BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for determining the location of cooking utensils on a cooking hob comprising a plurality of electrically powered thermal cells (resistors) distributed below a heat-resistant surface (for example of glass ceramic) on which at least one cooking utensil for the heat treatment (for example cooking, heating or thawing) of a food contained therein, the thermal cells being disposed in matrix formation. 
     2. Description of the Related Art 
     On these cooking hobs, known as high versatility hobs, the cooking utensil or utensils can be located at any desired point on them, for example depending on the space available, on the user&#39;s operating comfort, or even purely randomly. The heating elements which have to be operated depend on the position of the cooking utensil or utensils. Information relative to their position hence constitutes a basic element for the operation of a cooking hob of the considered type. 
     In WO 97/19298, information concerning this position is obtained by means for monitoring the thermal load associated with each of the heating elements. The drawback of such a solution is that it requires electrodes or similar means with relative cabling which, being located in proximity to the heating elements, is subjected to high temperature, to resist which it must be of dedicated type, and hence specific. To this must be added the fact that the large number of components and the complexity of the cabling represent costs which negatively affect the final cost of the product. 
     In co-pending U.S. patent application Ser. No. 09/981,035, filed Oct. 17, 2001, by Davide Gerola and Cristiano Pastore and assigned to the assignee of this application, relating to a cooking hob of matrix type describes, by way of example, a method for identifying the location of cooking utensils using a video camera which frames the cooking hob, and a touch-screen on which the cooking hob and the cooking utensils disposed on them appear. By touching with the finger the reproductions of these utensils on the screen, the user selects those heating elements underlying the cooking utensils. This although meritorious solution is complex and hence relatively costly besides having the drawback that the video camera is exposed to smoke and steam which can negatively affect its operability. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a device for determining the position, and the form and size (with suitable resolution), of cooking utensils placed on a cooking hob comprising a matrix arrangement of a plurality of heating elements, in order to power those which effectively need to be powered, i.e. those below the utensil, the device being simple, reliable and economically advantageous by comprising components intended for, or already present for, other purposes. 
     This and further objects which will be more apparent from the ensuing detailed description are attained by a determination device in accordance with the teachings of the accompanying claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more apparent from the detailed description of preferred embodiments thereof given hereinafter by way of non-limiting example and illustrated in the accompanying drawings, in which: 
     FIG. 1 is a basic schematic perspective view of a preferred embodiment of the determination device of the invention; 
     FIG. 2 is a schematic view of the device of the invention with its electrical-electronic circuitry, which also relates to the cooking hob; 
     FIG. 2A shows a practical embodiment of the device of the invention; and 
     FIG. 3 shows the basic scheme of a variant of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the figures, the reference numeral  1  indicates overall a schematically reproduced high-versatility cooking hob which, in conventional manner and as apparent for example from the aforesaid co-pending US patent application, comprises a conventional glass ceramic plate  2  on which conventional cooking utensils (saucepans, pans, frying pans, etc.) rest. Below the plate there are provided a plurality of heating elements  3 , represented by metal resistors disposed for example in spiral arrangement, distributed such as to overall cover the maximum useful surface of the glass ceramic plate  2 . The heating elements are arranged in matrix formation (as best seen from FIG.  2 ), of which conceptually each heating element can be considered an individually energizable “thermal cell”, by which definition, i.e. “thermal cell” they will be identified hereinafter. 
     Groups of cells  3  can be energized, where each group is dedicated to a different specific cooking utensil based on its peripheral outline, as described for example in the aforesaid co-pending US patent application. 
     In the embodiment of FIGS. 1,  2  and  2 A of the present invention, the location of the cooking utensil or utensils, for the purpose of selecting the heating elements to be made operative, is determined using the thermal cells  3  themselves to obtain an electromagnetic coupling with a separate means  4  formed from one or more conductive loops which, in a certain sense and in this example, surround the thermal cells overall. As is evident, the scope of the invention also includes the solution comprising groups of loops, each surrounding an assigned subassembly of thermal cells, for example as illustrated in FIG. 1 with dashed and dotted lines and carrying the reference numerals  4   a  and  4   b.    
     Each thermal cell, connectable to the power source  10 A via (FIG. 2) solid state switches  9  program-controlled by a microprocessor  8 , for example in accordance with the method stated in the aforesaid co-pending US patent application of the same applicant, can also be connected to an alternating signal source  6  via column selectors  7 A and row selectors  7 B (for example represented by multiplexers) both controlled by said microprocessor  8  such that the signal of the source  6  is applied, in cyclic succession, to different thermal cells  3 . The signal received by the loops  4  is different depending on whether a cooking utensil lies on a thermal cell receiving the signal of the source  6 . 
     If a utensil is present, the electromagnetic coupling between the thermal cell  3  and the loop  4  undergoes a variation. The variation is measured by a circuit  10  (for example comprising a band pass filter. amplifier, double half wave rectifier, envelope detector) the output of which reaches the microprocessor  8  via an AID converter (not shown). The microprocessor associates this signal variation with the presence/absence of the cooking utensil on the specific thermal cell which has produced it and effects such an association for each thermal cell on which the cooking utensil lies, and builds a memory map containing the overall the measurements relative to each cell. A suitable algorithm extracts from this map those thermal cells to be energized (via the solid state switches  9 ). The said mapping can be for example also used to display on a light-emitting panel the location of cooking utensils on the hob. It should be noted that in a preferred embodiment of the invention the thermal cell selection (for the purpose of applying to it the alternative signal of the source  6 ) takes place while the relative row and column are not powered with mains voltage via the aforesaid solid state switches  9 . In other words, the said algorithm (or another) coordinates the sequence in which the thermal cells  3  are powered by the power source  10 A (via the solid state switches  9   a ,  9   b ) with the sequence of selection operated via the multiplexer  7 A,  7 B. 
     FIG. 2 schematically shows the matrix formation of the thermal cells  3  of a cooking hob and the solid state switches  9  ( 9   a  for rows and  9   b  for columns, here exemplified as triacs) provided for energizing the selected thermal cells  3  via diodes  11 . The energy is supplied by the unfiltered full-wave rectified electrical source  10 A, in accordance with the aforesaid co-pending US patent application. 
     In the aforegoing it has been stated that the signal for determining the absence/presence of the cooking utensil is applied to the thermal cells  3  which hence act as a “transmitter” whereas the loops act as a “receiver” for the signal emitted by the cell itself. It is however evident that the scope of the invention includes the dual solution, in which the loops  4  act as the transmitter and the thermal cells  3  as the receiver. In this solution the loops  4  can be excited continuously or discontinuously (for example at predetermined intervals), the thermal cells  3  being enabled cyclically on receiving the signal during excitation of the loops  4 . 
     FIG. 2A represents a practical embodiment of the invention. This figure uses the same alphanumerical references as the preceding figures to indicate equal or corresponding parts. Here, each row switch  12   b  consists of an NPN transistor with its emitter earthed and a diode connected to the collector to which a positive source is connected via a resistor, whereas the column switch  12   a  consists of an NPN transistor with its emitter earthed, its collector connected to a positive source via a resistor and to the base of a PNP transistor with its collector connected to earth via a resistor and to a column diode, its emitter being connected to a positive source. The purpose of the diodes is to protect against overvoltage. In this embodiment the row concerned is connected to earth while the column concerned is driven or is in alternating current (or vice versa). 
     The embodiment of FIG. 3 in which equal or corresponding parts are indicated by the preceding reference numerals plus  100 , the thermal cell  103  itself constitutes an integral part of an oscillator  20  when a switching means (for example a triac), indicated by  21  and controlled by the microprocessor  108  is driven by this latter into the logic position A by which it is connected to the remaining part  20   a  of the oscillator, the oscillator being connected to a rectifier  20   b  and this to an integrator  20   c . The d.c. output indicated by  20   d  passes to the microprocessor  108 . If a cooking utensil is located on the thermal cell the oscillator characteristic varies, this variation acting on the microprocessor  108  in the sense of causing the switch  21  to assume the logic position B in which the thermal cell is connected to the power source  11  GA which energizes it, in accordance with the algorithm indicated in the aforesaid co-pending US patent application, which algorithm in alternating between energizing thermal cells  103  in logic position B and connecting thermal cells  103  as part of the oscillator in logic position A causes the switch to pass (during cut-out) to a floating position indicated by C to isolate the power source  110 A from the oscillator  20 . The switch  21  evidently represents a function and not the specific solution, which is obviously represented by electrical/electronic means implementing the described function. 
     Although the embodiment of FIG. 3 refers to a single thermal cell, it will be evident to the expert of the art how to adapt it to the plurality of thermal cells forming the cooking hob  1 . 
     The sensing part shown in FIG. 2 comprises the alternating signal source  6 , connected in series with a direct current source  13  (by which the signal of the source  6  plus the direct current source  13  will always be positive) and the selectors (multiplexers  7 A,  7 B) reproduced in the form of physical switches  12   a  and  12   b , as means which cyclically provide a pulse signal to the individual thermal cells  3 , as stated herein before. 
     The source  6  can be square wave and have a frequency of 80 kHz.
A device for determining the location of cooking utensils on a cooking hob comprising a plurality of thermal cells distributed in matrix formation below a heat-resistant surface on which the cooking utensil can be located in random manner, the determination of its location, form and dimensions enabling those thermal cells lying below the utensil to be energized, the same thermal cells being also individually used for this determination.