Differential pressure sensor having an alloy or metal isolator

A differential pressure sensor (10) having a housing 12 and a metal or alloy isolator (14) for stress isolation of a sensing cell (28) that is mounted to the isolator (14). The metal or alloy isolator (14) has an opening (16) therein for providing access to the sensing cell (28). The housing (12) is molded around the isolator (14) creating a seal therebetween and isolating the sensing cell (28).

TECHNICAL FIELD 
The present invention relates to differential pressure sensors and more 
particularly to an isolator for a differential pressure sensor. 
BACKGROUND ART 
Federal government regulations require close monitoring of automotive fuel 
system vapor pressure. Sensitive electronic components are used as part of 
an On-board Diagnostic System (OBD II) to detect hydrocarbon leaks in a 
fuel system. However, the fuel system environment is extremely harsh. 
Several attempts have been made to adapt integrated electronic sensors, 
normally used in ambient and atmospheric applications, so that they can 
withstand the hostile environment of an automotive fuel system. For 
example, placing the fragile sensing cell of a pressure sensor in a 
thermoplastic housing. 
U.S. Pat. No. 5,465,626 to Brown et al. provides a pressure sensor in which 
the pressure sensitive sensing cell is attached to a stress isolation 
platform using an adhesive that has a similar thermal coefficient of 
expansion and provides a hermetic seal between the isolation platform and 
the sensing cell. The stress isolation platform is attached to a plastic 
sensor housing with semi-rigid adhesive providing stress isolation and a 
hermetic seal between the plastic housing and the stress isolation 
platform. The hermetic seal between the housing and the stress isolation 
platform and the hermetic seal between the sensing cell and the stress 
isolation platform isolates the sensitive electronics of the sensor from 
the harsh environment it is exposed to. However, the adhesive used to 
hermetically seal the isolator is directly exposed to the harsh 
environment. 
The stress isolation platform is made of glass. A special blend of glass is 
required in order to withstand the thermal requirements of the sensor. 
Special bulk processing of the glass is required, which limits the number 
of suppliers capable of processing the glass. In addition, close 
tolerances of the glass after firing are required which adds significantly 
to the cost of the glass. The strength of glass is not easily 
quantifiable, adding to the potential for cracked glass during the sensor 
assembly process and leading to high scrap rates. 
In an attempt to avoid the problems associated with adhesive failure, U.S. 
Pat. No. 5,461,922 to Koen provides a pressure transducer that has a 
header housing made of a thermoplastic material. The housing has a 
resilient diaphragm made of a material that includes the same base 
material as the housing, but is flexible. The sensing cell is secured to a 
circuit board and the circuit board is secured to the header by and 
adhesive bond. The header and diaphragm provide a fully enclosed housing 
that is filled with a pressure transfer medium to couple the resilient 
diaphragm to the sensing cell, without exposing the sensing cell to any 
harsh chemicals. So while the housing of the pressure sensor protects the 
adhesive from exposure to harsh chemicals, the housing filled with 
transfer medium filled is extremely complicated and costly to manufacture. 
In addition, the reliability of the sensor is questionable due to the need 
for additional materials, such as the circuit board and the transfer 
medium, and the potential for the transfer medium to leak from the 
housing. 
SUMMARY OF THE INVENTION 
The present invention is a differential pressure sensor having a stress 
isolator made of a metal alloy. The isolator is molded inside of a 
one-piece thermoplastic housing. The shape of the isolator is such that 
optimal plastic flow is achieved around the isolator during the injection 
molding process, creating a seal between the isolator and the housing. 
Because the isolator is molded into the housing there is no need for epoxy 
to adhere the isolator to the housing. The isolator is a metal or alloy 
material. Thereby avoiding the inconsistencies associated with prior art 
glass isolators. 
The differential pressure sensor of the present invention is a one-piece 
thermoplastic housing with a metal alloy, isolator molded into it. A 
sensing cell is bonded to the topside of the metal alloy, isolator. The 
isolator is a disc, preferably circular, having an opening therein to 
provide access to the sensing cell. The underside of the isolator is in 
contact with the medium being sensed and the metal alloy, material remains 
resistant to the harsh chemical environment, such as a fuel system. 
In one embodiment of the isolator, the surface area of the isolator that 
comes in contact with the housing is rough to promote adhesion of the 
plastic when the housing is molded around the isolator. This feature and 
the circular shape of the isolator enhance the seal created between the 
isolator and the housing during the molding process. 
In another embodiment of the isolator, the outer periphery of the disc has 
a downward edge that further enhances the sealing between the isolator and 
the housing. 
It is an object of the present invention to improve the reliability of 
pressure sensors by eliminating the need for adhesive that is corroded by 
the harsh environment of a fuel tank system. It is another object of the 
present invention to improve the manufacturing process of pressure sensors 
by using a metal isolator thereby, reducing the cost and increasing the 
quality. It is still another object of the present invention to improve 
sensor performance by providing a reliable seal between the isolator and 
the housing. 
It is a further object of the present invention to provide an insert that 
is easily manufactured saving assembly time and thereby cost of the insert 
.

BEST MODES FOR CARRYING OUT THE INVENTION 
Referring to FIGS. 1 through 4, and in particular to FIG. 1, there is shown 
a portion of a pressure sensor 10 of the present invention. It has a 
one-piece thermoplastic housing 12. The housing 12 is injection molded and 
includes an isolator 14 that is preferably a metal or other alloy. 
A first embodiment of the isolator 14 is shown in FIGS. 1 and 2. The 
isolator 14 has a disc shape, preferably circular, having an opening 16 
therein. A first, or outer, diameter 18 and a second diameter 20 that is 
smaller than the first diameter 18, as best shown in FIG. 3, define a 
surface area 22 that is less than the entire surface area of the isolator 
14. In the embodiment of the isolator 14 shown in FIGS. 1 and 2, the 
surface area 22 between the first and second diameters 18 and 20 has a 
roughened or textured surface to promote adhesion of the isolator 14 to 
the thermoplastic of the housing 12 during manufacture. The rough surface 
area 22 also enhances the seal between the housing 12 and the isolator 14. 
The housing 12 is preferably injection molded. The isolator 14 is inserted 
into the injection molding die prior to molding, so that the housing 12 
surrounds the outer diameter of the isolator 14 creating a seal between 
the housing 12 and the isolator 14. 
In the embodiment shown in FIGS. 1 and 2, the outer periphery 25 of the 
isolator 14 has a downward extension or lip 26. The downward lip 26 on the 
underside of the isolator 14 performs the function of providing an 
enhanced seal between the housing 12 and the isolator 14. The downward lip 
26 prevents potential lateral movement of the isolator 14 within the 
housing 12. 
A portion of the underside 24 of the isolator 14 is directly exposed to the 
medium being monitored. The opening 16 provides access to the sensing 
electronics (not shown in FIG. 2) and the medium to be sensed. For 
example, the medium can consist of vapors in a fuel tank system (not 
shown). 
A top view of the entire pressure sensor 10 is shown in FIG. 3. FIG. 4 is a 
side view of the pressure sensor 10. In the sensor, a sensing cell 28 is 
bonded with a layer of epoxy 29 or other suitable adhesive, to the top 
surface 30 of the isolator 14. The opening 16 in the isolator 14 exposes 
the sensing cell 28 to the medium being monitored. 
In the embodiment shown in FIGS. 3 and 4, the isolator 14 does not have a 
downward lip but is shaped like a flat disc. A perspective view of the 
isolator 14 is shown in FIG. 5. The housing 12 captures the outer diameter 
18 of the isolator 14. The housing 12 extends over, contacts and creates a 
seal with the roughened surface area 22. 
The material of the isolator 14 is a metal alloy that has a low coefficient 
of thermal expansion. The sensing cell 28 is attached to the isolator 14 
by a layer of adhesive 29, such as an epoxy. It is desirable (but not 
necessary) for the coefficient of thermal expansion for the isolator 14 to 
match the coefficient of thermal expansion for the sensing cell 28 in 
order to avoid any undue stress on the sensing cell 28. 
The isolator 14 is easily formed from raw sheet metal stock. The properties 
of metal alloys are easily quantified, thereby providing a more reliable 
manufacturing process. The strength of metal and alloy is significantly 
higher than that of the glass used in the prior art, adding to the 
reliability of the manufacturing process and the pressure sensor 10. The 
ductile properties of metal alloy allow the isolator 14 to withstand 
shrinkage of the thermoplastic housing 12 as it cools after molding. Prior 
art glass isolators have a tendency to crack from mold shrinkage. Also, 
metal and alloy structures remain stable over the thermal operating range, 
typically -25.degree. C. to 125.degree. C. Prior art glass inserts are 
adversely affected by the thermal cycle and actually becomes unstable 
after being exposed to the heat necessary during the injection molding 
process. 
While particular embodiments of the invention have been shown and 
described, numerous variations and alternate embodiments will occur to 
those skilled in the art. Accordingly, it is intended that the invention 
be limited only in terms of the appended claims.