Acceleration sensor having fault diagnosing device

An acceleration sensor includes a piezoelectric element for outputting an electric signal corresponding to acceleration applied thereto, a signal processor for processing a signal outputted from the piezoelectric element, an AC signal outputting device for receiving an externally inputted signal and generating an AC signal which is synchronized with the period of the timing signal in response to receipt of the timing signal and for applying the AC signal to the piezoelectric element, and a capacitor connected between the AC signal outputting device and the piezoelectric element for detecting a fault in the acceleration sensor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an acceleration sensor for use 
in an air bag device carried by an automobile, and more particularly, to 
an acceleration sensor having a fault diagnostic device. 
2. Description of the Prior Art 
In an air bag device carried by an automobile, the air bag device operated 
in response to acceleration applied at the time of, for example, a 
collision. In order to ensure the operation of the air bag device, an 
acceleration sensor has been conventionally incorporated in the above 
described air bag device. As this type of acceleration sensor, an 
acceleration sensor using a piezoelectric element which is deformed in 
response to acceleration applied thereto to output an electric signal has 
been proposed, as disclosed in, for example, U.S. Pat. No. 4,700,973. 
One example of a conventionally known acceleration sensor utilizing a 
piezoelectric element will be described with reference to FIGS. 1 to 3. 
FIG. 1 is a schematic block diagram for explaining the construction of a 
conventional acceleration sensor. There is provided a piezoelectric 
element 1 outputting, when acceleration G is applied, an electric signal 
corresponding to the acceleration G. An impedance converter means 2 is 
electrically connected to the piezoelectric element 1. The impedance 
converter 2 converts the impedance of the electric signal applied from the 
piezoelectric element 1. A filter 3 having a band-pass filter is 
electrically connected to the impedance converter 2. In the filter 3, an 
unnecessary signal, that is, an out-of-band signal component is 
attenuated. An amplifier 4 is electrically connected to the filter 3. In 
the amplifier 4, an output signal applied from the filter 3 is amplified. 
This acceleration sensor has the piezoelectric element 1, the impedance 
converter 2, the filter 3 and the amplifier 4, and output a voltage signal 
corresponding to the acceleration G from an output terminal B. 
The voltage signal outputted from the output terminal B of the acceleration 
sensor is applied to a control unit 5 comprising a microcomputer arranged 
outside the acceleration sensor. The control unit 5 causes an air bag 
device for an automobile (not shown) to perform a necessary operation on 
the basis of the voltage signal applied. 
FIGS. 2 and 3 are respectively a plan sectional view showing the specific 
construction of the above described acceleration sensor and a cross 
sectional view taken along a line 3--3 shown in FIG. 2. In an acceleration 
sensor 6, a base plate and a cap 8 secured to the upper surface of the 
base plate constitute a package structure containing a housing space. 
Within the package structure, a hybrid IC 9 is secured on the base plate 7 
using adhesives (not shown). The hybrid IC 9 is used for constructing the 
impedance converter 2, the filter 3 and the amplifier 4 described above. A 
plurality of electrodes 9a to 9f for connection to outer portions are 
formed on the upper surface of the hybrid IC 9. Each of the electrodes 9a 
to 9d is electrically connected to a lead terminal 11 by a lead wire 10. A 
plurality of lead terminals 11 are passed through the base plate 7 and are 
extended downward. 
On other hand, a supporting base 12 is secured on the base plate 7 using 
adhesives (not shown) beside the hybrid IC 9. A piezoelectric element 1 
having electrodes (not shown) on both its major surfaces is secured on the 
supporting base 12 in a cantilevered shape. The electrode on the upper 
surface of the piezoelectric element 1 is electrically connected to the 
electrode 9f by a lead wire 10f, and the electrode on the lower surface of 
the piezoelectric element 1 is electrically connected to the supporting 
base 12. In addition, the supporting base 12 is electrically connected to 
the electrode 9e on the hybrid IC 9 by a lead wire 10e. 
The above described piezoelectric element 1 has a series type bimorph 
structure which is low in piezoelectric voltage. 
Meanwhile, the above described package structure constituted by the base 
plate 7 and the cap 8 is hermetically sealed, and an inert gas, for 
example, nitrogen gas is sealed into the package structure so as to 
prevent oxidation. When the above described acceleration sensor is 
incorporated in the air bag device for an automobile, the air bag device 
is operated in response to an output signal of the acceleration sensor. 
Accordingly, the acceleration sensor requires very high reliability, and a 
fault in the acceleration sensor must be quickly detected. Since a fault 
diagnostic function has not been conventionally added to the acceleration 
sensor itself, however, the fault diagnosis of the acceleration sensor is 
generally made by an outer fault diagnostic device which is provided 
separately from the acceleration sensor. When there occurs a fault such as 
the cracking of the piezoelectric element 1 in the acceleration sensor or 
the stripping of the piezoelectric element 1 from the supporting base 12, 
the fault may not, in some cases, be quickly found out because the 
acceleration sensor does not have a fault self-diagnostic function. 
Furthermore, the conventional acceleration sensor 6 shown in FIGS. 2 and 3 
also has the following disadvantages. That is, when an excessive shock is 
externally given, there occurs a state where the supporting base 12 fixed 
to the base plate 7 using adhesives is stripped from the base plate 7 and 
the piezoelectric element 1, along with the supporting base 12, is not 
fixed to the other members, as represented by an imaginary line in FIG. 3. 
Accordingly, acceleration cannot, in some cases, be accurately detected by 
the piezoelectric element 1. Even if such a fault occurs, the electrical 
connection between the piezoelectric element 1 and the hybrid IC 9 is 
ensured through the lead wires 10e and 10f so long as the piezoelectric 
element 1 is fixed to the supporting base 12, so that the occurrence of 
the fault may not, in some cases, be detected. 
SUMMARY OF THE INVENTION 
Therefore, the present invention has been made so as to overcome the above 
described disadvantages of the conventional acceleration sensor. Thus, and 
object of the present invention is to provide an acceleration sensor 
capable of reliably detecting the occurrence of a fault. 
In accordance with an aspect of the present invention, there is provided an 
acceleration sensor including a piezoelectric element outputting an 
electric signal corresponding to acceleration applied, a signal processor 
electrically connected to the above described piezoelectric element and 
for processing the electric signal outputted from the piezoelectric 
element, an AC signal outputting device electrically connected to the 
above described piezoelectric element so as to generate an AC signal which 
is synchronized with the period of a timing signal externally inputted in 
response to the timing signal and to apply the AC signal to the 
piezoelectric element, and a capacitor connected between the above 
described AC signal outputting device and the piezoelectric element. 
Furthermore, in accordance with another aspect of the present invention, 
there is provided an acceleration sensor including an acceleration 
detecting portion having a piezoelectric element having electrodes on both 
its major surfaces and outputting an electric signal corresponding to 
acceleration applied and a supporting base for supporting the above 
described piezoelectric element, the acceleration detecting portion being 
further provided with a pair of detecting portions arranged so as to be 
brought into contact with the piezoelectric element and spaced apart from 
each other by a predetermined distance, a signal processor electrically 
connected to one of the electrodes of the piezoelectric element and for 
processing the signal outputted from the piezoelectric element, and a 
signal outputting device electrically connected to one of the above 
described pair of detecting portions so as to cause a predetermined 
current to flow between the above described detecting portions and 
electrically connected to the above described signal processor so as to 
output to the above described signal processor a state signal representing 
the state of the acceleration detecting portion, the signal outputting 
device outputting the above described state signal in conformity with the 
variation of the current flowing between the detecting portions. 
In each of the acceleration sensors provided in accordance with the above 
described two aspects of the present invention, a fault self-diagnostic 
function is provided in the acceleration sensor, as apparent from the 
embodiments described later. Even when there occurs a fault such as the 
cracking of the piezoelectric element or the stripping of the 
piezoelectric element from a portion to which the piezoelectric element is 
fixed, therefore, the fault which occurred can be quickly detected. 
In accordance with still another aspect of the present invention, there is 
provided an acceleration sensor including a piezoelectric element 
outputting an electric signal corresponding to acceleration applied, and a 
signal processor electrically connected to the above described 
piezoelectric element and for processing the output signal of the 
piezoelectric element, at least a part of the above described signal 
processor being constituted by an electronic component having a flat upper 
surface, the above described piezoelectric element being fixed to the 
upper surface of the electronic component in a cantilevered arrangement. 
Furthermore, in accordance with a particular aspect of the present 
invention, an electrode for making connection to the piezoelectric element 
is formed on the upper surface of the above described electronic 
component, and the piezoelectric element is electrically connected to the 
electrode and is fixed to the upper surface of the electronic component. 
In this case, the area of a portion to which the piezoelectric element is 
fixed on the upper surface of the electronic component is smaller than the 
area of a portion to which the electronic component itself is fixed. 
Accordingly, when a shock or the like is applied, not the electronic 
component but the piezoelectric element supported on the electronic 
component is stripped from the electronic component. As a result, the 
electrical connection between the piezoelectric element and the electronic 
component is interrupted, thereby to make it possible to quickly detect a 
fault caused by the stripping of the piezoelectric element. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 4 is a schematic block diagram for explaining the construction of an 
acceleration sensor according to a first embodiment of the present 
invention, and FIG. 5 is a circuit diagram showing one example of an 
electric circuit of the acceleration sensor. 
As shown in FIGS. 4 and 5, in the acceleration sensor according to the 
present embodiment, there is provided a piezoelectric element 21 
outputting an electric signal corresponding to acceleration G. As the 
piezoelectric element 21, a piezoelectric element having a series type 
bimorph structure having electrodes on both its major surfaces, for 
example, is used because it is low in pyroelectric voltage. The 
piezoelectric element 21 is deformed in conformity with the acceleration G 
applied thereto to output an electric signal corresponding to the 
deformation. One of the electrodes of the piezoelectric element 21 is 
connected to a reference potential, as shown in FIG. 5. The other 
electrode of the piezoelectric element 21 is electrically connected to an 
impedance converter 22. The impedance converter 22 is provided so as to 
convert the impedance of the electric signal applied from the 
piezoelectric element 21 so that the impedance matches the impedance of 
the other circuit elements. The impedance convertor 21 can be constituted 
by, for example, a field effect transistor TR.sub.1 and resistors R.sub.1 
to R.sub.4 shown in FIG. 5. 
A filter 23 is provided in the succeeding stage of the impedance converter 
22. The filter 23 includes a band-pass filter for attenuating or removing 
an unnecessary signal component, and can be constituted by, for example, 
an operational amplifier D.sub.A1, resistors R.sub.5 to R.sub.7 and 
capacitors C.sub.1 to C.sub.3 as shown in FIG. 5. 
An Amplifier 24 is provided in the succeeding stage of the filter 23 so as 
to amplify an output signal applied from the filter 23. The amplifier 24 
can be constituted by, for example, an operational amplifier D.sub.A2 and 
variable resistors VR.sub.1 and VR.sub.2 as shown in FIG. 5. 
The impedance converter 22, the filter 23, and the amplifier 24 constitute 
a signal processor according to the present invention. An output terminal 
B of the amplifier 24 is an output terminal of the acceleration sensor 
according to the present embodiment. 
On the other hand, an AC signal outputting device 26 is electrically 
connected to the electrode, to which the impedance converter 22 is 
electrically connected, of the piezoelectric element 21 through a 
capacitor 25. The AC signal outputting device has an input terminal A, and 
a timing signal as described later is externally inputted to the input 
terminal A. The AC signal outputting device 26 is so constructed that an 
AC signal which is synchronized with the period of the timing signal 
inputted is outputted and the AC signal is applied to the piezoelectric 
element 21 through the capacitor 25. The AC signal outputting device 26 
can be constituted by, for example, a transistor TR.sub.2, resistors 
R.sub.8 and R.sub.9, and variable resistors VR.sub.3 and VR.sub.4 as shown 
in FIG. 5. 
Description is now made of a fault diagnostic operation in the acceleration 
sensor according to the first embodiment described with reference to FIGS. 
4 and 5. The fault diagnosis is made in cases, for example, a case where 
the acceleration sensor is used for an air bag device carried by an 
automobile and a case immediately before an automobile is driven. That is, 
when the engine of an automobile is started, for example, an instruction 
to start a fault diagnostic operation is issued to the acceleration sensor 
from an outer control unit 27 (see FIG. 4). 
When the fault diagnostic operation is started, a timing signal having a 
frequency f.sub.1 within a passband of the filter 23 is first outputted to 
the input terminal A of the AC signal outputting device 26 from the 
control device 27. When the above described timing signal is inputted to 
the input terminal A, the AC signal outputting device 25 outputs an AC 
signal which is synchronized with the period of the timing signal. The AC 
signal is applied to the piezoelectric element 21 through the capacitor 
The piezoelectric element 21 has a certain capacitance value. Accordingly, 
the capacitance value of the piezoelectric element 21 is taken as Q.sub.1, 
the capacitance value of the capacitor 25 is taken as Q.sub.2, and a 
voltage of the AC signal outputted from the AC signal outputting device 26 
is taken as V.sub.1. In this case, a voltage V.sub.2 applied to the 
impedance converter 22 is expressed by the following equation: 
EQU V.sub.2 =V.sub.1 . Q.sub.2 /(Q.sub.1 +Q.sub.2) . . . (I) 
A voltage signal having the above described voltage V.sub.2 is 
impedance-converted in the impedance converter 22 to an impedance that 
matches that of the other circuit elements, and an unnecessary signal 
component is attenuated or removed in the filter 23. The voltage signal is 
amplified by the amplifier 24 and is outputted as a voltage signal V.sub.S 
from the output terminal B. 
In this case, the AC signal outputting device 26 is previously adjusted so 
that the voltage signal V.sub.3 having a constant value is outputted from 
the output terminal B by inputting the timing signal having a frequency 
f.sub.1. This adjustment can be made by, for example, adjusting the 
variable resistors VR.sub.3 and VR.sub.4 in the circuit shown in FIG. 5. 
The voltage signal V.sub.3 outputted from the output terminal B of the 
acceleration sensor according to the present embodiment as described above 
is received in a measuring and operating device 28 incorporated in the 
microcomputer. 
On the other hand, when the above described voltage signal V.sub.3 is 
outputted from the output terminal B, a timing signal having a frequency 
f.sub.2 outside the passband of the filter 23 is continuously applied to 
the input terminal A of the AC signal outputting device 26. The above 
described same operation is repeated. In this case, the frequency f.sub.2 
of the timing signal inputted is outside the passband of the filter 23, so 
that a voltage signal V.sub.4 attenuated by the filter 23 and amplified by 
the amplifier 24 is outputted from the output terminal B. The voltage 
signal V.sub.4 is also received in the measuring and operating device 28. 
Furthermore, in the above described measuring and operating device 28, the 
respective variations of the voltage signal V.sub.3 corresponding to the 
timing signal having a frequency f.sub.1 and the voltage signal V.sub.4 
corresponding to the timing signal having a frequency f.sub.2 are 
measured. The fault diagnosis of the acceleration sensor is made by 
comparing the measured values of V.sub.3 and V.sub.4 with normal values of 
V.sub.3 and V.sub.4 stored in the microcomputor. 
In the acceleration sensor according to the present embodiment, the 
piezoelectric element 21 and signal processor including the impedance 
converter 22, the filter 23 and the amplifier 24 are connected in series, 
as apparent from FIG. 4. Even when an abnormality occurs in any structure, 
therefore, either one of the voltage signals V.sub.3 and V.sub.4 varies. 
Consequently, a fault can be quickly detected. 
Meanwhile, it has been generally considered that cracking occurring in the 
piezoelectric element 21 itself is not easily detected. In the 
acceleration sensor according to the present embodiment, however, when the 
piezoelectric element 21 is cracked, the capacitance value Q.sub.1 thereof 
is changed, so that the voltage V.sub.2 applied to the impedance converter 
22 varies, as apparent from the foregoing equation (I). Consequently, the 
voltage signals V.sub.3 and V.sub.4 vary, thereby to make it possible to 
reliably detect the cracking of the piezoelectric element 1. 
As described in the foregoing, in the acceleration sensor according to the 
present embodiment, the fault diagnostic function is added to the 
acceleration sensor by arranging the capacitor 25 between the AC signal 
outputting device 26 for outputting the AC signal which is synchronized 
with the period of the timing signal externally inputted and the 
piezoelectric element 21. Accordingly, a fault diagnostic device need not 
be provided outside the acceleration sensor, unlike the conventional 
acceleration sensor. 
It will be pointed out that the construction of a third embodiment 
described hereafter is applicable as the specific construction of the 
above described first embodiment. 
FIGS. 6 and 7 are respectively a block diagram for explaining the schematic 
construction of an acceleration sensor according to a second embodiment of 
the present invention and an electric circuit diagram showing one example 
of the specific circuit arrangement of the acceleration sensor. 
Referring to FIG. 5, there is provided an acceleration detecting portion 32 
in the present embodiment. The acceleration detecting portion 32 includes 
a piezoelectric element 31 having electrodes 31a and 31b on both its major 
surfaces, as shown in FIG. 7. As the piezoelectric element 31, a 
piezoelectric element having a series type bimorph structure which is low 
in pyroelectric voltage is preferably used, as in the first embodiment. 
The piezoelectric element 31 is fixed on a supporting base 33, as 
schematically shown in FIG. 7. A pair of detecting portions 34a and 34b is 
formed spaced apart from each other by a predetermined distance on the 
upper surface of the supporting base 33, and the pair of detecting 
portions 34a and 34b is electrically connected to the electrode 31a of the 
piezoelectric element 31. One of the detecting portions 34a is 
electrically connected to signal outputting device 35 as described 
hereafter. The other detecting portion 34b is electrically connected to a 
reference potential. The specific construction of the acceleration 
detecting portion 32 will be described hereafter with reference to, for 
example, FIG. 8. 
As can be seen from FIG. 7, the other electrode 31b of the piezoelectric 
element 31 is electrically connected to a signal processor 36. That is, as 
shown in FIG. 6, the acceleration detecting portion 32 is electrically 
connected to the signal processor 36 so that an output of the acceleration 
detecting portion 32 is applied to the signal processor 36. The signal 
processor 36 includes a impedance converter means 37, a filter 38, and an 
amplifier 39. The impedance converter 37 is provided so as to convert the 
impedance of a voltage signal applied from the acceleration detecting 
portion 32, to an impedance that matches that of the other circuit 
elements. The filter 38 includes a band-pass filter and is provided so as 
to attenuate or remove an out-of-band signal component, and the amplifier 
39 is provided so as to amplify a voltage signal applied from the filter 
38. Although one example of the specific construction of the signal 
processor 36 is as shown in FIG. 7, it can be constructed in approximately 
the same manner as the impedance converter 22, the filter 23 and the 
amplifier 24 in the above described first embodiment. In FIG. 7, 
therefore, the same circuit elements as the circuit elements shown in FIG. 
5 are assigned the same reference numerals and hence, the description 
thereof is not repeated. 
The signal outputting device 35 is connected to a power supply voltage Vcc, 
and is constituted by resistors R.sub.11 and R.sub.12 connected in series 
to each other. A node 40 between the resistors R.sub.11 and R.sub.12 is 
electrically connected to the signal processor 56. 
The specific construction of the above described acceleration detecting 
portion 32 will be described with reference to FIG. 8. A supporting base 
33 is constituted by a pair of supporting members 33a and 33b arranged 
spaced apart from each other by a predetermined distance. Detecting 
portions 34a and 34b are formed on the respective supporting members 33a 
and 33b by applying electrode films. That is, the pair of detecting 
portions 34a and 34b is arranged spaced apart from each other by a 
predetermined distance. A piezoelectric element 31 is fixed on the 
detecting portions 34a and 34b with one of electrodes 31a on the bottom, 
and the electrode 31a is electrically connected to the detecting portions 
54a and 34b. The signal outputting device 35 is electrically connected to 
the detecting portion 34a, as shown in FIG. 8, and the detecting portion 
34b is connected to a reference potential. 
A fault diagnostic operation in the acceleration sensor according to the 
second embodiment will be described with reference to FIGS. 7 and 8. 
In the acceleration sensor according to the second embodiment as described 
above, the signal outputting device 35 is connected in series to a 
predetermined potential source, and includes a pair of resistors R.sub.11 
and R.sub.12 for dividing the power supply voltage Vcc. In addition, 
respective input terminals of the filter 38 and the amplifier 39 are 
electrically connected to the node 40 between the resistors R.sub.11 and 
R.sub.12. On the other hand, one of the above described detecting portions 
34a is electrically connected to the downstream side of the resistors 
R.sub.11 and R.sub.12. Consequently, a predetermined current I.sub.1 flows 
from the node 40 to the filter 38 and the amplifier 39, and a 
predetermined current I.sub.2 flows from the downstream side of the 
resistors R.sub.11 and R.sub.12 to the detecting portion 34a. 
The above described signal outputting device 35 may be constructed, as in a 
modified example shown in FIG. 11A. That is, the signal outputting device 
35 may be so constructed that a circuit having resistors R.sub.13 to 
R.sub.15 and a transistor TR.sub.3 is provided in the succeeding stage of 
the node 40 between the resistors R.sub.11 and R.sub.12, and a current 
I.sub.3 corresponding to the predetermined current I.sub.1 from the node 
40 is applied to respective input terminals of the filter 38 and the 
amplifier 39. 
Meanwhile, in the above described acceleration detecting portion 32, the 
detecting portions 34a and 34b are electrically connected to each other by 
the electrode 31a on the lower surface of the piezoelectric element 31, as 
apparent from FIG. 8. Consequently, a predetermined current I.sub.2 
applied from the above described signal outputting device 35 flows between 
the detecting portions 34a and 34b. That is, the current 12 flowing 
between the detecting portions 34a and 34b is always monitored by the 
signal processor 36. If the current I.sub.2 varies, the current I.sub.1 
flowing from the node 40 between the resistors R.sub.11 and R.sub.12 
varies in conformity with the variation of the current I.sub.2. 
Accordingly, in, for example, a case where the piezoelectric element S1 is 
cracked or the piezoelectric element 31 is stripped from the supporting 
base 33, the current I.sub.2 flowing between the detecting portions 34a 
and 34b varies, so that the current I.sub.2 is reduced or does not flow. 
As a result, the current I.sub.1 flowing from the node 40 between the 
resistors R.sub.11 and R.sub.12 varies. Accordingly, a state signal 
representing the state of the acceleration detecting portion 32, that is, 
the cracking or the stripping of the piezoelectric element 31 constituting 
the acceleration detecting portion 32 is outputted from the signal 
outputting device 35 to the filter 38 and the amplifier 39 constituting 
the signal processor means 36 in conformity with the variation in value of 
the above described current I.sub.2. Consequently, a signal outputted 
outward from the output terminal B of the acceleration sensor varies, 
thereby to make it possible to make the diagnosis of a fault in the 
acceleration sensor by the variation of the output signal in a control 
unit 41 connected to the output terminal B. 
Although in the construction shown in FIG. 8, the supporting base 33 
constituting the acceleration detecting portion 32 includes a pair of 
supporting members 33a and 33b, and the electrode films constituting the 
detecting portions 34a and 34b are applied to the upper surfaces of the 
supporting members 33a and 33b, the detecting portions 34a and 34b may be 
provided separately from the electrode films formed on the supporting 
members 33a and 33b. In addition, the detecting portions 34a and 34b need 
not be respectively provided between the piezoelectric element and the 
supporting members 33a and 33b. For example, the detecting portions 34a 
and 34b may be arranged spaced apart from each other by a predetermined 
distance in another portion of the piezoelectric element 
Furthermore, the construction of the acceleration detecting portion 32 is 
not limited to the construction shown in FIG. 8. For example, the signal 
processor 36 and the like in the acceleration sensor according to the 
present embodiment can be usually incorporated in a single electronic 
component, for example, a hybrid IC. Consequently, as shown in FIG. 9, the 
supporting base 33 may be replaced with a hybrid IC 42. In the 
construction shown in FIG. 9, a notch 42a is formed in the center of the 
hybrid IC 42, a pair of supporting portions 42b and 42c is formed on both 
sides thereof, and detecting portions 34a and 34b are formed on the upper 
surface of the pair of the supporting portions 42b and 42c by applying 
electrode films. A piezoelectric element 31 is bonded and fixed on the 
detecting portions 34a and 34b with conductive adhesives. Consequently, an 
electrode 31a formed on one major surface of the piezoelectric element 31 
is electrically connected to the electrode films constituting the 
detecting portions 34a and 34b. 
Furthermore, as shown in FIG. 10, a piezoelectric element 31 may be fixed 
on a supporting base 33 in a cantilevered arrangement using conductive 
adhesives. In the construction shown in FIG. 10, a pair of electrode films 
is formed spaced apart from each other by a predetermined distance on the 
upper surface of the supporting base 33, thereby to constitute detecting 
portions 34a and 34b. On the other hand, an electrode 31a on the lower 
surface of the piezoelectric element 31 is formed to have a substantially 
U shape by cutting away its central part as represented by a broken line 
in FIG. 10, and both ends of the electrode 31a having a substantially U 
shape are electrically connected to the by detecting portions 34a and 34b, 
respectively. 
Although in the foregoing description of the second embodiment, the filter 
38 and the amplifier 39 constituting the signal processor 36 are 
electrically connected to the node 40 between the resistors R.sub.11 and 
R.sub.12 constituting the signal outputting device 35, the present 
invention is not limited to the same. That is, as shown in FIG. 11B, a 
fault in the acceleration detecting portion 32 may be diagnosed by 
directly connecting the signal outputting device 35 and control device X 
to each other, applying a current from the above described node 40 to the 
control unit X, and comparing the voltage signal applied from the output 
terminal B of the acceleration sensor and the state signal applied from 
the signal outputting device 35 with each other in the control device X. 
As described in the foregoing, the acceleration sensor according to the 
second embodiment includes the signal outputting device for outputting to 
the signal processor the state signal representing the state of the 
acceleration detecting portion 32 including the piezoelectric element 31 
and the supporting base 33, and the pair of detecting portions arranged 
spaced apart from each other by a predetermined distance in the 
acceleration detecting portion 32. The signal outputting device is 
constructed to output the above described state signal according to the 
variation of the current flowing between the detecting portions 34a and 
34b. Accordingly, when there occurs a fault such as the cracking of the 
piezoelectric element constituting the acceleration detecting portion or 
the stripping of the piezoelectric element from the supporting base, the 
current flowing between the above described detecting portions 34a and 34b 
varies, so that the state signal corresponding to the above described 
variation of the current is outputted from the signal outputting device, 
thereby to make it possible to quickly detect the fault which occurred. 
Consequently, also in the second embodiment, it is possible to reliably 
detect a fault, which has been conventionally difficult to detect, such as 
the cracking of the piezoelectric element or the stripping of the 
piezoelectric element from the supporting base. 
FIGS. 12 and 13 are respectively a plan sectional view for explaining an 
acceleration sensor according to a third embodiment of the present 
invention and a schematic cross sectional view taken along a line 13--13 
shown in FIG. 12. The third embodiment is characterized by a supporting 
structure of a piezoelectric element. Both the circuit arrangements in the 
above described first embodiment and second embodiment are applicable to 
the circuit arrangement in the third embodiment, and the acceleration 
sensor can be specifically constructed also in the first embodiment and 
the second embodiment using the construction of the third embodiment. 
Referring to FIGS. 12 and 13, an acceleration sensor 51 includes a base 
plate 52 and a cap 53 secured to the upper surface of the base plate 52 at 
the peripheral edge, and the base plate 52 and the cap 53 constitute a 
package structure. In a housing space constituted by the base plate 52 and 
the cap 53, a hybrid IC 54 is fixed on the base plate 52 using adhesives 
(not shown). The hybrid IC 54 has a structure containing the signal 
processor, the AC signal outputting device 26 and/or signal outputting 
device 35, and the like in the first and second embodiments. That is, in 
the third embodiment, circuit portions such as the signal processor 
device, the signal outputting device 35 and/or the AC signal outputting 
device 26, and the like are constructed by using the hybrid IC 54 serving 
as a single electronic component. 
The upper surface of the hybrid IC 54 is made flat, and electrodes 54a to 
54f for connection to outer portions are formed on the upper surface. Each 
of the electrodes 54a to 54d is electrically connected to a lead terminal 
56 by a lead wire 55. The lead terminal 56 is passed through the base 
plate 52 and is extended downward from the base plate 52 in a state where 
it is electrically insulated from the base plate 52. 
On the other hand, the electrode 54e is electrically connected to an 
electrode 31b on the upper surface of a piezoelectric element 31 by a lead 
wire 57. In addition, the piezoelectric element 31 is directly secured to 
the upper surface of the hybrid IC 54 with conductive adhesives, and an 
electrode 31a on the lower surface of the piezoelectric element 31 is 
electrically connected to the electrode 54f with the above described 
conductive adhesives. 
The area of the bottom surface of the hybrid IC 54 is made larger than the 
area of the bottom surface of the supporting base 12 in the conventional 
acceleration sensor shown in FIG. 2. That is, the hybrid IC 54 used for 
this type of application usually has an area of base larger than that of 
the supporting base 12 shown in FIG. 2. Consequently, the hybrid IC 54 can 
be secured to the base plate 52 more firmly, as compared with the 
supporting base 12 shown in FIG. 2. Accordingly, in the acceleration 
sensor 51 according to the third embodiment, the hybrid IC 54 is more 
difficult to strip from the base plate 52, as compared with the supporting 
base 12 shown in FIG. 2, thereby to effectively prevent such a failure 
that the piezoelectric element and a member supporting the piezoelectric 
element are stripped from the base plate. 
Furthermore, in the construction shown in FIGS. 12 and 13, the 
piezoelectric element 31 is secured to the upper surface of the hybrid IC 
54 in a cantilevered arrangement. Consequently, when a large mechanical 
shock is applied, the hybrid IC 54 is not stripped from the base plate 52 
but the piezoelectric element 31 is stripped from the upper surface of the 
hybrid IC 54 before the stripping. As a result, the electrical connection 
between the electrode 31a on the lower surface of the piezoelectric 
element 31 and the electrode 54f on the hybrid IC 54 shown in FIG. 12 is 
released, thereby to break an electric circuit. Consequently, when such a 
fault occurs, the occurrence of the fault is immediately detected by the 
operation of a fault self-diagnostic circuit incorporated in the hybrid IC 
54. 
Although in FIGS. 12 and 13, the piezoelectric element 31 is secured to the 
flat upper surface of the hybrid IC 54 in a cantilevered arrangement, the 
piezoelectric element 31 may be secured to the upper surface of the hybrid 
IC in such arrangement that its both ends are supported thereon, as in the 
modified example shown in FIG. 9 of the second embodiment. 
Description is now made of an acceleration sensor according to a fourth 
embodiment of the present invention with reference to FIGS. 14 to 16. The 
acceleration sensor according to the fourth embodiment is characterized in 
that a temperature compensating capacitor is connected in parallel to a 
piezoelectric element. The constructions of the first to third embodiments 
are applicable to the other structures without any modification. In other 
words, the temperature compensating capacitor used in the fourth 
embodiment is used in the constructions of the first to third embodiments 
without any modification, thereby to make it possible to produce the 
function and effect described hereafter. 
In an acceleration sensor using a piezoelectric element, an output voltage 
of the piezoelectric element is affected by the ambient temperature. 
Accordingly, temperature compensation has been conventionally achieved 
using a voltage amplifier. More specifically, in the conventional 
acceleration sensor, a piezoelectric element 61 is fixed on a supporting 
base 62 in a cantilevered shape within a case 60 represented by a broken 
line, as shown in FIG. 14. In addition, an impedance converter 63 is 
connected to the piezoelectric element 61, and a voltage amplifier 64 
arranged outside the case 60 is connected to the impedance converter 63. 
In the voltage amplifier 64, the temperature compensation of the 
piezoelectric element 61 is achieved. 
However, the ambient temperature of the voltage amplifier 64 does not 
necessarily coincide with the ambient temperature of the piezoelectric 
element 61. Consequently, there arises the problem of making it impossible 
to achieve proper temperature compensation. On the other hand, the 
inventors of the present application have found that temperature 
compensation can be suitably achieved if a temperature compensating 
capacitor 72 is connected in parallel to a piezoelectric element 71 in 
close proximity to the piezoelectric element 71. In FIG. 15, reference 
numeral 73 denotes a supporting base. The supporting base 73 may be 
replaced with an electronic component such as the above described hybrid 
IC. In addition, reference numeral 74 denotes an impedance converter 
means, and reference numeral 75 denotes a voltage amplifier. The voltage 
amplifier 75 can be constructed similarly to the amplifier in the above 
described first and second embodiments. Consequently, the impedance 
converter 74 and the voltage amplifier 75 can be constructed as a single 
electronic component, for example, a hybrid IC within a case 70. 
Description is now made of the reason why temperature compensation can be 
suitably achieved when the temperature compensating capacitor 72 is 
connected in parallel to the piezoelectric element 71. 
Examination made by the inventors of the present application shows that the 
reason why an output voltage of the piezoelectric element is affected by 
the temperature is that the rate of variation of capacitance Q with 
temperature (.DELTA.Qs/Qs) and the rate of variation of piezoelectric 
stress constant d.sub.31 with temperature (.DELTA.d.sub.31 /d.sub.31) 
differ from each other. In this case, Qs indicates the capacitance of the 
piezoelectric element, and d.sub.31 indicates the piezoelectric stress 
constant of the piezoelectric element in the 31 direction. 
If the above described rate of variation of capacitance with temperature 
and the above described rate of variation of piezoelectric stress constant 
with temperature are equal to each other, the amount of change in output 
due to the change in piezoelectric stress constant is canceled by the 
amount of change in capacitance even if the temperature changes, thereby 
to make it possible to avoid the variation of the output voltage. 
More specifically, when the output voltage of the piezoelectric element is 
taken as V, the rate of variation of output voltage in a case where the 
output voltage V is changed by .DELTA.V due to the rise in unit 
temperature is .DELTA.V/V. "The output voltage V is not affected by the 
change in temperature" means that the above described rate of variation of 
output voltage becomes zero (.DELTA.V/V=0). 
If the rate of variation of output voltage (.DELTA.V/V) is found, 
therefore, the output voltage V of the piezoelectric element is 
proportional to a stress (.alpha.G) produced by acceleration G at that 
time and the piezoelectric stress constant d.sub.31 of the piezoelectric 
element, and is inversely proportional to the capacitance Qs of the 
piezoelectric element. That is, the following equation holds: 
EQU V=.alpha.G . d.sub.31 /Qs . . . (2) 
On the other hand, the amount of change in output voltage .DELTA.V is as 
follows: 
EQU .DELTA.V=(V+.DELTA.V)-V . . . (3) 
Accordingly, the following equation holds: 
EQU .DELTA.V=.alpha.G (d.sub.31 
+.DELTA.d.sub.31)/(Qs+.DELTA.Qs)-.alpha.Gd.sub.31 /Qs . . . (4) 
.DELTA.d.sub.31 in the foregoing equation (4) indicates the amount of 
change per unit temperature in piezoelectric stress constant. 
If the above described equations (2) and (4) are substituted in respective 
terms in the rate of variation of output voltage (.DELTA.V/V) to eliminate 
common terms, the following equation holds: 
EQU .DELTA.V/V =[1+(.DELTA.d.sub.31 /d.sub.31)]/[1+(.DELTA.Qs/Qs)]-1 . . . (5) 
In order that the rate of variation of output voltage becomes zero in the 
above described equation (5), that is, .DELTA.V/V=0, the following 
equation may hold: 
EQU .DELTA.Qs/Qs=.DELTA.d.sub.31 /d.sub.31 . . . (6) 
That is, it is found that the rate of variation of capacitance of the 
piezoelectric element and the rate of variation of piezoelectric stress 
constant thereof may be equal to each other. 
In the temperature characteristics of the piezoelectric element, however, 
the rate of variation of piezoelectric stress constant d.sub.31 
(.DELTA.d.sub.31 /d.sub.31) and the rate of variation of capacitance Qs 
(.DELTA.Qs/Qs) actually differ from each other, as shown in FIG. 16. 
Accordingly, an output voltage which is not affected by the change in 
temperature is not obtained. 
Therefore, the inventors of the present application has found that if the 
above described temperature compensating capacitor 72 is connected in 
parallel to the piezoelectric element 71 in close proximity to the 
piezoelectric element 71, temperature compensation can be reliably 
achieved by selecting the capacitance of the capacitor 72. 
More specifically, if the rate of variation of capacitance of the 
piezoelectric element (.DELTA.Qs/Qs) and the rate of variation of 
piezoelectric stress constant thereof (.DELTA.d.sub.31 /d.sub.31) are 
equal to each other as shown in the equation (6), the rate of variation of 
output voltage of the piezoelectric element becomes zero (.DELTA.V/V=0). 
In the fourth embodiment, however, if the capacitance Qt at the reference 
temperature of the capacitor 72 and the rate of variation of capacitance 
with temperature thereof (.DELTA.Qt/Qt) are suitably selected, the rate of 
variation of piezoelectric stress constant of a parallel circuit and the 
rate of variation of capacitance thereof are approximately equal to each 
other, thereby to obtain an output voltage which is not affected by the 
ambient temperature. 
In the above described capacitor 72, the required capacitance Qt can be 
easily calculated as follows. That is, if the condition under which the 
rate of variation of output voltage becomes zero is found by the foregoing 
equation (6), the following equation holds because the total capacitance 
of a parallel circuit of the piezoelectric element 71 and the capacitor 72 
is the sum of the capacitance of the piezoelectric element 71 and the 
capacitance of the capacitor 72: 
EQU (.DELTA.Qs+.DELTA.Qt)/(Qs+Qt)=.DELTA.d.sub.31 /d.sub.31 . . . (7) 
If the equation (7) is changed for simplification, the following equation 
is obtained: 
EQU Qt/Qs=(.DELTA.Qs/Qs-.DELTA.d.sub.31 /d.sub.31)/(.DELTA.d.sub.31 /d.sub.31 
-.DELTA.Qt/Qt) . . . (8) 
The capacitance Qt of the capacitor 72 and the rate of variation of 
capacitance thereof (.DELTA.Qt/Qt) are unknown out of the respective terms 
in the equation (8). However, the rate of variation of capacitance of the 
capacitor 72 is determined depending on which type of capacitor is the 
capacitor 72. On the other hand, the other terms are all known. 
Therefore, letting the capacitance Qs (at 20.degree. C.) of the 
piezoelectric element 71=900 pF, the rate of variation of capacitance 
thereof (.DELTA.Qs/Qs)=0.0036, and the rate of variation of piezoelectric 
stress constant thereof (.DELTA.d.sub.31 /d.sub.31) =0.0023, the required 
capacitance Qt of each of capacitors exhibiting the following 
characteristics UJ, RH and CG: 
(1) In the capacitor having the characteristic UJ, the rate of variation of 
capacitance (.DELTA.Qt/Qt) is set to -0.00075, so that the required 
capacitance Qt of the capacitor 72 becomes 383.6 pF. 
(2) In the capacitor having the characteristic RH, the rate of variation of 
capacitance (.DELTA.Qt/Qt) is set to -0.00022, so that the required 
capacitance of the capacitor 72 becomes 464.3 pF. 
(3) In the capacitor having the characteristics CG, the rate of variation 
of capacitance (.DELTA.Qt/Qt) is set to approximately zero, so that the 
required capacitance Qt of the capacitor 72 becomes 508.7 pF. 
As described above, the type of capacitor 72 used is suitably selected to 
find the value of the rate of variation of capacitance of the capacitor 
72, and this value and the other known values are substituted in the above 
described equation (8) to find the required capacitance Qt of the 
capacitor 72. 
Consequently, according to the fourth embodiment, the rate of variation of 
piezoelectric stress constant and the rate of variation of capacitance of 
the parallel circuit of the piezoelectric element and the capacitor can be 
made approximately equal to each other by only selecting the capacitance 
of the capacitor 72 connected in parallel to the piezoelectric element 71 
in the above described manner. Consequently, even if the ambient 
temperature changes, the amount of change in output due to the change in 
piezoelectric stress constant is canceled by the amount of change in 
capacitance. As a result, an output voltage which is not affected by the 
ambient temperature is obtained. Since the above described capacitor 72 is 
provided in close proximity to the piezoelectric element 71, and the 
capacitor 72 is always operated at approximately the same temperature as 
the piezoelectric element 71, therefore, it is possible to accurately 
achieve temperature compensation. 
Referring now to FIGS. 17 to 21, description is made of a fifth embodiment 
of the present invention. The fifth embodiment is characterized in that in 
an acceleration sensor, constructed so that an electronic component 
containing a signal processing circuit and the like is arranged on a base 
plate, a cushioning member for absorbing a thermal shock produced between 
the base plate and the above described electronic component due to the 
difference in coefficient of thermal expansion or modulus of elasticity is 
provided between the base plate and the electronic component. The 
cushioning member is also applicable to each of the acceleration sensors 
in the above described first to fourth embodiments. In the fifth 
embodiment, the thermal shock produced due to the difference in 
coefficient of thermal expansion or modulus of elasticity is absorbed by 
the above described cushioning member, so that an excessive thermal stress 
is not exerted on the electronic component for constructing a signal 
processor and the like. Consequently, the possibility of causing damages 
to the electronic component due to the thermal shock is prevented, thereby 
to enhance the reliability of the acceleration sensor. 
Furthermore, the thermal shock is absorbed by the above described 
cushioning member. Accordingly, even if a wiring pattern or a resistor 
film is formed on the bottom surface of the above described electronic 
component, it is hardly damaged. Accordingly, it is possible to also form 
a more relieable wiring pattern or a resistor film on the bottom surface 
of the electronic component, thereby to make it possible for the 
electronic component used to be made smaller in size and higher in 
integration density. 
Referring to FIG. 17, the acceleration sensor according to the present 
embodiment includes a piezoelectric element 81 outputting an electric 
signal corresponding to acceleration and a hybrid IC 82 serving as an 
electronic component containing various types of signal processing 
circuits for processing a signal outputted from the piezoelectric element 
81. The piezoelectric element 81 has a strip shape, and the hybrid IC 82 
has a rectangular plate shape. 
The piezoelectric element 81 is held in the center of the hybrid IC 82 in a 
cantilevered arrangement. That is, a through hole 83 for mounting the 
piezoelectric element is formed in the center of the hybrid IC 82. A fixed 
end 81a of the piezoelectric element 81 is secured to an edge of the above 
described through hole 83, and a free end 81b thereof is so arranged as to 
face the through hole 83. 
A tip-type electronic component 84 and a wiring pattern (not shown) are 
secured or formed on the upper surface of the hybrid IC 82. A component 
such as a resistor film or a wiring pattern, an electrode and the like are 
also formed on the bottom surface of the hybrid IC 82, which are not 
shown. The hybrid IC 82 and the piezoelectric element 81 are connected to 
each other through a bonding wire 85. In addition, the upper surface of 
the hybrid IC 82 is covered with a cover body 87 made of a metal, a 
conductive resin or a material obtained by metal-plating a synthetic 
resin. 
Meanwhile, if the cover body 87 and the wiring pattern (not shown) formed 
on the hybrid IC 82 are connected to each other by bonding of the cover 
body 87, it is possible to further enhance the reliability of the 
electrical connection. A plurality of connecting terminals for connection 
to outer portions 88 are attached to one end of the hybrid IC 82, and are 
projected sideward from the hybrid IC 82. 
The acceleration sensor according to the present embodiment includes a base 
plate 89 made of a metal on which the above described hybrid IC 82 is 
mounted and an insulating resin film 90. This insulating resin film 90 
serves as a cushioning member in the present invention. 
The base plate 89 includes a mounting stage 89a for mounting the hybrid IC 
82 in its central part, and the insulating resin film 90 is affixed on the 
mounting stage 89a with, for example, epoxy adhesives. 
A polymer film such as a polyimide film having a coefficient of thermal 
expansion of 8.times.10.sup.-6 to 17.times.10.sup.-6 /.degree.C. and 
having modulus of elasticity of 380 kg/mm.sup.2 and a polyethylene 
terephthalate film having a coefficient of thermal expansion of 
30.times.10.sup.-6 to 50.times.10.sup.-6 /.degree.C. and having modulus of 
elasticity of 400 kg/mm.sup.2 are suitable as the insulating resin film 
90, and approximately 10 .mu.m is sufficient and particularly, 
approximately 30 .mu.m is suitable for the thickness thereof. 
The hybrid IC 82 is affixed on the insulating resin film 90 affixed to the 
mounting stage 89a through, for example, epoxy adhesives. In order to 
firmly bond the base plate 89, the insulating resin film 90, and the 
hybrid IC 82, it is preferable that both surfaces of the insulating resin 
film 90 are previously subjected to sandblasting. 
The area above the hybrid IC 82 positioned and fixed on the base plate 89 
is covered with a cap 91. The cap 91 is made of a metal or a conductive 
resin, and has dimensions covering the mounting stage 89a of the base 
plate 89. In addition, parts on the side of the bottom surface and one 
side surface of the cap 91 are opened. The lower parts of the side 
surfaces of the cap 91 are secured to the periphery of the mounting stage 
89a so that the cap 91 is fixed on the base plate 89. Consequently, the 
hybrid IC 82 is covered with the cap 91 in a state where the connecting 
terminals for connection to outer portions 88 are projected sideward from 
the opened part of the cap 91. In addition, the cap 91 is filled with an 
insulating resin 92 such as silicone resin for the purpose of hermetical 
sealing. Since the piezoelectric element 81 is surrounded by the cover 
body 87, however, a space for vibration A of the piezoelectric element 81 
is ensured. 
Furthermore, the hybrid IC 82 contains various circuits as described in the 
first and second embodiments, for example, signal processing circuits such 
as an impedance converting circuit, a filter circuit, an amplifying 
circuit and the like. 
Also in the acceleration sensor according to the present embodiment, the 
hybrid IC 82 and the base plate 89 differ from each other in coefficient 
of thermal expansion and modulus of elasticity (the coefficient of thermal 
expansion of the hybrid IC 82 is 2.times.10.sup.-6 to 7.times.10.sup.-6 
/.degree.C. and the modulus of elasticity thereof is 1.times.10.sup.4 to 
3.times.10.sup.4 kg/mm.sup.2, while the coefficient of thermal expansion 
of the base plate made of metal is 10.times.10.sup.-6 to 
30.times.10.sup.-6 /.degree.C. and the modulus of elasticity thereof is 
0.1.times.10.sup.4 to 2.times.10.sup.4 kg/mm.sup.2), as in the 
conventional acceleration sensor. Consequently, when the temperature 
changes suddenly, a thermal shock can be produced between the hybrid IC 82 
and the base plate 89. However, the above described insulating resin film 
90 is interposed therebetween, which functions as a cushioning member to 
absorb the above described thermal shock. Consequently, damage caused by 
cracking or chipping and stripping in the hybrid IC 82 is presented. 
Furthermore, the piezoelectric element 81 is mounted on the hybrid IC 82. 
The hybrid IC 82 is reliably fixed on the base plate 89 without being 
stripped by the function of the insulating resin film 90. Consequently, a 
fault making it impossible to measure acceleration, that is, the stripping 
of the piezoelectric element 81, does not easily occur. 
Additionally, the thermal shock is absorbed by the insulating resin film 
90, so that there is no possibility of damaging the wiring pattern or the 
resistor film formed on the bottom surface of the hybrid IC 82. Similarly, 
the surface of the wiring pattern or the resistor film is physically 
protected by the insulating resin film 90. Moreover, the electrical 
insulation between the bottom surface of the hybrid IC 82 and the base 
plate 89 is ensured. Accordingly, it is possible to form a more reliable 
wiring pattern or a resistor film on the bottom surface of the hybrid IC 
82, thereby to make it possible to make the hybrid IC 82 smaller in size 
and higher in integration density. 
Although in the above described fifth embodiment, the insulating resin film 
90 is used as a cushioning member, the present invention is not limited to 
the same. For example, insulating adhesives used for bonding can be also 
used as the above described cushioning member. In this case, the 
insulating adhesives must be so applied as to have a thickness of at least 
approximately 10 .mu.m in order to obtain the cushioning effect. 
Furthermore, the connecting terminals for connection to outer portions 88 
are directly mounted on the hybrid IC 82, and are not mounted on the base 
plate 89. Consequently, it is possible to omit complicated work such as 
work of mounting the connecting terminals for connection to outer portions 
88 so as to pass through the base plate 89 or work of insulating the 
connecting terminals for connection to outer portions 88 and the base 
plate 89 from each other. 
Additionally, the above described bonding wire 86 need not be necessarily 
used so as to connect the piezoelectric element 81 and the hybrid IC 82 to 
each other. For example, the piezoelectric element 81 and the hybrid IC 82 
may be connected to each other using solder, conductive adhesives or the 
like. 
Furthermore, as the piezoelectric element 81, a piezoelectric element of a 
bimorph type, a piezoelectric element of a unimorph type, and a 
piezoelectric element of a shear mode type (shearing type) may be used. In 
the piezoelectric element 81 of a shear mode type, the piezoelectric 
element 81 and the hybrid IC 82 can be connected to each other without 
using the bonding wire 86 from a structural point of view. 
Although in the embodiment shown in FIG. 17, the piezoelectric element 81 
is fixed to the upper surface of the hybrid IC 82 in a cantilevered 
arrangement, the fifth embodiment is not limited to the same. That is, as 
shown in an exploded perspective view of FIG. 19, a piezoelectric element 
81 may be fixed to a flat upper surface of a hybrid IC 82 so as to support 
both ends of the piezoelectric element 81 on both sides by edges opposed 
to each other of a through hole 83 in the hybrid IC 82. In this case, if 
grooves for positioning the piezoelectric element 81 and mounting the same 
are provided at both the edges of the through hole 83, the piezoelectric 
element 81 is easily positioned, and the necessity of making connection by 
a bonding wire is eliminated by only altering wiring on the hybrid IC 82, 
thereby to make it possible to further enhance reliability and 
productivity. 
Furthermore, as shown in an exploded perspective view of FIG. 20, a base 
plate 89 and a cap 91 may be integrated, and grooves for positioning 92 
may be respectively formed on the inside of a pair of side surfaces 
opposed to each other so as to extend in the horizontal direction. If the 
width of the grooves 92 is made approximately equal to the thickness of 
the hybrid IC 82, it is possible to simply mount the hybrid IC 82. 
Moreover, it is possible to reliably mount the hybrid IC 82. In addition, 
the necessity of bonding the base plate 89 and the cap 91 to each other is 
eliminated, thereby to increase the strength of the entire package 
structure. A member for preventing silicone resin or the like with which 
the cap 91 is filled from flowing in is mounted on the reverse surface of 
a mounting hole of a piezoelectric element (that is, a mounting hole 
corresponding to the through hole 83 shown in FIG. 17), which is not shown 
in FIG. 20. In addition, the hybrid IC 82 can be mounted with it being 
turned over by changing the position of the grooves for positioning 92, 
which is not shown. 
Additionally, as shown in an exploded perspective view of FIG. 21, the 
above described grooves for positioning 92 may be replaced with inward 
projections for positioning 93 extending in the horizontal direction, and 
the lower surface of the projection 93 may be utilized as a surface for 
positioning in inserting the hybrid IC 82. In this case, an insulating 
plate 94 having a predetermined thickness may be affixed to the reverse 
surface of the hybrid IC 82 so as to obtain positioning on the lower side 
of the hybrid IC 82. That is, the thickness of the above described 
insulating plate 94 may be selected so that the thickness of the 
insulating plate 94 and the thickness of the hybrid IC 82c are 
approximately equal to the distance between the lower surface of the 
projection 93 and the bottom surface of the cap 91. 
Meanwhile, in the above described all embodiments, it is possible to use, 
as the piezoelectric element, a suitable piezoelectric element such as not 
only a piezoelectric element of a bimorph type but also a piezoelectric 
element of a unimorph type, a piezoelectric element of a shear mode type 
or the like. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.