Gas sensor and gas sensor control method

Abstract

To accurately measure even measuring object gas with low concentration.SOLUTION: There are provided a gas sensor 100 for detecting measuring object gas in a measured gas and a method of controlling the gas sensor 100. The gas sensor includes a sensor element 101, and a control device 90. The sensor element 101 includes a main pump cell 21, an auxiliary pump cell 50, a measuring pump cell 41, and a reference electrode 42. The main pump cell 21 flows a main pump current Ip0 repeatedly turned on / off so as to make an auxiliary pump current Ip1 flowing in the auxiliary pump cell 50 become a predetermined target current value. The auxiliary pump cell 50 flows an auxiliary pump current Ip1 so as to make electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 become a predetermined target voltage value. The control device 90 includes a controlling power source 24 for flowing the main pump current Ip0 repeatedly turned on / off, and a setting portion 94 that sets the target voltage value of the auxiliary pump cell 50 on the basis of an electric potential difference V0 generated between an inner side main pump electrode 22 and the reference electrode 42.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas sensor and a method for controlling a gas sensor. [Background technology]

[0002] 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 gases to be measured, 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.

[0003] Known examples of such gas sensors include gas sensors that use an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2). For example, International Publication WO2020 / 196653 discloses a gas sensor that includes a main pump cell and an auxiliary pump cell that pump oxygen, and a measurement pump cell that pumps oxygen generated from a specific gas. When detecting NOx as a specific gas using the gas sensor, the main pump cell and the auxiliary pump cell first control the oxygen partial pressure in the measurement gas to a low level that does not substantially affect the measurement of NOx. The NOx in the measurement gas, whose oxygen partial pressure has been controlled, is reduced at the measurement electrode, and the resulting oxygen is pumped out by the measurement pump cell and detected as a current value.

[0004] Furthermore, International Publication WO2020 / 196653 discloses that at least one of the one or more pump cells provided in the gas sensor pumps oxygen by passing a pump current (pulse current) that is repeatedly turned on and off, thereby suppressing changes in the catalytic activity of the electrode that accompany use of the gas sensor. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication WO2020 / 196653 Summary of the Invention [Problem to be solved by the invention]

[0006] As automobile exhaust gas regulations become stricter, gas sensors are being required to accurately measure even lower concentrations of target gases. Here, "low concentration" means, for example, concentrations of less than 500 ppm, less than 400 ppm, less than 300 ppm, less than 200 ppm, or less than 100 ppm.

[0007] In a conventional gas sensor, as in the above-mentioned International Publication WO2020 / 196653, for example, a main pump cell and an auxiliary pump cell are used to control the oxygen partial pressure in the measurement gas to a low level that does not substantially affect the measurement of the target gas (e.g., NOx), and the measurement gas containing a predetermined low concentration of oxygen and the target gas reaches a measurement electrode. The measurement electrode then pumps oxygen generated by the target gas in the measurement gas using a measurement pump cell, thereby detecting a current value corresponding to the concentration of the target gas.

[0008] However, when measuring a low-concentration measurement target gas of about 10 to 500 ppm, variations in the detected value may occur. The inventors' investigations revealed that, during operation of the measurement gas sensor, the amount of oxygen in the measurement target gas that reaches the measurement electrode may fluctuate due to some factor, causing the current value detected in the measurement pump cell to fluctuate. When the amount of oxygen in the measurement target gas that reaches the measurement electrode fluctuates, the current value detected in the measurement pump cell fluctuates (shifts) regardless of the concentration of the measurement target gas. As a result, it was found that measurement accuracy may be affected, particularly when the concentration of the measurement target gas in the measurement target gas is low.

[0009] Therefore, an object of the present invention is to accurately measure even low concentrations of a measurement target gas, that is, to accurately measure a measurement target gas over a wide concentration range including low concentrations of the measurement target gas. [Means for solving the problem]

[0010] As a result of extensive research, the inventors have discovered that the following invention makes it possible to accurately measure even low concentrations of the measurement target gas by controlling the oxygen concentration in the measurement target gas to a predetermined concentration with higher precision.

[0011] The present invention includes the following inventions. (1) A gas sensor including a sensor element and a control device that controls the sensor element, The sensor element is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; 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 gas flow portion, and an outer main pump electrode disposed on the base portion at a position different from the measurement gas flow portion, the outer main pump electrode corresponding to the inner main pump electrode; an auxiliary pump cell including: an inner 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 auxiliary pump electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner auxiliary pump electrode; a measurement pump cell including: an inner 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 auxiliary pump electrode; and an outer measurement electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; a heater for heating the base portion; Including, the main pump cell supplies a main pump current that is repeatedly turned on and off so that the auxiliary pump current flowing through the auxiliary pump cell reaches a predetermined target current value; The auxiliary pump cell Inside flowing the auxiliary pump current so that the electromotive force between the auxiliary pump electrode and the reference electrode reaches a predetermined target voltage value; The control device a control power supply for supplying the main pump current, which is repeatedly turned on and off, to the main pump cell; a setting unit that sets the target voltage value of the auxiliary pump cell; Including, the setting unit acquires a potential difference generated between the inner main pump electrode and the reference electrode, and sets the target voltage value in the auxiliary pump cell based on the potential difference.

[0012] (2) The gas sensor according to (1), wherein the setting unit acquires the potential difference generated between the inner main pump electrode and the reference electrode during an off period in which the main pump current is not flowing in the main pump cell by the control power supply.

[0013] (3) The gas sensor according to (1) or (2), wherein the setting unit acquires the potential difference generated between the inner main pump electrode and the reference electrode during a stable period in which the main pump current is not flowing in the main pump cell by the control power supply and there is no change in the potential difference due to the main pump current flowing.

[0014] (4) The control power supply includes a pulse power supply that periodically turns on and off the main pump current, The gas sensor according to any one of (1) to (3) above, wherein the setting unit acquires the potential difference generated between the inner main pump electrode and the reference electrode for each period T of the pulsed power supply at a predetermined time point included in an off period during which the main pump current is not flowing due to the pulsed power supply.

[0015] (5) A method for controlling a gas sensor for detecting a target gas in a measurement gas, comprising: The gas sensor comprises: a sensor element and a control device that controls the sensor element; The sensor element is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; 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 gas flow portion, and an outer main pump electrode disposed on the base portion at a position different from the measurement gas flow portion, the outer main pump electrode corresponding to the inner main pump electrode; an auxiliary pump cell including: an inner 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 auxiliary pump electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner auxiliary pump electrode; a measurement pump cell including: an inner 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 auxiliary pump electrode; and an outer measurement electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; a heater for heating the base portion; Including, The control device a control power supply for supplying a main pump current to the main pump cell, the main pump current being repeatedly turned on and off; The control method includes: a potential difference occurring between the inner main pump electrode and the reference electrode is obtained, and based on the potential difference, Inside a setting step of setting a target voltage value of the electromotive force between the auxiliary pump electrode and the reference electrode; In the main pump cell, the control power supply is controlled to flow a main pump current that is repeatedly turned on and off so that the auxiliary pump current flowing in the auxiliary pump cell reaches a predetermined target current value, and in the auxiliary pump cell, Inside and adjusting the oxygen concentration by supplying the auxiliary pump current so that the electromotive force between the auxiliary pump electrode and the reference electrode becomes the target voltage value.

[0016] (6) The control method according to (5) above, wherein in the setting step, the potential difference generated between the inner main pump electrode and the reference electrode is acquired during an off period in which the main pump current is not flowing in the main pump cell by the control power supply.

[0017] (7) A control method according to (5) or (6) above, wherein in the setting step, the potential difference generated between the inner main pump electrode and the reference electrode is acquired during a stable period in which the main pump current is not flowing in the main pump cell by the control power supply and there is no change in the potential difference due to the main pump current flowing.

[0018] (8) The control power supply includes a pulse power supply that periodically turns on and off the main pump current, The control method according to any one of (5) to (7) above, wherein in the setting step, the potential difference generated between the inner main pump electrode and the reference electrode is acquired for each cycle T of the pulsed power supply at a predetermined time point included in an off period during which the main pump current is not flowing due to the pulsed power supply. [Effects of the Invention]

[0019] According to the present invention, even low concentrations of the measurement target gas can be measured with high accuracy. That is, according to the present invention, the measurement target gas over a wide concentration range, including low concentrations of the measurement target gas, can be measured with high accuracy. [Brief explanation of the drawings]

[0020] [Figure 1]1 is a schematic vertical cross-sectional view showing an example of the schematic configuration of a gas sensor 100 in the longitudinal direction. [Figure 2] 1 is a block diagram showing the electrical connection relationship between a control device 90 and each of pump cells 21, 50, 41, each of sensor cells 80, 81, 82, 83 of a sensor element 101, and a heater section 70. FIG. [Figure 3] 3 is a flowchart showing an example of a NOx concentration detection process in the gas sensor 100. [Figure 4] These are schematic diagrams showing the time changes of the main pump current Ip0 and the potential difference V0 when the main pump current Ip0 flows in the positive direction. FIG. 4(1) is a schematic diagram showing the time changes of the main pump current Ip0. The horizontal axis represents time t, and the vertical axis represents the main pump current Ip0. FIG. 4(2) is a schematic diagram showing the time changes of the potential difference V0. The horizontal axis represents time t, and the vertical axis represents the potential difference V0. [Figure 5] 5(a) and 5(b) are schematic diagrams showing the time changes of the main pump current Ip0 and the potential difference V0 when the main pump current Ip0 flows in the negative direction. FIG. 5(1) is a schematic diagram showing the time changes of the main pump current Ip0. The horizontal axis represents time t, and the vertical axis represents the main pump current Ip0. FIG. 5(2) is a schematic diagram showing the time changes of the potential difference V0. The horizontal axis represents time t, and the vertical axis represents the potential difference V0. [Figure 6] 1 is a schematic diagram showing an example in which the main pump current Ip0 is a burst pulse current, where the horizontal axis represents time t and the vertical axis represents the main pump current Ip0. DETAILED DESCRIPTION OF THE INVENTION

[0021] The gas sensor of the present invention includes a sensor element and a control device that controls the sensor element.

[0022] The sensor element included in the gas sensor of the present invention is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; 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 gas flow portion, and an outer main pump electrode disposed on the base portion at a position different from the measurement gas flow portion, the outer main pump electrode corresponding to the inner main pump electrode; an auxiliary pump cell including: an inner 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 auxiliary pump electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner auxiliary pump electrode; a measurement pump cell including: an inner 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 auxiliary pump electrode; and an outer measurement electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; a heater for heating the base portion; Including, the main pump cell supplies a main pump current that is repeatedly turned on and off so that the auxiliary pump current flowing through the auxiliary pump cell reaches a predetermined target current value; The auxiliary pump cell Inside The auxiliary pump current is applied so that the electromotive force between the auxiliary pump electrode and the reference electrode reaches a predetermined target voltage value.

[0023] The control device included in the gas sensor of the present invention includes: a control power supply for supplying the main pump current, which is repeatedly turned on and off, to the main pump cell; a setting unit that sets the target voltage value of the auxiliary pump cell; Including, The setting unit acquires a potential difference generated between the inner main pump electrode and the reference electrode, and sets the target voltage value in the auxiliary pump cell based on the potential difference.

[0024] The control power source may be a pulse power source or the like.

[0025] An example of an embodiment of the gas sensor of the present invention will be described in detail below.

[0026] [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 Fig. 1 as the reference, 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 rear end side.

[0027] In FIG. 1, a gas sensor 100 is an example of a NOx sensor that detects NOx in a measurement gas by a sensor element 101 and measures its concentration.

[0028] The gas sensor 100 also includes a control device 90 that controls the sensor element 101. The control device 90 includes a pulse power supply 24 as an example of a control power supply for supplying a main pump current Ip0, which is repeatedly turned on and off, to a main pump cell 21 (described later). Fig. 2 is a block diagram showing the electrical connection between the control device 90 and the sensor element 101.

[0029] (sensor element) The sensor element 101 is a long, plate-like element including a base portion 102 having a structure in which multiple oxygen-ion conductive solid electrolyte layers are stacked. The long, plate-like shape is also referred to as a long plate shape or a strip shape. 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). 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.

[0030] The sensor element 101 is manufactured by, for example, laminating ceramic green sheets corresponding to each layer after performing predetermined processing and printing a circuit pattern on them, and then firing the sheets to integrate them.

[0031] 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 includes, in the longitudinal direction from the gas inlet 10, a first diffusion-controlling section 11, a buffer space 12, a second diffusion-controlling section 13, a first internal space 20, a third diffusion-controlling section 30, a second internal space 40, a fourth diffusion-controlling section 60, and a third internal space 61, which are adjacently formed and communicate with each other in this order.

[0032] The gas inlet 10, the buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are spaces inside the sensor element 101, which are defined by hollowing out the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and sides defined by the side surfaces of the spacer layer 5.

[0033] The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 are each provided as two horizontally elongated slits (the openings have their longitudinal direction perpendicular to the plane of the drawing in FIG. 1). The first diffusion rate-controlling section 11 and the second diffusion rate-controlling section 13 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slits.

[0034] The fourth diffusion rate-controlling portion 60 is provided as a single horizontally elongated slit (the opening has its longitudinal direction perpendicular to the plane of the drawing in FIG. 1 ) between the spacer layer 5 and the second solid electrolyte layer 6. The fourth diffusion rate-controlling portion 60 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slit.

[0035] Furthermore, a reference gas introduction space 43 is provided at a position farther from the tip side than the measurement gas flow section 15, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and at a position defined at its 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. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0036] 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 so as to cover the reference electrode 42.

[0037] 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, the air introduction layer 48, which is connected to the reference gas introduction space 43, is provided around the reference electrode 42. That is, 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. As will be described later, 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. The reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO).

[0038] 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.

[0039] 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 rate-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.

[0040] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10.

[0041] The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling part 11 to the second diffusion rate-controlling part 13 .

[0042] The second diffusion rate-controlling portion 13 is a portion that applies a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20 .

[0043] It is sufficient that the amount of the measurement gas introduced into the first internal space 20 is within a predetermined range. That is, 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 that the first diffusion-controlling section 11 directly communicates with the first internal space 20, that is, the buffer space 12 and the second diffusion-controlling section 13 do not exist.

[0044] The buffer space 12 is a space provided to mitigate the influence of pressure fluctuations on the detected value when the pressure of the gas to be measured fluctuates.

[0045] When the measurement gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measurement gas is suddenly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations if the measurement gas is automobile exhaust gas), but is not introduced directly into the first internal space 20, but is introduced into the first internal space 20 after the pressure fluctuations of the measurement gas are canceled out through the first diffusion rate-controlling section 11, buffer space 12, and second diffusion rate-controlling section 13. As a result, the pressure fluctuations of the measurement gas introduced into the first internal space become almost negligible.

[0046] 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 part 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

[0047] 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 section 15, and an outer main pump electrode (in this embodiment, an outer pump electrode 23) disposed at a position on the base 102 different from the measurement gas flow section 15 (on the outer surface of the base 102 in FIG. 1 ) corresponding to the inner main pump electrode 22. The phrase "corresponding to the inner main pump electrode 22" means that the outer pump electrode 23 is provided on the inner main pump electrode 22 with a second solid electrolyte layer 6 interposed therebetween.

[0048] That is, the main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 having a ceiling electrode portion 22a provided on almost 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 so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0049] The inner main pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20 and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that 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 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that configure both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-like structure at the locations where the side electrode portions are provided.

[0050] The inner main pump electrode 22 and the outer pump electrode 23 are porous cermet electrodes (electrodes in which a metal component and a ceramic component are mixed). The ceramic component is not particularly limited, but it is preferable to use an oxygen ion conductive solid electrolyte, similar to the base portion 102. For example, ZrO2 can be used as the ceramic component.

[0051] The inner main pump electrode 22, which comes into contact with the measurement gas, is formed using a material with reduced reduction capability for the NOx component in the measurement gas. The inner main pump electrode 22 preferably contains a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd) and a noble metal (e.g., Au, Ag, etc.) that reduces the catalytic activity of the catalytically active noble metal for the measurement target gas (NOx in this embodiment). In this embodiment, the inner main pump electrode 22 is a porous cermet electrode made of Pt containing 1% Au and ZrO2.

[0052] The outer pump electrode 23 may contain any noble metal having catalytic activity as described above. Similarly, the reference electrode 42 may contain any noble metal having catalytic activity as described above. In this embodiment, the outer pump electrode 23 is a porous cermet electrode of Pt and ZrO2.

[0053] In the main pump cell 21, a pulse power supply 24, which is an example of a control power supply, causes a main pump current Ip0 to flow between the inner main pump electrode 22 and the outer pump electrode 23 in a positive or negative direction, thereby pumping oxygen from the first inner space 20 out to the external space or pumping oxygen from the external space into the first inner space 20. Here, the direction of the arrow of the main pump current Ip0 in FIG. 1 is defined as the positive direction.

[0054] In this embodiment, the pulsed power supply 24 serving as a control power supply is configured as a current source. The pulsed power supply 24 causes a main pump current Ip0 that is repeatedly turned on and off, i.e., an intermittent main pump current Ip0, to flow between the inner main pump electrode 22 and the outer pump electrode 23. The pulsed power supply 24 is configured to be able to cause the intermittent main pump current Ip0 to flow in either a positive or negative direction. The pulsed power supply 24 can be configured as appropriate by those skilled in the art. For example, the pulsed power supply 24 may be a pulsed power supply capable of generating a bidirectional pulsed current, or a combination of multiple pulsed power supplies that generate a unidirectional pulsed current.

[0055] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, an inner main pump electrode 22 and a second solid electrolyte 6 layer a spacer layer 5; and a first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 constitute an electrochemical sensor cell, that is, a main pump control oxygen partial pressure detection sensor cell 80.

[0056] By measuring the potential difference V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump, the oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined.

[0057] The third diffusion-controlling section 30 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled in the first internal space 20 by the operation of the main pump cell 21, which will be described later, and guides the measurement gas to the second internal space 40.

[0058] The second internal space 40 is provided as a space for adjusting with higher precision the oxygen partial pressure in the measurement gas introduced through the third diffusion-controlling part 30. The oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50.

[0059] In the second internal space 40, the oxygen concentration (oxygen partial pressure) is adjusted in advance in the first internal space 20, and then the third diffusion-controlling section 30 The oxygen partial pressure of the measurement gas introduced through the auxiliary pump cell 50 is further adjusted by the auxiliary pump cell 50. This makes it possible to keep the oxygen concentration in the second internal space 40 constant with high precision, thereby enabling the gas sensor 100 to measure the NOx concentration with high precision.

[0060] The auxiliary pump cell 50 is an electrochemical pump cell including an inner auxiliary pump electrode (auxiliary pump electrode 51 in this embodiment) disposed on the inner surface of the measurement gas flow section 15 at a position farther from the longitudinal end of the base section 102 than the inner main pump electrode 22, and an outer auxiliary pump electrode disposed on the base section 102 at a position different from the measurement gas flow section 15 and corresponding to the inner auxiliary pump electrode. In this embodiment, the outer pump electrode 23 disposed on the outer surface of the base section 102 also functions as the outer auxiliary pump electrode. The phrase "corresponding to the inner auxiliary pump electrode" means that the outer pump electrode 23 is provided on the auxiliary pump electrode 51 with a second solid electrolyte layer 6 interposed therebetween.

[0061] That is, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell that is composed of an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on 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, but any appropriate electrode on the outside of the sensor element 101 will suffice), and the second solid electrolyte layer 6.

[0062] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to the inner main pump electrode 22 disposed in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal space 40, forming a tunnel-shaped structure.

[0063] Like the inner main pump electrode 22, the auxiliary pump electrode 51 is also formed using a material with a weakened ability to reduce the NOx component in the measurement gas. Like the inner main pump electrode 22, the auxiliary pump electrode 51 preferably contains a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd) and a noble metal (e.g., Au, Ag, etc.) that reduces the catalytic activity of the catalytically active noble metal for the measurement gas (NOx in this embodiment). In this embodiment, the auxiliary pump electrode 51 is a porous cermet electrode of Pt containing 1% Au and ZrO2, like the inner main pump electrode 22.

[0064] 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, it is possible to pump oxygen from the atmosphere in the second internal space 40 into the external space or pump oxygen from the external space into the second internal space 40.

[0065] In addition, in order 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 configured by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3.

[0066] The main pump cell 21 adjusts the oxygen concentration in the measurement gas to the concentration to be introduced into the auxiliary pump cell 50, i.e., the concentration to be introduced into the second internal space 40, by repeatedly turning on and off the main pump current Ip0 so that the auxiliary pump current Ip1 flowing through the auxiliary pump cell 50 becomes a predetermined target current value; The auxiliary pump cell 50 adjusts the oxygen concentration in the measurement gas to the concentration to be introduced into the measurement pump cell 41 (described later), i.e., the third internal space 61, by flowing the auxiliary pump current Ip1 so that the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 becomes a predetermined target voltage value.

[0067] 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 the measurement of NOx.

[0068] At the same time, the auxiliary pump current Ip1 is used to control the pulse power supply 24 of the main pump cell 21. Specifically, the auxiliary pump current Ip1 or a control signal based on the auxiliary pump current Ip1 is input to the pulse power supply 24, and the pulse power supply 24 is controlled to cause the main pump current Ip0 to flow, thereby pumping oxygen from the first internal space 20 into the external space in the main pump cell 21, or pumping oxygen from the external space into the first internal space 20.

[0069] In this way, by operating the main pump cell 21 and the auxiliary pump cell 50, 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 always constant. When used as a NOx sensor, the main pump cell 21 and the auxiliary pump cell 50 work together to control the oxygen concentration in the second internal space 40. teeth The oxygen concentration in the second internal space 40 is preferably maintained at, for example, about 0.1 ppm or less, 0.01 ppm or less, 0.001 ppm or less, or 0.0001 ppm or less. In this embodiment, the oxygen concentration is maintained constant at about 0.001 ppm.

[0070] The fourth diffusion rate-controlling section 60 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been further controlled to a lower level by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas to the third internal space 61.

[0071] 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.

[0072] The measurement pump cell 41 is an electrochemical pump cell including an inner measurement electrode (measurement electrode 44 in this embodiment) disposed on the inner surface of the measurement gas flow section 15 at a position farther from the longitudinal tip of the base section 102 than the inner auxiliary pump electrode (auxiliary pump electrode 51 in this embodiment), and an outer measurement electrode disposed on the base section 102 at a position different from the measurement gas flow section 15 and corresponding to the inner measurement electrode. In this embodiment, the outer pump electrode 23 disposed on the outer surface of the base section 102 also functions as the outer measurement electrode. The phrase "corresponding to the inner measurement electrode" means that the outer pump electrode 23 is provided via the measurement electrode 44, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0073] That is, the measurement pump cell 41 is an electrochemical pump cell that includes 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, but any appropriate electrode on the outside of the sensor element 101 will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. The measurement pump cell 41 measures the NOx concentration in the measurement gas within the third internal space 61.

[0074] The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the third internal space 61. The measurement electrode 44 is an electrode containing a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd). It is preferable that the measurement electrode 44 does not contain a noble metal (e.g., Au, Ag, etc.) that reduces the catalytic activity of the catalytically active noble metal with respect to the measurement target gas (NOx in this embodiment). In this embodiment, the measurement electrode 44 is a porous cermet electrode of Pt, Rh, and ZrO2.

[0075] In addition, in order to detect the oxygen partial pressure around the measuring electrode 44, a second solid-state SolutionAn electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for controlling the measurement pump, is configured by the solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. 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.

[0076] The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 in the third internal space 61 through the fourth diffusion-controlling part 60 under conditions where the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measurement electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. The generated oxygen is then pumped by the measurement pump cell 41, and the voltage Vp2 of the variable power supply 46 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 measurement electrode 44 is proportional to the nitrogen oxide concentration in the measurement gas, the measurement pump current Ip2 in the measurement pump cell 41 is used to calculate the nitrogen oxide concentration in the measurement gas.

[0077] In addition, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the electromotive force Vref obtained by this sensor cell 83 makes it possible to detect the oxygen partial pressure in the measurement gas outside the sensor.

[0078] Furthermore, in order to enhance the oxygen ion conductivity of the solid electrolyte, the sensor element 101 is provided with a heater section 70 that adjusts the temperature by heating and maintaining the temperature of the sensor element 101. 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.

[0079] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power supply 77, which is an external power supply, it is possible to supply power to the heater section 70 from the outside.

[0080] 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 rear end side of the sensor element 101 in the longitudinal direction, and a through hole 73. The heater 72 generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0081] The heater 72 is connected to the first inner space 20 through the third inner space 21. 61 The solid electrolyte is embedded throughout the entire sensor element 101, making it possible to adjust the temperature of the entire sensor element 101 to a temperature at which the solid electrolyte is activated. The temperature needs to be adjusted so that the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 can operate. It is not necessary to adjust the entire sensor element 101 to the same temperature, and there may be a temperature distribution in the sensor element 101.

[0082] In the sensor element 101 of this embodiment, the heater 72 is embedded in the base portion 102, but the present invention is not limited to this. The heater 72 may be disposed so as to heat the base portion 102. That is, the heater 72 may be capable of heating the sensor element 101 to an extent that the sensor element 101 exhibits oxygen ion conductivity that enables the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 to operate. For example, the heater 72 may be embedded in the base portion 102 as in this embodiment. Alternatively, 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.

[0083] 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 for the purpose of obtaining 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.

[0084] The pressure release hole 75 penetrates the third substrate layer 3 and is formed so as to connect the heater insulating layer 74 and the reference gas introduction space 43. The pressure release hole 75 can mitigate an increase in internal pressure that accompanies an increase in temperature within the heater insulating layer 74. Note that the pressure release hole 75 may be omitted.

[0085] The above-described sensor element 101 is incorporated into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas.

[0086] (Control device) The gas sensor 100 of this embodiment includes the sensor element 101 described above and a control device 90 that controls the sensor element 101. In the gas sensor 100, the electrodes 22, 23, 51, 44, and 42 of the sensor element 101 are electrically connected to the control device 90 via lead wires (not shown). FIG. 2 is a block diagram showing the electrical connections between the control device 90 and the pump cells 21, 50, and 41 of the sensor element 101, the sensor cells 80, 81, 82, and 83, and the heater unit 70. The control device 90 includes the pulse power supply 24 (an example of the control power supply described above), the variable power supplies 46 and 52 described above, the heater power supply 77, and a control unit 91. The control unit 91 includes a drive control unit 92, a concentration calculation unit 93, and a setting unit 94.

[0087] The control unit 91 is realized by a general-purpose or dedicated computer, and the functions of the drive control unit 92, concentration calculation unit 93, and setting unit 94 are realized by a CPU, memory, etc. mounted on the computer. Note that when the gas sensor 100 measures NOx contained in exhaust gas from an automobile engine and the sensor element 101 is attached to an exhaust path, some or all of the functions of the control device 90 (particularly the control unit 91) may be realized by an ECU (Electronic Control Unit) mounted on the automobile.

[0088] The control unit 91 is configured to acquire the electromotive forces (V0, V1, V2, Vref) in the sensor cells 80, 81, 82, 83 of the sensor element 101, the pump currents (Ip0, Ip1, Ip2) in the pump cells 21, 50, 41, and the heater voltage Vh and heater current Ih in the heater unit 70. The control unit 91 is also configured to output control signals to the pulse power supply 24, the variable power supplies 52, 46, and the heater power supply 77, which are examples of control power supplies.

[0089] The drive control unit 92 is configured to control the heater unit 70, the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 so that the gas sensor 100 can measure the concentration of the gas to be measured (NOx in this embodiment).

[0090] The drive control unit 92 heats the heater 72 and maintains the temperature of the heater 72 at a desired temperature.

[0091] Various known control methods can be used to heat the heater 72. For example, the heater 72 can be heated by applying a constant voltage to it. The output of the heater power supply 77 can also be controlled based on the resistance value of the heater 72. 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.

[0092] For example, the drive control unit 92 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.

[0093] The drive control unit 92 operates the main pump cell 21 and the auxiliary pump cell 50 in conjunction with each other. That is, the drive control unit 92 controls the main pump cell 21 to control the auxiliary pump current Ip1 flowing through the auxiliary pump cell 50 to a predetermined target current value (target current value Ip1 SET ), and the main pump current Ip0 is repeatedly turned on and off so that the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 reaches a predetermined target voltage value (target voltage value V1 SET The auxiliary pump current Ip1 is controlled to flow so that the auxiliary pump current Ip1 becomes equal to the maximum value of the auxiliary pump current Ip1.

[0094] Specifically, the drive control unit 92 determines whether the electromotive force V1 in the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump is equal to or exceeds the target voltage value V1 SET The pump voltage Vp1 of the variable power supply 52 in the auxiliary pump cell 50 is feedback-controlled so that 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.

[0095] At the same time, the drive control section 92 controls the auxiliary pump current Ip1 in the auxiliary pump cell 50 to be a constant value (target current value Ip1 SET The main pump current Ip0 of the pulse power supply 24 of the main pump cell 21 is feedback-controlled based on the auxiliary pump current Ip1 so that the target current value Ip1 is the target current value Ip1. SET Although this can be determined appropriately by a person skilled in the art, it may be generally within the range of 1 to 20 μA. In other words, when a constant auxiliary pump current Ip1 flows in the auxiliary pump cell 50, the electromotive force V1 reaches the target voltage value V1. SETTherefore, the main pump cell 21 is controlled so that the oxygen partial pressure (oxygen concentration) near the inner main pump electrode 22 becomes a predetermined value. As a result, the main pump current Ip0 in the main pump cell 21 changes depending on the oxygen concentration in the measurement gas.

[0096] The drive control section 92 controls the electromotive force V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measurement pump to be a constant value (target value V2 SET The pump voltage Vp2 of the variable power supply 46 in the measuring pump cell 41 is feedback-controlled so that the measured voltage Vp2 is equal to the measured voltage Vp2. pole 4 In 4, nitrogen oxides in the measurement gas are reduced (2NO → N2 + O2) to generate oxygen. The drive control section 92 controls the generated oxygen so that the electromotive force V2 is equal to or greater than the set value V2 SET The measurement pump cell 41 is used to pump out the set value V2. SET can be set to a value that substantially completely decomposes NOx at the measuring electrode 44. SET By setting the above, substantially all of the NOx in the measurement gas is detected as the measurement pump current Ip2 in the measurement pump cell 41. To be precise, the measurement pump current Ip2 includes a current due to low-concentration oxygen after being controlled by the main pump cell 21 and the auxiliary pump cell 50, and a current due to oxygen derived from the NOx in the measurement gas. pole 4 By keeping the oxygen in the measurement gas that reaches the pump 4 constant as described above, the measurement pump current Ip2 can accurately measure the oxygen derived from NOx in the measurement gas. As a result, the measurement pump current Ip2 can be detected as a current value corresponding to the NOx concentration.

[0097] In addition, the target current value Ip1 SET is stored as a set current value (control current value) in the memory of the control unit 91 that functions as the drive control unit 92. SET and target value V2 SETare stored as set voltage values (control voltage values) in the memory of the control unit 91 functioning as the drive control unit 92. Based on these control values, the CPU of the control unit 91 functioning as the drive control unit 92 controls the drive of the gas sensor 100.

[0098] The concentration calculation unit 93 is configured to calculate and output the NOx concentration in the measurement gas.

[0099] The concentration calculation unit 93 acquires the measurement pump current Ip2 in the measurement pump cell 41, calculates the NOx concentration in the measurement gas based on a pre-stored conversion parameter (current-concentration conversion parameter) between the measurement pump current Ip2 and the NOx concentration in the measurement gas, and outputs the calculated value as a measurement value of the gas sensor 100. The current-concentration conversion parameter is pre-stored in the memory of the control unit 91 that functions as the concentration calculation unit 93. The current-concentration conversion parameter can be appropriately determined for the gas sensor 100 in advance through experiments or the like by a person skilled in the art. The current-concentration conversion parameter may be, for example, a coefficient of an approximate equation (such as a linear function) obtained through experiments, or may be a map showing the correspondence between the measurement pump current Ip2 and the NOx concentration in the measurement gas. The current-concentration conversion parameter may be a parameter unique to each gas sensor 100, or may be a parameter commonly used by multiple gas sensors.

[0100] The setting unit 94 acquires the potential difference V0 generated between the inner main pump electrode 22 and the reference electrode 42, and sets the target voltage value V1 in the auxiliary pump cell 50 based on the potential difference V0. SET The setting unit 94 acquires the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump, and sets the target voltage value V1 of the electromotive force V1 in the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, which is used in the drive control unit 92, based on the value of the acquired potential difference V0. SET The setting unit 94 calculates the calculated target voltage value V1 SETis set as a set voltage value (control voltage value) in the drive control unit 92. The drive control unit 92 sets the target voltage value V1 SET The above-mentioned control is performed based on the above.

[0101] Target voltage value V1 SET is set to a value such that the oxygen concentration in the vicinity of the auxiliary pump electrode 51, i.e., the residual oxygen in the measurement gas that reaches the measurement electrode 44, becomes a predetermined concentration. As described above, the drive control unit 92 controls the main pump cell 21 and the auxiliary pump cell 50 in conjunction with each other. As a result, the drive control unit 92 controls the oxygen concentration in the vicinity of the inner main pump electrode 22 to be constant. In other words, the target voltage value V1 SET If the target voltage V1 is set to a value such that the residual oxygen in the measurement gas that reaches the measurement electrode 44 has a predetermined concentration, the oxygen concentration in the vicinity of the inner main pump electrode 22 will be constant. Therefore, the target voltage V1 is set based on the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42 that occurs in accordance with the oxygen concentration in the vicinity of the inner main pump electrode 22. SET can be calculated.

[0102] Specifically, the target voltage value V1 SET is calculated based on the acquired potential difference V0 and target value calculation parameters stored in advance in the memory of the control unit 91 functioning as the setting unit 94. The target value calculation parameters are, for example, the potential difference V0, the time change amount of the potential difference V0, or the deviation of the potential difference V0 from the target value (i.e., the deviation of the oxygen concentration near the inner main pump electrode 22 from the target value), and the target voltage value V1 SET The target value calculation parameter can be appropriately determined in advance for the gas sensor 100 by a person skilled in the art through experiments or the like. The target value calculation parameter may be, for example, a relational expression or a map obtained from the experimentally determined relationship between the oxygen concentration near the inner main pump electrode 22 and the potential difference V0 or the experimentally determined relationship between the oxygen concentration at the auxiliary pump electrode 51 and the electromotive force V1.

[0103] [Detection of concentration of target gas]

[0104] Next, a method of using the gas sensor 100 configured as above will be described. A detection method for controlling the gas sensor 100 to measure the concentration of a target gas in a measurement gas will be described.

[0105] The gas sensor control method of this embodiment includes the steps of: a potential difference occurring between the inner main pump electrode and the reference electrode is obtained, and based on the potential difference, Inside a setting step of setting a target voltage value of the electromotive force between the auxiliary pump electrode and the reference electrode; In the main pump cell, the control power supply is controlled to flow a main pump current that is repeatedly turned on and off so that the auxiliary pump current flowing in the auxiliary pump cell reaches a predetermined target current value, and in the auxiliary pump cell, Inside and an oxygen concentration adjusting step of supplying the auxiliary pump current so that the electromotive force between the auxiliary pump electrode and the reference electrode reaches the target voltage value.

[0106] FIG. 3 is a flowchart showing an example of the NOx concentration detection process in the gas sensor 100.

[0107] The NOx concentration detection process is started, for example, when the gas sensor 100 receives a start signal (Dew point). When the gas sensor 100 is mounted on an automobile or the like, the start signal (Dew point) is a signal sent to the gas sensor 100 from, for example, an ECU or an exhaust gas treatment system of the automobile. The process may also be started by, for example, manually turning on the power of the control device 90.

[0108] When the NOx concentration detection process is started, the drive control unit 92 of the control unit 91 first energizes the heater 72 to start heating the heater 72 (step S10), and maintains the sensor element 101 at a drive temperature (e.g., about 800°C) at which the solid electrolyte is activated and the NOx concentration can be measured accurately.

[0109] Next, the drive control unit 92 starts pump control of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 (step S11). That is, the drive control unit 92 determines whether the auxiliary pump current Ip1 flowing through the auxiliary pump cell 50 in the main pump cell 21 reaches a predetermined target current value Ip1. SET The pulse power supply 24 of the control power supply is feedback-controlled to repeatedly turn on and off the main pump current Ip0, and the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 in the auxiliary pump cell 50 reaches the target voltage value V1. SET The oxygen concentration adjusting step is started in which the variable power supply 52 is feedback-controlled to supply the auxiliary pump current Ip1 so that the target value V2 is reached. SET Then, the variable power supply 46 is feedback-controlled so that the measurement pump current Ip2 is started to flow.

[0110] The pump controls of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 may all be started simultaneously, or the pump control of at least one pump cell may be started at a timing different from the pump control of the other pump cells. The pump controls of all pump cells may be started at different timings. Furthermore, step S11 may be performed after the sensor element 101 has reached the operating temperature, or may be performed at a temperature lower than the operating temperature.

[0111] Next, the setting unit 94 sets a predetermined target voltage value V1 SET The target voltage value V1 in the drive control unit 92 SET At the start of driving the gas sensor 100, the predetermined target voltage value V1 SET The drive control unit 92 may set the target voltage value V1 SET If the target voltage value V1 is set to V1, the process may proceed to the next step without performing step S12 when the gas sensor 100 starts to operate. SETcan be appropriately determined by a person skilled in the art as a value that will ensure that the oxygen concentration in the measurement gas that reaches the measurement electrode 44 is a concentration that does not affect the measurement of NOx.

[0112] While steps S11 and S12 are performed and pump control is continuing, the measurement gas passes through the gas inlet 10, the first diffusion-controlling section 11, the buffer space 12, and the second diffusion-controlling section 13 in this order to reach the first internal space 20, where the main pump cell 21 adjusts the oxygen concentration to the concentration to be introduced into the auxiliary pump cell 50, i.e., the concentration to be introduced into the second internal space 40. The measurement gas then passes through the third diffusion-controlling section 30 to reach the second internal space 40, where the auxiliary pump cell 50 further adjusts the oxygen concentration to the concentration to be introduced into the measurement pump cell 41, i.e., the concentration to be introduced into the third internal space 61. The measurement gas, whose oxygen concentration has been adjusted to a constant low concentration by the main pump cell 21 and the auxiliary pump cell 50, passes through the fourth diffusion-controlling section 60 to reach the third internal space 61. The NOx in the measurement gas that has reached the third internal space 61 is decomposed at the measurement electrode 44, and a measurement pump current Ip2 is passed through the measurement pump cell 41 in accordance with the amount of oxygen generated by the decomposition of NOx.

[0113] Next, the concentration calculation unit 93 acquires the measurement pump current Ip2 in the measurement pump cell 41, calculates the NOx concentration in the measurement gas based on the acquired measurement pump current Ip2 and a pre-stored conversion parameter (current-concentration conversion parameter) between the measurement pump current Ip2 and the NOx concentration in the measurement gas, and outputs the detected value of the NOx concentration in the measurement gas as the measurement value of the gas sensor 100 (step S13). The detected value of the NOx concentration is output as the measurement result of the gas sensor 100. S While the gas sensor 100 is performing measurements, the process 13 is performed continuously or in synchronization with the output of the gas sensor.

[0114] In parallel with step S13, i.e., in parallel with the gas sensor 100 detecting the NOx concentration, the setting unit 94 acquires the potential difference V0 generated between the inner main pump electrode 22 and the reference electrode 42, and sets a target voltage value V1 of the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 in the auxiliary pump cell 50 based on the potential difference V0. SET A setting step for setting the above is performed (steps S13, S23 to S25).

[0115] In this embodiment, the setting unit 94 acquires the potential difference V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump (step S23). The setting unit 94 determines whether the acquired potential difference V0 is a predetermined target value (step S24). If the potential difference V0 is a predetermined target value, the setting unit 94 sets the target voltage value V1 at the time of acquiring the potential difference V0. SET The drive control unit 92 continues to set the target voltage value V1 at the time of acquiring the potential difference V0. SET In this case, the process returns to step S23, and the setting unit 94 acquires the potential difference V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump.

[0116] The potential difference V0 is predetermined. Ta If the potential difference V0 is out of the target value, the setting unit 94 sets a new target voltage value V1 based on the value of the potential difference V0 or the deviation of the potential difference V0 from the target value and target value calculation parameters stored in advance in the memory of the control unit 91 functioning as the setting unit 94. SET (Step S25). The setting unit 94 calculates the calculated new target voltage value V1 SET is set as the control voltage value in the drive control unit 92 (step S12), and steps S13 and S23 onward are carried out. . new New target voltage value V1 SET By setting the above, the main pumping cell 21 and the auxiliary pumping cell 50 are controlled so that the potential difference V0 becomes the target value.

[0117] The setting unit 94 sets the target voltage value V1 based on the potential difference V0.SET By performing the above-described setting step of setting, it is possible to more accurately control the oxygen concentration near the auxiliary pump electrode 51. As a result, even when the NOx concentration in the measurement gas is low, the NOx concentration can be measured with high accuracy.

[0118] The setting unit 94 performs the setting step in parallel with the drive control unit 92 performing pump control and the concentration calculation unit 93 outputting the detected NOx concentration in the measurement gas. Therefore, while the gas sensor 100 continues to measure the target gas (NOx in this embodiment) in the measurement gas, the oxygen concentration near the auxiliary pump electrode 51, i.e., the concentration of oxygen in the measurement gas that reaches the measurement electrode 44, can be controlled with higher accuracy. As a result, even when the NOx concentration in the measurement gas is low, the NOx concentration can be measured continuously and with higher accuracy.

[0119] Here, the target voltage value V1 SET and the potential difference V0 will be described in detail.

[0120] An electromotive force is generated between the auxiliary pump electrode 51 and the reference electrode 42 due to the difference in oxygen concentration between the auxiliary pump electrode 51 and the reference electrode 42. Because the reference electrode 42 is in contact with the reference gas (air in this embodiment), the oxygen concentration near the reference electrode 42 can be considered to be always constant. Therefore, the electromotive force generated by the difference in oxygen concentration between the auxiliary pump electrode 51 and the reference electrode 42 is considered to be a value indicating the oxygen partial pressure near the auxiliary pump electrode 51.

[0121] Conventional gas sensors, such as those disclosed in the aforementioned International Publication WO2020 / 196653, disclose a technique in which the electromotive force between the auxiliary pump electrode and the reference electrode is kept constant, thereby controlling the oxygen partial pressure near the auxiliary pump electrode to a low partial pressure that does not substantially affect the measurement of NOx. The document discloses that the measurement gas controlled to a low oxygen concentration reaches the measurement electrode, and a measurement pump current corresponding to the NOx concentration is detected in the measurement pump cell.

[0122] However, when measuring a measurement target gas in a low concentration range of about 10 to 500 ppm, variations in the measurement value (i.e., the detected measurement pump current Ip2) may occur. The inventors' investigations have revealed that, while the measurement gas sensor is in operation, the amount of oxygen (residual oxygen) in the measurement gas that reaches the measurement electrode 44 may fluctuate due to some factor, causing the value of the measurement pump current Ip2 detected by the measurement pump cell 41 to fluctuate. When the amount of residual oxygen in the measurement gas that reaches the measurement electrode 44 fluctuates, the value of the measurement pump current Ip2 detected by the measurement pump cell 41 fluctuates (shifts) regardless of the NOx concentration in the measurement gas. As a result, it has been found that this can affect measurement accuracy, particularly when the concentration of the measurement target gas (NOx in this embodiment) in the measurement gas is low.

[0123] If the residual oxygen concentration in the measurement gas that reaches the measurement electrode 44 after being controlled to a low oxygen concentration by the main pump cell 21 and the auxiliary pump cell 50 is constant, the current due to the residual oxygen in the measurement pump current Ip2 is constant. pole 4 The value Ip2 fluctuates depending on the amount of oxygen generated by the reduction of NOx in the measurement gas. In other words, a nearly linear correlation is obtained between the measurement pump current Ip2 and the NOx concentration in the measurement gas. As a result, the measurement pump current Ip2 corresponding to the NOx concentration can be detected in the measurement pump cell 41, and the NOx concentration can be measured with high accuracy.

[0124] When the residual oxygen concentration in the measurement gas that reaches the measurement electrode 44 changes, the change ΔO2 in the residual oxygen concentration appears as a shift in the value of the measurement pump current Ip2 flowing through the measurement pump cell 41, regardless of the NOx concentration.

[0125] Contains high concentrations of NOx MeasuredWhen measuring gas, the measurement pump current Ip2 detected in the measurement pump cell 41 is relatively large, so the shift in the measurement pump current Ip2 due to the change ΔO2 in the residual oxygen concentration is relatively small. Therefore, it is considered that high measurement accuracy is maintained. On the other hand, when measuring gas containing low concentrations of NOx, Measured When measuring gas, the measurement pump current Ip2 detected in the measurement pump cell 41 is relatively small, so the shift in the measurement pump current Ip2 due to the change ΔO2 in the residual oxygen concentration tends to be relatively large, which tends to have a significant impact on the measurement accuracy.

[0126] The measurement pump current Ip2 includes a current due to residual oxygen in the measurement gas that reaches the measurement electrode 44 and a current due to oxygen generated at the measurement electrode 44 due to NOx in the measurement gas. However, it is not possible to measure the current due to residual oxygen and the current due to oxygen derived from NOx separately. Therefore, it is difficult to measure the current due to residual oxygen separately, especially for gases containing low concentrations of NOx. Measured When measuring gas, it is important to keep the change in residual oxygen concentration ΔO2 sufficiently small, that is, to keep the residual oxygen concentration constant. By keeping the residual oxygen concentration constant, the measurement pump cell 41 can accurately detect oxygen derived from NOx in the measurement gas.

[0127] The inventor further investigated the factors behind the change ΔO2 in the residual oxygen concentration in the measurement gas that reaches the measurement electrode 44, and found that when the gas sensor 100 is operating, the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 includes not only an electromotive force generated by the difference in oxygen concentration between the auxiliary pump electrode 51 and the reference electrode 42, but also other factors.

[0128] According to the study by the inventors, the electromotive force V1 is (1) The concentration difference electromotive force V(oxygen) generated by the difference in oxygen concentration between the auxiliary pump electrode 51 and the reference electrode 42, (2) the thermoelectric power V(thermal) generated by the temperature difference between the auxiliary pump electrode 51 and the reference electrode 42, and (3) The potential difference V(IR) caused by the auxiliary pump current Ip1 flowing through the auxiliary pump electrode 51, i.e., the potential difference V(IR) caused by the auxiliary pump current Ip1 and the resistance value of the auxiliary pump electrode 51. Furthermore, for example, when oxygen is pumped into the reference electrode 42 to control the reference gas atmosphere near the reference electrode 42, a current also flows through the reference electrode 42. In this case, the electromotive force V1 is considered to further include the potential difference V(IR)' generated by the current flowing through the reference electrode 42 and the resistance value of the reference electrode 42, in addition to the above.

[0129] To maintain a constant oxygen partial pressure near the auxiliary pump electrode 51, (1) the concentration difference electromotive force V(oxygen) should be kept constant. If (2) the thermoelectromotive force V(thermal) and (3) the potential difference V(IR) are constant, maintaining the electromotive force V1 constant means maintaining (1) the concentration difference electromotive force V(oxygen) constant.

[0130] However, if at least one of (2) thermoelectromotive force V(thermal) and (3) potential difference V(IR) fluctuates for some reason, (1) concentration difference electromotive force V(oxygen) may fluctuate even if electromotive force V1 is controlled to be constant. As a result, the oxygen partial pressure near the auxiliary pump electrode 51 may fluctuate. (2) thermoelectromotive force V(thermal) and (3) potential difference V(IR) may fluctuate due to, for example, changes in the temperature, flow rate, or flow velocity of the measured gas. Furthermore, for example, the auxiliary pump electrode 51 and the reference electrode 42 may change over time as the gas sensor is used, causing their respective resistance values to change (usually increase). Such changes in electrode resistance over time may also cause (2) thermoelectromotive force V(thermal) and (3) potential difference V(IR).

[0131] For example, when the proportions of (2) thermoelectromotive force V(thermal) and (3) potential difference V(IR) in electromotive force V1 increase, (1) concentration difference electromotive force V(oxygen), which should be controlled to a constant value, becomes relatively small. If electromotive force V1 is controlled to a constant value in this state, the oxygen concentration near auxiliary pump electrode 51 is controlled to be higher than the target oxygen concentration. In other words, the residual oxygen concentration in the measurement gas that reaches measuring electrode 44 is controlled to be higher.

[0132] Conversely, when the proportions of (2) thermoelectromotive force V(thermal) and (3) potential difference V(IR) in electromotive force V1 become smaller, (1) concentration difference electromotive force V(oxygen), which should be controlled to a constant value, becomes relatively larger. If electromotive force V1 is controlled to a constant value in this state, the oxygen concentration near auxiliary pump electrode 51 is controlled to be lower than the target oxygen concentration. In other words, the residual oxygen concentration in the measurement gas that reaches measuring electrode 44 is controlled to be lower.

[0133] As mentioned above, it is believed that the change in the residual oxygen concentration ΔO2 is influenced by the change in the proportion of (2) the thermoelectromotive force V(thermal) and (3) the potential difference V(IR) in the electromotive force V1.

[0134] (2) The thermoelectric power V (thermal) can be obtained, for example, by operating the gas sensor 100 in an atmospheric environment and measuring the electromotive force V1 generated between the auxiliary pump electrode 51 and the reference electrode 42 with the pump controls of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 all turned off.

[0135] (3) As described above, the potential difference V(IR) is generated according to the auxiliary pump current Ip1 and the resistance value of the auxiliary pump electrode 51. The resistance value of the auxiliary pump electrode 51 can be calculated, for example, from the electromotive force V1 generated when the gas sensor 100 is driven in the atmospheric air and the pump controls of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 are all turned off, and a constant auxiliary pump current Ip1 is passed through the auxiliary pump cell 50.

[0136] As described above, (2) thermoelectric power V(thermal) and (3) potential difference V(IR) are values obtained when pump control is not being performed, and therefore it is difficult to determine them when the gas sensor 100 is being driven to detect the NOx concentration.

[0137] As described above, the main pump cell 21 and the auxiliary pump cell 50 are controlled in an interlocked manner. That is, the drive control unit 92 controls the auxiliary pump cell 50 when the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 reaches the target voltage value V1. SET At the same time, the auxiliary pump current Ip1 in the auxiliary pump cell 50 is set to the target current value Ip1. SET In other words, as a result of flowing a constant auxiliary pump current Ip1 in the auxiliary pump cell 50, the electromotive force V1 reaches the target voltage value V1. SET As a result, the oxygen concentration in the vicinity of the inner main pump electrode 22 corresponds to the oxygen concentration in the vicinity of the auxiliary pump electrode 51.

[0138] Therefore, when a change ΔO2 occurs in the oxygen concentration near the auxiliary pump electrode 51, i.e., in the residual oxygen concentration in the measurement gas that reaches the measurement electrode 44, a change also occurs in the oxygen concentration near the inner main pump electrode 22. As described above, the oxygen concentration near the inner main pump electrode 22 can be detected by the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42 in the main pump control oxygen partial pressure detection sensor cell 80. Therefore, it has been found that a change ΔO2 in the residual oxygen concentration in the measurement gas that reaches the measurement electrode 44 can be detected by a change in the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42.

[0139] From this knowledge, the setting unit 94 acquires the potential difference V0 generated between the inner main pump electrode 22 and the reference electrode 42, and sets the target voltage value V1 of the electromotive force V1 between the auxiliary pump electrode 51 and the reference electrode 42 in the auxiliary pump cell 50 based on the potential difference V0. SET It has been found that by performing the setting step of setting the residual oxygen concentration, the change ΔO2 in the measurement gas reaching the measuring electrode 44 can be corrected to zero or substantially zero. A change ΔO2 in the residual oxygen concentration of substantially zero means that the residual oxygen concentration of the measurement gas reaching the measuring electrode 44 is substantially constant, that is, constant enough to not affect the accuracy of the measurement of the NOx concentration even in a low concentration range where the current value of the measurement pump current Ip2 is relatively small. Thus, it has been found that performing the above-described measurement step enables more accurate control of the residual oxygen concentration of the measurement gas reaching the measuring electrode 44. By maintaining the residual oxygen concentration constant with high accuracy, the measurement pump cell 41 can accurately detect oxygen derived from NOx in the measurement gas. As a result, the NOx concentration can be measured with high accuracy even in a low concentration range.

[0140] As described above, the drive control unit 92 causes the main pump current Ip0, which is repeatedly turned on and off by the pulse power supply 24, to flow through the main pump cell 21. The direction of the main pump current Ip0 that pumps oxygen out of the first internal space 20 is defined as positive, and the direction of the main pump current Ip0 that pumps oxygen into the first internal space 20 is defined as negative. The positive direction of the main pump current Ip0 is the direction of the arrow of the main pump current Ip0 in FIG. 1, which is the direction in which current flows from the inner main pump electrode 22 to the outer pump electrode 23 outside the sensor element 101.

[0141] 4A and 4B are schematic diagrams showing the time changes of the main pump current Ip0 and the potential difference V0 when oxygen is pumped out of the first internal space 20, i.e., when the main pump current Ip0 flows in the positive direction. FIG. 4(1) is a schematic diagram showing the time change of the main pump current Ip0. The horizontal axis represents time t, and the vertical axis represents the main pump current Ip0. The upward direction of the vertical axis represents the positive direction of the main pump current Ip0. FIG. 4(2) is a schematic diagram showing the time change of the potential difference V0. The horizontal axis represents time t, and the vertical axis represents the potential difference V0. The potential difference V0 is considered positive when the potential of the reference electrode 42 is higher than that of the inner main pump electrode 22. In FIG. 4(2), the upward direction of the vertical axis represents the positive direction. The value of the potential difference V0 decreases as the oxygen concentration near the inner main pump electrode 22 increases, and increases as the oxygen concentration near the inner main pump electrode 22 decreases.

[0142] In this embodiment, the main pump current Ip0 supplied by the pulse power supply 24 to the main pump cell 21 is a current with a pulse waveform that is repeatedly turned on and off with a period T, as shown in Fig. 4(1). For example, when the main pump current Ip0 is turned on at time t1, which is the start point of a certain period T, the main pump current Ip0 rises from 0 A to a maximum current Ip0max, and the on-period T ON This state continues until time t2 when the main pump current Ip0 is turned off. At time t2, the main pump current Ip0 drops from the maximum current Ip0max to 0 A, and the off period T OFF The main pump current Ip0 is 0 A until time t4 when the on-period T ON and the subsequent off period T OFF The actual main pump current Ip0 is calculated from the start time t1. Rise Although it takes a very short time for the main pump current Ip0 output from the pulsed power supply 24 to fall from time t1, it is shown as a rectangular wave in FIG. OFF Even if the pump current Ip0 is small, there may be cases where the main pump current Ip0 flows slightly due to the influence of noise or the like, but this is not shown in FIG. 4(1).

[0143] As described above, the drive control unit 92 controls the auxiliary pump current Ip1 in the auxiliary pump cell 50 to a predetermined value (target current value Ip1 SET ) based on the auxiliary pump current Ip1. In the main pump cell 21, the amount of oxygen pumped from the first internal space 20 by the main pump current Ip0 during one cycle (cycle T) is proportional to the average value Ip0ave of the main pump current Ip0 during one cycle (see the dashed line in FIG. 4(1)). Therefore, the drive control unit 92 calculates, for example, the average value Ip0ave of the main pump current Ip0 during the cycle T based on the auxiliary pump current Ip1. ON The drive control unit 92 outputs a control signal to the pulsed power supply 24 to change at least one of the parameters of the ratio (duty ratio), the period T, and the maximum current Ip0max, thereby changing the average value Ip0ave. The drive control unit 92 may output the average value Ip0ave as a control signal to the pulsed power supply 24, and the pulsed power supply 24 may change at least one of the parameters described above based on the control signal. Alternatively, the drive control unit 92 may output the auxiliary pump current Ip1 as a control signal to the pulsed power supply 24, and the pulsed power supply 24 may change at least one of the parameters described above based on the control signal. In this embodiment, the drive control unit 92 outputs the duty ratio (or the amount of change in the duty ratio) to the pulsed power supply 24 as a control signal, and the pulsed power supply 24 changes the duty ratio of the main pump current Ip0 based on the control signal. The duty ratio changes based on the control signal, but the maximum value of the duty ratio may be, for example, 90% or less. In this case, the ratio of the off period T to the period T may be OFF The minimum value of the ratio is 10% or more. Preferably, the maximum value of the duty ratio is 80% or less. In this case, the ratio of the off period T to the cycle T is OFF The minimum percentage is 20% or more.

[0144] The period T of the intermittent main pump current Ip0 in FIG. 4(1) is preferably short enough to keep the actual oxygen concentration in the first internal space 20 in an average controlled state during the period T. In other words, the ON period T ON and off period T OFFThe period T should be short enough so that a change in the actual oxygen concentration in the first internal space 20 between the first period T and the second period T does not affect the adjustment of the oxygen concentration in the auxiliary pump cell 50. By setting the period T in this manner, the actual oxygen concentration in the first internal space 20, i.e., the oxygen concentration in the measurement gas introduced into the second internal space 40, can be controlled to a predetermined concentration. The period T may be, for example, 0.1 s or less (frequency of 10 Hz or more), 0.02 s or less (frequency of 50 Hz or more), or 0.001 s or less (frequency of 100 Hz or more). In this embodiment, the period T is set to 0.0005 s (5 ms; frequency of 200 Hz).

[0145] Furthermore, for example, when the control device 90 outputs the derived NOx concentration to another device such as a vehicle engine ECU at predetermined intervals Tout, it is preferable to set the interval T to one-tenth or less of the interval Tout. By setting the interval T in this manner, the oxygen concentration in the measurement target gas can be controlled at an interval T that is sufficiently shorter than the interval Tout, and the concentration of the measurement target gas can be output more stably.

[0146] For example, the drive control unit 92 may adjust the auxiliary pump current Ip1 in the auxiliary pump cell 50 to the target current value Ip1 SET If it is larger, the duty ratio of the main pump current Ip0 is increased (the period T is not changed, and the on-period T ON (to lengthen the pulse power supply) 24 4(1), for example, the average value Ip0ave of the main pump current Ip0 is controlled to increase the oxygen concentration in the first internal space 20. ON (Time t1 to t2) On period T in the second cycle T ON4(2), a control is performed to double the length of the period (times t4 to t5) (double the duty ratio). In this case, the average value Ip0ave in the second period T is doubled compared to the average value Ip0ave in the first period T. Since the average value Ip0ave is doubled in the second period T, more oxygen is pumped out of the first internal space 20 than during the first period T. As a result, the oxygen concentration in the vicinity of the inner main pump electrode 22 decreases in the second period T, and as shown in FIG. 4(2), the potential difference V0b' at the end point of the second period T (time t6) is larger than the potential difference V0b at the end point of the first period T (time t4).

[0147] Next, the potential difference V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump will be described in detail. The potential difference V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump is the potential difference between the inner main pump electrode 22 and the reference electrode 42. The potential difference V0 is a potential difference generated mainly by the difference in oxygen concentration between the inner main pump electrode 22 and the reference electrode 42, and therefore has a value corresponding to the oxygen concentration in the first internal space 20 (near the inner main pump electrode 22). However, during the ON period T when the pulsed power supply 24 flows the maximum current Ip0max, ON During the ON period T, the maximum current Ip0max flows, causing a change in the potential of the inner main pump electrode 22. The amount of change in potential is approximately equal to the product of the maximum current Ip0max and the resistance value of the inner main pump electrode 22. ON In comparison with the potential difference V0 at OFF The potential difference V0 at the innermost pump electrode 22 has a value corresponding to the oxygen concentration in the vicinity of the innermost pump electrode 22.

[0148] Looking at the fluctuation of the potential difference V0 in more detail, the pulsed power supply 24 supplies the intermittent main pump current Ip0, which causes the potential of the inner main pump electrode 22 to change intermittently. As a result, the potential difference V0 also fluctuates periodically in accordance with the fluctuation of the potential of the inner main pump electrode 22. As shown in Fig. 4(2), the potential difference V0 also fluctuates periodically in a pulsating manner in conjunction with the on / off of the main pump current Ip0 shown in Fig. 4(1).

[0149] For example, in FIG. 4(2), the potential difference V0 starts to change (starts to rise) from time t1 due to the influence of the potential caused by the flow of the maximum pump current Ip0max. After that, the potential difference V0 reaches the most changed value V0a (maximum value) due to the influence of the potential caused by the flow of the maximum pump current Ip0max at time t2, and during the off period T OFF Since the main pump current Ip0 does not flow, the previous on-period T ON The influence of the potential due to the maximum pump current Ip0max flowing at time t0 disappears, and the change begins to settle (starts to fall). Then, at time t4, the potential difference V0 reaches the value V0b (minimum value) with the least change. The speed of the rise or fall of the potential difference V0 is considered to depend on the electrostatic capacitance (also called electric capacity or capacitance) of the inner main pump electrode 22 and the resistance value of the inner main pump electrode 22 (for example, the product of the electrostatic capacitance and the resistance value). Taking the rise or fall speed of the potential difference V0 into consideration, the ratio of the OFF period T to the period T is OFF The ratio of the off period T to the period T may be 10% or more (i.e., the duty ratio is 90% or less). OFF It is recommended that the ratio be 20% or more (i.e., the duty ratio be 80% or less).

[0150] The off period T of the main pump current Ip0 OFF is the off period T OFF The on-time T just before the start of ON The non-stable period Tnon-stable is the period until the change in the potential difference V0 caused by the main pump current Ip0 flowing in the ON and a stable period Tstable during which there is no change in the potential difference V0 due to the main pump current Ip0 flowing through the pump. Here, "no change in the potential difference V0 due to the main pump current Ip0 flowing through the pump" means that the change in the potential difference V0 is sufficiently small.

[0151] Last ON time T ONThe stable period Tstable, during which there is no change in the potential difference V0 due to the main pump current Ip0 flowing between the pumps, is defined as follows: The range of the potential difference V0 from V0b to V0a during one cycle T of the main pump current Ip0 is defined as 0% to 100%, and the stable period Tstable of the potential difference V0 is determined based on this range. Specifically, the stable period Tstable is defined as the period from the point at which the main pump current Ip0 is turned off and the potential difference V0 drops to a predetermined percentage until the start of the next cycle, i.e., the point at which the potential difference V0 begins to rise (increase) as the main pump current Ip0 is turned on. The predetermined percentage can be set appropriately by those skilled in the art. The predetermined percentage may be, for example, 1% to 50%. For example, it may be 50% or less, 40% or less, 30% or less, 20% or less, 15% or less, 10% or less, 5% or less, or 1% or less. In this embodiment, the stable period Tstable is defined as the period from when the main pump current Ip0 is turned off and the potential difference V0 becomes 10% or less until the main pump current Ip0 is turned on in the next cycle and the potential difference V0 starts to rise (times t3 to t4).

[0152] Alternatively, the stable period Tstable is defined as the period T when the potential difference V0 is off. OFF Alternatively, the period may be defined as the period during which the potential difference V0 at the

[0153] The non-stable period Tnon-stable is the off period T OFF The on-time T just before the start of ON The off period T is the period from when the change in the potential difference V0 caused by the main pump current Ip0 flowing through the off period T to when the value of the potential difference V0 converges. OFF This is the period from the start of the stable period Tstable to the start of the stable period Tstable (times t2 to t3 in FIG. 4(2)).

[0154] The length of the stable period Tstable is the on-time T in the period T ON This can vary depending on the length of the pulse (duty ratio), etc.

[0155] In Figure 4(2), the potential difference V0 is the main pump current Ip0 The largest change value V0a is reached at time t2 when the power supply is turned off. ON If is long, the value is V0a In some cases, the potential difference V0 reaches a value V0b at which it has no significant change until time t4, when the main pump current Ip0 of the next cycle is turned on. However, in other cases, the potential difference V0 reaches the value V0b before time t4 and remains in this state until time t4.

[0156] During the stable period Tstable, the immediately preceding on-time T ON Since there is no change in the potential difference V0 due to the main pump current Ip0 flowing through the inner main pump electrode 22, that is, the change is sufficiently small, the potential difference V0 during the stable period Tstable becomes a value that more accurately indicates the oxygen concentration near the inner main pump electrode 22.

[0157] In this way, the potential difference V0 varies under the influence of the main pump current Ip0. OFF During the stable period Tstable, the change in the potential difference V0 due to the flow of the main pump current Ip0 becomes small, and therefore the potential difference V0 indicates the oxygen concentration in the vicinity of the inner main pump electrode 22. Furthermore, during the stable period Tstable, the change in the potential difference V0 due to the flow of the main pump current Ip0 becomes small. ON Since there is no change in the potential difference V0 due to the main pump current Ip0 flowing through the inner main pump electrode 22, that is, the change in the potential difference V0 is sufficiently small, the potential difference V0 becomes a value that more accurately indicates the oxygen concentration near the inner main pump electrode 22.

[0158] As described above, in step S23, the setting unit 94 acquires the potential difference V0 in the main pump control oxygen partial pressure detection sensor cell 80. The timing for acquiring the potential difference V0 can be determined as appropriate. However, the setting unit 94 acquires the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42 during the OFF period T when the main pump current Ip0 is not flowing through the main pump cell 21. OFFThis makes it possible to obtain the potential difference V0 that is less affected by the main pump current Ip0. That is, it is possible to obtain the potential difference V0 that indicates the oxygen concentration in the vicinity of the inner main pump electrode 22.

[0159] More preferably, the setting unit 94 acquires the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42 during a stable period Tstable when the main pump current Ip0 is not flowing through the main pump cell 21 and there is no change in the potential difference V0 due to the main pump current Ip0 flowing. This allows the potential difference V0 to be acquired with less influence from the main pump current Ip0. In other words, the potential difference V0 indicating the oxygen concentration near the inner main pump electrode 22 can be acquired more accurately.

[0160] 4(2), the change in the potential difference V0 due to the flow of the main pump current Ip0 tends to become smaller as the potential difference V0 approaches the end point (time t4) of the stable period Tstable. Therefore, the setting unit 94 may obtain the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42, for example, at any point in the latter half of the stable period Tstable.

[0161] Further, for example, the setting unit 94 may set the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42 during the OFF period T when the main pump current Ip0 is not flowing through the main pump cell 21. OFF The predetermined time point may be, for example, during the off period T OFF Alternatively, the off period T may be set to a point a predetermined time before the end of the off period T OFFThe closer to the end of the predetermined time point, the more preferable. The predetermined time point should be set so as to be included in the stable period Tstable during which there is no change in the potential difference V0 due to the flow of the main pump current Ip0. For example, the time change in the potential difference V0 at a predetermined or set maximum duty ratio may be experimentally determined, and any time point included in the stable period Tstable at the maximum duty ratio may be set as the predetermined time point. The potential difference V0 indicating the oxygen concentration near the inner main pump electrode 22 can be obtained more accurately without monitoring the time change in the potential difference V0.

[0162] Up to this point, we have taken the example of pumping oxygen out of the first internal space 20, i.e., the case where the main pump current Ip0 flows in the positive direction. However, depending on the composition of the measurement gas, such as when the oxygen concentration in the measurement gas is very low or when the measurement gas contains a large amount of hydrocarbons (HC), oxygen may be pumped into the first internal space 20, i.e., the main pump current Ip0 may flow in the negative direction. Figure 5 is a schematic diagram showing the time changes in the main pump current Ip0 and the potential difference V0 when the main pump current Ip0 flows in the negative direction. Figure 5(1) is a schematic diagram showing the time changes in the main pump current Ip0. The horizontal axis represents time t, and the vertical axis represents the main pump current Ip0. As with Figure 4(1), the upward direction of the vertical axis represents the positive direction of the main pump current Ip0. Figure 5(2) is a schematic diagram showing the time changes in the potential difference V0. The horizontal axis represents time t, and the vertical axis represents the potential difference V0. The potential difference V0 is defined as positive when the potential of the reference electrode 42 is higher than that of the inner main pump electrode 22. In Fig. 5(2), the upward direction of the vertical axis is defined as positive, as in Fig. 4(2).

[0163] When the main pump current Ip0 flows in the negative direction, for example, when the main pump current Ip0 is turned on at time t1, which is the start point of a certain period T, the main pump current Ip0 goes from 0 A to the maximum current Ip0max (-maximum current Ip0max) in the negative direction, and the ON period T ON This state continues until time t2 when the main pump current Ip0 is turned off. When the main pump current Ip0 is turned off at time t2, the main pump current Ip0 drops from the maximum current Ip0max to 0 A, and the off period T OFFThe main pump current Ip0 is 0 A until time t4 when the time t1 has elapsed. In this embodiment, the absolute values of the maximum current Ip0max flowing in the positive direction and the maximum current flowing in the negative direction (-maximum current Ip0max) are the same, but the absolute values in the positive and negative directions may be different.

[0164] The potential difference V0 also fluctuates intermittently under the influence of the main pump current Ip0, which flows intermittently in the negative direction. For example, in FIG. 5(2), the potential difference V0 starts to change (begins to fall) under the influence of the potential caused by the flow of the maximum negative current Ip0max (-maximum current Ip0max) from time t1. Thereafter, the potential difference V0 reaches the most changed value V0c (minimum value) under the influence of the potential caused by the flow of the maximum negative current Ip0max (-maximum current Ip0max) at time t2, and during the off period T from time t2 OFF Since the main pump current Ip0 does not flow, the previous on-period T ON The influence of the negative maximum current Ip0max (-maximum current Ip0max) flowing through the potential disappears, and the change begins to settle down (starts to rise). Then, at time t4, it reaches the value V0d (maximum value) with the least change.

[0165] For example, the drive control unit 92 may adjust the auxiliary pump current Ip1 in the auxiliary pump cell 50 to the target current value Ip1 SET If it is larger, the duty ratio of the negative main pump current Ip0 is reduced, or the pulse power supply is adjusted to flow the positive main pump current Ip0. 24 is controlled, and the oxygen concentration in the first internal space 20 is further reduced.

[0166] Further, for example, the drive control unit 92 may control the auxiliary pump current Ip1 in the auxiliary pump cell 50 to be equal to or greater than the target current value Ip1 SET If it is smaller, the pulse power supply is set to increase the duty ratio of the negative main pump current Ip0. 24 5(1), for example, the ON period T in the first cycle T ON (Time t1 to t2) On period T in the second cycle TON 5B shows a case where control is performed to double the length of the period (times t4 to t5) (to double the duty ratio). In this case, the average negative value Ip0ave in the second period T is doubled compared to the average negative value Ip0ave in the first period T. Since the average negative value Ip0ave is doubled in the second period T, more oxygen is pumped into the first internal space 20 than during the first period T. As a result, the oxygen concentration near the inner main pump electrode 22 increases in the second period T, and as shown in FIG. 5B, the potential difference V0d' at the end of the second period T is smaller than the potential difference V0d at the end of the first period T.

[0167] Even when the main pump current Ip0 flows in the negative direction, the potential difference V0 during one cycle T of the main pump current Ip0 is set to 0% to 100% from the value V0c to the value V0d, and the OFF period T is set based on this. OFF The stable period Tstable and the non-stable period Tnon-stable of the potential difference V0 in the pump current Ip0 are defined. Specifically, the stable period Tstable is defined as the period from when the main pump current Ip0 is turned off and the potential difference V0 increases to a predetermined percentage until the start of the next cycle, i.e., when the main pump current Ip0 is turned on and the potential difference V0 starts to fall (decreasing). The predetermined percentage can be appropriately set by those skilled in the art, as in the case of the main pump current Ip0 flowing in the positive direction. The predetermined percentage may be, for example, 50% to 99%. For example, the predetermined percentage may be 50% or more, 60% or more, 70% or more, 80% or more, 85% or more, 90% or more, 95% or more, or 99% or more. In this embodiment, the stable period Tstable is defined as the period (times t3 to t4) from when the main pump current Ip0 is turned off and the potential difference V0 increases to 90% or more until the potential difference V0 starts to fall when the main pump current Ip0 is turned on in the next cycle.

[0168] Alternatively, the stable period Tstable is defined as the period T when the potential difference V0 is off. OFF Alternatively, the period may be defined as a period during which the potential difference V0 at the

[0169] The non-stable period Tnon-stable is the off period T OFF The on-time T just before the start of ON The off period T is the period from when the change in the potential difference V0 caused by the main pump current Ip0 flowing through the off period T to when the value of the potential difference V0 converges. OFF This is the period from the start of the stable period Tstable to the start of the stable period Tstable (times t2 to t3 in FIG. 5(2)).

[0170] The setting unit 94 determines whether the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42 is at a predetermined target value. If the potential difference V0 is at the target value, no change ΔO2 in the residual oxygen concentration has occurred, and therefore the target voltage value V1 at the time of acquiring the potential difference V0 is SET is still used. The potential difference V0 being at the target value includes the meaning that the deviation from the target value is within a predetermined range. The deviation from the target value being within a predetermined range means that no substantial change ΔO2 in the residual oxygen concentration occurs. If the potential difference V0 is out of the target value, a change ΔO2 in the residual oxygen concentration occurs, so the setting unit 94 sets a new target voltage value V1 based on the value of the potential difference V0 or the deviation of the potential difference V0 from the target value. SET The potential difference V0 being out of the target value includes the meaning that the deviation from the target value exceeds a predetermined range.

[0171] The target value of the potential difference V0 can be determined appropriately by those skilled in the art. The target value may be set so that, as a result of a constant auxiliary pump current Ip1 flowing through the auxiliary pump cell 50, the oxygen concentration near the auxiliary pump electrode 51, i.e., the residual oxygen concentration in the measurement gas reaching the measuring electrode 44, reaches a predetermined value. The target value may be set so that the change ΔO2 in the residual oxygen concentration in the measurement gas reaching the measuring electrode 44 falls within a range allowable for the desired measurement accuracy of the NOx concentration. For example, the target value of the potential difference V0 may be 150 mV to 300 mV, taking into account the fluctuation range of the residual oxygen concentration in the measurement gas reaching the measuring electrode 44, which will be described later.

[0172] The predetermined range of deviation from the target value can be determined as appropriate by those skilled in the art. It may be determined based on the range of change ΔO2 in the residual oxygen concentration in the measurement gas that reaches the measuring electrode 44 that is acceptable for the desired measurement accuracy of the NOx concentration. Alternatively, it may be determined based on the degree of minute control fluctuations in the pump control. For example, taking into account the fluctuation range of the residual oxygen concentration in the measurement gas that reaches the measuring electrode 44, which will be described later, the deviation of the potential difference V0 from the target value may be within 5 mV to 30 mV.

[0173] Alternatively, a target range of the potential difference V0 may be set in advance. When the potential difference V0 between the inner main pump electrode 22 and the reference electrode 42 is within the target range, the change ΔO2 in the residual oxygen concentration does not substantially occur, so the target voltage value V1 at the time of acquiring the potential difference V0 is set to 0. SET If the potential difference V0 is outside the target range, a change ΔO2 in the residual oxygen concentration has actually occurred, so the setting unit 94 sets a new target voltage value V1 based on the value of the potential difference V0. SET Calculate.

[0174] In this way, the target voltage value V1 is set based on the potential difference V0. SET By performing the above setting step of setting the target voltage value V1, it is possible to substantially prevent a change ΔO2 in the oxygen concentration near the auxiliary pump electrode 51, i.e., the residual oxygen concentration in the measurement gas that reaches the measurement electrode 44. In other words, it is possible to control the residual oxygen concentration in the measurement gas that reaches the measurement electrode 44 to be constant. SETAs a result of the setting step, the residual oxygen concentration in the measurement gas reaching the measuring electrode 44 can take a value of approximately 300 mV to 450 mV. The residual oxygen concentration in the measurement gas reaching the measuring electrode 44 is preferably kept constant, for example, at 0.1 ppm or less, 0.01 ppm or less, 0.001 ppm or less, or approximately 0.0001 ppm or less. It is sufficient that the residual oxygen concentration in the measurement gas reaching the measuring electrode 44 be kept within a certain range (within a predetermined fluctuation range) to the extent that the object of the present invention, which is to accurately measure a measurement target gas (NOx in this embodiment) with a low concentration of approximately 10 to 500 ppm, can be achieved. It is preferable that the residual oxygen concentration be controlled within as small a range (fluctuation range) as possible. To control the residual oxygen concentration in the measurement gas reaching the measuring electrode 44 to 0.001 ppm, for example, the fluctuation range of the residual oxygen concentration should be 0.0001 ppm or less (1 / 10 or less) or 0.00005 ppm or less (1 / 20 or less). The fluctuation range of the residual oxygen concentration can be appropriately determined by those skilled in the art depending on the intended use of the gas sensor 100.

[0175] If the potential difference V0 is deviated from a predetermined target value, the setting unit 94 sets a new target voltage value V1 based on the value of the potential difference V0 or the deviation of the potential difference V0 from the target value. SET The setting unit 94 calculates a new target voltage value V1 based on the value of the potential difference V0 acquired in each period T (for example, the value during the stable period Tstable). SET Alternatively, a new target voltage value V1 may be calculated based on the value of the potential difference V0 acquired in each of the multiple periods T (for example, based on the average value of multiple potential differences V0 acquired in different periods). SET For example, the setting unit 94 may calculate a new target voltage value V1 based on only the potential difference V0 acquired in one period. SET When calculating the target voltage value V1, the acquired potential difference V0 may become different from the value indicating the oxygen concentration near the inner main pump electrode 22 due to instantaneous fluctuations in the potential difference V0 or electrical noise. In this case, the setting unit 94 sets the target voltage value V1 SET There is a possibility that the target voltage V1 will be changed excessively. SETIf the target voltage value V1 is excessively changed, the auxiliary pump current Ip1 flowing through the auxiliary pump cell 50 and the main pump current Ip0 (average value Ip0ave) flowing through the main pump cell 21 by the pulse power supply 24 may change excessively, which may cause unstable control, such as vibration or oscillation. SET When the target voltage V1 is calculated, the target voltage V1 is calculated in accordance with the actual oxygen concentration in the vicinity of the inner main pump electrode 22. SET In this embodiment, the setting unit 94 stores the potential difference V0 at a predetermined timing during the stable period Tstable for each period T in the memory of the control unit 91 that functions as the setting unit 94, and calculates the target voltage value V1 based on the average value of the potential difference V0 for multiple (e.g., three) most recently stored times. SET The following was calculated.

[0176] While the gas sensor 100 for detecting the NOx concentration in a measurement gas has been described above as an example of an embodiment of the present invention, the present invention is not limited to this embodiment. The present invention may include gas sensors including various configurations of sensor elements and control devices, as long as they achieve the object of the present invention of accurately measuring a wide range of measurement gas concentrations, including low concentrations of the measurement gas.

[0177] In the above-described embodiment, the intermittent main pump current Ip0 is a current having one rectangular wave per cycle (rectangular single pulse current) as shown in FIG. 4(1), but is not limited to this. For example, the pulse power supply 24 may be an intermittent main Pump current Ip 0 6 may be used as the burst pulse current. In this case, the drive control section 92 determines the oscillation period T A (duty ratio), period T, number of pulses in one period (in Figure 6, 4A control signal may be output to the pulse power supply 24 to change at least one of the parameters of the number of oscillations (pulses), the time Ta of one oscillation (pulse), the pulse period (Ta+Tb), and the maximum current Ip0max, thereby changing the average value Ip0ave of the main pump current Ip0 during one period.

[0178] When the main pump current Ip0 is a burst pulse current as shown in Figure 6, the change period Tchange of the potential difference V0 is equal to the oscillation period T A The stable period Tstable is included in the non-oscillation period T B More specifically, when the main pump current Ip0 is a burst pulse current, the oscillation period T A during the period when the main pump current Ip0 is on (Fig. 4 Time t1~t 2 ) and the non-oscillation period T B during the period when the main pump current Ip0 is off (Fig. 4 At time t 2 ~t 4 ), and define the stable period Tstable in the same manner as in the above-described embodiment.

[0179] In the above-described embodiment and the above-described burst pulse current, the pulse power supply 24 flows a pulse current of a rectangular wave as shown in FIG. 4(1) as the main pump current Ip0, but the pulse current is not limited to a rectangular wave (square wave), and may be a pulse current of a half sine wave, a triangular wave, a sawtooth wave, a waveform during discharge, or the like, or may be a pulse current of a waveform that is a combination of one or more of these.

[0180] In FIGS. 4(1), 5(1), and 6, the pulse current of the main pump current Ip0 is illustrated as a rectangular shape. However, as described above, the actual main pump current Ip0 requires a very short time for rising and falling. That is, the pulse current actually has a rise time and a fall time, and the pulse current is not a perfect rectangle. Therefore, for example, if the pulse width of the pulse current is too small (for example, if the time Ta of one oscillation (pulse) in a burst pulse current is too short), the actual peak value of the pulse current may not reach the peak value of an ideal rectangular waveform due to the influence of the rise time. In this case, the drive control unit 92 controls the auxiliary pump current Ip1 in the main pump cell 21 to a constant value (target current value Ip1 SET ) and outputting a control signal to the pulsed power supply 24 to set the average value Ip0ave to a certain target value, the actual average value Ip0ave will be lower than the theoretical average value Ip0ave. As a result, the actual average value Ip0ave will deviate from the target value, and the oxygen concentration in the measured gas may not be accurately controlled. On the other hand, if the pulse width of the pulsed current is large, the actual peak value of the pulsed current will reach the peak value of the ideal rectangular waveform even if there is a rise time. In this case, deviation between the actual average value Ip0ave and the target value is unlikely to occur.

[0181] Therefore, it is preferable that the drive control unit 92 controls the main pump cell 21 so that the pulse width of the main pump current Ip0 reaches a value that matches the peak value of an ideal rectangular waveform. In other words, it is preferable that the drive control unit 92 controls the main pump current Ip0 so that the pulse width of the main pump current Ip0 is equal to or greater than a predetermined lower limit.

[0182] In the above-described embodiment, the pulsed power supply 24 is configured as a current source, but a voltage source may also be used. An intermittent main pump current Ip0 may be caused to flow by applying an intermittent voltage Vp0 between the inner main pump electrode 22 and the outer pump electrode 23 by the pulsed power supply 24.

[0183] In the above-described embodiment, the setting unit 94 sets the target voltage value V1 of the auxiliary pump cell 50 based on the value of the potential difference V0 or the deviation of the potential difference V0 from the target value. SET The target voltage value V1 in the auxiliary pump cell 50 is changed as needed. SET In addition, the target current value Ip1 SET For example, if the potential difference V0 is significantly different from the target value, the target voltage value V1 SET and target current value Ip1 SET By changing both of these, it may be possible to control the potential difference V0 to reach the target value more quickly.

[0184] In the above-described embodiment, the gas sensor 100 detects the NOx concentration in the measurement gas, but the measurement gas is not limited to NOx. For example, the measurement gas may be an oxide gas other than NOx (e.g., carbon dioxide CO2, water HO, etc.). When the measurement gas is an oxide gas, the measurement gas containing the oxide gas itself is introduced into the third internal space 61, as in the above-described embodiment for detecting the NOx concentration, and the oxide gas in the measurement gas is reduced at the measurement electrode 44 to generate oxygen. The generated oxygen can be obtained as the measurement pump current Ip2 in the measurement pump cell 41, and the measurement gas can be detected.

[0185] Alternatively, the measurement target gas may be a non-oxide gas such as ammonia (NH3). When the measurement target gas is a non-oxide gas, the non-oxide gas is converted to an oxide gas (for example, ammonia (NH3) is converted to NO), and the measurement target gas containing the converted oxide gas is introduced into the third internal space 61. The converted oxide gas in the measurement target gas is reduced at the measurement electrode 44 to generate oxygen. The generated oxygen can be detected by acquiring the measurement pump current Ip2 in the measurement pump cell 41. The non-oxide gas can be converted to an oxide gas by at least one of the inner main pump electrode 22 and the auxiliary pump electrode 51 functioning as a catalyst.

[0186] In the above-described embodiment, the auxiliary pump cell 50 and the measurement pump cell 41 are controlled to flow continuous currents by the variable power supplies 52, 46, but this is not limiting. In addition to the main pump cell 21, at least one of the auxiliary pump cell 50 and the measurement pump cell 41 may be controlled to flow intermittently a pump current. For example, the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 may all be controlled to flow intermittently a pump current. That is, a pulsed power supply may be used as a current source instead of the variable power supply 52 of the auxiliary pump cell 50, and a pulsed power supply may be used as a current source instead of the variable power supply 46 of the measurement pump cell 41. In this case, in the auxiliary pump cell 50, the electromotive force V1 in the auxiliary pump control oxygen partial pressure detection sensor cell 81 is controlled to a target voltage value V1. SET The auxiliary pump current Ip1 of the pulsed power supply in the auxiliary pump cell 50 is feedback-controlled so that the auxiliary pump current Ip1 reaches the target current value Ip1 SET In the main pump cell 21, the main pump current Ip0 of the pulsed power supply 24 is feedback-controlled so that the measurement pump current Ip2 can be detected as a current value corresponding to the NOx concentration in the measurement gas, as in the above-described embodiment.

[0187] 1 , the sensor element 101 has three internal cavities, namely, the first internal cavities 20, the second internal cavities 40, and the third internal cavities 61, and the inner main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 are respectively disposed in the internal cavities. However, the present invention is not limited to this. For example, the sensor element 101 may have two internal cavities, namely, the first internal cavities 20 and the second internal cavities 40, and the inner main pump electrode 22 is disposed in the first internal cavities 20, and the auxiliary pump electrode 51 and the measurement electrode 44 are disposed in the second internal cavities 40. In this case, for example, a porous protective layer covering the measurement electrode 44 may be formed as a diffusion rate-controlling part between the auxiliary pump electrode 51 and the measurement electrode 44.

[0188] In the gas sensor 100 of the above-described embodiment, the outer pump electrode 23 functions as three electrodes: the outer main pump electrode in the main pump cell 21, the outer auxiliary pump electrode in the auxiliary pump cell 50, and the outer measurement electrode in the measurement pump cell 41. However, this is not limiting. For example, the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be formed as separate electrodes. For example, one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be provided separately from the outer pump electrode 23 on the outer surface of the base portion 102 so as to be in contact with the gas to be measured. Alternatively, one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be used as a reference electrode. 42 It may also serve as both.

[0189] As described above, according to the present invention, the residual oxygen concentration in the measurement gas that reaches the measurement electrode can be controlled with high precision, so that the measurement gas can be measured accurately even in a low concentration range of about 10 to 500 ppm. As a result, the measurement gas can be measured accurately over a wide concentration range (e.g., 10 to 5000 ppm), including low concentrations.

[0190] Furthermore, according to the present invention, the gas sensor 100 measures the NOx concentration while calculating the target voltage value V1 based on the potential difference V0. SET Therefore, according to the present invention, the residual oxygen concentration in the measurement gas that reaches the measurement electrode can be controlled with high precision while the gas sensor 100 continues to measure the target gas (NOx in this embodiment) in the measurement gas. As a result, even when the NOx concentration in the measurement gas is low, the NOx concentration can be measured continuously and accurately. [Explanation of symbols]

[0191] 1 First board layer 2 Second board layer 3 Third board layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion-controlled section 12 Buffer space 13 Second diffusion-controlled section 15 Measurement gas flow section 20 1st internal void 21 Main pump cell 22 Inner main pump electrode 22a (Inner main pump electrode) ceiling electrode part 22b Bottom electrode part (of inner main pump electrode) 23 Outer pump electrode 24 Pulse power supply 30 Third diffusion-controlled section 40 Second internal void 41 Measuring pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measuring electrode 46 Variable power supply (for measuring pump cell) 47 Switching Unit 48 Atmospheric introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a (auxiliary pump electrode) ceiling electrode part 51b (auxiliary pump electrode) bottom electrode part 52 Variable power supply (for auxiliary pump cell) 60 4th diffusion-controlled section 61 3rd internal void 70 Heater section 71 Heater electrode 72 Heater 73 through holes 74 Heater insulator 75 Pressure relief hole 76 Heater lead 77 Heater power supply 80 Oxygen partial pressure detection sensor cell for main pump control 81 Oxygen partial pressure detection sensor cell for auxiliary pump control 82 Oxygen partial pressure detection sensor cell for measuring pump control 83 Sensor Cell 90 Control device 91 Control Unit 92 Drive control unit 93 Concentration calculation section 94 Settings 100 Gas Sensor 101 Sensor element 102 Base

Claims

1. A gas sensor including a sensor element and a control device that controls the sensor element, The sensor element is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; 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 gas flow portion, and an outer main pump electrode disposed on the base portion at a position different from the measurement gas flow portion, the outer main pump electrode corresponding to the inner main pump electrode; an auxiliary pump cell including: an inner 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 auxiliary pump electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner auxiliary pump electrode; a measurement pump cell including: an inner 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 auxiliary pump electrode; and an outer measurement electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; a heater for heating the base portion; Including, the main pump cell supplies a main pump current that is repeatedly turned on and off so that the auxiliary pump current flowing through the auxiliary pump cell reaches a predetermined target current value; the auxiliary pump cell causes the auxiliary pump current to flow so that an electromotive force between the inner auxiliary pump electrode and the reference electrode reaches a predetermined target voltage value; The control device a control power supply for supplying the main pump current, which is repeatedly turned on and off, to the main pump cell; a setting unit that sets the target voltage value of the auxiliary pump cell; Including, the setting unit acquires a potential difference generated between the inner main pump electrode and the reference electrode, and sets the target voltage value in the auxiliary pump cell based on the potential difference.

2. 2. The gas sensor according to claim 1, wherein the setting unit acquires the potential difference generated between the inner main pump electrode and the reference electrode during an off period in which the main pump current is not flowing in the main pump cell by the control power supply.

3. 3. The gas sensor according to claim 1, wherein the setting unit acquires the potential difference generated between the inner main pump electrode and the reference electrode during a stable period in which the main pump current is not flowing in the main pump cell by the control power supply and there is no change in the potential difference due to the main pump current having flowed.

4. the control power supply includes a pulse power supply that periodically turns on and off the main pump current; 4. The gas sensor according to claim 1, wherein the setting unit acquires the potential difference generated between the inner main pump electrode and the reference electrode for each cycle T of the pulsed power supply at a predetermined time point included in an off period during which the main pump current is not flowing due to the pulsed power supply.

5. 1. A method for controlling a gas sensor for detecting a measurement target gas in a measurement target gas, comprising: The gas sensor comprises: a sensor element and a control device that controls the sensor element; The sensor element is a long plate-shaped substrate including an oxygen ion conductive solid electrolyte layer; 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 gas flow portion, and an outer main pump electrode disposed on the base portion at a position different from the measurement gas flow portion, the outer main pump electrode corresponding to the inner main pump electrode; an auxiliary pump cell including: an inner 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 auxiliary pump electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner auxiliary pump electrode; a measurement pump cell including: an inner 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 auxiliary pump electrode; and an outer measurement electrode disposed at a position different from the measurement gas flow portion of the base portion, corresponding to the inner measurement electrode; a reference electrode disposed inside the base portion so as to be in contact with a reference gas; a heater for heating the base portion; Including, The control device a control power supply for supplying a main pump current to the main pump cell, the main pump current being repeatedly turned on and off; The control method includes: a setting step of acquiring a potential difference generated between the inner main pump electrode and the reference electrode, and setting a target voltage value of an electromotive force between the inner auxiliary pump electrode and the reference electrode in the auxiliary pump cell based on the potential difference; an oxygen concentration adjusting step of controlling the control power supply to flow a main pump current that is repeatedly turned on and off in the main pump cell so that the auxiliary pump current flowing in the auxiliary pump cell reaches a predetermined target current value, and of flowing the auxiliary pump current in the auxiliary pump cell so that an electromotive force between the inner auxiliary pump electrode and the reference electrode reaches the target voltage value.

6. 6. The control method according to claim 5, wherein in the setting step, the potential difference generated between the inner main pump electrode and the reference electrode is acquired during an off period in which the main pump current is not flowing in the main pump cell by the control power supply.

7. 7. The control method according to claim 5, wherein in the setting step, the potential difference generated between the inner main pump electrode and the reference electrode is acquired during a stable period in which the main pump current is not flowing in the main pump cell by the control power supply and there is no change in the potential difference due to the main pump current having flowed.

8. the control power supply includes a pulse power supply that periodically turns on and off the main pump current; 8. The control method according to claim 5, wherein in the setting step, the potential difference generated between the inner main pump electrode and the reference electrode is acquired for each cycle T of the pulsed power supply at a predetermined time point included in an off period during which the main pump current is not flowing due to the pulsed power supply.

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