Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2000-11-03
2003-04-15
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S514380
Reexamination Certificate
active
06546800
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an acceleration sensor element for three-dimensionally determining the direction and the rate of acceleration, applied from the exterior, using a piezoelectric body, to an acceleration sensor therewith, and to the manufacture thereof.
BACKGROUND OF THE INVENTION
In the automobile industry and the machinery industry, demands for sensors capable of accurately determining physical quantities such as force, acceleration, and magnetic force are-increasing. In particular, small sensors capable of detecting physical quantities for each component in two-dimensions or in three-dimensions are required.
For example, a sensor in which a plurality of piezoelectric bodies are mounted on a flexible plate having an operation body is disclosed in Japanese Unexamined Patent Application Publication No. 5-26744.
The sensor is constructed so that the flexible plate may be deflected corresponding to physical quantities applied to the operation body from the exterior, and can three-dimensionally determine the direction and the rate of the physical quantities by electric charges generated in the piezoelectric body, depending on the deflection of the flexible plate using a single sensor element.
The sensor element will be described, using as an example, an acceleration sensor having a weight as the operation body. As shown in
FIG. 2
, when acceleration a is applied to the sensor element from the exterior, an inertial force f is applied to a weight
10
in a direction opposite to the acceleration a, and as a result, deflection
14
due to the inertial force f is generated at a flexible plate
12
horizontally disposed between the weight
10
and a support
11
. Since electric charges depending on the direction and the rate of the deflection
14
are generated at a piezoelectric body
13
constructed over the flexible plate
12
, the acceleration applied from the exterior can be three-dimensionally determined by measuring the electric charges.
In the sensor element as shown in
FIG. 3
, the center of a base of the cylindrical weight
10
, on which the flexible plate
12
is horizontally disposed, is specified as the origin O, a surface including the origin O, which is parallel to the flexible plate
12
, is specified as an X-Y plane, the X-axis and Y-axis are specified so as to perpendicularly intersect each other in the X-Y plane, and the Z-axis is specified to include the origin O to perpendicularly intersect the X-Y plane.
In this instance, when it is assumed that a part sandwiched by one set of an upper electrode and a lower electrode of the piezoelectric body
13
is “one piezoelectric element,” for example, four elements of piezoelectric bodies and electrodes, each corresponding to the X-axis and the Y-axis, and eight elements of piezoelectric bodies and electrodes corresponding to the Z-axis can be arranged on the flexible plate
12
.
In this case, the magnitude of each inertial force f applied to the weight
1
by the acceleration a applied from the exterior is determined as follows: the magnitude of an inertial force f
X
in the X-axis direction as shown in FIG.
4
(
a
) is determined by the amounts of the electric charges generated at “piezoelectric elements” E
1
to E
4
; the magnitude of an inertial force f
Y
in the Y-axis direction is determined by the amounts at E
5
to E
8
(not shown); and the magnitude of an inertial force f
Z
in the Z-axis direction as shown in FIG.
4
(
b
) is determined by the amounts at “piezoelectric elements” E
9
to E
12
and E
13
to E
16
(not shown).
In addition, the directions of the inertial forces are each determined from the polarity patterns of the electric charges (e.g., for a piezoelectric body top surface in FIG.
4
(
a
), [+−+−] from the left; and for a piezoelectric body top surface in FIG.
4
(
b
), [+−−+] from the left).
The inertial force f as a resultant force of f
X
, f
Y
, and f
Z
determined as described above, and the direction and the rate of the acceleration a applied from the exterior can be three-dimensionally determined by a single small sensor element.
In the aforesaid sensor element, if the flexible plate is deflected easily, the amount of the electric charge generated will be increased even if the same acceleration is applied to the weight, and accordingly, it is obvious that the flexibility of the flexible plate and the sensor sensitivity are correlated.
However, since the entire flexible plate does not deflect uniformly, the deflection stresses instead being uneven, even if the flexibility of the flexible plate is uniform, the amount of electric charges generated is decreased depending on the arrangement of the piezoelectric body, so that sensor sensitivity may be degraded.
In addition, in the aforesaid sensor element, the flexible plate is required to have high flexibility in order to have sufficient sensitivity, while the weight and support are required to have high rigidity and to be unlikely to deflect in order to accurately detect the acceleration applied thereto.
If the sensor element is constructed by assembling the members such as the weight, the support, and the flexible plate, which were manufactured separately, the aforesaid incompatible characteristics can be satisfied; however, there are problems in that many components and steps are required, resulting in lower productivity.
As a means for reducing the number of components and steps and improving productivity, integral formation using the same material is possible. When integral formation using ceramics is used as an example, the following methods may be mentioned: a method of cutting a green ceramic body
23
formed by filling a mold
20
with a ceramic power
21
and by conducting uniaxial pressing, etc., as shown in
FIG. 5
; a method of filling the ceramic powder
21
into the formed mold
20
for a sensor
34
, and then performing uniaxial pressing as shown in
FIG. 6
; a method of injection molding ceramic slurry
22
as shown in
FIG. 7
; and a method of molding by slip casting.
In any of the aforesaid methods, however, since the strength of the flexible part of the integral compact is extremely low and the dispersion of the density distribution is large, it is difficult to make the flexible plate thin and to precisely control the thickness of the flexible plate. That is, in sensor elements formed by the above methods, the flexible plate is unlikely to deflect and the degree of deflection differs for each flexible plate or depending on portions thereof.
Accordingly, in addition to the drawback of low sensitivity, there is a case in which the sensitivity differs for each sensor element even if the same acceleration is applied, and also when the sensitivity differs for each axis, the direction and the rate of the acceleration obtained from the resultant force thereof are inaccurate, so that the sensor accuracy is reduced.
The present invention is made in consideration of the above problems, and objects thereof are to further improve sensor sensitivity and sensor accuracy while offering the advantages of a sensor element that is small and can measure three-dimensional physical quantities using a single sensor element, and to provide a manufacturing method in which the sensor element can be easily manufactured.
SUMMARY OF THE INVENTION
The present invention provides an acceleration sensor element (hereinafter referred to as “a sensor element”) including a weight, a support having a hollow part, which is disposed around the weight as a center, and a flexible plate having piezoelectric elements in which a piezoelectric body is sandwiched by at least one set of electrodes. The flexible plate is disposed horizontally across the support so as to suspend the weight at the center of the hollow part of the support Acceleration applied from the exterior is converted to a deflection of the flexible plate based on the behavior of the weight arising corresponding to the acceleration, and the direction and the rate of the acceleration arc three-dimensionally deter
Namerikawa Masahiko
Shibata Kazuyoshi
Burr & Brown
Moller Richard A.
NGK Insulators Ltd.
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