JP2008309641A - Gas concentration detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas concentration detector which enhances the detection precision of the current flowing to a gas sensor element. <P>SOLUTION: The gas concentration detector 1 has a pump cell, a sensor cell 2B and a monitor cell. Constant-current means E1-E3 for allowing a current of a predetermined value to flow to ground potential are respectively provided to the output terminal of a sensor cell differential amplifying operational amplifier OP3 for detecting the potential difference between both terminals of the sensor current detecting resistor Rs provided to the measuring target gas side electrode 203 of the sensor cell 2B, the output terminal of a monitor cell differential amplifying operational amplifier OP4 for detecting the potential difference between both terminals of a monitor current detecting resistor Rm provided to the measuring target 205 of the monitor cell and the output terminal of an NOx concentration detecting operational amplifier OP5 for outputting a voltage based on the NOx concentration performing the differential amplification of the output voltage of the monitor cell differential amplifying operational amplifier OP4 and the output voltage of the sensor cell differential amplifying operational amplifier OP3. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a gas concentration detection device that detects a NOx concentration, and more particularly to a circuit that detects a current flowing in a sensor cell or a monitor cell.

  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.

  For example, in the gas concentration detection device of Patent Document 1, in order to detect a current flowing through a pump cell (first cell) that discharges excess oxygen in a gas to be detected, an operational amplifier constituting a differential amplifier circuit is used. The potential difference between both ends of the resistor connected to the pump cell is detected. In addition, in order to detect a current flowing in a sensor cell (second cell) that flows a current corresponding to the concentration of a specific component from a gas component after excess oxygen is discharged, an operational amplifier that constitutes a differential amplifier circuit is used for the sensor cell. The potential difference between both ends of the connected resistor is detected.

By the way, the output terminal of a general operational amplifier is taken out between the resistance provided at the emitter terminal of the NPN output stage transistor and the resistance provided at the collector terminal of the PNP output stage transistor in the internal circuit of the operational amplifier. ing. Therefore, for example, when detecting the current flowing in the sensor cell, the monitor cell, etc., it is difficult to set the output voltage of the operational amplifier to 0 V, and the voltage Vce between the collector and the emitter of the output stage transistor remains in the output voltage.
When the output voltage of the operational amplifier exceeds the lower limit value of the voltage Vce, an error in detecting the current hardly occurs. On the other hand, when the output voltage of the operational amplifier is equal to or lower than the lower limit value of the voltage Vce, the voltage Vce. Becomes an error in detecting the current, which causes a deterioration in current detection accuracy.
In particular, in a gas concentration detection device that detects NOx concentration, in order to detect NOx concentration on the order of ppm, it is necessary to detect a minute current of about several nA to several hundred nA as a current flowing in a sensor cell, a monitor cell, or the like. In addition, deterioration of current detection accuracy becomes a serious problem.

JP 2000-171439 A

  The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a gas concentration detection device capable of improving the detection accuracy of a current flowing in a gas sensor element.

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 sensor cell has a reference voltage source connected to the reference gas side electrode, and a sensor current detection circuit connected to the measured gas side electrode.
The sensor current detection circuit includes a command voltage source,
An operational amplifier for detecting a sensor current for applying a voltage from the command voltage source to the measured gas side electrode of the sensor cell;
A sensor current detection resistor provided between an output terminal of the sensor current detection operational amplifier and a measured gas side electrode of the sensor cell;
A differential amplification of the voltage at both ends of the sensor current detection resistor, and a sensor cell differential amplification operational amplifier for detecting a potential difference at both ends of the sensor current detection resistor;
In the gas concentration detection apparatus, constant current means for flowing a predetermined value of current to the ground potential is provided at the output terminal of the operational amplifier for differential amplification of the sensor cell.

The gas concentration detection device of the present invention detects the NOx (nitrogen oxide) concentration in the gas to be measured, and devise to improve the detection accuracy of the current flowing through the sensor cell (hereinafter referred to as sensor current). Yes.
Specifically, in the gas concentration detection device of the present invention, in the sensor cell, when a differential voltage between the voltage by the reference voltage source and the output voltage of the sensor current detection operational amplifier (voltage by the command voltage source) is applied to the sensor cell. For example, a minute sensor current of several nA to several hundred nA flows through the sensor current detection resistor in the sensor current detection circuit. Then, a differential amplification of the voltage at both ends of the sensor current detection resistor is performed by the sensor cell differential amplification operational amplifier, and a potential difference at both ends of the sensor current detection resistor is detected.

At this time, the output terminal of the operational amplifier for differential amplification of the sensor cell of the present invention is provided with a constant current means for flowing a predetermined current to the ground potential. By this constant current means, a predetermined value of current can be passed from the output stage transistor in the internal circuit of the sensor cell differential amplification operational amplifier to the ground. Thereby, the lower limit value of the collector-emitter voltage Vce of the output stage transistor can be lowered, and the lower limit value of the voltage that can be output by the sensor cell differential amplification operational amplifier can be lowered.
In a control microcomputer or the like that controls the gas concentration detection device, the sensor current flowing through the sensor cell is accurately obtained based on the potential difference between both ends of the sensor current detection resistor and the resistance value of the sensor current detection resistor. Can do.

  Therefore, according to the gas concentration detection apparatus of the present invention, it is possible to improve the detection accuracy of the current (sensor current) flowing through the gas sensor element (sensor cell).

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 has a reference voltage source connected to the reference gas side electrode and a monitor current detection circuit connected to the measured gas side electrode.
The monitor current detection circuit includes a command voltage source,
A monitor current detection operational amplifier for applying a voltage from the command voltage source to the measured gas side electrode of the monitor cell;
A monitor current detection resistor provided between the output terminal of the monitor current detection operational amplifier and the measured gas side electrode of the monitor cell;
A monitor cell differential amplification operational amplifier for performing differential amplification of the voltage at both ends of the monitor current detection resistor and detecting a potential difference at both ends of the monitor current detection resistor;
The output terminal of the operational amplifier for differential amplification of the monitor cell is provided with a constant current means for allowing a predetermined current to flow to the ground potential (Claim 2).

The gas concentration detection device of the present invention detects the NOx (nitrogen oxide) concentration in the gas to be measured, and the monitor current flowing through the monitor cell (hereinafter, the current flowing through the monitor cell is distinguished from the current flowing through the sensor cell). Is called monitor current).
Specifically, in the gas concentration detection device of the present invention, in the monitor cell, when a differential voltage between the voltage from the reference voltage source and the output voltage of the monitor current detection operational amplifier (voltage from the command voltage source) is applied to the monitor cell. For example, a small monitor current of several nA to several hundreds of nA flows through the monitor current detection resistor in the monitor current detection circuit. Then, the monitor cell differential amplification operational amplifier performs differential amplification of the voltage at both ends of the monitor current detection resistor, and detects a potential difference at both ends of the monitor current detection resistor.

At this time, the output terminal of the operational amplifier for differential amplification of the monitor cell of the present invention is provided with a constant current means for flowing a predetermined value of current to the ground potential. By this constant current means, a predetermined value of current can be passed from the output stage transistor in the internal circuit of the monitor cell differential amplification operational amplifier to the ground. Thereby, the lower limit value of the collector-emitter voltage Vce of the output stage transistor can be lowered, and the lower limit value of the voltage that can be output by the monitor cell differential amplification operational amplifier can be lowered.
In a control microcomputer or the like that controls the gas concentration detection device, the monitor current flowing through the monitor cell must be accurately obtained based on the potential difference between both ends of the monitor current detection resistor and the resistance value of the monitor current detection resistor. Can do.

  Therefore, according to the gas concentration detection apparatus of the present invention, it is possible to improve the detection accuracy of the current (monitor current) flowing through the gas sensor element (monitor cell).

The third invention has a gas sensor element comprising electrodes 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 sensor cell has a reference voltage source connected to the reference gas side electrode, and a sensor current detection circuit connected to the measured gas side electrode.
The sensor current detection circuit includes a command voltage source,
An operational amplifier for detecting a sensor current for applying a voltage from the command voltage source to the measured gas side electrode of the sensor cell;
A sensor current detection resistor provided between an output terminal of the sensor current detection operational amplifier and a measured gas side electrode of the sensor cell;
A differential amplification of the voltage at both ends of the sensor current detection resistor, and a sensor cell differential amplification operational amplifier for detecting a potential difference at both ends of the sensor current detection resistor;
The monitor cell has a reference voltage source connected to the reference gas side electrode and a monitor current detection circuit connected to the measured gas side electrode.
The monitor current detection circuit includes a command voltage source,
A monitor current detection operational amplifier for applying a voltage from the command voltage source to the measured gas side electrode of the monitor cell;
A monitor current detection resistor provided between the output terminal of the monitor current detection operational amplifier and the measured gas side electrode of the monitor cell;
A monitor cell differential amplification operational amplifier for performing differential amplification of the voltage at both ends of the monitor current detection resistor and detecting a potential difference at both ends of the monitor current detection resistor;
The sensor current detection circuit performs differential amplification between the output voltage of the monitor cell differential amplification operational amplifier and the output voltage of the sensor cell differential amplification operational amplifier, and outputs a voltage based on the NOx concentration. Has an operational amplifier,
The output terminal of the operational amplifier for NOx concentration detection is provided with a constant current means for flowing a current of a predetermined value to the ground potential (Claim 3).

The gas concentration detection device of the present invention detects NOx (nitrogen oxide) concentration in the gas to be measured, and is based on the difference between the current flowing through the monitor cell (monitor current) and the current flowing through the sensor cell (sensor current). The device is designed to improve the detection accuracy of the required NOx concentration.
Specifically, in the gas concentration detection device of the present invention, in the sensor cell, when a differential voltage between the voltage by the reference voltage source and the output voltage of the sensor current detection operational amplifier (voltage by the command voltage source) is applied to the sensor cell. For example, a minute sensor current of several nA to several hundred nA flows through the sensor current detection resistor in the sensor current detection circuit. Then, a differential amplification of the voltage at both ends of the sensor current detection resistor is performed by the sensor cell differential amplification operational amplifier, and a potential difference at both ends of the sensor current detection resistor is detected.

  In the gas concentration detection device of the present invention, when a voltage difference between the voltage from the reference voltage source and the output voltage of the monitor current detection operational amplifier (voltage from the command voltage source) is applied to the monitor cell, A small monitor current of, for example, several nA to several hundreds of nA flows through the monitor current detection resistor in the detection circuit. Then, the monitor cell differential amplification operational amplifier performs differential amplification of the voltage at both ends of the monitor current detection resistor, and detects a potential difference at both ends of the monitor current detection resistor.

The NOx concentration detection operational amplifier performs differential amplification between the output voltage of the monitor cell differential amplification operational amplifier and the output voltage of the sensor cell differential amplification operational amplifier, and outputs a voltage based on the NOx concentration.
At this time, the output terminal of the operational amplifier for NOx concentration detection of the present invention is provided with a constant current means for flowing a predetermined current to the ground potential. By this constant current means, a predetermined value of current can be passed from the output stage transistor in the internal circuit of the NOx concentration detection operational amplifier to the ground. Thereby, the lower limit value of the collector-emitter voltage Vce of the output stage transistor can be lowered, and the lower limit value of the voltage that can be output by the monitor cell differential amplification operational amplifier can be lowered.
In a control microcomputer or the like that controls the gas concentration detection device, the NOx concentration in the gas to be measured can be accurately obtained based on the output voltage of the NOx concentration detection operational amplifier.

  Therefore, according to the gas concentration detection apparatus of the present invention, it is possible to improve the detection accuracy of the NOx concentration obtained from the difference between the currents flowing through the gas sensor elements (the difference between the currents flowing between the monitor cell and the sensor cell).

A preferred embodiment of the first to third inventions described above will be described.
In the first to third inventions, the resistance value of the sensor current detection resistor and the resistance value of the monitor current detection resistor can be several hundred to several thousand kΩ.
Further, two or more of the first to third inventions can be employed in combination with the same gas concentration detection device.

The constant current means can be configured using a constant current circuit or a resistor. In this case, the constant current means can be easily formed.
As the constant current circuit, for example, a current mirror circuit using two NPN transistors, a constant current circuit including a constant current diode such as an FET (field effect transistor), or the like can be used. In addition, various resistors having excellent temperature characteristics can be used as the resistor.
Further, when a constant current circuit is used as the constant current means, it is possible to reduce the variation in the range that can be output by the operational amplifier. On the other hand, when a resistor is used as the constant current means, it is inexpensive and the area occupied by the constant current means can be reduced, and the output range by the operational amplifier can be made closer to 0V.

Hereinafter, embodiments of the gas concentration detection apparatus 1 according to the present invention will be described with reference to the drawings.
As shown in FIGS. 1 to 3, 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 FIG. 3, the sensor cell 2 </ b> B of this example has a reference voltage source 51 connected to the reference gas side electrode 204 and a sensor current detection circuit 5 connected to the measured gas side electrode 203. The sensor current detection circuit 5 includes a command voltage source 52, a sensor current detection operational amplifier OP1 for applying a voltage from the command voltage source 52 to the measured gas side electrode 203 of the sensor cell 2B, and an output of the sensor current detection operational amplifier OP1. The sensor current detection resistor Rs provided between the terminal and the measured gas side electrode 203 of the sensor cell 2B and the voltage at both ends of the sensor current detection resistor Rs are differentially amplified, and the sensor current detection resistor Rs It has a sensor cell differential amplification operational amplifier OP3 for detecting a potential difference at both ends.
The output terminal of the operational amplifier OP3 for differential amplification of the sensor cell is provided with a constant current means E1 for allowing a predetermined current to flow to the ground potential.

As shown in the figure, the monitor cell 2C of this example has a reference voltage source 51 (shared with the reference voltage source 51 of the sensor cell 2B) connected to the reference gas side electrode 206, and the gas to be measured. The monitor current detection circuit 6 is connected to the side electrode 205. The monitor current detection circuit 6 includes a command voltage source 61, a 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 an output of the monitor current detection operational amplifier OP2. The monitor current detection resistor Rm provided between the terminal and the measured gas side electrode 205 of the monitor cell 2C and the voltage at both ends of the monitor current detection resistor Rm are differentially amplified, and the monitor current detection resistor Rm And a monitor cell differential amplification operational amplifier OP4 for detecting a potential difference between both ends.
The output terminal of the monitor cell differential amplification operational amplifier OP4 is provided with a constant current means E2 for flowing a predetermined current to the ground potential.

As shown in FIG. 3, the sensor current detection circuit 5 performs differential amplification of the output voltage of the monitor cell differential amplification operational amplifier OP4 and the output voltage of the sensor cell differential amplification operational amplifier OP3 to obtain a voltage based on the NOx concentration. It has an operational amplifier OP5 for detecting NOx concentration. The output terminal of the NOx concentration detecting operational amplifier OP5 is provided with a constant current means E3 for flowing a predetermined current to the ground potential.
As shown in FIG. 6, the constant current means E1 to E3 of this example are current mirror circuits as constant current circuits.

  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 monitor current detection circuit 6, and a switching circuit 65 described later. FIG. 3 shows the periphery of the sensor cell 2B, the sensor current detection circuit 5, the monitor cell 2C, and the monitor current detection circuit 6. FIG. 4 is a diagram showing a circuit configuration around 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 constant current means E1 to E3.

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)
As shown in FIGS. 2 and 3, the monitor cell 2 </ b> C of this example 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 monitor current detection circuit 6 is connected to the measured gas side electrode 205 of the monitor cell 2C.
The monitor current detection circuit 6 of this example includes an admittance measurement resistor Ra connected in parallel with the monitor current detection resistor Rm between the output terminal of the monitor current detection operational amplifier OP2 and the measured gas side electrode 205 of the monitor cell 2C. The switching wiring 601 provided with the admittance measurement resistor Ra has a switching element TR1 for switching between a conductive state and a non-conductive state.

  As shown in FIG. 2, the command voltage source 61 includes a sweeping means for applying an AC voltage for measuring the element admittance to the monitor current detecting operational amplifier OP2 in a state where the switching wiring 601 is conducted by the switching element TR1. 62 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.

  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 monitor current detection operational amplifier OP2 constitutes a voltage follower that applies the voltage from the command voltage source 61 to the measured gas side electrode 205 of the monitor cell 2C. The monitor current detection resistor Rm is connected between the output terminal of the monitor current detection operational amplifier OP2 and the measured gas side electrode 205 of the monitor cell 2C.
A protective resistor R1 for the monitor current detection operational amplifier OP2 is connected between the inverting input terminal of the monitor 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 configured by connecting two N-channel MOS type FETs (MOS type field effect transistors) (TR1A, B) with their drain terminals connected to each other. The gate terminal of the MOS FET is connected to the output port OUT3 of the control microcomputer 8 via a switching circuit 65 described later.
The two MOS FETs (TR1A, B) are downstream of the current flow in the monitor 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 monitor 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.
Further, between the output terminal of the monitor 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 monitor current detection resistor Rm and the admittance measurement resistor Ra. It is provided.

As shown in FIG. 2, the command voltage source 61 in the monitor current detection circuit 6 detects the monitor current in the monitor cell 2C, and the non-inverting input terminal of the monitor current detection operational amplifier OP2 and the measured gas side of the monitor cell 2C. A voltage (5 V in this example) from the circuit power supply Vcc is converted to 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 to the electrode 205. .
A current limiting resistor R9 is provided between the non-inverting input terminal of the monitor 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.

As shown in FIG. 3, when the monitor current is detected in the monitor cell 2C, the 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. A voltage (4 V in this example) from the command voltage source 61 is applied to the 2C measured gas side electrode 205.
The downstream end of the current flow of the monitor current detection resistor Rm is connected to the inverting input terminal of the monitor cell differential amplification operational amplifier OP4, and the non-inverting input terminal of the monitor cell differential amplification operational amplifier OP4 is connected to the inverting input terminal of the monitor cell differential amplification operational amplifier OP4. The upstream end of the current flow of the monitor current detection resistor Rm is connected via the buffer operational amplifier OP6 constituting the voltage follower.

  Voltage output based on monitor current (residual oxygen current, current flowing due to residual oxygen present in measured gas side electrode 205) flowing to monitor current detection resistor Rm (flowing to monitor cell 2C) by monitor cell differential amplification operational amplifier OP4 It is configured to The output terminal of the monitor cell differential amplification operational amplifier OP4 is connected to the A / D conversion input port IN2 of the control microcomputer 8. The control microcomputer 8 can detect and monitor the residual oxygen concentration in the monitor cell 2C. It has become.

More specifically, as shown in FIG. 3, the upstream end of the monitor 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 of the buffer operational amplifier OP6. The terminal is connected to the non-inverting input terminal of the monitor cell differential amplification operational amplifier OP4 via the resistor R37. On the other hand, the downstream end of the monitor current detection resistor Rm is connected to the inverting input terminal of the monitor cell differential amplification operational amplifier OP4 via the resistor R36.
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 monitor cell differential amplification operational amplifier OP4, and the non-inverting input of the monitor cell differential amplification operational amplifier OP4. The terminal is connected to a predetermined offset voltage Vd (in this example, 0.2 V). 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 monitor cell differential amplification operational amplifier OP4 has a size of the resistor R38 (R39) with respect to the resistor R36 (R37). The potential difference between both ends of the monitor current detection resistor Rm is output at an amplification factor determined by

The output terminal of the monitor cell differential amplification 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 monitor current i flowing from the measured gas side electrode 205 of the monitor cell 2C to the output terminal of the monitor current detection operational amplifier OP2 via the monitor current detection resistor Rm (the flow of the current i in FIG. 2). Is determined based on the resistance value (1.5 MΩ in this example) of the monitor current detection resistor Rm and the difference between the voltages at both ends of the monitor current detection resistor Rm.
The upstream voltage of the monitor 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 monitor 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 sensor 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 sensor 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 sensor current detection resistor Rs. Note that a noise absorbing capacitor C6 is connected in parallel to both ends of the sensor current detecting resistor Rs, and the inverting input terminal of the sensor current detecting operational amplifier OP1 is a protective resistor for the sensor current detecting operational amplifier OP1. It is connected to the measured gas side electrode 203 via R25.

Further, both ends of the sensor current detection resistor Rs are connected to the inverting input terminal and the non-inverting input terminal of the sensor cell differential amplification operational amplifier OP3 constituting the differential amplifier circuit, and this sensor cell differential amplification operational amplifier OP3. To output a voltage based on a sensor current (NOx current, current that flows after NOx present in the measured gas side electrode 203 reacts with residual oxygen) flowing in the sensor current detection resistor Rs (flowing in the sensor cell 2B). It is configured.
The upstream end of the sensor 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. The sensor cell differential amplification operational amplifier OP3 is connected to the non-inverting input terminal. On the other hand, the downstream end of the sensor current detection resistor Rs is connected to the inverting input terminal of the sensor cell differential amplification operational amplifier OP3 via the resistor R27.

Further, as shown in FIG. 3, a resistor R29 and a capacitor C7 for improving characteristics are connected in parallel between the inverting input terminal and the output terminal of the operational amplifier OP3 for differential amplification of the sensor cell. The non-inverting input terminal of the operational amplifier OP3 is connected to a predetermined offset voltage Vd (0.2 V in this example). 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 operational amplifier OP3 for sensor cell differential amplification has a size of the resistor R29 (R30) with respect to the resistor R27 (R28). The potential difference between both ends of the sensor current detection resistor Rs is output at an amplification factor determined by
The output terminal of the operational amplifier OP3 for differential amplification of the sensor cell 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. It has become.

(Detection of NOx concentration)
As shown in the figure, the output of the operational amplifier OP3 for differential amplification of the sensor cell and the output of the operational amplifier OP4 for differential amplification of the monitor cell are connected to the non-inverting input terminal of the operational amplifier OP5 for detecting NOx concentration constituting the differential amplification circuit. It is connected to the inverting input terminal. The NOx concentration detection operational amplifier OP5 calculates a difference between a voltage based on the monitor current (residual oxygen current) and a voltage based on the sensor current (NOx current), and outputs a voltage based on the difference. is there. The output terminal of the NOx concentration detection operational amplifier OP5 is connected to the A / D conversion input port IN1 of the control microcomputer 8, and the control microcomputer 8 can detect and monitor the NOx concentration in the gas G to be measured. It has become.

More specifically, as shown in FIG. 3, the output terminal of the operational amplifier OP3 for sensor cell differential amplification is connected to the non-inverting input terminal of the operational amplifier OP5 for NOx concentration detection via a resistor R32. The output terminal of the amplification operational amplifier OP4 is connected to the inverting input terminal of the operational amplifier OP5 for detecting NOx concentration via a resistor R31.
Further, a resistor R33 and a capacitor C8 for improving characteristics are connected in parallel between the inverting input terminal and the output terminal of the NOx concentration detecting operational amplifier OP5. The non-inverting input terminal of the NOx concentration detecting operational amplifier OP5 is Are connected to a predetermined offset voltage Vd (in this example, 0.2 V). 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 operational amplifier OP5 for detecting NOx concentration depends on the size of the resistor R33 (R34) relative to the resistor R31 (R32). A voltage corresponding to the NOx concentration is output at a fixed amplification factor.

(Constant current means E1-E3)
As shown in FIG. 6, the constant current means E1 to E3 in this example are current mirror circuits configured by using a pair of NPN bipolar transistors TR10 and TR11. In the current mirror circuit of this example, the collector terminal of one transistor TR10 is connected to a circuit power supply Vcc via a current limiting resistor R50, and the collector terminal of the other transistor TR11 is connected to a sensor cell differential amplification operational amplifier. It is connected to the output terminal of OP3. Further, the base terminal of one transistor TR10 is connected to the base terminal of the other transistor TR11, and the collector terminal and base terminal of one transistor TR10 are connected. The emitter terminal of one transistor TR10 and the emitter terminal of the other transistor TR11 are connected to the ground potential.

Then, the current flowing between the collector and the emitter of one transistor TR10 and the current flowing between the collector and the emitter of the other transistor TR11 are limited by the resistor R50, and the output terminal of the operational amplifier OP3 for sensor cell differential amplification Thus, a predetermined amount of current determined by the voltage of the circuit power supply Vcc and the value of the resistor R50 can be extracted.
In FIG. 6, the configuration of the constant current means E1 connected to the output terminal of the sensor cell differential amplification operational amplifier OP3 has been described. However, the configuration of the constant current means E2 connected to the monitor cell differential amplification operational amplifier OP4, and NOx The same applies to the configuration of the constant current means E3 connected to the concentration detection operational amplifier OP5.

In addition, the current mirror circuit can have various configurations such as one using a diode and a transistor and one using a MOS FET (insulated gate field effect transistor). In addition, the constant current circuit can be configured using various FETs (field effect transistors), resistors, diodes, and the like, and can be various circuits capable of flowing a predetermined amount of current.
The constant current means E1 to E3 can also be configured as a constant current circuit using a constant current diode or the like using an FET.
Further, as shown in FIG. 7, the constant current means E1 to E3 are resistors that connect the output terminals of the sensor cell differential amplification operational amplifier OP3 (monitor cell differential amplification operational amplifier OP4, NOx concentration detection operational amplifier OP5) to the ground potential. R51 etc. can also be used.

Although not shown, the constant current means E3 is provided only at the output terminal of the NOx concentration detection operational amplifier OP5, and the output terminal of the sensor cell differential amplification operational amplifier OP3 and the output terminal of the monitor cell differential amplification operational amplifier OP4 include The constant current means E1 and E2 can be omitted.
Further, instead of providing the operational amplifier OP5 for detecting NOx concentration, the difference between the output voltage from the monitor cell differential amplification operational amplifier OP4 and the output voltage from the sensor cell differential amplification operational amplifier OP3 is determined by a program in the control microcomputer 8. It can also be calculated. In this case, the constant current means E1 can be provided at the output terminal of the sensor cell differential amplification operational amplifier OP3, and the constant current means E2 can be provided at the output terminal of the monitor cell differential amplification operational amplifier OP4.

(Sweep means 62)
As shown in FIG. 2, 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 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 65)
As shown in FIG. 2, the two N-channel MOS type FETs (TR1A, B) constituting the switching element TR1 of this example are switched between a conductive state and a non-conductive state by a switching circuit 65 connected to each gate terminal. Is possible.
The switching circuit 65 receives the output signal from the control microcomputer 8 and receives the output state of the first-stage transistor TR6 that switches between the conduction state and the non-conduction state, and the output state of the first-stage transistor TR6, and turns on and off. The second stage transistor TR7 that switches to the state is used. The first-stage transistor TR6 and the second-stage transistor TR7 in this example are NPN-type bipolar transistors, and the gate terminal of the second-stage transistor TR7 is connected to the collector terminal of the first-stage transistor TR6.

  More specifically, in the switching circuit 65, the output port OUT3 of the control microcomputer 8 is connected to the base terminal of the first-stage transistor TR6 via the current-limiting resistor R40. A resistor R41 for current feedback is connected between the base terminal and the emitter terminal. A current limiting resistor R42 is provided between the circuit power source Ve and the collector terminal of the first-stage transistor TR6, and the emitter terminal of the first-stage transistor TR6 is connected to the ground potential. The resistor R42 and the collector terminal of the first stage transistor TR6 are connected to the base terminal of the second stage transistor TR7 via a current limiting resistor R43.

A resistor R45 for current feedback is connected between the base terminal and the emitter terminal of the second stage transistor TR7. A current limiting resistor R44 is provided between the battery power supply VB and the collector terminal of the second-stage transistor TR7, and the emitter terminal of the second-stage transistor TR7 is connected to the ground potential. The resistor R44 and the collector terminal of the second-stage transistor TR7 are connected to the gate terminal of the downstream MOS FET (TR1B) via a current limiting resistor R46.
The resistor R46 and the collector terminal of the second-stage transistor TR7 are connected to the ground potential by a Zener diode D3 whose anode terminal is on the ground potential side. The zener diode D3 protects the two MOS FETs (TR1A, B) from the back electromotive force.

As shown in FIG. 2, when the monitor current flowing through the monitor cell 2C is detected by the monitor current detection circuit 6, the output port of the control microcomputer 8 is in the OFF (Low) state, and one stage of the switching circuit 65 The collector-emitter of the eye transistor TR6 is nonconductive. Further, a voltage (8.4 V in this example) from the circuit power source Ve is applied to the base terminal of the second-stage transistor TR7, and the collector-emitter of the second-stage transistor TR7 is in a conductive state.
A voltage of almost 0 V is applied from the switching circuit 65 to the gate terminals of the two MOS FETs (TR1A, B) (the gate terminals of the two MOS FETs (TR1A, B) are in the low state). The two MOS FETs (TR1A, B) are in a non-conductive state.

  On the other hand, as shown in the figure, 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 the switching circuit 65 The collector-emitter of the first stage transistor TR6 becomes conductive. At this time, the voltage applied to the base terminal of the second-stage transistor TR7 becomes approximately 0V, and the collector-emitter of the second-stage transistor TR7 becomes non-conductive. A voltage (12 V in this example) from the battery power supply VB is applied from the collector terminal of the second-stage transistor TR7 to the base terminals of the two MOS FETs (TR1A, B).

As a result, the drain-source between the two MOS FETs (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 FETs (TR1A, B) can be made conductive by applying to the gate terminal of B).

(Admittance measurement circuit 7)
As shown in FIGS. 2 and 4, in the monitor cell 2C, the admittance measurement circuit 7 switches from detection of the monitor current to measurement of the element admittance, and uses the time zone during which the monitor current is not detected, to the monitor cell 2C. The element admittance is obtained by detecting a change in the alternating current flowing through the monitor cell 2C.
The element admittance signal is taken into the control microcomputer 8 from the A / D conversion input port IN3.

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.

  Further, as shown in FIG. 2, a voltage monitoring operational amplifier OP10 constituting a voltage follower is branched and connected between the source terminal S of the upstream side MOS FET (TR1A) and the admittance measurement resistor Ra. . The output terminal of the voltage monitoring operational amplifier OP10 is connected to the input port IN5 of the control microcomputer 8, and the control microcomputer 8 is configured to monitor the voltage at the downstream end of the admittance measurement resistor Ra. .

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. 8 schematically shows an operation when measuring 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 (output voltage of the voltage monitoring 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 device 1 of this example detects the NOx (nitrogen oxide) concentration in the gas G to be measured, and includes a monitor current flowing through the monitor cell 2C, a sensor current flowing through the sensor cell 2B, and a monitor current and a sensor current. The device is designed to improve the detection accuracy of the NOx concentration obtained from the difference between.
Specifically, in the gas concentration detection device 1 of this example, in the sensor cell 2B, the difference voltage between the voltage from the reference voltage source 51 and the output voltage of the sensor current detection operational amplifier OP1 (voltage from the command voltage source 52) is the sensor cell. When applied to 2B, a small sensor current of several nA to several hundred nA flows through the sensor current detection resistor Rs in the sensor current detection circuit 5, for example. Then, the differential amplification of the voltage at both ends of the sensor current detection resistor Rs is performed by the sensor cell differential amplification operational amplifier OP3, and the potential difference at both ends of the sensor current detection resistor Rs is detected.

At this time, constant current means E1 is provided at the output terminal of the operational amplifier OP3 for differential amplification of the sensor cell of the present example to flow a predetermined current to the ground potential. This constant current means E1 allows a current of a predetermined value to flow from the output stage transistors TR63 and 64 in the internal circuit of the sensor cell differential amplification operational amplifier OP3 to the ground. Thus, the collector-emitter voltage Vce of the output stage transistors TR63 and TR64 can be lowered, and the lower limit value of the voltage that can be output by the sensor cell differential amplification operational amplifier OP3 can be lowered (see FIG. 9).
In the control microcomputer 8, the sensor flowing in the sensor cell 2 B based on the output voltage of the sensor cell differential amplification operational amplifier OP 3 taken in from the A / D conversion input port IN 7 and the resistance value of the sensor current detection resistor Rs. The current can be obtained with high accuracy.

  Further, in the gas concentration detection device 1 of this example, in the monitor cell 2C, the differential voltage between the voltage from the reference voltage source 51 and the output voltage of the monitor current detection operational amplifier OP2 (voltage from the command voltage source 61) is applied to the monitor cell 2C. When this is done, a small monitor current of, for example, several nA to several hundreds of nA flows through the monitor current detection resistor Rm in the monitor current detection circuit 6. Then, the monitor cell differential amplification operational amplifier OP4 performs differential amplification of the voltage at both ends of the monitor current detection resistor Rm to detect a potential difference at both ends of the monitor current detection resistor Rm.

At this time, a constant current means E2 for supplying a predetermined value of current to the ground potential is provided at the output terminal of the monitor cell differential amplification operational amplifier OP4 of this example. This constant current means E2 allows a current of a predetermined value to flow from the output stage transistors TR63 and 64 in the internal circuit of the monitor cell differential amplification operational amplifier OP4 to the ground. As a result, the collector-emitter voltage Vce of the output stage transistors TR63 and TR64 can be lowered, and the lower limit value of the voltage that can be output by the monitor cell differential amplification operational amplifier OP4 can be lowered (see FIG. 9).
In the control microcomputer 8, the monitor flowing through the monitor cell 2 C based on the output voltage of the monitor cell differential amplification operational amplifier OP 4 taken in from the A / D conversion input port IN 2 and the resistance value of the monitor current detection resistor Rm. The current can be obtained with high accuracy.

The NOx concentration detection operational amplifier OP5 performs differential amplification of the output voltage of the monitor cell differential amplification operational amplifier OP4 and the output voltage of the sensor cell differential amplification operational amplifier OP3, and outputs a voltage based on the NOx concentration.
At this time, a constant current means E3 is provided at the output terminal of the NOx concentration detection operational amplifier OP5 of this example to flow a predetermined current to the ground potential. By this constant current means E3, a predetermined value of current can be supplied from the output stage transistors TR63 and 64 in the internal circuit of the NOx concentration detecting operational amplifier OP5 to the ground. Thus, the collector-emitter voltage Vce of the output stage transistors TR63 and TR64 can be lowered, and the lower limit value of the voltage that can be output by the NOx concentration detection operational amplifier OP5 can be lowered (see FIG. 9).
The control microcomputer 8 can accurately determine the NOx concentration in the measurement gas G based on the output voltage of the NOx concentration detection operational amplifier OP5 taken in from the A / D conversion input port IN1.

  Therefore, according to the gas concentration detection apparatus 1 of this example, the NOx obtained from the detection accuracy of the sensor current flowing through the sensor cell 2B, the detection accuracy of the monitor current flowing through the monitor cell 2C, and the difference between the currents flowing through the monitor cell 2C and the sensor cell 2B. The density detection accuracy can be improved.

  FIG. 9 simply shows an internal circuit of a general operational amplifier. In the figure, in the internal circuit of the operational amplifier, an inverting input terminal Vin− and a non-inverting input terminal Vin + are formed by the base terminals of a pair of input stage transistors TR61 that perform differential amplification. Amplifying by TR62, an output terminal Vout is formed between a resistor R61 provided at the emitter terminal of the output stage NPN transistor TR63 and a resistor R62 provided at the collector terminal of the output stage PNP transistor TR64. . The power supply of the operational amplifier is indicated by V + and V−.

  FIG. 10 is a graph showing the relationship between the input voltage (V) on the horizontal axis and the output voltage (V) on the vertical axis. As shown in the figure, when the constant current means E1 to E3 are not used, the collector-emitter voltage Vce remains in the output stage transistors TR63 and 64, and the output voltage is the lower limit of the voltage Vce. When the voltage becomes lower than the value, the voltage Vce becomes an error of the output voltage. Therefore, it is difficult to accurately output a voltage in the vicinity of 0 V according to the input voltage applied to the inverting input terminal Vin− and the non-inverting input terminal Vin +.

On the other hand, when the constant current means E1 to E3 of this example are used, the lower limit of the collector-emitter voltage Vce is lowered by drawing a predetermined amount of current from the output terminal Vout of the operational amplifier. Can do. Thereby, the lower limit value of the voltage that can be output by the operational amplifier can be lowered.
Therefore, it is possible to output a voltage closer to 0 V with high accuracy according to the input voltage applied to the inverting input terminal Vin− and the non-inverting input terminal Vin +.

The block diagram which shows schematically the whole structure of the gas concentration detection apparatus in an Example. Explanatory drawing which shows the circuit structure of the periphery of a monitor cell, a sensor current detection circuit, and a switching circuit in an Example. Explanatory drawing which shows the circuit structure of the periphery of the circuit structure of the periphery of a sensor cell, a sensor current detection circuit, a monitor cell, and a monitor current detection circuit in an Example. Explanatory drawing which shows the circuit structure of the admittance measurement circuit in an Example. Explanatory drawing which shows the circuit structure of a pump cell and a heater element in an Example. Explanatory drawing which shows the detail of a constant current means in an Example. Explanatory drawing which shows the detail of the other constant current means in an Example. Explanatory drawing which shows schematically the operation | movement at the time of measuring an element admittance in an Example. Explanatory drawing which shows schematically the internal circuit of the general operational amplifier in an Example. The graph which shows the relationship between both taking an input voltage on a horizontal axis and taking an output voltage on a vertical axis | shaft in an Example.

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 Sensor current detection circuit 51 Reference voltage source 52 Command voltage source 6 Monitor current detection Circuit 61 Command voltage source 62 Sweep means 7 Admittance measurement circuit 8 Control microcomputer OP1 Sensor current detection operational amplifier OP2 Monitor current detection operational amplifier OP3 Sensor cell differential amplification operational amplifier OP4 Monitor cell differential amplification operational amplifier OP5 NOx concentration detection operational amplifier Rs sensor Current detection resistance Rm Monitor current detection resistance Ra Admittance measurement resistance E1-E3 Constant current means

Claims (4)

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 sensor cell has a reference voltage source connected to the reference gas side electrode, and a sensor current detection circuit connected to the measured gas side electrode.
The sensor current detection circuit includes a command voltage source,
An operational amplifier for detecting a sensor current for applying a voltage from the command voltage source to the measured gas side electrode of the sensor cell;
A sensor current detection resistor provided between an output terminal of the sensor current detection operational amplifier and a measured gas side electrode of the sensor cell;
A differential amplification of the voltage at both ends of the sensor current detection resistor, and a sensor cell differential amplification operational amplifier for detecting a potential difference at both ends of the sensor current detection resistor;
A gas concentration detecting device, wherein a constant current means for flowing a current of a predetermined value to a ground potential is provided at an output terminal of the operational amplifier for differential amplification of the sensor cell. 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 has a reference voltage source connected to the reference gas side electrode and a monitor current detection circuit connected to the measured gas side electrode.
The monitor current detection circuit includes a command voltage source,
A monitor current detection operational amplifier for applying a voltage from the command voltage source to the measured gas side electrode of the monitor cell;
A monitor current detection resistor provided between the output terminal of the monitor current detection operational amplifier and the measured gas side electrode of the monitor cell;
A monitor cell differential amplification operational amplifier for performing differential amplification of the voltage at both ends of the monitor current detection resistor and detecting a potential difference at both ends of the monitor current detection resistor;
A gas concentration detecting device, wherein a constant current means for supplying a current of a predetermined value to a ground potential is provided at an output terminal of the operational amplifier for differential amplification of the monitor cell. 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 sensor cell has a reference voltage source connected to the reference gas side electrode, and a sensor current detection circuit connected to the measured gas side electrode.
The sensor current detection circuit includes a command voltage source,
An operational amplifier for detecting a sensor current for applying a voltage from the command voltage source to the measured gas side electrode of the sensor cell;
A sensor current detection resistor provided between an output terminal of the sensor current detection operational amplifier and a measured gas side electrode of the sensor cell;
A differential amplification of the voltage at both ends of the sensor current detection resistor, and a sensor cell differential amplification operational amplifier for detecting a potential difference at both ends of the sensor current detection resistor;
The monitor cell has a reference voltage source connected to the reference gas side electrode and a monitor current detection circuit connected to the measured gas side electrode.
The monitor current detection circuit includes a command voltage source,
A monitor current detection operational amplifier for applying a voltage from the command voltage source to the measured gas side electrode of the monitor cell;
A monitor current detection resistor provided between the output terminal of the monitor current detection operational amplifier and the measured gas side electrode of the monitor cell;
A monitor cell differential amplification operational amplifier for performing differential amplification of the voltage at both ends of the monitor current detection resistor and detecting a potential difference at both ends of the monitor current detection resistor;
The sensor current detection circuit performs differential amplification between the output voltage of the monitor cell differential amplification operational amplifier and the output voltage of the sensor cell differential amplification operational amplifier, and outputs a voltage based on the NOx concentration. Has an operational amplifier,
A gas concentration detecting device, characterized in that a constant current means for supplying a current of a predetermined value to a ground potential is provided at an output terminal of the operational amplifier for NOx concentration detection.   4. The gas concentration detection device according to claim 1, wherein the constant current means is configured using a constant current circuit or a resistor.

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