JP2646415B2 - Gas concentration detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to an electronic gas concentration detecting device, and more particularly to a gas sensor having an electric characteristic in which a capacitance component C and an electric resistance component R change according to, for example, a gas concentration. The present invention relates to an electronic gas concentration detecting device using the same.

[0002]

2. Description of the Related Art Conventionally, this type of electronic gas concentration detecting device is configured as shown in FIG. 4 when a gas sensor having electric characteristics of a capacitance change type is used, for example. When a gas sensor having the electrical characteristics of the mold is used, the gas sensor is configured as shown in FIG.

That is, both gas concentration detecting devices are C-MOS
Inverters 11 and 12 connected in series, such as a Schmidt inverter of a type, a resistor R1 inserted in a feedback circuit of the inverter 11 of the preceding stage, and a capacitance connected to the input side of the inverter 11 of the preceding stage as well. In the gas concentration detecting device shown in FIG. 4, the CR oscillation circuit 10 composed of an element (capacitor) C1 is used. In the gas concentration detecting device shown in FIG. Is used in series, and this gas sensor Ch is inserted in series between the input side of the inverter 11 at the preceding stage and the capacitance element C1 so that the CR oscillation circuit 10
Is composed.

The gas concentration detecting device shown in FIG. 4 changes the oscillation frequency of the CR oscillation circuit 10 correspondingly by changing the capacitance of a gas sensor Ch, which is an oscillation frequency determining element, according to a change in gas concentration. The oscillation output, that is, the frequency signal is sent to, for example, a microcomputer (not shown), and the microcomputer performs arithmetic processing to detect the oscillation frequency and calculate the gas concentration corresponding to the frequency.

Here, from the CR oscillation circuit 10 of FIG.
The capacitance value of the resistance value and the capacitance element C1 of the resistor R1 is constant, the next frequency F _C is output corresponding to the electrostatic capacitance value of the gas sensor Ch.

[0006]

(Equation 1) Where k is a constant,

[0007]

(Equation 2) It is. The resistor R2 connected in parallel with the gas sensor Ch is for preventing a DC voltage component from being applied to the gas sensor Ch. Usually, since a sensor made of aluminum oxide or the like is used as a gas sensor, if a DC voltage component is applied, the characteristics of the sensor are likely to be deteriorated due to polarization, dielectric breakdown, and the like, so that this is prevented.

On the other hand, in the gas concentration detecting device shown in FIG. 5, a resistance change type gas sensor Rh whose electric resistance changes according to a change in gas concentration is used as a gas sensor. The CR oscillating circuit 10 is configured by inserting this variable resistance gas sensor Rh as an oscillation frequency determining resistance element instead of the resistor R1 inserted in the feedback circuit of the inverter 11 in the preceding stage. This gas concentration detection device also changes the oscillation frequency of the CR oscillation circuit 10 by changing the electric resistance of the gas sensor Rh, which is an oscillation frequency determination element, according to the change in gas concentration. That is, the oscillation frequency of the CR oscillation circuit 10 is detected from the frequency signal as described above, and the gas concentration corresponding to the frequency is calculated.

From the CR oscillation circuit 10 of FIG. 5, since the capacitance value of the capacitance element C1 is constant, the gas sensor R
The next frequency F _R is output according to the electric resistance value of h.

[0010]

(Equation 3)

[0011]

However, in the conventional gas concentration detecting device having the above structure, the electric characteristics of the two gas sensors Ch and Rh are generally as shown in FIG. The range of the gas concentration that can be measured by the apparatus shown in FIG. 4 using the gas sensor Ch is not less than g1 in FIG. 3, and the gas concentration below g1 cannot be measured.
Further, in the apparatus shown in FIG. 5 using the resistance change type gas sensor Rh, the measurable gas concentration ranges from 0 to g2 in FIG. 3, and the gas concentration of g2 or more cannot be measured. Therefore, both gas concentration detection devices have a narrow range of gas concentration that can be measured,
There is a disadvantage that the measurement range is limited.

Accordingly, an object of the present invention is to provide a single CR oscillation circuit that oscillates in response to a change in the electrical resistance value of a gas sensor having the same circuit configuration in which the oscillation frequency changes in response to a change in the capacitance value of the gas sensor. It is an object of the present invention to provide an electronic gas concentration detecting device capable of measuring a gas concentration with high accuracy over a wide range by using a single gas sensor by switching to a circuit configuration in which the frequency changes.

[0013]

The above object is achieved by a gas concentration detecting device according to the present invention. In summary, the present invention provides a gas sensor having an electrical characteristic in which a capacitance component and an electric resistance component change according to a gas concentration, a CR oscillation circuit including the gas sensor as an element for determining an oscillation frequency, and a CR oscillation circuit. Switching means for switching the circuit between a circuit configuration in which the oscillation frequency changes in accordance with a change in the capacitance value of the gas sensor and a circuit configuration in which the oscillation frequency changes in accordance with a change in the electric resistance value of the gas sensor; And an arithmetic control means for detecting the frequency of the oscillation output from the CR oscillation circuit and converting the frequency to a gas concentration, and for controlling the switching operation of the switching means, wherein the arithmetic control means switches the switching means. The operation is automatically controlled, and the gas concentration can be measured over a wide range using the capacitance characteristics and electric resistance characteristics of the gas sensor. A gas concentration detecting apparatus being characterized in that the so that.

[0014]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing an embodiment of a gas concentration detecting device according to the present invention. In the gas concentration detecting device of the present embodiment, two inverters 1 connected in series
A CR oscillation circuit 10 is composed of 1 and 12, a resistor R1 inserted in a feedback circuit of the inverter 11 in the preceding stage, and a capacitance element C1 connected to the input side of the inverter 11 in the preceding stage.

In this embodiment, a gas sensor GS having an electric characteristic in which a capacitance component C and an electric resistance component R change according to the gas concentration is used, and the first, second and third gas sensors having the same configuration are used. When the gas sensor is used as a capacitance change type gas sensor by using three analog switches X, Y, and Z, the gas sensor GS is connected in series between the input side of the inverter 11 at the preceding stage and the capacitance element C1. When used as a variable resistance sensor, the gas sensor G
S is switched and connected so that S is inserted instead of the resistor R1 in the feedback circuit of the inverter 11 in the preceding stage, and FIG.
And the CR oscillation circuit shown in FIG.

The first, second, and third analog switches X, Y, and Z each have one movable contact XC, YC, ZC.
And two fixed contacts X0 and X1, Y0 and Y1, Z0 and Z1. The first analog switch X has a movable contact XC having a resistor R2 for removing a DC voltage and a capacitance element C2.
1, the first fixed contact X0 is not connected, and the second fixed contact X1 is connected to the input side of the inverter 11 in the preceding stage. Therefore, in this embodiment, the first analog switch X may be an on / off switch. The second analog switch Y has its movable contact Y
C is connected to the other terminal of the gas sensor GS, the first fixed contact Y0 is connected to the connection point between the DC voltage removing resistor R2 and the capacitance element C1, and the second fixed contact Y1 is connected to the third fixed contact Y1. Is connected to the second fixed contact Z1 of the analog switch Z. Further, the movable contact ZC of the third analog switch Z is connected to the output side of the inverter 11 at the preceding stage, and the first fixed contact Z0 is connected to the resistor R1.

In the above configuration, if the movable contacts XC, YC, and ZC of the three analog switches X, Y, and Z are connected to the first fixed contacts X0, Y0, and Z0 as shown in FIG. R1 is inserted into the feedback circuit of the preceding inverter 11 and the gas sensor GS is connected to the preceding inverter 11
Is connected in series with the capacitance element C1 on the input side of the capacitor R1, and a DC voltage removing resistor R is connected between both ends of the gas sensor GS.
2 are connected in parallel to form a capacitance measurement type circuit configuration, and a CR oscillator 10 whose oscillation frequency changes in accordance with the capacitance value of the gas sensor GS, which is the same as in FIG. 4, is configured.

On the other hand, three analog switches X,
When the movable contacts XC, YC, ZC of Y and Z are connected to the second fixed contacts X1, Y1, Z1, the gas sensor GS is inserted in the feedback circuit of the inverter 11 in the preceding stage instead of the resistor R1. The resistor R2 for removing the DC voltage on the input side of the inverter 11 in the preceding stage is short-circuited by the first analog switch X, and the capacitance element C1 is connected to the input side of the inverter 11 in the preceding stage by the first analog switch X. Since the connection is made through X, the CR oscillator 10 whose oscillation frequency changes in accordance with the electric resistance value of the gas sensor GS same as that of FIG.

An oscillation output from the CR oscillation circuit 10, ie, a frequency signal, is sent to a microcomputer 20, where it is subjected to arithmetic processing to detect an oscillation frequency, and a gas concentration corresponding to the frequency is calculated as a measured gas concentration. .

In this embodiment, C-MOS type Schmitt inverters are used as the inverters 11 and 12,
From the CR oscillation circuit 10, a pulse signal whose frequency changes in response to a change in the capacitance value or a change in the electric resistance value of the gas sensor GS is output and sent to the microcomputer 20. The microcomputer 20 is counter unit 21 for counting the number of pulses of the frequency signal F _C or F _R that is input
And a storage unit 22 including a ROM (Read Only Memory) storing an arithmetic processing program, and a storage unit 2 based on the number of pulses within a predetermined time counted by the counter unit 21.
2, an arithmetic processing unit 23 for detecting the frequency and converting the detected frequency to a gas concentration, and transmitting control commands from the arithmetic processing unit 23 to respective control lines LX, LX.
And a control port 24 for supplying the analog switches X, Y, and Z through Y and LZ to control the connection of these switches.

The operation of the C-MOS type Schmitt inverters 11 and 12 will be described briefly.
Both Schmitt inverter has two threshold voltage of the high level of the threshold voltage V _TH and a low-level threshold voltage V _TL, when the input voltage is lower than the threshold voltage V _TH of the high levels generates an output voltage V _H of the high-level When the input voltage reaches the high-level threshold voltage V _TH , the output voltage switches from the high level V _H to the low level V _L , and keeps the low level until the input voltage drops to the low-level threshold voltage V _TL. and holding the output voltage V _L, the input voltage is an output voltage when the drop of the low-level threshold voltage V _TL operates to switches from the low level V _L to the high level V _H. Therefore, C
From the R oscillation circuit 10, a pulse voltage having a cycle corresponding to the charging and discharging of the capacitance of the capacitance element C1 or the series circuit of the capacitance element C1 and the gas sensor GS is output.

FIG. 2 shows an example of an algorithm for controlling the measuring operation of the gas concentration detecting device of the present embodiment. First,
When the gas concentration measurement operation is started, in step S1, a control signal is sent from the control port 24 of the microcomputer 20 to the analog switches X, Y, and Z via the control lines LX, LY, and LZ. Movable contacts XC, YC, ZC of the second fixed contact X1,
The CR oscillation circuit 10 is connected to the Y1 and Z1 sides so that the gas concentration is measured by the resistance change characteristic of the gas sensor GS.
Is configured. With this configuration, the gas concentration is measured and CR
The frequency of the oscillation output from the oscillation circuit 10 is measured (Step S2). Next, a corresponding gas concentration g is calculated from the frequency measured in Step S3, and Step S4
It is determined whether or not the gas concentration g calculated in step 2 is higher than g2 (see FIG. 3). When the calculated gas concentration g is smaller than g2 (NO), the measurement of the gas concentration is terminated.

On the other hand, when the calculated gas concentration g is larger than g2 (YES), the measurement result based on the resistance change characteristic of the gas sensor GS lacks reliability. Control signals are sent to the respective analog switches through the control lines of the above, and the movable contacts of these switches are switched and connected to the first fixed contacts X0, Y0, Z0,
The CR oscillation circuit 10 is configured to measure the gas concentration based on the capacitance change characteristics of the gas sensor GS. With this configuration, the gas concentration is measured, and the frequency of the oscillation output from the CR oscillation circuit 10 is measured (step S6). Then
The corresponding gas concentration g is calculated from the frequency measured in step S7, and the measurement of the gas concentration ends.

As described above, in this embodiment, the switching operation of the three analog switches is automatically controlled by the microcomputer, so that the gas concentration detecting device (CR oscillation circuit) using the gas sensor GS can be used as the gas sensor G.
Automatically switched between a circuit configuration for measuring gas concentration using the electrical characteristics of the capacitance change type of S and a circuit configuration for measuring the gas concentration using the electrical characteristics of the resistance change type. Therefore, the gas concentration can be automatically measured over a wide range by using a single gas concentration detection device. In addition, since the CR oscillation circuit can be automatically switched with a simple circuit configuration, there is no increase in cost, and the gas concentration can be measured in a highly reliable part over the entire measurement range, resulting in high accuracy. There is an advantage that measurement can be performed.

In the above embodiment, the analog switch is used as the switching means for switching the circuit configuration. However, other means may be used. For example, an electronic switch such as an IC or a transistor may be used, or a relay may be used.

The above embodiment is merely an example of the present invention, and the circuit configuration, elements used, and the like can be arbitrarily changed as necessary. For example, an inverter other than the C-MOS type Schmitt inverter or another circuit element can be used, and an arithmetic control element other than the microcomputer may be used. Further, the frequency output generated from the CR oscillation circuit does not necessarily need to be a pulse output.

[0028]

As described above, according to the present invention,
With a simple circuit configuration, the CR oscillation circuit of a single gas concentration detection device changes the oscillation frequency according to the change in the electric resistance value of the same gas sensor as the configuration in which the oscillation frequency changes according to the change in the capacitance value of the gas sensor. Since the configuration can be automatically switched to the changing configuration, measurement of gas concentration in a wide range can be automatically performed by using a single gas concentration detection device. In addition, since the gas concentration can be measured in a highly reliable portion over the entire measurement range, there are many remarkable effects such as high-precision measurement.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a gas concentration detecting device according to the present invention.

FIG. 2 is a flowchart illustrating an example of an algorithm for controlling a measurement operation of the gas concentration detection device in FIG. 1;

FIG. 3 is a diagram showing electrical characteristics of a gas sensor used in a gas concentration detection device.

FIG. 4 is a circuit configuration diagram showing an example of a conventional gas concentration detection device.

FIG. 5 is a circuit configuration diagram showing another example of a conventional gas concentration detection device.

[Explanation of symbols]

 Reference Signs List 10 CR oscillation circuit 11, 12 Schmitt inverter 20 Microcomputer 21 Counter unit 22 Storage unit 23 Operation processing unit 24 Control port GS Gas sensor X, Y, Z Analog switch

Claims (3)

(57) [Claims] 1. A gas sensor having an electric characteristic in which a capacitance component and an electric resistance component change according to a gas concentration, a CR oscillation circuit including the gas sensor as an oscillation frequency determining element, and the CR oscillation circuit. Switching means for switching between a circuit configuration in which the oscillation frequency changes in accordance with a change in the capacitance value of the gas sensor and a circuit configuration in which the oscillation frequency changes in accordance with a change in the electric resistance value of the gas sensor; and Operation control means for detecting the frequency of the oscillation output from the CR oscillation circuit to convert the frequency into a gas concentration and controlling the switching operation of the switching means, wherein the operation control means automatically performs the switching operation of the switching means. So that the gas concentration can be measured over a wide range using the capacitance characteristics and electric resistance characteristics of the gas sensor. A gas concentration detecting device, characterized in that: 2. The method according to claim 1, wherein the CR oscillation circuit comprises two C-MOSs.
A Schmitt inverter of a type, a resistance element inserted in a feedback circuit of the inverter, and a series circuit of the gas sensor and the capacitance element also connected to the input side of the inverter. Item 7. The gas concentration detection device according to Item 1. 3. The CR oscillation circuit includes a C-MOS type Schmitt inverter, the gas sensor inserted in a feedback circuit of the inverter, and a capacitance element also connected to an input side of the inverter. The gas concentration detection device according to claim 1, wherein