Abstract:
An induction cooking system including one or more methods or apparatus for identifying items of cookware. The identifying may include identifying whether in use, the temperature of the outside surface of the element of cookware is significantly lower than the temperature of the inside surface. The methods may include one or more of measuring impedance, communicating information by RF radiation, or measuring reactance.

Description:
BACKGROUND 
       [0001]    This specification describes an induction cooking system. Some conventional cooking systems deliver heat to a cooking utensil (e.g., a pan, pot, skillet, etc.) by for example a gas flame or electric resistance coil. In these cooking systems, any material that lies between the heat source and the cooking utensil (e.g., a glass cooktop) is also heated. Induction cooking systems work differently. In an induction cooking system, an alternating current in an induction coil produces a time dependent magnetic field that induces eddy currents in electrically conductive materials near the coil, such as a ferromagnetic component (or the target material) of induction cooking utensils. As eddy currents flow within the target material, it becomes hot via a joule heating mechanism. Heat in the target is conducted through the body of the cooking utensil to the food surface, and the food is cooked. Unlike gas or electric cooking systems, induction cooking systems will not directly heat non-conductive materials (such as a glass cooktop) that are placed between the induction coil and the target material. However, any such non-conductive materials placed between the induction coil and the target material may be indirectly heated by the radiant, convective, or conductive heat emanating from the hot target material. 
       SUMMARY 
       [0002]    In one aspect, an item of cookware for use with an induction cooking system includes an element selected from a group consisting of ferrite chips, a passive resonant circuit, a material with a curie point that is in the temperature range of the operation of the induction cooking system, and a permanent magnet. The element is for coacting with the induction cooking system to identify the item of cookware. The item of cookware may contain more than one of ferrite chips, a passive resonant circuit, a material with a curie point that is in the temperature range of operation of the induction cooking system, and a permanent magnet for coacting with the induction cooking system to identify the item of cookware. The temperature of the outer surface of the cookware may be, in use, significantly lower than the temperature of the inner surface of the cookware. 
         [0003]    In another aspect, a method for identifying induction cookware includes providing in material properties of the cookware, an indication that in use, the outer surface of the cookware is significantly lower than the inside surface of the cookware. The method may further include identifying the indication and conducting current in the coil to provide a time dependent magnetic field that induces eddy currents in the cookware to heat a surface of the cookware. The identifying the indication may be performed by a coil. The current in the coil may be dependent on the presence or absence of the indication. The material properties providing the indication may include the impedance signature of the cookware. The identifying the indication may include measuring the impedance of the element of cookware at a number of frequencies. The identifying the indication may include measuring the impedance of a passive resonant circuit in the cookware. The identifying the indication may include measuring the impedance of the element of cookware by measuring electrical parameters of a secondary coil. The providing may include inserting a passive resonant circuit in the cookware. The providing may include including in the cookware materials with varying curie points to provide an impedance signature. The providing may comprise including in the cookware a permanent magnet. The providing may include embedding ferrite chip in the cookware. The material properties may include the resonance frequency of a coil embedded in the cookware. The material properties may include the reactance of the cookware. 
         [0004]    In another aspect, a process for operating an induction cooking system includes a plurality of methods for identifying an item of cookware. The plurality of methods for identifying the item of cookware may include a first method for identifying an item of cookware including one of transmitting RF radiation to a wireless network element in an item of cookware; measuring the reactance of the item of cookware; and detecting the presence of a permanent magnet in the element of cookware. The plurality of methods for identifying the item of cookware may include a second method for identifying cookware including one of transmitting RF radiation to a radio frequency identification (RFID) tag in the element of cookware; radiating RF radiation to a wireless communication element in the element of cookware; measuring the impedance of the cookware; measuring the reactance of the cookware; detecting the presence of a permanent magnet in the item of cookware; and detecting a resonant frequency of a resonant coil in the element of cookware. 
         [0005]    Other features, objects, and advantages will become apparent from the following detailed description, when read in connection with the following drawing, in which: 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0006]      FIG. 1  is a diagrammatic view of an induction cooking system; 
           [0007]      FIG. 2  is a block diagram of a process for operating an induction cooking system; 
           [0008]      FIGS. 3 ,  4 , and  5 A are diagrammatic views of an induction cooking system; 
           [0009]      FIG. 5B  is a circuit diagram of a the induction cooking system of  FIG. 5A ; 
           [0010]      FIG. 6  is a diagrammatic view of an induction cooking system; 
           [0011]      FIG. 7A  is a diagrammatic view of a portion of the induction cooking system of  FIG. 6 ; 
           [0012]      FIG. 7B  is a view of elements of the view of  FIG. 7A ; 
           [0013]      FIGS. 7C and 7D  are circuit diagrams of some elements of the induction cooking system of  FIG. 7A ; 
           [0014]      FIG. 7E  is a circuit diagram of a circuit for measuring reactance; and 
           [0015]      FIG. 8  is a block diagram of an induction cooking system. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Though the elements of several views of the drawing may be shown and described as discrete elements in a block diagram and may be referred to as “circuitry”, unless otherwise indicated, the elements may be implemented as one of, or a combination of, analog circuitry, digital circuitry, or one or more microprocessors executing software instructions. Operations may be performed by analog circuitry or by a microprocessor executing software that performs the mathematical or logical equivalent to the analog operation. 
         [0017]      FIG. 1  shows an induction cooking system. Power electronics circuitry  2  is operationally coupled to a primary induction coil  4 , system control circuitry  6 , and cookware identifier circuitry  8 . Cookware identifier circuitry  8  may be operationally coupled to a data base  10  including a cookware identity (ID) list and to system control circuitry  6 . A user interface (UI)  12  is operationally coupled to system control circuitry  6 . Cooktop  14  interfaces the primary induction coil  4  with cookware  16 .  FIGS. 1 ,  3 ,  4 ,  5 A, and  6  show the system in logical form. In an actual implementation, the elements may share components, and elements that perform the activities of a block may be physically separated. For example, cookware identifier circuitry  8  may include elements of the power electronics circuitry  2 . 
         [0018]    The cookware  16  may be of the type described in U.S. patent application Ser. No. 12/031,214, U.S. patent application Ser. No. 12/031,220 or in U.S. patent Ser. No. 12/031,226 incorporated by reference in their entirety. An outer wall this type of cookware can be relatively cool to the touch, even if an inner wall is at a temperature sufficient to cook food. Such cookware will be hereinafter referred to as “cool cookware”. If the cookware  16  is non-cool cookware, the outer wall may be nearly as hot as the inner wall. If a user mistakes non-cool cookware for cool cookware, burns and/or damage to surfaces on which the cookware is placed may result. The use of cool cookware permits greater flexibility in the design of the cooktop  14 . The cooktop does not need to be as heat tolerant as the cooktop of a conventional induction cooking surface, which permits the use of different materials for the cooktop, dimensions of the cooktop, and other advantages. Cookware  16  is provided with an identifying characteristic so that the cookware identifying circuitry  8  can identify the cookware. Identifying characteristics may include a specific impedance or impedance signature, an element that can respond to a radio frequency (RF) signals or can interact with a wireless network, a characteristic reactance, or a characteristic resonance frequency. More detailed specific examples of identifying characteristics will be described below. 
         [0019]    The operation of the system of  FIG. 1  is described in  FIG. 2 . At block  18  the process is initiated, for example, by the cooking system sensing the presence of the cookware  16  or by a user entering a command through the user interface  12  which is communicated to the system control circuitry  6 . At block  20 , the cookware identifier circuitry attempts to identify the cookware. In attempting to identify the cookware, the cookware identifier circuitry may interact with the cookware as indicated by the dashed line between the cookware identifier circuitry  8  and the cookware  16  in  FIG. 1 . The nature of the interaction will be described below. At block  22 , it is determined if the cookware has been identified. If the cookware has been identified at block  22 , at block   27 , it is determined if the cookware is suitable for full functionality operation of the induction heating system as will be described below. If the cookware is not identified at block  22 , at block  26  the system control circuitry may cause the system to operate with reduced functionality. If it is determined at block  27  that the cookware is not suitable for full functionality operation, at block  29  the system control circuitry may cause the system to operate with reduced functionality. If it is determined at block  27  that the cookware is suitable for full functionality operation, at block  24  the system control circuitry directs the power electronics circuitry to operate with full functionality, by supplying power to the primary induction coil  4 , which creates a magnetic field which causes eddy currents in cookware  16 , which causes the cookware to heat. “Reduced functionality” may include operating with reduced maximum power or providing no power to the primary induction coil, limitations on some features of the induction cooking system, or a warning to the user. The reduced functionality provided at block  26  may be the same or different than the reduced functionality provided at block  29 . 
         [0020]    In one embodiment, “identified” means a binary identification of the cookware as being or not being cool cookware. This embodiment (hereinafter referred to as a “binary identification embodiment”) does not permit as sophisticated a tailoring of the reduced functionality as the embodiment described below, but can operate with simpler cookware identifier circuitry  8  and does not require the cookware ID list  10 . 
         [0021]    In another embodiment, “identified” means that the specific identity, for example, a specific product code, manufacturer, model number or the like, of the cookware has been determined. If the specific identity of the cookware has been determined, the specific identity of the cookware can be compared with the cookware identity list  10  to determine the features of the cookware. Alternatively, the features of the cookware can be indicated directly by the identification scheme. For example, a specific identification scheme could indicate that the item of cookware is cool cookware and could also directly indicate the manufacturer, the dimensions, and other information about the cookware. This embodiment (hereinafter referred to as a “specific identification embodiment”) permits full functionality operation to include more sophisticated features and also permits a sophisticated tailoring of the reduced functionality of the operation at block  29 . For example, if it the cookware is not suitable for induction cooking, then the system control circuitry  6  can cause the power electronics circuitry  2  to provide no power to the induction coil  4  and provide visual and/or auditory indication that the cookware is not suitable for induction cooking; if the cookware is suitable for induction cooking, but is not cool cookware, the system control circuitry  6  can limit the maximum power that the power electronics circuitry  2  provides to the coil  4  and/or provide visual or auditory indication that the cookware is not cool cookware; or if the cookware is cool cookware (which includes being suitable for induction cooking), the system control circuitry  6  can provide full maximum power to the coil  4 . 
         [0022]    Many other embodiments of varying degrees of complexity and sophistication are possible. For example, a binary identification embodiment can make binary classifications instead of or in addition to whether or not the cookware is cool cookware. In some cases, the type of binary classification may lend itself to measurement with specific types of sensors. 
         [0023]    An induction cooking system according to  FIGS. 1 and 2  may prevent or lessen the risk of burns or damage to surfaces by preventing the conventional cookware from heating at all; by reducing the maximum power to the primary induction coil, which permits the cookware to heat, but not to a temperature that would cause serious burns; or by warning the user that the cookware may be hot. Additionally, if the cookware is non-cool cookware, the cooking system may limit or eliminate some features. 
         [0024]      FIG. 3  shows an implementation of the induction cooking system of  FIG. 1 , with the cookware identifier circuitry  8  in more detail. The cookware identifier circuitry  8  includes an impedance detector  28 . The cookware  16  may include a passive resonant circuit  36  and the cookware identifier circuitry may include a secondary coil  30 , that is, coil in addition to the primary induction coil whose function is something other than producing a magnetic field that induces eddy currents in electrically conductive materials near the coil. In one implementation of the cooking system, the impedance detector determines an impedance of the cookware  16  by measuring the voltage across and the current through the primary induction coil  4  at a frequency. In one variation, the impedance detector determines an impedance signature by measuring the impedance at a plurality of frequencies. In another variation, the impedance detector measures the impedance of the optional passive resonant circuit  36 . In another variation, the impedance detector detects the impedance at a plurality of temperatures. In another variation, the impedance detector measures the impedance by measuring the current through and the voltage across the secondary coil  30 . The variations can be combined; for example, the impedance detector may measure, at a number of frequencies, current through and voltage across a secondary coil to determine the impedance of passive resonant circuits to determine an impedance signature of the cookware; or the impedance detector can measure the impedance at a number of frequencies and at a number of temperatures. Measuring the impedance by measuring the current and voltage across the primary induction coil  4  at a single frequency requires the fewest components and the simplest circuitry. The variations may require more components and more complex circuitry, but permit more sophisticated identification schemes. 
         [0025]    The impedance of the cookware may be altered to produce a unique impedance or impedance signature in a number of ways. The dimensions and geometry of the cookware can be modified; the material of the cookware can be varied; ferrite chips may be embedded in the cookware; and in other ways, such as inserting a resonant circuit in the cookware. 
         [0026]    An example of modifying the geometry of the cookware is constructing the cookware so that there is a gap between the induction target and the cooktop. Examples of varying the material of the cookware include using layers of dielectric material in the cookware and using materials, for example alloys of nickel, chromium, and iron, with varying curie points. The curie point is the temperature at which a ferromagnetic material loses its ferromagnetic properties. The loss of ferromagnetic properties results in a loss of the ability to support low frequency (20-30 kHz) induction heating, leading to a dramatic change in the system impedance. Ferrite chips, for example low temperature ferrites can be incorporated into the cookware, for example by adhering them to the bottom of the target portion of the cookware. A low temperature ferrite has a low curie temperature which results in a characteristic low temperature impedance response. 
         [0027]    In the implementation of  FIG. 3 , if the system includes a cookware ID list, the entries of the cookware ID may be cataloged according to impedances or impedance signatures. The implementation of  FIG. 3  is suitable for either a specific identification embodiment, as defined above, but is especially suitable for a binary identification embodiment, as defined above. Impedance can be measured relatively simply, using elements (for example the coil  4 ) that have other functions, such as providing the time dependent magnetic field that induces eddy currents in electrically conductive materials near the coil. Since a binary identification embodiment does not require the transfer of large amount of data, a relatively simple coding scheme can be used so that a material property of the cookware, such as the impedance or impedance signature, can be used to transmit information in addition to the material property itself. Stated differently a material property such as the impedance or impedance signature can be used to encode information that is independent of the material property. For example, a specific impedance profile could indicate whether or not the cookware is cool cookware. Conventional induction cooking cookware detects information about the material properties that is directly related to the material property. For example, some conventional induction cooking systems measure the impedance signature of the cookware to determine if the cookware includes magnetic material or not. In this case, the impedance signature is a direct indicator of whether or not the cookware contains magnetic material. 
         [0028]      FIG. 4  shows another implementation of the induction cooking system of  FIG. 1 . In the implementation of  FIG. 4 , the cookware identifier circuitry includes an RF device  32 . The cookware  16  includes an identifier  34  that can coact with the RF device to identify the cookware. 
         [0029]    In one form of the implementation of  FIG. 4 , the RF device  32  is an RFID transponder and the identifier  34  is an RFID tag. In another form of the implementation of  FIG. 4 , the RF device  32  and the identifier  34  are both elements of a wireless communications system, for example, a “Bluetooth” system (url www.bluetooth.org or www.bluetooth.org) or a Zigbee system (Advantage Electronics Product Development, Inc. of Broomfield, Colo., USA www.advantage-dev.com). 
         [0030]    An implementation incorporating RFID devices or elements of a wireless communication system is also suitable for a binary identification embodiment, but is particularly suitable for a specific identification embodiment, because the more extensive communication capabilities of an RFID or wireless communications network permits the efficient transmission of larger amounts of information than does a simpler scheme such as measuring impedance signatures. 
         [0031]    In another form of the implementation of  FIG. 4 , the RF device  32  may be a receiver, antenna, or some other device that detects electromagnetic radiation from the identifier  34 . An example of this form of implementation, with other elements, is shown in  FIG. 5A . 
         [0032]    In the example of  FIG. 5A , the identifier  34  of  FIG. 4  is a resonant circuit  34 ′, for example a resonant coil, embedded in, or attached to, the cookware. RF device  32  of  FIG. 4  may be a receiver coil  32 ′. Resonant circuit  34 ′ is in the form of a coil with a shunting capacitance. The coil has a characteristic inductance that is generally defined by its mean diameter and quantity of turns. The resulting circuit with the inductance of the coil in parallel with the capacitance has a theoretical resonant frequency f of 
         [0000]    
       
         
           
             
               f 
               = 
               
                 1 
                 
                   2 
                    
                   π 
                    
                   
                     LC 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where the resonant frequency is expressed in Hz, the inductance L is expressed in Henries, and the capacitance C is expressed in Farads. The resonant frequency of an actual example may differ slightly due to non-theoretical behavior and tolerance differences. In one example, L is 6 μH, C is 1000 pF, and the coil has five turns with a diameter of about 11.4 cm (4.5 inches), so that f is approximately 2 MHz. In another example, L is 2.5 μH, C is 100 pF, and the coil has four turns with a diameter of about 8.9 cm (3.5 inches), so that f is approximately 10 MHz. 
         [0033]      FIG. 5B  is a schematic diagram of some elements of the system of  FIG. 5A . Some of the reference numbers identify the corresponding elements of  FIG. 5A . Inductance  48  represents the inductance of the target material of the cookware  16  of  FIG. 5A . Resonant circuit  34 ′ of  FIG. 5A  is represented in  FIG. 5B  as an inductance  41  and a capacitance  43 . Receiver coil  32 ′ of  FIG. 5A  is represented as an inductance  33 , a noise filter  35 , and an amplifier  37 . 
         [0034]    In operation, the induction coil  4  is powered at a very low power level so that that the coil  4  radiates at a fundamental frequency (for example 30 kHz) and also radiates harmonic and noise spectra. Typically, the receiver coil  32 ′ detects a reference signal level of about −60 dB at output terminal  47 . However, the noise at the resonance frequency of the resonant circuit  34 ′ causes the resonant circuit  34 ′ to radiate at the resonant frequency, in this example 1 MHz, many times greater in magnitude than the magnitude of the noise at that frequency. So at 1 MHz, the receiver coil  32 ′ detects a reference signal of −30 dB at output terminal  47 , providing a binary identification of the cookware  4 . A specific identification scheme (or a more robust binary identification system) could be developed by providing additional resonant circuits with different resonance frequencies. 
         [0035]    Using an identification method that includes an identifier  34  or  34 ′, such as an RFID tag or a resonant coil, is particularly suited to use with cool cookware. The heat tolerance of the identifier does not need to be a consideration and the identifier can be positioned anywhere on a cool surface. For example, the identifier can be centered on the bottom surface of the cookware. When used with conventional induction cooking cookware, an identifier would either need to be heat tolerant or would need to be placed in a location that is cool in use, for example in a handle. Placing the identifier in a handle is undesirable because a handle is typically several centimeters from the cooktop and therefore from the identification circuitry and because the handle, in use, may be in different orientations relative to the identification circuitry. 
         [0036]    In the implementation of  FIG. 6 , the cookware identifier circuitry  8  includes a reactance (capacitive impedance) detector  42 . The reactance detector  42  is operationally coupled to the cookware  16  as indicated by line  140 . 
         [0037]    As described in U.S. patent application Ser. No. 12/031,214, U.S. patent application Ser. No. 12/031,220 or in U.S. patent Ser. No. 12/031,226, an induction cooking system equipped to use cool cookware may have several physical characteristics that are different from induction cooking systems that are not equipped to use cool cookware. The physical characteristics may cause the capacitive impedance (hereinafter “reactance”) to differ from conventional induction cooking systems. The cool cookware may have a non-conductive outer surface including a dielectric material such as glass ceramic, glass, or plastic. The cool cookware may include a vacuum or inert gas layer between the outer surface  44  and the cookware conductive layer. The cooktop may be made of different material and have different dimensions than cooktops of conventional induction cooking systems because cooktops designed for usage with cool cookware do not need to be as heat resistant as cooktops designed for usage with conventional cookware. The combined effect of the non-conductive layers (which may include a vacuum or inert gas layer) may mean that the conductive layer of the cookware is farther from the induction coil than conventional induction cooking cookware, which also affects reactance. The different reactance of cool cookware system components permits an induction cooking system as shown in  FIG. 6  to use a reactance detector  42  as a cookware identifier. 
         [0038]      FIG. 7A  shows a cutaway diagrammatic view of some elements of  FIG. 6  in greater detail. The cookware has a conductive layer  46  of a material suitable for induction cooking and may have an outer layer  44  of a dielectric material. The cookware may have intervening layers represented by layer  48 , for example vacuum layers, inert gas layers, and reflective layers as described in U.S. patent application Ser. No. 12/031,214, U.S. patent application Ser. No. 12/031,220 or in U.S. patent Ser. No. 12/031,226. Reactance detection circuitry including reactance detector  42 , reactance sensing targets  38 A and  38 B, reactance guard rings  39 A and  39 B, and reactance detector leads  40 A and  40 B may be positioned so that the cooktop  14  is between the reactance sensing targets and the conductive layer  46 . Collectively, layers between sensing targets  38 A and  38 B and conductive layer  46 , in this view cooktop  14 , outer layer  44 , and intervening layers  48 , are referred to in  FIG. 7D  below as collective layer  50 . The coupling of the reactance detector and the reactance guard rings  39 A and  39 B will be shown below in  FIG. 7E . 
         [0039]      FIG. 7B  shows a diagrammatic view of the reactance sensing target  38 A or  38 B hand guard rings  39 A or  39 B as viewed in the direction indicated by indicator  51  of  FIG. 7A . The guard ring  39 A or  39 B surrounds the sensing target  38 A or  38 B to eliminate stray capacitance to ensure that the reactance measurement is along path  52  of  FIG. 7A . 
         [0040]      FIG. 7C  shows the equivalent electrical circuit of the arrangement of  FIG. 7A . The reference numbers with the “′” (prime) indicator refer to the electrical equivalent of the like numbered elements of  FIGS. 7A and 7B . The circuit is equivalent to two capacitors in series, each with a capacitance of C 50′ . As shown in  FIG. 7D , the two capacitors in series can be represented by a single capacitance  54 , with a capacitance C P  of 
         [0000]    
       
         
           
             
               C 
               P 
             
             = 
             
               
                 1 
                 
                   
                     1 
                     
                       C 
                       
                         50 
                         ′ 
                       
                     
                   
                   + 
                   
                     1 
                     
                       C 
                       
                         50 
                         ′ 
                       
                     
                   
                 
               
               = 
               
                 
                   
                     C 
                     
                       50 
                       ′ 
                     
                   
                   2 
                 
                 . 
               
             
           
         
       
     
         [0000]    The capacitance  54  has a reactance signature substantially different than the reactance signature of the equivalent circuit of a cookware item that does not have the physical features shown in  FIG. 7A , or in which the metal layer is a different distance from the cooktop. An element of cookware according to the arrangement of  FIG. 7A  can therefore be identified by its reactance pattern. 
         [0041]    In  FIGS. 7A-7D , reactance detector  42  is shown as a functional block in a block diagram. One circuit for measuring the reactance is shown schematically in  FIGS. 7E . Current sensor  42  includes a sinusoidal voltage source  54 A coupled to capacitance target  38 A and sinusoidal voltage source  54 B coupled to capacitance target  38 B with the polarity reversed from the coupling of voltage source  54 A and capacitance target  38 A. The current in leads  40 A and  40 B is sensed by current sensors  56 A and  56 B, respectively, differentially summed at element  58  to provide a sensed current I sense . The capacitance can then be determined according to 
         [0000]    
       
         
           
             
               C 
               P 
             
             = 
             
               
                 
                   I 
                   sense 
                 
                 
                   2 
                    
                   
                     V 
                     s 
                   
                    
                   jω 
                 
               
               . 
             
           
         
       
     
         [0000]    The frequency of the sinusoidal voltage source is varied to obtain a reactance pattern. 
         [0042]    In the implementation of  FIG. 8 , one or more permanent magnets  62  are embedded in, or attached to, the cookware  16 . The permanent magnets  62  result in the existence of a DC magnetic field, that is a magnetic field that is time independent. The cookware identifier circuitry  8  includes a DC magnetic field detector  63 . The DC magnetic field detector  63  is operationally coupled to the permanent magnets  62  as indicated by lines  64 . The DC magnetic field detector may include, for example, one or more Hall Effect sensors. An implementation including more than one permanent magnet  62  with varying size, geometry and magnetic field strength and including more than one Hall Effect sensor permits a more reliable, robust, and sophisticated identification of the cookware. For an implementation using permanent magnets, it is desirable to select the magnetic material and other system components so that the permanent magnets  62  do not perturb AC parameters and thereby overheat. For example, the magnetic material should have low resistivity so that eddy currents do not form in the permanent magnets. 
         [0043]    Some of the identification methods described above may be prone to misidentification, either due to “spoofing”, that is, intentionally designing an item of cookware so that it is mistakenly identified, or due to coincidental similarity or due to an item of non-cool cookware coincidentally having similar material characteristics to cool cookware. For example, if the identification system is the reactance measuring system described in  FIGS. 6 and 7 , an item of non-cool cookware could be designed so that it has a reactance that is similar to an item of cool cookware or it could coincidentally have a similar reactance. The probability of misidentification can be alleviated by using more than one method of identification, for example, the reactance method of  FIGS. 6-7B  and in addition, determining the resonant frequency of a resonant coil in the item of cookware, as described in  FIGS. 5A and 5B . 
         [0044]    A number of embodiments of the invention have been described. Modification may be made without departing from the spirit and scope of the invention, and accordingly, other embodiments are in the claims.