Patent Publication Number: US-8110783-B2

Title: Tubular heater

Description:
FIELD OF THE INVENTION 
     The present invention relates to a tubular heater designed to generate heat when energized via lead wires. 
     BACKGROUND OF THE INVENTION 
     Heater may take various shapes depending on the shape of an object to be heated by the heater. A tubular shaped heater is disclosed in, for example, Japanese Patent Application Laid-Open Publication (JP-A) No. 2006-349513, and a plate-like heater is disclosed in, for example, Japanese Patent Application Laid-Open Publication (JP-A) No. 2005-332628. 
     The tubular heater disclosed in JP 2006-349513A, owned by the present assignee, as shown in  FIG. 7  hereof, includes a tubular body  201  forming a ceramic heater incorporated in a gas sensor  202 . When energized via conducting wires  203 ,  203 , the ceramic heater  201  generates heat to thereby prevent dew condensation from occurring in a detection chamber of the gas sensor  202 . In  FIG. 7 , the conducting wires  203 ,  203  are shown as if they are disposed in opposed relation to each other. However, this is only for purposes of illustration. In reality, the conducting wires  203 ,  203  are disposed side by side or in lateral juxtaposition on one radial side of a central axis of the tubular ceramic heater  201 . 
     The thus arranged ceramic heater  201  is not fully satisfactory in that the temperature in the vicinity of the two juxtaposed conducing wires  203 ,  203  is relatively low, while the temperature at a portion diametrically opposed to the two juxtaposed conducting wires  203 ,  203  is relatively high. Thus the prior ceramic heater  201  cannot generate heat with a uniform temperature distribution. Furthermore, the conducting wires  203 ,  203  are disposed side by side and, hence, they are likely to cause a short circuit during manufacture or assembly of the ceramic heater  201 . 
       FIG. 8  hereof shows a thermosensor  221  disclosed in JP 2005-332628A. The thermosensor  221  includes a rectangular printed-circuit board  222  on which a resistance pattern  223  and a connection pattern  224  are formed by printing. Core wires  225  are connected to the connection pattern  224 . The connection pattern  224  facilitates easy connection of the core wires  225  to the thermosensor  221 . The resistance pattern  223  can be used as a resistance pattern of a heater in which instance the core wires  225  are connected directly to the resistance pattern  223 . 
     When the heater having the resistance pattern  223  is energized via the core wires  225 , the temperature of the resistance pattern  223  is relatively high at a central portion thereof and relatively low in the vicinity of the core wires  225 . Thus, regional temperature variations of the conventional heater are relatively large. 
     SUMMARY OF THE INVENTION 
     With the foregoing drawbacks of the prior art in view, an object of the present invention is to provide a tubular heater which is able to generate heat with less temperature variations and free of a short circuit between lead wires. 
     According to the present invention, there is provided a tubular heater comprising a tubular insulating substrate, a continuous heat-generating resistance element formed in a predetermined pattern on one surface of the insulating substrate, and a first lead wire connected to one end of the heat-generating resistance element and a second lead wire connected to an opposite end of the heat-generating resistance element, the first and second lead wires extending from one end of the tubular insulating substrate in a common axial direction of the tubular insulating substrate. The first and second lead wires are disposed in diametrically opposed relation to each other about a central axis of the tubular insulating substrate. 
     With this arrangement, since the first and second lead wires, which constitute non-heat-generating portions and tend to lower the temperature, are disposed in diametrically opposed relation about the central axis of the tubular insulating substrate, it is possible to reduce the regional temperature variations to an greater extent as compared to a convention tubular heater in which two lead wires are arranged side by side or in lateral juxtaposition on one radial side of the central axis of the tubular heater. 
     Furthermore, the first and second lead wires, which are disposed in diametrically opposed relation to each other about the central axis of the tubular insulating substrate is substantially free from a short circuit. 
     Preferably, when viewed in a development view, the pattern of the heat-generating resistance element is arranged such that the heat-generating resistance element runs from one of the first and second lead wires in a direction away from the other of the first and second lead wires and returns to the other of the first and second lead wires. 
     In one preferred form of the present invention, the heat-generating resistance element has a first meandering portion extending from the first lead wire toward the second lead wire, a second meandering portion extending from the second lead wire in a direction away from the first lead wire, and a linear connecting portion extending between ends of the first and second meandering portions which are located remote from the first and second lead wires, respectively. With this arrangement, the heat-generating resistance element is able to provide heating with a highly uniform temperature distribution. 
     In another preferred form of the present invention, the heat-generating resistance element has a first meandering portion extending from the first lead wire in an axial direction of the tubular insulating substrate, and a second meandering portion extending from the second lead wire in the axial direction of the tubular insulating substrate. The first and second meandering portions are disposed side by side in a circumferential direction of the tubular insulating substrate with respective one of the first and second lead wires disposed therebetween. By thus arranging the heat-generating resistance element, heating with less regional temperature variations can be achieved. One of the first and second meandering portions may include a meandering section extending in the circumferential direction of the tubular insulating substrate. 
     In still another preferred form of the present invention, the heat-generating resistance element has a series meandering portions arranged in a circumferential direction of the tubular insulating element and extending in an axial direction of the tubular insulating substrate. One endmost meandering portion is connected to the first lead wire, and another endmost meandering portion is connected to the second lead wire. The second lead wire is disposed between two adjacent ones of the meandering portions which are disposed between said two endmost meandering portions. The thus arranged heat-generating resistance element is also able to achieve heating with a highly uniform temperature distribution. 
     Preferably, the heat-generating resistance element is formed on an inner peripheral surface of the tubular insulating substrate, and the first and second lead wires are disposed on the inner peripheral surface of the tubular insulating substrate. By thus mounting the heat-generating resistance element and the first and second lead wires on the inner peripheral surface of the tubular insulating substrate, the tubular heater is allowed to have a circular cylindrical outer surface without projection, which is particularly advantageous when the heater is incorporated in a gas sensor. Furthermore, the lead wires disposed on the inner peripheral surface of the tubular insulating substrate does not increase an outside diameter of the tubular insulating substrate. 
     In one preferred form of the present invention, the tubular heater includes a dehumidifying agent incorporated therein. The tubular heater having such built-in dehumidifying agent is particularly useful when assembled in a gas sensor such as hydrogen sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain preferred embodiments of the present invention will be described in detail below, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1A  is a plan view of a tubular heater according to a first embodiment of the present invention; 
         FIG. 1B  is a cross-sectional view taken along line  1 B- 1 B of  FIG. 1A ; 
         FIG. 2  is a development view showing a pattern of a heat-generating resistance element of the tubular heater; 
         FIG. 3A  is a plan view of a tubular heater according to a second embodiment of the present invention; 
         FIG. 3B  is a cross-sectional view taken along line  3 B- 3 B of  FIG. 3A ; 
         FIG. 4  is a cross-sectional view of a gas sensor in which the tubular heater of the second embodiment is incorporated; 
         FIG. 5  is a view similar to  FIG. 2 , but showing a heat-generating resistance element having a different pattern according to a modification of the present invention; 
         FIG. 6  is a view similar to  FIG. 2 , but showing a heat-generating resistance element having a different pattern according to another modification of the present invention; 
         FIG. 7  is a schematic cross-sectional view of a conventional tubular heater incorporated in a gas sensor; and 
         FIG. 8  is a perspective view of a conventional plate-like heater. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and  FIGS. 1 and 2  in particular, there is shown a tubular heater  11  according to a first embodiment of the present invention. The tubular heater  11  is designed to heat a tubular object and develop heat when energized via two lead wires  13 ,  14 . 
     More particularly, the tubular heater  11  generally comprises an insulating tube  21  having a predetermined outside diameter D ( FIG. 1A ) and a predetermined axial length H ( FIG. 1B ), a continuous heat-generating resistance element  22  contained in the insulating tube  21 , and the lead wires  13 ,  14  connected to opposite ends of the heat-generating resistance element  22  and drawn from one end of the insulating tube  21  in a common axial direction (as indicated by a double-headed arrow shown in  FIG. 1B ). The lead wires  13 ,  14  are disposed in diametrically opposed relation to each other about a central axis C of the insulating tube  21 . In other words, the lead wires  13 ,  14  are spaced in a circumferential direction (indicated by the double-headed arrow shown in  FIG. 1A ) of the insulating tube  22  by an angle of 180-degrees. 
     The insulating tube  21  is composed of an outer insulating member  25  and an inner insulating member  26 , which are so configured as to jointly accommodate the heat-generating resistance element  22  and cover joint portions  28 ,  31  of the respective lead wires  13 ,  14  connected to the opposite ends of the heat-generating resistance element  22 . The outer insulating member  25  forms a tubular insulating substrate according to the present invention. The outer insulating member (tubular insulating substrate)  25  has an axial length Hu ( FIG. 1 ). 
     The outer and inner insulating members  25 ,  26  initially have elongated rectangular sheet-like configurations and after they are assembled together with the heat-generating resistance element  22  and the lead wires  13 ,  14  held therebetween, the outer and inner insulating members  25 ,  26  are rolled into a tubular form. By joining mating end edges  32 ,  33  ( FIG. 1   a ) of the tube, the insulating tube  21  is completed. Due to such forming process, the insulating tube  21  has an axial joint portion  34  ( FIG. 1A ). 
       FIG. 2  is a development view showing the outer insulating member (tubular insulating substrate)  25  and the heat-generating resistance element  22  formed, for example, by printing on an inner peripheral surface of the outer insulating member (tubular insulating substrate)  25 . The heat-generating resistance element  22  has a predetermined pattern. As shown in  FIG. 2 , when the insulating tube  21  ( FIG. 1A ) is in a developed state, the outside insulating member (tubular insulating substrate)  25  takes the form of a flat strip-shaped insulating substrate having one end edge (corresponding to one mating end edge  32  of the insulating tube  21 ) and an opposite end edge (corresponding to the other mating end edge  33  of the insulating tube  21 ). The strip-shaped insulating substrate  25  has a length L corresponding to a perimeter of the outer insulating member (tubular insulating substrate)  25 . The outer insulating member  25  is formed from a resinous material, preferably a highly thermal conductive resin. 
     As shown in  FIG. 2 , the first lead wire  13  is disposed adjacent to one end edge  32  of the strip-shaped insulating substrate  25 , and the second lead wire  14  is disposed on an intermediate portion between the one end edge  32  and the opposite end edge  33  of the strip-shaped insulating substrate  25 . More specifically, the second lead wire  14  is located at a position Pm which is spaced from the first lead wire  13  by a distance equal to one-half of the length L of the strip-shaped insulating substrate  25 . The strip-shaped insulating substrate  25  has a first surface region  25  extending between the one end edge  32  and a middle portion M of the strip-shaped insulating substrate  25 , and a second surface region  25   b  extending between the middle portion M and the opposite end edge  33  of the strip-shaped insulating substrate  25 . 
     The heat-generating resistance element  22  is formed by printing on one surface  36  of the strip-shaped insulating substrate  25 , which is corresponding to the inner peripheral surface  36  ( FIG. 1A ) of the tubular insulating substrate  25 . The heat-generating resistance element  22  has one end  38  connected to the joint portion  28  of the first lead wire  13 , and an opposite end  41  connected to the joint portion  31  of the second lead wire  14 . The heat-generating resistance element  22  has a pattern extending over the entire area of the surface  36  of the strip-shaped insulating substrate  25  such that the heat-generating resistance element  22  runs from the second lead wire  14  in a direction away from the first lead wire  13  and returns to the first lead wire  13 . Stated in other words, the pattern of the heat-generating resistance element  22  is arranged such that the heat-generating resistance element  22  runs from the first lead wire  13  in a direction toward the second lead wire  14 , further advances beyond the second lead wire  14 , and returns to the second lead wire  14 . 
     More specifically, the heat-generating resistance element  22  has a first meandering portion  44  formed on the first surface region  25   a  of the strip-shaped substrate  25  and extending in a lengthwise direction of the strip-shaped substrate  25  (corresponding to the circumferential direction of the tubular heater  11 ) between the joint portion  28  of the first lead wire  13  and the middle portion M of the strip-shaped insulating substrate  25 , a second meandering portion  46  formed on the second surface region  25   b  and extending in the lengthwise direction of the strip-shaped substrate  25  between the joint portion  31  of the second lead wire  14  and the opposite end edge  33  of the strip-shaped insulating substrate  25 , and a linear connecting portion  45  formed on the second surface region  25   b  and extending linearly between ends of the first and second meandering portions  44  and  46  which are located remote from the associated joint portions  28 ,  31 . The first meandering portion  44  has an amplitude approximately equal to the axial length Hu of the outer insulating member (insulating substrate)  25 , and the second meandering portion  46  has an amplitude substantially equal to one-half of the axial length Hu of the insulating substrate  25 . The linear connecting portion  45  extends linearly between the opposite end edge  33  and the middle portion M of the insulating substrate  25 . 
     The pattern of the heat-generating resistance element  22  is arranged such that a part of the heat-generating resistance element  22  which is formed on the first surface region  25   a  of the strip-shaped insulating substrate  25  is equal in length to a part of the heat-generating resistance element  22  which is formed on the second surface region  25   b  of the strip-shaped insulating substrate  25 . In the embodiment shown in  FIG. 2 , the length of the first meandering portion  44  of the heat-generating resistance element  22  is substantially equal to the sum of the length of the linear connecting portion  45  and the length of the second meandering portion  46  of the heat-generating resistance element  22 . 
     A mechanism to reduce regional temperature variations of the tubular heater  11  will be described below in conjunction with operation of the tubular heater  11  of the foregoing construction. When the tubular heater  11  is energized via the lead wires  13 ,  14 , the heat-generating resistance element  22  generates heat and increases its own temperature. In this instance, since the heat-generating resistance element  22  is arranged in a pattern distributed substantially uniformly over the entire surface (inner peripheral surface)  36  of the tubular insulating substrate  25 , regional temperature variations of the tubular heater become small. 
     The heat-generating resistance element  22  formed by printing on the surface  36  of the insulating substrate  25  generally has a heat-generating characteristic that the temperature becomes high at a portion which is located remote from each lead wire  13 ,  14 . This means that the temperature becomes relatively low at a portion located in the vicinity of each of the lead wires  13 ,  14 . This is because the lead sires  13 ,  14  and the joint portions  28 ,  31  thereof do not form a heat-generating element. To deal with this problem, according to the present invention, the first and second lead wires  13 ,  14  and their joint portions  28 ,  31  are disposed in diagrammatically opposed relation to each other about the central axis C ( FIG. 1A ) of the tubular heater  11 . By thus spacing the first and second lead wires  13 ,  14  in a circumferential direction of the tubular heater  11 , it is possible to reduce the temperature variations in the circumferential direction of the tubular heater  11 . 
     As understood from  FIG. 2 , when viewed in a direction from the one end edge  32  toward the opposite end edge of the insulating substrate  25 , the heater  11  includes non-heat-generating portions and heat-generating portions arranged alternatively. Stated more specifically, the first lead wire  13  including the joint portion  28 , which is located adjacent to the one end edge  32  of the insulating substrate  25 , forms a first non-heat-generating portion, and the first meandering portion  44  of the heat-generating resistance element  22 , which extends between the first lead wire  13  and the middle portion M of the insulating substrate  25 , forms a first heat-generating portion. Similarly, the second lead wire  14  including the joint portion  31 , which is located adjacent to the middle portion M of the insulating substrate  25 , forms a second non-heat-generating portion, and a combination of the linear connecting portion  45  and the second meandering portion  46 , which extends between the middle portion M and the opposite end edge  33  of the insulating substrate  25 , forms a second heat-generating portion. Since the first and second lead wires  13 ,  15  are spaced in the circumferential direction of the tubular heater  11  by an angle of 180-degrees, this arrangement can eliminate local concentration of the non-heat-generating portions (which may occur when the lead wires  13 ,  14  including their respective joint portions  28 ,  31  are disposed in lateral juxtaposition on one radial side of the central axis of the tubular heater). By thus arranging the lead wires  13 ,  14 , regional temperature variations or differences of the tubular heater  11  can be reduced. 
     Furthermore, since the first lead wire  13  disposed adjacent to the one end edge  32  of the insulating substrate  25  is also located near the linear connecting portion  45  and the second meandering portion  46  of the heat-generating resistance element  22 , heat from the linear connecting portion  45  and the second meandering portion  46  transfers to the joint portion  28  of the first lead wire  13 . As a result, temperature averaging is made between a temperature in the vicinity of the first lead wire  13  and a temperature in a central region  51  defined between the linear connecting portion  45  and the second meandering portion  46 , a temperature in the vicinity of the second lead wire  14 , and a temperature in a central region  48  defined by the first meandering portion  44 . With this temperature averaging, regional temperature variations of the tubular heater  11  can be reduced to a minimum. Furthermore, the insulating substrate (outer insulating member)  25  is formed from a highly thermal conductive resin and hence can efficiently transmit heat from the heat-generating resistance element  22  to an outer circumferential surface  53  ( FIGS. 1A and 1B ) of the tubular heater  11 . 
     The first and second lead wires  13 ,  14  are disposed on the inner peripheral surface  36  of the tubular insulating substrate (outer insulating member)  25 . This arrangement makes it possible to reduce the outside diameter D of the insulating tube  21 . Furthermore, since the first and second lead wires  13  and  14  drawn from one end of the tubular insulating substrate  25  are disposed in diametrically opposed relation to each other about the central axis of the tubular insulating substrate  25 , this arrangement can effectively preclude a short circuit between the lead wires  13 ,  14  which might otherwise occur during manufacture or assembly of the tubular heater  11 . 
       FIGS. 3A and 3B  are views similar to  FIGS. 1A and 1B , respectively, but showing a tubular heater  11 B according to a second embodiment of the present invention. The tubular heater  11 B is substantially the same in structure and function as the tubular heater  11  of the first embodiment with the exception that a dehumidifying agent  56  is mounted on an inner circumferential surface of the tubular heater  11 B, and the tubular heater  11 B is incorporated in a gas sensor  61  shown in  FIG. 4 . These parts which are similar or corresponding to those described above with reference to the first embodiment shown in  FIGS. 1A and 1B  are designated by the same reference characters, and further description thereof can be omitted. 
     As shown in  FIG. 4 , the gas sensor  61  is a hydrogen sensor designed to detect hydrogen gas flowing in the direction of arrow a 2 . The gas sensor  61  includes the tubular heater  11 B, a sensor element  62  disposed within a cylindrical detection chamber defined in the tubular heater  11 B, a printed circuit board  63  to which the lead wires  11   a ,  14  of the tubular heater  11 B are connected, and a case  64  configured to cover the printed circuit board  63  and the tubular heater  11 B. The dehumidifying agent  56  mounted on the inner circumferential surface  54  of the tubular heater  11 B defines part of the detection chamber and adsorbs fluid or moisture entering the detection chamber. 
     The tubular heater  11 B is provided to heat the detection chamber to thereby keep the detection chamber free from dew condensation. Since the dehumidifying agent  56  is mounted on the circumferential surface  54  of the tubular heater  11 B, it is readily possible to control the temperature and hence the moisture adsorbing capacity or power of the dehumidifying agent  56 . Furthermore, since the lead wires  13 ,  14  are disposed on the inner circumferential surface  54  of the insulating tube  21 , the insulating tube  21  is allowed to have a circular cylindrical outside surface. This will simplify the configuration of an outer cylindrical portion  66  of the gas sensor  61 , ensuring easy attachment of the gas sensor  61  to a vehicle body, for example. 
       FIG. 5  is a development view similar to  FIG. 2 , but showing a heat-generating resistance element  22 C having a pattern according to a first modification of the present invention. These parts which are similar or corresponding to those described above with reference to  FIG. 2  are designated by the same reference characters and no further description is needed. As shown in  FIG. 5 , the strip-shaped insulating substrate  25  has a first surface region  25   a  extending between one end edge  32  and a middle portion M of the strip-shaped insulating substrate  25 , and a second surface region  25   b  extending between the middle portion and the opposite end edge  33  of the strip-shaped insulating substrate  25 . 
     The heat-generating resistance element  22 C of a modified tubular heater  11 C includes a first meandering portion  71  formed on the first surface region  25   a  of the strip-shaped insulating substrate  25  and extending in a widthwise direction of the strip-shaped insulating substrate  25  (corresponding to the axial direction of the tubular heater  11 C) between the first lead wire  13  and the middle portion M of the strip-shaped insulating substrate  25 , and a second meandering portion  73  formed on the second surface region  25   b  of the strip-shaped insulating substrate  25  and extending in the widthwise direction of the strip-shaped insulating substrate  25  (corresponding to the axial direction of the tubular heater  11 C) between the second lead wire  14  and the middle portion M of the strip-shaped insulating substrate  25 . The first and second meandering portions  71 ,  73  have an amplitude approximately equal to one-half of the length L of the strip-shaped insulating substrate  25 . The second meandering portion  73  includes a longitudinally meandering section  73   a  extending in the lengthwise direction of the strip-shaped insulating substrate with an amplitude substantially equal to one-sixth of the width of the strip-shaped insulating substrate  25  (corresponding to the axial length Hu of the tubular insulating substrate  25 ). 
     The second lead wire  14  is disposed between the first and second meandering portions  71  and  73 . In a rolled or assembled state of the tubular heater  11 C, the first lead wire  13  is also disposed between the first and second meandering portions  71 ,  73 . The total length of the heat-generating resistance element  22 C is divided into two equal parts at the middle portion M of the strip-shaped insulating substrate  25 . This means that the length of the first meandering portion  71  formed on the first surface region  25   a  is equal to the length of the second meandering portion  73  formed on the second surface region  25 . 
     Operation and advantageous effects achieved by the modified tubular heater  11 C are substantially the same as those achieved by the tubular heater  11  of the first embodiment, and further description thereof can be omitted. 
       FIG. 6  is a development view similar to  FIG. 2 , but showing a heat-generating resistance element  22 D having a pattern according to a second modification of the present invention. These parts which are similar or corresponding to those described above with reference to  FIG. 2  are designated by the same reference characters and no further description is needed. As shown in  FIG. 6 , the heat-generating resistance element  22 D of a modified tubular heater  11 D includes a series of meandering portions (five in the illustrated embodiment)  81 - 85  arranged side by side along the length of a strip-shaped insulating substrate  25  and extending in a widthwise direction of the strip-shaped insulating substrate  25 . Each respective meandering portion  81 - 85  is integrally connected to an adjacent one of the meandering portions  81 - 85 , and the endmost two meandering portions  81  and  85  are connected to a first lead wire  13  and a second lead wire  14 , respectively. The second lead wire  14  is disposed between the third and fourth meandering portions  83  and  84  disposed between the two endmost meandering portions  81  and  85 . 
     The first to fourth meandering portions  81 - 84  have an amplitude nearly equal to one-ninth of the length L of the strip-shaped insulating substrate  25  (corresponding to the perimeter of the tubular insulating substrate  25 ), and the fifth meandering portion  85  has an amplitude nearly equal to one-sixth part of the length L of the strip-shaped insulating substrate  25 . The first and second meandering portions  81  and  82  and a major part of the third meandering portion  83  are formed on the first surface region  25   a  of the strip-shaped insulating substrate  25 , while the fourth and fifth meandering portions  84  and  85  and the remaining part of the third meandering portion  83  are formed on the second surface region  25   b  of the strip-shaped insulating substrate  25 . The total length of the heat-generating resistance element  22 D is halved at the middle portion M of the strip-shaped insulating substrate  25 . 
     Operation and advantageous effects achieved by the modified tubular heater  11 D are substantially the same as those achieved by the tubular heater  11  of the first embodiment, and further description thereof can be omitted. 
     It should be appreciated that the constructions, shapes, positional relationships have been explained above in relation to various examples only to the extent that the present invention can be appropriately understood and carried out, and that the numerical values and materials given above are just illustrative. Namely, the present invention should not be construed as limited to the above-described embodiment and examples and may be modified variously unless it departs from the technical scope indicated by the appended claims.