Piezoelectric actuator

Details Piezoelectric actuator Piezoelectric actuator

1998-09-01

2001-08-14

Budd, MArk O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S330000, C310S340000, C310S354000, C310S366000

Reexamination Certificate

active

06274966

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric actuator, and, more particularly, to an improvement in a d
31
type layered piezoelectric actuator which generates a large displacement.
2. Description of the Related Art
A conventional d
31
type layered piezoelectric actuator
1
, which is related to the present invention, is shown in FIG.
8
. The d
31
type layered piezoelectric actuator
1
is planar and substantially rectangular. One end of the actuator
1
is fixed to a fixing portion
2
such as a base and the other end of the actuator
1
is unsupported and free to experience displacement.
The piezoelectric actuator
1
includes an actuator body
6
having a layered structure including a plurality of piezoelectric layers
3
and a plurality of inner electrodes
4
and
5
that are disposed between the piezoelectric layers
3
. The actuator body
6
has a first major surface
7
and a second major surface
8
, as well as, a first end surface
9
and a second end surface
10
. The first and second major surfaces
7
and
8
are defined by the outwardly facing major surfaces of the two piezoelectric layers
3
a
and
3
b
that form the outermost layers of the actuator body
1
. The end surfaces
9
and
10
are defined by the end surfaces of the plurality of piezoelectric layers
3
.
A first external electrode
11
and a second external electrode
12
are disposed on the first end surface
9
and the second end surface
10
of the actuator body
6
, respectively. In order to actuate the piezoelectric actuator, a voltage is applied to the first and the second external electrodes
11
and
12
from outside of the actuator body
6
.
The aforementioned inner electrodes
4
are referred to as first inner electrodes
4
and are connected to the first external electrode
11
on the first end surface
9
. The inner electrodes
5
are referred to as second inner electrodes
5
and are connected to the second external electrode
12
on the second end surface
10
. Applying a voltage to the external electrodes
11
and
12
causes the first inner electrodes
4
and the second inner electrodes
5
to have different polarities. The first inner electrodes
4
and the second inner electrodes
5
are alternately disposed in the direction in which they are stacked upon each other.
At the side of the first end surface
9
of the actuator body
6
, a piezoelectrically active area
13
is defined by stacking the first inner electrodes
4
and the second inner electrodes
5
upon each other. The piezoelectrically active area
13
is located closer to the free end of the actuator
1
than the fixed end of the actuator attached to the fixing portion
2
. At the side of the second end surface
10
of the actuator body
6
, a piezoelectrically inactive area is defined and has a size or area which is equal to the fixing area to be fixed to the fixing portion
2
. The fixing area is fixed so that the piezoelectric actuator
1
is not dislodged or removed from the fixing portion
2
when the actuator body
6
is undergoing displacement.
For the purpose of increasing displacement, the piezoelectric layers
3
a
and
3
b
, being the outer most layers of the illustrated piezoelectric actuator
1
, are formed so as to be piezoelectrically active within at least the piezoelectric active area
13
. More specifically, the first external electrode
11
has an extension portion
14
and an extension portion
15
which extend to the first major surface
7
and the second major surface
8
, respectively, with the extension portion
14
facing, through the piezoelectric layer
3
a
at the first major surface
7
side, the second inner electrodes
5
having a polarity which is different from that of the first outer electrode
11
, and the extension portion
15
facing, through the piezoelectric layer
3
b
at the second major surface
8
side, the second inner electrodes
15
.
The displacement of the piezoelectric actuator
1
is shown in FIG.
9
. In
FIG. 9
, the piezoelectric actuator
1
is deformed to the shape indicated by broken lines, causing displacement shown by arrow
16
at one end portion of the piezoelectric actuator
1
.
The above-described piezoelectric actuator, though displaced by only a small amount, is displaced easily and quickly. As a result, the actuator can be advantageously used in inkjet printers. However, when a high-performance printer is to be provided, it is necessary to discharge ink in large amounts. In such a case, an actuator which can is arranged to experience a large displacement is desired.
To achieve such an actuator, it has been attempted to develop a new material having a large d
31
constant to produce a highly efficient d
31
type actuator. However, even this piezoelectric actuator does not provide sufficient performance or large enough displacement. In addition, in developing the new material having a large d
31
, considerable effort and time are required to increase the d
31
constant by only a few percent.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, the preferred embodiments of the present invention provide a piezoelectric actuator which can very easily be displaced by a larger amount without relying on the development of a new material.
In a preferred embodiment of the present invention, a d
31
type layered piezoelectric actuator includes an actuator body having a layered structure including a plurality of piezoelectric layers and a plurality of inner electrodes disposed between the piezoelectric layers. The actuator body has a first major surface and a second major surface, as well as, a first end surface and a second end surface. The first and second major surfaces are defined by outwardly facing major surfaces of the two piezoelectric layers which are the outermost layers of the actuator body. The first and the second end surfaces are defined by the end surfaces of the piezoelectric layers The piezoelectric actuator also includes a first external electrode and a second external electrode, which are located on at least the first end surface and the second end surface of the actuator body. The inner electrodes include first inner electrodes which are connected to the first external electrode on the first end surface and second inner electrodes which are connected to the second external electrode on the second end surface. The first inner electrodes and the second inner electrodes are alternately disposed in a direction in which they are stacked upon each other. The actuator body has a piezoelectrically active area defined by placing the first inner electrodes and the second inner electrodes upon each other. The first external electrode has an extension portion which extends to the first major surface of the actuator body. In the piezoelectrically active area, the second inner electrodes face, through the piezoelectric layer that is the outermost layer at the first major surface side, the extension portion of the first external electrode, whereby the piezoelectric layer that is the outermost layer at the first major surface side becomes piezoelectrically active, so that a large displacement occurs. The piezoelectric layer that is the outermost layer at the second major surface side is piezoelectrically inactive.
Therefore, the displacement value caused by bending is added to the d
31
component displacement, making it possible to increase the efficiency of displacement and to increase the displacement amount as a whole.
Accordingly, when the piezoelectric actuator of preferred embodiments of the present invention is applied, for example, to an inkjet printer, a larger amount of ink can be discharged, resulting in improved printer performance.
In the above described piezoelectric actuator, when either one of the first external electrode and the second external electrode has an extension portion which extends to the second major surface, the piezoelectric layer that is the outermost layer at the second major surface side is piezoelectrically inactive. Therefore, when, at the piezoele

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