NASA CASE NO.

PRINT FIG. _?J_c)_

NOEICE

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to

grant licenses to commercial concerns. Although NASA encourages nonexclusive

licensing to promote competition and achieve the widest possible utilization, NASA

will provide the necessary incentive to the licensee to achieve early practical

application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 212, Hampton, Virginia 23681-0001.

Serial No.: 08/319,142

10/04/94 LaRC

(NASA-Case-LAR-14240-I) VACUUM

HOLDING FIXTURE AND METHOD FOR

FABRICATING PIEZOELECTRIC POLYMER

ACOUSTIC SENSORS Patent Application

(NASA. Langley Research Center)

25 p

/

N95-20410

Unclas

G3/27 0039293

/ / /
NASA Case No. LAR 14240-1 

VACUUM HOLDING FIXTURE AND METHOD FOR 

FABRICATING PIEZOELECTRIC POLYMER ACOUSTIC SENSORS 

The present invention relates to a vacuum holding fixture and method for fabricating 

piezoelectric polymer acoustic sensors. 

As shown in FIGS. 2(A)-2(C), the vacuum holding fixture includes a top portion 38 and 

a base portion 40 which can be mated together to form a cavity. A first film 20 and a 

second film 22 are bonded together inside the cavity. A wire 50 is sandwiched between 

the first film 20 and the second film 22. The bonded first film 20 and second film 22 are 

removed from the vacuum holding fixture and form a sensor having an internal 

connection port created by wire 50. An additional embodiment is illustrated in FIGS. 

4(A)-6(B), in which an internal connection tab is sandwiched between the first film 20 

and the second film 22 to form a sensor having an internal connection tab instead of an 

internal connection port. The center conductor wire of a coaxial cable can be 

connected to the inner surfaces of the first film 20 and the second film 22 via the 

internal connection port or the internal connection tab. 

The vacuum holding fixture and method for forming the sensors provides several 

advantages. The films are held flat and a uniform bond line is obtained. The sensor 

also allows easy connection to the center conductor of a coaxial cable. 

Inventor: E. Thomas Hall, Jr. Timothy D. Bryant 

Address: 21 6 Wildwood Court 6994 Abbey Road 

Newport News. VA 23602 Gloucester. Virainia 23062 

SS#: SS#: 

Employer: NASA LaRC U.S. Army (NASA LaRC) 

Initial Evaluator: Wayne A. Wright 

Serial No.: 08/31 9,142 

Filed: 1 0/04/94 
LAR 14240-1 -1- PATENT APPLICATION

VACUUM HOLDING FIXTURE AND METHOD FOR

FABRICATING PIEZOELECTRIC POLYMER ACOUSTIC SENSORS

5

10

Ori,qin of the Invention

The invention described herein was made by employees of the United

States Goverment and may be manufactured and used by or for the

Government for governmental purposes without the payment of any royalties

thereon or therefor.

Back.qround of the Invention

Field of the Invention

15 The present invention relates to a vacuum holding fixture and method

for fabricating piezoelectric polymer acoustic sensors.

Description of the Related Art

20 A fetal monitoring system which monitors the status of a fetus inside

an expectant mother is disclosed in U.S. Patent No. 5,140,992. The fetal

monitoring system receives pressure pulses emitted from a fetus inside the

mother and filters out pressure pulses originating from sources other than the

fetus, such as pulses originating from the maternal heart. The fetal

25 monitoring system uses a belt which mounts several piezoelectric polymer

acoustic sensors. The belt is worn by an expectant mother to press the

sensors against the mother's body to allow the sensors to detect the status of

the fetus.

FIG. 1 illustrates a sensor as used by the fetal monitoring system

30 disclosed in U.S. Patent No. 5,140,992. The sensor is formed by two pieces
LAR 14240-1 -2- PATENT APPLICATION

of film (hereinafter a "first film 20" and a "second film 22") which are bonded

together by an adhesive 23. First film 20 and second film 22 are each a strip

of 0.004" thick polyvinylidene fluoride (PVF2)piezoelectric film. First film 20

and second film 22 each have an inner surface with a metal coating 24, 26

5 and an outer surface with a metal coating 28, 30. Each surface of first film 20

and second film 22 has a respective positive or negative dipole moment so

that each film has a positive dipole moment on one surface and a negative

dipole moment on the other surface. Such films are well-known in the art and

are fully described in U.S. Patent No. 5,140,992. An internal connection port

10 32 is formed between first film 20 and second film 22 and is used to connect

the sensor to the center conductor wire 34 of a coaxial cable 35. Center

conductor wire 34 is inserted into internal connection port 32 and is

electrically connected to the inner surface metal coatings 24, 26 of the

bonded first film 20 and second film 22. The outer conductor 36 of coaxial

15 cable 35 is electrically connected to the metal coatings 28, 30 on the outer

surfaces of first film 20 and second film 22.

Thus, a sensor is formed by bonding together two pieces of

piezoelectric polymer film, such as polyvinylidene fluoride (PVF2).

Conventional methods of bonding together two pieces of piezoelectric

20 polymer film include simply taping or pressing the two pieces of film together

with an adhesive. However, such methods do not maintain uniform thickness

of the adhesive between the films and do not maintain uniform "flatness" of

the film. Therefore, the resulting sensor does not have a uniform thickness.

Also, such conventional methods of bonding do not maintain a uniform

25 adhesive bond line between the films.

Summary of the Invention

Accordingly, it is an object of the present invention to provide an

30 apparatus and method for bonding two pieces of film so that a uniform
LAR 14240-1 -3- PATENT APPLICATION

thickness of the adhesive is obtained, the film is held flatly in place, and a

uniform adhesive bond line is obtained to produce a sensor of uniform

thickness.

It is a further object of the present invention to provide a method and

5 apparatus of bonding two pieces of film so that an internal connection port is

formed between the films to allow the center conductor of a coaxial cable

conductor to be connected to the inner surfaces of the films.

It is an additional object of the present invention to provide an

improved sensor formed by two pieces of film and having an internal

10 connection tab bonded between the two pieces of film, instead of having an

internal connection port.

It is a further object of the present invention to provide a method and

apparatus of forming the improved piezoelectric acoustic sensor.

Additional objects and advantages of the invention will be set forth in

15 part in the description which follows and, in part, will be obvious from the

description, or may be learned by practice of the invention.

The above objects of the present invention are achieved by providing

an apparatus which bonds a first film to a second film to form a sensor. The

apparatus comprises a base section which has a mating surface for holding

20 the first film; a top section which has a mating surface for holding the second

film, the top section and base section being mateable together to form a

cavity between the top section mating surface and the base section mating

surface with the first film and the second film pressed together inside the

cavity to bond the first film to the second film; and internal connection port

25 forming means for forming an internal connection port between the first film

and the second film when the base section and top section are mated

together with the first film and the second film pressed together inside the

cavity.

The above objects of the present invention are also achieved by

30 providing a method for forming a sensor by bonding a first film to a second
LAR 14240-1 -4- PATENT APPLICATION

film, the first film and second film each having an inner and an outer surface.

The method comprises the steps of (1) maintaining the first and second films

in flat positions with the inner surface of the first film adjacent to the inner

surface of the second film; (2) applying an adhesive to the inner surface of at

5 least one of the first and second films; (3) positioning an internal connection

port forming device between the inner surfaces of the first and second films;

(4) pressing the first and second films together to bond the inner surfaces of

the first and second films together, thereby bonding the first and second films

together; and (5) removing the internal port forming device from between the

10 inner surfaces of the first and second films to form an internal connection port

between the first and second films which allows an electrical connection to

the inner surfaces of the bonded first and second films.

Moreover, the above objects of the present invention are achieved by

providing an apparatus comprising a base section which has a mating

15 surface for holding the first film; a top section which has a mating surface for

holding the second film, the top section and base section being mateable

together to form a cavity between the top section mating surface and the

base section mating surface with the first film and the second film pressed

together inside the cavity to bond the first film to the second film; and internal

20 connection tab forming means for forming an internal connection tab between

the first film and the second film when the base section and top section are

mated together with the first film and the second film pressed together inside

the cavity.

In addition, the objects of the present invention are achieved by

25 providing a method for forming a sensor having a first film and a second film

bonded together with an internal connection tab therebetween, the first film

and second film each having an inner and an outer surface. The method

comprises the steps of (1) maintaining the first and second films in flat

positions with the inner surface of the first film adjacent to the inner surface of

30 the second film; (2) positioning the internal connection tab between the inner
LAR 14240-1 -5- PATENT APPLICATION

surfaces of the first and second films; (3) applying a conductive adhesive to

at least one of the internal connection tab, the inner surface of the first film

and the inner surface of the second film, the conductive adhesive being

applied to a location which will allow the inner surfaces of the first and

5 second films and the internal connection tab to be electrically connected via

the adhesive when the sensor is formed; (4) applying an insulative adhesive

to the inner surface of at least one film, the insulative adhesive being applied

to a location which does not cover the conductive adhesive; and (5) pressing

the first and second films together to bond the inner surfaces of the first and

10 second films together with the internal connection tab therebetween, thereby

bonding the first and second films and the internal connection tab together to

form the sensor.

In addition, the objects of the present invention are achieved by

providing a sensor comprising an internal connection tab; first and second

15 films, each film having an inner surface and an outer surface, the inner

surface of the first film being adjacent to the inner surface of the second film;

and an insulative adhesive which bonds together the first film, the second film

and the internal connection tab and a conductive adhesive which provides an

electrical connection between the inner surfaces of the first and second films

20 and the internal connection tab.

Brief Description of the Drawin.qs

These and other objects and advantages of the invention will become

25 apparent and more readily appreciated from the following description of the

preferred embodiments, taken in conjunction with the accompanying

drawings, of which:

FIG. 1 (prior art) is a diagram of a prior art sensor.

FIGS. 2(A), 2(B) and 2(C) are diagrams illustrating a vacuum holding

30 fixture according to a first embodiment of the present invention.
LAR 14240-1 -6- PATENT APPLICATION

FIGS. 3(A), 3(B) and 3(C) are diagrams illustrating a vacuum holding

fixture according to a second embodiment of the present invention.

FIGS. 4(A) and 4(B) are diagrams of a top section of a vacuum holding

fixture according to a third embodiment of the present invention.

FIGS. 5(A) and 5(B) are diagrams of a base section of a vacuum

holding fixture according to the third embodiment of the present invention.

FIGS. 6(A) and 6(B) are diagrams of a vacuum holding fixture

according to the third embodiment of the present invention.

10 Detailed Description of the Preferred Embodiments

Reference will now be made in detail to the present preferred

embodiments of the present invention, examples of which are illustrated in

the accompanying drawings, wherein like reference numerals refer to like

15 elements throughout.

FIGS. 2(A), 2(B) and 2(C) are diagrams illustrating a vacuum holding

fixture according to a first embodiment of the present invention. As illustrated

in FIG. 2(A), the vacuum holding fixture comprises a top section 38 having a

mating surface 39 and a base section 40 having a mating surface 41. Top

20 section 38 and base section 40 can be mated together and held in place by

wing-nut screws 42. Mating surfaces 39 and 41 are machined so that a

%"x1.5"x0.020" cavity is formed when top section 38 and base section 40 are

mated together. Vacuum inlet ports 43 are located in the center of the cavity

in top section 38 and the center of the cavity in base section 40. Clear,

25 flexible plastic tubing, such as PVC lines, serve as vacuum discharge lines

44. Vacuum discharge lines 44 are connected to vacuum discharge tubes 46

in top section 38 and base section 40 for applying a vacuum to the vacuum

holding fixture. Vacuum inlet ports 43 extend through top section 38 and

base section 40 to a side of top section 38 and base section 40, respectively,

30 and are connected to vacuum discharge lines 44. A Y-jumper (not illustrated)
LAR 14240-1 -7- PATENT APPLICATION

connects vacuum discharge lines 44 to a single line (not illustrated). The

single line is connected to a vacuum gauge (not illustrated) and vacuum

system (not illustrated). A vacuum gauge that measures to 30" Hg is suitable

for the bonding process. A 0.012" diameter wire 50 extends 1⁄4" into the

5 cavity. A 1⁄4" wide by 1/8"deep channel 52 is machined in base section 40 on

each side of the cavity to provide a place for excessive adhesive to squeeze

out.

As illustrated in FIG. 2(B), first film 20 is placed on mating surface 39

of top section 38. Wire 50 extends from top section 38 and first film 20 is

10 placed underneath wire 50. Second film 22 is placed on mating surface 41 of

base section 40. First film 20 and second film 22 are each, preferably,

%"x11⁄4" rectangular shaped and .004" thick. First film 20 and second film 22

are oriented so that the positive or negative charged dipole moments face

each other (positive-positive or negative-negative). Vacuum is applied to the

15 vacuum holding fixture to hold first film 20 and second film 22 flatly in place

through the remainder of the bonding operation. Fast setting insulative epoxy

adhesive is applied to the inner surface of each film and should cover the

entire surface of each film.

As illustrated in FIG. 2(C), top section 38 is then mated to base section

20 40. Top section 38 and base section 40 are clamped together with wing-nut

screws 42. Preferably, wing-nut screws 42 are used to facilitate ease of

clamping. Excessive adhesive will squeeze out into channels 52. When top

section 38 and base section 40 are mated together with first film 20 and

second film 22 therebetween, wire 50 is sandwiched between first film 20 and

25 second film 22. The purpose of wire 50 is to form internal connection port 32

(see FIG. 1 ) for use when first film 20 and second film 22 are removed from

the vacuum holding fixture. Thus, one end of wire 50 is sandwiched between

first film 20 and second film 22 while top section 38 and base section 40 are

mated together, and the other end of wire 50 is connected to top portion 38.

30 After approximately ten minutes, the epoxy is cured enough to
LAR 14240-1 -8- PATENT APPLICATION

disassemble the vacuum holding fixture. Wing-nut screws 42 are removed

and top section 38 should be gently tapped with a soft-faced hammer until top

section 38 slides off base section 40. Top section 38 and base section 40

should be gently separated. First film 20 and second film 22 are now bonded

5 together and should slide off wire 50. First film 20 and second film 22,

bonded together, form a sensor. Any excess adhesive should be removed

from the sensor edges. The fixture can be cleaned and will then be ready for

bonding of another sensor. A release agent is preferably sprayed on mating

surfaces 39 and 41 before top section 38 and base section 40 are mated

10 together. The release agent acts to ease the separation of top section 38

from base section 40 and also eases removal of first film 20 and second film

22 from top section 38 and base section 40.

Internal connection port 32 (see FIG. 1) is formed by removing the

sensor from the vacuum holding fixture and sliding the sensor off wire 50. As

15 previously discussed and illustrated in FIG. 1, internal connection port 32 is a

cavity which allows center conductor wire 34 of coaxial cable 35 to be

connected to inner surfaces metal coatings 24, 26 of the bonded first film 20

and second film 22. Thus, a sensor having an internal connection port 32 for

connection to a coaxial cable is formed by this embodiment of the present

20 invention, wherein two pieces of piezoelectric film are bonded together with a

fast-setting epoxy adhesive.

Thus, wire 50 acts as an internal connection port forming mechanism.

However, such an internal connection port forming mechanism does not have

to be a wire. It could be a rod or any other type of device which forms a

25 suitable cavity when first film 20 and second film 22 are pressed together.

In summary, the vacuum holding fixture holds two pieces of film in

place. Adhesive is applied to the films. The fixture is clamped together and

the adhesive cures. This embodiment of the present invention maintains a

uniform adhesive bond line between the two pieces of film, provides an

30 internal connection port between the inner surface metal coatings of each film
LAR 14240-1 -9- PATENT APPLICATION

and eliminates excessive adhesive clean-up. The vacuum holding fixture

according to the embodiments of the present invention also maintains

uniform thickness of the adhesive between the films and maintains uniform

"flatness" of the films resulting in uniform thickness of the sensor.

5 FIGS. 3(A), 3(B) and 3(C) are diagrams illustrating a vacuum holding

fixture according to a second embodiment of the present invention. This

embodiment is similar to the embodiment illustrated in FIGS. 2(A), 2(B) and

2(C) except that the second embodiment forms hexagonal shaped sensors

from hexagonal shaped pieces of film, as opposed to the rectangular shaped

10 sensors formed from rectangular shaped pieces of film as illustrated in FIG.

2(B). In FIGS. 3(A) and 3(B), the mating surfaces 39, 41 of top section 38

and base section 40 are hexagonal shaped to form a hexagonal shaped

cavity when top section 38 and base section 40 are mated together. This can

be compared to the rectangular shaped mating surfaces 39, 41 illustrated in

15 FIGS. 2(A) and 2(B). In other respects, the embodiment illustrated in FIGS.

3(A), 3(B) and 3(C) is similar to the embodiment illustrated in FIGS. 2(A), 2(B)

and 2(C). The formation of hexagon shaped sensors allows a plurality of

sensors to be positioned very close to each other on the mother's body so

that the sensors are all positioned within the same, small diameter circle. For

20 example, seven hexagonal shaped sensors can be arranged in a smaller

diameter circle as compared to seven rectangular shaped sensors having the

same surface area as the hexagon shaped sensors. Thus, by using several

sensors on the mother's body, as opposed to a single sensor, a fetal

monitoring system produces more accurate results.

25 The sensors formed by the first and second embodiments of the

vacuum holding fixture have an internal connection port 32 for connecting the

respective sensor to the center conductor of a coaxial cable. However, FIGS.

4(A), 4(B), 5(A), 5(B), 6(A) and 6(B) illustrate a third embodiment of a vacuum

holding fixture which forms an improved sensor. The improved sensor has an

30 internal connection tab 56 (see FIG. 6(A)) formed between first film 20 and
LAR 14240-1 -10- PATENT APPLICATION

second film 22, instead of having internal connection port 32 formed between

first film 20 and second film 22. Internal connection tab 56 acts as a

conductor for connecting to the inner surfaces of first film 20 and second film

22. Thus, there is no need to perform extra steps of attaching the center

5 conductor of a coaxial cable to the sensor through internal connection port

32. Also, a much stronger electrical connection to the inner surfaces of the

films is formed by the improved sensor, as compared to the sensor illustrated

in FIG. 1.

FIGS. 4(A) and 4(B) illustrate top section 38 of the vacuum holding

10 fixture according to the third embodiment of the present invention and FIGS.

5(A) and 5(B) illustrate base section 40 of the vacuum holding fixture

according to the third embodiment of the present invention. Top section 38

and base section 40 of the third embodiment of the present invention are

similar to the top section and the base section of the second embodiment of

15 the present invention, except that the third embodiment does not include a

wire for forming an internal connection port. The wire is not necessary since

an internal connection port is not formed in the third embodiment of the

present invention and, instead, an internal connection tab 56 is bonded

between first film 20 and second film 22. Also, channels 52 are illustrated as

20 formed in top section 38. However, channels 52 can be formed in either top

section 38 or base section 40. Preferably, internal connection tab 56 is .003"

X 0.0625" X 1" and is made of a copper coated "KAPTON" brand film.

"KAPTON" brand film is well-known and widely available. Copper coated

"KAPTON" brand film is preferred since the film does not melt when heat is

25 applied for soldering connections. Also, a copper coating is the preferred

coating. However, other brands of film can be used and other types of

solderable coatings can be used. It is significantly easier make the electrical

connections to internal connection tab 56 than to connect the center

conductor of the coaxial cable to the inner surfaces of first film 20 and

30 second film 22 through the internal connection port 32 of the sensor
LAR 14240-1 -11- PATENT APPLICATION

illustrated in FIG. 1.

According to the third embodiment of the present invention, first film 20

is hexagonal shaped film and is placed on top section 38. Second film 22 is

also hexagonal shaped and is placed on base section 40. Vacuum is applied

5 to top section 38 and base section 40 as in the previously described

embodiments to hold first film 20 tightly and flatly against top section 38 and

to hold second film 22 tightly and flatly against base section 40. Internal

connection tab 56 is then placed on base section 40 so that internal

connection tab 56 extends, preferably, about half the length of second film

10 22. A small amount of conductive epoxy is then placed in the inner surface of

second film 22 so that the epoxy contacts internal connection tab 56 and

second film 22 and so that the epoxy will also contact first film 20 when first

film 20 and second film 22 are bonded together. A preferable conductive

epoxy is Chomerics 548 Part B and Hardener 29, which is widely known and

15 available. Before first film 20 and second film 22 are bonded together, an

insulative epoxy is applied to the inner surface of second film 22 so that no

gaps will be present between first film 20 and second film 22 when first film

20 and second film 22 are bonded together. The insulative epoxy should not

cover the conductive epoxy or else first film 20, second film 22 and internal

20 conduction tab 56 will not be electrically connected together when the sensor

is formed. After the conductive epoxy and the insulative epoxy are applied,

top section 38 and base section 40 are mated together as illustrated in FIGS.

6(A) and 6(B) and wing-nut screws 42 are inserted into top section 38 and

base section 40. Wing-nut screws 42 should be "finger tight" to facilitate

25 ease of clamping. Excess insulative epoxy will flow into channels 52. After

approximately ten minutes, the conductive epoxy and insulative epoxy should

be cured enough to open the fixture. The vacuum should then be released

and top section 38 and base section 40 should be separated. First film 20

and second film 22, bonded together with internal connection tab 56

30 therebetween, form one sensor.
LAR 14240-1 -12- PATENTAPPLICATION

In summary, according to the third embodiment of the present

invention, first film 20 and second film 22 are bonded together with internal

connection tab 56 formed therebetween. A small amount of conductive epoxy

is used to maintain electrical contact between the metal coatings on the inner

5 surfaces of first film 20, second film 22 and internal connection tab 56.

Insulative epoxy exists between first film 20 and second film 22 to bond and

fill in the gaps between first film 20 and second film 22 so that an electrical

contact region formed by the conductive epoxy is surrounded and protected

by the insulative epoxy and, preferably, no gaps exist between first film 20

10 and second film 22 which are not filled with insulative epoxy. There should

not be an electrical connection between the inner and outer surfaces of either

first film 20 or second film 22. The use of the insulative epoxy serves to

prevent such a connection. Preferably, the metal coating 24, 26, 28, 30 on

the inner surfaces and outer surfaces of first film 20 and second film 22 is

15 made of nickel and covers the entire portion of each side of first film 20 and

second film 22.

FIGS. 4(A), 4(B), 5(A) and 5(B) show specific measurements for top

section 38 and base section 40; however, the embodiments of the present

invention are not intended to be limited to these measurements and a person

20 skilled in the art would recognize that various different measurements and

sizes could be used. Also, the sensors are described as being either

rectangular shaped or hexagonal shaped; however, the embodiments of the

present invention are not intended to be limited to these particular shapes

and a person skilled in the art would recognize that various shapes and sizes

25 of sensors, and the corresponding shapes and sizes of the vacuum holding

fixture, could be used.

Also, the sensors are not intended to be limiting only for use in an

apparatus which monitors the status of a fetus. There are many other uses

for such sensors. For example, the sensors could be used to detect the

30 heartbeat of the mother or other signals or pulses originating in a human or
10

LAR 14240-1 -13- PATENT APPLICATION

animal body. The sensors may also be useful for sensing signals, pulses and

noise in virtually any other type of body or material. Sensors described

herein can also be called "bimorph" sensors.

Although a few preferred embodiments of the present invention have

been shown and described, it would be appreciated by those skilled in the art

that changes may be made in these embodiments without departing from the

principles and spirit of the invention, the scope of which is defined in the

claims and their equivalents.

What is claimed is:
LAR 14240-1 PATENT APPLICATION

Abstract of the Disclosure

An apparatus and method for bonding a first film to a second film to

form a sensor. The first and second films are piezoelectric films. A base

section has a mating surface for holding the first film. A top section has a

5 mating surface for holding the second film, the top section and base section

being mateable together to form a cavity between the top section mating

surface and the base section mating surface with the first film and the second

film pressed together inside the cavity to bond the first film to the second film.

An internal connection port forming mechanism forms an internal connection

10 port between the first film and the second film when the base section and top

section are mated together with the first film and the second film pressed

together inside the cavity. Alternately, an internal connection tab forming

mechanism can be used to form a sensor having an internal connection tab

between the first and second film, instead of having an internal connection

15 port.
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