Piezoelectric sensor for monitoring kinetic momentum

A piezoelectric kinetic momentum sensor employs a thin oscillable diaphragm member on which a thin plate form piezoelectric sensor element is mounted. The entire circumferential edge of the oscillable diaphragm member is supported by the sensor casing. To avoid fluctuation of sensitivity due to the exerting direction of kinetic energy, the diaphragm member and the piezoelectric sensor element are formed in a coaxial thin disc shaped configuration, in the preferred construction. In the further preferred construction, the sensor casing is composed of two seperable bottomed cylindrical components, each of whcih has its circumferential cylindrical wall section free edge mating with the other. The circumferential edge of the diaphragm member is sandwiched between the mating free edges of the cylindrical components to define a sealed and enclosed internal space within the sensor casing.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a piezoelectric sensor for 
monitoring kinetic momentum of movable mechanical constructions. More 
specifically, the invention relates to a piezoelectric-type momentum 
sensor suitable to use as an accelerometer, such as that employed for an 
automotive control system. Further the invention relates to a 
piezoelectric sensor specifically adapted for monitoring relative 
displacement between a vehicle body and a wheel axel in an automotive 
suspension system. 
2. Description of the Background Art 
The U.S. Pat. No. 4,696,489, issued on Sept. 27, 1987, to Takeshi FUJISHIRO 
et al, and assigned to the common assignee of the present invention 
discloses an automotive suspension control system in which an 
accelerometer is incorporated for monitoring vertical acceleration of the 
vehicle body. The monitored vertical acceleration of the vehicle body is 
utilized as one of the control parameters in controlling damping 
characteristics of the vehicular suspension system. In the construction 
shown in the aforementioned U.S. Patent, the accelerometer employs an 
inertial member causing deformation of a resiliently deformable member and 
a strain gauge for detecting deformation magnitude and speed and whereby 
detecting the vertical acceleration of the vibrating vehicle body. This 
type of accelerometer may be replaced with a piezoelectric-type 
accelerometer for performing the same or similar acceleration monitoring 
operation. 
One of the typical constructions of piezoelectric-type accelerometers which 
employ a piezoelectric element as a sensor element, has been disclosed in 
the Japanese Patent First (unexamined) Publication (Tokkai) Showa 
59-23223, for example. This conventional piezoelectric accelerometer 
employs a piezoelectric sensor element supported in a housing in 
cantilever fashion. In this construction, the piezoelectric sensor element 
may vibrate about the one supported end when vibration energy is exerted. 
This causes the concentration of distortion stress around the supported 
end and results in fatigue or uneven exhaustion. Therefore, such 
concentrated stress may make it difficult to stably maintain reasonable 
performance and may shorten the life of the accelerometer. 
In addition, when the supporting structure, supporting the, piezoelectric 
sensor element, is asymmetric in construction, the sensitivity of the 
sensor element tends to be fluctuate depending upon the exerting direction 
of the vibration energy. In order to avoid this, it has been required that 
the conventional cantilever type piezoelectric sensor element supporting 
structure be precisely symmetric in construction. This requires high 
accuracy machining, such as by of laser machining apparatus. Such high 
accuracy machining requires substantial cost. 
SUMMARY OF THE INVENTION 
Therefore, it is a principle object of the present invention to provide a 
piezoelectric-type kinetic momentum sensor which can solve the problem in 
the prior art set forth above. 
Another object of the present invention is to provide a piezoelectric 
electric acceleration sensor which does not require precisely symmetric 
supporting structure for the piezoelectric sensor element. 
A further object of the invention is to provide a piezoelectric 
accelerometer which does not have the deficiencies resulting from a 
cantilever fashion piezoelectric sensor element supporting structure. 
A still further object of the invention is to provide a piezoelectric 
accelerometer which is specifically adapted to be employed in an 
automotive suspension control system. 
A yet further object of the invention is to provide a piezoelectric 
accelerometer which is designed to detect a predetermined frequency range 
of acceleration. 
In order to accomplish the aforementioned and other objects, a 
piezoelectric kinetic momentum sensor, according to the present invention, 
employs a thin oscillable diaphragm member on which a thin plate form 
piezoelectric sensor element is mounted. The overall circumferential edge 
of the oscillable diaphragm member is supported on a sensor casing. 
To avoid fluctuation of sensitivity due to the exerting direction of 
kinetic energy, the diaphragm member and the piezoelectric sensor element 
are formed in a coaxial thin disc shaped configuration, in the preferred 
construction. In the further preferred construction, the sensor casing is 
composed of two separable bottomed cylindrical components, each of which 
has its circumferential cylindrical wall section free edge mating with the 
other. The circumferential edge of the diaphragm member is sandwiched 
between the mating free edges of the cylindrical components to define a 
sealed and enclosed internal space within the sensor casing. 
According to one aspect of the invention, a piezoelectric sensor for 
monitoring kinetic momentum of a movable member, comprises a sensor casing 
defining an enclosed internal space therein, a piezoelectric sensor 
assembly composed of a flexible plate member having at least one plane 
surface and a piezoelectric element fitted onto the plane surface of the 
flexible plate member, and a sensor support member secured onto the inner 
periphery of a circumferential wall of the sensor casing and engaging 
circumferential edge of the piezoelectric sensor assembly so as to 
oscillably support the latter within the enclosed internal space of the 
sensor casing. 
According to another aspect of the invention, a piezoelectric sensor for 
monitoring the kinetic momentum of a movable member, comprises a sensor 
casing made of insulating material and defining an enclosed internal o 
space of circular cross section, a piezoelectric sensor assembly composed 
of a disc-shaped flexible plate member having at least one plane surface 
and a disc-shaped piezoelectric element fitted onto the plane surface of 
the flexible plate member, and a sensor support member secured onto the 
inner periphery of a circumferential wall of the sensor casing and 
engaging the entire circumferential edge of the piezoelectric sensor 
assembly so as to oscillably support the latter within the enclosed 
internal space of the sensor casing. 
According to a further aspect of the invention, a piezoelectric, 
accelerometer for monitoring acceleration of a vibrating member, comprises 
a sensor casing including separate first and second components which have 
circular edge sections, in which the first and second components are made 
of insulating material and are assembled to each other with sealed secured 
mating edges, the sensor casing defining an enclosed internal space of 
circular cross section, a piezoelectric sensor assembly composed of a 
disc-shaped flexible plate member made of an electrically conductive 
material and having at least one plane surface and a disc-shaped 
piezoelectric element fitted onto the plane surface of the flexible plate 
member, and a sensor support member made of an elastic material and 
secured onto the inner periphery of a circumferential wall of the sensor 
casing and engaging the entire circumferential edge of the piezoelectric 
sensor assembly so as to oscillably support the latter within the enclosed 
internal space of the sensor casing. 
The sensor support member is made of an elastic material. The sensor 
support member is interposed between the first and second components and 
secured thereto for establishing an air-tight seal therebetween. The 
piezoelectric sensor assembly has its circumferential edge firmly engaging 
the sensor support member. 
On the other hand, the first and second components are formed into bottomed 
cylindrical configuration to define the enclosed internal space of a 
circular cross-section, and the flexible plate member and the 
piezoelectric elements are formed into disc-shaped configurations, the 
flexible plate member having a diameter essentially conforming or slightly 
greater than the internal diameter of the circular internal space of the 
sensor casing. The first and second components of the sensor casing 
defines a recess for receiving the sensor support member on the inner 
periphery. 
In the preferred construction, the disc-shaped piezoelectric element has a 
smaller diameter than that of the flexible plate member. 
The flexible plate member may be made of a material which has a thermal 
expansion rate approximately the same as the thermal expansion rate of the 
piezoelectric element. 
The piezoelectric sensor may further comprise means for equalizing pressure 
in chambers defined in the internal space of the sensor casing and 
separated by the piezoelectric sensor assembly. In the preferred 
construction, the pressure equalizing means comprises a through opening 
formed through the flexible plate member. 
The sensor casing also receives a sensor circuit assembly within the 
enclosed internal space adjacent the piezoelectric sensor assembly. The 
sensor circuit assembly is so located as to be close enough to the 
piezoelectric sensor assembly so that the piezoelectric element of the 
piezoelectric sensor assembly can be connected to the sensor circuit by 
means of a short lead wire. The sensor circuit assembly comprises a 
circuit board and a plurality of sensor circuit components including an 
amplifier element and a filter element and is buried by a potting material 
.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, particularly to FIGS. 1 through 3, the 
preferred embodiment of a piezoelectric kinetic momentum sensor assembly 
includes a oscillable piezoelectric sensor 1. The detailed construction of 
the piezoelectric sensor 1 will be discussed later. 
The piezoelectric sensor 1 is within a sensor casing which is composed of a 
lower casing component 2 and an upper casing component 3. Both the lower 
and upper casing components 2 and 3 are made of non-conductive synthetic 
resin, such as polyacetals or the like. The lower and upper casing 
components 2 and 3 have cylindrical circumferential wall sections 2a and 
3a mating at free end edges thereof. An annular and cross-sectionally 
square clearance is defined between the mating edges of the lower and 
upper casing components 2 and 3 to receive therein an annular sensor 
support member 4. The sensor support member 4 is made of an elastic 
material, such as polyurethane resin. The sensor support member 4 engages 
with the edge of the oscillable piezoelectric sensor, 1 for oscillably 
supporting the latter within the internal space of the sensor casing. 
The lower and upper casing components 2 and 3 are sealed and rigidly bonded 
or are welded in such a way that the enclosed internal space of the sensor 
casing is completely sealed. 
A circuit board 5 on which various electric and/or electronic circuit 
components 6, such as a filter, an amplifier and so forth, are mounted. 
The circuit board 5 lies substantially parallel to the bottom 2b of the 
lower casing component 2 and is rigidly secured thereonto by means of 
fastening screws 5a. The circuit board 5 and the circuit components 6 are 
buried in a potting resin layer 7 which is filled after rigidly fastening 
the circuit board 5 onto the bottom 2b of the lower casing component 2. 
The circuit board 5 is connected to an external circuit via a lead wire 8 
which extends through a grommet 8a which is used as a seal and which is 
engaged on the outer periphery of the circumferential wall section 2a of 
the lower casing component 2. 
The piezoelectric sensor 1 comprises a thin disc-shaped metal plate 10 
which has a circumferential edge portion rigidly engaged with the sensor 
support member 4. A pair of piezoelectric elements 11 and 12 have a thin 
disc-shaped configuration with a diameter smaller than the diameter of the 
metal plate 10. The piezoelectric elements 11 and 12 are arranged 
concentrically with the metal plate 10 and are bonded or adhered on both 
sides of the metal plate 10 by epoxy resin or similar adhesive. 
As clearly seen from FIGS. 2 and 3, since the piezoelectric elements 11 and 
12 are smaller in diameter than metal plate 10, the circumferential edges 
of the piezoelectric elements 11 and 12 are placed away from the inner 
circumferential edge of the sensor support member 4. As a consequence, an 
annular portion of the metal plate 10, adjacent to the circumferential 
edge, is exposed. 
The piezoelectric elements 11 and 12 are connected to the circuit board via 
lead wires 13 and 14 respectively. As is well known, since the 
piezoelectric elements 11 and 12 have a substantially high impedance, the 
length of the lead wires 13 and 14 will affect the kinetic momentum 
sensing ability by creating static electric capacities. This affect of the 
static electric capacities can be minimized by shortening length of the 
leads 13 and 14. In the shown embodiment, since the piezoelectric sensor 1 
and the circuit board 5 are housed in the single sensor casing, with a 
substantially short distance therebetween, shortening of the lead wires 13 
and 14 becomes possible. 
In the preferred construction, the pair of piezoelectric elements 11 and 12 
have mutually opposite output characteristics relative to variation of the 
temperature. Namely, since the piezoelectric elements 11 and 12 have 
temperature dependent variable output characteristics, the influence of 
variation of the output characteristics of the piezoelectric elements can 
be compensated by providing the opposite temperature dependent variation 
characteristics. 
On the other hand, in order to avoid the influence of thermal expansion of 
the metal plate 10 and the piezoelectric elements 11 and 12, materials for 
the metal plate and the piezoelectric element will be selected so that the 
thermal expansion rates are as close as possible. In practice, when P.Z.T. 
is selected as a material for the piezoelectric elements 11 and 12, the 
material of the metal plate 10 may be selected among Ni.Fe alloys which 
have thermal expansion rates close to that of the piezoelectric elements 
11 and 12. With this construction, the influence of the difference of the 
thermal expansion rates of the metal plate 10 and the piezoelectric 
elements 11 and 12 can be minimized. 
On the other hand, the lead wires 13 and 14 are made of tin plated mild 
copper and are formed by aggregating a plurality of thin copper lines into 
a wire. The lead wires 13 and 14 are covered by a polyulethane cover 
layer. The lead wires 13 and 14 are fixed to piezoelectric elements 11 and 
12, respectively by soldering. The joining portions between the lead wires 
13 and 14 and the piezoelectric elements 11 and 12 are coated by an 
elastic shock absorbing material 15 and 17, such as polyulethane. 
Furthermore, lead wires 13 and 14 are further secured to the annular bare 
portion of the metal plate 10 by elastic shock absorbing material 16 and 
18. Similar to the shock absorbing materials 15 and 17, the elastic shock 
absorbing materials 16 and 18 are made of polyulethane, for example. 
Between the mutually associated elastic shock absorbing materials 15, 16 
and 17, 18, extra length of the lead wires 13 and 14 are provided to 
provide sag. 
The sensor support member 4 is formed of from an elastic synthetic resin, 
such as polyulethane resin. This sensor support member 4 is formed as a 
pre-assembly with the piezoelectric sensor 1. Namely, the piezoelectric 
sensor 1 is set in a molding for forming the sensor support member 4. 
After setting the piezoelectric sensor 1 in the molding, the molten 
material of the sensor support member is filled into the molding. The 
pre-assembly of the sensor support member 4 and the piezoelectric sensor 1 
is removed from the molding after solidifying of the resin of the sensor 
support material. 
The pre-assembly consisting of the sensor support member 4 and the 
piezoelectric sensor 1 is assembled with the lower and upper casing 
components 2 and 3 of the sensor casing by placing the sensor support 
member 4 within the annular space defined between the lower and upper 
casing components. After assembly, the sensor support member 4 may 
elastically supports the piezoelectric sensor relative to the sensor 
casing. The sensor support member 4 also serves as, sealing means for 
establishing an air- and/or liquid tight seal. Therefore, vapor in the 
atmospheric air and so forth will never enter into the internal space of 
the sensor casing. Vapor or moisture may otherwise cause leakage of the 
sensor voltage. 
Therefore, in the shown construction, the 
overall circumferential edge of the generally disc-shaped piezoelectric 
sensor 1 is supported on the inner periphery of the sensor casing. This 
construction allows the piezoelectric sensor 1 to be oscillated in 
response to the kinetic energy exerted parallel to the axis of the sensor 
casing. Namely, the piezoelectric sensor 1 is not responsive to the 
kinetic energy laterally exerted in a direction substantially 
perpendicular to the axis of the sensor casing. On the other hand, when 
the kinetic energy is exerted in oblique relative to the axis of the 
sensor casing, the piezoelectric sensor 1 is responsive only to the axial 
component of the kinetic energy. 
When the axial kinetic energy is exerted on the piezoelectric sensor 1, the 
sensor undergoes deformation and creates output voltages from the 
piezoelectric elements 11 and 12. The output voltages of the piezoelectric 
elements vary depending upon the magnitude of the kinetic energy exerted. 
The output voltages of the piezoelectric elements 11 and 12 are amplified 
and filtered by the circuit on the circuit board and then outputted, 
through the lead wire 8, as the kinetic momentum indicative sensor signal. 
At this time, since the overall circumferential edge of the pre-assembly 
consisting of the piezoelectric sensor 1 and the sensor support member 4 
is sealed and secured onto the inner periphery of the circumferential wall 
of the sensor casing, the enclosed internal space of the sensor casing is 
separated into two chambers. When both chambers are not in communication 
with each other, a pressure difference may be created by deformation of 
the piezoelectric sensor 1. This pressure difference serves as resistance 
against deformation of the piezoelectric sensor 1 and lowers the voltage 
to be created. This clearly degrades the precision of measurement of the 
kinetic momentum. Furthermore, this separated chamber tends to cause 
deformation of the piezoelectric sensor 1 when a temperature difference 
between the two chambers occurs. Namely, when a temperature difference 
occurs, pressures in the chambers become different and cause deformation 
of the piezoelectric sensor toward the chamber of lower pressure In order 
to avoid this problem, one or more small openings 19 are formed through 
the annular bare section of the metal plate 10. This through openings 19 
establish communication between both chambers and thus serve for 
equalization of the pressures of the chambers. 
As set forth, since the sensor casing is sealed and enclosed in a 
liquid-tight and air-tight fashion, entry of dust, moisture, corrosive gas 
and so forth is completely prevented. Therefore, as set forth above, 
leakage of the output voltage due to the presence of moisture is perfectly 
prevented. Since the piezoelectric sensor 1 of the shown embodiment of the 
kinetic momentum sensor is provided precise directionality in kinetic 
energy or momentum sensibility, kinetic energy or momentum in the 
direction different from a axial direction will not serve as a noise 
creating factor. This assures high precision of measurement of the kinetic 
momentum in the predetermined direction. 
In addition, since no corrosive gas may enter into the internal space of 
the sensor casing, corrosion of the piezoelectric elements and/or the 
metal plate is successfully prevented. Also, since the bending stress to 
be exerted on the metal plate is evenly distributed over the overall 
circumferential edge, stress concentrations are avoided. Therefore, life 
of the piezoelectric sensor can be remarkably expanded long enough to 
employ in the automotive suspension control system, for example. 
FIGS. 4 and 5 show a modified construction of the piezoelectric kinetic 
momentum sensor which is specifically designed to be employed in the 
suspension control system of an automotive vehicle. This modification has 
a sensible frequency range of kinetic energy, i.e. vibration energy. 
In order to specify the sensible frequency range, mass weight members 21 
and 22 are fitted on the piezoelectric elements 11 and 12 of the 
piezoelectric sensor 1. These mass weight members 21 and 22 will serve a 
pre-load for the piezoelectric sensor. In addition, since the resonance 
frequency of the vibrating member is determined by the mass weight 
thereof, such mass weight members 21 and 22, which serve to increase the 
mass weight of the piezoelectric sensor, lower the resonance frequency of 
the piezoelectric sensor. 
As set forth above, since the resonance frequency range of the 
piezoelectric sensor is variable depending upon the mass weight thereof, 
the resonance frequency range can be adjusted by adjusting the weight of 
the mass weight members 21 and 22. 
By adjusting the resonance frequency range, the kinetic momentum sensor of 
the present invention may be applicable to monitoring relative 
displacement stroke and acceleration of the vehicle body and the wheel 
axle in an automotive suspension system. 
As will be appreciated herefrom, the present invention fulfills all of the 
objects and advantages sought therefor.