Abstract:
In a cooktop with a cooktop panel, in particular, made of glass ceramic, beneath which at least one heating element is disposed for heating up a cooking vessel to be placed on the cooktop panel, and with a temperature sensor for sensing temperature of the cooktop panel, in particular, the underside thereof within the heating element and is connected to a control unit for controlling the heating power of the heating element, the temperature sensor is connected to a heat-conducting element, which is in heat-conducting connection with the underside of the cooktop panel, and the element completely covers the temperature sensor in the upward direction toward the cooktop panel, to achieve sufficient measurement accuracy with a high standard of safety.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application is a continuation of copending International Application No. PCT/EP01/01384, filed Feb. 8, 2001, which designated the United States and was not published in English.  
         BACKGROUND OF THE INVENTION  
       Field of the Invention  
         [0002]    The present invention relates to a cooktop or hob with a cooktop panel, in particular, made of glass ceramic, beneath which at least one heating element is disposed for heating up a cooking vessel to be placed on the cooktop panel, and a temperature sensor for sensing the temperature of the cooktop panel, and also relates to a corresponding heating element and a suitable element.  
           [0003]    German Patent DE 37 03 768 C2, corresponding to U.S. Pat. No. 4,851,645 to Wolf et al., discloses a cooktop having a device for sensing the temperature of a glass-ceramic panel heated up by heating windings or halogen lamps with a temperature sensor. This sensor emits a signal corresponding to the temperature of the glass ceramic for a control circuit. The heating windings or halogen lamps are disposed in the interior space of a cup-like insulating base and heat up the glass-ceramic panel by direct radiation. The edge of the insulating base bears under resilient stress against the underside of the glass-ceramic panel, and the temperature sensor is disposed outside the interior space of the insulating base, but within the heating element. The temperature sensor is also in heat-conducting connection with the underside of the glass-ceramic panel, the temperature sensor being disposed in a receptacle in the edge of the insulating base. The receptacle is disposed at a distance x from the inner side of the edge of the insulating base, the minimum value of which is chosen such that the brief temperature changes arising when the heating windings or halogen lamps are switched on and off have only a negligible influence on the temperature sensor. The maximum value of the distance x is chosen such that the delay caused by the thermal conductivity of the glass-ceramic panel produces a small hysteresis in the control characteristic. Widths of from 3 mm to 6 mm have proven to be advantageous as the distance x. The temperature sensor is fitted in the receptacle that has been made or pressed into the upper side of the attachment that protrudes into the interior space of the insulating base, and is in heat-conducting connection with the underside of the glass-ceramic panel. The temperature sensor is held indirectly under resilient stress against the underside of the glass-ceramic panel, to keep the heat transfer resistance between the glass-ceramic panel and the temperature sensor small.  
           [0004]    Furthermore, European Patent Application EP 0 021 107 A1 discloses a heating element for a cooking unit with a temperature sensor. To maintain complete heating of the entire surface area of the heating element, and, nevertheless, couple the temperature sensor of the controller closely to the heating device, a heat-transfer element in the form of a metal sheet is used, the sheet being disposed between the heating elements and the glass-ceramic panel such that it partly covers the heated region, but protrudes from the heating element and is in connection there with the temperature sensor of the controller. The heat-transfer element is fastened by secure clamping on the edge of the shell carrying the heating device and normally bears against the underside of the glass-ceramic panel. An outer portion protrudes from the heat-sensing region of the heat-transfer element outward beyond the edge of the heating element. It is formed in one piece with the aforementioned region, is substantially parallel to the latter, but offset downward somewhat by a bend, so that the outer portion does not bear against the underside of the glass-ceramic panel. The sensor cell of the temperature sensor is pressed by a compression spring against the underside of the heat-transfer face of the heat-transfer element, which is supported on a holding mechanism that guides the sensor cell and is attached to the outer portion of the heat-transfer element. However, other types of sensor and ways of attaching it are also possible. For example, an electrical NTC or PTC sensor, which is pressed resiliently into contact or securely attached to the outer portion of the heat-transfer element, may also be used. The transfer element can be grounded if desired, providing protection against electric shock.  
           [0005]    Furthermore, U.S. Pat. No. 4,447,710 to McWilliams discloses a glass-ceramic cooktop in which an insulator on which a temperature sensor, for example, a thermocouple, is mounted is disposed in the edge region of the heating element. The thermocouple is kept in good thermal contact with the underside of the glass-ceramic panel by the insulating block.  
         SUMMARY OF THE INVENTION  
         [0006]    It is accordingly an object of the invention to provide a cooktop with temperature sensor and a corresponding heating element that overcome the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that have, at the same time, a high safety standard and good measuring accuracy.  
           [0007]    With the foregoing and other objects in view, there is provided, in accordance with the invention, a cooktop, including a cooktop panel having an underside, at least one heating element disposed beneath the cooktop panel for heating up a cooking vessel to be placed on the cooktop panel, the at least one heating element having an inside, a control unit electrically connected to the at least one heating element for controlling a heating power of the at least one heating element, a temperature sensor sensing a temperature of the underside of the cooktop panel within the at least one heating element, the temperature sensor electrically connected to the control unit, a heat-conducting element connected to the temperature sensor, the heat-conducting element heat-conductively connected to the underside of the cooktop panel and completely covering the temperature sensor in an upward direction toward the cooktop panel.  
           [0008]    The invention provides that the temperature sensor is connected to a heat-conducting element, which is in heat-conducting connection with the underside of the cooktop panel, and that the element completely covers the temperature sensor in the upward direction toward the cooktop panel. The heat-conducting element has the effect, on one hand, that the temperature sensor is thermally coupled well to the underside of the cooktop panel and, on the other hand, the safety regulations with respect to the 4 mm clearance and 8 mm leakage distance between live parts and the glass-ceramic cooktop panel are observed. Complex implementation of the temperature-measuring configuration, for example, using safety extra-low voltage technology, is not required.  
           [0009]    Preferably, the heat-conducting element completely covers the temperature sensor when viewed from the cooktop panel.  
           [0010]    In accordance with another feature of the invention, to improve the safety of the configuration, the element also covers the electrical leads of the temperature sensor in the upward direction toward the cooktop panel or when viewed from the cooktop panel.  
           [0011]    In accordance with a further feature of the invention, the element is formed in an approximately shroud-shaped manner, at least in the region of the temperature sensor, and has an upper wall and skirt-like side walls. In such a case, the 8 mm clearance and leakage distance can be maintained without the base area of the shroud having to be chosen to be too large. This is important, in particular, because the shroud together with the temperature sensor is preferably to be shielded from the thermal radiation of the heating device of the heating element, but the shielding region must not be chosen to be too large.  
           [0012]    In accordance with an added feature of the invention, to be able to cast the temperature sensor into the shroud easily, for example, with a high-temperature-resistant, heat-conducting ceramic adhesive, the shroud is configured such that it is at least partly closed on the bottom side.  
           [0013]    To allow the element to be fitted quickly and without any errors, the heat-conducting element is fastened, in particular, screwed, in the region of the outer circumferential wall of the heating element or of the insulating base directly or with the aid of an intermediate fitting part. In such a case, in particular, the intermediate fitting part is fastened in the bottom region of the heating element and extends into the region of the outer circumferential wall of the heating element, in which the element is, in turn, screwed to the intermediate fitting part. To allow good setting of the bearing pressure or bearing area of the element, and, consequently, inter alia, the thermal coupling of the element to the underside of the cooktop panel, the element can be screwed on the outer circumferential wall of the heating element at various heights.  
           [0014]    According to a preferred embodiment, the temperature sensor is fastened on the underside of the element. As a result, on one hand, a large planar resting area can be realized, to improve the heat conduction from the underside of the glass ceramic panel to the temperature sensor. On the other hand, the temperature sensor is mechanically protected better by the element of a larger surface area in the fitting process, for example, in the event of the element/temperature sensor unit falling down.  
           [0015]    To make fitting easier, the element may have a receiving portion for the temperature sensor and a fitting portion for the fastening of the element, in particular, on the heating element, the receiving portion being radially offset laterally with respect to the fitting portion. This is important, in particular, whenever the temperature sensor is to be fitted in the direct vicinity of a temperature limiter present at the heating element. This is because the temperature limiter restricts the fitting space in the region of the outer circumferential wall of the heating element; on the other hand, however, it is favorable if the various electrical connections of the temperature limiter and of the temperature sensor are as close together as technical safety considerations allow.  
           [0016]    The element is advantageously formed in at least two parts. A receiving part for the temperature sensor is, in such a case, of a softer material, to allow the receiving part to be geometrically shaped optimally, with specific regard to technical aspects of the application and safety. The rest of the element may be of another material, a spring material being suitable, in particular, to allow the element to be pressed in a defined manner against the underside of the glass-ceramic panel.  
           [0017]    In accordance with an additional feature of the invention, at least one of the heat-conducting element and shroud thereof is of a material selected from the group consisting of X7 steel and X10 steel at least in the region of the temperature sensor.  
           [0018]    In accordance with yet another feature of the invention, it is particularly favorable from technical aspects of production and assembly/fitting if the element is formed as a torsion spring, the torsion region of the spring element being provided substantially outside the heating element and, consequently, in a cooler region.  
           [0019]    In accordance with yet a further feature of the invention, the heat-conducting element is a spring element and is pressed in a region of the temperature sensor against the underside of the cooktop panel.  
           [0020]    According to a preferred embodiment, the element is formed such that it is electrically conductive and is grounded, to conform optimally to the safety regulations in a simple construction. Preferably, the heat-conducting element is grounded outside the at least one heating element.  
           [0021]    In accordance with yet an added feature of the invention, the heat-conducting element has an ohmic resistance less than approximately 0.1 ohm.  
           [0022]    In accordance with yet an additional feature of the invention, to obtain adequate measuring accuracy, both the temperature sensor and the element are adequately shielded by an insulator against thermal radiation emanating from a heating device of the heating element.  
           [0023]    In accordance with again another feature of the invention, to make fitting easier, and, in particular, for strain relief, the electrical lines of the temperature sensor are connected to a first connection portion of the element or a connection piece mounted there. In a corresponding way, the element may also have a second connection portion, to which a ground line of the element is connected.  
           [0024]    In accordance with again a further feature of the invention, the heat-conducting element has an inside and the temperature sensor is fastened to the inside of the heat-conducting element by casting.  
           [0025]    In accordance with again an added feature of the invention, the cooktop panel is of glass ceramic.  
           [0026]    In accordance with again an additional feature of the invention, the heat-conducting element is a removable part of the at least one heating element.  
           [0027]    With the objects of the invention in view, in a cooktop having a cooktop panel with an underside, at least one heating element disposed beneath the cooktop panel for heating up a cooking vessel to be placed on the cooktop panel, a control unit electrically connected to the at least one heating element for controlling a heating power of the at least one heating element, and a temperature sensor sensing a temperature of the underside of the cooktop panel within the at least one heating element, the temperature sensor electrically connected to the control unit, there is also provided a temperature sensor holder including a heat-conducting element connected to the temperature sensor, the heat-conducting element heat-conductively connected to the underside of the cooktop panel and completely covering the temperature sensor in an upward direction toward the cooktop panel.  
           [0028]    With the objects of the invention in view, in a cooktop having a cooktop panel with an underside, a control unit, and a temperature sensor electrically connected to the control unit and sensing a temperature of the cooktop panel, there is also provided a heater including at least one heating element having an inside, the at least one heating element disposed beneath the cooktop panel for heating up a cooking vessel to be placed on the cooktop panel and electrically connected to the control unit for controlling a heating power of the at least one heating element, the temperature sensor sensing a temperature of the underside of the cooktop panel within the at least one heating element, and a heat-conducting element connected to the temperature sensor, the heat-conducting element heat-conductively connected to the underside of the cooktop panel and completely covering the temperature sensor in an upward direction toward the cooktop panel.  
           [0029]    Other features that are considered as characteristic for the invention are set forth in the appended claims.  
           [0030]    Although the invention is illustrated and described herein as embodied in a cooktop with temperature sensor, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0031]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is a fragmentary, cross-sectional view through line I-I in FIG. 2 of a cooktop with a heating element according to the invention;  
         [0033]    [0033]FIG. 2 is a partial fragmentary, perspective view from above of a heating element according to the invention;  
         [0034]    [0034]FIG. 3 is an enlarged, perspective view from below of a heat-conducting element from FIGS. 1 and 2 without a temperature sensor;  
         [0035]    [0035]FIG. 4 is a simplified, fragmentary, cross-sectional view of a portion of a second embodiment of the heating element of FIGS. 1 and 2;  
         [0036]    [0036]FIG. 5 is a block circuit diagram of the cooktop according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a cooktop  1  has a cooktop panel  3  (FIG. 1), made, in particular, of glass ceramic. Provided beneath the cooktop panel  3 , in a conventional way, are various heating elements  5  of the cooktop, which are pressed in a conventional way (not shown) against the underside of the cooktop panel  3 . In the region of the heating element  5 , the cooktop panel  3  is usually decorated appropriately on its upper side. In such a heated region, a cooking vessel  6  can be placed. In the cold state, the bottom of the cooking vessel  6  often rests on the cooktop panel  3  only in an annular area in the edge region of the heating element  5 , while in the remaining central region of the bottom of the pot it is kept at a distance away from the panel by an air gap (FIG. 1). In the heated state, the air gap is reduced or ideally approaches zero as a result of the conventional thermally induced movement of the bottom of the pot. The heating element  5  has a dish-like sheet-metal cup  7 , in which a circular disk-shaped insulating panel  9  lies. Furthermore, an inner insulating ring  11  and an outer insulating ring  13  are provided within the sheet-metal cup  7 , on the insulating panel  9 , in a way corresponding to a two-circuit heating configuration. As a result, the interior space of the heating element  5  is separated into an inner heating region and an outer heating region, in which a strip heating conductor  15  respectively extends (FIG. 1, FIG. 2). In a conventional way, fastened in the region of the outer circumferential wall of the sheet-metal cup  7  is a heating-conductor connection part  17 , which, on one hand, is connected in a conducting manner to the strip heating conductors  15  and, on the other hand, can be connected to non-illustrated electrical supply lines of the cooktop  1  (FIG. 2). The heating element  5  also has a conventional temperature limiter  19 , the bar of which extends transversely over the heated region of the heating element. The connection block of the temperature limiter  19  has the conventional and customary, laterally brought-out flat contact pins for connection to the voltage supply line or to the heating-conductor connection part  17  of the heating element  5 . An insulating block  21  is disposed between the inner insulating ring and the outer insulating ring  13  in the region of the temperature limiter  19 . The insulating block  21  may serve the purpose of thermally shielding the temperature limiter  19  in the region of portions of the strip heating conductor  15  taken underneath the insulating block  21  with respect to such portions. A receiving depression has been milled into the edge region of the insulating block  21 , in the upper side of the insulating block  21 . In the depression, a heat-conducting element  25  is disposed with its element shroud  27  (FIGS. 1, 2,  3 ). It should be ensured, in this respect, that the shroud  27  does not rest directly on the bottom of the depression so that the shroud  27  can yield slightly in the event of the cooktop panel  3  being subjected to impact. The yielding allows damage to or breakage of the panel  3  to be avoided, in particular, if it is made of glass or glass-ceramic material.  
         [0038]    A PT-500 measuring sensor  29  is embedded with its sensor lines  30  in the receiving space formed by the element shroud  27 , by a non-illustrated temperature-resistant and heat-conducting ceramic adhesive, and is fastened and guided as such. The material of the element shroud  27  is X7 steel and the shroud  27  is configured in respect thereto as a bending part. The shroud material must have adequately good heat-conducting properties, must be able to deform well, as explained below, but be adequately stable mechanically in the entire temperature range of up to 350-400° C., and retain its properties even at these temperatures. From the portion of the element shroud  27  serving as a top wall there are two side walls  31  bent away downward substantially at right angles (FIG. 3). Likewise bent away at right angles with respect to the side walls  31 , bottom walls  33  delimit a base of the element shroud  27  that is open in a slit-shaped manner. At the end face, the receiving space of the shroud is closed by an end wall  35 , which is bent away at right angles from the top wall. It is ensured by the shroud-shaped configuration of the element  25  that the clearance and leakage distance from the live temperature sensor  29  prescribed by safety regulations are maintained in the event of breakage of the cooktop panel  3 , without the base area of the element  25  or of the shroud  27 , and, consequently, of the insulating block  21 , having to be made all that large. More precise details on the geometrical construction and configuration of the temperature sensor  29 , of the element  25 , and of the insulating block  21  are given in connection with the description of the second exemplary embodiment, sketched in FIG. 4. The shroud  27  is securely connected, preferably welded, to a steel shroud support  37 , of a substantially L-shaped construction. For such a purpose, the element shroud  27  is mounted on a connecting portion  39  of the shroud support  37  (FIG. 3). As a result, the top wall of the element shroud  27  is slightly elevated with respect to the upper side of the shroud support  37  and defines and delimits an area region A in which the element  25  bears in a heat-conducting manner against the underside of the cooktop panel  3  (FIGS. 1, 2,  4 ). The overlapping connection of the shroud  27  and shroud support  37  also increases the stability of the connection. While the shroud support  37  is of a material 0.8 mm thick, to conform to regulations for the plug-in grounding connections described below, the element shroud  27  is of thinner material, which additionally makes it easier to shape.  
         [0039]    The shroud support  37  merges in a resilient portion  41  with a fitting portion  43  (FIGS. 2, 3). The resilient portion  41  is disposed substantially outside the heated region of the heating element  5  or of the outer insulating ring  13 . The fitting portion  43  of the shroud support  37  has a fitting plate  45 , which is bent away downward at right angles and has fitting openings  47 . The fitting openings  47  allow the heat-conducting element  25  to be fastened adjustably in height on the outer circumferential wall of the sheet-metal cup  7  by an intermediate fitting part  48  (FIG. 2). For such a purpose, the intermediate fitting part  48  is, on one hand, screwed on the underside of the sheet-metal cup  7  in the base thereof (not shown). The part  48  extends in an approximately L-shaped manner from the base of the heating element up to its side wall  7 . In the side wall region, the heat-conducting element  25  is then screwed to (see, i.e., screw  50  in FIG. 3) the intermediate fitting part  48  and, consequently, the position of the heat-conducting element  25  can be fixed in a defined manner in terms of height. The configuration dispenses with the need for troublesome screwing openings in the side wall of the sheet-metal cup  7  and allows the openings that are always already present in the base of the sheet-metal cup to be used. Alternatively, the heat-conducting element  25  may, however, also be screwed to the outer wall of the sheet-metal cup  7  in the region of the fitting openings  47 . It is also possible to fasten in the fitting openings  47  a non-illustrated connection part, to which, on one hand, the electrical sensor lines  30  of the temperature sensor  29  can be connected, for example, can be plugged on, and to which, on the other hand, electrical connecting lines of a control unit  101  (FIG. 5) of the cooktop  1  are connected. This provides reliable strain relief for the sensor lines  30 . It is also to be ensured by the connection part that the electrical connections of the PT temperature sensor  29  are insulated from ground and from the grounded shroud support  37 . The temperature sensor and the sensor lines  30  are covered on the top side over their entire length by the heat-conducting element  25 . For better guidance of the lines  30 , they may be adhesively attached on the underside of the element  25  in the region of the shroud support  37  and/or be held by guiding elements formed on the support  37 . Furthermore, the fitting plate  45  has a flat pin  49 , on which a ground line  51  or its standardized AMP plug of the cooktop can be directly fitted. As a result, the heat-conducting element  25  is connected to ground potential. It must be ensured in such a case that the ohmic resistance of the element  25  lies at a value of 0.1 ohm or less, to be able to withstand a continuous current load of at least 25 A. Furthermore, the heat-conducting element  25  must also not be made too rigid, to allow it to yield suitably under mechanical loading or movement of the cooktop panel  3 . Otherwise, excessively rigid abutment of the element  25  or the element shroud  27  against the cooktop panel  3  would give rise to the risk of the cooktop panel flaking away on the underside of the panel  3  or possibly even of it breaking. It should also be noted that an improvement in the heat conduction from the underside of the cooktop panel  3  to the heat-conducting element  25  could be achieved if the intermediate spaces between the studs formed on the underside of the glass-ceramic panel are filled with a heat-conducting paste or a suitable adhesive.  
         [0040]    According to the second exemplary embodiment as shown in FIG. 4, the heat-conducting element  85 , shaped, for example, in the form of a shroud, has an element shroud  87 , which corresponds to that of the first exemplary embodiment. By contrast with the first exemplary embodiment, however, a fitting portion  89  of the shroud support  37  is not disposed radially offset laterally with respect to the receiving portion of the element shroud  87 . Rather, the fitting portion  89  extends vertically downward as a continuation of the element shroud  87  without any radial offset along the outer wall of the sheet-metal cup  7 . In FIG. 4 it is schematically represented in which area region A the heat-conducting element  85  is thermally in contact with the underside of the cooktop panel  3 . The size of the area is in this case approximately 50 to 100 mm 2 . It is also represented that the contact area A is approximately about 10 times larger than a base area B of the temperature sensor  29 . As a result, it is ensured, inter alia, that the temperature on the underside of the cooktop panel is not determined by the temperature sensor, as it were, at a point, but in an integrating manner over a relatively large area region. This is important, in particular, because the respective pan diameter and the nature of its bottom are not precisely known and, in addition, may vary from pan type to pan type. A minimum lateral distance a of the element  85  from the edge region of the insulating material  21  is about 8 mm. This provides an optimum geometry, which has the following advantages for the accurate control of the heating power or the temperature, in particular, in the case of frying of braising operations in pans  6  placed on the cooktop panel  3 . The temperature sensor  29  and the element shroud  27  are, on one hand, adequately shielded by the insulating block  21  against the thermal radiation emanating from the strip heating conductor  15 . On the other hand, the insulating block is still small enough to be able to avoid disadvantageous shadowing of the vessel bottom  6  during heating or frying/braising, and the resultant undesirably uneven heat distribution in the bottom of the pan. In particular, the heat-conducting element  25  is still thermally coupled adequately well to the region of the cooktop panel that is heated directly by the thermal radiation of the heating device  15 . This is achieved, moreover, in the case of the first and second exemplary embodiments, the temperature sensor  29  at the same time being covered with respect to the cooktop panel  3  by a grounded protective element  27 , while conforming to the 4 mm clearance and 8 mm leakage distance required by regulations. It is also achieved by the enlargement of the area thermally in contact with the underside of the cooktop panel  3  that, in spite of all assembly/fitting tolerances, adequately good thermal contact is established between the temperature sensor, of a smaller surface area, and the cooktop panel  3 . This is important, in particular, whenever a glass-ceramic cooktop panel  3  that is studded on the underside is used and the geometry of the studs is of the same order of magnitude as the temperature sensor  29 . The above statements concerning the shaping of the geometries, distances and relative sizes apply to all exemplary embodiments. If appropriate, the measuring area A is coupled by a high-temperature lubricant to the underside of the cooktop panel, which is, in particular, of glass-ceramic material, in order to achieve improved heat transfer and improved damping under impact loading.  
         [0041]    A block diagram that shows the most important components of the cooktop is schematically shown in FIG. 5. The control unit  101  regulates the heating power of the strip heating conductor  15  in a way corresponding to the measured values of the temperature sensor  29  to the setpoint value predetermined by an input unit  103 . This achieves the effect, in particular, that burning during frying/braising is virtually ruled out.