JP2001108508A - Frequency type liquid level detecting method with ultrasonic wave and its device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily and surely detect liquid level independent of materials of a container, the kind of liquid, and temperatures of them by detecting the surface of the liquid contained in a container with an ultrasonic wave. SOLUTION: This invention is based on such new finding that a reflecting ultrasonic wave propagating through a no-liquid space part of the container is in a low frequency region of 0.3-0.6 MHz, and a reflecting ultrasonic wave propagating through liquid is in a high frequency region of 1.5-2.0 MHz. The frequency type liquid level detecting method relating to this invention is characterized that an ultrasonic pulse is applied to the outer surface of the container in which liquid is contained, reflecting waves of the ultrasonic pulse are received, and when the frequency of the reflected signal is in the low frequency region, judgment is made that the reflecting signal is propagated through the no-liquid space part in the container, and when the frequency is mainly in the high frequency region, judgment is made that the reflecting signal is propagated through the liquid in the container, and the position of the liquid surface is detected based on these judgments. A liquid level detecting device based on this method is also provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid level detecting method using ultrasonic waves, and more particularly, to a liquid level detecting method utilizing the fact that the ultrasonic waves propagating in liquid and in a non-liquid space have different frequencies. The present invention relates to a frequency-type liquid level detection method for detecting the liquid level of a liquid and a device therefor.

[0002]

2. Description of the Related Art Conventionally, there has been known a method shown in FIGS. 15 to 17 as a method for detecting a liquid surface position in a container in which a liquid such as a liquefied gas is sealed.

[0003] An ultrasonic transmitter / receiver 20 composed of a piezoelectric element has both functions of an ultrasonic transmitter and an ultrasonic receiver. The liquid 24 is stored in the metal tank 22, and the liquid surface 24a is at the position shown in the figure.

The ultrasonic transmitter / receiver 20 indicated by a dotted line is
a, that is, the position of the liquid-free space 26. When ultrasonic waves are transmitted at this position, the ultrasonic waves
Does not propagate, and travels around the peripheral wall of the container 22 with multiple reflections. The appearance of the peripheral wall multiple reflection is shown in FIG.

The ultrasonic receiver / transmitter 20 shown by a solid line
It is slightly lower than a. When an ultrasonic wave is transmitted at this position, most of the ultrasonic wave propagates into the liquid 24, and FIG.
The light is reflected back and forth as shown in FIG. In addition, the remaining small amount of ultrasonic waves is reflected multiple times on the peripheral wall of the container 22.

[0006] The speed of the ultrasonic wave that multiple-reflects the peripheral wall of the container is as high as about 6 km / s, and the time τ ₁ from transmission to reception is extremely short. On the other hand, the ultrasonic velocity transmitted through a liquid, for example, a liquefied gas is as low as about 1.5 km / s, and the reception time τ
₂ is slightly longer. Therefore, there has been proposed a method of detecting a position where the length of the reception time τ changes as a liquid level.

However, the ultrasonic waves transmitted through the peripheral wall and in the liquid repeat the cycle several times while the intensity is reduced, and are reflected not only in the shortest distance but also in many paths.
There is a drawback that noise is mixed in the reflected signal of the ultrasonic wave, which complicates the waveform and causes misreading of the waveform.

Therefore, in order to improve these disadvantages, the invention described in Japanese Patent Application Laid-Open No. 5-45201 has been proposed. The present invention is based on the premise that the reception times τ ₁ and τ ₂ of both ultrasonic waves are determined as long as the sound velocity in the peripheral wall and in the liquid is known. In other words, instead of determining the total length of the reflected signal,
A gate processing circuit is provided to extract only signals near the reception times τ ₁ , τ ₂ or an integer multiple thereof to accurately read the peripheral wall reflection signal and the liquid reflection signal.
In other words, this is a proposal for a technique for determining the liquid level based on whether or not there is a reflected signal at a position where it should appear without looking at noise.

[0009]

However, even with this improved invention, the reception times τ ₁ and τ ₂ become inaccurate unless the speed of sound is accurately determined. The sound wave propagating through the peripheral wall of the container is called a plate wave, and the speed of sound is not always taken to be the only value due to mode conversion between longitudinal wave and transverse wave.
Also, if the material of the container or the type of liquid changes, the speed of sound also changes. Further, if the temperature of the container or the liquid changes, the speed of sound propagating through them also changes naturally.

Therefore, since the sound speed changes depending on the measurement conditions, it is no exaggeration to say that the reception times τ ₁ and τ ₂ are different for each container to be measured. Even when gate processing is performed on these ultrasonic reflected signals, the gate time must be changed each time the values of the reception times τ ₁ and τ ₂ change. In other words, it is considered that the conventional signal detection method based on the knowledge of the speed of sound has no universality, and the fundamental disadvantage remains without being improved.

Accordingly, it is an object of the present invention to discover a new principle of detecting a reflected signal of an ultrasonic wave propagating through a peripheral wall and a liquid, and to provide a new liquid level detecting method applying the principle.

[0012]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks, and the invention of claim 1 is to transmit an ultrasonic pulse to the outer surface of a container containing a liquid, and The reflected wave of the sound wave pulse is received, and when the frequency of the reflected signal is in the low-frequency region, it is determined that the reflected signal has propagated through the non-liquid space portion in the container, and when the frequency is mainly in the high-frequency region, the liquid in the container is determined. A frequency type liquid level detection method is characterized in that it is determined that the liquid has propagated through the inside, and the position of the liquid level is detected from these determinations.

[0013] According to a second aspect of the present invention, the low frequency region is set to 0.1.
3 to 0.6 MHz, and the high frequency range is 1.5 to
2. The frequency type liquid level detecting method according to claim 1, wherein the frequency is 2.0 MHz.

According to a third aspect of the present invention, there is provided an ultrasonic transmitter for transmitting an ultrasonic pulse to an outer surface of a container containing a liquid, an ultrasonic receiver for receiving a reflected wave of the ultrasonic pulse, and a reflected signal. A high-pass filter that passes high-frequency signals, and a detector that detects high-frequency signals from the output signals of the high-pass filters, and detects the liquid level from changes in the presence or absence of high-frequency signals. It is a liquid level detection device.

According to a fourth aspect of the present invention, there is provided an ultrasonic transmitter for transmitting an ultrasonic pulse to an outer surface of a container containing a liquid, an ultrasonic receiver for receiving a reflected wave of the ultrasonic pulse, and a reflection signal for the ultrasonic wave. A low-pass filter that passes a low-frequency signal, and a detector that detects a low-frequency signal from an output signal of the low-pass filter, and detects a liquid level from a change in the strength of the low-frequency signal. This is a mold level detecting device.

According to a fifth aspect of the present invention, there is provided an ultrasonic transmitter for transmitting an ultrasonic pulse to an outer surface of a container containing a liquid, an ultrasonic receiver for receiving a reflected wave of the ultrasonic pulse, and a reflected signal. Is a frequency-type liquid level detection device comprising a Fourier transformer for converting the frequency level into a frequency distribution, and detecting the liquid level from the presence or absence of a high-frequency signal or a change in the strength of a low-frequency signal.

According to a sixth aspect of the present invention, the low-frequency signal is 0.1.
3 to 0.6 MHz, the high frequency signal is 1.5 to 2.0 M
6. The frequency type liquid level detecting device according to claim 3, which is in a frequency range of Hz.

[0018]

DETAILED DESCRIPTION OF THE INVENTION The present inventors have conducted intensive studies to discover the characteristics of ultrasonic waves propagating through the peripheral wall and liquid, and as a result, have found that there is a very characteristic difference in the frequency of ultrasonic waves. Was. That is, the ultrasonic wave propagating through the peripheral wall is 0.3 to
Ultrasonic waves propagating through a liquid have a high frequency of 1.5 to 2.0 MHz, whereas a low frequency of 0.6 MHz.

FIG. 1 is a reflection signal diagram of an ultrasonic wave propagating on the peripheral wall of an iron container. In other words, it is a reflection signal diagram of the channel 1 (Ch1) at the position of the liquid-free space portion 26 of the container 20, where 1 horizontal division is 100 μs and 1 vertical division is 100 mV. It can be seen that there are _four reflected pulse groups A _{1 to} A _4, which are gradually attenuated.

[0020] FIG. 2 is an enlarged view of a reflection signal A ₂ in FIG. 1. Ch1 signal is displayed and horizontal 1 division is 5
μs, 1 vertical division is 100 mV. Signal one
Since the period t, that is, the peak interval is 3.17 μs, the frequency f is f = 1 / 3.17 × 10 ⁻⁶ = 0.315
(MHz).

FIG. 3 is a reflection signal diagram of an ultrasonic wave propagating through the liquid propane contained in the container 22. However, 1MHz
The output signal of the high-pass filter that cuts the following signals is displayed as Ch1. One horizontal division of Ch1 is 100 μs, and one vertical division is 100 mV. It can be seen that the positions of the reflected pulses A ₁ ′ and A ₂ ′ are shifted in the delay direction because the sound velocity of the liquid is lower than that in FIG.

FIG. 4 is an enlarged view of a main part of the reflection signal of FIG. It is displayed as Ch1. One horizontal division is 1 μs and one vertical division is 100 mV. One cycle t ′ of the signal, that is, the peak interval is 600 ns = 0.600 μs, and the frequency f is f = 1 / 0.600 × 10 ⁻⁶ =
1.67 (MHz).

When no high-pass filter is used in FIG. 3, most of transmitted ultrasonic waves travel through liquid propane, but the rest of the ultrasonic waves travel around the peripheral wall as plate waves. Therefore, a 1.67 MHz wave and a 0.315 MHz wave are mixed in the reflected signal. However, the high frequency component of 1.67 MHz is the main part, and the low frequency component of 0.315 MHz is weak.

When a power spectrum is obtained by Fourier-transforming the reflected signal from the container peripheral wall, a strong spectral line is formed at a position of 0.315 MHz. If the power spectrum of the reflected signal of the liquid is similarly calculated without using a high-pass filter,
A weak spectral peak was observed at a position of 0.315 MHz and a strong spectral peak was observed at a position of 1.67 MHz.

Thus, in order to analyze reflected ultrasonic waves,
There are a method of analyzing a time-series signal and a method of analyzing a frequency distribution thereof, for example, a power spectrum. In the time-series signal, the period and the frequency are obtained from the interval between the peaks, while the frequency can be directly detected in the frequency distribution.

As can be seen from the results of FIGS. 2 and 4, while the ultrasonic frequency of the peripheral wall is 0.315 MHz, the ultrasonic frequency of the liquid is as high as 1.67 MHz and as low as 0.315 MHz.
MHz. Based on this result, the present inventors have argued that the frequency of the peripheral wall made of metal or the like is in a low frequency range, and the frequency of the liquid is mainly in a high frequency range.

To determine whether this hypothesis can be generalized, the ultrasonic frequencies of liquids such as liquid propane, liquid oxygen, liquid nitrogen, and liquid carbonic acid were measured. Also, changing the material of the container to accommodate them to stainless steel and steel,
The ultrasonic frequency of the surrounding wall was measured. As a result, the general conclusion was reached that the frequency of the container peripheral wall was in the low frequency range of 0.3 to 0.6 MHz, and the frequency of the liquid was mainly in the high frequency range of 1.5 to 2.0 MHz.

From the above findings, the following liquid level detection method can be considered. First, from the time series signal of reflected ultrasound,
The position where the amplitude of the low frequency changes strongly and weakly, and the position where the high frequency appears second is the liquid level.

The detection of the liquid surface position can be further enhanced by filtering the time series signal. If the low frequency is cut by the high-pass filter, the signal becomes only the high-frequency signal, so that the change point of the presence or absence of the signal can be determined to be the liquid level. When high frequency is cut by low pass filter,
Since the signal is only a low-frequency signal, it is possible to determine the level of change in the amplitude of the signal as the liquid level without worrying about the high-frequency signal.

Next, the liquid level can be detected from the frequency distribution obtained by Fourier-transforming the time series signal. First
Second, it is determined that the changing point of the presence or absence of the high frequency peak is the liquid level, and secondly, the changing point of the peak height of the low frequency peak is the liquid level.

Further, when the time-series signal is subjected to Fourier transform after passing through a filter, the liquid level can be easily detected. That is, only a low frequency peak or a high frequency peak appears in the frequency distribution,
Both peaks do not appear at the same time. Therefore, a change in the presence or absence of a high frequency peak or a change in the height of a low frequency peak can be easily detected.

FIG. 5 is a block diagram of the first embodiment of the present invention. Upon receiving the pulse signal from the pulse transmitter P, the ultrasonic transmitter / receiver S transmits an ultrasonic pulse to the container 22. The ultrasonic transmitter / receiver S receives the reflected ultrasonic wave and sends the reflected signal to the amplifier A. Next, only the high-frequency signal is selected by the high-pass filter HF, and the presence or absence of this high-frequency signal is detected by the detection circuit C, and the subsequent circuit is notified of the liquid level information from the notification circuit I. The subsequent circuit is, for example, a management center circuit that centrally controls the liquid levels of many containers 22.

FIG. 6 is a waveform change diagram of the output signal of the high-pass filter. Since the low-frequency signal is cut by the high-pass filter HF, there is no reflected signal K in the non-liquid space, but the high-frequency signal K 'appears suddenly at the liquid position. While changing the ultrasonic wave receiver S in the height direction of the container 22,
The change point of the presence / absence of the high-frequency signal K 'can be positioned on the liquid surface.

FIG. 7 is a flowchart for realizing the first embodiment by program control. When the measurement is started in step n1, an ultrasonic wave is transmitted (n2), and the presence or absence of a reflected signal is confirmed (n3). When there is no reflected signal, it is displayed that the space is a liquid-free space (n4), and an ultrasonic pulse is transmitted again while moving the ultrasonic transmitter / receiver S downward (n4).
2) It is done. When it is determined that there is a reflected signal for the first time (n3), the position of the ultrasonic wave receiver S is set to the liquid level detection position (n
5) and sends this liquid level information to the central control center (n
6) Yes.

FIG. 8 is a block diagram of a second embodiment of the present invention. Upon receiving the pulse signal from the pulse transmitter P, the ultrasonic transmitter / receiver S transmits an ultrasonic pulse to the container 22. The ultrasonic transmitter / receiver S receives the reflected ultrasonic wave and sends the reflected signal to the amplifier A. Next, only the low-frequency signal is selected by the low-pass filter LF. In order to detect the level change of the low frequency signal, it is compared with the reference voltage of the detection circuit C. If the reference voltage is higher than the reference voltage, it is determined that it is strong, and if it is lower, it is determined that it is weak. This detection signal is notified by the notification circuit I to, for example, a central management center.

FIG. 9 is a waveform change diagram of the output signal of the low-pass filter LF. Since the high-frequency signal is cut by the low-pass filter LF, the low-frequency signal L becomes a strong low-frequency signal L in the non-liquid space portion 26, but suddenly changes to a weak low-frequency signal L 'at the liquid position. While changing the ultrasonic wave transmitter / receiver S in the height direction of the container 22, the detection circuit C determines the point where the low-frequency signal changes in intensity as the liquid level position.

FIG. 10 is a flowchart for realizing the second embodiment by program control. When the measurement is started in step n11, an ultrasonic wave is transmitted (n12),
The strength of the reflected signal is confirmed (n13). When the reflected signal is strong, it is displayed that the liquid-liquid space 26 is present (n14), and the ultrasonic pulse is transmitted again (n12) while moving the ultrasonic transmitter / receiver S downward. When it is determined that the reflected signal has become weak for the first time (n13), the position of the ultrasonic transmitter / receiver S is set as the liquid level detection position (n15), and this liquid level information is transmitted to the central control center (n16).

FIG. 11 is a block diagram of a third embodiment of the present invention. Upon receiving the pulse signal from the pulse transmitter P, the ultrasonic transmitter / receiver S transmits an ultrasonic pulse to the container 22. The ultrasonic transmitter / receiver S receives the reflected ultrasonic waves and amplifies the reflected signals with the amplifier A. The amplified reflected signal is Fourier-exchanged by a Fourier exchanger FT to derive a power spectrum. The high frequency component of the power spectrum is detected by the detection circuit C. The position of the ultrasonic transmitter / receiver S in which the high frequency component has changed from nothing to present is determined to be the liquid level. This detection signal is notified to the central management center by the notification circuit I.

FIG. 12 is a waveform change diagram of the power spectrum. In the non-liquid space 26, the low frequency component H 'is strong and there is no high frequency component H. In the liquid 24, the low frequency component H' is weak and the high frequency component H appears strongly. Therefore, the position where the high-frequency component has changed from absent to present can be determined as the liquid level. Further, a point where the low frequency component changes from strong to weak may be determined as the liquid level.
In any case, these changes are detected by the detection circuit C.

FIG. 13 is a flowchart for implementing the third embodiment by program control. When the measurement is started in step n31, an ultrasonic wave is transmitted (n32),
The power spectrum of the reflected signal is derived (n33). If there is no high frequency component (n34) in this power spectrum, it is determined that there is no liquid space (n35).
6). This liquid level information is notified to the central management center (n37).

FIG. 14 is a block diagram for centrally managing liquid level information. n containers 22-1 to 22-n
Is notified to the central control center MC through the notification circuits I-1 to In. The liquid level information includes not only the liquid level position but also various kinds of information such as the safety state of the container 22 and the operation state of the ultrasonic transmitter / receiver S. A large number of liquid level information can be collectively managed by this centralized management method.

The present invention is not limited to the above-described embodiment, and the technical idea of the present invention is not limited. For example, the transmitter and the receiver may be separate or integrated. Various modifications and design changes without departing from the scope of the invention are included in the technical scope.

[0043]

According to the first aspect of the present invention, the liquid-free space and the inside of the liquid are determined by the low-frequency signal and the high-frequency signal of the reflected ultrasonic waves. Independently, the liquid level can be easily detected.

According to the second aspect of the present invention, since the low-frequency region and the high-frequency region are completely separated without any overlapping portion,
A liquid level detection method capable of reliably detecting the liquid level position can be provided.

According to the third aspect of the present invention, since the low-frequency signal is removed by the high-pass filter, the liquid level can be easily detected only by the change in the presence or absence of the remaining high-frequency signal. Since it is a digital judgment of presence / absence change, detection is extremely accurate, and is not easily affected by changes in the measurement environment.

According to the fourth aspect of the present invention, since the high-frequency signal is removed by the low-pass filter, the liquid surface position can be easily detected only by the strength change of the remaining low-frequency signal.

According to the fifth aspect of the present invention, the presence / absence and strength of the high frequency component and the low frequency component are determined based on the frequency distribution derived by performing the Fourier transform on the time signal. The position can be detected immediately.

According to the sixth aspect of the present invention, since the low frequency region and the high frequency region have no overlapping portion and are completely separated from each other, it is possible to provide a liquid level detecting device capable of reliably detecting the liquid level position.

[Brief description of the drawings]

FIG. 1 is a reflection signal diagram of an ultrasonic wave propagating on a peripheral wall of an iron container.

FIG. 2 is an enlarged view of a main part of the reflection signal of FIG.

FIG. 3 is a reflection signal diagram of an ultrasonic wave propagating through liquid propane stored in a container.

FIG. 4 is an enlarged view of a main part of the reflection signal of FIG. 3;

FIG. 5 is a block diagram of the first embodiment of the present invention.

FIG. 6 is a waveform change diagram of an output signal of the high-pass filter of FIG. 5;

FIG. 7 is a flowchart for implementing the first embodiment by program control.

FIG. 8 is a block diagram of a second embodiment of the present invention.

FIG. 9 is a waveform change diagram of an output signal of the low-pass filter of FIG.

FIG. 10 is a flowchart for implementing a second embodiment by program control.

FIG. 11 is a block diagram of a third embodiment of the present invention.

FIG. 12 is a waveform change diagram of the power spectrum of FIG. 11;

FIG. 13 is a flowchart for implementing a third embodiment by program control.

FIG. 14 is a block diagram for centrally managing liquid level information.

FIG. 15 is a cross-sectional view for detecting a liquid surface position in a container in which liquid is sealed.

FIG. 16 is a multiple reflection diagram of a plate wave orbiting the peripheral wall of the container.

FIG. 17 is a longitudinal wave reflection diagram for reciprocating reflection in liquid in a container.

[Explanation of symbols]

Reference numeral 20 denotes an ultrasonic wave transmitter / receiver, 22 · 22-1 to 22-n denotes a container, 24 denotes a liquid, 24a denotes a liquid surface, 26 denotes a non-liquid space, A
Is an amplifier, C is a detection circuit, FT is a Fourier transformer, HF
Is a high-pass filter, I is a notification circuit, LF is a low-pass filter, P is a pulse circuit, and S is an ultrasonic transmitter / receiver.

Claims (6)

[Claims] 1. An ultrasonic pulse is transmitted to an outer surface of a container containing a liquid, a reflected wave of the ultrasonic pulse is received, and when the frequency of the reflected signal is in a low-frequency region, a liquid-free space in the container is provided. It is determined that the liquid has propagated through the part, and when the frequency is mainly in the high-frequency region, it is determined that the liquid has propagated in the liquid in the container, and the position of the liquid surface is detected from these determinations. Frequency type liquid level detection method. 2. The low frequency range is 0.3 to 0.6 MHz.
The method of claim 1, wherein the high frequency range is 1.5 to 2.0 MHz. 3. An ultrasonic transmitter for transmitting an ultrasonic pulse to an outer surface of a container containing a liquid, an ultrasonic receiver for receiving a reflected wave of the ultrasonic pulse, and a high-frequency signal among the reflected signals. A frequency-type liquid level detecting apparatus using ultrasonic waves, comprising a high-pass filter that passes therethrough, and a detector that detects a high-frequency signal from an output signal of the high-pass filter, and detects a liquid level from a change in the presence or absence of the high-frequency signal. 4. An ultrasonic transmitter for transmitting an ultrasonic pulse to an outer surface of a container containing a liquid, an ultrasonic receiver for receiving a reflected wave of the ultrasonic pulse, and a low-frequency signal of the reflected signal. A low-pass filter that passes a signal, and a detector that detects a low-frequency signal from the output signal of the low-pass filter, and detects a liquid level from a change in the strength of the low-frequency signal. Surface detection device. 5. An ultrasonic transmitter for transmitting an ultrasonic pulse to an outer surface of a container containing a liquid, an ultrasonic receiver for receiving a reflected wave of the ultrasonic pulse, and converting the reflected signal into a frequency distribution. A frequency-type liquid level detecting device using ultrasonic waves, comprising: a Fourier transformer for detecting a liquid level based on the presence or absence of a high-frequency signal or a change in the strength of a low-frequency signal. 6. The low frequency signal has a frequency of 0.3 to 0.6 MHz.
6. The frequency type liquid level detecting device using ultrasonic waves according to claim 3, wherein the high frequency signal is in a frequency range of 1.5 to 2.0 MHz.