DE4416960A1 - Method and device for determining the state of cooking of a foodstuff in a closed space - Google Patents

Abstract

The invention relates to a method for determining the state of cooking of a foodstuff 3 which is arranged in a housing 1 which is closed and heated to a specific temperature, the foodstuff 3 emitting ultra-high-frequency waves in a predetermined frequency range during its heating, characterised in that, in the case of the walls 10, 11, 12, 13 of the housing 1, which reflect the ultra-high-frequency waves, the method contains at least the following steps: - receiving the ultra-high-frequency waves 50, 51, 52 emitted by the foodstuff 3 and transmitted to a radiometer 21, 2 directly or via reflection at the walls 10, 11, 12, 13, by means of the radiometer 21, 2; - converting the power of the ultra-high-frequency waves 50, 51, 52 which are received by the radiometer 21, 2 into a signal representing the global temperature TR of the foodstuff 3; and - proceeding from this signal, representing the global temperature TR of the foodstuff 3, determining the state of cooking of this foodstuff. <IMAGE>

Description

The invention relates to a method for determining a Cooking state of a food in a closed Space, especially in an oven.

The invention also relates to a device for executing this procedure.

The main problem to which the invention relates is is the optimal cooking time for each food to determine in a particularly automatic manner.

It is easy to see that there are numerous in this provision Factors or parameters are included.

One can first of all "objective" factors and "subjective" fac differentiate gates.

The taste of the user flows into the last category with a.  

In the first category you can without the enumeration is exhaustive that the locked room in which the Food is ordered, specify its own characteristics, the nature of the food, its weight Volume, its surface and of course the tem temperature of the room and the initial temperature of the food by means of.

These different parameters when determining themselves Many users rely on their intuition Applicant through experiments and various simulations certainly.

This is assumed below for the purpose of simplification conditions that the cooking space is an oven with metal walls. In the frame However, other devices can be used in the invention described later.

It is also noted that in the following Description of the term "cooking" for simplicity as Synonymous with all heat-based preparation types from Food is used, e.g. B. baking, braising, thin cooking, etc.

Under the above parameters, the characteristics of the Furnace can be determined once and for all in its conception.

The oven can be preheated, as is common, and it is numerous methods for regulating the temperature reached known. The older methods use a thermostat or a corresponding organ. The more advanced procedures use in connection with a temperature probe electro African circuits that act on the heating medium. This Methods are outside the scope of the invention.

It is also well known the cooking temperature to the food medium type, even to a special recipe for this Nah means of adaptation. One can say that everyone  Furnaces are provided with means for regulating the temperature. Other, more sophisticated devices contain regulation means Kind of a "menu", so they generally have a touchta on the function selection. Each key is special Assigned cooking program that via parameters, e.g. B. tempera structure, pre-setting of the heating time, etc., the program adapted a special type of cooking. The parameter "food Mittelart "can be partially treated by this procedure become.

One can also check the weight of the food before cooking to find out. However, it is necessary to do one beforehand Carrying out weighing, on the one hand, the provision of a Libra entails and on the other hand for some Nah Types of agents, e.g. B. for liquids, as expensive and uncomfortable.

On the other hand, it is not common in current practice to Volume as well as the surface of a food to be cooked tels, e.g. B. a roast to determine.

In general, the user relies on his experience tion, and it determines a certain number of these parameters consciously or unconsciously intuitive and introduces the necessary inputs positions, e.g. B. temperature or cooking time. He leaves also refer to tables or recipes and improve the provisional settings through successive "tests", the z. B. consist of the food in the course of Try cooking or optically monitor its condition chen.

The invention relates to the disadvantages of the known to improve the prior art, some of which be were written.

To this end, the invention first sees a measurement of the average temperature of the Nah placed in the oven agent.  

One could provide this temperature with the help of a probe to measure with a thermocouple or similar, which in Inside of the food is arranged.

This method would have several disadvantages as well it's not very efficient. The most obvious of these Disadvantages is that a mobile measurement is necessary organ, which is the execution of work steps (Inserting into the food) every time cooking pulls. It can also be seen that the temperature inside the Food of several combined with the food which parameters depend on: thermal conductivity, volume, Surface, etc. Finally, the position of the probe due to the gradient "outside temperature - inside temperature" a very critical parameter.

Temperature sensors are also used in other fields of application known, which are based on ultra-high frequency radiometry. These devices contain an antenna, which is featured is seen from an area of one to be examined Body given. Detect heat radiation. This before directions are particularly in infrared thermography used.

With this method you can not only measure the temperature on the Surface of a body, but also according to the Fre frequency range of the waves used at a shallow depth Measure on the order of 1 cm or more.

The method commonly used in these devices can be summarized as follows:

The ultra-high frequency signal detected by the antenna, the one Result of the thermal radiation of the object to be examined and is proportional to its temperature, becomes alternate compared to a signal from a heat Calibration resistance comes. One uses a low frequency  commutator to alternate the above signals to transmit an ultra-high frequency amplifier.

The alternating signal is then detected at the output of the amplifier kers, the signal being proportional to the temperature difference between the temperature of the object to be measured and the Temperature of the heat noise source. You have it an error signal. If you have the value of the calibration resistance the two temperatures can be set to the same Set the value, and since the temperature of the heat Calibration resistance, you know the absolute value the temperature of the object to be measured. This setting can take place automatically through feedback.

Various devices have been proposed that carry out such a procedure or a similar procedure ren. This applies in particular to those in French Pa Device FR-A-2 561 769 described device. This Patent application describes an improved device of the type mentioned above.

Another improved device is in the French described patent application No. 01 91344, which on February 1, 1991 was filed.

The invention takes advantage of the features and characteristics of such Devices to measure the mean temperature of one in one Oven arranged to measure food.

However, it should be noted that the procedure according to the invention has specific features that are not of the mentioned, adapted to other areas of application Devices can be provided.

Namely, the foods placed in the oven take generally large volumes and their masses usually between 500 g and 1 kg or more.  

It is according to one of the features of the method of the invention but necessary the average or global temperature of the in knowing the mass of food placed in the oven.

The known ultra-high frequency radiometric methods and Devices are not designed for this application.

According to another feature of the Erfin method The stove (or more generally the heating room) with the Ultra-high frequency wave reflecting walls, and on the one hand you arrange the antenna and on the other hand the near means in such a way that the antenna from the entire surface of the food mass radiation directly or through subsequent reflection receives the walls. The received signal is therefore a ge important signal that the average temperature of a Nah food in a shallow depth in the food poses.

By measuring the development of the above means ren temperature can be according to an additional feature of Invention determine the cooking state of the food, if you know the time that has already passed.

The invention also aims to automatically increase the cooking time determine and also ver automatically for this purpose to estimate different parameters from those in the oven depend on the food brought in.

For this purpose, various parameters are measured, the with the development of temperature depending on the Time, especially the slope of the curve after the start of cooking.

In general, these measurements are not feasible; it is necessary to carry out filtering: it will be ar steps like calculating linear regression or Higher order calculations performed.  

The results of this filtering, especially the amount of detected thermal radiation, enable the determination of the Permeability of the space used for ultra high frequency quartz waves. A space permeable to waves (e.g. from Glass) generates a basic heat radiation that is 1.5 to 2 times less than that of a room impervious to waves (e.g. made of metal).

A second parameter is also measured, which is the Reflection coefficient of the food for ultra high is frequency waves.

The experiments carried out by the applicant have been successful shows that if you look at the nature of the food knows and these parameters in a suitable way biniert, automatically the weight and the optimized cooking time of the food regardless of its volume and its Surface can determine.

The aim of the invention is therefore a method for determination a cooking state of a food in a Housing is arranged, which is completed and on a certain temperature is warmed up, the food when heating ultra high frequency waves in a pre emits certain frequency range, characterized in that at walls reflecting the ultra high frequency waves of the housing the method at least the following steps contains:

• - Reception of the dispensed and released from the food a radiometer directly or via reflection on the walls ultra-high frequency waves transmitted by the radiometer;
• - Conversion of the power received by the radiometer Ultra high frequency waves into a the global temperature of the Signal representing food; and
• - from this the global temperature of the food determination of a cooking state by means of a representative signal of this food.

The invention also has an apparatus for carrying out a such a process.

In this way, the invention enables the average temperature to determine the temperature of a food mass without a organ in contact with the food necessary is.

The measured temperature is a global indication of the in a "seen" temperature a few millimeters deep the entire heated surface.

The invention also enables the development of tempera recognizable.

It finally enables, depending on the previous ones certain parameters taking into account the executed ten measurements to determine the cooking state and automatically to decide on the end of cooking.

The following Be serves to explain the invention writing, from which further features and advantages of the Invention result. In the attached to the description Drawings show:

Fig. 1 shows schematically an oven containing an apparatus according to the invention;

Figures 2 and 3 curves of temperature rise of a food according to the method of the invention.

Figure 4 is a flow diagram illustrating the process steps of the invention in a special case of cooking a food such as beef or lamb;

Fig. 5 shows an embodiment of the furnace containing the device according to the invention;

Fig. 6 is an electrical schematic of the main elements of a radiometer, that can be used in the invention; and

Fig. 7 schematically shows an embodiment of the elec tronic signal processing circuits for use in the method according to the invention.

It appears of interest first, roughly the main ones to recall appearances that the ultra-high frequency frequency radiometry and its application for measuring tempera determine the temperature of a heated body.

For this purpose, reference is made to FIG. 1, which schematically represents a furnace 1 in section, in which a device according to the invention can be used.

According to a feature of the invention, the food 3 , the temperature of which it is desired to measure, is arranged on a plate 4 which is permeable to ultra-high frequency waves in a later specified range.

Each food 3 heated in the oven 1 and, more generally, each heated body emits an ultra-high frequency radiation, for the power PH of which the following relationship applies:

P _H = K · T _R · .DELTA.F (1)

in which
K the Boltzmann constant,
T _R the temperature of the body 3 ,
ΔF is the pass band.

The emitted frequencies are in a frequency band between around 0.1 GHz and 20 GHz. Appearance is of a stochastic nature.

In the context of the invention is generally for restricted frequency range, usually from 2 to 4 GHz.

According to a further feature of the invention, the walls of the closed space of the furnace 1 , four walls 10 , 11 , 12 and 13 of which are shown in FIG. 1, reflect the frequencies of this region. Of course, this also applies to the walls which are not visible in FIG. 1 and which are referred to as the front and rear walls.

According to a further feature of the invention, the direct radiation 50 or the single or multiple reflected radiation 52 , 51 from the antenna 21 of a radiometer 2 it captures. The antenna 21 is connected via any suitable connecting element 20 to the radiometer 2 , for. B. via a coaxial cable. Of course, the pass band of the antenna 21 and more generally of the radiometer 2 must be matched to the band of the ultra-high frequency to be detected (e.g. 2 to 4 GHz, as shown above).

As already mentioned, the operation of such a radiometer 2 is known per se. For a more detailed description, reference is made to the French patent applications mentioned above. A circuit example will be described with reference to FIG. 6.

With reference again to the relationship ( 1 ), the temperature T _{R of} the receiving element, that is to say the antenna 21 , in turn now satisfies the following relationship:

T _R = (1-R) · T A _M -A (1-R) · T₀ + T₀ (2)

in which
R the reflection coefficient of the antenna 21 ,
A is the percentage of ultra-high frequency energy absorbed by the closed space of furnace 1 and material 3 ,
T₀ the reference temperature of the radiometer 2 and
T _{M is} the measured temperature of the material (see (1)).

There are two main cases:

• - the enclosed space is perfect, which means that in particular fully reflect the walls and that the  Antenna has a reflection coefficient of zero;
• - the metallic space is not completely reflective, which is the real case.

The theory and experiments show, however, that in these two cases, taking into account the material forming the body 3 , i.e. the food to be cooked, the measured radiometer temperature depends essentially on the properties of the body 3 , in particular on the percentage of the absorbed ultra-high frequency energy (see (2)) and in the opposite way from the reflection coefficient of the body 3 . In the following, the reflection coefficient is referred to as RHO.

The errors due to the elements forming the furnace 1 and due to the radiometer 2 can be neglected in general. The emission characteristics of the walls, the antenna, etc. are constant in any case and can be determined once and for all when designing the furnace. Correction coefficients can be derived from them, which are to be used selectively in later measurements.

Due to the method according to the invention, the average temperature of the food mass 3 arranged in the oven 1 can be determined at any time. This measured temperature has the advantage that it is a global indication of the temperature seen at a depth of a few millimeters, which speaks to the ultra-high frequency emission of the entire surface of the heated food mass 3 . The "depth" of the temperature measurement depends on the frequency range used. In a range from 2 to 4 GHz, the temperature is measured with a penetration depth of approximately 10 mm.

So you can already monitor the temperature rise of the food mass 3 as a function of time.

Fig. 2 shows a diagram illustrating the development of the temperature of a piece of meat (roast pork) with a weight of about 1 kg and a thickness of 6.5 cm.

The vertical axis is divided into ° C and the time t Representing horizontal axis is divided into minutes. The temperature in ° C is obtained from a calibration which, depending on the measured signal, have an equiva limit value in ° C.

Curve A represents the course over time of the temperature measured by means of radiometry in accordance with the main feature of the invention. Curve B represents solely as a comparison the course of the temperature, which is measured using an inside of the body 3 , that is to say in the example shown in FIG the piece of meat, thermocouple used would be measured. Apart from the parts already mentioned, you can see that curve B is very different from curve A. This is due to the thermal inertia, which has the material of the body 3 .

Before the meat is introduced, i.e. at a time t <t₀, the signal from the oven 1 (in the example shown) corresponds to a temperature of 25 ° C, then this temperature drops sharply to approximately 8 ° C at t = t₀, it then grows strongly for a few ten minutes, in order to finally approach a limit temperature T _{lim of} the order of 100 ° C. with an ever lower rate of growth.

The strong drop in t₀ is due to the fact that the door of the furnace 1 is opened (which causes a signal loss) and that the material introduced is absorbent.

You can benefit from this phenomenon. A strong Ab fall of the measured signal in the course of cooking can näm deliberately or unintentionally opening the oven  show door. An alarm signal can therefore be provided and take any corrective measures.

The limit temperature T _lim depends of course on the temperature to which the furnace is heated.

The initial slope of the curve, in particular in a period of a few minutes after the time t₀, where t₀ is determined as the closing of the oven door, depends not only on T _lim , but also on the properties of the food 3 , z. B. mass, quality, surface, etc.

The applicant has experimentally found that one starting from the parameters of the curve of the development of the measured temperature and in particular based on the Knowing the initial slope determine the cooking state and from this the optimized cooking time for can derive numerous food categories.

However, curve A, which represents the temperature measured by means of radiometry, cannot be directly evaluated, as the diagram in FIG. 2 suggests, since it has numerous fluctuations with a greater or lesser amplitude. It is necessary to filter out these fluctuations, i.e. to smooth the curve.

In the first section of the curve (after t = t₀) one calculates a linear regression over a few minutes. The mathema that enables this calculation to be carried out tical methods are well known, which is why they are not here be repeated.

Referring again to FIG. 2, it can be seen that the first part of curve A of the development of the measured radiometer temperature can be represented in a first approximation by a straight line C (linear regression).

The time required to calculate linear regression Section is on the order of 6 minutes. The Of course, specifying this value means no on limitation of the invention, and this value has been experimented tell by the applicant using conventional, ovens available on the market.

Under certain measuring conditions, erroneous measuring points can occur give. To increase accuracy, the standard ab deviation between the measuring points and between the re points calculated.

With this method you can get a more precise, parallel slope not define the temperature measurements is influenced, the value of which is greater than the calculated value or less accuracy in terms of standard deviation deviates.

The determination of the standard deviation also makes it possible to determine the background heat noise, with the knowledge of which one can determine a parameter which is related to the nature of the container containing the material 3 : Low heat noise is characteristic of a container permeable to waves, while a strong heat noise is characteristic of a container impermeable to the waves. As already stated, the size ratio between the amplitudes of this background heat noise of the two types of containers is approximately 1.5 to 2. The absolute amplitude depends on a large number of parameters, such as the radiometer used, etc.

In a preferred embodiment of the method according to of the invention are the various for this method necessary provisions in an automated manner with the help executed by signal processing circuits that follow be described.  

To determine the slope, do the following preparation steps carried out:

• a) The reflection coefficient RHO of the food is tested using 3 in the oven 1 . At the moment the food 3 is introduced into the oven 1, this falls below a calibration value which depends on the value of the reflection coefficient of the empty oven 1 . The reflection coefficient for the empty oven is e.g. B. greater than 0.350.
• b) One measures a number of successive, increasing the points of radiometer temperature, usually 4 µm to rule out any inaccuracy or ambiguity. It it is determined that this time is the starting time t′₀ # t₀ of cooking forms.

Starting from t = t′₀, the temperature measurements performed serve on the one hand to determine the curve of the temperature profile (in FIG. 2 curve A) and on the other hand to calculate the linear regression (curve C).

For many foods, knowing the nature of the food, which allows selection of a heating temperature of the oven, and knowing the initial slope obtained from the linear regression, allows determining the optimized cooking time for the food mass effectively put into the oven 1 .

Experience has also shown that the mean reflection coefficient RHO must be known for some types of food. For this purpose, this value is calculated from the point in time at which the linear regression begins. This value represents the thermal load, which of course depends on the volume of the food, its exchange surface with the heating environment inside the oven 1 and the initial temperature. The larger the exchange area, the more the food 3 tends to absorb the waves. It follows that RHO is relatively high for a low surface area element, and vice versa.

For these foods, the cooking time is not just under Taking into account the slope, but also taking into account determined by RHO.

For most foods, the calculation of the Cooking time not very accurate. In contrast, are close agents such. B. beef or lamb very sensitive against changes in cooking time.

For this type of food, the linear regression shows in generally does not have sufficient accuracy. If the left linear regression becomes less precise, one calculates in it Case of second degree polynomial regression.

One can also see the curve of temperature rise in two or Subdivide more time intervals. Generally there are two Sufficient time intervals. A linear re is calculated gression in the two curve sections.

Fig. 3 shows a diagram illustrating a temperature rise curve A₁ and two straight lines C₁ and C₂, which approach this curve A₁ in the areas T₁: t₀ to t₁ or T₂: t <t₁.

In the example shown it is assumed that t₀ = 7 minu ten and t₁ = 21 minutes.

In addition, those for determining the cooking time are close the necessary steps of numerous and complex xer.

It is generally necessary to make a number of comparisons determine between the measured value RHO and experimentally threshold values. These values will be of course once for all the following cases for one given food type and a specific oven Right.  

Since this procedure is the most general, an example is illustrated below with reference to the flow chart of FIG. 4 for further explanation of the method according to the invention.

The task addressed by the example shown exists in starting the optimal cooking time of a roast beef of its various characteristics, mainly weight and thickness, to be determined, the characteristics not in advance are known and are therefore to be determined automatically.

The start of the cycle is labeled 100. From there The beginning of the cooking time is determined (at 101) by the two preparatory steps specified under a) and b) above steps are carried out: test of RHO and measurement of four (or a predetermined number) on top of each other following rising points of temperature. This time point determines the time t = t₀.

Now the actual cycle of determining the cooking time t _c begins.

The first step (at 102) is a multiple step Comparing the calculated value of RHO and the Thresholds 0.175; 0.18 and 0.19.

If RHO is less than 0.175, it is determined (at 103) that the weight of the roast is about 1 kg and in particular that the optimal cooking time is 30 minutes. The cycle of loading the cooking time is then finished (at 120).

If RHO is greater than 0.190, the same way (at 104) determines that the weight is on the order of 500 g and that the cooking time is 15 minutes. As above, the cycle is then ended (at 120).

The two remaining cases are more complex.  

The first of these concerns a RHO between 0.175 and 0.18. It is determined (at 105) that the weight is of the order of 500 g and that the cooking time must be between 15 and 20 minutes. The accuracy of this range is insufficient for the food under consideration. Now the linear regression and the slope are calculated (at 106) by measuring the radiometer temperature during a certain time t _ps ("elapsed time", usually 6 minutes, which was determined experimentally). The initial slope p can be calculated from the following relationship:

T _R = p × t _ps + cte (3),

where T _{R is} the measured radiometer temperature.

A slope test is performed (at 107) by is compared with a threshold: in the illustrated Example 0.145.

If p is greater than or equal to this threshold (0.145), the optimal cooking time t _c (at 109) is 15 minutes and the cycle is complete (at 120).

In the opposite case, the polynomial regression becomes two determined in order.

Based on this calculation and more precisely based on it of the coefficients a₀, a₁ and a₂ of this regression (Coefficients zero, first and second order) calc you net the radiometer temperature at the end of a certain Time tps (elapsed time). The experiment showed that you have an elapsed time of usually 300 s can expect.

Based on the following relationship, one calculates (at 108) the "corrected" radiometer temperature T ′ _R :

T ′ _R = a₀ + a₁ · tps + a₂ · tps² (4)

Thereafter, based on a threshold of (at 110) 80 ° C a comparison was made.

If T ' _{R is} greater than or equal to 80 ° C, it is determined (at 112) that the optimal cooking time is 15 minutes and the cycle is complete (at 120). Otherwise, the optimal cooking time (at 118) is 20 minutes and the cycle is also finished (at 120).

The second complex case is the case that 0.18 <RHO <0.19 is.

It is determined (at 106) that the weight of the roast is of the order of 1 kg and that the cooking time t _{c is} in the following range: 20 min. T _c 30 min.

In order to reduce the distance between these values, one carries out (at 114) the calculation of the second order polynomial regression and the calculation of the corrected radio meter temperature T ′ _{R in} accordance with step 108, to which reference is made.

Now (at 115) a multiple comparison between the Thresholds of 78 ° C and 85 ° C are carried out.

If T ' _{R is} less than 78 ° C, the cooking time t _c (at 116) is 30 minutes and the cycle is complete (at 120); if T ' _{R is} greater than 78 ° C and less than 85 ° C, the cooking time t _c (at 117) is 25 minutes and the cycle is ended (at 120); when T ' _{R is} finally greater than 85 ° C, the cooking time t _c (at 118) is 20 minutes and the cycle is complete (at 120).

It should be noted that all threshold values have been predetermined experimentally or by calculation based on the characteristics of the furnace 1 and (in particular) the radiometer 2 . Of course, they depend on the food under consideration and the antenna 21 .

Generally expressed for different foods similar categories and subcategories guided. As shown above, these processes can be seen in Ab dependence on the food under consideration is simplified the, in particular in that to determine the cooking time only the value of the slope p (linear regression) is taken into account is done.

In an automatic process, the temperature of the oven 1 , the various threshold values and the steps to be carried out specifically for a specific food item are taken into account from step 100 of the structure diagram of FIG. 4, which is designated by the general name “Start”.

With reference to FIGS . 5 to 7, a practical embodiment of the invention will now be described.

In Fig. 5, a furnace 1 is illustrated, in which the device is used according to the invention. Without restricting the invention, the following outer dimensions of the furnace are used to illustrate the example:

Height 285 mm
Width 450 mm
Depth 335 mm

as well as the following inside dimensions of the furnace:

Height 200 mm
Width 315 mm
Depth 290 mm.

The inner walls 10 , 11 , 12 , 13 and 15 (bottom) of the furnace are made of metal. You can use sheet steel or aluminum sheet. In the case of a microwave oven, the door 14 can be shielded in order to prevent a loss of the measurement signal.

If the door 14 is , as usual, completely or partially transparent, it is provided with a metallic coating on the inside, which allows a view of the inside of the oven. The coating is arranged on the glass of the door 14 .

In the illustrated embodiment, the antenna 21 contains a waveguide with a rectangular cross section with an optimization of the transition "coaxial connection 20 - waveguide 21 ". The adjustment of the waveguide can be improved by extending one of its sides. The waveguide is in the upper section of the furnace z. B. fastened by screwing, or it is obtained by pulling.

However, the waveguide antenna 21 can also be arranged on another wall, in particular on the side walls 10 or 12 .

The pass band used by the radiometer is, as already shown, 2 to 4 GHz, which enables a penetration depth and thus a temperature determination at a depth of approximately 10 mm. The waveguide 21 is dimensioned accordingly.

For a special version, the dimensions of the The waveguide is exemplary and not restrictive the following values:

Height H 32 mm
Width L 90 mm
Depth P 85 mm.

In FIG. 6, an electrical schematic is shown illustrating the major elements of the radiometer 2 and the explanation of the method of preservation of the radiometer allows temperature T _R and the coefficient RHO indicative signal.

The calculation of the last coefficient is not carried out by measuring the value of the reflection coefficient for ultra-high frequency waves from food 3 ( FIG. 1), which is not directly known, but it is based on the changes in the reflection coefficient of the antenna 21 , in particular between the state "empty oven" and the state "oven with food" inferred.

The circuits of such a radiometer correspond to those in the above-mentioned French patent application 91 01344 disclosed circuits on which for more detailed loading writing is referenced.

Apart from the radiometer 2 , which determines the radiometer temperature T _{R of} the food 3 and the coefficient RHO, the device contains signal processing circuits, as shown schematically in FIG. 7.

The signal processing circuits 6 can advantageously be formed from a commercially available standard microprocessor, e.g. B. starting from a microprocessor of the type COP800, which is sold by the company "National Semi-Conductor".

This circuit is in the illustrated embodiment, a keyboard for entering data or a similar organ, for. B. function keys etc., and a display element 8 , z. B. indicator lights, liquid crystal screen, etc., assigned.

In the following it is assumed that the circuits 6 are based on a microprocessor. As is well known, this type of circuit generally includes at least one arithmetic logic unit or central processing unit 60 , an ar memory 61 , a read-only memory 62 and input / output circuits 63 .

There are also others for the operation of the microprocessor Circuits (not shown) necessary, e.g. B. clock, internal registers, etc.

In a preferred embodiment of the invention, the read-only memory 62 is a programmable memory, e.g. B. an EPROM.

In this memory, on the one hand, the instructions and routines are stored, which are necessary for the general common functions of the microprocessor, and, on the other hand, the specific instructions and routines of the method according to the invention: the steps to be carried out for each type of food (e.g. the steps. of Figure 4 (for a beef roast), the parameters and thresholds temperature, reflection coefficient, etc.), etc.

The memory 61 is provided to temporarily store the specific programs and temporary data read from the memory 62 , which is well known.

The central unit 60 is connected to the input / output circuits 63 via a bidirectional bus 600 and to the memories 61 and 62 on the one hand via a bidirectional data bus 601 and on the other hand via an address bus 602 .

The data input keyboard 7 is finally connected to the central unit 60 via a bus 631 , which is connected to one of the connections of the input / output circuits 63 . This applies in the same way to the display element 8 , which is connected via a bus 632 to one of the connections of the input / output circuits 63 .

After the oven 1 and the associated devices (radiometer 2 and the electronic circuits 6 , 7 and 8 and the circuits 9 shown above) have been switched on, data relating to the food to be cooked can be acquired, e.g. B. Oven temperature.

In a preferred embodiment, this detection is carried out in an interactive manner. The display element 8 displays a menu, e.g. B. with options such as "starter", "vegetables", "meat", etc. An option is selected by pressing a number or a letter or a function key.

At this time, the display organ can display a list of submenus. If you e.g. B. in the above step selected an option with the name "pastry", the display organ 8 can display the following options: "pastries with low baking temperature", pastries with medium baking temperature "and" pastries with maximum baking temperature ".

The various menus and submenus as well as the processes necessary for their display are stored in the conventional memory 62 in a conventional manner.

The final choice can be made and confirmed in several ways, e.g. B. by pressing a special key on the keyboard 7 , whereby the corresponding program for the selected food is loaded into the working memory 61 and the corresponding steps of this program take into account the peculiarities of the selected food in the same manner as with reference has been described in Fig. 4.

The ultra-high frequency waves emitted by the food 3 are detected by the antenna 21 , transmitted via the connection 22 to the radiometer 2 and converted into an electrical signal which represents the radiometer temperature T _R. It is assumed in the example shown that the radiometer contains an interface card (not shown) which is capable of converting the analog signals into serial or parallel digital signals which comply with the standards of the input-output circuits 63 of the microprocessor 6 speak. For a more detailed explanation of the invention, it is assumed that these signals are converted into digital signals which are transmitted via the connection 630 to one of the parallel input connections of the circuits 63 . The conversion circuits, which are based on an analog-to-digital converter, and the interface circuits are well known to the person skilled in the art, which is why they are not shown. The respective interface card can also be assigned to the circuits 6 as well.

In return, the radiometer 2 can receive various arrangements and instructions via the same bus 630 , in particular for carrying out the necessary signal detection, as described with reference to FIG. 6.

Once the necessary cooking time (step 120, FIG. 4) is determined, this information can be displayed by the display organs 8 .

It can also be the time remaining until the end of cooking are displayed.

A more comprehensive display of information is also possible. Referring again to FIG. 4, e.g. B. At the end of the determination cycle (step 120), the display organ displays one of the following messages:

"small roast" (if t _c = 15 min.)
"medium roast" (if t _c = 20 min.)
"large roast" (if t _c = 25 min.)
"very large roast" (if t _c = 30 min.).

It can also be provided that the user, controlled via the keyboard 7 , is able to adjust the cooking time manually if he assumes that the automatic process leads to a cooking which does not correspond to his personal taste. It is Z. B. sufficient to provide a function key "time correction".

The linear regression or second order regression calculations, as well as all other calculations and / or comparisons, are performed by the central processing unit 60 , which may be connected to a mathematical coprocessor (not shown) to increase the computing speed.

The end of the cycle (after determining the cooking time t _c ) is carried out under the supervision of the microprocessor, which switches off the heating means (not shown) when the calculated cooking time t _{c has} expired and can trigger a sound signal at the end of the cooking.

In a preferred embodiment, the microprocessor 6 can also be used to control the heating means in a known manner. It is sufficient to provide a temperature probe. The temperature of the furnace is then regulated to a reference value, the z. B. automatically in the preparatory steps of selecting a menu and submenu be determined, as shown above. Means and / or processes can also be provided which make it possible to change this predetermined temperature manually. With this possibility, however, there is the danger that it is generally compatible with the process of automatically determining the course of the cooking time t _c or that it at least leads to very complex programs. However, this possibility can be provided for an optional operation of the furnace 1 in a manual operating mode.

In Fig. 7, the scarf generally designated 9 obligations include various control circuits of the method according to the invention by the micro-processor can be controlled within the framework of steps 6 or which are intended to perform other conventional functions. The circuits can be connected to the central processing unit 60 via a connection 633 and z. B. with a serial port of the standard R5232 or any other standard of the input / output circuitry 63 in connection. Among the executed controls are the temperature control or the triggering of the above-mentioned sound signal as well as various safety functions. The keyboard 7 can of course also serve to enter the control circuits 9 related inputs (commands or other), and the display member 8 can serve to display various messages and information which are also related to the circuits 9 and the functions they perform gene.

The invention is not limited to the exact examples, in particular with reference to FIGS . 1 to 4 described Ausfüh. It is pointed out in particular that the numerical examples (threshold values, reference temperatures, etc.) were only given to better illustrate the invention and not to limit its scope.

At the circuits used by the method can count rich changes that are easy to make with the selected technology related, e.g. B. Standard Micro processor or specialized circuits.

Finally, the invention can be applied to any completed Ge Housing applied to waves in the ultra high frequency range from 0.1 GHz to 20 GHz or in the more limited band of 2 to 4 GHz reflected without losses and with Heizele ment is provided. This applies to every food item container with lid, its inner surfaces and the underside of the lid meet the above requirements.

Claims (23)

1. A method for determining a cooking state of a Nah approximately ( 3 ), which is arranged in a housing ( 1 ), which is completed and heated to a certain temperature, the food ( 3 ) when heated ultra-high frequency waves in a predetermined frequency range emits, characterized in that the method contains at least the following steps in the ultra-high frequency wave reflecting walls ( 10 , 11 , 12 , 13 ) of the housing ( 1 ):

• - Reception of the ultra-high frequency waves ( 50 , 51 , 52 ) emitted by the food ( 3 ) and transmitted to a radiometer ( 21 , 2 ) directly or via reflection on the walls ( 10 , 11 , 12 , 13 ) by the radiometer ( 21 , 2 );
• - converting the power of the ultra-high frequency waves ( 50 , 51 , 52 ) received by the radiometer ( 21 , 2 ) into a signal representing the global temperature (T _R ) of the food ( 3 ); and
• - Starting from this signal representing the global temperature (T _R ) of the food ( 3 ), determining a cooking state of this food.

2. The method according to claim 1, characterized in that it contains an additional first step, which consists in the curve (A) of the change in global temperature (T _R ) as a function of time (t) during a certain time interval after To construct time (t₀), which represents the beginning of the cooking of the food ( 3 ), and to calculate the linear regression (c) of this curve and to calculate the slope (p) therefrom, and that the step of determining the cooking state is carried out thereby is that the value of the slope (p) is determined, a cooking time (t _c ) being determined from this value for a food of a certain nature. 3. The method according to claim 2, characterized in that it contains an additional step for calculating the linear regression (c), which consists in determining the standard deviation in order to eliminate the values of the global temperature (T _R ), whose value deviates upwards or downwards by a certain value with respect to the standard deviation. 4. The method according to claim 3, characterized in that, since the food is contained in a container, the calculation of the standard deviation makes it possible to find out the relative permeability of this container to the ultra-high frequency waves ( 50 , 51 , 52 ), wherein the standard deviation for an impermeable container is significantly higher than for a permeable container. 5. The method according to any one of claims 1 to 4, characterized in that it also includes a step of determining the mean value of the reflection coefficient (RHO) of the wel len of food ( 3 ). 6. The method according to claim 5, characterized in that it an additional step of comparing the reflection coefficients (RHO) with at least a certain smolder lenwert contains and that following this step according  determined the result of these comparisons or additional steps to be performed. 7. The method according to claim 6, characterized in that the additional steps make a comparison with the values of the Inclination (p) of the linear regression (c) contains. 8. The method according to claim 6, characterized in that it also includes a calculation of the second order polynomial regression, which is carried out based on the values of the curve of the change in global temperature (T _R ), and that based on the polynomial regression second Order to output a certain time interval a corrected value (T ′ _R ) of this global temperature (T _R ) is calculated. 9. The method according to claim 8, characterized in that it also includes a step of comparing the corrected value (T ' _R ) with at least a certain threshold and that the cooking time (t _c ) is determined based on the result of the comparison. 10. The method according to any one of claims 2 to 9, characterized in that the following preparatory steps are carried out to determine the time (t des) of the start of cooking:

• - Determination of the reflection coefficient (RHO) of the waves from the food ( 3 ) and comparison with a threshold value which represents the reflection coefficient of the empty housing ( 1 ); and
• - Detection of a predetermined number of successive, increasing values of the global temperature (T _R ); wherein the time (t₀) of the start of cooking is the time at which the reflection coefficient (RHO) becomes smaller than the threshold value, and at the same time the detection is carried out.

11. The method according to any one of claims 1 to 10, characterized characterized in that the threshold values be experimentally be true. 12. The method according to any one of claims 2 to 11, characterized in that it contains an additional step which consists in detecting a sharp drop in the signal representing the global temperature (T _R ) of the food ( 3 ), this drop being the Opening the closed housing ( 1 ) indicates. 13. The method according to any one of claims 1 to 12, characterized characterized in that the frequency range is the frequency inter vall contains between 2 and 4 GHz. 14. Device for carrying out the method according to one of claims 1 to 13, characterized in that it contains at least a radiometer ( 2 ) which is provided with an antenna ( 21 ) which extends into the housing ( 1 ), and means for converting the power of the ultra-high frequency waves ( 51 , 52 , 53 ) captured by the antenna ( 21 ) into a signal representing the global temperature (T _R ) of the food ( 3 ), the food being carried on a carrier ( 14 ) is arranged. 15. The apparatus according to claim 14, characterized in that the antenna ( 21 ) is a waveguide with a rectangular cross section, which opens into an opening which is formed in the upper wall ( 11 ) of the housing, and that the antenna via a coaxial connection ( 20 ) with the radiometer ( 2 ) is connected. 16. Device according to one of claims 14 or 15, characterized in that it also contains electronic circuits ( 6 ) for processing the global temperature (T _R ) of the approximate means ( 3 ) signal representing. 17. The apparatus according to claim 16, characterized in that the circuits ( 6 ) for processing the global tem perature (T _R ) of the food signal representing an arithmetic logic unit ( 60 ) and a read-only memory ( 62 ) contain routines save which process steps are assigned to specific food categories. 18. Device according to one of claims 16 or 17, characterized in that the electronic circuits ( 6 ) are connected to means ( 7 ) for inputting data or instructions which act on the operation of the electronic circuits ( 6 ). 19. The device according to one of claims 16 to 18, characterized in that the electronic circuits ( 6 ) on display means ( 8 ) control. 20. Device according to one of claims 18 or 19, characterized in that the means ( 7 ) for entering data or instructions and the display means ( 8 ) cooperate in order to enable the interactive selection of at least one element of a menu, so that one to be executed Operating mode or a food type to be cooked is selected. 21. The device according to one of claims 16 to 20, characterized in that the electronic circuits ( 6 ) are formed starting from a standard microprocessor. 22. Device according to one of claims 16 to 21, characterized in that the housing ( 1 ) is a furnace whose entire inner walls ( 10 , 11 , 12 , 13 , 14 ) reflect the waves ( 50 , 51 , 52 ). 23. Device according to one of claims 16 to 21, characterized in that the housing ( 1 ) is a food container provided with a lid, the inner walls and underside of the lid reflecting the waves.