Abstract:
An improved manufacturing method for a gas sensor is provided which is capable of establishing a required hermetic seal in a body of the gas sensor. The method includes preparing a sensor assembly including a housing, an air cover, an insulation porcelain, and a sensor element, pressing the air cover against the housing to fit an end of the air cover on an end of the housing to form an overlap thereof, and welding the air cover to the housing over the overlap. The welding is accomplished while pressing the air cover against the housing, thereby compressing an elastic member in the air cover to establish a hermetic seal between the sensor element and the housing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field of the Invention  
         [0002]     The present invention relates generally to a manufacturing method of a gas sensor which may be used in air-fuel ratio control of automotive internal combustion engines and a manufacturing machine therefore, and more particularly to a manufacturing method and machine for such a gas sensor which is capable of establishing a hermetic seal in the gas sensor reliably.  
         [0003]     2. Background Art  
         [0004]     Japanese Patent Publication No. 2847418 discloses a typical gas sensor used to measure an oxygen content of exhaust gasses of an automotive internal combustion engine. In this sensor, a cup or cover is joined to a sensor holder by staking, therefore, a sufficient degree of sealing between a sensor element and the atmosphere, i.e., the outside of the cover is not obtained.  
       SUMMARY OF THE INVENTION  
       [0005]     It is therefore an object of the invention to provide an improved manufacturing system for a gas sensor which is capable of establishing a required hermetic seal in the gas sensor.  
         [0006]     According to the first aspect of the invention, there is provided a gas sensor manufacturing method which comprise the steps of: (1) preparing an assembly which has a length and includes (a) a housing which has a length made up of a first end portion, a second end portion, and a flange between the first and second end portions, (b) a cover which is made up of a small-diameter portion, a large-diameter portion, and a shoulder formed between the small-diameter portion and the large-diameter portion, (c) a first insulation porcelain disposed in the large-diameter portion of the cover in contact with the shoulder through an elastic member, (d) a second insulation porcelain disposed in the housing in contact of an end thereof with an end of the first insulation porcelain in alignment with each other, and (e) a laminated sensor element disposed in the second insulation porcelain; (2) pressing the cover against the housing in a lengthwise direction of the assembly to fit an end of the large-diameter portion of the cover on the first end portion of the housing to form an overlap; (3) tacking the large-diameter portion of the cover to the first end portion of the housing at the overlap while pressing the cover against the housing; (4) rotating the assembly about the length thereof; and (5) welding the large-diameter portion of the cover to the first end portion of the housing over the overlap.  
         [0007]     In the preferred mode of the invention, the pressing steps presses the cover against the housing while compressing the elastic member to urge the second insulation porcelain against an inner wall of the housing elastically to establish a hermetic seal between an outer wall of the second insulation porcelain and the inner wall of the housing.  
         [0008]     The welding step is performed while pressing the cover against the housing.  
         [0009]     The pressure exerted on the cover may alternatively be released after the tacking step.  
         [0010]     The welding step is performed by laser welding.  
         [0011]     The tacking step makes at least two tack welds in the overlap of the cover and the housing.  
         [0012]     According to the second aspect of the invention, there is provided a gas sensor manufacturing method which comprises the steps of: (1) preparing an assembly which has a length and includes (a) a housing which has a length made up of a first end portion, a second end portion, and a flange between the first and second end portions, (b) a cover which is made up of a small-diameter portion, a large-diameter portion, and a shoulder formed between the small-diameter portion and the large-diameter portion, and (c) an insulation porcelain disposed in the large-diameter portion of the cover in contact with the shoulder through an elastic member, (d) a cup-shaped sensor element disposed in the housing; (2) pressing the cover against the housing in a lengthwise direction of the assembly to fit an end of the large-diameter portion of the cover on the first end portion of the housing to form an overlap; (3) tacking the large-diameter portion of the cover to the first end portion of the housing at the overlap while pressing the cover against the housing; (4) rotating the assembly about the length thereof; and (5) welding the large-diameter portion of the cover to the first end portion of the housing over the overlap.  
         [0013]     In the preferred mode of the invention, the pressing steps presses the cover against the housing while compressing the elastic member to urge the sensor element against an inner wall of the housing elastically to establish a hermetic seal between an outer wall of the sensor element and the inner wall of the housing.  
         [0014]     The welding step is performed while pressing the cover against the housing.  
         [0015]     The pressure exerted on the cover may alternatively be released after the tacking step.  
         [0016]     The welding step is performed by laser welding.  
         [0017]     The tacking step makes at least two tack welds in the overlap of the cover and the housing.  
         [0018]     According to the third aspect of the invention, there is provided a gas sensor manufacturing method which comprises the steps of: (1) preparing an assembly which has a length and includes (a) a housing which has a length made up of a first end portion, a second end portion, and a flange between the first and second end portions, (b) a cover which is made up of a small-diameter portion, a large-diameter portion, and a shoulder formed between the small-diameter portion and the large-diameter portion, (c) a first insulation porcelain disposed in the large-diameter portion of the cover in contact with the shoulder through an elastic member, (d) a second insulation porcelain disposed in the housing in contact of an end thereof with an end of the first insulation porcelain in alignment with each other, and (e) a laminated sensor element disposed in the second insulation porcelain; (2) pressing the cover against the housing in a lengthwise direction of the assembly to fit an end of the large-diameter portion of the cover on the first end portion of the housing to form an overlap; and (3) welding the large-diameter portion of the cover to the first end portion of the housing at the overlap while pressing the cover against the housing.  
         [0019]     In the preferred mode of the invention, the pressing steps presses the cover against the housing while compressing the elastic member to urge the second insulation porcelain against an inner wall of the housing elastically to establish a hermetic seal between an outer wall of the second insulation porcelain and the inner wall of the housing.  
         [0020]     The welding step is performed while rotating the cover and the housing.  
         [0021]     The welding step may alternatively be performed while fixing the cover and the housing.  
         [0022]     The welding step welds the large-diameter portion of the cover to the first end portion of the housing around an overall periphery of the overlap through laser welding.  
         [0023]     According to the fourth aspect of the invention, there is provided a gas sensor manufacturing method which comprises the steps of: (1) preparing an assembly which has a length and includes (a) a housing which has a length made up of a first end portion, a second end portion, and a flange between the first and second end portions, (b) a cover which is made up of a small-diameter portion, a large-diameter portion, and a shoulder formed between the small-diameter portion and the large-diameter portion, and (c) an insulation porcelain disposed in the large-diameter portion of the cover in contact with the shoulder through an elastic member, (d) a cup-shaped sensor element disposed in the housing; (2) pressing the cover against the housing in a lengthwise direction of the assembly to fit an end of the large-diameter portion of the cover on the first end portion of the housing to form an overlap; and (3) welding the large-diameter portion of the cover to the first end portion of the housing at the overlap while pressing the cover against the housing.  
         [0024]     In the preferred mode of the invention, the welding step is performed while rotating the cover and the housing.  
         [0025]     The welding step may alternatively be performed while fixing the cover and the housing.  
         [0026]     The welding step welds the large-diameter portion of the cover to the first end portion of the housing around an overall periphery of the overlap through laser welding.  
         [0027]     According to the fifth aspect of the invention, there is provided a gas sensor manufacturing method which comprise the steps of: (1) preparing an assembly which has a length and includes (a) a housing which has a length made up of a first end portion, a second end portion, and a flange between the first and second end portions, (b) a cover which is made up of a small-diameter portion, a large-diameter portion, and a shoulder formed between the small-diameter portion and the large-diameter portion, (c) a first insulation porcelain disposed in the large-diameter portion of the cover in contact with the shoulder through an elastic member, (d) a second insulation porcelain disposed in the housing in contact of an end thereof with an end of the first insulation porcelain in alignment with each other, and (e) a laminated sensor element disposed in the second insulation porcelain; (2) pressing the cover against the housing in a lengthwise direction of the assembly until a pressure exerted on the housing reaches a given pressure level to fit an end of the large-diameter portion of the cover on the first end portion of the housing to form an overlap; and (3) welding the large-diameter portion of the cover to the first end portion of the housing at the overlap while pressing the cover against the housing.  
         [0028]     In the preferred mode of the invention, the pressing steps presses the cover against the housing while compressing the elastic member to urge the second insulation porcelain against an inner wall of the housing elastically to establish a hermetic seal between an outer wall of the second insulation porcelain and the inner wall of the housing.  
         [0029]     The given pressure level is 1.2 times greater than or equal to an elastic pressure produced by the elastic member.  
         [0030]     According to the sixth aspect of the invention, there is provided a manufacturing method which comprise the steps of: (1) preparing an assembly which has a length and includes (a) a housing which has a length made up of a first end portion, a second end portion, and a flange between the first and second end portions, (b) a cover which is made up of a small-diameter portion, a large-diameter portion, and a shoulder formed between the small-diameter portion and the large-diameter portion, and (c) an insulation porcelain disposed in the large-diameter portion of the cover in contact with the shoulder through an elastic member, (d) a cup-shaped sensor element disposed in the housing; (2) pressing the cover against the housing in a lengthwise direction of the assembly until a pressure exerted on the housing reaches a given pressure level to fit an end of the large-diameter portion of the cover on the first end portion of the housing to form an overlap; and (3) welding the large-diameter portion of the cover to the first end portion of the housing at the overlap while pressing the cover against the housing.  
         [0031]     In the preferred mode of the invention, the pressing steps presses the cover against the housing while compressing the elastic member to urge the sensor element against an inner wall of the housing elastically to establish a hermetic seal between an outer wall of the sensor element and the inner wall of the housing.  
         [0032]     The given pressure level is 1.2 times greater than or equal to an elastic pressure produced by the elastic member.  
         [0033]     According to the seventh aspect of the invention, there is provided a gas sensor manufacturing machine designed to produce a gas sensor using an assembly which has a length and includes (a) a housing which has a length made up of a first end portion, a second end portion, and a flange between the first and second end portions, (b) a cover which is made up of a small-diameter portion, a large-diameter portion, and a shoulder formed between the small-diameter portion and the large-diameter portion, (c) a first insulation porcelain disposed in the large-diameter portion of the cover in contact with the shoulder through an elastic member, (d) a second insulation porcelain disposed in the housing in contact of an end thereof with an end of the first insulation porcelain in alignment with each other, and (e) a laminated sensor element disposed in the second insulation porcelain. The machine comprises: (1) a first annular press plate fitted on the first end portion of the housing in engagement with the flange; (2) a second annular press plate fitted on the small-diameter portion of the cover in engagement with the shoulder of the cover; and (3) a pressing means for pressing the cover against the housing in a lengthwise direction of the assembly to fit an end of the large-diameter portion of the cover on the first end portion of the housing to form an overlap; (4) a tacking means for tacking the large-diameter portion of the cover to the first end portion of the housing at the overlap while pressing the cover against the housing; (5) a rotating means for rotating the assembly; and (6) a welding means for welding the large-diameter portion of the cover to the first end portion of the housing over the overlap.  
         [0034]     In the preferred mode of the invention, the tacking means and the welding means are implemented by a welding machine including welding heads which are rotatable around the overlap.  
         [0035]     According to the eighth aspect of the invention, there is provided a gas sensor manufacturing machine designed to produce a as sensor using an assembly which has a length and includes (a) a housing which has a length made up of a first end portion, a second end portion, and a flange between the first and second end portions, (b) a cover which is made up of a small-diameter portion, a large-diameter portion, and a shoulder formed between the small-diameter portion and the large-diameter portion, and (c) an insulation porcelain disposed in the large-diameter portion of the cover in contact with the shoulder through an elastic member, (d) a cup-shaped sensor element disposed in the housing. The machine comprises: (1) a first annular press plate fitted on the first end portion of the housing in engagement with the flange; (2) a second annular press plate fitted on the small-diameter portion of the cover in engagement with the shoulder of the cover; (3) a pressing means for pressing the cover against the housing in a lengthwise direction of the assembly to fit an end of the large-diameter portion of the cover on the first end portion of the housing to form an overlap; (4) a tacking means for tacking the large-diameter portion of the cover to the first end portion of the housing at the overlap while pressing the cover against the housing; (5) a rotating means for rotating the assembly, and (6) a welding means for welding the large-diameter portion of the cover to the first end portion of the housing over the overlap.  
         [0036]     In the preferred mode of the invention, the tacking means and the welding means are implemented by a welding machine including welding heads which are rotatable around the overlap. 
     
    
     BRIEF DESPCRIPTION OF THE DRAWINGS  
       [0037]     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.  
         [0038]     In the drawings:  
         [0039]      FIG. 1  is a longitudinal sectional view which shows a laminated sensor element-equipped gas sensor to be manufactured by a method provided by the invention;  
         [0040]      FIG. 2  is a partially sectional view which shows the process of making a sensor assembly;  
         [0041]      FIG. 3  is a sectional view which shows the process of inserting an insulation porcelain into a housing to form a sensor assembly;  
         [0042]      FIG. 4  is a sectional view which shows the process of pressing the sensor assembly of  FIG. 3  for welding an air cover to a housing securely;  
         [0043]      FIG. 5  is a horizontal sectional view which shows welding heads placed around the gas assembly of  FIG. 3  to join an air cover to a housing;  
         [0044]      FIG. 6  is a longitudinal sectional view which shows a cup-shaped sensor element-equipped gas sensor to be manufactured by a method provided by the invention;  
         [0045]      FIG. 7  is a horizontal sectional view as taken along the line D-D in  FIG. 6 ;  
         [0046]      FIG. 8  is a sectional view which shows the process of inserting a sensor element into a housing to form a sensor assembly; and  
         [0047]      FIG. 9  is a sectional view which shows the process of pressing the sensor assembly of  FIG. 8  for welding an air cover to a housing securely. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]     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 first embodiment of the invention which may be employed in a burning control system or air-fuel ratio control system of an automotive vehicle to measure concentrations of components such as NOx, CO, HC, and O 2  contained in exhaust gasses of the engine.  
         [0049]     The gas sensor  1  generally includes a sensor element  2 , a first insulation porcelain  12 , a second insulation porcelain  11 , a hollow cylindrical housing  19 , and an air cover  10 . The sensor element  2  is made of a laminated plate consisting of a solid electrolyte body, ceramic layers, and electrode layers. For example, U.S. Pat. No. 5,573,650, issued on Nov. 12, 1996 to Fukaya et al. teaches a typical laminated sensor element, disclosure of which is incorporated herein by reference. The second insulation porcelain  11  is made of a hollow cylindrical member and is fitted within the housing  19  and holds therein the sensor element  2 . The first insulation porcelain  12  is mounted on the second insulation porcelain  11  in alignment with each other and surrounds a base portion of the sensor element  2 .  
         [0050]     The housing  19  has a large-diameter portion or flange  190 . The air cover  10  is fitted at an end thereof on a small-diameter portion (also referred to as a base end portion below) of the housing  19  to cover the first insulation porcelain  12 . The gas sensor  1  also includes a protective cover assembly  191  consisting of an outer cover and an inner cover. The protective cover assembly  191  is installed on a head of the housing  19  to define a gas chamber into which a gas to be measured is admitted through gas holes  150  formed in the outer and inner covers.  
         [0051]     The air cover  10  is made up of a large-diameter portion  105  greater in diameter than the first insulation porcelain  12 , a small-diameter portion  106  smaller in diameter than the first insulation porcelain  12  and a shoulder  102  between the large-diameter portion  105  and the small-diameter portion  106 .  
         [0052]     The first insulation porcelain  12  is made of a hollow cylindrical insulating member and retained between an upper end, as viewed in  FIG. 1 , of the second insulation porcelain  11  and the shoulder  102  of the air cover  10 . Between an inner wall of the shoulder  102  and a base end  129  of the first insulation porcelain  12 , an annular elastic ring  13  made of a conical spring is disposed to urge the first insulation porcelain  12  into firm engagement with the second insulation porcelain  11 , thereby establishing a hermetic seal in a gap between the second insulation porcelain  11  and the housing  19 .  
         [0053]     The assembling of the gas sensor  1  will be described below in brief.  
         [0054]     First, an assembly  100  having a given length made up of the air cover  10 , the elastic ring  13 , the first insulation porcelain  12 , the sensor element  2 , the second insulation porcelain  11 , the housing  19 , and the protective cover assembly  191  is, as shown in  FIG. 4 , prepared. The fabrication of the assembly  100  will be described later in detail.  
         [0055]     A manufacturing machine used in producing the gas sensor  1  includes an annular head plate  31 , an annular base plate  32  which are, as clearly shown in  FIG. 4 , laid in parallel, a press mechanism  400 , and a rotary mechanism  500 .  
         [0056]     After the assembly  100  is prepared, the annular head plate  31  is fitted on the head of the housing  19  in surface contact with an end surface of the flange  190 . The annular base plate  32  is fitted on the small-diameter portion  106  of the air cover  10  in surface contact with the shoulder  102 . Next, the annular head plate  31  and the annular base plate  32  are pressed by the press mechanism  400  in approach directions to fit the base end portion of the housing  19  into the end of the air cover  10  until a given overlap  15  is formed. The air cover  10  and the housing  19  are tacked, as described later in detail, at the overlap  15  while they are being pressed. The press mechanism  400  may be implemented by a known press machine using, for example, hydraulic cylinders. The structure itself is not essential part of the invention, and explanation thereof in detail will be omitted here.  
         [0057]     The assembly  100  is rotated about a longitudinal center line thereof by the rotary mechanism  500  which may be implemented by a known structure consisting of a gear set and an electric motor (not shown). The end of the air cover  19  is welded at an overall periphery of the overlap  15  to the base end portion of the housing  19 . The assembly  100  is preferably kept pressed by the annular head plate  31  and the annular base plate  32  during the welding of the air cover  19 , thereby minimizing a positional shift between the air cover  19  and the housing  19 .  
         [0058]     Referring back to  FIG. 1 , the air cover  10  is, as described above, mounted on the base end portion of the housing  19 . An outer air cover  181  is provided around the air cover  10  and staked or crimped to retain a water-repellent filter  182  on the small-diameter portion  106  of the air cover  10 .  
         [0059]     The second insulation porcelain  11  is retained within the housing  19  hermetically through a metallic packing ring  192  placed on an annular tapered shoulder formed on an inner wall of the housing  19  and holds therein the sensor element  2  through a glass sealing member  193 .  
         [0060]     The first insulation porcelain  12  is, as described above, mounted directly on the base end  119  of the second insulation porcelain  11  and surrounded by the air cover  10 . The elastic ring  13  is disposed between the shoulder  102  and the annular base end  129  of the first insulation porcelain  12  to elastically urge the first insulation porcelain  12  into constant engagement with the second insulation porcelain  11 . The annular base end  129  of the first insulation porcelain  12  is formed around a cylindrical projecting part or boss  128  on the end wall of the hollow cylindrical body  127 . The elastic ring  13  is, as described above, made of a conical spring and placed on the base end  129  of the first insulation porcelain  12  with the periphery thereof, as clearly shown in  FIG. 4 , oriented upward before the first insulation porcelain  12  is inserted into the air cover  10  in the assembling process.  
         [0061]     The first insulation porcelain  12  has disposed therein two pairs of leads  211  and  212  (only two are shown for the simplicity of illustration) each of which is made of a metal wire folded elastically to make an electric contact at one end with an electrode terminal (not shown) formed on the sensor element  2 . The leads  211  and  212  extend at the other end through holes formed in the boss  128  of the first insulation porcelain  12  and connect with four leads  180  through connectors  184 , respectively, for transmission of sensor signals between the sensor element  2  and an external device and supply of electric power to a heater installed on the sensor element  2 .  
         [0062]     The first insulation porcelain  12  has four vertical ribs  125  formed on an inner wall thereof at regular intervals. Between adjacent two of the vertical ribs  125 , each of the leads  211  and  212  is retained and insulated from the adjacent ones.  
         [0063]     The assembling of the gas sensor  1  will be described bellow in detail.  
         [0064]     First, the assembly  100  is prepared in the following steps.  
         [0065]     The sensor element  2  is, as clearly shown in  FIG. 2 , inserted into the second insulation porcelain  11 . The glass sealing member  193  is fitted in the chamber of the second insulation porcelain  11  to establish a hermetic seal between the sensor element  2  and the second insulation porcelain  11 . The glass sealing member  193  also serves to retain the sensor element  2  within the second insulation porcelain  11  rigidly.  
         [0066]     The leads  211  and  212  connecting with the leads  180  through the connectors  184  are installed in the first insulation porcelain  12 . The base portion of the sensor element  2  is inserted into the first insulation porcelain  12  to establish an electric connection the electrodes formed on the sensor element  2  with the leads  211  and  212 . The leads  211  and  212  are, as described above, made of a spring member and thus hold the base portion of the sensor element  2  elastically within the first insulation porcelain  12 .  
         [0067]     Next, the protective cover assembly  191  is, as clearly shown in  FIG. 3 , installed in the end of the housing  19  by staking. The metallic packing ring  192  is placed on the inner shoulder of the housing  19 . The elastic ring  13  is put in the air cover  10 . The air cover  10  is then put on the first insulation porcelain  12  and moved downward, as indicated by an arrow in  FIG. 3 , without compressing the metal packing ring  192  tightly, thereby inserting the second insulation porcelain  11  into the housing  19  to complete the assembly  100 . In the assembly  100 , the metal packing ring  192  does not yet establish a hermetic seal between the second insulation porcelain  11  and the housing  19 . The elastic ring  13  is not pressed sufficiently to exert the elastic pressure on the base end  129  of the first insulation porcelain  12  . Specifically, the elastic ring  13  is, as can be seen in  FIG. 3 , oriented at the periphery thereof upward.  
         [0068]     After the assembly  100  is prepared, the annular head plate  31  is, as described above in  FIG. 4 , fitted on the head of the housing  19  into contact with an end wall  195  of the flange  190  and then retained firmly in the manufacturing machine. Next, the annular base plate  32  is fitted on the small-diameter portion  106  of the air cover  10  and moved downward, as viewed in  FIG. 4 , by the press mechanism  400  at a given high speed into contact with the shoulder  102 .  
         [0069]     After hitting on the shoulder  102  of the air cover  10 , the annular base plate  32  is moved at a decreased speed and pushed downward to exert the pressure  39  on the shoulder  102  of the air cover  10  until the pressure  39  reaches about 650 kg. This causes the base end portion of the housing  19  to be forced into the end of the air cover  10 , thereby forming the overlap  15  and the elastic ring  13  to be flattened, thereby exerting the elastic pressure on the first insulation porcelain  12  downward in a longitudinal direction of the assembly  100 . The metal packing ring  192  is also compressed to hermetically seal a gap between the second insulation porcelain  11  (i.e., the sensor element  2 ) and the housing  19 . It is advisable that the pressure  39  be 1.2 times greater than the elastic pressure produced by the elastic ring  13  in order to force the housing  19  into the open end of the housing  19  reliably and to compress the metal packing ring  192  through the first insulation porcelain  11  for increasing the adhesion of the metal packing ring  192  to the surfaces of the first insulation porcelain  11  and the housing  19  to form a hermetic seal between the first insulation porcelain  11  and the housing  19  and lower than 7.8 kN in order to avoid the breakage of the first and second insulation porcelain  11  and  12 .  
         [0070]     The assembly  100  is held under a pressure of 650 kg.  
         [0071]     Two welding heads  4 , as shown in  FIG. 5 , are placed around the overlap  15  of the air cover  10  and the housing  19  and opposed diametrically to each other. Next, the welding heads  4  are turned together in a direction, as indicated by an arrow A, by an electric motor  600  to make two tack welds or joints  150  in the overlap  15 . Each of the joints  150  extends in a circumferential direction of the overlap  15  and has a length of 7 mm. The joints  150  are formed at radially symmetrical positions with respect to the center of the assembly  100 . A single joint  150  may be formed in the overlap, but however, at least two joints  150  are preferable in order to maintain the positional relation between the air cover  10  and the housing  19 . The joints  150  may alternatively be provided by spot welds. Additionally, the diameter of the open end of the air cover  10  is preferably smaller than that of the base end portion of the housing  19 . Specifically, it is advisable that the base end portion of the housing  19  be press fit in the open end of the air cover  10  in order to avoid an undesirable lift of the air cover  10  from the housing  19  during the tacking.  
         [0072]     The welding heads  4  are connected to a typical laser welding machine (not shown) and emit laser beams to the overlap  15 .  
         [0073]     After the air cover  10  and the housing  19  are joined by the laser welding, the pressure exerted on the assembly  100  is decreased down to 10 kg. The assembly  100  is turned up to a speed of 1500 mm/minute. The welding heads  4  are, like the above, placed around the overlap  15  again and held stationary. Laser beams are radiated from the welding heads  4  to weld the air over  10  to the housing  19  over the whole of the periphery of the overlap  15 .  
         [0074]     The assembly  100  is stopped from rotating. The annular base plate  32  is removed from the shoulder  102  of the air cover  10 . The assembly  100  is removed from the annular head plate  21 .  
         [0075]     Finally, the outer air cover  182  is fitted on the small-diameter portion  106  of the air cover  10  through the water-repellent filter  182  and pressed or crimped radially to join the outer air cover  182  to the air cover  10  firmly.  
         [0076]     As described above, the air cover  10  and the housing  19  are tacked at the overlap  15 , after which they are welded over the periphery of the overlap  15  to join the air cover  10  to the housing  19  completely. The tack welding serves to secure a desired positional relation between the air cover  10  and the housing  19 , thus avoiding any shift between the air cover  10  and the housing  19  in the circumferential direction.  
         [0077]     The tack welding is performed while the pressure is being exerted on the assembly  100  through the annular head and base plates  31  and  32 , thus causing the elastic ring  13  to be flattened and held between the shoulder  102  of the air cover  20  and the base end  129  of the first insulation porcelain  12 , which results production of pressure great enough to deform the metal packing ring  192  for sealing the gap between the outer wall of the second insulation porcelain  11  and the inner wall of the housing  19  hermetically.  
         [0078]     The second embodiment will be described below which joins the air cover  10  and the housing  19  without making the tack welds.  
         [0079]     The assembly  100  is prepared in the same manner as in the first embodiment. The annular head plate  31  is fitted on the head of the housing  19  into contact with the end wall  195  of the flange  190  and then retained firmly in the manufacturing machine. Next, the annular base plate  32  is fitted on the small-diameter portion  106  of the air cover  10  and moved downward, as viewed in  FIG. 4 , at a given high speed into contact with the shoulder  102 .  
         [0080]     After hitting on the shoulder  102  of the air cover  10 , the annular base plate  32  is moved at a decreased speed and pushed downward to exert the pressure  39  on the shoulder  102  of the air cover  10  until the pressure  39  reaches about 650 kg. This causes the base end portion of the housing  19  to be forced into the end of the air cover  10 , thereby forming the overlap  15  and the elastic ring  13  to be flattened, thereby exerting the elastic pressure on the first insulation porcelain  12  downward in a longitudinal direction of the assembly  100 . The metal packing ring  192  is also compressed to hermetically seal a gap between the second insulation porcelain  11  (i.e., the sensor element  2 ) and the housing  19 .  
         [0081]     The assembly  100  is held under a pressure of 650 kg. The assembly  100  is turned up to a constant speed of 1500 mm/minute. The welding heads  4 , as shown in  FIG. 5 , are placed around the overlap  15  of the air cover  10  and the housing  19  and opposed diametrically to each other. Laser beams are radiated from the welding heads  4  held stationary to weld the air over  10  to the housing  19  over the periphery of the overlap  15  of the rotating assembly  100 .  
         [0082]     After the air cover  10  and the housing  19  are joined by the laser welding, the rotation of the assembly  100  is stopped. The annular base plate  32  is removed from the shoulder  102  of the air cover  10 . The assembly  100  is removed from the annular head plate  21 .  
         [0083]     Finally, the outer air cover  182  is fitted on the small-diameter portion  106  of the air cover  10  through the water-repellent filter  182  and pressed or crimped radially to join the outer air cover  182  to the air cover  10  firmly.  
         [0084]     The third embodiment will be described below which turns the welding heads  4  to join the air cover  10  and the housing  19  without making the tack welds.  
         [0085]     The assembly  100  is prepared in the same manner as in the first embodiment. The annular head plate  31  is fitted on the head of the housing  19  into contact with the end wall  195  of the flange  190  and then retained firmly in the manufacturing machine. Next, the annular base plate  32  is fitted on the small-diameter portion  106  of the air cover  10  and moved downward, as viewed in  FIG. 4 , at a given high speed into contact with the shoulder  102 .  
         [0086]     After hitting on the shoulder  102  of the air cover  10 , the annular base plate  32  is moved at a decreased speed and pushed downward to exert the pressure  39  on the shoulder  102  of the air cover  10  until the pressure  39  reaches about 650 kg. This causes the base end portion of the housing  19  to be forced into the end of the air cover  10 , thereby forming the overlap  15  and the elastic ring  13  to be flattened, thereby exerting the elastic pressure on the first insulation porcelain  12  downward in a longitudinal direction of the assembly  100 . The metal packing ring  192  is also compressed to hermetically seal a gap between the second insulation porcelain  11  (i.e., the sensor element  2 ) and the housing  19 .  
         [0087]     The assembly  100  is held under a pressure of 650 kg. The welding heads  4 , as shown in  FIG. 5 , are placed around the overlap  15  of the air cover  10  and the housing  19  and opposed diametrically to each other. The welding heads  4  are turned at a given speed. Laser beams are then radiated from the welding heads  4  to weld the air over  10  to the housing  19  over the periphery of the overlap  15  of the assembly  100  held stationary.  
         [0088]     After the air cover  10  and the housing  19  are joined by the laser welding, the welding heads  4  are stopped from rotating. The annular base plate  32  is removed from the shoulder  102  of the air cover  10 . The assembly  100  is removed from the annular head plate  21 .  
         [0089]     Finally, the outer air cover  182  is fitted on the small-diameter portion  106  of the air cover  10  through the water-repellent filter  182  and pressed or crimped radially to join the outer air cover  182  to the air cover  10  firmly.  
         [0090]     Since the assembly  100  is held stationary, the pressure exerted on the assembly  100  is kept constant, thereby keeping the elastic ring  13  flat during the welding of the air cover  10  to the housing  19 , which establishes a firm seal between the second insulation porcelain  11  and the housing  19 .  
         [0091]      FIG. 6  shows a gas sensor  1  equipped with a cup-shaped sensor element  3 .  
         [0092]     The sensor element  3  consists of a cup-shaped solid electrolyte body  30  and a bar-shaped heater  35 . The solid electrolyte body  30  is retained in a hollow cylindrical housing  19 . The heater  35  is disposed within the solid electrolyte body  30 . The housing  19 , like the first embodiment, has the flange  190 .  
         [0093]     The sensor element  3  and the housing  19  are hermetically sealed by the packing ring  192  placed on an annular shoulder formed on an inner wall of the housing  19 .  
         [0094]     The solid electrolyte body  30  has a measuring electrode formed on an outer wall thereof and a reference gas electrode formed on an inner wall thereof. The gas measuring electrode and the reference gas electrode are known, for example, in European Patent Application EP 0918215 A2 assigned to the same assignee as that of this application, disclosure of which is incorporated herein by reference.  
         [0095]     The first insulation porcelain  12  has formed therein, as clearly shown in  FIG. 7 , a cylindrical bore in which four grooves are formed radially at regular intervals. The first insulation porcelain  12  is placed at a lower end thereof, as viewed in  FIG. 6 , on an upper end, as indicated by hatched lines S in  FIG. 7 , of the solid electrolyte body  30 . Within spaces  227  defined between the grooves and the outer wall of the first insulation porcelain  12 , metal terminal plates  213  and  214  are disposed which connect the measuring and reference gas electrodes of the sensor element  3  and the connectors  189 . Other arrangements are identical with those in the one shown in  FIG. 1 , and explanation thereof in detail will be omitted here.  
         [0096]     The assembling of the gas sensor  1  is accomplished in the following steps.  
         [0097]     First, the protective cover assembly  191  is fitted at an end thereof in an annular groove formed in the end of the housing  19  and fixed by staking a peripheral wall extension formed around the groove. The peripheral wall extension is then welded to the end of the protective cover assembly  191 . The heater  35  is fitted in the solid electrolyte body  30  of the sensor element  3 . The metal terminal plates  213  and  214  are installed on the solid electrolyte body  30 .  
         [0098]     Next, the sensor element  3  is built in the first insulation porcelain  12 . The leads  180  are coupled with the metal terminal plates  213  and  214  through the connectors  189 . The elastic ring  13  is placed on the first insulation porcelain  12 . The air cover  10  is fitted on the first insulation porcelain  12 .  
         [0099]     The metallic packing ring  192  is, as clearly shown in  FIG. 8 , put in the housing  19  and placed on the annular shoulder  300  formed on the inner wall of the housing  19 . The air cover  10  is fitted on the first insulation porcelain  12  to form the assembly  100 , as shown in  FIG. 9 .  
         [0100]     After the assembly  100  is prepared, the annular head plate  31  is, as shown in  FIG. 9 , fitted on the head of the housing  19  into contact with an end wall  195  of the flange  190  and then retained firmly in the manufacturing machine. Next, the annular base plate  32  is fitted on the small-diameter portion  106  of the air cover  10  and moved downward to fit the end of the air cover  10  on the base end portion of the housing  19 .  
         [0101]     Next, the end of the air cover  10  is welded to the base end portion of the housing  19  in the same manner as one of the above described first to third embodiment.  
         [0102]     Other assembling steps are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0103]     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.