Tubular heater

A tubular heater includes a continuous heat-generating resistance element formed in a predetermined pattern on one surface of a tubular insulating substrate, and first and second lead wires connected opposite ends of the heat-generating resistance element and 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.

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 inFIG. 7hereof, includes a tubular body201forming a ceramic heater incorporated in a gas sensor202. When energized via conducting wires203,203, the ceramic heater201generates heat to thereby prevent dew condensation from occurring in a detection chamber of the gas sensor202. InFIG. 7, the conducting wires203,203are shown as if they are disposed in opposed relation to each other. However, this is only for purposes of illustration. In reality, the conducting wires203,203are disposed side by side or in lateral juxtaposition on one radial side of a central axis of the tubular ceramic heater201.

The thus arranged ceramic heater201is not fully satisfactory in that the temperature in the vicinity of the two juxtaposed conducing wires203,203is relatively low, while the temperature at a portion diametrically opposed to the two juxtaposed conducting wires203,203is relatively high. Thus the prior ceramic heater201cannot generate heat with a uniform temperature distribution. Furthermore, the conducting wires203,203are disposed side by side and, hence, they are likely to cause a short circuit during manufacture or assembly of the ceramic heater201.

FIG. 8hereof shows a thermosensor221disclosed in JP 2005-332628A. The thermosensor221includes a rectangular printed-circuit board222on which a resistance pattern223and a connection pattern224are formed by printing. Core wires225are connected to the connection pattern224. The connection pattern224facilitates easy connection of the core wires225to the thermosensor221. The resistance pattern223can be used as a resistance pattern of a heater in which instance the core wires225are connected directly to the resistance pattern223.

When the heater having the resistance pattern223is energized via the core wires225, the temperature of the resistance pattern223is relatively high at a central portion thereof and relatively low in the vicinity of the core wires225. 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings andFIGS. 1 and 2in particular, there is shown a tubular heater11according to a first embodiment of the present invention. The tubular heater11is designed to heat a tubular object and develop heat when energized via two lead wires13,14.

More particularly, the tubular heater11generally comprises an insulating tube21having a predetermined outside diameter D (FIG. 1A) and a predetermined axial length H (FIG. 1B), a continuous heat-generating resistance element22contained in the insulating tube21, and the lead wires13,14connected to opposite ends of the heat-generating resistance element22and drawn from one end of the insulating tube21in a common axial direction (as indicated by a double-headed arrow shown inFIG. 1B). The lead wires13,14are disposed in diametrically opposed relation to each other about a central axis C of the insulating tube21. In other words, the lead wires13,14are spaced in a circumferential direction (indicated by the double-headed arrow shown inFIG. 1A) of the insulating tube22by an angle of 180-degrees.

The insulating tube21is composed of an outer insulating member25and an inner insulating member26, which are so configured as to jointly accommodate the heat-generating resistance element22and cover joint portions28,31of the respective lead wires13,14connected to the opposite ends of the heat-generating resistance element22. The outer insulating member25forms a tubular insulating substrate according to the present invention. The outer insulating member (tubular insulating substrate)25has an axial length Hu (FIG. 1).

The outer and inner insulating members25,26initially have elongated rectangular sheet-like configurations and after they are assembled together with the heat-generating resistance element22and the lead wires13,14held therebetween, the outer and inner insulating members25,26are rolled into a tubular form. By joining mating end edges32,33(FIG. 1a) of the tube, the insulating tube21is completed. Due to such forming process, the insulating tube21has an axial joint portion34(FIG. 1A).

FIG. 2is a development view showing the outer insulating member (tubular insulating substrate)25and the heat-generating resistance element22formed, for example, by printing on an inner peripheral surface of the outer insulating member (tubular insulating substrate)25. The heat-generating resistance element22has a predetermined pattern. As shown inFIG. 2, when the insulating tube21(FIG. 1A) is in a developed state, the outside insulating member (tubular insulating substrate)25takes the form of a flat strip-shaped insulating substrate having one end edge (corresponding to one mating end edge32of the insulating tube21) and an opposite end edge (corresponding to the other mating end edge33of the insulating tube21). The strip-shaped insulating substrate25has a length L corresponding to a perimeter of the outer insulating member (tubular insulating substrate)25. The outer insulating member25is formed from a resinous material, preferably a highly thermal conductive resin.

As shown inFIG. 2, the first lead wire13is disposed adjacent to one end edge32of the strip-shaped insulating substrate25, and the second lead wire14is disposed on an intermediate portion between the one end edge32and the opposite end edge33of the strip-shaped insulating substrate25. More specifically, the second lead wire14is located at a position Pm which is spaced from the first lead wire13by a distance equal to one-half of the length L of the strip-shaped insulating substrate25. The strip-shaped insulating substrate25has a first surface region25extending between the one end edge32and a middle portion M of the strip-shaped insulating substrate25, and a second surface region25bextending between the middle portion M and the opposite end edge33of the strip-shaped insulating substrate25.

The heat-generating resistance element22is formed by printing on one surface36of the strip-shaped insulating substrate25, which is corresponding to the inner peripheral surface36(FIG. 1A) of the tubular insulating substrate25. The heat-generating resistance element22has one end38connected to the joint portion28of the first lead wire13, and an opposite end41connected to the joint portion31of the second lead wire14. The heat-generating resistance element22has a pattern extending over the entire area of the surface36of the strip-shaped insulating substrate25such that the heat-generating resistance element22runs from the second lead wire14in a direction away from the first lead wire13and returns to the first lead wire13. Stated in other words, the pattern of the heat-generating resistance element22is arranged such that the heat-generating resistance element22runs from the first lead wire13in a direction toward the second lead wire14, further advances beyond the second lead wire14, and returns to the second lead wire14.

More specifically, the heat-generating resistance element22has a first meandering portion44formed on the first surface region25aof the strip-shaped substrate25and extending in a lengthwise direction of the strip-shaped substrate25(corresponding to the circumferential direction of the tubular heater11) between the joint portion28of the first lead wire13and the middle portion M of the strip-shaped insulating substrate25, a second meandering portion46formed on the second surface region25band extending in the lengthwise direction of the strip-shaped substrate25between the joint portion31of the second lead wire14and the opposite end edge33of the strip-shaped insulating substrate25, and a linear connecting portion45formed on the second surface region25band extending linearly between ends of the first and second meandering portions44and46which are located remote from the associated joint portions28,31. The first meandering portion44has an amplitude approximately equal to the axial length Hu of the outer insulating member (insulating substrate)25, and the second meandering portion46has an amplitude substantially equal to one-half of the axial length Hu of the insulating substrate25. The linear connecting portion45extends linearly between the opposite end edge33and the middle portion M of the insulating substrate25.

The pattern of the heat-generating resistance element22is arranged such that a part of the heat-generating resistance element22which is formed on the first surface region25aof the strip-shaped insulating substrate25is equal in length to a part of the heat-generating resistance element22which is formed on the second surface region25bof the strip-shaped insulating substrate25. In the embodiment shown inFIG. 2, the length of the first meandering portion44of the heat-generating resistance element22is substantially equal to the sum of the length of the linear connecting portion45and the length of the second meandering portion46of the heat-generating resistance element22.

A mechanism to reduce regional temperature variations of the tubular heater11will be described below in conjunction with operation of the tubular heater11of the foregoing construction. When the tubular heater11is energized via the lead wires13,14, the heat-generating resistance element22generates heat and increases its own temperature. In this instance, since the heat-generating resistance element22is arranged in a pattern distributed substantially uniformly over the entire surface (inner peripheral surface)36of the tubular insulating substrate25, regional temperature variations of the tubular heater become small.

The heat-generating resistance element22formed by printing on the surface36of the insulating substrate25generally has a heat-generating characteristic that the temperature becomes high at a portion which is located remote from each lead wire13,14. This means that the temperature becomes relatively low at a portion located in the vicinity of each of the lead wires13,14. This is because the lead sires13,14and the joint portions28,31thereof do not form a heat-generating element. To deal with this problem, according to the present invention, the first and second lead wires13,14and their joint portions28,31are disposed in diagrammatically opposed relation to each other about the central axis C (FIG. 1A) of the tubular heater11. By thus spacing the first and second lead wires13,14in a circumferential direction of the tubular heater11, it is possible to reduce the temperature variations in the circumferential direction of the tubular heater11.

As understood fromFIG. 2, when viewed in a direction from the one end edge32toward the opposite end edge of the insulating substrate25, the heater11includes non-heat-generating portions and heat-generating portions arranged alternatively. Stated more specifically, the first lead wire13including the joint portion28, which is located adjacent to the one end edge32of the insulating substrate25, forms a first non-heat-generating portion, and the first meandering portion44of the heat-generating resistance element22, which extends between the first lead wire13and the middle portion M of the insulating substrate25, forms a first heat-generating portion. Similarly, the second lead wire14including the joint portion31, which is located adjacent to the middle portion M of the insulating substrate25, forms a second non-heat-generating portion, and a combination of the linear connecting portion45and the second meandering portion46, which extends between the middle portion M and the opposite end edge33of the insulating substrate25, forms a second heat-generating portion. Since the first and second lead wires13,15are spaced in the circumferential direction of the tubular heater11by an angle of 180-degrees, this arrangement can eliminate local concentration of the non-heat-generating portions (which may occur when the lead wires13,14including their respective joint portions28,31are disposed in lateral juxtaposition on one radial side of the central axis of the tubular heater). By thus arranging the lead wires13,14, regional temperature variations or differences of the tubular heater11can be reduced.

Furthermore, since the first lead wire13disposed adjacent to the one end edge32of the insulating substrate25is also located near the linear connecting portion45and the second meandering portion46of the heat-generating resistance element22, heat from the linear connecting portion45and the second meandering portion46transfers to the joint portion28of the first lead wire13. As a result, temperature averaging is made between a temperature in the vicinity of the first lead wire13and a temperature in a central region51defined between the linear connecting portion45and the second meandering portion46, a temperature in the vicinity of the second lead wire14, and a temperature in a central region48defined by the first meandering portion44. With this temperature averaging, regional temperature variations of the tubular heater11can be reduced to a minimum. Furthermore, the insulating substrate (outer insulating member)25is formed from a highly thermal conductive resin and hence can efficiently transmit heat from the heat-generating resistance element22to an outer circumferential surface53(FIGS. 1A and 1B) of the tubular heater11.

The first and second lead wires13,14are disposed on the inner peripheral surface36of the tubular insulating substrate (outer insulating member)25. This arrangement makes it possible to reduce the outside diameter D of the insulating tube21. Furthermore, since the first and second lead wires13and14drawn from one end of the tubular insulating substrate25are disposed in diametrically opposed relation to each other about the central axis of the tubular insulating substrate25, this arrangement can effectively preclude a short circuit between the lead wires13,14which might otherwise occur during manufacture or assembly of the tubular heater11.

FIGS. 3A and 3Bare views similar toFIGS. 1A and 1B, respectively, but showing a tubular heater11B according to a second embodiment of the present invention. The tubular heater11B is substantially the same in structure and function as the tubular heater11of the first embodiment with the exception that a dehumidifying agent56is mounted on an inner circumferential surface of the tubular heater11B, and the tubular heater11B is incorporated in a gas sensor61shown inFIG. 4. These parts which are similar or corresponding to those described above with reference to the first embodiment shown inFIGS. 1A and 1Bare designated by the same reference characters, and further description thereof can be omitted.

As shown inFIG. 4, the gas sensor61is a hydrogen sensor designed to detect hydrogen gas flowing in the direction of arrow a2. The gas sensor61includes the tubular heater11B, a sensor element62disposed within a cylindrical detection chamber defined in the tubular heater11B, a printed circuit board63to which the lead wires11a,14of the tubular heater11B are connected, and a case64configured to cover the printed circuit board63and the tubular heater11B. The dehumidifying agent56mounted on the inner circumferential surface54of the tubular heater11B defines part of the detection chamber and adsorbs fluid or moisture entering the detection chamber.

The tubular heater11B is provided to heat the detection chamber to thereby keep the detection chamber free from dew condensation. Since the dehumidifying agent56is mounted on the circumferential surface54of the tubular heater11B, it is readily possible to control the temperature and hence the moisture adsorbing capacity or power of the dehumidifying agent56. Furthermore, since the lead wires13,14are disposed on the inner circumferential surface54of the insulating tube21, the insulating tube21is allowed to have a circular cylindrical outside surface. This will simplify the configuration of an outer cylindrical portion66of the gas sensor61, ensuring easy attachment of the gas sensor61to a vehicle body, for example.

FIG. 5is a development view similar toFIG. 2, but showing a heat-generating resistance element22C 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 toFIG. 2are designated by the same reference characters and no further description is needed. As shown inFIG. 5, the strip-shaped insulating substrate25has a first surface region25aextending between one end edge32and a middle portion M of the strip-shaped insulating substrate25, and a second surface region25bextending between the middle portion and the opposite end edge33of the strip-shaped insulating substrate25.

The heat-generating resistance element22C of a modified tubular heater11C includes a first meandering portion71formed on the first surface region25aof the strip-shaped insulating substrate25and extending in a widthwise direction of the strip-shaped insulating substrate25(corresponding to the axial direction of the tubular heater11C) between the first lead wire13and the middle portion M of the strip-shaped insulating substrate25, and a second meandering portion73formed on the second surface region25bof the strip-shaped insulating substrate25and extending in the widthwise direction of the strip-shaped insulating substrate25(corresponding to the axial direction of the tubular heater11C) between the second lead wire14and the middle portion M of the strip-shaped insulating substrate25. The first and second meandering portions71,73have an amplitude approximately equal to one-half of the length L of the strip-shaped insulating substrate25. The second meandering portion73includes a longitudinally meandering section73aextending 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 substrate25(corresponding to the axial length Hu of the tubular insulating substrate25).

The second lead wire14is disposed between the first and second meandering portions71and73. In a rolled or assembled state of the tubular heater11C, the first lead wire13is also disposed between the first and second meandering portions71,73. The total length of the heat-generating resistance element22C is divided into two equal parts at the middle portion M of the strip-shaped insulating substrate25. This means that the length of the first meandering portion71formed on the first surface region25ais equal to the length of the second meandering portion73formed on the second surface region25.

Operation and advantageous effects achieved by the modified tubular heater11C are substantially the same as those achieved by the tubular heater11of the first embodiment, and further description thereof can be omitted.

FIG. 6is a development view similar toFIG. 2, but showing a heat-generating resistance element22D 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 toFIG. 2are designated by the same reference characters and no further description is needed. As shown inFIG. 6, the heat-generating resistance element22D of a modified tubular heater11D includes a series of meandering portions (five in the illustrated embodiment)81-85arranged side by side along the length of a strip-shaped insulating substrate25and extending in a widthwise direction of the strip-shaped insulating substrate25. Each respective meandering portion81-85is integrally connected to an adjacent one of the meandering portions81-85, and the endmost two meandering portions81and85are connected to a first lead wire13and a second lead wire14, respectively. The second lead wire14is disposed between the third and fourth meandering portions83and84disposed between the two endmost meandering portions81and85.

The first to fourth meandering portions81-84have an amplitude nearly equal to one-ninth of the length L of the strip-shaped insulating substrate25(corresponding to the perimeter of the tubular insulating substrate25), and the fifth meandering portion85has an amplitude nearly equal to one-sixth part of the length L of the strip-shaped insulating substrate25. The first and second meandering portions81and82and a major part of the third meandering portion83are formed on the first surface region25aof the strip-shaped insulating substrate25, while the fourth and fifth meandering portions84and85and the remaining part of the third meandering portion83are formed on the second surface region25bof the strip-shaped insulating substrate25. The total length of the heat-generating resistance element22D is halved at the middle portion M of the strip-shaped insulating substrate25.

Operation and advantageous effects achieved by the modified tubular heater11D are substantially the same as those achieved by the tubular heater11of 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.