Abstract:
A sensing element for determining a physical property of a gas mixture, in particular the exhaust gas of internal combustion engines, includes a sensor element, arranged in a housing and connected to at least one electrical cable, as well as a molded piece, which seals the housing and is made of an elastically deformable material, which encloses the at least one cable in a gas-tight manner by radial compression. To ensure the gas-tightness of the cable feed-through also under a higher temperature load, which results in decreased elasticity of the molded piece, a spring element is arranged inside the molded piece, which is able to be tensioned by the radial compression and in the tensioned state generates a force component that acts on the cables in a radial direction.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a sensing element for determining a physical property of a gas mixture, in particular the exhaust gas of internal combustion engines. 
   BACKGROUND INFORMATION 
   Such sensing elements may be designed as gas sensors for determining the concentration of a gas component of a gas mixture, in particular the oxygen concentration in the exhaust gas of an internal combustion engine, or as sensor for measuring the temperature or the pressure of the gas mixture, in particular the temperature or the pressure of the exhaust gas of an internal combustion engine. 
   In a known gas sensing element, in particular for determining the oxygen concentration of the exhaust gas of internal combustion engines (German Published Patent Application No. 41 26 378), the elastic, plug-type molded piece that is used to feed the connector cable out of the housing in a gas-tight manner is made of a heat-resistant material such as PTFE. However, materials such as silicon rubber or fluorelastomers, for instance FKM or FFKM, are used as well. By radial compression of the molded piece, which is brought about by an all-around tamping of the housing, the molded piece is pressed onto the insulation covering of the cable and then has a sealing effect; the sealing effect may be optimized further by the shape of the axial feed-through hole for the cable and by the surface roughness of the insulation covering of the cable. 
   Under temperature load, the characteristics of the elastomers exposed to mechanical pressure change in a disadvantageous manner with respect to the sealing effect. Depending on the type of elastomer used, it will soften or harden, the hardening even leading to embrittlement in extreme cases. An adequate sealing effect will then no longer be ensured in all these cases. As a result, in sensing elements where higher thermal demands are made on the cable exit, the use of elastomeric molded pieces has already been abandoned and other measures are taken to seal the cable exit point. 
   SUMMARY OF THE INVENTION 
   The sensing element according to the present invention has the advantage that, due to the spring element which is prestressed during installation, the contact pressure of the molded piece on the cable is kept virtually constant even with decreasing elasticity of the molded piece as a result of high temperature stress, so that the sealing effect of the molded piece is maintained in unchanged form. This makes it possible to utilize elastomers as material for the molded piece even at temperatures that are 20–50° C. above the temperature to which the cable exit, which is usually sealed by an elastomeric molded piece, is allowed to be exposed in known sensing elements. The use of elastomers in turn results in considerable cost savings compared to other, high-temperature-resistant cable feed-throughs. 
   According to an advantageous specific embodiment of the present invention, the molded piece has a blind hole having a circular inner cross section and at least two feed-through holes for each cable, which are preferably arranged equidistantly on a divider circle that is concentric with respect to the blind hole. The molded element is designed as clamping sleeve, which is rolled up in the shape of a helical spring and inserted into the blind hole. When the metallic housing is tamped all-around, the clamping sleeve is prestressed in that the sleeve, while reducing its inner diameter, slides over itself, so that the “helical spring” is tensioned. Due to the tensioned sleeve, the material of the molded piece is pressed against the insulation covering of the cables in a radial manner. If the elasticity of the material decreases, the contact pressure will remain virtually unchanged because of the acting spring force of the clamping sleeve, thereby maintaining the sealing effect of the molded piece. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows in excerpted form, a longitudinal section of a sensing element for determining a physical property of a gas mixture. 
       FIG. 2  shows a longitudinal section of a molded piece for the feed-through of a cable in the sensing element according to  FIG. 1 , including an inserted spring element. 
       FIG. 3  shows a view of the molded piece in the direction of arrow III in  FIG. 2 . 
       FIG. 4  shows a longitudinal section of the molded piece with inserted spring element according to an additional exemplary embodiment. 
       FIG. 5  shows a view of the molded piece in the direction of arrow V in  FIG. 4 . 
       FIG. 6  shows an enlarged plan view of the spring element in the molded piece according to  FIGS. 4 and 5  in the untensioned (a) and tensioned (b) state. 
       FIG. 7  shows a longitudinal section of the form element according to a third exemplary embodiment. 
       FIG. 8  shows a spring element for insertion into the molded piece according to  FIG. 7  in the untensioned state. 
       FIG. 9  shows the spring element according to  FIG. 8  in the tensioned state. 
       FIG. 10  shows a longitudinal section of the molded piece with inserted spring element according to a fourth exemplary embodiment. 
       FIG. 11  shows a view of the molded piece in the direction of arrow XI in  FIG. 10 . 
       FIG. 12  shows a longitudinal section of the molded piece with inserted spring element according to a fifth exemplary embodiment. 
       FIG. 13  shows a view of the molded piece in the direction of arrow XIII in  FIG. 12 . 
   

   DETAILED DESCRIPTION 
   The sensing element—shown in a cut-away view in longitudinal section—for determining a physical property of a gas mixture, for example the oxygen concentration in the exhaust gas of an internal combustion engine, has a sensor element  11  whose one end is exposed to the gas mixture, i.e., the exhaust gas, and at whose other end a contacting of at least one electrical cable  12 , via which sensor element  11  is connected to a control unit, takes place. In the exemplary embodiment of  FIGS. 1 to 3 , a total of four cables is connected, these cables being combined to form a connector cable  10 . Each cable  12  has an electrical conductor  13  and a cable insulation  14  surrounding electrical conductor  13 . 
   Sensor element  11  is accommodated in a housing  15 , which is made up of a solid metal body (not shown here) and a metallic protective sleeve  151  affixed to the metal body. Sensor element  11  is conducted through the metal body in a gas-tight manner and in its contact region is enclosed with radial clearance by protective sleeve  151 , which also extends across a section of cables  12  connected to sensor element  11 . For a gas-tight cable feed-through of cables  12  out of housing  15 , an elastically deformable, plug-type molded piece  16  is inserted in the end of protective sleeve  151  facing away from the solid metal body, this molded piece  16  enclosing cables  12  in a gas-tight manner by radial compression. Silicon rubber or fluorelastomers are used as material for molded piece  16 ; the radial tamping is brought about by an all-around compression  17  of metallic protective sleeve  151 . 
   In the exemplary embodiment of  FIGS. 1 to 3 , molded piece  16  has four feed-through holes  19 , which are arranged equidistantly on a divider circle  18  and through which one of the altogether four cables  12  of connector cable  10  is fed in each case. A blind hole  20 , which has bulges  201  extending into the spaces between feed-through holes  19  in the manner of a finger, is introduced in molded piece  16 , coaxially with respect to divider circle  18 . Blind hole  20  is introduced from the front end of molded piece  16 , which faces sensor element  11 . The number of bulges  201  of blind hole  20  corresponds to the number of feed-through holes  19  arranged on divider circle  18  and presupposes that at least three feed-through holes  19  are provided in molded piece  16 . Bulges  201  are shaped such that each extends along two adjacent feed-through holes  19  across an approximately 90° circumferential angle, in parallel to the hole wall of feed-through holes  19 , so that each feed-through hole  19  is enclosed by blind hole  20  having bulges  201  across an approximately 180° circumferential angle. In the altogether four feed-through holes  19  provided in molded piece  16  in this case, blind hole  20  therefore has an approximately clover-shaped hole cross-section. Accommodated in blind hole  20 , in a form-locking manner, is a spring element  21 , which is configured as clamping sleeve  22  having a form that corresponds to the contour of blind hole  20  having bulges  201 . Clamping sleeve  22  is made of thin-walled spring steel and composed of a plurality of layers, which are spot-welded to hold them together. The thickness of a layer is less than 0.1 mm, for instance. 
   During installation of the sensing element, after cables  12  have been guided through feed-through holes  19  and after molded piece  16  has been inserted into the end region of protective sleeve  151 , metallic protective sleeve  151  is reduced in diameter by all-around tamping  17  of metallic protective sleeve  151 , such tamping being produced, for instance, with the aid of a tool which has stamps that act in the radial direction. In this way spring element  21  situated in blind hole  20  is tensioned, namely by overall compression of clamping sleeve  22 , a radial contact pressure of molded piece  16  on cables  12  being generated simultaneously via spring element  21 , so that a gas-tight sealing of cables  12  in feed-through holes  19  is ensured. Spring element  21 , tensioned during all-around tamping  17 , will maintain this contact pressure on a long-term basis even when the elastic property of the material of the molded piece lessens as a result of high temperature stresses. 
   Molded piece  16 , shown in longitudinal section and in a view from below in  FIGS. 4 and 5 , is identical to molded piece  16  according to  FIGS. 2 and 3 , so that identical components have been provided with matching reference numerals. In this case spring element  21  is not configured as clamping sleeve having a plurality of radial fingers or bulges, but is designed as clamping sleeve  23  rolled up in the manner of a helical spring, which is likewise inserted into a now circular blind hole  24  in molded piece  16 . Drawing a of  FIG. 6  shows clamping sleeve  23 , rolled up in the manner of a helical spring, in the untensioned state, while drawing b shows it in the tensioned state. Clamping sleeve  23  rolled up in the way of a helical spring is inserted into blind hole  20  with slight prestressing ( FIG. 6   a ). If metallic protective sleeve  151  is then subjected to all-around tamping during the afore-described installation procedure, helical-spring-shaped clamping sleeve  23  is tensioned and assumes the form shown in  FIG. 6   b , in which it exerts an even radial pressure on the hole walls of blind hole  20 . This radial pressure of helical-spring-shaped clamping sleeve  23  provides for a constant contact pressure of the material of the molded piece on cable insulation  14  of cables  12 , this being the case even when the elasticity of the molded piece material decreases. 
   Molded piece  16 ′, shown in  FIG. 7  as additional exemplary embodiment in longitudinal section, is suited for the through-feeding of only a single cable  12 . Molded piece  16 ′ has a central feed-through hole  19  for cable  12  and an annular groove  25  concentrically surrounding through-feed hole  19 , this groove being introduced from the direction of the particular front end of molded piece  16  that will point to the interior of housing  15  once molded piece  16 ′ has been installed, i.e., point toward sensor element  11 . Inserted in annular groove  25  is spring element  21  shown in  FIG. 8 , which is configured as clamping sleeve  26  having a multitude of axially extending spring arms  27 , which are held together by a sleeve ring  28 . Each spring arm  27  has an outer spring leg  271 , which is an integral part of sleeve ring  28 , and an inner spring leg  272 , which is bent off from outer spring leg  271  at its sleeve-ring-remote end, the inner spring leg being guided back in parallel with outer spring leg  271 , up to sleeve ring  28 . The mutual clearance between the two spring legs  271 ,  272  corresponds approximately to the width of annular groove  325 . Clamping sleeve  26  is inserted into annular groove  25  in molded piece  16 ′, outer spring legs  271  coming to rest against outer groove wall  251  and inner spring legs coming to rest against inner groove wall  252 . 
   If metallic protective sleeve  15  is then tamped all-around after molded piece  16 ′ has been installed, spring legs  271 ,  272  are pressed together, their mutual distance being reduced in the process. This tensions clamping sleeve  26  and generates a restoring force acting on inner groove wall  252 , which provides for a pressure-tight contacting of the material of the molded piece with respect to cable insulation  14  of cable  12 .  FIG. 9  shows clamping sleeve  26  tensioned by all-around tamping. 
   The exemplary embodiment of molded piece  16  introduced in  FIGS. 10 and 11  is identical to molded piece  16  according to  FIGS. 4 and 5 , so that identical components have been provided with matching reference numerals. Here, too, molded piece  16  has a total of four feed-through holes  19  for cables  12 , these holes being arranged equidistantly on a divider circle  18 , and it also has a central blind hole  24 , which is coaxial with respect to divider circle  19 . In this case, spring element  21  situated in blind hole  24  is a hollow cylinder  29 , which is closed at the front end and encloses an air volume. Hollow cylinder  29  acts as air spring and is pressed together during the all-around tamping performed after molded piece  16  has been installed, so that the air volume is compressed and exerts an even radial pressure on the cylinder walls of hollow cylinder  29 . This radial pressure is in turn transmitted to the molded piece material, so that it is pressed against cable insulation  14  of cables  12  in a gas-tight manner. 
   The exemplary embodiment of molded piece  16  shown in longitudinal section and in a view from below in  FIGS. 12 and 13  is identical to molded piece  16  according to  FIGS. 2 and 3 . Here, too, blind hole  20  has finger-type bulges  201 , which extend between feed-through holes  19  arranged equidistantly on a divider circle  18 . Inserted in blind hole  20  having finger-type bulges  201  is spring element  21  in the form of a hollow body  30  closed at the front end, whose form is adapted to the contour of blind hole  20  having bulges  201 , so that hollow body  30  is lying in blind hole  20  with form-locking. Hollow body  30  in turn encloses an air volume, which is compressed by the radial compression of molded piece  16  resulting from the all-around tamping. In this way, hollow body  30  acts as tensioned air spring having a restoring force, so as to press the material of the molded piece against cables  12  in a radial manner.