Apparatus for controlling heating time utilizing humidity sensing

Apparatus for controlling heating time for food placed in a cooking apparatus such as a microwave oven in which heating time required for the humidity of the food which varies with the heating of the food to reach a representative humidity value after abrupt change with a positive gradient is measured, and the product of the measured heating time multiplied by a predetermined heating time coefficient which is inherent in the particular food is added to the measured heating time so that the sum represents the total required heating time for the food.

The present invention relates to an apparatus for automatically controlling 
heating time for a food depending on the food to be heated in a cooking 
apparatus such as a microwave oven. 
In microwave heating, the optimum heating time for a food to be heated is 
determined by various factors such as the initial temperature of the food 
to be heated, the volume of the food, the temperature to which the food is 
to be heated, the specific heat of the food and the microwave power to be 
supplied. 
Heretofore, the heating time in the microwave oven has been determined by 
setting a standard heating time which was experimentarily determined 
depending on the type and volume of the food. 
Such a heating time setting method however involved the disadvantage that 
no special attention was paid to other factors for determining the heating 
time such as the initial temperature of the food, the specific heat of the 
food, the destination temperature and the microwave power, and hence 
proper heating or cooking of the food was not attained. This is because a 
main factor that determines the finished state of the food is not the 
heating time but the temperature rise of the food to be heated per se. 
Thus, if the temperature rise of the food being heating can be detected by 
some means, an optimum heating and cooking of the food will be attained, 
the finished state of which will not be influenced by the initial 
temperature of the food, the volume and the specific heat of the food and 
the microwave power supplied. 
As a method for sensing the temperature rise of the food, it has been 
proposed to insert a temperature sensor directly into the food and to 
sense the temperature rise of the food by a non-contact temperature 
sensor. However, applications thereof are limited because the former 
method requires the direct contact of the temperature sensor with the food 
and the latter method cannot always provide accurate sensing of the 
temperature. On the other hand, it has been known to sense the temperature 
of the food or degree of heating by measuring the change of humidity which 
takes place as the food is heated. For example, in most foods, water 
included therein abruptly evaporates when the temperature of the food 
reaches 100.degree. C and a large amount of water vapor appears in the 
oven. By detecting such change of humidity by a humidity sensor, the time 
at which the humidity abruptly changes can be related to the time at which 
the food has reached 100.degree. C. 
The present invention makes use of such a relation between the food 
temperature and the humidity appearing thereat. 
A method for detecting the humidity generated from the food to control the 
power of a magnetron is disclosed in U.S. Pat. No. 3,839,616 issued to 
Risman. This patent, however, uses a humidity sensor in order to 
periodically interrupt the heating of the food to prevent overheating of 
the food and does not provide automatic heating and cooking as in the 
present invention. 
It is a first object of the present invention to eliminate the setting 
operation of heating time in an oven as represented by a microwave oven, 
which heating time is normally determined taking the volume of the food to 
be heated into consideration. 
It is a second object of the present invention to eliminate the troublesome 
operation of taking a correction of the heating time into consideration, 
which correction is otherwise needed due to the variation of the initial 
temperature of the food. 
It is a third object of the present invention to eliminate the troublesome 
operation of taking a correction of the heating time into consideration, 
which correction is otherwise needed due to the variation in the capacity 
of a thermal energy source such as a magnetron and the variation of the 
microwave absorption factor of the food. 
It is a fourth object of the present invention to enable the detection of 
the temperature of the food under heating without requiring direct contact 
with the food. 
It is a fifth object of the present invention to eliminate a timer setting 
operation for the reasons set forth in connection with the above objects. 
It is a sixth object of the present invention to provide a function which 
enables external adjustment of the heating time so that a user can 
determine the heating time as he desires.

Referring to FIG. 1a, in microwave heating of the food, the change of 
humidity near the food being heating with the heating time generally rises 
abruptly, after the elapse of a certain period of time, with a different 
gradient than the previous one. 
The time at which such abrupt change of gradient appears in reheating the 
food approximates the time at which the temperature of the food reaches an 
optimum temperature, and in many cases it approximates the reheating time 
which has been specified in a prior art microwave oven for the particular 
food according to experience. 
It has been known from the experiment of heating and cooking of the food 
that a certain type of food must be further heated after the humidity has 
reached H.sub.1, for a time period determined by the volume of the food to 
be heated and the particular cooking method therefor. 
Since the time at which the humidity start to increase already includes the 
influence factors such as the amount of the food, the initial temperature 
of the food and the microwave power, there is no need for further taking 
the initial temperature and the volume into consideration when such 
humidity change is related to the temperature of the food and it is thus 
possible to automatically control the heating time. 
Referring now to FIG. 1b, the heating time T.sub.0 for the food is 
generally given by the sum of a time T.sub.1 required to reach a humidity 
value H.sub.1 which represents an abrupt rise of the humidity and a time 
T.sub.2 following T.sub.1, which is determined by the volume of food and 
the type of cooking. That is; 
EQU T.sub.0 = T.sub.1 + T.sub.2 (1) 
since the time T.sub.2 is determined by T.sub.1 and the volume of the food 
and the type of cooking, it can be represented by; 
EQU T.sub.2 = kT.sub.1 (2) 
where k is a coefficient inherent to the particular food. From the formulas 
(1) and (2), 
EQU T.sub.0 = T.sub.1 + kT.sub.1 (3) 
it is thus possible to determine the total required heating time by 
measuring the time T.sub.1 required for the humidity to reach the 
appropriate value H.sub.1 on the steep gradient and obtaining the sum of 
the time T.sub.1 and the product of T.sub.1 multiplied by the factor k 
which is determined by the type of the food and the type of cooking. 
A process for determining the total required heating time can be realized, 
in principle, by the combination of humidity sensing means, a counter for 
counting the time T.sub.1, a multiplier circuit for producing the product 
of T.sub.1 .times. k, a memory for the coefficient k for each type of 
cooking and a counter for counting the time T.sub.2, and according to the 
present invention it can be accomplished by the following simple 
construction. 
Assuming that the period of a clock signal is .tau. (frequency 1/.tau.) and 
n clock signals are counted in T.sub.1 seconds, then 
EQU T.sub.1 = .tau..sub.n (sec.) (4) 
By putting the formula (4) to the formula (2), 
EQU T.sub.2 = k.tau..sub.n (sec.) (5) 
When an up-down counter is used to count the number n wherein counting of 
T.sub.1 is effected in count-up mode while counting of T.sub.2 is effected 
in count-down mode and the circuit is arranged such that the content of 
the counter after the counting in the count-down mode reaches zero at the 
time T.sub.0 = T.sub.1 + T.sub.2, then the heating time T.sub.0 can be 
counted only with the up-down counter. 
When such a counting system is used, the content in the count-up mode and 
the content in the count-down mode are equal to each other. Therefore, in 
order to satisfy the relation of T.sub.2 = kT.sub.1, the period of the 
clock signal in the count-down mode should be set to be k times as large 
as the period of the clock signal in the count-up mode, as seen from the 
formulas (4) and (5). 
The frequency of the clock signal may be changed by changing circuit 
constants which determine the frequency of a clock signal generating 
circuit, such as a capacitance C or resistor R. In other words, the 
coefficient k which is inherent to the particular food to be heated can be 
related to the magnitude of the circuit constant C or R. 
A particular circuit configuration based on the above principle is shown in 
FIG. 2b. 
In FIG. 2b, a humidity sensor 1 has a characteristic which exhibits a 
decrease of resistance with an increase of humidity as shown in FIG. 2a. 
The humidity sensor 1 is mounted at a suitable location in an oven to 
detect the humidity in the heating cavity. For example, the sensor may be 
located in the path of exhaust air flow from the heating cavity. As a 
typical example, a titanium oxide (TiO.sub.2) ceramic humidity sensor has 
an excellent response, stability and reliability. An amplifier circuit 2 
converts the change in the resistance of the humidity sensor 1 to a 
voltage and amplifies the same. A level comparator circuit 3 compares the 
output magnitude Ha of the amplifier circuit 2 with a preset reference 
magnitude H.sub.1 and produces a binary signal of either high level i.e. a 
"1" signal or low level i.e. a "0" signal depending on the relation 
Ha.gtoreq.H.sub.1 or Ha<H.sub.1. Reference numeral 4 designates an 
inverter circuit, and 5 and 6 designate three-input AND gates, the outputs 
of which are applied to an up-down counter 7 as counting input signals. 
The up-down counter 7 operates in the count-up mode when the output of the 
AND gate 5 is UP and in the count-down mode when the output of the AND 
gate 6 is DN (down). A CLA signal clears the contents of the counter. 
The counter is a binary counter and the states of the respective bits are 
taken out. A decoder 8 receives the output signals for the respective bits 
of the counter 7 and the output ZERO thereof assumes the "1" state only 
when all of the bit output signals are "0". 
A two-input AND gate 12 produces a "1" output only when both of the signals 
at HDET and ZERO outputs are in the "1" state. 
Reference numeral 9 designates a flip-flop and the output signal OUT of 
which assumes the "1" state in response to a start signal STA and assumes 
the "0" state in response to the output signal from the AND gate 12. 
The output signal OUT of the flip-flop 9 is applied to a drive circuit 13 
for a magnetron 14. The CLA signal also clears the flip-flop 9 to render 
the output signal OUT to assume the "0" state. 
Reference numerals 10 and 11 designate pulse generator circuits, the 
oscillation frequencies of which are varied with the magnitudes of 
resistors and capacitors. Typically they may be astable multivibrators. 
Switches S.sub.1, S.sub.2, S.sub.3 . . . S.sub.n are food group selection 
switches which select a desired resistor which is one of the parameters to 
determine the pulse period, in order to relate the coefficient k 
determined by the particular food to the period of the clock pulse as 
described above. The operation of the circuit will now be described with 
reference to FIGS. 1 and 2. 
The CLA signal clears the flip-flop 9 and the counter 7. This may be 
effected by a circuit arrangement which automatically produces the CLA 
signal upon the power being turned on, although such a circuit is not a 
part of the present invention. 
The selection switches S.sub.1, S.sub.2, S.sub.3 . . . S.sub.n select one 
of the clock pulses CLDN having a period which is k times as large as the 
period T of the clock pulse CLUP produced by the pulse generator circuit 
10. In other words, the selection switches select one item of food to be 
cooked. After the item of food has been selected, a heating start switch 
is depressed so that a start signal STA is developed to set the flip-flop 
9. Thus, the output signal OUT assumes the "1" state. When this occurs, 
the drive circuit 13 powers the magnetron 14 so that the food is subjected 
to the heating condition. 
On the other hand, the output HDET of the level comparator remains "0" 
until Ha reaches H.sub.1 and the output HDETN of the inverter 4 remains 
"1". Since the OUT signal is "1", the AND gate 5 opens so that the clock 
pulses CLUP are applied to the up-down counter as the count-up input 
signal UP. Thus the pulses CLUP are serially counted up. 
As heating proceeds, humidity increases. When the input signal Ha of the 
level comparator circuit 3 reaches H.sub.1, the HDET assumes the "1" state 
and the HDETN assumes the "0" state. At the sametime, the AND gate 5 is 
closed and the count-up input signal UP ceases, and the AND gate 6 is 
opened so that the clock pulses CLDN are applied to the counter 7 as the 
count-down input signal DN. Thus, the contents of the counter 7 are 
counted up until the time at which H.sub.1 is reached and are thereafter 
counted down by the pulses CLDN. The period of the pulses CLDN is selected 
by the selection switches and it corresponds to k. 
The contents of the counter 7 are applied to the decoder 8 which monitors 
the "0" content of the counter 7. As the contents of the counter are 
counted up until the time T.sub.1 and are thereafter counted down, the 
contents of the counter 7 reach "0" when the time kT.sub.1 = T.sub.2, 
which is determined by the period of the clock pulses CLDN, has elapsed. 
At this time, the output ZERO of the decoder 8 is in the "1" state. On the 
other hand, after the time T.sub.1, the HDET is at the "1" state. Thus, 
the AND gate 12 is actuated and the flip-flop 9 is reset. That is, the 
output OUT is inverted to the "0" state so that the drive circuit 13 
ceases to supply power to the magnetron resulting in a cessation of 
heating. Accordingly, by merely depressing one of the cook item selection 
switches, the user can heat the food automatically for an appropriate 
heating time (T.sub.1 + kT.sub.1) without using a timer. 
However, the palate of human beings differs from person to person and the 
cooked state of the food heated in the above automatic heating system is 
an average. If the heating time can be externally controlled within an 
appropriate range in the above automatic heating system depending on the 
requirements of individuals, more satisfactory heating will be obtained. 
Thus, in the formula (3), T.sub.1 is determined by the humidity. If k can 
be varied in the range of .+-. .DELTA.k around the center value k.sub.o, 
then the total heating time T.sub.0 is represented by: 
##EQU1## 
where k .congruent. k.sub.o .+-. .DELTA.k. Thus, the total heating time 
can be adjusted by the amount .+-. .DELTA.kT.sub.1. The coefficient k 
corresponds to the period of the clock pulses CLDN, which in turn 
corresponds to the magnitude of the resistance which is the parameter for 
the period. Therefore, by changing the resistance corresponding to the 
coefficient k to assume R.sub.0 .+-. .DELTA.R, the formula (6) can be 
satisfied. FIG. 3 shows an embodiment of such a clock pulse generator. In 
FIG. 3a, a period .tau..sub.p of the pulses generated is given by the 
following formula: 
EQU .tau..sub.p = 0.7 (R.sub.A + 2R.sub.B)k (7) 
where 
EQU k = k.sub.v r (0.ltoreq.k.sub.v .ltoreq. 1) (8) 
FIG. 3b shows the internal configuration of the element 16 in FIG. 3a, and 
it corresponds to a standard timer IC 555 (Integrated Circuit). 
FIG. 4a shows a particular circuit wherein the functions of the counter 7 
and the decoder 8 in the embodiment of FIG. 2b are combined. It is 
realized by a standard MSI (binary up/down counter) 74193. The decoder 
produces an output when the contact of the counter is all "0". NC in FIG. 
4a designates non-connected terminal. 
FIG. 4b shows a timing chart for the standard MSI/74193. 
Referring to FIG. 5 which shows a cross-section of a microwave oven, a fan 
20 driven by a motor 21 is used to supply cooling air flow forcibly and 
also to cause the air flow to be exhausted from an exhaust port 22 through 
a heating cavity 24 along an air-flow path 23. The humidity sensor 1 is 
located, for example, down stream of the air-flow path 23. Alternatively, 
cooling air flow which is also used to cool a magnetron and other 
electrical devices may be utilized.