former, it is well known that the sound velocity in the medium changes with temperature. For example, as shown in FIG. 2, the temperature detection output from the temperature detection element (for example, thermistor) attached to the bottom of the container is supplied to the amplifier circuit 9. It is possible to correct the sound speed for calculating the liquid level by referring to the temperature detection input on the microcomputer 100 side.

  However, when the reflected wave of the side lobe SL is to be used for liquid level measurement, the following problem occurs with respect to the temperature correction of the sound velocity. That is, as shown in FIG. 3, the reflection propagation path of the side lobe SL includes an in-liquid propagation portion and a propagation portion at the bottom of the metal container. Naturally, since the speed of sound and the temperature dependence thereof are different between the liquid and the metal, it is necessary to specify the ratio of both propagation portions in the reflection propagation path of the side lobe SL. However, there is a certain spread in the transmission angle of the side lobe SL and the reflection propagation path, and it is generally difficult to accurately specify the ratio of each propagation part by calculation.

  On the other hand, the influence of the liquid composition on the speed of sound will be considered. LPG (liquefied petroleum gas), as commonly called “propane gas”, has propane as its main component, but often contains hydrocarbon components other than propane, particularly butane as the second component. The composition ratio of butane varies depending on the LPG supplier (particularly due to the production area), and the butane composition ratio of LPG that is generally distributed often has a distribution in the range of up to about 30% of the whole. Naturally, the speed of sound propagating through the LPG also varies depending on the butane composition ratio.

Therefore, if the ultrasonic transducer 2 attached to the container 30 measures the propagation time by the main lobe ML under conditions in which the temperature, the liquid composition, and the liquid level height (that is, the liquid amount) are known in advance, The speed of sound in the liquid at the liquid composition and temperature can be estimated. Further, the propagation time of the side lobe SL generated at this time is measured together. If such measurement is performed for each of various liquid compositions at a first temperature T ₀ (eg, 0 ° C.), the set of the sound speed and the propagation time of the side lobe SL for each liquid composition at the first temperature T ₀ is obtained. Obtained as a measurement result. Further, by performing the same measurement at a first temperature T ₀ second temperature T _a which is different from the, said second temperature T acoustic velocity of each liquid composition of _a and side lobe SL propagation times set is obtained. Such a set of measurement results is stored in the ROM 97 of FIG. 2 as a composition / sound velocity map (liquid type / sound velocity correlation information). Table 1 shows an example of the composition / sound velocity map. The first temperature T ₀ and direct reading possible propagation time in a different second temperature T _a is (t1, t2, t3 ‥) is able to be crowded combined with each other, in terms of performing the following interpolation calculation smoothly desirable.

If such a composition / sound velocity map is used, the liquid level in the container can be measured as described below in an improved form. That is, in FIG. 2, the microcomputer 100 switches the changeover switch to the drive circuit 105 side according to the switch control signal, so that the ultrasonic transducer 2 is connected to the drive circuit 105. Then, the frequency f _{ms at} which the reception intensity of the side reflection waveform is maximized is set as a driving frequency, and the ultrasonic transducer 2 is driven in bursts to transmit measurement ultrasonic waves (main lobe + side lobe). Next, the changeover switch is switched to the reflected ultrasonic measurement unit 103 side by a switch control signal, and the ultrasonic transducer 2 is connected to the reflected ultrasonic measurement unit 103. Thereby, the reflected waveform as shown in the lower part of FIG. 6 derived from the ultrasonic waves for measurement is received by the ultrasonic transducer 2. In the reflected ultrasonic measurement unit 103, only the waveform component in the vicinity of the drive frequency is extracted by the band pass filter 103a, amplified by the amplifier 103b, and then input to the signal processing unit 103d through the detection circuit 103c.

  Since the aforementioned reverberation signal appears at the beginning of the received waveform, it is not detected by signal masking. Then, after canceling the signal masking, the waveform that appears first is the side reflection waveform B, and a zero cross point (first zero cross point) of a predetermined order (for example, third) of the waveform is detected by the signal processing unit 103d in FIG. The detection signal is input to the microcomputer 100. In the microcomputer 100, the time from when the burst drive of the ultrasonic transducer 2 is cut off until the first zero cross point is detected is specified as the propagation time tr of the side lobe SL.

  Subsequently, the main reflected waveform A having a larger amplitude than the side reflected waveform appears in the received waveform. The signal processing unit 103 d also detects a zero cross point (second first zero cross point) of the main reflected waveform A, and inputs the detection signal to the microcomputer 100. In the microcomputer 100, the time from when the burst drive of the ultrasonic transducer 2 is cut off until the second zero cross point is detected is specified as the propagation time tx of the main lobe SL. The liquid temperature T detected by the temperature detection element 8 is also specified from the input value.

In the composition / sound velocity map in Table 1, only two levels of Ta = 20 ° C. and T0 = 0 ° C. were used for the temperature. However, the composition / sound velocity map after the third temperature is used together, or the composition It is also possible to use a two-dimensional map of the sound speed with parameters of temperature and temperature (for example, the temperature is set to about 2 ° C. and the butane composition ratio α is set to about 5%). It is also possible to store an approximate expression instead of a map and perform the calculation. For example, when the butane composition ratio α is 0% (that is, propane 100%), the approximate expression representing the sound velocity V ₁₀₀ is:
V ₁₀₀ ≈−6.6631T + 883.63 (m / sec) (9)
(A graph of this is shown in FIG. 5). Further, when the butane composition ratio α is 10% (that is, propane 90%), the approximate expression representing the sound velocity V ₉₀ is
V ₉₀ ≈−6.4945 · T + 900.04 (m / sec) (10)
Such an approximate expression may be prepared for each of various butane composition ratios (for example, in increments of 5%).

Receiving the result of the above, in the microcomputer 100, Table in one composition-sound speed map, measured sidelobes SL propagation time tr on each map temperature T ₀ and T _a sound speed V ₀ and V _a corresponding Find out. Then, using both acoustic velocity V ₀ and V _a, the sound velocity V corresponding to the detected fluid temperature T is calculated by linear interpolation. This speed of sound V is a value that has been corrected by both the liquid composition and the temperature. By dividing 1/2 of the propagation time tx of the main lobe SL by this value V, the final liquid level height is reduced. It is calculated and output to the monitor 94 in a form converted into the residual liquid amount in the container by a well-known method.

Further, the butane composition ratios α ₀ and α _a corresponding to the sound velocity V ₀ (temperature T ₀ ) and V _a (temperature T _a ) indicated by the propagation time tr of the side lobe SL are read from the composition / sound velocity map of Table 1. Can do. Similarly, the butane composition ratio α ₀ , α _a can be linearly interpolated with the ratio according to the measurement temperature, whereby the butane composition ratio α of the liquid in the container can be calculated. The calculation result of the butane composition ratio α is also output to the monitor 94.

The propagation time tr of the side lobe is Lc as its propagation path length.
tr = tc (Constant) + Lc / (2 · Vc) (11)
Can be expressed as tc represents the propagation time at the bottom of the container, but this reflection time reflected from the reflecting plate always includes this value and is common. Moreover, the temperature coefficient of the sound speed of iron forming the bottom of the container is about three orders of magnitude lower than the temperature coefficient of the sound speed of LPG, and the change in propagation speed due to the temperature in the metal can be ignored within the operating temperature range. For this reason, for example, when liquids with different blending ratios (propane: butane = 100−α: α) are measured at the same temperature with respect to the propagation time at 100% propane at 20 ° C., the propagation time t Since the sound speed Va is determined from _α = tc + Lc / (2 · Va), the butane composition ratio can be calculated from the sound speed Va from a map or approximate expression, and display output to the monitor 94 is possible.

Moreover, since the true speed of sound Va of the liquid is determined, it can be calculated accurate fluid volume by dividing the propagation time t _x of the main lobe at Va. For example, at a temperature Ta = 20 ° C., when the liquid butane composition ratio is α and the sound velocity is Va = Y [m / s],
t _X = tc + Lc / (2 · Y) (12)
So,
Y = Va = Lc / [2 · {t _X −tr + Lc / (2 · Vc)}] (13)
It becomes. The sound velocity Y is obtained from the above-described map or approximate expression, and the butane composition ratio α corresponding to Y (= Va) can be calculated.

In the above embodiment, the ultrasonic transducer 2 is burst-driven only once at the natural frequency at which the amplitude of the side reflection waveform is maximized. However, the ultrasonic transducer 2 receives the main reflection waveform. The frequency f _mm at which the intensity is maximized (the above-mentioned equation (4): first plate thickness natural frequency) and the frequency f _mS at which the reception intensity of the side reflection waveform is maximized (the above-mentioned equation (5): second) It may be configured to sequentially drive at the plate thickness natural frequency). In this case, only the main reflection waveform (FIG. 6: propagation time tx) is specified in the reception waveform when the ultrasonic transducer 2 is driven at f _mm , and the side in the reception waveform when it is also driven at f _mS . Only the reflected waveform (FIG. 6: propagation time tr) is specified. As a result, the S / N ratio of the main reflection waveform can be improved, and the liquid level can be measured with higher accuracy.

On the other hand, the ultrasonic transducer 2 maximizes the frequency f _mm (the above-described equation (4): first plate thickness natural frequency) at which the reception intensity of the main reflection waveform is maximized and the reception intensity of the side reflection waveform. The ultrasonic transducer 2 may be driven by a combined drive waveform obtained by combining the drive waveforms based on the frequency f _mS (the above-described equation (5): second plate thickness natural frequency). In this case, the main reflection waveform is specified from the waveform component by f _mm and the side reflection waveform is specified from the waveform component by f _mS among the reception waveforms obtained when the ultrasonic transducer 2 is driven by the composite drive waveform. In this way, both the main reflection waveform and the side reflection waveform can be detected simultaneously in a form in which the reception intensity is optimized by driving the ultrasonic transducer 2 once with the above composite drive waveform. It is possible to efficiently improve the S / N ratio of the reflected waveform. In FIG. 2, the second reflected ultrasonic measurement unit 103 ′ is added, and the reception intensity of the main reflected waveform is maximized in the pass band of the band pass filter 103 a included in the first reflected ultrasonic measurement unit 103. By adjusting to the frequency f _mm, and adjusting the pass band of the band pass filter 103′a included in the second reflected ultrasonic measurement unit 103 ′ to the frequency f _{mS at} which the reception intensity of the side reflection waveform is maximized, Each waveform is configured to be detected simultaneously.

1 Ultrasonic liquid level gauge 2 Ultrasonic transducer 8 Temperature detection element (temperature measurement means)
30 container 30K reflector (reflective member)
94 monitor (liquid level position information output means, liquid type output means)
100 microcomputer (reflected waveform specifying means, main lobe liquid level propagation time measuring means, side lobe propagation time measuring means, sound speed calculating means, liquid level height calculating means, liquid type / sound speed correlation information acquiring means, liquid type specifying means, temperature Correction means, minimum liquid level judgment means, drive frequency adjustment / setting means)
105 Drive circuit (ultrasonic drive means)
200 containers

Claims (11)

An ultrasonic transducer that is attached to a container bottom in a system to be measured that includes a container that contains a liquid, sends ultrasonic waves for measurement into the liquid via the container bottom, and receives reflected waves of the ultrasonic waves for measurement; ,
In the received waveform by the ultrasonic transducer, a reflected waveform specifying means for separating and specifying a main reflected waveform derived from the main lobe of the measurement ultrasonic wave and a side reflected waveform derived from the side lobe;
Liquid level position information generating means for generating information on the liquid level position in the container based on both the main reflection waveform and the side reflection waveform;
Liquid level position information output means for outputting the generated liquid level position information;
An ultrasonic liquid level gauge comprising: The ultrasonic transducer is attached to a position where the liquid surface reflected wave of the main lobe can be directly received with respect to the back surface of the bottom of the plate-shaped container.
The side lobe is passed through the inner surface of the side wall of the container so that the reflected wave is incident on the bottom of the container away from the receiving surface of the ultrasonic transducer while passing the main lobe toward the liquid surface. Reflective reflective member is attached,
The reflected waveform specifying means uses the liquid surface reflection waveform of the main lobe as the main reflection waveform, enters the container bottom, and multi-reflects the inside of the container while reaching the receiving surface of the ultrasonic transducer. The ultrasonic liquid level meter according to claim 1, wherein a lobe waveform is specified as the side reflection waveform. The liquid surface position information generating means includes
A main lobe liquid surface propagation time measuring means for measuring the liquid surface reciprocation propagation time of the main lobe from the time when the measurement ultrasonic wave is transmitted to the time when the main reflection waveform is detected;
A side lobe propagation time measuring means for measuring the propagation time of the side lobe from sending out the measurement ultrasonic wave to detecting the side reflection waveform;
The liquid surface position information generating means calculates the liquid surface height in the container as information on the liquid surface position based on both the liquid surface reciprocating propagation time of the main lobe and the propagation time of the side lobe. The ultrasonic liquid level gauge according to claim 2, wherein   The liquid surface position information generating means includes a liquid surface height calculating means for calculating a liquid surface height in the container based on the liquid surface reciprocating propagation time of the main lobe, and the calculated liquid surface height The ultrasonic liquid level meter according to claim 3, further comprising a liquid level height correcting unit configured to correct based on a propagation time of the side lobe.   The liquid surface position information generating means includes a sound speed calculating means for calculating a sound speed in the liquid based on the propagation time of the side lobe, the liquid surface reciprocation propagation time of the main lobe, and the propagation time of the side lobe. The ultrasonic liquid level meter according to claim 3 or 4, further comprising a liquid level height calculating means for calculating a liquid level height in the container based on a sound speed calculated based on the sound speed. Liquid type / sound speed correlation information acquisition means for acquiring a liquid type / sound speed correlation information for specifying a correlation between the type of liquid that can be stored in the container and the sound speed of each liquid;
A liquid type specifying means for specifying the liquid type corresponding to the sound speed calculated by the sound speed calculating means with reference to the liquid type / sound speed correlation information;
The ultrasonic liquid level meter according to claim 5, further comprising a liquid type output unit that outputs the specified liquid type. Temperature measuring means for measuring the temperature of the liquid in the container;
The temperature correction means for correcting the sound speed calculated by the sound speed calculation means or the liquid level height calculated by the liquid level calculation means with the measured temperature. The ultrasonic level gauge as described. The reflecting member is attached to the lowest liquid level position detectable by the main reflected waveform or a position lower than that,
The liquid surface position information generation means determines that the liquid surface height is less than the minimum liquid surface position on condition that both the main reflection waveform and the side reflection waveform are not detected. The ultrasonic liquid level meter according to any one of claims 3 to 7, further comprising means.   By driving the ultrasonic transducer while changing the frequency within a predetermined band, the reception intensity of the side reflection waveform at each frequency is specified, and the frequency at which the reception intensity is maximized is found. The ultrasonic liquid level meter according to any one of claims 1 to 8, further comprising a driving frequency adjustment / setting unit that sets the driving frequency of the ultrasonic transducer. Ultrasonic drive for sequentially driving the ultrasonic transducer with a first plate thickness natural frequency and a second plate thickness natural frequency different from each other, with the natural frequency relating to the longitudinal wave vibration in the plate thickness direction of the container bottom as the plate thickness natural frequency. Means are provided,
The reflected waveform specifying means specifies only the main reflected waveform in the received waveform when the ultrasonic transducer is driven at the first plate thickness natural frequency, and is also driven at the second plate thickness natural frequency. The ultrasonic liquid level meter according to any one of claims 1 to 9, wherein only the side reflection waveform is specified in the received waveform. The natural frequency related to the longitudinal wave vibration in the thickness direction of the container bottom is defined as the thickness natural frequency, and a first plate thickness natural frequency and a second plate thickness natural frequency that are different from each other are determined, and according to the first plate thickness natural frequency. Ultrasonic drive means for driving the ultrasonic transducer by a composite drive waveform obtained by combining a drive waveform and a drive waveform by the second plate thickness natural frequency is provided,
The reflected waveform specifying means specifies the main reflected waveform from a waveform component based on the first plate thickness natural frequency among received waveforms obtained when the ultrasonic transducer is driven by the synthesized drive waveform, The ultrasonic liquid level meter according to any one of claims 1 to 9, wherein the side reflection waveform is specified from a waveform component based on a plate thickness natural frequency.