Piezoelectric actuator

A piezo-electric actuator comprising: piezo-electric element 1a having piezo-electric body 3a which is provided with at least two opposing surfaces, wherein the surfaces perform an expanding and contracting motion in accordance with the state of an electric field; a constraint member 21a for constraining piezo-electric element 1a on at least one of the two surfaces, a supporting member disposed around constraint member 21a, and a plurality of beam members 22a each having both ends fixed to constraint member 21a and supporting member 4a, respectively, wherein each beam member has a neutral axis for bending in a direction substantially parallel with the constrained surface, wherein the constraint member vibrates by vibration which is generated by the constraining effect between the constraint member and the piezo-electric element, and is amplified by the beam members.

TECHNICAL FIELD

The present invention relates to a small-size piezo-electric actuator which is used in electronic devices.

BACKGROUND ART

Electromagnetic actuators have been generally utilized as driver components for acoustic elements such as speakers, due to their easy handling. An electromagnetic actuator comprises a permanent magnet, a voice coil, and a diaphragm, and causes a low-stiffness diaphragm that is made of an organic film and is fixed to the coil to vibrate, through the operation of a magnetic circuit in a stator which uses the magnet. Therefore, they present a reciprocal vibration mode and can provide large vibration amplitude.

By the way, the demand for power-saving actuators has been increasing, together with an increased demand for cellular phones and personal computers in recent years. However, electromagnetic actuators have the problem that the reduction in power consumption is difficult due to the large amount of current which flows in the voice coil to generate magnetic force. Further, despite the need for a reduction in size of actuators for mounting in a cellular phone or a personal computer, it is difficult to reduce the thickness due to its configuration, because, if a permanent magnet in an electromagnetic actuator, which is one of the components of the actuator, is reduced in thickness, orientation of the magnetic poles will not align, causing failure in ensuring stable a magnetic field, and thus resulting in difficulties in controlling the synchronization of the vibrating film and the voice coil. Further, magnetic flux may leak from the voice coil and may induce malfunctions in other electronic components which constitute the electronic device. Thus, an electromagnetic shield is required when applying the actuator to an electronic device. However, this shield requires a large space. For this reason as well, an electromagnetic actuator is not suitable for use in small devices such as a cellular phone. Additionally, there is the problem that if a voice coil is made of thinner wire, and has increased resistance, the voice coil may be burnt due to the large amount of current, which features the electromagnetic acoustic element, to drive the coil.

Thus, a piezo-electric actuator which employs a piezo-electric element as a driver component, having such features as small size, light weight, low power consumption, no leakage of magnetic flux, and so on, is desired as a thin vibration element, instead of an electromagnetic type vibration element. A piezo-electric actuator generates vibration through the expanding and contracting motion or the bending motion of a piezo-electric element that is in the shape of a thin plate. A piezo-electric actuator is fabricated by bonding a piezo-electric ceramic element to a base, as disclosed in the specification of Japanese Patent Laid-open Publication No. 168971/86.

An example of a conventional piezo-electric actuator is illustrated inFIGS. 1A,1B.FIG. 1Aillustrates an exploded perspective view of a piezo-electric actuator. Piezo-electric body203made of piezo-electric ceramics is fixed to the central region of circular base202to form piezo-electric element201. The outer periphery of base202is supported by circular supporting member204. As a predetermined AC voltage is applied to piezo-electric body203, piezo-electric body203performs an expanding and contracting motion. A bending motion is induced in base202in an out-of-plane direction to generate vibration through the constraining effect of the fixed portion between piezo-electric body203and base202. As illustrated inFIG. 1B, base202vibrates in an out-of-plane direction, with supporting member204fixed (as node) and the central portion moving as an antinode.

By the way, because a piezo-electric ceramic has high stiffness, a piezo-electric actuator has the problem that it vibrates only in small average amplitude as compared with an electromagnetic actuator. In particular, a piezo-electric actuator, which is fixed along its periphery and which has an arc-shaped vibration mode in which the central portion deforms dominantly, deforms only in small amplitude on average, making it even more difficult to achieve sufficient amplitude of vibration. Further, due to the high stiffness of the piezo-electric ceramic, the amplitude of vibration varies significantly around the resonance frequency, so that it is difficult to achieve vibration amplitude having flat frequency characteristic.

Further, the resonance frequency of the piezo-electric actuator largely depends on its shape. When a piezo-electric actuator is applied to low frequency acoustic components such as a loud speaker, the piezo-electric ceramic element must be either enlarged in area or extremely reduced in thickness in order to lower the resonance frequency. However, due to the brittleness of the ceramic material, enlargement in area or reduction in thickness may causes deterioration in reliability such as cracking during handling, breakage due to dropping, and the like. This makes the piezo-electric actuator unsuitable for practical use in many cases.

Additionally, when the actuator is applied to an electronic device, due to the large vibration reaction force of a piezo-electric ceramic, vibration tends to propagate to a housing, which contains the piezo-electric actuator, through support members. This leakage of vibration may cause the disadvantage that the housing generates abnormal sound.

Thus, to address the foregoing problems, the specification of Japanese Patent Laid-open Publication No. 2000-140759 discloses a technique in which a vibrator having a piezo-electric ceramic and a base is supported by springs along the periphery of the housing. The resonance frequency of the spring structure is set at near the resonance frequency of the vibrator. Since a large amount of energy is carried in the spring structure, large amplitude of vibration can be obtained.

For similar purposes, the specification of Japanese Patent Laid-open Publication No. 2001-17917 discloses a technique in which slits are provided in the peripheral region of a base along its circumference to form leaf springs in order to provide a similar function.

DISCLOSURE OF THE INVENTION

According to the technique disclosed in the specification of Japanese Patent Laid-open Publication No. 2000-140759, displacement of the vibration of the piezo-electric body is largely increased. However, since springs have to be arranged in a direction perpendicular to the plane of the vibrator to allow perpendicular movement of the vibrator, the thickness of the piezo-electric actuator is increased. Therefore, this technique is less suitable for a reduction in thickness. Further, since springs and a diaphragm are inserted in the housing according to the configuration in this patent document, it is very difficult to arrange the diaphragm at an optimal position.

On the other hand, in the technique disclosed in the specification of Japanese Patent Laid-open Publication No. 2001-17917, it is necessary that a circular base is combined with circular piezo-electric ceramic or rectangular piezo-electric ceramic, because it is difficult to form leaf springs if the base is substantially not circular. In the former case, since the piezo-electric ceramic has to be machined into a circular shape, the fabrication steps and the cost will increase because of machining the ceramic into a circular shape, and because forming the larger extra portion in advance worsens yield rate, etc. On the other hand, in the latter case, since the piezo-electric ceramic cannot be arranged on the peripheral region of the base in an effective fashion, vibration does not transmit efficiently to the base, making it difficult to obtain sufficient vibration displacement. Further, in both cases, slits that are formed on a disk to form leaf springs induce rotational motion in the support member for the piezo-electric ceramic during operation. This causes distortion in sound when a vibratory film is attached for use as an acoustic element.

In view of the foregoing situations, it is an object of the present invention to provide a small and thin piezo-electric actuator which is capable of generating vibration at a large amplitude, is adjustable for resonance frequency, is provided with high reliability, and is applicable to electronic devices, without causing an increase in dimensions.

To solve the aforementioned problems, a piezo-electric actuator of the present invention has a piezo-electric element having a piezo-electric body with at least two opposing surfaces which perform expanding and contracting motions in accordance with the state of an electric field, a constraint member for constraining the piezo-electric element on at least one of the two surfaces, a supporting member disposed around the constraint member, and a plurality of beam members each having both ends that are fixed to the constraint member and the supporting member, respectively, and each having a neutral axis for bending in a direction substantially parallel with the constrained surface.

In the piezo-electric actuator thus configured, vibration is caused by the constraining effect between the constraint member and the piezo-electric element, and is amplified by the beam members. Then the constraint member vibrates. Specifically, if vibration is induced at a resonance frequency, which is determined by physical properties, shape, number of constraint member, weight of the piezo-electric body, etc., the constraint member is significantly displaced, while deformation of the piezo-electric body, which has a limited capacity of deformation, is restricted. Thus, it is possible to cause the entire piezo-electric body to vibrate relative to the supporting members at a large amplitude. Further, the resonance frequency can be easily controlled by adjusting the physical properties (material), number etc. of the constraint member. Accordingly, the present invention can provide a piezo-electric actuator that is thin and small, is capable of generating large vibration amplitude, is adjustable for resonance frequency without changing outer dimensions, and has high reliability.

The beam members may be straight beams. The constraint member may have a base for constraining the piezo-electric element, and a plurality of arms which extend from the base and constitute the beam members.

The constraint member may also be a second piezo-electric element which differs in vibrating direction from the piezo-electric body.

Also, the piezo-electric element may have a plurality of piezo-electric bodies and a plurality of electrode layers for applying an electric field to the piezo-electric bodies, wherein each piezo-electric body and each electrode layer is alternately laminated.

Further, the piezo-electric element may have a rectangular parallelepiped shape.

An acoustic element of the present invention has the piezo-electric actuator described above, and a vibrating film coupled to the piezo-electric actuator for radiating sound by vibration that is transmitted from the piezo-electric actuator.

Also, the acoustic element of the present invention may further have a vibration transmitting member sandwiched between the piezo-electric actuator and the vibrating film.

An electronic device of the present invention has the piezo-electric actuator or acoustic element described above.

An acoustic apparatus of the present invention has a plurality of acoustic elements which have resonance frequencies that are different from each other for smoothing frequency response of sound pressure. Also, an electronic device of the present invention has the acoustic apparatus.

As described above, according to the piezo-electric actuator of the present invention, the entire piezo-electric body vibrates at a large amplitude relative to the supporting members mainly through displacement of the constraint member. Also, the resonance frequency can be easily controlled by adjusting the physical property (material), number etc. of the constraint member. Further, even in case that an electronic device which contains the piezo-electric actuator is dropped, the constraint member, made of an elastic material, can mitigate the impact to the piezo-electric body by absorbing the impact energy. In this way, according to the present invention, a piezo-electric actuator can be provided that is thin and small, is capable of generating large vibration amplitude, is adjustable for resonance frequency without changing outer dimensions, and has high reliability.

DESCRIPTION OF REFERENCE NUMERALS

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, embodiments of the present invention will be described with reference to the drawings.FIG. 2is an exploded perspective view of a piezo-electric actuator according to a first embodiment of the present invention. Piezo-electric element1ahas upper electrode layer31aand lower electrode layer32athat adhere to the opposing surfaces of piezo-electric body3amade of ceramic. As an adhesive, an epoxy-based adhesive, for example, may be used. Piezo-electric body3a, which is substantially in a rectangular parallelepiped shape, is polarized in the thickness direction indicated by a white arrow in the figure. Piezo-electric body3ais fixed to rectangular base21avia lower electrode layer32a. Specifically, the piezo-electric element has a piezo-electric body which includes at least two opposing surfaces that perform an expanding and contracting motion in accordance with the state of an electric field, and base21ais a constraint member for constraining the piezo-electric element by at least one of the two surfaces. Base21amay be made of a variety of materials which have a lower stiffness than ceramic material which constitutes piezo-electric body3a, such as metals including an aluminum alloy, phosphor bronze, titanium, a titanium alloy etc., and such as resins including epoxy, acrylic, polyimide, polycarbonate resin etc. Piezo-electric body3aneed not be in a rectangular parallelepiped shape, but may be in other shapes such as a cylindrical shape, for example, depending on the relationship to the mounting space.

Supporting member4aprovided with a rectangular hole therein is arranged around the periphery of base21a. Beam members22aconnect supporting member4aand base21a. Beam members22aextend from each side of base21ato the opposing side of supporting member4a, with both ends fixed to base21aand supporting member4a, respectively at the joints. Beam member22amay be fabricated of a material similar to that of base21a.

However, supporting member4ais not limited to a particular shape. For example, an annular member (seeFIG. 12) may be used instead of a rectangular shape with a hole. As another alternative, beam members22aand base21amay be integrated without fabricating the members separately. For example, cross-shaped base21bmay be used. As shown inFIG. 3, piezo-electric element la is arranged in the intersecting region, and four straight arms (beam members22b) which surround the region and extend from the respective sides thereof are fixed to surrounding supporting member4b, whereby each arm functions as beam member22b, and similar effects can be obtained. In such a configuration, beam members22bcan be integrally formed as part of base21bby just cutting away four corners of a rectangular base material, thereby improving productivity of piezo-electric actuators, and reliability as well, because the joints that connect the region for piezo-electric element1awith beam members22bare less susceptible to aging deteriorations.

Beam members22abend and deform such that entire piezo-electric element1avibrates in the out-of-plane direction of base21a. The vibration system consisting of piezo-electric element1aand beam members22ahas a natural frequency for bending vibration in the out-of-plane direction of base21a, and resonates and vibrates at a natural frequency in the up-and-down direction at a large amplitude. The natural frequency is determined by the physical properties (mainly, Young's modulus), cross-sectional shape, length, and the number of beam members22a, as well as the weights of the base and piezo-electric body3a, and so on. A detailed description will be given next on the mechanism to generate vibration.

First, as an AC electric field is applied to upper electrode layer31aand lower electrode layer32aof piezo-electric element1a, piezo-electric element1aperforms an expanding and contracting motion. Specifically, piezo-electric element1aalternately repeats, in accordance with the orientation of the electric field, a deformation mode in which piezo-electric body3ais compressed (a deformation mode in which the surfaces, to which upper electrode layer31aand lower electrode layer32aare fixed, are expanded, while the height of piezo-electric body3a(the spacing between upper electrode layer31aand lower electrode layer32a) is reduced) and a deformation mode in which piezo-electric body3aelongates in the height direction (a deformation mode in which the surfaces, to which upper electrode layer31aand lower electrode layer32aare fixed, are contracted, while the height of piezo-electric body3ais increased). As a result, when the fixing surfaces expand, the surface of base21adeforms to bend in a direction opposite to piezo-electric body3aby the constraint between base21aand piezo-electric body3a. Conversely, when the fixing surfaces contract, the surface of base21adeforms to bend towards piezo-electric body3a. With these motions, the peripheral edge of base21avibrates up and down, which motions are transmitted to a plurality of beam members22aattached to base21a. Since beam members22aare fixed to supporting member4abeam members22aand piezo-electric element1a, supported by beam members22a, vibrate in the up-and-down direction at a large amplitude about fixed supporting member4a.

FIG. 4conceptually illustrates the vibration mode of the piezo-electric actuator. Since the deformation of beam members22ais relatively large, while the deformation of piezo-electric body3ais relatively small, the resulting vibration mode presents a piston type, rather than an arc-shaped vibration mode as illustrated inFIG. 1B. In this way, a large reciprocal movement of piezo-electric element1ain the vertical direction can be induced without causing large deformation or distortion in piezo-electric body3a.

The piezo-electric actuator of the present invention further has the following advantages.

First, the vibration characteristics of the piezo-electric actuator of the present invention can be easily adjusted by changing the material characteristics, the number, the width, and the length etc. of beam members22a. Therefore, when a piezo-electric actuator having different vibration characteristics is fabricated, the resonance frequency can be easily changed simply by modifying beam members22a, without changing the outer dimensions. Further, the standardization and the common use of the elements in a wider range contribute to a reduction in cost as well.

Secondly, since there is less limitation for the configuration of piezo-electric element3aand supporting member4a, the piezo-electric actuator of the present invention excels at being adaptable to the space of a device in which the piezo-electric actuator is installed. Particularly, the piezo-electric actuator of the present invention excels in productivity, as compared to a piezo-electric actuator with a circular piezo-electric element, because the piezo-electric actuator of the present invention utilizes piezo-electric element3ain a rectangular shape, and thus base21aand beam members22acan be formed in simple shapes as well.

Thirdly, since the resonance frequency of the piezo-electric actuator can be lowered without significantly reducing the thickness of an expensive piezo-electric element, the strength of the piezo-electric element can be readily ensured. Further, the conventional piezo-electric actuator is susceptible to breakage such as cracks due to impact distortion that the ceramic part receives when an electronic device which contains the piezo-electric actuator is dropped, whereas, in the present invention, the impact distortion to the ceramic portion can be avoided because the impact distortion is absorbed mainly by beam members22a, resulting in higher mechanical reliability. Because of these advantages, low-frequency acoustic elements can be easily produced at a low cost.

Fourthly, since beam members22aare completely bonded and fixed to supporting member4a, the joints serve as vibration nodes when the piezo-electric actuator vibrates. Consequently, the vibration is less apt to propagate from the piezo-electric actuator toward an electronic device through the joints, resulting in higher reliability with less possibility of fatigue fracture and generation of abnormal sound due to vibration of the joints.

As described above, according to the present invention, a piezo-electric actuator can be provided which has a simple structure, high reliability and productivity as well as the capability of easily generating vibration at a large amplitude.

Additionally, application of the piezo-electric actuator of the present invention is not limited to cellular phones. The piezo-electric actuator of the present invention, for example, can provide functional components such as camera modules with a highly accurate zooming function and a focus adjusting function against hand shaking and so on by adjusting the displacement or the vibration amplitude by the amount of electricity applied to the piezo-electric actuator. Accordingly, the industrial value of electronic devices which contain the piezo-electric actuator of the present invention will be enhanced as well.

FIG. 5illustrates a conceptual cross-sectional view of a piezo-electric actuator according to a second embodiment of the present invention.FIG. 6illustrates a vibration mode of the piezo-electric actuator of this embodiment. A piezo-electric actuator which is formed by adhering together two piezo-electric bodies that are polarized in the thickness direction of the piezo-electric bodies is generally called a bimorph. This embodiment is an application of the concept of the present invention to a bimorph. As illustrated inFIG. 5, piezo-electric element1cis a laminated structure in which upper piezo-electric body3cand lower piezo-electric body3c′ are bonded, with insulating layer36sandwiched in between. More specifically, upper piezo-electric body3cis sandwiched between upper electrode layer31cand lower electrode layer32c. Lower piezo-electric body3c′ is sandwiched between upper electrode layer31c′ and lower electrode layer32c′. Insulating layer36is disposed between lower electrode layer32cand upper electrode layer31c′. In other words, this piezo-electric actuator has a second piezo-electric body which has lower piezo-electric body3c′, upper electrode layer31c′, and lower electrode layer32c′. Further, upper piezo-electric body3cand lower piezo-electric body3c′ are polarized in directions opposite to each other, as indicated by white arrows in the figure. In an alternative embodiment, base21amay be used as insulating layer36. Specifically, the piezo-electric actuator may have the structure that upper electrode layer31a, piezo-electric body3a, and lower electrode layer32aare arranged in mirror symmetry under base21ain the first embodiment.

As an AC electric field is applied to piezo-electric element1c, either of upper piezo-electric body3cor lower piezo-electric body3c′ expands while the other contracts, so that piezo-electric element1ccan perform self bending vibration through a mutual constraining effect between upper piezo-electric body3cand lower piezo-electric body3c′, as illustrated inFIG. 6. Therefore, there is no need for a base in piezo-electric element1cof this embodiment. Further, when the same AC voltage as the first embodiment is applied to each electrode, the field strength and the driving force doubles, respectively, and vibration amplitude quadruples.

FIG. 7illustrates a conceptual cross-sectional view of a piezo-electric actuator according to a third embodiment of the present invention. Though only piezo-electric elements are depicted, beam members and supporting members can be configured in a similar manner, for example, to the first embodiment. Piezo electric element1dis formed in a laminated structure in which piezo-electric bodies3dand electrode layers31dare alternately laminated. Each piezo-electric body3dis polarized in an alternately opposite direction, and also is electrically connected such that the electric fields are oriented in an alternately opposite direction. Thus, as electric fields are applied, all piezo-electric bodies3ddeform in the same manner, and as a result, the vibration amplitude increases in proportion to the number of piezo-electric body layers.

FIG. 8illustrates a conceptual cross-sectional view of a piezo-electric actuator according to a fourth embodiment of the present invention. This embodiment is made by providing the second embodiment with insulating layers on both sides of piezo-electric bodies and in the central portion of the actuator. Specifically, upper piezo-electric body3eis sandwiched between upper electrode layer31eand lower electrode layer32e, and lower piezo-electric body3e′ is sandwiched between upper electrode layer31e′ and lower electrode layer32e′. Next, upper insulating layer33eis disposed on upper electrode layer31e, and lower insulating layer33e′ is disposed under lower electrode layer32e′. Further, intermediate insulating layer35is disposed between lower electrode layer32eand upper electrode layer31e′. Such a layer configuration prevents electric leakage to the base even if a metal base is used for bonding, and allows for safe handling.

FIG. 9illustrates a conceptual cross-sectional view of a piezo-electric actuator according to a fifth embodiment of the present invention. Piezo-electric element1fof this embodiment includes vibrating film34that is bonded to the underside of base21f. Paper or an organic film such as polyethylene terephthalate may be used as a base material for vibrating film34. Vibrating film34suppresses sharp variation in vibration amplitude around the resonance frequency, making it possible to produce acoustic elements, such as a loud speaker and a receiver, with flat sound pressure and frequency characteristics. If an organic film, which is an insulating material, is used as a base material for vibrating film34, a metal wire that is connected to piezo-electric element21fcan be formed on the base material by a plating technique or the like, so that the metal wire can be utilized as an electric terminal lead. This configuration improves reliability, as well, because electric conduction through electrode materials can be avoided. Alternatively, vibrating film34may be disposed between piezo-electric element1fand base21f.

A vibrating film may be bonded to base21fvia a material that transmits vibration such as rubber, foamed rubber or the like. Higher effects for flattening frequency characteristics can be accomplished. Alternatively, a plurality of piezo-electric actuators which differ in resonance frequency to each other may be bonded to a vibrating film for application to an electric device. The resulting acoustic device can exhibit a flat sound pressure over a wide range of frequencies.

EXAMPLES

In order to evaluate the effects of the present invention, the characteristics of the piezo-electric actuator of the present invention were evaluated based on the following Examples 1-9 and Comparative Examples 1-4. Evaluation Items are as follows.(Evaluation 1) Measurement of resonance frequency: The resonance frequency was measured when 1V AC voltage was applied.(Evaluation 2) Maximum amplitude of the vibration velocity: The maximum amplitude of the vibration velocity at the resonance frequency was measured when 1V AC voltage was applied.(Evaluation 3) Average amplitude of the vibration velocity: As illustrated inFIG. 10, amplitudes of the vibration velocity were measured at20measured points (indicated by1-20in the figure) equally spaced in the longitudinal direction of piezo-electric element1, and an average value for them was calculated.(Evaluation 4) Vibration Mode: As illustrated inFIGS. 11A,11B, a vibration mode was evaluated, using a vibration velocity ratio that is defined as the average amplitude of the vibration velocity Vm divided by maximum amplitude of the vibration velocity Vmax. Curves in the figures represent the distribution of vibration velocity amplitudes. A small vibration velocity ratio means a bending (arc-shaped) motion as shown inFIG. 11A. A large vibration velocity ratio means a reciprocal (piston-type) motion as shown inFIG. 11B. In this specification, motion was defined to be reciprocal when the vibration velocity ratio was 80% or more, while it was defined to be bending when the vibration velocity ratio was less than 80%.(Evaluation 5) Q-value: The Q-value at the resonance frequency was measured when 1V AC voltage was applied. The frequency characteristic of sound pressure becomes flatter as the Q-value becomes lower.(Evaluation 6) Measurement of sound pressure level: The sound pressure level was measured when 1V AC voltage was applied.(Evaluation 7) Drop impact test: A cellular phone to which a piezo-electric actuator was mounted was dropped from just above 50 cm five times to perform a drop impact stability test. Specifically, a visual inspection was made for fractures such as cracks following the drop impact test, and additionally, sound pressure characteristic was measured after the test.

A piezo-electric actuator illustrated inFIGS. 12A,12B was fabricated.FIG. 12Aillustrates a top plan view of a base, beam members, and a supporting member. Values in the figure are in units of millimeters.FIG. 12Bin turn illustrates an exploded perspective view of the piezo-electric element. The piezo-electric actuator of Example 1 has piezo-electric element101a, base121a, supporting member104a, and beam members122a. Piezo-electric element101ais bonded to base121awith epoxy-based adhesive, while base121ais connected to supporting member104avia four beam members122a.

As illustrated inFIG. 12B, piezo-electric element101ais a single-layer type piezo-electric element consisting of upper insulating layer133a, upper electrode layer131a, piezo-electric body103a, lower electrode layer132a, and lower insulating layer133a′. Upper insulating layer133aand lower insulating layer133a′ have a length of 10 mm, a width of 10 mm, and a thickness of 50 μm. Piezo-electric body103ahas a length of 10 mm, a width of 10 mm, and a thickness of 300 μm. Upper electrode layer131aand lower electrode layer132ahave each a thickness of 3 μm. Therefore, piezo-electric element101ais in the form of a 10 mm square and has a thickness of approximately 0.4 mm.

Lead zirconate titanate based ceramic was used for piezo-electric body103a, upper insulating layer133a, and lower insulating layer133a′, while a silver/palladium alloy (in weight ratio of 70%:30%) was used for upper electrode layer131aand lower electrode layer132a. The piezo-electric element was manufactured by a green sheet method, and was sintered at 1100° C. for two hours in the atmosphere. Then, silver electrodes with a thickness of 8 μm were formed as external electrodes that were connected to the electrode layers, then piezo-electric body103awas polarized. Then electrode pads136athat were formed on the surface of upper insulating layer133awere connected together by copper foils with a thickness of 8 μm, then two electrode terminal lead lines115with a diameter of 0.2 mm were bonded to the pads through solder portions (not shown) having a diameter of 1 mm and a height of 0.5 mm.

Base121ais made of phosphor bronze with a thickness of 0.05 mm. Base121awas formed into the shape shown inFIG. 12Athrough cutting. Four beam members122aattached to base121aare made of SUS304, and all members have the same shape with a width of 4 mm, a length of 4 mm, and a thickness of 0.2 mm. Beam members122aare connected to annular supporting member104a.

The piezo-electric actuator of this example fabricated in the foregoing manner is a small and thin piezo-electric actuator in a circular shape having a diameter of 16 mm and a thickness of 0.45 mm. This piezo-electric actuator provided a reciprocal vibration mode as illustrated inFIG. 11B, with a resonance frequency of 529 HZ, a maximum amplitude of the vibration velocity of 180 mm/s, and a maximum vibration velocity ratio of 0.83.

Comparative Example 1

In order to confirm the effects of Example 1, a conventional piezo-electric actuator illustrated inFIG. 13was fabricated. Piezo-electric element1101ahaving a length of 16 mm, a width of 8 mm, and a thickness of 0.4 mm was fabricated in a way similar to Example 1, then metal plate1105(phosphor bronze, with a thickness of 0.1 mm) was bonded to fabricate the piezo-electric actuator, then both ends were connected by supporting member1104a.

The fabricated piezo-electric actuator provided an arc-shaped vibration mode as illustrated inFIG. 11A, with a resonance frequency of 929 HZ, a maximum amplitude of the vibration velocity of 1480 mm/s, and a maximum vibration velocity ratio of 0.47.

It was confirmed from the comparison between Example 1 and Comparative Example 1, that a piezo-electric actuator having a low resonance frequency, large vibration amplitude, and a flat vibration amplitude can be provided.

The piezo-electric actuator of Example 2 has base121b, supporting member104b, and beam members122b. In Example 2, the number of beam members122battached to the base was changed from four in Example 1 to two in order to confirm the degree of reduction in the resonance frequency. As illustrated inFIG. 14, conditions were the same as in Example 1 except for the number of beam members122b. The piezo-electric actuator had a circular form having a diameter of 16 mm and a thickness of 0.45 mm. Values in the figure are in units of millimeters. The piezo-electric actuator provided a reciprocal vibration mode, with a resonance frequency of 498 HZ, a maximum amplitude of the vibration velocity of 172 mm/s, and a maximum vibration velocity ratio of 0.86.

It was confirmed from the comparison between Examples 1 and 2, that the resonance frequency can be lowered by changing the number of beam members without causing a large change in the vibration mode or in the vibration velocity amplitude.

In Example 3, the configuration of Example 2 was used, while the material of the base was changed from phosphor bronze to SUS304. The other conditions are the same as in Example 2. The piezo-electric actuator provided a reciprocal vibration mode, with a resonance frequency of 572 HZ, and a maximum amplitude of the vibration velocity of 189 mm/s.

It was confirmed from the comparison between Examples 2 and 3, that the resonance frequency can be adjusted by changing the material of the base without causing a large change in the shape, vibration mode, and maximum amplitude of the vibration velocity of the actuator.

In Example 4, a bimorph type piezo-electric actuator was fabricated using two piezo-electric elements which differed in vibrating direction. As illustrated inFIG. 15, the piezo-electric actuator of Example 4 has a base121cand piezo-electric element101cwhich has piezo-electric bodies103c,103c′ which are in the same shape and are bonded such that they vibrate in different directions. Piezo-electric bodies103c,103c′ are in the form of a 10 mm square having a thickness of 0.2 mm. Therefore, piezo-electric element101cis the same as Example 2 in shape. Also, the configuration except for the piezo-electric element is the same as Example 2.

The piezo-electric actuator provided a reciprocal vibration mode, with a resonance frequency of 487 HZ, and a maximum amplitude of the vibration velocity of 352 mm/s.

It was confirmed from the comparison between Examples 2 and 4, that the maximum vibration displacement can be largely increased by using a bimorph type piezo-electric element which has two piezo-electric plates that are bonded together and vibrate in different directions.

In Example 5, the piezo-electric element was changed from the single type in Example 2 to laminated layers. The laminate type piezo-electric element101dof this example is a three-layer type. As illustrated inFIG. 16, it consists of upper insulating layer133d, four electrode layers131d, three piezo-electric bodies103d, and lower insulating layer133d′ which are laminated. Upper insulating layer133dand lower insulating layer133d′ are in the form of a 10 mm square with a thickness of 80 .mu.m. Piezo-electric bodies103dare in the form of a 10 mm square with a thickness of 80 .mu.m. Electrode layers131dare in the form of a 10 mm square with a thickness of 3 .mu.m. Therefore, piezo-electric element101dis in the form of a 10 mm square having a thickness of approximately 0.4 mm. Further, the piezo-electric actuator has a circular shape with a diameter of 16 mm and a thickness of 0.45 mm, which is the same as Example 2.

Lead zirconate titanate based ceramic was used for upper insulating layer133d, lower insulating layer133d′, and piezo-electric bodies103d, while a silver/palladium alloy (in weight ratio of 70%:30%) was used for electrode layers131d. The piezo-electric element104dwas manufactured by a green sheet method, and was sintered at 1100° C. for two hours in the atmosphere. Then, similar toFIG. 12, silver electrodes that were connected to the electrode layers were formed, then piezo-electric bodies103dwere polarized. Then electrode pads, not shown, that were formed on the surface of upper insulating layer133dwere connected together by copper foils.

The piezo-electric actuator provided a reciprocal vibration mode with a resonance frequency of 495 HZ, and a maximum amplitude of the vibration velocity of 518 mm/s.

It was confirmed from the comparison between Examples 2 and 5, that the maximum amplitude of the vibration velocity can be largely increased by using a piezo-electric element in a laminated structure without causing change in the resonance frequency.

In this example, insulating layer135ewas disposed between two piezo-electric plates of the bimorph piezo-electric element of Example 4, as illustrated inFIG. 17. A polyethylene terephthalate (PET) film with a thickness of 0.1 mm was used for insulating layer135edisposed between the two piezo-electric plates103eand103e′ of the bimorph piezo-electric element101e. The piezo-electric element as illustrated inFIG. 17also has base121e. The configuration of Example 6 is the same as that of Example 4, except that insulating layer135ewas added. The thickness of the piezo-electric actuator of this example is 0.55 mm which represents an increase of 0.1 mm as compared with Example 2, due to the thickness of insulating layer135e.

The piezo-electric actuator provided a reciprocal vibration mode with a resonance frequency of 442 HZ, and a maximum amplitude of the vibration velocity of 186 mm/s. Further, none of the 50 samples that were manufactured under the same conditions presented electric leakage, thus safety handling was confirmed.

It was confirmed from the comparison between Example 4 and 6, that a piezo-electric actuator with large vibration displacement, which suppresses electric leakage even when a metal base is used and can be safely handled, is provided by inserting an insulating layer in the piezo-electric element.

As illustrated inFIG. 18, in this example, vibrating film134fwas bonded to the piezo-electric actuator of Example 2 to create acoustic element39, which then was operated to radiate sound by the vibration that was transmitted to vibrating film134f. Specifically, vibrating film134fmade of a polyethylene terephthalate (PET) film with a thickness of 0.05 mm was attached to the back side of base121f. The piezo-electric actuator illustrated inFIG. 18comprises the piezo-electric element101f.

The acoustic element presented a resonance frequency of 483 HZ, Q-value of 8.76, and a sound pressure level of 98 dB.

Comparative Example 2

In order to compare the effects of the piezo-electric actuator of Example 7, a conventional piezo-electric acoustic element was fabricated, as illustrated inFIG. 19. This acoustic element has a piezo-electric element1101a′, a metal plate1105a′, and vibrating film134fthat is similar to that of Example 7, and was attached to the piezo-electric actuator (seeFIG. 13) of Comparative Example 1. The fabricated acoustic element presented a resonance frequency of 796 HZ, a Q-value of 37, and a sound pressure level of 79 dB.

It was confirmed from the comparison between Example 7 and Comparative Example 2, that an acoustic element can be provided that has a wide frequency range, the flat frequency characteristic of sound pressure, and a high sound pressure level.

In this example, as illustrated inFIG. 20A, conical coil spring38was interposed as a vibration transmitting member between piezo-electric element101gand vibrating film34gof acoustic element39of Example 7. Coil spring38has a thickness of 0.2 mm, a minimum coil radius of 2 mm, and a maximum coil radius of 4 mm, and is formed of a stainless steel wire, as illustrated inFIG. 20B. Coil spring38is bonded to base121gat the minimum coil radius plane, and is bonded to vibrating film34gat the maximum coil radius plane with epoxy-based adhesive. This example has the same configuration as that of Example 7 except that coil spring38is provided. The acoustic element of Example 7 has a thickness of 0.7 mm, by adding the thickness of coil spring38, i.e., 0.2 mm to the thickness of the element of Example 2.

The fabricated acoustic element presented a resonance frequency of 457 HZ, a Q-value of 9.8, and a sound pressure level of 108 dB.

It was confirmed from the comparison between Examples 7 and 8, that the resonance frequency can be lowered while the sound pressure level can be increased by interposing a vibration transmitting member between the vibrating film and the piezo-electric actuator.

As illustrated inFIG. 21, acoustic element39of Example 7 was mounted in cellular phone51, then the sound pressure level and the frequency characteristic of sound pressure of acoustic element39was measured at a distance of 30 cm. The resonance frequency was 501 HZ, the frequency characteristic of sound pressure was flat, the Q-value was 8.12, and the sound pressure level was 95 dB. Further, as a result of a drop impact test, no cracks were found in the piezo-electric element even after dropping five times, and the sound pressure level was found to be 94 dB after the test.

Comparative Example 3

The piezo-electric acoustic element of Comparative Example 2 was mounted in cellular phone51. The sound pressure level and the frequency characteristic of sound pressure of the acoustic element were measured at a distance of 30 cm in a manner similar to that in Example 9. The resonance frequency was 821 HZ, the frequency characteristic of sound pressure was very rough, and the sound pressure level was 75 dB. As the result of a drop impact test, a crack was found in the piezo-electric element after dropping cellular phone51twice, and the sound pressure was found to be 60 dB or lower at that time.

It was confirmed from the comparison between Example 9 and Comparative Example 3, that a cellular phone can be provided that reproduces sound over a wide frequency range with large sound pressure and flat frequency characteristic of sound pressure, by mounting the acoustic element of Example 9 in the cellular phone. It was also confirmed that the acoustic element of the present invention has a resistance to damage when dropped.

Comparative Example 4

As illustrated inFIG. 22, electromagnetic acoustic element61was mounted in a cellular phone. The acoustic element of this comparative example has permanent magnet62, voice coil63, and diaphragm64. A magnetic force was generated by voice coil63when a current was applied from electric terminal65. Diaphragm64was repeatedly attracted and repulsed by the generated magnetic force to generate a sound. Diaphragm64is connected to housing67by coupling member66at the periphery. The acoustic element of Comparative Example 4 has a circular shape having a diameter of 20 mm and a thickness of 2.5 mm. Sound pressure level and frequency characteristic of sound pressure of the acoustic element were measured at a distance of 30 mm in a manner similar to that of Example 9. The resultant resonance frequency was 730 HZ, and the sound level was 73 dB.

It was confirmed from the comparison between Example 9 and Comparative Example 4, that reproduction of sound over a wider frequency range with higher sound pressure as compared with the conventional electromagnetic acoustic element can be provided by mounting the acoustic element of the present invention in a cellular phone.

As described above in detail in BEST MODE FOR CARRYING OUT THE INVENTION, and in the results of Examples 1-9 and Comparative Examples 1-4, the present invention provides a piezo-electric actuator which is thin and small, is capable of providing large vibration amplitude, is adjustable for resonance frequency without changing the outer dimensions, and has high reliability, so that it can be applied to a wide range of electronic devices and so on.