Abstract:
A fluid control device includes a vibrating plate unit, a driver, a flexible plate, and a base plate. The vibrating plate unit includes a vibrating plate including first and second main surfaces, and a frame plate surrounding the surrounding of the vibrating plate. The driver is bonded to the first or the second main surface of the vibrating plate and vibrates the vibrating plate. The flexible plate includes a hole provided therein, and is bonded to the frame plate so as to face the vibrating plate. The base plate is bonded to the main surface of the flexible plate on a side opposite to the vibrating plate. A size relationship between the coefficients of linear expansion of the material of the base plate and the frame plate is equal to a size relationship between the coefficients of linear expansion of the material of the vibrating plate and the driver.

Description:
CROSS REFERENCE 
       [0001]    This non-provisional application claims priority under 35 U.S.C. §119(a) to Patent Application No. 2011-194429 filed in Japan on Sep. 6, 2011, the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a fluid control device which performs fluid control. 
         [0004]    2. Description of the Related Art 
         [0005]    International Publication No. 2008/069264 discloses a conventional fluid pump (see  FIGS. 1A to 1E ).  FIG. 1A  to  FIG. 1E  show operations of the conventional fluid pump in a tertiary mode. The fluid pump, as shown in  FIG. 1A , includes a pump body  10 ; a vibrating plate  20  in which the outer peripheral portion thereof is attached to the pump body  10 ; a piezoelectric element  23  attached to the central portion of the vibrating plate  20 ; a first opening  11  formed on a portion of the pump body  10  that faces the approximately central portion of the vibrating plate  20 ; and a second opening  12  formed on either one of a region intermediate between the central portion and the outer peripheral portion of the vibrating plate  20  or a portion of the pump body  10  that faces the intermediate region. 
         [0006]    The vibrating plate  20  is made of metal. The piezoelectric element  23  has a size so as to cover the first opening  11  and a size so as not to reach the second opening  12 . 
         [0007]    In the above mentioned fluid pump, by applying voltage having a predetermined frequency to the piezoelectric element  23 , a portion of the vibrating plate  20  that faces the first opening  11  and a portion of the vibrating plate  20  that faces the second opening  12  are bent and deformed in opposite directions, as shown in  FIG. 1A  to  FIG. 1E . This causes the fluid pump to draw fluid from one of the first opening  11  and the second opening  12  and to discharge the fluid from the other opening. 
         [0008]    The above mentioned fluid pump, as is shown in  FIG. 1A  with a conventional structure, has a simple structure, and thus the thickness of the fluid pump can be made thinner. Such a fluid pump is used, for example, as an air transport pump of a fuel cell system. 
         [0009]    At the same time, electronic equipment and apparatuses into which the fluid pump is incorporated have tended to be miniaturized. Therefore, it is necessary to further miniaturize the fluid pump without reducing the pump performance (the discharge flow rate and the discharge pressure) of the fluid pump. 
         [0010]    However, the performance of the fluid pump decreases as the fluid pump becomes smaller. Therefore, there are limitations to miniaturizing the fluid pump having the conventional structure while maintaining the pump performance. 
         [0011]    Accordingly, the inventors of the present invention have devised a fluid pump having a structure shown in  FIG. 2 . 
         [0012]      FIG. 2  is a sectional view showing a configuration of a main portion of the fluid pump  901 . The fluid pump  901  is provided with a base plate  39 , a flexible plate  35 , a spacer  37 , a vibrating plate  31 , and a piezoelectric element  32 . The fluid pump  901  is provided with a structure in which the components are layered in that order. 
         [0013]    In the fluid pump  901 , the piezoelectric element  32  and the vibrating plate  31  bonded to the piezoelectric element  32  constitute an actuator  30 . A ventilation hole  35 A is formed in the center of the flexible plate  35 . The end of the vibrating plate  31  is fixed to the end of the flexible plate  35  by means of an adhesive via the spacer  37 . This means that the vibrating plate  31  is supported at a location spaced away from the flexible plate  35  by the thickness of the spacer  37 . 
         [0014]    The base plate  39  is bonded to the flexible plate  35 . A cylindrical opening  40  is formed in the center of the base plate  39 . A portion of the flexible plate  35  is exposed to the side of the base plate  39  through the opening  40  of the base plate  39 . The circular exposed portion of the flexible plate  35  can vibrate at a frequency that is substantially the same as a frequency of the actuator  30  through the pressure fluctuation of fluid accompanied by the vibration of the actuator  30 . In other words, through the configuration of the flexible plate  35  and the base plate  39 , the portion of the flexible plate  35  that faces the opening  40  serves as a movable portion  41  that is capable of bending and vibrating. Furthermore, a portion on the outside of the movable portion  41  of the flexible plate  35  serves as a fixing portion  42  fixed to the base plate  39 . 
         [0015]    In the above structure, when driving voltage is applied to the piezoelectric element  32 , the vibrating plate  31  bends and vibrates as a result of the expansion and contraction of the piezoelectric element  32 . Furthermore, the movable portion  41  of the flexible plate  35  vibrates with vibration of the vibrating plate  31 . This causes the fluid pump  901  to suction or discharge air through the ventilation hole  35 A. Consequently, since the movable portion  41  vibrates with the vibration of the actuator  30 , the amplitude of vibration of the fluid pump  901  is effectively increased. This allows the fluid pump  901  to produce a high discharge pressure and a large discharge flow rate despite the small size and low profile design thereof. 
         [0016]    However, the fluid pump  901  is provided with a structure in which the components are layered. Each of the components is fixed by means of the adhesive agent. For this reason, as the temperature of the fluid pump  901  increased due to heat generation at a time of driving the fluid pump  901  or increases in an environmental temperature, each of the components bends according to differences in each of coefficients of linear expansion. As a result, a distance between the vibrating plate  31  and the flexible plate  35  varies. Here, the distance between the vibrating plate  31  and the flexible plate  35  is an important factor which affects the pressure-flow rate characteristics of the fluid pump  901 . 
         [0017]    Therefore, a problem exists with the fluid pump  901  in which the pressure-flow rate characteristics of the fluid pump  901  will vary depending on changes in temperature. In other words, the temperature characteristics of the fluid pump  901  are poor. 
       SUMMARY OF THE INVENTION 
       [0018]    Preferred embodiments of the present invention provide a fluid control device that significantly reduces and prevents variations in pressure-flow rate characteristics caused by changes in temperature. 
         [0019]    A fluid control device according to a preferred embodiment of the present invention includes a vibrating plate unit, a driver, a flexible plate, and a base plate. The vibrating plate unit includes a vibrating plate including a first main surface and a second main surface, and a frame plate surrounding the surrounding of the vibrating plate. The driver is bonded to either one of the first main surface or the second main surface of the vibrating plate and vibrates the vibrating plate. The flexible plate includes a hole provided on the flexible plate, and is bonded to the frame plate to face the vibrating plate. The base plate is bonded to the main surface of the flexible plate on the side opposite to the vibrating plate. A size relationship between the coefficient of linear expansion of the material of the base plate and the coefficient of linear expansion of the material of the frame plate is equal to a size relationship between the coefficient of linear expansion of the material of either the vibrating plate or the driver, whichever is closer to the flexible plate, and the coefficient of linear expansion of the material of either the vibrating plate or the driver, whichever is farther from the flexible plate. 
         [0020]    This configuration includes a first configuration and a second configuration in which the vibrating plate unit, the driver, the flexible plate, and the base plate all bend in different directions. 
         [0021]    In the first configuration, either the vibrating plate or the driver, whichever is closer to the flexible plate, is made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of either the vibrating plate or the driver, whichever is farther from the flexible plate. Then, the base plate is made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the frame plate. 
         [0022]    On the other hand, in the second configuration, either the vibrating plate or the driver, whichever is closer to the flexible plate, is made of a material having a coefficient of linear expansion that is smaller than the coefficient of linear expansion of either the vibrating plate or the driver, whichever is farther from the flexible plate. Then, the base plate is made of a material having a coefficient of linear expansion that is smaller than the coefficient of linear expansion of the frame plate. 
         [0023]    With this configuration, the vibrating plate unit, the driver, the flexible plate, and the base plate are bonded to each other at a temperature higher than a normal temperature. 
         [0024]    For this reason, in the first configuration, after the bonding at the normal temperature, the vibrating plate bends and forms a convex curve on the first main surface on the side opposite to the base plate due to the difference in the coefficients of linear expansion of the vibrating plate unit and the driver while the flexible plate bends and forms a convex curve on the main surface on the side provided with the driver (that is, the side opposite to the base plate) due to the difference in the coefficients of linear expansion of the vibrating plate unit and the base plate. On the other hand, in the second configuration, after the bonding at the normal temperature, the vibrating plate bends and forms a convex curve on the second main surface on the side of the base plate due to the difference in the coefficients of linear expansion of the vibrating plate unit and the driver, and the flexible plate bends and forms a convex curve on the main surface on the side of the base plate due to the difference in the coefficients of linear expansion of the vibrating plate unit and the base plate. 
         [0025]    Therefore, with this configuration, in a case where the difference between the coefficient of linear expansion of the vibrating plate unit and the coefficient of linear expansion of the driver is nearly the same as the difference in the coefficients of linear expansion of the vibrating plate unit and the base plate, as the temperature of the fluid control device changes due to heat generated during the drive or due to changes in environmental temperature, both the bending of the vibrating plate as well as the flexible plate reduces by approximately the same amount. 
         [0026]    Thus, with this configuration, as each material is selected for use in the vibrating plate unit, the driver, the flexible plate, and the base plate, even if the vibrating plate unit, the driver, the flexible plate, and the base plate deform, due to differences in coefficients of linear expansion when changes in temperature occur, the distance between the vibrating plate and the flexible plate will always remain approximately constant. 
         [0027]    Consequently, the fluid control device can significantly reduce and prevent variations in the pressure-flow rate characteristics by changes in temperature. 
         [0028]    Preferably, the driver is bonded to the first main surface of the vibrating plate on the side opposite to the base plate, and the flexible plate is bonded to the frame plate so as to face the second main surface of the vibrating plate on the side of the base plate, and the vibrating plate unit is made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the driver, and the base plate is made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the vibrating plate unit. 
         [0029]    This configuration is included in the above described first configuration. With this configuration, after the bonding, at the normal temperature, the vibrating plate bends and forms a convex curve on the first main surface of the vibrating plate on the side of the driver due to the difference in the coefficients of linear expansion of the vibrating plate unit and the driver, and the flexible plate bends and forms a convex curve on the main surface on the side of the driver due to the difference in the coefficients of linear expansion of the vibrating plate unit and the base plate. 
         [0030]    Preferably, the driver is bonded to the second main surface of the vibrating plate on the side of the base plate, and the flexible plate is bonded to the frame plate so as to face the second main surface of the vibrating plate on the side of the base plate, and the driver is made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the vibrating plate unit, and the base plate is made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the vibrating plate unit. 
         [0031]    This configuration is included in the above described first configuration. With the configuration, after the bonding at the normal temperature, the vibrating plate bends and forms a convex curve on the first main surface opposite to the driver due to the difference between the coefficient of linear expansion of the vibrating plate unit and the coefficient of linear expansion of the driver, and the flexible plate bends and forms a convex curve on the main surface on the side of the driver due to the difference between the coefficient of linear expansion of the vibrating plate unit and the coefficient of linear expansion of the base plate. 
         [0032]    Preferably, the vibrating plate unit may further include a link portion which links the vibrating plate and the frame plate, and elastically supports the vibrating plate against the frame plate. 
         [0033]    With this configuration, the vibrating plate is flexibly and elastically supported against the frame plate by the link portion. For this reason, the bending vibration of the vibrating plate generated by expansion and contraction of the piezoelectric element cannot be blocked at all. Therefore, in the fluid control device, there will be a reduction in the loss caused by the bending vibration of the vibrating plate. 
         [0034]    In addition, the flexible plate is preferably made of a material having a coefficient of linear expansion that is larger than the vibrating plate unit. 
         [0035]    Also with this configuration, at the normal temperature, the flexible plate bends and forms a convex curve on the side of the driver due to the differences in the coefficients of linear expansion of the vibrating plate unit, the flexible plate, and the base plate. Additionally, both the bending of the vibrating plate and the flexible plate are reduced as the temperature of the fluid control device increases due to heat generation at the time of driving the fluid control device or by changes of environmental temperature. 
         [0036]    Preferably, the vibrating plate forms a convex curve on the side opposite to the base plate, and is elastically supported by the link portion against the frame plate, and the flexible plate forms a convex curve on the side of the driver, and is bonded to the base plate. 
         [0037]    With this configuration, at the normal temperature, the vibrating plate bends and forms a convex curve on the side of the driver due to the difference in the coefficients of linear expansion of the vibrating plate unit and the driver, and the flexible plate bends and forms a convex curve on the main surface on the side of the driver due to the difference in the coefficients of linear expansion of the vibrating plate unit and the base plate. Thus, both the bending of the vibrating plate and the flexible plate are reduced as the temperature of the fluid control device increases due to heat generation at the time of driving the fluid control device or due to changes in environmental temperature. 
         [0038]    Also it is preferable for the vibrating plate and the link portion to be thinner than the thickness of the frame plate, so that surfaces of the vibrating plate and the link portion on the side of the flexible plate separate from the flexible plate. 
         [0039]    With this configuration, the surface of the link portion on the side of the flexible plate is spaced away from the flexible plate by a predetermined distance. Therefore, even if the adhesive agent flows into a gap between the link portion and the flexible plate when the frame plate and the flexible plate are fixed preferably by the adhesive agent, the fluid control device can prevent the link portion and the flexible plate from adhering to each other. 
         [0040]    Similarly, with this configuration, the surface of the vibrating plate on the side of the flexible plate is spaced away from the flexible plate by a predetermined distance. For this reason, even if an excess amount of the adhesive agent flows into a gap between the vibrating plate and the flexible plate when the frame plate and the flexible plate are fixed preferably by the adhesive agent, the fluid control device can prevent the vibrating plate and the flexible plate from adhering to each other. 
         [0041]    Thus, the fluid control device can prevent the vibration of the vibrating plate from being blocked and can prevent the vibrating plate, the link portion, and the flexible plate from adhering to each other. 
         [0042]    Moreover, it is preferable for a hole portion to be formed in a region of the flexible plate facing the link portion. 
         [0043]    With this configuration, when the frame plate and the flexible plate are fixed preferably by the adhesive agent, an excess amount of the adhesive agent flows into the hole portion. For that reason, the fluid control device can further prevent the vibrating plate and the link portion, and the flexible plate from adhering to one another. In other words, the fluid control device can further prevent the vibration of the vibrating plate from being blocked by the adhesive agent. 
         [0044]    In addition, preferably, the vibrating plate and the driver constitute an actuator, and the actuator has a disk shaped configuration. 
         [0045]    With this configuration, the actuator vibrates in a rotationally symmetric pattern (a concentric circular pattern). For this reason, an unnecessary gap is not generated between the actuator and the flexible plate. Therefore, the fluid control device enhances operation efficiency as a pump. 
         [0046]    Preferably, the flexible plate includes a movable portion that is positioned in the center or near the center of the region of the flexible plate on a side facing the vibrating plate and can bend and vibrate, and a fixing portion that is positioned outside the movable portion in the region and is substantially fixed. 
         [0047]    According to this configuration, the movable portion vibrates with vibration of the actuator. For this reason, in the fluid control device, the amplitude of vibration is effectively increased. Thus, the fluid control device can achieve a higher discharge pressure and a larger discharge flow rate despite the small size and low profile design thereof. 
         [0048]    The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0049]      FIG. 1A  to  FIG. 1E  are cross-sectional views of a main part of a conventional fluid pump. 
           [0050]      FIG. 2  is a cross-sectional view of a main portion of a fluid pump  901  according to a comparative example of the present invention. 
           [0051]      FIG. 3  is an external perspective view of a piezoelectric pump  101  according to a preferred embodiment of the present invention. 
           [0052]      FIG. 4  is an exploded perspective view of the piezoelectric pump  101  as shown in  FIG. 3 . 
           [0053]      FIG. 5  is a cross-sectional view of the piezoelectric pump  101  as shown in  FIG. 3  taken along line T-T. 
           [0054]      FIG. 6A  is a cross-sectional view of a main portion of the piezoelectric pump  101  as shown in  FIG. 3  at normal temperature, and  FIG. 6B  is a cross-sectional view of the main portion of the piezoelectric pump  101  as shown in  FIG. 3  at high temperature. 
           [0055]      FIG. 7  is a plan view of a bonding body of the vibrating plate unit  160  and the flexible plate  151  as shown in  FIG. 4 . 
           [0056]      FIG. 8A  is a cross-sectional view of a main portion of a piezoelectric pump  201  at normal temperature according to another preferred embodiment of the present invention, and  FIG. 8B  is a cross-sectional view of the main portion of the piezoelectric pump  201  at high temperature according to another preferred embodiment of the present invention. 
           [0057]      FIG. 9A  is a cross-sectional view of a main portion of a piezoelectric pump  301  at normal temperature according to another preferred embodiment of the present invention, and  FIG. 9B  is a cross-sectional view of the main portion of the piezoelectric pump  301  at high temperature according to another preferred embodiment of the present invention. 
           [0058]      FIG. 10A  is a cross-sectional view of a main portion of a piezoelectric pump  401  at normal temperature according to another preferred embodiment of the present invention, and  FIG. 10B  is a cross-sectional view of the main portion of the piezoelectric pump  401  at high temperature according to another preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0059]    Hereinafter, a piezoelectric pump  101  will be described according to a first preferred embodiment of the present invention. 
         [0060]      FIG. 3  is an external perspective view of the piezoelectric pump  101  according to the first preferred embodiment of the present invention.  FIG. 4  is an exploded perspective view of the piezoelectric pump  101  as shown in  FIG. 3 .  FIG. 5  is a cross-sectional view of the piezoelectric pump  101  as shown in  FIG. 3  taken along line T-T. 
         [0061]    As shown in  FIG. 3  to  FIG. 5 , the piezoelectric pump  101  preferably includes a cover plate  195 , a base plate  191 , a flexible plate  151 , a vibrating plate unit  160 , a piezoelectric element  142 , a spacer  135 , an electrode conducting plate  170 , a spacer  130 , and a lid portion  110 . The piezoelectric pump  101  is provided with a structure in which the above components are layered in that order. 
         [0062]    A vibrating plate  141  has an upper surface facing the lid portion  110 , and a lower surface facing the flexible plate  151 . 
         [0063]    The piezoelectric element  142  is fixed to the upper surface of the vibrating plate  141  preferably by an adhesive agent. The upper surface of the vibrating plate  141  is equivalent to the “first main surface” according to a preferred embodiment of the present invention. Both the vibrating plate  141  and the piezoelectric element  142  preferably are disc shaped. In addition, the vibrating plate  141  and the piezoelectric element  142  define a disc shaped actuator  140 . The vibrating plate unit  160  that includes the vibrating plate  141  is formed of a metal material which has a coefficient of linear expansion greater than the coefficient of linear expansion of the piezoelectric element  142 . By applying heat to cure the vibrating plate  141  and the piezoelectric element  142  at time of adhesion, an appropriate compressive stress can be left on the piezoelectric element  142  which allows the vibrating plate  141  to bend and form a convex curve on the side of the piezoelectric element  142 . This compressive stress can prevent the piezoelectric element  142  from cracking. For example, it is preferred for the vibrating plate unit  160  to be formed of SUS430. For example, the piezoelectric element  142  may be made of lead titanate zirconate-based ceramics. The coefficient of linear expansion for the piezoelectric element  142  is nearly zero, and the coefficient of linear expansion for SUS430 is about 10.4×10 −6  K −1 . 
         [0064]    It should be noted that the piezoelectric element  142  is equivalent to the “driver” according to a preferred embodiment of the present invention. 
         [0065]    The thickness of the spacer  135  may preferably be the same as, or slightly thicker than, the thickness of the piezoelectric element  142 . 
         [0066]    The vibrating plate unit  160  preferably includes the vibrating plate  141 , the frame plate  161 , and a link portion  162 . The vibrating plate unit  160  is preferably integrally formed by etching a metal plate, for example. The vibrating plate  141  has the frame plate  161  provided therearound. The vibrating plate  141  is linked to the frame plate  161  by the link portion  162 . Additionally, the frame plate  161  is fixed to the flexible plate  151  preferably by the adhesive agent. 
         [0067]    The vibrating plate  141  and the link portion  162  are preferably thinner than the thickness of the frame plate  161  so that the surfaces of the vibrating plate  141  and the link portion  162  on the side of the flexible plate  151  may separate from the flexible plate  151 . The vibrating plate  141  and the link portion  162  are preferably made thinner than the thickness of the frame plate  161  by half etching the surfaces of the vibrating plate  141  and the link portion  162  on the side of the flexible plate  151 . Accordingly, a distance between the vibrating plate  141  and the link portion  162 , and the flexible plate  151  is accurately determined to a predetermined size (15 μm, for example) by the depth of the half etching. The link portion  162  has an elastic structure having the elasticity of a small spring constant. 
         [0068]    Therefore, the vibrating plate  141  is flexibly and elastically supported preferably at three points against the frame plate  161  by three link portions  162 , for example. For this reason, the bending vibration of the vibrating plate  141  cannot be blocked at all. In other words, the piezoelectric pump  101  has a structure in which the peripheral portion of the actuator  140  (as well as the central portion) is not substantially fixed. 
         [0069]    It is to be noted that the flexible plate  151 , an adhesive agent layer  120 , the frame plate  161 , the spacer  135 , the electrode conducting plate  170 , the spacer  130 , and the lid portion  110  constitute a pump housing  180 . Additionally, the interior space of the pump housing  180  is equivalent to a pump chamber  145 . 
         [0070]    The spacer  135  is adhesively fixed to an upper surface of the frame plate  161 . The spacer  135  preferably is made of resin. The thickness of the spacer  135  is the same as or slightly thicker than the thickness of the piezoelectric element  142 . Additionally, the spacer  135  constitutes a portion of the pump housing  180 . Moreover, the spacer  135  electrically insulates the electrode conducting plate  170 , described below, with the vibrating plate unit  160 . 
         [0071]    The electrode conducting plate  170  is adhesively fixed to an upper surface of the spacer  135 . The electrode conducting plate  170  is preferably made of metal. The electrode conducting plate  170  includes a frame portion  171  which is a nearly circular opening, an inner terminal  173  which projects into the opening, and an external terminal  172  which projects to the outside. 
         [0072]    The leading edge of the inner terminal  173  is soldered to the surface of the piezoelectric element  142 . The vibration of the inner terminal  173  can be significantly reduced and prevented by setting a soldering position to a position equivalent to a node of the bending vibration of the actuator  140 . 
         [0073]    The spacer  130  is adhesively fixed to an upper surface of the electrode conducting plate  170 . The spacer  130  is preferably made of resin. The spacer  130  is a spacer that prevents the soldered portion of the inner terminal  173  from contacting the lid portion  110  when the actuator  140  vibrates. The spacer also prevents the surface of the piezoelectric element  142  from coming too close to the lid portion  110 , thus preventing the amplitude of vibration from reducing due to air resistance. For this reason, the thickness of the spacer  130  may be equivalent to the thickness of the piezoelectric element  142 . 
         [0074]    The lid portion  110  with a discharge hole  111  formed thereon is bonded to an upper surface of the spacer  130 . The lid portion  110  covers the upper portion of the actuator  140 . Therefore, air sucked through a ventilation hole  152 , to be described below, of the flexible plate  151  is discharged from the discharge hole  111 . 
         [0075]    Here, the discharge hole  111  is a discharge hole which releases positive pressure in the pump housing  180  which includes the lid portion  110 . Therefore, the discharge hole  111  need not necessarily be provided in the center of lid portion  110 . 
         [0076]    An external terminal  153  is arranged on the flexible plate  151  to connect electrically. In addition, a ventilation hole  152  is formed in the center of the flexible plate  151 . 
         [0077]    On a lower surface of the flexible plate  151 , the base plate  191  is attached preferably by the adhesive agent. A cylindrical opening  192  is formed in the center of the base plate  191 . A portion of the flexible plate  151  is exposed to the base plate  191  at the opening  192  of the base plate  191 . The circularly exposed portion of the flexible plate  151  can vibrate at a frequency substantially the same as a frequency of the actuator  140  through the fluctuation of air pressure accompanying the vibration of the actuator  140 . In other words, with the configuration of the flexible plate  151  and the base plate  191 , a portion of the flexible plate  151  facing the opening  192  serves as the circular movable portion  154  capable of bending and vibrating. The movable portion  154  corresponds to a portion in the center or near the center of the region facing the actuator  140  of the flexible plate  151 . Furthermore, a portion positioned outside the movable portion  154  of the flexible plate  151  serves as the fixing portion  155  that is fixed to the base plate  191 . The characteristic frequency of the movable portion  154  is designed to be the same as or slightly lower than the driving frequency of the actuator  140 . 
         [0078]    Accordingly, in response to the vibration of the actuator  140 , the movable portion  154  of the flexible plate  151  also vibrates with large amplitude, centering on the ventilation hole  152 . If the vibration phase of the flexible plate  151  is a vibration phase delayed (for example, 90 degrees delayed) from the vibration of the actuator  140 , the thickness variation of a gap between the flexible plate  151  and the actuator  140  increases substantially. Through this, the piezoelectric pump  101  can improve pump performance (the discharge pressure and the discharge flow rate). 
         [0079]    The cover plate  195  is bonded to a lower surface of the base plate  191 . Three suction holes  197  are provided in the cover plate  195 . The suction holes  197  communicate with the opening  192  through a passage  193  formed in the base plate  191 . 
         [0080]    The flexible plate  151 , the base plate  191 , and the cover plate  195  are preferably made of a material having a coefficient of linear expansion that is greater than a coefficient of linear expansion of the vibrating plate unit  160 . In addition, the flexible plate  151 , the base plate  191 , and the cover plate  195  are preferably made of a material having approximately the same coefficient of linear expansion. For example, it is preferable to have the flexible plate  151  that is made of substances such as beryllium copper. It is preferable to have the base plate  191  that is made of substances such as phosphor bronze. It is preferable to have the cover plate  195  that is made of substances such as copper. These coefficients of linear expansion are approximately 17×10 −6  K −1 . Moreover, it is preferable to have the vibrating plate unit  160  that is made of SUS430. The coefficient of linear expansion of SUS430 is about 10.4×10 −6  K −1 . 
         [0081]    In this case, due to the differences in the coefficients of linear expansion of the flexible plate  151 , the base plate  191 , and the cover plate  195  in relation to the frame plate  161 , by applying heat to cure the flexible plate  151  at a time of adhesion, a tension which makes the flexible plate  151  bend and form a convex curve on the side of the piezoelectric element  142 , is applied to the flexible plate  151 . Thus, a tension which makes the movable portion capable of bending and vibrating is adjusted on the movable portion  154 . Furthermore, the vibration of the movable portion  154  is not blocked due to any slack on the movable portion  154 . It is to be understood that since the beryllium copper which constitutes the flexible plate  151  is a spring material, even if the circular movable portion  154  vibrates with large amplitude, there will be no permanent set in fatigue or similar symptoms. In other words, beryllium copper has excellent durability. 
         [0082]    In the above structure, when a driving voltage is applied to the external terminals  153 ,  172 , the actuator  140  of the piezoelectric pump  101  concentrically bends and vibrates. Furthermore, in the piezoelectric pump  101 , the movable portion  154  of the flexible plate  151  vibrates from the vibration of the vibrating plate  141 . Thus, the piezoelectric pump  101  sucks air from the suction hole  197  to the pump chamber  145  through the ventilation hole  152 . Then, the piezoelectric pump  101  discharges the air in the pump chamber  145  from the discharge hole  111 . In this state of the piezoelectric pump  101 , the peripheral portion of the vibrating plate  141  is not substantially fixed. For that reason, the piezoelectric pump  101  achieves significantly lower loss caused by the vibration of the vibrating plate  141 , while being small and low profile, and can obtain a high discharge pressure and a large discharge flow rate. 
         [0083]    In addition, in the piezoelectric pump  101 , the surface of the link portion  162  on the side of the flexible plate  151  is separated from the flexible plate  151 . Therefore, the piezoelectric pump  101  can prevent the link portion  162  and the flexible plate  151  from adhering to each other even if an excess amount of the adhesive agent flows into a gap between the link portion  162  and the flexible plate  151 . 
         [0084]    Similarly, in the piezoelectric pump  101 , the lower surface of the vibrating plate  141  on the side of the flexible plate  151  is separated from flexible plate  151 . For that reason, the piezoelectric pump  101  can prevent the vibrating plate  141  and the flexible plate  151  from adhering to each other even if the excess amount of the adhesive agent flows into a gap between the vibrating plate  141  and the flexible plate  151 . Here, the lower surface of the vibrating plate  141  is equivalent to the “second main surface” according to a preferred embodiment of the present invention. 
         [0085]    Thus, the piezoelectric pump  101  can prevent the vibrating plate  141  and the link portion  162  and the flexible plate  151  from adhering to each other and blocking the vibration of the vibrating plate  141 . 
         [0086]    Additionally, in the piezoelectric pump  101 , a difference between the thickness of the vibrating plate  141  and the thickness of the frame plate  161  is equivalent to a distance between the vibrating plate  141  and the flexible plate  151 . In other words, in the piezoelectric pump  101 , the distance that affects the pressure-flow rate characteristics is determined by the depth of the half etching to the vibrating plate  141 . 
         [0087]    It is possible to achieve precise setting of the depth of the half etching. Thus, the piezoelectric pump  101  can prevent the pressure-flow rate characteristics from fluctuating with each piezoelectric pump  101 . 
         [0088]      FIG. 6A  is a cross-sectional view of the main portion at normal temperature of the piezoelectric pump  101  as shown in  FIG. 3 , and  FIG. 6B  is a cross-sectional view of the main portion at high temperature of the piezoelectric pump  101  as shown in  FIG. 3 . Here, for illustrative purposes,  FIG. 6A  highlights the bending of the bonding body of the vibrating plate unit  160 , the piezoelectric element  142 , the flexible plate  151 , the base plate  191 , and the cover plate  195  in a scale that is larger than reality. Additionally, in  FIGS. 6A and 6B , the lid portion  110 , the spacer  130 , the electrode conducting plate  170 , and the spacer  135  are omitted in the drawing for illustrative purposes. 
         [0089]    In the piezoelectric pump  101 , the piezoelectric element  142 , the vibrating plate unit  160 , the flexible plate  151 , the base plate  191 , and the cover plate  195  are bonded, for example, by an adhesive agent at a temperature (about 120 degrees, for example) higher than a normal temperature (about 20 degrees) (see  FIG. 6B ). Thus, after the bonding at the normal temperature, the vibrating plate  141  bends and forms a convex curve on the side of the piezoelectric element  142  due to the difference between the coefficient of linear expansion of the vibrating plate unit  160  and the coefficient of linear expansion of the piezoelectric element  142 . Furthermore, the flexible plate  151  bends and forms a convex curve on the side of the piezoelectric element  142  due to the difference between the coefficient of linear expansion of the above mentioned vibrating plate unit  160  and the coefficient of linear expansion of the base plate  191  (see  FIG. 6A ). 
         [0090]    At the normal temperature, the vibrating plate  141  and the flexible plate  151  bend and form convex curves on the side of the piezoelectric element  142  by approximately the same amount. Then, both the bending of the vibrating plate  141  and the flexible plate  151  are reduced by approximately the same amount as the temperature of the piezoelectric pump  101  increases due to heat generation at the time of driving the piezoelectric pump  101  or due to changes in environmental temperature. 
         [0091]    Therefore, even if the vibrating plate unit  160 , the piezoelectric element  142 , the flexible plate  151 , and the base plate  191  deform by the difference in each of the coefficients of linear expansion due to changes in temperature, the distance between the vibrating plate  141  and the flexible plate  151  is always maintained constant by selecting each material for the vibrating plate unit  160 , the piezoelectric element  142 , the flexible plate  151 , and the base plate  191  as described above. 
         [0092]    Consequently, the piezoelectric pump  101  can significantly reduce and prevent a variation in the pressure-flow rate characteristics caused by changes in temperature. That is, the piezoelectric pump  101  can maintain proper pressure-flow rate characteristics of a pump over a wide temperature range. 
         [0093]      FIG. 7  is a plan view of a bonding body of the vibrating plate unit  160  and the flexible plate  151  as shown in  FIG. 4 . 
         [0094]    As shown in  FIG. 4  to  FIG. 7 , it is preferable that a hole portion  198  is provided in the region facing the link portion  162  in the flexible plate  151  and the base plate  191 . Thus, when the frame plate  161  and the flexible plate  151  are fixed preferably by the adhesive agent, an excess amount of the adhesive agent flows into the hole portion  198 . 
         [0095]    Therefore, the piezoelectric pump  101  can further prevent the vibrating plate  141  and the link portion  162  and the flexible plate  151  from adhering to each other. In other words, the piezoelectric pump  101  can further prevent the vibration of the vibrating plate  141  from being blocked. 
         [0096]    It is to be noted that in the piezoelectric pump  101 , the lid portion  110  may be fixed to the spacer  130  using a silicone adhesive having low elasticity, for example. Alternatively, in place of the lid portion  110  and the spacer  130 , a bulb structure defined by a resin molded article, rubber, and other suitable material may be fixed to the electrode conducting plate  170  using the silicone adhesive having low elasticity, for example. With the former configuration, generation of thermal stress between the lid portion  110  and the spacer  130  is suppressed with by the silicone adhesive of low elasticity. Moreover, with the latter configuration, generation of thermal stress between the bulb structure and the electrode conducting plate  170  is suppressed by the silicone adhesive of low elasticity. 
         [0097]    As described above, when the generation of thermal stress is significantly reduced and prevented, the deformation of the vibrating plate unit  160  and the base plate  191  due to changes in the temperature of the piezoelectric pump  101  cannot be blocked. In other words, the effects of the lid portion  110  and the bulb structure are eliminated. For that reason, the piezoelectric pump  101  can further reduce and prevent variations in the pressure-flow rate characteristics by changes in temperature. 
       Other Preferred Embodiments 
       [0098]    In the above described preferred embodiments, as shown in  FIG. 6A  and  FIG. 6B , while the actuator  140  is configured preferably by bonding the piezoelectric element  142  to the upper surface of the vibrating plate  141  on the side opposite to the flexible plate  151 , the configuration is not limited thereto. In a piezoelectric pump  201  as shown in  FIG. 8A  and  FIG. 8B , for example, an actuator  240  may be configured by bonding the piezoelectric element  142  to the lower surface of the vibrating plate  141  on the side of the flexible plate  151 . However, in the piezoelectric pump  201  as shown in  FIG. 8A  and  FIG. 8B , the piezoelectric element  142  is preferably made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the vibrating plate unit  160 . 
         [0099]    While the actuator  140  having a unimorph type structure and undergoing bending vibration was preferably provided in the above mentioned preferred embodiments, the structure is not limited thereto. For example, it is possible to attach a piezoelectric element  142  on both sides of the vibrating plate  141  so as to have a bimorph type structure and undergo bending vibration. 
         [0100]    Moreover, in the above described preferred embodiments, while the actuator  140  which undergoes bending vibration by expansion and contraction of the piezoelectric element  142  was preferably provided, the method is not limited thereto. For example, an actuator which electromagnetically undergoes bending vibration may be provided. 
         [0101]    In the preferred embodiments, while the piezoelectric element  142  is preferably made of lead titanate zirconate-based ceramics, the material is not limited thereto. For example, an actuator may be made of a piezoelectric material of non-lead based piezoelectric ceramics such as potassium-sodium niobate based or alkali niobate based ceramics. 
         [0102]    While the above-mentioned preferred embodiment shows an example in  FIG. 6A  in which the vibrating plate unit  160 , the flexible plate  151 , and the base plate  191  preferably form convex curves on the side of the piezoelectric element  142  at normal temperature, the structure is not limited thereto. For example, even if the vibrating plate unit  160 , the piezoelectric element  142 , the flexible plate  151 , and the base plate  191  deform due to the difference in each of the coefficients of linear expansion caused by changes in temperature, as long as the distance can always remain constant between the vibrating plate  141  and the flexible plate  151 , the configuration such as the piezoelectric pump  301  as shown in  FIG. 9A  may be used. In other words, as shown in  FIG. 9A , at normal temperature, the vibrating plate unit  160 , the flexible plate  151 , and the base plate  191  may form convex curves on the sides opposite to the piezoelectric element  142 . However, in the piezoelectric pump  301  as shown in  FIG. 9A  and  FIG. 9B , the piezoelectric element  142  is preferably made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the vibrating plate unit  160 , and the vibrating plate unit  160  is preferably made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the base plate  191 . 
         [0103]    In addition, in the piezoelectric pump  301  as shown in  FIG. 9A  and  FIG. 9B , while the actuator  140  is configured preferably by bonding the piezoelectric element  142  to the upper surface of the vibrating plate  141  on the side opposite to the flexible plate  151 , the configuration is not limited thereto. In a piezoelectric pump  401  as shown in  FIG. 10A  and  FIG. 10B , for example, the actuator  240  may be configured by bonding the piezoelectric element  142  to the lower surface of the vibrating plate  141  on the side of the flexible plate  151 . However, in the piezoelectric pump  401  as shown in  FIG. 10A  and  FIG. 10B , the piezoelectric element  142  is preferably made of a material having a coefficient of linear expansion that is larger than the coefficient of linear expansion of the vibrating plate unit  160 . 
         [0104]    Additionally, while the above described preferred embodiments showed an example in which the piezoelectric element  142  and the vibrating plate  141  preferably have roughly the same size, there are no limitations to the size. For example, the vibrating plate  141  may be larger than the piezoelectric element  142 . 
         [0105]    Moreover, although the disc shaped piezoelectric element  142  and the disc shaped vibrating plate  141  were preferably included in the above mentioned preferred embodiments, there are no limitations to the shape. For example, either of the piezoelectric element  142  or the vibrating plate  141  can be a rectangle or a polygon. 
         [0106]    In addition, while a thickness of the entire vibrating plate  141  is preferably thinner than the thickness of the frame plate  161 , there are no limitations to the thickness. For example, the thickness of at least a portion of the vibrating plate  141  may be preferably thinner than the thickness of the frame plate  161 . However, a portion of the vibrating plate  141  is preferred to be an end of the vibrating plate, of the entire vibrating plate  141 , nearest to an adhesion portion between the flexible plate  151  and the frame plate  161 . 
         [0107]    Moreover, in the above described preferred embodiment, while the link portion  162  is preferably provided at three spots, the number of places is not limited thereto. For example, the link portion  162  may be provided at only two spots or the link portion  162  may be provided at four or more spots. Although the link portion  162  does not block vibration of the actuator  140 , the link portion  162  does more or less affect the vibration of the actuator  140 . Therefore, the actuator  140  can be held naturally by linking (holding) the actuator at three spots, for example, and the position of the actuator  140  is held accurately. The piezoelectric element  142  can also be prevented from cracking. 
         [0108]    Furthermore, the actuator  140  may be driven in an audible frequency band in various preferred embodiments of the present invention if it is used in an application in which the generation of audible sounds does not cause problems. 
         [0109]    In addition, while the above described preferred embodiments show an example in which one ventilation hole  152  is disposed at the center of a region facing the actuator  140  of the flexible plate  151 , there are no limitations to the number of holes. For example, a plurality of holes may be disposed near the center of the region facing the actuator  140 . 
         [0110]    Further, while the frequency of driving voltage in the above mentioned preferred embodiments is determined so as to make the actuator  140  vibrate in a primary mode, there are no limitations to the mode. For example, the driving voltage frequency may be determined so as to vibrate the actuator  140  in other modes such as a tertiary mode. 
         [0111]    In addition, while air is used as fluid in the above mentioned preferred embodiments, the fluid is not limited thereto. For example, any kind of fluid such as liquids, gas-liquid mixture, solid-liquid mixture, and solid-gas mixture can be applied to the above preferred embodiment. 
         [0112]    While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.