JP2008309640A - Gas concentration detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas concentration detector capable of enhancing the detection precision of a sensor current, forcibly the detection precision of the concentration of NOx in a monitor cell which performs the detection of the sensor current and the measurement of element impedance so as to change over them. <P>SOLUTION: When the sensor current flowing to the sensor current detecting resistor Rm provided to a sensor current detection circuit 6 is detected in the monitor cell 2C of the gas concentration detector 1, voltage of potential almost the same to that of a source terminal of an upstream side MOS type FET (TR1A) is applied to the gate terminal of the upstream-side MOS type FET and the gate terminal of a downstream-side MOS type FET (TR1B) is set to a ground potential to set two MOS type FETs to a non-continuity state. When the measurement of element admittance is performed by detecting a change in the current flowing through the admittance measuring resistor Ra provided to the sensor current detection circuit 6 is detected, a voltage higher than the voltage of a command voltage source 61 is applied to the gate terminals of two MOS type FETs to set two MOS type FETs to a continuity state. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a gas concentration detection device that detects NOx concentration, and more particularly, to a gas concentration detection device in which a monitor cell is also used to measure element admittance (or element impedance).

  For example, when detecting the concentration of NOx (nitrogen oxide) contained in a gas to be measured such as exhaust gas from a vehicle, the gas sensor element in which electrodes are provided on both surfaces of the solid electrolyte body respectively, A pump cell for adjusting the oxygen concentration, a sensor cell for measuring the NOx concentration in the measured gas after the oxygen concentration is adjusted by the pump cell, and a measured gas after the oxygen concentration is adjusted by the pump cell A gas concentration detection device comprising a monitor sensor cell for monitoring the oxygen concentration therein is used. In the gas concentration detection device, the pump cell reduces the residual oxygen concentration in the gas to be measured as much as possible. When the gas to be measured after the oxygen concentration adjustment is supplied to the sensor cell and the monitor cell, the sensor current flowing in the sensor cell The NOx concentration is obtained by detecting the difference from the sensor current flowing through the monitor cell.

Further, in the gas concentration detection device, in order to stabilize the sensor output by the gas sensor elements of the pump cell, sensor cell, and monitor cell, the heater element is energized to keep the temperature of the gas sensor element at a predetermined temperature.
For example, in the gas concentration detection device of Patent Document 1, a pump cell (first cell) that discharges excess oxygen in a gas to be detected, and a sensor cell that passes a current according to the concentration of a specific component from the gas component after discharging excess oxygen. (Second cell) and a gas concentration sensor including a heater that heats the pump cell and the sensor cell, the control circuit detects the internal resistance (element impedance) of the sensor cell, and the heater is energized based on the detected internal resistance. Is disclosed. The control circuit detects the element impedance of the sensor cell by changing the applied voltage of the sensor cell in an alternating manner, and controls the energization of the heater based on the detected element impedance. According to this, it is possible to always ensure a predetermined gas concentration detection accuracy even if there is a temperature variation or a flow velocity change of the gas to be detected.

  Further, in the gas concentration detection device of Patent Document 2, using the timing when the current flowing through the monitor cell is not detected, the sensor resistance is switched from the detection of the sensor current to the detection of the element resistance (element impedance) by the element resistance detection unit. In contrast, the element resistance of the monitor cell is detected by sweeping a predetermined AC voltage. Further, it is disclosed that a current detection resistor formed by connecting two types of large and small resistors in parallel is connected to the monitor cell, and a resistance value for detecting the sensor current and a resistance value for detecting the element resistance are switched. Has been.

By the way, in the circuit for detecting the sensor current flowing in the monitor cell as in the cited document 2, a resistor for detecting the sensor current and a resistor for measuring the element impedance are provided to detect the sensor current and the element. In the impedance measurement, the following problems may occur when switching the resistors using a switching element.
Specifically, when a MOS FET is used as the switching element, a Zener diode for protecting the MOS FET from an overvoltage due to static electricity or the like is provided between the gate terminal and the source terminal of the MOS FET. It is often provided in both bias directions. Therefore, a MOS FET is an insulated gate type field effect transistor, and it is considered that almost no leakage current (leakage current) is generated between the gate and the source. On the other hand, when the sensor current is detected in the monitor cell, switching is performed. In spite of the non-conducting state of the element (MOS type FET), a leak current may be generated from the source terminal to the gate terminal via the Zener diode.

  In this case, the detection accuracy of the sensor current in the monitor cell may be deteriorated. In particular, in a gas concentration detection device that detects NOx concentration, the sensor current needs to detect a minute current of several nA to several hundreds nA in order to detect the NOx concentration on the order of ppm, and the detection accuracy of the sensor current Deterioration becomes a serious problem.

JP 2000-171439 A JP 2002-372514 A

  The present invention has been made in view of such a conventional problem, and in a monitor cell that switches between detection of sensor current and measurement of element impedance, the detection accuracy of sensor current can be improved, and in addition, NOx. An object of the present invention is to provide a gas concentration detection device capable of improving the concentration detection accuracy.

The first invention has a gas sensor element in which electrodes are provided on both surfaces of a solid electrolyte body having oxygen ion permeability,
A pump cell for adjusting the oxygen concentration in the gas to be measured provided with an NOx inactive electrode by the gas sensor element, and a gas in the gas to be measured after adjusting the oxygen concentration by the pump cell provided with the NOx active electrode. Gas concentration comprising a sensor cell for measuring the NOx concentration and a monitor cell for monitoring the oxygen concentration in the gas to be measured after the NOx inert electrode is provided and the oxygen concentration is adjusted by the pump cell. In the detection device,
The monitor cell is a cell that also serves to measure the element admittance (or element impedance, the same applies hereinafter). A reference voltage source is connected to the reference gas side electrode of the monitor cell, and the measured gas side electrode of the monitor cell is connected to the monitor cell. Is connected to the sensor current detection circuit,
The sensor current detection circuit includes a command voltage source,
An operational amplifier for current detection for applying a voltage from the command voltage source to the measured gas side electrode of the monitor cell;
A sensor current detection resistor provided between the output terminal of the current detection operational amplifier and the measured gas side electrode of the monitor cell;
Sensor current detection means for detecting a current flowing through the sensor current detection resistor;
An admittance measurement resistor connected in parallel with the sensor current detection resistor between the output terminal of the current detection operational amplifier and the measured gas side electrode of the monitor cell;
A switching element for switching between a conductive state and a non-conductive state of the switching wiring provided with the admittance measurement resistor;
The command voltage source is provided with sweeping means for applying an AC voltage for measuring the element admittance to the current detection operational amplifier in a state where the switching wiring is made conductive by the switching element.
An admittance measurement circuit for measuring a change in the current flowing through the admittance measurement resistor by the application of the alternating voltage by the sweep means is connected to the switching wiring,
The switching element is configured using two N-channel MOS FETs configured by connecting drain terminals to each other,
The two MOS FETs are downstream of the sensor current detection resistor, and are connected to the output terminal of the current detection operational amplifier, and upstream of the sensor current detection resistor. It consists of an upstream MOS type FET connected to the measured gas side electrode of the monitor cell,
Each MOS type FET is provided with a zener diode for protecting each MOS type FET from overvoltage due to static electricity or the like between the gate terminal and the source terminal in both bias directions.
When the current flowing through the sensor current detection resistor is detected by the sensor current detection means, a voltage (approximately the same potential as the source terminal of the upstream MOS FET is connected to the gate terminal of the upstream MOS FET. A voltage of approximately the same potential as the voltage from the command voltage source), the gate terminal of the downstream MOS FET is set to a substantially ground potential, and the two MOS FETs are made non-conductive,
When detecting a change in current flowing through the admittance measurement resistor by the admittance measurement circuit, a voltage higher than the voltage of the command voltage source is applied to the gate terminals of the two MOS FETs, and the two A gas concentration detection apparatus is characterized in that the MOS type FET is made to conduct (claim 1).

  The gas concentration detection apparatus of the present invention detects NOx (nitrogen oxide) concentration, and in a monitor cell, switches between detection of sensor current and measurement of element admittance (or element impedance, the same applies hereinafter). It is something that can be done. Then, when detecting the sensor current, a contrivance is made to suppress current leakage that has an adverse effect on the detection.

Specifically, the switching element provided in the switching wiring for switching between the conductive state and the non-conductive state of the admittance measurement resistor is configured by connecting two N-channel MOS FETs with their drain terminals connected to each other. . The two MOS FETs are provided with a Zener diode in both bias directions between the gate terminal and the source terminal for protecting each MOS FET from an overvoltage caused by static electricity or the like.
The switching element is configured by connecting two drain terminals of two MOS FETs, and when detecting the sensor current, the parasitic diode formed in the forward direction between the source and the drain causes the switching element to Leakage current can be prevented from occurring in the switching wiring.

When detecting the sensor cell current flowing through the monitor cell (when detecting the current flowing through the sensor current detecting resistor by the sensor current detecting means), the two MOS FETs are made non-conductive and the switching wiring is connected. Leave in a non-conductive state. At this time, a voltage having substantially the same potential as that of the source terminal of the upstream MOS FET (a voltage having approximately the same potential as that of the command voltage source) is applied to the gate terminal of the upstream MOS FET. The gate terminal of the type FET is set to a substantially ground potential.
Next, when a differential voltage between the voltage by the reference voltage source and the voltage by the command voltage source is applied to the monitor cell, the sensor current detection resistor in the sensor current detection circuit has a minute resistance of, for example, several nA to several hundred nA. Sensor current flows. The current flowing through the sensor current detection resistor can be detected by the sensor current detection means.

  At this time, a voltage having substantially the same potential as that of the source terminal of the upstream MOS type FET (voltage having substantially the same potential as that of the command voltage source) is applied to the gate terminal of the upstream side MOS FET. There is almost no potential difference between the gate and source of the upstream MOS FET. Thereby, when detecting the sensor current flowing through the monitor cell, it is possible to prevent the leak current from being generated in the Zener diode in the upstream MOS type FET. Therefore, this leakage current can be prevented from flowing to the sensor current detection resistor, and the detection accuracy of the sensor current by the sensor current detection means can be improved. And when calculating | requiring NOx density | concentration from the difference of the electric current value of the sensor electric current which flows into a sensor cell, and the electric current value of the sensor electric current which flows into a monitor cell, this NOx density | concentration detection accuracy can be improved.

  When the sensor current is detected, there is a potential difference between the command voltage source and the ground potential between the gate and the source of the downstream MOS type FET, but there is a possibility that it may occur in a Zener diode in the downstream MOS type FET. The leak current is the current after passing through the sensor current detection resistor. For this reason, even if a leak current is generated in the Zener diode in the downstream MOS type FET, the leak current hardly affects the detection accuracy of the sensor current by the sensor current detecting means.

Also, when measuring element admittance in the monitor cell (when detecting the current flowing through the admittance measurement resistor by the admittance measurement circuit), the two MOS FETs are turned on and the switching wiring is turned on. Put it in a state. At this time, a voltage higher than the voltage of the command voltage source is applied to the gate terminals of the two MOS type FETs to make the two MOS type FETs conductive.
Next, an AC voltage is applied to the current detection operational amplifier by the sweep means provided in the command voltage source. At this time, a voltage resulting from the difference between the voltage from the reference voltage source and the AC voltage from the sweep means is applied to the monitor cell, and a current flows through the monitor cell and the admittance measurement resistor.
And the element admittance of a monitor cell can be stably measured by detecting the voltage change by the electric current which flows into the resistance for admittance measurement using an admittance measurement circuit.

  As described above, according to the gas concentration detection device of the present invention, the detection accuracy of the sensor current can be improved in the monitor cell that switches between the detection of the sensor current and the measurement of the element admittance. Detection accuracy can be improved.

The second invention has a gas sensor element in which electrodes are provided on both surfaces of a solid electrolyte body having oxygen ion permeability,
A pump cell for adjusting the oxygen concentration in the gas to be measured provided with an NOx inactive electrode by the gas sensor element, and a gas in the gas to be measured after adjusting the oxygen concentration by the pump cell provided with the NOx active electrode. Gas concentration comprising a sensor cell for measuring the NOx concentration and a monitor cell for monitoring the oxygen concentration in the gas to be measured after the NOx inert electrode is provided and the oxygen concentration is adjusted by the pump cell. In the detection device,
The monitor cell is a cell that also serves to measure the element admittance (or element impedance, the same applies hereinafter). A reference voltage source is connected to the reference gas side electrode of the monitor cell, and the measured gas side electrode of the monitor cell is connected to the monitor cell. Is connected to the sensor current detection circuit,
The sensor current detection circuit includes a command voltage source,
An operational amplifier for current detection for applying a voltage from the command voltage source to the measured gas side electrode of the monitor cell;
A sensor current detection resistor provided between the output terminal of the current detection operational amplifier and the measured gas side electrode of the monitor cell;
Sensor current detection means for detecting a current flowing through the sensor current detection resistor;
An admittance measurement resistor connected in parallel with the sensor current detection resistor between the output terminal of the current detection operational amplifier and the measured gas side electrode of the monitor cell;
A switching element for switching between a conductive state and a non-conductive state of the switching wiring provided with the admittance measurement resistor;
The command voltage source is provided with sweeping means for applying an AC voltage for measuring the element admittance to the current detection operational amplifier in a state where the switching wiring is made conductive by the switching element.
An admittance measurement circuit for measuring a change in current flowing through the admittance measurement resistor by application of the AC voltage by the sweep means is connected to the switching wiring,
The switching element is configured by using two P-channel MOS FETs configured by connecting drain terminals to each other,
The two MOS FETs are downstream of the sensor current detection resistor, and are connected to the output terminal of the current detection operational amplifier, and upstream of the sensor current detection resistor. It consists of an upstream MOS type FET connected to the measured gas side electrode of the monitor cell,
Each MOS type FET is provided with a zener diode for protecting each MOS type FET from overvoltage due to static electricity or the like between the gate terminal and the source terminal in both bias directions.
When the current flowing through the sensor current detection resistor is detected by the sensor current detecting means, a voltage (approximately the same potential as that of the source terminal of the upstream MOS FET is connected to the gate terminals of the two MOS FETs. A voltage of substantially the same potential as the voltage from the command voltage source), and the two MOS FETs are made non-conductive,
When detecting a change in current flowing through the admittance measurement resistor by the admittance measurement circuit, a voltage lower than the voltage of the command voltage source is applied to the gate terminals of the two MOS FETs, and the two A gas concentration detection apparatus is characterized in that the MOS FET is made conductive.

  The gas concentration detection apparatus of the present invention also detects NOx (nitrogen oxide) concentration, and in the monitor cell, switches between detection of sensor current and measurement of element admittance (or element impedance, the same shall apply hereinafter). It is something that can be done. Then, when detecting the sensor current, a contrivance is made to suppress current leakage that has an adverse effect on the detection.

Specifically, the switching element provided in the switching wiring for switching between the conductive state and the non-conductive state of the admittance measurement resistor is configured by connecting two P-channel MOS type FETs with their drain terminals connected to each other. . The two MOS FETs are provided with a Zener diode in both bias directions between the gate terminal and the source terminal for protecting each MOS FET from an overvoltage caused by static electricity or the like.
The switching element is configured by connecting two drain terminals of two MOS FETs, and when detecting the sensor current, the parasitic diode formed in the forward direction between the source and the drain causes the switching element to Leakage current can be prevented from occurring in the switching wiring.

When detecting the sensor cell current flowing through the monitor cell (when detecting the current flowing through the sensor current detecting resistor by the sensor current detecting means), the two MOS FETs are made non-conductive and the switching wiring is connected. Leave in a non-conductive state. At this time, a voltage having substantially the same potential as the voltage of the command voltage source is applied to the gate terminals of the two MOS FETs.
Next, when a differential voltage between the voltage by the reference voltage source and the voltage by the command voltage source is applied to the monitor cell, the sensor current detection resistor in the sensor current detection circuit has a minute resistance of, for example, several nA to several hundred nA. Sensor current flows. The current flowing through the sensor current detection resistor can be detected by the sensor current detection means.

  At this time, a voltage having substantially the same potential as that of the source terminal of the upstream-side MOS FET (a voltage having approximately the same potential as that of the command voltage source) is applied to the gate terminals of the two MOS FETs. There is almost no potential difference between the gate and source of two MOS FETs. Thereby, when detecting the sensor current flowing through the monitor cell, it is possible to prevent a leak current from occurring in the Zener diodes in the two MOS FETs. Therefore, this leakage current can be prevented from flowing to the sensor current detection resistor, and the detection accuracy of the sensor current by the sensor current detection means can be improved. And when calculating | requiring NOx density | concentration from the difference of the electric current value of the sensor electric current which flows into a sensor cell, and the electric current value of the sensor electric current which flows into a monitor cell, this NOx density | concentration detection accuracy can be improved.

Also in the present invention, when the element admittance is measured in the monitor cell (when the current flowing through the admittance measurement resistor is detected by the admittance measurement circuit), the two MOS FETs are turned on, Turn on the switching wiring. At this time, a voltage lower than the voltage of the command voltage source is applied to the gate terminals of the two MOS type FETs to make the two MOS type FETs conductive.
Next, similarly to the first aspect of the invention, the element admittance of the monitor cell can be stably measured by detecting the voltage change caused by the current flowing through the admittance measurement resistor using the admittance measurement circuit.

  As described above, even with the gas concentration detection device of the present invention, the detection accuracy of the sensor current can be improved in the monitor cell that switches between the detection of the sensor current and the measurement of the element admittance, and the detection of the NOx concentration. Accuracy can be improved.

A preferred embodiment in the first and second inventions described above will be described.
In the first and second inventions, it is preferable that the admittance measurement resistor has a resistance value smaller than that of the sensor current detection resistor. In particular, the sensor current flowing through the sensor current detection resistor is about several nA to several hundred nA, whereas the sweep current (alternating current) flowing through the admittance measurement resistor is about several mA to several tens mA. The admittance measurement resistance and the sensor current detection resistance can be resistance values corresponding to the difference between the sensor current and the sweep current. For example, the resistance value of the admittance measurement resistor can be several hundred Ω, and the resistance value of the sensor current detection resistor can be several hundred to several thousand kΩ.
The element impedance (Ω) is the reciprocal of the element admittance (S), and either can be measured in the admittance measurement circuit.

In the first and second inventions, when measuring the element admittance, the sweeping means in the command voltage source applies the measured gas side electrode of the monitor cell (from the current detection operational amplifier to the measured gas of the monitor cell). It is considered that the AC voltage (applied to the side electrode) needs to swing at least ± 0.2V. Therefore, the voltage from the command voltage source must be at least 0.2 V or more. Considering that it is difficult to output near 0 V as the output characteristic of the operational amplifier, the voltage from the command voltage source is preferably several V (for example, 2 to 8 V).
In order to stabilize the detection of the sensor current, it is preferable that the voltage by the reference voltage source is about 0.4 V higher than the voltage by the command voltage source.

In the first invention, when the upstream MOS type FET is branched and connected to the source terminal of the upstream MOS type FET and the two MOS type FETs are made non-conductive, the upstream side of the upstream side MOS type FET is connected to the upstream side. It is preferable that the voltage extracted by the voltage extracting operational amplifier is applied to the gate terminal of the side MOS type FET.
In this case, when detecting the sensor current by a simple devise using an operational amplifier, the gate terminal of the upstream MOS type FET is connected to a voltage (command) having the same potential as the source terminal of the upstream MOS type FET. The voltage of the voltage source is approximately the same potential as that of the voltage source).

Further, the upstream MOS FET can be switched between a conductive state and a non-conductive state by an upstream switching circuit connected to the gate terminal, and the downstream MOS FET is connected to the gate terminal. The switching circuit can be switched between a conduction state and a non-conduction state by the downstream side switching circuit. Each of the switching circuits receives an output signal from the control microcomputer and switches to a conduction state and a non-conduction state. An output terminal of the voltage extraction operational amplifier, comprising: a stage transistor; and an NPN-type second stage transistor that switches between a conductive state and a non-conductive state in response to an output state of the first stage transistor. Is connected to the emitter terminal of the second-stage transistor in the upstream switching circuit, and the second-stage transistor is in a conductive state. When in, the gate terminal of the upstream MOS type FET, it is preferable to be configured to apply a source terminal and a voltage of substantially the same potential of the upstream-side MOS type FET (claim 3).
In this case, when detecting the sensor current by a simple device using an operational amplifier and a transistor, the voltage of the gate terminal of the upstream MOS type FET is approximately the same as that of the source terminal of the upstream MOS type FET. (A voltage having substantially the same potential as that of the command voltage source).

In the second invention, when the voltage extracting operational amplifier constituting the voltage follower is branched and connected to the source terminal of the upstream side MOS type FET, and the two MOS type FETs are kept non-conductive, It is preferable that the voltage extracted by the voltage extracting operational amplifier is applied to the gate terminals of the two MOS FETs.
In this case, it is possible to keep the gate terminals of the two MOS FETs at a voltage substantially equal to the voltage of the command voltage source when detecting the sensor current by a simple device.

In the first and second inventions, the sensor current detection resistor preferably has a resistance value of 100 kΩ to 2 MΩ, and the admittance measurement resistor preferably has a resistance value of 100 Ω to 1 kΩ. (Claim 6).
In this case, a minute sensor current of about several nA to several hundred nA can be stably detected by a sensor current detection resistor having a large resistance value of 100 kΩ to 2 MΩ. Further, a change in current during admittance measurement of several mA to several tens of mA can be stably detected by an admittance measurement resistor having a small resistance value of 100Ω to 1 kΩ.

If the resistance value of the sensor current detection resistor is less than 100 kΩ, it is necessary to increase the amplification factor of an operational amplifier or the like that detects the sensor current, which may increase detection error. On the other hand, when the resistance value of the sensor current detection resistor exceeds 2 MΩ, the upper limit of the sensor current detection range may be small (the detection range is narrow).
In addition, when the resistance value of the admittance measurement resistor is less than 100Ω, it is necessary to increase an amplification factor of an operational amplifier or the like that measures the element admittance, which may increase a measurement error. On the other hand, when the resistance value of the admittance measurement resistor exceeds 1 kΩ, the upper limit of the element admittance measurement range may be small (the detection range is narrow).

Embodiments of the gas concentration detection apparatus according to the present invention will be described below with reference to the drawings.
Example 1
As shown in FIGS. 1 to 5, the gas concentration detection device 1 of this example includes a gas sensor element 2 in which electrodes are provided on both surfaces of a solid electrolyte body having oxygen ion permeability, and a heater element 3 that generates heat when energized. And have. The gas concentration detection apparatus 1 includes a pump cell 2A for adjusting the oxygen concentration in the gas G to be measured, which includes NOx inactive electrodes 201 and 202 by the gas sensor element 2, a NOx active electrode 203, and a NOx inactive electrode. 204, the sensor cell 2B for measuring the NOx concentration in the gas G to be measured after the oxygen concentration is adjusted by the pump cell 2A, and the oxygen concentration is adjusted by the pump cell 2A. A monitor cell 2C for monitoring the oxygen concentration in the gas G to be measured after having been performed is configured. The pump cell 2A, the sensor cell 2B, the monitor cell 2C, and the heater element 3 can be input / output controlled by the control microcomputer 8.

As shown in FIGS. 1 and 2, the monitor cell 2 </ b> C is a cell that also serves to measure its element admittance (or element impedance, the same applies hereinafter). The reference voltage source 51 is connected to the reference gas side electrode 206 of the monitor cell 2C, and the sensor current detection circuit 6 is connected to the measured gas side electrode 205 of the monitor cell 2C.
The sensor current detection circuit 6 includes a command voltage source 61, a current (monitor current) detection operational amplifier OP2 for applying a voltage from the command voltage source 61 to the measured gas side electrode 205 of the monitor cell 2C, and a current detection operational amplifier OP2. Sensor current detection resistor Rm provided between the output terminal of the monitor cell 2C and the measured gas side electrode 205 of the monitor cell 2C, sensor current detection means 63 for detecting the current flowing through the sensor current detection resistor Rm, and current detection An admittance measurement resistor Ra connected in parallel with the sensor current detection resistor Rm between the output terminal of the operational amplifier OP2 and the measured gas side electrode 205 of the monitor cell 2C, and a switching wiring 601 provided with the admittance measurement resistor Ra Switching element TR1 for switching between the conductive state and the non-conductive state.

  As shown in FIG. 2, the command voltage source 61 has a sweep means 62 for applying an AC voltage for measuring the element admittance to the current detection operational amplifier OP2 in a state where the switching wiring 601 is made conductive by the switching element TR1. Is provided. As shown in FIG. 4, an admittance measurement circuit 7 is connected to the switching wiring 601 for measuring a change in the current flowing through the admittance measurement resistor Ra by application of an AC voltage by the sweep means 62. Here, the symbol A in FIG. 2 indicates that it is connected to the symbol A in FIG.

As shown in FIG. 6, the switching element TR1 is configured using two N-channel MOS type FETs (TR1A, B) (MOS type field effect transistors) configured by connecting the drain terminals D to each other. . The two MOS FETs (TR1A, B) are downstream of the current flow in the sensor current detection resistor Rm, and are connected to the output terminal of the current detection operational amplifier OP2. And an upstream MOS type FET (TR1A) connected to the measured gas side electrode 205 of the monitor cell 2C on the upstream side of the current flow in the sensor current detection resistor Rm. The two MOS FETs (TR1A, B) operate using the source terminal S of the upstream MOS type FET (TR1A) as the drain terminal and the source terminal S of the downstream side MOS FET (TR1B) as the source terminal. To do.
Each MOS FET (TR1A, B) has a Zener diode D2 ′ for protecting the MOS FET (TR1A, B) from an overvoltage caused by static electricity between the gate terminal G and the source terminal S. D3 ′ is provided in both bias directions (one zener diode D2 ′ is connected with the anode terminal on the gate terminal G side, and the other zener diode D3 ′ is connected with the anode terminal on the source terminal S side. ).

  When the gas concentration detecting device 1 detects the current flowing through the sensor current detecting resistor Rm by the sensor current detecting means 63, the gate terminal G of the upstream MOS type FET (TR1A) is detected by the switching circuits 65A and 65B described later. Is applied with a voltage having substantially the same potential as the source terminal S of the upstream MOS FET (TR1A) (a voltage having approximately the same potential as that of the command voltage source 61), and the gate terminal of the downstream MOS FET (TR1B). G is configured to be a ground potential so that the two MOS FETs (TR1A, B) are in a non-conducting state. On the other hand, when the gas concentration detection device 1 detects the current flowing through the admittance measurement resistor Ra by the admittance measurement circuit 7, a voltage higher than the voltage of the command voltage source 61 is set by the switching circuits 65A and 65B described later. The two MOS type FETs (TR1A, B) are applied to the gate terminals G of the two MOS type FETs (TR1A, B) to make the two MOS type FETs (TR1A, B) conductive.

In addition, about TR1A and B which show two MOS type FETs, a code | symbol is shown in parenthesis in order to make it easy to see.
FIG. 1 is a diagram schematically showing the overall configuration of the gas concentration detection apparatus 1. FIG. 2 shows a circuit configuration around the monitor cell 2C, the sensor current detection circuit 6, and switching circuits 65A and B described later. FIG. 3 shows the periphery of the sensor cell 2B, the sensor current detection circuit 6, and a NOx current detection circuit 5 described later. FIG. 4 is a diagram illustrating a peripheral circuit configuration of the admittance measurement circuit 7. FIG. 5 is a diagram schematically showing the circuit configuration of the pump cell 2 </ b> A and the heater element 3. FIG. 6 is a diagram showing details of the periphery of the switching element TR1 of the sensor current detection circuit 6.

Hereinafter, the gas concentration detection device 1 of this example will be described in detail with reference to FIGS.
The gas concentration detection apparatus 1 of this example detects the NOx concentration in the exhaust gas (measured gas) G flowing through the exhaust pipe of an internal combustion engine such as an engine.
The gas sensor 10 including the pump cell 2A, the sensor cell 2B and the monitor cell 2C as the gas sensor element 2 and the heater element 3 can be variously configured. In this example, as shown in FIG. 1, the pump cell 2A is provided with a measured gas side electrode 201 exposed to the measured gas G on one surface of the first solid electrolyte body 21A and opposed to the electrode 201. The reference gas side electrode 202 exposed to the reference gas F (atmospheric gas or the like) is provided on the other surface of the first solid electrolyte body 21A. The pair of electrodes 201 and 202 of the pump cell 2A is configured using a NOx inactive material.

  The sensor cell 2B is provided with an electrode 203 exposed to the gas G to be measured on one surface of the second solid electrolyte body 21B laminated on the first solid electrolyte body 21A via the spacer 23, and opposed to the electrode 203. The electrode 204 exposed to the reference gas F (atmospheric gas or the like) is provided on the other surface of the second solid electrolyte body 21B. In the sensor cell 2B, the electrode 203 exposed to the gas G to be measured is configured using a NOx active material, and the electrode 204 exposed to the reference gas F is configured using a NOx inactive material. is there.

The monitor cell 2C is provided with an electrode 205 exposed to the gas G to be measured on one surface of the second solid electrolyte body 21B, and on the other surface of the second solid electrolyte body 21B so as to face the electrode 205. An electrode 206 that is exposed to a reference gas F (atmospheric gas or the like) is provided. The pair of electrodes 205 and 206 of the monitor cell 2C is configured using a NOx inactive material.
The sensor cell 2B and the monitor cell 2C are provided adjacent to both surfaces of the second solid electrolyte body 21B, and the electrodes 204 and 206 exposed to the respective reference gases F are shared.

As shown in FIG. 1, the heater element 3 is formed by sandwiching a heater conductor 3 made of platinum or the like between insulating ceramic substrates 31. The heater element 3 is laminated on the first solid electrolyte body 21A.
A chamber 24 to which a gas to be measured G is supplied is formed between the first solid electrolyte body 21A and the second solid electrolyte body 21B, and the pump cell 2A, sensor cell 2B, and monitor cell 2C are formed in the chamber 24. The electrodes 201, 203, and 205 on the measured gas G side are exposed. Further, the gas to be measured G is supplied into the chamber 24 via the porous diffusion layer 22 and the pinhole 211 provided in the second solid electrolyte body 21B.

Further, the gas concentration detection device 1 is configured to drive the pump cell 2A, the sensor cell 2B, the monitor cell 2C, and the heater element 3 by the sensor driving circuit 100.
In the pump cell 2A, the sensor driving circuit 100 applies a voltage to the pair of electrodes 201 and 202 to influence the oxygen concentration in the gas G to be measured on the detection of the NOx concentration in the order of ppm in the sensor cell 2B and the monitor cell 2C. The oxygen concentration is within a predetermined target range that does not reach.

(Circuit configuration of pump cell 2A)
Specifically, as shown in FIG. 5, the reference gas side electrode 202 of the pump cell 2A receives a pulse width modulation signal (PWM signal) from the output port (ON / OFF port) OUT4 of the control microcomputer 8, A pump cell output circuit 41 configured to apply a voltage within a predetermined range for maintaining the limit current characteristic is connected to the pump cell 2A. Further, a reference voltage source 43 and a pump cell input circuit 42 are connected to the measured gas side electrode 201 of the pump cell 2A. A differential voltage between the output voltage of the pump cell output circuit 41 and the voltage of the reference voltage source 43 is applied to the pump cell 2A.

  Then, the control microcomputer 8 obtains the oxygen concentration in the measured gas G in the pump cell 2A based on the current value flowing through the pump current detection resistor Rp of the pump cell input circuit 42 taken in from the A / D conversion input port IN4. Thus, the oxygen concentration is monitored. Further, the control microcomputer 8 sets the duty of the pulse width modulation signal within a range in which the limit current characteristic of the pump cell 2A is maintained so that the oxygen concentration in the pump cell 2A is within the target range (the current value is within the target range). The ratio is changed.

  Further, the sensor drive circuit 100 applies a predetermined voltage indicating a limit current characteristic to the sensor cell 2B and the monitor cell 2C, detects a minute current of several nA to several hundred nA flowing through the sensor cell 2B and the monitor cell 2C, and detects these currents. The NOx concentration is obtained by obtaining the difference between the current values.

(Circuit configuration of monitor cell 2C)
2 and 3, the reference voltage source 51 is connected to the reference gas side electrode 206 of the monitor cell 2C, and the sensor current detection circuit 6 is connected to the measured gas side electrode 205 of the monitor cell 2C. It is.
The reference voltage source 51 is configured using a buffer operational amplifier OP11 that constitutes a voltage follower. That is, the voltage (4.4 V in this example) determined by the values of the resistors R21 and R22 with respect to the resistors R20 to R22 (4.4 V in this example) is input to the non-inverting input terminal of the buffer operational amplifier OP11. Then, a voltage (4.4 V in this example) is applied from the output terminal of the buffer operational amplifier OP11 to the reference gas side electrode 204 of the sensor cell 2B through the current limiting resistor R23. Note that a capacitor C5 for improving characteristics is connected between the inverting input terminal and the output terminal of the buffer operational amplifier OP11, and the inverting input terminal of the buffer operational amplifier OP11 is a reference through a protective resistor R24. It is connected to the gas side electrode 204.

As shown in FIG. 2, in the monitor cell 2C, the current detection operational amplifier OP2 constitutes a voltage follower that applies a voltage from the command voltage source 61 to the measured gas side electrode 205 of the monitor cell 2C. The sensor current detection resistor Rm is connected between the output terminal of the current detection operational amplifier OP2 and the measured gas side electrode 205 of the monitor cell 2C.
A protective resistor R1 for the current detection operational amplifier OP2 is connected between the inverting input terminal of the current detection operational amplifier OP2 and the measured gas side electrode 205 of the monitor cell 2C.
The switching element TR1 provided in the switching wiring 601 is obtained by connecting two N-channel MOS type FETs (TR1A, B) in series. The gate terminals G of the two MOS type FETs (TR1A, B) are described later. Are connected to the output port OUT3 of the control microcomputer 8 via the switching circuits 65A and 65B. Further, between the output terminal of the sensor current detection circuit 6 and the measured gas side electrode 205 of the monitor cell 2C, there is a noise absorbing capacitor C1 in parallel with the sensor current detection resistor Rm and the admittance measurement resistor Ra. It is provided.

When the sensor current is detected in the monitor cell 2C, the command voltage source 61 in the sensor current detection circuit 6 supplies a circuit power source to the non-inverting input terminal of the current detection operational amplifier OP2 and the measured gas side electrode 205 of the monitor cell 2C. The voltage Vcc (5 V in this example) is converted into a voltage (4 V in this example) determined by the values of the resistors R7 and R8 for the resistors R6 to R8 and applied.
Further, a current limiting resistor R9 is provided between the non-inverting input terminal of the sensor current detection circuit 6 and the voltage dividing intermediate point between the resistors R6 and R7. The non-inverting input terminal is connected to the ground potential via a noise absorbing capacitor C2.

When the sensor current is detected in the monitor cell 2C, a voltage (4.4 V in this example) from the reference voltage source 51 is applied to the reference gas side electrode 206 of the monitor cell 2C, and the measured gas side of the monitor cell 2C is measured. A voltage (4 V in this example) from the command voltage source 61 is applied to the electrode 205.
The sensor current detection means 63 is constituted by a residual oxygen operational amplifier OP4 that constitutes a differential amplifier circuit. The downstream end of the current flow of the sensor current detection resistor Rm is connected to the inverting input terminal of the residual oxygen operational amplifier OP4, and a voltage follower is configured at the non-inverting input terminal of the residual oxygen operational amplifier OP4. The upstream end of the current flow of the sensor current detection resistor Rm is connected via the buffer operational amplifier OP6.

  The residual oxygen operational amplifier OP4 is configured to output a voltage based on the residual oxygen current flowing through the sensor current detection resistor Rm (flowing through the monitor cell 2C) (current flowing through the residual oxygen existing in the measured gas side electrode 205). It is. The output terminal of the residual oxygen operational amplifier OP4 is connected to the A / D conversion input port IN2 of the control microcomputer 8, and the control microcomputer 8 can detect and monitor the residual oxygen concentration in the monitor cell 2C. Yes.

More specifically, the upstream end of the sensor current detection resistor Rm is connected to the non-inverting input terminal of the buffer operational amplifier OP6 via the resistor R35, and the output terminal of the buffer operational amplifier OP6 is connected to the resistor R37. To the non-inverting input terminal of the residual oxygen operational amplifier OP4. On the other hand, the downstream end of the sensor current detection resistor Rm is connected to the inverting input terminal of the residual oxygen operational amplifier OP4 via the resistor R36.
In addition, a resistor R38 and a capacitor C9 for improving characteristics are connected in parallel between the inverting input terminal and the output terminal of the residual oxygen operational amplifier OP4. Are connected to the offset voltage Vd (0.2 V in this example). The resistance values of the resistor R36 and the resistor R37 and the resistance values of the resistor R38 and the resistor R39 are the same, and the residual oxygen operational amplifier OP4 is determined by the size of the resistor R38 (R39) with respect to the resistor R36 (R37). A potential difference between both ends of the sensor current detection resistor Rm is output at an amplification factor.

The output terminal of the residual oxygen operational amplifier OP4 is connected to the A / D conversion input port IN2 of the control microcomputer 8. In this way, the control microcomputer 8 causes the sensor current i (the current i to flow in FIG. Is obtained based on the resistance value (1.5 MΩ in this example) of the sensor current detection resistor Rm and the voltage difference between both ends of the sensor current detection resistor Rm.
The upstream voltage of the sensor current detection resistor Rm (the output voltage of the buffer operational amplifier OP6) is connected to the A / D conversion input port IN6 of the control microcomputer 8. The control microcomputer 8 is configured to monitor the voltage on the upstream side of the sensor current detection resistor Rm.

(Circuit configuration of sensor cell 2B)
As shown in FIG. 3, the reference voltage source 51 is connected to the reference gas side electrode 204 of the sensor cell 2B, and the NOx current detection circuit 5 is connected to the measured gas side electrode 203 of the sensor cell 2B. The reference gas side electrode 204 of the sensor cell 2B is shared with the reference gas side electrode 206 of the monitor cell 2C, and the reference voltage source 51 shares what is used for the monitor cell 2C.
The NOx current detection circuit 5 is configured using a sensor current detection operational amplifier OP1 that constitutes a voltage follower. That is, a voltage (4 V in this example) determined by the value of the resistor R22 with respect to the resistors R20 to R22 (4 V in this example) is input to the non-inverting input terminal of the sensor current detection operational amplifier OP1. A voltage (4 V in this example) is applied to the measured gas side electrode 203 of the sensor cell 2B from the output terminal of the sensor current detection operational amplifier OP1 via the NOx current detection resistor Rs. Note that a noise absorbing capacitor C6 is connected in parallel to both ends of the NOx current detection resistor Rs, and the inverting input terminal of the sensor current detection operational amplifier OP1 is a protection resistor for the sensor current detection operational amplifier OP1. It is connected to the measured gas side electrode 203 via R25.

Further, both ends of the NOx current detection resistor Rs are connected to the inverting input terminal and the non-inverting input terminal of the NOx operational amplifier OP3 constituting the differential amplifier circuit, and the NOx operational amplifier OP3 is used to detect the NOx current. The voltage output is based on the NOx current flowing through the resistor Rs (flowing through the sensor cell 2B) (current flowing after NOx present in the measured gas side electrode 203 reacts with the remaining oxygen).
The upstream end of the NOx current detection resistor Rs is connected to the non-inverting input terminal of the buffer operational amplifier OP12 constituting the voltage follower via the resistor R26, and the output terminal of the buffer operational amplifier OP12 is connected to the resistor R28. And connected to the non-inverting input terminal of the NOx operational amplifier OP3. On the other hand, the downstream end of the NOx current detection resistor Rs is connected to the inverting input terminal of the NOx operational amplifier OP3 via the resistor R27.

  Further, as shown in FIG. 3, a resistor R29 and a characteristic improving capacitor C7 are connected in parallel between the inverting input terminal and the output terminal of the NOx operational amplifier OP3, and the non-inverting input of the NOx operational amplifier OP3. The terminal is connected to a predetermined offset voltage Vd (in this example, 0.2 V). The resistance values of the resistor R27 and the resistor R28 and the resistance values of the resistor R29 and the resistor R30 are the same, and the NOx operational amplifier OP3 is determined by the magnitude of the resistor R29 (R30) relative to the resistor R27 (R28). The potential difference between both ends of the NOx current detection resistor Rs is output at a rate.

(Detection of NOx concentration)
Further, as shown in the figure, the output of the NOx operational amplifier OP3 and the output of the residual oxygen operational amplifier OP4 are connected to the non-inverting input terminal and the inverting input terminal of the verification operational amplifier OP5 constituting the differential amplifier circuit. It is. By this operational amplifier OP5 for comparison, the difference between the voltage based on the NOx current and the voltage based on the residual oxygen current is calculated, and the voltage based on this difference is output. The output terminal of the verification operational amplifier OP5 is connected to the A / D conversion input port IN1 of the control microcomputer 8. The control microcomputer 8 can detect and monitor the NOx concentration in the gas to be measured. Yes.

More specifically, as shown in FIG. 3, the output terminal of the NOx operational amplifier OP3 is connected to the non-inverting input terminal of the verification operational amplifier OP5 via the resistor R32, and the output terminal of the residual oxygen operational amplifier OP4. Is connected to the inverting input terminal of the verification operational amplifier OP5 through a resistor R31.
Further, a resistor R33 and a characteristic improving capacitor C8 are connected in parallel between the inverting input terminal and the output terminal of the verification operational amplifier OP5, and the non-inverting input terminal of the verification operational amplifier OP5 has a predetermined offset. It is connected to a voltage Vd (0.2 V in this example). The resistance values of the resistor R31 and the resistor R32 and the resistance values of the resistor R33 and the resistor R34 are the same, and the verification operational amplifier OP5 is determined by the magnitude of the resistor R33 (R34) relative to the resistor R31 (R32). A voltage corresponding to the NOx concentration is output at a rate.
The output terminal of the NOx operational amplifier OP3 is connected to the A / D conversion input port IN7 of the control microcomputer 8. The control microcomputer 8 can detect and monitor the sensor current in the sensor cell 2B. Yes.

As shown in FIG. 2, in the monitor cell 2C, the admittance measurement circuit 7 switches from sensor current detection to element admittance measurement, and uses a time zone during which no sensor current is detected, to the monitor cell 2C with a predetermined AC voltage. Is applied, and the change in the alternating current flowing through the monitor cell 2C is detected to obtain the element admittance.
The element admittance signal is taken into the control microcomputer 8 from the A / D conversion input port IN3.

(Sweep means 62)
As shown in the figure, the sweep means 62 configured in the command voltage source 61 includes a plus-side switching element (MOS FET) TR2 and a resistor R4 provided in parallel with the resistor R6, and a resistor R7 and a resistor R8. A negative side switching element (MOS type FET) TR3 and a resistor R5 are provided. The gate terminals G of the switching elements TR2 and TR3 are connected to the output ports OUT1 and OUT2 of the control microcomputer 8 via resistors R2 and R3, respectively.

When the element admittance is measured in the monitor cell 2C, the switching element TR1 is turned on by the output signal from the output port OUT3 of the control microcomputer 8 and then output from the output port OUT1 of the control microcomputer 8 The signal is turned ON to make the plus side switching element TR2 conductive. As a result, a voltage (4.2 V in this example) obtained by dividing the parallel resistance of the resistor R4 and the resistor R6 and the series resistance of the resistor R7 and the resistor R8 is applied to the measured gas side electrode 205 of the monitor cell 2C. . Next, after a predetermined time has elapsed, the output signal from the output port OUT1 of the control microcomputer 8 is turned off, and the output signal from the output port OUT2 of the control microcomputer 8 is turned on to make the minus side switching element TR3 conductive. . As a result, a voltage (3.8 V in this example) obtained by dividing the resistance R6 and the parallel resistance of the resistance R5, the resistance R7, and the resistance R8 is applied to the measured gas side electrode 205 of the monitor cell 2C. Thereafter, the output signal from the output port OUT2 of the control microcomputer 8 is turned OFF.
Thus, the sweep means 62 can apply an AC voltage for measuring element admittance to the measured gas side electrode 205 of the monitor cell 2C.

(Switching circuit 65A, B)
As shown in FIG. 2, the two N-channel MOS FETs (TR1A, B) constituting the switching element TR1 of this example are turned on and off by the switching circuits 65A, B connected to the respective gate terminals G. Switching to a state is possible. The switching circuits 65A and 65B include an upstream switching circuit 65A that applies a voltage to the gate terminal G of the upstream MOS FET (TR1A) and a downstream side that applies a voltage to the gate terminal G of the downstream MOS FET (TR1B). And a switching circuit 65B.
Each of the switching circuits 65A and 65B receives the output signal from the control microcomputer 8, and changes the output state of the first-stage transistors TR6A and B6 and the first-stage transistors TR6A and B that switch between the conductive state and the non-conductive state. In response to this, the second stage transistors TR7A and B7 that are switched between a conductive state and a non-conductive state are used. The first-stage transistors TR6A and B and the second-stage transistors TR7A and B in this example are NPN bipolar transistors, and the gate terminals G of the second-stage transistors TR7A and B are collectors of the first-stage transistors TR6A and B. It is connected to the terminal.

  More specifically, in the upstream switching circuit 65A, the output port OUT3 of the control microcomputer 8 is connected to the base terminal of the first-stage transistor TR6A via the current-limiting resistor R40A, and the first-stage transistor A resistor R41A for current feedback is connected between the base terminal and the emitter terminal of TR6A. A current limiting resistor R42A is provided between the circuit power source Ve and the collector terminal of the first-stage transistor TR6A, and the emitter terminal of the first-stage transistor TR6A is connected to the ground potential. The resistor R42A and the collector terminal of the first stage transistor TR6A are connected to the base terminal of the second stage transistor TR7A via a current limiting resistor R43A.

As shown in FIG. 2, a current feedback resistor R45A is connected between the base terminal and the emitter terminal of the second-stage transistor TR7A. A current limiting resistor R44A is provided between the battery power source VB and the collector terminal of the second-stage transistor TR7A, and a current-limiting resistor is provided between the resistor R44A and the collector terminal of the second-stage transistor TR7A. It is connected to the gate terminal G of the upstream MOS type FET (TR1A) via R46A.
The resistor R46A and the collector terminal of the second-stage transistor TR7A are connected to the ground potential by a Zener diode D3A whose anode terminal is on the ground potential side. The Zener diode D3A protects the upstream MOS FET (TR1A) from the back electromotive force.

As shown in FIG. 2, a voltage extraction operational amplifier OP10 constituting a voltage follower is branched and connected between the source terminal S of the upstream MOS type FET (TR1A) and the admittance measurement resistor Ra.
The source terminal S of the upstream MOS FET (TR1A) and the admittance measurement resistor Ra are connected to the non-inverting input terminal of the voltage extraction operational amplifier OP10 via the resistor R47, and the voltage extraction operational amplifier OP10. Is connected to the emitter terminal of the second-stage transistor TR7A.
The output terminal of the voltage extraction operational amplifier OP10 is connected to the input port IN5 of the control microcomputer 8. The control microcomputer 8 is configured to monitor the voltage at the downstream end of the admittance measurement resistor Ra. is there.

  In the downstream switching circuit 65B, the output port OUT3 of the control microcomputer 8 is connected to the base terminal of the first-stage transistor TR6B via the current limiting resistor R40B, and the base terminal of the first-stage transistor TR6B. A resistor R41B for current feedback is connected between the emitter terminal and the emitter terminal. A current limiting resistor R42B is provided between the circuit power source Ve and the collector terminal of the first-stage transistor TR6B, and the emitter terminal of the first-stage transistor TR6B is connected to the ground potential. The resistor R42B and the collector terminal of the first stage transistor TR6B are connected to the base terminal of the second stage transistor TR7B via a current limiting resistor R43B.

A current feedback resistor R45B is connected between the base terminal and the emitter terminal of the second-stage transistor TR7B. A current limiting resistor R44B is provided between the battery power supply VB and the collector terminal of the second-stage transistor TR7B, and the emitter terminal of the second-stage transistor TR7B is connected to the ground potential. The resistor R44B and the collector terminal of the second-stage transistor TR7B are connected to the gate terminal G of the downstream MOS FET (TR1B) via a current limiting resistor R46B.
The resistor R46B and the collector terminal of the second-stage transistor TR7B are connected to the ground potential by a Zener diode D3B whose anode terminal is on the ground potential side. The Zener diode D3B protects the downstream MOS FET (TR1B) from back electromotive force.

As shown in FIG. 2, when the sensor current detection circuit 6 detects the sensor current flowing through the monitor cell 2C, the output port of the control microcomputer 8 is in the OFF (Low) state, and each switching circuit 65A, B The first-stage transistors TR6A, B are in a non-conductive state between the collector and the emitter. In addition, a voltage (8.4 V in this example) from the circuit power source Ve is applied to the base terminals of the second-stage transistors TR7A and B, and the collector-emitters of the second-stage transistors TR7A and B are brought into conduction. It has become.
A voltage of approximately 0 V is applied from the downstream switching circuit 65B to the gate terminal G of the downstream MOS FET (TR1B) (the gate terminal G of the downstream MOS FET (TR1B) is in a low state. The downstream MOS type FET (TR1B) is in a non-conductive state.

  Also, the gate terminal G of the upstream side MOS type FET (TR1A) from the upstream side switching circuit 65A is supplied with substantially the same voltage (current detection operational amplifier) as the source terminal S of the upstream side MOS type FET (TR1A) by the buffer operational amplifier OP10. A voltage (substantially the same voltage as that applied to the admittance measurement resistor Ra from OP2) (4 V in this example) is applied. Since there is no potential difference (almost 0V) between the gate terminal G of the upstream MOS FET (TR1A) and the source terminal S of the upstream MOS FET (TR1A), the upstream MOS FET ( The gate terminal G of TR1A) is in a Low state, and the upstream side MOS FET (TR1A) is in a non-conducting state.

  On the other hand, as shown in FIG. 2, when the admittance measurement circuit 7 measures a change in the current flowing through the admittance measurement resistor Ra, the output port of the control microcomputer 8 is turned on (High), and each switching circuit 65A. , B first stage transistors TR6A, the collector-emitter of the B becomes conductive. At this time, the voltage applied to the base terminal of each second stage transistor TR7A, B becomes substantially 0V, and the collector-emitter between each second stage transistor TR7A, B becomes non-conductive. A voltage (12 V in this example) from the battery power source VB is applied from the collector terminals of the second-stage transistors TR7A and B to the base terminals of the MOS FETs (TR1A and B).

As a result, the drain-source of each MOS FET (TR1A, B) becomes conductive, and the switching wiring 601 provided with the admittance measurement resistor Ra becomes conductive.
Thus, when the admittance measurement circuit 7 detects the current flowing through the admittance measurement resistor Ra, the voltage VB generated by the battery power supply VB, which is higher than the voltage of the command voltage source 61, is converted into two MOS FETs (TR1A, The two MOS type FETs (TR1A, B) can be made conductive by being applied to the gate terminal G of B).

(Admittance measurement circuit 7)
As shown in FIG. 4, the admittance measurement wiring 701 constituting the admittance measurement circuit 7 of the present example has a downstream end (between the admittance measurement resistance Ra and the switching element TR1) of the current flow of the admittance measurement resistance Ra. Is connected to. Note that the symbol A in FIG. 4 is connected to the symbol A in FIGS.
In the admittance measurement wiring 701, a buffer operational amplifier OP7, a high-pass filter 71, a diode D1, a peak hold means 72, and an amplification operational amplifier OP9 constituting an amplification circuit are provided in this order from the upstream side of the current flow.

  The high-pass filter 71 is a so-called primary filter, and includes a capacitor C3 provided in the admittance measurement wiring 701, and a resistor R10 provided between the downstream side of the capacitor C3 and the ground potential. In the admittance measurement wiring 701, a current limiting resistor R11 is provided on the downstream side of the high-pass filter 71. The downstream side of the resistor R11 is connected to the ground potential via a diode D1 whose anode terminal is on the ground potential side. Is connected to.

  As shown in FIG. 4, in the admittance measurement wiring 701, the downstream side of the connection position of the diode D1 is connected to the peak hold means 72 via a current limiting resistor R12. The peak hold means 72 of this example includes a peak hold operational amplifier OP8 constituting a voltage follower, a peak voltage detection capacitor C4 connected between the output terminal of the peak hold operational amplifier OP8 and the ground potential, and a peak voltage detection. A diode D2 for preventing current leakage from the capacitor C4 to the output terminal of the peak hold operational amplifier OP8. A current limiting resistor R13 is provided between the diode D2 and the peak voltage detecting capacitor C4.

In the admittance measurement wiring 701, the downstream side of the peak voltage detecting capacitor C4 is connected to a non-inverting input terminal of an amplifying operational amplifier OP9 constituting a non-inverting amplifying circuit. The amplifying operational amplifier OP9 has an amplification factor determined by a resistor R15 provided between the inverting input terminal and the ground potential and a resistor R16 provided between the inverting input terminal and the output terminal. The peak voltage held at C4 is amplified. The output terminal of the amplification operational amplifier OP9 is connected to the A / D conversion input port IN3 of the control microcomputer 8.
In this way, the control microcomputer 8 has the peak voltage held by the peak hold means of the admittance measurement circuit 7 and the resistance value (511Ω in this example) of the admittance measurement circuit 7 during the voltage sweep. It is comprised so that it may obtain | require based on.

As shown in FIG. 4, in the admittance measurement wiring 701, the admittance measurement wiring 701 is set to the ground potential when the element admittance measurement is not performed in the admittance measurement circuit 7 on the upstream side of the peak hold operational amplifier OP8. A measurement stop switching element TR4 is provided to keep the peak voltage detection capacitor C4 downstream of the peak hold operational amplifier OP8 when the element admittance measurement is completed in the admittance measurement circuit 7. A reset switching element TR5 for resetting the voltage (substantially zero) is provided.
The base terminals of the measurement stop switching element TR4 and the reset switching element TR5 are connected to the output ports OUT6 and OUT7 of the control microcomputer 8.

When element admittance is detected in the monitor cell 2C, the voltage applied to the non-inverting input terminal of the peak hold operational amplifier OP8 prevents current leakage to the output terminal of the peak hold operational amplifier OP8 by the diode D2. In the state, it is stored in the peak voltage detecting capacitor C4. Then, the signal is taken into the A / D conversion input port IN3 of the control microcomputer 8 while being amplified to a predetermined amplification factor by the amplification operational amplifier OP9.
Further, when resetting the voltage stored in the peak voltage detecting capacitor C4, the reset switching element TR5 can be turned on by the output signal from the output port OUT6 of the control microcomputer 8 to measure the element admittance. When not performed, the measurement stop switching element TR4 can be made conductive by an output signal from the output port OUT7 of the control microcomputer 8.

FIG. 7 schematically shows the operation when measuring the element admittance.
The upper part of the figure shows the sweep voltage generated by the sweep means 62 when measuring the element admittance, and the middle part of the figure shows the change voltage (the output voltage of the voltage monitor operational amplifier OP10) generated in the admittance measurement resistor Ra. The lower part of the figure shows the peak voltage in the peak hold means 72 (the output voltage of the peak hold operational amplifier OP8). The voltage of the reference voltage source 51 of this example is 4.4V, and the voltage actually applied to the admittance measurement resistor Ra varies between 0.2V and 0.6V. The control microcomputer 8 can determine the element admittance of the monitor cell 2C based on the peak voltage and the resistance value of the admittance measurement resistor Ra.

(Temperature control of gas sensor element 2)
As shown in FIG. 1, in the control microcomputer 8, a relationship map (graph) between the temperature T (° C.) of the gas sensor element 2 (monitor cell 2C) and the element admittance (or element impedance) Adm (S or Ω) of the monitor cell 2C. ) Is formed. The control microcomputer 8 receives the output voltage from the peak hold means 72 in the admittance measurement circuit 7, and the element admittance (or element impedance) of the monitor cell 2C from this output voltage and the admittance measurement resistor Ra (511Ω in this example). The temperature of the gas sensor element 2 at the time of measurement is detected from the value of the element admittance (or element impedance).

As shown in FIG. 5, the energization voltage source VB is connected to one terminal 302 of the heater element 3, and the output port (ON / OFF) of the control microcomputer 8 is connected to the other terminal 301 of the heater element 3. A heater output circuit 32 that performs an ON / OFF operation so as to energize the heater element 3 in response to a pulse width modulation signal (PWM signal) from the port OUT5 is provided.
The control microcomputer 8 changes the duty ratio of the pulse width modulation signal to the heater output circuit 32 based on the value of the element admittance, so that the temperature of the gas sensor element 2 can stably exhibit the limit current characteristics. To a predetermined target temperature. The control microcomputer 8 performs feedback control such as PID control and performs pulse width modulation control so that the temperature of the gas sensor element 2 becomes a target temperature.
The control microcomputer 8 is configured to perform electrical communication with a host ECU (electronic control unit).

  The gas concentration detection apparatus 1 of this example detects NOx (nitrogen oxide) concentration, and in the monitor cell 2C, it can switch between detection of sensor current and measurement of element admittance (or element impedance). It can be done. Then, when detecting the sensor current, a contrivance is made to suppress current leakage that has an adverse effect on the detection.

Specifically, as shown in FIG. 6, the switching element TR1 provided in the switching wiring 601 for switching between the conduction state and the non-conduction state of the admittance measurement resistor Ra includes two N-channel MOS FETs (TR1A, B ) Is formed by connecting the drain terminals D to each other. The two MOS FETs (TR1A, B) are provided between the gate terminal G and the source terminal S so as to protect the MOS FETs (TR1A, B) from overvoltage caused by static electricity or the like. , D3 ′ are provided in both bias directions.
The switching element TR1 is formed in the forward direction between the source and the drain when detecting the sensor current by configuring the two MOS FETs (TR1A, B) by connecting the drain terminals D to each other. Due to the parasitic diode D1 ′, leakage current can be prevented from occurring in the switching wiring 601.

When detecting the sensor cell current flowing through the monitor cell 2C (when detecting the current flowing through the sensor current detection resistor Rm by the sensor current detecting means 63), the two MOS FETs (TR1A, B) are made non-conductive. Then, the switching wiring 601 is kept in a non-conductive state. At this time, the gate terminal G of the upstream MOS FET (TR1A) has a voltage substantially equal to that of the source terminal S of the upstream MOS FET (TR1A) (approximately the same potential as that of the command voltage source 61). Voltage) is applied, and the gate terminal G of the downstream-side MOS FET (TR1B) is set to the ground potential.
Next, when a differential voltage (0.4 V in this example) between a voltage (4.4 V in this example) from the reference voltage source 51 and a voltage (4 V in this example) from the command voltage source 61 is applied to the monitor cell 2C For example, a minute sensor current of several nA to several hundreds of nA flows through the sensor current detection resistor Rm in the sensor current detection circuit 6. The sensor current detection means 63 can detect the current flowing through the sensor current detection resistor Rm.

  At this time, the gate terminal G of the upstream MOS FET (TR1A) has a voltage substantially equal to that of the source terminal S of the upstream MOS FET (TR1A) (a voltage approximately equal to the voltage by the command voltage source 61). ) (4 V in this example) is applied, so there is almost no potential difference between the gate and source of the upstream MOS FET (TR1A). Thereby, when detecting the sensor current flowing through the monitor cell 2C, it is possible to prevent leakage current from occurring in the Zener diodes D2 'and D3' in the upstream MOS FET (TR1A). Therefore, this leakage current can be prevented from flowing to the sensor current detection resistor Rm, and the sensor current detection accuracy by the sensor current detection means 63 can be improved. Then, when obtaining the NOx concentration from the difference between the current value of the sensor current flowing through the sensor cell 2B and the current value of the sensor current flowing through the monitor cell 2C, the detection accuracy of the NOx concentration can be improved.

When detecting the sensor current, a potential difference (4 V in this example) between the command voltage source 61 and the ground potential is generated between the gate and the source of the downstream MOS FET (TR1B). The leakage current that may occur in the Zener diodes D2 ′ and D3 ′ in the MOS FET (TR1B) is the current after passing through the sensor current detection resistor Rm. Therefore, even if a leakage current occurs in the Zener diodes D2 ′ and D3 ′ in the downstream MOS type FET (TR1B), this leakage current does not adversely affect the detection accuracy of the sensor current by the sensor current detection means 63. rare.
When the NOx concentration is detected, the monitor cell 2C needs to detect a minute sensor current of several nA to several hundred nA, but the Zener diode D2 ′ in the upstream MOS type FET (TR1A), By preventing the leakage current from occurring at D3 ′, it is possible to prevent the leakage current from adversely affecting the detection accuracy of the sensor current.

  In the monitor cell 2C, when the element admittance is measured (when the current flowing through the admittance measurement resistor Ra is detected by the admittance measurement circuit 7), the two MOS FETs (TR1A, B) are turned on. Then, the switching wiring 601 is made conductive. At this time, the voltage (12 V in this example) from the battery power supply VB, which is higher than the voltage of the command voltage source 61 (4 V in this example), is applied to the gate terminals G of the two MOS FETs (TR1A, B). Applied, the two MOS FETs (TR1A, B) are made conductive.

Next, an AC voltage is applied to the current detection operational amplifier OP <b> 2 by the sweep means 62 provided in the command voltage source 61. At this time, a voltage resulting from the difference between the voltage from the reference voltage source 51 and the AC voltage from the sweep means 62 is applied to the monitor cell 2C, and a current flows through the monitor cell 2C and the admittance measurement resistor Ra.
The element admittance of the monitor cell 2C can be stably measured by detecting a voltage change caused by the current flowing through the admittance measurement resistor Ra using the admittance measurement circuit 7.

  As described above, according to the gas concentration detection apparatus 1 of this example, the detection accuracy of the sensor current can be improved in the monitor cell 2C that switches between the detection of the sensor current and the measurement of the element admittance. The density detection accuracy can be improved.

(Example 2)
In this example, as shown in FIG. 8, the switching element TR1X provided in the switching wiring 601 of the sensor current detection circuit 6 includes two P-channel MOS type FETs (TR1A, TR1A, This is an example configured using B).
The configuration of the two MOS type FETs (TR1A, B) of this example is the same as that of Example 1 except that there is a difference between the N channel and the P channel as compared to the two MOS type FETs (TR1A, B) of Example 1. It is the same. That is, each MOS type FET (TR1A, B) has a Zener diode D2 ′ for protecting the MOS type FET (TR1A, B) from an overvoltage caused by static electricity between the gate terminal G and the source terminal S. D3 ′ is provided in both bias directions.

In the gas concentration detection apparatus 1 of this example, when the current flowing through the sensor current detection resistor Rm is detected by the sensor current detection means 63, the gate terminals G of the two P-channel MOS type FETs (TR1A, B) are connected to each other. Applies a voltage of approximately the same potential as the source terminal S of the upstream MOS type FET (TR1A) (a voltage of approximately the same potential as the voltage from the command voltage source 61) (4 V in this example), and the two MOS types The FET (TR1A, B) is kept non-conductive.
Further, when the gas concentration detection device 1 of this example detects the current flowing through the admittance measurement resistor Ra by the admittance measurement circuit 7, the voltage concentration of the command voltage source 61 (4V in this example) is lower (almost). 0V) is applied to the gate terminals G of the two P-channel MOS type FETs (TR1A, B) to make the two MOS type FETs (TR1A, B) conductive.

As shown in FIG. 8, a voltage follower is connected to the source terminal S (between the source terminal S of the upstream MOS type FET (TR1A) and the admittance measurement resistor Ra) of the upstream side MOS FET (TR1A) of this example. A voltage extracting operational amplifier OP14 is branched and connected. When the drain and source of the two P-channel MOS type FETs (TR1A, B) are kept non-conductive, a voltage extracting circuit is connected to the gate terminal G of the two MOS type FETs (TR1A, B). The voltage extracted by the operational amplifier OP14 is applied.
The switching circuit 65C for switching between the drain and source of the two P-channel MOS type FETs (TR1A, B) of this example between the conductive state and the non-conductive state is switched by the output signal from the output port OUT3 of the control microcomputer 8. This is a simple configuration using an operating transistor (NPN transistor in this example) TR8 and a voltage extracting operational amplifier OP14.

The output port OUT3 of the control microcomputer 8 of this example is connected to the base terminal B of the transistor TR8, and the collector terminal C of the transistor TR8 is the gate terminal G of two P-channel MOS type FETs (TR1A, B). Is connected to. The emitter terminal E of the transistor TR8 is connected to the ground potential.
The downstream end of the admittance measurement resistor Ra of this example is connected to the non-inverting input terminal of the voltage extraction operational amplifier OP14 via the resistor R50, and the output terminal of the voltage extraction operational amplifier OP14 is a current limiting resistor. It is connected to the collector terminal C of the transistor TR8 via R51.

As shown in FIG. 8, when the sensor cell current flowing through the monitor cell 2C is detected (when the current flowing through the sensor current detecting resistor Rm is detected by the sensor current detecting means 63), the output port of the control microcomputer 8 is used. The output signal of OUT3 is set to an OFF (Low) state. At this time, the voltage (4 V in this example) by the command voltage source 61 is applied to the two P-channel MOS FETs (TR1A, B) and the admittance detection resistor Ra by the current detection operational amplifier OP2.
At this time, the gate terminals G of the two P-channel MOS FETs (TR1A, B) have substantially the same potential as the source terminal S of the upstream MOS FET (TR1A) drawn out by the voltage extraction operational amplifier OP14. A voltage (a voltage having substantially the same potential as that of the command voltage source 61) (4 V in this example) is applied.

  As a result, there is almost no potential difference between the gate and source of the two MOS FETs (TR1A, B). Then, when detecting the sensor current flowing through the monitor cell 2C, it is possible to prevent leakage current from occurring in the Zener diodes D2 'and D3' in the two MOS FETs (TR1A and B). Therefore, this leakage current can be prevented from flowing to the sensor current detection resistor Rm, and the sensor current detection accuracy by the sensor current detection means 63 can be improved. Then, when obtaining the NOx concentration from the difference between the current value of the sensor current flowing through the sensor cell 2B and the current value of the sensor current flowing through the monitor cell 2C, the detection accuracy of the NOx concentration can be improved.

Also in this example, when the element admittance is measured in the monitor cell 2C (when the current flowing through the admittance measurement resistor Ra is detected by the admittance measurement circuit 7), the output port OUT3 of the control microcomputer 8 is used. Is set to ON (High) state. At this time, a current flows between the collector and emitter of the transistor TR8 from the output terminal of the voltage extraction operational amplifier OP14 via the resistor R51 and is applied to the gate terminals G of the two P-channel MOS FETs (TR1A, B). The voltage becomes almost 0V.
As a result, the drain-source of the two P-channel MOS FETs (TR1A, B) is brought into conduction, and the switching wiring 601 is brought into conduction. As in the first embodiment, the element admittance of the monitor cell 2C can be stably measured by detecting the voltage change caused by the current flowing through the admittance measurement resistor Ra using the admittance measurement circuit 7.

As described above, the gas concentration detection apparatus 1 of this example can also improve the detection accuracy of the sensor current in the monitor cell 2C that switches between the detection of the sensor current and the measurement of the element admittance. Detection accuracy can be improved.
Although the switching circuit of this example is configured simply, this switching circuit can have various configurations. For example, the transistor TR8 (a plurality of transistors can be used) is operated using the circuit power supply Vcc, the battery power supply VB, etc., and is non-conductive to the gate terminals G of the two P-channel MOS type FETs (TR1A, B). Sometimes, various configurations may be employed in which a voltage (4 V in this example) having substantially the same potential as that of the source terminal S of the upstream-side MOS FET (TR1A) is applied and substantially 0 V is applied during conduction.
Also in this example, the other configuration of the gas concentration detection device 1 is the same as that of the first embodiment, and the same effects as those of the first embodiment can be obtained.

1 is a configuration diagram schematically showing an overall configuration of a gas concentration detection device in Embodiment 1. FIG. FIG. 3 is an explanatory diagram illustrating a circuit configuration around a monitor cell, a sensor current detection circuit, and a switching circuit in the first embodiment. FIG. 3 is an explanatory diagram illustrating a circuit configuration around a sensor cell, a sensor current detection circuit, and a NOx current detection circuit in the first embodiment. FIG. 3 is an explanatory diagram illustrating a circuit configuration of an admittance measurement circuit in the first embodiment. FIG. 3 is an explanatory diagram illustrating circuit configurations of a pump cell and a heater element in the first embodiment. FIG. 3 is an explanatory diagram illustrating details of the periphery of the switching element of the sensor current detection circuit in the first embodiment. FIG. 3 is an explanatory diagram schematically illustrating an operation when measuring element admittance in the first embodiment. FIG. 6 is an explanatory diagram showing the periphery of a sensor current detection circuit and a switching circuit in Example 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Gas concentration detection apparatus 2 Gas sensor element 2A Pump cell 2B Sensor cell 2C Monitor cell 21 Solid electrolyte body 3 Heater element 41 Pump cell output circuit 42 Pump cell input circuit 43 Reference voltage source 5 NOx current detection circuit 51 Reference voltage source 6 Sensor current detection circuit 601 For switching Wiring 61 Command voltage source 62 Sweep means 63 Sensor current detection means 7 Admittance measurement circuit 8 Control microcomputer OP2 Current detection operational amplifier OP4 Residual oxygen operational amplifier Rm Sensor current detection resistance Ra Admittance measurement resistance TR1 Switching element TR1A Upstream MOS type FET
TR1B Downstream MOS type FET
D2 ', D3' Zener diode F Reference gas G Gas to be measured (exhaust gas)

Claims (6)

It has a gas sensor element in which electrodes are provided on both surfaces of a solid electrolyte body having oxygen ion permeability,
A pump cell for adjusting the oxygen concentration in the gas to be measured provided with an NOx inactive electrode by the gas sensor element, and a gas in the gas to be measured after adjusting the oxygen concentration by the pump cell provided with the NOx active electrode. Gas concentration comprising a sensor cell for measuring the NOx concentration and a monitor cell for monitoring the oxygen concentration in the gas to be measured after the NOx inert electrode is provided and the oxygen concentration is adjusted by the pump cell. In the detection device,
The monitor cell is a cell that also serves to measure the element admittance (or element impedance, the same applies hereinafter). A reference voltage source is connected to the reference gas side electrode of the monitor cell, and the measured gas side electrode of the monitor cell is connected to the monitor cell. Is connected to the sensor current detection circuit,
The sensor current detection circuit includes a command voltage source,
An operational amplifier for current detection for applying a voltage from the command voltage source to the measured gas side electrode of the monitor cell;
A sensor current detection resistor provided between the output terminal of the current detection operational amplifier and the measured gas side electrode of the monitor cell;
Sensor current detection means for detecting a current flowing through the sensor current detection resistor;
An admittance measurement resistor connected in parallel with the sensor current detection resistor between the output terminal of the current detection operational amplifier and the measured gas side electrode of the monitor cell;
A switching element for switching between a conductive state and a non-conductive state of the switching wiring provided with the admittance measurement resistor;
The command voltage source is provided with sweeping means for applying an AC voltage for measuring the element admittance to the current detection operational amplifier in a state where the switching wiring is made conductive by the switching element.
An admittance measurement circuit for measuring a change in current flowing through the admittance measurement resistor by application of the AC voltage by the sweep means is connected to the switching wiring,
The switching element is configured using two N-channel MOS FETs configured by connecting drain terminals to each other,
The two MOS FETs are downstream of the sensor current detection resistor, and are connected to the output terminal of the current detection operational amplifier, and upstream of the sensor current detection resistor. It consists of an upstream MOS type FET connected to the measured gas side electrode of the monitor cell,
Each MOS type FET is provided with a zener diode for protecting each MOS type FET from overvoltage due to static electricity or the like between the gate terminal and the source terminal in both bias directions.
When the current flowing through the sensor current detection resistor is detected by the sensor current detection means, a voltage (approximately the same potential as the source terminal of the upstream MOS FET is connected to the gate terminal of the upstream MOS FET. A voltage of approximately the same potential as the voltage from the command voltage source), the gate terminal of the downstream MOS FET is set to a substantially ground potential, and the two MOS FETs are made non-conductive,
When detecting a change in current flowing through the admittance measurement resistor by the admittance measurement circuit, a voltage higher than the voltage of the command voltage source is applied to the gate terminals of the two MOS FETs, and the two A gas concentration detecting device characterized in that a MOS type FET is made conductive. In Claim 1, a voltage extraction operational amplifier constituting a voltage follower is branched and connected to the source terminal of the upstream MOS type FET.
A gas concentration detecting device configured to apply a voltage extracted by the voltage extracting operational amplifier to the gate terminal of the upstream MOS FET when the two MOS FETs are kept non-conductive. . 3. The upstream MOS FET according to claim 2, wherein the upstream MOS FET can be switched between a conductive state and a non-conductive state by an upstream switching circuit connected to the gate terminal thereof. The downstream switching circuit connected to can be switched between a conductive state and a non-conductive state,
Each of the switching circuits receives an output signal from the control microcomputer, receives an NPN first-stage transistor that switches between a conductive state and a non-conductive state, and receives an output state of the first-stage transistor, It is configured using an NPN-type second stage transistor that switches to a non-conductive state,
The output terminal of the operational amplifier for voltage extraction is connected to the emitter terminal of the second-stage transistor in the upstream-side switching circuit, and when the second-stage transistor is in a conductive state, the upstream-side MOS FET A gas concentration detection apparatus configured to apply a voltage having substantially the same potential as a source terminal of the upstream MOS type FET to a gate terminal. It has a gas sensor element in which electrodes are provided on both surfaces of a solid electrolyte body having oxygen ion permeability,
A pump cell for adjusting the oxygen concentration in the gas to be measured provided with an NOx inactive electrode by the gas sensor element, and a gas in the gas to be measured after adjusting the oxygen concentration by the pump cell provided with the NOx active electrode. Gas concentration comprising a sensor cell for measuring the NOx concentration and a monitor cell for monitoring the oxygen concentration in the gas to be measured after the NOx inert electrode is provided and the oxygen concentration is adjusted by the pump cell. In the detection device,
The monitor cell is a cell that also serves to measure the element admittance (or element impedance, the same applies hereinafter). A reference voltage source is connected to the reference gas side electrode of the monitor cell, and the measured gas side electrode of the monitor cell is connected to the monitor cell. Is connected to the sensor current detection circuit,
The sensor current detection circuit includes a command voltage source,
An operational amplifier for current detection for applying a voltage from the command voltage source to the measured gas side electrode of the monitor cell;
A sensor current detection resistor provided between the output terminal of the current detection operational amplifier and the measured gas side electrode of the monitor cell;
Sensor current detection means for detecting a current flowing through the sensor current detection resistor;
An admittance measurement resistor connected in parallel with the sensor current detection resistor between the output terminal of the current detection operational amplifier and the measured gas side electrode of the monitor cell;
A switching element for switching between a conductive state and a non-conductive state of the switching wiring provided with the admittance measurement resistor;
The command voltage source is provided with sweeping means for applying an AC voltage for measuring the element admittance to the current detection operational amplifier in a state where the switching wiring is made conductive by the switching element.
An admittance measurement circuit for measuring a change in current flowing through the admittance measurement resistor by application of the AC voltage by the sweep means is connected to the switching wiring,
The switching element is configured by using two P-channel MOS FETs configured by connecting drain terminals to each other,
The two MOS FETs are downstream of the sensor current detection resistor, and are connected to the output terminal of the current detection operational amplifier, and upstream of the sensor current detection resistor. It consists of an upstream MOS type FET connected to the measured gas side electrode of the monitor cell,
Each MOS type FET is provided with a zener diode for protecting each MOS type FET from overvoltage due to static electricity or the like between the gate terminal and the source terminal in both bias directions.
When the current flowing through the sensor current detection resistor is detected by the sensor current detection means, a voltage (approximately the same potential as the source terminal of the upstream MOS FET is connected to the gate terminals of the two MOS FETs. A voltage of substantially the same potential as the voltage from the command voltage source), and the two MOS FETs are made non-conductive,
When detecting a change in current flowing through the admittance measurement resistor by the admittance measurement circuit, a voltage lower than the voltage of the command voltage source is applied to the gate terminals of the two MOS FETs, and the two A gas concentration detecting device characterized in that a MOS type FET is made conductive. In Claim 4, the voltage extraction operational amplifier which comprises a voltage follower is branched and connected to the source terminal of the said upstream MOS type FET,
A gas concentration characterized in that when the two MOS FETs are made non-conductive, the voltage drawn by the voltage extracting operational amplifier is applied to the gate terminals of the two MOS FETs. Detection device.   6. The sensor current detection resistor according to claim 1, wherein the sensor current detection resistor has a resistance value of 100 kΩ to 2 MΩ, and the admittance measurement resistor has a resistance value of 100Ω to 1 kΩ. A gas concentration detection device characterized by that.

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