Acceleration sensor

Details Acceleration sensor Acceleration sensor

2003-07-14

2004-08-24

Kwok, Helen (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Acceleration determination utilizing inertial element

C310S324000

Reexamination Certificate

active

06779403

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acceleration sensor, and more particularly to an acceleration sensor for detecting an acceleration by transforming oscillation levels into electrical signals.
2. Description of the Related Art
In general, the acceleration sensor now in use includes various types such as an electro magnetic type, a piezoelectric element type, and a semiconductor type which are known as detecting an acceleration applied thereto. Among these types of acceleration sensor, the piezoelectric element type of the acceleration sensor has a piezoelectric element deformable in response to the acceleration to detect the acceleration. These piezoelectric element types of the acceleration sensor are applied to automotive vehicles and used for controlling knocking of engine and air bag.
A conventional piezoelectric element type of the acceleration sensor herein raised for example is shown in
FIG. 25
to comprise an oscillation plate having a central portion fixed. This type is called “the center-fixed type of acceleration sensor”, i.e., the first conventional acceleration sensor. This center-fixed type of acceleration sensor
100
comprises a fixed metal case
101
having a central bottom portion from which projects a supporting protrusion
101
a
integrally formed with the central portion. Onto the supporting protrusion
101
a
is welded and securely connected an oscillation plate
102
made of a metal and in the form of a thin disc shape to facilitate resonance motion of the oscillation plate
102
as shown in FIG.
26
. On the upper surface of the oscillation plate
102
is mounted a piezoelectric element
103
in a doughnut shape in a manner that the piezoelectric element
103
is held in axial alignment with the oscillation plate
102
. The piezoelectric element
103
has upper and lower surfaces on which are respectively mounted a pair of electrodes
104
axially aligned with the piezoelectric element
103
. One of the electrodes
104
is electrically connected with the oscillation plate
102
, while the other one of the electrodes
104
is soldered at
105
a
and thus electrically connected with a metal wire
105
by way of, for instance, wire bonding. The acceleration sensor
100
further comprises an output terminal
107
having one end electrically connected with the metal wire
105
and the other end electrically connected with an exterior connector, not shown, and a cover member
106
in the form of a bowl shape and made of a resin material. The fixed case
101
and the cover member
106
have peripheral edge portions
101
c
and
106
c
, respectively, which are firmly coupled with each other to define a closed space
109
having the oscillation plate
102
and the piezoelectric element
103
received therein. Between the peripheral edge portions
101
c
of the fixed case
101
and
106
c
of the cover member
106
is disposed an O-ring which serves to hermetically seal the closed space
109
.
Another conventional piezoelectric element type of the acceleration sensor herein raised for example, i.e., the second conventional acceleration sensor is shown in FIG.
27
. The acceleration sensor
110
comprises a fixed case
111
made of a metal and having a peripheral ledge portion
111
c
, and a metal base member
112
in the form of a disc shape and also having a peripheral edge portion
112
c
. The metal base member
112
is welded to and thus securely mounted on the fixed case
111
with the peripheral ledge portion
111
c
being in registry with the peripheral edge portion
112
c
so that the fixed case
111
is covered and closed by the metal base member
112
. On the metal base member
112
is mounted a connector member
116
also in the form of a disc shape and having a peripheral edge portion
116
c
fixedly engaged with the peripheral ledge portion
111
c
of the fixed case
111
. The connector member
116
has an output terminal
107
securely mounted thereon and is electrically connected with an exterior connector, not shown. The fixed case
111
, the metal base member
112
and the connector member
116
collectively define a closed space
109
in which the oscillation plate
102
and the piezoelectric element
103
are accommodated. The metal base member
112
has a central portion from which downwardly extends a protrusion
112
a
having the oscillation plate
102
supported thereon, compared with the protrusion
110
a
of the case base
101
upwardly projected and having the oscillation plate
102
supported thereon as shown in FIG.
25
. Both of the oscillation plate
102
and the piezoelectric element
103
are in the form of a doughnut shape and securely supported by the protrusion
112
a
of the metal base member
112
to ensure that the oscillation plate
102
is oscillatable with respect to the fixed case
111
. The connector member
116
is made of a resin material and serves to electrically insulate the metal base member
112
from the fixed case
111
. The output terminal
107
securely mounted on the connector member
116
extends through the protrusion
112
a
of the metal base member
112
and has a lower end electrically connected with one of the electrodes
104
on the piezoelectric element
103
by way of a connecting disc plate
115
soldered at
115
a
to the lower end of the output terminal
107
and one of the electrode
104
. The acceleration sensor
110
comprises an O-ring
118
disposed between the inner peripheral face of the fixed case
111
and the outer peripheral face of the metal base member
112
to hermetically seal the closed space
109
. It is preferable that the connecting disc plate
115
has a rigidity as small as possible so that the oscillation plate
102
and the piezoelectric element
103
are not prevented from being oscillated. The connecting disc plate
115
may be replaced with a metal wire having one end electrically connected to the output terminal
107
and the other end electrically connected to one of the electrode
104
on the piezoelectric element
103
in a manner that the oscillation plate
102
is welded on the protrusion
112
a
of the metal base member
112
.
The first and second conventional acceleration sensors
100
and
110
respectively have lower portions formed with male screws
101
b
and
111
b
each screwed in to an oscillation object such as an automotive engine or the like to ensure that the oscillation plate
102
is oscillated with respect to the fixed cases
101
and
111
when the oscillation object is oscillated for some reason. The oscillation of the oscillation plate
102
causes the piezoelectric elements
103
to be deformed and energized to generate voltage levels which are outputted to the output terminal
107
through one of the electrodes
104
with the fixed case
101
or
111
and the metal base member
112
grounded.
In general, the piezoelectric element
103
has a capacity C between the electrodes
104
which can produce an electric charge Q when the oscillation plate
102
is oscillated and deformed to produce a stress deformation in the piezoelectric element
103
by exterior oscillations, i.e. accelerations. The electric charge Q thus caused by the stress deformation of the oscillation plate
102
can be measured as voltage V that is represented by the following equation:
V=Q/C
It is considered that the oscillation plate
102
has the maximum oscillation amplitude at around its outer peripheral end while the piezoelectric element
103
has the maximum stress deformation value at around its central portion, resulting from the fact that the piezoelectric element
103
is extended and contracted.
The acceleration sensor
100
or
110
has a frequency characteristic under a predetermined level of oscillation corresponding to a predetermined level of acceleration as shown in FIG.
28
.
FIG. 28
indicates that the output voltage V
0
is high at a frequency of the resonance point f
0
, hereinlater referred to as “resonance frequency f
0
”, while being flat and low at frequency points in other areas such as

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