JPH0450529B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to an absolute humidity sensing device, and more particularly to an absolute temperature sensing device that enables cost reduction.

Prior Art It is known to use an absolute humidity sensor (hereinafter referred to as AH sensor) as a detection element in an automatic food finishing control device for a microwave oven. 1st
As shown in the figure, the AH sensor 10 is composed of a closed sensor 13 and an open sensor 12.
A thermistor 13a is connected and supported between two terminals 13c, 13c via a wire 13b inside a closed container 13e filled with dry air. Further, a thermistor 12a is connected to two terminals 12 through a wire 12b inside a container 12e that is open to the outside.
c, 12c and supported. The thermistors 13a and 12a have the same characteristics. Further, the containers 13e and 12e are connected by a heat equalizing tube 14 made of a heat conductive material in order to equalize the heat between the closed container 13e and the open container 12e. As shown in FIG. 2, the AH sensor thus formed is supported on the terminal block 11 and covered with a wire mesh 15 through which air can freely circulate. Lead wires 16, 16, 17, 17 connected to the above-mentioned terminals 13c, 13c, 12c, 12c, respectively, pass through the terminal block 11 and are respectively connected to four terminals 18 protruding below the terminal block 11. .

The thermistor AH sensor measures absolute humidity using the difference in thermal conductivity between humid air and dry air, and the principle circuit for humidity measurement is shown in Figure 3. In FIG. 3, _R1 is a thermistor corresponding to the open sensor 12 equipped with a ventilation hole 12d,
R ₂ is a temperature compensation thermistor corresponding to the sealed sensor 13 filled with dry air. E is a constant voltage power supply, R ₃ and R ₄ are resistors forming bridge circuit B, and 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 will be around 200 degrees Celsius due to Joule heat.

Now, the humidity sensor consisting of resistors R ₁ and R ₂ is placed in a dry air atmosphere, and the resistance value of resistor R ₄ is adjusted so that the output voltage V _B of bridge circuit B becomes zero. Next, place this sensor in a humid atmosphere and examine the relationship between its output voltage V _B and absolute humidity. An example of the absolute humidity-output characteristics measured in this way is shown in FIG. As is clear from Figure 4,
Absolute humidity and sensor output voltage have a nearly linear relationship, and absolute humidity can be read directly from sensor output voltage.

If we consider this qualitatively, the thermistor 12 of the AH sensor used in microwave ovens
a and 13a (Fig. 1) are both self-heated to 150 to 200°C by Joule _heat . ), and its thermal conductivity is the thermistor 13a of the sealed sensor 13 containing dry air.
That is, compared to the compensation side thermistor _R2 , it varies greatly depending on the amount of water vapor itself. In other words, the thermistor _R1 exposed to humid air cools and its resistance increases, causing the bridge to become unbalanced. This imbalance results in an output that is linear and corresponds to the absolute humidity.

5 and 6 are a plan view and a front view, respectively, of a microwave oven using the above-mentioned AH sensor, and show a magnetron 6 disposed inside the microwave oven 30.
The food 34 placed on the turntable 33 is caused by the electromagnetic waves emitted from the waveguide 32 due to the oscillation of the
is heated. Gas containing water vapor generated from the food 34 by this heating is sent into an exhaust duct 36 by a fan 35, and is discharged from the exhaust duct 36 to the outside of the microwave oven 30 via an exhaust port 37. AH sensor 1 on the inside surface of the exhaust duct 36
0 is attached and the absolute humidity of the gas passing through the exhaust duct 36 is detected. As shown in FIG. 7, the front surface of this microwave oven is provided with an operation panel 1, and on this operation panel 1 are provided a menu selection key 2 and a heating start key 3.

FIG. 8 shows an example of a specific circuit configuration of a microwave oven using the above-mentioned AH sensor. In addition, the 8th
In the drawings, the same reference numbers as in FIGS. 2, 3, 5, 6, and 7 indicate the same or equivalent parts.

In FIG. 8, 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 19.
The interface circuit for interfacing with the microcomputer or microprocessor 19 includes a CPU that is the main body of calculation and control.
19a, a ROM 19b for storing programs for arithmetic and control processing, table data based on the table in FIG. 4, and the like, and a RAM 19c for storing external programs and for registers, flags, and the like. 20 is a transistor that operates according to the output of the microprocessor 19; 21 is a transistor 2;
A relay with a relay coil connected in series to 0 and its contact point is the commercial power supply line 22 that supplies power to the magnetron 6.
is interposed.

In operation, first, the menu selection key 2 on the operation panel 1 is pressed, and then the heating start key 3 is pressed. The microprocessor 19 receives this start signal and sends a "High" level signal to the transistor 20 via the interface circuit 18.
Turn on relay 21. Then, magnetron 6
oscillates and heats the food. At this time, the AH sensor 10 disposed in the exhaust passage 9 detects water vapor emitted from the food. The output voltage of the AH sensor 10 is sent to the DC amplifier 1 via the bridge circuit B.
6, converted into a digital signal by an A/D converter 17, and inputted to a microprocessor 19 via an interface circuit 18. The input data is finally compared and determined by the CPU 19a with the optimum finishing level value for each temperature stored in advance in the ROM 19b, and unless the input data reaches that level, the magnetron 6
continue to operate. 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 finishing level, the microprocessor 19 gives a "Low" level signal to the transistor 20 via the interface circuit 18. .
The relay 21 is then 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, details will be omitted.

If the temperature characteristics, so-called B constants, of the two thermistors R ₁ and R ₂ of the AH sensor are uniform, the AH
No matter how much the ambient temperature T of the sensor changes, the thermistors R ₁ and R ₂ both exhibit the same change. Therefore,
The output voltage V _B of bridge circuit B (Fig. 3) is
As shown by the solid line in the figure, it does not vary depending on the ambient temperature T. In this case, the AH sensor has a good so-called zero balance in which no temperature drift occurs.

By the way, the above explanation assumes that the zero balance of the AH sensor is maintained well, but it is generally difficult to make the temperature characteristics of the two thermistors R ₁ and R ₂ uniform from a manufacturing standpoint. . In other words,
Zero balance is usually off. If the zero balance is unbalanced, an output voltage V _B is generated in the bridge circuit from the beginning due to temperature drift, as shown by the dashed line and dashed line in FIG. Moreover, the larger the output voltage V _B is, the larger the amount of change in the output voltage V _B will be as the ambient temperature T rises. Looking at this in terms of the output voltage V _AH of the absolute atmosphere detection device, the factors that change the output voltage V _AH are not only the water vapor generated from the food but also the heating atmosphere temperature T.
The change in the output voltage _VB of the bridge circuit due to the rise in VB is also superimposed. For this reason, the 10th
As shown in the figure, the change in the output voltage V _AH of the absolute humidity detection device with the heating time u becomes a curve as shown by a dashed line or a broken line. That is, the standard level
If the time to reach V _AHO is shorter than the optimal heating time u ₁ when zero balance is good,
It can become a long time. In other words, a problem arises in that the finished quality of the food becomes unstable.

Conventionally, the only way to maintain consistent food quality due to the above-mentioned imbalance in the zero balance of the AH sensor was to select a thermistor for the AH sensor with a good zero balance.
Therefore, the yield of AH sensors deteriorates, and even if they are mass-produced, there is a limit to what can be used, and the cost of AH sensors increases.

Purpose The present invention was made in view of the above problems, and its purpose is to provide an absolute humidity detection device that can be used even with an AH sensor with poor zero balance.

In order to achieve the above-mentioned object, the absolute humidity detection device of the present invention includes a first thermistor exposed to humid air, a second thermistor exposed to dry air, and an opposite side relationship between these two thermistors. and a bridge circuit having two resistors, and a signal corresponding to the absolute temperature of the humid air is generated at the output terminal of the bridge circuit from the temperature change of the thermistor caused by the difference in thermal conductivity between the humid air and the dry air. In the absolute humidity detection device configured to obtain
In series with the thermistor of the bridge circuit, the above 2
It is characterized by connecting a low resistance to correct whether the output of each thermistor occurs on the + side or on the - side depending on the temperature characteristics.

Example An embodiment of the present invention will be described below.

The results of an experiment using a sensor with good zero balance in the circuit shown in Figure 3 above are shown in the 11th section.
As shown in the figure. In this case, since the zero balance is good, the weekly voltage of bridge circuit B is
V _B is zero volts as shown by the solid line. Therefore, if a low resistance of, for example, IΩ or less is inserted between the input terminal B ₁ of the bridge circuit B and the detection side thermistor R ₁ , the output voltage V _B of the bridge circuit B with respect to a change in temperature T will be shown by the dashed line. Drift to the + side like this. Conversely, if a low resistance of 1Ω or less is inserted between the compensation side thermistor R ₂ and input terminal B ₂ ,
The output voltage V _B drifts to the negative side as shown by the broken line. On the contrary, this result shows that the zero balance is poor.
This shows that when using an AH sensor, temperature drift can be corrected by inserting a low resistance into the bridge circuit.

The bridge circuit shown in FIGS. 12 and 13 is
This circuit performs correction using the method described above. This bridge circuit is replaced with bridge circuit B of the microwave oven circuit shown in FIG. That is, a DC constant voltage source E is connected between the input terminals B ₁ and B ₂ , the output terminal B ₃ is connected to the - input terminal of the DC amplifier 16, and the output terminal B ₄ is connected to the + input terminal of the DC amplifier 16. do. The output terminal of the DC amplifier 16 is connected to an A/D converter 17 (FIG. 8). In this case, the AH sensor 10 has poor zero balance,
The detection side thermistor R ₁ exposed to humid air and the compensation side thermistor R ₂ exposed to dry air have uneven temperature characteristics. These two thermistors
Resistors R ₃ and R ₄ are connected opposite R ₁ and R ₂ .

Now, if the AH sensor 10 experiences a temperature drift on the + side with respect to temperature changes, as shown by curve a in Fig. 14, as shown in Fig. 12, the compensation side thermistor R ₂ of the bridge circuit is input. Insert a low resistance R _C in series with thermistor R ₂ between terminal B ₂ . In addition, if the AH sensor exhibits a negative temperature drift with respect to temperature changes, as shown by curve b in Figure 14, the input terminal _B1 of the bridge circuit and the detection side thermistor are connected as shown in Figure 13. Insert a low resistance R _C between R ₁ and in series with the thermistor R ₁ .
In this way, the output voltage of the bridge circuit V _B
becomes zero volts as shown by curve c in FIG. 14 as the temperature changes. In other words, by inserting a correction resistor into the bridge circuit, the deviation due to the temperature drift of the AH sensor itself and the deviation due to the inserted resistor cancel each other out, and the bridge circuit responds to temperature changes like a sensor with good zero balance. The output voltage of will be zero volts.

FIG. 15 shows the amount of correction for temperature changes when the above-mentioned correction resistor R _c is inserted into the detection thermistor side or the compensation thermistor side, using the resistance value of the correction resistor R _c as a parameter. The larger the resistance value of the correction resistor R _C is, the larger the correction amount becomes.

Figure 16 shows the heating pattern of food in a microwave oven equipped with a conventional absolute humidity detection device (shown by the solid line) and the heating pattern of food in a microwave oven equipped with an absolute humidity detection device with a correction resistor inserted into the bridge circuit (shown by the broken line). ), and when a correction resistor is inserted, since the resistance value of the correction resistor is small, it has no effect on humidity detection sensitivity, and the food is heated in the same pattern as when no correction is performed.

As mentioned above, the zero balance correction of the AH sensor using this low resistance has no effect on the humidity detection sensitivity when food is heated, and can only be corrected for the zero balance due to the problematic temperature drift. can.

Effects As explained above, in the present invention, a bridge circuit having two thermistors exposed to humid air and dry air is used to prevent humidity from changing in temperature of the thermistor due to the difference in thermal conductivity between humid air and dry air. In an absolute humidity detection device that detects the absolute humidity of air, a low resistance is connected in series with the thermistor of the bridge circuit, and the output is in the + direction or - direction due to temperature drift caused by the uneven temperature characteristics of the two thermistors. Since this phenomenon is corrected by the low resistance mentioned above, even two thermistors with uneven temperature characteristics can be used as an AH sensor in a microwave oven.
The yield of the AH sensor is improved and the price of microwave ovens equipped with this AH sensor can be reduced. Furthermore, it is possible to improve the finish of food cooked in a microwave oven.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of the AH sensor, Figure 2 is the AH sensor
Figure 3 shows the state in which the sensor is assembled to the terminal block.
The figure is a circuit diagram showing the principle circuit of humidity measurement, Figure 4 is a graph showing absolute humidity-output characteristics, Figures 5 and 6 are diagrams showing the schematic configuration of a microwave oven using an AH sensor, and Figure 7 is a diagram showing the front configuration of the microwave oven, Figure 8 is a circuit diagram showing the circuit configuration of a microwave oven using an AH sensor, and Figure 9 is a diagram showing the ambient temperature -
Graph showing bridge circuit output voltage characteristics, 10th
The figure is a graph showing the heating time-absolute humidity detection device output characteristics, Figure 11 is a graph showing the temperature-output voltage characteristics when using an AH sensor with good zero balance, and Figures 12 and 13 are graphs showing the output characteristics of the present invention. The circuit diagram showing one example, Figure 14, has poor zero balance.
A graph showing the temperature-output voltage characteristics when correction is performed using the AH sensor. Figure 15 is a graph showing the temperature-output voltage characteristics using the resistance value of the correction resistor as a parameter. It is a graph showing a heating pattern of food. R ₁ , R ₂ ... Thermistor, R ₃ , R ₄ ... Resistor, R _C
...Low resistance.

Claims (1)

[Claims] 1 a first thermistor exposed to humid air;
The bridge circuit includes a second thermistor exposed to dry air and two resistors on opposite sides of the two thermistors. In an absolute humidity detection device that obtains a signal corresponding to the absolute humidity of humid air at the output terminal of the bridge circuit from a temperature change, the above-mentioned signal generated on the + side or - side depending on the temperature characteristics is connected in series to the thermistor of the bridge circuit. An absolute humidity detection device characterized in that a low resistance is connected to correct temperature drift in the output of two thermistors.