Patent Publication Number: US-11391275-B2

Title: Fluid control apparatus

Description:
This is a continuation of International Application No. PCT/JP2018/044654 filed on Dec. 5, 2018 which claims priority from Japanese Patent Application No. 2018-025663 filed on Feb. 16, 2018. The contents of these applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a fluid control apparatus for controlling flow rate of fluid. 
     Various fluid control apparatuses equipped with a driving device, such as a piezoelectric device, have been in practical use. 
     Patent Document 1 describes a fluid control apparatus having a pump chamber and a valve chamber. The pump chamber is defined by a top plate that also partially defines the valve chamber and by a vibrating plate to which a driving device is directly attached. The top plate and the vibrating plate vibrate in opposite phase, thereby controlling fluid flow. 
     Patent Document 1: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2012-528981 
     BRIEF SUMMARY 
     However, with the structure of the fluid control apparatus according to Patent Document 1, the center of gravity of the fluid control apparatus may oscillate largely. 
     In addition, in the case of the fluid control apparatus being fixed to an external housing, vibrations may be transmitted to the external housing. This may cause the fixation portion of the fluid control apparatus to become loose, which degrades the performance of the fluid control apparatus. 
     The present disclosure provides a fluid control apparatus that can reduce oscillation of the center of gravity. 
     A fluid control apparatus according to the present disclosure includes a valve and a pump. The valve includes a first main plate, a second main plate having one principal surface that opposes one principal surface of the first main plate, and a side plate that connects the first main plate and the second main plate to each other. The valve has a valve chamber surrounded by the first main plate, the second main plate, and the side plate. The first main plate has a first aperture through which the valve chamber communicates with the outside of the valve chamber, and the second main plate has a second aperture through which the valve chamber communicates with the outside of the valve chamber. The valve further includes a valve diaphragm disposed inside the valve chamber. The valve diaphragm is configured to switch between a state in which the first aperture and the second aperture communicate with each other and a state in which the first aperture and the second aperture do not communicate with each other. 
     The pump includes a vibration unit that has a piezoelectric device and a vibrating plate and is disposed so as to oppose the other principal surface of the second main plate. The pump has a pump chamber that is defined by the vibration unit and the second main plate. The pump chamber communicates with the valve chamber through the second aperture. 
     In addition, in flexural vibration of the vibration unit, a frequency coefficient of the first main plate is greater than a frequency coefficient of the second main plate. 
     With this configuration, the first main plate having a greater frequency coefficient is less flexible than the second main plate. Accordingly, the first main plate and the vibration unit vibrate in opposite phase, which counteracts the vibration of the fluid control apparatus caused by the vibration of the vibration unit. As a result, the fluctuation of the center of gravity of the fluid control apparatus is reduced, which improves the reliability of the fluid control apparatus. 
     A fluid control apparatus according to the present disclosure includes a valve and a pump. The valve includes a first main plate, a second main plate having one principal surface that opposes one principal surface of the first main plate, and a side plate that connects the first main plate and the second main plate to each other. The valve has a valve chamber surrounded by the first main plate, the second main plate, and the side plate. The first main plate has a first aperture through which the valve chamber communicates with the outside of the valve chamber, and the second main plate has a second aperture through which the valve chamber communicates with the outside of the valve chamber. The valve further includes a valve diaphragm disposed inside the valve chamber. The valve diaphragm is configured to switch between a state in which the first aperture and the second aperture communicate with each other and a state in which the first aperture and the second aperture do not communicate with each other. 
     The pump includes a vibration unit that has a piezoelectric device and a vibrating plate and is disposed so as to oppose the other principal surface of the second main plate. The pump has a pump chamber that is defined by the vibration unit and the second main plate. The pump chamber communicates with the valve chamber through the second aperture. 
     In addition, the first main plate and the second main plate are made of the same material, and the thickness of the first main plate is greater than the thickness of the second main plate in a direction normal to respective principal surfaces. 
     With this configuration, the first main plate and the vibration unit vibrate in opposite phase, which counteracts the vibration of the fluid control apparatus caused by the vibration of the vibration unit. This improves the reliability of the fluid control apparatus. 
     In the fluid control apparatus of the present disclosure, the first main plate and the vibrating plate can displace in opposite phase. 
     With this configuration, the first main plate and the vibration unit vibrate in opposite phase. The influence of vibration of the first main plate on the center of the gravity of the apparatus counteracts the influence of vibration of the vibration unit on the center of gravity of the apparatus, which improves the reliability of the fluid control apparatus. 
     In addition, the fluid control apparatus according to the present disclosure can include an external housing to which the valve is fixed by using the first main plate. 
     With this configuration, the valve is fixed to the external housing, and the valve is not readily detached since the center of gravity of a structure formed of the pump and the valve scarcely oscillates. 
     The fluid control apparatus of the present disclosure is applied to a medical apparatus. 
     The performance of the medical apparatus is thereby improved. The medical apparatus is, for example, a sphygmomanometer, a massage machine, an aspirator, a nebulizer, or a device for negative pressure wound therapy. 
     Accordingly, the present disclosure can provide a reliable fluid control apparatus that can reduce transmission of vibrations caused by the oscillation of the center of gravity of the fluid control apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1A  is a perspective view illustrating the exterior of a fluid control apparatus  10  according to a first embodiment of the present disclosure when the fluid control apparatus  10  is viewed from the side of a valve  20 .  FIG. 1B  is a perspective view illustrating the exterior of the fluid control apparatus  10  according to the first embodiment of the present disclosure when the fluid control apparatus  10  is viewed from the side of a pump  30 . 
         FIG. 2  is an exploded perspective view illustrating the fluid control apparatus  10  according to the first embodiment of the present disclosure. 
         FIG. 3  is a cross-sectional side view illustrating the fluid control apparatus  10  according to the first embodiment of the present disclosure. 
         FIG. 4A  to  FIG. 4F  are cross-sectional side views conceptually illustrating oscillation of center of gravity of the fluid control apparatus  10  according to the first embodiment of the present disclosure. 
         FIG. 5  is a graph depicting displacement percentage with respect to frequency coefficient ratio of the fluid control apparatus  10  according to the first embodiment of the present disclosure. 
         FIG. 6  is a graph depicting rate of change in fluctuation of the center of gravity with respect to frequency coefficient ratio of the fluid control apparatus  10  according to the first embodiment of the present disclosure. 
         FIG. 7  is a cross-sectional side view illustrating the fluid control apparatus  10  according to the first embodiment of the present disclosure when a structure constituted by a valve  20  and a pump  30  is fixed to an external housing. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A fluid control apparatus according to a first embodiment of the present disclosure will be described with reference to the drawings.  FIG. 1A  is a perspective view illustrating the exterior of a fluid control apparatus  10  according to the first embodiment of the present disclosure when the fluid control apparatus  10  is viewed from the side of a valve  20 .  FIG. 1B  is a perspective view illustrating the exterior of the fluid control apparatus  10  according to the first embodiment of the present disclosure when the fluid control apparatus  10  is viewed from the side of a pump  30 .  FIG. 2  is an exploded perspective view illustrating the fluid control apparatus  10  according to the first embodiment of the present disclosure.  FIG. 3  is a cross-sectional side view of the fluid control apparatus  10 , which is taken along line S-S of  FIG. 1A  and of  FIG. 1B .  FIG. 4A  to  FIG. 4F  are cross-sectional side views conceptually illustrating fluctuation of the center of gravity of the fluid control apparatus  10  according to the first embodiment of the present disclosure.  FIG. 5  is a graph depicting relative displacement with respect to frequency coefficient ratio of the fluid control apparatus  10  according to the first embodiment of the present disclosure.  FIG. 6  is a graph depicting rate of change in fluctuation of the center of gravity with respect to frequency coefficient ratio of the fluid control apparatus  10  according to the first embodiment of the present disclosure.  FIG. 7  is a cross-sectional side view illustrating the fluid control apparatus  10  according to the first embodiment of the present disclosure when a structure formed of a valve  20  and a pump  30  is fixed to an external housing. Note that some reference signs are omitted and part of a structure is exaggerated for the purpose of easy recognition. 
     As illustrated in  FIGS. 1A, 1B, 2, and 3 , the fluid control apparatus  10  includes a valve  20  and a pump  30 . The valve  20  has multiple first apertures  201  that open at a top surface of the valve  20 . The first apertures  201  are ventholes. 
     A structure of the valve  20  will be described first. The valve  20  includes a first main plate  21 , a second main plate  22 , a side plate  23 , and a valve diaphragm  24 . Note that a thickness t 1  of the first main plate  21  is greater than a thickness t 2  of the second main plate  22 . 
     As illustrated in  FIGS. 1A, 2, and 3 , the first main plate  21  and the second main plate  22  are shaped like discs. The side plate  23  is shaped like a cylinder. 
     The side plate  23  is disposed between the first main plate  21  and the second main plate  22  and connects these plates to each other so as to enable the first main plate  21  and the second main plate  22  to oppose each other. More specifically, the center of the first main plate  21  and the center of the second main plate  22  coincide with each other as viewed in plan. The side plate  23  connects outer peripheral regions of the first main plate  21  and the second main plate  22 , which are disposed as described above, along the entire circumferences. 
     According to this configuration, the valve  20  has a valve chamber  200  that is a columnar space surrounded by the first main plate  21 , the second main plate  22 , and the side plate  23 . Note that the side plate  23  may be integrally formed with the first main plate  21  or with the second main plate  22 . In this case, the first main plate  21  or the second main plate  22  may be shaped like a recess. 
     The valve diaphragm  24  is disposed inside the valve chamber  200 . 
     As described, the first main plate  21  has the first apertures  201  that are formed so as to penetrate the first main plate  21 . The valve diaphragm  24  also has multiple second apertures  202  that are formed so as to penetrate the valve diaphragm  24  at the same positions as the first apertures  201  as viewed in plan. 
     Moreover, the second main plate  22  has multiple third apertures  203  that are formed so as to penetrate the second main plate  22 . The third apertures  203 , however, are formed so as not to overlap the first apertures  201  nor the second apertures  202  as viewed in plan. The valve chamber  200  of the valve  20  communicates with a pump chamber  300  of the pump  30  through the third apertures  203 . 
     Next, a structure of the pump  30  will be described. As illustrated in  FIGS. 1B, 2, and 3 , the second main plate  22  also serves as a component of the pump  30 . The pump  30  is formed of the second main plate  22 , a pump side plate  31 , a pump bottom plate  32 , and a vibration unit  33 . The vibration unit  33  is formed of a vibrating plate  331  and a piezoelectric device  332 . The vibrating plate  331  has a thickness t 3 . 
     In addition, the pump bottom plate  32  is formed integrally with the vibrating plate  331 . More specifically, when the pump  30  is viewed from the second main plate  22 , the pump bottom plate  32  and the vibrating plate  331  are connected by connection portions  35  so as to be flush with each other. In other words, the pump bottom plate  32  has multiple pump bottom apertures  34  with a predetermined opening width at positions arranged along the outer periphery of the pump bottom plate  32 , and the pump bottom apertures  34  separates the vibrating plate  331  from the pump bottom plate  32 . With this configuration, the pump bottom plate  32  holds the vibrating plate  331  so as to enable the vibrating plate  331  to vibrate. 
     The pump side plate  31  is shaped like a ring as viewed from the first main plate  21 . The pump side plate  31  is disposed between the second main plate  22  and the pump bottom plate  32  and connects these plates to each other. More specifically, the center of the second main plate  22  and the center of the pump bottom plate  32  coincide with each other. The pump side plate  31  connects outer peripheral regions of the second main plate  22  and the pump bottom plate  32 , which are disposed as described above, along the entire circumferences. 
     According to this configuration, the pump  30  has a pump chamber  300  that is a columnar space surrounded by the second main plate  22 , the pump bottom plate  32 , and the pump side plate  31 . 
     The piezoelectric device  332  is constituted by a disc-like piezoelectric member and electrodes for driving the piezoelectric member. The electrodes are formed on respective principal surfaces of the disk-like piezoelectric member. 
     The piezoelectric device  332  is disposed on a surface of the vibrating plate  331  that is opposite to the surface facing the pump chamber  300 , in other words, disposed on the outside surface of the pump  30 . The center of the piezoelectric device  332  and the center of the vibrating plate  331  substantially coincide with each other as viewed in plan. 
     The piezoelectric device  332  is coupled to a control unit (not illustrated). The control unit generates drive signals and applies them to the piezoelectric device  332 . The drive signals displaces the piezoelectric device  332 , and the displacement generates stresses in the vibrating plate  331 . This causes the vibrating plate  331  to vibrate flexurally. For example, the vibration of the vibrating plate  331  produces a wave form of Bessel function of the first kind. 
     The flexural vibration of the vibrating plate  331  (i.e., vibration unit  33 ) changes the volume and the pressure of the pump chamber  300 . Accordingly, a fluid drawn in through the pump bottom apertures  34  is discharged through the third apertures  203 . 
     With the above configuration of the valve  20 , the fluid flowing in through the third apertures  203  moves the valve diaphragm  24  toward the first main plate  21 . As a result, the fluid is discharged out through the second apertures  202  and the first apertures  201 . On the other hand, if the fluid tries to flow from the third apertures  203  to the pump bottom apertures  34 , the fluid moves the valve diaphragm  24  toward the second main plate  22 , and the valve diaphragm  24  thereby plugs the third apertures  203 . Accordingly, the fluid control apparatus  10  serves to rectify fluid flow. 
     Note that the first main plate  21  and the second main plate  22  are made of such a material and a thicknesses that enable the first main plate  21  and the second main plate  22  to vibrate in a direction normal to the principal surfaces. For example, the material of the first main plate  21  and the second main plate  22  is a stainless steel. 
     The first main plate  21  and the second main plate  22  will be compared below by using frequency coefficients obtained from a specific formula in a condition where the thickness t 1  of the first main plate  21  &gt;the thickness t 2  of the second main plate  22  according to the present embodiment. The frequency coefficient is a coefficient representing flexibility of the first main plate  21  and the second main plate  22  that vibrate. More specifically, the frequency coefficient is expressed in the following formula, where in a vibrating plate, t is the thickness of the plate, E is the modulus of longitudinal elasticity (i.e., Young&#39;s modulus) of the plate, and ρ is the material density of the plate. 
     
       
         
           
             
               
                 
                   
                     frequency 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     coefficient 
                   
                   = 
                   
                     t 
                     × 
                     
                       
                         E 
                         ρ 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
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                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     When the material of the first main plate  21  is the same as that of the second main plate  22 , a frequency coefficient F 1  of the first main plate  21  is greater than a frequency coefficient F 2  of the second main plate  22  since the thickness t 1  of the first main plate  21  is greater than the thickness t 2  of the second main plate  22 . In other words, the first main plate  21  is less flexible than the second main plate  22 . 
       FIGS. 4A to 4F  are cross-sectional side views of the fluid control apparatus  10  conceptually depicting fluctuation of the center of gravity of the fluid control apparatus  10 . In  FIGS. 4A to 4F , t 1  denotes the thickness of the first main plate  21 , and t 2  denotes the thickness of the second main plate  22 . Note that positions of the center of gravity are only for example. 
       FIGS. 4A to 4C  are conceptual illustrations depicting the fluctuation in a fluid control apparatus having a known configuration. In this case, the thickness t 1  of the first main plate  21  is equal to the thickness t 2  of the second main plate  22 . 
     On the other hand,  FIGS. 4D to 4F  are conceptual illustrations depicting the fluctuation in the fluid control apparatus according to the present embodiment. In this case, the thickness t 1  of the first main plate  21  is greater than the thickness t 2  of the second main plate  22 . 
     In  FIGS. 4A to 4F , some elements and some reference signs are omitted, and the state of vibration is exaggerated for the purpose of clear understanding. 
     To begin with, fluctuation of the center of gravity of the fluid control apparatus  10  will be described schematically in the case of the fluid control apparatus  10  having a known configuration.  FIG. 4A  is a conceptual illustration of the fluid control apparatus  10  when the fluid control apparatus  10  stops. In this case, the center of gravity of the fluid control apparatus  10  is denoted by P 1 . 
       FIG. 4B  is a conceptual illustration of the fluid control apparatus  10  when the fluid control apparatus  10  draws a fluid. In this case, the center of gravity of the fluid control apparatus  10 , which is denoted by P 2 , is largely shifted toward the first main plate  21 . 
       FIG. 4C  is a conceptual illustration of the fluid control apparatus  10  when the fluid control apparatus  10  discharges the fluid. In this case, the center of gravity of the fluid control apparatus  10 , which is denoted by P 3 , is largely shifted toward the second main plate  22 . 
     In the fluid control apparatus  10  with the known configuration, the center of gravity P 2  shifts largely toward the first main plate  21 , while the center of gravity P 3  shifts largely toward the second main plate  22 , with respect to the center of gravity P 1 , which is the position when the fluid control apparatus  10  stops (as is the case in  FIG. 4A ). 
     Next, fluctuation of the center of gravity of the fluid control apparatus  10  according to the present embodiment will be described schematically.  FIG. 4D  is a conceptual illustration of the fluid control apparatus  10  when the fluid control apparatus  10  stops. 
     In this case, the center of gravity of the fluid control apparatus  10  is denoted by P 4 . 
       FIG. 4E  is a conceptual illustration of the fluid control apparatus  10  when the fluid control apparatus  10  draws a fluid. In this case, the center of gravity of the fluid control apparatus  10 , which is denoted by P 5 , is located substantially at the same position as the center of gravity P 4 . 
       FIG. 4F  is a conceptual illustration of the fluid control apparatus  10  when the fluid control apparatus  10  discharges the fluid. In this case, the center of gravity of the fluid control apparatus  10 , which is denoted by P 6 , is located substantially at the same position as the center of gravity P 4 . 
     In the case of the fluid control apparatus  10  according to the present embodiment, the center of gravity P 5  and the center of gravity P 6  are located substantially at the same position as the center of gravity P 4 , which is the position when the fluid control apparatus  10  stops (as is the case in  FIG. 4D ). 
     Accordingly, when the fluid control apparatus  10  vibrates, the center of gravity is caused to stay substantially at the same position by setting the thickness t 1  of the first main plate  21  to be greater than the thickness t 2  of the second main plate  22 . In other words, the center of gravity is caused to stay substantially at the same position by setting a frequency coefficient F 1  to be greater than a frequency coefficient F 2 . A large oscillation of the center of gravity is thereby suppressed. In the case of a structure formed of the valve  20  and the pump  30  being mounted on another member, stress is generated at the mounting portion due to the fluctuation of the center of gravity. However, with this configuration, the stress can be reduced. Thus, the reliability of the fluid control apparatus  10  is improved. 
       FIG. 5  is a graph depicting simulation results of displacement percentage with respect to frequency coefficient ratio in the fluid control apparatus  10 . 
     In the case illustrated in  FIG. 5 , the thickness t 2  of the second main plate  22  is set to be 0.5 mm, and the thickness t 3  of the vibrating plate  331  is set to be 0.4 mm. The thickness t 1  of the first main plate  21  is varied in a range between 0.3 mm and 0.7 mm. 
     The transverse axis represents frequency coefficient ratio. The frequency coefficient ratio is obtained from the following formula: (frequency coefficient of first main plate  21 )/(frequency coefficient of second main plate  22 ). The vertical axis represents relative displacement. The relative displacement of the first main plate  21  and the relative displacement of the second main plate  22  are expressed as the displacement relative to the vibrating plate  331 . 
     When the relative displacement is 0% or more, the first main plate  21  displaces in phase with the vibrating plate  331 . When the relative displacement is less than 0%, the first main plate  21  displaces in opposite phase to the vibrating plate  331 . 
     When thickness t 1  of first main plate  21 &lt;thickness t 2  of second main plate  22 , the first main plate  21  and the vibrating plate  331  vibrate in phase. When thickness t 1  of first main plate  21  thickness t 2  of second main plate  22 , the first main plate  21  and the vibrating plate  331  vibrate in opposite phase. 
     In other words, in the condition where thickness t 1  of first main plate  21 &gt;thickness t 2  of second main plate  22 , the vibration of the first main plate  21  and the vibration of the vibrating plate  331  are in opposite phase. 
     Note that the oscillation of the center of gravity can be reduced when the phase difference θ is in a range of 120°&lt;0&lt;240°. In the case of the phase difference θ being in a range of 152°&lt;0&lt;208°, the amplitude of the oscillation of the center of gravity can be reduced by half. 
     The phase difference θ can be measured, for example, by a displacement meter employing the laser Doppler method. In this case, the external housing to which the fluid control apparatus  10  is fixed may be perforated to enable laser light to enter and illuminate measurement targets. The measurement targets are, for example, the surface of piezoelectric device  332  of the vibrating plate  331  and the surface of the first main plate  21  near the perforated hole. Even if the external housing is perforated for measurement, the state of vibration is not affected. 
     Next, the graph of  FIG. 6  will be explained based on the results illustrated in  FIG. 5 .  FIG. 6  is a graph showing simulation results of rate of change in fluctuation of the center of gravity with respect to frequency coefficient ratio in the fluid control apparatus  10 . 
     In the case illustrated in  FIG. 6 , the thickness t 2  of the second main plate  22  is set to be 0.5 mm, and the thickness t 3  of the vibrating plate  331  is set to be 0.4 mm. The thickness t 1  of the first main plate  21  is varied in a range between 0.3 mm and 0.7 mm. 
     The transverse axis represents frequency coefficient ratio. The frequency coefficient ratio is obtained from the following formula: (frequency coefficient of first main plate  21 )/(frequency coefficient of second main plate  22 ). The vertical axis represents rate of change in oscillation of the center of gravity. The rate of change in fluctuation of the center of gravity represents how the vibrations of the first main plate  21  and the second main plate  22  counteract the vibration of the vibrating plate  331 . 
     The following explains how the rate of change in oscillation of the center of gravity is calculated. The rate of change in oscillation of the center of gravity is expressed in the equation below, where t 1  is the thickness of the first main plate  21 , t 2  is the thickness of the second main plate  22 , t 3  is the thickness of the vibrating plate  331 , ρ 1  is the material density of the first main plate  21 , ρ 2  is the material density of the second main plate  22 , ρ 3  is the material density of the vibrating plate  331 , A 1  is the center displacement amplitude of the first main plate  21 , A 2  is the center displacement amplitude of the second main plate  22 , and A 3  is the center displacement amplitude of the vibrating plate  331 . In this case, the material density ρ 1  of the first main plate  21  is equal to the material density ρ 2  of the second main plate  22  and is also equal to the material density ρ 3  of the vibrating plate  331 . 
     [Math. 2]
 
rate of change in fluctuation of the center of gravity=(( t 1×ρ1× A 1)+( t 2×ρ2 ×A 2)+( t 3×ρ3 ×A 3))/( t 3×ρ3 ×A 3)
 
     The center displacement amplitude A 1  of the first main plate  21 , the center displacement amplitude A 2  of the second main plate  22 , and the center displacement amplitude A 3  of the vibrating plate  331  take positive values when the corresponding vibrations are in phase with the vibration of the vibrating plate  331 . The center displacement amplitude A 1  of the first main plate  21 , the center displacement amplitude A 2  of the second main plate  22 , and the center displacement amplitude A 3  of the vibrating plate  331  take negative values when the corresponding vibrations are in opposite phase to the vibration of the vibrating plate  331 . 
     In other words, when the rate of change in oscillation of the center of gravity takes a positive value, the first main plate  21  and the second main plate  22  amplify the oscillation of the center of gravity. Conversely, when the rate of change in oscillation of the center of gravity takes a negative value, the first main plate  21  and the second main plate  22  attenuate the oscillation of the center of gravity. 
     Accordingly, as illustrated in  FIG. 6 , when thickness t 1  of first main plate  21 &lt;thickness t 2  of second main plate  22 , the rate of change in oscillation of the center of gravity takes a positive value, and the oscillation of the center of gravity is amplified. On the other hand, when thickness t 1  of the first main plate  21  thickness t 2  of second main plate  22 , the rate of change in oscillation of the center of gravity takes a negative value, and the oscillation of the center of gravity is attenuated. 
     Thus, the oscillation of the center of gravity of the fluid control apparatus  10  is attenuated by setting the thickness t 1  of the first main plate  21  to be equal to or greater than the thickness t 2  of the second main plate  22 , which improves the reliability of the fluid control apparatus  10 . 
     In the case of the fluid control apparatus  10  having an external housing, the fluid control apparatus  10  may have, for example, the following configuration.  FIG. 7  is a cross-sectional side view illustrating the fluid control apparatus according to the present embodiment when a structure formed of a valve  20  and a pump  30  is fixed to an external housing. 
     The first main plate  21  has an extension portion  25  that is extended therefrom. For example, the fluid control apparatus  10  is fixed to a first external housing  40  via the extension portion  25  by using adhesion, screw fixation, interlocking, or the like. The external housing is formed of the first external housing  40  and a second external housing  50  that is disposed so as to abut the first external housing  40  and surround the structure. 
     In other words, the structure of the fluid control apparatus  10  is disposed in the space defined by the first external housing  40  and the second external housing  50 . 
     As described, the oscillation of the center of gravity of the fluid control apparatus  10  is attenuated by setting the thickness t 1  of the first main plate  21  to be equal to or greater than the thickness t 2  of the second main plate  22 . As a result, even if the first main plate  21  is fixed to the first external housing  40 , the influence of the oscillation of the center of gravity on the extension portion  25 , in other words, which is the portion fixed to the external housing, can be reduced. 
     In the above description, the structure is fixed to the first external housing  40 . The second main plate  22  of the structure may be fixed to the first external housing  40 . Note that the reliability is improved more in the case of the first main plate  21  of the structure being fixed to the first external housing  40  since the vibration amplitude of the first main plate  21  is smaller than that of the second main plate  22 . 
     The external housing is described, by way of example, as being formed of the first external housing  40  and the second external housing  50 . However, the external housing may be formed integrally or formed of three or more housing parts. The external housing is not limited to these configurations. It is sufficient that the external housing has a shape to which the structure can be fixed. 
     The shapes of the valve  20  and the pump  30  of the fluid control apparatus  10  have been described as substantially disc-like shapes. However, the shapes of the valve  20  and the pump  30  of the fluid control apparatus  10  are not limited to the disc-like shapes but may be polygon-like shapes. 
     In addition, the first main plate  21  and the second main plate  22  have been described as being made of the same material, for example, a stainless steel. However, the material of the first main plate  21  and the material of second main plate  22  need not be the same. A different material may be used insofar as the material provides the first main plate  21  with flexibility and with the frequency coefficient greater than that of the second main plate  22 . The same advantageous effects can be thereby obtained. 
     The above-described fluid control apparatus is applied, for example, to a medical apparatus, such as a sphygmomanometer, a massage machine, an aspirator, a nebulizer, or a device for negative pressure wound therapy. The fluid control apparatus can improve efficiency of such a medical apparatus. 
     Note that in the above, the first main plate and the second main plate have been described as flat plates having uniform thicknesses. However, in the case of the first main plate and the second main plate each having uneven thickness, the average thickness of the first main plate and the average thickness of the second main plate can be compared and be set so as to satisfy the following inequality: average thickness t 1   a  of first main plate  21 &gt;average thickness t 2   a  of second main plate  22 . 
     REFERENCE SIGNS LIST 
     A 1 , A 2 , A 3  center displacement amplitude 
     F 1 , F 2  frequency coefficient 
     P 1 , P 2 , P 3 , P 4 , P 5 , P 6  center of gravity 
     t 1 , t 2 , t 3  thickness 
       10  fluid control apparatus 
       20  valve 
       21  first main plate 
       22  second main plate 
       23  side plate 
       24  valve diaphragm 
       25  extension portion 
       30  pump 
       31  pump side plate 
       32  pump bottom plate 
       33  vibration unit 
       34  pump bottom aperture 
       35  connection portion 
       40  first external housing 
       50  second external housing 
       200  valve chamber 
       201  first aperture 
       202  second aperture 
       203  third aperture 
       300  pump chamber 
       331  vibrating plate 
       332  piezoelectric device