1. Field of the Invention
A capacitive pressure sensor for use in the measurement of pressure in a fluid medium, which medium may be either liquid or gas.
2. Prior Art
Capacitive pressure sensors as known in the prior art generally teach a pressure responsive diaphragm forming one plate or electrode of a capacitor. This electrode or capacitive plate is subject to deformation, generally spherical, and is mated or compared to a second electrode means of a stable or unmoving capacitive plate. As the spherical deformation expands or contracts the distance between the diaphragm capacitor plate and the second capacitor plate produces an electrical or an electrically translatable signal which varies with the radial distance from the center of the first electrode. This spherically variable distance between electrodes requires adjustments and calibrations to accurately define the signal produced or for utilization of the signal as a measure of the deforming force. Further, such material on which the diaphragm capacitor electrode means is mounted requires a merger of physical properties, specifically the merger of electrical and elastic modulous as well as a degree of bonding between two dissimilar materials. The production of capacitor sensors or transducers is well-known as is shown in the following patents.
There are certain inherent problems in the capacitive structures taught in today's technology: (1) calibration; (2) outside noise; (3) output as a function of small diaphragm deflections; (4) close tolerance of manufacture; and, (5) relatively high costs of materials. Indicative of both the complexity and steps taken to overcome these problems is the capacitance-type pressure transducer taught in U.S. Pat. No. 3,634,727 (Polye) wherein the sensor has a hollow capsule formed by a pair of discs of single crystal silicon-doped material to make it electrically conductive. A layer of silicon dioxide is deposited on the surfaces of the discs which are then selectively etched to form a cavity. The pair of discs are insulated from each other and cooperate to form the plates of a condenser with varying capacities responsive to pressure changes. This device is particularly useful in airborne applications. However, it highlights the materials complexity and those steps, including doping, thin film deposition and selective etching, to which others have gone to overcome the problem of sensing pressure by springs and diaphragms. The use of single crystal materials or single crystal silicon disclosed herein is an extreme example of an uneconomical material not readily available for general use. Also noted in this patent is the hysteresis characteristics which such sensors operate to overcome. U.S. Pat. No. 3,808,480 (Johnston) discloses a pressure transducer with a concave flexible pressure responsive diaphragm that is hermetically sealed and cooperates with a central electrode to define a variable capacitor. The flexible diaphragm and central electrode are encased and the space between the central electrode and flexible diaphragms is evacuated to provide a benign environment for the circuitry. Recognition of the problem of the flexing of the substrate and the stress therein during the operation of the device is noted. A composite laminar structure or substrate is provided to minimize the stray capacitance between the electrodes and the covers or housing. The operative structure is secured by screws which pass through substrate and provision is made to utilize such screws for external wiring terminals.
U.S. Pat. No. 3,858,097 (Polye) teaches a pressure sensing capacitor to change pressure outputs to electrical signals. A reference capacitor and pressure sensing capacitor may be connected in the arms of a bridge circuit, alternatively, one of the capacitors may be connected to the input of an amplifier and the other capacitor to a feedback path around the amplifier such that the signal is proportional to the ratio of these capacitors as a function of pressure. This particular invention is designed to overcome variations caused by temperature changes in the transducer. This transducer device includes a hollow body with opposing walls supported at the edges. The portions of the walls spaced from the edges, that is toward the center of the hollow, being deflectable relative to one another and responsive to pressure changes. The wall portions are substantially stable relative to one another and measure pressure change on the deflectable wall portions while maintaining the electrical conducting means on the nondeflectable portions. This device teaches a dual capacitor arrangement, one being a reference capacitor which is relatively stable and the second having a deflectable capacitor to measure the pressure change and compare such pressure change to the reference capacitor.
U.S. Pat. No. 3,859,575 (Lee et al.) teaches a variable capacitance sensor having capacitor plates connected at their center. The conducting plates or surfaces are insulated from a central connection and each other. The structure provides a stress relieving means which eliminates hysteresis effects. In this patent the pressure responsive diaphragm forms one plate of the capacitor, and the charge and separation between capacitor plates varies radially as in prior pressure sensor devices.
U.S. Pat. No. 3,952,234 teaches or discloses the very structural problem which the present invention serves to overcome, that is, an electrode formed on one surface of the diaphrgm of such a pressure sensor or transducer forming one of the electrical electrodes and producing an arched or spherical capacitance change requiring intergration of such varying signal across a radius. The deflection of such diaphragm and electrode is a problem recognized earlier in U.S. Pat. No. 3,859,575--Lee et al. patent which further produces hysteresis conditions that are found to be unfavorable.
U.S. Pat. No. 4,064,550 to Dias et al. teaches a capacitive fluid pressure transducer with electrodes on quartz bodies and diaphragms in a structure shown to produce the spherical condition noted in their FIGS. 5A and 5B. However, this patent clearly indicates one of the basic criteria of producing a capacitive transducer, that is, ". . . transducer is generally difficult to produce, it is very rugged and reliable . . . ". The desirability of a parallel plate capacitor is clearly noted in this patent at column 4, line 25 and, further, it is noted that these individuals were unable to develop the concept to produce such an item, but again had to utilize a calibration or adjustment based on a predictable function of the pressure as a function of the elastic properties of the material of the diaphragm, its diameter and thickness, and the quality of the diaphragm clamp. This latter recognition of the problems associated with past transducer assemblies is therefore again recognized in the art. U.S. Pat. No. 4,089,036 (Geronime) teaches a capacitive-type load cell with a support and diaphragm member mounted thereto. The diaphragm member again has a conductive surface and capacitor plate attached thereto, so that upon deflection of the diaphragm there will be relative movement between the diaphragm and the capacitor plate. This distance between plates or plate and conductive surface is the means of producing a measured signal calibratible to a pressure associated therewith. The diaphragm of the present device of Geronime provides free edge bending at the outer edges and between the diaphragm and load support to provide reduced bending stresses in such diaphragm. Although Geronime '036 has reduced such bending stresses the deflection again causes differences in plate separation with changes in radius from the diaphragm center. This patent does recognize the usefulness of having such diaphragms made of metal, however, and the disclosure provides such surface forming a conductive surface of such diaphragm. The device taught in Geronime U.S. Pat. No. 4,089,036 recognizes its limitations and acknowledges the existence of the change in total capacitive measurement for a given load as unequal bending occurs in the diaphragm and is felt to be averaged-out by the capacitor plate. Such load cells are frequently used for the measurement of large changes in large mass-type operations, such as, basic oxygen furnace measurements, measurements dealing with seismic changes, and require a larger or seismic mass to operate. Although such limitations are not noted in the Geronime U.S. Pat. No. 4,089,036, U.S. Pat. No. 4,125,027 (Clark) teaches a gage for remote indication of pressure in a subterranean formation. This apparatus measures such changes in pressure utilizing a variable capacitor or capacitance arrangement. The apparatus has a tubular housing with a deformable end forming a diaphragm which deflects inwardly, the deflection of such diaphragm causes a separation between the diaphragm and a capacitance or conductive plate which provides a change in a capactive load or measurement, thereby providing the signal for the pressure measurement. However, this device teaches that such deformation is or will be concave inwardly and the capacitive relationship between the conductive surface and the deformable end changes to provide an indication of ambient pressure. Such changes based on this concavity are again subject to variation along the radius from a central point and this disclosure acknowledges such changes and provides for a circuitry to compensate for such variation. They refer to such circuitry as a buffer and acknowledges that such buffering improves the signal and provides a measurable quantity.
U.S. Pat. No. 4,295,376 (Bell) teaches a capacitive pressure transducer to provide a substantially linear relationship between transducer output and input variations. Again, a force is applied to a diaphragm whose opposite surface includes a pair of electrodes at different positions. The force provides a variation or differential deflection of such diaphragm mounted electrodes. The deflection of the capacitive or capacitor plates is again provided in an arcuate or spherical shape as shown in FIG. 6 of this patent. Electronics are again utilized to buffer or correct the signal provided. A plate is provided in this device to move with the diaphragm and to maintain a fixed spacing of these spacer elements which vary in opposite senses. Therefore, the diaphragm having some of the electrodes flexes, while the relatively rigid plate stands firm. The capacitance variations at a radial distance are still inherent in such device.
The objective of all of the above devices is to provide a capacitance or capacitor-type signal to measure force or, more generally, pressure. This measurement is proportional to or a function of a change in distance between capacitor plates. The problems alluded to above include materials, construction, separation, hysteresis, cost, environment, temperature, and the flexure or variation in the diaphragm generally utilized in such structures. This flexure provides bending and torsional forces or stresses, and this condition has been accommodated in both the Bell U.S. Pat. No. 4,295,376 and Lee et al, disclosures above. Further, the attainment of a substantially constant parallel separation between the plates of the capacitors or capacitor of such transducer devices has been alluded to in both the Bell U.S. Pat. No. 4,295,376 and the Dias et al., U.S. Pat. No. 4,064,550. There is one last condition which is provided by the use of materials which are more responsive or which can accommodate larger deflections. Larger diaphragm deflections provide a greater range over which to calibrate a device and such broader range can lead to more accurate or a more easily measured signal over a narrower range of pressure differentials.