Oxygen concentration sensor
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims priority from Japanese Patent Application Hei 7-70758, filed on Mar. 3, 1995, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oxygen concentration sensor used for air-fuel ratio control of an automobile engine. 
     2. Description of Related Art 
     A conventional oxygen concentration sensor, which is disclosed in U.S. Pat. No. 5,039,972 (corresponding to JPU 2-146363) and shown in FIGS. 15 and 16, includes a housing 10, a plate sensor element 12 inserted in housing 10, and a ceramic holding member 9 having a rectangular holding hole 90 for supporting plate sensor element 12 and disposed between housing 10 and plate sensor element 12. 
     Powder 13 filled and pressurized by a pressurizing member 14 placed above of housing 10 is disposed on ceramic holding member 9. 
     As shown in FIG. 16, ceramic holding member 9 has a tubular main body and holding hole 90 for holding plate sensor element 12. The circumference of an upper opening end 91 of holding hole 90 is a horizontal surface 93 and a tapered surface 92 is disposed around horizontal plane 93. As shown in FIG. 15, both horizontal plane 93 and conical surface 92 receive powder 13. 
     As shown in FIG. 15, however, at the time of filling and pressurizing powder 13 in a mounting process of plate sensor element 12, an impact is applied to the direction of arrows B around a boundary contacting plate sensor element 12, ceramic holding member 9 and powder 13, causing cracking of plate sensor element 12. The cracking of the element occurs more often when the impact is applied to a shorter (thinner) side of plate sensor element 12 as shown by an arrow T in FIG. 2B. 
     Therefore, the pressurizing speed has to be slowed down in order to prevent the cracking of the element in a conventional method. Although excessively large impact on plate sensor element 12 is avoided by this method, it takes a long time to mount plate sensor element 12. Controlling the pressurizing speed makes the mounting process of plate sensor element 12 more complicated. Furthermore, the conventional method is still ineffective in preventing force applied in the direction indicated by the arrow B to plate sensor element 12. Thus, the cracking of the element cannot be prevented completely. 
     SUMMARY OF THE INVENTION 
     In light of the above-described problem, a main object of the present invention is to provide an oxygen concentration sensor which is free from cracking of an element through a simple mounting process. 
     Another object of the present invention is to provide an oxygen concentration sensor in which the ceramic holding member has a inclined surface extending outward directly from an upper opening end of the holding hole to provide a moderate boundary along an interface between the inclined surface and the sensor element so that the powder can flow outward smoothly from the plate sensor element along the inclined surface. 
     The inclined surface is preferably a conical surface, and the holding hole has preferably upper opening sides in a same horizontal plane. The holding hole can be polygonal and the inclined surface can be formed around the upper opening sides of the polygonal holding hole. 
     The inclined surface can be formed only in a direction in parallel with a shorter side of the plate sensor element. It is possible to reduce the force applied to the direction which is more breakable in the plate sensor element so that the cracking of the element is prevented. 
     According to the feature described above, the pressurized powder can flow smoothly along the inclined surface, preventing stress from exerting on the plate sensor element. 
     It is also possible to use the upper opening end as a fixing surface when setting the holding member in a baking furnace, therefore, a freedom of positioning increases. 
     Also, as described in the above, in accordance with the decrease of defect caused by the cracking of the element, the productivity of oxygen concentration sensors is improved. 
     Also, the plate sensor element can be fixed in the housing with a pressure greater than that of before without increasing the stress, so that air tightness of the oxygen concentration sensor can be enhanced. 
     Since there is no need of slowing down the pressurizing speed for filling the powder, mounting speed of the sensor can be increased without sacrificing the precision of the plate sensor element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
     FIG. 1 is a cross-sectional view illustrating an oxygen concentration sensor according to a first embodiment of the present invention; 
     FIGS. 2A and 2B are a front view and a side view illustrating a plate sensor element of the oxygen concentration sensor according to the first embodiment respectively; 
     FIGS. 3A and 3B are partial cross-sectional front and side views illustrating a ceramic holding member according to the first embodiment; 
     FIGS. 4A, 4B and 4C are a plan view, a sectional view taken along lines 4B--4B of FIG. 3 and a sectional view taken along the line 4C--4C of FIG. 3 of the ceramic holding member according to the first embodiment respectively; 
     FIG. 5 is a perspective view illustrating the ceramic holding member according to the first embodiment; 
     FIG. 6 is a cross-sectional view illustrating the major part of the oxygen concentration sensor according to the first embodiment; 
     FIG. 7 is a perspective view illustrating the plate sensor element having an oval cross section and the ceramic holding member according to the first embodiment; 
     FIGS. 8A and 8B are a partial cross-sectional front and side views illustrating a ceramic holding member according to a second embodiment of the present invention; 
     FIG. 9 is a perspective view illustrating the ceramic holding member according to the second embodiment; 
     FIGS. 10A, 10B and 10C are a plan view, a cross-sectional view taken along lines 10B--10B of FIG. 8 and a sectional view taken along lines 10C--10C of FIG. 8 illustrating the ceramic holding member according to the second embodiment respectively; 
     FIGS. 11A and 11B are a partial cross-sectional front and side views illustrating a ceramic holding member according to a third embodiment of the present invention; 
     FIG. 12 is a perspective view illustrating the ceramic holding member according to the third embodiment; 
     FIGS. 13A and 13B are a partial cross-sectional front and side views illustrating a ceramic holding member according to a fourth embodiment; 
     FIG. 14 is a perspective view illustrating a plate sensor element having a polygonal cross section supported by a ceramic holding member according to a fifth embodiment of the present invention; 
     FIG. 15 is a cross sectional view illustrating the major part of an oxygen concentration sensor of a prior art; and 
     FIG. 16 is a perspective view illustrating a ceramic holding member of the above oxygen concentration sensor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     An oxygen concentration sensor according to the present invention will be explained with reference to FIG. 1 through FIG. 6. 
     An oxygen concentration sensor 1, as shown in FIG. 1, includes a housing 16, a plate sensor element 12 inserted into housing 10 and a ceramic holding member 2 having a longitudinal holding hole 20 for supporting plate sensor element 12. 
     Powder 13 is filled under pressure in a space between ceramic holding member 2 and a pressurizing member 14. Ceramic holding member 2 has a conical surface 22 which has an upper opening end 21 of holding hole 20 at the center thereof and receives pressurized powder 13. Conical surface 22 is a part of cone-shaped surface directly descending from upper opening end of holding hole 20 as shown in FIG. 3B and indicated by a dotted line in FIG. 5, so that the angle formed by plate sensor element 12 and conical surface 13 becomes obtuse and provides continuous flow surface for powder 13 as shown in FIG. 6. 
     As shown in FIGS. 2A and 2B, plate sensor element 12 has a main body having a rectangular cross-section and a pair of flanges 120 which are supported by a receiving surface 25 of ceramic holding member 2 at a central portion of the main body. 
     An arrow C indicates a longer side or width-wise direction and an arrow T indicates a shorter side or thickness-wise direction respectively in plate sensor element 12. 
     As shown in FIGS. 3A and 3B, holding hole 20 is formed in the center of holding member to receive and hold plate sensor element 12. Receiving face 25 is formed to support flanges 120 of plate sensor element 12 as shown in FIG. 3B, FIG. 4A, FIG. 4B and FIG. 4C. 
     In FIG. 1, reference numerals 16, 17, 18 and 19 indicate respectively a cushion packing, a protecting cover, a metallic ring and a protection cover. 
     Next, a function and an effect according to the first embodiment is explained mainly with reference to FIG. 6. 
     Plate sensor element 12 is inserted into holding hole 20 of holding member 2, and powder 13 is filled in housing 10 and pressurized via pressurizing member 14 from outside. Powder 13 can be formed into a predetermined shape in advance and inserted in housing 10. 
     When pressure is applied on powder 13 via pressurizing member 14, powder 13 flows along conical surface 22 from the vicinity of plate sensor element 12 to the side of housing 10 smoothly as indicated by arrows A in FIG. 6. Therefore, high stress is not applied to plate sensor element 12. 
     As a result, plate sensor element 12 is fixed in holding hole 20 without receiving an excessively large impact thereto even at the time of pressurizing, thus cracking of the element is prevented. 
     Also, since stress applied to plate sensor element 12 decreases, plate sensor element 12 can be mounted in housing 10 with greater force so that a more air-tight oxygen concentration sensor can be obtained. 
     Furthermore, since the pressurizing speed of the powder pressurization step can be increased without increasing stress on the plate sensor element, the productivity is increased without reducing the precision of the sensor. 
     A ceramic holding member 28 with an oval hole 218 for supporting plate sensor element 128 having the same oval cross-section is shown in FIG. 7. An upper opening end of the holding hole is indicated as a reference numeral 218. 
     Second Embodiment 
     An oxygen concentration sensor according to a second embodiment of the present invention is described with reference to FIGS. 8 through FIGS. 10A, 10B and 10C. 
     A ceramic holding member 3 has inclined surfaces formed only in a direction (indicated by arrow T in FIG. 2B) parallel to the shorter side of holding hole 30. Longer sides of a rectangular upper opening end 31 of holding hole 30 are placed in a same horizontal plane as the shorter sides thereof. 
     Inclined surfaces 32 descend outward from opposite longer sides of upper opening end 31. On the other hand, horizontal surfaces 320 are formed on opposite shorter sides of the same. Both inclined surfaces 32 and horizontal surfaces 320 receive the pressurized powder. 
     Other elements are the same as described in the first embodiment. 
     The plate sensor element tends to be broken more in the direction parallel to the shorter side indicated by arrow T (thickness-wise) of FIG. 2B than in the direction parallel to the longer side indicated by arrow C of FIG. 2A when the plate sensor element is particularly thin. Therefore, in the oxygen concentration sensor having ceramic holding member 3, inclined surfaces 32 are formed to extend from the longer sides of upper opening end 31 so that powder can flow smoothly on the inclined surfaces to decrease the force applied to the thickness-wise direction. 
     Third Embodiment 
     In a ceramic holding member 2, a horizontal surface 220 is formed on an outer circumference of a conical surface 221 as shown in FIGS. 11A, 11B and FIG. 12. 
     Conical surface 221 is formed around an upper opening end 21 of ceramic holding member 2. Both conical and horizontal surfaces 221 and 220 receive the pressurized powder. Other elements are the same as described in the first embodiment. 
     In the oxygen concentration sensor having ceramic holding member 2 described in this embodiment, there is no portion which constitutes an acute angle and an even pressurized density of the powder can be obtained. 
     Fourth Embodiment 
     As shown in FIG. 13, parabolic surface 222 receives the pressurized powder. Other elements are the same as described in the first embodiment. 
     In the oxygen concentration sensor having ceramic holding member 2 according to this embodiment, the powder can be evenly filled as in the third embodiment. 
     Fifth Embodiment 
     As shown in FIG. 14, a ceramic holding member 29 having a holding hole of a polygonal cross-section for supporting a plate sensor element 129 which has the same polygonal cross section. 
     Inclined surfaces 229 are formed to descend from respective sides of upper opening end 219. Other elements are the same as described in the first embodiment. 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the present invention as defined by the appended claims.