GB2266961A - Capacitive differential pressure detector - Google Patents

Abstract

A capacitive differential pressure transducer includes a pair of electrode plates (12, 17) and a diaphragm (10) between these plates. Each electrode plate has a central fluid pressure supply hole (25, 26) and, in order to prevent closing off of this hole in the event of an excessive pressure moving the diaphragm into contact with either plate, each plate is formed with at least one groove (12a, 17a) extending from the supply hole to the periphery of the electrode plate. There may be a pair of such grooves, intersecting at right angles at the supply hole (Fig. 7 not shown). <IMAGE>

Description

1 2266961 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 ringlike groove 23 surrounding the first conductive plate 12. The support 21 is couple d 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 2 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 the 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.

Fig. 2 is a cross sectional view of a key portion of another conventional capacitive differential pressure detector.

3 In Fig. 2, reference symbols 100A designates a diaphragm made of silicon, 2A and 3A represent fixed electrodes, which are respectively coupled with the diaphragm 100A by means of glass bonding portions parts interposed therebetween. Reference symbol 8A stands for a gap between the diaphragm 100A and the fixed electrode 2A, and 9A, a gap between the diaphragm 100A and the fixed electrode 3A. 6A denotes a through-hole formed in the fixed electrode 2A for introducing pressure P1 into the gap 8A. 7A denotes a through-hole formed in the fixed electrode 3A for introducing pressure P2 into the gap 9A.

The diaphragm 100A and the fixed electrode 2A cooperate to form a first capacitor whose capacitance is pulled out through lead pins A and C. The diaphragm 100A and the fixed electrode 3A cooperate to form a second capacitor whose capacitance is pulled out through lead pins B and C.

When the pressures P1 and P2 are differentially applied to the diaphragm, the diaphragm displaces in accordance with a differential pressure. The capacitances 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. 25 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 non-compressive fluid, e.g., silicone oil. through which pressure transfers. In these circumstances, the gaps 8A and 9A, and the pressure guide holes 6A and 7A are filled with silicone oil.

4 The conventional pressure detectors as mentioned above have a problem of slow response. When an excessive pressure is suddenly removed, the displacement of the diaphragm cannot quickly ffillow the removal of the pressure. In other words, a response of the diaphragm displacement is poor.

Fig. 3 shows a sectional view of a key portion of the differential pressure detector of the prior art, when it is operating. As seen, a difference of the pressures introduced through the holes 25 and 26 (in the illustration, the pressure introduced through the hole 26 is much larger than that through the hole 25) bend the diaphragm 10 to the left, and the left side of the diaphragm is in contact with the right side of the conductive plate 12.

Under ideal conditions the contained liquid, such as silicone oil, is perfectly non-compressive and the surfaces of the diaphragm 10 and the conductive plate 12 are finished perfectly flat. When those surfaces come in contact with each other, no liquid will leak through the interface between them, the diaphragm 10 will further be bent after the central portion of the left side of the diaphragm is bent to close the right opening of the hole 25. Accordingly, the contact area of the diaphragm 10 with the conductive plate 12 is kept slightly larger than the right opening of the hole 25 even if the differential pressure increases.

Fig. 4 is a graphical representation of a variation of a contact area between the diaphragm and the fixed electrode vs. differential pressure. In the graph, the abscissa represents differential pressure P, and the ordinate a contact area 5. As seen, in a region where the differential pressure is small, the contact area S is zero. At Pa of the differential pressure, the contact area is Sa slightly larger than the right opening of the hole 25. Under the ideal conditions as mentioned above., the contact area 5 is kept at Sa (see an alternate long and short dash line) even if the differential pressure P further increases. Actually, the ideal conditions are not present, and the contact area 5 increases as indicated by a solid line with respect to increase of the differential pressure P from the pressure Pa.

Accordingly, when an excessive differential pressure acts on the diaphragm 10, the diaphragm 10 and the conductive plate 12 contact each other over a relatively large area. When the excessive pressure is suddenly removed, the central portion of the diaphragm 10 displaces to the right, to break the sealing of the right opening of the pressure guide hole 25. Then, the pressure introduced through the hole 25 acts on the left side of the diaphragm 10, to facilitate the return action of the displacement of the diaphragm 10. Thus, in the conventional differential pressure detector, the return of the diaphragm 10 cannot quickly follow the abrupt decrease or removal of the excessive pressure. In other words, a response of the displacement of the diaphragm 10 is poor or slow.

It is an object to the present invention to provide a capacitive differential pressure detector which is responsive to diaphragm displacement following an abrupt decrease of an excessive differential pressure.

A capacitive differential pressure detector according to the invention comprises: a diaphragm h-aving opposite side surfaces; fixed electrodes disposed adjacent each of said opposite side surfaces of said diaphragm, each of said fixed electrodes including a pressure guide hole passing 6 through the central portion thereof; wherein each of said fixed electrodes has at least one groove formed on a surface closer to said diaphragm, said at least one groove intersects said pressure guide hole.

In such a capacitive differential pressure detector, when an excessive pressure presses a part of one of the sides of the diaphragm against the surface of the fixed electrode, the pressure introduced through the fixed electrode is also applied to the surface of the diaphragm, through the groove intersection the pressure guide hole. The pressure applied to the diaphragm surface facilitates the return action of the displaced diaphragm, thereby improving the response of the diaphragm.

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 an example of a prior art differential pressure transducer;

Fig. 2 is a cross-sectional view of another example of a prior art transducer;

Fig. 3 is an enlarged cross-sectional view showing the transducer of Figure 1 in operation; 7 Fig. 4 is a graph showing a characteristic variation of diaphragm to fixed electrode contact area of a transducer of Fig. 1 plotted against differential pressure; Fig. 5 is a cross-sectional view like Fig. 1, but showing a transducer in accordance with the present invention; Figs. 6 and 7 are front views of two alternative electrode plate configurations which could be employed in the example shown in Fig. 5; and Fig. 8 is an enlarged cross-sectional view like Fig. 3, but showing the example of Fig. 5.

Fig. 5 shows a cross sectional view of the embodiment, and Fig. 6 shows a front view of a part of the embodiment. In Figs. 5 and 6, the instant embodiment is different from the prior art shown in Fig.1 in that grooves 1 2a and 1 7e are diametrically formed in the surfaces of the conductive plates 12 and 17, which face the diaphragm 10. The grooves 12a and 1 7e extend through the pressure guide holes 25 and 27. Like reference symbols are used to designate like or equivalent portions in Fig. 1.

An operation of the third embodiment will be described with reference to Fig. 8. In the figure, there is illustrated a cross section showing in detail a portion of the pressure detector where the diaphragm 10, when receiving an excessive differential pressure, displaces and comes in contact with the conductive plate 12, and its peripheral portion. In the figure, the center portion of the left side of the diaphragm 10 is pressed against and in contact with the right side of the conductive plate 12. It is 8 noted that unlike the prior art of Fig. 3, a pressure guided by the hole 25 is also exerted on the left side of the diaphragm 10 through the groove 12a. With the pressure acting on the left side of the diaphragm 10, when the excessive pressure is removed, the diaphragm 10 quickly returns to the original position. In other words, the diaphragm 10 returns with a good response.

To improve the return action or response of the displacement of the diaphragm 10, it is advantageous to widen the gap 12a as wide as possible. When the groove 12a is made wide, the surface area of the conductive plate 12 is proportionally reduced. Accordingly, the capacitance between the conductive plate 12 and diaphragm 10 reduces. To cope with this, compromise is made between the response of the diaphragm 10 in its displacement and the capacitance. The same thing is true for the conductive plate 17.

Fig. 7 shows a front view of each of the conductive plates 12 and 17 of another configuration. In this arrangement, grooves 12 and 12b, each of which are diametrically formed on the surface of the conductive plate, cross with each other. The pressure guide hole 25 is located at the intersection of the crossed grooves 12a and 12b. The grooves 1 7a and 1 7b, and the pressure guide hole 26 of the conductive plate 17 are similarly formed and arranged. With such structures, the guided pressure is applied to different locations on the diaphragm, so that the return action or response of the diaphragm 10 is superior to that of the previous embodiment. In this embodiment., the disadvantage of capacitance due to the crossed grooves exists. Practically., therefore, the compromise between the response and the capacitance is required in design.

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Claims (3)

1 A capacitive differential transducer 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 including: a pressure guide hole passing through the central portion thereof; wherein each of said fixed electrodes has at least one groove formed on a surface closer to said diaphragm, said at least one groove intersects said pressure guide hole.

2. A transducer as claimed in Claim 1 wherein a plurality of grooves are formed on a surface of each of said fixed electrodes closer to said diaphragm, each of said grooves intersecting said pressure guide hole.

3. A capacitive differential transducer substantially as hereinbefore described with reference to and as shown in Figures 5, 8 and 6 or 7 of the accompanying drawings.