Method for controlling a microwave oven, microwave oven and its use for cooking or heating food in accordance with the method

Abstract: A method for controlling a microwave oven, a microwave oven and its use for automatic cooking or heating of food are disclosed. Food-category information, which is based on the type of food and its initial state, is supplied to the oven for the selection of a corresponding preprogrammed control program. The control program is divided into time periods, during which the microwave radiation source of the oven, a microwave-activable crisp plate (4) and a grill element, if any, are activated at power levels determined by the program. The implementation of the program is monitored by one or more sensors for sensing humidity emission or other substance emission from the food and/or the temperature of the food.

Description: This invention relates to a method for controlling a procedure for cooking or heating food in a microwave oven by using the microwave radiation source of the oven and a microwave-activable bottom heater in the form of a receptacle, a plate or a carrier on which the food is placed, which act as separate heat sources for the food that can be controlled via the control unit of the oven. The invention further concerns a microwave oven for implementing the method, as well as the use of such a microwave oven for cooking or heating various sorts of food by automatically-controlled procedures. The microwave radiation source and the associated microwave-feed system generate a microwave distribution in the oven cavity that brings about so-called volume heating of the food. The microwave oven may comprise an additional source of heat for the food in the form of a microwave-activable, so-called crisp or browning plate. A crisp plate may consist of an aluminium plate which has small thermal mass and good thermal conductivity and on whose underside is provided a microwave-absorbing layer. SE Patent Specification 9003104-8 discloses such a crisp plate. A browning plate has larger thermal mass and may consist of a plate of ceramics or glass provided with a microwave-absorbing layer. In general, both the crisp plate and the browning plate are arranged to rotate during cooking or heating. The microwave oven may have yet another source of heat in the form of a so-called "grill element" or an equivalent IR-radiation element, which usually is arranged in the ceiling of the cavity (see, for instance, SE Patent Specification 9201786-2). A microwave oven equipped with these three types of heat sources basically has access to a volume heater, a bottom heater and a top heater, in which case the heat emission from the two former is controlled by regulating the microwave feed to the cavity, and the latter is controlled directly as regards activation time and power level. In order to optimise the heating or cooking of a specific type of food, the three heat sources have to be used in an appropriate manner, i.e. be activated at adjusted power levels for a suitable period of the cooking or heating procedure. However, the problem is that the activation times and power levels for different sorts of food vary considerably. For the average user, this is much too complicated, and he therefore is usually not given the opportunity to control the cooking or heating procedure in this fashion. Microwave ovens are instead so constructed that there is only one power balance, determined by the manufacturer, between the different sources of heat. In some countries, the power consumption of apparatus with single-phase connection is restricted, which among other things means that the microwave radiation source and the grill element cannot be activated at the same time. In turn, this places special requirements on the utilisation of the heat sources depending on the type of food at issue, and tends to complicate matters further for the user. One object of the invention is, therefore, to provide a microwave oven which does not suffer from any of the drawbacks mentioned above, enables advantageous utilisation of the heat sources, and at the same time is more user-friendly. By optimal utilisation of the heat sources, the microwave oven becomes more useful in general, so that it may serve to cook or heat also such food and dishes as today should be heated in conventional ovens, e.g. in order not to lose their crispiness. According to the invention, this object is achieved by a method which is of the type mentioned by way of introduction and is characterised by the steps of supplying the oven with food-category information on the food, selecting a cooking or heating program, divided into one or more time periods, on the basis of the food-category information supplied, activating, during each time period, said heat sources at power levels in accordance with the selected program, calculating, repeatedly during each time period, a control variable on the basis of one or more sensing parameters obtained from one or more sensors for sensing the state of the food, and on the basis of parameter information determined by the food category, decrementing the time periods to zero from a maximum value determined by the food category, after starting the program, ending each time period when it has been decremented to zero or when the calculated value of the control variable exceeds a value determined by the food category, and ending the program when all the time periods have been run through. The method according to the invention drastically facilitates handling of the microwave oven, since the user merely has to input information on the food category involved, e.g. via the control panel of the oven. Thereafter, the oven may be started automatically or by operating a starter button. The oven then selects an appropriate program in accordance with the food category and monitors the cooking or heating procedure fully automatically. By food category or food-category information is here meant information corresponding to a cooking or heating program specially designed for a specific type of food, such as potato products, pizza and chicken, and the state of the food when put in the oven, such as fresh or deep-frozen raw products, and deep-frozen semi-processed products. The food-category information can be supplied by inputting e.g. a numerical value indicating the type of food and its state, or by inputting a first value indicating the type of food and a second value indicating its state. Preferably, the food-category information is inputted by operating a control button specific for the food category, or by operating specific buttons for the type of food and the initial state thereof. A microwave oven according to the invention comprises a cavity, a microwave radiation source for feeding microwaves to the cavity, a microwave-activable bottom heater in the form of a receptacle, a plate or a carrier on which the food is placed for heating or cooking, a control unit for controlling the bottom heater and the feeding of microwaves to the cavity, and one or more sensors for sensing, and supplying to the control unit, one or more sensing parameters indicating the state of the food, and is characterised in that it has means for supplying the control unit with food-category information on the food, and that the microprocessor of the control unit is programmed to implement the following functions: selecting a cooking or heating program, divided into one or more time periods, on the basis of the food-category information supplied, activating the microwave radiation source, and hence the bottom heater, at power levels in accordance with the selected program, calculating, repeatedly during each time period, a control variable on the basis of parameter values obtained from said one or more sensors, and on the basis of one or more associated constants determined by the food category, decrementing the time periods to zero from a maximum value determined by the food category, after starting the program, ending each time period when it has been decremented to zero or when the calculated value of the control variable exceeds a value determined by the food category, and ending the program when all the time periods have been run through. In a preferred embodiment of the invention, the microwave oven is provided with a top heater, for instance a grill element arranged in the ceiling of the cavity, which is controlled by the selected program. The design of the inventive microwave oven is based on the insight that the average user mostly employs the oven for cooking or heating certain kinds of food, e.g. heating deep-frozen ready-cooked dishes or bread and potato products, and that this food can be divided into food categories on the basis of the type of food and its initial state. Furthermore, it has been found that optimum utilisation of the heat sources available in a modern oven, i.e. the microwave field in the cavity, the bottom-heating crisp plate and the top-heating grill element, may improve the cooking results and make it possible to use the oven for a broader range of food or dishes, the user-friendliness of the oven being maintained or even improved. According to the invention, the use of a microwave oven, which has a rotary crisp plate arranged in the oven cavity, a grill element arranged in the ceiling of the cavity, and sensors for sensing substance emission from the food as well as temperature values indicating the state of the food, is distinguished by the fact that the food is cooked or heated automatically according to preprogrammed cooking or heating programs that can be selected on the basis of food-category information based on the type of food and its initial state and supplied to the oven. Further distinctive features of the method and the microwave oven according to the invention are recited in the appended claims. A preferred, non-restricting embodiment of the invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 shows an inventive microwave oven whose door is open; Fig. 2 is a block diagram illustrating cooperating functional units of the oven during implementation of the inventive method; and Fig. 3 is a flow chart illustrating a preprogrammed cooking or heating program for controlling the microprocessor of the control unit. The microwave oven illustrated in Fig. 1 has an external casing 1, and an oven door 2 for closing the cavity 3, in which a crisp plate 4 is arranged. The crisp plate 4 is made of sheet aluminium of small thermal mass, and its underside is provided with a microwave-absorbing layer of rubber-borne ferrite. Preferably, the ferrite material employed has a Curie point at which ceases the layer's energy absorption from the microwaves, which means that the temperature of the upper side of the crisp plate that comes into contact with the food stabilises in a temperature range of 130-230°C. The crisp plate is adapted to rotate during the cooking or heating procedure. SE Patent Specification 9003104-8, as well as the applicant's microwave ovens of type designations VIP20 and VIP27, illustrates a conceivable construction of the crisp plate as well as its rotary mechanism. The microwaves are fed to the cavity 3 through one or more feed openings (not shown) which communicate via wave guides with the microwave radiation source 20 (normally a magnetron) of the oven (see Fig. 2). In the illustrated oven, the magnetron, the associated wave guide system, the power unit 19 for operating the magnetron, and the control unit 15 are disposed behind the control panel 5. In a preferred embodiment of the microwave-feed system, use is made of an upper and a lower feed opening, which are provided in the right-hand lateral wall of the cavity, whereas the remainder of the feed system is designed to feed polarised microwaves through these openings. For more detailed information on the construction of the microwave-feed system, reference is made to SE Patent Specification 9003012-3, as well as to the applicant's microwave ovens mentioned in the foregoing. A grill element (not shown) is arranged in the ceiling of the cavity, e.g. in the manner described in SE Patent Specification 9201786-2. The grill element may be a so-called "grill tube", a quartz tube or a halogen-radiation source. Instances of concrete designs are found in the applicant's microwave oven of type designation VIP20. The control panel 5 has a display 6 which, controlled by the control unit 15 (see Fig. 2), shows, among other things, symbols or plain-text messages for selected programs, and remaining cooking or heating time, i.e. verifies the user's selections made via the control panel as well as provides other information on how the cooking or heating proceeds. The control button 7 is used for selecting heating through the feeding of microwaves to the cavity. The button 8 is used for activating the grill element of the oven, and the desired time is set by the knob 9. These basic options have their equivalence in a conventional oven and supplement the inventive option to select from a given number of preprogrammed cooking or heating programs for specific types of food while utilising the interplay of the direct-acting microwaves, the crisp plate and the grill element. For instance, the removable crisp plate 4 may be replaced with an ordinary rotary bottom plate in the case heating involves direct-acting microwaves only. The buttons 10 and 11 are provided for, respectively, starting and switching off the oven. The keyset 12 serves to input the food-category information and, hence, to select a preprogrammed cooking or heating program. For instance, the buttons may, from top to bottom, concern the heating of deep-frozen pizza, the heating of deep-frozen pie, the heating of deep-frozen potato products, such as chips, and the heating of deep-frozen chicken parts. The operation of a button entails simultaneous input of complete food-category information, i.e. information on the type of food and its initial state (e.g. deep-frozen). In a modified embodiment of the oven, the buttons are divided into two sets, in which case the information on the type of food is inputted via one set and the information on the initial state of the food is inputted via the other set. In the simplest embodiment of the oven, only one food category is used, which means that only one button is required. All the buttons, as well as the knob and the display, are in communication with the control unit 15. On the upper side of the oven, there are provided ventilation holes 13 communicating with the evacuation channel (not shown) of the cavity that is disposed in the space between the ceiling of the cavity 3 and the external casing 1. A humidity sensor and a temperature sensor are arranged in the evacuation channel to sense the humidity and the temperature of the ventilating air, these bearing a direct relation to the state of the food being cooked in the cavity. In general, the humidity sensor may be a sensor capable of sensing substance emission from the food, humidity, gas and so forth (see SE Patent Specification 9201314-2). Choosing the sensors and positioning them in the evacuation channel are but measures of convenience, which will not be described in more detail here. In view of single-phase connection to the mains, the oven has a flex 14 with a plug. The block diagram of Fig. 2 shows the control unit 15 and a microprocessor and its associated program store 16. The user information is inputted to the control unit via the block 17, which represents the control buttons and the knob described above. The control unit controls the display 6. Via a driver 18 and a microwave power unit 19, the control unit 15 controls the microwave radiation source 20, and hence the feeding of microwaves to the cavity 3. Via a driver 21, the control unit 15 controls the grill element 22, and hence the IR radiation fed to the cavity 3. From the cavity, information is transmitted to the control unit 15 via the humidity sensor 23 and the temperature sensor 24 which sense the air humidity and the temperature, respectively, of the ventilating air in the evacuation channel of the cavity. Thus, the control of the cooking or heating procedure is performed by complete feedback, so that the sensitivity of the cooking or heating procedure to variations in food raw products, starting temperature and mains voltage is minimised. For more detailed information on the construction of these functional units, reference is made to the above-mentioned patents and microwave ovens manufactured by the applicant. In Fig. 3, the flow chart illustrating a preprogrammed cooking or heating program comprises three successive time periods T1, T2, T3. Each period is assigned a maximum duration determined by the food category. During the respective periods, the feeding of microwaves and, hence, the crisp plate as well as the grill element are activated at power levels determined by the food category. The power levels for, respectively, the microwaves and the grill element during the pertaining periods are designated, respectively, PM1, PM2, PM3 and PG1, PG2, PG3. On the basis of information obtained from the humidity sensor and the temperature sensor, a control variable c is continuously calculated during the respective periods as follows during T1 c = a1 • humidity increase + b1 • temperature during T2 c = a2 • humidity increase + b2 • temperature during T3 c = a3 • humidity increase + b3 • temperature wherein a1, a2, a3 and b1, b2, b3 are constants determined by the food category. During the respective periods, the period time is decremented towards zero. When the time has been decremented to zero or when the control variable exceeds a value c1, c2 and c3, respectively, determined by the food category, the period is ended and the next is begun. When the period T3 has been run through, the program is ended. The power levels PM1, PM2, PM3 and PG1, PG2, PG3, respectively, are optimised for each food category and may, in a further development of the oven, vary during the time periods. Likewise, all the constants a, b, c are optimised for the respective food categories. The supplied microwave power may be distributed between different feed openings to enable energy to be distributed between bottom heat from the crisp plate and microwave heating of the interior of the food. Such distribution can be achieved by using a microwave-feed system of the type described in SE Patent Specification 9302302-6. The program illustrated in the flow chart of Fig. 3 runs through the following steps, where Y represents "yes" and N represents "no". s1 Start. s2 Setting of period times T1, T2, T3 on the basis of the food category. s3 Microwaves at the power PM1. Grill on the power PG1. Decrementation of T1. s4 T1 = 0? If Y, proceed to s6. If N, proceed to s5. s5 Calculation of c. c ≥ c1? If N, return to s3. If Y, proceed to s6. s6 Microwaves at the power PM2. Grill on the power PG2. Decrementation of T2. s7 T2 = 0? If Y, proceed to s9. If N, proceed to s8. s8 Calculation of c. c ≥ c2? If Y, proceed to s9. If N, return to s6. s9 Microwaves at the power PM3. Grill on the power PG3. Decrementation of T3. s10 T3 = 0? If Y, proceed to s12. If N, proceed to s11. s11 Calculation of c. c ≥ c3? If Y, proceed to s12. If N, return to s9. s12 Ending of the program. Step s13 concerns a possible further development of the control program, in the event that the microwave oven is provided with means for supplying the control unit 15 with information on the weight of the food. For instance, this can be done by so arranging the knob 9 in Fig. 1 as to serve also as means for setting the weight of the food when use is made of the automatic control programs according to the invention. Alternatively, the oven may be provided with a weight sensor 25 (see Fig. 2) adjacent to the rotary crisp plate, e.g. of the design described in Patent Application 9300291-3. On the basis of the weight information w, the microprocessor calculates the expected duration T1e, T2e, T3e of the respective time periods of the program as follows T1e = k1 • w T2e = k2 • w T3e = k3 • w wherein k1, k2, k3 are constants depending on the food category. It will be appreciated that those skilled in the art are well qualified to further develop the described program within the scope of the invention. For instance, the temperature of the ventilating air may be sensed when starting the program, and the parameter information used during implementation of the program may then be corrected in view of the temperature sensed. One thus eliminates any effect of the oven being heated at the start. Another possibility would be to introduce a temperature sensor for sensing the temperature of the crisp plate and use the sensed temperature for controlling the microwave radiation source and the grill element. When implementing the method, use can be made of so-called fuzzy logic decisions.

Claims: A method for controlling a procedure for cooking or heating food in a microwave oven by using the microwave radiation source (20) of the oven and a microwave-activable bottom heater (4) in the form of a receptacle, a plate or a carrier on which the food is placed, which act as separate heat sources for the food that can be controlled via the control unit (15) of the oven, characterised by the steps of supplying the oven with food-category information on the food, selecting a cooking or heating program, divided into one or more time periods (T1, T2,...), on the basis of the food-category information supplied, activating, during each time period, said heat sources at power levels in accordance with the selected program, calculating, repeatedly during each time period, a control variable (c) on the basis of one or more sensing parameters obtained from one or more sensors for sensing the state of the food, and on the basis of parameter information determined by the food category, decrementing the time periods to zero from a maximum value determined by the food category, after starting the program, ending each time period when it has been decremented to zero or when the calculated value of the control variable exceeds a value (c1, c2,...) determined by the food category, and ending the program when all the time periods have been run through. A method as set forth in claim 1, in which the microwave oven further comprises a top heater, which is in the form of a grill element or some other equivalent electric IR-radiation element, as an additional controllable heat source, characterised by the further step of activating the top heater in accordance with the selected program. A method as set forth in claim 1, in which the program is divided into time periods T1, T2,...Tn during each of which one or more sensing parameters x1, x2,...xk are sensed, characterised by selecting for each sensing parameter, as said parameter information, an associated constant a1, a2,...an; b1, b2,...bn; k1,...kn during the respective time periods. A method as set forth in claim 3, characterised by calculating the control variable c during the time period Tn as follows c = an • xl + bn • x2 + ... + kn • xk A method as set forth in claim 4, in which the program comprises two time periods T1, T2 and in which two sensing parameters x1, x2 are sensed, characterised by calculating the control variable c as follows c = al • xl + bl • x2, during the time period T1, and as follows c = a2 • xl + b2 • x2, during the time period T2. A method as set forth in claim 1, characterised by using the humidity increase and the temperature of the ventilating air from the oven cavity as sensing parameters. A method as set forth in claim 1, in which the temperature of the ventilating air from the oven cavity is used as sensing parameter, characterised by sensing the initial temperature of the ventilating air and introducing corrections based thereon into said parameter information. A method as set forth in claim 1, in which the microwave oven further comprises means for inputting or sensing the weight of the food, characterised by calculating, with the aid of the control unit, the expected duration of each time period Ti of the cooking or heating program on the basis of information on the weight w of the food and a constant ki determined by the food category, as follows Tie = ki • w wherein i = 1,2,3,... Tie is the expected duration of the time period ki is a constant determined by the food category w is the weight of the food and using the calculated value of the expected duration of the time period for supplementary monitoring of the cooking or heating procedure. A microwave oven comprising a cavity (3), a microwave radiation source (20) for feeding microwaves to the cavity, a microwave-activable bottom heater (4) in the form of a receptacle, a plate or a carrier on which the food is placed for heating or cooking, a control unit (15) for controlling the bottom heater and the feeding of microwaves to the cavity, and one or more sensors (23, 24) for sensing, and supplying to the control unit, one or more sensing parameters indicating the state of the food, characterised in that it has means (12) for supplying the control unit with food-category information on the food, and that the microprocessor (16) of the control unit is programmed to implement the following functions: selecting a cooking or heating program, divided into one or more time periods (T1,T2,...), on the basis of the food-category information supplied, activating the microwave radiation source, and hence the bottom heater, at power levels in accordance with the selected program, calculating, repeatedly during each time period, a control variable (c) on the basis of parameter values obtained from said one or more sensors, and on the basis of one or more associated constants determined by the food category, decrementing the time periods to zero from a maximum value determined by the food category, after starting the program, ending each time period when it has been decremented to zero or when the calculated value of the control variable exceeds a value (c1, c2,...) determined by the food category, and ending the program when all the time periods have been run through. A microwave oven as set forth in claim 9, which is provided with an evacuation channel for ventilating the cavity, characterised in that a temperature sensor (24) and a humidity sensor (23) are arranged in the evacuation channel to sense, respectively, the temperature and the humidity of the ventilating air as said sensing parameters, the sensors being maintained in activated state during the cooking or heating procedure. A microwave oven as set forth in claim 10, characterised in that the microprocessor (16) of the control unit (15) is programmed to derive, from the temperature sensor (24), a value of the temperature of the ventilating air when the cooking or heating program is started, and to adjust the program in view of the temperature value derived. A microwave oven as set forth in claim 9, characterised in that the means (12) for supplying food-category information are adapted to supply a parameter which is based on the type of food and the state of the food when put in the oven, and which corresponds to a cooking or heating program stored in the control unit. A microwave oven as set forth in claim 9, characterised in that the means (12) for supplying food-category information include first means for supplying a food-type parameter and second means for supplying a food-state parameter, the combination of these parameters corresponding to a cooking or heating program stored in the control unit. A microwave oven as set forth in claim 9, characterised in that it has a control panel (5) which is provided with a display (6) adapted to show a designation, a symbol or a plain-text message indicating the cooking or heating program selected. A microwave oven as set forth in claim 9, which is provided with a top heater in the form of a grill element or some other equivalent electric IR-radiation element which is controllable via the control unit (15), characterised in that the top heater is adapted to be controlled in accordance with the selected program in respect of activation time and heating effect. A microwave oven as set forth in claim 15, characterised in that it has a rotary crisp plate (4) which is made of metal of small thermal mass and good thermal conductivity and on whose underside is provided a microwave-absorbing layer, that a grill element is arranged in the ceiling of the oven cavity, and that the oven is equipped with a microwave-feed system having a feed opening on a level with the crisp plate (4) and adapted to feed, to said microwave-absorbing layer, polarised microwaves whose E- or H-vector is directed substantially in the plane of said layer. A microwave oven as set forth in claim 9, characterised in that the control unit (15) is adapted to use the weight (w) of the food as an additional control parameter, information on the weight of the food being supplied from a weight sensor (25), which is disposed adjacent to the receptacle, plate or carrier (4), or from a means (9) for inputting a measured value of the weight, and the control unit being adapted to calculate the expected duration of a time period (T1e, T2e,...) of the selected program on the basis of the weight of the food and a constant (k1, k2,...) determined by the food category. The use of a microwave oven according to claim 9, which has a rotary crisp plate (4) arranged in the oven cavity (3), a grill element arranged in the ceiling of the cavity, and sensors (23, 24) for sensing substance emission from the food as well as temperature values indicating the state of the food, for automatic cooking or heating of the food according to preprogrammed cooking or heating programs that can be selected on the basis of food-category information which is based on the type of food and its initial state and is supplied to the oven.

Inventors: IDEBRO MATS GUNNAR, KIDBLAD INA BIRGITTA, IDEBRO, MATS GUNNAR, KIDBLAD, INA BIRGITTA, IDEBRO, MATS GUNNAR, C/O WHIRLPOOL ITALIA S.R.L., Kidblad, Ina Birgitta c/o Whirlpool Italia s.r.l.
Applicants: WHIRLPOOL EUROP, WHIRLPOOL EUROPE B.V.
Application year: 1995
Main classification: H05B6
IPC codes: H05B6