Abstract:
A system and method for limiting the temperature of a burner for a cooking appliance without the use of a temperature sensor. The method includes the step of sensing the conduction state of a thermal switch and feeding back the sensed signal to control the duty-cycle (and thus “on” time) of bang-bang thermal limiting control. The power to the burner is reduced until the sensed duty-cycle (near  100 %) cycling is reduced (lower frequency and amplitude) resulting in smoother power and temperature control. Preferably, the control system and method is implemented for controlling power applied to a burner for a glass-ceramic cooktop.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to temperature control systems for cookware and, particularly, to a novel thermal limiting system and method for controlling application of thermal energy to a burner element of a cookware apparatus.  
           [0003]    2. Discussion of the Prior Art  
           [0004]    The life of the glass ceramic material forming a cooking surface or burner in a cookware apparatus is dependent on the temperature it is subjected to. Therefore, the power to a burner must be limited to prevent premature failure of the glass. The temperature of the glass is a function of time, burner power and the properties of the cooking utensil place on it (e.g. flatness, reflectivity, contents, etc.) consequently a method of dynamically adjusting the power to prevent overheating is needed, i.e. thermal limiting control.  
           [0005]    In conventional systems, the temperature is limited in two ways: 1) by using of a temperature switch that interrupts power to the burner at excessive temperatures such as described in U.S. Pat. No. 6,150,641, the whole contents and disclosure of which is incorporated by reference as if fully set forth herein; or, 2) by directly sensing the temperature and applying appropriate feedback control such as described in U.S. Pat. No. 6,285,012, the whole contents and disclosure of which is incorporated by reference as if fully set forth herein.  
           [0006]    The first thermal limiting approach  10 , as described in U.S. Pat. No. 6,150,641, and illustrated in FIG. 1( a ), includes implementing a thermal switch and bang-bang thermal limiting to control the temperature  18  of the cookware burner  12 , and incorporates a power control component  14  receiving the power command signal  16  which, in this approach, constitutes the user power command signal. This approach is inexpensive but results in large swings in power and temperature of the cooking utensil. That is, in this first approach, a thermal switch is used to provide bang-bang temperature control when the temperature exceeds the predetermined limit. This type of control results in the frequent cycling of the power causing corresponding swings in the pan temperature.  
           [0007]    [0007]FIG. 2( a ) illustrates an example simulation of bang-bang thermal control implemented for a ceramic burner. In the example simulation, the thermal switch is modeled as a relay with an arbitrary 30° C. of hysteresis, and the thermal response of the burner (e.g., glass temperature output) is modeled as a first order linear model (derived empirically). Initially, as shown in FIG. 2( a ), the user-demanded power setting (user power command signal) is about one-half (50%) of the maximum power. At this initial setting, thermal limiting does not engage as indicated in FIG. 2( b ). At the time indicated at  141 , the user increases the power to 100% (FIG. 2( a )) causing the conduction state  145  of the thermal switch (e.g., bi-metallic switch) to change in accordance with bang-bang thermal limiting at time indicated as time  142 . In FIG. 2( b ), the conduction on/off states, i.e., engagement of bang-bang thermal limiting, is represented as the plot  145 . At this setting, the glass temperature of the burner increases to the thermal limit  182 , e.g., the safety thermal limit of a glass burner, as shown in FIG. 2( c ). Finally, the user reduces the power back to its initial one-half power level and thermal limiting ceases, as indicated at time  143  in FIG. 2( a ).  
           [0008]    The second thermal limiting approach  20 , as described in U.S. Pat. No. 6,285,012, and illustrated in FIG. 1( b ), includes implementing a thermal limiting controller component  22  that limits thermal heating of burner  12 ′ in accordance with the user power command signal  16 ′, a predetermined thermal limit signal  25 , and an instantaneous sensed temperature  28  that is feedback from a temperature sensor element included with the burner  12 ′. As described in U.S. Pat. No. 6,285,012, the controller includes proportional plus integral control, minimum selector and anti wind-up control elements (not shown) to provide thermal limiting for a burner  12 ′ implementing a sensor. The output  15  of the thermal limit controller  22  is input to a further power control unit for adjusting, e.g., quantizing the thermal limiter power output. This approach provides for very smooth power and temperature profiles but the temperature sensor is often expensive.  
           [0009]    It would thus be highly desirable to provide a thermal limiting system and method for providing thermal limiting control to a cooktop burner of an electric cooking device, that provides for very smooth power without the use of an expensive thermal sensor.  
         SUMMARY OF THE INVENTION  
         [0010]    A system and method for smoothly limiting the temperature of a burner of a cooking appliance, e.g. a stove ceramic burner, without the use of a temperature sensor. The method includes the steps of sensing the conduction state of a thermal switch in a bang-bang thermal limiting burner, and feeding back a signal representing this switch conduction state to control duty-cycle (and thus “on” time) of the applied power. The power to the burner is reduced until the sensed duty-cycle cycling is reduced (lower frequency and amplitude) resulting in smoother power and temperature control.  
           [0011]    Preferably, this sensed duty-cycle cycling is increased to near 100%, i.e., the thermal switch conducting state is almost always on, i.e., off-time is reduced.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Details of the invention disclosed herein shall be described below, with the aid of the figures listed below, in which:  
         [0013]    [0013]FIG. 1( a ) is a block diagram illustrating a typical thermal limiting architecture using bang-bang thermal limiting control;  
         [0014]    [0014]FIG. 1( b ) is a block diagram illustrating a typical thermal limiting architecture using temperature feedback control to provide thermal limiting;  
         [0015]    FIGS.  2 ( a )- 2 ( c ) illustrate exemplary, simulation results of a cooking appliance burner implementing bang-bang thermal limiting control;  
         [0016]    [0016]FIG. 3 is a high-level block diagram of the thermal limiting architecture of the present invention implementing bang-bang thermal limiting;  
         [0017]    [0017]FIG. 4 is a detailed block diagram of the thermal limiting architecture of the present invention according to a first embodiment;  
         [0018]    FIGS.  5 ( a )- 5 ( c ) illustrates an example simulation of bang-bang thermal control including power command, thermal switch conduction state and glass temperature implemented for a ceramic burner according to the first embodiment;  
         [0019]    [0019]FIG. 6 is a detailed block diagram of the thermal limiting architecture of the present invention according to a second embodiment;  
         [0020]    FIGS.  7 ( a )- 7 ( c ) illustrates an example simulation of bang-bang thermal control including power command, thermal switch conduction state and glass temperature implemented for a ceramic burner according to the second embodiment;  
         [0021]    [0021]FIG. 8 is a detailed block diagram of the thermal limiting architecture of the present invention according to a third embodiment; and,  
         [0022]    FIGS.  9 ( a )- 9 ( c ) illustrates an example simulation of bang-bang thermal control including power command, thermal switch conduction state and glass temperature implemented for a ceramic burner according to the third embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    As now described with respect to FIG. 3, the present invention is a system and method  100  for reducing the power cycling by modifying the power applied to a ceramic burner  120 , which uses bang-bang thermal limiting. The bang-bang controller works by interrupting power to the burner when the temperature exceeds a preset limit and restoring it again when it drops, usually with some hysteresis. Typically this is implemented with a thermal switch, e.g., a bimetallic switch (not shown).  
         [0024]    As illustrated in FIG. 3, the conduction state of the switch represented as the “on/off” time signal output  260  representative of bang-bang thermal limiting, is fed back to a novel thermal limiting controller component  190 , which also receives a desired user power command signal  160 . The thermal limiting controller device  190  in response, outputs a minimum power value, that is, a power command signal generated either by the user from user manipulation of a burner control knob, for example, or the thermal controller. The power to the burner is reduced until the sensed duty-cycle is equal to a reference duty cycle  250  (that is, on the average). A power control element  140 , typically an AC switch (e.g., a TRIAC), is actuated to receive the power command signal  150  output from the thermal limiting controller  200  and reduce the power via either cycle skipping, phase control, or the like, to provide power at fine resolutions for heating the burner. It is understood that one skilled in the art may implement other techniques for applying power in fine resolutions. A detailed description of a preferred mechanism for providing power control via cycle skipping is described in commonly-owned, co-pending U.S. patent application Ser. No. 10/000,275 entitled APPARATUS FOR CYCLE SKIPPING POWER CONTROL. Choosing a sufficiently large reference duty-cycle (near 100%) reduces thermal cycling (lower frequency and amplitude) and thus, provides smoother power and temperature control. Thus, if the user desires more power than the system can deliver, the invention will detect this power request, and the temperature controller will generate a power command signal  150  designed to limit the power the user asks for. According to the first embodiment, the temperature controller generates a signal causing application of power to the burner at a higher duty cycle (e.g., near 100% on time) either (at or below) the upper temperature safety limit. In this manner, the maximum power is being run without excessive bang-bang control engagement.  
         [0025]    [0025]FIG. 4 illustrates one embodiment of the thermal limiting system and method of the invention depicted generally in FIG. 3. As shown in FIG. 4, the system  101  includes the following primary elements: the thermal limiter controller  200 , including a duty cycle controller  210 , anti-windup controller  220 , and a duty cycle estimator  250 . In this first embodiment, the thermal limiter controller  200  receives a signal  202  representing a desired duty cycle. For example, a signal  202  representing 100% duty cycle may comprise a pre-defined d.c. voltage while a signal  202  representing 50% duty cycle may be one-half of that pre-defined d.c. voltage level, etc.. The duty cycle estimator  250  estimates the instantaneous duty-cycle by timing the “on” and “off” durations of the sensed conduction state, i.e., times when thermal limiting is engaged. Specifically, integrator circuits  252   a,    252   b  receive a signal  253  representative of the on/off bang-bang control engagement cycle, i.e., conduction state of the thermal switch. There are many ways to obtain the conduction state of the thermal switch. For example: 1) by measuring the voltage across a small resistor in series with the burner load; 2) by measuring the voltage across the thermal switch; or, 3) by measuring the voltage across the TRIAC, etc. Care must be taken to measure the voltages when the AC switch in the power control  140  is conducting (unless some form of linear power regulation is employed rather than an AC switch is used for power control.  
         [0026]    In the duty cycle estimator  250 , respective integrator  252   a  integrates the signal to determine an “on” time proportional value, while the integrator  252   b  integrates the inverse of signal  253 , i.e., representative of the “off” time, to determine an “off” time proportional value. Circuitry  255  adds the on time and off time proportional values to determine a total time. The circuit then computes the instantaneous bang-bang control duty cycle estimate  256  comprising the “on” time over the total time. At each cycle, i.e., each on/off transition resets the integrators  252   a,    252   b  and resets a latch  258  which holds the duty cycle estimate of the prior cycle. The output signal  259  of the duty cycle estimator is the maximum of the instantaneous duty cycle estimate for the current cycle or the latched duty cycle estimate of the immediate prior cycle.  
         [0027]    Thus, in the embodiment depicted in FIG. 4, the duty-cycle estimate is formed by averaging the thermal limiting conduction state. There is a heuristic modification as follows: 1) the instantaneous duty-cycle estimate is formed by the ratio of the cumulative “on” time to the cumulative total time (i.e. the instantaneous average) since the last thermal limiting cycle began (i.e. “on” to “off” transition); 2) at the end of the thermal limit cycle the instantaneous estimate is latched and held constant over the next interval as the previous cycle&#39;s estimate of duty-cycle; and, 3) the duty-cycle estimate is the maximum of the previously latched estimate and the current instantaneous estimate. This increases the responsiveness of the estimate when the duty-cycle is increasing.  
         [0028]    Further, as shown in FIG. 4, the duty cycle estimate output signal  259  is input to the duty cycle controller  210  where it is compared to the desired duty cycle command signal  202  to provide a feedback signal which is input to an integral controller  212 . The duty cycle controller  210  employs integral control  212  to regulate the duty-cycle to the desired value. The generated power command signal  150  is the minimum of the integrator output and the user specified power command  160 . It is understood that the integrator  212  employed is reset when the user changes power.  
         [0029]    As further shown in FIG. 4, an anti-windup controller  220  is employed to smooth transitions from the user power command to closed loop control, i.e., prevent the integrator  212  from winding up. The anti-windup controller circuit  220  comprises summer device  214  and amplifier device  216  for tracking the user power command. The summer device  214  receives the duty cycle controller thermal limiter input  149  and, the thermal limited power command signal  150  output of the minimum block  213  which comprises either one of the duty cycle controller thermal limiter input  149  to the minimum block  213  or, the user power command signal  160 , and generates the difference. When the duty cycle controller thermal limiter input  149  is the minimum, this difference is zero the anti-wind up controller output is zero. However, the anti-wind up controller will track a difference signal when the user power command is in control. The difference signal is fed back to the duty cycle (integral) controller to form another control loop for tracking user power command and preventing integrator wind-up.  
         [0030]    As further shown in FIG. 4, the controller circuit  200  further includes a change detector device  225  which resets when the user changes power. That is, the change detector device  225  takes the derivative of the user power. If the derivative is below some threshold, indicating user power reduction (when in the negative direction), the integrator is reset. It is understood that, a user power change in a positive direction may be also be detected to initiate further circuit correction.  
         [0031]    [0031]FIG. 5( a ) illustrates an example simulation of bang-bang thermal control implemented for a ceramic burner according to the first embodiment of FIG. 4. In the example simulation, the thermal switch is modeled as a relay with an arbitrary 30° C. of hysteresis. The thermal response of the burner (e.g., glass temperature output) is modeled as a first order linear model (derived empirically). Initially, as shown in FIG. 5( a ), the user-demanded power setting (user power command signal) is about one-half (50%)of the maximum power. At this initial setting, thermal limiting does not engage as indicated in FIG. 5( b ). At the time indicated at  151 , the user increases the power to 100% (FIG. 5( a )) causing the conduction state  155  of the thermal switch (e.g., bi-metallic switch) to change in accordance with bang-bang thermal limiting at time indicated as time  152  in FIG. 5( b ) and thermal limiting is engaged. In FIG. 5( b ), the conduction on/off states, i.e., engagement of bang-bang thermal limiting, according to the first embodiment of the invention, is represented as the plot  155 . At the point in time indicated at time  153 , the output power command signal  150  of the duty cycle controller becomes less than the user power command (the output of the minimum block of the duty cycle controller is generated from the duty cycle controller which is now in command to reduce the power to the burner). The power command  150  smoothly decreases to a value in close proximity above the power needed to maintain the temperature at the thermal limit, and the duty cycle of the bang-bang control, i.e., “on” state of the thermal switch, increases according to the pre-set duty cycle signal  202 , which is less than but approaching 100%. This preset value may be, e.g., 96%, or any appropriate value as long as the on time is significantly longer than the cycle off time and will vary depending upon the application. At this setting, the glass temperature of the burner increases to the thermal limit  182 , e.g., the safety thermal limit of the burner, as shown in FIG. 5( c ). As shown in FIG. 5( c ), there are longer periods  158  of the thermal switch being in a conduction state. Finally, the user reduces the power back to its initial one-half power level and thermal limiting ceases, as indicated at time  156  in FIG. 5( a ). In sum, as shown in FIG. 5( b ), the duty cycle control of bang-bang thermal limiting for the example simulation according to the first embodiment demonstrates a slow response time due to the duty cycle estimation processing, but achieves a smooth power decrease as shown in FIG. 5( a ).  
         [0032]    It should be understood that the duty cycle estimator circuit  250  of FIG. 4, may be configured in a variety of ways known to skilled artisans. In a simple embodiment (not shown) the duty-cycle estimator may be simply replaced with a low pass filter having a time constant tau (τ) greater than the typical “on” time (i.e., tau&gt;typical on time) of the thermal limiting cycle to form the duty-cycle estimate  259 . This may increase the controller response time, but the estimation circuit (duty cycle averaging) is simplified.  
         [0033]    It should be further understood that in another embodiment (not shown) the duty-cycle estimation employed may be programmed in software operating under computer, e.g., microprocessor, control.  
         [0034]    The same integral control described with respect to the first embodiment of FIG. 4, may be used without explicitly estimating duty-cycle of the conduction state. Thus, in a second embodiment of the invention, depicted in FIG. 6, a thermal limiting system and method  102  includes the following primary elements: the thermal limiter controller  300 , including a duty cycle controller  310 , and an anti-windup controller  320 . In this second embodiment, the conduction state  353  of the thermal switch (not shown) is directly fed back to the controller  300  which, as in the first embodiment, performs an averaging function. That is, the integrator  312  in the duty cycle controller circuit  310  intrinsically estimates the duty-cycle by averaging the conduction state signal  353  (the desired duty cycle minus the conduction state signal). Specifically, the integral control drives the difference between the desired duty cycle  302  and the average of the conduction state (i.e., estimate of the bang-bang engagement duty cycle) to zero. This control provides faster response (no explicit duty cycle estimator circuit) at the expense of saw-tooth like power cycling, which may be beneficial in some applications.  
         [0035]    [0035]FIG. 7( a ) illustrates an example simulation of bang-bang thermal control implemented for a ceramic burner according to the second embodiment of FIG. 6. In the example simulation, the user-demanded power setting (user power command signal) is about one-half (50%)of the maximum power. At this initial setting, thermal limiting does not engage as indicated in FIG. 7( b ). At the time indicated at  171 , the user increases the power to 100% (FIG. 7( a )) causing the conduction state  175  of the burner&#39;s thermal switch (e.g., bimetallic switch) to change in accordance with bang-bang thermal limiting at time indicated as time  172  in FIG. 7( b ) and thermal limiting is engaged. In FIG. 7( b ), the conduction on/off states, i.e., engagement of bang-bang thermal limiting, according to the second embodiment of the invention, is represented as the plot  175 . At the point in time indicated at time  173 , the duty cycle controller  300  is activated for limiting output power, and the power command signal  150  starts decreasing (becomes less than the user power command). As shown in FIG. 7( b ), as bang-bang control is engaged, the power command signal  150  again increases when the conduction state is on and decreases when the conduction state is off in a saw-tooth fashion according to the conduction state. This is because the input to the integral controller  312  is only one of two values: the desired duty cycle  202  minus zero, i.e., when the conduction state is zero (0), or the desired duty cycle  202  minus one, i.e., when the conduction state is one (1), as the conduction state is directly fed back to the controller. This power command thus will always have two different values increasing or decreasing at two different slopes (never zero). Thus, as the integrator integrates up or down, the power command  150  oscillates to maintain burner temperature at or about the thermal limit. This results in the glass temperature oscillating about the thermal limit temperature  182 , i.e., the safety thermal limit of the burner, as shown in FIG. 7( c ). Finally, the user reduces the power back to its initial one-half power level and thermal limiting ceases, as indicated at time  176  in FIG. 7( a ). As shown in FIG. 7( b ), the duty cycle control of bang-bang thermal limiting of the example simulation according to the second embodiment responds more quickly than the controller circuit of the first embodiment of  5 ( b ), however at the expense of greater power fluctuation as shown in FIG. 7( a ).  
         [0036]    In a third embodiment of the invention, depicted in FIG. 8, a thermal limiting system and method  103  is provided for directly calculating power needed to maintain the temperature at the thermal limit, or else apply the user power, whichever is smaller. Thus, in the third embodiment of the invention, depicted in FIG. 8, the power command controller element  400  includes: a duty cycle estimator circuit which may be the estimator circuit  250  according to the first embodiment, a low pass filter, or like software or hardware implemented duty cycle averaging device; a thermal limiting power estimator device  410  including a multiplier device  413  and an averaging circuit  411  for averaging how much power it estimates is being applied to the burner based on the product of the estimated instantaneous duty cycle  407  and the average of the power command signal  150  being requested; and, a periodic reset logic circuit  420  for periodically calculating and applying the power needed to maintain temperature at the thermal limit. That is, by itself this method would cycle only once and consequently stop responding to changing thermal conditions (e.g. pan removal, contents added to pan, etc.). Periodic recomputation is necessary and is achieved by resetting power to the user power command whenever the estimated duty-cycle is greater than a predetermined threshold  421  as performed by comparator circuit  422 . The value of the threshold  421  sets the period of the re-computation and functions similar to the desired duty cycle in the first and second embodiments. Thus, if the current latched duty cycle estimate signal  408  output from the duty cycle estimator  250  is greater than the duty cycle threshold value, e.g., typically a fixed value between 90% to 99.9% dependent upon a specific application, and for exemplary purposes is 0.96, then the lesser of the full power value or user power command value  160  (at the minimum block  213 ) will be applied to maintain the burner at the thermal limit as indicated by a switch  425 . Otherwise, the predicted power  415  at the thermal limit will be applied. Preferably, the predicted thermal limiting power  415  is the product of the duty-cycle and the average power over the last cycle and which has been held constant (latched) by latch device  412  over the current cycle. The output  415  of the thermal limiting power estimator device  410  is the predicted power at the thermal limit and is input to the switch device  425  provided in the periodic reset logic circuit  420 . The switch device  425  outputs either full power, or, the predicted power  415  at the thermal limit output from the estimator that is the power required to maintain the burner at the thermal safety limit. The reset logic interacts to periodically compute the estimate of the power required to just maintain the temperature at the thermal limit  415 .  
         [0037]    [0037]FIG. 9( a ) illustrates an example simulation of bang-bang thermal control implemented for a ceramic burner according to the third embodiment of FIG. 8. In the example simulation, the user-demanded power setting (user power command signal) is about one-half (50%)of the maximum power. At this initial setting, thermal limiting does not engage as indicated in FIG. 9( b ). At the time indicated at  191 , the user increases the power to 100% (FIG. 9( a )) causing the conduction state  195  of the burner&#39;s thermal switch (e.g., bi-metallic switch) to change in accordance with bang-bang thermal limiting at time indicated as time  192  in FIG. 9( b ) and thermal limiting is engaged. According to this embodiment, at least one cycle of bang-bang control is needed to estimate what the average power was over that cycle. In FIG. 9( b ), the conduction on/off states, i.e., engagement of bang-bang thermal limiting, according to the third embodiment of the invention, is represented as the plot  195 . At the point in time indicated at time  193 , after the one cycle duration in which the power estimate has been made, the power command is decreased to that estimated power value. That is, returning to FIG. 8, in the power command controller element  400 , the predicted power level  415  is computed for the first time, and thus the output of minimum block  213  changes to reduce output power from the user power command  160 , to the predicted power  415  required to maintain temperature at the thermal limit. As shown in FIG. 9( b ), bang-bang control thermal limit cycles are periodically re-engaged, for example, at steps  196   a  and  196   b,  etc. At each of these periodic intervals, the controller element  400  switches the power back to what the user has requested, and after the bang-bang thermal control limit cycle, the power command is re-set to the predicted power level (i.e., average-power that was applied) to maintain burner temperature at or about the thermal limit. This results in the glass temperature varying about the thermal limit temperature  182 , i.e., the safety thermal limit of the burner, as shown in FIG. 9( c ). Finally, the user reduces the power back to its initial one-half power level and thermal limiting ceases, as indicated at time  197  in FIG. 9( a ). As shown in FIG. 9( b ), the duty cycle control of bang-bang thermal limiting of the example simulation according to the third embodiment responds more quickly than the controller circuit of the first embodiment of  5 ( b ), however at the expense of greater power fluctuation as shown in FIG. 9( a ).  
         [0038]    While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.