Capacitive pressure transducer

#This invention relates to pressure transducers and more particularly to pressure transducers utilizing glasseous sealing materials to join a flexible diaphragm to another part of the pressure transducer.

#There exists in the prior art several types of capacitive and strain gage pressure transducers. These pressure transducers often comprise a first flexible diaphragm that is sealed to a second portion of the pressure transducers such as another flexible diaphragm or a more rigid base structure by a peripheral glasseous or epoxy seal. These pressure transducers have at least one common deficiency. The deficiency is that after prolonged exposure of the transducer to a cyclically varying pressure or exposure to high levels of pressure the seal will tend to crack or otherwise develop hairline fissures therethrough. These cracks or fissures permit the volume between the diaphragm and seal to become contaminated, leak or otherwise lose its evacuated pressure characteristics. The degradation of the physical integrity of the peripheral seal is a result of the combined compressive and tensile forces which are imparted thereto during periods involving pressure loading. In addition, the prior art transducers are subject to hysteresis due to the frictional or sliding deformation resulting from the movement of the diaphragms. The movement of the diaphragms further causes a zero or null shift therein changing the sensitivity of the pressure transducer. These deficiencies are further amplified when the pressure transducer is required to measure pressures that exceed atmospheric pressure and which may reach or exceed 1000 psi.

#It is a broad object of the present invention to remedy those deficiencies in the prior art. More specifically, it is an object of the present invention to provide a pressure transducer having an improved long lasting seal. Another object of the present invention is to reduce the tensile and compressive stresses developed on the seal due to the applied pressure environment. A further object of the present invention is to provide a pressure capsule having a wide dynamic range and having a high uniform sensitivity.

#In the drawings:

#FIG. 1 is a cross-sectional view of a flat plate capacitive pressure sensor not incorporating the present invention;

#FIG. 2 is a top plan view of a pressure sensing capacitor incorporating the present invention;

#Reference is now made to FIG. 1 which illustrates a cross-sectional view of a flat plate pressure transducer 20. More particularly, the transducer 20 is shown in a stressed position resulting from the application of a pressure environment (P) to be measured. The pressure transducer 20 is fabricated using at least one flexible diaphragm 22 or the plurality of flexible diaphragms 22 and 24. Preferrably these diaphragms are fabricated from a material exhibiting zero or minimal hysteresis and further exhibiting a low temperature coefficient of expansion that is compatible with the sealing material and other parts of the transducer 20. Typical of materials exhibiting this behavior are: quartz, Pyrex and the like. The diaphragms 22 and 24 are spaced apart by a single annular spacer 26 which serves to establish a cavity 28. The circumferential spacer 26 defines the pressure sensitive regions of the adjacent diaphragms and provides a peripheral seal therebetween. The transducer 20 further comprises a plurality of conductive electrodes deposited on the surfaces of the respective diaphragms 22 and 24 within the cavity 28 and under the pressure sensitive regions. In the embodiment illustrated in FIG. 1 the electrodes comprise centrally located circular portions 40 and 42 which are placed upon the respective diaphragms (22 24) and located coaxially with the spacer 26. In addition, the transducer 20 further includes a second pair of electrodes 44 and 46 which are deposited to the undersurfaces of the diaphragms 22 and 24 near the extremes of the pressure sensitive regions in a manner so that they too are coaxially situated to the electrode pair 40,42 and the seal 26. The transducer 20 further includes a plurality of lead wires 50, 52 and 54 connecting electrodes 44, 40 and the combination 42 and 46 respectively. These wires (50, 52, and 54) are connected to the appropriate electrodes through feedthrough passages 56 which are later sealed by an epoxy 58 or like sealing material to protect and seal the environment within cavity 28. The precise relationship of the electrodes is not important to the present invention. A variety of electrode relationships such as the one illustrated in FIG. 1 which illustrates the use of a sensing capacitor, Cs, comprising the electrode pair 40 and 42 and a reference capacitor Cr comprising the electrode pair 44 and 46. An inspection of FIG. 1 illustrates that when the transducer 20 is in a compressed position due to the application of the pressure force to be measured, the innermost portions 60 of the peripheral spacer 26 are in compression while the outermost surface 62 of the seal 26 are in tension. One should appreciate that the continued stressing of the seal 26 due to the combined compressive and tensile forces resulting from the continued flexing of the transducer 20 will cause premature failure of the seal material. This failure, as previously mentioned, may provide leakage paths for the external environment to flow into cavity 28 which destroy the structural integrity of the seal thereby changing the zero and scale factor of the transducer 20.

#Reference is now made to FIGS. 2 and 3 which illustrate respectively a top plan view and cross-sectional view of an improved capacitive pressure transducer 70. The capacitor 70 includes a pair of substantially flat, spaced apart, flexible diaphragms or plates 72 and 74. The preferred embodiment of the invention utilizes fused quartz because of its hysteresis free and low temperature coefficient characteristics. However, materials such as Pyrex or other ceramics, glasses or metals are applicable to practice the present invention. The use of quartz is highly advantageous in that quartz is chemically inert so that it will not corrode in adverse environments and furthermore, quartz will withstand considerable compressive forces which make it desirable to be incorporated within a high level pressure sensing transducer such as transducer 70. In addition, quartz exhibits substantially no hysteresis characteristics when subjected to periodic stresses and strains. Each diaphragm or plate 72 and 74 is preferably circular, while other shapes may be substituted, except for a small peripheral portion or cut-out 76 which has been removed from each plate. The purpose of truncating the diaphragms 72 or 74 is to permit convenient access to the electrodes and to eliminate the need for passthrough holes 56 and sealing material 58 as discussed in FIG. 1. It should be noted that depending upon the orientation and number of electrodes, each capacitive plate may contain additional cut-outs 76 located about the periphery of the diaphragms 72 and 74. As previously mentioned, the diaphragms 72 and 74 are fabricated from flexible plates of fused quartz or glass and that these plates 72, 74 may be polished or cut to a substantially uniform thickness and flatness.

#Diaphragms 72 and 74 further include a layer of conductive material of a known type which may be applied to a thickness of several thousand angstroms to form the electrodes of the pressure sensitive capacitor. These layers are formed on the inner surfaces 78 and 80 of the respective diaphragms 72 and 74 by screen printing, vacuum evaporation, sputtering or other methods. Preferably the major portions of these electrodes are located within the pressure sensitive regions 106 of the diaphragms 72 and 74. In the embodiment illustrated in FIGS. 2 and 3, the electrodes comprise two circular main electrodes 90 and 92 having electrode leads 94 and 96 radially extending to the periphery of the diaphragms 92 and 94.