Patent Application
20150373787
The present invention relates to temperature sensing in inductive cooking systems.
In one embodiment, the invention provides an inductive cooking system including a non-ferromagnetic cooking surface; an induction coil disposed adjacent to the cooking surface; a contact-based temperature sensing device thermally coupled to the cooking surface; and a non-contact temperature sensing device positioned to collect heat energy from an underside of the cooking surface.
In another embodiment the invention provides a method of inductive cooking using an inductive cooking system. The inductive cooking system includes a non-ferromagnetic cooking surface and an induction coil disposed adjacent to the cooking surface. The method includes the steps of obtaining a measurement from a contact-based temperature sensing device thermally coupled to the cooking surface; and obtaining a measurement from a non-contact temperature sensing device positioned to collect heat energy from an underside of the cooking surface.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.
a and 1b show IR transmission curves for ceramitized glass.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.
Induction cooking systems may use contact-based temperature sensing mechanisms or infrared temperature sensing mechanisms. Each mechanism alone has certain drawbacks, as discussed below.
After a period of time has elapsed during a typical warming or cooking cycle on an induction cooktop, and particularly at temperatures below 250° F., the cooking vessel (e.g. a pan) and the glass cooking surface reach a thermal equilibrium provided there is sufficient physical contact between the pan and glass. This allows for relatively accurate temperature measurement of the pan by using a contact temperature sensor such as a resistive temperature device (RTD), placed in physical contact with the ceramitized glass, as a temperature sensor. The RTD measures the temperature of the glass, which accurately reflects the temperature of the pan provided that the pan has sufficient contact with the glass. Thus, one approach that may be used to monitor the temperature of a pan in an induction heating system may be using an RTD sensor alone. Nevertheless, this approach has several potential drawbacks:
Since most cooking is done at temperatures in excess of 300° F., it is desirable to have a method by which cooking vessel temperature can be accurately measured and controlled. Present temperature measurement methods which use RTD readings alone (which measure the temperature of the glass cooking surface and not the cooking vessel) do not provide sufficient control over the temperature of the cooking vessel and thus are not conducive to cooking applications. Given the lack of accurate temperature control, typical control algorithms for inductive cooking systems allow for significant temperature overshoot when trying to obtain the response times required by many cooking applications. That is, in an attempt to quickly heat a food item (e.g. a pot of liquid such as soup), typical control algorithms will apply a high level of heat until the glass temperature reaches a desired level. However, given the delay or lag in the glass temperature relative to the pan temperature as well as a possible additional lag time due to the response time of the contact-based temperature sensing device, the pan in many cases will have exceeded the desired temperature by the time the temperature sensing mechanisms actually detect that the glass has reached the desired temperature. Nonetheless, this overshoot can be reduced, if not eliminated, by combining the use of an RTD with infrared temperature sensor measurements as disclosed herein.
Heated objects emit energy in the form of infrared radiation (light with wavelength ranging generally from 0.75 μm to about 15 μm) and thus measuring infrared energy can be used to determine the temperature of an object from a distance without making contact with the object. However, the poor transmissive properties of glass in the infrared spectrum have so far prevented the use of infrared (IR) energy-based sensing as a sole modality for measuring the temperature of a cooking or warming vessel (i.e. generally, but not exclusively, made of ferromagnetic materials) typically used in induction cooking surfaces. To the extent that IR temperature sensing has been used in induction cooking systems, this has involved creating a hole in the glass cooking surface and filling the hole with a material that is transparent to IR energy. This technique allows for reasonable temperature sensing for systems that are restricted for use at lower heating temperatures (<250° F.) because systems that are limited to operating in this low temperature range can simply use tempered glass, which is capable of withstanding the presence of the hole.
However, for applications in which a cooking temperature greater than 250° F. is desired, ceramitized glass is desirable because it provides the low thermal expansion and thermal shock resistance required for use at such elevated temperatures. For a cooking surface that is intended to be used at temperatures above 250° F., it is not possible to place a “window” of alternate material (i.e. material that is transparent to IR wavelengths) in ceramitized glass and still maintain the required material strength (i.e. prevent the glass from breaking during standardized impact tests). Therefore, the IR sensor must “view” the thermal load through the ceramitized glass (instead of through an IR-transparent window), the transmissivity of which varies according to wavelength and which is limited in certain wavelength ranges. That is, the ceramitized glass affects the transmissivity of IR energy, which adversely affects the accuracy of temperature calculations based on IR readings.
a shows the percent transmission of IR energy through ceramitized glass as a function of wavelength.
An IR sensor for monitoring temperature in an induction heating system is located below the glass cooking surface and collects IR energy from the cooking vessel and the glass, as the glass is heated by the cooking vessel. Given the relatively small amount of IR energy emitted by cooking vessels at lower temperatures and the relatively low percent transmission in the thermal energy band, the amount of IR energy transmitted through the ceramitized glass is very low when the cooking vessel temperature is less than about 200° F. Thus, at low temperatures (e.g. 200° F.), most of the IR energy collected by the IR sensor is from the glass and only a small amount from the cooking vessel. This makes obtaining an accurate temperature calculation of the cooking vessel through ceramitized glass difficult at temperatures below 200° F. using IR readings. With increasing cooking vessel temperatures, however, the amount of IR energy that is transmitted through the ceramitized glass increases as a direct function of the cooking vessel temperature. At temperatures of 250° F. and above, the amount of thermal (IR) energy that is transmitted through the ceramitized glass from the heated cooking vessel shifts towards wavelength ranges having a greater percentage of transmission through the glass (e.g. towards the 10 μm wavelength range, see e.g.
Accordingly,
The non-contact (e.g. infrared-based) temperature sensing device 140 is located under the cooking surface 120 (e.g. within the area circumscribed by the induction coil 110) and is positioned to collect IR energy emitted by the cooking surface 120 and any items that are on the cooking surface 120. In certain embodiments the induction coil 110 is designed so that it includes an opening 112 through which the non-contact (e.g. IR) temperature sensing device 140 can “see” the energy being emitted from a cooking vessel 150 placed on the cooking surface 120. In one embodiment, the non-contact (e.g. IR) temperature sensing device 140 has a conical field of view (e.g. at a 15° angle) aimed at the underside of the cooking surface 120. In various embodiments, the non-contact (e.g. IR) temperature sensing device 140 detects IR energy from a circle of approximately 1.25 inches in diameter near the center of the induction coil 110. Given that the non-ferromagnetic (e.g. glass) cooking surface 120 is relatively thin (e.g. 4-6 mm), the area of the cooking vessel 150 from which the non-contact (e.g. IR) temperature sensing device 140 receives energy is also a circle having a diameter of about 1.25 inches. One or more of the induction coil 110, the non-contact (e.g. IR) temperature sensing device 140, the contact-based temperature-sensing device 130, and a user interface may be operatively connected to a controller which carries out the operations disclosed herein.
In use, a cooking vessel (e.g. a pan), typically made of a ferromagnetic material, is placed on the cooking surface and is heated by magnetic induction from the induction coil. As the cooking vessel is heated, some of the heat of the cooking vessel is transferred to the cooking surface by conductive heat transfer, the efficiency of which depends on the amount of contact between the cooking vessel and the cooking surface. The heated cooking vessel and cooking surface both emit IR energy which is collected by the IR sensor. As shown in
The total energy detected by the IR sensor (ETotal) shown in the arrangement in
E
Total
=E
Pan
+E
Glass (1)
where EPan is the thermal radiation energy of the cooking vessel that is transmitted through the ceramitized glass and EGlass is the thermal radiation energy emitted by the glass itself.
The energy measured by the IR sensor (ETotal) has two components, one contributed by the pan (EPan) and the other by the glass (EGlass). The RTD sensor tracks the temperature of the glass. Glass temperature readings can be used to determine EGlass and thus will account for the energy contributed to the IR sensor measurement from the glass. Therefore, it is possible to calculate the temperature of the pan by measuring the total energy received by the IR sensor, subtracting the energy contributed by the glass, and calculating pan temperature from the amount of energy emitted by the pan and collected by the IR sensor.
In various embodiments, the disclosed methods are suitable for use at a wide range of temperatures, including temperatures over 200° F., and are particularly well suited for use at temperatures above 225° F. For temperatures below 200° F., it is possible to use the RTD as the sole temperature sensor since the amount of IR energy transmitted from the pan through the glass is often too low to allow reliable IR sensor measurement of the pan temperature. Nevertheless, testing by the present inventors has shown that using the IR sensor to monitor glass temperatures below 200° F. provides reliability improvements over the use of the RTD alone. In particular, direct sensing of the glass temperature by the IR energy sensor at temperatures below 200° F. provides a faster response time to changes in the glass temperature since the lag time due to the response of the contact-based temperature sensing device is not present. In addition, at temperatures below 200° F. virtually all of the energy received by the IR sensor is from the glass itself and little or none is from the pan: the ratio of IR energy from the glass to IR energy from the pan in this temperature range varies from 20:1 to 40:1, so less than 5% of the IR energy below 200° F. is from the pan. Therefore, at these lower temperatures it is possible to calculate the glass temperature based on the IR energy readings. Accordingly, even at temperatures below 200° F. the combined sensor system disclosed herein offers advantages over known systems.
Thus, by combining the signals from two sensors—an RTD temperature sensor and an IR energy sensor—one can correct for the presence of a ceramitized glass cooking surface and accurately measure pan temperature at a broad range of temperatures. This allows for accurate monitoring and control of cooking vessel temperatures over a wide range, including temperatures above 300° F. that are often used in cooking applications. This level of temperature control has not previously been achieved in other systems, particularly at higher temperatures.
Combining information from the RTD sensor and the IR sensor provides at least two advantages. A first advantage is that the presence of both sensors allows for a calibration of the pan temperature, as described below, which permits the system to compensate for widely varying levels of emissivity of cookware. A second advantage is that using a combination of sensors allows for compensation for the effect of the ceramitized glass temperature when measuring at temperatures above approximately 250° F. In particular, the ability to subtract out the energy radiated by the ceramitized glass from the total energy measured by the IR sensor enables an accurate determination of the pan temperature, even at elevated temperatures at which RTD measurements alone are far less accurate.
Emissivity Calibration
When heating a cooking vessel (e.g. a pan) to a temperature above 250° F., the pan temperature and glass temperature will initially track one another well (i.e. are approximately equal) up to about 200° F., although the pan often reaches a given temperature before the glass does since the glass is heated indirectly by absorbing heat from the pan. Up to temperatures of approximately 200° F., the IR sensor receives only a small amount of energy from the pan that is transmitted through the glass. As discussed above, using the glass temperature that is obtained from the RTD sensor measurement allows one to subtract out the amount of energy contributed by radiation from the glass to determine the amount of energy from the pan. Using this information allows calculation of an estimated pan temperature which, at an RTD-measured glass temperature of 200° F., is expected to be equal to or slightly greater than 200° F. However, if the estimated pan temperature is below 200° F. when the RTD-measured glass temperature is 200° F., then this anomalously low estimated pan temperature is taken as an indication that the pan is made of a shiny material which has a relatively low emissivity. While only a small amount of IR energy collected by the IR sensor at about 200° F. is from the cooking vessel (less than 5%), this is sufficient to obtain an estimate of the cooking vessel temperature.
Pans with low emissivity emit less IR energy at a given temperature than pans with high emissivity. The emissivity of most pans falls into one of two basic ranges—for shiny pans, typical emissivity values of around 0.6 have been measured, while non-shiny cookware has emissivity values between 0.92 and 1. Therefore, to compensate for differences in emissivity of the cooking vessel, an emissivity value of 0.6 is used and appropriate adjustments are made to the temperature calculations when the estimated pan temperature is below 200° F. when the glass temperature is 200° F., as discussed below. In various embodiments, other emissivity values can be used other than 0.6 when the estimated pan temperature is lower than expected. In some embodiments, a different emissivity value is used when the estimated pan temperature is higher than the RTD-measured glass temperature (e.g. for a standard pan), for example an emissivity value of 0.92. Using one or both of these emissivity correction values improves the accuracy of the IR measurement for all temperatures above the calibration point (e.g. 200° F.).
Conversion Between Energy Values and Temperatures
Equation (2) is a modified form of the equation (1):
E
Measured
=τE
Pan+EGlass (2)
where EMeasured is the energy measured by the IR sensor, EPan is the energy radiated by the pan, EGlass is the energy radiated by the glass and τ is the transmissivity of the glass (e.g. as shown in
The IR sensor data can be used to compute a temperature based on the thermal energy that is detected, where thermal energy is proportional to the temperature measured to the fourth power. Radiated energy, in its simplest form is
E=σT
4 (3)
where σ is the well-known Stefan-Boltzmann constant and T is the temperature of the radiating object.
Each of the energies in equation (2) can be represented by this temperature relationship. Thus, equation (2) becomes:
σTMeasured4=τσTPan4+σTGlass4 (4)
Solving the above for TPan yields:
Therefore, the temperature of the pan is calculated using the measured temperature from the IR sensor (TMeasured, which can be computed from the measured energy, EMeasured, using equation (3)), the glass temperature (TGlass, which can be computed from the RTD or other contact sensor readings), and the transmissivity of the glass τ. In some embodiments, the calculations of equation (5) including the determination of the fourth root may be performed using lookup tables.
Equation (3) assumes that the body that is radiating energy is a black body which has an emissivity of 1.0. However, as discussed above many pans have emissivity values less than 1.0 and certain types (e.g. shiny pans) have relatively low emissivity values around 0.6. Thus, an object with less than perfect emissivity (i.e. an object that does not exhibit black body radiation behavior) will emit less energy than expected at a given temperature and therefore calculated temperatures will be underestimates of the actual temperature of the body. Therefore, to correct for emissivity of the cooking vessel, the temperature that is calculated in equation (5) may be divided by an emissivity correction factor. As described above, the emissivity correction factor in some embodiments may be set to 0.6 for pans that are determined to have particularly low emissivity (e.g. due to an anomalously low estimated temperature at about 200° F.). In other embodiments the temperature determined in equation (5) may be divided by an emissivity correction factor of 0.92 when it is determined that the cooking vessel has a relatively high emissivity. In either case, dividing by the emissivity correction factor will increase the final calculated temperature since the factor is less than 1.
Various features and advantages of the invention are set forth in the following claims.
This application claims priority to U.S. Provisional Application Ser. No. 62/015,755, the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62015755 | Jun 2014 | US |
