Abstract:
An operating control device, such as for an appliance, is disclosed comprising sensor elements with a capacitive sensing function, the elements being located underneath a metallic surface that is used as the operating field. An insulating layer is situated between the elements and the metallic surface. When pressure is applied to the metallic surface the capacitance of the sensor element is altered by capacitive coupling. This alteration of capacitance can be determined by a corresponding evaluation circuit or the like for detecting activation of the device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from PCT application no. PCT/EP2005/000186, filed Jan. 12, 2005, which is based on German Application No. 102004002825.7, which was filed Jan. 12, 2004, of which the contents of both are hereby incorporated by reference. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     The invention generally relates to an operating device for an electrical appliance, as well as to an electrical appliance incorporating such an operating device.  
         [0003]     Various types of operating devices for operating electrical appliances that are actuated by placing a finger or exerting a slight pressure with a finger are known. For example, EP 859 467 A and EP 859 468 A disclose an operating principle in which a capacitance change can be brought about in a capacitive sensor element, without exerting a significant pressure or a covered operating path. This can be evaluated as an actuation.  
         [0004]     It is also known from DE 198 11 372 A to place beneath a panel of an electrical appliance on the metal frame of a glass ceramic cooking hob or electrical cooktop, a switch having a piezoelectric element. If a finger is placed on the panel area and a certain pressure exerted, said pressure can be detected by the piezoelectric element and evaluated as an actuation. However, the use of piezoelectric elements may be disadvantageous in that piezoelectric elements are relatively expensive and in part mechanically sensitive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     Embodiments of the invention are described in greater detail hereinafter and the attached drawings, wherein:  
         [0006]      FIG. 1  illustrates a section through a sandwich structure of an operating device according to the invention;  
         [0007]      FIG. 2  illustrates a plan view of a layer of the operating device of  FIG. 1  with several juxtaposed sensor elements;  
         [0008]      FIG. 3  illustrates a plan view of the insulating layer; and  
         [0009]      FIG. 4  illustrates an oblique view of a hob, integrated into the frame of an operating device as in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0010]     One problem to be solved by the invention is to provide an aforementioned operating device and an electrical appliance equipped with such an operating device making it possible to avoid the prior art shortcomings and in particular so as to increase the range of uses of capacitive sensor elements, whilst simultaneously achieving robustness and reliability in daily use.  
         [0011]     This problem is solved by an operating device having the features of claim  1  and an electrical appliance having the features of claim  20 . Advantageous and preferred developments of the invention form the subject matter of further claims and are explained in greater detail hereinafter. By express reference the wording of the claims is made into part of the content of the description.  
         [0012]     According to one embodiment of the invention, the operating device has a variable-shaped or elastic operating field, which typically is a planar surface of a material that can be slightly deformed by a pressing action onto an area, which can be accomplished by the application of a finger with pressure thereto. The pressure triggering the operation is in the form of a force of a few Newtons. Under the operating field is a capacitive, preferably flat sensor element in the form of an electrically conductive, lower sensor surface. The latter forms a capacitor plate of the capacitive sensor element. Between the operating field and sensor element/sensor surface is a dielectric layer. The operating field is metallic or otherwise electrically conductive on the outside or top. If in the aforementioned manner, pressure is exerted on the operating field, the latter approaches the lower sensor surface, and the dielectric layer becomes thinner and causing a capacitance change on the sensor element. This capacitance change from an initial specific value is monitored by a corresponding evaluation circuitry. The capacitance change generally corresponds to a specific applied pressure causing a specific change of the operating field, which is detected as the user&#39;s operation of the appliance.  
         [0013]     An advantage of the invention is that a capacitive sensor element with the advantages of its control and evaluation can also be provided with a metallic surface of the associated operating field. It is in particular possible to integrate this into a larger metallic surface, whereof a non-separated area, which can be optically marked, is used as the operating field. It is in particular also possible to provide several different sensor elements or operating fields beneath a through metallic surface, without there being mutual influencing or interference between the sensor elements.  
         [0014]     The operating field or the material from which the sensor is made can be electrically conductive throughout. It is preferably constituted by a single material layer. If it is constructed in uninterrupted manner in the vicinity of the operating device or the one or more operating fields, it is possible to create an easily cleaned and relatively insensitive surface for the operating device. It is possible to construct an operating field from thin metal, for example having a metal foil with a thickness of less than 1 mm and in particular as thin as 100 μm. The material can be constituted by various metal alloys having an adequate elasticity for the indicated requirements. It is in particular possible to use high-grade steel or a chromium-plated metal which, as a result of their characteristics in connection with corrosion resistance, offer important advantages.  
         [0015]     The dielectric layer can be constructed in numerous different ways. The capacitance, as an absolute value on the sensor element, can be influenced by means of the associated dielectric constant. The main requirement made on the dielectric is compressibility as a result of the aforementioned, advantageous actuating force. It is also possible to use air as the dielectric layer. For this purpose, a certain spacing is advantageously left between the sensor surface or element and the operating field. The dielectric layer can be thinner than 1 mm and is preferably as thin as 100 μm.  
         [0016]     In a preferred embodiment of the invention, there are several sensor elements each having a sensor surface and operating fields over the same, as well as dielectric layers between the same, which are juxtaposed and have a certain mutual spacing. A simplification can be brought about if the operating fields are made from a material, particularly flat material, which is used throughout, such as a metal plate or foil. Advantageously, there are no openings, so that the surface is water-tight and can be easily cleaned. Interruptions, recesses, etc., in order to achieve the limited thickness for elasticity for compression purposes, are preferably provided on the underside, i.e. towards the sensor elements.  
         [0017]     A sensor surface can be surrounded at a certain distance, particularly a few millimetres, by a conductive surface, which forms a grounding surface. In the case of an operating device with several juxtaposed sensor elements, it is advantageous to provide a common, electrically connected or continuous grounding surface. During the manufacturing process, this surface can be formed from the same metal layer as the sensor surface faces and electrical separation is brought about by appropriate structure. In certain circumstances, a spacing layer can be provided in order to separate the sensor surface and any possible grounding surface from an electrically conductive underside of the operating fields, as well as to create space between the sensor surface and operating field for the dielectric layer. The latter should comprise insulating flat material, which can also be applied as a workable layer. Recesses can be provided in the vicinity of the sensor surfaces, so as to form the air dielectric layer. It is alternatively possible to form the spacing layer from a material having a suitable dielectric constant and appropriate thickness, which is then located between the sensor surface and operating field and which must be adequately compressible.  
         [0018]     According to a further development of the invention, the operating field can be made from a flat material or cover, which is considerably larger than the operating field. This makes it possible to construct an operating device, such as an electrical appliance, with one or more sensor elements and therefore also one or more operating fields in a cover or side wall. Thus, an operating device constructed in this way can be integrated into such a wall and, in particular, also into an electrical appliance. It is possible make the cover or its material thinner in the vicinity of the operating field than in other areas. To this end, it is possible to make material recesses on the underside by material-removing methods or by deforming methods, such as punching or stamping. Thus, a wall which offers a hold or protection can be made thinner in certain zones or areas, in order to locate the operating fields of an operating device. One possibility for such a cover is a panel of an electrical appliance, for example a front panel or frame.  
         [0019]     An electrical appliance according to the invention has at least one operating device, which corresponds to one of the above-described constructions. For this purpose, the electrical appliance advantageously has an operating area, at which the operating device is located with the one or more operating fields. For example, the electrical appliance can be a cooktop or stove with a side part, which on at least one side can be in the form of a panel or an all-round frame. The operating device is integrated into said side part or frame, for example with sensor elements located beneath the side part or frame.  
         [0020]     These and further features can be gathered from the claims, description and drawings and the individual features, both singly or in the form of subcombinations, can be implemented in an embodiment of the invention and in other fields and can represent advantageous, independently protectable constructions for which protection is claimed here. The subdivision of the application into individual sections and also the subheadings in no way restrict the general validity of the statements made thereunder.  
         [0021]      FIG. 1  diagrammatically shows in section an operating device  11  having an operating field  13  on which is placed a finger  14  exerting a slight pressure for operation or triggering an operation. On the top surface of the metal surface  25  near the operating field  13 , optically detectable markings (e.g., indicia) such as inscriptions, impressions, etc may be indicated. There can also be a corresponding surface structuring, for example using symbols by means of depressions or elevations.  
         [0022]     The operating device  11  has a sandwich structure, which will be explained hereinafter. A metal coating  17  is applied to a printed circuit board  15  as a support. The metal coating  17  is subdivided into sensor elements  17   a , whose flat shape with connection can be seen in the plan view of  FIG. 2  and which form the aforementioned sensor surfaces. In place of a square shaping of the sensor elements  17   a , it is obviously also possible to use round, oval and elongated shapes. The sensor elements  17   a  are worked out of the metal coating  17  through frame cutouts  18 , whose corresponding structure is also visible in  FIG. 2 . The surrounding metal surface  17   b  forms a type of grounding surface. The metal coating  17  can for example be applied as a copper coating using conventional methods to the circuit board  15 . Structuring the shapes can also take place in known manner.  
         [0023]     To the metal coating  17  is applied an insulating layer  19  with recesses  20 , which correspond in shape to the sensor elements  17   a , but are somewhat larger so as to surround the sensor elements like the frame cutouts  18 . The insulating layer  19  can be a plastic film, for example a polyester film. It can be self-adhesive on one or both sides so as to make it easier to produce the structure.  
         [0024]     A metal surface  25  is applied to the insulating layer  19  and can be constituted by a high-grade steel foil, a thin high-grade steel plate, etc., which more particularly when the surface is exposed so as to form the operating field  13 , can be cleaned and cared for in an advantageous, practical manner.  
         [0025]     At least in the vicinity of operating device  11  or operating fields  13 , the metal surface  25  has a uniform thickness throughout. In alternative constructions, a thicker metal layer or plate can be used and is thinned out over the operating fields  13 .  
         [0026]     A gap, filled with an air layer  22 , is formed between metal foil  25  and sensor element  17   a  and is represented as an air filling by dotted lines. The air layer  22  forms a dielectric between the metallic operating field  13  and sensor element  17   a  and its function will be described in greater detail hereinafter. In place of an air layer  22  as the dielectric, it is also possible to use some other dielectric, provided that it is compressible or can be compressed in order to bring the operating field  13  closer to the sensor element  17   a . This sag of the operating field  13  is illustrated in the vicinity of finger  14  by the downwardly curved broken line.  
         [0027]     A terminal  28  emanating from the downwardly projecting ends of the sensor elements  17  in  FIG. 2  leads to the sensor element  17   a , for example for the control circuitry or evaluation circuitry of operating device  11 . The metal surface  25 , as illustrated in broken line form, can be connected to ground  29 . However, this may not be necessary, and if so, then there is only a capacitive coupling to sensor element  17   a  or the surrounding surface  17   b  of metal coating  17 .  
         [0028]      FIG. 4  shows a cooktop  31 , such as those incorporating a glass ceramic heating elements. In conventional manner, the latter has an all-round frame  33 , which is constructed on its left-hand, lower side as a front panel directed towards a user. The frame  33  has various operating fields  13 , namely to the far left, an ON-OFF switch and in the central area, a power adjusting switch for the individual hotplates and which are diagrammatically represented in circular form. Through pressure on the operating fields  13  it is possible to initiate the associated function for each heating element. In this construction, an operating device similar to that of  FIG. 1  is located beneath the frame  33 . The surface of the frame  33  can be made from a metal coating, plate or foil throughout and corresponds to the metal surface  25  of  FIG. 1 . As shown in  FIG. 1 , the sandwich structure is only provided below the operating fields  13 . In the remaining area of the frame  33 , the metal can be solidly lined.  
         [0029]     Alternatively, the frame can be made from an metallic flat material that is several millimeters thick. The material may be thinned out from below in the vicinity of operating fields  13  and an associated operating device of  FIG. 1  can be housed underneath in such thinned out areas. As an alternative to thinning out a thicker metal coating from below, there can also be a thinning out from above. In this case a resulting depression could be used as a structuring for a precise application of a finger or the like.  
         [0030]     In connection with the dimensions in  FIG. 1 , it is pointed out that these can be varied. The thicknesses of the individual layers can be varied as advantageous for circuit board  15 . The metal coating  17  can be of various thickness, for example approximately 0.1 mm, such as is conventional for copper-coated circuit boards or the like. The insulating layer can also be 0.1 mm thick. The metal surface  25  above it has been used in tests with a thickness of 0.2 mm in the form of a high-grade steel foil and good results have been obtained. The exaggerated sag illustrated by the broken lines in  FIG. 1  with respect to the metal surface  25  can be in the μm range, for example 10 μm or less. In the case of a test structure with a 0.2 mm thick high-grade steel foil as metal foil  25 , a weight of 230 grams gave rise to a sag such that it was possible to detect a capacitance change of 18% on sensor element  17   a . The surface of a sensor element  17   a  or operating field is advantageously approximately 1 to 3 cm per side. This leads to a relatively well defined application surface, together with the resulting sag, on applying a finger  14 .  
         [0000]     Function  
         [0031]     Fundamentally, the operating device  11  according to the invention is based on the evaluation of a capacitive sensor element  17   a , a capacitance change resulting from the operation. An example of an evaluating circuit for such capacitive sensor elements, but in the case of a somewhat differently constructed sensor element, is known from EP 859 468 A. This evaluating circuit can also be used here in principle.  
         [0032]     Due to the compressive force on applying a finger  14  to operating field  13 , there is a certain sagging or deformation of the latter or of the associated metal surface  25 . This modifies the spacing between sensor element  17   a , which functionally represents a capacitor surface, and the metal surface  25  above it. This leads to a clear capacitance change on sensor element  17   a  and said change can be detected and evaluated by the associated circuitry. As a result of the interposed dielectric  22 , the effect can be intensified by increasing the capacitance and in this way detection of the change is improved.  
         [0033]     Another advantage of determining the capacitance change as a relative change and not only as the absolute quantity, is avoiding false detections resulting as a function of any time-based downward sagging of the metal surface  25  and the resulting change to the absolute capacitance.  
         [0034]     Thus, the invention creates an operating device, which has a capacitive sensor element or capacitive operating principle. It is simultaneously possible to use particularly high-grade steel surfaces combined therewith on the operating fields for the operating device metal surfaces. These are particularly advantageous for practical reasons as such steel surfaces occur in numerous electrical appliances, particularly domestic appliances. Only a slight pressure with the finger is needed for operating purposes. In addition, very short operating paths are sufficient, but would not be sufficient as the switching path for mechanical switches.