Abstract:
Disclosed is a piezoelectric pump. The piezoelectric pump includes a pump body, a drive unit, a detection section, and a control unit. The pump body includes a hole for introducing a fluid from outside thereinto and jetting the fluid from inside, a wall portion disposed so as to face the hole, and a piezoelectric body provided on the wall portion to vibrate the wall portion. The drive unit drives the piezoelectric body. The detection section detects a signal corresponding to a discharge rate of the fluid from the hole. The control unit controls a drive voltage and a drive frequency of the drive unit on the basis of the signal.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2007-336076 filed in the Japanese Patent Office on Dec. 27, 2007, the entire contents of which being incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a piezoelectric pump, a cooling device, and an electronic apparatus used for cooling a hard disk drive or the like mounted in a portable electronic apparatus such as a portable video camera. 
         [0004]    2. Description of the Related Art 
         [0005]    As a technique for cooling an electronic component mounted in a portable electronic apparatus, there has been disclosed, for example, a “jet generating device” for vibrating a vibration plate which uses a piezoelectric body within a chamber having a nozzle for jetting gas to jet the gas for cooling from the nozzle (see, Japanese Patent Application Laid-open No. 2006-297295 (paragraphs [0029]-[0031], FIG. 2)). 
         [0006]    Since each cooling device using the piezoelectric body has a different natural frequency, in order to maximize the discharge rate and suction rate, it is necessary to control the drive voltage of the piezoelectric body at an optimal value and adjust the drive frequency of the piezoelectric body to the natural frequency. In the past, in a manufacturing line, the drive frequency has been adjusted to match the natural frequency of the cooling device and the adjusted value has been used as a fixed value. 
       SUMMARY OF THE INVENTION 
       [0007]    However, the cooling device has a property of changing its natural frequency depending on the temperature change of the piezoelectric body. Therefore, when the drive frequency is fixed as in the past, there is a problem in that the discharge rate and suction rate of the cooling device decrease with the change of an ambient temperature. 
         [0008]    In view of the above-mentioned circumstances, it is desirable to provide a piezoelectric pump, a cooling device, and an electronic apparatus whose discharge rate is properly controllable. 
         [0009]    According to an embodiment of the present invention, there is provided a piezoelectric pump including a pump body, a drive unit, a detection means, and a control unit. The pump body includes a hole for introducing a fluid from outside thereinto and jetting the fluid from inside, a wall portion disposed so as to face the hole, and a piezoelectric body provided on the wall portion to vibrate the wall portion. The drive unit drives the piezoelectric body. The detection means detects a signal corresponding to a discharge rate of the fluid from the hole. The control unit controls a drive voltage and a drive frequency of the drive unit on the basis of the signal. 
         [0010]    In the embodiment of the present invention, even if the natural frequency of the piezoelectric body changes, it is possible to detect the signal corresponding to the discharge rate of the fluid from the hole by the detection means and control the drive voltage and drive frequency of the drive unit on the basis of the signal, so that the discharge rate of the fluid from the hole can be controlled at a proper rate (its maximum). 
         [0011]    The drive unit includes a pair of electrodes disposed to sandwich the piezoelectric body therebetween, a transformer having a primary winding connected to the control unit and a secondary winding whose both ends are connected to the respective electrodes, and an electronic variable reactance element connected in parallel with the secondary winding. The control unit includes means for varying a reactance of the electronic variable reactance element such that the discharge rate is maximized on the basis of the signal to control the drive frequency. 
         [0012]    Thus, it is possible to vary the reactance of the electronic variable reactance element on the basis of the signal corresponding to the discharge rate such that the discharge rate is maximized. 
         [0013]    The control unit further includes an alternating current oscillator configured to generate an alternating current to be supplied to the primary winding, and means for performing such control that a process of linearly varying and resetting an oscillation frequency of the alternating current oscillator is repeated in a predetermined cycle. The detection means includes means for varying a reference voltage in synchronization with the cycle, means for comparing the reference voltage and a voltage based on the signal, and means for, on the basis of a result of the comparison by the comparing means, detecting a predetermined oscillation frequency of the alternating current oscillator at which the discharge rate is maximized. The drive voltage of the drive unit is controlled by setting the oscillation frequency of the alternating current oscillator to the detected predetermined oscillation frequency. 
         [0014]    Consequently it is possible to control the drive voltage of the drive unit by repeating the process of linearly varying and resetting the oscillation frequency in the predetermined cycle, varying the reference voltage in synchronization with the cycle, comparing the reference voltage and the voltage based on the signal, detecting, on the basis of a result of the comparison, the predetermined oscillation frequency of the alternating current oscillator at which the discharge rate is maximized, and setting the oscillation frequency of the alternating current oscillator to the predetermined oscillation frequency. 
         [0015]    The detection means includes a discharge rate detection unit configured to detect the discharge rate of the fluid from the hole. Thus, it is possible to detect the discharge rate by the discharge rate detection unit and control the drive voltage and drive frequency on the basis of a signal corresponding to the detected value. 
         [0016]    The detection means includes a sound detection unit configured to detect a sound corresponding to the discharge rate. Consequently, it is possible to detect the sound by the sound detection unit and control the drive voltage and drive frequency on the basis of the signal corresponding to the detected value. 
         [0017]    The control means includes means for detecting a disturbance on the basis of the signals detected by the detection means when the piezoelectric body is being driven and when the driving is stopped. Thus, it is possible to judge whether there is the disturbance and precisely control the discharge rate. 
         [0018]    Another piezoelectric pump according to an embodiment of the present invention is a piezoelectric pump including a plurality of piezoelectric pumps connected in parallel, each of the plurality of piezoelectric pumps including a pump body having a hole for introducing a fluid from outside thereinto and jetting the fluid from inside, a wall portion disposed so as to face the hole, and a piezoelectric body provided on the wall portion to vibrate the wall portion, a drive unit configured to drive the piezoelectric body, a detection means for detecting a signal corresponding to a discharge rate of the fluid from the hole, and a control unit configured to control a drive voltage and a drive frequency of the drive unit on the basis of the signal. 
         [0019]    In the embodiment of the present invention, by adjusting the drive frequency of the piezoelectric body to the natural frequency of a piezoelectric pump corresponding to a position where an object to be cooled is located, the object to be cooled can be selectively and efficiently cooled. 
         [0020]    A cooling device according to an embodiment of the present invention includes a pump body, a drive unit, a detection means, and a control unit. The pump body has a hole for introducing a fluid from outside thereinto and jetting the fluid from inside, a wall portion disposed so as to face the hole, and a piezoelectric body provided on the wall portion to vibrate the wall portion. The drive unit drives the piezoelectric body. The detection means detects a signal corresponding to a discharge rate of the fluid from the hole. The control unit controls a drive voltage and a drive frequency of the drive unit on the basis of the signal. 
         [0021]    An electronic apparatus according to an embodiment of the present invention includes a piezoelectric pump and an electronic component. The piezoelectric pump includes a pump body having a hole for introducing a fluid from outside thereinto and jetting the fluid from inside, a wall portion disposed so as to face the hole, and a piezoelectric body provided on the wall portion to vibrate the wall portion, a drive unit configured to drive the piezoelectric body, a detection means for detecting a signal corresponding to a discharge rate of the fluid from the hole, and a control unit configured to control a drive voltage and a drive frequency of the drive unit on the basis of the signal. The electronic component is cooled by the fluid jetted from the piezoelectric pump. 
         [0022]    The piezoelectric pump is used also as a speaker of the electronic apparatus. Thus, the speaker with the excellent reliability of a connection between voltage application wiring and an electrode can be obtained. 
         [0023]    As described above, according to the embodiments of the present invention, the piezoelectric pump whose discharge rate is controllable can be provided. 
         [0024]    These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of best mode embodiments thereof, as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is an exploded perspective view of a principal portion showing the constitution of a portable electronic apparatus according to an embodiment of the present invention; 
           [0026]      FIG. 2  is a sectional view of the portable electronic apparatus including a microphone and a lens; 
           [0027]      FIG. 3  is an external view of the portable electronic apparatus; 
           [0028]      FIG. 4  is a perspective view of a cooling device; 
           [0029]      FIG. 5  is a perspective view of the bottom surface side of the cooling device; 
           [0030]      FIGS. 6A and 6B  are an enlarged plan view and a perspective view showing a portion where an electrode formed in a piezoelectric element and an annular connection portion are connected; 
           [0031]      FIG. 7  is a sectional view taken along the line A-A of  FIG. 6 ; 
           [0032]      FIG. 8  is a sectional view showing the constitution of the cooling device; 
           [0033]      FIG. 9  is an exploded perspective view of a piezoelectric pump body; 
           [0034]      FIG. 10  is a block diagram showing a schematic configuration of a control system of the piezoelectric element; 
           [0035]      FIG. 11  is a diagram showing a circuit of the piezoelectric element side; 
           [0036]      FIG. 12  is a circuit diagram showing details of an optimal drive frequency search circuit of  FIG. 10 ; 
           [0037]      FIG. 13  is a diagram showing a relation between the drive frequency of the piezoelectric element and the voltage (flow velocity, discharge rate) detected by a flow sensor; 
           [0038]      FIG. 14  is a partial plan view of the flow sensor; 
           [0039]      FIG. 15  is a sectional view taken along the line B-B of the flow sensor of  FIG. 14 ; 
           [0040]      FIG. 16  is a diagram showing a relation between the drive frequency of the piezoelectric element and time (cycle); 
           [0041]      FIG. 17  is a diagram showing a relation between a voltage V ref  serving as a reference and time (cycle); 
           [0042]      FIG. 18  is a diagram showing a relation between the voltage of the flow sensor and the drive frequency of the piezoelectric element; 
           [0043]      FIG. 19  is a diagram showing a relation between the output voltage of a comparator and the drive frequency; 
           [0044]      FIG. 20  is a diagram showing a relation between voltage and time; 
           [0045]      FIG. 21  is a perspective view of a cooling device of a second embodiment; and 
           [0046]      FIG. 22  is a perspective view of another cooling device. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0047]    Hereinafter, embodiments of the present invention will be described below with reference to the drawings. 
         [0048]      FIG. 1  is an exploded perspective view of a principal portion showing the constitution of a portable electronic apparatus according to an embodiment of the present invention,  FIG. 2  is a sectional view of the portable electronic apparatus including a microphone and a lens, and  FIG. 3  is an external view of the electronic apparatus. A portable video camera is taken here for an example of the portable electronic apparatus, but of course a portable phone or other electronic apparatuses are also available. 
         [0049]    As shown in these figures, in a portable video camera  1 , a captured subject image is video-recorded/reproduced by an HDD unit  5  mounted in a housing  3  of a camera body  2 . 
         [0050]    As shown in  FIG. 3 , the portable video camera  1  includes a lens  4  of a camera portion, a microphone  6  for collecting sound when an image is captured, a display portion  7  such as an LCD, which is rotatably and pivotally fitted to the camera body  2  and doubles as a monitor and a finder, an eyepiece  8 , and an operation button group  9 , and a cooling device  16  is disposed at a bottom portion  10  of the housing  3 . 
         [0051]    In many cases, the obverse side (underside in  FIG. 1 ) of a casing  5   a  of the HDD unit  5  is a metal portion cast by aluminum die casting or the like, and the reverse side (upper side in  FIG. 1 ) thereof is constituted by a printed-circuit board for driving, and therefore the casing  5   a  is joined to a heat transfer portion  20  made of metal such as copper and having high heat conductivity with a heat transfer sheet  21   a  therebetween with the obverse-side metal portion down. 
         [0052]    The heat transfer portion  20  has a shape formed by bending both ends of a band-shaped copper plate in opposite directions to each other, and the heat transfer sheet  21   a  is joined to one bent piece  20   a  and fixed to the HDD unit  5 . A heat transfer sheet  21   b  is also joined to the other bent piece  20   b  and joined to an upper surface of a sealed casing  19 , and as shown in  FIG. 2 , the HDD unit  5  is fixed onto the sealed casing  19 . 
         [0053]    The sealed casing  19  and the cooling device  16  are fixed to the bottom portion  10  of the housing  3  by screws  23 . Accordingly, heat raised to a high temperature by the HDD unit  5  is transferred to the heat transfer portion  20  and cooled by the cooling device  16 . 
         [0054]      FIG. 4  is a perspective view of the cooling device  16 . 
         [0055]    As shown in  FIG. 4 , the cooling device  16  includes a piezoelectric pump body  31 , a drive circuit substrate  32  integrally connected to the piezoelectric pump body  31 , and a flow sensor (discharge rate sensor, flow velocity sensor)  140 . 
         [0056]    A discharge hole  33  for jetting gas is formed nearly in the center of the piezoelectric pump body  31 . Holes  34  serving as gas passages are formed at four corners of the piezoelectric pump body  31 . 
         [0057]    The flow sensor  140  is used for detecting the discharge rate (flow velocity) of gas discharged from the discharge hole  33 . The flow sensor  140  is provided near the discharge hole  33  so as to protrude from an upper surface of the piezoelectric pump body  31 . The flow sensor  140  may have the same constitution as that of the related art as described later. 
         [0058]    A microtransformer  35  and so on which form part of a drive circuit for driving the piezoelectric pump body  31  are mounted on the drive circuit substrate  32 . 
         [0059]      FIG. 5  is a perspective view of the bottom surface side of the cooling device  16 ,  FIGS. 6A and 6B  are an enlarged plan view and a perspective view showing a portion where an electrode formed in a piezoelectric element and an annular connection portion are connected, and  FIG. 7  is a sectional view taken along the line A-A of  FIG. 6A . 
         [0060]    As shown in  FIG. 5 , an opening  61  is formed in a bottom surface of the piezoelectric pump body  31 , and a diaphragm  36  described later is exposed from the opening. The diaphragm  36  is provided with an electrode  38  with a piezoelectric element  37  therebetween. 
         [0061]    An annular connection portion  39  is led out from the drive circuit substrate  32  via a lead-out portion  39   d , and wiring  41  is routed in the annular connection portion  39  and so on. The annular connection portion  39  (wiring  41 ) is connected to the surface of the electrode  38  at a plurality of positions by solders  40  or the like. 
         [0062]    As shown in  FIGS. 6A and 6B , the solders  40  are provided on the circumference of a circle with a diameter L 2 , while the piezoelectric element  37  and the electrode  38  have a diameter L 1 . The circumference of the circle with the diameter L 2  corresponds to, for example, the positions of vibration nodes of the piezoelectric element  37  when the piezoelectric element  37  is being driven as described later. The connection positions of the plurality of solders  40  are provided at even intervals in a circumferential direction of the annular connection portion  39 . 
         [0063]    As shown in  FIG. 7 , a metal plate, for example, having the same shape as the piezoelectric element  37  is used for the electrode  38 . Through-holes are formed in the drive circuit substrate  32 , and the electrode  38  and the wiring  41  are connected by the solders  40  in the through-holes. 
         [0064]      FIG. 8  is a sectional view showing the constitution of the cooling device  16 , and  FIG. 9  is an exploded perspective view of the piezoelectric pump body  31 . It should be noted that the flow sensor  140  is not shown in  FIG. 9 . 
         [0065]    As shown in  FIG. 8 , the cooling device  16  includes the piezoelectric pump body  31  and the drive circuit substrate  32  integrally connected to the piezoelectric pump body  31 . 
         [0066]    As shown in  FIG. 8 , the piezoelectric pump body  31  includes, in order from the top, a top plate  43 , an insertion plate  46  for forming a passage  42 , a top plate  45  of a pump chamber, an insertion plate  46  for forming a space in the pump chamber, the diaphragm  36 , the piezoelectric element  37  attached to a rear surface of the diaphragm  36 , the electrode  38 , a protection ring  47 , and a ring  47 B. 
         [0067]    As shown in  FIG. 9 , the top plate  43  has, for example, an approximately box shape, and the discharge hole  33  for discharging gas jetted from the pump chamber toward the bent piece  20   b  is provided in the center of the top plate  43 . It should be noted that the flow sensor  140  is provided near the discharge hole  33  so as to protrude from an upper surface of the top plate  43 . 
         [0068]    The insertion plate  44  has, for example, an approximately rectangular shape and includes a cross-shaped punched portion  48  to form a passage. A Venturi nozzle portion is provided at a position corresponding to the discharge hole  33 , that is, in the center of the insertion plate  44  (an intersection portion of the cross-shaped punched portion  48 ). 
         [0069]    The top plate  45  of the pump chamber has, for example, a square shape, and holes  50  are formed at respective corners and form the passage  42 . The top plate  45  is provided with a hole  53  for introducing the gas into the pump chamber from the passage  42  and jetting the gas from inside the pump chamber toward the bent piece  20   b  through the discharge hole  33 . 
         [0070]    In the insertion plate  46  for forming the space in the pump chamber, a hole  55  for forming the space in the pump chamber is formed in its center and holes  56  are respectively formed at four corners to form the passage  42 . The insertion plate  46  has a thickness sufficient to form the space serving as the pump chamber. 
         [0071]    The diaphragm  36  also has the same shape as the top plate  45 , and holes  57  are formed at respective corners to form the passage  42 . 
         [0072]    The piezoelectric element  37  has, for example, a circular shape and vibrates in response to an applied voltage (alternating voltage). A piezoelectric material such as lead zirconate titanate (PZT) is used for the piezoelectric element  37 . 
         [0073]    The protection ring  47  has the same shape as the above insertion plate  46  and so on, and holes  58  are formed at respective corners to form the passage  42 . 
         [0074]    A metal plate, for example, having the same shape as the piezoelectric element is used for the electrode  38 . It should be noted that in place of the electrode  38 , the electrode may be formed by coating the surface of the piezoelectric element  37  with silver paste. 
         [0075]    The ring  47 B has the same shape as the protection ring  47 , and cutout portions  60  are formed on the inside of the holes  50  at respective corners. The protection ring  47  and the ring  47 B have a combined thickness sufficient to prevent the diaphragm  36 , the piezoelectric element  37 , and the annular connection portion  39  to hit the bottom portion  10  of the housing  3  by the vibration of the piezoelectric element  37  during the vibration. 
         [0076]      FIG. 10  is a block diagram showing a schematic configuration of a control system of the piezoelectric element  37 , and  FIG. 11  is a diagram showing a circuit of the piezoelectric element  37  side. 
         [0077]    A control system  100  of the piezoelectric element  37  includes a drive control circuit  35 A and a drive section  35 B. 
         [0078]    As shown in  FIG. 11 , the drive section  35 B includes the microtransformer  35 , a variable capacitor  79 , the diaphragm  36 , and the electrode  38 . The drive section  35 B has a configuration in which a secondary coil  35   b , the diaphragm  36 , and the electrode  38  are connected in series, and the variable capacitor  79  is connected in parallel therewith. Note that portions other than these are not shown in  FIG. 11 . The variable capacitor  79  includes input/output electrodes  79   a  and  79   b  and control electrodes  79   c  and  79   d  for changing the dielectric constant of the variable capacitor  47  to control the capacitance. For example, the control electrode  79   d  is connected to an output terminal of the drive circuit  35 A. The piezoelectric element  37  is disposed between the diaphragm  36  and the electrode  38 . 
         [0079]    The drive control circuit  35 A includes an oscillator (OSC)  70 , an AND circuit  71 , a DIV circuit  72 , a counter circuit  73 , a DAC (digital/analog convertor)  74 , a VCO (voltage-controlled oscillator)  75 , FETs  76  and  77 , an optimal drive frequency search circuit  80 , a power control circuit  81 , a voltage control circuit  82 , an AND circuit  83 , a DIV  84 , a counter circuit  85 , a DAC  86 , and a BUF (amplifier)  87 . 
         [0080]    A clock pulse signal of a predetermined frequency oscillated by the oscillator  70  is inputted to one input end of the AND circuit  71 . The other input end of the AND circuit  71  is connected to an output end of the optimal drive frequency search circuit  80  described later. The clock pulse signal corresponding to a control output of the optimal drive frequency search circuit  80  is inputted to the DIV circuit (frequency dividing circuit)  72 , where the frequency of the clock pulse signal is divided at a predetermined frequency division ratio, and inputted to the counter circuit  73 . The counter circuit  73  counts output pulses of the DIV circuit (frequency dividing circuit)  72  and inputs a result of the counting not only to the DAC (digital/analog converter)  74  but also to the optimal drive frequency search circuit  80 . The DAC  74  converts the count value into an analog signal and outputs the analog signal to the VCO  75 . 
         [0081]    Two output terminals of the VCO  75  are connected to gates of the FETs  76  and  77  respectively, and drains of the FETs  76  and  77  are connected to both ends of a primary coil  35   a  of the microtransformer. The VCO  75  performs on/off switching of the FETs  76  and  77  at a frequency corresponding to a control voltage from the DAC  74 . Thus, the frequency of an alternating current flowing through the primary coil  35   a  of the microtransformer is controlled. 
         [0082]      FIG. 12  is a circuit diagram showing details of the optimal drive frequency search circuit  80  of  FIG. 10 , and  FIG. 13  is a diagram showing a relation between the drive frequency of the piezoelectric element  37  and the voltage (flow velocity, discharge rate) detected by the flow sensor  140 . 
         [0083]    When a drive frequency f of the piezoelectric element  37  is changed, a voltage V corresponding to the flow velocity (discharge rate) detected by the cooling device  16  shows such a characteristic as shown in  FIG. 13 . That is to say, when the piezoelectric element  37  is being driven at a predetermined drive frequency f 0 , the discharge rate of the flow sensor  140  is maximized. The optimal drive frequency search circuit  80  is a circuit for detecting the drive frequency f 0  of the piezoelectric element  30  at which the voltage V (flow velocity, discharge rate) of the flow sensor  140  is maximized. The optimal drive frequency search circuit  80  is used for detecting the drive frequency f 0  at which the discharge rate (flow velocity) is maximized and fixing the drive frequency f to the drive frequency f 0 . 
         [0084]    As shown in  FIG. 12 , the optimal drive frequency search circuit  80  includes the flow sensor  140 , an amplifier (AMP)  152 , a low-pass filter (LPF)  153 , an envelope detector (DET)  154 , a comparator (CMP)  155 , a latch circuit  156 , a counter circuit  157 , and a DAC (digital/analog converter)  158 . 
         [0085]      FIG. 14  is a partial plan view of the flow sensor  140 , and  FIG. 15  is a sectional view taken along the line B-B of the flow sensor  140  of  FIG. 14 . 
         [0086]    As shown in  FIG. 14  and  FIG. 15 , the flow sensor  140  is a sensor capable of detecting a change in so-called gas particle velocity, for example, a change at a particle level in gas flow velocity (discharge rate), heat conduction amount, or the like. For example, the flow sensor  140  has a structure in which when air moves, the resistance values of resistors S 1  and S 2  of the flow sensor  140  change. In the flow sensor  140 , a diaphragm  62 , the resistors S 1  and S 2 , and a common C 1  which is a platinum resistor are formed on a semiconductor wafer  90 . 
         [0087]      FIG. 16  is a diagram showing a relation between the drive frequency f of the piezoelectric element  37  and time (cycle),  FIG. 17  is a diagram showing a relation between a voltage V ref  serving as a reference and time (cycle), and  FIG. 18  is a diagram showing a relation between the voltage V of the flow sensor  140  and the drive frequency f of the piezoelectric element  37 . 
         [0088]    A detection signal detected by the flow sensor  140  is amplified by the amplifier  152  and inputted to the low-pass filter  153 . In the low-pass filter  153 , a low-frequency component is extracted from an output of the amplifier  152  and inputted to the envelope detector (DET)  154 . The envelope detector (DET)  154  detects an amplitude value of an output of the low-pass filter  153  and outputs a voltage V A  corresponding to the detected amplitude value to the comparator  155 . 
         [0089]    An output from the counter circuit  73  is inputted to the counter circuit  157 . The counter circuit  157  counts output pulses of the counter circuit  73  and inputs a result of the counting to the DAC  158 . The DAC  158  converts the count value into an analog signal and outputs the voltage V ref  corresponding to the analog signal obtained by the conversion to the comparator  155 . 
         [0090]    At this time, as shown in  FIG. 16 , the drive frequency f of the piezoelectric element  37  repeats the process of increasing linearly and being reset in a predetermined cycle T 0  and is swept in a saw-tooth pattern, and on the basis of the count value of the counter circuit  157 , the voltage V ref  decreases by a predetermined value in synchronization with each cycle T 0  as shown in  FIG. 17  and  FIG. 18 . 
         [0091]    As shown in  FIG. 18 , the comparator  155  compares the inputted voltage V A  and the voltage V ref  and outputs a result of the comparison to the latch circuit  156 . 
         [0092]      FIG. 19  is a diagram showing a relation between the output voltage of the comparator  155  and the drive frequency f, and  FIG. 20  is a diagram showing a relation between voltage and time. 
         [0093]    When the voltage V ref  is α, the voltage V ref  is constant even if the drive frequency f is changed as shown in  FIG. 19 . At this time, the latch circuit  156  shown in  FIG. 12  outputs an “on” signal to the AND circuit  71  and outputs an “off” signal to the AND circuit  83 . 
         [0094]    When the voltage V ref  is β, the voltage V ref  changes when the drive frequency f is f 0  if the drive frequency f is changed as shown in  FIG. 19 . At this time, the latch circuit  156  outputs an “off” signal to the AND circuit  71  and outputs an “on” signal to the AND circuit  83 . 
         [0095]    At this time, an output of the AND circuit  71  shown in  FIG. 10  is turned off and the increase of the count value of the counter circuit  73  is stopped. As a result, a predetermined count value is converted into an analog signal by the DAC  74 , and the on/off switching of the FETs  76  and  77  is performed at the predetermined drive frequency f 0  corresponding to the control voltage from the DAC  74 . In other words, the sweeping of the drive frequency f is stopped and fixed at the optimal drive frequency f 0  (the drive voltage optimal for driving the piezoelectric element  37  is fixed). 
         [0096]    The voltage control circuit  82  detects voltages at points A and B of a secondary-side circuit and variably controls the capacitance of the variable capacitor  79  on the basis of the detected voltages. More specifically, the voltage control circuit  82  switches a logical value inputted to the AND circuit  83 . 
         [0097]    Input ends of the AND circuit  83  are connected to a control output of the voltage control circuit  82 , an output of the oscillator  70 , and a control output of the optimal drive frequency search circuit  80 . The control output of the voltage control circuit  82  is a variable output of the ratio between the time of H (logical value) and the time of L (logical value) according to a target controlled variable. Thus, a clock pulse signal corresponding to the control output of the voltage control circuit  82  and the control output of the optimal drive frequency search circuit  80  is inputted to the DIV  84 . The frequency of the clock pulse signal inputted to the DIV  84  is divided at a predetermined frequency division ratio by the DIV  84 , and the clock pulse signal is inputted to the counter circuit  85 . The counter circuit  85  counts output pulses of the DIV  84  and inputs a result of the counting to the DAC  86  and the voltage control circuit  82 . The DAC  86  converts the count value into an analog signal and outputs the analog signal to the BUF (amplifier)  87 . The BUF (amplifier)  87  amplifies the signal from the DAC  86  and outputs the amplified signal to the variable capacitor  79 . Thus, the capacitance of the variable capacitor  79  can be controlled. 
         [0098]    The power control circuit  81  outputs a signal for controlling duty to the VCO  75 . 
         [0099]    As described above, according to this embodiment, even if the natural frequency of the piezoelectric element  37  changes, for example, due to the change of the ambient temperature, the drive voltage of the piezoelectric element  37  can be controlled by detecting the signal corresponding to the gas discharge rate of the cooling device  16  by the flow sensor  140 , detecting the predetermined drive frequency f 0  at which the discharge rate is maximized on the basis of the voltage V A  corresponding to the detected signal, and setting the oscillation frequency of the drive control circuit  35 A to the drive frequency f 0 , so that the discharge rate of the gas from the discharge hole  33  can be controlled at a proper rate (its maximum). 
         [0100]    At this time, by repeating the process of linearly increasing and resetting the oscillation frequency in the predetermined cycle T 0  as shown in  FIG. 16 , decreasing the reference voltage V ref  in synchronization with each cycle T 0  as shown in  FIG. 17 , comparing the reference voltage V ref  and the voltage V A  based on the signal from the flow sensor  140  by the comparator  155 , detecting the predetermined drive frequency f 0  at which the discharge rate is maximized on the basis of a result of the comparison, and setting the oscillation frequency of the drive control circuit  35 A to the drive frequency f 0 , the drive voltage of the piezoelectric element  37  is controlled. 
         [0101]    Moreover, it is possible to change the drive frequency by the drive control circuit  35 A to control the capacitance of the variable capacitor  79  such that the discharge rate is maximized on the basis of the signal from the flow sensor  140 , whereby it is possible to control the drive frequency at the natural frequency of the piezoelectric element  37  and increase (maximize) the discharge rate. It is only necessary that, for example, when the flow velocity (discharge rate) does not increase after the drive frequency is changed, the drive frequency be fixed at a value before the change. 
         [0102]    Next, a practical example of the cooling device  16  will be described. 
         [0000]    (1) Volume of Pump Chamber (area, height) 
         [0000]                                                    Diameter   φ16   mm           Height   0.15   mm                        
(2) Area of Inlet Hole of Air from Outside
 
         [0103]    Diameter φ1.2 mm (four positions) 
         [0000]    (3) Area of Outlet Hole of Air (discharge hole  33 , hole  53 ) 
         [0000]                                                    Pump chamber hole 53   Diameter   φ0.6 mm           Discharge hole 33   Diameter   φ0.8 mm                        
(4) Total Volume of Cooling Device  16  (area, height)
 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Area 
                 20 mm × 20 mm 
               
               
                   
                 Height 
                 1.6 mm 
               
               
                   
                   
               
             
          
         
       
     
         [0104]    (Height without nozzle. When the nozzle is provided, the nozzle height is 0.8 mm, and the total height is 2.4 mm.) 
       (5) Amplitude of Piezoelectric Element  37   
       [0105]    Approximately ±6 μm 
       (6) Vibration Frequency of Piezoelectric Element  37   
       [0106]    Approximately 24 kHz 
       (7) Diameter L 1  of Piezoelectric Element  37   
       [0107]    6 mm 
       (8) Diameter L 2  of Annular Connection Portion  39   
       [0108]    (distance between solders  40 ) 
         [0109]    4 mm 
       (9) Flow Rate 
       [0110]    1.0 L/min 
         [0111]    A cooling device according to another embodiment of the present invention will be described. In this embodiment and the subsequent embodiments, the same numerals and symbols will be used to designate the same components as those in the above embodiment, the description thereof will be omitted, and different points will be mainly described. 
         [0112]      FIG. 21  is a perspective view of a cooling device of a second embodiment. 
         [0113]    An electronic apparatus  200  of this embodiment differs in including a substrate  160  having a region a, a region b, a region c, a region d, and a region e on which electronic components (not shown) with different heating values are mounted, respectively, and a plurality of cooling devices  170 A,  170 B,  170 C,  170 D, and  170 E which are arranged corresponding to the respective regions a to e and connected in parallel. 
         [0114]    On the respective regions a to e of the substrate  160 , electronic components having higher heating values in this order are mounted. That is to say, an electronic component having the highest heating value is mounted on the region a of the substrate  160 , and an electronic component having the lowest heating value is mounted on the region e. 
         [0115]    The natural frequency of the cooling device  170 A is set to f 1 , the natural frequency of the cooling device  170 B to f 2 , the natural frequency of the cooling device  170 C to f 3 , the natural frequency of the cooling device  170 D to f 4 , and the natural frequency of the cooling device  170 E to f 5 . The natural frequencies f 1  to f 5  are set, for example, to have a relation of f 1 &gt;f 2 &gt;f 3 &gt;f 4 &gt;f 5 . 
         [0116]    According to the above constitution, by making the drive frequency of the cooling device  170 A the same as the natural frequency f 1  of a piezoelectric element of the cooling device  170 A, the largest amount of gas can be discharged from the cooling device  170 A to the region a on which the electronic component having the highest heating value is mounted to efficiently cool the electronic component on the region a, and the electronic component of the region b can be cooled by another cooling device  170 B. 
         [0117]    Moreover, since the drive frequency can be controlled by the drive control circuit  35 A, for example, it is possible to sequentially change the drive frequency to match the natural frequency of a cooling device corresponding to an electronic component on another region and efficiently cool only a specific region. 
         [0118]      FIG. 22  is a perspective view of another cooling device. 
         [0119]    A cooling device  300  shown in  FIG. 22  differs in that in order to detect a signal corresponding to the discharge rate of the cooling device  300 , a flow sensor  140 ′ is disposed to face a surface (surface from which the piezoelectric element and the diaphragm are exposed) opposite to the side on which the discharge hole  33  of the piezoelectric pump body  31  is disposed, with a predetermined space therebetween. 
         [0120]    In this case, even if the size of the flow sensor  140 ′ increases, the flow sensor  140 ′ and the piezoelectric pump body  31  can be provided so as to overlap each other with the predetermined space therebetween, whereby the thickness of the cooling device can be easily reduced. 
         [0121]    It is to be understood that the present invention is not intended to be limited to the above embodiments, and various changes may be made within the scope of the technical idea of the present invention. 
         [0122]    For example, by using a microphone for detecting a sound corresponding to a discharge rate instead of using the flow sensor  140 , the discharge rate may be controlled on the basis of a signal from the microphone. 
         [0123]    The drive control circuit  35 A may detect a disturbance on the basis of signals detected by the flow sensor  140  when the piezoelectric element  37  is being driven and when the driving is stopped. Thus, the influence of the disturbance can be eliminated and the discharge rate can be controlled more precisely. 
         [0124]    For example, the diaphragm  36  may be used also as a speaker of the portable video camera  1 . Thus, lower cost and reduced size and thickness can be realized. 
         [0125]    In the above embodiments, the example in which the drive frequency or the like of the cooling device  16  is controlled by cooling the electronic component such as the HDD unit  5  by jetting gas and detecting the signal based on the discharge rate of the gas is shown. However, the present invention is not limited to this example, and for example, it is also possible to control the drive frequency or the like of the cooling device  16  by cooling the electronic component by jetting water or the like from the cooling device  16  and detecting a signal based on the discharge rate of the water or the like by the same flow sensor  140  or the like.