NOx sensor and its operation method

Abstract

This invention provides a NOx sensor that protects the sensor element when used in a fuel-rich atmosphere. The NOx sensor controller determines whether a threshold value, which is an indicator of oxygen intake in the electrochemical pump unit (excluding the measurement pump unit), exceeds within a predetermined determination time. If the threshold value is not exceeded, the NOx sensor is controlled in a basic mode that maintains a constant oxygen concentration in each of the multiple internal cavities and determines the NOx concentration based on the pump current flowing between the measurement electrode and the outer electrode due to NOx reduction at the measurement electrode. If the threshold value is exceeded, the basic mode is stopped, and the NOx sensor is controlled in a protection execution mode to protect the measurement electrode and prevent over-reduction. The electrochemical pump unit includes: an inner electrode disposed in each of the multiple internal cavities of the sensor element; an outer electrode disposed outside the cavity; and a solid electrolyte between the two electrodes.

Description

Technical Field

[0001] This invention relates to a limiting current type NOx sensor, and particularly to the operation control of the NOx sensor when used in a fuel-rich atmosphere. Background Technology

[0002] It is well known that NOx sensors employ a limiting current type sensor element with a solid electrolyte, such as yttrium oxide-stabilized zirconium oxide, as the main component, which has oxygen ion conductivity. In this NOx sensor, the gas to be measured is sequentially introduced into a plurality of cavities (internal cavities) continuously arranged inside the sensor element. Furthermore, control is performed to maintain the potential difference between each of the plurality of inner electrodes arranged facing each internal cavity and a reference electrode arranged inside the element and in contact with the reference gas at a predetermined value corresponding to the desired oxygen concentration in the cavity.

[0003] In short, the control is performed as follows: a pumping voltage is applied between the two electrodes in an electrochemical pump unit consisting of inner electrodes, an outer electrode positioned outside the cavity (outer cavity electrode), and a solid electrolyte region between the two electrodes, thereby facilitating the intake or extraction of oxygen between the inner and outer cavities. By applying this pumping voltage, an oxygen pumping current, corresponding to the oxygen concentration within the cavity, flows between the inner and outer electrodes in a direction appropriate to the oxygen concentration.

[0004] In particular, the potential difference between the inner electrode (the measuring electrode) and the reference electrode, located in the innermost cavity, is controlled to a value at which all oxygen generated by the reduction of NOx is absorbed. In the NOx sensor, the NOx concentration is determined based on the magnitude of the oxygen pumping current flowing between the measuring electrode and the outer electrode during this control.

[0005] As an example of such a gas sensor, there is also a gas sensor that has an outer electrode on the outer surface of the sensor element and a ceramic layer is provided such that a slit is formed around the outer electrode to apply a specified diffusion resistance (see, for example, Patent Document 1).

[0006] In addition, it is already known that there are gas sensors 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 between them, and the detected gas is introduced into the interior through a diffusion rate control section formed by a porous material provided in a part of the insulating layer (see, for example, Patent Document 2).

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2021-162465

[0010] Patent Document 2: Japanese Patent Application Publication No. 2012-173146 Summary of the Invention

[0011] Sometimes, the limiting current type NOx sensor described above is used in environments where fuel-rich gas with an air-fuel ratio lower than the stoichiometric air-fuel ratio can be introduced into the element, such as midway through the exhaust path of a gasoline engine.

[0012] In this case, when fuel-rich gas is introduced into the internal cavity, in the electrochemical pump unit, an action is typically performed to draw oxygen from the outside of the element into the internal cavity (pumping action) in order to maintain a constant oxygen concentration within the cavity. That is, a pumping voltage is applied in a manner that draws oxygen into the internal cavity (in a manner that moves oxygen ions from the outside of the element into the internal cavity), thereby causing a corresponding oxygen pumping current to flow between the inner and outer electrodes.

[0013] During oxygen intake, there is a trend that the greater the amount of fuel-rich gas introduced into the internal cavity, the higher the pumping voltage and the greater the oxygen pump current. However, if the fuel enrichment of the gas being measured is too high, it becomes difficult to draw oxygen from the outside as the pumping voltage increases. In most cases, the measuring electrode located at the innermost part of the gas flow section is over-reduced due to the fuel-rich gas. Once this over-reduction occurs at the measuring electrode, even if the gas being measured introduced into the internal cavity is in a state of stoichiometric air-fuel ratio or lean fuel, it takes time (e.g., about 20 minutes) for the measuring electrode to stabilize, during which time the NOx concentration cannot be accurately measured. It should be noted that the stability of the measuring electrode can be judged based on the stability of the oxygen pumping current flowing between the measuring electrode and the outer electrode.

[0014] The present invention was made in view of the above-mentioned problems, and its object is to provide a NOx sensor that can protect the measuring electrode when used in a fuel-rich atmosphere.

[0015] To address the aforementioned issues, a first aspect of the present invention is a NOx sensor configured to monitor NOx in a gas to be measured, characterized by comprising: a sensor element composed of a solid electrolyte with oxygen ion conductivity; and a controller that controls the operation of the NOx sensor. The sensor element comprises: a plurality of internal cavities sequentially connected from the inlet of the gas to be measured under a predetermined diffusion resistance, and each cavity has an inner electrode; an external pump electrode disposed in a location other than the plurality of internal cavities; and a reference electrode configured to contact a reference gas. The device also includes: a plurality of electrochemical pump units configured to perform oxygen intake or exhaust between corresponding internal cavities in the plurality of internal cavities and the outside of the sensor element by applying a pump voltage between each of the inner electrodes and the outer pump electrode of the cavity with a predetermined pump power supply; and a plurality of electrochemical sensor units configured to generate a potential difference between each of the inner electrodes and the reference electrode corresponding to the oxygen concentration in the corresponding internal cavity, wherein one of the inner electrodes of each of the plurality of internal cavities is a measuring electrode capable of reducing NOx, and the plurality of electrochemical pump units include: a measuring pump unit, the measuring pump unit comprising... The plurality of electrochemical sensor units include: a measuring electrode; and at least one oxygen concentration control pump unit, the at least one oxygen concentration control pump unit including an inner electrode other than the measuring electrode; the plurality of electrochemical sensor units include: a measuring sensor unit including the measuring electrode; and at least one oxygen concentration monitoring sensor unit including an inner electrode other than the measuring electrode; the controller judges whether a predetermined judgment target value exceeds a predetermined threshold within a predetermined judgment time; the predetermined judgment target value is an indicator of the oxygen inhalation action in the judgment target pump unit included in the at least one oxygen concentration control pump unit; as long as the judgment... If the target value does not exceed the predetermined threshold, the NOx sensor is controlled in a basic mode. In this basic mode, the oxygen concentration in each of the multiple internal cavities is kept constant by activating the multiple electrochemical pump units. The NOx concentration is determined based on the measurement pump current flowing between the measurement electrode and the external pump electrode in the measurement pump unit, corresponding to the NOx concentration, due to NOx reduction at the measurement electrode. If the target value exceeds the predetermined threshold, the basic mode is stopped to protect the measurement electrode and prevent over-reduction. This is a protection execution mode that controls the NOx sensor.

[0016] The second aspect of the present invention, based on the NOx sensor involved in the first aspect, is characterized in that the predetermined threshold is a stop threshold used to determine whether the at least one oxygen concentration control pump unit needs to be stopped. When the value of the determined object exceeds the stop threshold, the controller starts the protection execution mode. In the protection execution mode, the operation of the at least one oxygen concentration control pump unit is stopped, and the measuring pump unit is allowed to inhale oxygen. Furthermore, the monitoring of the potential difference in the sensor unit corresponding to the determined object pump unit in the at least one oxygen concentration monitoring sensor unit is started. When the monitored potential difference is lower than the predetermined restart threshold, the control of the NOx sensor is restarted in the basic mode.

[0017] The third aspect of the present invention, based on the NOx sensor involved in the first aspect, is characterized in that the prescribed threshold is a change threshold, which is used to determine whether the target value of the potential difference in the at least one oxygen concentration monitoring sensor unit needs to be changed when controlling the application of the pump voltage in the at least one oxygen concentration control pump unit. When the value of the determined object exceeds the change threshold, the controller starts the protection execution mode. In the protection execution mode, after changing the target value to a value larger than the normal value, the controller continues to control the at least one oxygen concentration control pump unit, so that the measuring pump unit inhales oxygen. Furthermore, the controller starts monitoring the value of the pump voltage or current in the at least one oxygen concentration control pump unit, including the determined object pump unit, i.e., the pump unit action value. When the pump unit action value is lower than the recovery threshold, the target value is restored to the normal value, thereby restoring the control of the NOx sensor to the basic mode.

[0018] The fourth aspect of the present invention, based on the NOx sensor involved in the first aspect, is characterized in that the prescribed threshold is a change threshold, which is used to determine whether the target value of the potential difference in the corresponding at least one oxygen concentration monitoring sensor unit needs to be changed when controlling the application of the pump voltage in the at least one oxygen concentration control pump unit. When the determined target value exceeds the change threshold, the controller initiates the protection execution mode. In the protection execution mode, after changing the target value to a value larger than the normal value, the control of the at least one oxygen concentration control pump unit continues, causing the measuring pump unit to inhale oxygen, and the pump voltage or current in at least one oxygen concentration control pump unit, including the determined target pump unit, is monitored. The value of the pump unit action value is monitored. When the pump unit action value is lower than the recovery threshold, the target value is restored to the normal value, thereby restoring the control of the NOx sensor to the basic mode. As long as the pump unit action value is not lower than the recovery threshold and the judgment object value does not exceed the stop threshold which is greater than the change threshold, the monitoring of the pump unit action value continues. If the pump unit action value is not lower than the recovery threshold and the judgment object value further exceeds the stop threshold, the monitoring of the potential difference in the sensor unit corresponding to the judgment object pump unit in at least one oxygen concentration monitoring sensor unit begins. When the monitored potential difference is lower than the specified restart threshold, the control of the NOx sensor in the basic mode is restarted.

[0019] The fifth embodiment of the present invention is based on the NOx sensor involved in any of the first to fourth embodiments, characterized in that the value of the judgment object is the measured value of the potential difference in the sensor unit corresponding to the judgment object pump unit in the at least one oxygen concentration monitoring sensor unit.

[0020] The sixth embodiment of the present invention is based on the NOx sensor involved in any of the first to fourth embodiments, characterized in that the value of the judgment object is the pump current when oxygen is inhaled in the judgment object pump unit.

[0021] The seventh embodiment of the present invention, based on the NOx sensor involved in any of the first to sixth embodiments, is characterized in that: the plurality of internal cavities are a first internal cavity, a second internal cavity, and a third internal cavity connected sequentially through a diffusion resistance section; the inner electrodes, excluding the measuring electrode, are a main pump electrode disposed in the first internal cavity and an auxiliary pump electrode disposed in the second internal cavity; the measuring electrode is disposed in the third internal cavity; the at least one oxygen concentration control pump unit is a main pump unit for controlling the oxygen concentration in the first internal cavity and an auxiliary pump unit for controlling the oxygen concentration in the second internal cavity; and the at least one oxygen concentration monitoring sensor unit is configured to be located in the main pump... The controller comprises a main sensor unit that generates a potential difference between the auxiliary 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. At least in the basic mode, the controller operates the main pump unit and the auxiliary pump unit in such a manner that the oxygen concentrations in the first internal cavity and the second internal cavity are respectively maintained at predetermined constant values. Furthermore, the controller determines the NOx concentration based on the magnitude of the measuring pump current flowing through the measuring pump unit, which corresponds to the concentration of NOx contained in the measured gas, adjusted to the oxygen concentration introduced into the third internal cavity.

[0022] The eighth aspect of the present invention is a method of operating a NOx sensor, the NOx sensor comprising a sensor element made of a solid electrolyte with oxygen ion conductivity, and configured to monitor NOx in a gas to be measured, characterized in that the sensor element comprises: a plurality of internal cavities connected sequentially from an inlet into which the gas to be measured is introduced, with a predetermined diffusion resistance; a plurality of inner electrodes disposed facing each of the internal cavities; an external pump electrode disposed in a location other than the internal cavities; and a reference electrode disposed to be in contact with a reference gas; the sensor element further comprises... The device comprises: a plurality of electrochemical pump units configured to perform oxygen intake or exhaust between corresponding internal cavities in the plurality of internal cavities and the exterior of the sensor element by applying a pump voltage between each of the inner electrodes and the outer pump electrode of the cavity using a predetermined pump power supply; and a plurality of electrochemical sensor units configured to generate a potential difference between each of the inner electrodes and the reference electrode corresponding to the oxygen concentration in the corresponding internal cavity, wherein one of the inner electrodes of each of the plurality of internal cavities is a measuring electrode capable of reducing NOx, and the plurality of electrochemical pump units further comprises: a measuring pump unit, the measuring pump unit including the measuring... The plurality of electrochemical sensor units include: a measuring sensor unit including the measuring electrode; and at least one oxygen concentration control pump unit including the inner electrode other than the measuring electrode. The plurality of electrochemical sensor units further include: a measuring sensor unit including the measuring electrode; and at least one oxygen concentration monitoring sensor unit including the inner electrode other than the measuring electrode. In this case, a judgment step is included to determine whether a predetermined judgment target value exceeds a predetermined threshold within a predetermined judgment time. The predetermined judgment target value is an indicator of oxygen inhalation action in the judgment target pump unit included in the at least one oxygen concentration control pump unit. In the judgment step, as long as the judgment target value does not exceed the predetermined threshold, the NOx sensor is controlled in the basic mode. In this basic mode, the oxygen concentration in each of the plurality of internal cavities is kept constant, and the NOx concentration is determined based on the measurement pump current flowing between the measurement electrode and the external pump electrode in the measurement pump unit, corresponding to the NOx concentration, due to the reduction of NOx at the measurement electrode. In the judgment step, if the judgment target value exceeds the predetermined threshold, the basic mode is stopped, and the NOx sensor is activated by a protection execution mode to protect the measurement electrode and prevent over-reduction.

[0023] The ninth aspect of the present invention, based on the NOx sensor operation method involved in the eighth aspect, is characterized in that the predetermined threshold is a stop threshold used to determine whether the at least one oxygen concentration control pump unit needs to be stopped. In the determination step, when the value of the determined object exceeds the stop threshold, the protection execution mode is started. In the protection execution mode, the operation of the at least one oxygen concentration control pump unit is stopped, and the measuring pump unit is allowed to inhale oxygen. Furthermore, the monitoring of the potential difference in the sensor unit corresponding to the determined object pump unit in the at least one oxygen concentration monitoring sensor unit is started. When the monitored potential difference is lower than the predetermined restart threshold, the NOx sensor is restarted to operate in the basic mode.

[0024] The tenth aspect of the present invention, based on the NOx sensor operation method involved in the eighth aspect, is characterized in that the prescribed threshold is a change threshold, which is used to determine whether the target value of the potential difference in the at least one oxygen concentration monitoring sensor unit needs to be changed when controlling the application of the pump voltage in the at least one oxygen concentration control pump unit. In the determination step, if the value of the determined object exceeds the change threshold, the protection execution mode is started. In the protection execution mode, after changing the target value to a value larger than the prescribed normal value, the operation of the at least one oxygen concentration control pump unit continues, so that the measuring pump unit inhales oxygen. Furthermore, monitoring of the pump voltage or current value, i.e., the pump unit operation value, in the at least one oxygen concentration control pump unit, including the determined object pump unit, begins. When the pump unit operation value is lower than the recovery threshold, the target value is restored to the prescribed normal value, thereby restoring the operation of the NOx sensor to the basic mode.

[0025] The eleventh aspect of the present invention, based on the NOx sensor operation method involved in the eighth aspect, is characterized in that the prescribed threshold is a change threshold, which is used to determine whether the target value of the potential difference in the at least one oxygen concentration monitoring sensor unit needs to be changed when controlling the application of the pump voltage in the at least one oxygen concentration control pump unit. In the determination step, if the value of the determined object exceeds the change threshold, the protection execution mode is started. In the protection execution mode, after changing the target value to a value larger than the normal value, the operation of the at least one oxygen concentration control pump unit continues, causing the measuring pump unit to inhale oxygen, and the value of the pump voltage in the at least one oxygen concentration control pump unit, including the determined object pump unit, is started. The monitoring of the current value, i.e., the pump unit operation value, is performed. When the pump unit operation value is lower than the recovery threshold, the target value is restored to the normal value, thereby restoring the control of the NOx sensor to the basic mode. As long as the pump unit operation value is not lower than the recovery threshold and the judgment object value does not exceed the stop threshold which is greater than the change threshold, the monitoring of the pump unit operation value continues. If the pump unit operation value is not lower than the recovery threshold and the judgment object value further exceeds the stop threshold, the monitoring of the potential difference in the sensor unit corresponding to the judgment object pump unit in at least one oxygen concentration monitoring sensor unit is started. When the monitored potential difference is lower than the specified restart threshold, the NOx sensor is restarted to operate in the basic mode.

[0026] The twelfth aspect of the present invention is based on the NOx sensor operation method involved in any of the eighth to eleventh aspects, characterized in that the judgment object value is set as the measured value of the potential difference in the sensor unit corresponding to the judgment object pump unit in the at least one oxygen concentration monitoring sensor unit.

[0027] The thirteenth aspect of the present invention is based on the NOx sensor operation method involved in any of the eighth to eleventh aspects, characterized in that the value of the judgment object is set as the pump current when oxygen is inhaled in the judgment object pump unit.

[0028] Invention Effects

[0029] According to the first to thirteenth aspects of the present invention, even when using a NOx sensor in an environment where the gas being measured may be a fuel-rich gas with a low air-fuel ratio, it is possible to effectively avoid a situation where oxygen concentration cannot be controlled due to difficulty in drawing oxygen into the internal cavity. Furthermore, it is possible to effectively suppress excessive reduction of the measuring electrode due to fuel-rich gas, thus enabling the measuring electrode to quickly become usable when NOx measurement is restarted. Attached Figure Description

[0030] Figure 1 This is a diagram that schematically illustrates an example of the configuration of a gas sensor 100.

[0031] Figure 2 This is a diagram showing the action flow in the first mode of component protection.

[0032] Figure 3 This is a diagram illustrating the action flow in the second mode of component protection.

[0033] Figure 4 This is a diagram showing the action flow in the third mode of component protection.

[0034] Figure 5 This is a diagram showing the action flow in the third mode of component protection.

[0035] Figure 6 This is a diagram illustrating an example of the configuration of the gas sensor 100B.

[0036] Explanation of reference numerals in the attached figures

[0037] 1…First substrate layer, 2…Second substrate layer, 3…Third substrate layer, 4…First solid electrolyte layer, 5…Isolation layer, 6…Second solid electrolyte layer, 10…Gas inlet, 11…First diffusion rate control unit, 13…Second diffusion rate control unit, 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 unit, 40…Second internal cavity, 41…Measurement pump unit, 42…Reference electrode, 43…Reference gas inlet space, 44…Measurement electrode, 50…Auxiliary pump unit, 51…Auxiliary pump electrode, 60…Fourth diffusion rate control unit, 61…Third internal cavity, 70…Heater unit, 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 Implementation

[0038] <Overview of the composition of a gas sensor>

[0039] Figure 1This diagram schematically illustrates an example of the configuration of the gas sensor 100 according to this embodiment. The gas sensor 100 is a limiting current type NOx sensor that monitors NOx and measures its concentration using a sensor element 101. Furthermore, the gas sensor 100 also includes a controller 110, which controls the operation of each component and determines the NOx concentration based on the NOx current flowing through the sensor element 101. Figure 1 A vertical cross-sectional view along the length direction including sensor element 101.

[0040] The sensor element 101 is a flat (elongated) ceramic element body, which has a structure obtained by stacking six solid electrolyte layers in the following order from bottom to top: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, an isolation layer 5, and a second solid electrolyte layer 6, each containing zirconium oxide (ZrO2) (e.g., yttrium-stabilized zirconium oxide (YSZ)) as an oxygen ion conductive solid electrolyte. Furthermore, the solid electrolyte forming these six layers is a dense and gas-tight solid electrolyte. Additionally, hereinafter, it will sometimes be referred to as... Figure 1 The upper surface of each of the six layers is referred to as the upper surface, and the lower surface is referred to as the lower surface. In addition, the entire portion of the sensor element 101 containing the solid electrolyte is collectively referred to as the substrate.

[0041] For example, the sensor element 101 can be manufactured in the following manner: the ceramic green sheets corresponding to each layer are processed in a specified manner and the circuit pattern is printed, then they are stacked and fired to achieve integration.

[0042] At one end of the sensor element 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, a first diffusion rate control unit 11, a buffer space 12, a second diffusion rate control unit 13, a first internal cavity 20, a third diffusion rate control unit 30, a second internal cavity 40, a fourth diffusion rate control unit 60, and a third internal cavity 61, which also serve as gas inlet 10, are formed adjacent to each other in this order.

[0043] The buffer space 12, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 are the internal spaces (regions) of the sensor element 101, which are formed by hollowing out the isolation layer 5. The upper part of this internal space is defined by the lower surface of the second solid electrolyte layer 6, the lower part by the upper surface of the first solid electrolyte layer 4, and the side part by the side of the isolation layer 5. It should be noted that the gas inlet 10, similarly, can be provided on the end face (left end in the drawing) of the sensor element 101 by hollowing out the isolation layer 5, unlike the first diffusion rate control unit 11. In this case, the first diffusion rate control unit 11 is formed adjacent to the gas inlet 10, closer to the interior.

[0044] The first diffusion velocity control unit 11, the second diffusion velocity control unit 13, the third diffusion velocity control unit 30, and the fourth diffusion velocity control unit 60 are all configured as two horizontally elongated slits (forming the length direction of the opening perpendicular to the drawing). Furthermore, the portion from the gas inlet 10 to the third internal cavity 61 is also referred to as the gas flow section.

[0045] Furthermore, a reference gas introduction space 43 is provided at a location farther from the end side than the gas flow section, between the upper surface of the third substrate layer 3 and the lower surface of the isolation layer 5, and at a position defined by the side of the first solid electrolyte layer 4. For example, atmospheric air is introduced into the reference gas introduction space 43 as a reference gas for measuring NOx concentration.

[0046] The atmosphere introduction layer 48 is a layer composed of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. In addition, the atmosphere introduction layer 48 is formed to cover the reference electrode 42.

[0047] The reference electrode 42 is an electrode formed by being sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, an atmospheric inlet layer 48 communicating with the reference gas inlet space 43 is provided around it. 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.

[0048] In the gas flow section, the gas inlet 10 (first diffusion rate control section 11) is an opening relative to the external space, through which the gas to be measured is introduced from the external space into the sensor element 101.

[0049] The first diffusion rate control unit 11 is a part that applies a predetermined diffusion resistance to the introduced gas to be measured.

[0050] The buffer space 12 is a space provided for guiding the measured gas introduced from the first diffusion rate control unit 11 to the second diffusion rate control unit 13.

[0051] The second diffusion rate control unit 13 is a part that applies a predetermined diffusion resistance to the gas being measured introduced from the buffer space 12 into the first internal cavity 20.

[0052] When the gas to be measured is introduced from outside the sensor element 101 into the first internal cavity 20, the gas being measured, which is rapidly introduced into the sensor element 101 from the gas inlet 10 due to pressure fluctuations in the external space (pulsations in exhaust pressure in the case of automobile exhaust), is not directly introduced into the first internal cavity 20. Instead, it is introduced into the first internal cavity 20 after the concentration fluctuations of the gas being measured are eliminated by the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13. As a result, the concentration fluctuations of the gas being measured introduced into the first internal cavity 20 are almost negligible.

[0053] The first internal cavity 20 is configured as a space for adjusting the oxygen partial pressure in the gas to be measured, which is introduced through the second diffusion rate control unit 13. The main pump unit 21 operates to adjust this oxygen partial pressure.

[0054] The main pump unit 21 is an electrochemical pump unit consisting of an inner pump electrode 22, an outer (outside the cavity) pump electrode 23, and a second solid electrolyte layer 6 sandwiched between the inner pump electrode 22 and the outer (outside the cavity) pump electrode 23. The inner pump electrode 22 has a top electrode portion 22a disposed on the lower surface of the second solid electrolyte layer 6, facing the entire surface of the first internal cavity 20. The outer (outside the cavity) pump electrode 23 is disposed on the upper surface of the second solid electrolyte layer 6 (a main surface of the sensor element 101) in a manner that exposes it to the external space in the area corresponding to the top electrode portion 22a.

[0055] The inner pump electrode 22 is formed on the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that divide the first internal cavity 20. Specifically, a top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that constitutes the top surface of the first internal cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that constitutes the bottom surface. The top electrode portion 22a and the bottom electrode portion 22b are connected by conductive portions provided on the side wall surfaces (inner surfaces) of the insulating layers 5 that constitute the two side walls of the first internal cavity 20 (illustration omitted).

[0056] The top electrode portion 22a and the bottom electrode portion 22b are arranged to be rectangular when viewed from above. However, it is also possible to have only the top electrode portion 22a or only the bottom electrode portion 22b.

[0057] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes. Specifically, the inner pump electrode 22, which is in contact with the gas being measured, is formed using a material that reduces the reducing power of NOx components in the gas being measured. For example, it is formed as a cermet electrode of Au-Pt alloy and ZrO2 with a porosity of 5% to 40% and containing approximately 0.6 wt% to 1.4 wt% Au, with a thickness of 5 μm to 20 μm. The weight ratio of Au-Pt alloy to ZrO2 only needs to be approximately Pt:ZrO2 = 7.0:3.0 to 5.0:5.0.

[0058] On the other hand, the outer pump electrode 23 is formed, for example, as a Pt or its alloy with ZrO2 cermet electrode, and is rectangular when viewed from above.

[0059] 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 using a variable power supply 24, and the main pump current Ip0 flows between the inner pump electrode 22 and the outer pump electrode 23 in either a positive or negative direction. This allows oxygen to be drawn from the first internal cavity 20 into the external space, or oxygen from the external space to be drawn into the first internal cavity 20. Furthermore, 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.

[0060] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, a main sensor unit 80 is constructed as an electrochemical sensor unit, consisting of an inner pump electrode 22, a second solid electrolyte layer 6, an isolation layer 5, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42.

[0061] The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 is known by measuring the potential difference, i.e., the electromotive force V0, between the inner pump electrode 22 and the reference electrode 42 in the main sensor unit 80.

[0062] Furthermore, the controller 110 performs feedback control on the main pump voltage Vp0 to keep the electromotive force V0 constant, thereby controlling the main pump current Ip0. As a result, the oxygen concentration in the first internal cavity 20 is maintained at a predetermined constant value.

[0063] The third diffusion rate control unit 30 is a component that applies a predetermined diffusion resistance to the gas to be measured after the oxygen concentration (oxygen partial pressure) has been controlled in the first internal cavity 20 by the operation of the main pump unit 21, and introduces the gas to be measured into the second internal cavity 40.

[0064] The second internal cavity 40 is configured as a space for further adjusting the oxygen partial pressure in the gas to be measured, which is introduced through the third diffusion rate control unit 30. This oxygen partial pressure is adjusted by operating the auxiliary pump unit 50. Within the second internal cavity 40, the oxygen concentration of the gas to be measured can be adjusted with greater precision.

[0065] In the second internal cavity 40, the oxygen concentration (oxygen partial pressure) of the gas to be measured, which has been pre-adjusted in the first internal cavity 20 and then introduced through the third diffusion rate control unit 30, is further adjusted by the auxiliary pump unit 50.

[0066] The auxiliary pump unit 50 is an auxiliary electrochemical pump unit consisting of an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode on the outside of the sensor element 101), and a second solid electrolyte layer 6. The auxiliary pump electrode 51 has a top electrode portion 51a that is generally disposed on the lower surface of the second solid electrolyte layer 6 and facing the second internal cavity 40.

[0067] 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, a top electrode portion 51a is formed relative to the second solid electrolyte layer 6 constituting the top surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on 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 to be rectangular when viewed from above, and are connected by conductive portions provided on the side wall surfaces (inner surfaces) of the isolation layers 5 constituting the two side walls of the second internal cavity 40 (illustration omitted).

[0068] Furthermore, the auxiliary pump electrode 51 is also formed using a material that reduces the reduction ability of NOx components in the measured gas, just like the inner pump electrode 22.

[0069] For 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, thereby enabling oxygen in the atmosphere inside the second internal cavity 40 to be drawn out to the external space, or oxygen to be drawn from the external space into the second internal cavity 40.

[0070] In addition, to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, an auxiliary sensor unit 81, comprising an auxiliary pump electrode 51, a reference electrode 42, a second solid electrolyte layer 6, an isolation layer 5, a first solid electrolyte layer 4, and a third substrate layer 3, serves as an electrochemical sensor unit. The auxiliary sensor unit 81 detects the potential difference, i.e., the electromotive force V1, generated between the auxiliary pump electrode 51 and the reference electrode 42 corresponding to the oxygen partial pressure within the second internal cavity 40.

[0071] The auxiliary pump unit 50 pumps using a variable power supply 52, which controls the voltage based on the electromotive force V1 detected by the auxiliary sensor unit 81. As a result, the oxygen partial pressure in the atmosphere within the second internal cavity 40 is controlled by feedback to a low partial pressure that has virtually no impact on NOx measurement.

[0072] Additionally, 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 to the main sensor unit 80 as a control signal and controls its electromotive force V0, thereby ensuring that the gradient of oxygen partial pressure in the gas to be measured, introduced from the third diffusion rate control unit 30 into the second internal cavity 40, remains constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of approximately 0.001 ppm due to the action of the main pump unit 21 and the auxiliary pump unit 50.

[0073] The fourth diffusion rate control unit 60 is a part that applies a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the auxiliary pump unit 50 in the second internal cavity 40, and guides the gas to be measured to the third internal cavity 61.

[0074] The third internal cavity 61 is configured as a space for measuring the concentration of nitrogen oxides (NOx) in the gas to be measured, which is introduced through the fourth diffusion rate control unit 60. In the third internal cavity 61, the NOx concentration is measured by operating the measuring pump unit 41. Since the gas to be measured, whose oxygen concentration has been precisely adjusted in the second internal cavity 40, is introduced into the third internal cavity 61, the gas sensor 100 can perform highly accurate NOx concentration measurements.

[0075] The measuring pump unit 41 is used to measure the NOx concentration of the gas to be measured introduced into the third internal cavity 61. The measuring pump unit 41 is an electrochemical pump unit composed of a measuring electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, an isolation layer 5, and a first solid electrolyte layer 4. The measuring electrode 44 is disposed on the upper surface of the first solid electrolyte layer 4, facing the third internal cavity 61 and separated from the third diffusion rate control unit 30.

[0076] The measuring electrode 44 is a porous cermet electrode composed of a noble metal and a solid electrolyte. For example, it is a cermet electrode formed of Pt or an alloy of Pt and other noble metals such as Rh with ZrO2, which is the constituent material of the sensor element 101. The measuring electrode 44 also functions as a NOx reduction catalyst to reduce NOx present in the atmosphere within the third internal cavity 61.

[0077] For the measuring pump unit 41, under the control of the controller 110, it can absorb the oxygen generated by the decomposition of NOx in the atmosphere of the third internal cavity 61, and can detect its generation amount as pump current Ip2.

[0078] In addition, to detect the oxygen partial pressure around the measuring electrode 44, a measuring sensor unit 82, which serves as an electrochemical sensor unit, is constructed from the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42. Based on the potential difference, i.e., the electromotive force V2, generated between the measuring electrode 44 and the reference electrode 42 corresponding to the oxygen partial pressure within the third internal cavity 61 detected by the measuring sensor unit 82, the variable power supply 46 is subjected to feedback control.

[0079] NOx in the gas being measured, introduced into the third internal cavity 61, is reduced by the measuring electrode 44 (2NO→N2+O2) to generate oxygen. This generated oxygen is then pumped by the measuring pump unit 41. During this process, the voltage (measuring pump voltage) Vp2 of the variable power supply 46 is controlled to keep the electromotive force V2 detected by the measuring sensor unit 82 constant. The amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of NOx in the gas being measured; therefore, the concentration of NOx in the gas being measured is calculated using the pump current Ip2 in the measuring pump unit 41. Hereinafter, this pump current Ip2 will also be referred to as the NOx current Ip2.

[0080] Furthermore, if the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form an oxygen partial pressure detection mechanism as an electrochemical sensor unit, the electromotive force corresponding to the following difference can be detected, thereby allowing the determination of the concentration of NOx in the measured gas. This difference refers to the difference between the amount of oxygen generated by the reduction of NOx in the atmosphere surrounding the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere.

[0081] In addition, the electrochemical sensor unit 83 is composed of the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23 and the reference electrode 42. The electromotive force Vref can be obtained by using the sensor unit 83, and the oxygen partial pressure in the gas to be measured outside the sensor can be detected by using the electromotive force Vref.

[0082] The sensor element 101 also includes a heater section 70, which performs the function of heating and maintaining the temperature of the sensor element 101 in order to improve the oxygen ion conductivity of the solid electrolyte constituting the substrate.

[0083] The heater section 70 mainly includes a heater electrode 71, a heater component 72, a heater conductive section 72a, a through hole 73, a heater insulating layer 74, a pressure diffusion hole 75, and Figure 1 The heater resistance detection conduction section shown in the figure is omitted. In addition, except for the heater electrode 71, the heater section 70 is embedded in the base of the sensor element 101.

[0084] The heater electrode 71 is an electrode formed in contact with the lower surface of the first substrate layer 1 (the other main surface of the sensor element 101).

[0085] The heater component 72 is a resistive heating element disposed between the second substrate layer 2 and the third substrate layer 3. Figure 1 The heater power supply (not shown) located outside the sensor element 101 supplies power to the heater member 72 via the heater electrode 71, through-hole 73, and heater conduction portion 72a, which serve as the power path, thereby heating the heater member 72. The heater member 72 is formed of Pt or is formed with Pt as the main component. The heater member 72 is embedded in a predetermined range on the side of the sensor element 101 containing the gas passage, opposite to the gas passage in the element thickness direction. The heater member 72 is provided to have a thickness of approximately 10 μm to 20 μm.

[0086] For sensor element 101, current flows through heater electrode 71 to heater component 72, thereby heating heater component 72 and heating each part of sensor element 101 to a predetermined temperature and maintaining that temperature. Specifically, sensor element 101 is heated so that the temperature of the solid electrolyte near the gas flow section and the electrodes reaches approximately 700°C to 900°C. This heating improves the oxygen ion conductivity of the solid electrolyte constituting the matrix of sensor element 101. Furthermore, the heating temperature of heater component 72 when using gas sensor 100 (when sensor element 101 is driven) is called sensor element driving temperature.

[0087] The degree of heating of the heater component 72 (heater temperature) is controlled by the magnitude of the resistance value of the heater component 72 (heater resistance).

[0088] It should be noted that, although Figure 1 The diagram is omitted, but on one main surface side of the sensor element 101, for the purpose of protecting the outer pump electrode 23, an electrode protective layer covering the outer pump electrode 23 may be provided.

[0089] Alternatively, a single or multiple porous layer, i.e., a heat-shock-resistant protective layer, can be provided on the outer periphery of a defined range at one end of the sensor element 101 (the left end in the attached figure) to cover the sensor element 101. The purpose of this heat-shock-resistant protective layer is to prevent thermal shock caused by moisture in the gas being measured adhering to and condensing on the sensor element 101 during use, which could lead to cracks in the sensor element 101, and to prevent poisoning substances mixed in the gas being measured from entering the interior of the sensor element 101. Furthermore, a layered void (void layer) can be formed between the sensor element 101 and the heat-shock-resistant protective layer.

[0090] Furthermore, the sensor element 101 is housed in a metal housing (not shown) with the gas inlet 10 side and the reference gas inlet space 43 side sealed in an airtight manner. The sensor element 101 and the housing constitute the main body of the gas sensor 100. In actual use, the gas sensor 100 is mounted at a location such as an engine exhaust pipe. Wiring is extended from the housing to ensure electrical connection to various parts of the sensor element 101 internally; this wiring is appropriately connected to the controller 110, various power supplies, etc.

[0091] <Actions in normal mode>

[0092] When measuring the NOx concentration in the gas sensor 100 with the above configuration, the main pump unit 21 and the auxiliary pump unit 50 are activated to perform feedback control that keeps the oxygen concentration in the first internal cavity 20 and the second internal cavity 40 constant. The gas to be measured with a constant oxygen concentration is introduced into the third internal cavity 61 and reaches the measuring electrode 44. For example, when the gas to be measured is a lean fuel atmosphere, the gas to be measured with an oxygen partial pressure sufficiently reduced to a level that has no substantial effect on the NOx measurement (e.g., 0.0001 ppm to 1 ppm) is introduced into the third internal cavity 61.

[0093] Then, at the measuring electrode 44, the NOx in the measured gas is reduced, thereby producing oxygen. This oxygen is drawn out by the measuring pump unit 41, and the NOx current Ip2 flowing during this extraction has a constant functional relationship with the concentration of NOx in the measured gas (hereinafter referred to as sensitivity characteristics).

[0094] Before actual use of the gas sensor 100, the aforementioned sensitivity characteristics are determined using various sample gases with known NOx concentrations, and the data is stored in the controller 110. Furthermore, during actual use of the gas sensor 100, a signal representing the value of the NOx current Ip2 flowing corresponding to the NOx concentration in the gas being measured is continuously provided to the controller 110. Based on this value and the determined sensitivity characteristics, the controller 110 sequentially calculates the NOx concentration and outputs it as the NOx sensor detection value. Accordingly, the gas sensor 100 can approximately determine the NOx concentration in the gas being measured in real time.

[0095] In this embodiment, the operation of the gas sensor 100 related to the determination of NOx concentration is referred to as the operation of the gas sensor 100 in its normal mode.

[0096] It should be noted that, in the above-described normal mode, the target values ​​of the electromotive forces V0, V1, and V2 in the main sensor unit 80, auxiliary sensor unit 81, and measuring sensor unit 82 when feedback control is performed on the main pump unit 21, auxiliary pump unit 50, and measuring pump unit 41, respectively, can be appropriately set according to the specific configuration, size, usage conditions, and methods of the various parts of the sensor element 101. However, as an example, it is assumed that the target values ​​of the electromotive forces V0, V1, and V2 are set to 250mV, 385mV, and 400mV, respectively. The above values ​​are approximately standard values ​​set when the oxygen ion conductive solid electrolyte constituting the sensor element 101 is zirconium oxide.

[0097] <Action in Component Protection Mode>

[0098] Assumption: The gas sensor 100 involved in this embodiment mainly operates in the above-described normal mode when the gas to be measured, such as a lean fuel atmosphere, contains relatively sufficient oxygen, that is, it determines the concentration of NOx in the gas to be measured.

[0099] More specifically, when the gas sensor 100 operates in normal mode, the main pump unit 21 operates in such a way that the electromotive force V0 generated by the main sensor unit 80 reaches a predetermined value corresponding to the desired value of the oxygen concentration (or oxygen partial pressure) in the first internal cavity 20. At this time, the oxygen concentration in the gas to be measured introduced from the external space into the first internal cavity 20 changes constantly, so the main pump unit 21 can perform oxygen extraction or intake.

[0100] In contrast, the auxiliary pump unit 50 and the measuring pump unit 41 are also capable of oxygen intake in terms of their configuration. However, the set values ​​of the electromotive force V1 in the auxiliary sensor unit 81 and the electromotive force V2 in the measuring sensor unit 82, which are the control target values ​​for the operation of each pump unit, are set based on the premise of oxygen extraction in the NOx concentration measurement principle. That is, when the gas sensor 100 operates in normal mode, the auxiliary pump unit 50 and the measuring pump unit 41 are dedicated to oxygen extraction.

[0101] However, the gas sensor 100 may not be used in an atmosphere containing sufficient oxygen. Sometimes it is used in environments where the main body of the gas sensor 100 is installed in, for example, the exhaust path of a gasoline engine and the exhaust gas from that engine is used as the measured gas. In other cases, it may be used in environments where the atmosphere is a fuel-rich gas with a low air-fuel ratio. In such situations, the measured gas introduced into the sensor element 101 is also a fuel-rich gas. At this time, the main pump unit 21 ensures the oxygen concentration value within the first internal cavity 20 by drawing oxygen from the outside.

[0102] However, if the fuel in the gas being measured introduced into the sensor element 101 is too concentrated, even if the main pump voltage Vp0 applied to the main pump unit 21 is increased, it will be impossible to draw in an amount of oxygen corresponding to that main pump voltage Vp0 from the outside. This could result in an uncontrollable oxygen concentration in the first internal cavity 20, where the target oxygen concentration cannot be achieved. Furthermore, it could also lead to an undesirable situation where the measuring electrode 44 located at the innermost part of the gas flow section is over-reduced due to the fuel-rich gas.

[0103] It should be noted that the situation described above, where oxygen cannot be properly drawn in from the outside, is more likely to occur when the gas sensor disclosed in Patent Document 1 controls the inflow and outflow rate of the atmospheric gas around the part where oxygen is introduced from the outside space, i.e., the outer pump electrode 23, with a specified diffusion resistance, as a gas sensor disclosed in Patent Document 1. Figure 6This diagram schematically illustrates an example of the configuration of a gas sensor 100B, which is one embodiment of the aforementioned gas sensor 100. The gas sensor 100B further comprises a ceramic layer 7 and a porous region 8 on top of the second solid electrolyte layer 6, and also has... Figure 1 The gas sensor 100 shown has a common configuration. The porous region 8 is made of a porous material (e.g., alumina) with a porosity of approximately 30% to 60%, covering the outer pump electrode 23 and exposed at both ends in the element width direction (not shown). The ceramic layer 7 is made of a ceramic (e.g., zirconium oxide, alumina, etc.) dense to the same degree as the second solid electrolyte layer 6, covering the entire upper surface of the second solid electrolyte layer 6, including the porous region 8. In this gas sensor 100B, the inlet and outlet rates of the atmospheric gas around the outer pump electrode 23 are controlled by the diffusion resistance applied by the porous region 8.

[0104] Based on the above aspects, when the fuel in the gas being measured is too rich, the gas sensor 100 according to this embodiment can perform an element protection mode operation to avoid the situation where the oxygen concentration cannot be controlled and to prioritize the protection of the measuring electrode 44 and the sensor element 101 by avoiding excessive reduction of the measuring electrode 44.

[0105] This component protection mode has three different processing sequences. These three sequences will be explained in turn below.

[0106] (First method)

[0107] Figure 2 This is a diagram illustrating the operation flow in the first mode of component protection. The first mode is, in general, in the case where the gas being measured is in an atmosphere excessively enriched with fuel, temporarily stopping the NOx measurement operation of the gas sensor 100, including the intake operation of oxygen from the outside, thereby preventing excessive reduction of the measuring electrode.

[0108] In this method, firstly, the gas sensor 100 is set to operate in the initial component protection mode (step S1-1). This setting is achieved, for example, by the user (operator) of the gas sensor giving appropriate setting instructions to the controller 110 through a specified interface not shown. Alternatively, the gas sensor 100 can be set to always operate in component protection mode.

[0109] After the component protection mode is activated, NOx concentration measurements are continuously performed under the control of the controller 110, just as in the normal mode. This mode of operation, in which NOx concentration measurements are performed in the same manner as in the normal mode even during component protection mode, is specifically referred to as the basic mode. It should be noted that the operations including those in the normal mode are sometimes referred to as operations in the basic mode. However, when the component protection mode is activated, the controller 110 begins monitoring the specified judgment target value while performing the operations in this basic mode (steps S1-2).

[0110] In this method, the judgment target value refers to the value used as an indicator to determine when to stop the NOx measurement operation. Specifically, the measured value of the electromotive force V0 is used. If the oxygen concentration of the gas to be measured introduced into the first internal cavity 20 is less than the predetermined target oxygen concentration of the first internal cavity 20, oxygen is drawn in by the main pump unit 21 in order to maintain the electromotive force V0 at the target value. However, if the gas to be measured is introduced with excessively rich fuel, sufficient oxygen cannot be drawn in, and the measured value of the electromotive force V0 deviates from the target value (greater than the target value). The value of the electromotive force V0 at the limit of this deviation is preset as the stop threshold. The stop threshold is set to, for example, 350mV.

[0111] Continue monitoring the value of the object to be judged until the pre-set time (judgment time) has elapsed (end) (steps S1-3). The judgment time is set to, for example, about 10 seconds.

[0112] Then, at the moment when the judgment time has elapsed (end) (Yes in step S1-3), the controller 110 judges whether the value of the judgment object exceeded the predetermined stop threshold during the judgment time period (step S1-4). Alternatively, it can judge whether the value of the judgment object exceeded the predetermined stop threshold at the end of the judgment time.

[0113] If the value of the object being measured does not exceed the specified stop threshold within the judgment time (No in step S1-4), the controller 110 allows the gas sensor 100 to continue its measurement operation (step S1-5). For this measurement operation, the component protection mode can be ended and the operation can continue in the normal mode, or the component protection mode can be restarted and the operation can continue in the basic mode.

[0114] On the other hand, if the value of the measured object exceeds the specified stop threshold within the judgment time (Yes in step S1-4), the controller 110 enters a protection execution mode that stops the NOx measurement operation. Specifically, the pump control (feedback control) in the main pump unit 21 and the auxiliary pump unit 50 is stopped, and the measurement pump unit 41 is switched to operation under measurement electrode protection control (step S1-6). Here, measurement electrode protection control refers to a control method in which the measurement pump unit 41 is operated to draw oxygen from the outside into the third internal cavity 61 in order to protect the measurement electrode 44 and prevent excessive reduction due to the fuel being too rich in the measured gas. However, in order to avoid cracking of the sensor element 101, the absolute value of the suction voltage (measurement pump voltage) Vp2 applied to the measurement pump unit 41 during measurement electrode protection control is 2000mV or less.

[0115] In this case, neither the main pump unit 21 nor the auxiliary pump unit 50 operates. Therefore, the gas to be measured introduced from the gas inlet 10 directly enters the third internal cavity 61 through the second internal cavity 40 from the first internal cavity 20. However, oxygen is drawn into the third internal cavity 61 through the measuring pump unit 41. Therefore, the excessive reduction of the measuring electrode 44 due to the fuel-rich gas to be measured is well suppressed.

[0116] When the pump control of the main pump unit 21 and the auxiliary pump unit 50 is stopped, the measured values ​​of the electromotive forces V0 and V1 in the main sensor unit 80 and the auxiliary sensor unit 81, which were previously controlled to achieve a predetermined constant value corresponding to the desired oxygen concentration, are respectively values ​​corresponding to the oxygen concentration of the measured gas flowing into the first internal cavity 20 and the second internal cavity 40 without adjustment. The electromotive force generated in each sensor unit under the above-mentioned pump control stop state is specifically referred to as the OPEN electromotive force.

[0117] The OPEN electromotive force varies in relation to the oxygen concentration of the gas being measured flowing into each internal cavity; the higher the oxygen concentration, the larger the value. Therefore, the respective OPEN electromotive force can be used as an indicator of the oxygen concentration of the gas being measured in the internal cavity corresponding to each sensor unit when the pump control is stopped.

[0118] The controller 110 initiates the shutdown of pump control in the main pump unit 21 and auxiliary pump unit 50, and executes protection control for the measuring electrode targeting the measuring pump unit 41. It also begins monitoring the OPEN electromotive force (EMF) (steps S1-7) and determines whether the OPEN EMF is below a predetermined restart threshold (steps S1-8). At this time, the OPEN EMF in any sensor unit other than the measuring sensor unit 82 (e.g., the main sensor unit 80) can be monitored, and its relationship with the restart threshold can be determined. Monitoring of the OPEN EMF continues as long as it is not below the restart threshold (No in step S1-8).

[0119] The restart threshold is set to a value that determines when the oxygen concentration of the gas being measured flowing into each internal cavity increases to a level where, even if the pump control in each pump unit is restarted, oxygen intake or exhaust in each pump unit can proceed without problems once the OPEN electromotive force reaches that value. For example, this restart threshold can be set based on a pre-determined correlation (functional relationship) between the air-fuel ratio of the gas being measured and the OPEN electromotive force value.

[0120] For example, if the exhaust gas from a gasoline engine is used as the measured gas, even if at some point the exhaust gas, which is too fuel-rich, flows into the sensor element 101 as the measured gas, since this inflow is usually not permanent, after a period of time the oxygen concentration of the exhaust gas will return to the level where each pump unit is working well.

[0121] When the OPEN electromotive force in the main sensor unit 80 is the monitored object, setting the restart threshold to 450mV is a preferred example. It has been previously confirmed that when the OPEN electromotive force is below 450mV, the atmosphere inside the first internal cavity 20 is either stoichiometric or lean fuel composition.

[0122] If the OPEN electromotive force is determined to be lower than the specified restart threshold (Yes in steps S1-8), the controller 110 restarts the pump control operation that was stopped before (steps S1-9). That is, the measurement electrode protection control for the measurement pump unit 41 is stopped, and the target values ​​of the electromotive forces V0, V1, and V2 in the main sensor unit 80, auxiliary sensor unit 81, and measurement sensor unit 82 are reset in a manner that makes the oxygen concentration in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 reach the desired value. Each pump unit then operates again to achieve the aforementioned target value.

[0123] Finally, when the pump unit reaches a state where it can perform feedback control based on the target values ​​of the electromotive forces V0, V1, and V2, the NOx measurement operation is restarted (steps S1-10). For this measurement operation, the component protection mode can be ended and the operation can continue in the normal mode, or the component protection mode can be restarted and the operation can continue in the basic mode.

[0124] In this method, to avoid stopping the NOx measurement operation due to the inability to control the oxygen concentration, oxygen is drawn into the third internal cavity 61 by the measurement pump unit 41 under the protection control of the measurement electrode 44 to prevent over-reduction. Therefore, when the NOx measurement operation is restarted, the measurement electrode 44 quickly returns to a state in which normal feedback control can be performed.

[0125] In this method, when fuel-rich gas is introduced into the sensor element 101 as the measured gas, the operation of the main pump unit 21 and the auxiliary pump unit 50 is stopped to prevent the oxygen concentration from becoming uncontrollable. On the other hand, to suppress excessive reduction of the measuring electrode 44, oxygen is drawn into the third internal cavity 61 by the measuring pump unit 41. Accordingly, even when fuel-rich measured gas reaches the third internal cavity 61, excessive oxygen intake can be prevented, thus protecting the pump unit and effectively suppressing excessive reduction of the measuring electrode 44. Furthermore, when it is determined that the oxygen concentration of the introduced measured gas has recovered to a level suitable for oxygen intake by the pump unit, restarting the NOx measurement allows the measuring electrode to quickly become usable.

[0126] (Second method)

[0127] Figure 3 This is a diagram illustrating the operation flow in the second mode of component protection. The second mode is as follows: In summary, when the gas being measured is an atmosphere with excessively concentrated fuel, excessive intake into the main pump unit 21 is suppressed by temporarily reducing the target oxygen concentration in the first internal cavity 20, and excessive reduction of the measuring electrode is avoided.

[0128] Steps S2-1 to S2-5 in the second method are largely the same as steps S1-1 to S1-5 in the first method. Furthermore, the fact that the value to be determined is specifically the measured value of the electromotive force V0 is also the same as in the first method.

[0129] However, in this approach, the judgment target value is used as an indicator when judging changes in the target oxygen concentration in the first internal cavity 20. More specifically, it is used as an indicator when judging changes in the target value of the electromotive force V0 of the main sensor unit 80 corresponding to the target oxygen concentration. It should be noted that the larger the target value of the electromotive force V0, the smaller the target oxygen concentration in the first internal cavity 20.

[0130] That is, in this method, the value of the electromotive force V0 at the limit of the allowable deviation when the measured value of the measured electromotive force V0 deviates from the target value due to the introduction of the measured gas with excessively concentrated fuel into the first internal cavity 20 is preset as the change threshold. Then, the controller 110 starts the component protection mode (basic mode) (step S2-1) in the same way as in the first method, and starts monitoring the value of the judgment object (step S2-2). At the moment when the judgment time has elapsed (end) (Yes in step S2-3), it judges whether the value of the judgment object has exceeded the prescribed change threshold during the judgment time period (step S2-4). The change threshold may be the same as or different from the stop threshold in the first method.

[0131] If the value of the object being judged does not exceed the specified change threshold within the judgment time (No in step S2-4), similarly to the first method, the controller 110 allows the measurement operation in the gas sensor 100 to continue in either the normal mode or the component protection mode (basic mode) (step S2-5).

[0132] On the other hand, if the value of the object being judged exceeds the prescribed change threshold within the judgment time (Yes in step S2-4), this method also switches from the basic mode to the protection execution mode. However, in this mode, the controller 110 changes the control reference value in the pump control performed during NOx concentration measurement relative to the value (normal value) in the basic mode (normal mode) (step S2-6).

[0133] Specifically, the controller 110 modifies the target value of the electromotive force V0 of the main sensor unit 80, which serves as the pump control reference in the main pump unit 21, to be greater than its normal value (and thus, greater than the change threshold). For example, the target value of the electromotive force V0 is changed from 250mV in the normal mode to 350mV. This modified target value of the electromotive force V0 is referred to as the modified reference value. By modifying the target value of the electromotive force V0 in this way, the target oxygen concentration in the first internal cavity 20 is reduced. In addition, the control reference value for pump control in the auxiliary pump unit 50 can also be modified relative to the value in the normal mode.

[0134] Then, the controller 110 begins controlling the main pump unit 21 (or the auxiliary pump unit 50) based on the modified reference value, while switching the measuring pump unit 41 to operation under measuring electrode protection control (steps S2-7). As a result, in this mode, NOx measurement is also stopped in the protection execution mode. Furthermore, in this mode, the absolute value of the suction voltage (measuring pump voltage) Vp2 applied to the measuring pump unit 41 during measuring electrode protection control is also made to be 2000mV or less.

[0135] By setting the target value of the electromotive force V0 to a modified reference value that is larger than the usual value, the target oxygen concentration in the first internal cavity 20 is reduced, making the difference between this target oxygen concentration and the oxygen concentration in the measured gas, where the fuel is excessively enriched, less than in the normal mode. Therefore, when each pump unit is controlled based on the modified reference value, the amount of oxygen drawn into the first internal cavity 20 through the main pump unit 21 is reduced compared to before the modification in order to achieve the target oxygen concentration. As a result, excessive increases in the main pump current Ip0 or the main pump voltage Vp0 in the main pump unit 21 are suppressed.

[0136] Furthermore, in the first mode, when the value of the object being judged exceeds the stop threshold and enters the protection execution mode, the main pump unit 21 and the auxiliary pump unit 50 are stopped. However, in this mode, the pump control operation itself in the above-mentioned pump unit continues. Therefore, compared with the first mode, the time required from the end of the protection execution mode to the measured restart is shorter.

[0137] The controller 110 causes the measuring pump unit 41 to start performing the action based on the measuring electrode protection control, and starts monitoring the pump unit action value (step S2-8), and determines whether the pump unit action value is lower than the recovery threshold (step S2-9).

[0138] The pump unit operating value is specifically the main pump current Ip0 or the main pump voltage Vp0. The pump unit operating value increases with the amount of oxygen drawn in. Furthermore, the recovery threshold is set to a value that allows the determination of when the oxygen concentration of the measured gas flowing into each internal cavity increases to a level where, even if the control reference value is restored from the changed reference value to its normal value, oxygen can still be drawn in or drawn out of each pump unit without any problems when the pump unit operating value reaches this level.

[0139] As long as the pump unit action value is not lower than the recovery threshold (No in step S2-9), the monitoring of the pump unit action value will continue.

[0140] On the other hand, if it is determined that the pump unit's operating value is lower than the specified recovery threshold (Yes in step S2-9), the controller 110 restores the control reference value from the changed reference value to the normal value (step S2-10). Then, the operation of the measurement pump unit 41 based on the measurement electrode protection control is stopped, and instead, the NOx measurement operation based on the normal control reference value is restarted (step S2-11). For this measurement operation, the component protection mode can be ended and the operation can continue in the normal mode, or the component protection mode can be restarted to continue.

[0141] In this configuration, when fuel-concentrated gas is introduced into the sensor element 101 as the measured gas, the target oxygen concentration in the first internal cavity 20 is temporarily reduced to prevent uncontrollable oxygen concentration, thereby suppressing excessive intake in the main pump unit 21. On the other hand, oxygen is drawn into the third internal cavity 61 by the measurement pump unit 41 to suppress over-reduction of the measurement electrode 44. Therefore, similar to the first configuration, even when fuel-concentrated measured gas reaches the third internal cavity 61, over-reduction of the measurement electrode 44 can be well suppressed. Furthermore, unlike the first configuration, the operation of the main pump unit 21 and the auxiliary pump unit 50 is not stopped. Therefore, when NOx measurement is restarted after determining that the oxygen concentration of the introduced measured gas has recovered to a level suitable for oxygen intake by the pump unit, the measurement electrode can be quickly made usable.

[0142] (Third method)

[0143] Figure 4 and Figure 5 This is a diagram illustrating the operation flow in the third mode of component protection. The third mode is a method that combines the first and second modes. In the case where the measured gas is in an atmosphere where the fuel is too rich, the response to avoid over-reduction of the measuring electrode and to address the situation where the oxygen concentration in the main pump unit 21 cannot be controlled is carried out in two stages, depending on the degree of the situation.

[0144] In short, when the gas being measured is an atmosphere with excessively concentrated fuel, firstly, similar to the second method, the measuring pump unit 41 is subjected to measuring electrode protection control. On the other hand, excessive intake in the main pump unit 21 is suppressed by temporarily reducing the target oxygen concentration in the first internal cavity 20. Furthermore, even if the oxygen concentration in the first internal cavity 20 does not sufficiently recover, similar to the first method, the NOx measuring operation of the gas sensor 100, including the intake operation of oxygen drawn from the outside, is temporarily stopped.

[0145] Steps S3-1 to S3-7 of the third method are the same as steps S2-1 to S2-7 of the second method. That is, in the third method, similarly to the second method, the controller 110 first starts the component protection mode (basic mode) (step S3-1) and begins monitoring the value of the judgment object (step S3-2). Then, at the moment when the judgment time has elapsed (end) (Yes in step S3-3), it judges whether the value of the judgment object exceeded the specified change threshold during the judgment time period (step S3-4). It should be noted that in this method, the value of the judgment object is specifically the measured value of the electromotive force V0.

[0146] If the value of the object being judged does not exceed the specified change threshold within the judgment time (No in step S3-4), similarly to the first and second methods, the controller 110 allows the measurement operation in the gas sensor 100 to continue in normal mode or component protection mode (basic mode) (step S3-5).

[0147] On the other hand, if the value of the object being judged exceeds the prescribed change threshold within the judgment time (Yes in step S3-4), in this method, the system also switches from the basic mode to the protection execution mode. The controller 110 changes the control reference value of the main pump unit 21 (or the auxiliary pump unit 50) relative to its value in the basic mode (normal mode) (step S3-6), causing it to begin control based on the changed reference value. Meanwhile, for the measuring pump unit 41, the system switches to operation under measuring electrode protection control (step S3-7). As a result, NOx measurement is stopped. Furthermore, in this method, the absolute value of the suction voltage (measuring pump voltage) Vp2 applied to the measuring pump unit 41 during measuring electrode protection control is also ensured to be below 2000mV.

[0148] In addition, monitoring of the pump unit's operating value begins simultaneously (step S3-8). If the pump unit's operating value is determined to be below the specified recovery threshold (Yes in step S3-9), the controller 110 restores the control reference value from the changed reference value to the normal value (step S3-10), stopping the operation of the measurement pump unit 41 based on the measurement electrode protection control. Instead, the NOx measurement operation based on the normal control reference value is restarted in normal mode or component protection mode (basic mode) (step S3-11), which is the same as the second method.

[0149] On the other hand, if it is determined that the pump unit's operating value is not lower than the recovery threshold (No in step S3-9), it is determined whether the value of the judgment object exceeds the specified stop threshold (step S3-12). It should be noted that in this method, the stop threshold is set to a value larger than the change threshold.

[0150] If the value of the determined object does not exceed the stop threshold (No in step S3-12), return to step S3-8, restart the monitoring of the pump unit's operating value, and repeat the processing after step S3-9. Therefore, if the pump unit's operating value is above the specified recovery threshold and the determined object value is below the stop threshold, repeat the cycle of step S3-8 → step S3-9 → step S3-12 → step S3-8 → ... This means that if the main pump unit 21 excessively inhales oxygen due to the introduction of fuel-rich gas into the sensor element 101 to a degree that can be avoided by reducing the target oxygen concentration, the response will continue in this manner. In this situation, the protection control of the measuring electrode targeting the measuring pump unit 41 also continues.

[0151] If the value of the target object exceeds the stop threshold after the control reference value is changed (Yes in step S3-12), the same steps as in the first method are performed. The intention of performing this step is to achieve more reliable protection when simply reducing the target oxygen concentration is insufficient to adequately protect the pump unit.

[0152] Specifically, firstly, the controller 110 stops the pump control in the main pump unit 21 and the auxiliary pump unit 50 (steps S3-13). Therefore, in this state, only the measurement pump unit 41, which implements the measurement electrode protection control, operates. That is, even if it is difficult to avoid the intake of excessive oxygen by the main pump unit 21 by reducing the target oxygen concentration, the operation of the pump units is stopped in order to protect the main pump unit 21 and the auxiliary pump unit 50.

[0153] The controller 110 further begins monitoring the OPEN electromotive force (EMF) in sensor units other than the measuring sensor unit 82 (step S3-14), and determines whether the OPEN EMF is lower than a predetermined restart threshold (step S3-15). When the OPEN EMF in the main sensor unit 80 is the monitored object, setting the restart threshold to 450mV is a preferred example. Monitoring of the OPEN EMF continues as long as the OPEN EMF is not lower than the restart threshold (No in step S3-15).

[0154] If the OPEN electromotive force is determined to be lower than the restart threshold (Yes in step S3-15), the controller 110 will stop the measurement electrode protection control for the measurement pump unit 41, and then reset the control reference value to the normal value for each pump unit, restarting the pump control operation (step S3-16). Finally, when the state becomes such that feedback control based on electromotive forces V0, V1, and V2 can be performed in each pump unit, the NOx measurement operation in the normal mode or component protection mode (basic mode) will be restarted (step S3-17).

[0155] In this method, the first and second methods are combined, and the response method for preventing uncontrollable oxygen concentration in the protection execution mode is switched in stages according to the fuel enrichment level of the gas to be measured introduced into the sensor element 101. On the other hand, the measuring electrode 44 is protected to prevent over-reduction, just like in the first and second methods. Accordingly, even if the gas to be measured with excessive fuel enrichment reaches the third internal cavity 61, over-reduction of the measuring electrode 44 can be well suppressed. On the other hand, the shutdown of the main pump unit 21 and the auxiliary pump unit 50 is kept to a minimum. In addition, when it is determined that the oxygen concentration of the introduced gas to be measured has recovered to a level that allows oxygen to be drawn in by the pump unit, the NOx measurement can be restarted, and the measuring electrode can be quickly made usable.

[0156] As explained above, according to this embodiment, even when using a gas sensor in an environment where the measured gas, such as exhaust gas from a gasoline engine, may be a fuel-rich gas with a low air-fuel ratio, it is possible to effectively avoid a situation where oxygen cannot be drawn into the internal cavity, thus preventing the oxygen concentration from becoming uncontrollable. Furthermore, the excessive reduction of the measuring electrode due to the fuel-rich gas is well suppressed; therefore, when NOx measurement is restarted, the measuring electrode can be quickly made usable again.

[0157] <Variation Example>

[0158] In the above embodiments, the measured value of the electromotive force V0 is used as the judgment object value. However, it is also possible to use the value of the main pump current Ip0 when oxygen intake occurs in the main pump unit 21 as the judgment object value instead. This main pump current Ip0 increases correspondingly to the amount of oxygen intake toward the first internal cavity 20, and therefore can serve as an indicator of the degree of oxygen intake toward the first internal cavity 20. The stop threshold in the first method and the change threshold in the second and third methods are set according to the judgment object value used.

[0159] In addition, in the basic modes of normal mode and component protection mode, the gas sensor 100 can be activated by a control method that uses constant Ip1 control. This control method involves controlling the auxiliary pump unit 50 by allowing a constant auxiliary pump current Ip1 to flow. In this case, the measured value of the auxiliary pump current Ip1 can replace the measured value of the electromotive force V0, or be used in combination with the measured value of the electromotive force V0, as the judgment object value in the protection execution mode of the component protection mode. During the period when the oxygen concentration in the gas to be measured is maintained at a predetermined value in the gas introduced from the first internal cavity 20 to the second internal cavity 40, the auxiliary pump current Ip1 when the auxiliary pump unit 50 draws out oxygen is maintained at a preset constant value, thereby controlling the oxygen concentration in the second internal cavity 40 to a predetermined value. However, if the measured gas with an excessively concentrated fuel is introduced into the first internal cavity 20, making it impossible to adjust the oxygen concentration in the first internal cavity 20, the measured gas with a lower oxygen concentration will enter the second internal cavity 40. Consequently, the auxiliary pump unit 50 will be unable to extract oxygen from the second internal cavity 40, the measured value of the electromotive force V1 will increase relative to the constant value, and the measured value of the auxiliary pump current Ip1 will decrease relative to the set constant value. Therefore, the measured value of the auxiliary pump current Ip1 can also be used as a judgment value.

[0160] In the above embodiments, the gas sensor is designed to have a sensor element with three cavities inside. However, the configuration of the sensor element is not limited to the configuration of the above embodiments, but may be blackened due to the introduction of the measured gas with excessive fuel enrichment into the interior of the sensor element.

[0161] In this case, similarly, as long as the gas sensor has a sensor element with an internal cavity that maintains a constant oxygen concentration by allowing oxygen to be drawn in or drawn out through an electrochemical pump unit, the first to third methods described above can be appropriately modified and applied according to the element configuration as needed, thereby avoiding situations where the oxygen concentration cannot be controlled in the sensor element. Furthermore, it is also possible to effectively suppress excessive reduction of the measuring electrode due to fuel-rich gas. That is, the sensor element can be well protected in a fuel-rich gas atmosphere.

Claims

1. A NOx sensor configured to monitor NOx in a gas to be measured. The NOx sensor is characterized by having: The sensor element is composed of a solid electrolyte with oxygen ion conductivity; and A controller that controls the operation of the NOx sensor. The sensor element includes: Multiple internal cavities are connected sequentially from the inlet of the gas to be measured under a specified diffusion resistance, and are provided with inner electrodes. An external pump electrode, disposed in a location other than the plurality of internal cavities; and A reference electrode configured to contact a reference gas; The sensor element also includes: Multiple electrochemical pump units are configured to enable oxygen intake or exhaust between corresponding internal cavities in the multiple internal cavities and the exterior of the sensor element by applying a pump voltage between each of the inner electrodes and the outer pump electrode of the cavity using a predetermined pump power supply; and Multiple electrochemical sensor units are configured to generate a potential difference between each inner electrode and the reference electrode that corresponds to the oxygen concentration in the corresponding internal cavity. Each of the plurality of internal cavities has one of its internal electrodes, which is a measuring electrode capable of reducing NOx. The plurality of electrochemical pump units include: A measuring pump unit, comprising the measuring electrode; and At least one oxygen concentration control pump unit, the at least one oxygen concentration control pump unit including the inner electrode other than the measuring electrode, The plurality of electrochemical sensor units include: A measuring sensor unit, comprising the measuring electrode; and At least one oxygen concentration monitoring sensor unit, the at least one oxygen concentration monitoring sensor unit including the inner electrode other than the measuring electrode, The controller determines whether a specified judgment object value exceeds a specified threshold within a specified judgment time. The specified judgment object value is an indicator of the oxygen inhalation action in the judgment object pump unit included in the at least one oxygen concentration control pump unit. As long as the value of the target does not exceed the predetermined threshold, the NOx sensor is controlled in a basic mode. In this basic mode, the oxygen concentration in each of the multiple internal cavities is kept constant by activating the multiple electrochemical pump units. The NOx concentration is determined based on the measurement pump current flowing between the measurement electrode and the external pump electrode in the measurement pump unit, corresponding to the NOx concentration, due to NOx reduction at the measurement electrode. If the value of the determined object exceeds the specified threshold, the basic mode is stopped, and a protection execution mode is used to control the NOx sensor to protect the measuring electrode and prevent over-reduction.

2. The NOx sensor according to claim 1, characterized in that, The specified threshold is a stop threshold used to determine whether the at least one oxygen concentration control pump unit needs to be stopped. When the value of the determined object exceeds the stop threshold, the controller initiates the protection execution mode. In this mode, the operation of at least one oxygen concentration control pump unit is stopped, and the measuring pump unit is forced to draw in oxygen. Furthermore, monitoring of the potential difference in the sensor unit corresponding to the determined object pump unit among the at least one oxygen concentration monitoring sensor unit begins. When the monitored potential difference falls below a predetermined restart threshold, the NOx sensor is restarted and controlled in the basic mode.

3. The NOx sensor according to claim 1, characterized in that, The specified threshold is a change threshold, which is used to determine whether the target value of the potential difference in the at least one oxygen concentration monitoring sensor unit needs to be changed when controlling the application of the pump voltage in the at least one oxygen concentration control pump unit. When the value of the target object exceeds the change threshold, the controller initiates the protection execution mode. In this mode, after changing the target value to a value larger than the usual value, it continues to control the at least one oxygen concentration control pump unit, causing the measuring pump unit to draw in oxygen. Furthermore, it begins monitoring the pump voltage or current value (i.e., the pump unit's operating value) of the at least one oxygen concentration control pump unit, including the target object pump unit. When the pump unit's operating value falls below the recovery threshold, the target value is restored to the normal value, thereby restoring the control of the NOx sensor to the basic mode.

4. The NOx sensor according to claim 1, characterized in that, The specified threshold is a change threshold, which is used to determine whether the target value of the potential difference in the at least one oxygen concentration monitoring sensor unit needs to be changed when controlling the application of the pump voltage in the at least one oxygen concentration control pump unit. When the value of the target object exceeds the change threshold, the controller initiates the protection execution mode. In this mode, after changing the target value to a value larger than the usual value, it continues to control the at least one oxygen concentration control pump unit, causing the measuring pump unit to draw in oxygen. Furthermore, it begins monitoring the pump voltage or current value (i.e., the pump unit's operating value) of the at least one oxygen concentration control pump unit, including the target object pump unit. When the pump unit's operating value falls below the recovery threshold, the target value is restored to its normal value, thereby restoring the NOx sensor's control to the basic mode. As long as the pump unit action value does not fall below the recovery threshold and the judgment object value does not exceed the stop threshold which is greater than the change threshold, the monitoring of the pump unit action value will continue. If the pump unit's operating value does not fall below the recovery threshold and the value of the target being judged further exceeds the stop threshold, monitoring of the potential difference in the sensor unit corresponding to the target pump unit among the at least one oxygen concentration monitoring sensor unit begins. When the monitored potential difference falls below a predetermined restart threshold, the NOx sensor is restarted and controlled in the basic mode.

5. The NOx sensor according to any one of claims 1 to 4, characterized in that, The value of the object to be judged is the measured value of the potential difference in the sensor unit corresponding to the object to be judged pump unit among the at least one oxygen concentration monitoring sensor unit.

6. The NOx sensor according to any one of claims 1 to 4, characterized in that, The value of the judgment object is the value of the pump current when oxygen is drawn into the judgment object pump unit.

7. The NOx sensor according to claim 1, 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 a diffusion resistance section. The inner electrodes, other than the measuring electrode, are the main pump electrode disposed in the first internal cavity and the auxiliary pump electrode disposed in the second internal cavity. The measuring electrode is disposed in the third internal cavity. The at least one oxygen concentration control pump unit is a main pump unit for controlling the oxygen concentration in the first internal cavity and an auxiliary pump unit for controlling the oxygen concentration in the second internal cavity. The at least one oxygen concentration monitoring sensor unit comprises: 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. At least in the basic mode, the controller operates the main pump unit and the auxiliary pump unit in such a way that the oxygen concentrations in the first internal cavity and the second internal cavity are respectively maintained at a predetermined constant value, and determines the NOx concentration based on the magnitude of the measuring pump current flowing through the measuring pump unit corresponding to the concentration of NOx contained in the measured gas, which has been adjusted to the oxygen concentration introduced into the third internal cavity.

8. A method for operating a NOx sensor, the NOx sensor comprising a sensor element made of a solid electrolyte with oxygen ion conductivity, and configured to monitor NOx in a gas to be measured. The operating method of the NOx sensor is characterized by the following: The sensor element includes: Multiple internal cavities are connected sequentially from the inlet into which the gas to be measured is introduced, under a specified diffusion resistance. Multiple inner electrodes are disposed facing each of the multiple inner cavities; An external pump electrode, disposed in a location other than the plurality of internal cavities; and A reference electrode configured to contact a reference gas; The sensor element also includes: Multiple electrochemical pump units are configured to enable oxygen intake or exhaust between corresponding internal cavities in the multiple internal cavities and the outside of the sensor element by applying a pump voltage between each of the inner electrodes and the outer pump electrode of the cavity with a specified pump power supply. as well as Multiple electrochemical sensor units are configured to generate a potential difference between each inner electrode and the reference electrode that corresponds to the oxygen concentration in the corresponding internal cavity. Each of the plurality of internal cavities has one of its internal electrodes, which is a measuring electrode capable of reducing NOx. The plurality of electrochemical pump units include: A measuring pump unit, comprising the measuring electrode; and At least one oxygen concentration control pump unit, the at least one oxygen concentration control pump unit including the inner electrode other than the measuring electrode, The plurality of electrochemical sensor units include: A measuring sensor unit, comprising the measuring electrode; and At least one oxygen concentration monitoring sensor unit, the at least one oxygen concentration monitoring sensor unit including the inner electrode other than the measuring electrode, In this case, there is a judgment step that determines whether a specified judgment object value exceeds a specified threshold within a specified judgment time. The specified judgment object value is an indicator of the oxygen inhalation action in the judgment object pump unit included in the at least one oxygen concentration control pump unit. In the judgment step, as long as the value of the judgment object does not exceed the predetermined threshold, the NOx sensor is controlled in the basic mode. In this basic mode, the oxygen concentration in each of the plurality of internal cavities is kept constant, and the NOx concentration is determined based on the measurement pump current flowing between the measurement electrode and the external pump electrode in the measurement pump unit, corresponding to the NOx concentration, due to the reduction of NOx at the measurement electrode. In the judgment step, if the value of the judgment object exceeds the specified threshold, the basic mode is stopped, and the NOx sensor is activated by a protection execution mode to protect the measuring electrode and prevent over-reduction.

9. The method for operating a NOx sensor according to claim 8, characterized in that, The specified threshold is a stop threshold used to determine whether the at least one oxygen concentration control pump unit needs to be stopped. In the judgment step, if the value of the judgment target exceeds the stop threshold, the protection execution mode is initiated. In the protection execution mode, the operation of at least one oxygen concentration control pump unit is stopped, and the measuring pump unit is allowed to draw in oxygen. Furthermore, monitoring of the potential difference in the sensor unit corresponding to the judgment target pump unit among the at least one oxygen concentration monitoring sensor unit begins. When the monitored potential difference falls below a predetermined restart threshold, the NOx sensor is restarted in the basic mode to activate.

10. The method for operating a NOx sensor according to claim 8, characterized in that, The specified threshold is a change threshold, which is used to determine whether the target value of the potential difference in the at least one oxygen concentration monitoring sensor unit needs to be changed when controlling the application of the pump voltage in the at least one oxygen concentration control pump unit. In the judgment step, if the value of the judgment target exceeds the change threshold, the protection execution mode is initiated. In the protection execution mode, after changing the target value to a value larger than the predetermined normal value, the operation of at least one oxygen concentration control pump unit continues, causing the measuring pump unit to draw in oxygen. Furthermore, monitoring of the pump voltage or current value (i.e., the pump unit operation value) of at least one oxygen concentration control pump unit, including the judgment target pump unit, begins. When the pump unit's operating value falls below the recovery threshold, the target value is restored to the specified normal value, thereby restoring the NOx sensor's operation to the basic mode.

11. The method for operating a NOx sensor according to claim 8, characterized in that, The specified threshold is a change threshold, which is used to determine whether the target value of the potential difference in the at least one oxygen concentration monitoring sensor unit needs to be changed when controlling the application of the pump voltage in the at least one oxygen concentration control pump unit. In the judgment step, if the value of the judgment target exceeds the change threshold, the protection execution mode is initiated. In the protection execution mode, after changing the target value to a value larger than the normal value, the operation of at least one oxygen concentration control pump unit continues, causing the measuring pump unit to draw in oxygen. Furthermore, monitoring of the pump voltage or current value (i.e., the pump unit operation value) of at least one oxygen concentration control pump unit, including the judgment target pump unit, begins. When the pump unit's operating value falls below the recovery threshold, the target value is restored to its normal value, thereby restoring the NOx sensor's control to the basic mode. As long as the pump unit action value does not fall below the recovery threshold and the judgment object value does not exceed the stop threshold which is greater than the change threshold, the monitoring of the pump unit action value will continue. If the pump unit's operating value does not fall below the recovery threshold and the value of the target being judged further exceeds the stop threshold, monitoring of the potential difference in the sensor unit corresponding to the target pump unit among the at least one oxygen concentration monitoring sensor unit begins. When the monitored potential difference falls below a predetermined restart threshold, the NOx sensor is restarted in the basic mode to activate.

12. The method of operating the NOx sensor according to any one of claims 8 to 11, characterized in that, The value of the judgment object is set as the measured value of the potential difference in the sensor unit corresponding to the judgment object pump unit in the at least one oxygen concentration monitoring sensor unit.

13. The method of operating the NOx sensor according to any one of claims 8 to 11, characterized in that, The value of the judgment object is set to the value of the pump current when oxygen is drawn into the judgment object pump unit.

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