U.S. patent number 3,736,436 [Application Number 05/195,750] was granted by the patent office on 1973-05-29 for electret pressure transducer.
Invention is credited to Roger C. Crites.
United States Patent |
3,736,436 |
Crites |
May 29, 1973 |
ELECTRET PRESSURE TRANSDUCER
Abstract
An electret pressure transducer consisting of a very thin
insulating coating applied to the surface of a body to be
investigated when under stress, a thin layer of conductive paint or
metallic deposit configured to extend over the area to be
investigated, a layer of a solid elastic material, the
electret-film containing a permanent electric charge, and a
metallic deposit on the electret film. The layer of conductive
paint and the metallic deposit constitute the two plates of a
parallel-plate capacitor, and the elastic material in conjunction
with the electric charge provided by the electret determines the
sensitivity, linearity, and frequency response of the
transducer.
Inventors: |
Crites; Roger C. (Florissant,
MO) |
Family
ID: |
22722636 |
Appl.
No.: |
05/195,750 |
Filed: |
November 4, 1971 |
Current U.S.
Class: |
307/400;
381/191 |
Current CPC
Class: |
H04R
19/01 (20130101) |
Current International
Class: |
H04R
19/00 (20060101); H04R 19/01 (20060101); H01g
007/02 (); H04r 019/00 () |
Field of
Search: |
;307/88ET ;179/111R,111E
;317/262F |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Urynowicz, Jr.; Stanley M.
Claims
What is claimed is:
1. An electret pressure transducer comprising a body consisting of
an electret film adjacent an imperforate solid elastic substrate,
and said film and substrate sandwiched between imperforate
conductive layers, the film, substrate and layers constituting a
parallel plate capacitor in which the thickness of said substrate
fluctuates as the applied pressure fluctuates, while the effective
electrical charge, due to the electret, remains substantially
constant.
2. The transducer set forth in claim 1 wherein said electret film,
substrate and conductive layers are flexible and capable of
conforming to the shape of a body under investigation for applied
pressure.
3. The transducer set forth in claim 1 wherein said conductive
layers are applied conductive films, having a combined thickness of
about one mil.
4. The transducer set forth in claim 1 wherein said film, substrate
and layers have a substantially coextensive area and are in
intimate contact.
5. As an article of manufacture in the form of an electret pressure
transducer body for placement on an imperforate surface to be
investigated for applied surface pressure, said body being subject
to the applied pressure and consisting at least of two electrically
conductive imperforate and flexible layers and an intervening layer
of a compliant imperforate material, said compliant layer
containing an electric charge and responding to applied pressure to
change the capacitance of said article as a function of the
response of said compliant layer to pressure applied on the surface
to be investigated, and said electrically conductive layers and
said compliant layer being in surface to surface contact.
6. As an article of manufacture set forth in claim 5, the electret
pressure transducer thereof in which the plurality of layers in
stacked engagement include an electrical insulating coating.
7. The article of manufacture set forth in claim 5 wherein said
electret pressure transducer body is flexible and conformable to
the configuration of the surface area to be investigated.
Description
BACKGROUND OF THE INVENTION
An electret device is generally understood to be a dielectric body
in which a permanent electric charge has been established. Early
usage of electrets has been made in microphones as a transducer for
converting the pressure generated by sound waves on a diaphragm
into electrical signals for reproduction in audio systems. The
common electret microphone was constructed in a manner that
operated in response to changes of pressure in a trapped volume of
air.
The typical electret microphone of the prior art using a trapped
volume of air is far too bulky and not at all adaptable to large
surface areas that require pressure response investigation, and are
incapable of use on surfaces that have compound curvature
characteristics. The early electret involved use of dielectric
materials that exhibited the electret phenomena, but almost all the
materials producing electrets are unstable. That is to say the
electric field produced thereby decayed rapidly to an insignificant
intensity within a few weeks. Since the early work with electrets
was almost entirely with microphones, it limited the thinking to
the concept that electrets should be flat and used with a volume of
trapped air so that high sensitivity could be obtained through
large changes in the air gap for very small changes in the external
pressure.
SUMMARY OF THE INVENTION
This invention relates to improvements in electret pressure
responsive transducers, and more specifically to transducers that
have very thin, free-surface characteristics adaptable to a broad
range of uses for measuring fluctuating pressures or detecting
sudden changes in fluctuating pressure intensity, or where any
small, large, flat or highly curved surface, or where the
transducers must not interfere with the flow of a fluid relative to
a body.
The transducer herein consists of a thin film body in which the
prior art air gap between the electret and a backplate is replaced
by a solid elastic material which provides an improved range of
dynamic characteristics by the thickness and stiffness of the
elastic substrate. In a preferred form of the improvement, the
transducer includes a very thin film of electrical insulating
material, a very thin layer of conductive material such as paint or
vacuum deposited metal, a layer of elastic solid material, the
electret material such as FEP Teflon or K-1 Polycarbonate, and a
metallization film on the electret. The transducer is intended to
be incorporated in an impedence-matching electronic circuit.
The objects of this invention are to provide an improved transducer
employing a thin film electret having a high order of sensitivity
to fluctuating pressure, to provide a transducer with an elastic
solid state electret film capable of use on large area surfaces
having compound curvature characteristics, to provide an electret
transducer that can measure sound pressure levels of a very low
decible value, and to provide an electret transducer having a
linear response to sound pressure and improved frequency response
through control of the electret substrate thickness and elastic
modulus.
Other objects and advantages of this invention will appear from the
following description of a preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiment of this invention and its circuit
application is shown in the accompanying drawings wherein:
FIG. 1 is a perspective view of a body to be investigated for
surface pressure fluctuations showing a typical electret transducer
installation;
FIG. 2 is a sectional elevational view, greatly exaggerated, of a
typical improved transducer taken at line 2--2 in FIG. 1 and
embodying a solid state electret in association with an
electrically conductive body;
FIG. 3 is a sectional elevational view similar to FIG. 2 but
showing a modification involving an electrically insulative body;
and
FIG. 4 is a diagram of an impedence matching electronic circuit for
the electret transducer of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An electret is a dielectric material which produces an electrical
field about itself in the absence of an external voltage source. In
this manner, an electret is analogous to a magnet, which produces a
magnetic field about itself in the absence of an external current
source. Electrets are made by imposing a permanent electrostatic
charge onto the surfaces of a dielectric material.
Heretofore electrets have been widely used in microphones and have
been designed invariably utilizing a thin film of fluorocarbon,
such as FEP Teflon or other films such as K-1 Polycarbonate
metalized on one side. The metal deposit on the electret serves as
one conductor of a parallel plate capacitor. An electret of this
character is generally stretched over a metallic backplate, thus
trapping a thin layer of air, and the metal backplate serves as the
other conductor in the capacitor. If the thickness of the electret
material is neglected (for clarity) the capacitance of this
arrangement is given by the relationship:
c = e.sub.o A/d
In the above relationship e.sub.o is the electric permittivity of
free space, A is the surface area of a capacitor, and d is the
distance between the two parallel conductors which is essentially
the thickness of the air gap in electret transducers heretofore
designed.
As the pressure over the electret fluctuates, the air gap distance
d fluctuates because the electret film essentially has no
stiffness, whereby the distance d must change just enough for the
pressure in the air gap to equal the external pressure acting on
the electret at each instance in time. The capactiance will,
therefore, fluctuate as a function of the external pressure. The
electret emits a constant electric field that is produced by a
constant charge Q, which, together with the value of capacitance C,
determines the voltage developed across the output terminals
according to the definition:
V = Q/C
The voltage, thereof, will fluctuate as the capacitance C
fluctuates, which also will fluctuate as the distance between the
two parallel conductors d fluctuates, which, in turn, fluctuates as
the external pressure fluctuates. It is understood from the
foregoing relationships that voltage developed across the output
terminals of the electret capacitor is a function of the pressure
acting on the electret surface.
The present electret pressure transducer is an improvement upon the
previous devices as it does away with the air gap and incorporates
a solid elastomer or elastic material which imparts to the
resulting transducer extreme flexibility and broadens the field of
application to many more uses. The dynamic characteristics of the
solid elastic material electret transducer are determined by the
thickness and elastic modulus of the elastic substrate. A typical
installation of the improved transducer is shown in FIG. 1 where a
group 10, 11, 12 and 13 is applied on an electrically conductive
metallic body 14 upon which it is desired to measure fluctuating
pressure. Each transducer 10, 11, 12 and 13 is secured in place on
the body 14 with leads 15, 16, 17 and 18 supported on the body and
extending to a contact pick-up pin suitably mounted in an insulated
terminal pot 19. A common ground lead 20 is in contact with the
body 14, and jumper leads 21, 22, 23 and 24 connect the respective
transducers 10, 11, 12 and 13 to this ground. The arrangement shown
in FIG. 1 is for obtaining individual reading of each transducer.
However, if the pick-up pins in the terminal pot 19 are connected
together a composite reading can be obtained which would be the
average reading for the total area covered by the group of
transducers. Thickness of transducers shown in the figure are
greatly exaggerated.
Turning now to FIG. 2 the transducer 10 which is typical of the
others comprises a thin film or coating 2 of electrical insulating
paint which is applied to the surface of the body 14. Adjacent the
electrical insulating film is a very thin film 3 of conductive
paint which may be vacuum-deposited material to form one electrode
in the electrical capacitor. A solid elastic material substrate 4
is in contact with the conductive film 3 and replaces the prior art
air gap forming means between the electret metalization film 3 and
the film 5 which is the electret material which may comprise a
fluorocarbon film or other suitable films. The other conductor of
the electret capacitor is a metalized film 6 on the electret
material film 5. In the transducer 10 thus described the electret
metalizing film 6 is grounded by jumper lead 21 to the ground lead
20 on the conductive body 14 whereby the conductive film 3 is
shielded from stray electric fields and the signal is then taken
between film 3 and ground 6 as indicated at the output terminals 7.
The electret transducer 10 of FIG. 2 undergoes capacitance changes
as a linear function of the compression response of the elastic
substrate layer 4. The charge on the capacitor remains constant, as
determined by the electret material 5, so that the voltage across
the capacitor must fluctuate as the pressure fluctuates, generating
a fluctuating voltage without any power supply.
In FIG. 3 the electret pressure transducer 10A is shown as applied
to a body or a surface 8 which is electrically insulating. In this
arrangement one additional layer or film of conductive material 9
is necessary in order to provide a bottom film layer which may be a
very thin metalization. In other respects the view of FIG. 3 shows
the stacked arrangement of layers 2, 3, 4, 5 and 6 described in
connection with FIG. 2.
Electret transducers of the type described in FIGS. 2 and 3 are
inherently high-impedance devices. It is necessary, therefore, to
utilize an electronic follower which has impedance matching
characteristics. A suitable example is a simple FET source follower
with more than 10.sup.8 ohms imput impedance and such a circuit is
shown in FIG. 4. The electronic follower circuit is connected at
terminal 7 to the transducer 10 or 10A and includes a FET
transistor 22, diode 23 and resistor 24 in the negative power
return lead 25. The output terminals 26 are connected to any
suitable amplifier (not necessary to show). Calibration of the
elastic substrate electret transducer heretofore described may be
carried out in the reflected wave tube, commercially employed for
the calibration of conventional fluctuating pressure
transducers.
It has been found that the sensitivity of the electret transducers
of this invention is typically two to three orders of magnitude
greater than conventional fluctuating pressure transducers. The
herein improved electret transducers are capable of measuring
further pressure levels 40 to 60 decibels (dB) lower than
conventional transducers at the same signal-to-noise ratio, and 10
to 20 dB higher pressure levels than prior art electret
microphones. This makes the improved electret suitable for
application in low speed wind tunnels where sound pressure levels
are commonly less than 120 dB, as well as supersonic wind tunnels
with sound pressure levels as high as 190 dB.
Electret pressure transducers using a solid electric substrate or
solid elastomer as heretofore described have been developed where
the transducer has a thickness of approximately 1 mil. and results
in dynamic characteristics which are independent of sensor surface
area configuration. Because of this thickness the present
transducers are well suited to the measurement of fluctuating
pressures over any aerodynamic surface of interest either small or
large, flat or highly curved. The sensor area may be easily
configured into narrow thin strips for the establishment of
boundary layer transition and areas for investigation. Furthermore,
the transducer may be produced as an integral tape in which all of
the layers are combined with the layer 2 or 9 having adhesive
characteristics for direct application to the bodies 14 or 8 as
heretofore noted. Also, the electret film may be consolidated with
the solid elastic substrate. When so produced the transducer must
have the metallic shielding 3 and 6 on each surface of the electret
material. The solid elastomer or solid elastic substrate 4 may be
separate or combined with the electret.
The transducer heretofore described may also be one large area to
provide direct measurement of the integral or average of
fluctuation of pressure over that area for buffet or aerodynamic
force study. Electret transducers herein described can have
resonant frequencies in excess of 100 KH.sub.z, regardless of size
or shape, and they have demonstrated excellent linearity,
acceptable frequency response, and a dynamic sensitivity which
exceeds that of most conventional fluctuating pressure transducers
by more than two orders of magnitude. A particularly important
characteristic of the present electret pressure transducer is that
the material costs involved are negligible, so that the use thereof
in place of conventional instruments provides a substantial cost
saving.
The present electret pressure transducer provides cheap, high
response, fluctuating pressure instrumentation for aerodynamic
tests, and may be applied to small or large aerodynamic surfaces,
subject to fluctuating pressure forces which require
investigation.
The improved transduction mechanism of this electret transducer can
be used in an electrostatic speaker, and because it requires no
high voltage bias supply it is more compatable with modern low
voltage transistor audio circuits than other electrostatic
speakers. Also, because it uses no air gaps it is better suited to
high sound level output, particularly in the ultrasonic frequency
domain.
The electret pressure transducer herein above described clearly
expands the use thereof so that it may be incorporated, for
example, in ultra-sensitive intruder alarms, micro-switches,
ultra-thin keyboards, chamber pressure monitors for small arms, and
particle and hail detectors.
The field of use is also expanded by the present electret pressure
transducers to include medical uses involving vital function alarms
for intensive care patents and baby incubators; implantable
pressure sensors; orthopedic responses and bio-switches which are
made to respond to various controllable muscles of the human body,
thereby permitting victims of paralysis to control servo
mechanisms.
* * * * *