JP2011007763A - Ultrasonic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrasonic flowmeter having high accuracy of measurement and high safety.SOLUTION: In the ultrasonic flowmeter 1, a hole is provided at a measurement pipe 2 (outer diameter of 50 mm, inner diameter of 40 mm, and length of 100 mm) made of stainless steel, and an inflow part 4 and an outflow part 5 that are the pipes (outer diameter of 12 mm and inner diameter of 8 mm) made of the same stainless steel are attached thereto by wielding. Diaphragms 8a, 8b made of stainless steel are welded to both ends of the measurement pipe. Composite piezoelectric elements 6a, 6b are joined to the central parts of the diaphragms 8a, 8b made of stainless steel with an epoxy resin.

Description

  The present invention relates to an ultrasonic flowmeter that propagates ultrasonic waves into a liquid to be measured and measures the flow velocity and flow rate of the liquid from the difference in propagation time between the upstream direction and the downstream direction of the flow path.

  Conventionally, an ultrasonic wave is propagated to the liquid in the flow channel to be measured, and the flow velocity (flow rate) of the liquid is measured using the difference in propagation time when the ultrasonic wave propagates in both the upstream and downstream of the flow channel. Sonic flow meters have been used.

  Patent Document 1 describes the configuration of a conventional typical ultrasonic flowmeter. A liquid inflow portion and an outflow portion are attached to both ends of the linear measurement pipe line, and an ultrasonic transducer is attached to the outside of each of the inflow portion and the outflow portion so as to face each other. The flow velocity of the liquid flowing in the flow velocity measuring line is measured from the time required for propagation of the ultrasonic wave propagating between the pair of ultrasonic transducers.

  A cylindrical ultrasonic transducer composed of piezoelectric elements 11 such as PZT (lead zirconate titanate) repeats expansion and contraction in the thickness direction at the frequency of the ultrasonic band. However, when viewed in detail, the shape of the ultrasonic transducer changes as shown in FIG. That is, in a stationary state when not in operation, as shown in A, it has a cylindrical shape. In the reduced state in the thickness direction, it extends in the radial direction and becomes the shape of a concave lens as shown in FIG. Further, in the stretched state in the thickness direction, it shrinks in the radial direction, and becomes a shape like a convex lens as shown by emphasizing C. As a result, as shown in FIG. 13D, the closer to the center of the cylinder, the greater the amount of movement in the thickness direction (axial direction of the cylinder), and the reciprocating speed V also increases. As a result, the energy of the emitted ultrasonic waves increases toward the center.

  In general, the flow velocity of the liquid flowing in the flow velocity measuring pipe is not uniform in the cross section of the pipe, and the distribution is such that the central portion of the pipe is large and decreases as it approaches the pipe wall. In general, a low-speed laminar flow has a parabolic velocity distribution as shown in FIG. 14A, and a high-speed turbulent flow has a velocity distribution as shown in FIG. 14B. Conventionally, the flow velocity of this ultrasonic wave is measured by concentrating the ultrasonic wave propagation path at the center of the cross section of the flow velocity measurement pipe, and this is multiplied by the correction coefficient of the flow velocity distribution corresponding to laminar flow and turbulent flow. Thus, the average flow velocity in the cross section of the flow path was calculated.

  As described above, in the method of measuring the flow velocity at the central portion of the flow path, as described with reference to FIG. 13, the vibration velocity of the central portion is large, and the energy of the emitted ultrasonic waves is increased toward the central portion. It has been desired to use a simple ultrasonic transducer.

  In the above-described method of measuring the flow velocity at the central portion of the flow path and multiplying this by the correction coefficient of the flow velocity distribution to calculate the average flow velocity, the flow velocity distribution may not always match the theoretical one. Whether the flow is laminar or turbulent is determined by the Reynolds number, but an error may occur in this determination near the boundary, which causes a reduction in measurement accuracy.

  Further, as shown in FIG. 13, since the ultrasonic end face of the ultrasonic transducer is uneven, the ultrasonic wave radiated in the normal direction of the end face is not parallel to the axis of the pipe line. Will be inclined to. As a result, as shown in FIG. 15, a part of the ultrasonic wave radiated from the ultrasonic transducer propagates in the vicinity of the inner wall of the pipe or is reflected by the inner wall. As a result, the accuracy of correction for the flow velocity distribution performed under the assumption that the ultrasonic wave propagates near the center of the flow path is reduced.

  Furthermore, since the reflected ultrasonic component has a longer propagation time as indicated by the dotted arrow in the figure, it is delayed from the component received directly without being reflected as indicated by the solid arrow in the figure. Received. As a result, there is a problem that the reception waveform becomes dull, causing a delay in the reception time point and reducing the detection accuracy.

  In order to solve the above problems, an ultrasonic flow meter of Patent Document 2 has been proposed. In the ultrasonic flowmeter of Patent Document 2, a liquid inflow portion and an outflow portion are attached to both ends of a linear flow velocity measurement pipe, and an ultrasonic transducer is attached to each of the inflow portion and the outflow portion so as to face each other. The flow velocity and flow rate of the liquid flowing in the flow velocity measurement pipe are measured from the time required for propagation of the ultrasonic waves propagating between the ultrasonic transducers inside the measurement pipe. In this ultrasonic flow meter, each ultrasonic transducer is composed of a composite vibrator having a cross-sectional shape larger than the cross-sectional area of the flow velocity measuring pipe.

  In the ultrasonic flowmeter of Patent Document 2, the ultrasonic transducer is composed of a composite vibrator having a cross-sectional shape larger than the cross-sectional shape of the flow velocity measuring pipe, so that the supersonic energy of uniform energy can be obtained over the cross-section of the flow path. Sound waves propagate almost parallel to the axial direction of the flow path. As a result, correction for the flow velocity distribution in the cross section of the flow path is unnecessary, and a highly accurate flow meter can be realized.

  JP 2000-171478 A   JP 2004-198339 A

  Tadashi Shiogama, “Manufacture and Application of New Piezoelectric Materials”, CMC Corporation, December 1987, p99-109

  However, since the ultrasonic flowmeter of Patent Document 2 uses a composite vibrator having a cross-sectional shape larger than the cross-sectional shape of the measurement pipe as the ultrasonic transducer, when the pipe diameter of the measurement pipe is large, for example, 10 mm In the case of exceeding this, since a larger composite vibrator must be used, the manufacturing cost becomes high.

  In addition, when the diameter of the composite vibrator is large, the ultrasonic vibration of the composite vibrator propagates to the measurement pipe. Therefore, when the measurement pipe is a metal, the ultrasonic wave propagating the liquid However, there is a problem that accurate reception is not possible due to the influence of ultrasonic vibration propagating through the measurement pipeline.

  The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide an inexpensive ultrasonic flowmeter capable of measuring a flow rate with high accuracy regardless of the material of the measurement pipe line. It is in.

According to the present invention, a pair of ultrasonic transducers are arranged upstream and downstream in the flow direction of the measurement pipe through which the liquid to be measured flows, and flows in the measurement pipe from the time required for propagation of ultrasonic waves between the ultrasonic transducers. In an ultrasonic flowmeter for measuring the flow rate or flow velocity of a liquid to be measured, a composite piezoelectric material having a diaphragm joined to both ends of the measurement pipe and having a diameter of less than 1 time and 0.1 times or more the inner diameter of the measurement pipe. The element is an ultrasonic transducer.

  In the present invention, the composite piezoelectric element includes nine or more piezoelectric elements.

  In the present invention, the measurement pipe and the diaphragm are made of metal, and the liquid does not come into contact with anything other than metal.

  The present invention also uses welding, soldering, or a metal seal to join the measurement pipe and the diaphragm.

  According to the ultrasonic flowmeter of the present invention, the flow velocity or flow rate of liquid can be measured with high accuracy regardless of the material of the measurement pipe line.

It is sectional drawing which shows 1st Embodiment of the ultrasonic flowmeter by this invention. It is a top view which shows the shape of a composite piezoelectric element. It is a figure which shows the cross section in the AA of FIG. It is a received wave when there is no water in the ultrasonic flowmeter of FIG. It is a received wave when water exists in the ultrasonic flowmeter of FIG. It is a top view of a metal diaphragm. It is a figure which shows the cross section in the AA of FIG. 8 is a frequency characteristic of an ultrasonic transducer in which a piezoelectric element is bonded to the diaphragm shown in FIGS. It is a propagation characteristic when using the ultrasonic transducer of FIG. 8 is a frequency characteristic of an ultrasonic transducer in which a composite piezoelectric element is bonded to the diaphragm shown in FIGS. It is a propagation characteristic when the ultrasonic transducer of FIG. 10 is used. It is a figure explaining the circuit which measures the flow volume of an ultrasonic flowmeter. It is a figure explaining the vibration displacement of the conventional piezoelectric element. It is a conceptual diagram for demonstrating the mode of distribution of the flow velocity in a measurement pipe line. It is a conceptual diagram for demonstrating a mode that an ultrasonic wave propagates in the inner wall vicinity of a pipe line, or is reflected there. It is sectional drawing which shows 2nd Embodiment of the ultrasonic flowmeter by this invention.

  The basic configuration of the first embodiment is shown in the sectional view of FIG.

  The ultrasonic flow meter 1 shown in FIG. 1 is provided with a hole in a stainless steel measurement pipe 2 (outer diameter 50 mm, inner diameter 40 mm, length 100 mm), and an inflow that is also a stainless steel pipe (outer diameter 12 mm, inner diameter 8 mm). The part 4 and the outflow part 5 are attached by welding. Here, in addition to welding, soldering or metal sealing can also be used. And stainless steel diaphragm 8a, 8b is joined to both ends of measurement piping by welding. Here, in addition to welding, soldering or metal sealing can also be used. By making all the materials that come into contact with the liquid metal in this way, even flammable and explosive liquids can be handled safely. Then, the composite piezoelectric elements 6a and 6b are joined to the central portions of the stainless steel diaphragms 8a and 8b with an epoxy resin. The ultrasonic transducers 3a and 3b are diaphragms 8a and 8b in which composite piezoelectric elements 4a and 4b are joined.

  A diaphragm 8 made of stainless steel having a diameter of 50 mm and a thickness of 2 mm is bonded to covers 10 a and 10 b having a ring having an outer diameter of 50 mm and an inner diameter of 40 mm, the same as the measurement pipe 2. Further, the covers 10a and 10b press the composite piezoelectric elements 6a and 6b obtained by bonding the backing materials 7a and 7b to the back surface by the springs 9a and 9b.

  The composite piezoelectric elements 6a and 6b joined to the central portions of the stainless steel diaphragms 8a and 8b by an epoxy resin have a diameter of 20 mm and a thickness of 3 mm. 2 and 3 show details of the composite piezoelectric element 6. FIG. The composite piezoelectric element has a diameter of 20 mm and a thickness of 3 mm, each piezoelectric element 11 has a 2 mm square and a length of 3 mm, and the width of the epoxy resin 12 joining the piezoelectric element 11 and the piezoelectric element 11 is 1 mm. . The piezoelectric element 11 is a lead-based piezoelectric ceramic, and is a so-called Low Q material.

  A composite piezoelectric element is a material in which a piezoelectric body such as a piezoelectric ceramic and a resin are structurally combined, and has characteristics not found in piezoelectric ceramics and piezoelectric polymers. Although described in detail in Non-Patent Document 1, a composite piezoelectric vibrator has been developed to obtain characteristics that cannot be realized with a single piezoelectric element, and is currently expensive for medical use. Mainly used. Composite piezoelectric vibrators are classified into ceramic and polymer composites based on how many dimensions (in which direction) each ceramic and polymer are self-bonded. That is, an mn composite is represented by a dimension number m in which the piezoelectric ceramic as a piezoelectric active component is continuous in the composite and a dimension n in which a polymer as a piezoelectric inactive component is continuous. Among the composite piezoelectric vibrators, it is the most practical 1-3 composite and was used in the present invention.

Here, the propagation characteristics of the ultrasonic wave between the ultrasonic transducers when the ultrasonic flowmeter 1 shown in FIG. 1 is filled with water and when there is no water were compared.
A six-tone tone burst voltage of 474 KHz and 102 Vp-p is applied to the composite piezoelectric element. Then, the reception voltage generated in the composite piezoelectric element is observed.

  FIG. 4 shows a case where there is no water in the measurement pipe, and the received waveform 13 having a measurable size reaches the stainless steel measurement pipe 2 having a length of 100 mm in 29.6 μsec. The speed of sound of the received waveform 13 is about 3400 m / s, which is considered to be a cylindrical wave propagating through the stainless steel measurement pipe 2. Thereafter, the received signal continues, but it is presumed to be due to the wave reciprocating through the measurement pipe 2. Further, since air exists in the measurement pipe 2, a wave propagating through the air having a sound velocity of about 340 m / s is also received, but compared to the magnitude of the received signal of the wave propagating through the measurement pipe 2, It is so small that it cannot be measured in practice. Therefore, it cannot be used as an ultrasonic flowmeter for measuring the gas flow rate.

  FIG. 5 shows a case where the measurement pipe 2 is filled with water, and the received waveform 13 having a measurable size reaches the stainless steel measurement pipe having a length of 100 mm in 67.2 μsec. The speed of sound is about 1490 m / s, which is a wave propagating through the water in the stainless steel measurement pipe 2. And no waves other than water propagating are received. Therefore, the inventor has found that the ultrasonic transducer of this configuration can propagate more ultrasonic waves to the water than the measurement pipe.

  This is because only the portion of the diaphragm joined with the composite piezoelectric element mainly vibrates in the longitudinal direction. And most of this ultrasonic vibration propagates to water. Therefore, it is thought that it is because it does not propagate to the measurement pipe.

  The shape of the diaphragm and the composite piezoelectric element propagates ultrasonic vibration to the liquid. If the diameter of the composite piezoelectric element is less than 0.1 times the diameter of the diaphragm, the energy for propagating the ultrasonic vibration to the liquid is small. . Further, when the diameter of the composite piezoelectric element is larger than 1 times the diameter of the diaphragm, the ultrasonic vibration is propagated through the measurement pipe.

  Therefore, the diameter of the composite piezoelectric element is desirably 0.1 times or more and 1 time or less with respect to the diameter of the diaphragm.

  Further, FIG. 8 shows frequency characteristics of an ultrasonic transducer in which an ordinary piezoelectric element 11 having a diameter of 20 mm and a thickness of 3 mm is prepared by using an epoxy resin for the diaphragm 8 shown in FIGS. There are more than 10 peaks up to 500 KHz, and it can be seen that there are many natural vibration modes. As a result, when a tone bust wave is applied to the piezoelectric element 11, many natural vibration modes are excited, and the received waveform may be complicated. And the said ultrasonic transducer was arrange | positioned on both sides of the measurement pipe | tube 2 (outside diameter 50mm, internal diameter 40mm, length 100mm) made from stainless steel, the measurement pipe | tube was filled with water, and the propagation characteristic shown in FIG. 9 was measured. Note that two tone bust waves of 124 KHz and 74 Vp-p were applied to the piezoelectric element. When three or more tone bust waves are applied, the decay time becomes longer. As a result, the propagation sound velocity is about 3200 m / s, which is considered to be a cylindrical wave propagating through the measurement pipe. Therefore, it is difficult to measure the wave propagation through water. In addition, the numerical value in FIG. 6, FIG. 7 shows length, and a unit is mm.

  FIG. 10 shows frequency characteristics of an ultrasonic transducer formed by joining the composite piezoelectric element 6 having a diameter of 20 mm and a thickness of 3 mm to be compared to the diaphragm shown in FIGS. 6 and 7 using an epoxy resin. There is only a large peak at about 303 KHz. This is a natural vibration mode in the thickness direction. As a result, when a tone bust wave is applied to the composite piezoelectric element 6, only the natural vibration mode of longitudinal vibration is excited. The reception waveform can be simplified. Then, the ultrasonic transducer was arranged on both sides of a stainless steel measurement pipe (outer diameter 50 mm, inner diameter 40 mm, length 100 mm), the measurement pipe was filled with water, and the propagation characteristics shown in FIG. 11 were measured. Note that six tone bust waves of 318 KHz and 99 Vp-p were applied to the piezoelectric element. As a result, the propagation sound velocity is about 1470 m / s, which is considered to be a longitudinal wave propagating through the water in the measurement pipe.

  The operation of the ultrasonic flowmeter having the above configuration will be described below with reference to FIG.

  When a voltage signal is applied to an electrode provided on one composite piezoelectric element 6a of the pair of ultrasonic transducers 3a and 3b, the composite piezoelectric element 6a expands and contracts based on the voltage signal, and the composite piezoelectric element 6a Ultrasonic waves are radiated from the ultrasonic wave transmitting / receiving surface. This ultrasonic wave propagates to the diaphragm and is radiated to the water 24. On the other hand, the ultrasonic wave propagating through the water 24 propagates to the composite piezoelectric element 6b through the diaphragm 8b and generates a voltage signal between the electrodes.

  By repeatedly performing the above operations while alternately switching the transmission and reception waves by the pair of ultrasonic transducers 3a and 3b, the difference in propagation time of the ultrasonic waves propagated in the opposite direction along the same path is measured. Calculate the flow velocity based on the measured value. And a flow volume is calculated | required from the measured flow velocity and the cross-sectional area of piping.

  Hereinafter, the operation of this ultrasonic flowmeter will be described in more detail with reference to FIG.

  Consider a case in which liquid hydrogen flows through a pipe as a liquid to be measured. The drive frequency of the ultrasonic transducer and the ultrasonic transducer is about 300 KHz. The control unit 21 starts the time measurement of the timer 19 at the same time as outputting the transmission start signal to the drive circuit 17.

  Upon receiving the transmission start signal, the drive circuit 17 drives the ultrasonic transducer 3a and transmits an ultrasonic pulse. The transmitted ultrasonic pulse propagates through the liquid hydrogen inside the measurement pipe 2 and is received by the ultrasonic transducer 3b. The received ultrasonic pulse is converted into an electrical signal by the ultrasonic transducer 3 b and output to the received wave detection circuit 18.

  The reception detection circuit 18 determines the reception timing of the reception signal and stops the timer 19. The calculation unit 20 calculates the propagation time t1.

  Next, the switching circuit 16 switches the ultrasonic transducer 3a and the ultrasonic transducer 3b connected to the drive circuit 17 and the received wave detection circuit 18. Then, again, the control unit 21 outputs a transmission start signal to the drive circuit 17 and simultaneously starts time measurement of the timer 19.

  Contrary to the measurement of the propagation time t1, an ultrasonic pulse is transmitted by the ultrasonic transducer 3b, received by the ultrasonic transducer 3a, and the propagation time t2 is calculated by the calculation unit 20.

  The propagation times t1 and t2 are obtained by measurement. Since the distance L is known, the flow rate can be determined from the flow velocity V by measuring the times t1 and t2.

  In such an ultrasonic flowmeter, the propagation times t1 and t2 are preferably measured by a method called a zero cross method.

  In the ultrasonic flowmeter of the present invention, ultrasonic waves go straight through the liquid. And, almost no ultrasonic wave propagates to the metal measuring pipe. Therefore, the above measurement can measure the flow rate with higher accuracy than the conventional ultrasonic flowmeter. Also, since the liquid is completely surrounded by metal, the flow rate can be measured safely even with flammable or explosive liquids.

  A basic configuration according to the second embodiment is shown in a sectional view of FIG.

  The ultrasonic flowmeter 1 shown in FIG. 16 is provided with a hole in a stainless steel measurement pipe 2, and the stainless steel pipes 25a and 25b are attached by welding. Then, the ultrasonic transducers 3a and 3b are mounted therein.

  The ultrasonic transducers 3a and 3b have stainless steel diaphragms at the distal ends of stainless steel guide parts 16a and 16b, and the composite piezoelectric elements 6a and 6b are joined to the central part of the diaphragms by epoxy resin. The stainless steel guide portions 16a and 16b and the stainless steel diaphragms 8a and 8b were manufactured from a single material by machining such as a lathe.

  As with the ultrasonic flow meter of the first embodiment, the ultrasonic flow meter above measures the flow rate of liquid with high accuracy because the ultrasonic wave travels straight through the liquid and hardly propagates to the measurement pipe. it can.

  The metal ultrasonic flowmeter has been described above, but the same effect can be obtained with plastic measurement pipes and diaphragms.

DESCRIPTION OF SYMBOLS 1 Ultrasonic flow meter 2 Measurement piping 3 Ultrasonic transducer 4 Inflow part 5 Outflow part 6 Composite piezoelectric element 7 Backing material 8 Diaphragm 9 Spring 10 Cover 11 Piezoelectric element 12 Epoxy resin 13 Reception waveform 14 Drive waveform 15 Admittance characteristic 16 Guide part 17 Diaphragm ring 18 Switching circuit 19 Drive circuit 20 Received wave detection circuit 21 Timer 22 Calculation unit 23 Control unit 24 Water 25 Pipe

Claims (4)

  A pair of ultrasonic transducers are arranged upstream and downstream in the flow direction of the measurement pipe through which the liquid to be measured flows, and the liquid to be measured flowing inside the measurement pipe from the time required for propagation of ultrasonic waves between the ultrasonic transducers. In an ultrasonic flowmeter for measuring a flow rate or a flow velocity, a diaphragm is joined to both ends of the measurement pipe, and a composite piezoelectric element having a diameter of less than 1 time and at least 0.1 times the inner diameter of the measurement pipe is ultrasonically connected to the diaphragm. It is characterized by being a transducer.   The ultrasonic flowmeter according to claim 3, wherein the composite piezoelectric element includes nine or more piezoelectric elements.   The ultrasonic flowmeter according to claim 1, wherein the measurement pipe and the diaphragm are made of metal, and the liquid does not come into contact with anything other than metal.   The ultrasonic flowmeter according to claim 3, wherein welding, soldering, or a metal seal is used to join the measurement pipe and the diaphragm.

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