Pressure Sensitive Transducer

This invention relates in general to pressure responsive transducers and, more particularly, to an improved and more economical capacitative diaphragm type pressure transducer.

In U.S. Pat. No. 3,808,480 there is disclosed an improved pressure transducer utilized to obtain electrical signals corresponding to pressure variations existing between variably sized chambers formed by a pair of diaphragms and either a chamber surrounding the diaphragms or the ambient environment. The transducer is an unusually compact and effective arrangement for detecting pressure variations and the two diaphragms forming the variable chambers are each soldered or otherwise hermetically secured to a respective electrode on a nonconductive substrate.

The electrode encircles and is spaced from a respective inner electrode or fixed capacitor plate located on the nonconductive substrate and an opening in the substrate communicates the chambers formed by the diaphragms on opposite side of the substrate to equalize the pressure therebetween. The diaphragms thus also form one plate of a capacitor and their spacing from the fixed plates on the substrate is determined by the difference in pressure between the variable chambers and the external space, which pressure difference is signalled by extending respective leads from the diaphragm electrodes and the fixed capacitor plates.

In the described arrangement, at least one of the diaphragms was pierced, usually along its central axis for forming a vacuum between the diaphragms. The hole in the pierced diaphragm was then required to be soldered or sealed in a vacuum chamber or alternatively a connection to an external vacuum or source of pressure has to be provided by a fitting usually connected at the rim wall of the diaphragm.

A problem is therefore created not only by the economics and reliability of the process, but also by the response of the diaphragm due to the distortions introduced by the opening and the seal in the diaphragm and by the differences in the reaction of one diaphragm from the other.

Alternatively, the space between the diaphragms may be at ambient pressure and the entire transducer in turn placed in a containment chamber, which is either subject to a vacuum or some other pressure source, but this undesirably requires an additional chamber.

Another problem relates to the need for extending a lead from the fixed capacitor plates since this lead usually passes between a diaphragm rim and the adjacent substrate and must be insulated from the diaphragm thereby creating an undesirable capacitative relationship.

This invention provides a diaphragm type pressure transducer in which communication and/or sealing of the variably sized chamber formed by each diaphragm is provided without affecting the response of either diaphragm.

Basically this is accomplished by a sandwich or substrate assembly including an intermediate substrate having a radially extending slot therein to form a conduit or passageway through which communication with the variable chambers formed by the diaphragms is easily established at the boundary or edge of the substrate and leaves the diaphragms unstrained and in similar condition. When the intermediate substrate is placed between two opposing or spaced substrates, each carrying a respective diaphragm and fixed capacitor plate, the conduit is closed except at the end or edge of the intermediate substrate and a metal layer or deposit on portions of each substrate adjacent the boundary end of the slot may be soldered to either close the end of the passageway or to enable an appropriate fitting to be secured thereto as desired without introducing any strain or undesired condition in the diaphragms.

Electrical connections to and between the diaphragms are established by conductive deposits or leads formed on supporting surfaces of the outer substrates and on aligned passageways extending through all the substrates. The fixed capacitor plates are connected to an external connection by a lead extending through the passageway between the diaphragm chambers to thereby use this passageway for two purposes and connected to a lead on the intermediate substrate extending to a position external to the diaphragm peripheries to thereby space the fixed plate leads from the diaphragms and their leads by a uniformly large distance and to minimize distortions in capacitative response.

FIG. 1 is an exploded isometric view of the pressure transducer embodying the principals of the present invention; and

FIG. 2 is a a sectional view taken generally through the line 2--3 in FIG. 1.

In FIG. 1, an assembly forming a capacitative pressure transducer is indicated by the reference character 10. The transducer 10, as described in the aforementioned patent may be referred to as a capsule and be secured in a casing or exposed to an ambient environment condition as required by its use. The transducer 10 includes a pair of spaced cup shaped metallic diaphragms 12 and 14 with a substrate assembly 16 therebetween.

The substrate assembly 16 includes an upper plate or outer substrate 18 and intermediate plate or substrate 20 and a lower plate or outer substrate 22. The substrates 18, 20 and 22 are each preferably formed of a ceramic material such as 94% Al.sub.2 O.sub.3 sold by American Lava Corporation, a subsidiary of 3M Company and located at Chattanooga, Tennessee. The substrate material is described in advertising literature entitled AlSiMg and in a publication entitled Design Consideration for Multilayer Ceramic Substrates, both published by the aforementioned Company.

The substrates 18, 20 and 22 are shaped in the form of polygons such as hexagons, rectangles or squares with sheared corners having coincident edges or boundaries when stacked. Each outer substrate 18 and 22 is between 0.010 inch and 0.030 inch thick, preferably between 0.020 inch and 0.030 inch thick, and has printed or otherwise hermetically sealed and secured on the outer or supporting surface 24 thereof metallized portions comprising or forming an inner or fixed capacitor plate 26 and an outer annular electrode or lead 28 spaced from and encircling the fixed plate 26. An aligned hole or passageway 30 in each fixed plate 26 and each substrate 18, 20 and 22 extending along the central axis of each substrate 18, 20 and 22 has a conductive material or lead 32 deposited and bonded on the walls thereof as seen in FIG. 2. The lead 32 interconnects each fixed capacitor plate 26 with a conductive deposit or lead 34 formed and secured on one surface of the plate 20, separated from lead 28 by the thickness of substrate 18, and extending to a second series of aligned or registering so-called via holes or passageways 36 radially outwardly of or beyond the periphery of the diaphragms 12 and 14. The holes 36 each have a suitable metal deposit or lead on the walls thereof interconnecting the fixed capacitor plates 26 with the terminals 40 on the supporting surfaces 24 of substrates 18 and 22 at a position beyond the periphery of the diaphragms 12 and 14.

Each cup shaped diaphragm 12 and 14 has a peripheral rim or wall portion 42 which is soldered or otherwise hermetically secured or bonded to a respective annular electrode 28 so that each diaphragm is hermetically secured to a respective supporting surface 24 to form a respective chamber 44 which communicates through the passageways 30 to equalize pressure in the chamber. A metal deposit or lead 46 bonded on the outer supporting surface 24 of each outer substrate 18 and 22 extends radially outwardly from the electrode 28 to a terminal on each supporting surface and these are connected in common by metal deposited lead bonded to the wall of so-called via holes or passageway 48 in each substrate to enable the diaphragms to be connected in common to an external lead.

The intermediate substrate or plate 20 is substantially 0.030 inch thick and has a radially extending passageway or slot 50 formed therein and extending radially outwardly from the passageway 30 to one edge portion or boundary 52 of plate 20. The edge portion 52, which is radially outward of the periphery of diaphragms 12 and 14 has a suitable metal deposit 55 formed and bonded thereon as to adjacent edge or boundary portions 54 and 56 and adjacent supporting surfaces 24 of substrates 18 and 22. The respective edge or boundary portions of substrates 18 and 22 are coincident with edge portion 52 and adjacent the radial outward end of slot 50. Thus when the plates are stacked and and fired to provide a bond and a hermetic seal therebetween, the slot 50 and adjacent substrates, which overlap the slot 50, form a fluid passageway, port or orifice which communicates with the central passageways 30 and the hermetically sealed variable sized chamber formed by the diaphragms and the substrates. The fluid passageway may be used to establish a vacuum in chambers 44 and then sealed by applying solder, for example, to the adjacent metallized portions 52 and 54 and 56 of the substrates or a conduit fitting may be secured thereto thereby eliminating problems created by an opening in one of the diaphragms. The metallized portions 52, 54 and 56 on the boundary or periphery of the substrate assembly and on supporting surface 24, thus enable control of the fluid passage through the passageway 50.

The lead 34 extends radially outwardly at 180.degree. from lead 46 and is, of course, uniformly spaced by a relatively large distance from the diaphragms and lead 46 to minimize capacitative problems. If required, the conduit or slot 50 may be provided with a metal deposit to facilitate a connection to the fixed capacitor plate instead of utilizing lead 34.

The substrates 18, 20 and 22 are provided with suitable spaced and aligned openings 58 for mounting purposes and a pair of passageways 60 are provided in the plates to provide fluid communication between opposite supporting surfaces and external diaphragm surfaces in the event the transducer is mounted in a container or in a vessel without means for equalizing the external pressure on the diaphragms.

Preferably, in forming the transducer 10, the ceramic substrates 18, 20 and 22 are first formed and punched to form the passageways or openings and metallized in the described configurations after which the substrates may be fired to fuse the same and hermetically seal or bond the same to each other. The metallized portions may also be plated with other metals such as silver, gold, tin, copper or solder to facilitate subsequent diaphragm attachment. The diaphragm may be attached by soldering, welding, brazing or thermo compression or adhesive technique to the electrode 28. Termination tabs for leads 34 and 46 may be similarly provided.

The foregoing is a description of an improved capacitative pressure transducer whose inventive concepts are believed set forth in the accompanying claims.