Abstract:
Boil dry conditions are detected in utensils heated on a glass-ceramic cooking appliance having at least one heating unit disposed under a glass-ceramic plate by monitoring the level of power supplied to the heating unit. A power level detector is electrically connected to the heating unit and generates a signal representative of the power level. A controller for controlling the heating unit so as to prevent the glass-ceramic plate from exceeding a maximum temperature is arranged to receive the power signal. The controller provides a boil dry indication in response to a decrease in the power signal. Alternatively, the controller can monitor the level of power being supplied to the heating unit by monitoring a signal generated to control the level of power, thereby eliminating the power level detector.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to detecting a boil dry condition in a utensil being heated on a cooking appliance and more particularly to boil dry detection in glass-ceramic cooking appliances. 
     The use of glass-ceramic plates as the cooking surface in cooking appliances such as cooktops and ranges is well known. Such cooking appliances (referred to herein as glass-ceramic cooktop appliances) typically include a number of heating units mounted under the glass-ceramic plate, one or more sensors for measuring the glass-ceramic temperature, and an electronic or electro-mechanical controller. The glass-ceramic plate presents a pleasing appearance and is easily cleaned in that its smooth, continuous surface lacks seams or recesses in which debris can accumulate. The glass-ceramic plate also prevents spillovers from falling onto the heating units below. The controller controls the power applied to the heating units in response to user input and input from the temperature sensors. 
     In one known type of glass-ceramic cooktop appliance, the glass-ceramic plate is heated predominantly by radiation from one or more of the heating units disposed beneath the plate. The glass-ceramic plate is sufficiently heated by the heating unit to heat utensils placed on it primarily by conduction from the heated glass-ceramic plate to the utensil. Another type of glass-ceramic cooktop appliance uses a heating unit that radiates substantially in the infrared region in combination with a glass-ceramic plate that is substantially transparent to such radiation. In these appliances, a utensil placed on the cooking surface is heated partially by radiation transmitted directly from the heating unit to the utensil, in addition to conduction from the glass-ceramic plate. Such radiant glass-ceramic cooktop appliances are more thermally efficient than other glass-ceramic cooktop appliances and have the further advantage of responding more quickly to changes in the power level applied to the heating unit. Yet another type of glass-ceramic cooktop appliance inductively heats utensils placed on the cooking surface. In this case, the heating unit is a coil connected to an RF generator; the coil emits RF energy when activated. The utensil, which comprises an appropriate material, absorbs the RF energy and is thus heated. 
     In each type of glass-ceramic cooktop appliances, provision must be made to avoid overheating the glass-ceramic plate. For most glass-ceramic materials, the operating temperature should not exceed approximately 600-700°C. for any prolonged period. Under normal operating conditions, the temperature of the glass-ceramic plate will generally remain below this limit. However, conditions can occur which can cause this temperature limit to be exceeded. Commonly occurring examples include operating the appliance with a small load or no load (i.e., no utensil) on the cooking surface, using badly warped utensils that make uneven contact with the cooking surface, and operating the appliance with a shiny and/or empty utensil. 
     To protect the glass-ceramic plate from extreme temperatures, a control system is utilized in which temperature sensors provide a signal indicative of the glass-ceramic temperature to the appliance&#39;s controller. If the glass-ceramic plate approaches its maximum temperature, a special control mode, known as the thermal limiter mode, is activated. In the thermal limiter mode, the controller reduces power to the heating units to maintain the temperature of the glass-ceramic cooking surface at a relatively constant, safe temperature. 
     Another concern with cooking appliance generally is a boil dry condition. A boil dry condition occurs when all the liquid contents of a heated utensil evaporate during the boil phase. This commonly happens when a utensil is inadvertently left on a hot cooking surface or otherwise overheated. A boil dry condition can cause burned food, utensil damage and potential fire hazards. Accordingly, automatic detection of a boil dry condition is a desirable feature in cooking appliances. 
     In glass-ceramic cooktop appliances, it is known to use the glass ceramic temperature to determine when a utensil has boiled dry. Specifically, when a utensil containing water or another liquid is placed on a glass-ceramic cooking surface and the burner is turned on, the glass-ceramic temperature initially increases rapidly. The glass-ceramic temperature will continue to rise until the utensil contents come to a boil. During the boil phase, the utensil contents will boil off at a steady temperature and remove excess heat via evaporation. With this steady heat removal, the glass-ceramic temperature also reaches a steady state value some time after the contents come to a boil. However, when the liquid completely boils off, there is a sudden drop in heat removal from the pan, and consequently, the glass-ceramic temperature increases rapidly. This temperature rise is thus indicative of the boil dry condition. 
     This method of boil dry detection generally works well while the cooking appliance is in its standard operating mode. But under the thermal limiter mode, the glass-ceramic plate is being maintained at a relatively constant temperature by the controller. Therefore, the glass-ceramic temperature will not rise when upon a boil dry condition. Accordingly, it would be desirable to be able to automatically detect boil dry conditions while in the thermal limiter mode. 
     BRIEF SUMMARY OF THE INVENTION 
     The above-mentioned need is met by the present invention, which provides a boil dry detection system for a glass-ceramic cooking appliance having at least one heating unit disposed under a glass-ceramic plate and a power source for providing power to the heating unit. The boil dry detection system includes means for providing a signal representative of the level of power being supplied to the heating unit. A controller for controlling the power source so as to prevent the glass-ceramic plate from exceeding a maximum temperature is arranged to receive the power level signal. The controller provides a boil dry indication in response to a decrease in the power level signal. 
     The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
     FIG. 1 is a perspective view of a glass-ceramic cooktop appliance incorporating a preferred embodiment of the present invention. 
     FIG. 2 is partly schematic view of a glass-ceramic cooktop appliance showing one of its burner assemblies in cross-section. 
     FIG. 3 is a graph plotting glass-ceramic temperature as a function of time. 
     FIG. 4 is a graph plotting power level as a function of time. 
     FIG. 5 is a partly schematic view of a glass-ceramic cooktop appliance showing an alternative embodiment of a burner assembly in cross-section. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 shows a glass-ceramic cooktop appliance  10  having a glass-ceramic plate  12  that provides a cooking surface. The appliance  10  can be any type of cooktop appliance including a range having an oven and a cooktop provided thereon or a built-in cooktop unit without an oven. Circular patterns  14  formed on the cooking surface of the plate  12  identify the positions of each of a number (typically, but not necessarily, four) of burner assemblies (not shown in FIG. 1) located directly underneath the plate  12 . A control panel  16  is also provided. As is known in the field, the control panel  16  includes knobs, touch pads or the like that allow an operator of the appliance  10  to individually control the power applied to the burner assemblies. 
     Turning to FIG. 2, an exemplary one of the burner assemblies, designated generally by reference numeral  18 , is shown located beneath the glass-ceramic plate  12  so as to heat the plate  12  and/or a utensil  15  placed thereon. The burner assembly  18  includes a controllable heating unit  20  in the form of an open coil electrical resistance element, which is designed when fully energized to radiate primarily in the infrared region of the electromagnetic energy spectrum. It should be noted that other types of heating units could be used in place of the resistance element. The heating unit  20  is arranged in an effective heating pattern such as a concentric coil and is secured to the base of an insulating liner  22  which is supported in a sheet metal support pan  24 . The insulating liner  22  includes an annular, upwardly extending portion  26  that serves as an insulating spacer between the heating unit  20  and the glass-ceramic plate  12 . The support pan  24  is typically spring loaded upwardly, forcing the annular portion  26  into abutting engagement with the underside of the glass-ceramic plate  12 , by conventional support means (not shown). 
     The heating unit  20  is coupled to a power source  28  (typically a standard 240 volt, 60 Hz AC power source) via suitable power lines  30 . A power source control means such as a triac  32  is provided to regulate the level of power delivered to the heating unit  20 . The triac  32  is a conventional semiconductor device capable of conducting current in either direction across its main terminals when triggered by either a positive or negative voltage or signal  34  applied to its gate terminal  36 . An electronic controller  38  supplies the gate signal  34 . The controller  38  controls the power applied to the heating unit  20  by controlling the rate at which gate signals  34  are applied to the triac gate terminal  36 . The gate signal pulse rate is dictated by the power setting selections for the burner assembly  18  entered by user actuation of the control panel  16 . Although not shown in FIG. 2, other heating units included in the appliance  10  are connected to the power source  28  in the same manner as, and in parallel with, the illustrated heating unit  20 . 
     A power level detector  40  is electrically connected to the heating unit  20 , preferably between the power source  28  and the heating unit  20 , and generates a signal  42  that is directly or indirectly representative of the amount of power applied to the heating unit  20 . In this sense, the power level detector  40  does not necessarily detect power directly, but is referred to herein as a “power level detector” because it provides the system with the means to detect the power level. The power level signal is fed to the controller  38 . In one preferred embodiment, the power level detector  40  is any suitable circuit or device capable of measuring RMS voltage and generating a signal corresponding to the measured voltage. This controller  38  uses the measured voltage and the resistance of the heating unit  20  to determine the power level applied to the heating unit  20 . Thus, the power signal  42  would be indirectly representative of the power level. 
     For purposes of the power calculation, the heating unit resistance is assumed to be constant. Although the heating unit resistance actually varies significantly over the entire operating range of the appliance  10 , this resistance is fairly constant while the appliance is in its thermal limiter mode, which is described below. As also described below, the controller  38  uses the power signal primarily during thermal limiter mode operation. Thus, the assumption of a constant heating unit resistance is valid for the power calculation. This value of the heating unit resistance used in the calculation is a predetermined value that is based on a mean resistance value typically provided by the manufacturer. 
     Alternatively, the power level detector  40  could comprise conventional circuitry designed to measure the instantaneous current and instantaneous voltage and use these values to determine the power level. In this case, the power signal  42  is directly representative of the power level. However, such circuitry is generally more expensive. 
     A temperature sensor  44  is provided to detect the temperature of the glass-ceramic plate  12 . In one preferred embodiment, the temperature sensor  44  is a resistive element, such as a resistance temperature detector, having a resistance that is very sensitive to temperature. The resistive element  44  is placed within the heater cavity  46  defined by the insulating liner  22  and the glass-ceramic plate  12 . Alternatively, the temperature sensor  44  could be a resistance temperature detector or similar device attached directly to the underside of the glass-ceramic plate  12 . In any event, the temperature sensor  44  generates a signal  48  indicative of temperature that is fed to the controller  38 . 
     During normal operation, a user selects the desired cooking setting via manipulation of the control panel  16 , and the controller  38  supplies gate signals  34  to the triac gate terminal  36  at an appropriate rate so as to provide the necessary level of power from the power source  28  to the heating unit  20 . However, overheating of the glass-ceramic plate  12  should be avoided to insure long life. Thus, the controller  38  monitors the temperature signal  48  provided by the temperature sensor  44  to insure that the glass-ceramic temperature does not exceed a maximum safe level. Specifically, as the utensil  15  is being heated, the temperature of the glass-ceramic plate  12  will generally increase. If the glass-ceramic temperature reaches a preset value, which is typically in the range of 600-700° C., then the controller  38  will activate its thermal limiter mode to protect the glass-ceramic plate  12  from overheating. Under the thermal limiter mode, the controller  38  controls the pulse rate of the gate signals  34  such that the power supplied to the heating unit  20  is reduced to maintain the glass-ceramic temperature below the maximum safe level. Accordingly, the glass-ceramic temperature is maintained at a relatively constant level during the thermal limiter mode. 
     The controller  38  also provides a boil dry detection function. As mentioned previously, a boil dry condition occurs when all the liquid contents of a heated utensil are boiled off. During normal operation, the controller  38  detects a boil dry condition based on temperature signal  48 . This is illustrated by referring to FIG. 3, which shows a plot of the temperature signal  48  as a function of time. The utensil  15  is placed on the glass-ceramic plate  12  and the appliance  10  is turned on, at time t 0 , causing the glass-ceramic temperature to increase from room temperature. The glass-ceramic temperature continues to rise until the utensil contents come to a boil. During the boil phase, the utensil contents will boil off at a steady temperature and remove excess heat via evaporation. With this steady heat removal, the glass-ceramic temperature also reaches a steady state value at time t 1 , which is a short time after the contents have come to a boil. If the heating is continued, the liquid contents will eventually completely boil off, as shown at time t 2 . At this point, there is a sudden drop in heat removal from the utensil  15 , and consequently, the glass-ceramic temperature increases rapidly. This rise in the temperature signal  48  is thus indicative of the boil dry condition. In response to detecting a boil dry condition, the controller  38  shuts off power to the heating unit  20  and optionally sends a triggering signal to an alarm  50  (FIG.  2 ). 
     In the thermal limiter mode, the temperature signal  48  is maintained at a steady state. This means that the temperature signal  48  will not rise if a boil dry condition occurs while the appliance  10  is operating under the thermal limiter mode. The controller  38  provides boil dry detection during the thermal limiter mode by monitoring the level of power applied to the heating unit  20  via the power signal  42  provided by the power level detector  40 . Referring to FIG. 4, which shows a plot of the power signal  42  as a function of time, time t 0  again represents the point at which the utensil  15  is placed on the glass-ceramic plate  12  and the appliance  10  is turned on. The power level is determined by the desired cooking setting selected by the user via manipulation of the control panel  16  and generally remains constant as long as the cooking setting is unchanged by the user. The glass-ceramic temperature will increase and at some point, represented by time t L  in FIG. 4, can reach the preset value causing the controller  38  to activate the thermal limiter mode. At this point, the controller  38  will reduce the power level supplied to the heating unit  20  so as to maintain the glass-ceramic plate  12  at a safe temperature. 
     At some point in the heating process (which could be either before of after the time t L  when the thermal limiter mode is activated), the utensil contents come to a boil. During the boil phase, the utensil contents will boil off at a steady temperature and remove excess heat via evaporation at a steady rate. With this steady heat removal, the power level supplied to the heating unit  20  in order to maintain the glass-ceramic temperature at its safe level will generally remain steady, although there may be slight fluctuations in the power level due to changes in room temperature and the like. Continued heating will result in the liquid contents eventually being completely boiled off, as shown at time t 2 . At this point, there is a sudden drop in heat removal from the utensil  15  meaning less power is required to maintain the glass-ceramic temperature. Therefore, the power signal  42  will show an abrupt drop that will be indicative of the boil dry condition. As before, the controller  38  will shut off power to the heating unit  20  and optionally send a signal to an alarm  50 . Generally, the abrupt drop in the power signal  42  need be only on the order of about 2-3% over a period of a few seconds to trigger a boil dry indication. 
     FIG. 5 shows an alternative embodiment in which boil dry detection in the thermal limiter mode is accomplished by monitoring the level of power applied to the heating unit. However, in this embodiment, a separate power level detector is not used. Specifically, FIG. 5 shows a burner assembly  118  located beneath a glass-ceramic plate  112  so as to heat the plate  112  and/or a utensil  115  placed thereon. The burner assembly  118  includes a controllable heating unit  120 , which is the same or similar to that of the first described embodiment. The heating unit  120  is coupled to a power source  128  (typically a standard 240 volt, 60 Hz AC power source) via suitable power lines  130 . 
     A power source control means such as a triac  132  is provided to regulate the level of power delivered to the heating unit  120 . The triac  132  is a conventional semiconductor device capable of conducting current in either direction across its main terminals when triggered by either a positive or negative voltage or signal  134  applied to its gate terminal  136 . An electronic controller  138  supplies the gate signal  134 . The controller  138  controls the power applied to the heating unit  120  by controlling the rate at which gate signals  134  are applied to the triac gate terminal  136 . The gate signal pulse rate is dictated by the power setting selections for the burner assembly  118  entered by user actuation of the control panel  116 . A temperature sensor  144 , which is the same as or similar to the temperature sensor of the first described embodiment, is provided to detect the temperature of the glass-ceramic plate  112 . In any event, the temperature sensor  144  generates a signal  148  indicative of temperature that is fed to the controller  138 . 
     Boil dry detection during normal operation is accomplished in the same manner as described above. That is, the controller  138  monitors the temperature signal  148  where a rapid rise in the temperature signal  144  is indicative of a boil dry condition. Boil dry detection during the thermal limiter mode is accomplished monitoring the level of power applied to the heating unit  120 , wherein an abrupt drop in the power level is indicative of a boil dry condition. Instead of monitoring the power level with a power level detector, the controller  138  monitors the gate signal pulse rate, which is controlled by, and thus “known” by, the controller  138 . Because the power level is a function of the gate signal pulse rate, the gate signal  134 , like the power signal  42  of the first described embodiment, is representative of the level of power being supplied to the heating unit  120 . As before, the controller  138  shuts off power to the heating unit  120  and optionally sends a triggering signal to an alarm  150  in response to detecting a boil dry condition. 
     The foregoing has described a method and system for automatically detecting boil dry conditions, including monitoring the power level to detect boil dry conditions while operating in the thermal limiter mode. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.