U.S. patent application number 10/391064 was filed with the patent office on 2004-02-26 for capacitive pressure sensor having encapsulated resonating components.
Invention is credited to Pinto, Gino A., Vaughan, Kevin.
Application Number | 20040035211 10/391064 |
Document ID | / |
Family ID | 23456017 |
Filed Date | 2004-02-26 |
United States Patent
Application |
20040035211 |
Kind Code |
A1 |
Pinto, Gino A. ; et
al. |
February 26, 2004 |
Capacitive pressure sensor having encapsulated resonating
components
Abstract
A capacitive pressure sensor for measuring a pressure applied to
an elastic member includes a capacitive plate disposed adjacent to
the elastic member so as to define a gap between a planar
conductive surface of the elastic member and a corresponding planar
surface of the capacitive plate. The gap, capacitive plate and
elastic member together define a capacitor having a characteristic
capacitance. The sensor further includes an elongated electrical
conductor characterized by an associated inductance value. The
conductor is fixedly attached to and electrically coupled with the
capacitive plate. The gap between the capacitive plate and the
elastic member varies as a predetermined function of the pressure
applied to the elastic member so as to vary the characteristic
capacitance. The capacitor and the electrical conductor together
form an electrical resonator having a characteristic resonant
frequency. Varying the capacitance of this tank circuit varies the
resonant frequency of the tank circuit. Thus, the resonant
frequency of the tank circuit is indicative of the pressure applied
to the elastic member. The close physical proximity of the
capacitor and the electrical conductor equalizes the effects of
environmental influences such as temperature variations, vibration
and shock, thus making such effects more predictable.
Inventors: |
Pinto, Gino A.; (Milford,
MA) ; Vaughan, Kevin; (Bedford, MA) |
Correspondence
Address: |
Mark G. Lappin, P.C.
McDERMOTT, WILL & EMERY
28 State Street
Boston
MA
02109
US
|
Family ID: |
23456017 |
Appl. No.: |
10/391064 |
Filed: |
March 18, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10391064 |
Mar 18, 2003 |
|
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09369573 |
Aug 6, 1999 |
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6532834 |
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Current U.S.
Class: |
73/718 |
Current CPC
Class: |
G01L 9/0072
20130101 |
Class at
Publication: |
73/718 |
International
Class: |
G01L 009/12 |
Claims
What is claimed is:
1. A capacitive pressure sensor comprising: an elastic member
having a first side and a second side, said elastic member
comprising a conductive surface at said first side; a support
member supporting an edge of said elastic member, wherein said
support member and said elastic member separate two pressure
regions across said elastic member, a first pressure region
contiguous with said first side of said elastic member and a second
pressure region contiguous with said second side of said elastic
member, wherein a central portion of said elastic member is
displaceable along an axis in response to a pressure differential
between said two pressure regions across said elastic member; an
electrically conductive, capacitive plate disposed substantially
adjacent to said elastic member so as to define a gap between said
first side of said elastic member and said capacitive plate,
wherein said gap, capacitive plate and elastic member define a
capacitor having a characteristic capacitance; and, an elongated
electrical conductor characterized by an associated inductance
value, said conductor being fixedly attached, along a substantial
portion of its entire length, to said capacitive plate; wherein
said capacitor and said electrical conductor are electrically
coupled to form a resonant tank circuit; wherein said gap varies as
a predetermined function of the pressure differential between said
two pressure regions across said elastic member so as to vary said
characteristic capacitance, and consequently vary a resonant
frequency of said tank circuit.
2. A capacitive pressure sensor according to claim 1, wherein said
first pressure region is a sealed region.
3. A capacitive pressure sensor according to claim 2, wherein said
first sealed pressure region is vacuum.
4. A sensor according to claim 1, wherein said electrical conductor
is disposed in a spiral configuration within a plane substantially
parallel to said capacitive plate.
5. A sensor according to claim 1, further including an insulator
disposed between and fixedly attached to said capacitive plate and
said electrical conductor.
6. A sensor according to claim 1, further including a stiffening
element fixedly attached to said electrical conductor.
7. A sensor according to claim 6, wherein said stiffening element
includes a ceramic material.
8. A sensor according to claim 1, wherein said electrical conductor
includes at least two layers of electrical conductors separated by
alternating layers of insulating material, each said at least two
layers of electrical conductors being spirally disposed within a
plane parallel to other said layers of electrical conductors and
being electrically coupled to adjacent layers of electrical
conductors.
9. A sensor according to claim 1, wherein said elastic member is
electrically conductive.
10. A sensor according to claim 1, wherein said elastic member is
non-electrically conductive and includes a plated surface forming
said conductive surface.
11. A sensor according to claim 1, wherein said resonant tank
circuit includes a parallel coupling of said capacitor and said
elongated conductor.
12. A sensor according to claim 1, wherein said resonant tank
circuit includes a series coupling of said capacitor and said
elongated conductor.
13. A sensor according to claim 1, wherein said first pressure
region comprises a chamber having at least one port.
14. A sensor according to claim 1, wherein said second pressure
region comprises a chamber having at least one port.
15. A sensor according to claim 1, wherein a central portion of
said capacitive plate is supported by a rigid structure extending
from said central portion of said elastic member.
16. A sensor according to claim 1, wherein said capacitive plate is
supported by way of a rigid structure connected to said support
member.
17. A capacitive pressure sensor comprising: an elastic member
having a first side and a second side, said elastic member
comprising a conductive surface at said first side; a housing for
supporting said elastic member, said elastic member and said
housing forming a first pressure region contiguous with said first
side of said elastic member, and a second pressure region
contiguous with said second side of said elastic member, wherein
said first pressure region and said second pressure region generate
a pressure differential across said elastic member, wherein a
central portion of said elastic member is displaceable along an
axis in response to the pressure differential; a capacitive plate
disposed substantially parallel to said elastic member so as to
define a gap between said conductive surface and a corresponding
surface of said capacitive plate, said gap, capacitive plate and
elastic member defining a capacitor having a characteristic
capacitance; and, an elongated electrical conductor characterized
by an inductance value and fixedly attached, along a substantial
portion of its entire length, to said capacitive plate; wherein
said capacitor and said electrical conductor are electrically
coupled to form a resonant tank circuit; wherein said gap varies as
a predetermined function of said pressure differential across said
elastic member so as to vary said characteristic capacitance, and
consequently vary a resonant frequency of said tank circuit.
18. A capacitive pressure sensor according to claim 17, wherein
said first pressure region is characterized by a constant and
controlled pressure value.
19. A sensor according to claim 17, wherein said electrical
conductor is disposed in a spiral configuration within a plane
substantially parallel to said capacitive plate.
20. A sensor according to claim 17, further including an insulator
disposed between and fixedly attached to said capacitive plate and
said electrical conductor.
21. A sensor according to claim 17, further including a stiffening
element fixedly attached to said electrical conductor.
22. A sensor according to claim 21, wherein said stiffening element
includes a ceramic material.
23. A sensor according to claim 17, wherein said electrical
conductor includes at least two layers of electrical conductors
separated by alternating layers of insulating material, each said
at least two layers of electrical conductors being spirally
disposed within a plane parallel to other said layers of electrical
conductors and being electrically coupled to adjacent layers of
electrical conductors.
24. A sensor according to claim 17, wherein said elastic member is
electrically conductive.
25. A sensor according to claim 17, wherein said elastic member is
non-electrically conductive and includes a plated surface forming
said conductive surface.
26. A sensor according to claim 17, wherein said resonant tank
circuit includes a parallel coupling of said capacitor and said
elongated conductor.
27. A sensor according to claim 17, wherein said resonant tank
circuit includes a series coupling of said capacitor and said
elongated conductor.
28. A sensor according to claim 17, wherein said first pressure
region comprises a chamber having at least one port.
29. A sensor according to claim 17, wherein said second pressure
region comprises a chamber having at least one port.
30. A sensor according to claim 17, wherein a central portion of
said capacitive plate is supported by a rigid structure extending
from said central portion of said elastic member.
31. A sensor according to claim 17, wherein said capacitive plate
is supported by way of a rigid structure connected to said
housing.
32. A sensor for measuring a pressure differential across an
elastic member having at least a first substantially planar,
electrically conductive surface and being supported by at least one
edge, comprising: a housing for supporting said elastic member by
said edge, forming (i) a first pressure region disposed on a first
side of said elastic member corresponding to said first planar
surface, and (ii) a second pressure region disposed on a second
side of said elastic member opposite said first side, wherein said
first pressure region and said second pressure region generate said
pressure differential across said elastic member, and wherein a
central portion of said elastic member is displaceable along an
axis in response to said pressure differential; an electrically
conductive, capacitive plate disposed substantially adjacent to
said elastic member so as to define a gap between said first planar
surface and a corresponding planar surface of said capacitive
plate, said gap, capacitive plate and elastic member defining a
capacitor having a characteristic capacitance; and, an elongated
electrical conductor characterized by an associated inductance
value, disposed, along a substantial portion of its entire length,
upon a substrate in a substantially planar configuration; wherein
said capacitor and said electrical conductor are electrically
coupled to form a resonant tank circuit; wherein said gap varies as
a predetermined function of said pressure differential so as to
vary said characteristic capacitance, and consequently vary a
resonant frequency of said tank circuit.
33. A sensor according to claim 32, wherein said capacitive plate
is disposed within said housing and said elongated conductor is
disposed outside of said housing.
34. A sensor according to claim 32, wherein said capacitive plate
and said elongated conductor are each electrically coupled to a
conductive post.
35. A sensor according to claim 32, wherein said conductive post
extends through and is fixedly attached to said housing via an
electrically non-conductive sleeve, said capacitive plate is
fixedly attached to a portion of said post extending into said
housing, and said elongated conductor is fixedly attached to a
portion of said post extending out of said housing.
36. A sensor according to claim 32, wherein said elongated
conductor is fixedly attached to an insulating substrate.
37. A sensor according to claim 32, wherein said resonant tank
circuit includes a parallel coupling of said capacitor and said
elongated conductor.
38. A sensor according to claim 32, wherein said resonant tank
circuit includes a series coupling of said capacitor and said
elongated conductor.
39. A sensor according to claim 32, wherein said first pressure
region comprises a chamber having at least one port.
40. A sensor according to claim 32, wherein said second pressure
region comprises a chamber having at least one port.
41. A sensor according to claim 32, wherein a central portion of
said capacitive plate is supported by a rigid structure extending
from said central portion of said elastic member.
42. A sensor according to claim 32, wherein said capacitive plate
is supported by way of a rigid structure connected to said housing.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation in part of U.S. patent
application Ser. No. 09/369,573, filed Aug. 6, 1999, the disclosure
of which is incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a pressure sensor, and more
particularly, a pressure sensor which relies on changes in
capacitance to indicate pressure fluctuations.
BACKGROUND OF THE INVENTION
[0003] Capacitive pressure sensors are well known in the prior art.
Such sensors typically include a fixed element having a rigid,
planar conductive surface forming one plate of a substantially
parallel plate capacitor. A displaceable (relative to the fixed
element) conductive member, such as a metal diaphragm, or a plated
non-conductive member, such as a metalized ceramic diaphragm, forms
the other plate of the capacitor. Generally, the diaphragm is
edge-supported so that a central portion is substantially parallel
to and opposite the fixed plate. Because the sensor generally has
the form of a parallel plate capacitor, the characteristic
capacitance C of the sensor may be approximated by the equation: 1
C = A d ( 1 )
[0004] where .epsilon. is the permittivity of the material between
the parallel plates, A is the surface area of the parallel plate
and d represents the gap between the plates. The characteristic
capacitance is inversely proportional to the gap between a central
portion of the diaphragm and the conductive surface of the fixed
element. In order to permit a pressure differential to develop
across the diaphragm, the region on one side of the diaphragm is
sealed from the region on the opposite side.
[0005] In practice, the diaphragm elasticity is selected so that
pressure differentials across the diaphragm in a particular range
of the interest cause displacements of the central portion of the
diaphragm. These pressure differential-induced displacements result
in corresponding variations in the gap, d, between the two
capacitor plates, and thus in capacitance variations produced by
the sensor capacitor. For relatively high sensitivity, such sensors
require large changes of capacitance in response to relatively
small gap changes. Regarding equation (1), if .epsilon. and A are
held constant, the greatest slope of the d verses C plot occurs
when d is small. Thus, for the greatest sensitivity, the gap is
made as small as possible when the device is in equilibrium and the
sensor is designed so that the gap d changes as pressure is
applied. The multiplicative effect of .epsilon. and A increases the
sensitivity of the d to C relationship, so .epsilon. and A are
maximized to achieve the highest possible sensitivity.
[0006] In a typical prior art embodiment, the sensor capacitor
formed by the fixed conductive surface and the diaphragm is
electrically coupled via conductors to an oscillator circuit. The
oscillator circuit typically includes an inductor that forms a tank
circuit with the remotely located sensor capacitor. This LC tank
circuit provides a frequency reference for the oscillator circuit;
the output frequency of which is a direct function of the resonant
frequency of the tank circuit. The resonant frequency of the tank
circuit is in turn a direct function of the inductance L of the
inductor and the capacitance C of the sensor capacitor. It is well
known to those in the art that the resonant frequency .omega..sub.0
of a simple LC tank circuit is given by 2 0 = 1 L C .
[0007] As long as the values of the inductor and the capacitor both
remain fixed, the output frequency of the oscillator circuit
remains constant. However, since the capacitance of the sensor
capacitor varies as a function of the pressure applied to the
diaphragm, the output frequency of the of the oscillator circuit
also varies as a direct function of the applied pressure.
[0008] Such a configuration produces a signal whose frequency is
indicative of the pressure applied to the remote sensor. One
disadvantage to this configuration is that having the capacitive
sensor located remotely can introduce environmentally induced
errors in the expected resonant frequency of the tank circuit. For
example, it is well known to those in the art that the inductance
value L of an inductor and the capacitance value C of a capacitor
are each temperature dependent to some extent, depending upon the
design of each particular physical component. The effect of the
temperature on the capacitance or inductance of a particular
component is often quantified as the "temperature coefficient"
associated with that component. It is possible to design a
component so as to minimize the temperature coefficient, thus
rendering the value of the device relatively insensitive to
temperature, but commercially available components typically do
have a measurable temperature coefficient which affects the
component performance. It is also possible to choose components
whose temperature coefficients are complementary, such that the net
effect of a temperature change to the components together is
nominally zero. However, when two components are not located
together, such as the capacitive sensor and the inductor in the
oscillator circuit, the ambient temperatures are often different,
and complementary temperature coefficients do not produce a
nominally zero sensitivity to temperature changes.
[0009] Another disadvantage to having a remotely located capacitive
sensor is that the conductors used to electrically couple the
sensor to the oscillator circuit introduce stray capacitances and
inductances to the basic LC tank circuit. This disadvantage could
be mitigated and thus acceptable if the stray values remained
constant, but the stray values can change with environmental
factors, physical movement of the conductors, etc.
[0010] It is an object of the present invention to substantially
overcome the above-identified disadvantages and drawbacks of the
prior art.
SUMMARY OF THE INVENTION
[0011] The foregoing and other objects are achieved by the
invention which in one aspect comprises a capacitive sensor for
measuring a pressure differential across a conductive, elastic
member, or a plated non-conductive elastic member, having at least
a first substantially planar surface and being supported on at
least one edge. The elastic member extends about a central axis.
The sensor includes a support member or a housing extending about
the central axis for supporting the elastic member by its edge,
thereby forming (i) a first pressure region disposed on the side of
the elastic member corresponding to the first planar surface, and a
second pressure region disposed on the side of the elastic member
opposite said first side. The first pressure region and the second
pressure region generate the pressure differential across the
elastic member. The sensor also includes a capacitive plate
disposed substantially adjacent to the elastic member so as to
define a gap between the first planar surface and a corresponding
planar surface of the capacitive plate. The gap, capacitive plate
and elastic member together define a capacitor having a
characteristic capacitance. The sensor further includes an
elongated electrical conductor characterized by an associated
inductance value. The conductor is fixedly attached to and
electrically coupled with the capacitive plate. The gap between the
capacitive plate and the elastic member varies as a predetermined
function of the pressure differential across the elastic member so
as to vary the characteristic capacitance. The capacitor and the
electrical conductor together form a tank circuit having a
characteristic resonant frequency; varying the capacitance of this
tank circuit varies the resonant frequency of the tank circuit.
Thus, the resonant frequency of the tank circuit is indicative of
the pressure differential across the elastic member.
[0012] In another embodiment of the invention, the pressure applied
to the elastic member is generated by a pressure differential
across (i) the first planar surface of the elastic member and (ii)
a second planar surface of the elastic member disposed
substantially parallel to the planar surface. In one embodiment,
this pressure differential is the result of a constant, controlled
environment being in contact with the first planar surface, along
with a fluid under pressure being in contact with the second planar
surface of the elastic member. In an alternative embodiment, the
two planar surfaces of the elastic member can be respectively
connected to two fluid mediums, and the pressure sensor can be used
to measure the pressure difference between the two fluid
mediums.
[0013] In another embodiment, the electrical conductor is disposed
in a spiral configuration within a plane substantially parallel to
the capacitive plate.
[0014] In a further embodiment, the sensor further includes an
insulator disposed between the capacitor plate and the electrical
conductor. The insulator may be fixedly attached to either the
capacitor plate, the electrical conductor, or both.
[0015] In another embodiment, the sensor further includes a
stiffening element fixedly attached to the capacitive plate and the
conductive element.
BRIEF DESCRIPTION OF DRAWINGS
[0016] The foregoing and other objects of this invention, the
various features thereof, as well as the invention itself, may be
more fully understood from the following description, when read
together with the accompanying drawings in which:
[0017] FIG. 1 shows a sectional view of one preferred embodiment of
a capacitive pressure sensor;
[0018] FIG. 2A shows the capacitive sensor of FIG. 1 with an
elastic member having a neutral displacement;
[0019] FIG. 2B shows the capacitive sensor of FIG. 1 with a higher
pressure in the second pressure region than the first pressure
region;
[0020] FIG. 3A shows a bottom view of the capacitor plate;
[0021] FIG. 3B shows a top view of the inductor coil;
[0022] FIG. 4A shows the capacitor and the inductor coil connected
as a series resonant tank circuit;
[0023] FIG. 4B shows the capacitor and the inductor coil connected
as a parallel resonant tank circuit;
[0024] FIG. 5 shows the tank circuit of FIG. 4A connected to an
oscillator circuit;
[0025] FIG. 6 shows a closing-gap embodiment of the pressure sensor
in accordance with the present invention;
[0026] FIG. 6A shows an alternative embodiment of the pressure
sensor of FIG. 6;
[0027] FIG. 7 shows a sensor including a stiffening element
attached to the electrode assembly in accordance with one
embodiment of present invention;
[0028] FIG. 8 shows an alternate, multiple layer embodiment of the
inductor coil from the sensor in accordance with one embodiment of
the present invention;
[0029] FIG. 9 shows another view of the multiple layer inductor
coil shown in FIG. 8;
[0030] FIG. 10 shows another embodiment of the sensor in accordance
with the present invention;
[0031] FIG. 10A shows an alternative embodiment of the pressure
sensor of FIG. 10; and
[0032] FIG. 11 is a schematic view of one embodiment of the present
invention showing the pressure sensor being used to measure the
pressure difference between two fluid mediums.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0033] FIG. 1 shows a sectional view of one preferred embodiment of
a capacitive pressure sensor 100 constructed in accordance with the
present invention, which produces a characteristic capacitance
proportional to a pressure (e.g., pressure via a fluid medium)
applied to the sensor 100. Sensor 100 includes a first body member
103A, a second body member 103B, and an electrically conductive,
elastic member 102 that forms a physical boundary between a first
pressure region 106 and a second pressure region 104 defined by the
two body members 103A and 103B. The elastic member 102, the first
body member 103A, and the second body member 103B extend about a
central axis X.
[0034] The elastic member 102 is supported at its periphery 108 by
a support member 110. The support member 110 may include, or be
integral with, the pressure sensor 100 housing, as is disclosed and
described in detail in U.S. Pat. No. 5,442,962, assigned to the
assignee of the subject invention and is hereby incorporated by
reference.
[0035] In this embodiment, the planar surface of the elastic member
102 is substantially circular, although alternate embodiments may
incorporate other shapes. A connection post 112 extending along the
central axis X for supporting an electrode assembly 114 is fixedly
attached to the elastic member 102. The connection post 112 may be
attached to the elastic member 102 by brazing, soldering, welding,
gluing, press fit, stud mount, or by other securing methods known
to those in the art. The cross section of the elastic member 102
(shown in FIG. 1) is somewhat greater (i.e., thicker) at the
center, as compared to the perimeter, to provide a foundation for
attaching the connection post 112. Other elastic member 102 cross
sections may be used to provide similar results. Similarly, the
electrode assembly 114 may be attached to the connection post 112
by brazing, soldering, gluing, press fit, stud mount, or by other
methods of securing components known to those in the art.
[0036] The electrode assembly 114 includes a capacitor plate 116,
an insulator 118 and a planar inductor coil 120. The capacitor
plate 116, a bottom view of which is shown in FIG. 3A, is shaped,
sized and contoured to substantially match the planar surface of
the electrically conductive elastic member 102. In a preferred
embodiment, the capacitor plate 116 includes a sheet of copper,
silver or gold bonded to an insulating base 117 such as fiberglass,
polyimide, glass, or ceramic, although other electrically
conductive materials and other insulating materials known to those
in the art may be used to form the capacitor plate 116 and the
insulating base 117, respectively. Alternately, the capacitor plate
116 may be etched from a copper-clad substrate, or screened and
fired using thick-film techniques, using procedures well known for
the fabrication of printed circuits.
[0037] The insulator 118 may include a separate piece of insulating
material bonded to and contiguous with the capacitor plate 116 and
the inductor coil 120, or it may include an extension of the
insulating base from the capacitor plate 116. The insulator 118 may
include fiberglass, polyimide, ceramic, or other insulating
materials known to those in the art.
[0038] A preferred embodiment of the inductor coil 120, a top view
of which is shown in FIG. 3B, includes an elongated electrical
conductor wound in a spiral form within a plane that is
substantially parallel to the capacitor plate 116. As with the
capacitive plate 116, the inductor coil 120 may be etched from a
sheet of conductive foil bonded to an insulator 118, using printed
circuit board techniques well known to those in the art.
Alternatively, the coil may be screened and fired using thick-film
techniques well known to those in the art. In other embodiments,
the coil 120 may include a single long conductor, wound in the
shape shown in FIG. 3B and bonded to an insulator 118. Other
methods of fabricating the coil 120 known to those in the art
(e.g., vapor deposition, photoetching, etc.) may also be used, as
long as the resulting coil 120 provides the inductive properties
described herein. The end of the coil 120 shown in FIG. 3B is
electrically coupled to a plated through-hole 128 that passes
through the insulator 118. The plated through-hole 128 is also
electrically coupled to the capacitor plate 116; the coil 120 is
thus electrically coupled to the capacitor plate 116. In alternate
embodiments, this electrical coupling between the coil 120 and the
capacitive plate 116 may be accomplished by an electrical conductor
passing through the insulator 118, by a conductor wrapping around
the side of the insulator 118, or by other methods known to those
in the art.
[0039] The capacitive plate 116, the conductive elastic member 102
and the gap 126 formed between the capacitive plate 116 and the
elastic member 102 form a capacitor 130 having a characteristic
capacitance. In general, the characteristic capacitance of such a
structure is directly proportional to the areas of the capacitive
plate 116 and the elastic member 102, and inversely proportional td
the distance between the capacitive plate 116 and the elastic
member 102.
[0040] In a preferred embodiment of the invention, the pressure
sensor 100 senses a pressure differential generated by the first
pressure region 106 and the second pressure region 104 across the
elastic member 102. The pressure in the first pressure region 106
may be ambient atmospheric pressure (i.e., simply exposed to the
"open air"), or it may be more precisely controlled with respect to
a constant pressure reference. In another preferred form, the first
pressure region 106 is a vacuum chamber, and the pressure in the
first pressure region 106 substantially equals zero.
[0041] In one preferred embodiment, the device 100 can be used to
measure the pressure of a fluid medium. In this embodiment, the
second pressure region 104 is preferably constructed as a chamber
having a port 105 for receiving air or another fluid whose pressure
is to be measured. The first pressure region 106 can be a sealed
chamber or an opened chamber having a port 107, and the pressure in
the first pressure region 106 is preferably maintained
constant.
[0042] In another preferred embodiment, the device can be used to
measure the pressure difference between two fluid mediums that are
respectively introduced into the first pressure region 106 and the
second pressure region 104. In this embodiment, both pressure
regions 104 and 106 have ports 105 and 107 for connecting with two
fluid mediums, between which, the pressure difference is to be
measured. A difference in pressure across the two regions 104 and
106 produces a net differential pressure 124 on the elastic member
102. When the second pressure region 104 is greater than the first
pressure region 106, the direction of the elastic member
displacement (along the central axis X) is from the second pressure
region 104 to the first pressure region 106, as shown in FIG. 2B. A
change of ambient pressure in any of the pressure regions 104 and
106 produces a corresponding change in the amount of displacement
of the elastic member 102. FIG. 2A shows the elastic member 102 in
a neutral displacement position; i.e., when the differential
pressure across the elastic member 102 is substantially zero. In
the neutral displacement position, a substantially uniform gap 126
exists between the capacitive plate 116 and the elastic member 102.
FIG. 2B shows the elastic member 102 displaced (along the central
axis X) toward the first pressure region 106, such that the elastic
member 102 presents a convex surface in the first pressure region
106. In this convex displacement position, a non-uniform gap 126
exists between the capacitive plate 116 and the elastic member 102.
The width of the non-uniform gap 126 near the connection post 112
is substantially the same as the uniform gap 126 in the neutral
displacement position, and the width of the non-uniform gap 126
increases as the distance from the post 112 increases. The increase
in the gap 126 distance as the elastic member 102 displaces toward
the first pressure region 106 produces a decrease in the
characteristic capacitance. Thus, the characteristic capacitance of
the capacitor 130 formed by the capacitive plate 116, the
conductive elastic member 102 and the gap between them is inversely
proportional to the magnitude of the differential pressure 124
across the elastic member 102.
[0043] In one embodiment of the invention, the capacitor 130 is
electrically coupled in series to the inductive coil 120 so as to
form a series resonant tank circuit 132 having a resonant frequency
3 0 = 1 L C
[0044] as shown schematically in FIG. 4A. Alternately, the
capacitor 130 may be electrically coupled in parallel to the
inductive coil 120 so as to form a parallel resonant tank circuit
132 having a resonant frequency 4 0 = 1 L C
[0045] as shown schematically in FIG. 4B. In either case, the tank
circuit (132 or 134) is electrically coupled to an oscillator
circuit 136 that uses the tank circuit 132 as a frequency
reference, as shown in FIG. 5 for a series resonant tank circuit
132. The oscillator circuit 136 is electrically coupled to the tank
circuit 132 via conductors electrically coupled to inductor
terminal 129, which is insulated from the first body member 103A by
an insulator 129A, and capacitor terminal 131. The output of the
oscillator circuit is a signal SOUT having a frequency of 5 OUT = 1
L C ,
[0046] thus the capacitance C is a function of the frequency; i.e.,
C 6 C = 1 out 2 L .
[0047] Since the characteristic capacitance of the capacitor 130 is
directly proportional to the magnitude of the differential pressure
124 across the elastic member 102, the frequency .omega..sub.OUT of
the output signal S.sub.OUT is also a function of the magnitude of
the differential pressure 124. The close mutual proximity of the
inductive coil 120 and the capacitor 130 ensures similar
environmental conditions for both components of the tank circuit
132A closing-gap embodiment of a pressure sensor 200, shown in FIG.
6, includes an electrically conductive elastic member 202 extending
about the central axis X and secured about its perimeter 208 by a
housing 210. In this form of the invention, the housing 210
includes an upper portion 210a and a lower portion 210b, and the
elastic member 202 is secured between the two portions at its
perimeter 208. The elastic member may be secured by a bonding
technique known in the art such as brazing, welding, gluing, etc.,
or the elastic member may be secured by pressure (i.e., clamping)
between the upper portion 210a and the lower portion 210b of the
housing 210. As with the embodiment shown in FIG. 1, the elastic
member 202 forms a physical boundary between a first pressure
region 206 and a second pressure region 204. In the closing-gap
embodiment, however, the electrode assembly 214 is not mechanically
coupled to the elastic member 202 via a connection post. Rather,
the electrode assembly 214 is suspended from the housing 210 by a
suspension post 212, such that the electrode assembly 214 is
disposed substantially adjacent to the elastic member 202. Because
the electrode assembly 214 is not attached to the elastic member
202 in this embodiment, the cross section of the elastic member 202
can be relatively uniform as shown in FIG. 6, as opposed to the
non-uniform cross section (i.e., thicker at the center and tapering
out toward the perimeter) of the elastic member 102 shown in FIG.
1.
[0048] The construction of the electrode assembly 214 in this
embodiment is essentially the same as for the form of the invention
shown in FIG. 1; the electrode assembly 214 includes a capacitor
plate 216, an insulator 218 and a planar inductor coil 220. The
inductor coil 220 and the capacitor plate 216 are electrically
coupled via the plated through-hole 228. A capacitor 230 having a
characteristic capacitance C is formed by the capacitor plate 216,
the conductive elastic member 202 and the variable gap 226 formed
between the plate 216 and the member 202. Since the areas of the
capacitive plate 216 and the elastic member 202 do not vary, the
characteristic capacitance C varies only as a function of the gap
226. As a differential pressure 224 is applied across the elastic
member 202 in a direction from the second pressure region 204
toward the first pressure region 206, the elastic member deflects
toward the electrode assembly 214, so as to be substantially convex
in the first pressure region 206. This pressure induced deflection
toward the electrode assembly closes the variable gap 226, thereby
increasing the characteristic capacitance C. The characteristic
capacitance C is thus directly proportional to the magnitude of the
differential pressure 224 across the elastic member 202 for this
embodiment of the invention. Electrical access to the capacitor 230
is gained by a first electrical terminal 229 and a second
electrical terminal 231. In one preferred embodiment, the first
electrical terminal 229 is electrically coupled to the inductor
coil 220 through an electrically conductive suspension post 212,
and the second electrical terminal 231 is electrically coupled to
the elastic member 202 at its perimeter 208. The second pressure
region 204 preferably includes a port 205 for receiving fluid
medium whose pressure is to be measured. The first pressure region
can be a sealed chamber, as shown in FIG. 6. In a preferred
embodiment, the sealed chamber 206 is vacuum. In another preferred
embodiment, as shown in FIG. 6A, the first pressure region 206 also
includes a port 207 for receiving another fluid medium. The port
207 can be connected to a fluid medium with a constant pressure to
maintain a constant pressure in the first pressure region 206.
Also, the port 207 can be connected to a second fluid medium and
the device can measure the pressure difference between the first
fluid medium that is introduced into the second pressure region 204
and the second fluid medium in the first pressure region 206. FIG.
11 schematically shows that the device 100, 200, or 300 is
connected to two fluid mediums to measure the pressure difference
between the two mediums having pressures P1 and P2. The first
pressure region and the second pressure region each may have more
than one ports for connecting with pressurized mediums.
[0049] In one embodiment, the electrode assembly 214 includes a
stiffening element 140 as shown in FIG. 7. The stiffening element
140 prevents flexure of the overall electrode assembly, which in
turn maintains the capacitor plate 116 within its nominal plane
142. The stability of capacitor 130 of FIG. 1, formed in part by
the variable gap 126, is dependant upon the capacitor plate 116
being substantially planar. Flexure of the plate 116 due to
temperature variations or other environmental forces (such as
vibration and shock) may corrupt the measured value of the
characteristic capacitance of the capacitor 130. Any corruption of
the characteristic capacitance translates directly to a corruption
of the resonant frequency .omega..sub.0 of the tank circuit 132 and
thus to a corruption of the measurement of the differential
pressure 124. The stiffening element 140 may include ceramics or
other materials that are known to exhibit small amounts of
expansion or contraction with respect to ambient temperature
variations.
[0050] In another embodiment of the invention, the inductor coil
120 of FIG. 1 may include a multi-layer inductive coil. The coil
150 shown in FIG. 8 includes two layers of electrical conductor
electrically coupled in series via a plated through-hole 152,
although alternate embodiments may include any number of layers.
The two layers of electrical conductor are bonded to opposite sides
of an insulating layer 154, similar to the construction of a
multi-layered printed circuit board. One utility of a multiple
layer inductive coil 150 is a higher characteristic inductance
value due to the increase in the length of the conductor. Another
utility of the multiple layer inductive coil 150 is the ability to
compensate a variation of the coil's characteristic inductance with
respect to temperature variations. It is well known to those in the
art that as a planar spiral coil 150 expands in its spiraling plane
and the distance d1 between adjacent turns of a single coil
increases, the characteristic inductance L of the coil increases
(see FIG. 9). It is also well known that as the distance d2 between
two coils increases, the characteristic inductance L of the coils
decreases. An expansion of the insulating layer due to a
temperature change results in a corresponding increase in both d1
and d2. By choosing the appropriate initial dimensions d1 and d2,
and by choosing a material for the insulating layer 154 having an
appropriate expansion coefficient (with respect to temperature),
the changes in characteristic inductance of the coil 150 due to the
changes in d1 and d2 can be made to cancel.
[0051] In yet another form of the invention, as shown in FIG. 10,
the capacitor 330 portion of the electrode assembly 314 is located
within the housing 310, formed by upper portion 310a and lower
portion 310b, while the insulator 318 and the inductor 320 portions
are disposed outside of the housing 310. An electrically conductive
post 312 extends through the upper portion 310a of the housing 310,
and is secured in place by a non-conductive sleeve 322. This sleeve
322 electrically isolates the conductive post from the housing 310.
Electrical access to the resonator formed by the inductor 320 and
the capacitor 330 is gained via a first terminal 329 and a second
terminal 331. The first terminal 329 is electrically coupled to the
diaphragm 302 at the perimeter 308. The second terminal 331 is
electrically coupled to a first end of the inductor 320. The second
end of the inductor 320 is electrically coupled to the conductive
post 312, through which to the capacitive plate 316. Thus, the
conductive post serves not only to support the capacitive plate 316
and the inductor 320, but also to electrically couple the inductor
320 to the capacitor 330. The second pressure region 304 includes a
port 305 for receiving fluid medium whose pressure is to be
measured. Similar to previously described embodiments, the first
pressure region 306 can be a sealed chamber, as shown in FIG. 10,
and also can be an opened chamber having a port 307, as shown in
FIG. 10A.
[0052] The present invention may be used as different types of
pressure sensors, including "gauge" pressure sensor, "vacuum gauge"
pressure sensor, "compound gauge" pressure sensor, "scaled gauge"
pressure sensor, "absolute" pressure sensor, and "vacuum" pressure
sensor.
[0053] A "gauge" pressure sensor is defined when the pressure P1 in
the first pressure region is allowed to equal atmospheric pressure
by virtue of having the first pressure region vented to the
atmosphere and the measured pressure P2 in the second pressure
region is typically greater than atmospheric pressure.
[0054] A "vacuum gauge" pressure sensor is defined when the
pressure P1 in the first pressure region is allowed to equal
atmospheric pressure by virtue of having the first pressure region
vented to atmosphere and the applied pressure P2 in the second
pressure region is typically lower than atmospheric pressure.
[0055] A "compound gauge" pressure sensor is defined when the
pressure P1 in the first pressure region is allowed to equal
atmospheric pressure by virtue of having the first pressure region
vented to atmosphere and the applied pressure P2 is allowed to be
either higher or lower than atmospheric pressure.
[0056] A "sealed gauge" pressure sensor is defined when the
pressure P1 contains a sealed finite reference pressure where the
reference pressure may be subject to change in response to changes
in temperature if the sealed media approximates an ideal gas.
[0057] An "absolute" pressure sensor is defined when the first
pressure region is substantially evacuated and sealed, causing
pressure P1 in the first pressure region to approach 0 psia.
[0058] A "vacuum" pressure sensor is fabricated substantially the
same way as an "absolute" pressure sensor, but the full-scale
pressure ranges are typically lower than atmospheric pressure.
[0059] The invention may be embodied in other specific forms
without departing from the spirit or essential characteristics
thereof. The present embodiments are therefore to be considered in
respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims rather than by the
foregoing description, and all changes which come within the
meaning and range of the equivalency of the claims are therefore
intended to be embraced therein.
* * * * *