Gas sensor and method of operation of a gas sensor

Abstract

The present application provides a gas sensor and a method for operating a gas sensor, which can protect a sensor element when used in a fuel-rich atmosphere. A controller of the gas sensor judges whether a judgment object value, which is an index of oxygen intake operation in an electrochemical pump cell, exceeds a threshold value within a prescribed judgment time, controls the gas sensor in a basic mode in which oxygen concentration in at least one internal cavity is kept constant by operating at least one electrochemical pump cell as long as the judgment object value does not exceed the threshold value, and controls the gas sensor in a protection execution mode in which at least one electrochemical pump cell is protected from excessive oxygen intake operation when the judgment object value exceeds the threshold value. The electrochemical pump cell includes an inner electrode disposed facing at least one internal cavity provided in a sensor element into which a measured gas is introduced, a cavity outer pump electrode disposed at a position other than the internal cavity, and a solid electrolyte between the two electrodes.

Description

TECHNICAL FIELD

[0001] The present application relates to a gas sensor of limiting current type, and particularly to operation control of the gas sensor when used in a fuel-rich atmosphere. BACKGROUND

[0002] A gas sensor (for example, a NOx sensor, an oxygen sensor) of limiting current type employing a solid electrolyte of oxygen ion conductivity such as yttria-stabilized zirconia as a main constituent component is known. In this gas sensor, a measured gas is introduced into a cavity (internal cavity) provided inside a sensor element. Also, a potential difference between an inner electrode provided facing the internal cavity and a reference electrode provided inside the element and in contact with a reference gas is controlled to be maintained at a prescribed value corresponding to a desired oxygen concentration in the cavity.

[0003] In summary, in an electrochemical pump cell constituted by the inner electrode, an outer electrode (cavity-external electrode) provided outside the cavity, and a solid electrolyte region existing between the two electrodes, a pumping voltage is applied between the two electrodes, and oxygen is taken in or taken out between the internal cavity and the outside. By the application of the pumping voltage, an oxygen pumping current of a size and direction corresponding to the oxygen concentration in the cavity flows between the inner electrode and the outer electrode.

[0004] As an example of such a gas sensor, a gas sensor provided with an outer electrode on the outer surface of a sensor element and provided with a ceramic layer in such a manner that a slit portion imparting a prescribed diffusion resistance is formed around the outer electrode is also known (for example, see Patent Literature 1).

[0005] In addition, a gas sensor provided with a sensor element configured such that an oxygen concentration detection unit and an oxygen pump unit are stacked in the element thickness direction with an insulating layer interposed therebetween, and a measured gas is introduced into the inside through a diffusion speed control portion formed of a porous body provided in a portion of the insulating layer is also known (for example, see Patent Literature 2).

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURE

[0008] Patent Literature 1: Japanese Patent Application Laid-Open No. 2021-162465

[0009] Patent Literature 2: Japanese Patent Application Laid-Open No. 2012-173146 SUMMARY

[0010] Sometimes, the gas sensor of the limiting current type as described above is used in an environment where a rich fuel gas having an air-fuel ratio smaller than the stoichiometric air-fuel ratio can be introduced into the inside of the element, such as in the middle of an exhaust path from a gasoline engine.

[0011] In this case, when a rich fuel gas is introduced into the internal cavity, in the electrochemical pump cell, generally, in order to make the oxygen concentration in the cavity constant, an action of sucking oxygen from the outside of the element into the internal cavity (pumping action) is performed. That is, a pumping voltage is applied in such a manner that oxygen is sucked into the internal cavity (in such a manner that oxygen ions move from the outside of the element into the internal cavity), and in correspondence therewith, an oxygen pumping current flows between the inner electrode and the outer electrode.

[0012] There is a tendency that, in this oxygen sucking, the more the amount of the rich fuel gas introduced into the internal cavity, the greater the pumping voltage, and the greater the oxygen pumping current. However, if the fuel enrichment degree of the measured gas is too great, it becomes difficult to suck oxygen from the outside as the pumping voltage increases, and instead, it is possible that so-called blackening in which oxygen is taken away from the solid electrolyte occurs. Blackening is an irreversible phenomenon, and once it occurs, it results in the gas sensor being unusable any more.

[0013] The above blackening has a tendency that, like the gas sensor of which the outer electrode is covered with a ceramic layer as disclosed in Patent Document 1, the greater the diffusion resistance around the outer electrode, the more likely blackening occurs.

[0014] The present application is implemented in view of the above-described problems, and aims to provide a gas sensor that can protect the sensor element when used in a rich fuel atmosphere.

[0015] To solve the above problems, a first aspect of the present application is a gas sensor configured to monitor a predetermined gas component in a measured gas, characterized by comprising: a sensor element composed of a solid electrolyte having oxygen ion conductivity; and a controller that controls the operation of the gas sensor, the sensor element including: at least one internal cavity that communicates with a gas inlet of the measured gas under a predetermined diffusion resistance and is provided with an inner electrode; an extra-cavity pump electrode disposed at a position other than the at least one internal cavity; a reference electrode disposed so as to be able to contact a reference gas; the sensor element further including: at least one electrochemical pump unit configured to be able to take in or release oxygen between 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 extra-cavity pump electrode with a predetermined pump power source; and at least one electrochemical sensor unit configured to generate a potential difference between the inner electrode and the reference electrode corresponding to the oxygen concentration in the corresponding at least one internal cavity, the controller judging whether a judgment target value, which is an index of oxygen intake operation in a judgment target pump unit included in the at least one electrochemical pump unit, exceeds a predetermined threshold value within a predetermined judgment time, and controlling the gas sensor in a basic mode in which the oxygen concentration in the at least one internal cavity is maintained constant by causing the at least one electrochemical pump unit to operate, as long as the judgment target value does not exceed the predetermined threshold value, and in a protection execution mode in which the at least one electrochemical pump unit is protected to prevent excessive oxygen intake operation, in the case where the judgment target value exceeds the predetermined threshold value.

[0016] A second aspect of the present application is based on the gas sensor of the first aspect, characterized in that the predetermined threshold value is a stop threshold value for judging whether the at least one electrochemical pump unit needs to be stopped, and the controller starts the protection execution mode in the case where the judgment target value exceeds the stop threshold value, in which the operation of the at least one electrochemical pump unit is stopped, and monitoring of the potential difference in the electrochemical sensor unit corresponding to the judgment target pump unit in the at least one electrochemical sensor unit is started, and the gas sensor is controlled in the basic mode at the time when the monitored potential difference is lower than a restart threshold value.

[0017] The third aspect of the present application is based on the gas sensor according to the first aspect, characterized in that the prescribed threshold value is a change threshold value for judging whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the control of the application of the pump voltage in the at least one electrochemical pump cell needs to be changed, the controller starts the protection execution mode in a case where the judged value exceeds the change threshold value, in the protection execution mode, the control of the gas sensor is continued after the target value is changed to a value larger than a normal value, and monitoring of a pump cell action value, which is a value of the pump voltage or a value of the current in the at least one electrochemical pump cell including the judged pump cell, is started, at a timing at which the pump cell action value is lower than a return threshold value, the target value is returned to the normal value, and thereby the control of the gas sensor is returned to the basic mode.

[0018] The fourth aspect of the present application is based on the gas sensor according to the first aspect, characterized in that the prescribed threshold value is a change threshold value for judging whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the control of the application of the pump voltage in the at least one electrochemical pump cell needs to be changed, the controller starts the protection execution mode in a case where the judged value exceeds the change threshold value, in the protection execution mode, the control of the gas sensor is continued after the target value is changed to a value larger than a normal value, and monitoring of a pump cell action value, which is a value of the pump voltage or a value of the current in the at least one electrochemical pump cell including the judged pump cell, is started, at a timing at which the pump cell action value is lower than a return threshold value, the target value is returned to the normal value, and thereby the control of the gas sensor is returned to the basic mode, as long as the pump cell action value is not lower than the return threshold value and the judged value does not exceed a stop threshold value larger than the change threshold value, the monitoring of the pump cell action value is continued, in a case where the pump cell action value is not lower than the return threshold value and the judged value further exceeds the stop threshold value, the action of the at least one electrochemical pump cell is stopped, and monitoring of the potential difference in the electrochemical sensor cell corresponding to the judged pump cell in the at least one electrochemical sensor cell is started, at a timing at which the monitored potential difference is lower than a restart threshold value, the control of the gas sensor in the basic mode is restarted.

[0019] The gas sensor according to the fifth aspect of the present application can be any one of the gas sensors according to the first to fourth aspects of the present application, and is characterized in that the judgment target value is a measured value of the potential difference in the electrochemical sensor cell corresponding to the judgment target pump cell among the at least one electrochemical pump cell.

[0020] The gas sensor according to the sixth aspect of the present application can be any one of the gas sensors according to the first to fourth aspects of the present application, and is characterized in that the judgment target value is a value of a pump current when oxygen is taken in by the judgment target pump cell.

[0021] The gas sensor according to the seventh aspect of the present application can be any one of the gas sensors according to the first to sixth aspects of the present application, and is characterized in that the at least one internal cavity is a plurality of internal cavities that are connected in series, the inner electrode is a plurality of inner electrodes that are respectively 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 at least an index of oxygen intake action in an electrochemical pump cell corresponding to an internal cavity closest to the inlet among the plurality of internal cavities, among the plurality of electrochemical pump cells.

[0022] The gas sensor according to the eighth aspect of the present application can be the gas sensor according to the seventh aspect of the present application, and is characterized in that one of the plurality of inner electrodes is a measurement electrode for monitoring the prescribed gas component, the plurality of electrochemical pump cells include a measurement pump cell including the measurement electrode and at least one oxygen concentration control pump cell other than the measurement pump cell, the at least one oxygen concentration control pump cell including any one of the plurality of inner electrodes other than the measurement electrode, the plurality of electrochemical sensor cells include a measurement sensor cell including the measurement electrode and at least one oxygen concentration monitoring sensor cell other than the measurement sensor cell, the at least one oxygen concentration monitoring sensor cell including any one of the plurality of inner electrodes other than the measurement electrode, and the controller determines the concentration of the prescribed gas component based on a measurement pump current that flows between the measurement electrode and the cavity outer pump electrode in the measurement pump cell corresponding to the concentration of the prescribed gas component, at least in the basic mode.

[0023] The ninth aspect of the present application is based on the gas sensor according to the eighth aspect, characterized in that the plurality of internal cavities are a first internal cavity, a second internal cavity, and a third internal cavity that are sequentially connected through the diffusion resistance portion, the plurality of internal 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, the measurement electrode is provided in the third internal cavity, the at least one oxygen concentration control pump unit is a main pump unit that controls the oxygen concentration in the first internal cavity and an auxiliary pump unit that controls the oxygen concentration in the second internal cavity, the at least one oxygen concentration monitoring sensor unit is a main sensor unit configured to generate a potential difference corresponding to the oxygen concentration in the first internal cavity between the main pump electrode and the reference electrode and an auxiliary sensor unit configured to generate a potential difference corresponding to the oxygen concentration in the second internal cavity between the auxiliary pump electrode and the reference electrode, and the controller causes the main pump unit and the auxiliary pump unit to operate in such a manner that the oxygen concentrations in the first internal cavity and the second internal cavity are maintained at predetermined constant values in the basic mode, and determines the concentration of the predetermined gas component based on the size of the measurement pump current that flows through the measurement pump unit in correspondence with the concentration of the predetermined gas component included in the measured gas whose oxygen concentration has been adjusted that is introduced into the third internal cavity.

[0024] The tenth aspect of the present application is a method for operating a gas sensor having a sensor element composed of a solid electrolyte having oxygen ion conductivity and configured to be capable of monitoring a prescribed gas component in a measured gas, characterized in that the sensor element has: at least one internal cavity that communicates with a gas inlet of the measured gas under a prescribed diffusion resistance and is provided with an inner electrode; a cavity-external pump electrode that is disposed at a position other than the at least one internal cavity; a reference electrode that is disposed so as to be capable of contacting a reference gas; at least one electrochemical pump cell configured to be capable of taking in or out oxygen between 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 cavity-external pump electrode with a prescribed pump power supply; and at least one electrochemical sensor cell configured to generate a potential difference between the inner electrode and the reference electrode corresponding to the oxygen concentration in the corresponding at least one internal cavity, in which case, there is a judgment process in which it is judged whether or not a judgment target value that is an index of oxygen intake operation in a judgment target pump cell included in the at least one electrochemical pump cell exceeds a prescribed threshold value in a prescribed judgment time, and the gas sensor is operated in a basic mode in which the oxygen concentration in the at least one internal cavity is maintained constant by causing the at least one electrochemical pump cell to operate, as long as the judgment target value does not exceed the prescribed threshold value in the judgment process, and the gas sensor is operated in a protection execution mode in which the at least one electrochemical pump cell is protected to prevent excessive oxygen intake operation from occurring, in the case where the judgment target value exceeds the prescribed threshold value in the judgment process.

[0025] The eleventh aspect of the present application is based on the method for operating a gas sensor according to the tenth aspect, characterized in that the prescribed threshold value is a stop threshold value for judging whether or not the at least one electrochemical pump cell needs to be stopped, and the protection execution mode is started in the case where the judgment target value exceeds the stop threshold value in the judgment process, in which the operation of the at least one electrochemical pump cell is stopped, and monitoring of the potential difference in the electrochemical sensor cell corresponding to the judgment target pump cell in the at least one electrochemical sensor cell is started, and the gas sensor is operated in the basic mode at the time when the monitored potential difference falls below a restart threshold value.

[0026] The twelfth aspect of the present application is based on the operation method of the gas sensor according to the tenth aspect, characterized in that the prescribed threshold value is a change threshold value for judging whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the application of the pump voltage in the at least one electrochemical pump cell needs to be changed, the protection execution mode is started in a case where the judged value exceeds the change threshold value in the judging step, in the protection execution mode, the operation of the gas sensor is continued after the target value is changed to a value larger than a normal value, and monitoring of a pump cell operation value, which is a value of the pump voltage or a value of the current in the at least one electrochemical pump cell including the judged pump cell, is started, the target value is returned to the normal value at a time when the pump cell operation value is lower than a return threshold value, and thereby the operation of the gas sensor is returned to the basic mode.

[0027] The thirteenth aspect of the present application is based on the operation method of the gas sensor according to the tenth aspect, characterized in that the prescribed threshold value is a change threshold value for judging whether or not a target value of the potential difference in the at least one electrochemical sensor cell corresponding to the application of the pump voltage in the at least one electrochemical pump cell needs to be changed, the protection execution mode is started in a case where the judged value exceeds the change threshold value in the judging step, in the protection execution mode, the operation of the gas sensor is continued after the target value is changed to a value larger than a normal value, and monitoring of a pump cell operation value, which is a value of the pump voltage or a value of the current in the at least one electrochemical pump cell including the judged pump cell, is started, the target value is returned to the normal value at a time when the pump cell operation value is lower than a return threshold value, and thereby the operation of the gas sensor is returned to the basic mode, the monitoring of the pump cell operation value is continued as long as the pump cell operation value is not lower than the return threshold value and the judged value does not exceed a stop threshold value larger than the change threshold value, in a case where the pump cell operation value is not lower than the return threshold value and the judged value further exceeds the stop threshold value, the operation of the at least one electrochemical pump cell is stopped, and monitoring of the potential difference in the electrochemical sensor cell corresponding to the judged pump cell in the at least one electrochemical sensor cell is started, the gas sensor is restarted in the basic mode at a time when the monitored potential difference is lower than a restart threshold value.

[0028] The fourteenth aspect of the present application is characterized in that, in the operation method of the gas sensor according to any one of the tenth to thirteenth aspects, the judgment target value is set to the measured value of the potential difference in the electrochemical sensor cell corresponding to the judgment target pump cell among the at least one electrochemical sensor cell.

[0029] The fifteenth aspect of the present application is characterized in that, in the operation method of the gas sensor according to any one of the tenth to thirteenth aspects, the judgment target value is set to the value of the pump current when oxygen is sucked in the judgment target pump cell.

[0030] Effects of the Invention

[0031] According to the first to fifteenth aspects of the present application, even in the case where the gas sensor is used in an environment where the measured gas is a rich fuel gas having a small air-fuel ratio, it is possible to well avoid the difficulty of sucking oxygen into the internal cavity, which leads to blackening of the solid electrolyte constituting the sensor element. Accordingly, it is possible to prevent the gas sensor from malfunctioning due to use in a rich fuel gas atmosphere. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is a diagram schematically showing an example of the configuration of the gas sensor 100.

[0033] Figure 2 is a diagram showing the operation flow in the first mode of the element protection mode.

[0034] Figure 3 is a diagram showing the operation flow in the second mode of the element protection mode.

[0035] Figure 4 is a diagram showing the operation flow in the third mode of the element protection mode.

[0036] Figure 5 is a diagram showing the operation flow in the third mode of the element protection mode.

[0037] Figure 6 is a diagram schematically showing an example of the configuration of the gas sensor 100B.

[0038] Explanation of Symbols

[0039] 1…first substrate layer, 2…second substrate layer, 3…third substrate layer, 4…first solid electrolyte layer, 5…separation layer, 6…second solid electrolyte layer, 10…gas introduction port, 11…first diffusion rate control section, 13…second diffusion rate control section, 20…first internal cavity, 21…main pump unit, 22…inner pump electrode, 23…outer pump electrode, 24, 46, 52…variable power supply, 30…third diffusion rate control section, 40…second internal cavity, 41…measurement pump unit, 42…reference electrode, 43…reference gas introduction space, 44…measurement electrode, 50…auxiliary pump unit, 51…auxiliary pump electrode, 60…fourth diffusion rate control section, 61…third internal cavity, 70…heater section, 80…main sensor unit, 81…auxiliary sensor unit, 82…measurement sensor unit, 100…gas sensor, 101…sensor element, Ip0…main pump current, Ip1…auxiliary pump current, Ip2…NOx current. DETAILED DESCRIPTION

[0040]

[0041] Figure 1 is a diagram that schematically shows an example of the configuration of a gas sensor 100 according to the present embodiment. The gas sensor 100 is a limit current type NOx sensor that monitors and measures the concentration of NOx using a sensor element 101. In addition, the gas sensor 100 is provided with a controller 110 that controls the operation of each part and determines the NOx concentration based on the NOx current flowing through the sensor element 101. Figure 1 is a vertical cross-sectional view along the length direction of the sensor element 101.

[0042] The sensor element 101 is a ceramic element body in a flat plate shape (long strip plate shape) that has a structure in which six solid electrolyte layers are stacked in the order of a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a separation layer 5, and a second solid electrolyte layer 6, each of which contains zirconium oxide (Zr02) (for example, yttrium-stabilized zirconium oxide (YSZ) or the like) as an oxygen ion-conducting solid electrolyte, from the lower side in the drawing. In addition, the solid electrolytes that form the six layers are dense and airtight solid electrolytes. Hereinafter, the six layers in the sensor element 101 will be referred to as the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the separation layer 5, and the second solid electrolyte layer 6, respectively. Figure 1 The upper side of each of the six layers in the sensor element 101 will be referred to as the upper surface, and the lower side will be referred to as the lower surface. In addition, the entire portion of the sensor element 101 that contains the solid electrolytes will be referred to as the base portion.

[0043] ​The sensor element 101 is manufactured, for example, by performing prescribed processing on ceramic green sheets corresponding to the respective layers and printing of circuit patterns and the like, then laminating them, and further performing firing to integrate them.

[0044] At one end portion of the sensor element 101 and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a first diffusion speed control portion 11 serving also as the gas introduction port 10, a buffer space 12, a second diffusion speed control portion 13, a first internal cavity 20, a third diffusion speed control portion 30, a second internal cavity 40, a fourth diffusion speed control portion 60, and a third internal cavity 61 are formed adjacently in that order so as to communicate sequentially.

[0045] The buffer space 12, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 are internal spaces (regions) of the sensor element 101 provided by hollowing out the separation layer 5, the upper portion of which is demarcated by the lower surface of the second solid electrolyte layer 6, the lower portion of which is demarcated by the upper surface of the first solid electrolyte layer 4, and the side portion of which is demarcated by the side surface of the separation layer 5. Note that the gas introduction port 10 can also be provided by hollowing out the separation layer 5 at the end surface (left end in the drawing) of the sensor element 101, separately from the first diffusion speed control portion 11. In this case, the first diffusion speed control portion 11 is formed adjacently more inward than the gas introduction port 10.

[0046] The first diffusion speed control portion 11, the second diffusion speed control portion 13, the third diffusion speed control portion 30, and the fourth diffusion speed control portion 60 are each provided as two long slits (lengthwise direction of the opening in the direction perpendicular to the drawing). Further, the portion from the gas introduction port 10 to the third internal cavity 61, which is the innermost internal cavity, is also referred to as a gas flow-through portion.

[0047] Further, at a position farther from the end portion than the gas flow-through portion, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the separation layer 5, and at a position demarcated by the side surface of the first solid electrolyte layer 4 on the side portion. For example, the atmosphere is introduced as a reference gas for measuring the concentration of NOx to the reference gas introduction space 43.

[0048] The atmosphere introduction layer 48 is a layer composed of porous alumina, and the reference gas is introduced to the atmosphere introduction layer 48 through the reference gas introduction space 43. Further, the atmosphere introduction layer 48 is formed so as to cover the reference electrode 42.

[0049] The reference electrode 42 is an electrode formed in a manner that it is sandwiched by the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and, as described above, the atmospheric gas introduction layer 48 that communicates with the reference gas introduction space 43 is provided around the reference electrode 42. In addition, as described later, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 and the second internal cavity 40 can be measured using the reference electrode 42.

[0050] In the gas flow passage, the gas introduction port 10 (the first diffusion speed control section 11) is a portion that is opened with respect to the outside space, and the measured gas is introduced into the sensor element 101 from the outside space through the gas introduction port 10.

[0051] The first diffusion speed control section 11 is a portion that applies a prescribed diffusion resistance to the introduced measured gas.

[0052] The buffer space 12 is a space provided to guide the measured gas introduced from the first diffusion speed control section 11 to the second diffusion speed control section 13.

[0053] The second diffusion speed control section 13 is a portion that applies a prescribed diffusion resistance to the measured gas introduced from the buffer space 12 to the first internal cavity 20.

[0054] When the measured gas is introduced from the outside of the sensor element 101 into the first internal cavity 20, the measured gas that is sharply introduced into the inside of the sensor element 101 from the gas introduction port 10 due to the pressure variation of the measured gas of the outside space (pulsation of the exhaust gas pressure in the case where the measured gas is the exhaust gas of an automobile) is not directly introduced into the first internal cavity 20, but is introduced into the first internal cavity 20 after the concentration variation of the measured gas is eliminated by the first diffusion speed control section 11, the buffer space 12, and the second diffusion speed control section 13. Thus, the concentration variation of the measured gas introduced into the first internal cavity 20 reaches a level that can be almost ignored.

[0055] The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the measured gas introduced through the second diffusion speed control section 13. The main pump unit 21 operates to adjust the oxygen partial pressure.

[0056] The main pump unit 21 is an electrochemical pump unit composed of an inner pump electrode (main pump electrode) 22, an outer (cavity-outer) pump electrode 23, and the second solid electrolyte layer 6 sandwiched by the inner pump electrode (main pump electrode) 22 and the outer (cavity-outer) pump electrode 23, wherein the inner pump electrode (main pump electrode) 22 has a top electrode portion 22a provided on substantially the entire surface of the lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, and the outer (cavity-outer) pump electrode 23 is provided on the region of the upper surface (one main surface of the sensor element 101) of the second solid electrolyte layer 6 corresponding to the top electrode portion 22a in a manner exposed to the outside.

[0057] The inner pump electrode 22 is formed in the solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) above and below the first internal cavity 20. Specifically, the top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 constituting the top surface of the first internal cavity 20, and the bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 constituting the bottom surface. The top electrode portion 22a and the bottom electrode portion 22b are connected by the conductive portions provided on the side wall surfaces (inner surfaces) of the partition layer 5 constituting the side wall portions of the first internal cavity 20 (omitted from the drawing).

[0058] The top electrode portion 22a and the bottom electrode portion 22b are provided in a rectangular shape in plan view. However, a configuration in which only the top electrode portion 22a is provided or a configuration in which only the bottom electrode portion 22b is provided can also be adopted.

[0059] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous metal ceramic electrodes. In particular, the inner pump electrode 22 which contacts the gas to be measured is formed using a material in which the reduction ability against the NOx components in the gas to be measured is weakened. For example, as a metal ceramic electrode of Au-Pt alloy containing Au at a content of 0.6 wt% to 1.4 wt% or so and ZrO2, having a porosity of 5% to 40%, and having a thickness of 5 μm to 20 μm. The weight ratio of the Au-Pt alloy and ZrO2 can be Pt:ZrO2 = 7.0:3.0 to 5.0:5.0 or so.

[0060] On the other hand, the outer pump electrode 23 is formed as a metal ceramic electrode of Pt or an alloy thereof and ZrO2 in a rectangular shape in plan view, for example.

[0061] For the main pump unit 21, a desired pump voltage Vp0 is applied between the inner pump electrode 22 and the outer pump electrode 23 by the variable power supply 24, and a main pump current Ip0 is caused to flow in the positive direction or the negative direction between the inner pump electrode 22 and the outer pump electrode 23, whereby oxygen in the first internal cavity 20 can be drawn out to the outside or oxygen from the outside can be drawn into the first internal cavity 20. Further, the pump voltage Vp0 applied between the inner pump electrode 22 and the outer pump electrode 23 in the main pump unit 21 is referred to as the main pump voltage Vp0.

[0062] Further, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, a main sensor unit 80 as an electrochemical sensor unit is constituted by the inner pump electrode 22, the second solid electrolyte layer 6, the separation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42.

[0063] The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 is obtained by measuring the potential difference, i.e., electromotive force V0, between the inner pump electrode 22 and the reference electrode 42 in the main sensor unit 80.

[0064] Further, the controller 110 performs feedback control of the main pump voltage Vp0 so that the electromotive force V0 is constant, whereby the main pump current Ip0 is controlled. Thus, the oxygen concentration in the first internal cavity 20 is maintained at a prescribed constant value.

[0065] The third diffusion rate control unit 30 is a site that applies a prescribed diffusion resistance to the measured gas after the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 is controlled by the action of the main pump unit 21, and that introduces the measured gas into the second internal cavity 40.

[0066] The second internal cavity 40 is provided as a space for further adjusting the oxygen partial pressure in the measured gas introduced by the third diffusion rate control unit 30. The oxygen partial pressure is adjusted by the action of the auxiliary pump unit 50. In the second internal cavity 40, the oxygen concentration of the measured gas can be adjusted with higher accuracy.

[0067] In the second internal cavity 40, the measured gas introduced again by the third diffusion rate control unit 30 after the oxygen concentration (oxygen partial pressure) is adjusted in the first internal cavity 20 is further subjected to adjustment of the oxygen partial pressure by the auxiliary pump unit 50.

[0068] The auxiliary pump unit 50 is an auxiliary electrochemical pump unit composed of an auxiliary pump electrode 51, the outer pump electrode 23 (not limited to the outer pump electrode 23, but any appropriate electrode on the outer side of the sensor element 101), and the second solid electrolyte layer 6. The auxiliary pump electrode 51 has a top electrode portion 51a provided on substantially the entire bottom surface of the second solid electrolyte layer 6 facing the second internal cavity 40.

[0069] The auxiliary pump electrode 51 is disposed in the second internal cavity 40 in the same manner as the inner pump electrode 22 previously disposed in the first internal cavity 20. That is, the top electrode portion 51a is formed with respect to the second solid electrolyte layer 6 constituting the top surface of the second internal cavity 40, and the bottom electrode portion 51b is formed with respect to the first solid electrolyte layer 4 constituting the bottom surface of the second internal cavity 40. Both the top electrode portion 51a and the bottom electrode portion 51b are formed in a rectangular shape in plan view, and are connected (omitted from the drawing) by a conductive portion provided on the side wall surface (inner surface) of the separation layer 5 constituting the two side wall portions of the second internal cavity 40.

[0070] Further, the auxiliary pump electrode 51 is also formed of a material that has reduced reduction ability with respect to NOx components in the measured gas, like the inner pump electrode 22.

[0071] With respect to the auxiliary pump unit 50, under the control of the controller 110, a desired voltage (auxiliary pump voltage) Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23, whereby oxygen in the atmosphere in the second internal cavity 40 can be pumped out to the outside space, or oxygen from the outside space can be pumped into the second internal cavity 40.

[0072] Further, in order to control the oxygen partial pressure in the atmosphere in the second internal cavity 40, an auxiliary sensor unit 81 as an electrochemical sensor unit is composed of the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the separation layer 5, the first solid electrolyte layer 4, and the third substrate layer 3. In the auxiliary sensor unit 81, a potential difference, i.e., electromotive force V1, generated between the auxiliary pump electrode 51 and the reference electrode 42 corresponding to the oxygen partial pressure in the second internal cavity 40 is detected.

[0073] The auxiliary pump unit 50 pumps using a variable power source 52 that controls the voltage based on the electromotive force V1 detected by the above-described auxiliary sensor unit 81. Thereby, the oxygen partial pressure in the atmosphere in the second internal cavity 40 is feedback-controlled to a lower partial pressure that has substantially no influence on the measurement of NOx.

[0074] In addition, at the same time, the auxiliary pump current Ip1 is used to control the electromotive force of the main sensor unit 80. Specifically, the auxiliary pump current Ip1 is input as a control signal to the main sensor unit 80, and controls the electromotive force Vo, whereby the gradient of the oxygen partial pressure in the measured gas introduced into the second internal cavity 40 from the third diffusion rate control section 30 is always constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of about 0.001 ppm or so due to the effects of the main pump unit 21 and the auxiliary pump unit 50.

[0075] The fourth diffusion rate control section 60 is a site that applies a prescribed diffusion resistance to the measured gas whose oxygen concentration (oxygen partial pressure) is controlled in the second internal cavity 40 by the action of the auxiliary pump unit 50, and guides the measured gas to the third internal cavity 61.

[0076] The third internal cavity 61 is provided as a space (measuring internal cavity) that performs a process of measuring the concentration of nitrogen oxides (NOx) in the measured gas introduced through the fourth diffusion rate control section 60. In the third internal cavity 61, the measurement of the NOx concentration is performed by the action of the measurement pump unit 41. The third internal cavity 61 is introduced with the measured gas whose oxygen concentration is adjusted with high precision in the second internal cavity 40, and thus the gas sensor 100 is able to perform a high-precision measurement of the NOx concentration.

[0077] The measurement pump unit 41 is used to measure the NOx concentration of the measured gas introduced into the third internal cavity 61. The measurement pump unit 41 is an electrochemical pump unit composed of a measurement electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a separation layer 5, and a first solid electrolyte layer 4, in which the measurement electrode 44 is provided on the upper surface of the first solid electrolyte layer 4 at a position facing the third internal cavity 61 and separated from the third diffusion rate control section 30.

[0078] The measurement electrode 44 is a porous metal ceramic electrode of a noble metal and a solid electrolyte. For example, it is formed as a metal ceramic electrode of Pt or an alloy of Pt and other noble metals such as Rh and Zr02, 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 cavity 61.

[0079] With respect to the measurement pump unit 41, under the control of the controller 110, oxygen generated by the decomposition of NOx in the atmosphere in the third internal cavity 61 can be sucked out, and the amount of generation thereof can be detected as a pump current Ip2.

[0080] Further, a measurement sensor unit 82 as an electrochemical sensor unit is configured by the second solid electrolyte layer 6, the separation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42, in order to detect the oxygen partial pressure around the measurement electrode 44. The variable power source 46 is feedback controlled based on the electromotive force V2 generated between the measurement electrode 44 and the reference electrode 42, which corresponds to the oxygen partial pressure in the third internal cavity 61, detected by the measurement sensor unit 82.

[0081] The NOx in the measured gas introduced into the third internal cavity 61 is reduced (2NO→N2+O2) by the measurement electrode 44 to generate oxygen. The generated oxygen is pumped by the measurement pump unit 41, and at this time, the voltage (measurement pump voltage) Vp2 of the variable power source 46 is controlled so that the electromotive force V2 detected by the measurement sensor unit 82 is constant. The amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of NOx in the measured gas, and thus the NOx concentration in the measured gas is calculated using the pump current Ip2 in the measurement pump unit 41. Hereinafter, such a pump current Ip2 is also referred to as an NOx current Ip2.

[0082] Further, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to configure an oxygen partial pressure detection mechanism as an electrochemical sensor unit, the electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere can be detected, and thus the concentration of the NOx component in the measured gas can also be obtained.

[0083] Further, an electrochemical sensor unit 83 is configured by the second solid electrolyte layer 6, the separation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42, and the electromotive force Vref can be obtained using the sensor unit 83, and the oxygen partial pressure in the measured gas outside the sensor can be detected using the electromotive force Vref.

[0084] The sensor element 101 further has a heater portion 70 that performs a temperature adjustment function of heating and keeping the sensor element 101 warm, in order to improve the oxygen ion conductivity of the solid electrolyte configuring the base portion.

[0085] The heater portion 70 mainly has a heater electrode 71, a heater member 72, a heater conduction portion 72a, a through-hole 73, a heater insulating layer 74, a pressure release hole 75, and Figure 1 The heater resistance detection conduction portion is omitted in the figure. Further, the heater portion 70 is buried in the base portion of the sensor element 101 except for the heater electrode 71.

[0086] The heater electrode 71 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).

[0087] The heater member 72 is a resistance heating body provided between the second substrate layer 2 and the third substrate layer 3. From Figure 1 The unillustrated heater power source provided outside the sensor element 101 is omitted from illustration, but the heater member 72 is supplied with power to generate heat by the heater electrode 71, the through-hole 73, and the heater conduction portion 72a as a power supply path. The heater member 72 is formed of Pt or mainly composed of Pt. The heater member 72 is embedded in a prescribed range of the side of the sensor element 101 having the gas flow-through portion, in a manner so as to oppose the gas flow-through portion in the element thickness direction. The heater member 72 is provided so as to have a thickness of about 10 to 20 μm.

[0088] With the sensor element 101, a current is passed through the heater electrode 71 to the heater member 72, whereby the heater member 72 generates heat, so that each portion of the sensor element 101 can be heated to a prescribed temperature and kept at the temperature. Specifically, the sensor element 101 is heated so that the temperature of the solid electrolyte and the electrodes in the vicinity of the gas flow-through portion reaches about 700 to 900°C. The oxygen ion conductivity of the solid electrolyte constituting the base portion in the sensor element 101 is improved by such heating. Further, the heating temperature of the heater member 72 when the gas sensor 100 is used (when the sensor element 101 is driven) is referred to as the sensor element driving temperature.

[0089] The degree of heat generation (heater temperature) of the heater member 72 is grasped by the magnitude of the resistance value (heater resistance) of the heater member 72.

[0090] Note that, although Figure 1 In the sensor element 101, an electrode protection layer covering the outside pump electrode 23 can be provided on the side of one main surface for the purpose of protecting the outside pump electrode 23, although illustration is omitted.

[0091] Further, a single-layer or multi-layer porous layer, i.e., a heat shock resistance protective layer, which covers the sensor element 101, can be further provided on the outer periphery of a prescribed range on the side of one end portion (the left end portion in the drawing) of the sensor element 101. The heat shock resistance protective layer is provided to prevent cracks from occurring in the sensor element 101 due to heat shock accompanying condensation of moisture contained in the measured gas adhering to the sensor element 101 during use of the gas sensor 100, and to prevent a poisoning substance mixed in the measured gas from entering the inside of the sensor element 101. Further, a scheme in which a layered gap (gap layer) is formed between the sensor element 101 and the heat shock resistance protective layer can also be adopted.

[0092] Further, the sensor element 101 is housed in a metal housing member (housing) not shown in the drawing in a manner such that the side of the gas introduction port 10 and the side of the reference gas introduction space 43 are hermetically sealed. The sensor element 101 and the housing member constitute a main body portion of the gas sensor 100. Further, the main body portion is attached to a use site such as an engine exhaust pipe during actual use of the gas sensor 100. Further, wires for electrically connecting the respective portions of the sensor element 101 inside the housing member are drawn from the housing member, and these wires are appropriately connected to the controller 110, various power sources, and the like.

[0093] <Operation in Normal Mode>

[0094] When the concentration of NOx is measured in the gas sensor 100 having the above-described configuration, the main pump unit 21 and the auxiliary pump unit 50 are operated to perform feedback control to make the oxygen concentration in the first internal cavity 20 and the second internal cavity 40 constant, and the measured gas having a constant oxygen concentration is introduced into the third internal cavity 61 and reaches the measurement electrode 44. For example, in the case where the measured gas is lean fuel atmosphere, the measured gas in which the oxygen partial pressure is sufficiently reduced to a degree (for example, 0.0001 ppm to 1 ppm) at which the measurement of NOx is substantially not affected is introduced into the third internal cavity 61.

[0095] Then, at the measurement electrode 44, the NOx in the measured gas that has reached there is reduced to thereby generate oxygen. The oxygen is sucked by the measurement pump unit 41, and the NOx current Ip2 flowing at the time of the suction has a constant functional relationship (hereinafter referred to as sensitivity characteristic) with the concentration of NOx in the measured gas.

[0096] Before the gas sensor 100 is actually used, the above-described sensitivity characteristics are determined using a plurality of sample gases of which the NOx concentration is known in advance, and the data thereof are stored in the controller 110. Also, in the actual use of the gas sensor 100, a signal indicating the value of the NOx current Ip2 that flows in correspondence with the NOx concentration in the gas to be measured is supplied to the controller 110 at all times. In the controller 110, the NOx concentration is sequentially calculated on the basis of the value and the determined sensitivity characteristics, and is output as a NOx sensor detection value. Accordingly, in the gas sensor 100, the NOx concentration in the gas to be measured can be grasped substantially in real time.

[0097] In the present embodiment, the above-described operation of the gas sensor 100 related to the determination of the NOx concentration is referred to as the operation of the gas sensor 100 in the normal mode.

[0098] Note that the target values of the electromotive forces V0, V1, and V2 in the main sensor unit 80, the auxiliary sensor unit 81, and the measurement sensor unit 82 at the time of feedback control of the main pump unit 21, the auxiliary pump unit 50, and the measurement pump unit 41 in the above-described normal mode can be appropriately set in accordance with the configuration, dimensions, and the like of the respective portions of the sensor element 101, the use conditions and the use mode of the gas sensor 100, and the like, but in the following, as an example of a proposed scheme, a case where the above-described target values of the electromotive forces V0, V1, and V2 are set to 250 mV, 385 mV, and 400 mV, respectively, is assumed. The above-described values are values that are substantially set as standards in the case where the oxygen-ion-conductive solid electrolyte that constitutes the sensor element 101 is zirconia.

[0099] <Operation in the element protection mode>

[0100] Assumption: The gas sensor 100 according to the present embodiment mainly operates in the above-described normal mode in a state where the gas to be measured contains relatively sufficient oxygen, such as a lean fuel atmosphere, that is, the concentration of NOx in the gas to be measured is determined.

[0101] More specifically, when the gas sensor 100 operates in the normal mode, the main pump unit 21 operates in such a manner that the electromotive force V0 generated by the main sensor unit 80 reaches a prescribed value corresponding to a desired value as the oxygen concentration value (or the oxygen partial pressure value) in the first internal cavity 20, at this time, the oxygen concentration in the gas to be measured introduced from the outside space into the first internal cavity 20 changes at all times, and thus, in the main pump unit 21, the oxygen can be sucked out or sucked in.

[0102] On the other hand, the auxiliary pump unit 50 and the measurement pump unit 41 are also capable of taking in oxygen from a constitutional aspect, but the set values of the electromotive force VI in the auxiliary sensor unit 81 and the electromotive force V2 in the measurement sensor unit 82 as control target values when each pump unit is operated are set on the premise of taking out oxygen in the principle of measurement of the NOx concentration. That is, when the gas sensor 100 is operated in the normal mode, the auxiliary pump unit 50 and the measurement pump unit 41 exclusively take out oxygen.

[0103] However, the gas sensor 100 is not necessarily used in an atmosphere containing sufficient oxygen, and the main body portion of the gas sensor 100 is mounted to, for example, an exhaust passage of a gasoline engine to take exhaust gas from the engine as a measurement target gas, and the like, and is sometimes used in an environment where the atmospheric gas can be a rich fuel gas with a small air-fuel ratio. In this case, the measurement target gas introduced into the inside of the sensor element 101 is also a rich fuel gas. At this time, the main pump unit 21 ensures the oxygen concentration value in the first internal cavity 20 by taking in oxygen from the outside.

[0104] However, if the fuel in the measurement target gas introduced into the inside of the sensor element 101 is excessively rich, even if the main pump voltage Vp0 applied to the main pump unit 21 is increased, oxygen corresponding to the main pump voltage Vp0 cannot be taken in from the outside, and a situation in which the oxygen concentration in the first internal cavity 20 cannot be controlled to a target value can occur. Furthermore, an adverse situation in which blackening in which oxygen is taken out of the solid electrolyte constituting the main pump unit 21 occurs, and the gas sensor 100 no longer functions can occur.

[0105] Note that, for the above situation in which oxygen cannot be well taken in from the outside, the more the speed of the inflow and outflow of the atmospheric gas around the outer side pump electrode 23, which is a site at which oxygen is introduced from the outside space, is controlled by a prescribed diffusion resistance, like the gas sensor disclosed in Patent Document 1, the more likely the situation is to occur. Figure 6 is a diagram schematically showing one example of the constitution of the gas sensor 100B as one scheme of the above-described gas sensor 100. The gas sensor 100B further has a ceramic layer 7 and a porous body region 8 above the second solid electrolyte layer 6, and has the same constitution as the above-described gas sensor 100 except for this. Figure 1The gas sensor 100 shown is a general constitution. The porous body region 8 is provided with a porous body (for example, alumina or the like) having a porosity of about 30% to 60% so as to cover the outer side pump electrode 23 and be exposed at both ends in the element width direction, not shown. The ceramic layer 7 is provided with a ceramic (for example, zirconia, alumina or the like) densified to the same degree as the second solid electrolyte layer 6 or the like so as to cover the entire upper surface of the second solid electrolyte layer 6 including the porous body region 8. In this gas sensor 100B, the diffusion resistance applied by the porous body region 8 controls the speed of entry and exit of the ambient gas around the outer side pump electrode 23.

[0106] Based on the above, the gas sensor 100 according to the present embodiment can perform operation in the element protection mode that avoids a condition in which the oxygen concentration cannot be controlled and protects the sensor element 101 when the fuel in the measured gas is excessively rich.

[0107] The element protection mode has three modes that differ in the processing sequence. Hereinafter, the three modes will be described in order.

[0108] (First Mode)

[0109] Figure 2 is a diagram showing the flow of operation in the first mode of the element protection mode. The first mode is a method in which, in the case where the measured gas is an atmosphere in which the fuel is excessively rich, the NOx measurement operation of the gas sensor 100 including the intake operation that takes in oxygen from the outside is temporarily stopped, whereby blackening of the sensor element 101 is avoided.

[0110] In this mode, first, the gas sensor 100 is set to start operation in the element protection mode (step S1-1). This setting is achieved, for example, by the user (operator) of the gas sensor giving an appropriate setting instruction to the controller 110 through a prescribed interface, not shown. Alternatively, the gas sensor 100 can be set to always operate in the element protection mode.

[0111] After the element protection mode is started, basically, the NOx concentration measurement is also continuously performed under the control of the controller 110 in the same way as in the normal mode. The operation mode in which the NOx concentration measurement is performed in the same way as in the normal mode even when the element protection mode is being executed is particularly referred to as the basic mode. Note that the operation in the basic mode is sometimes referred to as the operation in the basic mode including the operation in the normal mode. However, when the element protection mode is started, the controller 110 starts monitoring of a prescribed judgment object value (step S1-2) at the same time as the operation in the basic mode.

[0112] In this mode, the judgment target value refers to a value that is an index for judging the stop of the NOx measurement operation. Specifically, the measured value of the electromotive force Vo is used. In the case where the oxygen concentration of the measured gas introduced into the first internal cavity 20 is lower than the target oxygen concentration of the first internal cavity 20, oxygen is taken in by the main pump unit 21 in order to maintain the electromotive force Vo at the target value, but in the case where the measured gas introduced into the fuel is excessively rich, oxygen cannot be sufficiently taken in, and the measured value of the electromotive force Vo deviates from the target value (is greater than the target value). The value of the electromotive force Vo at the limit at which this deviation is permitted is set in advance as the stop threshold value. The stop threshold value is set to, for example, 350 mV.

[0113] The monitoring of the judgment target value is continued until a predetermined time (judgment time) set in advance has elapsed (ends) (step S1-3). The judgment time is set to, for example, about 10 seconds.

[0114] Then, at the time when the judgment time has elapsed (ends) (Yes in step S1-3), the controller 110 judges whether or not the judgment target value exceeds the predetermined stop threshold value during the judgment time (step S1-4). Alternatively, it can be a mode in which it is judged whether or not the judgment target value at the end of the judgment time exceeds the predetermined stop threshold value.

[0115] In the case where the judgment target value does not exceed the predetermined stop threshold value during the judgment time (No in step S1-4), the controller 110 causes the measurement operation of the gas sensor 100 to continue (step S1-5). As for the measurement operation, the element protection mode can be ended, and the normal mode can be continued, or the element protection mode can be restarted, and the basic mode can be continued.

[0116] On the other hand, in the case where the judgment target value exceeds the predetermined stop threshold value during the judgment time (Yes in step S1-4), the controller 110 stops the pump control (feedback control) of the main pump unit 21, the auxiliary pump unit 50, and the measurement pump unit 41 (step S1-6), and enters the protection execution mode. This means that the measurement operation of NOx in the gas sensor 100 is stopped.

[0117] Accordingly, the measured gas introduced from the gas introduction port 10 directly enters the third internal cavity 61 from the first internal cavity 20 via the second internal cavity 40, but since none of the main pump unit 21, the auxiliary pump unit 50, and the measurement pump unit 41 is operating, a situation in which an excessively large pump voltage is applied in order to take in oxygen is avoided.

[0118] When the pump control of each pump unit is stopped, the measured values of the electromotive forces V0, V1, and V2 in the main sensor unit 80, the auxiliary sensor unit 81, and the measurement sensor unit 82, which were controlled to reach the prescribed constant values corresponding to the desired oxygen concentration, are values corresponding to the oxygen concentration of the measured gas flowing into the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61, which is not adjusted. The electromotive forces generated in each sensor unit in the state where the above-described pump control is stopped are particularly referred to as OPEN electromotive forces.

[0119] The OPEN electromotive force varies in correspondence with the oxygen concentration of the measured gas flowing into each internal cavity, and the greater the oxygen concentration, the greater the value. Therefore, each OPEN electromotive force can be used as an index indicating the magnitude of the oxygen concentration of the measured gas present in the internal cavity corresponding to each sensor unit in the state where the pump control is stopped.

[0120] The controller 110 stops the pump control in each pump unit, and starts monitoring the above-described OPEN electromotive force (step S1-7), and judges whether or not the OPEN electromotive force is lower than a prescribed restart threshold value (step S1-8). At this time, it can be a manner of monitoring the OPEN electromotive force in at least one sensor unit (for example, the main sensor unit 80) and judging the magnitude relationship with the restart threshold value. During the period where the OPEN electromotive force is not lower than the restart threshold value (No in step S1-8), the monitoring of the OPEN electromotive force is continued.

[0121] The restart threshold value is set to a value capable of judging that the oxygen concentration of the measured gas flowing into each internal cavity is increased to a degree where, at the stage where the OPEN electromotive force reaches the value, the intake or discharge of oxygen in each pump unit can be performed without any problem even if the pump control in each pump unit is restarted. For example, the restart threshold value can be set based on a correspondence relationship (functional relationship) between the air-fuel ratio of the measured gas and the OPEN electromotive force value, which is determined in advance through experiments.

[0122] For example, in the case of exhaust gas of a gasoline engine as the measured gas, even if exhaust gas in which fuel is excessively rich flows into the inside of the sensor element 101 at some timing, since this inflow is not permanent in general, the oxygen concentration of the exhaust gas returns to a degree where each pump unit operates well after a certain period of time.

[0123] In the case where the OPEN electromotive force in the main sensor unit 80 is the monitoring target, the restart threshold value is set to 450 mV, which is an example of a preferable value. It has been confirmed in advance that, in the case where the OPEN electromotive force is 450 mV or less, the atmosphere inside the first internal cavity 20 is stoichiometric composition or lean fuel composition.

[0124] In the case where it is determined that the open electromotive force is lower than the prescribed restart threshold (Yes in step S1-8), the controller 110 restarts the pump control operation that was stopped before (step S1-9). That is, the target values of the electromotive forces V0, V1, and V2 in the main sensor unit 80, the auxiliary sensor unit 81, and the measurement sensor unit 82 are respectively reset in such a manner that the oxygen concentrations in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 reach desired values, and the pump units are operated again to achieve the above target values.

[0125] Finally, when it becomes a state where feedback control based on the target values of the electromotive forces V0, V1, and V2 can be performed in the pump units, the measurement operation of NOx is restarted (step S1-10). For the measurement operation, the element protection mode can be ended to continue in the normal mode, or can be restarted to continue in the basic mode.

[0126] As such, in the present mode, in the case where the gas in which the fuel is excessively rich is introduced into the sensor element 101 as the measurement gas, in order to avoid a situation where the oxygen concentration cannot be controlled, the measurement of NOx is temporarily stopped by stopping the operation of the pump units. Then, at the time when the oxygen concentration of the introduced measurement gas is determined to have recovered to a degree where the intake of oxygen can be performed by the pump units, the measurement of NOx is restarted by restarting the operation of the pump units. Thereby, it is possible to prevent the occurrence of the excessive oxygen intake operation, thereby protecting the pump units, and further, it is possible to well avoid the occurrence of blackening in the sensor element 101. That is, the sensor element 101 is properly protected.

[0127] (Second Mode)

[0128] Figure 3 is a diagram showing the operation flow in the second mode of the element protection mode. The second mode is a method in which, in the case where the measurement gas is an atmosphere in which the fuel is excessively rich, excessive intake in the main pump unit 21 is suppressed by temporarily lowering the target oxygen concentration in the first internal cavity 20, thereby continuing the measurement of NOx, and blackening of the sensor element 101 is avoided.

[0129] Steps S2-1 to S2-5 of the second mode are substantially the same as steps S1-1 to S1-5 of the first mode, respectively. In addition, the determination target value is the measured value of the electromotive force V0, which is the same as the first mode.

[0130] However, in the present mode, the judged value is used as an index when judging the change in the target oxygen concentration in the first internal cavity 20. More specifically, it is used as an index when changing the target value of the electromotive force Vo of the main sensor unit 80 corresponding to the target oxygen concentration. Note that there is a relationship in which the greater the target value of the electromotive force Vo, the smaller the target oxygen concentration in the first internal cavity 20.

[0131] That is, in the present mode, the value of the electromotive force Vo at the limit at which the deviation of the measured value of the electromotive force Vo is permitted when the measured gas in which the fuel is excessively rich is introduced into the first internal cavity 20 and the measured value deviates from the target value is set in advance as the change threshold value. Then, the controller 110 starts the element protection mode (basic mode) as in the first mode (step S2-1), and after starting the monitoring of the judged value (step S2-2), at the time when the judged time has elapsed (Yes in step S2-3), it judges whether or not the judged value exceeds the prescribed change threshold value during the judged time (step S2-4). The change threshold value can be the same as the stop threshold value in the first mode, or it can be different.

[0132] In the case where the judged value does not exceed the prescribed change threshold value during the judged time (No in step S2-4), as in the first mode, the controller 110 causes the measurement operation in the gas sensor 100 to continue in the normal mode or the element protection mode (basic mode) (step S2-5).

[0133] On the other hand, in the case where the judged value exceeds the prescribed change threshold value during the judged time (Yes in step S2-4), in the present mode as well, the basic mode is entered into the protection execution mode. However, in the present mode, the controller 110 changes the control reference value in the pump control at the time of NOx concentration measurement with respect to the value (normal value) at the time of the basic mode (normal mode) (step S2-6).

[0134] Specifically, the controller 110 performs a change in which at least the target value of the electromotive force Vo of the main sensor unit 80, which is the reference for the pump control, in the main pump unit 21 is made greater than the normal value (and further, greater than the change threshold value). For example, the target value of the electromotive force Vo is changed from 250 mV at the time of the normal mode to 350 mV. The target value of the electromotive force Vo after the change is referred to as the changed reference value. By changing the target value of the electromotive force Vo in this way, the target oxygen concentration in the first internal cavity 20 is decreased. In addition to this, the control reference value at the time of the pump control in the auxiliary pump unit 50 and the measurement pump unit 41 can also be changed with respect to the value at the time of the normal mode.

[0135] Then, the controller 110 starts the control of the pump units based on the changed reference value. That is, in the case of the present embodiment, even in the protection execution mode, the measurement of NOx is continued (step S2-7).

[0136] By making the target value of the electromotive force Vo the changed reference value which is larger than the normal value, the target oxygen concentration in the first internal cavity 20 is lowered so that the difference between the target oxygen concentration and the oxygen concentration in the measured gas which is excessively rich in fuel entering the first internal cavity 20 is smaller than in the case of the normal mode. Thus, in the case where the pump units are controlled based on the changed reference value, the amount of oxygen which needs to be taken into the first internal cavity 20 by the main pump unit 21 in order to achieve the target oxygen concentration is reduced as compared with before the change. As a result, excessive increase of the main pump current Ip0 or the main pump voltage Vp0 in the main pump unit 21 and blackening are suppressed.

[0137] Also, in the case of the first embodiment, when the judged value exceeds the stop threshold value and enters the protection execution mode, the measurement of NOx is stopped, but in the case of the present embodiment, the pump control action itself for the measurement of NOx is continued even in the protection execution mode, and therefore, it is possible to avoid occurrence of a blank in the measurement.

[0138] However, the changed reference value is preferably set within a range in which an increase in the pumping burden in the auxiliary pump unit 50 or the measurement pump unit 41 or a decrease in the measurement accuracy of NOx does not occur significantly.

[0139] Simultaneously with the start of the measurement based on the changed reference value, the controller 110 starts the monitoring of the pump unit action value (step S2-8), and judges whether or not the pump unit action value is lower than the return threshold value (step S2-9).

[0140] The pump unit action value is specifically the main pump current Ip0 or the main pump voltage Vp0. The pump unit action value is a value which is larger the more the amount of oxygen taken in. Also, the return threshold value is set to a value at which it is possible to judge that the oxygen concentration of the measured gas flowing into each internal cavity is increased to such a degree that even if the control reference value is returned from the changed reference value to the normal value, the taking in or taking out of oxygen in each pump unit can be performed without any problem.

[0141] As long as the pump unit action value is not lower than the return threshold value (No in step S2-9), the monitoring of the pump unit action value is continued.

[0142] On the other hand, in a case where it is determined that the pump unit operation value is lower than the prescribed return 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 control reference value (step S2-11). As for the measurement operation, the element protection mode can be ended to continue in the normal mode, or the element protection mode can be restarted to continue in the basic mode.

[0143] In this way, in the present mode, in a case where the fuel excessively rich gas is introduced into the sensor element 101 as the measurement gas, the excessive intake in the main pump unit 21 is suppressed by temporarily lowering the target oxygen concentration in the first internal cavity 20. Then, at the time when the oxygen concentration of the introduced measurement gas is determined to have recovered to a degree at which the oxygen intake by the pump unit is possible, the target oxygen concentration in the first internal cavity 20 is restored. Thereby, it is possible to well avoid that the main pump voltage Vp0 increases to a degree at which the oxygen intake is not possible, and even the sensor element 101 is blackened. That is, the sensor element 101 is properly protected. Further, unlike the first mode, the measurement of the NOx concentration is not interrupted.

[0144] (Third Mode)

[0145] Figure 4 and Figure 5 is a diagram showing the operation flow in the third mode of the element protection mode. The third mode is a method in which the first mode and the second mode are combined, and the response in the protection execution mode when the measurement gas is the fuel excessively rich atmosphere is performed in 2 stages according to the degree, thereby avoiding the blackening of the sensor element 101.

[0146] Briefly, in a case where the measurement gas is the fuel excessively rich atmosphere, first, as in the second mode, the excessive intake in the main pump unit 21 is suppressed by temporarily lowering the target oxygen concentration in the first internal cavity 20, and even if the oxygen concentration in the first internal cavity 20 does not sufficiently recover, as in the first mode, the NOx measurement operation in the gas sensor 100 including the intake operation of the oxygen intake from the outside is temporarily stopped, thereby avoiding the blackening due to the increase in the main pump voltage Vp0.

[0147] The steps S3-1 to S3-7 of this third mode are the same as the steps S2-1 to S2-7 of the second mode, respectively. That is, in the third mode, first, as in the second mode, the controller 110 starts the element protection mode (basic mode) (step S3-1), and after starting the monitoring of the judgment target value (step S3-2), at the time when the judgment time has elapsed (Yes in step S3-3), it judges whether or not the judgment target value has exceeded the prescribed change threshold value during the judgment time (step S3-4). Note that in this mode, the judgment target value is specifically the actually measured value of the electromotive force V0.

[0148] In the case where the judgment target value has not exceeded the prescribed change threshold value during the judgment time (No in step S3-4), as in the first and second modes, the controller 110 continues the measurement operation in the gas sensor 100 in the normal mode or the element protection mode (basic mode) (step S3-5).

[0149] On the other hand, in the case where the judgment target value has exceeded the prescribed change threshold value during the judgment time (Yes in step S3-4), in this mode as well, the protection execution mode is entered from the basic mode. The controller 110 changes the control reference value in the pump control during the NOx concentration measurement with respect to the value (normal value) at the time of the basic mode (normal mode) (step S3-6). Then, based on the changed reference value, the control of each pump unit is performed, and the measurement of NOx is continued (step S3-7). That is, the measurement of NOx is continued in a state where the amount of oxygen that needs to be taken in by the main pump unit 21 into the first internal cavity 20 in order to achieve the target oxygen concentration is reduced.

[0150] In addition, along with the change of the control reference value, the monitoring of the pump unit operation value is started (step S3-8), and in the case where it is judged that the pump unit operation value is below the prescribed return threshold value (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 this control reference value is continued in the normal mode or the element protection mode (basic mode) (step S3-11), as in the second mode as well.

[0151] On the other hand, in the case where it is judged that the pump unit operation value is not below the return threshold value (No in step S3-9), it is judged whether or not the judgment target value has exceeded the prescribed stop threshold value (step S3-12). Note that in this mode, the stop threshold value is set to a value larger than the change threshold value.

[0152] In the case where the judgment target value does not exceed the stop threshold value (No in step S3-12), the processing returns to step S3-8, and the monitoring of the pump unit operation value is restarted, and the processing after step S3-9 is performed again. Therefore, in the case where the pump unit operation value is equal to or higher than the prescribed return threshold value and the judgment target value is equal to or lower than the stop threshold value, the cycle of step S3-8→step S3-9→step S3-12→step S3-8→... is repeated. This means that, in the case where the excessive intake of oxygen into the sensor element 101 by the introduction of the rich fuel gas into the sensor element 101 causes the main pump unit 21 to excessively suck oxygen to the extent that can be coped with by lowering the target oxygen concentration in the same manner as in the second mode, the coping is continued in that manner.

[0153] In the case where the judgment target value exceeds the stop threshold value after the control reference value is changed (Yes in step S3-12), the same sequence as in the first mode is performed. The intention of performing this sequence is to more reliably achieve the protection of the pump units in the case where the lowering of the target oxygen concentration alone cannot sufficiently perform the protection.

[0154] Specifically, first, the controller 110 stops the pump control (feedback control) in the main pump unit 21, the auxiliary pump unit 50, and the measurement pump unit 41 (step S3-13). By this, the NOx measurement operation in the gas sensor 100 that has been continued based on the changed reference value is stopped.

[0155] The controller 110 further starts the monitoring of the OPEN electromotive force (step S3-14), and judges whether or not the OPEN electromotive force is lower than a prescribed restart threshold value (step S3-15). In the case where the OPEN electromotive force in the main sensor unit 80 is the monitoring target, the restart threshold value is preferably set to 450 mV as one example. During the period in which the OPEN electromotive force is not lower than the restart threshold value (No in step S3-15), the monitoring of the OPEN electromotive force is continued.

[0156] In the case where the OPEN electromotive force is judged to be lower than the restart threshold value (Yes in step S3-15), the controller 110 re-sets the control reference value to the normal value, and restarts the pump control operation that has been stopped (step S3-16). Finally, when the state in which the feedback control based on the electromotive forces V0, V1, and V2 can be performed in each pump unit is achieved, the NOx measurement operation in the normal mode or the element protection mode (basic mode) is restarted (step S3-17).

[0157] In this way, in the present embodiment, the first and second modes are combined, and the response method in the protection execution mode is switched in stages depending on the degree of fuel enrichment of the measured gas introduced into the sensor element 101. This controls the conditions under which NOx measurement is stopped to a minimum, and enables the occurrence of blackening to be prevented in cases where fuel-enriched measured gas is introduced into the inside of the sensor element 101.

[0158] As explained above, according to the present embodiment, even in cases where the gas sensor is used in an environment where the measured gas, such as exhaust gas from a gasoline engine, is likely to be a fuel-enriched gas with a small air-fuel ratio, it is possible to prevent blackening of the solid electrolyte constituting the sensor element from occurring due to the oxygen being difficult to be taken into the internal cavity. This enables the gas sensor to be prevented from malfunctioning due to use in a fuel-enriched gas atmosphere.

[0159] <Modification Example>

[0160] In the above-described embodiment, the measured value of the electromotive force V0 is used as the judgment target value, but instead, a scheme can be adopted in which the value of the main pump current Ip0 at the time when oxygen is taken in by the main pump unit 21 is used as the judgment target value. This main pump current Ip0 increases in correspondence with the amount of oxygen taken in toward the first internal cavity 20, and thus can serve as an index of the degree of oxygen intake toward the first internal cavity 20. The stop threshold in the first mode and the change threshold in the second and third modes are set in accordance with the judgment target value used.

[0161] In addition, in the basic mode of the normal mode and the element protection mode, the gas sensor 100 can be caused to operate in a control mode in which Ip1 is controlled to be constant, that is, the auxiliary pump unit 50 is controlled in such a manner that a constant amount of auxiliary pump current Ip1 flows, and in this case, the measured value of the auxiliary pump current Ip1 can be used instead of or in combination with the measured value of the electromotive force V0 as the judgment target value in the protection execution mode of the element protection mode. The oxygen concentration in the measured gas introduced from the first internal cavity 20 into the second internal cavity 40 is maintained at a predetermined value, and the auxiliary pump current Ip1 when the auxiliary pump unit 50 takes in oxygen is maintained at a constant value set in advance, whereby the oxygen concentration in the second internal cavity 40 can be controlled to be the predetermined value. However, if fuel-enriched measured gas is introduced into the first internal cavity 20, causing the oxygen concentration in the first internal cavity 20 to be unable to be adjusted, and oxygen-poor measured gas enters the second internal cavity 40, the auxiliary pump unit 50 is unable to take in oxygen from the second internal cavity 40, the measured value of the electromotive force V1 increases with respect to the constant value, and the measured value of the auxiliary pump current Ip1 decreases with respect to the constant value set. Therefore, the measured value of the auxiliary pump current Ip1 can also be used as the judgment target value.

[0162] In the above-described embodiments, the gas sensor is targeted at a case where the sensor element has three cavities inside, but the configuration of the sensor element that can be blackened by the introduction of a measured gas that is excessively rich in fuel into the inside of the sensor element is not limited to the configuration of the above-described embodiments.

[0163] In addition, there are cases where the monitoring target component in the measured gas is set to a component other than NOx. In such a case, even in the normal mode, the oxygen intake operation can sometimes be dominant.

[0164] In any case, as long as the gas sensor has a sensor element having an internal cavity in which the oxygen concentration is kept constant by the intake or discharge of oxygen by an electrochemical pump unit, the above-described first to third modes can be appropriately modified and applied in correspondence with the element configuration as needed, thereby enabling the solid electrolyte constituting the sensor element to be prevented from being blackened. That is, it is possible to prevent the gas sensor including the sensor element from malfunctioning.

Claims

1. A gas sensor configured to be capable of monitoring a prescribed gas component in a measured gas, the gas sensor characterized by comprising: a sensor element composed of a solid electrolyte of oxygen ion conductivity; and a controller that controls the operation of the gas sensor, the sensor element comprising: at least one internal cavity that communicates with a gas inlet of the measured gas under a prescribed diffusion resistance and is provided with an inner electrode; an extra-cavity pump electrode that is disposed at a position other than the at least one internal cavity; a reference electrode that is disposed so as to be capable of contacting a reference gas; the sensor element further comprising: at least one electrochemical pump unit configured to be capable of taking in or out oxygen between 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 extra-cavity pump electrode with a prescribed pump power supply; and at least one electrochemical sensor unit configured to generate a potential difference between the inner electrode and the reference electrode corresponding to the oxygen concentration in the corresponding at least one internal cavity, the controller judging whether or not a judgment target value, which is an index of oxygen intake operation in a judgment target pump unit included in the at least one electrochemical pump unit, exceeds a prescribed threshold value in a prescribed judgment time, controlling the gas sensor in a basic mode in which the oxygen concentration in the at least one internal cavity is maintained constant by causing the at least one electrochemical pump unit to operate, as long as the judgment target value does not exceed the prescribed threshold value, controlling the gas sensor in a protection execution mode in which the at least one electrochemical pump unit is protected from excessive oxygen intake operation, in the case where the judgment target value exceeds the prescribed threshold value, the prescribed threshold value being a change threshold value for judging whether or not a target value of the potential difference in the corresponding at least one electrochemical sensor unit needs to be changed when controlling the application of the pump voltage in the at least one electrochemical pump unit, the controller starting the protection execution mode in the case where the judgment target value exceeds the change threshold value, in which the target value is changed to a value larger than a normal value and the control of the gas sensor is continued, and starting monitoring of a pump unit operation value, i.e., a value of the pump voltage or a value of the current in the at least one electrochemical pump unit including the judgment target pump unit, returning the target value to the normal value at a time when the pump unit operation value is lower than a return threshold value, thereby returning the control of the gas sensor to the basic mode.

2. A gas sensor configured to be capable of monitoring a prescribed gas component in a measured gas, the gas sensor characterized by comprising: a sensor element composed of a solid electrolyte of oxygen ion conductivity; and a controller that controls the operation of the gas sensor, the sensor element has: at least one internal cavity that communicates with an inlet for the gas to be measured at a prescribed diffusion resistance, and that is provided with an inner electrode; a cavity-external pump electrode that is disposed at a position other than the at least one internal cavity; a reference electrode that is disposed so as to be able to contact a reference gas; the sensor element further has: at least one electrochemical pump unit that is configured to be able to take in or out oxygen between 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 cavity-external pump electrode with a prescribed pump power supply; and at least one electrochemical sensor unit that is configured to generate a potential difference between the inner electrode and the reference electrode that corresponds to the oxygen concentration in the corresponding at least one internal cavity, the controller judges whether a judgment target value, which is an index of oxygen intake operation in a judgment target pump unit included in the at least one electrochemical pump unit, exceeds a prescribed threshold value within a prescribed judgment time, as long as the judgment target value does not exceed the prescribed threshold value, the gas sensor is controlled in a basic mode in which the oxygen concentration in the at least one internal cavity is maintained constant by causing the at least one electrochemical pump unit to operate, in the case where the judgment target value exceeds the prescribed threshold value, the gas sensor is controlled in a protection execution mode in which the at least one electrochemical pump unit is protected from excessive oxygen intake operation, the prescribed threshold value is a change threshold value for judging whether a target value of the potential difference in the corresponding at least one electrochemical sensor unit needs to be changed when controlling the application of the pump voltage in the at least one electrochemical pump unit, the controller starts the protection execution mode in the case where the judgment target value exceeds the change threshold value, in which the target value is changed to a value that is larger than a normal value, and then continues the control of the gas sensor, and starts monitoring of a pump unit operation value, which is a value of the pump voltage or a value of the current in the at least one electrochemical pump unit including the judgment target pump unit, at the time when the pump unit operation value falls below a return threshold value, the target value is returned to the normal value, thereby returning the control of the gas sensor to the basic mode, as long as the pump unit operation value does not fall below the return threshold value and the judgment target value does not exceed a stop threshold value that is larger than the change threshold value, the monitoring of the pump unit operation value is continued, In a case where the pump unit operation value does not fall below the return threshold value and the judgment target value further exceeds the stop threshold value, the operation of the at least one electrochemical pump unit is stopped, and monitoring of the potential difference in the electrochemical sensor unit corresponding to the judgment target pump unit among the at least one electrochemical sensor unit is started, At a timing at which the monitored potential difference falls below a restart threshold value, the control of the gas sensor in the basic mode is restarted.

3. The gas sensor according to claim 1 or 2, wherein the judgment target value is a measured value of the potential difference in the electrochemical sensor unit corresponding to the judgment target pump unit among the at least one electrochemical sensor unit.

4. The gas sensor according to claim 1 or 2, wherein the judgment target value is a value of a pump current when oxygen is sucked in the judgment target pump unit.

5. The gas sensor according to claim 1 or 2, wherein the at least one internal cavity is a plurality of internal cavities that are sequentially communicated, the inner electrode is a plurality of inner electrodes that are respectively provided in the plurality of internal cavities, the at least one electrochemical pump unit is a plurality of electrochemical pump units, the at least one electrochemical sensor unit is a plurality of electrochemical sensor units, the judgment target value is at least one of the plurality of electrochemical pump units and is an index of oxygen suction operation of an electrochemical pump unit that is provided corresponding to an internal cavity closest to the guide port among the plurality of internal cavities.

6. The gas sensor according to claim 5, wherein one of the plurality of inner electrodes is a measurement electrode for measuring the prescribed gas component, the plurality of electrochemical pump units include: a measurement pump unit that includes the measurement electrode; and at least one oxygen concentration control pump unit other than the measurement pump unit, the at least one oxygen concentration control pump unit including any one of the plurality of inner electrodes other than the measurement electrode, the plurality of electrochemical sensor units include: a measurement sensor unit that includes the measurement electrode; and at least one oxygen concentration monitoring sensor unit other than the measurement sensor unit, the at least one oxygen concentration monitoring sensor unit including any one of the plurality of inner electrodes other than the measurement electrode, the controller determines the concentration of the prescribed gas component based on a measurement pump current that flows between the measurement electrode and the cavity outer pump electrode in the measurement pump unit corresponding to the concentration of the prescribed gas component, at least in the basic mode.

7. The gas sensor according to claim 6, wherein the plurality of internal cavities are a first internal cavity, a second internal cavity, and a third internal cavity that are sequentially communicated across 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, the measurement electrode is provided in the third internal cavity, The at least one oxygen concentration control pump unit is a main pump unit configured to control the oxygen concentration in the first internal cavity, and an auxiliary pump unit configured to control the oxygen concentration in the second internal cavity, The at least one oxygen concentration monitoring sensor unit is a main sensor unit configured to generate a potential difference between the main pump electrode and the reference electrode corresponding to the oxygen concentration in the first internal cavity, and an auxiliary sensor unit configured to generate a potential difference between the auxiliary pump electrode and the reference electrode corresponding to the oxygen concentration in the second internal cavity, The controller causes the main pump unit and the auxiliary pump unit to operate in such a manner that the oxygen concentrations in the first internal cavity and the second internal cavity are maintained at predetermined constant values, respectively, at least in the basic mode, and determines the concentration of the predetermined gas component included in the measured gas whose oxygen concentration introduced into the third internal cavity has been adjusted, based on the magnitude of the measurement pump current flowing through the measurement pump unit in correspondence with the concentration of the predetermined gas component included in the measured gas whose oxygen concentration introduced into the third internal cavity has been adjusted.

8. A method for operating a gas sensor that includes a sensor element composed of a solid electrolyte having oxygen ion conductivity and is configured to be capable of monitoring a predetermined gas component in a measured gas, The method for operating the gas sensor is characterized in that The sensor element includes: at least one internal cavity that communicates with an introduction port of the measured gas under a predetermined diffusion resistance and is provided with an internal electrode; an inter-cavity pump electrode that is disposed at a position other than the at least one internal cavity; a reference electrode that is disposed so as to be capable of contacting a reference gas; The sensor element further includes: at least one electrochemical pump unit that is configured to be capable of taking in or out 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 inter-cavity pump electrode with a predetermined pump power source; and at least one electrochemical sensor unit that is configured to generate a potential difference between the internal electrode and the reference electrode corresponding to the oxygen concentration in the corresponding at least one internal cavity, In the case where the sensor element is configured as described above, the method for operating the gas sensor includes a judgment process in which it is judged whether or not a judgment target value that is an index of oxygen intake operation in a judgment target pump unit included in the at least one electrochemical pump unit exceeds a predetermined threshold value in a predetermined judgment time, The predetermined threshold value is a change threshold value for judging whether or not a target value of the potential difference in the corresponding at least one electrochemical sensor unit when the pump voltage is applied in the at least one electrochemical pump unit needs to be changed, The gas sensor is operated in a basic mode in which the oxygen concentrations in the at least one internal cavity are maintained constant by causing the at least one electrochemical pump unit to operate, as long as the judgment target value does not exceed the change threshold value in the judgment process, In a case where the judgment object value exceeds the change threshold value in the judgment process, the gas sensor is caused to operate in a protection execution mode in which the at least one electrochemical pump unit is protected to prevent the intake of excess oxygen, In the protection execution mode, after the target value is changed to a value larger than the normal value, the operation of the gas sensor is continued, and monitoring of a pump cell operation value, which is a value of the pump voltage or a value of the current in the at least one electrochemical pump unit including the judgment object pump unit, is started, At a timing at which the pump cell operation value is lower than a return threshold value, the target value is returned to the normal value, and thereby the operation of the gas sensor is returned to the basic mode.

9. An operation method of a gas sensor that includes a sensor element composed of a solid electrolyte having oxygen ion conductivity and is configured to be able to monitor a prescribed gas component in a measured gas, The operation method of the gas sensor is characterized in that The sensor element includes: at least one internal cavity that communicates with a guide port of the measured gas under a prescribed diffusion resistance and is provided with an inner electrode; an inter-cavity pump electrode that is disposed at a position other than the at least one internal cavity; a reference electrode that is disposed to be able to contact a reference gas; The sensor element further includes: at least one electrochemical pump unit that is configured to be able to intake or exhaust oxygen between 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 inter-cavity pump electrode with a prescribed pump power supply; and at least one electrochemical sensor unit that is configured to generate a potential difference between the inner electrode and the reference electrode corresponding to the oxygen concentration in the corresponding at least one internal cavity, In a case where the sensor element is configured as described above, the operation method of the gas sensor includes a judgment process in which a judgment object value that is an index of oxygen intake operation in a judgment object pump unit included in the at least one electrochemical pump unit is judged as to whether or not the judgment object value exceeds a prescribed threshold value in a prescribed judgment time, the prescribed threshold value being a change threshold value for judging whether or not a target value of the potential difference in the corresponding at least one electrochemical sensor unit that controls the application of the pump voltage in the at least one electrochemical pump unit needs to be changed, and the gas sensor is caused to operate in a basic mode in which the at least one internal cavity is maintained to have a constant oxygen concentration by causing the at least one electrochemical pump unit to operate as long as the judgment object value does not exceed the change threshold value in the judgment process, In a case where the judgment object value exceeds the change threshold value in the judgment process, the gas sensor is caused to operate in a protection execution mode in which the at least one electrochemical pump unit is protected to prevent the intake of excess oxygen, In the protection execution mode, after the target value is changed to a value larger than a normal value, the operation of the gas sensor is continued, and monitoring of a pump cell operation value, i.e., a value of the pump voltage or a value of the current in the at least one electrochemical pump cell including the judgment object pump cell, is started, At a time when the pump cell operation value is lower than a return threshold value, the target value is returned to the normal value, thereby returning the operation of the gas sensor to the basic mode, The monitoring of the pump cell operation value is continued as long as the pump cell operation value is not lower than the return threshold value and the judgment object value does not exceed a stop threshold value larger than the change threshold value, In a case where the pump cell operation value is not lower than the return threshold value and the judgment object value further exceeds the stop threshold value, the operation of the at least one electrochemical pump cell is stopped, and monitoring of the potential difference in the electrochemical sensor cell corresponding to the judgment object pump cell in the at least one electrochemical sensor cell is started, At a time when the monitored potential difference is lower than a restart threshold value, the operation of the gas sensor is restarted in the basic mode.

10. The operation method of the gas sensor according to claim 8 or 9, characterized in that, the judgment object value is set to a measured value of the potential difference in the electrochemical sensor cell corresponding to the judgment object pump cell in the at least one electrochemical sensor cell.

11. The operation method of the gas sensor according to claim 8 or 9, characterized in that, the judgment object value is set to a value of the pump current when oxygen is sucked in the judgment object pump cell.

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