Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR CONTROLLING A MICROWAVE OVEN, MICROWAVE
OVEN AND ITS USE FOR COOKING OR HEATING FOOD
IN ACCORDANCE WITH THE METHOD
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 sourGe 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-activ-
able, 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 Speci-
fication 9003104-8 discloses such a crisp plate. A brown-
ing 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 thebrowning 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 arrang-
ed in the c~;l; n~ 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 con-
trolled 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 cook-
ing 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 on~ power balance, deter-
mined by the manufacturer, betwee~ thé different sources
of heat.
In some countries, the power consumption of appara-
tus 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 re~uirements on
the utilisation of the heat sources dep~n~; ng on the type
of food at issue, and tends to complicate matters further
for the user.
One object of the invention is, therefore, to pro-
vide a microwave oven which does not suffer from any of
the drawbacks mentioned above, enables advantageous uti-
lisation 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 gene-
ral, 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.
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According to the invention, this object is achiev-
ed 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 con-
trol variable on the basis of one or more sensing para-
meters 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 ca~tegory, 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 cate-
gory, 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 auto-
matically.
By food category or food-category information is
here meant information correspon~;~g to a cooking or
heating program specially designed for a specific type
of food, such as potato products, pizza and chicken, and
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the state of the food when put in the oven, such as fresh
or deep-frozen raw products, and deep-frozen semi-pro-
cessed 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 com-
prises 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 cavi~y, 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 con-
trol variable on the basis of parameter values obt~; nP~
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
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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 foo~d can be divided into
food categories on the basis of t~e type of food and its
initial state. Furthermore, it has been found that opti-
mum utilisation of the heat sources available in a modernoven, 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
25 user-frienAl ;n~s 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 c~;l; ng of the
cavity, and sensors for sensing substance emission from
the food as well as temperature values indicating the
state of the food, is distingl~;shPA 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.
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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 micropro-
cessor 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 ~ is arranged. The crisp
plate 4 is made of sheet aluminiu~ 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 130-230C. The crisp plate is
adapted to rotate during the cooking or heating proce-
dure. SE Patent Specification 9003104-8, as well as the
applicant's microwave ovens of type designations VIP20
and VIP27, illustrates a con~P;vable construction of the
crisp plate as well as its rotary r~ch~n; ~m,
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 (nor-
mally a magnetron) of the oven (see Fig. 2). In the illu-
strated oven, the magnetron, the associated wave guidesystem, the power unit 19 for operating the magnetron,
and the control unit 15 are disposed behind the control
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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 rpm~in~r of the feed system is
designed to feed polarised microwaves through these open-
ings. 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 appli-
cant's microwave ovens mentioned in the foregoing.
A grill element (not shown) is arranged in the ceil-
ing of the cavity, e.g. in the m~nn~r 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, con-
trolled by the control unit 15 (see Fig. 2), shows, among
other things, symbols or plain-text méssages for selected
programs, and rPm~; n; ng 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 but-
ton 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, respective-
ly, starting and switr-h;~g off the oven. The keyset 12
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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. infor-
mation 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
ch~nn~l ( not shown) of the cavity that is disposed in the
space between the c~il;ng of the cavity 3 and the exter-
nal casing 1. A humidity sensor and a temperature sensor
are arranged in the evacuation ~h~nnel to sense the humi-
dity and the temperature of the ventilating air, thesebearing 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 Spe-
cification 9201314-2). Choosing the sensors and position-
ing 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
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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
S 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 sanse the air humidity and
the temperature, respectively, of the ventilating air in
the evacuation ch~nn~l of the cavity. Thus, the control
of the cooking or heating procedure is performed by com-
plete feedback, so that the sensitivity of the cooking orheating procedure to variations in food raw products,
starting temperature and mains voltage is mi n; m; sed. For
more detailed information on the ~onstruction of these
functional units, reference is made to the above-men-
tioned patents and microwave ovens manufactured by theapplicant.
In Fig. 3, the flow chart illustrating a prepro-
grammed cooking or heating program comprises three suc-
cessive time periods T1, T2, T3. Each period is assigned
a m~; ml7m duration determined by the food category.
During the respective periods, the f~;ng 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 micro-
waves and the grill element during the pert~;n;ng periodsare designated, respectively, PM1, PM2, PM3 and PG1, PG2,
PG3. On the basis of information obt~;n~7 from the humi-
dity sensor and the temperature sensor, a control vari-
able c is continuously calculated during the respective
periods as follows
..
,
during Tl
c = al humidity increase + bl temperature
during T2
c = a2 humidity increase + b2 temperature
during T3
c = a3 humidity increase + b3 temperature
wherein
al, a2, a3 and bl, b2, b3 are constants deter-
mined by the food category.
During the respective periods, the period time is
decremented towards zero. When the time has been decre-
mented to zero or when the control variable exceeds a
value cl, 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 PMl, PM2, P~3 and PGl, 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 dif-
ferent 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".
sl Start.
s2 Setting of period times Tl, T2, T3 on the basis
of the food category.
s3 Microwaves at the power PMl. Grill on the power PGl.
Decrementation of Tl.
c;~ ~ 21~i932
.
11
s4 T1 = 0? If Y, proceed to s6. If N, proceed to s5.
s5 Calculation of c. c > cl? 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.
slO T3 = 0? If Y, proceed to s12. If N, proceed to sll.
sll Calculation of c. c > c3? If Y, proceed to s12. If
N, return to s9.
s12 ~n~; ng 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 supply~ng 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 micro-
processor calculates the expected duration Tle, T2e, T3e
of the respective time periods of the program as follows
Tle = kl w
T2e = k2 w
T3e = k3 w
wherein
kl, k2, k3 are constants depPn~i ng on the food cate-
gory.
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It will be appreciated that those skilled in the art
are well qualified to further develop the described pro-
gram 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 correct-
ed in view of the temperature sen$ed. 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. Any modifications derived therefrom are but
measures of convenience to be regarded as ~nromrassed by
the scope of the appended claims.
