Driving method for piezoelectric actuator

Abstract

To provide a driving method for a piezoelectric actuator that ensures both operational reliability and displacement amount.SOLUTION: In a driving method for a piezoelectric actuator 1 comprising a piezoelectric section 4 including a piezoelectric element 2 and a diaphragm 3 bonded to the piezoelectric element 2, a drive voltage is applied to the piezoelectric section 4 such that the direction of the drive voltage applied to the piezoelectric section 4 is aligned with the polarization direction of the piezoelectric element 2, and such that an electric field strength exceeding the coercive field (+Ec) is applied to the piezoelectric element 2.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a method for driving a piezoelectric actuator. [Background technology]

[0002] Piezoelectric elements have the property of deforming due to the reverse voltage effect, and are used as piezoelectric actuators that convert electrical signals into physical deformation (see, for example, Patent Document 1). One application of piezoelectric actuators is piezoelectric valves. Piezoelectric valves are attached to compressed air pipes used in factories, for example, and have the function of maintaining a constant pressure within the pipes. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-133372 Summary of the Invention [Problem to be solved by the invention]

[0004] The piezoelectric element used in a piezoelectric actuator is made of, for example, a sintered dielectric ceramic body. A high DC voltage is applied to the sintered piezoelectric element to align the spontaneous polarization within the element (a polarization process). When a driving voltage is applied from the outside to the poling-processed piezoelectric element, the charges within the element attract or repel the external charges, causing displacement of the element.

[0005] When such a piezoelectric actuator is operated with an AC voltage that does not contain a DC component, a voltage component opposite to the polarization direction of the element body is applied to the piezoelectric element. Therefore, it is necessary to limit the applied voltage so that polarization reversal due to this opposite voltage component does not occur, making it difficult to operate at high power, i.e., to ensure a large amount of displacement. Furthermore, even when the piezoelectric actuator is operated with a voltage that contains a DC component, if polarization reversal occurs due to the applied voltage component, the element body will no longer be displaced, which could reduce the operational reliability of the piezoelectric actuator.

[0006] The present disclosure has been made to solve the above-mentioned problems, and has an object to provide a method for driving a piezoelectric actuator that can satisfactorily ensure both operational reliability and displacement amount. [Means for solving the problem]

[0007] The gist of the present disclosure is as follows.

[0008] [1] A method for driving a piezoelectric actuator having a piezoelectric part comprising a piezoelectric element and a vibration plate bonded to the piezoelectric element, wherein the direction of application of a drive voltage to the piezoelectric part is aligned with the polarization direction of the piezoelectric element, and the drive voltage is applied to the piezoelectric part so that an electric field strength exceeding the coercive electric field is applied to the piezoelectric element.

[0009] In this method of driving a piezoelectric actuator, the direction of the drive voltage applied to the piezoelectric section is aligned with the polarization direction of the piezoelectric element. In this state, by applying a drive voltage to the piezoelectric section so that an electric field strength exceeding the coercive field is applied to the piezoelectric element, an electric field exceeding the coercive field on the negative side is not applied to the piezoelectric element, and it is possible to apply a high electric field exceeding the coercive field on the positive side. Therefore, this method of operating a piezoelectric actuator can avoid polarization reversal due to the applied voltage component, and can ensure both operational reliability and displacement.

[0010] [2] The method for driving a piezoelectric actuator according to [1], wherein a voltage including a DC component is applied as the drive voltage to the piezoelectric section, in which case the direction of the drive voltage applied to the piezoelectric section can be more reliably aligned with the polarization direction of the piezoelectric element.

[0011] [3] The method for driving a piezoelectric actuator according to [1] or [2], wherein the piezoelectric part is warped so that the piezoelectric element side is concave when no drive voltage is applied to the piezoelectric element. By warping the piezoelectric part in a concave shape in advance in this way, the piezoelectric part is more likely to warp in a concave shape when a drive voltage is applied. Therefore, a larger displacement can be ensured with a low voltage.

[0012] [4] The method for driving a piezoelectric actuator according to [3], wherein the amount of warping of the piezoelectric portion is made larger than the thickness of the diaphragm when no drive voltage is applied to the piezoelectric element. By providing a sufficient amount of warping to the piezoelectric portion when no drive voltage is applied to the piezoelectric element, a larger amount of displacement can be ensured at a low voltage.

[0013] [5] A method for driving a piezoelectric actuator according to [3] or [4], wherein the amount of warping of the piezoelectric part when a drive voltage is applied to the piezoelectric element is made larger than the amount of warping of the piezoelectric part when no drive voltage is applied to the piezoelectric element. By making the direction of warping of the piezoelectric part when a drive voltage is applied to the piezoelectric element coincide with the direction of warping of the piezoelectric part when no drive voltage is applied to the piezoelectric element, a larger amount of displacement can be ensured at a low voltage. [Effects of the Invention]

[0014] According to the present disclosure, both the operational reliability and the displacement amount of the piezoelectric actuator can be ensured satisfactorily. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a perspective view illustrating a configuration of a piezoelectric actuator according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a side view of the piezoelectric actuator shown in FIG. [Figure 3] FIG. 1(a) is a side view showing the state of polarization processing of the piezoelectric actuator, and FIG. 1(b) is a side view showing the state of voltage application of the piezoelectric actuator after polarization processing. [Figure 4] FIG. 10 is a diagram showing the relationship between the electric field strength and the degree of polarization in a piezoelectric element. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, a preferred embodiment of a method for driving a piezoelectric actuator according to one aspect of the present disclosure will be described in detail with reference to the drawings.

[0017] Fig. 1 is a perspective view showing the configuration of a piezoelectric actuator according to one embodiment of the present disclosure. Fig. 2 is a side view thereof. The piezoelectric actuator 1 shown in Figs. 1 and 2 is configured as a bending element that is applied to, for example, a piezoelectric valve. A piezoelectric valve is a component that is attached to a compressed air pipe used in, for example, a factory, and has the function of maintaining a constant pressure within the pipe.

[0018] The piezoelectric actuator 1 is configured with a piezoelectric part 4 that includes a piezoelectric element 2 and a vibration plate 3 bonded to the piezoelectric element 2. The vibration plate 3 has a rectangular shape, for example, in a plan view. The thickness of the vibration plate 3 is approximately the same as or slightly larger than the thickness of the piezoelectric element 2. The vibration plate 3 is made of a conductive material such as metal.

[0019] The material constituting the diaphragm 3 may be carbon fiber reinforced plastic, 42 alloy, etc. One longitudinal side of the diaphragm 3 is held by, for example, adhering it to another member with an adhesive, forming a hole through which a fixing member can be inserted, or clamping it with another member. As a result, one longitudinal side of the diaphragm 3 becomes a fixed end, and the other longitudinal side of the diaphragm 3 becomes a free end.

[0020] In this embodiment, a notch 3b is provided at one of the corners on the other side in the longitudinal direction of the diaphragm 3. The notch 3b serves as a mark for visually distinguishing the front and back of the diaphragm 3, and contributes to improving the ease of attachment work when holding the diaphragm 3 to another member. In the example of FIG. 1, the shape of the notch 3b (the shape of the corner cut out by the notch 3b) is triangular, but the shape of the notch 3b is not limited to this and may be other shapes such as rectangular or arcuate.

[0021] In this embodiment, the piezoelectric section 4 has a plurality of plate-shaped piezoelectric elements 2. Each piezoelectric element 2 has an element body 5 and a pair of electrodes 6, 6. The element body 5 does not have internal electrodes and is composed of a single piezoelectric layer. The piezoelectric layer is made of a piezoelectric material such as a piezoelectric ceramic. Examples of piezoelectric ceramic materials include PZT [Pb(Zr,Ti)O3], PT(PbTiO3), PLZT [(Pb,La)(Zr,Ti)O3], and barium titanate (BaTiO3).

[0022] The electrodes 6 are provided on both sides of the piezoelectric element 2. The electrodes 6 are made of a metal plating film formed by, for example, sputtering or vapor deposition. The electrodes 6 may also be made of a multi-layer metal plating film. Examples of materials for the electrodes 6 include chromium (Cr) / copper-nickel alloy (Ni-Cu) / gold (Au). The planar shape of the electrodes 6 on both sides of the piezoelectric element 2 is a rectangle that is slightly smaller than the planar shape of the base body 5, for example. As a result, the electrodes 6 are arranged in a region inside the outline of the piezoelectric element 2 in a planar view of the piezoelectric element 2, and a frame-shaped electrode-free region R is formed around the edge of the piezoelectric element 2.

[0023] The piezoelectric section 4 is a laminate of the above-described vibration plate 3 and multiple piezoelectric elements 2. The piezoelectric section 4 is configured, for example, by stacking multiple piezoelectric elements 2 having different longitudinal dimensions on the vibration plate 3 in descending order of size. In the example of FIGS. 1 and 2, piezoelectric elements 2A and 2B are stacked in this order parallel to each other on one surface of the vibration plate 3. An electrode 6 on one surface of the piezoelectric element 2A contacts one surface of the vibration plate 3, and an electrode 6 on the other surface of the piezoelectric element 2A contacts an electrode 6 on one surface of the piezoelectric element B. As a result, the vibration plate 3 and the piezoelectric element 2A are electrically connected to each other, and the piezoelectric elements 2A and 2B are electrically connected to each other. In this embodiment, the thickness of the piezoelectric element 2A and the thickness of the piezoelectric element 2B are equal to each other. An embodiment in which one of the thicknesses of the piezoelectric element 2A and the piezoelectric element 2B is greater than the other may also be used.

[0024] The piezoelectric element 2A overlapping the diaphragm 3 is an element that mainly contributes to the displacement of the piezoelectric portion 4. The longitudinal dimension of the piezoelectric element 2A is smaller than the longitudinal dimension of the diaphragm 3. This allows the position of the piezoelectric element 2A to be adjusted freely in the longitudinal direction of the diaphragm 3. The extension length L1 of one side (fixed end side) of the diaphragm 3 from the piezoelectric element 2A and the extension length L2 of the other side (free end side) of the diaphragm 3 from the piezoelectric element 2A may be equal to or different from each other. Furthermore, the lateral dimension of the piezoelectric element 2A is slightly smaller than the lateral dimension of the diaphragm 3. Due to this dimensional difference, a margin for forming an adhesive portion 7A, which will be described later, is provided on one side of the diaphragm 3.

[0025] 1 and 2, the piezoelectric element 2A is biased toward the fixed end on one surface of the vibration plate 3, and the protrusion length L1 on one side of the vibration plate 3 is smaller than the protrusion length L2 on the other side of the vibration plate 3. According to this embodiment, it is possible to suppress variations in the amount of displacement for each piezoelectric actuator 1, which are caused, for example, by manufacturing variations in the piezoelectric elements 2A and 2B. On the other hand, the piezoelectric element 2A may be biased toward the free end on one surface of the vibration plate 3, and the protrusion length L2 on the other side of the vibration plate 3 may be smaller than the protrusion length L1 on one side of the vibration plate 3. In this case, the center of gravity of the piezoelectric portion 4 is located on the free end side, which makes it possible to increase the power (torque) when the piezoelectric portion 4 is displaced.

[0026] The piezoelectric element 2B overlapping the piezoelectric element 2A is an element that mainly contributes to the power (torque) of the piezoelectric portion 4. The longitudinal dimension of the piezoelectric element 2B is smaller than the longitudinal dimension of the piezoelectric element 2A. In the example of FIGS. 1 and 2, the piezoelectric element 2B is located on the other surface side of the piezoelectric element 2A toward the free end, to the extent that the electrode-free region R on the other longitudinal side (free end side) of the piezoelectric element 2A is slightly exposed. This positions the center of gravity of the piezoelectric portion 4 toward the free end, further increasing the power (torque) when the piezoelectric portion 4 is displaced. Furthermore, the lateral dimension of the piezoelectric element 2B is slightly smaller than the lateral dimension of the piezoelectric element 2A. Due to this dimensional difference, an allowance for forming an adhesive portion 7B (described later) is provided in the electrode-free region R of the piezoelectric element 2A.

[0027] Adhesive sections 7 are provided to bond the diaphragm 3 to the piezoelectric element 2A and to bond the piezoelectric element 2A to the piezoelectric element 2B. To prevent short circuits, the adhesive sections 7 are made of an adhesive material that does not contain conductive components such as conductive particles. Examples of adhesive materials that make up the adhesive sections 7 include thermosetting epoxy resins.

[0028] 2, adhesive portion 7A used to bond diaphragm 3 to piezoelectric element 2A is disposed over the entire surface of piezoelectric element 2A, connecting one surface of diaphragm 3 to electrode 6 on one surface of piezoelectric element 2A. Adhesive portion 7B used to bond piezoelectric element 2B to piezoelectric element 2A is disposed over the entire surface of piezoelectric element 2B, connecting electrode 6 on the other surface of piezoelectric element 2A to electrode 6 on one surface of piezoelectric element 2B. As described above, adhesive portion 7 is made of an adhesive material that does not contain conductive components. However, because the driving voltage of piezoelectric actuator 1 includes a high voltage of, for example, several hundred volts, the electrical connection between diaphragm 3 and piezoelectric element 2A and the electrical connection between piezoelectric element 2A and piezoelectric element 2B are maintained even with adhesive portion 7 interposed.

[0029] As shown in Fig. 2, the piezoelectric actuator 1 having the above-described configuration has a wiring section 11 used for polarization of the piezoelectric element 2 and for driving the piezoelectric section 4. The wiring section 11 is electrically connected to a power supply section (not shown) that applies a predetermined drive voltage to the piezoelectric actuator 1. The piezoelectric elements 2A and 2B are electrically connected in parallel via the electrode 6, and are connected to the positive side of the power supply section by the wiring section 11. The diaphragm 3 is electrically connected in series with the piezoelectric elements 2A and 2B via the electrode 6, and is connected to the negative side of the power supply section by the wiring section 11.

[0030] The wiring portion 11 is electrically connected to one surface of the diaphragm 3, the electrode 6 on the other surface of the piezoelectric element 2A, and the electrode 6 on the other surface of the piezoelectric element 2B by joints 12. The joints 12 are formed by, for example, soldering. Examples of the solder material used for the joints 12 include Sn-Ag-Cu (M705, manufactured by Senju Metal Industry Co., Ltd.).

[0031] In this embodiment, the bonded portion 12A between the vibration plate 3 and the wiring portion 11, the bonded portion 12B between the electrode 6 of the piezoelectric element 2A and the wiring portion 11, and the bonded portion 12C between the electrode 6 of the piezoelectric element 2B and the wiring portion 11 are all located on one longitudinal end side (fixed end side) of the piezoelectric portion 4. In the example of FIG. 2 , the bonded portion 12A is provided on one surface of the vibration plate 3 in a region that protrudes toward the fixed end from the longitudinal end of the piezoelectric element 2A, and the bonded portion 12B is provided on the electrode 6 on the other surface of the piezoelectric element 2A in a region that protrudes toward the fixed end from the longitudinal end of the piezoelectric element 2B. The bonded portion 12C is provided on the electrode 6 on the other surface of the piezoelectric element 2B in a region closer to the fixed end. By arranging these bonded portions 12A to 12C on the fixed end side, excessive stress is prevented from being applied to the bonded portions 12A to 12C when the piezoelectric portion 4 is displaced, and breakage of the wiring portion 11 can be preferably prevented.

[0032] FIG. 3(a) is a side view showing the polarization process of the piezoelectric actuator 1. In the piezoelectric actuator 1, a high DC voltage is applied to the element body 5 to perform a polarization process that aligns the direction of spontaneous polarization within the element body 5. By applying an external drive voltage to the piezoelectric element 2 after polarization process, the charges within the element body 5 attract or repel the external charges, resulting in displacement of the element body 5. When performing polarization process on a piezoelectric actuator in which a diaphragm is bonded to a piezoelectric element, generally, the piezoelectric element is polarized before bonding to the diaphragm. In contrast, in this embodiment, as shown in FIG. 3(a), the piezoelectric element 2 is bonded to the diaphragm 3 and then polarization process is performed on the piezoelectric element 2.

[0033] In the example of Figure 3(a), after bonding the piezoelectric element 2 to one surface of the diaphragm 3, a DC voltage is applied to the base body 5 of the piezoelectric element 2 so that the piezoelectric element 2 side is positive and the diaphragm 3 side is negative. The positive charges in the base body 5 are biased toward the negative potential side, and the negative charges in the base body 5 are biased toward the positive potential side. This causes the piezoelectric element 2 to displace in the contraction direction, and the piezoelectric part 4 takes on a warped shape so that the piezoelectric element 2 side is concave when no drive voltage is applied to the piezoelectric element 2.

[0034] The amount of warping T1 of the piezoelectric part 4 can be measured, for example, by setting one end or the other end in the longitudinal direction of the piezoelectric part 4 as a reference end and measuring the height of the other end from the horizontal plane when the reference end is placed on a horizontal plane. In this embodiment, when no drive voltage is applied to the piezoelectric element 2, the amount of warping T1 of the piezoelectric part 4 is greater than the thickness T2 of the diaphragm 3. There are no particular restrictions on the relationship between the amount of warping T1 of the piezoelectric part 4 and the thickness T2 of the diaphragm 3, and this relationship can be set appropriately depending on the configuration and use of the piezoelectric actuator 1 to which it is applied.

[0035] To drive the piezoelectric actuator 1 after polarization processing, a voltage containing a DC component is applied as a drive voltage to the piezoelectric part 4. The direction of the drive voltage applied to the piezoelectric part 4 is made to coincide with the polarization direction of the piezoelectric element 2. The voltage containing a DC component may be a voltage containing only a DC component, or a voltage in which an AC component is superimposed on a DC component. In the latter case, the superimposed voltage is set so that it does not cross between positive and negative.

[0036] In this embodiment, as shown in Fig. 3(a), during polarization, a DC voltage is applied to the base body 5 of the piezoelectric element 2 so that the piezoelectric element 2 side is positive and the diaphragm 3 side is negative. Therefore, when the piezoelectric actuator 1 is driven, a drive voltage is also applied to the piezoelectric element 2 so that the piezoelectric element 2 side is positive and the diaphragm 3 side is negative, as shown in Fig. 3(b). This causes the direction of displacement of the piezoelectric part 4 during driving to match the direction of displacement of the piezoelectric part during polarization, and the amount of warping T1' of the piezoelectric part 4 when a drive voltage is applied to the piezoelectric element 2 is greater than the amount of warping T1 of the piezoelectric part 4 when no drive voltage is applied to the piezoelectric element 2.

[0037] Furthermore, when driving the piezoelectric actuator 1 after polarization processing, the direction of the drive voltage applied to the piezoelectric portion 4 is aligned with the polarization direction of the piezoelectric element 2, and the drive voltage is applied to the piezoelectric portion 4 so that an electric field strength exceeding the coercive electric field is applied to the piezoelectric element 2. FIG. 4 is a diagram showing the relationship between the electric field strength and the degree of polarization in the piezoelectric element 2. As shown in FIG. 4, the relationship between the electric field strength and the degree of polarization in the piezoelectric element 2 is represented by a hysteresis curve H. In the hysteresis curve H, the coercive electric field refers to the electric field strength at which the degree of polarization becomes 0. In FIG. 4, the positive electric field strength at which the degree of polarization reverses from negative to positive is referred to as the coercive electric field +Ec, and the negative electric field strength at which the degree of polarization reverses from positive to negative is referred to as the coercive electric field -Ec.

[0038] As a comparative example, when the piezoelectric actuator 1 is driven by an AC voltage that does not contain a DC component, a voltage component opposite to the polarization direction of the element body 5 is applied to the piezoelectric element 2, and the electric field strength applied to the piezoelectric element 2 by this AC voltage falls within a certain range R1 between positive and negative values, straddling 0. For this reason, in order to prevent polarization reversal due to the opposite voltage component, it is necessary to limit the applied voltage so that the range R1 of the electric field strength does not exceed the negative coercive field −Ec or the positive coercive field +Ec, making it difficult to operate at high power, i.e., to ensure a large displacement.

[0039] In contrast, in this embodiment, a voltage including a DC component is applied to the piezoelectric portion 4 as a drive voltage, and the direction of the drive voltage applied to the piezoelectric portion 4 is aligned with the polarization direction of the piezoelectric element 2. The drive voltage is then applied to the piezoelectric portion 4 so that an electric field strength exceeding the positive coercive field +Ec is applied to the piezoelectric element 2. That is, in this embodiment, the electric field strength applied to the piezoelectric element 2 is in a range R2 that is equal to or greater than 0 and exceeds the positive coercive field +Ec, as shown in FIG. 4 . In this range R2, an electric field exceeding the negative coercive field −Ec is not applied to the piezoelectric element 2, and a high electric field exceeding the positive coercive field +Ec can be applied. Therefore, polarization reversal due to the applied voltage component can be avoided, ensuring both operational reliability and the amount of displacement.

[0040] As explained above, in this method of driving a piezoelectric actuator, the direction of the drive voltage applied to the piezoelectric section 4 is aligned with the polarization direction of the piezoelectric element 2. In this state, by applying a drive voltage to the piezoelectric section 4 so that an electric field strength exceeding the positive coercive field +Ec is applied to the piezoelectric element 2, it is possible to apply a high electric field exceeding the coercive field +Ec on the positive side without applying an electric field exceeding the negative coercive field -Ec to the piezoelectric element 2. Therefore, in this method of operating a piezoelectric actuator, it is possible to avoid polarization reversal due to the applied voltage component, and to ensure both operational reliability and displacement.

[0041] In this embodiment, a voltage containing a DC component is applied as the drive voltage to the piezoelectric portion 4. This makes it possible to more reliably align the direction of the drive voltage applied to the piezoelectric portion 4 with the polarization direction of the piezoelectric element 2.

[0042] In this embodiment, the piezoelectric portion 4 is warped so that the piezoelectric element 2 side is concave when no drive voltage is applied to the piezoelectric element 2. By warping the piezoelectric portion 4 in a concave shape in advance in this way, the piezoelectric portion 4 is more likely to warp in a concave shape when a drive voltage is applied. Therefore, a larger amount of displacement can be ensured at a low voltage.

[0043] In this embodiment, when no drive voltage is applied to the piezoelectric element 2, the amount of warping T1 of the piezoelectric portion 4 is set to be larger than the thickness T2 of the diaphragm 3. By providing a sufficient amount of warping to the piezoelectric portion 4 when no drive voltage is applied to the piezoelectric element 2, a larger amount of displacement can be ensured at a low voltage.

[0044] In this embodiment, the amount of warping T1' of the piezoelectric portion 4 when a drive voltage is applied to the piezoelectric element 2 is set to be larger than the amount of warping T1 of the piezoelectric portion 4 when no drive voltage is applied to the piezoelectric element 2. By matching the direction of warping of the piezoelectric portion 4 when a drive voltage is applied to the piezoelectric element 2 with the direction of warping of the piezoelectric portion 4 when no drive voltage is applied to the piezoelectric element 2, a larger amount of displacement can be ensured at a low voltage.

[0045] The present disclosure is not limited to the above-described embodiments. For example, in the above-described embodiments, a piezoelectric valve is given as an example of the application of the piezoelectric actuator 1, but the piezoelectric actuator 1 is not limited to piezoelectric valves and can also be applied to other devices such as acoustic devices such as speakers and buzzers, and print heads for inkjet printers.

[0046] In the above embodiment, the piezoelectric portion 4 is formed by a laminate in which piezoelectric elements 2A, 2B having different longitudinal dimensions are stacked, but the longitudinal dimensions of the piezoelectric elements 2A, 2B may be the same. Furthermore, the number of layers of piezoelectric elements 2 stacked on the vibration plate 3 is not limited to two, and may be a single layer or three or more layers. [Explanation of symbols]

[0047] 1...piezoelectric actuator, 2 (2A, 2B)...piezoelectric element, 3...diaphragm, 4...piezoelectric part, T1...amount of warping of the piezoelectric part when no voltage is applied, T1'...amount of warping of the piezoelectric part when voltage is applied, T2...thickness of the diaphragm, +Ec...coercive electric field.

Claims

1. A method for driving a piezoelectric actuator having a piezoelectric part including a piezoelectric element and a vibration plate bonded to the piezoelectric element, A method for driving a piezoelectric actuator, wherein the direction of application of a drive voltage to the piezoelectric portion is aligned with the polarization direction of the piezoelectric element, and the drive voltage is applied to the piezoelectric portion so that an electric field strength exceeding the coercive electric field is applied to the piezoelectric element.

2. 2. The method for driving a piezoelectric actuator according to claim 1, wherein a voltage including a DC component is applied as the drive voltage to the piezoelectric portion.

3. 3. The method for driving a piezoelectric actuator according to claim 1, wherein the piezoelectric portion is warped so that the piezoelectric element side is concave when no drive voltage is applied to the piezoelectric element.

4. 4. The method for driving a piezoelectric actuator according to claim 3, wherein the amount of warping of the piezoelectric portion is made larger than the thickness of the vibration plate when no drive voltage is applied to the piezoelectric element.

5. 4. The method for driving a piezoelectric actuator according to claim 3, wherein the amount of warping of the piezoelectric portion when a drive voltage is applied to the piezoelectric element is made larger than the amount of warping of the piezoelectric portion when no drive voltage is applied to the piezoelectric element.

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