CA1197592A - Cooking appliance - Google Patents

Abstract

ABSTRACT

This invention provides a cooking appliance using a gas fuel as a heat source and having an appliance body (1), comprising a temperature sensor (6) installed in a heating chamber, a control circuit (18) adapted to be actuated by the output from the temperature sensor (6) and to operate according to three working temperatures, namely, a preset temperature, an upper limit temperature and a lower limit temperature, the arrangement being such that when the temperature in the heating chamber being detected by the temperature sensor reaches the upper limit temperature, main burners (2, 3) are completely closed; when it reaches the preset temperature, the firing of the main burners (2, 3) is reduced by half; and when it reaches the lower limit temperature, the main burners (2, 3) are fully opened, thereby effecting fine temperature control, the firing rate being automatically adjusted according to variations in external conditions affecting the heating chamber temperature, thereby maintaining the preset temperature throughout the heating operation to provide satisfactory results of cooking.

Description

SPECIFICATION
TITLE: Cot~king Appliance BACKGROUND OF THE INVENTION
Field o~ the I~vention The present invention relates to a composite cooking appliance consisting of a yas oven and a microwave oven, or a cooking appliance such as a gas oven, and it particular-1~ rela~es to a temperature control device therefor uti-lizing gas.combus~ion.
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BRIEF DESCRIPTION QF THE DRAWINGS
Fig. 1 is a perspective view of a cooking appliance according to an embodiment of the present invention; Fig. 2 is a layout view of said cooking appliance; Fig. 3 is a block diagram for explaining the operation of a microcomputer incorporated in said heating appliance;
Figs. 4 and 5 are a detection temperature characteristic diagram and a combustion heat generation rate charac-teristic diagram of a conventional cooking appliance;
Figs~ 6, 7 and 8 are a detection temperature charac-teristic diagram, a combustion heat generation rate characteristic diagram, and a heating chamber temperature characteristic diagram in the "strong" state oE ~ cooking appliance according to an embodiment o~ the invention;
Figs. 9, 10 and 11 are a detection temperature charac-teristic diat3ram9 a combustion heat generation rate characteristlc diagram and a heating chamber temperature characteristic diagram in the "medium" state of said appliance; and Figs. 12, 13 and 14 are a detection .~3~5~

temperature characteristic diagram, a combustion heat generation rate characteristic diagram and a heating chamber temperature characteristic diagram in the "weak" state of said appliance.

DESCRIPTION OF THE PRIOR ART
The ramarkable advance o~ semiconductor technology has resulted insophisticat~on of control circuits, miniaturization of such circuits by higherscale mtegration, alld reduction of the costs of such circuits by mass-production, and these electronic control circuits have come to be also wl~ely used in household electric equip-.' 3 ment.
Th~ technique using intelligence based on thiselectronic control is making rapid inroads into various heating apparatuses including electric ovens, microwave ovens, gas ovens, and combinations thereof.
One of the most important factors in cooking appli-ance~ based on gas combustion i5 exact temperature control.
Since the quality of cooking depends on temperature, i~

is important that a preset temperature suitable for a given heating load be maintained accurately. Conven-tional cooking appliances using gas have temperature control means, most of which have been liquid expansion control methods and bimetalsO
A gas oven will be taken up by way of example. There i~ an arrangement wherein a main burner is fired to produce hot air, which is fed into the heating chamber ~ ..

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of the oven to cook a heating load therein. The operation o a gas oven oE this arrangement will be described with reference to Figs. 4 and 5. When the user sets the knob at a certain temperature, two temperatures, upper and lower limit temperatures TL and T2, are set by a control circuit. When cooking is started, the main burners start firing and eventually the upper limit tem-perature Tl will be reached at time t2. Then, the main burners stop iring, so that the temperature in the oven heating chamber gxadually lowers until the lower limit temperature T2 is reached at ~ime t3~ Then, the main burners start firing again`, repeating this cycle henceforth.
If the preset temperature is changed to a higher one after passage of time t7 (the upp~r and lower limit tem-peratures being T3 and T4, respectively)~ the upper limit temperature T~ is reached at time t8, as shown in Fig. 4, and henceorth the same operation as described above is repeated~
- Thus, the conventional gas oven is designed to control temperature by the on-off operation of the main burners to maintain the oven heating chamber temperature at a preset value, but this design has the following drawbacksO
The pressure of household gas differs with districts.
Even in the same district, the gas pressure available for th~ gas oven installed in a home will always very ,_ .. , -.1~97S~

owing to the turning on and off of gas in other homes ol in another room in the same home. Thus, if -the gas pre~sure drops below the normal value, this decreases the rate of heat generation by ~he main burners, thus requiring a longer time than usual to reach the preset temperature and hence a longer cooking time.
Further, since the ambient temperature of the air ` surrounding the gas stove differs greatly between midsummer and midwinter, the cooking time will be different in mid-summer, particularly in a drafty room.
Furthermore, because of the design which turns off allof the main burners upon reaching the upper limit tem-.perature and turn~.loP..all of the main burners upon reaching of the lower limit temperature, the actuaLdifference in temperature between the two extremes is as great as about 10C, showing that this design fails to attain the purpose of maintaining a temperature at a constant value. Furthermore, the need for frequently turning on and off the main burners entails the drawback of shortening the life of the control circuit system, particularly the relay.

SUMMARY OF THE ~NVENTION
The present invention eliminates such conventional drawbacks~ .

Accordingly, an object of the invention is to provide a heating appliance having an automatic control . function, wherein a control circuit automatically com-:

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pensates for external condi-tions which affect the gas oven hea~ing chamber temperature. More parti~ularly, it provides a cooking appliance comprising a heating chamber for receiving a heating load, a heating means for heating said heating chamber, a tempe;rature sensox for detecting the temperature in said heating cha~er, and a control means including a microcomputer for controlling the output and heating time of sa:id heating means in response to signals from said temperature sensor, wherein temperatures detectible by said temperature sensor are classified into at least three levels, namely, upper limit, middle and lower limit temperatures according to a preset heating temperature and the output of said heating means is changed stepwise according to the temperatures at the respective levels so as to control the heating chamber with respect to said preset heating temperature.
It is so d~signed that i~ the capacity of a main burner in operation is insufficient for a preset temperature, another main burner is fired so as -to maintain the preset temperature as constant as possible, and when the preset temperature is attained, a minimum number of main burners are operated intermittently or .in an on-off manner to maintain the temperature at the preset valueO

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DES~RIPTION OF THE PREFERRE~) EMBODIMENT
An embodiment of the invention will now be described.
A cooking appliance body 1 shown in Fig. 1 represents a composite cooking appliance comprising a combination of a microwave oven and a gas ovenl but only the gas oven section will be taken up for explanation.
In Fig. 2, turning a gas cock knob 8 opens a gas cock ~ and simultaneously turns on a gas cock switch 10, actuating a high voltage spark generator 11 to cause an ignition plug 12 to pxoduce a spark. On the other hand, .
the gas entering through a hose end 13 passes successively through the gas cock 9, a now-opened main solenoid valve 14, a governor 15 and a pilot gas passage 16, and enters a pilot burner 7. The latter is ignited by said spark and a thermocouple 17 is thereby heated, signaling a microcomputer 18 to open ~irst and second solenoid valves 4 and 5. The ga~ flows through main gas passages 19 and 20 into main burners 2 and 3, where it is ignited by said pilot burner 7. Henceforth a temperature sensor 6 detects the temperature in the heating chamber (not shown), signaling the microcomputer 18 to turn the first and second solenoid valves 4 and 5 on and off so as to control the oven heating chamber temperature to maintain it at a preset heatinq temPerature.
The configuration of the microcomputer 18 ~of Fig. 2 . .

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is shown in Fig. 3. The output from the temperature sensor 6 and the output from a temperakure setti.ng circuit .. . ... . .
`~ 22 which has been preset by the user are compared in a . . .
comparator circuit 23. If the temperature detected by the temperature sensox 6 is higher than the preset temperature, the comparator circuit 23 actuates a driver circuit 24 to cut of the second solenoid valve 5. As a result, one main burner 3 stops firing.

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If the temperature further rises by about 3C, a shift circuit 25 is actuated to cause a comparator circuit 26 to make a comparison between the output from the temperature sensor 6 and t.he output from the temperatur~ s~tting circuit 22. If the temperature detected by the temperature sensor 6 is higher, the comparator circuit 26 actuates the driver circuit 2~ to cut off the ~irst solenoid valve 4 As a result, the other main burner 2 stops firing.
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cQoking in the High Temperature Region Figs. 6-8 illustrate a situation where the heating chamber temperature is set in the 250-300C range which is a high cooking temperature region for cooking loads includi~g fish and sweet potatoes whi.ch require 300C.

In the case where the preset temperature is 300C, different valves of heating chamber temperature are indicated by Rm3, (Rm3 + about 3C) by Rh3, and (Rm3 -, .. . .

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about 3C) by R13. Similarly, temperatures de,tectedby the temperature sensor 6 in the case of said preset temperature of 300C are indicated by the middle tem-perature Tm3, upper limit temperature Th3 and lower limit temperature T13. The preset temperature is established by the user adjusting the knob to 300C;
thus, three temperatures are clesignated, two of which are shifted 3C a~ove and below said temperature of 300C.
In order to detect heat.ing chamber temp~ratures more accurately it is 50 arranged that the temperature sensor 6 will detect them at points about 5C higher.
This is because there is a discrepancy between the heating chamber temperature and the temperature detec-ted by the sensor 6. That is:
Middle temperature Tm3 = Rm3 ~ about ~C
The same is true of the upper limit temperature Th3 and the lower limit temperature T13. Detection of tem-peratures by the temperature sensor provides on-off control o~ the main burners 2 and 3, whereby the heating chamber temperature is adjusted. Where the pilot burnex 7 is iring at all times, let the combustion heat genera-tion rate be indicated by C2 Kcal/h when the two main burners 2 and 3 are all fired, by Cl Kcal/h when the _ : ........
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g main burner 2 alone is fired,.and by C0 Kcal/h for the pilot burner 7 alone~
In the initial stage of combustion, -the m~in burners

2 and 3 are fired, rap.idly heating the heating chamber with the combustion heat generation rate of C2. Upon lapse of tl minutes, when the temperature being detected by the temperature sensor 6 reaches the middle temperature Tm3C, the microcomputer 18 turns off the second solenoid val~e 5, putting out the main buxner 3~ At this time, the main hurner 2 remains fixing. Thus, the combustion heat generation rate lowers to Cl Kcal/h. However, the heating chamber temperature drops after it has overshot for some time owing to the remaining heat. When the temperature being detected by the temperature .sensor 6 reaches the lower limit temperature T13, the microcomputer 18 turns on the second solenoid valve 5, igniting the main burner 3. Thereafter, the aforesaid control is repeated until completion of cooking.
Thus, when the temperature being detected by the temperature sensor 6 reaches the middle temperature Tm3C~ .
one of the two burners, or the burner 3, is automatically put out, and it depends on the value of the preset tem-perature whether the heating chamber temperature further rises, remains as it is, or drops. More particularly, in the case of cooking in the high temperature region around 300C, stopping one burner 3 xesults in the h~ating ., :

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1.0 chamber temperature tending to lower a-t a rate dependent on the heating chamber heat capacity and burner capacity, until it reaches the lower limit temperature Tl30 There-upon, the control circuit is actuated again -to ignite the previously stopped main burner 3~ Since the other main burner 2 remains firing during this period of time, the temperature in the heating chamber is kept high, thus eliminating the drawback of the conventional control means causing a large difference in temperature owing to the act that all the burners simultaneously turn on and off repeatedly. Theoretically, the size of temperature change is half that for the conventional control means.
It is so arranged that if the door 21 should be opened in the course of cooking, as at t5, the flames of the main burners 2 and 3 will be put out for safety and the hot air circulation fan (not shown) will be stopped;
, but this will rapidly lower the temperature in the heating chamber. The door 21 is closed and heating is restarted, when the temperature in the heating chamber is S'C, which i5 below the lower limit temperature Rl3~ while the temperatures SC detected by the temperature sensor 6 at ~his time is also below the lowex limit temperature Tl3.
As a result, the main burners 2 and 3 are ignited. The hot air circulation fan is also operated again. The normal contrvl will be repeated henceEorth.
In Fig. 6, if the temperature sensor 6 detects the .. . :

upper limit temperature Th3 for some reason or o-ther (which detection is notillustrated), the main burners 2 and 3 are put out, with the pilot burner 7 alone firing.
As described above, in the "high" range between 250C and 300~C, three detectlon levels are assigned to the temperature sensor 6 to detect the lower limit, middle, and upper limit temperatures. The two main burners 2 and 3 are usedi.n the initial stage of heating and when the middle temperature Tm3 is reached, the main burner 3 is put out, with only one being used for heating~
When the lower limit temperature T13 is reached, both burnexs take part in.heatingj but when the upper limit temperature Th3 is reached, both of the main burners 2 and 3 are put out, with the pilot burner 7 alone firing~
The aforsaid detection levels o~ the temperature sensor 6 are stGred in the microcomputer 1~ in advance, and when the preset temperature value is inputted into the microcomputer, the optimum upper limit, middle and lower limit temperatures are selected and control is effected.
Cooking in the Middle Temperature Region ... .. .. ..
A description will be given of a manner of control which is effected where the preset heating temperature is in the "middlel' range of about 200-250C. As shown in ~igsO 9-11, the two main burners 2 and 3 are used in the initial stage of heating, and when the temperature being detectled by thetemperature sensor 6 reaches the ..

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~ 12 middle temperature Tm2, one main burner 3 is put out.
Upon lapse of t6 minutes during which overshoOting takes place, the lowe.r limit ternpera-ture tl2 is reached, whereupon the main burner 3 is ignited. The control r continues with this operation repeated, but when the upper limit temperature Th2 is reached, both main burners are put out.
Thus, in the case where the preset temperature is in the middle region, the temperature in -the heating 1 J chamber will not change so much even if one main burner

3 is put outr thus allowing the other main burner 2 alone to continue firingO If sueh a combus-tion state proceeds until the upper limit temperature Th2 or lower limit temperature T12 is reached, the control circuit performs !-` the same control operation as d~scribed above to maintain .. the preset temperature~
Cooking in the'.Low Temperature Region A descriptiorl will be given of a manner of control in thecase of cooking where the preset heating temperature is in the '710w" range of about 150-200C. As shown in Fig. 12-14, in a preheating pexiod from the start to time t~, both of the two main burners 2 and 3 are fired, but when the temperature being de-tected reaches the preset middle temperature Tml, one main burner 3 stops firing.
However, since the combustion capacity of one burner is large as compared with -the size of the heating chamber, .. . .
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~ 13 the remaining one burner 2 i.n opera-tion is sufficient for the heating chamber temperature to keep on rising until at time t3 it reaches the upper limit temperature Thl. At this point, the main burner 2 is also put out;
that is, none of the two main burners 2 and 3 are firing, with the pilot burner alone firing. As a result, the temperature lowers, and at t.ime t4 -the middle temperature Tml is reached whereupon one main burner 3 is fired again. Henceforth such an on-off operation is repeated.
In this case also, since it is not all main burners but only one main burner 3 that is on-off operated, the tem-perature change is small.
In the case o cooking in the low temperature region, normally $he temperature in the heating chamber is on the increase even after one main burner 3 is turned off, but under special conditions as when the gas pressure is so low that the use of a single burner alone is insuffi-cient or when the ambient temperature is extremely low as in midwinter, if one main burner 3 is turned off, the heating chamber temperature will soon lower to the lower limit temperature Tll. In this case, therefore, one main burner 3, now put out, is ignited again to keep the temperature rising. This operation is the sclme as -the one that was described with re~erence to cooking i.n the high temperature region; thus, even if the setting is in the low temperature region, the control circuit will .. . . .

3 75~, autom~tically come into operation depending upon a change inthe external conditions including gas pressure and ambient temperature, so as to provide the optimum conditions for maintaining the preset temperature.
Operations similar to the one described above will be performed in the case o~ cooking in the medium and high temperature regions. In addition, the num~er of main burners to be used may be optionally determined, as occasion demands.
As is clear from the foregoing description, the present embodiment assigns three levels o detection ~o the temperature sensor 6 for detecting the upper limit temperature, middle temperature and lower limit temperature, wherein upon detection of the upper limit temperature, all main burners are turnad off; upon detection of the middle temperature, the combustion heat generation rate of the main burners is reduced approximately by hal~;
and upon detection of the lower limit temperature, all the main burners are turned off. Thus, the following effects are obtained.
1. Generally, theflow rate varies to a large extent with the types of the gas, nozzle and governor, and in the - .
~ case of coal gas, the calculated flow rate decreases by -- . . . . . .. .
~ about 40 percentat worst, which accounts for the fact :_ . .. .... .
that with theconventional heating control system, the combustion heat generation rate is insufficient, leading ~ - .

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to a failure in cooking or to protraction of the cooking time. In contrast, in the present embodiment, the microcomputer control automatically compensates the combustion heat generation rate for variations in the input gas flow rate, so that the proper combustion heat generation rate can be maintained for any preset tem-perature.
2. Since the ternperature sensor, providing selec-tive use of three levels of heating power, makes it possible to finely control heating, the size of variations in the heating chamber temperature is small, ensuring satisfactory results of cooking.
3. Since full heating power is developed in the inita~l stage of heating, the initial rise of tempera-ture is quick. Further, even if the door is opened during heat, the initial re-rise oE temperature is also quick because of the development of the full heating power.

4. During heating for initial rise of temperature one of the main burners is put out when the central temperature Tm is reached, so that less overshooting takes place and no preheating is required and hence the cooking time is shortened.

5. Since the frequency of on-off actions of the solenoid valve is reduced, the durability of the solenoid valve is improved and low-noise operation is possible.

6. Complex control in multistage, such as three .

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stages "strong, medium, weak" is made possible by computer con-trol.

As has been described so far, according to the present invention, any preset temperature can be automatically compensated for insu~ficient gas flow ra-tes dependent on the type of the gas or due to various troubles to the piping, and sufficient gas flow rates can be attalned~
Further, the main burners are controlled so that they develop ~heir full power, about half the full power or zero power when the temperature being detected by the temperature sensor reaches one of the at least three temperatures, namely, the upper limit tempexature, middle temperature and lower limit temperature, no matter what the preset heating temperature may be. Thus, it is possible to provide a cooking appliance causing smaller size of variations in the heating chamber temperature, thus ensuring satisfactory results of cooking.

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Claims (2)

CLAIMS: - 17 -

1. A cooking appliance comprising: a heating chamber for receiving a heating load, a heating means for heating said heating chamber, an input means for providing a preset temperature input, a temperature sensor for detecting the temperature in said heating chamber, and a control means including a microcomputer for controlling the heat output and heating time of said heating means in response to signals from said temperature sensor and said input means, wherein temperature detectible by said temper-ature sensor are classified by means contained within said micro-computer into at least three levels, namely, an upper limit temperature, a middle temperature, and a lower limit temperature according to said preset heating temperature and wherein said heat output of said heating means is changed in a stepwise fashion according to said temperatures at the respective levels so as to control said heating chamber with respect to said preset heating temperature; and wherein when said temperature being de-tected by said temperature sensor is respectively said upper limit temperature, said middle temperature and said lower limit temper-ature, said heating means develops no heat output, half its full heat output and its full heat output, respectively, thereby controlling the atmosphere in said heating chamber so as to be at said preset heating temperature.

2. A cooking appliance as set forth in Claim 1, wherein an initial stage of heating, said heating means develops its full output, but it develops half of its full output when said middle temperature is first reached and henceforth it varies its output with respect to said detected temperature, thereby controlling the atmosphere in the heating chamber so that it is at said preset heating temperature.