Gas sensor and method for diagnosing the moisture absorption state of the gas sensor

Abstract

To diagnose a moisture absorption state around a reference electrode.SOLUTION: A gas sensor 100 comprises a sensor element 101 and a controller. The sensor element 101 includes: an element body (layers 1 to 6) inside which a gas-to-be-measured flow portion is provided; a measurement electrode 44 disposed on the gas-to-be-measured flow portion; an outer pump electrode 23 provided on the element body to be in contact with gas to be measured; a reference electrode 42 disposed inside the element body; a reference gas introduction portion 49 that causes reference gas to flow from outside of the element body to the reference electrode 42; and a reference gas adjustment pump cell 90 configured to include the outer pump electrode 23 and the reference electrode 42. The controller performs moisture absorption state diagnostic processing for diagnosing a moisture absorption state around the reference electrode 42 on the basis of a pump current Ip3 flowing through the reference gas adjustment pump cell 90 when controlling the reference gas adjustment pump cell 90 to pump oxygen out from around the reference electrode 42 to around the outer pump electrode 23.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a gas sensor and a method for diagnosing the moisture absorption state of the gas sensor.

Background Art

[0002] Conventionally, a sensor element used in a gas sensor for detecting the concentration of a specific gas such as NOx in a measured gas such as automotive exhaust gas is known. For example, in Patent Document 1, an element main body having an oxygen ion conductive solid electrolyte layer and provided therein with a measured gas flow portion for introducing and flowing the measured gas, a measurement electrode disposed on the inner peripheral surface of the measured gas flow portion, a reference electrode disposed inside the element main body, and a reference gas introduction portion for introducing a reference gas (for example, air) serving as a reference for detecting the concentration of a specific gas in the measured gas and flowing it to the reference electrode are described. The reference gas introduction portion has a porous reference gas introduction layer. The concentration of a specific gas in the measured gas can be detected based on the electromotive force generated between the reference electrode and the measurement electrode of this sensor element.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, for example, during a period when the sensor element is not being driven, the reference gas introduction portion may adsorb external water. When the driving is started, since the sensor element is heated, the water in the reference gas introduction portion becomes a gas and escapes to the outside from the reference gas introduction portion. However, until the water escapes, the presence of gaseous water causes the oxygen concentration around the reference electrode to decrease. As a result, the detection accuracy of the specific gas concentration may decrease until the water escapes. Therefore, there has been a desire to diagnose the moisture absorption state around the reference electrode.

[0005] This invention was made to solve these problems, and its main purpose is to diagnose the moisture absorption state around a reference electrode. [Means for solving the problem]

[0006] To achieve the main objectives described above, the present invention employs the following means.

[0007] The gas sensor of the present invention is A gas sensor for detecting the concentration of a specific gas in a gas being measured, The element body includes an oxygen ion conductive solid electrolyte layer and has an internal gas flow section for introducing and circulating the gas to be measured, The measuring electrode is disposed in the gas flow section to be measured, The element body is provided with an electrode on the side of the gas to be measured so as to be in contact with the gas to be measured, A reference electrode disposed inside the main body of the element, A reference gas introduction unit that flows a reference gas, which serves as a reference for detecting the concentration of the specific gas in the gas to be measured, from outside the element body to the reference electrode, A reference gas adjustment pump cell comprising the aforementioned electrode on the gas side to be measured and the aforementioned reference electrode, A sensor element having, A control unit performs a moisture absorption state diagnostic process that diagnoses the moisture absorption state around the reference electrode based on the pump current flowing through the reference gas adjustment pump cell when the reference gas adjustment pump cell is controlled to pump oxygen from around the reference electrode to around the measured gas side electrode, It is something that is provided.

[0008] In this gas sensor, the control unit diagnoses the moisture absorption state around the reference electrode based on the pump current flowing through the reference gas adjustment pump cell when it controls the reference gas adjustment pump cell to pump oxygen from around the reference electrode to around the measured gas electrode. Here, the pump current flowing when the reference gas adjustment pump cell pumps oxygen from around the reference electrode to around the measured gas electrode changes depending on the amount of moisture around the reference electrode. Therefore, the moisture absorption state around the reference electrode can be diagnosed based on this pump current. The control unit may also determine whether or not there is a lot of moisture around the reference electrode during the moisture absorption state diagnosis process.

[0009] In the gas sensor of the present invention, the control unit may, in the moisture absorption state diagnosis process, diagnose the moisture absorption state around the reference electrode based on the pump current when a predetermined control voltage higher than the voltage in the limit current region of the reference gas adjustment pump cell is applied between the electrode on the gas to be measured and the reference electrode. Applying a voltage higher than the voltage in the limit current region makes the moisture around the reference electrode more easily decomposed, so the amount of moisture around the reference electrode easily affects the pump current. Therefore, by using the pump current when such a voltage is applied, the moisture absorption state of the reference electrode can be diagnosed more appropriately.

[0010] In this case, the control unit may diagnose the moisture absorption state around the reference electrode in the moisture absorption state diagnosis process based on a comparison between the pump current and the limit current of the reference gas adjustment pump cell. The greater the moisture around the reference electrode, the greater the difference between the pump current and the limit current, so by comparing these, the moisture absorption state around the reference electrode can be diagnosed more appropriately. In this case, the control unit may diagnose the moisture absorption state around the reference electrode in the moisture absorption state diagnosis process based on the difference or ratio between the pump current and the limit current.

[0011] In a gas sensor of the present invention that compares a pump current with a limit current, the control unit may include a storage unit that stores the value of the limit current, and the control unit may compare the pump current with the limit current stored in the storage unit during the moisture absorption state diagnostic process. In this case, it is not necessary to measure the limit current during the moisture absorption state diagnostic process.

[0012] In the gas sensor of the present invention, which compares the pump current and the limit current, the control unit may, in the moisture absorption state diagnostic process, compare the pump current with the limit current measured by applying a voltage in the limit current region to the reference gas adjustment pump cell. In this way, since not only the pump current but also the limit current is measured in the moisture absorption state diagnostic process, a more accurate diagnosis can be performed.

[0013] In the gas sensor of the present invention, the predetermined control voltage may be a voltage of 0.8V or more and 1.5V or less. If the control voltage is 0.8V or higher, the pump current when a voltage in this range is applied is easily affected by the amount of moisture around the reference electrode, making it suitable for moisture absorption state diagnostic processing. If the control voltage is 1.5V or lower, blackening of the sensor element can be suppressed.

[0014] In the gas sensor of the present invention, a heater for heating the element body is provided, and the control unit may perform the moisture absorption state diagnostic process after energizing the heater and the heater temperature reaches a predetermined temperature or higher. In this way, since the moisture absorption state diagnostic process is performed after the heater temperature has risen, the reference gas adjustment pump cell can be operated in a state where the solid electrolyte layer is activated and exhibits oxygen ion conductivity. Therefore, the moisture absorption state diagnostic process can be performed at an appropriate timing.

[0015] The present invention provides a method for diagnosing the moisture absorption state of a gas sensor, A method for diagnosing the moisture absorption state of a gas sensor that detects the concentration of a specific gas in a gas to be measured, The aforementioned gas sensor is An element body including an oxygen ion conductive solid electrolyte layer, and having a measured gas flow portion provided therein for introducing and flowing the measured gas; A measurement electrode disposed in the measured gas flow portion; A measured gas side electrode provided on the element body so as to contact the measured gas; A reference electrode disposed inside the element body; A reference gas introduction portion for flowing a reference gas, which serves as a reference for detecting the concentration of the specific gas in the measured gas, from the outside of the element body to the reference electrode; A reference gas adjustment pump cell including the measured gas side electrode and the reference electrode; A sensor element having the above; A moisture absorption state diagnosis process for diagnosing the moisture absorption state around the reference electrode based on the pump current flowing through the reference gas adjustment pump cell when the reference gas adjustment pump cell is controlled to pump out oxygen from around the reference electrode to around the measured gas side electrode; It includes the above.

[0016] In the method for diagnosing the moisture absorption state of this gas sensor, similar to the above-described gas sensor, the moisture absorption state around the reference electrode can be diagnosed. In addition, in the method for diagnosing the moisture absorption state of this gas sensor, various aspects of the above-described gas sensor may be adopted, or processes for realizing each function of the above-described gas sensor may be added.

Brief Description of Drawings

[0017] [Figure 1] A longitudinal sectional view of the gas sensor 100. [Figure 2] A schematic cross-sectional view schematically showing an example of the configuration of the sensor element 101. [Figure 3] A block diagram showing the electrical connection relationship between the control device 95 and each cell. [Figure 4] A graph showing the relationship between the voltage Vp3 and the pump current Ip3 of the reference gas adjustment pump cell 90. [Figure 5] A flowchart showing an example of a control routine. [Figure 6] A graph showing the relationship between time t and voltage V2open. [Figure 7] A schematic cross-sectional diagram showing the configuration around the reference gas introduction section 249 of the modified example. [Figure 8] A schematic cross-sectional view of the modified sensor element 201. [Modes for carrying out the invention]

[0018] Next, embodiments of the present invention will be described with reference to the drawings. Figure 1 is a longitudinal cross-sectional view of a gas sensor 100, which is one embodiment of the present invention. Figure 2 is a schematic cross-sectional view showing an example of the configuration of a sensor element 101 provided in the gas sensor 100. Figure 3 is a block diagram showing the electrical connection relationship between the control device 95 and each cell. The sensor element 101 has a long rectangular parallelepiped shape, and the longitudinal direction of the sensor element 101 (left-right direction in Figure 2) is defined as the front-back direction, and the thickness direction of the sensor element 101 (up-down direction in Figure 2) is defined as the up-down direction. In addition, the width direction of the sensor element 101 (direction perpendicular to the front-back and up-down directions) is defined as the left-right direction.

[0019] As shown in Figure 1, the gas sensor 100 comprises a sensor element 101, a protective cover 130 that protects the front end of the sensor element 101, and a sensor assembly 140 that includes a connector 150 that is electrically connected to the sensor element 101. As shown in the figure, the gas sensor 100 is attached to a pipe 190, such as the exhaust gas pipe of a vehicle, and is used to measure the concentration of specific gases such as NOx and O2 contained in the exhaust gas, which is the gas to be measured. In this embodiment, the gas sensor 100 measures the NOx concentration as the specific gas concentration.

[0020] The protective cover 130 comprises a bottomed cylindrical inner protective cover 131 that covers the front end of the sensor element 101, and a bottomed cylindrical outer protective cover 132 that covers the inner protective cover 131. The inner protective cover 131 and the outer protective cover 132 have multiple holes formed therein to allow the gas to be measured to flow into the protective cover 130. A sensor element chamber 133 is formed as a space surrounded by the inner protective cover 131, and the front end of the sensor element 101 is located within this sensor element chamber 133.

[0021] The sensor assembly 140 includes an element encapsulant 141 for enclosing and fixing the sensor element 101, bolts 147 and an outer cylinder 148 attached to the element encapsulant 141, and a connector 150 that contacts and is electrically connected to connector electrodes (only the heater connector electrode 71, which will be described later, is shown in Figure 2) formed on the surface (upper and lower surfaces) of the rear end of the sensor element 101.

[0022] The element encapsulant 141 comprises a cylindrical main body fitting 142, a cylindrical inner cylinder 143 welded and fixed coaxially to the main body fitting 142, and ceramic supporters 144a-144c, compacted powder 145a, 145b, and a metal ring 146 sealed within through holes inside the main body fitting 142 and the inner cylinder 143. The sensor element 101 is located on the central axis of the element encapsulant 141 and penetrates the element encapsulant 141 in the front-rear direction. The inner cylinder 143 has a diameter-reducing portion 143a for pressing the compacted powder 145b in the direction of the central axis of the inner cylinder 143, and a diameter-reducing portion 143b for pressing the ceramic supporters 144a-144c and compacted powder 145a, 145b forward via the metal ring 146. The pressing force from the reduced diameter portions 143a and 143b compresses the compacted powder bodies 145a and 145b between the main fitting 142 and the inner cylinder 143 and the sensor element 101. As a result, the compacted powder bodies 145a and 145b seal the space between the sensor element chamber 133 in the protective cover 130 and the space 149 in the outer cylinder 148, while also fixing the sensor element 101 in place.

[0023] The bolt 147 is fixed coaxially with the main fitting 142 and has a male threaded portion formed on its outer surface. The male threaded portion of the bolt 147 is inserted into a fixing member 191 which is welded to the pipe 190 and has a female threaded portion on its inner surface. As a result, the gas sensor 100 is fixed to the pipe 190 with the front end of the sensor element 101 and the portion of the protective cover 130 of the gas sensor 100 protruding into the pipe 190.

[0024] The outer cylinder 148 surrounds the inner cylinder 143, the sensor element 101, and the connector 150, with multiple lead wires 155 connected to the connector 150 extending outwards from the rear end. These lead wires 155 are electrically connected to each electrode (described later) of the sensor element 101 via the connector 150. The gap between the outer cylinder 148 and the lead wires 155 is sealed by a rubber stopper 157. The space 149 inside the outer cylinder 148 is filled with a reference gas (atmosphere in this embodiment). The rear end of the sensor element 101 is located within this space 149.

[0025] As shown in Figure 2, the sensor element 101 is a laminated element having six layers stacked in this order from the bottom in the drawing view: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each consisting of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO2). Furthermore, the solid electrolytes forming these six layers are dense and airtight. Such a sensor element 101 is manufactured, for example, by performing predetermined processing and printing circuit patterns on ceramic green sheets corresponding to each layer, stacking them, and then firing them to integrate them.

[0026] At one end of the sensor element 101 (left side in Figure 2), a gas inlet 10, a first diffusion rate-limiting section 11, a buffer space 12, a second diffusion rate-limiting section 13, a first internal cavity 20, a third diffusion rate-limiting section 30, a second internal cavity 40, a fourth diffusion rate-limiting section 60, and a third internal cavity 61 are formed adjacent to each other in this order, communicating with each other.

[0027] The gas inlet 10, the buffer space 12, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 are spaces inside the sensor element 101, provided in a manner in which the spacer layer 5 has been hollowed out, with the upper part partitioned by the lower surface of the second solid electrolyte layer 6, the lower part partitioned by the upper surface of the first solid electrolyte layer 4, and the sides partitioned by the side surface of the spacer layer 5.

[0028] The first diffusion rate-limiting section 11, the second diffusion rate-limiting section 13, and the third diffusion rate-limiting section 30 are all provided as two horizontally elongated slits (with their longitudinal openings perpendicular to the drawing). The fourth diffusion rate-limiting section 60 is provided as a single horizontally elongated slit (with its longitudinal openings perpendicular to the drawing) formed as a gap with the lower surface of the second solid electrolyte layer 6. The section from the gas inlet 10 to the third internal cavity 61 is also referred to as the gas flow section under measurement.

[0029] The sensor element 101 is equipped with a reference gas introduction section 49 that allows a reference gas to flow from outside the sensor element 101 to the reference electrode 42 for measuring NOx concentration. The reference gas introduction section 49 has a reference gas introduction space 43 and a reference gas introduction layer 48. The reference gas introduction space 43 is a space provided inward from the rear end surface of the sensor element 101. The reference gas introduction space 43 is located between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, and is provided at a position where its sides are partitioned by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 opens to the rear end surface of the sensor element 101, and this opening functions as the inlet 49a of the reference gas introduction section 49. The inlet 49a is exposed within the space 149 (see Figure 1). The reference gas is introduced into the reference gas introduction space 43 from this inlet 49a. The reference gas introduction section 49 introduces the reference gas introduced from the inlet section 49a to the reference electrode 42 while imparting a predetermined diffusion resistance to it. In this embodiment, the reference gas is the atmosphere (the atmosphere in space 149 in Figure 1).

[0030] The reference gas introduction layer 48 is provided between the upper surface of the third substrate layer 3 and the lower surface of the first solid electrolyte layer 4. The reference gas introduction layer 48 is a porous material made of ceramics such as alumina. A portion of the upper surface of the reference gas introduction layer 48 is exposed within the reference gas introduction space 43. The reference gas introduction layer 48 is formed to cover the reference electrode 42. The reference gas introduction layer 48 allows the reference gas to flow from the reference gas introduction space 43 to the reference electrode 42.

[0031] The reference electrode 42 is formed in such a manner that it is sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, a reference gas introduction layer 48 connected to the reference gas introduction space 43 is provided around it. Furthermore, as will be described later, it is possible to measure the oxygen concentration (partial pressure of oxygen) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 using the reference electrode 42. The reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode made of Pt and ZrO2).

[0032] In the gas flow section, the gas inlet 10 is a part that opens to the external space, and the gas to be measured is taken into the sensor element 101 from the external space through the gas inlet 10. The first diffusion rate-limiting section 11 is a part that imparts a predetermined diffusion resistance to the gas to be measured taken in from the gas inlet 10. The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate-limiting section 11 to the second diffusion rate-limiting section 13. The second diffusion rate-limiting section 13 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20. When the gas to be measured is introduced from outside the sensor element 101 into the first internal cavity 20, the gas to be measured, which is rapidly drawn into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the gas to be measured in the external space (pulsations of exhaust pressure if the gas to be measured is automobile exhaust gas), is not directly introduced into the first internal cavity 20. Instead, the pressure fluctuations of the gas to be measured are canceled out through the first diffusion rate-limiting unit 11, the buffer space 12, and the second diffusion rate-limiting unit 13 before being introduced into the first internal cavity 20. As a result, the pressure fluctuations of the gas to be measured introduced into the first internal cavity 20 become almost negligible. The first internal cavity 20 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured introduced through the second diffusion rate-limiting unit 13. This partial pressure of oxygen is adjusted by the operation of the main pump cell 21.

[0033] The main pump cell 21 is an electrochemical pump cell comprising an inner pump electrode 22 having a ceiling electrode portion 22a provided over almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, an outer pump electrode 23 provided in a manner exposed to the external space (sensor element chamber 133 in Figure 1) in a region corresponding to the ceiling electrode portion 22a on the upper surface of the second solid electrolyte layer 6, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0034] The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that partition the first internal cavity 20, and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. A side electrode portion (not shown) is formed on the side wall surface (inner surface) of the spacer layer 5 that constitutes both side walls of the first internal cavity 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and is arranged in a tunnel-shaped structure at the location where the side electrode portion is installed.

[0035] The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, cermet electrodes made of Pt containing 1% Au and ZrO2). The inner pump electrode 22, which comes into contact with the gas to be measured, is formed using a material with reduced reducing ability to NOx components in the gas to be measured.

[0036] In the main pump cell 21, by applying a desired voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23, and flowing a pump current Ip0 in the positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23, it is possible to pump oxygen from the first internal cavity 20 to the external space, or pump oxygen from the external space into the first internal cavity 20.

[0037] Furthermore, in order to detect the oxygen concentration (partial pressure of oxygen) in the atmosphere in the first internal cavity 20, an electrochemical sensor cell, i.e., a main pump control oxygen partial pressure detection sensor cell 80, is constructed from an inner pump electrode 22, a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42.

[0038] The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be determined by measuring the electromotive force (voltage V0) in the oxygen partial pressure detection sensor cell 80 for main pump control. Furthermore, the pump current Ip0 is controlled by feedback control of the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes a target value. As a result, the oxygen concentration in the first internal cavity 20 can be maintained at a predetermined constant value.

[0039] The third diffusion rate-limiting section 30 is a part that applies a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (partial pressure of oxygen) is controlled by the operation of the main pump cell 21 in the first internal cavity 20, and guides the gas to be measured to the second internal cavity 40.

[0040] The second internal cavity 40 is provided as a space for further adjustment of the oxygen partial pressure by the auxiliary pump cell 50 for the gas to be measured, which is introduced through the third diffusion rate-limiting unit 30 after the oxygen concentration (oxygen partial pressure) has been adjusted in the first internal cavity 20. As a result, the oxygen concentration in the second internal cavity 40 can be kept constant with high precision, enabling highly accurate NOx concentration measurement in the gas sensor 100.

[0041] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell composed of an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided over substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, any suitable electrode on the outside of the sensor element 101 is sufficient), and the second solid electrolyte layer 6.

[0042] The auxiliary pump electrode 51 is disposed within the second internal cavity 40 in a tunnel-shaped structure similar to that of the inner pump electrode 22 provided within the first internal cavity 20. Specifically, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 which forms the ceiling surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 which forms the bottom surface of the second internal cavity 40. Side electrode portions (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both walls of the spacer layer 5 which forms the side walls of the second internal cavity 40, creating a tunnel-shaped structure. The auxiliary pump electrode 51 is also formed using a material with weakened reduction ability for NOx components in the gas being measured, similar to the inner pump electrode 22.

[0043] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, it is possible to pump oxygen from the atmosphere in the second internal cavity 40 to the outside space, or pump oxygen from the outside space into the second internal cavity 40.

[0044] Furthermore, in order to control the partial pressure of oxygen in the atmosphere within the second internal cavity 40, an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 81 for auxiliary pump control, is constructed using an auxiliary pump electrode 51, a reference electrode 42, a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, and a third substrate layer 3.

[0045] Furthermore, the auxiliary pump cell 50 is pumped by a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the oxygen partial pressure detection sensor cell 81 for auxiliary pump control. As a result, the oxygen partial pressure in the atmosphere inside the second internal cavity 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0046] Furthermore, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for main pump control. Specifically, the pump current Ip1 is input to the oxygen partial pressure detection sensor cell 80 for main pump control as a control signal, and the aforementioned target value of its voltage V0 is controlled so that the gradient of the oxygen partial pressure in the gas to be measured, introduced from the third diffusion rate-limiting 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 through the action of the main pump cell 21 and the auxiliary pump cell 50.

[0047] The fourth diffusion rate-limiting section 60 is the part that applies a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (partial pressure of oxygen) is controlled by the operation of the auxiliary pump cell 50 in the second internal cavity 40, and guides the gas to be measured to the third internal cavity 61. The fourth diffusion rate-limiting section 60 plays a role in limiting the amount of NOx flowing into the third internal cavity 61.

[0048] The third internal cavity 61 is provided as a space for performing processing related to the measurement of nitrogen oxide (NOx) concentration in the gas to be measured, which is introduced through the fourth diffusion rate-limiting unit 60 after the oxygen concentration (partial pressure of oxygen) has been adjusted in advance in the second internal cavity 40. The NOx concentration is measured mainly in the third internal cavity 61 by the operation of the measuring pump cell 41.

[0049] The measuring pump cell 41 measures the NOx concentration in the gas to be measured within the third internal cavity 61. The measuring pump cell 41 is an electrochemical pump cell composed of a measuring electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, and the first solid electrolyte layer 4. The measuring electrode 44 is a porous cermet electrode made of a material that has a higher reduction capacity for NOx components in the gas to be measured than the inner pump electrode 22. The measuring electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere within the third internal cavity 61.

[0050] In the measuring pump cell 41, oxygen generated by the decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 44 can be pumped out, and its amount can be detected as the pump current Ip2.

[0051] Furthermore, in order to detect the partial oxygen pressure around the measuring electrode 44, an electrochemical sensor cell, namely an oxygen partial pressure detection sensor cell 82 for controlling the measuring pump, is formed by the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for controlling the measuring pump.

[0052] The gas to be measured, introduced into the second internal cavity 40, reaches the measuring electrode 44 in the third internal cavity 61 via the fourth diffusion rate-limiting section 60 under controlled conditions of oxygen partial pressure. Nitrogen oxides in the gas to be measured surrounding the measuring electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. This generated oxygen is then pumped by the measuring pump cell 41, and at this time, the voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for measuring pump control remains constant (target value). Since the amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the nitrogen oxide concentration in the gas to be measured is calculated using the pump current Ip2 in the measuring pump cell 41.

[0053] Furthermore, an electrochemical sensor cell 83 is constructed from a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, a third substrate layer 3, an outer pump electrode 23, and a reference electrode 42. The electromotive force (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the partial pressure of oxygen in the gas being measured outside the sensor.

[0054] Furthermore, the electrochemical reference gas adjustment pump cell 90 is composed of a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, a third substrate layer 3, an outer pump electrode 23, and a reference electrode 42. This reference gas adjustment pump cell 90 pumps oxygen when a control current (pump current Ip3) flows due to a control voltage (voltage Vp3) applied by a power supply circuit 92 connected between the outer pump electrode 23 and the reference electrode 42. As a result, the reference gas adjustment pump cell 90 can draw oxygen from the space around the outer pump electrode 23 (sensor element chamber 133 in Figure 1) to the area around the reference electrode 42, and draw oxygen from the area around the reference electrode 42 to the area around the outer pump electrode 23.

[0055] In a gas sensor 100 having such a configuration, the gas to be measured, whose oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50, is supplied to the measuring pump cell 41. Therefore, the NOx concentration in the gas to be measured can be determined based on the pump current Ip2 that flows as oxygen generated by the reduction of NOx is pumped out from the measuring pump cell 41, which is approximately proportional to the NOx concentration in the gas to be measured.

[0056] Furthermore, the sensor element 101 is equipped with a heater section 70 that plays a role in temperature control by heating and maintaining the sensor element 101 to enhance the oxygen ion conductivity of the solid electrolyte. The heater section 70 comprises a heater connector electrode 71, a heater 72, a through-hole 73, a heater insulating layer 74, a pressure relief hole 75, and a lead wire 76.

[0057] The heater connector electrode 71 is an electrode formed in such a manner that it is in contact with the lower surface of the first substrate layer 1. By connecting the heater connector electrode 71 to an external power supply, power can be supplied to the heater unit 70 from an external source.

[0058] The heater 72 is an electrical resistor formed sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater connector electrode 71 via lead wires 76 and through-holes 73, and generates heat when power is supplied from the outside through the heater connector electrode 71, thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element 101.

[0059] Furthermore, the heater 72 is embedded throughout the entire area from the first internal cavity 20 to the third internal cavity 61, making it possible to adjust the entire sensor element 101 to a temperature at which the solid electrolyte is activated.

[0060] The heater insulating layer 74 is an insulating layer made of porous alumina formed by an insulator such as alumina on the upper and lower surfaces of the heater 72. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72.

[0061] The pressure relief holes 75 are provided so as to penetrate the third substrate layer 3 and the reference gas introduction layer 48, and are formed for the purpose of mitigating the rise in internal pressure due to the rise in temperature within the heater insulating layer 74.

[0062] As shown in Figure 3, the control device 95 comprises the variable power supplies 24, 46, and 52 described above, a heater power supply 78, the power supply circuit 92 described above, and a control unit 96. The control unit 96 is a microprocessor equipped with a CPU 97, RAM (not shown), and a memory unit 98. The memory unit 98 is a non-volatile memory, a device that stores various programs and various data, for example. The control unit 96 receives the voltages V0 to V2 and voltage Vref from each sensor cell 80 to 83 as input. The control unit 96 receives the pump currents Ip0 to Ip2 and pump current Ip3 flowing through each pump cell 21, 50, 41, and 90 as input. The control unit 96 controls the voltages Vp0 to Vp3 output by the variable power supplies 24, 46, and 52 and the power supply circuit 92 by outputting control signals to them, thereby controlling each pump cell 21, 41, 50, and 90. The control unit 96 controls the power supplied by the heater power supply 78 to the heater 72 by outputting a control signal to the heater power supply 78, thereby adjusting the temperature of the sensor element 101. The memory unit 98 stores target values ​​V0*, V1*, V2* and target current Ip1*, which will be described later. The CPU 97 of the control unit 96 controls each of the cells 21, 41, and 50 by referring to these target values ​​V0*, V1*, V2* and target current Ip1*.

[0063] The control unit 96 performs auxiliary pump control processing to control the auxiliary pump cell 50 so that the oxygen concentration in the second internal cavity 40 reaches a target concentration. Specifically, the control unit 96 controls the auxiliary pump cell 50 by feedback control of the voltage Vp1 of the variable power supply 52 so that the voltage V1 becomes a constant value (referred to as the target value V1*). The target value V1* is defined as a value such that the oxygen concentration in the second internal cavity 40 becomes a predetermined low concentration that does not substantially affect the measurement of NOx.

[0064] The control unit 96 performs main pump control processing to control the main pump cell 21 so that the pump current Ip1 flowing when the auxiliary pump cell 50 adjusts the oxygen concentration in the second internal cavity 40 by the auxiliary pump control processing becomes a target current (referred to as target current Ip1*). Specifically, the control unit 96 sets a target value of voltage V0 (referred to as target value V0*) based on the pump current Ip1 so that the pump current Ip1 flowing due to voltage Vp1 becomes a constant target current Ip1* (feedback control). Then, the control unit 96 feedback controls the voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes the target value V0* (i.e., so that the oxygen concentration in the first internal cavity 20 becomes the target concentration). This main pump control processing ensures that the gradient of the oxygen partial pressure in the gas to be measured introduced into the second internal cavity 40 from the third diffusion rate limiting unit 30 is always constant. The target value V0* is set to a value such that the oxygen concentration in the first internal cavity 20 is higher than 0% and is low oxygen concentration. Furthermore, the pump current Ip0 that flows during this main pump control process changes according to the oxygen concentration of the gas to be measured (i.e., the gas to be measured around the sensor element 101) flowing into the gas flow section from the gas inlet 10. Therefore, the control unit 96 can also detect the oxygen concentration in the gas to be measured based on the pump current Ip0.

[0065] The main pump control process and auxiliary pump control process described above are collectively referred to as the adjustment pump control process. The first internal cavity 20 and the second internal cavity 40 are collectively referred to as the oxygen concentration adjustment chamber. The main pump cell 21 and the auxiliary pump cell 50 are collectively referred to as the adjustment pump cell. The control unit 96 performs the adjustment pump control process, causing the adjustment pump cell to adjust the oxygen concentration in the oxygen concentration adjustment chamber.

[0066] Furthermore, the control unit 96 performs a measurement pump control process to control the measurement pump cell 41 so that the voltage V2 becomes a constant value (target value) (i.e., so that the oxygen concentration in the third internal cavity 61 becomes a predetermined low concentration). Specifically, the control unit 96 controls the measurement pump cell 41 by feedback control of the voltage Vp2 of the variable power supply 46 so that the voltage V2 becomes the target value V2*. Through this measurement pump control process, oxygen is pumped out from the third internal cavity 61.

[0067] The measurement pump control process is performed so that oxygen is pumped out of the third internal cavity 61 so that the oxygen generated by the reduction of NOx in the gas being measured in the third internal cavity 61 becomes virtually zero. The control unit 96 then acquires the pump current Ip2 as a detection value corresponding to the oxygen generated in the third internal cavity 61 originating from the specific gas (in this case, NOx), and calculates the NOx concentration in the gas being measured based on this pump current Ip2.

[0068] The memory unit 98 stores relationships between the pump current Ip2 and NOx concentration, such as relational equations (e.g., linear or quadratic functions) or maps. Such relational equations or maps can be determined in advance through experiments.

[0069] The control unit 96 controls the power supply circuit 92 so that a voltage Vp3 is applied to the reference gas adjustment pump cell 90, thereby supplying a pump current Ip3 to the reference gas adjustment pump cell 90. The control unit 96 controls the magnitude and direction of the pump current Ip3 by changing the magnitude and sign of the voltage Vp3. This allows the control unit 96 to control the direction of oxygen movement between the reference electrode 42 and the outer pump electrode 23 (pumping in oxygen to the area around the reference electrode 42 or pumping out oxygen from the area around the reference electrode 42) and to control the amount of oxygen moved. In this embodiment, the voltage Vp3 is a DC voltage such that the pump current Ip3 becomes a predetermined value (a constant DC current).

[0070] The control unit 96 performs a reference gas adjustment process by controlling the reference gas adjustment pump cell 90 to adjust the oxygen concentration around the reference electrode 42 by drawing oxygen from around the outer pump electrode 23 to around the reference electrode 42. Here, the gas to be measured is introduced into the gas inlet 10 and other gas flow parts of the sensor element 101 from the sensor element chamber 133 shown in Figure 1. On the other hand, the reference gas (atmosphere) in the space 149 shown in Figure 1 is introduced into the reference gas introduction part 49 of the sensor element 101. The sensor element chamber 133 and the space 149 are separated by the sensor assembly 140 (particularly the compacted powder 145a, 145b) and sealed to prevent gas flow between them. However, in cases where the pressure on the gas to be measured side is high, the gas to be measured may slightly enter the space 149, causing the oxygen concentration in the space 149 to decrease. In this case, if the oxygen concentration drops to the level of the oxygen concentration around the reference electrode 42, the reference potential, which is the potential of the reference electrode 42, will change. By performing a reference gas adjustment process, the decrease in oxygen concentration around the reference electrode 42 can be compensated for.

[0071] Furthermore, the control device 95, including the variable power supplies 24, 46, and 52 and the power supply circuit 92 shown in Figure 2, is actually connected to each electrode inside the sensor element 101 via lead wires (not shown) formed inside the sensor element 101, the connector 150 and lead wire 155 shown in Figure 1.

[0072] Incidentally, during periods when the sensor element 101 is not being driven, the reference gas introduction section 49 may adsorb water from outside the sensor element 101 (in this case, within the space 149). In this regard, the inventors investigated the relationship between the moisture absorption state of the reference gas introduction section 49 and the pump current Ip3 flowing to the reference gas adjustment pump cell 90. First, the sensor element 101 was driven by the control device 95. Specifically, with the gas sensor 100 placed in an atmospheric environment, the heater 72 was energized from the heater power supply 78 to heat the sensor element 101, and the temperature of the sensor element 101 was maintained at 800°C. After waiting for 0.5 hours in this state, the gas sensor 100 was in a state where the amount of moisture absorbed by the reference gas introduction section 49 was low. Subsequently, with the gas sensor 100 placed in an atmospheric environment, the value of the pump current Ip3 was measured when the voltage Vp3 applied by the power supply circuit 92 to the reference gas adjustment pump cell 90 was gradually changed from 0mV to 1000mV. The voltage Vp3 was applied in the direction that the reference gas adjustment pump cell 90 pumped oxygen from around the reference electrode 42 to around the outer pump electrode 23. The relationship between the voltage Vp3 and pump current Ip3 in the gas sensor 100 with low moisture absorption, as measured in this way, is shown as a solid line graph L1 in Figure 4. Next, the gas sensor 100 was stored in a constant temperature and humidity chamber at 40°C and 85% humidity for one week to adsorb water onto the reference gas introduction section 49, thereby creating a gas sensor 100 with high moisture absorption. This gas sensor 100 was placed in an atmospheric environment, and the temperature of the sensor element 101 was maintained at 800°C by a heater 72. In this state, the value of the pump current Ip3 was measured when the voltage Vp3 was gradually changed from 0mV to 1000mV, as described above. The relationship between the voltage Vp3 and pump current Ip3 in the gas sensor 100 with high moisture absorption, as measured in this way, is shown as a dashed line graph L2 in Figure 4.

[0073] As shown in Figure 4, in both graphs L1 and L2, in the region where the voltage Vp3 is between 100mV and 700mV, the pump current Ip3 remained almost constant even as the voltage Vp3 increased. In other words, the pump current Ip3 was the limiting current. The value of the limiting current is determined by, for example, the diffusion resistance of the reference gas introduction section 49. This region in which the pump current Ip3 hardly changes even when the voltage Vp3 changes (for example, the region where the voltage Vp3 is between 100mV and 700mV in Figure 4) is called the limiting current region. Furthermore, in both graphs L1 and L2, in the region where the voltage Vp3 is higher than the limiting current region, there was a tendency for the pump current Ip3 to increase with increasing voltage Vp3. This is thought to be because, as the voltage Vp3 increases, moisture inside the reference gas introduction section 49, especially around the reference electrode 42, is decomposed and oxygen is generated, and this oxygen is also pumped out from around the reference electrode 42. Furthermore, in both the limit current region and the region where the voltage Vp3 is higher than that, the pump current Ip3 value was larger in graph L2 than in graph L1. In other words, it was confirmed that the gas sensor 100, when the amount of moisture absorbed by the reference gas introduction section 49 is high, tends to have a higher pump current Ip3 value. Therefore, it is considered that even when the voltage Vp3 in the limit current region is applied, decomposition of moisture around the reference electrode 42 occurs. In particular, in the region where the voltage Vp3 is higher than the limit current region (for example, the region where the voltage Vp3 is 800mV or higher in Figure 4), the difference in the pump current Ip3 values ​​between graph L2 and graph L1 was more pronounced. For example, the difference between the pump current Ip3 value B1 in graph L1 and the pump current Ip3 value B2 in graph L2 when the voltage Vp3 is 1000mV (=B2-B1) was larger than the difference between the pump current Ip3 value A1 in graph L1 and the pump current Ip3 value A2 in graph L2 (=A2-A1) when the voltage Vp3 is 400mV, which is within the limit current range.

[0074] Thus, the pump current Ip3 that flows when the reference gas adjustment pump cell 90 pumps oxygen from around the reference electrode 42 to around the outer pump electrode 23 changes depending on the amount of moisture around the reference electrode 42. Specifically, the more moisture there is around the reference electrode 42, the larger the pump current Ip3 becomes. Therefore, the control unit 96 of this embodiment performs a moisture absorption state diagnosis process to diagnose the moisture absorption state around the reference electrode 42 based on this pump current Ip3. More specifically, as an example of the moisture absorption state diagnosis process, the control unit 96 of this embodiment performs a moisture determination process to determine whether or not there is a lot of moisture around the reference electrode 42 based on the pump current Ip3. Details of the moisture determination process will be described later.

[0075] Next, an example of the process by which the control unit 96 of the gas sensor 100 measures the NOx concentration will be described. Figure 5 is a flowchart showing an example of a control routine executed by the control unit 96. The control unit 96 stores this routine in, for example, the memory unit 98. When the control unit 96 receives a start command from, for example, an engine ECU (not shown), it starts this control routine.

[0076] When the control routine starts, the CPU 97 of the control unit 96 first outputs a control signal to the heater power supply 78 to start heater control processing, which controls the temperature of the heater 72 so that it reaches a target temperature (e.g., 800°C) (step S100). Here, the temperature of the heater 72 can be expressed as a linear function of the resistance value of the heater 72. Therefore, in the heater control processing of this embodiment, the CPU 97 calculates the resistance value of the heater 72 as a value that can be considered as the temperature of the heater 72 (a value that can be converted to temperature), and feedback controls the heater power supply 78 so that the calculated resistance value becomes the target resistance value (the resistance value corresponding to the target temperature). The CPU 97 can, for example, acquire the voltage of the heater 72 and the current flowing through the heater 72, and calculate the resistance value of the heater 72 based on the acquired voltage and current. The CPU 97 may calculate the resistance value of the heater 72 using, for example, the three-terminal method or the four-terminal method. The CPU 97 outputs a control signal to the heater power supply 78 so that the calculated resistance value of the heater 72 becomes the target resistance value, thereby feedback-controlling the power supplied by the heater power supply 78. When energizing the heater 72, the heater power supply 78 adjusts the power supplied to the heater 72, for example, by changing the value of the voltage applied to the heater 72.

[0077] Next, the CPU 97 determines whether the heater temperature has reached a predetermined temperature or higher through heater control processing (step S110). This predetermined temperature is predetermined as a value less than or equal to the target temperature of the heater control processing described above and is stored in the memory unit 98. The predetermined temperature is predetermined as the temperature at which the solid electrolyte of the sensor element 101 is activated and oxygen pumping by the reference gas adjustment pump cell 90 becomes possible. The predetermined temperature may be a value less than the target temperature. The predetermined temperature may be a value of 80% or more of the target temperature, or 90% or more of the target temperature. In this embodiment, the predetermined temperature was set to 90% of the target temperature. In this embodiment, as described above, the CPU 97 uses the resistance value as a value representing the temperature of the heater 72, so the determination in step S110 is also made using the resistance value of the heater 72.

[0078] If the CPU 97 determines that step S110 is negative, it repeats step S110 until it determines that it is positive. That is, it waits until the temperature of the heater 72 reaches a predetermined temperature or higher. If the CPU 97 determines that it is positive in step S110, it performs the following steps S120 and S130 as a moisture detection process.

[0079] In the moisture detection process, the CPU 97 first applies a voltage Vp3 to the reference gas adjustment pump cell 90 and obtains the pump current Ip3 that flows at that time (step S120). The value of the voltage Vp3 applied at this time is denoted as voltage Vha, and the value of the obtained pump current Ip3 is denoted as pump current Iph. The voltage Vha is applied in the direction that the reference gas adjustment pump cell 90 pumps oxygen from around the reference electrode 42 to around the outer pump electrode 23. The value of voltage Vha may be a value within the range of the limit current region explained in Figure 4, but it is preferable to use a voltage higher than the limit current region. For example, it is preferable that the voltage Vha be 0.8V or higher. The voltage Vha may also be 1.5V or lower. In this embodiment, the voltage Vha was set to 1.0V.

[0080] Next, the CPU 97 determines the moisture absorption state around the reference electrode 42 based on the acquired pump current Iph, specifically whether there is a lot of moisture around the reference electrode 42 (step S130). In this embodiment, the CPU 97 makes this determination based on a comparison between the pump current Iph and the limit current Iplim of the reference gas adjustment pump cell 90. More specifically, the CPU 97 determines whether there is a lot of moisture around the reference electrode 42 based on whether the difference ΔI between the pump current Iph and the limit current Iplim is greater than the threshold Iref. The limit current Iplim of the reference gas adjustment pump cell 90 is the same as the limit current explained in Figure 4, and is the limit current when a voltage Vp3 is applied to the reference gas adjustment pump cell 90 in the direction of pumping oxygen from around the reference electrode 42 to around the outer pump electrode 23. In this embodiment, the value of the limit current at the sensor element 101 when the amount of moisture absorption of the reference gas introduction section 49 is low, as measured experimentally beforehand (for example, value A1 in Figure 4), is stored in the storage unit 98 as the limit current Iplim. Therefore, the CPU 97 calculates the difference ΔI between the pump current Iph obtained in step S120 and the limit current Iplim stored in the memory unit 98, and determines whether the difference ΔI is greater than or equal to the threshold Iref. As described above, the more moisture there is around the reference electrode 42, the larger the pump current Iph becomes, and therefore the larger the difference ΔI becomes. For example, the threshold Iref is set as the value of the difference ΔI when the amount of moisture around the reference electrode 42 is considered to be the upper limit that does not affect the detection accuracy of NOx concentration. For example, in the example in Figure 4, when there is a lot of moisture around the reference electrode 42, the difference ΔI = B2 - A1, and when there is little moisture around the reference electrode 42, the difference ΔI = B1 - A1, so the threshold Iref is set to be a value between these two.

[0081] If the result in step S130 is positive, the CPU 97 executes a moisture concentration reduction process (step S140) which controls the reference gas adjustment pump cell 90 to reduce the moisture concentration around the reference electrode 42. In the moisture concentration reduction process of this embodiment, the CPU 97 applies a voltage Vp3 to the reference gas adjustment pump cell 90 in the direction of pumping oxygen from around the reference electrode 42 to around the outer pump electrode 23. The value of this voltage Vp3 is denoted as voltage Vhc. As described above, by applying a voltage Vp3 to the reference gas adjustment pump cell 90 in the direction of pumping oxygen from around the reference electrode 42, the moisture around the reference electrode 42 can be decomposed, thereby reducing the moisture concentration around the reference electrode 42. The value of voltage Vhc may be within the range of the limit current region, or it may be a voltage higher than the limit current region. For example, voltage Vhc may be 0.3V or more and 1.5V or less. Voltage Vhc may be 0.8V or more. The voltage Vhc may be 1.0V or less. The voltage Vhc may be the same value as the voltage Vha in the moisture determination process described above. In this embodiment, the voltage Vhc was set to 1.0V. The execution time for the moisture concentration reduction process is preferably 5 seconds or more and 300 seconds or less.

[0082] If the determination in step S130 is negative, or after the moisture concentration reduction process in step S140 is executed, the CPU 97 starts the normal control process, which is the control process for normal operation, i.e., when measuring NOx concentration (step S150). Specifically, the CPU 97 starts the main pump control process, auxiliary pump control process, measuring pump control process, and reference gas adjustment process described above, and then terminates this routine. After starting the normal control process, the CPU 97 acquires the value of the pump current Ip2 at predetermined intervals, for example, and derives the NOx concentration in the gas to be measured based on the acquired pump current Ip2 and the correspondence stored in the storage unit 98. The CPU 97 outputs the derived NOx concentration value to the engine ECU or stores it in the storage unit 98.

[0083] An example of performing the moisture concentration reduction process in step S140 in an atmospheric environment will be explained using Figure 6. Figure 6 is a graph showing the relationship between time t and voltage V2open, where time t=0 is defined as the time when the heater 72 reaches the predetermined temperature in step S110. Voltage V2open is the value of voltage V2 in the open state, i.e., when no control is performed to flow current through the measuring electrode 44 and the reference electrode 42. The graph of the embodiment shown by the solid line in Figure 6 was obtained as follows. First, similar to the measurement of graph L2 in Figure 4, a gas sensor 100 with a high moisture absorption amount in the reference gas introduction section 49 was prepared and placed in an atmospheric environment. Then, the heater control process was started, and the moisture concentration reduction process was started from the timing when the heater 72 reached the predetermined temperature in step S110 (time t=0) and executed until time t=t1 in Figure 6. The period from time t=0 to time t=t1, i.e., the execution time of the moisture concentration reduction process, was set to a predetermined time of 5 seconds to 300 seconds. The voltage Vhc was set to 1.0V. From time t=t1 onward, the reference gas adjustment pump cell 90 was not operated, and the connection between the measuring electrode 44 and the reference electrode 42 was kept open. Then, the voltage V2open was measured every 0.1 seconds from time t=0, and the graph of the embodiment shown by the solid line in Figure 6 was obtained. Between time t=0 and time t=t1, the moisture concentration reduction process was momentarily stopped, and the voltage V2open was measured. Furthermore, the same measurements as in the embodiment were performed, except that the reference gas adjustment pump cell 90 was not operated at all, and the connection between the measuring electrode 44 and the reference electrode 42 was kept open, and the graph of the comparative example shown by the dashed line in Figure 6 was obtained.

[0084] As can be seen from Figure 6, in both the example and the comparative example, the voltage V2open decreased as time elapsed from time t=0, and then tended to stabilize. However, in the comparative example, where no moisture concentration reduction treatment was performed, the voltage V2open stabilized more slowly than in the example. In addition, in the comparative example, the voltage V2open temporarily became negative. This is thought to be because the moisture around the reference electrode 42 is heated by the heater 72 and turns into a gas, temporarily lowering the oxygen concentration around the reference electrode 42 to a level lower than the oxygen concentration of the atmospheric atmosphere. In such a state, the potential of the reference electrode 42 (reference potential) is unstable, causing errors in the values ​​of the voltages V0, V1, and V2 measured with the reference potential as a reference, thus reducing the accuracy of NOx concentration detection. In contrast, in the example where the moisture concentration reduction treatment was performed, the voltage V2open stabilized earlier than in the comparative example. This is thought to be because the moisture around the reference electrode 42 is decomposed by the moisture concentration reduction treatment between time t=0 and time t=t1, suppressing the decrease in oxygen concentration around the reference electrode 42 due to the vaporization of moisture. In this case, the reference potential stabilizes quickly, thus suppressing the decrease in the detection accuracy of NOx concentration compared to the comparative example. In both the example and the comparative example, the voltage V2open decreases as time elapses from time t=0, which is thought to be because the voltage V2open includes the thermoelectric force between the reference electrode 42 and the measuring electrode 44, and this thermoelectric force decreases over time. For example, if there is temperature variation within each electrode of the reference electrode 42 and the measuring electrode 44, the thermoelectric force between the reference electrode 42 and the measuring electrode 44 will increase. As the temperature within each electrode becomes more uniform over time, the thermoelectric force will decrease.

[0085] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. In this embodiment, the first substrate layer 1, second substrate layer 2, third substrate layer 3, first solid electrolyte layer 4, spacer layer 5, and second solid electrolyte layer 6 correspond to the element body of the present invention, the measuring electrode 44 corresponds to the measuring electrode, the outer pump electrode 23 corresponds to the electrode on the side of the gas to be measured, the reference electrode 42 corresponds to the reference electrode, the reference gas introduction unit 49 corresponds to the reference gas introduction unit, the reference gas adjustment pump cell 90 corresponds to the reference gas adjustment pump cell, the sensor element 101 corresponds to the sensor element, and the control unit 96 corresponds to the control unit. In addition, the heater 72 corresponds to the heater, and the memory unit 98 corresponds to the memory unit. Furthermore, in this embodiment, an example of a method for diagnosing the moisture absorption state of the gas sensor of the present invention is also clarified by explaining the operation of the control device 95.

[0086] As described above, with respect to the gas sensor 100 of this embodiment, the control device 95 diagnoses the moisture absorption state around the reference electrode 42 based on the pump current Iph flowing through the reference gas adjustment pump cell 90 when the control device 95 controls the reference gas adjustment pump cell 90 to pump oxygen from around the reference electrode 42 to around the outer pump electrode 23. As described above, the pump current Ip3 (=Iph) that flows when the reference gas adjustment pump cell 90 pumps oxygen from around the reference electrode 42 to around the outer pump electrode 23 changes depending on the amount of moisture around the reference electrode 42, so the moisture absorption state around the reference electrode 42 can be diagnosed based on the pump current Iph. Furthermore, as a moisture absorption state diagnosis process, the control device 95 performs a moisture determination process to determine whether or not there is a lot of moisture around the reference electrode 42. If the moisture determination process determines that there is a lot of moisture, the control device 95 performs a moisture concentration reduction process. In this way, it is possible to appropriately decide whether or not to perform the moisture concentration reduction process based on the diagnosis result of the moisture absorption state diagnosis process. Furthermore, by performing a moisture concentration reduction process, the moisture concentration around the reference electrode 42 can be rapidly reduced, thereby suppressing a decrease in the detection accuracy of specific gas concentrations caused by moisture around the reference electrode 42.

[0087] Furthermore, in the moisture absorption state diagnosis process, the control device 95 diagnoses the moisture absorption state around the reference electrode 42 based on the pump current Iph obtained when a predetermined control voltage (voltage Vha) higher than the voltage in the limit current region of the reference gas adjustment pump cell 90 is applied between the outer pump electrode 23 and the reference electrode 42. Applying a voltage Vha higher than the voltage in the limit current region to the reference gas adjustment pump cell 90 makes the moisture around the reference electrode 42 more easily decomposed, so the amount of moisture around the reference electrode 42 easily affects the pump current Iph. Therefore, by using the pump current Iph obtained when such a voltage Vha is applied, the moisture absorption state around the reference electrode 42 can be diagnosed more appropriately.

[0088] Furthermore, in the moisture absorption state diagnosis process, the control device 95 diagnoses the moisture absorption state around the reference electrode 42 based on a comparison between the pump current Iph and the limit current Iplim of the reference gas adjustment pump cell 90. When a voltage Vha higher than the voltage in the limit current region is applied to the reference gas adjustment pump cell 90, the difference between the pump current Iph and the limit current Iplim becomes larger as the amount of moisture around the reference electrode 42 increases. By comparing these two values, the moisture absorption state around the reference electrode 42 can be diagnosed more appropriately.

[0089] Furthermore, in the moisture absorption state diagnostic process, the control device 95 compares the pump current Iph with the limit current Iplim stored in the memory unit 98. This eliminates the need to measure the limit current Iplim during the moisture absorption state diagnostic process.

[0090] Furthermore, if the voltage Vha is 0.8V or higher, the pump current Iph when a voltage within this range is applied is easily affected by the amount of moisture around the reference electrode 42, making it suitable for performing moisture absorption state diagnostic processing. If the voltage Vha is greater than 1.5V, oxygen ions in the solid electrolyte of the sensor element 101 may become depleted, leading to electron conduction of the solid electrolyte and potentially causing the sensor element 101 to blacken and become unusable. However, if the voltage Vha is 1.5V or lower, blackening of the sensor element 101 can be suppressed.

[0091] Furthermore, the control device 95 energizes the heater 72 and performs a moisture absorption state diagnostic process after the heater 72 reaches a predetermined temperature or higher. In this way, since the moisture absorption state diagnostic process is performed after the heater 72's temperature has risen, the reference gas adjustment pump cell 90 can be operated in a state where the solid electrolyte layer is activated and exhibits oxygen ion conductivity. Therefore, the moisture absorption state diagnostic process can be performed at an appropriate timing.

[0092] It goes without saying that the present invention is not limited in any way to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of the present invention.

[0093] For example, in the embodiment described above, the limit current Iplim stored in the memory unit 98 in advance was used in the moisture absorption state diagnosis process, but it is not limited to this. For example, the control device 95 may, in the moisture absorption state diagnosis process, apply a voltage Vp3 in the limit current range to the reference gas adjustment pump cell 90 and measure the pump current Ip3 that flows at this time as the limit current Iplim. The value of the voltage Vp3 applied at this time is denoted as voltage Vhb. Voltage Vhb may be predetermined as a value within the limit current range (for example, in the example of Figure 4, a value within the range of 100mV to 700mV). Alternatively, the control device 95 may, in the moisture absorption state diagnosis process, gradually change the value of the voltage Vhb while measuring the value of the pump current Ip3, and measure the value at which the pump current Ip3 can be considered to have stopped changing as the limit current Iplim. By measuring not only the pump current Iph but also the limit current Iplim in the moisture absorption state diagnosis process in this way, a more accurate determination can be made.

[0094] In the embodiments described above, the moisture absorption state around the reference electrode 42 was diagnosed based on the difference between the pump current Iph and the limiting current Iplim, but the invention is not limited to this. Diagnosis can be performed by comparing the pump current Iph and the limiting current Iplim, and for example, diagnosis may be performed based on the ratio of the pump current Iph to the limiting current Iplim. Alternatively, diagnosis may be performed based on the pump current Iph, and the limiting current Iplim may not be used for diagnosis. For example, the pump current Iph may be compared with a predetermined threshold, and if the pump current Iph exceeds the threshold, it may be determined that there is a lot of moisture around the reference electrode 42.

[0095] In the embodiment described above, the control device 95 performed the moisture absorption state diagnosis process in step S110 when the temperature of the heater 72 reached a predetermined temperature or higher. However, the moisture absorption state diagnosis process may be performed not only immediately after the temperature of the heater 72 reaches a predetermined temperature or higher, but also after a predetermined time has elapsed since the temperature of the heater 72 reached a predetermined temperature or higher. Alternatively, the control device 95 may perform the moisture absorption state diagnosis process after a predetermined time has elapsed since the start of energizing the heater 72, without determining whether the temperature of the heater 72 has reached a predetermined temperature or higher.

[0096] In the embodiment described above, the control device 95 performed a moisture determination process to determine whether there was a lot of moisture around the reference electrode 42 as a moisture absorption state diagnosis process. However, it is not limited to this, and any moisture absorption state around the reference electrode 42 can be diagnosed. For example, the control device 95 may calculate the amount of moisture around the reference electrode 42 based on the pump current Iph as a moisture absorption state diagnosis. For example, the relationship between the pump current Iph and the amount of moisture around the reference electrode 42, or the relationship between the difference ΔI and the amount of moisture around the reference electrode 42, may be investigated in advance by experiment and stored in the storage unit 98, and the control device 95 may calculate the amount of moisture in step S130 based on the relationship between the pump current Iph and the storage unit 98.

[0097] In the embodiment described above, the control device 95 used the diagnostic results of the moisture absorption state diagnostic process to decide whether or not to perform a moisture concentration reduction process. However, the diagnostic results are not limited to this and may be used for other purposes. For example, since the potential of the reference electrode 42 (reference potential) changes depending on the amount of moisture around the reference electrode 42, the control device 95 may predict the change in the reference potential based on this amount of moisture and change the control of each pump cell 21, 50, 41, 90. Specifically, the control device 95 may calculate the amount of moisture around the reference electrode 42 in the moisture absorption state diagnostic process and change one or more of the target values ​​V0*, V1*, V2* according to the calculated amount of moisture, or change the voltage Vp3 applied to the reference gas adjustment pump cell 90 in the reference gas adjustment process.

[0098] In the embodiment described above, the control device 95 performed the moisture absorption state diagnostic process in steps S120 and S130 before starting the normal control process in step S150, but it is not limited to this. The control device 95 may perform the moisture absorption state diagnostic process after starting the normal control process. For example, the control device 95 may perform the moisture absorption state diagnostic process at predetermined time intervals. In this case, the normal control process may be temporarily stopped while the moisture absorption state diagnostic process is being performed.

[0099] In the embodiment described above, the voltage Vp3 was set to a DC voltage, but it is not limited to this and may be a pulsed voltage or other voltage that is repeatedly switched on and off. Even in this case, the control device 95 can still perform the moisture absorption state diagnosis process, the moisture concentration reduction process, and the reference gas adjustment process. If the voltage Vp3 is a voltage that is repeatedly switched on and off, the control device 95 may measure the voltages V0, V1, and V2 during the period when the voltage Vp3 is off (in other words, during the period when the pump current Ip3 is not flowing) and use them for the normal control process. In this way, the moisture absorption state diagnosis process and the normal control process can be performed in parallel without temporarily stopping the normal control process while the moisture absorption state diagnosis process is being executed.

[0100] In the embodiment described above, the control device 95 does not need to perform the reference gas adjustment process.

[0101] In the embodiment described above, the reference gas introduction section 49 included a reference gas introduction space 43 and a reference gas introduction layer 48, but it is sufficient to include at least one of the reference gas introduction space 43 and the reference gas introduction layer 48. Since the reference gas introduction layer 48 readily adsorbs moisture, the presence of the reference gas introduction section 49 with the reference gas introduction layer 48 makes the moisture absorption state diagnostic processing of the present invention particularly meaningful. For example, in the embodiment described above, the reference gas introduction section 249 shown in Figure 7 may be used instead of the reference gas introduction section 49. The reference gas introduction section 249 does not include a reference gas introduction space 43, but it does include a reference gas introduction layer 48. The reference gas introduction layer 48 in Figure 7 is arranged from around the reference electrode 42 to the rear end surface of the element body of the sensor element 101. The portion of the reference gas introduction layer 48 in Figure 7 that is exposed to the rear end surface of the element body of the sensor element 101 functions as the inlet 49a of the reference gas introduction section 249. The inlet 49a is exposed in the space 149 outside the sensor element 101.

[0102] In the embodiment described above, the sensor element 101 of the gas sensor 100 is provided with a first internal cavity 20, a second internal cavity 40, and a third internal cavity 61, but is not limited to this. For example, as shown in the modified sensor element 201 in Figure 8, the third internal cavity 61 may not be provided. In the modified sensor element 201 shown in Figure 8, a gas inlet 10, a first diffusion rate-limiting section 11, a buffer space 12, a second diffusion rate-limiting section 13, a first internal cavity 20, a third diffusion rate-limiting section 30, and a second internal cavity 40 are formed adjacent to each other in this order, communicating between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measuring electrode 44 is disposed on the upper surface of the first solid electrolyte layer 4 within the second internal cavity 40. The measuring electrode 44 is covered by a fourth diffusion rate-limiting section 45. The fourth diffusion rate-limiting section 45 is a film made of a porous ceramic material such as alumina (Al2O3). Similar to the fourth diffusion rate-limiting section 60 in the embodiment described above, the fourth diffusion rate-limiting section 45 plays a role in limiting the amount of NOx flowing into the measuring electrode 44. The fourth diffusion rate-limiting section 45 also functions as a protective film for the measuring electrode 44. The ceiling electrode portion 51a of the auxiliary pump electrode 51 is formed up to directly above the measuring electrode 44. Even with a sensor element 201 configured in this way, the NOx concentration can be detected by the measuring pump cell 41, similar to the embodiment described above. In the sensor element 201 of Figure 8, the area around the measuring electrode 44 functions as a measurement chamber. That is, the area around the measuring electrode 44 plays a similar role to the third internal cavity 61.

[0103] In the embodiment described above, the front surface of the sensor element 101 including the outer pump electrode 23 (the portion exposed to the sensor element chamber 133) may be covered with a porous protective layer made of ceramics such as alumina.

[0104] In the embodiment described above, the sensor element 101 is used to detect the NOx concentration in the gas to be measured, but it is not limited to this, as long as it detects the concentration of a specific gas in the gas to be measured. For example, the specific gas concentration may be other oxide concentrations, not just NOx. If the specific gas is an oxide, oxygen is generated when the specific gas itself is reduced in the third internal cavity 61, as in the embodiment described above, so the measuring pump cell 41 can detect the specific gas concentration by obtaining a detection value (e.g., pump current Ip2) corresponding to this oxygen. Alternatively, the specific gas may be a non-oxide such as ammonia. If the specific gas is a non-oxide, oxygen is generated when the converted gas is reduced in the third internal cavity 61 by converting the specific gas to an oxide (e.g., converting ammonia to NO), so the measuring pump cell 41 can detect the specific gas concentration by obtaining a detection value (e.g., pump current Ip2) corresponding to this oxygen. For example, the inner pump electrode 22 of the first internal cavity 20 functions as a catalyst, so ammonia can be converted to NO in the first internal cavity 20.

[0105] In the embodiment described above, the element body of the sensor element 101 is a laminate having a plurality of solid electrolyte layers (layers 1 to 6), but it is not limited to this. The element body of the sensor element 101 only needs to include at least one solid electrolyte layer that conducts oxygen ions. For example, in Figure 2, layers 1 to 5 other than the second solid electrolyte layer 6 may be structural layers made of a material other than a solid electrolyte layer (for example, layers made of alumina). In this case, each electrode of the sensor element 101 should be arranged on the second solid electrolyte layer 6. For example, the measuring electrode 44 in Figure 2 should be arranged on the lower surface of the second solid electrolyte layer 6. Alternatively, the reference gas introduction space 43 may be provided in the spacer layer 5 instead of the first solid electrolyte layer 4, the reference gas introduction layer 48 may be provided between the second solid electrolyte layer 6 and the spacer layer 5 instead of between the first solid electrolyte layer 4 and the third substrate layer 3, and the reference electrode 42 may be provided behind the third internal cavity 61 and on the lower surface of the second solid electrolyte layer 6.

[0106] In the embodiment described above, the outer pump electrode 23 served as an outer main pump electrode located on the part of the main pump cell 21 that is exposed to the gas to be measured outside the sensor element 101, an outer auxiliary pump electrode located on the part of the auxiliary pump cell 50 that is exposed to the gas to be measured outside the sensor element 101, an outer measuring electrode located on the part of the measuring pump cell 41 that is exposed to the gas to be measured outside the sensor element 101, and a measured gas side electrode located on the part of the reference gas adjustment pump cell 90 that is exposed to the gas to be measured outside the sensor element 101, but is not limited to this. One or more of the outer main pump electrode, outer auxiliary pump electrode, outer measuring electrode, and measured gas side electrode may be provided outside the sensor element 101 separately from the outer pump electrode 23. Furthermore, the electrode on the side of the gas to be measured in the reference gas adjustment pump cell 90 only needs to be provided on the sensor element 101 so as to be in contact with the gas to be measured. For example, it may be located on the inside of the sensor element 101, not just the outside, and more specifically, it may be located in the gas flow section of the sensor element 101. For example, the inner pump electrode 22 may serve as both the electrode of the main pump cell 21 (inner main pump electrode) and the electrode on the side of the gas to be measured in the reference gas adjustment pump cell 90, and the reference gas adjustment pump cell 90 may pump in or out oxygen between the area around the inner pump electrode 22 and the area around the reference electrode 42.

[0107] The inventors investigated the relationship between the voltage Vhc and execution time of the moisture concentration reduction process and the time until the reference potential stabilized as follows. First, the sensor element 101 and gas sensor 100 of the above-described embodiment, as explained using Figures 1 to 3, were prepared. The gas sensor 100 was stored in a constant temperature and humidity chamber at 40°C and 85% humidity for one week to adsorb water onto the reference gas introduction layer 48. Next, the gas sensor 100 was attached to the piping. A model gas was prepared with nitrogen as the base gas, an oxygen concentration of 0%, and an NO concentration of 1500 ppm, and this was flowed through the piping as the gas to be measured. In this state, the sensor element 101 was driven by the control device 95 to perform the heater control process and the moisture concentration reduction process. The moisture concentration reduction process was performed from the time when the heater 72 reached a predetermined temperature (time t=0) after the heater control process was started until time t=t1. The moisture concentration reduction process was performed by controlling the reference gas adjustment pump cell 90 to pump oxygen from around the reference electrode 42. After the moisture concentration reduction process was completed, the control device 95 performed normal control processing, continuing to control each pump cell and acquire the voltages V0, V1, V2, and Vref from each sensor cell. Normal control processing continued until 60 minutes after the start of operation (heating) of the sensor element 101, and the pump current Ip2 was continuously measured during this time. The value of the pump current Ip2 60 minutes after the start of operation of the sensor element 101 was set as the reference value (100%), and the rate of change of the pump current Ip2 value 10 minutes after the start of operation of the sensor element 101 relative to the reference value was calculated. The calculation of the rate of change using the above procedure was performed by varying the voltage Vhc and execution time of the moisture concentration reduction process as shown in Table 1, resulting in Experimental Examples 1 to 14. The voltage Vhc was varied within the range of 0.3V to 1.5V. The execution time of the moisture concentration reduction process (time from time t=0 to time t=t1) was varied within a range of 5 seconds to 300 seconds. Furthermore, the rate of change of the pump current Ip2 was calculated in the same manner as in Experimental Examples 1-14, except that the normal control process was started from time t=0 without executing the moisture concentration reduction process. This was designated as Experimental Example 15. In all of Experimental Examples 1-15, the reference gas adjustment pump cell 90 was not operated during the normal control process; that is, the reference gas adjustment process was not performed.As described above, if moisture is present around the reference electrode 42, it is heated by the heater 72 and turns into a gas, temporarily destabilizing the potential of the reference electrode 42. Therefore, until the potential of the reference electrode 42 stabilizes, the pump current Ip2 will not stabilize even if the NOx concentration of the gas being measured is constant. The smaller the rate of change of the pump current Ip2, the less moisture is present around the reference electrode 42 after 10 minutes from the start of operation, and the more stable the potential of the reference electrode 42 is considered to be. Therefore, the length of the stabilization time, which is the time from the start of operation of the sensor element 101 until the potential of the reference electrode 42 stabilizes, can be evaluated by the magnitude of the rate of change of the pump current Ip2. A shorter stabilization time is preferable. For each of experimental examples 1 to 15, if the calculated rate of change was 3% or less, it was judged to be a very short stabilization time ("A"). If the calculated rate of change was between 3% and 5%, it was judged to be a short stabilization time ("B"). If the calculated rate of change was above 5%, it was judged to be a long stabilization time ("F"). Table 1 shows the evaluation results for voltage Vhc, execution time, and stabilization time for each of the experimental examples 1 to 15. As shown in Table 1, it was confirmed that experimental examples 1 to 14, in which the moisture concentration reduction treatment was performed, had a shorter stabilization time compared to experimental example 15, in which the moisture concentration reduction treatment was not performed. Furthermore, from the results of experimental examples 1 to 14, it was confirmed that the greater the voltage Vhc and the longer the execution time of the moisture concentration reduction treatment, the shorter the stabilization time can be.

[0108] [Table 1]

[0109] This specification also discloses technical concepts in which, in the original claim 7, "the gas sensor described in any one of claims 2 to 4" was changed to "the gas sensor described in any one of claims 2 to 6", in the original claim 8, "the gas sensor described in any one of claims 1 to 4" was changed to "the gas sensor described in any one of claims 1 to 7", and in the original claim 15, "a method for diagnosing the moisture absorption state of a gas sensor described in any one of claims 9 to 12" was changed to "a method for diagnosing the moisture absorption state of a gas sensor described in any one of claims 9 to 14". [Industrial applicability]

[0110] This invention can be used in gas sensors that detect the concentration of specific gases, such as NOx, in a gas to be measured, such as automobile exhaust gas. [Explanation of symbols]

[0111] 1 First substrate layer, 2 Second substrate layer, 3 Third substrate layer, 4 First solid electrolyte layer, 5 Spacer layer, 6 Second solid electrolyte layer, 10 Gas inlet, 11 First diffusion-controlled section, 12 Buffer space, 13 Second diffusion-controlled section, 20 First internal cavity, 21 Main pump cell, 22 Inner pump electrode, 22a Ceiling electrode section, 22b Bottom electrode section, 23 Outer pump electrode, 24 Variable power supply, 30 Third diffusion-controlled section, 40 Second internal cavity, 41 Measurement pump cell, 42 Reference electrode, 43 Reference gas introduction space, 44 Measurement electrode, 45 Fourth diffusion-controlled section, 46 Variable power supply, 48 Reference gas introduction layer, 49, 249 Reference gas introduction section, 49a Inlet section, 50 Auxiliary pump cell, 51 Auxiliary pump electrode, 51a Ceiling electrode section, 51b Bottom electrode section, 52 Variable power supply, 60 Fourth diffusion rate-limiting section, 61 Third internal cavity, 70 Heater section, 71 Heater connector electrode, 72 Heater, 73 Through hole, 74 Heater insulating layer, 75 Pressure relief hole, 76 Lead wire, 78 Heater power supply, 80 Oxygen partial pressure detection sensor cell for main pump control, 81 Oxygen partial pressure detection sensor cell for auxiliary pump control, 82 Oxygen partial pressure detection sensor cell for measuring pump control, 83 Sensor cell, 90 Reference gas adjustment pump cell, 92 Power supply circuit, 95 Control device, 96 Control unit, 97 CPU, 98 Memory unit, 100 Gas sensor, 101, 201 Sensor element, 130 Protective cover, 131 Inner protective cover, 132 Outer protective cover, 133 Sensor element chamber, 140 Sensor assembly, 141 Element encapsulant, 142 Main body fitting, 143 Inner cylinder, 143a, 143b; Reduced diameter section, 144a~144c; Ceramic supporter, 145a, 145b; Compacted powder, 146; Metal ring, 147; Bolt, 148; Outer cylinder, 149; Space, 150; Connector, 155; Lead wire, 157; Rubber stopper, 190; Piping, 191; Fixing component.

Claims

1. A gas sensor for detecting the concentration of a specific gas in a gas being measured, The element body includes an oxygen ion conductive solid electrolyte layer and has an internal gas flow section for introducing and circulating the gas to be measured, The measuring electrode is disposed in the gas flow section to be measured, The element body is provided with an electrode on the side of the gas to be measured so as to be in contact with the gas to be measured, A reference electrode disposed inside the main body of the element, A reference gas introduction unit that flows a reference gas, which serves as a reference for detecting the concentration of the specific gas in the gas to be measured, from outside the element body to the reference electrode, A reference gas adjustment pump cell comprising the aforementioned electrode on the gas side to be measured and the aforementioned reference electrode, A sensor element having, A control unit performs a moisture absorption state diagnostic process that diagnoses the moisture absorption state around the reference electrode based on the pump current flowing through the reference gas adjustment pump cell when the reference gas adjustment pump cell is controlled to pump oxygen from around the reference electrode to around the measured gas side electrode, A gas sensor is included.

2. The control unit diagnoses the moisture absorption state around the reference electrode based on the pump current when a predetermined control voltage higher than the voltage in the limit current region of the reference gas adjustment pump cell is applied between the electrode on the gas to be measured and the reference electrode during the moisture absorption state diagnosis process. The gas sensor according to claim 1.

3. The control unit, in the moisture absorption state diagnosis process, diagnoses the moisture absorption state around the reference electrode based on a comparison between the pump current and the limit current of the reference gas adjustment pump cell. The gas sensor according to claim 2.

4. The control unit diagnoses the moisture absorption state around the reference electrode based on the difference or ratio between the pump current and the limit current in the moisture absorption state diagnosis process. The gas sensor according to claim 3.

5. The control unit includes a storage unit that stores the value of the limit current, The control unit compares the pump current with the limit current stored in the memory unit during the moisture absorption state diagnosis process. The gas sensor according to claim 3 or 4.

6. The control unit, in the moisture absorption state diagnosis process, compares the pump current with the limit current measured by applying the voltage in the limit current region to the reference gas adjustment pump cell. The gas sensor according to claim 3 or 4.

7. The predetermined control voltage is a voltage of 0.8V or more and 1.5V or less. The gas sensor according to any one of claims 2 to 4.

8. A gas sensor according to any one of claims 1 to 4, A heater for heating the main body of the element, Equipped with, The control unit energizes the heater and, after the heater's temperature reaches a predetermined temperature or higher, performs the moisture absorption state diagnostic process. Gas sensor.

9. A method for diagnosing the moisture absorption state of a gas sensor that detects the concentration of a specific gas in a gas to be measured, The aforementioned gas sensor is The element body includes an oxygen ion conductive solid electrolyte layer and has an internal gas flow section for introducing and circulating the gas to be measured, The measuring electrode is disposed in the gas flow section to be measured, The element body is provided with an electrode on the side of the gas to be measured so as to be in contact with the gas to be measured, A reference electrode disposed inside the main body of the element, A reference gas introduction unit that flows a reference gas, which serves as a reference for detecting the concentration of the specific gas in the gas to be measured, from outside the element body to the reference electrode, A reference gas adjustment pump cell comprising the aforementioned electrode on the gas side to be measured and the aforementioned reference electrode, A sensor element having, A moisture absorption state diagnostic process that diagnoses the moisture absorption state around the reference electrode based on the pump current flowing through the reference gas adjustment pump cell when the reference gas adjustment pump cell is controlled to pump oxygen from around the reference electrode to around the measured gas side electrode, A method for diagnosing the moisture absorption state of a gas sensor, including a gas sensor.

10. In the moisture absorption state diagnosis process, the moisture absorption state around the reference electrode is diagnosed based on the pump current when a predetermined control voltage higher than the voltage in the limit current region of the reference gas adjustment pump cell is applied between the electrode on the gas to be measured and the reference electrode. A method for diagnosing the moisture absorption state of a gas sensor according to claim 9.

11. In the moisture absorption state diagnosis process, the moisture absorption state around the reference electrode is diagnosed based on a comparison between the pump current and the limit current of the reference gas adjustment pump cell. A method for diagnosing the moisture absorption state of a gas sensor according to claim 10.

12. In the moisture absorption state diagnosis process, the moisture absorption state around the reference electrode is diagnosed based on the difference or ratio between the pump current and the limit current. A method for diagnosing the moisture absorption state of a gas sensor according to claim 11.

13. The gas sensor includes a storage unit that stores the value of the limit current, In the moisture absorption state diagnosis process, the pump current is compared with the limit current stored in the memory unit. A method for diagnosing the moisture absorption state of a gas sensor according to claim 11 or 12.

14. In the moisture absorption state diagnostic process, the pump current is compared with the limit current measured by applying the voltage in the limit current region to the reference gas adjustment pump cell. A method for diagnosing the moisture absorption state of a gas sensor according to claim 11 or 12.

15. A method for diagnosing the moisture absorption state of a gas sensor according to any one of claims 9 to 12, The gas sensor is equipped with a heater for heating the element body, After energizing the heater and the heater's temperature reaches a predetermined temperature or higher, the moisture absorption state diagnostic process is performed. A method for diagnosing the moisture absorption status of a gas sensor.

Images