CERAMIC PRESSURE SENSOR CAPSULE

Abstract: A capacitive pressure sensor capsule has a housing with a ceramic cover sealingly attached to a ceramic base that defines an interior chamber. The sensing medium enters the chamber through an inlet port. A glass sensor is attached to an integral pillar that extends from the housing into the chamber. A conducting member extends through the pillar into the housing and carries the signals from the sensor. The combination of the glass sensor and ceramic housing provides high accuracy over a wide range of temperature, because the thermal coefficient of expansion of glass and ceramic are similar. Hermetic sealing of the sensor electrical circuitry ensures long term immunity from the sensed environment.

Description: CERAMIC PRESSURE SENSOR CAPSULE REFERENCE TO COPENDING APPLICATION This application claims the benefit of U.S. Provisional Application No. 60/040,824 filed March 20, 1997. TECHNICAL FIELD This invention relates generally to pressure sensors or transducers and in particular to glass sensors. BACKGROUND OF THE INVENTION Ohnesorge et al, U.S. Patent No. 4,422,335 discloses a pressure transducer made using quartz plates and Bernot, U.S. Patent No. 5,189,591 discloses a pressure transducer using glass plates. Whether glass or quartz, these sensors have generally been mounted to headers made of aluminum or stainless steel. Besides being expensive to machine, another disadvantage to these aluminum and stainless steel headers is that it is difficult to hermetically seal the electrical connections to these sensors. Without a hermetic seal liquid and gas contaminants in the sensing medium can enter and ruin the sensor. As a result these types of sensors can only be used where the pressure medium is contaminant free such as dry gasses. Accordingly, a need exist for a more robust capacitance sensor where the electrical connections can be hermetically sealed. SUMMARY OF THE INVENTION In view of the above, it is an object for this invention to provide a more robust glass sensor that can be used is sensing mediums containing liquid or gas contaminants. The present invention achieves these objects by providing a capacitive pressure sensor capsule that includes a housing with a ceramic cover sealingly attached to a ceramic base to define an interior chamber. The sensing medium enters the chamber through an inlet port. A glass sensor is attached to the end of a tube member that extends from the housing into the chamber. Conducting pins or film extend through the hole in the ceramic header carry the signals from the sensor. These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view of a glass pressure transducer. FIG. 2 is a side view of the pressure transducer of FIG. 1. FIG. 3 is a side view of a glass accelerometer. FIG. 4 is a schematic of the sensor of FIG. 1 mounted in a ceramic housing as contemplated by the present invention. DESCRIPTION OF THE INVENTION Bernot, U.S. Patent No. 5,189,591 which issued on February 23, 1993 and is assigned to AlliedSignal Inc. is hereby incorporated by reference. This patent discloses a capacitive pressure transducer made of alumino silicate glass. A glass pressure transducer 10 contemplated by the present invention is shown in Figs. 1 and 2. The transducer or sensor 10 includes a top diaphragm 12 and a bottom diaphragm 14 which may be of different thicknesses made from a glass preferably aluminosilicate glass such Corning #1723 or # 1737F. Ground shields 18 and 24 are deposited on the outer surfaces of the diaphragms 12 and 14. Electrodes 20 and 22 are deposited in the inner surfaces of the diaphragms 12 and 14. A crossover metal tab 26 preferably made of silver acts as a conductor from the electrode 20 to a top electrode lead 21. Alternatively, the crossover tab 26 can be made from other metals such as platinum, palladium, or gold. The electrode 22 is connected to a bottom electrode lead 23. A ground lead 25 may also be added. The leads 21, 23, and 25 are preferably made of a noble metal, preferably platinum. Wires, not shown, may be connected to these leads. The geometry of the electrodes 20, and 22 can be round, square, or other shape. In addition the electrodes can be configured as multiple reference capacitor or pressure capacitors. The ground shields 18, 24 and the electrodes 20, 22 are made of a noble metal preferably platinum. The diaphragms 12 and 14 are bonded together such that the electrodes 20 and 22 form a capacitor. The bond is formed by hydrate bonding, glass friting, or other adhesives. The frit glass 30 acts as a spacer between the diaphragms 12 and 14 and also as a seal to form a cavity 32 between the diaphragms. The cavity 32 can be evacuated to form either a vacuum, some other reference pressure, or left open for gauge pressure measurement. Referring to FIG. 3, a three piece glass accelerometer 40 can be formed from the pressure transducer 10 by mounting a glass, preferably aluminum silicate, seismic mass 42 between the top and bottom diaphragms 12,14. The seismic mass 42 has an electrode 44 in opposed relationship with electrode 20 and an electrode 46 in opposed relationship with electrode 22. The seismic mass 42 is bonded to the diaphragms 12,14 by frit 48 in such a way as to form cavity 49. Referring to FIG. 4, the sensor 10 is mounted within a two part glass ceramic housing having a cover 24 and a header or base 26 that define an interior chamber 25. The cover 24 and header 26 can be made by aqueous ceramic injection molding or a dry process similar to that currently used to make spark plugs. Alternatively, the cover 24 can be made of metal or polymer. The cover 24 and base 26 can be sealed together with glass frit or organic adhesive or using a mechanical sealing method. The cover 24 has a pressure inlet port 28 that is in fluid communication with a source of fluid pressure to be measured and with the chamber 25. In a manner familiar to those skilled in the art, the base 26 is mounted to electronic circuitry 23. The base 26 has at least one two holes through the integral pillar 50 which extends into the chamber 25. The top diaphragm 12 is mounted to the end of the pillar 50 with glass frit or other adhesive. At least two metal conducting pins 52 or films, electrically connect top diaphragm 12 through the pillar 50 to optional electronic circuitry on the base 26. The cover 24 and base 26 can have a variety of shapes such as square, rectangular or circular. The combination of the glass diaphragms and ceramic housing provides high accuracy over a wide range of temperature, because the thermal coefficient of expansion of glass and ceramic are matched. Hermetic sealing of the sensor electrical circuitry ensures long term compatibility with the sensed environment. Capsule mounting techniques minimizes mechanically induced stresses and hysteresis thus maximizing sensor performance. Thus the sensor is made more robust than those in the prior art and can be used with sensing mediums that contain gas or liquid contaminants.

Claims: What is claimed is: 1. A capacitive pressure sensor capsule comprising: a housing having a ceramic cover sealingly attached to a ceramic base to define an interior chamber, said housing further including an inlet port in fluid communication with a source of fluid pressure to be measured and said chamber and a tube member extending into said chamber, a glass sensor attached to said tube member and disposed in said chamber; and at least one conducting member extending from said sensor and through said pillar member and into said housing. 2. The capsule of claim 1 wherein said cover is metal. 3. The capsule at claim 1 wherein said cover is a polymer. 4. The capsule of claim 1 wherein said cover and base are sealed together with an adhesive. 5. The capsule of claim 4 wherein said adhesive is glass frit. 6. The capsule of claim 1 wherein said glass sensor comprises: a first diaphragm with a first electrode thereon; a second diaphragm with a second electrode thereon; and said first and second diaphragms bonded together so that said first and second electrodes cooperate to form a capacitor. 7. The capsule of claim 4 wherein said diaphragms are of different thicknesses. 8. The capsule of claim 6 wherein said diaphragms are bonded together with frit, said frit spacing said first diaphragm from said second diaphragm to define a cavity therebetween. 9. The capsule of claim 8 wherein said cavity is evacuated to a preselected pressure. 10. The capsule of claim 8 wherein said cavity is open to ambient. 11. The capsule of claim 6 further comprising a crossover tab connecting said first electrode to a lead. 12. The capsule of claim 6 further comprising a ground shield on each of said diaphragms.

Inventors: BERNOT ANTHONY J, HUGHES GRENVILLE, LINDBERG LAURA J, BERNOT, ANTHONY, J., HUGHES, GRENVILLE, LINDBERG, LAURA, J.
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL INC.
Application year: 1998
Main classification: G01L9
IPC codes: G01L9