Capacitive pressure sensor having petal electrodes

Details Capacitive pressure sensor having petal electrodes Capacitive pressure sensor having petal electrodes

1999-11-15

2001-07-10

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Fluid pressure gauge

Diaphragm

C073S724000, C361S283400

Reexamination Certificate

active

06257068

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not Applicable
REFERENCE TO MICROFICHE APPENDIX
Not Applicable
FIELD OF THE INVENTION
The present invention relates to a pressure sensor, and more particularly, a pressure sensor which relies on changes in capacitance to indicate pressure fluctuations.
BACKGROUND OF THE INVENTION
Capacitive pressure sensors are well known in the prior art. Such sensors typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A displacable (relative to the fixed element) conductive member, such as a metal foil diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. Because the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance C of the sensor is approximated by the equation:
C
=
ε
⁢
 
⁢
A
d
(
1
)
where &egr; is the permittivity of the material between the parallel plates, A is the surface area of the parallel plate and d represents the gap between the plates. The characteristic capacitance is inversely proportional to the gap between a central portion of the diaphragm and the conductive surface of the fixed element. In order for there to permit a pressure differential across the diaphragm, the region on one side of the diaphragm is sealed from the region on the opposite side.
In practice, the diaphragm elasticity is selected so that pressure differentials across the diaphragm in a particular range of the interest cause displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates, and thus in capacitance variations produced by the sensor capacitor. For relatively high sensitivity, such sensors require large changes of capacitance in response to relatively small gap changes. Regarding equation (1), if &egr; and A are held constant, the greatest slope of the d verses C plot occurs when d is small. Thus for the greatest sensitivity, the gap is made as small as possible when the device is in equilibrium and the sensor is designed so that the gap d decreases as pressure is applied. The multiplicative effect of &egr; and A increases the sensitivity of the d to C relationship, so &egr; and A are maximized to achieve the highest possible sensitivity.
In high pressure applications, the diaphragm must have a relatively small diameter and be relatively rigid to prevent rupture at the high pressure interface. The small diameter reduces A in equation (1) relative to conventional sensors, and the rigidity of the diaphragm reduces the range of d. These characteristics together tend to reduce the sensitivity of conventional sensors in high pressure applications.
There is therefore a need for a high pressure capacitive sensor that can attain the levels of sensitivity currently demonstrated by conventional capacitive sensors used in relatively low pressure environments.
It is therefore an object of the invention to provide a high pressure capacitive sensor that can attain the levels of sensitivity currently demonstrated by conventional capacitive sensors which are designed to operate in relatively low pressure environments.
Other objects and advantages of the present invention will become apparent upon consideration of the appended drawings and description thereof.
SUMMARY OF THE INVENTION
The foregoing and other objects are achieved by the invention which in one aspect comprises a capacitive pressure sensor for use in high pressure applications, such as injection molding and plastic extrusion. The capacitive pressure sensor includes a rigid housing disposed about a central axis, with at least one aperture extending about the central axis in a plane perpendicular to the axis. The sensor further includes a first electrode assembly which spans the aperture of the housing and is secured to the housing at the electrode's perimeter. This electrode consists of a planar, electrically conductive elastic member with multiple petal electrodes extending substantially perpendicularly from a first side of the elastic member's surface from locations disposed about the central axis. These electrodes are sometimes referred to below as “petal” electrodes. A second side of the elastic member is exposed to the environment whose pressure is to be measured. When secured to the housing, the first electrode is oriented so that its petal electrodes are enclosed within the housing. The sensor further includes a second central electrode assembly which extends along the central axis and has a conductive portions on its outer surface disposed about the central axis. The central electrode is positioned so that it is surrounded by the petal electrodes and at least a portion of each petal electrode overlaps the conductive surface of the central electrode along the central axis. All of the petal electrodes are electrically connected to each other so that each in effect forms a capacitor with its underlying portion of the central electrode, where all of the capacitors are coupled in parallel. As a consequence, the total capacitance between the group of petal electrodes and the central electrode is the sum of the capacitance between each of the petal electrodes and the central electrode.
The entire sensor is positioned so that the elastic member is disposed between two isolated regions. As pressure on the second side of the elastic member increases relative to that on the first side, the elastic member deforms so that the side from which the petal electrodes becomes convex, causing the petal attachment point to displace and rotate. This rotation causes the distal tip of the petal electrodes to spread apart, and thus away from the second electrode's conductive surface. As the petal electrodes spread, the distance d measured from each petal electrode to the central electrode increases, but not necessarily in a uniform manner. The spreading causes the end of the petal electrode not attached to the elastic member (i.e. its distal tip) to be farther away from the central electrode than the end of the petal electrode attached to the elastic member. Because of this distinctive geometry, the resulting capacitance cannot be represented by a simple mathematical representation such as equation (1) for the parallel plate configuration. However, similar to a parallel plate capacitor, the resulting capacitance between a given point along a petal electrode and the central electrode varies directly with permittivity &egr; and the area A, and varies inversely with the distance d between them. Relatively long petal electrodes result in a correspondingly large amount of surface area exposed to the central electrode relative to the elastic member surface area, and a small deflection in the elastic member results in a relatively large change in distance between the petal electrode and the central electrode, especially at the end of the petal electrode not attached to the elastic member. The result of this configuration is a large change in capacitance for a relatively small elastic member deflection.


REFERENCES:
patent: 4408194 (1983-10-01), Thompson
patent: 4784577 (1988-11-01), Riston et al.
patent: 4944187 (1990-07-01), Frick et al.
patent: 5194819 (1993-03-01), Briefer

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