Gas sensor and method of operating the gas sensor

Abstract

To provide a gas sensor capable of protecting a sensor element when being used under a rich atmosphere.SOLUTION: A controller of a gas sensor comprises an electrochemical pump cell including an inner electrode included in a sensor element and provided to face at least one internal space into which a measurement object gas is introduced, a pump electrode outside the space disposed at a place other than the internal space, and a solid electrolyte provided between both electrodes. The controller: determines whether a determination target value as an index of oxygen pumping operation in the electrochemical pump cell exceeds a threshold in a prescribed determination time; as long as the determination target value does not exceed the threshold, controls the gas sensor in a basic mode for keeping oxygen concentration in the at least one internal space constant by operating at least one electrochemical pump cell; and when the determination target value exceeds the threshold, controls the gas sensor in a protection execution mode for protecting the at least one electrochemical pump cell from excessive oxygen pumping operation.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a limiting current type gas sensor, and more particularly to control of the operation of the gas sensor when used in a rich atmosphere. [Background technology]

[0002] Limiting current gas sensors (e.g., NOx sensors, oxygen sensors) using a sensor element whose main component is an oxygen-ion conductive solid electrolyte such as yttria-stabilized zirconia are already known. In such gas sensors, a gas to be measured is introduced into a cavity (internal cavity) provided inside the sensor element. The potential difference between an inner electrode facing the internal cavity and a reference electrode provided inside the element in contact with a reference gas is controlled to be kept at a predetermined value corresponding to the desired oxygen concentration in the cavity.

[0003] This control is generally achieved by applying a pumping voltage between the electrodes of an electrochemical pump cell consisting of an inner electrode, an outer electrode (outside the cavity) provided outside the cavity, and a solid electrolyte region between the two electrodes to pump oxygen in and out between the inner cavity and the outside. The application of this pumping voltage causes an oxygen pumping current to flow between the inner and outer electrodes, the magnitude and direction of which depend on the oxygen concentration in the cavity.

[0004] As an example of such a gas sensor, a gas sensor is already known in which an outer electrode is provided on the outer surface of the sensor element and a ceramic layer is provided so that a slit portion that provides a predetermined diffusion resistance is formed around the outer electrode (see, for example, Patent Document 1).

[0005] In addition, a gas sensor having a sensor element in which an oxygen concentration detection cell and an oxygen pump cell are stacked in the element thickness direction via an insulating layer, and the detection gas is introduced into the interior through a diffusion-controlling part made of a porous body provided in part of the insulating layer, is also already known (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2021-162465 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-173146 Summary of the Invention [Problem to be solved by the invention]

[0007] The limiting current type gas sensor described above may be used in an environment where rich gas having an air-fuel ratio lower than the stoichiometric air-fuel ratio may be introduced into the element, for example, in the exhaust path from a gasoline engine.

[0008] In such a case, when the rich gas is introduced into the internal cavity, the electrochemical pump cell usually performs an operation of pumping oxygen from the outside of the element into the internal cavity (pumping operation) to maintain a constant oxygen concentration in the cavity. That is, a pumping voltage is applied to pump oxygen into the internal cavity (to move oxygen ions from the outside of the element to the internal cavity), and a corresponding oxygen pumping current flows between the inner electrode and the outer electrode.

[0009] During this oxygen pumping, the pumping voltage and oxygen pump current tend to increase as the amount of rich gas introduced into the internal cavity increases. However, if the measured gas becomes too rich, it becomes difficult to pump oxygen from the outside in response to the increase in pumping voltage. Instead, oxygen in the solid electrolyte may be extracted, which is known as blackening. Blackening is an irreversible phenomenon, and once it occurs, the gas sensor becomes unusable.

[0010] Such blackening tends to occur more easily when the diffusion resistance around the outer electrode is larger, as in the gas sensor disclosed in Patent Document 1, in which the outer electrode is covered with a ceramic layer.

[0011] The present invention has been made in view of the above-mentioned problems, and has as its object to provide a gas sensor capable of protecting a sensor element when used in a rich atmosphere. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems, a first aspect of the present invention is a gas sensor configured to be able to detect a predetermined gas component in a measurement gas, comprising: a sensor element made of an oxygen ion conductive solid electrolyte; and a controller for controlling the operation of the gas sensor, wherein the sensor element comprises at least one internal cavity in communication with an inlet for the measurement gas at a predetermined diffusion resistance and provided with an inner electrode; an outer-cavity pump electrode disposed at a location other than the at least one internal cavity; and a reference electrode provided so as to be able to come into contact with a reference gas, and wherein oxygen can be pumped in or out of the at least one internal cavity and the outside of the sensor element by applying a pump voltage between the inner electrode and the outer-cavity pump electrode by a predetermined pump power source; and and at least one electrochemical sensor cell configured to generate a potential difference between itself and the reference electrode according to the oxygen concentration in the at least one corresponding internal space, wherein the controller determines whether a determination target value, which is an index of an oxygen pumping operation in a determination target pump cell included in the at least one electrochemical pump cell, exceeds a predetermined threshold value at a predetermined determination time, and controls the gas sensor in a basic mode in which the oxygen concentration in the at least one internal space is kept constant by operating the at least one electrochemical pump cell unless the determination target value exceeds the predetermined threshold value, and controls the gas sensor in a protection execution mode in which the at least one electrochemical pump cell is protected from excessive oxygen pumping when the determination target value exceeds the predetermined threshold value. It is something , the predetermined threshold is a change threshold for determining whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the at least one electrochemical pump cell needs to be changed when controlling application of the pump voltage to the at least one electrochemical pump cell, and the controller starts the protection execution mode when the determination target value exceeds the change threshold, and in the protection execution mode, changes the target value to a value greater than a normal value and continues control of the gas sensor, and starts monitoring a pump cell operating value, which is the value of the pump voltage or the value of the current in at least the electrochemical pump cell including the determination target pump cell, and when the pump cell operating value falls below a return threshold, returns the target value to the normal value and returns control of the gas sensor to the basic mode. It is characterized by:

[0015] The present invention2 The embodiment of A gas sensor configured to be able to detect a predetermined gas component in a gas to be measured, comprising: a sensor element made of an oxygen ion conductive solid electrolyte; and a controller for controlling the operation of the gas sensor, wherein the sensor element comprises at least one internal cavity in which an inner electrode is provided and which communicates with an inlet for the gas to be measured under a predetermined diffusion resistance; an outer-cavity pump electrode disposed at a location other than the at least one internal cavity; and a reference electrode provided so as to be able to come into contact with a reference gas, and wherein the sensor element comprises at least one electrochemical pump cell configured to be able to pump oxygen in and out of the at least one internal cavity and the outside of the sensor element by applying a pump voltage between the inner electrode and the outer-cavity pump electrode by a predetermined pump power supply; and at least one electrochemical sensor cell configured to generate a potential difference corresponding to the oxygen concentration in the at least one internal space, wherein the controller determines whether a determination target value, which is an index of an oxygen pumping operation in a determination target pump cell included in the at least one electrochemical pump cell, exceeds a predetermined threshold value at a predetermined determination time, and controls the gas sensor in a basic mode in which the oxygen concentration in the at least one internal space is kept constant by operating the at least one electrochemical pump cell unless the determination target value exceeds the predetermined threshold value, and controls the gas sensor in a protection execution mode in which the at least one electrochemical pump cell is protected from excessive oxygen pumping operation when the determination target value exceeds the predetermined threshold value. The predetermined threshold is a change threshold for determining whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the at least one electrochemical pump cell needs to be changed when controlling application of the pump voltage to the at least one electrochemical pump cell, and the controller starts the protection execution mode when the determination target value exceeds the change threshold, and in the protection execution mode, changes the target value to a value greater than a normal value and continues control of the gas sensor, and starts monitoring a pump cell operating value, which is a value of the pump voltage or a value of a current in at least the electrochemical pump cell including the determination target pump cell, and resets the target value to the previous value when the pump cell operating value falls below a recovery threshold. the pump cell operating value is restored to the normal value, and control of the gas sensor is restored to the basic mode; monitoring of the pump cell operating value is continued unless the pump cell operating value falls below the restoration threshold and the judgment target value exceeds a stop threshold that is greater than the change threshold; if the pump cell operating value does not fall below the restoration threshold and the judgment target value further exceeds the stop threshold, operation of the at least one electrochemical pump cell is stopped, and monitoring of the potential difference in an electrochemical sensor cell corresponding to the judgment target pump cell among the at least one electrochemical sensor cell is started; and control of the gas sensor in the basic mode is resumed when the monitored potential difference falls below a restart threshold.

[0016] The present invention 3 The first aspect is or No. 2 attitude Like In this gas sensor, the determination target value is an actual measurement value of the potential difference in an electrochemical sensor cell corresponding to the determination target pump cell among the at least one electrochemical sensor cell.

[0017] The present invention 4 The first aspect is or No. 2 attitude LikeIn the gas sensor, the determination target value is a value of a pump current when oxygen is pumped into the determination target pump cell.

[0018] The present invention 5 The first to second aspects are 4 a gas sensor according to any one of the above aspects, characterized in that the at least one internal cavity is a plurality of internal cavities that are sequentially connected to one another, the inner electrode is a plurality of inner electrodes provided in the plurality of internal cavities, the at least one electrochemical pump cell is a plurality of electrochemical pump cells, the at least one electrochemical sensor cell is a plurality of electrochemical sensor cells, and the judgment target value is an indicator of the oxygen pumping operation of at least one of the plurality of electrochemical pump cells that is provided corresponding to the internal cavity among the plurality of internal cavities that is provided closest to the inlet.

[0019] The present invention 6 The embodiment is 5 a gas sensor according to the embodiment of the present invention, characterized in that one of the plurality of inner electrodes is a measurement electrode for detecting the predetermined gas component, the plurality of electrochemical pump cells comprise a measurement pump cell including the measurement electrode and at least one oxygen concentration control pump cell other than the measurement pump cell, including any of the plurality of inner electrodes other than the measurement electrode, the plurality of electrochemical sensor cells comprise a measurement sensor cell including the measurement electrode and at least one oxygen concentration detection sensor cell other than the measurement sensor cell, including any of the plurality of inner electrodes other than the measurement electrode, and the controller, at least in the basic mode, determines the concentration of the predetermined gas component based on a measurement pump current flowing in the measurement pump cell between the measurement electrode and the outside-space pump electrode in accordance with the concentration of the predetermined gas component.

[0020] The present invention 7 The embodiment is 6In the gas sensor according to the aspect of the present invention, the plurality of internal cavities are a first internal cavity, a second internal cavity, and a third internal cavity, which are sequentially connected via a diffusion resistance portion, the plurality of inner electrodes other than the measurement electrode are a main pump electrode provided in the first internal cavity and an auxiliary pump electrode provided in the second internal cavity, and the measurement electrode is provided in the third internal cavity, the at least one oxygen concentration control pump cell is a main pump cell that controls the oxygen concentration in the first internal cavity and an auxiliary pump cell that controls the oxygen concentration in the second internal cavity, and the at least one oxygen concentration detection sensor cell is a ... and an auxiliary pump cell that controls the oxygen concentration in the first internal cavity and an auxiliary pump cell that controls the oxygen concentration in the second internal cavity and an auxiliary pump cell that controls the oxygen concentration in the first internal cavity and an auxiliary pump cell that controls the oxygen concentration in the second internal cavity and an auxiliary pump cell that controls the oxygen concentration in the first internal cavity and an auxiliary pump cell that controls the oxygen concentration in the second internal and an auxiliary sensor cell configured to generate a potential difference between the auxiliary pump electrode and the reference electrode according to the oxygen concentration in the second internal space, and the controller, at least in the basic mode, operates the main pump cell and the auxiliary pump cell so that the oxygen concentrations in the first internal space and the second internal space are maintained at predetermined constant values, respectively, and determines the concentration of the predetermined gas component based on the magnitude of the measurement pump current flowing through the measurement pump cell according to the concentration of the predetermined gas component contained in the measurement gas whose oxygen concentration has been adjusted and introduced into the third internal space.

[0021] The present invention 8a gas sensor including a sensor element made of an oxygen ion conductive solid electrolyte and configured to be capable of detecting a predetermined gas component in a measurement gas, the gas sensor further comprising: at least one internal cavity in which an inner electrode is provided and which communicates with an inlet for the measurement gas under a predetermined diffusion resistance; an outer-cavity pump electrode disposed at a location other than the at least one internal cavity; and a reference electrode disposed so as to be in contact with a reference gas; at least one electrochemical pump cell configured to be able to pump oxygen in or out of the corresponding at least one internal cavity and the outside of the sensor element by applying a pump voltage between the inner electrode and the outer-cavity pump electrode from a predetermined pump power source; and at least one electrochemical sensor cell configured to generate a potential difference between the inner electrode and the reference electrode according to the oxygen concentration in the corresponding at least one internal cavity; the predetermined threshold is a change threshold for determining whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the at least one electrochemical pump cell needs to be changed when the pump voltage is applied to the at least one electrochemical pump cell; In the determination step, the determination target value is change The gas sensor is operated in a basic mode in which the oxygen concentration in the at least one internal space is kept constant by operating the at least one electrochemical pump cell unless the threshold value is exceeded, and in the determining step, the determination target value is determined to be change If the threshold is exceeded, the at least one electrochemical pump cell is put into a protective run mode to protect the at least one electrochemical pump cell from excessive oxygen pumping. evening the gas sensor of operation In the protection execution mode, the target value is changed to a value greater than the normal value, and the operation of the gas sensor is continued. At the same time, monitoring of a pump cell operation value, which is the value of the pump voltage or the value of the current in at least one electrochemical pump cell including the target pump cell, is started. When the pump cell operation value falls below a restoration threshold, the target value is restored to the normal value, and the operation of the gas sensor is restored to the basic mode. It is characterized by making

[0024] The present invention 9 The embodiment of a gas sensor including a sensor element made of an oxygen ion conductive solid electrolyte and configured to be capable of detecting a predetermined gas component in a measurement gas, the gas sensor further comprising: at least one internal cavity in which an inner electrode is provided and which communicates with an inlet for the measurement gas under a predetermined diffusion resistance; an outer-cavity pump electrode disposed at a location other than the at least one internal cavity; and a reference electrode provided so as to be in contact with a reference gas; at least one electrochemical pump cell configured to be able to pump oxygen in or out of the corresponding at least one internal cavity and the outside of the sensor element by applying a pump voltage between the inner electrode and the outer-cavity pump electrode from a predetermined pump power source; and at least one electrochemical sensor cell configured to generate a potential difference between the inner electrode and the reference electrode according to the oxygen concentration in the corresponding at least one internal cavity; the predetermined threshold is a change threshold for determining whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the at least one electrochemical pump cell needs to be changed when controlling application of the pump voltage to the at least one electrochemical pump cell; operating the gas sensor in a basic mode in which the oxygen concentration in the at least one internal space is kept constant by operating the at least one electrochemical pump cell unless the determination target value exceeds the change threshold value in the determination step;When the determination target value exceeds the change threshold value in the determination step for , the above Protecting at least one electrochemical pump cell from excessive oxygen pumping Protected Execution Mode Operation of the gas sensor according to and in the protection execution mode, the gas sensor continues to operate after changing the target value to a value greater than the normal value, and starts monitoring a pump cell operation value, which is the pump voltage or current value, of at least one electrochemical pump cell including the target pump cell. When the pump cell operation value falls below a restoration threshold, the target value is restored to the normal value, and the operation of the gas sensor returns to the basic mode. The pump cell operation value is continued to be monitored unless the pump cell operation value falls below the restoration threshold and the target value for determination exceeds a stop threshold that is greater than the change threshold. When the pump cell operation value does not fall below the restoration threshold and the target value for determination further exceeds the stop threshold, the operation of the at least one electrochemical pump cell is stopped, and the potential difference of an electrochemical sensor cell corresponding to the target pump cell among the at least one electrochemical sensor cell is started to be monitored. When the monitored potential difference falls below a restart threshold, the gas sensor resumes operation in the basic mode.

[0025] The present invention 10 The embodiment is 8 or No. 9 attitude Like The method for operating such a gas sensor is characterized in that the judgment target value is an actual measurement value of the potential difference in an electrochemical sensor cell corresponding to the judgment target pump cell among the at least one electrochemical sensor cell.

[0026] The present invention 11 The embodiment is 8 or No. 9 attitude Like The method for operating such a gas sensor is characterized in that the target value for judgment is a value of a pump current when oxygen is pumped into the target pump cell. [Effects of the Invention]

[0027] The first to third aspects of the present invention 11 According to the above aspect, even when the gas sensor is used in an environment where the gas to be measured may be a rich gas with a low air-fuel ratio, it is possible to preferably avoid blackening of the solid electrolyte constituting the sensor element, which would otherwise be caused by difficulty in pumping oxygen into the internal cavity, thereby preventing failure of the gas sensor due to use in a rich gas atmosphere. [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a diagram schematically illustrating an example of the configuration of a gas sensor 100. FIG. [Figure 2] FIG. 10 is a diagram showing an operation flow in a first aspect of the element protection mode. [Figure 3] FIG. 10 is a diagram showing an operation flow in a second aspect of the element protection mode. [Figure 4] FIG. 10 is a diagram showing an operation flow in a third aspect of the element protection mode. [Figure 5] FIG. 10 is a diagram showing an operation flow in a third aspect of the element protection mode. [Figure 6] FIG. 2 is a diagram schematically illustrating an example of the configuration of a gas sensor 100B. DETAILED DESCRIPTION OF THE INVENTION

[0029] <Outline of gas sensor configuration> 1 is a diagram schematically illustrating an example of the configuration of a gas sensor 100 according to this embodiment. The gas sensor 100 is a limiting current type NOx sensor that detects NOx and measures its concentration using a sensor element 101. The gas sensor 100 also includes a controller 110 that controls the operation of each component and determines the NOx concentration based on the NOx current flowing through the sensor element 101. FIG. 1 includes a vertical cross-sectional view of the sensor element 101 along its longitudinal direction.

[0030] The sensor element 101 is a flat (long) 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 solid electrolyte layer is made of zirconia (ZrO), 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. 1 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.

[0031] 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.

[0032] At one 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, there are formed adjacent to each other and in communication with each other in this order: a first diffusion-controlling section 11 which also serves 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.

[0033] The buffer space 12, the first internal void 20, the second internal void 40, and the third internal void 61 are spaces (regions) inside the sensor element 101, defined by an upper portion cut out from the spacer layer 5 and defined 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 tip end surface (left end in the drawing) of the sensor element 101, separate from the first diffusion-controlling section 11. In this case, the first diffusion-controlling section 11 is formed adjacent to and inside the gas inlet 10.

[0034] 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.

[0035] Furthermore, at a position farther from the tip side than the gas flow section, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, at a position defined on the side by the side surface of the first solid electrolyte layer 4. 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] 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.

[0038] 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.

[0039] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the taken-in measurement gas.

[0040] 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 .

[0041] 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 .

[0042] 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.

[0043] 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.

[0044] The main pump cell 21 is an electrochemical pump cell comprising an inner pump electrode (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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by a 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 out 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.

[0050] 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.

[0051] 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.

[0052] Furthermore, the controller 110 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] In the auxiliary pump cell 50, under the control of the controller 110, 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] In the measuring pumping cell 41, under the control of the controller 110, 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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. 1). The heater section 70, except for the heater electrode 71, is embedded in the base of the sensor element 101.

[0074] 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).

[0075] 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. 1) 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 side of the sensor element 101 where the gas flow section is provided, 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 20 μm.

[0076] In the sensor element 101, 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 that constitutes 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.

[0077] 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).

[0078] Although not shown in FIG. 1, 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.

[0079] Furthermore, a thermal shock-resistant protective layer, which is a single-layer or multi-layer porous layer that covers the sensor element 101, may be further provided on a predetermined area of ​​the outer periphery of one tip end side (left end side in the drawing) of the sensor element 101. The thermal shock-resistant protective layer is provided for the purposes of preventing cracks from occurring in the sensor element 101 due to thermal shock that occurs when moisture contained in the measured gas adheres to and condenses on the sensor element 101 during use of the gas sensor 100, and for the purposes of preventing poisoning substances present in the measured gas from penetrating into the sensor element 101. Note that a layered void (void layer) may be formed between the sensor element 101 and the thermal shock-resistant protective layer.

[0080] The sensor element 101 is housed in a metal housing member (casing) (not shown) in such a manner that the gap between the gas inlet 10 side and the reference gas introduction space 43 side is airtightly sealed. The sensor element 101 and the housing member constitute the main body of the gas sensor 100. When the gas sensor 100 is actually used, the main body is attached to the location where it is to be used, such as an engine exhaust pipe. Wiring that ensures electrical connection with each part of the sensor element 101 inside is drawn out from the housing member, and these wirings are connected appropriately to the controller 110, various power sources, etc.

[0081] <Operation in normal mode> When the concentration of NOx is measured in the gas sensor 100 having the above configuration, feedback control is performed to maintain constant oxygen concentrations in the first internal space 20 and the second internal space 40 by operating the main pump cell 21 and then the auxiliary pump cell 50, 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 measurement of NOx (for example, 0.0001 ppm to 1 ppm).

[0082] 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.

[0083] 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 controller 110. 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 controller 110. The controller 110 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.

[0084] In this embodiment, the above-described operation of the gas sensor 100 for specifying the concentration of NOx is referred to as operation of the gas sensor 100 in the normal mode.

[0085] In the normal mode, the target values ​​of the electromotive forces V0, V1, and V2 in the main sensor cell 80, the auxiliary sensor cell 81, and the measurement sensor cell 82 when feedback-controlling the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41, respectively, may be set appropriately depending on the specific configuration and size of each part of the sensor element 101, as well as the usage conditions and usage manner of the gas sensor 100. However, hereinafter, as an exemplary embodiment, it is assumed that the target values ​​of the electromotive forces V0, V1, and V2 are set to 250 mV, 385 mV, and 400 mV, respectively. These values ​​are generally standard values ​​set when the oxygen-ion conductive solid electrolyte constituting the sensor element 101 is zirconia.

[0086] <Operation in element protection mode> The gas sensor 100 according to this embodiment is intended to operate in the normal mode described above, i.e., to identify the concentration of NOx in the measured gas, mainly under conditions where the measured gas contains a relatively sufficient amount of oxygen, such as a lean atmosphere.

[0087] More specifically, when the gas sensor 100 is operating in normal mode, the main pump cell 21 operates so that the electromotive force V0 generated in the main sensor cell 80 becomes a predetermined value corresponding to the desired oxygen concentration value (or oxygen partial pressure value) in the first internal space 20. However, since the oxygen concentration in the measurement gas introduced from the external space into the first internal space 20 changes from moment to moment, the main pump cell 21 can both pump out and pump in oxygen.

[0088] On the other hand, although the auxiliary pump cell 50 and the measuring pump cell 41 are configured to be capable of pumping oxygen, the set values ​​of the electromotive force V1 in the auxiliary sensor cell 81 and the electromotive force V2 in the measuring sensor cell 82, which are control target values ​​when operating each pump cell, are set on the assumption that oxygen will be pumped out, based on the principle of measuring NOx concentration. In other words, when the gas sensor 100 operates in the normal mode, the auxiliary pump cell 50 and the measuring pump cell 41 mainly pump out oxygen.

[0089] However, the gas sensor 100 is not necessarily used in an atmosphere containing sufficient oxygen. For example, the gas sensor 100 may be used in an environment where the ambient gas may be rich gas with a low air-fuel ratio, such as when the main body of the gas sensor 100 is attached to the exhaust path of a gasoline engine and the exhaust gas from the engine is used as the measurement gas. In such a case, the measurement gas introduced into the sensor element 101 is also rich gas. In such a case, the main pump cell 21 attempts to maintain the oxygen concentration value in the first internal space 20 by pumping in oxygen from the outside.

[0090] However, if the measurement gas introduced into the sensor element 101 is excessively rich, even if the main pump voltage Vp0 applied to the main pump cell 21 is increased, it may become impossible to pump in an amount of oxygen corresponding to the main pump voltage Vp0 from the outside, and the target oxygen concentration may not be achieved in the first internal space 20, resulting in an uncontrollable situation of the oxygen concentration. Moreover, oxygen may be extracted from the solid electrolyte constituting the main pump cell 21, causing blackening, and causing the gas sensor 100 to no longer function.

[0091] Such a situation where oxygen cannot be suitably pumped in from the outside is more likely to occur in a case where the rate of flow of atmospheric gas around the outer pump electrode 23, which is the point at which oxygen is taken in from the external space, is controlled by a predetermined diffusion resistance, as in the gas sensor disclosed in Patent Document 1, for example. FIG. 6 is a diagram schematically illustrating an example of the configuration of a gas sensor 100B as one embodiment of such a gas sensor 100. The gas sensor 100B has a common configuration with the gas sensor 100 shown in FIG. 1, except that it further includes a ceramic layer 7 and a porous region 8 on the second solid electrolyte layer 6. The porous region 8 is made of a porous material (e.g., alumina) with a porosity of approximately 30% to 60%, and is provided so as to cover the outer pump electrode 23 and to be exposed at both ends in the widthwise direction of the element (not shown). The ceramic layer 7 is made of, for example, a ceramic (e.g., zirconia, alumina, etc.) as dense as the second solid electrolyte layer 6, and is provided so as to cover the entire upper surface of the second solid electrolyte layer 6 including the porous region 8. In the gas sensor 100B, the rate of entry and exit of the ambient gas around the outer pump electrode 23 is determined by the diffusion resistance provided by the porous region 8.

[0092] In consideration of the above, the gas sensor 100 according to this embodiment is capable of operating in an element protection mode when the measured gas is excessively rich, in order to protect the sensor element 101 while avoiding a situation in which the oxygen concentration cannot be controlled.

[0093] The element protection mode has three different modes with different processing procedures, which will be explained below in order.

[0094] (First aspect) 2 is a diagram showing an operation flow in the first mode of the element protection mode. The first mode is a method for preventing blackening of the sensor element 101 by temporarily stopping the NOx measurement operation of the gas sensor 100, including the pumping of oxygen from outside, when the measurement gas becomes an excessively rich atmosphere.

[0095] In this embodiment, first, the gas sensor 100 is set to start operating in the element protection mode (step S1-1). This is realized, for example, by a user (operator) of the gas sensor giving an appropriate setting instruction to the controller 110 through a predetermined interface (not shown). Alternatively, the gas sensor 100 may be set to operate in the element protection mode at all times.

[0096] Even after the element protection mode is initiated, the NOx concentration is continuously measured in the same manner as in the normal mode under the control of the controller 110. In this manner, the operation mode in which the NOx concentration is measured in the same manner as in the normal mode even during the element protection mode is particularly referred to as the basic mode. Note that the operation in the normal mode may also be referred to as the operation in the basic mode. However, when the element protection mode is initiated, the controller 110 starts monitoring a predetermined determination target value in parallel with the operation in the basic mode (step S1-2).

[0097] In this embodiment, the judgment target value is a value that serves as an index when determining whether to stop the NOx measurement operation. Specifically, the actual measured value of the electromotive force V0 is used. When the oxygen concentration of the measurement gas introduced into the first internal space 20 is lower than a predetermined target oxygen concentration for the first internal space 20, the main pump cell 21 pumps in oxygen to maintain the electromotive force V0 at the target value. However, when an excessively rich measurement gas is introduced, the pumping of oxygen becomes insufficient, and the actual measured value of the electromotive force V0 deviates from the target value (becomes larger than the target value). The value of the electromotive force V0 at the limit at which such deviation is tolerable is preset as a stop threshold. The stop threshold is set to, for example, 350 mV.

[0098] The monitoring of the judgment target value continues until a predetermined time (judgment time) has elapsed (ended) (step S1-3). The judgment time is set to, for example, about 10 seconds.

[0099] Then, when the determination time has elapsed (ended) (Yes in step S1-3), the controller 110 determines whether the determination target value has exceeded a predetermined stop threshold during the determination time (step S1-4). Alternatively, it may be configured to determine whether the determination target value has exceeded a predetermined stop threshold at the end of the determination time.

[0100] If the determination target value does not exceed the predetermined stop threshold during the determination time (No in step S1-4), the controller 110 continues (step S1-5) the measurement operation in the gas sensor 100. The measurement operation may be continued in the normal mode after terminating the element protection mode, or may be continued in the basic mode by restarting the element protection mode.

[0101] On the other hand, if the determination target value exceeds the predetermined stop threshold during the determination time (Yes in step S1-4), the controller 110 transitions to a protection execution mode in which it stops pump control (feedback control) of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 (step S1-6). This means that the NOx measurement operation in the gas sensor 100 is stopped.

[0102] As a result, the measurement gas introduced from the gas inlet 10 flows directly from the first internal space 20 through the second internal space 40 into the third internal space 61, but since the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 are all inactive, a situation in which an excessively large pump voltage is applied to pump in oxygen is avoided.

[0103] When the pump control of each pump cell is stopped, the actual measured values ​​of the electromotive forces V0, V1, and V2 in the main sensor cell 80, auxiliary sensor cell 81, and measurement sensor cell 82, which had been controlled to predetermined constant values ​​corresponding to the desired oxygen concentration, respectively, become values ​​corresponding to the oxygen concentration of the measurement gas whose oxygen concentration has not been adjusted and which flows into the first internal space 20, second internal space 40, and third internal space 61. These electromotive forces generated in each sensor cell when the pump control is stopped are particularly referred to as open electromotive forces.

[0104] The open electromotive force varies depending on the oxygen concentration of the measurement gas flowing into each internal space, and the higher the oxygen concentration, the larger the value. Therefore, each open electromotive force can be used as an index of the oxygen concentration of the measurement gas present in the internal space corresponding to each sensor cell when pump control is stopped.

[0105] The controller 110 stops pump control of each pump cell and starts monitoring the open electromotive force of each pump cell (step S1-7), and determines whether the open electromotive force is below a predetermined restart threshold (step S1-8). At this time, the open electromotive force of at least one sensor cell (e.g., the main sensor cell 80) may be monitored to determine whether it is higher or lower than the restart threshold. As long as the open electromotive force does not fall below the restart threshold (No in step S1-8), the monitoring of the open electromotive force continues.

[0106] The restart threshold is set to a value at which it can be determined that the oxygen concentration of the measurement gas flowing into each internal space has increased to such an extent that each pump cell can pump oxygen in or out without any problems even if the pump control of each pump cell is resumed when the OPEN electromotive force reaches that value. The restart threshold can be set, for example, based on a correspondence relationship (functional relationship) between the air-fuel ratio of the measurement gas and the OPEN electromotive force value, which has been experimentally determined in advance.

[0107] For example, when exhaust gas from a gasoline engine is used as the measurement gas, even if excessively rich exhaust gas flows into the sensor element 101 as the measurement gas at a certain timing, such inflow is usually not permanent, and after a certain period of time, the oxygen concentration of the exhaust gas will recover to a level at which each pump cell can operate properly.

[0108] In a case where the open electromotive force of the main sensor cell 80 is to be monitored, a suitable example is to set the restart threshold to 450 mV. It has been confirmed in advance that when the open electromotive force is 450 mV or less, the atmosphere inside the first internal space 20 has a stoichiometric composition or a lean composition.

[0109] If it is determined that the OPEN electromotive force has fallen below the predetermined restart threshold (Yes in step S1-8), the controller 110 restarts the pump control operation that had been stopped (step S1-9). That is, the target values ​​of the electromotive forces V0, V1, and V2 in the main sensor cell 80, auxiliary sensor cell 81, and measurement sensor cell 82 are reset, respectively, so that the oxygen concentrations in the first internal space 20, the second internal space 40, and the third internal space 61 become desired values, and the pump cells are operated again so that these target values ​​are realized.

[0110] Finally, when the pump cells are in a state where feedback control based on the target values ​​of the electromotive forces V0, V1, and V2 is possible, the NOx measurement operation is resumed (step S1-10). This measurement operation may be continued in the normal mode after terminating the element protection mode, or may be continued in the basic mode by restarting the element protection mode.

[0111] In this manner, in the present embodiment, when excessively rich gas is introduced into the sensor element 101 as the measurement gas, the operation of each pump cell is stopped to temporarily halt the NOx measurement in order to avoid a situation in which the oxygen concentration becomes uncontrollable. Then, when it is determined that the oxygen concentration of the measurement gas being introduced has recovered to a level at which the pump cell can pump oxygen, the operation of the pump cell is restarted to resume the NOx measurement. This prevents excessive oxygen from being pumped into the sensor element 101, protecting the pump cell and further preventing blackening of the sensor element 101. In other words, the sensor element 101 is appropriately protected.

[0112] (Second aspect) 3 is a diagram showing an operation flow in the second mode of the element protection mode. The second mode is, roughly speaking, a method for preventing excessive pumping into the main pump cell 21 by temporarily reducing the target oxygen concentration in the first internal space 20 when the measurement gas becomes an excessively rich atmosphere, thereby avoiding blackening of the sensor element 101 while continuing to measure NOx.

[0113] Steps S2-1 to S2-5 of the second embodiment are generally similar to steps S1-1 to S1-5 of the first embodiment, respectively. Also, the determination target value is specifically the measured value of the electromotive force V0, which is the same as in the first embodiment.

[0114] However, in this embodiment, the judgment target value is used as an index when judging whether to change the target oxygen concentration in the first internal space 20. More specifically, it is used as an index when changing the target value of the electromotive force V0 of the main sensor cell 80 according to the target oxygen concentration. Note that there is a relationship in which the larger the target value of the electromotive force V0, the smaller the target oxygen concentration in the first internal space 20.

[0115] That is, in this embodiment, when an excessively rich measurement target gas is introduced into the first internal space 20, the actual measured value of the electromotive force V0 deviates from the target value. The value of the electromotive force V0 at the limit of allowable deviation is preset as the change threshold. The controller 110 starts the element protection mode (basic mode) (step S2-1) as in the first embodiment, starts monitoring the determination target value (step S2-2), and then, when the determination time has elapsed (ended) (Yes in step S2-3), determines whether the determination target value has exceeded a predetermined change threshold during the determination time (step S2-4). The change threshold may be the same as or different from the stop threshold in the first embodiment.

[0116] If the judgment target value does not exceed the predetermined change threshold during the judgment time (No in step S2-4), as in the first aspect, the controller 110 continues the measurement operation in the gas sensor 100 in normal mode or element protection mode (basic mode) (step S2-5).

[0117] On the other hand, if the determination target value exceeds the predetermined change threshold during the determination time (Yes in step S2-4), the mode also shifts from the basic mode to the protection execution mode in this embodiment. However, in this embodiment, the controller 110 changes the control reference value in the pump control performed when measuring the NOx concentration from the value (normal value) in the basic mode (normal mode) (step S2-6).

[0118] Specifically, the controller 110 at least changes the target value of the electromotive force V0 of the main sensor cell 80, which is the reference for pump control in the main pump cell 21, so that it is larger than the normal value (and even larger than the change threshold value). For example, the target value of the electromotive force V0 is changed from 250 mV, which is the value in the normal mode, to 350 mV. The target value of the electromotive force V0 after such change is referred to as the changed reference value. By changing the target value of the electromotive force V0 in this manner, the target oxygen concentration in the first internal space 20 is lowered. In addition, the control reference values ​​for pump control in the auxiliary pump cell 50 and the measurement pump cell 41 may also be changed from the values ​​in the normal mode.

[0119] Then, the controller 110 starts controlling each pump cell based on the changed reference value. That is, in this embodiment, the measurement of NOx continues even in the protection execution mode (step S2-7).

[0120] The target value of the electromotive force V0 is set to a changed reference value that is greater than the normal value, and the target oxygen concentration in the first internal space 20 is lowered. This reduces the difference between the target oxygen concentration and the oxygen concentration in the excessively rich measurement gas that has entered the first internal space 20 compared to the normal mode. Therefore, when each pump cell is controlled based on the changed reference value, the amount of oxygen that needs to be pumped into the first internal space 20 by the main pump cell 21 to achieve the target oxygen concentration is reduced compared to before the change. As a result, an excessive increase in the main pump current Ip0 or the main pump voltage Vp0 in the main pump cell 21 and the occurrence of blackening are suppressed.

[0121] Furthermore, in the first embodiment, when the judgment target value exceeds the stop threshold and the system transitions to protection execution mode, NOx measurement is stopped, but in the present embodiment, the pump control operation for measuring NOx continues even in protection execution mode, thereby avoiding measurement gaps.

[0122] However, it is desirable that the changed reference value be set within a range that does not significantly increase the pumping load on the auxiliary pump cell 50 or the measuring pump cell 41, or that does not significantly decrease the accuracy of measuring NOx.

[0123] When the measurement based on the changed reference value is started, the controller 110 starts monitoring the pump cell operating value (step S2-8) and determines whether the pump cell operating value falls below the recovery threshold value (step S2-9).

[0124] Specifically, the pump cell operating value is the main pump current Ip0 or the main pump voltage Vp0. The pump cell operating value increases as the amount of oxygen pumped in increases. The restoration threshold is set to a value that allows the oxygen concentration of the measurement gas flowing into each internal space to be determined to be high enough that each pump cell can pump in or out oxygen without any problems, even if the control reference value is restored from the changed reference value to the normal value when the pump cell operating value reaches that value.

[0125] As long as the pump cell operating value does not fall below the recovery threshold (No in step S2-9), the pump cell operating value continues to be monitored.

[0126] On the other hand, if it is determined that the pump cell operating value has fallen below the predetermined restoration threshold (Yes in step S2-9), the controller 110 returns the control reference value from the changed reference value to the normal value (step S2-10) and continues the NOx measurement operation based on the changed control reference value (step S2-11). This measurement operation may be continued in the normal mode after terminating the element protection mode, or in the basic mode by restarting the element protection mode.

[0127] In this manner, in the case of this embodiment, when an excessively rich gas is introduced into the sensor element 101 as the measurement gas, the target oxygen concentration in the first internal space 20 is temporarily reduced to suppress excessive pumping into the main pump cell 21. Then, when it is determined that the oxygen concentration of the measurement gas being introduced has recovered to a level at which the pump cell can pump oxygen, the target oxygen concentration in the first internal space 20 is restored to its original level. This effectively prevents the main pump voltage Vp0 from increasing to a level at which oxygen cannot be pumped in, and further prevents blackening of the sensor element 101. In other words, the sensor element 101 is appropriately protected. Additionally, unlike the first embodiment, the measurement of the NOx concentration is not interrupted.

[0128] (Third aspect) 4 and 5 are diagrams showing the operation flow in the third aspect of the element protection mode. The third aspect is a method of avoiding blackening of the sensor element 101 by combining the first and second aspects and taking two steps in the protection execution mode when the measured gas becomes an excessively rich atmosphere depending on the degree of the condition.

[0129] In summary, when the measurement gas becomes an excessively rich atmosphere, first, the target oxygen concentration in the first internal space 20 is temporarily reduced to suppress excessive pumping in the main pump cell 21, as in the second embodiment. If the oxygen concentration in the first internal space 20 is still not sufficiently restored, the NOx measurement operation in the gas sensor 100, including the pumping of oxygen from the outside, is temporarily stopped, as in the first embodiment, to reduce the main pump voltage Vp 0 To avoid blackening due to an increase in

[0130] Steps S3-1 to S3-7 of the third aspect are the same as steps S2-1 to S2-7 of the second aspect, respectively. That is, in the third aspect as well, as in the second aspect, the controller 110 first starts the element protection mode (basic mode) (step S3-1), starts monitoring the determination target value (step S3-2), and then, when the determination time has elapsed (ended) (Yes in step S3-3), determines whether or not the determination target value has exceeded a predetermined change threshold during the determination time (step S3-4). Note that in this aspect as well, the determination target value is specifically the actual measured value of the electromotive force V0.

[0131] If the judgment target value does not exceed the predetermined change threshold during the judgment time (No in step S3-4), as in the first and second aspects, the controller 110 continues the measurement operation in the gas sensor 100 in normal mode or element protection mode (basic mode) (step S3-5).

[0132] On the other hand, if the judgment target value exceeds the predetermined change threshold during the judgment time (Yes in step S3-4), the present embodiment also transitions from the basic mode to the protection execution mode. The controller 110 changes the control reference value for the pump control performed when measuring the NOx concentration from the value (normal value) in the basic mode (normal mode) (step S3-6). Then, each pump cell is controlled based on the changed reference value, and the NOx measurement continues (step S3-7). That is, the NOx measurement continues in a state where the amount of oxygen that needs to be pumped into the first internal space 20 by the main pump cell 21 to achieve the target oxygen concentration is reduced.

[0133] Also, as with the second aspect, monitoring of the pump cell operating value is started in response to the change in the control reference value (step S3-8), and if it is determined that the pump cell operating value has fallen below a predetermined return threshold (Yes in step S3-9), the controller 110 returns the control reference value from the changed reference value to the normal value (step S3-10), and the NOx measurement operation based on the control reference value continues in normal mode or element protection mode (basic mode) (step S3-11).

[0134] On the other hand, if it is determined that the pump cell operating value is not below the restoration threshold (No in step S3-9), it is determined whether the determination target value exceeds a predetermined stop threshold (step S3-12). In this embodiment, the stop threshold is set to a value greater than the change threshold.

[0135] If the determination target value does not exceed the stop threshold (No in step S3-12), the process returns to step S3-8, where monitoring of the pump cell operating value is resumed, and the process from step S3-9 onward is repeated. Therefore, in a situation where the pump cell operating value is equal to or greater than the predetermined return threshold but the determination target value is equal to or less than the stop threshold, a loop of step S3-8 → step S3-9 → step S3-12 → step S3-8 → ... is repeated. This means that if the excessive oxygen pumping by the main pump cell 21 caused by the introduction of rich gas into the sensor element 101 can be addressed by reducing the target oxygen concentration, as in the second mode, the process continues in this mode.

[0136] If the determination target value after changing the control reference value exceeds the stop threshold (Yes in step S3-12), the same procedure as in the first mode is executed. This procedure is intended to more reliably protect the pump cell when simply reducing the target oxygen concentration is not sufficient to protect the pump cell.

[0137] Specifically, first, the controller 110 stops the pump control (feedback control) of the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 (step S3-13). As a result, the NOx measurement operation of the gas sensor 100, which had been continued based on the changed reference value until then, is stopped.

[0138] The controller 110 further starts monitoring the OPEN electromotive force (step S3-14) and determines whether the OPEN electromotive force is below a predetermined restart threshold (step S3-15). If the OPEN electromotive force in the main sensor cell 80 is to be monitored, a suitable example is to set the restart threshold to 450 mV. As long as the OPEN electromotive force does not fall below the restart threshold (No in step S3-15), the monitoring of the OPEN electromotive force continues.

[0139] If it is determined that the OPEN electromotive force has fallen below the restart threshold (Yes in step S3-15), the controller 110 resets the control reference value to the normal value and restarts the pump control operation that had been stopped (step S3-16). Finally, when feedback control based on the electromotive forces V0, V1, and V2 becomes possible in each pump cell, the NOx measurement operation in the normal mode or element protection mode (basic mode) is restarted (step S3-17).

[0140] In this manner, the present embodiment combines the first and second embodiments, and the way of dealing with the problem in the protection execution mode is switched in stages depending on the richness of the measurement gas introduced into the sensor element 101. This minimizes the occurrence of situations where NOx measurement is stopped, and effectively avoids the occurrence of blackening when an excessively rich measurement gas is introduced into the sensor element 101.

[0141] As described above, according to this embodiment, even when the gas sensor is used in an environment where the gas to be measured may be a rich gas with a low air-fuel ratio, such as exhaust gas from a gasoline engine, it is possible to effectively avoid blackening of the solid electrolyte constituting the sensor element, which would otherwise be caused by difficulty in pumping oxygen into the internal cavity. This makes it possible to prevent failure of the gas sensor due to use in a rich gas atmosphere.

[0142] <Modification> In the above-described embodiment, the actual measured value of the electromotive force V0 is used as the judgment target value. However, instead of this, an embodiment may be such that the value of the main pump current Ip0 when oxygen is being pumped into the main pump cell 21 is used as the judgment target value. The main pump current Ip0 increases in accordance with the amount of oxygen being pumped into the first internal space 20, and therefore can be an index of the degree of oxygen being pumped into the first internal space 20. The stop threshold in the first embodiment and the change threshold in the second and third embodiments are set depending on the judgment target value used.

[0143] In the basic modes of the normal mode and the element protection mode, the gas sensor 100 may be operated in a control mode called constant Ip1 control, in which the auxiliary pump cell 50 is controlled so that a constant magnitude of the auxiliary pump current Ip1 flows. In this case, the actual measured value of the auxiliary pump current Ip1 may be used as a determination value in the protection execution mode of the element protection mode, instead of or in combination with the actual measured value of the electromotive force V0. While the oxygen concentration of the measurement gas introduced from the first internal space 20 to the second internal space 40 is maintained at a predetermined value, the auxiliary pump current Ip1 when the auxiliary pump cell 50 pumps oxygen can be maintained at a predetermined constant value, thereby controlling the oxygen concentration in the second internal space 40 to the predetermined value. However, if an excessively rich measurement gas is introduced into the first internal space 20, making it impossible to adjust the oxygen concentration in the first internal space 20, and as a result, a measurement gas with a low oxygen concentration also enters the second internal space 40, the auxiliary pump cell 50 will be unable to pump oxygen from the second internal space 40, and the actual measured value of the electromotive force V1 will increase from its constant value, and the actual measured value of the auxiliary pump current Ip1 will decrease from the set constant value. Therefore, the actual measured value of the auxiliary pump current Ip1 can also be used as a target value for judgment.

[0144] In the above-described embodiment, the gas sensor is configured to have a sensor element having three voids therein, but the configuration of the sensor element in which blackening may occur due to excessively rich gas being taken into the sensor element to be measured is not limited to that of the above-described embodiment.

[0145] In addition, there are cases where the target component in the measurement gas is other than NOx. In such cases, the pumping of oxygen may become dominant even in the normal mode.

[0146] In either case, as long as the gas sensor includes a sensor element having an internal cavity in which the oxygen concentration is kept constant by pumping oxygen in or out using an electrochemical pump cell, it is possible to avoid blackening of the solid electrolyte constituting the sensor element by applying the first to third aspects described above, with appropriate modifications if necessary depending on the element configuration, thereby preventing failure of the gas sensor including the sensor element. [Explanation of symbols]

[0147] 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 13 Second diffusion-controlled section 20 1st internal void 21 Main pump cell 22 Inner pump electrode 23 Outer pump electrode 24, 46, 52 variable 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 50 Auxiliary pump cell 51 Auxiliary pump electrode 60 4th diffusion-controlled section 61 3rd internal void 70 Heater section 80 Main sensor cell 81 Auxiliary sensor cell 82 Measuring sensor cell 100 Gas Sensor 101 Sensor element Ip0 Main pump current Ip1 Auxiliary pump current Ip2 NOx current

Claims

1. A gas sensor configured to be able to detect a predetermined gas component in a measurement gas, a sensor element made of an oxygen ion conductive solid electrolyte; a controller for controlling the operation of the gas sensor; Equipped with The sensor element is at least one internal cavity communicating with the inlet for the gas to be measured at a predetermined diffusion resistance and having an internal electrode provided therein; an extra-cavity pumping electrode disposed outside the at least one internal cavity; a reference electrode provided so as to be in contact with the reference gas; Equipped with at least one electrochemical pump cell configured to be able to pump oxygen between the corresponding at least one internal cavity and the outside of the sensor element by applying a pump voltage between the internal electrode and the external pump electrode of the cavity by a predetermined pump power supply; at least one electrochemical sensor cell configured to generate a potential difference between the inner electrode and the reference electrode according to the oxygen concentration in the corresponding at least one internal space; Furthermore, The controller determining whether a target value serving as an index of an oxygen pumping operation in a target pump cell included in the at least one electrochemical pump cell exceeds a predetermined threshold value within a predetermined determination time; controlling the gas sensor in a basic mode to maintain a constant oxygen concentration in the at least one internal space by operating the at least one electrochemical pump cell unless the determination target value exceeds the predetermined threshold; If the determination value exceeds the predetermined threshold, the gas sensor is controlled in a protective running mode to protect the at least one electrochemical pump cell from excessive oxygen pumping operation, the predetermined threshold is a change threshold for determining whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the at least one electrochemical pump cell needs to be changed when controlling application of the pump voltage to the at least one electrochemical pump cell; The controller When the judgment target value exceeds the change threshold, the protection execution mode is initiated, and in the protection execution mode, the target value is changed to a value greater than the normal value, and control of the gas sensor is continued, and monitoring of a pump cell operating value, which is a value of the pump voltage or a value of the current, of at least one electrochemical pump cell including the judgment target pump cell is initiated; When the pump cell operating value falls below the restoration threshold value, the target value is restored to the normal value, and the control of the gas sensor is restored to the basic mode. A gas sensor characterized by:

2. A gas sensor configured to be able to detect a predetermined gas component in a gas to be measured, a sensor element made of an oxygen ion conductive solid electrolyte; a controller for controlling the operation of the gas sensor; Equipped with The sensor element is at least one internal cavity communicating with the inlet for the gas to be measured at a predetermined diffusion resistance and having an internal electrode provided therein; an extra-cavity pumping electrode disposed outside the at least one internal cavity; a reference electrode provided so as to be in contact with the reference gas; Equipped with at least one electrochemical pump cell configured to be able to pump oxygen between the corresponding at least one internal cavity and the outside of the sensor element by applying a pump voltage between the internal electrode and the external pump electrode of the cavity by a predetermined pump power supply; at least one electrochemical sensor cell configured to generate a potential difference between the inner electrode and the reference electrode according to the oxygen concentration in the corresponding at least one internal space; Furthermore, The controller determining whether a target value serving as an index of an oxygen pumping operation in a target pump cell included in the at least one electrochemical pump cell exceeds a predetermined threshold value within a predetermined determination time; controlling the gas sensor in a basic mode to maintain a constant oxygen concentration in the at least one internal space by operating the at least one electrochemical pump cell unless the determination target value exceeds the predetermined threshold; If the determination value exceeds the predetermined threshold, the gas sensor is controlled in a protective running mode to protect the at least one electrochemical pump cell from excessive oxygen pumping operation, the predetermined threshold is a change threshold for determining whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the at least one electrochemical pump cell needs to be changed when controlling application of the pump voltage to the at least one electrochemical pump cell; The controller When the judgment target value exceeds the change threshold, the protection execution mode is initiated, and in the protection execution mode, the target value is changed to a value greater than the normal value, and control of the gas sensor is continued, and monitoring of a pump cell operating value, which is a value of the pump voltage or a value of the current, of at least one electrochemical pump cell including the judgment target pump cell is initiated; When the pump cell operating value falls below the restoration threshold, the target value is restored to the normal value, and the control of the gas sensor is restored to the basic mode. Continue monitoring the pump cell operation value as long as the pump cell operation value does not fall below the restoration threshold and the determination target value does not exceed a stop threshold that is greater than the change threshold; If the pump cell operation value is not below the recovery threshold value and the target value for judgment exceeds the stop threshold value, the operation of the at least one electrochemical pump cell is stopped, and monitoring of the potential difference in an electrochemical sensor cell corresponding to the target pump cell among the at least one electrochemical sensor cell is started; resume control of the gas sensor in the basic mode when the monitored potential difference falls below a resume threshold. A gas sensor characterized by:

3. 3. The gas sensor according to claim 1, the target value is an actual measurement value of the potential difference in an electrochemical sensor cell corresponding to the target pump cell among the at least one electrochemical sensor cell; A gas sensor characterized by:

4. 3. The gas sensor according to claim 1, the target value is a pump current value when the target pump cell pumps oxygen; A gas sensor characterized by:

5. 5. The gas sensor according to claim 1, the at least one internal cavity is a plurality of internal cavities that are sequentially connected to one another; the inner electrode is a plurality of inner electrodes provided in the plurality of internal cavities, the at least one electrochemical pumping cell is a plurality of electrochemical pumping cells; the at least one electrochemical sensor cell is a plurality of electrochemical sensor cells; the determination target value is an index of an oxygen pumping operation in at least one of the plurality of electrochemical pump cells, the electrochemical pump cell being provided corresponding to an internal space among the plurality of internal spaces that is provided nearest to the inlet; A gas sensor characterized by:

6. 6. The gas sensor according to claim 5, one of the plurality of inner electrodes is a measurement electrode for detecting the predetermined gas component; the plurality of electrochemical pump cells a measurement pump cell including the measurement electrode; At least one oxygen concentration control pump cell other than the measurement pump cell, the oxygen concentration control pump cell including any one of the plurality of inner electrodes other than the measurement electrode; It consists of the plurality of electrochemical sensor cells a measurement sensor cell including the measurement electrode; at least one oxygen concentration detection sensor cell other than the measurement sensor cell, the oxygen concentration detection sensor cell including any one of the plurality of inner electrodes other than the measurement electrode; It consists of the controller, at least in the basic mode, determines the concentration of the predetermined gas component based on a measurement pump current flowing in the measurement pump cell between the measurement electrode and the outer-space pump electrode in accordance with the concentration of the predetermined gas component. A gas sensor characterized by:

7. 7. The gas sensor according to claim 6, the plurality of internal spaces are a first internal space, a second internal space, and a third internal space, which are sequentially connected to each other via a diffusion resistance portion; the plurality of inner electrodes other than the measurement electrode are a main pump electrode provided in the first inner space and an auxiliary pump electrode provided in the second inner space, The measurement electrode is provided in the third internal space, the at least one oxygen concentration control pump cell is a main pump cell that controls the oxygen concentration in the first internal space and an auxiliary pump cell that controls the oxygen concentration in the second internal space, the at least one oxygen concentration detection sensor cell is a main sensor cell configured so that a potential difference corresponding to the oxygen concentration in the first internal space is generated between the main pump electrode and the reference electrode, and an auxiliary sensor cell configured so that a potential difference corresponding to the oxygen concentration in the second internal space is generated between the auxiliary pump electrode and the reference electrode, the controller, at least in the basic mode, operates the main pump cell and the auxiliary pump cell so as to maintain the oxygen concentrations in the first internal space and the second internal space at predetermined constant values, respectively, and determines the concentration of the predetermined gas component based on the magnitude of the measurement pump current flowing through the measurement pump cell in accordance with the concentration of the predetermined gas component contained in the measurement target gas whose oxygen concentration has been adjusted and which has been introduced into the third internal space. A gas sensor characterized by:

8. A method for operating a gas sensor having a sensor element made of an oxygen ion conductive solid electrolyte and configured to be able to detect a predetermined gas component in a measurement gas, comprising: The sensor element is at least one internal cavity communicating with the inlet for the gas to be measured at a predetermined diffusion resistance and having an internal electrode provided therein; an extra-cavity pumping electrode disposed outside the at least one internal cavity; a reference electrode provided so as to be in contact with the reference gas; Equipped with at least one electrochemical pump cell configured to be able to pump oxygen between the corresponding at least one internal cavity and the outside of the sensor element by applying a pump voltage between the internal electrode and the external pump electrode of the cavity by a predetermined pump power supply; at least one electrochemical sensor cell configured to generate a potential difference between the inner electrode and the reference electrode according to the oxygen concentration in the corresponding at least one internal space; In the case where the further a determining step of determining whether a determination target value, which is an index of an oxygen pumping operation in a determination target pump cell included in the at least one electrochemical pump cell, exceeds a predetermined threshold value within a predetermined determination time; Equipped with the predetermined threshold is a change threshold for determining whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the at least one electrochemical pump cell needs to be changed when the pump voltage is applied to the at least one electrochemical pump cell; operating the gas sensor in a basic mode in which the oxygen concentration in the at least one internal space is kept constant by operating the at least one electrochemical pump cell unless the determination target value exceeds the change threshold value in the determination step; If the determination value exceeds the change threshold in the determination step, an operation of the gas sensor in a protective execution mode is initiated to protect the at least one electrochemical pump cell from excessive oxygen pumping operation; In the protection execution mode, the target value is changed to a value greater than the normal value, and the gas sensor continues to operate, and starts monitoring a pump cell operation value, which is a pump voltage value or a pump current value, of at least one electrochemical pump cell including the target pump cell; When the pump cell operating value falls below the restoration threshold value, the target value is restored to the normal value, and the operation of the gas sensor is restored to the basic mode. A method for operating a gas sensor comprising:

9. A method for operating a gas sensor having a sensor element made of an oxygen ion conductive solid electrolyte and configured to be able to detect a predetermined gas component in a gas to be measured, comprising: The sensor element is at least one internal cavity communicating with the inlet for the gas to be measured at a predetermined diffusion resistance and having an internal electrode provided therein; an extra-cavity pumping electrode disposed outside the at least one internal cavity; a reference electrode provided so as to be in contact with the reference gas; Equipped with at least one electrochemical pump cell configured to be able to pump oxygen between the corresponding at least one internal cavity and the outside of the sensor element by applying a pump voltage between the internal electrode and the external pump electrode of the cavity by a predetermined pump power supply; at least one electrochemical sensor cell configured to generate a potential difference between the inner electrode and the reference electrode according to the oxygen concentration in the corresponding at least one internal space; In the case where the further a determining step of determining whether a determination target value, which is an index of an oxygen pumping operation in a determination target pump cell included in the at least one electrochemical pump cell, exceeds a predetermined threshold value within a predetermined determination time; Equipped with the predetermined threshold is a change threshold for determining whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the at least one electrochemical pump cell needs to be changed when controlling application of the pump voltage to the at least one electrochemical pump cell; operating the gas sensor in a basic mode in which the oxygen concentration in the at least one internal space is kept constant by operating the at least one electrochemical pump cell unless the determination target value exceeds the change threshold value in the determination step; If the determination value exceeds the change threshold in the determination step, an operation of the gas sensor in a protective execution mode is initiated to protect the at least one electrochemical pump cell from excessive oxygen pumping operation; In the protection execution mode, the target value is changed to a value greater than the normal value, and the gas sensor continues to operate, and starts monitoring a pump cell operation value, which is a pump voltage value or a pump current value, of at least one electrochemical pump cell including the target pump cell; When the pump cell operating value falls below the restoration threshold, the target value is restored to the normal value, and the operation of the gas sensor is restored to the basic mode. Continue monitoring the pump cell operation value as long as the pump cell operation value does not fall below the restoration threshold and the determination target value does not exceed a stop threshold that is greater than the change threshold; If the pump cell operation value is not below the recovery threshold value and the target value for judgment exceeds the stop threshold value, the operation of the at least one electrochemical pump cell is stopped, and monitoring of the potential difference in an electrochemical sensor cell corresponding to the target pump cell among the at least one electrochemical sensor cell is started; resuming operation of the gas sensor in the fundamental mode when the monitored potential difference falls below a resume threshold. A method for operating a gas sensor comprising:

10. A method for operating the gas sensor according to claim 8 or claim 9, comprising: the target value is an actual measurement value of the potential difference in an electrochemical sensor cell corresponding to the target pump cell among the at least one electrochemical sensor cell; A method for operating a gas sensor comprising:

11. A method for operating the gas sensor according to claim 8 or claim 9, comprising: The judgment target value is a pump current value when the judgment target pump cell pumps in oxygen. A method for operating a gas sensor comprising:

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