Three-piece pressure sensor with high pressure stainless steel sensor element

An improved pressure sensor assembly includes a low-cost pressure port comprising a casing formed of cold rolled steel, a small stainless steel insert and an O-ring. The casing has a threaded slug at one end for attaching the sensor assembly to the wall of a pressure vessel and an annular cup at the other end. The annular cup has a hex-shaped periphery to facilitate installation and removal of the sensor assembly, and defines a cylindrical interior cavity including an annular recess in which the O-ring is disposed. The stainless steel insert has a base portion disposed within the cylindrical cavity atop the annular recess so as to compress the O-ring between the insert and casing, and a neck portion that extends out of the cavity to form a pedestal to which the stainless steel pressure sensor element is welded. The insert is retained relative to the casing by a staking operation in which a portion of the casing is deformed against the neck of the insert. The casing and insert have central axial openings which align when so assembled, placing the sensor element in communication with the interior of the pressure vessel. A connector assembly covering the sensor element environmentally seals the sensor element and provides access to its electrical pressure signal output. Compared to previously known sensor assemblies, the stainless steel content and required machining are significantly reduced, contributing to an overall cost reduction of the sensor assembly.

FIELD OF THE INVENTION 
This invention relates to a high pressure sensor for an automotive system, 
and more particularly to a low cost three-piece assembly having a 
stainless steel pressure sensor element. 
BACKGROUND OF THE INVENTION 
It is necessary for automotive controls to obtain a measure of certain 
pressure parameters such as engine oil pressure, transmission fluid 
pressure or brake pressure. To this end it is customary to attach pressure 
sensors to a pressure vessel. High pressure applications generally utilize 
a stainless steel pressure sensor element welded to a pressure port which 
attaches to the vehicle by a threaded fitting, for example. To permit 
welding, the pressure port is also formed of stainless steel material. 
Such pressure ports are expensive due to the high cost of stainless steel 
relative to other materials and the difficulty of machining stainless 
steel. 
Any pressure port design must accommodate a hermetic seal for the sensor 
(and therefore must be weldable to stainless steel) and must withstand the 
mechanical stresses such as pressure and torque to which the sensor will 
be subjected during installation and subsequent use. Proposed alternatives 
to the one-piece stainless steel pressure port have included multiple 
piece stainless steel pressure ports. Although this reduces the cost of 
the pressure port, the cost of the complete sensor is increased due to 
additional pieces to assemble and the reliability is decreased. 
SUMMARY OF THE INVENTION 
The present invention provides an improved pressure sensor assembly 
including a low-cost pressure port comprising a casing formed of cold 
rolled steel, a small stainless steel insert and an O-ring. The casing has 
a threaded slug at one end for attaching the sensor assembly to the wall 
of a pressure vessel and an annular cup at the other end. The annular cup 
has a hex-shaped periphery to facilitate installation and removal of the 
sensor assembly, and defines a cylindrical interior cavity including an 
annular recess in which the O-ring is disposed. The stainless steel insert 
has a base portion disposed within the cylindrical cavity atop the annular 
recess so as to compress the O-ring between the insert and casing, and a 
neck portion that extends out of the cavity to form a pedestal to which 
the stainless steel pressure sensor element is welded. The insert is 
retained relative to the casing by a staking operation in which a portion 
of the casing is deformed against the neck of the insert. The casing and 
insert have central axial openings which align when so assembled, placing 
the sensor element in communication with the interior of a pressure vessel 
to which the assembly is attached. A connector assembly covering the 
sensor element environmentally seals the sensor element and provides 
access to its electrical pressure signal output. Compared to previously 
known sensor assemblies, the stainless steel content and required 
machining are significantly reduced, contributing to an overall cost 
reduction of the sensor assembly.

DESCRIPTION OF THE INVENTION 
FIGS. 1-4 variously depict a pressure sensor assembly 10, or pressure port 
10' according to this invention. As indicated, a pressure sensor element 
18 is mounted on the pressure port 10' to form a pressure sensor assembly 
10. 
The pressure sensor assembly 10 is designed to be threaded into a pressure 
vessel wall 62, such as the housing of a brake master cylinder, engine 
crankcase, etc., as shown in FIGS. 3A-3B. The assembly 10 comprises a 
casing 12, an insert 14, an O-ring 16 and a pressure sensor 18. The casing 
12 is formed of inexpensive cold rolled steel, whereas insert 14 is formed 
of stainless steel (preferably 304 or 316 stainless steel for weldability) 
so that the sensor 18 can be welded to it. The casing 12 has a threaded 
slug portion 20 at one end for attachment to the pressure vessel wall 62, 
and an annular cup portion 22 at the other end. The annular cup portion 22 
has a hex-shaped periphery 24 adapted to be engaged by a tool (not shown) 
when the assembly 10 is installed in or removed from the pressure vessel 
wall 62. Additionally, the cup portion 22 defines a cylindrical interior 
cavity 26, and a central annular recess 28 formed in the bottom of the 
cavity 26 for retaining the O-ring 16. The insert 14 has a base portion 30 
and a neck portion 32. The base portion 30 is disposed within the cavity 
26 atop the annular recess 28 so as to compress the O-ring 16 between the 
insert 14 and casing 12, and the neck portion 32 extends out of the cavity 
26 to form a pedestal 34 to which the sensor element 18 is welded. The 
insert 14 is retained relative to casing 12 by a staking operation in 
which a raised boss 36 of casing 12, best seen in the exploded view of 
FIG. 1, is staked or otherwise deformed against the neck portion 32 of 
insert 14. Subsequent to the staking operation, the boss 36 is flattened 
as best shown in FIG. 3B, leaving a generally circular recess 38. The 
casing 12 and insert 14 have central axial openings 40 and 42 which align 
when assembled as shown, placing the underside of sensor element 18 in 
communication with the interior of the pressure vessel. 
The stainless steel pressure sensor element 18 comprises a cylindrical wall 
46 and a thin diaphragm 48 with a central mass 50. The sensor wall 46 is 
seated on a narrow step 52 formed on the periphery of pedestal 34 so that 
the mass 50 is axially aligned with the central openings 40 and 42. The 
sensor element 18 is welded onto the pedestal 34 by a laser or electron 
beam welder, providing a hermetic seal. A good weld is possible due to the 
stainless steel composition of both insert 14 and sensor 18. Stress 
sensitive elements formed on the exposed surface of diaphragm 48 between 
the mass 50 and the wall 46, and connected in a bridge arrangement, have a 
resistance characteristic that changes with the pressure coupled to the 
underside of sensor 18 through the central openings 40 and 42. These 
elements are coupled (by wire bonding, for example) to an integrated 
circuit chip (not shown) which may be mounted on the outer surface of 
diaphragm 48 opposite mass 50. The integrated circuit detects and 
amplifies the resistance variations due to the sensed pressure, and 
provides an electrical signal output indicative of the sensed pressure. A 
plastic terminal assembly (not shown) with terminal pins coupled to the 
integrated circuit is seated on the cup portion 22, retained by a 
peripheral flange 54, and is secured thereon to complete the sensor 
assembly 10. 
As described above, the assembly of the present invention admits of reduced 
material and fabrication costs compared to one-piece and two-piece 
stainless steel pressure ports and yet maintains high reliability. While 
the invention has been described in reference to the illustrated 
embodiment, it will be understood that various modifications will occur to 
those skilled in the art, and that sensor assemblies incorporating such 
modifications may fall within the scope of this invention, which is 
defined by the appended claims.