GB2266960A

GB2266960A - Capacitive differential pressure detector

Info

Publication number: GB2266960A
Application number: GB9314964A
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: 1993-07-29
Publication date: 1993-11-17
Anticipated expiration: 2013-07-29
Also published as: GB2266963A; GB2266961B; GB9315065D0; GB2266963B; GB9314964D0; GB9314965D0; GB9315064D0; GB2266962B; GB2266960B; GB2266962A; GB2266961A

Abstract

A capacitive differential pressure transducer includes a diaphragm 10 mounted between two composite fixed electrodes (85, 86) each such fixed electrode comprises a first plate (12, 17), an annular support (21, 22) bonded to the diaphragm, an insulating plate (83, 88) bonded to one side of the annular support and the first plate, and a second plate bonded to the other side of the insulating plate, the diaphragm, the first and second plates and the insulating plate having substantially equal co-efficients of thermal expansion. As described the insulating plates (83, 88) are made of a mixture of cordierite and mullite to give an expansion coefficient matched to that of silicon. <IMAGE>

Description

CAPACITIVE DIFFERENTIAL PRESSURE DETECTOR 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 ring-like 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 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.

Prior art capacitive differential pressure detectors, as will be described in detail below, have a drawback in that the span characteristic and the linearity of the detectors are adversely influenced by variation of an ambient temperature. In other words, the temperature characteristic of the detector degrades. Here, the span characteristic is a variation span of the capacitance relative to the 100% variation span of a differential pressure, that is, the characteristic variation of a displacement of the diaphragm.

Each electrode can be considered as a kind of bimetal formed by laminating plate-like members of different expansion coefficients. The electrodes have a three-layered structure consisting of the first and second conductive plates made of silicon and the insulating plate made of cordierite, which is sandwiched by the first and second conductive plates. Each electrode deforms when ambient temperature changes, so that in the diaphragm made of silicon peripherally fastened, as stress is developed in the radial direction of the diaphragm. A displacement of the diaphragm due to the stress degrades the linearity of the differential pressure signal due to the original displacement of the diaphragm caused by the differential pressure.

The radial stress in the diaphragm due to the ambient temperature change and the displacement of the diaphragm due to the stress will be described in detail.

In Fig. 1 a composite thermal expansion coefficient ocof the fixed electrodes 15 and 20 is oc = K1 (A-K2/ss)(oc1-02) + 0t2 (id) where: α1 and α2 = thermal expansion coefficients of cordierite and silicon respectively; El and E2 = Young's moduli of cordierite and silicon respectively; H1 and H2 = thickness of the cordierite and silicon layers respectively; and H3 = thickness of supports 21 and 22.

In the above equation, K1 and K2 are constants that are determined by El, E2, H1 and H2. Further, A and B are A = (H1 + 2H3)/2, and B = 1/(H1 El).

If El = 8,000, E2 = 15,300 (kg/mm2), α1 = 1.1 (10-6/C ), α2 = 3.1(10- 6/C"), H1 = 0.5 (mm), H2 = 1.5 (mm), and H3 = 1.5 (mm), then we have cc = 2.53 x 10-6/C0.

Accordingly, when a change of the ambient temperature is DT, a radial stress "d" developed in the diaphragm is # = E. Dα.DT/(1-#) (2d) where E and # are Young's modulus and Poisson's ratio respectively, Dcc = difference between the thermal coefficients of the fixed electrode and the diaphragm.

A displacement W of the diaphragm containing a radial stress C developed therein when a differential pressure P is applied thereto, is W = P/[K + (4H/R2) < t (3d) where H and R = thickness and radius of the diaphragm respectively, and K = constant determined by E, u, H and R.

As seen from the equation (3d), the displacement W is determined by the first factor involving material and size of the diaphragm, and a second factor involving the radial stress. The equation (3d) further teaches that to measure a minute differential pressure P, it is necessary to reduce the thickness H of the diaphragm, and that the stress ci hinders the differential pressure measurement of good sensitivity.

Fig. 3 shows a graphical representation of a characteristic variation of a value WIG with respect to thermal stress o. A measurement to collect the data plotted in the graph was conducted under water-gauge pressures of 0.1 m and 3.2 m. In the graph, a solid line indicates a variation of the W/G when the water-gauge pressure is 0.1 m, and a broken line, a variation of the W/G when the water-gauge pressure is 3.2 m. G indicates a gap width between the diaphragm and the fixed electrode when the differential pressure applied is zero.

Specifically, when the ambient temperature changes within a range of + 60"C (120"C), the equation (2d) shows that the thermal stress "d" changes by 0.62 kg/mm2. Due to the change of the thermal stress, the W/G concerning the diaphragm displacement is approximately 82% for the 0.1 m water-gauge pressure, and approximately 6% for the 3.2 m watergauge pressure.

One object of the present invention is to provide a capacitive differential pressure detector with a good temperature characteristic. Specifically, it is an object of the present invention to provide a capacitive differential pressure detector which minimizes the adverse effects of temperature variation upon the span characteristic and the linearity of a differential signal.

A capacitive differential pressure transducer according to the present invention comprises: a diaphragm having opposite side surfaces; fixed electrodes disposed adjacent each of said opposite side surfaces of said diaphragm, each of said fixed electrodes comprising: a first plate adjacent a central portion of one of said opposite side surfaces of said diaphragm, said first plate having a thermal expansion coefficient substantially equal to that of said diaphragm; an annular support bonded to a peripheral edge portion of said diaphragm and disposed separated from and around a peripheral end face of said first plate; an insulating plate bonded over a surface of said annular support and a surface of said first plate opposite a surface facing said diaphragm, said insulating plate having a thermal expansion coefficient substantially equal to that of said first conductive plate; and a second plate bonded to said insulating plate on a surface on a surface opposite said first plate, said second plate electrically connected to said first plate.

In such a capacitive differential pressure transducer, the first and second conductive plates of the fixed electrodes disposed on both sides of the diaphragm, and the insulating plates interposed therebetween are made of the materials of equal or near thermal expansion coefficients.

Accordingly, no radial stress is developed in the diaphragm when the ambient temperature varies. Accordingly, no displacement occurs due to this stress. Therefore, the span characteristic and the linearity of the differential signal are kept good even if ambient temperature varies.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

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 an example of the present invention; Fig. 3 is a graph showing a characteristic variation of a diaphragm displacement with respect to thermal stress; and Fig. 4 is a graph showing a characteristic variation of thermal expansion coefficient of cordierite and mullite with respect to percentage of those materials.

Fig. 2 shows a cross sectional view of the example. In Fig. 2, insulating plates 83 and 88 contained in fixed electrodes 85 and 86 are made of two types of ceramics of different thermal expansion coefficients which are mixed and baked. The resultant thermal coefficient is approximately equal to that of silicon. Other members are substantially the same as those already described, and hence like reference symbols are used to designate them. A mixing ratio of cordierite and mullite was variously changed. For those different mixing ratios, differential values between the thermal expansion coefficients of the mixed ceramics and that of silicon were collected, and plotted in a graph of Fig. 4.In the figure, the abscissa represents a percentage C of mullite (%), and the crdinate represents the difference ss between the thermal expansion coefficient of the mixed ceramics and that of silicon. As seen from Fig. 4, when the percentage C of the mullite exceeds 50%, the difference ss is below + 106/C .

By using the equation (led), oc = 2.71 x 10-6/C is worked out. By using the equation (2d), C = 0.43 kg/cm~2 is worked out. By using those figures and the equation (3), W/G was worked out. At the 0.1 m of water-gauge pressure for measuring a minute differential pressure, it was 47% against 1 200C. This figure is approximately half of 82% of the prior detector.

In Fig. 4, when the percentage C of mullite is 80%, the thermal expansion coefficient ss is approximately zero. The influence by temperature variation is further suppressed compared with the previous case. According, the temperature characteristic is improved.

As described above, the diaphragm, the first and second conductive plates of the fixed electrodes disposed on both sides of the diaphragm, and the insulating plates interposed therebetween are made of materials of equal or near equal thermal expansion coefficients. Accordingly, no radial stress develops in the diaphragm when the ambient temperature varies. Accordingly, no displacement occurs due to this stress.

Therefore, the span characteristic and the linearity of the differential signal are kept good. The influence of the ambient temperature upon the span characteristic and the linearity may be minimized. In this respect, the temperature characteristic of the detector is improved.

Claims

1. A capacitive differential pressure transducer for measuring a pressure on the basis of capacitances comprising: a diaphragm having opposite side surfaces; fixed electrodes disposed adjacent each of said opposite side surfaces of said diaphragm, each of said fixed electrodes comprising: a first plate adjacent a central portion of one of said opposite side surface of said diaphragm, said first plate having a thermal expansion coefficient substantially equal to that of said diaphragm; an annular support bonded to a peripheral edge portion of said diaphragm and disposed separated from and around a peripheral end face of said first plate; an insulating plate bonded over a surface of said annular support and a surface of said first plate opposite a surface facing said diaphragm, said insulating plate having a thermal expansion coefficient substantially equal to that of said first conductive plate; and a second plate bonded to said insulating plate on a surface opposite said first plate, said second plate electrically connected to said first plate.

2. A capacitive differential pressure transducer substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.