JP3079811B2 - Piezo actuator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piezoelectric actuator used for an automobile or the like.

[0002]

2. Description of the Related Art In recent years, instead of an actuator using electromagnetic force, for example, Japanese Patent Application Laid-Open No. 62-291187,
As disclosed in Japanese Utility Model Application Laid-Open No. 64-30865 and the like, a piezoelectric actuator using piezoelectric ceramics such as lead zirconate titanate (hereinafter referred to as PZT) has been proposed.

The piezoelectric actuator is very promising as various types of mechanical drive elements because of its low power consumption, low heat generation, small size and high-speed drive. However, since mechanical displacement due to the piezoelectric effect is extremely small in nature, in order to obtain a large displacement, a piezoelectric laminate having a structure in which plate-like piezoelectric bodies and electrode plates are alternately and multiply laminated is provided.

[0004]

However, for example, PZ
The piezoelectric actuator using the T piezoelectric body has a problem that the displacement amount is reduced (deterioration of displacement) particularly during use under high temperature and high load such as for an automobile, and a desired displacement amount cannot be obtained. Therefore, in order to keep the displacement constant, a complicated control power supply circuit for controlling the voltage or current of the drive power supply is required, and this is an obstacle to expanding the use of piezoelectric actuators from the viewpoint of cost and the like.

The inventors of the present invention have conducted intensive studies on the cause of the decrease in displacement during use of the piezoelectric actuator, and
It has been found that a major cause is that the electromechanical coupling coefficient Kp (hereinafter referred to as Kp) of the piezoelectric body decreases during use. It has also been found that Kp decreases due to loads such as pressure, temperature, and voltage during use, and that this decrease in Kp is mainly caused by 90 ° switching of the domain.
However, no means has yet been found to prevent a decrease in Kp.

The present invention has been made in view of such circumstances, and has as its object to prevent a displacement amount during use from being reduced.

[0007]

A piezoelectric actuator according to the present invention for solving the above-mentioned problems is a piezoelectric actuator composed of a piezoelectric material formed of piezoelectric ceramics, and has a relative permittivity ε _r of the piezoelectric material in an initial state. There is configured lower than the normal value, by the product of the square root of Kp and epsilon _r in use increase in epsilon _r in use compensates for the decrease in Kp value is substantially constant, the displacement amount approximately It is characterized by being kept constant.

[0008]

The piezoelectric body is used after being subjected to a polarization treatment. The condition of the polarization is preferably that the relative permittivity is such that the arrangement of the spontaneous polarization of the piezoelectric element is in the most adjusted state. It is polarized under the conditions of 50 kv / cm and a temperature of 80 to 130 ° C. Relative permittivity ε obtained under these conditions
_max (ε ₃₃ ^T / ε ₀ ) varies depending on the material composition. In general, a piezoelectric substance polarized at such a high temperature has almost no change in the relative dielectric constant during use.

[0009] However the present inventors have, by setting smaller than the relative dielectric constant epsilon _r epsilon _max of the piezoelectric body in the initial state, the dielectric constant epsilon _r is equal to epsilon _max increases during use I found that. Further, time course of displacement during use as a piezoelectric actuator, that there is a correlation between Kp and the relative dielectric constant epsilon _r of the square root of epsilon _r ^1/2 of the product ^{_{(Kp · ε r 1/2),}} Kp · ε It also became clear that the _r ^1/2 value could be used as a proxy for displacement.

[0010] In the present invention, therefore, the dielectric constant epsilon _r of the piezoelectric body in the initial state a value lower than the normal value. Because this way during use to increase the specific dielectric constant epsilon _r, can be made substantially constant Kp and epsilon _r ^1/2 of the product in use, substantially constant amount of displacement during use of the resulting piezoelectric actuator It can be. The reduction range from normal values of the relative permittivity epsilon _r, it is desirable to 5% to 20% lower by about the value of the normal value. When the decrease width is less than 5%, the relative permittivity during use hardly increases, and the product of Kp and ε _r ^1/2 is reduced, and the displacement is reduced. On the other hand, when the relative permittivity epsilon _r decreases from the normal value of 20% or more, for the initial value of the relative dielectric constant epsilon _r is too low, it can not be obtained the desired displacement. In addition, as a method of lowering the relative dielectric constant in the initial stage, there are a method of polarizing at room temperature, an aging treatment, and the like.

[0011]

The present invention will be described more specifically with reference to test examples, examples and conventional examples. The piezoelectric material used was Pb _0.89 Sr
PZT of composition of _0.11 (Zr _0.55 Ti _0.44 Nb _0.01 ) O ₃
It is a piezoelectric body. (Test Example) A silver paste was printed on both front and back surfaces of a disk-shaped PZT piezoelectric body having a diameter of 15 mm to form electrodes on the entire surface.
Polarization was performed at temperatures of 60 ° C., 60 ° C., and 100 ° C., and the electromechanical coupling coefficient Kp and the relative permittivity ε _r were measured. FIG. 4 shows the results. The polarization conditions were such that the PZT piezoelectric body was immersed in silicon oil at each temperature, and a voltage of 3 kv / mm was applied in the thickness direction and held for 5 minutes. Note that the relative permittivity ε
_r is ε ₃₃ ^T / measured in a direction parallel to the polarization direction.
ε ₀ .

FIG. 4 shows that as the polarization temperature decreases, ε
It is clear that _r is decreasing. On the other hand, Kp shows a tendency to increase as the polarization temperature decreases. Therefore, in the following embodiment, PZ polarized at room temperature of 22 ° C.
A T piezoelectric plate is used. (Example) Next, a PZT piezoelectric body polarized at 22 ° C. was subjected to a durability test in which an initial load of 30 MPa was applied in an oil bath at 100 ° C. and a voltage of 0 to 800 V was repeatedly applied. The subsequent Kp and ε _r were measured.
The results are shown in FIG.

FIG. 1 shows that as the number of times of durability increases, K
p is decreasing. Lowering ε _r initially, as in the present invention, results in an increased Kp value, which is also advantageous from this point. On the other hand, ε _r is 26 in the initial state.
Although it is as low as 00 or less, it rapidly increases after the start of durability and becomes stable after reaching the same level as the conventional one. And Kp and ε
_From the value of _r , the product of Kp and ε _r ^1/2 was calculated for each endurance cycle, and the results are shown in FIG. As a result, the range of variation of the product of Kp and ε _r ^1/2 from the initial stage to after the endurance is about 2%, which indicates that the product is extremely stable. From this result, this P
It is estimated that a piezoelectric actuator having a constant displacement can be obtained by using a ZT piezoelectric body.

Therefore, a PZT piezoelectric body polarized at 22 ° C.
And a copper electrode plate having a diameter of 15 mm and a thickness of 0.03 mm
Are alternately stacked, and every other electrode plate has the same polarity.
To form a pair of external electrodes
Eta, and the same endurance test as above
Was examined for changes. The result is shown in FIG. According to FIG.
The piezoelectric actuator according to the embodiment has an initial displacement and a displacement after the endurance.
It can be seen that the fluctuation is small and shows a stable displacement.
You. FIG. 2 and FIG. 3 agree well, and the displacement amount is Kp.
ε_r ^1/2It can be seen that the product of
Is. (Conventional example) PZT piezoelectric material polarized at 100 ° C
And apply a voltage of 0 to 800 V in a 100 ° C oil bath.
A durability test was conducted by repeatedly applying the Kp.
And ε _rWas measured. The results are shown in FIG.

As shown in FIG. 1, Kp decreases as the number of endurance increases as in the embodiment. Further, Kp is lower as a whole as compared with the embodiment. On the other hand, ε _r shows a high value from the beginning and hardly changes even after running. And Kp and ε
_From the value of _r , the product of Kp and ε _r ^1/2 was calculated for each endurance cycle, and the results are shown in FIG. As a result, the variation width of the product of Kp and ε _r ^1/2 from the initial stage to after the endurance is about 15.7%, which is larger than that of the embodiment.

Next, 50 PZT piezoelectric bodies polarized at 100 ° C. are laminated via an electrode plate in the same manner as in the embodiment to form a laminated piezoelectric actuator. The changes were examined. The result is shown in FIG. As shown in FIG. 3, the displacement of the piezoelectric actuator of the conventional example is large at the initial stage and after the endurance. That is, it is understood that a stable displacement amount cannot be obtained. This result agrees well with FIG. 2, and it can be seen from this conventional example that the displacement can be substituted by the product of Kp and ε _r ^1/2 .

Although a PZT-based piezoelectric material is used in this embodiment, the present invention is not limited to this.
O ₃ system, PbTiO ₃ system, or PbTiO ₃ -Pb
Piezoelectric ceramics such as ZrO ₃ —Pb (Mg _1/3 Nb _2/3 ) O ₃ can also be used. The relative permittivity ε _r
Not only ε ₃₃ ^T / ε ₀ but also ε ₂₂ ^T / ε ₀ and ε ₁₁ ^T / ε ₀ can be used.

[0018]

That is, according to the piezoelectric actuator of the present invention, the variation in the amount of displacement over time can be reduced, so that the piezoelectric actuator can be driven with a stable displacement. Therefore, a complicated control power supply circuit for controlling the voltage or current of the driving power supply is not required, and the cost can be reduced.

[Brief description of the drawings]

1 is a graph showing the relationship between the endurance and Kp and epsilon _r.

FIG. 2 is a graph showing the relationship between the number of endurance times and the product of Kp and ε _r ^1/2 .

FIG. 3 is a graph showing the relationship between the number of endurance times and the amount of displacement.

4 is a graph showing the relationship between the polarization temperature and Kp and epsilon _r.

Claims (1)

(57) [Claims] 1. A piezoelectric actuator configured from a piezoelectric body formed of piezoelectric ceramic, the dielectric constant epsilon _r of the piezoelectric member in the initial state is configured lower than the normal value, the dielectric constant in use epsilon The increase in _r compensates for the decrease in the electromechanical coupling coefficient Kp, and the electromechanical coupling coefficient Kp in use.
A piezoelectric actuator characterized in that a product of the square root of the relative permittivity ε _r is substantially constant, so that a displacement amount is kept substantially constant.