JPS6359053B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a microwave oven that automatically stops heating food in response to a detection signal from a built-in sensor.

Conventional technology There are microwave ovens that use a so-called "automatic heating" method that detects the doneness of food and automatically stops heating, but in conventional methods, the doneness of the food is determined by a detection element such as an infrared sensor, gas sensor, or humidity sensor. I am trying to detect it with . However, infrared sensors are subject to restrictions such as having to place the food in the center of the turntable, and gas sensors are affected by the external environment due to the presence of miscellaneous gases.
Furthermore, even the most desirable humidity sensor has drawbacks such as no detection signal being output when the atmosphere exceeds 70°C, and automatic shutdown in high temperature conditions not being possible.

To be more specific about the latter type of humidity sensor, the ones currently in use are those that use metal oxide ceramics, those that use organic polymer thin films, and those that are made of electrolyte materials such as lithium chloride, all of which are relative. This is a relative humidity sensor (hereinafter referred to as "RH sensor") that detects changes in humidity as changes in resistance. We installed such an RH sensor in the oven exhaust passage of a microwave oven and configured the circuit as shown in Figure 1, and investigated how the output signal of the RH sensor changes when food (e.g., water) is heated. This is the graph in Figure 2. As you can see from the graph, when the water gets close to the boiling point, the resistance value of the RH sensor suddenly decreases and the output signal drops. Conventionally, this decreasing change has been processed by circuit means including a microcomputer and the like to control the heating stop.

However, the atmosphere in which the RH sensor is installed reaches a high temperature of nearly 200 degrees Celsius, for example after cooking in an oven or on a grill. Therefore, when changing the ambient temperature and conducting the same investigation as before, as shown in FIG. 2, the higher the ambient temperature, the worse the detection sensitivity becomes. That is, as the temperature increases, the level change of the output signal gradually becomes smaller, and there is a considerable time delay in the level change even though the food (water) is boiling. This property is unique to the RH sensor and is based on the principle that as the temperature rises, the amount of saturated water vapor increases, while the amount of water vapor emitted from food remains constant and the relative humidity inevitably decreases. Therefore, no matter what kind of RH sensor is provided, the desired operation cannot be expected substantially when the ambient temperature is 70° C. or higher.

In addition, the RH sensor is used during refreshment work (maintenance work) to prevent contamination from food vaporization gas and oil smoke.
This has the drawback that it requires a long stabilization time during refreshing, and the response during moisture absorption and dehumidification is also slow due to chemical adsorption of moisture on the sensor surface (especially when dehumidifying when transitioning from high humidity to low humidity). (significant), and its hysteresis is also large. Furthermore, since the RH sensor output changes exponentially with changes in relative humidity, a logarithmic compression amplifier circuit is essential to linearize the sensor output, and changes in resistance are based on changes in conductivity due to chemisorption. Therefore, the voltage applied to the RH sensor must be an AC power source as shown in Figure 1 to avoid electrolytic effects. There was a problem in that the circuit configuration around the Ikioi sensor became complicated, resulting in a cost disadvantage.

OBJECTS OF THE INVENTION An object of the present invention is to provide a microwave oven that has a simple circuit configuration around a humidity sensor, is low in cost, and can be heated normally even if the ambient temperature around the humidity sensor is high.

Summary of the Invention In order to achieve the above object, the present invention is characterized by providing a microwave oven that automatically stops heating food according to a detection signal from a built-in humidity sensor, wherein the humidity sensor is an absolute humidity sensor. can be,
This absolute humidity sensor includes a first heat-sensitive element housed in a closed container containing dry air and fixed to a terminal via a wire rod, and a first heat-sensitive element housed in an open container and fixed to the terminal via a wire rod. a heat conductive member comprising a heat sensitive element and a second heat sensitive element having the same characteristics and connecting the closed container and the open container to equalize heat; and a heat conductive member fixed to the outside of the heat conductive member. The structure includes a porous member through which air can freely circulate.

Hereinafter, the present invention will be explained with reference to embodiments shown in the accompanying drawings.

Embodiment In the microwave oven of the embodiment, as shown in FIG. 3a, a menu selection key 2 and a heating start key 3 are provided on the operation panel 1, and as shown schematically in FIG. A microwave box 5 equipped with a mounting table 4 at the bottom on which food to be heated in a container is placed.
A magnetron 6 provided above the microwave box 5 heats the food 7. During heating, outside air is sent into the microwave box 5 by a fan 8, and oil smoke and steam from the food 7 are exhausted to the outside through an exhaust passage 9.
An absolute humidity sensor 10 (absolute humidity sensor, hereinafter referred to as "AH sensor") installed in the AH sensor detects the finish of the food.

As shown in FIG. 4, the external structure of this AH sensor 10 has an open area above a terminal block 11 that forms a mounting part for the exhaust passage 9 and fixes four lead terminals, allowing air to freely flow inside. A type sensor 12 and a closed type sensor sensor 13 whose interior is isolated from the outside air are provided in contact with each other, and in order to equalize the heat of these two sensors, a cylindrical heat equalizing tube 14 made of, for example, brass is used to connect the pair of sensors. 12 and 13 are connected, and a wire mesh 15 is covered to protect the sensor section.

FIG. 5 shows the internal structures of the open type sensor 12 and the closed type sensor 13 in detail.

Both the open-type sensor 12 and the closed-type sensor 13 use glass-coated bead-type thermistors 12a and 13a of about 0.5φ as the sense part,
Two thermistors with well-matched characteristics are selected. The bead type thermistors 12a, 13a are attached to hermetic terminals 12c, 13c via thin platinum wires 12b, 13b, respectively. The open type sensor 12 is covered with a metal case 12e in which a ventilation hole 12d is formed, and the joints are welded to form a humidity detection element, while the closed type sensor 1
No. 3 has the same structure as the open-type sensor described above, but a metal case 13e without ventilation holes is attached, dry air is admitted therein, and the joints are welded to form a temperature compensating element. Note that the heat exchanger tube 14 has a ventilation hole 12d.
A hole 14a is formed at a location corresponding to the hole 14a.

The above-mentioned thermistor humidity sensor measures absolute humidity using the difference in thermal conductivity between humid air and dry air, and the principle of its measurement circuit is shown in FIG.

In FIG. 6, _R1 is a humidity detection element corresponding to the open sensor 12 equipped with a ventilation hole 12d,
R ₂ is a temperature compensating element corresponding to the sealed sensor 13 filled with dry air. E is a constant voltage power supply, R ₃ and R ₄ are resistors that constitute bridge circuit B,
R _s is a resistor connected in series with bridge circuit B.
The resistor R _s functions to limit the amount of current that flows, and in this circuit the resistance value is selected so that the temperature of the thermistor is approximately 200°C.

Now, the humidity sensor consisting of resistors R ₁ and R ₂ is placed in a dry air atmosphere, and the resistance values of resistors R ₃ and R ₄ are adjusted so that the output V of bridge circuit B becomes zero. Next, this sensor is placed in a humid atmosphere and the relationship between its output V and absolute humidity is examined. An example of insulation humidity-output characteristics measured in this way is shown in FIG. As is clear from FIG. 7, the absolute humidity and the sensor output have a substantially linear relationship, and the absolute humidity can be directly read from the sensor output.

Considering this qualitatively, the thermistors 12a and 13a (Fig. 5) of this sensor are both at 150°C.
Since it is self-heated to ~200°C, the thermistor 12a of the sensor 12 with ventilation holes receives air containing water vapor (humid air) due to changes in humidity, and its thermal conductivity is higher than that of the thermistor 12a of the sensor 13 containing dry air. Compared to 13a, the amount of water vapor itself varies greatly. In other words, the thermistor 12a exposed to humid air is cooled and the resistance value of _R1 increases, causing the bridge to become unbalanced. This imbalance results in a linear absolute humidity output.

Next, when the AH sensor 10 having the above characteristics is installed and the relationship between the sensor ambient temperature and the sensor output is examined, the characteristics shown in FIG. 8 are obtained. AH
The sensor output voltage exhibits almost the same heating time rise characteristics and heating pattern characteristics even at 200 degrees Celsius as at room temperature (30 degrees Celsius), even if the ambient temperature is high.
This is based on the fact that the amount of moisture generated from the food (absolute amount of moisture) does not change even if the temperature inside the oven box increases. Therefore, unlike conventional RH sensors, it does not become unusable at high temperatures.
At high temperatures of 200°C or higher, the AH sensor responds sharply as long as there is a change in absolute humidity. Since the amount of moisture released from food corresponds to changes in absolute temperature, AH sensors are most suitable for automating food heating.

Additionally, while conventional humidity sensors utilize chemical changes caused by moisture adsorption, thermistors utilize physical phenomena mediated by heat conduction, making them extremely stable and also subject to problems such as surface contamination. No maintenance work is required. Furthermore, since the thermistor itself is in the form of a glass-coated bead, stability is doubled. In addition, there is no hysteresis during moisture absorption and dehumidification, and the response is superior to RH sensors. In addition, since the voltage source can be DC and the output is linear with respect to absolute temperature, there is no need for a special circuit configuration for output conversion, and all of the problems caused by the RH sensor mentioned above are solved.
This can be solved with a sensor.

FIG. 9 shows an example of a specific circuit of a microwave oven using this AH sensor. In FIG. 9, the same reference numerals as those in FIGS. 3, 4, and 6 indicate one or equivalent components.

In FIG. 9, E is a DC constant voltage source, 16 is a DC amplifier, 17 is an A/D converter that converts the analog output of the DC amplifier 16 into a digital value, and 18
is a microcomputer or microprocessor 1
An interface circuit for interfacing with 9, a microcomputer or a microprocessor 19 has a CPU that is the main body of calculation and control.
19a, and stores programs for arithmetic and control processing, table data based on the table in FIG. 7, etc.
It includes a ROM 19b and a RAM 19c for storing external programs and used for registers, flags, and the like. 20 is a transistor that operates according to the output of the microprocessor 19; 21 is a transistor 20;
The relay has a relay coil connected in series with the relay, and its contacts are connected to the commercial power supply line 22.

In operation, first, the menu selection key 2 on the operation panel 1 is pressed, and then the heating start key 3 is pressed. Microprocessor 19 receives this start signal, sends a high level signal to transistor 20 via interface circuit 18, and turns on relay 21. Then, the magnetron 6 oscillates and the food is heated. At this time, the AH sensor 10 (FIG. 3b) disposed in the exhaust passage 9 constantly detects water vapor emitted from the food. AH
The output voltage from the sensor 10 is connected to the bridge circuit B.
The signal is amplified by a DC amplifier 16 via an A/D converter 17, converted into a digital signal, and input to a microprocessor 19 via an interface circuit 18. The input data is prepared in advance by the CPU 19a.
A point-by-point comparison is made with the optimum finishing level value for each temperature stored in the ROM 19b, and the operation of the magnetron 6 is continued until that level is reached. When a large amount of water vapor is generated from the food and the AH sensor 10 detects its absolute amount and reaches the optimal finish level, the microprocessor 19 provides a low level signal to the transistor 20 via the interface circuit 18. Relay 21 is deenergized and its contacts open. Immediately, the oscillation of the magnetron 6 stops, and cooking heating is stopped. Heating stop control is generally performed as described above, but since the capture of sensor output, processing of sensor data in the microprocessor 19, etc. are already known, the details will be omitted.

Although the above embodiments refer only to microwave ovens, the present invention can also be applied to general food heating devices such as gas ovens that require automatic finishing control.

Effects of the Invention As described above, the present invention automatically controls the finishing of food by heating based on the output of an absolute humidity sensor that outputs a sensor output corresponding to absolute humidity. In addition to being able to control the heating stop at any time, it also simplifies the circuit configuration around the humidity sensor, which has great cost advantages.

[Brief explanation of drawings]

Figure 1 is a circuit configuration diagram when using a relative humidity sensor (RH sensor) for heating control, and Figure 2 is a circuit configuration diagram when using a relative humidity sensor (RH sensor) for heating control.
Characteristic diagram of RH sensor according to ambient temperature, 3rd
The figures show examples, a is a schematic diagram of the external structure, and b is a schematic diagram of the internal structure. FIG. 4 shows the external structure of the absolute humidity sensor (AH sensor), and FIG. 5 shows its internal structure. Fig. 6 is a circuit diagram for explaining the principle of measurement by the AH sensor, Fig. 7 is a graph showing an example of its characteristics, Fig. 8 is a graph examining the characteristics of the example in a high temperature atmosphere,
FIG. 9 is a block diagram of the circuit section of the embodiment. 9...Exhaust passage, 10...Absolute humidity sensor,
12... Open type sensor, 13... Closed type sensor, 12a, 13a... Glass coated bead type thermistor, 19... Microcomputer or microprocessor.

Claims (1)

[Claims] 1. In a microwave oven that automatically stops heating food according to a detection signal from a built-in humidity sensor, the humidity sensor is an absolute humidity sensor,
This absolute humidity sensor includes a first heat-sensitive element housed in a closed container containing dry air and fixed to a terminal via a wire rod, and a first heat-sensitive element housed in an open container and fixed to the terminal via a wire rod. a heat conductive member comprising a heat sensitive element and a second heat sensitive element having the same characteristics and connecting the closed container and the open container to equalize heat; and a heat conductive member fixed to the outside of the heat conductive member. A microwave oven characterized by having a structure including a porous member through which air can freely circulate. 2. The microwave oven according to claim 1, wherein the heat-sensitive element is a glass-coated bead-type thermistor.