Patent Publication Number: US-8530798-B2

Title: Hob having a temperature sensor

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a hob having at least one heating zone and a temperature sensor according to the preamble of claim  1  and to a method for operating a hob according to the preamble of claim  8 . 
     A hob having a heating zone and a temperature sensor located in the center of the heating zone is known from DE 10 2006 057 885 A1. A method is described by which an instant at which the liquid in the cookware element reaches a boiling point is predicted as accurately as possible. The cookware contents are prevented from boiling by reducing the supply of heating energy before boiling point is reached. Predicting is based on evaluating characteristic temperature curves recorded in the past. 
     Different liquids&#39; boiling points can, though, in practice differ greatly depending on, for instance, their composition and/or the prevailing atmospheric pressure. The same applies to the shape of temperature curves, which is unpredictable for the hob&#39;s control units also through the addition of ingredients during the heating-up process. However, precisely predicting the boiling point is important for realizing an effective simmer operation of the hob during which the contents of the cookware element are kept at a temperature just below boiling point. A large amount of energy can be saved by simmering compared with vigorous boiling because the evaporation energy released as a result of boiling can be very high. If, though, the temperature of what is being cooked is too low and the difference between said temperature and boiling point is too great, the cooking process will be protracted and/or lead to undesired results. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is hence in particular to enable an energy-saving simmer operation at a target temperature precisely coordinated with the cookware contents&#39; boiling point. Said object is achieved in particular by means of the features of the independent claims. Advantageous embodiments and developments of the invention will emerge from the subclaims. 
     The invention proceeds in particular from a hob having at least one heating zone, a temperature sensor for detecting the temperature of a cookware element placed on the heating zone, and a control unit for operating the heating zone. The control unit is designed so that in at least one operating mode it will heat up the cookware element during a heating-up phase and regulate the cookware element&#39;s temperature to a target temperature during a holding phase. 
     It is proposed for the control unit to be designed to detect a boiling point of the liquid in the cookware element during the heating-up phase and determine the target temperature as a function of the boiling point. The purpose is accordingly for the boiling point to be measured during the heating-up phase itself and not, say, in a series of complex trials with different cookware contents. Detecting the boiling point directly will make it possible to dispense with error-prone predictions and estimations of the boiling point. The heating-up phase will accordingly last at least until the boiling point has been reached. The boiling point will be determined very precisely because errors in predicting or estimating it can be avoided through measuring it directly. The target temperature during the heating phase, which can in particular be a simmer temperature, can thus be precisely determined as a function of the boiling point. 
     In particular the target temperature can be selected as being a predefined temperature difference lower than the boiling point. The energy consumption accompanying vigorous boiling can be avoided thereby and a fast cooking process ensured. The temperature difference can be in particular between 2° and 7° C. 
     For detecting the boiling point the control unit can record a temperature curve of the cookware element particularly during the heating-up phase and detect a substantially constant section along the temperature curve. The temperature of the cookware contents will not increase further on reaching the boiling point, which will result in a constant temperature on the outside of the cookware. A temperature averaged within the constant section can be used as the measurement value for the boiling point. The temperature sensor&#39;s signal can be filtered and/or averaged or subjected to suitable scale transforming generally in a manner appearing appropriate to a person skilled in the relevant art. 
     For detecting the constant section of the temperature curve the control unit can in particular form a gradient of the temperature curve. The temperature curve can be classified as “substantially constant” if the gradient is below a specific threshold. 
     Safety shutdown can be ensured if the control unit is designed to deactivate the operating mode and produce a warning signal if the temperature registered by the temperature sensor exceeds a maximum value. Said maximum value can be for example approximately 150° C. Exceeding the maximum value indicates that the cookware element is empty so that boiling cannot take place. 
     Another aspect of the invention relates to a method for operating a hob having at least one heating zone, a temperature sensor for detecting the temperature of a cookware element placed on the heating zone, and a control unit. In at least one operating mode the cookware element is heated up during a heating-up phase and the cookware element&#39;s temperature is regulated to a target temperature during a holding phase. 
     It is inventively proposed for a boiling point of the liquid in the cookware element to be detected during the heating-up phase and the target temperature to be determined as a function of the boiling point. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and characteristic features of the invention will emerge from the following description of the figures. An exemplary embodiment of the invention is shown in the figures. The drawings, the description, and the claims contain numerous features in combination. A person skilled in the relevant art will expediently also consider said features individually and combine them further to purposeful effect. 
         FIG. 1  is a schematic of an induction hob having a temperature sensor and a cookware element placed on a heating zone, 
         FIG. 2  shows the curves of a heating power, a wall temperature of the cookware element, and a temperature of something being cooked according to an exemplary embodiment of the invention, and 
         FIG. 3  is a flowchart of a method for operating an inventive hob. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION 
       FIG. 1  shows a hob having a heating zone  10 , a temperature sensor  12  for detecting the wall temperature of a cookware element  14  placed on the heating zone, and a control unit  16 . Heating zone  10  is an area that is marked on a cover plate  18  of the hob and whose position and size correspond to the position and size of an inductor  20  located beneath cover plate  18 . The hob is an induction hob and inductor  20  receives a high-frequency heating current from an inverter  22 . Located between a domestic power-supply terminal  34  and inverter  22  is a rectifier that is not shown here. Control unit  16  determines the frequency and/or amplitude of the heating current produced by inverter  22  in such a way that, averaged over time, a specific heating power will be produced. Via inductor  20 , the heating current produces a high-frequency alternating magnetic field which in turn produces high-frequency eddy currents in the base of cookware element  14 . Cookware element  14  is heated through the dissipation of said eddy currents. 
     Temperature sensor  12  is an infrared temperature sensor that projects tower-like beyond the topside of cover plate  18  and detects infrared radiation being emitted from a side wall of cookware element  14 . The signal detected by sensor  12  is processed by a sensor read-out unit  24  and forwarded to control unit  16 . Sensor read-out unit  24  can perform, for example, low-pass filtering and/or scale-transforming. 
     In contrast to cover plate  18  having a tower-like temperature sensor, other exemplary embodiments of the invention are conceivable in which the temperature sensor is embodied as being an NTC element located beneath cover plate  18  or as being an infrared sensor located beneath the cover plate. It is also conceivable for temperature sensor  12  to be fixed directly to the wall of cookware element  14 . 
     Control unit  16  is a universally programmable computing unit that performs a software-implemented method for operating the hob. The method has different operating modes. In a special operating mode, which could also be called a simmer mode, cookware element  14  is heated up during a heating-up phase  26  until a liquid  28  in cookware element  14  reaches its boiling point TB. 
     Control unit  16  keeps cookware element  14  at boiling point TB only until said point has been determined with sufficient accuracy. Control unit  16  then switches from heating-up phase  26  to a holding phase  32  during which the temperature of cookware element  14  or, as the case may be, liquid  28  will be regulated to a target temperature TS. The feedback from temperature sensor  12  is used for forming a closed control loop. 
     The correlation between the temperature of liquid  28  and the temperature of cookware element  14  or, as the case may be, the temperature of the radial outer wall of cookware element  14  can be ascertained by way of an empirically determined function. The outer wall&#39;s radiation losses as a rule result in there being a proportionality between the wall temperature of cookware element  14  and the temperature of liquid  28 , which proportionality can be expressed by a constant factor. For the invention it is of secondary importance what value the boiling point TB of liquid  28  itself has. What matters is to precisely determine what the external temperature of cookware element  14  is when boiling point TB has been reached. Both temperatures TB, TS can be used equivalently owing to their proportionality. 
       FIG. 2  shows the time curve of a heating power (continuous line), the temperature of liquid  28  (dashed line), and the temperature of the wall of cookware element  14  (dotted line). Liquid  28  reaches boiling point TB at an instant t 1  during heating-up phase  26  and its temperature in cookware element  14  will be substantially constant along a section  30  of the temperature curve. Control unit  16  averages the temperature measured in said section  30  by temperature sensor  12  and stores said temperature as the boiling point TB or, as the case may be, as the wall temperature, assigned to the boiling point, of cookware element  14 . Control unit  16  switches at an instant t 2  to holding phase  32  during which liquid  28  is kept as constantly as possible at a target temperature TS. That takes place in a closed control loop. If, say, at an instant t 3  something more requiring to be cooked or more liquid  28  is added to cookware element  14 , the liquid&#39;s temperature will briefly drop but will be raised again to the target temperature TS. If the drop in temperature is too great, in an exemplary embodiment of the invention the boiling point can be detected again to possibly allow for a changed composition of the liquid. 
     Control unit  16  determines the target temperature by subtracting a predefined stored value from the previously detected boiling point TB. This subtracted temperature difference can be, for instance, 5° C. so that for pure water at standard atmospheric pressure the target temperature will be 95° C. Food is cooked in substantially the same way at 95° C. as in water that is vigorously boiling at 100° C. so that evaporation energy can be saved with little adverse effect on the cooking process. 
       FIG. 3  is a flowchart of performing the inventive method. Temperature sensor  12  that can be lowered into cover plate is activated at a step S 1  and moved up from its lowered position into an activation position. Heating-up phase  26  is started at a step S 2 . The temperature of the outer wall of cookware element  14  is measured at a step S 3  and at a step S 4  an estimation of the temperature of liquid  28  is ascertained from said temperature by multiplying it by a constant. Control unit  16  judges at a step S 5  whether liquid  28  is boiling or not. Control unit  16  for that purpose assesses the temperature recorded after the last measurements and checks whether except for unavoidable fluctuations it is constant. If it is, the boiling point TB has been reached. If it is not, the method will branch back to step S 3  and perform another measurement. 
     Emergency shutdown (not shown) will take place if a temperature that is above a maximum temperature is detected at step S 5 . 
     If it is established at step S 5  that boiling point TB has been reached, control unit  16  will at a step S 6  compute target temperature TS for liquid  28  and proceed at a step S 7  to holding phase  32 . In a closed control loop the temperature of cookware element  14  is measured at a step S 8 , the temperature of liquid  28  is computed from the temperature of cookware element  14  at a step S 9 , and the heating power is regulated at a step S 10  as a function of the result. If the temperature of liquid  28  is above the target temperature, the heating power of inductor  20  will be reduced by varying the frequency of inverter  22 . The heating energy of inductor  20  will be increased if the temperature is below target temperature TS. The method will branch back to step S 8  when the heating energy has been adjusted. 
     REFERENCE CHARACTERS 
     
         
           10  Heating zone 
           12  Temperature sensor 
           14  Cookware element 
           16  Control unit 
           18  Cover plate 
           20  Inductor 
           22  Inverter 
           24  Sensor read-out unit 
           26  Heating-up phase 
           28  Liquid 
           30  Section 
           32  Holding phase 
           34  Domestic power-supply terminal 
         TS Target temperature 
         TB Boiling point