Patent Publication Number: US-7902720-B2

Title: High-voltage driver and piezoelectric pump with built-in driver

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a high-voltage driver, and a piezoelectric pump unit incorporating a piezoelectric pump and its control board in the same housing. 
     2. Related Art 
     A piezoelectric pump has a variable volume chamber (liquid pump chamber) formed between a flat piezoelectric vibrator and a housing, and causes the piezoelectric vibrator to vibrate to thereby change the volume of the variable volume chamber and achieve the pumping action. More specifically, in a pair of paths connected to the variable volume chamber, a pair of check valves for different flow directions (a check valve which allows fluid flow into the variable volume chamber and a check valve which allows fluid flow from the variable volume chamber) are provided, respectively, and, when the volume of the volume variable chamber is changed by the vibration of the piezoelectric vibrator, the operation of opening one of the pair of check valves and closing the other is repeated, thereby achieving the pumping action. Because such a piezoelectric pump is used as a cooling water circulating pump for, for example, a water-cooled notebook computer, reducing the size and the thickness of the pump becomes a key issue. 
     [Patent Document 1] JP 6-109068 A 
     [Patent Document 2] JP 8-205563 A 
     [Patent Document 3] JP 2000-60847 A 
     In order to reduce the size of the piezoelectric pump, it is advantageous to contain the piezoelectric vibrator and a control board (driver) feeding this piezoelectric vibrator with a drive signal in the same housing. Further, as the control board of the piezoelectric pump generates high voltage, in terms of obtaining UL (Underwriters Laboratories Inc.) approval as well, it is essential to contain the control board in the housing. However, with a conventional control board, drive control parts for a piezoelectric vibrator, such as a waveform generating circuit generating a sinusoidal signal for drive control use, a booster circuit boosting an input signal from a power supply, and a high-voltage control circuit feeding the piezoelectric vibrator with a high-voltage drive signal obtained by synthesizing the boosted voltage signal and the sinusoidal signal, are composed of analog circuits. Therefore, the circuit scale is too large to be contained in the housing. JP 8-205563 A discloses the use of a transmitter composed of such analog circuits, as a reference pulse transmitter. In contrast to this, when the drive control parts on the control board are composed of digital circuits, although their smaller circuit scale enables a smaller and thinner control board, a sinusoidal signal for drive control use is generated by a digital waveform, and therefore, a steep voltage change occurs locally, resulting in a non ideal sinusoidal wave (see  FIG. 8C ). When a high-voltage drive signal obtained by synthesizing this non smooth sinusoidal digital signal and the boosted voltage signal is fed to the piezoelectric vibrator, the piezoelectric vibrator responds to the steep voltage change of the high-voltage drive signal, resulting in noise generation. Although, in order to remove the steep voltage change of the sinusoidal digital signal, it is possible to enhance the resolution of the time axis and the voltage axis when generating the sinusoidal digital signal, this is not realistic because the circuit scale becomes enormous in order to achieve such resolution. Further, although JP 6-109068 A and JP 2000-60847 A disclose configurations that use a digital/analog converter to generate signals for a piezoelectric actuator and a piezoelectric ceramic plate simply in order to cancel vibration of an engine of an automobile and external environmental noise, such configurations are used merely to cancel vibration and noise in large-size apparatuses like an automobile and a bioacoustic detecting apparatus, and these patent documents fail to recognize the problems to be overcome when a small-size electronic device is to be used by integrating into a piezoelectric pump that constantly causes a diaphragm to vibrate. 
     SUMMARY 
     The inventors focused on the fact that a smaller and thinner control board can be achieved by configuring the electric drive control parts for the piezoelectric vibrator with the digital circuits, the fact that high-frequency components of the sinusoidal digital signal (steep voltage change portions) cause noise, and the fact that noise can be reduced by removing these high-frequency components and bringing the sinusoidal digital signal closer to an ideal sinusoidal signal. 
     A high-voltage driver according to an aspect of the present invention has a digital waveform generating circuit that has a DC voltage signal as an input and generates a sinusoidal digital signal, an active filter that extracts only low frequency components from the sinusoidal digital signal generated in the digital waveform generating circuit, and a high-voltage control circuit that employs the sinusoidal digital signal after passing through the active filter and generates a high-voltage drive signal. 
     A piezoelectric pump with an integrated driver according to another aspect of the present invention contains, in a single housing, a piezoelectric vibrator and a control board on which drive control parts for the piezoelectric vibrator are mounted, forms a liquid pump chamber on at least one of front and rear faces of the piezoelectric element, and causes the piezoelectric vibrator to vibrate to supply and exhaust a liquid to and from the liquid pump chamber to thereby conduct the pump action. On the control board, a digital waveform generating circuit generating a sinusoidal digital signal for drive control use, an active filter extracting only low-frequency components from the sinusoidal digital signal generated in the digital waveform generating circuit, and a high-voltage control circuit generating a high-voltage drive signal using the sinusoidal digital signal after passing through the active filter and feeding the high-voltage drive signal to the piezoelectric vibrator, are provided. 
     The high-voltage driver can generate a smooth signal waveform without step-like steep voltage changes. In addition, the piezoelectric pump with the integrated driver can reduce noise during the pump operation, and also downsizing can be achieved. 
    
    
     
       BRIEF DESCRIPTION THE DRAWINGS 
         FIG. 1  is a plane view showing a piezoelectric pump according to an embodiment of the invention; 
         FIG. 2  is a back view showing the piezoelectric pump; 
         FIG. 3  is a cross section taken along line III-III of  FIG. 1  and  FIG. 2 ; 
         FIG. 4  is a cross section taken along line IV-IV of  FIG. 1  and  FIG. 2 ; 
         FIG. 5  is an exploded perspective view of the piezoelectric pump; 
         FIG. 6  is a block diagram explaining a drive control system of the piezoelectric pump; 
         FIG. 7  is a circuit configuration example of a second-order active filter of  FIG. 6 ; 
         FIGS. 8A ,  8 B,  8 C,  8 D, and  8 E show signal waveforms at points a-e in  FIG. 6 , respectively; and 
         FIG. 9  is a graph showing a relationship between a drive frequency and a noise value of the piezoelectric pump. 
     
    
    
     LIST OF REFERENCE NUMERALS 
       100  piezoelectric pump,  10  piezoelectric vibrator,  14  first electric supply line,  15  second electric supply line,  18  conductive rubber member,  20  housing,  20 A upper cover,  20 B main housing,  20 C lower cover,  41  circular recessed portion,  45  and  46  electric supply line-containing grooves,  50  drive board,  51  board-containing recessed portion,  52  large cutout,  53  electronic circuit parts,  54  external communication passages,  500  power supply,  501  booster circuit,  502  digital waveform generating circuit,  503  second-order active filter,  504  high-voltage control circuit, A air chamber, DC 1  DC voltage signal (low-voltage signal), DC 2  DC voltage signal (high-voltage signal), P liquid pump chamber, S 1  sinusoidal digital signal, S 2  sinusoidal digital signal (low frequency components only), S 3  high-voltage drive signal. 
     DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  through  FIG. 6  show the entire configuration of a piezoelectric pump  100  according to an embodiment of the present invention. This piezoelectric pump  100  includes a piezoelectric vibrator  10 , a housing  20 , and a drive board  50 . The housing  20  is composed of an upper cover (upper housing)  20 A, a main housing  20 B, and a lower cover (lower housing)  20 C, and, in the main housing B, a circular recessed portion  41  having an opening to the upper housing  20 A side is formed (see  FIG. 3  and  FIG. 5 ), and a board-containing recessed portion  51  having an opening to the lower housing  20 C side is formed (see  FIG. 4  and  FIG. 5 ). Around the circumference of the circular recessed portion  41 , an o-ring containing annular groove  41   a is formed concentrically. 
     As shown in  FIG. 3  and  FIG. 5 , the piezoelectric vibrator  10  has a circular metal shim  11  and a circular piezoelectric body  12  formed on one of the front and back faces of this shim  11 . In this embodiment, the shim  11  faces the liquid pump chamber P side, and the piezoelectric body  12  faces the air chamber A side. 
     The shim  11  is a conductive thin metal plate made of, for example, stainless steel and 42Alloy having a thickness of approximately 30to 300μm, and the piezoelectric body  12  is made of a piezoelectric material such as PZT(Pb(Zr,Ti)O 3 ) having a thickness of approximately 50to 300μm, and has been subjected to polarization processing in the front-rear direction thereof. Such a piezoelectric vibrator is well known. When an alternating electric field (high-voltage drive signal) is applied on the front and back of the piezoelectric body  12 , the cycle in which one of the front and back of the piezoelectric body  12  expands and the other contracts is repeated to thereby cause the shim  11  (piezoelectric vibrator  10 ) to vibrate. 
     As shown in  FIG. 5 , in the piezoelectric vibrator  10 , a first power supply line (lead material) is conductively connected to the circumference of the front face of the piezoelectric body  12  via a conductive rubber member  18 . The conductive rubber member  18  is made of a conductive rubber in which rubber property is maintained and a volume resistivity value is made small. Further, a second electric supply line  15  is connected to a wiring connecting projection  11   c  integrally molded so as to project along the radius direction of the shim  11 . 
     The o-ring  27  is inserted in the o-ring containing annular groove  41   a , and the piezoelectric vibrator  10  is inserted in the circular recessed portion  41  of the main housing  20 B. Then, by placing the upper housing  20 A on the main housing  20 B while providing a circular guide  28  on the circumference of the piezoelectric vibrator  10 , the piezoelectric vibrator  10  is tightly supported in between in a fluid-tight manner. The liquid pump chamber P is provided between this piezoelectric vibrator  10  and the circular recessed portion  41 , and the air chamber (air pump chamber) A is formed between the piezoelectric vibrator  10  and the upper housing  20 A. 
     In the circular recessed portion  41  of main housing  20 B, a suction side liquid-pool chamber  42  and a discharge side liquid-pool chamber  43  are formed and located in positions which are eccentric and symmetric with respect to the plane center of the piezoelectric vibrator  10  (circular recessed portion  41 ). A suction side check valve  32  and a discharge side check valve  33  are provided between the suction side liquid-pool chamber  42  and the liquid pump chamber P, and between the discharge side liquid-pool chamber  43  and the liquid pump chamber P, respectively. Further, in the main housing  20 B, a suction port  24  and a discharge port  25  communicating with these suction side liquid-pool chamber  42  and discharge side liquid-pool chamber  43 , respectively, are formed. 
     The suction side check valve  32  is a suction side check valve that allows fluid flow from the suction port  24  to the liquid pump chamber P and does not allow fluid flow in the reverse direction, and the discharge side check valve  33  is a discharge side check valve that allows fluid flow from the liquid pump chamber P to the discharge port  25  and does not allow fluid flow in the reverse direction. 
     The check valves  32  and  33  have the same configurations and are formed such that umbrellas made of elastic material are mounted on perforated boards  32   a and  33   a bonded to the flow path in a fixed manner, respectively. 
     In the main housing  20 B, electric supply line-containing grooves  45  and  46  are formed in a tubular portion  44  located around the circular recessed portion  41 , each indifferent positions along the circumferential direction of the circular recessed portion  41  ( FIG. 4  and  FIG. 5 ). The electric supply line-containing grooves  45  and  46  allow the first electric supply line  14  and the second electric supply line  15  to pass therethrough, respectively, and have large cross-sections so that sufficient air circulation spaces are secured even when the first electric supply line  14  and the second electric supply line  15  pass respectively therethrough. 
     In the main housing  20 B, a large cutout (air chamber passage or through hole)  52  allowing the air chamber A and the board-containing recessed portion  51  to communicate with each other through the electric supply line-containing grooves  45  and  46  is formed ( FIG. 4  and  FIG. 5 ). As is clear from  FIG. 4 , the top surface of this large cutout  52  is covered by the upper housing A placed on the main housing  20 B. 
     In the main housing  20 B, external communication passages (holes)  54  allowing the board-containing recessed portion  51  to communicate externally are formed. As such, the board-containing recessed portion  51  is in communication with the air chamber A through the large cutout  52  and the electric supply line-containing grooves  45  and  46 , and is externally communicated through the external communication passages  54 . As such, the air chamber A is externally communicated even when the board-containing recessed portion  51  of the main housing  20 B is set with the drive board  50  and is covered by the lower housing  20 C. In other words, when the piezoelectric vibrator  10  vibrates to thereby contract the volume of the air chamber A, an outward flow passing through the electric supply line-containing grooves  45  and  46 , the large cutout  52 , the board-containing recessed portion  51 , and the external communication passage  54  is generated, while when the volume of the air chamber A is expanded, an inward flow passing through the external communication passage  54 , the board-containing recessed portion  51 , the large cutout  52 , and the electric supply line-containing grooves  45  and  46  is generated. 
     On the drive board  50 , electronic circuit parts  53  controlling drive of the piezoelectric vibrator  10  ( FIG. 4  and  FIG. 5 ) and a printed circuit (not shown) connecting these electronic circuit parts  53  are formed. The first electric supply line  14  and the second electric supply line  15 , which are guided outside the air chamber A (circular recessed portion  41 ) through the electric supply line-containing grooves  45  and  46 , are connected to the drive board  50 . Heat generation from the electric circuit parts  53  on the drive board  50  is released outside by the outward air flow passing through the electric supply line-containing grooves  45  and  46 , the large cutout  52 , the board-containing recessed portion  51 , and the external communication passage  54 , or by the inward air flow passing through the external communication passage  54 , the board-containing recessed portion  51 , the large cutout  52 , and the electric supply line-containing grooves  45  and  46 . 
     Next, referring to  FIG. 6  to  FIG. 9 , drive control of the piezoelectric vibrator  10 , which is a feature of the present invention, will be described. 
       FIG. 6  is a block diagram showing the drive control system (electronic circuit parts  53 ) of the piezoelectric vibrator  10 . This drive control system has a power supply  500 , a booster circuit  501 , a digital waveform generating circuit  502 , a second-order active filter  503 , and a high-voltage control circuit  504 . 
     The booster circuit  501  boosts a DC voltage signal (low-voltage signal) DC 1  input from the power supply  500  and outputs a DC voltage signal (high-voltage signal) DC 2  which is higher than this DC voltage signal DC 1  to the high-voltage control circuit  504 . In this embodiment, for example, a DC voltage signal DC 1  of 5V is boosted to a DC voltage signal DC 2  of 200V. Waveforms of the DC voltage signals DC 1  and DC 2  are shown in  FIG. 5A  and  FIG. 8B , respectively. In  FIG. 8 , the longitudinal axis represents a voltage and the horizontal axis represents time. This booster circuit  501  may be provided in the high-voltage control circuit  504 . 
     The digital waveform generating circuit  502  inputs the DC voltage signal DC 1  from the power supply  500  and generates a sinusoidal digital signal S 1  for controlling drive of the piezoelectric vibrator  10 . Frequency and amplitude of the sinusoidal digital signal S 1  can be set appropriately according to the drive behavior of the piezoelectric vibrator  10 .  FIG. 8C  shows a waveform of the sinusoidal digital signal S 1 . Because the sinusoidal digital signal S 1  has a sinusoidal waveform expressed by discontinuous digital values (voltage values), as shown in  FIG. 5C , step-like voltage changes along the time axis, that is, steep voltage changes, occur locally. Although it is possible to bring this sinusoidal digital signal S 1  closer to an ideal continuous sinusoidal waveform by enhancing the resolution in the time axis and the voltage axis, there is a limitation due to the configuration of the digital waveform generating circuit  502 . For the sinusoidal digital signal S 1  in this embodiment, the maximum amplitude (amplitude from a positive peak to a negative peak) Vpp is set to 3V. 
     The second-order active filter  503  has, as an input, the sinusoidal digital signal S 1  generated in the digital waveform generating circuit  502 , cuts off frequency components higher than a predetermined cutoff frequency fc, and extracts only low frequency components equal to or lower than the same cutoff frequency from this sinusoidal digital signal S 1 .  FIG. 8D  shows a signal waveform of a sinusoidal digital signal S 2  after passing through the second-order active filter  503 . The sinusoidal digital signal S 2 , from which the high frequency components are removed by the second-order active filter  503 , has no step-like steep voltage change and has a smooth signal waveform to thereby be closer to an ideal sinusoidal waveform. This sinusoidal digital signal S 2  has a maximum amplitude Vpp of 3V, which is the same as that of the sinusoidal digital signal S 1  before passing through the second-order active filter  503 .  FIG. 7  shows a specific circuit configuration of the second-order active filter  503  composed of an op-amp, resisters R 1  and R 2 , and capacitors C 1  and C 2 . In this case, a cutoff frequency Fc of the second-order active filter  503  is determined as follows: 
     
       
         
           
             
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                 ⁢ 
                 
                   
                     C 
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                     1 
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                     C 
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                     ⁢ 
                     2 
                     ⁢ 
                     r 
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                     1 
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                     r 
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     The high-voltage control circuit  504  synthesizes the DC voltage signal DC 2  boosted in the booster circuit  501  with the smooth sinusoidal digital signal S 2  after passing through the second-order active filter  503 , generates a high-voltage drive signal S 3  at a level that can drive the piezoelectric vibrator  10 , and outputs this high-voltage drive signal S 3  to the piezoelectric vibrator  10 .  FIG. 8E  shows a signal waveform of the high-voltage drive signal S 3 . The high-voltage drive signal S 3  has a smooth signal waveform (sinusoidal waveform) without stepwise steep voltage changes, like the sinusoidal digital signal S 2 . The high-voltage control circuit  504  of this embodiment generates a high-voltage drive signal S 3  having an amplitude (amplitude from 0V to one of positive and negative peaks) Vop of 170V. 
     When the high-voltage drive circuit  504  outputs the high-voltage drive signal S 3 , the piezoelectric vibrator  10  vibrates (elastically deforms) reciprocally based on the high-voltage drive signal S 3 . In the piezoelectric pump  100 , during the process in which the volume of the liquid pump chamber P is expanded by the vibration of the piezoelectric vibrator  10 , the suction side check valve  32  is opened and the discharge side check valve  33  is closed to thereby cause the fluid to flow from the suction port  24  into the liquid pump chamber P, while during the process in which the volume of the liquid pump chamber P is contracted, the discharge side check valve  33  is opened and the suction side check valve  32  is closed to thereby cause the fluid to flow from the liquid pump chamber P to the discharge port  25 . In such a manner, the pumping action is achieved. Because, during this pumping action, the high-voltage drive signal S 3  has the smoothed signal waveform (sinusoidal waveform) without step-like steep voltage change as described above, the vibrations of the piezoelectric vibrator  10  are repeated smoothly and noise is reduced. 
     In the above drive control system, the power supply  500 , the booster circuit  501 , the digital waveform generating circuit  502 , and the second-order active filter  503  constitute a low-voltage section for processing a low-voltage signal (DC voltage signal DC 1 ), and the high-voltage control circuit  504  constitutes a high-voltage section for processing the high-voltage signal (DC voltage signal DC 2 ). The second-order active filter  504  may be provided in the high-voltage section. However, when the second-order active filter  504  is provided in the high-voltage section, lower frequency components are extracted from the high-voltage signal boosted in the booster circuit, and the number of filter component parts thus becomes more than when extracting the lower frequency components from the low-voltage signal (sinusoidal digital signal) before being boosted in the booster circuit, resulting in a disadvantage in reducing the size. In addition, the use of heavy high-voltage parts in the filter component parts becomes essential, and this results in increase in cost. It is therefore desirable to provide the second-order active filter  504  in the low-voltage section like the present embodiment. The drive board on which the above drive control system is mounted is formed as small as 20mm×31mm and 4.5mm in thickness. As such, the piezoelectric pump  100  contains the drive board  50  in the housing  20  and achieves a size as small as 20mm×31mm and 4.5mm in thickness. 
       FIG. 9  shows the result of measuring noise values [dBA] during the pump operation while changing a cutoff frequency fc [Hz] of the second-order active filter  503 . 
     In  FIG. 9 , the dotted line and the dash-dot line are comparative examples and indicate noise values generated when an oscillator is operated at frequencies of 30Hz and 60Hz, respectively. These noise values are measured by amplifying the outputs of the oscillator using an amplifier. As the oscillator, DF1905 from NF Corporation is employed, and as the amplifier, M-2601 from Mess-Tek Co., Ltd. is employed. The noise values measured by this oscillator are approximately 16.9dBA at a drive frequency of 30 Hz and approximately 18.4dBA at a drive frequency of 60 Hz. 
     Further, in  FIG. 9 , the thick solid line (solid line in the horizontal direction in the figure) is a comparative example indicating a noise value generated when the piezoelectric vibrator  10  is vibrated by a conventional drive control system having no second-order active filter  503 . Here, driving the piezoelectric vibrator  10  by the conventional drive control system means that the piezoelectric vibrator  10  is driven by a high-voltage drive signal generated using a sinusoidal digital signal S 1  output from the digital waveform generating circuit  503 . That is, steep voltage changes occur locally in the high-voltage drive signal for driving the piezoelectric vibrator  10  (a state in which high frequency components of the sinusoidal digital signal S 1  are included). In this case, a noise value is 42.8dBA. 
     In  FIG. 9 , the line charts are examples and show the relationship between cutoff frequencies fc and noise values when the piezoelectric vibrator  10  is vibrated at drive frequencies of 30Hz and 60Hz, respectively, by the drive control system (the power supply  50   o , the booster (amplifier) circuit  501 , the digital waveform generating circuit  502 , the second-order active filter  503 , and the high-voltage control circuit  504 ) of the present embodiment. 
     Referring to  FIG. 9 , it will be understood that, when the piezoelectric vibrator  10  is driven at either of the drive frequencies 30Hz and 60Hz, the noise value during the pump operation is much lower than when the piezoelectric vibrator is driven by the drive control system having no second-order active filter. As such, it is obvious that noise during the pump operation can be reduced by employing the second-order active filter  503 . Referring to  FIG. 9  in more detail, it is understood that, when the cutoff frequency fc is lower than 1.6kHz, noise during the pump operation becomes smaller than the noise value by the oscillator, and when the cutoff frequency fc is equal to or greater than 1.6kHz, noise during the pump operation becomes greater than the noise value by the oscillator. This trend is the same both when the piezoelectric vibrator  10  is driven at a drive frequency of 30Hz and when the piezoelectric vibrator  10  is driven at a drive frequency of 60Hz. In the present embodiment, the noise value by the oscillator serves as a reference noise level, and the cutoff frequency fc of the second-order active filter  503  is set so that a noise value during the pump operation does not exceed this reference noise level. In other words, the cutoff frequency fc is set so as to have an upper-limit frequency of 1.6kHz at which a noise value during the pump operation is the same as the reference noise level. It is preferable to set a lower limit cutoff frequency fc to a level that does not influence the drive frequency region of the piezoelectric vibrator  10 . Further, although in this example the second-order active filter was employed, it is preferable to employ the first active filter when a difference between a drive frequency of the piezoelectric pump and a frequency of target noise is large and to employ a second- or higher-order active filter when a difference between a drive frequency and a frequency of target noise is small. However, because the circuit scale becomes larger as the order of the active filter is higher, it is preferable to employ an active filter having a lower order. 
     As described above, because the present embodiment has the second-order active filter  503  which cuts off high frequency components causing noise during the pump operation and extracts low frequency components from non smooth sinusoidal digital signal S 1  having steep voltage changes locally, it is possible to generate the high-voltage drive signal S 3  having a smooth sinusoidal waveform without steep voltage changes using the sinusoidal digital signal S 2  (low frequency components only) after passing through the second-order active filter  503 . This high-voltage drive signal S 3  then causes the piezoelectric vibrator  10  to vibrate and repeats the vibration of the piezoelectric vibrator  10  smoothly to thereby reduce noise during the pump operation. Because it is possible to reduce noise during the pump operation in such a manner, by containing the control board  50  on which the drive control parts for the piezoelectric vibrator  10  are configured with the digital circuits, and reduced in size and thickness, and the piezoelectric vibrator  10  in the single housing  20 , it is possible to thereby achieve a small-sized piezoelectric pump with integrated a driver.