Method for humidity-emission control of a microwave oven, and microwave oven with humidity-sensor control according to the method

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INVENTOR INVENTOR

Whirlpool Europe BV, Veldhoven NL Mats Gunnar Idebro, Linköping SE

OMBUD AWAPATENT AB

NAME Procedure for moisture delivery control of a microwave oven and microwave oven with moisture sensor control according to the procedure PUBLICATIONS QUOTED: - - SUMMARY:

In a microwave oven, a cooking / heating process is controlled by a food product using one or more moisture sensors (11), which sense the moisture output from the food or dish. The control is based on detecting relative humidity changes and requires access to a starting or reference moisture value, which is determined by a calculation process, which eliminates the effect of residual residual moisture in the oven and at the same time saves time at the start of the cooking / heating process.

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The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code.

50.2 880

The present invention relates to a method of a microwave oven for controlling a cooking or heating process for a food or dish by sensing the moisture output from the food or dish, the oven comprising an oven cavity, a microwave source, a control unit for controlling the supply of the microwave oven, for returning a sensed moisture value as a control parameter to the controller, and a device controllable for ventilating the cavity. The invention also relates to a microwave oven which is arranged to operate according to the method.

Microwave ovens with one or more moisture sensors to sense the moisture output from a food or dish and thereby control a cooking or heating process, e.g. thawing of a frozen food, is known in the art. Examples of such microwave ovens are found in patent SE 8604868-3. Common in these contexts is that the humidity in the ventilation air from the cavity is used as a control parameter. Since the humidity of the ventilation air is affected by the humidity of the ambient air, which in turn varies in the environments where the microwave oven is used, the humidity change during the cooking or heating process is usually used as a control parameter. This type of humidity control requires that a relevant reference value for the humidity at the start of the process is available in the microwave oven control unit. An incorrect moisture reference value results in a degraded cooking result.

One method of accessing a moisture reference value is to measure the humidity of the ventilation air at the start of the cooking or heating process. An example of utilization of this method can be found in

502 8.80 patent EP 0.289.000, where in the program flow diagram of Fig. 13 one begins by sensing an initial value of the humidity. However, this initial value can be misleading if the oven cavity is not dry at start-up but contains residual moisture and condensation from previous cooking processes. The humidity sensor will then sense a humidity value which is primarily a combination of residual moisture and the ambient air moisture value. Using this combined moisture value as the initial moisture value for the moisture control gives incorrect information to the controller and adversely affects the cooking.

Another known method involves using the lowest measured humidity value during the heating process as the moisture reference value. Examples of this method are found in the patent EP 0.078.607, in particular Figures 4A-4D. The problem then is that one can obtain an incorrect lowest humidity value if the moisture output from the dish or food, placed in the cavity for heating, begins even before the remaining residual moisture has been vented. Even in this case, the cooking results in a deteriorating result.

These deficiencies of the known ovens could be avoided by activating the oven's ventilation device before starting the cooking / heating process.

ie normally a fan, for expansion of the cavity and to allow the expansion to continue until the humidity has ceased to decrease and assumed a stable minimum value. However, such a solution would result in an extension of the cooking / heating process which can vary from approx. 20 sec. up to 3 min. This longer time is experienced negatively by the user.

The object of the invention is to eliminate the relevant disadvantages of the known microwave ovens without adding any new disadvantage in the form of extra time required for the cooking / heating process and at the same time enabling the determination of a relevant moisture reference value.

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The object of the invention is achieved by a method of the type described initially, which according to the invention is characterized by the measures: activating the ventilation device for initial dilution of residual moisture in the furnace, calculating an expected humidity final value of residual moisture during the initial dilution based on sensed moisture values ​​and a moisture calculation algorithm, and using the calculating humidity final value as the use of the derived heat input / output value at the execution of moisture sensor.

The method according to the invention involves a time saving compared to the time required for complete dilution of all residual moisture in the oven. The time savings are achieved by calculating an expected final value of the humidity, ie the value that would have been achieved if the expansion had to continue. This calculation is based, inter alia, on on the realization that the residual moisture in the cavity decreases exponentially with respect to the elongation time. By using such a calculated humidity end value as the moisture reference value for the subsequent process, an improved cooking result is achieved without noticeable extra time.

A microwave oven according to the invention for cooking or heating a food or dish, and comprising an oven cavity, a microwave source, a control unit for controlling the microwave supply to the cavity, a moisture sensor for sensing the moisture output from the food or dish and returning a sensing feed to a sensing feed the control unit; characterized in that the control unit is arranged to activate the ventilation device for initial evaporation of residual moisture from the furnace at start-up of a moisture sensor controlled cooking / heating process, that calculating means are arranged to calculate an expected humidity final value for the residual humidity during the initial evacuation, and

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the control unit is arranged to use the calculated humidity end value as the moisture reference value for the humidity sensor control.

The controller in such a microwave oven usually includes a microprocessor which may be programmed to execute one or more automatic cooking programs. The merit effort to implement the invention in such a microwave oven is therefore limited to a somewhat expanded control program for the microprocessor. This, in turn, means that the microwave oven according to the invention enables an improved cooking result without significant increase in the oven's production cost in longer series.

Further features of the method and microwave according to the invention are set forth in the appended claims.

In the following, the method of the invention is described in connection with a non-limiting embodiment of a microwave oven with reference to the drawings, in which:

Fig. 1 shows a block diagram of the affected functional units in the microwave oven,

Fig. 2 shows a moisture-time diagram illustrating the time course of the residual moisture and the moisture output from a food or dish, and

Fig. 3 shows a program flow diagram for the moisture sensing control method according to the invention.

Since the detailed mechanical structure of the microwave oven is of no significance for a closer understanding of the invention, this is not disclosed here other than by reference to the microwave-manufactured ovens available on the market, e.g. oven types VIP20 and V27 and the patent applications filed by the applicant with no. 9402061-7, 9402062-5 and 9402063-3.

The block diagram shown in Fig. 1 comprises the controller 1 with the microprocessor and associated program memory 2. The input of user information to the controller is via the block 3, which represents control means which

502 880 is located on the microwave control panel, which also includes a display 4, which is controlled by the controller. The display 4 can, for example, be used to return to the user information about the choices made on the control panel, as well as information about an ongoing cooking / heating process, e.g. remaining time.

Via a drive stage 5 and a microwave power unit 6, the microwave source 7 is controlled by the control unit 1 and thereby the microwave supply to the oven cavity 8. Via a drive stage 9, a grill element 10 arranged in the ceiling of the cavity is controlled and thereby the IR radiation in the cavity 8. the functional units represented are referred to the microwave oven types specified above by the applicant.

The residual moisture at the start of the microwave oven as well as the moisture output from a food or dish during cooking is sensed by a moisture sensor 11 which is arranged in the evacuation duct for the ventilation air from the cavity 8. The sensing information from the moisture sensor 11 is returned as a control parameter to the control unit 1.

In the humidity-time diagram of Fig. 2, the vertical axis represents the humidity h and the horizontal axis the time t. Curve 2 represents moisture release from a food item or dish during heating in the cavity. Initially, the moisture value of the curve 2 is at a constant level equal to the moisture value of the ambient air. At the time marked by arrow 4, the heating process is started and some time thereafter, curve 2 shows a knee at point c, which indicates that the moisture release from the food or dish starts. As can be seen from the diagram, the decreasing residual moisture curve 1 has a flattening process and finally assumes a value that corresponds to an initial constant moisture level for the curve 2, ie the ambient air moisture value. Curve 3 represents the utility signal sensed by

502 880 moisture sensor. Curve 3 corresponds to knee point c in curve 2 with decreasing curve 1 for residual moisture, and after said knee point with the sum of the relative contributions, relative to the ambient air moisture level, from the increasing moisture output from the food and the contribution from the declining residual moisture level.

The diagram in Fig. 2 illustrates the measurement errors that occur when using prior art according to the initially referenced patents, ie in the case of EP 0.289.000, the value of the residual moisture curve at point a would be recognized as the incorrect reference value and in the application of EP 0.078.607 the value of the residual moisture curve in point b would be sensed as an incorrect lowest moisture value.

In applying the invention, an expected final value of the residual moisture is calculated which corresponds to the decaying level of the residual moisture curve 1, ie a value which substantially corresponds to the humidity level of the ambient air. Even when applying such an improved reference value according to the invention, the humidity sensor will initially, after starting the process, sense an excessively high moisture level until the residual moisture has decayed and no longer makes measurable contributions. According to the invention, this measurement error can be eliminated by correcting the sensed moisture values ​​with respect to the relative contributions of residual moisture. In principle, this can be done with arbitrary accuracy since the exponentially decreasing residual moisture curve 1 can be calculated with knowledge of two points on the curve. However, a more practical way of eliminating this measurement error is to introduce a correction based on the residual moisture curve derivatives at the start time at arrow 4 multiplied by the time to the time of the moisture sensor sensing.

The program flow diagram shown in Fig. 3 illustrates the calculation of a final value for residual moisture in the oven and its determination as moisture reference value for a subsequent moisture sensor controlled cooking process. The program goes through the following steps, where Y stands for yes and N stands for

502 880 for no:

sl. Launch the program.

s2. Start the ventilation fan.

s3. Wait 2s.

s4. Does the humidity level increase? If Y, return to s3. If N go to s5.

s5. Choose a predetermined first execution time for the calculation. Go to s6.

s6. Use the selected first execution time, or an estimated expected value of the remaining execution time from s9. Count down the implementation time to zero. Calculate the humidity time derivatives. Go to s7.

S7. Every second; Counted employee time 4 0? If Y, go to slO. If N, go to s8.

S8. Countdown time / 4 = integer? If N, return to s6. If Y, go to p9.

s9. Calculate the expected value of the remaining execution time. Return this to s6.

SLO. Calculate the expected humidity end value.

Select the humidity end value as the reference value for the humidity sensor control. Go to sll.

socket. Exit the program.

As the first execution time, a predetermined value is selected, e.g. 100 s, of sufficient size for the program to enter its main loop, ie steps s6, s7, s8, s9.

The step s8 is based on the chosen assumption that a new calculation of the expected value of the remaining execution time in the step s9 takes place every four seconds. Choosing another calculation periodicity is of course possible.

The calculation of the expected value of the remaining execution time in step s9 is based on the sensed value of the humidity's time derivatives, counted down time and a time calculation algorithm as follows:

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IT = c »(a / Dt + B · | dh (t) / dt |) where

IT is the calculated expectation value of the remaining execution time h (t) is the time function of the decreasing residual moisture

It is counted some time since step s5, ie when the humidity has stopped increasing a, b, c are empirically determined constants

For each calculation, the remaining execution time receives an ever lower value, depending on the total time Dt and the planar residual moisture curve's smaller derivatives.

An expected final value of the residual moisture is calculated on the basis of sensed humidity, the current expected value of the remaining time, the last sensed value of the moisture derivative, and the use of the following moisture calculation algorithm:

EH = HO - f · | dh (t) / dt | · (E · IT - d) there

EH is the calculated expected final value of the humidity

HO is the humidity at the time of computation IT is currently calculated expectation value of the remaining lead time d, e, f are empirically determined constants h (t) is the time function of the decreasing residual moisture.

The above-mentioned time function for calculating the moisture derivative can in a practical application be simplified by calculating the time derivative as the decrease of the humidity signal during the last for example 10 s. In the algorithms above, then dh (t) / dt is replaced by Ah / At.

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It will be appreciated that it is within the skill of the person skilled in the art to propose modifications to the process described above within the scope of the invention. Such a modification can mean that the described microprocessor program flows are realized through so-called Fuzzy Logic decisions. Any necessary changes to enable the use of Fuzzy Logic are skilled in the art and are considered to be within the scope of the subsequent claims.

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