JP3431822B2 - Nitrogen oxide concentration detector - Google Patents

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 oxides, which are harmful components discharged from various combustion equipment such as an internal combustion engine.

[0002]

2. Description of the Related Art Conventionally, as a nitrogen oxide concentration detecting device, for example, European Patent Application Publication No. 06787 has been disclosed.
40A1, SAE paper No. 960334
As disclosed in P137-142 1996 and the like, a first measurement chamber communicated with the measured gas side via a first diffusion rate controlling layer and a first measurement chamber communicating with this first measurement chamber via a second diffusion rate controlling layer. The second measurement chamber is formed of a solid electrolyte layer having oxygen ion conductivity, and the first measurement chamber has a solid electrolyte layer sandwiched between porous electrodes to measure the first oxygen pumping cell and the oxygen concentration. And a sensor constituted by forming a second oxygen pumping cell by sandwiching a solid electrolyte layer between porous electrodes in the second measurement chamber. It is known that the concentration of nitrogen oxides (NOx) in exhaust gas is detected.

In this type of nitrogen oxide concentration detector, the first oxygen pumping cell is energized so that the output voltage from the oxygen concentration measuring cell becomes a preset constant value, and the oxygen is removed from the first measuring chamber. By pumping out 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 to the second measurement chamber) at a constant concentration, the oxygen concentration in the second oxygen pumping cell is increased. Oxygen is further pumped out from the second measurement chamber by applying a constant voltage. Then, the NOx concentration in the measured gas is detected from the value of the current flowing through the second oxygen pumping cell.

That is, in the exhaust gas from the internal combustion engine, which is the gas to be measured, other gas components such as oxygen, carbon monoxide, carbon dioxide, etc. 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.
By applying a constant voltage to the second oxygen pumping cell in the direction of pumping out oxygen in the second measurement chamber on the side of the second measurement chamber into which the measured gas from which oxygen is extracted flows,
Due to the catalytic function of the porous electrode that constitutes the oxygen pumping cell, NOx in the gas to be measured is decomposed into nitrogen and oxygen, and oxygen is extracted from the second measurement chamber. At that time, the pump current flowing through the second oxygen pumping cell is By detecting, the NOx concentration in the measured gas can be detected without being affected by other gas components in the measured gas.

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

That is, as a method for controlling the temperature of the sensor, as disclosed in, for example, Japanese Patent Laid-Open Nos. 59-163556 and 59-214756,
How to control the amount of electricity to the heater so that the amount of heat generated by the heater is constant, how to control the electricity to the heater so that the temperature of the sensor reaches the target temperature with a preset control pattern, and to detect the sensor temperature. Various methods are known, such as a method of controlling the amount of electricity supplied to the heater, and it is conceivable to use such a conventional control method also in the nitrogen oxide concentration detecting device.

[0008]

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

Particularly, 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 energization of the first oxygen pumping cell, As a result, the oxygen concentration in the second measurement chamber changes and the detection result of the NOx concentration greatly changes. Therefore, in order to ensure the detection accuracy of the NOx concentration, it is necessary to change the first measurement chamber to the second measurement chamber by changing the temperature of the sensor. It is required that temperature control can be executed efficiently with a simple configuration so that the oxygen concentration of the measured gas that flows into the measurement chamber does not change. The temperature control method to obtain it has not been established, and a nitrogen oxide concentration detection device that has a simple structure and ensures temperature characteristics has not been realized.

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

The present invention has been made in view of these problems, and in the above-mentioned nitrogen oxide concentration detecting device, the sensor temperature is efficiently changed with a simple structure so that the detection result of the NOx concentration does not change. The first purpose is to enable control, and to improve the NOx concentration detection accuracy by correcting the NOx concentration detection error due to the temperature change of the measured gas that cannot be controlled even by this temperature control. The purpose is 2.

[0012]

A nitrogen oxide concentration test according to claim 1, which was carried out in order to achieve the first object.
The discharge device has a first oxygen pumping cell and an oxygen concentration measuring cell in which an oxygen ion conductive solid electrolyte layer is sandwiched by porous electrodes, and is connected to the measured gas side through the first diffusion controlling layer. And a second oxygen pumping cell in which a solid electrolyte layer having oxygen ion conductivity is sandwiched between porous electrodes, and is communicated with the first measurement chamber via a second diffusion control layer. a second measurement chamber, and a sensor body with a, the
In front of the first oxygen pumping cell and the oxygen concentration measuring cell
In the state where the electrodes on the first measurement chamber side are connected to each other, the acid
Raw on the electrode of the elementary concentration measurement cell on the side opposite to the first measurement chamber.
By detecting the voltage Flip, the oxygen and detects an output voltage of the concentration measuring cell, said first measurement chamber of the first oxygen pumping cell so that the output voltage becomes constant value
By injecting a current toward the electrode on the side opposite to,
Pump current control means for pumping oxygen from the first measurement chamber by the first oxygen pumping cell and controlling the oxygen concentration of the gas to be measured flowing from the first measurement chamber to the second measurement chamber to a constant value. In the second oxygen pumping cell, the second
Constant voltage applying means for applying a constant voltage in the direction of pumping oxygen from the measurement chamber, and detecting the nitrogen oxide concentration in the gas to be measured based on the current value flowing in the second oxygen pumping cell by applying the constant voltage. Nitrogen oxide concentration detecting means,
A heater for heating the sensor body to a temperature at which the nitrogen oxide concentration can be detected, a temperature detecting means for detecting the temperature of the oxygen concentration measuring cell , and the oxygen concentration measuring cell detected by the temperature detecting means. as the temperature of the preset target temperature, characterized by comprising a heater energization control means for controlling the energization of the heater.

The invention according to claim 2 made in order to achieve the second object is, in the nitrogen oxide concentration detecting device according to claim 1, the oxygen concentration detected by the temperature detecting means. Correcting means for compensating the detected value of the nitrogen oxide concentration by correcting the nitrogen oxide concentration detected by the nitrogen oxide concentration detecting means according to the deviation of the temperature of the measuring cell from the target temperature. Is provided.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The nitrogen oxide concentration detector according to claim 1 is similar to the conventional nitrogen oxide concentration detector described above in that an oxygen ion conductive solid electrolyte layer is sandwiched between porous electrodes. A first measuring chamber having a first oxygen pumping cell and an oxygen concentration measuring cell, which communicate with the gas to be measured through a first diffusion-controlling layer, and a solid electrolyte layer having oxygen ion conductivity, which is a porous electrode. A second oxygen measuring cell having a second oxygen pumping cell sandwiched between the second measuring chamber and a second measuring chamber communicating with the first measuring chamber via a second diffusion controlling layer. The NOx concentration is detected, and the pump current control means includes a first oxygen pumping cell and an oxygen concentration.
The state where the electrodes on the first measurement chamber side of the measurement cell are connected to each other
At the electrode on the side opposite to the first measurement chamber of the oxygen concentration measurement cell.
The output voltage of the oxygen concentration measuring cell is detected by detecting the generated voltage, and the output voltage is opposite to the first measuring chamber of the first oxygen pumping cell so that the output voltage becomes a constant value.
By flowing an electric current toward the electrode on the side, oxygen is pumped out from the first measurement chamber by the first oxygen pumping cell, and the oxygen concentration of the measured gas flowing into the second measurement chamber from the first measurement chamber is made constant. Control. Then, the constant voltage applying means,
A constant voltage is applied to the second oxygen pumping cell in the direction of pumping oxygen from the second measurement chamber, and the nitrogen oxide concentration detecting means is based on the current value flowing in the second oxygen pumping cell by the application of the constant voltage, The NOx concentration in the measured gas is detected.

The nitrogen oxide concentration detector of the present invention is also provided with a heater for heating the sensor body to a temperature at which the NOx concentration can be detected, as in the conventional device. The heater energization control unit controls the temperature of the oxygen concentration measurement cell detected by the temperature detection unit to be a preset target temperature.

Therefore, according to the nitrogen oxide concentration detecting apparatus of the present invention, the oxygen concentration measuring cell has the greatest effect on the detection accuracy of the NOx concentration.
It is possible to accurately detect the oxygen concentration of the measured gas flowing from the measurement chamber to the second measurement chamber. Further, if 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, by controlling the energization of the first oxygen pumping cell by the pump current control means, the second measuring chamber can be changed from the first measuring chamber to the second measuring chamber.
The oxygen concentration of the measured gas flowing into the measurement chamber can be controlled to a preset constant concentration, and the NOx concentration in the measured gas can be accurately detected from the current flowing through the second oxygen pumping cell.

As described above, in the nitrogen oxide concentration detector of the present invention, of the three types of cells forming the sensor body, the oxygen concentration that most affects the NOx concentration detection accuracy (specifically, the first measurement By controlling the amount of current supplied to the heater for heating the sensor main body so that the temperature of the oxygen concentration measuring cell for detecting (the oxygen concentration of the measured gas flowing into the second measuring chamber from the chamber) becomes the target temperature, The NOx concentration can be accurately detected without being affected by the temperature change of the sensor body, and the temperature detecting means for detecting the temperature of the oxygen concentration measuring cell and the heater for controlling the energization of the heater according to the detection result. Since the temperature control system of the sensor body can be configured only by providing the energization control means, the temperature control of the sensor body can be efficiently performed with a simple structure. It can be provided.

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

Next, in the nitrogen oxide concentration detecting device according to the second aspect of the invention, the correcting means changes the nitrogen oxide concentration according to the deviation of the temperature of the concentration measuring cell detected by the temperature detecting means from the target temperature. The nitrogen oxide concentration detected by the substance concentration detecting means is corrected. Therefore, according to the present invention (Claim 2), if the temperature of the sensor body cannot be controlled to reach the target temperature even though the temperature control is being performed by the heater energization control unit, the correction unit By N
The detection result of Ox concentration can be temperature-compensated, and NOx
The density detection accuracy can be further improved.

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

Here, the temperature detecting means for detecting the temperature of the oxygen concentration measuring cell can be realized by, for example, providing an element for temperature detection in the vicinity of the oxygen concentration measuring cell. Is complicated, and it is difficult to accurately detect the temperature of the oxygen concentration measuring cell itself.

Therefore, as described in claim 3, the temperature detecting means is constituted so as to detect the internal resistance of the oxygen concentration measuring cell, and the heater energization control means is adapted to detect the inside 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 it changes according to the temperature of the oxygen concentration measuring cell (the higher the temperature, the lower the internal resistance), as described in claim 3, if the internal resistance of the oxygen concentration measuring cell is detected, From the detected internal resistance, it becomes possible to accurately detect the temperature of the oxygen concentration measuring cell without separately providing a temperature detecting element in the sensor body, and the temperature control of the sensor body can be performed easily and with high temperature. It can be executed with accuracy.

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

However, when detecting the internal resistance of the oxygen concentration measuring cell as described above, the pump current controlling means temporarily cuts off the connection between the pump current controlling means and the oxygen concentration measuring cell. 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 becomes a value that does not correspond to the oxygen concentration of the measured gas flowing into the second measuring chamber from the first measuring chamber, and at that time the pump current 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, such erroneous control is performed when the internal resistance of the oxygen concentration measurement cell is detected. It is desirable to stop the control operation by the pump current control means so that the above problem does not occur.

Next, the oxygen concentration measuring cell is for detecting the oxygen concentration of the gas to be measured flowing into the second measuring chamber from the first measuring chamber, and a pair of porous electrodes constituting this cell are provided. Of these, 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 having a constant oxygen concentration (for example, the atmosphere) 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 make the porous electrode on the side of the oxygen concentration measuring cell opposite to the first measuring chamber (that is, the side not in contact with the gas to be measured) the reference oxygen concentration, as described in claim 4, In the sensor 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 to the outside through the leak resistance portion. On the pump current control means side, a small current is caused to flow to the oxygen concentration measuring cell in the direction of pumping out the oxygen in the first measurement chamber into the closed space, and the closed space is made to function 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, according to this structure, it is not necessary to provide a space for introducing the reference gas in the sensor body, and the structure of the sensor body can be simplified.

In this case, in order to measure the internal resistance of the oxygen concentration measuring cell by the temperature detecting means, the connection between the pump current control means and the oxygen concentration measuring cell is periodically performed as described in claim 4. At the time of interruption, an internal resistance detection current larger than the minute current for generating the internal oxygen reference source is made to flow in the oxygen concentration measuring cell in the opposite direction of the minute current, and at that time, the electrode of the oxygen concentration measuring cell 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 between the cells.

That is, the internal resistance detection current can be made to flow in the same direction as the minute current, but in order to suppress deterioration of the electrode, a current is made to flow in the opposite direction to the normal direction to activate the electrode. It can have the effect of When a current is applied to the oxygen concentration measuring cell, the voltage (inter-electrode voltage) generated by the oxygen concentration measuring cell depends not only on the internal resistance of the oxygen concentration measuring cell but also on the ratio of the oxygen concentration on each electrode side. The oxygen concentration on each electrode side of the oxygen concentration measuring cell (that is, the oxygen concentration of the measured gas flowing into the second measuring chamber from the first measuring chamber and the oxygen concentration in the closed space) varies depending on the electromotive force generated. , Respectively, the electromotive force during the energization period of the internal resistance detection current is approximately constant, because they are substantially constant due to the energization of the minute current and the energization control of the first oxygen pumping cell by the pump current control means, respectively. Since the counter 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 measuring cell, according to the present invention, the oxygen concentration can be measured without being affected by the change in the electromotive force. SE It can detect the internal resistance of.

In this way, when an internal resistance detecting current is passed through the oxygen concentration measuring cell in order to detect the internal resistance of the oxygen concentration measuring cell, the oxygen concentration measuring cell is a solid electrolyte body or a solid electrolyte body. Since the direction of polarization changes between the electrodes, it is immediately possible to generate an electromotive force according to the ratio between the oxygen concentration in the first measurement chamber and the oxygen concentration in the closed space, as before measurement. However, the electromotive force is slightly off. Therefore, it takes some time for the polarization to be eliminated and the oxygen concentration of the gas to be measured flowing into the second measurement chamber from the first measurement chamber to be accurately detected by the oxygen concentration measurement cell. Even if the control operation of the pump current control means is restarted immediately after the detection, the NOx concentration cannot be detected accurately.

The time until the NOx concentration can be accurately detected after detecting the internal resistance (detection stop time)
In order to shorten the value, as described in claim 5, after the internal resistance detection current is passed through the oxygen concentration measuring cell to detect the internal resistance, the internal resistance detection current is passed in the opposite direction. The temperature detecting means may be configured.

That is, at the time of detecting the internal resistance of the oxygen concentration measuring cell, if the internal resistance detecting current is alternately passed through the oxygen concentration measuring cell once in a pulse shape, each electrode side of the oxygen concentration measuring cell The oxygen concentration will quickly return to the stable state before the internal resistance was detected.
It is possible to shorten the time until the Ox concentration can be accurately detected.

Next, in the present invention, of the three types of cells constituting the sensor body, the oxygen concentration that most affects the detection accuracy of the NOx concentration (that is, from the first measurement chamber to the second concentration chamber).
The temperature of the oxygen concentration measuring cell that detects (the oxygen concentration of the measured gas flowing into the measurement chamber) is detected as the temperature of the sensor body to control the current flowing to the heater. It is 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 obtain the effect of the temperature control of the present invention more favorably, the first oxygen pumping cell, the oxygen concentration measuring cell, and the second oxygen pumping cell in the sensor body are provided as described in claim 6. Each oxygen pumping cell
The first and second measurement chambers are formed on different thin plate-shaped solid electrolyte layers, and the solid electrolyte layers having the first and second oxygen pumping cells are disposed outside in the predetermined measurement chambers and the second measurement chambers, respectively. The heater is formed by stacking with a gap therebetween, and the heater is a thin plate-like 2 having heater wiring formed on a substrate.
Sheets made from the heater board by the heaters substrates respectively arranged with a predetermined gap in the stacking direction on both sides of the solid electrolyte layer in the sensor body, is heatable constructed sensor body, moreover, the first diffusion rate-determining It is desirable that the layer is formed at a location including a position facing the central portion of the heater wiring formed on the heater substrate in the solid electrolyte layer in which the first oxygen pumping cell is formed.

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

As a result, according to the present invention (claim 6),
It is possible to make it difficult for the temperature of each cell in the sensor body to fluctuate, and to make each cell less susceptible to the temperature of the gas to be measured, thereby further improving the NOx concentration detection accuracy. 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 the solid electrolyte layers, and in the vicinity of the second diffusion-controlling layer,
By disposing the oxygen concentration measuring cell, the temperature of the sensor main body and the gas to be measured inside the sensor main body can be reliably controlled by the target temperature and the detection accuracy of the NOx concentration can be improved.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing the overall configuration of a nitrogen oxide concentration detecting apparatus of an embodiment to which the present invention is applied, and FIG. 2 is a NOx sensor 2 used in this nitrogen oxide concentration detecting apparatus.
FIG.

As shown in FIG. 1, the nitrogen oxide concentration detecting device includes 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 measurement. Cell (hereinafter Vs
(Hereinafter referred to as a cell) 6, a drive circuit 40 for energizing and switching the energization path, and a second oxygen pumping cell (hereinafter referred to as a second pump cell) constituting the NOx sensor 2.
A constant voltage is applied to 8 and the current that flows at that time (hereinafter referred to as the second
A detection circuit 42 for detecting IP2 (referred to as pump current), and
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, the drive circuit 40 and the heater energizing circuit 44 are driven and controlled, and an electronic control circuit (hereinafter referred to as ECU) composed of a microcomputer for calculating the NOx concentration in the gas to be measured based on the detection signal VIP2 from the detection circuit 42. )
And 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 the plate-shaped solid electrolyte layer 4a.
And its lead portions 4bl, 4cl are formed, and a round hole is formed in the solid electrolyte layer 4a so as to penetrate the central portions of the porous electrodes 4b, 4c, and a porous filler is filled in the round hole. The diffusion-controlling layer 4d is formed by filling.

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,
The circular porous electrodes 6b and 6c and the lead portions 6bl and 6cl thereof are formed, and round holes are formed in the solid electrolyte layer 6a so as to penetrate through the central portions of the porous electrodes 6b and 6c. By filling the round holes with a porous filler,
The diffusion-controlling layer 6d is formed.

Then, the porous electrode 6 of this Vs cell 6 is used.
b, 6c and the porous electrodes 4b, 4c of the first pump cell 4 have substantially the same center position 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 controlling layers 6d and 4d are arranged 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,
An insulating film made of alumina or the like is formed on the front and back surfaces of the Vs cell 6 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. A leakage resistance portion 6f is formed between the portions 6bl and 6cl for leaking a part of oxygen pumped into the porous electrode 6c side to the porous electrode 6b side by energization control described later.

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

Also on the porous electrode 6c side of the Vs cell 6,
A solid electrolyte layer 22 having the same shape as each of the solid electrolyte layers 4a and 6a is laminated. Then, in the solid electrolyte layer 22, a circular hole having the same size is formed at the same position as the diffusion controlling layer 6d of the Vs cell 6, and the porous hole is filled with a porous filling material. 22d is formed.

On the other hand, like 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 plate-shaped solid electrolyte layer 8a, respectively. Is formed. Then, the second pump cell 8 is laminated on the solid electrolyte layer 22 laminated on the Vs cell 6 via the 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.

On both sides of the laminated body of the first pump cell 4, the Vs cell 6, and the second pump cell 8 thus laminated, that is, on the outside of the first pump cell 4 and the second pump cell 8, spacers 28, The heaters 12 and 14 are laminated at a predetermined interval by 29.

The heaters 12, 14 have heater substrates 12a, 12 of the same shape as the solid electrolyte layers 4a, 6a ,.
c, 14a, 14c and the heater substrates 12a and 12c, and between the heater substrates 14a and 14c, respectively,
Heater wiring 1 formed by being embedded in each heater substrate
2b and 14b and their lead portions 12bl and 14bl, and the spacers 28 and 29 are such that the heaters 12 and 14 are
Porous electrode 4 of the first pump cell 4 and the second pump cell 8
b and 8c are respectively 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,
Typical solid electrolytes that compose ... are solid solutions of zirconia and yttria and zirconia and calcia, but other solid solutions of hafnia, perovskite type oxide solid solutions, trivalent metal oxide solid solutions, etc. are also used. it can. Further, it is preferable to use platinum or rhodium or its alloy having a catalytic function for the porous electrode provided on the surface of each solid electrolyte layer 4a, 6a, 8a. Then, as the forming method, for example, a thick film forming method in which platinum powder is mixed with powder of the same material as the solid electrolyte layer to form a paste, screen-printed on the solid electrolyte layer, and then sintered, or thermal spraying A method for forming a coating film is known. Further, it is preferable that the diffusion-controlling layers 4d, 6d, 22d be made of ceramics having thin through holes 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 thereof are made of platinum or a platinum alloy in order to reduce the resistance value and reduce the electrical loss in the lead portion. preferable. Also, the heater substrate 1
2a, 12b, 14a, 14c and spacers 28, 29
Alumina, spinel, forsterite, steatite, zirconia or the like can be used as the material.

In particular, when zirconia is used as the material for the heater substrate and the spacer, the heater and each pump cell can be integrated and sintered at the same time.
This is suitable for manufacturing 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. (Alumina or the like) is provided.

When alumina is used for the heater substrate, a porous material is used as the spacer in order to prevent the occurrence of cracks due to the difference in shrinkage ratio and the difference in coefficient of thermal expansion during sintering with each pump cell. Good to use. It is also possible to sinter the heater and each pump cell separately, and later bond them by using an inorganic material such as cement as a bonding material that also serves as a spacer.

Next, as shown in FIG. 1, the NOx sensor 2
The porous electrodes 4c and 6b of the first pump cell 4 and the Vs cell 6 on the first measurement chamber 20 side are grounded through the resistor R1, and the other porous electrodes 4b and 6c are connected to the drive circuit 40. It is connected. A constant voltage VCP is applied to one end of the drive circuit 40, and the other end is connected to V
a resistor R2 connected to the porous electrode 6c of the s cell 6,
Vs cell 6 via the open / close switch SW1 to the-side input terminal
Porous electrode 6c and one end of the capacitor Cp are connected, the reference voltage VCO is applied to the + side input terminal, and the output terminal is the porous electrode 4 of the first pump cell 4 via the resistor R0.
and a control unit 4 including a differential amplifier AMP connected to b
With 0a. The other end of the capacitor Cp is grounded.

This control section 40a has an open / close switch SW1.
When is on, it operates as follows. First, oxygen in the first measurement chamber 20 is pumped to the porous electrode 6c side of the Vs cell 6 by causing a constant minute current iCP to flow through the Vs cell 6 via the resistor R2. This porous electrode 6c is
Since it is closed by the solid electrolyte layer 22 and communicates with the porous electrode 6b side through the leakage resistance portion 6f, the closed space in the porous electrode 6c has a constant oxygen concentration due to the application of the minute current iCP. , Acts as an internal oxygen reference source.

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

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

That is, the control section 40a functions as the pump current control means of the present invention, so that the oxygen concentration of the measured gas flowing from the first measurement chamber 20 into the second measurement chamber 26 becomes constant. The control for pumping oxygen from the first measurement chamber 20 to the outside is executed. The first controlled in this way
The oxygen concentration of the measurement gas flowing from the measurement chamber 20 into the second measurement chamber 26 does not decompose the NOx component in the measurement gas in the first measurement chamber 20 by energizing the first pump current IP1. As described above, the oxygen concentration is set to a low oxygen concentration (for example, about 100 ppm), and the reference voltage VCO that determines the oxygen concentration is 100 mV to
A value of about 200 mV is set. In addition, the differential amplifier A
The resistor R0 provided between the output of MP and the porous electrode 4b is for detecting the first pump current IP1, and the voltage across the resistor VIP1 is the ECU 50 as a detection signal of the first pump current IP1. Entered in.

Next, in the drive circuit 40, the control section 40
In addition to a, a constant current circuit 40b that is connected to the porous electrode 6c of the Vs cell 6 via the open / close switch SW2 and flows 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 through the open / close switch SW3 and causes a constant current to flow between the porous electrodes 6b-6c in the same direction as the minute current iCP.
c and are provided.

Each of the constant current circuits 40b and 40c has V
This is for detecting the internal resistance RVS of the s cell 6.
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 on the ECU 50 side by applying the constant current. The constant currents passed by the constant current circuits 40b and 40c are set to the same current value except that the current directions are different. This current value is larger than the minute current iCP supplied to the Vs cell 6 via the resistor R2.

Further, the open / close switches SW1 to SW3 provided between the control section 40a, the constant current circuits 40b and 40c, and the porous electrode 6c of the Vs cell 6 are the ECU 50.
Only when the control unit 40a is activated by turning on / off the control signal from the NOx concentration to perform the NOx concentration detection operation, 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 which constitutes the detection circuit 42. The application direction of this constant voltage VP2 is
In the second pump cell 8, a current flows from the porous electrode 8c to the 8b side, and oxygen in the second measurement chamber 26 is pumped out to the outside, so that the porous electrode 8c side is the positive electrode and the porous electrode 8
It is set such that the side b is a negative electrode. The constant voltage VP2 is applied to the diffusion rate controlling layers 6d and 22 from the first measuring chamber 20.
The voltage is set to, for example, 450 mV, which is capable of decomposing the NOx component in the gas to be measured in the second measurement chamber that flows in via d and pumping out the oxygen component.

The resistor R3 has a second pump current I flowing through the second pump cell 8 when the constant voltage VP2 is applied.
It is for converting P2 into voltage VIP2 and inputting it to the ECU 50 as a detection signal of the second pump current IP2. In the nitrogen oxide concentration detecting apparatus of the present 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 measurement gas flowing from the first measurement chamber 20 into the second measurement chamber 26 is controlled to be a constant oxygen concentration, the second pump current IP2 flowing through the second pump cell 8 is the oxygen concentration in the measurement gas. Is almost unaffected by the NOx concentration and changes according to the NOx concentration.
The NOx concentration in the measured gas is detected from the detection signal VIP2 (in other words, the second pump current IP2) by reading the detection signal VIP2 of the second pump current IP2 on the side of 50 and executing a predetermined calculation process. be able to.

By the way, 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 measuring 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 reaches 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 energization amount from the heater energizing circuit 44 to the heaters 12 and 14 is controlled so that the temperature of the Vs cell 6 becomes the target temperature.

The control process executed by the ECU 50 for the temperature control and the NOx concentration detection will be described below with reference to the flow charts shown in FIGS. 3 and 4. In addition, in FIG.
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 of the heaters 12 and 14.
0 represents an internal resistance detection process executed as an interrupt process at a constant time T0 (for example, 1 sec.).

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

This activation determination process is performed by, for example, the internal resistance R of the Vs cell 6 detected by the internal resistance detection process described later.
It is executed by determining whether VS has become equal to or lower 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, it is determined whether or not the element temperature has reached a predetermined activation temperature by determining whether or not the internal resistance RVS of the Vs cell 6 has become equal to or lower than the activation determination value after the energization of the heaters 12 and 14 is started. To judge.

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

Next, when it is determined in S100 that the NOx sensor 2 is activated, the process proceeds to S110 and the detection circuit 4
The second pump current IP2 is detected by reading the detection signal VIP2 input from the second resistor R3. In 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 subsequent 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 control by the drive circuit 40 prevents the NOx component in the gas to be measured in the first measurement chamber 20 from being decomposed from the first measurement chamber 20. The oxygen concentration of the measured gas flowing into the second measurement chamber 26 is controlled to be a low oxygen concentration.
In the measurement chamber 26, not only the NOx in the gas to be measured but also the first
Oxygen remaining in the measurement substance also flows in. Therefore,
The second pump current IP2 changes according to the NOx concentration in the measured gas, but is slightly affected by the oxygen concentration in the measured gas. FIG. 6 shows a first graph when the apparatus is operated with a test gas containing no NOx as a measured gas.
An example of the measurement results of the pump current IP1 and the second pump current IP2 is shown, but as is clear from this figure, the first
The pump current IP1 changes with a constant slope corresponding to the oxygen concentration in the gas to be measured, and the second pump current IP2 also changes under the influence of the oxygen concentration in the gas to be measured. And, the influence becomes stronger when the oxygen concentration is low.

Therefore, in this embodiment, the second pump current IP2
In order to correspond to the NOx concentration in the measured gas,
The value of the second pump current IP2 corresponding to the oxygen concentration obtained when measuring the measured gas that does not contain 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. It is stored in advance, the oxygen concentration in the gas to be measured is detected from the first pump current IP1, and the offset value corresponding to this oxygen concentration is read out from the offset value data stored in advance and used as the reference correction amount. I am trying to set it.

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

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

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

For example, in FIG. 7, the NOx concentration in the exhaust gas of an internal combustion engine is detected by using the detection device of this embodiment.
It shows an example of the measurement result of measuring the temperature change of the NOx sensor 2 when the x sensor 2 is attached to the exhaust pipe of the internal combustion engine and the device is operated. As is clear from this figure, in the detection apparatus of the present embodiment, the NOx sensor temperature temporarily drops with the increase of the intake air amount during acceleration of the internal combustion engine, although the temperature control described later is performed. Or, when decelerating the internal combustion engine, the amount of intake air decreases and NOx
When the sensor temperature temporarily rises, the first pump current IP1 and the second pump current IP2 both change due to the influence of the temperature change, especially the second pump current IP2.
Will take about 1 minute to return to a stable state. This is because once the oxygen concentration of the measured gas flowing from the first measurement chamber 20 into the second measurement chamber 26 deviates from the target concentration due to the influence of the exhaust temperature on the first pump current IP2, the This is because it takes time to return the oxygen concentration to the target concentration.

Therefore, in the present embodiment, in order to accurately detect the NOx concentration from the second pump current IP2 even if the temperature of the gas to be measured changes suddenly, the internal resistance RVS of the Vs cell 6 to the Vs cell 6 is obtained, and the second pump current I is calculated using a map for calculating the temperature correction amount as shown in FIG.
The temperature correction amount for P2 is calculated.

The map shown in FIG. 8 is set to obtain the temperature correction amount from the element temperature of the Vs cell 6, but a map for calculating the temperature correction amount using the internal resistance RVS of the Vs cell 6 as a parameter. By setting in advance, the temperature correction amount can be directly obtained from the internal resistance RVS without converting the internal resistance RVS into temperature. In addition, for example, a map having a deviation between the element temperature and the target temperature (the target temperature is 850 ° C. in FIG. 8) as a parameter is set in advance, and the deviation (deviation) of the element temperature from the target temperature is set. The temperature correction amount may be obtained, or the internal resistance R
It is also possible to preset a map that uses the deviation between VS and the target resistance value corresponding to the target temperature as a parameter, and obtain the temperature correction amount from the 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, and the second pump current is added by adding the reference correction amount and the temperature correction amount to the second pump current IP2 detected in S110. Correct IP2. Then, in subsequent S170, the second pump current IP2 after the correction is set to NO.
The x density is output, and the process proceeds to S110 again.

In this embodiment, the second pump current IP2
The processing of S150 and S160 for correcting the above according to the temperature of the NOx sensor 2 (specifically, the Vs cell 6) corresponds to the correcting means of the present invention. Then, in the present embodiment, in the NOx concentration detection processing, the second pump current I is determined according to the oxygen concentration in the measured gas based on the first pump current IP1.
It is described 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 calculated to correct the second pump current IP2. However, for example, by setting a map for calculating the reference correction amount for each temperature of the Vs cell 6 and switching the map used for calculating the reference correction amount 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 obtained. Alternatively, a two-dimensional map for calculating the correction amount using (or the internal resistance RVS) as a parameter 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. It should be noted 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 energization controlling means for controlling the amount of energizing current 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, the voltage Vs on the porous electrode 6c side of the Vs cell 6 is read in S210, and this is read as the basic detection voltage VS1 of the Vs cell 6.
Set as. Then, in the following S220, the open / close switch SW that has been turned on for detecting the NOx concentration.
1 is turned off and the open / close switch SW2 connected to the constant current circuit 40b is turned on, so that the Vs cell 6 has a direction opposite 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 passed in the direction of pumping oxygen to the measuring chamber 20 side.

In subsequent S230, it is determined whether or not a predetermined time T1 (for example, 60 μsec.) Has elapsed after the detection process is started, and the predetermined time T1 is waited until the predetermined time T1 is reached. When the time elapses, in S240,
The voltage Vs on the porous electrode 6c side of the Vs cell 6 is read, and this is set as the resistance detection voltage VS2 of the Vs cell 6.

When the resistance detection voltage VS2 is set in this way,
After the detection process is started, the process proceeds to S250, and it is determined whether or not a predetermined time T2 (for example, 100 μsec.) Has elapsed, thereby waiting for the predetermined time T2 to elapse, and when the predetermined time T2 elapses, In S260, the Vs cell 6 is turned on by turning off the open / close switch SW2 that has been in the on state for the fixed time T2 after the activation of the detection process and turning on the open / close switch SW3 connected to the constant current circuit 40c.
Then, a constant current is passed in the same direction as the minute current iCP (that is, the direction in which oxygen in the first measurement chamber 20 is pumped to the closed space side).

When the open / close switch SW3 is turned on in this way, the process proceeds to S270, and this time, it is determined whether or not a predetermined time T3 (for example, 200 μsec.) Has elapsed after the start of the detection processing, thereby determining a predetermined time. Waiting for the time T3 to elapse, and when the predetermined time T3 elapses, the open / close switch SW3 is turned off in S280. As a result, the drive circuit 40
All the open / close switches SW1 to SW3 are turned off.

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

In step S310, the heaters 12 and 14 are operated 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 to control the amount of current supplied from the heater energizing circuit 44 to the heaters 12 and 14 as heater energizing control means. .

In this heater energization control, when the heater energization circuit 44 is composed of a switching circuit capable of switching the energization / non-energization to the heaters 12 and 14 at high speed, the energization / non-energization is switched. It suffices to control the duty ratio of the drive pulse to be performed, and the heater energization circuit 44 is composed of a voltage control circuit capable of detecting the energization current to the heaters 12 and 14 and controlling the output voltage to the heaters 12 and 14. In this case, the target voltage that is the target value of the heater current may be output.

When the heater control signal is output in this way, this time, the process proceeds to S320, and it is determined whether or not a predetermined time T4 (for example, 500 μsec.) Has elapsed after the activation of the detection process, Waiting for the predetermined time T4 to elapse, and when the predetermined time T4 elapses, in S330,
After the detection process is activated, the open / close switch SW1 that has been in the off state for the fixed time T4 is turned on, and the detection process is ended, thereby restarting the NOx concentration detection operation.

In the internal resistance detection processing described above, FIG.
As shown in (4), when the process is started (time point t1), the open / close switch SW1 in the drive circuit 40 is turned off, and the Vs cell 6
To the Vs cell 6 by turning on / off the switch SW2 while stopping the energization of the minute current iCP to the
A constant current flows in the direction opposite to the minute current iCP. Then, when a certain time T1 has passed thereafter (time point t2), the voltage Vs on the porous electrode 6c side at that time is set as the resistance detection voltage VS2, and the resistance detection voltage VS2 and the porous electrode 6c side at the time of starting the detection process. 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 reason for this will be described below.

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

Further, the oxygen concentration on the side of each electrode 6b, 6c of the Vs cell 6 becomes substantially constant due to the pump current control and the application of the minute current ICP, so that the electromotive force of the Vs cell 6 also becomes substantially constant. Become. Therefore, by applying a constant current 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 measuring chamber 20 into the second measuring chamber 26 is
Since it is controlled by the feedback control of the pump current, it fluctuates due to the response delay of the control system and the like, and is not fixed to a constant concentration. Further, the oxygen concentration of the measured gas flowing from the first measurement chamber 20 into 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 applying a constant current for detecting the internal resistance RVS to the s cell 6, an error will occur in the internal resistance RVS, albeit slightly.

Therefore, in this embodiment, in order to detect the internal resistance RVS of the Vs cell 6 and further the element temperature more accurately, a constant current for detecting the internal resistance RVS is applied to the Vs cell 6. To a predetermined time (for example, 60 μsec.), The amount of change (deviation ΔVs) in the voltage Vs on the porous electrode 6c side is detected, and the internal resistance RVS is obtained from the deviation ΔVs to obtain the first measurement chamber. 20 to the second measuring chamber 26
Even if the oxygen concentration of the gas to be measured flowing into is deviated from the target concentration, the internal resistance RVS of the Vs cell 6, and consequently 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. You can do this. Next, in the internal resistance detection processing of the present embodiment, after the start-up, a fixed time T1 has elapsed and the resistance detection voltage VS2 is set (time t2), and then a predetermined time (for example, 40 μsec.) Has elapsed. , Time point t3 when the elapsed time after the detection process is activated reaches T2
Then, by turning off the open / close switch SW2 of the drive circuit 40 and turning on the open / close switch SW3, a constant current is caused to flow in the Vs cell 6 in the same direction as the minute current iCP, and a certain period of time (for example, 100 μsec.) Elapses. Then, when the elapsed time after starting the detection process reaches T3 (time point t4), the open / close switch SW3 is turned off.

As a result, in the present embodiment, the polarization between the solid electrolyte body of the Vs cell 6 or the electrode and the solid electrolyte body for the detection of the internal resistance RVS can be promptly relaxed, and the Vs cell 6 can be relaxed. Can quickly function as an oxygen concentration battery cell. Therefore, after starting the process,
The time T4 before entering the NOx concentration detection operation 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 NOx concentration detection operation. It is possible to detect with high accuracy.

As described above, in the nitrogen oxide concentration detector of the present embodiment, the oxygen concentration that has the greatest influence on the NOx concentration detection accuracy (specifically, the first measurement chamber 2
The temperature of the NOx sensor 2 is detected from the internal resistance RVS of the Vs cell 6 that detects the oxygen concentration of the measured gas flowing into the second measurement chamber 26 from 0, and this temperature is the target temperature (for example, 8
If the detected internal resistance RVS or the element temperature obtained from this internal resistance RVS deviates from the target value, the deviation is controlled so that the temperature becomes 50 ° C.). The NOx concentration detection result is temperature-compensated by correcting the second pump current IP2 representing the NOx concentration detection result with a temperature correction amount according to the above. Therefore, according to the nitrogen oxide concentration detecting apparatus 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 particular, in this embodiment, the NOx sensor 2
Is the first pump cell 4, the Vs cell 6, the second pump cell 8
And the heaters 12 and 14 are laminated on both sides in the stacking direction, and when the NOx sensor 2 is projected from the stacking direction, the diffusion rate controlling layer 4d and the diffusion rate controlling layers 6d and 22d are formed. Are overlapped with each other, and the heater wirings 12b and 14b of the heaters 12 and 14 are arranged so as to sandwich these diffusion rate controlling layers at substantially the center position. Therefore, in the present embodiment, the structure of the NOx sensor 2 allows the heaters 12 and 14 to be used to efficiently heat the cells 4 to 8, and the first measurement chamber 20 via the diffusion rate controlling layers. Also, the gas to be measured flowing into the second measurement chamber 26 can be efficiently heated. Therefore, according to the present embodiment, by controlling the temperature of the Vs cell 6, it becomes possible to more reliably control the temperature of each cell forming the NOx sensor 2 to the target temperature, and to detect the NOx concentration. The accuracy can be improved.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modes can be adopted. For example, in the above-described 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 plate-shaped solid electrolyte layer 6a. Since it suffices to be able to detect the oxygen concentration of the gas to be measured flowing into the second measurement chamber 26 from the first measurement chamber 20, it is not always necessary to configure as in the above embodiment, and for example, FIG.
It may be configured as shown in (a) and (b).

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

In the above embodiment, the first pump current I
From the first measurement chamber 20 to the second measurement chamber 2 so that the NOx component in the measured gas is not decomposed by the energization of P1
The oxygen concentration of the measured gas flowing into 6 has been described as being controlled to a low oxygen concentration in which a small amount of oxygen exists.
Since the detection accuracy of the NOx concentration by the second pump cell 8 can be improved as the oxygen in the measured gas flowing from the measurement chamber 20 into the second measurement chamber 26 is reduced, for example,
The first pump cell 4 may pump 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, a first pump current IP1 that decomposes the NOx component in the first measurement chamber 20 is caused to flow through the first pump cell 4, and the first measurement current is passed from the first measurement chamber 20 to the second measurement chamber 26. The oxygen concentration in the measured gas that flows in is made sufficiently small. And if you do this,
The offset value of the second pump current IP2 is reduced to
It is possible to improve the x-density detection accuracy.

However, even with such a configuration, the oxygen in the gas to be measured flowing into the second measurement chamber 26 cannot be made zero. Therefore, when the NOx concentration is detected, it is different from that in the above embodiment. Similarly, an offset value corresponding to the oxygen concentration obtained from the first pump current IP1 is obtained as a reference correction amount, and using this reference correction amount and the temperature correction amount obtained based on the internal resistance RVS of the Vs cell 6, It is desirable to obtain the NOx concentration by correcting the 2 pump current IP2.

In this case, the first pump current IP1 is
Since the current component generated by decomposing the NOx component in the measured gas is also included, in addition to the map for obtaining the offset value of the first pump current IP1 from the second pump current IP2, the first pump cell 4 NO decomposed by
A map for obtaining a gain for compensating for 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 calculated.

NOx concentration = gain × (IP2−offset value) This expression is an expression for correcting the second pump current IP2 based on the offset value and the gain obtained as the reference correction amount from the first pump current IP1. Therefore, when the second pump current IP2 is corrected using the temperature correction amount described above,
It is necessary to further correct the NOx concentration obtained by this equation with the temperature correction amount.

[Brief description of drawings]

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

FIG. 2 is an exploded perspective view showing the configuration of the NOx sensor of the embodiment.

FIG. 3 is an N repeatedly executed in the ECU of the embodiment.
It is a flow chart showing Ox concentration detection processing.

FIG. 4 is a flowchart showing 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 the relationship between the internal resistance of the oxygen concentration measuring cell and the element temperature.

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

FIG. 7 is a time chart showing changes in the first pump current and the second pump current caused by changes 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.

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

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

[Explanation of symbols]

2 ... NOx sensor 4 ... 1st 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 rate controlling layer 6f ... Leakage resistance portions 12a, 12c, 14a, 14c ... Heater substrates 12b, 14b ... Heater wiring 20 ... First measurement chamber
26 ... 2nd measurement chamber 40 ... Drive circuit 40a ... Control part 40b, 40c
... constant current circuit 42 ... detection circuit 44 ... heater energizing circuit AMP ...
Differential amplifiers R0, R1, R2, R3 ... Resistors SW1, SW2, SW3 ... Open / close switches

Continuation of the front page (56) Reference JP-A-10-142194 (JP, A) JP-A-10-232220 (JP, A) JP-A-8-271476 (JP, A) JP-A-6-27078 (JP , A) JP-A-59-163556 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/416 G01N 27/41 G01N 27/419

Claims (7)

(57) [Claims] 1. A first oxygen pumping cell having an oxygen ion conductive solid electrolyte layer sandwiched between porous electrodes and an oxygen concentration measuring cell, which communicates with a gas to be measured side through a 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 connected to the first measurement chamber via a second diffusion control layer. A second measuring chamber, a first oxygen pumping cell and an oxygen concentration measuring cell
In the state where the electrodes on the side of the first measurement chamber are connected to each other,
Electrode on the side of the oxygen concentration measuring cell opposite to the first measuring chamber
To by detecting the voltage produced, and detects an output voltage of said oxygen concentration measuring cell, measuring the first of said first oxygen pumping cell so that the output voltage becomes constant value
By applying an electric current to the electrode on the side opposite to the fixed chamber,
Ri, the pumped out oxygen from the first measurement chamber by the first oxygen pumping cell, and a pump current control means for controlling the oxygen concentration in the measurement gas flowing into the second measurement chamber from said first measurement chamber to a constant Constant voltage applying means for applying a constant voltage to the second oxygen pumping cell in a direction of pumping oxygen from the second measurement chamber, and a constant voltage applying means for applying a constant voltage to the second oxygen pumping cell based on a current value flowing in the second oxygen pumping cell. detecting the NOx concentration detecting means for detecting an NOx concentration in the measurement gas, a heater for heating said sensor body to detectable temperature nitrogen oxide concentration, the temperature of the oxygen concentration measuring cell A heater energization control that controls energization to the heater so that the temperature of the oxygen concentration measuring cell detected by the temperature detecting means and the oxygen concentration measuring cell reaches a preset target temperature. Nitrogen oxide concentration detection apparatus comprising: the means. 2. The nitrogen oxide concentration detected by the nitrogen oxide concentration detecting means is corrected according to the deviation of the temperature of the oxygen concentration measuring cell detected by the temperature detecting means from the target temperature. The nitrogen oxide concentration detecting device according to claim 1, further comprising a correcting means for temperature-compensating the detected value of the nitrogen oxide concentration. 3. The temperature detecting means detects the temperature of the oxygen concentration measuring cell by detecting the internal resistance of the oxygen concentration measuring cell, and the heater energization control means detects the inside of the detected oxygen concentration measuring cell. The nitrogen oxide concentration detecting device according to claim 1 or 2, wherein energization to the heater is controlled so that the resistance has a predetermined value corresponding to the target temperature. 4. In the sensor body, the porous electrode on the opposite side of the oxygen concentration measuring cell from the first measurement chamber is closed, and a part of oxygen in the closed space is leaked through a leak resistance portion. The pump current control means applies a minute current to the oxygen concentration measuring cell in a direction for pumping oxygen in the first measuring chamber into the closed space, and the closed space is formed. While functioning as an internal oxygen reference source, the amount of current flowing through the first oxygen pumping cell is controlled so that the electromotive force generated in the oxygen concentration measuring cell has a constant value, and the temperature detecting means controls the pump current control. The connection between the means and the oxygen concentration measuring cell is periodically interrupted, and at the time of interruption, an internal resistance detection current larger than the minute current is passed through the oxygen concentration measuring 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 detecting current through the oxygen concentration measuring cell, and then reverses the internal resistance detecting current through the oxygen concentration measuring cell. 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 different thin plate-shaped solid electrolyte layers, and the first measuring chamber and the first measuring chamber The two measurement chambers are formed by stacking the respective solid electrolyte layers with a predetermined gap, with the solid electrolyte layers having the first and second oxygen pumping cells formed on the outside, and the heater being a substrate. Thin plate-shaped 2 with heater wiring formed on
The heater main body is composed of a single heater substrate, and the heater main body is configured to be heatable by arranging the heater substrates on both sides of the solid electrolyte layers in the sensor main body in the stacking direction with a predetermined gap therebetween . One diffusion rate controlling layer is formed at a location including a position facing the central portion of the heater wiring formed on the heater substrate in the solid electrolyte layer in which the first oxygen pumping cell is formed. 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 body is projected from the stacking direction of the solid electrolyte layers, and the second diffusion-controlling layer is formed. 7. The nitrogen oxide concentration detecting device according to claim 6, wherein the oxygen concentration measuring cell is arranged in the vicinity of the diffusion-controlling layer.