Patent Description:
As a piezoelectric actuator used for a mass flow controller, an XY table precision positioning device, and the like, for example, a multi-layered piezoelectric body disclosed in <CIT> (hereinafter referred to as Patent Document <NUM>) is known. The multi-layered piezoelectric body includes a piezoelectric element and a cylindrical case that houses the piezoelectric element inside. In the multi-layered piezoelectric body, a strain gauge is adhered to the surface of a beam-like portion of the case to detect a displacement of the multi-layered piezoelectric body. <CIT> and <CIT> disclose exemplary piezoelectric actuators, each comprising a multi-layered piezoelectric element that is housed within a tubular bellows type case, wherein a strain gauge is arranged on a side surface of the piezoelectric element. In the latter document, an additional strain gauge is arranged outside the case and mounted on, and extending between, end plates of a tubular bellows.

A piezoelectric actuator of the present invention includes a piezoelectric element having a longitudinal direction, a case including a lid portion, a bottom portion, and a tubular portion, the case housing the piezoelectric element inside, and a strain gauge positioned at the tubular portion. The tubular portion includes a plurality of bent portions in the longitudinal direction, the bent portion bending in response to extension and contraction of the piezoelectric element, and the strain gauge is positioned at the bent portion.

Embodiments of a piezoelectric actuator according to the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the embodiments that will be described below.

A piezoelectric actuator <NUM> illustrated in <FIG> includes a piezoelectric element <NUM> and a case <NUM>. The case <NUM> houses the piezoelectric element <NUM> inside and includes a bottom portion <NUM>, a tubular portion <NUM>, and a lid portion <NUM>.

As illustrated in <FIG>, the piezoelectric element <NUM> forming the piezoelectric actuator <NUM> is a piezoelectric element provided with a layered body including, for example, an active portion in which a plurality of piezoelectric layers and internal electrode layers are alternately layered, and an inactive portion including the piezoelectric layers layered at both ends in a layering direction of the active portion. Here, the active portion is a portion where the piezoelectric layers extend or contract in the layering direction during driving, and the inactive portion is a portion where the piezoelectric layers do not extend or contract in the layering direction during driving.

The layered body forming the piezoelectric element <NUM> is formed in a rectangular parallelepiped shape, for example, having a length of approximately <NUM> to <NUM>, a width of approximately <NUM> to <NUM>, and a height of approximately <NUM> to <NUM>. The layered body may also have, for example, a hexagonal column shape or an octagonal column shape.

The piezoelectric layers forming the layered body include piezoelectric ceramic having piezoelectric characteristics, and the piezoelectric ceramic has an average particle size of, for example, <NUM> to <NUM>. Examples of the piezoelectric ceramic that can be used include perovskite-type oxides made from lead zirconate titanate (PbZrO<NUM>-PbTiO<NUM>), lithium niobate (LiNbO<NUM>), lithium tantalate (LiTaO<NUM>), and the like.

Further, the internal electrode layers forming the layered body mainly contains, for example, a metal such as silver, silver-palladium, silver-platinum, or copper. For example, positive electrodes and negative electrodes are alternately disposed in the layering direction. The positive electrodes are drawn out to one side surface of the layered body, and the negative electrodes are drawn out to another side surface thereof. With this configuration, in the active portion, a drive voltage can be applied to the piezoelectric layer sandwiched between the internal electrode layers adjacent to each other in the layering direction.

Note that the layered body may include a metal layer or the like that is a layer for mitigating stress and does not function as the internal electrode layer.

Then, external electrodes are provided on each of a pair of opposing side surfaces of the layered body from which the positive electrodes or the negative electrodes (or ground electrodes) of the internal electrode layers are drawn out, and the external electrodes are electrically connected to the drawn-out internal electrode layers. The external electrode is a metallization layer containing, for example, silver and glass.

Both positive and negative electrodes (or ground electrodes) of the internal electrode layers are exposed on another pair of opposing side surfaces of the layered body, and these side surfaces are provided with a covering layer including an insulator as necessary. By providing the covering layer, it is possible to prevent creeping discharge between the electrodes from occurring when a high voltage is applied during driving. Examples of the insulator that serves as the covering layer include a ceramic material, and in particular, a material that can be deformed by stress can be used so as to follow drive deformation (extension and contraction) of the layered body, which occurs when the piezoelectric actuator is driven, the material being deformable by stress so as to eliminate a possibility that the creeping discharge occurring as a result of the covering layer peeling off. Specifically, examples of the material include a ceramic material such as partially stabilized zirconia that can deform through local phase transformation and volume change when stress is generated, and Ln<NUM>-XSiXAlO<NUM>+<NUM>. 5X (Ln represents at least one selected from the group consisting of Sn, Y, La, Ce. Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb. x = <NUM> to <NUM>), and a piezoelectric material such as barium titanate and lead zirconate titanate in which the interionic distance inside the crystal lattice changes so as to mitigate the generated stress. Such covering layer is formed, for example, into an ink form, then applied to the side surfaces of the layered body by dipping or screen printing, and sintered.

The case <NUM> forming the piezoelectric actuator <NUM> includes the bottom portion <NUM>, the tubular portion <NUM>, and the lid portion <NUM>. Then, the case <NUM> houses the piezoelectric element <NUM> inside, the lower end surface of the piezoelectric element <NUM> is in contact with the upper surface of the bottom portion <NUM>, and the upper end surface of the piezoelectric element <NUM> is in contact with the lower surface of the lid portion <NUM>.

The bottom portion <NUM>, the tubular portion <NUM>, and the lid portion <NUM> may be, for example, metal bodies such as SUS304 and SUS316L. Further, the bottom portion <NUM>, the tubular portion <NUM>, and the lid portion <NUM> may be integrally formed, or may be formed by separate bodies being bonded to each other using a known technique such as welding or adhering, for example.

The tubular portion <NUM> is a tubular body extending vertically and having openings at both ends. The tubular portion <NUM> includes a bent portion <NUM> that bends due to the extension and contraction of the piezoelectric element <NUM>. Due to the expansion and contraction of the piezoelectric element <NUM>, the bent portion <NUM> bends so as to extend in the longitudinal direction to extend and contract the case <NUM> as a whole. The bent portion <NUM> is formed, for example, by forming a seamless tube having a predetermined shape and then forming the seamless pipe into a bellows shape by rolling or hydrostatic pressing. The tubular portion <NUM> has a predetermined spring constant such that the extension and contraction of the piezoelectric element <NUM> can be followed when a voltage is applied to the piezoelectric element <NUM>, and the spring constant is adjusted according to the thickness, the shape of grooves, and the number of grooves. For example, the thickness of the tubular portion <NUM> is, for example, from <NUM> to <NUM>. Note that the tubular portion <NUM> may include a straight portion in addition to the bent portion <NUM>. Further, a plurality of the bent portions <NUM> are provided in the longitudinal direction. In this way, when the case <NUM> extends and contracts, stress generated at the bent portion <NUM> can be dispersed. This makes it possible to reduce the possibility that the bent portion <NUM> is damaged as a result of the stress being concentrated therein.

The lid portion <NUM> is formed such that the outer diameter is approximately the same as the inner diameter of the opening on one end side of the tubular portion <NUM>. The lid portion <NUM> is fitted through the opening on the one end side of the tubular portion <NUM>, and the side surface (outer periphery) of the lid portion <NUM> is bonded to the inner wall of the tubular portion <NUM> in the vicinity of the opening on the one end side by welding, for example. At this time, a bonded portion between the tubular portion <NUM> and the bottom portion <NUM> is referred to as a welded portion.

The bottom portion <NUM> includes, for example, a bottom plate portion and an annular protruding portion vertically provided at the bottom plate portion. The bottom plate portion has a circular plate shape, and in the example illustrated in the drawings, a peripheral edge portion thereof is thinner than other portions thereof. Note that two through holes through which lead pins <NUM> can be inserted are formed in the bottom portion <NUM>, and the lead pins <NUM> are inserted through the through holes. Then, a gap in the through hole is filled with, for example, soft glass <NUM>, whereby the lead pin <NUM> is fixed. A lead wire <NUM> is connected to the tip of the lead pin <NUM>, and this lead wire <NUM> is attached to the external electrode of the piezoelectric element <NUM> by solder <NUM> to apply a drive voltage to the piezoelectric element <NUM> through the lead wire <NUM>.

The strain gauge <NUM> is a member that detects a displacement of the case <NUM>. The strain gauge <NUM> is, for example, a plate-shaped member having a metal wire inside. The strain gauge <NUM> has a shape having a longitudinal direction and having a length of <NUM> to <NUM> and a width of from <NUM> to <NUM>, for example. The strain gauge <NUM> may be a disc-shaped member having a diameter of <NUM> to <NUM>, for example. The strain gauge <NUM> is adhered to the case <NUM> to be measured by a resin adhesive material, for example. In accordance with the extension and contraction of the case <NUM>, the metal wire provided inside the strain gauge <NUM> extends and contracts, and as a result, a resistance value of the metal wire changes. By measuring the change in the resistance, a strain of the case <NUM> can be measured. The strain gauge <NUM> is adhered to the tubular portion <NUM> of the case <NUM>.

As illustrated in <FIG>, according to the piezoelectric actuator <NUM> of the present invention, the strain gauge <NUM> is positioned at the bent portion <NUM>. Since the tubular portion <NUM> includes the bent portion <NUM> at which the tubular portion <NUM> is bent, an adhesive can be easily accumulated in the bent portion <NUM>. Thus, the adhesion strength between the strain gauge <NUM> and the case <NUM> can be increased. As a result, the possibility that the strain gauge <NUM> comes off from the case <NUM> can be reduced.

Further, as illustrated in <FIG>, the strain gauge <NUM> may be positioned outside the case <NUM>. When the strain gauge <NUM> is provided inside the case <NUM>, the adhesive adhering the strain gauge <NUM> to the case <NUM> may react with oxygen to generate moisture that causes a short circuit of the piezoelectric element <NUM>. However, when the strain gauge <NUM> is positioned outside the case <NUM>, the possibility of generating the moisture that causes the short circuit can be reduced. As a result, durability of the piezoelectric element <NUM> can be increased.

Note that the strain gauge <NUM> need not necessarily be entirely provided at the bent portion <NUM>, and as illustrated in <FIG>, the strain gauge <NUM> may be provided from the bent portion <NUM> to the straight portion.

Further, as illustrated in <FIG>, the strain gauge <NUM> may be positioned inside the case <NUM>. When the strain gauge <NUM> is positioned outside the case <NUM>, depending on the external environment, the adhesive of the strain gauge <NUM> may be oxidized, which may reduce the adhesiveness. However, when the strain gauge <NUM> is positioned inside the case <NUM>, and an influence of the external environment can be reduced. This makes it possible to suppress a decrease in adhesiveness.

When the strain gauge <NUM> is positioned inside the case <NUM>, the lead wire <NUM> may be inserted through the through hole, through which the lead pin <NUM> is inserted, of the bottom portion <NUM>. In this case, even if the number of strain gauges <NUM> is increased, the number of through holes in the bottom portion <NUM> is not required to be increased, and thus, the strain gauge <NUM> can be energized without reducing the strength of the bottom portion <NUM>.

Further, as illustrated in <FIG>, the tubular portion <NUM> has the bellows shape, and the strain gauge <NUM> may be positioned at a bellows-shaped portion of the tubular portion <NUM>. Here, the bellows shape is a shape in which a plurality of protruding portions or recessed portions are repeatedly formed, but may include a straight portion. Further, the bellows shape may be a shape in which a plurality of grooves are provided in the circumferential direction. As a result of the tubular portion <NUM> having the bellows shape, stress generated at the bent portion <NUM> can be dispersed. Thus, when the piezoelectric actuator <NUM> is driven, the stress applied to the bent portion <NUM> can be reduced. As a result, durability of the piezoelectric actuator <NUM> can be increased.

In particular, in recent years, the piezoelectric actuator <NUM> has been required to be used in a high temperature environment. Thus, during long-term use, there is a possibility that an extension and contraction behavior of the bellow-shaped portion may be locally changed due to metal fatigue. As a result of the strain gauge <NUM> being positioned at the bellows-shaped portion, the local change in the extension and contraction behavior can be detected. As a result, the durability of the piezoelectric actuator <NUM> can be increased.

Further, as illustrated in <FIG>, the strain gauge <NUM> may be positioned at the protruding surface of the bent portion <NUM>. The curvature of the strain gauge <NUM> increases as the case <NUM> extends, and decreases as the case <NUM> contracts. That is, the strain gauge <NUM> itself bends in response to the extension and contraction of the case <NUM>. Thus, the local stress of the protruding portion can be measured.

Note that in <FIG>, for ease of understanding, the strain gauge <NUM> and the tubular portion <NUM> are partially illustrated, and other members such as the lead wire <NUM> and the adhesive are omitted.

Further, as illustrated in <FIG>, the strain gauge <NUM> may be positioned at the recessed surface of the bent portion <NUM>. In this case, compared to the case in which the bent portion <NUM> is positioned at the protruding surface of the bent portion <NUM>, the outer diameter or the inner diameter of the case <NUM> can be reduced. Thus, the size of the piezoelectric actuator <NUM> can be reduced. In particular, when the strain gauge <NUM> is positioned inside the case <NUM>, as a result of the strain gauge <NUM> being positioned at the recessed surface of the bent portion <NUM>, a possibility of the piezoelectric element <NUM> coming into contact with the strain gauge <NUM> and being short circuited can be reduced.

Furthermore, since the strain gauge <NUM> is sandwiched between the protruding surfaces of the case <NUM>, the strain gauge <NUM> is less likely to come off from the case <NUM>. In addition, when the case <NUM> is compressed by stress from the outside to an extreme extent, as a result of the strain gauge <NUM> being sandwiched between the protruding surfaces to detect the strain, such that the case <NUM> is prevented, in advance, from being damaged. Further, the entire strain gauge <NUM> may be positioned to be closer to the recessed portion than to an apex of the protruding portion in a direction perpendicular to the longitudinal direction of the piezoelectric element <NUM>.

Further, as illustrated in <FIG>, the strain gauge <NUM> may be positioned from the protruding surface to the recessed surface of the bent portion <NUM>. In this case, the strain gauge <NUM> can detect the displacement of half a cycle of the protrusion and recess of the bellows shape. Thus, by multiplying the detected numerical value of the displacement by twice the number of bellows, the displacement of the entire case <NUM> can be calculated. That is, since the displacement of the entire case <NUM> can be identified without increasing the adhesion area between the case <NUM> and the strain gauge <NUM>, the stress between the case <NUM> and the strain gauge <NUM> is reduced, and the possibility that the strain gauge <NUM> comes off from the case <NUM> can be reduced.

Further, as illustrated in <FIG>, the strain gauge <NUM> may be provided from the protruding portion to the protruding portion. As a result, the strain gauge <NUM> itself is formed into a spring shape, and can extend and contract in the same manner as the element. Thus, the strain gauge <NUM> can be prevented from coming off from the case. At the same time, since the strain gauge <NUM> is locally strained at three locations including the protruding portion, the recessed portion, and the protruding portion, the stress generated at a bonded portion between the strain gauge <NUM> and the case <NUM> can be dispersed. As a result, the strain gauge <NUM> is prevented from coming off from the case <NUM> can be reduced. In addition, since shearing stress generated at an inflection point between the protruding portion and the recessed portion can also be detected, detection sensitivity is improved.

Similarly, the strain gauge <NUM> may be provided from the recessed portion to the recessed portion. In this case also, the same effects as described above can be obtained.

Further, as illustrated in <FIG>, plural strain gauges <NUM> may be positioned at symmetrical positions with respect to the piezoelectric element <NUM>. Although the stress tends to be concentrated at a portion, of the bent portion <NUM>, at which the strain gauge <NUM> is provided, even if the strain gauge <NUM> inhibits the deformation of the case <NUM>, the displacement can be generated symmetrically with respect to the case <NUM> as a whole, and thus, the stress generated in the case <NUM> can be dispersed. As a result, durability of the piezoelectric actuator <NUM> can be increased.

Further, as illustrated in <FIG>, the strain gauge <NUM> may be a plate-shaped member having a longitudinal direction, and the longitudinal direction may be positioned so as to overlap with the circumferential direction of the case <NUM>. As a result, stress, in a twist direction, generated by the extension and contraction of the piezoelectric element <NUM> can be detected.

Further, the strain gauge <NUM> may also be provided in the vicinity of the welded portion. This makes it possible to detect a strain in the vicinity of the welded portion, which is not supposed to be deformed under normal circumstances. Specifically, as illustrated in <FIG>, the strain gauge <NUM> may be positioned at a portion closest to the welded portion among the plurality of bent portions <NUM>.

Further, by attaching a plurality of strain gauges <NUM>, it is possible to detect local changes over time in the case <NUM> and suppress the damage of the piezoelectric actuator <NUM>.

Further, as illustrated in <FIG>, the strain gauge <NUM> may be provided on the surface of the case <NUM>, and a second strain gauge <NUM> may be provided at the side surface of the piezoelectric element <NUM>. As a result, the strain of the case <NUM> can be detected, and also, the changes over time of the piezoelectric element <NUM> can be detected. Thus, for example, when the piezoelectric element <NUM> is not strained but only the case <NUM> is strained, when the case <NUM> is strained as a result of the piezoelectric element <NUM> being strained, or when one of the strain gauge <NUM> provided at the piezoelectric element <NUM> or the second strain gauge <NUM> provided at the case <NUM> is detached, strain of the case <NUM> can be immediately detected.

Claim 1:
A piezoelectric actuator (<NUM>) comprising:
a piezoelectric element (<NUM>) having a longitudinal direction;
a case (<NUM>) comprising a lid portion (<NUM>), a bottom portion (<NUM>), and a tubular portion (<NUM>), the case (<NUM>) being configured to house the piezoelectric element (<NUM>) inside; and
a strain gauge (<NUM>) positioned at the tubular portion (<NUM>),
characterised in that
the tubular portion (<NUM>) comprises a plurality of bent portions (<NUM>) in the longitudinal direction, each of the plurality of bent portions (<NUM>) being configured to bend in response to extension and contraction of the piezoelectric element (<NUM>), and
the strain gauge (<NUM>) is positioned at a bent portion (<NUM>) of the plurality of bent portions (<NUM>).