Abstract:
An improved structure of a gas sensor is provided which is designed to establish firm electric connections between electrode terminals formed on opposed major surfaces of a sensor element and lead wires leading to an external device through a connector. The connector includes terminal connecting springs and holding members working to clamp the sensor element through the terminal connecting springs elastically to establish elastic contact of the terminal connecting springs with the electrode terminals of the sensor element. This structure is easy to manufacture and secures firm electrical connections between the terminal connecting springs and the electrode terminals.

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 equipped with an electric connector designed to ensure electric connections between a sensor element and lead wires leading to an external device.  
           [0003]    2 Background Art  
           [0004]    Gas sensors equipped with a sensor element such as an oxygen sensor as taught in Japanese Utility Model Second Publication No. 8-1493 are known for use in burning control of fuel in internal combustion engines of modern automotive vehicles. Gas sensors of this type generally have disposed therein a connector establishing electrical connections between lead wires leading to an external controller and electrodes provided on the sensor element for use in picking up a sensor output and supplying the power to a heater provided on the sensor element. For instance, the connector is made up of terminal connecting conductors making electrical connections between the lead wires and terminals leading to the electrodes of the sensor element and a holder retaining therein the terminal connecting conductors.  
           [0005]    Connectors which are easy to manufacture and designed to retain the terminal connecting conductors firmly to ensure the electrical connections between the lead wires and the terminals of the sensor element are sought.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the invention to provide an improved structure of a gas sensor constructed to secure electric connections between electrode terminals of a sensor element and lead wires leading to an external device such as a controller and to be manufactured easily.  
           [0007]    According to one aspect of the invention, there is provided a gas sensor which comprises: (a) a sensor element having a length and electrical terminals formed on an end portion thereof; and (b) a connector working to establish electrical connections between the electrical terminals of the sensor element and conductors extending from inside to outside the gas sensor. The connector includes terminal connecting members and at least two holding members. The holding members work to retain therein the terminal connecting members and the end of the sensor element to make the electrical connections between the electrical terminals of the sensor element and the conductors. The terminal connecting members and the holding members are so configured geometrically as to establish mechanical engagement therebetween.  
           [0008]    In the preferred mode of the invention, each of the terminal connecting members has a protrusion. Each of the holding members has formed therein recesses within which the protrusions of the terminal connecting members are fitted to establish the mechanical engagement between the terminal connecting members and the holding members.  
           [0009]    The protrusions of the terminal connecting members may be implemented by bends formed on lengths of the terminal connecting members, respectively.  
           [0010]    The bends project perpendicular to the lengths of the terminal connecting members, respectively.  
           [0011]    Each of the terminal connecting members may alternatively have a plurality of protrusions. Each of the holding members may have formed therein recesses within which the protrusions of the terminal connecting members are fitted to establish the mechanical engagement between the terminal connecting members and the holding members.  
           [0012]    Each of the terminal connecting members is made up of a supporting portion, a bent portion, and an elastic contact portion placed in electrical contact with one of the electrical terminals of the sensor element. Each of the elastic contact portions continues from an end of the support portion through the bent portion and is turned at the bent portion toward the support portion. The support portion has the protrusion. The protrusion is located farther from the bent portion than the elastic contact portion.  
           [0013]    According to the second aspect of the invention, there is provided a gas sensor which comprises: (a) a sensor element having a length and electrical terminals formed on an end portion thereof; (b) at least two holding members joined together to define a chamber therein; (c) terminal connecting spring members leading to conductors extending from inside to outside the gas sensor, the terminal connecting spring members being retained within the chamber of the holding members in electrical contact with the electrical terminals of the sensor element so as to add elastic pressures to the sensor element in a direction perpendicular to the length of the sensor element, respectively, to hold the end portion of the sensor element within the chamber of the holding members; and (d) a clamping spring mechanism disposed on an outer periphery of the holing members. The clamping spring mechanism works to add an elastic pressure F 2  to the holding members to clamp the holding members together. The elastic pressure F 1  is lower than or equal to an elastic pressure F 2  that is a sum of the elastic pressures produced by the terminal connecting spring members. This ensures electrical contact of the terminal connecting spring members with the terminals of the sensor element.  
           [0014]    In the preferred mode of the invention, the clamping spring mechanism is made up of at least two springs fitted on the holding members.  
           [0015]    If a plane is defined which extends along the length of the sensor element, a vector of the elastic pressure F 1  and a vector of the elastic pressure F 2  have the same position on the plane.  
           [0016]    According to the third aspect of the invention, there is provided a gas sensor which comprises: (a) a plate-shaped sensor element having a length and electrical terminals formed on an end portion thereof; (b) terminal connecting spring members leading to conductors extending from inside to outside the gas sensor, each of the terminal connecting members is made up of a supporting portion, an elastic contact portion, and a bent portion connecting between the supporting portion and the elastic contact portion, the bent portion having one of substantially a U-shape and substantially a V-shape and directing the elastic contact portion toward the supporting portion so as to produce elasticity which allows the elastic contact portion to be deformed toward the supporting portion; and (c) at least two clamping members working to clamp the end portion of the gas sensor through the terminal connecting spring members so as to establish elastic contact of each of the terminal connecting spring members with one of the electrical terminals of the sensor element.  
           [0017]    In the preferred mode of the invention, each of the terminal connecting spring members is made of one of a plate and a round bar.  
           [0018]    A surface of each of the terminal connecting spring members is plated with gold.  
           [0019]    Each of the elastic contact portion has a protrusion facing a corresponding one of the electrical terminals of the sensor element.  
           [0020]    The gas sensor also includes a spring mechanism which produces an elastic pressure oriented perpendicular to the length of the gas sensor to clamp the clamping members together.  
           [0021]    The spring mechanism may be made up of two or more springs.  
           [0022]    The clamping members have electrical insulation properties. 
       
    
    
     BRIEF DESPCRIPTION OF THE DRAWINGS  
       [0023]    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.  
         [0024]    In the drawings:  
         [0025]    [0025]FIG. 1 is a longitudinal sectional view of a gas sensor according to the invention;  
         [0026]    [0026]FIG. 2 is a transverse sectional view which shows an internal structure of an electric connector;  
         [0027]    [0027]FIG. 3( a ) is a plane view which shows one of a pair of clamping spring plates;  
         [0028]    [0028]FIG. 3( b ) is a side view of FIG. 3( a );  
         [0029]    [0029]FIG. 4( a ) is a plane view which shows a clamping spring plate of the type different from the one of FIGS.  3 ( a ) and  3 ( b );  
         [0030]    [0030]FIG. 4( b ) is a side view of FIG. 4( a );  
         [0031]    [0031]FIG. 5 is a partial plane view which shows terminal connecting strips establishing electrical contact with terminals of a sensor element;  
         [0032]    [0032]FIG. 6( a ) is a partial side view which shows a terminal connecting strip;  
         [0033]    [0033]FIG. 6( b ) is a partial side view which shows a terminal connecting strip of the type different from the one in FIG. 6( a );  
         [0034]    [0034]FIG. 7 is a partially enlarged view which shows elastic contact between the terminal connecting strip of FIG. 6( b ) and a gas sensor;  
         [0035]    [0035]FIG. 8 is a plane view which shows an internal structure of a holding member;  
         [0036]    [0036]FIG. 9( a ) is a vertical sectional view as taken along the line a-a in FIG. 8;  
         [0037]    [0037]FIG. 9( b ) is a vertical sectional view as taken along the line b-b in FIG. 8;  
         [0038]    [0038]FIG. 10 is a plane view which shows an outer structure of the holding member of FIG. 8;  
         [0039]    [0039]FIG. 11( a ) is a partial side view which shows a modified form of the terminal connecting strip of FIG. 6( a );  
         [0040]    [0040]FIG. 11( b ) is a plane view of FIG. 11( a );  
         [0041]    [0041]FIG. 12 is a plane view which shows a modified form of the holding member of FIG. 8;  
         [0042]    [0042]FIG. 13 is a partial side view which shows a modified form of the terminal connecting strip of FIG. 6( a );  
         [0043]    [0043]FIG. 14( a ) is a partial side view which shows a modified form of the terminal connecting strip of FIG. 6( a );  
         [0044]    [0044]FIG. 14( b ) is a plane view as viewed from a longitudinal direction of the terminal connecting strip of FIG. 14( a );  
         [0045]    [0045]FIG. 15 is a graph which shows a calibration curve indicating a relation between a load applied to an elastic member and a resultant flexure;  
         [0046]    [0046]FIG. 16 is an explanatory view which shows flexture of a clamping spring plate;  
         [0047]    [0047]FIG. 17 is an explanatory view which shows flexture of a terminal connecting strip;  
         [0048]    [0048]FIG. 18 is a plane view for explaining how to determine an elastic pressure produced in a case where holding members are clamped only by one clamping spring plate;  
         [0049]    [0049]FIG. 19 is a plane view for explaining how to determine an elastic pressure produced in a case where holding members are clamped by two clamping spring plates;  
         [0050]    [0050]FIG. 20 is an explanatory view for explaining how to determine an elastic pressure produced by terminal connecting strips; and  
         [0051]    [0051]FIG. 21 is an explanatory view which shows location where elastic pressures produced by terminal connecting strips and clamping spring plates act. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0052]    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.  
         [0053]    The gas sensor  1  includes a sensor element  29  with two opposed major surfaces, as clearly shown in FIG. 5, each having four terminals  291  and  292  affixed thereto (i.e., a total of eight terminals). The gas sensor  1  also includes an electrical connector consisting of electrical terminal connecting strips  51  and  52  and holding members  61  and  62  working as a clamper, as will be described later in detail, to clamp the sensor element  29  through the terminal connecting strips  51  and  52 . The terminal connecting strips  51  and  52  work to connect through electric connectors  41  the terminals  291  and  292  with lead wires  41  extending from outside to inside the gas sensor  1  through an elastic insulator  4 .  
         [0054]    Each of the terminal connecting strips  51  and  52 , as shown in FIGS.  5  to  7 , has a locking protrusion  500  facing the holding members  61  and  62 , as shown in FIGS. 1 and 2. Each of the holding members  61  and  62 , as clearly shown in FIG. 8, has formed in a surface facing the terminal connecting strips  51  and  52  recesses  600  in which the locking protrusions  500  are to be fitted or locked.  
         [0055]    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.  
         [0056]    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.  
         [0057]    The heater and the cells are joined electrically to an external controller (not shown) through the terminals  291  and  292  mounted on end portions of the side surfaces of the sensor element  29 . Specifically, electric power and voltage are inputted to the heater and each cell through the terminals  291  and  292 . Additionally, outputs of each cell is picked up by the controller through the terminals  291  and  292 .  
         [0058]    The gas sensor  1  has, as described above, the three cells and the one heater and thus needs the eight terminals  291  and  292  in total for supplying the power to the heater and transmitting outputs of the cells to the external controller. The terminals  291  and  292  are coupled electrically to the lead wires  41  through the connectors  42  and the terminal connecting strips  51  and  52 , respectively.  
         [0059]    The sensor element  29 , as clearly shown in FIGS. 2 and 5, has the total of the four terminals  291  and  292  affixed to each of the opposed major surfaces. The total of the four electrical terminal connecting strips  51  and  52  are, thus, arrayed at each side of the sensor element  29 . 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.  
         [0060]    The gas sensor  1 , as shown in FIG. 1, also includes a hollow cylindrical metallic housing  10 , a double-walled protective cover assembly  109  made up of an outer and an inner cover, and an 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 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 to the housing  10 . The second cover  112  is installed on the periphery of the small-diameter portion of the first cover  111  and crimped to retain a water-repellent filter  113  around the small-diameter portion of the first cover  111 .  
         [0061]    A ceramic-made insulation porcelain  2  is retained within the housing  10 . The insulation porcelain  2  has a tapered shoulder  102 . The housing  10  has an inner shoulder  101  tapering off to the cover assembly  109 . The shoulder  102  of the insulation porcelain  2  is placed on the inner shoulder  101  of the housing  10  through a metallic packing ring  200  in an air-tight fashion.  
         [0062]    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.  
         [0063]    Each of the terminal connecting strips  51  and  52 , as shown in FIGS.  6 ( a ) and  6 ( b ), includes a support  50 , an elastic contact  502 , and a bend  501  which is of substantially a U-shape to provide elasticity to the elastic contact  502 . The elastic contact  502  serves to make an electric contact with a corresponding one of the terminals  291  and  292 . The holding members  61  and  62  are, as will be described below in detail, clamped together to elastically deform the elastic contacts  502  of the terminal connecting strips  51  and  52  toward the supports  50 , as clearly shown in FIG. 7, to secure electric connections between the elastic contacts  502  and the terminals  291  and  292 .  
         [0064]    Two clamping spring plates  31  and  32 , as shown in FIG. 2, are fitted over outer peripheries of the holding members  61  and  62  elastically to provide an elastic pressure thereto in a radius direction of the gas sensor  1  (i.e., a direction perpendicular to the length of the sensor element  29 ). The holding members  61  and  62  are each made up of an insulating ceramic material and form an air-side insulation porcelain  3  which works to establish electric insulation between the terminal connecting strips  51  and  52 .  
         [0065]    The clamping spring plate  31  is, as clearly shown in FIGS.  3 ( a ) and  3 ( b ), made up of a rectangular plate  310  and legs  319 . The plate  310  is curved slightly outward and has formed in a central portion thereof an opening  318  for saving weight and increasing flexibility thereof. The legs  319  extend substantially perpendicular to the plate  310  from four corners thereof in the form of a C-shape, as shown in FIG. 3( a ). An end of each of the legs  319  is bent outward.  
         [0066]    A solid line in FIG. 3( a ) indicates the profile of the legs  319  before the clamping spring plate  31  is fitted on the holding members  61  and  62 . A broken line indicates the profile of the legs  319  after the clamping spring plate  31  is fitted on the holding members  61  and  62  to elastically couple them together, as shown in FIG. 2.  
         [0067]    The clamping spring plate  32  is, as clearly shown in FIGS.  4 ( a ) and  4 ( b ), made up of a rectangular plate  320  and a pair of legs  329 . The legs  329  extend from sides of the plate  320  and serve to couple the holding members  61  and  62  together elastically. An end of each of the legs  329  is bent outward. The clamping spring plate  32  also includes a pair of anchoring legs  321  which extend, as clearly shown in FIGS.  4 ( b ) and  1 , from the legs  329  so as to establish elastic engagement with an inner wall of the first cover  111  of the air cover assembly  11 , thereby anchoring the holding members  61  and  62  within the first cover  111 .  
         [0068]    A solid line in FIG. 4( a ) indicates the profile of the legs  329  before the clamping spring plate  32  is fitted on the holding members  61  and  62 . A broken line indicates the profile of the legs  329  after the clamping spring plate  32  is fitted on the holding members  61  and  62  to elastically couple them together, as shown in FIG. 2.  
         [0069]    Each of the terminal connecting strips  51  and  52  is, as shown in FIGS.  5  to  7 , made up of the support  50 , the locking protrusion  500  formed on the support  50 , the elastic contact  502 , and the bend  501  formed between the support  50  and the elastic contact  502 .  
         [0070]    The support  50  of the terminal connecting strip  52 , as shown in FIGS. 5 and 6( b ), extends straight in parallel to a length of the sensor element  29  and ends at the bend  501 . The elastic contact  502  is bent in a direction opposite a direction in which the locking protrusion  500  bulges out at an angle θ to the support  50  and extends toward the base side, as shown in FIG. 1, of the gas sensor  1 .  
         [0071]    The support  50  of the terminal connecting strip  51 , as shown in FIGS. 5 and 6( a ), includes a vertical portion A extending in parallel to the length of the sensor element  29  and an L-shaped portion B extending at right angles to the vertical portion A and then straight in parallel to the vertical portion A. The L-shaped portion B leads to the elastic contact  502  through the bend  501 . The bend angle θ between the support  50  and the elastic contact  502  is an acute angle.  
         [0072]    Each of the elastic contacts  502  has, as clearly shown in FIGS.  6 ( a ) and  6 ( b ), a second bend  505  to define a first contact portion  503  between the first bend  501  and the second bend  505  and a second contact portion  504  between the second bend  505  and the end of the elastic contact  502 . The angle φ which the second contact portion  504  makes with the first contact portion  503  is an obtuse angle.  
         [0073]    The terminal connecting strips  51  and  52  make, as shown in FIGS. 5 and 7, electrical connections with the terminals  291  and  292  of the sensor element  29 . Specifically, the terminal connecting strips  51  abut to the terminals  291 , while the terminal connecting strips  51  abut to the terminals  292 .  
         [0074]    Each of the terminal connecting strips  51  and  52  is, as described above, urged elastically by the clamping spring plates  31  and  32  through the holding members  61  and  62  so that it is deformed, as indicated by a broken line in FIG. 7, in the radius direction of the gas sensor  1  to establish constant engagement with one of the terminals  291  and  292 .  
         [0075]    The terminal connecting strips  51  and  52  are different in distance to the terminals  291  and  292 , but the above described elastic deformation thereof absorbs such a variation to secure the electrical connections to the terminals  291  and  292 .  
         [0076]    The holding members  61  and  62  are each made of an insulating ceramic material and joined to each other by the clamping spring plates  31  and  32  to form the air-side insulation porcelain  3  with a vertical extending chamber which is octagonal in cross section, as clearly shown in FIG. 2, and works to establish electric insulation between the terminal connecting strips  51  and  52 . FIG. 2 illustrates the air-side insulation porcelain  3  as viewed from the base side of the gas sensor  1 .  
         [0077]    [0077]FIG. 8 shows an inside structure of the holding member  61  facing the terminal connecting strips  51  and  52 . The holding member  61  has formed therein grooves  601  within which the terminal connecting strips  51  are to be disposed and grooves  602  within which the terminal connecting strips  52  are to be disposed. FIG. 9( a ) is a sectional view of the holding member  61  as taken along the line a-a in FIG. 8. FIG. 9( b ) is a sectional view of the holding member  62  as taken along the line b-b in FIG. 8.  
         [0078]    The grooves  601  are similar in configuration to the supports  50  of the terminal connecting strips  51 . The grooves  602  are similar in configuration to the supports  50  of the terminal connecting strips  52 . Each of the grooves  601  and  602  has formed therein the recess  600  in which the locking protrusion  500  of a corresponding one of the terminal connecting strips  51  and  52  is to be fitted or locked.  
         [0079]    The holding member  62  is identical in structure with the holding member  61 , and explanation thereof in detail will be omitted here.  
         [0080]    Each of the locking protrusions  500  of the terminal connecting strips  51  and  52  is, as clearly shown in FIGS.  6 ( a ) and  6 ( b ), of a U-shape and located farther from the bend  501  than the end  506  of the elastic contact  502 .  
         [0081]    [0081]FIG. 10 shows an outer structure of the holding member  61  which has formed therein recesses  605  and  606  serving to hold the clamping spring plates  31  and  32  from moving undesirably. The clamping spring plate  31  is fitted within the recesses  605 . The clamping spring plate  32  is fitted within the recess  606 . The holding member  62  is identical in outer structure with the holding member  61 , and explanation thereof in detail will be omitted here.  
         [0082]    Each of the terminal connecting strips  51 , as shown in FIGS.  11 ( a ) and  11 ( b ), may also have a protrusion  505  which is formed on the first contact portion  503  of the elastic contact  501  by punching or pressing.  
         [0083]    Each of the holding members  61  and  62  may alternatively have an inner structure, as illustrated in FIG. 12, which has a recess  607  configured to fit the terminal connecting strips  51  and  52  therewithin.  
         [0084]    Each of the terminal connecting strips  51  and  52  may have, as shown in FIG. 13, two locking protrusions  500 .  
         [0085]    Each of the terminal connecting strips  51  and  52  may alternatively have, as shown in FIGS.  14 ( a ) and  14 ( b ), a C-shaped locking member  507  which has a pair of strips  508  extending perpendicular to the length of the support  50  to establish tight engagement with the recess  600 .  
         [0086]    As apparent from the above discussion, the elastic contacts  502  of the terminal connecting strips  51  and  52  are configured to be deformable in the radius direction of the gas sensor  1  (i.e., the sensor element  29 ) and thus serve to secure electrical connections with the terminals  291  and  292  with aid of elastic pressure produced by the clamping spring plates  31  and  32 . Additionally, an unwanted shift of the terminal connecting strips  51  and  52  in a lengthwise direction thereof is avoided by the engagement of the locking protrusions  500  with the recess  600  of the holding members  61  and  62 .  
         [0087]    The pressure F 1  produced by the terminal connecting strips  51  and  52  to hold or clamp the end portion of the sensor element  29  in a desired location and orientation within the air-side insulation porcelain  3  is lower than or equal to the pressure F 2  produced by the clamping spring plates  31  and  32  to clamp the holding members  61  and  62  (i.e., F 1 ≦F 2 ) together.  
         [0088]    The four terminal connecting strips  51  and  52  are, as described above, arrayed on each side of the sensor element  29  and urged by the clamping spring plates  31  and  32  to press the four terminals  291  and  292  elastically to retain the sensor element  29  within the air-side insulation porcelain  3 . For instance, the pressure produced by each of the clamping spring plates  31  and  32  is more than or equal to one half of the pressure F 1  produced by all of the terminal connecting strips  51  and  52 . Specifically, the pressure F 2  produced by the clamping spring plates  31  and  32  is set substantially equal to or higher than the pressure F 1 . This ensures electrical contact between each of the terminal connecting strips  51  and  52  and a corresponding one of the terminals  291  and  292  of the sensor element  29  without any clearances.  
         [0089]    The pressures F 1  and F 2  may be determined in the following manner.  
         [0090]    Usually, an elastic force is determined by measuring the degree of deformation of an elastic member, magnetostriction, piezo-electricity, or characteristic frequency of an ossilator, and comparing it with a calibration curve.  
         [0091]    [0091]FIG. 15 shows an example of a calibration curve defined by a load applied to a spring and a resultant deflection or flexture of the spring measured actually. In the shown example, the load is in direct proportion to the flexture, but they may bear another relation depending upon the type of a spring.  
         [0092]    Each of the legs  319  of the clamping spring plate  31  takes a form, as indicated by a solid line in FIG. 16, when subjected to no loads. Application of load K 1  causes the legs  319  to be deflected outward, as indicated by broken lines. The degree of fluxture of the clamping spring plate  31  may be expressed by distance a minus distance b (i.e., a−b). Therefore, the elastic pressure produced by the clamping spring plate  31  when clamping the holding members  61  and  62 , as illustrated in FIG. 2, may be determined by measuring a load applied to the legs  319  and a resultant interval between the legs  319  (i.e., the distance a) to define a calibration curve, like the one in FIG. 15, and finding a load corresponding to the width of the assembly of the holding members  61  and  62  (i.e., the distance a between the legs  319  after fitted on the holding members  61  and  62 ) minus the distance b by look-up using the calibration curve. The elastic pressure produced by the clamping spring plate  32  may be determined in the same manner.  
         [0093]    The elastic contact  502  of each of the terminal connecting strips  51  takes a form, as indicated by a solid line in FIG. 17, when subjected to no loads. Application of load K 2  causes the elastic contact  502  to be deflected to the support  50 , as indicated by a broken line. The degree of fluxture of the elastic contact  502  may be expressed by distance c minus distance d (i.e., c−d). Therefore, the elastic pressure produced by each of the terminal connecting strips  51  when urged by the clamping spring plates  31  and  32  through the holding members  61  and  62 , as illustrated in FIG. 2, into constant engagement with one of the terminals  291  and  292  may be determined by measuring a load applied to the elastic contact  502  and a resultant displacement thereof (i.e., c−d) to define a calibration curve, and finding a load corresponding to the interval between the elastic contact  502  and the support  50  (i.e., the distance c) minus a clearance between the support  50  and a corresponding one of the terminals  291  and  292  after the terminal connecting strip  51  is installed within the holding members  61  and  62  (i.e., the distance d) by look-up using the calibration curve. The elastic pressure produced by the terminal connecting strips  52  may be determined in the same manner.  
         [0094]    The manner in which the pressures F 1  and F 2  are determined will also be described below in more detail with reference to FIGS.  18  to  20 .  
         [0095]    The holding members  61  and  62  may be clamped, as shown in FIG. 18, only by the clamping spring plate  31 . The distance between innermost portions of the legs  319 , that is, points  610  of contact with the outer surfaces of the holding members  61  and  62  after the clamping spring plate  31  is fitted on the holding members  61  and  62  is defined as f. The distance between the innermost portions  611  of the clamping spring plate  31  when the clamping spring plate  31  is not fitted on the holding members  61  and  62  is defined as e. The pressure produced by the clamping spring plate  31  may be determined as a function of the distance f minus the distance e by look-up using the calibration curve, as illustrated in FIG. 13. This pressure corresponds to the pressure F 2  in a case where the holding members  61  and  62  are clamped only by the clamping spring plate  31 . Each of the terminal connecting strips  51  and  52  is so selected that the pressure F 1  produced by all of the terminal connecting strips  51  and  52  may be lower than the pressure F 2  produced by the clamping spring plate  31 .  
         [0096]    [0096]FIG. 19 illustrates for a case where the holding members  61  and  61  are clamped using both the clamping spring plates  31  and  32 .  
         [0097]    The pressure produced by the clamping spring plate  31  may be determined based on the distance f 1  minus the distance e 1  in the same manner as described above. Similarly, the pressure produced by the clamping spring plate  32  may be determined based on the distance f 2  minus the distance e 2 . The sum of these two pressures is equivalent to the pressure F 2 .  
         [0098]    The sensor element  29  may be, as shown in FIG. 20, retained within the holding members  61  and  62  only by the terminal connecting strips  51 . The distance d between the support  50  and the elastic contact  502  after the connecting strips  51  are installed in the holding members  61  and  62  is given by dividing the distance h between the inner walls  613  of the holding members  61  and  62  minus the thickness g of the sensor element  29  by two (i.e., (h−d)/2). Thus, the elastic pressure produced by each of the terminal connecting strips  51  to hold the sensor element  29  in a desired position within the holding members  61  and  62  may be determined by look-up using the calibration curve, like the one of FIG. 15, based on the distance c between the support  50  and the elastic contact  502  before the connecting strips  51  are installed minus the distance d.  
         [0099]    The center of a total holding pressure given by the terminal connecting strips  51  and  52  (i.e., the pressure F 1 ) and the center of a total clamping pressure given by the clamping spring plates  31  and  32  (i.e., the pressure F 2 ) will be described below.  
         [0100]    The sensor element  29  is rectangular in cross section and, as can be seen in FIG. 5, has the four terminals  291  and  292  on each of the opposed major surfaces. Four of the terminal connecting strips  51  and  52  are placed in contact with the terminals  291  and  292  on each of the surfaces of the sensor element  29 .  
         [0101]    A plane including one of the major surfaces of the sensor element  29  is, as shown in FIG. 21, defined as H. The origin O is defined on any point on the plane H. Points on the plane H to which contacts between the elastic contacts  502  of the terminal connecting strips  51  and  52  and the terminals  291  and  292  of the sensor element  29  are projected are expressed by x,y coordinates (x1, y1), (x2, y2), (x3, y3), and (x4, y4), respectively. The center of points on the plane H to which portions of the holding members  61  and  62  pressed by the legs  319  of the clamping spring plate  31  and the legs  329  of the clamping spring plate  32  are projected is expressed by x,y coordinates (xw, yw).  
         [0102]    If pressures produced by the terminal connecting strips  51  and  52  acting on the points (x1, y1), (x2, y2), (x3, y3), and (x4, y4) are defined as P 1 , P 2 , P 3 , and P 4  and a pressure produced by the clamping spring plates  31  and  32  acting on the point (xw, yw) is defined as W (P 1  to P 4  are vectors, and W is a vector sum of the pressures produced by the legs  319  of the clamping spring plate  31  and the legs  329  of the clamping spring plate  32 ), x,y coordinates (Xp, Yp) of the center (i.e., a vector sum) of the pressures P 1 , P 2 , P 3 , and P 4  (i.e., coordinates of the pressure F 1 ) are given below.  
           Xp =( P   1 · x   1 + P   2 · x   2 + P   3 · x   3 + P   4 · x   4 )/( P   1 + P   2 + P   3 + P   4 )  
           Yp =( P   1 · y   1 + P   2 · y   2 + P   3 · y   3 + P   4 · y   4 )/( P   1 + P   2 + P   3 + P   4 )  
         [0103]    X,Y coordinates of the pressure W (i.e., the pressure F 2 ) are, as apparent from the above, xw and yw.  
         [0104]    In this embodiment, the pressures F 1  and F 2  are selected to be identical in position with each other. Thus, Xp=xw, and Yp=yw. The clamping spring plates  31  and  32  and the holding members  61  and  62  are so designed as to meet such relations.  
         [0105]    The coordinates (xw, yw) of the pressure W may be determined using points on the plane H to which portions of the holding members  61  and  62  pressed by the clamping spring plates  31  and  32  are projected.  
         [0106]    Each of the terminal connecting strips  51  and  52  is made of a plate member, but may alternatively be formed by a round bar member.  
         [0107]    The surface of the terminal connecting strips  51  and  52  may be plated with gold.  
         [0108]    The bend  501  of each of the terminal connecting strips  51  and  52  is of substantially a U-shape, but may have a substantially a V-shape.  
         [0109]    The air-side insulation porcelain  3  consists of the two holding members  61  and  62 , but may be made up of three or more parts.  
         [0110]    The holding members  61  and  62  may also be clamped together by three or more springs.  
         [0111]    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.