Patent Publication Number: US-2012031189-A1

Title: Pressure sensor

Description:
BACKGROUND 
     1. Technical Field 
     The invention relates to a pressure sensor, and more particularly, to a pressure sensor which uses two pressure sensitive elements and operates the two pressure sensitive elements differentially to improve detection sensitivity, temperature property, and the like. 
     2. Related Art 
     In the related art, pressure sensors which use a piezoelectric vibrating element as a pressure sensitive element are known, such as a hydraulic pressure meter, a barometer, a differential pressure meter, and the like. In a pressure sensor using a piezoelectric vibrating element, when pressure is applied to the piezoelectric vibrating element in the detection axis direction thereof, the resonance frequency of the piezoelectric vibrating element changes, and pressure applied to the pressure sensor is detected from a change in the resonance frequency. 
     JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534 disclose pressure sensors in which a piezoelectric vibrating element is used as a pressure sensitive element. When pressure is applied to a bellows through a pressure inlet opening, a force corresponding to the effective area of the bellows is applied to a piezoelectric vibrating element as a compressive force or a tensile force (extensional force) F through a force transmission member in which a pivot (flexible hinge) is used as a fulcrum. A stress corresponding to the force F occurs in the piezoelectric vibrating element, and the resonance frequency of the piezoelectric vibrating element changes due to the stress. The pressure sensor can calculate the applied pressure by detecting a change in the resonance frequency of the piezoelectric vibrating element. 
       FIG. 10  is a cross-sectional view showing a configuration of a pressure sensor disclosed in JP-A-56-119519. The pressure sensor includes a housing  104  having first and second pressure inlet openings  102  and  103 , and a force transmission member  105  disposed inside the housing  104 . A first bellows  106  and a second bellows  107  are connected with one end of the force transmission member  105  disposed therebetween. Moreover, an opening on the other end of the first bellows  106  is connected to the first pressure inlet opening  102 , and an opening on the other end of the second bellows  107  is connected to the second pressure inlet opening  103 . Furthermore, a double-ended vibrating element  109  serving as a pressure sensitive element is disposed between the other end of the force transmission member  105  and an end portion of a substrate  108  which is on the opposite end of a pivot (fulcrum) of the substrate  108 . 
     When detecting pressure with high accuracy, liquid is filled in the bellows. As the liquid, silicone oil or the like having high viscosity is generally used in order to prevent bubbles from entering and accumulating inside the bellows or between the folds of the bellows. That is, viscous oil  110  is filled in the first bellows  106 . When liquid is a pressure measurement target, the oil  110  contacts and faces the liquid through an opening  111  opened to the first pressure inlet opening  102 . The size of the opening  111  is set so as to prevent leakage of the oil  110 . 
       FIG. 11  is a cross-sectional view showing a configuration of a pressure sensor disclosed in JP-A-2-228534. A pressure sensor  150  shown in  FIG. 11  includes a housing  120 , a pressure inlet opening  121 , and bellows  122   a  and  122   b.  A force transmission member  125  is connected to the bellows  122   a  and  122   b,  and a pressure sensitive element  130  is attached and fixed between a flexible portion  125   a  and a fixing portion  125   b  of the force transmission member  125 . When pressure is applied to the bellows  122   a  and  122   b  through the pressure inlet opening  121  of the pressure sensor  150 , force corresponding to the effective area of the bellows  122   a  and  122   b  is applied to the force transmission member  125  in the vertical direction. Force corresponding to differential pressure is applied to the pressure sensitive element  130  as a compressive force or a tensile force (extensional force) with a pivot  135  used as a fulcrum. The resonance frequency of the pressure sensitive element  130  changes with this force, and the pressure sensor  150  measures pressure by detecting the change in the resonance frequency. 
     The bellows  122   a  and  122   b,  the force transmission member  125 , the pressure sensitive element  130 , and the housing  120  are formed of different materials. As a result, thermal deformation occurs due to a change in the temperature of the use environment, which deteriorates pressure measurement accuracy. Therefore, a supporting portion of the pressure sensitive element  130  is arranged to be separated from a flexible portion  125   a  of the force transmission member  125  and the force transmission member  125 . Moreover, the supporting portion is cross-linked to a fixing member  140  of the pressure sensitive element  130  provided in the housing  120 . In this way, thermal deformation due to a change in ambient temperature is prevented from affecting the pressure sensitive element  130 . 
     Moreover, the thermal deformation of the bellows line, the force transmission member, a force transmission member supporting portion, and the pressure sensitive element fixing portion were separated and analyzed for thermal deformation. For example, stainless steel, nickel, phosphor bronze, and quartz crystal were used for the housing, the bellows, the force transmission member, and the pressure sensitive element, respectively, and the respective linear expansion coefficients were used in the analysis. JP-A-2-228534 describes that when the dimensions of the respective members are set, and the linear expansion coefficient of the fixing member of the pressure sensitive element  130  is set, it is possible to calculate the optimum length of the fixing member and to realize a pressure sensor which is not affected by thermal deformation. 
     However, in the pressure sensor  101  disclosed in JP-A-56-119519, the oil  110  filled in the first bellows  106  shown in  FIG. 10  has a higher thermal expansion coefficient than other constituent elements, for example, the force transmission member  105 , the double-ended vibrating element  109 , and the like. Thus, the oil  110  causes thermal deformation in the respective constituent members of the pressure sensor  101  due to a change in temperature. Stress due to this thermal deformation is superimposed on the signal of the double-ended vibrating element  109  as noise, and measurement accuracy of the pressure sensor deteriorates. 
     Moreover, the oil  110  filled in the first bellows  106  contacts and faces the liquid which is the pressure measurement target. However, depending on a method of installing the pressure sensor, the oil may flow toward the liquid which is the pressure measurement target, and the liquid may flow into the first bellows  106 . Thus, there is a possibility that bubbles are formed in the oil  110  filled in the first bellows  106 . If bubbles are formed in the oil  110 , the bubbles absorb pressure, and the oil does not properly function as a pressure transmission medium. Thus, there is a possibility that errors occur in the measured pressure value. 
     Furthermore, since the oil  110  is in contact with the liquid which is the pressure measurement target, depending on a method of installing the pressure sensor, there is a possibility that the oil  110  flows toward the liquid which is the pressure measurement target. Thus, a pressure sensor which uses the oil  110  as in the related art may not be used for fluid-pressure measurement where mixing of foreign materials is to be prevented. 
     Moreover, in the pressure sensors disclosed in JP-A-56-119519 and JP-A-2-228534, the force transmission members  105  and  125  have a complex structure, which makes it difficult to miniaturize the pressure sensors. Moreover, the force transmission members  105  and  125  are components which require a flexible hinge having a narrow waist portion, which increases the manufacturing costs of the pressure sensor. 
     In order to solve such a problem, JP-A-2010-48798 discloses a pressure sensor  210  as shown in the cross-sectional view of  FIG. 12 . The pressure sensor  210  includes a housing  212 , a pressure receiving member (diaphragm  224 ) which seals an opening  222  of the housing  212  and includes a flexible portion (central region  224   a ) and a peripheral region  224   c  positioned on the outer side of the flexible portion, and in which one principal surface of the flexible portion is a pressure receiving surface, and a pressure sensitive element  240  which includes a pressure sensing portion and first and second base portions  240   a  and  240   b  respectively connected to both ends of the pressure sensing portion, and in which an arrangement direction of the first and second base portions  240   a  and  240   b  is parallel to a displacement direction of the diaphragm  224 . In the pressure sensor  210 , the first base portion  240   a  is connected to a central region  224   a  of the diaphragm  224 , which is the rear side of the pressure receiving surface, and the second base portion  240   b  is connected to the peripheral region  224   c  on the rear side, or to an inner wall of the housing  212  facing the first base portion  240   a,  through a connecting member  242 . 
     With this configuration, force corresponding to the displacement of the pressure receiving member can be directly applied to the pressure sensitive element  240  as compressive force without through the flexible hinge described above. Thus, it is possible to improve sensitivity. Moreover, it is possible to widen measurement targets since it does not use oil. In addition, in the pressure sensitive element  240 , since the first base portion  240   a  is fixed to the pressure receiving member and the second base portion  240   b  is fixed to the side of the pressure receiving member through the connecting member  242 , it is possible to alleviate a thermal deformation problem. Moreover, since the connecting member  242  and the pressure sensitive element  240  are integrally formed using a piezoelectric material, it is possible to alleviate thermal deformation further. The above configuration can be used as a fluid pressure sensor which measures fluid pressure with reference to atmospheric pressure by making the inside of the housing  212  open to atmospheric pressure. In this case, tensile force as well as compressive force can be applied to the pressure sensitive element  240 . 
     However, in the above configuration, since thermal expansion and thermal contraction occur due to a change in the temperature of the pressure sensitive element and the connecting member, there is a problem in that it is difficult to suppress a change in the resonance frequency of the pressure sensitive element due to the thermal expansion. There is also a problem in that the resonance frequency of the pressure sensitive element changes with time. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides a pressure sensor capable of measuring pressure stably by suppressing problems associated with a change in temperature, aging, and the like. 
     APPLICATION EXAMPLE 1 
     This application example of the invention is directed to a pressure sensor including: a pressure receiving member having a flexible portion that is displaced in response to force and a peripheral portion connected to an outer periphery of the flexible portion; and first and second pressure sensitive elements which have a pressure sensing portion and a pair of base portions connected to both ends of the pressure sensing portion, and which have a detection axis parallel to a line connecting the base portions, and in which the detection axis is parallel to a displacement direction of the flexible portion, wherein one base portion of the first pressure sensitive element is fixed to the flexible portion, and the other base portion is fixed to a first supporting member that is supported by the peripheral portion, and wherein one base portion of the second pressure sensitive element is fixed to the peripheral portion, and the other base portion is fixed to a second supporting member that is supported by the flexible portion. 
     With this configuration, when the flexible portion is displaced toward the outer side of the housing, the first pressure sensitive element receives tensile stress from the flexible portion and the first supporting member that is supported by the peripheral portion, and the second pressure sensitive element receives compressive stress from the flexible portion through the second supporting member that is supported by the flexible portion. In contrast, when the flexible portion is displaced toward the inner side of the housing, the first pressure sensitive element receives compressive stress from the first supporting member, and the second pressure sensitive element receives tensile stress from the flexible portion through the second supporting member. The resonance frequencies of the respective pressure sensitive elements increase in response to tensile stress and decrease in response to compressive stress. Therefore, the pressure applied to the flexible portion can be detected by calculating a difference between the resonance frequencies of the first and second pressure sensitive elements. If the first and second pressure sensitive elements are the same constituent elements, since they have the same temperature property and the same aging property with respect to the resonance frequency, these characteristics are canceled in relation to the difference. Therefore, the pressure sensor can measure pressure stably regardless of the temperature property, the aging property, and the like. Moreover, since pressure is measured based on the difference between the resonance frequencies of two pressure sensitive elements, it is possible to obtain higher sensitivity than when using one pressure sensitive element. Furthermore, since at least one base portion of the first and second pressure sensitive elements is fixed to the side of the pressure receiving member, it is possible to decrease the overall size of the pressure sensor. 
     APPLICATION EXAMPLE 2 
     In the pressure sensor of the above application example, the pressure sensing portion may include at least one columnar beam. 
     With this configuration, when the pressure sensing portion is formed of one columnar beam, since the stress applied to the beam increases, it is possible to improve sensitivity of the pressure sensor. 
     APPLICATION EXAMPLE 3 
     In the pressure sensor of the above application example, the first and second pressure sensitive elements and the first and second supporting members may be integrally formed of a piezoelectric material. 
     With this configuration, since the respective pressure sensitive elements and the respective supporting members have the same thermal expansion coefficient, it is possible to prevent thermal deformation between the respective pressure sensitive elements and the respective supporting members and to improve the temperature property. Moreover, by integrally forming the respective pressure sensitive elements and the respective supporting members, it is possible to decrease the number of components of the pressure sensor, increase the assembly efficiency of the pressure sensor, and achieve cost reduction. 
     APPLICATION EXAMPLE 4 
     In the pressure sensor of the above application example, the first and second pressure sensitive elements and the first and second supporting members may be formed so that end portions thereof on the sides connected to the pressure receiving member are arranged on a straight line that is vertical to the displacement direction of the flexible portion. 
     With this configuration, since the respective pressure sensitive elements and the respective supporting members will not receive thermal deformation from the pressure receiving member, the pressure sensor can measure pressure with high accuracy stably against a change in temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a perspective cross-sectional view of a pressure sensor according to a first embodiment, taken along the XZ plane. 
         FIGS. 2A and 2B  are cross-sectional views of the pressure sensor according to the first embodiment, taken along the XZ and YZ planes, respectively. 
         FIGS. 3A to 3D  show schematic views when a diaphragm is formed of metal. 
         FIGS. 4A to 4E  show schematic views when a diaphragm is formed of quartz crystal. 
         FIGS. 5A and 5B  show modification examples when a diaphragm is formed of quartz crystal. 
         FIGS. 6A and 6B  are cross-sectional views of a pressure sensor according to a modification example of the first embodiment, taken along the XZ and YZ planes, respectively. 
         FIG. 7  is a perspective cross-sectional view of a pressure sensor according to a second embodiment, taken along the XZ plane. 
         FIGS. 8A and 8B  are cross-sectional views of the pressure sensor according to the second embodiment, taken along the XZ and YZ planes, respectively. 
         FIGS. 9A to 9E  show schematic views when an integral member that integrates first and second pressure sensitive elements and first and second supporting members is formed of quartz crystal. 
         FIG. 10  is a cross-sectional view showing a configuration of a pressure sensor disclosed in JP-A-56-119519. 
         FIG. 11  is a cross-sectional view showing a configuration of a pressure sensor disclosed in JP-A-2-228534. 
         FIG. 12  is a cross-sectional view showing a configuration of a pressure sensor disclosed in JP-A-2010-48798. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A pressure sensor according to the invention will be described in detail below with reference to embodiments shown in the accompanying drawings. Note that constituent elements, types, combinations, shapes, relative positions, and the like described in the embodiments are not intended to limit the range of this invention, but are only examples unless there is a specific statement. 
     First Embodiment 
       FIG. 1  is a perspective cross-sectional view of a pressure sensor according to a first embodiment, taken along the XZ plane.  FIGS. 2A and 2B  are cross-sectional views of the pressure sensor according to the first embodiment, taken along the XZ and YZ planes, respectively. Here, the X, Y, and Z axes shown in  FIGS. 1 ,  2 A, and  2 B constitute an orthogonal coordinate system, and the same is applied to the drawings referred hereinafter. A pressure sensor  10  according to the first embodiment includes a housing  12 , a diaphragm  24  serving as a pressure receiving member, first and second pressure sensitive elements  40  and  42 , and first and second supporting members  44  and  46 . The pressure sensor  10  has a structure in which the housing  12  and the diaphragm  24  serve as a container, and the first and second pressure sensitive elements  40  and  42  are accommodated in the accommodation space of the container having the diaphragm  24 . For example, the pressure sensor  10  can be used as a fluid pressure sensor in which the inside of the housing  12  is opened to the atmosphere, and which receives fluid pressure from outside the diaphragm  24  with reference to atmospheric pressure. 
     The housing  12  includes a circular flange portion  14 , a circular ring portion  16 , a supporting shaft  18 , and cylindrical side surfaces (side walls)  20 . 
     The flange portion  14  includes an outer peripheral portion  14   a  that is in contact with the end portions of the cylindrical side surfaces (side walls)  20  and an inner peripheral portion  14   b  that is formed on the outer peripheral portion  14   a  to be concentric to the outer peripheral portion  14   a  so as to protrude in a ring shape having the same diameter as the ring portion  16 . The ring portion  16  includes a circular opening  22  which is formed by the inner peripheral edge thereof. The diaphragm  24  is connected to the opening  22  so as to seal the opening  22 . 
     Holes  14   c  and  16   a  in which supporting shafts  18  are inserted are formed at predetermined positions of the inner peripheral portion  14   b  of the flange portion  14  and the mutually facing surfaces of the ring portion  16 . Moreover, the holes  14   c  and  16   a  are formed at the mutually facing positions. Therefore, when the supporting shafts  18  are inserted into the holes  14   c  and  16   a,  the flange portion  14  and the ring portion  16  are connected by the supporting shafts  18 . The supporting shafts  18  are rod-like members having predetermined rigidity and extending in the ±Z direction. The supporting shafts  18  are disposed inside the container which includes the housing  12  and the diaphragm  24 . When one set of ends of the supporting shafts  18  is inserted into the holes  14   c  of the flange portion  14  and the other set of ends thereof is inserted into the holes  16   a  of the ring portion  16 , predetermined rigidity is obtained between the flange portion  14 , the supporting shafts  18 , and the ring portion  16 . Although a plurality of supporting shafts  18  is used, the arrangement thereof is optional depending on the design of the positions of the respective holes. 
     Moreover, hermetic terminals  36  are attached to the flange portion  14 . The hermetic terminals  36  are configured to be capable of electrically connecting electrode portions (not shown) of the first and second pressure sensitive elements  40  and  42  described later and an integrated circuit (IC, not shown). The IC is used for oscillating the first and second pressure sensitive elements  40  and  42  and calculating a difference between the resonance frequencies of the first and second pressure sensitive elements  40  and  42  and is attached to the outer surface of the housing  12  or is disposed outside the housing  12  to be separated from the housing  12 . 
     Although two hermetic terminals  36  are depicted in  FIGS. 1 ,  2 A, and  2 B, the hermetic terminals  36  are attached to the flange portion  14  in accordance with the total number of electrode portions of the first and second pressure sensitive elements  40  and  42  described later. Moreover, an air inlet opening  14   e  is formed on the flange portion  14  so that the inside of the housing  12  can be opened to the atmosphere. The hermetic terminals  36  and the air inlet opening  14   e  are disposed at any positions of the flange portion  14  such that they do not interfere with each other. 
     Since both sets of ends of the side surfaces  20  are respectively connected to the outer periphery  14   d  of the inner peripheral portion  14   b  of the flange portion  14  and the outer periphery  16   b  of the ring portion  16  of which the opening  22  is covered by the diaphragm  24 , the container is sealed. The flange portion  14 , the ring portion  16 , and the side surfaces  20  are preferably formed of metal such as stainless steel. The supporting shafts  18  are preferably formed of ceramics or the like having predetermined rigidity and a low thermal expansion coefficient. 
     One principal surface of the diaphragm  24  facing the outer surface of the housing  12  is configured as a pressure receiving surface. The pressure receiving surface has a flexible portion which is bent and deformed in response to pressure of a pressure measurement environment (for example, liquid). When the flexible portion is bent and deformed to be displaced toward the inner or outer side (Z-axis direction) of the housing  12 , the diaphragm  24  transmits Z-axis direction compressive or tensile force to the first and second pressure sensitive elements  40  and  42 . Moreover, the diaphragm  24  includes the flexible portion which includes a central region  24   a  that is displaced by pressure from the outside, and a flexible region  24   b  that is disposed on the outer periphery of the central region  24   a  so as to be bent and deformed by the pressure from the outside so as to allow the displacement of the central region  24   a,  and a peripheral portion  24   c  that is disposed on the outer side of the flexible portion, namely on the outer periphery of the flexible region  24   b  and is bonded and fixed to the inner wall of the opening  22  formed in the ring portion  16 . Ideally, the peripheral portion  24   c  is not displaced and the central region  24   a  is not deformed even when pressure is applied thereto. 
     The surface of the central region  24   a  of the diaphragm  24  on the opposite side of the pressure receiving surface is connected to one end in the longitudinal direction (detection axis direction) of the first pressure sensitive element  40  described later. The surface of the central region  24   a  opposite the pressure receiving surface is attached to the second supporting member  46  described later by an adhesive agent or the like. One end (first base portion  40   a ) of the first pressure sensitive element  40  described later is fixed to the second supporting member  46  by a fixing material such as an adhesive agent. The surface of the peripheral portion  24   c  of the diaphragm  24  opposite the pressure receiving surface is connected to the first supporting member  44  described later and the fixing portion  48  described later by a fixing material such as an adhesive agent. The first and second supporting members  44  and  46  and the fixing portion  48  are preferably formed of the same material as the diaphragm  24 . 
     The diaphragm  24  is preferably formed of a material having excellent corrosion resistance such as metal (for example, stainless steel) or ceramics and may be formed of a single crystalline body (for example, quartz crystal) or another amorphous body. For example, when the diaphragm  24  is formed of metal, it may be formed by pressing a base metal material. 
     When the diaphragm  24  is formed of metal, the base metal material (not shown) may be pressed from both surfaces thereof by a pair of pressing plates (not shown) having recesses (not shown) which correspond to wavy concentric circular shapes of the flexible region  24   b  of the diaphragm  24 . 
       FIGS. 3A to 3E  show schematic views when the diaphragm is formed of metal.  FIG. 3D  is a bottom view of  FIG. 3C . In order to suppress the diaphragm  24  from vibrating with a vibration of the first pressure sensitive element  40 , the central region  24   a  of the diaphragm  24  may be made thicker than other regions. In this case, a base metal material  30  is prepared ( FIG. 3A ), and is subjected to half-etching while leaving the central region  24   a  ( FIG. 3B ). Then, the etched base metal material  30  is pressed by a pair of pressing plates (not shown) having a shape corresponding to the shapes of the central region  24   a,  the flexible region  24   b,  and the peripheral portion  24   c,  whereby the diaphragm  24  is formed ( FIG. 3C ). After that, the first and second supporting members  44  and  46  and the fixing portion  28  are connected to predetermined positions of the diaphragm  24  by a fixing material such as an adhesive agent as shown in  FIGS. 1 ,  2 A, and  2 B. 
       FIGS. 4A to 4E  show schematic views when the diaphragm is formed of quartz crystal. When the diaphragm  24  is formed of quartz crystal, similarly, it is preferable to form the diaphragm  24  by photolithographic etching. In this case, a base substrate  32  as a material is prepared and a positive photoresist  34  is applied on the surface of the base substrate  32  ( FIG. 4A ). Subsequently, exposure is preformed using a photomask  35  corresponding in arrangement and shape to the central region  24   a,  the flexible region  24   b,  and the peripheral region (not shown) so as to expose the photoresist  34  ( FIG. 4B ). Subsequently, development is performed so as to remove the exposed photoresist  34   a  ( FIG. 4C ). Subsequently, a region on which the base substrate  32  is exposed is subjected to half-etching, whereby the central region  24   a,  the flexible region  24   b,  and the peripheral region (not shown) are integrally formed ( FIG. 4D ). Finally, the photoresist  34  is removed ( FIG. 4E ), whereby the diaphragm  24  is formed. 
       FIGS. 5A and 5B  show modification examples when the diaphragm is formed of quartz crystal. As a modification example of photolithographic etching of the diaphragm  24 , it is preferable to etch only one surface of the flexible region  24   b  as shown in  FIG. 5A , and it is also preferable to etch the front and rear surfaces of the flexible region  24   b  at the mutually facing positions as shown in  FIG. 5B . 
     In addition, the surface of the diaphragm  24  exposed to the outside may be coated with an anti-corrosion film so as not to be corroded by liquids, gases, or the like. For example, if the diaphragm  24  is formed of metal, the diaphragm  24  may be coated with a nickel compound. Moreover, if the diaphragm  24  is formed of a piezoelectric crystal body such as quartz crystal, the diaphragm  24  maybe coated with silicon. 
     As shown in  FIGS. 1 ,  2 A, and  2 B, the first supporting member  44  is configured to fix the second base portion  40   b  of the first pressure sensitive element  40  described later. The first supporting member  44  includes a pedestal portion  44   a  that is fixed to the peripheral portion  24   c  of the diaphragm  24 , a supporting column portion  46   b  that extends from the pedestal portion  44   a  in a displacement direction (Z-axis direction) of the central region  24   a  of the diaphragm  24 , and an arm portion  44   c  that extends from the distal end of the supporting column portion  46   b  toward the central region  24   a  to be connected to and support the second base portion  40   b  of the first pressure sensitive element  40 . 
     The supporting member  46  is configured to fix the second base portion  42   b  of the second pressure sensitive element  42  described later and the first base portion  40   b  of the first pressure sensitive element. The second supporting member  46  includes a pedestal portion  46   a  which is fixed to the central region  24   a  of the diaphragm  24  and to which the first base portion  40   a  of the first pressure sensitive element  40  is fixed, a supporting column portion  46   b  that extends from the pedestal portion  46   a  in a displacement direction of the central region  24   a  of the diaphragm  24 , and an arm portion that extends from the distal end of the supporting column portion  46   b  toward the peripheral portion  24   c  to be connected to and support the second base portion  42   b  of the second pressure sensitive element  42 . 
     The fixing portion  48  is fixed to the peripheral portion  24   c  of the diaphragm  24  at a position facing the distal end of the arm portion of the second supporting member, and the first base portion  42   a  of the second pressure sensitive element  42  is fixed to the fixing portion  48 . It is assumed that the first and second supporting members  44  and  46  and the fixing portion  48  have predetermined rigidity, and will not be deformed in directions other than the displacement direction of the central region  24   a  of the diaphragm  24 . 
     The materials of the first and second supporting members  44  and  46  are not particularly limited as long as predetermined rigidity can be obtained between the pedestal portion  44   a,  the supporting column portion  44   b,  and the arm portion  44   c,  and between the pedestal portion  46   a,  the supporting column portion  46   b,  and the arm portion  46   c.  However, the first and second pressure sensitive elements  40  and  42  are preferably formed of the same material as these portions in order to suppress thermal stress applied to the first and second pressure sensitive elements  40  and  42 . Similarly, the fixing portion  48  is preferably formed of the same material as the pressure sensitive elements for the same reason. 
     The first and second pressure sensitive elements  40  and  42  can be formed of a piezoelectric material such as a quartz crystal, lithium niobate, or lithium tantalate. 
     As shown in  FIGS. 1 ,  2 A, and  2 B, the first pressure sensitive element  40  includes vibrating arms  40   c  and first and second base portions  40   a  and  40   b  which are formed at both ends of the vibrating arms  40   c.  Similarly, the second pressure sensitive element  42  includes vibrating arms  42   c  and first and second base portions  42   a  and  42   b  which are formed at both ends of the vibrating arms  42   c.  Furthermore, the first and second pressure sensitive elements  40  and  42  include excitation electrodes (not shown) which are formed on the vibrating arms  40   c  and  42   c  and the electrode portions (not shown) which are electrically connected to the excitation electrodes (not shown). 
     The first pressure sensitive element  40  is disposed so that the longitudinal direction (Z-axis direction) thereof, namely the arrangement direction of the first and second base portions  40   a  and  40   b,  is coaxial to or parallel to the displacement direction of the diaphragm  24 , and the displacement direction thereof is used as the detection axis. The first base portion  40   a  of the first pressure sensitive element  40  is fixed to the pedestal portion  46   a  of the second supporting member  46  and is in contact with the central region  24   a  of the diaphragm  24 . Moreover, the second base portion  40   b  which is on the opposite side of the first base portion  40   a  with the vibrating arms  40   c  disposed therebetween is connected to the distal end of the arm portion  44   c  of the first supporting member  44 . 
     Similarly to the first pressure sensitive element  40 , the second pressure sensitive element  42  includes the vibrating arms  42   c  and the first and second base portions  42   a  and  42   b  formed at both ends of the vibrating arms  42   c.  The second pressure sensitive element  42  has its detection axis which is in parallel to a line connecting the first and second base portions  42   a  and  42   b,  similarly to the first pressure sensitive element  40 . Moreover, it is assumed that the material and dimensions of the second pressure sensitive element  42  are the same as those of the first pressure sensitive element  40 , and the two pressure sensitive elements have the same temperature property and the same aging property. The second pressure sensitive element  42  is disposed in parallel to the first pressure sensitive element  40 , and the first base portion  42   a  is connected to the fixing portion  48  fixed to the peripheral portion  24   c  and is in contact with the peripheral portion  24   c.  Furthermore, the second base portion  42   b  of the second pressure sensitive element  42  is connected to the distal end of the arm portion  46   c  of the second supporting member  46 . 
     In addition, since the first and second pressure sensitive elements  40  and  42  are fixed to the first and second supporting members  44  and  46  and the fixing portion  48 , the respective pressure sensitive elements can be easily fixed to the side of the diaphragm  24 . Moreover, since the first and second pressure sensitive elements  40  and  42  are not bent in directions other than the detection axis direction, it is possible to prevent the first and second pressure sensitive elements  40  and  42  from moving in directions other than the detection axis direction and to improve the sensitivity in the detection axis direction of the first and second pressure sensitive elements  40  and  42 . 
     The first and second pressure sensitive elements  40  and  42  are electrically connected to the IC (not shown) through wires  38  and the hermetic terminals  36  described above and vibrate at a natural resonance frequency in response to an alternating voltage supplied from the IC (not shown). Moreover, the resonance frequencies of the first and second pressure sensitive elements  40  and  42  change when they receive extensional stress or compressive stress from the longitudinal direction thereof. In the present embodiment, a double-ended tuning fork vibrator can be used as the vibrating arms  40   c  and  42   c  serving as the pressure sensing portion. The double-ended tuning fork vibrator has characteristics such that the resonance frequency thereof changes substantially in proportion to tensile stress (extensional stress) or compressive stress which is applied to the two vibrating beams which are the vibrating arms  40   c  and  42   c.  Moreover, a double-ended tuning fork piezoelectric vibrator is ideal for a pressure sensor which has such an excellent resolution as to detect a small pressure difference since a change in the resonance frequency to extensional and compressive stress is very large as compared to a thickness shear vibrator or the like, and a variable width of the resonance frequency is large. In the double-ended tuning fork piezoelectric vibrator, the resonance frequency of the vibrating arm increases when it receives extensional stress, whereas the resonance frequency of the vibrating arm decreases when it receives compressive stress. 
     Moreover, in the present embodiment, the pressure sensing portion is not limited to one which has two rod-like vibrating beams, but a pressure sensing portion having one vibrating beam (single beam) may be used. If the pressure sensing portion (the vibrating arms  40   c  and  42   c ) is configured as a single-beam vibrator, the displacement thereof is doubled when the same amount of stress is applied from the longitudinal direction (detection axis direction). Therefore, it is possible to obtain a pressure sensor which is more sensitive than one having a double-ended tuning fork vibrator. In addition, among the piezoelectric materials described above, a quartz crystal having excellent temperature property is preferred as the material of a piezoelectric substrate of a double-ended or single-beam piezoelectric vibrator. 
     The pressure sensor  10  of the first embodiment is assembled in the following manner. First, the diaphragm  24  is connected to the ring portion  16 , and the first and second supporting members  44  and  46  and the fixing portion  48  are connected to predetermined positions of the diaphragm  24 . Moreover, the first base portion  40   a  of the first pressure sensitive element  40  is connected to the pedestal portion  46   a  of the second supporting member  46 , and the second base portion  40   b  is connected to the arm portion  46   c  of the first supporting member  44 . Furthermore, the first base portion  42   a  of the second pressure sensitive element  42  is connected to the fixing portion  48 , and the second base portion  42   b  is connected to the arm portion  46   c  of the second supporting member  46 . 
     Subsequently, the supporting shaft  18  is fixed by inserting into the hole  16   a  of the ring portion  16 , and the other end of the supporting shaft  18  of which one end thereof has been inserted into the ring portion  16  is fixed by inserting into the hole  14   c  of the flange portion  14 . Moreover, the portions of the hermetic terminals  36  disposed inside the housing  12  are electrically connected to the electrode portions (not shown) of the first pressure sensitive element  40  and the second pressure sensitive element  42  by the wires  38 . In this case, the portions of the hermetic terminals  36  disposed outside the housing  12  are connected to the IC (not shown). Finally, the side surfaces  20  are inserted from the side of the ring portion  16  so as to be bonded to the inner periphery and outer periphery  14   d  of the flange portion  14  and the outer periphery  16   b  of the ring portion  16 . In this way, the housing  12  is formed, and the pressure sensor  10  is assembled. 
     Next, the operation of the pressure sensor  10  according to the first embodiment will be described. In the first embodiment, when measuring fluid pressure with reference to atmospheric pressure, the central region  24   a  of the diaphragm  24  is displaced toward the inner side of the housing  12  if the fluid pressure is lower than the atmospheric pressure. In contrast, the central region  24   a  is displaced toward the outer side of the housing  12  if the fluid pressure is higher than the atmospheric pressure. 
     Moreover, when the central region  24   a  of the diaphragm  24  is displaced toward the outer side of the housing  12 , the first pressure sensitive element  40  receives tensile stress from the central region  24   a  and the first supporting member  44  that is supported by the peripheral portion  24   c  (the fixing portion  48 ), and the second pressure sensitive element  42  receives compressive stress from the central region  24   a  through the second supporting member  46  that is supported by the central region  24   a  of the diaphragm  24 . In contrast, when the central region  24   a  is displaced toward the inner side of the housing  12 , the first pressure sensitive element  40  receives compressive stress from the first supporting member  44 , and the second pressure sensitive element  42  receives tensile stress from the central region  24   a  through the second supporting member  46 . 
     The resonance frequencies of the respective pressure sensitive elements increase in response to tensile stress and decrease in response to compressive stress. Therefore, the pressure applied to the central region  24   a  can be detected by calculating a difference between the resonance frequencies of the first and second pressure sensitive elements  40  and  42 . If the first and second pressure sensitive elements  40  and  42  are the same constituent elements, since they have the same temperature property and the same aging property with respect to the resonance frequency, these characteristics are canceled in relation to the difference. 
     Therefore, the pressure sensor  10  can measure pressure stably regardless of the temperature property, the aging property, and the like. Moreover, since pressure is measured based on the difference between the resonance frequencies of two pressure sensitive elements, it is possible to obtain higher sensitivity than when using one pressure sensitive element. Furthermore, since at least one base portion of the first and second pressure sensitive elements  40  and  42  is fixed to the side of the diaphragm  24 , it is possible to decrease the overall size of the pressure sensor  10 . 
     Here, a change in the resonance frequency of the first pressure sensitive element  40  relative to the second pressure sensitive element  42  will be discussed. A change ΔF in the resonance frequency of each pressure sensitive element can be expressed as the sum of a frequency change ΔF(P) due to pressure P applied from the diaphragm, a frequency change ΔF(T) due to temperature T, a frequency change ΔF(τ) due to aging (τ), and a frequency change ΔF(μ) due to air viscosity (μ). That is, the resonance frequency changes ΔF 1  and ΔF 2  of the first and second pressure sensitive elements  40  and  42  are expressed by Expression (1) below. 
     
       
         
           
             
               
                 
                   { 
                   
                     
                       
                         
                           
                             Δ 
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                              
                             
                               F 
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                           = 
                           
                             
                               Δ 
                                
                               
                                   
                               
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                                   F 
                                   1 
                                 
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                                   ( 
                                   P 
                                   ) 
                                 
                               
                             
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                                   1 
                                 
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                                   ( 
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                   ( 
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     Here, since the first and second pressure sensitive elements  40  and  42  are formed of elements having the same property, although frequency changes ΔF(T), ΔF(τ), and ΔF(μ) are the same, the frequency changes ΔF(P) thereof due to pressure P have different signs because of their structure in the present embodiment. That is, Expression (2) below is satisfied. 
     
       
         
           
             
               
                 
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                                 2 
                               
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                                 ( 
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                                 2 
                               
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                   ( 
                   2 
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     Therefore, when Expression (2) is substituted into Expression (1), the difference between the resonance frequency changes ΔF 1  and ΔF 2  of the first and second pressure sensitive elements  40  and  42  is calculated as Expression (3) below. 
       Δ F   1   −ΔF   2 =2Δ F   1 ( P )   (3)
 
     Therefore, when the difference between the resonance frequencies of the first and second pressure sensitive elements  40  and  42  is calculated, only the frequency change component ΔF(P) due to pressure P remains, and the other components are canceled. Thus, it can be understood that errors in pressure values due to changes in temperature and changes with time of the respective pressure sensitive elements and the effect of air viscosity can be eliminated. Furthermore, since the ΔF(P) component is doubled, it can be understood that the pressure measurement sensitivity is improved so as to be double. 
       FIGS. 6A and 6B  show a pressure sensor according to a modification example of the first embodiment.  FIGS. 6A and 6B  are cross-sectional views taken along the XZ and YZ planes, respectively. In  FIGS. 1 ,  2 A, and  2 B, the first and second pressure sensitive elements  40  and  42  are connected to the side of the arm portion. However, as shown in  FIGS. 6A and 6B , the first base portion  40   a  of the first pressure sensitive element  40  may be connected to the end portion of the pedestal portion  47   a  of the second supporting member  47 , and the second base portion  40   b  may be connected to the side of the end portion of the arm portion  45   c  of the first supporting member  45 . Similarly, the first base portion  42   a  of the second pressure sensitive element  42  may be connected to the end portion of the fixing portion  49  of the first base portion  42   a,  and the second base portion  42   b  may be connected to the end portion of the arm portion  47   c  of the second supporting member  47 . In addition, either one of the first and second pressure sensitive elements  40  and  42  may be connected as shown in  FIGS. 6A and 6B . 
     Second Embodiment 
       FIG. 7  shows a perspective cross-sectional view of a pressure sensor according to a second embodiment taken along the XZ plane.  FIGS. 8A and 8B  show cross-sectional views of the pressure sensor according to the second embodiment, taken along the XZ and YZ planes, respectively. In a pressure sensor  50  according to the second embodiment, although the housing  12  and the diaphragm  24  are the same as those of the first embodiment, first and second pressure sensitive elements  52  and  54  and first and second supporting members  56  and  58  are integrally formed of a piezoelectric material different from the first embodiment. 
     When integrally forming the first and second pressure sensitive elements  52  and  54  and the first and second supporting members  56  and  58 , a first base portion  52   a  of the first pressure sensitive element  52  is integrated with a pedestal portion  58   a  of the second supporting member  58 , and a second base portion  52   b  of the first pressure sensitive element  52  is integrated with the distal end of an arm portion  56   c  of the first supporting member  56 . Moreover, a second base portion  54   b  of the second pressure sensitive element  54  is integrated with the distal end of an arm portion  58   c  of the second supporting member  58 . 
     With this configuration, since the respective pressure sensitive elements and the respective supporting members have the same thermal expansion coefficient, it is possible to prevent thermal deformation between the respective pressure sensitive elements and the respective supporting members and to improve the temperature property. Moreover, by integrally forming the respective pressure sensitive elements and the respective supporting members, it is possible to decrease the number of components of the pressure sensor  50 , increase the assembly efficiency of the pressure sensor  50 , and achieve cost reduction. 
     Furthermore, the first base portion  52   a  (the pedestal portion  58   a  of the second supporting member  58 ) of the first pressure sensitive element  52 , the first base portion  54   a  of the second pressure sensitive element  54 , and the pedestal portion  56   a  of the first supporting member  56  are formed so that the end portions thereof on the sides connected to the diaphragm  24  are arranged on the same straight line. In addition, in the integral member formed by these portions, the above-mentioned portions are connected to the diaphragm  24  (fixing portions  60 ,  62 , and  64  described later) so that the straight line is vertical to the displacement direction of the diaphragm  24 . 
     With this configuration, since the respective pressure sensitive elements and the respective supporting members will not receive thermal deformation from the diaphragm  24 , the pressure sensor  50  can measure pressure with high accuracy stably against a change in temperature. 
     The fixing portion  60  for fixing the integral member is fixed to the central region  24   a  by an adhesive agent or the like, and the fixing portions  62  and  64  are fixed to the peripheral portion  24   c  by an adhesive agent or the like. The fixing portion  60  is connected to the pedestal portion  58   a  (the first base portion  52   a  of the first pressure sensitive element  52 ) of the second supporting member  58 . The fixing portion  62  is connected to the pedestal portion  56   a  of the first supporting member  56 . The fixing portion  62  is connected to the first base portion  54   a  of the second pressure sensitive element  54 . These fixing portions  60 ,  62 , and  64  are preferably formed of the same material as the diaphragm  24  similarly to the first embodiment. 
       FIGS. 9A to 9E  show schematic views when the integral member that integrates the first and second pressure sensitive elements  52  and  54  and the first and second supporting members  56  and  58  is formed of quartz crystal. When the integral member is formed of quartz crystal, it is preferable to form the integral member by photolithographic etching similarly to the diaphragm  24  of the first embodiment. In this case, a base substrate  66  serving as a material is prepared and a positive photoresist  68  is applied on the surface of the base substrate ( FIG. 9A ). Subsequently, exposure is preformed using a photomask (not shown) corresponding in shape to the first and second pressure sensitive elements  52  and  54  and the first and second supporting members  56  and  58  so as to expose the photoresist  68  ( FIG. 9B ). Subsequently, development is performed so as to remove the exposed photoresist  68   a  ( FIG. 9C ). Subsequently, a region on which the base substrate  66  is exposed is subjected to etching, whereby the first and second pressure sensitive elements  52  and  54  and the first and second supporting members  56  and  58  are integrally formed ( FIG. 9D ). Finally, the photoresist  68  is removed ( FIG. 9E ), whereby the integral member is formed. 
     The pressure sensor  50  of the second embodiment is assembled essentially similarly to the first embodiment. That is, the diaphragm  24  is connected to the ring portion  16 , the fixing portion  60  is connected to the central region  24   a,  and the fixing portions  62  and  64  are connected to predetermined positions of the peripheral portion  24   c.  Moreover, the pedestal portion  58   a  (the first base portion  52   a  of the first pressure sensitive element  52 ) of the second supporting member  58  is connected to the side surface of the fixing portion  60 , the first base portion  54   a  of the second pressure sensitive element  54  is connected to the side surface of the fixing portion  62 , and the pedestal portion  56   a  of the first supporting member  56  is connected to the side surface of the fixing portion  64 . In this case, the end portions of the pedestal portion  56   a,  the first base portion  54   a,  and the pedestal portion  58   a  disposed close to the diaphragm  24  may be in contact with the diaphragm  24 . 
     The entire disclosure of Japanese Patent Application No. 2010-178591, filed Aug. 9, 2010 is expressly incorporated by reference herein.