Insert-molded pressure sensor with high pressure stainless steel sensing element

[0002] This invention relates to a high pressure sensor, and more particularly to a low cost stainless steel pressure sensor assembly that is integrated into a molded plastic component.

[0003] Motor vehicle controls frequently require measurement of certain pressure parameters such as engine oil pressure, fuel pressure, transmission fluid pressure or brake pressure. For reliability and durability, such applications usually require the use of a stainless steel pressure sensor element. In a typical application, the sensor element is welded to stainless steel a pressure port that is attached to the pressure vessel by a threaded fitting, for example. Due to the high cost of stainless steel relative to other materials, and the difficulty of machining stainless steel, various pressure port configurations have been developed for mininizing the required amount of stainless steel. See, for example, the U.S. Pat. Nos. 5,939,637 and 6,050,147, both of which are assigned to the assignee of the present invention. However, there is a growing interest in integrating a pressure sensor with the pressure vessel instead of attaching the pressure sensor to the pressure vessel by a threaded fitting, particularly in the case of molded plastic pressure vessels. Accordingly, what is needed is a pressure sensor assembly that minimizes the usage of stainless steel while being amenable to integration with molded plastic assemblies.

[0004] The present invention provides an improved pressure sensor assembly incorporating a stainless steel pressure sensing element, wherein the pressure sensing element is welded to a stainless steel pressure port that is insert-molded into a molded plastic pressure vessel. Sealing between the pressure port and the pressure vessel is enhanced by a O-ring seated in a groove on the outer periphery of the pressure port, or by a sealant applied to the periphery of the pressure port prior to the insert molding operation. According to the invention, the pressure port is insert-molded with the pressure vessel so as to expose an axial end of the pressure port having a stepped annular surface, and the pressure sensor element is welded to the exposed end of the pressure port. In a preferred embodiment, the plastic material of the pressure vessel extends into a central axial bore of the pressure port to minimize leakage by maximizing a potential leakage path for the medium contained by the pressure vessel, and over a shoulder of the pressure port to secure the pressure port in the body of the pressure vessel.

[0005] The single drawing FIGURE is a cross-sectional view of a stainless steel pressure sensor integrated with a molded plastic pressure vessel according to this invention.

[0006] Referring to the single drawing FIGURE, the reference numeral 10 designates the wall of a plastic pressure vessel containing a high pressure liquid or gaseous medium, and the reference numeral 12 generally designates a pressure sensor assembly integrated into the pressure vessel wall 10 by an insert molding operation. In addition to the pressure vessel wall 10, the pressure sensor assembly 12 includes a stainless steel pressure sensing element 14, a stainless steel pressure port 16, a sealing device such as an O-ring 18, and a terminal cover 20 which is only partially illustrated in the drawing FIGURE.

[0007] The stainless steel pressure sensing element 14 includes a cylindrical diaphragm 14awith a central mass i4b,and an annular side-wall 14cdepending from the rim of diaphragm 14a.Stress sensitive elements are formed on the exposed surface of diaphragm 14abetween the central mass 14band the side-wall 14c,and connected in a bridge arrangement to detect stress due to a pressure difference across the diaphragm 14a.Typically, 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 14aopposite central mass 14b.The integrated circuit detects and amplifies resistance variations due to the sensed pressure, and provides an electrical signal output which is coupled to metallic terminals (not shown) which may be integrated into the cover 20.

[0008] The stainless steel pressure port 16 is cylindrical cross-section, with a central axial bore 16a.The outer periphery of the pressure port 16 is stepped as shown to define upper and lower portions 16b,16c,the lower portion 16bhaving a larger outside diameter than the upper portion 16c.A circumferential groove 16dformed on the outer periphery of the lower portion 16breceives the O-ring 18, and a step 16eformed on the outer periphery of the upper portion 16caccommodates the annular side-wall 14cof the sensor element 14.

[0009] During manufacture of the pressure vessel, the stainless steel pressure port 16 is secured in the mold used to form the pressure vessel wall 10 such that plastic material injected into the mold surrounds and encases the pressure port 16, leaving part of the exterior periphery of upper portion 16cexposed, as shown. In particular, the plastic material extends into the axial bore 16aas indicated by the reference numeral 10a,around the O-ring 18 and the exterior periphery of the lower portion 16bas indicated by the reference numeral 10b,and around part of the exterior periphery of the upper portion 16cas indicated by the reference numeral 10c.This minimizes leakage by maximizing a potential leakage path for the medium contained by the pressure vessel, and secures the pressure port 16 in the pressure vessel wall 10. Additionally, the injection pressure compresses the O-ring 18 into the circumferential groove 16dto maximize its sealing capability. As indicated above, the O-ring may be replaced (or supplemented) by a sealing material applied to the periphery of pressure port 16 prior to the molding operation, if so desired. When the molding operation is completed, the pressure vessel and integrated pressure port 16 are removed from the mold, and the pressure sensing element 14 is seated on the step 16eand welded in place as shown. In usage, the medium contained by the pressure vessel is applied directly to the inboard surface of the sensor diaphragm 14avia the pressure wall portion 10a.

[0010] In summary, the insert-molded pressure sensor assembly of the present invention contributes to reduced cost by minimizing the usage of stainless steel and integrating the pressure port with a plastic pressure vessel containing the medium to be measured. While the invention has been described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. Accordingly, it will be understood that sensor assemblies incorporating such modifications may fall within the scope of this invention, which is defined by the appended claims.