WO1991015742A2

WO1991015742A2 - Pressure sensing capacitor, and transducer

Info

Publication number: WO1991015742A2
Application number: PCT/GB1991/000523
Authority: WO
Inventors: Richard Dennis Shelton
Original assignee: Richard Dennis Shelton
Priority date: 1990-04-07
Filing date: 1991-04-04
Publication date: 1991-10-17
Also published as: GB9007948D0; WO1991015742A3; AU7583291A

Abstract

A pressure sensing capacitor utilising two planar layers (41, 141) of conductive material disposed substantially parallel to each other, but separated by a narrow gap. The conductive layers are formed as coatings on two imperforate insulative plates (11, 111), at least one of the plates being a diaphragm adapted to flex when subjected to a change in pressure. The diaphragm is supported by a first wall portion (20) which extends rearwardly away from the front surface of the diaphragm, and by an inwardly facing edge (22) of the peripheral portion (14) to which it is integrally connected. The capacitor can be electrically connected to electronic circuitry adapted to condition the capacitor output signal to an indication of pressure, or pressure difference, and so can be part of a transducer.

Description

PRESSURE SENSING CAPACITOR, AND TRANSDUCER

FIELD OF INVENTION

This invention relates to a pressure sensing capacitor, and transducer.

DESCRIPTION OF THE PRIOR ART

Pressure sensing capacitors are widely used to determine the pressure of gases and liguids. Usually the capacitor has output leads to electronic circuitry, to provide a transducer permitting the electrical output signal from the capacitor to be conditioned in known fashion.

One known type of pressure sensing capacitor utilises two planar layers of conductive material disposed substantially parallel to each other, but separated by a narrow dielectric gap. The conductive layers are formed as coatings on two imperforate insulative plates, at least one of the plates being a diaphragm adapted to flex when subjected to a change in ambient pressure. Movement of one capacitor plate towards the other decreases the dielectric gap and so increases the capacitance. The plates are sealed together around their (outer) peripheries to form an internal cavity in which there is a reference pressure. By comparing the capacitance with a reference capacitance, the ambient pressure may then be determined.

For some applications, the absolute pressure is measured, wit the reference pressure in the cavity between the plates being as near absolute vacuum as possible. For other applications th gauge pressure is measured, usually with the reference pressur being atmospheric pressure. For yet other applications th difference between two pressures is usually measured, with th reference pressure selected so that is similar to but lower tha the pressure to be measured.

in a "single diaphragm" capacitor, one capacitor plate i rigid, whereas the other is a diaphragm plate able to flex; in "double diaphragm" capacitor, both plates are diaphragms able t flex.

The plates are selected to have small temperature coefficient of expansion, substantially zero mechanical hysteresis, as wel as being impermeable, and may typically be of alumina, fuse silica, or glass such as Pyrex. The conductive layers can b applied in a manner known in the art, and so may be sputtered, o screened and fired on the plates; however there are know problems in the firing process of ceramic members having sudde changes in cross section, and one feature of the invention seek to reduce this problem. The seal between the plates is of material having substantially the same thermal coefficient o expansion as that of the plate material, and suitably is glas frit.

Bunker Ramo Corporation British Patent 1,455,375 discloses a variable capacitance pressure responsive device utilising a thin, flat, plate-like electrically non-conductive substrate to the inner (in use) face of which first and second spaced-apart electrodes are bonded. The first electrode and a facing diaphragm of metal construction form the plates of a capacitor. The sensor is positioned in a metal casing. The cavity between the substrate and diaphragm can be evacuated, so that the device can be responsive to pressure external to the diaphragms, usually atmospheric but if the transducer is enclosed in an external casing then it is responsive to the pressure within the casing.

Polye US Patent 3,858,097 also discloses a capacitance type pressure transducer. Polye teaches providing a reference capacitor as an integral part of the sensor, so that both are subjected to the same temperatures, to limit the errors caused by temperature changes. The insulative plates are supported at their edges; there is a first electrically conducting layer on a portion of the plates spaced from the edges and which are thus deflectable upon ambient pressure changes, and there is a second conducting layer on a portion of the plates adjacent the edges and which are thus substantially non-deflectable. To avoid the introduction of stray capacitance in the electrical measurements, a thin film metallic coating may be provided on external surfaces of the sensor. The edges are peripheral portions thickened compared to the remainder of the plates, but Polye teaches thickening by an amount less than the plate separation; the gap between the plates is substantially equal to the thickness of the peripheral edge portions. The main plate portions are in a different plane to the plate edges, being spaced "rearwardly" therefrom, the plates thus being cup-like members. The edges are sealed by an annular circumferential sealing ring of the full edge width, so that the seal extends to the outer edge of the plate; during diaphragm flexing the sealing ring is highly stressed, and this can mean that diaphragm flexure is unduly influenced by the properties of the sealant and the quality of the bond, perhaps therefore requiring individual calibration of each capacitor.

Irwin US patent 4,974,117 discloses a pressure transducer, wherein the pressure sensing capacitor is electrically connected to an electronic circuit, so that the capacitor signal can be conditioned as required. The capacitor plates at their peripheral portion are sealed and spaced by an annular sealing ring of glass frit, generally of the width of the respective peripheral edge portions but interrupted by an inner porting channel, and an outer transfer channel. Each capacitor plate comprises a thick radially-outer peripheral portion and a thin radially-inner portion, the inner portion being in substantially the same plane as the plate sealing surface. The inner portion which forms the diaphragm can be bevelled or rounded at its outer circumference to lessen strain. A reference pressure may be provided in the gap between the plates, with the ambient or test pressure to be measured applied to the external face of the flexible diaphragm member(s). The sensor is mounted in a metal casing, against which it is sealed by O-rings located in grooves formed in the plate edge portion.

The known pressure sensing capacitors have one or more recognised disadvantages. Thus they can be expensive to manufacture, subject to long term stability and drift problems, can have significant inherent temperature coefficients resulting in measurement errors with change in temperature, and can be labour intensive to assemble into commercial products.

SUMMARY OF THE INVENTION

The present invention seeks to provide a pressure sensing capacitor, and transducer, which aims to reduce or avoid some or all of the diadvantages mentioned in the preceding paragraph.

According to one feature of the invention we provide a pressure sensing capacitor which includes :

a first insulative member which includes a first wall with a front first surface and rear first surface, and a first peripheral portion with a front first edge and a rear first edge, the peripheral portion being radially outwards of the first wall, the thickness distance between said first edges being greater than the spacing between said first surfaces whereby the first wall can flex relative to said peripheral portion in response to changes in pressure at said rear first surface, a first electrically conductive coating on said front first surface forming a first capacitive layer so that the said first wall is a first capacitor plate, the front first surface being substantially co-planar with the front first edge;

a second insulative member which includes a second wall with a front second surface and a rear second surface, and a second peripheral portion with front second edge, the peripheral portion being radially outwards of the second wall, a second electrically conducting coating on said front second surface forming a second capacitive layer so that said second wall is a second capacitor plate, the front second surface being substantially co-planar with the front second edge; the second insulative member neing disposed opposite to said first insulative member with the first and second walls overlying and with the capacitor plates facing one towards the other;

sealing means between said first and second insulative members which together with said first and second insulative members define an inner cavity, the sealing means also spacing apart the said front first and front second edges of the peripheral portions; and

first and second electrical contact areas electrically connected to said respective first and second conductive coatings;

characterised by support means for the first wall comprising

{a} a first wall portion which extends rearwardly away from the front first surface, (b) an inwardly facing edge portion of the first peripheral portion, said inwardly facing edge portion being of lesser length than the remainder of the peripheral portion, and (c) an integral connection between the said first wall portion and said inwardly facing edge portion located rearwardly of the said front first edge of the peripheral portion whereby to form an annular forwardly facing groove located between the first wall and the first peripheral portion.

Alternatively stated the outer periphery of the first wail extends rearwardly away from the second insulative member before joining the first peripheral edge portion, notwithstanding that the front first surface and the front first edge are substantially co-planar.

Preferably the support means is multi-function, being used not only to support the wall and help control wall flexure but also to help control sensor performance.

Usefully the second insulative member is of a ceramic material and is formed identical to the first insulative member, also of ceramic material, with the respective first and second walls both being flexible diaphragms. In a valuable embodiment a number of insulative (ceramic) members can be stacked in overlying relationship, with inter-connected outputs from the respective capacitor plates. The sensor can be mounted directly on a substrate which may include circuit components, and connected as a transducer, though we do not exclude positioning the circuit components either within the cavity, or "above or below" the sensor assembly.

There can be first port means for connecting said inner cavity with a first external (reference) pressure source, the first port means usefully communicating directly with the groove. There can be second port means communicating with a chamber formed between the rear first surface and a cover therefore, for connecting said chamber with a second external (test) pressure source.

The first and second conductive coatings can be connected by way of respective first and second contact areas with electrical contact means and thus with circuit means for determining capacitance, change in capacitance, or differential capacitance, usually in each case as an interim step to a pressure measurement. The coatings will be radially inwards of the sealing means, whilst the contact areas will be radially outwards of the sealing means; thus a coating and respective area will be laid down on the insulative member, with an interconnecting track (to be covered by the sealing means), prior to the provision of the sealing means. The contact means can be pins or wires located in the respective insulative member, or a pin located in a notch in the external face of the insulative member, the contact means in both cases being generally perpendicular to the contact area.

Alternatively the contact means can be a J-clip having one arm which fits in the gap between two members to engage the contact area on one of the members, the contact area on the other of the members (overlying and opposed thereto) being angularly spaced so that it is engaged by another J-clip. Thus the arm will be generally parallel to the contact area. The peripheral front first edge of one or both facing insulative members can be angled to permit easier access for the contact arm of the J-clip, though in a preferred embodiment the front surface of the respective first or second wall is co-planar in its normal rest position with the peripheral front edge, rather than being substantially co-planar therewith.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described by way of example, with reference to the accompanying drawings, in which:-

Fig.l is a sectional corner to corner view of a ceramic member according to the invention;

Fig.2 is a plan view of the ceramic member of Fig.l; Fig.3 is a sectional view along line X-X of Fig.2;

Fig.4 is a part-sectional view of an embodiment of a "single-diaphragm capacitor";

Fig.5 is a part-sectional view of an embodiment of a "double-diaphragm capacitor";

Fig.6 is a part sectional view of an embodiment of a multi-sensor capacitor;

Fig. is a plan view of a preferred arrangement of conductive coating, as applied to a diaphragm member;

Fig.8 is a plan view of a diaphragm member with combination sealing and spacing means;

Fig.9 is of a sensor arrangement with a

"single-diaphragm capacitor" employing a rigid base of standard LSI (large scale integration) configuration;

Fig.10 is of a transducer arrangement with a

"single-diaphragm capacitor" employing a rigid base on which are mounted electronic devices;

Fig.11 is of an embodiment of a ported capacitor, suitable for the pressure sensing of hot corrosive fluids;

Fig.12 is of the capacitor Fig.11, employing a base of LSI configuration, to form a ported sensor; and, Fig.13 is of an embodiment of a ported transducer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First member 10 comprises a one-piece insulative block, in the embodiment of Fig.l of a ceramic material, specifically high purity Alumina, formed to provide an element which is square in plan (Fig.2), a shape suitable for current auto "pick and place" machines, though the element could be of other shapes e.g. circular. High purity Alumina has a low temperature coefficient of expansion, a small change in modulus of elasticity with temperature and is substantially impermeable. It will also accept a conductive coating. Typically the selected ceramic material is pressed into shape from powder, to form a "green" shape, for firing. After firing the material can be machined, for instance to obtain the desired first wall or diaphragm 11 thickness necessary for the full scale pressure values.

Thus first member 10 includes a first wall 11 formed as a flexible diaphragm, and in different embodiments can be of thickness as measured by the distance separating or spacing apart of front first surface 12 and rear first surface 13 of between 0.127mm and l.27mm; first wall 11 is thus thin enough to flex when a test pressure to be measured, liquid or gaseous, is applied to rear first surface 13. First member 10 also includes a first peripheral portion 14 radially outwards of the first wall 11, and which provides a stiff base frame structure mounting the first wall 11; the thickness of the peripheral portion as measured between front first edge 15 and rear first edge 16 is several times the thickness of first wall 11, preferably at least ten times the thickness of wall 11, and so the peripheral portion is substantially inflexible under applied test pressure.

In use, the thicker frame part 14 can if required be located to compress a resilient (sealing) gasket, for instance for mounting a capacitor assembly (Fig.4) in a housing, or for isolating the capacitor assembly from vibrations, without unduly affecting the bending stresses at the first wall or diaphragm 11.

Support means are provided for the first wall comprising (a) a first wall portion 20 which extends rearwardly away from the front first surface 12, {b} an inwardly facing edge portion 22a of the first peripheral portion 14, said inwardly facing edge portion 22a being of lesser length than the remainder of the peripheral portion, and {c} an integral connection between the said first wall portion and said inwardly facing edge portion located rearwardly of the said front first edge 15 of the peripheral portion whereby to form an annular forwardly facing groove 23 located between the first wall 11 and the first peripheral portion 14. The length of the inwardly facing edge portion 22a is that measured in the thickness direction i.e. between the front first edge 15 and front rear edge 16. Thus the flexing properties of the diaphragm are determined by the continuous insulative material section, rather than (perhaps erratically) by an intermediate bonding medium between the first wall portion 20 and the peripheral portion 14, or by bondiing annulus 50 (Fig.4, Fig.8).

The inwardly facing edge portion 22a is radially outwards of the first wall 11. It is also radially outwards of edge portion 22b, which in conjunction therewith and base 22c provide a groove which in this embodiment is substantially square in section. Thus the integral connection between the first wall 11 and the first peripheral portion 14, in this embodiment substantially aligned with edge portion 22b, is of lesser length than the thickness of the first peripheral portion between its front 15 and rear 16 edges, the base 22c being rearward of the front first edge 15.

First wall 11, which can act as a flexible diaphragm, is substantially co-planar with front first edge 15 i.e. first wall 11 is aligned with front first edge 15 rather than with rear first edge 16. Furthermore, in the specific embodiments of the figures, in the inoperative or rest condition of the diaphragm, the diaphragm is co-planar with the front first edge 15; in an alternative embodiment, front first edge 15 can be angled relative to the diaphragm, to provide easier access for connecting means such as . J-clips 71 (Fig.9) to conductive areas such as 43a (Fig.7). When under load, typically from pneumatic pressure acting against rear first surface 13, the highest stresses of the supported first wall or diaphragm 11 are concentrated at its outer edge. Thus in known fashion the the first wall 11 and first wall portion 20 are joined by an integral bevelled section 25, this section 25 being rounded to lessen strain at this position as the first wall 11 flexes. However we have found that this simple expedient if used alone reduces the sensitivity and linearity, since the diaphragm area is not only made smaller but remains directly supported around its edge in a substantially fully constrained manner. Thus one purpose of providing a first wall portion 20 is to permit useful additional deflection to the diaphragm without the expected strain increase of a fully constrained edge.

In a preferred embodiment, first wall portion 20, forming a sidewall to the diaphragm, has the same section thickness as the supported diaphragm. In the embodiment shown, due to the moulding and manufacturing methods used, first wall portion has a thickness up to three times that of the body of first wall 11, but preferably below two times. Specifically, in the embodiment of Fig.2, the first wall portion 11 has a thickness of 0.245mm and a sidewall 20 thickness of 0.5mm. The height of sidewall 20 is between three and ten times the thickness of first wall 11.

A further advantage arising from groove or undercut 23 is that the behaviour of the diaphragm under test pressures becomes more dependent on the mechanical properties of the ceramic material, since the highest bending stresses are in the sidewall region. Thus as compared to other sensors, particularly those comprising a flat or substantially flat disc bonded and sealed to a similar disc, the deflection of the diaphragm is less affected by the properties of the bonding material, which in turn is freed from many of the bending stresses. Specifically, the first wall portion is not bonded to the peripheral portion (which could be effected for instance with a bonding material similar to that used to join the members 10,110); thus as a feature of the invention the first wall portion is formed integral with the peripheral portion, so that flexure of the supported diaphragm in response to test pressure is determined by the characteristics of the ceramic or other insulative material of which e.g. the member 10 is made, and so that repeatable results can be achieved with a sensor and between sensors, whereby to reduce the calibration regime for both newly assembled sensors and for sensors already in service, and reduce the rejected proportion.

Provided for a purpose to be described below, adjacent the corners 30 of the member 10 and in front first edge 15 are undercuts 31; in this embodiment the undercuts 31 are arcuate in plan (Fig.2) with their hypothetical centre at the adjacent corner 30 of the insulative member. The undercuts each circumscribe an aperture 32 extending through the peripheral portion from its front first edge 15 to its rear first edge 16. One embodiment of capacitor 40 is shown in Fig.4. In this embodiment the front first surface 12 has a first conductive coating 41, so that the first wall 11 is a first capacitor plate; at one angular section position, as shown, the coating 41 is continued as a narrow strip or track 42 (Fig.7) across the groove 23, towards the outer edge of insulative member 10, to provide a contact area 43 which can be connected to external circuitry (Figs.9,10) to make a transducer.

Opposite to the first conductive coating 41 and in overlying relationship therewith is a second conductive coating 141 bonded to front second surface 112 of second insulative e.g. ceramic, member 110 to form a second capacitor plate, and connected by an integral narrow strip 142 of conductive material to a contact area 143. The rear second surface l±3 is substantially co-planar with the rear second edge 116 of the peripheral portion 114 (and in this embodiment is coterminous therewith) so that the second insulative member 110 is a substantially inflexible block of ceramic material. Thus this capacitor is a "single diaphragm" capacitor.

in the Fig.4 embodiment, pins 35 (shown prior to fitting) of length slightly greater than the thickness of capacitor 40 are located in apertures 32 and provide first and second contact means, electrically connected to said respective first and second conductive coatings 41,141 by way of conductive areas 43,143 (which are angularly spaced) . In an alternative embodiment, connector pads (not shown) fit into notches 39 electrically to connect to conductive areas such as 43a (Fig.7). In a third embodiment, J-connectors such as 71 (Fig.9) can embrace 5 radially-extended peripheral portions of the members, with one end of the J-connector contacting a simlarly extended thin "track" portion of the coating, and the other end soldered onto a P.C.B. In yet a further embodiment, one J-member can have the said one end soldered, for instance to the conductive area 43, or 10 43a, outside sealing ring 50, with another J-member connected similarly to conductive area 143 or to an extended thin track as above.

The sealing and spacing and bonding means 50 surrounds the groove or undercut 23. In this embodiment this sealing means is a

15 thin layer of glass frit, of similar temperature coefficient to that of the ceramic substrate, and loaded with particles of ceramic of a maximum dimension equal to the desired spacing between the ceramic members 10,110; thus when the capacitor is "fired" to seal the cavity or (low) pressure chamber 52, the 0 bonding material sets the gap between the members 10 and 110. Alternatively, prior to firing, shims or spacers can be provided at each corner 30 to set this gap.

An extra purpose for groove 23 is to provide an inner control boundary for the seal material 50, to help to ensure that seal 5 material does not contaminate the critical area of the flexing diaphragm, which could modify its performance or prevent it from working. Thus there is an inbuilt safety margin for the positioning of the sealing material 50, and for its possible subsequent capillary flow whilst the capacitor is being made. Thus in this way also the groove 23 can act to improve the repeatability of the calibration of an individual capacitor, and ease the manufacturing problems involved in producing capacitors of similar calibration and performance, again to reduce the rejection rate.

A vent 60 is provided in member 10. This vent 60 is a first port means which permits the evacuation of the cavity or inner pressure chamber 52, as when the capacitor is to be used as an "absolute pressure sensor" with a substantially zero reference pressure in the cavity. The vent 60 also permits the reference pressure in cavity 52 to be set just below the test pressure to be measured (by deflection of one or more diaphragms as above described), as when the capacitor is to be used as a "differential pressure sensor". In the former case, when the chamber 52 has been evacuated, the vent 60 can be sealed in known fashion, in this embodiment by plug 62, typically a seal of glass material, or of the solder braze type; alternatively, the reference pressure can be set whilst the first and second ceramic members are being sealed under vacuum conditions. In the further alternative use of the capacitor as a "gauge pressure sensor" the vent 60 is left unsealed, with ambient pressure in the cavity, and usually then the vent 60 will be connected through a filter. The vent 60 in this embodiment is connected to the groove 23. This greatly eases manufacturing and usage problems. The vent does not impinge upon and so weaken the first wall 11, with possible stress failures around the vent opening 61 as the diaphragm flexes. The vent does not need to be adjacent to or to penetrate through the seal material, which might otherwise close aperture 61 and make it impossible to set the required reference pressure, so rendering the assembled capacitor unusable. Greater tolerance in seal material positioning is therefore permitted by positioning the vent aperture in groove 23.

The vent 60 can be made by laser drilling selected and otherwise standard insulative e.g. ceramic, members.

A further and valuable advantage of the groove 23 is that it permits a significant increase in the volume of the cavity 52. Thus problems are known to arise in the use of capacitors with "small volume" cavities, typically of longer term drift and "instability" in the capacitance recorded, and with groove 23 these can be proportionally reduced, particularly for "absolute pressure sensors" requiring evacuation of the cavity. This is because subsequent out-gassing from the ceramic, bonding and metallising materials, and the ingress of helium and the like gas atoms, have less proportional effect.

In the embodiment of Fig.5, the second ceramic member 110 has its second wall ill as a diaphragm, with a front second surface 112 having a conductive coating 141, and a rear second surface 113. The outer frame comprises a second peripheral portion 114, with a front second edge 115. The second ceramic member 110 is disposed opposite to and facing the first ceramic member, with the first and second walls overlying, and with the coatings 41,141 and thus the capacitor plates facing one towards the other.

Whilst Fig.5 uses similar first and second ceramic members, to form a "double-diaphragm capacitor" such that the capacitance change for a given pressure change will be twice that of the

"single-diaphragm capacitor" of Fig.4, it may be necessary for only one of the members to have a vent aperture 60.

In the embodiment of Fig.6, one "double-diaphragm" capacitor (insulative e.g. ceramic, member pair) is stacked on another, though additional capacitors ("single" or "double" diaphragm or mixed) can be so stacked if desired; the two capacitors are aligned by contact pins or wires 35 which extend through respective apertures 32, as above described, electrically to connect the respective first and second capacitor plates at contact areas 43. The spacer 55, conveniently of glass frit, is not necessarily continuous between the capacitors, so that an ambient (test) pressure could affect all four diaphragms. The sensitivity of the assembly, that is the smallest pressure change that will register as a measurable capacitance change, increases with the number of capacitors used, this number being decided for the application required.

Typically the spacing between the facing electrodes 41,141 will be 0.05mm, reducing to 0.0127mm at full sensor design pressure. The coatings 41,141 can however be protected by a thin dielectric layer, to prevent short-circuiting of the plates if these touch under excess pressure. The assembled capacitors can be coated with a metallic screen, to limit stray capacitative effects.

As clearly shown in Fig.7 and Fig.8 the apertures 32 for receiving contact pins or wires 35 are outside the annular sealing ring 50. The purpose of undercut 31 can also be seen, this being to set an outer boundary for the sealing material, so that it does not for instance clog or seal a pin aperture 32.

When pins 35 have been inserted in respective apertures 32, the electrical connection (and sensor assembly) can both be completed by applying electrical solder of known composition into the gap between the ceramic members 10,110, whereby electrically to connect a pin to the area of coating. Additionally, assembled capacitors can be stacked using pins 35 both as alignment aids, and as common conductors when soldered. Thus an assembly of capacitors can be selected to achieve a desired full scale value.

As also shown in Fig.7, as an alternative or additional connection means, notches 39 can provide connection pads (from above and below respectively) with connections to areas 43a, 47a of the electrodes or capacitor plates on the respective members.

In the Fig.7 alternative embodiment the front first surface 12 of the first wall has a reference coating 45 electrically 5 isolated from the sensor coating 41, the pattern of the coatings being seen in the plan view. The coating 45 is continued as a thin strip or track 46 to contact area 47, enclosing a pin aperture 32d; electrical continuity is maintained across groove 23 and undercut 31, and this can be achieved by various 0 deposition techniques, including thin film, pad printing, spraying or brushing. The front first surface 112 of the other ceramic member is identical in plan view, so that the respective contact areas 143,147 will be electrically connected at the other diagonally-disposed pin apertures 32a,32c. The respective

15 connection pins 35 are soldered to their contact areas as above described.

The central region of the diaphragm 11 with the inward coating 41, will deflect further under pressure change than will the region of the diaphragm covered by outward coating 45. If the

20 second coating, as applied to the second diaphragm ill of a double-diaphragm capacitor has corresponding inner and outer coatings, connected respectively to the pin apertures 32a,32c, the capacitance change, under applied pressure, as measured between the two inward • coatings 41 i.e. at pin apertures

25 32a,32b, will be greater than that change as measured between the

SUBSTITUTE SHEET two outer coatings 45 i.e. at apertures 32c,32d. The capacitance effective between the apertures 32c,32d, can thus be used as a reference capacitance, whilst the capacitance between apertures 32a,32b can be used as the active capacitance. When the pressure outside the inner pressure chamber is the same aε the pressure inside, the diaphragm(s) will be flat (planar), as in Fig.l, and the active and reference capacitance can be identical. As the pressure outside the inner pressure chamber changes with respect to the pressure inside, the diaphragm(s) will deflect, and the active and reference capacitances will differ. The difference between the active and reference capacitances will thus give a direct indication of the degree of deflection of the diaphragm(s) , and hence a direct indication of the pressure difference between the inside and outside of the cavity or inner pressure chamber.

The two electrode diaphragm areas thus permit a differential measurement technique. If a microprocessor is used as part of the signal conditioning circuitry, calibration curves can be held in memory, to permit a true, integrated, intelligent transducer.

Fig.8 shows the preferred arrangement of the sealing and spacing means 50; utilising the safety features provided by groove 23 and undercut 31 as above described, the sealing means can be laid on the respective peripheral portions by an automatic process. Fig.9 shows a capacitor assembly employing a rigid base 70, of similar outer dimensions to a standard LSI (Large Scale Integration) device, as widely used in the electronics industry. However, in this instance base 70 has four standard output connections 71 and corresponding connections (not shown) on the opposed edge; the outer pairs of connections are used, with inner pairs being redundant in thiε application. Base 70 can be a block of ceramic material, as in the Fig.4 embodiment.

Fig.10 is of an embodiment wherein the electronic components are mounted on an extended ceramic substrate base 170. The substrate is rigid, with a conductive coating being applied to produce a capacitance between it and the capacitor diaphragm 13, and with electrical connections at the corners 30. The electronic components are adapted to modify the capacitance measurements from the capacitor into usable electrical signals, and so the whole unit forms a transducer 54.

Fig.11 shows a valuable embodiment of a possible ported capacitor, as could be used to measure the difference between an absolute reference pressure in the cavity or inner pressure chamber 52, and a test pressure in an outer pressure chamber 152. In this embodiment the outer pressure chamber is formed by sealingly connecting a cover 56 over a "single-diaphragm" capacitor, such as that of Fig. 4; the sealing annulus 57 can be of solder, or of glass frit, so avoiding the need for perishable resilient O-rings. An apertured coupling 58 forming second port

SUBSTITUTE SHEET means passes through the cover 56, and this can be connected to a tube or other passage means, so that the pressure of a remote source can be measured; in an alternative embodiment, apertured coupling 58 can be connected through a greatly thickened sealing annulus 57, since it will be understood that cover 56 does not comprise a capacitor plate and so sealing annulus 57 can be several times the thickness of sealing annulus 50. In a further alternative embodiment, a variable reference pressure can be supplied through an additionally ported vent or first port means 60, so that the assembly is a pressure difference capacitor. With this embodiment in particular, the pressure of hot solvents and the like, normally corrosive to resilient gaskets or "0"-ring materials, can be measured, as well as being safely contained.

Fig.12 is of an embodiment utilising the ported capacitor of Fig.11, and employs a rigid base 70 of LSI device dimensions. Surface mount "J" bend type connectors 171 are used for combined electrical and mechanical connection to electronic circuitry.

In an alternative embodiment the electrical conditioning circuit can be mounted on the cover 56, so that a rigid extended base, such as base 170 (Fig.10), is not necessary. This arrangement could be particularly useful for "single-diaphragm capacitors" such as that of Fig.4, utilising a ported cover 56 or protected by a solid cover with apertured sealing annulus as described for the Fig.11 embodiment.

SUBSTITUTE SHEET Fig.13 is a further embodiment utilising the ported sensor of Fig.11 wherein the substrate 110 forms the second electrode, and also is extended at 170 to carry the microelectronics necessary for conditioning the output signal from the sensor. A capacitance to frequency circuit can therefore interface directly with the sensor by way of the conductive areas provided by connection pads 74 and the coated areas 143; whilst printed resistors 73 can be laser trimmed to provide full scale, and zero calibration of the complete sensor assembly, the notches 39 not being used for electrical connections in this embodiment since the lead throughs for the (lower) electrodes of member 110 are already printed on the lower substrate. The conductive area 74 is thus used to connect through to the (upper) electrode, for instance by way of contact area 43 (Fig.4).

It will be understood that during manufacture of a capacitive member, substantial thermal gradients can arise between the thick frame and the thin diaphragm. Thus during metallisation, each firing stage of the printed conductive paste typically requires a temperature cycle from ambient to about 950 degrees C, and back to ambient, in one hour. Thus the member is subjected to considerable thermal shock, with a temperature change of about 30 degrees C each minute, and this is a particular problem when the component has large changes in section. If the ceramic member is not cooled evenly after firing, then large stresses can build up at the section boundary, which can cause cracking. The recess or undercut 23 near to the diaphragm provides a more tapered and

SUBSTITUTE SHEET gradual profile to the cross section, so that mechanical and thermal stresses are distributed more evenly, even when a temperature gradient exists. Tests have shown that the presence of the groove 23 (and thus of the disclosed support means) considerably reduces the risk of thermal cracking, permitting reliable manufacture of thick frame, thin diaphragm profile sections.

SUBSTITUTE SHEET

Claims

1. A pressure sensing capacitor which includes :

a first insulative member (10) which includes a first wall (11) with a front first surface (12) and rear first surface (13), and a first peripheral portion (14) with a front first edge (15) and a rear first edge (16), the peripheral portion being radially outwards of the first wall, the thickness distance between said first edges (15,16) being greater than the spacing between said first surfaces (12,13) whereby the first wall can flex relative to said peripheral portion in response to changes in pressure at said rear first surface, a first electrically conductive coating (41) on said front first surface forming a first capacitive layer so that the said first wall is a first capacitor plate, the front first surface being substantially co-planar with the front first edge;

a second insulative member (110) which includes a second wall (111) with a front second surface (112) and a second peripheral portion (114) with front second edge (115), the said front second edge being radially outwards of the second wall, a second electrically conducting coating (141) on said front second surface forming a second capacitive layer so that said second wall is a second capacitor plate, the front second surface being substantially co-planar with the front second edge, the second insulative member being disposed opposite to said first insulative member with the first and second walls overlying and with the capacitor plates facing one towards the other;

sealing means (50) between said first and second insulative members which together with said first and second insulative members define an inner cavity (52), the sealing means also spacing apart the said front first and front second edges of the peripheral portions; and

first and second electrical contact areas (43,43a,143) electrically connected to said first (41) and second (141) conductive coatings;

characterised by support means for the first wall comprising {a} a first wall portion (20) which extends rearwardly away from the front first surface (12), (b) an inwardly facing edge portion of the first peripheral portion (14), said inwardly facing edge portion being of lesser length than the thickness of the remainder of the peripheral portion, and (c) an integral connection between the said first wall portion and said inwardly facing edge portion located rearwardly of the said front first edge (15) of the peripheral portion whereby to form an annular forwardly facing groove (23) located between the first wall (11) and the first peripheral portion (14).

2. A capacitor according to Claim 1 characterised in that the

SUBSTITUTE SHEET first and second insulative members are of a ceramic material, and in that a vent (60) connects the groove (23) to the rear first edge (16) of the peripheral portion (14), said vent being a first port means.

3. A capacitor according to Claim 1 characterised in that the groove (23) has annular sidewalls (22a,22b), a base (22c) joining the sidewalls and perpendicular thereto, and in that the sidewalls (22a,22b) have a length less than half the spacing between the front first edge (15) and the rear first edge (16) of the peripheral portion (14).

4. A capacitor according to Claim 1 characterised in that the support means for the first wall (11) is located between the first wall and at least one undercut (31) in the front first edge of the first peripheral section, and further characterised in that annular sealing means (50) is positioned between the support means and the said undercuts (31).

5. A capacitor according to Claim 1 characterised in that the said first insulative member is square in plan, with four corners (30) each bounded by an undercut (31) in the front first edge of the first peripheral portion, the undercut being arcuate in plan and having its hypothetical centre at the respective corner, in that the first conductive coating is electrically connected to one of said corners by a track (42) of the coating which traverses the undercut and connects to a conductive area (43)

SUBSTITUTE SHEET between the undercut and the respective corner, in that a pin aperture (32) extends from the said front first edge to the rear first edge, said aperture intercepting said first conductive coating, and in that the first insulative member has an active coating (41) and electrically isolated therefrom a reference coating (45), the reference coating being electrically connected to a different corner of the first insulative member to that to which the active coating (41) is connected.

6. A capacitor according to Claim 1 characterised in that the first wall and the first wall portion are joined by an integral bevelled section (25).

7. A capacitor according to Claim 1 characterised in that the first insulative member (10) is bonded to an identical second insulative member (110), in that each wall carries an active capacitive layer (41,141) and a reference capacitive layer (45,145), each of the reference layers being electrically isolated from its associated active layer, the respective active and reference layers being electrically connected to four separate electrical outputs (32a,32b,32c,32d) , in that said outputs are one of pins or wires, in that the first and second insulative members have overlying edge portions separable by removable shims adapted to set the gap between the first and second capacitor plates prior to setting of a bonding annulus (50) sealing a cavity between the plates.

SUBSTITUTE SHEET - -

8. A capacitor according to Claim 1 characterised in that a plurality of respective pairs of first and second members according to Claim 1 are stacked in overlying array, and in that said respective first and second members are electrically connected and aligned by a respective common first and second pins (35), in that each first and second surface has both an active coating (41) and a reference coating (45), with the respective refence coatings being electrically connected by third and fourth pins (35), said third and fourth pins also being alignment pins.

9. A capacitor according to Claim 1 characterised by an outer shape and dimensions substantially the same as the standardised dimensions of electronic industry LSI (Large Scale Integration) devices.

10. A capacitor according to Claim 1 characterised in that a cover (56) is sealingly fitted to a rigid peripheral portion (14) of the sensor, and in that a second port means (58) extends through a sealing annulus (57) between the cover and the rigid portion.

11. A capacitor according to Claim 1 characterised in that a cover (56) is directly bonded to a rigid peripheral portion (14) of the sensor, and in that a second port means (58) extends through said cover.

SUBSTITUTE SHEET

12. A pressure indicating transducer having electronic circuitry adapted to condition an output signal from a capacitor characterised in that the electronic circuitry is mounted on an extended base (170), or mounted above or below the capacitor, in that the capacitor is constructed according to any of claims 1-11, and in that the electronic circuitry includes means for determining capacitance.

13. A method of making a capacitor as claimed in any of claims 1-11 characterised by the steps of compressing insulative powder to the shape of the insulative member (10) of Figs 1-3, firing the powder, preparing the insulative member, and bonding the prepared member to another prepared insulative member.

14. A capacitor member formed as a one-piece insulative block for use in a capacitor as claimed in any of Claims 1-11, comprising a flexible diaphragm (11) surrounded by a rigid frame (14), the frame having an edge surface (15), the diaphragm being co-planar with the said edge surface, the edge surface having a mounting and sealing section spaced outwardly of the diaphragm, and means (20) to mount the diaphragm flexibly to the frame.

SUBSTITUTE SHEET