EP0294872A1 - Microwave oven having the capacity to detect when a product is fully thawed - Google Patents

Abstract

Microwave oven comprising a microwave source and a sensor placed in the oven close to a product to be treated, the absorbed microwave energy being distributed between the sensor and the product by causing their temperature to rise, the temperature of the sensor being measured by a measuring device. It includes a calculation and control device which determines the end of the defrosting of the product by calculating the values of the second derivative of the temperature rise curve of the sensor as a function of time. The calculation and control device acts on the operation of the oven at the end of defrosting when the values of the second derivative are lower than a predetermined value.

Description

The invention relates to a microwave oven comprising a microwave source and a sensor placed in the oven near a product to be treated, the absorbed microwave energy being distributed between the sensor and the product, causing their temperature rise, the temperature of the sensor being measured by a measuring device.

Microwave ovens are now widely used to defrost and reheat food previously placed in a freezer. This defrosting is generally carried out by an empirical method: the user determines an approximate weight of the food to be defrosted and deduces therefrom an approximate time too during which his microwave oven must operate. It follows a more or less complete thawing, even a beginning of cooking. On the other hand, it is known in the literature that water, an essential constituent of most foods, absorbs microwaves very differently around 2.45 GHz depending on whether its temperature is below or above 0 ° C. Below 0 ° C the ice is very substantially transparent to microwaves, on the other hand for a temperature above 0 ° C water very strongly absorbs microwaves. This phenomenon is due to variations in dielectric losses of water as a function of temperature. Document FR 2 571 830 describes a microwave oven provided with a standard load placed in the oven next to the food to be treated. The standard load absorbs microwave energy according to a distribution taking place as a function of the loads constituted by the standard load and the load of the food to be treated.

It is thus possible, from the heating of the standard load, to know the quantity of food in the oven and automatically determine the cooking time. According to this document, the heating speed of the standard load is practically independent of the temperature of the sensor.

Although the defrosting operation is named, no means is revealed which makes it possible to follow and control this delicate transition which concerns the passage for the element from a frozen state to a defrosted state.

The technical problem posed by the invention is therefore to follow the evolution of the temperature of the product to be thawed and to determine the end of thawing in order to be able to operate the next step.

The solution to this technical problem is that the oven includes a calculation and control device which determines the end of the thawing of the product by calculating the values of the second derivative of the rise in temperature of the sensor as a function of time, and which acts on the operation of the oven at the end of defrosting when the values of the second derivative are less than a predetermined value.

In this situation, the calculation and control device acts on the operation of the oven at the end of defrosting when the values of the second derivative are less than a predetermined value. The oven can thus be programmed either manually or automatically for a subsequent cooking stage, or be stopped if only the defrosting stage was planned.

In a microwave oven the temperature rise of a load as a function of time follows a law of the calorimetric type:
Δϑ = P.Δt / mc
where Δϑ is the temperature variation during the time difference Δt for a mass m of a specific heat body c. P is the microwave power available in the oven.

It has been verified experimentally that this law is verified when this mass is separated into two masses m₁ and m₂ such that m = m₁ + m₂.

• (1)      m₁Δϑ₁ + m₂Δϑ₂ = mΔϑ
  Δϑ₁ et Δϑ₂ sont alors respectivement les élévations de tempé­rature des deux masses m₁ et m₂ et Δϑ serait l'élévation de température de la masse m si elle avait été soumise aux micro-­ondes dans le four dans les mêmes conditions que les masses m₁ et m₂, en particulier pendant le même temps de chauffage. Cette loi se vérifie encore lorsque deux masses de chaleurs spécifiques différentes sont introduites dans le four :
• (2)      m₁c₁Δϑ₁ + m₂c₂Δϑ₂ = mcΔϑ.

The law then becomes:

• (1) m₁Δϑ₁ + m₂Δϑ₂ = mΔϑ
  Δϑ₁ and Δϑ₂ are then respectively the temperature rises of the two masses m₁ and m₂ and Δϑ would be the temperature rise of the mass m if it had been subjected to microwaves in the oven under the same conditions as the masses m₁ and m₂ , especially during the same heating time. This law is still verified when two different specific heat masses are introduced into the oven:
• (2) m₁c₁Δϑ₁ + m₂c₂Δϑ₂ = mcΔϑ.

From these laws it follows that if two charges are introduced at the same time in a microwave oven, the total available power is distributed in the two charges to raise the temperature of each charge by a temperature value inversely proportional to their mass and their heat capacity. Thus, if the thermodynamic characteristics of one of the charges are known, the temperature variation of the defrosting sensor will be dependent on the presence and the thermodynamic state of the product to be thawed. The sensor must have determined and stable thermodynamic parameters.

But the law represented by the relations (1) or (2) concerns substances for which the absorption of microwaves is the same. If this is not the case, the temperature rises of the substance of mass m₁ and of the substance of mass m₂ will be modified accordingly. In particular, and this is the situation exploited by the invention, when one of the substances is ice, its absorption coefficient is very low. The microwave energy will therefore be absorbed mainly by the sensor itself, which is designed to have a sufficient absorption coefficient. The transition of the substance from ice to water will cause the substance to gradually absorb more and more more microwave energy therefore heating up more and more. The energy absorbed by the sensor will gradually decrease. Thus the evolution of the temperature of the sensor will make it possible to follow the evolution of the temperature of the product being thawed placed nearby. The heating rate of the sensor will then not be practically independent of its temperature as indicated in document FR 2 571 830 but on the contrary will be indicative of the change in thermodynamic state of the substance of the product.

The temperature rise of the sensor will depend on the state of the product to be thawed. In particular if the product which by its nature contains a lot of water leaves the freezer at a temperature of around -20 ° C, it will only very slightly absorb microwaves. Therefore all the power available in the microwave will be used to raise the temperature of the sensor. As soon as the thawing process of the product is started, it will absorb more and more microwave power and consequently the temperature rise of the sensor will be slower. The slope (first derivative) of the temperature rise curve of the sensor as a function of time will therefore constantly decrease until all the ice present in the product to be thawed is completely transformed into water. Thereafter in accordance with the calorimetric law of temperature rise in a microwave oven as a function of time, the temperature rise of the product will be a linear function of time if the thermodynamic characteristics of the product do not vary.

To determine the temperature of the sensor, the temperature measuring device delivers an electrical signal whose variations as a function of time are determined by a calculation and control device. These variations are processed by the calculation device which compares said variations as a function of time at successive instants. It thus determines the values of the second derivative of the curve representing the evolution over time of the temperature of the sensor measured by the measuring device. The device then intervenes to control the operating cycle of the microwave source when two successive values of said variations are substantially equal, that is to say when the values of the second derivative are less than a predetermined value.

The presence of the sensor eliminates the need for the oven energy change switch. It suffices to operate the oven at the start with a low repetition rate of waves and then measure the slope (first derivative) of the temperature rise curve of the sensor as a function of time. If this slope is decreasing (with an absolute value of the second derivative greater than the predetermined value) the product present in the oven is being thawed. If this slope is small (with an absolute value of the second derivative lower than the predetermined value) the oven can automatically increase its microwave emission cycle because the product present in the oven is already thawed and is therefore only to be reheated .

The criterion for stopping the defrosting function must take into account the fact that if the product to be defrosted is essentially composed of ice, the second derivative may be constant and therefore resemble that of an already thawed product. The distinction is then made by the value of this second derivative:
- if it is substantially equal to that of the sensor alone, the product present in the oven is frozen,
- if it is significantly lower, the product in the oven is therefore thawed.

When it is necessary to have a very good detection sensitivity at the start of defrosting, it is possible to use as a substance a liquid whose calorific capacity and / or microwave absorption very sharply decreases with temperature, from l oil for example. Under these conditions when the product is still frozen, the temperature of the liquid will rise very quickly and as soon as the defrosting begins there will appear a very clear plateau on the curve of evolution of the temperature of the sensor as a function of time. This phenomenon is explained by the very strong decrease in the product mcΔϑ of the sensor. It is also possible to use several sensors with different thermodynamic characteristics.

As the product to be defrosted is generally formed of a large part of ice, the substance of the defrosting sensor must have losses greater than the dielectric losses of the ice.

The substance of the sensor can be a liquid such as water, oil or a solid solid or deposited on a non-absorbent support. It can be placed in a box that lets microwaves pass.

The defrost sensor can be removable or integral with the microwave oven. When it is removable it can easily be removed for cleaning and positioned anywhere in the cavity. It can also be integral with the oven and form an integral part of the oven itself. In this case, it may consist of a liquid substance which circulates in a pipeline, the member for measuring temperature variations determining the temperature difference between the inlet and the outlet of the pipeline. Circulation can be ensured by a pump.

• figure 1a : les courbes de variations de tempéra­ture d'un capteur de masse m₁ = 100 grammes et d'un produit de masse m₂ formés tous deux d'eau à l'état liquide en fonction de la masse m₂.
• figure 1b : les courbes de variations montrant la concordance des mesures expérimentales de température effec­tuées sur une masse m₁+m₂ et celles déduites du calcul de l'é­ quation 1.
• figure 2a : les courbes de la température et des variations de la température en fonction du temps d'un capteur d'eau placé à côté d'un produit à décongeler formé d'une masse de glace au cours de la décongélation de la masse de glace.
• figure 2b : une courbe analogue à celle de la fi­gure 2a représentant la fin de décongélation d'un produit mais avec un pas de calcul plus fin pour la mesure des dérivées première et seconde.
• figure 3 : une représentation schématique d'un capteur.
• figure 4a, figure 4b, figure 4c : trois représen­tations schématiques d'un four à micro-ondes utilisant diffé­rents capteurs.
• figure 5 : un schéma électrique selon l'invention pour la mise en oeuvre du contrôle du fonctionnement de la source micro-ondes à partir des mesures effectuées par le cap­teur afin de contrôler la décongélation.

The invention will be better understood using the following figures given by way of nonlimiting examples and which represent:

• FIG. 1a: the curves of temperature variations of a mass sensor m₁ = 100 grams and of a product of mass m₂ both formed of water in the liquid state as a function of the mass m₂.
• Figure 1b: the variation curves showing the agreement of the experimental temperature measurements carried out on a mass m₁ + m₂ and those deduced from the calculation of the e equation 1.
• FIG. 2a: the curves of the temperature and of the variations in temperature as a function of time of a water sensor placed next to a product to be thawed formed from a mass of ice during the thawing of the mass of ice.
• FIG. 2b: a curve similar to that of FIG. 2a representing the end of thawing of a product but with a finer calculation step for the measurement of the first and second derivatives.
• Figure 3: a schematic representation of a sensor.
• Figure 4a, Figure 4b, Figure 4c: three schematic representations of a microwave oven using different sensors.
• FIG. 5: an electrical diagram according to the invention for implementing the control of the operation of the microwave source from measurements made by the sensor in order to control the defrosting.

FIG. 1a represents the variations in temperature 10 of a sensor formed by a mass m₁ of 100 grams of water and the variations in temperature 11 in a product formed by a mass m₂ of water both placed in a microwave oven for temperatures above ambient temperature and this for a period determined as a function of the mass m₂. When the mass m₂ increases the temperature rise of the two masses decreases. The mass m₁ of the sensor undergoes a rise in temperature greater than that undergone by the higher mass m₂.

FIG. 1b represents the variations in temperature 12 of a mass of m₁ + m₂ grams of water. The curve 13 is formed of the points obtained by calculating from equation 1 the elevation that a mass of m₁ + m₂ grams of water would undergo. We see that the two curves overlap. This makes it possible to demonstrate that the microwave energy dissipated in thermal form is distributed in the two loads for raise their temperature inversely proportional to their mass and their specific heat. The rise in temperature of the sensor will therefore make it possible to follow the rise in temperature of the product located nearby and in particular to follow the thawing steps.

FIG. 2a indicates the variations in temperature 21 as a function of time of a sensor formed of water during the thawing of a mass of 200 grams of ice. The slope (first derivative) of curve 21 is represented by curve 22. The slope of curve 22 (the second derivative of curve 21) is represented by curve 25. We note that at the time of departure this first derivative presents in absolute value a high value which decreases first slowly then quickly enough finally to stabilize. This stabilization will be taken to decide on the end of defrosting and used by the calculation and control device. The second derivative 25, presented here in the form of line segments, begins by increasing and then decreasing, in absolute value, during the thawing step. When this is finished, the second derivative has a low value. When this value becomes lower than a predetermined value, the calculation and control device can intervene to position the oven for a new operation: cooking, slow reheating, off, etc.

Figure 2b shows a curve similar to that of Figure 2a. The first and second derivatives are determined with a finer calculation step. Curve 1 represents the temperature variation of the sensor. Curve 2 represents the first derivative of curve 1. Curve 3 represents the second derivative of curve 1. The zeros of the scales for curves 2 and 3 are indicated on the right.

FIG. 3 represents a nonlimiting example of embodiment of a defrosting sensor 30. It represents a substance 31 which can absorb microwaves, the subs tance being in contact with a member 32 for measuring its temperature. This can be formed by a thermocouple, a thermistor, a semiconductor detector or any other temperature measuring element. It is connected to the outside by the connections 33. The substance 31 can be liquid. It is then placed in a housing 34 or a container. Substance 31 can be solid. It may or may not be placed in a housing 34. The substance can also be deposited on a support which does not absorb or only slightly absorb microwaves.

The liquid substance can be water, oil or any other liquid having sufficient dielectric losses to ensure usable heating of the sensor.

The solid substance can be a ferrite, a solid partially containing metal ions or any other solid having sufficient losses to ensure usable heating of the sensor.

FIG. 4a represents a microwave oven 40 provided with a defrosting sensor 30. It is placed next to the product to be defrosted 41. A microwave source 42 emits microwaves which are picked up by the product 41 and the sensor 30. The temperature measurement carried out on the sensor 30 is transmitted to a calculation and control device 43 which acts on the microwave source to modify its operation.

FIG. 4b represents another microwave oven in which the defrosting sensor consists of the substance 31 which is separated from the temperature measurement member 32. This consists of an infrared light radiation detector of pyroelectric type. It thus remotely determines the temperature of the substance 31. The measurement is itself transmitted to the calculation and control device 43 which acts on the microwave source 42.

FIG. 4c represents another microwave oven 40 in which the sensor consists of a pipe containing a liquid, a part being located in the cavity of the oven. Circulation can be ensured by a pump 45. The temperature measurement device takes the temperatures at the inlet and at the outlet of the pipe portion located in the cavity and delivers its data to the calculation and control device 43 which acts on the microwave source 42.

FIG. 5 represents an electrical diagram for the implementation of the control of the operation of the microwave source from measurements made by the sensors. The electrical signals coming from the sensor 30 penetrate into the calculation and control device 43. According to an exemplary embodiment, it is formed by an A / D converter 51 joined to a microprocessor 52 provided with a memory 53. It operates with an operating clock 54. The microprocessor 52 will determine the variations in slope of the electrical signal received and store the values in memory 53. The value at time t is compared to that determined at time t-1, and when the two successive values are substantially equal, the microprocessor intervenes on the supply 55 of the magnetron 56 which constitutes the microwave source. An alarm 57 can warn of the progress of operations.

The operating mechanism is as follows. The temperature of the sensor is converted into an electrical signal which is transformed into a digital signal via an analog to digital converter. This signal is subsequently memorized by a RAM memory and processed by the microprocessor. The processing consists, in the case of defrosting, of measuring the temperature at a fixed time interval and of comparing the different measurements with one another in order to determine a slope (first derivative) of the temperature rise curve of the sensor as a function of the time then to determine the evolution (second derivative) of said slope. For example, during a complete defrost, a measurement point can be taken every two seconds, and a measurement of the slope of the temperature rise can be made every 100 points by a method such as least squares method. Such a measurement then shows a variation in slope in function tion of time, the characteristics of which may be as follows in the case of a body containing a lot of water.
- At first, the load is frozen. The temperature rise of the sensor is rapid and follows a curve which would be that if the sensor were alone. Under these conditions the slope measured by the method of least squares is substantially a straight line substantially parallel to the time axis.
- Then the load begins to thaw. The temperature rise of the sensor is slower. The curve of the slope as a function of time then has a negative derivative.
- When the load is completely thawed, the temperature rise of the sensor becomes monotonous again with a slope lower than the slope at the start of the experiment if no other change of state occurs, boiling for example. This phenomenon is revealed on the least squares curve by a stabilization of the curve which is found parallel to the time axis. It is this new stabilization which is recognized by the microprocessor as an end of thawing. The microprocessor can then, via appropriate input / output interfaces, stop the microwave radiation source and possibly warn the user or start a warming phase.

Claims (7)

1. Microwave oven comprising a microwave source and a sensor placed in the oven near a product to be treated, the absorbed microwave energy being distributed between the sensor and the product causing their temperature rise , the temperature of the sensor being measured by a measuring device, characterized in that it comprises a calculation and control device which determines the end of the thawing of the product by calculating the values of the second derivative of the elevation curve of the temperature of the sensor as a function of time, and which acts on the operation of the oven at the end of defrosting when the values of the second derivative are less than a predetermined value. 2. Microwave oven according to claim 1, characterized in that the calculation and control device comprises an analog-to-digital converter, connected to a microprocessor provided with a memory. 3. Microwave oven according to one of claims 1 or 2, characterized in that the sensor comprises a solid material absorbing the microwaves. 4. Microwave oven according to one of claims 1 or 2, characterized in that the sensor comprises a liquid material absorbing the microwaves. 5. Microwave oven according to claim 4, characterized in that the liquid circulates in a pipe, the member for measuring temperature variations determining the temperature difference between the inlet and the outlet of the pipe. 6. Microwave oven according to one of claims 1 to 5, characterized in that the member for measuring temperature rises is a thermistor, a thermocouple or a semiconductor detector. 7. Microwave oven according to one of claims 1 to 4, characterized in that the member for measuring temperature rises consists of a detector of infrared light radiation of pyroelectric type.

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