Patent Publication Number: US-2022228894-A1

Title: Ultrasonic transceiver and ultrasonic flow meter

Description:
TECHNICAL FIELD 
     The present disclosure relates to an ultrasonic transceiver that transmits and receives an ultrasonic wave by using a piezoelectric element or the like, and an ultrasonic flow meter using the same. 
     BACKGROUND ART 
     In recent years, an ultrasonic flow meter that measures a time taken for an ultrasonic wave to travel in a propagation path, thereby measuring a moving speed of a fluid, and consequently measures a flow rate has been being used for gas meters and the like. In an ultrasonic flow meter that performs flow rate measurement by measuring a propagation time of an ultrasonic wave, a pair of ultrasonic transceivers are generally disposed upstream and downstream of a measurement flow path through which a fluid to be measured flows, and the propagation time of the ultrasonic wave is measured by transmission and reception of an ultrasonic wave between the ultrasonic transceivers. 
     When the fluid to be measured is a gas, the difference in acoustic impedance between the gas and the piezoelectric element is large, and the ultrasonic wave is therefore easily reflected at an interface between the piezoelectric element and the gas. Therefore, in the ultrasonic transceiver, an acoustic matching layer is provided at the interface between the piezoelectric element and the gas in order to enable the ultrasonic wave to efficiently enter the gas from the piezoelectric element. 
     Further, in order to improve the radiation efficiency, there is used an acoustic matching layer configured by stacking a first acoustic matching layer having a high density and a high sound speed and a second acoustic matching layer having a lower density and a lower sound speed than the first acoustic matching layer. There is proposed an ultrasonic transceiver that can be matched with a gas having a sufficiently small acoustic impedance by arranging on the gas side a second acoustic matching layer that is acoustic impedance-matched with the gas, which is a fluid to be measured (for example, see PTL 1). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Patent No. 3552054 
       
    
     SUMMARY OF THE INVENTION 
     However, in the conventional configuration, since a reverberation in the received wave of the ultrasonic wave is large, there is a possibility that a reference point for measuring a propagation time cannot be accurately detected and the gas flow rate is erroneously detected. 
     Next, a method of measuring a propagation time in an ultrasonic flow meter will be described with reference to  FIGS. 9 and 10 . 
     A conventional flow rate measurement device for a fluid of this type is generally a device as shown in  FIG. 9 . Flow rate measurement device  100  includes: first ultrasonic transceiver  102  and second ultrasonic transceiver  103  installed in flow path  101  through which a fluid to be measured flows; and switching unit  104  that switches first ultrasonic transceiver  102  and second ultrasonic transceiver  103  between transmission and reception. In addition, flow rate measurement device  100  includes: transmitter  105  that drives first ultrasonic transceiver  102  and second ultrasonic transceiver  103 ; and receiver  106  that receives a received signal that is received by the ultrasonic transceiver on the reception side and has passed through switching unit  104 . Flow rate measurement device  100  further includes: amplifier  107  that amplifies the received signal to a predetermined amplitude; and reference comparator  108  that compares a voltage of the received signal amplified by amplifier  107  with a reference voltage. 
     Further, flow rate measurement device  100  includes: reference voltage setting unit  109  that sets the reference voltage to be compared by reference comparator  108 ; determination unit  110  that determines a reference point for measuring time on the basis of a comparison result of reference comparator  108 ; and time counter  111  that measures a propagation time of an ultrasonic wave on the basis of a result of determination unit  110 . Flow rate measurement device  100  further includes: flow rate calculator  112  that calculates a flow rate of the fluid to be measured on the basis of the propagation time measured by time counter  111 ; and controller  113  that is configured with a microcomputer and the like and performs overall control. 
     Next, a method of measuring the propagation time of the ultrasonic wave by time counter  111  will be described with reference to  FIG. 10 .  FIG. 10  illustrates a drive signal D of the ultrasonic transceiver (first ultrasonic transceiver  102  or second ultrasonic transceiver  103 ) assigned to a transmission side by switching unit  104 , and further illustrates a received signal S that is received by the ultrasonic transceiver (first ultrasonic transceiver  102  or second ultrasonic transceiver  103 ) assigned to a reception side by switching unit  104  and that is amplified by amplifier  107  such that the maximum amplitude becomes a predetermined amplitude. 
     Reference comparator  108  compares the received signal S with a reference voltage Vr set by reference voltage setting unit  109 , and determination unit  110  detects a zero-crossing point R 1  of the received signal after the received signal S becomes larger than the reference voltage Vr. 
     Here, a reference voltage Vr is set to such a voltage that the third wave of the received signal S can be detected, but this setting is based on the following assumption: when amplifier  107  amplifies the received signal S such that the maximum amplitude of the received signal S becomes a predetermined amplitude, the amplitude of the third wave is also amplified by the same amplification factor, and also the third wave therefore becomes a voltage in a predetermined range. 
     Time counter  111  can calculate a propagation time TPO by measuring a time TP from a start point TO to the zero-crossing point R 1  and by subtracting from the time TP a time TR that is from a reception start point R 0  to the zero-crossing point R 1 . 
     Then, by switching between the transmission and the reception of first ultrasonic transceiver  102  and second ultrasonic transceiver  103  by using switching unit  104 , it is possible to obtain, by the above-described method, a propagation time t 1  from first ultrasonic transceiver  102  to second ultrasonic transceiver  103  and a propagation time t 2  from second ultrasonic transceiver  103  to first ultrasonic transceiver  102 . 
     Then, a flow rate Q can be obtained by the following equation, where v is a flow velocity of the fluid to be measured, S is a cross-sectional area of a flow path, φ is a sensor angle, and L is a propagation distance. 
         Q=S·v=S·L/ 2·cos φ·( n/t 1− n/t 2)  (Equation 1)
 
     Flow rate calculator  112  calculates the flow rate by further multiplying Equation 1 by a coefficient corresponding to the flow rate. 
     However, it has been found that when the ultrasonic transceiver having the conventional configuration is used as the ultrasonic transceiver, the waveform of the received signal is distorted due to reverberation, and thus the zero-crossing point R 1  cannot be correctly detected in some cases. 
       FIGS. 11A and 11B  each illustrate a configuration of a conventional ultrasonic transceiver.  FIG. 11A  is a cross-sectional view of ultrasonic transceiver  200 , and  FIG. 11B  is a plan view of ultrasonic transceiver  200 . In ultrasonic transceiver  200 , piezoelectric element  202  is bonded to top face inner part  201   a  of metal sensor case  201  having a capped cylindrical shape. In addition, first acoustic matching layer  203  having a disk shape and second acoustic matching layer  204  having a disk shape having the same outer diameter as first acoustic matching layer  203  are stacked and joined to a top face outer part  201   b  of sensor case  201 . 
       FIG. 12  illustrates an example of a received waveform in a case where ultrasonic transceiver  200  is used. As illustrated in  FIG. 12 , relatively large reverberation waves are seen after a normal maximum amplitude A, and in some cases, an amplitude B that is equivalent to the maximum amplitude A or may be larger than the maximum amplitude A is generated. 
     The waveform of the ultrasonic wave transmitted and received by the ultrasonic transceiver is mainly affected by a thickness and shape of the piezoelectric element, a material, thickness, and shape of the sensor case, shapes, thicknesses, and acoustic impedances of the first acoustic matching layer and the second acoustic matching layer, and the like. 
     Further, it has been found from our study that the received ultrasonic waveform illustrated in  FIG. 12  is affected by the shapes in the surface direction of the acoustic matching layers. In a case where an area of a joining surface of first acoustic matching layer  203  is the same as an area of a joining surface of second acoustic matching layer  204  as in ultrasonic transceiver  200  illustrated in  FIG. 11A , or in a case where the area of the joining surface of second acoustic matching layer  204  is larger than the area of the joining surface of first acoustic matching layer  203 , the following phenomenon occurs. That is, the ultrasonic wave radiated to the fluid to be measured generates direct wave  205  in which a vibration of piezoelectric element  202  propagates to the fluid to be measured by the shortest distance via sensor case  201 , first acoustic matching layer  203 , and second acoustic matching layer  204 . In addition, the vibration of piezoelectric element  202  propagates in a circumferential direction of first acoustic matching layer  203 , and indirect wave  206  having a phase delay is generated by reflection by joint part  207  between first acoustic matching layer  203  and second acoustic matching layer  204 , and by other causes. The vibration of piezoelectric element  202  is a combination of direct wave  205  and indirect wave  206 , and indirect wave  206  is considered to generate reverberation. 
     Note that indirect wave  206  illustrated in  FIG. 11  is schematically illustrated, and the illustrated propagation paths are not limitative. 
     When the position of the maximum amplitude varies due to the influence of such a large reverberation wave as described above, the third wave to be detected does not have an assumed amplitude even if amplifier  107  amplifies the maximum amplitude to have the predetermined amplitude. Therefore, the preceding and following second and fourth waves are detected, and the correct propagation time cannot be measured. That is, the propagation times t 1  and t 2  in Equation 1 cannot be accurately measured, and a measurement accuracy of the flow rate decreases. 
     For example, in the received waveform illustrated in  FIG. 12 , when the amplitude B becomes larger than the normal maximum amplitude A, amplifier  107  amplifies the received signal such that the amplitude B becomes the predetermined amplitude. Then, the third wave originally desired to be detected is not amplified to the reference voltage or more, and reference comparator  108  therefore detects the fourth wave. 
     The present disclosure enables stable measurement of an ultrasonic signal by suppressing reverberation of a received ultrasonic waveform. 
     An ultrasonic transceiver of the present disclosure includes a piezoelectric element and a plurality of acoustic matching layers stacked on and joined to the piezoelectric element directly or via another layer, where at least a part of a joining part between the acoustic matching layers is inside an outer periphery of a joining surface of the acoustic matching layer disposed on a piezoelectric element side. 
     With this configuration, it is possible to suppress propagation of the indirect wave from the acoustic matching layer on the piezoelectric element side to the next acoustic matching layer, and when this ultrasonic receiver is used as an ultrasonic flow meter, a reverberation level of the ultrasonic transceiver can be reduced, and stable measurement of the propagation time of the ultrasonic wave can be achieved. 
     The ultrasonic transceiver of the present disclosure can reduce the reverberation of the received ultrasonic waveform, and erroneous measurement of the propagation time in flow rate measurement can be reduced, thereby achieving stable flow rate measurement. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view of an ultrasonic transceiver in a first exemplary embodiment. 
         FIG. 1B  is a plan view of the ultrasonic transceiver in the first exemplary embodiment. 
         FIG. 2  is an image diagram of a received waveform of an ultrasonic receiver in the first exemplary embodiment. 
         FIG. 3A  is a cross-sectional view illustrating a variation of the ultrasonic transceiver in the first exemplary embodiment. 
         FIG. 3B  is a cross-sectional view illustrating a variation of the ultrasonic transceiver in the first exemplary embodiment. 
         FIG. 4A  is a plan view illustrating a variation of the ultrasonic transceiver in the first exemplary embodiment. 
         FIG. 4B  is a cross-sectional view illustrating a variation of the ultrasonic transceiver in the first exemplary embodiment. 
         FIG. 5A  is a perspective view of an ultrasonic transceiver used for an ultrasonic flow meter in a second exemplary embodiment. 
         FIG. 5B  is a perspective view of the ultrasonic transceiver used for the ultrasonic flow meter in the second exemplary embodiment. 
         FIG. 5C  is a plan view of the ultrasonic transceiver used for the ultrasonic flow meter in the second exemplary embodiment. 
         FIG. 5D  is a cross-sectional view taken along line  5 D- 5 D of  FIG. 5C . 
         FIG. 6A  is a perspective view of a flow path block used for the ultrasonic flow meter in the second exemplary embodiment. 
         FIG. 6B  is a view taken in a direction of arrow  6 B of  FIG. 6A . 
         FIG. 6C  is a perspective view of a sensor fixing member used for the ultrasonic flow meter in the second exemplary embodiment. 
         FIG. 7A  is a side view of an ultrasonic flow meter in the second exemplary embodiment. 
         FIG. 7B  is a main-part cross-sectional view taken along line  7 B- 7 B of  FIG. 7A . 
         FIG. 8  is a graph for illustrating a width of a short side of a second acoustic matching layer, an output of an ultrasonic wave, and a magnitude of reverberation in the second exemplary embodiment. 
         FIG. 9  is a block diagram illustrating a configuration of a conventional ultrasonic flow meter. 
         FIG. 10  is an image diagram of a drive wave and a received wave for describing a method of measuring a propagation time in a conventional ultrasonic flow meter. 
         FIG. 11A  is a cross-sectional view of the conventional ultrasonic transceiver. 
         FIG. 11B  is a plan view of the conventional ultrasonic transceiver. 
         FIG. 12  is an image view illustrating a received waveform of the conventional ultrasonic receiver. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, exemplary embodiments will be described in detail with reference to the drawings. However, unnecessarily detailed description is omitted in some cases. For example, a detailed description of already well-known matters and a redundant description of substantially the same configuration is omitted in some cases. 
     Note that the attached drawings and the following description are provided for those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter as described in the appended claims. 
     First Exemplary Embodiment 
       FIG. 1A  is a cross-sectional view of an ultrasonic transceiver in a first exemplary embodiment.  FIG. 1B  is a plan view of the ultrasonic transceiver in the first exemplary embodiment. 
     With reference to  FIGS. 1A and 1B , ultrasonic transceiver  10  includes case  11  having conductivity and a capped cylindrical shape, piezoelectric element  12  joined to top face inner part  11   a  of case  11  via joining part  19 , first acoustic matching layer  13  joined to top face outer part lib of case  11  via joining part  18 , and second acoustic matching layer  14  joined to first acoustic matching layer  13  via joining part  17 . 
     Electrodes  12   a  and  12   b  are provided on opposite surfaces of piezoelectric element  12 , electrode  12   a  is conductively joined to case  11  via joining part  19 , and when an AC voltage is applied across electrode  12   b  and case  11 , piezoelectric element  12  is deformed according to the voltage. The deformation generated in piezoelectric element  12  propagates to a fluid to be measured via first acoustic matching layer  13  and second acoustic matching layer  14 . 
     In the present exemplary embodiment, both first acoustic matching layer  13  and second acoustic matching layer  14  have a disk shape, a diameter of second acoustic matching layer  14  is smaller than a diameter of first acoustic matching layer  13 , and first acoustic matching layer  13  and second acoustic matching layer  14  are concentrically stacked. With this configuration, joining part  17  between first acoustic matching layer  13  and second acoustic matching layer  14  is located inside an outer periphery of joining surface  13   a  of first acoustic matching layer  13  bonded on a side of piezoelectric element  12 . 
     In this arrangement, as illustrated in  FIG. 1A , in ultrasonic transceiver  10  of the present exemplary embodiment, direct wave  15  that is a part of the ultrasonic wave radiated to the fluid to be measured is mainly propagated to the fluid to be measured, but the ultrasonic wave other than direct wave  15  attenuates because first acoustic matching layer  13  and the fluid to be measured are not matched. Therefore, it is possible to suppress propagation of indirect waves  16  to the fluid to be measured. 
     Note that indirect waves  16  illustrated in  FIG. 1A  are schematically illustrated, and these propagation paths of indirect waves  16  are not limitative. 
       FIG. 2  is an image diagram of a received waveform of the ultrasonic receiver in the first exemplary embodiment. As illustrated in the drawing, it can be seen that a reverberation part attenuates more rapidly than the conventional received wave illustrated in  FIG. 11 . 
     As described above, an influence of the reverberation wave can be suppressed by ultrasonic transceiver  10  of the present exemplary embodiment; therefore, by using ultrasonic transceiver  10  of the present exemplary embodiment for the ultrasonic flow meter illustrated in  FIG. 9 , amplifier  107  can amplify an ultrasonic wave such that the maximum amplitude becomes the predetermined amplitude, the third wave to be detected therefore becomes to have the assumed amplitude, and the correct propagation time can be measured. 
     Note that a thickness t 1  of first acoustic matching layer  13  and a thickness t 2  of second acoustic matching layer  14  illustrated in  FIG. 1A  preferably have a thickness of about ¼ of a wavelength of the sound wave propagating in the acoustic matching layers. Assuming that a frequency for driving the ultrasonic transceiver is f and a sound velocity in the acoustic matching layer is V, a wavelength A of the sound wave is obtained by λ=V/f. Therefore, a thickness d of the acoustic matching layer is obtained by d=1/4·λ. With such a structure, it is possible to align phases of ultrasonic waves reflected inside the sensor and to efficiently transmit and receive ultrasonic waves. 
     In order to efficiently propagate the vibration of piezoelectric element  12  to the fluid, it is important to match the acoustic impedances. In the present exemplary embodiment, first acoustic matching layer  13  is attached to top face outer part lib of case  11 , and second acoustic matching layer  14  is joined to first acoustic matching layer  13 . Assuming that an acoustic impedance of first acoustic matching layer  13  is Za and an acoustic impedance of second acoustic matching layer  14  is Zb, a relationship of Za&gt;Zb is satisfied. The impedance Za is smaller than the acoustic impedance of piezoelectric element  12 . Since the plurality of acoustic matching layers having such acoustic characteristics are provided, the vibration of the piezoelectric element can be efficiently propagated into the fluid to be measured. 
     (Variations) 
       FIG. 3A  is a cross-sectional view illustrating a variation of the ultrasonic transceiver in the first exemplary embodiment.  FIG. 3B  is a cross-sectional view illustrating a variation of the ultrasonic transceiver in the first exemplary embodiment.  FIG. 4A  is a plan view illustrating a variation of the ultrasonic transceiver in the first exemplary embodiment.  FIG. 4B  is a cross-sectional view illustrating a variation of the ultrasonic transceiver in the first exemplary embodiment. 
     In order to achieve matching with a substance which is a fluid to be measured and to which an ultrasonic wave is propagated, it is known to stack a plurality of acoustic matching layers (not limited to two layers), and  FIG. 3A  illustrates a side cross-sectional view of ultrasonic transceiver  20 , where first acoustic matching layer  23 , second acoustic matching layer  24 , and third acoustic matching layer  25  are stacked in this order as an acoustic matching layer and are joined to top face outer part lib of case  11 , so that the acoustic matching layer is formed as three layers. 
     In ultrasonic transceiver  20 , joining part  27  between first acoustic matching layer  23  and second acoustic matching layer  24  is located inside an outer periphery of joining surface  23   b  of first acoustic matching layer  23 . Further, joining part  25   b  between second acoustic matching layer  24  and third acoustic matching layer  25  is located inside an outer periphery of joining surface  24   b  of second acoustic matching layer  24 . With this configuration, it is possible to reduce propagation of indirect waves  16   a ,  16   b  from piezoelectric element  12  to third acoustic matching layer  25 , and the reverberation can therefore be suppressed. 
     Similarly, also in a case where more than three acoustic matching layers are provided, when a joining part of an acoustic matching layer is located on an inner side of a joining surface inside an outer periphery of a joining surface of an acoustic matching layer disposed on the piezoelectric element side, it is possible to suppress the propagation of indirect waves  16  ( 16   a ,  16   b ), and the reverberation can therefore be suppressed. 
       FIG. 3B  illustrates a side cross-sectional view of ultrasonic transceiver  30 . Instead of second acoustic matching layer  14  in ultrasonic transceiver  10  illustrated in  FIG. 1A , second acoustic matching layer  34  having a truncated cone shape is used. In ultrasonic transceiver  30 , an area of radiation surface  34   b  of second acoustic matching layer  34  through which an ultrasonic wave propagates is substantially the same as an area of the joining surface  13   b  of first acoustic matching layer  13 ; however, since joining part  17  between first acoustic matching layer  13  and second acoustic matching layer  34  is located inside joining surface  13   b  of first acoustic matching layer  13 , it is possible to avoid the propagation of indirect waves  16  to second acoustic matching layer  34 , and the reverberation can therefore be suppressed. 
       FIG. 4A  illustrates a plan view of ultrasonic transceiver  70 , and second acoustic matching layer  74  is used in which a shape of a matching surface (a radiation surface of an ultrasonic wave) of second acoustic matching layer  14  in ultrasonic transceiver  10  illustrated in  FIG. 1A  is changed from a circle to a rectangle and in which a length in a short-side direction is shorter than a diameter of first acoustic matching layer  13 . In ultrasonic transceiver  70 , regardless of a length in a long-side direction of second acoustic matching layer  74 , it is possible to avoid the propagation of a phase-delayed indirect wave from side parts C of first acoustic matching layer  13  to second acoustic matching layer  74 , and the reverberation can therefore be suppressed. 
       FIG. 4B  illustrates a side cross-sectional view of ultrasonic transceiver  80 , in which case  11  of ultrasonic transceiver  10  illustrated in  FIG. 1A  is removed and first acoustic matching layer  13  is directly joined to piezoelectric element  12 . Also in this case, it is possible to reduce the propagation of ultrasonic waves other than direct wave  15  from piezoelectric element  12  to second acoustic matching layer  14 , and the reverberation can therefore be suppressed. 
     As described above, with the ultrasonic transceivers according to the present exemplary embodiment, at least a part of the joining part between the acoustic matching layers is located inside the outer periphery of the joining surface of the acoustic matching layer disposed on the piezoelectric element side, so that the reverberation can therefore be suppressed. 
     Note that, in the present exemplary embodiment, various configurations are illustrated as examples by  FIGS. 3A, 3B, 4A, and 4B , but it is needless to say that when at least a part of the entire periphery of the joining part is located inside the outer periphery of the joining surface of the acoustic matching layer disposed on the piezoelectric element side, it is possible to reduce the propagation of the ultrasonic wave other than the direct wave from the piezoelectric element to the next acoustic matching layer; and the shape of the acoustic matching layer can be appropriately selected. 
     In addition, in order to suppress the propagation of the indirect wave to the next acoustic matching layer, it needless to say that the entire joining part between the acoustic matching layers are preferably located inside the outer periphery of the joining surface of the acoustic matching layer disposed on the piezoelectric element side. 
     Second Exemplary Embodiment 
     Next, an ultrasonic flow meter using the ultrasonic transceiver described in the first exemplary embodiment will be described with reference to  FIGS. 5, 6, and 7 . 
       FIG. 5A  is a perspective view of an ultrasonic transceiver used for an ultrasonic flow meter in a second exemplary embodiment.  FIG. 5B  is a perspective view of the ultrasonic transceiver used for the ultrasonic flow meter in the second exemplary embodiment.  FIG. 5C  is a plan view of the ultrasonic transceiver used for the ultrasonic flow meter in the second exemplary embodiment.  FIG. 5D  is a cross-sectional view taken along line  5 D- 5 D of  FIG. 5C . 
     As illustrated in the drawings, in ultrasonic transceiver  40 , electrode surface  42   a  of piezoelectric element  42  is conductively joined to top part inner side  41   a  of metal case  41  having a capped cylindrical shape, and lead wire  46  is joined to electrode surface  42   b  by solder  49 . In addition, lead wire  47  is joined to case  41  by welding, and piezoelectric element  42  vibrates at a predetermined frequency by applying an AC voltage to lead wire  46  and lead wire  47 . 
     Flange  41   d  is formed at an open end of case  41 , and vibration-proof member  48  is integrally formed, by molding, on an outer periphery of flange  41   d . Lead wire  46  and lead wire  47  are held by case  41  by vibration-proof member  48 . 
     Disk-shaped first acoustic matching layer  43  having a diameter of 10.8 mm is joined to top part outer side  41   b  of case  41 , and second acoustic matching layer  44  is joined to first acoustic matching layer  43 . Here, second acoustic matching layer  44  is made to have a substantially rectangular shape with a long-side length of 9.5 mm and a short-side length of 5.5 mm such that second acoustic matching layer  44  is joined to first acoustic matching layer  43  inside an outer periphery of a joining surface of first acoustic matching layer  43 . Note that thicknesses of first acoustic matching layer  43  and second acoustic matching layer  44  are set to optimum values, as described above, depending on the frequency of the ultrasonic wave to be propagated. 
     Piezoelectric element  42  has a rectangular parallelepiped shape, and the joining surface via which piezoelectric element  42  and case  41  are joined together is configured to be smaller than an outer diameter of second acoustic matching layer  44 . In addition, piezoelectric element  42  is disposed such that a longitudinal direction of piezoelectric element  42  coincides with a longitudinal direction of second acoustic matching layer  44 . Further, piezoelectric element  42  has slit  42   c  to improve an excitation efficiency in a longitudinal vibration mode. 
     Further, in flange  41   d , a pair of recesses  41   c  for positioning are formed at parts of the outer periphery of flange  41   d  that are in the longitudinal direction of second acoustic matching layer  44 . 
       FIG. 6A  is a perspective view of the flow path block used for the ultrasonic flow meter in the second exemplary embodiment.  FIG. 6B  is a view taken in a direction of arrow  6 B in  FIG. 6A .  FIG. 6C  is a perspective view of a sensor fixing member used for the ultrasonic flow meter in the second exemplary embodiment. 
     As illustrated in  FIGS. 6A, 6B, and 6C , flow path block  50  includes measurement flow path  51  which has a cylindrical shape and has a rectangular cross-section and through which a fluid to be measured flows. Measurement flow path  51  is divided into three divided flow paths  53  (first divided flow path  53   a , second divided flow path  53   b , and third divided flow path  53   c ) by two partition plates  52  arranged in parallel along a flow direction of the fluid to be measured, so that a multilayer flow path is formed as a whole. Further, flow path block  50  has inlet  54  and outlet  55  for the fluid to be measured. 
     Flow path block  50  further includes: upstream-side sensor mounting portion  56   a  and downstream-side sensor mounting portion  56   b  each having an opening for mounting the ultrasonic transceiver; positioning projections  57  for the time of mounting the ultrasonic transceivers; locking portions  58  for sensor fixing members to be described later; and locking portions  59  for a circuit board. 
       FIG. 7A  is a side view of the ultrasonic flow meter in the second exemplary embodiment. As illustrated in  FIG. 7A , in ultrasonic flow meter  60 , a pair of ultrasonic transceivers  40  are mounted on upstream-side sensor mounting portion  56   a  and downstream-side sensor mounting portion  57   b  of flow path block  50 . Here, after ultrasonic transceivers  40  are mounted such that recesses  41   c  (see  FIG. 5C ) are positioned at positioning projections  57  (see  FIG. 6A ), hinges  61   a  (see  FIG. 6C ) of sensor fixing members  61  (see  FIG. 6C ) are engaged with locking portions  58  provided on flow path block  50 , so that the ultrasonic transceivers are pressed against and fixed to upstream-side sensor mounting portion  56   a  (see  FIG. 6A ) and downstream-side sensor mounting portion  56   b  (see  FIG. 6A ). 
       FIG. 7B  is a main-part cross-sectional view taken along line  7 B- 7 B of  FIG. 7A .  FIG. 7B  illustrates the relationship between second acoustic matching layer  44  and partition plates  52 . As illustrated in the drawing, second acoustic matching layer  44  is disposed at a position facing second divided flow path  53   b  of the multilayer flow path, and a width W (short-side length) of second acoustic matching layer  44  is made to be matched with outer sides of two partition plates  52  and is made to be larger than a distance X (2.9 mm) between the partition plates. 
       FIG. 8  is a graph for illustrating a width of a short side of a second acoustic matching layer, an output of an ultrasonic wave, and a magnitude of reverberation in the second exemplary embodiment. The graph illustrated in  FIG. 8  is an image in which the horizontal axis represents the width of the short side of second acoustic matching layer  44 , and the output of the ultrasonic signal and the magnitude of the reverberation are shown as a graph. As illustrated in the drawing, the output increases as the width of second acoustic matching layer  44  increases, but the reverberation increases after the width reaches a certain size, so that the optimum dimension is determined to be 5.5 mm. 
     As described above, by setting the width (short-side length) of second acoustic matching layer  44  to be inside the outer periphery of the joining surface of first acoustic matching layer  43 , it is possible to prevent the wave reflected on the side surface of first acoustic matching layer  43  from propagating to second acoustic matching layer  44 , and it is possible to suppress the reverberation of the ultrasonic signal and to efficiently propagate the ultrasonic signal to second divided flow path  53   b , which is at the center. 
     Circuit board  62  including a drive circuit for ultrasonic transceivers  40  and a measurement circuit that measures the propagation time to calculate the flow velocity and the flow rate of the fluid to be measured is placed on an upper part of flow path block  50  and is locked by locking portion  59 . Lead wires  46  and  47  of ultrasonic transceivers  40  are connected to circuit board  62  by soldering or the like. Further, board case  63  represented by a broken line is placed so as to cover circuit board  62 . 
     Then, the ultrasonic signal from upstream ultrasonic transceiver  40  on the upstream side is reflected, on a rout represented by arrow P 1 , by inner wall  51   a  of the opposing flow path, then passes through a route represented by arrow P 2 , and is received by ultrasonic transceiver  40  on the downstream side. 
     With the above configuration, ultrasonic flow meter  60  of the present exemplary embodiment can obtain the flow rate by the above-described Equation (1), as described with reference to  FIG. 9 . 
     As described above, an ultrasonic transceiver in a first disclosure includes: a piezoelectric element; and a plurality of acoustic matching layers stacked on and joined to the piezoelectric element directly or via another layer, wherein at least a part of a joining part between the acoustic matching layers is disposed inside an outer periphery of a joining surface of the acoustic matching layer disposed on a side of the piezoelectric element. 
     With this configuration, it is possible to suppress the propagation of the indirect wave from the acoustic matching layer on the piezoelectric element side to the next acoustic matching layer. 
     In an ultrasonic transceiver in a second disclosure may be configured, in the first disclosure, as follows. The plurality of acoustic matching layers includes: a first acoustic matching layer joined to the piezoelectric element directly or via another layer; and a second acoustic matching layer stacked on and joined to the first acoustic matching layer, wherein the first acoustic matching layer has a disk shape, the second acoustic matching layer has a rectangular shape, and a short side of a joining surface via which the second acoustic matching layer and the first acoustic matching layer are joined together is shorter than a diameter of the first acoustic matching layer. 
     An ultrasonic transceiver in a third disclosure may be configured, in the second disclosure, as follows. The piezoelectric element has a joining surface having a rectangular shape, and a direction of a long side of the piezoelectric element coincides with a long-side direction of the second acoustic matching layer. 
     An ultrasonic transceiver in a fourth disclosure may be configured, in the first disclosure, as follows. A thickness of each of the acoustic matching layers is about ¼ of a wavelength of an ultrasonic wave propagating through the each of the acoustic matching layers. 
     An ultrasonic transceiver in a fifth disclosure may be configured, in the second disclosure, as follows. A thickness of each of the acoustic matching layers is about ¼ of a wavelength of an ultrasonic wave propagating through the each of the acoustic matching layers. 
     An ultrasonic transceiver in a sixth disclosure may be configured, in the third disclosure, as follows. A thickness of each of the acoustic matching layers is about ¼ of a wavelength of an ultrasonic wave propagating through the each of the acoustic matching layers. 
     An ultrasonic transceiver in a seventh disclosure may be configured, in the first disclosure, as follows. An acoustic impedance of each of the acoustic matching layers is larger toward the piezoelectric element. 
     An ultrasonic transceiver in an eighth disclosure may be configured, in the second disclosure, as follows. An acoustic impedance of each of the acoustic matching layers is larger toward the piezoelectric element. 
     An ultrasonic transceiver in a ninth disclosure may be configured, in the third disclosure, as follows. An acoustic impedance of each of the acoustic matching layers is larger toward the piezoelectric element. 
     An ultrasonic flow meter in a tenth disclosure includes: a measurement flow path that has a rectangular cross-section and that a fluid to be measured flows through; a plurality of partition plates that are inserted in parallel between two opposing surfaces of the measurement flow path and divide the measurement flow path into multiple layers; a pair of ultrasonic transceivers disposed upstream and downstream of a surface, of the measurement flow path, different from the two opposing surfaces, and each of the pair of ultrasonic transceivers is the ultrasonic transceiver disclosed in any one of the second to ninth disclosures. 
     An ultrasonic flow meter in an eleventh disclosure may be configured, in the tenth disclosure, as follows. Each of the ultrasonic transceivers is disposed in the measurement flow path with a long-side direction of the second acoustic matching layer parallel to the partition plates. 
     An ultrasonic flow meter in a twelfth disclosure may be configured, in the eleventh disclosure, as follows. A length of the second acoustic matching layer in a short-side direction is equal to or larger than a distance between the partition plates. 
     INDUSTRIAL APPLICABILITY 
     As described above, with the ultrasonic transceiver according to the present disclosure, it is possible to reduce reverberation of an ultrasonic wave and to limit an ultrasonic propagation path, and the ultrasonic transceiver can also be applied to applications such as in-vehicle sensing devices and the like. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               10 ,  20 ,  30 ,  40 ,  70 ,  80 : ultrasonic transceiver 
               11 ,  41 : case 
               12 ,  42 : piezoelectric element 
               13 ,  23 ,  43 : first acoustic matching layer (acoustic matching layer) 
               14 ,  24 ,  34 ,  44 ,  74 : second acoustic matching layer (acoustic matching layer) 
               25 : third acoustic matching layer (acoustic matching layer) 
               51 : measurement flow path 
               52 : partition plate 
               60 : ultrasonic flow meter