Abstract:
A sensor assembly for glass-ceramic cooktop appliances includes an optical detector having an reference component and an active component. The active component is arranged to receive radiation from the glass-ceramic plate, and the reference component is insulated from radiation from the glass-ceramic plate. The sensor assembly further includes a temperature sensor and a heater located adjacent to the reference component and a controller having a first input connected to the optical detector and a second input connected to the temperature sensor. The controller is responsive to the optical detector and the temperature sensor to calibrate the sensor assembly. Calibration is accomplished by noting the temperature reading of the temperature sensor after the burner assembly has not been used for a predetermined period of time to obtain a first calibration point. Then, the burner assembly is activated so that the temperature of the glass-ceramic plate is raised, and the output of the optical detector is noted. Next, an exciting circuit is used to heat the reference component. When the output of the optical detector reaches zero, the temperature reading of the temperature sensor is noted and used with the noted optical detector output to obtain a second calibration point. The first and second calibration points are used to calibrate the sensor assembly.

Description:
BACKGROUND OF INVENTION 
     This invention relates generally to glass-ceramic cooktop appliances and more particularly to the long term calibration of a device for sensing properties relating to the appliance. 
     The use of glass-ceramic plates as cooktops in cooking appliances is well known. Such glass-ceramic cooktops have a smooth surface that presents a pleasing appearance and is easily cleaned in that the smooth, continuous surface prevents spillovers from falling onto the heating unit underneath the cooktop. 
     In one known type of glass-ceramic cooktop appliance, the glass-ceramic plate is heated by radiation from a heating unit, such as an electric coil or a gas burner, disposed beneath the plate. The glass-ceramic plate is sufficiently heated by the heating unit to heat utensils upon 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 cooktop is heated primarily by radiation transmitted directly from the heating unit to the utensil, rather than by conduction from the glass-ceramic plate. Such radiant glass-ceramic cooktops are more thermally efficient than other glass-ceramic cooktops and have the further advantage of responding more quickly to changes in the power level applied to the heating unit. 
     In both types of glass-ceramic cooktop appliances, provision must be made to avoid overheating the cooktop. For most glass-ceramic materials, the operating temperature should not exceed 700° C. for any prolonged period. During operation, conditions can occur which can cause this temperature limit to be exceeded. Commonly occurring examples include operating the appliance with no load, i.e., no utensil, on the cooktop surface, using warped utensils that make uneven contact with the cooktop surface, and operating the appliance with a shiny and/or empty utensil. 
     To protect the glass-ceramic from extreme temperatures, glass-ceramic cooktop appliances ordinarily have some sort of temperature sensing device that can cause the heating unit to be shut down if high temperatures are detected. In addition to providing thermal protection, such temperature sensors can be used to provide temperature-based control of the cooking surface and to provide a hot surface indication, such as a warning light, after a burner has been turned off. 
     One common approach to sensing temperature in glass-ceramic cooktop appliances is to place a temperature sensor directly on the underside of the glass-ceramic plate. With this approach, however, the temperature sensor is subject to the high burner temperatures and thus more susceptible to failure. Moreover, direct contact sensors detect an average flux across the contact and do not produce a direct measurement of the glass-ceramic temperature. Thus, it is desirable to use an optical sensor assembly that “looks” at the glass-ceramic surface from a remote location to detect the temperature of the surface. Remote sensor assemblies are also capable of “looking” through the glass-ceramic plate to detect characteristics of a utensil placed on the cooktop, such as the temperature, size or type of the utensil, the presence or absence of the utensil, or the properties, such as boiling state, of the utensil contents. 
     Remote sensor assemblies are calibrated such that the sensor output signal will accurately represent the cooktop related property being detected. Over time, however, the system will experience certain effects that will affect the calibration and performance of the sensor assembly. These long term effects include aging of the glass-ceramic plate resulting in changes in its emissivity and reflectivity, formation of deposits on the glass-ceramic plate, the aging effects of the system&#39;s optical components, and drifts and variations in system electronics. 
     Accordingly, there is a need for a remote sensor assembly that can monitor and compensate for long term changes. 
     SUMMARY OF INVENTION 
     The above-mentioned needs are met by the present invention which provides a sensor assembly for glass-ceramic cooktop appliances that includes an optical detector having a reference component and an active component. The active component is arranged to receive radiation from the glass-ceramic plate, and the reference component is insulated from radiation from the glass-ceramic plate. The sensor assembly further includes a temperature sensor located adjacent to the reference component, means for exciting the reference component, and a controller having a first input connected to the optical detector and a second input connected to the temperature sensor. The controller is responsive to the optical detector and the temperature sensor to calibrate the sensor assembly. Calibration is accomplished by noting the temperature reading of the temperature sensor after the burner assembly has not been used for a predetermined period of time to obtain a first calibration point. Then, the burner assembly is activated so that the temperature of the glass-ceramic plate is raised, and the output of the optical detector is noted. Next, the exciting means are used to heat the reference component. Alternatively, the reference component could be heated first, followed by heating the glass-ceramic plate. Either way, when the output of the optical detector reaches zero, the temperature reading of the temperature sensor is noted and used with the noted optical detector output to obtain a second calibration point. The first and second calibration points are used to calibrate the sensor assembly. 
     Other objects and advantages of the present invention will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The subject matter which 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 sectional view of a burner assembly having the optical sensor assembly of the present invention. 
     FIG. 2 is a schematic view of a portion of the sensor assembly of FIG. 1 
     FIG. 3 is a schematic view of one preferred embodiment of an optical detector arrangement. 
     FIG. 4 is a plot showing an exemplary initial transfer function. 
     FIG. 5 is a plot showing a first updated transfer function compared to the initial transfer function. 
     FIG. 6 is a plot showing a second updated transfer function compared to the initial transfer function. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 shows a burner assembly  10  of the type suitable for use in a glass-ceramic cooktop appliance, which typically includes a plurality of such burner assemblies. As used herein, the term “cooktop” is intended to refer to both the flat top of a range or stove and counter-top cooking apparatuses (either built-in or portable). Burner assembly  10  includes an open coil electrical resistance element  12 , which is designed when fully energized to radiate primarily in the infrared region of the electromagnetic energy spectrum. It should be noted that another type of heating unit, such as a gas burner, could be used in place of element  12 . Element  12  is arranged in an effective heating pattern such as a concentric coil and is secured to the base of an insulating liner  14  which is supported in a sheet metal support pan  16 . Insulating liner  14  includes an annular, upwardly extending portion  18  which serves as an insulating spacer between element  12  and a glass-ceramic plate  20  that provides the cooktop surface. Support pan  16  is spring loaded upwardly, forcing annular portion  18  into abutting engagement with the underside of glass-ceramic plate  20 , by conventional support means (not shown). 
     An optical sensor assembly  22  is provided to detect one or more characteristics relating to the cooking appliance (referred to herein as “cooktop related properties”), such as the temperature of glass-ceramic plate  20 , the presence or absence of a utensil on the cooktop, the temperature, size or type of utensil on the cooktop, or the properties or state of the utensil contents. Sensor assembly  22  includes a radiation collector  24  disposed in the interior of burner assembly  10  underneath glass-ceramic plate  20 . This location provides radiation collector  24  with a field of view of the desired sensing location (i.e., the portion of glass-ceramic plate  20  directly over burner assembly  10 ). Radiation gathered by radiation collector  24  is delivered to an optical detector  26  located at a relatively cool place outside of burner assembly  10  via a light pipe or waveguide  28 . Waveguide  28  allows detector  26  to be located where the thermal environment is more favorable. The use of waveguides also permits the co-location and sharing of detectors among several burner assemblies. 
     Waveguide  28  is preferably a metal tube having a highly reflective internal surface. More preferably, waveguide  28  is provided with an internal coating that is an excellent infrared reflector and has very low emissivity. Gold is one preferred internal coating material because of its high reflectivity and low emissivity. To prevent the tube material, which is preferably a metal such as copper, from bleeding into the internal coating, a barrier layer can be deposited between the metal tube and the internal coating. The barrier layer can comprise any suitable material, such as nickel or nichrome. 
     Waveguide  28  extends through the bottom of insulating liner  14  and support pan  16  so as to have a first or entry end  30  disposed in the interior of burner assembly  10  adjacent to radiation collector  24  and a second or exit end  32  located outside of burner assembly  10  adjacent to detector  26 . Preferably, waveguide  28  extends through the bottom of insulating liner  14  and support pan  16  at their respective center points so as not to interfere with element  12 . 
     As shown in FIG. 1, waveguide  28  is bent at a point intermediate its two ends so as to reflect radiation through a 90 degree turn. Thus, detector  26  is located not only below burner assembly  10  but also beyond its outer circumference. This configuration could alternatively be accomplished by providing waveguide  28  with a planar region formed at a 45 degree angle. Furthermore, it should be noted that waveguide  28  could also be straight, without any bend, so that detector  26  would be located directly below the point at which waveguide  28  extends through the bottom of insulating liner  14  and support pan  16 . The waveguide  28  could alternatively have several bends. 
     Referring now to FIG. 2, it is seen that optical detector  26  comprises an active component  34  that is exposed to radiation exiting exit end  32  of waveguide  28  and a reference component  36  that is isolated from the radiation. An excitation means  38  and a temperature sensor  40  are located adjacent to reference component  36 . Excitation means  38 , which can be any device capable of heating or otherwise exciting reference component  36 , is provided for selectively changing a condition (such as temperature) of reference component  36 . Temperature sensor  40  is provided for sensing the temperature of reference component  36 . 
     Active component  34  produces a first signal, and reference component  36  produces a second signal. These two signals are compared at a comparative junction  42 . The comparative junction  42  provides a detector output that is a function of the first and second signals. The detector output signal is fed to an electronic controller  44 , which is a common element used in many glass-ceramic cooktop appliances, via a multi-channel signal conditioner  46 . The output of temperature sensor  40  is also fed to controller  44  via signal conditioner  46 . Signal conditioner  46  is a conventional element comprising means for filtering or otherwise conditioning the signals as well as gain amplifying circuitry. Controller  44  provides a control signal to excitation means  38 , causing reference component  36  to be excited. 
     In one preferred embodiment, shown in FIG. 3, optical detector  26  is a thermopile, i.e., a plurality of series-connected thermocouples having hot junctions that function as active component  34  and cold junctions that function as reference component  36 . It should be noted that a thermopile is one possible optical detector and that a wide variety of thermal and quantum detectors could be used. The thermopile is arranged in a casing  48  such that hot junctions  34  are exposed to the radiation exiting exit end  32  of waveguide  28 , and cold junctions  36  are attached to casing  48  and isolated from the radiation. Accordingly, hot junctions  34  are heated to a temperature representative of the temperature of glass-ceramic plate  20 , and cold junctions  36  are at the temperature of casing  48 . Optical detector  26  produces a voltage or output signal, V opt , which is representative of the difference in the temperature of hot junctions  34  and cold junctions  36 . The output signal V opt  is a positive value when the temperature of hot junctions  34  exceeds the temperature of cold junctions  36 , is a negative value when the temperature of cold junctions  36  exceeds the temperature of hot junctions  34 , and is zero when hot junctions  34  and cold junctions  36  are at equal temperatures. The output signal V opt  is fed to controller  44  via signal conditioner  46  (not shown in FIG.  3 ). 
     In the embodiment of FIG. 3, a single element, a thermistor  50 , functions as both excitation means  38  and temperature sensor  40 . Specifically, thermistor  50  is located adjacent to the portion of casing  48  to which cold junctions  36  are attached. Thus, thermistor  50  produces an output signal, V c , which is fed to controller  44  and representative of the casing temperature, and hence the temperature of cold junctions  36 . Furthermore, thermistor  50  can be used to heat casing  48  and cold junctions  36 . Thermistor  50  is powered by a current source  52 . It should be noted that other devices, such as resistance temperature detectors and thermocouples, could be used as an alternative to a thermistor. 
     During normal operation of sensor assembly  22 , controller  44  monitors the output signals V opt  and V c  to determine the temperature of glass-ceramic plate  20 . Controller  44  utilizes a transfer function that relates the output signals V opt  and V c  to a corresponding temperature of glass-ceramic plate  20 . In an ideal case, the transfer function is given by the following equation: 
     
       
           V   opt =α( T   g   4   −γT   c   4 )+β 
       
     
     where T g  is the temperature of glass-ceramic plate  20 , T c  is the temperature of casing  48  (obtained from thermistor output signal V c ), α is the slope of the transfer function, β is the offset value, and γ is a constant generally assumed to be equal to one. The values of α and β are set during the initial calibration of sensor assembly  22 . The value of V opt  is obtained from optical detector  26  such that the equation can be solved for T g . 
     This concept is shown graphically in FIG. 4 in which the optical detector output signal V opt  is plotted against the glass-ceramic temperature T g . FIG. 4 shows an exemplary transfer function A (which for purposes of illustration is shown to be linear) that represents an initial calibration of sensor assembly  22  and has an initial slope α i  and an initial offset β i . Over time, long term effects such as aging of glass-ceramic plate  20  and the optical components of sensor assembly  22 , formation of deposits on glass-ceramic plate  20 , and drifts and variations in system electronics can cause changes in the transfer function. For instance, the offset can change, the transfer function slope can change, or the shape of the transfer function can change (i.e., it becomes non-linear). 
     By the method of the present invention, controller  44  monitors the transfer function for any such changes and makes appropriate corrections so as to maintain the accuracy of sensor assembly  22 . Generally, thermistor  50 , functioning as a temperature sensor, is used to determine the glass-ceramic temperature Tg independently of optical detector  26 . This is possible because the casing temperature Tc is equal to the glass-ceramic temperature Tg when the optical detector output signal V opt  is zero. Thus, the thermistor output V c  is representative of the glass-opt ceramic temperature Tg whenever a zero crossing occurs. By using such independent measurements of the glass-ceramic temperature Tg and monitoring the corresponding optical detector output signal V opt , at predetermined intervals, two or more new calibration points can be obtained, stored in controller  44 , and used for calibrating sensor assembly  22 . 
     Referring to FIG. 5, in which the optical detector output signal V opt  is again plotted against the glass-ceramic temperature Tg, the method of the present invention is described in more detail. A first new calibration point P 1  is obtained by carrying out a steady state glass-ceramic temperature measurement, i.e., after heating element  12  has not been energized for some predetermined time such that glass-ceramic plate  20  and casing  48  are at the same temperature, which would be room temperature. Because glass-ceramic plate  20  and casing  48  are at the same temperature, the optical detector output signal V opt  is zero and the glass-ceramic temperature Tg is determined from thermistor  50 . This measured temperature is set at a first value T 1 . Thus, as shown in FIG. 5, the first point P 1  has a value of zero for the output signal V opt  and the measured value T 1  for the glass-ceramic temperature Tg. These values are fed to and stored in controller  44 . Since the output signal V opt  is zero, the first calibration point P 1  will provide an indication of the offset β 1 , which can be compared to the original offset βi. If a change in offset has occurred, controller  44  will adjust the offset accordingly. 
     Next, a second new calibration point P 2  is obtained. This is done by first energizing heating element  12  to heat glass-ceramic plate  20  such that its temperature is increased above room temperature, resulting in a positive value of the optical detector output signal V opt . When glass-ceramic plate  20  reaches a constant temperature, the optical detector output signal V opt  is noted by controller  44  and stored as a second value V 2 . Then, controller  44  feeds a control signal to thermistor  50 , now functioning as an excitation means, causing it to heat casing  48  and cold junctions  36 . When another zero crossing occurs, this means the casing temperature Tc has reached the glass-ceramic temperature Tg. At this point, the output of thermistor  50  (which is again functioning as a temperature sensor) is used to determine the new temperature of glass-ceramic plate  20 , which is stored as a second value T 2 . It should be noted that a separate resistance heater could be used to heat casing  48 . That is, it is not necessary to use a single device to function as excitation means  38  and temperature sensor  40 . 
     Alternatively, calibration point P 2  could be obtained by first heating casing  48  and cold junctions  36  to an elevated temperature and then energizing heating element  12  to heat glass-ceramic plate  20 . When the zero crossing occurs, the optical detector output signal V opt  is noted by controller  44  and stored as second value V 2  and the output of thermistor  50  is stored as second value T 2 . 
     With either approach, the second point P 2  has an output signal of V 2  and a glass-ceramic temperature T 2 , as shown in FIG.  5 . Second point P 2  is fed to and stored in controller  44 . Controller  44  uses first and second points P 1  and P 2  to determine an updated transfer function B and compares its slope with the slope of the initial transfer function A (shown in FIG. 5 for comparison). If change in slope has occurred, controller  44  adjusts it accordingly. 
     Additional new calibration points can be determined by using the same heating process described above with respect to second point P 2 , but at different power levels. These additional points are then fed to and stored in controller  44 . For example, FIG. 6 shows a third new calibration point P 3  having an output signal of V 3  and a measured glass-ceramic temperature T 3 . Controller  44  uses all three points to determine an updated transfer function C and compares its shape to the shape of the initial transfer function A (shown in FIG. 6 for comparison). If a change in shape has occurred, controller  44  adjusts it accordingly. 
     The foregoing has described a remote sensor assembly for a burner in a glass-ceramic cooktop appliance that can monitor and compensate for long term calibration changes. 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.