JP2009109296A - Ultrasonic liquid level meter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic liquid level meter of a simple configuration and low power consumption capable of measuring the height h of a liquid surface up to a level close unlimitedly to zero with high accuracy. <P>SOLUTION: The behavior of a stationary wave formed by a continuous wave WC which is frequency-swept is measured by a change in electric impedance of a piezoelectric element 2, and the height h of the liquid surface is detected based on an electric impedance frequency characteristic of the piezoelectric element 2 acquired by the measurement. Even when the height h is low, a fault of a reduction in S/N ratio caused by burying of a burst reflected wave into reverberation does not essentially occur. Even when the height h is small, moreover even when the height h is close unlimitedly to zero, the height can be measured with high accuracy. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an ultrasonic liquid level gauge.

JP 2004-53504 A JP 2002-90210 A

  Conventionally, an ultrasonic transducer (ultrasonic transmitter and receiver) is disposed in such a manner that the ultrasonic transmission surface or reception surface is exposed on the inner surface of the tank, and the ultrasonic beam is reflected at the liquid surface position until it returns. There is known an ultrasonic liquid level gauge that can know the liquid level height by measuring time (Patent Documents 1 and 2).

  In the configuration of the ultrasonic liquid level gauge described above, when the transducer is driven in bursts to generate ultrasonic waves, forced vibration due to the burst drive and reflected ultrasonic waves generated at the transducer / container boundary or the container / liquid boundary are generated. The vibration gradually attenuates while overlapping in time. However, when the liquid level is low, the reflected wave at the liquid level that should be detected is buried during the reverberation continuation period of the forced vibration or boundary reflection vibration where the attenuation does not proceed sufficiently. There is a problem that sufficient detection accuracy cannot be secured. The lower limit of the detectable liquid level can be even smaller depending on factors such as the material and thickness of the container, the coating condition, the measurement environment (especially the environmental temperature), the transducer mounting condition, etc. The deterioration in detection accuracy (when the amount of liquid in the container is small) becomes even more significant. Conventionally, in order to improve detection accuracy when the liquid level is low, complicated signal processing such as controlling the number of times of ultrasonic driving, changing the gain of the amplifier, or changing the comparator level is necessary, and the signal processing circuit and software are complicated. In addition, it was difficult to avoid an increase in circuit power consumption and an improvement effect commensurate with the cost.

  An object of the present invention is to provide an ultrasonic liquid level meter that has a simple configuration, has low power consumption, and can measure the liquid level with high accuracy to a level close to zero.

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
In a system to be measured comprising a container containing a liquid, an ultrasonic transmission element that is attached to the bottom of the container and that transmits measurement ultrasonic waves toward the liquid level inside the container;
Continuous wave drive means for driving the ultrasonic transmission element so that the ultrasonic waves for measurement are sent out as a continuous wave while sweeping the drive frequency in a predetermined range;
A standing wave consisting of a piezoelectric element that is excited by a standing wave vibration generated in the liquid by a traveling wave sent to the liquid surface of the continuous wave and a reflected wave from the liquid surface, and that changes the electrical impedance by the excitation. An excited element;
Impedance frequency characteristic measuring means for measuring a frequency characteristic profile of the electrical impedance by monitoring a change in the electrical impedance of the standing wave excited element accompanying the continuous wave sweep;
A liquid level calculation means for calculating the liquid level in the container based on the frequency characteristic profile obtained by the measurement,
And a liquid level information output means for outputting the liquid level information.

  In the ultrasonic liquid level meter according to the present invention, a continuous wave (continuous sine wave) is used as a measurement ultrasonic wave instead of a conventional pulsed burst wave. The output continuous wave from the ultrasonic transmission element is a traveling wave from the bottom surface of the container toward the liquid surface, and is reflected by the liquid surface to generate a reflected wave. The traveling wave and the reflected wave are combined to generate a standing wave in the liquid.

In this case, the incident wave is a measurement ultrasonic wave, and depending on whether the measurement ultrasonic wave satisfies the resonance condition (that is, the natural frequency condition) with respect to the liquid level height h, The amplitude changes. In particular,
2h = n · λ = n · C / f _n (n = 1, 2,...) (1)
When the relationship is satisfied, the resonance condition is satisfied and the amplitude of the standing wave is maximized.
2h = n · λ ′ + λ ′ / 2 = n · C / f _n + C / (2f _n ) (n = 1, 2,...) (2)
When this relationship is established, the amplitude of the standing wave is minimized. Here, n is the order of the natural vibration. Therefore, if a standing wave excited element composed of a piezoelectric element excited by this standing wave vibration is provided at the bottom of the container, the electrical impedance of the piezoelectric element changes according to the excitation level due to the standing wave. To do. Therefore, if the frequency characteristic profile of this electrical impedance is measured, the liquid level in the container can be calculated from the measurement result.

  According to the liquid level meter of the present invention, since the liquid level is detected from the standing wave behavior formed by the continuous wave, even if the liquid level is low as in the conventional method, the burst reflected wave becomes reverberant. Even if the liquid level height is small, even if the liquid level height is infinitely close to zero, the problem that the S / N ratio drops and the S / N ratio does not occur essentially occurs. It can measure with high accuracy.

  Even when the ultrasonic transmission element is being driven with a continuous wave output, the impedance measurement can be performed without any problem. Therefore, the ultrasonic transmission element can be constituted by a piezoelectric element that is also used as a standing wave excited element. As a result, the total number of elements constituting the measurement system can be reduced, and the apparatus can be simplified.

In the ultrasonic liquid level gauge of the present invention,
Burst wave driving means for driving the ultrasonic transmission element so that measurement ultrasonic waves are transmitted as burst waves at a predetermined drive frequency;
A reflected wave receiving element for receiving the reflected wave at the liquid level of the burst wave at the bottom of the container;
Propagation time measuring means for measuring the propagation time of the burst wave from the transmission to reception by the reflected wave receiver when the burst wave is transmitted;
Second liquid level height calculating means for calculating the liquid level height based on the measured propagation time of the burst wave;
The measurement mode of the liquid level is the first measurement mode that measures the liquid level height based on the frequency characteristic profile obtained using the continuous wave, and the first that measures the liquid level height by measuring the burst wave propagation time. Measurement mode switching means for switching between the two measurement modes can be provided.

  In the method in which the standing wave by the continuous wave as described above is detected by the change in the electrical impedance of the standing wave excited element and the liquid level is detected, the measurement can be performed when the liquid level is large. However, even when the burst drive method is used, if the liquid level is sufficiently large, there is no concern that the reflected wave at the liquid level will be buried in reverberation vibration, and the propagation length for detecting reflected ultrasonic waves will be longer. The liquid level detection accuracy can be secured high. Therefore, as described above, the measurement mode of the liquid level is changed to the first measurement mode for measuring the liquid level based on the frequency characteristic profile obtained using the continuous wave, and the liquid level by measuring the propagation time of the burst wave. By providing a measurement mode switching means that switches between the second measurement mode that measures the height, depending on the liquid level height to be measured, the measurement mode that can be measured with higher accuracy can be switched at any time. This can greatly improve the usability as a liquid level gauge.

  In this case, in the first measurement mode in which the liquid level height is measured by changing the electrical impedance of the standing wave excited element, it is possible to perform measurement when the liquid level is large although the accuracy is slightly reduced. First, when the liquid level height is measured (roughly) in the first measurement mode, and the liquid level height obtained in the first measurement mode exceeds a predetermined threshold liquid level height It is preferable to provide measurement control means for switching to the second measurement mode and performing re-measurement of the liquid level. In this case, the liquid level information output means can be configured to output the remeasured liquid level information (in the second measurement mode). In other words, when the liquid level height measured in the first measurement mode is smaller than the threshold liquid level height, the measurement in the second measurement mode is originally difficult, so the measurement result is used as the liquid level height information as it is. If the liquid level measured in the first measurement mode is larger than the threshold liquid level, remeasurement is performed in the second measurement mode to improve the accuracy. It becomes possible to output surface height information. The threshold liquid level can be changed at any time (for example, according to deterioration with time of the ultrasonic transmission / reception element).

  Hereinafter, the specific content of the method of calculating the liquid level height using the electrical impedance change of the standing wave excited element will be described. As is clear from the above-described equations (1) and (2), the excitation level (amplitude) of the standing wave, and the change in the electrical impedance of the standing wave excited element reflecting this, is expressed by the following equations (1) and (2). There is a close relationship with the wavelength λ (or frequency ν) of the condition measurement ultrasound shown, and the frequency when the liquid level height h is twice (2h) divisible by the wavelength λ of the continuous wave ((1)), Frequency when a fraction of λ / 2 occurs ((2): λ / 4 for the propagation length of one-way ultrasonic wave; that is, equivalent to a case where the standing wave becomes antinode at the liquid surface position at the fixed end) Corresponding to the above, there is a feature that a maximum value and a minimum value appear alternately. That is, the frequency characteristic profile measured by the electrical impedance frequency characteristic measuring means has a saw-toothed section in which the maximum value and the minimum value derived from the standing wave of the electrical impedance appear alternately. Accordingly, the liquid level calculation means can be configured to calculate the liquid level based on the frequency difference between the maximum values or the minimum values adjacent to each other in the sawtooth-shaped section. By finding a sawtooth section in the obtained frequency profile of the electrical impedance and calculating the frequency difference between adjacent maximum values or minimum values in the sawtooth section, the liquid level height can be calculated easily. be able to. Specifically, the liquid level height calculation means calculates the liquid level height h by regarding 1/2 of the reciprocal of the frequency difference as the propagation time tp between the measurement ultrasonic wave container bottom and the liquid level. can do.

  In this case, if there is no liquid in the container, the frequency characteristic profile measured by the standing wave excited element is changed to the standing wave excitation as there is no liquid to generate the standing wave. No special saw blade section will appear. Therefore, by providing a liquid presence / absence determination unit that performs liquid presence / absence determination in the container based on the presence / absence of the sawtooth-shaped section in the frequency characteristic profile, and a liquid presence / absence determination result output unit that outputs the liquid presence / absence determination result, Not only the information related to the liquid level, but also information related to whether or not the liquid itself exists in the container can be clearly obtained.

There are a large number of sets of frequencies f _{n + 1} and f _n (hereinafter also referred to as adjacent frequencies) corresponding to adjacent maximum and minimum values that form a sawtooth-shaped section, depending on the order n of the natural vibration, The liquid level height h can be measured by using any of them, but the measurement accuracy becomes higher as the electric impedance difference value between the maximum value and the minimum value adjacent to each other forming the sawtooth-shaped section increases. There is a tendency.

  Here, the measurement output of the change in the electrical impedance of the standing wave excited element that detects the standing wave vibration is greatly influenced by the electrical or mechanical resonance characteristics of the piezoelectric element constituting the standing wave excited element. The Specifically, in the sawtooth section appearing in the frequency profile of the electrical impedance of the standing wave excited element, in the vicinity of the electrical or mechanical resonance point, the electrical impedance between the adjacent local maximum and minimum values. A difference stabilization section in which the difference value is stabilized (with a relatively large value) appears. The liquid level height calculating means detects the difference stabilizing section, and is configured to identify a frequency difference for calculating the liquid level height in the difference stabilizing section. Measurement accuracy can be increased. Specifically, the liquid level calculation means calculates the liquid level by calculating the liquid level based on the frequency difference between the adjacent maximum / minimum values corresponding to the detected frequency center of the difference stabilization section. The surface height h can be detected with higher accuracy.

  In the sawtooth section, a plurality of difference stabilization sections with different electrical impedance difference values to be stabilized may be detected. The ultrasonic beam for measurement has directivity that maximizes the beam intensity in the normal direction (beam central axis direction) of the ultrasonic transmission surface of the ultrasonic transmission element, and the liquid surface to be measured is the beam central axis direction. When the ultrasonic wave is transmitted so as to be at right angles, the amplitude of the standing wave that is generated is also maximized in this direction (hereinafter referred to as the main standing wave). However, the measurement ultrasonic beam has a spread of a wave packet with a constant intensity in the direction inclined from the beam center axis direction, and the spread wave packet component is incident on the liquid surface with a slight inclination. Along with this, another standing wave also appears in the inclined direction (hereinafter referred to as a secondary standing wave). The sub-standing wave derived from the spread of the wave packet component has a longer propagation length than the main standing wave that appears in the beam center axis direction by the amount of the propagation path, resulting in a frequency different from that of the main standing wave. Appears in the region with an amplitude smaller than that of the main standing wave. Therefore, when a plurality of difference stabilization sections having different electrical impedance difference values to be stabilized are detected in the sawtooth section, the difference stabilization section having the largest electrical impedance difference value (derived from the main standing wave) is detected. It is more advantageous to calculate the liquid level at 1 in terms of improving the measurement accuracy of the liquid level.

  As described above, the measurement output of the change in the electrical impedance of the standing wave driven element is greatly influenced by the electrical or mechanical resonance characteristics of the piezoelectric elements constituting the standing wave driven element. In this case, the frequency characteristic profile measured by the impedance frequency characteristic measuring means specifically corresponds to the base minimum point at the frequency position corresponding to the electrical resonance point of the standing wave excited element, and also to the mechanical resonance point. Each having a S-shaped base profile that generates a base maximum point at each frequency position where the maximum value and the minimum value derived from the standing wave are smaller than the frequency interval between the base minimum point and the base maximum point. Sawtooth-like sections appearing at intervals are acquired as superimposed. The liquid level calculation means specifies the frequency difference for calculating the liquid level height in the difference constant section detected in the sawtooth section located between the base minimum point and the base maximum point. If constituted so, the liquid level can always be measured with high accuracy and stability.

  Further, the level of the electrical impedance difference value between the maximum value and the minimum value in the difference stabilization section is also affected by the mounting state of the standing wave excited element, and the mounting state of the standing wave excited element (for example, the bottom surface of the container) The degree of electrical impedance difference tends to decrease as the degree of contact (such as the degree of close contact) deteriorates with time. Therefore, if an attachment state determination means for determining the attachment state of the standing wave excited element and an attachment state determination output means for outputting the determination result based on the level of the electrical impedance difference value are provided, This can help in the maintenance of the sonic level gauge. Specifically, the attachment state determination means includes an initial value level storage means for storing an initial value of the electrical impedance difference value, and compares the current value of the electrical impedance difference value acquired with the liquid level measurement with the initial value. The attachment state can be determined based on the comparison result. By monitoring how much the current value of the electrical impedance difference value has deteriorated compared to the initial value, the current mounting state of the standing wave excited element can be accurately grasped.

  An embodiment of a liquid detection device according to the present invention will be described with reference to the drawings. FIG. 1 shows an example of a container to be measured, and is configured as a metal tank 190 that stores LPG or LNG as the liquid h. The metal tank 190 is made of steel, and an ultrasonic transducer 2 composed of a piezoelectric element is attached to the bottom surface of the metal tank 190 via the acoustic impedance matching layer 3 and the grease coating layer 4. The acoustic impedance matching layer 2P has an acoustic impedance intermediate between the ultrasonic transducer 2 (piezoelectric element) and the container side wall 200 (steel) (preferably the geometric average value of both), and is pressed against the container side wall 200. It is made of a flexible elastic material (for example, a silicone resin) that can be deformed following the contact with the outer surface. Reference numeral 200p denotes a container coating portion.

  The liquid detection device 1 is configured so that the ultrasonic transducer 2 can measure the height of the liquid level LV (LV ′) in the metal tank 190 from the outside of the tank. FIG. 2 is a block diagram showing an electrical configuration of the liquid detection device 1. The ultrasonic transducer 2 includes a piezoelectric ceramic diaphragm 2p polarized in the thickness direction, and an electrode pair formed so as to cover each main surface of the piezoelectric ceramic diaphragm 21 with the piezoelectric ceramic diaphragm 21 interposed therebetween. 2e, 2e. The electrode pairs 2e and 2e serve as driving electrodes to which a driving voltage for ultrasonically vibrating the piezoelectric ceramic diaphragm 21 is applied during transmission of the measurement ultrasonic beam, and the piezoelectric ceramic diaphragm when receiving reflected ultrasonic waves. It becomes an output electrode which outputs an electric signal accompanying the vibration of 21. It also serves as a measurement electrode for measuring the electrical impedance of the piezoelectric ceramic diaphragm 21 (piezoelectric element).

  The electrode pairs 2e and 2e of the ultrasonic transducer 2 are connected to the input / output unit 99 of the microcomputer 100 via the changeover switch 104, the continuous wave drive circuit 101, the impedance analyzer 102, and the reflected ultrasonic measurement unit 103. The microcomputer 100 includes the input / output unit 99, a CPU 98 serving as a processing main body, a RAM 96 serving as a work area thereof, a control program 97 (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 storing the calculation processing control of the height. The input / output unit 99 is connected to a monitor 94 that displays and outputs the measured liquid level height h, and a keyboard 95 that is used to input a change in threshold liquid level height hx, which will be described later.

  The continuous wave drive circuit 101 receives the analog frequency instruction voltage from the D / A converter 101c that converts the frequency instruction value from the microcomputer 100 into an analog frequency instruction voltage, and changes the output frequency. An oscillation circuit (here, VCO (Voltage Controlled Oscillator) 101b) and a continuous sine wave signal output (FIG. 3: upper stage) of the oscillation circuit 101b are amplified and used as a drive signal to the ultrasonic transducer 2 (piezoelectric ceramic diaphragm 21). In addition, a burst wave driving circuit 105 is provided separately, and receives a control pulse signal from the microcomputer 100 as shown in the middle and lower stages of FIG. And output as a drive signal to the ultrasonic transducer 2.

  The continuous wave drive circuit 101 is driven so that the ultrasonic waves for measurement are transmitted as a continuous wave (continuous sine wave) WC (FIG. 1) while sweeping the drive frequency within a predetermined range (that is, continuous wave). Functions as a driving means). The drive frequency is swept by changing the input frequency instruction value to the D / A converter 101c on the microcomputer 100 side. The burst wave drive circuit 105 functions as burst wave drive means for driving the ultrasonic transmission element 2 so that the measurement ultrasonic wave is transmitted as a burst wave WB (FIG. 1) at a predetermined drive frequency. To do.

The ultrasonic transducer 2 is used not only as an ultrasonic transmission element but also as each of the following elements.
Standing wave excited element: In FIG. 1, the standing wave is excited by receiving standing wave vibration generated in the liquid L by the traveling wave toward the liquid level LV of the continuous wave WC and the reflected wave by the liquid level LV, The electrical impedance is changed by the excitation.
Reflected wave receiving element: Receives the reflected wave at the liquid level LV of the burst wave WB at the bottom of the container 200.

  Further, the microcomputer 100 outputs from the microcomputer 100 the amplifier 103a that amplifies the reflected wave signal at the liquid level LV received by the ultrasonic transducer 2 and the burst wave output timing to the ultrasonic transducer 2. A propagation time measuring circuit 103 including a propagation time measuring unit 103b that starts when receiving a trigger signal and measures a propagation time until receiving a reflected wave signal is connected. This propagation time measuring circuit 103 constitutes a propagation time measuring means.

  The measurement mode of the liquid height h by the ultrasonic transducer 2 includes the first measurement mode in which the microcomputer 100 measures the liquid level height h based on the frequency characteristic profile obtained using the continuous wave WC, and the burst wave WB. It is possible to switch between the second measurement mode in which the liquid level height h is measured by the propagation time measurement (measurement mode switching means).

  In the first measurement mode, the changeover switch 104 operates by the switch control signal from the microcomputer 100 so that the continuous wave drive circuit 101 and the impedance analyzer 102 are connected to the ultrasonic transducer 2. The ultrasonic transducer (ultrasonic transmission element) 2 is driven by the continuous wave drive circuit 101 so as to send out a continuous sine wave WC while the drive frequency is swept within a predetermined range. The traveling wave of the continuous wave WC toward the liquid level LV is overlapped with the reflected wave from the liquid level LV to generate a standing wave in the liquid L. The ultrasonic transducer 2 is excited by receiving the standing wave vibration and changes the electrical impedance by the excitation. This electrical impedance is measured by the impedance analyzer 102 for each frequency and output as a digital value. The digital output of the electrical impedance is sampled by the microcomputer 100, and the set is stored as frequency characteristic profile data of the electrical impedance in the RAM 96 in association with the numerical value. In the microcomputer 100, the frequency characteristic profile data is analyzed, and the liquid level height h in the container 200 is calculated (liquid level height calculating means).

  This will be described in more detail below. The excitation level (amplitude) of the standing wave generated in the liquid by the continuous ultrasonic wave is a resonance in which twice the liquid level height h is an integral multiple of the wavelength λ, as shown in the above equations (1) and (2). It becomes maximum when the condition is satisfied, and becomes minimum when a fraction of λ / 4 occurs in the one-way propagation length. As a result, as shown in FIG. 6, the frequency characteristic profile measured by the electrical impedance frequency characteristic measuring unit has a saw-toothed section in which the maximum value and the minimum value derived from the standing wave of the electrical impedance appear alternately. (The upper part shows an overall view, and the lower part shows an enlarged view of the main part. If the horizontal axis is the wavelength λ, the maximum value and the minimum value are alternated in a λ / 2 period).

That is, a sawtooth section is found in the obtained frequency profile of the electrical impedance, and as shown in FIG. 7, the maximum frequencies f _{n + 1} and f _n adjacent to each other are read in the saw blade section, and the frequency difference Δf The liquid level can be calculated based on = f _{n + 1} −f _n . It is also possible to calculate the liquid level height based on the frequency difference Δf ′ = f _{n + 1} ′ −f _n ′ by reading the adjacent minimum frequency f _{n + 1} ′ and f _n ′.

  The sawtooth-shaped section is extracted by analyzing the frequency profile data by the microcomputer 100. That is, the electrical impedance value of each data point constituting the profile is sequentially compared while changing the frequency in one direction to identify the peak point (maximum point / minimum point), and the electrical impedance difference between adjacent maximum / minimum points. A frequency section in which is equal to or greater than a predetermined threshold can be specified as a sawtooth section.

The sound velocity C of the ultrasonic waves for measurement is the same regardless of the frequency. If the liquid level is h, the continuous wave round-trip propagation time ts
ts = 2h / C (3)
The relationship is established. At the frequency f _{n + 1 of the} equation (1) where the standing wave amplitude is maximized after _{n + 1} , the total wave number propagating within the time ts of the equation (3) is
n + 1 = f _{n + 1} · ts = f _{n + 1} · (2h / C) (4)
Similarly, at the frequency f _{n of the} equation (2) in which the standing wave amplitude is maximized after the _{n-th order} , the total wave number propagating within the time ts of the equation (3) is
n = f _n · ts = f _n · (2h / C) (5)
It is. (4) Since the wave number difference in equation (5) is 1,
(F _{n + 1} −f _n ) · ts = (f _{n + 1} −f _n ) · (2h / C) = 1 (6)
If this is transformed,
ts = 2h / C = (f _{n + 1} −f _n ) ⁻¹ (6) ′
That is,
h = (C / 2) · (f _{n + 1} −f _n ) ⁻¹ = C · {(1/2) · (f _{n + 1} −f _n ) ⁻¹ }
= C · tp (where tp≡ts / 2 = {(1/2) · (f _{n + 1} −f _n ) ⁻¹ }) (6) ”
That is, it is clear that the liquid level height h can be calculated by regarding 1/2 of the reciprocal of the frequency difference as the (one-way) propagation time tp of the measurement ultrasonic wave between the container bottom and the liquid level.

The liquid level height h can be calculated using any adjacent frequencies f _{n + 1} and f _n included in the sawtooth-shaped section, but the measurement accuracy itself is the electric impedance for the adjacent frequencies f _{n + 1} and f _n . The higher the difference value, the higher. For example, the measurement output of the electrical impedance change of the ultrasonic transducer (standing wave excited element) 2 is greatly influenced by the electrical or mechanical resonance characteristics of the piezoelectric element (piezoelectric ceramic) that forms the main part of the element. In the impedance frequency characteristic profile shown in FIG. 6, the base minimum point at the frequency position corresponding to the electrical resonance point of the ultrasonic transducer 2 is also at the frequency position corresponding to the mechanical resonance point (electrically anti-resonance point). Each has a sigmoid base profile that generates base maxima, and local maxima and minima derived from standing waves appear at frequency intervals smaller than the frequency interval between the base minima and base maxima. The saw-toothed section is superimposed on this. And the sawtooth-shaped section located between the base minimum point and the base maximum point is a difference constant section. The difference stabilization section can be specified as a section in which the electrical impedance difference between adjacent maximum / minimum points falls within a predetermined range.

Then, one of the adjacent frequencies f _{n + 1} and f _n selected within the difference stabilization interval (for example, the frequency difference between the adjacent maximum value / minimum value corresponding to the detected frequency center of the difference stabilization interval) : The frequency difference Δf _{n + 1} = f _{n + 1} −f _n for calculating the liquid level height h is calculated using f _{n + 1} ′ and f _n ′ corresponding to the minimum value). Alternatively, the average value of the frequency differences Δf _{j + 1} = f _{j + 1} −f _j related to the adjacent frequencies f _{j + 1} and f _j within the difference stabilization section;
(Δf _{j + 1} ) _m = (1 / N) · Σ (f _{j + 1} −f _j ) (N is the total number of adjacent frequencies)
It is also possible to use.

  In the sawtooth section of the impedance frequency characteristic profile shown in FIG. 6, a plurality of difference stabilization sections having different electrical impedance difference values to be stabilized, specifically, a section between the base minimum point and the base maximum point. There are three difference stabilizing sections, an additional section A that appears on the low frequency side of the base minimum point and an additional section B that appears on the high frequency side of the base maximum point (hereinafter also referred to as the main section). The ultrasonic WC beam for measurement has directivity that maximizes the beam intensity in the normal direction (beam central axis direction) of the ultrasonic transmission surface of the ultrasonic transmission element 2, and the beam central axis direction should be measured. When an ultrasonic wave is transmitted so as to be perpendicular to the liquid level LV, the amplitude of the standing wave generated is maximum in this direction (hereinafter, this is referred to as a main standing wave). However, the measurement ultrasonic WC beam has a spread of a wave packet having a constant intensity even in a direction inclined from the beam center axis direction, and the spread wave packet component is incident with a slight inclination with respect to the liquid level LV. Along with this, another standing wave also appears in the inclined direction (hereinafter referred to as a sub-standing wave). The sub-standing wave derived from the spread of the wave packet component has a longer propagation length than the main standing wave that appears in the beam center axis direction by the amount of the propagation path, resulting in a frequency different from that of the main standing wave. In this region, it appears with an amplitude smaller than that of the main standing wave (if this is used, the measurement error of the liquid level increases as the propagation length increases). Therefore, when a plurality of difference stabilizing sections are detected as described above, it is advantageous to improve the measurement accuracy to calculate the liquid level height h in the main section derived from the main standing wave. .

  As shown in FIG. 8, if the liquid L does not exist in the container 200 (FIG. 1), the liquid is to be generated by the standing wave excited element as the liquid to generate the standing wave does not exist. In the frequency characteristic profile, a saw-toothed section peculiar to standing wave excitation does not appear. Therefore, the presence / absence of liquid in the container can be determined based on the presence / absence of a sawtooth-shaped section in the frequency characteristic profile. Specifically, it can be determined that there is no liquid if the electrical impedance difference between adjacent local maximum / minimum points is uniformly less than the threshold on the frequency characteristic profile. The determination result is output to the monitor 94 in FIG.

6 to 8 are based on the absolute value | Z | (= (R ² + X ² ) ^1/2 : where R is a DC resistance and X is an AC reactance) measured by an impedance analyzer. A frequency characteristic profile is used. However, the change in the electrical impedance of the piezoelectric element due to the standing wave excitation is caused by reactance term X (= j (ωL− (1 / ωC): complex impedance display), in particular, the capacitive reactance term (j (1 / ωC)). Therefore, this can be replaced by the capacitance measurement of the ultrasonic transducer 2 (piezoelectric element) In this case, as shown in FIG. It is obtained as a frequency characteristic profile.

Next, in the second measurement mode, the changeover switch 104 operates in accordance with the switch control signal from the microcomputer 100 so that the burst wave driving circuit 105 and the propagation time measuring circuit 103 are connected to the ultrasonic transducer 2. The ultrasonic transducer (ultrasonic transmission element) 2 is driven by a burst wave drive circuit 105 so that measurement ultrasonic waves are transmitted as a burst wave WB at a predetermined drive frequency (Patent Documents 1 and 2). The detailed description is omitted. The reflected wave of the burst wave WB at the liquid level LV is received by the ultrasonic transducer 2. This received waveform is amplified again by the amplifier 103a of the propagation time measuring circuit 103, and a zero-cross point at a predetermined rank is detected by a known zero-cross point detection circuit included in the propagation time measuring unit 103b. The propagation time measurement unit 103b measures the time from the trigger signal output timing of the burst wave to the detection timing of the zero cross point as the round-trip propagation time ts to the liquid level of the burst wave WB. If the speed of sound is C, the liquid level height h is
h = (ts / 2) · C (7)
(Second liquid level calculation means).

FIG. 5 is a graph showing the relationship between the liquid level height h and the frequency difference Δf = (f _{n + 1} −f _n ) ⁻¹ used for the calculation in the first measurement mode. It is clear that the frequency difference Δf decreases as the liquid level height h increases. In other words, the liquid level height measurement in the first measurement mode is more advantageous in terms of ensuring the measurement accuracy as the liquid level height h is smaller, and the measurement accuracy is better in the second measurement mode at a liquid level height above a certain level. It becomes easy to secure. Therefore, the measurement mode is automatically switched so that measurement in the first measurement mode is performed when the threshold liquid level height is less than hx, and measurement is performed in the first measurement mode when the threshold liquid level height hx is exceeded. It is convenient to do so.

  FIG. 12 shows the flow of measurement processing by the microcomputer 100 in that case. First, in S1, the first measurement mode is set, and the liquid level height h is roughened by sweeping the frequency of the continuous wave in such a manner that it is intermittently changed at a first frequency interval (for example, 2 to 10 kHz). To measure. In S2, it is determined whether or not the liquid level height h obtained in the rough first measurement mode exceeds a predetermined threshold liquid level height hx. If it is larger than the threshold liquid level height hx, the process proceeds to S3, where the measurement is switched to the second measurement mode and remeasured. If the result is h> hx, the re-measured liquid level height h (in the second measurement mode) is displayed and output as the final liquid level height (if h <hx in S4, Proceed to S5). When the liquid level height h measured in the first measurement mode is smaller than the threshold liquid level height hx, the measurement result h is displayed and output as it is because measurement in the second measurement mode is originally difficult. can do. However, in the present embodiment, the frequency of the continuous wave is swept in such a manner that it is intermittently changed at a second frequency interval (for example, 50 to 1000 Hz) smaller than the first frequency interval (for example, 2 to 10 kHz). The precision first measurement mode (S5) is performed, and the measurement result h of the precision measurement step S5 is displayed and output.

Next, the level of the electrical impedance difference value between the maximum value and the minimum value in the difference stabilization section is also affected by the state of attachment of the ultrasonic transducer 2. In particular, when the attachment state of the ultrasonic transducer 2 (for example, the degree of adhesion with the bottom surface of the container 200) deteriorates with time, the electrical impedance difference value also tends to decrease. Therefore, as shown in FIG. 10, the initial value ΔZ ₀ of this electrical impedance difference value (for example, the value at the time of shipment from the factory) is stored in the ROM 97 of the microcomputer 100 (initial value level storage means). the current value [Delta] Z _C of the obtained electrical impedance difference value compared to the initial value [Delta] Z _0, it is possible to perform attachment state determination based on the comparison result. Specifically, when ΔZ _C / ΔZ ₀ becomes smaller than a certain threshold value, it is determined that the mounting state is deteriorated and output to the monitor 94 or an alarm sound is output from a separately provided alarm output unit. This can be notified.

The schematic diagram which shows the principal part of the ultrasonic liquid level meter of this invention. The block diagram which shows the electrical structure of the ultrasonic liquid level meter of FIG. The schematic diagram which shows the form of the ultrasonic wave for a measurement. The figure which shows the concept of the measurement mode switching in the ultrasonic liquid level meter of FIG. The flowchart which shows the control flow of the microcomputer in the structure of FIG. The figure which shows the example of a measurement of the frequency characteristic profile of the electrical impedance in 1st measurement mode. The figure which expands and shows a part of FIG. The figure which shows the example of a measurement of the frequency characteristic profile of an electrical impedance concerning the case where a liquid does not exist. The figure which shows the example at the time of substituting the frequency characteristic profile of an electrical impedance with an electrostatic capacitance measurement result. The figure explaining a mode that the level of an electrical impedance difference value is influenced also by the attachment state of the ultrasonic transducer. The flowchart which shows the process example of the measurement mode switch by a microcomputer.

Explanation of symbols

1 Ultrasonic Level Meter 2 Ultrasonic Transducer (Ultrasonic Transmitting Element, Standing Wave Excited Element, Reflected Wave Receiving Element, Propagation Time Measuring Means)
94 Monitor (liquid level information output means)
100 microcomputer (liquid level height calculating means, second liquid level height calculating means, liquid presence / absence determining means, mounting state determining means)
101 Continuous wave drive circuit (continuous wave drive means)
102 Impedance analyzer (impedance frequency characteristic measuring means)
103 monitors (liquid level information output means, liquid presence / absence determination result output means, attachment state determination output means)
104 changeover switch (measurement mode changeover means)
105 Burst wave drive circuit (burst wave drive means)
200 container L liquid

Claims (13)

In the system to be measured consisting of a container containing a liquid, an ultrasonic transmission element that is attached to the bottom of the container and that transmits measurement ultrasonic waves toward the liquid surface inside the container;
Continuous wave drive means for driving the ultrasonic transmission element so that the measurement ultrasonic wave is transmitted as a continuous wave while sweeping a drive frequency in a predetermined range;
From a piezoelectric element that is excited by a standing wave vibration generated in the liquid by a traveling traveling wave of the continuous wave toward the liquid surface and a reflected wave from the liquid surface, and changes an electrical impedance by the excitation. Standing wave excited element,
Impedance frequency characteristic measuring means for measuring a frequency characteristic profile of the electric impedance by monitoring a change in electric impedance of the standing wave excited element accompanying the continuous wave sweep;
Liquid level height calculating means for calculating the liquid level height in the container based on the frequency characteristic profile obtained by the measurement,
Liquid level information output means for outputting liquid level information;
An ultrasonic liquid level gauge comprising:   The ultrasonic liquid level meter according to claim 1, wherein the ultrasonic transmission element is constituted by a piezoelectric element that is also used as the standing wave excited element. Burst wave drive means for driving the ultrasonic transmission element so that the measurement ultrasonic wave is transmitted as a burst wave at a predetermined drive frequency;
A reflected wave receiving element for receiving a reflected wave of the burst wave at the liquid surface at the bottom of the container;
Propagation time measuring means for measuring the propagation time of the burst wave from the transmission to reception by the reflected wave receiver when the burst wave is transmitted;
Second liquid level height calculating means for calculating the liquid level height based on the measured propagation time of the burst wave;
The measurement mode of the liquid level is a first measurement mode for measuring the liquid level based on the frequency characteristic profile obtained using the continuous wave, and the liquid level is measured by measuring the propagation time of the burst wave. Measurement mode switching means for switching between the second measurement mode for measuring the height;
The ultrasonic liquid level meter according to claim 1 or 2, wherein When the liquid level height is measured in the first measurement mode and the liquid level height obtained in the first measurement mode exceeds a predetermined threshold liquid level height, the second measurement is performed. Having a measurement control means for re-measuring the liquid level by switching to a mode;
4. The ultrasonic liquid level gauge according to claim 3, wherein the liquid level information output means outputs the remeasured liquid level information. The frequency characteristic profile measured by the electrical impedance frequency characteristic measuring means has a sawtooth-like section in which a maximum value and a minimum value derived from the standing wave of electrical impedance appear alternately,
5. The liquid level height calculation means calculates the liquid level height based on a frequency difference between maximum values or minimum values adjacent to each other in the sawtooth section. The ultrasonic liquid level meter according to item. Based on the presence or absence of the sawtooth-shaped section in the frequency characteristic profile, a liquid presence / absence determination unit that performs liquid presence / absence determination in the container;
A liquid presence / absence determination result output means for outputting the liquid presence / absence determination result;
The ultrasonic liquid level meter according to claim 5, wherein:   The liquid level calculation means calculates the liquid level by regarding 1/2 of the reciprocal of the frequency difference as a propagation time of the measurement ultrasonic wave between the container bottom and the liquid level. The ultrasonic liquid level meter according to claim 5 or 6.   The liquid level calculation means detects a difference stabilization section in which the electrical impedance difference value between the local maximum value and the local minimum value adjacent to each other in the sawtooth section is constant, and the difference level stabilization section The ultrasonic liquid level meter according to any one of claims 5 to 7, wherein the frequency difference for calculating the liquid level is specified.   The liquid level height calculating unit calculates the liquid level height based on a frequency difference between adjacent maximum values or adjacent minimum values corresponding to the detected frequency center of the difference stabilizing section. Ultrasonic level gauge.   The liquid surface height calculating means is configured to maintain a constant difference having the largest electrical impedance difference value when a plurality of the difference stabilizing sections differing in the electrical impedance difference value to be constant in the sawtooth section are detected. The ultrasonic liquid level meter according to claim 8 or 9, wherein the frequency difference for calculating the liquid level height is specified in a conversion section. The frequency characteristic profile measured by the impedance frequency characteristic measuring means has a base minimum point at a frequency position corresponding to an electrical resonance point of the standing wave excited element, and a base position at a frequency position corresponding to a mechanical resonance point. For the S-shaped base profiles that respectively generate the local maximum points, the local maximum value and the local minimum value derived from the standing wave appear at a frequency interval smaller than the frequency interval between the base local minimum point and the base local maximum point. The saw blade section is superimposed,
The liquid level calculation means calculates the liquid level in the difference constant section detected in a sawtooth section located between the base minimum point and the base maximum point. The ultrasonic liquid level gauge according to claim 10, wherein the frequency difference is specified. Based on the level of the electrical impedance difference value between the maximum value and the minimum value in the difference stabilization section, an attachment state determination unit that determines the attachment state of the standing wave excited element;
An attachment state determination output means for outputting the determination result;
The ultrasonic liquid level meter according to claim 8, comprising:   The attachment state determination means includes an initial value level storage means for storing an initial value of the electrical impedance difference value, and compares the current value of the electrical impedance difference value acquired with the liquid level measurement with the initial value. The ultrasonic liquid level gauge according to claim 12, wherein the attachment state determination is performed based on the comparison result.

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