WO2022008050A1

WO2022008050A1 - Backscatter rf communication for microwave oven

Info

Publication number: WO2022008050A1
Application number: PCT/EP2020/069295
Authority: WO
Inventors: Kristian LINDBERG-POULSEN; Henrik Schneider
Original assignee: Senserna AS
Current assignee: Senserna AS
Priority date: 2020-07-08
Filing date: 2020-07-08
Publication date: 2022-01-13
Anticipated expiration: 2023-01-08

Abstract

The present disclosure relates to a sensor assembly and a receiver assembly for a microwave oven. The sensor assembly can harvest energy from the microwave radiation and generate a modulated backscatter signal of a higher harmonic. The receiver assembly can receive and extract the modulated backscatter signal from the intense background microwave radiation.

Description

BACKSCATTER RF COMMUNICATION FOR MICROWAVE OVEN

The present disclosure relates to a sensor assembly for a cooking cavity of a microwave oven, a receiver assembly for a microwave oven, a microwave oven comprising a receiver assembly, and a kit of parts comprising a sensor assembly and a receiver assembly.

Further, the disclosure relates to a method of communication between a sensor assembly and a receiver assembly in a microwave oven.

BACKGROUND OF THE INVENTION

Microwave ovens have been known and used for many years, in industrial settings as well as in homes, for heating products containing polar molecules. A magnetron generates microwave radiation, which is directed into a shielded cavity, wherein the product to be heated is placed. Usually, the electromagnetic waves enter the cavity from a single entry point and are reflected from the insides of the cavity of the microwave oven such that a greater volume within the cavity is filled with the radiation. The frequencies at which a microwave oven is allowed to operate is regulated by law such that consumer ovens generally work at a frequency around 2.45 GHz, which is in the 2.4 GHz to 2.5 GHz ISM band, while industrial ovens generally use a frequency of 915 MHz.

Within the volume of the microwave oven cavity, so-called cold spots are formed, wherein either no or little radiation is present at a given time and a product to be heated would not be affected in the parts that is within that volume. In contrast, the volume where microwave radiation is present in greater intensity is called a hot spot. To alleviate the problem of uneven heating of the product, two solutions are generally employed: a rotating platter and/or a field stirrer. In the first solution, the product to be heated is placed on a platter, which rotates while the oven is operating to make more of the product pass through hot spots. In the second solution, the microwave radiation is deflected in varying directions by a so-called stirrer such that the location of the hot and cold spots within the cavity vary with time.

The microwave radiation heats the product placed in the cavity by exciting polar molecules, such as e.g. water, fat and sugars, as well as by inducing dielectric heating, thus producing thermal energy from within the product. This heating is usually very efficient and fairly uniform in the outer 2.5 - 3.8 cm of a homogeneous, high water content food item. However, products to be heated often vary greatly in e.g. size, shape and polar molecule composition, the latter also within the product itself. Further, when a user of a microwave oven wants to heat a product, information such as water content, viscosity, pressure, etc. could be a factor in how the product reacts to the exposure to the microwave radiation in the oven.

These above mentioned realities leading to uneven heating combined with the potentially very fast heating of some or all of the product and differing power of different ovens means that heating a product in a desired manner in a microwave oven, particularly one that a user is unfamiliar with, can be a challenge.

Therefore, it would be desirable to be able to obtain various types of sensor information from within the microwave oven cavity and/or the surface of the irradiated product and/or from within the irradiated product. Such information could then advantageously be communicated, for example, directly to the user or to the control system of the microwave oven. In the latter situation, the sensor information could be used in advanced cooking programs, which could adjust the operation of the microwave oven in response, such as turning off the radiation, either entirely or for a time period or such as reducing or increasing the intensity of the microwave radiation, etc.

However, when the microwave oven is operating, the cavity of the oven is a harsh radiation environment and the cavity itself is heavily shielded so as to greatly reduce any leakage of the microwave radiation to the outside of the cavity. Therefore, any electronic equipment within the cavity of the oven during its operation will need to be able to withstand the radiation at least to some degree. Further, it is generally difficult to reliably transmit wireless data signals out of the microwave oven during food preparation because of the previously discussed excessive strength of the microwave electromagnetic field inside the microwave oven chamber. The microwave electromagnetic field tends to interfere with RF signals carrying the wireless data signals. To worsen the situation the cooking cavity of microwave ovens acts essentially as a Faraday cage designed to block any emission of RF signals to avoid leakage of the potentially harmful microwave radiation to the outside and reach the users.

Thus, there is a need in the art for a microwave oven sensor assembly that can transmit sensor readings with up-to-date parameter values of a measured physical or chemical property of a product from inside the cooking cavity of a microwave oven. SUMMARY OF THE INVENTION

Disclosed herein is a sensor assembly for a microwave oven, where the sensor assembly is able to transmit signals within the cavity of the microwave oven, which may then be received by a receiver assembly within the microwave oven so as to facilitate communication from the sensor assembly e.g. to the control system of the oven, which may then react to the information, e.g. to initiate a change in the function of the oven or by communicating to the user using e.g. sound, light, display text or other means.

In a first aspect is provided a sensor assembly for a cooking cavity of a microwave oven, in a second aspect is provided a receiver assembly for a microwave oven, in a third aspect is provided a microwave oven comprising a receiver assembly, in a fourth aspect is provided a kit of parts comprising a sensor assembly and a receiver assembly and in a fifth aspect is provided a method of communication between a sensor assembly and a receiver assembly in a microwave oven.

In the first aspect, a sensor assembly for a cooking cavity of a microwave oven, that is configured to generate microwave radiation with a predetermined oven frequency (f1) within the cooking cavity, comprises:

- an RF energy receiver part configured to harvest energy from the microwave radiation within the cooking cavity and the RF energy receiver part comprises a sensor assembly microwave antenna circuit configured to generate an RF antenna signal,

- one or more sensors,

- a backscatter modulation part that is configured to generate a modulated backscatter signal within the cooking cavity, and

- an energy supplying part that is configured to supply energy from the RF energy receiver part to the one or more sensors and to the backscatter modulation part, and wherein the frequency (f2) of the modulated backscatter signal is a higher harmonic of the predetermined oven frequency (f1).

While the cavity of a microwave oven within which a product to be irradiated is placed is often referred to as the cooking cavity, it should be understood that the irradiation may be for other purposes than cooking such as e.g. for heating or disinfection.

Each of the one or more sensors may measure e.g. temperature, viscosity, pressure, colour, humidity, reflectivity, electric conductivity or any other relevant physical or chemical property. The sensor assembly may be placed within the microwave oven cavity separate from any product within the cavity to be irradiated, such as one or more food items, or it may be placed on top of or next to any such product, or it may be placed to contact a surface of such a product, or it may be placed partly within a product to be irradiated, such as at or close to the core of any such product. The sensor assembly may be comprised within a container suitable for use in a microwave oven. The sensor assembly may be attached to, or integrated with, the container in numerous ways and may be partially or fully embedded in a wall section, lid section, or bottom section of the container. In some embodiments, the sensor assembly may be comprised within a food probe. The food probe may comprise an elongate housing enclosing and protecting the sensor assembly.

During operation, i.e. while microwave radiation is powering the sensor assembly, the one or more sensors may record one or more physical or chemical properties within the microwave oven cavity and/or from the surface of the irradiated product and/or from within the irradiated product. A measurement, while in contact with a surface of a product to be irradiated, may be used to detect whether the surface of a particular product has reached a target or treatment temperature for hygienic or disinfection purposes. The product to be irradiated may be arranged in a suitable container or utensil, such as a cup, bottle, bag or plate etc. In some embodiments, one or more of the sensors may operate without physical contact to the product and instead remotely sense/measure the physical property of the product e.g. using an infrared (IR) temperature detector etc. The sensory portion of the sensor may alternatively or additionally measure or detect a chemical property of a product under heating, for example water content or the presence and/or concentration of certain chemical agents such as salt, sugar etc. in the product.

The product to be irradiated may e.g. be a food item, or a medical preparation or substance such as a dose of infusion fluid. If the product to be irradiated is a food item, the food item may comprise a liquid such as milk, water, baby formula, coffee, tea, juice or other drinkable substances or the food item may comprise solid or frozen food such as bread, meat or a dinner meal.

The sensor assembly may comprise multiple individual sensors of different types or comprise multiple individual sensors of the same type. Multiple individual sensors of different types may be configured to measure different physical properties and/or chemical properties of the product, while multiple sensors of the same type may be configured to measure the physical or chemical property in question, for example temperature, at different locations of the product, for example simultaneously at the core and at the surface of the product. In an embodiment, the backscatter modulation part is further configured to modulate the modulated backscatter signal in such a way that the modulated backscatter signal comprises data based on sensor readings produced by at least one of the one or more sensors.

The RF energy receiver part, optionally comprising an RF power limiter circuit, and the energy supplying part together form an energy harvesting circuit in which microwave radiation incident on the sensor assembly microwave antenna circuit is used to generate power such as a DC current, which may then power any energy consuming parts such as the one or more sensors and the backscatter modulation part. Thus, the sensor assembly is microwave powered and is able to operate without any battery source by relying on energy harvested from the microwave radiation in the microwave oven cavity. Due to the extremely EMI hostile environment inside the cooking cavity it may be unsafe to place batteries or similar chemical energy storage device for powering the sensor assembly inside the cavity. Furthermore, the need to replace any batteries in the sensor assembly from time to time makes it more difficult to make a housing of a battery powered sensor device or assembly hermetically sealed against the external environment.

By antenna circuit is meant a circuit comprising an antenna. Other components attached to an antenna may alter the properties of the antenna, for example an antenna will comprise one or more conducting elements, but any conducting element connected to an antenna, such as an electric wire connecting the antenna and other electrical components can alter the properties of the antenna. Thus, antenna circuit is used to mean that any component altering the properties of the antenna are comprised in the antenna circuit.

The sensor assembly microwave antenna circuit is responsive to the microwave radiation in the microwave oven cavity and may comprise at least one of: a monopole antenna, a dipole antenna, a loop antenna, a slot antenna, or a patch antenna. The sensor assembly microwave antenna circuit may be integrally formed in a wire or conductor pattern of a carrier or substrate, such as a printed circuit board, supporting the sensor assembly. A monopole microwave antenna is generally compact and omnidirectional.

To characterise the reception and transmission efficiency of an antenna circuit, the reflection coefficient may be used. The reflection coefficient represents how much power is reflected from an antenna circuit at a given frequency. By definition, the reflection coefficient is the ratio of the amplitude of the reflected wave to the amplitude of the incident wave and will therefore be a number between 0 and 1. However, for practical purposes the reflection coefficient is often given in dB as:

S₁₁(dB) = 2O - log_lo (0 dB, where

Sn(dB) = reflection coefficient in dB F = measured amplitude of reflected wave Fo = measured amplitude of incident wave

Thus, if the reflection coefficient in dB is zero, i.e. the amplitude of the reflected wave is equal to the amplitude of the incident wave, then all the power is reflected from the antenna circuit and nothing is radiated by the antenna circuit. In units of dB, zero

reflection and total reflection are respectively written: — = 0 => S_1;L(dB) = - dB, and

Fo

— = 1 => S_1;L(dB) = 0 dB. Further, the lower the reflection coefficient at a given

Fo frequency is, i.e. the larger the absolute value of the negative number is, the better the antenna circuit is at receiving or transmitting at that frequency. An antenna circuit that has a relatively low reflection coefficient at a given frequency is said to be tuned to that frequency and, conversely, an antenna circuit that has a relatively high reflection coefficient at a given frequency is said to be detuned from that frequency.

One of the parameters characterising an antenna is the physical dimension and, disregarding all other factors, the larger the tuning frequency of the antenna, the smaller the physical dimension of the antenna. As is well-known to the skilled person, the type of antenna circuit will determine, what may be done to tune an antenna. For example, to optimise a simple monopole or dipole antennas response to electromagnetic waves of a given wavelength the antenna could have physical dimensions comparable to a multiple of one quarter of the given wavelength. For the 2.45 GHz operating frequency of consumer microwave ovens, the microwave wavelength is approximately 12.2 cm and a dipole antenna optimised to receive the 2.45 GHz signal would advantageously have physical dimensions of around 3.05 cm. However, an antenna that is part of a circuit may be detuned by that circuit and the physical dimension of the antenna itself is therefore not the only factor in determining the tuning frequency of the antenna circuit. When an RF signal is received by a device and a portion of the energy that was radiated in the direction of the device is reflected back, the signal reflected back is said to be a backscatter signal. A modulated backscatter signal is generated, when the device modulates the reflected signal resulting in an amplitude and/or phase modulated reflection. As the sensor assembly is within the cooking cavity of a microwave oven when in operation, the modulated backscatter signal generated by the sensor assembly will propagate within the cooking cavity.

The oven’s microwave radiation field is both high intensity and noisy and therefore, a backscatter signal at the predetermined oven frequency (f1) would be extremely difficult to extract and filter from the oven’s microwave radiation. Therefore, the generated backscatter signal by the sensor assembly is designed to be a higher harmonic of the predetermined oven frequency (f1) and thus has a different frequency.

A harmonic frequency is a multiple of a fundamental frequency, /, and can therefore be expressed as: 2/, 3/, 4/, etc. A fundamental frequency, /, is also called the first harmonic, a frequency that is twice the first harmonic, i.e. 2/, is called the second harmonic, etc. Thus, as the frequency (f2) of the modulated backscatter signal is a higher harmonic of the predetermined oven frequency (f1), it may be any of a multiple of the predetermined oven frequency (f1), e.g. twice, three times, or four times, etc.

In the case of a consumer microwave oven, the predetermined oven frequency will be 2.45 GHz as this is the frequency wherein consumer microwave ovens are allowed to operate is regulated such that consumer ovens work at a frequency around 2.45 GHz, whereas industrial microwave ovens are regulated to operate at 915 MHz. For a consumer microwave oven, the first harmonic, i.e. fundamental frequency, is therefore 2.45 GHz, the second harmonic is 4.9 GHz, the third harmonic is 7.35 GHz, etc. For an industrial microwave oven, the first harmonic, i.e. fundamental frequency, is 915 MHz, the second harmonic is 1.83 GHz, the third harmonic is 2.745 GHz, etc. In an embodiment, the frequency (f2) of the modulated backscatter signal is the second harmonic of the predetermined oven frequency (f1).

To reduce the amount of microwave energy picked up by the sensor assembly microwave antenna circuit, the antenna circuit may have a relatively high reflection coefficient at the frequency of the predetermined oven frequency (f1). In an embodiment, the sensor assembly microwave antenna circuit has a reflection coefficient that is greater than -10 dB at the frequency of the predetermined oven frequency (f1), such as greater than -6 dB, such as greater than -3 dB. The generated modulated backscatter signal may be transmitted by the sensor assembly microwave antenna circuit or by another antenna circuit comprised in the sensor assembly. In an embodiment, the generated modulated backscatter signal is transmitted by a backscatter microwave antenna circuit having a reflection coefficient at the backscatter signal frequency (f2), which is lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the predetermined oven frequency (f1).

In some embodiments, the backscatter microwave antenna circuit has a reflection coefficient at the frequency of the higher harmonic (f2), which is at least 3 dB lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the frequency of the predetermined oven frequency (f1), such as at least 6 dB lower, such as at least 10 dB lower.

In an embodiment, the backscatter microwave antenna circuit is partly or wholly the same as the sensor assembly microwave antenna circuit.

If the generated modulated backscatter signal is transmitted by a backscatter microwave antenna circuit having a reflection coefficient at the backscatter signal frequency (f2), which is lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the predetermined oven frequency (f1), the backscatter microwave antenna circuit being the same as the sensor assembly microwave antenna circuit means that the single antenna circuit is detuned from the predetermined oven frequency and tuned to a higher harmonic.

An advantage of the sensor assembly microwave antenna circuit being detuned from the predetermined oven frequency is that it decreases the amount of microwave energy picked up by the sensor assembly microwave antenna circuit and therefore decreases the level of the RF antenna signal applied to either an RF power limiter circuit (if present) and/or to the energy supplying part and may assist in protecting the circuits in the latter against excessively high voltage or power levels of the RF antenna signal when the sensor assembly microwave antenna circuit is situated in a hot spot in the microwave oven cavity.

An advantage of the sensor assembly microwave antenna circuit being tuned to the higher frequency of a higher harmonic, compared to the predetermined oven frequency, is that the tuning/detuning can be, at least partially, achieved by a smaller form factor of the antenna circuit. The smaller physical dimensions leads to various benefits such as smaller dimensions of the sensor assembly and therefore simpler integration into the various kinds of equipment.

Typically, antenna circuits are designed to be low loss, i.e. have a low reflection coefficient, at the frequency/frequencies at which the antenna circuit should receive or transmit. The antenna circuit is said to be tuned to a frequency, when it has a relatively low reflection coefficient at that frequency. Conversely, an antenna circuit that is not tuned to a frequency is said to be detuned from that frequency. The intensity of the radiation in the cavity of the microwave oven is by far more than enough to power the sensor assembly via the energy harvesting circuit and, therefore, the sensor assembly microwave antenna circuit does not need to be tuned to the frequency of the oven’s microwave radiation, the predetermined oven frequency, i.e. it does not need to have a low reflection coefficient at that frequency. It is even advantageous for the sensor assembly microwave antenna circuit to have a high reflection coefficient at the predetermined oven frequency as this can drastically lower the otherwise possibly excessive base level of power received by the energy harvesting circuit. An excessive level of power received by the energy harvesting circuit may need to be limited.

Thus, in an embodiment, the RF energy receiver part further comprises an RF power limiter circuit coupled to the sensor assembly microwave antenna circuit and configured to limit an amplitude or power of the RF antenna signal to produce a limited RF antenna signal.

The RF power limiter may comprise a variable impedance circuit connected across the RF antenna signal, wherein the variable impedance circuit is configured to exhibit a decreasing input impedance with increasing amplitude or power of the RF antenna signal at the predetermined oven frequency to decrease a matching between the input impedance of the RF power limiter and an impedance of the sensor assembly microwave antenna circuit. The variable impedance circuit may be configured to exhibit a substantially constant input impedance at power or amplitude levels of the RF antenna signal below a threshold level; and exhibit a gradually, or abruptly, decreasing input impedance at power or amplitude levels of the RF antenna signal above the threshold level. The input impedance of the variable impedance circuit may for example gradually decrease with increasing input power of the RF antenna signal above the threshold level. The threshold level may be a power threshold or an amplitude threshold. The variable impedance circuit may comprise a PIN diode. In an embodiment, the energy supplying part comprises a rectifier circuit configured to rectify the RF antenna signal or the limited RF antenna signal.

The sensor assembly microwave antenna circuit generates an RF antenna signal and the energy supplying part can rectify and extract energy from either the limited RF antenna signal, or in case the sensor assembly lacks the RF power limiter circuit, directly from the received RF antenna signal. The power supply voltage generated by the energy supplying part may be connected to active electronic circuits and components of the sensor assembly and supply electrical power thereto. The energy supplying part may comprise one or more RF Schottky diode(s) for rectification of the limited RF antenna signal or of the received RF antenna signal (in case the sensor assembly lacks the RF power limiter circuit) as discussed in further detail below with reference to the appended drawings.

A rectifier circuit comprises semi-conductor devices (such as diodes) that only conduct current in one direction and only when the voltage on the one end is greater than the voltage at the other end. When a rectifier circuit rectifies an AC signal, harmonics are generated. In the sensor assembly the signal being rectified has the predetermined oven frequency (f1) and therefore the rectifier circuit will generate harmonics at integer multiples of the predetermined oven frequency (f1), rectifies the signal. By varying the power of the incident RF antenna signal, the intensity of the harmonics generated by the rectification, part of which are reflected back through the antenna, are also varied and modulated harmonics are generated. In an embodiment, the backscatter modulation part comprises a microcontroller and a control circuitry configured to allow the microcontroller to reduce or increase RF power incident to the rectifier circuit so as to modulate a backscatter signal generated by the rectifying circuitry.

By microcontroller is meant one of an off-the shelf microcontroller, an ASIC logic controller, optionally with a support circuit such as an non-volatile memory (NVM), a programmable logic unit or the like. The microcontroller is coupled to the energy supplying part for receipt of operating power and the one or more sensors may be coupled to the microcontroller via an input port of the microcontroller for receipt of measured parameter values of the physical or chemical property or properties of the product to be irradiated. A sensor may be configured to deliver the measured parameter values to the input port of the microcontroller in digital format or analog format. The control circuit may comprise the PIN diode in the RF power limiter circuit and use the PIN diode to perform the backscatter modulation by using an output pin of the microcontroller to drive a binary modulated current through the PIN diode, thereby binary modulating the impedance of the PIN diode as discussed in further detail below with reference to the appended drawings.

The sensor assembly is preferably enclosed by a housing. The sensor assembly microwave antenna circuit and the backscatter microwave antenna circuit are preferably partly or wholly arranged outside the housing if the latter comprises an electrically conducting material to allow the relevant electromagnetic radiation to reach or leave the antenna circuits substantially without significant attenuation. An electrically conductive housing may comprise a metal sheet or metal net, enclosing and shielding at least the RF power limiter and the energy supplying part against the microwave electromagnetic radiation. The housing may be hermetically sealed to protect these circuits and the one or more sensors enclosed therein against harmful liquids, gasses or other contaminants of the product to be irradiated or present within the microwave oven cavity. A sensory portion of any of the one or more sensors may protrude from the housing to allow the sensory portion to obtain physical contact with the product.

The individual electrical components in the sensor assembly and receiver assembly may have more than one function and may belong to more than one denoted circuit or part. For example, as described above, the PIN diode in the RF power limiter circuit of the sensor assembly may also be part of the control circuit, which modulates the backscatter signal. Thus, the PIN diode would belong to both the RF power limiter circuit and the backscatter modulation part.

In the following aspects, the terms and features relate to the terms and features having the same name in the first aspect and therefore the descriptions and explanations of terms and features given above apply also to the following aspects.

In the second aspect is presented a receiver assembly for a microwave oven, the microwave oven being configured to generate microwave radiation having a predetermined oven frequency (f1) within a cooking cavity, where the receiver assembly comprises:

- a receiver microwave antenna circuit that is configured to receive a modulated backscatter signal propagating within the cooking cavity of the microwave oven, and - an RF receiver part coupled to the receiver microwave antenna circuit, wherein the frequency (f2) of the modulated backscatter signal is a higher harmonic of the predetermined oven frequency (f1).

The antenna opening(s) of the receiver microwave antenna circuit is positioned such that it can sense the EM field within the cooking cavity of the microwave oven. The antenna circuit may be positioned so as to sense the EM field through an aperture in the Faraday cage of the microwave oven. A number of possible mounting locations exist. For example, most low-midrange priced microwave ovens use a rigid cover plate to obscure the hole for the magnetron waveguide. This cover could simply be lengthened a bit so as to also cover the antenna opening(s) of a receiver microwave antenna circuit mounted adjacent to the magnetron opening. Another option is to use a sidewall of the magnetron waveguide as a mounting position. In a more high-end priced microwave oven utilizing a field stirrer instead of a turntable, the oven may have a “false floor” with microwave transparent material obscuring the bottom shield wall, shield stirrer and waveguide opening. In this case, the antenna opening(s) of the receiver microwave antenna circuit could readily be mounted anywhere to the bottom shield wall and in this way be protected and also hidden from view. Yet another option is cutting an aperture in a sidewall of the cooking cavity to let the higher harmonic signal exit the cavity and reach a receiver microwave antenna circuit placed somewhere outside the cavity, for example near the main controller board of the oven. This option could potentially worsen the ovens EMI compatibility. However, it is possible that the cooking cavity is already “leaky” enough that the backscatter signal could escape the cavity without any additional cuts or holes in sidewalls, for example through the door seal, to be picked up by a receiver microwave antenna circuit with enough sensitivity.

The entire receiver assembly does not need to be mounted at the same location. For example, the receiver antenna circuit could be mounted in or near the cooking cavity, while the rest of the receiver assembly was mounted as part of the ovens main controller board, and with a coaxial cable connecting the antenna circuit to the rest. However, this would likely be more expensive and certainly more complicated with respect to assembly.

The receiver microwave antenna circuit may be backed by a shield, so that the oven’s microwave radiation field does not escape for EMC compliance reasons, but this may not be strictly necessary. The receiver assembly (except the antenna opening(s) of the receiver microwave antenna circuit) may be covered by a separate shield to minimize noise levels for improved signal reception.

The receiver microwave antenna circuit may comprise a slot antenna. Slot antennas can be easily integrated in a PCB (Printed Circuit Board), requires a minimal opening in the oven cavity wall with lax tolerance, and does not protrude within the chamber. This enables the receiver microwave antenna circuit to be mounted such that it is hidden from view and safe from any contaminants such as food splatter.

In an embodiment, the frequency (f2) of the modulated backscatter signal is the second harmonic of the predetermined oven frequency (f1).

In an embodiment, the receiver microwave antenna circuit has a lower reflection coefficient at the backscatter signal frequency (f2) than at the predetermined oven frequency (f1). The intensity of the microwave radiation having the predetermined oven frequency is many times higher, possibly hundreds of times higher, than the intensity of the backscatter signal frequency and it is advantageous if the receiver microwave antenna circuit is detuned from the predetermined oven frequency (f1) and tuned to the backscatter signal frequency (f2). In an embodiment, the reflection coefficient of the receiver microwave antenna circuit at the frequency of the higher harmonic (f2) is at least 10dB lower, such as at least 20 dB lower, such as at least 40dB lower, than the reflection coefficient of the receiver microwave antenna circuit at the predetermined oven frequency (f1). An advantage of the receiver microwave antenna circuit being tuned to the higher frequency of a higher harmonic (f2), compared to the predetermined oven frequency (f1), is that the tuning/detuning can be, at least partially, achieved by a smaller form factor of the antenna circuit. The smaller physical dimensions leads to various benefits such as smaller dimensions of the receiver assembly and therefore simpler integration, for example simpler integration into a microwave oven.

The microwave radiation at the predetermined oven frequency within the cooking cavity of the microwave oven is both noisy and has a high intensity, so the receiver assembly has to extract the higher harmonic that is the modulated backscatter signal, while preferably strongly rejecting the predetermined oven frequency. Therefore, in an embodiment, the receiver microwave antenna circuit further comprises a microwave filter, such as a bandpass filter or a high-pass filter, configured to reflect the signal at the predetermined oven frequency (f1) and transmit the signal at the backscatter signal frequency (f2). In some preferred embodiments, the tuning of the receiver microwave antenna circuit to the higher harmonic combines with the microwave filter to reject remnants of the usually hundreds of times stronger microwave radiation field generated by the oven’s magnetron. After the filtering to further isolate the data signal, the antenna signal can be received by the RF receiver part, and the data signal can be extracted. The microwave filter may for example be a strong bandpass filter around the frequency of the higher harmonic or a high-pass filter with a pass frequency substantially above the predetermined oven frequency and below or near the higher harmonic. Thus, the preferred embodiments may achieve the extraction and rejection by combining a moderately narrowband antenna circuit with a microwave filter having strong out-of- band rejection. The microwave filter may be a coupled line microstrip filter, which is a bandpass filter. Such a microstrip bandpass filter may be quite large and, to keep the form factor smaller, could be replaced with SMD integrated filter components with the trade-off that such components typically have a worse out of band rejection of at best around 40 dB, while the microstrip filter can achieve more than 60 dB rejection.

Alternatively, in some embodiments an extremely narrowband antenna circuit may be used without a microwave filter to achieve the extraction and rejection, but such an antenna circuit would be highly sensitive to manufacturing tolerances and would likely be more expensive than the combination of a moderately narrowband antenna circuit with a microwave filter having strong out-of-band rejection.

In an embodiment, the modulated backscatter signal comprises data based on sensor readings produced by at least one sensor. The receiver assembly may comprise a decoder coupled to the RF receiver circuit and configured to decode the modulated backscatter signal. In an embodiment, the decoder comprises a microcontroller. By microcontroller is meant one of an off-the shelf microcontroller, an ASIC logic controller, optionally with a support circuit such as an non-volatile memory (NVM), a programmable logic unit or the like. In an embodiment, the decoder or microcontroller is further configured to interpret the modulated backscatter signal as wholly or partly comprising data based on sensor readings produced by one or more sensors within the cooking cavity. Thus, the decoder or microcontroller will decode the modulated backscatter signal to extract the sensor data comprised in the signal. Alternatively, decoding of the modulated backscatter signal could take place in the main controller board or other control unit of the microwave oven. The main controller board or control unit may have wired connections directly to the outputs of RF receiver part of the receiver assembly. Within the cooking cavity, both the microwave radiation generated by the microwave oven and the higher harmonics are standing waves and the receiver microwave antenna circuit may be located at a position where the amplitude of the higher harmonic is low and thus be unable to receive the modulated backscatter signal. Whether this is the case will depend on, among other things, the position and orientation of the sensor assembly transmitting the backscatter signal, the shape of the oven cavity, the items being heated, etc.

In a microwave oven having a field stirrer, the field strength at a given point within the cavity of the microwave oven will vary with a fluctuation determined by the field stirrer.

In a microwave oven that has a rotating platter (also called a turntable) to distribute the heating of the product, the receiver microwave antenna circuit will be moved around by the rotating platter and experience varying levels of signal strength and possibly regularly experience very low signal level. Since the turntable typically revolves rather slowly, the antenna can thus risk being in a spot with very low signal level for several seconds at a time. In order to compensate for this, a preferred embodiment of the receiver assembly comprises two or more receiver microwave antenna circuits. By having the two antenna circuits placed at different locations and/or angles within the cavity of the microwave oven, it is likely that when one antenna circuit is in a spot with very low signal level, the other antenna circuit will not be.

Thus, in some embodiments, the receiver assembly further comprises:

- a second receiver microwave antenna circuit configured to receive the modulated backscatter signal propagating within the cooking cavity of the microwave oven,

- a second RF receiver circuit coupled to the second receiver microwave antenna circuit.

The two receiver microwave antenna circuits need separate receiver circuits, and filtering if present, because their combined signal, if summed before the receiver, would sum to zero whenever the phases of their received signals were inverse.

The receiver assembly may comprise a second decoder configured to decode the modulated backscatter signal and the second RF receiver part is coupled to the second decoder or the second RF receiver part is coupled to the same decoder as the RF receiver part. In an embodiment, the second decoder comprises a second microcontroller. In an embodiment, the second decoder or second microcontroller is further configured to interpret the modulated backscatter signal as wholly or partly comprising data based on sensor readings produced by one or more sensors within the cooking cavity.

The two receiver microwave antenna circuits could be affixed on a single PCB with a single microcontroller connected to both and indeed, all the components of the sensor assembly may be on that single PCB. In another embodiment, the two receiver microwave antenna circuits are affixed on two separate PCBs, which may then be mounted at different locations within the cooking cavity and possibly have separate microcontrollers for decoding. The use of two antenna circuits is more advantageous in a microwave oven that use a turntable as the locations of cold spots and hot spots are typically moving around much more quickly within the cavity of an oven using a field stirrer and it will be easier for a single antenna circuit to achieve a regular signal reception.

The second receiver microwave antenna circuit, the second microwave filter, the second decoder and the second microcontroller may be identical components to the receiver microwave antenna circuit, the microwave filter, the decoder and the microcontroller described above and the descriptions and explanations of terms and features given above apply equally.

In an embodiment, the second receiver microwave antenna circuit has a lower reflection coefficient at the backscatter signal frequency (f2) than at the predetermined oven frequency (f1).

In an embodiment, the reflection coefficient of the second receiver microwave antenna circuit at the frequency of the higher harmonic (f2) is at least 10 dB lower, such as at least 20 dB lower, such as at least 40dB lower, than the reflection coefficient of the second receiver microwave antenna circuit at the predetermined oven frequency (f1).

In an embodiment, the second receiver microwave antenna circuit further comprises a second microwave filter, such as a bandpass filter or a high-pass filter, configured to reflect the signal at the predetermined oven frequency (f1) and transmit the signal at the backscatter signal frequency (f2).

In an embodiment, the receiver assembly is configured to being coupled, upon installation, to a control unit, such as the main controller board, configured to control settings relating to the function of a microwave oven. The control unit may comprise a microprocessor. The control unit may be configured to receive data from the receiver assembly, such as parameter values decoded from the modulated backscatter signal, or to receive a signal, which it then decodes, and utilize the decoded information from the data in the operation of the microwave oven. The receipt of one or more parameter values could cause the control unit of the microwave oven to act in response and e.g. turn off the radiation, either entirely or for a time period, or reduce or increase the intensity of the oven’s microwave radiation, etc. This would allow for the advanced heating algorithms in which the operation of the microwave oven is adjusted regularly based on sensor readings from within the cavity of the microwave oven. The microwave oven may instead, or in addition, display information to the user on a display panel.

A third aspect relates to a microwave oven comprising a cooking cavity, the microwave oven being configured to generate microwave radiation having a predetermined oven frequency (f1) within the cooking cavity and the microwave oven further comprising a receiver assembly according to any of the above-described receiver assembly embodiments.

A fourth aspect relates to a kit of parts comprising a sensor assembly for a cooking cavity of a microwave oven according to any of the above-described sensor assembly embodiments and a receiver assembly for a microwave oven according to any of the above-described receiver assembly embodiments.

A fifth aspect relates to a method of communication between a sensor assembly and a receiver assembly in a microwave oven, the microwave oven being configured to generate microwave radiation having a predetermined oven frequency (f1) within a cooking cavity of the microwave oven, the sensor assembly comprising an RF energy receiver part configured to harvest energy from the microwave radiation within the cooking cavity, the RF energy receiver part comprising a sensor assembly microwave antenna circuit configured to generate an RF antenna signal, the sensor assembly being positioned within the cooking cavity and the sensor assembly further comprising a backscatter modulation part configured to generate a modulated backscatter signal within the cooking cavity, the receiver assembly comprising a receiver microwave antenna circuit configured to receive the modulated backscatter signal propagating within the cooking cavity, the method comprising:

- the backscatter modulation part generating a modulated backscatter signal, wherein the frequency (f2) of the modulated backscatter signal is a higher harmonic of the predetermined oven frequency (f1),

- the receiver microwave antenna circuit receiving the modulated backscatter signal generated by the backscatter modulation part.

The sensor assembly may be a sensor assembly according to any of the above- described sensor assembly embodiments and the receiver assembly may be a receiver assembly according to any of the above-described receiver assembly embodiments.

The microwave radiation at the predetermined oven frequency within the cooking cavity of the microwave oven is both noisy and high intensity, so it would be very difficult for the receiver assembly to extract a backscatter signal having the predetermined oven frequency. Instead, the method utilizes that the sensor assembly can both use the oven microwave radiation to power the sensor assembly and to generate a modulated backscatter signal of a higher harmonic. The higher harmonic can more easily be extracted from the immensely powerful microwave radiation background generated within the cooking cavity thus enabling a sensor signal to be transmitted from within the cavity to a receiver antenna circuit that senses the EM field through an aperture in the Faraday cage of the microwave oven.

In an embodiment, the sensor assembly further comprises one or more sensors, and the modulated backscatter signal comprises data based on sensor readings produced by the sensor.

In an embodiment, the receiver assembly further comprises a decoder configured to decode the modulated backscatter signal and the method further comprises:

- the decoder decoding the modulated backscatter signal.

In an embodiment, the microwave oven further comprises a control unit configured to control settings relating to the function of the microwave oven, the receiver assembly being coupled to the control unit, and the method further comprises:

- changing a setting of the microwave oven in response to the data comprised in the modulated backscatter signal.

In an embodiment, the generated modulated backscatter signal is being transmitted by a backscatter microwave antenna circuit having a reflection coefficient at the backscatter signal frequency (f2), which is lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the predetermined oven frequency (f1). In an embodiment, the receiver microwave antenna circuit has a lower reflection coefficient at the backscatter signal frequency (f2) than at the predetermined oven frequency (f1).

BRIEF DESCRIPTION OF THE DRAWINGS

In the following exemplary embodiments of the invention are described in more detail with reference to the appended drawings, wherein:

FIG. 1 is a simplified schematic block diagram of a sensor assembly for a cooking cavity of a microwave oven according to an embodiment of the invention,

FIG. 2 shows a simplified electrical circuit diagram of a sensor assembly according to an embodiment of the invention,

FIG. 3 is a simplified schematic block diagram of a receiver assembly according to an embodiment of the invention,

FIG. 4 is a simplified schematic block diagram of a receiver assembly according to another embodiment of the invention,

FIG. 5 and 6 schematically illustrate a microwave oven having a cooking cavity within which cavity is shown a sensor assembly and incorporated within the microwave oven is a receiver assembly.

FIG. 7 shows a flow diagram in accordance with exemplary embodiments of the invention.

DETAILED DESCRIPTION

In the following various exemplary embodiments are described with reference to the appended drawings. The skilled person will understand that the accompanying drawings are schematic and simplified for clarity and therefore merely show details which are essential to the understanding of the invention, while other details have been left out. Like reference numerals refer to like elements throughout. Like elements will, thus, not necessarily be described in detail with respect to each figure.

FIG. 1 shows a simplified schematic block diagram of a sensor assembly 1 suitable for use in any type of microwave ovens, industrial or consumer. A microwave oven will generate microwave radiation having a predetermined oven frequency (f1) within its cooking cavity during use. The sensor assembly 1 comprises an RF energy receiver part 3, which comprises a microwave antenna circuit 4 that is responsive to excitation created by the microwave radiation propagating within a cooking cavity of the microwave oven being used.

The microwave antenna circuit 3 generates an RF antenna signal in response to the excitation by the RF electromagnetic radiation in the cooking cavity. The RF antenna signal is electrically connected or coupled to the input of an optional RF power limiter circuit 11. The RF power limiter circuit 11 is configured to limiting a level such as amplitude, power or energy of the RF antenna signal in accordance with predetermined signal limiting characteristics of the RF power limiter circuit 11. The RF power limiter circuit 11 thereby produces a limited RF antenna signal VLIM at an output of the RF power limiter circuit 11. The predetermined signal limiting characteristics may for example comprise a linear behaviour at relatively small levels of the RF antenna signal, for example below a certain threshold level, and a non-linear behaviour above the threshold level. In this manner, the level of the RF antenna signal and the level of the limited RF antenna signal may be largely identical for RF antenna signals below the threshold level while the level of the limited RF antenna signal may be smaller than the level of the RF antenna signal above the threshold level.

The RF power limiter circuit 11 of the sensor assembly 1 is advantageous because the limiter circuit 11 protects the down-stream energy supplying part 5, electrically connected or coupled to the limited RF antenna signal, against over-voltage conditions created by excessively large power or amplitude of the RF antenna signal in response to the RF electromagnetic radiation in the cooking cavity. These excessive signal input conditions are quite contrary to the operation of normal wireless RF data communication equipment, where the challenge often is to obtain sufficient RF power to safely transmit or decode data signals modulated onto the carrier wave. In contrast, the sensor assembly 1 will often be placed very close to the source of the RF electromagnetic radiation in the cooking cavity leading to excessively large voltages and input power of the RF antenna signal. Furthermore, the strength of the microwave radiation in the cooking cavity is often highly variable throughout the cavity due to standing waves. These standing waves lead to the formation of so-called “hot spots” and “cold spots” inside the cavity during operation with highly varying field strengths of the oven’s microwave radiation. The sensor assembly 1 should be configured to on the one hand extract sufficient power from the microwave antenna circuit 4 to ensure proper operation when positioned in a cold spot and on the other hand be able to withstand very large amplitude RF antenna signals, when the microwave antenna circuit 4 is positioned in a hot spot. In the latter situation, the RF power limiter circuit 11 ensures that these large amplitude RF antenna signals are attenuated by reflecting a large portion of the incoming RF signal power back to the microwave antenna circuit 4 for emission as discussed in further detail below. To further reduce the amount of microwave energy picked up by the sensor assembly microwave antenna circuit, the antenna circuit is detuned from the predetermined oven frequency (f1).

The energy supplying part 5 is a dc power supply circuit configured to rectify the limited RF antenna signal VLIM and extract a dc power supply voltage VDD therefrom. The dc power supply circuit 5 may comprise one or more filter or smoothing capacitor(s) coupled to the output of a rectifying element. Several types of rectifying elements may be used such as semiconductor diodes or actively controlled semiconductor switches/transistors. In one embodiment, the rectifying element comprises a Schottky diode as schematically indicated on circuit block 5. The one or more filter or smoothing capacitor(s) serves to suppress voltage ripple and noise on the dc power supply voltage VDD and may further serve as an energy reservoir. The energy reservoir stores extracted energy for a certain time period and ensures that the dc power supply voltage remains charged or powered during short drop outs of the RF antenna signal. A power supply terminal or input of the one or more sensors 9 is connected to the dc power supply voltage VDD for receipt of operating power. The one or more sensors 9 may comprise various types of active digital and/or analog electronic circuitry that need power to function properly.

A product being irradiated within the cooking cavity of the microwave oven may be arranged in a suitable container or utensil during heating such as a cup or plate etc.

The sensor assembly has one or more sensors 9 and a sensory portion of a sensor 9 may be in physical contact with the product so as to measure or detect a physical property of the product during heating/preparation such as a temperature, viscosity, pressure, colour, humidity, electric conductivity etc. In the alternative, one or more sensors 9 may operate without physical contact to the product and instead measure the property of the food item by remote or non-contact sensing e.g. using an infrared (IR) temperature detector etc. The sensory portion of a sensor 9 may alternatively measure or detect a chemical property of a food item under heating for example water content or the presence and/or concentration of certain chemical agents salt, sugar etc. in the food item. The skilled person will understand that the sensor may be configured to measure or detect several different physical properties of the food item and/or one or more chemical properties. The sensor assembly 1 may comprise multiple individual sensors of different types to measure the different physical properties and/or chemical properties of the food item.

The sensor assembly 1 preferably comprises a housing or casing 13 surrounding and enclosing at least the RF power limiter circuit 11 , dc power supply circuit 5 and one or more sensors 9. The housing 13 may be hermetically sealed to protect these circuits and the sensor(s) enclosed therein against harmful liquids, gasses or other contaminants inside the oven chamber. The previously discussed sensory portion of any of the one or more sensors 9 may protrude from the housing 13 to allow the sensory portion to obtain physical contact with a product. The housing 13 may comprise an electrically conductive layer or shield, such as a metal sheet or metal net, enclosing preferably the RF power limiter circuit 11 and the dc power supply circuit 5, and optionally the one or more sensors 9, against the strong RF microwave electromagnetic field generated by the microwave oven during operation. The housing 13 is arranged such that the microwave antenna circuit 4 can receive the microwave radiation propagating within the oven cavity, while shielding any component that would be negatively affected by the RF oven field. In some embodiments, the microwave antenna circuit 4 is partly or wholly arranged outside the electrically shielded housing 13 to allow harvesting of the microwave energy from the microwave radiation or field. In other embodiments, e.g. where the microwave antenna circuit 4 comprises a slot antenna, the housing 13 may comprise an aperture arranged to allow the slot antenna to receive the oven microwave radiation by way of it. In embodiments, where the sensor assembly 1 comprises a separate backscatter microwave antenna circuit, the housing 13 is additionally arranged such that the backscatter microwave antenna circuit can transmit the modulated backscatter signal into the oven cavity.

In the embodiment shown in fig. 1 , the sensor assembly microwave antenna circuit 4 also acts as a backscatter microwave antenna circuit. When the rectifier circuit within the dc power supply circuit 5 rectifies the signal, harmonics are generated. As the signal being rectified has the predetermined oven frequency (f1), the rectifier circuit will generate harmonics at integer multiples of the predetermined oven frequency (f1), when it rectifies the signal. Part of the higher harmonics are reflected back through the microwave antenna circuit 4, as discussed below in additional detail, to propagate within the cooking cavity of the microwave oven. In this way, a backscatter signal, which is a higher harmonic of the predetermined oven frequency (f1), is generated and has a higher frequency (f2) than the oven’s microwave radiation. The sensor assembly microwave antenna circuit 4 has a reflection coefficient at the backscatter signal frequency (f2), which is lower than its reflection coefficient at the predetermined oven frequency (f1). The low reflection coefficient at the backscatter signal frequency (f2) will help achieve transmission of a strong backscatter signal. This has a synergetic effect as it is preferable to have a high reflection coefficient at the predetermined oven frequency (f1) in order to drastically lower the base level of power received by the energy harvesting circuit, as described above.

In reality, the generated signal will comprise several higher harmonics with decreasing intensity the higher the harmonic, but likely only the second harmonic will have a high enough power that it can later be received and extracted at a receiver antenna circuit.

To generate a modulated backscatter signal the power of the incident RF antenna signal to the rectifier is varied, which in turn varies the intensity of the harmonics generated by the rectification, as discussed in further detail below.

FIG. 2 shows a simplified electrical circuit diagram of an exemplary sensor assembly for use with industrial or consumer types of microwave ovens (not shown) according to an embodiment of the invention. The sensor assembly comprises an sensor assembly microwave antenna circuit 4, a rectifier circuit 6, an RF power limiter circuit 11, a microcontroller 29 and one or more sensors 9.

The power limiting function is achieved by a Zener diode D4 coupled from the output of the rectifier circuit 6, such that when the rectifier circuit 6 output voltage exceeds the Zener diode D4 voltage plus the PIN diode D1 forward voltage, a current starts conducting through the Zener diode D4 and PIN diode D1, thus dropping the PIN diode D1 impedance and thereby reflecting input energy from the antenna circuit 4, thus reducing power received by the rectifier circuit 6.

In the same manner, the current in the PIN diode D1 can be directly modulated from a digital output port 15 of a microcontroller 29, thus digitally modulating the power received by the rectifier circuit 6. By actively increasing and reducing the RF power incident to the rectifier circuit 6, also increased and reduced are the harmonics generated by the rectification, part of which are reflected back through the antenna circuit 4. Thus, a modulated backscatter signal is generated, where the signal comprises both the predetermined oven frequency and higher harmonics of the predetermined oven frequency.

Defined in the most general sense, harmonic generation is achieved by presenting a non-linear load, i.e. a load, where the current drawn is not strictly proportional to the voltage draw. The modulation is then achieved by switching between different levels of this load. The preferred embodiment shown in FIG. 2 achieves this by actively limiting the current that enters the rectifier circuit 6 by controlling the RF power limiter circuit 11. Alternative ways of achieving the modulation are: Modulating the current draw on the dc side of the rectifier circuit 6, e.g. by switching on and off a load, modulating the power input to the rectifier circuit 6 by a separate power limiter element in parallel or in series with the main RF power limiter circuit 11 , such as another PIN diode or other active switch like a MOSFET, or presenting and modulating an alternative non-linear element to the microwave antenna circuit 4 (or to a separate backscatter microwave antenna circuit) such as a PIN diode, transistor, varactor etc.

Further illustrated in FIG. 2 is an embodiment, wherein circuitry between the rectifier stage and microcontroller 29 allows for switching off of the connection between the rectifier output and the microcontroller supply during signal transmission such that the varying current consumption of the microcontroller 29 and/or energy storage circuitry/sensors does not create a varying current draw from the rectifiers, which would present a varying amplitude of the harmonics generated and may interfere with the generated modulated backscatter signal. In the embodiment shown the interruption of the connection between the rectifier output and the microcontroller supply is symbolized by a switch, but in reality this would likely be achieved in a different way than with a switch. The skilled person will be aware of several way to achieve the switching off of the connection.

Additionally, a load resistance is provided from the rectifier output to ground, which is connected during signal transmission so as to provide a constant load such that a constant current flow through the rectifiers is achieved. This helps ensure that the rectifiers continuously generate the harmonics. Since the rectifier stage only conducts current when the absolute value of the instantaneous RF voltage exceeds the DC output voltage plus the diode forward voltage, it is also beneficial to pull the DC output voltage of the rectifier stage down, such that the RF voltage only needs to exceed the diode forward voltage in order for harmonics to be generated. This allows for harmonics to be generated, and thus communication to be possible, in as many places in the microwave oven as possible, even where cold spots mean that the RF signal received is low.

FIG. 3 is a simplified schematic block diagram of a receiver assembly 21 suitable for installation in any type of microwave ovens, industrial or consumer. A microwave oven will generate microwave radiation having a predetermined oven frequency (f1) within its cooking cavity during use. The receiver assembly 21 comprises a receiver microwave antenna circuit 23 that is responsive to the frequency (f2) of a higher harmonic of the predetermined oven frequency (f1) and is installed within a microwave oven in such a way that the receiver microwave antenna circuit 23 can pick up a signal of the higher harmonic. The higher harmonic comprises a modulated backscatter signal such as the one transmitted by a sensor assembly as illustrated in figs. 1 and 2. The oven’s microwave radiation field is both high intensity and noisy and therefore, a backscatter signal at the predetermined oven frequency (f1) would be extremely difficult to extract and filter from the oven’s microwave radiation and instead, the higher harmonic is used for data transmission. To isolate the backscatter signal, the receiver microwave antenna circuit 23 is advantageously tuned to the higher harmonic and detuned from the fundamental frequency and the receiver microwave antenna circuit therefore has a lower reflection coefficient at the backscatter signal frequency (f2) than at the predetermined oven frequency (f1). This allows the receiver microwave antenna circuit 23 to better extract the backscatter signal from the intense and noisy background of the oven’s radiation field.

Further, in the embodiment in fig. 3 of a receiver assembly 21, the receiver microwave antenna circuit 23 has a microwave filter 25, such as a bandpass filter or a high-pass filter, that is configured to reflect the signal at the predetermined oven frequency (f1) and transmit the signal at the backscatter signal frequency (f2). The filtered signal is sent to the RF receiver part 27 such as a Schottky diode.

In the embodiment shown in fig. 3, the receiver assembly 21 has a microcontroller 29 to decode the received signal so as to extract the data within the modulated backscatter signal.

The receiver assembly 21 comprises a housing or casing 31 surrounding and enclosing at least the RF receiver part 27 and the microcontroller 29. The housing 31 of the receiver assembly 21 can have an electrically conductive layer, such as a metal sheet or metal net, enclosing at least the RF receiver part 27 and the microcontroller 29 and shielding them against the strong RF microwave electromagnetic field generated by the microwave oven during operation. The housing 31 is arranged such that the receiver microwave antenna circuit 23 can receive the backscatter signal, while shielding any component that would be negatively affected by the RF oven field. In some embodiments, the receiver microwave antenna circuit 23 could be partly or wholly arranged outside the electrically shielded housing 31 to allow reception of the modulated backscatter signal. In other embodiments, e.g. where the receiver microwave antenna circuit 23 comprises a slot antenna, the housing 31 may comprise an aperture arranged to allow the slot antenna to receive the backscatter signal by way of it.

FIG. 4 is a simplified schematic block diagram of a receiver assembly 21 suitable for installation in any type of microwave ovens, industrial or consumer. The receiver assembly 21 has the same components as the embodiment shown in fig. 3, but in addition, the embodiment of a receiver assembly in fig. 4 comprises a second receiver microwave antenna circuit 33, a second bandpass filter 35 and a second RF receiver part 37. The second receiver microwave antenna circuit 33, a second bandpass filter 35 and a second RF receiver part 37 have the same or similar properties such as e.g. reflection coefficient as the receiver microwave antenna circuit 23, a bandpass filter 25 and RF receiver part 27, respectively.

The second bandpass filter 35 is comprised in the second receiver microwave antenna circuit 33 and filters the signal received at the antenna circuit 33 to produce a filtered signal, where most of the signal having the predetermined oven frequency (f1) has been filtered out to extract the desired modulated backscatter signal. The signal filtered by the second bandpass filter 35 is sent to the second RF receiver part 37, which may comprise a Schottky diode.

In the embodiment shown in fig. 4, the RF receiver part 27 and the second RF receiver part 37 are connected to a microcontroller 29 to decode the received signal so as to extract the data within the modulated backscatter signal.

The two receiver microwave antenna circuits 23, 33, the two RF receiver parts 27, 37 and the microcontroller 29 could be affixed on a single PCB. Using two receiver antenna circuits 23, 33 is advantageous in some microwave ovens as it makes it more likely that at least one of the antenna circuits will be able to achieve a regular signal reception.

The receiver assembly 21 comprises a housing or casing 31 surrounding and enclosing at least the RF receiver part 27, the second RF receiver part 37 and the microcontroller 29. The housing 31 of the receiver assembly 21 can have an electrically conductive layer, such as a metal sheet or metal net, enclosing at least the RF receiver part 27, the second RF receiver part 37 and the microcontroller 29 and shielding them against the strong RF microwave electromagnetic field generated by the microwave oven during operation. The receiver microwave antenna circuit 23 and the second receiver microwave antenna circuit 33 are partly or wholly arranged outside the electrically shielded housing 31 to allow reception of the modulated backscatter signal.

FIGS. 5 and 6 schematically illustrate a microwave oven 61, wherein a receiver assembly 21 has been installed within the oven 61, and a sensor assembly 1 has been placed within the cooking cavity 63 of the microwave oven 61. The sensor assembly 1 is positioned within a container 69, which holds a product to be irradiated by the microwave radiation having a predetermined oven frequency (f1) generated by a magnetron within the microwave oven 61 when activated. During the irradiation, one or more sensors comprised in the sensor assembly 1 measure physical and/or chemical properties. The sensor readings are encoded in a modulated backscatter signal, which is transmitted by the sensor assembly 1 to propagate within the cooking cavity 63.

In fig. 5, the receiver assembly 21 is positioned on or partially behind a sidewall of the microwave oven 61. In fig. 6, the receiver assembly 21 is positioned on or partially behind the bottom shield wall of the microwave oven 61, possibly behind a “false floor” with microwave transparent material obscuring the bottom shield wall. A receiver microwave antenna circuit in the receiver assembly 21 picks up the modulated backscatter signal and either decodes the sensor readings in a microcontroller within the receiver assembly or sends the backscatter signal to a microcontroller within the microwave oven for decoding.

The receiver assembly 21 is connected with a main controller board 65, which controls the main functions of the microwave oven 61 and which is in turn connected to a user control panel 67 with a display 71. This allows for the main controller board 65 to either receive the sensor readings or a signal, which it can decode to extract the sensor readings. The main controller board 65 can then choose to adjust the operation of the microwave oven, such as turn off the microwave radiation, either entirely or for a time period, or reduce or increase the intensity of the oven’s microwave radiation, etc., and/or cause the display 71 to show information to the user.

FIG. 7 is a flow diagram of a method of communication between a sensor assembly and a receiver assembly in a microwave oven. The microwave oven is configured to generate microwave radiation with a predetermined oven frequency (f1) within a cooking cavity of the microwave oven.

The sensor assembly has an RF energy receiver part that is configured to harvest energy from the microwave radiation within the cooking cavity and the RF energy receiver part has a sensor assembly microwave antenna circuit that is configured to generate an RF antenna signal. The sensor assembly is positioned within the cooking cavity and the sensor assembly further has a backscatter modulation part that is configured to generate a modulated backscatter signal within the cooking cavity. The generated modulated backscatter signal is transmitted by a backscatter microwave antenna circuit that has a reflection coefficient at the backscatter signal frequency (f2), which is lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the predetermined oven frequency (f1).

The receiver assembly has a receiver microwave antenna circuit that is configured to receive the modulated backscatter signal propagating within the cooking cavity and the receiver microwave antenna circuit has a lower reflection coefficient at the backscatter signal frequency (f2) than at the predetermined oven frequency (f1).

The method comprises steps S10 - S20 and, optionally, one or more of steps S30 - S40.

In step S10 the backscatter modulation part generates a modulated backscatter signal, wherein the frequency (f2) of the modulated backscatter signal is a higher harmonic of the predetermined oven frequency (f1).

In step S20, the receiver microwave antenna circuit receives the modulated backscatter signal generated by the backscatter modulation part. The sensor assembly can have one or more sensors and the modulated backscatter signal can comprise data based on sensor readings produced by the one or more sensors.

The receiver assembly can have a decoder, which is configured to decode the modulated backscatter signal and in step S30 the decoder decodes the modulated backscatter signal.

The receiver assembly can be coupled to a control unit of the microwave oven, where the control unit is configured to control settings relating to the function of the microwave oven. In step S40, a setting of the microwave oven is changed in response to the data comprised in the modulated backscatter signal.

LIST OF REFERENCES

1 Sensor assembly

3 RF energy receiver part

4 Sensor assembly microwave antenna circuit 5 Energy supplying part

6 Rectifier circuit

7 Backscatter modulation part

9 Sensor

11 RF power limiter circuit

13 Electrically shielded housing of sensor assembly

15 Microcontroller output port

17 Microcontroller input port

19 Switch

21 Receiver assembly

23 Receiver microwave antenna circuit

25 Bandpass filter

27 RF Receiver part

29 Microcontroller/Decoder

31 Electrically shielded housing of receiver assembly

33 Second receiver microwave antenna circuit

35 Second bandpass filter

37 Second RF Receiver part

61 Microwave oven

63 Cooking cavity

65 Main controller panel

67 User control panel

69 Container

71 Display

Claims

1. A sensor assembly for a cooking cavity of a microwave oven, the microwave oven being configured to generate microwave radiation having a predetermined oven frequency (f1) within the cooking cavity, the assembly comprising:

- an RF energy receiver part configured to harvest energy from the microwave radiation within the cooking cavity, the RF energy receiver part comprising a sensor assembly microwave antenna circuit configured to generate an RF antenna signal,

- one or more sensors,

- a backscatter modulation part configured to generate a modulated backscatter signal within the cooking cavity, and

- an energy supplying part configured to supply energy from the RF energy receiver part to the one or more sensors and to the backscatter modulation part, wherein the frequency (f2) of the modulated backscatter signal is a higher harmonic of the predetermined oven frequency (f1), the generated modulated backscatter signal being transmitted by a backscatter microwave antenna circuit having a reflection coefficient at the backscatter signal frequency (f2), which is lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the predetermined oven frequency (f1), and the backscatter modulation part being further configured to modulate the backscatter signal in such a way that the modulated backscatter signal comprises data based on sensor readings produced by at least one of the one or more sensors.

2. A sensor assembly for a cooking cavity of a microwave oven, the microwave oven being configured to generate microwave radiation having a predetermined oven frequency (f1) within the cooking cavity, the assembly comprising:

- one or more sensors,

- an energy supplying part configured to supply energy from the RF energy receiver part to the one or more sensors and to the backscatter modulation part, wherein the frequency (f2) of the modulated backscatter signal is a higher harmonic of the predetermined oven frequency (f1).

3. A sensor assembly according to claim 2, wherein the RF energy receiver part further comprises an RF power limiter circuit coupled to the sensor assembly microwave antenna circuit and configured to limit an amplitude or power of the RF antenna signal to produce a limited RF antenna signal.

4. A sensor assembly according to any of claims 2-3, wherein the energy supplying part comprises a rectifier circuit configured to rectify the RF antenna signal or the limited RF antenna signal.

5. A sensor assembly according to claim 4, wherein the backscatter modulation part comprises a microcontroller and a control circuitry configured to allow the microcontroller to reduce or increase RF power incident to the rectifier circuit so as to modulate a backscatter signal generated by the rectifying circuitry.

6. A sensor assembly according to any of claims 2-5, wherein the sensor assembly microwave antenna circuit has a reflection coefficient that is greater than -10 dB at the frequency of the predetermined oven frequency (f1), such as greater than -6 dB, such as greater than -3 dB.

7. A sensor assembly according to any of claims 2-6, wherein the generated modulated backscatter signal is transmitted by a backscatter microwave antenna circuit having a reflection coefficient at the backscatter signal frequency (f2), which is lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the predetermined oven frequency (f1).

8. A sensor assembly according to claim 7, wherein the backscatter microwave antenna circuit has a reflection coefficient at the frequency of the higher harmonic (f2), which is at least 3 dB lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the frequency of the predetermined oven frequency (f1), such as at least 6 dB lower, such as at least 10 dB lower.

9. A sensor assembly according to any of claims 7-8, wherein the backscatter microwave antenna circuit is partly or wholly the same as the sensor assembly microwave antenna circuit.

10. A sensor assembly according to any of claims 2-9, wherein the backscatter modulation part is further configured to modulate the modulated backscatter signal in such a way that the modulated backscatter signal comprises data based on sensor readings produced by at least one of the one or more sensors.

11. A receiver assembly for a microwave oven, the microwave oven being configured to generate microwave radiation having a predetermined oven frequency (f1) within a cooking cavity, the receiver assembly comprising:

- a receiver microwave antenna circuit configured to receive a modulated backscatter signal propagating within the cooking cavity of the microwave oven, and

- an RF receiver part coupled to the receiver microwave antenna circuit, wherein the frequency (f2) of the modulated backscatter signal is a higher harmonic of the predetermined oven frequency (f1).

12. A receiver assembly according to claim 11 , wherein the receiver microwave antenna circuit has a lower reflection coefficient at the backscatter signal frequency (f2) than at the predetermined oven frequency (f1).

13. A receiver assembly according to any of claims 11-12, wherein the reflection coefficient of the receiver microwave antenna circuit at the frequency of the higher harmonic (f2) is at least 10dB lower, such as at least 20 dB lower, such as at least 40dB lower, than the reflection coefficient of the receiver microwave antenna circuit at the predetermined oven frequency (f1).

14. A receiver assembly according to any of claims 11-13, wherein the receiver microwave antenna circuit further comprises a microwave filter, such as a bandpass filter or a high-pass filter, configured to reflect the signal at the predetermined oven frequency (f1) and transmit the signal at the backscatter signal frequency (f2).

15. A receiver assembly according to any of claims 11-14, wherein the receiver assembly further comprises a decoder coupled to the RF receiver part and configured to decode the modulated backscatter signal.

16. A receiver assembly according to claim 15, wherein the decoder comprises a microcontroller.

17. A receiver assembly according to any of claims 15-16, wherein the decoder or microcontroller is further configured to interpret the modulated backscatter signal as wholly or partly comprising data based on sensor readings produced by one or more sensors within the cooking cavity.

18. A receiver assembly according to any of claims 11-17, wherein the receiver assembly further comprises:

19. A receiver assembly according to claim 18, wherein the second receiver microwave antenna circuit has a lower reflection coefficient at the backscatter signal frequency (f2) than at the predetermined oven frequency (f1).

20. A receiver assembly according to any of claims 18-19, wherein the reflection coefficient of the second receiver microwave antenna circuit at the frequency of the higher harmonic (f2) is at least 10 dB lower, such as at least 20 dB lower, such as at least 40dB lower, than the reflection coefficient of the second receiver microwave antenna circuit at the predetermined oven frequency (f1).

21. A receiver assembly according to any of claims 18-20, wherein the second receiver microwave antenna circuit further comprises a second microwave filter, such as a bandpass filter or a high-pass filter, configured to reflect the signal at the predetermined oven frequency (f1) and transmit the signal at the backscatter signal frequency (f2).

22. A receiver assembly according to any of claims 18-21, wherein the second RF receiver part is coupled to the decoder, or the receiver assembly comprises a second decoder configured to decode the modulated backscatter signal and the second RF receiver part is coupled to the second decoder.

23. A receiver assembly according to any of claims 18-22, wherein the second decoder comprises a second microcontroller.

24. A receiver assembly according to any of claims 18-23, wherein the second decoder or second microcontroller is further configured to interpret the modulated backscatter signal as wholly or partly comprising data based on sensor readings produced by one or more sensors within the cooking cavity.

25. A receiver assembly according to any of claims 11-24, wherein the receiver assembly is configured to being coupled, upon installation, to a control unit configured to control settings relating to the function of a microwave oven.

26. A microwave oven comprising a cooking cavity, the microwave oven being configured to generate microwave radiation having a predetermined oven frequency (f1) within the cooking cavity and the microwave oven further comprising a receiver assembly according to any of claims 11 - 25.

27. A microwave oven according to claim 26, wherein the microwave oven comprises a control unit connected to the RF receiver part and, if present, to the second RF receiver part, and the control unit is configured to decode the modulated backscatter signal.

28. A kit of parts comprising a sensor assembly for a cooking cavity of a microwave oven according to any of claims 2 - 10, and a receiver assembly for a microwave oven according to any of claims 11 - 25.

29. A method of communication between a sensor assembly and a receiver assembly in a microwave oven, the microwave oven being configured to generate microwave radiation having a predetermined oven frequency (f1) within a cooking cavity of the microwave oven, the sensor assembly comprising an RF energy receiver part configured to harvest energy from the microwave radiation within the cooking cavity, the RF energy receiver part comprising a sensor assembly microwave antenna circuit configured to generate an RF antenna signal, the sensor assembly being positioned within the cooking cavity and the sensor assembly further comprising a backscatter modulation part configured to generate a modulated backscatter signal within the cooking cavity, the receiver assembly comprising a receiver microwave antenna circuit configured to receive the modulated backscatter signal propagating within the cooking cavity, the method comprising:

30. The method of communication between a sensor assembly and a receiver assembly according to claim 29, wherein the sensor assembly further comprises one or more sensors, and the modulated backscatter signal comprises data based on sensor readings produced by the one or more sensors.

31. The method of communication between a sensor assembly and a receiver assembly according to any of claims 29-30, wherein the receiver assembly further comprises a decoder configured to decode the modulated backscatter signal and the method further comprises:

- the decoder decoding the modulated backscatter signal.

32. The method of communication between a sensor assembly and a receiver assembly according to any of claims 29-31, wherein the microwave oven further comprises a control unit configured to control settings relating to the function of the microwave oven, the receiver assembly being coupled to the control unit, and the method further comprises:

33. The method of communication between a sensor assembly and a receiver assembly according to any of claims 29-32, wherein the generated modulated backscatter signal is being transmitted by a backscatter microwave antenna circuit having a reflection coefficient at the backscatter signal frequency (f2), which is lower than the reflection coefficient of the sensor assembly microwave antenna circuit at the predetermined oven frequency (f1).

34. The method of communication between a sensor assembly and a receiver assembly according to any of claims 29-33, wherein the receiver microwave antenna circuit has a lower reflection coefficient at the backscatter signal frequency (f2) than at the predetermined oven frequency (f1).