U.S. patent application number 09/746346 was filed with the patent office on 2001-08-23 for piezoelectric sensor and acceleration sensor.
This patent application is currently assigned to Murata Manufacturing Co., Ltd.. Invention is credited to Ogiura, Mitsugu, Tabota, Jun, Unami, Toshihiko.
Application Number | 20010015103 09/746346 |
Document ID | / |
Family ID | 27475009 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015103 |
Kind Code |
A1 |
Tabota, Jun ; et
al. |
August 23, 2001 |
Piezoelectric sensor and acceleration sensor
Abstract
A piezoelectric sensor has a piezoelectric element having a
specific axis of sensitivity and a package for the piezoelectric
element. The package has a rectangular parallelopiped configuration
with opposite longitudinal end surfaces having a height-to-width
ratio of about 1:1. External lead electrodes are formed to cover at
least the longitudinal end surfaces. A method and apparatus for
detecting if the sensor is disposed in the proper posture is also
disclosed.
Inventors: |
Tabota, Jun; (Toyama-ken,
JP) ; Ogiura, Mitsugu; (Toyama-shi, JP) ;
Unami, Toshihiko; (Toyama-ken, JP) |
Correspondence
Address: |
OSTROLENK FABER GERB & SOFFEN
1180 AVENUE OF THE AMERICAS
NEW YORK
NY
100368403
|
Assignee: |
Murata Manufacturing Co.,
Ltd.
|
Family ID: |
27475009 |
Appl. No.: |
09/746346 |
Filed: |
December 22, 2000 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09746346 |
Dec 22, 2000 |
|
|
|
09170845 |
Oct 13, 1998 |
|
|
|
09170845 |
Oct 13, 1998 |
|
|
|
08683231 |
Jul 18, 1996 |
|
|
|
6043588 |
|
|
|
|
Current U.S.
Class: |
73/514.16 ;
73/514.34; 73/649 |
Current CPC
Class: |
H01L 41/1132 20130101;
G01P 15/0922 20130101; G01N 2291/02827 20130101 |
Class at
Publication: |
73/514.16 ;
73/514.34; 73/649 |
International
Class: |
G01P 015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 18, 1995 |
JP |
7-181675 |
Jul 27, 1995 |
JP |
7-191588 |
Aug 29, 1995 |
JP |
7-220334 |
Claims
What is claimed is:
1. A piezoelectric sensor, comprising: a piezoelectric element
having a specific axis of sensitivity; and a package in which said
piezoelectric element is provided; wherein said package has a
rectangular parallelopiped configuration with end surfaces having a
height-to-width ratio approximating 1:1, and wherein external lead
electrodes are formed on at least said end surfaces.
2. A piezoelectric sensor, comprising: a piezoelectric element
having a specific axis of sensitivity; and a package in which said
piezoelectric element is provided; wherein said piezoelectric
element has a pair of strip-shaped piezoelectric ceramic plates
joined to each other, and said package includes a pair of clamping
members for clamping at least one longitudinal end of said
piezoelectric element from upper and lower sides thereof so a to
support said piezoelectric element, and a pair of side cover
members secured to both sides of said clamping members so as to
cover left and right side surfaces of said piezoelectric element;
and wherein said package has a substantially rectangular
parallelopiped configuration with end surfaces having a
height-to-width ratio approximating 1:1, and external lead
electrodes are formed on at least said end surfaces.
3. An acceleration sensor comprising: a bimorph element having an
axis of highest sensitivity extending in a direction which
substantially coincides with a line normal to the plane of a
circuit board; and a case assembly for fixing and supporting both
longitudinal ends of said bimorph element, said case assembly being
adapted to be mounted on said circuit board at both its
longitudinal ends which are in support of both said longitudinal
ends of said bimorph element; wherein said bimorph element
comprises a pair of piezoelectric ceramic plates each having a
signal electrode and an intermediate electrode formed on opposite
major surfaces thereof, said piezoelectric ceramic plates being
joined to each other face to face at the surfaces having said
intermediate electrodes such that said intermediate electrodes are
coupled to each other; each said piezoelectric ceramic plate having
sections in the longitudinal direction of said bimorph element,
there being three sections including a central section and an end
section at each end of the central section, the sections being
positioned such that, when said bimorph element is deformed in
response to deflection of said circuit board, a quantity of charges
is generated in said central section equal to a sum of the
quantities of charges generated in both said end sections, said
central section and both said end sections of each said
piezoelectric ceramic plate being polarized thicknesswise of said
piezoelectric ceramic plate in opposite directions of polarization,
the directions of polarization of said central section and both
said end sections of one of said piezoelectric ceramic plates being
opposite to those of the other of said piezoelectric ceramic
plates.
4. An electronic part comprising: a polyhedral body having opposing
end surfaces, electrodes being disposed on said opposing end
surfaces for outputting voltages of different polarities, said
electronic part further comprising: a conductive film formed on one
surface of said body orthogonal to said end surfaces having said
electrodes at a predetermined position closer to one of said end
surfaces than to the other, said conductive film having an area
large enough to be simultaneously contacted by a pair of probe
terminals for applying a voltage.
5. An electronic part comprising: a polyhedral body having opposing
end surfaces, electrodes being disposed on said opposing end
surfaces for outputting voltages of different polarities, wherein
at least one surface of said polyhedral body orthogonal to said end
surfaces is provided at portions adjacent to said end surfaces with
electrodes formed in continuation from said electrodes on said end
surfaces, one of said electrodes on said at least one surface being
extended to a region contactable by a pair of probe terminals for
applying a voltage, while another electrode on said at least one
surface is not extended to the region contactable by said pair of
probe terminals.
6. A method of examining posture of an electronic part, the
electronic part comprising a polyhedral body having opposing end
surfaces, electrodes being disposed on said opposing end surfaces
for outputting voltages of different polarities, said electronic
part further comprising: a conductive film formed on one surface of
said body orthogonal to said end surfaces having said electrodes at
a predetermined position closer to one of said end surfaces than to
the other, said conductive film having an area large enough to be
simultaneously contacted by a pair of probe terminals for applying
a voltage, the electronic part being required to be placed in a
predetermined posture in terms of up and down, left and right and
front and back directions, said method comprising: bringing a pair
of probe terminals into contact with a portion of an upwardly
directed surface of said electronic part disposed in an examination
position in an arbitrary posture, said portion being closer to one
of the end surfaces having electrodes than to the other; applying a
voltage between said probe terminals; and determining, based on the
presence or absence of electrical current between said probe
terminals, whether or not the electronic part has been placed in a
correct posture.
7. A method of examining posture of an electronic part, the
electronic part comprising a polyhedral body having opposing end
surfaces, electrodes being disposed on said opposing end surfaces
for outputting voltages of different polarities, wherein at least
one surface of said polyhedral body orthogonal to said end surfaces
is provided at portions adjacent to said end surfaces with
electrodes formed in continuation from said electrodes on said end
surfaces, one of said electrodes on said at least one surface being
extended to a region contactable by a pair of probe terminals for
applying a voltage, another electrode on said at least one surface
not being extended to the region contactable by said pair of probe
terminals, the electronic part being required to be placed in a
predetermined posture in terms of up and down, left and right and
front and back direction, the method comprising: bringing a pair of
probe terminals into contact with a portion of an upwardly directed
surface of said electronic part disposed in an examination position
in an arbitrary posture, said portion being closer to one of the
end surfaces having electrodes than to the other; applying a
voltage between said probe terminals; and determining, based on the
presence or absence of electrical current between said probe
terminals, whether or not the electronic part has been placed in a
correct posture.
8. An apparatus for examining posture of an electronic part, the
electronic part comprising a polyhedral body having opposing end
surfaces, electrodes being disposed on said opposing end surfaces
for outputting voltages of different polarities, said electronic
part further comprising: a conductive film formed on one surface of
said body orthogonal to said end surfaces having said electrodes at
a predetermined position closer to one of said end surfaces than to
the other, said conductive film having an area large enough to be
simultaneously contacted by a pair of probe terminals for applying
a voltage, the electronic part being required to be placed in a
predetermined posture in terms of up and down, left and right and
front and back directions, said apparatus comprising: a pair of
probe terminals adapted to be brought into contact with a portion
of an upwardly directed surface of said electronic part disposed in
an examination position in an arbitrary posture, said portion being
closer to one of the end surfaces having electrodes than to the
other; a detector applying a voltage between said probe terminals
for detecting presence or absence of electrical current between
said pair of probe terminals; and a circuit for determining, based
on the results of the detection by said detector, whether or not
the electronic part has been placed in a correct posture.
9. An apparatus for examining posture of an electronic part, the
electronic part comprising a polyhedral body having opposing end
surfaces, electrodes being disposed on said opposing end surfaces
for outputting voltages of different polarities, wherein at least
one surface of said polyhedral body orthogonal to said end surfaces
is provided at portions adjacent to said end surfaces with
electrodes formed in continuation from said electrodes on said end
surfaces, one of said electrodes on said at least one surface being
extended to a region contactable by a pair of probe terminals for
applying a voltage; another electrode on said at least one surface
not being extended to the region contactable by said pair of probe
terminals, the electronic part being required to be placed in a
predetermined posture in terms of up and down, left and right and
front and back directions, the apparatus comprising: a pair of
probe terminals adapted to be brought into contact with a portion
of an upwardly directed surface, said electronic part disposed in
an examination position in an arbitrary posture, said portion being
closer to one of the end surfaces having electrodes than to the
other; a detector applying a voltage between said probe terminals
for detecting presence or absence of electrical current between
said pair of probe terminals; and a circuit for determining, based
on the results of the detection by said detector means, whether the
electronic part has been placed in a correct posture.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a piezoelectric sensor for
sensing acceleration or vibration and, more particularly, to a
piezoelectric sensor to be mounted on a substrate such as a printed
circuit board.
[0003] 2. Description of the Related Art
[0004] An acceleration sensor which is a kind of piezoelectric
sensor is usually used by packaging it or mounting it on a mounting
substrate such as a circuit board which has a circuit for
processing acceleration signals generated by the sensor.
[0005] Mounting of an acceleration sensor on a mounting substrate
has caused two manor problems in the prior art.
[0006] The first problem is as follows. The design of an
acceleration sensor to be mounted on a substrate is made such that
the sensor senses acceleration acting in directions perpendicular
to the plane of the mounting substrate or acceleration acting
parallel to the plane of the mounting substrate, depending on
factors such as the use of the sensor and specifications of the
circuit board. It is therefore necessary that the acceleration
sensor be mounted on the circuit board so as to be sensitive to
acceleration acting in either one of the above-mentioned
directions.
[0007] Meanwhile, the acceleration sensor has a rectangular
parallelopiped configuration having six surfaces, and is adapted to
be jointed to the circuit board at a specific one of the six
surfaces or a surface opposite to this surface. This is because the
acceleration sensor has a flattened rectangular parallelopiped
configuration having a rectangular bottom and top surfaces and side
surfaces of a height which is small as compared with the two sides
of the rectangle comprising the top and bottom surfaces, so that
the sensor can be soldered to the circuit board while being stably
held on the circuit board. For instance, the height ranges from 0.5
to 0.7 times as large as the length of the shorter side of the
rectangle defining the top and bottom surfaces.
[0008] Consequently, it has been necessary to prepare two types of
acceleration sensors: an acceleration sensor of the type which is
capable of sensing acceleration acting perpendicularly to the plane
of the circuit board and an acceleration sensor of the type which
is capable of sensing acceleration acting parallel to the plane of
the circuit board. An acceleration sensor of either one of these
two types is selected for use. This not only raises the costs of
production of acceleration sensors but also requires costs for
storing and administrating these two types of acceleration sensors,
resulting in a rise in the costs of various products incorporating
such acceleration sensors.
[0009] The second problem is as follows: The circuit board on which
an acceleration sensor is packaged may be deflected for any reason,
before or after the packaging of the acceleration sensor.
Specifically, the deflection of the circuit board causes a
corresponding deformation of the acceleration sensor. Such a
deformation of the acceleration sensor due to an external force
adversely affects the piezoelectric member which senses
acceleration, thus hampering sensing of acceleration. This problem
is particularly serious when the acceleration sensor is intended to
sense acceleration acting perpendicularly to the plane of the
mounting substrate, because in such a case the direction of
acceleration to be sensed coincides with the direction of
deflection of the mounting substrate. Consequently, the
acceleration sensor may be influenced by the deflection of the
circuit board so as to erroneously produce an acceleration signal
even when there is no acceleration acting on the sensor.
SUMMARY OF THE INVENTION
[0010] Accordingly, an object of the present invention is to
provide a piezoelectric sensor which can reduce adverse effects due
to the deflection of the substrate and a piezoelectric sensor which
can be mounted on a circuit board selectively either in a direction
to sense acceleration perpendicular to the plane of the circuit
board or in a direction to sense acceleration parallel to the plane
of the circuit board, thereby obviating at least one of the two
major problems encountered with the prior art.
[0011] To this end, according to one aspect of the present
invention, there is provided a piezoelectric sensor comprising: a
piezoelectric element having a specific axis of sensitivity; and a
package in which the piezoelectric element is packaged; wherein the
package has a rectangular parallelopiped configuration with end
surfaces having a height-to-width ratio approximating 1:1, and
wherein external lead electrodes are formed on at least the end
surfaces.
[0012] According to a second aspect of the present invention, there
is provided an acceleration sensor comprising: a bimorph element
having an axis of highest sensitivity extending in a direction
which substantially coincides with a line normal to the plane of a
circuit board; and a case assembly for fixing and supporting both
longitudinal ends of the bimorph element, the case assembly being
adapted to be mounted on the circuit board at both its longitudinal
ends which support both longitudinal ends of the bimorph element;
wherein the bimorph element has a pair of piezoelectric ceramic
plates each having a signal electrode and an intermediate electrode
formed on the opposite major surfaces thereof, the piezoelectric
ceramic plates being joined to each other face to face at their
surfaces having the intermediate electrodes such that the
intermediate electrodes are coupled to each other; each of the
piezoelectric ceramic plates being sectioned in the longitudinal
direction of the bimorph element into three sections including a
central section and both end sections at border lines which are
positioned such that when the bimorph element is deformed in
response to deflection of the circuit board, the quantity of
charges generated in the central section equals the sum of the
quantities of the charges generated in both the end sections, the
central section and both end sections of each piezoelectric ceramic
plates being polarized thicknesswise of the piezoelectric ceramic
plate in opposite directions, the directions of polarization of the
central section and both end sections of one of the piezoelectric
ceramic plates being opposite to those of the other of the
piezoelectric ceramic plates.
[0013] According to a third aspect of the present invention, there
is provided an electronic part having a polyhedral body on opposing
end surfaces of which are formed electrodes for outputting voltages
of different polarities, the electronic part comprising: a
conductive film formed on one of the surfaces of said body
orthogonal to the end surfaces having the electrodes at a
predetermined position closer to one of the end surfaces than to
the other, the conductive film having an area large enough to be
simultaneously contacted by a pair of probe terminals for applying
a voltage.
[0014] According to a fourth aspect of the present invention, there
is provided a method of examining posture of an electronic part
which has to be placed in a predetermined posture in terms of up
and down, left and right and front and back directions, the method
comprising: bringing a pair of probe terminals into contact with a
potion of an upwardly directed surface of the electronic part
disposed in an examination position in an arbitrary posture, the
portion being closer to one of the end surfaces having electrodes
than to the other; applying a voltage between the probe terminals;
and determining, based on the presence or absence of electrical
current between the probe terminals, whether the electronic part as
the examination object has been placed in a correct posture.
[0015] According to a fifth aspect of the invention, there is
provided an apparatus for examining posture or an electronic part
and which has to be placed in a predetermined posture in terms of
up and down, left and right and front and back directions, the
apparatus comprising: a pair of probe terminals adapted to be
brought into contact with a potion of an upwardly directed surface
of the electronic part disposed in an examination position in an
arbitrary posture, the portion being closer to one of the end
surfaces having electrodes than to the other; detecting means for
applying a voltage between the probe terminals and for detecting
presence or absence of electrical current between the pair of probe
terminals; and determining means for determining, based on the
results of the detection by the detecting means, whether the
electronic part as the examination object has been placed in a
correct posture.
[0016] The piezoelectric sensor in accordance with the present
invention is designed to be stably seated on a printed circuit
board regardless of the posture of mounting thereof on the printed
circuit board. Therefore, a single piezoelectric sensor can provide
a variety of directions of axis sensitivity, by changing the
posture of the piezoelectric sensor mounted on the printed circuit
board, thus widening the adaptability of the piezoelectric sensor.
The present invention therefore achieves a remarkable reduction in
the costs of production and management of piezoelectric sensors
and, hence, the price of the same.
[0017] The acceleration sensor of the present invention offers an
advantage in that, even if the bimorph element is deformed due to
the influence of deflection of the circuit board on which the
acceleration sensor is mounted, charges generated as a result of
the deformation are canceled by each other, so that no signal
charges are derived from the sensor when no acceleration is acting
thereon, whereby the influence of deflection of the circuit board
is eliminated. However, when acceleration acts on the acceleration
sensor, charges are generated in the central section and both end
sections of each piezoelectric ceramic plate, based on the
relationship between the directions of polarization of these
sections and the tensile and compression stresses caused by the
deformation. Such charges are picked up as output signal voltage,
without being canceled, thus providing a high level of output
signal. According to the invention, therefore, it is possible to
suppress influence caused by deflection of the circuit board, while
achieving a higher degree of reliability. Moreover, a high level of
sensor output can be obtained when acceleration actually acts on
the sensor.
[0018] The electronic part in accordance with the present invention
enables, with simple electronic processing, confirmation of the
posture of the electronic part which is to be mounted on a printed
circuit board or to be loaded in a tape carrier correctly in a
predetermined posture, with a high degree of accuracy while
avoiding increase in production cost.
[0019] The examination method and apparatus of the present
invention enables the confirmation of the electronic part under the
condition described above. In particular, the examination apparatus
offers a remarkable reduction in the installation cost as compared
with the conventional examination system which relies upon image
processing techniques.
[0020] The above and other objects, features and advantages of the
present invention will become clear from the following description
of the preferred embodiments when the same is read in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a partly-sectioned perspective view of an
embodiment of the acceleration sensor in accordance with the
present invention;
[0022] FIG. 2 is an exploded perspective view of the acceleration
sensor as shown in FIG. 1;
[0023] FIG. 3 is a schematic illustration of a piezoelectric
element deformed under action of acceleration;
[0024] FIGS. 4(a) and 4(b) are illustrations of the postures in
which the acceleration sensor of FIG. 1 is mounted on a printed
circuit board;
[0025] FIG. 5 is an illustration of the acceleration sensor of FIG.
1 carried by a tape carrier;
[0026] FIGS. 6(a) to 6(d) are perspective views of a second
embodiment of the acceleration sensor in accordance with the
present invention, showing positioning of the sensor relative to
probe terminals;
[0027] FIG. 7 is a block diagram schematically showing the
construction of an examining device used in combination with the
acceleration sensor of the second embodiment;
[0028] FIG. 8 is a perspective view of a modification of the
acceleration sensor;
[0029] FIG. 9 is a perspective view of another modification of the
acceleration sensor;
[0030] FIG. 10 is a perspective view of still another modification
of the acceleration sensor;
[0031] FIG. 11 is an illustration of an acceleration sensor carried
by a tape carrier;
[0032] FIG. 12 is a schematic sectional perspective view of the
third embodiment of the acceleration sensor;
[0033] FIG. 13 is an illustration of a bimorph element deformed
under action of an acceleration; and
[0034] FIG. 14 is a schematic illustration of an acceleration
sensor deformed due to deflection of a circuit board.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0035] First Embodiment
[0036] A first embodiment of a piezoelectric sensor of the present
invention, serving as an acceleration sensor sensitive to
acceleration, will be described with specific reference to FIGS. 1
to 5. FIG. 1 is a partly-sectioned perspective view of an
embodiment of the acceleration sensor in accordance with the
present invention. FIG. 2 is an exploded perspective view of the
acceleration sensor as shown in FIG. 1. FIG. 3 is a schematic
illustration of a piezoelectric element deformed under action of
acceleration. FIGS. 4(a) and 4(b) are illustrations of the postures
in which the acceleration sensor is mounted on a printed circuit
board. FIG. 5 is an illustration a tape carrier carrying the
acceleration sensor.
[0037] Referring to these Figures, an embodiment of the
acceleration sensor 1 has a piezoelectric element 2 of a type known
as a "bimorph piezoelectric element", a package 3 and external
leads 4a, 4b. The acceleration sensor 1 has a sensitivity axis
which, as indicated by an arrow, extends in the direction of
thickness according to a definition given later of the
piezoelectric element 2.
[0038] The piezoelectric element 2 has a pair of strip-shaped
piezoelectric ceramic plates 5, 5 each having on its major surfaces
a signal pickup electrode 6 and an intermediate electrode 7, the
piezoelectric ceramic plates 5, 5 being jointed face to face at
their intermediate electrodes 7, 7. The direction of stacking of
the pair of piezoelectric ceramic plates 5, 5 is referred to as the
direction of thickness of the piezoelectric element 2. Each of the
piezoelectric ceramic plates 5, 5 is divided in the longitudinal
direction into three sections 5a, 5b and 5b at border lines L, L.
The arrangement is such that a tensile stress or a compression
stress acts on each section 5a, 5b, 5b in response to acceleration
G acting on the sensor. The central sections 5a and both end
sections 5b of both piezoelectric ceramic plates 5, 5 are polarized
in opposite directions along the thicknesses of the piezoelectric
ceramic plates 5, as indicated by arrows A, B and C, D. More
specifically, the central sections 5a, 5a are polarized towards
each other as indicated by the arrows A and C, whereas both end
sections 5b, 5b; 5b, 5b are polarized away from each other, as
indicated by the arrows B and D.
[0039] The package 3 has a pair of clamping members 8, 9 each
having a substantially square-bottomed U-like form when viewed from
one side thereof, for clamping each longitudinal end portion of the
piezoelectric element 2 from the upper and lower sides of the
piezoelectric element 2. The package 3 also has a pair of cover
members 10, 11 each having a substantially square-bottomed U-shaped
cross-section, the cover members 10, 11 being secured to the left
and right ends of the clamping members 8, 9 so as to cover the left
and right side faces of the piezoelectric element 2. The package 3
generally has a rectangular parallelopiped configuration, with
opposing end surfaces having a height-to-width ratio of 1:1, i.e.,
a square configuration. Representing the height and the width of
the end surface by L1 and L2 (FIG. 1), respectively, according to
the present invention, it is preferred that the ratio L1/L2 is set
to about 1.0, i.e., to meet the condition of L1=L2, although the
invention does not exclude such a height-to-width ratio L1/L2 of
from about 0.9 to 1.1.
[0040] In accordance with the present invention, the external lead
electrodes 4a, 4b are provided in the form of layers laid on the
longitudinal end surfaces of the package 3 including the
longitudinal end surfaces of the piezoelectric element 2. One of
the external lead electrodes 4a, 4b is connected to one of the
signal pickup electrodes 6, 6 formed on the piezoelectric ceramic
plates 5, 5, while the other of the external lead electrodes 4a, 4b
is connected to the other of the signal pickup electrodes 6, 6.
[0041] The operation of the described acceleration sensor 1 is as
follows. It is assumed that the central portion of the
piezoelectric element 2 is mainly deflected to be convex upward as
illustrated in FIG. 3, in response to an acceleration G. In such a
case, positive (+) charges are generated on the outer surface of
the central section 5a of the piezoelectric ceramic plate 5 which
is on the outer side as viewed in the direction of deflection of
the piezoelectric element 2, based on the relationship between the
direction A of polarization and the tensile stress Pt. Similarly,
positive charges are generated on the outer surfaces of both end
sections 5b, 5b of the same piezoelectric ceramic plate 5, due to
the relationship between the direction B of polarization and the
compression stress Pc.
[0042] The positive charges generated on the outer major surfaces
of the central section 5a and both end sections 5b, 5b of the
above-mentioned piezoelectric ceramic plate 5 are delivered from
the signal pickup electrode 6 to the associated external lead
electrode 4b while being summed with each other.
[0043] In the meantime, negative (-) charges are generated on the
outer surface of the central section 5a of the piezoelectric
ceramic plate 5 which is on the inner side as viewed in the
direction of deflection of the piezoelectric element 2, based on
the relationship between the direction C of polarization and the
Compression stress Pc. Similarly, negative charges are generated on
the outer surfaces of both end sections 5b, 5b of the same
piezoelectric ceramic plate 5, due to the relationship between the
direction D of polarization and the compression stress Pt. These
negative charges are transferred to external lead electrodes 4a
from the signal pickup electrode 6 associated with this
piezoelectric ceramic plate 5.
[0044] Although negative charges and positive charges are generated
on the inner surfaces of the piezoelectric ceramic plates 5, 5,
respectively, the negative charges and the positive charges cancel
each other via the intermediate electrodes 7, 7.
[0045] A description will now be given as to why tension and
compression are applied to the piezoelectric ceramic plates 5, 5 of
the piezoelectric element 2 in response to the action of the
acceleration G. When the whole acceleration sensor 1 is accelerated
by the acceleration G, such an acceleration directly acts on the
package 3 tending to move the package 3 in the direction of the
acceleration G. However, the piezoelectric element 2 is not
directly subjected to such an acceleration so that the
piezoelectric element 2 tends to remain in the state before the
application of the acceleration. Namely, inertia force is generated
in response to the acceleration G so as to act on the piezoelectric
element 2. The end sections 5b, 5b of both piezoelectric ceramic
plates 5, 5 tend to move together with the package 3 by being
pulled by the package 3, whereas the central sections 5a, 5a tend
to remain at the instant position, whereby the central portion of
the piezoelectric element 2 is mainly deformed to deflect in the
direction of action of the acceleration G, such as to be convex
upward in the illustrated case. As a consequence, tensile stress Pt
is generated in the central section 5a of the piezoelectric ceramic
plate 5 which is on the outer side of the deflection, i.e., the
upper piezoelectric ceramic plate 5, while compression stress Pc is
generated in each of the end sections 5b of the same piezoelectric
ceramic plate 5, as will be seen from FIG. 3. Conversely, the
piezoelectric ceramic sheet 5 which is on the inner side of the
deflection, i.e., the lower piezoelectric ceramic plate 5, is
stressed such that compression stress Pc appears in the central
section 5a, while tensile stress Pt appears in each of the end
sections 5b, 5b of this piezoelectric ceramic plate 5.
[0046] The acceleration sensor 1 which has been described is
mounted on a printed circuit board 12 such that one of the four
major surfaces is directed upward, depending on the direction of
sensitivity axis according to the design requirement, in a manner
as shown in FIG. 4(a) or a manner as shown in FIG. 4(b). Thus, the
direction of the sensitivity axis is determined by the posture of
the piezoelectric sensor 1 on the circuit board. Turning sideways
or other movement of the acceleration sensor 1 during soldering due
to lack of stability is avoided regardless of the posture of the
piezoelectric sensor. In FIGS. 4(a) and 4(b), numerals 1a and 1b
denote upper and lower surfaces, while numerals 1c and 1d represent
right and left side surfaces. When the piezoelectric sensor 1 is
mounted in a manner shown in FIG. 4(a), the axis of sensitivity
extends in the direction parallel to the printed circuit board 12
as indicated by the arrow, whereas, when the same is mounted in the
posture as shown in FIG. 4(b), the axis of sensitivity extends in
the direction perpendicular to the plane of the printed circuit
board 12.
[0047] A process for packaging the acceleration sensor 1 typically
employs a tape carrier 20. FIG. 5 shows such a tape carrier 20 by
way of example. The tape carrier 20 is adapted to hold a plurality
of independent acceleration sensors 1 of the type described above.
More specifically, the tape carrier 20 has an embossed tape 21
having substantially square recesses 23 approximating the
configuration of the acceleration sensor 1, at a predetermined
pitch along the length thereof, and an upper tape 22 which is
bonded to the upper side of the embossed tape 21 so as to close the
above-mentioned recesses 23. All the acceleration sensors 1 on the
same tape carrier 20 are disposed in the same posture in regard to
the axis of sensitivity as indicated by arrows, depending on the
direction of sensitivity axis to be obtained when these sensors 1
are mounted on circuit boards or the like. For instance, the
posture in which the acceleration sensors 1 are accommodated in the
recesses 23 of the tape carrier 20 is determined such that the left
side surface id of each acceleration sensor 1 is exposed through an
opening of the recess 23 in the tape carrier 20.
[0048] Preparation of a plurality of acceleration sensors on the
tape carrier 20 in the described manner facilitates packaging of
the sensors 1 on the printed circuit boards. Namely, the packaging
can be performed by a simple process having the steps of peeling
the upper tape 22 off the tape carrier 20 so as to expose one
surface of the acceleration sensor 1 in each recess 23, picking up
the acceleration sensor 1 by, for example, a vacuum sucker (not
shown) acting on the exposed surface of the acceleration sensor 1,
and placing the acceleration sensor 1 on the target position on the
printed circuit board.
[0049] The described embodiment is only illustrative and various
changes and modifications may be made thereto. For instance, the
piezoelectric element 2 may be a so-called uni-morph type element
which is cantilevered by the clamping members 8, 9 at its one end,
rather than being supported at both its longitudinal ends as in the
described embodiment. Furthermore, the piezoelectric element 2 may
be constructed to serve as a vibration sensor, although an
acceleration sensor has been specifically mentioned.
[0050] In the illustrated embodiment, each external lead electrode
4 is formed to cover not only each end surface of the package but
also the portion of the peripheral surface of the package near each
longitudinal end of the package 3. The external lead electrode 4,
however, may be formed to cover only each end surface of the
package 3.
[0051] Furthermore, the directions of polarizations of the sections
of the piezoelectric ceramic plates 5 may be determined such that
the directions A and C of polarizations of the central sections 5a,
5a are away from each other, while the directions B and D of
polarizations of the end sections 5a, 5a; 5a, 5a are towards each
other.
[0052] Second Embodiment
[0053] The acceleration sensor 1 described as the first embodiment
of the present invention has a square cross-section perpendicular
to the longitudinal axis. It is therefore impossible to
discriminate the direction of the sensitivity axis based on the
appearance. This poses a risk that the acceleration sensor 1 when
mounted on a printed circuit board, for example, is placed in a
wrong posture. Such a risk can be overcome when the acceleration
sensor 1 of the first embodiment is mounted on the circuit board in
accordance with a method which will be described hereinunder. With
this method, it is possible to mount the acceleration sensor 1 such
that the sensitivity axis is extended in the desired direction
without fail, so that the advantage of the acceleration sensor,
which resides in alternative posture of mounting, can be fully
enjoyed.
[0054] Although the acceleration sensor of the first embodiment is
specifically mentioned in the following description, it is to be
noted that the following method can be carried out by using other
types of electronic parts. FIGS. 6(a) to 6(d) are perspective views
of the acceleration sensor, while FIG. 7 is a block diagram
schematically showing the construction of an acceleration sensor
examining device.
[0055] Although not shown, the acceleration sensor 1 has a
piezoelectric element arranged in a manner like a bridge and
packaged in an insulating package which has a rectangular
parallelopiped configuration. The direction of the axis of
sensitivity varies as indicated by arrows depending on the posture
of the piezoelectric element in the sensor. When the acceleration
sensor is placed in the posture as shown in FIG. 6(a), the axis of
sensitivity extends in the horizontal direction.
[0056] External lead electrodes 32, 33 for delivering voltages of
opposite polarity, i.e., positive and negative voltages, are
provided in the form of layers laid on both longitudinal ends of
the acceleration sensor 1. More particularly, the external lead
electrode 32 is laid on the front end surface 31a of the
acceleration sensor 1 and the regions of the upper, lower, left and
right surfaces 31c to 31f adjacent to the front end surface 31a of
the sensor 1. Likewise, the external lead electrode 33 is laid on
the rear end surface 31b and the regions of the upper, lower, left
and right surfaces 31c to 31f adjacent to the rear end surface 31b
of the sensor 1. More specifically, the external lead electrode 32a
on the front end surface 31a delivers a voltage of positive
polarity, while the external lead electrode 33 on the rear end
surface 31b delivers a voltage of negative polarity. A dummy
electrode 34 formed of a web-like conductive film is provided on
the upper surface 31c among the surfaces 31c to 31f, at a
predetermined position closer to the positive external lead
electrode 32 than to the negative external lead electrode 33. The
dummy electrode 34 extends over the entire width of the upper
surface 31c and has a size large enough to allow a pair of probe
terminals 36 to contact therewith for the purpose of applying a
voltage which will be described later. The dummy electrode 34
serves as a sign indicating that the acceleration sensor 1 should
be mounted on, for example, a printed circuit board (not shown) in
such a posture that the surface in which the dummy electrode 34 is
present is directed upward, and the surface on which this dummy
electrode is provided is determined depending on factors such as
the direction of the sensitivity axis, polarities of the external
lead electrodes 32, 33, and so forth. The dummy electrode 34 may be
formed, for example, by firing with silver, application of a
conductive paste or by plating.
[0057] Whether the acceleration sensor 1 has been situated in
correct posture is examined before the sensor 1 is packaged on, for
example, a printed circuit board. The examination is conducted by
using an examination device 35 which will now be described with
reference to FIG. 7.
[0058] The examination device 35 has a pair of probe terminals 36
for applying a voltage, locating/actuating device 37 for locating
the probe terminals 36 both in vertical and horizontal directions,
voltage application circuit 38 for applying a predetermined voltage
to the probe terminal 36, detector 39 for detecting any electrical
current between the probe terminals 36, discriminator 40 for
discriminating whether or not the acceleration sensor 1 is in
correct posture, based on the results of detection performed by the
detector 39, display 41 for displaying the results of the
discrimination performed by the discriminator 40, and control
circuit 42 for controlling the operations of the described
components in order to execute the examination processing. The
detector 39, which is in this embodiment a current-sensitive
detector, may be of the type which senses resistance or
capacitance. When such a detector is used, the discriminator 40
performs the discrimination based on the results of comparison
between the detected value and a predetermined reference value.
[0059] Using the examination device 35 having the described
construction, an examination as to whether the acceleration sensor
1 is in correct posture or not is examined in accordance with the
following procedure.
[0060] The acceleration sensor 1 as the examination object is
placed in a predetermined posture in regard to up and down, left
and right and front and back directions. When the posture of the
acceleration sensor 1 is correct, the upper surface 31c, i.e., the
surface carrying the dummy electrode 34, is directed upward and,
when the sensor 1 in such a posture is viewed in top plan view, the
front end surface 31a is directed to the south and the rear end
surface 31b is directed to the north, with the left and right side
surfaces 31e and 31f respectively directed to the east and west.
Thus, the dummy electrode 34 is positioned closer to the south end
of the sensor than to the north end.
[0061] The probe electrodes 36 are brought into contact with the
acceleration sensor 1. Namely, the locating/actuating device 37 is
activated to move the probe electrodes 36 to an area where the
acceleration sensor 31 is disposed. More specifically, the probe
electrodes 36 are moved to and located at a position where the
dummy electrode 34 should exist when the acceleration sensor 31 is
disposed in the correct posture, and are further moved into contact
with the dummy electrode 34.
[0062] Then, a predetermined voltage is applied between the probe
electrodes 36 across the sensor 1 so as to detect any electrical
current flowing between the probe electrodes 36.
[0063] If the acceleration sensor 1 is in the correct posture as
shown in FIG. 6(a), the probe electrodes 36 safely contact with the
dummy electrode 34 so that an electrical current flows between the
probe electrodes 36 through the dummy electrode 34 so as to be
detected by the detector 39. In response to the output from the
detector 39, the discriminator 40 determines that the acceleration
sensor 1 is in the correct posture, and the control circuit 42
gives instruction to the display 41 to cause the latter to display
that the examined acceleration sensor 1 is in the correct
posture.
[0064] However, if the acceleration sensor 1 is in a wrong posture,
e.g., turned upside down, falls sideways or rotated such that the
front end surface is directed improperly, as shown in FIGS. 6(b) to
6(d), no electrical current flows between the probe electrodes 35
because these electrodes do not contact with the dummy electrode 34
on the acceleration sensor 1. When no electrical current is
detected by the detector 39, the discriminator 40 determines that
the acceleration sensor 1 is in a wrong posture, so that the
control circuit 42 gives instruction to cause the display 41 to
display that the examined acceleration sensor 1 is in the wrong
posture.
[0065] The operator monitors the display on the display 41 to check
whether each acceleration sensor is in the correct posture. When
the acceleration sensor has been placed in the correct posture, the
assembly process advances to the next step for soldering, otherwise
the process skips to a step in which an operation is performed to
correct the posture of the acceleration sensor 1. Although in the
described embodiment, display 41 is employed to display the results
of the examination, such a display operation is not essential.
Namely, one of the above-mentioned two steps may be automatically
selected in accordance with the output from the discriminator.
[0066] Thus, the examination device 35 is required only to perform
a two-step operation applying a voltage through the probe terminals
36 and discriminating presence or absence of electrical current
flowing between the probe terminals 36. Thus, the described
examination device 35 handles much less amount of data as compared
with known examination apparatus which rely on image processing
techniques, whereby the costs of the facility for the examination
are greatly reduced. In addition, examination is performed with a
higher degree of reliability than by manual inspection and makes it
possible to realize an inexpensive examination system incorporated
in an automatic production line.
[0067] The dummy electrode 34 on the acceleration sensor 1 can be
formed simultaneously with the formation of the external lead
electrode 32, 33, without requiring any additional step in the
production process, so that cost of production of the acceleration
sensor 1 is not substantially raised.
[0068] The described embodiments are only illustrative and various
changes and modifications may be made thereto.
[0069] Firstly, it is to be noted that the square cross-section of
the acceleration sensor is not essential and other cross-sectional
shapes also may be employed. For instance, acceleration sensors as
modifications which will now be described have a rectangular
cross-section perpendicular to the longitudinal axis.
[0070] In these modifications, the positive external lead electrode
32 and the negative external lead electrode 33 provided on the
acceleration sensor 1 are made to have different areas as shown in
FIGS. 8 to 10 so as to provide information as to the posture,
instead of the dummy electrode 34 used in the described
embodiments.
[0071] More specifically, FIG. 8 shows a modification in which the
portion of the positive external lead electrode 32 on the upper
surface 31c of the acceleration sensor 1 is extended to a region
contactable with the probe electrodes 36, whereas the size and area
of the portions of the external lead electrode 32 on the lower
surface 31d and left and right side surface 31e, 31f, as well as
the size and area of the negative external lead electrode 33 on all
the surfaces 31c, 31d, 31e and 31f, are so designed that these
portions are not extended to regions contactable with the probe
electrodes 36.
[0072] In the modification shown in FIG. 9, the portion of the
external lead electrode 32 on the upper surface 31c is extended to
the same length as that in the first embodiment, but the portions
of this electrode 32 on the lower surface 31d and left and right
side surfaces 31e, 31f, as well as the portions of the negative
external lead electrode 33 on all the surfaces 31c, 31d, 31e and
31f, are retracted so as not to be contacted by the probe terminals
36.
[0073] In the modification shown in FIG. 10, the portion of the
positive external lead electrode 32 on the upper surface 31c of the
acceleration sensor 1 has the same size and area as that in the
embodiment shown in FIG. 9, whereas the portions of the positive
and negative external lead electrodes 32, 33 on both side surfaces
31e, 31f are omitted. The portion of the positive external lead
electrode 32 on the lower surface 31d, as well as the portions of
the negative external lead electrodes 33 on the upper and lower
surfaces 31c, 31d, is formed to extend over only half the width of
the acceleration sensor 1 so as not to be simultaneously contacted
by two probe electrodes 36.
[0074] Each of the modifications shown in FIGS. 8 to 10 can be
examined by using the examination device 35 described before.
Examinations of the modifications shown in FIGS. 9 and 10, however,
require that the positions at which the probe terminals 36 contact
the acceleration sensor are shifted from that in the cases shown in
FIGS. 6(a)-6(d) and 8.
[0075] Although in the described embodiments the external lead
electrodes 32, 33 are formed to cover not only the longitudinal
front and rear end surfaces 31a, 31b but also portions of the
upper, lower, left and right surfaces 31c to 31f adjacent to the
respective end surfaces, the present invention does not exclude
such an arrangement that the external lead electrodes 32, 33 are
formed to cover only the front and rear end surfaces 31a, 31b.
[0076] In the embodiment described in connection with FIGS. 6(a) to
10, the examination device 35 is used for the purpose of examining
the posture of the acceleration sensor 1 in the course of mounting
of the sensor on a printed circuit board. The same examination
device 35 also can be used for the purpose of examining the posture
of the acceleration sensor 1 when the sensor 1 is loaded on a tape
carrier 50 as shown in FIG. 11 which is used, as explained before,
for the purpose of conveying a plurality of acceleration sensors
1.
[0077] The tape carrier 50 is adapted to hold a plurality of
independent acceleration sensors 1 of the type described above.
More specifically, the tape carrier 50 has an embossed tape 51
having substantially square recesses 53 approximating the
configuration of the acceleration sensor 1, at a predetermined
pitch along the length thereof, and an upper tape 52 which is
bonded to the upper side of the embossed tape 51 so as to close the
above-mentioned recesses 53. All the acceleration sensors 1 on the
same tape carrier 50 are disposed in the same posture in regard to
the axis of sensitivity, depending on the direction of sensitivity
axis to be obtained when these sensors 1 are mounted on a circuit
boards or the like. For instance, the posture in which the
acceleration sensors 1 are accommodated in the recesses 53 of the
tape carrier 50 is determined such that the upper surface 31c of
each acceleration sensor 1 is exposed through the opening of the
recess 23 in the tape carrier 50. Loading of the acceleration
sensors 1 in the predetermined posture on the tape carrier 50 can
be executed advantageously and effectively by using the examination
device 35 which is capable of ascertaining that the acceleration
sensors 1 have been placed in the correct posture.
[0078] Third Embodiment
[0079] A third embodiment of the acceleration sensor in accordance
with the present invention will now be described with reference
FIG. 12, which is a schematic perspective view of a third
embodiment, FIG. 13 which is a schematic illustration of
deformation of a bimorph element under acceleration, and FIG. 14
which is a schematic illustration of the acceleration sensor
deformed due to deflection of a circuit board.
[0080] The third embodiment of the acceleration sensor is adapted
to be packaged on the surface of a circuit board 7 after being
correctly located. The acceleration sensor has a bimorph element 61
having a highest sensitivity axis S extending in a direction
substantially normal to the surface of the circuit board, a pair of
end case members 72 which support both longitudinal end portions of
the bimorph element 61 by clamping these ends at the upper and
lower surfaces of the bimorph element 61, and a pair of side case
members 77 which is integrated with the end case members 72 while
sealing both longitudinal side surfaces of the bimorph element
61.
[0081] The bimorph element 61 has a pair of strip-like
piezoelectric ceramic plates 64, 64 each having a thin-film signal
electrode 62 and a thin-film intermediate electrode 63 formed on
the opposite major surfaces thereof, the piezoelectric ceramic
plates 64, 64 being joined together at their surfaces having the
intermediate electrodes 63. Each piezoelectric ceramic plate 64 is
divided into three sections along the length thereof: namely a
central section 64a and both end sections 64b, 64b at a pair of
border lines L, L. The positions of the border lines L, L are so
determined that, when the bimorph element 61 has been deformed due
to a deflection of the circuit board 73, the quantity of the
charges generated in the central section 64a equals to the sum of
the quantities of charges generated in both end sections 64b, 64b.
The central section 64a and both end sections 64b, 64b have been
polarized thicknesswise of the ceramic plate 64 in opposite
directions as indicated by arrows A and B. Similarly, the other
piezoelectric ceramic plate 64 also is sectioned into a central
section 64a and end sections 64b, 64b, and these sections are
polarized thicknesswise in the directions opposite to those of the
corresponding sections of the first-mentioned piezoelectric ceramic
plate 64, as indicated by arrows C and D, respectively. Thus, the
central sections 64a, 64a of both piezoelectric ceramic plates 64,
64 of the bimorph element 61 are polarized inward, i.e., towards
each other, as indicated by arrows A and C in FIG. 12, whereas both
end sections 64b, 64b; 64b, 64b of both piezoelectric ceramic
plates 64, 64 are polarized outward, i.e., away from each other, as
indicated by arrows B and D. One of the pair of signal electrodes
62 presented on the outer surfaces of the piezoelectric ceramic
plates 64, 64 is connected to an external lead terminal 78 which
covers one longitudinal end of the case assembly 72, 77, while the
other of the signal electrodes 62 is connected to an external lead
terminal 78 which covers the other end of the case assembly 72, 77.
The acceleration sensor thus constructed senses acceleration based
on the quantity of charges generated by the bimorph element 61.
[0082] Deformation of the bimorph element 61 under acceleration
mainly appears in the central section 64a as schematically shown in
FIG. 13. In each of the piezoelectric ceramic plates 64, 64 forming
the bimorph element 13, tensile stress is generated in the central
section 64a, while both end sections 64b, 64b sustain compression
stress Pc. However, since the central section 64a and both end
sections 64b, 64b of each piezoelectric ceramic plate 64 are
polarized in opposite directions, charges generated in the central
section 64a due to tensile stress Pt and charged generated in both
end sections 64b, 64b due to compression stress Pc are not canceled
by each other, so that the bimorph element 61 produces a large
quantity of charges, i.e., a high level of output signal.
[0083] More specifically, in the piezoelectric ceramic plate 64
which is on the outer side of the bimorph element 61 as viewed in
the direction of the deflection, i.e., the upper piezoelectric
ceramic plate 64, positive (+) charges are generated on the outer
major surface of the central section 64a based on the relationship
between the polarization direction A and the tensile stress Pt.
Positive charges are also generated on the outer major surfaces of
both end sections 64b, 64b of the same piezoelectric ceramic plate
64, based on the relationship between the polarization direction B
and the compression stress Pc. Consequently, the positive charges
generated on the outer major surface of the central section 64a and
the positive charges generated on the outer major surfaces of both
end sections 64b, 64b are transferred from the signal electrode 62
to the associated external lead electrode 78 while being summed
with each other.
[0084] In the meantime, negative (-) charges are generated in the
piezoelectric ceramic plate 64 which is on the inner side of the
acceleration sensor 1 as viewed in the direction of deflection,
i.e., in the lower piezoelectric ceramic plate 64. More
specifically, negative (-) charges are generated in the outer major
surface of the central section 64a of this piezoelectric ceramic
plate 64, due to the relationship between the polarization
direction C and the compression stress Pc. Negative charges are
also generated in the outer major surfaces of both end sections
64b, 64b of this piezoelectric ceramic plate 64, due to the
relationship between the polarization direction D and the tensile
stress Pt. The negative charges generated on the central section
64a and the negative charges generated on both end sections 64b,
64b are transferred from the signal electrode 62 to the associated
external lead terminal 68 while enhancing each other. Consequently,
large quantities of positive and negative charges are generated in
the bimorph element 61 so that a high level of sensor output is
derived from this acceleration sensor.
[0085] In each of the piezoelectric ceramic plates 64, 64 under
acceleration, charges of polarity opposite to that of the charges
produced on the outer major surface are generated on the inner
major surface, i.e., on the surface facing the other piezoelectric
ceramic plate 64. These charges on the inner major surfaces of both
piezoelectric ceramic plates 64, 64 are of opposite polarities and,
hence, cancel each other through the electrical connection between
the intermediate electrodes 63, 63 on these piezoelectric ceramic
plate 64, 64.
[0086] FIG. 14 schematically shows the state in which a circuit
board 13 on which the acceleration sensor is mounted has been
deflected or a circuit board 73 after the mounting of the
acceleration sensor is deflected, so as to cause a deformation of
the whole bimorph element 61. Note that the deflection of the
circuit board 73 deforms the whole bimorph element 61, while the
deformation of the bimorph element 61 under acceleration mainly
appears in the central section 64a as schematically shown in FIG.
13.
[0087] In such a case, tensile stresses Pt are generated in all the
sections 64a, 64b, 64b of the ceramic plate 64 which is on the
outer side of the bimorph element 61 as viewed in the direction of
the deflection, while compression stresses Pc are generated in all
the sections 64a, 64b, 64b of the piezoelectric ceramic plate 64
which is on the inner side of the bimorph element 61 as viewed in
the direction of the deflection. As a result, positive charges and
negative charges are generated on the outer surfaces of section 64a
and sections 64b, respectively, of the piezoelectric ceramic plate
64 which is on the outer side of the bimorph element 61 as viewed
in the direction of the deflection. Moreover, negative charges and
positive charges are generated on the outer surfaces of section 64a
and sections 64b, respectively, of the piezoelectric ceramic plate
64 which is on the inner side of the bimorph element 61 as viewed
in the direction of the deflection.
[0088] As is explained above, since the piezoelectric ceramic
plates 64, 64 are divided into the central sections 64a and the end
sections 64b, 64c so that the amount of the charges generated in
the central section 64a equals to the sum of the amount of the
charges generated in both end sections 64b, 64b, the charges
generated in response to the deformation of the bimorph element 61
cancel each other, so that no electrical signal is derived from the
acceleration sensor based on the deformation of the bimorph element
caused by deflection of the circuit board. Accordingly, in the case
where the bimorph element 61 deflected due to the deflected circuit
board is also subject to an acceleration, the bimorph element 61
can output signals only in response to the acceleration, and detect
the degree of acceleration correctly.
[0089] Although the invention has been described through its
specific form, it is to be noted that the described embodiment is
only illustrative and may be changed or modified within the scope
of the invention which is limited solely by the appended
claims.
* * * * *