Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-10-04
2002-03-26
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06360603
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an acceleration sensor and an acceleration detecting device. More particularly, the present invention relates to an acceleration sensor of the piezoelectric bimorph type and an acceleration detecting device using the acceleration sensor.
2. Description of the Related Art
Various acceleration sensors using bimorph type piezoelectric element are known. For example, in Japanese Unexamined Patent Publication No. 6-324073, an acceleration sensor
51
, as shown in
FIG. 5
, is disclosed.
In the acceleration sensor
51
, piezoelectric plates
52
,
53
are used. The piezoelectric plates
52
,
53
are of a rectangular shape, and the central portions
52
a
,
53
a
viewed from the longitudinal direction are polarized in opposite directions from each other in the thickness direction, as shown by the arrows.
Further, external areas
52
b
,
52
c
, shown by broken lines, are polarized, as shown by the arrows, in the thickness direction so as to be opposite to that of the central area
52
a
. Also in the piezoelectric plate
53
, external areas
53
b
,
53
c
arranged on both sides of the central area
53
a
, and shown by broken lines are polarized, as shown by the arrows, in the thickness direction so as to be opposite to the central area
53
a
. Accordingly, the external area
52
b
and external area
53
b
, and the external area
52
c
and external area
53
c
facing each other are polarized in the thickness direction so as to be opposite to each other, respectively.
On each of the external main surfaces of the piezoelectric plates
52
,
53
, signal output electrodes
54
,
55
are formed respectively. Further, at the portion where the piezoelectric plate
52
and piezoelectric plate
53
facing each other are joined and connected an intermediate electrode
56
is formed. However, the intermediate electrode
56
is formed so as not to extend to both ends of the piezoelectric plates
52
,
53
.
On the other hand, the signal output electrode
54
is led out to one end of the piezoelectric plate
52
, and the signal output electrode
55
is led out to the end portion opposite to the side to which the signal output electrode
54
is led.
On the outsides of the piezoelectric plates
52
,
53
, supporting members
57
,
58
are joined and connected respectively. The supporting members
57
,
58
support the piezoelectric plates
52
,
53
in the vicinity of both ends of the plates.
Further, on one end surface of the piezoelectric plates
52
,
53
and the supporting members
57
,
58
, an external electrode
59
is formed, and the external electrode
59
is electrically connected to the signal output electrode
55
. In the same way, also on the end surface opposite to the side with the external electrode
59
, an external electrode (not illustrated) is formed, and this external electrode is electrically connected to the signal output electrode
54
.
In the acceleration sensor
51
, when acceleration acts in the direction of an arrow A, the piezoelectric plates
52
,
53
are bent and the electric charge produced by the bending is output through the electrodes
54
,
55
, and accordingly the acceleration is able to be detected. As the acceleration sensor
51
is constructed in such a way that the piezoelectric plates
52
,
53
are supported in the vicinity of both ends, the amount of electric charge to be generated at the time when acceleration acts is increased and, because of this, even if the acceleration sensor
51
is made of small size, it is said that the detection sensitivity is not likely to be lowered.
Although it is possible to make the acceleration sensor
51
small-sized and to improve the detection sensitivity, the capacitance between the electrodes is small and accordingly there is a problem that acceleration at low frequencies is difficult to be measured. This is explained in detail below.
When acceleration acts in the direction of the arrow A, the generated voltage and the capacitance between the signal output electrode
54
and the intermediate electrode
56
, between the intermediate electrode
56
and signal output electrode
55
, and between a pair of external electrodes are represented by V
1
, V
2
, V
p
, C
1
, C
2
, and C
p
respectively as shown in Table 1.
TABLE 1
Generated
Between electrodes
voltage
Capacitance
Signal output electrode 54-
V
1
C
1
Intermediate electrode 56
Intermediate electrode 56-
V
2
C
2
Signal output electrode 55
Between a pair of external
V
p
C
p
electrodes
Here, suppose the thickness, length, and width of the piezoelectric plates
52
,
53
are the same, the relation of V
1
=V
2
and C
1
=C
2
results. Therefore, when V
1
and V
2
are respectively represented by V
0
, and C
1
and C
2
are respectively represented by C
0
, since the piezoelectric plates
52
,
53
are connected in series, the generated voltage V
p
at the time when acceleration acts in the direction of the arrow A in the acceleration sensor
51
becomes V
p
=2 V
0
, and the capacitance C
p
becomes C
p
=C
0
/2.
When acceleration is detected by using the above acceleration sensor
51
, because the acceleration sensor
51
has a relatively high impedance, it is common to use a voltage amplifier or a charge amplifier.
FIG. 6
is a circuit diagram showing an acceleration detection circuit having such a voltage amplifier connected.
In
FIG. 6
, a leak resistor R is connected in parallel to the acceleration sensor
51
. Further, the output side of the acceleration sensor
51
is connected to one input terminal of a voltage follower
60
. Further, the output terminal and the other input terminal of the voltage follower
60
are connected.
In the above acceleration detecting device, the following relation is established: Output voltage V
OUT
=Input voltage to the amplifier V
i
=Generated voltage V
p
in the acceleration sensor. The output of the voltage follower
60
converts the output to a sufficiently low impedance.
However, in an operational amplifier and FET constituting the above voltage follower
60
, for example, because there is bias current i
B
flowing out of the input terminal, the above-mentioned leak resistor R is required. That is, unless the leak resistor R is present, the capacitance of the acceleration sensor
51
continues to be charged and the voltage becomes saturated. Accordingly, the leak resistor R is required.
But the leak resistor R causes the electric charge generated at the piezoelectric plates
52
,
53
to leak. That is, when the acceleration is slowly changed, or when the acceleration is not changed, the electric charge completely leaks before any voltage V
p
is generated. Therefore, no predetermined detection voltage can be obtained. This is expressed by a frequency characteristic as shown in FIG.
7
.
FIG. 7
shows the relation between the frequency of acting acceleration at the time when the above-mentioned acceleration detecting circuit is used and the voltage V
i
to be input to the voltage follower
60
.
In
FIG. 7
, f
c
represents a cutoff frequency. Here, the cutoff frequency f
c
is given by
f
c
=1/(2&pgr;RC
p
)
Therefore, in order to measure acceleration at lower frequencies than the above cutoff frequency f
c
, resistance R and/or capacitance C
p
is required to increase. But if resistance R is increased, the offset voltage of the voltage follower
60
increases and in order to reduce the offset voltage an operational amplifier having small bias current is required to be used as a voltage follower, which results in high cost.
Further, even if an operational amplifier of low bias voltage is available, when, for example, a high leak resistance R exceeding 10 M&OHgr; is connected, advanced measures for humidity resistance are required including for the printed-circuit board to which the leak resistor R is connected. As a result, there are various restrictions even if the resistance of the leak resistor R were to be increased.
On the other hand, the capacitance C
p
is determined by the configuration of the
Moller Richard A.
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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