Measuring and testing – Fluid pressure gauge – Diaphragm
Patent
1997-08-25
1999-07-06
Felber, Joseph L.
Measuring and testing
Fluid pressure gauge
Diaphragm
73718, 3612834, G01L 912
Patent
active
059200154
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to pressure transducers or sensors for measurement of pressures derived from fluids of various kinds, both gases and liquids, and methods of producing such pressure transducers or sensors.
BACKGROUND OF THE INVENTION AND STANDPOINT OF THE ART
Sensor elements for pressure sensors based on ceramics and constructed as dilatation or strain sensors or capacitive sensors can conventionally comprise various ceramic materials. Often, then, ceramics is used based on aluminum oxide but also glass ceramics is used. There are some problems associated with the use of such sensors and some of them will be discussed hereinafter with reference to FIG. 1 and FIGS. 2a-2d.
A capacitive sensor element 1 of conventional type based on ceramics, for example glass ceramics, is illustrated in the sectional view of FIG. 1. The sensor element comprises a thick house part 1, in the shape of a plate 3 comprising an annular projection or platform 5 located at the circumference thereof on one side. At the annular projection a thin plate 7, a pressure diaphragm, is attached, which thus has a considerably lesser thickness than the house part 1. The house part 3 and the diaphragm 7 carry on their interior, opposite surfaces at the central portions thereof electrically conducting areas 9 and 11 respectively in the shape of thin layers, which normally have the same configuration and are located opposite each other and at a small distance of each other. When the distance between the opposite surfaces on the two conductive areas 9, 11 in the capacitor formed thereof is varied, the capacitor will have a varying capacitance, which is readily detected by means of suitable electronic circuits. For an optimally designed sensor element then the capacitance or some function derived therefrom in a simple way, for instance the inverted value of the capacitance, should be a linear function of for example the distance of the plates in the capacitor formed. The sensor element can then be used for pressure measurement and then, in the corresponding way, the electronically detected capacitance or some other quantity derived in a simple way should be proportional to the pressure acting on the diaphragm 7. However, deviations from the linear behavior are always present and will be described hereinafter.
The presupposition that a linear function exists or should exist, is based on the theory of the capacitance between two flat electrically conducting plates which are parallel to each other. However, for the pressure sensor according to FIG. 1, the movable diaphragm 7 and hereby the electrode plate or the electrode area 11, which is located thereon, will have some curved profile, when the diaphragm is exposed to an exterior pressure deviating from the pressure on the chamber formed between the house 3 and the diaphragm 7. The electrodes 9, 11 in the plate capacitor formed are thus then not flat and not parallel to each other. This deflection effect can be calculated numerically and has generally a small importance in a complete or finished pressure sensor having the sensor element attached in a house. If required, however, this effect can be compensated in an electronic way.
Another basic condition for the presupposition of a linear dependence of an output signal and the pressure acting on the diaphragm is that the magnitude of the deviation or deflection of the diaphragm is proportional to the applied pressure force. It is valid for small deviations from the equilibrium position of the diaphragm, where the equilibrium or rest position can be the state of the diaphragm for equally large pressures on the two sides thereof. However, for larger deflections from the rest position the deflection will not follow the pressure force proportionally but is less than what would be obtained in an ideal proportional case. The deflection of the diaphragm can be calculated in different ways, such as for example be calculated approximatively by means of a theory such as "Large Deflection Theory, LDT". This non-linear effect can
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patent: 4628403 (1986-12-01), Kuisma
patent: 4823603 (1989-04-01), Ferran et al.
patent: 4831492 (1989-05-01), Kuisma
patent: 4935841 (1990-06-01), Jonsson et al.
patent: 5189591 (1993-02-01), Bernot
Hallberg Nils Gunno
Jonsson Staffan
Cecap AB
Felber Joseph L.
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