EP0093172A1

EP0093172A1 - High frequency heating device

Info

Publication number: EP0093172A1
Application number: EP82901428A
Authority: EP
Inventors: Akihiko Ueno; Kiyoshige Watanabe; Mitsuo Akiyoshi; Kenji Watanabe
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1981-11-06
Filing date: 1982-05-13
Publication date: 1983-11-09
Also published as: EP0093172A4; JPS5880427A; WO1983001674A1

Abstract

A high frequency heating device which automates cooking in combination with a microcomputer and various sensors. A heating element (15) is provided in the vicinity of a suction port (16) of a heater housing (12), and a temperature sensor (20) and a moisture sensor (19) are provided in the vicinity of an exhaust port (17). The heating element (15) is controlled in response to temperature signals from the sensor (20), thereby maintaining the exhaust temperature constant, and vapor from food is detected by the sensor (19) placed in the vapor. The detections are hardly affected by the influence of the conditions in a kitchen, and this heating device can thus cook more types of foods automatically than the conventional type.

Description

TECHNICAL FIELD

The present invention relates to a high frequency heating apparatus intended for automation of cooking by using a combination of a microcomputer and various sensors.

BACKGROUND ART

With development and cost reduction ot microcomputers and with development of various snesors such as temperature sensors and humidity sensors, microwave ovens capable of automatic cooking have made their advent and are in the limelight. Further, there is a proposal to control the finished state of food.
Above all,the humidity sensor used in microwave ovens designed for automatic cooking by detecting vapor from food, as compared with other gas sensors, has a stabilized characteristic hardly responsive to seasonings, alcohols and other impurities ditterent from vapor, but since the temperature ot the atmosphere around the humidity sensor varies with the progress of cooking, even if changes in humidity due to vapor from food can be detected, it has been impossible to detect a predetermined amount of change. In the case where the amount of vapor from food is very small as in thawing, rises in the temperature ot the atmosphere around the humidity sensor relatively lower the rate ot change of relative humidity, thus making it impossible to detect a predetermined amount of change.
There is a method wherein in an effort to eliminate these drawbacks, a wrap is applied to tood or a special lidded container is used to temporarily suppress generation of vapor from food, and at about the time of boiling, the vapor is allowed to emit at a time, and is detected to effect control. With this method, cooking takes much time and labor, and if the operator makes a mistake in handling the wrap, etc., this leads directly to malfunction. Further, since the finished state of cooking is like baking, the drawback is that the external appearance of cooked food is poor particularly in the case of meat and cake. If the humidity of the atmosphere in the room is high, water vapor trom the food causes the relative humidity to be saturated and the humidity sensor loses its function, involving the danger of overheating or carbonizing the food or of causing a tire. Further, when frozen food is thawed, it is impossible to detect the vapor, which is very small in amount, from the food, so that this method cannot be applied to thawing. Thus, it has been impossible to effect automatic thawing by heating using a humidity sensor.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide an arrangement wherein the temperature of the exhaust from the heating chamber is made constant and the humidity is detected, thereby accurately detecting the time required to reach a tixed amount of change in relative humidity due to water vapor from food, so as to effect automatic cooking and automatic thawing without using any wrap or special container for food.
Another object of the invention is to provide an arrangement wherein high frequency oscillation is intermittently controlled to control the output so as to heat a large food uniformly to the interior thereof, while the output of a heater for making constant the exhaust temperature is switched to a low output when the high frequency oscillator is oscillated at the time of intermittent control and to a high output when it is not oscillated, thereby keeping down the maximum consumption current and consumption power to cut down the cost, and wherein the apparatus is adapted to be used with a household plug receptacle and is convenient to use.
A further object of the invention is to provide an arrangement wherein the atmosphere temperature at the start of cooking is detected and one of predetermined exhaust control temperatures is selected according to the size ot said atmosphere temperature to avoid reckless use of high temperature for control, and wherein rises in the temperatures of electric parts including a magnetron are used as an auxiliary heat source to reduce the power cost.
To achieve said objects, a high frequency heating apparatus of the present invention comprises a heater installed adjacent the suction port of the heating chamber, a temperature sensor and a humidity sensor which are installed adjacent the exhaust port, wherein said heater is controlled in response to detection signals from said temperature sensor to keep the exhaust temperature at a constant value, while the vapor emitting from the food in the atmosphere adjacent the exhaust port is detected by the humidity sensor and the time required for relative humidity to reach a predetermined amount of change due to said vapor emitting from the food is calculated by a microcomputer, and said tood is cooked or frozen food is thawed according to one of heating patterns determined according to the kind of food with said time used as a function, enabling automatic cooking of not only foods in general but also frozen foods.
According to the invention, the following effects can be obtained.

(1) Because ot relative humidity detected with a constant temperature, the amount of change of relative humidity due to vapor from food can be accurately found and therefore by measuring the time required to reach a predetermined amount of change, it is possible to know the weight and volume of the food. Thus, with this time as a function, cooking foods according to respective heating patterns based on the kind of tood enables automatic cooking which provides satisfactory finish.
(2) In the case of thawing frozen food, since the apparatus is responsive to even a very small amount of vapor emitting from the food, the frozen food can be satisfactorily thawed without overheating the same.
(3) Since the exhaust temperature is kept constant, the operation-is hardly intluenced by the environment of the room. Since the suction port is provided with the heater, the relative humidity in the heating chamber can be kept low and hence the danger of malfunction due to condensation of vapor from food can be eliminated. As a result, the amount of change of relative humidity due to vapor from food can be set in a wide range from low to high humidity and it becomes possible to satisfactorily identify the characteristic of water vapor generation with respect to heating time which differs with the kind and weight of food.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a perspective view of an automatic microwave oven, showing an embodiment of the present invention; Fig. 2 is a front view of the operating section of the same; Fig. 3 is a plan view of the heating chamber of the same; Fig. 4 is a perspective view of the humidity sensor of the same; Fig. 5 is the circuit diagram of the control device of the same; and Figs. 6 and 7 are graphs showing humidity characteristics when the cooking and thawing of beet are effected.

BEST MODE FOR CARRYING OUT THE INVENTION

Fig. 1 shows an automatic microwave oven according to an embodiment of the present invention, having an operating section 1 and a door 2, with an exhaust port 3 formed in a top plate at a corner thereot. Fig. 2 is a front view ot the operating section 1. When no cooking program is set, a display section 4 indicates the time. For the setting of the time, the time can be inputted by a clock switch 5 and numeral keys 6. For normal manual cooking, a high frequency output is selected by a power key 7 and then an optional time is inputted by numeral keys 6, and cooking is started by a start key 8. Keys denoted by 9 and 10 are an automatic cooking key and an automatic thawing key based on humidity, sensor control which is the point of the present invention. By tapping these keys, a numeral corresponding to the food, e.g., A1, is indicated in the display section 4, and then by pushing the start key 8 it is possible to automatically effect cooking or thawing. A canceling key denoted by 11 is a key used to interrupt cooking or cancel the program.
Fig. 3 shows a plan view of a heating chamber 12, indicating the path of flow of cooling air. The air from a cooling fan driven by a fan motor 13, after cooling a magnetron 14, is passed across a heater 15 and then through a suction port 16 in the heating chamber 12 to enter the latter, and it is exhausted together with the water vapor emanating from the food (not shown), through an exhaust port 17 in the heating chamber 12 and it is concentrated by an exhaust guide 18, passes a humidity sensor 19 and a temperature sensor 20 and is tinally discharged to the outside through the exhaust port 3 in the top plate of the body.
Fig. 4 shows the humidity sensor 19 composed of a refreshing heater 19-a and a detecting element 19-b, having a refreshing function such that the dirt sticking to the surface of the detecting element 19-b is burnt up by elevating the surface temperature to about 500°C by the refreshing heater 19-a so as to maintain constant the humidity characteristic of the detecting element 19-b.
Fig. 5 shows the circuit diagram of the automatic microwave oven main body showing an embodiment ot the invention. A power source plug 21 is connected to a low voltage transformer 23 through a fuse 22 to feed power to a control section 24. The input-output relation of the control section 24 is such as to control, by a microcomputer inside the control section 24, the display section 4 for indicating the time when the microwave oven is not used or for indicating the contents of cooking and the remaining time for cooking, a key board 25 tor inputting cooking time, the temperature sensor 20 for detecting the temperature of exhaust air adjacent to the exhaust port 17, the humidity detecting element 19-b for detecting the humidity in the exhaust section, the refreshing heater 19-a for removing dirt from the humidity detecting element 19-b, a power relay 27 for opening and closing the power source to the heater 15 and a high voltage transformer 26, a temperature control relay 28 for on-off controlling the heater 15 in response to signals from the temperature sensor 20 so as to maintain the exhaust temperature at a constant value, a heater changeover relay 29 for changeover between a 400 W heater 15-a and a 100 W heater 15-b, a high voltage reed switch 30 for on-off controlling the oscillation of the magnetron and controlling high frequency output, and a buzzer (not shown) for reporting the completion of cooking and the situation of input.
Further, 31, 32 and 33 denote a first latch switch, a second latch switch and a short switch, the role of the short switch 33 being to blow the tuse 22 to cause a fault on the safety side if the door 2 is opened when the first latch switch 31 is in a fused or other abnormal state. The power for the magnetron 14 is supplied by half-wave double-voltage rectification ot high voltage on the secondary side of the high voltage transformer 26 and through the high voltage reed switch 30.
For manual cooking, a high trequency output and a cooking time are inputted from the key board 25, whereupon the cooking time is indicated on the display section 4, and pushing the start key 8 causes the cooking time on the display section 4 to be counted down, while closing the power relay 27 to energize the high voltage transformer 26. The high voltage reed switch is closed or intermittently or on-off operated according to the high trequency output which is set. In this case, the temperature control relay 28 remains opened, that is, the heater 15 is not energized. However, in order to prevent dirt from accumulating on the temperature sensor 19, the refreshing action is periodically performed by energizing the refreshing heater 19-a.
The operation of the present automatic microwave oven will now be described.
Fig. 6 shows a humidity detection characteristic where automatic cooking is performed. The humidity detecting element 19-b is responsive to relative humidity and exhibits a negative resistance characteristic with respect to temperatures above 150°C, so that it is possible to maintain the refreshing temperature at a constant value by reading the resistance value of the humidity detecting element 19-b during retreshing. In the case ot automatic cooking and automatic thawing, the refreshing operation is performed immediately after start, and then a humidity detecting state is established. In Fig. 6, point a at the time ot start shows the relative humidity of the room, and the exhaust section temperature is read at the same time by the temperature sensor 20. On the basis ot the exhaust section temperature thus read, the exhaust section temperature to be controlled is determined. The need for maintaining the exhaust temperature at a high value arises from the tact that where the atmosphere in the room has a high relative humidity, it cannot further contain the water vapor from the food. That is, it becomes saturated, making it impossible to know the amount of change of humidity. For example, if the atmosphere in the chamber is 20"C, 100%, heating it to 50°C by the heater 15 changes the relative humidity to about 20%, enabling the atmosphere to turther contain 70 g/m³, so that the humidity sensor 19 can detect water vapor up to 70 g/m³ generated from the food. However, where the atmosphere temperature in the chamber is low, elevating the exhaust temperature to, e.g., 50°C at all times involves a problem concerning the capacity of the heater 15 and makes it necessary to keep down the maximum combined power consumption of the magnetron and heater during oscillation of the magnetron 14, and since the output of the heater 15 is reduced to 100 W, it is ditticult to maintain its temperature of 50°C with a low output. Further, from the standpoint of minimizing the wasteful use of power, it is desirable to set the exhaust control temperature to as low a value as possible. In the present embodiment, exhaust control temperatures are classified into four temperatures, 35°C, 40°C, 50°C, and 55°C, and an optimum temperature is selected therefrom according to the room temperature. Another reason for keeping the exhaust temperature at a high value is to give ample room for humidity detection by keeping the exhaust temperature at a high value in advance since control becomes impossible if the exhaust control temperature is exceeded owing to temperature rises of the food and electric parts from when the heater 15 starts to control the exhaust temperature to a constant value and the minimum value of relative humidity is stored to when a certain tixed amount of change of relative humidity is obtained to detect vapor from the food.
At the cooking start point a in the figure, the 400 W heater 15-a of greater output included in the heater 15, and the refreshing heater 15-b are energized. Since the refreshing heater 19-a is integrally installed in close vicinity to the humidity detecting element 19-b, the relative humidity of the atmosphere around the humidity detecting element 19-b sharply decreases, reaching point b near 0%. When the atmosphere temperature exceeds 150°C, the humidity detecting element 19-b exhibits a negative resistance characteristic, so that when it is about 500°C, it reaches point c. Thereafter, the refreshing heater 19-a is deenergized and hence the temperature lowers. When the temperature is below 150°C, the atmosphere around the refreshing heater 19-b becomes dried, reaching point d. However, the relative humidity from point b to point d is shown by'a characteristic represented by a straight line connecting points b and d. Thereafter, the atmosphere around the humidity detecting element 19-b is cooled. In a state before the 400 W heater 15-a is continuously energized to control the exhaust temperature to a constant value, it once returns to point e, and when the exhaust temperature starts to be controlled, the oscillation start point f is reached where the magnetron 14 starts to oscillate and the counting of the time till vapor detection is started. From start point a to oscillation start point f, the continuous energization of the 400 W heater 15-a accelerates arrival at point f. After point f is reached, in accordance with the situation of oscillation of the magnetron 14 being intermittently oscillated, i.e., while switching to the 100 W heater 15-b at the time of stoppage and to the 100 W heater 15-b at the time of oscillation, the exhaust temperature is controlled by the temperature control relay 28. The reason for using the 400 W heater 15-b after point f is that the use of the 100 W heater 15-a alone is insufficient to maintain the exhaust temperature.
The method of detecting humidity comprises the steps of storing the lowest value of relative humidity which takes plce after point t, counting the time until a predetermined amount of change of relative humidity is obtained, and progressing automatic cooking on the basis of it.
The characteristics of humidity change shown in the figure refer to the cooking ot 1 Kg, 2 Kg and 3 Kg of beef, respectively indicated by characteristic curves g, h and i. Further, j indicates an amount of change of relative humidity for determining a reference detection time T₁ which is a function of a heating program, the j changing with the kind of food and exhaust control temperature. This is because the amount of saturated water vapor differs with temperature and because the amount of change which affects relative humidity varies even if the same weight of water evaporates from food. Further, as shown in the figure, the reference detection time T₁with respect to the weight of food varies the more clearly and stabilizes the more, the greater the amount of change j of relative humidity, so that it is important that the relative humidity be minimized when cooking is started. For example, a heating program for cooking roast beef using the reference detection time T₁ detected in this way is as follows.
The high frequency output is made a medium output (500 W) until vapor is detected, whereupon the control of exhaust temperature is stopped and with this output maintained heating is effected for T₂ = T₁ x 2 hours. Upon the lapse of time T_2' the high frequency output is made a medium low output (350 W) and heating is effected for time T₃. The method of calculating time T₃ differs with the time T₁ which is detected; when T_I=5 (min.), T₃= T₁ x 2 (min.), and when T₁>5 (min.), T₃= 10 + T₁x 5 (min.). This is a program empirically tound when roast beef is to be medium-finished, but similar procedures may be applied to various menus, and heating patterns, such as the one described above, are stored in the automatic cooking keys 9 according to the kinds of menues.
Fig. 7 shows a humidity detection characteristic where thawing of frozen beef is performed. This is the same as the aforesaid automatic cooking up to point f where a predetermined control temperature is reached by using the heater 15, but after point f, in the case ot automatic thawing, the 100 Wheater 15-b is used without employing the 400 W heater 15-a. In the case of automatic thawing, control is performed without using so high a temperature setting with respect to the exhaust temperature detected at the start of cooking. This is because the temperature of the food is so low that a natural rise in the exhaust temperature due to rise in the temperatures of the electric parts rarely occurs before vapor is detected, and because it is only necessary to detect a very small change in relative humidity, so that there is no need to reduce the humidity at point f to a great extent. However, in order to shorten the cooking time; continuous operation is performed up to point f by using the 400 W heater 15-a to hasten the arrival time, and after point f control is performed using the 100 W heater 15-b alone. Thus, the humidity once increases to point k, and thereafter at point f the magnetron 14 starts to oscillate, so that the heat therefrom is also added to contribute to normalization toward point m. Detection of the lowest value of relative humidity is effected after point k, and when the amount of change reaches point j, the reference detection time T₁ is obtained. After detection, as in the case of automatic cooking, the temperature control relay 28 is opened and control of exhaust temperature is not performed. As is clear from Fig. 7, the amount of change j of humidity which is set is much smaller than in the case of automatic cooking, and, since ripples of exhaust temperature must be minimized, the 400 W heater 15-b, which has a large amount ot overshoot, cannot be used. That is, in the case ot automatic thawing, it exhaust temperature control is performed by intermittently operating the 400 W heater 15-a, the ripples of temperature that occur would affect the humidity detection time; thus, the 100 W heater 15-a alone is used. The heating pattern obtained from the results ot experiments of cooking in the case of thawing trozen beef using T₁ thus obtained is as follows.
Heating is effected with low output (180 W) from point f to vapor detection, and after vapor detection the heating time is changed to T₂ or T₃. If T₁ is less than 15 minutes, the heating time skips T₂ and goes to T₃, and the tood is heated with warmth retention output (70 W) for T₃= T₁ (minutes) to complete thawing. In the case where T₁ exceeds 15 minutes, it is heated with low output (180 W) tor T₂ = (T₁-15) x 2 (minutes), whereupon the heating time output (70 W) for T₃ = 15 + (T₁-15) x 2 (minutes) to complete thawing. This procedure can be applied also to the thawing of other frozen foods, and moreover, by combining i with other high frequency outputs as a function of time T,, it is possible to automate all operation from thawing to cooking.
The aforesaid changeover of the heater capacity could be made by phase control using control rectifier element such as triacs, but considered microscopically it would be impossible to keep down instantaneous maximum consumption power and current; thus, it cannot be said to be a very preferable method. Therefore, an arrangement, as in the present embodiment, wherein two heaters, a large one (400 W) and a small one (100 W), are separately provided for changeover may be said to be most advantageous at present.

INDUSTRIAL APPLICABILITY

As has been described so tar, according to the present invention, the exhaust temperature is made constant by heaters installed adjacent the suction port so as to detect relative humidity, so that accurate amounts of change of humidity can be found, and by utilizing the humidity detection time it is possible to attain automation of cooking, thawing, and trom thawing to cooking ot a variety of foods. Further, since there is no need to use wraps or special containers, there is no danger of spoiling the external appearance and taste of food, nor is the danger of failure in cooking or overheating tood or causing a fire. Thus, a high trequency heating apparatus which is convenient to use can be provided.

Claims

1. A high frequency heating apparatus comprising a heating chamber for receiving food, a high frequency oscillator for feeding high frequency waves into said heating chamber, a control circuit including a microcomputer for controlling said high frequency oscillator, a suction section tor suction into the heating chamber and an exhaust section tor exhaust from the heating chamber, a heating device for elevating the temperature in said heating chamber to a preset value, and a temperature sensor for detecting the temperature of the atmosphere in said heating chamber or in said exhaust section and a humidity sensor for detecting the humidity, the arrangement being such that said heater is controlled in response to detection signals from said temperature sensor to keep said heating chamber at a constant temperature, while the amount of change of humidity in said atmosphere is detected by said humidity sensor and the time required to reach a predetermined amount of change of humidity is detected, and with said time as a function said control circuit controls said high frequency oscillator.

2. A high trequency heating apparatus as set forth in Claim 1, wherein the heating device is disposed adjacent the suction port and the exhaust temperature is controlled so that it is constant.

3. A high frequency heating apparatus as set forth in Claim 1, wherein the temperature of the atmosphere in the exhaust section at the start of cooking is detected by the temperature sensor and the temperature in the heating chamber to be controlled can be changed in steps.

4. A high frequency heating apparatus as set forth in Claim 2, wherein cooling air which has cooled electric parts including the high frequency oscillator is heated by the heating device and is sucked into the heating chamber through the suction port.

5. A high frequency heating apparatus as set forth in Claim 1 or 2, wherein the temperature sensor and humidity sensor are disposed very close to each other in the exhaust section to detect the temperature and humidity of the exhaust.

6. A high frequency heating apparatus as set forth in Claim 1 or 2, wherein the high frequency oscillator is intermittently controlled to control the output and the output of the heating device is changed at the time of oscillation and non-oscillation ot said high frequency oscillator.

7. A high frequency heating apparatus as set forth in Claim 1 or 2, wherein the humidity sensor detects the humidity of the atmosphere subsequently to a refreshing action at the start of cooking.