JPH10257787A - Vibration actuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To excite a vertical vibration and a torsion vibration by one kind of piezoelectric elements. SOLUTION: A vibration actuator has two elastic members that can be obtained by dividing a column-shaped member by a dividing plane that is roughly in parallel with a specific axis and a vibrator where first vibration generation sources 42 and 62 and second vibration generation sources 44, 45, 64, and 65 where an electromechanical conversion element that performs a stretching vibration including a deformation constituent in a specific axis direction is laminated in a direction that orthogonally crosses a specific axis are arranged between the dividing surfaces of the elastic members. The first vibration generation source excites a vertical vibration for vibrating in a direction in parallel with a specific axis in a vibrator by performing a nearly identical stretching vibration to all electromechanical conversion elements for constituting the first vibration generation source in-phase, while the second vibration generation source excites a twisting vibration around a specific axis in the vibration by performing a stretching vibration that is nearly identical to the other to at least one electromechanical conversion element for constituting the second vibration generation source with a phase that differs by nearly 180 degrees.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vibration actuator that generates a driving force by using vibration of a vibrator.

[0002]

2. Description of the Related Art Conventionally, a vibration actuator of this type has been disclosed in, for example, Japanese Patent Application Laid-Open No.
There is a vibration actuator disclosed in 140377. FIG. 6 shows a vibration actuator disclosed in JP-A-8-140377 (hereinafter referred to as "conventional vibration actuator").
7 is a perspective view showing the stator of the vibration actuator shown in FIG.

As shown in FIG. 6, a conventional vibration actuator has a stator 107 and a moving element 106 on a shaft 107.
Is attached, and the moving element 106 is brought into pressure contact with the stator 101. The mover 106 is a mover base material 1
06-1 and a sliding member 106-2 provided on a surface of the moving element base material that comes into contact with the stator 101 and sliding with the stator 101. In addition, as shown in FIG.
It comprises elastic bodies 102 and 103 obtained by dividing a thick cylindrical member into two, and two types of plate-like piezoelectric elements 104 and 105. The stator 101 includes a piezoelectric element 104
And 105 are sandwiched between elastic bodies 102 and 103,
It is assembled so as to have a substantially cylindrical shape as a whole.

[0004] Here, the piezoelectric element 104, when a voltage is applied in the thickness direction, a device using the piezoelectric constant d ₃₁ to perform telescopic deformation along the central axis A1 of the stator 101. on the other hand,
The piezoelectric element 105, when a voltage is applied in the same way, a device using the piezoelectric constant d ₁₅ of the shear deformation along the central axis A1. In a conventional vibration actuator, a predetermined frequency voltage is applied to the piezoelectric elements 104 and 105 with a phase difference of 90 degrees, and a longitudinal vibration in a direction along the center axis A1 and a direction of twisting about the center axis A1 are applied to the stator 101. Excites torsional vibration. As a result, the end face (hereinafter referred to as “drive face”) D1 of the stator 101 performs an elliptical motion, and a part of the elliptical motion is transmitted to the moving element 106, and the moving element 10
6 rotates in a certain direction.

[0005]

However, in the above-described conventional vibration actuator, the longitudinal vibration is excited by using the expansion and contraction deformation of the piezoelectric element, and the torsional vibration is excited by using the shear deformation of the piezoelectric element. Therefore, in the conventional vibration actuator, always, the piezoelectric element is polarized to use a piezoelectric constant d _31, the two piezoelectric elements of the polarized piezoelectric element to use the piezoelectric constant d ₁₅ was prepared There was a problem that had to be.

It is an object of the present invention to provide a vibration actuator having a novel structure which can excite both longitudinal vibration and torsional vibration using only one kind of piezoelectric element.

[0007]

In order to solve the above-mentioned problems, the invention according to claim 1 has two elastic members obtained by dividing a columnar member along a division plane substantially parallel to a predetermined axis. ,
A first vibration generation source and a second vibration generation source in which electromechanical transducers that perform expansion and contraction vibration including a deformation component in the direction of the predetermined axis are stacked in a direction intersecting the predetermined axis; A vibrator arranged between the divided surfaces of the members, wherein the first vibration source applies substantially the same stretching vibration in phase to all of the electromechanical transducers constituting the first vibration source. By performing the vibration, the vibrator excites a longitudinal vibration that vibrates in a direction parallel to the predetermined axis, and the second vibration source is an electromechanical transducer that constitutes the second vibration source. A vibrating actuator characterized in that at least one of the vibrators is caused to vibrate torsional vibration around the predetermined axis by causing at least one of the vibrators to perform substantially the same expansion and contraction vibrations at different phases.

According to a second aspect of the present invention, in the vibration actuator according to the first aspect, the second vibration source is at least one of the electromechanical transducers constituting the second vibration source. Approximately 18 stretching vibrations that are almost the same as others
A vibration actuator characterized in that the vibration is performed with a phase difference of 0 degrees.

The present invention according to claim 1 or 2 has two elastic members obtained by dividing the columnar member along a division plane substantially parallel to a predetermined axis, and is provided in the predetermined axial direction. A vibrator in which a first vibration source and a second vibration source in which piezoelectric elements that expand and contract are stacked in a direction intersecting with the predetermined axis are disposed between divided surfaces of the elastic member; The first vibration source excites the vibrator to generate longitudinal vibrations parallel to the predetermined axis by causing the laminated piezoelectric elements to undergo the same expansion and contraction deformation, and the second vibration generation source ,
A vibration actuator characterized in that at least one of the laminated piezoelectric elements undergoes expansion and contraction deformation different from that of the other piezoelectric elements to excite the vibrator to torsional vibration around the predetermined axis. There may be.

According to a third aspect of the present invention, there is provided a vibration actuator including a vibrator for performing a longitudinal vibration parallel to a predetermined axis and a torsional vibration about the predetermined axis, wherein the vibrator comprises:
The vibration source includes a vibration generation source in which expansion and contraction members that perform expansion and contraction vibration including a deformation component in the direction of the predetermined axis are stacked in a direction intersecting the predetermined axis. When performing the vibration, the bending amount is performed by creating a state in which the deformation amount of at least one elastic member is different from the deformation amount of the other elastic member, and the torsional vibration is excited using the bending vibration. Vibration actuator.

According to a third aspect of the present invention, there is provided a vibration actuator including a vibrator for performing longitudinal vibration parallel to a predetermined axis and torsional vibration about the predetermined axis, wherein the vibrator includes the predetermined vibration. An elastic member that performs elastic deformation including an elastic component in the direction of an axis is laminated in a direction intersecting the predetermined axis, and the vibration source includes at least one of the elastic members and another elastic member. A vibration actuator may be characterized in that a bending deformation is performed by performing expansion and contraction deformation different from that of the expansion and contraction member, and the torsional vibration is excited using the bending deformation.

According to a fourth aspect of the present invention, in the vibration actuator according to the third aspect, the vibrator has two or more elastic members obtained by dividing the columnar member by a dividing surface substantially parallel to the predetermined axis. The vibration source is a vibration actuator, which is disposed between the divided surfaces. The invention according to claim 5 is claim 3 or claim 4.
3. The vibration actuator according to claim 1, wherein the elastic member is a piezoelectric element.

[0013]

An embodiment according to the present invention will be described below with reference to the drawings. In the following description, an ultrasonic motor using an ultrasonic vibration region will be described as an example. FIG. 1 is a side view showing an embodiment of the vibration actuator according to the present invention. As shown in FIG. 1, the ultrasonic motor 10 of the present embodiment mainly includes a pressing member 16, a moving element 20, and a vibrator 30. In addition,
An alternate long and short dash line A represents a central axis of the ultrasonic motor 10.

The moving element 20 is a columnar member that rotates around a central axis A by receiving a driving force from the vibrator 30. The moving element 20 includes a moving element member 22 and a sliding member 26. The mover member 22 is a member made of stainless steel, an aluminum alloy, or the like. The sliding member 26 is provided on the lower surface of the moving member 22 and is a member that contacts and slides on the driving surface D of the vibrator 30. The sliding material 26 contains a polymer material or the like as a main component in order to improve the sliding characteristics.

The pressing member 16 connects the moving element 20 to the vibrator 30
, And specifically, a member such as a disc spring, a coil spring, or a leaf spring is used.
The moving element 20 receives a pressing force from the pressing member 16 and comes into pressure contact with the driving surface D of the vibrator 30.

FIG. 2 is an exploded perspective view of the vibrator 30.
The vibrator 30 is a member that generates elliptical motion on the drive surface D by performing primary longitudinal vibration displaced in the direction of the central axis A and secondary torsional vibration displaced around the central axis A.
As shown in FIG. 2, the vibrator 30 includes two elastic members 3
It has a structure that sandwiches three layers composed of a piezoelectric element and an electrode plate between 2 and 34. The piezoelectric element, the electrode plate, and the elastic body are bonded to each other with an adhesive, and have a substantially cylindrical shape as a whole. If the bonding strength between the members by the adhesive is insufficient, the elastic members 32 and 3
The vibrator 30 may be provided with a fastening member (not shown) for fastening the piezoelectric element 4 from the outside in the laminating direction of the piezoelectric element and the electrode plate.

The two elastic members 32 and 34 are provided in the circumferential groove 30a.
This is a member having a shape obtained by dividing a cylindrical member having a shape into two along a plane along the central axis, and is made of a metal material such as iron and steel, stainless steel or phosphor bronze. The circumferential groove 30a locally changes the stiffness of the elastic body so that the resonance frequency of the longitudinal vibration and the torsional vibration excited in the vibrator 30 substantially match, and causes the vibrator 30 to cause so-called degeneration. It is provided.

As described above, three layers are disposed between the elastic bodies 32 and 34. The first layer and the third layer are all layers made of a piezoelectric element. Among these piezoelectric elements, the piezoelectric elements 42 and 62 are vibration sources for exciting the vibrator 30 to longitudinal vibration in a direction parallel to the central axis A. The piezoelectric elements 42 and 62 are elements arranged at the center of the vibrator 30 and have the same shape as the length of the elastic body 32 and the elastic body 32 at the position where the circumferential groove 30a is provided. Is a rectangle whose width is equal to the diameter of.

The piezoelectric element 44 is paired with the piezoelectric element 64, and the piezoelectric element 45 is paired with the piezoelectric element 65 to excite the vibrator 30 torsional vibration displaced around the center axis A. It is a source of vibration to perform. These piezoelectric elements are arranged on the side surface of the piezoelectric element 42 or 62 and between the driving surface D and the circumferential groove 30a. Further, these piezoelectric elements are placed in the elastic body 3 while being arranged as described above.
It has a shape that fills the gap between 2 and 34.

The electrode plate 52 is for applying a voltage to the piezoelectric elements 42 and 62. The electrode plate 52 has substantially the same surface shape as the piezoelectric elements 42 and 62, and is arranged between these piezoelectric elements. On the other hand, the electrode plates 54 and 55
Piezoelectric elements 44 and 64 or piezoelectric elements 45 and 6 respectively
5 for applying a voltage. These electrode plates have substantially the same surface shape as the piezoelectric element 45 and the like.
4 is disposed between the piezoelectric elements 44 and 64, and the electrode plate 55 is disposed between the piezoelectric elements 45 and 65. These electrode plates 5
2, 54 and 55 are arranged in an electrically independent state from each other.

The piezoelectric elements 41, 43, 61 and 63 are elements arranged on the side surface of the piezoelectric element 42 or 62 and below the circumferential groove 30 a of the vibrator 30. These piezoelectric elements mainly fill the gap between the elastic bodies 32 and 34. Similarly, the electrode plates 51 and 53 are disposed between the piezoelectric elements 41 and 43 or the piezoelectric elements 61 and 63, respectively, to prevent a gap from being generated between these piezoelectric elements. As described above, the provision of the piezoelectric element 41 and the like and the electrode plate 51 and the like prevents a situation in which proper vibration cannot be excited in the vibrator 30 due to the presence of the gap between the elastic bodies 32 and 34. That's why. Therefore, the vibrator 30
If the piezoelectric element 41 can secure the appropriate vibration of
Or a member made of another material instead of the electrode plate 51 or the like may be arranged at the same position.

FIG. 3 is an explanatory view showing the polarization directions of the piezoelectric elements 42 and the like, and the deformations that occur in these piezoelectric elements when a voltage is applied. FIG. 3A shows a cross section of the piezoelectric element 42 and the like perpendicular to the center axis A of the vibrator 30, and the direction of polarization of each piezoelectric element is indicated by an arrow. On the other hand, FIGS. 3B to 3D show a state in which a voltage is applied to the electrode plate 52 or 54 and the elastic members 32 and 3 that are grounded are applied.
FIG. 4 is a diagram showing, with arrows and the like, a state of deformation appearing in a piezoelectric element 42 and the like when an electric field is generated between the piezoelectric element 42 and the like.

In this embodiment, all the piezoelectric elements are
It is polarized so as to perform expansion and contraction deformation in the direction of the central axis A when a voltage is applied in the thickness direction, a device using the piezoelectric constant d _31. However, in the present embodiment, the longitudinal vibration is caused by the piezoelectric elements 42 and 62 arranged at the center of the vibrator,
In order to excite torsional vibration by the piezoelectric elements 44 and the like arranged on the outer peripheral surface side of the vibrator, the direction of polarization of each piezoelectric element is
It depends on the position where it is placed.

That is, as shown in FIG. 3A, the piezoelectric elements 42 and 62 are both polarized toward the electrode plate 62. Therefore, when a positive voltage is applied to the electrode plate 62, for example, as shown in FIG. 3B, the piezoelectric elements 42 and 62 extend and deform in the same direction in the center axis A. Further, when a frequency voltage is applied to the electrode plate 62, the piezoelectric elements 42 and 62 perform the same expansion and contraction vibration, thereby exciting the vibrator 30 to longitudinal vibration.

On the other hand, the piezoelectric elements 44 and 64
As can be seen in FIG. 3 (a), all are polarized downward in the figure. Therefore, when, for example, a positive voltage is applied to the electrode 64, the piezoelectric element 44 extends and deforms along the central axis A as shown in FIG.
4 shrinks and deforms. For this purpose, the piezoelectric element 44
And 64 bend toward the piezoelectric element 64 side. Conversely, the electrode plate 54
When a negative voltage is applied to the piezoelectric element 44, the relationship between the extension and the contraction of the piezoelectric element is reversed.
As shown in (d), it bends to the piezoelectric element 44 side. here,
The bending deformation performed by the piezoelectric elements 44 and 64 is a form similar to the deformation of the secondary torsional vibration that the vibrator 30 can perform. Therefore, when a frequency voltage is applied to the electrode plate 54,
The bending deformations shown in FIGS. 3C and 3D occur periodically in the piezoelectric elements 44 and 64.

On the other hand, the piezoelectric elements 45 and 65
As shown in (a), each is polarized upward in the drawing. This is the direction opposite to the polarization direction of the piezoelectric elements 44 and 64. Thus, when the same voltage as that applied to the electrode plate 64 is applied to the electrode plate 65, the piezoelectric elements 45 and 65 bend in the opposite direction to the piezoelectric elements 44 and 64. As a result, the piezoelectric elements 45 and 65
The elastic bodies 32 and 34 are twisted in the same direction as 4 and 64. Therefore, when the same frequency voltage is applied to the electrode plates 65 and 64, one torsional vibration is generated by the two flexural deformations generated in the two sets of piezoelectric elements.
It will be excited to zero.

Next, a method of driving the ultrasonic motor 10 will be described. FIG. 4 is a schematic diagram showing a drive circuit of the ultrasonic motor and connection of the drive circuit to the vibrator. The drive circuit 70 according to the present embodiment mainly includes an oscillator 72, a phase shifter 74, and amplifiers 76 and 78. Oscillator 72
Is a circuit for oscillating a frequency voltage. The frequency voltage output from the oscillator 72 is applied to the electrode plate 52 after being amplified by the amplifier 76. On the other hand, the electrode plates 54 and 55 have
The output of the oscillator is applied via the phase shifter 74 and the amplifier 78. The phase shifter 74 is a circuit that outputs a signal having a predetermined phase difference with respect to the input signal. In the case of the present embodiment, the phase shifter 74 determines whether the input frequency voltage is
A frequency voltage having a phase difference of ° is output. Therefore,
A frequency voltage having a phase difference of 90 ° from that applied to the electrode plate 52 is applied to the electrode plates 54 and 55. In addition,
The amplifier 78 is the same circuit as the amplifier 76.

When a frequency voltage is applied to the electrode plates 52, 54 and 55, the primary longitudinal vibration and the secondary torsional vibration are excited in the vibrator 30 as already described with reference to FIG. Further, as described above, there is a phase difference of 90 ° between the frequency voltages applied to the electrode plate 52 and the electrode plate 54 and the like. °
Is generated.

As described above, between the longitudinal vibration and the torsional vibration, 9
The reason for having a phase difference of 0 ° is to generate an elliptical motion on the driving surface D of the vibrator 30. Hereinafter, it will be described that an elliptical motion is generated on the driving surface D of the vibrator 30 in the present embodiment, and the movable element 20 is rotationally driven in a certain direction by the elliptical motion. FIG. 5 is an explanatory diagram showing that elliptical motion occurs on the driving surface of the vibrator. FIG. 5 is a schematic diagram showing the time, the displacement of the torsional vibration, the displacement of the longitudinal vibration, the combined displacement of the torsional vibration and the longitudinal vibration, and the movement of the vibrator that appear on the drive surface D in order from the left column. I have. Here, the angular frequency when the driving frequency is f is ω (= 2πf).

First, at time t = 0, the displacement of the torsional motion is maximum on the left side, and the displacement of the longitudinal vibration is zero. In this state, the moving element 20 is in contact with the driving surface D of the vibrator 30 by the pressing member 16. From this state, from t = 0 to (4/4) × (π / ω), the torsional vibration is displaced from the left maximum to the right maximum, and the longitudinal vibration is displaced from zero to the upper maximum, Return to zero again. Therefore, the driving surface D of the vibrator 30 rotates rightward while pressing the movable element 20, and the movable element 20 rotates. Next, t = (4/4) × (π / ω) to (8/4) × (π /
Up to ω), the torsional vibration is displaced from the maximum on the right to the maximum on the left, and the longitudinal vibration is displaced from zero to the maximum below and returns to zero again. Here, since the natural frequency of the pressing member 16 is set to be lower than the driving frequency of the vibrator, the moving element 20 can be driven by the vibrator 30 even if the moving member 20 is pressed by the pressing member.
Does not follow the shrinkage of Therefore, the driving surface D of the vibrator 30 rotates leftward while moving away from the movable element 20, and the movable element 20 is not driven.

As described above, in the present embodiment, since the torsional vibration and the longitudinal vibration are oscillating with a phase difference of 90 °, an elliptical motion is generated on the driving surface D. In addition, since only a part of the elliptical motion of the drive surface D is transmitted to the movable element 20, the movable element 20 performs a rotational movement in a certain direction. If the frequency of the drive signal output from the oscillator 72 is set to a value close to the natural frequency of the longitudinal primary vibration mode and the natural frequency of the torsional secondary vibration mode of the vibrator 30, the longitudinal vibration amplitude and the torsional vibration amplitude become Both are large, and a large elliptical motion is obtained on the drive surface D.

[0032] As described above, in the ultrasonic motor of the present embodiment, by utilizing the deformation of a piezoelectric element according to the piezoelectric constant d _31, not longitudinal vibration only, are excited in the vibrator even torsional vibrations . Therefore, in the present embodiment, it is not necessary to separately prepare two types of piezoelectric elements for longitudinal vibration and for torsional vibration as in the related art, but prepare one type of piezoelectric element that is polarized in the thickness direction. Only by this, it is possible to manufacture an ultrasonic motor.

(Other Embodiments) The present invention is not limited to the above embodiment. The above embodiment is an exemplification, and has substantially the same configuration as the technical idea described in the scope of the claims of the present invention. It is included in the technical scope of the invention.

For example, in FIG. 3, the piezoelectric elements 42 and 62
Has been described in the case where both are polarized in the direction of the electrode plate 52, but this may naturally be the case where both the piezoelectric elements 42 and 62 are polarized in the direction opposite to the electrode plate 52. In this embodiment, a case has been described in which a plate member is used as the piezoelectric element and the plate member is bonded between the elastic members. It may be formed directly. Thus, directly forming the thin film of the piezoelectric material on the elastic body is a particularly effective means for promoting the miniaturization of the ultrasonic motor. In the above embodiment, the case where the elastic body is excited by using the piezoelectric element has been described. However, this does not mean to limit the types of the vibration sources that can be used in the present invention. The vibration generating source may be not only a piezoelectric element but also another electromechanical conversion element that converts electric energy into mechanical displacement, such as an electrostrictive element or a magnetostrictive element.

[0035]

As described above in detail, according to the first or second aspect of the present invention, only one type of piezoelectric element that expands and contracts longitudinal vibration and torsional vibration in a predetermined axial direction is used. It became possible to excite. According to the third aspect of the present invention, the vibration generation source bends by creating a state where the amount of deformation of at least one elastic member is different from the amount of deformation of the other elastic member when each elastic member performs elastic vibration. Since the torsion vibration is excited by using the bending vibration, it is possible to provide a vibration actuator having a novel structure and to enrich the technology in the field of the vibration actuator. According to the fourth aspect of the present invention, since the vibration source is arranged between the divided surfaces of the elastic member and excites the elastic member, it is easy to extract the rotational motion from the torsional vibration, and it is easy to reduce the size. It is possible to provide a vibration actuator in a form. According to the fifth aspect of the invention, since the piezoelectric element is used as the elastic member, it is possible to reduce the size and torque of the vibration actuator.

[Brief description of the drawings]

FIG. 1 is a side view showing one embodiment of a vibration actuator according to the present invention.

FIG. 2 is an exploded perspective view of a vibrator 30.

FIG. 3 is an explanatory view showing the polarization directions of the piezoelectric elements and the like, and the deformations that occur in these piezoelectric elements when a voltage is applied.

FIG. 4 is a schematic diagram showing a drive circuit of an ultrasonic motor and connection of the drive circuit to a vibrator.

FIG. 5 is an explanatory diagram showing that elliptical motion occurs on a driving surface of a vibrator.

FIG. 6 is a sectional view showing a vibration actuator disclosed in Japanese Patent Application Laid-Open No. 8-140377.

FIG. 7 is a perspective view showing a stator of a vibration actuator disclosed in Japanese Patent Application Laid-Open No. 8-140377.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Ultrasonic motor 16 Pressurization member 20 Moving element 30 Vibrator 30a Peripheral groove 32, 34 Elastic body 41-45 Piezoelectric element 51-55 Electrode plate 61-65 Piezoelectric element 70 Drive circuit 72 Oscillator 74 Phase shifter 76, 78 Amplifier D drive surface

Claims (5)

[Claims] 1. An electromechanical converter having two elastic members obtained by dividing a columnar member along a division plane substantially parallel to a predetermined axis, and performing stretching vibration including a deformation component in a direction of the predetermined axis. A vibrator in which a first vibration source and a second vibration source in which elements are stacked in a direction intersecting the predetermined axis are arranged between divided surfaces of the elastic member; The source causes the vibrator to vibrate in a direction parallel to the predetermined axis by causing all of the electromechanical transducers constituting the first vibration source to perform substantially the same stretching vibration in phase. By exciting longitudinal vibration, the second vibration source causes at least one of the electromechanical transducers constituting the second vibration source to perform substantially the same stretching vibration at a different phase as the other. Exciting the torsional vibration around the predetermined axis to the vibrator. And a vibration actuator. 2. The vibration actuator according to claim 1, wherein the second vibration source has at least one of the electromechanical transducers constituting the second vibration source that is substantially the same as the other. A vibration actuator, wherein vibration is performed with a phase difference of about 180 degrees. 3. A vibration actuator comprising a vibrator that performs longitudinal vibration parallel to a predetermined axis and torsional vibration about the predetermined axis, wherein the vibrator generates a deformation component in a direction of the predetermined axis. A vibration generation source in which telescopic members that perform telescopic vibrations are stacked in a direction intersecting the predetermined axis, wherein the vibration generation source includes at least one telescopic member when each of the telescopic members performs telescopic vibration. A vibration actuator that produces a state in which the amount of deformation of the elastic member is different from the amount of deformation of the other elastic member, performs bending vibration, and excites the torsional vibration using the bending vibration. 4. The vibration actuator according to claim 3, wherein the vibrator has two or more elastic members obtained by dividing a columnar member by a division surface substantially parallel to the predetermined axis. And a vibration actuator, which is arranged between the divided surfaces. 5. The vibration actuator according to claim 3, wherein the expansion and contraction member is a piezoelectric element.