JPH10332615A - Detection circuit for carbon dioxide gas sensor and detecting apparatus for carbon dioxide gas concentration - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatically calibrate a concentration reference value without depending on a measuring atmosphere and to detect a concentration over a wide range by a method wherein a second impedance measuring means by which the impedance of a carbon dioxide gas sensor is measured at a frequency for calibration is installed and the concentration reference value is computed at a prescribed timing so as to be updated. SOLUTION: First, when a calibration-timing judgment means 303 judges the passage of, e.g. 30 minutes, an impedance measuring means 306 measures the impedance of a carbon dioxide gas sensor 302 at 1 MHz. Then, a reference-value storage means 307 computes a reference value up to 100 kHz on the basis of a measured value so as to be stored. Then, an impedance measuring means 304 measures a sensor output value at 100 kHz so as to be stored on a sensor-output-value storage means 305. Then, on the basis of the reference value which is stored on the means 307 and on the basis of the sensor output value which is stored on the means 305, a concentration computing means 308 computes the concentration of carbon dioxide in a measuring atmosphere so as to be output. After that, the elapse state of a prescribed time is judged by the means 303. When the time has elapsed, it is calibrated, and the reference value is updated. When the time has not elapsed, the reference value is not calibrated so as to continue a measurement.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon dioxide sensor used for measuring and controlling carbon dioxide concentration for indoor air conditioning, environmental hygiene, fresh food preservation, plant cultivation, disaster prevention, industrial use, and the like. The present invention relates to a technology for detecting the concentration of carbon dioxide used.

[0002]

2. Description of the Related Art As one type of carbon dioxide sensor, there is an impedance-variable carbon dioxide sensor having a capacitor structure in which electrodes are formed on a ceramic element. As disclosed in, for example, Japanese Patent Application Laid-Open No. 9-15179, this carbon dioxide gas sensor transmits and receives signals to and from the firing disk, one or more electrodes provided on or on the surface of the firing disk. Wherein the fired disk has at least one oxide selected from the group consisting of alkaline earth metals, transition metals, and lanthanoid elements;
It has the composition of a composition comprising at least one alkaline earth metal carbonate.

This carbon dioxide sensor detects a change in carbon dioxide concentration based on a change in impedance or capacitance. In order to measure the absolute value of the carbon dioxide concentration, the impedance at the carbon dioxide concentration as a reference is measured. The value must be known. Therefore, generally, a technique of exposing the carbon dioxide sensor to a known concentration of carbon dioxide to calibrate the initial impedance value is adopted.

However, in such a carbon dioxide gas sensor, when the carbon dioxide gas concentration is measured more accurately or when the capacitance value of the carbon dioxide gas sensor changes with time, the above-mentioned calibration work must be frequently performed. However, there is a problem that the cost and the working time for that are taken.

Therefore, as a means for solving the calibration problem, the concentration of carbon dioxide in the clean atmosphere is constant (about 350 pp).
Japanese Patent Application Laid-Open No. H5-249073 discloses a technique for correcting a change with time by utilizing the fact that m). This technology utilizes the fact that the electromotive force of the solid electrolyte type carbon dioxide sensor always decreases with the passage of time, and sequentially stores the maximum value of the carbon dioxide sensor within a certain period of time, and stores this stored value when the carbon dioxide concentration is 350 ppm. By using the reference value, the difference between the signal of the carbon dioxide sensor at the concentration of carbon dioxide being detected and the reference value is calculated to calculate the carbon dioxide concentration.

FIG. 10 is a block diagram showing the structure of a conventional carbon dioxide concentration detecting device. In FIG. 10, in the conventional carbon dioxide concentration detecting device, the maximum value of the signal of the carbon dioxide sensor 1001 during a predetermined time a is stored in the maximum output value storage means a.
The maximum value is stored in the maximum output value storage means b1003 for a predetermined time b. Each of the maximum value groups stored in the maximum output storage means b1003 is weighted by its variance and weighted by the difference with the reference value, and the reference value calculation means 1004 calculates a reference value with improved accuracy. It is stored in the reference value storage means 1005. Then, the concentration calculator 1006 calculates the concentration of carbon dioxide based on the output value of the carbon dioxide sensor 1001 and the stored reference value, and outputs the concentration to the outside by the concentration output unit 1007.

Although the above-mentioned invention uses a solid electrolyte type carbon dioxide gas sensor which detects by electromotive force, the invention is also applied to an impedance change type carbon dioxide gas sensor whose output tends to shift to a low concentration side with the passage of time. It is possible in principle.

[0008]

However, in the conventional carbon dioxide concentration detecting device, the change in the impedance of the carbon dioxide sensor with respect to the change in the carbon dioxide concentration can be obtained with only a small sensor output. The measurement was sometimes difficult.

In the conventional carbon dioxide concentration detecting device,
Since the sensor output is corrected using the clean air, there is a problem that the sensor output can be used only in a place where a clean air as a reference can be obtained. That is, in an environment where it is difficult to obtain a clean atmosphere by natural ventilation such as a highly airtight building or house, the sensor output cannot be corrected to an appropriate value, so that the carbon dioxide concentration detecting device cannot be used. Was.

Furthermore, in the conventional carbon dioxide concentration detecting device, since the carbon dioxide concentration in the clean atmosphere is set to be higher than the standard, the carbon dioxide concentration is higher than 350%.
There is a problem that the method cannot be applied to an application that needs to detect a carbon dioxide gas concentration of ppm or less.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a carbon dioxide gas concentration measuring technique capable of amplifying a change in impedance of a carbon dioxide gas sensor and detecting it with a high SN ratio.

Another object of the present invention is to provide a carbon dioxide concentration measuring technique capable of calibrating a reference value of the carbon dioxide concentration without depending on a measurement atmosphere such as a clean atmosphere.

An object of the present invention is to provide a carbon dioxide concentration measuring technique capable of detecting a carbon dioxide concentration over a wide range.

[0014]

In order to solve this problem, a carbon dioxide sensor detection circuit according to the present invention comprises a first inverter, and an output from the first inverter to an input to the first inverter. A carbon dioxide gas sensor and a resistor, which are electrically connected in series and have a resistance component, and are disposed between the carbon dioxide gas sensor and the resistor with respect to an input of the first inverter, and an output of the first inverter is provided; A second inverter electrically connected to the input unit;
Frequency counting means for electrically measuring the oscillation frequency output from the first inverter, wherein the frequency counting means measures the oscillation frequency output from the first inverter. It is configured to measure from the measured oscillation frequency, whereby it is possible to amplify the impedance change of the carbon dioxide sensor and detect it with a high SN ratio.

In this carbon dioxide sensor detection circuit, by connecting a compensating capacitor between the output of the first inverter and the carbon dioxide sensor, it is possible to stably detect the capacitance value of the carbon dioxide sensor. In addition, the resistance is constituted by a fixed resistance and a variable resistance, and the oscillation frequency output from the first inverter is adjusted by changing the resistance value of the variable resistance, thereby correcting the variation in the impedance characteristic of the carbon dioxide sensor. Can be.

A carbon dioxide gas concentration detecting device according to the present invention comprises a carbon dioxide gas sensor for detecting a carbon dioxide concentration, a heating means for heating the carbon dioxide gas sensor to an operating temperature, and a calibration timing judging means for judging a calibration timing of the carbon dioxide gas sensor. A first impedance measuring means for measuring the impedance of the carbon dioxide gas sensor at a measurement frequency when the calibration timing determining means determines not to calibrate, and a sensor output value storage for storing an output value from the first impedance measuring means Means, a second impedance measuring means for measuring the impedance of the carbon dioxide sensor at the frequency for calibration when the calibration timing judging means judges to calibrate, and receiving a reference value by receiving an output value from the second impedance measuring means. The reference value storage means for calculating and storing and the sensor output value storage means A concentration calculating means for calculating the carbon dioxide concentration from the sensor output value obtained and the reference value stored in the reference value storing means, and a concentration outputting means for outputting the carbon dioxide concentration obtained by the concentration calculating means. And
The reference value of the carbon dioxide concentration can be automatically calibrated without depending on the measurement atmosphere such as a clean atmosphere, and the carbon dioxide concentration can be detected over a wide range regardless of the measurement atmosphere.

In this carbon dioxide concentration detecting device, the first
And the second impedance measuring means are constituted by the above-described carbon dioxide sensor detection circuits in which one carbon dioxide sensor is switched by the circuit switching means and mutually shared, thereby amplifying the impedance change of the carbon dioxide sensor. It becomes possible to detect with a high SN ratio. Further, the first and second impedance measuring means are integrated such that at least two or more resistances are provided and any one of these resistances is selectively electrically connected by the resistance switching means. By using the above-described carbon dioxide sensor circuit, it is possible to reduce the size and cost of the device.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the first aspect of the present invention, a first inverter and an output section of the first inverter are electrically connected in series to an input section of the first inverter. A carbon dioxide sensor comprising a capacitor having a resistance component, a resistor, and an input portion of the first inverter disposed between the carbon dioxide sensor and the resistor, and an output portion electrically connected to the input portion of the first inverter. The second
And an frequency converter that is electrically connected to the output of the first inverter and measures an oscillation frequency output from the first inverter. The impedance of the carbon dioxide sensor depends on the carbon dioxide concentration. This is a carbon dioxide sensor detection circuit that measures from the oscillation frequency measured by the frequency counting means, and has the effect of amplifying the impedance change of the carbon dioxide sensor and detecting it at a high SN ratio.

According to a second aspect of the present invention, in the first aspect of the invention, there is provided a carbon dioxide sensor detection circuit in which a compensation capacitor is connected between the output of the first inverter and the carbon dioxide sensor. Since the circuit oscillates regardless of the magnitude of the resistance between the resistance component and the resistance of the carbon dioxide gas sensor, the capacitance value of the carbon dioxide gas sensor can be stably detected.

According to a third aspect of the present invention, in the first or second aspect, the resistor is constituted by a fixed resistor and a variable resistor, and the first resistor is changed by changing the resistance value of the variable resistor.
This is a carbon dioxide gas sensor detection circuit in which the oscillation frequency output from the inverter can be adjusted. Since the oscillation frequency can be adjusted by a variable resistor, it has the effect of correcting variations in the impedance characteristics of the carbon dioxide gas sensor.

According to a fourth aspect of the present invention, there is provided a carbon dioxide sensor for detecting a carbon dioxide concentration, heating means for heating the carbon dioxide sensor to an operating temperature, and a calibration timing determination for determining a calibration timing of the carbon dioxide sensor. Means, first impedance measuring means for measuring the impedance of the carbon dioxide gas sensor at a measuring frequency when the calibration timing judging means judges not to calibrate, and a sensor output for storing an output value from the first impedance measuring means. Value storage means, second impedance measurement means for measuring the impedance of the carbon dioxide gas sensor at the calibration frequency when the calibration timing determination means determines that calibration is to be performed, and a reference receiving the output value from the second impedance measurement means. The reference value storage means for calculating and storing the value, and the value stored in the sensor output value storage means. A concentration calculating means for calculating the carbon dioxide concentration from the sensor output value obtained and the reference value stored in the reference value storing means, and a concentration output means for outputting the carbon dioxide concentration obtained by the concentration calculating means. A detection device that can automatically calibrate the reference value of carbon dioxide concentration without depending on the measurement atmosphere such as clean air, and can detect the carbon dioxide concentration over a wide range regardless of the measurement atmosphere It has the action of:

According to a fifth aspect of the present invention, in the invention of the fourth aspect, the first and second impedance measuring means are mutually switched by switching one carbon dioxide sensor by the circuit switching means. A carbon dioxide gas concentration detecting device comprising the shared carbon dioxide gas sensor detecting circuit according to claim 1, 2 or 3, wherein a capacitor having a resistance component is used as the carbon dioxide gas sensor.
High S by amplifying the impedance change of carbon dioxide sensor
This has the effect that detection can be performed at the N ratio.

According to a sixth aspect of the present invention, in the fourth aspect of the invention, the first and second impedance measuring means are provided with at least two or more resistors and one of these resistors is provided. 4. A carbon dioxide gas concentration detecting device comprising a carbon dioxide gas sensor circuit according to claim 1, wherein one is integrated so as to be selectively electrically connected by resistance switching means. Since the calibrated impedance and the uncalibrated impedance can be measured, it has an effect that the size and cost of the device can be reduced.

Hereinafter, an embodiment of the present invention will be described with reference to FIG.
This will be described with reference to FIG. In these drawings, members having the same functions are denoted by the same reference numerals, and redundant description is omitted.

(Embodiment 1) FIG. 1 is a block diagram showing a carbon dioxide sensor detection circuit according to Embodiment 1 of the present invention.

As shown in FIG. 1, a carbon dioxide sensor detection circuit according to the present embodiment comprises an unstable multivibrator circuit 102 and this unstable multivibrator circuit 102.
And a frequency counting means 109 for measuring the oscillation frequency from the input.

The astable multivibrator circuit 102
An inverter (first inverter) 106; a carbon dioxide gas sensor 101 and a resistor 104 which are electrically connected in series from an output portion P4 to an input portion P3 of the inverter 106
have. Also, the carbon dioxide sensor 101 and the resistor 10
4 to the input P3 of the inverter.
And an inverter (second inverter) 105 whose input part P1 is connected to the resistor 103 is electrically connected in series. Further, an input unit is electrically connected to an output unit P4 of the inverter 106 to provide an inverter 107, and an output unit of the inverter 107 is electrically connected to the frequency counting unit 109. Therefore, the output of the inverter 106 is electrically connected to the frequency counting means 109 via the inverter 107. In addition,
Between the output of the inverter 107 and the frequency counting means 109, a capacitor 108, one of which is connected to the ground,
Is connected.

Here, the resistor 103 has a function of electrically protecting the inverter 105, and the capacitor 108 has a function of removing generated noise. Note that the carbon dioxide sensor 101 is composed of a capacitor having a resistance component.

In the carbon dioxide sensor detection circuit having such a structure, a change in impedance of the carbon dioxide sensor 101 is detected as follows.

In FIG. 1, when the power is turned on, the output portion P4 of the inverter 106 becomes a HIGH potential (hereinafter, simply referred to as "H") because no electric charge is stored in the carbon dioxide sensor 101. Then, the carbon dioxide sensor 101
, A charging current flows through the resistor 104. Since the output P4 of the inverter 106 is H, the input P1 of the inverter 105 is still H at this time.

The current is supplied to the carbon dioxide sensor 101 and the resistor 104.
, The carbon dioxide gas sensor 101 is charged, and the voltage of the input unit P1 starts to decrease. When the potential of the input section P1 falls below 1/2 VDD (VDD is the power supply voltage) or less, the input section P1 is inverted from H to a LOW potential (hereinafter simply referred to as "L"). When the input P1 of the inverter 105 becomes L, its output P2, that is, the inverter 1
The input of 06 becomes H, and the output P4 is inverted to L.

As a result, the reverse current flows out, and the carbon dioxide sensor 101 is charged in the reverse direction.
Then, after a lapse of a predetermined time, the potential of the input section P1 becomes 1/2 VDD or more and the input section P1 is inverted to H, whereby the output section P2 and the input section P3 are set to L, and the output section P4 is set to H. Flip to By repeating such operations alternately, the unstable multivibrator circuit 102 oscillates.

This oscillation frequency is measured by the frequency counting means 109 using the inverter 107 as a buffer. Here, the oscillation frequency is proportional to the product of the capacitance value C of the carbon dioxide sensor 101 and the resistance value R of the resistor 104, and is approximately (Equation 1).
Satisfy the relationship.

[0034]

(Equation 1)

Here, k is a proportional constant. As can be seen from (Equation 1), since the proportional constant k and the resistance value R are constant, the oscillation frequency depends on the capacitance value C of the carbon dioxide sensor. Therefore, the capacitance value C of the carbon dioxide sensor 101 can be detected by measuring the transmission frequency of the unstable multivibrator circuit 102 by the frequency counting means 109.

The resistor 104 may be constituted by a combination of a fixed resistor and a variable resistor. In that case, the oscillation frequency of the unstable multivibrator circuit can be adjusted by the variable resistor (see Equation 1). Thereby, for example, even if there is a variation in the impedance characteristic of the carbon dioxide sensor 101 during mass production, the influence can be reduced.

As described above, the carbon dioxide gas sensor 101 composed of a capacitor is disposed in the unstable multivibrator circuit 102 as described above, and the oscillation frequency of the unstable multivibrator circuit 102, which is a self-excited oscillation circuit, is measured to obtain the carbon dioxide gas. It is possible to detect the capacitance value C of the carbon dioxide sensor 101 that changes according to the gas concentration.

Here, the reason why the impedance change of the carbon dioxide sensor 101 can be amplified and detected will be described.

The above-described impedance change type carbon dioxide sensor has an equivalent circuit in which a capacitance component C and a resistance component R are connected in parallel or in series. When a circuit analysis is performed by the HP4194A impedance analyzer, the impedance change type carbon dioxide sensor 101 can be simplified to an equivalent circuit in which the capacitance Cp and the resistance (resistance component) Rp are connected in parallel. Therefore, since the impedance change type carbon dioxide sensor is not an ideal capacitor, the carbon dioxide sensor detection circuit of FIG. 1 actually performs a complicated operation.

That is, in FIG.
01 is charged, a current flows through the resistance Rp inside the carbon dioxide gas sensor 101, so that the voltage of the carbon dioxide gas sensor 101 becomes １ ／ compared to the case of an ideal capacitor.
The time required to reach VDD becomes longer and the oscillation frequency becomes lower. That is, when the carbon dioxide gas sensor 101 detects the carbon dioxide gas and the resistance Rp increases or decreases, the oscillation frequency changes due to the increase or decrease of the leak current accompanying this. Further, since the change in the oscillation frequency according to Equation 1 due to the change in the capacitance Cp is added, the change rate of the oscillation frequency of the astable multivibrator circuit 102 is determined by the CO 2 gas sensor 10 measured by the impedance analyzer.
It becomes larger than the impedance change rate of 1.

The results of measuring the rate of change of the sensor output with respect to the concentration of carbon dioxide using the same carbon dioxide sensor are shown in Table 1 separately for the case of the carbon dioxide sensor detection circuit of the present invention and the case of measurement by the impedance analyzer. .
In addition, in (Table 1), the changes are summarized based on the output at 350 ppm.

[0042]

[Table 1]

As shown in (Table 1), according to the carbon dioxide sensor detection circuit of the present invention, an output more than twice as large as that in a normal impedance measurement can be obtained. That is, the change in the impedance of the carbon dioxide sensor 101 is doubled. For example, if the concentration of carbon dioxide is
At 00 ppm, the sensor output of the present invention changes by 97%, while the impedance measured conventional sensor output changes by only 47%.

As described above, according to the carbon dioxide sensor detection circuit of the present embodiment, since the carbon dioxide sensor 101 composed of a capacitor having a resistance component is arranged in the unstable multivibrator circuit 102, the carbon dioxide sensor 101
Can be amplified and detected at a high SN ratio.

When the resistance Rp becomes smaller than the resistance value of the resistor 104, the carbon dioxide sensor 101 is not charged to more than 1/2 VDD, so that the circuit of FIG. 1 does not oscillate. Therefore, the internal resistance Rp of the carbon dioxide sensor 101 is
04 must be greater than the resistance.

(Embodiment 2) FIG. 2 is a block diagram showing a carbon dioxide sensor detection circuit according to Embodiment 2 of the present invention.

The carbon dioxide sensor detection circuit according to the present embodiment is different from the carbon dioxide gas detection circuit according to the first embodiment in that the compensation capacitor 201 is connected in series between the output of the inverter 106 and the carbon dioxide sensor 101. Is different. In other respects, it is the same as that shown in the first embodiment, and therefore basically performs substantially the same operation as the circuit in the first embodiment.

However, in the present embodiment,
Since the compensating capacitor 201 is arranged as described above, a stable oscillating operation can be performed.
That is, unlike the circuit shown in the first embodiment, when the resistance Rp inside the carbon dioxide sensor 101 becomes smaller than the resistance value of the resistor 104, the compensation capacitor 201 cuts off the current, so that the unstable multivibrator circuit 102
Does not stop oscillation.

Therefore, according to the carbon dioxide gas sensor detection circuit of the present embodiment, the unstable multivibrator circuit 102 oscillates regardless of the magnitude of the internal resistance Rp of the carbon dioxide gas sensor 101 and the resistance value of the resistance 104, so that it is stable. Thus, the capacitance value C of the carbon dioxide sensor 101 can be detected.

In this embodiment, the resistance 10
4 may be a combination of a fixed resistor and a variable resistor. In this case, since the oscillation frequency of the unstable multivibrator circuit 102 can be adjusted by the variable resistor, even if the impedance characteristics of the carbon dioxide sensor 101 vary during mass production, the influence can be reduced.

(Embodiment 3) FIG. 3 is a block diagram showing a configuration of a carbon dioxide concentration detecting apparatus according to Embodiment 3 of the present invention.

As shown in the figure, in the carbon dioxide concentration detecting apparatus according to the present embodiment, the heating means 301 for raising the temperature of the carbon dioxide sensor 302 for detecting the carbon dioxide concentration to the operating temperature, and the calibration timing of the carbon dioxide sensor 302 Measuring means 303 for measuring the impedance of the carbon dioxide sensor 302 at the measurement frequency (first impedance measuring means)
304, a sensor output value storage unit 305 for storing the measured value, an impedance measuring unit (second impedance measuring unit) 306 for measuring the impedance of the carbon dioxide sensor 302 at a calibration frequency, and a reference value storing unit for storing the measured value 307. Further, the sensor output value storage means 305 and the reference value storage means 307
Has a concentration calculating means 308 for calculating the concentration of carbon dioxide gas from the reference value stored in the storage section, and a concentration output means 309 for outputting the obtained carbon dioxide gas concentration.

As described above, in the carbon dioxide concentration detecting device according to the present embodiment, the impedance of the carbon dioxide sensor 302 is measured by the impedance measuring means 304 at the measurement frequency, and the impedance measuring means 306 is measured.
To measure at the calibration frequency.

The operation principle of the carbon dioxide concentration detecting apparatus according to the present embodiment will be described in detail below.

The impedance change rate of the impedance change type carbon dioxide sensor is expressed by (Equation 2).

[0056]

(Equation 2)

Is represented by Here, Sx is an impedance change rate, Zx is an impedance value at an arbitrary carbon dioxide gas concentration, and Z0 is an impedance value at a reference concentration.

Therefore, if the relationship between the concentration of carbon dioxide and the rate of change in impedance is known in advance, the concentration of carbon dioxide can be uniquely determined by measuring Zx and using the known Z0. However, a problem common to general carbon dioxide gas sensors is that Z0 changes with time. Therefore, in the present embodiment,
This is to timely calibrate Z0 using the frequency dependence of the impedance of the carbon dioxide sensor and the rate of change in impedance.

FIG. 4 shows the measurement principle. FIG. 4 is a graph showing the frequency dependency of carbon dioxide concentration detection in the carbon dioxide sensor used in the carbon dioxide concentration detecting device of FIG. In the case shown in the drawing, the impedance change rate of the carbon dioxide sensor at 1000 ppm based on 350 ppm is measured using an HP4194A impedance analyzer manufactured by YHP, and the relationship between the impedance change rate and the measurement frequency is shown.

As shown in FIG. 4, the rate of change in impedance gradually decreases when the measurement frequency becomes 100 kHz or higher. For example, the rate of change in impedance becomes substantially 1 at 1 MHz. This means that at frequencies above 1 MHz, the impedance of the carbon dioxide sensor is no longer sensitive to the carbon dioxide concentration. Thus, for example, 100
When the impedance value at kHz is measured as Zx, the impedance value at 1 MHz is measured, and the reference value Z0 at 100 kHz is calculated from the frequency dependence of the known impedance, so that the reference value Z0 is independent of the surrounding carbon dioxide concentration. Is obtained.

FIG. 5 shows the change over time in the frequency characteristic of the impedance at a concentration of 350 ppm of the carbon dioxide sensor. FIG. 5 is a graph showing a frequency characteristic of impedance before and after a change over time in the carbon dioxide sensor used in the carbon dioxide concentration detecting device of FIG.

As shown in the figure, even when the impedance changes with time, the impedance only shifts up and down with respect to the same frequency, and the frequency dependence of the impedance does not change. Therefore, even if the carbon dioxide sensor changes over time, the reference value Z0 at 100 kHz can be calculated from the above-mentioned 1 MHz impedance value using the initial frequency dependence before the change over time. Alternatively, without calculating the reference value Z0, the 100 kH
The impedance at z is Za, the impedance at 1 MHz is Zb, and the carbon dioxide concentration can be calculated using the ratio Za / Zb.

FIG. 6 shows a change in Za / Zb when the carbon dioxide gas concentration is changed. FIG. 6 is a graph showing the relationship between the output of the carbon dioxide sensor according to the present invention used in the carbon dioxide concentration detector of FIG. 3 and the carbon dioxide concentration.
As shown in the figure, since the value of Za / Zb continuously changes with respect to the carbon dioxide concentration, it is understood that this value can be a reference for the measured value of the carbon dioxide concentration.

Next, the operation of detecting the concentration of carbon dioxide in the carbon dioxide concentration detector according to the present embodiment will be described with reference to FIG.

The carbon dioxide gas detector is turned on, and the heating means 301 heats the carbon dioxide gas sensor 302 to a predetermined temperature to perform a warm-up operation. Then, the carbon dioxide gas detection device is installed in a normal measurement atmosphere. Then, the following operation is repeated until the power is turned off.

First, the calibration timing determining means 303 determines whether a predetermined time (for example, about 30 minutes) has elapsed (401). If the predetermined time has elapsed, the impedance measuring means 306 sets the frequency to 1 MHz.
Is measured (402). Next, a reference value at 100 kHz is calculated from the impedance value measured in step 402 and stored in the reference value storage means 307 (403).

Then, the sensor output value of the carbon dioxide sensor 302 at 100 kHz is measured by the impedance measuring means 304, and the value is stored in the sensor output value storing means 305.
(404).

Next, from the reference value stored in the reference value storage means 307 and the sensor output value stored in the sensor output value storage means 305, the carbon dioxide concentration in the measurement atmosphere is calculated by the concentration calculation means 308 (405). , Concentration output means 3
09, the carbon dioxide concentration is output to the outside (40
6).

Thereafter, the calibration timing means 303 determines whether or not a predetermined time (for example, about 24 hours) has elapsed (407).
After the calibration is performed and the reference value is updated, the operations of 403 and below are executed. If the predetermined time has not elapsed, the process returns to 404, the impedance is measured without calibration, and thereafter, the operation of 405 and below is executed.

As described above, according to the carbon dioxide concentration detecting apparatus of the present embodiment, the impedance measuring means 304 is used for normal measurement, and the impedance measuring means 3 is used for calibration.
Impedance measurement means 3 whose measurement frequency is higher than 04
In step 06, the reference value is calculated by measuring the impedance of the carbon dioxide sensor 302, so that the reference value can be calibrated irrespective of the concentration of carbon dioxide in the measurement atmosphere such as clean air.

Since the measurement can be performed regardless of the carbon dioxide concentration in the measurement atmosphere, the carbon dioxide concentration can be detected over a wide range.

In this embodiment, the case where the measuring frequency of the impedance measuring means 304 is 100 kHz and the calibration frequency of the impedance measuring means 306 is 1 MHz has been described as an example, but the present invention is not limited to this. Instead, an arbitrary frequency can be set.

(Embodiment 4) FIG. 8 is a block diagram showing a carbon dioxide concentration detecting apparatus according to Embodiment 4 of the present invention.

In FIG. 8, the carbon dioxide concentration detecting apparatus of the present embodiment has substantially the same impedance measuring means (first impedance measuring means) 304 and impedance measuring means (first impedance measuring means) as those described in the second embodiment. 2 impedance measuring means) 306 is provided. However, in the impedance measuring means 304 and the impedance measuring means 306, the circuit switching means 505
The carbon dioxide sensor 302 is shared via the. Then, the impedance of the carbon dioxide sensor 302 is measured by the impedance measuring unit 304 at the measurement frequency,
The impedance measuring means 306 measures at the calibration frequency. Note that the impedance measuring means 304 and the impedance measuring means 306 having the configuration shown in Embodiment 1 can be used.
2 and the resistor 504 can be composed of a fixed resistor and a variable resistor.

Further, the present carbon dioxide concentration detecting apparatus is provided with a calibration timing determining means 303 for determining whether or not it is necessary to calibrate the impedance value of the carbon dioxide sensor 302. Is connected to one of the impedance measuring means 304 and 306 by the circuit switching means 505 according to the following. A heating unit 301 is provided to heat the carbon dioxide sensor 302 that detects the carbon dioxide concentration by a change in the impedance value to a predetermined temperature.

The impedance measuring means 304 is provided with a sensor output value storing means 305 for storing the measured value of the impedance measuring means 304.
Reference numeral 6 is electrically connected to reference value storage means 307 for storing the measured value of impedance measurement means 306. The sensor output value storage unit 305 and the reference value storage unit 307 are connected to a concentration calculation unit 308 that calculates the carbon dioxide concentration from these reference values. further,
A concentration output means 309 is connected to the concentration calculation means 308, and the obtained carbon dioxide gas concentration is calculated by the concentration output means 30.
At 9 the signal is output to the outside.

As described above, in the carbon dioxide concentration detecting apparatus according to the present embodiment, the unstable multivibrator circuit (first unstable multivibrator circuit) 501 and the impedance measuring means 3 constituting the impedance measuring means 304 are provided.
06 and a non-stable multivibrator circuit (second non-stable multivibrator circuit) 503 constituting
By switching by the circuit switching means 505 and electrically connecting one of them to the carbon dioxide sensor 302, the impedance of the carbon dioxide sensor 302 is measured by the two unstable multivibrator circuits 501 and 503.

For example, at a concentration of 350 ppm carbon dioxide in clean air, the unstable multivibrator circuit 5
The resistor 502 of 01 is set so that the oscillation frequency becomes 100 kHz, and the resistor 504 of the astable multivibrator circuit 503 is set so as to become 1 MHz. Then, as described in the third embodiment, when the impedance is measured by the impedance measuring unit 306, the change in the oscillation frequency with respect to the carbon dioxide concentration is small, so that it can be used as a measuring unit for calibration.

Next, the operation of the carbon dioxide concentration detecting device according to the present embodiment will be described. This apparatus performs the same operation as the flowchart of FIG. 7 shown in the third embodiment, and steps 401 to 407 are repeated.

That is, in FIG. 8, when the calibration timing determining means 303 determines that calibration is necessary, the output terminal of the carbon dioxide sensor 302 is connected to the impedance measuring means 306 by the circuit switching means 505. Thereby, the impedance of the carbon dioxide sensor 302 is measured by the impedance measuring means 306, and the measured value is sent to the reference value storing means 307. When the calibration timing determination unit 303 determines that calibration is not necessary,
The output terminal of the carbon dioxide sensor 302 is connected to the impedance measuring means 304 by the circuit switching means 505. Thereby, the impedance of the carbon dioxide sensor 302 is measured by the impedance measuring means 304, and the measured value is sent to the sensor output value storing means 307.

Next, the measured values stored in the sensor output value storing means 305 and the reference value storing means 307 are sent to the concentration calculating means 308 to calculate the carbon dioxide gas concentration in the measurement atmosphere. Output to

Thereafter, if the predetermined time has elapsed, the reference value is updated, and the subsequent operations are executed. If the time has not elapsed, the impedance is measured without performing calibration, and the subsequent operation is executed.

Here, the oscillation frequency of the impedance measuring means 304 and the impedance measuring means 306 is adjusted.
As shown in FIG. 8, in addition to arranging the compensating capacitor 201, the resistance values of the resistors 502 and 504 are set to a predetermined level, and the adjusting capacitor is connected in parallel to the carbon dioxide sensor 302. Can be done by However, when the compensating capacitor 201 is used or an adjusting capacitor is connected, the settable oscillation frequency range is narrow and the detection sensitivity of the carbon dioxide gas sensor is reduced. Therefore, the adjustment is performed by adjusting the resistance values of the resistors 502 and 504. Is desirable. The same applies to the following fifth embodiment.

As described above, according to the carbon dioxide concentration detecting apparatus of the present embodiment, the impedance measuring means 3 for measurement is used.
04 and an impedance measuring means 306 for calibration are provided, the impedance value of the carbon dioxide sensor 302 is switched and measured, and this is appropriately calibrated.
The reference value of the carbon dioxide concentration can be calibrated without depending on the measurement atmosphere such as the clean atmosphere.

Since the measurement can be performed regardless of the carbon dioxide concentration in the measurement atmosphere, the carbon dioxide concentration can be detected over a wide range.

Further, since a capacitor having a resistance component is used as the carbon dioxide sensor 302, it is possible to amplify the impedance change of the carbon dioxide sensor 302 and detect it with a high SN ratio.

(Embodiment 5) FIG. 9 is a block diagram showing a carbon dioxide concentration detecting apparatus according to Embodiment 5 of the present invention.

The carbon dioxide concentration detecting device shown in the present embodiment is calibrated by switching the resistance 502 and the resistance 504 provided between the carbon dioxide sensor 302 and the output section of the inverter 106 by the resistance switching means 601. The impedance of the case where it is set and the case where it is not set are measured. Therefore, the present carbon dioxide concentration detecting apparatus has a structure in which the first and second unstable multivibrator circuits 501 and 503 in the fourth embodiment are integrated.
The first and second impedance measuring units 304 and 306 in the fourth embodiment are integrated into one impedance measuring unit, that is, one assembling into one astable multivibrator circuit 603. is there. Note that three or more resistors may be provided.

Since two kinds of impedances are measured by the single astable multivibrator circuit 603, the count value corresponding to the measured impedance is stored in the sensor output value storage means 305 or the reference value. The output switching means 602, whose output destination is switched by the calibration timing determining means 303, is sent from the frequency counting means 109 to the sensor output value storing means 305 and the reference value storing means 307 in order to send to any of the value storing means 307.
Is provided on the data path leading to.

In other respects, the structure is substantially the same as that of the carbon dioxide concentration detecting device shown in the fourth embodiment.

Next, the operation of the carbon dioxide concentration detecting device according to the present embodiment will be described. This device also performs the same operation as the flowchart of FIG. 7 shown in the third embodiment, and steps 401 to 407 are repeated.

That is, in FIG. 9, when the calibration timing determination means 303 determines that calibration is necessary, the resistance switching means 601 connects the resistor 502 to the unstable multivibrator circuit 603, and this unstable multivibrator circuit 603 Operate to measure the calibrated impedance. Also, the calibration timing determination means 3
In response to the output of the number 03, the output switching means 602 connects the output of the frequency counting means 109 to the reference value storage means 307. As a result, the measured value based on the calibrated impedance of the carbon dioxide sensor 302 is sent to the reference value storage unit 307.

On the other hand, when the calibration timing determining means 303 determines that calibration is not necessary, the resistance switching means 6
01 designates a resistor 504 as an unstable multivibrator circuit 60;
3 and the non-stable multivibrator circuit 603
Operates to measure the uncalibrated impedance. Further, in response to the output of the calibration timing determining means 303, the output switching means 602 connects the output of the frequency counting means 109 to the sensor output value storing means 305. As a result, the measured value of the uncalibrated impedance of the carbon dioxide sensor 302 is sent to the sensor output value storage means 305.

Next, the measured values stored in the sensor output value storage means 305 and the reference value storage means 307 are sent to the concentration calculation means 308 to calculate the concentration of carbon dioxide in the measurement atmosphere. Output to If the predetermined time has elapsed, the reference value is updated, and if not, the subsequent operation is executed without calibration.

As described above, according to the carbon dioxide concentration detecting device of the present embodiment, the resistance value of the carbon dioxide gas sensor 302 is switched and measured by switching between the resistance 502 and the resistance 504 by the resistance switching means 601. Therefore, the reference value of the carbon dioxide concentration can be calibrated without depending on the measurement atmosphere such as the clean atmosphere.

Further, since the measurement can be performed regardless of the carbon dioxide concentration in the measurement atmosphere, the carbon dioxide concentration can be detected over a wide range.

Further, since a capacitor having a resistance component is used as the carbon dioxide gas sensor 302, it is possible to amplify the impedance change of the carbon dioxide gas sensor 302 and detect it with a high SN ratio.

Since the impedance calibrated by one non-stable multivibrator circuit 603 and the uncalibrated impedance can be measured, the size and cost of the device can be reduced.

[0099]

As described above, according to the carbon dioxide gas sensor detection circuit of the present invention, since the carbon dioxide gas sensor comprising the capacitor having the resistance component is arranged, the impedance change of the carbon dioxide gas sensor is amplified to achieve a high SN ratio. An effective effect of being able to detect is obtained.

Further, according to the carbon dioxide sensor detection circuit of the present invention, by connecting a compensating capacitor between the output of the first inverter and the carbon dioxide sensor, the resistance value of the resistance component and the resistance of the carbon dioxide sensor is obtained. The circuit oscillates irrespective of the magnitude of the above, an effective effect that the capacitance value of the carbon dioxide sensor can be stably detected.

According to the carbon dioxide gas sensor detection circuit of the present invention, the oscillation frequency can be adjusted by the variable resistance because the resistance is composed of the fixed resistance and the variable resistance, so that the variation of the impedance characteristic of the carbon dioxide gas sensor can be reduced. An effective effect of being able to correct is obtained.

According to the carbon dioxide concentration detecting device of the present invention,
During normal measurement, the first impedance measuring means measures the impedance of the carbon dioxide gas sensor during calibration, and the second impedance measuring means measures the reference value to calculate the reference value. An effective effect that the reference value can be calibrated regardless of the above is obtained. In addition, since the calibration is performed automatically and easily in a short time, an effective effect that this calibration work can be performed as needed can be obtained. Furthermore, even when the measured value changes suddenly, an effective effect of being able to determine whether the change is due to a concentration change or a failure is obtained. Since the measurement can be performed irrespective of the concentration of carbon dioxide in the measurement atmosphere, an effective effect that the concentration of carbon dioxide can be detected over a wide range can be obtained.

According to the carbon dioxide concentration detecting device of the present invention,
The first and second impedance measuring means are constituted by the above-described carbon dioxide sensor detection circuit in which one carbon dioxide sensor is switched by the circuit switching means and shared with each other, and a capacitor having a resistance component is used as the carbon dioxide sensor. As a result, there is obtained an effective effect that it is possible to amplify the impedance change of the carbon dioxide gas sensor and detect it with a high SN ratio.

According to the carbon dioxide concentration detecting device of the present invention,
The above-mentioned first and second impedance measuring means are integrated such that at least two or more resistors are provided and any one of these resistors is selectively electrically connected by the resistance switching means. With the configuration using the carbon dioxide gas sensor circuit, the impedance that is calibrated and the impedance that is not calibrated by a single circuit can be measured. Therefore, an effective effect that the size and cost of the apparatus can be reduced can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a carbon dioxide sensor detection circuit according to Embodiment 1 of the present invention.

FIG. 2 is a block diagram showing a carbon dioxide sensor detection circuit according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of a carbon dioxide concentration detecting device according to a third embodiment of the present invention.

FIG. 4 is a graph showing frequency dependency of carbon dioxide concentration detection in a carbon dioxide sensor used in the carbon dioxide concentration detecting device of FIG. 3;

FIG. 5 is a graph showing frequency characteristics of impedance before and after a change over time in a carbon dioxide sensor used in the carbon dioxide concentration detecting device of FIG. 3;

6 is a graph showing the relationship between the output of a carbon dioxide sensor used in the carbon dioxide concentration detecting device of FIG. 3 and the carbon dioxide concentration.

FIG. 7 is a flowchart showing an operation of detecting the concentration of carbon dioxide in the carbon dioxide concentration detector of FIG. 3;

FIG. 8 is a block diagram showing a carbon dioxide concentration detecting device according to a fourth embodiment of the present invention.

FIG. 9 is a block diagram showing a carbon dioxide concentration detecting device according to a fifth embodiment of the present invention.

FIG. 10 is a block diagram showing the structure of a conventional carbon dioxide concentration detecting device.

[Explanation of symbols]

Reference Signs List 101 Carbon dioxide sensor 104 Resistance 105 Inverter (second inverter) 106 Inverter (first inverter) 109 Frequency counting means 201 Compensation capacitor 301 Heating means 302 Carbon dioxide sensor 303 Calibration timing determination means 304 Impedance measurement means (First impedance measurement) Means) 305 Sensor output value storage means 306 Impedance measurement means (second impedance measurement means) 307 Reference value storage means 308 Density calculation means 309 Density output means 505 Circuit switching means 601 Resistance switching means

Continued on the front page (72) Inventor Shinji Morimoto 1006 Kazuma Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (6)

[Claims] A first inverter; a carbon dioxide gas sensor comprising a capacitor having a resistance component and a resistor electrically connected in series from an output of the first inverter to an input of the first inverter; A second inverter disposed between the carbon dioxide sensor and the resistor with respect to an input unit of the first inverter, and an output unit electrically connected to an input unit of the first inverter; Frequency counting means for electrically measuring the oscillation frequency output from the first inverter, the frequency counting means being configured to measure the impedance of the carbon dioxide sensor depending on the carbon dioxide concentration. A carbon dioxide sensor detection circuit, wherein the measurement is performed from the oscillation frequency measured by the means. 2. The carbon dioxide gas sensor detecting circuit according to claim 1, wherein a compensating capacitor is connected between an output of said first inverter and said carbon dioxide gas sensor. 3. The method according to claim 1, wherein said resistor comprises a fixed resistor and a variable resistor, and the oscillation frequency output from said first inverter can be adjusted by changing the resistance value of said variable resistor. The carbon dioxide sensor detection circuit according to claim 1 or 2, wherein: 4. A carbon dioxide sensor for detecting a carbon dioxide concentration, a heating unit for heating the carbon dioxide sensor to an operating temperature, a calibration timing determining unit for determining a calibration timing of the carbon dioxide sensor, and the calibration timing determining unit. A first impedance measuring unit that measures the impedance of the carbon dioxide gas sensor at a measurement frequency when it is determined that calibration is not to be performed; a sensor output value storing unit that stores an output value from the first impedance measuring unit; A second impedance measuring means for measuring the impedance of the carbon dioxide sensor at a calibration frequency when the calibration timing judging means judges to calibrate, and calculating a reference value by receiving an output value from the second impedance measuring means The reference value storage means for storing, and the sensor output value storage means. Concentration calculation means for calculating a carbon dioxide concentration from the stored sensor output value and the reference value stored in the reference value storage means; and concentration output means for outputting the carbon dioxide concentration obtained by the concentration calculation means. A carbon dioxide concentration detecting device, comprising: 5. The carbon dioxide sensor according to claim 1, wherein said first and second impedance measuring means are each shared by one of said carbon dioxide sensors being switched by circuit switching means. The carbon dioxide concentration detecting device according to claim 4, wherein the device is a circuit. 6. The first and second impedance measuring means are provided such that at least two or more of the resistances are provided and any one of these resistances is selectively electrically connected by resistance switching means. The carbon dioxide gas sensor according to claim 4, wherein the carbon dioxide gas sensor circuit is integrated with the carbon dioxide gas sensor circuit according to claim 1, 2 or 3.