WO2008064446A1

WO2008064446A1 - Device and method for controlling a heating element of a glass-ceramic cooktop

Info

Publication number: WO2008064446A1
Application number: PCT/BR2007/000324
Authority: WO
Inventors: Márcio TIBIRIÇÁ; Cesário ZIMMERMANN JUNIOR; Davide Gerola; Davide Parachini; Cristiano Pastore
Original assignee: Whirlpool S.A
Priority date: 2006-12-01
Filing date: 2007-11-23
Publication date: 2008-06-05
Also published as: BRPI0605541A; WO2008064446A8

Abstract

Device and method for controlling a heating element (20) of a glass-ceramic cookplate (10), having at least two conductive metal tracks (38,40), to achieve a selected temperature and to automatically determine the resistance of the glass-ceramic cookplate (10) between the tracks, or the resistance between two points spaced apart by a known length of one of the tracks, depending upon the value of the selected temperature by applying an AC voltage (54) to said tracks, when one desires to determine the resistance of the glass-ceramic cookplate (10), and to only one of the tracks (38, 40), when the track resistance is to be determined. The temperature of the cookplate (10) is computed from the value of the determined resistance and compared with the selected temperature to turn the heating element (20) on/off to achieve the selected temperature.

Description

"DEVICE AND METHOD FOR CONTROLLING A HEATING ELEMENT OF A GLASS-CERAMIC COOKTOP" Field of the Invention

This invention relates generally to glass-ceramic cooktop appliances and particularly to a device and a method for electronically controlling the powering of a heating element for such cooktop appliances. The control device operates to maintain a cookplate, supporting a cooking utensil, at a desired temperature over a wide range of cooking temperatures. Background of the Invention

Use of glass-ceramic cookplates for cooktop appliances is well known. Advantages of this type of cookplate include the smooth surface with a pleasing appearance and ease of cleaning. In such conventional cooktop appliances, the glass-ceramic cookplate is heated by a resistive heating element, such as an open coil electric resistive heating element or a gas burner disposed beneath the cookplate. A domestic cooking utensil placed on the glass-ceramic cookplate is heated when the heating element is turned on, by

^■ conduction from the cookplate, and mainly by radiation of the heat generated by the heating element.

Currently available cooktops with glass-ceramic cookplates have relatively poor temperature control for the cooking operation. Prior art temperature controls for these cooktops have been of various types. One of said controls uses electro-mechanical bimetallic thermostats to turn the heating element on and off. Temperature values ranging from zero to a temperature close to a maximum one are obtained by mechanically changing the operation of the bimetallic thermostat by means of a rotatory knob. The electromechanical on-off control provokes a high temperature hysteresis in the ceramic glass which results in a large dead zone, often greater than 50⁰C, between the set and actual temperatures.

Further, this type of control is not very accurate, because it depends on mechanical devices, whose characteristics can depend on one or more components, vary over time and, in some measure, depend upon ambient temperature. This component variation requires said control to be designed for a lower temperature range and with a higher hysteresis, leading to a safer device.

Another type of control uses a fixed thermostat in the heating element to limit the maximum temperature on the surface made of glass-ceramic material to about 550⁰C. Intermediate values of cooking temperature are obtained by utilizing an electronic on-off control . The control turns on and off the resistive heating element in a determined frequency and on/closed time, so as to maintain a temperature level in the glass- ceramic plate. The cooking temperature is defined by the thermal characteristics of the load (pan) and the food being prepared. This kind of control is very poor, because temperature varies for different types of pan, food and cooking conditions. Often, the food can be overcooked or undercooked, due to the fact that there is not a closed mesh control of the glass temperature. In this control, the power levels are defined, that is, the time in which the resistive heating element remains on is previously defined by using a standard load (pan with food) . This causes, from the moment the cooktop user increases or reduces the weight on the glass, alterations in the cooking characteristics because the control does not know there has been an increase or decrease of weight on the ceramic glass. Another type of temperature control uses the principle of the variation of the electrical resistance of the glass-ceramic cookplate with temperature. To accomplish this, as shown in figures IA and IB, a pair of strips, or tracks, 12 and 14 of an electrically conductive metal, such as gold, is deposited on the lower surface of a glass-ceramic cookplate 10. A current Im of known value is applied to one of the tracks 12 and the voltage drop Vd across the two tracks is measured. The glass resistance is determined from the Ohm's Law formula.

The glass resistance varies with temperature according to the formula (Rasch and Hinrichesen) :

Log (Rt) = A + B (1)

T where

A = 1,922

B = 4561.61 and

T = the glass temperature in ⁰C

Rt = the glass resistance The logarithmic relation of the glass electrical resistance to the temperature of the glass-ceramic cookplate of a cooktop is shown by line A of figure 3. As seen, said glass electrical resistance starts significantly decreasing its value at temperatures above 350⁰C. Nevertheless, the use of measuring the glass resistance for control of temperatures below 350⁰C is very useful and desirable for a cooktop temperature control circuit. However, the system of measuring the temperature through its resistance variation does not offer the possibility of controlling several useful cooking functions at low temperatures, such as melting chocolate, making a fondue, and a "Keep Warm" function. However, measurement of glass resistance allows a fairly precise control for temperatures above 350⁰C up to the maximum glass temperature (620⁰C for Ceran Suprema products from Schott) .

Another type of control technique uses what is called "track resistance". In this case, a track of a conductive material, such as gold, of a known length and cross-section is deposited under a surface of the cookplate and the resistance of the length of the track between two points, such as the track opposite ends, is measured. This is sometimes called "gold track" resistance and is shown in figures 2A and 2B, where a U-shaped track of gold 16 is deposited on the lower surface of the glass-ceramic cookplate 10. The track resistance is measured by the voltage drop (Vd) produced between the track terminals by the applied measurement current (Im) of known value at one of the terminals 16a. A leakage current (Id) can be expected at high temperatures due to glass resistance reduction. The voltage drop across the ends 16a and 16b of the track 16 is measured, allowing the track resistance to be calculated.

The track resistance varies according to the well known equation of resistivity for metals :

R = p x L (2)

A where

R = track resistance p = the resistivity of the track material in ohms/meter

L = the track length in meters A = the track length cross-sectional area in square meters

The track resistance variation over temperature follows the equation:

R_tf = Rto [ 1 + α (t_f - t₀) ] (3) where

CX = angular coefficient

R_tf = resistance in the track at the final instant R_to = resistance in the track at the initial instant t_f = final time t₀ = initial time

As it can be seen from line B of figure 3, the track resistance variation is quite linear over the temperature range from ambient temperature (Tamb) to the maximum glass operational temperature (about 600⁰C) . However, this linear characteristic of track resistance variation is not totally useful for measurement of cookplate temperatures higher than 45°C, because from this point the measured value of the track resistance stops rising linearly. This occurs because the glass resistance falls drastically, preventing the resistance between the ends 16a and 16b of the track 16 from rising linearly as the above described equation. Accordingly, a better and more precise control of temperature is desirable to make the food preparation process more predictable and stable. This control would also permit new features to be added to the cooktop, as well as the improvement of the efficiency and quickness of the cooking process.

Summary of the Invention

The present invention provides a novel device and method for controlling the cooking temperature of a glass-ceramic element, by selectively measuring either the glass resistance or the track resistance, depending upon the cooking temperature selected by the user. In operation, the cooking temperature is set by the cooktop user. A microprocessor determines if the selected temperature is in one of two temperature ranges that could be more precisely controlled by one of two different measuring techniques. If the temperature selected by the user is in a range higher than 450⁰C, in a preferred embodiment of the present invention, the control is accomplished by the measurement of the glass resistance and, if said temperature is in another temperature range under 450⁰C, the control is accomplished by the measurement of the track resistance. Other temperature ranges can be used. In its implementation, the temperature control system has at least two generally metallic strips or tracks, which are usually U-shaped (or circular or other geometric form) and parallel, preferably made of gold and one placed within the other on the lower surface of the glass-ceramic cookplate . The microprocessor controls, through an AC excitation section, the application of an excitation drive current in an alternating current (AC) of known value, which passes through a multiplexer switch in the appropriate position, to one or both tracks to permit either measurement of track resistance or glass resistance, depending on the selected cooking temperature . An analog signal detection circuit detects the voltage generated in the terminals and, from the detected voltage, the microprocessor computes a voltage value proportional to the value of the glass resistance or the track resistance. Then, the microprocessor uses the computed glass resistance or track resistance to determine the cookplate temperature in a look up table stored in the microcontroller, which basically corresponds to the curves of Fig. 3. The measured temperature is compared to the selected cooking temperature and the microcontroller controls the turn- on or turn-off of the cookplate heating element, to first achieve and then maintain the selected cooking temperature during the cooking operation. Using dual control of the heating element based upon different ranges of selected cooking temperature results in a more precise temperature control for the cookplate in the whole variation range of the glass temperature .

Brief Description of the Drawings

Other objects and advantages of the present invention will become more apparent upon reference to the following description and enclosed drawings, given by^¬ way of example of possible embodiments of the invention and in which:

Figures IA and IB are schematic diagrams showing the measurement of the cookplate temperature using glass resistance;

Figures 2A and 2B are schematic diagrams showing the measurement of the cookplate temperature using track resistance;

Figure 3 is a graph showing the cookplate temperature correlated to the measurement of the glass resistance and to the measurement of the track resistance; Figure 4A is a side elevational view showing a cookplate presenting the tracks in the lower part of the glass-ceramic plate; Figure 4B is a bottom plan view of the cookplate, taken along the lines IV-IV of figure 4A; Fig. 5 is a schematic diagram of the illustrative electronic circuit of the device of the present invention; and Fig. 6 is a flow chart showing the operation of the device of the present invention. Detailed Description of the Invention

In figures 4A and 4B, a heating element 20 of the invention includes a supporting box 26 of a heat insulating material having side walls 28, which support a solid glass-ceramic cookplate 10. There may be other supports near the center of the cookplate 10, as needed. The heating element 20 is usually built into a kitchen countertop cooktop that has a plurality of such heating elements, often four of the same or different sizes. The cookplate 10 of a cooktop is relatively thin and may be a large single plate or a series of either two medium plates or four smaller plates. All of these constructions are conventional in the prior art. The glass-ceramic cookplate 10 material is crystalline glass, generally opaque, or of milk- white appearance, and typically is of lithia-alumina- silicates having a very low coefficient of thermal expansion. Examples of such material are sold under trademarks as PYROCERAM, CER-VIT and HERCUVIT. The glass-ceramic material of the cookplate 10 is electrically insulating (at low temperatures) and relatively thermally transmissive . While the term glass-ceramic material or crystalline glass material is used throughout the whole description, it should be understood that the present invention encompasses other materials with similar characteristics, such as quartz, high-silica glass, high-temperature glasses and different ceramic materials. Under the cookplate 10, there is provided an electrical heating element shown as a coil 22, although a gas burner or another resistive element can be used. As best seen in figure 4B, bonded to the lower side 10a of the cookplate 10, in a region of maximum temperature indicated by the circle 32, are attached at least two U-shaped metalized conducting tracks 38 and 40. The two tracks have parallel sides and base ends and are spaced apart one within the other with a constant distance between the sides and the base ends. The two free ends of the outer U-shaped track 38 are defined as Xl and X4 , and the two free ends of the inner U-shaped track 40 are defined as X2 and X3. The four free ends of the U-shaped tracks 38 and 40 extend beyond the maximum temperature region 32 of the heated cookplate 10 and preferably terminate in a cool region 34. Due to fact that the track ends are relatively cool, suitable connections (not shown) leading to an external control circuit are provided to the track free ends, by leads or foil tracks, by welding, spring loaded contacts, or other suitable contact arrangement .

The U-shaped tracks 38 and 40 are preferably made of precious metal, such as gold, palladium, gold- palladium combinations or the like. It is important to emphasize that the shape can be altered according to the convenience of the design, such as circular or other shape . In the preferred embodiment of the present invention, the conducting tracks 38 and 40 are of gold and may be silk screened and fired onto the surface of the cookplate 10 at a temperature of about

1300⁰F (704⁰C) . The two tracks are built up to a desired thickness, for example, of about 50 to 100 angstroms . The tracks are spaced from each other by a suitable distance, for example, of about 0.4 in (1.016 cm) . The width of each track is of approximately 0.2 in (0.508 cm) . This construction gives a finite measurable resistance value for the resistance of each track 38, 40 and for the glass resistance between the two tracks. The overall track resistance of each of the tracks is not critical and any value between about

0.1 ohm and 300 ohms can advantageously be used. The dimensions are selected so that there is a useful resistance, which can be measured between the tracks 38 and 40, that is the glass resistance, and along the length of each track, that is the track resistance. A preferred embodiment of the device of the invention is shown in figure 5, which illustrates a cookplate 10 with two tracks ^'38 and 40 on its lower surface as described above. There is provided a conventional microcontroller 50 presenting various input and output terminals as indicated and also preferably has at least an internal analog to digital (A/D) converter. The term microcontroller is used in its broadest meaning and includes any suitable electronic device, such as a microprocessor which can be and is programmed to perform the various functions described below. There is also provided an external keypad 52, into which a user keys in a selected cooking temperature and this value is entered into the microcontroller 50. The keypad 52 usually permits selection of other functions, such as cooking time duration, start time, etc. A display (not shown) is also usually associated with the keypad 52 to inform the user of the cookplate 10 actual temperature. All these elements are of conventional use in the prior art .

The circuit used in the device has two major parts, these being an AC excitation drive section, which is the upper part of the circuit, as shown in figure 5, and a signal detection section, which is the lower part of the illustrated circuit . The AC excitation drive section supplies a drive current of a known value to either the track 38 or track 40, to develop the voltage drop needed to make the glass resistance measurement, and to both tracks to make the track resistance measurement, as described above with respect to figures IA, IB and 2A, 2B. The AC voltage is used in order to minimize the electro-migration phenomenon that occurs when a thin conducting metal layer, conducting a high current density, is submitted to high temperatures. This phenomenon occurs both in DC and AC current, but in AC the electro-migration is higher than in DC. However, a DC excitation could be used rather that an AC excitation. The AC excitation drive section has a DC voltage supply 54 controlled by the microcontroller 50, to supply a DC output voltage to the input of each of first and second operational amplifiers 56a and 56b, which set the current to an appropriate known value. Each operational amplifier 56a and 56b passes its output signal through a respective pair of resistors 58a and 62a, and 58b and 62b connected in series. A respective capacitor 60a and 60b connects each pair of resistors to circuit ground. The capacitors 60a and 60b smooth the AC signal to remove noise.

The AC output of the resistor 62a is applied through a capacitor 64a to a first terminal 70a of a multiplexer 74 controlled by an output from the microcontroller 50 over the line 73. Similarly, the AC signal at the output end of the resistor 62b is applied through a capacitor 64b to a second terminal 70b of the multiplexer 74. The multiplexer 74 essentially is an electronic switch controlled by the microcontroller 50 that consists in a bank of single-pole, single-throw switches Sl, S2, S3 and S4. The number of multiplexer switches can be greater for a measurement system with a larger amount of tracks. One terminal of each of the switches Sl and S3 is connected to the first terminal 70a of the multiplexer 74 and one terminal of each of the switches S2 and S4 is connected to the second terminal 70b of the multiplexer 74. The electronic equivalent of the multiplexer 74 for the free end of each of the switches Sl, S2 , S3 and S4 contacts a corresponding end Xl, X2 , X3 and X4 of the tracks 38 and 40. The terminals 70a and 70b of the multiplexer 74 and the switches have both an input function to supply the AC current to a certain one or both tracks 38 and 40 and also have the output function to assist in providing the signal detection section of the circuit, with voltage produced from each of the glass resistance and track resistance measurements . The signal detection section of the circuit includes a load resistor 76 connected across the two terminals 70a and 70b of the multiplexer 74 and across which a voltage measurement value is produced in a manner to be described below. There is an upper output signal line 80a connected to the first terminal 70a of the multiplexer 74 and to the upper end of the load resistor 76. A lower output signal line 80b is connected to the second terminal 70b of the multiplexer 74 and to the lower end of the load resistor 76. A DC voltage, taken from a voltage divider of the resistors 86a and 86b connected between a source of positive voltage and ground, is applied to the lower output signal line 80b. The output of the upper signal line 80a is connected to one end of a resistor 88a, whose other end is connected to ground through a capacitor 89a to eliminate noise from the output signal. Similarly, the output of the lower signal line 80b is connected to one end of a resistor 88b, whose other end is connected to ground through a capacitor 89b.

A diode bridge rectifier circuit is formed by four diodes 90a, 90b, 90c and 9Od connected as shown and whose function is to convert AC voltage into DC voltage. The AC voltage on the upper signal line 80a is supplied through a resistor 88a to the junction of the diodes 90b and 90c, while the AC voltage on the lower signal line 80b is supplied through the resistor 88b to the junction of the diodes 90a and 9Od. The anodes of the diodes 90a and 90b is connected to ground and the cathodes of the diodes 90c and 9Od is connected to the source of DC voltage.

The circuit of figure 5 is controlled by the microcontroller 50, to measure either the glass resistance of the cookplate 10 or the track resistance under the glass. After the user sets the temperature in the keypad 52, the microcontroller 50 automatically makes a determination, based upon the selected temperature, as to whether the measurement and control of the heating element is to be based upon glass resistance measurement (for a high temperature range) or track resistance (for a lower temperature range) . All of the known values and. equations needed for computation are programmed in the microcontroller 50.

If the temperature set by the user in the keypad 52 is in the higher temperature range, as determined by the microcontroller 50, the control of said heating element must be carried out, using measurement of the glass resistance and, then, the microcontroller 50 applies a signal over line 73 to the multiplexer 74, to close either set of switches Sl and S2 or set of switches S3 and S4. In this case, AC current is applied to both tracks 38 and 40, so as to produce an AC voltage drop through the output load resistor 76 that is applied to the signal detection section. The diode bridge circuit allows a DC analog output voltage to be applied to the two input terminals InI and In2 of the microcontroller 50 for conversion into a digital value, proportional to the resistance value.

The microcontroller 50 uses the digital value in accordance with the equation (1) above, to compute the glass resistance. The computed glass resistance is compared with temperature values in a stored lookup table, to determine the cookplate 10 temperature. This temperature is compared by the microcontroller 50 with the temperature selected by the user, to determine whether the heating element of the cookplate 10 is to be turned on or off to achieve and/or maintain the selected temperature, and the microcontroller 50 produces the appropriate signal on line 93. If track resistance is to be measured, the microcontroller 50 operates the multiplexer 74 by a signal on line 73 to close the switches Sl and S4 , applying an AC current in the outer conductive track 38. An AC voltage is produced across the load resistor 76, which appears at the upper and lower junctions of the diode bridge circuit, to produce a positive output voltage that is applied to the input terminals InI and In2 of the microcontroller 50, supplying a DC voltage to the analog to digital converter of the microcontroller 50, which produces a digital value from the DC voltage. This digital value is used by the microcontroller 50, to compute the track resistance in accordance with the equation (3) . The computed track resistance is used in the microcontroller 50 lookup table to determine the cookplate 10 temperature, based upon the track resistance measurement . Depending upon whether or not the measured temperature agrees with the selected temperature, the microcontroller 50 operates to produce an output signal on line 93 to control the heating element.

The microcontroller 50 also operates the multiplexer 74 to close the switches S2 and S3, so that the resistance of the inner track 40 can be measured in the same manner as described above. The digital value from this second track resistance measurement can be used sequentially with the first measured value for control of the heating element or else the two can be averaged for control of the heating element. Figure 6 is a flow chart showing the device operation. In SlOl, the device is powered to allow temperature selection for a cookplate 10. In this figure 6, three cooking function keys with different levels of desired cooktop temperature are shown as being accessible in S105, S107 and S109. In S103, if it is determined that the temperature selection function key has been actuated and if there is a determined function defined as YES, then the glass-ceramic temperature is set by the function key S105, S107 or S109, pressed by the user. That is, the user can choose which function he desires for controlling the system. The function chosen by the user generates a temperature value which is sent to the microcontroller 50. For example, if the user presses the function key S105, the temperature value in S106 is sent to the microcontroller 50. If neither of the functions keys is pressed, the user must select the desired cooking temperature in S123. All of this data is supplied to the microcontroller 50, which in S113 judges whether the temperature is in one of the two ranges, here illustratively, one range with a temperature equal or above 450⁰C and the other below 450⁰C. If the decision is YES, then in S117 the microcontroller 50 operates the circuit of figure 5 to measure the glass resistance, as described above. If the decision in S113 is NO, then in S115 the microcontroller 50 operates the circuit of figure 4 to measure the track resistance. In S119, the microcontroller 50 determines whether or not the temperature measured by the selected glass resistance or track resistance is greater than or equal to the temperature set in the steps S106, S108, SIlO or S123. If the decision is YES, then in S120 the heating element 20 is turned off, and if the decision is NO, then in S122 the heating element is turned on. The procedure above is made to achieve the selected temperature. This cycle is repeated to make the cookplate 10 reach and maintain the set temperature. The circuit can be used to set and control, at the same time, the temperature for two or more heating elements in the cookplate, that is, the cookplate can have many heating elements, allowing several domestic utensils to be used at the same time. The present invention provides a number of advantages. There is provided a more accurate temperature control in the cooktop operating range. High temperatures are controllable and can provide a faster boiling water function. Low temperatures also are controllable and allow the preparation of delicate food, such as melted chocolate or fondue. The present invention also affords use of a control algorithm to detect broken tracks and a function of detecting the presence of a pan on a cookplate. Said device, created to control the temperature of the glass-ceramic cookplate, carries out the resistance reading and controls the resistive element (typically called resistance or heater) so as to maintain the glass temperature close to a desired value. Specific features of the invention are shown in one or more of the drawings for convenience only, as each feature may be combined with other features in accordance with the invention. Alternative embodiments will be recognized by those skilled in the art and are intended to be included within the scope of the claims. Accordingly, the above description should be construed as illustrating and not limiting the scope of the invention. All such obvious changes and modifications are within the patented scope of the appended claims .

Claims

1. A method for controlling a heating element of a glass-ceramic cookplate (10) , characterized in that it comprises the steps of: - selecting a temperature; measuring the temperature of the cookplate (10) using a predetermined one of a plurality of techniques, depending upon the value of the selected temperature; and - controlling the heating element, based on the temperature measured by said predetermined technique, to achieve the selected temperature.

2. The method, as set forth in claim 1, characterized in that it further comprises the step of providing at least two spaced apart electrically conductive metal tracks (38, 40) on a lower surface (10a) of the cookplate (10) , said plurality of techniques comprising a technique of determining the glass resistance between said tracks, or the resistance of a track of known length of at least one of said tracks, and of computing the temperature based on the resistance determined.

3. The method, as set forth in claim 2, characterized in that said step of measuring comprises determining the glass resistance in case the ^"selected temperature is greater than 450⁰C, and determining the resistance of the track (38, 40) in case the selected temperature is lower than 450⁰C.

4. The method, as set forth in claim 3, characterized in that said step of providing said tracks (38, 40) comprises providing U-shaped tracks disposed one within the other.

5. The method, as set forth in claim 4, characterized in that said tracks are of gold.

6. A device for controlling a heating element of a glass-ceramic cookplate (10) to achieve a temperature selected for the cookplate (10) , characterized in that it comprises:

- at least two electrically conductive tracks (38, 40) equally spaced apart on a surface of the cookplate

(10) ;

- a source of voltage; and a switching circuit responsive to the selected temperature, to apply the voltage from said source to said tracks, in order to produce a first voltage drop across the glass-ceramic cookplate (10) between said two tracks (38, 40), or apply the voltage from said source to two points spaced- apart by a known length along the length of one of said tracks, to produce a second voltage drop between said two points.

7. The device, as set forth in claim 6, characterized in that it further comprises: a microcontroller (50) to compute the resistance of the glass-ceramic cookplate (10) in response to said first voltage drop and to compute the track resistance in response to said second voltage drop.

8. The device, as set forth in claim 7, characterized in that said microcontroller (50) further computes the temperature of the cookplate (10) in response to the computed resistance of the glass-ceramic cookplate (10) or of the track.

9. The device, as set forth in claim 8, characterized in that said microcontroller (50) compares the computed temperature of the cookplate (10) to the selected temperature to produce a signal to control the heating element, so as to achieve the selected temperature.

10. The device, as set forth in claim 9, characterized in that it further comprises a keypad (52) connected to said microcontroller (50) for a user to enter a selected temperature of the cookplate (10) or to select a function with a predetermined temperature.

11. The device, as set forth in claim 6, characterized in that the tracks (38, 40) are U-shaped and disposed one within the other.

12. The device, as set forth in claim 6, characterized in that said source of voltage produces a DC or AC voltage .

13. The device, as set forth in claim 12, characterized in that it further comprises:

- an- element on which said first and second voltage drops are produced, said voltage drops preferably being in AC, but which can also be in DC;

- a rectifier to convert the AC voltage drop to a DC voltage; and a microcontroller (50) including an analog to digital converter, to which the DC voltage is applied and which converts the DC voltage to a digital value, the microcontroller (50) computing the resistance of the glass-ceramic cookplate (10) , in response to said first voltage drop, and computes the track resistance, in response to said second voltage drop.

14. The device, as set forth in claim 6, characterized in that said switching circuit further applies a voltage from said source between two points of the other of said tracks (38, 40), to produce an additional second voltage drop.

15. The device, as set forth in claim 14, characterized in that it further comprises: a microcontroller (50) to compute the resistance of the glass-ceramic cookplate (10) , in response to said first voltage drop and to compute the track resistance, in response to each of said second voltage drop and said additional second voltage drop.