Patent Description:
In the field of cooking appliances, particularly inductive cooking appliances, it is advantageous to determine, contactlessly, the temperature of cookware, for example a cooking pot or saucepan, because the inductive cooking appliance, for example a stove or hob, does not possess a heating element per se in the sense of an element that heats up, and so the temperature of the heating "source" cannot be measured directly; only the amount of energy induced in the cookware is being controlled, not the resulting cookware temperature.

Several solutions aimed at measuring the temperature of cookware placed on an inductive cooking top exist. Some of these solutions comprise measuring the temperature of the cookware using a contactless infrared temperature sensor placed underneath the cooking top. Examples of such solutions can be found in patent documents <CIT>, <CIT>, <CIT> and <CIT>, and in the article "<NPL>.

However, in order to be able to use multiple, different items of cookware, such temperature measuring solutions need to measure accurately a value for the infrared emissivity of the different items of cookware being measured. Quantitatively, emissivity is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature, as given by the Stefan-Boltzmann law. The ratio varies from <NUM> to <NUM>, and can be measured based on the equation: emissivity = <NUM> - reflectance. Thus, by measuring the amount of known radiation reflected by a body the emissivity can be calculated.

The infrared emissivity of a base of an item of cookware largely depends on many characteristics of the surface of the base of the item of cookware, including for example material, colour and processing conditions. Additionally, variations exist between cookware of the identical design, because the infrared emissivity changes with the presence of contamination, for example cooking oil or dirt, adhering to the base of the item of cookware.

<CIT> describes a circuit for measuring emissivity using an infrared Light Emitting Diode (LED) configured to emit infrared light towards cookware, through a cooking top, and a phototransistor both arranged in an electrical circuit to generate a time-varying emission of infrared light and to process received reflected infrared light for a processing unit to determine emissivity.

Two possible states of the cookware can exist: the cooking top has nothing disposed thereon, and the cooking top has an item of cookware disposed thereon. In both scenarios, it is challenging to measure emissivity accurately owing to variations in the gain of the photodetector device.

According to a first aspect of the present invention, there is provided a method of a method of measuring reflectivity in a heating appliance comprising a substrate for receiving an item of cookware, the method comprising: emitting a time-varying electromagnetic signal from a first side of the substrate, a portion of the time-varying electromagnetic signal propagating through the substrate; receiving electromagnetic radiation at the first side of the substrate, the received electromagnetic radiation comprising a background ambient component of electromagnetic radiation received and a reflected component of the time-varying electromagnetic signal reflected by the substrate; applying a gain factor to translate the received electromagnetic radiation to a receive electrical signal, the gain factor applied being responsive to the background ambient component of electromagnetic radiation received; identifying an offset signal component from the receive electrical signal, the offset signal component arising from the background ambient component of electromagnetic radiation received; estimating the gain factor from the offset signal component using a characterisation of the offset signal component; and calculating the reflectivity using the receive electrical signal and the estimated gain factor.

Calculating the reflectivity may further comprises: identifying the reflected component of the time-varying electromagnetic signal; and compensating the identified reflected component of the time-varying electromagnetic signal using the estimated gain factor.

Calculating the reflectivity may further comprises: attenuate the identified offset signal component from the receive electrical signal.

The method may further comprise: identifying the reflected component of the time-varying electromagnetic signal; and using the estimated gain factor to calculate a parameter of emission of the time-varying electromagnetic signal; the calculated parameter of emission of the time-varying electromagnetic signal may compensate the identified reflected component of the time-varying electromagnetic signal.

The method may further comprise: analysing the receive electrical signal and repeatedly extracting the offset signal component from the receive electrical signal.

According to a second aspect of the present invention, there is provided a method of measuring reflectivity of a substrate of an inductive cooking appliance, the method comprising: measuring reflectivity of the substrate in the presence of the background ambient component of electromagnetic radiation as set forth above in relation to the first aspect of the present invention.

The measured reflectivity of the substrate may be a reference reflectivity.

According to a third aspect of the present invention, there is provided a method of measuring the reflectivity of an item of cookware disposed on a substrate of an inductive cooking appliance, the method comprising: measuring reflectivity of the substrate as set forth above in relation to the second aspect of the present invention; placing the item of cookware on the substrate; and emitting another time-varying electromagnetic signal from the first side of the substrate, a portion of the another time-varying electromagnetic signal propagating through the substrate; receiving further electromagnetic radiation at the first side of the substrate, the further received electromagnetic radiation comprising a first reflected component of the time-varying electromagnetic signal reflected by the substrate and a second reflected component of the time-varying electromagnetic signal reflected by the item of cookware; translating the further received electromagnetic radiation to another receive electrical signal; and calculating the reflectivity additionally using the another receive electrical signal.

The method may further comprise: applying the gain factor to translate the further received electromagnetic radiation to another receive electrical signal; the gain factor may be applied responsive to the background ambient component of electromagnetic radiation received.

The time-varying electromagnetic signal and the another time-varying electromagnetic signal may share a common time-varying electrical drive signal; and calculating the reflectivity may further comprise: calculating a fraction of the time-varying electromagnetic signal that is received as the second reflected component of the time-varying electromagnetic signal.

Calculating the fraction may comprise using the gain factor, the receive electrical signal, and the another receive electrical signal, and a predetermined reflectivity value of the substrate.

The reflectivity may be calculated using the following equation: <MAT>
where Ameas2 is an amplitude of the another receive electrical signal, Ameas1 is an amplitude of the receive electrical signal compensated using the estimated gain factor, and Rs is the predetermined reflectivity value of the substrate.

According to a fourth aspect of the present invention, there is provided a method of heating cookware inductively, the method comprising: measuring reflectivity of an item of cookware as set forth above in relation to the third aspect of the present invention; receiving a demand for inductive heating after measuring the reflectivity of the item of cookware; and generating an inductive electromagnetic heating signal in response to the demand for inductive heating.

The substrate may be a cooktop. An emissivity may be calculated using the measured reflectivity of the item of cookware.

According to a fifth aspect of the present invention, there is provided a reflectivity measurement apparatus comprising: an electromagnetic illumination circuit configured to emit a time-varying electromagnetic signal from a first side of a substrate of a heating appliance, the time-varying electromagnetic signal propagating, when in use, through the substrate; a photodetector circuit configured to receive electromagnetic radiation at the first side of the substrate, the received electromagnetic radiation comprising a background ambient component of electromagnetic radiation and a reflected component of the time-varying electromagnetic signal reflected by the substrate; the photodetector circuit is configured to apply a gain factor to translate the received electromagnetic radiation to a receive electrical signal, the gain factor applied being responsive to the background ambient component of electromagnetic radiation received; a signal processing unit configured to identify an offset signal component from the receive electrical signal, the offset signal component arising from the background ambient component of the received electromagnetic radiation; wherein the signal processing unit is configured to estimate the gain factor from the offset signal component using a characterisation of the offset signal component; and the signal processing unit is configured to calculate the reflectivity using the receive electrical signal and the estimated gain factor.

According to a sixth aspect of the present invention, there is provided an inductive heating apparatus comprising: a reflectivity measurement apparatus as set forth above in relation to the fifth aspect of the present invention.

It is thus possible to provide an apparatus and method that provide a measure of reflectance with improved accuracy. The measurement of reflectance is not distorted by the presence of ambient light received by an optical detector unit. By improving the accuracy of the measurement of reflectance, emissivity can be calculated with greater accuracy and thus temperature of an item to be measured can be calculated with greater accuracy.

At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:.

Throughout the following description, identical reference numerals will be used to identify like parts.

Referring to <FIG>, a cooking appliance <NUM>, for example an inductive cooking stove or hob, comprises a substrate <NUM>, for example a cooking top, for receiving one or more items of cookware thereon. In this example, a single cooking station is described, but the skilled person will appreciate that the cooking appliance <NUM> can comprise a greater number of cooking stations to receive multiple items of cookware, respectively. The substrate <NUM> is formed from a ceramic material, but any other suitable material can be employed. Although not shown, the cooking appliance <NUM> comprises a heating unit, in this example an induction coil operably coupled to a control and drive system.

A measurement apparatus of the cooking appliance <NUM> comprises an illumination unit <NUM> and an optical detector unit <NUM>, respectively disposed at a first side of the substrate <NUM>.

Turning to <FIG>, the measurement apparatus comprises a measurement circuit <NUM> comprising a positive supply operably coupled to an anode of an infrared LED <NUM>. A cathode of the infrared LED <NUM> is coupled to ground potential via a controllable current source <NUM>, for example a Field Effect Transistor (FET) or to a bipolar junction transistor. An LED driver unit <NUM> has a first output coupled to an input of the controllable current source <NUM>, for example a gate terminal of the FET transistor or a base terminal of the bipolar transistor mentioned above. The LED driver unit <NUM> is arranged to control the current flowing through the controllable current source <NUM>. This can be achieved, for example in the case of a FET transistor, by controlling the gate voltage of the FET transistor. The infrared LED <NUM>, the controllable current source <NUM> and the LED driver unit <NUM> constitute the illumination unit <NUM>.

A photodiode <NUM>, is arranged in a reverse bias configuration, having an anode terminal connected to ground potential and a cathode terminal coupled to a first input terminal of a signal processing unit <NUM>. Although a photodiode arrangement is specifically described above, the skilled person should appreciate that, in other examples, other types of photodetector device and configurations can be employed.

The signal processing unit <NUM> comprises a signal separation circuit <NUM> having a first input coupled to the input of the signal processing unit <NUM>. The signal separation unit <NUM> serves to output separately AC and the DC components of an output signal of the photodiode <NUM>. The signal processing unit <NUM> further comprises an emissivity calculation unit <NUM> operably coupled to the signal separation unit <NUM>, an AC output of the signal separation unit <NUM> and a DC output of the signal separation unit <NUM> being coupled to an AC input and a DC input of the emissivity calculation unit <NUM>, respectively. An output of the emissivity calculation unit <NUM> is coupled to an emissivity output terminal <NUM> of the signal processing unit <NUM>. The emissivity output terminal <NUM> of the signal processing unit <NUM> is coupled to a processing resource, for example a microprocessor (not shown).

The LED driver unit <NUM> is operably coupled to the signal processing unit <NUM>, thereby enabling providing a bi-directional communication link <NUM> between the LED diver unit <NUM> and the signal processing unit <NUM>.

In operation (<FIG>), and in the state of <FIG>, cookware is not disposed on the cooking top <NUM>. Upon being powered up, the infrared LED <NUM> of the measurement circuit <NUM> emits (Step <NUM>) pulsed infrared light <NUM> under the control of the LED driver unit <NUM>, which is directed towards the cooking top <NUM>. In this regard, the LED driver unit <NUM> generates a pulsed drive waveform, for example a square waveform constituting a time-varying electromagnetic signal and having a predetermined duty cycle of alternating ON and OFF portions. A portion of the emitted pulsed infrared light <NUM> propagates through the cooking top <NUM>, hereinafter referred to as transmissive light <NUM>. Another portion of the emitted pulsed infrared light <NUM> is reflected by the cooking top <NUM>, hereinafter referred to as reflected light <NUM>. A further portion of the emitted infrared light <NUM> is scattered by the cooking top <NUM>, hereinafter referred to as scattered light <NUM>. Additionally, background ambient light <NUM> is present in varying intensities, part of which propagates through the cooking top <NUM>, as transmissive ambient light <NUM>, but can also originate from sources beneath the cooking top <NUM>.

Substantially contemporaneously, the photodiode <NUM> receives (Step <NUM>) light reflected by the cooking top <NUM> and received through the cooking top <NUM>, for example the reflected light <NUM> and the transmissive ambient light <NUM>. The light received by the photodiode <NUM> is translated (Step <NUM>) to an electrical signal <NUM> (<FIG>), for example an electrical current signal, IPD. In this regard, it has been discovered that the gain factor applied by the photodiode <NUM>, or indeed other photodetector devices, is influenced by the receipt of the transmissive ambient light. The transmissive light is typically constant and so constitutes a DC signal. As such, the gain characteristic of the photodiode <NUM> is characterised with respect to DC signal level a priori and used to correct for the drift in the gain factor applied by the photodiode <NUM> as will be described later herein. As such, the signal separation unit <NUM> receives, through the communication link <NUM> (Step <NUM>), the pulsed drive waveform generated by the LED driver unit <NUM> and the electrical signal, IPD, received at the first input terminal of the signal processing unit <NUM>. Although, in this example, the electrical output signal, IPD, of the photodiode <NUM> is an electrical current, the skilled person should appreciate other configurations are possible, for example by way of presenting a voltage signal to the signal separation unit <NUM>.

Upon receipt of both the electrical current signal, IPD, from the photodiode <NUM> and the pulsed drive waveform from the LED driver unit <NUM>, the signal separation unit <NUM> separates the DC component and AC component of the electrical current signal, IPD, received at the first input terminal of the signal processing unit <NUM>. The DC component can also be considered as the level of the electrical current signal, IPD, during the OFF portion of the pulsed drive waveform, whereas the AC component can be considered as the electrical current signal, IPD, with the DC component removed.

The signal separation unit <NUM> analyses (Step <NUM>) the pulsed drive waveform received from the LED driver unit <NUM> and identifies an OFF portion of the pulsed drive waveform and measures (Step <NUM>) a corresponding amplitude of the electrical signal, which corresponds to a period of time when the light received by the photodiode <NUM> is attributable to the transmissive ambient light <NUM> and not the emissions of the infrared LED <NUM>, i.e. the DC component as mentioned above. Additionally, while the signal separation unit <NUM> is analysing (Step <NUM>) the pulsed drive waveform, the signal separation unit <NUM> identifies the ON portion of pulsed drive waveform that does not correspond to the DC component, and thereby the signal separation unit <NUM> obtains the AC component of the electrical current signal, IPD, received at the first input terminal of the signal processing unit <NUM>.

It should be appreciated the above-described technique for obtaining the DC and AC components of the electrical current signal, IPD, is just one of a number of different techniques to obtain the components of the electrical current signal, IPD, using the pulsed drive waveform generated by the LED driver unit <NUM>, and other suitable techniques can be employed. In any event, by separating the DC and AC components of the electrical current signal, IPD, it is possible to obtain an estimate of the background illumination (DC component) and the amplitude of the pulsed signal (AC component) as received by the photodiode <NUM>.

In response to the measurement of the amplitude of the electrical current signal, IPD, during the OFF portion of the pulsed drive waveform, the emissivity calculation unit <NUM> receives (Step <NUM>) a offset measurement signal <NUM> (<FIG>) from the signal separation unit <NUM>, the offset measurement signal <NUM> being a measure of amplitude corresponding to the measured amplitude of the electrical signal. The offset measurement signal <NUM> is maintained until the amplitude of the electrical current signal, IPD, is measured again during, in this example, a subsequent OFF portion of the pulsed drive waveform. The above process is repeated (Steps <NUM> to <NUM>) while emissivity measurements are required.

At the emissivity calculation unit <NUM>, the AC component of the electrical current signal, IPD, <NUM> is received and processed (Step <NUM>) so as to amplify the AC component of received electrical current signal, IPD, <NUM>. Thereafter, the offset measurement signal <NUM> received from the signal separation unit <NUM> is applied by the emissivity calculation unit <NUM> in order to remove (Step <NUM>) bias present in the received electrical current signal, IPD, <NUM> attributable to the presence of the transmissive ambient light <NUM>, thereby yielding a partially compensated electrical signal <NUM> (<FIG>).

Following generation of the partially compensated electrical signal <NUM>, the emissivity calculation unit <NUM> corrects (Step <NUM>) the partially compensated electrical signal <NUM> for the gain drift experienced during generation of the received electrical current signal, IPD, <NUM> as a result of the presence of the transmissive ambient light <NUM>. In this regard, the emissivity calculation unit <NUM> comprises the a priori characterisation of the gain of the photodiode <NUM> and uses the offset measurement signal <NUM> to obtain a gain factor, which is subsequently applied to the partially compensated electrical signal <NUM> in order to compensate the partially compensated electrical signal further. Thereafter, the emissivity calculation unit <NUM> calculates (Step <NUM>) an amplitude, Ad, of the time-varying electromagnetic signal emitted by the photodiode <NUM>. In this regard, the reflectivity of the substrate <NUM>, Rs, is known and the amplitude of the compensated electrical signal, Ameas1, is measured by the emissivity calculation unit <NUM>. Consequently, the amplitude, Ad, is calculated by the emissivity calculation unit <NUM> using the following equation: <MAT>.

The above process (Steps <NUM>, <NUM>, and <NUM> to <NUM>) are repeated for as long as measurement is required (Step <NUM>).

The above example relates to measurement of the amplitude, Ad, of the time-varying electromagnetic signal emitted by the photodiode <NUM> when an item of cookware is not disposed on the cooking top <NUM>. Referring to <FIG>, in another example, the above apparatus and method are employed when the item of cookware <NUM> is disposed on the cooking top <NUM> and where measurement of reflectivity suffers from bias attributable to the presence of the transmissive ambient light <NUM>. In this regard, the amplitude, Ad, of the time-varying electromagnetic signal calculated in respect of the substrate <NUM> without the presence of the item of cookware <NUM> is used when calculating the reflectivity of the item of cookware <NUM> and hence emissivity thereof.

Having previously calculated the amplitude, Ad, and measured the amplitude of the compensated electrical signal, Ameas1, the item of cookware <NUM> can be placed on the cooking top <NUM> for measurement. In this regard, the item of cookware <NUM> is placed on the substrate <NUM> and the infrared LED <NUM> emits the pulsed infrared light <NUM> as the time-varying electromagnetic signal as previously described, but the time-varying electromagnetic radiation is now computationally treated as another distinct time-varying electromagnetic signal from the first side of the substrate <NUM>, but the time-varying electrical drive signal used to generate the electromagnetic signals is nevertheless common to both electromagnetic signals. Again, a portion of the pulsed infrared light <NUM> propagates through the substrate <NUM>.

While the infrared LED <NUM> emits the pulsed infrared light <NUM>, the photodiode <NUM> receives further electromagnetic radiation (distinct from the electromagnetic radiation received when the item of cookware <NUM> was not placed on the cooking top <NUM>) at the first side of the substrate <NUM>, the further received electromagnetic radiation comprising a first reflected component of the time-varying electromagnetic signal reflected by the substrate <NUM> and a second reflected component of the time-varying electromagnetic signal reflected by the item of cookware <NUM>. Assuming ambient light is still present, the gain factor is re-calculated and then applied to translate the further received electromagnetic radiation to another received electrical current signal, which has compensation applied thereto. Of course, where ambient light is below a threshold value considered not materially to affect measurement of reflectivity within predetermined tolerances, the skilled person will appreciate that the gain factor is not re-calculated and applied. The AC component of the received electric current signal attributable to the received further electromagnetic radiation is provided by the signal separation unit <NUM> and used by the emissivity calculation unit <NUM> to measure a second amplitude of the received electrical current signal, Ameas2. Once the second amplitude of the received electrical current signal, Ameas2 has been measured, the reflectivity of the item of cookware <NUM> can then be calculated as follows.

To calculate the reflectivity of the item of cookware <NUM>, the emissivity calculation unit <NUM> calculates what fraction of the time-varying electromagnetic signal is received as the second reflected component of the time-varying electromagnetic signal <NUM>. The reflectivity of the item of cookware <NUM> is therefore calculated using the following equation: <MAT>.

From the reflectivity calculated in respect of the item of cookware <NUM>, the emissivity can be calculated by subtracting the reflectivity calculated from unity.

In this regard, by using equation (<NUM>), the reflectivity of the substrate <NUM> is used as a reference reflectivity. As such, any changes over time to the measurement setup, for example due to temperature variations and/or ageing, are automatically compensated.

The calculated emissivity is then communicated to the processing resource for calculation of the temperature of the item of cookware <NUM> using the calculated emissivity.

The reflectivity calculated can thus be employed in a heating appliance (not shown), for example a cooking appliance, such as an inductive heating appliance, such as a stove. The appliance receives a demand for inductive heating from a user after measuring the reflectivity of an item of cookware <NUM> placed on the cooking top <NUM>. The appliance then generates an inductive electromagnetic heating signal in response to the demand for inductive heating from the user and can control the inductive heating signal to ensure a desired temperature of the item of cookware <NUM> is attained.

Claim 1:
A method of measuring reflectivity in a heating appliance (<NUM>) comprising a substrate (<NUM>) for receiving an item of cookware (<NUM>), the method comprising:
emitting (<NUM>) a time-varying electromagnetic signal (<NUM>) from a first side of the substrate, a portion of the time-varying electromagnetic signal (<NUM>) propagating through the substrate (<NUM>);
receiving (<NUM>) electromagnetic radiation (<NUM>, <NUM>) at the first side of the substrate (<NUM>), the received electromagnetic radiation (<NUM>, <NUM>) comprising a background ambient component of electromagnetic radiation (<NUM>) received and a reflected component of the time-varying electromagnetic signal (<NUM>) reflected by the substrate (<NUM>);
applying (<NUM>) a gain factor to translate the received electromagnetic radiation (<NUM>, <NUM>) to a receive electrical signal (<NUM>), the gain factor applied being responsive to the background ambient component of electromagnetic radiation (<NUM>) received;
identifying (<NUM>) an offset signal component (<NUM>) from the receive electrical signal (<NUM>), the offset signal component (<NUM>) arising from the background ambient component of electromagnetic radiation (<NUM>) received;
estimating the gain factor from the offset signal component (<NUM>) using a characterisation of the offset signal component (<NUM>); and
calculating the reflectivity using the receive electrical signal (<NUM>) and the estimated gain factor.