Piezoelectric actuator and sound wave generator

Abstract

To reduce the amount of displacement and stress of a vibrating membrane 1 during resonance.SOLUTION: A piezoelectric actuator 100, 101 with a cantilever structure includes: a vibration membrane 1 having a fixed end 1a and a free end 1b; a membrane support portion 2 to which the fixed end 1a of the vibration membrane 1 is connected; and a piezoelectric element 3A arranged on a main surface 1A of the vibration membrane 1. The piezoelectric element 3A includes a first element portion 3A formed in an area of the main surface 1A of the vibration membrane 1, extending from the fixed end 1a toward the free end 1b for a predetermined length (b). A ratio (b / L) of the predetermined length (b) of the first element part 3A to a length (L) of the vibration membrane 1 from the fixed end 1a to the free end 1b is set according to resonance modes M1 to M4 specific to the piezoelectric actuator 100, 101.SELECTED DRAWING: Figure 1B

Description

[Technical field]

[0001] The present invention relates to a piezoelectric actuator and a sound wave generating device. [Background technology]

[0002] Conventionally, transducers that transmit or receive sound waves or ultrasonic waves are known (see Patent Document 1). A cantilever-type piezoelectric actuator that can be used as a speaker that transmits sound waves has a piezoelectric body arranged over as large an area as possible on the main surface of the diaphragm in order to obtain the maximum output (displacement of the diaphragm) for the applied voltage. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-52305

[0004] [overview] However, if the operating frequency band of the piezoelectric actuator includes a resonance frequency specific to the piezoelectric actuator, the vibration membrane is significantly displaced during resonance, causing a large stress to be applied, which may damage the piezoelectric actuator.

[0005] An object of the present disclosure is to provide a piezoelectric actuator and a sound wave generating device in which the displacement and stress of the vibrating membrane during resonance are reduced.

[0006] In order to solve the above-mentioned problems, one aspect of the present disclosure is a piezoelectric actuator having a cantilever structure, which includes a vibration membrane having a fixed end and a free end, a membrane support part to which the fixed end of the vibration membrane is connected, and a piezoelectric element arranged on a main surface of the vibration membrane. The piezoelectric element has a first element part formed in a region of the main surface of the vibration membrane from the fixed end toward the free end to a predetermined length. The ratio of the predetermined length of the first element part to the length of the vibration membrane from the fixed end to the free end is set according to a resonance mode specific to the piezoelectric actuator. [Brief description of the drawings]

[0007] [Figure 1A] FIG. 1A is a plan view showing the configuration of a piezoelectric actuator 100 according to a first embodiment. [Figure 1B] FIG. 1B is a cross-sectional view showing the configuration of the piezoelectric actuator 100 according to the first embodiment. [Diagram 2] FIG. 2 is a graph showing the change in maximum displacement of the free end 1b of the vibrating membrane 1 in the piezoelectric actuator 100 according to the vibration frequency of the vibrating membrane 1. As shown in FIG. [Diagram 3] FIG. 3 is a graph showing the displacement shape of the vibrating membrane 1 when the piezoelectric actuator 100 resonates in a plurality of resonance modes. [Figure 4] FIG. 4 is a contour diagram showing the displacement shape of the diaphragm 1 when resonating in a butterfly mode BF, which is an example of a plurality of resonance modes. [Figure 5A] FIG. 5A is a graph showing the results of calculating the maximum displacement of vibrating membrane 1 by changing the ratio (b / L) in piezoelectric actuator 100 of FIGS. 1A and 1B. [Figure 5B] FIG. 5B is a graph showing the results of calculating the maximum value of the stress generated in the vibrating membrane 1 by changing the ratio (b / L) in the piezoelectric actuator 100 of FIGS. 1A and 1B. [Figure 6A] FIG. 6A is a plan view showing the configuration of a piezoelectric actuator 101 having a vibration membrane 1 with different planar dimensions compared to piezoelectric actuator 100 in FIGS. 1A and 1B. [Figure 6B] FIG. 6B is a cross-sectional view showing the configuration of a piezoelectric actuator 101 having a vibration membrane 1 with different planar dimensions compared to the piezoelectric actuator 100 in FIGS. 1A and 1B. [Figure 7A] FIG. 7A is a graph showing the results of calculating the maximum value of the displacement of vibrating membrane 1 by changing the ratio (b / L) in piezoelectric actuator 101 of FIGS. 6A and 6B. [Figure 7B]FIG. 7B is a graph showing the results of calculating the maximum value of the stress generated in the vibrating membrane 1 by changing the ratio (b / L) in the piezoelectric actuator 101 of FIGS. 6A and 6B. [Figure 8A] FIG. 8A is a perspective view (part 1) showing a configuration example of a vibration membrane 1 and a piezoelectric element 3A according to a modified example. [Figure 8B] FIG. 8B is a perspective view (part 2) showing a configuration example of the vibration membrane 1 and the piezoelectric elements 3A and 3B according to a modified example. [Figure 8C] FIG. 8C is a perspective view (part 3) showing a configuration example of the vibration membrane 1 and the piezoelectric elements 3A, 3Ba, and 3Bb according to a modified example. [Figure 8D] FIG. 8D is a perspective view (part 4) showing a configuration example of the vibration membrane 1 and the piezoelectric elements 3A, 3Ba, and 3Bb according to a modified example. [Figure 8E] FIG. 8E is a perspective view (part 5) showing a configuration example of the vibration membrane 1 and the piezoelectric elements 3A, 3Ba, and 3Bb according to a modified example. [Figure 9] FIG. 9 is a graph showing the results of calculating the maximum stress when the planar patterns of piezoelectric elements 3A, 3B, 3Ba, and 3Bb shown in FIGS. 8A to 8E are resonated in butterfly mode BF. [Figure 10] FIG. 10 is a block diagram showing the configuration of a sound wave generating device 200 according to the second embodiment.

[0008] [Detailed Description] Hereinafter, a piezoelectric actuator and a sound wave generating device according to a plurality of embodiments will be described in detail with reference to the drawings. Note that the embodiments described below are comprehensive or specific examples. The numerical values, shapes, materials, components, installation positions and connection forms of the components shown in the following embodiments are merely examples and are not intended to limit the present disclosure. In addition, among the components in the following embodiments, components that are not described in the independent claims showing the highest concept are described as optional components. Furthermore, the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. In addition, the following embodiments and their modified examples may include similar components, and the similar components are given common symbols and redundant explanations are omitted.

[0009] (First embodiment) Fig. 1A is a plan view showing the configuration of a piezoelectric actuator 100 according to a first embodiment. Fig. 1B is a cross-sectional view showing the configuration of the piezoelectric actuator 100 according to the first embodiment. The cross-sectional view of Fig. 1B shows a cross-section of the piezoelectric actuator 100 taken along the A-A' cut surface of Fig. 1A.

[0010] 1A and 1B, a piezoelectric actuator 100 having a cantilever structure includes a vibration membrane 1 having a fixed end 1a and a free end 1b, a membrane support part 2 to which the fixed end 1a of the vibration membrane 1 is connected, and a piezoelectric element 3A arranged on a main surface 1A of the vibration membrane 1. The piezoelectric actuator 100 is an example of a MEMS (Micro Electro Mechanical Systems) that can be applied to a sound wave generating device such as a speaker that generates sound waves in an audible range or an ultrasonic generator that generates ultrasonic waves. The sound wave generating device will be described later.

[0011] The diaphragm 1 is a flexible thin film made of, for example, a semiconductor such as Si (silicon). The planar shape is not particularly limited, but for example, it has a rectangular shape as shown in Figures 1A and 1B. In this case, the fixed end 1a is located on one of the four sides, and the free end 1b is located on the side opposite to this one side.

[0012] The direction from the fixed end 1a to the free end 1b (X direction) is the length direction of the vibrating membrane 1. In the main surface 1A of the vibrating membrane 1, the direction perpendicular to the X direction (Y direction) is the width direction of the vibrating membrane 1. The length (L) and width (W) of the vibrating membrane 1 are not particularly limited, but in the piezoelectric actuator 100 shown in Figures 1A and 1B, the length (L) is 3 mm and the width (W) is 2 mm. Piezoelectric actuators 101 with different lengths (L) and widths (W) will be described later with reference to Figures 6A and 6B.

[0013] 1A, the membrane support part 2 has a cylindrical shape surrounding the periphery of the diaphragm 1. A slit 5 (gap) is formed between the inner periphery of the membrane support part 2 and the outer periphery (three sides) of the diaphragm 1 excluding the fixed end 1a (one side) of the diaphragm 1. The slit 5 allows the diaphragm 1, whose fixed end 1a is fixed to the membrane support part 2, to freely deform relative to the membrane support part 2.

[0014] 1B, the membrane support part 2 has a laminated structure of an upper layer part 2b arranged in the same Y direction position as the diaphragm 1, and a lower layer part 2a connected to the upper layer part. The Y direction is the normal direction of the main surface 1A of the diaphragm 1.

[0015] The diaphragm 1 and the membrane support portion 2 can be formed simultaneously from one semiconductor substrate. The semiconductor substrate is etched from its lower surface side (the lower layer portion 2a side) to form a cavity surrounded by the lower layer portion 2a and the diaphragm 1, as shown in FIG. 1B. Furthermore, the semiconductor substrate between the periphery (three sides) of the diaphragm 1, excluding the fixed end 1a, and the upper layer portion 2b is etched. This separates the periphery (three sides) of the diaphragm 1 from the upper layer portion 2b, and forms a slit 5.

[0016] When an SOI substrate is used as the semiconductor substrate, the Si active layer of the SOI substrate can be used as the diaphragm 1 and the upper layer 2b, and the Si support substrate of the SOI substrate can be used as the lower layer 2a. The insulating layer between the Si active layer and the Si support substrate functions as an etching stopper layer when forming a cavity. The insulating layer may be included in either the upper layer 2b or the lower layer 2a.

[0017] The piezoelectric element 3A has a first element portion 3A formed in a region of the main surface 1A of the vibration membrane 1 from the fixed end 1a toward the free end 1b for a predetermined length (b). In the first embodiment, the piezoelectric element 3A is composed of only the first element portion 3A. That is, the first element portion 3A is formed in a region of the main surface 1A of the vibration membrane 1 for a predetermined length (b) from the fixed end 1a toward the free end 1b, and no piezoelectric element is formed in a region from the end portion 3b on the free end 1b side of the first element portion 3A to the free end 1b. In FIG. 1A and FIG. 1B, the protective film covering the vibration membrane 1 and the piezoelectric element is omitted. Therefore, the main surface 1A of the vibration membrane 1 is exposed in the region where the piezoelectric element is not formed.

[0018] The ratio (b / L) of the predetermined length (b) of the first element portion 3A to the length (L) of the vibration membrane 1 from the fixed end 1a to the free end 1b is set according to a resonance mode specific to the piezoelectric actuator 100. The ratio (b / L) set according to the resonance mode and the resonance mode specific to the piezoelectric actuator 100 will be described later with reference to Figures 2 to 4, 5A, and 5B.

[0019] Here, "the length (L) of the vibration membrane 1 from the fixed end 1a to the free end 1b" refers to the length from the fixed end 1a to the free end 1b of the protruding part (upper layer part 2b) of the cantilever. On the other hand, the range of the predetermined length (b) at which the first element part 3A is formed is 0 or more and less than the length (L) of the vibration membrane 1 in the calculation described later. However, in reality, the outer shapes of the vibration membrane 1 and the piezoelectric element 3A are patterned by different processes. Due to restrictions on the pattern processing accuracy such as mask alignment accuracy, the maximum outer shape of the piezoelectric element 3A formed on the main surface 1A of the vibration membrane 1 is narrowed by a certain margin with respect to the outer shape of the vibration membrane 1. Therefore, the predetermined length (b) at which the first element part 3A is formed is a value smaller than the length (L) of the vibration membrane 1.

[0020] 1A and 1B, the piezoelectric actuator 100 may include a Si subframe 4 for reinforcing the strength of the membrane support portion 2. The Si subframe 4 is connected to the upper end portion of the membrane support portion 2.

[0021] Although not shown in the figure, the piezoelectric element 3A has a laminated structure of a pair of electrodes and a piezoelectric film sandwiched between the pair of electrodes. Each of the pair of electrodes is formed of a thin film of a conductive metal such as aluminum or copper. Alternatively, the lower electrode of the pair of electrodes may have a laminated structure in which titanium (Ti) and platinum (Pt) are laminated in this order. The upper electrode of the pair of electrodes may have a laminated structure in which iridium oxide (IrO2) and iridium (Ir) are laminated in this order. The piezoelectric film is composed of, for example, a lead zirconate titanate (PZT) film. The piezoelectric film may be made of aluminum nitride (AlN), zinc oxide (ZnO), lead titanate (PbTiO3), potassium sodium niobate (KNN), or the like, other than lead zirconate titanate. When an AC voltage is applied to the pair of electrodes, an electric field is generated between the pair of electrodes. This electric field causes the piezoelectric film to expand or contract in a direction parallel to the XY plane, and the free end 1b of the vibration film 1 to be displaced so as to warp in the Y direction or the direction opposite to the Y direction. By repeatedly applying a voltage, the diaphragm 1 alternately repeats displacement in the Y direction and displacement in the direction opposite to the Y direction. This vibration of the diaphragm 1 causes the air around the diaphragm 1 to vibrate, and the vibration of the air is output as a sound wave. The frequency of the sound wave is controlled by the frequency of the AC voltage.

[0022] Fig. 2 is a graph showing the change in the maximum displacement of the free end 1b of the vibrating membrane 1 according to the vibration frequency of the vibrating membrane 1 in the piezoelectric actuator 100 of Figs. 1A and 1B. As shown in Fig. 2, the piezoelectric actuator 100 of Fig. 1 has multiple resonance modes M1, M2, BF, and M3, in order from the lowest frequency, in which the maximum displacement is maximized at a specific vibration frequency. BF is a resonance mode characterized by the shape of the displacement in the width direction (Z direction) of the vibrating membrane 1. To distinguish it from other resonance modes (bending modes) characterized by the shape of the displacement in the length direction (X direction) of the vibrating membrane 1, it will hereinafter be referred to as the "butterfly mode".

[0023] FIG. 3 is a graph showing the displacement shape of the diaphragm 1 when the piezoelectric actuator 100 resonates in a plurality of resonance modes. The horizontal axis indicates the coordinate (X coordinate) in the length direction (X direction) of the diaphragm 1, and the vertical axis indicates the Y-direction component of the displacement of the diaphragm 1. Hereinafter, the "Y-direction component of the displacement" is simply referred to as the "amount of displacement." In any resonance mode, the X coordinate of the fixed end 1a of the diaphragm 1 is normalized to 0, and the X coordinate of the free ends T1 and T2 is normalized to 1. Furthermore, the displacement amounts of the free ends T1 and T2 of the diaphragm 1 are also normalized to 1. The free ends T1 and T2 in FIG. 3 are synonymous with the free end 1b in FIG. 1A and FIG. 1B.

[0024] 3 shows a first-order bending mode M1, a second-order bending mode M2, a third-order bending mode M3, and a fourth-order bending mode M4 as examples of multiple resonance modes. These bending modes are resonance modes characterized by the shape of the displacement in the length direction (X direction) of the diaphragm 1.

[0025] In the first-order bending mode M1, no portion where the Y-direction component of the displacement becomes an extreme value (hereinafter referred to as "antinode of vibration") is formed between the fixed end and the free end T1. In the second-order bending mode M2, one antinode of vibration M2-1 is formed between the fixed end and the free end T2. The X coordinate of the antinode of vibration M2-1 is 0.471. In the third-order bending mode M3, two antinodes of vibration M3-1 and M3-2 are formed between the fixed end and the free end T1. The X coordinates of the antinodes of vibration M3-1 and M3-2 are 0.291 and 0.692, respectively. In the fourth-order bending mode M4, three antinodes M4-1, M4-2, and M4-3 are formed between the fixed end and the free end T2. The X coordinates of the antinodes of vibration M4-1, M4-2, and M4-3 are 0.208, 0.501, and 0.780, respectively.

[0026] 4 is a contour diagram showing the displacement shape of the diaphragm 1 when resonating in butterfly mode BF as an example of multiple resonance modes. A central portion 1b-1 of one side of the diaphragm 1 forming the free end 1b is displaced in the Y direction relative to the frame 6, and both end portions 1b-2 and 1b-3 of one side of the diaphragm 1 forming the free end 1b are displaced in the opposite direction to the Y direction relative to the frame 6. Note that the frame 6 indicates the position where the Y coordinate is 0, i.e., the XZ plane.

[0027] The inventors of the present application calculated the maximum displacement of the vibrating membrane 1 and the maximum stress generated in the vibrating membrane 1 by changing the ratio (b / L) in the piezoelectric actuator 100 of Figures 1A and 1B. The results are shown in Figures 5A and 5B. The horizontal axis shows the ratio (b / L), and the vertical axis shows the maximum displacement of the vibrating membrane 1 after normalization (Figure 5A) and the maximum stress of the vibrating membrane 1 after normalization (Figure 5B). Figures 5A and 5B show the calculation results for the higher-order bending modes M2, M3, and M4.

[0028] As shown by reference symbol m21 in Fig. 5A, in the second-order bending mode M2, when the ratio (b / L) is around 0.5, the maximum displacement of the diaphragm 1 decreases to close to 0 and takes a minimum value. Also, as shown by reference symbols m31 and m32, in the third-order bending mode M3, when the ratio (b / L) is around 0.3 or 0.67, the maximum displacement of the diaphragm 1 decreases to close to 0 and takes a minimum value. Furthermore, as shown by reference symbols m41, m42, and m43, in the fourth-order bending mode M4, when the ratio (b / L) is around 0.2, 0.5, or 0.78, the maximum displacement of the diaphragm 1 decreases to take a minimum value.

[0029] As shown by reference symbol n21 in Fig. 5B, in the second bending mode M2, when the ratio (b / L) is around 0.5, the maximum stress of the diaphragm 1 decreases to nearly 0 and takes a minimum value. Also, as shown by reference symbols n31 and n32, in the third bending mode M3, when the ratio (b / L) is around 0.3 or 0.67, the maximum stress of the diaphragm 1 decreases to nearly 0 and takes a minimum value. Furthermore, as shown by reference symbols n41, n42, and n43, in the fourth bending mode M4, when the ratio (b / L) is around 0.2, 0.5, or 0.78, the maximum stress of the diaphragm 1 takes a minimum value.

[0030] According to the calculation results of Fig. 5A and Fig. 5B, it was found that the maximum displacement and maximum stress of the vibrating membrane 1 each take a minimum value when the ratio (b / L) is approximately the same value. It was also found that the ratio (b / L) at which the minimum value is taken is approximately the same as the X-coordinate of the antinode of vibration shown in Fig. 3. The X-coordinate of the antinode of vibration shown in Fig. 3 is a value obtained by normalizing the X-coordinate of the free end to 1. For this reason, it was found that the maximum displacement and maximum stress in the resonance modes M2 to M4 are simultaneously minimized when the end 3b on the free end 1b side of the piezoelectric element (first element portion) 3A is located at the antinode parts M2-1, M3-1, M3-2, M4-1, M4-2, and M4-3 of the vibration shown in Fig. 3.

[0031] The end 3b on the free end 1b side of the piezoelectric element (first element portion) 3A is aligned with the vibration antinodes M2-1, M3-1, M3-2, M4-1, M4-2, and M4-3 shown in FIG. 3. This causes the movement of the diaphragm 1 in the area where the piezoelectric element 3A is not formed to be out of sync with the movement of the diaphragm 1 in the area where the piezoelectric element 3A is formed, and the movements cancel each other out. This makes it possible to reduce the maximum displacement and maximum stress during resonance. In this way, by optimizing the position of the piezoelectric element 3A on the main surface 1A of the diaphragm 1, it is possible to suppress the vibration of the diaphragm 1 at a specific resonance frequency.

[0032] The inventors of the present application performed calculations similar to those in Fig. 5A and Fig. 5B for piezoelectric actuator 101 having different planar dimensions of vibration membrane 1. As shown in Fig. 6A and Fig. 6B, a piezoelectric element (first element portion) 3A is formed only in a part of vibration membrane 1 on the fixed end 1a side, and no piezoelectric element is formed in the remaining part of vibration membrane 1 on the free end 1b side.

[0033] The only difference between piezoelectric actuator 100 and piezoelectric actuator 101 is the planar dimensions of vibration membrane 1 shown in Fig. 6A. Piezoelectric actuator 101 shown in Fig. 6A and Fig. 6B has a length (L) of 2 mm and a width (W) of 3 mm. The other structures and dimensions are the same.

[0034] The inventors of the present application calculated the maximum displacement of the vibrating membrane 1 and the maximum stress generated in the vibrating membrane 1 by changing the ratio (b / L) in the piezoelectric actuator 101 of Figures 6A and 6B. The results are shown in Figures 7A and 7B. The horizontal axis shows the ratio (b / L), and the vertical axis shows the maximum displacement of the vibrating membrane 1 after normalization (Figure 7A) and the maximum stress of the vibrating membrane 1 after normalization (Figure 7B). Figures 7A and 7B show the calculation results for the higher-order bending modes M2, M3, and M4.

[0035] As shown by reference symbol m21 in Fig. 7A, in the second-order bending mode M2, when the ratio (b / L) is around 0.5, the maximum displacement of the diaphragm 1 decreases to close to 0 and takes a minimum value. Also, as shown by reference symbols m31 and m32, in the third-order bending mode M3, when the ratio (b / L) is around 0.3 or 0.7, the maximum displacement of the diaphragm 1 decreases to close to 0 and takes a minimum value. Furthermore, as shown by reference symbols m41, m42, and m43, in the fourth-order bending mode M4, when the ratio (b / L) is around 0.2, 0.5, or 0.78, the maximum displacement of the diaphragm 1 decreases to take a minimum value.

[0036] As shown by reference symbol n21 in Fig. 7B, in the second bending mode M2, when the ratio (b / L) is around 0.5, the maximum stress of the diaphragm 1 decreases to nearly 0 and takes a minimum value. Also, as shown by reference symbols n31 and n32, in the third bending mode M3, when the ratio (b / L) is around 0.3 or 0.7, the maximum stress of the diaphragm 1 decreases to nearly 0 and takes a minimum value. Furthermore, as shown by reference symbols n41, n42, and n43, in the fourth bending mode M4, when the ratio (b / L) is around 0.2, 0.5, or 0.78, the maximum stress of the diaphragm 1 takes a minimum value.

[0037] According to the calculation results of FIGS. 7A and 7B, it was found that the same findings as those obtained from the calculation results of FIGS. 5A and 5B can be obtained regardless of the planar dimensions of the diaphragm 1 and the first element portion 3A.

[0038] (Modification) A modified example of the piezoelectric actuator 100 shown in Fig. 1A and Fig. 1B will be described with reference to Fig. 8A to Fig. 8E and Fig. 9. The modified example will be a specific example of suppressing the maximum stress generated in the vibration membrane 1 in the butterfly mode BF shown in Fig. 4. Here, the piezoelectric actuator 100 is exemplified, but it goes without saying that the same can be applied to the piezoelectric actuator 101 in Fig. 6A and Fig. 6B.

[0039] 8A to 8E are perspective views of the vibration membrane 1 and the piezoelectric elements 3A and 3B taken out from the piezoelectric actuator 100 shown in Fig. 1A and Fig. 1B. In Fig. 8A, the ratio (b / L) of a given length (b) from the end 3a on the fixed end side of the first element portion 3A to the end 3b on the free end 1b side to the length (L) of the vibration membrane 1 from the fixed end 1a to the free end 1b is 1 / 2. Note that the ratio (b / L) is just an example and may be a value other than 1 / 2.

[0040] 8B to 8E have the same ratio (b / L) as FIG. 8A, but differ in that the piezoelectric element further includes a second element portion 3B. The second element portion 3B is narrower than the first element portion 3A, and extends from the end portion 3b on the free end 1b side of the first element portion 3A toward the free end 1b of the vibration membrane 1. The second element portion 3B and the first element portion 3A have the same layered structure, and are formed integrally by adjusting the patterning shape.

[0041] In the example shown in FIG. 8B, the number of second element parts 3B is one, and they are arranged in the center of the width direction of the diaphragm 1. The width of the second element part 3B is, for example, 1 / 4 of the width of the diaphragm 1. In the examples shown in FIG. 8C to FIG. 8E, the number of second element parts 3Ba, 3Bb is two. The width of each of the second element parts 3Ba, 3Bb is, for example, 1 / 8 of the width of the diaphragm 1. In FIG. 8C, the distance between the second element part 3Ba and the second element part 3Bb is short, but in FIG. 8D, the distance between the second element part 3Ba and the second element part 3Bb is long, and in FIG. 8E, the second element part 3Ba and the second element part 3Bb extend along the end of the diaphragm 1 in the width direction. In all of FIG. 8B to FIG. 8E, the second element part 3B extends to the free end 1b of the diaphragm 1, but the end of the second element part 3B may be located in front of the free end 1b of the diaphragm 1.

[0042] Fig. 9 is a graph showing the results of calculating the maximum stress when the planar patterns of piezoelectric elements 3A and 3B shown in Fig. 8A to Fig. 8E are resonated in butterfly mode BF. The vertical axis of Fig. 9 shows the maximum stress normalized by setting the maximum stress when the ratio (b / L) is 1 as 1. 8A to 8E on the horizontal axis correspond to the planar patterns of Fig. 8A to Fig. 8E, respectively.

[0043] 9, the maximum stress after normalization in all of the patterns shown in Figures 8A to 8E is smaller than 1. Therefore, compared to when the ratio (b / L) is 1, that is, when the piezoelectric elements are arranged over the largest possible area of ​​the main surface 1A of the vibrating membrane 1, all of the planar patterns of the piezoelectric elements 3A, 3B shown in Figures 8A to 8E can reduce the maximum stress during resonance in the butterfly mode BF.

[0044] Comparing Fig. 8A, which does not have the second element portion 3B, with Fig. 8B to Fig. 8E, which have the second element portion 3B, the normalized maximum stress in Fig. 8E is smaller than that in Fig. 8A. Therefore, when the second element portion 3B is added to the first element portion 3A, by extending the second element portion 3B along the end portion in the width direction of the vibrating membrane 1, the maximum stress during resonance in the butterfly mode BF can be further reduced.

[0045] Moreover, by extending the second element portion 3B to the free end 1b of the vibration membrane 1, a region where the piezoelectric element 3B is not formed and a region where the piezoelectric element 3B is formed are formed at the free end 1b of the vibration membrane 1. The butterfly mode BF (see FIG. 4) mainly occurs on the free end 1b side of the vibration membrane 1. Therefore, the movement of the vibration membrane 1 in the region where the piezoelectric element 3B is not formed is no longer synchronized with the movement of the vibration membrane 1 in the region where the piezoelectric element 3B is formed, and the movements cancel each other out. In this way, the maximum displacement and maximum stress of the butterfly mode BF (see FIG. 4) which mainly occurs on the free end 1b side of the vibration membrane 1 can be reduced.

[0046] Second embodiment As described above, the piezoelectric actuators 100 and 101 described in the first embodiment and the modified example are applicable to sound wave generating devices such as speakers that generate sound waves in an audible range or ultrasonic generators that generate ultrasonic waves. For example, as shown in FIG. 10, a sound wave generating device 200 according to the second embodiment may include at least the piezoelectric actuator 100 and a driving unit 150 that applies a predetermined AC voltage to the piezoelectric elements 3A and 3B included in the piezoelectric actuator 100. The AC voltage applied by the driving unit 150 includes at least an AC voltage of a vibration frequency of the vibration membrane in a resonance mode specific to the piezoelectric actuator 100. The driving unit 150 applies an AC voltage to a pair of electrodes of the piezoelectric element based on a weak audio signal input to the sound wave generating device. The piezoelectric actuator 100 vibrates the vibration membrane 1 at a predetermined frequency by the AC voltage, and the vibration of the vibration membrane 1 is transmitted as vibration of the air, and sound waves or ultrasonic waves in an audible range are generated and output. Although not shown in the figure, the sound wave generating device can further include a control unit such as a microcontroller that controls the driving unit 150, thereby making it possible to adjust the overall volume or the volume for each frequency.

[0047] (Appendix 1) A piezoelectric actuator 100, 101 having a cantilever structure, A vibrating membrane 1 having a fixed end 1a and a free end 1b; A membrane support part 2 to which a fixed end 1a of the vibration membrane 1 is connected; A piezoelectric element is disposed on the main surface 1A of the vibration membrane 1, The piezoelectric element has a first element portion 3A formed in a region of the main surface 1A of the vibration membrane 1, the region extending from the fixed end 1a toward the free end 1b by a predetermined length (b); The ratio (b / L) of the predetermined length (b) of the first element portion 3A to the length (L) of the vibration membrane 1 from the fixed end 1a to the free end 1b is set according to the resonance modes M1 to M4 specific to the piezoelectric actuators 100 and 101. Piezoelectric actuator.

[0048] In order to obtain maximum displacement, the cantilever type piezoelectric actuator arranges the piezoelectric body in as wide an area as possible on the main surface of the vibration membrane (cantilever portion). However, when the actuator resonates, the vibration membrane is displaced greatly, increasing the stress generated in the vibration membrane, which may damage the element. Therefore, in Appendix 1, the area on the main surface 1A of the vibration membrane 1 where the piezoelectric element (first element portion 3A) is formed is set to a predetermined length (b) from the fixed end 1a of the vibration membrane 1 toward the free end 1b. As a result, the movement of the vibration membrane 1 in the area where the piezoelectric element (first element portion 3A) is not formed is no longer in sync with the movement of the vibration membrane 1 in the area where the piezoelectric element (first element portion 3A) is formed, and the movements cancel each other out. This makes it possible to reduce the maximum displacement and maximum stress of the vibration membrane 1 during resonance. In this way, by optimizing the arrangement position of the piezoelectric element on the main surface 1A of the vibration membrane 1, it is possible to suppress the vibration of the vibration membrane 1 at a specific resonance frequency (see FIG. 2).

[0049] (Appendix 2) In piezoelectric actuators 100 and 101 described in Supplementary Note 1, the ratio (b / L) is set so that end 3b on the free end 1b side of first element portion 3A is located at the antinode of vibration in the natural resonance mode.

[0050] The inventors calculated the maximum displacement and maximum stress of the vibrating membrane 1 by changing the ratio (b / L), and discovered that when the end 3b on the free end 1b side of the first element portion 3A is located at the antinode of vibration in the natural resonance mode, the maximum displacement and maximum stress are simultaneously minimized. According to Supplementary Note 2, the ratio (b / L) is set so that the end 3b on the free end 1b side of the piezoelectric element (first element portion 3A) is located at the antinode of vibration in the natural resonance mode, thereby reducing the maximum displacement and maximum stress of the vibrating membrane 1. Specifically, the ratio (b / L) is set to a numerical range including the ratio (b / L) having the minimum value in Figs. 5A, 5B, 7A, and 7B, centered on the antinode of vibration in each of the high-order resonance modes M2, M3, and M4 shown in Fig. 3. This allows the maximum displacement and maximum stress of the vibrating membrane 1 to be reduced. The numerical ranges for each of the high-order bending modes will be described below.

[0051] (Appendix 3) In the piezoelectric actuator 100, 101 described in Supplementary Note 1 or 2, the inherent resonance mode includes a bending secondary mode M2 ​​in which one antinode M2-1 is formed between the fixed end 1 a and the free end 1 b, The ratio (b / L) is set to 0.416 or more and 0.526 or less for the second bending mode M2. According to Supplementary Note 3, the displacement and stress of the vibrating membrane 1 during resonance in the second bending mode M2 ​​can be reduced.

[0052] (Appendix 4) In the piezoelectric actuator 100, 101 according to any one of Supplementary Notes 1 to 3, The inherent resonance modes include a third-order bending mode M3 in which two antinodes M3-1 and M3-2 are formed between the fixed end 1a and the free end 1b. The ratio (b / L) is set to 0.256 or more and 0.326 or less, or 0.657 or more and 0.727 or less, for the third bending mode M3. According to Appendix 4, the displacement and stress of the vibrating membrane 1 during resonance in the third bending mode M3 can be reduced.

[0053] (Appendix 5) In the piezoelectric actuator 100, 101 according to any one of Supplementary Notes 1 to 4, The inherent resonance modes include a fourth-order bending mode M4 in which three antinodes M4-1 to M4-3 are formed between the fixed end 1a and the free end 1b, The ratio (b / L) is set to 0.183 to 0.233, 0.476 to 0.526, or 0.755 to 0.805 for the fourth bending mode M4. According to Appendix 5, the displacement and stress of the vibrating membrane 1 during resonance in the fourth bending mode M4 can be reduced.

[0054] (Appendix 6) In the piezoelectric actuator 100, 101 according to any one of Supplementary Notes 1 to 5, The piezoelectric element further includes a second element portion 3B narrower in width than the first element portion 3A and extending from an end portion 3b on the free end 1b side of the first element portion 3A toward the free end 1b.

[0055] (Appendix 7) In the piezoelectric actuator 100, 101 described in Supplementary Note 6, The second element portion 3B extends along the end portion in the width direction of the vibrating membrane 1. This can reduce the maximum stress during resonance in the butterfly mode BF.

[0056] (Appendix 8) In the piezoelectric actuator 100 or 101 according to Supplementary Note 6 or 7, The second element portion 3B extends to the free end 1b. According to Supplementary Note 8, an area where the piezoelectric element 3B is not formed and an area where the piezoelectric element 3B is formed are formed at the free end 1b of the vibration membrane 1. The butterfly mode BF (see FIG. 4) mainly occurs on the free end 1b side of the vibration membrane 1. Therefore, the movement of the vibration membrane 1 in the area where the piezoelectric element 3B is not formed is not synchronized with the movement of the vibration membrane 1 in the area where the piezoelectric element 3B is formed, and the movements cancel each other out. In this way, the maximum displacement and maximum stress of the butterfly mode BF (see FIG. 4) mainly occurring on the free end 1b side of the vibration membrane 1 can be reduced.

[0057] (Appendix 9) A piezoelectric actuator 100, 101 according to any one of Supplementary Notes 1 to 8; A driving unit 150 that applies an AC voltage having a vibration frequency of the vibration membrane 1 in at least a natural resonance mode to the piezoelectric element; The sound wave generating device 200 includes:

[0058] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the present disclosure is not limited to the embodiments described in the present disclosure. One or more elements of one embodiment can be combined with one or more elements of another embodiment. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended to be illustrative and does not have any limiting meaning on the present disclosure. [Explanation of symbols]

[0059] 1. Diaphragm 1a fixed end 1A main surface 1b free end 1b-1 Central part 1b-2, 1b-3 both ends 2 Membrane support part 2a Lower part 2b upper level 3A First element part 3b Free end 3B, 3Ba, 3Bb 2nd element part 4Si subframe 5 Slit 100, 101 Piezoelectric actuator 150 Drive unit 200 Sonic Generator M1 bending mode M2 Secondary bending mode M3 third bending mode M4 Fourth bending mode M2-1, M3-1, M3-2, M4-1, M4-2, M4-3 Belly T1, T2 free end W width

Claims

1. A piezoelectric actuator having a cantilever structure, a diaphragm having a fixed end and a free end; A membrane support part to which a fixed end of the vibration membrane is connected; A piezoelectric element is disposed on a main surface of the vibration membrane, the piezoelectric element has a first element portion formed in a region of a main surface of the vibration membrane, the region extending from the fixed end toward the free end to a predetermined length, a ratio of the predetermined length of the first element portion to the length of the vibration membrane from the fixed end to the free end is set according to a resonance mode specific to the piezoelectric actuator. Piezoelectric actuator.

2. 2. The piezoelectric actuator according to claim 1, wherein the ratio is set so that an end portion on the free end side of the first element portion is located at an antinode portion of vibration in the natural resonance mode.

3. the inherent resonance mode includes a bending second-order mode in which one antinode is formed between the fixed end and the free end; The piezoelectric actuator according to claim 1 , wherein the ratio is set to be equal to or greater than 0.416 and equal to or less than 0.526 for the secondary bending mode.

4. the inherent resonance mode includes a third bending mode in which two antinodes are formed between the fixed end and the free end; The piezoelectric actuator according to claim 1 , wherein the ratio is set to be equal to or greater than 0.256 and equal to or less than 0.326, or equal to or greater than 0.657 and equal to or less than 0.727, for the third bending mode.

5. the inherent resonance mode includes a bending fourth-order mode in which three antinodes are formed between the fixed end and the free end; The piezoelectric actuator according to claim 1 , wherein the ratio is set to be equal to or greater than 0.183 and equal to or less than 0.233, equal to or greater than 0.476 and equal to or less than 0.526, or equal to or greater than 0.755 and equal to or less than 0.805 for the fourth bending mode.

6. 2. The piezoelectric actuator according to claim 1, wherein the piezoelectric element further comprises a second element portion narrower in width than the first element portion, the second element portion extending from an end portion on the free end side of the first element portion toward the free end.

7. The piezoelectric actuator according to claim 6 , wherein the second element portion extends along an end portion in a width direction of the vibration membrane.

8. The piezoelectric actuator according to claim 6 , wherein the second element portion extends to the free end.

9. A piezoelectric actuator according to any one of claims 1 to 8; A drive unit that applies an AC voltage having a vibration frequency of the vibration membrane in at least the inherent resonance mode to the piezoelectric element; A sound wave generating device comprising:

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