JPH10288595A - Nitrogen oxide concentration detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitrogen oxide concentration detector capable of efficiently controlling sensor temperature with a simple structure and capable of accurately detecting NOx concentration even if the sensor temperature is changed. SOLUTION: This NOx sensor 2 is provided with the first measuring chamber 20 communicated with the measured gas side via a diffusion rate controlling layer 4d and the second measuring chamber 26 communicated with the first measuring chamber 20 via diffusion limiting layers 6d, 22d. When the first pump current IP1 is controlled so that the output of a Vs cell 6 becomes the reference voltage VC0, the oxygen concentration of the measured gas flowing into the second measuring chamber 26 from the first measuring chamber 20 is controlled constant. A constant voltage is applied to the second pump cell 8, the NOx component in the second measuring chamber 26 is decomposed to pump out oxygen, and the NOx concentration is detected based on the second pump current IP2. The sensor temperature is detected from the internal resistance of the Vs cell 6, and the excitation current quantities of heaters 12, 14 are controlled. When the temperature of the measured gas is abruptly changed, the sensor temperature is changed, and the detected second pump current IP2 is corrected in response to the drift of the detected sensor temperature from the target value. The NOx concentration can be detected with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitrogen oxide concentration detecting device for detecting the concentration of nitrogen oxide, which is a harmful component, discharged from various combustion devices such as internal combustion engines.

[0002]

2. Description of the Related Art Conventionally, as a nitrogen oxide concentration detector, for example, European Patent Application Publication No. 06787
40A1, SAE paper No. 960334
P137-142 1996 and the like, a first measurement chamber communicated with the measured gas side via a first diffusion-controlling layer, and a communication with the first measurement chamber via a second diffusion-controlling layer. The second measurement chamber formed is formed of an oxygen ion-conductive solid electrolyte layer, and the first measurement chamber is provided with a first oxygen pumping cell and an oxygen concentration measurement by sandwiching the solid electrolyte layer between porous electrodes. A second oxygen pumping cell by sandwiching the solid electrolyte layer with a porous electrode in the second measurement chamber, and using a sensor constituted by forming a second oxygen pumping cell. There is known an apparatus for detecting the concentration of nitrogen oxides (NOx) in exhaust gas.

In this type of nitrogen oxide concentration detecting device, the first oxygen pumping cell is energized so that the output voltage from the oxygen concentration measuring cell becomes a predetermined constant value, and the oxygen is supplied from the first measuring chamber to the first oxygen pumping cell. By pumping the oxygen into the second oxygen pumping cell while controlling the oxygen concentration in the first measurement chamber (in other words, the oxygen concentration of the gas to be measured flowing from the first measurement chamber into the second measurement chamber) to a constant concentration. A constant voltage is applied to pump more oxygen from the second measurement chamber. Then, the NOx concentration in the gas to be measured is detected from the current value flowing through the second oxygen pumping cell.

That is, in the exhaust gas from the internal combustion engine or the like, which is the gas to be measured, other gas components such as oxygen, carbon monoxide, and carbon dioxide exist in addition to NOx. Then, a current is passed through the first oxygen pumping cell to extract most of the oxygen in the first measurement chamber, and further,
By applying a constant voltage to the second oxygen pumping cell in the direction of pumping oxygen in the second measurement chamber on the side of the second measurement chamber into which the gas to be measured from which the oxygen has been extracted flows,
By the catalytic function of the porous electrode constituting the oxygen pumping cell, NOx in the gas to be measured is decomposed into nitrogen and oxygen and oxygen is extracted from the second measuring chamber. At that time, the pump current flowing through the second oxygen pumping cell is measured. By detecting, the NOx concentration in the measured gas can be detected without being affected by other gas components in the measured gas.

In this type of nitrogen oxide detector,
In order to accurately detect the NOx concentration by the above detection method, it is necessary to heat the sensor to a predetermined activation temperature (for example, 800 ° C. or higher) to activate each cell.
A heater for heating the sensor is separately provided.

As a method of controlling the sensor to a predetermined temperature by using a heater as described above, an oxygen sensor for detecting the oxygen concentration in exhaust gas by an oxygen concentration measuring cell in which a solid electrolyte layer is sandwiched between porous electrodes is used. It is conceivable to use a generally used control method.

That is, as a method of controlling the temperature of the sensor, for example, as disclosed in Japanese Patent Application Laid-Open Nos. 59-163556 and 59-214756,
A method of controlling the amount of current supplied to the heater so that the amount of heat generated by the heater is constant, a method of controlling the amount of power supplied to the heater by a preset control pattern so that the temperature of the sensor reaches the target temperature, and detecting the sensor temperature. Various methods are known, such as a method of controlling the amount of power supplied to the heater by using a conventional method. It is conceivable that such a conventional control method is used in a nitrogen oxide concentration detection device.

[0008]

However, unlike the oxygen sensor, the nitrogen oxide concentration detecting device is provided with three cells in the sensor, and further, a heater is provided for each cell to reduce the temperature. Since it is difficult to control, it has been a problem how to apply the above-mentioned conventional control method.

In particular, in the nitrogen oxide concentration detecting device, when the sensor temperature changes, the oxygen concentration of the gas to be measured flowing from the first measuring chamber to the second measuring chamber controlled by energizing the first oxygen pumping cell, Since the oxygen concentration in the second measurement chamber changes, and the detection result of the NOx concentration greatly changes, in order to secure the detection accuracy of the NOx concentration, the temperature of the sensor changes from the first measurement chamber to the second measurement chamber. It is required that the temperature control be performed efficiently with a simple structure so that the oxygen concentration of the gas to be measured flowing into the measurement chamber does not change. A temperature control method to obtain the temperature has not been established, and a nitrogen oxide concentration detection device having a simple structure and ensuring temperature characteristics has not been realized.

Further, as described above, in the nitrogen oxide concentration detecting device, the detection result greatly changes due to the change in the sensor temperature, and the detection result may be affected by the temperature change of the gas to be measured. Even if the temperature of the sensor can be accurately controlled, there is a problem that the detection result temporarily fluctuates during a transition when the temperature of the gas to be measured changes.

The present invention has been made in view of such a problem, and in the above-mentioned nitrogen oxide concentration detecting device, the sensor temperature is efficiently adjusted with a simple configuration so that the detection result of the NOx concentration does not change. The first object of the present invention is to improve the detection accuracy of the NOx concentration by correcting a detection error of the NOx concentration due to a temperature change of the gas to be measured, which cannot be completely controlled by the temperature control. This is the purpose of 2.

[0012]

Means for Solving the Problems In order to achieve the first object, the invention according to the first aspect is directed to a method of forming a first oxygen solution comprising an oxygen ion conductive solid electrolyte layer sandwiched between porous electrodes. A first measurement chamber having a pumping cell and an oxygen concentration measurement cell, which is communicated to the measured gas side via a first diffusion-controlling layer, and an oxygen ion-conductive solid electrolyte layer sandwiched between porous electrodes A sensor main body including a second oxygen pumping cell, a second measurement chamber communicated with the first measurement chamber via a second diffusion-controlling layer, and an output voltage of the oxygen concentration measurement cell being constant. The first oxygen pumping cell pumps oxygen from the first measurement chamber so that
Pump current control means for controlling the oxygen concentration of the gas to be measured flowing from the measurement chamber into the second measurement chamber at a constant level;
Constant voltage applying means for applying a constant voltage to the oxygen pumping cell in a direction of pumping oxygen from the second measurement chamber; and applying a constant voltage to the oxygen pumping cell based on a current value flowing through the second oxygen pumping cell. A nitrogen oxide concentration detecting device comprising: a nitrogen oxide concentration detecting means for detecting a nitrogen oxide concentration; and a heater for heating the sensor body to a temperature at which the nitrogen oxide concentration can be detected. Temperature detection means for detecting the temperature of the measurement cell, and heater energization for controlling the energization of the heater such that the temperature of the oxygen concentration measurement cell detected by the temperature detection means becomes a preset target temperature. And control means.

According to a second aspect of the present invention, which has been made to achieve the second object, in the nitrogen oxide concentration detecting apparatus according to the first aspect, the oxygen concentration detected by the temperature detecting means is provided. Correction means for correcting the detected value of the nitrogen oxide concentration by correcting the nitrogen oxide concentration detected by the nitrogen oxide concentration detecting means in accordance with the deviation of the temperature of the measuring cell from the target temperature. Is provided.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The nitrogen oxide concentration detecting device according to the first aspect of the present invention has an oxygen ion conductive solid electrolyte layer sandwiched between porous electrodes, similarly to the above-described conventional nitrogen oxide concentration detecting device. A first measurement chamber, which has a first oxygen pumping cell and an oxygen concentration measurement cell, and is connected to the gas to be measured via a first diffusion-controlling layer, and a porous electrode comprising an oxygen ion-conductive solid electrolyte layer. And a second measurement chamber connected to the first measurement chamber via a second diffusion-controlling layer, using a sensor body having a second oxygen pumping cell sandwiched between The NOx concentration is detected, and pump current control means pumps oxygen from the first measurement chamber with the first oxygen pumping cell so that the output voltage of the oxygen concentration measurement cell becomes a constant value, and the first measurement chamber Gas to be measured flowing into the second measurement chamber Controlling the oxygen concentration constant. Then, the constant voltage applying means applies a constant voltage to the second oxygen pumping cell in a direction of pumping oxygen from the second measurement chamber, and the nitrogen oxide concentration detecting means applies the second oxygen pumping cell by applying the constant voltage. The NOx concentration in the gas to be measured is detected based on the current value flowing through the cell.

Further, the nitrogen oxide concentration detecting apparatus of the present invention is provided with a heater for heating the sensor body to a temperature at which the NOx concentration can be detected, similarly to the conventional apparatus. The heater energization control means controls the temperature of the oxygen concentration measurement cell detected by the temperature detection means so as to reach a preset target temperature.

Therefore, according to the nitrogen oxide concentration detecting apparatus of the present invention, the oxygen concentration measuring cell has the largest influence on the NOx concentration detection accuracy (specifically, the first concentration).
The oxygen concentration of the gas to be measured flowing from the measurement chamber to the second measurement chamber can be accurately detected. Further, when the temperature of the oxygen concentration measuring cell is controlled to the target temperature by the heater as in the present invention, the temperatures of the first oxygen pumping cell and the second oxygen pumping cell can be controlled to be substantially constant. As a result,
According to the present invention, the second current from the first measurement chamber is controlled by controlling the energization of the first oxygen pumping cell by the pump current control means.
The oxygen concentration of the gas to be measured flowing into the measurement chamber can be controlled to a predetermined constant concentration, and the NOx concentration in the gas to be measured can be accurately detected from the current flowing through the second oxygen pumping cell.

As described above, in the nitrogen oxide concentration detecting apparatus of the present invention, of the three types of cells constituting the sensor main body, the oxygen concentration (specifically, the first measurement) that most affects the detection accuracy of the NOx concentration. By controlling the amount of current supplied to the heater that heats the sensor body, the temperature of the oxygen concentration measurement cell that detects the oxygen concentration of the gas to be measured flowing from the chamber into the second measurement chamber) reaches the target temperature. Temperature detection means for detecting the temperature of the oxygen concentration measurement cell and a heater for controlling the energization of the heater according to the detection result so that the NOx concentration can be accurately detected without being affected by the temperature change of the sensor body. Since the temperature control system of the sensor body can be configured simply by providing the power supply control means, the nitrogen oxide concentration detection device can efficiently control the temperature of the sensor body with a simple configuration. It can be provided.

When measuring the oxygen concentration of the gas to be measured flowing from the first measurement chamber to the second measurement chamber, it is necessary to pay attention to the arrangement of the electrodes of the oxygen concentration measurement cell. That is, the first measurement is performed by arranging the electrode on the side in contact with the gas to be measured (in other words, the electrode on the first measurement chamber side) among the electrodes of the oxygen concentration measurement cell as close to the second diffusion-controlling layer as possible. The oxygen concentration of the gas to be measured flowing from the chamber into the second measurement chamber can be accurately measured.

According to a second aspect of the present invention, in the nitrogen oxide concentration detecting apparatus, the correcting means detects the nitrogen oxide concentration in accordance with a deviation of the temperature of the concentration measuring cell from the target temperature detected by the temperature detecting means. The nitrogen oxide concentration detected by the substance concentration detecting means is corrected. Therefore, according to the present invention (claim 2), when the temperature of the sensor body cannot be controlled to reach the target temperature even though the temperature control is performed by the heater energization control means, the correction means By N
The detection result of the Ox concentration can be temperature compensated and NOx
The detection accuracy of the concentration can be further improved.

That is, for example, when the temperature of the gas to be measured suddenly changes, the temperature of the sensor main body cannot be completely controlled by the temperature control by the heater energization control means. Although the temperature may temporarily change, according to the present invention, even in such a case, the NOx concentration can be accurately detected, and N
This makes it possible to further improve the detection accuracy of the Ox concentration.

Here, the temperature detecting means for detecting the temperature of the oxygen concentration measuring cell can be realized, for example, by providing a temperature detecting element near the oxygen concentration measuring cell. Becomes complicated, and it is also difficult to accurately detect the temperature of the oxygen concentration measuring cell itself.

Therefore, the temperature detecting means is configured to detect the internal resistance of the oxygen concentration measuring cell, and the heater energization control means detects the internal resistance of the detected oxygen concentration measuring cell. It is desirable to control the energization of the heater so that the resistance has a predetermined value corresponding to the target temperature.

That is, the internal resistance of the oxygen concentration measuring cell is
Since the internal resistance changes according to the temperature of the oxygen concentration measuring cell (the internal resistance decreases as the temperature increases), if the internal resistance of the oxygen concentration measuring cell is detected as described in claim 3, From the detected internal resistance, the temperature of the oxygen concentration measuring cell can be accurately detected without separately providing a temperature detecting element in the sensor main body, so that the temperature control of the sensor main body can be performed more easily and more easily. It can be executed with high accuracy.

When the internal resistance of the oxygen concentration measuring cell is detected by the temperature detecting means, a constant voltage for detecting the internal resistance is applied to the oxygen concentration measuring cell. At this time, the amount of current flowing through the oxygen concentration measurement cell may be detected, or a constant current for detecting internal resistance may be passed through the oxygen concentration measurement cell, and the voltage across the oxygen concentration measurement cell may be detected at that time. .

However, when the internal resistance of the oxygen concentration measuring cell is detected as described above, the connection between the pump current control means and the oxygen concentration measuring cell is temporarily interrupted, and the first operation by the pump current control means is performed. It is necessary to stop the energization control of the oxygen pumping cell. That is, when the oxygen concentration measuring cell is energized to detect the internal resistance, the voltage across the cell has a value that does not correspond to the oxygen concentration of the gas to be measured flowing from the first measuring chamber to the second measuring chamber. If the control operation of the control means is continued, the oxygen concentration of the gas to be measured flowing from the first measurement chamber to the second measurement chamber is erroneously controlled. Therefore, when the internal resistance of the oxygen concentration measurement cell is detected, such erroneous control is performed. It is desirable to stop the control operation by the pump current control means so as not to cause the above.

Next, the oxygen concentration measuring cell is for detecting the oxygen concentration of the gas to be measured flowing from the first measuring chamber into the second measuring chamber, and comprises a pair of porous electrodes constituting the cell. Among them, the oxygen concentration on the electrode side not in contact with the gas to be measured needs to be constant. For this purpose, for example, a reference gas (for example, air) having a constant oxygen concentration may be introduced to the electrode side.
In order to introduce the reference gas from the outside in this way, a gap for introducing the reference gas must be provided in the sensor body,
The structure of the sensor body becomes complicated.

Therefore, in order to set the porous electrode on the side opposite to the first measurement chamber of the oxygen concentration measurement cell (that is, the side not in contact with the gas to be measured) to the reference oxygen concentration, as described in claim 4, In the sensor main body, the porous electrode on the side opposite to the first measurement chamber of the oxygen concentration measurement cell is closed, and a part of oxygen in the closed space is formed so as to be able to leak outside through the leak resistance portion, On the pump current control means side, a minute current is supplied to the oxygen concentration measurement cell in a direction to pump oxygen in the first measurement chamber into the closed space, and the closed space functions as an internal oxygen reference source while the oxygen concentration is measured. The amount of current flowing through the first oxygen pumping cell may be controlled so that the electromotive force generated in the measurement cell has a constant value. That is, with this configuration, it is not necessary to provide a gap for introducing the reference gas in the sensor main body, and the structure of the sensor main body can be simplified.

In this case, in order to measure the internal resistance of the oxygen concentration measuring cell by means of the temperature detecting means, the connection between the pump current control means and the oxygen concentration measuring cell is periodically made. When the current is cut off, a current for detecting the internal resistance, which is larger than the minute current for generating the internal oxygen reference source, flows in the oxygen concentration measuring cell in the opposite direction to the minute current. It is desirable to configure the temperature detecting means so as to detect the internal resistance of the oxygen concentration measuring cell from the voltage generated therebetween.

In other words, the current for detecting the internal resistance can flow in the same direction as the minute current. However, in order to suppress the deterioration of the electrode, the current is flowed in the opposite direction to the normal direction to periodically activate the electrode. This can have the effect of causing When a current is passed through the oxygen concentration measurement cell, the voltage (inter-electrode voltage) generated by the oxygen concentration measurement cell depends on not only the internal resistance of the oxygen concentration measurement cell but also the ratio of the oxygen concentration on each electrode side. Although it varies depending on the generated electromotive force, the oxygen concentration on each electrode side of the oxygen concentration measurement cell (that is, the oxygen concentration of the gas to be measured flowing from the first measurement chamber to the second measurement chamber and the oxygen concentration in the closed space) is , Respectively, are substantially constant by the control of the supply of the minute current and the control of the supply of current to the first oxygen pumping cell by the pump current control means, so that the electromotive force during the supply of the current for detecting the internal resistance is approximately constant, and According to the present invention, the back electromotive force generated by the internal resistance detection current is relatively large with respect to the change in the electromotive force of the oxygen concentration measurement cell. C It can detect the internal resistance of.

As described above, when a current for detecting internal resistance is supplied to the oxygen concentration measuring cell in order to detect the internal resistance of the oxygen concentration measuring cell, the oxygen concentration measuring cell is connected to the solid electrolyte or the solid electrolyte. Since the direction of polarization changes between the electrodes, the electromotive force can be generated immediately according to the ratio between the oxygen concentration in the first measurement chamber and the oxygen concentration in the closed space, as before the measurement. However, the electromotive force is slightly out of value. Therefore, it takes some time for the polarization to be eliminated and for the oxygen concentration measurement cell to be able to accurately detect the oxygen concentration of the gas to be measured flowing from the first measurement chamber to the second measurement chamber, and the internal resistance is to be reduced. Even if the control operation of the pump current control means is restarted immediately after the detection, the NOx concentration cannot be accurately detected.

After detecting the internal resistance, the time until the NOx concentration can be accurately detected (detection stop time)
As described in claim 5, in order to shorten the internal resistance detection current, an internal resistance detection current is caused to flow through the oxygen concentration measurement cell to detect the internal resistance, and then the internal resistance detection current is caused to flow in the reverse direction. What is necessary is just to comprise a temperature detection means.

That is, at the time of detecting the internal resistance of the oxygen concentration measuring cell, if the current for detecting the internal resistance is alternately applied to the oxygen concentration measuring cell once in a pulsed manner, the electrode side of each electrode of the oxygen concentration measuring cell can be detected. The oxygen concentration will quickly return to the stable state before the detection of the internal resistance.
The time until the Ox concentration can be accurately detected can be shortened.

Next, in the present invention, of the three types of cells constituting the sensor main body, the oxygen concentration (that is, the first measurement chamber to the second
The temperature of the oxygen concentration measurement cell for detecting the oxygen concentration of the gas to be measured flowing into the measurement chamber) is detected as the temperature of the sensor main body, and the current supplied to the heater is controlled. It is also conceivable that the temperatures of the first oxygen pumping cell and the second oxygen pumping cell cannot be controlled near the target temperature.

Therefore, in order to better obtain the effect of the temperature control of the present invention, the first oxygen pumping cell, the oxygen concentration measuring cell, and the second The oxygen pumping cells are
The first measurement chamber and the second measurement chamber are formed in different thin plate-like solid electrolyte layers, and the first measurement chamber and the second measurement chamber are arranged such that the solid electrolyte layers in which the first and second oxygen pumping cells are formed are located outside. The heater is formed by laminating through a gap.
The sensor body is configured to be heatable by arranging each heater substrate on both sides of each solid electrolyte layer of the sensor body in the stacking direction with a predetermined gap therebetween, and further comprising a first diffusion layer. Is preferably formed in a portion of the solid electrolyte layer in which the first oxygen pumping cell is formed, including a position facing a central portion of a heater wiring formed on the heater substrate.

That is, if the sensor body and the heater are formed as described above, the solid electrolyte layer in which the oxygen concentration measuring cell is formed is replaced with the solid electrolyte layer in which the first oxygen pumping cell and the second oxygen pumping cell are formed. Since the heater substrates are disposed on both sides in the stacking direction, the first oxygen pumping cell and the second oxygen pumping cell are controlled by controlling the temperature of the oxygen concentration measuring cell to the target temperature by controlling the power supply to the heater. Can be more reliably controlled to the target temperature, and the gas to be measured flowing into the first measurement chamber from the first diffusion layer can be sufficiently heated by the heater.

As a result, according to the present invention (claim 6),
The temperature variation of each cell in the sensor body hardly occurs, and each cell is hardly affected by the temperature of the gas to be measured, so that the detection accuracy of the NOx concentration can be further improved. In this case, as described in claim 7,
The second diffusion-controlling layer is formed so as to overlap at least a part of the first diffusion-controlling layer when the sensor body is projected from the stacking direction of each solid electrolyte layer, and in the vicinity of the second diffusion-controlling layer,
If the oxygen concentration measurement cell is arranged, the temperature of the sensor main body and the gas to be measured in the sensor main body can be reliably controlled by the target temperature, and the detection accuracy of the NOx concentration can be improved.

[0037]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing the entire configuration of a nitrogen oxide concentration detection device according to an embodiment to which the present invention is applied, and FIG. 2 is a NOx sensor 2 used in the nitrogen oxide concentration detection device.
FIG. 3 is an exploded perspective view of FIG.

As shown in FIG. 1, the nitrogen oxide concentration detecting device comprises a NOx sensor 2 as a sensor body, a first oxygen pumping cell (hereinafter referred to as a first pump cell) 4 constituting the NOx sensor 2, and an oxygen concentration measuring device. Cell (hereinafter, Vs
A drive circuit 40 for energizing the cell 6 and switching the energization path, and a second oxygen pumping cell (hereinafter, referred to as a second pump cell) constituting the NOx sensor 2
8 and a current flowing at that time (hereinafter referred to as a second
A detection circuit 42 for detecting IP2 (called a pump current);
A heater energizing circuit 4 for energizing a pair of heaters 12 and 14 provided in the Ox sensor 2 to heat the cells 4, 6 and 8.
4, an electronic control circuit (hereinafter referred to as ECU) comprising a microcomputer for controlling the drive of the drive circuit 40 and the heater energizing circuit 44 and calculating the NOx concentration in the gas to be measured based on the detection signal VIP2 from the detection circuit 42. )
50.

As shown in FIG. 2, in the NOx sensor 2, the first pump cell 4 has rectangular porous electrodes 4b and 4c on both sides of a solid electrolyte layer 4a formed in a plate shape.
And the lead portions 4bl and 4cl thereof, and further, a round hole is formed in the solid electrolyte layer 4a so as to penetrate the central portion of the porous electrodes 4b and 4c, and a porous filler is filled in the round hole. By filling, the diffusion-controlling layer 4d is formed.

The Vs cell 6 is provided on both sides of the solid electrolyte layer 6a having the same shape as the solid electrolyte layer 4a of the first pump cell 4.
Circular porous electrodes 6b, 6c and their lead portions 6bl, 6cl are formed, respectively, and further, a round hole is formed in the solid electrolyte layer 6a so as to penetrate through the center of the porous electrodes 6b, 6c. By filling the round hole with a porous filler,
This is one in which a diffusion-controlling layer 6d is formed.

Then, the porous electrode 6 of the Vs cell 6
b, 6c and the porous electrodes 4b, 4c of the first pump cell 4 have substantially the same center positions on the solid electrolyte layers 4a, 6a, and when the Vs cell 6 and the first pump cell 4 are stacked, each diffusion The rate limiting layers 6d and 4d are configured to face each other. The circular porous electrodes 6b and 6c formed in the Vs cell 6 are smaller than the rectangular porous electrodes 4b and 4c formed in the first pump cell 4. Also,
On the front and back surfaces of the Vs cell 6, an insulating film made of alumina or the like is formed so as to cover the lead portions 6bl and 6cl from the outside in order to prevent current leakage from the lead portions 6bl and 6cl. Between the portions 6bl and 6cl, there is formed a leakage resistance portion 6f that allows a part of oxygen pumped into the porous electrode 6c to leak to the porous electrode 6b by the energization control described later.

The first pump cell 4 and the Vs cell 6 formed in this manner are stacked via the solid electrolyte layers 18 having the same shape as the solid electrolyte layers 4a and 6a. A rectangular hole larger than the porous electrode 4c is formed in the solid electrolyte layer 18 at a position facing each of the porous electrodes 4c and 6b, and the hole functions as the first measurement chamber 20. I do.

Also, the porous electrode 6c side of the Vs cell 6
The solid electrolyte layers 22 having the same shape as the solid electrolyte layers 4a and 6a are stacked. The solid electrolyte layer 22 is provided with a round hole of the same size at the same position as the diffusion controlling layer 6d of the Vs cell 6, and the porous hole is filled in the round hole to form a diffusion controlling layer. 22d are formed.

On the other hand, similarly to the first pump cell 4, the second pump cell 8 has rectangular porous electrodes 8b and 8c and lead portions 8bl and 8cl on both sides of the solid electrolyte layer 8a formed in a plate shape. Is formed. Then, the second pump cell 8 is stacked on the solid electrolyte layer 22 stacked on the Vs cell 6 via a solid electrolyte layer 24 formed exactly like the solid electrolyte layer 18. As a result, the rectangular hole formed in the solid electrolyte layer 24 functions as the second measurement chamber 26.

Further, on both sides of the stacked body of the first pump cell 4, the Vs cell 6, and the second pump cell 8 stacked in this manner, that is, outside the first pump cell 4 and the second pump cell 8, the spacers 28, 29, the heaters 12, 14 are stacked at a predetermined interval.

The heaters 12, 14 have the same shape as the solid electrolyte layers 4a, 6a,.
c, 14a, 14c, and between the heater substrates 12a and 12c and between the heater substrates 14a and 14c, respectively.
Heater wiring 1 buried in each heater substrate
2b, 14b and their lead portions 12bl, 14bl, the spacers 28, 29
Porous electrode 4 of first pump cell 4 and second pump cell 8
b and 8c are arranged between the heaters 12 and 14 and the first pump cell 4 and the second pump cell 8 so as to face each other with a gap therebetween.

Here, each of the solid electrolyte layers 4a, 6a,
As a solid electrolyte constituting ..., a solid solution of zirconia and yttria or a solid solution of zirconia and calcia is typical. In addition, a solid solution of hafnia, a perovskite-type oxide solid solution, and a trivalent metal oxide solid solution are also used. it can. For the porous electrodes provided on the surfaces of the solid electrolyte layers 4a, 6a, 8a, it is preferable to use platinum, rhodium, or an alloy thereof having a catalytic function. As a forming method, for example, a thick film forming method in which a mixture of platinum powder and a powder of the same material as that of the solid electrolyte layer is made into a paste, screen-printed on the solid electrolyte layer, and then sintered, There is known a method of forming a film by using the method described above. Further, it is preferable that the diffusion controlling layers 4d, 6d, and 22d use ceramics having a small through hole or porous ceramics.

On the other hand, the heater wiring 12 of the heaters 12 and 14
b and 14b are made of a composite material of ceramics and platinum or a platinum alloy, and the lead portions 12bl and 14bl are preferably made of platinum or a platinum alloy in order to reduce the resistance value and reduce the electric loss in the lead portion. preferable. Also, the heater substrate 1
2a, 12b, 14a, 14c and spacers 28, 29
For example, alumina, spinel, forsterite, steatite, zirconia, or the like can be used.

In particular, when zirconia is used as the material of the heater substrate and the spacer, the heater and each pump cell can be simultaneously integrated and sintered, so that NO
This is suitable for producing the x sensor 2. In this case, an insulating layer is provided between the heater wiring 12b and its lead portion 12bl and the heater substrates 12a and 12c, and between the heater wiring 14b and its lead portion 14bl and the heater substrates 14a and 14c, respectively. (Made of alumina or the like).

When alumina is used for the heater substrate, a porous body is used as a spacer in order to prevent the occurrence of cracks due to a difference in contraction rate and a difference in thermal expansion coefficient during sintering with each pump cell. Good to use. In addition, the heater and the pump cells can be separately sintered and then joined by using an inorganic material such as cement as a joining material also serving as a spacer.

Next, as shown in FIG.
Of the first pump cell 4 and the Vs cell 6 on the first measurement chamber 20 side are grounded via the resistor R1, and the other porous electrodes 4b and 6c are connected to the drive circuit 40. It is connected. The drive circuit 40 has one end to which a constant voltage VCP is applied, and the other end to which V.sub.
a resistor R2 connected to the porous electrode 6c of the s cell 6,
The Vs cell 6 is connected to the negative input terminal via the open / close switch SW1.
Is connected to one end of the capacitor Cp, the reference voltage VCO is applied to the + input terminal, and the output terminal is connected to the porous electrode 4 of the first pump cell 4 via the resistor R0.
b, a differential amplifier AMP connected to
0a. The other end of the capacitor Cp is grounded.

The control unit 40a includes an open / close switch SW1
Operates as follows when is in the ON state. First, oxygen in the first measurement chamber 20 is pumped into the porous electrode 6c side of the Vs cell 6 by flowing a constant minute current iCP to the Vs cell 6 via the resistor R2. This porous electrode 6c is
The closed space in the porous electrode 6c has a constant oxygen concentration due to the passage of the minute current iCP because it is closed by the solid electrolyte layer 22 and communicates with the porous electrode 6b through the leak resistance portion 6f. , Functions as an internal oxygen reference source.

As described above, the porous electrode 6 of the Vs cell 6
When the c-side functions as the internal oxygen reference source, the Vs cell 6 stores the oxygen concentration near the diffusion-controlling layer 6d in the first measurement chamber 20 (in other words, the second oxygen concentration from the first measurement chamber 20 via the diffusion-controlling layer 6d). An electromotive force is generated in accordance with the ratio between the oxygen concentration of the gas to be measured flowing into the measurement chamber 26) and the oxygen concentration on the internal oxygen reference source side, and the voltage Vs on the porous electrode 6c side is changed from the first measurement chamber 20 to the second 2 The voltage is in accordance with the oxygen concentration of the gas to be measured flowing into the measurement chamber 26. And this voltage is the differential amplifier AMP
, A voltage corresponding to the difference between the reference voltage VCO and the input voltage (VCO−input voltage) is output from the differential amplifier AMP, and this output voltage is output via the resistor R0. The voltage is applied to the porous electrode 4b of one pump cell 4.

As a result, a current (hereinafter, referred to as a first pump current) IP1 flows through the first pump cell 4, and the first pump cell 4
The pump current IP1 causes the electromotive force generated in the Vs cell 6 to be a constant voltage (in other words, the oxygen concentration of the gas to be measured flowing from the first measurement chamber 20 to the second measurement chamber 26 is constant). ) Is controlled.

In other words, the control section 40a functions as the pump current control means of the present invention, and controls the oxygen concentration of the gas to be measured flowing from the first measurement chamber 20 to the second measurement chamber 26 to a constant concentration. Control for pumping out oxygen from the first measurement chamber 20 to the outside is executed. It should be noted that the first controlled in this manner is
The oxygen concentration of the gas to be measured flowing from the measurement chamber 20 to the second measurement chamber 26 does not decompose the NOx component in the gas to be measured in the first measurement chamber 20 by applying the first pump current IP1. Thus, the oxygen concentration is set so as to be a low oxygen concentration (for example, about 100 ppm) where a little oxygen exists, and the reference voltage VCO for determining the oxygen concentration is 100 mV to 100 mV.
A value of about 200 mV is set. Also, the differential amplifier A
A resistor R0 provided between the output of the MP and the porous electrode 4b is for detecting the first pump current IP1, and the voltage VIP1 across the resistor is provided as a detection signal of the first pump current IP1 by the ECU 50. Is input to

Next, the drive circuit 40 includes the control unit 40
a constant current circuit 40b connected to the porous electrode 6c of the Vs cell 6 via the open / close switch SW2 and flowing a constant current between the porous electrodes 6b-6c in a direction opposite to the minute current iCP; A constant current circuit 40 which is connected to the porous electrode 6c of the Vs cell 6 via the open / close switch SW3, and allows a constant current to flow between the porous electrodes 6b-6c in the same direction as the minute current iCP.
c is provided.

Each of these constant current circuits 40b, 40c
This is for detecting the internal resistance RVS of the s cell 6.
Then, the voltage Vs on the porous electrode 6c side is input to the ECU 50 so that the internal resistance RVS of the Vs cell 6 can be detected by the ECU 50 by applying the constant current. The constant currents flowing through the constant current circuits 40b and 40c are set to the same current value except for the direction of the current. This current value is larger than the minute current iCP supplied to the Vs cell 6 via the resistor R2.

The open / close switches SW1 to SW3 provided between the control unit 40a, the constant current circuits 40b and 40c, and the porous electrode 6c of the Vs cell 6, respectively, are connected to the ECU 50.
In the normal state where the operation is performed by the control signal from the controller and the detection operation of the NOx concentration is performed, only the opening / closing switch SW1 is turned on and the control unit 40a operates to detect the internal resistance RVS of the Vs cell 6. , Open / close switch SW
1 is turned off, and the open / close switches SW2 and SW3 are sequentially turned on.

On the other hand, the second pump cell 8 of the NOx sensor 2
A constant voltage VP2 is applied between the porous electrodes 8b and 8c via a resistor R3 as a constant voltage applying means constituting the detection circuit 42. The application direction of this constant voltage VP2 is
In the second pump cell 8, the porous electrode 8c side is a positive electrode and the porous electrode 8 is so configured that an electric current flows from the porous electrode 8c to the 8b side and oxygen in the second measurement chamber 26 is pumped out.
The b side is set to be the negative electrode. The constant voltage VP2 is supplied from the first measurement chamber 20 to the diffusion-controlling layers 6d and 22d.
The voltage is set to a voltage that can decompose the NOx component in the gas to be measured in the second measurement chamber flowing through d and pump out the oxygen component, for example, 450 mV.

The resistor R3 applies a second pump current I through the second pump cell 8 by applying the constant voltage VP2.
This is for converting P2 into a voltage VIP2 and inputting it to the ECU 50 as a detection signal of the second pump current IP2. In the nitrogen oxide concentration detection device of this embodiment configured as described above, if the open / close switch SW1 in the drive circuit 40 is turned on and the open / close switches SW2 and SW3 are turned off, the operation of the control unit 40a causes Since the oxygen concentration of the gas to be measured flowing from the first measurement chamber 20 to the second measurement chamber 26 is controlled to a constant oxygen concentration, the second pump current IP2 flowing through the second pump cell 8 is determined by the oxygen concentration in the gas to be measured. And is changed according to the NOx concentration.
On the 50 side, the detection signal VIP2 of the second pump current IP2 is read, and a predetermined calculation process is executed to detect the NOx concentration in the gas to be measured from the detection signal VIP2 (in other words, the second pump current IP2). be able to.

Incidentally, in order to ensure the detection accuracy of the NOx concentration, the temperature of each of the cells 4, 6 and 8, particularly the temperature of the Vs cell 6 for detecting the oxygen concentration in the first measurement chamber 20, is controlled to be constant. For this purpose, it is necessary to control the amount of current supplied from the heater power supply circuit 44 to each of the heaters 12 and 14 so that the temperature of the Vs cell 6 becomes the target temperature. Therefore, in the present embodiment, the ECU 50 detects the temperature of the Vs cell 6 from its internal resistance RVS by switching the on / off states of the open / close switches SW1 to SW3, and the detected internal resistance RVS has a constant value (that is, The amount of power supplied from the heater power supply circuit 44 to the heaters 12 and 14 is controlled so that the temperature of the Vs cell 6 becomes the target temperature).

Hereinafter, control processing executed by the ECU 50 for such temperature control and NOx concentration detection will be described with reference to the flowcharts shown in FIGS. FIG. 3 is a graph showing the relationship between E and E to detect the NOx concentration.
FIG. 4 shows the NOx concentration detection processing repeatedly executed in the CU 50. FIG. 4 shows the ECU 5 for detecting the internal resistance RVS of the Vs cell 6 and controlling the energization to the heaters 12 and 14.
0 indicates an internal resistance detection process executed as an interrupt process every fixed time T0 (for example, 1 sec.).

As shown in FIG. 3, in the NOx concentration detection processing, first, in S100 (S represents a step), after the detection device is started, whether the NOx sensor 2 is activated by energizing the heaters 12 and 14 is determined. By deciding whether or not the activation is performed, an activation determination process for waiting for activation of the NOx sensor 2 is executed.

This activation determination processing is performed, for example, by detecting the internal resistance R of the Vs cell 6 detected in the internal resistance detection processing described later.
This is executed by determining whether or not VS has become equal to or less than a preset activation determination value. That is, as shown in FIG. 5, the internal resistance RVS of the Vs cell 6 decreases as the element temperature rises and the Vs cell 6 is activated.
Then, after the energization of the heaters 12 and 14 is started, it is determined whether or not the internal resistance RVS of the Vs cell 6 has become equal to or less than an activation determination value, thereby determining whether or not the element temperature has reached a predetermined activation temperature. Is to judge.

Immediately after the start of the detection device, the open / close switch SW1 in the drive circuit 40 is controlled to be in the on state and the open / close switches SW2 and SW3 in the drive circuit 40 are controlled to be in the off state by an initialization process (not shown). Until the NOx sensor 2 rises to near the activation temperature by the activation determination process, the operation of the differential amplifier AMP in the drive circuit 40 is stopped.

Next, when it is determined in S100 that the NOx sensor 2 has been activated, the flow shifts to S110, where the detection circuit 4
The second pump current IP2 is detected by reading the detection signal VIP2 input from the second resistor R3. In the subsequent S120, the first pump current IP1 is detected by reading the detection signal VIP1 input from the resistor R0 of the drive circuit 40. Then, in S130, a reference correction amount for the second pump current IP2 is calculated based on the detected first pump current IP1.

That is, in this embodiment, the pump current is controlled by the drive circuit 40 so that the NOx component in the gas to be measured in the first measurement chamber 20 is not decomposed so that the NOx component is not decomposed. The oxygen concentration of the gas to be measured flowing into the second measurement chamber 26 is controlled to a low oxygen concentration.
The measurement chamber 26 contains not only NOx in the gas to be measured but also the first gas.
Oxygen remaining in the measurement quality also flows in. Therefore,
Although the second pump current IP2 changes according to the NOx concentration in the gas to be measured, it is also slightly affected by the oxygen concentration in the gas to be measured. FIG. 6 shows a first example in which the apparatus was operated using a test gas containing no NOx as a gas to be measured.
An example of the measurement results of the pump current IP1 and the second pump current IP2 is shown. As is clear from FIG.
The pump current IP1 changes at a constant slope corresponding to the oxygen concentration in the measured gas, and the second pump current IP2 also changes under the influence of the oxygen concentration in the measured gas. The effect becomes stronger when the oxygen concentration is low.

Therefore, in this embodiment, the second pump current IP2
To correspond only to the NOx concentration in the gas to be measured,
The value of the second pump current IP2 corresponding to the oxygen concentration obtained when measuring the gas to be measured containing no NOx as described above is stored in a storage medium such as a ROM as an offset value for correcting the second pump current IP2. The oxygen concentration in the gas to be measured is detected in advance from the first pump current IP1, and an offset value corresponding to the oxygen concentration is read out from the offset value data stored in advance, and is stored as the reference correction amount. They try to set it.

When the reference correction amount is actually calculated, a map storing an offset value (that is, a reference correction amount) corresponding to the first pump current IP1 is used, and the first pump current IP1 is used as a parameter. The reference correction amount is directly obtained from the first pump current IP1 by searching this map.

When the reference correction amount has been calculated in this manner, the process shifts to S140 to read the internal resistance RVS of the Vs cell 6 obtained by an internal resistance detection process described later. Then, in S150, a temperature correction amount for the second pump current IP2 is calculated based on the read internal resistance RVS.

That is, in this embodiment, the internal resistance RVS of the Vs cell 6 is detected in the internal resistance detection processing described later so that the internal resistance RVS becomes a predetermined value (in other words, the temperature of the NOx sensor 2 is changed). Is controlled to a predetermined target temperature), and energization of the heaters 12 and 14 is controlled. If the temperature of the gas to be measured suddenly changes, the temperature control should follow the temperature change of the gas to be measured. And the temperature of the NOx sensor 2 may change due to a change in the temperature of the gas to be measured.

For example, FIG. 7 shows that the detection device of this embodiment is used to detect the NOx concentration in the exhaust gas of an internal combustion engine.
4 shows an example of a measurement result obtained by measuring a temperature change of the NOx sensor 2 when the x sensor 2 is attached to an exhaust pipe of an internal combustion engine and the device is operated. As is apparent from this figure, in the detection device of the present embodiment, the temperature of the NOx sensor temporarily decreases as the intake air amount increases during acceleration of the internal combustion engine, despite the temperature control described later. Or NOx due to the decrease in intake air amount when the internal combustion engine decelerates
When the sensor temperature temporarily rises, the first pump current IP1 and the second pump current IP2 both change under the influence of the temperature change, and in particular, the second pump current IP2
Takes about one minute to return to a stable state. Note that this is because once the oxygen concentration of the gas to be measured flowing from the first measurement chamber 20 to the second measurement chamber 26 deviates from the target concentration once the first pump current IP2 is affected by the exhaust gas temperature, This is because it takes time to return the oxygen concentration to the target concentration.

Therefore, in this embodiment, the internal resistance RVS of the Vs cell 6 is changed to the Vs cell so that the NOx concentration can be accurately detected from the second pump current IP2 even if the temperature of the gas to be measured changes suddenly. 6, and using the map for calculating the temperature correction amount as shown in FIG.
The temperature correction amount for P2 is determined.

Although the map shown in FIG. 8 is set so as to obtain the temperature correction amount from the element temperature of the Vs cell 6, a map for calculating the temperature correction amount using the internal resistance RVS of the Vs cell 6 as a parameter. Is set in advance, the temperature correction amount can be directly obtained from the internal resistance RVS without converting the internal resistance RVS into temperature. Further, for example, a map is set in advance using a deviation between the element temperature and the target temperature (the target temperature is 850 ° C. in FIG. 8) as a parameter, and the deviation (deviation) of the element temperature from the target temperature is determined. The temperature correction amount may be obtained, or the internal resistance R
A map may be preset using a deviation between VS and a target resistance value corresponding to the target temperature as a parameter, and the temperature correction amount may be obtained from a deviation (deviation) of the internal resistance RVS from the target resistance value.

Next, when the temperature correction amount is calculated in S150, the process proceeds to S160, where the reference correction amount and the temperature correction amount are added to the second pump current IP2 detected in S110, thereby obtaining the second pump current. Correct IP2. At S170, the corrected second pump current IP2 is set to NO.
The x density is output, and the process returns to S110.

In this embodiment, the second pump current IP2
Are corrected according to the temperature of the NOx sensor 2 (specifically, the Vs cell 6) in steps S150 and S160. In the present embodiment, in the NOx concentration detection processing, the second pump current I P1 is determined according to the oxygen concentration in the gas to be measured based on the first pump current I P1.
A description will be given assuming that the reference correction amount for correcting P2 and the temperature correction amount for correcting the second pump current IP2 according to the temperature of the Vs cell 6 are individually obtained, and the second pump current IP2 is corrected. However, for example, a map for calculating the reference correction amount is set for each temperature of the Vs cell 6, and the map used for calculating the reference correction amount is switched according to the temperature of the Vs cell 6. A correction amount for correcting the second pump current IP2 may be obtained according to the oxygen concentration in the measurement gas and the temperature of the Vs cell 6, or the first pump current IP2 and the temperature of the Vs cell 6 may be determined. (Or an internal resistance RVS) as a parameter, a two-dimensional map for calculating a correction amount may be set in advance, and the correction amount for the second pump current IP2 may be obtained using this map.

Next, the internal resistance detection processing shown in FIG. 4 will be described. Note that this internal resistance detection processing is simply performed by the Vs cell 6.
Not only as a temperature detecting means for detecting the internal resistance RVS, but also as a heater energizing control means for controlling the amount of current supplied to the heaters 12 and 14 via the heater energizing circuit 44 based on the detection result.

As shown in FIG. 4, when this process is started, in step S210, the voltage Vs on the porous electrode 6c side of the Vs cell 6 is read, and this is read as the basic detection voltage VS1 of the Vs cell 6.
Set as Then, in the subsequent S220, the open / close switch SW that has been turned on to detect the NOx concentration
1 is turned off, and the on / off switch SW2 connected to the constant current circuit 40b is turned on, so that the Vs cell 6 has the opposite direction to the minute current iCP (that is, from the closed space side which has been the internal oxygen reference source until now). (1) A constant current is supplied in the direction of pumping oxygen into the measurement chamber 20).

In S230, after the detection process is started, it is determined whether or not a predetermined time T1 (for example, 60 μsec.) Has elapsed, so that the predetermined time T1 elapses. After elapse, at S240,
The voltage Vs on the porous electrode 6c side of the Vs cell 6 is read and set as the resistance detection voltage VS2 of the Vs cell 6.

When the resistance detection voltage VS2 is set in this way,
The process proceeds to S250 and waits for the predetermined time T2 to elapse by determining whether or not the predetermined time T2 (for example, 100 μsec.) Has elapsed after the start of the detection processing. In S260, after the start of the detection processing, the on / off switch SW2, which has been on for a predetermined time T2, is turned off, and the on / off switch SW3 connected to the constant current circuit 40c is turned on.
Then, a constant current flows in the same direction as the minute current iCP (that is, the direction in which oxygen in the first measurement chamber 20 is pumped into the closed space side).

When the open / close switch SW3 is turned on in this manner, the flow shifts to S270. This time, after the detection process is started, it is determined whether a predetermined time T3 (for example, 200 μsec.) Has elapsed. Wait for the time T3 to elapse, and when the predetermined time T3 elapses, turn off the open / close switch SW3 in S280. As a result, the driving circuit 40
All of the open / close switches SW1 to SW3 are turned off.

Then, in S290, the basic detection voltage VS1 set immediately after the start of the detection processing and the predetermined time T1
Deviation from resistance detection voltage VS2 set after elapse ΔVs (=
VS1-VS2), and in S300, the internal resistance RVS of the Vs cell 6 is calculated from the deviation ΔVs, and the process proceeds to S310. The method of calculating the internal resistance RVS in this embodiment will be described later in detail.

In step S310, the heaters 12 and 14 are supplied to the heaters 12 and 14 based on the deviation between the calculated internal resistance RVS of the Vs cell 6 and the target value or the deviation between the temperature of the Vs cell 6 and the target temperature obtained from the internal resistance RVS. A control signal for increasing / decreasing the amount of energizing current is output to the heater energizing circuit 44, and processing as heater energizing control means for controlling the amount of current supplied from the heater energizing circuit 44 to the heaters 12, 14 is executed. .

In this heater energization control, when the heater energization circuit 44 is constituted by a switching circuit capable of switching between energization and non-energization to the heaters 12 and 14 at high speed, the energization / non-energization switching is performed. The duty ratio of the driving pulse to be performed may be controlled, and the heater energizing circuit 44 includes a voltage control circuit capable of detecting an energizing current to the heaters 12 and 14 and controlling an output voltage to the heaters 12 and 14. In such a case, a target voltage serving as a target value of the heater current may be output.

When the heater control signal is output in this manner, the process proceeds to S320 to determine whether or not a predetermined time T4 (for example, 500 μsec.) Has elapsed after the start of the detection processing. Wait for the predetermined time T4 to elapse, and when the predetermined time T4 elapses, in S330,
After the start of the detection process, the open / close switch SW1, which has been in the OFF state for the predetermined time T4, is turned on, and the detection process is terminated, thereby restarting the NOx concentration detection operation.

In the internal resistance detection processing described above, FIG.
As shown in (3), when the process is started (time t1), the open / close switch SW1 in the drive circuit 40 is turned off, and the Vs cell 6
Supply of the small current iCP to the Vs cell 6 and the pump current control are stopped, and the open / close switch SW2 is turned on.
A constant current flows in a direction opposite to the minute current iCP. Then, after a lapse of a predetermined time T1 (time t2), the voltage Vs on the porous electrode 6c side at that time is set as a resistance detection voltage VS2, and the resistance detection voltage VS2 and the porous electrode 6c side at the time of starting the detection processing are set. Voltage Vs (that is, basic detection voltage VS1)
The internal resistance RVS of the Vs cell 6 is detected from the deviation ΔVs from the above. Hereinafter, the reason will be described.

First, when a constant current for detecting the internal resistance is applied to the Vs cell 6, the voltage V
s is not only the internal resistance RVS of the Vs cell 6 but also the
It also changes depending on the electromotive force generated according to the ratio of the oxygen concentration on the b, 6c side. Therefore, in this embodiment, in order to make the voltage Vs on the side of the porous electrode 6c for detecting the internal resistance less affected by the electromotive force, a current larger than the minute current ICP flows, and the internal resistance RVS of the Vs cell 6 is increased. The voltage drop due to is increased.

The oxygen concentration on the side of each of the electrodes 6b and 6c of the Vs cell 6 becomes substantially constant by the pump current control and the application of the minute current ICP, respectively, so that the electromotive force of the Vs cell 6 becomes substantially constant. Become. Therefore, a constant current is applied to the Vs cell 6,
Even if the voltage Vs on the porous electrode 6c side at that time is detected, the internal resistance RVS of the Vs cell 6 can be obtained almost accurately from this voltage value.

However, more strictly, the oxygen concentration of the gas to be measured flowing from the first measurement chamber 20 to the second measurement chamber 26 is
Since it is controlled by the feedback control of the pump current, it fluctuates due to a response delay of the control system or the like, and is not fixed at a constant concentration. The oxygen concentration of the gas to be measured flowing from the first measurement chamber 20 to the second measurement chamber 26 is:
It also changes depending on the temperature of the NOx sensor 2. Therefore, V
When the internal resistance RVS is obtained from the voltage Vs detected by flowing a constant current for detecting the internal resistance RVS through the s cell 6, an error occurs in the internal resistance RVS, though slightly.

Therefore, in this embodiment, a constant current for detecting the internal resistance RVS is applied to the Vs cell 6 in order to more accurately detect the internal resistance RVS of the Vs cell 6 and thus the element temperature. , A change amount (deviation ΔVs) of the voltage Vs on the porous electrode 6c side until a predetermined time (for example, 60 μsec.) Elapses, and the internal resistance RVS is determined from the deviation ΔVs to obtain the first measurement chamber. 20 to the second measurement chamber 26
Therefore, even if the oxygen concentration of the gas to be measured flowing into the Vs cell deviates from the target concentration, the internal resistance RVS of the Vs cell 6 and thus the element temperature can be accurately obtained.

In calculating the internal resistance RVS, a map storing the internal resistance RVS corresponding to the deviation ΔVs is set in advance, and the internal resistance RVS is calculated using this map. What should I do? Next, in the internal resistance detection processing of the present embodiment, after a certain period of time T1 has elapsed after activation and the resistance detection voltage VS2 is set (time t2), a further predetermined time (for example, 40 μsec.) Has elapsed after that. The time t3 when the elapsed time after the start of the detection processing reaches T2.
By turning off the open / close switch SW2 of the drive circuit 40 and turning on the open / close switch SW3, a constant current flows in the Vs cell 6 in the same direction as the minute current iCP, and after a certain time (for example, 100 μsec.) Elapses. When the elapsed time after the start of the detection process reaches T3 (time t4), the open / close switch SW3 is turned off.

As a result, in the present embodiment, the polarization between the solid electrolyte or the electrode and the solid electrolyte of the Vs cell 6 can be quickly alleviated to detect the internal resistance RVS, and the Vs cell 6 Can quickly function as an oxygen concentration battery cell. Therefore, after starting the process,
The time T4 before the operation for detecting the NOx concentration can be made extremely short, for example, 500 μsec., And the internal resistance RVS of the Vs cell 6 can be increased without affecting the operation for detecting the NOx concentration. It is possible to detect with high accuracy.

As described above, in the nitrogen oxide concentration detecting apparatus according to the present embodiment, the oxygen concentration which has the greatest influence on the NOx concentration detection accuracy (specifically, the first measuring chamber 2)
From 0, the temperature of the NOx sensor 2 is detected from the internal resistance RVS of the Vs cell 6 for detecting the oxygen concentration of the gas to be measured flowing into the second measurement chamber 26, and this temperature is set to the target temperature (for example, 8
50 ° C.), the amount of current supplied to the heaters 12 and 14 is controlled, and if the detected internal resistance RVS or the element temperature obtained from the internal resistance RVS deviates from the target value, the deviation is calculated. The temperature of the NOx concentration detection result is compensated by correcting the second pump current IP2 representing the NOx concentration detection result with a temperature correction amount corresponding to the above. For this reason, according to the nitrogen oxide concentration detection device of the present embodiment, the NOx concentration can always be detected with high accuracy without being affected by the temperature of the NOx sensor 2.

In this embodiment, in particular, the NOx sensor 2
Are the first pump cell 4, the Vs cell 6, and the second pump cell 8
And the heaters 12 and 14 are stacked on both sides in the stacking direction, and when the NOx sensor 2 is projected from the stacking direction, the diffusion-controlling layer 4d and the diffusion-controlling layers 6d and 22d Are overlapped, and heater wirings 12b and 14b of the heaters 12 and 14 are arranged so as to sandwich these diffusion control layers at substantially the center position. Therefore, in the present embodiment, with the structure of the NOx sensor 2, the cells 4 to 8 can be efficiently heated using the heaters 12 and 14, and the first measurement chamber 20 can be heated via the diffusion-controlling layers. The gas to be measured flowing into the second measurement chamber 26 can also be efficiently heated. Therefore, according to the present embodiment, by controlling the temperature of the Vs cell 6, it is possible to more reliably control the temperature of each cell constituting the NOx sensor 2 to the target temperature, and to detect the NOx concentration. Accuracy can be improved.

The embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and can take various forms. For example, in the above embodiment, in the NOx sensor 2, the porous electrodes 6b and 6c of the Vs cell 6 have been described as being formed on both sides of the solid electrolyte layer 6a formed in a plate shape. Since it is sufficient that the oxygen concentration of the gas to be measured flowing from the first measurement chamber 20 to the second measurement chamber 26 can be detected, it is not always necessary to configure as in the above embodiment.
It may be configured as shown in (a) and (b).

That is, in the NOx sensor shown in FIG. 10A, the porous electrode 6b arranged on the first measurement chamber 20 side of the Vs cell 6 is connected to the solid electrolyte layer 6a facing the first measurement chamber 20.
The NOx sensor shown in FIG. 10B has a porous electrode 6b and a hollow solid electrolyte layer 6a in which the diffusion-controlling layer 6d is formed. Although it is formed on the inner wall surface, even if the porous electrode 6b of the Vs cell 6 is arranged in this way, the first measurement chamber 20 and the second measurement chamber are located between the electrodes 6b-6c of the Vs cell 6. Since a voltage corresponding to the oxygen concentration of the gas to be measured flowing into the side 26 is generated, NO
x concentration can be detected. Therefore, even with the NOx sensor configured as shown in FIGS. 10A and 10B, the same effects as in the above embodiment can be obtained by applying the present invention in the same manner as in the above embodiment. .

In the above embodiment, the first pump current I
The first measuring chamber 20 is connected to the second measuring chamber 2 so that the NOx component in the gas to be measured is not decomposed by energizing P1.
Although the oxygen concentration of the gas to be measured flowing into 6 has been described as being controlled to a low oxygen concentration where a little oxygen exists, the first
As the oxygen in the gas to be measured flowing from the measurement chamber 20 to the second measurement chamber 26 is reduced, the accuracy of detecting the NOx concentration by the second pump cell 8 can be improved.
The first pump cell 4 may pump the oxygen in the first measurement chamber 20 to the extent that the NOx component in the gas to be measured is decomposed. Specifically, the reference voltage VCO
Is set to, for example, about 350 mV, so that a first pump current IP1 enough to decompose the NOx component in the first measurement chamber 20 flows through the first pump cell 4, and from the first measurement chamber 20 to the second measurement chamber 26. The oxygen concentration in the gas to be measured is made sufficiently small. And if you do this,
By reducing the offset value of the second pump current IP2,
It becomes possible to improve the detection accuracy of the x concentration.

However, even with such a configuration, the oxygen in the gas to be measured flowing into the second measurement chamber 26 cannot be reduced to zero. Similarly, an offset value corresponding to the oxygen concentration obtained from the first pump current IP1 is obtained as a reference correction amount, and the reference correction amount and the temperature correction amount obtained based on the internal resistance RVS of the Vs cell 6 are used. It is desirable to obtain the NOx concentration by correcting the two pump currents IP2.

In this case, the first pump current IP1 includes:
Since the current component generated by decomposing the NOx component in the gas to be measured is also included, the first pump cell 4 is added to the map for obtaining the offset value between the first pump current IP1 and the second pump current IP2. NO decomposed
A map for obtaining a gain for compensating the x component from the first pump current IP1 is set in advance by experiments or the like,
Using the offset value and the gain obtained from each of these maps, the second pump current IP2 is corrected as in the following equation, and N
The Ox concentration may be determined.

NOx concentration = gain × (IP 2 −offset value) This equation is for correcting the second pump current IP 2 based on the offset value and the gain obtained as the reference correction amount from the first pump current IP 1. Therefore, when correcting the second pump current IP2 using the above-described temperature correction amount,
It is necessary to further correct the NOx concentration obtained by this equation with the temperature correction amount.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating a configuration of an entire nitrogen oxide concentration detection device according to an embodiment.

FIG. 2 is an exploded perspective view illustrating a configuration of a NOx sensor according to the embodiment.

FIG. 3 shows N repeatedly executed by the ECU of the embodiment.
It is a flowchart showing an Ox density | concentration detection process.

FIG. 4 is a flowchart illustrating an internal resistance detection process executed as an interrupt process at predetermined time intervals in the ECU of the embodiment.

FIG. 5 is a graph showing a relationship between an internal resistance of an oxygen concentration measurement cell and an element temperature.

FIG. 6 is a graph showing a relationship between an oxygen concentration of a gas to be measured containing no NOx and a first pump current and a second pump current.

FIG. 7 is a time chart showing changes in a first pump current and a second pump current caused by a change in exhaust gas temperature during acceleration / deceleration of the internal combustion engine.

FIG. 8 is a graph showing an example of a map used when obtaining a temperature correction amount of a second pump current.

FIG. 9 is a time chart for explaining the operation of the internal resistance detection processing shown in FIG. 4;

FIG. 10 is a cross-sectional view illustrating another configuration example of the NOx sensor to which the present invention can be applied.

[Explanation of symbols]

2 NOx sensor 4 First pump cell 6 Vs
Cell 8 Second pump cell 12, 14 Heater 4a, 6a, 8a, 18, 22, 24 Solid electrolyte layer 4b, 4c, 6b, 6c, 8b, 8c Porous electrode 4d, 6d, 22d Diffusion controlling layer 6f: leakage resistance portion 12a, 12c, 14a, 14c: heater substrate 12b, 14b: heater wiring 20: first measurement chamber
26 second measurement chamber 40 driving circuit 40a control units 40b and 40c
... constant current circuit 42 ... detection circuit 44 ... heater energization circuit AMP ...
Differential amplifier R0, R1, R2, R3 ... resistor SW1, SW2, SW3 ... open / close switch

Claims (7)

[Claims] A first oxygen pumping cell and an oxygen concentration measuring cell in which an oxygen ion conductive solid electrolyte layer is sandwiched between porous electrodes, and the first oxygen pumping cell and the oxygen concentration measuring cell communicate with the gas to be measured via the first diffusion-controlling layer. And a second oxygen pumping cell in which an oxygen ion-conductive solid electrolyte layer is sandwiched between porous electrodes, and is communicated with the first measurement chamber via a second diffusion-controlling layer. A sensor main body including a second measurement chamber, and oxygen pumped from the first measurement chamber by the first oxygen pumping cell so that an output voltage of the oxygen concentration measurement cell becomes a constant value. Pump current control means for controlling the oxygen concentration of the gas to be measured flowing from the measurement chamber into the second measurement chamber at a constant level; and applying a constant voltage to the second oxygen pumping cell in a direction to pump oxygen from the second measurement chamber. Constant voltage applying means for applying; A nitrogen oxide concentration detecting means for detecting a nitrogen oxide concentration in the gas to be measured based on a current value flowing through the second oxygen pumping cell by applying the constant voltage; and the sensor body can detect the nitrogen oxide concentration. A heater for heating to a temperature; and a nitrogen oxide concentration detection device comprising: a temperature detection unit configured to detect a temperature of the oxygen concentration measurement cell; and a temperature detection unit configured to detect a temperature of the oxygen concentration measurement cell detected by the temperature detection unit. A heater energization control means for controlling energization of the heater so that the temperature becomes a preset target temperature; and a nitrogen oxide concentration detection device. 2. The nitrogen oxide concentration detected by the nitrogen oxide concentration detecting means is corrected in accordance with a deviation of the temperature of the oxygen concentration measuring cell detected by the temperature detecting means from the target temperature. 2. The nitrogen oxide concentration detecting apparatus according to claim 1, further comprising a correction unit for performing temperature compensation on the detected value of the nitrogen oxide concentration. 3. The temperature detecting means detects a temperature of the oxygen concentration measuring cell by detecting an internal resistance of the oxygen concentration measuring cell, and the heater energizing control means detects an internal resistance of the oxygen concentration measuring cell. 3. The nitrogen oxide concentration detecting device according to claim 1, wherein energization of the heater is controlled so that a resistance has a predetermined value corresponding to the target temperature. 4. 4. In the sensor body, a porous electrode of the oxygen concentration measuring cell opposite to the first measuring chamber is closed, and a part of oxygen in the closed space is leaked through a leak resistance portion. The pump current control means supplies a small current to the oxygen concentration measurement cell in a direction to pump oxygen in the first measurement chamber into the closed space, Controlling the amount of current flowing through the first oxygen pumping cell so that the electromotive force generated in the oxygen concentration measuring cell becomes a constant value while functioning as an internal oxygen reference source; Means for periodically disconnecting the connection between the means and the oxygen concentration measurement cell, and at the time of the interruption, flowing a current for detecting an internal resistance larger than the minute current to the oxygen concentration measurement cell in a direction opposite to the minute current. And that Nitrogen oxide concentration detection apparatus according to claim 3, characterized in that for detecting the internal resistance of said oxygen concentration measuring cell from a voltage generated between the electrodes of the oxygen concentration measurement cell. 5. The temperature detecting means detects the internal resistance by flowing the internal resistance detection current to the oxygen concentration measurement cell, and then supplies the internal resistance detection cell to the oxygen concentration measurement cell in a direction opposite to the internal resistance detection current. The nitrogen oxide concentration detecting device according to claim 4, wherein a current larger than the minute current is passed. 6. In the sensor body, the first oxygen pumping cell, the oxygen concentration measuring cell, and the second oxygen pumping cell are respectively formed in mutually different thin plate-like solid electrolyte layers, and the first measuring chamber and the second (2) The measurement chamber is configured by stacking the solid electrolyte layers via a predetermined gap with the solid electrolyte layers forming the first and second oxygen pumping cells being outside, and the heater is provided on a substrate. 2 with heater wiring formed on it
A plurality of heater substrates, and each heater substrate is arranged on both sides of the solid electrolyte layer of the sensor body in the stacking direction with a predetermined gap therebetween, so that the sensor body can be heated. The method according to claim 1, wherein the first diffusion layer is formed in a portion of the solid electrolyte layer in which the first oxygen pumping cell is formed, including a position facing a central portion of a heater wiring formed on the heater substrate. The nitrogen oxide concentration detection device according to any one of claims 1 to 5. 7. The second diffusion-controlling layer is formed so as to overlap at least a part of the first diffusion-controlling layer when the sensor main body is projected from a direction in which the solid electrolyte layers are stacked. 7. The nitrogen oxide concentration detecting device according to claim 6, wherein the oxygen concentration measuring cell is arranged near the diffusion-controlling layer.