Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2000-11-03
2003-01-21
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06508127
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 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 FIGS.
3
(
a
) and
3
(
b
), 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 and 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, the entire flexible plate does not deflect uniformly; instead, the deflection stress rises rapidly, particularly at a boundary portion of the weight or the support and the flexible plate. Accordingly, when the piezoelectric elements are disposed at the boundary portion, the electric charges generated at the piezoelectric elements rapidly increase or decrease depending on slight deviations in the arrangement positions of the piezoelectric elements or variations in the shape of the weight or the support, so that it is possible to cause not only a variation of the output from each axis but also degradation in the sensor accuracy.
In addition, when the piezoelectric elements are disposed at a boundary portion, it is possible to cause degradation in sensor reliability due to the following deficiencies.
First, when the sensor element receives a rapid acceleration, a larger bending stress is applied to the boundary portion compared to other parts, and as a result, a particularly large force will be exerted not only on the piezoelectric elements but also on the piezoelectric body at the boundary portion Since the piezoelectric body Is made of piezoelectric ceramics such as PZT and PMN, whose mechanical characteristics such as strength and toughness are generally inferior, there is a concern that the piezoelectric body may be damaged. Second, since the behaviors of the pliable flexible plate and the rigid support or weight when the impact is applied are different, there is a concern that the piezoelectric elements may peel off from the flexible plate even if the piezoelectric body Is not damaged with the construction in which lower surfaces of the piezoelectric elements contact both the flexible plate and the support or the weight.
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 opposed characteristics can be satisfied; however, there are problems in that many components and steps are required, resulting in lowered productivity.
As means for reducing the number of components and steps and for improving productivity, integral forming by the same material can be possible. When integral forming by ceramics is used as an example, the following methods may be mentioned: a ago method of cutting a ceramic green body
33
formed by filling a mold
30
with a ceramic power
31
, and by conducting uniaxial pressing, etc., as shown in
FIG. 5
; a method of filling a mold
30
with a ceramic powder
31
and forming for a sensor
34
, and then performing uniaxial pressing as shown in
FIG. 6
; a method of injection molding ceramic slurry
32
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 manufacture the flexible plate thinly and to precisely control the thickness of the flexible plate. That is, in the sensor element formed by the above method, the flexible plate is unlikely to deflect and the degree of deflection differs for each flexible plate or depending on a portion 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 sensor
Namerikawa Masahiko
Shibata Kazuyoshi
Burr & Brown
Moller Richard A.
NGK Insulators Ltd.
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