This invention relates to a temperature control for cooking appliances having a single heating chamber in which a food is cooked by high-frequency dielectric heating as well as by heating ambient air therein with a heater means which comprises a plurality of sensors and a microcomputer in combination.

With the development and decrease of cost of microcomputers, there has recently been a vogue of microcomputer-controlled cooking appliances having a variety of control functions. However, so long as a cooking program is inputted on a manual mode, the imputting operation is complicated, the user being forced to follow a series of setting procedures so that the multiple functional capabilities of a microcomputer cannot be fully utilized.

It has, thus, become necessary to develop an automatic cooking system incorporating a number of sensors adapted to detect the current status of cooking, to improve the manipulability of the appliance and further to broaden the scope of "menu" which the appliance is capable of taking care of.

Further, as cooking is automated more and more, it has become difficult to predict the end point of cooking, with the result that a-keep-warm function for maintaing the cooked food at a palatable temperature has become an essential requisite.

However, from technical and economic points of view, the microcomputer has its own limitations as to memory capacity and the available number of input and output terminals and these must be utilized with efficiency. Moreover, the use of a plurality of sensors requires an interface-circuit and -other elements for each sensor and this means an inevitable increase-in the number of circuit elements, complexity of the circuit and a reduced reliability of functions.

Moreover, when a temperature sensor probe adapted to directly detect the temperature of food is employed as sensor means, the probe must be detachable and the cooking program must be changed after judging if the probe is on the food or not. As means for this judgement, there has for example been employed a change-over switch associated with the attachment and detachement of the sensor probe.

However, this mehtod entails an increase of cost and a decrease of reliability, due to the increased number of parts required, and moreover, has the problem that the association of the change-over switch with the presence or absence of the sensor probe in the food demands an increased force for an attachment or detachment of the probe, thus detracting from the ease of using the appliance.

It is a principal object of this invention to provide a temperature control-comprising a combination of a microcomputer with a plurality of temperature sensors having different characteristics,said temperature control being such that the detection-temperature signals from the respective temperature sensors -are compared with the output voltage of a reference voltage generator to judge the presence or-absence of the sensor probe in the food and the presence or absence of a failure of other sensors according to the reference voltage level to thereby realize a simplication of the required circuitry, a decrease of production cost and an increase of reliability.

It is another object of this invention to improve the safety of a cooking appliance without entailing increases in the number of parts and in production cost.

It is still another object of this invention to further improve the accuracy of control of the air temperature in the heating chamber and of the temperature of a cooking load (food) by the incorporation of an attenuator in the control circuit.

This invention will hereinafter be described in further detail by way of a preferred embodiment depicted in the accompanying drawings.

• Fig. 1 is a perspective view showing a cooking appliance as-an embodiment of this -invention;
• Fig. 2 is'an enlarged front view showing a control segment as a principal part of the same appliance;
• Fig. 3 is a perspective view showing a temperature sensor probe;
• Fig. 4 is an electric circuit-diagram of the same cooking appliance; and
• Fig. 5 is a circuit diagram of a control circuit which constitutes a principal part of the same cooking appliance.

In Fig. 1 there is shown a cooking appliance having a body 1 defining a heating chamber-(1) (not shown) therein and a door 2 fitted across the front opening of said heating chamber in such a manner that it may be opened or closed with respect to the heating chamber.

There is further disposed an operation panel 3 on the front of the body 1 and alongside the door 2, and the top face of the body 1 is provided with vents 4 and 5 for letting out the air from the heating chamber.

As illustrated in Fig. 2, said-operation panel 3 has a display segment 6 in a top position thereof for displaying 4-digit figures. The figure 12:59 represents 12 minutes and 59 seconds. There are also-disposed time setting keys 7a, 7b, 7c and 7d in positions corresponding to digits for entering a 4 digit figure into the display segment at the-corresponding digital positions. Thus, 7a represents the 10-minute digit, 7b the one-minute digit, 7c the 10-second digit, and 7d the one-second digit.

- The operation panel 3-further carries output selection keys 8a, 8b and 8c. Entries can be made with these keys, i.e. 8a representing a high output (HIGH), 8b an intermediate output (MED) and 8c a low output (LOW).

There are further provided a temperature setting key 9, a heater key 10 which is used only on the heater- cooking mode; a keep-warm key 11, a reset key 12 for stopping or cancelling the cooking, a start key for initiating a cooking operation and a door button 14 for opening and closing the door 2.

When the high frequency heating mode only is employed, the output is selected by depressing one of the output selection keys 8a, 8b and 8c, then the time setting keys 7a through 7b are respectively depressed to set the cooking time and have the time displayed in the display segment 6, and the start key 13 is finally depressed to initiate cooking.

For high frequency cooking through direct detection of the temperature of the cooking load with the temperature sensor probe of Fig. 3 fitted into the cooking load (not shown), a plug 16 of the probe 15 is set in a jack 17 disposed in the heating chamber, then an output selection is made-with output selection keys 8a to 8c, the temperature setting key 9 is tapped until a desired temperature is displayed in the display segment 6, and the start key 13 is-depressed to start cooking.

When cooking is to be carried out with the heater, the heater key 10 is tapped to-set the temperature and have the temperature in the heating chamber displayed in the display segment 6. Then, the cooking time is set by tapping-the time setting keys 7a through 7b, whereupon the temperature display disappears from the display segment 6 and, instead, the cooking time is displayed in said segment 6. As illustrated in Table 1, temperatures from 100°C to 230°C can be displayed at intervals of 10°C. For example, cooking with the heater can be carried out at 200°C for one hour. As to-the keep-warm setting, the keep-warm temperature can be controlled over the range of 40°C to 95°C at intervals of 5°C as will be more fully described hereinafter.

When heating with the heater is to be controlled with the temperature sensor probe 15, the heating chamber temperature is set by tapping the heater key 10 and the desired temperature of food is then set by tapping the temperature setting key 9.- Thus, by way of illustration, cooking can be carried out at the heating chamber temperature of 200°C and the internal food temperature of 80°C. When the food is cooked with the temperature sensor probe 15, the range of temperatures that can be set with the temperature setting-key 9 is 40°C-to 95°C at intervals of 5°C..

Cooking with the temperature sensor probe 15 is convenient in that the-setting can be made in disregard of the weight of food but has the disadvantage that it is then difficult to predict the time-of completion of cooking, and it is necessary that the cooked food be kept warm until it is taken out from the heating chamber. In such cases, by inputting the keep-warm key 11 immediately following the temperature setting with the temperature setting key 9, the cooked food is maintained at the temperature designated by the temperature setting key 9 until the door 2 is opened.

As regards the method of keeping food warm, the food cooked by high frequency heating can be easily kept warm by making and breaking the high frequency energy in response to the detection temperature of the temperature sensor probe. This is because high frequency heating causes the inside of food to be simultaneously heated and, therefore, ensures a good thermal response of the probe.

On the other hand, it is very difficult to keep warm the food cooked with the heater just because of the reverse of the above-mentioned reason.

Therefore, in accordance with this invention, it is ensured that the heating chamber temperature may be automatically set at the temperature set by the temperature sensor probe 15.

In Fig. 4, there is illustrated an electric circuit of the cooking appliance depicted in Fig. 1., showing the condition after closure of the door 2 and before start of cooking. The main circuit of the cooking appliance comprises a current fuse 18, a first latch switch adapted to open and close in association with the operation of the door 2, a power relay contact 20, a second latch switch 21, a bilateral control rectifying element 22 and a high-voltage transformer having a primary winding 24 connected in series with an AC power source. A secondary side 25 of said high-voltage transformer 23 is connected to a magnetron 28 through a voltage doubling rectifying circuit consisting of a high-voltage capacitor 26 and a diode 27, and the heater of the magnetron 28 is connected to a heater winding 29 of the high-voltage transformer 23. For cooking with the heater means, there is provide a heater 30 one end of which is connected to an end of said AC source, with the other end of the heater being connected to the connection between the second latch switch 21 and the anode of the bilateral control rectifying element 22. There also is provided a fan motor 32, one end of which is connected to one end of the AC source with the other end being connected to the anode side of the bilateral control rectifying element 31. There is also disposed a short switch 31,- one end of which is connected to one end.of the AC power source, with the other end being connected to the connection between the power relay contact 20 and second latch switch 21, and its ON-and OFF-operation is reversely-associated with said first and second latch switches 19 and 21.

Thus, it is opened on closure of the first and second latch switches 19 and 21, while it is closed as the latter switches are opened. When at-least one of said first latch switch 19 and said power relay contact 20 is fused, the main circuit-is short-circuited to break the current fuse 16. An oven lamp is connected via a lamp switch 35 which is operatively associated with said first and second latch switches 19 and 21. The primary side of low-voltage transformer 36 is connected to one end of the AC source and between the current fuse 18 and first latch switch 19, while its secondary side is connected as a power source to a control circuit 37. The control circuit 37 controls the power relay contact 20 and bilateral control rectifying elements 22 and 31 to thereby.control the output of the magnetron 28. The control circuit 37 has a door switch associated with said first and second latch switches 19 and 21, and the operation signal from the door 2 is detected and inputted by-said door switch 38. Connected in parallel with-the.primary side of said low-voltage transformer 36 is a variable resistor 39 for the purpose of preventing erratic actions due to noise.

In the above arrangement,-the control circuit 37 is supplied with power from the low-voltage transformer 36 at all times, and as a cooking program is set by the procedure explained with reference to Fig. 2 and the start key 13-is depressed, the power relay contact 20 is closed because the signal of the door 2 has been inputted by door switch 38 into-the control circuit 37, whereupon the main circuit is closed, the oven lamp 34 is lit through the lamp switch 35 and the fan motor 32 starts rotating. And on the high-frequency heating mode, the bilateral-control-rectifying element 22 becomes ON to supply-power to the high-voltage transformer 23 so that the magnetron 28 is excited.

Therefore, the high frequency output is controlled as the bilateral control rectifying element 22 is turned ON and OFF by the control circuit 37.

Then, on the heater cooking mode, the output is controlled as the bilateral control rectifying element 31 is turned ON and OFF by the control circuit 37.

Fig. 5 shows a schematic view of the control circuit 37. The operation of the circuit 37 will now be described below. In response to a 5-bit signal of outputs R₁₀ to R₁₄ of a microcomputer 40, a reference voltage generator 41 which is formed by matrixing on inverter and resistors generates a voltage varibale in 32 steps by a combination of inputs. The voltage is directly fed to the minus (-) input of a comparator 42, while it is divided by resistors 44 and 45 and fed to the minus (-) input of a comparator 43. Moreover, the minus (-) -input voltage of the comparator 42 can be tapped through a resistor 47 by the output R₁₅ of the^-microcomputer-40 and an inverter 46.

The reference numeral 48 designates a high-temperature thermister (temperature sensor)-for detecting the internal temperature of the heating chamber and is connected to the plus (+) input of the comparator 42 in voltage-dividing relation with a resistor 51.

When the internal temperature of the heating chamber is controlled in the above arrangement, the temperature signal divided by the high-temperature thermister 48 and resistor 49 and the output of the reference voltage generator 41 are compared by the comparator 42 and the result is read into K₂ through resistor 52 and transistor 53 at the timing of RO output of the microcomputer 40, whereby the heater 30 is controlled.

When temperature control is effected with the temperature sensor probe 15, the temperature signal voltage divided by low-temperature thermister 50 and resistor 51 and the output voltage of the reference voltage generator 41 via an attenuator consisting of resistors 44 and 45 are compared by the comparator 43 and the result of comparison is read into K₂ at the timing of R₁ output of the microcomputer .40 via resistor 54 and transistor 55 whereby the output of the heater 30 or magnetron 28 is controlled. As previously mentioned, the temperature control range of the low-temperature thermister 50 is 40°C to 95°C and that of the high-temperature thermister is 100°C to 230°C. However, when the cooked food is to be kept warm by the heater 30, the temperature must be controlled at 40°C to 95°C with the high-temperature thermister 48. In this case, the R₁₅ output of microcomputer 40 is HIGH, the output of the inverter 46 is LOW and the resistor 47 is connected in parallel with the output of the reference voltage generator 41 so that it functions as an attenuator. Temperature control at 40°C to 95°C is thus enabled by comparing this voltage with the temperature signal of the-high-temperature thermister 48. The relationship of such reference voltage steps with temperature signals is set as shown in Table 1.

It will be seen from Table 1 that by programming the microcomputer with such parameters as the presence or absence of the temperature sensor probe in food and broken-wire and-short-circuit troubles, the corresponding detection functions can be imparted to the control. It is for preventing erroneous operations due to an overshoot of food, etc. that -in Table 1, the -temperature control range and the trouble detection range are not adjacent but are separated-discretely.

Table 1 shows-a continuous array of temperature steps in line with reference voltage steps but such an arrangement is not essential.- Particularly because the high-temperature thermister 48 effects control by detecting the temperature in the neighborhood-of the heater and estimates the temperature in the central zone of the heating chamber, the relationship between detection signal and temperature is complicated by the influence of a difference between the temperature detected by the thermister and the temperature in the central zone of the heating chamber. To solve this problem, the temperature setting may be made discontinuous with respect to the reference voltage steps. A glance at Table 1 might suggest that the characteristics of a single thermister, i.e. the high-temperature thermister, are sufficient to cover the function of the low-temperature thermister for the temperature sensor probe but, actually, there would arise the problem of inferior temperature accuracy. Thus, the low-temperature thermister for setting the end-point temperature of food must be very high in accuracy while the high-temperature thermister is selected with reference to high-temperature characteristics. Therefore, these thermisters are preferably used in.the manner described hereinbefore.

As described hereinbefore, this invention ensures an automation of cooking through a combined use of a microcomputer with a plurality of temperature sensors having dissimilar characteristics and a comparison of the temperature signals detected by the respective temperature sensors with the output voltage of a single reference voltage generator. And the presence or absence of the temperature sensor probe in the food and the presence or absence of a trouble at a different temperature sensor can be judged according to reference voltage levels. In this manner, it is now possible to simplify the control circuit, decrease the number of necessary parts, reduce the cost of production and improve the safety and reliability of the control system.

It should be understood that by incorporating an attenuator in the control circuit for the microcomputer, the accuracy of control over the internal temperature of the heating chamber and over the food can both be improved.