TECHNICAL FIELD
[0001] This invention relates to an automatic high frequency heating appliance which cooks
food by high frequency heating and especially senses state of food being cooked and
automatically determines heating time.
BACKGROUND ART
[0002] The time length of high frequency heating of food is determined by various factors
of food including initial temperature, amount, final temperature, specific heat, absorption
factor of high frequency energy, etc. A conventional manner to preset heating time
in microwave ovens is that the user presets time with a timer, based upon the kind
and amount of food to be heated. In this case, since the initial temperature of food
or the like is not taken into consideration, the user feels an increased fear of erroneous
cooking and the inconvenience that he should calculate and preset heating time whenever
the amount of the food is varied.
[0003] Automatic microwave ovens which automatically preset cooking time and complete cooking
through the use of a control relying mainly upon a sensor or sensors monitoring the
progress of food cooking and a microcomputer in an attempt to overcome the above disadvantages
have been developed and have been growing into the leading type of ovens in the industry
of microwave ovens. The automtic microwave ovens are designed to trace variances in
relative humidity, food temperature, odor, gas, etc. resulting from heating of food,
using various kinds of sensors and achieves automatic cooking for only limited items
of food. Those ovens are still unable to perform automatic cooking on some of food
items which faced great difficulties as before with the conventional types of ovens,
for example, eggs.
[0004] Figs. 1 through 4 will give a better understanding of the operating principle and
the progress of cooking when a pot-steamed hotchpotch, a typical example of egg dishes,
is prepared in the conventional type of automatic microwave oven. In Fig. 1, a transformer
1 is connected to energize a magnetron 1 together with a rectifying circuit consisitng
of a capacitor 3 and a diode 4. Food'6 is received in a heating chamber 5. While the
food is cooked with microwave radiations, moisture-ladden air from the food is discharged
via an air outlet 7. A humidity sensor 8 of which characteristic is illustrated in
Fig. 2 is disposed in an exhaust air passageway around the air outlet 7. A voltage
variance across a resistor 9 in series with the humidity sensor is used as a humidity
readout signal. There are further provided a standard signal source 10, a pre-amplifier
11, a minimum value holding circuit 12 and a subtractor 13 for evaluating a humidity
increase with regard to the mininum value and providing a signal C which is proportional
to (A - B), where A is the varying humidity signal h, B is the minumum value h
min and C is the humidity variance Δh as shown in Fig. 3(a).
[0005] A voltage comparator 14 compares the difference signal C with a reference voltage
Vh and provides an output signal when C is in excess of Vh. A switch circuit 15 responsive
to the output signal from the voltage comparator 14 is provided for a contact 16 which
switches on and off a power supply upon starting and discontinuing of cooking.. The
contact 16 is closed upon receipt of a cook start signal STA and opened upon passage
of time T in Fig. 3(a). While the food 6 is heated with the microwave radiations from
the magnetron 2, water vapor is generated and moisture- ' ladden air is discharged.
The humidity sensor 8 senses this discharge air so that the control circuits 11, 12,
13, 14 and 15 place the contact 16 into open position and automatically ends cooking
when an increase in the readout voltage from its minimum level exceeds Vh.
[0006] The progress of heating and cooking will now be discussed in more detail.
[0007] The food 6 hotchpotch material rises gradually in temperature after heating has been
started. Because of the inherent tendency that microwave readiations would be absorbed
by the peripheral portion of the food 6 first, the zone D in Fig. 3(c) demonstrates
a temperature rise earlier than the zone E due to rapid heating by the microwave radiations.
With further advance of heating, the temperature in the zone D reaches up to 70°C
to 90°C for time T
2 in Fig. 3(a) and water vapor begins generating gradually from the zone D as depicted
in Fig. 3(b). However, the readout signal still continues falling during this period.
This is because the microwave radiations continue being generated at a high output
level and the relative humidity of the air passing through the air outlet 7 continues
falling due to a temperature rise at the magnetron 2, the heating chamber 5, etc.
The temperature in the zone D exhibits a sharp increase during the second half of
the time T
-. This is a phenomenon peculiar to egg cooking. There is a trend for protein such
as hovalbumin and lipovitellin, the predominant constituent of eggs to solidify at
about 60°C to 70°C. In the case of pot-steamed hotchpotch, eggs and stock are generally
mixed in the ratio 1:3 to 1:4 and heated together with seasoning. It is noted that
the solidifying point of such mixture is between 75°C and 85°C. The egg-containing
liquid in the zone D where the solidifying point is exceeded, becomes completely solidified
so that convention within the bowl 17 is completely interrrupted with a resulting
sharp rise in temperature.
[0008] No increase in the readout signal is viewed and heating is further advanced since
the relative humidity does not rise though water vapor is slightly generated under
these circumstances. If heating is further kept on for a period T
3 of time, then the temperature in the zone D rises up to about 100°C and a large amount
of water vapor is generated suddenly. This amount of water vapor overrides a decrease
in the relative humidity originating from the temperature rise in the air within the
heating chamber 5 and therefore is developed as the increase Δh in the readout signal,
thus automatically discontinuing heating. At this moment the mixture 6 in the zone
D contains lots of bubbles and becomes hard and solid and disagreeable to the taste.
The mixture in the zone E, on the other hand, is still at 60°C to 70°C and fully in
liquid phase. This section is therefore inedible. If "miso soup" is re-heated and
the boiling point is approached, then good heat conduction is maintained throughout
heating due to convention together with even heat distribution because of no possibility
that "miso soup" may become solidified. No problem is entertained with "miso soup"
additionally because it is usually mixed with something else when people drink it.
In addition, if fresh vegetables are wrapped with a thin sheet and heated, then the
spaces in the vegetables in the wrapping material are full of water vapor during heating
and uniformly heated. For this reason the heating procedure and humidity decision
as described previously present no problem with cooking of vegetables. As noted already,
egg cooking especially pot-steamed hotchpotch has many outstanding problems: final
cooking temperature should be as low as possible because of the amount of water vapor
or gas generated being relatively small; allowance of the final cooking temerature
is very limited; convection is interruped with results in uneven temperature distribution
because eggs become solidified at about 80°C, etc. The conventional types of automatic
microwave ovens are unable to put automated processes of egg cooking into practice.
DISCLOSURE OF THE INVENTION
[0009] It is therefore an object of the present invention to provide a high frequency heating
appliance which makes possible automated egg cooking which was considered as difficult
to achieve in the past years, thanks to skillful design of heating patterns or sequences
and a control circuit taking full advantage of the peculiar nature of egg cooking,
the relation between water vapor and relative humidity, the relation between food
temperature and generated gas and the operating characteristics of the high frequency
heating appliance. Pursuant to the present invention, a heating output is switched
between high and low levels during high frequency cooking of eggs and decision as
to a readout signal from a sensor means is carried out to determine heating item while
heating is kept on at the low output level. The present invention is usefull especially
for pot-steamed hotchpotch or other egg cooking menus.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010]
Fig. 1 is a block diagram of a conventional high frequency heating appliance;
Fig. 2 is a graph showing characteristics of a humidity sensor;
Figs. 3(a), 3(b), 3(c) and 3(d) are graphs for explaining operation of the conventional
appliance;
Fig. 4 is a cross sectional view for explaining cooking state of food;
Fig. 5 is a block diagram of a high frequency heating appliance designed accordign
to an embodiment of the present invention;
Figs. 6(a), 6(b), 6(c) and 6(d) are graphs for explaining operation of the illustrated
embodiment;
Figs. 7(a), 7(b) and 7(c) are illustrations of heating sequences;
Fig. 8 is a front view of an operational panel; and
Fig. 9 is a front view of a modification in the panel illustrated in Fig. 8.
BEST MODE FOR CARRYING OUT THE INVENTION
[0011] Principal components 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 and 16 in
Fig. 5 for sensing water vapor from food 6 are similar to those in the conventional
appliance and detailed descriptions thereof are omitted herein. In Fig. 5, there are
further provided a menu selector 18 for selection of an intended cooking item and
presetting food amounts and a heating sequence memory and selector 19 which also serves
as a timer circuit. A menu selection and amount setting signal is designated IN and
a display 20 is provided to display a menu and an amount selected.
[0012] Referring to Fig. 6, the progress of heating will now be described. When the user
selects "pot-steamed hotchpotch" and presets the amount of food, the signal IN is
fed to the menu selector. Immediately after a start signal STA has been fed to the
menu selector 18 and the relay driver 15, high frequency energy of the maximum output
level, i.e., 600 W, is radiated toward the food 6 for a length T
1 of time. The length T1 of time is selected such that heating is completed before
the food 6 or pot-steamed hotchpotch reaches the solidifying point or 75°C to 85°C.
If the given time length T
1 contained in the time sequence memory and selector has gone by, then the relay driver
15 is supplied with a signal to switch on and off the contact 16 and permit heating
to continue with an intermittent output level or a low output level.
[0013] Attention is invited to the amount of water vapor which the readout signal is developed
and the temperature of the food. The readout signal h traces an abrupt decrease during
heating T1 with the high output level but settles down during heating with the low
output level. This is because a temperature rise in the magnetron 2 and the heating
chamber 5 is very small and minimized by the action of a cooling fan 21 and no variance
in the relative humidity is viewed. Moreover, since the food 6 is not solidified and
heated gradually with the low output level, satisfactory convention is maintained
in . the bowl 17 in Fig. 4 with a minimum of differential temperature between the
zones D and E. If the temperature of the whole of the food 6 reaches 75°C to 80°
C with further advance of heating, the food begins solidifying with generation of a
slight amount of water vapor. Under these circumstances, the relative humidity in
the heating chamber 5 in stable state starts increasing due to the slight amount of
water vapor from the food 6 and such increase is detected as the increase Ah in the
readout signal which in turn provides a signal for the relay driver 15 to place the
relay contact 16 in the full-open position for discontinuation of heating.
[0014] Fig. 7 illustrates heating sequences provided for different amounts of pot-steamed
hotchpotch, in which sequences the maximum output length T
1 beginning with the start of heating is varied depending upon the amount of the food.
The length T
1 is correlated as T
1< T
α where T
a is the time at which the hotchpotch becomes solidified, whatever initial cooking
temperature, surrounding atmosphere and other conditions. Though the amount of the
food is varied, heating is effected automatically with the maximum output level before
the given temperature is reached and thereafter continued with the low .output level
to enalbe only a slightest amount of water vapor to be sensed. Eventually, the humidity
sensor 8 automatically terminates heating. Furthermore, the cycle of intermittent
delivery of the high frequency output is varied based upon the amount setting during
heating with the low output level. By this method of control, it is possible to effect
compensation so as to make the increase in the food temperature constant during the
low output level heating as shown in Fig. 6(c) even if the amount of the food is varied.
This makes possible automatic cooking with stability of final cooking temperature
and uniformity of temperature distribution at all times.
[0015] Fig. 8 shows an operational panel on which cooking items can be selected and the
amount of the food be preset at the same time through single operation to thereby
ensure simplicity of manual operation. Fig. 9 shows an example by which the amount
of the food is preset through rotation of a knob 22 and displayed on the display 21.
The latter of examples demands- only one key for each of the-cooking items on the
operational panel and permits a much more number of the item keys to be mounted on
a limited space of the operational panel. Whereas the foregoing description has set
forth humidity variance, the same effect is available when a gas sensor or an odor
sensor is used.
INDUSTRIAL APPLICABILITY
[0016] As is clear from the foregoing description, it is possible to catch a slight amount
of water vapor, gas or odor which is generated at a final cooking temperature ranging
from 70°C to 85°C during egg cooking, as long as heating is carried out with an appropriate
output level depending upon the amount of the food. Accordingly, the present invention
makes readily possible automatic cooking of eggs which had great difficulties as experienced
in the past years.
[0017] Furthermore, since the optimum condition during heating is switched between the high
and low output levels automatically depending upon the amount of the food, egg cooking
is completed in satisfactory and delicious state most promptly whatever amount of
the food. It is obvious that almost all of the various control circuits used with
the present invention may be implemented with a single microcomputer recently in wide-spread
use and without no substantial money expenditures.
1. A high frequency heating appliance comprising a heating chamber in which food is
received, a high frequency_generator means for generating high frequency radiations
for heating the food, a sensor means for sensing a variance in humidity, gas, odor
or the like in said heating chamber, an output control means for controlling the output
of the high frequency radiations in response to a signal from said sensor means and
a cooking item presetting means, wherein heating sequences as defined by high freguency
output level settings for individual cooking items are selectable and wherein decision
as to said sensor output signal is carried out to control a total of heating time
during egg cooking with an output level lower than the maximum level of the high frequency
radiations.
2. A high frequency heating appliance as set forth in Claim 1 further including a
control means for conducting heating with the maximum output level at least at the
initial phase thereof during egg cooking.
3. A high frequency heating appliance as set forth in Claim 1 further including an
amount presetting means and a control means for selecting and controlling the heating
sequences depending upon the amount presetting, wherein heating is patterned with
the high output level during the first half of heating and with the low output level
during the second half of heating for egg cooking and the length of time where the
high output level heating is carried out is determined automatically depending upon
said amount presetting, and wherein the end of heating is determined automatically
by the output from said sensor means.
4. A high frequency heating appliance as set forth in Claim 3 wherein the low output
level during the second half of heating is selected and controlled automatically depending
upon said amount presetting.
5. A high frequency heating appliance as set forth in Claim 3 further including a
display for displaying cooking items and so forth, said amount presetting being displayed
on said display.