CA1036832A - Pressure sensitive capacitance sensing element - Google Patents
Pressure sensitive capacitance sensing elementInfo
- Publication number
- CA1036832A CA1036832A CA196,605A CA196605A CA1036832A CA 1036832 A CA1036832 A CA 1036832A CA 196605 A CA196605 A CA 196605A CA 1036832 A CA1036832 A CA 1036832A
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- CA
- Canada
- Prior art keywords
- core
- sensing element
- electrodes
- capacitance sensing
- dielectric
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
- G01L9/0041—Transmitting or indicating the displacement of flexible diaphragms
- G01L9/0072—Transmitting or indicating the displacement of flexible diaphragms using variations in capacitance
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Measuring Fluid Pressure (AREA)
Abstract
Abstract of the Disclosure A diaphragm element includes thin outer conducting layers integrally bonded to the opposite sides of a core layer of silicone or other dielectric which is a resilient, flexible material, to form a single, integral, resilient, flexible structure.
The conducting layers are a matrix of the material of the dielectric core layer with carbon or like conductive particles embedded therein. One conducting layer is spaced peripherally from the edge of the core layer to thereby pre-vent electrical contact between the two conducting layers.
The total unit is rigidly mounted about the peripheral edge with metallic contacts connected to the conducting layers as by a silver conducting paint applied to the outer surface of the layers. The element will thus flex to the side of the lower pressure with a change in the effective conducting area of the opposite conducting layers and a simultaneous decrease in the thickness of the dielectric inner core.
Consequently, the capacitance of the unit varies as a func-tion of the deflection and thereby in accordance with the pressure differential applied across the diaphragm element.
The conducting layers are a matrix of the material of the dielectric core layer with carbon or like conductive particles embedded therein. One conducting layer is spaced peripherally from the edge of the core layer to thereby pre-vent electrical contact between the two conducting layers.
The total unit is rigidly mounted about the peripheral edge with metallic contacts connected to the conducting layers as by a silver conducting paint applied to the outer surface of the layers. The element will thus flex to the side of the lower pressure with a change in the effective conducting area of the opposite conducting layers and a simultaneous decrease in the thickness of the dielectric inner core.
Consequently, the capacitance of the unit varies as a func-tion of the deflection and thereby in accordance with the pressure differential applied across the diaphragm element.
Description
~0:~68:~2 Back~round of the Invention This invention relates to a novel pressure sensitive capacitance sensing'element and particularly 'to such an ele-ment for sensing a fluid pressure condition.
5In many control andtor monitoring systems pressure contitions are detected by transducing to an electrical output.
Generally a pressure to electr~cal signal transtucer employs a mech~nical movement initiated by the pressure contition, with the movement effecting a corresponding change in an electrical output unit as a result ofthe change in a resistance, inductance, capacitance or other similar electrical characteristic.
A capacitance type pressure to el'ectrical signal trans-ducer is a very common form of a transducer employed in modern technology. Generaily, they take either one of two forms. In one form, a fixed or stationary electrode forms a base struc-ture for the sensing unit. A dielectric material, which may be a fluid, is supported by the stationary electrote. A
conductive outer plate is movably mounted abutting the di-' electric material and exposed to the pressure contition to cause flexing of the dielectric material with a corréspondingchange in the capacitance characteristic. In an alternative construction a conductive diaphragm is movably mounted between a pair of stationary electrode plates for movement parallel to the plates and coupled to the pressure source for corres-ponding positioning between the two fixed plates to therebyvary the capacitance. A fuller explanation of the broad field of pressure to electrical signal transduction and the various unit constructions including capacitance type is found in the Handbook of Transducers for Electronic Measuring Systems by Harry N. Norton which was published by Prentice-Hall, Inc. in 1969.
. ~036B3Z
Capacitive'sensing units are desirable because of the stability of their characteristic as long as the inte-grity of the sensing elements are maintained.
Although satisfactory pressure sensitive capacitance transducers are available, the electrode plates are relative-ly fragile members and present some difficulty in connection with production, particularly in a mass production process.
Further, a trapped dielectric fluid is often employed be-tween the plates. As a result of the relative moving of an element in the un~t, the fluid is sub~ect to exposure to the environment and contamination by environmental borne dust and the'like. This may change the characteristic of the sensor and may, in fact, result in actual shorting of the electrode plates. Further, the fluid d,ielectric medium may be lost, at least in part, resulting in a distinct change in the charac-teristic of the sensor.
SummarY of' the'P'r'es'eh't''Ihv'en'tion The pre8ent invention is partlcularly directed to a pressure sensitive capacitance sensing unit which is readily adapted to production on a commercial basis and which provides long life with high stability in use. Generally, in accord-ance with the present invention the capacitance sensing unit includes an inner core and outer electrode plates having an elastic portion which is subject to elastic deformation as a result of a pressure or force differential impressed across the unit and with the capacitance directly related to the deformation. More particularly, the capacitor sensing unit includes an inner elastic diaphragm core formed of a dielec-tric material with opposite conducting elastic layers firmly connected to the outer opposed surfaces of the dielectric ~ 03683Z
diaphragm core. The Unit i8 provided with laterally spaced mounting means to create a diaphragm element therebetween which will flex or bulge in accordance with a pressure tifferential which is applied across the diaphragm element between the mounting means. The element will thus flex to the site of lower pressure with a change in the effective conducting area o~ the opposite conducting layers and a simultaneous decrease in the thickness of the dielectric inner core. In accordance with known phenomena, the capaci-tance of a capacitor i8 directly proportional to the conduct-~ng plate area and inversely proportional to the thickness.
Consequently, the capacitance of the unit will vary as a function of the deflection and thereby in accordance with t~e pressure differential applied across the diaphragm ele-ment. At zero pressure, the sensing element is in an un-~tressed state and the capacitance i9 dependent upon the bafiic area of the conductive layer and the thickness of the tielectric core as well as the permittivity of the dielectric ~ ~ core. The sensitivity in turn is dependent on the relative size of the element and the composite elastic dulus of the total diaphragm element which, in turn, is a composite of the elastic modulus of the inner core and the outer two conducting layers.
The diaphragm element of the present invention prefer-ably includes the integral attachment of thin outer conduct~ng layers to the opposite sides of a sheet of the dielectric core material. Thus the conducting layers are preferably deposited on the core and integrally bonded throughout the mating faces to form a single integral flexible structure. The thickness of the conducting layers as well as the elastic modulus of the conducting layers for optimum results should be less than or equal to the corresponding characteristic of the core layer.
The'conducting layers may advantageously be formed of a matrix which is similar to the dielectric core layer or another material which can be integrally bonded thereto. Dis-po~ed within the conducting layers are suitable electrically contuctive particles' such as carbon ant the like, with the amount of the particles selected to provide the desired conduc-tivity without significantly interfering with the elastic char-lO- acteristics, stability, and response to pressure change of the total unit.
Thé core dielectric material as weIl as the outer flexible conductors should have excellent resistance against 'plastic deformation such'thit the element may provide reliable ant repeatable response to pressure changes. Further, one of the conducting layers is spaced peripherally from the edge of the core layer to the~eby prevent electrical contact between the two con-tucting layers. The total unit is rigidly mounted about the peripheral edge of the element. The rigid mounting for the units 2Q' preferably includes metallic contacts cGnnected to the conducting layers by a silver conducting paint applied to the outer surface of the layers. Suitable lead wires are connected to the contacts and connected in a circuit such that the capacitance characteristic ; of the'element provides' a change in output or a detectible output with a change'in pressure.
In accordance with a particular feature of the present invention, the inner core is formed of a silicone elastomer which has a dielectric constant of between l.l to ~5 and preferably approximately 3. The elastic modulus may also vary 30' significantly for example between 1 to l,OOO,OOO psi but optimum ~036~3Z
results have been found em~loying materials w~th an elastic modulus of 50 to 1,000 psi. The core layer thickness can also vary widely with a range of from 0.0001 ~nch to 0.1000 inch.
S Applicants have found that the'flexi~le core with the bondet outer flexible electrodes' produces a highly sensi-tive'1exible membrane or tiaphragm construction which can be readily protuced. Further, an integrally bonded construction es8entially prevents any danger of air pocket developments or 10' the entry of dust and other forei'gn material into the dielec-trlc portion of the unit and thereby not only prevents the possibility of shorting the capacitance plates but prevents relative changes in the'characteristic in the capacitor's dielectric and therefore'in the'capacitance characteristic.
, Br'i'e'f De's'c'riP't'i'on''of''t~e'~'rawing The'trawing furnished herewith illustrates the best mode'pre~ently contemplated by the inventors for carrying out the sub~ect inYention in whfch'thé abo~e'advantages and fea-tures are'clearly disclosed as well as others which will be readily understood from the description of such'illustrated embodiment.
In the drawing:-Fig. l is a plan view of a pressure sensitive capa-citance sensing element constructed in accordance with the present invention;
Fig. 2,is a vertical section taken generally on line 2-2 of Fig. l;
Fig. 3 is an enlarged fragmentary section taken generally on line'3-3 of Fig. 1 illustrating a highly satis-factory construction of the elements forming the flexible ~0~U6~32 'portion of the sensing element;
-Fig. 4 is an illustration of the'sensing element's response to a pressure'differential when connected in a suit-able'eIectric output circuit;
Fig. 5 is a view similar to Fig. 1 illustrating an alternative'embodiment of the'invention; and Fig. 6 is a fragmentary section taken generally on line 6-6 of Flg. 5.
DescriPti'on'o'f Il'lustrated Embodiment 10' Referring to the'drawing and particularly to Figs.
1 and 2, i pressure sensitive'capacitance sensing unit con-structed in accordance with'the present ~nvent~on is shown including an outer annular pair of supporting rings 1 and 2 with a flexible preasure'sens'itive'diaphragm 3 rigidly clamped therebetween. The rings 1 and 2 may be'interconnected by a plurality of equicircumferentially distributed clamping screws 4 which,' when drawn up, form a firm integrated structure and in particuiar rigidly support the flexible diaphragm 3 com-pletely about the periphery thereof in the illustrated embodi-2Q ment.
The diaphragm eIement thus defines a pair of distinctopposite sides or surfaces, one of which is sub~ected to a first or reference pressure 5 and the opposite side of which is connected to a second pressure 6 to be sensed, as dia-grammatically illustrated in Fig. 2.
According to the present invention, diaphragm 3 isespecially constructed as a flexible assembly which includes a central core 7 shown as a sheet-like member of a dielectric material with flexible'electrode layers 8 and 9 firmly attached in abutting relation to the'opposite'faces' of the'core 7, In the embodiment of Figs. 1 - 4 the'three'layer diaphragm proJects laterally with'all three'layers be~ng firmly held by the'rings l and 2. ' The core'7 is formed of a dieIectric material and the electrodes 8 and 9 are'formed-of a conductive material and all three have'an elastic modulus to permit the elastic movement from the'rigid mounting position.
m e'flexible diaphragm is such, therefore, that a differential pressure arising as a result of a difference 10' ' in the signal pressure'6 and the reference pressure 5 re-sults in a corresponding movement of the'diaphragm 3. As a result of the rigîd holding of the'periphery of the'three layers and a firm interconnection between the core 7 and the conductive layers 8 and 9, the'unit bulges and flexes in accordance with the differential pressure, as shown in phantom at lO'in Fig. 2. The'area of a curved surface is greater than the pro~ected flat area ant the'movement of the diaphragm there-fore effectively increases the'area of the'electrode layers 8 ' and 9. This increase in differential pressure'also results in 20' a thinning of the multilayer dlaphragm, including its core 7 which is similarly flexed.simultaneously reducing the core thick-ness and therefore reducing the'spacing between the electrode - plates.
The deflection of diaphragm 3 to the phantom line position clearly indicates the increased area of layers 8 and 9 and decreased thickness of the'dielectric material 7. Both the increased area of the'conducting layers 8 and 9 and the reduced thickness of the core'7 correspondingly function to vary the capacitance in the same'direction and thereby have 30' been found to provide'a hi'ghly sensitive'capacitance sensing l0~3æelement producing a reliable transduction o pressure to electrical signals.
The'capacitance of a capacitor is basically given - by the'equation: C = e x A ' t where'---S C ~ the'capacitance e - the permittivity of the dielectric core A ~ the'area of the capacitor plates or electrodes t ~ the'thickness of the dielectric between the'capacitor plates.
Consequently~ the capacitance wili increase with the bulging vement of the'diaphragm 3, as -a result of the in-creased area A and also as a result of the'decreased thick-ness t and will correspondingly decrease as the capacitance diaphragm moves' back to thé'full-line,' balanced position.
Thé'outer electrodes 8 and 9 are for optimu~ results integrally bonded throughout their interfaces lla to the ad~acent core layer 7 such that the element ves as a single ~ntegrated element. The top electrode 8 is shown formed of a smaller diameter with'the'periphery within the line of bolts 4. m is prevents short~ng of the'plates. Any other suitable means can, of course, be used.
The dielectric core layer 7 is, of course, electrical-ly insulating and formed of an elastic material which is ad-vantageously relatively non-porous to the fluid medium to the opposite sides of the diaphragm. A silicone elastomer having a dielectrlc constant of about 3, an elastic modulus of about 500 psi and a resistivity of the order of 1 x 1ol5 microhm cm has been found to be'a satisfactory material for the dielectric core. Applicants have found that the dielectric constant of 30' the'core'can readily vary from a dielectric constant of 1~1 to 25.
~ 036832 S~milarly, the elastic modulus of the core layer can readily vary between 1 psi and 1,000,000 psi with more typical values ranging from 50 psi to 1,000 psi. A typical resistivity of the insulating core is about 1 x lol5 microhm cm. The thick-ness of core layer 7 may readily vary from 0.0001 inch to 0.1000 inch.' In practical applications, Applicants ha~e found a range of 0.0003 inch to 0.0200 lnch to provide particularly useful results.
Although the conducting layers or electrodes may be formed of any suitable material, they are advantageously formed as a matrix with a base which'is similar to the tielectric core layer material such as silicone elastomer.
Highly electrically conductive'particles 11 such as graphi-tized carbon and deoxidized carbon are dispersed through the conducting electrodes such'that they define conductive plates of an elastic nature.
The conducting electrodes 8 and 9 may be constructed in any ~uitable manner, for example,' by dispersing of graphi-tized carbon through a silicone elastomer, as moré fully dis-20' closed in U.S. Patent 3,582,728. The one electrode layer 8 or9 can readily be cast as a film on a clean glass plate. The dielectric core 7 is thereafter applied on the film and inter-connected to the electrode by cross linking. ~ubsequently, the outer electrode is cast as an appropriate film over the core layer and interconnected by cross linking.
Optimum results are obtained by maintaining the elastic modulus of the'conducting layers 8 and 9 at values less than or equal to that of the core layer 7 and similarly of a thIckness which'is less than or equal to that of the core 30' layer.
_q_ iO36WZ
Further, the diaphragm 3 should, of course, be formed of a material which also has excellent resistance to plastic deformation such that the element returns to the balanced pressure position to maintain accurate and repeatable outputs with varying pressure input. If it does not, the unit will, of course, plastically deform with pressure changes and result ln a variation in a subsequent response of the element to the ~me pressure change.
As the capacitor unit of the invention is particu-10' larly adapted to a fluid pressure medium, the conductinglayers and the core'layer are'formed of a suitable material which is relatively non-porous to the fluit medium. If the layers are porous, the pressure would, of course, be trans-mitted through the unit and corres'pondingly reduce'the sensi-tivlty of the unit to pressure'changes.
In the illustrated Pmhodiment of the invention, thebottom conducting layer 9 is coextensive with the dielectric core la~er 7. The opposite or top conducting layer 8 i8 spaced from the peripheral edge of the dielectric core layer 20' 7. This ensures effective'spacing between the two electrodes 8 and 9 and minimizes creation of electrical contact there-between with the resulting shorting o the electrode plates.
Circuit connections are made to the conducting elec-trodes in any suitable manner. As shown in Figs. 1 and 3, the conducting layers 8 and 9 within the clamping rings 1 and 2 may be provided with a spot coating of a silver conducting paint 12 and 12a to provide connection to the conductive particles 11 within the layers 8 and 9. Metallic contacts 13 and 14 within the rigid mounting elements 1 and 2 are aligned with and make contact with the silver conducting paint 12 and 12a. Lead l0~6~æ
wires 15 and 16, in turn, are connected to the metallic con-tacts and provide connection of the capacitance sensing ele-ment in any suitable system. The pressure sensing element, for example, may be connected into a leg of an alternating current bridge circuit, not shown, which is energized from suitable alternating current source. The balanced condi-tion, and therefore, the output, o~ the bridge is responsive to the capacitance of the capacitance element. The output Is such that it can be readily detected and amplified by a suitable amplifier or the like to energize a suitable detec-tion or control load.
The characteristic of a unit such as shown in Figs.
1 and 2 is shown ~n Fig. 4. The characteristic of Fig. 4 is illustrative of a unit having an active conducting area in the balanced position as shown in Fig. 2 of 0.60 square lnches. The dielectric core 7 was 0.004 inch thick and the outer conducting layers 8 and 9 were of a corresponding thickness, and were formed as sheet-like layers bonded to the core as pre~iously described with reference t~ Patent No. 3,582,728.
The core layer was a silicone elaætomer with a dielectric constant of 3Ø The conducting layers were - similarly a silicone elastomer matrix with a conductive carbon particle uniformly dispersed therein. The test was run with atmosphere as a reference pressure and with air pressure supplied to the opposite side of the capacitance element. The circuit was energized from an alternating cur-rent source of a thousand cycles per second and readings taken at increments of 1 inch of water. As shown, the capacitance changes essentially in accordance with a straight ' 103683Z
line function between approximately 328 picofarads at zero sig~al pressure to 365 picofarads at fifteen inches of water. The sensitivity of the unit correspon~s essentially to the slope of the illustrated curve and was essentially
5In many control andtor monitoring systems pressure contitions are detected by transducing to an electrical output.
Generally a pressure to electr~cal signal transtucer employs a mech~nical movement initiated by the pressure contition, with the movement effecting a corresponding change in an electrical output unit as a result ofthe change in a resistance, inductance, capacitance or other similar electrical characteristic.
A capacitance type pressure to el'ectrical signal trans-ducer is a very common form of a transducer employed in modern technology. Generaily, they take either one of two forms. In one form, a fixed or stationary electrode forms a base struc-ture for the sensing unit. A dielectric material, which may be a fluid, is supported by the stationary electrote. A
conductive outer plate is movably mounted abutting the di-' electric material and exposed to the pressure contition to cause flexing of the dielectric material with a corréspondingchange in the capacitance characteristic. In an alternative construction a conductive diaphragm is movably mounted between a pair of stationary electrode plates for movement parallel to the plates and coupled to the pressure source for corres-ponding positioning between the two fixed plates to therebyvary the capacitance. A fuller explanation of the broad field of pressure to electrical signal transduction and the various unit constructions including capacitance type is found in the Handbook of Transducers for Electronic Measuring Systems by Harry N. Norton which was published by Prentice-Hall, Inc. in 1969.
. ~036B3Z
Capacitive'sensing units are desirable because of the stability of their characteristic as long as the inte-grity of the sensing elements are maintained.
Although satisfactory pressure sensitive capacitance transducers are available, the electrode plates are relative-ly fragile members and present some difficulty in connection with production, particularly in a mass production process.
Further, a trapped dielectric fluid is often employed be-tween the plates. As a result of the relative moving of an element in the un~t, the fluid is sub~ect to exposure to the environment and contamination by environmental borne dust and the'like. This may change the characteristic of the sensor and may, in fact, result in actual shorting of the electrode plates. Further, the fluid d,ielectric medium may be lost, at least in part, resulting in a distinct change in the charac-teristic of the sensor.
SummarY of' the'P'r'es'eh't''Ihv'en'tion The pre8ent invention is partlcularly directed to a pressure sensitive capacitance sensing unit which is readily adapted to production on a commercial basis and which provides long life with high stability in use. Generally, in accord-ance with the present invention the capacitance sensing unit includes an inner core and outer electrode plates having an elastic portion which is subject to elastic deformation as a result of a pressure or force differential impressed across the unit and with the capacitance directly related to the deformation. More particularly, the capacitor sensing unit includes an inner elastic diaphragm core formed of a dielec-tric material with opposite conducting elastic layers firmly connected to the outer opposed surfaces of the dielectric ~ 03683Z
diaphragm core. The Unit i8 provided with laterally spaced mounting means to create a diaphragm element therebetween which will flex or bulge in accordance with a pressure tifferential which is applied across the diaphragm element between the mounting means. The element will thus flex to the site of lower pressure with a change in the effective conducting area o~ the opposite conducting layers and a simultaneous decrease in the thickness of the dielectric inner core. In accordance with known phenomena, the capaci-tance of a capacitor i8 directly proportional to the conduct-~ng plate area and inversely proportional to the thickness.
Consequently, the capacitance of the unit will vary as a function of the deflection and thereby in accordance with t~e pressure differential applied across the diaphragm ele-ment. At zero pressure, the sensing element is in an un-~tressed state and the capacitance i9 dependent upon the bafiic area of the conductive layer and the thickness of the tielectric core as well as the permittivity of the dielectric ~ ~ core. The sensitivity in turn is dependent on the relative size of the element and the composite elastic dulus of the total diaphragm element which, in turn, is a composite of the elastic modulus of the inner core and the outer two conducting layers.
The diaphragm element of the present invention prefer-ably includes the integral attachment of thin outer conduct~ng layers to the opposite sides of a sheet of the dielectric core material. Thus the conducting layers are preferably deposited on the core and integrally bonded throughout the mating faces to form a single integral flexible structure. The thickness of the conducting layers as well as the elastic modulus of the conducting layers for optimum results should be less than or equal to the corresponding characteristic of the core layer.
The'conducting layers may advantageously be formed of a matrix which is similar to the dielectric core layer or another material which can be integrally bonded thereto. Dis-po~ed within the conducting layers are suitable electrically contuctive particles' such as carbon ant the like, with the amount of the particles selected to provide the desired conduc-tivity without significantly interfering with the elastic char-lO- acteristics, stability, and response to pressure change of the total unit.
Thé core dielectric material as weIl as the outer flexible conductors should have excellent resistance against 'plastic deformation such'thit the element may provide reliable ant repeatable response to pressure changes. Further, one of the conducting layers is spaced peripherally from the edge of the core layer to the~eby prevent electrical contact between the two con-tucting layers. The total unit is rigidly mounted about the peripheral edge of the element. The rigid mounting for the units 2Q' preferably includes metallic contacts cGnnected to the conducting layers by a silver conducting paint applied to the outer surface of the layers. Suitable lead wires are connected to the contacts and connected in a circuit such that the capacitance characteristic ; of the'element provides' a change in output or a detectible output with a change'in pressure.
In accordance with a particular feature of the present invention, the inner core is formed of a silicone elastomer which has a dielectric constant of between l.l to ~5 and preferably approximately 3. The elastic modulus may also vary 30' significantly for example between 1 to l,OOO,OOO psi but optimum ~036~3Z
results have been found em~loying materials w~th an elastic modulus of 50 to 1,000 psi. The core layer thickness can also vary widely with a range of from 0.0001 ~nch to 0.1000 inch.
S Applicants have found that the'flexi~le core with the bondet outer flexible electrodes' produces a highly sensi-tive'1exible membrane or tiaphragm construction which can be readily protuced. Further, an integrally bonded construction es8entially prevents any danger of air pocket developments or 10' the entry of dust and other forei'gn material into the dielec-trlc portion of the unit and thereby not only prevents the possibility of shorting the capacitance plates but prevents relative changes in the'characteristic in the capacitor's dielectric and therefore'in the'capacitance characteristic.
, Br'i'e'f De's'c'riP't'i'on''of''t~e'~'rawing The'trawing furnished herewith illustrates the best mode'pre~ently contemplated by the inventors for carrying out the sub~ect inYention in whfch'thé abo~e'advantages and fea-tures are'clearly disclosed as well as others which will be readily understood from the description of such'illustrated embodiment.
In the drawing:-Fig. l is a plan view of a pressure sensitive capa-citance sensing element constructed in accordance with the present invention;
Fig. 2,is a vertical section taken generally on line 2-2 of Fig. l;
Fig. 3 is an enlarged fragmentary section taken generally on line'3-3 of Fig. 1 illustrating a highly satis-factory construction of the elements forming the flexible ~0~U6~32 'portion of the sensing element;
-Fig. 4 is an illustration of the'sensing element's response to a pressure'differential when connected in a suit-able'eIectric output circuit;
Fig. 5 is a view similar to Fig. 1 illustrating an alternative'embodiment of the'invention; and Fig. 6 is a fragmentary section taken generally on line 6-6 of Flg. 5.
DescriPti'on'o'f Il'lustrated Embodiment 10' Referring to the'drawing and particularly to Figs.
1 and 2, i pressure sensitive'capacitance sensing unit con-structed in accordance with'the present ~nvent~on is shown including an outer annular pair of supporting rings 1 and 2 with a flexible preasure'sens'itive'diaphragm 3 rigidly clamped therebetween. The rings 1 and 2 may be'interconnected by a plurality of equicircumferentially distributed clamping screws 4 which,' when drawn up, form a firm integrated structure and in particuiar rigidly support the flexible diaphragm 3 com-pletely about the periphery thereof in the illustrated embodi-2Q ment.
The diaphragm eIement thus defines a pair of distinctopposite sides or surfaces, one of which is sub~ected to a first or reference pressure 5 and the opposite side of which is connected to a second pressure 6 to be sensed, as dia-grammatically illustrated in Fig. 2.
According to the present invention, diaphragm 3 isespecially constructed as a flexible assembly which includes a central core 7 shown as a sheet-like member of a dielectric material with flexible'electrode layers 8 and 9 firmly attached in abutting relation to the'opposite'faces' of the'core 7, In the embodiment of Figs. 1 - 4 the'three'layer diaphragm proJects laterally with'all three'layers be~ng firmly held by the'rings l and 2. ' The core'7 is formed of a dieIectric material and the electrodes 8 and 9 are'formed-of a conductive material and all three have'an elastic modulus to permit the elastic movement from the'rigid mounting position.
m e'flexible diaphragm is such, therefore, that a differential pressure arising as a result of a difference 10' ' in the signal pressure'6 and the reference pressure 5 re-sults in a corresponding movement of the'diaphragm 3. As a result of the rigîd holding of the'periphery of the'three layers and a firm interconnection between the core 7 and the conductive layers 8 and 9, the'unit bulges and flexes in accordance with the differential pressure, as shown in phantom at lO'in Fig. 2. The'area of a curved surface is greater than the pro~ected flat area ant the'movement of the diaphragm there-fore effectively increases the'area of the'electrode layers 8 ' and 9. This increase in differential pressure'also results in 20' a thinning of the multilayer dlaphragm, including its core 7 which is similarly flexed.simultaneously reducing the core thick-ness and therefore reducing the'spacing between the electrode - plates.
The deflection of diaphragm 3 to the phantom line position clearly indicates the increased area of layers 8 and 9 and decreased thickness of the'dielectric material 7. Both the increased area of the'conducting layers 8 and 9 and the reduced thickness of the core'7 correspondingly function to vary the capacitance in the same'direction and thereby have 30' been found to provide'a hi'ghly sensitive'capacitance sensing l0~3æelement producing a reliable transduction o pressure to electrical signals.
The'capacitance of a capacitor is basically given - by the'equation: C = e x A ' t where'---S C ~ the'capacitance e - the permittivity of the dielectric core A ~ the'area of the capacitor plates or electrodes t ~ the'thickness of the dielectric between the'capacitor plates.
Consequently~ the capacitance wili increase with the bulging vement of the'diaphragm 3, as -a result of the in-creased area A and also as a result of the'decreased thick-ness t and will correspondingly decrease as the capacitance diaphragm moves' back to thé'full-line,' balanced position.
Thé'outer electrodes 8 and 9 are for optimu~ results integrally bonded throughout their interfaces lla to the ad~acent core layer 7 such that the element ves as a single ~ntegrated element. The top electrode 8 is shown formed of a smaller diameter with'the'periphery within the line of bolts 4. m is prevents short~ng of the'plates. Any other suitable means can, of course, be used.
The dielectric core layer 7 is, of course, electrical-ly insulating and formed of an elastic material which is ad-vantageously relatively non-porous to the fluid medium to the opposite sides of the diaphragm. A silicone elastomer having a dielectrlc constant of about 3, an elastic modulus of about 500 psi and a resistivity of the order of 1 x 1ol5 microhm cm has been found to be'a satisfactory material for the dielectric core. Applicants have found that the dielectric constant of 30' the'core'can readily vary from a dielectric constant of 1~1 to 25.
~ 036832 S~milarly, the elastic modulus of the core layer can readily vary between 1 psi and 1,000,000 psi with more typical values ranging from 50 psi to 1,000 psi. A typical resistivity of the insulating core is about 1 x lol5 microhm cm. The thick-ness of core layer 7 may readily vary from 0.0001 inch to 0.1000 inch.' In practical applications, Applicants ha~e found a range of 0.0003 inch to 0.0200 lnch to provide particularly useful results.
Although the conducting layers or electrodes may be formed of any suitable material, they are advantageously formed as a matrix with a base which'is similar to the tielectric core layer material such as silicone elastomer.
Highly electrically conductive'particles 11 such as graphi-tized carbon and deoxidized carbon are dispersed through the conducting electrodes such'that they define conductive plates of an elastic nature.
The conducting electrodes 8 and 9 may be constructed in any ~uitable manner, for example,' by dispersing of graphi-tized carbon through a silicone elastomer, as moré fully dis-20' closed in U.S. Patent 3,582,728. The one electrode layer 8 or9 can readily be cast as a film on a clean glass plate. The dielectric core 7 is thereafter applied on the film and inter-connected to the electrode by cross linking. ~ubsequently, the outer electrode is cast as an appropriate film over the core layer and interconnected by cross linking.
Optimum results are obtained by maintaining the elastic modulus of the'conducting layers 8 and 9 at values less than or equal to that of the core layer 7 and similarly of a thIckness which'is less than or equal to that of the core 30' layer.
_q_ iO36WZ
Further, the diaphragm 3 should, of course, be formed of a material which also has excellent resistance to plastic deformation such that the element returns to the balanced pressure position to maintain accurate and repeatable outputs with varying pressure input. If it does not, the unit will, of course, plastically deform with pressure changes and result ln a variation in a subsequent response of the element to the ~me pressure change.
As the capacitor unit of the invention is particu-10' larly adapted to a fluid pressure medium, the conductinglayers and the core'layer are'formed of a suitable material which is relatively non-porous to the fluit medium. If the layers are porous, the pressure would, of course, be trans-mitted through the unit and corres'pondingly reduce'the sensi-tivlty of the unit to pressure'changes.
In the illustrated Pmhodiment of the invention, thebottom conducting layer 9 is coextensive with the dielectric core la~er 7. The opposite or top conducting layer 8 i8 spaced from the peripheral edge of the dielectric core layer 20' 7. This ensures effective'spacing between the two electrodes 8 and 9 and minimizes creation of electrical contact there-between with the resulting shorting o the electrode plates.
Circuit connections are made to the conducting elec-trodes in any suitable manner. As shown in Figs. 1 and 3, the conducting layers 8 and 9 within the clamping rings 1 and 2 may be provided with a spot coating of a silver conducting paint 12 and 12a to provide connection to the conductive particles 11 within the layers 8 and 9. Metallic contacts 13 and 14 within the rigid mounting elements 1 and 2 are aligned with and make contact with the silver conducting paint 12 and 12a. Lead l0~6~æ
wires 15 and 16, in turn, are connected to the metallic con-tacts and provide connection of the capacitance sensing ele-ment in any suitable system. The pressure sensing element, for example, may be connected into a leg of an alternating current bridge circuit, not shown, which is energized from suitable alternating current source. The balanced condi-tion, and therefore, the output, o~ the bridge is responsive to the capacitance of the capacitance element. The output Is such that it can be readily detected and amplified by a suitable amplifier or the like to energize a suitable detec-tion or control load.
The characteristic of a unit such as shown in Figs.
1 and 2 is shown ~n Fig. 4. The characteristic of Fig. 4 is illustrative of a unit having an active conducting area in the balanced position as shown in Fig. 2 of 0.60 square lnches. The dielectric core 7 was 0.004 inch thick and the outer conducting layers 8 and 9 were of a corresponding thickness, and were formed as sheet-like layers bonded to the core as pre~iously described with reference t~ Patent No. 3,582,728.
The core layer was a silicone elaætomer with a dielectric constant of 3Ø The conducting layers were - similarly a silicone elastomer matrix with a conductive carbon particle uniformly dispersed therein. The test was run with atmosphere as a reference pressure and with air pressure supplied to the opposite side of the capacitance element. The circuit was energized from an alternating cur-rent source of a thousand cycles per second and readings taken at increments of 1 inch of water. As shown, the capacitance changes essentially in accordance with a straight ' 103683Z
line function between approximately 328 picofarads at zero sig~al pressure to 365 picofarads at fifteen inches of water. The sensitivity of the unit correspon~s essentially to the slope of the illustrated curve and was essentially
2.5 picofarads per inch of water.
An alternative embodiment is shown in Figs. S
ant 6 having three bonded elements,and elements corres-ponting to the elements of the first embodiment are similar-ly numbered for simplicity and clarity of explanation.
In the alternative embodiment the one electrode plate, shown as the top plate 17, is especially formed and applied to minimize the projection beneath the clamping ring l.
This minimizes the hysteresis of the element and further contributes to the stability and performance of the unit.
lS More particularly, in Figs. 5 and 6, the electrode plate 17 is a circular element of a slightly smaller dia-meter than the inner diameter of the ring 1. The plate 17 i8 bonded or otherwise suitably affixed to the core with a circumferential space or gap 18 extending essentially com-pletely about the unit. A small tab-like extension 19 is integrally formed on the edge of plate 17 and aligned with the contact 13,shown only in Fig. 3, of the ring 1 to pro-vide connection to the electrode.
The bonded plate construction is particularly sig-nificant in the practical construction and application ofthe capacitive sensor. Thus, the construction provides a simple mechanism which is relatively stable and relatively rugged. Thus, the outer carbon plates tend to be somewhat sensiti~e to tearing and would be subject to breakage. How-ever, the interconnected bonding to the interior dielectricprovides significant support to the plates.
10;~6B3Z
The capacitance sensing element of the present invention thus provides a relatively simply constructed mechanism which provides a long reliable life.with repeat-able outputs or output characteristics.
.
10 ' , ., ..
An alternative embodiment is shown in Figs. S
ant 6 having three bonded elements,and elements corres-ponting to the elements of the first embodiment are similar-ly numbered for simplicity and clarity of explanation.
In the alternative embodiment the one electrode plate, shown as the top plate 17, is especially formed and applied to minimize the projection beneath the clamping ring l.
This minimizes the hysteresis of the element and further contributes to the stability and performance of the unit.
lS More particularly, in Figs. 5 and 6, the electrode plate 17 is a circular element of a slightly smaller dia-meter than the inner diameter of the ring 1. The plate 17 i8 bonded or otherwise suitably affixed to the core with a circumferential space or gap 18 extending essentially com-pletely about the unit. A small tab-like extension 19 is integrally formed on the edge of plate 17 and aligned with the contact 13,shown only in Fig. 3, of the ring 1 to pro-vide connection to the electrode.
The bonded plate construction is particularly sig-nificant in the practical construction and application ofthe capacitive sensor. Thus, the construction provides a simple mechanism which is relatively stable and relatively rugged. Thus, the outer carbon plates tend to be somewhat sensiti~e to tearing and would be subject to breakage. How-ever, the interconnected bonding to the interior dielectricprovides significant support to the plates.
10;~6B3Z
The capacitance sensing element of the present invention thus provides a relatively simply constructed mechanism which provides a long reliable life.with repeat-able outputs or output characteristics.
.
10 ' , ., ..
Claims (24)
1. A pressure sensitive capacitance sensing element comprising a core layer formed of a dielectric material, a plurality of electrically conductive electrode means inter-connected with the opposite sides of said core layer to define capacitor plates and defining a capacitor, said core and elec-trodes including an elastic portion deforming in response to a differential pressure across said portion and with the defor-mation producing a change in the capacitance of the capacitor.
2. A pressure sensitive capacitance sensing ele-ment comprising a core formed of a dielectric material, a plurality of electrically conductive electrodes inter-connected with the opposite sides of said core layer to define capacitor plates and defining a capacitor unit, said capacitor unit being elastic and being fixedly mountable at spaced points of the capacitor unit with the unit elastically moving with respect to the said fixed mounting and correspondingly elastically deforming the capacitor unit with an increased conductive area of the electrodes with respect to said mounting and a decreased dielectric layer thickness to thereby provide a change in the capacitance of the element.
3. The sensing element of Claim 2 wherein said dielectric core and said electrically conductive electrodes are flexible solid elements and the electrodes are integrally bonded throughout the interface to the dielectric core.
4. The capacitance sensing element of Claim 2 wherein each of said electrodes include electrically con-ductive particles distributed throughout an electrically insulating material.
5. The capacitance sensing element of Claim 4 wherein said particles are graphitized carbon.
6. The capacitance sensing element of Claim 2 wherein each of said conductive electrodes are formed of the same material as said core and having electrically conductive particles distributed throughout the electrode.
7. The capacitance sensing element of Claim 2 wherein the one conducting layer is coextensive with the dielectric core and the opposite electrode is spaced from the periphery of the core, and having a peripheral mounting means extending over at least one of the electrodes to establish said fixed mounting of said spaced points.
8. The sensing element of Claim 7 wherein said opposite electrode is further essentially completely spaced from the mounting means.
9. The sensing element of Claim 8 wherein said opposite electrode has a small tab extending into the mounting means and defining a circuit connection means.
10. The capacitance sensing element of Claim 2 having a peripheral mounting means extending over and being attached to both of the electrodes and defining fixed mount-ing at spaced points.
11. The sensing element of Claim 2 wherein said di-electric core and said electrically conductive electrodes are integrally bonded throughout their interface, said core being
11. The sensing element of Claim 2 wherein said di-electric core and said electrically conductive electrodes are integrally bonded throughout their interface, said core being
Claim 11 continued....
formed of a silicone elastomer, each of said conductive elec-trodes being formed of a silicone elastomer and having elec-trically conductive particles distributed throughout the rubber to define a conductive plate.
formed of a silicone elastomer, each of said conductive elec-trodes being formed of a silicone elastomer and having elec-trically conductive particles distributed throughout the rubber to define a conductive plate.
12. The sensing element of Claim 11 wherein the individual electrode thickness is no greater than the thick-ness of the core.
13. The capacitance sensing element of Claim 2 wherein the core is a solid flexible element having a di-electric constant of essentially 1.1 to 25, an elastic modulus of essentially 1 to 1,000,000 pounds per square inch and a thickness of essentially 0.0001 inch to 0.1000 inch.
14. The capacitance sensing element of Claim 13 wherein the elastic modulus and the thickness of the electrodes are not greater than the elastic modulus and the thickness of the core.
15. The capacitance sensing element of Claim 13 wherein the core has a dielectric constant of three.
16. The capacitance sensing element of Claim 13 wherein the core and electrodes have an elastic modulus between 50 and 1,000 pounds per square inch.
17. The capacitance sensing element of Claim 13 wherein the core thickness is constant and between 0.0003 and 0.0200 inch thick.
18. A pressure sensitive capacitance sensing element comprising a sheet-like core formed of a flexible solid di-electric material, a plurality of electrically conductive and flexible solid electrodes interconnected with the opposite sides of said core to define a capacitor diaphragm, a fixed mounting means connected to said electrodes and rigidly supporting of spaced
18. A pressure sensitive capacitance sensing element comprising a sheet-like core formed of a flexible solid di-electric material, a plurality of electrically conductive and flexible solid electrodes interconnected with the opposite sides of said core to define a capacitor diaphragm, a fixed mounting means connected to said electrodes and rigidly supporting of spaced
Claim 18 continued....
portions of the diaphragm and with the diaphragm elastically moving with respect to the fixed mounting and correspondingly deforming the capacitor with an increased conductive area of the electrodes with respect to said mounting and a decreased dielectric layer thickness to thereby provide a change in the capacitance of the element in response to a differential fluid force applied to opposite sides of the diaphragm.
portions of the diaphragm and with the diaphragm elastically moving with respect to the fixed mounting and correspondingly deforming the capacitor with an increased conductive area of the electrodes with respect to said mounting and a decreased dielectric layer thickness to thereby provide a change in the capacitance of the element in response to a differential fluid force applied to opposite sides of the diaphragm.
19. The sensing element of Claim 18 wherein said dielectric core and said electrically conductive electrodes are integrally bonded throughout the interface to the dielec-tric core, and said mounting means extends completely about the periphery to correspondingly flexibly mount the diaphragm about the periphery.
20. The capacitance sensing element of Claim 19 wherein the thickness and the elastic modulus of each of salt conductive electrodes is no greater than the thickness and elastic modulus of said core.
21. The capacitance sensing element of Claim 19 wherein the diaphragm core and electrodes are formed of a material which is essentially free of plastic deformation within the normal differential pressure conditions.
22. The pressure sensitive capacitance sensing element of Claim 19 wherein the mounting-means includes a pair of ring-like members secured in clamping relationship to the peripheral edge portion of the diaphragm, wherein at least one ring-like member is of an insulating materialand contact means in each of said ring-like members firmly abutting the adjacent electrode.
23. The pressure sensitive capacitance sensing element of Claim 22 wherein one of said electrodes is smaller
23. The pressure sensitive capacitance sensing element of Claim 22 wherein one of said electrodes is smaller
Claim 23 continued....
in area than the other and wherein said ring-like member of an insulating material is positioned on the same side as said smaller electrode and is spaced therefrom except for a limited contact portion as said contact means.
in area than the other and wherein said ring-like member of an insulating material is positioned on the same side as said smaller electrode and is spaced therefrom except for a limited contact portion as said contact means.
24. The sensing element of Claim 22 wherein said electrodes include a conducting point on the surface in alignment with said contact means.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US00361813A US3814998A (en) | 1973-05-18 | 1973-05-18 | Pressure sensitive capacitance sensing element |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1036832A true CA1036832A (en) | 1978-08-22 |
Family
ID=23423544
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA196,605A Expired CA1036832A (en) | 1973-05-18 | 1974-04-02 | Pressure sensitive capacitance sensing element |
Country Status (2)
| Country | Link |
|---|---|
| US (1) | US3814998A (en) |
| CA (1) | CA1036832A (en) |
Families Citing this family (15)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3962921A (en) * | 1972-02-04 | 1976-06-15 | The Garrett Corporation | Compensated pressure transducer |
| US4158217A (en) * | 1976-12-02 | 1979-06-12 | Kaylico Corporation | Capacitive pressure transducer with improved electrode |
| FR2442438A1 (en) * | 1978-11-24 | 1980-06-20 | Vaisala Oy | Aneroid capsule pressure gauge - has central plate between corrugated membranes and distortion is measured by capacitance change |
| US4234361A (en) * | 1979-07-05 | 1980-11-18 | Wisconsin Alumni Research Foundation | Process for producing an electrostatically deformable thin silicon membranes utilizing a two-stage diffusion step to form an etchant resistant layer |
| US5090246A (en) * | 1990-09-19 | 1992-02-25 | Johnson Service Corp. | Elastomer type low pressure sensor |
| US5542300A (en) * | 1994-01-24 | 1996-08-06 | Setra Systems, Inc. | Low cost, center-mounted capacitive pressure sensor |
| JP5568206B2 (en) * | 2006-09-15 | 2014-08-06 | 東海ゴム工業株式会社 | Deformation sensor |
| US8079269B2 (en) * | 2007-05-16 | 2011-12-20 | Rosemount Inc. | Electrostatic pressure sensor with porous dielectric diaphragm |
| JP5496446B2 (en) * | 2007-07-12 | 2014-05-21 | 東海ゴム工業株式会社 | Capacitive sensor |
| FR2940904B1 (en) * | 2009-01-13 | 2012-08-31 | Urgo Laboratoires | INTERFACE PRESSURE MEASURING SYSTEM |
| US10420374B2 (en) | 2009-09-18 | 2019-09-24 | Altria Client Services Llc | Electronic smoke apparatus |
| ES2608458T5 (en) * | 2009-09-18 | 2022-04-04 | Altria Client Services Llc | Electronic cigarette |
| US8303897B2 (en) | 2011-02-28 | 2012-11-06 | Colla Jeannine O | Capacitive sensor for organic chemicals comprising an elastomer and high dielectric materials with titanate |
| WO2012151586A1 (en) * | 2011-05-05 | 2012-11-08 | Eksigent Technologies, Llc | Gel coupling for electrokinetic delivery systems |
| EP3339814B1 (en) * | 2016-12-21 | 2020-06-03 | Fimcim S.P.A. | Assembly installable in an air conditioning and/or heating system, air conditioning and/or heating system comprising the assembly and method of controlling the assembly |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR741189A (en) * | 1931-09-16 | 1933-02-08 | ||
| US3177967A (en) * | 1962-09-05 | 1965-04-13 | Louis A Wilson | Taxi control systems |
| GB1234083A (en) * | 1968-04-04 | 1971-06-03 | ||
| US3582728A (en) * | 1969-03-06 | 1971-06-01 | Johnson Service Co | Capacitance humidity sensing element |
-
1973
- 1973-05-18 US US00361813A patent/US3814998A/en not_active Expired - Lifetime
-
1974
- 1974-04-02 CA CA196,605A patent/CA1036832A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| US3814998A (en) | 1974-06-04 |
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