JP2012230095A - 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, an inflow part 4 and an outflow part 5 that are pipes made of PFA (a thermoplastic fluorocarbon resin) are attached to piping blocks 6a, 6b also made of PFA by welding. A measurement pipe 2 (outer diameter of 14 mm, inner diameter of 10 mm, and length of 100 mm) made of PFA are welded to the piping blocks 6a, 6b. Furthermore, rectangular composite piezoelectric elements 3a, 3b each having a size of 5.5 mm square and a thickness of 2 mm are joined to respective end faces of the piping blocks 6a, 6b using an epoxy resin.

Description

  The present invention relates to an ultrasonic flowmeter that propagates ultrasonic waves into a fluid to be measured and measures the flow velocity and flow rate of the fluid 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 fluid in the flow channel to be measured, and the flow velocity (flow rate) of the fluid is measured using the difference in propagation time when the ultrasonic wave propagates in both the upstream and downstream directions of the flow channel. Sonic flow meters have been used.

  Patent Document 1 describes the configuration of a conventional typical ultrasonic flowmeter. A fluid inflow portion and an outflow portion are attached to both ends of the linear measurement pipe, 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 fluid 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 a piezoelectric element 33 such as PZT (lead zirconate titanate) repeats expansion and contraction in the thickness direction at an ultrasonic band frequency. 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 indicated by the arrow D in FIG. 7, the amount of movement in the thickness direction (the axial direction of the cylinder) increases as the distance from the center of the cylinder increases, 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 fluid flowing in the flow velocity measuring pipe is not uniform in the cross section of the pipe, and has a distribution in which 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. 8A, and a high-speed turbulent flow has a velocity distribution as shown in FIG. 8B. 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. 7, 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. 9, since the end face of the ultrasonic wave of the ultrasonic transducer has an uneven shape, the ultrasonic wave radiated in the normal direction of the end face does not become parallel to the axis of the pipe line. Will be inclined to. As a result, as shown in FIG. 9, a part of the ultrasonic wave radiated from the ultrasonic transducer propagates in the vicinity of the inner wall of the duct 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 fluid 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 fluid 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, so that the ultrasonic vibration propagating through the fluid is super There is a problem that accurate reception is not possible due to the influence of sound wave vibration.

  In addition, since the composite vibrator is disk-shaped, the energy of the emitted ultrasonic waves increases toward the center, and the ultrasonic energy does not become uniform within the cross section of the flow path, and at the boundary between laminar flow and turbulent flow. An error occurs in the measurement accuracy of the flow rate.

  In addition, since the composite vibrator has a disk shape, when the number of piezoelectric elements constituting the composite vibrator is rectangular and five or more, the outer piezoelectric element and the inner piezoelectric element have different shapes, so that they propagate in the fluid. There is a problem that a distribution is generated in the wave, and as a result, the received waveform becomes dull and the detection accuracy is lowered.

  The present invention has been made in view of the above circumstances, and its main object is to provide an inexpensive ultrasonic flowmeter capable of measuring a flow rate with high accuracy regardless of the material of the measurement pipe. 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 that measures the flow rate or flow velocity of a liquid to be measured, the ultrasonic transducer is a rectangular composite piezoelectric element.

  The present invention also provides the ultrasonic wave according to the above, wherein the shape of the composite piezoelectric element is rectangular, and the length of the diagonal line is not more than the inner diameter of the measurement pipe and not less than ½ of the length of the inner diameter of the measurement pipe. This is a flow meter.

  The present invention also provides an ultrasonic flowmeter as described above comprising a composite piezoelectric element having a thickness equal to or longer than the longest side of a rectangular piezoelectric element.

  The present invention also provides the ultrasonic flowmeter as described above, wherein the flow path cross section is rectangular.

  According to the ultrasonic flowmeter of the present invention, the flow velocity or flow rate of a fluid can be measured with high accuracy regardless of the flow velocity distribution in the pipeline and the material of the measurement pipeline.

It is sectional drawing which shows 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 side view which shows the shape of a composite piezoelectric element. It is a top view which shows the shape of a disk shaped composite piezoelectric element. It is a top view which shows the shape of a rectangular-shaped composite piezoelectric element. 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 disk shaped 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.

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

  The ultrasonic flowmeter 1 shown in FIG. 1 attaches the inflow part 4 and the outflow part 5 which are the pipes made from PFA similarly to welding to the piping blocks 6a and 6b made from PFA (thermoplastic fluorine resin). Then, the PFA measurement pipe 2 (outer diameter 14 mm, inner diameter 10 mm, length 100 mm) is attached to the pipe blocks 6a and 6b by welding. Further, 5.5 mm square rectangular composite piezoelectric elements 3a and 3b having a thickness of 2 mm are joined to the end faces of the piping blocks 6a and 6b using an epoxy resin. Note that the end surfaces of the piping blocks 6a and 6b to be joined to the composite piezoelectric elements 3a and 3b were chemically treated in advance in order to strengthen the adhesion by the epoxy resin. Thus, by using all the materials that come into contact with the fluid as PFA, even a strongly acidic fluid can be handled safely. Here, the ultrasonic transducer is a composite piezoelectric element 3a, 3b.

  Details of the composite piezoelectric element 3 are shown using the plan view of FIG. 2 and the side view of FIG. The rectangular composite piezoelectric elements 3a and 3b are regular quadrangular prisms having a side length of 5.5 mm and a thickness of 2 mm, and are polarized in the thickness direction. The individual piezoelectric elements 7 constituting the composite piezoelectric element 3 are 1.5 mm square and 2 mm thick, and the width of the epoxy resin 8 joining the piezoelectric elements 7 and 7 is about 0.5 mm. . The piezoelectric element 7 is a lead-based piezoelectric ceramic, and is a so-called LowQ material.

  Here, a general composite piezoelectric element will be described. 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. The composite piezoelectric vibrator is a composite of ceramic and polymer, and is classified according to how many dimensions of the ceramic and the polymer are self-bonded in several dimensions (in which direction). 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, it will be described that the rectangular composite piezoelectric vibrator 32 is superior by comparing the shapes of the rectangular composite piezoelectric vibrator 32 and the disc-shaped composite piezoelectric vibrator 31. As shown in the plan view of FIG. 4, each piezoelectric element 7 of the disc-shaped composite piezoelectric vibrator 31 has a shape in which a rectangle is missing at the outer periphery. For example, in FIG. 4, there are three types of piezoelectric elements 7a, 7b, and 7c. These three types of piezoelectric elements 7a, 7b, and 7c have different natural frequencies. Therefore, each propagates in the fluid with a different propagation waveform. As a result, there is a problem that the received waveform becomes dull and the detection accuracy is lowered.

  On the other hand, in the rectangular composite piezoelectric element 32 shown in FIG. 5, all the piezoelectric elements 7 have the same shape, all the piezoelectric elements 7 have the same natural frequency, and propagate with the same propagation waveform. Therefore, there is no propagation time error, and an accurate flow rate can be measured.

  Further, as shown in FIG. 8, the flow velocity distribution in the pipe is different between the laminar flow and the turbulent flow. Since the conventional ultrasonic flowmeter 1 cannot measure the entire flow velocity in the pipe, an error occurs in the flow rate measurement at the boundary between the laminar flow and the turbulent flow.

  Therefore, a circular tube and a rectangular tube are compared in the shape of the measurement flow tube 2. There are two types, an ultrasonic flowmeter in which a disk-shaped composite piezoelectric vibrator 31 is bonded to the entire area of the inner diameter of a circular tube, and a rectangular composite piezoelectric vibrator 32 bonded to the inside of a rectangular tube. In the piezoelectric vibrator 31, as described above, the individual piezoelectric elements 7a, 7b, and 7c are not the same shape, so that the received waveform becomes dull and the detection accuracy is lowered.

  On the other hand, since the individual piezoelectric elements 7 constituting the rectangular composite piezoelectric vibrator 32 have the same shape, the received waveforms have the same phase, so that accurate measurement can be performed. Therefore, only the ultrasonic flowmeter 1 in which the rectangular composite piezoelectric vibrator 32 is joined inside the rectangle of the rectangular tube is the only ultrasonic flowmeter that can measure an accurate flow rate regardless of the flow velocity distribution of laminar flow and turbulent flow. 1.

  The operation of the ultrasonic flowmeter 1 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 3a which is a pair of ultrasonic transducers, the composite piezoelectric element 3a expands and contracts based on the voltage signal, and the ultrasonic wave of the composite piezoelectric element 3a Ultrasonic waves are radiated from the transmitting and receiving surfaces. This ultrasonic wave propagates to the PFA surface and is radiated to the ultrapure water 9. On the other hand, the ultrasonic wave that has propagated through the ultrapure water 9 propagates to the composite piezoelectric element 3b via the PFA surface 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 ultrapure water 8 flows through the measurement pipe 2 as a fluid to be measured. The drive frequency of the composite piezoelectric elements 3a and 3b is about 1 MHz. The controller 15 starts the time measurement of the timer 13 at the same time as outputting the transmission start signal to the drive circuit 11.

  Upon receiving the transmission start signal, the drive circuit 11 drives the composite piezoelectric element 3a and transmits an ultrasonic pulse. The transmitted ultrasonic pulse propagates through the ultrapure water 9 inside the measurement pipe 2 and is received by the composite piezoelectric element 3b. The received ultrasonic pulse is converted into an electrical signal by the composite piezoelectric element 3 b and output to the received wave detection circuit 12.

  The reception detection circuit 12 determines the reception timing of the reception signal and stops the timer 13. The calculator 14 calculates the propagation time t1.

  Next, the composite piezoelectric element 3 a and the composite piezoelectric element 3 b connected to the drive circuit 11 and the received wave detection circuit 12 are switched by the switching circuit 10. Then, again, the control unit 15 outputs a transmission start signal to the drive circuit 11 and simultaneously starts time measurement of the timer 13.

  Contrary to the measurement of the propagation time t1, an ultrasonic pulse is transmitted by the composite piezoelectric element 3b, received by the composite piezoelectric element 3a, and the propagation time t2 is calculated by the calculation unit 14.

  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.

  As described above, the ultrasonic flowmeter of the present invention is highly accurate because the ultrasonic wave travels straight through the fluid in a uniform waveform due to the uniform vibration of the rectangular composite piezoelectric element, so the variation in the phase of the detected waveform is small. The flow rate can be measured. In addition, by making the measurement pipe rectangular, it is possible to measure the flow rate with high accuracy regardless of the flow velocity distribution.

  As described above, the ultrasonic flowmeter of the present invention has been described with respect to an example in which PFA, which is an organic material, is used as the measurement pipe 2. However, the measurement pipe 2 may be a metal material such as stainless steel. Since the ultrasonic vibration propagating through the pipe 2 is large, according to the present invention, the ultrasonic vibration propagating through the measurement pipe can be greatly reduced, so that it can be used more suitably.

DESCRIPTION OF SYMBOLS 1 Ultrasonic flowmeter 2 Measurement piping 3 Composite piezoelectric element 4 Inflow part 5 Outflow part 6 Piping block 7 Piezoelectric element 8 Epoxy resin 9 Ultrapure water 10 Switching circuit 11 Drive circuit 12 Received wave detection circuit 13 Timer 14 Calculation part 15 Control part 31 Disc-shaped composite piezoelectric element 32 Rectangular composite piezoelectric element 33 Disc-shaped piezoelectric element

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, the ultrasonic transducer is a rectangular composite piezoelectric element.   2. The composite piezoelectric element according to claim 1, wherein the shape of the composite piezoelectric element is rectangular, and the length of the diagonal line is equal to or less than the inner diameter of the measurement pipe and is equal to or more than ½ of the length of the inner diameter of the measurement pipe. Ultrasonic flow meter.   The ultrasonic flowmeter according to claim 1, wherein the ultrasonic flowmeter is a composite piezoelectric element having a thickness equal to or longer than the longest side of the rectangular piezoelectric element.   The ultrasonic flowmeter according to claim 1, wherein the flow path section has a rectangular shape.

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