JPH05164722A - Humidity detecting circuit - Google Patents

Abstract

PURPOSE:To obtain a humidity detecting circuit enabling improvement of precision in detection and a yield. CONSTITUTION:In order to correct nonuniformity in the characteristic of two temperature-sensitive elements Rs and Rr, the potential at a joint of a resistor R3 and a bridge circuit constructed of the temperature-sensitive elements Rs and Rr and resistors R1 and R2 is detected by a control part 10. The control part 10 changes the resistance value of a resistor R5 in accordance with this detected potential and sets the resistance value of the resistor R5 at an optimum resistance value at which no temperature drift occurs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a humidity detecting circuit, and more particularly to an improvement of a humidity detecting circuit for detecting the degree of finishing of an object to be cooked by dielectric heating in a cooking apparatus such as a microwave oven. is there.

[0002]

2. Description of the Related Art In a conventional cooking device such as a microwave oven, an absolute humidity detector or the like is installed in the vicinity of an exhaust port of a cooking cabinet to generate water vapor generated by heating an object to be cooked. Known means for detecting and controlling the finish of the food to be cooked is used.

FIG. 5 shows an example of a conventional humidity detecting circuit in the case where an element having a positive temperature characteristic such as a platinum resistor is used as a temperature sensitive element of such a detector.
As shown in the figure, the circuit includes two temperature sensitive elements Rs and Rr.
The first temperature sensitive element Rs is exposed to the atmosphere,
The second temperature-sensitive element Rr is hermetically sealed, the first and second temperature-sensitive elements Rs, Rr are self-heated to substantially the same temperature, and a change in the heat transfer coefficient due to a change in humidity in the atmosphere is first sensed. A humidity detector that detects the absolute humidity while detecting the change in the resistance value of the temperature element Rs and compensating the temperature change of the atmosphere with the second temperature sensitive element Rr is configured. Further, while a power supply of a power supply voltage Eo for operating the humidity detector is provided, first and second two which form a bridge circuit for detecting the resistance ratio of the two temperature sensing elements Rs and Rr are provided. Resistors R1, R2
And a voltage conversion circuit, which will be described later, for converting the resistance ratio of the temperature sensitive elements Rs and Rr into a voltage.
Rr is connected in series, one terminal is connected to the power supply of the power supply voltage Eo, the other terminal is connected to the ground side, and the first and second resistors R are connected in parallel to the two temperature sensing elements Rs and Rr.
A bridge circuit is configured by connecting 1 and R2 in series, and the connection point of the first and second temperature sensitive elements Rs and Rr and the connection point of the first and second resistors R1 and R2 are connected to the voltage. It is connected to a conversion circuit and detects the absolute humidity amount from the output of the voltage conversion circuit.

Here, the midpoint between the two temperature sensitive elements Rs and Rr is connected to the inverting input terminal of the operational amplifier OP by the input resistance R of OP.
connected via c, while two resistors R1,
The middle point of R2 is connected to the non-inverting input terminal of the operational amplifier OP. Further, a negative feedback resistor Rf of OP is connected between the midpoint of the operational amplifier OP and the input resistance Rc of OP and the output terminal of the operational amplifier OP. OP, Rc, R
The voltage conversion circuit is configured by f.
When the output voltage of the OP in such a configuration is represented by Vout, it is represented by the following equation (1). However, Av is an amplification factor and is represented by Av = Rf / Rc.

FIG. 6 shows an example of a conventional temperature detecting circuit in the case where an element having a negative temperature characteristic such as a thermistor is used as the temperature sensitive element of the detector. As can be seen in the figure, the resistance of an element with negative characteristics such as a thermistor decreases at high temperature, and when driven by a constant voltage power supply, the self-heating temperature rises sharply with increasing ambient temperature, and in the worst case, The current limiting resistor Rd is usually inserted between the power source and the power source because it leads to element destruction. Other configurations are substantially the same as those of the platinum resistor described above. That is, it is equipped with two temperature sensitive elements Rs and Rr, the first temperature sensitive element Rs is exposed to the atmosphere, the second temperature sensitive element Rr is sealed, and the first and second temperature sensitive elements are arranged. R
s and Rr are self-heated to approximately the same temperature, and a change in the heat transfer coefficient due to a change in humidity in the atmosphere is detected by a change in the resistance value of the first temperature sensing element Rs, and a temperature change in the atmosphere is detected as a second temperature sensing A humidity detector that detects the absolute humidity while compensating with the element Rr is configured. Further, while a power source of a power source voltage Eo for operating the humidity detector is provided, the two temperature sensing elements R are provided.
First and second two resistors R1 and R2 that form a bridge circuit for detecting the resistance ratio of s and Rr, and a current limiting resistor Rd that limits the current value flowing in the humidity detector.
And a voltage conversion circuit, which will be described later, for converting the resistance ratio of the temperature sensitive elements Rs and Rr into a voltage.
Rr is connected in series and one terminal is a current limiting resistor Rd
, The other terminal is connected to the ground side, and the first and second resistors Rs and Rr are connected in parallel with each other.
1, R2 are connected in series to form a so-called bridge circuit, and the other terminal of the current limiting resistor Rd is connected to the power supply voltage E.
connected to the power source of 0, the connection point of the first and second temperature sensitive elements Rs and Rr and the connection point of the first and second resistors R1 and R2 to the voltage conversion circuit, and the voltage conversion Circuit output and first,
An arithmetic processing unit 3 for arithmetically processing the midpoint voltage V _{T of} the second resistors R1 and R2 and an output of the arithmetic processing unit 3 detect the absolute humidity amount.

Also in the voltage conversion circuit, the midpoint of the two temperature sensitive elements Rs and Rr is connected to the inverting input terminal of the operational amplifier OP through the input resistor Rc, as in the case of the platinum resistor described above, and The middle point of the two resistors R1 and R2 is connected to the non-inverting input terminal of the operational amplifier OP, and the OP negative terminal is provided between the intermediate point between the operational amplifier OP and the input resistor Rc and the output terminal of the operational amplifier OP. The feedback resistor Rf is connected to form a voltage conversion circuit. Further, the output voltage Vout of the output OP of this voltage conversion circuit and the midpoint voltage V _{T of the} two resistors R1 and R2 are processed. The output Vout / V _T obtained by the arithmetic processing by the unit 3 is subjected to arithmetic correction to obtain an absolute humidity output. Note that Vf is a bridge circuit voltage.

The output voltage V of the OP in such a configuration
out is represented by the following equation (2). However, Av is the above amplification factor, Av = Rf / Rc, Vf is the bridge circuit voltage, and Vf = Eo × Zs / (Zs
+ Rd) Zs is a combined resistance value of the bridge circuit, and Zs = (Rs + R
r) × (R1 + R2) / (Rs + Rr + R1 + R2). However, in the equation (2), since Vf changes due to changes in thermistor resistance values Rs and Rr due to temperature changes,
In order to correct this value, for example, JP-A-60-322.
As seen in Japanese Patent No. 88, the midpoint voltage V of R1 and R2
_The absolute humidity output is obtained by calculating Vout / VT by the following equation (3) using _T. However, C is a coefficient. As shown in the above equations (1) to (3), the ratio of the first temperature sensitive element Rs and the second temperature sensitive element Rr is converted into a voltage to detect the absolute humidity amount.

[0008]

FIG. 7 and FIG. 8 show a case where the ambient temperature rises from Ta to Tb to Tc and the absolute humidity amount is constant between Ta and Tb, and the humidity gradually increases between Tb and Tc. 2 shows the Rr-Rs characteristics of the platinum and thermistor.
In an ideal state, the relationship between Rs and Rr between Ta and Tb is Rs =
With Rr, as is clear from the above equation, even if the humidity changes, the output is constant as long as the absolute humidity does not change. However, it is very difficult to select an element having a combination of Rs and Rr having such a relationship, and in reality, Rs
= A × Rr + B (A and B are coefficients). In extreme cases, the linear relationship may not be maintained. Therefore, a so-called temperature drift occurs in which the output changes even when the absolute humidity does not change and only the temperature changes.

Therefore, as measures against this, the following (a), (b)
The conventional measures have been taken. That is, (a) There is a means for adding a resistance for correction to either Rs or Rr as seen in JP-A-60-14149. In the above relationship of Rs = A × Rr + B, A is regarded as 1, and when B is positive, B is set to Rr.
A correction resistor corresponding to B is connected in series, and a correction resistor corresponding to B is connected in series with Rs when B is negative. (B) As seen in Japanese Patent Laid-Open No. 60-203811, etc., the value of the current flowing through the element is controlled by the resistance change values of Rs and Rr due to the temperature change. For example, if the atmospheric temperature decreases, the current is increased to increase the element. The temperature resistance of the device is suppressed, and if the ambient temperature rises, the current is reduced to suppress the temperature rise of the element,
As a result, changes in resistance values of Rs and Rr due to changes in ambient temperature are reduced, and temperature drift is reduced.

However, the above countermeasures (a) and (b) have the following drawbacks, and neither is sufficient.
That is, the countermeasure (a) is satisfied only when A = 1 and it is difficult to actually set the correction resistance values individually. Therefore, in practice, the allowable range of the temperature drift is set in advance. The correction resistance value is set so that it falls within the range. Therefore, it is necessary to select Rs and Rr in advance to some extent. In the countermeasure of (b), although the correction effect is recognized, a plurality of parameters are required for determining the circuit constant for setting the current control amount, and the number of parts of the device is large, so that the cost and the design are reduced. There were difficulties. Therefore, it is an object of the present invention to provide a humidity circuit capable of preventing temperature drift by a simple means and enabling accurate humidity detection regardless of ambient temperature conditions.

[0011]

SUMMARY OF THE INVENTION To achieve the above object, the present invention comprises a first temperature sensitive element exposed to a humidity detection atmosphere and a second temperature sensitive element sealed for isolation from the atmosphere. From humidity detecting means connected in series, first and second resistors connected in series connected in parallel to the humidity detecting means, and the humidity detecting means and the first and second resistors A third resistor connected in series to the configured bridge circuit, fourth and fifth resistors connected in series connected to the bridge circuit and the third resistor in parallel, and the third resistor Based on the potential at the connection point between the resistor and the bridge circuit, the voltage drop in the fourth or fifth resistor is corrected so that the characteristic variation between the first temperature sensitive element and the second temperature sensitive element is corrected. And a voltage drop adjusting means for adjusting .

[0012]

In the humidity detecting circuit of the present invention configured as described above, the voltage drop adjusting means is provided in the fourth or fifth resistor based on the potential at the connection point between the third resistor and the bridge circuit. Since the voltage drop is adjusted, variations in the characteristics of the two temperature sensitive elements are compensated. Therefore, it is possible to improve the detection accuracy. Further, since the adjustment of the voltage drop can be performed within a certain range, the yield is also improved.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of a humidity detecting circuit according to the present invention, and FIG. 2 is a detailed diagram of the circuit shown in FIG. In terms of the configuration of this circuit, the difference from the conventional circuit shown in FIG. 5 is that a control unit is provided and the resistor R
5 is variable according to the output of this control unit. Therefore, the same members as those shown in FIG. 5 or 6 are designated by the same reference numerals. As shown, the temperature sensitive elements Rs, Rr and the resistors R1, R
2 forms a bridge circuit. This bridge circuit is designed to take out a change in humidity as an unbalanced voltage, and this unbalanced voltage is input to the voltage conversion circuit 1, where it is amplified to an appropriate voltage and output to the outside. This output changes according to changes in humidity. It should be noted that one of the two temperature-sensitive elements described above is arranged so as to be exposed in the humidity detection atmosphere, and the remaining one is hermetically sealed so as to be shielded from this atmosphere. Only the temperature of is detected. A resistor R3 (Rd), which is a third resistor, is connected in series to this bridge circuit. Further, a resistor R4 as a fourth resistor and a resistor R5 as a fifth resistor connected in series are connected in parallel to the bridge circuit and the resistor R3 connected in series.

The resistor R5 is conceptually a variable resistor, and specifically, it is composed of a plurality of fixed resistors which can be connected in parallel to the resistor R5 as shown in FIG. That is, the apparent resistance value of the resistor R5 is changed by selecting the resistor connected to the resistor R5 by turning on / off the illustrated switch. The voltage comparison circuit 4 is connected to the connection point between the resistors R4 and R5 and the connection point between the resistor R3 and the bridge circuit. The potential difference between these connection points is compared by the comparator 5 shown in FIG. The operation of the current control element 6 is controlled based on the result. The potential VB at the connection point between the resistor R3 and the bridge circuit is input to the control unit 10. In the control unit 10, according to the voltage value of this potential VB,
The switch is turned on and off to select a resistor connected in parallel with the resistor R50 shown in FIG. That is, this control unit 1
This operation of 0 will compensate for temperature drift.

The humidity detecting circuit of the present invention configured as described above operates as follows to perform temperature drift compensation and humidity detection. First, until the humidity measurement is started, the switch S built in the control unit 10 is
W1 and SW2 are off. Therefore, only the resistor R50 is connected in series to the resistor R4, and the combined resistance Rs + of the temperature sensitive elements Rs and Rr is determined according to the resistance ratio determined by the resistor R4 and the resistor R50.
r will be determined. At this time, the voltage value VB across the bridge circuit changes as shown in FIG. 3 depending on the magnitude of the initial value of the combined resistance Rs + r.
This is because when the initial resistance value is large as shown in FIG. 4, it is necessary to increase the energization current to the temperature sensitive element (increase to the negative side) in order to obtain a certain resistance value.

Therefore, the value of VB obtained under the preset ambient temperature indicates the initial resistance value of Rs + r. Rs + set by R4 and R50
Assuming that r is in the vicinity of the upper limit of Rs + r to be used, the switches SW1 and SW2 are selectively turned on / off according to the value of VB, and the combined resistance value obtained by the resistors 50, R51, and R52 is changed to initialize The operating temperature is controlled to be almost constant regardless of the resistance value. The operating points of SW1 and SW2 are predetermined in the control unit 10, and three operating modes as shown in FIG. 3 are set.

In this embodiment, R51 and R52 are exemplified as the resistors for drift compensation, but the present invention is not limited to this, and three or four resistors may be used.
In this case, more precise drift compensation can be performed, but it is necessary to provide the same number of switches as the provided resistors. Once the control unit 10 selects the switch setting as described above, this state is maintained. For example, if this setting is made at the time of manufacture, this state will be retained thereafter. According to Rs + r set in this way,
When the humidity is detected, the temperature sensitive element is controlled to a constant temperature, so that the humidity can be detected while preventing the temperature drift.

As described above, according to the present invention, the temperature drift caused by the inconsistency of the characteristics of the two temperature sensitive elements can be easily adjusted at the production site or the like, so that the detection accuracy and the yield can be improved. You will be able to plan.
Although the temperature drift is compensated by changing the resistance value in this embodiment, the resistance R50 is changed from an external circuit in order to change the voltage across the resistance R50.
A similar purpose can be achieved by applying a reverse bias to 50.

[0019]

As is apparent from the above description, according to the present invention, the difference in characteristics of the first and second temperature sensitive elements is compensated by the voltage drop adjusting means. The detection accuracy can be improved. Further, since this compensation is automatically performed, the manufacturing yield can be improved.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a humidity detection circuit according to the present invention.

FIG. 2 is a detailed view of the circuit shown in FIG.

FIG. 3 is a diagram for explaining the operation of the circuit shown in FIG.

FIG. 4 is a diagram for explaining the operation of the circuit shown in FIG.

FIG. 5 is a schematic configuration diagram of a conventional absolute humidity detection circuit (when a platinum low antibody is used).

FIG. 6 is a schematic configuration diagram of a conventional absolute humidity detection circuit (when a thermistor is used).

FIG. 7 is a characteristic diagram showing a relationship between Rr and Rs in the case of a platinum low antibody.

FIG. 8 is a characteristic diagram showing a relationship between Rr and Rs in the case of a thermistor.

[Explanation of symbols]

 1 voltage conversion circuit 4 voltage comparison circuit 6 current control element 10 control unit (voltage drop adjusting means) Rs, Rr: first and second temperature sensitive elements (humidity detecting means)

Claims (1)

[Claims] 1. Humidity detection means comprising a first temperature sensitive element exposed to a humidity detection atmosphere and a second temperature sensitive element sealed for isolation from the atmosphere, connected in series, and the humidity detection means. A third resistor connected in series to a bridge circuit composed of first and second resistors connected in series to the means and connected in series, and the humidity detecting means and the first and second resistors. A body, fourth and fifth series-connected resistor bodies connected in parallel to the bridge circuit and the third resistor body, and based on a potential at a connection point between the third resistor body and the bridge circuit Voltage drop adjusting means for adjusting the voltage drop in the fourth or fifth resistor so as to correct the characteristic variation between the first temperature sensitive element and the second temperature sensitive element,
A humidity detecting circuit comprising: