gas sensor

Abstract

This gas sensor 100 comprises a sensor element 101 and a control device. The control device determines whether a refresh of the sensor element 101 is necessary on the basis of at least one of an undershoot and an overshoot of pump current Ip2. When determined that a refresh is necessary, the control device performs a refresh process including at least one from among: a refresh-time adjustment pump control process for controlling an adjustment pump cell (main pump cell 21 or auxiliary pump cell 50) such that more oxygen is pumped out from an oxygen concentration adjustment chamber (first interior void 20 and second interior void 40) than a normal-time adjustment pump control process; and a refresh-time measurement pump control process for controlling a measurement pump cell 41 so as to pump out more oxygen from a third internal void 61 than a normal-time measurement pump control process.

Description

Technical Field

[0001] The present invention relates to a gas sensor.

Background Art

[0002] Conventionally, gas sensors for detecting the concentration of specific gases such as NOx in a measured gas such as automotive exhaust gas are known. For example, the NOx sensor described in Patent Document 1 includes an electrochemical pump cell including an oxygen ion conductive solid electrolyte and a measurement electrode having NOx reducing ability. This NOx sensor converts NOx gas in the measured gas into O2 gas at the measurement electrode, and outputs the NOx concentration in the measured gas based on the pump current flowing through the electrochemical pump cell that changes according to the concentration of the converted O2 gas.

[0003] Further, in Patent Document 1, in such a NOx sensor, for example, when the fuel cut for stopping the fuel supply to the engine is performed, or when the NOx concentration changes to zero, an undershoot occurs in which the output signal of the NOx concentration temporarily decreases excessively compared to the ideal signal. When such an undershoot occurs, the measurement accuracy of the NOx concentration tends to deteriorate, which is not preferable. In Patent Document 2, it is described that such a phenomenon is caused by a change in the moisture in the measured gas. Further, Patent Document 1 describes that the undershoot can be suppressed by performing a rich treatment in which the sensor element of the NOx sensor is treated at a temperature of 500 °C or higher for 15 minutes or more in a rich atmosphere. The rich atmosphere at this time contains hydrocarbons, and the concentration of NO is 0.05% or more and 1.0% or less by volume ratio, and the air excess ratio (λ) is 0.80 to 0.9999.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] Incidentally, even with gas sensors that did not exhibit the aforementioned pump current undershoot during manufacturing, the undershoot sometimes increased with use. Furthermore, similar to undershoot, overshoot sometimes increased with use.

[0006] This invention was made to solve these problems, and its main objective is to suppress the increase in undershoot and overshoot of the measuring pump current that occurs when using a gas sensor. [Means for solving the problem]

[0007] To achieve the main objectives described above, the present invention employs the following means.

[0008] [1] The gas sensor of the present invention 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, A measuring pump cell comprising an outer measuring electrode provided on the outside of the element body so as to be in contact with the gas to be measured, and an inner measuring electrode disposed in the measuring chamber of the gas flow section to be measured, wherein oxygen is pumped from around the inner measuring electrode to around the outer measuring electrode, A control pump cell for adjusting the oxygen concentration in the oxygen concentration adjustment chamber located upstream of the measurement chamber in the gas flow section to be measured, A reference electrode is disposed inside the element body so as to be in contact with a reference gas that serves as a reference for detecting the concentration of a specific gas in the gas to be measured, A measuring voltage detection sensor cell for detecting the measuring voltage between the reference electrode and the inner measuring electrode, A sensor element having, A normal adjustment pump control process operates the adjustment pump cell, and a normal measurement pump control process controls the measurement pump cell so that the measurement voltage of the sensor element reaches a target value and pumps oxygen from the measurement chamber. A specific gas concentration detection unit detects the specific gas concentration in the gas to be measured based on the measurement pump current flowing through the measurement pump cell by the normal measurement pump control process. A refresh necessity determination unit performs a refresh necessity determination process to determine whether or not the sensor element needs to be refreshed based on at least one of the undershoot and overshoot when the measuring pump current changes abruptly. A refresh control unit that performs a refresh operation when the refresh necessity determination process determines that a refresh is necessary, and which includes at least one of the following: a refresh adjustment pump control process that controls the adjustment pump cell to pump out more oxygen from the oxygen concentration adjustment chamber compared to the normal adjustment pump control process; and a refresh measurement pump control process that controls the measurement pump cell to pump out more oxygen from the measurement chamber compared to the normal measurement pump control process. It is something that is provided.

[0009] This gas sensor performs two processes: a normal adjustment pump control process that operates the adjustment pump cell, and a normal measurement pump control process that controls the measurement pump cell so that the measurement voltage of the sensor element reaches a target value, thereby pumping oxygen from the measurement chamber. Based on the measurement pump current flowing through the measurement pump cell by the normal measurement pump control process, the concentration of a specific gas in the gas being measured is detected. When the gas sensor is used to detect a specific gas concentration in this way, the undershoot and overshoot of the measurement pump current may increase with use due to sudden changes in the measurement pump current. Therefore, this gas sensor performs a refresh necessity determination process to determine whether the sensor element needs to be refreshed based on at least one of the undershoot and overshoot of the measurement pump current during sudden changes, and performs a refresh process if it is determined that a refresh is necessary. The refresh process includes at least one of the following: a refresh adjustment pump control process that controls the adjustment pump cell to pump more oxygen from the oxygen concentration adjustment chamber compared to the normal adjustment pump control process, and a refresh measurement pump control process that controls the measurement pump cell to pump more oxygen from the measurement chamber compared to the normal measurement pump control process. The inventors have found that by performing such a refresh process, the increased undershoot and overshoot associated with the use of the gas sensor can be reduced. In this gas sensor, a refresh determination is made, and if a refresh is deemed necessary, the refresh process is performed, thereby suppressing the increase in undershoot and overshoot of the measuring pump current associated with the use of the gas sensor.

[0010] Here, the refresh necessity determination process is not limited to a process that makes a determination based on the measuring pump current itself, but also includes a process that makes a determination based on a value that can be converted to the measuring pump current or a value that can be considered equivalent to the measuring pump current. For example, the refresh necessity determination process may be performed based on at least one of the undershoot and overshoot when the value of the specific gas concentration detected based on the measuring pump current changes abruptly. Furthermore, it is preferable that the specific gas concentration detection unit does not perform the normal measuring pump control process during the refresh process, and it is even more preferable that it does not perform the normal adjustment pump control process. During the refresh process, it is preferable that the measuring pump cell is not controlled to draw oxygen into the measurement chamber, and it is even more preferable that the adjustment pump cell is not controlled to draw oxygen into the oxygen concentration adjustment chamber.

[0011] [2] In the gas sensor described above (the gas sensor described in [1] above), the refresh process may include the refresh adjustment pump control process. The refresh adjustment pump control process has a higher effect of refreshing the sensor element compared to the refresh measurement pump control process. Therefore, by performing at least the refresh adjustment pump control process in the refresh process, the effect of suppressing undershoot and overshoot of the measurement pump current can be increased, and the refresh process can be performed in a shorter time. In this case, the refresh process does not have to include the refresh measurement pump control process.

[0012] [3] In this case (in the gas sensor described in [2] above), the oxygen concentration adjustment chamber has a first internal cavity and a second internal cavity provided downstream of the first internal cavity and upstream of the measurement chamber, the adjustment pump cell has a main pump cell for adjusting the oxygen concentration of the first internal cavity and an auxiliary pump cell for adjusting the oxygen concentration of the second internal cavity, and the refresh adjustment pump control process may include at least one of the following: a process for controlling the main pump cell to pump out more oxygen from the first internal cavity compared to the normal adjustment pump control process, and a process for controlling the auxiliary pump cell to pump out more oxygen from the second internal cavity compared to the normal adjustment pump control process.

[0013] [4] In the gas sensor described above (the gas sensor described in any of [1] to [3] above), the gas to be measured is exhaust gas from an internal combustion engine, and the necessity determination unit may perform the refresh necessity determination process based on the behavior of the measuring pump current when it changes abruptly due to fuel cut-off of the internal combustion engine. Undershoot and overshoot of the measuring pump current are likely to occur when there is a sudden change in the H2O concentration (moisture) in the gas to be measured. And there is a high possibility that the H2O concentration is changing abruptly when the fuel cut-off of the internal combustion engine occurs. For this reason, by making a necessity determination based on the behavior of the measuring pump current when it changes abruptly due to fuel cut-off of the internal combustion engine (at least one of undershoot or overshoot), the necessity of refreshing can be determined more appropriately.

[0014] [5] In the gas sensor described above (the gas sensor described in any of [1] to [4] above), the necessity determination unit may determine that a refresh is necessary if at least one of the undershoot amount and overshoot amount during a sudden change in the measuring pump current falls outside the acceptable range. In this way, the necessity of a refresh can be appropriately determined based on at least one of the undershoot amount and overshoot amount.

[0015] [6] In the gas sensor described above (the gas sensor described in any of [1] to [5] above), the refresh control unit may perform the refresh process when the gas to be measured in the gas flow section can be considered to contain carbon. Here, the refresh process must be performed when the gas to be measured contains carbon. Therefore, by performing the refresh process when the gas to be measured in the gas flow section can be considered to contain carbon, the refresh can be performed effectively. This makes it less likely that, for example, undershoot and overshoot will not be reduced even after the refresh process is performed, requiring the refresh process to be performed again. Here, "the gas to be measured contains carbon" also includes cases where the gas to be measured contains molecules that have carbon. For example, if the gas to be measured contains one or more of carbon (C), carbon monoxide (CO), carbon dioxide (CO2), and hydrocarbons (HC), then the gas to be measured can be said to contain carbon.

[0016] [7] In the gas sensor described above (the gas sensor described in any of [1] to [6] above), the processing time for the refresh process may be 1 second or more and 10 seconds or less. By setting the processing time to 1 second or more, the sensor element can be refreshed more reliably. Furthermore, the refresh effect does not increase significantly even if the refresh process is performed for a long period of time, and the refresh effect is relatively high in the first 10 seconds from the start. Also, since the specific gas concentration cannot be correctly detected during the refresh process, it is preferable to shorten the processing time for the refresh process. By setting the processing time to 10 seconds or less, the sensor element can be refreshed efficiently while shortening the time during which the specific gas concentration cannot be correctly detected. [Brief explanation of the drawing]

[0017] [Figure 1] Schematic cross-sectional view of gas sensor 100. [Figure 2] A block diagram showing the electrical connection relationships between the control device 90 and each cell and heater 72. [Figure 3] A graph showing the undershoot and overshoot of the pump current Ip2. [Figure 4] Graph showing the increase in the undershoot and overshoot of the pump current Ip2 in the sensor element 101. [Figure 5] Graph showing the state of undershoot and overshoot before and after the refresh process. [Figure 6] Graph showing the relationship between the processing time of the refresh process and the reduction effect of the undershoot amount. [Figure 7] Flowchart showing an example of the control routine. [Figure 8] Schematic cross-sectional view of the modified sensor element 201. [Figure 9] Graph showing the relationship between the processing time of the refresh process and the reduction effect of the undershoot amount.

Mode for Carrying Out the Invention

[0018] Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic cross-sectional view schematically showing an example of the configuration of a gas sensor 100 according to an embodiment of the present invention. FIG. 2 is a block diagram showing the electrical connection relationship between the control device 90, each cell, and the heater 72. This gas sensor 100 is attached to a pipe such as an exhaust gas pipe of an internal combustion engine such as a gasoline engine or a diesel engine. The gas sensor 100 detects the concentration of a specific gas such as NOx in the measured gas using the exhaust gas of the internal combustion engine as the measured gas. The gas sensor 100 includes a sensor element 101 having a long rectangular parallelepiped shape, each cell 15, 21, 41, 50, 80 to 83 configured to include a part of the sensor element 101, a heater unit 70 provided inside the sensor element 101, and a control device 90 that controls the entire gas sensor 100.

[0019] The sensor element 101 is a laminated element having six layers stacked in this order from the bottom as seen in the drawing: 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.

[0020] On the tip side of the sensor element 101 (the left end side in Figure 1), 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.

[0021] 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.

[0022] 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.

[0023] Furthermore, a reference gas introduction space 43 is provided at a position further from the tip side than the gas flow section to be measured, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, with its sides partitioned by the side surface of the first solid electrolyte layer 4. For example, air is introduced into the reference gas introduction space 43 as the reference gas when measuring the NOx concentration.

[0024] The atmospheric introduction layer 48 is a layer made of porous ceramics, and a reference gas is introduced into the atmospheric introduction layer 48 through a reference gas introduction space 43. The atmospheric introduction layer 48 is also formed to cover the reference electrode 42.

[0025] The reference electrode 42 is an electrode 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, an atmospheric 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 (oxygen partial pressure) 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).

[0026] 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.

[0027] 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 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.

[0028] 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.

[0029] 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.

[0030] In the main pump cell 21, by applying a desired pump 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.

[0031] 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 using 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.

[0032] 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 pump 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.

[0033] 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 oxygen pressure) 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] Specifically, the measuring electrode 44 is an electrode containing at least one of Pt and Rh, which are noble metals having catalytic activity. Preferably, the measuring electrode 44 is an electrode made of a cermet containing at least one of Pt and Rh and an oxide having oxygen ion conductivity (in this case, ZrO2). Furthermore, it is preferable that the measuring electrode 44 is a porous material. In this embodiment, the measuring electrode 44 is a porous cermet electrode made of Pt, Rh and ZrO2.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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, and a pressure relief hole 75.

[0051] 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.

[0052] 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 a through-hole 73, and generates heat when power is supplied through the heater connector electrode 71 by the heater power supply 76 (see Figure 2), thereby heating and maintaining the temperature of the solid electrolyte forming the sensor element 101.

[0053] 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.

[0054] The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 using an insulator such as alumina. The heater insulating layer 74 is formed to provide electrical insulation between the second substrate layer 2 and the heater 72, and between the third substrate layer 3 and the heater 72.

[0055] The pressure relief hole 75 is a portion that penetrates the third substrate layer 3 and the atmospheric introduction layer 48 and is provided to communicate with the reference gas introduction space 43, and is formed for the purpose of mitigating the rise in internal pressure due to the rise in temperature within the heater insulating layer 74.

[0056] As shown in Figure 2, the control device 90 comprises the variable power supplies 24, 46, and 52 described above, a heater power supply 76, and a control unit 91. The control unit 91 is a microprocessor equipped with a CPU 92 and a memory unit 94, etc. The memory unit 94 is a device that stores, for example, various programs and various data. The control unit 91 receives the following inputs: voltage V0 detected by the oxygen partial pressure detection sensor cell 80 for main pump control, voltage V1 detected by the oxygen partial pressure detection sensor cell 81 for auxiliary pump control, voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for measuring pump control, voltage Vref detected by the sensor cell 83, pump current Ip0 detected by the main pump cell 21, pump current Ip1 detected by the auxiliary pump cell 50, and pump current Ip2 detected by the measuring pump cell 41. Furthermore, the control unit 91 controls the pump voltages Vp0, Vp1, and Vp2 output by the variable power supplies 24, 46, and 52 by outputting control signals to them, thereby controlling the main pump cell 21, the measuring pump cell 41, and the auxiliary pump cell 50. The control unit 91 controls the power supplied by the heater power supply 76 to the heater 72 by outputting control signals to the heater power supply 76. The memory unit 94 also stores target values ​​V0*, V0r*, V1*, V1r*, V2*, V2r*, etc., which will be described later. The CPU 92 of the control unit 91 controls each cell 21, 41, and 50 by referring to these target values ​​V0*, V0r*, V1*, V1r*, V2*, V2r*.

[0057] The control unit 91 performs normal auxiliary pump control processing to control the auxiliary pump cell 50 so that the oxygen concentration in the second internal cavity 40 reaches the target concentration. Specifically, the control unit 91 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.

[0058] The control unit 91 performs normal main pump control processing, which controls 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 normal auxiliary pump control processing becomes a target current (referred to as target current Ip1*). Specifically, the control unit 91 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 91 feedback-controls the pump 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). Through this normal main pump control processing, 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% but is low. In addition, the pump current Ip0 that flows during this normal 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) that flows into the gas flow section from the gas inlet 10. Therefore, the control unit 91 can also detect the oxygen concentration in the gas to be measured based on the pump current Ip0.

[0059] The above-mentioned normal main pump control process and normal auxiliary pump control process are collectively referred to as the normal 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 91 performs the normal adjustment pump control process, causing the adjustment pump cell to adjust the oxygen concentration in the oxygen concentration adjustment chamber.

[0060] Furthermore, the control unit 91 performs normal measurement pump control processing to control the measurement pump cell 41 so that the voltage V2 becomes a constant value (referred to as the target value V2*) (that is, so that the oxygen concentration in the third internal cavity 61 becomes a predetermined low concentration). Specifically, the control unit 91 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 normal measurement pump control processing, oxygen is pumped out from the third internal cavity 61.

[0061] During normal operation, 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 91 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.

[0062] The memory unit 94 stores relationships between the pump current Ip2 and NOx concentration, such as relational equations (e.g., linear functions) or maps. Such relational equations or maps can be determined in advance through experiments.

[0063] Next, we will explain the undershoot and overshoot that occur in the pump current Ip2 of the sensor element 101. Figure 3 is a graph showing the undershoot and overshoot of the pump current Ip2. Figure 3 shows an example of the behavior of the pump current Ip2 when the fuel cut of the internal combustion engine occurs while the gas sensor 100 is measuring the NOx concentration in the gas being measured, with a solid line. The behavior of the ideal pump current Ip2 is shown with a dashed line.

[0064] When fuel cut-off begins, the NOx concentration in the measured gas decreases rapidly to almost zero, and when fuel cut-off ends, the NOx concentration in the measured gas increases rapidly again. Therefore, in principle, as shown by the dashed line in Figure 3, the pump current Ip2, which is the value corresponding to the NOx concentration, should quickly follow the start of fuel cut-off and change to the corresponding value (changing from I1 to I0 in Figure 3). Also, when fuel cut-off ends, it should quickly follow and change to the corresponding value (changing from I0 to I1 in Figure 3). However, in reality, as shown by the solid line in Figure 3, the pump current Ip2 experiences excessive output fluctuations that differ from the actual change in NOx concentration. Specifically, at the start of fuel cut-off, the pump current Ip2 experiences an undershoot, where its value decreases excessively once before rising to the value corresponding to the actual NOx concentration. Also, at the end of fuel cut-off, the pump current Ip2 experiences an overshoot, where its value increases excessively once before falling to the value corresponding to the actual NOx concentration. For example, in a gas sensor 100 immediately after manufacturing, such overshoot and undershoot of the pump current Ip2 may not occur, but the overshoot and undershoot tend to increase with use of the gas sensor 100. For example, the amount of undershoot and overshoot shown in Figure 3 tend to increase with use of the gas sensor 100. In particular, when the sensor element 101 is exposed to high temperatures and atmospheric conditions, such as when the sensor element 101 is heated to a high temperature by the heater 72 and the vehicle is stopped, such an increase in overshoot and undershoot is likely to occur. The amount of undershoot can be calculated, for example, as the difference between the lowest value when the pump current Ip2 drops excessively and the value when the pump current Ip2 stabilizes afterward. The amount of overshoot can be calculated, for example, as the difference between the highest value when the pump current Ip2 rises excessively and the value when the pump current Ip2 stabilizes afterward. Alternatively, the undershoot and overshoot amounts can be calculated as the maximum difference between the value of the pump current Ip2 when excessive output fluctuations occur and the ideal pump current Ip2 value.

[0065] The inventors have found that by performing a process to control the sensor element 101 so as to pump out more oxygen from the gas flow section to be measured compared to when measuring a specific gas concentration (under normal conditions), the increased undershoot and overshoot associated with the use of the gas sensor 100 can be reduced. This process is called a refresh process. Specifically, the refresh process includes at least one of the following: a refresh-time adjustment pump control process that controls the adjustment pump cell so as to pump out more oxygen from the oxygen concentration adjustment chamber compared to the normal-time adjustment pump control process; and a refresh-time measurement pump control process that controls the measurement pump cell 41 so as to pump out more oxygen from the third internal cavity 61 compared to the normal-time measurement pump control process.

[0066] In this embodiment, the pump control process for measurement during refresh is the same as the pump control process for measurement during normal operation, except that the voltage Vp2 of the variable power supply 46 is feedback-controlled so that the voltage V2 becomes a target value V2r* which is higher than the target value V2* described above. The voltage V2 is a value related to the difference in oxygen concentration between the area around the reference electrode 42 and the third internal cavity 61. The lower the oxygen concentration in the third internal cavity 61, the larger the oxygen concentration difference and the higher the voltage V2 becomes. Therefore, a target value V2r* being higher than the target value V2* means that when the pump control process for measurement during refresh is executed, the target value of the oxygen concentration in the third internal cavity 61 is set to a lower value compared to when the pump control process for measurement during normal operation is executed. Accordingly, in this pump control process for measurement during refresh, the measurement pump cell 41 is controlled to pump out more oxygen from the third internal cavity 61 compared to the pump control process for measurement during normal operation.

[0067] The pump control process for adjustment during refresh includes at least one of the following: a main pump control process during refresh that controls the main pump cell 21 to pump out more oxygen from the first internal cavity 20 compared to the main pump control process during normal operation; and an auxiliary pump control process during refresh that controls the auxiliary pump cell 50 to pump out more oxygen from the second internal cavity 40 compared to the auxiliary pump control process during normal operation. In this embodiment, the main pump control process during refresh and the auxiliary pump control process during refresh are processes that set the target value of the feedback control to a higher value than during normal operation, similar to the pump control process for measurement during refresh. Specifically, the main pump control process during refresh is a process that feedback controls the pump voltage Vp0 of the variable power supply 24 so that the voltage V0 becomes a predetermined target value V0r* that is higher than the target value V0* described above. Note that in the main pump control process during normal operation, the target value V0* is set (changed) based on the pump current Ip1, but in the main pump control process during refresh, the target value V0r* is not changed based on the pump current Ip1 and a predetermined value is used. The auxiliary pump control process during refresh is the same as the auxiliary pump control process during normal operation, except that the voltage Vp1 of the variable power supply 52 is feedback-controlled so that the voltage V1 becomes a predetermined target value V1r* which is higher than the target value V1*.

[0068] During the refresh process, it is preferable not to perform the normal measurement pump control process, and it is even more preferable not to perform the normal adjustment pump control process. During the refresh process, it is preferable not to perform control that causes the measurement pump cell 41 to pump oxygen into the third internal cavity 61, and it is even more preferable not to perform control that causes the main pump cell 21 to pump oxygen into the first internal cavity 20, and control that causes the auxiliary pump cell 50 to pump oxygen into the second internal cavity 40. For example, if the measurement pump control process is performed during the refresh, it is naturally impossible to perform the normal measurement pump control process, and it is also preferable not to perform the normal adjustment pump control process. For example, if the measurement pump control process is performed during the refresh, it is preferable that no voltage is applied to the variable power supply 24 and the variable power supply 52, and that the main pump cell 21 and the auxiliary pump cell 50 are inactive. Similarly, if the main pump control process is performed during the refresh, it is preferable that no voltage is applied to the variable power supply 52 and the variable power supply 46, and that the auxiliary pump cell 50 and the measurement pump cell 41 are inactive. When performing auxiliary pump control processing during refresh, it is preferable that no voltage is applied to the variable power supply 24 and variable power supply 46, and that the main pump cell 21 and the measuring pump cell 41 are inactive. In this way, it is preferable that the pump cells among the main pump cell 21, auxiliary pump cell 50, and measuring pump cell 41 that are not used for the refresh process are not subjected to normal pump control processing or control such as oxygen pumping, and that the pump cells are inactive.

[0069] Figure 4 is a graph showing the increase in undershoot and overshoot of the pump current Ip2 in the actual sensor element 101. This graph was obtained as follows: The sensor element 101 was attached to a pipe, and a model gas was flowed through the pipe using a model gas apparatus. The control device 90 controlled the sensor element 101 to measure the NOx concentration. First, a first model gas (NO concentration 500 ppm, H2O concentration 12%, oxygen concentration 0%, base gas nitrogen) was flowed, then a second model gas (NO concentration 0 ppm, H2O concentration 0%, oxygen concentration 21%, base gas nitrogen) was flowed, and then the first model gas was flowed again. This rapidly changed the NO concentration, H2O concentration, and oxygen concentration of the gas being measured, simulating a fuel cut-off. The flow rate of the model gas was 100 L / min, and the temperature was 120°C. The time change of the pump current Ip2 at this time was measured. The dashed line in Figure 4 shows the time variation of the pump current Ip2 in the sensor element 101 in its initial state (immediately after manufacturing). The solid line in Figure 4 shows the time variation of the pump current Ip2 in the sensor element 101, simulating the state after use from the initial state. A sensor element 101 simulating the state after use was prepared by energizing the sensor element 101 in its initial state with the heater 72 and leaving it in the air for 5 minutes while maintaining a temperature of approximately 700°C to 800°C. As shown in Figure 4, there was almost no overshoot and undershoot in the sensor element 101 in its initial state, whereas the overshoot and undershoot were increased in the sensor element 101 simulating the state after use.

[0070] Next, a refresh process was performed on the sensor element 101, which simulated the post-use state described above. A model gas with a CO2 concentration of 15%, an H2O concentration of 15%, and nitrogen as the base gas was used as the gas to be measured and flowed through the piping to which the sensor element 101 was attached. In this state, the refresh process was performed as described above, which involves controlling the main pump during the refresh. The target value V0r* for the main pump control process during the refresh was set to 1000mV, and the processing time was set to 300 seconds. During the refresh process, no voltage was applied to the variable power supply 46 and variable power supply 52, and the measuring pump cell 41 and auxiliary pump cell 50 were kept inactive. After the refresh process, the time change of the pump current Ip2 was measured for the sensor element 101, simulating a fuel cut-off state as in Figure 4. The results are shown in Figure 5. The solid line in Figure 5 represents the time change of the pump current Ip2 in the sensor element 101 before the refresh process. The dashed line in Figure 5 represents the time change of the pump current Ip2 in the sensor element 101 after the refresh process. As shown in Figure 5, the refresh process reduced both undershoot and overshoot, and the sensor element 101 recovered to almost the same state as the initial sensor element 101 shown in Figure 4. Since undershoot and overshoot occur in the waveform of the pump current Ip2, it is thought that the refresh process refreshes the measurement electrode 44 of the sensor element 101 in particular.

[0071] Next, in the case where the refresh process was performed as a refresh main pump control process, similar to Figure 5, the time change of the pump current Ip2 was measured with different processing times, similar to Figure 5, and the relationship between processing time and the effect of reducing the amount of undershoot was investigated. Similarly, the relationship between processing time and the effect of reducing the amount of undershoot was investigated when the refresh process was performed as an auxiliary pump control process, and when the refresh process was performed as a measurement pump control process. The results are shown in Figure 6 and Table 1. Note that the target value V1r* when performing the refresh auxiliary pump control process was set to 1000mV, and the main pump cell 21 and measurement pump cell 41 were not operating during this process. The target value V2r* when performing the refresh measurement pump control process was set to 1000mV, and the main pump cell 21 and auxiliary pump cell 50 were not operating during this process. The vertical axis of Figure 6 shows the difference ΔUS between the initial value and the amount of undershoot after the refresh process, expressed as a percentage relative to the initial value, with the initial value being the amount of undershoot of the sensor element 101 (a small value close to zero). The closer this difference ΔUS is to 0%, the more the undershoot is reduced and the more the sensor element 101 recovers (refreshes) to the same state as the initial state. In Figure 6, the data when the main pump control process is performed during refresh is shown by a solid line (indicated as "V0" in the legend), the data when the auxiliary pump control process is performed during refresh is shown by a dashed line (indicated as "V1" in the legend), and the data when the measurement pump control process is performed during refresh is shown by a dashed line (indicated as "V2" in the legend).

[0072] [Table 1]

[0073] As can be seen from Figure 6 and Table 1, the difference ΔUS was approximately -450% (undershoot amount approximately 5.5 times the initial value) for the sensor element 101 with a refresh processing time of 0 seconds, i.e., the sensor element 101 simulating the post-use state described above, whereas it was confirmed that the difference ΔUS tended to approach 0% as the processing time increased. Furthermore, it was confirmed that the difference ΔUS approached 0% in a shorter time when the main pump control processing and the auxiliary pump control processing were performed during the refresh (data shown in the legends "V0" and "V1" in Figure 6 and Table 1) compared with the case where the measurement pump control processing during the refresh was performed (data shown in the legends "V2" in Figure 6 and Table 1). In other words, it was confirmed that the adjustment pump control processing during the refresh (here, the main pump control processing and the auxiliary pump control processing during the refresh) is more effective in refreshing the sensor element 101 than the measurement pump control processing during the refresh. Therefore, it is preferable that the refresh processing includes the adjustment pump control processing during the refresh. Furthermore, as can be seen from Figure 6 and Table 1, when the processing time is 10 seconds or less, the difference ΔUS approaches 0% rapidly, and thereafter tends to approach 0% relatively slowly. Therefore, even if the refresh process is performed for a long time, the refreshing effect does not increase significantly, and the refreshing effect is considered to be relatively high in the first 10 seconds from the start. Accordingly, it is considered that the sensor element 101 can be refreshed more efficiently by performing multiple short refresh processes rather than performing one long refresh process. In addition, the processing time for the refresh process is preferably 1 second or longer.

[0074] Here, the refresh process must be performed when the gas being measured in the gas flow section contains carbon. "The gas being measured contains carbon" also includes cases where the gas being measured contains molecules that have carbon. For example, if the gas being measured contains one or more of the following: carbon (C), carbon monoxide (CO), carbon dioxide (CO2), or hydrocarbons (HC), then the gas being measured can be said to contain carbon. For example, in the example described above, the refresh process was performed using a model gas with a CO2 concentration of 15%, an H2O concentration of 15%, and nitrogen as the base gas. However, when the refresh process was performed using a model gas that did not contain CO2, no reduction in undershoot and overshoot was observed. On the other hand, when the refresh process was performed using a model gas that contained ethylene (C2H4) instead of CO2, a reduction in undershoot and overshoot was confirmed.

[0075] The reason why the refresh treatment is effective when the gas being measured contains carbon is thought to be as follows. First, in the initial state (immediately after manufacturing), at least one of carbon (C), carbon monoxide (CO), and hydrocarbons (HC) is attached to the measuring electrode 44 of the sensor element 101, which is thought to prevent or reduce undershoot and overshoot. Then, as the above substances attached to the measuring electrode 44 decrease with use of the sensor element 101, it is thought that undershoot and overshoot will increase. When the refresh treatment is performed, more oxygen from the gas being measured in the gas flow section is drawn out than usual, resulting in a more reducing atmosphere than usual. As a result, the reduction of carbon dioxide (CO2) in the gas being measured to produce carbon (C) and carbon monoxide (CO), or the oxidation of carbon (C) and carbon monoxide (CO) in the gas being measured to carbon dioxide (CO2), is suppressed. Therefore, it is thought that the refresh treatment makes it easier for at least one of carbon (C) and carbon monoxide (CO) to adhere to the measuring electrode 44. As a result, the measuring electrode 44 can be restored (refreshed) to the same state as the initial state, and it is believed that undershoot and overshoot are reduced. Furthermore, if hydrocarbons (HC) are present in the gas being measured, the refreshing treatment creates a reducing atmosphere in the gas being measured as described above, thus suppressing the oxidation of hydrocarbons (HC) to produce water (H2O) and carbon dioxide (CO2). Therefore, it is believed that the refreshing treatment makes it easier for hydrocarbons (HC) to adhere to the measuring electrode 44. Consequently, in this case as well, it is believed that the measuring electrode 44 can be restored (refreshed) to the same state as the initial state, and undershoot and overshoot are reduced.

[0076] Furthermore, when the pump control process for measurement during refresh is performed, more oxygen is drawn from around the measuring electrode 44 than under normal conditions. For example, some of the oxygen extracted from water (H2O) in the gas being measured may oxidize the carbon (C) and carbon monoxide (CO) around the measuring electrode 44, generating carbon dioxide (CO2). In contrast, when the pump control process for adjustment during refresh is performed, the gas being measured reaches the measuring electrode 44 after oxygen has already been drawn out, so it is thought that almost no carbon dioxide (CO2) is generated around the measuring electrode 44. Due to this difference, it is thought that the amount of carbon (C) and carbon monoxide (CO) adhering to the measuring electrode 44 is less when the pump control process for measurement during refresh is performed compared to when the pump control process for adjustment during refresh is performed. For this reason, as shown in Figure 6, it is thought that the pump control process for adjustment during refresh is more effective in refreshing the sensor element 101 than the pump control process for measurement during refresh.

[0077] Next, an example of when the control unit 91 of the gas sensor 100 measures the NOx concentration and performs refresh processing will be described. Figure 7 is a flowchart of an example of a control routine executed by the control unit 91. The control unit 91 stores this routine in, for example, the storage unit 94. The control unit 91 controls, for example, the power supplied to the heater 72 by the heater power supply 76, and starts this control routine when the temperature of the heater 72 reaches a target temperature (for example, 800°C).

[0078] When the control routine is started, the CPU 92 of the control unit 91 first starts the normal control processing for measuring the NOx concentration (step S100). In the normal control processing, the CPU 92 performs the normal adjustment pump control processing (normal main pump control processing and normal auxiliary pump control processing) as described above, as well as the normal measurement pump control processing described above. Then, the CPU 92 calculates the NOx concentration in the gas to be measured based on the pump current Ip2 that flows due to the normal measurement pump control processing.

[0079] Next, the CPU 92 determines whether or not it is the timing to determine whether or not to refresh the sensor element 101 (step S110). The timing to determine whether or not to refresh is the timing at which there is a possibility of undershoot or overshoot in the pump current Ip2. As described in Patent Document 2, undershoot and overshoot are likely to occur when the H2O concentration in the gas being measured changes. Therefore, it is preferable to set the timing to determine whether or not to refresh at a time when it can be considered that the H2O concentration in the gas being measured in the gas flow section has changed abruptly. In this embodiment, the timing to determine whether or not to refresh is set to the start of fuel cut of the internal combustion engine. When fuel cut is started, the gas being measured becomes similar to the atmosphere and there is a high possibility that the H2O concentration will change abruptly, making it suitable as the timing to determine whether or not to refresh. Note that when fuel cut is started, the NOx concentration in the gas being measured also changes abruptly. For example, the control unit 91 detects the start of fuel cut based on fuel cut execution information obtained from the engine ECU (not shown) of the internal combustion engine. For example, the CPU 92 determines at predetermined intervals whether or not it has received fuel cut execution information from the engine ECU indicating that a fuel cut has been performed. When it receives the fuel cut execution information, it determines that the fuel cut has started and that it is time to determine whether or not it is necessary.

[0080] If it is determined in step S110 that it is time to determine whether a refresh is necessary, the CPU 92 calculates the amount of undershoot of the pump current Ip2 and performs a refresh necessity determination process to determine whether the calculated value is within the acceptable range (step S120). In this embodiment, since it is determined that it is time to determine whether a refresh is necessary at the start of fuel cut, immediately afterward the pump current Ip2 decreases rapidly as shown in Figures 3-5 due to the fuel cut. The CPU 92 calculates the amount of undershoot from the behavior (waveform) of the pump current Ip2 at this time. The determination in step S120 may be made by comparing the calculated undershoot amount itself with the acceptable range, or by comparing the difference between the undershoot amount in the initial state of the sensor element 101 (initial value) and the calculated undershoot amount with the acceptable range. For example, the upper limit of the acceptable range may be set as the value of the pump current Ip2 corresponding to a NOx concentration of 5 ppm, and it may be determined that the undershoot amount itself is outside the acceptable range if it exceeds this upper limit. Alternatively, it may be determined that the undershoot amount is outside the acceptable range if the difference between the initial value and the current undershoot amount exceeds this upper limit. Information regarding the acceptable range and initial values ​​can be stored in the memory unit 94 beforehand. If the amount of undershoot is outside the acceptable range in step S120, the CPU 92 determines that a refresh process is necessary and sets flag F to value 1 (step S130). Flag F is set to value 1 when a refresh process is necessary, and is set to value 0 when a refresh process is not necessary or when a control routine starts.

[0081] After step S130, if step S110 determines that it is not the timing to determine whether a refresh is necessary, or if the undershoot amount is within an acceptable range in step S120, the CPU 92 determines whether it is the timing to execute the refresh process (step S140). The timing to execute the refresh process is the timing at which the gas to be measured in the gas flow section can be considered to contain carbon. In this embodiment, the execution timing is set to during the operation of the internal combustion engine (except during fuel cut). Except during fuel cut, the gas to be measured basically contains carbon (especially CO2), making it suitable for the refresh process. For example, the CPU 92 determines whether it is the timing to execute the refresh process based on fuel cut execution information obtained from the engine ECU (not shown) of the internal combustion engine. For example, the CPU 92 determines at predetermined intervals whether it has obtained fuel cut execution information from the engine ECU indicating that a fuel cut has been performed, and if it has not obtained fuel cut execution information, it determines that it is currently operating outside of fuel cut time and is the timing to execute the refresh process.

[0082] If it is determined in step S140 that it is time to perform the refresh process, the CPU 92 checks whether flag F is valued at 1 (step S150). If it is valued at 1, it stops the normal control process started in step S100 (step S160) and performs the refresh process (step S170). In other words, when it is time to perform the refresh process and it is determined that a refresh is necessary (flag F is valued at 1), the CPU 92 performs the refresh process. For example, as part of the refresh process, the CPU 92 performs the refresh main pump control process for 10 seconds. In this case, since the normal control process is stopped in step S160, the normal auxiliary pump control process and the normal measuring pump control process are not performed during the refresh process. When the refresh process is completed, the CPU 92 sets flag F to value 0 (step S180), restarts the normal control process (step S190), and starts detecting the NOx concentration.

[0083] After starting the normal control process in step S190, if it is determined in step S140 that it is not time to perform the refresh process, or if flag F is not valued at 1 in step S150, the CPU 92 executes the processes from step S110 onward. As described above, by the CPU 92 performing the control routine shown in Figure 7, it is possible to measure the NOx concentration while performing the refresh process as needed. Thus, the refresh process of the present invention can be performed not only during the manufacturing or maintenance of the gas sensor 100, but also during the use of the gas sensor 100 (for example, during the operation of an internal combustion engine).

[0084] Here, the correspondence between the components of this embodiment and the components of the present invention will be clarified. The laminate in which the six layers of the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6 of this embodiment are stacked in this order corresponds to the element body of the present invention, the outer pump electrode 23 corresponds to the outer measuring electrode, the third internal cavity 61 corresponds to the measuring chamber, the measuring electrode 44 corresponds to the inner measuring electrode, the measuring pump cell 41 corresponds to the measuring pump cell, the first internal cavity 20 and the second internal cavity 40 correspond to the oxygen concentration adjustment chamber, the main pump cell 21 and the auxiliary pump cell 50 correspond to the adjustment pump cell, the reference electrode 42 corresponds to the reference electrode, the oxygen partial pressure detection sensor cell 82 for measuring pump control corresponds to the measuring voltage detection sensor cell, the sensor element 101 corresponds to the sensor element, the pump current Ip2 corresponds to the measuring pump current, and the control device 90 corresponds to the specific gas concentration detection unit, the necessity determination unit, and the refresh control unit.

[0085] As described in detail above, the gas sensor 100 of this embodiment includes a refresh necessity determination process that determines whether the sensor element 101 needs to be refreshed based on the undershoot when the pump current Ip2 changes abruptly, and performs a refresh process if it determines that a refresh is necessary. The refresh process includes at least one of the following: a refresh-time adjustment pump control process that controls the adjustment pump cells (here, the main pump cell 21 and the auxiliary pump cell 50) to pump out more oxygen from the oxygen concentration adjustment chamber (here, the first internal cavity 20 and the second internal cavity 40) compared to the normal adjustment pump control process, and a refresh-time measurement pump control process that controls the measurement pump cell 41 to pump out more oxygen from the third internal cavity 61 compared to the normal measurement pump control process. By performing such a refresh process, the increase in undershoot and overshoot of the pump current Ip2 associated with the use of the gas sensor 100 can be suppressed.

[0086] Furthermore, the refresh process includes a pump control process for adjustment during the refresh. Compared to the pump control process for measurement during the refresh, the pump control process for adjustment during the refresh is more effective in refreshing the sensor element 101. Therefore, by performing at least the pump control process for adjustment during the refresh process, the effect of suppressing undershoot and overshoot of the pump current Ip2 can be enhanced, and the refresh process can be performed in a shorter time.

[0087] Furthermore, the oxygen concentration adjustment chamber has a first internal cavity 20 and a second internal cavity 40 located downstream of the first internal cavity 20 and upstream of the third internal cavity 61. The adjustment pump cell also has a main pump cell 21 that adjusts the oxygen concentration in the first internal cavity 20 and an auxiliary pump cell 50 that adjusts the oxygen concentration in the second internal cavity 40. The refresh adjustment pump control process includes at least one of the following: a process that controls the main pump cell 21 to pump out more oxygen from the first internal cavity 20 compared to the normal adjustment pump control process, and a process that controls the auxiliary pump cell 50 to pump out more oxygen from the second internal cavity 40 compared to the normal adjustment pump control process.

[0088] Furthermore, the gas being measured is exhaust gas from an internal combustion engine, and the control unit 91 performs a refresh necessity determination process based on the behavior of the pump current Ip2 during sudden changes associated with the fuel cut-off of the internal combustion engine. As mentioned above, undershoot of the pump current Ip2 is more likely to occur when there is a sudden change in the H2O concentration rather than a sudden change in the NOx concentration in the gas being measured. And, when the fuel cut-off of the internal combustion engine, the gas being measured temporarily becomes similar to the atmosphere, so there is a high possibility that the H2O concentration is changing rapidly. Therefore, by making a necessity determination based on the behavior of the pump current Ip2 during sudden changes associated with the fuel cut-off of the internal combustion engine, the necessity of refreshing can be determined more appropriately. For example, even if the NOx concentration changes rapidly, if the H2O concentration does not change rapidly, the undershoot and overshoot of the pump current Ip2 may not be very large, even if the sensor element 101 is actually in a state where refresh processing is necessary. If the refresh necessity determination process is performed in this case, it may be determined that refresh processing is unnecessary, and the necessity determination may not be performed appropriately. In contrast, by performing a refresh necessity determination process during fuel cut-off, when the H2O concentration is likely to change rapidly, it is possible to more accurately determine whether a refresh is necessary.

[0089] Furthermore, the control device 90 determines that the sensor element 101 needs to be refreshed if the amount of undershoot during a sudden change in the pump current Ip2 falls outside the acceptable range. This allows for an appropriate determination of whether or not a refresh is necessary based on the amount of undershoot.

[0090] The control device 90 then performs a refresh process when it can be determined that the gas to be measured in the gas flow section contains carbon. Since the refresh process must be performed when the gas to be measured contains carbon, performing the refresh process when it can be determined that the gas to be measured in the gas flow section contains carbon allows for effective refreshing. This makes it less likely that, for example, undershoot and overshoot will not be reduced even after the refresh process, requiring another refresh process.

[0091] Furthermore, the processing time for the refresh process is between 1 second and 10 seconds. By setting the processing time to 1 second or more, the sensor element 101 can be refreshed more reliably. Also, the refresh effect does not increase significantly even if the refresh process is performed for a long period of time, and the refresh effect is relatively high in the first 10 seconds from the start. In addition, since the NOx concentration cannot be correctly detected during the refresh process, it is preferable to shorten the processing time for the refresh process. By setting the processing time to 10 seconds or less, the sensor element 101 can be refreshed efficiently while shortening the time during which the NOx concentration cannot be correctly detected. However, if the processing time for the refresh process is 10 seconds or less, the undershoot and overshoot of the pump current Ip2 may not be sufficiently reduced in a single refresh process. However, in this case, the next refresh necessity determination process (for example, step S120 after step S170 in Figure 7) will determine that a refresh process is necessary, and the refresh process will be executed again. Therefore, the refresh process will be repeatedly executed until the refresh necessity determination process determines that a refresh process is unnecessary, and the undershoot and overshoot will be reduced to the extent that a refresh process is ultimately deemed unnecessary.

[0092] Furthermore, by setting the refresh processing time to 10 seconds or less, the light-off time of the sensor element 101 can be shortened. For example, in step S190 of Figure 7, it may take some time for the pump current Ip2 to reach a value corresponding to the NOx concentration after the normal control processing has started. This time is called the light-off time, and the NOx concentration cannot be measured correctly until the light-off time has elapsed. The longer the refresh processing time, the longer the light-off time tends to be. By setting the refresh processing time to 10 seconds or less, the light-off time of the sensor element 101 can be shortened, and the measurement of the NOx concentration can be started quickly after the refresh processing.

[0093] 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.

[0094] For example, in the embodiment described above, in step S120 of Figure 7, the CPU 92 determined whether a refresh process was necessary based on the amount of undershoot of the pump current Ip2. However, the determination is not limited to this, and can be made based on the undershoot of the pump current Ip2 in any other way. For example, the determination may be made based on the minimum value of the undershoot of the pump current Ip2. Alternatively, the determination may be made based on the undershoot time, which is the time from when the pump current Ip2 reaches the minimum undershoot value until the pump current Ip2 stabilizes. Furthermore, the CPU 92 may determine whether a refresh process is necessary based on the overshoot of the pump current Ip2. For example, the determination may be made based on the amount of overshoot of the pump current Ip2. The determination may also be made based on the maximum value of the overshoot of the pump current Ip2. Alternatively, the determination may be made based on the overshoot time, which is the time from when the pump current Ip2 reaches the maximum overshoot value until the pump current Ip2 stabilizes. Furthermore, CPU92 may determine whether a refresh process is necessary based on both the undershoot and overshoot of the pump current Ip2.

[0095] When determining whether a refresh process is necessary based on the amount of overshoot of the pump current Ip2, the timing for determining whether a refresh process is necessary in step S110 may be a timing at which an overshoot is likely to occur, for example, at the end of the fuel cut. The end of the fuel cut can be detected based on the fuel cut execution information described above, similar to the start of the fuel cut. For example, the CPU 92 may determine at predetermined intervals whether or not it has acquired fuel cut execution information from the engine ECU indicating that a fuel cut has been performed, and when it stops acquiring fuel cut execution information for the first time since acquiring fuel cut execution information, it may determine that the fuel cut has ended and that it is time to determine whether a refresh process is necessary.

[0096] In the embodiment described above, the need for a refresh was determined based on the undershoot of the pump current Ip2. However, the determination is not limited to the pump current Ip2 itself; it may also be determined based on a value that can be converted to the pump current Ip2 or a value that can be considered equivalent to the pump current Ip2. For example, the need for a refresh may be determined based on the behavior of the NOx concentration [ppm] value (at least one of undershoot and overshoot) based on the pump current Ip2.

[0097] In the embodiment described above, the refresh process in step S170 was defined as the main pump control process during refresh. However, as mentioned above, the refresh process only needs to include at least one of the adjustment pump control process during refresh and the measurement pump control process during refresh. Therefore, in step S170, it is sufficient to execute one or more of the main pump control process during refresh, the auxiliary pump control process during refresh, and the measurement pump control process during refresh. However, as explained using Figure 6 and Table 1, the adjustment pump control process during refresh has a higher refresh effect than the measurement pump control process during refresh. Therefore, it is preferable that the refresh process includes at least one of the main pump control process during refresh and the auxiliary pump control process during refresh.

[0098] In the above-described embodiment, the refresh-time main pump control process, the refresh-time auxiliary pump control process, and the refresh-time measurement pump control process all involve raising the target value compared to the normal state. However, any process that controls the pump to draw out more oxygen from the gas flow section compared to the normal state is acceptable. For example, the refresh-time control process may be a process that performs constant voltage control of the pump cell without performing feedback control using the target value, or a process that performs constant current control of the pump cell. For example, the refresh-time main pump control process may be a process in which the CPU 92 controls the variable power supply 24 so that the pump voltage Vp0 becomes a predetermined constant voltage higher than the value during the normal main pump control process, without performing feedback control. Alternatively, the refresh-time main pump control process may be a process in which the CPU 92 controls the variable power supply 24 so that the pump current Ip0 becomes a predetermined constant current higher than the value during the normal main pump control process. The same applies to the refresh-time auxiliary pump control process and the refresh-time measurement pump control process.

[0099] In the embodiments described above, the refresh process is performed during operation other than when the fuel cut-off of the internal combustion engine is used as an example of the execution timing. However, it is not limited to this, as long as the timing is such that the gas to be measured in the gas flow section can be considered to contain carbon. For example, if the vehicle to which the gas sensor 100 is attached is a hybrid vehicle, the execution timing may be during electric operation of the hybrid vehicle (when the engine is stopped) and when the exhaust gas is stationary (when exhaust gas remains around the sensor element 101). Alternatively, the execution timing may be such that the gas to be measured can be considered not to be in an atmospheric environment. Furthermore, the refresh process may be performed without determining whether or not it is the execution timing, that is, whether or not the gas to be measured in the gas flow section can be considered to contain carbon. For example, when the sensor element 101 is performing the control routine in Figure 7, the internal combustion engine is basically running, and the effect of performing the refresh process can be obtained. Therefore, step S140 may be omitted. In this case, for example, if the refresh process is performed during fuel cut-off instead of during operation, it may not be possible to reduce the undershoot and overshoot of the pump current Ip2. However, in this case, the next refresh necessity determination process will determine that a refresh is necessary, and the refresh will be executed again. If the internal combustion engine is running at that time, the effect of the refresh will be obtained. Thus, even if it is not determined whether or not it is time to perform a refresh, undershoot and overshoot can ultimately be reduced by performing the refresh multiple times. However, as mentioned above, NOx concentration cannot be measured correctly during the refresh process, so it is preferable to determine whether or not it is time to perform the refresh in order to minimize such time.

[0100] In the embodiment described above, the CPU 92 detected the start of a fuel cut or that it was not a fuel cut based on fuel cut execution information, but these could also be detected based on the pump current Ip0. As described above, the pump current Ip0 that flows during normal main pump control processing changes according to the oxygen concentration of the gas to be measured (i.e., the gas to be measured around the sensor element 101) that flows into the gas flow section from the gas inlet 10. When the gas to be measured is exhaust gas from an internal combustion engine, the oxygen concentration in the gas to be measured becomes about the same as the oxygen concentration in the atmosphere during a fuel cut. Therefore, the CPU 92 can determine whether or not a fuel cut is in progress by checking whether or not the pump current Ip0 that flows during normal main pump control processing corresponds to a predetermined concentration range (e.g., 20-22%) that can be considered the same as the oxygen concentration in the atmosphere. For example, in step S110, the CPU 92 may determine whether or not it is the start of a fuel cut based on the pump current Ip0, and if it is the start, it may determine that it is the timing for determining necessity. Alternatively, in step S140, the CPU 92 may determine whether or not fuel cut is in progress based on the pump current Ip0, and if it is not in progress, it may determine that it is time to perform the refresh process. If the CPU 92 determines whether or not a refresh process is necessary in step S120 based on the amount of overshoot of the pump current Ip2, the CPU 92 may determine in step S110 whether or not the fuel cut has ended based on the pump current Ip0, and if it has ended, it may determine that it is time to determine whether or not a refresh process is necessary.

[0101] Although not described in the embodiments described above, there are cases where fuel cut-off is initiated during the refresh process in step S170. In this case, the refresh process may be terminated without waiting for the processing time (10 seconds in the embodiments described above) to elapse.

[0102] In the embodiment described above, the oxygen concentration adjustment chamber had a first internal cavity 20 and a second internal cavity 40, but it is not limited to this, for example, the oxygen concentration adjustment chamber may have another internal cavity, or one of the first internal cavity 20 and the second internal cavity 40 may be omitted. Similarly, in the embodiment described above, the adjustment pump cell had a main pump cell 21 and an auxiliary pump cell 50, but it is not limited to this, for example, the adjustment pump cell may have another pump cell, or one of the main pump cell 21 and the auxiliary pump cell 50 may be omitted. For example, if the oxygen concentration of the gas to be measured can be sufficiently lowered with only the main pump cell 21, the auxiliary pump cell 50 may be omitted. In this case, the CPU 92 may, as a normal adjustment pump control process, perform a process to control the main pump cell 21 so that the oxygen concentration in the oxygen concentration adjustment chamber (first internal cavity 20) becomes the target concentration. More specifically, a target value V0* may be predetermined, and the CPU 92 may control the main pump cell 21 by feedback-controlling the pump 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).

[0103] In the embodiment described above, the outer pump electrode 23 served as an outer main pump electrode, which is part of the main pump cell 21 and is located on the outside of the sensor element 101 in contact with the gas to be measured; an outer auxiliary pump electrode, which is part of the auxiliary pump cell 50 and is located on the outside of the sensor element 101 in contact with the gas to be measured; and an outer measuring electrode, which is part of the measuring pump cell 41 and is located on the outside of the sensor element 101 in contact with the gas to be measured. However, it is not limited to this. One or more of the outer main pump electrode, outer auxiliary pump electrode, and outer measuring electrode may be provided outside the sensor element 101 separately from the outer pump electrode 23.

[0104] In the embodiment described above, the outer pump electrode 23 is exposed to the outside of the sensor element 101, but the outer pump electrode 23 is not limited to this and can be provided on the outside of the element body (layers 1 to 6) so as to be in contact with the gas to be measured. For example, the sensor element 101 may have a porous protective layer covering the element body (layers 1 to 6), and the outer pump electrode 23 may also be covered by the porous protective layer.

[0105] 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 sensor element 201 of Figure 8, it may not have a third internal cavity 61. 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, for example, based on the pump current Ip2, similar to the embodiment described above. In this case, the area around the measuring electrode 44 functions as a measurement chamber.

[0106] 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 oxygen ion conductive solid electrolyte layer and have a gas to be measured flow section inside. For example, in Figure 1, 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 (for example, layers made of alumina). In this case, each electrode of the sensor element 101 should be arranged in the second solid electrolyte layer 6. For example, the measuring electrode 44 in Figure 1 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 atmosphere 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 space 61 and on the lower surface of the second solid electrolyte layer 6.

[0107] In the embodiment described above, the control device 90, in the normal main pump control process, sets a target value V0* for the voltage V0 based on the pump current Ip1 so that the pump current Ip1 becomes the target current Ip1* (feedback control), and then feedback controls the pump voltage Vp0 so that the voltage V0 becomes the target value V0*. However, other control methods may be used. For example, in the normal main pump control process, the control device 90 may feedback control the pump voltage Vp0 based on the pump current Ip1 so that the pump current Ip1 becomes the target current Ip1*. That is, the control device 90 may omit the acquisition of the voltage V0 from the oxygen partial pressure detection sensor cell 80 for main pump control and the setting of the target value V0*, and directly control the pump voltage Vp0 (and consequently the pump current Ip0) based on the pump current Ip1.

[0108] In the embodiment described above, the gas sensor 100 detected the NOx concentration as the specific gas concentration, but other oxide concentrations may also be used as the specific gas concentration. 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 CPU 92 can detect the specific gas concentration based on the detected value corresponding to this oxygen. Alternatively, the specific gas may be a non-oxide such as ammonia. If the specific gas is a non-oxide, the specific gas is converted to an oxide in the first internal cavity 20 (for example, ammonia is oxidized to NO), and oxygen is generated when the converted oxide is reduced in the third internal cavity 61, so the CPU 92 can obtain a detected value corresponding to this oxygen and detect the specific gas concentration. Thus, whether the specific gas is an oxide or a non-oxide, the gas sensor 100 can detect the specific gas concentration based on the oxygen generated in the third internal cavity 61 originating from the specific gas.

[0109] In the embodiment described above, the target value V0r* in the main pump control process during refresh is set to a value higher than the target value V0*, and in Figure 6, the target value V0r* is set to 1000mV. Preferably, the target value V0r* is between 700mV and 1100mV. The inventors investigated the relationship between the refresh processing time and the effect of reducing the amount of undershoot when the target value V0r* is varied within the range of 700mV to 1100mV. The results are shown in Figure 9. The horizontal and vertical axes in Figure 9 are the same as those in Figure 6, and the graph in Figure 9 was measured under the same conditions as the graph shown by the solid line (labeled "V0" in the legend) in Figure 6, except for the point where the target value V0r* was varied. Figure 9 shows the results when the target value V0r* is changed in five ways: 700mV, 800mV, 900mV, 1000mV, and 1100mV. The results in Figure 9 when the target value V0r* is 1000mV (the dashed line graph in Figure 9) are almost the same as the solid line graph in Figure 6, where the target value V0r* is also 1000mV. As can be seen from Figure 9, the larger the target value V0r*, the more the difference ΔUS tends to approach 0% in a shorter time, confirming that the effect of refreshing the sensor element 101 is high. Furthermore, since almost no effect of refreshing the sensor element 101 was obtained when the target value V0r* was 700mV, it is considered preferable for the target value V0r* to exceed 700mV, and even more preferable for 800mV or higher. In addition, the higher the target value V0r*, the higher the voltage Vp0 during the main pump control process during refresh. If the voltage Vp0 is too high, reduction of the solid electrolyte (zirconia in this case) of the sensor element 101 may occur. For this reason, it is preferable for the target value V0r* to be 1100mV or less. For the target value V1r* for the auxiliary pump control process during refresh and the target value V2r* for the measuring pump control process during refresh, it is preferable that they exceed 700mV and are 1100mV or less, and more preferably 800mV or more, similar to the target value V0r*.

[0110] This application is based on the priority claim of Japanese Patent Application No. 2021-137094, filed on 25 August 2021, the entire contents of which are incorporated herein by reference. [Industrial applicability]

[0111] 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]

[0112] 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 Atmospheric introduction layer, 50 Auxiliary pump cell, 51 Auxiliary pump electrode, 51a Ceiling electrode section, 51b Bottom electrode section, 52 Variable power supply, 60 Fourth diffusion-controlled section, 61 Third internal cavity, 70 Heater section, 71 Connector electrode, 72 Heater, 73 Through hole, 74 Heater insulating layer, 75 Pressure relief hole, 76 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 Control device, 91 Control unit, 92 CPU, 94 Memory unit, 100 Gas sensor, 101, 201 Sensor elements.

Claims

1. 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, A measuring pump cell comprising an outer measuring electrode provided on the outside of the element body so as to be in contact with the gas to be measured, and an inner measuring electrode disposed in the measuring chamber of the gas flow section to be measured, wherein oxygen is pumped from around the inner measuring electrode to around the outer measuring electrode, A control pump cell for adjusting the oxygen concentration in the oxygen concentration adjustment chamber located upstream of the measurement chamber in the gas flow section to be measured, A reference electrode is disposed inside the element body so as to be in contact with a reference gas that serves as a reference for detecting the concentration of a specific gas in the gas to be measured, A measuring voltage detection sensor cell for detecting the measuring voltage between the reference electrode and the inner measuring electrode, A sensor element having, A normal adjustment pump control process operates the adjustment pump cell, and a normal measurement pump control process controls the measurement pump cell so that the measurement voltage of the sensor element reaches a target value and pumps oxygen from the measurement chamber. A specific gas concentration detection unit detects the specific gas concentration in the gas to be measured based on the measurement pump current flowing through the measurement pump cell by the normal measurement pump control process. A refresh necessity determination unit performs a refresh necessity determination process to determine whether or not the sensor element needs to be refreshed based on at least one of the undershoot and overshoot of the measuring pump current, A refresh control unit that performs a refresh operation when the refresh necessity determination process determines that a refresh is necessary, and which includes at least one of the following: a refresh adjustment pump control process that controls the adjustment pump cell to pump out more oxygen from the oxygen concentration adjustment chamber compared to the normal adjustment pump control process; and a refresh measurement pump control process that controls the measurement pump cell to pump out more oxygen from the measurement chamber compared to the normal measurement pump control process. A gas sensor is included.

2. The refresh process includes the pump control process for adjustment during the refresh process. The gas sensor according to claim 1.

3. The oxygen concentration adjustment chamber has a first internal cavity and a second internal cavity located downstream of the first internal cavity and upstream of the measurement chamber. The adjustment pump cell comprises a main pump cell that adjusts the oxygen concentration in the first internal cavity and an auxiliary pump cell that adjusts the oxygen concentration in the second internal cavity. The refresh adjustment pump control process includes at least one of the following: a process to control the main pump cell so as to pump out more oxygen from the first internal cavity compared to the normal adjustment pump control process; and a process to control the auxiliary pump cell so as to pump out more oxygen from the second internal cavity compared to the normal adjustment pump control process. The gas sensor according to claim 2.

4. The gas being measured is exhaust gas from an internal combustion engine. The necessity determination unit performs the refresh necessity determination process based on at least one of the undershoot and overshoot of the measuring pump current associated with the fuel cut of the internal combustion engine. A gas sensor according to any one of claims 1 to 3.

5. The necessity determination unit determines that a refresh is necessary if at least one of the undershoot and overshoot amounts of the measuring pump current falls outside the acceptable range. A gas sensor according to any one of claims 1 to 3.

6. The refresh control unit performs the refresh process when the gas to be measured in the gas flow section to be measured is deemed to contain carbon. A gas sensor according to any one of claims 1 to 3.

7. The processing time for the refresh process is 1 second or more and 10 seconds or less. A gas sensor according to any one of claims 1 to 3.

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