Abstract:
An improved structure of a gas sensor is provided which may be employed in an oxygen measuring device of an air-fuel ratio control system measuring an oxygen content in exhaust gasses of an internal combustion engine of automotive vehicles. The structure includes a holding mechanism disposed within an air cover installed on an end of a sensor housing to cover a base of a gas-sensing element. The holding mechanism is made of a spring member or a rigid member to retain an insulation porcelain within the air cover elastically or rigidly. The use of the holding mechanism provides for ease of installation of the insulation porcelain, results in an increase in durability of the gas sensor, and allows the overall size of the gas sensor to be decreased.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates generally to an improvement on a gas sensor which is employed, for example, in an oxygen measuring device of an air-fuel ratio control system to measure an oxygen content in exhaust gasses of an internal combustion engine for automotive vehicles, and more particularly to an improved structure of such a gas sensor which provides for ease of installation of an insulation porcelain.  
           [0003]    2. Background Art  
           [0004]    It is known in the art that burning control of fuel in internal combustion engines based on an oxygen content in exhaust gasses as a parameter indicating an air-fuel ratio is effective in energy saving and emission control. As gas sensors measuring the concentration of oxygen in exhaust gasses, gas sensors equipped with a sensing element made of a solid electrolyte such as zirconia are known.  
           [0005]    [0005]FIG. 25 shows one example of conventional gas sensors.  
           [0006]    The gas sensor  9  consists of a sensing element  910 , a housing  94 , and a protective cover  92 . The sensing element  910  has a portion  911  exposed to a gas to be measured and is retained within the housing  94 . The protective cover  92  is installed on the housing  94  and surrounds the gas-exposed portion  911  of the sensing element  910 . A packing or sealing member  918  is disposed between the housing  94  and the sensing element  910 .  
           [0007]    The sensing element  910  is of a cup-shape and has defined therein a reference gas chamber  912  leading to the atmosphere. A gas chamber  913  into which the gas to be measured is admitted is defined in the protective cover  92  around the sensing element  910 . The sensing element  910  has a reference electrode exposed to the reference gas chamber  912  and a measuring electrode exposed to the gas chamber  913 . The sensing element  910  also has a heater  915  disposed therein.  
           [0008]    The sensing element  910  is electrically connected to leads  981  and  991  through coupling terminals  982  and  992  and metallic terminals  983  and  993 , respectively. The metallic terminals  983  and  993  are in contact with terminals formed on the sensing element  910  leading to the reference and measuring electrodes, respectively. A lead  971  is connected to the heater  915  for supplying the power thereto.  
           [0009]    Bases of the leads  981  and  991  connected to the coupling terminals  982  and  992  are retained within an insulation porcelain  95 . The insulation porcelain  95  is held by an inner cover  931  and an outer cover  932  of an air cover assembly. Specifically, the inner cover  931  supports at an end thereof a lower portion of the insulation porcelain  95 , while the outer cover  932  presses at a shoulder thereof the insulation porcelain  95  downward against the end of the inner cover  931 . A disc spring  956  is disposed between the shoulder of the outer cover  932  and the insulation porcelain  95 .  
           [0010]    An air cover  933  surrounds an upper portion of the outer cover  932  through a cylindrical water-repellent filter  938  An elastic insulator  945  is fitted within an open end portion of the air cover  933  which holds the leads  971 ,  981 , and  991  therein.  
           [0011]    The air cover  933  and the outer cover  932  have first air vents  936  and second air vents  937 , respectively, which lead to the reference gas chamber  912  through the water-repellent filter  938  for inducting the air into the reference gas chamber  912 .  
           [0012]    The elastic insulator  945 , the outer cover  932  of the air cover assembly, and the air cover  933  are joined together by crimping.  
           [0013]    The gas sensor  9  produces at the sensing element  910  an electromotive force as a function of a difference in concentration between the air in the reference gas chamber  912  and the gas in the gas chamber  913  to be measured and outputs it through the leads  981  and  991 .  
           [0014]    The above described structure of the gas sensor  9  contributes to the improvement of control ability in automotive internal combustion engines, however, it is being still required for decreasing the overall size and manufacturing costs, and improving the durability of the gas sensor  9 .  
         SUMMARY OF THE INVENTION  
         [0015]    It is therefore a principal object of the invention to avoid the disadvantages of the prior art.  
           [0016]    It is another object of the invention to provide an improved structure of a gas sensor which is compact in size, but possesses high durability and may be manufactured at low costs.  
           [0017]    According to one aspect of the invention, there is provided an improved structure of a gas sensor designed to measure a given component content in a gas. The gas sensor comprises: (a) a housing having a first end and a second end; (b) a sensing element disposed in the housing, the sensing element having a base portion and a gas-sensing portion, the base portion projecting from the first end of the housing, the gas-sensing portion projecting from the second end of the housing; (c) a first cover installed on the first end of the housing to cover the base portion of the sensing element; (d) a plurality of electric terminals connected to the sensing element for establishing electric communication between the sensing element and an external device; (e) an insulator in which the electric terminals are disposed, the insulator including a body and a flange, the flange having an groove formed in a peripheral wall thereof; (f) a second cover installed on the second end of the housing to cover the gas-sensing portion of the sensing element and retain the insulator therein, the second cover including a small-diameter portion, a large-diameter portion, and a shoulder connecting between the small-diameter portion and the large-diameter portion, the small-diameter portion being greater in diameter than the body of the insulator and smaller in diameter than the flange of the insulator, the large-diameter portion being greater in diameter than the flange of the insulator; and (g) an elastic holding mechanism disposed between the groove of the flange of the insulator and an inner wall of the large-diameter portion of the second cover to be deformable elastically in a radius direction of the second cover to hold the insulator within the second cover firmly.  
           [0018]    In the preferred mode of the invention, the elastic holding mechanism includes a plurality of springs disposed in the groove of the flange at regular intervals away from each other.  
           [0019]    Each of the springs is made of a corrugated plate which is disposed between the groove of the flange and the inner wall of the large-diameter portion of the second cover so as to urge the flange of the insulator elastically inwardly of the second cover, thereby holding the insulator within the second cover.  
           [0020]    Each of the corrugated plate may have an extension wall which is placed in surface contact with the inner wall of the second cover and welded at a portion thereof to the inner wall of the second cover.  
           [0021]    The elastic holding mechanism may alternatively be made of a spring ring disposed in the groove of the flange of the insulator.  
           [0022]    The spring ring has disposed on a periphery wall thereof a plurality of members which are so folded as to produce elastic pressure between the flange of the insulator and the inner wall of the second cover for holding the insulator within the second cover firmly.  
           [0023]    According to the second aspect of the invention, there is provided a gas sensor measuring a given component content in a gas which comprises: (a) a housing having a first end and a second end; (b) a sensing element disposed in the housing, the sensing element having a base portion and a gas-sensing portion, the base portion projecting from the first end of the housing, the gas-sensing portion projecting from the second end of the housing; (c) a first cover installed on the first end of the housing to cover the base portion of the sensing element; (d) a plurality of electric terminals connected to the sensing element for establishing electric communication between the sensing element and an external device; (e) an insulator in which the electric terminals are disposed, the insulator including a body and a flange; (f) a second cover installed on the second end of the housing to cover the gas-sensing portion of the sensing element and retain the insulator therein, the second cover including a small-diameter portion, a large-diameter portion, and a shoulder connecting between the small-diameter portion and the large-diameter portion, the small-diameter portion being greater in diameter than the body of the insulator and smaller in diameter than the flange of the insulator, the large-diameter portion being greater in diameter than the flange of the insulator; and (g) an elastic holding mechanism disposed in a gap between the insulator and an inner wall of the second cover so as to produce elastic pressure which holds the insulator elastically within the second cover.  
           [0024]    In the preferred mode of the invention, the elastic holding mechanism is disposed in the gap between the body of the insulator and an inner wall of the large-diameter portion of the second cover in contact with the body of the insulator and the inner wall of the large-diameter portion of the second cover to urge the flange elastically into constant engagement with an inner wall of the shoulder of the second cover, thereby retaining the insulator within the second cover firmly.  
           [0025]    The elastic holding mechanism is made of a ring base and a plurality of elastic deformable members installed on the ring base. Each of the elastic deformable members is placed in contact with the body of the insulator and the inner wall of the large-diameter portion of the second cover to urge the flange elastically into constant engagement with an inner wall of the shoulder of the second cover, thereby retaining the insulator within the second cover firmly.  
           [0026]    The elastic holding mechanism may alternatively include a base and an elastically deformable member. The base is placed in contact with one of opposed surfaces of the flange of the insulator remote from an inner wall of the shoulder of the second cover. The elastically deformable member is press fit within the gap between the flange of the insulator and the inner wall of the large-diameter portion of the second cover in surface contact with the inner wall of the large-diameter portion of the second cover to urge the flange inwardly in the radius direction of the second cover elastically, thereby retaining the insulator within the second cover firmly.  
           [0027]    The elastic holding mechanism may alternatively include a ring base and an elastically deformable member installed on the ring base. The ring base is placed in contact with one of the opposed surfaces of the flange of the insulator remote from the inner wall of the shoulder of the second cover. The elastically deformable member is press fit within the gap between the flange of the insulator and the inner wall of the large-diameter portion of the second cover to urge the flange inwardly in the radius direction of the second cover elastically, thereby retaining the insulator within the second.  
           [0028]    The elastically deformable member may have a V-shape or a U-shape.  
           [0029]    The elastic holding mechanism may alternatively include a ring base and a plurality of elastically deformable members installed on the ring. The ring base is placed in contact with one of the opposed surfaces of the flange of the insulator remote from the inner wall of the shoulder of the second cover. The elastically deformable members is press fit within the gap between the flange of the insulator and the inner wall of the large-diameter portion of the second cover to urge the flange inwardly in a radius direction of the second cover elastically, thereby retaining the insulator within the second cover firmly.  
           [0030]    The elastic holding mechanism may alternatively include a base and an elastically deformable member extending from the base. The base is placed between the inner wall of the shoulder of the second over and the surface of the flange of the insulator. The elastically deformable member is press fit within the gap between the flange of the insulator and the inner wall of the large-diameter portion of the second cover to urge the flange inwardly in the radius direction of the second cover elastically, thereby retaining the insulator within the second cover firmly.  
           [0031]    The elastically deformable member may be made of a corrugated spring plate extending longitudinally of the insulator in contact with the flange of the insulator and the inner wall of the large-diameter portion of the second cover.  
           [0032]    The base of the elastic holding mechanism may be made of a ring. The corrugated spring plate is of an annular shape and extends from a periphery of the ring.  
           [0033]    The elastic holding mechanism may alternatively include a ring base and elastically deformable corrugated members connected to the ring base. The ring base is placed between the inner wall of the shoulder of the second over and the surface of the flange of the insulator. Each of the elastically deformable corrugated members is press fit within the gap between the flange of the insulator and the inner wall of the large-diameter portion of the second cover to urge the flange inwardly in the radius direction of the second cover elastically, thereby retaining the insulator within the second cover firmly.  
           [0034]    The elastic holding mechanism may alternatively include a protrusion provided on an inner wall of the large-diameter portion of the second cover and an elastically deformable member disposed between the protrusion and the surface of the flange of the insulator to urge the flange elastically into constant engagement with the shoulder of the second cover, thereby retaining the insulator within the second cover firmly.  
           [0035]    The protrusion is made of a ring connected to the inner wall of the large-diameter portion of the second cover. The elastically deformable member is made of an annular spring which is substantially of S-shape in cross section.  
           [0036]    The elastic holding mechanism may alternatively include a plurality of corrugated spring plates and a ridge formed on the inner wall of the large-diameter portion of the second cover on which the flange of the insulator is placed. The corrugated spring plates are disposed in the gap between the inner wall of the shoulder of the second cover and one of the opposed surfaces of the flange to produce elastic pressure which urges the flange elastically against the ridge, thereby holding the insulator within the second over firmly.  
           [0037]    The elastic holding mechanism may alternatively include a spring ring having a corrugated shape in cross section and a ridge formed on the inner wall of the large-diameter portion of the second cover on which the flange of the insulator is placed. The spring ring is disposed in the gap between an inner wall of the shoulder of the second cover and one of the opposed surfaces of the flange to produce elastic pressure which urges the flange elastically against the ridge, thereby holding the insulator within the second over firmly.  
           [0038]    The elastic holding mechanism may alternatively include a plurality of springs each made up of an outer plate, an inner plate, and a base connecting between the outer and inner plates. The outer plate is in elastic contact with the inner wall of the large-diameter portion of the second over. The inner plate is in elastic contact with an outer wall of the body of the insulator, thereby bringing the base into constant engagement with one of the opposed surfaces of the flange of the insulator to urge the flange elastically against an inner wall of the shoulder of the second cover.  
           [0039]    The outer plate may have an extension wall which is placed in surface contact with the inner wall of the second cover and welded at a portion thereof to the inner wall of the second cover.  
           [0040]    The elastically deformable member of the elastic holding mechanism may have an extension wall which is placed in surface contact with the inner wall of the second cover and welded at a portion thereof to the inner wall of the second cover.  
           [0041]    The elastic holding mechanism may alternatively be made of a spring plate having a first and a second end. The first end is connected to an inner wall of the large-diameter portion of the second cover. The second end is in elastic contact with one of opposed surfaces of the flange of the insulator, thereby urging the flange of the insulator into constant engagement of the other opposed surface of the flange with an inner wall of the shoulder of the second cover.  
           [0042]    According to the third aspect of the invention, there is provided a gas sensor measuring a given component content in a gas which comprises: (a) a housing having a first end and a second end; (b) a sensing element disposed in the housing, the sensing element having a base portion and a gas-sensing portion, the base portion projecting from the first end of the housing, the gas-sensing portion projecting from the second end of the housing; (c) a first cover installed on the first end of the housing to cover the base portion of the sensing element; (d) a plurality of electric terminals connected to the sensing element for establishing electric communication between the sensing element and an external device; (e) an insulator in which the electric terminals are disposed, the insulator including a body and a flange; (f) a second cover installed on the second end of the housing to cover the gas-sensing portion of the sensing element and retain the insulator therein, the second cover including a small-diameter portion, a large-diameter portion, and a shoulder connecting between the small-diameter portion and the large-diameter portion, the small-diameter portion being greater in diameter than the body of the insulator and smaller in diameter than the flange of the insulator, the large-diameter portion being greater in diameter than the flange of the insulator; and (g) a holding mechanism disposed between an end of the body of the insulator and the first end of the housing to retain the insulator within the second cover.  
           [0043]    In the preferred mode of the invention, the holding mechanism is made of a spring.  
           [0044]    The holding mechanism may alternatively be made of a cylindrical rigid member.  
           [0045]    According to the fourth aspect of the invention, there is provided a gas sensor measuring a given component content in a gas which comprises: (a) a housing having a first end and a second end; (b) a sensing element disposed in the housing, the sensing element having a base portion and a gas-sensing portion, the base portion projecting from the first end of the housing, the gas-sensing portion projecting from the second end of the housing; (c) a first cover installed on the first end of the housing to cover the base portion of the sensing element; (d) a plurality of electric terminals connected to the sensing element for establishing electric communication between the sensing element and an external device; (e) an insulator in which the electric terminals are disposed, the insulator including a body and a flange; (f) a second cover installed on the second end of the housing to cover the gas-sensing portion of the sensing element and retain the insulator therein, the second cover including a small-diameter portion, a large-diameter portion, and a shoulder connecting between the small-diameter portion and the large-diameter portion, the small-diameter portion being greater in diameter than the body of the insulator and smaller in diameter than the flange of the insulator, the large-diameter portion being greater in diameter than the flange of the insulator; and (g) a holding member holding the insulator within the second cover, the holding member being made of a wedge-shaped member and fitted between the flange of the insulator and an inner wall of the large-diameter portion of the second cover to retain the insulator firmly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]    The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.  
         [0047]    In the drawings:  
         [0048]    [0048]FIG. 1 is a longitudinal sectional view which shows a gas sensor according to the first embodiment of the invention;  
         [0049]    [0049]FIG. 2 is a longitudinal sectional view which shows an insulation porcelain installed in the gas sensor of FIG. 1;  
         [0050]    [0050]FIG. 3( a ) is a perspective view which shows a holding member used to retain the insulation porcelain of FIG. 2 in the gas sensor of FIG. 1;  
         [0051]    [0051]FIG. 3( b ) is a perspective view which shows holding members fitted in the insulation porcelain of FIG. 2;  
         [0052]    [0052]FIG. 3( c ) is a partial sectional view which shows a holding member fitted in a groove formed in a flange of the insulation porcelain shown in FIG. 3( b );  
         [0053]    [0053]FIG. 4 is a vertical sectional view which shows another type of a gas sensor with which the holding member in FIGS.  1  to  3 ( c ) may be used;  
         [0054]    [0054]FIG. 5 is a perspective which shows a modification of a holding member of the first embodiment;  
         [0055]    [0055]FIG. 6( a ) is a perspective view which shows a holding member according to the second embodiment of the invention;  
         [0056]    [0056]FIG. 6( b ) is a perspective view which shows a modification of the holding member of FIG. 6( a );  
         [0057]    [0057]FIG. 7( a ) is a perspective view which shows a holding member according to the third embodiment of the invention;  
         [0058]    [0058]FIG. 7( b ) is a perspective view which shows holding members attached to an insulation porcelain in the third embodiment;  
         [0059]    [0059]FIG. 7( c ) is a partially vertical sectional view which shows installation of the insulation porcelain of FIG. 7( b ) in an air cover of a gas sensor;  
         [0060]    [0060]FIG. 8 is a plane view which shows a modification of the third embodiment shown in FIGS.  7 ( a ) to  7 ( c );  
         [0061]    [0061]FIG. 9( a ) is a perspective view which shows a holding member in the fourth embodiment of the invention;  
         [0062]    [0062]FIG. 9( b ) is a partial sectional view which shows installation of the holding member of FIG. 9( a );  
         [0063]    [0063]FIG. 10 is a perspective view which shows a modification of the holding member shown in FIG. 9( a );  
         [0064]    FIGS.  11 ( a ) and  11 ( b ) show modifications of a holding member in the fourth embodiment;  
         [0065]    [0065]FIG. 12 is a perspective view which shows a modification of a holding member in the fourth embodiment;  
         [0066]    [0066]FIG. 13( a ) is a perspective view which shows a holding member in the fifth embodiment of the invention;  
         [0067]    [0067]FIG. 13( b ) is a partial sectional view which shows installation of the holding member of FIG. 13( a );  
         [0068]    [0068]FIG. 14 is a perspective view which shows a modification of a holding member in the fifth embodiment;  
         [0069]    [0069]FIG. 15 is a perspective view which shows another modification of a holding member in the fifth embodiment;  
         [0070]    [0070]FIG. 16 is a partially vertical sectional view which shows an insulator-holding member according to the sixth embodiment of the invention;  
         [0071]    [0071]FIG. 17( a ) is a partially vertical sectional view which shows a modification of the structure of FIG. 16;  
         [0072]    [0072]FIG. 17( b ) is a perspective view which shows an insulator-holding member of FIG. 17( a );  
         [0073]    [0073]FIG. 18( a ) is a partially vertical sectional view which shows a mount structure of an insulation porcelain according to the seventh embodiment of the invention;  
         [0074]    [0074]FIG. 18( b ) is a perspective view which shows an annular support member for installation of the insulation porcelain in FIG. 18( a );  
         [0075]    [0075]FIG. 19( a ) is a partially vertical sectional view which shows a mount structure of an insulation porcelain according to the eighth embodiment of the invention;  
         [0076]    [0076]FIG. 19( b ) is a partially perspective view which shows the mount structure of FIG. 19( a );  
         [0077]    [0077]FIG. 20 is a perspective view which shows another holding member which may be used in the mount structure in FIGS.  19 ( a ) and  19 ( b );  
         [0078]    [0078]FIG. 21 is a partially vertical sectional view which shows holding members for installation of an insulation porcelain within an air cover according to the ninth embodiment of the invention;  
         [0079]    [0079]FIG. 22( a ) is a partially vertical sectional view which shows the tenth embodiment of the invention which is a modification of the first embodiment shown in FIGS.  1  to  3 ( c );  
         [0080]    [0080]FIG. 22( b ) is a partially vertical sectional view which shows the tenth embodiment of the invention which is a modification of the third embodiment shown in FIGS.  7 ( a ) to  7 ( c );  
         [0081]    [0081]FIG. 23( a ) is a partially vertical sectional view which shows a modification of the fourth embodiment as shown in FIGS.  9 ( a ) and  9 ( b );  
         [0082]    [0082]FIG. 23( b ) is a partially vertical sectional view which shows a modification of the fifth embodiment shown in FIGS.  13 ( a ) and  13 ( b );  
         [0083]    [0083]FIG. 24( a ) is a perspective view which shows a holding member which is a modification of the one shown in FIG. 22( b );  
         [0084]    [0084]FIG. 24( b ) is a partially vertical view which shows installation of an insulation porcelain within an air cover using the holding member of FIG. 24( a ); and  
         [0085]    [0085]FIG. 25 is a longitudinal sectional view which shows a conventional gas sensor. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0086]    Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIGS.  1  to  5 , there is shown a gas sensor  1  according to the first embodiment of the invention which is designed to be installed in an exhaust system of an automotive internal combustion engine to measure an oxygen content in exhaust gasses. Note that the present invention is not limited to an oxygen sensor and may alternatively be used with a variety of gas sensors such as HC, CO, and NOx sensors.  
         [0087]    The gas sensor  1  generally includes a sensing element  15 , a hollow cylindrical housing  10 , a measured gas cover assembly  13 , terminals  151  electrically leading to the sensing element  15 , an insulation porcelain  2 , and an air cover  121 . The sensing element  15  is retained in the housing  10  and surrounded at a head (i.e., a gas-sensing portion) thereof by the measured gas cover assembly  13 . The insulation porcelain  2  insulates the terminals  151  from each other. The air cover  121  is installed at an end thereof on a flange of the housing  10  and covers the insulation porcelain  2  and a base of the sensing element  15 .  
         [0088]    The insulation porcelain  2  is made of a hollow cylindrical member which, as clearly shown in FIG. 2, consists of a body  21  and a flange  22 . The air cover  121  has, as shown in FIG. 1, a small-diameter portion  127 , a large-diameter portion  128 , and a shoulder  129 . The small-diameter portion  127  has an inner diameter which is greater than an outer diameter of the body  21  of the insulation porcelain  2  and smaller than an outer diameter of the flange  22  of the insulation porcelain  2 . The large-diameter portion  128  has an inner diameter greater than the outer diameter of the flange  22 . The shoulder  129  is formed between the small-diameter portion  127  and the large-diameter portion  128 .  
         [0089]    The insulation porcelain  2  has, as shown in FIGS. 1 and 2, a groove  23  formed in a peripheral surface  220  of the flange  22 . Holding members  31 , as will be described later in detail, are disposed within the groove  23  of the insulation porcelain  2  in elastic contact with an inner wall of the air cover  121 . The holding members  31  are each made of a spring designed to expand and contract in a radius direction of the air cover  121 .  
         [0090]    The measured gas cover assembly  13  is, as described above, installed at an end thereof in an groove formed in the bottom of the housing  10 . The measured gas cover assembly  13  is made up of an inner cover  131  and an outer cover  132  both of which have gas inlets  130  through which the gas to be measured is admitted into a gas chamber defined around the sensing element  15 .  
         [0091]    The air cover  121  is welded to a base portion of the housing  10 . An outer air cover  14  is installed around the small-diameter portion  127  of the air cover  121  through a cylindrical water-repellent filter  140 . The air cover  121  and the outer air cover  14  have formed therein air vents  149  which lead to a reference gas chamber defined in the sensing element  15  for inducting the air into the reference gas chamber.  
         [0092]    An elastic insulator  16  is fitted within an open end portion of the air cover  121  which holds a pair of leads  153  and a pair of leads  163  (only one is shown in FIG. 1 for the brevity of illustration) in holes formed therein. The leads  163  are connected to a heater  16  through connectors  161  for supplying the power thereto. The leads  153  are connected to the sensing element  15  to provide sensor signals to an external device which are used in determining the concentration of oxygen contained in a gas. This technique is well known in the art, and explanation thereof in detail will be omitted here. For instance, U.S. application Ser. No. 09/196,693, filed on Nov. 20, 1998, assigned to the same assignee as that of this application teaches a gas measuring method in this type of gas sensor, and disclosure of which is incorporated herein by reference.  
         [0093]    The sensing element  15  is made of a cup-shaped solid electrolyte body  150  which defines therein the reference gas chamber as described above. The heater  16  is made of a bar-shaped resistance member and disposed within the reference gas chamber for heating measuring and reference electrodes formed on the sensing element  15  up to a temperature at which the oxygen concentration can be measured correctly. The measuring and reference electrodes are connected to the terminals  151 , respectively. The terminals  151  are connected to the leads  153  through connectors  152 , respectively.  
         [0094]    The insulation porcelain  2  which is made of a ceramic material (alumina) is, as described above, disposed within the air cover  121 . The insulation porcelain  2 , as clearly shown in FIG. 2, has formed therein four through holes  291  into which the leads  153  and  163  are inserted and a cavity  290  which communicates with the through holes  291  and opens downward, as viewed in the drawing. Within the cavity  290 , the base of the sensing element  15  is disposed.  
         [0095]    The insulation porcelain  2  has, as already described, the flange  22 . The annular groove  23  is formed in the peripheral wall of the flange  22 . The holding members  31  are installed in the grooves  23  to retain the insulation porcelain  2  within the air cover  121 , as will be described in detail below. The holding members  31  are each made of a heat-resisting metal such as stainless steel. Instead of the annular groove  23 , discrete recesses may be formed in the periphery of the flange  22 .  
         [0096]    Each of the holding members  31  is, as shown in FIG. 3( a ), implemented by an N-shaped spring plate which is so designed as to deformable elastically in a direction, as indicated by an arrow. The three holding members  31  are, as can be seen in FIG. 3( b ), disposed in the groove  23  at regular intervals. Each of the holding members  31  is fitted within the groove  23  in contact with upper and lower walls of the groove  23  in order to avoid undesirable play in a vertical direction, as viewed in the drawing (i.e., a width-wise direction of the groove  23 ). Specifically, the length L of the holding members  31 , as shown in FIG. 3( a ), is set substantially equal to the width of the groove  23 .  
         [0097]    The width W of the holding members  31  when subjected to no load is set greater than the interval between the bottom of the groove  23  and the inner wall  120  of the air cover  121 .  
         [0098]    The installation of the insulation porcelain  2  in the air cover  121  is accomplished by placing the holding members  31  in the groove  23 , as shown in FIGS.  3 ( b ) and  3 ( c ), compressing the holding members  31  inwardly of the insulation porcelain  2 , and inserting the insulation porcelain  2  into the air cover  121 . Upon insertion of the insulation porcelain  2 , spring pressures are produced by the holding members  31  which are oriented in the radius direction of the air cover  121  to retain the insulation porcelain  2  in the air cover  121  in elastic engagement with the inner wall  120  of the air cover  121 .  
         [0099]    The holding members  31  are, as described above, made of a spring and thus need not be formed with high dimensional accuracy. Accumulated dimensional errors of the groove  23 , the insulation porcelain  2 , and the air cover  121  in the radius direction of the gas sensor  1  are absorbed by the elasticity of the holding members  31 , thus eliminating the need for machining those parts with high dimensional accuracy and resulting in an increase in durability of the gas sensor  1 . The gas sensor  1  may thus be fabricated easily at low costs.  
         [0100]    Moreover, the installation of the insulation porcelain  2  within the single-walled air cover  121  allows the overall length of the gas sensor  1  to be decreased as compared with the conventional structure shown in FIG. 25.  
         [0101]    The sensing element  15  may be replaced with another type of element such as one indicated at  159  in FIG. 4 which is made of a lamination of a heater layer and electrode layers. U.S. Pat. No. 5,573,650, issued on Nov. 12, 1996 to Fukaya et al. teaches such a structure, disclosure of which is incorporated herein by reference.  
         [0102]    The holding members  31  may alternatively be, as shown in FIG. 5, made of a W-shaped spring strip which is so designed as to expand and contract in a direction as indicated by an arrow.  
         [0103]    [0103]FIG. 6( a ) shows the second embodiment of the invention in which a holding member  311  is used instead of the holding members  31  in the first embodiment.  
         [0104]    The holding member  311  is made of a single ring which is folded to have, like the first embodiment, an N-shape in cross section.  
         [0105]    A holding member  312 , as shown in FIG. 6( b ), may alternatively be used which consists of a ring  313  and three V-shaped spring plates  314 . The spring plates  314  are joined to the periphery of the ring  313  at regular intervals to possess the elasticity in a radius direction of the ring  313 .  
         [0106]    Other arrangements are identical with those of the first embodiment, and explanation thereof in detail will be omitted here.  
         [0107]    FIGS.  7 ( a ) to  7 ( c ) show the third embodiment of the invention.  
         [0108]    The insulation porcelain  2  does not have the groove  23  formed in the flange  22  in the first embodiment and is, as clearly shown in FIG. 7( c ), retained within the air cover  121  by three holding members  32 .  
         [0109]    Each of the holding members  32  is made of substantially a C-shaped spring strip which consists of a base  320 , an inner claw  321 , and an outer claw  322 . The inner and outer claws  321  and  322  are bent from ends of the base  320  in the same direction.  
         [0110]    The insulation porcelain  2  is, like the first embodiment, installed within the air cover  121 . The holding members  32  are, as shown in FIG. 7( b ), disposed on a lower surface  222  of the flange  22  at regular intervals and elastically fitted in a gap between the large-diameter portion  229  of the insulation porcelain  2  and the inner wall  120  of the air cover  121 . Specifically, the inner claw  321  of each of the holding members  32  extends from the base  320  downward and to the left, as viewed in FIG. 7( c ), and elastically engages the large-diameter portion  229  of the insulation porcelain  2 , while the outer claw  322  extends from the base  320  downward and to the right and elastically engages the inner wall  120  of the air cover  121 , thereby producing a horizontal spring pressure to hold the insulation porcelain  2  in the radius direction of the air cover  121  and a vertical spring pressure to urge the flange  22  into constant engagement of the upper surface  221  with the inner wall of the shoulder  129  of the air cover  121 , thereby holding the insulation porcelain  2  in a longitudinal direction of the air cover  121 .  
         [0111]    The installation of the insulation porcelain  2  is accomplished by inserting the insulation porcelain  2  into the air cover  121  and forcing the holding members  32  into the gap between the inner wall  120  of the air cover  121  and the large-diameter portion  229  of the insulation porcelain  2  to hold the flange  22  between the base  320  of each of the holding members  32  and the shoulder  129  of the air cover  121 .  
         [0112]    Like the first embodiment, accumulated dimensional errors of the insulation porcelain  2  and the air cover  121  in the radius direction of the gas sensor  1  are absorbed by the elastic deformation of the inner and outer claws  321  and  322  of the holding members  32  when fitted in the air cover  121 , thus eliminating the need for machining those parts with high dimensional accuracy, resulting in ease of fabrication of the gas sensor  1 .  
         [0113]    Instead of the holding members  32 , a one-piece holding member, as shown in FIG. 8, may be used which consists of a ring  323  and inner and outer claws  321  and  322 . The inner and outer claws  321  and  322  extend from the ring  323  diagonally in the same direction, like the ones shown in FIG. 7( a ).  
         [0114]    FIGS.  9 ( a ) and  9 ( b ) show the fourth embodiment of the invention which is different from the third embodiment of FIGS.  7 ( a ) and  7 ( b ) only in that three holding members  33  are fitted within a gap between the flange  22  and the inner wall  120  of the air cover  121  at regular intervals. Other arrangements are identical, and explanation thereof in detail will be omitted here.  
         [0115]    Each of the holding members  33  consists of a base  331  and a U-shaped elastic portion  332  projecting vertically from the base  331 . A total thickness of the elastic portion  332  is slightly greater than the interval between the peripheral surface  220  of the flange  22  and the inner wall  120  of the air cover  121  so as to produce the spring pressure in the radius direction of the insulation porcelain  2  when installed in the air cover  121 .  
         [0116]    The elastic portion  332  of each of the holding members  33  is press fit within the gap between the peripheral surface  220  of the flange  22  and the inner wall  120  of the air cover  121  to hold the insulation porcelain  2  in the radius direction of the air cover  121 , while the base  331  engages the lower surface  222  of the flange  22  to urge the flange  22  into constant engagement of the upper surface  221  with the inner wall of the shoulder  129  of the air cover  121 , thereby holding the insulation porcelain  2  in the longitudinal direction of the air cover  121 .  
         [0117]    In this embodiment, the elastic portion  332  of each of the holding members  33  has wider contact areas with the inner wall  120  of the air cover  121  and the peripheral surface  220  of the flange  22 , thereby producing a greater frictional pressure between the inner wall  120  of the air cover  121  and the peripheral surface  220  of the flange  22  to retain the insulation porcelain  2  firmly.  
         [0118]    The installation of the insulation porcelain  2  is accomplished in the following manner. First, each of the holding members  33  is joined to the flange  22  in engagement of the base  331  and the elastic portion  332  with the lower surface  222  and the peripheral surface  220  of the flange  22 , respectively. Next, the insulation porcelain  2  is forced into the air cover  121  until the flange  22  hits on the shoulder  129  of the air cover  121 .  
         [0119]    Like the above embodiments, a total dimensional error of the insulation porcelain  2  and the air cover  121  in the radius direction of the gas sensor  1  is absorbed by the elastic deformation of the elastic portions  332  of the holding members  32  when fitted in the air cover  121 , thus eliminating the need for machining those parts with high dimensional accuracy, resulting in ease of fabrication of the gas sensor  1 . Moreover, the installation of the insulation porcelain  2  within the single-walled air cover  121  allows the overall length of the gas sensor  1  to be decreased as compared with the conventional structure shown in FIG. 25.  
         [0120]    The elastic portion  332  of each of the holding members  33  may alternatively be machined, as shown in FIG. 10, to a V-shape.  
         [0121]    Instead of the holding members  33 , a one-piece holding member, as shown in FIG. 11 ( a ) or  11  ( b ), may be used. The holding member of FIG. 11 ( a ) consists of a ring-shaped disc  331  and a double-walled annular member  332 . the annular member  332  is, like the one shown in FIG. 9( a ), of a U-shape in cross section. The holding member of FIG. 11 ( b ) is different from the one shown in FIG. 11 ( b ) only in that the annular member  332  is of a V-shape in cross section.  
         [0122]    The bodies  331  of the holding members  33  in FIGS.  9 ( a ) and  9 ( b ) may alternatively be formed, as shown in FIG. 12, integrally with a ring-shaped disc  336 .  
         [0123]    FIGS.  13 ( a ) and  13 ( b ) show the fifth embodiment of the invention which is different from the fourth embodiment of FIGS.  9 ( a ) and  9 ( b ) in configuration of holding members. Other arrangements are identical, and explanation thereof in detail will be omitted here.  
         [0124]    Three holding members  34  are used to retain the insulation porcelain  2  within the air cover  121 . Each of the holding members  34  consists of a flat base  341  and a W-shaped elastic portion  342  extending vertically from the base  341 . The flat base  341  is, as clearly shown in FIG. 13( b ), held between the upper surface  221  of the flange  22  and the inner wall of the shoulder  129 . The elastic portion  342  is formed by waving a portion of the holding member  34  to be deformable elastically in the longitudinal direction of the insulation porcelain  2  (i.e., a vertical direction as viewed in FIGS.  13 ( a ) and  13 ( b )) and press fit within a gap between the peripheral surface  220  of the flange  22  and the inner wall  120  of the air cover  121  so as to produce the spring pressure in the radius direction of the insulation porcelain  2 , thereby holding the insulation porcelain  2  within the air cover  121  firmly.  
         [0125]    The installation of the insulation porcelain  2  is accomplished in the following manner. First, each of the holding members  34  is attached to the flange  22  of the insulation porcelain  2  in engagement of the base  341  and the elastic portion  342  with the upper surface  221  and the peripheral surface  220  of the flange  22 , respectively. Next, the insulation porcelain  2  is forced into the air cover  121  until the flange  22  hits on the shoulder  129  of the air cover  121 .  
         [0126]    Like the above embodiments, a total dimensional error of the insulation porcelain  2  and the air cover  121  in the radius direction of the gas sensor  1  is absorbed by the elastic deformation of the elastic portions  342  of the holding members  34  when fitted in the air cover  121 , thus eliminating the need for machining those parts with high dimensional accuracy, resulting in ease of fabrication of the gas sensor  1 . Moreover, the installation of the insulation porcelain  2  within the single-walled air cover  121  allows the overall length of the gas sensor  1  to be decreased as compared with the conventional structure shown in FIG. 25.  
         [0127]    The base  341  of each of the holding members  34  is, as described above, interposed between the shoulder  129  of the air cover  121  and the upper surface  221  of the flange  22  of the insulation porcelain  2 , thereby defining gaps therebetween which work as air passages establishing communication between the air vents  149  and the reference chamber within the sensing element  15 , thus facilitating ease of air flow into the reference chamber.  
         [0128]    Instead of the holding members  34 , a holding member  345 , as shown in FIG. 14, or a holding member  346 , as shown in FIG. 15, may be used. The holding member  345  is made of a one-piece cylindrical member which consists of a ring-shaped base  341  and an accordion-folded annular elastic portion  342 . The holding member  346  consists of a ring-shaped base  341  and three W-shaped spring plates  342  attached to the periphery of the base  341  at regular intervals.  
         [0129]    [0129]FIG. 16 shows the sixth embodiment of the invention.  
         [0130]    The insulation porcelain  2  has substantially the same structure as that in the first embodiment except that the groove  23  is not formed in the flange  22 .  
         [0131]    A holding member  351  which is made of a coil spring is disposed vertically within the air cover  121  to retain the insulation porcelain  2  firmly. Specifically, the holding member  351  is fitted at one end on the large-diameter portion  229  of the insulation porcelain  2  and attached at the other end to the upper end of the housing  10  to urge the flange  22  elastically into constant engagement with the inner wall of the shoulder  129  of the air cover  121 .  
         [0132]    The installation of the insulation porcelain  2  is accomplished by fitting the holding member  351  fixed on the upper end of the housing  10  on the large-diameter portion  229  of the insulation porcelain  2 , inserting these into the air cover  12   1 , and joining the housing  10  and the air cover  121  together. This facilitates ease of fabrication of the gas sensor  1 , thereby resulting in a decrease in manufacturing cost.  
         [0133]    The use of the single holding member  351  for installation of the insulation porcelain  2  results in an increase in durability of the gas sensor  1 . Moreover, the installation of the insulation porcelain  2  within the single-walled air cover  121  allows the overall length of the gas sensor  1  to be decreased as compared with the conventional structure shown in FIG. 25.  
         [0134]    Instead of the holding member  351 , a rigid holding member  352 , as shown in FIG. 17( b ), may be used. The holding member  352  is made of a metallic or ceramic hollow cylinder. The holding member  352  is, as clearly shown in FIG. 17( a ), fitted at one end on the large-diameter portion  229  of the insulation porcelain  2  in contact with the lower surface  222  of the flange  22  and placed at the other end on the upper end of the housing  10  to bring the flange  22  into constant engagement with the shoulder  129  of the air cover  121 .  
         [0135]    [0135]FIG. 18( a ) shows the seventh embodiment of the invention.  
         [0136]    A ring-shaped mount base  366  is welded at a portion, as indicated at  366 , to the inner wall of the air cover  121  within a gap between the large-diameter portion  229  and the inner wall  120  of the air cover  121 . A holding member  36  which is, as shown in FIG. 18( b ), made of an annular spring having an S-shape in cross section is disposed between the mount base  366  and the lower surface of the flange  22  of the insulation porcelain  2  to urge the flange  22  elastically into constant engagement with the inner wall of the shoulder  129  of the air cover  121 , thereby retaining the insulation porcelain  2  in the air cover  121  firmly. The holding member  36  also works to absorb vertical vibrations of the insulation porcelain  2  within the air cover  121 , thereby resulting in an increase in total durability of the gas sensor  1 .  
         [0137]    The installation of the insulation porcelain  2  within the air cover  121  is accomplished by placing the insulation porcelain  2  in contact of the flange  22  with the shoulder  129 , putting the holding member  36  on the lower surface  222  of the flange  22 , forcing the mount base  366  into the gap between the large-diameter portion  229  of the insulation porcelain  2  and the inner wall of the air cover  121  against the spring pressure of the holding member  36 , and welding the mount base  366  to a given portion of the large-diameter portion  128  of the air cover  121 , thereby retaining the insulation porcelain  2  within the air cover  121  firmly.  
         [0138]    Each of the holding member  36  and the mount base  366  may alternatively be made up of a plurality of elements.  
         [0139]    Other arrangements and effects of this embodiment are identical with those in the sixth embodiment, and explanation thereof in detail will be omitted here.  
         [0140]    FIGS.  19 ( a ) and  19 ( b ) show the eighth embodiment of the invention.  
         [0141]    Three holding members  37  (only one is shown for the brevity of illustration) are disposed at regular intervals between the upper surface  221  of the flange  22  of the insulation porcelain  2  and the inner wall of the shoulder  129  of the air cover  121 . Each of the holding members  37  is made of a corrugated plate and designed to be deformable elastically between the upper surface  221  of the flange  22  and the shoulder  129  vertically, as viewed in the drawings. The large-diameter portion  128  of the air cover  121  has formed on the inner wall thereof an annular ridge  375  projecting inwardly. The flange  22  of the insulation porcelain  2  is placed at a corner thereof on the annular ridge  375  against the spring force of the holding members  37 , thereby retaining the insulation porcelain  2  within the air cover  121  firmly. The holding members  37  also work to absorb vertical vibrations of the insulation porcelain  2  within the air cover  121 , thereby resulting in an increase in total durability of the gas sensor  1 .  
         [0142]    Instead of the annular ridge  375 , a plurality of discrete ridges may be formed on the inner wall of the large-diameter portion  128  of the air cover  121 .  
         [0143]    The installation of the insulation porcelain  2  within the air cover  121  is accomplished by placing the holding members  37  on the inner wall of the shoulder  129 , inserting the insulation porcelain  2  into the air cover  121  against the spring pressure of the holding members  37 , and pressing the large-diameter portion  128  of the air cover  121  inwardly to form the annular ridge  375 , thereby holding the flange  22  elastically against the spring pressure of the holding members  37 .  
         [0144]    Other arrangements and effects of this embodiment are identical with those in the sixth embodiment, and explanation thereof in detail will be omitted here.  
         [0145]    Instead of the holding members  37 , an annular holding member  375 , as shown in FIG. 20, may be used which is made by pressing an annular plate to a W-shape in cross section.  
         [0146]    [0146]FIG. 21 shows the ninth embodiment of the invention.  
         [0147]    A holding member  38  (only one is shown for the brevity of illustration) which is made of a wedge-shaped ring is press fit within a gap between the inner wall  120  of the air cover  121  and the peripheral surface  220  of the flange  22  so as to bring the flange  22  into constant engagement with the inner wall of the shoulder  129  of the air cover  121  and welded, as indicated at  381 , to the inner wall  120 .  
         [0148]    The holding member  38  is in contact with the inner wall  120  of the air cover  121  and the peripheral surface  220  of the flange  22 , thereby producing a frictional pressure therebetween to retain the insulation porcelain  2  firmly.  
         [0149]    The holding member  38  may alternatively be formed by a plurality of discrete wedges.  
         [0150]    Other arrangements and effects of this embodiment are identical with those in the sixth embodiment, and explanation thereof in detail will be omitted here.  
         [0151]    [0151]FIG. 22( a ) shows the tenth embodiment of the invention which is a modification of the first embodiment shown in FIGS.  1  to  3 ( c ).  
         [0152]    The holding member  31  has an extension  318  continuing straight from the outer wall thereof. The extension  318  is welded at a portion, as indicated at  318 , to the inner wall of the air cover  121 . Other arrangements are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0153]    [0153]FIG. 22( b ) shows a modification of the third embodiment shown in FIGS.  7 ( a ) to  7 ( c ).  
         [0154]    The holding member  32  has an extension  329  continuing from the outer claw  322 . The holding member  32  is forced into the gap between the insulation porcelain  2  and the inner wall of the air cover  121  in surface contact of the extension  329  with the inner wall of the air cover  121  and welded at a portion of the extension  329 , as indicated at  328 . Other arrangements are identical with those in the third embodiment, and explanation thereof in detail will be omitted here.  
         [0155]    [0155]FIG. 23( a ) shows a modification of the fourth embodiment as shown in FIGS.  9 ( a ) and  9 ( b ).  
         [0156]    The holding member  33  has an extension  339  continuing straight from the outer wall thereof. The extension  339  is welded at a portion, as indicated at  338 , to the inner wall of the air cover  121 . Other arrangements are identical with those in the fourth embodiment, and explanation thereof in detail will be omitted here.  
         [0157]    [0157]FIG. 23( b ) shows a modification of the fifth embodiment shown in FIGS.  13  ( a ) and  13 ( b ).  
         [0158]    The holding member  34  has an extension  349  continuing from the W-shaped elastic portion  342 . The extension  349  is placed in surface contact with the inner wall of the air cover  121  and welded at a portion, as indicated at  348 . Other arrangements are identical with those in the fifth embodiment, and explanation thereof in detail will be omitted here.  
         [0159]    FIGS.  24 ( a ) and  24 ( b ) show the eleventh embodiment of the invention which is a modification of the one shown in FIG. 22( b ).  
         [0160]    Three holding members  39  (only one is shown for the sake of simplicity of illustration) are attached to the inner wall  120  of the air cover  121  to retain the insulation porcelain  2  firmly within the air cover  121 .  
         [0161]    Each of the holding members  39  is made of a spring plate consisting of a base  390 , a mount portion  399 , and a connecting portion  391  connecting between the base  390  and the mount portion  399 . The mount portion  399  is attached in surface contact with the inner wall  120  of the air cover  121  and welded thereto to elastically urge the base  390  into constant engagement with the lower surface  222  of the flange  22 , thereby retaining the insulation porcelain  2  firmly within the air cover  121 .  
         [0162]    Instead of the holding members  39 , a one-piece annular spring having the same sectional shape as that of the holding members  39  may be used. This spring may be welded to the whole of a circumferential portion or discrete portions of the inner wall  120  of the air cover  121 .  
         [0163]    Other arrangements an effects of this embodiment are identical with those in the embodiment in FIG. 22( b ), and explanation thereof in detail will be omitted here.  
         [0164]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments witch can be embodied without departing from the principle of the invention as set forth in the appended claims.