JP2000249683A - Gas-detecting apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas-detecting apparatus which can stably detect with a high detection accuracy. SOLUTION: A DC-DC converter 2 is set. A connecting terminal (1) of a reference electrode of a carbon dioxide sensor 71 and a minus power source terminal of an operational amplifier 96 constituting a buffer circuit 83 are electrically insulated from a minus terminal of a d.c. power source respectively. An insulating circuit 4 is set to electrically insulate an output side of the buffer circuit 83 from the minus terminal of the d.c. power source 85. Even when an impedance between connecting terminals (1) and (2) of the carbon dioxide sensor 71 is high, and one end of an electric heater 72 is connected to the minus terminal of the d.c. power source 85 via a resistor R4 of several hundreds Ωor lower, a leakage current is prevented from being generated between the electric heater 72 and a sensor main body 74 inside or at a surface of a base 73 or pedestal of the carbon dioxide sensor 71. Decrease and instability of a detection accuracy because of the leakage current can be avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detector, and more particularly to a gas detector using a gas sensor having a solid electrolyte and a metal salt and having an electric heater.

[0002]

2. Description of the Related Art Conventionally, a concentration cell type gas detector using a solid electrolyte element having sodium ion conductivity has been known, and the gas detector is used for general-purpose applications, environmental control, medical technology, facility horticulture, and the like. In a wide technical field such as the fermentation industry, it is used for gas detection and concentration measurement, and various controls based on the detection results and concentration measurement results.

As one example of such a gas detecting device, a carbon dioxide (C) disclosed in Japanese Patent Application Laid-Open No. Hei 10-73560 is disclosed.
O ₂ ) sensors are known. The carbon dioxide sensor described in the publication includes a base body containing a heating element, a reference electrode, an ion conductor layer made of a solid electrolyte, a detection electrode, and a metal carbonate layer formed in this order. , Reference electrodes, and detection electrodes, and are indirectly attached to the pedestal via conductive connection members and the ends of the respective wirings led out to the surface or side surface of the base. As the heating element, a planar electric heater capable of heating the ion conductor layer to a temperature range of about 200 to 800 ° C. and holding the same uniformly is used.

FIG. 4 shows a carbon dioxide sensor 7 described in the publication.
FIG. 2 is a block circuit diagram showing a configuration of a conventional carbon dioxide gas detection device 81 using the first embodiment. The carbon dioxide sensor 71 includes a base 73.
The sensor main body 74 is attached and formed. The base 73 contains a planar electric heater 72 as a heating element. The sensor body 74 has a reference electrode, an ion conductor layer, a detection electrode, and a metal carbonate layer (all not shown) laminated in this order. From the sensor body 74, a connection terminal for the reference electrode and a connection terminal for the detection electrode are led out.

The carbon dioxide gas detector 81 includes a heater temperature control circuit 82, a buffer circuit 83, and a microcomputer 8
4. It comprises a DC power supply 85. The heater temperature control circuit 82 includes resistors R1 to R8, CMOS circuits 91 and 9,
2. It comprises an operational amplifier 93.

The electric heater 72 of the carbon dioxide sensor 71 and the resistors R1 to R3 constitute a Wheatstone bridge 94. The connection point between the resistors R1 and R2 is connected to the positive terminal of a DC power supply 85 to apply a DC voltage. The connection point between the electric heater 72 and the resistor R3 is connected to the negative terminal of the DC power supply 85 via the resistor R4 and grounded.

The operational amplifier 93 is supplied with power from a DC power supply 85 to perform a single power supply operation, and a differential amplifier circuit 95 is composed of the operational amplifier 93 and the resistors R5 to R8. Then, the differential amplifier circuit 95 determines the voltage at the connection point between the resistors R1 and R3 and the connection point A between the resistor R2 and the electric heater 72.
And differentially amplify the voltage.

Each of the CMOS circuits 91 and 92 has a PM
Each of the CMOS circuits 91 and 92 is composed of an OS transistor and an NMOS transistor.
5 are connected to the plus side terminal and the minus side terminal. That is, the sources of the PMOS transistors of the CMOS circuits 91 and 92 are connected to the positive terminal of the DC power supply 85, and the sources of the NMOS transistors of the CMOS circuits 91 and 92 are connected to the negative terminal of the DC power supply 85. Grounded. The output of the CMOS circuit 91 (the drain of each transistor) is connected to the connection point A between the resistor R2 and the electric heater 72, and the output of the CMOS circuit 92 (the drain of each transistor) is connected to the connection point between the electric heater 72 and the resistor R4. ing.

The buffer circuit 83 comprises a voltage follower comprising an operational amplifier 96. The operational amplifier 96 is supplied with power from the DC power supply 85 and performs a single power supply operation. The connection terminal of the detection electrode of the carbon dioxide sensor 71 is connected to the non-inverting input terminal of the operational amplifier 96. The connection terminal of the reference electrode of the carbon dioxide sensor 71 is connected to the negative terminal of the DC power supply 85 and grounded.

The microcomputer 84 includes a CPU, R
OM, RAM, and I / O circuits are provided, and power is supplied from a DC power supply 85. The microcomputer 84 is connected to the gates of the respective transistors of the CMOS circuits 91 and 92 and receives the outputs of the differential amplifier circuit 93 and the buffer circuit 83. And
The microcomputer 84 includes a heater temperature control circuit 82
And a detection signal Vo of the concentration of carbon dioxide is generated from the output of the buffer circuit 83.

Next, the operation of the carbon dioxide gas detecting device 81 configured as described above will be described. FIG.
FIG. 4 is a characteristic diagram showing a time displacement of a voltage at a connection point A of the electric heater 72 and the electric heater 72.

The microcomputer 84 is provided for each CMOS.
A pseudo AC is generated by switching on / off of each transistor of the circuits 91 and 92, and the pseudo AC is caused to flow through the electric heater 72. That is, the PMOS transistor of the CMOS circuit 91 and the NM of the CMOS circuit 92
The OS transistor is turned on, and the NMOS transistor of the CMOS circuit 91 and the PM of the CMOS circuit 92 are turned on.
When the OS transistor is turned off, a current flows through the electric heater 72 from the connection point A toward the resistor R4. Then, since the connection point A is pulled up via the PMOS transistor of the CMOS circuit 91, the voltage at the connection point A becomes the voltage Vcc (for example, the voltage at the plus terminal of the DC power supply 85, as shown in a period T1 in FIG. 12V).

Conversely, when the PMOS transistor of the CMOS circuit 91 and the NMOS transistor of the CMOS circuit 92 are turned off and the NMOS transistor of the CMOS circuit 91 and the PMOS transistor of the CMOS circuit 92 are turned on, the resistance of the electric heater 72 is reduced. A current flows from R4 toward the connection point A. Then, the connection point A is CM
Since the pull-down is performed via the NMOS transistor of the OS circuit 91, as shown in a period T2 in FIG.
Is equal to the voltage of the negative terminal of the DC power supply 85 (ground voltage = 0 V).

Here, when the periods T1 and T2 shown in FIG. 5 are made equal and the periods T1 and T2 are alternately repeated, a pseudo alternating current is generated in one cycle of the period T1 + T2 (= 2T1 = 2T2). . Then, as shown in FIG. 5, the microcomputer 84 applies the pseudo alternating current to the electric heater 72 by turning off all the transistors of the CMOS circuits 91 and 92 during a period T3 in which the pseudo alternating current flows to the electric heater 72. And the period T4 during which the current is not passed through is alternately repeated. The voltage Vr at the connection point A during this period T4 (for example, 2.5 V)
Are the voltage Vcc of the positive terminal of the DC power supply 85 and the resistance R
4 and each resistor R1 constituting the Wheatstone bridge 94
R3 and the respective resistance values of the electric heater 72.

In a period T4, the differential amplifier circuit 95 amplifies the difference between the voltage at the connection point of each of the resistors R1 and R3 and the voltage at the connection point A. The microcomputer 84 uses the resistance value measurement method by the Wheatstone bridge 94 based on the output of the differential amplifier circuit 95 (the difference between the voltage at the connection point of each of the resistors R1 and R3 and the voltage at the connection point A), and Heater 7
2 is detected. Here, since the temperature of the electric heater 72 is uniquely determined by its resistance value, a predetermined resistance value when the electric heater 72 reaches a set temperature (for example, 450 ° C.) can be obtained.

The microcomputer 84 controls each of the periods T3 and T3 so that the electric heater 72 has a predetermined resistance value.
By appropriately setting the time 4, the electric heater 72 is controlled to reach the set temperature. For example, the electric heater 7
2, set temperature = 450 ° C., DC power supply 85 voltage Vcc = 1
2V, voltage Vr at node A in period T4 = 2.5
V, if the period T3 + T4 = 535 msec, the period T3
+ T4 to period T3 (duty ratio = 100
× T3 / (T3 + T4)) is 61.3%.

By controlling the electric heater 72 at a constant temperature in this manner, the connection terminal of the detection electrode in the carbon dioxide gas sensor 71 is connected to the above-mentioned publication (JP-A-10-7).
3560), an output corresponding to the concentration of carbon dioxide in the atmosphere in which the carbon dioxide gas 71 is installed can be obtained. The output of the connection terminal of the detection electrode is input to the microcomputer 84 via the buffer circuit 83.

The reason why the buffer circuit 83 is provided is as follows.
Since the impedance between the connection terminals of the carbon dioxide sensor 71 is high, the output of the connection terminal of the detection electrode is reliably transmitted to the microcomputer 84 by lowering the impedance.

Here, the output of the connection terminal of the detection electrode in the carbon dioxide sensor 71 is logarithmically output with respect to the carbon dioxide concentration. However, in order to measure the carbon dioxide concentration, it is desirable that a linear output be obtained with respect to the carbon dioxide concentration. Therefore, the microcomputer 84 converts the output of the connection terminal of the detection electrode, which is logarithmically output with respect to the carbon dioxide concentration, to a linear output with respect to the carbon dioxide concentration, and performs an appropriate correction to obtain a linear output. A corresponding detection signal Vo is generated.

[0020]

In the conventional carbon dioxide gas detector 81, the impedance between the respective connection terminals of the carbon dioxide sensor 71 is high, and the connection terminal of the reference electrode is connected to the minus terminal of the DC power supply 85. In addition, one end of the electric heater 72 is connected to the minus terminal of the DC power supply 85 via the resistor R4 and is grounded.

The resistor R4 is provided at the connection point A.
This is because the voltage of the connection point A can be easily detected by the differential amplifier circuit 95 even when the resistance value of the electric heater 72 is low. Therefore, the resistance value of the resistor R4 is set to several hundred Ω or less.

Therefore, in the carbon dioxide gas sensor 71, a leak current may be generated between the electric heater 72 and the sensor body 74 inside or on the surface of the base 73 or the pedestal (not shown). When the leak current occurs, there is a problem that the detection accuracy of the carbon dioxide concentration is reduced. Further, since the leak current increases as the humidity increases, the output of the connection terminal of the detection electrode also changes with the change of the leak current with respect to the change of the humidity, and the detection signal Vo of the microcomputer 84 is incorrect and unstable. There is also the problem of becoming.

By replacing the metal carbonate layer in the carbon dioxide sensor 71 with a metal salt layer that dissociates with the gas to be detected, a gas sensor that detects the concentration of a specific gas corresponding to the metal salt is realized. can do. For example, by replacing the metal carbonate layer on the metal nitrate layer, it is possible to achieve a NO _X gas sensor for detecting the NO _X gas concentration. Further, by replacing the metal carbonate layer with the metal sulfate layer, it is possible to realize an SO _X gas sensor that detects the SO _X gas concentration.

As described above, in the various gas detection devices using the gas sensor constituted by replacing the metal carbonate layer of the carbon dioxide gas sensor 71 with an appropriate metal salt layer, the carbon dioxide gas detection device 81 using the above-described carbon dioxide gas sensor 71 is also used. Had similar problems. The present invention has been made in order to solve the above problems, and an object of the present invention is to provide a gas detection device that has high detection accuracy and can perform stable detection.

[0025]

Means for Solving the Problems According to the first aspect of the present invention, which has been made to achieve the above object, a reference electrode, an ion conductor layer composed of a solid electrolyte, a detection electrode, and a metal salt layer are provided. The electric heater and the gas sensor are electrically insulated from the gas sensor main body formed in this order by electrically insulating the gas sensor to which the electric heater is attached and the ground side of the gas sensor main body from the ground side of the electric heater. The gist of the present invention is to provide a gas detection device provided with an insulating means for preventing a leak current from being generated between the main body and the main body.

Therefore, according to the first aspect of the present invention,
Even if the impedance of the gas sensor body is high, since a leak current can be prevented from being generated between the electric heater and the gas sensor body inside or on the surface of the gas sensor,
Since the detection accuracy of the gas concentration does not decrease due to the leak current, and the detection accuracy does not change with the change of the leak current with respect to the change of the humidity, the detection accuracy is high and stable detection is possible.

According to a second aspect of the present invention, in the gas detection device according to the first aspect, heater temperature control means for controlling the temperature of the electric heater by controlling energization of the electric heater. Gas concentration detection means for detecting a gas concentration based on an electromotive force generated between a reference electrode and a detection electrode of the gas sensor; power supply means for supplying power to the heater temperature control means and the gas concentration detection means; Is provided. The insulating means electrically insulates the ground side of the gas sensor main body from the ground side of the electric heater by electrically insulating the gas concentration detecting means from the ground side of the power supply means. A leak current may be prevented from being generated between the electric heater and the gas sensor main body via the heater temperature control unit, the power supply unit, and the gas concentration detection unit.

Next, a third aspect of the present invention provides a gas sensor body comprising a reference electrode, an ion conductor layer made of a solid electrolyte, a detection electrode, and a metal salt layer formed in this order. A gas sensor having an electric heater attached thereto and a potential difference between a ground side potential of the gas sensor body and a ground side potential of the electric heater are eliminated, thereby preventing a leak current from being generated between the electric heater and the gas sensor body. The gist of the present invention is a gas detection device provided with an electric potential setting unit that performs the operation.

Therefore, the third aspect of the invention can provide the same effects as the first aspect of the invention. According to a fourth aspect of the present invention, in the gas detection device according to the third aspect, a heater temperature control unit that controls a temperature of the electric heater by controlling energization of the electric heater, and the gas sensor. Gas concentration detecting means for detecting a gas concentration based on an electromotive force generated between the reference electrode and the detection electrode. The electric potential setting means generates an offset voltage equal to an effective voltage applied between both terminals of the electric heater by the heater temperature control means, and applies the offset voltage to the reference electrode, whereby the gas sensor A potential difference between the ground side potential of the main body and the ground side potential of the electric heater may be eliminated to prevent a leak current from occurring between the electric heater and the gas sensor main body.

According to a fifth aspect of the present invention, in the gas detecting device according to the fourth aspect, the gas concentration detecting means subtracts the offset voltage from the electromotive force to generate an original electromotive force of the gas sensor. May be returned. By the way, as in the invention according to claim 6, in the gas detection device according to any one of claims 1 to 5,
The metal salt layer of the gas sensor may be composed of a metal carbonate, a metal nitrate, or a metal sulfate that dissociates with the gas to be detected.

Therefore, according to the invention described in claim 6,
Carbon dioxide gas using a carbon dioxide sensor with a metal carbonate layer
(CO_Two) Carbon dioxide detector to detect concentration, metal nitric acid
NO with salt layer_XNO using gas sensor_XGas concentration
NO to detect_XGas detection device, S with metal sulfate layer
O_XSO using gas sensor_XSO for detecting gas concentration _X
A gas detection device can be provided.

In the embodiments of the invention described below, the "heater temperature control means" described in claims or means for solving the problems is a heater temperature control circuit 8
2, the "gas concentration detecting means" is composed of the buffer circuit 83, the differential amplifier circuit 14, and the microcomputer 84, the "power supply means" is equivalent to the DC power supply 85, and the "insulating means" is also equivalent. The "potential setting means" is composed of a resistance voltage dividing circuit 12 and a buffer circuit 13.

[0033]

Embodiments of the present invention will be described below with reference to the drawings. In each embodiment,
The same components as those of the conventional embodiment shown in FIGS. 4 and 5 have the same reference numerals, and detailed description thereof will be omitted.

(First Embodiment) FIG. 1 is a block circuit diagram showing a configuration of a carbon dioxide gas detecting device 1 according to a first embodiment using a carbon dioxide gas sensor 71. The carbon dioxide detection device 1
In addition to the components (heater temperature control circuit 82, buffer circuit 83, microcomputer 84, DC power supply 85) of the conventional carbon dioxide detection device 81, it is composed of a DC-DC converter 2, a constant voltage circuit 3, and an insulation circuit 4. ing.

The DC-DC converter 2 having a general circuit configuration includes an insulated power transformer in which a primary winding and a secondary winding are insulated, a switching element, and a smoothing circuit (all not shown). ing. One end of the primary winding of the insulated power transformer is connected to the positive terminal of the DC power supply 85, and a switching element is connected between the other end of the primary winding and the negative terminal of the DC power supply 85. And
By the switching operation of the switching element, the DC supplied from the DC power supply 85 is converted into an AC, the AC is smoothed by a smoothing circuit to generate a DC, and the DC is applied to the plus output terminal 2a of the DC-DC converter. The signal is output from the negative output terminal 2b.

As described above, since the DC-DC converter 2 uses the insulated power transformer, its negative output terminal 2b and the negative terminal of the DC power supply 85 are electrically insulated. If the negative output terminal 2b and the negative terminal of the DC power supply 85 are electrically insulated, the DC-DC converter 2 is not limited to the above circuit configuration, but may have any circuit configuration. .

The constant voltage circuit 3 has a general circuit configuration of a shunt type or a series type using a voltage control transistor, and each output terminal 2a,
The DC voltage output from 2b is stabilized, and the DC is output from the plus output terminal 3a and the minus output terminal 3b. Therefore, the negative output terminal 3b of the constant voltage circuit 3 and the negative terminal of the DC power supply 85 are electrically insulated.

Operational Amplifier 9 Constituting Buffer Circuit 83
The power supply 6 is not supplied from the DC power supply 85 but is supplied from the constant voltage circuit 3 to perform a single power supply operation. That is, the positive power supply terminal of the operational amplifier 96 is connected to the positive output terminal 3a of the constant voltage circuit 3, and the negative power supply terminal of the operational amplifier 96 is connected to the negative output terminal 3b of the constant voltage circuit 3. Therefore, the buffer circuit 83 and the DC power supply 85
Is electrically insulated from the negative terminal.

The connection terminal of the reference electrode in the carbon dioxide sensor 71 is not the negative terminal of the DC power supply 85,
It is connected to the negative side output terminal 3b of the constant voltage circuit 3. Therefore, the connection terminal of the reference electrode and the negative terminal of the DC power supply 85 are electrically insulated.

The output of the buffer circuit 83 is input to the microcomputer 84 via the insulating circuit 4. The insulating circuit 4 is provided to prevent the output side of the buffer circuit 83 and the negative terminal of the DC power supply 85 from being electrically connected via the internal circuit of the microcomputer 84.

For example, to configure the insulating circuit 4 using a signal transmission transformer in which the primary winding and the secondary winding are insulated, the primary winding of the signal transmission transformer is connected to the buffer circuit 83. Connected to the output (output terminal of the operational amplifier 96) and the negative output terminal 3b of the constant voltage circuit 3,
The secondary winding of the signal transmission transformer may be connected to the input of the microcomputer 84 and the negative terminal of the DC power supply 85.

To configure the insulating circuit 4 using a photocoupler including a photodiode and a phototransistor, the photodiode is connected to the output of the buffer circuit 83 (the output terminal of the operational amplifier 96) and the constant voltage circuit 3
And the phototransistor may be connected to the input of the microcomputer 84 and the negative terminal of the DC power supply 85.

If the output side of the buffer circuit 83 and the negative terminal of the DC power supply 85 can be prevented from being electrically connected via the internal circuit of the microcomputer 84, the insulating circuit 4 has the above-described configuration. The configuration is not limited to this, and any configuration may be used. As described above, in the carbon dioxide detection device 1 of the first embodiment, by providing the DC-DC converter 2, the connection terminal of the reference electrode in the carbon dioxide sensor 71 and the negative side of the operational amplifier 96 forming the buffer circuit 83 are provided. The power supply terminal is electrically insulated from the negative terminal of the DC power supply 85. By providing the insulating circuit 4, the buffer circuit 83
Is electrically insulated from the negative terminal of the DC power supply 85. Therefore, the connection terminal of the reference electrode (the ground side of the sensor main body 71) and the connection point of the electric heater 72 and the resistor R4 (the ground side of the electric heater 72) are electrically insulated.

Therefore, according to the carbon dioxide sensor 1, the impedance between the connection terminals of the carbon dioxide sensor 71 is high, and one end of the electric heater 72 has a resistance R4 of several hundred Ω or less.
The ground 7 is connected to the negative terminal of the DC power supply 85 via the
A leak current can be prevented from occurring between the electric heater 72 and the sensor body 74 inside or on the surface of the base 3 or the pedestal (not shown). Therefore, the detection accuracy of the carbon dioxide concentration does not decrease due to the leak current, and the output of the connection terminal of the detection electrode does not change with the change of the leak current with respect to the change of humidity. Vo can be made accurate and stable.

(Second Embodiment) FIG. 2 is a block circuit diagram showing a configuration of a carbon dioxide gas detecting device 11 of a second embodiment using a carbon dioxide sensor 71. Carbon dioxide detector 11
Includes a resistance voltage dividing circuit 1 in addition to the components (heater temperature control circuit 82, buffer circuit 83, microcomputer 84, DC power supply 85) of the conventional carbon dioxide gas detecting device 81.
2, a buffer circuit 13 and a differential amplifier circuit 14.

The resistor voltage dividing circuit 12 is composed of resistors R11 and R12 connected in series between a positive terminal and a negative terminal of the DC power supply 85.
An offset voltage Voff described later is generated from the second connection point B. The buffer circuit 13 is constituted by a voltage follower including an operational amplifier 15. Operational amplifier 15
Is supplied with power from the DC power supply 85 and performs a single power supply operation. The connection point B of the buffer circuit 13 is connected to a non-inverting input terminal of the operational amplifier 15, and an offset voltage Voff is applied. The output terminal of the operational amplifier 15 is connected to the connection terminal of the reference electrode of the carbon dioxide sensor 71.

Therefore, the offset voltage Voff generated by the resistance voltage dividing circuit 12 is applied to the connection terminal of the reference electrode in the carbon dioxide gas sensor 71 via the buffer circuit 13. The reason why the buffer circuit 13 is provided is as follows.
This is for applying an accurate and stable offset voltage Voff to the connection terminal of the reference electrode without being affected by the impedance between the connection terminals of the carbon dioxide sensor 71 and the resistance values of the resistors R11 and R12 of the resistance voltage dividing circuit 12. is there.

The differential amplifier circuit 14 includes an operational amplifier 16 and resistors R13 to R16. The operational amplifier 16 is supplied with power from a DC power supply 85 and performs a single power supply operation. Then, the differential amplifier circuit 14 differentially amplifies the outputs of the buffer circuits 13 and 83. That is, the output of the differential amplifier circuit 14 is obtained from the output of the connection terminal of the detection electrode of the carbon dioxide sensor 71 (the output of the buffer circuit 83).
It is a value obtained by subtracting the offset voltage Voff (output of the buffer circuit 13).

The microcomputer 84 generates a detection signal Vo of the concentration of carbon dioxide from the output of the differential amplifier circuit 14. As described above, the carbon dioxide detection device 1 of the second embodiment
1, the resistor voltage dividing circuit 12 and the buffer circuit 1
By providing 3, the offset voltage Voff is applied to the connection terminal of the reference electrode in the carbon dioxide gas sensor 71. By providing the differential amplifier circuit 14,
The offset voltage Voff is subtracted from the output of the connection terminal of the detection electrode of the carbon dioxide gas sensor 71 to return to the original output (electromotive force) of the carbon dioxide sensor 71.

Therefore, in the carbon dioxide detection device 11,
If the offset voltage Voff is set so as to be equal to the effective voltage applied between the two terminals of the electric heater 72, the potential of the connection point between the electric heater 72 and the resistor R4 (the ground side of the electric heater 72) and the reference electrode Connection terminal (sensor body 71
Potential difference from the ground side) can be eliminated. Therefore, although the impedance between the connection terminals of the carbon dioxide sensor 71 is high and one end of the electric heater 72 is connected to the minus terminal of the DC power supply 85 via a resistor R4 of several hundred Ω or less and grounded, Carbon dioxide sensor 7
It is possible to prevent a leak current from being generated between the electric heater 72 and the sensor main body 74 inside or on the surface of the base 73 or the pedestal (not shown).

As a result, according to the carbon dioxide detection device 11 of the second embodiment, the carbon dioxide detection device 1 of the first embodiment can be used.
Similarly to the above, the detection accuracy of the carbon dioxide concentration does not decrease due to the leak current, and the output of the connection terminal of the detection electrode does not change with the change of the leak current with respect to the change of humidity. Detection signal V
o can be made accurate and stable.

Further, in the carbon dioxide detection device 11 of the second embodiment, the DC-DC converter 2 and the insulation circuit 4 which have high component costs, unlike the carbon dioxide detection device 1 of the first embodiment, are not used. Since the resistance voltage dividing circuit 12, the buffer circuit 13, and the differential amplifier circuit 14, which have relatively low component costs, are used, the cost can be reduced as compared with the carbon dioxide detection device 1 of the first embodiment.

Incidentally, the offset voltage Voff can be set as follows. FIG. 3 is a main part circuit diagram of the carbon dioxide gas detection device 11 shown in FIG. In a period T1 shown in FIG. 5, as shown in FIG.
Current leaking from the sensor to the connection terminal side of the detection electrode
1 ". In a period T2 shown in FIG.
As shown in FIG.
The current leaking to the second side is defined as “i2”. Further, in a period T4 shown in FIG. 5, a current leaking from the electric heater 72 to the connection terminal side of the detection electrode is “i3” as shown in FIG. Then, the electromotive force (the voltage between the connection terminals) of the carbon dioxide gas sensor 71 is set to “Vemf”. Further, the insulation resistance of the base 73 and the pedestal of the carbon dioxide sensor is represented by “R”.

Then, the voltage Vcc of the DC power supply 85, the offset voltage Voff, the electromotive force Vemf of the carbon dioxide gas sensor 71, the insulation resistance R, and the voltage V of the connection point A in the above-described period T4.
By substituting r into the following equations (1) to (3),
The respective currents i1 to i3 are obtained. i1 = (Vcc−Voff−Vemf) / R (1) i2 = (− Voff−Vemf) / R (2) i3 = (Vr−Voff−Vemf) / R (2) Equation 3) Here, in the period T3 + T4 shown in FIG. 5 (that is, one cycle of the pseudo AC flowing through the electric heater 72),
The amount of charge (the amount of leaked charge) moving between the electric heater 72 and the sensor main body 74 is defined as “Q”. Then, each of the periods T3 and T4 and each of the currents i1 to i3 are calculated by the following equation (4).
To obtain the charge amount Q. Q = i1 · T3 / 2 + i2 · T3 / 2 + i3 · T4 (Equation 4) In Expression (4), the current i is calculated by the first and second terms on the right side.
1. The reason for multiplying i2 not only by T3 but also by １ ／ is that the electric current supplied to the electric heater 72 is a pseudo alternating current. i1. This is because i2 flows.

Then, the charge amount Q and each period T3, T4
Into the following equation (5), the average leak current I is obtained. I = Q / (T3 + T4) (Equation 5) Therefore, the change of the average leak current I with respect to the change of the offset voltage Voff is simulated by the equation (5). Then, the offset voltage Voff when the average leak current I becomes zero may be obtained. The offset voltage Voff becomes equal to the effective voltage applied between both terminals of the electric heater 72.

For example, the set temperature of the electric heater 72 = 45
0 ° C., voltage Vcc of DC power supply 85 = 12 V, voltage Vr of connection point A in period T4 = 2.5 V, period T3 + T4 =
535 msec, the ratio of the period T3 to the period T3 + T4 (duty ratio = 100 × T3 / (T3 + T4)) =
61.3%, electromotive force Vemf of carbon dioxide sensor 71 = 30
In the case of 0 mV, when the change of the average leak current I with respect to the change of the offset voltage Voff is simulated by the equation (5), the offset voltage Voff when the average leak current I becomes zero is 4.3 V.

Therefore, the offset voltage Voff is 4.3 V
Each of the resistors R11 and R12 of the resistor voltage dividing circuit 12
When the offset voltage Voff is applied to the connection terminal of the reference electrode of the carbon dioxide gas sensor 71, the leak current of the carbon dioxide gas sensor 71 hardly changes even if the humidity increases, and the microcomputer 84 Has a constant value irrespective of a change in humidity.

The present invention is not limited to the above embodiments, but may be modified as follows. Even in such a case, the same operation and effect as those of the above embodiments can be obtained. (1) The heater temperature control circuit 82 is not limited to the above circuit configuration as long as the electric heater 72 can be controlled to the set temperature.
Any circuit configuration may be used.

(2) As described above, the microcomputer 84 converts the output of the connection terminal of the detection electrode, which is logarithmically output with respect to the carbon dioxide concentration, to a linear output with respect to the carbon dioxide concentration, and converts the output into a linear output. Has been generated. However, instead of converting the logarithmic output to a linear output by the microcomputer 84, the output of the connection terminal of the detection electrode may be directly output to the outside as the detection signal of the gas detection devices 1 and 11 via the buffer circuit 83. In that case, the insulating circuit 4 can be omitted in the first embodiment.

(3) In the second embodiment, if the accurate and stable offset voltage Voff can be applied to the connection terminal of the reference electrode of the carbon dioxide sensor 71, the buffer circuit 13 may be omitted. (4) Not only the carbon dioxide detection devices 1 and 11 using the carbon dioxide sensor 71 but also various gas sensors configured by replacing the metal carbonate layer of the carbon dioxide sensor 71 with a metal salt layer that dissociates and equilibrates with the gas to be detected. The present invention may be applied to various types of gas detection devices used. For example, by replacing the metal carbonate layer on the metal nitrate layer, it is possible to achieve a NO _X gas sensor for detecting the NO _X gas concentration. Also, by replacing the metal carbonate layer with a metal sulfate layer, S
An SO _X gas sensor that detects the O _X gas concentration can be realized. In such a NO _X NO _X gas detector using a gas sensor or SO _X gas sensor or SO _X gas detector, by the action similar to the above carbon dioxide sensing device 1 and 11, accurate and stable detection of the gas concentration It can be carried out.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram showing a configuration of a first embodiment embodying the present invention.

FIG. 2 is a block circuit diagram showing a configuration according to a second embodiment of the present invention;

FIG. 3 is a main part circuit diagram for explaining the operation of the second embodiment.

FIG. 4 is a block circuit diagram showing a configuration of a conventional mode.

FIG. 5 is a characteristic diagram for explaining the operation of the conventional embodiment and the first and second embodiments.

[Explanation of symbols]

Reference numerals 1, 11: carbon dioxide detection device 2: DC-DC converter 4: insulation circuit 12: resistor voltage divider 13, 83:
Buffer circuit 14 ... Differential amplifier circuit 82 ... Heater temperature control circuit 84 ... Microcomputer 85 ... DC power supply

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Noboru Ishida No. 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi F-term (reference) 2G004 BB04 BJ03 BL08 BL20 BM07 ZA04

Claims (6)

[Claims] A gas sensor having an electric heater attached to a gas sensor body having a reference electrode, an ion conductor layer made of a solid electrolyte, a detection electrode, and a metal salt layer formed in this order; Insulating means for electrically insulating the ground side of the gas sensor main body from the ground side of the electric heater to prevent a leak current from being generated between the electric heater and the gas sensor main body. Gas detector. 2. The gas detection device according to claim 1, wherein a heater temperature control unit that controls the temperature of the electric heater by controlling energization of the electric heater; and a reference electrode and a detection electrode of the gas sensor. A gas concentration detecting unit for detecting a gas concentration based on an electromotive force generated therebetween; and a power supply unit for supplying power to the heater temperature control unit and the gas concentration detecting unit. By electrically insulating the detection means and the ground side of the power supply means, the ground side of the gas sensor body and the ground side of the electric heater are electrically insulated, and the heater temperature control means and the power supply Gas leakage between the electric heater and the gas sensor main body is prevented from occurring through the means and the gas concentration detecting means. Location. 3. A gas sensor having an electric heater attached to a gas sensor body in which a reference electrode, an ion conductor layer made of a solid electrolyte, a detection electrode, and a metal salt layer are formed in this order. Potential setting means for preventing a leak current from being generated between the electric heater and the gas sensor main body by eliminating a potential difference between a ground side electric potential of the gas sensor main body and a ground side electric potential of the electric heater. Characteristic gas detection device. 4. The gas detection device according to claim 3, wherein a heater temperature control unit that controls the temperature of the electric heater by controlling energization of the electric heater; and a reference electrode and a detection electrode of the gas sensor. Gas concentration detecting means for detecting a gas concentration based on an electromotive force generated therebetween, wherein the potential setting means is an offset voltage equal to an effective voltage applied between both terminals of the electric heater by the heater temperature control means. And applying the offset voltage to the reference electrode eliminates the potential difference between the ground side potential of the gas sensor body and the ground side potential of the electric heater. A gas detection device for preventing generation of a leak current. 5. The gas detection device according to claim 4, wherein the gas concentration detection means subtracts the offset voltage from the electromotive force to return to the original electromotive force of the gas sensor. 6. The gas detection device according to claim 1, wherein the metal salt layer of the gas sensor includes a metal carbonate, a metal nitrate, or a metal sulfate that dissociates with a gas to be detected. A gas detection device comprising: