Abstract:
A method for controlling an induction heating system of a cooking appliance provided with an induction coil, particularly for controlling it in connection with a predetermined working condition, comprises measuring the value of one electrical parameter of the induction heating system, feeding a computing model with actual switching frequency signals in order to estimate a temperature indicative of the thermal status of the heating system and to provide an estimated value of the electrical parameter, and comparing the measured electrical parameter with the estimated one and tuning the computing model on the basis of such comparison.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for controlling an induction heating system of a cooktop provided with an induction coil, particularly for controlling it in connection with a predetermined working condition. 
     More specifically the invention relates to a method to estimate the temperature of a cooking utensil placed on the cooktop and the temperature of the food contained therein, as well as the food mass. 
     2. Description of the Related Art 
     With the term “heating system” we mean not only the induction coil, the driving circuit thereof and the glass ceramic plate or the like on which the cooking utensil is placed, but also the cooking utensil itself, the food content thereof and any element of the system. As a matter of fact in the induction heating systems it is almost impossible to make a distinction between the heating element, on one side, and the cooking utensil, on the other side, since the cooking utensil itself is an active part of the heating process. 
     The increasing need of cooktops performances in food preparation is reflected in the way technology is changing in order to meet customer&#39;s requirements. Technical solutions related to the evaluation of the cooking utensil or “pot” temperature derivative are known from EP-A-1732357 and EP-A-1420613, but none discloses a quantitative estimation of the pot temperature. 
     Information is available in scientific literature about algorithms concerning state estimation (Recursive Least Square, Kalman Filter, Extended Kalman Filter [EKF] etc.); none of them relates to an industrial application focused on induction cooking appliances. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method according to which a temperature value connected to the temperature of the pot and/or of the food contained therein or of the induction heating system or of the glass surface placed under the pot can be assessed in a reliable way, particularly with reference to a heating condition in which the temperature of the food has to be kept substantially constant (boiling condition, simmering or the like). 
     According to the invention, the above object is reached thanks to the features listed in the appended claims. 
     The control method according to the present invention is used for estimating the temperature of a pot, pan or a griddle (in the following indicated simply as “pot”), used onto the induction cooktop, food thermodynamics state inside the pot (mass and temperature/enthalpy/entropy/internal energy, etc.) and induction coil temperature by the knowledge of the switching frequency of the induction heating system and of at least another measured electrical parameter of the induction heating system. 
     In general, the estimation reliability (roughly such reliability could be assumed a function of the difference between the actual value and the estimated value) gets better and better as the number of available electrical measurements increases. 
     Moreover, the estimation reliability gets better and better as the number of switching frequencies at which the electrical measurement(s) is acquired increases. 
     According to the invention, no constrain is imposed on the way the switching frequency(-ies), at which the electrical measurement(s) is acquired, is chosen. The estimated pot temperature can be used e.g. to monitor or control the temperature. The estimated food temperature can be used e.g. to monitor or control the temperature or the cooking phase (as boil detection, boil control, in case the ‘food’ is ‘water’ or similar kind of liquids). The estimated food mass can be used e.g. to monitor or control the cooking phase. The estimated coil temperature can be used e.g. to prevent damages due to overheating. The parameters of a simplified equivalent electrical circuit that describes the behaviour of the process are useful to estimate the temperature of the pot, to detect a dynamic mismatching, and the pot quality as well. 
     Another aspect of the present invention is to provide a method that non only allow to evaluate the temperature of the pot or of the food contained wherein (and eventually its mass), but also that is able to compensate different noise factors. Some noise factors that can affect the estimation are for example the initial pot/food temperature and initial food mass, the voltage fluctuation of the electrical network, the tolerances/drift of the components, the use of different pots and the possible movements of the pot away from its original position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages according to the present invention will become clear from the following detailed description with reference to the annexed drawings in which: 
         FIG. 1  is a schematic view of an induction cooktop; 
         FIG. 2  is a sketch showing how the model according the invention works; 
         FIG. 3  is a schematic view of an electric circuit of one possible equivalent models; 
         FIG. 4  shows one of the possible implementation of the method according to the invention; 
         FIG. 5  shows a diagram comparing the actual and the estimated values of the equivalent resistance of the primary circuit; 
         FIG. 6  is a figure similar to  FIG. 5  and relates to a comparison between the actual and the estimated temperature values of the pot; 
         FIG. 7  is similar to  FIG. 5  and shows the comparison with and without voltage compensation; and 
         FIG. 8  is similar to  FIG. 6  and shows the comparison with and without voltage compensation. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 2 , the method comprises one (or more) electrical measurement of an electrical parameter, a mathematical model that provides at least an estimation of the electrical measurement(s) and one or more temperatures as a function of the switching frequency, and any kind of algorithm that tunes on-line the mathematical model in function of the difference between estimated and measured electrical parametes. 
     The on-line tuning of the model represents a way to compensate:
         the initial state uncertainty—i.e. if the model is based on differential equations, the initial state of the solution is required but it could be unknown;   measurement errors—measurements are usually affected by noises;   model uncertainties—i.e. each model is a simplified representation of the reality and so it is always affected by “model uncertainties”.       

     The ability to compensate the above uncertainties and errors comes from a model based approach that combines the model and the tuning thereof by a feedback on the difference between prediction end measures. Many algorithms are available in literature to fix these kinds of problems (Recursive Least Square, Kalman Filter, Extended Kalman Filter [EKF]) and therefore no detailed description of these is deemed necessary here. 
     As the effect of the temperature of the pot is usually appreciable only on a small subset of the model parameters, the on-line tuning of the algorithm can be split up in two steps. In the first step part of the model parameters (eventually all or none of them) are tuned on the basis of a first set of data; in the second step only the subset of model parameters that are affected by temperature variations are tuned on the basis of the data collected during the cooking phase. 
     To improve the performances of this method, the first step of the on-line tuning can be repeated during the cooking process whenever a modification on the process is detected (e.g. when a pot mismatching is detected), so giving the opportunity to compensate detectable noises. 
     As a consequence of the approach described above, a possible implementation of the method according to the invention is as follows. 
     EXAMPLE 
     
         
         
           
             the current circulating in the induction coil (i) is measured; 
             the simplified mathematical model described by the following differential equations (Eq. 1) and shown in  FIG. 3  is used: 
             in order to complete the method proposed in this example, the Extended Kalman Filter is used as on-line tuning algorithm. 
           
         
       
    
     The model proposed in this example is described by the following differential equations (Eq. 1), in which the suffix “p” stands for the primary circuit (i.e. the induction coil, and the capacitors) and the suffix “s” stands for the secondary circuit (i.e. the metal pot). These equations are an example of the relation between the input voltage, the current in the primary circuit and the current in the secondary circuit: 
                   {                 L   P     ⁢       ⅆ     i   p         ⅆ   t         +     M   ⁢       ⅆ     i   s         ⅆ   t         +       R   P     ⁢     i   p       +       1   C     ⁢     ∫         i   p     ⁡     (   τ   )       ⁢     ⅆ   τ             =       V     I   ⁢           ⁢   N       ⁡     (     t   ,   f     )                       M   ⁢       ⅆ     i   p         ⅆ   t         +       L   s     ⁢       ⅆ     i   s         ⅆ   t         +       R   s     ⁢     i   s         =   0                 R   s     =       R   0     (     1   +     α   ⁡     (       T   pot     -     T   0       )                           (     Eq   .           ⁢   1     )               
where:
         C → equivalent capacitance of the primary circuit;   R p  → equivalent resistance of the primary circuit;   L p  → equivalent self-inductance of the primary circuit;   L s  → equivalent self-inductance of the secondary circuit;   M → equivalent mutual inductance;   R s  → equivalent resistance of the secondary circuit;   V in  → input voltage of the primary circuit;   i p  → current circulating in the primary circuit;   i s  → current circulating in the secondary circuit;   R 0  → equivalent resistance of the primary circuit when T pot =T 0 ;   T pot  → Temperature of the pot bottom   T 0  → Reference temperature   α → Adimensional parameter       

     The model provides an estimation of different electrical variables of interest (in this case i p , i s ), at least one of which must be measurable (i p ), and the estimation of the temperature of the pot ({circumflex over (T)} pot ) and uses the switching frequency f. For the on-line estimation of the model parameters it is possible to take advantage of the measures that are usually available on the appliance. For sake of simplicity, in the rest of the description of the invention it will be assumed to have the measure of the root mean square of the current circulating in the coil (i p ); however, an analogous process can be used having different electrical measures or different measurement points. 
     As a result, the general sketch shown in  FIG. 2  can be modified as in  FIG. 4 , where the element “K” represents the Kalman Matrix. 
     In this model the temperature of the pot is affecting only the R s  parameter; hence the on-line tuning of the algorithm in this case can be split up in two steps:
         part of the model parameters—C , R p , L p , L s , M and R s —(eventually all or none of them) are tuned on the basis of a first set of data;   only the subset of model parameters that are affected by temperature variations—R s —is tuned on the basis of the data collected during the cooking phase.       

     Theoretically, the parameters C, R p  and L p  should be known by the manufacturer but the tolerances/drift of the components and the model imprecision require usually an on-line estimation of these parameters together with M , L s  and R s . However, if the resulting error is tolerated, one could skip the first part of the on-line tuning assuming that all the parameters are known. 
     In the present example, in the former step of the on-line tuning all of the model parameters have been optimized by using a line search algorithm on the basis of six acquisition of i p  at six different frequencies. In the second step of the on-line tuning the R s  parameter has been tuned with a Kalman filter using the current i p  acquired at a known frequency that can eventually change during the cooking process. 
     Even though the optimized parameters are different from the actual ones (cfr.  FIG. 5 ), as can be seen in  FIG. 6  the temperature of the pot is correctly estimated. In this particular case, the model is not able to compensate the initial state temperature error but the use of a more sophisticated model that takes into account also the thermal dynamics of the food can do this type of compensation. 
     The results of the previous example can be improved by introducing the voltage measure. In a further example the inlet voltage drifts from 230 V rms at the beginning of the simulation to 232.3 V rms (1% in 100 s) at the end whereas all the other simulation parameters are equal to the ones of the previous example. As shown in  FIG. 7  and  FIG. 8 , in which the results obtained with and without using the voltage information are compared, the voltage variation can be compensated only if this information is available. 
     As it is clear from the above description, the present invention can be used to improve the performances of an induction cooktop, to provide more information about the status of the cooking phase and to enable new product features. In particular the expected benefits are:
         the estimated pot temperature can be used e.g. to monitor or control the the temperature;   the estimated food temperature can be used e.g. to monitor or control the the temperature or the cooking phase (as boil detection, boil control, in case the ‘food’ is ‘water’ or similar kind of liquids);   by knowing the type of food, the computing model is able to detect a predetermined optimal working condition, for instance the optimal temperature for the Maillard reaction (if the food is meat or the like);   the estimated food mass can be used e.g. to monitor or control the cooking phase;   the estimated coil temperature can be used e.g. to prevent damages due to overheating; and   the parameters of a simplified equivalent electrical circuit that describes the behaviour of the process are useful to estimate the temperature of the pot, to detect a dynamic mismatching and the pot quality.       

     Even if the control method according to the present invention is primarily for applications on cooktops or the like, it can be used also in induction ovens as well.