JP2005345445A - Ultrasonic flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic flowmeter having both a wide measurement range and high measurement accuracy while having a compact composition. <P>SOLUTION: In the ultrasonic flowmeter 100, ultrasonic sensors 2a and 2b are arranged in an upstream side and a downstream side of a communication pipe 10 for communicating a fluid. The ultrasonic sensors 2a and 2b have a structure wherein a piezoelectric element 13 is joined to an acoustic match layer 15. The piezoelectric element 13 has a form of a rectangular parallelepiped wherein electrode forming faces 13p and 13q positioned on both sides in a polarization axis direction comprise a rectangle (excluding a square). A resonance frequency fr<SB>1</SB>of an elongation vibration mode in a long edge direction of the piezoelectric element 13 and a resonance frequency fr<SB>2</SB>of an elongation vibration mode in a short edge direction are selectively used during a high flow rate and a low flow rate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an ultrasonic flow meter and an ultrasonic sensor.

  Conventionally, an ultrasonic flowmeter is known as an apparatus for measuring the flow rate of a fluid such as city gas or water. An ultrasonic sensor having a structure in which an acoustic matching layer is bonded to an element (piezoelectric element) made of piezoelectric ceramic such as PZT is used for the transducer of the ultrasonic flowmeter.

In the ultrasonic flowmeter, it is known that there is a technique for changing the frequency of the ultrasonic wave according to the flow rate as a technique for performing highly accurate measurement over a wide flow range. In order to make the frequency variable, conventionally, an ultrasonic sensor having a structure in which a plurality of piezoelectric elements having different characteristics are stacked, or an ultrasonic sensor having a structure in which piezoelectric elements having different characteristics are arranged in an array are employed. ing.
JP 2000-346685 A

  By the way, in ultrasonic flowmeters for city gas applications, it is technically difficult to employ a so-called clamp-on type in which an ultrasonic sensor is attached to the outside of the distribution pipe, and the ultrasonic sensor is used for the fluid to be measured (gas). A structure that is directly exposed to the passage is adopted. For this reason, there are restrictions on the space and arrangement available for the ultrasonic sensor, and it is difficult to employ an ultrasonic sensor having a plurality of piezoelectric elements. In addition, when an ultrasonic sensor having a plurality of piezoelectric elements is employed, there is a problem in that control and correction become complicated and cost increases. Complicated control and correction hinder measurement accuracy improvement. In addition, since it is important for an ultrasonic flowmeter that requires battery driving to have low power consumption, control complexity should be avoided if possible.

  In addition, it is known that the reflection of ultrasonic waves by the side wall of the flow pipe is often an obstacle to the cause of S / N reduction, that is, measurement accuracy. Using sharp directional ultrasonic waves reduces the effect of reflection in the flow pipe, but this time sacrifices the measurable flow range. This is because sharper directional ultrasonic waves are more likely to deviate from the range that can be transmitted and received by the flow of fluid, and the flow rate limit that makes measurement impossible comes earlier.

  As described above, highly accurate measurement over a wide flow range is still one of the main problems of the ultrasonic flowmeter. An object of the present invention is to provide an ultrasonic flowmeter having a wide flow range and high measurement accuracy while having a compact configuration. Moreover, the ultrasonic sensor suitable for the ultrasonic flowmeter with such a subject is provided.

Means for Solving the Problems and Effects of the Invention

  In order to solve the above problems, an ultrasonic flowmeter of the present invention is an ultrasonic flowmeter in which ultrasonic sensors are arranged on the upstream side and the downstream side of a flow pipe through which a fluid flows. Is bonded to the acoustic matching layer, and the piezoelectric element has a rectangular parallelepiped shape in which the electrode forming surfaces located on both sides of the polarization axis direction form a rectangle (except for a square), and has a high flow rate. The main feature is that the resonance frequency of the extension vibration mode in the long side direction of the piezoelectric element and the resonance frequency of the extension vibration mode in the short side direction are selectively used depending on the time and the low flow rate.

  In the present invention, an ultrasonic sensor configured to take out the extension vibration mode in two directions of the piezoelectric element in the direction perpendicular to each direction is used. When the electrode forming surface has a rectangular shape, as shown in FIGS. 4A and 4B, the piezoelectric element 13 has an extension vibration mode in the long side direction and an extension vibration mode in the short side direction, and the resonance frequency of each vibration mode. Is different. That is, only one pair of ultrasonic sensors can be used to perform measurement with different frequencies at the high flow rate and the low flow rate from the same point, and the measurable flow rate range is widened accordingly. In addition, since a plurality of ultrasonic sensor pairs corresponding to the flow rate range are not required, the space occupied by the ultrasonic sensors is small. Further, control and correction are simpler than when a plurality of ultrasonic sensor pairs are used, which is advantageous in improving measurement accuracy, reducing the number of measurements, and reducing power consumption. In order to efficiently use the extension vibration mode in the short side direction and the extension vibration mode in the long side direction, the ratio of the short side to the long side of the electrode forming surface is approximately 1: 3. It is desirable that the dimensions of the piezoelectric element be adjusted.

  In one preferred embodiment, the cross section perpendicular to the fluid flow direction of the flow pipe has a rectangular shape in which the side where the ultrasonic sensor is located corresponds to the short side, and the short side direction of the cross section of the flow pipe and the piezoelectric element An ultrasonic sensor is attached to the sensor attachment portion of the flow pipe so that the long side direction of the electrode forming surface is parallel. As shown in FIG. 3, a piezoelectric element having a rectangular parallelepiped shape in which the electrode formation surface is rectangular exhibits anisotropy in the direction of ultrasonic wave emission at a certain frequency in the short side direction and the long side direction. Specifically, the directivity in the short side direction is dull (FIG. 3A), and the directivity in the long side direction is sharp (FIG. 3B). This tendency continues even if the frequency used changes. Therefore, in the case where the short side direction of the cross section of the flow pipe is parallel to the long side direction of the electrode forming surface of the piezoelectric element, it is possible to suppress the spread of ultrasonic waves in the short side direction of the flow pipe. Unnecessary noise due to is suppressed, and the S / N is also good. In addition, ultrasonic waves with relatively low directivity are propagated in the fluid flow direction. For this reason, the oscillated ultrasonic wave is less likely to deviate from the ultrasonic sensor on the reception side, a stable reception sensitivity characteristic is obtained, and the upper limit of the flow rate (flow velocity) that can be measured is increased.

  Although it is ideal that the short side direction of the cross-section of the flow pipe and the long side direction of the electrode forming surface of the piezoelectric element are perfectly parallel, the significant effect of the present invention can be obtained even if there is a slight deviation due to assembly errors. Can be obtained. For example, the angle formed by the short side direction of the cross-section of the flow pipe and the long side direction of the electrode forming surface of the piezoelectric element can be regarded as parallel by about ± 5 °.

  In order to solve the problem, the second of the ultrasonic flowmeters of the present invention is an ultrasonic flowmeter in which ultrasonic sensors are arranged on the upstream side and the downstream side of a flow pipe through which a fluid flows. Has a structure in which a piezoelectric element is bonded to an acoustic matching layer. The piezoelectric element has a rectangular (square-shaped) electrode forming surface on one side in the polarization axis direction and opposite to the side connected to the acoustic matching layer. A groove extending in a direction substantially perpendicular to the longitudinal direction of the piezoelectric element is formed on the side connected to the acoustic matching layer, and the electrode forming surface is divided into left and right sides of the groove. It has a rectangular parallelepiped shape, and the resonance frequency of the longitudinal vibration mode of the piezoelectric element and the resonance frequency of the thickness direction vibration mode of the piezoelectric element are properly used at high flow rate and low flow rate. Main features.

  In the ultrasonic flowmeter of the present invention, the ultrasonic waves are taken out from the electrode forming surfaces divided on the left and right of the groove of the piezoelectric element. With regard to the lengthwise extension vibration mode, by providing the groove, the extension vibration mode in the long side direction is significantly reduced. That is, by providing the groove, the long-side vibration is reduced, and only the short-side vibration is efficiently extracted, and the short-side vibration and the thickness vibration are selectively used. Long-side vibration is reduced to a level at which the harmonics hardly affect the used frequency band (in particular, it does not affect thickness vibration). As a result, it is possible to perform measurement at a high flow rate with the longitudinal vibration (short side direction vibration) of the piezoelectric element as a low frequency side, and perform measurement at a low flow rate with the thickness direction vibration as a high frequency side. Since a plurality of ultrasonic sensor pairs corresponding to the flow rate range to be measured is not required, the space occupied by the ultrasonic sensors is small. In addition, the control and correction are simpler than when a plurality of ultrasonic sensor pairs are used, which is advantageous in improving measurement accuracy, reducing the number of measurements, and reducing power consumption.

  In addition, it is first described that the ultrasonic sensor is attached to the sensor attachment portion of the flow pipe so that the short side direction of the cross section of the flow pipe is parallel to the long side direction of the electrode forming surface of the piezoelectric element. For this reason, that is, from the viewpoint of suppressing the reflection of ultrasonic waves by the channel side wall as much as possible. As shown in FIGS. 7 and 8, also when the groove 23m is provided in the piezoelectric element 23 and the vibration in the short side direction is taken out efficiently, the vibrating surfaces 23p and 23p vibrate in the same phase. Therefore, although the vibration mode is in the short side direction, the sound wave is emitted as if the long side is long. As a result, the directivity of the ultrasonic wave with respect to the short side direction of the flow pipe is maintained relatively sharp, and reflection on the wall surface is suppressed. In addition, it has a dull directivity in the flow direction.

In order to solve the problem, the third of the ultrasonic flowmeters of the present invention is an ultrasonic flowmeter in which ultrasonic sensors are arranged on the upstream side and the downstream side of the flow pipe through which the fluid flows. The plate-like piezoelectric element has a structure in which the acoustic matching layer is disposed on the opposite side in a form of interposing the partition wall part, directly or via an adhesive, and the piezoelectric element when no load is applied. When the fundamental resonance frequency of the thickness vibration mode of the element is f ₀ and the wavelength of the ultrasonic wave propagating through the partition wall at the frequency f ₀ is λ, the partition wall portion is split so that the basic resonance frequency of the thickness vibration mode of the piezoelectric element is split. Is adjusted to a thickness deviating from n * λ / 2 (n: natural number).

  Generally, when a load is bonded in the thickness direction of a piezoelectric element having a polarization axis direction in the plate thickness direction, the resonance frequency of the thickness vibration mode changes. It is known that how the resonance frequency of the thickness vibration mode changes mainly depends on the material properties of the load to be joined (particularly the Young's modulus of the material) and the thickness of the load. However, when the thickness of the load is an integral multiple of λ / 2, it matches the no-load state regardless of the material properties, and the resonance frequency does not change theoretically. From these findings, when the case partition wall is interposed between the piezoelectric element and the acoustic matching layer, the thickness of the case partition wall can be adjusted to λ / 2 where the attenuation loss is minimized or can be ignored. Usually it is thinned to a certain extent.

  In other words, the present invention focuses on positively changing (dispersing) the resonance frequency of the thickness vibration mode. According to this, measurement with different frequencies can be easily performed with only one pair of ultrasonic sensors, and the measurable flow range can be widened. Further, since a plurality of ultrasonic sensor pairs are not required, the space occupied by the ultrasonic sensors is small. In addition, the control and correction are simpler than when a plurality of ultrasonic sensor pairs are used, which is advantageous in improving measurement accuracy, reducing the number of measurements, and reducing power consumption.

In order to solve the problem, the ultrasonic sensor according to the present invention has a plate-like piezoelectric element fixed to the partition wall of the housing case directly or via an adhesive, and acoustically matched to the opposite side through the partition wall. When the basic resonance frequency of the thickness vibration mode of the piezoelectric element at no load is f ₀ and the wavelength of the ultrasonic wave propagating through the partition wall at the frequency f ₀ is λ, In order to split the fundamental resonance frequency of the thickness vibration mode, the main feature is that the partition wall is adjusted to a thickness deviated from n * λ / 2 (n: natural number).

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 shows a basic configuration of an ultrasonic flow meter 100 used as a general residential gas meter or the like. A flow path 1 is formed in the circulation pipe 10. A gas flows through the flow path 1 along the flow direction axis O in the illustrated flow direction (average flow velocity v). A pair of sensor mounting recesses 10 a and 10 b are formed on the upstream side and the downstream side of the flow pipe 10. Ultrasonic sensors 2a and 2b are individually attached to the sensor attachment recesses 10a and 10b. The ultrasonic sensors 2a and 2b are exposed to the flow path 1 and are in direct contact with the gas flowing through the flow path 1. In the embodiment of FIG. 1, the ultrasonic sensors 2 a and 2 b are located on the opposite side across the flow path 1, but there is also a form located on the same side.

  In the flow pipe 10, the flow direction axis O is linear between the ultrasonic sensor 2a and the ultrasonic sensor 2b, and the shape and cross-sectional area of the axial cross section are the same in the flow direction. When the measurement object is gas, any of a circular shape, an elliptical shape, a square shape, a rectangular shape, and the like may be adopted as the axial cross-sectional shape of the flow pipe 10. In the present embodiment, as shown in FIG. 5, the flow pipe 10 has a rectangular cross section perpendicular to the flow direction axis O, and the sides where the ultrasonic sensors 2a and 2b are located are short sides 11a and 11b. It hits.

  The ultrasonic sensors 2a and 2b are transducers composed of piezoelectric elements, acoustic matching layers, etc., and transmitting means 22 composed of a drive voltage circuit or the like for oscillating these ultrasonic sensors 2a and 2b, or ultrasonic waves It is connected to receiving means 32 comprising a voltage detection circuit for detecting the voltage generated by the sensors 2a, 2b. Switching of the connection destinations of the ultrasonic sensors 2a and 2b is performed by a switching unit 3 constituted by an analog switch or the like. The switching means 3 is controlled by the microcomputer 9. For example, when transmitting an ultrasonic wave from the upstream side in the flow direction (ultrasonic sensor 2a side) to the downstream side in the flow direction (ultrasonic sensor 2b side), the ultrasonic sensor 2a becomes the transmission side (oscillation source). Therefore, the switching unit 3 first connects the transmission unit 22 and the ultrasonic sensor 2a, and connects the reception unit 32 and the ultrasonic sensor 2b.

  The ultrasonic flowmeter 100 includes, as a measurement unit, an amplifier 5 that amplifies the ultrasonic reception output obtained by the ultrasonic sensors 2a and 2b, and a zero cross point detection unit 6 that detects an ultrasonic arrival time from an output waveform by a zero cross method. And a microcomputer 9. The received wave is amplified by the amplifier 5 and input to the zero cross point detection means 6. The zero cross point detection unit 5 detects whether or not the ultrasonic wave oscillated by one of the ultrasonic sensors 2a and 2b has been received by the other. A so-called zero cross method is employed for detection of the received wave. The zero cross point detector 5 inputs a signal indicating that the received wave has been detected to the microcomputer 9. The microcomputer 9 serving as the time measuring means is based on the signal acquisition from the zero cross point detecting means 6 and directly after one of the ultrasonic sensors 2a and 2b oscillates the ultrasonic wave until the other receives the ultrasonic wave. The arrival time is measured, and the flow rate is calculated from the measurement result.

The ultrasonic flowmeter 100 is configured as a device that measures a flow rate by a reciprocal propagation time difference method. In FIG. 1, the average flow velocity of gas is v, the velocity of sound propagating in the gas is c, the ultrasonic traveling direction (line connecting the ultrasonic sensors 2a and 2b) and the gas flow direction (flow direction axis O). When the angle is θ and the distance between the ultrasonic sensor 2a and the ultrasonic sensor 2b is H, the forward arrival time Td and the reverse arrival time Tu when the ultrasonic wave propagates by the distance H are respectively expressed as follows. .
Td = H / (c + v · cos θ) (1)
Tu = H / (cv · cos θ) (2)
Taking the reciprocal of equations (1) and (2) and taking the difference, the following equation is obtained.
1 / Td−1 / Tu = 2v · cos θ / H (3)
Therefore, from the measurement of the forward arrival time Td and the reverse arrival time Tu, the average gas flow velocity v and flow rate Q are obtained by the following equations. “A” is a cross-sectional area of the flow path 1.
v = (1 / 2Td−1 / 2Tu) H / cos θ (4)
Q = v · A (5)
In this way, by eliminating the sound velocity c depending on the gas temperature and the contained components from the equation (4), the flow velocity v is obtained from the measured values (arrival times Td, Tu) and the constant values (H, θ). Has the advantage of being

  Next, FIG. 2 shows a top view (upper part) and a cross-sectional view (lower part) of the main parts of the ultrasonic sensors 2a and 2b. The ultrasonic sensors 2 a and 2 b have a structure in which the piezoelectric element 13 is bonded to the acoustic matching layer 15. A cylindrical housing case 17 is fixed to the outer peripheral edge portion of the acoustic matching layer 15 so as to surround the piezoelectric element 13, and the side surface of the piezoelectric element 13 and the inner peripheral surface of the housing case 17 face each other. ing. The piezoelectric element 13 and the acoustic matching layer 15 are joined by an epoxy-based adhesive.

  The piezoelectric element 13 is composed of a PZT (lead zirconate titanate) -based piezoelectric ceramic, and the joint surface with the acoustic matching layer 15 and a surface parallel to the surface are electrode formation surfaces 13p and 13q (electrodes are formed). The surface). In addition to PZT, barium titanate, lead titanate, lead lanthanum zirconate titanate, etc. may be used. The polarization axis direction of the piezoelectric element 13 is a direction orthogonal to the electrode formation surfaces 13p and 13q. As the electrode material, generally a highly conductive metal such as silver is used. Leads for voltage application and reception wave extraction are connected to the electrode formation surfaces 13p and 13q by solder or the like, which is omitted in FIG. The piezoelectric element 13 is formed in a rectangular parallelepiped shape in which the electrode forming surfaces 13p and 13q form a rectangle (excluding a square).

  The acoustic matching layer 15 is formed by mixing a glass balloon into a resin material such as an epoxy resin and molding it into a disk shape. The acoustic impedance of the acoustic matching layer 15 is adjusted to be as close as possible to the geometric mean of the acoustic impedances of the piezoelectric element 13 and the medium (gas) in order to improve the transmission efficiency of ultrasonic waves. In order to obtain a target acoustic impedance, the acoustic matching layer 15 may be made of a resin material that does not contain glass balloons. The case 17 is formed by molding a metal material having excellent corrosion resistance, such as austenitic stainless steel or aluminum alloy, or an engineering plastic.

Now, the ultrasonic flowmeter 100 according to the present invention uses the ultrasonic sensors 2a and 2b shown in FIG. 2 to perform measurement with different ultrasonic frequencies at high flow rate and low flow rate. It is configured. Specifically, the resonance frequency fr ₁ of the extension vibration mode in the long side direction of the ultrasonic sensors 2a and 2b is used at a high flow rate, and the resonance frequency fr ₂ of the extension vibration mode in the short side direction is used at a low flow rate. Thereby, high measurement accuracy is obtained over a wide flow rate range from a low flow rate to a high flow rate. The longitudinal vibration mode in the long side direction is one of modes in which the vibration direction is perpendicular to the polarization axis direction as shown in FIG. Further, the extension vibration mode in the short side direction is another mode in which the vibration direction is perpendicular to the polarization axis direction as shown in FIG.

The resonance frequency normally used in the rectangular plate-like piezoelectric element 13 is that in the longitudinal vibration mode in the long side direction represented as the longitudinal vibration mode. In the present invention, not only the long side direction but also the resonance frequency fr ₂ of the extension vibration mode in the short side direction is positively used. Drive pulses corresponding to the resonance frequencies fr ₁ and fr ₂ are generated by the microcomputer 9 and the transmission means 22 controlled by the microcomputer 9. In this case, the transmission means 22 is configured to include a VCO (voltage controlled oscillator).

However, it is difficult to efficiently extract ultrasonic waves in each vibration mode under completely unobtrusive conditions. Therefore, by adjusting the size of each part of the ultrasonic sensors 2a and 2b as described below, it is possible to efficiently extract ultrasonic waves with the resonance frequencies of the two extension vibration modes. Here, the resonance frequency of the extension vibration mode in the long side direction on the low frequency side is fr ₁ , and the resonance frequency in the short side direction on the high frequency side is fr ₂ . The wavelength of the ultrasonic wave propagating through the acoustic matching layer 15 at the frequency fr ₂ is λ ′, and the thickness of the acoustic matching layer 15 is t1. As shown in FIG. 2, the short side direction is a direction parallel to the short side 13s of the electrode forming surface. The long side direction is a direction parallel to the long side 13r of the electrode formation surface.

Considering the matching conditions of the acoustic matching layer 15, when each component is configured to satisfy the following expressions (A) and (B), two different frequencies are simultaneously applied to the thickness of one acoustic matching layer 15. Align. That is, the acoustic matching layer 15 is adjusted to a thickness t1 that matches the resonance frequency fr ₁ on the low frequency side and the resonance frequency fr ₂ on the high frequency side.
fr ₂ ≈ (2m ₁ +1) * fr ₁ (m ₁ : natural number) (A)
t1≈ (2m ₂ +1) * λ ′ / 4 (m ₂ : natural number) (B)

In the ultrasonic flowmeter 100 of the present invention, it is desirable to use ultrasonic waves in the KHz band and to make the thickness t1 of the acoustic matching layer 15 as thin as possible in order to reduce the attenuation loss. Considering this, it is realistic to adjust m ₁ = 1 and m ₂ = 1, that is, the frequency ratio is approximately 1: 3 and the thickness t1 of the acoustic matching layer 15 is adjusted to 3λ ′ / 4. It is ideal that fr ₂ = 3 * fr ₁ is satisfied, but the object of the present invention can be achieved even if there is a certain width. The values of the resonance frequencies fr ₁ and fr ₂ and the thickness t1 of the acoustic matching layer 15 slightly change depending on various factors such as the material characteristics of the acoustic matching layer 15, the medium to be measured, the bonding adhesive, and the electrode. Therefore, considering the situation that it is difficult to determine in general, for example, the range of fr ₂ = (3 ± 0.18) * fr ₁ , t1 = (3/4 ± 0.022) λ ′ is the purpose of the invention. It can be said that it is a realistic range that can be achieved.

As a more specific standard, a commercially available PZT piezoelectric element for an ultrasonic sensor is used, the resonance frequency fr ₁ in the extension vibration mode in the long side direction is set to 120 KHz to 190 KHz, and the resonance frequency fr in the extension vibration mode in the short side direction. _{When 2} is set to 330 KHz to 530 KHz and the resonance frequency difference is set to 220 KHz to 420 KHz, for example, an optimal point may be searched for under the following conditions.

<Piezoelectric element>
Long side dimension: L1 = 8.5 mm to 15.5 mm
Short side dimension: W1 = 3.0 mm to 6.3 mm
Element thickness: T1 = 0.5 mm to 3.75 mm
<Acoustic matching layer>
Acoustic ^{impedance: Z1 = 0.65Kg / m 2 ·} s~2.70Kg / m 2 · s
Matching layer thickness: t1 = 1.5 mm to 5.5 mm

As described above, it is difficult to uniformly derive the optimum resonance frequencies fr ₁ and fr ₂ and the thickness t1 of the acoustic matching layer 15, but the ratio between the short side 13s and the long side 13r of the piezoelectric element 13 is difficult. Is set to 1: 2.7 to 1: 3.2 (approximately 1: 3), the resonance frequency difference (fr ₂ −fr ₁ ) is 220 KHz to 420 KHz, the extension vibration mode in the short side direction, and the long side direction It is comparatively easy to find the conditions that can be taken out efficiently in both of the stretching vibration modes. When the resonance frequency difference is in the range of 220 KHz to 420 KHz, it is easy to achieve both high-precision measurement and a wide flow rate range. Further, in the above dimension range, the thickness T1 of the piezoelectric element 13 may coincide with or exceed the short side dimension W1, but it is possible to find suitable conditions even in such a case. That is, the piezoelectric element 13 is not limited to a plate shape.

  When the piezoelectric element 13 described above is used, the emitted ultrasonic wave has anisotropy. FIG. 3 conceptually shows this state. FIG. 3A shows the directivity in the short side direction, and FIG. 3B shows the directivity in the long side direction. The directivity in the long side direction is sharp, and the directivity in the short side direction is dull. In such a case, the posture of the piezoelectric element 13 with respect to the flow pipe 10 with the ultrasonic sensors 2a and 2b attached to the sensor attachment portions 10a and 10 is important.

  As described above with reference to FIG. 5, it has been described that the flow pipe 10 has a rectangular cross section, and the ultrasonic sensors 2 a and 2 b are disposed at positions corresponding to the short sides 11 a and 11 b of the cross section. FIG. 6 is an exploded perspective view of the flow pipe 10 and the ultrasonic sensors 2a and 2b when viewed from a different viewpoint from FIG. As shown in FIG. 6, the sensor mounting portions 10a and 10b of the flow tube 10 are arranged such that the short side direction WL of the cross section of the flow tube 10 and the long side 13r of the electrode forming surface 13p of the piezoelectric element 13 are parallel. The ultrasonic sensors 2a and 2b are attached to the head.

  This makes it difficult for the oscillated ultrasonic wave to deviate from the ultrasonic sensor on the reception side, so that stable reception sensitivity characteristics can be obtained and the upper limit of the flow rate range that can be measured is increased. At the same time, unnecessary noise due to the channel wall surface is suppressed, S / N is improved, and measurement accuracy is increased. Of course, the directivity also changes depending on the frequency of the ultrasonic wave used, but the tendency shown in FIG. 3 continues. Accordingly, the arrangement shown in FIG. 6 is suitable regardless of whether the frequency of ultrasonic waves used is plural or single.

  Further, if the following arrangement is adopted in combination with the arrangement described with reference to FIG. 6, the problem that the ultrasonic wave deviates from the ultrasonic sensor on the reception side at a high flow rate can be alleviated. As shown in FIG. 1 and the like, (1) the transmission / reception surface of the upstream ultrasonic sensor 2a (the main surface of the acoustic matching layer 15) and the transmission / reception surface of the downstream ultrasonic sensor 2b are arranged in parallel. (2) Consider a case where the upstream and downstream ultrasonic sensors are arranged so that the angle formed by the same side of the flow pipe 10, that is, the transmission / reception surfaces of the pair of ultrasonic sensors, is 90 °. In these cases (1) and (2), when the flow is zero, the reception sensitivity peaks.

  Therefore, as shown in FIG. 12, the normal direction of the transmission / reception surface of the upstream ultrasonic sensor 2a and the transmission / reception surface of the ultrasonic sensor 2b on the downstream side so that the peak of the reception sensitivity arrives at a constant flow velocity. The arrangement is shifted from the normal direction. In the case of arranging the upstream and downstream ultrasonic sensors on the same side of the flow pipe 10, the normal direction of the transmission / reception surface of the upstream ultrasonic sensor and the method of the transmission / reception surface of the downstream ultrasonic sensor The arrangement of each ultrasonic sensor is adjusted so that the angle formed with the line direction is smaller than 90 °. In this way, in combination with the configuration described in FIG. 6, it is possible to expect an increase in the measurement limit on the high flow rate side.

(Second embodiment)
Next, another ultrasonic flowmeter 200 according to the present invention will be described. The basic configuration is the same as that of the ultrasonic flowmeter 100 described above, as shown in FIG. The main difference is in the vibration modes of the ultrasonic sensors 5a and 5b and the piezoelectric element 23. FIG. 7 shows a top view and a schematic cross-sectional view of the main parts of the ultrasonic sensors 5a and 5b. The ultrasonic sensors 5a and 5b have a structure in which the piezoelectric element 23 is bonded to the acoustic matching layer 25 with an adhesive. In the piezoelectric element 23, the electrode forming surfaces 23p, 23p on the side to be joined to the acoustic matching layer 25 are divided into a plurality of portions (in this embodiment, left and right equal divisions) by the grooves 23m. If another groove is formed in parallel with the longitudinal direction ND of the piezoelectric element 23, the electrode forming surface is divided into four, but this form can also be adopted. Other structures are substantially the same as those of the ultrasonic sensors 2a and 2b in FIG. 2, but the dimensions of each component are different.

  The electrode formation surfaces 23p and 23p divided into two regions by the groove 23m are substantially square. On the other hand, the electrode forming surface 23q opposite to the side bonded to the acoustic matching layer 25 is rectangular (except for a square). The groove 23m is formed to extend in a direction substantially orthogonal to the longitudinal direction ND (see FIG. 8) of the piezoelectric element 23. The piezoelectric element 23 has a rectangular parallelepiped shape as a whole. In the case of such a structure, as shown in FIG. 8, ultrasonic waves extracted in the longitudinal vibration mode of the piezoelectric element 23 are transmitted to the acoustic matching layer 25 from the individual electrode forming surfaces 23p and 23p. Then, regarding the longitudinal direction vibration mode, by providing the groove 23m, the long side direction vibration can be reduced and only the short side direction vibration can be efficiently extracted. Since the ultrasonic waves of the short side direction vibration are relatively dull in the flow direction, they are suitable for measurement at high flow rates.

On the other hand, when the flow rate is low, sharp directional ultrasonic waves are used to suppress reflection on the wall surface of the flow pipe 10 as much as possible, thereby improving S / N and increasing measurement accuracy. When the flow rate is low, the vibration mode with a high resonance frequency can be used because the influence of the flow field is small. Specifically, as shown in the graph of FIG. 9, by using the thickness direction vibration mode having a resonance frequency fr ₂ higher than the resonance frequency fr ₁ of the lengthwise extension vibration mode, at a high flow rate and a low flow rate. Different ultrasonic frequencies.

Of course, as described above, a practical ultrasonic sensor cannot be obtained if each component has a clear dimension. Therefore, by adjusting the component dimensions and the like of the ultrasonic sensors 5a and 5b as described below, it is possible to efficiently extract ultrasonic waves with the resonance frequencies of the two vibration modes. The resonance frequency fr 1 longitudinal vibration _mode, fr 2 the resonant frequency of the thickness direction vibration _mode, the wavelength of the ultrasonic wave propagating in the acoustic matching layer 25 at a frequency fr ₂ lambda ', the thickness of the acoustic matching layer 25 t2 And

Considering the matching conditions of the acoustic matching layer 15, when each component is configured to satisfy the following expressions (A) and (B), two different frequencies are simultaneously applied to the thickness of one acoustic matching layer 15. Align. That is, the acoustic matching layer 15 is adjusted to a thickness t2 that matches the resonance frequency fr ₁ on the low frequency side and the resonance frequency fr ₂ on the high frequency side.
fr ₂ ≈ (2m ₁ +1) * fr ₁ (m ₁ : natural number) (A)
t2≈ (2m ₂ +1) * λ ′ / 4 (m ₂ : natural number) (B)

Considering the fact that it is difficult to determine the values of the resonance frequencies fr ₁ and fr ₂ and the thickness t2 of the acoustic matching layer 25, for example, fr ₂ = (3 ± 0.20) * fr ₁ , t2 = (3 / It can be said that the range of 4 ± 0.025) λ ′ is a practical range in which the object of the invention can be achieved.

As one guideline, a commercially available PZT piezoelectric element for an ultrasonic sensor is used, and the resonance frequency fr ₁ in the extension vibration mode in the short side direction is set to 120 KHz to 190 KHz, and the resonance frequency fr ₂ in the extension vibration mode in the long side direction is set. In the case where 330 KHz to 530 KHz and the resonance frequency difference are set to 220 KHz to 420 KHz, an optimal point may be searched for under the following conditions.

<Piezoelectric element>
Long side dimension: L2 = 18 mm to 24 mm (preferably 20 mm to 22 mm)
Short side dimension: W2 = 7.5 mm to 12.5 mm (preferably 9 mm to 11 mm)
Element thickness: T2 = 2.75 mm to 5.45 mm (preferably 3 mm to 4 mm)
Groove width d1: About 30% to 40% of element thickness T2 Groove depth d2: About 55% to 65% of element thickness T2 <Acoustic matching layer>
Acoustic ^{impedance: Z = 0.65Kg / m 2 ·} s~2.70Kg / m 2 · s
Matching layer thickness: t2 = 1.5 mm to 5.5 mm

If the ratio of the length of the piezoelectric element 13 in the width direction (direction parallel to the groove 23m) and the length in the longitudinal direction is set to 1: 2.7 to 1: 3.2 (approximately 1: 3), the resonance frequency difference ( fr ₂ −fr ₁ ) 220 KHz to 420 KHz, and it is easy to find a condition for efficiently extracting ultrasonic waves. When the resonance frequency difference is in the range of 220 KHz to 420 KHz, it is easy to achieve both high-precision measurement and a wide flow rate range. Further, in the above dimension range, the thickness T2 of the piezoelectric element 23 may coincide with or exceed the short side dimension W2, but it is possible to find suitable conditions even in such a case. That is, the piezoelectric element 23 is not limited to a plate shape.

  Further, the posture of the piezoelectric element 23 when the ultrasonic sensors 5a and 5b are attached to the sensor attachment portions 10a and 10b of the flow pipe 10 is also important as in the previous embodiment. That is, as described with reference to FIG. 6, the sensor mounting recesses 10 a and 10 b of the flow pipe 10 are superposed so that the short side direction WL of the cross section of the flow pipe 10 and the longitudinal direction ND of the piezoelectric element 23 are parallel to each other. The sonic sensors 5a and 5b are attached.

  Then, the ultrasonic waves emitted from the ultrasonic sensors 5a and 5b have a sharp directivity in the short side direction WL of the cross section of the flow tube 10 and a broad directivity in the flow direction. Then, unnecessary noise due to the channel wall surface is suppressed and the measurement accuracy is improved, and since the ultrasonic wave is less likely to deviate from the ultrasonic sensor on the reception side, a stable reception sensitivity characteristic is obtained, and the flow rate limit that can be measured increases. Both effects can be achieved. Coupled with this, since ultrasonic waves having different frequencies are used separately at low flow rates and high flow rates, further improvement in measurement accuracy and expansion of the measurable flow rate range can be expected.

(Third embodiment)
Next, another ultrasonic flow meter 300 according to the present invention will be described. As shown in FIG. 1, the basic configuration is common to the ultrasonic flowmeters 100 and 200 described above. The main difference is in the vibration modes of the ultrasonic sensors 7 a and 7 b and the piezoelectric element 33. FIG. 10 is a schematic cross-sectional view of the ultrasonic sensors 7a and 7b. The ultrasonic sensors 7a and 7b have a structure in which a plate-like piezoelectric element 33 is fixed to a partition wall portion 37k of a case 37, and an acoustic matching layer 35 is disposed on the opposite side with the partition wall portion 37k interposed.

  The shape of the piezoelectric element 33 may be square or circular as long as it is plate-shaped, the thickness direction is the polarization axis direction, and the surfaces positioned on both sides of the thickness direction are electrode formation surfaces 33p and 33q. It has become. A conductive adhesive is used for bonding the case 37 and the piezoelectric element 33. Since the case 37 is made of a metal material such as stainless steel or aluminum alloy, the one electrode forming surface 33p of the piezoelectric element 33 and the case 37 are electrically connected. On the other hand, the adhesive that fixes the acoustic matching layer 35 to the case 37 does not need conductivity.

  The case 37 has a bottomed cylindrical shape that accommodates the piezoelectric element 33, and the bottom part is a partition part 37 k that fixes the piezoelectric element 33. A cylindrical vibration suppressing member 45 is disposed in the case 37 so as to surround the piezoelectric element 33. The vibration suppressing member 45 is made of a rubber material such as urethane rubber. A gel material is filled as a backing material so as to fill a gap between the vibration suppressing member 45 and the piezoelectric element 33. A terminal plate 43 having substantially the same diameter as the inner diameter of the case 37 is disposed at a position facing the piezoelectric element 33. The terminal plate 43 is made of a metal material having good conductivity equivalent to or equivalent to that of the case 37 and is directly connected to the case 37. A ground terminal 41 extending to the outside of the case 37 is connected to the terminal plate 43. The ground terminal 41 is electrically connected to the electrode forming surface 33 p of the piezoelectric element 33 through the terminal plate 43 and the case 37. Therefore, one electrode forming surface 33p of the piezoelectric element 33 has a structure that does not require attachment of the lead wire and routing of the lead wire. A lead wire 46 extending from the power supply terminal 42 is solder-connected to the other electrode forming surface 33q.

Here, the partition portion 37k of the case 37 that separates the piezoelectric element 33 and the acoustic matching layer 35 is adjusted to a special thickness. Specifically, when the fundamental resonance frequency of the thickness vibration mode of the piezoelectric element 33 at no load is f ₀ , and the wavelength of the ultrasonic wave propagating through the partition wall portion 37k at the frequency f ₀ is λ, the thickness vibration of the piezoelectric element 33 is The partition wall 37k is adjusted to a thickness D that deviates from n * λ / 2 (n: natural number) so that the fundamental resonance frequency of the mode is split. In general, when ultrasonic waves are oscillated in the thickness vibration mode, even if the piezoelectric element and the acoustic matching layer are directly bonded, or even if a case is interposed, the thickness of the interposed part affects the basic resonance frequency of the piezoelectric element. Make it very thin so as not to reach it. As shown in FIG. 11, the thickness vibration mode is a vibration mode in a direction parallel to the polarization axis direction.

  The present invention is characterized in that the thickness of the partition wall 37k of the case 37 is adjusted so as to positively affect the basic resonance frequency of the piezoelectric element 33. Specifically, the thickness of the partition wall portion 37k is adjusted to be smaller than λ / 2 so that the basic resonance frequency of the thickness vibration mode of the piezoelectric element 33 is divided into two. If the thickness deviates from an integral multiple of λ / 2, it is possible to peak the resonance frequency where the thickness vibration mode is dominant, but considering the attenuation loss of the ultrasonic wave, the partition wall portion 37k This is because the thickness should be as thin as possible.

As in the above-described embodiment, the basic resonance frequency of the thickness vibration mode divided into two at high flow rate and low flow rate is used properly. Specifically, the measurement is performed using the resonance frequency fr ₁ on the low frequency side when the flow rate is high, and using the resonance frequency fr ₂ on the high frequency side when the flow rate is low. Of course, it is necessary to appropriately adjust the dimensions of each component. First, assuming that the acoustic impedance of the piezoelectric element 33 is Z ₀ , the acoustic impedance of the case 37 (including the partition wall 37 k) is Z ₁ , and the acoustic impedance of the acoustic matching layer 35 is Z ₂ , Z ₂ << Z ₀ ≦ Z ₁ It is assumed that.

Considering the matching condition of the acoustic matching layer 35, when each component is configured to satisfy the following expressions (A) and (B), the frequency divided into two is matched to the thickness of one acoustic matching layer 35. To do. That is, the acoustic matching layer 35 is adjusted to a thickness t3 that matches the resonance frequency fr ₁ on the low frequency side and the resonance frequency fr ₂ on the high frequency side.
fr ₂ ≈ (2m ₁ +1) * fr ₁ (m ₁ : natural number) (A)
t3≈ (2m ₂ +1) * λ ′ / 4 (m ₂ : natural number) (B)

The dispersion of the thickness direction vibration mode varies depending on what thickness D the partition wall portion 37k is adjusted to. Furthermore, the dispersion mode of the thickness direction vibration mode varies depending on the type of adhesive used, the thickness of the adhesive layer, and the material physical properties of the case. Therefore, it is difficult to uniquely determine the thickness D of the partition wall portion 37k. Considering such circumstances, for example, the ranges of fr ₂ = (3 ± 0.30) * fr ₁ , t3 = (3/4 ± 0.030) λ ′ are practical ranges in which the object of the invention can be achieved. It can be said that. This is based on the fact that fr ₁ : fr ₂ = 1: 3 is ideal, but the deviation of fr ₂ to about 10% can be considered at a practical level. This is also true for the two embodiments (FIGS. 2 and 7) described above. Therefore, the range described in the previous two embodiments is a suitable range, and it can be said that the range can actually be expanded to the above-mentioned extent.

As a specific guideline, the material of the case 37 is austenitic stainless steel (for example, SUS304), an epoxy resin material is used for the base material of the conductive adhesive, and the thickness of the conductive adhesive layer is 3.5 μm. When adjusting to ˜15 μm, the partition wall 37k of the case 37 is fixed to one point within the range of 1.1 mm to 1.5 mm, and the shape and dimensions of the piezoelectric element 33 and the acoustic matching layer 35 are then used. By adjusting the basic resonance frequency of the thickness direction vibration mode which has been divided into _{two, an} ultrasonic sensor in which the resonance frequency fr ₁ on the low frequency side is 230 KHz to 260 KHz and the resonance frequency fr ₂ on the high frequency side is 670 KHz to 700 KHz. It can be produced relatively easily.

  As mentioned above, although several embodiment was shown in this specification, it is possible to combine them mutually within the range which does not deviate from the summary of invention. In addition, although the same thing is used in each embodiment about the notation of a resonant frequency and a wavelength, it is not necessarily meaning that these mutually correspond.

Explanatory drawing which shows the basic composition of the ultrasonic flowmeter of this invention. The principal part top view and cross-sectional schematic diagram of an ultrasonic sensor. The conceptual diagram which shows the directivity of an ultrasonic wave. Explanatory drawing of the vibration mode of a piezoelectric element. The perspective view which shows the cross-sectional shape of a flow pipe. The exploded perspective view explaining the direction of the piezoelectric element with respect to a flow pipe. The principal part top view and cross-sectional schematic diagram of the ultrasonic sensor of a 2nd form. Explanatory drawing of the vibration mode of a piezoelectric element. The graph which shows typically the frequency characteristic of the ultrasonic sensor of FIG. The cross-sectional schematic diagram of the ultrasonic sensor of a 3rd form. Explanatory drawing of the vibration mode of a piezoelectric element. The schematic diagram which shows suitable arrangement | positioning of an ultrasonic sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Flow path 2a, 2b, 5a, 5b, 7a, 7b Ultrasonic sensor 10 Flow pipe 10a, 10b Sensor attachment recessed part 13, 23, 33 Piezoelectric element 13s Short side 13r Long side 13p, 13q, 23p, 23q Electrode formation surface 15 , 25, 35 Acoustic matching layers 17, 27, 37 Case 23m Groove 37k Bulkhead 100, 200, 300 Ultrasonic flow meter O Flow direction axis WL Short side direction ND Longitudinal direction of flow pipe

Claims (8)

In the ultrasonic flowmeter in which ultrasonic sensors are arranged on the upstream side and the downstream side of the flow pipe through which the fluid flows,
The ultrasonic sensor has a structure in which a piezoelectric element is bonded to an acoustic matching layer,
The piezoelectric element has a rectangular parallelepiped shape in which electrode forming surfaces located on both sides in the polarization axis direction form a rectangle (excluding a square),
The ultrasonic flow rate is characterized in that the resonance frequency of the extension vibration mode in the long side direction of the piezoelectric element and the resonance frequency of the extension vibration mode in the short side direction are selectively used at high flow rate and low flow rate. Total. In the ultrasonic flowmeter in which ultrasonic sensors are arranged on the upstream side and the downstream side of the flow pipe through which the fluid flows,
The ultrasonic sensor has a structure in which a piezoelectric element is bonded to an acoustic matching layer,
In the piezoelectric element, the electrode forming surface on one side in the direction of the polarization axis and the side opposite to the side connected to the acoustic matching layer forms a rectangle (except for a square), while on the side connected to the acoustic matching layer A groove extending in a direction substantially perpendicular to the longitudinal direction of the piezoelectric element is formed, and the electrode forming surface is divided into right and left of the groove, and as a whole has a rectangular parallelepiped shape,
An ultrasonic flowmeter, wherein the resonance frequency of the longitudinal vibration mode of the piezoelectric element and the resonance frequency of the thickness direction vibration mode are selectively used at high flow rate and low flow rate. The cross section of the flow pipe perpendicular to the fluid flow direction has a rectangular shape in which the side where the ultrasonic sensor is located corresponds to the short side,
The ultrasonic sensor is attached to a sensor attachment portion of the flow pipe so that a short side direction of the cross section of the flow pipe is parallel to a long side direction of the electrode forming surface of the piezoelectric element. The ultrasonic flowmeter according to 1 or 2. In the ultrasonic flowmeter in which ultrasonic sensors are arranged on the upstream side and the downstream side of the flow pipe through which the fluid flows,
The ultrasonic sensor has a structure in which a plate-like piezoelectric element is fixed to a partition wall portion of a housing case directly or via an adhesive, and an acoustic matching layer is disposed on the opposite side with the partition wall interposed. ,
When the fundamental resonance frequency of the thickness vibration mode of the piezoelectric element at no load is f ₀ , and the wavelength of the ultrasonic wave propagating through the partition wall at the frequency f ₀ is λ, the fundamental resonance frequency of the thickness vibration mode of the piezoelectric element is The ultrasonic flowmeter is characterized in that the partition wall is adjusted to have a thickness deviating from n * λ / 2 (n: natural number) so as to split.   The ultrasonic flowmeter according to claim 4, wherein a thickness of the partition wall is adjusted to be smaller than λ / 2 so that a basic resonance frequency of a thickness vibration mode of the piezoelectric element is divided into two.   6. The ultrasonic flowmeter according to claim 4, wherein measurement is performed using a low frequency side of the split fundamental resonance frequency of the thickness vibration mode at a high flow rate and using a high frequency side at a low flow rate. When the resonance frequency on the low frequency side is fr ₁ , the resonance frequency on the high frequency side is fr ₂ , the wavelength of the ultrasonic wave propagating through the acoustic matching layer at the frequency f ₂ is λ ′, and the thickness of the acoustic matching layer is t The ultrasonic flowmeter according to any one of claims 1 to 6, wherein dimensions of each component are adjusted so as to satisfy the following expressions (A) and (B).
fr ₂ ≈ (2m ₁ +1) * fr ₁ (m ₁ : natural number) (A)
t≈ (2m ₂ +1) * λ ′ / 4 (m ₂ : natural number) (B) The plate-like piezoelectric element is fixed to the partition wall of the housing case directly or via an adhesive,
Having a structure in which an acoustic matching layer is disposed on the opposite side with the partition wall interposed therebetween,
When the fundamental resonance frequency of the thickness vibration mode of the piezoelectric element at no load is f ₀ , and the wavelength of the ultrasonic wave propagating through the partition wall at the frequency f ₀ is λ, the fundamental resonance frequency of the thickness vibration mode of the piezoelectric element is The ultrasonic sensor is characterized in that the partition wall is adjusted to have a thickness shifted from n * λ / 2 (n: natural number).

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