EP0739508A1

EP0739508A1 - An apparatus for controlling the operating temperature of a cooking zone

Info

Publication number: EP0739508A1
Application number: EP95901363A
Authority: EP
Inventors: Johan Axelson; Jan E. Dahl; Daniel Schneider
Original assignee: Electrolux AB
Current assignee: Electrolux AB
Priority date: 1993-12-06
Filing date: 1994-12-05
Publication date: 1996-10-30
Also published as: AU1064195A; WO1995016230A1

Abstract

Pot detection, determination of type of pot, and determination of a fault condition may be performed by suitable signal processing and statistics on the measurements of thermal radiation from the top, possibly vitroceramic, surface of a cooking zone. The measurements are converted to indicators of thermal load over time on the radiating elements and classified according to stored limits possibly determined during calibration runs. The power supplied to the heating elements is used as a correction factor.

Description

AN APPARATUS FOR CONTROLLING THE OPERATING TEMPERATURE OF A COOKING ZONE

The present invention relates to an apparatus for controlling the operating temperature of a cooking zone heated from below by energy supplied in pulses by detecting and classifying into neighbouring ranges the thermal load on its surface, and comprising means for determining the average temperature of at least part of the cooking zone.

It is known to apply "pot detection" to cooking zones, in particular cooking zones on ceramic cooking surfaces in order to determine the presence or non-presence of a vessel to be heated. Such determination frequently occurs while energy is supplied to the cooking zone and is necessary since the sudden removal of a pot may lead to overheating of the cooking zone. Hence there may be said to exist two basic operating ranges, one termed "an acceptable operating range" and one termed "a non-acceptable operating range".

Similarly the concept "pot detection" covers the determination whether a cooking vessel present on a cooking zone fulfils the criteria needed in order for it to represent an acceptable load, viz. its material, the flatness of its bottom, even whether the vessel covers a sufficient area of the heated cooking zone or whether water has boiled away entirely.

It is known to detect parameters indicating the suitablility of the load to the energy supply condition, e.g. by optical "pot presence detection" or magnetically or capacitively related to the metal or dielectric properties of a pot. However such detection methods are not suitable in practical use, because soiled surfaces may generate spurious signals. In order to overcome such problems it is known to measure the presence or

SUBSTITUTESHEET non-presence of a cooking vessel by special active circuits comprising a transmitter and a receiver which however contribute considerably to the complexity of a cooking zone.

It is the purpose of the invention to provide means for controlling the temperature of a cooking zone which more directly relates to the principle of heating, i.e. that the cooking vessel is a thermal load on the cooking zone. This is obtained by means of an apparatus according to the invention which is particular in that at least one measurement value contributing to a representation of the momentary thermal load condition is extracted from the function of temperature over time during transient heating and/or cooling phases, of which at least one value is stored representing the limit between neighbouring operational ranges, that the measurement value is compared to the stored value, and that a control signal is generated which represents the particular range of the applied thermal load, which control signal controls the supply of energy.

In this manner it may be said that the thermal load is determined by means of the step response of the load to a known input energy step function. In this way all the problems of interpreting parameters related to non-thermal detection of the physical presence of a cooking vessel are eliminated.

This general principle is advantageously applied to obtain a safe shut-off by an adjustment of the values according to claim 2.

An advantageous simplification of the evaluation is obtained by synchronizing the evaluation to the pulses of energy according to claim 3. The rate of change of the temperature over time is compared to a predetermined value which is in itself a function of the desired operating temperature of the cooking zone which is set by the user.

SUBSTITUTESHEET The basis for the measurement of a thermal load is the stability of temperature. This is obtained and optimized by the measures of claim 4 by which means the hysteresis is kept to a minimum.

According to claim 5 a further simplification is obtained by performing the measurements and evaluation during a phase where no energy is supplied to the thermal load. The determination may be based on a differential decrease of temperature which corresponds to only two points of the curve of decrease or on a monitoring of the curve, in which case the rate of decrease in optional points may be determined.

According to claim 6 an advantageous operation is obtained when a time delay is introduced from the time of determining the too small load and to the actual switching off of the energy because a further measurement may indicate that there is a load after all, the user was simply slow in putting a pot on the cooking zone. In case a second measurement indicates no change in the potentially dangerous no-load situation the control circuits switches off, and the cooking zone control will have to be reset.

According to claim 7 the measurement of a temperature representative of the thermal load is performed when a sufficient degree of equilibrium of temperature based on the heat loss (thermal load) has been obtained, that is as late in a period of no energy supply as is possible. In this way, in particular influences from supply voltage fluctuations are avoided, which is important because the influence of such fluctuations follow a square law.

A cooking zone controlled by means of an apparatus according to claim 8 will have an advantageous fast heating because the energy supply is uninterrupted until the intended operating temperature is reached.

An further advantageous embodiment of the invention

SUBSTITUTESHEET is defined in claim 9 which enables a more precise determination of the degree of overlap between a cooking vessel and the cooking zone.

In claim 10 an advantagous embodiment is defined which enables evaluation to take place even if the changes in operating conditions are vague or even conflicting from measuring area to another, such as would be the case if a cooking vessel is moved about or briefly removed and replaced on the cooking zone.

In claim 11 a further advantageous embodiment is defined which provides a greater flexibility in use, in that the calibration phase permits a personalization of the set of pots in use, thereby allowing the limit between acceptable and non- acceptable operating ranges to be determined with greater precision.

The invention will be described in detail in the following with reference to the drawing, in which

Fig. 1 shows the basic concept of the present invention in connection with a schematic diagram,

Fig. 2 shows the experimental determination of temperature over time following the switching on of a heating element, in the cases "no pot", "unsuitable pot", and "normal pot",

Fig. 3 shows a first embodiment of the invention according to the general schematic of Fig. 1, utilizing the different types of measurement results of Fig. 2,

Fig. 4 shows the first derivative over time of the measurements of Fig. 2,

Fig. 5 shows a preferred embodiment of the invention according to the general schematic of Fig. 1, utilizing the different results of Fig. 4,

Fig. 6 shows the classification of the representation of the thermal load on a two-dimensional graph of "maximum of temperature function derivative" over "limit temperature obtained", using the supplied power as a parameter,

SUBSTITUTESHEET Fig. 7 shows a further preferred embodiment of the invention utilizing the classification of Fig. 6,

Fig. 8 schematically shows an advantageous distribution of temperature sensors in order to obtain information relating to quality of pot and its degree of off-axis placement,

Fig. 9 shows the function of the cooking zone temperature in a stable situation where the temperature is controlled by a thermostat, and indicates two different measures representing characteristic features which are influenced by the quality of pot or its degree of off-axis placement.

Fig. 1 shows schematically in a signal flow and functional block diagram the principal features of an apparatus according to the invention.

Heating elements 1 for a glass ceramic cooking zone 3 are connected by means of a switch 5 to the electrical supply 7 which may supply electric energy at predetermined rates which is indicated by the arrow. There may also be variations due to voltage fluctuations in the mains supply. At least one continuous temperature sensor 9 is provided which may be of a conventional construction and which may be integrated with the cooking zone 3, placed directly below the cooking zone, supported in the volume V between the heating elements and the cooking zone or connected to a heat conductive support which follows the temperature or the average temperature of the volume it occupies. The temperature sensor gives a signal S9 at its output dependent on the temperature th(t) which is a function of time t.

The output signal S9 of the sensor 9 is an input to the evaluation circuit 11. According to the invention it is determined whether the cooking zone 3 is loaded thermically in a permitted or non-permitted manner. Permitted or "acceptable" is the placement of a cooking pot or other utensil on the cooking zone, and

SUBSTITUTESHEET non-permitted would be the condition of no load.

At the switching on or off of the electrical supply to the heating elements 1 a transent phase is initiated as shown schematically by the switch 5. When the supply is switched on, the cooking zone 3 acts as a heat sink until the heat supplied by the heating elements 1 equals the heat transported to the surroundings by radiation and conduction. The switching on phase may be described by means of a function of the type (1- exp(-at/tau) ) .

At the switching off of the heating elements 1 the cooking zone 3 acts as a heat source, and its temperature decreases according to a function of the type exp(bt/tau) . As shown schematically, a control signal S5 from the switch controlling the supply of energy to the heating elements 1 starts the evaluation circuit 11.

Generally the evaluation occurs during the transient phases during which certain representations related to the signal S9 which are to be described further in the following are determined before the transient conditions have settled. Such determinations may occur by extrapolation of the temperature functions determined from the starting conditions or by determining characterizing representations which are essentially independent of an end condition which is approached but not reached.

As may be readily seen by the person skilled in the art, the temperature function in the above mentioned transient phases is not only dependent on the load determined by the user but also by the instantaneous power delivered by the heating elements 1. In order to be able to determine the load in a manner which will be described further in the following by means of the function of the signal S9 independently of the instantaneous power and its fluctuations, the evaluation in the evaluation circuit 11 occurs, as is shown in Fig.

SUBSTITUTESHEET 1, by using the power P as another input, e.g. by using the applied voltage SP fed to the heating elements 1.

On the output of the evaluation circuit 11 one or several signals which represent characteristic values X(th(t)) of the temperature function appear and are fed to a comparator 13. In a data store 15 certain, possibly empirically obtained, characteristic values Xo(th(t)) are stored and function as limit values for determining the allowability of the load on the cooking zone 3.

In the comparator 13 the instantaneous determined values X are compared to the stored values Xo, and dependent on the result a signal S13 is obtained at its output indicating the allowability which may be used as a control signal.

Generally it is the different temperature functions in the transient heating which enables distinguishing between "pot present" and "no pot". This function may be explained by the fact that any pot placed on the cooking zone will reflect heat and will establish a high temperature faster than if there is not pot present.

In Fig. 2 the temperature function th(t) of the heating zone 3 is shown, whereby the function oT corresponds to "no pot", the function sT corresponds to "unsuitable pot", i.e. in the present case a pot with a concave bottom, and the function nT corresponds to a normal pot with an essentially flat bottom. It will be seen that the increase in temperature over time is slower in the case of oT as compared to sT or nT.

As is shown on Fig. 3, it is quite possible to use the results of Fig. 2 in an evaluation circuit which uses a dividing circuit 16 to introduce a correction relating to the nominal power P fed to the heating element of the signal S9 from the temperature sensing element 9. The signal derived by correction from S9 may be stored as thN(t) in the store 11a. In a comparator unit 13 it is compared to a previously stored function

SUBSTITUTESHEET thθ(t) taken from a similar store 15 and produces the output signal S13 which is then used as a control signal for the power supply to the cooking zone.

In a further preferred embodiment it is not the whole time function during the transient phase which is used for evaluating the thermal load situation but rather a derived function which creates a significant descriptor.

In view of the fact that the law governing the temperature drop is exponential, one descriptor could be the first derivative at the start of the temperature drop. However, the exponential function is only an approximation, in particular because it is not a first order system. This may be realized in view of the different thermal time constants present in the system, such as for the heating element, the ceramic plate on which the cooking zone is defined and the pot, and may be seen from the time function given in Fig. 2 as oT.

In Fig. 4 the first derivative function of the time functions oT, St, and nT are shown, indicated by the apostrophy. It may be seen that the maxima of the respective functions very clearly give the desired discrimination between various thermal load conditions.

In Fig. 5 is given a schematic diagram of an apparatus which will perform an analysis of the signal according to the observations discussed in relation to Fig. 4. A zero in the second derivative function indicates a maximum, and at this moment the switch K connects the first derivative signal from the differentiator 19 to the comparator 13 in which this value is compared to a previously stored value taken from the store 15. In particular a value above the value G on Fig. 4 indicates the presence of a pot. Similar to the case of Fig. 3 the output S13 will be the control signal for the power supply. Several similar signal processing means may be used to obtain reliable

SUBSTITUTESHEET indication.

The determination of the quality of the pot or its excentric placement on the zone is more exacting but it may be shown that it is quite feasible to use a particular combination of derivatives of the temperature function. In Fig. 6 the desired or set temperature thoo is taken as the abscissa, and the value S13 which is (dth/dt)MAX is taken as the ordinate. During variation of the power P supplied to the heating elements as shown in Fig. 1 measured points are traced relating to a "no pot" condition. These points are indicated oT in Fig. 6 as well as the variation of P.

Subsequently an unsuitable pot with a concavity of 14 o/oo was placed centrally on the cooking zone. In the present connection the concavity is defined as the distance between the top of the concavity and the cooking zone divided by the diameter of the bottom of the pot. Using the same variation of the power P the measured points indicated by crosses were obtained which belong to the group termed sT. Finally a series of measurements were made with a well-fitting pot placed centrally, and the corresponding points are shown as * on Fig. 6. It will now be seen that two limit curves Fl and F2 divide the whole operating area into three well-defined areas corresponding to the three classifications.

This realization may be used in a circuit which is represented schematically in Fig. 7 in order to obtain a precise indication of the thermal load and to control precisely the temperature of the cooking zone. In analogy with Fig. 5 the output signal S9 of the temperature sensor 9 of Fig. 1 is taken to the unit 19 which determines the first time derivative and to the unit 21 which determines the second time derivative. A null-detector 23 determines the time of of obtaining a null of the second derivative, and the maximum of the

SUBSTITUTESHEET first derivative which this corresponds to is transferred by means of the switch Ql as the signal S19 to the comparator unit 13.

A null of the second derivative corresponding to the first minimum of the first derivative is irrelevant for the present purpose, while the minimum value of the first derivative after the maximum is relevant for determining the end of the transient switched-off state. Hence further circuit elements 25, Q3 and Q5 are employed in order to eliminate the influence of the first minimum.

The signal S25 represents the temperature thoo at the minimum of the first derivative after the maximum, and so the end temperature reached at the end of the switched-off phase. In the storage unit 15 value pairs corresponding to the limit curves Fl and F2 are stored. By comparing the momentary value of the signals S25 and S19 with the stored values corresponding to Fl and F2 an unambiguous signal representing the condition is obtained, i.e. "acceptable pot present", "no pot", or "unacceptable pot".

The determination which is described above is preferably performed digitally, the signals from the temperature sensors being converted in A/D converters.

The apparatus herein described is independent on the temperature sensor in use, however precision is increased by a more continuous function of temperature over time when several temperature sensors are distributed in proximity to the cooking zone. Such temperature sensors may combined, optionally via suitable weighting, into one signal which has been discussed as S9.

A particularly suitable arrangement of temperature sensors is shown schematically on Fig. 8. Three temperature sensors 9a, 9b, 9c are disposed along the outer perimeter but excentrically, and these will give

SUBSTITUTESHEET rise to a number of curves corresponding to a large number of excentricities of an acceptable pot, which is hence a further parameter relating to the type of load which may be determined according to the invention.

The circuit of Fig. 1 lends itself directly to the control of the temperature of the cooking zone. When the corresponding maximum temperature Po has been reached during a phase of power supply, the transient switched-off phase begins, and when the minimum acceptable temperature Pu has been reached, a phase of power supply begins again. This enables a simpler way of determining the thermal load, i.e. the quality of a pot and its placement.

Instead of or in addition to the general method of operation described above and specified in claims 1 and 3 it may be desirable and more efficient to determine the load condition during a phase where the energy supply to the heater is switched off. Although the time constant tau of the cooling as indicated on Fig. 9 is a parameter which is well known and easy to determine in connection with a truly exponential decrease over time, it has been determined that results become more reliable when the relative decrease in the later part of the switched-off period is determined. This is also shown on Fig. 9. That is, the measurement of the decrease is performed with the end point immediately before the energy is again supplied to the heating elements. The reason for this may be found in that the influence of fluctuations in the mains voltage during the preceding period of energy supply will have averaged out at the beginning of the measurement. In practice the decrease is measured as the duration of the decrease from the desired temperature which occurs at the time tl and to a predertmined lower limit which occurs at the time t2.

SUBSTITUTESHEET

Claims

P A T E N T C L A I M S

1. An apparatus for controlling the operating temperature of a cooking zone heated from below by energy supplied in pulses by detecting and classifying into neighbouring ranges the thermal load on its surface, and comprising means for determining the average temperature of at least part of the cooking zone, c h a r a c t e r i z e d i n that at least one measurement value contributing to a representation of the momentary thermal load condition is extracted from the function of temperature over time during transient heating and/or cooling phases, of which at least one value is stored representing the limit between neighbouring operational ranges, that the measurement value is compared to the stored value, and that a control signal is generated which represents the particular range of the applied thermal load, which control signal controls the supply of energy.

2. An apparatus according to claim 1, c h a r a c t e r i z e d i n that said neighbouring ranges represent acceptable and non-acceptable ranges of operation.

3. An apparatus according to claim 1, c h a r a c t e r i z e d i n that the temperature change over time following the beginning or end of an energy pulse is monitored and that a rate of change above or below a predetermined value or values as a function of the operating temperature indicates the thermal load on the cooking zone, said indication controlling the power supply to the cooking zone. . An apparatus according to any of the claims 1 to 3, c h a r a c t e r i z e d i n that the control parameters of the control circuit utilizing the control signal are adjusted such that a stable operating temperature is obtained for the cooking zone with a predetermined hysteresis. 5. An apparatus according to any of the claims 1 to 4, c h a r a c t e r i z e d i n that the temperature decrease over time in the time period just before an energy pulse is monitored, and that a rate of decrease above a limit predetermined by the actual operating temperature indicates a state of a too small load and that the control signal switches off the energy supply.

6. An apparatus according to claim 5, c h a r a c t e r i z e d i n that a time delay is introduced from the indication of a too small load and to the switching off of the energy supply.

7. An apparatus according to claim 5, c h a r e c t e r i z e d i n that the operating temperature is determined when the lower hysteresis level is approached and just before the onset of an energy pulse, said temperature determining the limit value.

8. An apparatus according to claim 3, c h a r a c t e r i z e d i n that the pulsed supply of energy is not activated before reaching the intended operating temperature during a phase of unpulsed supply of energy.

9. An apparatus according to claim 1, c h a r a c t e r i z e d i n that the momentary average temperaure is determined in a number of locations in conjunction with the cooking zone sufficient that a horizontal profile and classfication of the thermal load is enabled.

10. An apparatus according to claim 1 and 7, c h a r a c t e r i z e d i n that the values of temperature over time are sampled and digitized for use as a first input to an evaluation circuit, that the timing and power of the energy pulses are sampled and digitized as a second input to an evaluation circuit, and that the evaluation circuit functions as a fuzzy logic device generating a control output corresponding to the acceptability of the operating range.

11. An apparatus according to any of the preceding claims, c h a r a c t e r i z e d i n that the evaluation circuit has a first operating mode (learning mode) in which the thermal load characteristics of particular cooking vessels are determined and stored, and a second operating mode in which the stored thermal load characteristics are used for determinining the acceptance or non-acceptance of the load.