Patent Publication Number: US-2004045822-A1

Title: Gas sensor having increased number of lead wire through holes

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Technical Field of the Invention  
       [0002] The present invention relates generally to a gas sensor which may be employed in burning control of automotive engines, and more particularly to a such gas sensor designed to have an increased number of lead wire through holes without increasing the diameter of the gas sensor.  
       [0003] 2. Background Art  
       [0004]FIG. 5 shows a conventional gas sensor installed in an exhaust pipe of an automotive internal combustion engine for use in air-fuel ratio control of the engine.  
       [0005] The gas sensor  9  consists of a sensor element  920  disposed within a cylindrical housing  91  through an insulation porcelain  92 , an air cover  911  installed on a base end of the housing  91 , lead wires  923  extending from outside to inside the air cover  911  for applying the voltage to and picking up an output from the sensor element  920 , and an elastic insulator  93  fitted within a base end of the air cover  911 .  
       [0006] The elastic insulator  923  has formed therein lead through holes  930  through which the lead wires  923  pass, respectively.  
       [0007] The gas sensor  9  is assembled by inserting the lead wires  923  through the holes  930  and then crimping the air cover  911  from outside it to clamp the lead wires  923  firmly in the holes  930  and secure the elastic insulator  93  within the air cover  911 .  
       [0008] The air cover  911  is made up of a first cover  913  joined to the base end of the housing  91  and an outer cover  914  surrounding the base end of the first cover  913  through a water-repellent filter  915 .  
       [0009] The crimping of the air cover  913  is achieved at an area of contact between the first cover  913  and the outer cover  914  which is closer to the end of the air cover  913  than the water-repellent filter  915 . The holes  930  are, as clearly shown in FIG. 6, arranged at corners of a square defined about the center of the elastic insulator  93 .  
       [0010] In recent years, use of sensor elements requiring many lead wires for electrical connections to an external device such as the type which has a heater built therein and a grounding wire and the type which has a plurality of electrochemical cells has increased.  
       [0011] Increasing the holes in the elastic insulator without decreasing the distance between adjacent two of the holes and the distance between an outer periphery of the elastic insulator and each of the holes, that is, reducing the rigidity of the elastic insulator requires increasing the diameter of the elastic insulator. The increasing of the diameter of the elastic insulator results in an increased size of the gas sensor.  
       [0012] In a case where the gas sensor is used in burning control of automotive internal combustion engines, the gas sensor is usually installed in a very small space within an exhaust pipe. Large-diameter gas sensors are, therefore, unsuitable for such installation.  
       [0013] Recently, automotive systems in which a plurality of gas sensors are installed in the exhaust pipe of the engine is being used. In this case, small-diameter gas sensors are very useful in terms of the efficiency of installation thereof.  
       [0014] Reduction in diameter of the gas sensor may be achieved by decreasing the distance between adjacent two of the holes in the elastic insulator and the distance between the outer periphery of the elastic insulator and each of the holes, that is decreasing the wall thickness of the elastic insulator. However, crimping such an elastic insulator through which the lead wires are disposed from outside the air cover causes compressive stress arising from the crimping to concentrate on a thin-walled portion of the elastic insulator (e.g., a portion between each of the holes and the outer periphery of the elastic insulator). This results in great deformation of the thin-walled portion of the elastic insulator. Thus, when such an elastic insulator is exposed to a high-temperature atmosphere, it will result in increased permanent set, thus leading to reduction in ability of sealing between the holes and the lead wires. This may cause moisture to intrude into the air cover easily through a clearance between the lead wires and the holes if water splashes the gas sensor. The intrusion of moisture may result in cracks in the sensor element, thus leasing to failure or reduction in performance of the gas sensor.  
       [0015] Additionally, the decrease in wall thickness of the elastic insulator may also result in decreases in strength and elasticity.  
       SUMMARY OF THE INVENTION  
       [0016] It is therefore a principal object of the invention to avoid the disadvantages of the prior art.  
       [0017] It is another object of the invention to provide an improved structure of a gas sensor designed to have a large number of lead wire through holes without increasing the diameter or size of the gas sensor.  
       [0018] According to one aspect of the invention, there is provided a gas sensor which comprises: (a) a housing; (b) a sensor element disposed within the housing; (c) a cover having a first and a second end, the cover extending at the first end from an end portion of the housing; (d) at least four lead wires extending from outside to inside the cover to establish electrical connections with the sensor element; (e) an elastic member retained within the second end of the cover. The elastic member has formed therein at least four holes through which the lead wires pass. One of the holes has a center substantially coinciding with that of the elastic member on a plane extending perpendicular to a length of the gas sensor.  
       [0019] In the preferred mode of the invention, the second end of the cover has a portion crimped to retain the elastic member within the cover. The elastic member experiences a 10% to 20% reduction in outer diameter due to crimping of the second end of the cover.  
       [0020] A minimum distance between adjacent two of the holes on the plane extending perpendicular to the length of the gas sensor is 1 mm or more. A minimum distance between an outer periphery of the elastic member and one of the holes closest to the periphery of the elastic member is 1 mm or more.  
       [0021] The holes may have formed on an inner wall thereof a rib which projects in a radius direction of the holes to establish elastic abutment to the lead wires and is opposed to a portion of the second end of the cover crimped to retain the elastic member within the cover in the radius direction of the holes.  
       [0022] The elastic member is made of an insulating material.  
       [0023] The holes other than the one formed in the center of the elastic member are arranged at regular intervals in a circle defined about the center of the elastic member. 
     
    
    
     BRIEF DESPCRIPTION OF THE DRAWINGS  
     [0024] 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.  
     [0025] In the drawings:  
     [0026]FIG. 1 is a longitudinal sectional view of a gas sensor according to the invention;  
     [0027]FIG. 2 is a transverse sectional view, as taken along the line A-A in FIG. 1, which shows an internal structure of an elastic insulator installed in an end of the gas sensor of FIG. 1;  
     [0028] FIGS.  3 ( a ),  3 ( b ),  3 ( c ), and  3 ( d ) show modified forms of an elastic insulator;  
     [0029]FIG. 4( a ) is a partially vertical sectional view which shows a modified form of an elastic insulator having two annular ribs formed on an inner wall of a hole;  
     [0030]FIG. 4( b ) is a partially vertical sectional view which shows a hole of the elastic insulator of FIG. 4( a ) after a lead wire is inserted into the hole, and the elastic insulator is crimped through an air cover;  
     [0031]FIG. 4( c ) is a partially vertical sectional view which shows a modified form of an elastic insulator having three saw-edged annular ribs formed on an inner wall of a hole;  
     [0032]FIG. 4( d ) is a partially vertical sectional view which shows a hole of the elastic insulator of FIG. 4( c ) after a lead wire is inserted into the hole, and the elastic insulator is crimped through an air cover;  
     [0033]FIG. 5 is a longitudinal sectional view of a conventional gas sensor; and  
     [0034]FIG. 6 is a transverse sectional view, as taken along the line B-B in FIG. 5, which shows an internal structure of an elastic insulator installed in an end of the gas sensor of FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0035] Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIG. 1, there is shown a gas sensor  1  according to the invention which may be employed in a burning control system for automotive vehicles to measure concentrations of components such as NOx, CO, HC, O 2  contained in exhaust gasses of the engine.  
     [0036] The gas sensor  1  includes a sensor element  29 , an air cover assembly  11 , lead wires  40 , and a cylindrical elastic insulator  4 . The sensor element  29  is disposed within the housing  10  through an insulation porcelain  2 . The air cover assembly  11  is made up of a first cover  111  and a second cover  112 . The first cover  111  has an upper small-diameter portion, as viewed in the drawing, and an open end thereof stacked or crimped to the housing  10 . The second cover  112  is installed on the periphery of the small-diameter portion of the first cover  1   11  and crimped to retain a water-repellent filter  113  around the small-diameter portion of the first cover  111 . The lead wires  40  extend from outside to inside the air cover assembly  11  through the elastic insulator  4  installed within an end of the first cover  111 . The lead wires  40  lead to an external device such as a controller and are used in applying the voltage to and picking up a sensor output from the sensor element  29  and supplying the power to a heater provided on the sensor element  29 .  
     [0037] The elastic insulator  4  is made of, for example, fluororubber, silicone rubber, or acrylic rubber which is suitable for use in high temperature atmosphere such as the inside of an exhaust pipe of automotive engines and, as clearly shown in FIG. 2, has formed therein eight holes  41  and  42  through which the eight lead wires  40  pass respectively. The hole  41  is located so that the center thereof coincides with the center of the elastic insulator  4  on a plane extending perpendicular to the length (i.e., a longitudinal center line) of the gas sensor  1  (or the sensor element  29 ). In other words, the hole  41 , the elastic insulator  4 , and the sensor element  29  are aligned in longitudinal center lines thereof with each other.  
     [0038] The gas sensor  1  is designed to be installed in an exhaust pipe of an automotive engine to measure the concentration of O 2  and NOx to determine the air-fuel ratio of a mixture within a combustion chamber of the engine.  
     [0039] The sensor element  29  is made of a typical laminated ceramic plate which has a monitor cell working to monitor the concentration of oxygen within a gas chamber defined in the laminated ceramic plate, an oxygen pump cell working to regulate the concentration of oxygen within the gas chamber, and a sensor cell working to measure the concentration of NOx within the gas chamber. The ceramic plate also includes a heater which heats the ceramic plate up to a temperature required to be sensitive to gases to be measured correctly. Gas sensors of this type are well known in the art, and structure and operation thereof in detail will be omitted here.  
     [0040] The electric power and voltage are inputted to the heater and each cell through electrode terminals (not shown) affixed to the surface of the sensor element  29 . Additionally, outputs of each cell is picked up by the controller through the electrode terminals.  
     [0041] The gas sensor  1  has, as described above, the three cells and the one heater and thus needs the eight lead wires  40  in total for supplying the power to the heater, applying the voltage, and transmitting outputs of the cells to the external controller.  
     [0042]FIG. 1 is a longitudinal sectional view of the gas sensor  1  and does not show all of the lead wires  41  for the brevity of illustration.  
     [0043] The gas sensor  1 , as shown in FIG. 1, also includes a hollow cylindrical heat-resistant metallic housing  10 , a double-walled protective cover assembly  109  made up of an outer and an inner cover, and the air cover assembly  11 . The protective cover assembly  109  is installed on a head of the housing  10  to define a gas chamber into which gases to be measured are admitted through gas holes formed in the outer and inner covers. The air cover assembly  11  is, as described above, made up of the first cover  111  and the second cover  112 .  
     [0044] The ceramic-made insulation porcelain  2  is retained within the housing  10 . The insulation porcelain  2  consists of a large-diameter portion, a small-diameter portion, and a tapered shoulder therebetween. The housing  10  has an inner shoulder tapering off to the cover assembly  109 . The shoulder of the insulation porcelain  2  is placed on the inner shoulder of the housing  10  through a metallic packing ring in an air-tight fashion.  
     [0045] A disc spring  21  is mounted on an upper end, as viewed in FIG. 1, of the insulation porcelain  2 . A press assembly  22  is fitted over the upper end of the insulation porcelain  2  through the disc spring  21 . The press assembly  22  is made up of a press plate  221  and an annular leg  222  extending vertically from the periphery of the press plate  221 . The leg  222  is, for example, press fit over the periphery of the insulation porcelain  2  and retains the press plate  221  tightly so as to press the disc spring  21  elastically to apply an elastic pressure to the insulation porcelain  2 , so that the insulation porcelain  2  is installed within the housing  10  in the air-tight fashion.  
     [0046] An insulation porcelain  3  is disposed above the insulation porcelain  2  within the air cover assembly  11 . The insulation porcelain  3  has retained therein terminal springs (not shown) which establish electrical contact with the electrode terminals of the sensor element  29 . The terminal springs are connected electrically to the lead wires  40  through connectors  409 , respectively.  
     [0047] The elastic insulator  4  is, as described above, fitted on the open end of the first cover  112  far from the housing  10  and has the holes  42  arranged around the center hole  41 .  
     [0048] The hole  41 , as described above, has the longitudinal center line coinciding with that of the elastic insulator  4 . “O” in FIG. 2 indicates the longitudinal center lines of the hole  41  and the elastic insulator  4 . The other holes  42  are so arrayed that the centers R 1  (i.e., longitudinal center lines) thereof lie on a circle R whose center is defined on the longitudinal center line O. The holes  42  are arranged preferably at regular intervals, so that intervals between the adjacent holes  42  and intervals between the outer peripheral wall of the elastic insulator  4  and the holes  42  will be equal to each other, thereby minimizing the degree of concentration of stress arising from the crimping of the air cover assembly  11 .  
     [0049] The formation of the hole  41  to have the center coinciding with the center of the elastic insulator  4  on a plane extending perpendicular to the length of the gas sensor  1  permits a sectional area of the elastic insulator  4  extending perpendicular to the length of the elastic insulator  4  to be minimized without sacrificing required mechanical strength, elasticity, etc. Specifically, as compared with the conventional structure, as illustrated in FIG. 6, in which no hole is formed in the center of the elastic insulator  93 , the structure of this embodiment allows intervals between adjacent two of the holes  42  surrounding the center hole  41  and between the outer periphery of the elastic insulator  4  and each of the holes  42  to be increased or the elastic insulator  4  to be decreased in diameter without changing the above intervals, that is, without reducing the mechanical properties of the insulator  4  as long as the holes  930  in the conventional structure and the holes  41  and  42  in this embodiment are identical in number.  
     [0050] The elastic insulator  4  may alternatively be made of a polygonal member. In this case, the center hole  41  is preferably formed in the center of gravity of the insulator  4 .  
     [0051] An allowable shift between the center of the hole  41  and the center of the elastic insulator is within a range of 20% of the diameter of the hole  41 .  
     [0052] If a minimum wall thickness or distance between the holes  42  and an outermost surface  408  of the elastic insulator  4  is defined as t1, a minimum distance between adjacent two of the holes  42  is defined as t2, and the distance between the center hole  41  and the holes  42  is defined as t3, they are all more than or equal to 1 mm for ensuring resistance to deformation of the elastic insulator  4  and required degree of liquid-tightness between the holes  41  and  42  and the lead wires  40 . A preferable upper limit of the distances t1, t2, and t3 is 3 mm in terms of compactness of the gas sensor  1 . For instance, t1=1.3 mm, t2=1.8 mm, and t3=2.35 mm.  
     [0053] The air cover assembly  11  is, as described above, crimped at a portion, as indicated at numeral  49  in FIG. 1, in a radius direction thereof to secure the elastic insulator  4  therewithin. This crimping preferably results in 10% to 20% reduction in outer diameter of the elastic insulator  4 . If the reduction is less than 10%, it may result in a lack of compressive pressure acting on the elastic insulator  4 , thus leading to a lack of liquid-tightness between the lead wires  40  and the holes  41  and  42 . Alternatively, if the reduction is more than 20%, it may result in excess of compressive pressure acting on the elastic insulator  4 , thus leading to cracks in the elastic insulator  4 . For instance, the outer diameter of the elastic insulator  4  before and after installed within the air cover assembly  11  is 12.8 mm and 10.8 mm, respectively.  
     [0054] FIGS.  3 ( a ),  3 ( b ),  3 ( c ), and  3 ( d ) show modifications of the elastic insulator  4 .  
     [0055]FIG. 3( a ) illustrates for a case where the gas sensor  1  uses seven lead wires  40 . The elastic insulator  4 , thus, has formed therein six holes  42  around the center hole  41 .  
     [0056]FIG. 3( b ) illustrates for a case where the gas sensor  1  uses six lead wires  40 . The elastic insulator  4 , thus, has formed therein five holes  42  around the center hole  41 .  
     [0057]FIG. 3( c ) illustrates for a case where the gas sensor  1  uses five lead wires  40 . The elastic insulator  4 , thus, has formed therein four holes  42  around the center hole  41 .  
     [0058]FIG. 3( d ) illustrates for a case where the gas sensor  1  uses four lead wires  40 . The elastic insulator  4 , thus, has formed therein three holes  42  around the center hole  41 .  
     [0059] Symbols t1, t2, and t3 in FIGS.  3 ( a ) to  3 ( d ) indicate the same dimensions as used in FIG. 2. Other arrangements and effects in each of the modifications are the same as those in the first embodiment, and explanation thereof in detail will be omitted here.  
     [0060] FIGS.  4 ( a ),  4 ( b ),  4 ( c ), and  4 ( d ) show modifications of the elastic insulator  4  which have inner ribs.  
     [0061] In a case, as illustrated in FIG. 4( a ), the hole  41  has formed on an inner wall thereof two annular ribs  411  which project in a radius direction thereof and are laid in the lengthwise direction of the hole  41 . The ribs  411  are preferably opposed to the crimped portion  49  of the air cover assembly  11  in the radius direction of the gas sensor  1 . In this case, the crimping of the portion  49  results in or enhances, as shown in FIG. 4( b ), deformation of the ribs  411  to achieve elastic abutment to the lead wire  40 , thereby providing an liquid- and air-tight seal therebetween.  
     [0062] In a case, as illustrated in FIG. 4( c ), the holes  41  has formed on the inner wall thereof three saw-edged annular ribs  412  which are laid in the lengthwise direction of the hole  41 . The ribs  412  are preferably opposed to the crimped portion  49  of the air cover assembly  11  in the radius direction of the gas sensor  1 . In this case, the crimping of the portion  49  results in or enhances, as shown in FIG. 4( d ), deformation of the ribs  412  to achieve elastic abutment to the lead wire  40 , thereby providing a liquid- and air-tight seal therebetween.  
     [0063] The ribs  411  or  412  may also be formed in each of the holes  42 .  
     [0064] In a case where the inner diameter of the ribs  411  or  412  is small enough to clamp the lead wire  40  firmly, they may alternatively be shifted from the crimped portion  49  of the air cover assembly  11  vertically.  
     [0065] The ribs  411  or  412 , as described above, may be formed at a location where the pressure produced by the crimping of the air cover assembly  11  acts on the ribs  411  or  412 . This enhances the hermetic sealing between the ribs  411  or  412  and the lead wire  40  and also reduces the need for concern about dimensional accuracy of the lead wires  40  and the holes  41 . Specifically, it is possible to form a certain degree of clearance between the lead wire  40  and the hole  41 , thus facilitating ease of installation of the lead wires  30  within the elastic insulator  4 .  
     [0066] 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.