JP7741752B2 - Gas sensor control method - Google Patents

Description

The present invention relates to a method for controlling a gas sensor.

Gas sensors are used to detect and measure the concentration of target gas components (oxygen _O2 , nitrogen oxides NOx, ammonia _NH3 , hydrocarbons HC, carbon dioxide _CO2, etc.) in measurement gases such as automobile exhaust gases. For example, the concentration of target gas components in automobile exhaust gases is measured, and the exhaust gas purification system installed in the automobile is optimally controlled based on the measured value.

Known examples of such gas sensors include a gas sensor having a sensor element that uses an oxygen ion conductive solid electrolyte such as zirconia ( _ZrO2 ). In order to detect a target gas component, the gas sensor is used by heating the sensor element to a temperature at which the solid electrolyte exhibits oxygen ion conductivity.

For example, Japanese Patent Publication No. 6208060 discloses that the sensor element of a gas sensor is equipped with a heater for heating the sensor element. It also discloses an embodiment in which the sensor element includes a main pump cell and an auxiliary pump cell for adjusting the oxygen concentration, and a measurement pumping cell for detecting the gas component to be measured (e.g., NOx).

Furthermore, Japanese Patent Publication No. 6208060 discloses a method for setting a temperature rise profile for raising the temperature of a sensor element to a predetermined operating temperature, and an example of the temperature rise profile (Figure 7).

Patent No. 6208060

Gas sensors are used to control exhaust gas purification systems installed in automobiles, and are required to accurately measure the concentration of target gases (oxygen _O2 , nitrogen oxides NOx, ammonia _NH3 , hydrocarbons HC, carbon dioxide _CO2 , etc.) Even at startup, the gas sensor is required to be able to perform accurate measurements in as short a time as possible.

A gas sensor requires that the sensor element be heated to a temperature that will cause the solid electrolyte to exhibit oxygen ion conductivity. Therefore, the time it takes for a gas sensor to start measuring accurately after receiving a dew point signal from an exhaust gas purification system, etc. (i.e., the start-up time) depends primarily on the time it takes for the sensor element to heat up.

During the gas sensor's startup time, it is unable to detect the target gas (nitrogen oxides, NOx, etc.), which means the exhaust gas purification system does not function properly, and nitrogen oxides, NOx, etc. contained in the exhaust gas may be emitted unpurified. To prevent this situation, there is a need to shorten the startup time.

Furthermore, when NOx and high concentrations of oxygen are present in the gas being measured, it generally tends to be difficult to maintain high accuracy in measuring the NOx concentration.

The present invention aims to provide a method for controlling a gas sensor during startup that can shorten the startup time from when the gas sensor is started until accurate measurement can begin.

Furthermore, the present invention aims to provide a method for controlling a gas sensor at startup that shortens the startup time from when the gas sensor is started until accurate measurement can begin, and that can maintain high NOx detection accuracy over long periods of use, even when the oxygen concentration in the measured gas is high.

In order to shorten the start-up time, the inventors investigated ways to shorten the time required to heat the sensor element to the desired temperature (i.e., the temperature rise time), as follows:

Increasing the heater's heating rate is one way to shorten the temperature rise time. However, it has been confirmed that if the heater's heating rate is too high, there is an increased risk of cracks occurring in the internal structure of the sensor element. This is discussed in more detail in Comparative Example 1 below.

On the other hand, by following the temperature rise profile disclosed in Japanese Patent No. 6208060 and decreasing the temperature rise rate as the temperature of the sensor element increases, the risk of cracks occurring in the internal structure of the sensor element can be reduced. However, because the temperature rise rate needs to be reduced in the high-temperature region of the sensor element (region close to the operating temperature), the temperature rise time tends to be longer.

In addition, one way to shorten the temperature rise time is to lower the operating temperature of the sensor element. However, if the operating temperature is too low, the oxygen ion conductivity of the solid electrolyte that makes up the sensor element deteriorates. As a result, it has been confirmed that the target gas (nitrogen oxides, NOx, etc.) cannot be accurately detected, particularly when the oxygen concentration of the target gas is high. This is described in more detail in Comparative Example 2 below.

After extensive research, the inventors discovered that the startup time from when the gas sensor was started until measurement could begin can be shortened by controlling the gas sensor so that the gas sensor's activity determination unit determines whether the sensor element is in an activated state where measurements can be made, starts pre-driving from the heater temperature (i.e., the activation temperature) when it is determined that the sensor element is in an activated state where measurements can be made, and then begins steady-state driving once the steady-state driving temperature is reached. Furthermore, by controlling the gas sensor with this control method, the inventors discovered that the NOx detection accuracy of the gas sensor can be maintained at a high level over long periods of use, even when the oxygen concentration in the gas being measured is high.

The present invention includes the following inventions.
(1) A method for controlling a gas sensor for detecting NOx in a measurement gas, comprising:
The gas sensor comprises:
A sensor element;
an activation determination unit that determines whether the sensor element is in a measurable activated state;
Including,
The sensor element is
a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers;
a measurement gas flow portion formed at one end of the base portion in the longitudinal direction;
a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode, the main pump cell adjusting oxygen in the measurement target gas to a desired concentration;
a measurement pump cell for detecting NOx in the measurement gas, the measurement pump cell including: a measurement electrode disposed on an inner surface of the measurement gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed on the base portion at a position different from the measurement gas flow portion and corresponding to the measurement electrode;
a heater for heating the base portion;
a reference electrode disposed inside the base portion so as to be in contact with a reference gas;
Including,
The control method includes:
a temperature raising step of heating the sensor element by the heater to an activation temperature at which the activation determining unit determines that the sensor element is in a measurable activated state;
a pre-driving step of raising the temperature of the sensor element from the activation temperature to a steady-state driving temperature by the heater and operating the main pump cell and the measuring pump cell to detect NOx in the measurement gas;
a steady-state driving step of maintaining the sensor element at the steady-state driving temperature by the heater and operating the main pump cell and the measuring pump cell to continuously detect NOx in the measurement gas;
A method for controlling a gas sensor for detecting NOx in a measurement gas, comprising:

(2) The control method described in (1) above, wherein the oxygen concentration adjusted by the main pump cell in the advance driving step is higher than the oxygen concentration adjusted by the main pump cell in the steady driving step.

(3) The sensor element further comprises:
an auxiliary pump cell including an auxiliary pump electrode disposed on an inner surface of the measurement gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement gas flow portion of the base portion and corresponding to the auxiliary pump electrode,
The control method according to (1) or (2) above, wherein the auxiliary pump electrode is disposed at a position closer to the one end in the longitudinal direction of the base portion than the measurement electrode.

(4) In the auxiliary pump cell, a pump current flowing through the auxiliary pump cell is controlled based on an electromotive force between the auxiliary pump electrode and the reference electrode;
The control method according to (3) above, further comprising controlling an electromotive force between the inner main pump electrode and the reference electrode in the main pump cell so that the pump current flowing through the auxiliary pump cell is constant.

(5) The control method described in (4) above, wherein, in the advance driving step, the electromotive force between the auxiliary pump electrode and the reference electrode is controlled to a value smaller than the electromotive force between the auxiliary pump electrode and the reference electrode in the steady driving step.

(6) The control method described in (4) or (5) above, wherein the pump current flowing through the auxiliary pump cell in the advance driving step is controlled to a value greater than the pump current flowing through the auxiliary pump cell in the steady driving step.

(7) The method includes controlling a pump current flowing through the measurement pump cell based on an electromotive force between the measurement electrode and the reference electrode in the measurement pump cell,
A control method described in any one of (1) to (6) above, wherein in the advance driving step, the electromotive force between the measurement electrode and the reference electrode is controlled to a value different from the electromotive force between the measurement electrode and the reference electrode in the steady-state driving step.

(8) The control method described in (7) above, wherein, in the advance driving step, the electromotive force between the measurement electrode and the reference electrode is controlled to a value greater than the electromotive force between the measurement electrode and the reference electrode in the steady driving step.

(9) A control method described in any one of (1) to (8) above, in which the activity determination unit determines whether the sensor element is in a measurable active state based on at least one selected from the group consisting of the heater temperature, the resistance value of the main pump cell, and the resistance value between the inner main pump electrode and the reference electrode.

(10) A control method described in any one of (1) to (9) above, wherein, in the advance driving step, the heater is heated from the activation temperature to the steady driving temperature based on a preset temperature rise rate in the advance driving step.

(11) The control method described in (10) above, in which the temperature rise rate in the advance driving step is set in multiple stages.

(12) A control method described in any one of (1) to (11) above, wherein, in the temperature-raising step, heating to the activation temperature is performed based on a preset temperature-raising rate in the temperature-raising step.

(13) The control method described in (12) above, wherein the temperature increase rate in the temperature increase step is set in multiple stages.

(14) A gas sensor including a sensor element and a control unit that controls the sensor element,
The sensor element is
a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers;
a measurement gas flow portion formed at one end of the base portion in the longitudinal direction;
a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode, the main pump cell adjusting oxygen in the measurement target gas to a desired concentration;
a measurement pump cell for detecting NOx in the measurement gas, the measurement pump cell including: a measurement electrode disposed on an inner surface of the measurement gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed on the base portion at a position different from the measurement gas flow portion and corresponding to the measurement electrode;
a heater disposed to heat the base portion;
a reference electrode disposed inside the base portion so as to be in contact with a reference gas;
Including,
The control unit
an activation determination unit that determines whether the sensor element is in a measurable activated state;
a heater control unit that controls the temperature of the heater;
a pump control unit that operates the main pump cell and the measurement pump cell to detect NOx in the measurement gas;
Including,
the activation determination unit determines whether the sensor element is in an activated state where measurement is possible;
the heater control unit heats the sensor element by the heater to an activation temperature at which the activation determination unit determines that the sensor element is in a measurable activated state, and further heats the sensor element to a steady-state driving temperature;
The pump control unit starts operation of the main pump cell and the measurement pump cell when the sensor element reaches the activation temperature, and further continues operation of the main pump cell and the measurement pump cell when the sensor element reaches the steady-state operating temperature.

This invention shortens the startup time from when the gas sensor is started until accurate measurements can begin.

Furthermore, according to the present invention, the startup time from when the gas sensor is started until accurate measurement can begin is short, and the gas sensor's NOx detection accuracy can be maintained at a high level over long periods of use, even when the oxygen concentration in the measured gas is high.

1 is a schematic vertical cross-sectional view showing an example of the schematic configuration of a gas sensor 100 in a longitudinal direction. 2 is a block diagram showing the connection relationship between the control unit 90 and each of the pump cells 21, 50, 41 of the sensor element 101 and the heater portion 70. FIG. 1 is a schematic diagram showing an example of the relationship between the change in heater temperature over time and the driving of the pump cell according to an embodiment of the present invention at the start of the gas sensor 100. The horizontal axis represents time (seconds), and the vertical axis represents heater temperature (°C). 10 is a schematic diagram showing another example of the relationship between the change in heater temperature over time and the driving of the pump cell according to the embodiment of the present invention at the start of the gas sensor 100. The horizontal axis represents time (seconds), and the vertical axis represents heater temperature (°C). 10 is a schematic diagram showing an example of the relationship between the change in heater temperature over time and the driving of the pump cell in a comparative example at the start of the gas sensor 100. The horizontal axis represents time (seconds), and the vertical axis represents heater temperature (° C.). 1 is a graph showing the change over time in the NOx concentration output value at the start of the gas sensors 100 of Examples 1 to 3 and Comparative Example 1. The horizontal axis represents time (seconds: s), and the vertical axis represents the NOx concentration output value (ppm) of the gas sensor 100.

The gas sensor of the present invention includes a sensor element and a control unit that controls the sensor element.

The sensor element included in the gas sensor of the present invention is
a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers;
a measurement gas flow portion formed at one end of the base portion in the longitudinal direction;
a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode, the main pump cell adjusting oxygen in the measurement target gas to a desired concentration;
a measurement pump cell for detecting NOx in the measurement gas, the measurement pump cell including: a measurement electrode disposed on an inner surface of the measurement gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed on the base portion at a position different from the measurement gas flow portion and corresponding to the measurement electrode;
a heater for heating the base portion;
a reference electrode disposed inside the base portion so as to be in contact with a reference gas;
Includes.

The control unit included in the gas sensor of the present invention comprises:
an activation determination unit that determines whether the sensor element is in a measurable activated state;
a heater control unit that controls the temperature of the heater;
a pump control unit that operates the main pump cell and the measurement pump cell to detect NOx in the measurement gas;
Includes.

An example embodiment of the gas sensor of the present invention is described in detail below.

[Outline of gas sensor configuration]
The gas sensor of the present invention will be described below with reference to the drawings. Fig. 1 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of the general configuration of a gas sensor 100 including a sensor element 101. In the following, with reference to Fig. 1, the upper side of Fig. 1 will be referred to as the top, the lower side as the bottom, the left side of Fig. 1 as the leading end side, and the right side as the trailing end side.

In Figure 1, the gas sensor 100 is an example of a limiting current type NOx sensor that uses a sensor element 101 to detect NOx in the measurement gas and measure its concentration.

The gas sensor 100 also includes a control unit 90. Figure 2 is a block diagram showing the connection between the control unit 90 and the sensor element 101.

(sensor element)
The sensor element 101 is a long, plate-shaped element including a base portion 102 having a structure in which multiple oxygen-ion conductive solid electrolyte layers are stacked. The long, plate-shaped structure is also referred to as a long plate or a strip-shaped structure. The base portion 102 has a structure in which six _layers are stacked in this order from bottom to top as viewed in the drawing: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is made of an oxygen-ion conductive solid electrolyte layer such as zirconia (ZrO 2 ). The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness or may have different thicknesses. The layers are bonded together via adhesive layers made of solid electrolyte, and the base portion 102 includes the adhesive layers. While FIG. 1 illustrates a layer structure consisting of six layers, the layer structure of the present invention is not limited to this and any number and layer structure may be used.

The sensor element 101 is manufactured, for example, by laminating ceramic green sheets corresponding to each layer, performing the required processing and printing circuit patterns on them, and then firing them to integrate them.

A gas inlet 10 is formed at one longitudinal end (hereinafter referred to as the tip) of the sensor element 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measurement gas flow section 15 is formed in the longitudinal direction from the gas inlet 10, with a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, a first internal cavity 20, a third diffusion-controlling section 30, a second internal cavity 40, a fourth diffusion-controlling section 60, and a third internal cavity 61 adjacently and communicating in this order.

The gas inlet 10, buffer space 12, first internal cavity 20, second internal cavity 40, and third internal cavity 61 are spaces within the sensor element 101, defined by hollowing out the spacer layer 5, with the upper portion defined by the underside of the second solid electrolyte layer 6, the lower portion defined by the upper surface of the first solid electrolyte layer 4, and the sides defined by the side surfaces of the spacer layer 5.

The first diffusion rate-limiting section 11, the second diffusion rate-limiting section 13, and the third diffusion rate-limiting section 30 are each provided as two horizontally elongated slits (with the openings extending in the direction perpendicular to the plane of the drawing in Figure 1). The first diffusion rate-limiting section 11, the second diffusion rate-limiting section 13, and the third diffusion rate-limiting section 30 may each have any shape that provides the desired diffusion resistance, and their shape is not limited to the slits.

The fourth diffusion-controlling portion 60 is provided between the spacer layer 5 and the second solid electrolyte layer 6 as a single horizontally elongated slit (with the opening extending longitudinally in the direction perpendicular to the plane of the drawing in Figure 1). The fourth diffusion-controlling portion 60 may have any shape that provides the desired diffusion resistance, and its shape is not limited to the slit.

Furthermore, at a position farther from the tip side than the measurement gas flow portion 15, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, at a position defined on the side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. Air, for example, is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

The air introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed to cover the reference electrode 42.

The reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, is surrounded by an air introduction layer 48 that connects to the reference gas introduction space 43. In other words, the reference electrode 42 is disposed so as to come into contact with the reference gas via the porous air introduction layer 48 and the reference gas introduction space 43. Furthermore, as described below, the reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20, the second internal space 40, and the third internal space 61.

In the measurement gas flow section 15, the gas inlet 10 is open to the external space, and the measurement gas is introduced into the sensor element 101 from the external space through the gas inlet 10.

In this embodiment, the measurement gas flow section 15 is configured such that the measurement gas is introduced through the gas inlet 10 opening at the tip end surface of the sensor element 101, but the present invention is not limited to this configuration. For example, the measurement gas flow section 15 does not need to have a recess for the gas inlet 10. In this case, the first diffusion-controlling section 11 essentially serves as the gas inlet.
Furthermore, for example, the measurement gas flow part 15 may have an opening in a side surface along the longitudinal direction of the base part 102, the opening communicating with the buffer space 12 or a position in the first internal space 20 close to the buffer space 12. In this case, the measurement gas is introduced from the side surface along the longitudinal direction of the base part 102 through the opening.
Furthermore, for example, the measurement gas flow portion 15 may be configured so that the measurement gas is introduced through a porous body.

The first diffusion-controlling section 11 is a section that imparts a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10.

The buffer space 12 is a space provided to guide the measurement gas introduced from the first diffusion-controlling section 11 to the second diffusion-controlling section 13.

The second diffusion rate-controlling section 13 is a section that provides a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20.

It is sufficient that the amount of measurement gas introduced into the first internal space 20 is within a predetermined range. In other words, it is sufficient that a predetermined diffusion resistance is imparted to the entire area from the tip of the sensor element 101 to the second diffusion-controlling section 13. For example, it is also possible for the first diffusion-controlling section 11 to be directly connected to the first internal space 20, i.e., for the buffer space 12 and the second diffusion-controlling section 13 not to exist.

The buffer space 12 is a space provided to mitigate the effect of pressure fluctuations on the detected value when the pressure of the gas being measured fluctuates.

When the measurement gas is introduced from outside the sensor element 101 into the first internal space 20, pressure fluctuations in the measurement gas in the external space (exhaust pressure pulsations, if the measurement gas is automobile exhaust gas) cause the measurement gas to be suddenly drawn into the sensor element 101 from the gas inlet 10. Instead of being introduced directly into the first internal space 20, the measurement gas is introduced into the first internal space 20 after the pressure fluctuations are canceled out through the first diffusion-controlling section 11, buffer space 12, and second diffusion-controlling section 13. This makes the pressure fluctuations of the measurement gas introduced into the first internal space almost negligible.

The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion-controlling section 13. This oxygen partial pressure is adjusted by the operation of the main pump cell 21.

The main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 disposed on the inner surface of the measurement gas flow portion 15, and an outer pump electrode 23 disposed at a position on the base portion 102 different from the measurement gas flow portion 15 (in FIG. 1, on the outer surface of the base portion 102) that corresponds to the inner main pump electrode 22. "Corresponding to the inner main pump electrode 22" means that the outer pump electrode 23 is disposed on the inner main pump electrode 22 via a second solid electrolyte layer 6.

In other words, the main pump cell 21 is an electrochemical pump cell comprising an inner main pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a and exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

The inner main pump electrode 22 is formed across the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that define the first internal space 20, and the spacer layer 5 that provides the sidewalls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6, which provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4, which provides the bottom surface. Side electrodes (not shown) are formed on the sidewall surfaces (inner surfaces) of the spacer layer 5 that form both sidewalls of the first internal space 20 to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-shaped structure at the locations where the side electrodes are located.

The inner main pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, a cermet electrode of Pt containing 1% Au and _ZrO2 ). The inner main pump electrode 22, which comes into contact with the measurement gas, is formed using a material with a weakened ability to reduce the NOx component in the measurement gas.

In the main pump cell 21, by applying a desired pump voltage Vp0 between the inner main pump electrode 22 and the outer pump electrode 23 using a variable power supply 24 and flowing a pump current Ip0 in either a positive or negative direction between the inner main pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen from the first internal space 20 out to the external space, or to pump oxygen from the external space into the first internal space 20.

In addition, to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner main pump electrode 22, the second solid electrolyte 6, the spacer layer 5, the first solid electrolyte 4, the third substrate layer 3, and the reference electrode 42 constitute an electrochemical sensor cell, i.e., the oxygen partial pressure detection sensor cell 80 for controlling the main pump.

The oxygen concentration (oxygen partial pressure) within the first internal space 20 can be determined by measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Furthermore, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 so that the electromotive force V0 remains constant. This allows the oxygen concentration within the first internal space 20 to be maintained at a predetermined constant value.

The third diffusion rate-controlling section 30 applies a predetermined diffusion resistance to the measurement gas, the oxygen concentration (oxygen partial pressure) of which has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas into the second internal space 40.

The second internal space 40 is provided as a space for more precisely adjusting the oxygen partial pressure in the measurement gas introduced through the third diffusion-controlling section 30. This oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50. The second internal space 40 and auxiliary pump cell 50 may be omitted. From the perspective of precision in adjusting the oxygen partial pressure, it is more preferable to have the second internal space 40 and auxiliary pump cell 50.

In the second internal space 40, the oxygen concentration (oxygen partial pressure) is first adjusted in the first internal space 20, and then the oxygen partial pressure of the measurement gas introduced through the third diffusion-controlling section is further adjusted by the auxiliary pump cell 50. This allows the oxygen concentration in the second internal space 40 to be kept constant with high precision, enabling the gas sensor 100 to measure NOx concentrations with high accuracy.

The auxiliary pump cell 50 is an electrochemical pump cell including an auxiliary pump electrode 51 disposed on the inner surface of the measurement gas flow portion 15 at a position farther from the longitudinal tip of the base portion 102 than the inner main pump electrode 22, and an outer pump electrode 23 disposed at a position on the base portion 102 different from the measurement gas flow portion 15 (in FIG. 1, on the outer surface of the base portion 102) that corresponds to the auxiliary pump electrode 51. "Corresponding to the auxiliary pump electrode 51" means that the outer pump electrode 23 is provided on the auxiliary pump electrode 51 via a second solid electrolyte layer 6.

In other words, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell composed of an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided over substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23; any suitable electrode between the sensor element 101 and an outer electrode will suffice), and the second solid electrolyte layer 6.

The auxiliary pump electrode 51 is disposed within the second internal cavity 40 in a tunnel-shaped structure similar to the inner main pump electrode 22 disposed within the first internal cavity 20. That is, a ceiling electrode portion 51a is formed on the lower surface of the second solid electrolyte layer 6, which provides the ceiling surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on the upper surface of the first solid electrolyte layer 4, which provides the bottom surface of the second internal cavity 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5, which provide the side walls of the second internal cavity 40, forming a tunnel-shaped structure.

Like the inner main pump electrode 22, the auxiliary pump electrode 51 is also made of a material with a reduced ability to reduce the NOx components in the measurement gas.

In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 using a variable power supply 52, oxygen in the atmosphere within the second internal space 40 can be pumped out to the external space, or oxygen can be pumped into the second internal space 40 from the external space.

In addition, to control the oxygen partial pressure in the atmosphere within the second internal space 40, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, is formed by the auxiliary pump electrode 51, reference electrode 42, second solid electrolyte layer 6, spacer layer 5, first solid electrolyte layer 4, and third substrate layer 3.

The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. This allows the oxygen partial pressure in the atmosphere within the second internal space 40 to be controlled to a low level that does not substantially affect NOx measurement.

In addition, the pump current Ip1 is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and by controlling its electromotive force V0, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-controlling section 30 into the second internal space 40 is controlled to be constant at all times. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and auxiliary pump cell 50.

The fourth diffusion-controlling section 60 provides a predetermined diffusion resistance to the measurement gas, whose oxygen concentration (oxygen partial pressure) has been further reduced by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas into the third internal space 61.

The third internal space 61 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the measurement gas introduced through the fourth diffusion-controlling section 60. The NOx concentration is measured by the operation of the measurement pump cell 41.

The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 disposed on the inner surface of the measurement gas flow portion 15 at a position farther from the longitudinal tip of the base portion 102 than the auxiliary pump electrode 51, and an outer pump electrode 23 disposed on the base portion 102 at a position different from the measurement gas flow portion 15 (in FIG. 1, on the outer surface of the base portion 102) that corresponds to the measurement electrode 44. "Corresponding to the measurement electrode 44" means that the outer pump electrode 23 is provided via the measurement electrode 44, second solid electrolyte layer 6, spacer layer 5, and first solid electrolyte layer 4.

That is, the measurement pump cell 41 measures the NOx concentration in the measurement gas within the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell composed of a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, an outer pump electrode 23 (not limited to the outer pump electrode 23; any suitable outer electrode will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

The measuring electrode 44 is a porous cermet electrode. The measuring electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere within the third internal space 61.

The measurement pump cell 41 pumps out oxygen produced by the decomposition of nitrogen oxides in the atmosphere surrounding the measurement electrode 44, and the amount of oxygen produced can be detected as a pump current Ip2.

In addition, to detect the oxygen partial pressure around the measurement electrode 44, the second solid electrolyte layer 6, spacer layer 5, first solid electrolyte layer 4, third substrate layer 3, measurement electrode 44, and reference electrode 42 constitute an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump. The variable power supply 46 is controlled based on the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump.

The measurement gas introduced into the second internal space 40 reaches the measuring electrode 44 in the third internal space 61 through the fourth diffusion-controlling part 60 under conditions in which the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measuring electrode 44 are reduced (2NO → _N2 + _O2 ) to generate oxygen. This generated oxygen is then pumped by the measuring pump cell 41, and the voltage Vp2 of the variable power supply is controlled so that the control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 remains constant. Because the amount of oxygen generated around the measuring electrode 44 is proportional to the nitrogen oxide concentration in the measurement gas, the pump current Ip2 in the measuring pump cell 41 can be used to calculate the nitrogen oxide concentration in the measurement gas.

Furthermore, by combining the measurement electrode 44, first solid electrolyte layer 4, third substrate layer 3, and reference electrode 42 to form an oxygen partial pressure detection means as an electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reduction of NOx components in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere, thereby making it possible to determine the concentration of NOx components in the measured gas.

The second solid electrolyte layer 6, spacer layer 5, first solid electrolyte layer 4, third substrate layer 3, outer pump electrode 23, and reference electrode 42 form an electrochemical sensor cell 83, and the electromotive force Vref obtained by this sensor cell 83 can be used to detect the oxygen partial pressure in the measured gas outside the sensor.

In the gas sensor 100 having this configuration, the main pump cell 21 and auxiliary pump cell 50 are operated to provide the measurement gas, whose oxygen partial pressure is always maintained at a constant low value (a value that has no substantial effect on NOx measurement), to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas can be determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41, which is approximately proportional to the NOx concentration in the measurement gas.

Furthermore, the sensor element 101 is equipped with a heater section 70 that adjusts the temperature by heating and keeping the sensor element 101 warm in order to increase the oxygen ion conductivity of the solid electrolyte. The heater section 70 includes a heater electrode 71, a heater 72, a heater lead 76, a through hole 73, a heater insulating layer 74, and a pressure release hole 75.

The heater electrode 71 is an electrode formed in contact with the underside of the first substrate layer 1. By connecting the heater electrode 71 to a heater power supply 77, which is an external power source, it is possible to supply power to the heater section 70 from outside.

The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via a heater lead 76 that is connected to the heater 72 and extends to the longitudinal rear end of the sensor element 101, and via a through-hole 73. When power is supplied from the outside through the heater electrode 71, the heater 72 generates heat, heating and keeping warm the solid electrolyte that forms the sensor element 101.

The heater 72 is embedded throughout the entire area from the first internal space 20 to the third internal space 61, making it possible to adjust the entire sensor element 101 to a temperature that activates the solid electrolyte. The temperature needs to be adjusted so that the main pump cell 21, auxiliary pump cell 50, and measurement pump cell 41 can operate. It is not necessary for these entire areas to be adjusted to the same temperature; there may be a temperature distribution within the sensor element 101.

In the sensor element 101 of this embodiment, the heater 72 is embedded in the base portion 102, but this is not a limitation. The heater 72 is only required to be disposed so as to heat the base portion 102. In other words, the heater 72 is only required to be capable of heating the sensor element 101 to an extent that it develops oxygen ion conductivity that enables the main pump cell 21, auxiliary pump cell 50, and measurement pump cell 41 to operate. For example, as in this embodiment, the heater may be embedded in the base portion 102. Alternatively, for example, the heater portion 70 may be formed as a heater substrate separate from the base portion 102 and disposed adjacent to the base portion 102.

The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed on the upper and lower surfaces of the heater 72 and heater lead 76. The heater insulating layer 74 is formed to provide electrical insulation between the second substrate layer 2 and the heater 72 and heater lead 76, and between the third substrate layer 3 and the heater 72 and heater lead 76.

The pressure relief hole 75 penetrates the third substrate layer 3 and is formed to connect the heater insulating layer 74 to the reference gas introduction space 43. The pressure relief hole 75 can mitigate the increase in internal pressure that accompanies a temperature rise within the heater insulating layer 74. Note that the pressure relief hole 75 may be omitted.

(control unit)
The gas sensor of this embodiment includes the above-described sensor element 101 and a control unit 90 that controls the sensor element 101. Fig. 2 is a block diagram showing the connection relationship between the control unit 90 and the pump cells 21, 50, 41 and heater unit 70 of the sensor element 101. The control unit 90 includes a heater control unit 91, an activity determination unit 92, a pump control unit 93, an oxygen concentration detection unit 94, and a NOx concentration detection unit 95. The control unit 90 is actually a microprocessor designed and configured to control the sensor element 101.

The control unit 90 is configured to acquire the electromotive forces (V0, V1, V2, Vref) in each sensor cell 80, 81, 82, 83 of the sensor element 101, the pump currents (Ip0, Ip1, Ip2) in each pump cell 21, 50, 41, and the heater voltage Vh and heater current Ih in the heater section 70. The control unit 90 is also configured to output control signals to the variable power supplies 24, 52, 46 and the heater power supply 77.

The heater control unit 91 is configured to be able to control the temperature of the heater 72. The heater control unit 91 heats the heater 72 at a desired temperature rise rate and also maintains the temperature of the heater 72 at a desired temperature.

Various known control methods can be used to heat the heater 72. For example, a constant voltage can be applied to the heater 72 to heat it. The output of the heater power supply 77 can be changed based on the resistance value of the heater 72 to control it to achieve the desired temperature rise rate. Alternatively, the output of the heater power supply 77 can be controlled based on at least one of the resistance values of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 to control it to achieve the desired temperature rise rate.

For example, the heater control unit 91 feedback-controls the control signal output to the heater power supply 77 based on the heater resistance value Rh (= Vh/Ih) calculated from the heater voltage Vh and heater current Ih in the heater 72 so that the heater 72 reaches the target temperature.

The activity determination unit 92 determines whether the sensor element 101 is in an active state where measurements can be taken. That is, it determines whether the main pump cell 21, auxiliary pump cell 50, and measurement pump cell 41 are in an active state where they can be driven. Here, "active state" refers to a state in which the solid electrolyte constituting each pump cell 21, 50, 41 is heated by the heater 72, resulting in oxygen ion conductivity being exhibited to an extent that pump cell control, as described below, is possible.

Whether the sensor element 101 is in a measurable active state can be determined, for example, by determining whether oxygen ion conductivity is sufficient to enable pump cell control, as described below. As oxygen ion conductivity increases, the resistance of each pump cell 21, 50, 41 decreases. The active state may be determined by detecting the resistance of any of the pump cells: the main pump cell 21 (inner main pump electrode 22 and outer pump electrode 23), the auxiliary pump cell 50 (auxiliary pump electrode 51 and outer pump electrode 23), and the measurement pump cell 41 (measurement electrode 44 and outer pump electrode 23). For example, the value of Ip0 when a constant Vp0 is applied to the main pump cell 21 is detected, and the resistance value Rp0 (= Vp0/Ip0) is calculated. The active state may be determined when the calculated resistance value Rp0 reaches a desired value.

The resistance value Rp0 of the main pump cell 21 that is determined to be in an active state may vary depending on the configuration of the sensor element 101. For example, the resistance value Rp0 that is determined to be in an active state may be between 300 Ω and 1000 Ω. It may also be between 600 Ω and 1000 Ω. Or, it may be between 300 Ω and 400 Ω.

As with the pump cells 21, 50, and 41, increasing oxygen ion conductivity also decreases the resistance value in each of the sensor cells 80, 81, 82, and 83. The active state may be determined by detecting the resistance value in any of the sensor cells: the main pump control oxygen partial pressure detection sensor cell 80 (inner main pump electrode 22 and reference electrode 42), the auxiliary pump control oxygen partial pressure detection sensor cell 81 (auxiliary pump electrode 51 and reference electrode 42), the measurement pump control oxygen partial pressure detection sensor cell 82 (measurement electrode 44 and reference electrode 42), and the sensor cell 83 (outer pump electrode 23 and reference electrode 42).

Furthermore, oxygen ion conductivity improves as the solid electrolyte temperature increases. Taking this relationship into consideration, it may be possible to determine whether the sensor element 101 is in a measurable active state based on the temperature of the sensor element 101 or the temperature of the heater 72.

The temperature at which the heater 72 is determined to be in an activated state may vary depending on the configuration of the sensor element 101. In this embodiment, for example, the temperature at which the heater 72 is determined to be in an activated state may be between 550°C and 700°C. It may also be between 550°C and 650°C. Alternatively, it may be between 650°C and 700°C.

For example, when determining whether the sensor element 101 is in a measurable activated state based on the temperature of the heater 72, the correlation between the temperature of the heater 72 in the sensor element 101 and oxygen ion conductivity may be obtained in advance, and the heater temperature at which the desired oxygen ion conductivity is achieved may be set based on that correlation. The activated state may be determined when the heater 72 reaches the set temperature. Whether the heater temperature is at the set temperature may be determined using the heater resistance value Rh, as in the heater control unit 91 described above.

While the heater control unit 91 heats the heater 72 at the desired temperature rise rate, the activity determination unit 92 periodically determines whether the sensor element 101 is in a measurable activated state. The determination by the activity determination unit 92 may be performed continuously or at appropriately determined timing. The determination may be performed using, for example, one or more of the resistance values of the pump cells 21, 50, and 41, the resistance values of the sensor cells 80, 81, 82, and 83, the temperature of the sensor element 101, and the temperature of the heater 72.

If the activity determination unit 92 determines that the sensor element 101 has not yet reached a measurable activation state, the heater control unit 91 continues to heat the heater 72 until the sensor element 101 reaches an activation state (the temperature increase step described below). If the activity determination unit 92 determines that the sensor element 101 has reached a measurable activation state, the heater control unit 91 continues to heat the heater 72, and the pump control unit 93 described below operates (the advance drive step described below).

The temperature of the heater 72 when the activation determination unit 92 determines that the sensor element 101 is in an activated state where it can be measured is referred to as the activation temperature.

The pump control unit 93 controls the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41.

The pump control unit 93 feedback-controls the pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 so that the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80 is kept at a constant value (referred to as a set value V0 _SET ). Since the electromotive force V0 indicates the oxygen partial pressure in the vicinity of the inner main pump electrode 22, maintaining the electromotive force V0 constant means maintaining the oxygen partial pressure in the vicinity of the inner main pump electrode 22 constant. As a result, the pump current Ip0 in the main pump cell 21 changes depending on the oxygen concentration in the measurement gas.

When the oxygen partial pressure in the measurement gas is higher than the oxygen partial pressure corresponding to the set value V0 _SET , the main pump cell 21 discharges oxygen from the first internal space 20. On the other hand, when the oxygen partial pressure in the measurement gas is lower than the oxygen partial pressure corresponding to the set value V0 _SET (for example, when hydrocarbons HC are contained), the main pump cell 21 pumps oxygen from the space outside the sensor element 101 into the first internal space 20. Therefore, the pump current Ip0 can be either positive or negative.

The pump control unit 93 feedback controls the pump voltage Vp1 of the variable power supply 52 in the auxiliary pump cell 50 so that the electromotive force V1 in the auxiliary pump control oxygen partial pressure detection sensor cell 81 becomes a constant value (referred to as a set value V1 _SET ). Since the electromotive force V1 indicates the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51, maintaining the electromotive force V1 constant means maintaining the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51 constant. In this way, the oxygen partial pressure in the atmosphere in the second internal space 40 is controlled to a low level that does not substantially affect the measurement of NOx.

In addition, feedback control is performed to set a set value V0 _SET of the electromotive force V0 based on the pump current Ip1 so that the pump current Ip1 in the auxiliary pump cell 50 remains constant (referred to as the set value Ip1 _SET ). Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and the electromotive force V0 is controlled to the set value V0 _SET based on the pump current Ip1, thereby controlling the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-controlling part 30 into the second internal space 40 to be always constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50. That is, it appears that the oxygen concentration in the measurement gas introduced from the fourth diffusion-controlling section 60 into the third internal space 61 is maintained at a constant value of about 0.001 ppm.

The pump control unit 93 feedback-controls the pump voltage Vp2 of the variable power supply 46 in the measurement pump cell 41 so that the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 becomes a constant value (referred to as the set value V2 _SET ). At the measurement electrode 44, nitrogen oxides in the measurement gas are reduced (2NO → N ₂ + O ₂ ) to generate oxygen. The generated oxygen is pumped out by the measurement pump cell 41 so that the electromotive force V2 becomes the set value V2 _SET . The set value V2 _SET can be set to a value that substantially all of the NOx is decomposed at the measurement electrode 44.

The oxygen concentration detection unit 94 acquires the pump current Ip0 in the main pump cell 21 and detects the oxygen concentration in the measurement gas based on a pre-stored correlation between the pump current Ip0 and the oxygen concentration in the measurement gas. Multiple correlations between the pump current Ip0 and the oxygen concentration in the measurement gas corresponding to the advance driving step and the steady driving step described below may be stored in advance. In the advance driving step, the oxygen concentration may be calculated based on the correlation in the advance driving step, and in the steady driving step, the oxygen concentration may be calculated based on the correlation in the steady driving step. Alternatively, the oxygen concentration detection unit 94 may be omitted.

The NOx concentration detection unit 95 acquires the pump current Ip2 in the measurement pump cell 41 and detects the NOx concentration in the measurement gas based on a pre-stored correlation between the pump current Ip2 and the NOx concentration in the measurement gas. Multiple correlations between the pump current Ip2 and the NOx concentration in the measurement gas corresponding to the advance driving step and the steady driving step described below may be stored in advance. In the advance driving step, the NOx concentration may be calculated based on the correlation in the advance driving step, and in the steady driving step, the NOx concentration may be calculated based on the correlation in the steady driving step.

[Gas sensor control method]
The control method of the present invention comprises:
a temperature raising step of heating the sensor element by a heater to an activation temperature at which an activation determining unit determines that the sensor element is in a measurable activated state;
a pre-driving step of heating the sensor element from the activation temperature to a steady-state driving temperature by the heater and operating a pump cell including the main pump cell and a measurement pump cell to detect NOx in the measurement gas;
a steady-state driving step of maintaining the sensor element at the steady-state driving temperature by the heater and operating a pump cell including the main pump cell and a measurement pump cell to continuously detect NOx in the measurement gas;
Includes.

The control performed during startup of the gas sensor 100 of this embodiment will be described in detail below. Figure 3 is a schematic diagram showing an example of the relationship between the change in heater temperature over time and the drive of the pump cell during startup of the gas sensor 100 according to embodiment 1 of the present invention. The horizontal axis represents time (seconds), and the vertical axis represents heater temperature (°C).

The control method for the gas sensor 100 at startup includes a temperature rise step, a pre-drive step, and a steady-state drive step.

The time required from starting the gas sensor 100 until accurate measurement can begin is referred to as the startup time. The startup time includes the temperature rise time required for the heater control unit 91 to heat the sensor element 101 to a temperature at which measurement is possible, and the time required for the pump control unit 93 to begin the above-mentioned control after temperature rise until the NOx concentration can be accurately measured.

In the temperature-raising step, the heater 72 heats the sensor element 101 until the activation temperature is reached, at which the activation determination unit 92 determines that the sensor element 101 is in a measurable activated state.

That is, in the temperature-raising step, the heater control unit 91 causes the heater 72 to heat the sensor element 101 to an activation temperature at which the activation determination unit 92 determines that the sensor element 101 is in an activated state where measurements can be made. In Figure 3, the gas sensor 100 is started up at the origin (0 seconds) of the horizontal axis.

When the gas sensor 100 receives a start signal (Dew point), the heater control unit 91 starts heating the sensor element 101. That is, the temperature-raising step begins. For example, if the gas sensor 100 is installed in an automobile or the like, a start signal is sent to the gas sensor 100 from an ECU (Electronic Control Unit), exhaust gas treatment system, or the like. The temperature-raising step may be started by supplying electricity to the control unit 90 of the gas sensor 100 from an external source.

If the activity determination unit 92 determines that the sensor element 101 has not yet reached a measurable activation state, the temperature rise step continues. Then, when the activity determination unit 92 determines that the sensor element 101 has reached a measurable activation state, the temperature rise step ends and the advance drive step, described below, begins.

In the temperature rise step of embodiment 1, the heater 72 is heated at a constant rate over time t1 from the start of the temperature rise step (0 seconds in Figure 3) until it reaches activation temperature T1. That is, at time t1, the temperature rise step ends and the advance drive step, described below, begins. As described below, the advance drive step allows NOx in the measurement gas to be detected. In embodiment 1, time t1 refers to the temperature rise time.

Activation temperature T1 may be, for example, 550°C to 700°C. It may also be, for example, 550°C to 650°C. Alternatively, it may be 650°C to 700°C. For example, activation temperature T1 may be 650°C. However, activation temperature T1 must be lower than steady-state operating temperature T2, which will be described later.

The rate of temperature rise when raising the temperature to the activation temperature T1 can be set appropriately so that cracks do not occur in the internal structure of the sensor element 101 during temperature rise (the probability of cracks occurring is low enough to withstand practical use). The rate of temperature rise does not need to be constant during the temperature rise step, and the rate of temperature rise may be set in multiple stages during the temperature rise step. The number of stages and the rate of temperature rise in each stage can be set appropriately.

The time t1 required to raise the temperature to the activation temperature T1 (=heat-up time) is typically, for example, 20 to 50 seconds in relation to the activation temperature T1. Within this range, it is possible to shorten the start-up time of the gas sensor by shortening the heat-up time.

The advance driving step begins when the activation determination unit 92 determines that the sensor element 101 has reached a measurable activation state. In other words, the advance driving step begins when the sensor element 101 reaches its activation temperature. Then, when the sensor element 101 reaches its steady-state driving temperature, the advance driving step ends and the steady-state driving step, described below, begins.

The advance drive step involves heating the sensor element 101 from the activation temperature to the steady-state drive temperature using the heater 72, and operating the main pump cell 21 and measurement pump cell 41 to detect NOx in the measurement gas.

That is, in the advance driving step, the heater control unit 91 causes the heater 72 to heat the sensor element 101 from the activation temperature to a predetermined steady-state driving temperature that has been set in advance.

At the same time, the pump control unit 93 starts operation of the pump cells, including the main pump cell 21 and the measurement pump cell 41, when the sensor element 101 reaches its activation temperature.

In the advance driving step of embodiment 1, the heater 72 is heated at a constant rate from the activation temperature T1 to the steady-state driving temperature T2. The time elapsed since the start-up of the gas sensor 100 is t2. The steady-state driving temperature T2 may be, for example, 750°C to 850°C. It may also be, for example, 750°C to 800°C. Or, it may be, for example, 800°C to 850°C.

The temperature rise rate in the advance driving step can be set appropriately so that cracks do not occur in the internal structure of the sensor element 101 during temperature rise (the probability of this occurring is low enough to withstand practical use). The temperature rise rate does not need to be constant during the advance driving step, and the temperature rise rate in the advance driving step may be set in multiple stages. The number of stages and the temperature rise rate in each stage can be set appropriately.

The time for the advance drive step, i.e., the time required to raise the temperature from the activation temperature T1 to the steady-state drive temperature T2 (t2-t1), can typically be, for example, 5 to 30 seconds, depending on the relationship between the activation temperature T1 and the steady-state drive temperature T2.

Furthermore, in the advance driving step of embodiment 1, when the heater 72 reaches the activation temperature T1, operation of each pump cell 21, 50, 41 begins, and NOx in the measurement gas can be detected throughout the entire advance driving step. Specifically, the pump control unit 93 performs various controls as described above. Furthermore, the oxygen concentration detection unit 94 and NOx concentration detection unit 95 detect the oxygen concentration and NOx concentration in the measurement gas.

The steady-state driving step begins when the sensor element 101 reaches the steady-state driving temperature. After the steady-state driving step begins, it typically continues until driving of the gas sensor 100 is terminated.

The steady-state drive step involves using the heater 72 to maintain the sensor element 101 at a steady-state drive temperature, while operating the pump cells, including the main pump cell 21 and the measurement pump cell 41, to continue detecting NOx in the gas being measured.

That is, in the steady-state drive step, the heater control unit 91 uses the heater 72 to maintain the sensor element 101 at the steady-state drive temperature.

At the same time, the pump control unit 93 continues to operate the pump cells, including the main pump cell 21 and the measurement pump cell 41, when the sensor element 101 reaches a steady-state operating temperature.

In the steady-state drive step of embodiment 1, the heater 72 is maintained at steady-state drive temperature T2.

Furthermore, when the heater 72 reaches the steady-state operating temperature T2, the pump cells 21, 50, and 41 continue to operate, so NOx in the measurement gas can continue to be detected even during the steady-state operating step. Specifically, the pump control unit 93 performs various controls as described above. Furthermore, the oxygen concentration detection unit 94 and NOx concentration detection unit 95 detect the oxygen concentration and NOx concentration in the measurement gas.

In this way, in embodiment 1, it is possible to detect NOx in the measurement gas after the start time t1 of the advance driving step.

As described above, the starting temperature of the advance driving step, i.e., the activation temperature T1, is lower than the steady-state driving temperature T2. The strength of the sensor element 101 (mainly the strength of the _ZrO2 constituting the base portion 102) increases as the temperature decreases. Therefore, it is possible to increase the temperature rise rate in the temperature rise step to a degree that suppresses cracks in the internal structure of the sensor element 101. As a result, the temperature rise time t1 can be shortened. As a result, the start-up time of the gas sensor 100 can be shortened.

In addition, by transitioning from the advance drive step to the steady drive step, each pump cell 21, 50, 41 can be operated at a temperature with higher oxygen ion conductivity, thereby maintaining high overall detection accuracy for NOx in the measured gas.

Furthermore, by making the operation of each pump cell 21, 50, 41 during the advance drive step different from the operation of each pump cell 21, 50, 41 during the steady drive step, it is possible to further improve the detection accuracy of NOx in the measurement gas.

Specifically, the oxygen concentration adjusted by the main pump cell 21 in the advance driving step may be set to a higher concentration than the oxygen concentration adjusted by the main pump cell 21 in the steady driving step.

As mentioned above, the starting temperature of the advance driving step, i.e., the activation temperature T1, is lower than the steady-state driving temperature T2. The higher the temperature of the solid electrolyte constituting the base portion 102 of the sensor element 101, the higher its oxygen ion conductivity. The oxygen ion conductivity at the activation temperature T1 is thought to be slightly lower than the oxygen ion conductivity at the normal driving temperature T2. In other words, the resistance value Rp0 of the main pump cell 21 at the activation temperature T1 is thought to be slightly higher than the resistance value Rp0 of the main pump cell 21 at the steady-state driving temperature T2. Therefore, when the oxygen concentration in the measured gas is particularly high (i.e., when the Ip0 that should flow through the main pump cell 21 is large), the pump voltage Vp0 applied to the main pump cell 21 is thought to be higher.

If the pump voltage Vp0 becomes too high, NOx will be decomposed at the inner main pump electrode 22. As a result, the current value Ip2 detected by the measuring pump cell 41 will be smaller than the value that should be detected. This will reduce the accuracy of NOx detection, especially when the oxygen concentration of the measured gas is high.

By setting the oxygen concentration adjusted by the main pump cell 21 in the advance drive step to a higher concentration than the oxygen concentration adjusted by the main pump cell 21 in the steady drive step, it is possible to reduce the Ip0 that should flow to the main pump cell 21 according to the oxygen concentration in the measured gas. As a result, it is possible to suppress increases in pump voltage Vp0, particularly when the oxygen concentration in the measured gas is high. This makes it possible to further improve the accuracy of detecting NOx in the measured gas.

Furthermore, in the advance driving step, the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 may be controlled to a value smaller than the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 in the steady driving step.

That is, in the advance driving step, the set value _V1SET of the electromotive force V1 in the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump may be set to a value smaller than the set value _V1SET in the steady driving step. The set value _V1SET in the steady driving step (the set value at the steady driving temperature T2) may be 350 mV to 430 mV.

At the start of the advance driving step, i.e., at the activation temperature T1, the set value V1 _SET may be set to 300 mV to 380 mV, provided that the set value V1 _SET at the activation temperature T1 is smaller than the set value V1 _SET in the steady driving step.

In the advance driving step, the set value V1 _SET may be changed continuously or stepwise from the set value V1 _SET at the activation temperature T1 to the set value V1 _SET at the steady driving temperature T2, or the set value V1 _SET at the activation temperature T1 may be always used.

Alternatively, in the advance driving step, the pump current Ip1 flowing through the auxiliary pump cell 50 may be controlled to a value greater than the pump current Ip1 flowing through the auxiliary pump cell 50 in the steady driving step.

That is, in the advance driving step, the set value Ip1 _SET of the pump current Ip1 in the auxiliary pump cell 50 may be set to a value greater than the set value Ip1 _SET in the steady driving step. The set value Ip1 _SET in the steady driving step may be set to 3 μA to 10 μA.

At the start of the advance driving step, i.e., at the activation temperature T1, the set value Ip1 _SET may be set to 5 μA to 15 μA, provided that the set value Ip1 _SET at the activation temperature T1 is greater than the set value Ip1 _SET in the steady driving step.

In the advance driving step, the set value Ip1 _SET may be changed continuously or stepwise from the set value Ip1 _SET at the activation temperature T1 to the set value Ip1 _SET at the steady driving temperature T2, or the set value Ip1 _SET at the activation temperature T1 may be always used.

Performing control based on such set value V1 _SET and/or set value Ip1 _SET results in the oxygen concentration adjusted by the main pump cell 21 in the above-mentioned advance driving step being higher than the oxygen concentration adjusted by the main pump cell 21 in the steady-state driving step.

As described above, if the oxygen concentration adjusted by the main pump cell 21 in the advance drive step is set to a higher concentration than the oxygen concentration adjusted by the main pump cell 21 in the steady drive step, the Ip0 that should flow to the main pump cell 21 can be reduced according to the oxygen concentration in the measured gas. As a result, the increase in pump voltage Vp0 can be suppressed, particularly when the oxygen concentration in the measured gas is high. This makes it possible to further improve the accuracy of detecting NOx in the measured gas.

In addition, because the oxygen concentration adjusted by the main pump cell 21 during the steady-state operation step is sufficiently low, it is believed that deterioration of the measurement electrode 44 can be further reduced even with long-term use. When the oxygen concentration near the measurement electrode 44 is sufficiently low, it is believed that oxidation and evaporation of the metal components of the measurement electrode 44 can be suppressed. Therefore, it is possible to prevent a decrease (deterioration) in the catalytic activity of the measurement electrode 44 to reduce (decompose) NOx. As a result, high detection accuracy can be maintained even with long-term use.

In the advance driving step, the electromotive force V2 between the measurement electrode 44 and the reference electrode 42 in the measurement pump control oxygen partial pressure detection sensor cell 82 may be controlled to a value different from the electromotive force V2 between the measurement electrode 44 and the reference electrode 42 in the steady driving step. Preferably, in the advance driving step, the electromotive force V2 between the measurement electrode 44 and the reference electrode 42 may be controlled to a value greater than the electromotive force V2 between the measurement electrode 44 and the reference electrode 42 in the steady driving step.

Before the advance driving step begins (up to t1 in Figure 3), the inside of the measurement gas flow section 15 is the gas atmosphere surrounding the gas sensor 100 (e.g., the air atmosphere). Therefore, before the advance driving step begins, the oxygen concentration in the third internal space 61 is usually higher than when the advance driving step is being performed. In other words, there is more oxygen near the measurement electrode 44 than when the advance driving step is being performed.

If the advance driving step is started in this state, immediately after the advance driving step starts, the measurement pump cell 41 will pump out oxygen derived from NOx in the measurement gas, as well as oxygen that has been present in the third internal space 61 since before the advance driving step started. Therefore, immediately after the advance driving step starts, the pump current Ip2 flowing through the measurement pump cell 41 may be greater than the current value corresponding to the NOx concentration in the measurement gas. As a result, immediately after the advance driving step starts, the NOx concentration calculated based on the pump current Ip2, i.e., the NOx concentration output value of the gas sensor 100, may be output as a value greater than the actual NOx concentration in the measurement gas.

In order to allow the measurement pump cell 41 to quickly pump out oxygen that is present in the third internal space 61 before the start of the advance driving step, the electromotive force V2 between the measurement electrode 44 and the reference electrode 42 in the advance driving step may be controlled to a value different from the electromotive force V2 between the measurement electrode 44 and the reference electrode 42 in the measurement pump cell 41 in the steady driving step. Preferably, it may be controlled to a value greater than the electromotive force V2 between the measurement electrode 44 and the reference electrode 42 in the measurement pump cell 41 in the steady driving step.

That is, in the advance driving step, the set value _V2SET of the electromotive force V2 in the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump may be set to a value greater than the set value _V2SET in the steady driving step. The set value _V2SET in the steady driving step (set value at steady driving temperature T2) may be 350 mV to 550 mV.

At the start of the advance driving step, i.e., at the activation temperature T1, the set value V2 _SET may be set to 400 mV to 1.2 V. However, the set value V2 _SET at the activation temperature T1 must be different from the set value V2 _SET in the steady driving step. More preferably, the set value V2 _SET at the activation temperature T1 must be greater than the set value V2 _SET in the steady driving step.

In the advance driving step, the set value V2 _SET may be changed continuously or stepwise from the set value V2 _SET at the activation temperature T1 to the set value V2 _SET at the steady driving temperature T2, or the set value V2 _SET at the activation temperature T1 may be always used.

By performing control based on this set value V2 _SET , the measurement pump cell 41 can more quickly pump out the oxygen present in the third internal space 61 before the start of the advance driving step. As a result, only the oxygen derived from NOx in the measurement gas can be detected as the pump current Ip2 in the measurement pump cell 41 from an earlier stage after the start of the advance driving step. Therefore, the gas sensor 100 can accurately detect the NOx concentration from an earlier stage after the start of the advance driving step. In other words, the time required for the sensor element 101 to be able to accurately measure the NOx concentration after the temperature of the sensor element 101 is increased can be further shortened.

In the advance driving step, the heater 72 may be heated from the activation temperature T1 to the steady-state driving temperature T2 based on a preset temperature rise rate.

In the above-described first embodiment, as shown in FIG. 3, the temperature rise rate of the heater 72 is constant throughout the entire advance driving step. As mentioned above, the temperature rise rate does not need to be constant, and the temperature rise rate may be set in multiple stages during the advance driving step. The number of stages and the temperature rise rate in each stage can be set as appropriate.

For example, Figure 4 shows an example of a second embodiment in which three stages of temperature rise rates are set in the advance drive step. Figure 4 is a schematic diagram showing the relationship between the change in heater temperature over time and the drive of the pump cell when the gas sensor 100 is started up according to the second embodiment. The horizontal axis represents time (seconds), and the vertical axis represents heater temperature (°C). As with Figure 3, the start time of the temperature rise step in Figure 4 is set to 0 seconds.

In embodiment 2, the temperature rise process of the heater 72 in the advance driving step is divided into three stages. Referring to FIG. 4, the first stage of the advance driving step begins from the activation temperature T1 (time t1) where the temperature rise step ends, and the temperature rises at a constant rate to temperature Ta (time ta). The second stage of the advance driving step begins from temperature Ta (time ta), and the temperature rises at a constant rate to temperature Tb (time tb). The third stage of the advance driving step begins from temperature Tb (time tb), and the temperature rises at a constant rate to steady driving temperature T2 (time t2).

In the advance driving step of embodiment 2, the temperature rise rate is switched between three successive stages. As shown in FIG. 4, it is preferable to raise the temperature of the heater 72 so that the temperature rise rate in each stage decreases in the order of stage 1, stage 2, and stage 3. With such temperature rise, the temperature rise rate can be further reduced when the temperature is high and the strength of the sensor element 101 is reduced. This further reduces the risk of cracks occurring in the internal structure of the sensor element 101.

[Comparative Forms 1 and 2]
5 is a schematic diagram showing an example of the relationship between the change in heater temperature over time and the driving step in Comparative Examples 1 and 2 at the start of the gas sensor 100. The horizontal axis represents time (seconds), and the vertical axis represents heater temperature (°C). In FIG. 5, as in FIG. 3, the start time of the temperature increase step is set to 0 seconds.

Comparative Example 1 is a configuration in which the temperature rise time is shortened to t1 by heating the heater 72 to the steady-state operating temperature T2 at a high temperature rise rate. In this configuration, the temperature rise time itself can be shortened.

However, if the temperature is increased to the steady-state operating temperature T2 at a rapid rate, there is a problem in that the risk of cracks occurring in the internal structure of the sensor element 101 increases.

More specifically, in Comparative Example 1, the sensor element 101 is heated by a heater 72 embedded inside. If the temperature rise rate of the heater 72 is too high, a temperature difference occurs between the heater 72 and a portion of the sensor element 101 that is distant from the heater 72 during the temperature rise process, which causes a problem of cracks occurring in the internal structure of the sensor element 101. In particular, when the temperature of the sensor element 101 is high, the strength of the solid electrolyte (e.g., ZrO ₂ ) that constitutes the sensor element 101 decreases, which causes a problem of more easily causing cracks in the internal structure of the sensor element 101.

Comparative embodiment 2 is an embodiment in which the heater 72 is heated to a temperature TL, which is set to a value lower than the activation temperature T1 in embodiments 1 and 2, and steady-state operation is performed from temperature TL, thereby shortening the temperature rise time to t1. Because temperature TL is lower than the conventional steady-state operation temperature T2, the temperature rise time can be shortened even when the temperature is raised at a conventional rate.

However, temperature TL is lower than the activation temperature T1 in embodiments 1 and 2. Therefore, the oxygen ion conductivity of the solid electrolyte constituting the base portion 102 at temperature TL is lower than the oxygen ion conductivity at activation temperature T1. Therefore, the resistance value Rp0 of the main pump cell 21 is likely to be larger. When the resistance value Rp0 of the main pump cell 21 is high, the pump voltage value Vp0 applied to the main pump cell 21 when adjusting the oxygen concentration of the measured gas is likely to be large. In particular, when the oxygen concentration of the measured gas is high, the pump voltage Vp0 becomes larger. In this case, when the oxygen concentration of the measured gas is particularly high, some of the NOx is decomposed at the inner main pump electrode 22 constituting the main pump cell 21. As a result, the current value Ip2 detected by the measuring pump cell 41 becomes smaller than the value that should be detected. This causes a problem of reduced accuracy in detecting NOx in the measured gas, particularly when the oxygen concentration of the measured gas is high.

Normally, when the gas sensor 100 is used for an extended period of time, the resistance value Rp0 of the main pump cell 21 tends to increase. In Comparative Example 2, as described above, the pump voltage value Vp0 of the main pump cell 21 is large. Therefore, in Comparative Example 2, there is a concern that the detection accuracy of NOx in the measurement gas will further decrease when the gas sensor 100 is used for an extended period of time.

The following describes specific examples of gas sensors. Note that the present invention is not limited to the following examples.

[Example 1]
As Example 1, a gas sensor 100 shown in FIGS. 1 and 2 was fabricated. Specifically, the sensor element 101 was fabricated as follows. First, zirconia particles containing 4 mol% yttria as a stabilizer were mixed with an organic binder, a dispersant, a plasticizer, and an organic solvent, and six ceramic green sheets corresponding to layers 1 to 6 were formed by tape casting. Next, the ceramic green sheets corresponding to layers 1 to 6 were processed to form holes for sheet positioning during printing and lamination, as well as holes for internal spaces 20, 40, and 61. Various patterns for forming electrodes and the like were also formed by screen printing. The green sheets corresponding to layers 1 to 6 were then stacked and pressure-bonded to form a laminate. The resulting laminate was then cut into individual sensor elements 101. The cut laminates were fired to obtain the sensor elements 101. The produced sensor element 101 was electrically connected to the power supplies and the control unit 90 to produce the gas sensor 100 of Example 1.

In the gas sensor 100 of Example 1, the set values V1 _SET , Ip1 _SET , and V2 _SET in the advance driving step and the set values V1 _SET , Ip1 _SET , and V2 _SET in the steady driving step were set as shown in Table 1.

[Examples 2 to 3]
In the gas sensors 100 of Examples 2 and 3, the set values V1 _SET , Ip1 _SET , and V2 _SET in the advance driving step and the set values V1 _SET , Ip1 _SET , and V2 _SET in the steady driving step were set as shown in Table 1. Other than that, the gas sensors 100 were fabricated in the same manner as the gas sensor 100 of Example 1.

[Comparative Example 1]
Except for not performing the preceding driving step, the gas sensor 100 was fabricated in the same manner as the gas sensor 100 of Example 1. The set values V1 _SET , Ip1 _SET and V2 _SET in the steady driving step were set as shown in Table 1.

In all of Examples 1 to 3 and Comparative Example 1, the heater control unit 91 was configured to perform the temperature increase as shown in Figure 3. The activation temperature T1 was set to 600°C, and the steady-state operating temperature T2 was set to 800°C. In other words, the way the temperature of the heater 72 increased was the same in all of Examples 1 to 3 and Comparative Example 1.

In all of Examples 1 to 3, the advance driving step is configured to begin when the temperature of heater 72 reaches 600°C.

[Start-up time measurement]
The start-up time was measured for the fabricated gas sensors of Examples 1 to 3 and Comparative Example 1. Specifically, the measurement was carried out as follows.

The gas sensors of Examples 1 to 3 and Comparative Example 1 were measured in a model gas device. The gas sensors of Examples 1 to 3 and Comparative Example 1 were attached to the measurement pipe of the model gas device. With a model gas with NO = 100 ppm and _O2 = 18% flowing through the measurement pipe, the gas sensors of Examples 1 to 3 and Comparative Example 1 were driven and the waveforms of the NOx concentration output values were measured. The gas components other than NO and _O2 in the model gas used for measurement were _H2O (3%) and _N2 (balance). All gas component units are based on volume.

Figure 6 shows the change in NOx concentration output value over time at the start of the gas sensors 100 of Examples 1 to 3 and Comparative Example 1. The horizontal axis represents time (seconds), and the vertical axis represents the NOx concentration output value (ppm) of the gas sensor 100. In Figure 6, as in Figure 3, the start time of the temperature rise step is set to 0 seconds. The start time was the time from the start of the gas sensor 100 until the NOx concentration output value fell within the range of 100 ppm ± 10 ppm (90 ppm to 110 ppm).

It was confirmed that Examples 1 to 3 all had significantly faster start-up times than Comparative Example 1.

In Example 3, the set values V1 _SET and Ip1 _SET in the advance driving step are the same as the set values V1 _SET and Ip1 _SET in the steady driving step. On the other hand, in Examples 1 and 2, the set value V1 _SET in the advance driving step is smaller than the set value V1 _SET in the steady driving step, and the set value Ip1 _SET in the advance driving step is larger than the set value Ip1 _SET in the steady driving step. That is, in the advance driving step, the oxygen concentration in the first internal space 20 adjusted by the main pump cell 21 is controlled to be higher than the oxygen concentration in the first internal space 20 in the steady driving step.

In Examples 1 and 2, the NOx concentration output value did not change slowly from approximately 90 ppm to the original 100 ppm as in Example 3. In Examples 1 and 2, the oxygen concentration in the first internal space 20 was controlled to a higher concentration in the advance drive step than in the steady drive step. This is thought to have caused the pump current Ip0 that should flow to the main pump cell 21 in accordance with the oxygen concentration in the measured gas to be smaller than in Example 3. This is thought to have prevented the pump voltage Vp0 from becoming too high. As a result, in Examples 1 and 2, the NOx concentration output value calculated from the current value Ip2 flowing through the measuring pump cell 41 is thought to have more quickly accurately indicated the NOx concentration in the measured gas.

In Example 1, the set value _V2SET in the advance driving step is set to be larger than the set value _V2SET in the steady driving step. As a result, the measurement pump cell 41 can more quickly pump out the oxygen that has been present in the third internal space 61 since before the advance driving step started, which is thought to have shortened the start-up time.

The present invention can reduce the temperature rise time of the sensor element 101 while resolving the problems described above in Comparative Examples 1 and 2. As a result, the start-up time of the gas sensor 100 can be reduced.

Therefore, according to the present invention, the gas sensor can be controlled to detect NOx in the gas being measured so as to shorten the startup time from when the gas sensor is started until accurate measurement can begin.

Furthermore, according to the present invention, the startup time from when the gas sensor is started until accurate measurement can begin is short, and the gas sensor can be controlled to detect NOx in the measured gas so that the NOx detection accuracy of the gas sensor is maintained high over long periods of use, even when the oxygen concentration in the measured gas is high.

1 First substrate layer 2 Second substrate layer 3 Third substrate layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion rate-controlling section 12 Buffer space 13 Second diffusion rate-controlling section 15 Measurement gas flow section 20 First internal space 21 Main pump cell 22 Inner main pump electrode 22a Ceiling electrode section 22b (of inner main pump electrode) Bottom electrode section 23 Outer pump electrode 24 (of main pump cell) Variable power supply 30 Third diffusion rate-controlling section 40 Second internal space 41 Measurement pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measurement electrode 46 (of measurement pump cell) Variable power supply 48 Air introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a Ceiling electrode section 51b (of auxiliary pump electrode) Bottom electrode section 52 (of auxiliary pump electrode) Variable power supply 60 Fourth diffusion rate-controlling portion 61 Third internal space 70 Heater portion 71 Heater electrode 72 Heater 73 Through hole 74 Heater insulator 75 Pressure diffusion hole 76 Heater lead 77 Heater power supply 80 Oxygen partial pressure detection sensor cell for controlling main pump 81 Oxygen partial pressure detection sensor cell for controlling auxiliary pump 82 Oxygen partial pressure detection sensor cell for controlling measurement pump 83 Sensor cell 90 Control unit 91 Heater control portion 92 Activation determination portion 93 Pump cell control portion 94 Oxygen concentration detection portion 95 NOx concentration detection portion 100 Gas sensor 101 Sensor element 102 Base portion

Claims (14)

A method for controlling a gas sensor for detecting NOx in a measurement gas, comprising:
The gas sensor comprises:
A sensor element;
an activation determination unit that determines whether the sensor element is in a measurable activated state;
Including,
The sensor element is
a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers;
a measurement gas flow portion formed at one end of the base portion in the longitudinal direction;
a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode, the main pump cell adjusting oxygen in the measurement target gas to a desired concentration;
a measurement pump cell for detecting NOx in the measurement gas, the measurement pump cell including: a measurement electrode disposed on an inner surface of the measurement gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed on the base portion at a position different from the measurement gas flow portion and corresponding to the measurement electrode;
a heater for heating the base portion;
a reference electrode disposed inside the base portion so as to be in contact with a reference gas;
Including,
The control method includes:
a temperature raising step of heating the sensor element by the heater to an activation temperature at which the activation determining unit determines that the sensor element is in a measurable activated state;
a preceding driving step in which the heater is used to monotonically raise the temperature of the sensor element from the activation temperature to a steady driving temperature without cooling it midway, and the main pump cell and the measuring pump cell are operated to detect NOx in the measurement gas;
a steady-state driving step of maintaining the sensor element at the steady-state driving temperature by the heater and operating the main pump cell and the measuring pump cell to continuously detect NOx in the measurement gas;
A method for controlling a gas sensor for detecting NOx in a measurement gas, comprising: The control method described in claim 1, wherein the oxygen concentration adjusted by the main pump cell in the advance driving step is higher than the oxygen concentration adjusted by the main pump cell in the steady driving step. The sensor element further comprises:
an auxiliary pump cell including an auxiliary pump electrode disposed on an inner surface of the measurement gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed at a position different from the measurement gas flow portion of the base portion and corresponding to the auxiliary pump electrode,
3. The control method according to claim 1, wherein the auxiliary pump electrode is disposed at a position closer to the one end in the longitudinal direction of the base portion than the measurement electrode. a pump current flowing through the auxiliary pump cell is controlled based on an electromotive force between the auxiliary pump electrode and the reference electrode in the auxiliary pump cell;
4. The control method according to claim 3, further comprising controlling an electromotive force between the inner main pump electrode and the reference electrode in the main pump cell so that the pump current flowing through the auxiliary pump cell is constant. The control method described in claim 4, wherein, in the advance driving step, the electromotive force between the auxiliary pump electrode and the reference electrode is controlled to a value smaller than the electromotive force between the auxiliary pump electrode and the reference electrode in the steady driving step. The control method described in claim 4 or 5, wherein the pump current flowing through the auxiliary pump cell in the advance driving step is controlled to a value greater than the pump current flowing through the auxiliary pump cell in the steady driving step. controlling a pump current flowing through the measurement pump cell based on an electromotive force between the measurement electrode and the reference electrode in the measurement pump cell;
7. The control method according to claim 1, wherein in the advance driving step, the electromotive force between the measurement electrode and the reference electrode is controlled to a value different from the electromotive force between the measurement electrode and the reference electrode in the steady driving step. The control method described in claim 7, wherein, in the advance driving step, the electromotive force between the measurement electrode and the reference electrode is controlled to a value greater than the electromotive force between the measurement electrode and the reference electrode in the steady driving step. A control method according to any one of claims 1 to 8, wherein the activation determination unit determines whether the sensor element is in a measurable activated state based on at least one selected from the group consisting of the heater temperature, the resistance value of the main pump cell, and the resistance value between the inner main pump electrode and the reference electrode. A control method according to any one of claims 1 to 9, wherein, in the advance driving step, the heater is heated from the activation temperature to the steady driving temperature based on a preset temperature rise rate in the advance driving step. The control method described in claim 10, wherein the temperature rise rate in the advance driving step is set in multiple stages. A control method according to any one of claims 1 to 11, wherein, in the temperature-raising step, the heater is heated to the activation temperature based on a preset temperature-raising rate in the temperature-raising step. The control method described in claim 12, wherein the temperature increase rate in the temperature increase step is set in multiple stages. A gas sensor comprising a sensor element and a control unit for controlling the sensor element,
The sensor element is
a long plate-shaped substrate including a plurality of stacked oxygen ion conductive solid electrolyte layers;
a measurement gas flow portion formed at one end of the base portion in the longitudinal direction;
a main pump cell including an inner main pump electrode disposed on an inner surface of the measurement target gas flow portion and an outer pump electrode disposed on the base portion at a position different from the measurement target gas flow portion and corresponding to the inner main pump electrode, the main pump cell adjusting oxygen in the measurement target gas to a desired concentration;
a measurement pump cell for detecting NOx in the measurement gas, the measurement pump cell including: a measurement electrode disposed on an inner surface of the measurement gas flow portion at a position farther from the one longitudinal end of the base portion than the inner main pump electrode; and an outer pump electrode disposed on the base portion at a position different from the measurement gas flow portion and corresponding to the measurement electrode;
a heater for heating the base portion;
a reference electrode disposed inside the base portion so as to be in contact with a reference gas;
Including,
The control unit
an activation determination unit that determines whether the sensor element is in a measurable activated state;
a heater control unit that controls the temperature of the heater;
a pump control unit that operates the main pump cell and the measurement pump cell to detect NOx in the measurement gas;
Including,
the activation determination unit determines whether the sensor element is in an activated state where measurement is possible;
the heater control unit heats the sensor element by the heater to an activation temperature at which the activation determination unit determines that the sensor element is in a measurable activated state, and further heats the sensor element monotonically to a steady driving temperature without cooling it down midway ;
The pump control unit starts operation of the main pump cell and the measurement pump cell when the sensor element reaches the activation temperature, and further continues operation of the main pump cell and the measurement pump cell when the sensor element reaches the steady-state operating temperature.