Piezoelectric actuator and mass flow controller

Abstract

This piezoelectric actuator comprises a piezoelectric element, a case, and a strain sensor. The piezoelectric element has a longitudinal direction. The case has a lid part, a bottom part, and a cylindrical part, and accommodates the piezoelectric element therein. The strain sensor is positioned in the cylindrical part. The cylindrical part has a bellows shape in which a plurality of recesses and protrusions are repeated along the longitudinal direction. The strain sensor is positioned so as to span over at least one of the recesses.

Description

Piezoelectric Actuator and Mass Flow Controller

[0001] The present disclosure relates to a piezoelectric actuator and a mass flow controller.

[0002] A mass flow controller and a piezoelectric actuator used in a precision positioning device of an XY table are known. This piezoelectric actuator includes a piezoelectric element and a case that houses such a piezoelectric element therein. Further, a strain sensor is attached to the surface of the case of this piezoelectric actuator, and thereby, the displacement of the piezoelectric actuator can be detected (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 63-66980

[0004] The piezoelectric actuator of the present disclosure includes a piezoelectric element, a case, and a strain sensor. The piezoelectric element has a longitudinal direction. The case has a lid portion, a bottom portion, and a cylindrical portion, and houses the piezoelectric element therein. The strain sensor is located in the cylindrical portion. The cylindrical portion has a bellows shape in which a plurality of concave portions and convex portions are repeated along the longitudinal direction. The strain sensor is located across at least one of the concave portions.

[0005] Figure 1 is a perspective view showing the overall configuration of a piezoelectric actuator according to an embodiment. Figure 2 is a cross-sectional view taken along the line A-A shown in Figure 1. Figure 3 is an enlarged cross-sectional view showing an example of the configuration of a strain sensor and its vicinity according to an embodiment. Figure 4 is an enlarged cross-sectional view showing another example of the configuration of a strain sensor and its vicinity according to an embodiment. Figure 5 is an enlarged cross-sectional view showing another example of the configuration of a strain sensor and its vicinity according to an embodiment. Figure 6 is a longitudinal cross-sectional view showing the configuration of a piezoelectric actuator according to another embodiment 1. Figure 7 is a longitudinal cross-sectional view showing the configuration of a piezoelectric actuator according to another embodiment 2. Figure 8 is a longitudinal cross-sectional view showing the configuration of a piezoelectric actuator according to another embodiment 3. Figure 9 is a longitudinal cross-sectional view showing the configuration of a piezoelectric actuator according to another embodiment 4. Figure 10 is a longitudinal cross-sectional view showing the configuration of a piezoelectric actuator according to another embodiment 5. Figure 11 is a perspective view showing the overall configuration of a piezoelectric actuator according to another embodiment 6. Figure 12 is a transverse cross-sectional view showing the configuration of a piezoelectric actuator according to another embodiment 6. Figure 13 is a transverse cross-sectional view showing the configuration of a piezoelectric actuator according to another embodiment 7. Figure 14 is a block diagram showing the configuration of a mass flow controller according to an embodiment.

[0006] The embodiments for implementing the piezoelectric actuator and mass flow controller according to this disclosure (hereinafter referred to as "Embodiments") will be described in detail below with reference to the drawings. However, this disclosure is not limited by these embodiments.

[0007] Furthermore, each embodiment can be combined as appropriate, provided that the processing content is not inconsistent. Also, the same parts are denoted by the same reference numerals in each of the following embodiments, and redundant explanations are omitted.

[0008] Furthermore, it should be noted that drawings are schematic representations, and the dimensional relationships and proportions of each element may differ from reality. Moreover, there may be discrepancies in dimensional relationships and proportions between drawings themselves.

[0009] Piezoelectric actuators are known for use in mass flow controllers and precision positioning devices for XY tables. These piezoelectric actuators comprise a piezoelectric element and a case housing the piezoelectric element. Furthermore, a strain sensor is attached to the surface of the case, allowing for the detection of the piezoelectric actuator's displacement.

[0010] However, with conventional technology, while displacement along the longitudinal direction of the piezoelectric element can be detected, it has been extremely difficult to detect strain caused by torsional stress when it occurs in the piezoelectric actuator.

[0011] Therefore, there is a need for a technology that can solve the above problems and detect distortion, including twisting, that occurs in the case of a piezoelectric actuator.

[0012] <Embodiment> First, the piezoelectric actuator 1 according to the embodiment will be described with reference to Figures 1 to 5. Figure 1 is a perspective view showing the overall configuration of the piezoelectric actuator 1 according to the embodiment, and Figure 2 is a cross-sectional view taken along the line A-A shown in Figure 1.

[0013] As shown in Figures 1 and 2, the piezoelectric actuator 1 according to this embodiment comprises a piezoelectric element 10, a pair of electrode plates 20, a pair of lead terminals 30, a case 40, and a strain sensor 50. The pair of electrode plates 20 includes electrode plate 20A and electrode plate 20B, and the pair of lead terminals 30 includes lead terminal 30A and lead terminal 30B.

[0014] As shown in Figure 2, the piezoelectric element 10 has a columnar shape. For example, the piezoelectric element 10 is a rectangular prism (cuboid) with dimensions of 0.5 mm to 10 mm in length, 0.5 mm to 10 mm in width, and 1 mm to 100 mm in height. However, the shape of the piezoelectric element 10 is not limited to a rectangular prism; it may also be a hexagonal prism, an octagonal prism, or a cylindrical shape.

[0015] As shown in Figure 2, the piezoelectric element 10 includes a piezoelectric body 11, an internal electrode 12, a planned fracture layer 13, and a pair of conductive layers 14. The pair of conductive layers 14 includes conductive layer 14A and conductive layer 14B.

[0016] The piezoelectric element 10 is constructed by stacking a plurality of piezoelectric bodies 11, internal electrodes 12, and planned fracture layers 13 in a predetermined order along the longitudinal direction L of the piezoelectric actuator 1. In other words, in this disclosure, the longitudinal direction L of the piezoelectric actuator 1 coincides with the stacking direction of the piezoelectric element 10.

[0017] The piezoelectric body 11 is made of a piezoelectric material having piezoelectric properties, for example, a piezoelectric ceramic. The material of such piezoelectric ceramic is, for example, lead zirconate titanate (PbZrO ３ -PbTiO ３ ) or potassium sodium niobate ((K,Na)NbO ３ ) Perovskite-type oxides consisting of lithium niobate (LiNbO ３ ) or lithium tantalate (LiTaO) ３ ) and so on.

[0018] The average particle size of such piezoelectric ceramics is, for example, 0.5 μm to 2.8 μm. The thickness of one layer of piezoelectric material 11 is, for example, 3 μm to 250 μm.

[0019] The internal electrode 12 is made of a conductive material and includes a plurality of first electrodes 12a and a plurality of second electrodes 12b. The first electrodes 12a are electrically connected to a conductive layer 14A. This conductive layer 14A is located along the longitudinal direction L on one side surface 10a of the piezoelectric element 10. A predetermined positive voltage is applied to the first electrodes 12a, for example, through the conductive layer 14A.

[0020] The second electrode 12b is electrically connected to the conductor layer 14B. This conductor layer 14B is located along the longitudinal direction L on the side 10b of the piezoelectric element 10 opposite to the side 10a. A predetermined negative voltage (or ground voltage) is applied to the second electrode 12b, for example, via the conductor layer 14B.

[0021] As shown in Figure 2, inside the piezoelectric element 10, the first electrode 12a, the second electrode 12b, and the piezoelectric body 11 are stacked such that the piezoelectric body 11 is positioned between the first electrode 12a and the second electrode 12b. This allows the piezoelectric element 10 to apply a driving voltage to the piezoelectric body 11 using the first electrode 12a and the second electrode 12b.

[0022] Furthermore, the piezoelectric element 10 according to the embodiment is composed of an active portion formed by alternately stacking a plurality of piezoelectric bodies 11 and internal electrodes 12, and an inactive portion located at both ends in the longitudinal direction L of the active portion and having a piezoelectric body 11.

[0023] The active portion is the part that expands or contracts (hereinafter also referred to as "expanding or contracting") in the lamination direction when a driving voltage is applied to the piezoelectric element 10 from the outside. On the other hand, the inactive portion is the part that does not expand or contract even when a driving voltage is applied to the piezoelectric element 10 from the outside.

[0024] Furthermore, in this disclosure, the end of the case 40 on the bottom 41 side is the base end 10c of the piezoelectric element 10, and the end of the case 40 on the lid 43 side is the tip end 10d of the piezoelectric element 10.

[0025] In the piezoelectric actuator 1 according to this embodiment, the base end portion 10c (i.e., the bottom portion 41) of the piezoelectric element 10 is fixed, while the tip portion 10d (i.e., the cover portion 43) of the piezoelectric element 10 is displaced along the longitudinal direction L.

[0026] The material of the internal electrode 12 is, for example, a metal mainly composed of silver, silver-palladium, silver-platinum, or copper. The internal electrode 12 can be formed, for example, by co-firing with the piezoelectric body 11. The thickness of the internal electrode 12 is, for example, 0.1 μm to 5 μm.

[0027] The planned fracture layer 13 is a layer for relieving the stress generated by the driving of the piezoelectric element 10. Examples of the planned fracture layer 13 include an extremely porous metal layer that does not function as an internal electrode 12, or an extremely porous piezoelectric layer. In the piezoelectric element 10 according to this embodiment, the planned fracture layer 13 may be omitted.

[0028] As described above, the pair of conductive layers 14 include a conductive layer 14A located on the side surface 10a of the piezoelectric element 10 and a conductive layer 14B located on the side surface 10b of the piezoelectric element 10. The conductive layer 14 is positioned to extend across the entire active portion of the piezoelectric element 10.

[0029] The material of the conductive layer 14 is, for example, a metal mainly composed of silver or copper. For example, a metallized layer made of a sintered body of the above-mentioned metal and glass can be used for the conductive layer 14. The thickness of the conductive layer 14 is, for example, 5 μm to 500 μm.

[0030] Although not shown in this disclosure, a coating layer made of an insulating material may be located on a pair of sides between sides 10a and 10b of the piezoelectric element 10. By arranging such a coating layer on the pair of sides between sides 10a and 10b, surface discharge between the two electrodes that occurs when a high voltage is applied during operation can be reduced.

[0031] Examples of insulating materials that form this coating layer include ceramic materials. Examples of such ceramic materials include materials that can follow the expansion and contraction of the piezoelectric element 10 when the piezoelectric actuator 1 is driven, and that can be deformed by stress so as not to cause the coating layer itself to peel off and generate surface discharge.

[0032] Specifically, the coating layer is a partially stabilized zirconia, Ln, which undergoes a localized phase transformation and volume change when stress is applied, allowing it to deform. １－Ｘ Si Ｘ AlO ３＋０．５Ｘ Examples of ceramic materials include the following. Note that Ln represents at least one selected from Sn, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb, and x = 0.01 to 0.3.

[0033] Alternatively, the coating layer may be a piezoelectric material such as barium titanate, lead zirconate titanate, or potassium sodium niobate, which changes the interionic distance within the crystal lattice to relieve the resulting stress.

[0034] The pair of electrode plates 20 includes electrode plate 20A and electrode plate 20B, and are electrically connected to a pair of conductor layers 14, respectively. Specifically, electrode plate 20A is electrically connected to conductor layer 14A, and electrode plate 20B is electrically connected to conductor layer 14B.

[0035] The electrode plate 20 is, for example, positioned along the stacking direction of the laminate as a whole, and locally meanders to intersect with the stacking direction. At least a portion of the electrode plate 20 that intersects with the stacking direction is bonded to the conductor layer 14 via a conductive bonding material (not shown).

[0036] Examples of such bonding materials include epoxy resins or polyimide resins containing highly conductive metal powders such as Ag powder or Cu powder.

[0037] The electrode plate 20 is made of a metal such as copper, iron, stainless steel, or phosphor bronze. The width of the electrode plate 20 is, for example, 0.5 mm to 10 mm, and the thickness of the electrode plate 20 is, for example, 0.01 mm to 1.0 mm. The surface of the electrode plate 20 may be coated with a plating film such as tin plating or silver plating to improve electrical and thermal conductivity.

[0038] The pair of lead terminals 30 includes lead terminal 30A and lead terminal 30B, as shown in Figures 1 and 2. The pair of lead terminals 30 are electrically connected to the pair of electrode plates 20, respectively. For example, as shown in Figure 2, lead terminal 30A is electrically connected to electrode plate 20A via a conductive member 31A and a conductive adhesive 32A, and lead terminal 30B is electrically connected to electrode plate 20B via a conductive member 31B and a conductive adhesive 32B.

[0039] As shown in Figure 2, the case 40 houses the piezoelectric element 10 and the electrode plate 20 inside. The case 40 has a bottom portion 41, a cylindrical portion 42, and a lid portion 43. The bottom portion 41 is columnar (for example, cylindrical) and may have a cylindrical portion 41a protruding in a cylindrical shape at one end (the upper end in Figure 2).

[0040] Further, the inner bottom surface 41b located at the bottom of the cylindrical portion 41a contacts the base end portion 10c of the piezoelectric element 10. Note that the inner bottom surface 41b of the bottom portion 41 and the base end portion 10c of the piezoelectric element 10 may be joined with a joining material (not shown).

[0041] Further, in the bottom portion 41, a pair of through holes 41d penetrating between the inner bottom surface 41b and the end surface 41c on the opposite side of the inner bottom surface 41b are located, and a pair of lead terminals 30 are respectively inserted into the pair of through holes 41d.

[0042] Then, an insulating material (for example, soft glass or the like) is filled in the gap between the through hole 41d in the bottom portion 41 and the lead terminal 30, whereby the lead terminal 30 is fixed to the bottom portion 41. Further, as shown in FIG. 2, the lead terminal 30 passes through the bottom portion 41 from the inside of the case 40 and protrudes outward from the end surface 41c of the bottom portion 41.

[0043] The cylindrical portion 42 has a cylindrical shape (for example, a circular cylindrical shape). Further, the cylindrical portion 42 has a bellows (bellows) shape in which a plurality of concave portions and convex portions are repeated, and the axial direction of the cylinder coincides with the stacking direction of the piezoelectric element 10. Thereby, the cylindrical portion 42 can expand and contract in the axial direction following the expansion and contraction of the piezoelectric element 10 in the stacking direction.

[0044] As shown in FIG. 2, the cylindrical portion 42 constitutes a bellows shape, for example, by repeating a plurality of extending portions 42a and a plurality of bending portions 42b. The extending portion 42a is a portion that extends along the axial direction (that is, the longitudinal direction L) of the cylindrical portion 42 in the cylindrical portion 42.

[0045] The bending portion 42b is a portion that bends toward the radial direction (for example, the central direction) of the cylindrical portion 42 in the cylindrical portion 42. As shown in FIG. 1, the bending portion 42b has, for example, a groove shape that extends along the circumferential direction C of the cylindrical portion 42 on the outer peripheral surface of the cylindrical portion 42.

[0046] Thus, in the cylindrical portion 42 according to this embodiment, on the outer circumferential surface, the bent portion 42b is a concave portion, and the extended portion 42a located between adjacent concave portions is a convex portion. Furthermore, in the cylindrical portion 42 according to this embodiment, on the inner circumferential surface, the bent portion 42b is a convex portion, and the extended portion 42a located between adjacent convex portions is a concave portion.

[0047] In this embodiment, for example, four extended portions 42a and five bent portions 42b are arranged alternately. However, the number of extended portions 42a and bent portions 42b is not limited to this example in this disclosure.

[0048] The cylindrical portion 42 has a predetermined spring constant so that it can follow the expansion and contraction of the piezoelectric element 10. The spring constant of the cylindrical portion 42 can be adjusted by the thickness of the cylindrical portion 42, the shape and number of bent portions 42b in the cylindrical portion 42, and so on. The thickness of the cylindrical portion 42 is, for example, 0.1 mm to 0.5 mm, and the diameter of the cylindrical portion 42 is, for example, 5 mm to 50 mm.

[0049] Furthermore, the end 42c of the cylindrical portion 42 on the bottom 41 side is welded, for example, while a compressive load is applied to the piezoelectric element 10. Alternatively, the cylindrical portion 42 may be welded after a flange portion that widens radially outward in a trumpet shape is provided at the end 42c on the bottom 41 side.

[0050] The cylindrical portion 42 is formed, for example, by preparing a seamless tube of a predetermined shape and then processing the seamless tube into a bellows shape by rolling or hydrostatic pressing.

[0051] The lid portion 43 is cylindrical in shape (for example, cylindrical) with one end closed. The lid portion 43 is fitted onto the end portion 42d of the cylindrical portion 42, and the inner wall of the end portion 42d and the side wall of the lid portion 43 are fixed together, for example, by welding.

[0052] Furthermore, the inner bottom surface 43a of the lid 43 is in contact with the tip 10d of the piezoelectric element 10. The inner bottom surface 43a of the lid 43 and the tip 10d of the piezoelectric element 10 may be joined together with a bonding material (not shown).

[0053] The strain sensor 50 is a component that detects the displacement of the case 40. The strain sensor 50 is, for example, a plate-shaped component having a metal wire inside. The strain sensor 50 is, for example, rectangular in shape with a length of 0.1 mm to 50 mm and a width of 2 mm to 120 mm.

[0054] Figure 3 is an enlarged cross-sectional view showing an example of the configuration of the strain sensor 50 and its vicinity according to the embodiment. As shown in Figure 3, the strain sensor 50 is attached to the case 40, which is the object to be measured, for example, by a bonding material 51. Note that the bonding material 51 is not shown in Figures 2 and 6 onward.

[0055] The strain sensor 50 works by expanding and contracting the metal wire inside it as the case 40 expands and contracts, causing a change in the resistance of the metal wire. The piezoelectric actuator 1 according to this embodiment can measure the strain of the case 40 by measuring this change in the resistance of the metal wire.

[0056] The strain sensor 50 is provided with, for example, multiple output terminals 60 (see Figure 1). The strain sensor 50 can measure the resistance value of the internal metal wires via these multiple output terminals 60.

[0057] In this embodiment, as shown in Figures 2 and 3, the strain sensor 50 may be positioned across a single bent portion 42b, which is a recess, on the outer circumferential surface of the case 40.

[0058] As a result, when torsional stress is applied to the case 40 and the relative positions of adjacent protrusions (in this case, extended portions 42a) shift in a direction perpendicular to the longitudinal direction L, this shift can be detected by strain sensors 50 directly connected between adjacent protrusions.

[0059] Therefore, according to this embodiment, it is possible to detect strain, including twisting, that occurs in the case 40 of the piezoelectric actuator 1.

[0060] In addition, in this embodiment, the bonding material 51 located between the cylindrical portion 42 and the strain sensor 50 may be made of resin. This allows the bonding material 51 to twist together with the strain sensor 50 when torsional stress is applied to the case 40 and the relative positions of adjacent protrusions shift in a direction perpendicular to the longitudinal direction L.

[0061] Therefore, according to this embodiment, strain can be detected without the strain sensor 50 peeling off, and thus the piezoelectric actuator 1 of this embodiment is highly reliable.

[0062] Furthermore, in this embodiment, as shown in Figure 3, the bonding material 51 located between the cylindrical portion 42 and the strain sensor 50 may be a first bonding material 52 located along the strain sensor 50.

[0063] As a result, the entire contact surface between the protrusion (in this case, the extended portion 42a) of the case 40 (see Figure 2) and the strain sensor 50 can be made into a bonding surface, so that strain can be detected without the strain sensor 50 peeling off. Therefore, the piezoelectric actuator 1 of this embodiment has excellent reliability.

[0064] The first bonding material 52 may be, for example, an epoxy resin or an acrylic resin, and is more preferably an epoxy resin. By using an epoxy resin for the first bonding material 52, good adhesive strength and heat resistance can be obtained.

[0065] Note that the form of the bonding material 51 in this disclosure is not limited to the example in Figure 3. Figures 4 and 5 are enlarged cross-sectional views showing another example of the configuration of the strain sensor 50 and its vicinity according to the embodiment.

[0066] As shown in Figure 4, the bonding material 51 located between the cylindrical portion 42 and the strain sensor 50 may be a second bonding material 53 located inside the recessed bent portion 42b. In this way, by filling the recessed bent portion 42b with the second bonding material 53, the strain sensor 50 can be installed almost flat.

[0067] Furthermore, in this embodiment, deformation of the recess, which could adversely affect strain due to torsional stress, can be mitigated by the second joining material 53 embedded in the recess. Therefore, the piezoelectric actuator 1 of this embodiment has high accuracy in measuring strain, including torsion, in the strain sensor 50.

[0068] The second bonding material 53 may be, for example, an epoxy resin, a silicone resin, or a polyolefin resin, and is more preferably an epoxy resin. By using an epoxy resin for the second bonding material 53, good adhesive strength and heat resistance can be obtained.

[0069] In this disclosure, the bonding material 51, the first bonding material 52, and the second bonding material 53 are not limited to being resins, but may be inorganic materials (for example, cement or glass).

[0070] Furthermore, in this embodiment, as shown in Figure 5, the joining material 51 located between the cylindrical portion 42 and the strain sensor 50 may have a first joining material 52 located along the strain sensor 50 and a second joining material 53 located inside the bent portion 42b.

[0071] The piezoelectric actuator 1 offers excellent reliability thanks to the first bonding material 52, and high accuracy in measuring strain in the strain sensor 50 thanks to the second bonding material 53.

[0072] Furthermore, in the embodiment, as shown in the example in Figure 5, the Young's modulus of the second bonding material 53 may be smaller than that of the first bonding material 52. This reduces the likelihood that the second bonding material 53 will hinder the expansion and contraction of the case 40 (see Figure 2) when the case 40 expands and contracts in response to the expansion and contraction of the piezoelectric element 10 (see Figure 2).

[0073] Therefore, the piezoelectric actuator 1 of this embodiment can detect strain, including twisting, of the case 40 while ensuring the amount of displacement during operation.

[0074] In this embodiment, the strain sensor 50 may be located on the outer circumferential surface of the case 40. Since the outer circumferential surface of the case 40 is further from the central axis of the case 40 than the inner circumferential surface of the case 40, the displacement of the outer circumferential surface will be greater than that of the inner circumferential surface even when the same rotational angle of twist is applied.

[0075] Therefore, when the strain sensor 50 is located on the outer circumferential surface of the case 40, the measurement accuracy of the strain, including torsion, in the strain sensor 50 is superior to when it is located on the inner circumferential surface.

[0076] <Another Embodiment 1> Next, various other embodiments will be described with reference to Figures 6 to 13. Figure 6 is a longitudinal cross-sectional view showing the configuration of the piezoelectric actuator 1 according to Another Embodiment 1. As shown in Figure 6, in Another Embodiment 1, the arrangement of the strain sensor 50 differs from that of the above-described embodiment.

[0077] Specifically, in another embodiment 1, the strain sensor 50 may be positioned across a plurality (two in the figure) of bent portions 42b, which are recesses, on the outer circumferential surface of the case 40.

[0078] In this disclosure, when a torsional stress is applied to the case 40, all the protrusions (in this case, the extended portions 42a) are affected by torsion. Therefore, by placing strain sensors 50 across multiple recessed, bent portions 42b, the torsional effect on a greater number of protrusions can be detected.

[0079] Therefore, the piezoelectric actuator 1 of another embodiment 1 has high accuracy in measuring strain, including torsion, in the strain sensor 50.

[0080] <Another Embodiment 2> Figure 7 is a longitudinal cross-sectional view showing the configuration of the piezoelectric actuator 1 according to another embodiment 2. As shown in Figure 7, in another embodiment 2, the arrangement of the strain sensor 50 differs from that of the above-described embodiment and another embodiment 1.

[0081] Specifically, in another embodiment 2, the strain sensor 50 may be positioned across all (five in the figure) the bent portions 42b that are recesses on the outer circumferential surface of the case 40. By positioning the strain sensor 50 across all the bent portions 42b in this way, the torsional effect on even more convex portions can be detected.

[0082] Therefore, the piezoelectric actuator 1 of another embodiment 2 has even higher accuracy in measuring strain, including torsion, in the strain sensor 50.

[0083] Furthermore, if the strain sensor 50 is installed so as to span only some of the recesses rather than all of them, localized strain in the case 40 can be detected. In particular, when the piezoelectric element 10 (see Figure 2) is driven for a long period of time, the area near the bottom 41 of the case 40 tends to heat up.

[0084] Therefore, for example, by providing a strain sensor 50 only on the bottom 41 side of the case 40 and detecting the strain of the case 40 over a long period of time, it is possible to detect changes in the amount of displacement caused by metal fatigue of the case 40.

[0085] <Another Embodiment 3> Figure 8 is a longitudinal cross-sectional view showing the configuration of the piezoelectric actuator 1 according to another embodiment 3. As shown in Figure 8, in another embodiment 3, the arrangement of the strain sensor 50 differs from that of the embodiments described above.

[0086] Specifically, in another embodiment 3, the strain sensor 50 may be positioned across a recessed extension 42a on the inner circumferential surface of the case 40.

[0087] As a result, when torsional stress is applied to the case 40 and the relative positions of adjacent protrusions (in this case, bent portions 42b) shift in a direction perpendicular to the longitudinal direction L, this shift can be detected by strain sensors 50 directly connected between adjacent protrusions.

[0088] Therefore, the piezoelectric actuator 1 of another embodiment 3 can detect strain, including twisting, that occurs in the case 40.

[0089] <Another Embodiment 4> Figure 9 is a longitudinal cross-sectional view showing the configuration of the piezoelectric actuator 1 according to another embodiment 4. As shown in Figure 9, in another embodiment 4, the arrangement of the strain sensor 50 is different from that of the other embodiment 3 described above.

[0090] Specifically, in another embodiment 4, the strain sensor 50 may be positioned across a plurality (two in the figure) of recessed extended portions 42a on the inner circumferential surface of the case 40.

[0091] In this disclosure, when a torsional stress is applied to the case 40, all the protrusions (in this case, the bent portions 42b) are affected by torsion. Therefore, by placing the strain sensor 50 across multiple recessed extended portions 42a, the torsional effect on a greater number of protrusions can be detected.

[0092] Therefore, the piezoelectric actuator 1 of another embodiment 4 has high accuracy in measuring strain, including torsion, in the strain sensor 50.

[0093] <Another Embodiment 5> Figure 10 is a longitudinal cross-sectional view showing the configuration of the piezoelectric actuator 1 according to another embodiment 5. As shown in Figure 10, in another embodiment 5, the arrangement of the strain sensor 50 is different from that of the other embodiments 3 and 4 described above.

[0094] Specifically, in another embodiment 5, the strain sensor 50 may be positioned across all (four in the figure) the recessed extended portions 42a on the inner circumferential surface of the case 40. By positioning the strain sensor 50 across all the extended portions 42a in this way, the torsional effect on even more convex portions can be detected.

[0095] Therefore, the piezoelectric actuator 1 of another embodiment 5 has even higher accuracy in measuring strain, including torsion, in the strain sensor 50.

[0096] <Another Embodiment 6> Figure 11 is a perspective view showing the overall configuration of the piezoelectric actuator 1 according to another embodiment 6. Figure 12 is a cross-sectional view showing the configuration of the piezoelectric actuator 1 according to another embodiment 6.

[0097] As shown in Figures 11 and 12, the strain sensor 50 may be positioned along the circumferential direction C of the cylindrical portion 42. In other words, the strain sensor 50 may be positioned in a way that forms an arc along the outer circumferential surface of the cylindrical portion 42 in a cross-sectional view.

[0098] Since the torsional stress applied to case 40 occurs circumferentially along the outer surface of case 40, the strain sensor 50 is positioned along the circumferential direction C of the cylindrical portion 42, resulting in high accuracy in measuring strain, including torsion, at the strain sensor 50.

[0099] <Another Embodiment 7> Figure 13 is a cross-sectional view showing the configuration of a piezoelectric actuator 1 according to another embodiment 7. As shown in Figure 13, the piezoelectric actuator 1 according to another embodiment 7 may have a plurality (three in the figure) of strain sensors 50.

[0100] Depending on how the piezoelectric actuator 1 is fixed, one side of the case 40 may be subjected to a compressive load, while the other side is subjected to a tensile load. Even in such cases, by positioning multiple strain sensors 50 on the case 40, it is possible to measure the strain of the case 40, including its torsion, with greater accuracy.

[0101] In this case, for example, by taking the average of the measurements from all strain sensors 50, it is possible to measure the strain of the case 40, including its twist, with greater accuracy.

[0102] In another embodiment 7, multiple strain sensors 50 of the same length in the longitudinal direction L (see Figure 2) may be arranged in a line along the circumferential direction C of the cylindrical portion 42. This makes it possible to detect the direction in which the case 40 is tilted when the piezoelectric actuator 1 expands or contracts diagonally.

[0103] In another embodiment 7, the multiple strain sensors 50 may be positioned at equal intervals along the circumferential direction C (for example, at 120° intervals if there are three strain sensors 50). This allows for accurate detection of the direction in which the case 40 is tilted when the piezoelectric actuator 1 expands or contracts diagonally.

[0104] Furthermore, this disclosure is not limited to the case where there are three strain sensors 50 provided on the piezoelectric actuator 1; there may be two or four or more strain sensors 50. In this case as well, it is possible to detect the direction in which the case 40 is tilted when the piezoelectric actuator 1 expands or contracts diagonally.

[0105] <Configuration of Mass Flow Controller, etc.> Next, an example of a mass flow controller 100 and a valve on which the piezoelectric actuator 1 according to the embodiment is mounted will be described with reference to Figure 14. Figure 14 is a block diagram showing the configuration of the mass flow controller 100 according to the embodiment.

[0106] As shown in Figure 14, the mass flow controller 100 comprises a flow path 101, a flow sensor unit 102, a flow control valve 103, and a control circuit unit 104. A fluid, such as gas, flows through the flow path 101. This fluid flows in through the inlet 101a and out through the outlet 101b.

[0107] A flow sensor unit 102 is connected to a portion of the flow path 101, for example, in a bypass configuration. This flow sensor unit 102 is configured to detect the flow rate (mass flow rate) of the fluid flowing through the flow path 101. The flow rate signal detected by the flow sensor unit 102 is amplified by an amplification circuit and transmitted to the control circuit unit 104.

[0108] The flow control valve 103 is equipped with a piezoelectric actuator 1 according to the embodiment and is configured to control the flow rate of the fluid flowing through the flow path 101. The flow control valve 103 can control the flow rate, for example, by the expansion and contraction of the piezoelectric actuator 1.

[0109] The control circuit unit 104 controls each part. For example, the control circuit unit 104 compares the flow rate signal transmitted from the flow rate sensor unit 102 with a flow rate signal that has been set in advance by the user or the like.

[0110] Then, a drive signal (drive voltage) that eliminates the difference between the transmitted flow rate signal and a preset flow rate signal is input to the piezoelectric actuator 1 provided on the flow control valve 103.

[0111] The piezoelectric actuator 1 expands and contracts in response to the input drive voltage, and this expansion and contraction controls the opening and closing amount of the flow control valve 103, thereby controlling the flow rate of the fluid flowing through the flow path 101. In the case of a valve, it is sufficient to open and close the flow control valve 103.

[0112] As described above, in this embodiment, since strain including twisting that occurs in the case 40 of the high-voltage actuator 1 can be detected, a highly reliable mass flow controller 100 and valve can be made.

[0113] Although the present disclosure has been described in detail above, this disclosure is not limited to the embodiments described above, and various modifications and improvements are possible without departing from the gist of this disclosure.

[0114] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. Indeed, the embodiments described above can be embodied in a variety of forms. Furthermore, the embodiments described above may be omitted, replaced, or modified in various ways without departing from the scope and spirit of the appended claims.

[0115] The present technology can take the following configurations: (1) A piezoelectric actuator comprising: a piezoelectric element having a longitudinal direction; a case having a lid portion, a bottom portion, and a cylindrical portion, and housing the piezoelectric element inside; and a strain sensor located in the cylindrical portion, wherein the cylindrical portion has a bellows shape with a plurality of recesses and protrusions repeated along the longitudinal direction, and the strain sensor is located across at least one of the recesses. (2) The piezoelectric actuator according to (1), further comprising: a bonding material located between the cylindrical portion and the strain sensor, wherein the bonding material is made of resin. (3) The piezoelectric actuator according to (2), wherein the bonding material has a first bonding material located along the strain sensor. (4) The piezoelectric actuator according to (3), wherein the bonding material has a second bonding material located in the recess. (5) The piezoelectric actuator according to (4), wherein the Young's modulus of the second bonding material is smaller than the Young's modulus of the first bonding material. (6) The strain sensor is a piezoelectric actuator according to any one of (1) to (5) above, located across a plurality of recesses. (7) The strain sensor is a piezoelectric actuator according to any one of (1) to (6) above, located across all of the recesses. (8) The strain sensor is a piezoelectric actuator according to any one of (1) to (7) above, located along the circumferential direction of the cylindrical portion. (9) The piezoelectric actuator according to any one of (1) to (8) above, comprising a plurality of strain sensors. (10) The piezoelectric actuator according to (9) above, comprising a plurality of strain sensors arranged in a line along the circumferential direction of the cylindrical portion. (11) A mass flow controller comprising: a flow path; a flow sensor unit for detecting the flow rate of fluid flowing in the flow path; a flow control valve having a piezoelectric actuator according to any one of (1) to (10) above, and controlling the flow rate of fluid flowing in the flow path by the expansion and contraction of the piezoelectric actuator; and a control circuit unit for controlling each part.

[0116] 1 Piezoelectric actuator 10 Piezoelectric element 40 Case 41 Bottom 42 Cylindrical part 42a Extended part (example of a convex or concave part) 42b Bent part (example of a concave or convex part) 43 Cover 50 Strain sensor 51 Joining material 52 First joining material 53 Second joining material 100 Mass flow controller 101 Flow path 102 Flow sensor part 103 Flow control valve 104 Control circuit part C Circumferential direction L Longitudinal direction

Claims

1. A piezoelectric actuator comprising: a piezoelectric element having a longitudinal direction; a case having a lid portion, a bottom portion, and a cylindrical portion, and housing the piezoelectric element inside; and a strain sensor located in the cylindrical portion, wherein the cylindrical portion has a bellows shape with a plurality of recesses and protrusions repeated along the longitudinal direction, and the strain sensor is located across at least one of the recesses.

2. The piezoelectric actuator according to claim 1, further comprising a bonding material located between the cylindrical portion and the strain sensor, wherein the bonding material is made of resin.

3. The piezoelectric actuator according to claim 2, wherein the bonding material is a first bonding material positioned along the strain sensor.

4. The piezoelectric actuator according to claim 3, wherein the joining material is a second joining material located within the recess.

5. The piezoelectric actuator according to claim 4, wherein the Young's modulus of the second bonding material is smaller than the Young's modulus of the first bonding material.

6. The piezoelectric actuator according to any one of claims 1 to 5, wherein the strain sensor is positioned across a plurality of recesses.

7. The piezoelectric actuator according to any one of claims 1 to 6, wherein the strain sensor is positioned across all of the recesses.

8. The piezoelectric actuator according to any one of claims 1 to 7, wherein the strain sensor is located along the circumferential direction of the cylindrical portion.

9. A piezoelectric actuator according to any one of claims 1 to 8, comprising a plurality of strain sensors.

10. The piezoelectric actuator according to claim 9, wherein the plurality of strain sensors are arranged in a line along the circumferential direction of the cylindrical portion.

11. A mass flow controller comprising: a flow path; a flow sensor unit for detecting the flow rate of a fluid flowing in the flow path; a flow control valve having a piezoelectric actuator according to any one of claims 1 to 10, which controls the flow rate of a fluid flowing in the flow path by the expansion and contraction of the piezoelectric actuator; and a control circuit unit for controlling each of the units.

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