Microwave heating apparatus having automatic heating period control
United States Patent Fukui et al. 1451 Apr. 1, 1975 MICROWAVE HEATING APPARATUS 3,662,140 5/1972 Jones et al. 219/1055 HAVING AUTOMATIC HEATING PERIOD 3,716,687 2/1973 Constable 219/1055 CONTROL P E B A R 1d rzmary xammerruce eyno s [75] Inventors Egg g??? l l j watanabe, Attorney, Agent, or FirmCraig &amp; Antonelli o ama, apan [73] Assignee: Hitachi, Ltd., Tokyo, Japan [57] ABSTRACT [22] Fil d; J e 13, 1973 A micrplwave heptingb appargtus coipprising a gnagnetron, a eatingc am er, an asma amounto mate- [21] Appl&#39; 369428 rial capable of absorbing microwave energy which is placed in the heating chamber, wherein microwave [30] F i A n ti P i i -D energy is supplied into the heating chamber until the June 16 1972 Japan 4769478 small amount of material is heated to a predetermined Mar 1973 48 30068 temperature and, when the temperature of the small amount of material reaches the predetermined level, 52 us. 01 219/1055 B the Supply of microwave energy is that the 51 1m. 01. H05b 9/06 heating Pehd is ahhmahcahy determined for any [58] Field of Search 219/1055-55/355 R 359 qhahhty hf the material to be heated hereby form an appropriate heating operation. Upon comple- [56] References Cited tion of the heating process, the material absorbing the microwave energy is cooled immediately to the orig- UNITED STATES PATENTS nial temperature, whereby the apparatus is rendered 73/359 ready for the next heating operation in a short time. e ro e a. 3,523,170 8/1970 Boehm 219/1055 10 Claims, 7 Drawing Figures POWER f5 SUPPLY CONTROL CIRCUIT Y PATEHTEDAPR&#39; H975 SiiiET 1 UF 4 FIG POWER SUPPLY CONTROL f CIRCUIT H minutes PAIEMEBAPR&#39; 1 Ma FIG.3  
 FIG.4  
  t minutes) POWER SUPPLY TO MDGPETRON 6 &#39;IIIIIIIIIIIIIIIIIIIIII F I G .7  
 POWER MICROWAVE HEATING APPARATUS HAVING AUTOMATIC HEATING PERIOD CONTROL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a microwave heating apparatus such as a microwave oven or more in particular to a microwave heating apparatus in which a material to be heated or an object material is appropriately heated by regulating the high frequency electric power supplied to a heating chamber in accordance with the temperature of the object material placed in the chamher.  
 2. Description of the Prior Art In prior art microwave heating apparatus including a microwave oven. the temperature of an object material is controlled by time with a timer. In other words, a heating period of time considered to be proper for the quantity and quality of an object material is set in advance on a timer. and high frequency electric poweris supplied into a heating chamber to heat the object material for the set period of time. Such a prior art method of controlling the temperature of the object material has the disadvantage that it is very difficult to set the timer accurately according to slight differences in the quantity or quality of the object material, often resulting in insufficient or excessive heating thereof. Although the insufficient heating can be compensated for by reheating the object material after resetting the timer, the excessive heating leads to damage of the object material in most cases.  
  In past attempts to obviate the above-mentioned disadvantages. a thermometer is inserted in the heating chamber or directly into the object material to detect the temperature thereof. In the former attempt. the fact that the temperature of the object material is measured by utilization of the convection of air in the heating chamber results in a low accuracy with which the temperature of the object material is detected, while the latter method. in spite of its high accuracy, has the disadvantage that the object material may be deformed. For these reasons, both the methods have yet to be applied commercially.  
 SUMMARY OF THE INVENTION An object of the invention is to provide a microwave heating apparatus which is capable of heating automatically an object material for an appropriate period of time in accordance with the quantity thereof.  
  Another object of the invention is to provide a microwave heating apparatus which is capable of heating an object material automatically as desired by merely setting a temperature to which the object material is required to be heated.  
  Still another object of the invention is to provide a microwave heating apparatus which is rendered ready for the next heating operation in a short period of time after performing the preceding heating operation to thereby perform the next heating operation accurately.  
  A further object of the invention is to provide means by which the generation of microwave energy is stopped automatically in a short period of time if the generated microwave energy is erroneously supplied into the heating chamber in the absence of an object material to be heated, thereby preventing the magnetron from being damaged.  
  In order to achieve the above-mentioned objects, the temperature of an object material is detected by the method mentioned below according to the invention.  
  An appropriate small amount of material which is capable of absorbing high-frequency electric power is placed in a heating chamber, and the temperature of the object material is estimated on the basis of the temperature of the small amount of material. When the object material is great in quantity, small energy is absorbed by the small amount of material and therefore the increase in the temperature thereof is delayed. When the object material is small in quantity, on the contrary, much energy is absorbed by the small amount of material, so that the temperature of the small amount of material is increased at a rapid rate. In view of the fact that a certain temperature of the small amount of material is reached in a short or long period of time depending on whether the object material to be heated is small or great in quantity respectively, it is possible to obtain a predetermined temperature of the object material by continuing to supply high-frequency electric power until the predetermined temperature of the small amount of material is reached.  
  Upon completion of a heating process, the small amount of material absorbing microwave energy is quickly cooled almost to the room temperature prior to the start of the next heating process. This quick cooling operation is effected interlacking with the stoppage of radiation of the high-frequency power into the oven or the opening operation of the door of the oven.  
 BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic block diagram showing the construction of an embodiment of the invention.  
  FIG. 2 is a graph showing the manner in which the temperatures of the small amount of material absorbing microwave energy and the object material are increased.  
  FIG. 3 is a circuit diagram showing an example of the control circuit employed in the apparatus of the inventron.  
  FIG. 4 is a graph showing the manner in which the temperatures of the small amount of material and the object material change when they are reheated.  
  FIGS. 5 and 7 are diagrams showing examples of a device for cooling the small amount of material upon completion of the heating process.  
  FIG. 6 is a circuit diagram showing the essential parts of an example of the power supply circuit and the control circuit.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, reference numeral 1 shows a heating chamber, numeral 6 a magnetron for generating microwave energy, numeral 6a an antenna for applying the microwave energy to the heating chamber 1, numeral 5 a power supply circuit for actuating the magnetron 6 which supplies electric power to the magnetron or cuts off the supply by means of an external control, numeral 7 an object material to be heated, numeral 8 a mount on which the object material 7 is placed, numeral 2 a temperature-detecting element, numeral 3 a small amount of material, mounted on the temperature detecting section of the element 2, for absorbing the microwave energy, and numeral 4 a control circuit for regulating the power supply circuit 5 in response to the temperature detected by the element 2. It sufficies if the small amount of material for absorbing the microwave energy be as small as l cm or less. The small amount of material 3 and the element for detecting the temperature of the small amount of material 3 are placed inside of the heating chamber 1, while on the other hand the output terminal of the temperature detecting element 2 is connected to the control circuit 4. The output of the control circuit 4 is in turn connected to the power supply circuit 5 of the magnetron 6 for controlling the power applied to the magnetron 6. The small amount of material for absorbing the microwave energy is attached to the detecting section of the temperature detecting element 2. Microwave energy produced by the magnetron 6 and applied to the heating chamber 1 through the antenna 6a is absorbed partly by the object material 7 on the mount 8 and partly by the small amount of material 3. As a result, the object material 7 and the small amount of material 3 are heated. The energy absorption ratio between object material 7 and small amount of material 3 depends on the quantity of the object material 7. The greater the quantity of the object material 7, the more the energy absorption by the object material increases. In that case, less energy is absorbed by the small amount of material 3 accordingly.  
  Thus the temperature increase of the small amount of material 3 depends on the quantity of the object material 7, thereby making it possible to detect the temperature of the object material 7 by measuring the temperature of the small amount of material 3. The graph of FIG. 2 shows the result of measurement of the temperature increase with ferrite rubber and water employed as the small amount of material 3 and the object material 7 respectively. The reason why water is employed as the object material is that a major portion of almost all food products consists of water. In the graph, the abscissa shows heating time in minutes, while the ordinate represents the temperature in C. The solid line 11 shows the temperature of the water employed as the object material 7, while the one-point chain 12 indicates the temperature of the ferrite rubber used as the small amount of material 3. When the quantity of the water is doubled, the result is as shown by the solid line 11&#39; and the one-point chain 12 which represent the temperatures of the water and ferrite rubber respectively. As will be apparent from the graph, in spite of the difference of the quantity of the water or object material, the temperatures Tp and T C of the ferrite rubber for different quantities of the water almost agree with each other at the same water temperature, say, T C to which the water is heated. When it is desired to heat the object material 7 to T C, therefore, the temperature of the small amount ofmaterial 3 is detected by the temperature detecting element 2 so that the power supply circuit 5 is controlled by the control circuit 4 thereby to cut off the generation of microwave by the magnetron 6 at the temperature Tp &#34;C of the small amount of material 3. In this way, it is possible to automatically and accurately regulate the temperature of the object material 7 without damage to the shape thereof.  
  The control circuit 4 is provided for the purpose of stopping the power supply to the magnetron 6 when the temperature detected by the temperature detecting element reaches a predetermined level, and any circuit may be employed therefor if it functions to energize a switch at a predetermined temperature. As an example, such a circuit may take the form as shown in FIG. 3 which employs a bridge circuit including a thermistor 2&#39; as the temperature detecting element 2. The thermistor 2 in combination with resistors 13, 14 and 15 constitute a bridge circuit that is well known as a circuit for measuring the temperature of an object material. Upon application of a voltage to the bridge circuit from the power supply circuit 16, a voltage is obtained between points a and b as a function of the temperature detected by the thermistor 2&#39;. Between the points a and b are inserted a series-connected circuit comprising a diode 17, resistor 18 and a temperature-setting power supply 19. The resistance value of thermistor 2 changes with temperature, and when the voltage between points a and b exceeds that of the temperaturesetting power supply 19, current flows through the diode l7, resistor 18 and power supply 19, with the result that a voltage is generated across the resistor 18. The voltage thus produced across the resistor 18 is increased by means of the amplifier 20 to a level required for the controlling of the magnetron 6 and then supplied to the power supply circuit 5 for the magnetron 6, whereby a relay is energized to cut off the power supply thereby to stop the oscillation of the magnetron 6. The temperature detected by the thermistor 2&#39; for stopping the oscillation is set by changing the voltage of the temperature-setting power supply 19. Various degrees of the temperature of the object material corresponding to voltages of the temperature-setting power supply 19 are calibrated on a knob for adjusting the voltage, whereby it is possible to effect an appropriate heating operation simply by designating a desired temperature irrespective of the weight of the object material. If it is desired to heat the object material 7 immediately after taking it out of a refrigerator instead of from room temperature, the appropriate heating is effected by designating a temperature which is greater than the desired temperature by the difference between the room temperature and the temperature in the refrigerator.  
  The problem mentioned below arises when there is insufficient time before the reuse of the microwave heating apparatus. The graph of FIG. 4 shows the result of measurement of temperature increase of the small amount of material 3 and the water that is the object material as in the case of FIG. 2 which is heated a short time after the preceding heating operation.  
  In the preceding heating operation, the small amount of material 3 and water are heated from room temperature T C as in the case shown in FIG. 2. In the graph under consideration, numerals 12a and 11a show the temperatures of the small amount of material 3 and the water respectively. When the small amount of material 3 reaches the predetermined temperature T C corresponding to the desired temperature T C of the water or the object material, the oscillation of the magnetron is stopped thereby to heat the water to T C as mentioned already. At this time, the small amount of material 3 rediates heat and as a result its temperature decreases as indicated by l2c. If, however, the water of the same quantity begins to be heated again, say, 3 minutes after the completion of the preceding heating operation when the temperature of the small amount of material 3 stands at, say, T C without being reduced to the room temperature of T C, the increase in the temperature of the small amount of material 3 follows line 12b, so that it reaches the temperature of Tp C before a minute. In the meantime, the water is heated as shown by 11b and reaches the temperature of only T C when the generation of microwave is terminated, making it impossible to heat the object material to the desired temperature. This problem is solved by providing means for cooling the small amount of material immediately after the preceding heating operation.  
  Examples of such means are shown in FIGS. 5 and 7. In FIG. 5, reference numeral 4 shows a control circuit, numeral 21 a fan, numeral 22 a motor. and numeral 23 a ventilation flue made of a low-loss dielectric material. When the temperature-detecting element 2 detects the fact that the small amount of material 3 has been heated to T]: C, the power supply circuit 5 is controlled by the control circuit 4&#39; in such a manner as to terminate the high-frequency oscillation of the magnetron 6, while at the same time actuating the motor 22. Then the fan 21 connected to the motor 22 is rotated, so that accelerated air flow from outside of the heating chamber is supplied through the ventilation flue 23. The accelerated air flow in the ventilation flue 23 is applied to the small amount of material 3 thereby to forcibly cool it. The reduction of the temperature of the small amount of material 3 to T C of the preheating state is detected by the temperature-detecting element 2, and therefore by utilizing the same element 2, the control is effected to stop the rotation of the motor 22. An example of the circuit which effects the control operation on the above-mentioned principle is illustrated in FIG. 6.  
  The exemplary circuit of FIG. 6 includes only the essential parts for controlling the power supply circuit for the magnetron 6 and the motor 22 of a microwave oven. In the drawing, reference numeral 50 shows a transformer for supplying a high voltage and a filament voltage for actuating the magnetron 6. Symbols S and S show main switches connected to a commercial power supply. 5;, a switch which is provided inside of the heating chamber and energized in response to the opening and closing operation of the door of the apparatus, and S, a switch often, in the form of a push button which is depressed by the operator to start the heating operation. Symbol S shows a switch for stopping the heating operation, which is energized in the apparatus according to the invention when a predetermined temperature of the small amount of material 3 is detected. Symbols 5., and S show contacts for connecting and cutting off the power supply to the transformer 50, and numeral 51 a relay for closing and opening the contacts 5,, and S Symbol S shows a contact for the selfmaintenance of the contacts 5,, and 8,. With the closing of the main switches S and S and the switch S for starting the heating operation, current flows in the relay 51 thereby to close the contacts S S and 8,. As a result, power is supplied to the transformer 50, and the magnetron connected to the secondary winding of the transformer 50 is energized for oscillation. Under this condition, the contact 8,, is in the closed state and therefore the relay 51 continues to be energized even if the switch S, is opened, supplying the electric power to the transformer 50.  
  With the increase in the temperature of the small material 3 by the continued heating thereof, an output is obtained from the amplifier included in the control circuit comprising a bridge circuit, whereby the transistor 53 is turned on and a current flows in the relay 52 thereby to open the switch S The result is the cutting off of current in the relay 5], so that the contacts 5,, and S; are opened and the power supply to the transformer 50 is terminated to complete the heating process. With the energization of the relay 52, the switch S is opened, while at the same time closing the switch S A DC voltage appropriately divided by resistors is applied to a set input terminal of the flip-flop circuit 44, whereby the relay 45 is energized and the switch S is closed thereby to start the motor 22. The rotation of the motor 22 causes the fan to be rotated and as a result the small amount of material for absorbing microwave energy is cooled at a rapid rate. When the output of the amplifier 20 disappears with the decrease in the temperature of the small amount of material 3, the transistor 53 is cut off and the relay 52 is de-energized. The switch S is opened and the input to the input terminal of the flip-flop circuit 44 is reduced to ground potential. In spite of this, the cooling of the small amount of material is continued because a current continues to flow in the relay 45 until an input signal is applied to the reset input terminal 43.  
  In the case where the points a and b of the bridge circuit of FIG. 3 are connected to the emitter and base of the transistor 41 respectively, the transistor 41 conducts thereby to maintain a low collector voltage thereof, as long as the bridge is unbalanced due to the fact that the temperature detected by the temperature detecting element 2 is higher than room temperature. When the temperature of the small amount of material 3 is reduced almost to room temperature. the potential difference between point a and b approximates to zero and the transistor 41 is cut off. The potential of the collector of the same transistor is increased to the source voltage, which is applied to the reset input terminal 43 of the flip-flop circuit 44 so as to reset the same, thereby cutting off the current in the relay 45. As a result, the switch S is opened to stop the motor 22.  
  Instead of detecting the reduction in the temperature of the small amount of material 3, the flip-flop circuit 44 may be replaced by a monostable multivibrator in order that a signal produced at the input terminal 42 is utilized as a trigger signal to continue the energization of the relay 45 for a predetermined sufficient period of time to cool the small amount of material upon completion of a heating process. In such a case, the transistor 41 may be omitted.  
  By connecting in series with the relay 51 an additional switch dependent on the relay 45 in such a manner that the switch remains open as long as the relay 45 is energized, the next heating operation is prevented from being started before the small amount of material 3 is sufficiently cooled, and thus an erroneous operation of the apparatus is avoided. As will be apparent from the above description, after a heating process is completed, the small amount of microwave absorbing material 3 can be cooled in a short time by means of the accelerated air flow from the fan 21, thus preventing an erroneous operation of the apparatus which often occurs when it is used again before a sufficient time elapsed after the preceding heating operation.  
  Another example of the cooling means is shown in FIG. 7, in which it will be seen that an arm 25 with teeth is provided on the door 24 on one side of the heating chamber 1. The teeth of the arm 25 are in mesh with a gear 26 the turning effort of which is imparted to the fan 21 through the belt 27 hung on the gear 26. In this arrangement, the opening of the door 24 for retrieving the object material 7 out of the heating chamber 1 upon completion of the heating thereof causes the arm connected to the door 24 to slide. The sliding of the arm 25 in turn causes the rotation of the gear 26 in mesh with the arm 25, so that the rotation of the gear is imparted through the belt 27 to the fan 21 thereby to rotate the same, with the result that the accelerated air flow is allowed into the ventilation flue 23 thereby to forcibly cool the small amount of microwave absorbing material 3. Another gear or gears may be used in place of the belt 27.  
  Although ferrite rubber is used as the small amount of microwave absorbing material 3 in the abovedescribed embodiments, the materials of the small amount of microwave absorbing material 3 are not limited to ferrite rubber but any material may be used so far as it is heated by absorbed microwave energy, a material with high dielectric loss being more accurately controlled in temperature. Also, the thermistor used as the temperature detecting element 2 may be replaced by any other means including a thermocouple with a corresponding circuit on condition that an electrical constant of such a means changes with temperature.  
  Further, if the apparatus is used erroneously without any object material being inserted therein, all the microwave energy generated is concentrated on the small amount of material 3 and therefore its temperature is increased instantaneously to such a degree as to stop the oscillation of the magnetron, thereby leading to the advantage that operation of the apparatus in the absence of any load is prevented.  
 We claim:  
  1. A microwave heating apparatus of the type heating an object in a heating chamber by microwaves, said apparatus comprising:  
 detection means for detecting temperature in the heating chamber, said detection means including a microwave energy-absorbing member disposed in said heating chamber for absorbing a portion of said microwaves, and a detector for detecting a temperature of said microwave energy-absorbing member, said temperature of said microwave energy-absorbing member changing dependent on the quantity of said object, thereby detecting a temperature of said object to be heated, and  
 control means for controlling the supply of microwaves to said heating chamber as a function of the detection of a predetermined temperature of said object by said detection means.  
  2. A microwave heating apparatus according to claim 1, further comprising cooling means for reducing the temperature of said microwave energy-absorbing member in said heating chamber to a predetermined level prior to heating a second object to be heated.  
 3. A microwave heating apparatus comprising:  
 a heating chamber for containing an object material to be heated;  
 means for generating and supplying microwave energy to said heating chamber for heating the object material;  
 detecting means for detecting a temperature including an energy-absorbing material located within said heating chamber for absorbing a part of said microwave energy in said heating chamber, said perature detected by said detecting means upon the absorption of said part of said microwave enenergy-absorbing material being heated to a tem- I ergy. said part of said microwave energy being inversely dependent on the quantity of said object material; means for generating an electrical signal according to the temperature of said energy-absorbing material; means for generating a reference voltage; means for comparing said electrical signal and said reference voltage so as to generate a control signal when said electrical signal is larger than said reference voltage; and means for applying said control signal to said microwave energy generating means so as to stop the generation of the microwave energy. 4. A microwave heating apparatus according to claim 3, further comprising:  
 means for cooling said energy-absorbing material;  
  and means for energizing said cooling means when the generation of said microwave energy is stopped. 5. A microwave heating apparatus according to claim 4, further comprising:  
  means for de-energizing said cooling means when the electrical signal generated by said electrical signal generating means is reduced to a predetermined level. a 6. A microwave heating apparatus according to claim 4, further comprising:  
 means for de-energizing said cooling means when said cooling means has completed a cooling operation for a predetermined period of time. 7. A microwave heating apparatus according to claim 4, wherein said means for cooling includes:  
 a ventilation flue connected to said heating chamber, a fan for supplying cooling air through said ventilation flue into said heating chamber for cooling said energy-absorbing material so that any effect of the temperature of said energy-absorbing material detecting the temperature of a first object material is eliminated prior to detecting a temperature of a second object material, and a motor operable in response to said control signal for rotating said fan. 8. A microwave heating apparatus according to claim 7, further comprising:  
  means for generating a second control signal when the temperature of said energy-absorbing material is reduced to a predetermined temperature; and means for supplying said second control signal to said motor for stopping the operation of said motor. 9. A microwave heating apparatus according to claim 3, further comprising:  
 a ventilation flue connected to said heating chamber, a fan for supplying cooling air through said ventilation flue into said heating chamber for cooling said energy-absorbing material so that any effect of the temperature of said energy-absorbing material detecting the temperature of a first object material is eliminated prior to detection of a temperature of a second object material, a motor for rotating said fan; and a switch adapted to be closed by said control signal for a predetermined period of time for supplying a power to said motor to rotate said fan for said predetermined period of time. 10. A microwave heating apparatus comprising: a heating chamber having a door for containing an object material to be heated;  
 means for generating and supplying microwave energy to said heating chamber for heating the object material;  
 detecting means for detecting a temperature including an energy-absorbing material located within said heating chamber for absorbing a part of said microwave energy in said heating chamber, said energy-absorbing material being heated to a temperature detected by said detecting means upon the absorption of said part of said microwave en ergy, said part of said microwave energy being inversely dependent upon the quantity of said object material;  
 means for generating an electrical signal according to the temperature of said energy-absorbing material;  
 means for generating a reference voltage;  
 means for comparing said electrical signal and said reference voltage so as to generate a control signal when said electrical signal is larger than said reference voltage;  
 means for applying said control signal to said microwave energy generating means so as to stop the generation of the microwave energy;  
 an arm having a rack the movement of which is operatively associated with the opening and closing of said door of said heating chamber;  
 a pinion being in mesh with said rack of saidarm;  
 a fan adapted to be rotated by the rotation of said pinion;  
 means for generating an air flow by the use of said fan; and a ventilation flue for guiding said air flow to said energy-absorbing material from outside of said heating chamber so that any effect of the temperature of said energy-absorbing material detecting the temperature of a first object material is eliminated prior to detection of a temperature of a second object material.  
  Disclaimer 3,875,361.-Yuki0 Fukml and Mitsum Watanabe, Yokohama, J apan. MICRO- WAVE HEATING APPARATUS HAVING AUTOMATIC HEATING PERIOD CONTROL. Patent dated Apr. 1, 1975. Disclaimer filed J an. 11, 1978, by the inventors, the assignee, H itachz&#39;, Ltd.,  
  assenting. Hereby enter this disclaimer to claim 1 of said patent.  
 [Oyficz&#39;al Gazette M amh Q8, 1978.]  
 Disclaimer 3,87 5,361.Y ukio Fukm&#39; and M itsum Watanabe, Yokohama, Japan. MICRO- WAVE HEATING APPABATUS HAVING AUTOMATIC HEATING PERIOD CONTROL. Patent dated Apr. 1, 197 5. Disclaimer filed J an. 11, 1978, by the inventors, the assignee, H itachz&#39;, Ltol, assenting.  
 Hereby enter this disclaimer to claim 1 of said patent.  
 [Ofiicial Gazette March 28, 1978.]