GB2266962A

GB2266962A - Capacitive differential pressure detector

Info

Publication number: GB2266962A
Application number: GB9315064A
Authority: GB
Inventors: Mitsura Tamai; Tadanori Yuhara; Kimihiro Nakamura; Kazuaki Kitamura; Toshiyuki Takano; Teizo Takahama; Mikihiko Matsuda; Shinichi Souma
Original assignee: Fuji Electric Co Ltd
Current assignee: Fuji Electric Co Ltd
Priority date: 1989-04-14
Filing date: 1993-07-20
Publication date: 1993-11-17
Anticipated expiration: 2013-07-20
Also published as: GB2266961B; GB2266961A; GB2266963B; GB9315065D0; GB2266960B; GB9314965D0; GB2266960A; GB2266962B; GB9315064D0; GB9314964D0; GB2266963A

Abstract

A capacitive differential pressure transducer (Fig. 3) includes a diaphragm (10) mounted between two fixed electrodes (15 and 20). The assembly is mounted in a housing as in Fig. 4, with pressures P1 and P2 being introduced via passages (57 and 73), and central openings (25 and 26) in the fixed electrodes. To obviate non-linearity being introduced by deformation of the fixed electrode (15) by a very high pressure P2, the electrode is mounted on a substrate (80) and spaced therefrom by a bonding portion (42). The substrate (80) can thus be deflected by P2 acting on its outer face, without affecting the electrode (15) since the latter is subject on both sides only to the pressure P1. <IMAGE>

Description

1 22669C32 CAPACITIVE DIFFERENTIAL PRESSURE DETECTOR This application is

divided from our application No 9008565.5 The present invention relates to a capacitive differential pressure detector. In particular, the capacitive differential pressure detector of the present invention may be adapted as a gauge pressure detector if one of the applied pressures is atmospheric pressure. The capacitive differential detector of the present invention may also be adapted as an absolute pressure detector if one of the introduced pressures is a vacuum.

Fig. 1 is a cross sectional view showing a structure of a conventional capacitive differential pressure detector. As shown, fixed electrodes 15 and 20 are respectively mounted to both sides of a diaphragm 10. The fixed electrode 15 is made up of a first conductive plate 12 disposed confronting the diaphragm 10, an insulating plate 13 coupled with the first conductive plate 12, and a second conductive plate 14 coupled with the insulating plate 13. The first and second conductive plates 12 and 14 are electrically interconnected by a conductive film 27 layered over the inner surface of a pressure guide hole 25. Pressure guide hole 25 also acts as a through hole.

The fixed electrode 15 is provided with a ring-like or annular support 21 which is coupled with the insulating plate 13 and disposed around a ringlike groove 23 surrounding the first conductive plate 12. The support 21 is coupled with the diaphragm 10 at a glass bonding portion 11 of predetermined thickness. The first conductive plate 12 and the support 21 are electrically insulated from each other. The support member 21 may be made of either insulating material or conductive 2 material. The pressure guide hole 25, which is formed passing through the fixed electrode 15, introduces pressure P1 into a gap 29, which exists between the fixed electrode and the diaphragm 10.

A structure of the fixed electrode 20 resembles the structure of the fixed electrode 15 as mentioned above. Hence, only necessary portions of it will be referred to. A pressure guide hole 26, which is formed passing through the fixed electrode 20, introduces pressure P2 into a gap 30, which exists between the fixed electrode and the diaphragm 10.

The diaphragm 10 and the fixed electrode 15 cooperate to form a first capacitor whose capacitance Ca is taken out through lead pins A and C. Similarly, the diaphragm 10 and the fixed electrode 20 cooperate to form a second capacitor whose capacitance Cb is taken out through lead pins B and C.

When the pressures P1 and P2 are differentially applied to the diaphragm 10, the diaphragm displaces in accordance with a differential pressure. The capacitances Ca and Cb vary depending on a displacement of the diaphragm. The differential pressure can be measured on the basis of the variations of the capacitances.

The pressure detector shown in Fig. 1 is accommodated within a housing sealed by two sealing diaphragms (not shown), which respectively receive the pressures P1 and P2. The housing is filled with a noncompressive fluid, e.g., silicone oil, through which pressure transfers. Under this condition, the gaps 29 and 30, and the pressure guide holes 25 and 26 are filled with silicone oil.

3 The prior art pressure detectors describes above have the following problem:

When the differential pressure (= P2 - P1) is very large, the fixed electrode 15 is bent or displaced to the right. The capacitance between the diaphragm 10 and each of the fixed electrodes 15 and 20 varies depending on the displacement of the fixed electrode alone. This fact can be readily understood by replacing a detector 50 in Fig. 2 with prior art detector shown in Fig. 1. In Fig. 2, the fixed electrode designated by no reference symbol on the left side (corresponding to the fixed electrode 15 in Fig. 1) is subjected to a pressure P2 applied from the left side and a pressure P1 from the right side. The instance of Fig. 2 will subsequently be described in detail in connection with a first embodiment of the present invention.

In a state that a displacement "De" of the fixed electrode 15 due to the differential pressure, and the displacement of "D" as already mentioned concurrently occur, capacitances Cle and C2e between the diaphragm 10 and the fixed electrodes 15 and 20 are Cl e = 8. AAT-D-De) (1 c) C2e = C2 = s. AAT + D) (2c) Exactly, the variations of the capacitances Cle and C2e are not differential. Accordingly, a differential pressure signal Fe, which is calculated by using the equation (15), is given by Fe = (Cle-C2e2Cle+C2e) = QD+DO/QD-De) (3c) 4 As seen from the equation (3c), when "De" is not negligible compared to "D", the differential pressure signal Fe is not proportional to the differential pressure (= P2 - P1). In other words, the linearity of the signal Fe is lost.

It is an object of the present invention to provide a capacitive differential pressure detector which produces a differential pressure signal with good linearity even if the differential pressure is large.

A capacitive differential pressure transducer according to the present invention measures a pressure on the basis of capacitances of capacitors formed between a diaphragm displaced in response to said pressure and each of first and second fixed electrodes with pressure guide holes disposed on both sides of said diaphragm, said transducer comprising a substrate with a pressure guide hole spaced by a predetermined distance from said first fixed electrode but bonded at the peripheral edge portion to the outside surface of said first fixed electrodes, whereby a pressure introduced from the second fixed electrode is also applied to a surface of said substrate opposite said first fixed electrode.

In such a capacitive differential pressure transducer, a differential pressure applied to both sides of the substrate displaces the substrate. However. the pressure., applied to both sides of each fixed electrode are equal, so that it will not be displaced. Accordingly, the capacitances of the capacitors formed by the diaphragm and the fixed electrodes vary in an exact differential manner.

The accompanying drawings illustrate an embodiment of the invention and together with the description serve to explain the principles of the invention.

Fig. 1 is a cross sectional view of a prior art pressure detecting apparatus;

Fig. 2 is a cross sectional view of a pressure detecting apparatus incorporating an embodiment of the invention forming the subject of the parent application; Fig. 3 is a cross sectional view of an embodiment of the present invention; and Fig. 4 is a cross sectional view of a pressure detecting apparatus incorporating the embodiment of figure invention.

An illustrative embodiment of a capacitive differential pressure detector according to the invention will be described with reference to the accompanying drawings. It-should be noted that Fig. 2 which shows a construction outside the scope of the present invention is included herein for reference only.

Fig. 3 shows a cross sectional view of the embodiment. In the figure, the embodiment is different from the prior art of Fig. 6 in that a substrate 80 having a pressure guide hole 81 at the central portion is bonded to the peripheral edge portion of the left side of the conductive plate 14 contained in the fixed electrode 15 through a glass bonded portion 42, and that in place of the conductive layer 31, a conductive layer 34 is provided on the peripheral end faces of the conductive plate 14 and the substrate 80. In Fig. 1, like reference symbols are used to designate like or equivalent portions in Fig. 3.

6 The substrate 80 may be made of either insulating material or conductive material. In this instance, the same conductive material, e.g., silicon, as that of the conductive plate 14 is used because it is easy to manufacture and with the intention of suppressing the influence by temperature change. The glass bonding portion 42 may be made of AI - Si eutectic. The use of the conductive layer 34 places the substrate 80 and the conductive plate 14 at an equal potential.

Fig. 4 shows a cross sectional view of a differential pressure detecting apparatus into which the above embodiment is assembled. in the figure, the present differential pressure detecting apparatus is different from the apparatus shown in Fig. 2, in that a tubular member 71 with a bottom is used in place of the tubular member 51 with a bottom, a chamber 72 replaces the insulating chamber 52, a through hole 73 replaces the through hole 60. The difference arises from the fact that the horizontal dimension of the capacitive differential pressure detector 82 is longer than the previous detector 50 by the length of the substrate 80.

An operation of the embodiment will be described with reference to mainly Fig. 3, and supplementally Fig. 4.

In Fig. 3, it is assumed that a pressure P2 applied from the right side is much higher than a pressure P1 from the left side. The pressure P2 also acts on the peripheral outer surface on the detector 82, as shown in Fig. 4. Accordingly, the pressure P2 acts on the left side of the substrate 80, while the pressure P1 acts on the right side. A difference between the applied pressure (= P2 - P1) bends the substrate 80 to the right. On the other hand, the fixed electrodes 15 and 20 are not bent or displaced because the pressure P1 and P2 are equally applied to both sides of each 7 of the fixed electrodes 15 and 20.

As a result, the capacitances formed by the diaphragm 10 and the fixed electrodes 15 and 20 exactly differentially vary. Accordingly, a differential pressure signal derived from the detector 82 exactly linearly varies in proportion to a variation of the differential pressure.

8

Claims

A capacitive differential pressure transducer for measuring a pressure on the basis of the capacitances of capacitors formed between a diaphragm displaced in response to said pressure and each of first and second fixed electrodes with pressure guide holes disposed on both sides of said diaphragm, said capacitive differential pressure transducer comprising a substrate with a pressure guide hole spaced by a predetermined distance from said first fixed electrode but bonded at the peripheral edge portion to the outside surface of said first fixed electrodes, whereby a pressure introduced from the second fixed electrode is also applied to a surface of said substrate opposite said first fixed electrode.
2. A capacitive differential pressure transducer substantially as hereinbefore described with reference to and as shown in Figs. 3 and 4 of the accompanying drawings.