GB2101331A - Capacitive differential pressure transducer - Google Patents

Abstract

A capacitive pressure transducer (20) comprises: a central member (22) having a hole (24) therethrough; a pair of electrically insulative elastic diaphragms (30, 32), deflectable in response to pressure applied thereto, disposed parallel to one another about the hole (24) and bonded to the sides of said central member by glass-like seals (34, 36) forming a sealed cavity (40) therebetween; a post (50) slidably received through the hole and attached to the diaphragms for ensuring corresponding displacement of the diaphragms; a first pair (80) of conductive layers applied to each side of the central member (22), and a second pair of conductive members (60a, b) applied respectively to the inside surfaces of the diaphragms thus forming first and a second pressure variable capacitors such that the changes in capacitance of the first and second capacitors are substantially equal and opposite. <IMAGE>

Description

SPECIFICATION Improved quartz differential pressure transducer Background and Summary of the Invention This invention relates to differential pressure transducers and in particular to capacitive pressure transducers.

Pressure sensors often comprise at least one flexible diaphragm having a metalized portion or electrode thereon that cooperates with a base plate of another diaphragm having a similar metalized portion to form a pressure sensitive capacitor. These pressure transducers have one inherent shortcoming; that is, the variation of capacitance with pressure is non-linear. Prior art pressure capacitive pressure transducers and/or systems have utilized various techniques to make this variation of capacitance with pressure a more linear function. These techniques include: removing portions of the electrode to create a complicated electrode pattern, utilizing electronics to match eletronic non-linearities to the inherent non-linear function of the pressure sensor such that the overall output is more linear.By incorporating sophisticated electrode patterns and complicated electronics the complexity of the sensor is increased, the sensor is more costly and its reliability is lowered.

The present invention relates to a capacitive pressure sensor or transducer comprising a central member having a hole therethrough and a pair of electrically insulative elastic diaphragms that are deflectable in response to the pressure differential applied across these diaphragms and disposed in a stacked alignment relative to one another on either side of the central member about the hole. These diaphragms are bonded, in a spaced apart relationship, to the sides of the central member by a glasslike seal, forming a sealed cavity therebetween.The transducer further includes post means slidably received through the hole and attached to the undersides of each diaphragm for causing the displacement of each of the diaphragms to be dependant upon one another and a first pair of conductive layers or electrodes applied to each side of the central member and a second pair of conductive layers applied to the inside surface of each diaphragm to form a first, C51, and a second, Cs2, pressure responsive capacitor in cooperation with the corresponding conductive layers disposed upon the central member, such that, the change in capacitance of the first and second capacitors is substantially equal and opposite.The transducer further includes a signal generating means, responsive to the value of the first and the second capacitors, for generating an output signal to minimize the non-linear component of the variation in capacitance as a function of pressure whereby the output signal generated is substantially more linear in response to the pressure differential applied across the elastic diaphragms.

Accordingly, it is a broad object of the present invention to provide a differential pressure transducer that has a more linear output signal. A further object of the present invention is to provide a capacitive pressure transducer which substantially reduces the above mentioned non-linearities and which does not require complex compensation electronics. Another object of the present invention is to provide a capacitive pressure transducer having increased sensitivity.

An advantage of the present invention is derived from the symmetry of the diaphragms and its corresponding electrodes. This relationship permits the present invention to display an insensitivity to temperature changes, as well as a high degree of common mode rejection.

Brief Description of the Drawings In the Drawings: FIGURE 1 is a cross-sectional view of a capacitive differential pressure transducer embodying the present invention.

FIGURE 2 is a plan view of one of the elastic diaphragms illustrating the electrode pattern thereon.

FIGURE 3 is a plan view illustrating one configuration of electrodes disposed on the central member.

FIGURE 4 illustrates electrode configuration for the central member.

FIGURE 5 is a cross-sectional view illustrating the pressure transducer in a flexed position.

FIGURE 6 illustrates a capacitive instrumentation bridge.

FIGURE 7 illustrates a diode instrumentation bridge.

Detailed Description of the Drawings Reference is now made to FIGURES 1 through 4 which illustrate the major components of a pressure transducer 20. The pressure transducer 20 comprises a central support 22 having an opening 24 therethrough. In the preferred embodiment the central support is a circular disc that is adapted to be supported at its edge. The edge support is not illustrated in FIGURE 1, but is schematically illustrated in FIGURE 5. The central support 22 is preferably fabricated of material that has a substantially zero hysteresis characteristic and a low thermal co-efficient of expansion. Materials of this type include aumina, Pyrex and fused quartz. While the preferred embodiment of the invention utilizes a circular cenral support 22, other geometric shapes are workable.The transducer further includes two smaller discs comprising the flexible diaphragms 30 and 32 that are preferably co-axially situated relative to the center 26 of the central member 22. The flexible diaphragms 30 and 32 are preferably fabricated from the same material as the central support 22. The diaphragms are maintained in a spaced apart parallel relationship to one another and to the central support 22 by a pair of peripheral or annular seals of a known variety such as the annular glass frit seals 34 and 36. The annular seals 34 and 36 may be deposited to the central support in an known manner such as by silk screening, vacuum deposition and/or sputtering.

To prevent the central support 22 from bending when pressure is applied to the outer surfaces of the diaphragms 30 and 32, it is preferable to fabricate the central support 22 to be more rigid than each of the diaphragms 30 and 32. In addition, to minimize thermal effects, it is desirable to fabricate the diaphragms and central support from the same material. In addition, to further reduce the mechanical stresses developed within the sensor, it is desirable that the mechanical characteristics of the material utilized for the seals 34 and 36 should be similar to the characteristics of the diaphragms and central support. One typical sealing material is a fused quartz glass such as a glass transfer tape G 1015 manufactured by the Vitta Corporation of Damberry, Connecticut.The diaphragms 30 and 32, the seals 34 and 36 and the central support 22 cooperate to provide a hermetically sealed chamber 40 therebetween.

The pressure capsule 20 further includes a central spacer, link or post 50 that is received through the opening 24 and is connected to the undersides of the diaphragms 30 and 32. The post 50 is joined to the diaphragms by the glass-like seals 52 and 54. The length of the post 50 and the thickness of the seals 52 and 54 are controlled in a known manner such that in a quiscient situation, i.e., when no pressure is applied to the diaphragms, the diaphragms are maintained in a parallel substantially unstressed relationship. The transducer 20 further includes a plurality of electrodes that are bonded or otherwise attached to the surfaces of the central member 22 and to the diaphragms 30 and 32. These electrodes form at least two pressure responsive capacitors. The specific configuration of the electrodes which form these pressure sensing capacitors may vary.For sake of illustration, two such electrode configurations are illustrated. It is appreciated, however, that the electrode configurations between the diaphragm 30 and the central supporter 22 and between the diaphragm 32 and the central support 22 will be substantially identical for any specific configuration.

Reference is now made to FIGURE 2 which illustrates a plan view of a typical electrode 60a disposed on the inner surfaces 56 of the diaphragm 30. An identical electrode 60b, that is shown in FIGURE 1 would be disposed on the inner surface 58 and the diaphragm 32. More specifically, there is illustrated a plan view of the underside of the diaphragm 30. The diaphragm 30 contains a centrically situated annular electrode 60a that is positioned in surrounding relationship to the seal 34. The electrode 60a includes the electrodes leads 62a and 64a. The leads 62a and 64a extend from the annular electrode 60a to the edges of the diaphragm 30. FIGURE 2 further illustrates the relationship between the seal 52 and the electrode 60a.

In one embodiment of the invention, the electrode configuration on the central support and diaphragms define a first, C51, and a second, C3 pressure sensing capacitor. In an alternative embodiment, the electrode configuration defines a first and a second reference capacitors Cri, Cr2 in combination with a first and second pressure sensing capacitors Cs1 and C52.

Reference is made to FIGURE 3 which illustrates an electrode configuration that may be used to form two pressure sensing capacitors Cs1 and Cs2 in combination with the electrodes 60a and 60b. This configuration would be utilized with either bridge circuit shown in FIGURES 6 or 7. In this configuration, either side of the central support 22 would include a centrally situated annular electrode 80 connected to a radially extending lead 82. The electrode 80 is situated within the annular seals 34 and/or 36 and surrounds the hole 24.

FIGURE 4 illustrates an aternate electrode configuration yielding two pressure sensing capacitors Cs1 and Cs2 and two reference capacitors C,1 and Cr2. More particularly, either side central support 22 would contain a centrally located annular electrode 90 that is positioned and surrounding relationship relative to the opening 24. A radially extending lead extends the electrode 90 to the edge of the support 22. The support 22 further includes a second substantially annular, C-shaped electrode 94 that partially encircles the annular electrode 90. The electrode 94 further includes another radially extending lead 96.

It can be shown that the deflection between the electrodes 60 and 90 will be greater than the deflection between the electrodes 94 and 60. Consequently, the capacitor formed between the electrodes 94 and 60 is designated as a reference capacitor C, since its capacitance will not change appreciably and the capacitor formed between the electrodes 90 and 60a, which is centrally located within the pressure responsive region on the diaphragm 30 or 32 is designated as a pressure sensing capacitor Cs. During the fabrication of the pressure transducer 20, it is preferable to orient the leads (82, 92, 94) in either electrode configuration to be equally spaced from the leads 62 and 64 that are located on the undersides of the diaphragms 30 and 32 respectively.

Reference is made to FIGURES 6 and 7 which illustrate two instrumentation bridges that may be connected in a known manner to the electrodes forming the capacitors of the transducer 20. The instrumentation bridge 100 illustrated in FIGURE 6 comprises four capacitors C1-C4. The instrumentation bridge circuit illustrated in FIGURE 6 may be used with either capacitive electrode configuration. As illustrated in FIGURE 6, capacitors C1 and C2 correspond to the pressure sensing capacitors Cs1 and Cs2. The capacitors C3 and C4 correspond to the reference capacitors which may comprise the reference capacitors situated within the transducer 20 or may comprise reference capacitors that are external to the transducer. It can be shown that the output voltage of the bridge 100 is proportional to its capacitance values.This relationship is given in equation (1) below: E0 a 1/C,(1/C, + 1/C2) - 1/C3/(1/C3 + 1/C4) (1) FIGURE 7 illustrates another instrumentation bridge 100 comprising inter alia the diodes D1-D4 that are connected to the pressure sensing capacitors C, and C2. The output voltage of this bridge is similarly proportional to its capacitance values and is given by equation (2) below: 1 1 E0 a ---- -- ---- (2) Cs1 Cs2 The instrumentation bridges of FIGURES 6 and 7 can be used to measure the variation in the capacitance of a pressure transducer having a single pressure sensing capacitor as well as the multiple pressure sensing capacitors as illustrated by the present invention.In the case of the prior art pressure sensor having a single pressure sensing capacitor, the output voltage of the capacitance bridge of FIGURE 6 is given by equation (1). In addition since the right hand term of equation (1) is a constant H has been combined with the output voltage to form a modified output voltage E'o, Eo Equation (3) shows the resulting output voltage relationship.

Recalling that the variable capacitance C1 can be expressed as: A C1 d1 + Ad1 (4) wherein A is a constant of proportionality and Ad1 is the change in the nominal spacing d between the plates forming capacitor C1.

Substituting equation (4) into equation (3) and introducing the term: A (d + Ad) X=- C2 (5) equation (3) may be rewritten as: 1 1 E'o a 1 + A/C2 (d1 + Ad1) 1 + X (6) Expanding equation (6) by using the binomial expansion yields equation (7) which emphasizes that the output voltage, E'o, generated by the prior art pressure sensor varies non-linearly within displacement.

E'o = 1X +X2X3 + X4. . . (7) It can be shown that the separation between the capacitor plates is proportional to the applied pressure hence, equation (7) further illustrates that the output voltage varies nonlinearly with pressure.

In contrast, the output voltage generated by the transducer 20 in combination with either bridge circuit of FIGURES 6 or 7 varies much more linearly as a function of the change in separation of the capacitor plates.

As an example, if the transducer 20 is connected to the bridge circuit 100 of FIGURE 6 and if C, and C2 are equated to Cs1 and Cs2 respectively the output voltage Ec, given by equation (8):

Recalling that the initial values of the capacitors Cs1 and Cs2 are equal; this relationship implies that the constant of proportionality A1 and A2, as well as the initial displacements d1 and d2 are also equal. In addition, it can be shown that the displacements Ad1 and Ad2 are equal and opposite.

The output voltage E0, as expressed in equation (8) can now be expressed as:

Equation (9) illustrates that the output voltage E0 is linearly proportional to the change in the spacing between the plates of each capacitor Cs1 and/or Cos2. Alternatively, it can be seen that the output voltage is proportional to the applied pressure differential. A similar relationship, showing the improved linearity in the output voltage can be derived for the diode bridge circuit of FIGURE 7.

In addition, a very desirable attribute of the present invention is that it displays a high degree of common mode rejection, that is, the pressure capsule and associated electronics respond only to pressure differences and not to the level of the applied absolute pressure. This attribute is provided by the symmetry of the diaphragms 30 and 32 and its associated electrodes which yield a first order cancellation of the deflection of the diaphragms relative to the support 22. In many operational situations, the capsule is subjected to a high base pressure and is required to measure deviations about this base pressure. This operating situation is schematically illustrated in FIGURE 8 wherein the applied or base pressures P1 and P2 are virtually identical. In response to these pressures, the diaphragms will deform as illustrated.Since the pressure sensing capacitors Cs1 and Cs2 will change equally the bridge circuits 100 and 110 will remain balanced and an output signal will not be generated until the applied pressures vary slightly to produce a pressure differential.

This situation is contrasted by the operation of the a prior art exemplified by those transducers having a single pressure sensing capacitor disposed between one diaphragm such as the diaphragm 30 or 32 and a support such as the support 22. In the common mode situation, that is, with the applied pressures P1 and P2 being equal, the transducer will generate an output signal due to the change in the value of the single pressure sensing capacitor.

By utilizing the pressure capsule 20 in combination with the bridge 100 wherein the electrode configuration, as illustrated in FIGURES 3 and 4, is disposed upon the diaphragms 30 and 32 and upon the support 22, and furthermore, if the nominal values of the reference, Cri and Cr2 and the pressure sensitive Cs1 and Cs2 capacitors are equal, then in response to temperature deviations all our capacitors will change by the same amount and in the same direction; thus again maintains the bridge 100 in a balanced condition. The above temperature compensating effect is similarly displayed by the bridge 100.

In operation, the pressure capsule 20 is placed within an housing in a similar manner as taught by an Anastasia in U.S. S.N. 1 64,758, which is herein expressly incorporated by reference to provide an edge clamp about the central support 22 and to provide two pressure receiving chambers separated by the support 22, such that a first pressure is communicated to the pressure receiving surface on the diaphragm 30 and a second pressure is communicated to a pressure receiving surface on the diaphragm 32. When no pressure or when equal pressures are applied to the diaphragms, the diaphragms will be maintained at equal distances d1 and d2 from the central support 22. If a pressure differential exists, the diaphragms 30 and 32 will deflect as illustrated in FIGURE 5. As an example, if the pressure P, is greater than the pressure P2 the spacing between the diaphragm 30 and the central support will decrease while the spacing between the diaphragm 32 and the central support 22 would increase. An output voltage will be generated in accordance with the above equations yielding a substantially more linear relationship between the change in capacitance and the applied pressure or pressure differential.

Many changes and modifications in the above-described embodiments of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, that scope is intended to be limited only by the scope of the appended claims.

Claims (9)

1. A capacitive pressure transducer (20) characterized in that it comprises: a central member (22) having a hole (24) therethrough; a pair of electrically insulative elastic diaphragms (30, 32), deflectable in response to pressure applied thereto, disposed in vertical alignment relative to one another and disposed about said hole and bonded to the sides of said central member by a glass-like seal (34, 36) in a spaced apart relationship forming a sealed cavity (40) therebetween; post means (50) slidably received through said hole and attached to said diaphragms for causing the displacement of each of said diaphragms to be dependant upon the displacement of the other;; a first pair (60a,b) of conductive layers applied to each side of said central member, and a second pair of conductive members (80, 90, 92) applied to the inside surface of each of said diaphragms forming a first and a second pressure variable capacitor in cooperation with the corresponding conductive layers disposed upon said central member such that the change in capacitance of said first and second capacitors is substantially equal and opposite.

2. The transducer as defined in Claim 1 characterized in that the value of capacitance of said first and said second pressure variable capacitors, with no or equal pressures applied to said diaphragms, is substantially equal.

3. The transducer as defined in Claim 2, characterized in that it further includes reference capacitor means, disposed to said diaphragms and said central member for providing a pair of capacitors that are substantially insensitive to the applied pressure.

4. The transducer as defined in Claim 3 characterized in that it further includes housing means for supporting said central member and for defining, in cooperation with said central member means for communicating a first applied pressure to one of said diaphragms and for communicating a second applied pressure to the other of said diaphragms.

5. A capacitive pressure transducer (20) characterized in that it comprises: a central member (22) having a hole (24) therethrough; a pair of electrically insulative elastic diaphragms (30, 32), deflectable in response to pressure applied thereto, disposed in vertical alignment relative to one another and disposed about said hole and bonded to the sides of said central member by a glass-like seal (34, 36) in a spaced apart relationship forming a sealed cavity therebetween; post means (50) slidably received through said hole and attached to said diaphragms for causing the displacement of each of said diaphragms to be dependant upon the displacement of the other; a first pair of conductive layers (60) applied to each side of said central member;; a second pair of conductive members (80, 90, 92) applied to the inside surface of each of said diaphragms forming a first and a second pressure variable capacitor in cooperation with the corresponding conductive layers disposed upon said central member such that the change in capacitance of said first and second capacitors is substantially equal and opposite; and signal means, responsive to the values of said first and said second capacitors for generating an output signal to minimize the non-linear component of the variation in capacitance as a function of pressure whereby the output signal so generated is substantially more linear in response to the pressure differential applied to said elastic diaphragms.

6. The transducer as defined in Claims 5 characterized in that said transducer further includes reference capacitor means (94, 96) associated with each of said pressure variable capacitors.

7. The transducer as defined in Claim 6 characterized in that said reference capacitor means are situated within said sealed cavity.

8. The transducer as defined in Claim 7 characterized in that it further includes housing means for supporting said central member and for defining, in cooperation with said central member means for communicating a first applied pressure to one of said diaphragms and for communicating a second applied pressure to the other of said diaphragms.

9. A capacitive transducer (20) characterized in that it comprises: a central member (22) having a hole (24) therethrough; a pair of electrically insulative elastic diaphragms (30, 32), deflectable in response to pressure applied thereto, disposed in vertical alignment relative to one another and disposed about said hole and bonded to the sides of said central member by a glass-like seal (34, 36) in a spaced apart relationship forming a sealed cavity therebetween; post means (50) slidably received through said hole and attached to said diaphragm for causing the displacement of each of said diaphragms to be dependant upon the displacement of the other; electrode means (60, 90, 92) disposed upon the undersides of said diaphragms for informing the plates of at least two pressure sensing capacitors; and sensing means including a capacitance bridge circuit (100) connected to each of said at least two pressure sensing capacitors for generating an output signal in response to the change in the spacing between said at least two pressure sensing capacitors wherein said capacitance bridge circuit is maintained in a balance configuration such that the output signal is zero when the corresponding spacing of said at least two pressure sensing capacitors is substantially equal.

1 0. A capacitive pressure transducer substantially as described and as shown in the accompanying drawings.