Gas sensor and concentration correction method therefor

Abstract

To provide a gas sensor which can suppress deterioration of measurement accuracy, even when a pollutant gas occurs on a reference gas space side.SOLUTION: A controller of a gas sensor comprises: a density identification part which identifies density of a prescribed gas component, on the basis of a measurement pump current which flows according to the density of the prescribed gas component between a measurement electrode and an outside-of-gap pump electrode, by application of a prescribed pump voltage; and a correction processing part for correcting the density of the prescribed gas component identified by the density identification part. The correction processing part corrects the density of the prescribed gas component, on the basis of a correlation between an offset current value being magnitude of the measurement pump current when a measured gas including no prescribed gas component flows, the measured gas being identified in advance, or a standardized value of the offset current value, and an output variation generated on the measurement pump current in start of the gas sensor.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a gas sensor having a ceramic sensor element, and more particularly to ensuring measurement accuracy in a gas sensor. [Background technology]

[0002] Gas sensors in which a sensor element is formed using an oxygen ion conductive solid electrolyte ceramic such as zirconia (ZrO2) have been known for some time as devices for measuring the concentration of a specific gas component in a measurement gas such as combustion gas or exhaust gas from an internal combustion engine such as an automobile engine.

[0003] Such gas sensors typically have a main body in which a long, plate-shaped ceramic sensor element (detection element) is fixed inside (hollow portion of) a metal housing member by a ceramic supporter and a ceramic powder compact such as talc, and the powder compact hermetically seals between one end side, which is equipped with a gas inlet for introducing the gas to be measured into the element interior equipped with a measurement electrode, etc., and the other end side, through which a reference gas (atmosphere) is introduced (see, for example, Patent Document 1).

[0004] The other end of the housing member is a cylindrical member also known as an outer cylinder, and a rubber stopper is fitted into the tip of the cylinder as a sealing member. The space surrounded by the outer cylinder and the rubber stopper is the reference gas space. The rubber stopper has through holes through which several lead wires are inserted to electrically connect the center element to the outside. The reference gas is usually the air that is present in the outer cylinder when the rubber stopper is fitted into the outer cylinder during the gas sensor manufacturing process. However, air that subsequently enters the outer cylinder through the gap between the through hole in the rubber stopper and the lead wires can also be used as the reference gas.

[0005] On the other hand, at the other end of the sensor element, a reference gas is introduced from the reference gas space into the element interior, and a reference electrode for providing a reference potential is provided so as to be in contact with the reference gas. By the reference electrode coming into contact with a reference gas having a constant oxygen concentration, the reference potential is kept constant, and a potential difference corresponding to the atmosphere around the electrode is generated between the reference electrode and other electrodes provided in the sensor element such as the measurement electrode.

[0006] Various electrodes provided in the sensor element are electrically connected to connection terminals provided at the element end by electrode leads disposed inside the element or on the element side surface. However, a sensor element in which a reference electrode lead connecting between the reference electrode and an electrode pad as such a connection terminal is porous is already known (for example, see Patent Document 2).

[0007] In addition, a gas sensor element in which the width W1 of the measurement gas chamber into which the measurement gas is introduced and the width W2 of the porous diffusion resistance layer disposed at the gas inlet to the measurement gas chamber satisfy the relationship W1 < W2 is already known (for example, see Patent Document 3).

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] The gas sensor disclosed in Patent Document 1 is used in a high-temperature environment, with its main body disposed in the exhaust path from the engine and exposed to exhaust gases. Furthermore, the sensor element itself is heated by a heater. Therefore, when the main body is heated to a high temperature after use, the reference gas may be contaminated by contaminant gases generated by the evaporation of oil adhering to the inner surface of the outer tube or the generation of gas from the rubber stopper. If such contaminant gases reach the reference electrode, the reference potential, which should be kept constant, may change, potentially disrupting the measurement accuracy of the gas sensor. This situation is considered more likely to occur when the reference electrode lead is porous, as disclosed in Patent Document 2.

[0010] In the gas sensor disclosed in Patent Document 1, an electrochemical pump cell is composed of an outer electrode, a reference electrode, and a solid electrolyte between the two. By applying a predetermined voltage between the two electrodes, oxygen can be pumped from outside the element into the reference gas space. By pumping oxygen in this way, the reference potential can be kept constant even if the reference gas is contaminated.

[0011] However, the above-mentioned contaminant gas may reach not only the reference electrode but also the measurement electrode or even an internal space closer to the gas inlet via an electrode lead such as a measurement electrode lead. In this case, the measurement gas, which is supposed to be introduced through the gas inlet and then reach the measurement electrode after its oxygen concentration has been adjusted to a predetermined value, may be contaminated by the contaminant gas that has infiltrated through the lead, resulting in a deterioration in the measurement accuracy of the gas sensor. Even if the amount of infiltration is small, its effect cannot be ignored if the concentration of the measurement target gas component is low.

[0012] For example, Patent Document 3 discloses a configuration in which, when the width W1 of the measurement gas chamber is reduced compared to conventional configurations and the width of the electrodes provided in the gas sensor element is also reduced, the width of the electrode leads connected to each electrode is set relatively large to prevent a large change in the impedance of the electrode portion including the lead portions, thereby adjusting the electrode impedance to a predetermined value. If the width of the lead portions is increased in this way, the diffusion resistance of the lead portions decreases, increasing the risk of an increase in the amount of contaminant gas reaching the electrode portion via the lead portions.

[0013] Furthermore, adopting porous electrode leads such as those disclosed in Patent Document 2 for electrodes other than the reference electrode lead can reduce the amount of platinum used, leading to cost savings, but there is a risk that the amount of contaminant gas that penetrates through the electrode lead may increase.

[0014] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a gas sensor in which deterioration of measurement accuracy is suppressed even when contaminant gas is generated on the reference gas space side. [Means for solving the problem]

[0015] In order to achieve the above object, a first aspect of the present invention is a gas sensor capable of detecting a predetermined gas component in a measurement gas, the gas sensor comprising: a sensor element having a long, plate-shaped base portion made of an oxygen ion conductive solid electrolyte and having a detection portion on one end side; a casing in which the sensor element is housed and fixed; and a controller for controlling the operation of the gas sensor, wherein the casing comprises a reference gas space in which a reference gas is present, an outer cylinder having the other end side of the sensor element protruding into the reference gas space, and a seal member fitted into the end of the outer cylinder for sealing the reference gas space, and the sensor element comprises at least one oxygen partial pressure adjusting internal cavity which communicates with an inlet for the measurement gas provided on the one end side at a predetermined diffusion resistance, a measurement internal cavity which further communicates with the at least one oxygen partial pressure adjusting internal cavity, a cavity outer pump electrode disposed at a location other than the at least one oxygen partial pressure adjusting internal cavity and the measurement internal cavity, and a measurement electrode lead portion including: a measurement electrode provided facing a space between the measurement electrode and the outer-space pump electrode; a measurement pump cell in which a measurement pump current corresponding to the concentration of the predetermined gas component flows between the measurement electrode and the outer-space pump electrode when a predetermined pump voltage is applied; a measurement electrode lead portion extending from the measurement electrode and capable of electrically connecting the outside of the sensor element to the measurement electrode; and a measurement lead insulating layer covering the measurement electrode lead; wherein the controller comprises: a concentration specifying portion that specifies the concentration of the predetermined gas component based on the measurement pump current; and a correction processing portion that corrects the concentration of the predetermined gas component specified by the concentration specifying portion, wherein the correction processing portion corrects the concentration of the predetermined gas component based on a correlation between an offset current value or a normalized value of the offset current value, which is the magnitude of the measurement pump current when a measurement gas not containing the predetermined gas component flows, which is specified in advance.

[0016] A second aspect of the present invention is the gas sensor according to the first aspect, characterized in that the correction processing unit includes a temperature estimation unit that estimates the temperature of the sealing member based on pre-specified temperature estimation information, and corrects the concentration of the specified gas component when it is determined that the temperature of the sealing member has exceeded a specified threshold temperature after the gas sensor is started.

[0017] A third aspect of the present invention is a gas sensor according to the first or second aspect, characterized in that the correction processing unit includes a time measurement unit that measures the time during which correction of the concentration of the specified gas component is performed, and the correction of the concentration of the specified gas component is stopped after the cumulative value of the time measured by the time measurement unit reaches a predetermined maximum correction time.

[0018] A fourth aspect of the present invention is a concentration correction method for a gas sensor capable of detecting a predetermined gas component in a measurement gas, the gas sensor comprising: a sensor element having a long, plate-shaped base portion made of an oxygen ion conductive solid electrolyte and having a detection portion on one end side; and a casing in which the sensor element is housed and fixed, the casing comprising: an outer cylinder having a reference gas space inside which a reference gas is present, the other end side of the sensor element protruding into the reference gas space; and a seal member fitted into the end of the outer cylinder and sealing the reference gas space, the sensor element comprising: at least one oxygen partial pressure adjusting internal cavity that communicates with an inlet for the measurement gas provided on the one end side at a predetermined diffusion resistance; a measurement internal cavity that further communicates with the at least one oxygen partial pressure adjusting internal cavity; an outer cavity pump electrode arranged at a location other than the at least one oxygen partial pressure adjusting internal cavity and the measurement internal cavity; and a measurement electrode provided facing the measurement internal cavity. A predetermined pump voltage is applied between the measurement electrode and the pump electrode outside the cavity.a measurement electrode lead portion including a measurement pump cell through which a measurement pump current corresponding to the concentration of the predetermined gas component flows, a measurement electrode lead extending from the measurement electrode and capable of electrically connecting the outside of the sensor element to the measurement electrode, and a measurement lead insulating layer covering the measurement electrode lead; and a concentration determination process for determining the concentration of the predetermined gas component based on the measurement pump current; and a correction process for correcting the concentration of the predetermined gas component determined in the concentration determination process, wherein the correction process corrects the concentration of the predetermined gas component based on a correlation between an offset current value or a normalized value of the offset current value, which is the magnitude of the measurement pump current when a measured gas not containing the predetermined gas component flows, and an output fluctuation occurring in the measurement pump current when the gas sensor is started up.

[0019] A fifth aspect of the present invention is a concentration correction method for a gas sensor according to the fourth aspect, characterized in that the correction processing step includes a temperature estimation step of estimating the temperature of the sealing member based on pre-specified temperature estimation information, and the concentration of the specified gas component is corrected when it is determined in the temperature estimation step that the temperature of the sealing member has exceeded a specified threshold temperature after the gas sensor is started.

[0020] A sixth aspect of the present invention is a concentration measurement method for a gas sensor according to the fourth or fifth aspect, characterized in that the correction processing step includes a time measurement step of measuring the time during which correction of the concentration of the specified gas component is performed, and after the cumulative value of the time measured by the time measurement step reaches a predetermined maximum time required for correction, correction of the concentration of the specified gas component is not performed. [Effects of the Invention]

[0021] According to the first to sixth aspects of the present invention, it is possible to suppress deterioration in the measurement accuracy of the NOx concentration caused by the generation of pollutant gas in the reference gas space when the gas sensor is started up. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a cross-sectional view of a main part of a gas sensor 100 taken along the longitudinal direction. [Figure 2] 1 is a vertical cross-sectional view taken along the longitudinal direction of a sensor element 101, schematically illustrating an example of the configuration of the sensor element 101. FIG. [Figure 3] FIG. 1 is a block diagram showing functional components implemented in a controller 110. [Figure 4] 1 is a plan view showing the arrangement of the electrode lead portion including the electrode leads extending from each of the inner pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44. FIG. [Figure 5] FIG. 2 is a plan view showing the arrangement of the electrode lead portion including the electrode lead extending from the outer pump electrode 23. [Figure 6] 10 is an example of a graph showing a change in NOx current Ip2 from the start of operation, together with a change in temperature of a rubber plug 106, when the gas sensor is operated in a model gas atmosphere with a constant NOx concentration. [Figure 7] FIG. 10 is a diagram showing an example of the correlation between Ip2 offset and the amount of output fluctuation of NOx current Ip2. [Figure 8] FIG. 10 is a diagram for explaining a method for evaluating an output fluctuation amount. [Figure 9] 11 is a diagram showing a specific procedure of the correction processing performed in the correction processing unit 1103. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0023] <Gas sensor configuration> FIG. 1 shows an embodiment of the present invention. relating to1 is a longitudinal cross-sectional view of a main portion of the gas sensor 100 (more specifically, of its main body). In this embodiment, the gas sensor 100 is attached to an exhaust path of an internal combustion engine (e.g., an automobile engine) (not shown) and detects, with a sensor element 101 provided therein, a predetermined gas component (e.g., NOx) contained in exhaust gas (a measurement gas) flowing through the exhaust path. In FIG. 1, the vertical direction is shown as the z-axis direction, and the longitudinal direction of the gas sensor 100 coincides with the z-axis direction (this also applies to the following figures).

[0024] In the following description, the internal combustion engine is an automobile engine, and the gas component to be detected by the gas sensor 100 is NOx. However, the method for specifying and correcting the NOx concentration described in this embodiment can also be applied to other gas species that can be measured and corrected based on similar principles.

[0025] The gas sensor 100 mainly comprises a sensor element 101, a ring part 120 mounted around the sensor element 101, a cylindrical body 130 which is further mounted around the ring part 120 and houses the ring part 120, and these are covered with a protective cover 102, a fixing bolt 103, and an outer cylinder 104. In other words, the gas sensor 100 is roughly configured such that the sensor element 101 passes through the cylindrical body 130 in the axial direction at the axial center position inside the cylindrical body 130, and the ring part 120 is mounted around the sensor element 101 inside the cylindrical body 130. The cylindrical body 130, the protective cover 102, and the outer cylinder 104 mainly form a casing (housing member) for the sensor element 101.

[0026] The sensor element 101 is a long, columnar or thin-plate member whose main constituent material is an element body made of oxygen-ion conductive solid electrolyte ceramics such as zirconia. The sensor element 101 is disposed on the central axis along the longitudinal direction of the cylindrical body 130. Hereinafter, the extending direction of the central axis, which coincides with the longitudinal direction of the cylindrical body 130, will also be referred to as the axial direction. In FIG. 1 and the following figures, the axial direction coincides with the z-axis direction.

[0027] The sensor element 101 has a detection section including a gas inlet and an internal cavity on the side of the first tip portion 101a, and is configured with various electrodes and wiring patterns on the surface and inside of the element body. In the sensor element 101, a test gas introduced into the internal cavity is reduced or decomposed in the internal cavity to generate oxygen ions. In the gas sensor 100, the amount of oxygen ions flowing inside the element is proportional to the concentration of the gas component in the test gas, and therefore the concentration of the gas component can be determined.

[0028] A predetermined area of the surface of the sensor element 101 in the longitudinal direction from the first tip portion 101a is covered with a protective film 111. The protective film 111 is provided to protect the area around the first tip portion 101a of the sensor element 101, which includes an internal cavity and electrodes, from thermal shock due to exposure to water, and is also referred to as a thermal shock-resistant protective layer. The protective film 111 is a porous film made of, for example, Al2O3 and having a thickness of approximately 10 μm to 2000 μm. In light of its purpose, the protective film 111 is preferably formed to withstand a force of up to approximately 50 N. However, the formation area of the protective film 111 in FIG. 1 and the subsequent figures is merely an example, and the actual formation area is determined appropriately depending on the specific structure of the sensor element 101.

[0029] The protective cover 102 is a substantially cylindrical exterior member that protects the first tip portion 101a, which is the portion of the sensor element 101 that comes into direct contact with the test gas during use. The protective cover 102 is fixed by welding to the outer circumferential end portion (the outer periphery of a reduced diameter portion 131, which will be described later) on the lower side (negative side in the z-axis direction) of the cylindrical body 130 as viewed in the drawing.

[0030] 1, the protective cover 102 has a two-layer structure consisting of an outer cover 102a and an inner cover 102b. The outer cover 102a and the inner cover 102b are each provided with a plurality of through-holes H1, H2, H3, and H4 through which gas can pass. Note that the type, number, position, and shape of the through-holes shown in FIG. 1 are merely examples and may be determined appropriately taking into account the manner in which the measurement gas flows into the protective cover 102.

[0031] The fixing bolt 103 is an annular member used to fix the gas sensor 100 to a measurement position. The fixing bolt 103 includes a threaded bolt portion 103a and a holding portion 103b that is held when the bolt portion 103a is screwed. The bolt portion 103a is screwed into a nut provided at the installation position of the gas sensor 100. For example, by screwing the bolt portion 103a into a nut provided in an exhaust pipe of an automobile, the gas sensor 100 is fixed to the exhaust pipe in a manner that the protective cover 102 side is exposed inside the exhaust pipe.

[0032] The outer cylinder 104 is a cylindrical metal member, one end of which (the lower end as viewed in the drawing) is welded to the outer peripheral end of the cylindrical body 130 on the upper side as viewed in the drawing (the positive side in the z-axis direction). A connector 105 is disposed inside the outer cylinder 104. A rubber stopper 106 is fitted into the other end (the upper end as viewed in the drawing) of the outer cylinder 104 as a sealing member. In the gas sensor 100, the space between the cylindrical body 130 and the rubber stopper 106 and surrounded by the outer cylinder 104 serves as a reference gas space SP. In other words, the reference gas space SP is sealed by the rubber stopper 106. A second tip portion 101b of the sensor element 101 protrudes into the reference gas space SP. For example, air is introduced into the reference gas space SP as a reference gas when measuring the NOx concentration.

[0033] The connector 105 is provided with a plurality of contact members 151 that come into contact with a plurality of terminal electrodes (not shown) provided on the second tip portion 101b of the sensor element 101. The contact members 151 are connected to lead wires 107 that are inserted through a rubber plug 106. The lead wires 107 are connected to a controller 110 and various power sources (not shown in FIG. 1) outside the gas sensor 100.

[0034] Although only two contact members 151 and two lead wires 107 are shown in FIG. 1, this is merely an example.

[0035] The cylindrical body 130 is a metallic cylindrical member also called a metal shell. The sensor element 101 and the ring part 120 are housed inside the cylindrical body 130. In other words, the cylindrical body 130 is mounted around the ring part 120 which is mounted around the sensor element 101.

[0036] The cylindrical body 130 mainly comprises a thick-walled main portion 130M having a cylindrical inner space formed by a cylindrical inner surface 130a parallel to the axial direction, a tapered portion 131 which is located at the lower axial end (negative side in the z-axis direction) as viewed in the drawing and is thicker than the main portion 130M, a thin-walled crimped portion 132 which extends further upward from the end face 130c of the main portion 130M which is located at the upper end in the axial direction as viewed in the drawing and is bent in a direction toward the axial center, and a locking portion 133 which protrudes outward in the circumferential direction.

[0037] The crimping portion 132 is bent to press and fix (restrain) the ring component 120 (specifically, the second ceramic supporter 123) disposed inside from above in the drawing. As will be described later, the crimping portion 132 is against The ring member 120 is bent after being mounted on the ring member 120.

[0038] The annular component 120 is made up of a first ceramic supporter 121 , a powder compact 122 , and a second ceramic supporter 123 .

[0039] The first ceramic supporter 121 and the second ceramic supporter 123 are insulators made of ceramic. More specifically, a rectangular through-hole (not shown) corresponding to the cross-sectional shape of the sensor element 101 is provided at the axial center position of the first ceramic supporter 121 and the second ceramic supporter 123, and the sensor element 101 is inserted into the through-hole, whereby the first ceramic supporter 121 and the second ceramic supporter 123 are annularly mounted on the sensor element 101. The first ceramic supporter 121 is engaged with the tapered surface 130b of the cylindrical body 130 at the bottom as viewed in the drawing.

[0040] On the other hand, the powder compact 122 is made by molding ceramic powder such as talc, and like the first ceramic supporter 121 and the second ceramic supporter 123, the sensor element 101 is inserted into the through-hole, and the powder compact 122 is disposed inside the cylindrical body 130 in a state of being annularly mounted around the sensor element 101, and then further compressed to form an integrated body. More specifically, the ceramic particles that make up the powder compact 122 are surrounded by the first ceramic supporter 121, the second ceramic supporter 123, and the cylindrical body 130, and are densely packed in the space through which the sensor element 101 passes.

[0041] In the gas sensor 100, the sensor element 101 and the enclosure part 120 are fixed inside the cylindrical body 130 by engagement with the tapered surface 130b of the first ceramic supporter 121 and pressure from the upper side in the drawing by the crimping portion 132 of the second ceramic supporter 123. In addition, airtight sealing is achieved between the first tip end portion 101a side and the second tip end portion 101b side of the sensor element 101 by compressing and filling the powder compact 122.

[0042] <Schematic configuration of sensor element and controller> Fig. 2 is a vertical cross-sectional view taken along the longitudinal direction of the sensor element 101, schematically illustrating an example of the configuration of the sensor element 101. In Fig. 2, the protective film 111 provided on the first tip portion 101a side of the sensor element 101 is not shown, but a controller 110 is also shown that controls the operation of each part of the gas sensor 100 and determines the NOx concentration based on the NOx current (measured pump current) flowing through the sensor element 101.

[0043] The controller 110 includes at least one processor (not shown) and memory (not shown), and the functions of the controller 110 are realized by the processor executing software stored in the memory. The memory is, for example, a non-volatile or volatile semiconductor memory.

[0044] 3 is a block diagram showing functional components realized in the controller 110. The controller 110 includes, as functional components, an element operation control unit 1101 that controls the operation of each unit of the sensor element 101, a NOx concentration specifying unit 1102 that specifies the NOx concentration, and a correction processing unit 1103 that corrects the NOx concentration specified by the NOx concentration specifying unit 1102 in a predetermined situation, which will be described later. The correction processing unit 1103 also includes a seal member temperature (rubber stopper temperature) estimation unit 1103A that is responsible for estimating the temperature of the rubber stopper 106, a correction time measurement unit 1103B that is responsible for measuring the time of the correction processing and storing the accumulated value (accumulated correction time), and a correction execution unit 1103C that actually executes the correction processing.

[0045] In this embodiment, the controller 110 includes an ECU (electronic control unit, not shown) to which the gas sensor 100 is electrically connected, for controlling the operation of each part of the automobile.

[0046] The sensor element 101 is a flat (long plate-like) ceramic element having a structure in which six solid electrolyte layers—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—are stacked in this order from bottom to top as viewed in the drawing, each of which is made of zirconia (ZrO2), an oxygen-ion conductive solid electrolyte (e.g., yttria-stabilized zirconia (YSZ)). The solid electrolyte forming these six layers is dense and airtight. Hereinafter, the upper and lower surfaces of these six layers in FIG. 2 may be simply referred to as the upper and lower surfaces, respectively. The entire portion of the sensor element 101 made of the solid electrolyte is collectively referred to as the base portion.

[0047] The sensor element 101 is manufactured, for example, by laminating ceramic green sheets corresponding to each layer, performing predetermined processing and printing circuit patterns (e.g., electrodes, electrode leads, lead insulating layers, etc.), and then firing the sheets to integrate them.

[0048] On the first tip portion 101a side 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, a first diffusion-controlling section 11 also serving as a gas inlet 10, 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 are formed adjacent to each other and communicate with each other in this order.

[0049] The buffer space 12, the first internal space 20, the second internal space 40, and the third internal space 61 are spaces (regions) inside the sensor element 101, defined by an upper portion cut out from the spacer layer 5 by the underside of the second solid electrolyte layer 6, a lower portion cut out from the upper surface of the first solid electrolyte layer 4, and a side portion cut out from the side surface of the spacer layer 5. Similarly, the gas inlet 10 may be defined by a portion cut out from the spacer layer 5 at the first tip portion 101a, separately from the first diffusion rate-controlling portion 11. In this case, the first diffusion rate-controlling portion 11 is formed adjacent to and inside the gas inlet 10.

[0050] The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, the third diffusion rate-controlling section 30, and the fourth diffusion rate-controlling section 60 are each provided as two horizontally elongated slits (with the openings extending longitudinally in the direction perpendicular to the drawing). The region extending from the gas inlet 10 to the third internal space 61, which is the innermost internal space, is also referred to as the gas flow section.

[0051] Furthermore, a reference gas introduction space 43 is provided on the second tip portion 101b side of the sensor element 101 between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, at a position defined on the side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 opens to the reference gas space SP in the outer cylinder 104, and air serving as a reference gas is introduced from the reference gas space SP.

[0052] 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.

[0053] Reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of third substrate layer 3 and first solid electrolyte layer 4, and as described above, is surrounded by air introduction layer 48 that connects to reference gas introduction space 43. Furthermore, as will be described later, reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in first internal space 20 and second internal space 40.

[0054] In the gas flow section, the gas inlet 10 (first diffusion rate-controlling section 11) is a section that opens to the external space, and the measurement gas is taken into the sensor element 101 from the external space through the gas inlet 10.

[0055] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the taken-in measurement gas.

[0056] 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 .

[0057] 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 .

[0058] 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 passes through the first diffusion rate-controlling section 11, buffer space 12, and second diffusion rate-controlling section 13, where the concentration fluctuations of the measurement gas are canceled out, before being introduced into the first internal space 20. As a result, the concentration fluctuations of the measurement gas introduced into the first internal space 20 become almost negligible.

[0059] 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.

[0060] The main pump cell 21 is an electrochemical pump cell comprising an inner pump electrode (also referred to as a 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 (outside the space) 23 provided in a region corresponding to the ceiling electrode portion 22a on the upper surface of the second solid electrolyte layer 6 (one of the main surfaces of the sensor element 101) so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0061] The inner pump electrode 22 is formed on 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. 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. The ceiling electrode portion 22a and the bottom electrode portion 22b are connected by conductive portions (not shown) that are provided on the side wall surfaces (inner surfaces) of the spacer layer 5 that form the both side wall portions of the first internal space 20.

[0062] The ceiling electrode portion 22a and the bottom electrode portion 22b are provided in a rectangular shape in a plan view, but may be provided with only the ceiling electrode portion 22a or only the bottom electrode portion 22b.

[0063] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes. In particular, the inner pump electrode 22, which comes into contact with the measurement gas, is formed using a material with a weakened ability to reduce the NOx components in the measurement gas. For example, it is formed as a cermet electrode of ZrO2 and an Au-Pt alloy containing approximately 0.6 wt% to 1.4 wt% Au, with a porosity of 5% to 40%, and a thickness of 5 μm to 20 μm. The weight ratio of the Au-Pt alloy to ZrO2 may be approximately Pt:ZrO2=7.0:3.0 to 5.0:5.0.

[0064] On the other hand, the outer pump electrode 23 is formed as a cermet electrode of, for example, Pt or an alloy thereof and ZrO2, and has a rectangular shape in plan view.

[0065] In the main pump cell 21, under the control of the element operation control unit 1101, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by the variable power supply 24, and a main pump current Ip0 is caused to flow between the inner pump electrode 22 and the outer pump electrode 23 in a positive or negative direction, thereby pumping oxygen from the first inner space 20 to the external space or pumping oxygen from the external space into the first inner space 20. The pump voltage Vp0 applied between the inner pump electrode 22 and the outer pump electrode 23 in the main pump cell 21 is also referred to as the main pump voltage Vp0.

[0066] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 constitute a main sensor cell 80, which is an electrochemical sensor cell.

[0067] By measuring the electromotive force V0, which is the potential difference between the inner pump electrode 22 and the reference electrode 42 in the main sensor cell 80, the oxygen concentration (oxygen partial pressure) in the first inner space 20 can be determined.

[0068] Furthermore, the element operation control section 1101 controls the main pump current Ip0 by feedback controlling the main pump voltage Vp0 so that the electromotive force V0 is constant, thereby maintaining the oxygen concentration in the first internal space 20 at a predetermined constant value.

[0069] The third diffusion control section 30 is a section that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas to the second internal space 40.

[0070] The second internal space 40 is provided as a space for further adjusting the oxygen partial pressure in the measurement gas introduced through the third diffusion-controlling part 30. The oxygen partial pressure is adjusted by operating the auxiliary pump cell 50. In the second internal space 40, the oxygen concentration of the measurement gas is adjusted with even higher precision.

[0071] 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 oxygen partial pressure of the measurement gas introduced through the third diffusion-controlling section 30 is further adjusted by the auxiliary pump cell 50.

[0072] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell that includes 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 suitable electrode between the sensor element 101 and an outer electrode will suffice), and the second solid electrolyte layer 6.

[0073] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a similar manner to the inner 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. The ceiling electrode portion 51a and the bottom electrode portion 51b are rectangular in plan view, and are connected by conductive portions (not shown) that are provided on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side wall portions of the second internal space 40.

[0074] Like the inner 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.

[0075] In the auxiliary pump cell 50, under the control of the element operation control unit 1101, by applying a desired voltage (auxiliary pump voltage) Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, it is possible to pump oxygen in the atmosphere within the second internal space 40 out to the external space or pump oxygen from the external space into the second internal space 40.

[0076] In order to control the oxygen partial pressure in the atmosphere in the second internal space 40, an auxiliary sensor cell 81, which is an electrochemical sensor cell, 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. In the auxiliary sensor cell 81, an electromotive force V1, which is a potential difference generated between the auxiliary pump electrode 51 and the reference electrode 42 in accordance with the oxygen partial pressure in the second internal space 40, is detected.

[0077] 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 sensor cell 81. As a result, the oxygen partial pressure in the atmosphere within the second internal space 40 is feedback-controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0078] At the same time, the auxiliary pump current Ip1 is used to control the electromotive force of the main sensor cell 80. Specifically, the auxiliary pump current Ip1 is input as a control signal to the main sensor cell 80, and the electromotive force V0 is controlled to always maintain a constant gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-controlling part 30 into the second internal space 40. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

[0079] 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 controlled 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.

[0080] The third internal space 61 is provided as a space (measurement internal space) for performing processing related to measurement of the nitrogen oxide (NOx) concentration in the measurement gas introduced through the fourth diffusion-controlling part 60. The NOx concentration is measured by operating the measurement pump cell 41 in the third internal space 61. Since the measurement gas, the oxygen concentration of which has been adjusted with high precision in the second internal space 40, is introduced into the third internal space 61, the gas sensor 100 can measure the NOx concentration with high precision.

[0081] The measurement pump cell 41 is used to measure the NOx concentration of the measurement gas introduced into the third internal space 61. The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61 and spaced apart from the third diffusion-controlling part 30, an outer pump electrode 23, the second solid electrolyte layer 6, a spacer layer 5, and the first solid electrolyte layer 4.

[0082] The measurement electrode 44 is a porous cermet electrode of a precious metal and a solid electrolyte. For example, it is formed as a cermet electrode of Pt or an alloy of Pt and another precious metal such as Rh, and ZrO2, which is a constituent material of the sensor element 101. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the third internal space 61.

[0083] In the measuring pumping cell 41, under the control of the element operation control section 1101, oxygen generated by the decomposition of NOx in the atmosphere in the third internal space 61 is pumped out, and the amount of oxygen generated can be detected as a pump current Ip2.

[0084] Furthermore, in order to detect the oxygen partial pressure around the measurement electrode 44, the second 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 constitute a measurement sensor cell 82, which is an electrochemical sensor cell. The variable power supply 46 is feedback-controlled based on the electromotive force V2, which is the potential difference generated between the measurement electrode 44 and the reference electrode 42 in response to the oxygen partial pressure in the third internal space 61, detected by the measurement sensor cell 82.

[0085] The NOx in the measurement gas introduced into the third internal space 61 is reduced by the measurement electrode 44 (2NO → N2 + O2) to generate oxygen. The generated oxygen is then pumped by the measurement pump cell 41, and the voltage (measurement pump voltage) Vp2 of the variable power supply 46 is controlled so that the electromotive force V2 detected by the measurement sensor cell 82 remains constant. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the NOx concentration in the measurement gas, the NOx concentration in the measurement gas is calculated using the pump current Ip2 in the measurement pump cell 41. Hereinafter, this pump current Ip2 will also be referred to as the NOx current Ip2 or the measurement pump current Ip2.

[0086] Furthermore, by combining the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 to form an oxygen partial pressure detection means as an electrochemical sensor cell, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by reduction of the NOx components in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere, thereby making it possible to determine the concentration of the NOx components in the measured gas.

[0087] 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.

[0088] 2, for the sake of simplicity, wiring for electrically connecting the electrodes constituting the various pump cells and sensor cells to various parts outside the sensor element 101 is shown only schematically. However, in the actual sensor element 101, electrode leads that connect the electrodes of the respective pump cells and sensor cells to terminal electrodes are provided inside or on the side surfaces.

[0089] Fig. 4 is a plan view showing, as an example, the arrangement of electrode lead portions including electrode leads extending from the main pump electrode 22 (more specifically, the bottom electrode portion 22b), the auxiliary pump electrode 51 (more specifically, the bottom electrode portion 51b), and the measurement electrode 44. Fig. 5 is a plan view showing the arrangement of electrode lead portions including the electrode leads extending from the outer pump electrode 23.

[0090] The bottom electrode portion 22b, the bottom electrode portion 51b, and the measurement electrode 44 are all provided on the first solid electrolyte layer 4. A main pump electrode lead 22L surrounded by a main pump electrode lead insulating layer 22I extends from the bottom electrode portion 22b toward the second tip portion 101b. An auxiliary pump electrode lead 51L surrounded by an auxiliary pump electrode lead insulating layer 51I extends from the bottom electrode portion 51b toward the second tip portion 101b. On the other hand, the outer pump electrode 23 is provided on the second solid electrolyte layer 6. An outer pump electrode lead 23L surrounded by an outer pump electrode lead insulating layer 23I extends from the outer pump electrode 23 toward the second tip portion 101b. Each insulating layer insulates each electrode lead from the solid electrolyte constituting the sensor element 101.

[0091] Hereinafter, the region including the main pump electrode lead 22L and the main pump electrode lead insulating layer 22I and surrounded by the solid electrolyte (mainly the first solid electrolyte layer 4 and the spacer layer 5) will be collectively referred to as the main pump electrode lead portion 22P. Similarly, the region including the auxiliary pump electrode lead 51L and the auxiliary pump electrode lead insulating layer 51I and surrounded by the solid electrolyte (mainly the first solid electrolyte layer 4 and the spacer layer 5) will be collectively referred to as the auxiliary pump electrode lead portion 51P. Furthermore, the region including the measurement electrode lead 44L and the measurement electrode lead insulating layer 44I and surrounded by the solid electrolyte (mainly the first solid electrolyte layer 4 and the spacer layer 5) will be collectively referred to as the measurement electrode lead portion 44P.

[0092] The outer pump electrode lead 23L is connected to the terminal electrode 23T provided on the second tip portion 101b. Although the outer pump electrode lead insulating layer 23I is provided linearly (in a line) along the outer pump electrode lead 23L in Fig. 5, the outer pump electrode lead insulating layer 23I may be provided in a planar manner (in a layer) between the outer pump electrode 23 and the terminal electrode 23T.

[0093] Meanwhile, the main pump electrode lead 22L, the auxiliary pump electrode lead 51L, and the measurement electrode lead 44L are each interlayer wirings, with most of the lead buried between the first solid electrolyte layer 4 and the spacer layer 5, but are bent toward the side of the element near the second tip 101b. Furthermore, although not shown, the main pump electrode lead 22L, the auxiliary pump electrode lead 51L, and the measurement electrode lead 44L are surrounded by the main pump electrode lead insulating layer 22I, the auxiliary pump electrode lead insulating layer 51I, and the measurement electrode lead insulating layer 44I, respectively, and connected to each other via through holes in either the upper or lower direction in the thickness direction near the element-side end, and then connected to one of a plurality of terminal electrodes (not shown) similar to the terminal electrode 23T provided at the second tip 101b via other interlayer wirings. Note that in this embodiment, "surrounding each electrode lead with a lead insulating layer" includes a mode in which the electrode lead is covered with the lead insulating layer and a mode in which the electrode lead is sandwiched between the lead insulating layers.

[0094] The main pump electrode lead 22L, the auxiliary pump electrode lead 51L, the measurement electrode lead 44L, and the outer pump electrode lead 23L are preferably made of platinum, and the main pump electrode lead insulating layer 22I, the auxiliary pump electrode lead insulating layer 51I, the measurement electrode lead insulating layer 44I, and the outer pump electrode lead insulating layer 23I are preferably made of alumina.

[0095] The sensor element 101 further includes a heater portion 70 that serves to adjust the temperature by heating and keeping the sensor element 101 warm in order to increase the oxygen ion conductivity of the solid electrolyte that constitutes the base portion.

[0096] The heater section 70 mainly includes a heater electrode 71, a heater element 72, a heater lead 72a, a through hole 73, a heater insulating layer 74, a pressure release hole 75, and a heater resistance detection lead (not shown in Fig. 2). The heater section 70, except for the heater electrode 71, is embedded in the base of the sensor element 101.

[0097] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).

[0098] The heater element 72 is a resistance heating element provided between the second substrate layer 2 and the third substrate layer 3. The heater element 72 generates heat when power is supplied from a heater power source (not shown in FIG. 2) provided outside the sensor element 101 through a current path including the heater electrode 71, the through hole 73, and the heater lead 72a. The heater element 72 is made of Pt or contains Pt as its main component. The heater element 72 is embedded in a predetermined area on the gas flow section side of the sensor element 101 so as to face the gas flow section in the element thickness direction. The heater element 72 is provided to have a thickness of approximately 10 μm to 30 μm.

[0099] In the sensor element 101, under the control of the element operation control unit 1101, current is passed through the heater electrode 71 to the heater element 72, causing the heater element 72 to generate heat, thereby heating and maintaining each part of the sensor element 101 at a predetermined temperature. Specifically, the sensor element 101 is heated so that the temperature of the solid electrolyte and electrodes near the gas flow portion reaches approximately 700°C to 900°C. This heating increases the oxygen ion conductivity of the solid electrolyte constituting the base portion of the sensor element 101. The heating temperature by the heater element 72 when the gas sensor 100 is used (when the sensor element 101 is driven) is referred to as the sensor element driving temperature.

[0100] The degree of heat generation by the heater element 72 (heater temperature) is determined by the magnitude of the resistance value of the heater element 72 (heater resistance).

[0101] Although not shown in FIG. 2, one main surface of the sensor element 101 may be provided with an electrode protection layer that covers the outer pump electrode 23 in order to protect the outer pump electrode 23.

[0102] When the NOx concentration is measured in the gas sensor 100 having the above configuration, the element operation control unit 1101 operates the main pump cell 21 and then the auxiliary pump cell 50, thereby performing feedback control to maintain constant oxygen concentrations in the first internal space 20 and then the second internal space 40, and the measurement gas with the constant oxygen concentration is introduced into the third internal space 61 and reaches the measurement electrode 44. For example, when the measurement gas is a lean atmosphere, the measurement gas is introduced into the third internal space 61 with its oxygen partial pressure sufficiently reduced to a level that does not substantially affect the NOx measurement (for example, 0.0001 ppm to 1 ppm).

[0103] At the measuring electrode 44, NOx in the measurement gas that has reached the electrode is reduced to generate oxygen. The oxygen is pumped out by the measuring pump cell 41, and the NOx current Ip2 that flows during this pumping has a certain functional relationship (hereinafter referred to as sensitivity characteristic) with the concentration of NOx in the measurement gas.

[0104] The sensitivity characteristic is determined in advance of actual use of the gas sensor 100 using a plurality of model gases with known NOx concentrations, and the data is stored in the NOx concentration determining unit 1102 (more specifically, in memory). During actual use of the gas sensor 100, a signal representing the value of the NOx current Ip2 that flows in response to the NOx concentration in the measurement gas is continuously supplied to the NOx concentration determining unit 1102. The NOx concentration determining unit 1102 successively calculates the NOx concentration based on the value and the determined sensitivity characteristic, and outputs the calculated NOx concentration as a NOx sensor detection value. This allows the gas sensor 100 to grasp the NOx concentration in the measurement gas in almost real time.

[0105] <Generation of pollutant gas in the reference gas space> In the case of a gas sensor like the gas sensor 100 according to the present embodiment, which has a configuration in which the reference gas space SP is surrounded by the outer cylinder 104 and the rubber stopper 106 and is used in a high-temperature environment, oil adhering to the inner surface 104a of the outer cylinder 104 may volatilize, or gas may be generated from the rubber stopper 106, resulting in the generation of contaminated gas in the reference gas space SP. As a result, the NOx current Ip2 may change. The generation of such contaminated gas is particularly likely to occur under severe environmental conditions such as high-temperature gas conditions or where heat accumulates around the main body.

[0106] FIG. 6 is an example of a graph showing the change in the NOx current Ip2 from the start of operation together with the temperature change of the rubber stopper 106 when such a gas sensor is operated in a model gas atmosphere with a constant NOx concentration. In FIG. 6, the NOx current Ip2, which should be constant, temporarily decreases after the temperature of the rubber stopper 106 rises. Then, after the temperature of the rubber stopper 106 stabilizes, it increases to approximately the same peak value as at the beginning and becomes substantially constant.

[0107] The temporary decrease in the NOx current in FIG. 6 is considered to be due to the above-described contaminated gas generated during the process of heating the gas sensor entering the third internal space 61 provided with the measurement electrode 44 through the measurement electrode lead portion described later. And the increase in the value of the NOx current Ip2 after a certain period of time is considered to be due to the fact that all the contaminated gas has been released as a result of the outer cylinder 104 and the rubber stopper 106 being sufficiently heated.

[0108] Such fluctuations in the NOx current Ip2 naturally become a factor that reduces the measurement accuracy of the NOx concentration in the gas sensor, even temporarily. Also, the generation of contaminated gas does not necessarily occur only after the start of use as shown in FIG. 6.

[0109] <Correlation between Ip2 offset and output fluctuation amount> In the gas sensor 100 according to this embodiment, the NOx current Ip2, which fluctuates due to the generation of pollutant gases as described above, is corrected in the correction processing unit 1103, thereby ensuring the measurement accuracy of the NOx concentration. For such correction, an Ip2 offset is used.

[0110] Here, the Ip2 offset refers to the magnitude (offset current value) of the pump current Ip2 that flows through the measurement pump cell 41 when the measurement gas does not contain NOx. As described above, the pump current Ip2 flows when the measurement pump cell 41 pumps out oxygen produced by decomposition of NOx at the measurement electrode 44 in the measurement gas that has reached the third internal space 61 and is almost oxygen-free. Therefore, in reality, the pump current Ip2 should not flow when the measurement gas does not contain NOx. However, in reality, the pump current Ip2 flows even when the measurement gas does not contain NOx because trace amounts of oxygen remaining in the measurement gas and oxygen entering the third internal space 61 through the measurement electrode lead 44P are pumped out. The magnitude of the pump current Ip2 in such a case is specifically referred to as the Ip2 offset.

[0111] The fact that a certain gas sensor 100 has a high rate of intrusion of contaminant gas into the third internal space 61 through the measurement electrode lead 44P also means that the gas sensor 100 is configured to easily allow gas to infiltrate (flow) from the reference gas space SP into the third internal space 61 through the measurement electrode lead 44P. Therefore, for such a gas sensor 100, the fluctuation of the NOx current Ip2 increases when contaminant gas is generated, and the Ip2 offset also increases when the measured gas does not contain NOx. In other words, there is a positive correlation between the Ip2 offset and the output fluctuation of the NOx current Ip2.

[0112] FIG. 7 is a diagram showing an example of the correlation between the Ip2 offset and the amount of output fluctuation of the NOx current Ip2. In FIG. 7, the Ip2 offset and the amount of output fluctuation of the NOx current Ip2 are evaluated for six different gas sensors 100, and the horizontal axis represents the Ip2 offset and the vertical axis represents the amount of output fluctuation of the NOx current Ip2 ("Ip2 fluctuation" in FIG. 7). transformation It can be seen from Figure 7 that there is a strong positive correlation between the Ip2 offset and the output fluctuation amount of the NOx current Ip2. The coefficient of determination R, which is the square of the correlation coefficient R, 2 was 0.9965.

[0113] The Ip2 offset value was evaluated using a model gas that did not contain either oxygen or NOx (O2 = 0%, NO = 0%, H2O = 3%, balance N2) as the measurement gas.

[0114] On the other hand, the output fluctuation amount of the NOx current Ip2 is roughly as shown in Fig. 6 This was determined by obtaining the profile of the NOx current Ip2 as shown in

[0115] 8 is a diagram for explaining how to evaluate the output fluctuation amount. Specifically, when the temperature is 1050°C, the air ratio λ is 1.05, and the NOx concentration is 100 ppm Each gas sensor was started in a model gas atmosphere of 300°C, and the NOx current Ip2 was continuously measured while controlling the heating by the heater unit 70 so that the temperature of the rubber plug 106 was ultimately kept constant at 300°C. As a result, a profile Ip2(t) corresponding to the function of the NOx current Ip2 with respect to time was obtained, as shown in Figure 8.

[0116] Although the degree of the results obtained differs for each gas sensor, 6 A profile Ip2(t) similar to the example shown in Fig. 1 was obtained. That is, the NOx current Ip2 becomes substantially constant soon after the gas sensor starts operating, then decreases, and becomes substantially constant again as pollutant gas is generated. After a while, the NOx current Ip2 starts to increase, and finally becomes substantially constant at a value substantially the same as when the operation started.

[0117] However, in FIG. 8, for ease of understanding, the profile Ip2(t) is illustrated as a solid-line broken line. Actually, the profile Ip2(t) had some fluctuations as schematically shown by a broken line.

[0118] When evaluating the output fluctuation amount for such a profile Ip2(t), first, the average value av1 of the value of the NOx current Ip2 at a predetermined time Δt1 (for example, 10 minutes) immediately before the transition from the substantially constant state after the decrease to the increase was specified. The reason for taking the average value is that the value regarded as substantially constant actually fluctuates.

[0119] Next, after the transition to the increase, the difference value Δ1(t) of the value of the NOx current Ip2 with respect to the average value av1 and the difference value Δ2(t) between the maximum value and the minimum value of the NOx current Ip2 in the substantially constant state after the transition to the increase were obtained. Since both these difference values Δ1(t) and difference value Δ2(t) are dynamic values that vary according to the fluctuation of the value of the NOx current Ip2 after the increase, they are expressed as functions of time.

[0120] And between the difference value Δ1(t) and the difference value Δ2(t), Difference value Δ1(t)>2 × Difference value Δ2(t) ····(1) When the state where the formula (1) holds is reached at the time point when a predetermined observation time Δtz (for example, 60 minutes) has elapsed, it is determined that the fluctuation associated with the polluted gas has been eliminated and the value of the NOx current Ip2 has become substantially constant at its original value. Then, the average value av2 of the value of the NOx current Ip2 at a predetermined time Δt2 (for example, 10 minutes) immediately before the time point of such determination is specified, and the difference value between the two is Output fluctuation amount = Average value av2 - Average value av1 ···(2) Calculated as

[0121] <Correction of NOx Concentration> 7 exists between the output fluctuation amount of the NOx current Ip2 and the Ip2 offset, and the correction processing unit 1103 corrects the NOx concentration fluctuation due to the generation of pollutant gas in the reference gas space SP, for example, when the gas sensor 100 starts operating. That is, while the Ip2 offset is a value specific to each gas sensor 100, the correlation between the output fluctuation amount of the NOx current Ip2 and the Ip2 offset is a general relationship that exists among many gas sensors 100. Therefore, when each gas sensor 100 is in use, the NOx concentration can be corrected by applying the offset value of that gas sensor 100 to the correlation that has been specified in advance.

[0122] In the case of industrially mass-produced gas sensors 100, some of the many manufactured gas sensors 100 are sampled to determine the output fluctuation amount of the NOx current Ip2 and the Ip2 offset, and a correlation such as that shown in FIG. 7 is identified from the obtained values. The identified correlation is stored in the correction processing unit 1103 (more specifically, in memory) as correction mapping information. Meanwhile, for each individual gas sensor 100, the Ip2 offset is also identified when the above-mentioned sensitivity characteristic is identified immediately after manufacture, and is stored in the NOx concentration identifying unit 1102 as part of the sensitivity characteristic. Then, at the timing to execute the correction processing, the correction processing unit 1103 applies the Ip2 offset obtained from the NOx concentration identifying unit 1102 to the pre-stored correlation to identify the output fluctuation amount of the NOx current Ip2, and corrects the NOx concentration based on the identified output fluctuation amount.

[0123] FIG. 9 is a diagram showing a specific procedure of the correction processing performed by the correction processing unit 1103.

[0124] First, when the gas sensor 100 is started, the seal temperature estimator 1103A starts estimating the temperature (seal temperature) of the rubber plug 106 (step S1). The gas sensor 100 is usually started at the same time as the start of the automobile.

[0125] The seal temperature varies depending on external factors such as the temperature of exhaust gas from the engine (the gas to be measured), the temperature around the main body of the gas sensor 100, and the flow velocity around the main body, which varies depending on the traveling speed of the vehicle. In consideration of this, in this embodiment, the relationship between the actual seal temperature and characteristic values (seal temperature estimation factors) that affect the seal temperature when the vehicle is traveling (i.e., when the gas sensor 100 is operating), such as exhaust gas temperature, traveling speed, and torque, is experimentally determined in advance, and the determined relationship is stored in the seal temperature estimation unit 1103A of the correction processing unit 1103 as mapping information (seal temperature estimation information) for estimating the seal temperature. Then, upon startup of the gas sensor 100, the seal temperature estimation unit 1103A starts acquiring actual measurement values of the seal temperature estimation factors and continuously compares the actual measurement values with the seal temperature estimation information, thereby estimating the seal temperature in almost real time. The outer cylinder 104 is made of metal and has higher thermal conductivity than the rubber stopper 106, so it is estimated that the temperature of the outer cylinder 104 is higher than that of the rubber stopper 106 when the gas sensor 100 is in use.

[0126] The sealing member temperature estimation unit 1103A determines whether the estimated sealing member temperature exceeds a predetermined threshold temperature (correction execution threshold temperature) (step S2). Here, the correction execution threshold temperature is a predetermined minimum temperature at which it is determined that correction of the NOx concentration is necessary as the temperature of the rubber plug 106 and further the outer cylinder 104 increases. For example, if the NOx current Ip2 changes over time as shown in Fig. 6, the temperature at which the NOx current Ip2, which once increased, decreases by a predetermined value from its peak value is set as the correction execution threshold temperature.

[0127] As long as the seal member temperature estimation unit 1103A determines that the seal member temperature does not exceed the correction threshold temperature (NO in step S2), it continues to estimate the seal member temperature. This is because it is considered that pollutant gases are not generated in a temperature range where the seal member temperature is lower than the correction threshold temperature, and therefore it is not necessary to correct the NOx concentration. Note that the estimation of the seal member temperature by the seal member temperature estimation unit 1103A continues until the correction process is stopped.

[0128] On the other hand, if the sealing member temperature estimation unit 1103A determines that the sealing member temperature has exceeded the correction execution threshold temperature, the correction time measurement unit 1103B acquires the accumulated correction time stored at that time (step S3), and determines whether the accumulated correction time has reached the maximum correction time that is set in advance when the gas sensor 100 is manufactured and recorded in the correction time measurement unit 1103B (more specifically, in the memory) (step S4).

[0129] Here, the cumulative correction time is the cumulative value of the execution time of the correction process to deal with the generation of pollutant gases that has been executed up until that point. Each time such a correction process is executed, the correction time measurement unit 1103B counts the execution time and stores the cumulative correction time obtained by integrating the values.

[0130] The decrease (fluctuation) in the NOx current Ip2 due to the generation of pollutant gases on the rubber stopper 106 or the inner surface 104a of the outer cylinder 104 is merely a temporary phenomenon that occurs when the gas sensor 100 is started up shortly after the start of use, and it no longer occurs after the pollutant substances have volatilized and evaporated. For the gas sensor 100 after such a pollution source has disappeared, even if the temperature is raised each time the gas sensor 100 is used (started up), pollutant gases are not generated, and therefore no correction processing is required. Therefore, in this embodiment, the maximum time (maximum required correction time) for which such correction processing is deemed necessary is experimentally determined in advance and stored in the correction time measurement unit 1103B. In other words, this means that the correction processing unit 1103 needs to perform correction each time the gas sensor 100 is started up until the maximum required correction time has elapsed.

[0131] If the correction time measurement unit 1103B determines that the accumulated correction time has already reached the maximum correction time (YES in step S4), no further processing is performed by the correction processing unit 1103. This means that as a result of the correction processing performed up to that point, correction of the NOx concentration for the purpose of dealing with the generation of polluting gases is not required.

[0132] If the correction time measurement unit 1103B determines that the cumulative correction time has not reached the maximum correction required time (NO in step S4), the correction execution unit 1103C starts correcting the NOx concentration value identified by the NOx concentration specifying unit 1102 (step S5). That is, the correction execution unit 1103C applies the value of the Ip2 offset stored in the NOx concentration specifying unit 1102 to mapping information indicating the correlation between the output fluctuation amount of the NOx current Ip2 and the Ip2 offset to specify a correction amount for the NOx current Ip2. Then, the correction execution unit 1103C applies the correction amount to the sensitivity characteristics stored in the NOx concentration specifying unit 1102 and corrects the NOx concentration identified by the NOx concentration specifying unit 1102 with the correction amount obtained thereby, thereby correcting the NOx concentration value identified moment by the NOx concentration specifying unit 1102 in almost real time. As a result, the NOx concentration value output from the gas sensor 100 is preferably free from the influence of fluctuations in the NOx current Ip2 caused by the generation of polluting gases.

[0133] In this manner, when the correction execution unit 1103C starts correcting the NOx concentration value, the correction time measurement unit 1103B starts counting the correction execution time (step S6). Also, the seal member temperature estimation unit 1103A continuously determines whether the estimated seal member temperature is still higher than the correction execution threshold temperature (step S7).

[0134] Then, if the sealing member temperature estimation unit 1103A determines that the estimated sealing member temperature exceeds the correction execution threshold temperature (YES in step S7), the correction time measurement unit 1103B determines whether the accumulated correction time that is increasing due to the correction being performed has exceeded the maximum correction required time (step S8).

[0135] After the correction execution starts, the seal member temperature reaches the correction execution threshold. temperature If the cumulative correction time is equal to or less than the maximum correction required time (NO in step S7), or if the cumulative correction time has exceeded the maximum correction required time (YES in step S8), it is no longer necessary to correct the NOx concentration, and therefore the correction execution unit 1103C stops executing the correction (step S9). At the same time, the correction time measurement unit 1103B updates the cumulative correction time by adding the correction time counted up to that point (step S10). This ends the correction processing in the correction processing unit 1103.

[0136] If the cumulative correction time has exceeded the maximum correction time (YES in step S8), and the execution of correction is stopped as a result, the updated cumulative correction time will match the maximum correction time. In this case, the next time the gas sensor 100 is started, correction of the NOx concentration will no longer be executed.

[0137] As described above, according to this embodiment, when the gas sensor is started up, fluctuations in the NOx concentration caused by the generation of pollutant gas in the reference gas space, which is the space surrounded by the outer tube and the rubber stopper, are corrected based on the correlation between a pre-specified Ip2 offset and the output fluctuation amount of the NOx current Ip2, thereby suppressing temporary deterioration in the measurement accuracy of the NOx concentration.

[0138] <Modification> In the above-described embodiment, the NOx concentration is corrected based on the correlation between the Ip2 offset and the output fluctuation amount of the NOx current Ip2. However, instead of the Ip2 offset, the correlation between the output fluctuation amount of the NOx current Ip2 and a normalized value obtained by dividing the Ip2 offset by the NOx current Ip2 when a measurement gas having a known NOx concentration (for example, 500 ppm) is passed through may be specified, and the correlation may be stored in the correction processing unit 1103 as correction mapping information, and the NOx concentration may be corrected based on this correlation.

[0139] The Ip2 offset is a value corresponding to the amount of oxygen remaining in the measurement gas that flows in through the gas inlet 10 and reaches the third internal space 61 while oxygen is being pumped out, and the amount of oxygen that has entered the third internal space 61 from the reference gas space SP through the measurement electrode lead 44P. The former depends on the diffusion resistance of the gas flow section from the gas inlet 10 to the third internal space 61. Therefore, by using the above-mentioned ratio instead of the Ip2 offset itself, the correlation with the output fluctuation of the NOx current Ip2 obtained becomes higher in correlation coefficient. Correction of the NOx concentration using such a correlation becomes more accurate. [Explanation of symbols]

[0140] 1~3 1st~3rd substrate layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion-controlled section 13 Second diffusion-controlled section 20 1st internal void 21 Main pump cell 22 Inner pump electrode 22P Main pump electrode lead 23 Outer pump electrode 23T terminal electrode 30 Third diffusion-controlled section 40 Second internal void 41 Measuring pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measuring electrode 44P Measurement electrode lead 48 Atmospheric introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51P Auxiliary pump electrode lead 60 4th diffusion-controlled section 61 3rd internal void 70 Heater section 100 Gas Sensor 101 Sensor element 102 Protective cover 103 Fixing bolt 104 Outer cylinder 105 Connector 106 Rubber stopper 107 Lead Wire 120 Ring Parts 151 Contact parts H1~H4 through hole SP Reference gas space

Claims

1. A gas sensor capable of detecting a predetermined gas component in a measurement gas, a sensor element having a long plate-shaped base portion made of an oxygen ion conductive solid electrolyte and having a detection portion on one end side; a casing in which the sensor element is housed and fixed; a controller for controlling the operation of the gas sensor; Equipped with The casing is an outer cylinder having a reference gas space in which a reference gas is present, the other end of the sensor element protruding into the reference gas space; a seal member fitted to an end of the outer cylinder and sealing the reference gas space; Equipped with The sensor element is at least one internal space for adjusting the oxygen partial pressure, which is in communication with the inlet for the gas under measurement provided at the one end side under a predetermined diffusion resistance; a measuring internal cavity further communicating with the at least one oxygen partial pressure adjusting internal cavity; an extra-void pump electrode disposed at a location other than the at least one oxygen partial pressure adjusting internal cavity and the measurement internal cavity; a measurement electrode provided facing the measurement internal space; a measurement pump cell to which a predetermined pump voltage is applied, so that a measurement pump current corresponding to the concentration of the predetermined gas component flows between the measurement electrode and the outside-space pump electrode; a measurement electrode lead portion including a measurement electrode lead extending from the measurement electrode and capable of electrically connecting the outside of the sensor element to the measurement electrode, and a measurement lead insulating layer covering the measurement electrode lead; Equipped with The controller: a concentration determination unit that determines the concentration of the predetermined gas component based on the measured pump current; a correction processing unit that corrects the concentration of the predetermined gas component identified by the concentration identifying unit; Equipped with the correction processing unit corrects the concentration of the predetermined gas component based on a correlation between an offset current value, which is the magnitude of the measured pump current when a predetermined measurement gas not containing the predetermined gas component flows, or a normalized value of the offset current value, and an output fluctuation occurring in the measured pump current at the start of the gas sensor. A gas sensor characterized by:

2. 2. The gas sensor according to claim 1, The correction processing unit a temperature estimation unit that estimates the temperature of the sealing member based on pre-specified temperature estimation information; Equipped with correcting the concentration of the predetermined gas component when it is determined that the temperature of the sealing member has exceeded a predetermined threshold temperature after the gas sensor is started; A gas sensor characterized by:

3. 3. The gas sensor according to claim 1, The correction processing unit a time measurement unit that measures the time during which the correction of the concentration of the predetermined gas component is performed; Equipped with and stopping correction of the concentration of the predetermined gas component after the cumulative value of the time measured by the time measuring unit reaches a preset maximum correction time. A gas sensor characterized by:

4. A concentration correction method for a gas sensor capable of detecting a predetermined gas component in a measurement gas, comprising: The gas sensor a sensor element having a long plate-shaped base portion made of an oxygen ion conductive solid electrolyte and having a detection portion on one end side; a casing in which the sensor element is housed and fixed; and The casing is an outer cylinder having a reference gas space in which a reference gas is present, the other end of the sensor element protruding into the reference gas space; a seal member fitted to an end of the outer cylinder and sealing the reference gas space; and The sensor element is at least one internal space for adjusting the oxygen partial pressure, which is in communication with the inlet for the gas under measurement provided at the one end side under a predetermined diffusion resistance; a measuring internal cavity further communicating with the at least one oxygen partial pressure adjusting internal cavity; an extra-void pump electrode disposed at a location other than the at least one oxygen partial pressure adjusting internal cavity and the measurement internal cavity; a measurement electrode provided facing the measurement internal space; a measurement pump cell to which a predetermined pump voltage is applied, so that a measurement pump current corresponding to the concentration of the predetermined gas component flows between the measurement electrode and the outside-space pump electrode; a measurement electrode lead portion including a measurement electrode lead extending from the measurement electrode and capable of electrically connecting the outside of the sensor element to the measurement electrode, and a measurement lead insulating layer covering the measurement electrode lead; In the case where the device is provided with a concentration determining step of determining the concentration of the predetermined gas component based on the measured pump current; a correction process step of correcting the concentration of the predetermined gas component identified in the concentration identifying step; Equipped with In the correction processing step, the concentration of the predetermined gas component is corrected based on a correlation between an offset current value, which is the magnitude of the measured pump current when a measurement gas not containing the predetermined gas component flows, or a normalized value of the offset current value, and an output fluctuation occurring in the measured pump current at the start of the gas sensor. A method for correcting concentration in a gas sensor.

5. 5. A method for correcting a concentration in a gas sensor according to claim 4, comprising: The correction processing step a temperature estimation step of estimating the temperature of the sealing member based on pre-specified temperature estimation information; Equipped with In the temperature estimation step, when it is determined that the temperature of the sealing member exceeds a predetermined threshold temperature after the gas sensor is started, the concentration of the predetermined gas component is corrected. A method for correcting concentration in a gas sensor.

6. 6. A method for correcting a concentration in a gas sensor according to claim 4, comprising: The correction processing step a time measurement step of measuring a time during which the correction of the concentration of the predetermined gas component is performed; Equipped with after the cumulative value of the time measured by the time measurement step reaches a preset maximum time required for correction, correction of the concentration of the predetermined gas component is not performed; A method for correcting concentration in a gas sensor.

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