JP3362346B2 - Piezo actuator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric actuator for converting electric energy into mechanical energy such as displacement or force. More specifically, the piezoelectric actuator body is enclosed in a metal case so as not to be affected by the operating environment. The present invention relates to a sealed piezoelectric actuator.

[0002]

2. Description of the Related Art Conventionally, piezoelectric actuators have been used for minute position control in machining equipment such as precision grinding and precision cutting and semiconductor manufacturing equipment.

This piezoelectric actuator is a displacement element utilizing the solid strain of piezoelectric ceramics, and can be controlled by a micro displacement of micrometer order depending on the magnitude of the applied voltage, and the generated force at that time is a unit area. Since it is several kilograms / mm ² per hit, it is suitable for applications such as ultra-precision machining and minute positioning such as steppers in semiconductor manufacturing equipment.

By the way, in a machining device for performing precision grinding or precision cutting, since a water-based lubricating liquid is widely used in order to improve the machining performance, it is avoided that the lubricating liquid wets the piezoelectric actuator. Can not. Under such usage conditions, the insulation resistance of the piezoelectric actuator is easily lowered in a high humidity environment, and in some cases dielectric breakdown occurs. Therefore, when used in a high-humidity environment that is exposed to moisture, the piezoelectric actuator main body is resin-coated or the piezoelectric actuator main body is sealed in a metal case in order to prevent the lubricating liquid from touching the piezoelectric actuator. Measures such as a sealed structure are taken.

Further, the stepper of the semiconductor manufacturing apparatus is driven in a clean environment, and the temperature and humidity under the environment are controlled to be constant. Usually, the semiconductor manufacturing apparatus itself is very expensive and the operating rate thereof is high. Therefore, if the piezoelectric actuator fails, the damage to the semiconductor manufacturing line will be very large. Therefore, piezoelectric actuators used in applications that require such high reliability have a sealed structure in which the piezoelectric actuator body is enclosed in a metal case so as not to be adversely affected by the assembly of semiconductor manufacturing equipment and changes in the surrounding environment. Is used.

By the way, various types of piezoelectric actuators having such a sealed structure are commercially available, and these piezoelectric actuators are slightly bonded to each other without joining the metal case and the stem so as not to restrain the displacement amount of the piezoelectric actuator body. Examples of the structure include a structure in which a hollow space is formed and a rubber O-ring is sandwiched in the space, or a part of the side surface of the metal case has a bellows structure that is flexible in the displacement direction.

[0007]

In the case of the above-mentioned sealed structure piezoelectric actuator, if the piezoelectric actuator body is coated with a resin or a rubber O-ring is provided between the metal case and the stem, moisture will not enter. This cannot be completely prevented, and there is a problem that the insulation resistance of the piezoelectric actuator is still reduced when the usage time exceeds several hundred hours.

Further, in the case of the sealed structure in which the piezoelectric actuator body is enclosed in a metal case, the metal case has a bellows structure which has an effect of preventing the intrusion of moisture but does not restrain the displacement amount of the piezoelectric actuator body. In addition to being expensive, there is a problem that the manufacturing cost becomes too expensive because the process of encapsulating the piezoelectric actuator body in the metal case becomes complicated.

The present invention has been made to solve the above problems, and its technical problem is that it is possible to sufficiently prevent the invasion of moisture and to manufacture the piezoelectric actuator body at a low cost without restraining the displacement amount thereof. Another object of the present invention is to provide a piezoelectric actuator having a sealed structure.

[0010]

According to the present invention, one end surface of a piezoelectric actuator body extending in a predetermined direction, which is orthogonal to the predetermined direction, is fixed to one surface portion of a metal stem, and the other end surface of the metal case is fixed. In a piezoelectric actuator having a hermetically sealed structure in which the piezoelectric actuator body is enclosed in the metal stem and the metal case, the side surface extending in the length direction of the metal case being fixed to an inner end surface parallel to a predetermined direction and orthogonal to the length direction. A piezoelectric actuator is obtained in which at least one annular groove portion is formed on each of the inner peripheral surface and the outer peripheral surface of the portion so as to go around the peripheral surface.

Further, according to the present invention, in the above piezoelectric actuator, when the thickness of the metal case is represented by t, the depth of the annular groove portion of the outer peripheral surface is represented by a, and the depth of the annular groove portion of the inner peripheral surface is represented by b. And t ≦ a + b <2t, and further, with the one surface of the metal stem as a reference surface, the distance from the reference surface to the annular groove portion of the inner peripheral surface is c, and the distance from the reference surface to the annular groove portion of the outer peripheral surface is d. A piezoelectric actuator having a structure satisfying the relationship of t / 2 <| cd− ≦ t when expressed by

On the other hand, according to the present invention, one end surface of the piezoelectric actuator body extending in the predetermined direction orthogonal to the predetermined direction is fixed to one surface portion of the metal stem, and the other end surface is parallel to the predetermined direction in the metal case. In a piezoelectric actuator having a hermetically sealed structure in which the piezoelectric actuator main body is sealed in the metal stem and the metal case, the other end surface of the piezoelectric actuator main body in the metal case is fixed to the side where the other end surface is fixed. At least one annular groove is formed concentrically around the central axis parallel to the length direction of the metal case on the inner surface and the outer surface excluding the fixed portion of the end surface portion to be positioned. When the wall thickness of the portion is t ', the depth of the outer annular groove is a', and the depth of the inner annular groove is b ', then t'≤a'
+ B ′ <2t ′, and further, when the diameter of the annular groove portion on the outer surface is represented by c ′ with respect to the center of the central axis and the diameter of the annular groove portion on the inner surface is represented by d ′ with reference to the center of the central axis, c It is possible to obtain a piezoelectric actuator having a structure satisfying the relation of '<d' and t '/ 2 <| c'-d' | ≤t '.

[0013]

According to the sealed structure type piezoelectric actuator of the present invention, even if the piezoelectric actuator main body is displaced and tensile stress acts on the metal case, the ring-shaped piezoelectric actuator formed on the inner peripheral surface of the side surface extending in the length direction of the metal case. Since a rotational moment is generated in the thick portion between the groove and the annular groove formed on the outer peripheral surface, it is possible to prevent the metal case from being damaged by fatigue without restraining the displacement of the piezoelectric actuator body. In addition, such an effect is obtained by using a metal actuator for other sealed structure type piezoelectric actuators with respect to an inner surface and an outer surface excluding a fixed portion of an end surface portion located on a side where the other end surface of the piezoelectric actuator body is fixed in the metal case. The same can be obtained when at least one annular groove is formed concentrically around a central axis parallel to the length direction of the case.

[0014]

The piezoelectric actuator of the present invention will be described below in detail with reference to the drawings. Figure 1
FIG. 3 is a side sectional view showing the basic structure of the piezoelectric actuator according to the first embodiment of the present invention.

In this piezoelectric actuator, one end surface of a piezoelectric actuator body 11 extending in a predetermined direction, which is orthogonal to the predetermined direction, is fixed to one surface portion of the metal stem 12, and the other end surface is in the length direction (predetermined direction) of the metal case 13. The piezoelectric actuator main body 11 is enclosed in the metal stem 12 and the metal case 13 so that the piezoelectric actuator body 11 has a sealed structure.

Of these, on the outer peripheral surface and the inner peripheral surface of the side surface portion extending in the lengthwise direction of the metal case 13, annular groove portions 14 and 15 each having a V-shaped cross section are formed so as to go around the peripheral surface. There is.

FIG. 2 is an enlarged partial sectional view of the annular groove portions 14 and 15 formed on the outer and inner peripheral surfaces of the side surface of the metal case 13. These annular groove portions 14, 15
Is a metal case 13 having a wall thickness t and an annular groove 14 on the outer peripheral surface.
Has a depth of a and the depth of the annular groove portion 15 of the inner peripheral surface is represented by b, and t ≦ a + b <2t, and one surface of the metal stem 12 to which one end surface of the piezoelectric actuator body 11 is fixed. When the distance from the reference surface to the annular groove portion 15 on the inner peripheral surface is represented by c and the distance from the reference surface to the annular groove portion 14 on the outer peripheral surface is represented by d with respect to the portion as a reference surface, t / 2 <| c
The relationship of −d | ≦ t is established.

Where a + b <t and t <| c-d
If the relationship is |, the rigidity of the metal case 13 cannot be reduced, and as a result, the piezoelectric actuator body 11
Will constrain the displacement amount of. Also, 2t <a +
If it is b, the risk that the metal case 13 will be cut by the annular groove portions 14 and 15 becomes high, so that it becomes difficult to form the annular groove portions 14 and 15 in the metal case 13.
Further, if | c−d | ≦ t / 2, the annular groove portions 14, 1
When the piezoelectric actuator body 11 is repeatedly expanded and contracted due to the distance between the respective tips of 5 being too small, tensile stress is concentrated on the tips of the annular groove portions 14 and 15, and the metal case 13 is easily broken by metal fatigue. turn into.

Therefore, if the annular groove portions 14 and 15 are formed so as to satisfy the above relationship, even if the piezoelectric actuator body 11 is displaced and tensile stress acts on the metal case 13, the wall thickness between the annular groove portions 14 and 15 is increased. A rotational moment is generated in the portion 21, and the metal case 13 can be prevented from being damaged by fatigue without restraining the displacement of the piezoelectric actuator body 11.

FIG. 3 is a side sectional view showing the basic structure of the piezoelectric actuator of Comparative Example 1. In this piezoelectric actuator, only the outer peripheral surface of the side surface extending in the lengthwise direction of the metal case 13a has a cross-section V so as to go around the peripheral surface.
The piezoelectric actuator has the same configuration as the piezoelectric actuator of the first embodiment except that the character-shaped annular groove portion 14 is formed. When the annular groove portion 14 is formed only on the outer peripheral surface of the metal case 13 as described above, when the piezoelectric actuator body 11 is repeatedly expanded and contracted, tensile stress is concentrated on the tip 31 of the annular groove portion 14 and the metal case 13a is destroyed by metal fatigue. It is easy to be done. This also applies when the annular groove 15 is formed only on the inner peripheral surface instead of the annular groove 14.

The manufacturing process and durability of the hermetically sealed piezoelectric actuator according to the first embodiment will be briefly described below. In the manufacturing process of this piezoelectric actuator, first the chemical composition formula of Pb (Ni.N
b) ₀ . ₅ Ti ₀ . ₃₅ Zr ₀ . ₁₅ O ₃ , a lead zirconate titanate porcelain powder represented by ₁₅ O ₃ is used as a raw material, and internal electrodes and piezoelectric ceramics are laminated in the length direction by a green sheet lamination method, and the cross-sectional dimension is 5 mm × 5 mm. The piezoelectric actuator body 11 having a length of 70 mm is manufactured.

Next, one end face in the length direction of the piezoelectric actuator body 11 is provided with a disc-shaped metal stem 12 having an outer diameter of 20 mm.
After fixing to the center of the
The other end surface of which has an outer diameter of 10 mm, a length of 70.5 mm, a wall thickness of 0.5 mm, and a depth of one round of the outer peripheral surface and the inner peripheral surface of the side surface portion in the length direction. 0.3 m
The m-shaped annular groove portions 14 and 15 are fixed to the inner end surface of the metal case 13 formed one by one such that the tips of the annular groove portions 14 and 15 are separated by 0.3 mm. As a result, the piezoelectric actuator body 11 is enclosed in the metal stem 12 and the metal case 13. After that, the metal case 13 and the metal stem 12 are attached to each other, and as a lead wire for applying a voltage to the piezoelectric actuator body 11, a two-terminal hermetically sealed terminal is attached to a predetermined position on the side surface of the metal case 13. The piezoelectric actuator was finished by attaching and allowing electrical connection.

By the way, in order to investigate the performance of the piezoelectric actuator thus obtained, the piezoelectric actuator main body 11 before and after being enclosed in the metal case 13 is described as follows.
When the measurement of the displacement amount under the condition of applying a DC voltage of 0 V and the measurement of the aging failure rate% under the condition of continuously applying a DC voltage of 250 V at a temperature of 85 ° C. and a humidity of 90% RH were performed, The results are shown in Table 1.

[0024]

[Table 1]

From Table 1, there is almost no change in the displacement amount of the piezoelectric actuator body 11 regardless of whether the piezoelectric actuator body 11 is sealed in the metal case 13 or not. It can be seen that, for example, it was completely suppressed, and there was no effect due to aging.

On the other hand, with respect to the piezoelectric actuator according to Example 1 and the piezoelectric actuator according to Comparative Example 1 (shown in FIG. 3) obtained by the above-described manufacturing process, the frequency is 200 Hz, the duty is 50%, and 0 to 250 V, respectively. Under the condition that the pulse current of
When the fracture fatigue of 13a was examined, the results shown in Table 2 were obtained.

[0027]

[Table 2]

From Table 2, the piezoelectric actuator of Example 1 in which the annular grooves 14 and 15 are formed on the outer and inner peripheral surfaces of the side surface of the metal case 13, the annular groove only on the outer peripheral surface of the side surface of the metal case 13. As compared with the piezoelectric actuator of Comparative Example 1 in which No. 14 is formed, it is found that the influence of fracture fatigue due to aging is not recognized at all.

Therefore, from Tables 1 and 2, the piezoelectric actuator according to the first embodiment has no durability even if it is continuously used under high temperature and high humidity conditions, and the metal case 13 has no fatigue fracture. It turns out that it will be excellent.

In the piezoelectric actuator of the first embodiment described above, the case where the annular groove portions 15 and 14 are formed on the inner surface and the outer peripheral surface of the side surface of the metal case 13 respectively has been described. For example, it is possible to provide a plurality of annular groove portions on each of the side surface portion and the outer peripheral surface of the metal case 13, and the same effect can be obtained in this case as well, so the present invention is not limited to the first embodiment.

FIG. 4 is a side sectional view showing the basic structure of the piezoelectric actuator according to the second embodiment of the present invention.

The basic structure of this piezoelectric actuator is also the same as that of the first embodiment, that is, the metal stem 12 and the metal case 13.
′ Has a sealed structure in which the piezoelectric actuator body 11 is enclosed, but the outer surface (surface) of the end surface portion located on the side where the other end surface of the piezoelectric actuator body 11 in the metal case 13 ′ is fixed, and the fixed portion are On the inner surface to be removed, annular groove portions 14 ', 1 each having a V-shaped cross section are concentrically formed around a central axis parallel to the length direction of the metal case 13'.
5'is formed.

FIG. 5 is an enlarged partial sectional view of annular groove portions 14 'and 15' formed on the outer and inner surfaces of the end surface portion of the metal case 13 '. These annular grooves 1
4 ′ and 15 ′ have a wall thickness t of the metal case 13 ′ of t.
′, When the depth of the outer annular groove portion 14 ′ is represented by a ′ and the depth of the inner annular groove portion 15 ′ is represented by b ′, t ′ ≦ a ′ +
b '<2t', and further, the diameter of the annular groove portion 14 'on the outer surface is represented by c'with reference to the center of the central axis, and the diameter of the annular groove portion 15' on the inner surface is represented by d'with reference to the center of the central axis. C '<d', and t '/ 2 <| c'-d'
It is formed so that the relationship | ≦ t ′ holds.

Here, a '+ b'<t'andt'<
With the relationship of | c′−d ′ |, the rigidity of the metal case 13 ′ cannot be reduced, and as a result, the displacement amount of the piezoelectric actuator body 11 is restricted.
Further, if 2t '<a' + b ', the risk of the metal case 13' being cut by the annular groove portions 14 ', 15' increases, so that the metal case 13 'has annular groove portions 14', 15 '.
Is difficult to form. Further, if c '≧ d', the annular groove portions 14 ', 1 in the metal case 13' are
No rotational moment is generated in the thick portion 21 ′ between the 5 ′ and only tensile stress acts, and as a result, the metal case 13 ′ is easily broken by the thick portion 21 ′ due to metal fatigue. In addition, if | c′−d ′ | ≦ t ′ / 2, when the distance between the tips of the annular groove portions 14 ′ and 15 ′ becomes too small and the piezoelectric actuator body 11 is repeatedly expanded and contracted, The tensile stress is concentrated on the tips of the groove portions 14 'and 15', and the metal case 13 'is easily broken due to metal fatigue.

Therefore, if the annular groove portions 14 'and 15' are formed so as to satisfy the above relationship, even if the piezoelectric actuator body 11 is displaced and tensile stress acts on the metal case 13 ', the annular groove portions 14' and 15 'are formed. A rotational moment is generated in the thick portion 21 'between the piezo and the piezoelectric actuator body 11
It is possible to prevent the metal case 13 'from being damaged by fatigue without restraining the displacement of the metal case 13'.

FIG. 6 is a side sectional view showing the basic structure of the piezoelectric actuator of Comparative Example 2. This piezoelectric actuator has the same configuration as that of the piezoelectric actuator of the second embodiment except that the annular groove 14 'having a V-shaped cross section is formed only on the outer surface of the end surface of the metal case 13a'. When the annular groove portion 14 'is formed only on the outer surface of the end surface portion of the metal case 13a' as described above, when the piezoelectric actuator body 11 is repeatedly expanded and contracted, the annular groove portion 14 'is formed.
The tensile stress concentrates on the tip 31 ′ of the metal case 13 a, and the metal case 13 a ′ is easily broken due to metal fatigue. This also applies to the case where the annular groove 15 'is formed only on the inner surface of the end face portion of the metal case 13a' instead of the annular groove 14 '.

The manufacturing process and durability characteristics of the sealed piezoelectric actuator according to the second embodiment will be briefly described below. In the manufacturing process of this piezoelectric actuator, first the chemical composition formula of Pb (Ni.N
b) ₀ . ₅ Ti ₀ . ₃₅ Zr ₀ . ₁₅ O ₃ , a lead zirconate titanate-based porcelain powder is used as a raw material, and a structure in which internal electrodes and piezoelectric ceramics are laminated in the length direction by a green sheet lamination method, and the cross-sectional dimensions are 5 mm × 5 mm, The piezoelectric actuator body 11 having a length of 70 mm is manufactured.

Next, one end face of the piezoelectric actuator body 11 in the length direction is provided with a disc-shaped metal stem 12 having an outer diameter of 20 mm.
After fixing to the center of the
The other end surface of the end surface portion having an outer diameter of 10 mm, a length of 70.5 mm, a wall thickness of 0.5 mm, and located on the side to which the other end surface of the piezoelectric actuator body 11 is fixed,
Also, with respect to the inner surface excluding the fixed portion, the annular groove portions 14 ', 15' having a depth of 0.3 mm are concentrically centered on the central axis parallel to the lengthwise direction, and the tips of the annular groove portions 14 ', 15' are separated by 0.3 mm. The metal case 13 'thus formed is fixed to the inner end surface of the metal case 13'. As a result, the piezoelectric actuator body 11 is enclosed in the metal stem 12 and the metal case 13 '. After that, the metal case 13 ′ and the metal stem 12 are joined together, and as a lead wire for applying a voltage to the piezoelectric actuator body 11, a two-terminal hermetically sealed terminal is provided at a predetermined position on the side surface of the metal case 13 ′. The piezoelectric actuator was finished by mounting it on the substrate and allowing electrical connection.

By the way, in order to investigate the performance of the piezoelectric actuator thus obtained, regarding the piezoelectric actuator main body 11 before and after being enclosed in the metal case 13 ', 2
When the measurement of the displacement amount under the condition of applying a DC voltage of 50 V and the measurement of the aging defect rate% under the condition of continuously applying a DC voltage of 250 V at a temperature of 85 ° C. and a humidity of 90% RH were performed, The results are shown in Table 3.

[0040]

[Table 3]

From Table 3, there is almost no change in the amount of displacement of the piezoelectric actuator body 11 regardless of whether it is sealed in the metal case 13 '. However, when aging failure occurs, the piezoelectric actuator body 11 is moved to the metal case 13'. It can be seen that if it is encapsulated, it is completely suppressed, and that there is no effect due to aging.

On the other hand, for the piezoelectric actuator of Example 2 and the piezoelectric actuator of Comparative Example 2 (shown in FIG. 6) obtained by the above-described manufacturing process, a frequency of 200 was used.
When the fracture fatigue of the metal cases 13 ', 13a' was examined under the condition that the pulse current of 0 Hz, duty of 50% and 0 to 250 V was continuously applied, the results shown in Table 4 were obtained.

[0043]

[Table 4]

From Table 4, the piezoelectric actuator of Example 2 in which the annular groove portions 14 'and 15' are formed on the outer and inner surfaces of the end surface portion of the metal case 13 'has an annular shape only on the outer surface of the end surface portion of the metal case 13a'. It can be seen that the influence of fracture fatigue due to aging is not recognized at all as compared with the piezoelectric actuator of Comparative Example 2 in which the groove portion 14 'is formed.

Therefore, it can be seen from Tables 3 and 4 that the piezoelectric actuator according to the second embodiment, like the piezoelectric actuator according to the first embodiment, does not cause a failure even when continuously used under high temperature and high humidity conditions. Moreover, it has been found that the metal case 13 'has excellent durability without fatigue fracture.

In the piezoelectric actuator according to the second embodiment, the case where the annular groove portions 15 'and 14' are formed on the inner surface and the outer surface of the end surface of the metal case 13 'has been described. Metal case 1 if filled
It is possible to provide a plurality of annular groove portions on the inner surface and the outer surface of the end surface portion 3 ', respectively, and the same effect can be obtained in this case, so the present invention is not limited to the second embodiment.

[0047]

As described above, according to the present invention, moisture can be sufficiently prevented from entering, and the piezoelectric actuator body can be sealed in a metal case without restraining the displacement amount, and the manufacturing cost is low. Thus, it becomes possible to realize a piezoelectric actuator having a sealed structure. That is, since this piezoelectric actuator can guarantee excellent durability without depending on the use environment and without being affected by a change over time, high reliability can be ensured.

[Brief description of drawings]

FIG. 1 is a side sectional view showing a basic configuration of a piezoelectric actuator according to a first embodiment of the present invention.

FIG. 2 is a partial cross-sectional view showing, on an enlarged scale, annular grooves formed on the inner and outer peripheral surfaces of the side surface of the metal case of the piezoelectric actuator shown in FIG.

FIG. 3 is a side sectional view showing a basic configuration of a piezoelectric actuator according to Comparative Example 1.

FIG. 4 is a side sectional view showing the basic structure of a piezoelectric actuator according to a second embodiment of the present invention.

5 is a partial cross-sectional view showing, on an enlarged scale, annular groove portions formed on the inner and outer surfaces of the end surface portion of the metal case of the piezoelectric actuator shown in FIG.

FIG. 6 is a side sectional view showing a basic configuration of a piezoelectric actuator according to a comparative example 2.

[Explanation of symbols]

11 Piezoelectric actuator body 12 metal stem 13, 13a, 13 ', 13'a Metal case 14,15,14 ', 15' Annular groove 21,21 'Thick part

Claims (3)

(57) [Claims] 1. A piezoelectric actuator main body extending in a predetermined direction has one end surface orthogonal to the predetermined direction fixed to one surface portion of a metal stem, and the other end surface orthogonal to a length direction of the metal case parallel to the predetermined direction. In a piezoelectric actuator having a hermetically sealed structure in which the piezoelectric actuator main body is enclosed in the metal stem and the metal case, the inner surface and the outer surface of the side surface extending in the length direction of the metal case are fixed. Is at least 1 that goes around each circumference
A piezoelectric actuator having an annular groove formed in a book. 2. The piezoelectric actuator according to claim 1, wherein the thickness of the metal case is represented by t, the depth of the annular groove portion of the outer peripheral surface is represented by a, and the depth of the annular groove portion of the inner peripheral surface is represented by b. Sometimes, t ≦ a + b <2t, and further, the distance from the reference surface to the annular groove portion of the inner peripheral surface is c with one surface portion of the metal stem as the reference surface, and the annular groove portion of the outer peripheral surface is from the reference surface. T / when the distance to is represented by d
A piezoelectric actuator having a structure satisfying a relationship of 2 <| c−d | ≦ t. 3. A piezoelectric actuator body extending in a predetermined direction has one end surface orthogonal to the predetermined direction fixed to one surface portion of a metal stem, and the other end surface orthogonal to a length direction of the metal case parallel to the predetermined direction. In the piezoelectric actuator having a hermetically sealed structure in which the piezoelectric actuator body is sealed in the metal stem and the metal case, the end surface located on the side where the other end surface of the piezoelectric actuator body in the metal case is fixed. On the inner surface and the outer surface excluding the fixed portion of the portion, at least one each is concentrically formed around a central axis parallel to the length direction of the metal case.
When a plurality of annular groove portions are formed and the thickness of the end surface portion of the metal case is represented by t ′, the depth of the outer surface annular groove portion is represented by a ′, and the inner surface annular groove portion is represented by b ′, then t is represented. ′ ≦ a ′
+ B ′ <2t ′, further, the diameter of the annular groove portion of the outer surface is c ′ with reference to the center of the central axis, and the diameter of the annular groove portion of the inner surface is d ′ with reference to the center of the central axis.
Is represented by c ′ <d ′, and t ′ / 2 <| c ′
A piezoelectric actuator having a structure satisfying a relationship of −d ′ | ≦ t ′.