CAPACITIVE PRESSURE SENSOR

Abstract: A capacitive pressure sensor (10) comprises: a housing (12) including first and second spaced apart diaphragms (114, 116), matched in compliance to vibrate and deflect in substantial unison in response to vibratory and acceleration forces; said first diaphragm (114) being an outer pressure responsive diaphragm, movably cooperating with said housing to bound a reference-pressure chamber (106, 108) within which is movably disposed said second diaphragm (116), which carries on its surface facing said first diaphragm (114) a first pair of spaced apart planar electrodes (52, 54) each having a predetermined geometric shape, and a first planar shield electrode (56) having a portion (58) circumscribing said first pair of electrodes (52, 54) and a cross bar portion (60) interposing between said first pair of electrodes; said first diaphragm (114) carrying on its surface facing said second diaphragm (116) a second pair of spaced apart planar electrodes (92, 94) each having said predetermined geometric shape; said first pair of electrodes (52, 54) and said second pair of electrodes (92, 94) being congruent to define a respective pair of variable capacitors, having substantially identical value and varying in capacitance value simultaneously in union as said first diaphragm (114) moves in response to external fluid pressure, whereby said variable capacitors remain of substantially identical value in response to external fluid pressure acting on said sensor, and said capacitance values being substantially immune from said vibration and acceleration forces.

Description: CAPACITIVE PRESSURE SENSOR The present invention relates generally to a pressure sensor providing a variable capacitance indicative of variable pneumatic pressure acting on the sensor. More particularly, the present invention relates to an improved capacitive pressure sensor, having a dual variable capaci¬ tance providing a highly accurate indication of pressure acting on the sensor. Pressure sensors are known in the avionics field for sensing an air pressure related to aircraft status, and producing a capacitance signal indicative of, for example, aircraft altitude, mach number, air speed, angle of attack, and yaw angle. Because the derived information must be highly accurate, the sensor must also be accurate, reliable and relatively immune from electrical interference. A system for measuring pressure usually includes an externally mounted pitot tube, a pressure conduit, a capacitive sensor which is physically acted upon by the pressure in the conduit, and electronic circuitry which has as an input the variable capacitance from the pressure sensor. Technological developments have reduced the size of the conventional sensor system to a volume of 5 to 10 3 cubic inches (82 to 164 cm ). Further, the technology of micromachining of silicon has progressed so that the feasibility of producing a capacitive pressure sensor comparable in size to an integrated circuit has been demonstrated. The present invention provides a capacitive pressure sensor comprising: a housing including first and second spaced apart diaphragms, matched in compliance to vibrate and deflect in substantial unison in response to vibratory and acceleration forcesj said first diaphragm being an outer pressure respon¬ sive diaphragm, movably cooperating with said housing to bound a reference-pressure chamber within which is movably disposed said second diaphragm, which carries on its surface facing said first diaphragm a first pair of spaced apart planar electrodes each having a predetermined geometric shape, and a first planar shield electrode having a portion circumscribing said first pair of electrodes and a cross bar portion interposing between said first pair of electrodes; said first diaphragm, carrying on its surface facing said second diaphragm, a second pair of spaced apart planar electrodes each having said predetermined geometric shape; said first pair of electrodes and said second pair of electrodes being congruent to define a respective pair of variable capacitors, having substantially identical value and varying in capacitance value simultaneously in union as said first diaphragm moves in response to external fluid pressure, whereby said variable capacitors remain of substantially identical value in response to external fluid pressure acting on said sensor and said capacitance values being substantially immune from said vibration and acceleration forces. Advantageously, said second diaphragm (116) also carries on its surface facing away from said first diaphragm, a third pair of spaced apart planar electrodes each having said predetermined geometric shape, said first pair of electrodes and said third pair of electrodes being inter¬ connected electrically, and a second planar shield electrode having a portion circumscribing said third pair of elec¬ trodes and a diametral cross bar portion interposing between said third pair of electrodes, said first shield electrode and said second shield electrode being interconnected electrically. Preferably, said second diaphragm defines a pair of through passages each opening at opposite ends on a respective one of said first pair of electrodes and on a respective one of said third pair of electrodes, and a conductive metal coating on each of said pair of through passages conductively connecting one electrode of each pair of electrodes, to its respective electrode of the other pair of electrodes. Generally, the present invention, in its preferred embodiment, includes two pairs of electrodes facing one another and mounted within an evacuated housing. The housing is constructed from homogeneous fused silica glass or fused quartz material and is hermetically sealed. A variety of other materials, including silicon single crystal material or insulated metallic material, may also be used in constructing a sensor according to the present invention. The facing electrodes define capacitor plates. Physical deformation of this housing under the influence of external pressure results in a change of the physical separation of each pair of electrodes. Thus, a variation in capacitance results from changes in external pressure. Throughout the entire range of pressures for which the housing is designed, the capacitance exhibited by each pair of electrodes varies simultaneously so that the capacitances of the electrode pairs are substantially identical. More particularly, the invention provides, accord¬ ing to a particularly preferred embodiment, a pressure sensor having a housing defining an exteriorly exposed pressure-responsive diaphragm and an internal acceleration- compensating diaphragm, each of the diaphragms carries a metal surface coating on each opposite face thereof. The metallized surfaces of confronting faces of the two diaphragms are divided into two matching semi-circular areas. These matching and confronting semi-circular areas define a pair of capacitors which vary in capacitance as the spacing between the two diaphragms changes. Another portion of the metallized surface defines a shield which is grounded to prevent electromagnetic interference being manifested within the capacitors. This shield also prevents relative movement between the capacitor electrodes and any potential charge-holding structures from being mani¬ fested as a capacitance variation in the sensor. The compliance of the two diaphragms of the sensor is carefully matched, so that acceleration and vibration will cause them to move in unison rather than differentially. As a result, the capacitance value of the pair of capacitors is substantially immune from influence by vibration and acceler¬ ation forces. Also, the capacitance values of the two capacitors change in unison and by like amounts in response to external pressure acting upon the externally exposed diaphragm in concert with a reference pressure within the pressure sensor. Consequently, the capacitance values of the pair of capacitors are at all times substantially equal. Further advantages of the invention will become apparent from the following description and claims, and from the accompanying drawings wherein: FIG. 1 is a side view of the capacitive pressure transducer; FIG. 2 is an exploded isometric view of the capacitive pressure transducer of FIG. 1; FIG. 3 is a top view of a portion of the capacitive pressure transducer of FIG. 1 viewed along line 3-3; FIG. 4 is a top view of another portion of the capacitive pressure transducer of FIG. 1 taken along line 4-4 of FIG. 1; FIG. 5 is a bottom view of a portion of the capacitive pressure transducer of FIG. 1 taken along line 5-5 of FIG. 1; FIG. 6 is a bottom view of another portion of the capacitive pressure transducer of FIG. 1 taken along lines 6-6 of FIG. 1; FIG. 7 is a bottom view of the capacitive pressure transducer of FIG. 1; FIG. 8 is a fragmentary somewhat schematic cross- sectional view of the capacitive pressure transducer of FIGS. 1 and 7, taken along line 8-8 of FIG. 7; FIG. 9 is a fragmentary somewhat schematic cross- sectional view of the capacitive pressure transducer of FIGS. 1 and 7 taken along line 9-9 of FIG. 7; and FIG. 10 is a somewhat schematic cross-sectional view of the capacitive pressure sensor taken along circular line 10-10 of FIG. 7. FIG. 1 depicts a side view of a sensor (10) embodying the present invention. The sensor (10) includes a body (12) of generally cylindrical shape having both a flat top surface (14) and a flat lower surface (16) . A mounting stem (18) is centrally disposed upon the body (12) and projects axially from the lower surface (16) in order to provide a means for physically attaching the sensor (10) to a mounting structure, such as a circuit board or chassis. Also protruding from the lower surface (16) of the housing (12) are a plurality of electrical pin mounting anchors (20) each carrying an axially extending electrical lead (22) or electrical pin. The pin mounting anchors (20) and electri- cal leads (22) are five in number, and each of the latter includes an axially extending shape-retaining portion (24). The electrical leads (22) also include provision for axial and radial compliance, as will be more fully described hereinbelow. Further consideration of FIGS. 1 to 7 will reveal that the sensor (10) includes a band (26) of exterior surface metalization encompassing the body (12) on a side surface (28) thereof adjacent to the lower surface (16). The band (26) of exterior surface metalization is circum- ferentially continuous, and a pair of diametrically oppositely disposed axially extending portions (30) of metallized exterior surface (only one of which is visible in FIG. 1) connect with the band (26) and extend therefrom axially along the side surface (28) to the top surface (14) . As will be more fully explained hereinbelow, the exterior metalization on the sensor (10) covers almost all of the base surfaces (16), and almost all of the top surface (14) as well. The exterior metallized surface on sensor (10) is electrically continuous with one of the pin mounting anchors (20) and the respective electrical lead (22) extend- ing therefrom. It will be seen viewing FIGS. 1 to 7 that the housing (12) includes three axially stacked and sealingly intersecured portions. A lower portion (32) carries the pin mounting anchors (20) and mounting stem (18) as well as the band of metallized surface (26). On the lower portion (32) , are sequentially stacked a middle layer (34) , which is much thinner in axial extent than the lower portion (32) . and a top layer (36) , which is substantially equal in axial dimension with the middle layer (34). Turning now in particular to FIG. 2, it will be seen that the lower portion (32) defines a planar upper surface (38) facing the middle layer (34) . The lower portion (32) also defines five circumferentially spaced passages (40) extending therethrough to open on the lower surface (16). Each one of the passages (40) communicates with a respective one of the pin mounting anchors (20) , as will be more fully explained hereinafter. Received within each one of the passages (40) is a respective one of five flexible electrical leads (42) ' . These leads (42) are electrically continuous with a respective one of the pin anchors (20) and electrical leads (22). FIG. 2 also reveals that the middle layer (34) includes a radially outer rim portion (44) as well as a central upper recess (46) which faces the top layer (36). This top layer (36) carries a central exterior metallized surface portion (48), which is electrically connected with the axially extending portions (30) of exterior metallized surface on the side surfaces (28) by a pair of oppositely radially extending metallized surface portions (50). Keeping in mind that which has been depicted and described with reference to FIGS. 1 and 2, reference now particularly to FIGS. 3 to 6 in conjunction will show that the upper face of middle layer (34), facing the top layer (36) carries three electrically separate metallized surface portions (52), (54) and (56) (viewing particularly FIG. 4). The metallized surface portions (52) and (54) are spaced apart segments of a circle, that is, they are semi¬ circular. On the other hand, the metallized surface portion (56) comprises an annular part (58) , and a cross-bar part (60), joined at each end to the annular portion (58), and separating the metallized surface portions (52) and (54). As an assistance to the viewer, alphabetic alignment charac¬ ters have been placed upon FIGS. 3 to 6, in order to assist in visualizing the congruent features which provide for electrical connection between electrical leads (22) , carried upon the lower portion (32) of the sensor, and the various internal features thereof. Viewing FIG. 5 it will be seen that the lower face of middle layer (34) also includes a radially outer annular rim portion (62) like rim portion (44) of the opposite upper face of middle layer (34). Similarly, the lower face of the middle layer (34) includes a recess (64) within the rim portion (62) , and three portions of metallized surface (66), (68) and (70), which are congruent and substantially identical in shape with the portions of metallized surface (52) to (56) of the top face of this layer. However, the metallized surface portions (66) and (68) include respective radially projecting portions (72) and (74), which align with respective ones of the passages (40) in lower portion (32), and receive electrical connection with the respective flexible electrical lead (42) thereof (alignment references 'a' and 'b'). It will be noted that the metallized surface portion (70), similarly to metallized surface portion (56), includes an annular portion (76) and a cross-bar part (78). Viewing FIG. 5, it will be noted that the intersection of cross-bar part (78) with annular part (76) of metallized surface (70) corresponds at the right end thereof with one of the passages (40) of lower portion (32) (alignment reference 'c'). Accordingly, the connector (42) received in this passage connects with the metallized portion (70) adjacent this intersection. It will also be noted that the middle layer (34) defines four through passages (80), (82), (84) and (86). Two of these through passages (80) and (82) open in the metallized surface portions (52) and (66), and (54) and (68) , respectively. In order to electrically connect these metallized surface portions, the cylindrical surfaces of the two through passages (80) and (82) also have metallized surf ces. The other two through passages (84) and (86), it will be seen, each encroach upon the annular part of metallized surface portions (56) and (70). The through passages (84) and (86) are each larger in diameter than the through passages (80) and (82), and also include a chamfer region adjacent to the opposite faces of the middle layer (34) . In order to electrically connect the metallized surface portions (56) and (70) , the conical chamfer surfaces of the through passages (84) and (86) also have metallized surfaces. Turning now to FIG. 6, it will be seen that the underside of top layer (36) also includes an annular rim (88). Within the rim (88), the top layer (36) defines a recess (90), congruent with the recesses (46) and (64) of the middle layer (34). Within recess (90), the top layer (36) carries two portions of metallized surface (92) and (94) which are similarly semi-circular in shape, which are confrontingly spaced from and overlay the metallized surface portions (52) and (54) , and which at least partially overlap with the metallized portion (56). Similarly with the metallized surface portions (66) and (68) , the metallized surface portions (92) and (94) include areas (96) and (98) aligned with the through passages (84) and (86) of middle layer (34) , and provide for electrical connection of a respective lead (42) to each one of the metallized portions (92) and (94) (alignment references 'd' and 'e'). Viewing the alignment features previously referred to, the character 'a' in FIG. 3 indicates the position of a passage (40) and electrical lead (42), which on FIG. 5 aligns with the radially extending portion (72) of metallized surface (66) so that the lead (42) may connect therewith. Similarly the character 'b' on FIG. 3 indicates the position of a passage (40) and lead (42) which aligns with the projecting portion (74) of metallized surface (68) shown in FIG. 5, so that the lead (42) may connect thereto. The character 'c' shown in FIG. 3 adjacent to the passage (40) and electrical lead (42), align with the intersection of cross-bar part (78) and annular part (76) of metallized surface (70) shown in FIG. 5, so that the lead (42) may connect thereto. Also shown in FIG. 3, the characters 'd' and 'e' indicate the positions of passages (40) and respec- tive electrical leads (42), which align with passages (84) and (86) respectively of the middle layer (34) , as seen in FIGS. 4 and 5, and pass therethrough to connect with the metallized surface (92) and (94) of upper layer (36), as seen in FIG. 6 at the respective areas (96) and (98) thereof. In order to complete this description of the physical structure of sensor (10), the reader's attention is called to FIG. 7, wherein a view of the lower surface of the sensor is depicted. It will be seen viewing FIG. 7 that each one of the electrical leads (22) includes a straight portion (100) connecting the axially extending portion (24) of the lead with a curved portion (102). The curved portion (102) is seen to circumscribe and be spaced from the pin mounting anchor (20) while extending therearound through substantially 360° before attaching mechanically and elec¬ trically thereto at a joint (104). This construction of the electrical leads (22) provides for both radial and axial movement of the axially extending portion (24) of the lead relative to the pin mounting anchors (20) without trans- mitting significant loads into the body (12) of the sensor (10). Turning now to FIGS. 8, 9 and 10 in conjunction, the cross-sectional views of the sensor (10) are depicted somewhat schematically in order to present the structural and functional cooperation of the components depicted and described heretofor. As an assistance to the reader, the alignment alphabetic key letters 'a 1 through 'e' used previously to depict the congruence and cooperation of features depicted in FIGS. 3 to 6 are carried over in FIGS. 8 to 10. FIGS. 8 to 10 illustrate that the recess (60) of middle layer (34) cooperates with the planar upper surface (38) of the lower portion (32) of the sensor to define a chamber (106). Similarly, the recesses (46) and (90) of the middle layer (34) and top layer (36) cooperate to define a chamber (108). Of course, the chambers (106) and (108) communicate with one another via the through passages (80) , (82), (84) and (86). The chambers (106) and (108) coopera¬ tively define a reference pressure chamber for the sensor (10). The chambers (106) and (108) are partially evacuated, so that capacitance value of the sensor is an indication of absolute pressure level acting thereon. Preferably, as close to an ideal vacuum as possible is created in chambers (106,108). It will be seen viewing FIGS. 8 to 10 that the pin mounting anchors (20) are depicted as conductive cup-like members sealingly securing to the lower surface (16) of the lower portion (32) of the sensor (10), and spanning the through passages (40) to close the latter and chambers (106) and (108) from atmospheric pressure. The lower surface (16) of sensor (10) is substantially completely covered with exterior metallized surface (110) which is continuous with the band (26). The metallized surface (110) immediately circumscribes the one through passage (40) at alignment reference 'c'. The pin mounting anchor (20) at this through passage is secured to the metallized surface (110), as is the lead (42) connecting with metallized portion (70) on sensor portion (34). Consequently, a grounded shield structure is completed, which includes metallized surface portions (26), (30), (48), (50), (56), and (70). The metallized surface (110) circumscribes the remaining passages (40) at some distance therefrom. Sur¬ rounding each of the remaining through passages (40), on surface (16) (alignment references a, b, d, and e) , is a respective annular area of metallized surface generally referenced with the numeral (112). Each of the mounting pin anchors (20) at these passages is secured to its respective annulus of metallized surface, as is the respec¬ tive lead (42) thereof. Of course, it is understood that the electrical leads (22) depicted in FIGS. 1 and 7 respec- tively connect to the exterior surface of the pin mounting anchors (20) as has been previously described. Viewing now most particularly FIG. 10, it will be understood that the top layer (36) of the sensor defines a flexible diaphragm part (114) spanning the recess (90) thereof. Similarly, the middle layer (34) defines a flex- ible diaphragm part (116) spanning the recesses (46) and (60) thereof. The flexible diaphragm parts (114) and (116) of the layers (34) and (36) are substantially the same in compliance. Therefore, should the sensor (10) depicted in FIG. 10 be subjected to acceleration or vibration forces oriented vertically with respect to the figure, the dia¬ phragm parts (114) and (116) will deflect or vibrate sub¬ stantially in unison. Consequently, the spacing between the metallized portions carried upon the top layer and middle layer (34) and (36), respectively, will not change. In this way it is seen that the sensor (10) is substantially immune to influence from vibration and acceleration forces. On the other hand, should the sensor (10) be exposed to a fluid pressure, as is depicted by arrow (118), it is easily understood that the diaphragm part (114) of top layer (36) is bowed convexly inward toward the chamber (108) , to result in the metallized layers carried upon the diaphragm parts being forced closer together. Consequently the variable capacitors defined by the respective congruent metallized portions change in value predictably in accord with the level of the fluid pressure applied externally to the sensor (10). It further is to be noted that the capaci¬ tors defined by the metallized surface layers are sub¬ stantially identical with respect to the area of their confronting capacitor plates, and that the change in spacing of these capacitor plates in response to applied external pressure is substantially the same for each of the two capacitors. Therefore, the capacitance values of the pair of capacitors change by the same amount, so that at all times and in response to the full range of fluid pressures for which the sensor (10) is designed, the capacitance values are substantially identical. Further to the above, it will be seen that each of the diaphragms (114) and (116) carries substantially equal areas of metallized surface on each of its opposite faces. This feature is important in maintaining calibration of the transducer over time, as the metallized films age and stresses therein relax. Without substantially equal metallized areas on its two opposite faces, a diaphragm will bow in response to a bi-metal effect as the stresses in the metal film relax over time. The present invention provides diaphragms with substantially balanced surface metaliza¬ tion. Also, it is to be noted that the cross-bar portions (60) and (78) of the diaphragms lie on a plane of bilateral symmetry for the sensor (10). That is, each structure on one side of the plane is substantially duplicated on the other side of the plane. This feature is important in achieving equal capacitance change for each of the variable capacitors as external pressure on the sensor changes.

Claims: CLAIMS 1. A capacitive pressure sensor (10) comprising: a housing (12) including first and second spaced apart diaphragms (114, 116), matched in compliance to vibrate and deflect in substantial unison in response to vibratory and acceleration forces; said first diaphragm (114) being an outer pressure responsive diaphragm, movably 10 cooperating with said housing to bound a reference-pressure chamber (106, 108) within which is movably disposed said second diaphragm (116) , which carries on its surface facing said first diaphragm (114) a first pair of spaced apart planar electrodes (52, 54) each having a predetermined -^ geometric shape, and a first planar shield electrode (56) having a portion (58) circumscribing said first pair of electrodes (52, 54) and a cross bar portion (60) interposing between said first pair of electrodes; 2υ said first diaphragm (114) carrying on its surface facing said second diaphragm (116) a second pair of spaced apart planar electrodes (92, 94) each having said predetermined geometric shape; 25 said first pair of electrodes (52, 54) and said second pair of electrodes (92, 94) being congruent to define a respective pair of variable capacitors, having sub¬ stantially identical value and varying in capacitance value simultaneously in union as said first diaphragm (114) moves 30 in response to external fluid pressure, whereby said variable capacitors remain of substantially identical value in response to external fluid pressure acting on said sensor, and said capacitance values being substantially immune from said vibration and acceleration forces. 35 2. A sensor according to claim 1 characterized in that said second diaphragm (116) also carries on its surface facing away from said first diaphragm, a third pair of spaced apart planar electrodes (66, 68) each having said predetermined geometric shape, said first pair (52, 54) of electrodes and said third pair of electrodes (66, 68) being interconnected electrically, and a second planar shield electrode (70) having a portion (76) circumscribing said third pair of electrodes and a diametral cross bar portion (78) interposing between said third pair of electrodes (66, 68) , said first shield electrode (56) and said second shield electrode (70) being interconnected electrically. 3. A sensor according to claim 2 characterized in that said second diaphragm (116) defines a pair of through passages (80, 82) each opening at opposite ends on a respective one of said first pair of electrodes (52, 54) and on a respective one of said third pair of electrodes (66, 68) and a conductive metal coating on each of said pair of through passages (80, 82) conductively connecting one electrode of each pair of electrodes, to its respective electrode of the other pair of electrodes. 4. A sensor according to any one of claims 1 to 3 characterized in that a metallic film shield electrode (110, 26, 30) covers at least a portion of the external surface of said housing. 5. A sensor according to claim 4 characterized in that said metallic film shield electrode includes a portion (48, 50) carried upon said first diaphragm (114) and congruent with said first (52, 54) and second (92, 94) pairs of electrodes. 6. A sensor according to claim 2 characterized in that said second diaphragm (116) defines a through passage (84, 86) opening at opposite ends on said first and said second shield electrodes (56, 70), with a metallic film coating on said passage electrically interconnecting said shield electrodes. 7. A sensor according to any one of claims 1 to 6 characterized in that said housing (12) comprises a disc-like base portion (32), a disc-like yieldable middle portion (34), and a disc-like yieldable top portion (36) said portions being stacked and sealingly united to define said reference pressure chamber (106, 108), said housing top portion (36) defining said first diaphragm (114), and said housing middle portion (34) defining said second diaphragm (116). 8. A sensor according to claim 7 characterized in that said base portion (32) carries conductive means (20, 40, 42) for electrical connection individually with each one of said first pair of electrodes (52, 54), with each one of said second pair of electrodes (92, 94), and with said first (56) and second (70) shield electrodes together. 9. A sensor according to claim 8 characterized in that said conductive connection means includes a plurality of through passages (40) defined by said base portion (32) , a like plurality of flexible conductors each (42) received in a respective through passage, and each electrically connecting at its inner end with a respective electrode, and a like plurality of cup-like conductive anchor members (20), each sealingly engaging said base portion to close a respective through passage, and electrically connecting internally thereof with the outer end of a respective flexible conductor. 10. A sensor according to claim 9 characterized in that said conductive connection means further includes an electrical lead (22) connecting with one of said anchor members (20) , said electrical lead including a shape retaining axially extending portion (24) having a free end 5 for interface with electrical circuitry external to said sensor, a curved resilient portion (102) joining with said axially extending portion (24), said resilient portion (102) extending circularly around and being spaced apart from said anchor member (20) through substantially 360°C to J_ Q yieldably provide for axial and radial relative movement of said axially extending portion, and an inner part (104) of said electrical lead anchoring said resilient portion with said anchor member externally thereon. 5 0 5 0 5

Inventors: LIPS DONALD C, MCCORMACK WILLIAM H, SHKEDI ZVI, LIPS, DONALD, C., MCCORMACK, WILLIAM, H., SHKEDI, ZVI
Applicants: ALLIED SIGNAL INC, ALLIED-SIGNAL INC.
Application year: 1989
Main classification: G01L9
IPC codes: G01L9, G01L19