Electricity: electrical systems and devices – Electrostatic capacitors – Variable
Patent
1989-10-04
1990-06-19
Griffin, Donald A.
Electricity: electrical systems and devices
Electrostatic capacitors
Variable
73724, G01L 912, H01G 700
Patent
active
049358413
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention is related to a capacitive pressure sensor.
BACKGROUND OF THE INVENTION
Capacitive pressure sensors of the type concerned with herein consist of a hermetically closed housing having a reference pressure therein. One side of the housing has the shape of a flexible diaphragm. On the diaphragm a capacitor electrode is located and on an opposite surface, inside the housing, another capacitor electrode is located. Both of the capacitor electrodes are connected to an electrical measuring circuit. When the pressure acting on said diaphragm varies the diaphragm is flexed. Thereby the distance between said both capacitor electrodes is varied whereby the magnitude of the capacity will vary in the capacitor formed by these two capacitor electrodes. Other locations of said electrodes are also possible.
For a pressure sensor of this type the capacitance of the capacitor electrodes is composed of a capacitance varying inversely proportionally to the distance between the capacitor electrodes and a further partial capacitance, not varying in this way and deriving, from among other things, stray capacitances and border effects in the flat capacitor formed by said electrodes. When the capacitance of the capacitor electrodes is measured in order to obtain a measure of the bending of said diaphragm and consequently of value of the pressure acting on said diaphragm this further partial capacitance is a source of errors.
Pressure sensors of this prior art type are described in e.g. U.S. Pat. No. 4 207 504, and U.S. Pat. No. 3 948 102.
To reduce said further partial capacitance and simultaneously constitute an electrical shielding of said capacitor electrodes it is previously known to provide a conducting layer on the outer surface of the pressure sensor, see for instance US A 3 858 097, EP AI 0 095 966, US A 3 750 476, though these documents do not directly describe a pressure sensor of the type set forth above. Thus the first of these documents is concerned with flat pressure sensors having two equally thick diaphragms located adjacent to one another and the third document is concerned with a pressure sensor having a cylindrical diaphragm.
SUMMARY OF THE INVENTION
However, it has been shown that error-producing partial capacitance can be reduced even more and the linearity of the pressure sensor be increased even more, as is proposed according to the invention by, providing in said thicker housing a recess adjacent to an electrode located inside said housing. This recess will also then be coated with a conducting material. The bottom of said recess is preferably flat and parallel to the adjacent capacitor electrode. The lateral width of said bottom surface, moreover, should be larger than said adjacent capacitor electrode.
It can be emphasized that said recess is not allowed to be so deep that the remaining material in said housing or the thickest part of the sensor also will operate as a flexible diaphragm. Further the recess has a diameter being maximally 2.5 of the diameter of said adjacent capacitor electrode. The recess further has conveniently the same shape as the adjacent capacitor plate, i.e. it is in most cases circular or cylindrical.
By the fact that the electrically conducting layer of the surrounding metal shielding, due to said recess, will be located more closely to one electrode the electrical conditions around this capacitor electrode will be more homogeneous. It can be supposed that this causes the mentioned effect of significantly improving the linearity of the pressure sensor.
DESCRIPTION OF THE FIGURES
FIG. 1 is a section of a prior art shielded pressure sensor,
FIG. 2 is a view from above of the housing or the base part of the pressure sensor of FIG. 1,
FIG. 3 is a section of a shielded pressure sensor according to the invention and
FIG. 4 is a view from above of the base portion of the pressure sensor of FIG. 3.
In FIGS. 1 to 4 is illustrated how the electrical shielding of a capacitive pressure sensor can be realized. Thus, in FIGS. 1 and 2 is illustrated
REFERENCES:
patent: 4562742 (1986-01-01), Bell
patent: 4875134 (1989-10-01), Kuisma
Jonsson Staffan
Olsson Ray
Griffin Donald A.
Hydrolab AB
Johnsson & Billquist Development AB
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