JP2006017639A - Ultrasonic flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic flowmeter capable of improving lowering of an S/N ratio caused by wall face reflection, and capable of securing a stable reception sound wave along a flow direction, without being accompanied with a problem of increasing a power consumption, a cost or the like. <P>SOLUTION: This flowmeter is provided with a flow pipe 10 having a cross-section perpendicular to a gas flowing direction, and sensor attaching recesses 10a, 10b are provided in a form to be opened to a flow passage 1, in an upstream and a downstream of a position corresponding to a short side 10r of the cross-section. Ultrasonic sensors 2a, 2b are attached to the sensor attaching recesses 10a, 10b to be positioned in a place where all of ultrasonic transmitting and receiving faces 2ap, 2bp are separated by at least a distance H from the flow passage 1. When a reference axis OH is defined to penetrate perpendicularly the substantial center of the ultrasonic transmitting and receiving faces 2ap, 2bp of the ultrasonic sensors 2a, 2b, a distance Dh from inner circumferential faces 10ap, 10bp of the sensor attaching recesses 10a, 10b up to the reference axis OH is gradually made long between the transmitting and receiving faces 2ap, 2bp of the ultrasonic sensors 2a, 2b to the flow passage 1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an ultrasonic flow meter.

Conventionally, there is an ultrasonic flow meter as a device for measuring the flow rate of a fluid such as city gas or water. In an ultrasonic flowmeter, it is known that reflection of ultrasonic waves on the side wall of a flow pipe through which fluid flows often becomes a cause of S / N reduction, that is, an obstacle to measurement accuracy. In order to deal with these problems, 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. Has been done.
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. Moreover, since complicated control leads to an increase in power consumption, it is not preferable for an ultrasonic flowmeter that requires battery driving.

  An object of the present invention is to provide an ultrasonic flowmeter that can improve the S / N decrease due to wall surface reflection without causing problems such as an increase in power consumption and cost, and in which a stable received sound wave is secured in the flow direction. It is in.

Means for Solving the Problems and Effects of the Invention

  In order to solve the above problems, the first of the ultrasonic flowmeter of the present invention is provided with a flow pipe having a rectangular cross section perpendicular to the flow direction of the fluid whose flow rate is measured, and upstream of the position corresponding to the short side of the cross section. Cylindrical sensor attachment portions are provided on the side and downstream sides so as to open to the flow path, and the ultrasonic transmission surface or reception surface (referred to as transmission / reception surface) is at least a distance H from the flow passage in the sensor attachment portion. The ultrasonic sensor is mounted so that it is located only at a distance, and when defining a reference axis that penetrates substantially the center of the transmitting / receiving surface of the ultrasonic sensor vertically, from the inner peripheral surface of the sensor mounting portion to the reference axis The main feature is that the distance between the transmission and reception surfaces gradually increases from the transmission / reception surface to the flow path.

  Similarly, in order to solve the problem, the second of the ultrasonic flowmeter of the present invention includes a flow pipe having a rectangular cross section perpendicular to the flow direction of the fluid whose flow rate is measured, and upstream of the position corresponding to the short side of the cross section. A cylindrical sensor mounting portion is provided on the side and the downstream side so as to open to the flow path, and an ultrasonic transmission surface or reception surface (referred to as a transmission / reception surface) is inclined with respect to the fluid flow direction in the sensor mounting portion. When the ultrasonic sensor is mounted in a different posture and a reference axis that passes through substantially the center of the transmission / reception surface of the ultrasonic sensor is defined vertically, the distance from the inner peripheral surface of the sensor mounting portion to the reference axis is the transmission / reception surface. It is characterized by gradually increasing from the point to the channel.

  Similarly, in order to solve the problem, the third of the ultrasonic flowmeter of the present invention includes a flow pipe having a rectangular cross section perpendicular to the flow direction of the fluid whose flow rate is measured, and upstream of the position corresponding to the short side of the cross section. Cylindrical sensor attachment portions are provided on the side and downstream sides so as to open to the flow path, and the ultrasonic transmission surface or reception surface (referred to as transmission / reception surface) is at least a distance H from the flow passage in the sensor attachment portion. The main feature is that the ultrasonic sensor is mounted so that it is located only at a distance, and the diameter of the sensor mounting part gradually increases as it goes to the flow path from the transmission / reception surface to the flow path. To do.

  According to the above-described present invention, even if there is an influence of wall surface reflection, a behavior closer to a pseudo open space can be realized by adjusting the near field from the ultrasonic sensor to the flow path. Specifically, the ultrasonic sensor is offset by an appropriate amount from the flow path, and the shape of the sensor mounting portion is gradually widened as it moves toward the flow path. By adopting such a configuration, even if the short side dimension (in short, width) of the distribution pipe having a rectangular cross section is narrowed, the behavior of the open space is ensured, so that stable measurement can be realized, and the dimensions of the distribution pipe itself can be reduced. This makes it possible to reduce the size of the entire apparatus. In addition, according to the present invention, a behavior close to that of a pseudo open space is obtained by structural ingenuity, so that the control is not complicated, and the power consumption is not increased. Therefore, it is particularly suitable for a battery-driven ultrasonic flow meter. Moreover, according to the structure of this invention, the directivity gain in the direction where the flow path width is narrow (the short side direction of the flow pipe) can be reduced. Therefore, the energy of sound in the direction of the side wall of the distribution pipe can be reduced. In other words, unnecessary noise in the side wall direction is reduced, and it plays a role in improving S / N. In this case, improvement in detection reproducibility can be expected, and highly accurate flow rate detection at a minute flow rate can be performed.

  Further, regarding the first invention and the second invention, it is preferable that the inner peripheral surface of the sensor mounting portion is curved in a cross section parallel to the fluid flow direction and including the reference axis. According to such a configuration, it is easy to control the directivity of ultrasonic waves.

  By the way, when an ultrasonic flowmeter capable of measuring a large flow rate is considered, it is necessary to increase the cross-sectional area of the flow pipe. This is because if the cross-sectional area of the flow pipe is increased, it is possible to flow the fluid without causing pressure loss. However, by increasing the cross-sectional area of the flow pipe, the average flow velocity and the average flow rate per unit cross-sectional area are reduced. In the ultrasonic flowmeter having such a configuration, it is difficult to ensure measurement accuracy at a low flow rate and a small flow rate. In order to compensate for this, it is conceivable to increase the distance between the ultrasonic sensors. This is because if the distance between the ultrasonic sensors is increased, the time during which the ultrasonic waves are exposed to the flow velocity and flow rate due to the flow of the fluid becomes longer, so it is easy to ensure measurement accuracy at a small flow rate.

Therefore, in the connection position between the flow pipe and the sensor mounting portion, than the short side dimension W of the cross section of the flow pipe, as towards the width D ₃ of the sensor attachment portion is large, the size of the flow tube and the sensor mounting portion Adjustments can be made. According to such a configuration, it is possible to reduce the width (length in the short side direction) of the flow pipe, which is advantageous for increasing the distance between the ultrasonic sensors.

  Waveforms that exceed or fall below a preset threshold voltage are used as trigger waves, and the ultrasonic wave propagation time is obtained based on detecting the zero cross point where the amplitude of this trigger wave is zero. In addition, a flow rate measuring unit configured to use a zero cross point at a later stage can be further provided. Compared with high flow rate and low flow rate, the low flow rate is more easily affected by reflected waves. Therefore, more accurate detection can be expected by changing the zero cross point to be used depending on whether the flow velocity (flow rate) is equal to or higher than a predetermined value.

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. In the flow path 1, the gas flows in the flow direction shown in the drawing along the flow direction axis O (average flow velocity V). A pair of sensor mounting recesses 10 a and 10 b communicating with the flow pipe 10 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 arranged in direct contact with the gas flowing through the flow path 1 through the short-range sound field FE formed by the sensor mounting recesses 10a and 10b.

  In the embodiment of FIG. 1, the ultrasonic sensor 2 a and the ultrasonic sensor 2 b are located on the opposite side across the flow path 1. However, as shown in the schematic diagram of FIG. 4, the ultrasonic sensor 2 a and the ultrasonic sensor 2 b may be located on the same side. is there. If the ultrasonic sensors 2a and 2b are arranged in a V shape as shown in FIG. 4, there is an advantage that the propagation length can be doubled if the width of the flow pipe is the same. In other words, the width of the distribution pipe can be widened even if the propagation length is the same. Then, since reflection of the ultrasonic wave in the side wall of a flow pipe can be suppressed, it is advantageous for S / N improvement.

  As can be seen from FIG. 1, in the flow tube 10, the flow direction axis O is linear between the ultrasonic sensor 2 a and the ultrasonic sensor 2 b, and the shape and cross-sectional area of the axial cross section are the same in the flow direction. . In the present embodiment, the flow pipe 10 has a rectangular cross section perpendicular to the flow direction axis O. As shown in FIG. 3 from another angle, the sides where the ultrasonic sensors 2a and 2b are located in the cross section of the flow tube 10 are the short sides 10r and 10r. The direction parallel to the short side 10r is the short side direction WL (also referred to as the width direction WL). In addition, although the cross-sectional shape of the flow pipe 10 can be a square, it is preferable that the rectangular shape has a short side and a long side in terms of increasing the distance between the ultrasonic sensors 2a and 2b.

  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 flow measurement unit, an amplifier 4 that amplifies 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 4 and input to the zero cross point detector 6. The zero cross point detector 6 detects whether or not the ultrasonic wave oscillated by one of the ultrasonic sensors 2a and 2b has been received by the other. The zero cross point detector 6 inputs a signal indicating that the received wave has been detected to the microcomputer 9. The microcomputer 9 serving as a time measuring means is based on the signal acquisition from the zero cross point detection unit 6 and directly from when one of the ultrasonic sensors 2a and 2b oscillates an ultrasonic wave until the other receives the ultrasonic wave. The arrival time is measured, and the flow rate is calculated from the measurement result.

In the zero-cross method, a waveform portion that exceeds (or falls below) a threshold value V _TH input and set to the differential comparator 22 (see FIG. 2) is used as a trigger wave, and the amplitude (or phase) of this trigger wave becomes zero. In this method, the zero cross point is detected by the zero cross point pulse generation circuit 24 on the waveform of the amplified signal Va (or a derivative signal thereof).

  As shown in the block diagram of FIG. 2, the ultrasonic reception outputs by the ultrasonic sensors 2 a and 2 b are voltage amplified (for example, non-inverted amplification) by an amplifier 4 (for example, an operational amplifier), and the amplified signal Va is input to the zero cross point detection unit 6. Is done. In the zero cross point detector 6, the amplified signal Va is input (for example, non-inverted input) to the zero cross type comparator 21 (first comparator) and input (for example, inverted input) to the differential type comparator 22 (second comparator). The comparator outputs Vb and Vc are input to the ports #S and #R of the RS flip-flop circuit 23, respectively. By means of the port #Q output Vd of the RS flip-flop circuit 23, the zero cross point pulse generation circuit 24 constituted by a monostable multivibrator or the like detects the arrival time of the ultrasonic wave in the output waveform Va, and the zero cross point detection signal Ve is time measuring means. The output is directed to the microcomputer 9. The microcomputer 9 knows the propagation time of the ultrasonic wave by inputting the zero cross point detection signal Ve.

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 through 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 L, the forward arrival time Td and the reverse arrival time Tu when the ultrasonic wave propagates by the distance L are respectively expressed as follows. .
Td = L / (c + V · cos θ) (1)
Tu = L / (c−V · 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 / L (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) L / cos θ (4)
Q = V · A (5)
In this way, by eliminating the sound velocity c depending on the gas temperature / containing component from the equation (4), the flow velocity V is obtained from the measured values (arrival times Td, Tu) and the constant values (L, θ). Has the advantage of being

  As shown in FIG. 1, the sensor mounting recesses 10a and 10b of the ultrasonic flowmeter 1 are made of a cylindrical member made of the same material as that of the flow pipe 10, for example, stainless steel or aluminum alloy. And is provided integrally. The sensor mounting recesses 10 a and 10 b are opened in the flow path 1. FIG. 5 is a cross section of the ultrasonic flow meter 100 of FIG. 1 including a reference axis OH that is parallel to the gas flow direction and penetrates substantially the center of the transmission / reception surfaces 2ap and 2bp of the ultrasonic sensors 2a and 2b. Is schematically represented. FIG. 5A is an overall view, and FIG. 5B is an enlarged view.

  As shown in FIG. 5 (a), the ultrasonic sensors 2a and 2b are configured so that all of the surfaces (hereinafter referred to as transmission / reception surfaces 2ap and 2bp) for transmitting and receiving ultrasonic waves are short on the upper surface or the lower surface of the flow channel 1, that is, the flow pipe 10. The sensor mounting recesses 10a and 10b are attached so as to be located at least a distance H from the surface forming the side 10r. In this embodiment, the mounting posture in which the ultrasonic transmission / reception surfaces 2ap and 2bp are inclined with respect to the gas flow direction axis O is employed, but the normal direction of the transmission / reception surfaces 2ap and 2bp and the gas flow direction axis O It is also possible to consider a form in which the

  As can be understood from FIGS. 1 and 3, the sensor mounting recesses 10 a and 10 b have a circular shape in a cross section perpendicular to the reference axis OH, and an oval shape in a cross section parallel to both the flow direction axis O and the short side direction WL. The diameter gradually increases from the ultrasonic sensor 10a, 10b side toward the flow pipe 10 side. That is, as shown in the cross-sectional view of FIG. 5B, the inner peripheral surfaces 10ap and 10bp of the sensor mounting recesses 10a and 10b draw a smooth curve, from the inner peripheral surfaces 10ap and 10bp to the reference axis OH. The distance Dh continuously increases from the transmission / reception surfaces 2ap and 2bp of the ultrasonic sensors 2a and 2b to the flow path 1.

  As described above, when the sensor mounting recesses 10a and 10b that are gradually expanded in diameter on the flow pipe 10 side are employed, the width direction WL (short side) of the flow pipe 10 is larger than when a straight cylindrical sensor mounting recess is employed. Direction) can be focused. Furthermore, as shown in FIG. 5A and the like, the ultrasonic sensors 2a and 2b are disposed at a position that is moderately deep from the flow path 1, that is, the distance H (offset amount) from the flow path 1 to the transmission / reception surfaces 2ap and 2bp. By adjusting H), it is possible to control the directivity of the ultrasonic wave and propagate a plane wave uniform in the width direction WL into the flow path 1.

  Specifically, it is desirable that the sensor mounting recesses 10a and 10b have an exponential horn shape. Referring to FIG. 5B, in the case of the exponential type sensor mounting recesses 10a and 10b, the distance Dh in the figure is Dh = exp (ax) + α using the distance x from the transmission / reception surfaces 2ap and 2bp. It can be expressed as “Α” is an initial value of the distance Dh where the transmission / reception surfaces 2ap and 2bp of the ultrasonic sensors 2a and 2b are located, and “a” is a constant that varies depending on the design.

As shown in the schematic diagram of FIG. 13, if the diameter is the same horn (sensor mounting recess), horn longer (length H ₂ than in FIG. 13 length H ₁ is large) is ultrasonic spread In addition to being able to suppress, the ultrasonic wave can be brought closer to a plane wave having the same phase wavefront. Then, the reflection on the wall surface of the flow pipe 10 can be reduced, and the sound pressure is concentrated, so that the attenuation is also reduced. In the case of an exponential horn, the ultrasonic wave can be converged most efficiently, and the above-described effect can be obtained to the maximum.

In addition, as shown in FIG. 3, in this embodiment, the sensor mounting recess 10a is larger than the width W (short side dimension) of the cross section of the flow tube 10 at the connection position between the flow tube 10 and the sensor mounting recesses 10a and 10b. Trip width _{D 3} of 10b is large. In order to make both the propagation distance L of the ultrasonic wave as long as possible and the cross section small so that a relatively stable flow velocity can be obtained even if the flow rate is small, a sensor is attached as shown in FIGS. It is preferable that the recesses 10a and 10b are shaped so as to protrude in the short side direction of the flow pipe 10b.

In the present embodiment shown in FIG. 5 and the like, the sensor mounting recess 10a, which is a cylindrical member in which the cross section including the transmitting / receiving surfaces 2ap, 2bp of the ultrasonic sensors 2a, 2b and perpendicular to the reference axis OH is circular is shown. 10b. As shown in FIG. 9 (a), when the ultrasonic sensor 2a, 2b of the transmitting and receiving surface 2ap, _{D 1} the diameter of 2 bp, sensor mounting recess 10a, the inner diameter of 10b was _{D 2, D 2 / D 1} = 0 .8～1.5, in the relationship with the _{D 3} shown in FIG. _3, if D ₃ / D 2 = 1.0~2.5, the easy configuration control directivity. Note that D ₂ <D ₁ is allowed because the ends of the transmission / reception surfaces 2ap and 2bp of the ultrasonic sensors 2a and 2b do not contribute much to vibration and are covered to some extent by sensor mounting recesses. Also based on good reasons.

In addition, various forms can be adopted. For example, as shown in FIG. 9 (b), it is possible to employ a sensor mounting recess composed of a cylindrical member that includes an ultrasonic sensor transmission / reception surface and whose cross section perpendicular to the reference axis OH is elliptical. is there. In the embodiment of FIG. 9B, the transmission / reception surface of the ultrasonic sensor is circular as in the case of FIG. In this embodiment, it is preferable to provide the sensor mounting recess so that the short side direction WL of the flow pipe 10 and the major axis direction of the ellipse coincide with each other because it becomes easy to obtain a pseudo open space behavior. It is appropriate that the elliptical sensor mounting recess has D ₄ / D ₅ = 1.5 to 4.0 when the major axis is D ₄ and the minor axis is D ₅ . In this way, the sensor mounting recess does not become too large while realizing a pseudo open space behavior.

  In addition, it is also possible to employ a sensor mounting recess formed of a cylindrical member whose cross section perpendicular to the reference axis OH is rectangular or square. Further, regarding the ultrasonic sensor, the transmission / reception surface is not limited to a circular shape, and for example, those having an elliptical shape as shown in FIG. 9C and those having various shapes such as a rectangle are suitably employed. it can.

  Moreover, a form as shown in FIG. 6 is employable as a sensor attachment recessed part. For example, as shown to Fig.6 (a), the sensor attachment recessed part 10c which has a cone shape is also employable. In this case, the inner peripheral surface 10cp of the sensor mounting recess 10c shows a substantially straight line in the cross section. Further, as shown in FIGS. 6B and 6C, the offset amount H is set to zero, and the ends of the ultrasonic sensors 2a and 2b coincide with the connection positions of the flow pipe 10 and the sensor mounting recesses 10a and 10b. Also good. However, even in the configurations of FIGS. 6B and 6C, the inner peripheral surfaces 10dp and 10ep of the sensor mounting recesses 10d and 10e are shaped to expand linearly or curvedly toward the flow path 1. .

  FIG. 8 shows how the directivity of the ultrasonic wave (200 KHz) changes between the arrangement where the offset amount H shown in FIG. 5A is zero and the arrangement of 12 mm by computer simulation. It is a graph which shows the result. In this simulation, a straight cylindrical sensor mounting recess was used as a condition. Also, the offset amount H being zero means that the ultrasonic sensors 2a and 2b are arranged closer to the flow path 1 until the transmission / reception surfaces 2ap and 2bp reach the boundary between the near field FE and the flow path 1. .

As is clear from this result, the directivity of the ultrasonic beam can be sharpened by appropriately adjusting the offset amount H. Specifically, when the offset amount H is zero, the directivity angle (θ _B / 2) is about 15 °, but when the offset amount H is 12 mm, the directivity angle (θ _B / 2) is about 7 °. It changed from ° to 12 °. Thus, by controlling the offset amount H, the directivity of the ultrasonic wave can be controlled.

  Next, FIG. 7 shows the result of experimentally examining the transition of the zero cross point when the short side dimension W of the flow pipe 10 and the offset amount H of the ultrasonic sensors 2a and 2b are changed. The reference time for zero-cross detection was set to the case where the short side dimension W of the flow tube 10 was 6 mm. The detection points were examined for both the second zero cross and the third zero cross. The offset amount H was 0 mm, 6 mm, 9 mm, and 11.5 mm. The frequency of ultrasonic waves was 200 KHz.

In the graph of FIG. 7, when the change in the deviation time of the zero cross detection is large, it means that it is strongly influenced by the reflected wave. That is, the following information (1) and (2) can be read from the graph of FIG.
(1) When the short side dimension W of the flow pipe is 6 mm to 11 mm and the offset amount H is 6 mm, 9 mm, and 11.5 mm, the change in the shift time is small as a whole. The fact that the zero cross detection is not greatly affected even if the short side dimension W of the flow pipe changes means that a pseudo open space behavior is realized.
(2) When the short side dimension W of the flow pipe is sufficiently large (for example, 15 mm or more), relatively stable zero-cross detection can be performed regardless of the offset amount H.

  Here, in consideration of the fact that the short side dimension W of the distribution pipe 10 cannot be made wide in an actual product, the above-described method (1), that is, the configuration of the present invention is adopted in order to realize stable zero cross detection. It turns out that it is suitable.

  Moreover, the graph of FIG. 10 is a graph which shows the result of having investigated by computer simulation where the wave reflected once in the flow path overlaps when the short side dimension W of the flow pipe 10 is changed. The computer simulation was performed when the offset amount H was 0 mm, 6 mm, 9 mm, and 11.5 mm, respectively. The frequency of the ultrasonic wave was 200 KHz, the medium was air, and the temperature was 20 ° C. For example, when the offset amount H is 0 mm, the first zero cross with a short side dimension of 8 mm to 10.5 mm (phase angle 0.8π to 1.4π) and the second zero cross with a short side dimension of 12.0 mm to 14.0 mm ( The graph of FIG. 10 shows a state in which the reflected wave overlaps with the third zero cross (phase angle 2.8π to 3.4π) with a short side dimension of 15.0 mm to 17.0 mm and a phase angle of 1.8π to 2.4π). Can be read from.

  In general, when the fluid to be measured is a gas, it is necessary to secure the short side dimension W of at least about 6.0 mm. In the graph of FIG. 10, the phase angle Δθ is 0.8π, that is, the short side dimension W at which the first zero cross of the direct reaching wave starts to be affected increases as the offset amount H increases. It can be seen that the range of the short side dimension W allowed in design is widened by taking the offset amount H appropriately large.

  A similar computer simulation was performed with an ultrasonic frequency of 160 KHz and an offset amount H of 12.0 mm. Other conditions are the same as in the previous simulation. The results are shown in the graph of FIG. From this graph, it can be seen that the reflected wave is superimposed on the 0 to 0.8π area of the first incoming received sound wave, and these are added for each π. The lower the frequency, the greater the offset amount, and the further the zero cross point that is the detection point, the more room in the channel width direction that shows the same behavior as a pseudo open space even if it is affected by reflected waves. Is obtained. From this and the result of computer simulation, if the short side dimension W of the flow pipe 10 is set to W = 6 mm to 9 mm (preferably 7 mm to 8 mm), the flow path length varies in the vicinity thereof, and the part dimensions depend on the temperature. Even when it changes, high accuracy can be maintained for detection at a minute flow rate.

  As described above, even when the frequency to be used is low (for example, 100 KHz to 200 KHz), the reflected wave increases as the offset amount H of the ultrasonic sensors 2a and 2b is appropriately set and the zero cross point to be detected is set backward. Even under the influence of, it becomes easy to obtain the same behavior as a pseudo open space. That is, it is desirable to change the zero cross point detected according to the flow rate. Specifically, as shown in FIG. 11 (b), the front zero cross point (first or second zero cross point) is detected when the flow rate is large, and the rear zero cross point (third, third) is detected when the flow rate is low. 4th, 5th or 6th zero cross point).

Further, the phase difference Δθ between the ultrasonic wave that directly reaches the reception-side ultrasonic sensor from the transmission-side ultrasonic sensor and the reflected wave from the wall surface of the flow tube 10 is expressed by the following equation (i).
Δθ = 2π * (f ₀ * ΔL) / V (i) (where f _{0 is the} ultrasonic frequency, V is the flow velocity)

Since the phase difference Δθ depends on the path difference ΔL depending on the frequency and the propagation length, even if ΔL and V change due to characteristic variations of the ultrasonic sensors 2a and 2b and temperature changes during use, the phase difference Δθ is driven to correct it. It is preferable that fine adjustment is performed by changing the frequency f ₀ within a range of about ± 40 KHz so that stable zero-cross detection can be performed. When the driving frequency is changed, it is desirable to use a piezoelectric element constituting the ultrasonic sensors 2a and 2b having a relatively low mechanical resonance sharpness Q. Specifically, a piezoelectric element having a single resonance of Q = 4 to 7 is suitable.

  In addition, the mode in which the ultrasonic sensors 2a and 2b, which are touched slightly in FIG. As shown in FIG. 4, the configuration in which the ultrasonic sensors 2 a and 2 b are arranged so that the ultrasonic propagation path is V-shaped has the following merits in addition to the fact that the propagation length can be increased. First, in the case of the Z-type arrangement as shown in FIG. 1, the reflection in the wall surface (short side) direction is a surface reflection. This reflected wave arrives at the ultrasonic sensor on the receiving side with a delay from the direct arrival wave, and has a great influence on zero cross detection. On the other hand, as shown in FIG. 14, in the case of the V-shaped arrangement, the reflection of the ultrasonic wave on the wall surface is close to the line reflection at the corner. In this case, the influence of the reflected wave on the zero cross detection is significantly reduced as compared with the case of surface reflection. For this reason, it can be said that the V-type arrangement is more advantageous than the Z-type arrangement in order to reduce the influence of the reflected wave and improve the S / N ratio.

  However, when the ultrasonic frequency is set to several hundred KHz (for example, about 500 KHz), it is difficult to increase the propagation length L due to the problem of detection sensitivity. Therefore, the V-shaped arrangement as shown in FIG. 4 is the most suitable form when the object to be measured is a gas such as gas and the frequency of the ultrasonic wave used is a low frequency of about 100 KHz to 200 KHz.

Explanatory drawing which shows the basic composition of the ultrasonic flowmeter of this invention. The block diagram of a flow measurement part provided with an amplifier and a zero crossing point detection part. The cross-sectional perspective view of a distribution pipe. The schematic diagram explaining the form which arrange | positions an ultrasonic sensor in the same side of a flow path. Sectional drawing explaining the shape of a sensor attachment recessed part, and the attachment position of an ultrasonic sensor. The schematic diagram explaining the other form of a sensor attachment recessed part. The graph which shows the result of the experiment which investigated the transition (deviation of detection time) of the zero crossing point when changing the short side dimension W and offset amount of a flow pipe. The graph which shows the result of having investigated the directivity of an ultrasonic wave by computer simulation. The schematic diagram for demonstrating the dimensional relationship of each component. The graph which shows the result of having investigated in the computer simulation where the reflected wave overlaps with the zero cross point. The graph following FIG. The figure explaining zero cross detection. The schematic diagram explaining the convergence effect of the ultrasonic wave with respect to the length of a horn. The schematic diagram explaining reflection of an ultrasonic wave when a V type arrangement is adopted.

Explanation of symbols

1 Flow path 2a, 2b Ultrasonic sensor 2ap, 2bp Transmission / reception surface 10 Flow pipe 10a, 10b Sensor mounting recess (sensor mounting portion)
10aq, 10bq Inner peripheral surface 10r of sensor mounting recess Short side 100 of flow pipe Ultrasonic flow meter O Flow direction axis OH Reference axis WL Short side direction of flow pipe

Claims (7)

  A flow pipe having a rectangular cross section perpendicular to the flow direction of the fluid whose flow rate is to be measured is provided, and a cylindrical sensor mounting portion opens into the flow path on the upstream side and the downstream side of the position corresponding to the short side of the cross section. An ultrasonic sensor is attached to the sensor mounting portion so that the entire ultrasonic transmission surface or reception surface (referred to as a transmission / reception surface) is located at a distance H from the flow path. When a reference axis that penetrates substantially the center of the transmission / reception surface of the ultrasonic sensor perpendicularly is defined, the distance from the inner peripheral surface of the sensor mounting portion to the reference axis reaches the flow path from the transmission / reception surface. Ultrasonic flowmeter characterized by gradually increasing during the period.   A flow pipe having a rectangular cross section perpendicular to the flow direction of the fluid whose flow rate is to be measured is provided, and a cylindrical sensor mounting portion opens into the flow path on the upstream side and the downstream side of the position corresponding to the short side of the cross section. The ultrasonic sensor is attached to the sensor mounting portion in a posture in which an ultrasonic transmission surface or a reception surface (referred to as a transmission / reception surface) is inclined with respect to the fluid flow direction. When a reference axis that penetrates substantially the center of the transmission / reception surface is defined vertically, the distance from the inner peripheral surface of the sensor mounting portion to the reference axis gradually increases from the transmission / reception surface to the flow path. An ultrasonic flowmeter characterized by   3. The ultrasonic flowmeter according to claim 1, wherein an inner peripheral surface of the sensor mounting portion is curved in a cross section parallel to the fluid flow direction and including the reference axis.   A flow pipe having a rectangular cross section perpendicular to the flow direction of the fluid whose flow rate is to be measured is provided, and a cylindrical sensor mounting portion opens into the flow path on the upstream side and the downstream side of the position corresponding to the short side of the cross section. An ultrasonic sensor is attached to the sensor mounting portion so that the entire ultrasonic transmission surface or reception surface (referred to as a transmission / reception surface) is located at a distance H from the flow path. The ultrasonic flowmeter is characterized in that the diameter of the sensor mounting portion gradually increases as it goes from the transmitting / receiving surface to the flow path toward the flow path. 5. The width D ₃ of the sensor attachment part is larger than the short side dimension W of the cross section of the circulation pipe at the connection position between the circulation pipe and the sensor attachment part. The ultrasonic flowmeter according to item 1.   The ultrasonic flowmeter according to claim 1, wherein the sensor attachment portion has an exponential horn shape.   Waveforms that exceed or fall below a preset threshold voltage are used as trigger waves, and the ultrasonic wave propagation time is obtained based on detecting the zero cross point where the amplitude of this trigger wave is zero. The ultrasonic flowmeter according to any one of claims 1 to 6, further comprising a flow rate measurement unit configured to use the zero cross point at a later stage.

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