DE10253198A1 - Method and device for thermal monitoring of an induction heatable cooking vessel - Google Patents

Abstract

The invention relates to a method for the thermal monitoring of an inductively heatable cooking vessel by monitoring the frequency (f) of the alternating current (I) causing the inductive heating. In addition, the invention relates to a corresponding device.

Description

The invention relates to a method and a device for the thermal monitoring of an inductively heatable Cooking vessel. Have induction hobs Glass ceramic surfaces to hold the cooking vessels and below arranged induction coils for heating them. The induction coils an alternating current in the frequency range of approx. 15 kHz to 80 KHz, in particular supplied about 20 kHz to 60 kHz. The resulting Alternating fields create eddy currents in the ferromagnetic bottoms of the cooking vessels to heat development to lead. Monitoring them is done by sensor systems that are complex and / or sluggish.

The invention is based on the object Method and device for the thermal monitoring of an inductively heatable To create a cooking vessel the simple and low inertia work.

According to the invention, this object is achieved by Features of the claims 1 and 14 solved. Advantageous training results from the subclaims.

On the process side, thermal monitoring takes place of the inductively heatable Cooking vessel through monitoring the frequency (f) of the alternating current causing inductive heating (I).

The advantages are as follows:

• - The thermal monitoring according to the invention (e.g. overheating) is low inertia. While it according to the state the technology can happen that the cooking vessel overheats or even fire occurs before the cooking zone is switched off by a monitoring solution that is too slow the solution according to the invention reacts a lot faster, d. that is, it provides a faster response.
• - The According to the invention, temperature becomes direct in the bottom of the cooking vessel measured and determined the food to be cooked, while according to the prior art a measurement is made elsewhere and the temperature is estimated.
• - It is not a separate transmission channel for measurement data from the cooking vessel to the hob required.
• - It no separate sensor or other separate hardware is required, as they are usually already in the power card of the inverter is implemented, d. that is, the additional cost is very small.

According to an initial training The supervision in particular qualitative or sudden changes in the temperature-time curve (T (t)) preferably as a monitor in particular qualitative or sudden changes in the frequency-time curve (f (t)) and / or for changes and / or absolute values of the first derivative (f '(t)) of the frequency-time profile (f (t)).

Monitoring takes place accordingly on the boiling of one in the cooking vessel Food to be cooked and / or to empty the cooking vessel or to dry it of the food to be cooked in it to monitor the frequency-time curve (f (t)) on certain qualitative or abrupt changes and / or for predetermined changes and / or Absolute values of the first derivative (f '(t)) of the temperature-time profile (f (t)).

The boil is monitored especially as surveillance

• - on a predetermined qualitative or abrupt change in the frequency-time curve (f (t)) from the direction of an essentially linearly decreasing frequency (f) towards a substantially constant frequency (f) and / or
• - on a predetermined change the first derivative (f '(t)) of the frequency-time curve (f (t)) from the direction of a negative Value in the direction of a value of approximately zero and / or
• - on a predetermined value of the first derivative (f '(t)) of the temperature-time profile (f (t)) he follows.

The alternating current (I) or the corresponding one Power (P) is reduced in particular to a predetermined value, if with this surveillance one or more changes were determined which correspond to one or more predetermined changes.

Monitoring for empty or Dry cooking takes place especially as a surveillance

• - on a predetermined qualitative or abrupt change in the frequency-time curve (f (t)) from the direction of a substantially constant frequency (f) towards a substantially linearly decreasing frequency (f) and or
• - on a predetermined change the first derivative (f '(t)) of the frequency-time profile (f (t)) from the direction of a value of about zero towards a negative value and / or
• - on a predetermined value of the first derivative (f '(t)) of the frequency-time profile (f (t)) he follows.

The alternating current (I) or the corresponding one Power (P) is particularly switched off when this monitoring one or more changes were determined which correspond to one or more predetermined changes.

The predetermined qualitative or abrupt change in the frequency-time profile (f (t)) lies in particular in or immediately after the transition from the falling frequency profile (f (t)) the constant frequency response (f (t)) and / or from the constant frequency curve (f (t)) into the falling frequency curve f (t).

According to a second training monitoring takes place to absolute temperatures (T) by assigning predetermined frequencies (f) and / or frequency ratios (f / f-start) at predetermined temperatures (T) and / or vice versa.

The assignment of frequencies (f) and / or frequency ratios (f / f-start) to the temperatures (T) is preferably dependent of predetermined properties, in particular material properties, the cooking vessel, the especially for the Use specified or queried by the user, for example by specifying a selection list. The assignment of the frequencies (f) and / or frequency ratios (f / f-start) to the temperatures (T) can also depend on predetermined services (P) take place.

The temperature of the cooking vessel will especially to a desired one Value set or kept at a desired value by

• - the Frequency (f) (continuously) monitored becomes,
• - the Frequency (f) and / or the frequency ratio (f / f-start) depending of the currently assigned power (P) and the currently assigned Cooking vessel (continuous) a temperature (T) is assigned,
• - the Deviation between the assigned and the desired temperature (T) (continuous) is determined and
• - corresponding the deviation one of the assignable services is selected.

Overall, there is also a monitoring procedure and / or control of the cooking process in an inductively heatable Cooking vessel available at the while of cooking temperatures and / or temperature changes as frequencies and / or frequency changes one is inductive heating effecting alternating current are recorded and evaluated in order to To signal the temperature of the cooking vessel and / or to influence.

The thermal monitoring device according to the invention an induction heatable cooking vessel, in particular to carry out of the above-described method

• - first Means for providing induction heating Alternating current
• - second Means for determining the frequency of the AC and
• - third Means for signaling and / or controlling the temperature of the Cooking vessel depending on the frequency of the alternating current.

The third funds point in particular first storage means with which predetermined qualitative or erratic changes of frequency-time profiles (f (t)) and / or predetermined changes and / or Absolute values of first derivatives (f '(t)) of frequency-time profiles (f (t)) can be stored, which in particular boiling as reference data one in the cooking vessel Food to be cooked and / or an empty boil of the cooking vessel or a dry boil of the food in it.

The third means can also have second storage means with which predetermined frequency-temperature profiles (f (T)) and / or frequency ratio temperature profiles (f / f-start (T)) can be stored are, in particular per cooking vessel property and / or performance.

The third means are particular Assigned input means with which the user desired temperatures and / or programs, in particular cooking programs containing temperatures, and / or services and / or cooking vessels or their properties can enter.

Finally, the third means in particular computing means with which with the second means determined frequencies and those with the first and / or second storage means stored and the data entered with the input means to alternating currents or corresponding services are billable, the temperature of the Cooking vessel accordingly influence the entered and / or saved data.

Influencing the temperature can in their predetermined reduction in determined boiling and / or determined empty or Dry cooking and / or in their adjustment or maintenance exist according to user data entered.

A cooking device according to the invention has at least one of the devices described above.

The invention is illustrated below of an embodiment explained in more detail. In the associated The drawings show:

1 2 shows the block diagram of a device for monitoring and / or controlling the temperature in an inductively heatable base of a cooking vessel, for example a pan, a deep fryer or a pot,

2 an alternating current causing inductive heating and its measurement,

3 a principle of regulation,

4 Frequency-temperature curves for different outputs,

5 basic temporal profiles of the temperature, the frequency and the first derivative of the frequency when a food to be cooked is boiling,

6 Principal temporal courses of the temperature, the frequency and the first derivation of the frequency when cooking the cooking vessel empty or when cooking a food in the cooking vessel dry.

7 a principle of temperature control ner frying pan and the like

To in a cooking vessel or one Cooking appliance one desired Setting the temperature is the output of the cooking vessel Controlled cooking zone. Therefor it is necessary to measure the temperature in the cooking pan. Usually, the user has this manually on power-controlled cooking zones Change performance level to a certain one in the cooking vessel To maintain temperature.

In the following, a method and a device are described for placing in the bottom of a cooking vessel or cooking device standing on an induction cooking zone K. 8th Measure temperatures T or temperature changes and use the information to regulate the power P of the induction cooking zone K, so that automatic cooking functions are possible.

The resonant circuit of the induction heating system according to 1 insists on an inverter 5 and an induction coil 6 that under a cooking device or cookware 8th load-bearing surface 9 , for example a glass ceramic plate, is arranged. The bottom of the dishes 8th is ferromagnetic. In it is due to eddy currents caused by the magnetic field of the induction coil 6 be induced, heat is generated. The oscillation current (size and frequency) through the induction coil is subject to the influence of various elements of the resonant circuit, one of which is the impedance (Z = R + jωL) of the induction coil system 6 /Dishes 8th is. This impedance Z is influenced by the temperature T in the base of the dishes, in particular due to changes in the electromagnetic properties of the base material. It in turn influences the oscillation frequency f of the inverter 5 , which is ultimately used to measure the temperature T in the bottom of the dishes.

The coil current I is by means of a current transformer 7 measured. Its basic appearance shows 2 , The oscillation frequency f (20 to 60 kHz) is modulated with the rectified mains power supply. This signal leads to a frequency measurement circuit 3 , which analyzes the signal and determines the oscillation frequency f. The size of the mains voltage supply is determined by a voltage measurement circuit 10 measured and the information to a control logic 2 guided. The control logic contains control algorithms A and ensures filtering of the signal if it is noisy. The algorithm A calculates the suitable power P for the cooking zone K in order to ensure the desired functionality, based on the information about the frequency f, the voltage supply U and the user settings N (entered via the user interface). 3 ).

The operating frequency of the induction inverter 5 moves in the range between 20 and 60 kHz, depending on the performance level. The frequency f (ie, the reversal of the period of a cycle) also varies over the rectified sinusoidal period (ie, over a period of 10 ms) by approximately 10%. Therefore the frequency is measured when the mains voltage is at its maximum. A time switch with a delay time of 5 ms, which is triggered in the zero crossing of the mains voltage, triggers the frequency measurement after the delay time has expired. In 2 the triggering of the time switch is marked with ZT and the triggering of the frequency measurement with FM, the time delay signal with ZV. The signal of the current transformer 7 is sampled with an A / D converter and stored in a memory (500 samples in an interval of 666 ns result in 25-75 samples per period). From this, the period of the vibrations (6–20, depending on the frequency) is calculated and the center frequency in turn. Frequency influences by changes in the mains power supply (by the mains power supply device 4 ) by measuring the voltage (by the voltage measuring device 10 ) and offsetting with empirically determined values or formulas. The corrected and / or uncorrected frequency is then used in the algorithms to signal temperatures.

In order to maintain the relationship between the frequency f and the temperature T, a calibration curve f (T) must be recorded or registered. This is done by measuring the temperature T in the crockery bottom (using separate temperature sensors) and simultaneously registering the temperature T and the frequency f with a fixed power setting P. The measurement data can then be adapted to an accepted accuracy using a curve fitting method, for example a polynomial fitting. At the end of this section, a relation f (T) is obtained for a specific dish. However, since the frequency f depends on the power level P, all of this has to be repeated for the other power levels to be used in order to finally have a relationship f (T, P), which is stored in the control logic as a calibration curve (for a specific tableware). 4 shows a family of declining f (T) calibration curves for outputs P1, P2, P3 and P4.

If these dishes (pan, saucepan, Deep fryer and the like) later is used, the control logic connects the measured frequency f with a temperature T for a certain level of performance P. This then makes it possible to desired Functions according to the algorithms stored in the control logic perform.

This procedure requires identification of the dishes used 8th ,

There are differences between different ones Pans / pots with respect to the f (T, P) behavior, both the relative level as well as the rise or the first derivative f '(t) at a certain temperature can vary.

Assuming the pan is too Start is at room temperature T-start, to which a start frequency f-start is assigned, the measured value is expressed as a relative frequency f-rel = f / f-start, so that the deviations between different pans due to the relative frequency level can be avoided. In any case, the value f-rel for 250 ° C is approximately 0.83 for a wide register of pans, which can be recommended to the user when using this method.

In the following, the tax or Control algorithms A described.

In order to recognize boiling or boiling, it is not necessary to know the absolute value of the temperature T, ie to have a relationship between the frequency f and the temperature T. Control algorithm A monitors the change in frequency response f (t) over time t ( 5 ). If the first derivative f '(t) changes from a negative value to approximately zero, the bottom of the pot has reached a constant temperature, so that the time t at which the food is boiling or boiling can be derived. In this case, the power P or the current I in the induction coil 6 reduced to a predetermined value. Without prejudice to that 5 that a temperature curve T (t), which changes its previously linearly increasing curve in the area of the boiling point to an approximately constant thereafter, a frequency curve f (t) that changes its previously approximately linearly decreasing curve in the area of the boiling point to an approximately constant thereafter permanent course changes, is assignable and vice versa.

While cooking empty or dry ( 6 ) a drop in frequency f occurs when, for example, water has escaped from the pot. This can be recognized as a sudden drop or a sudden change in the first derivative f '(t) or as a specific value of the first derivative f' (t), corresponding to a temperature rise in an empty pot. For slow dry cooking processes (food with a low or medium water content) the temperature T does not rise as quickly. Then the stable frequency level during boiling (cooking) can be used as a reference level to carry out a calibration with respect to 100 ° C. When the water has escaped, the frequency drops and the power of the cooking zone is switched off at a certain relative value f1 with respect to 100 ° C.

Without prejudice to that 6 that a temperature profile T (t) which changes its profile from a previously approximately linearly increasing to a subsequently approximately constant profile in the area of the boiling point and its previously constant profile to a subsequently linearly increasing profile in the area of cooking empty or dry, a frequency profile f (t) can be assigned, which changes its previously linearly decreasing profile to a subsequently approximately constant profile in the boiling point and its previously approximately constant profile to an approximately linearly decreasing profile in the area of cooking empty or dry. Both in the T (t) course and in the f (t) course, the fast empty or dry cooking is marked with B1 and the slow empty or dry cooking with B2. In the course of f '(t), S indicates the span f' (t), which corresponds to the heating of an empty pot.

When regulating the temperature of a pan or a pot to a constant temperature according to a user setting N (particularly used for frying pans, frying and deep-frying in oil), a number of performance levels are used. According to what has been described above, calibration curves f (T, P) are for this 4 for the performance levels intended for use.

The temperature control could then be carried out, for example, as a thermostat function. The power of the cooking zone can be set to two different levels P-high and P-low (for example P1 and P2), which are connected to two calibration curves f (T, P-high) and f (T, P-low). The frequency f is then measured continuously and the temperature T is derived from the calibration curves, depending on which power level is used. If the calculated temperature T-meas is higher than the set temperature T-set, the lower power P-low is supplied to the cooking zone. If T-meas is smaller than T-set, the cooking zone receives the higher power P-high ( 7 ). In this way, the pan or pot is kept at a clearly constant temperature level.

Overall, frequencies and / or frequency ratios and / or frequency changes and / or frequency profiles and / or their first derivatives of temperatures and / or temperature relationships and / or temperature changes and / or temperature profiles and / or assign their first derivatives and / vice versa.

The user is thus in the manner according to the invention device properties to disposal, which is both functionality as well as positively affect security. These characteristics are generally:

• - Boiling detection: The Performance becomes low inertia reduced to a predetermined level or switched off when the The contents of the cooking vessel boil or boils. The fast boiling detection also works as overcooking protection.
• - empty or dry cook detection: The performance becomes low inertia switched off when the water escaped from the cooking vessel and / or the temperature the cooking container is too high. Fire and vascular damage avoided.
• - temperature control: The Temperature becomes low inertia kept at a constant temperature level once set, without the user having to intervene manually after the first setting got to. This can be done especially use to
• - one stable temperature for to get the roast
• - one stable temperature during of deep frying in oil (at about 180 ° C, for example) to get what's also called overheating protection acts, and
• - the Limit temperature to lower levels to prevent burns and stick to the bottom of the vessel avoid.

Claims (21)

Process for the thermal monitoring of an inductively heatable Cooking vessel through monitoring the frequency (f) of the alternating current causing inductive heating (I). The method of claim 1, wherein the monitoring in particular qualitative or sudden changes in the temperature-time profile (T (t)) as surveillance in particular qualitative or sudden changes in the frequency-time curve (f (t)) and / or changes and / or absolute values of the first derivative (f '(t)) of the frequency-time profile (f (t)) he follows. The method of claim 1 or 2, wherein the monitoring on the boiling or boiling of a food in the cooking vessel and / or on the boiling of the cooking vessel or the dry cooking of the food in it as monitoring of the frequency-time curve (f (t)) to predetermined qualitative or sudden changes and / or for predetermined changes and / or absolute values of the first derivative (f '(t)) of the temperature-time profile (f (t)) he follows. The method of claim 3, wherein the monitoring on boiling or boiling as a monitor - on a predetermined qualitative or abrupt change in the frequency-time curve (f (t)) from the direction of an essentially linearly decreasing frequency (f) towards a substantially constant frequency (f) and / or - on a predetermined change the first derivative (f '(t)) of the frequency-time curve (f (t)) from the direction of a negative Value in the direction of a value of approximately zero and / or - on one predetermined value of the first derivative (f '(t)) of the temperature-time profile (f (t)) he follows. The method of claim 4, wherein the alternating current (I) or the corresponding power (P) to a predetermined Value is reduced if one or more changes occur during monitoring were determined which correspond to one or more predetermined changes. The method of claim 3, wherein the monitoring on empty or dry cooking as a monitoring - on a predetermined qualitative or abrupt change in the frequency-time curve (f (t)) from the direction of a substantially constant frequency (f) towards a substantially linearly decreasing frequency (f) and or - on a predetermined change the first derivative (f '(t)) of the frequency-time profile (f (t)) from the direction of a value of about zero towards a negative value and / or - on one predetermined value of the first derivative (f '(t)) of the frequency-time profile (f (t)) he follows. The method of claim 5, wherein the alternating current (I) or the corresponding power (P) is switched off when when monitoring a or more changes were determined which correspond to one or more predetermined changes. Method according to one of claims 3 to 7, wherein the predetermined (s) qualitative or abrupt change (s) in the frequency-time curve (f (t)) in or immediately after the transition from the declining Frequency response (f (t)) into the constant frequency response (f (t)) and / or from the constant frequency response (f (t)) into the falling frequency response f (t) lie. Method according to one of claims 1 to 8, wherein the monitoring to temperatures (T) by assigning predetermined frequencies (f) and / or frequency ratios (f / f-start) at predetermined temperatures (T) and / or vice versa. The method of claim 9, wherein the assignment the frequencies (f) and / or frequency ratios (f / f-start) to the Temperatures (T) depending of predetermined properties, in particular material properties, of the cooking vessel. The method of claim 10, wherein the properties the cooking vessel for use specified or queried by the user, for example by Specification of a selection list. Method according to one of claims 9 to 11, wherein the assignment the frequencies (f) and / or frequency ratios (f / f-start) to the temperatures (T) depending of predetermined powers (P). Method according to one of Claims 9 to 12, in which the temperature of the cooking vessel is set to a desired value or is kept at a desired value by - the frequency (f) being monitored continuously, - the frequency (f) and / or the frequency ratio (f / f-start) a temperature (T) is continuously assigned as a function of the currently assigned power (P) and the currently assigned cooking vessel, - the deviation between the assigned and the desired temperature (T) is continuously determined and - accordingly the deviation one of the assignable services is selected. Device for the thermal monitoring of an induction heatable cooking vessel, in particular for carrying out the method according to one of claims 1 to 13, with - first means ( 4 . 5 ) to provide an alternating current (I) which induces induction heating, - second means ( 3 ) to determine the from the cooking vessel ( 8th ) influenced frequency (f) of the alternating current (I) and - third means ( 2 ) for signaling and / or controlling the temperature (T) of the cooking vessel ( 8th ) depending on the frequency (f) of the alternating current (I). The apparatus of claim 14, wherein the third means ( 2 ) have first storage means with which predetermined qualitative or abrupt changes in frequency-time profiles (f (t)) and / or predetermined changes and / or absolute values of first derivatives (f '(t)) of frequency-time profiles (f ( t)) can be stored which, as reference data, in particular involves boiling or boiling one in the cooking vessel ( 8th ) the food to be cooked and / or boiling the cooking vessel empty ( 8th ) or represent dry cooking of the food in it. Apparatus according to claim 14 or 15, wherein the third means ( 2 ) have second storage means with which predetermined frequency-temperature profiles (f (T)) and / or frequency ratio temperature profiles (f / f-start (T)) can be stored. Device according to Claim 16, in which the frequency-temperature profiles (f (T)) and / or frequency ratio-temperature profiles (f / f-start (T)) per cooking vessel property and / or power (P1, P2, P3, P4) can be stored. Device according to one of claims 14 to 17, in which the third ( 2 ) Means input means ( 1 ) with which the user can select the desired temperatures (T) and / or programs, in particular cooking programs containing temperatures (T), and / or outputs (P) and / or cooking vessels ( 8th ) or enter their properties. Device according to one of Claims 15 to 18, in which the third means ( 2 ) Have computing means with which the second means ( 3 ) determined frequencies (f) and the data stored with the first and / or second storage means and the data with the input means ( 1 ) entered data can be offset against alternating currents (I) or corresponding powers (P), which the temperature (T) of the cooking vessel ( 8th ) according to the entered and / or saved data. Apparatus according to claim 19, wherein the influencing the temperature (T) in its predetermined reduction when determined Boiling and / or determined empty or dry cooking and / or in their setting or maintenance accordingly entered User data exists. cooker with at least one device according to one of claims 14 to 20th