US20110014069A1

US20110014069A1 - Piezoelectric fan device and air-cooling apparatus using the piezoelectric fan device

Info

Publication number: US20110014069A1
Application number: US12/889,439
Authority: US
Inventors: Hiroaki Wada; Gaku Kamitani
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 2008-03-25
Filing date: 2010-09-24
Publication date: 2011-01-20
Also published as: WO2009119431A1; JPWO2009119431A1; CN101978172A; JP5170238B2

Abstract

A piezoelectric fan device includes four piezoelectric fans that are arranged side-by-side in their width direction, and ends of the piezoelectric fans opposite to other ends in which the blades extend are coupled side-by-side and supported by a support. The piezoelectric fans include piezoelectric vibrators arranged to flexurally vibrate when a voltage is applied thereto and blades coupled to the piezoelectric vibrators such that the blades can be excited by the piezoelectric vibrators. Two central piezoelectric fans are driven in the same phase and the other two piezoelectric fans are driven in the opposite phase by a voltage applying device. Accordingly, not only vibrations of the centroid acting on the support but also moments about three axes are cancelled out.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a piezoelectric fan device that produces wind using a blade that is flexurally displaced to a large extent by flexurally vibrating a piezoelectric vibrator coupled to the blade.
2. Description of the Related Art
Recently, for portable electronic equipment, with the advancement of miniaturization and high-density mounting of components, devices for dissipating heat generated inside the electronic equipment have become an issue. An example of a device that efficiently dissipates heat in such electronic equipment is an air-cooling apparatus that uses a piezoelectric fan.
Japanese Unexamined Patent Application Publication No. 2-19700 discloses a piezoelectric fan that includes a piezoelectric bimorph vibrator including a pair of plate piezoelectric elements and a thin metal plate disposed therebetween and bonded thereto, a thin elastic plate is bonded to both ends of the piezoelectric bimorph vibrator so as to extend in a direction perpendicular to the piezoelectric bimorph vibrator, and the central portion of the piezoelectric bimorph vibrator is supported and fixed by a support portion.
With this structure, because the central portion of the single piezoelectric bimorph vibrator is supported by the support portion, the piezoelectric vibrator on both sides of the support portion is deformed in a bilaterally symmetrical manner. That is, when the left portion with respect to the support portion is deformed so as to project upward, the right portion is also deformed so as to project upward, and the centroid of the left portion and that of the right portion with respect to the support portion always move in the same direction in a direction perpendicular to the piezoelectric vibrator surface. For a reaction force caused by vibration of a piezoelectric member to the support portion, the reaction force caused by movement of the left portion and the reaction force caused by movement of the right portion act in the same direction, so the support portion experiences twice as much vibration as when only a single side exists. As a result, the support portion vibrates very easily, the vibration is conveyed to other portions which adversely affects the reliability of other portions and connections. When that piezoelectric fan is used to discharge warm air in the gaps of many dissipating fins of a heat sink, for example, the configuration of the piezoelectric fan is relatively large which restricts the locations at which the piezoelectric fan can be arranged.
Japanese Unexamined Patent Application Publication No. 2002-339900 discloses a piezoelectric fan that causes an air generating vibrator to vibrate in a similar manner as a round fan using a piezoelectric element to discharge warm air in the gaps of many dissipating fins of a heat sink. In this case, the piezoelectric fan includes an air generating plate that is fixed between a pair of piezoelectric elements displaced in opposite directions, the air generating plate protrudes a relatively large distance from a first end of the piezoelectric elements, and a second end of the piezoelectric elements is fixed to a casing (see FIG. 2 of Japanese Unexamined Patent Application Publication No. 2002-339900). Thus, the centroid of the entire piezoelectric fan largely vibrates together with the vibration of the air generating plate. Accordingly, a large vibration and moment act on a support portion that supports the piezoelectric elements, and the vibrations of the piezoelectric elements are conveyed directly to the body of the casing. This arrangement produces noise and decreases the durability of the casing. If the piezoelectric elements are fixed to the casing with an elastic body, such as rubber, disposed therebetween, the influence of the vibrations on the casing can be suppressed. However, the rigidity of the support portion is reduced, such that the amplitude of the air generating plate is significantly reduced and thus a desired volume of air cannot be obtained.
Japanese Unexamined Utility Model Registration Application Publication No. 62-122199 discloses a blower in which a plurality of piezoelectric fans are supported in a parallel arrangement and phases of alternating voltages supplied to the piezoelectric fans are alternately opposite to each other. In this case, the piezoelectric fans arranged in the width direction are driven in alternately opposite phases, such that the volume of air can be increased as compared to when they are driven in the same phase. In addition to vibrations of the centroid, moments about three axes including the longitudinal-direction axis, width-direction axis, and thickness-direction axis act on a support that supports the plurality of piezoelectric fans. In Japanese Unexamined Utility Model Registration Application Publication No. 62-122199, when the piezoelectric fans are driven in alternately opposite phases and the number of the piezoelectric fans is an even number, vibrations of the centroid in vertical directions cancel each other out and the moments about the width-direction and thickness-direction axes also substantially cancel each other out. However, the moment about the longitudinal-direction axis is not cancelled out, such that a load is imposed on the support. This creates a problem in that the support may rotationally vibrate, and the vibration may influence other elements through the support or may produce noise. Additionally, the vibration of the support means that a portion of vibration energy produced by the piezoelectric fans is lost.
FIG. 13 illustrates three axes for a blower related to Japanese Unexamined Utility Model Registration Application Publication No. 62-122199. For the sake of simplifying the description, four piezoelectric fans 101 to 104 are used. In FIG. 13, X is the longitudinal-direction axis, Y is the width-direction axis, and Z is the thickness-direction axis. As indicated by the arrows D1 to D4, when the piezoelectric fans 101 to 104 are driven in mutually opposite phases, the moments about the Y-axis and Z-axis are substantially zero, and virtually no load is imposed on a support 105. In contrast, about the X-axis, a counterclockwise moment ML caused by the second and third piezoelectric fans 102 and 103 occurs, and a clockwise moment MR caused by the first and fourth piezoelectric fans 101 and 104 occurs. However, the distance from the X-axis to the first and fourth piezoelectric fans 101 and 104 is greater than the distance to the second and third piezoelectric fans 102 and 103, such that the moment MR is greater than the moment ML. The difference in moment produces rotational vibration in the support 105.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of the present invention provide a piezoelectric fan device that includes a plurality of small piezoelectric fans and that reduces vibration of a support caused by a piezoelectric vibrator.
A preferred embodiment of the present invention provides a piezoelectric fan device including a plurality of piezoelectric fans arranged side-by-side in a width direction thereof, each of the plurality of piezoelectric fans including a piezoelectric vibrator and a blade, the piezoelectric vibrator being arranged to flexurally vibrate in response to the application of a voltage, the blade being coupled to or integral with the piezoelectric vibrator, the blade being capable of being excited by the piezoelectric vibrator, and a support that couples and supports ends of the plurality of piezoelectric fans side-by-side, the ends being opposite to other ends in which the blades extend. The piezoelectric fan device further includes a voltage applying device arranged to apply a voltage to each of the piezoelectric vibrators such that, with respect to a piezoelectric fan that is centrally located in the width direction or with respect to a gap between piezoelectric fans that are centrally located in the width direction, driving directions for the piezoelectric fans at both sides are axisymmetric and such that a driving direction of one half of the plurality of piezoelectric fans is in an opposite phase to a driving direction of a remaining one half of the plurality of piezoelectric fans.
If the plurality of piezoelectric fans having the same vibration characteristics are arranged side-by-side and supported by the support and the piezoelectric fans are driven in alternately opposite phases, vibrations of the centroid of the piezoelectric fans cancel each other out, and vibrations occurring in the support can be reduced. However, the moment about the longitudinal direction is not cancelled out, the support vibrates, and the vibrations influence other elements through the support. With preferred embodiments of the present invention, a voltage is applied to each of the piezoelectric vibrators such that, with respect to a centrally located piezoelectric fan of the plurality of piezoelectric fans (when the number of piezoelectric fans is odd) or with respect to a gap between centrally located piezoelectric fans thereof when the number of the piezoelectric fans is an even number, driving directions of the piezoelectric fans at both sides are axisymmetric and such that a driving direction of one half of the plurality of piezoelectric fans is in an opposite phase to a driving direction of a remaining one half of the plurality of piezoelectric fans. Accordingly, not only vibrations occurring in the support but also moments about the three axes can be effectively cancelled or reduced, such that rotational vibrations of the support is suppressed. This enables the influence of vibrations of the support produced by vibrations of the blades on, for example, the casing to be efficiently suppressed, such that a low-noise high-reliability piezoelectric fan is produced. Because the influence of vibrations of the piezoelectric fans on the outside is suppressed, electric energy input into the piezoelectric vibrators can be efficiently converted into vibrations of the blades, such that an increase in the volume of air and an improvement in cooling efficiency are achieved. Additionally, because a load resulting from vibrations to the support is small, even if the rigidity of the fixing portion that fixes the support to, for example, the casing is relatively low, each of the blades can be driven with large amplitude. Therefore, even if a slight vibration occurs in the support, the vibration can be absorbed by a vibration absorber, and the influence on the outside can be suppressed. That is, an increase in the volume of air and a suppression of adverse effects associated with the vibrations are achieved.
The piezoelectric fans included in preferred embodiments of the present invention preferably have the same vibration characteristics. The same vibration characteristics referred to herein indicate that the piezoelectric fans have substantially the same resonant frequency and amplitude characteristics obtained when each of the piezoelectric fans is vibrated alone. It is preferable that the piezoelectric fans have the same or substantially the same shape. If the width of a piezoelectric element is increased or reduced in accordance with an increase or reduction in the width of a corresponding blade, equal or substantially equal vibration characteristics can preferably be obtained. Therefore, as long as the same vibration characteristics are obtained, it is not necessary to use blades having the same width.
Each of the piezoelectric vibrators in preferred embodiments of the present invention is arranged to flexurally vibrate when an alternating voltage is applied thereto, and various configurations may be used. For example, a unimorph piezoelectric vibrator can be configured to include a blade and a single-plate piezoelectric element bonded to a first-end main surface of the blade. Alternatively, a bimorph piezoelectric vibrator can be configured to include a blade and two piezoelectric elements capable of expanding and contracting in opposite directions bonded to both surfaces of the blade. Alternatively, independent of a blade, a piezoelectric vibrator can be configured to include a single-plate piezoelectric element and a metal plate bonded together, and the piezoelectric vibrator can be fixed to the blade. Although the amplitude associated with flexural vibration of the piezoelectric vibrator is relatively small, the amplitude of the piezoelectric vibrator can be amplified many times by resonating of the blade coupled to the piezoelectric vibrator. The blade may be a metal plate, or may also be a resin plate, for example. The thickness, length, and Young's modulus of the blade can be appropriately set such that the blade can be primarily resonated by the vibration of the piezoelectric vibrator. The voltage applying device can apply voltages having opposite phases to the piezoelectric vibrators in order to drive the piezoelectric fans in mutually opposite phases. Alternatively, if the polarization directions of the piezoelectric elements included in the piezoelectric vibrators are opposite, the piezoelectric fans can be driven in opposite phases with the application of voltages having the same phase.
The number of piezoelectric fans is not limited to an even number, so may be an odd number. When an odd number of piezoelectric fans are provided, two halves of the piezoelectric fans except for the single piezoelectric fan located centrally in the width direction are driven in opposite phases. In such an odd-number configuration, the effects of vibration of the centroid are present. However, the effects are reduced with an increase in the number of piezoelectric fans. When the number of piezoelectric fans is even, it is preferable that the even number be a multiple of four, for example, 4, 8, or 12. In this case, an even number of piezoelectric fans is provided at each of both sides with respect to the center in the width direction, such that it is easy to cancel out vibrations of the centroid and moments about three axes.
When four piezoelectric fans are arranged side-by-side, it is preferable that the two central piezoelectric fans are driven in the same phase and the two piezoelectric fans at both ends are driven in the opposite phase to the two central piezoelectric fans. In this case, the structure can be simplified, and vibrations of the centroid and moments about three axes can be effectively cancelled out.
Each of the piezoelectric fans may preferably include an elongated strip blade and a piezoelectric element fixed to an end of the blade in the longitudinal direction thereof, the end of the blade in the longitudinal direction and the piezoelectric element may preferably define the piezoelectric vibrator, and the end of the blade in the longitudinal direction may preferably be coupled and supported by the support. In this case, the blade is directly supported by the support, and thus, the piezoelectric element is not restrained by the support, such that the piezoelectric element can be displaced more freely. The structure of the piezoelectric fan can be simple so that variations in the vibration characteristics of piezoelectric fans can be reduced.
Each of the piezoelectric vibrators may preferably include a first vibrator and a second vibrator, a first end of the first vibrator and a first end of the second vibrator in their longitudinal direction may be coupled together, a second end of the first vibrator in the longitudinal direction may be coupled to the blade, a second end of the second vibrator in the longitudinal direction may be supported by the support, and the voltage applying device may be connected so as to enable the first vibrator and the second vibrator to flexurally vibrate in opposite directions. In this case, the amplitude can be doubled by providing the two vibrators and the blade vibrates with the vibration, such that the amplitude of the blade can be further amplified. As a result, a large increase in the volume of air can be achieved. It is preferable that the piezoelectric fan device according to a preferred embodiment of the present invention and a heat sink be used in combination. That is, the piezoelectric fan device may preferably be arranged in the vicinity of a heat sink that includes a plurality of dissipating fins spaced apart and arranged side-by-side and the blades may preferably be disposed in the gaps of the dissipating fins such that directions in which the blades are displaced are parallel to the sides of the dissipating fins. In this case, warm air in the gaps of the dissipating fins can be pulled by flexural displacement of the blades and efficiently discharged in the longitudinal direction of the blades. Because the blades are isolated by the dissipating fins, interaction of the blades through the air can be prevented, and no load caused by an unexpected vibration mode is applied to the support.
As described above, with preferred embodiments of the present invention, because the plurality of piezoelectric fans are arranged side-by-side, supported by the support, and driven such that, with respect to the center in the width direction, driving directions for the piezoelectric fans at both sides are axisymmetric and such that one half of the plurality of piezoelectric fans is in an opposite phase to a remaining one half of the plurality of piezoelectric fans, both vibrations of the centroid and moments about three axes are suppressed. Accordingly, the amplitude of each of the blades can be increased, cooling efficiency can be improved, and vibration propagation to other portions caused by the vibrations through the support can be reduced. As a result, adverse effects on the reliability of other components and a casing are greatly reduced, and noise is greatly reduced.
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

FIG. 1 is a perspective view of a reference example of a piezoelectric fan device.

FIG. 2 is a perspective view of the piezoelectric fan device illustrated in FIG. 1 in a driven state.

FIG. 3 illustrates a vibration model of the piezoelectric fan device illustrated in FIG. 1.

FIG. 4 is a perspective view of a piezoelectric fan device according to a first preferred embodiment of the present invention.

FIG. 5 is a perspective view of the piezoelectric fan device illustrated in FIG. 4 in a driven state.

FIG. 6 is a cross-sectional view of an air-cooling apparatus in which the piezoelectric fan device illustrated in FIG. 4 and a heat sink are combined.

FIG. 7 is a side view of the air-cooling apparatus illustrated in FIG. 6 as seen from its longitudinal direction.

FIGS. 8A to 8C illustrate moments on a support of the piezoelectric fan device illustrated in FIG. 4.

FIGS. 9A to 9C illustrate an experimental structure, in which FIG. 9A is a plan view, FIG. 9B is a front view, and FIG. 9C is a right side view.

FIGS. 10A and 10B illustrate a comparison of amplitudes of an end of a blade when the thickness of a coupler is 0.3 mm and 0.6 mm.

FIGS. 11A and 11B illustrate how a piezoelectric fan device that includes eight piezoelectric fans is driven.

FIGS. 12A to 12D illustrate various configurations of a piezoelectric fan.

FIG. 13 is an illustration for describing three-axis moments for a conventional piezoelectric fan device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before a piezoelectric fan device according to preferred embodiments of the present invention is described, the basic structure of a piezoelectric fan device is described with reference to FIGS. 1 to 3.
In FIG. 1, two piezoelectric fans 1 a and 1 b having the same vibration characteristics are arranged side-by-side in their width direction on a support 6 and are coupled and fixed thereto. The piezoelectric fans 1 a and 1 b include blades 2 a and 2 b and piezoelectric vibrators 3 a and 3 b coupled to first ends of the blades 2 a and 2 b in their longitudinal direction, respectively. The blades 2 a and 2 b are capable of being freely flexurally displaced in their thickness direction. The piezoelectric vibrators 3 a and 3 b are flexurally vibrated when a voltage is applied. Weights 4 a and 4 b are fixed to free ends of the blades 2 a and 2 b, respectively. Each of the piezoelectric vibrators 3 a and 3 b preferably is a bimorph vibrator in which piezoelectric elements are bonded to both sides of a metal plate that defines an intermediate electrode. The piezoelectric vibrators 3 a and 3 b are electrically connected to a voltage applying device 5. Application of an alternating voltage from the voltage applying device 5 to the piezoelectric vibrators 3 a and 3 b causes the piezoelectric vibrators 3 a and 3 b to flexurally vibrate, causes the blades 2 a and 2 b to primarily resonate, and thus, causes the blades 2 a and 2 b to be flexurally displaced vertically to a greater extent than the piezoelectric vibrators 3 a and 3 b. Ends of the piezoelectric fans opposite to the direction in which the blades 2 a and 2 b extend, that is, ends at which the piezoelectric vibrators 3 a and 3 b are arranged, are coupled and supported side-by-side by a support 6. The support 6 is fixed to a fixing portion (not shown), such as a casing, for example. Reaction forces produced by movements of the blades 2 a and 2 b in opposite phases are conveyed in the width direction of the support 6 and an action in which the reaction forces cancel each other out occurs. Thus, the support 6 must have enough rigidity to convey the reaction forces.
The voltage applying device 5 includes an alternating-current power supply 5 a and lines 5 b and 5 c to supply signals having reversed phases from the power supply 5 a to the piezoelectric vibrators 3 a and 3 b therethrough. That is, a first end of the alternating-current power supply 5 a is connected to upper and lower electrodes of the piezoelectric vibrator 3 a and to an intermediate electrode of the piezoelectric vibrator 3 b through the line 5 b, and a second end of the alternating-current power supply 5 a is connected to an intermediate electrode of the piezoelectric vibrator 3 a and to upper and lower electrodes of the piezoelectric vibrator 3 b through the line 5 c. Accordingly, when the free end of the first piezoelectric vibrator 3 a is displaced downward, the free end of the second piezoelectric vibrator 3 b is displaced upward. As a result, as illustrated in FIG. 2, the blade 2 a, which is coupled to the free end of the first piezoelectric vibrator 3 a, and the blade 2 b, which is coupled to the free end of the second piezoelectric vibrator 3 b, are displaced in mutually opposite phases, and a current of air indicated by the arrow A in FIG. 2 occurs. Because the two piezoelectric fans 1 a and 1 b have the same or substantially the same vibration characteristics (e.g., length, thickness, and resonant frequency), both blades 2 a and 2 b have substantially the same vibration frequency and amplitude. It is to be noted that, because the weights 4 a and 4 b are fixed to the free ends of the blades 2 a and 2 b, the resonant frequency is lower and the amplitude is greater than those of a single blade.
The reason that vibrations of the centroid acting on the support 6 can be reduced by driving the two piezoelectric fans 1 a and 1 b in opposite phases is provided below. Vibrations of the piezoelectric vibrator 3 and the blade 2 cause the centroid of the single piezoelectric fan 1 to periodically move in the thickness direction (z) and the longitudinal direction (x). Simplified vibration modeling of a piezoelectric fan illustrated in FIG. 3 is discussed below. Here, it is assumed that the concentrated mass M (centroid) is located at the distance R from the support 6 and a piezoelectric fan vibrates with θ=Θ sin ωt. In this case, the movements of the centroid in the x and z directions can be represented as follows:
x=R cos(Θ sin ωt)
z=R sin(Θ sin ωt)
Here, the amplitude in the x direction is (R/2)sin Θ tan Θ, and the amplitude in the z direction is R sin Θ. That is, when Θ is small, the amplitude in the x direction is the square of a small quantity, so as to be negligible. In contrast, for a piezoelectric fan that vibrates in an opposite phase, the movements can be as follows:
x=R cos(−Θ sin ωt)=R cos(Θ sin ωt)
z=R sin(−Θ sin ωt)=R sin(Θ sin ωt)
The centroid when these two piezoelectric fans are combined can be calculated as follows:
$\begin{matrix} x = (MR \cos (Θ \sin ω t) + MR \cos (Θ \sin ω t)) / (M + M) \\ = R \cos (Θ \sin ω t) \end{matrix}$ $\begin{matrix} z = (MR \sin (Θ \sin ω t) + MR \sin (Θ \sin ω t)) / (M + M) \\ = 0 \end{matrix}$
Therefore, with a pair of the two piezoelectric fans 1 a and 1 b moving in mutually opposite phases, the centroid does not vibrate in the thickness direction. Accordingly, no force in the thickness direction acts on the support 6 supporting the pair of the two piezoelectric fans. Vibration in the longitudinal direction remains, but it is at a negligible level when Θ is small, as previously described. Thus, forces acting on the support 6 are substantially cancelled out.
As previously described, loads on the support 6 can be cancelled out by driving the two piezoelectric fans 1 a and 1 b in opposite phases, but vibrations caused by reaction forces resulting from vibrations of the blades 2 a and 2 b occur in the support 6. However, unlike a traditional piezoelectric fan, even if the support 6 is not strongly fixed to, for example, a casing, sufficient amplitude of each of the blades 2 a and 2 b is obtainable. That is, it is possible to provide a vibration absorber, such as a vibration absorber made of rubber, for example, between the support 6 and the casing. Accordingly, the influence of vibrations occurring in the support 6 on, for example, the casing can be effectively suppressed, and a low-noise high-reliability piezoelectric fan can be obtained.
FIG. 1 illustrates an example in which the piezoelectric fans 1 a and 1 b are fixed to the support 6 with the same orientation in the thickness direction and the voltage applying devices 5 applies voltages of opposite phases to the piezoelectric fans 1 a and 1 b. If the piezoelectric fans 1 a and 1 b are fixed to the support 6 in opposite orientations in the thickness direction, voltages of the same phase may be applied. Alternatively, if the piezoelectric vibrators 3 a and 3 b of the piezoelectric fans 1 a and 1 b have opposite characteristics, that is, if the piezoelectric elements included in the piezoelectric vibrators 3 a and 3 b have opposite polarization directions, even with the application of voltages of the same phase to the piezoelectric vibrators 3 a and 3 b, the piezoelectric fans 1 a and 1 b can be driven in opposite phases.

First Preferred Embodiment

FIGS. 4 to 7 illustrate a piezoelectric fan device according to a first preferred embodiment of the present invention that is used as an air-cooling apparatus for a heat sink. In FIG. 4, four piezoelectric fans 10 a to 10 d having the same or substantially the same vibration characteristics are spaced uniformly in their thickness direction on a support 11 and coupled and fixed thereto. The piezoelectric fans 10 a to 10 d have substantially the same structure as the piezoelectric fans 1 a and 1 b illustrated in FIG. 1. That is, the piezoelectric fans 10 a to 10 d include blades 12 a to 12 d and bimorph piezoelectric vibrators 13 a to 13 d coupled to first ends of the blades 12 a to 12 d in their longitudinal direction, respectively. The blades 12 a to 12 d are capable of being freely flexurally displaced in their width direction. The piezoelectric vibrators 13 a to 13 d are flexurally vibrated when a voltage is applied. Weights 14 a to 14 d are fixed to free ends of the blades 12 a to 12 d, respectively. The piezoelectric vibrators 13 a to 13 d are connected to the voltage applying device 15. Application of an alternating voltage from the voltage applying device 15 to each of the piezoelectric vibrators 13 a to 13 d causes the piezoelectric vibrators 13 a to 13 d to vibrate and causes the blades 12 a to 12 d to resonate. Ends of the piezoelectric vibrators 13 a to 13 d opposite to the direction in which the blades 12 a to 12 d extend are coupled and supported side-by-side by the support 11.
In this preferred embodiment, the voltage applying device 15 includes an alternating power supply 15 a and lines 15 b and 15 c to supply signals having opposite phases from the power supply 15 a to the piezoelectric vibrators 13 a to 13 d therethrough. That is, a first end of the alternating-current power supply 15 a is connected to upper and lower electrodes of each of the first and fourth piezoelectric vibrators 13 a and 13 d and to an intermediate electrode of each of the second and third piezoelectric vibrators 13 b and 13 c through the line 15 b, and a second end of the alternating-current power supply 15 a is connected to an intermediate electrode of each of the first and fourth piezoelectric vibrators 13 a and 13 d and to upper and lower electrodes of each of the second and third piezoelectric vibrators 13 b and 13 c through the line 15 c. Accordingly, when the first and fourth piezoelectric vibrators 13 a and 13 d are displaced downward, the second and third piezoelectric vibrators 13 b and 13 c are displaced upward. As a result, as illustrated in FIG. 5, the blades 12 a and 12 d, which are coupled to the first and fourth piezoelectric vibrators 13 a and 13 d, and the blades 12 b and 12 c, which are coupled to the second and third piezoelectric vibrators 13 b and 13 c, are displaced in mutually opposite phases. Because the piezoelectric fans 10 a to 10 d have the same or substantially the same vibration characteristics (e.g., length, thickness, and resonant frequency), all of the blades 12 a to 12 d also have an equal or substantially equal vibration frequency and amplitude.
A heat sink 20, which includes five dissipating fins 21 a to 21 e that are spaced apart and arranged side-by-side, is disposed in the vicinity of the piezoelectric fans 10 a to 10 d. The blades 12 a to 12 d are disposed in the gaps between the dissipating fins 21 a to 21 e and arranged such that the directions of displacement of the blades 12 a to 12 d are parallel or substantially parallel to sides of the dissipating fins 21 a to 21 e. As illustrated in FIGS. 6 and 7, the heat sink 20 is arranged so as to be thermally coupled to the upper surface of a heating element (e.g., central processing unit (CPU)) 23 mounted on a circuit board 22. Accordingly, heat produced from the heating element 23 is conveyed to the heat sink 20, and air in the gaps of the dissipating fins 21 a to 21 e is heated. Because the blades 12 a to 12 d disposed in the gaps of the dissipating fins 21 a to 21 e are displaced in parallel to the sides of the dissipating fins 21 a to 21 e, warm air in the gaps of the dissipating fins 21 a to 21 e is pulled by the blades and is discharged to the longitudinal direction of the blades 12 a to 12 d. As a result, as indicated by the arrows B in FIG. 6, due to a current of air along the longitudinal direction of the blades 12 a to 12 d, heat in the gaps of the dissipating fins 21 a to 21 e can be efficiently discharged, and an air-cooling apparatus having improved dissipation effects is obtained. Because neighboring blades are displaced in opposite phases, there is a possibility that an unexpected vibration mode, such as a twist, may be produced in the blades 12 a to 12 d by their interaction through the air. However, because the blades 12 a to 12 d are isolated by the dissipating fins 21 b to 21 d, as illustrated in FIG. 7, the interaction between the blades through the air is eliminated, such that no unexpected load is applied on the support 11.
In the present preferred embodiment, two halves of the four piezoelectric fans 10 a to 10 d vibrate in opposite phases. For the same reason illustrated in FIG. 3, vibrations of the centroid acting on the support 11 can be substantially zero. Additionally, with the present preferred embodiment, the two central piezoelectric fans 10 b and 10 c are driven in the same phase, and the two piezoelectric fans 10 a and 10 d at both ends are driven in an opposite phase to the two central piezoelectric fans 10 b and 10 c. Therefore, moments about three axes of the support 11 are cancelled out. The reason for this is described with reference to FIGS. 8A to 8C.
FIG. 8A illustrates the four piezoelectric fans 10 a to 10 d as seen from the longitudinal direction (X direction). When the fans 10 a to 10 d are driven in the directions indicated by the arrows D1 to D4 about the longitudinal-direction axis (X-axis), a clockwise moment is produced in the first and third piezoelectric fans 10 a and 10 c and a counterclockwise moment is produced in the second and fourth piezoelectric fans 10 b and 10 d. These moments are the same or substantially the same, such that the moments cancel each other out. Thus, the moments about the longitudinal-direction axis become zero or substantially zero.
FIG. 8B illustrates the piezoelectric fans 10 a to 10 d as seen from the width direction (Y direction). About the width-direction axis (Y-axis), a counterclockwise moment is produced in the first and fourth piezoelectric fans 10 a and 10 d and a clockwise moment is produced in the second and third piezoelectric fans 10 b and 10 c. Therefore, the moments cancel each other out, and the moments about the width-direction axis also become zero or substantially zero.
FIG. 8C illustrates the piezoelectric fans 10 a to 10 d as seen from the thickness-direction axis (Z direction). As previously described, when the piezoelectric fans 10 a to 10 d are driven, vibrations in the longitudinal direction of the piezoelectric fans are at a negligible level, such that the moments themselves about the thickness-direction axis (Z-axis) in the piezoelectric fans are small. Additionally, because the moments acting on the first and second fans 10 a and 10 b cancel each other out and the moments acting on the third and fourth fans 10 c and 10 d also cancel each other out, the moments about the thickness-direction axis (Z-axis) also become zero or substantially zero. In this manner, all of the moments about the three axes acting on the support 11 cancel each other out, such that a support structure that experiences much less vibrations and loads is obtained.
Note that it is preferable that, for a piezoelectric fan, the torsional rigidity of a coupler satisfy the following relationship in order to obtain large blade amplitude.
D>kmAf²LW
where
D: torsional rigidity [Nm²/rad]
m: mass of the fan other than the coupler [kg]
A: amplitude of the blade end (tip-to-tip) [m]
f: driving frequency [Hz]
L: length of the fan [m]
W: width of the coupler [m]
k: coefficient
When the three-axis moments are cancelled out, if the coefficient k is a value that is about 10 or greater, for example, propagation of vibration can be further reduced, and adverse effects on the reliability of other components and the casing are reduced and the noise is reduced.
FIGS. 9A to 9C illustrate an experimental structure prepared to check the cooling performance of a piezoelectric fan device. Four piezoelectric fans 30 a to 30 d include elongated strip blades 31 a to 31 d, respectively. First ends of these blades in the longitudinal direction are fixed to first ends of holders 33 a to 33 d. Piezoelectric elements 32 a to 32 d are fixed in the vicinity of the first ends of the blades fixed to the holders and define piezoelectric vibrators. Second ends of the holders 33 a to 33 d are coupled to a coupler (support) 34 extending in the width direction. The coupler 34 extends in one width direction and is fixed to a fixing portion 35.
For the piezoelectric fan device having the above structure, the amplitude of the blade end in cases having four relationships of phases of vibrations of the fans (when the phase of the fan 1 is 0°) shown in Table 1 was measured. A voltage applied to a piezoelectric member was constant at about 45 Vpp, 42Ni was used as the blade, and a glass epoxy plate was used as each of the holder and the coupler. To observe the effects of the torsional rigidity of a coupler, two different couplers having thicknesses of about 0.3 mm and about 0.6 mm were used. The dimensions of each component are provided in FIGS. 9A to 9C. CASE 1 is an example in which all fans are driven in the same phase; CASE 2 is an example in which one half of the fans at the right-hand side and the other half of the fans at the left-hand side with respect to the center are driven in opposite phases; CASE 3 is an example in which the fans are driven in alternately opposite phases; and CASE 4 is an example in which the fans are driven in axisymmetrically opposite phases with respect to the center in the width direction according to a preferred embodiment of the present invention.

TABLE 1

FAN 1	FAN 2	FAN 3	FAN 4

CASE 1	0°	0°	0°	0°
CASE 2	0°	0°	180°	180°
CASE 3	0°	180°	0°	180°
CASE 4	0°	180°	180°	0°

FIGS. 10A and 10B illustrate the amplitude of the blade end in CASE 1 to CASE 4 when the couplers having thicknesses of about 0.3 mm and about 0.6 mm are used. As shown in FIGS. 10A and 10B, the amplitude in CASE 2 is greater than in CASE 1, and the amplitude in CASE 3 is greater than in CASE 2. It is discovered that the amplitude of the blade in CASE 4, in which three-axis moments can be cancelled out, is the greatest.
For CASE 1, it is necessary to support vibrations of all of the fans by the coupler, such that large amplitude is not obtainable if the rigidity of the coupler is decreased. For CASE 2, the fans are supported by reaction forces from the fans moving in opposite phases through the coupler. However, only vibrations of the centroid cancel each other out, and the moments remain. Accordingly, if the coupler is rigid, vibrations of the centroid can be suppressed to some extent, and an amplitude greater than that in CASE 1 is obtainable. However, because the moments are not cancelled out and thus the coupler rotationally vibrates, if the rigidity of the coupler is decreased, the amplitude greatly decreases. CASE 3 is similar to CASE 2, but in CASE 3 the distance between the fans moving in opposite directions is less than in CASE 2. Thus, the moments are smaller and the rotational vibration is also smaller. Accordingly, with the same rigidity of the coupler, an amplitude greater than that in CASE 2 is obtainable. For CASE 4, both the vibrations of the centroid and vibrations of the moments can be cancelled out in the coupler, such that an amplitude greater than that in CASE 3 is obtainable.
The difference between the amplitudes of the blade in CASE 3 and in CASE 4 is relatively small for the coupler having a thickness of about 0.6 mm, whereas the amplitude difference is on the order of about 5% for the coupler having a thickness of about 0.3 mm. The amplitude difference between CASE 3 and CASE 4 increases with a reduction in the torsional rigidity of the coupler. An amplitude difference of about 5% is relatively small numerically, but it may result in a difference of approximately 15% in cooling performance, depending on the location of the fans relative to the heat source. Accordingly, if the rigidity of the coupler supporting the fans is reduced in order to suppress the influence of vibration on the outside, the cooling performance varies greatly.
FIGS. 11A and 11B illustrate an example of a driving method for use when eight piezoelectric fans are arranged side-by-side. First to eighth fans 41 to 48 are spaced uniformly in their width direction and coupled and supported by a support (not shown). The arrows D1 to D8 indicate driving directions thereof. The X-axis indicates the longitudinal-direction axis in the center in the width direction, and the Z-axis indicates the thickness-direction axis. In FIG. 11A, the two central fans 44 and 45 are driven in the same phase, and the other fans are driven in an opposite phase to its neighboring fan in an alternating manner. In this case, the vibrations of the centroid in the Z direction cancel each other out, and the moments about the X-axis, Y-axis, and Z-axis cancel each other out. Therefore, even with relatively low rigidity of the support coupling the fans 41 to 48, a large amplitude is obtainable. In FIG. 11B, the fans 41 and 48 at both ends and the two central fans 44 and 45 are driven in the same amplitude, and the second, third, sixth, and seventh fans 42, 43, 46, and 47 are driven in the opposite phase thereto. Also in this case, the vibrations of the centroid cancel each other out and the moments about three axes also cancel each other out, such that the rigidity of the support can be reduced and large amplitude is obtainable.
FIGS. 12A to 12D illustrate configurations of the piezoelectric fan structure. A piezoelectric fan 50 illustrated in FIG. 12A is an example of a unimorph vibrator in which a first-end main surface of a blade 52 made of a metal plate is bonded to a first main surface of a single-plate piezoelectric element 51. An end of the piezoelectric fan 50 opposite to another end at which the blade projects is fixed to a support 53. Application of an alternating voltage between the piezoelectric element 51 and the blade 52 causes the piezoelectric fan 50 to be flexurally deformed as a whole by the expanding and contracting piezoelectric element 51 and the unexpanded and uncontracted blade 52. In this case, a first electrode of the piezoelectric element 51 can be shared with the blade 52.
A piezoelectric fan 60 illustrated in FIG. 12B is an example of a bimorph vibrator in which piezoelectric elements 62 and 63 are bonded to both first-end main surfaces of a blade 61 made of a metal plate. An end of the piezoelectric fan 60 opposite to another end at which the blade projects is fixed to a support 64. When the piezoelectric elements 62 and 63 are polarized in the same thickness direction, the application of an alternating voltage between the blade 61 and each of the electrodes of both main surfaces causes the piezoelectric fan 60 to be flexurally deformed as a whole.
A piezoelectric fan 70 illustrated in FIG. 12C includes a first vibrator 71 and a second vibrator 72, a first end of the first vibrator 71 and a first end of the second vibrator 72 in the longitudinal direction are coupled together with a spacer 73 disposed therebetween so as to provide a U-shaped structure, a second end of the first vibrator 71 in the longitudinal direction is coupled to a blade 74, and a second end of the second vibrator 72 in the longitudinal direction is supported by a support 75. The first vibrator 71 and the second vibrator 72 are vibrators having the same or substantially the same vibration characteristics and are flexurally displaced in mutually opposite directions. For example, when the first vibrator 71 is flexurally displaced so as to project upward, the second vibrator 72 is flexurally displaced so as to project downward. Vibrations having twice the amplitude of that for each of the vibrators 71 and 72 are provided to the blade 74, and the amplitude of the blade 74 is increased correspondingly. Therefore, a large increase in the volume of air can be achieved.
A piezoelectric fan 80 illustrated in FIG. 12D is a variation of the piezoelectric fan 70 illustrated in FIG. 12C. The same reference numerals are used in FIG. 12D for the same components as in FIG. 12C, and redundant description is omitted. A blade 81 coupled to the second end of the first vibrator 71 in the longitudinal direction is bent into a V shape. In this case, the two vibrators 71 and 72 provide a U-shaped structure, and the blade 81 is bent with respect to the vibrators 71 and 72. Therefore, the length dimension can be reduced, and a compact piezoelectric fan as a whole can be provided.
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 the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.

Claims

1. A piezoelectric fan device comprising:

a plurality of piezoelectric fans arranged side-by-side in a width direction thereof, each of the plurality of piezoelectric fans including a piezoelectric vibrator and a blade, the piezoelectric vibrator being arranged to flexurally vibrate when a voltage is applied thereto, the blade being coupled to or integrally provided with the piezoelectric vibrator, the blade being arranged to be excited by the piezoelectric vibrator;

a support arranged to couple and support ends of the plurality of piezoelectric fans side-by-side, the ends being opposite to other ends from which the blades extend; and

a voltage applying device arranged to apply a voltage to each of the piezoelectric vibrators such that, with respect to a piezoelectric fan of the plurality of piezoelectric fans centrally located in the width direction or with respect to a gap between piezoelectric fans of the plurality of piezoelectric fans that are centrally located in the width direction, driving directions for the piezoelectric fans at both sides are axisymmetric and such that a driving direction for one half of the plurality of piezoelectric fans is in an opposite phase relative to a driving direction for a remaining one half of the plurality of piezoelectric fans.

2. The piezoelectric fan device according to claim 1, wherein a number of the plurality of piezoelectric fans is a multiple of four.

3. The piezoelectric fan device according to claim 2, wherein the piezoelectric fans are arranged side-by-side in a group or groups of four, two central piezoelectric fans are driven in the same phase, and two piezoelectric fans on both ends of the two central piezoelectric fans are driven in an opposite phase relative to the two central piezoelectric fans.

4. The piezoelectric fan device according to claim 1, wherein

each of the piezoelectric fans includes an elongated strip blade and a piezoelectric element fixed to an end of the elongate strip blade in a longitudinal direction thereof;

the end of the elongate strip blade in the longitudinal direction and the piezoelectric element define the piezoelectric vibrator; and

the end of the elongate strip blade in the longitudinal direction is coupled and supported by the support.

5. The piezoelectric fan device according to claim 1, wherein each of the piezoelectric vibrators includes a first vibrator and a second vibrator, a first end of the first vibrator and a first end of the second vibrator in longitudinal directions thereof are coupled together, a second end of the first vibrator in the longitudinal direction thereof is coupled to the blade, a second end of the second vibrator in the longitudinal direction thereof is supported by the support, and the voltage applying device is connected so as to enable the first vibrator and the second vibrator to flexurally vibrate in opposite directions.

6. An air-cooling apparatus in which the piezoelectric fan device according to claim 1 is arranged in the vicinity of a heat sink that includes a plurality of dissipating fins spaced apart and arranged side-by-side and the blades are disposed in gaps between the dissipating fins such that directions in which the blades are displaced are parallel or substantially parallel to sides of the dissipating fins.