Abstract:
A pressure sensor comprises a sensor diaphragm which on one of its surfaces can be acted upon by the fluid pressure to be defected. The sensor diaphragm is rigidly supported at its opposite surface. It is made of an elastomeric material of an electrically non-conducting material wherein fine particles of electrically conducting material are homogenously embedded and distributed in such a density that a compression of the sensor diaphragm caused by the fluid pressure results in a measurable change in its electrical resistivity.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a pressure sensor comprising a sensor diaphragm having a surface adapted to be exposed to a fluid under pressure. 
     BACKGROUND OF THE INVENTION 
     Pressure sensors exist in various types depending on their field of application. In automotive engineering applications only rugged types come into consideration which are capable of reliable operation over a broad temperature range for many years, whilst their absolute sensing accuracy is of less importance. The response of most pressure sensors is a function of pressure, i.e. the magnitude being a measure of the sensed pressure. An elastomeric sensor diaphragm exposed to a constant reference pressure on one side and to the fluid pressure to be sensed at the other responds to the difference in pressure by a deflection, the magnitude of which is convertible e.g. by a connected potentiometer into an electrical signal. Limit switches are actuated directly or indirectly by the response of the sensor diaphragm in triggering switching actions as a function of the pressure. In all cases, moving parts are needed which are exposed over lengthy periods to shock, wear and heavy temperature effects, the fluid separation between the two sides of a sensor diaphragm also being problematic under these circumstances. 
     SUMMARY OF THE INVENTION 
     The present invention provides a pressure sensor which due to its rugged design, very simple configuration and compact size is particularly suitable for automotive engineering applications in not only producing a pressure-proportional measured variable but also controlling switching actions as a function of pressure. 
     In accordance with the invention the sensor diaphragm is rigidly supported on its surface opposite the surface exposed to pressure. The sensor diaphragm is made of an elastomeric material in the electrically non-conducting mass of which fine particles of electrically conducting material are embedded so as to be homogenously distributed. A compression of the sensor diaphragm caused by the fluid pressure results in a measurable change in the surface resistance, or resistivity, of the sensor diaphragm because the particles of electrically conducting material are moved closer to each other as a result of which the probability of the neighboring particles coming into contact with each other is increased. Accordingly, the pressure sensor in accordance with the invention requires no moving parts whatsoever, thus eliminating the problems associated with moving parts such as susceptibility to failure, bulkiness, wear, high production costs and leakage problems. The electrical surface resistance of the sensor diaphragm can be determined by contact being made at two measurement points spaced away from each other. Conventional measurement circuits can be used. For generating a pressure-proportional measurement signal a bridge circuit is used to advantage. For controlling a switching action as a function of pressure simple differential amplifier circuits are suitable. 
     In the preferred embodiment of the invention the sensor diaphragm has two measurement contacts on the side of its rigidly supported surface, the sensor diaphragm as such thus ensuring a perfect separation of fluid. 
     The configuration of the pressure sensor is based preferably on the simple principle of the sensor diaphragm being clamped by a supporting block between annular components with sealing rings being interposed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and features of the invention read from the following description of one embodiment of the pressure sensor with reference to the attached drawings in which 
     FIG. 1 is a plan view of the pressure sensor on its fluid side; 
     FIG. 2 is a section taken along the line II—II as shown in FIG. 1; 
     FIG. 3 is a section taken along the line III—III as shown in FIG. 1; 
     FIG. 4 is a section taken along the line IV—IV as shown in FIG. 1; 
     FIG. 5 is a plan view of the pressure sensor on its base body side; 
     FIG. 6 is a section view of a preferred embodiment of the pressure sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 to  5  there is illustrated a pressure sensor identified in general by the reference numeral  10 , comprising as the main functional element a sensor diaphragm  12  made of an electrically insulating elastomer incorporating particles of an electrically conducting material embedded in the elastomer mass. The electrically conducting particles are provided in such a density that they endow the sensor diaphragm with a measurable volume resistance and thus also a surface resistance. This electrical volume resistance or surface resistance is measurably reduced on compression of the diaphragm mass by the conductive particles being bunched together to thus enhance the probability of them coming into contact with each other. A suitable elastomer is silicone rubber, in which electrically conducting particles are embedded homogeneously distributed. The density of the conductive particles may be determined empirically, suitable examples of which are particles of nickel, silver, copper or silver coated particles of copper or glass. 
     The sensor diaphragm  12  is a flat circular disk resting by one of its two surfaces on a face surface of a cylindrical supporting block  14  by which it is rigidly supported. In the supporting block  14  consisting of an electrically insulating material two contact pins  16  are embedded having widened ends facing the sensor diaphragm and which are in full contact with the sensor diaphragm  12  for the purpose of forming the measuring contacts. The opposite ends of the contact pins  16  protrude from the supporting block and form connecting pins for a connector. The supporting body  14  is snugly accommodated in a cylindrical depression of an annular base body  18 . 
     A compression ring  20  is provided on its two annular surfaces with three concentric grooves for reveiving O-rings and is supported by the surface of the sensor diaphragm  12  facing away from the supporting block  14 . By its opposite annular surface the compression ring  20  is clamped against a ring flange  24  at one end of a connecting port  26  by clamping bolts  22 . The assembly formed by the compression ring  20  and the ring flange  24  with the connecting port  26  is clasped by a clamping ring  28  which is clamped against the base body  18  by means of clamping bolts  30 , as a result of which the compression ring is urged against the surface of the sensor diaphragm  12 . This contact pressure is definable by inserting shims between the clamping ring  28  and the base body  18 . 
     Referring now to FIG. 4 there is illustrated how the annular base body  18  forms in its middle a connector depression  32  into which the contact pins  16  protrude, a radial cable conduit  34  adjoining the connector depression  32 . 
     Precise alignment of the assembled components of the pressure sensor is achieved by locating pins  36  inserted in correspondingly aligned holes in the clamping ring  28  and the base body  18 , these locating pins also precisely defining the rotary positions of the pressure sensor components relative to each other. 
     The connecting port  26  surrounds a fluid passage porting into the central opening of the compression ring  20  surrounding the exposed part of the sensor diaphragm surface. This sensor diaphragm surface is acted upon via the connecting port  26 ,with the fluid pressure to be determined. Separating the media is done by the sensor diaphragm itself in conjunction with the O-rings inserted in the grooves of the compression ring  20 . 
     The contact pins are connected via a connector (not shown) to an electronic circuit which depending on the particular application of the pressure sensor may be configured e.g. as a bridge circuit for generating a pressure-proportional measurement signal or as a differential amplifier for activating switching actions as a function of the pressure. By applying the fluid pressure infed via the connecting port  26  to the exposed surface of the sensor diaphragm  12  the mass of the sensor diaphragm  12  is compressed, as a result of which its electrical volume resistance and thus also its surface resistance available via the contact pins  16  is reduced. For maximizing the measurement signal the ends of the contact pins  16  are arranged in the region of the clamped peripheral rim of the sensor diaphragm  12 . 
     In the embodiment shown the sensor diaphragm  12  is a round flat disk of material. However, it may also be configured as a three-dimensionally shaped body adapted to special applications. 
     The embodiment as shown in FIGS. 1 to  5  is particularly suitable for testing since using the clamping bolts  30  makes for a releasable assembly. 
     Referring now to FIG. 6 there is illustrated an embodiment suitable for cost-effective series production. In this embodiment the connecting port  26  is provided with a female thread  26   a  configured integrally with a housing body  40  into which the supporting block  14  is inserted. The supporting block  14  is with the sensor diaphragm  12  mounted thereon is rigidly fixed by a crimped rim  42  of the housing body  40 . At its outer circumference the sensor diaphragm  12  comprises a molded ring bead  12   a  which is urged against an opposite ring surface area  40   a  in the interior of the housing body  40  to seal off the structure. The contact pins  16  protrude into a electronic module,  44  applied to the supporting block  14  from which contact pins  46  are brought out. The electronic module  44  contains at least the high-impedance components of the measuring circuit with which the changes in resistance of the sensor diaphragm  12  are converted into an electrical signal. 
     One of the contact pins  16  may be eliminated when the sensor diaphragm  12  is in contact—more particularly by its outer circumference—with a compression ring which may be embedded in the supporting block  14 . As an alternative the housing body  40  is made of an electrically conducting material, more particularly metal, and forms in all as well as with the ring surface area  40   a  a ground contact and simultaneously an eletrical shield.