JP4738897B2 - Ultrasonic flow meter - Google Patents

Description

  The present invention relates to an ultrasonic flowmeter using a propagation time difference method.

When measuring the flow rate or flow rate of a liquid with ultrasonic waves, if bubbles are mixed in the liquid, there is a weakness that the measurement accuracy is lowered or the output is not performed. This is because the bulk modulus of gas is four orders of magnitude smaller than that of liquid, so that the sound waves are scattered because the bubbles oscillate due to the sound pressure, and some of the energy of the sound waves is dissipated by the bubbles. Because. In normal use, a flow meter that uses ultrasonic waves is not used in a line (pipe system) that is always assumed to contain air bubbles. The ultrasonic flowmeter is used when bubbles are rarely mixed or when there are no other options, but in many cases, bubbles flow alone or as a group of bubbles.
As described above, part of the energy of the sound wave is dissipated by the vibration of the bubble, so the attenuation of the traveling sound wave increases in the liquid containing the bubble group, and at the boundary between the part without the bubble and the part containing the bubble. Reflects sound waves.
The reflection of sound waves generally occurs at the boundary between two media. If the density and sound speed of the medium on the incident side are ρi and ci, and the density and sound speed of the medium on the transmission side are ρo and co, then the reflectance of the sound pressure is
(Ρo · co−ρi · ci) / (ρo · co + ρi · ci)
It becomes. This indicates that the greater the difference between the product of density and sound speed, the easier it is to reflect.
For example, if the liquid contains a very small amount of bubbles with a volume ratio of about 10 ^−4, the average value of the bulk modulus decreases to about ½ and is attenuated, and the average density also decreases and reflection occurs. The ultrasonic waves that reach the receiving side are considerably small.

FIG. 7 is a diagram showing an example of an experiment in a pipe incorporating a certain ultrasonic flow meter. Here, although the pipe diameter is about 5 mm and the size of the bubbles is 1 mm or less, the size is visible.
FIG. 7A shows an example of a reception waveform when the reception level is adjusted to 100 mV with almost no bubbles. FIG. 7B is an example of a received waveform when a fluid (water) containing bubbles of about 1.7 × 10 ⁻³ in a volume ratio is flowed at a flow rate of about 0.4 m / s. And (c) is an example of a received waveform when a fluid containing bubbles having a volume ratio of about 10 ⁻³ in a pipe is flowed at a flow velocity of about 1.2 m / s.
The volume ratio is larger in the case of (b), but in the case of (c), the flow rate is faster, so that the amount of passage of bubbles per unit time is larger in (c). Therefore, the level of the received signal is lower in the case of (c) than in the case of (b).

With ultrasonic flowmeters, the voltage of the received signal is weak compared to the voltage of the transmitted signal, so if the ultrasonic wave attenuates due to air bubbles mixing in, the ultrasonic signal cannot be received or the arrival time is set at a certain voltage level of the received signal. When measurement is performed at (detection level), a deviation occurs and an erroneous arrival time is detected, which may reduce accuracy.
FIG. 8 is a diagram showing this state. In the figure, A indicated by a solid line indicates an example of a reception waveform when bubbles are not mixed, and B indicated by a broken line indicates a reception waveform whose level is lowered due to bubbles being mixed. As shown in this figure, when bubbles are mixed, the level of the received waveform is lowered, so that the timing for detecting the reception of the signal is shifted.

In order to avoid such mixing of bubbles, a bubble removing device that removes bubbles upstream so that bubbles do not enter the ultrasonic flowmeter has been proposed (Patent Document 1). This bubble removing device is smaller and less expensive than the conventional one, but it is still more expensive than the three-way switching valve.
Patent Document 2 proposes removing the influence of bubbles on the flow rate to be measured by detecting the mixing of bubbles and maintaining the output of the ultrasonic vortex flowmeter.
Further, Patent Document 3 proposes that when the mixing of bubbles is detected, the flow rate measurement is continued urgently by increasing the level of the received signal.
JP 2002-151458 A JP 2004-117283 A JP 07-159213 A Supervised by Junichi Miyoshi and two others, “Ultrasound Technology Handbook (new edition)”, Nikkan Kogyo Shimbun, December 30, 1984, p. 1285-1287

In the prior art described above, it was determined that bubbles were mixed in the liquid by monitoring the voltage of the received signal.
However, it is known that the voltage of the ultrasonic reception signal varies depending on the temperature of the fluid. This is because sound absorption by the fluid has temperature dependence.
FIG. 9 is a diagram showing temperature change characteristics of ultrasonic absorption of water (Non-Patent Document 1). Here, α represents the absorption rate, and f represents the frequency. As shown in this figure, the absorption rate decreases as the water temperature increases.
Therefore, as in the past, it is difficult to determine whether the voltage drop is due to a change in fluid temperature or the voltage drop due to the inclusion of bubbles simply by monitoring the voltage of the received signal. Could not be detected.

  Therefore, an object of the present invention is to provide an ultrasonic flow meter that can accurately determine the presence of bubbles.

In order to achieve the above object, an ultrasonic flowmeter of the present invention includes a measurement pipe, a pair of ultrasonic transducers provided so as to cross the measurement pipe, a control unit, A smoothing circuit for smoothing a received signal from the ultrasonic transducer, a gain control circuit for controlling the gain of the received signal from the ultrasonic transducer, and a waveform shaping circuit for shaping the output of the gain control circuit An ultrasonic flowmeter having a transmission circuit that outputs an ultrasonic signal under the control of the control unit, and a transmission / reception switching circuit that switches between transmission and reception of the pair of ultrasonic transducers, the control unit Calculates the sound speed from the propagation time between the pair of ultrasonic transducers, calculates the fluid temperature from the relationship between the sound speed and the fluid temperature based on the calculated sound speed, and responds from the calculated fluid temperature. Calculate the received signal level and calculate the received signal level. It is what is adapted to detect the inclusion of air bubbles on the basis of the level and the output of the smoothing circuit.
The storage unit stores a correspondence table indicating a relationship between the sound speed and the fluid temperature and a correspondence table indicating a relationship between the fluid temperature and the received signal level.

  According to such an ultrasonic flowmeter of the present invention, since the attenuation amount of the ultrasonic wave due to the fluid temperature can be grasped, it becomes possible to judge the mixing of bubbles with higher accuracy, and the reliability of the system. Can raise the sex.

FIG. 1 is a block diagram showing a configuration of an embodiment of an ultrasonic flowmeter of the present invention.
In this figure, 1 is a measurement pipe through which a fluid to be measured flows, 2 and 3 are ultrasonic transducers (ultrasonic transducers), and 4 is a control unit (hereinafter referred to as a “microcomputer”) including a microcomputer. 5 is a transmission circuit for outputting an ultrasonic signal, 6 is a transmission / reception switching circuit for switching between transmission / reception of the ultrasonic transducers 2 and 3, and 7 is a smoothing of a reception wave from the ultrasonic transducer 2 or 3 A smoothing circuit for outputting a signal corresponding to the average voltage level, 8 a gain control circuit for controlling the level of the received wave from the ultrasonic transducer 2 or 3 to a constant magnitude, and 9 for the gain control circuit. 8 is a waveform shaping circuit that shapes the waveform of the received signal controlled to a constant magnitude by 8 so that the microcomputer 4 can calculate the flow rate, and outputs the waveform.
A pair of ultrasonic transducers 2 and 3 are provided on the outside of both ends of the measurement pipe line 1 through which the non-measurement fluid flows, or facing the measurement pipe line 1 so as to cross the measurement pipe line 1. By controlling the transmission / reception switching circuit 6, ultrasonic signals are alternately transmitted and received from the ultrasonic transducers 2 and 3.
The smoothing circuit 7 smoothes one received wave and outputs a signal corresponding to the average voltage level to the microcomputer 4 in order to constantly monitor the magnitude of the received wave.
The microcomputer 4 outputs a control signal for changing the gain to the gain control circuit 8 in order to cancel the attenuation of the ultrasonic wave due to the temperature change of the fluid, and the gain control circuit 8 outputs the control signal to the ultrasonic transducer 2. Alternatively, the received waveform from 3 is amplified so as to be amplified to a certain magnitude and output to the waveform shaping circuit 9. For example, the microcomputer 4 determines a gain control signal to be output to the gain control circuit 8 based on the first output from the smoothing circuit 7.
The microcomputer 4 calculates the propagation time between the ultrasonic transducers based on the time when the signal is generated from the transmission circuit 5 and the signal received from the waveform shaping circuit 9, and the fluid is calculated from the propagation time. The flow rate Q is calculated and the flow rate Q is calculated by multiplying the cross-sectional area of the measurement pipe line 1.
As an output of this ultrasonic flowmeter, an output by analog (1-5 V or 4-20 mA), digital (frequency or integration pulse), alarm open collector, etc. is prepared, and a communication function may be added. .

In the ultrasonic flowmeter of the present invention, the microcomputer 4 obtains the sound velocity c in the fluid from the ultrasonic wave propagation time between the ultrasonic transducers 2 and 3.
Although the fluid temperature and sound velocity vary from fluid to fluid, there is a range that is uniquely related to the ambient temperature for most liquids. FIG. 2 is a diagram showing a temperature change of the sound speed c of water. As shown in this figure, for example, the sound speed and temperature of water are uniquely determined between 0.7 and 73.95 ° C. (minimum at 0.7 ° C. and maximum at 73.95 ° C.). To become).
Therefore, by registering the relationship between the sound speed c of the fluid to be measured and the temperature in a flash memory included in the microcomputer 4, the temperature of the fluid can be known by calculating the sound speed.
As described above, the attenuation of the ultrasonic wave propagating in the fluid is related to the temperature of the fluid. There are various causes of attenuation such as reflection, diffusion, and absorption. For example, it is known that absorption is related to fluid temperature (Non-Patent Document 1).
Therefore, the relationship between the fluid temperature and the received signal level is obtained through experiments.
FIG. 3 is a diagram showing an example of the relationship between the fluid temperature and the received signal level in a certain flow meter. Since the attenuation characteristic changes depending on the material of the pipe line and the like, the relationship between the fluid temperature and the received signal level is obtained individually and also stored in the flash memory included in the microcomputer 4.

Thus, in the present invention, in the memory provided in the microcomputer 4, the first correspondence table showing the relationship between the sound speed in the fluid and the fluid temperature, and the relationship between the fluid temperature and the received signal level are shown. 2 correspondence tables are stored. In addition, you may make it memorize | store the arithmetic expression corresponding to them, not a correspondence table.
As described above, if the fluid temperature is related to the calculated sound velocity and the received signal level is related to the fluid temperature, it may be due to the fluid temperature when the received signal level is lowered or a factor other than the fluid temperature is involved. You can see if
The decrease in the received signal level other than the fluid temperature is also caused by the viscosity of the fluid. However, if the relationship between the sound speed and the fluid temperature and the fluid temperature and the received signal level is related to each fluid, the received signal level will decrease depending on the fluid state. It turns out that.
Although it affects the received signal in the fluid state, the top of the head is air bubbles. As described above, there is no continuity because the bubbles are not always mixed as described above, and many bubbles flow alone or as a group of bubbles. If there is continuity, it can be judged as abnormal. Therefore, it is determined that bubbles are mixed when the level of the received signal is lower than the value of the received signal level that is derived from the sound speed.
In this judgment, if the level of the received signal remains low even after the time when it is assumed that the bubbles have flowed away, it is judged that an abnormality has occurred due to continued mixing of bubbles or other factors. Can do.

FIG. 4 is a flowchart showing the operation of the ultrasonic flowmeter of the present invention.
First, measurement processing is performed by controlling the transmission circuit 5 and the transmission / reception switching circuit 6 (step S1). The transmission / reception switching circuit 6 sets, for example, the ultrasonic transducer 2 on the transmission side and the ultrasonic transducer 3 on the reception side, and generates an ultrasonic pulse signal of a predetermined time width from the transmission circuit 5 a predetermined number of times. Let Then, the reception signal from the ultrasonic transducer 3 is smoothed by the smoothing circuit 7 to detect the level thereof, and the timing at which the ultrasonic pulse signal is output from the transmission circuit 5 and received from the waveform shaping circuit 9. Measure the average time until the signal shaping output is output. Next, the transmission / reception switching circuit 6 switches the ultrasonic transducer 3 to the transmission side and the ultrasonic transducer 2 to the reception side, similarly outputs an ultrasonic pulse signal a predetermined number of times, and propagates the propagation time. Measure the average value of.
Next, in step S2, the speed of sound c and the flow velocity v are obtained based on the propagation time measured in step S1.
When the distance between the ultrasonic transducers 2 and 3 is L, and the propagation delay times between the ultrasonic transducers are T1 and T2,
Speed of sound c = L / 2 × (1 / T1 + 1 / T2)
Flow velocity v = L / 2 x (1 / T1-1 / T2)
It is.

Next, with reference to the first correspondence table showing the relationship between the sound speed and temperature stored in the memory, the temperature of the corresponding fluid is obtained from the calculated sound speed c (step S3).
Then, referring to the second correspondence table showing the relationship between the temperature and the reception level stored in the memory, the reception level corresponding to the temperature is obtained (step S4).
Then, the level of the reception signal output from the smoothing circuit 7 is compared with the reception level obtained in step S4 (step S5).
As a result, when the difference between the actual received signal level and the received level corresponding to the temperature obtained from the sound speed is within a predetermined range (for example, within 80%), it is determined that it is within the normal range and is calculated in step S2. The flow rate data based on the measured flow velocity v is output (step S6).
On the other hand, when it is not within the normal range, for example, when the output of the smoothing circuit 7 is lower than 80% of the reception level obtained in step S4, abnormal processing is performed assuming that bubbles are mixed. That is, an alarm output indicating that air bubbles are mixed is output.
As described above, according to the present invention, the temperature of the fluid is obtained by measuring the sound speed of the fluid, and the amount of ultrasonic attenuation due to the temperature of the fluid is reflected. can do.

Next, a process after the detection of air bubbles as described above will be described.
Here, it can be divided into two methods according to the use of the user.
The first is a case where a small amount of air bubbles are mixed into the user and the amount of air bubbles does not greatly affect the system even if the accuracy is slightly reduced. In this case, priority is given not to stop the system without immediately outputting an alarm and maintaining the output until bubbles are removed.
For example, an inexpensive flow rate sensor such as a turbine type may be used for the flow rate control of the cooling water. However, a turbine type or impeller type with a moving part has a short life, and a vortex type or An ultrasonic flow meter is used.
FIG. 5 shows an example of a system for managing the flow rate of cooling water. In the cooling water flow management, cooling is performed by changing the flow rate using the normal temperature as a feedback source. That is, the temperature control valve 13 is controlled by the output of the temperature sensing element 12. This flow rate management is used to detect a failure based on the presence / absence of cooling water and whether it is too large or too small. In addition, after the temperature change of the cooling water due to the load change, the temperature control valve 13 is operated, and the flow rate change is performed according to the load change, but until the temperature of the cooling water returns to the original temperature, It takes several tens of seconds and usually several minutes. Therefore, it cannot be said that it is an appropriate operation that a warning is issued from the flow meter 11 immediately after air bubbles are mixed and the apparatus stops.
In this case, since there is a time delay until bubbles mixed in the measuring tube of the flow meter 11 are removed, importance is attached to the stability of the system, and the output of the flow rate is maintained until the time when bubbles are assumed to flow away. Becomes effective.
The time when the bubble is assumed to flow away is, for example, the passage time of the bubble using the flow rate value Q immediately before the level of the received signal decreases and the flow path cross-sectional area A and the distance L between the ultrasonic transducers. Ti,
From Q = AL / Ti, Ti = AL / Q
It can be set as the time which gave some margin (about 10-20%) to Ti.
However, when the bubbles are removed earlier than Ti or the bubbles disappear for some reason and transmission / reception becomes possible, the measurement is immediately performed and the maintenance of the output is released.

The second is a case where mixing of bubbles is not allowed. In this case, when it is determined that bubbles are mixed, it is important to change the output of the flow rate value immediately and change the dedicated output for mixing the bubbles separately from the output of the flow rate value to convey the message that bubbles are mixed. is there.
For example, in the case where a very small amount of fluid having a high viscosity is applied and coated in a film forming process or the like, if bubbles remain on the coating film, a uniform film cannot be formed, causing problems. For this reason, it is required to discharge a fluid without bubbles from the nozzle at a constant flow rate.

FIG. 6 is a diagram showing an example of a system for performing such coating. In this figure, 21 is an ultrasonic flowmeter, 22 is a three-way switching valve, 23 is a remote control valve, 24 is a tank, 25 is a liquid level sensor, 26 is a pump, and 27 is a nozzle. In order to control these, there are control devices such as sequencers and MPUs having control / calculation functions. In this system, a fixed amount of liquid is supplied to the suction side of the pump 26, the fixed amount is sucked by the pump 26 and sent to the nozzle 27.
In this system, a process of storing liquid without bubbles in the tank 24 will be described.
1. The remote control valve 23 is opened to guide the liquid to the tank 24.
2. The flow rate of the fluid guided to the tank 24 is integrated to monitor the liquid amount in the tank 24.
3. When bubbles pass through the measurement tube of the ultrasonic flowmeter 21, the three-way switching valve 22 is operated to discharge the liquid containing bubbles. At this time, the integration is stopped while the three-way switching valve 22 is operated and discharged.
4). When the bubbles are discharged, the three-way switching valve 22 is operated again to guide the liquid to the tank 24, and the integration is resumed.
5. When the integrated value reaches a specified amount, the remote control valve 23 is closed.
As a result, a certain amount of liquid without bubbles is stored in the tank 24. However, the remote control valve 23 does not open during the suction operation of the pump 26. In addition, when collecting in the tank 24, the device which the liquid which comes out of piping does not involve a bubble is made.

It is a block diagram which shows the structure of one Embodiment of the ultrasonic flowmeter of this invention. It is a figure which shows the temperature change characteristic of the sound speed of water. It is a figure which shows the example of the experimental value which showed the temperature dependence of a received wave. It is a flowchart for demonstrating operation | movement of the ultrasonic flowmeter of this invention. It is a figure which shows an example of the flow volume management system which uses the ultrasonic flowmeter of this invention. It is a figure which shows the other example of the flow volume management system which uses the ultrasonic flowmeter of this invention. It is a figure which shows an example of a reception waveform, (a) is a reception waveform when a reception level is set to 100 mV in the state with almost no bubble, (b) is about 1.7 * 10 < ^-3 > by volume ratio in piping. The received waveform when the flow rate is about 0.4 m / s, (c) is about 10 ^-3 bubbles in volume ratio in the pipe, and the flow rate is about 1.2 m / s It is a figure which shows the received waveform at the time of. It is a figure for demonstrating detection of a received waveform. It is a figure which shows the temperature change characteristic of the ultrasonic absorption of water.

Explanation of symbols

  1: measurement pipeline, 2, 3: ultrasonic transducer (ultrasonic transducer), 4: control unit (microcomputer), 5: transmission circuit, 6: transmission / reception switching circuit, 7: smoothing circuit, 8: gain Control circuit, 9: Waveform shaping circuit

Claims (2)

A measuring line;
A pair of ultrasonic transducers provided opposite to each other across the measurement pipeline;
A control unit;
A smoothing circuit for smoothing a received signal from the ultrasonic transducer;
A gain control circuit for controlling the gain of the received signal from the ultrasonic transducer;
A waveform shaping circuit for shaping the output of the gain control circuit;
A transmission circuit that outputs an ultrasonic signal under the control of the control unit;
An ultrasonic flowmeter having a transmission / reception switching circuit for switching between transmission and reception of the pair of ultrasonic transducers,
The controller calculates a sound speed from a propagation time between the pair of ultrasonic transducers,
Based on the calculated sound speed, the fluid temperature is calculated from the relationship between the sound speed and the fluid temperature,
Calculate a corresponding received signal level from the calculated fluid temperature,
An ultrasonic flowmeter for detecting the mixing of bubbles based on the calculated received signal level and the output of the smoothing circuit.   The ultrasonic flowmeter according to claim 1, further comprising a storage unit that stores a correspondence table indicating a relationship between the sound speed and the fluid temperature, and a correspondence table indicating a relationship between the fluid temperature and the reception signal level.

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