NOx Sensor

Abstract

To provide a NOx sensor that can improve the detection accuracy of NOx concentration by suppressing an influence such as a change in an ambient environment and more accurately performing correction based on moisture content in gas to be measured.SOLUTION: A NOx sensor S comprises: a sensor element 1 that has a first cell 1p which outputs a first signal Ip having a correlation to a H2O concentration and a second cell 1s which outputs a second signal Is in accordance with a NOx concentration and a H2O concentration; a heater H; and a sensor control section 10. The sensor control section 10 includes: a NOx concentration reference value calculation section 101 that calculates a NOx concentration reference value C1 based on the second signal Is; a H2O concentration information detection section 102 that detects H2O concentration information Ti based on the first signal Ip or external information; a second cell temperature information detection section 103 that detects second cell temperature information Ti; and a NOx concentration correction section 104 that corrects the NOx concentration reference value C1 using an offset correction value C2 based on the H2O concentration information Ci and the second cell temperature information Ti.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a NOx sensor for detecting the concentration of NOx in a measurement gas. [Background technology]

[0002] A NOx sensor for a vehicle includes a sensor element using a solid electrolyte, and detects the NOx concentration by introducing exhaust gas as the measured gas into the sensor element and measuring the current flowing through the solid electrolyte due to the decomposition of nitrogen oxides (NOx). In general, the sensor element has a plurality of cells with electrodes formed on the surface of the solid electrolyte, and for example, a pump cell provided upstream of the gas flow pumps oxygen (O2) in the measured gas to adjust it to a predetermined low oxygen concentration, and then a sensor cell provided downstream detects the NOx contained in the measured gas.

[0003] On the other hand, moisture (H2O) contained in the gas to be measured can be a cause of error in detecting the NOx concentration. As a countermeasure, Patent Document 1 proposes a method of estimating the amount of moisture in the exhaust gas from the excess air ratio or A / F value determined by the engine operating conditions, and correcting the gas concentration detection signal according to the estimated amount of moisture. In addition, for a gas sensor that utilizes the oxygen pumping action, a method has been proposed in which the signal obtained from the second cell that decomposes NOx is corrected using a map or on an analog circuit, utilizing the correlation between the signal obtained from the first cell that pumps out oxygen and the oxygen concentration and amount of moisture in the exhaust gas. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 3372186 Summary of the Invention [Problem to be solved by the invention]

[0005] The gas sensor of Patent Document 1 estimates the amount of moisture in exhaust gas or obtains a value corresponding to the amount of moisture from engine information or sensor information, and corrects the NOx gas concentration detection output using a correction amount corresponding to the amount of moisture. As a premise, it is shown that there is a linear relationship between the NOx gas concentration detection output and the NOx gas concentration, and it is said that if correction based on the amount of moisture is not made, the sensitivity will not be affected but the offset will change. However, it has been found that if the temperature conditions vary over a wider range depending on the load conditions, the offset based on the amount of moisture will not necessarily be constant. This is presumably because the relationship between the amount of moisture and the offset is affected by the temperature environment around the sensor and sensor control, which differ depending on the load conditions, and there is a risk that the detection accuracy of the NOx gas concentration will decrease because the error caused by the amount of moisture is not properly corrected.

[0006] The present invention has been made in consideration of such problems, and aims to provide a NOx sensor that can improve the detection accuracy of NOx concentration by suppressing the effects of changes in the surrounding environment, etc., and performing more accurate correction based on the amount of moisture contained in the measured gas. [Means for solving the problem]

[0007] One aspect of the present invention is A NOx sensor (S) comprising: a sensor element (1) for detecting NOx contained in a measurement gas (G); a heater (H) for heating the sensor element; and a sensor control unit (10) for controlling operations of the sensor element and the heater and for calculating a NOx concentration based on a signal from the sensor element, The sensor element is a measurement gas chamber (2) into which the measurement gas is introduced via a gas inlet (3) and which has a solid electrolyte body (11) as a chamber wall; a first cell (1p) having a first cell electrode (21) arranged in the measurement gas chamber on the upstream side of a gas flow introduced therein and outputting a first signal (Ip) having a correlation with an HO concentration in the measurement gas; and a second cell (1s) having a second cell electrode (22) arranged downstream of the first cell and outputting a second signal (Is) corresponding to the NOx concentration and HO concentration in the measurement gas, The sensor control unit is a NOx concentration reference value calculation unit (101) that calculates a NOx concentration reference value (C1) based on the second signal; an H2O concentration information detection unit (102) that detects H2O concentration information (Ci) including an H2O concentration or concentration information correlated with an H2O concentration based on the first signal or external information; a second cell temperature information detection unit (103) that detects second cell temperature information (Ti) having a correlation with the temperature (Ts) of the second cell; and a NOx concentration correction unit (104) that corrects the NOx concentration reference value by using an offset correction value (C2) based on the H2O concentration information and the second cell temperature information. Effect of the Invention

[0008] In the NOx sensor having the above configuration, the sensor control unit controls the operation of the sensor element and the operation of the heater, and calculates the NOx concentration based on a signal from the sensor element. When the second signal is input from the second cell, the NOx concentration reference value calculation unit calculates the NOx concentration reference value based on a previously known relationship between the second signal and the NOx concentration. The H2O concentration information detection unit detects H2O concentration information based on the first signal input from the first cell or external information of the sensor element. In addition, the second cell temperature information detection unit detects the temperature of the corresponding second cell based on the input second cell temperature information. The NOx concentration correction unit calculates an offset correction value based on the H2O concentration information and the second cell temperature information, and corrects the NOx concentration reference value.

[0009] Here, the surrounding environment of the sensor element varies depending on the exhaust environment of the measured gas, and the amount of moisture contained in the measured gas varies, and the temperature and flow rate of the measured gas also vary depending on the load conditions. Therefore, even if the temperature control by the heater is performed by the sensor control unit, the temperature of the second cell from which the second signal is output may deviate from the control temperature by the heater, which has been found to be a cause of the offset. Even in such a case, the NOx concentration correction unit calculates an offset correction value based on the H2O concentration information and the second cell temperature information, and corrects the NOx concentration reference value using the offset correction value that takes into account the temperature of the second cell, so that the NOx concentration can be calculated more accurately.

[0010] As described above, according to the above aspect, a NOx sensor can be provided that can suppress the effects of changes in the surrounding environment, etc., and perform more accurate correction based on the amount of moisture contained in the measured gas, thereby improving the detection accuracy of the NOx concentration. In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate a correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing the overall configuration of a NOx sensor according to a first embodiment. [Diagram 2] 2A and 2B are a longitudinal sectional view and a line II sectional view showing a schematic configuration of a sensor element which is a main part of a NOx sensor in the first embodiment. [Diagram 3] FIG. 2 is a block diagram showing the configuration of a main part of a sensor control unit of the NOx sensor in the first embodiment. [Figure 4] 1 is an overall configuration diagram showing an installation state of a NOx sensor according to a first embodiment, and a partially enlarged sectional view thereof. [Diagram 5] 4 is a graph showing the relationship between the pump cell current of the sensor element and the O 2 concentration and the H 2 O concentration in the first embodiment. [Figure 6] FIG. 4 is a graph showing the relationship between NOx output and H2O concentration before offset correction of a sensor element in the first embodiment. [Figure 7] FIG. 4 is a graph showing the relationship between NOx output and H2O concentration after offset correction of a sensor element in the first embodiment. [Figure 8] FIG. 4 is a diagram showing the relationship between the electrode material of the sensor element and the adsorption energy of gas species in the first embodiment. [Figure 9] 4 is a graph showing the relationship between the pump cell current of the sensor element and the O 2 concentration and the H 2 O concentration in the first embodiment. [Figure 10] FIG. 2 is a block diagram showing the configuration of a main part of a NOx concentration calculation unit of a sensor control unit in the first embodiment. [Figure 11] FIG. 4 is a graph showing the relationship between the offset correction value of the sensor element and the H2O concentration in the first embodiment. [Figure 12] FIG. 4 is a graph showing the relationship between the offset correction value of the sensor element and the O2 concentration in the first embodiment. [Figure 13] FIG. 2 is a schematic diagram of a test device for acquiring an offset correction value calculation map and a diagram showing gas composition formulas before and after combustion in the first embodiment. [Figure 14] FIG. 4 is a cross-sectional view of a main part of a sensor element for explaining power supply control by a heater control unit of a sensor control unit in the first embodiment. [Figure 15] FIG. 4 is a diagram showing the relationship between the O2 concentration and the H2O concentration in exhaust gas in the first embodiment. [Figure 16] FIG. 4 is a diagram showing the influence of fuel properties on the relationship between O 2 concentration and H 2 O concentration in exhaust gas in the first embodiment. [Figure 17] FIG. 11 is a flowchart showing a calculation process procedure performed by a NOx concentration calculation unit of a sensor control unit in the second embodiment. [Figure 18] FIG. 11 is a time chart showing the relationship between the voltage applied and the current output by the detection control unit of the sensor control unit in the second embodiment. [Figure 19] FIG. 11 is a schematic diagram showing the relationship between the voltage applied to the sensor element and the current output in the second embodiment. [Figure 20] FIG. 11 is a graph showing the relationship between the sensor cell temperature and the cell impedance in the second embodiment. [Figure 21]FIG. 11 is a block diagram showing the configuration of a main part of a NOx concentration calculation unit of a sensor control unit in a third embodiment. [Figure 22] 13 is a schematic diagram for explaining the principle of detection of sensor cell temperature information of a sensor element in the third embodiment. FIG. [Diagram 23] FIG. 11 is a graph showing the relationship between the sensor cell temperature and the monitor cell current in the third embodiment. [Figure 24] FIG. 11 is a graph showing the relationship between the voltage applied to a sensor cell and NOx output in the third embodiment. [Diagram 25] FIG. 11 is a graph showing the relationship between the sensor cell temperature and the heater resistance in the third embodiment. [Figure 26] 13 is a graph showing the relationship between heater resistance and heater temperature, and between the heater temperature and sensor cell temperature in the third embodiment. [Figure 27] FIG. 13 is a block diagram showing the configuration of a main part of a NOx concentration calculation unit of a sensor control unit in a fourth embodiment. [Figure 28] FIG. 13 is an O2 concentration conversion map used in a NOx concentration calculation unit of a sensor control unit in the fourth embodiment. [Figure 29] 13 is a H2O concentration conversion map diagram and an atmospheric H2O concentration conversion map diagram used in a NOx concentration calculation unit of a sensor control unit in embodiment 4. FIG. [Diagram 30] FIG. 13 is a cross-sectional view showing a mounting structure of a sensor element in a fifth embodiment. [Diagram 31] FIG. 13 is a graph showing the relationship between the sensor cell temperature of the sensor element and the exhaust gas temperature and exhaust gas flow velocity in the fifth embodiment. [Diagram 32] FIG. 13 is a diagram showing the relationship between the chamber structure of the sensor element and the NOx output offset in the fifth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] (Embodiment 1) A first embodiment of a NOx sensor will be described with reference to the drawings. As shown in Fig. 1, the NOx sensor S of this embodiment includes a sensor element 1, a heater H for heating the sensor element 1, and a sensor control unit 10. As shown in the overall view (right diagram) of Fig. 4, a sensor main body 1A in which the sensor element 1 is housed is installed, for example, in an exhaust gas passage EX of an internal combustion engine such as a vehicle engine, and detects nitrogen oxides (i.e., NOx) contained in exhaust gas G, which is a gas to be measured, using the sensor element 1. The sensor control unit 10 is installed outside the exhaust gas passage EX, and controls the detection operation by the sensor element 1 and the heating operation by the heater H, and calculates the NOx concentration based on the output from the sensor element 1.

[0013] 1 and 2, the sensor element 1 is configured as a limiting current sensor using, for example, a solid electrolyte body 11, and has a pump cell 1p as a first cell and a sensor cell 1s as a second cell. The sensor element 1 has a measurement gas chamber 2 with the solid electrolyte body 11 as a chamber wall formed inside one end side (for example, the lower end side in the right diagram of FIG. 4) that serves as a gas detection section, and exhaust gas G is introduced into the measurement gas chamber 2 from the outside of the sensor element 1 through a gas introduction section 3. The sensor element 1 can also have a monitor cell 1m as a third cell provided in parallel with the sensor cell 1s.

[0014] As shown in the left diagram of FIG. 4, exhaust gas G is introduced into the measurement gas chamber 2 from the gas inlet 3 provided at the tip of the sensor element 1 with the longitudinal direction of the sensor element 1 being the gas flow direction X. In the measurement gas chamber 2, the pump cell 1p is arranged on the upstream side of the gas flow and is configured to output a first signal, which is a signal corresponding to the oxygen concentration in the exhaust gas G and correlates with the H2O concentration, while adjusting the oxygen concentration in the exhaust gas G. The sensor cell 1s is arranged on the downstream side of the pump cell 1p and outputs a second signal corresponding to the NOx concentration and H2O concentration in the exhaust gas G with the adjusted oxygen concentration. A reference gas chamber 4 is arranged on the opposite side of the measurement gas chamber 2 across the solid electrolyte body 11. A porous protective layer 5 is formed on the outer surface of the sensor element 1 so as to cover the entire gas detection section.

[0015] As shown in FIG. 2, specifically, the pump cell 1p has a pump electrode 21, which is a first cell electrode, on the surface of the solid electrolyte body 11 facing the measurement gas chamber 2, and performs oxygen pumping to adjust the oxygen concentration in the exhaust gas G to a predetermined low concentration. At this time, oxygen (i.e., O2) contained in the exhaust gas G is reduced by the catalytic action of the pump electrode 21 to produce oxide ions (i.e., O 2- ), which is conducted inside the solid electrolyte body 11 and discharged to the reference gas chamber 4. At that time, a current Ip flowing through the pump cell 1p (hereinafter referred to as a pump cell current) corresponds to the oxygen concentration (hereinafter referred to as an O2 concentration, as appropriate) in the exhaust gas G before adjustment, and is output to the sensor control unit 10 as a first signal correlating with the H2O concentration.

[0016] The sensor cell 1s has a sensor electrode 22, which is a second cell electrode, on the surface of the solid electrolyte body 11 facing the measurement gas chamber 2, and detects NOx contained in the exhaust gas G whose O2 concentration has been adjusted. If the exhaust gas G contains H2O, the catalytic action of the sensor electrode 22 decomposes NOx and H2O, and oxide ions resulting from NOx and H2O are conducted inside the solid electrolyte body 11 and discharged to the reference gas chamber 4. A current Is flowing through the sensor cell 1s at this time (hereinafter referred to as a sensor cell current) corresponds to the NOx concentration and H2O concentration in the exhaust gas G, and is output to the sensor control unit 10 as a second signal.

[0017] The NOx sensor S is used, for example, in a selective catalytic reduction (i.e., SCR) system for purifying NOx, to detect the NOx concentration downstream of the catalyst and to control the supply amount of reducing agent to the upstream side of the catalyst. In that case, when moisture (i.e., H2O) is mixed in the exhaust gas G together with NOx, the output from the sensor element 1 is an output based on H2O added to the output based on NOx. H2O in the exhaust gas G is mainly caused by the combustion of fuel in the engine and has a correlation with O2 in the exhaust gas G. Therefore, it is possible to estimate the offset amount of the NOx output using the H2O concentration or the O2 concentration and correct the NOx concentration. Furthermore, the offset correction can be appropriately performed by considering the temperature characteristics of the output based on H2O.

[0018] As shown in Figs. 1 and 3, the sensor control unit 10 is configured to be able to communicate with the sensor element 1 or an external device, and functionally includes a NOx concentration calculation unit 100, a detection control unit 200, and a heater control unit 300. The NOx concentration calculation unit 100 corrects an error in the NOx output caused by H2O (hereinafter referred to as NOx output error) based on the first signal, the second signal, and external information output from the sensor element 1, to calculate the NOx concentration. The detection control unit 200 controls the detection operation by the sensor element 1 so that it is appropriately performed, and the heater control unit 300 controls the heating operation by the heater H so that the temperature of the sensor element 1 is suitable for detection. Details of the control by the detection control unit 200 and the heater control unit 300 will be described later.

[0019] The NOx concentration calculation unit 100 includes a NOx concentration reference value calculation unit 101, an H2O concentration information detection unit 102, a sensor cell temperature information detection unit 103 which is a second cell temperature information detection unit, and a NOx concentration correction unit 104. The NOx concentration reference value calculation unit 101 receives a sensor cell current Is which is a second signal, and calculates a NOx concentration reference value C1 based on the sensor cell current Is. The H2O concentration information detection unit 102 receives a pump cell current Ip which is a first signal, or external information, and detects H2O concentration information Ci which includes an H2O concentration or concentration information correlated with the H2O concentration, based on the pump cell current Ip or the external information. The sensor cell temperature information detection unit 103 detects sensor cell temperature information Ti which is correlated with a temperature Ts of the sensor cell 1s (hereinafter referred to as sensor cell temperature) as the second cell temperature information.

[0020] The NOx concentration correction unit 104 calculates an offset correction value C2 based on the detected H2O concentration information Ci and sensor cell temperature information Ti, and corrects the NOx concentration reference value C1 using this offset correction value C2. Specifically, the NOx concentration correction unit 104 has an offset correction value calculation unit 104A, and stores a correction map or a correction formula or the like for calculating the offset correction value C2 based on the relationship between the offset amount of the sensor cell current Is, which is acquired in advance, and the H2O concentration information Ci and the sensor cell temperature information Ti.

[0021] In this manner, the NOx concentration calculation unit 100 calculates the NOx concentration reference value C1 based on the sensor cell current Is, which is the output from the sensor cell 1s, while detecting H2O concentration information Ci based on the pump cell current Ip or external information. As shown in FIG. 5, the O2 concentration determined from the pump cell current Ip is correlated with the H2O concentration in the exhaust gas G, and is in a roughly inversely proportional relationship in which the O2 concentration decreases as the H2O concentration increases. By utilizing this relationship, the O2 concentration can be converted into the H2O concentration, or the H2O concentration based on external information can be obtained to obtain the H2O concentration information Ci. In addition, sensor cell temperature information Ti, which is correlated with the sensor cell temperature Ts, is detected, and the offset correction value C2 is calculated using the detected information.

[0022] As shown in Fig. 6, the NOx concentration (unit: ppm) based on the sensor cell current Is shows an offset caused by moisture in the atmosphere, and the sensor output increases with an increase in the H2O concentration (unit: %) even if the NOx concentration in the model gas is substantially zero. This NOx output error also varies with the atmospheric temperature, and shows a temperature characteristic in which the sensor output increases at the same H2O concentration as the temperature of the model gas increases to 70°C, 200°C, and 500°C. For this reason, in an operating environment where the temperature change of the exhaust gas G introduced is large, such as in the case of a vehicle NOx sensor S, sufficient correction accuracy may not be obtained by only correcting the output based on the H2O concentration.

[0023] Here, the sensor control unit 10 uses a built-in heater H to heat one end of the sensor element 1, which serves as a gas detection unit, to a predetermined temperature. However, as shown in Fig. 6, it has been found that the sensor cell temperature Ts fluctuates toward a higher temperature (for example, in a range of 600°C to 650°C) relative to the control temperature of the pump cell 1p (for example, 600°C). Due to this temperature fluctuation, while it is possible to control the sensor cell temperature Ts to the same temperature as the pump cell 1p under low load conditions such as idling, it becomes difficult to make the sensor cell temperature Ts follow the control temperature under higher load operating conditions.

[0024] In Fig. 1, the heater H faces the solid electrolyte body 11, which is the chamber wall of the measurement gas chamber 2, across the reference gas chamber 4, and is capable of heating the region in which the pump cell 1p and the sensor cell 1s are arranged by heat transfer. However, it is difficult to uniformly heat the entire solid electrolyte body 11, and in the configuration in which the pump cell 1p and the sensor cell 1s are arranged vertically in the gas flow direction X as in this embodiment, differences in heat transfer between the exhaust gas G and the surroundings occur, making the sensor cell susceptible to the temperature of the exhaust gas G. For this reason, when the temperature of the exhaust gas G increases, the sensor cell temperature Ts does not match the control temperature of the pump cell 1p, and it is presumed that the high temperature of the sensor cell 1s promotes the decomposition of H2O, thereby increasing the sensor cell current Is.

[0025] 7, when the NOx concentration correction unit 104 performs offset correction based on the H2O concentration information Ci and the sensor cell temperature information Ti, the sensor cell current Is substantially coincides with the NOx concentration of the model gas, regardless of the H2O concentration or the sensor cell temperature Ts. In this way, by focusing on the difference between the control temperature by the heater H and the temperature of the sensor cell 1s at which the NOx concentration is detected and using the offset correction value C2 taking the sensor cell temperature Ts into consideration when correcting the sensor output, the NOx concentration can be calculated with higher accuracy.

[0026] The H2O concentration information Ci detected by the H2O concentration information detection unit 102 may include H2O concentration or concentration information correlated with the H2O concentration, such as O2 concentration information. When the O2 concentration information is acquired based on the pump cell current Ip input from the sensor element 1, the O2 concentration information may be an air-fuel ratio (A / F), an air excess factor (λ=air-fuel ratio / stoichiometric air-fuel ratio), an equivalence ratio (φ=1 / λ), or the like, which have a certain relationship with the O2 concentration (%) in the exhaust gas G.

[0027] Furthermore, the H2O concentration information Ci is not limited to such internal information of the sensor element 1, but may be external information acquired outside the sensor element 1. The external information includes engine operation information, mounting environment information, and the like, and the H2O concentration may be calculated in the H2O concentration information detection unit 102 based on the external information, or an estimated value of the H2O concentration or O2 concentration estimated based on the external information may be acquired as the external information.

[0028] The sensor cell temperature information Ti detected by the sensor cell temperature information detection unit 103 may include information correlated with the sensor cell temperature Ts. Such information may include, for example, impedance information and resistance value information acquired inside the sensor element 1. The impedance information may be, for example, a detection value of the cell impedance Zac based on the sensor cell current Is input from the sensor cell 1s or the monitor cell current Im as a third signal input from the monitor cell 1m.

[0029] Furthermore, the information is not limited to the internal information of the sensor element 1, but may be external information acquired outside the sensor element 1. The external information may include, for example, engine operation information, exhaust gas information around the sensor element 1, etc., and the sensor cell temperature Ts may be calculated in the sensor cell temperature information detection unit 103 based on this external information, or an estimated value of the sensor cell temperature Ts estimated based on this external information may be acquired as the external information.

[0030] In the H2O concentration information detection unit 102 or the sensor cell temperature information detection unit 103, external information is input to the sensor control unit 10 as information from, for example, an engine control unit (i.e., Engine Control Unit; hereinafter referred to as ECU) for a vehicle (not shown). The sensor control unit 10 is configured as a sensor control unit including a communication unit, a microcomputer (hereinafter referred to as MCU), and various terminals connected to the sensor element 1 and the ECU, and the NOx concentration is calculated in the NOx concentration calculation unit 100 based on the internal information from the sensor element 1 or the external information from the ECU.

[0031] The ECU receives inputs of the amount of intake air detected by an air flow meter (not shown), as well as detection signals from an engine speed sensor, an accelerator opening sensor, etc., and based on this input information, it determines the operating state of the engine E and controls the entire vehicle.

[0032] In this way, in the sensor control unit 10, the NOx concentration calculation unit 100 is equipped with a NOx concentration correction unit 104 that performs offset correction taking into account the sensor cell temperature Ts, thereby making it possible to appropriately correct the NOx output error caused by the moisture in the exhaust gas G and accurately calculate the NOx concentration. An example of the configuration of the NOx sensor S and details of the control by the sensor control unit 10 will be described below.

[0033] (Overall configuration of NOx sensor S) 4, in a sensor body 1A of the NOx sensor S, the sensor element 1 is inserted and held inside a cylindrical housing S1, and an outer peripheral threaded portion of the housing S1 is fixed to a passage wall EX1 of the exhaust gas passage EX. The sensor body 1A is housed in an element cover S2 with a tip side of the sensor element 1 protruding into the inside of the exhaust gas passage EX, and is housed in an atmosphere cover S3 with a base side of the sensor element 1 protruding outside the exhaust gas passage EX. The sensor control unit 10 is electrically connected to the sensor element 1 via a lead wire S4 drawn out to the base end side of the sensor body 1A.

[0034] The element cover S2 has a double-cylinder structure having a bottom surface, and the sides and bottom surfaces of the outer cover and inner cover are provided with a plurality of gas circulation holes (not shown). This allows the exhaust gas G flowing through the exhaust gas passage EX to be taken into the element cover S2, and the tip side of the sensor element 1, which serves as the gas detection section, is exposed to the exhaust gas G, which is the gas to be measured. In addition, the side surface of the cylindrical atmosphere cover S3 is provided with a plurality of gas circulation holes (not shown). This allows the atmosphere A, which is the reference gas, to be taken into the atmosphere cover S3 and reach the base end side of the sensor element 1.

[0035] 4, exhaust gas G that has reached the tip side of the sensor element 1 is taken into the measurement gas chamber 2 through the gas inlet 3 and moves toward the base end side. In the measurement gas chamber 2, a pump cell 1p is disposed on the upstream side of the gas flow direction X, and a sensor cell 1s and a monitor cell 1m are disposed on the downstream side. NOx in the exhaust gas G is detected using the air A taken into the reference gas chamber 4 as a reference gas.

[0036] (Configuration of sensor element 1) 2, the sensor element 1 is a stacked element with a three-cell structure, in which a shielding layer 13, a spacer layer 12, a solid electrolyte body 11, and a heater insulating layer 61 are arranged in this order with the stacking direction being a direction perpendicular to the gas flow direction X (i.e., the up-down direction in FIG. 2). The spacer layer 12 and the shielding layer 13 are arranged on one surface side of the solid electrolyte body 11, forming a space that becomes the measured gas chamber 2. In addition, a space that becomes the reference gas chamber 4 is formed between the solid electrolyte body 11 and the heater insulating layer 61 that constitutes the heater H on the other surface side.

[0037] The measurement gas chamber 2 communicates with a space in the element cover S2 (see FIG. 4, for example) in which the exhaust gas G exists, via a diffusion resistance layer 31 embedded in the tip surface (i.e., the left end surface in FIG. 2) of the spacer layer 12. The diffusion resistance layer 31, together with a part of the porous protective layer 5 disposed on its outside, constitutes the gas inlet portion 3 to the measurement gas chamber 2, and is adjusted so that the exhaust gas G is introduced under a predetermined diffusion resistance. The porous protective layer 5 is formed to cover the entire surface on the tip side, which is the gas detection portion of the sensor element 1, and protects the sensor element 1 from poisoning substances and the like.

[0038] The gas detection section of the sensor element 1 is provided with a pump cell 1p, a sensor cell 1s, and a monitor cell 1m, and each cell has a pair of electrodes facing each other with a solid electrolyte body 11 in between. In the measured gas chamber 2, a pump electrode 21 of the pump cell 1p is arranged on the upstream side of the surface of the solid electrolyte body 11 in the gas flow direction X, and a sensor electrode 22 of the sensor cell 1s and a monitor electrode 23 of the monitor cell 1m are arranged on the downstream side of the pump electrode 21. In the reference gas chamber 4, a reference electrode 41 serving as a common electrode is arranged on the surface of the solid electrolyte body 11 at a position facing these electrodes 21, 22, and 23.

[0039] The reference gas chamber 4 in which the common reference electrode 41 is disposed extends to the base end side of the sensor element 1 (e.g., the right end side in FIG. 2) and communicates with a space inside the atmosphere cover S3 (e.g., see FIG. 4) in which the atmosphere A exists, via a reference gas inlet opening at an end face not shown. A heating element 62 that generates heat when electricity is passed through it is embedded inside a heater insulating layer 61 that forms the bottom wall of the reference gas chamber 4, constituting a heater H. The heating element 62 is disposed corresponding to the portions where the electrodes of the pump cell 1p, the sensor cell 1s, and the monitor cell 1s are formed, and is capable of heating the entire tip side of the element that serves as the gas detection portion.

[0040] As a result, one of the pair of electrodes of each cell, 21, 22, 23, is exposed to the exhaust gas G introduced into the measurement gas chamber 2, and the other electrode 41 is exposed to the atmosphere A introduced into the reference gas chamber 4. At this time, by applying a voltage between the pair of electrodes of each cell, oxygen pumping is possible, in which oxygen contained in the exhaust gas G is pumped to or from the reference gas chamber 4 through the solid electrolyte body 11 having oxide ion conductivity. In addition, by operating the heater H, each cell is heated to a temperature above the activation temperature, making it possible to stably perform oxygen pumping.

[0041] In the measurement gas chamber 2, the pump electrode 21 of the pump cell 1p is formed with a large area on the upstream side in the gas flow direction X. When the exhaust gas G that has passed through the diffusion resistance layer 31 comes into contact with the pump electrode 21, oxygen is decomposed by the catalytic action and discharged to the reference electrode 41 side. The exhaust gas G adjusted to a low oxygen concentration by the pump cell 1p reaches the sensor electrode 22 and the monitor electrode 23 on the downstream side, and at the sensor electrode 22, oxygen caused by NOx is discharged together with remaining oxygen by oxygen pumping. At the monitor electrode 23, the remaining oxygen is discharged by oxygen pumping.

[0042] The sensor electrode 22 and the monitor electrode 23 are arranged side by side at the same position in the gas flow direction X, and are formed to have the same shape and smaller area than the pump electrode 21. This places the sensor electrode 22 and the monitor electrode 23 under the same conditions with respect to the flow of exhaust gas G, and by comparing the outputs of the sensor cell 1s and the monitor cell 1m, the influence of the remaining oxygen contained in the NOx output can be eliminated.

[0043] The solid electrolyte body 11 is made of a zirconia-based solid electrolyte material having oxide ion conductivity. As the zirconia-based solid electrolyte material, for example, partially stabilized zirconia or stabilized zirconia containing a stabilizer such as yttria can be used. The diffusion resistance layer 31 and the porous protective layer 5 are made of a ceramic material such as alumina. The heater insulating layer 61, the shielding layer 13, and the spacer layer 12 can be made of an insulating ceramic material such as alumina.

[0044] The electrodes of the pump cell 1p, the sensor cell 1s, and the monitor cell 1m may be porous cermet electrodes containing a precious metal or a precious metal alloy material and a zirconia-based solid electrolyte. The sensor electrode 22 of the sensor cell 1s is made of an electrode material having decomposition activity for the NOx to be detected. For example, an electrode containing platinum and rhodium (hereinafter referred to as a Pt-Rh electrode) may be used as such an electrode.

[0045] As shown in Figure 8, the precious metal elements constituting the electrodes of each cell exhibit different adsorption energies (decomposition properties) for the gas species contained in the exhaust gas G, with the order of gas adsorption properties being Rh>Pt>Au. In addition, in terms of gas species, the order of adsorption energies (decomposition properties) for the precious metal elements is O2>NO>H2O. Therefore, by changing the combination and metal ratio of these precious metal elements and adjusting the voltage applied to each cell, it becomes possible to selectively decompose the desired gas in each cell.

[0046] The pump electrode 21 of the pump cell 1p is made of an electrode material having oxygen decomposition activity and low NOx decomposition activity. For example, an electrode containing platinum and gold (hereinafter referred to as a Pt-Au electrode) can be used as such an electrode. The monitor electrode 23 of the monitor cell 1m is also made of the same electrode material as the pump electrode 21. The reference electrode 41 can be made of an electrode containing platinum (hereinafter referred to as a Pt electrode).

[0047] Thus, by heating the sensor element 1 to a predetermined temperature with the heater H and applying a predetermined voltage between the electrodes of each cell, the sensor cell 1s can detect NOx in the exhaust gas G adjusted to a predetermined low oxygen concentration by the pump cell 1p. In addition, by utilizing the difference in gas adsorption properties between the Pt-Rh electrode used in the sensor electrode 22 and the Pt-Au electrode used in the monitor electrode 23, the difference in current output from the sensor cell 1s and the monitor cell 1m can be used as the NOx output, thereby canceling the effect of oxygen remaining in the exhaust gas G.

[0048] However, since Rh contained in the Pt-Rh electrode constituting the sensor electrode 22 has decomposition activity for H2O in addition to NOx and O2, when H2O is contained in the exhaust gas G that reaches the sensor electrode 22, an output error occurs due to the decomposition of these. This output error is corrected by the NOx concentration calculation unit 100 of the sensor control unit 10 using offset correction information obtained in advance. The control of each unit and the output correction by the sensor control unit 10 will be described next.

[0049] (Configuration of sensor control unit 10) 1, the sensor control unit 10 includes a current detection unit 10A that detects the output current from each cell of the sensor element 1, and a NOx concentration calculation unit 100 that calculates the NOx concentration based on the pump cell current Ip, the sensor cell current Is, and the monitor cell current Im detected by the current detection unit 10A. The sensor control unit 10 also includes a detection control unit 200 that controls the NOx detection by the sensor element 1, and a heater control unit 300 that controls the heating of the sensor element 1 by the heater H.

[0050] Each cell of the sensor element 1 is connected to the sensor control unit 10 via a detection terminal P- and detection terminals S-, M-, and both ends of the heating element 62 of the heater H are connected to a pair of heater terminals H+, H-. The sensor control unit 10 is connected to an ECU (not shown) via a pair of communication terminals CAN+, CAN-, and is capable of performing NOx detection processing and outputting the detection result upon receiving a command from the ECU. The sensor control unit 10 is also connected to a vehicle battery and ground (not shown) via a power supply terminal VB and a ground terminal GND, respectively, and is capable of supplying power to various circuits of the sensor control unit 10 and the heater H of the sensor element 1.

[0051] The NOx concentration calculation unit 100 is composed of a microcomputer or the like having a storage unit and a calculation unit, and executes a pre-stored program and performs a predetermined calculation using the internal information of the sensor element 1 input from the current detection unit 10A to calculate the NOx concentration. The current detection unit 10A is connected to the pump electrode 21 of the pump cell 1p via the detection terminal P-, and is also connected to the sensor electrode 22 of the sensor cell 1s and the monitor electrode 23 of the monitor cell 1m via the detection terminals S- and M-. The current detection unit 10A includes, for example, a detection circuit using a resistor for current detection, and is configured to be able to detect the output current of each cell from the potential difference between both ends of the resistor. The detected output current is converted into a digital signal in an analog / digital conversion circuit and input to each unit of the NOx concentration calculation unit 100.

[0052] The detection control unit 200 controls the detection operations of the pump cell 1p, the sensor cell 1s, and the monitor cell 1m. The detection control unit 200 includes, for example, a voltage application circuit for applying a voltage between a pair of electrodes of each cell, and can generate a predetermined voltage signal based on a control command and apply it to the common reference electrode 41 via the common terminal COM+. The applied voltage is adjusted within a range in which the current flowing through each cell exhibits a limiting current characteristic so that the O2 concentration in the measurement gas chamber 2 becomes a predetermined low concentration by oxygen pumping of the pump cell 1p.

[0053] The heater control unit 300 controls the supply of electricity to the heater H built in the sensor element 1. The heater control unit 300 includes, for example, a switch circuit connected to a power supply terminal VB, and is capable of controlling the supply of electricity to the heater H via the heater terminals H+ and H- by driving the switch circuit on and off based on a control command. As a result, each cell of the sensor element 1 heated by the heater H is maintained at a temperature suitable for detecting NOx.

[0054] 3, the NOx concentration calculation unit 100 includes a NOx concentration reference value calculation unit 101, an H2O concentration information detection unit 102, a sensor cell temperature information detection unit 103, and a NOx concentration correction unit 104. The NOx concentration calculation unit 100 calculates a NOx concentration reference value C1 in the NOx concentration reference value calculation unit 101 based on the sensor cell current Is and the monitor cell current Im detected by the current detection unit 10A. Specifically, the NOx concentration calculation unit 100 calculates a difference between the sensor cell current Is and the monitor cell current Im to calculate an NOx output with the influence of remaining oxygen removed, and calculates the corresponding NOx concentration reference value C1 based on the relationship between the NOx output and the NOx concentration obtained in advance by performing a test or the like.

[0055] As shown by the solid line in Fig. 9, the difference value [Is-Im], which is the NOx output from the sensor element 1, and the actual NOx concentration (measured value) measured by an analyzer or the like should have a relationship represented by a straight line passing through the zero point. On the other hand, as shown by the dotted line in Fig. 8, when the NOx output contains an error due to H2O, the difference value (detected value) based on the actually detected output current is an output in which the error is added to the measured value. Therefore, the NOx concentration reference value calculation unit 101 stores the correspondence relationship represented by the solid line in advance as a map or a calculation formula, and obtains a value corresponding to the difference value (detected value) as the NOx concentration reference value C1 before correcting the NOx output error.

[0056] The NOx concentration correction unit 104 corrects the error included in the NOx concentration reference value C1 to calculate the original NOx concentration. As described above, the error included in the NOx concentration reference value C1 increases or decreases depending on the H2O concentration and has temperature characteristics, so the NOx concentration correction unit 104 is provided with an offset correction value calculation unit 104A that calculates an offset correction value C2 based on these pieces of information.

[0057] 10, H2O concentration information Ci is detected by an H2O concentration information detection unit 102, and sensor cell temperature information Ti is detected by a sensor cell temperature information detection unit 103. The H2O concentration information detection unit 102 detects the H2O concentration information Ci based on, for example, a pump cell current Ip input from the pump cell 1p, and the sensor cell temperature information detection unit 103 detects the sensor cell temperature information Ti based on, for example, a monitor cell current Im input from the monitor cell 1m.

[0058] The H2O concentration information Ci may be, for example, the H2O concentration obtained from the pump cell current Ip, and the sensor cell temperature information Ti may be, for example, the cell impedance Zac of the monitor cell 1m obtained from the monitor cell current Im. These pieces of information Ci and Ti are input to the offset correction value calculation unit 104A and used to calculate an offset correction value C2 for performing offset correction.

[0059] 11, the offset correction value calculation unit 104A stores an offset correction value calculation map obtained in advance by performing tests or the like. As an example of the relationship between the H2O concentration information Ci and the sensor cell temperature information Ti and the offset correction value C2, the map shows the offset correction value C2 corresponding to the H2O concentration range (e.g., 0% to about 13%) in the exhaust gas G for each temperature in the mounting environment of the sensor element 1. As shown in the figure, the offset correction value C2 becomes larger as the H2O concentration increases and as the mounting environment temperature becomes higher.

[0060] By using such a calculation map, an offset correction value C2 corresponding to the H2O concentration information Ci and the sensor cell temperature information Ti can be obtained, and as described above, it becomes possible to appropriately correct the NOx output error (for example, see FIG. 5). Note that, for convenience, the relationship between the low and high temperature ranges corresponding to the upper and lower limits of the temperature range that the sensor cell temperature Ts can take in the installation environment and the intermediate temperature range therebetween is shown here, but it goes without saying that the correction may be performed using, for example, map values ​​corresponding to the entire installation environment temperature range.

[0061] As shown in Fig. 12, the calculation map may indicate an offset correction value C2 corresponding to an O2 concentration range (for example, 0% to about 21%) for each temperature in the mounting environment of the sensor element 1 based on the relationship between the H2O concentration and the O2 concentration in the exhaust gas G. In that case, contrary to the relationship in Fig. 11 described above, the offset correction value C2 becomes smaller as the O2 concentration increases, and becomes larger as the mounting environment temperature increases. The relationship between the H2O concentration and the O2 concentration in the exhaust gas G will be described later.

[0062] The calculation map can be obtained, for example, by using a test device 400 shown in FIG. 13. The test device 400 includes a gas mixing section 401 that generates a model gas G1 simulating the exhaust gas G, and an exhaust gas pipe 402 that is connected to the gas mixing section 401. A supply pipe 403 for various gases including N2 gas, O2 gas, and CO2 gas is connected to the gas mixing section 401, and the gas mixture ratio and gas flow rate in the model gas G1 can be adjusted by adjusting the opening of a valve 403A. A heater 404 is disposed around the outer periphery of the connection part with the gas mixing section 401 in the exhaust gas pipe 402, and the model gas G1 that flows in can be heated to a predetermined temperature. A sensor body 1A of the gas sensor S is attached to the exhaust gas pipe 402 downstream of the heater 404, and the temperature and various gas concentrations of the model gas G1 can be measured by a thermocouple 405 and a gas concentration measuring section 406 disposed downstream of the heater 404.

[0063] In such a test apparatus 400, the NOx concentration was measured by the gas sensor S under the following conditions, and the calculation map shown in FIG. 10 was obtained by comparing the NOx output with the measured value of the model gas G1. H2O concentration: Variable (0% to approx. 13%) Gas temperature: variable (70℃, 200℃, 500℃) Gas flow rate: set value Control temperature: 600℃ (pump cell temperature Tp) The gas temperature of model gas G1 was changed in three stages in the range of 70°C to 500°C so that the H2O concentration was in the range of 0% to about 13% (i.e., the O2 concentration was in the range of about 21% to 0%) by changing the gas mixture ratio. The gas flow rate was adjusted to be the same regardless of the H2O concentration by adjusting the flow rates of the various gases in model gas G1.

[0064] The gas sensor S was connected to a sensor control unit 10 (not shown), and was controlled by a heater control unit 300 so that the pump cell temperature Tp, which was the control temperature, was kept constant. When the control temperature was set to 600°C and the gas temperature was changed in the range of 70°C to 500°C, the sensor cell temperature Ts of the sensor element 1 changed as follows: Gas temperature: 70℃ → Sensor cell temperature Ts: 600℃ Gas temperature: 200℃ → Sensor cell temperature Ts: 620℃ Gas temperature: 500℃ → Sensor cell temperature Ts: 650℃

[0065] As shown in Fig. 14, the sensor control unit 10 can detect the pump cell temperature Tp based on, for example, the cell impedance Zp of the pump cell 1p. The heater control unit 300, for example, compares the detected pump cell temperature Tp with a predetermined control temperature to set a duty ratio for energizing the heater and duty-drives the heater H. At this time, the pump cell 1p is temperature-controlled, while the sensor cell 1s is temperature-controlled as the situation demands. This is due to (1) heat transfer to the passage wall EX1 (see, for example, Fig. 4) of the exhaust gas passage EX to which the sensor element 1 is attached, and (2) heat transfer between the sensor element 1 and the exhaust gas G present around it.

[0066] In the measurement gas chamber 2, the sensor cell 1s is located downstream of the gas flow and closer to the base end of the sensor element 1 than the pump cell 1p, and therefore the influence of factors (1) and (2) on the sensor cell 1s is different from that on the pump cell 1p. In addition, the heating of the pump cell 1p and the sensor cell 1s by the heating element 62 of the heater H is not necessarily uniform. For this reason, for example, when the gas temperature of the exhaust gas G is relatively high, if the temperature is controlled based on the pump cell 1p that is exposed to the exhaust gas G and thus easily exchanges heat, the heat from the sensor cell 1s is not easily removed, and a temperature difference may easily occur.

[0067] Even in such a case, the NOx concentration correcting section 104 can perform good offset correction using the above-mentioned offset correction value calculation map by taking into account the temperature characteristics of the NO output error based on the H2O concentration. Furthermore, the pump cell temperature Tp can be controlled with good responsiveness using the cell impedance Zp of the pump cell 1p, so that oxygen can be pumped out of the pump cell 1p well and the pump cell current Ip corresponding to the H2O concentration information Ci can be accurately detected. In this way, the NOx concentration can be calculated with good accuracy by performing offset correction based on the H2O concentration information Ci and the sensor cell temperature information Ti.

[0068] As shown in equation (1) in FIG. 13, the unburned mixture contains a hydrocarbon fuel (C m Hn O o ), and air in an amount according to the excess air ratio λ, and the air contains water vapor in an amount according to the atmospheric water vapor concentration x (hereinafter referred to as atmospheric water vapor). As shown in FIG. 13 as equation (2), in exhaust gas G, which is burned gas, oxygen is consumed by the combustion of fuel, leaving excess oxygen according to the excess air ratio λ, and carbon dioxide (i.e., CO2) and water vapor (hereinafter referred to as fuel-derived water vapor) are generated. The water vapor contained in exhaust gas G (hereinafter referred to as exhaust gas water vapor) is made up of fuel-derived water vapor and water vapor based on atmospheric water vapor (hereinafter referred to as air-derived water vapor), and the air-derived water vapor is determined according to the excess air ratio λ and the atmospheric water vapor concentration x.

[0069] From this relationship, as the excess air ratio λ becomes larger beyond 1, the O2 concentration in the exhaust gas G increases, and the H2O concentration decreases relatively. Alternatively, as the excess air ratio λ becomes smaller, the O2 concentration decreases, and the H2O concentration increases relatively. Therefore, by detecting the pump cell current Ip accompanying the discharge of oxygen in the pump cell 1p, the O2 concentration and further the H2O concentration in the exhaust gas G can be known. At this time, it is desirable to take into account the atmospheric water vapor concentration x, and although an actual measured value or an estimated value can be used, this is not necessarily easy. On the other hand, since the influence of the atmospheric water vapor concentration x (e.g., 0.5% to 3.5%) on the fluctuation range of the fuel-derived water vapor (e.g., about 13%) is relatively small, it is preferable to set a certain reference value (e.g., 2%) based on the surrounding environment.

[0070] As shown in FIG. 15, the relationship between the O2 concentration in the exhaust gas G and the H2O concentration in the exhaust gas G was calculated based on the relationship between the above formulas (1) and (2) with the atmospheric water vapor concentration x set to 0% or 2%, and compared with the evaluation results from an actual vehicle test. The vehicle test was carried out using the H / C ratio (C m H n O oThe actual H2O concentration in the exhaust gas G was measured by changing the excess air ratio λ and changing the O2 concentration in the exhaust gas G with the air excess ratio (n / m ratio in the exhaust gas G) set to 1.886. The result was almost consistent with the relationship shown by the characteristic line when the atmospheric water vapor concentration x was set to 2%. In this way, the H2O concentration can be easily calculated by setting the reference value of the atmospheric water vapor concentration x. Note that this reference value can be set arbitrarily depending on the installation environment of the gas sensor S, for example, taking into consideration the variation in the atmospheric water vapor concentration x that differs depending on the region and climate.

[0071] As shown in FIG. 16, when the fuel properties used in the engine are different, the water vapor in the exhaust gas also varies depending on the H / C ratio, and the relationship between the O2 concentration and the H2O concentration changes. In the formula (2) in FIG. 13, the fuel-derived water vapor is expressed as [n / 2], and the fuel-derived water vapor increases as the fuel does not contain oxygen and the H / C ratio increases. Therefore, the slope of the characteristic line in FIG. 16 changes according to the H / C ratio (for example, 1.8 to 2.2), and as shown by the arrow in the figure, the characteristic line shifts in an increasing direction as the atmospheric water vapor concentration x increases from 0% to 4%. Therefore, it is preferable to calculate the H2O concentration in the exhaust gas G in consideration of the fuel properties. Specifically, in the relationship between the standard O2 concentration and the H2O concentration, the water vapor in the exhaust gas can be calculated by adding the air-derived water vapor based on the following calculation formula to the air-derived water vapor term in the above formula (2). Air-borne water vapor = [m+(n / 4)-(O / 2)] x [x / (100-x) / 0.209]

[0072] In this manner, according to this embodiment, by using the H2O concentration information Ci and the sensor cell temperature information Ti, it is possible to control the pump cell 1p to a state suitable for oxygen pumping, while performing appropriate offset correction in response to temperature fluctuations in the sensor cell 1s due to changes in load conditions, etc. As a result, it is possible to correct the temperature characteristics of the output caused by the moisture in the exhaust gas G and calculate the NOx concentration with high accuracy over a wide range of operating conditions from low load conditions to high load conditions.

[0073] (Embodiment 2) A second embodiment of the NOx sensor will be described with reference to the drawings. In this embodiment, an example of a specific procedure for the detection control by the sensor control unit 10 and the NOx concentration calculation described above will be shown. FIG. 17 is a flow chart showing the procedure of offset correction performed by the NOx concentration calculation unit 100, and FIG. 18 shows a time chart of the applied voltage control performed by the detection control unit 200 for NOx detection. In the NOx sensor S of this embodiment, the basic configuration of the sensor element 1 and the basic control of the sensor control unit 10 are the same as those of the first embodiment, and the following description will focus on the differences. In addition, among the symbols used in the second and subsequent embodiments, the same symbols as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0074] 17, the sensor control unit 10 drives the heater control unit 300 shown in FIG. 1 to energize the heater H of the sensor element 1 and start feedback control based on the pump cell temperature Tp. The sensor control unit 10 also drives the detection control unit 200 to generate a predetermined voltage signal and apply it to the common electrode 41 side of the sensor element 1. In the flowchart of FIG. 17, steps S1 and S2 correspond to the H2O concentration information detection unit 102 and the sensor cell temperature information detection unit 103, respectively. Step S3 corresponds to the offset correction value calculation unit 104A, steps S4 and S5 correspond to the NOx concentration reference value calculation unit 101, and step S6 corresponds to the NOx concentration correction unit 104.

[0075] 17 is started, in step S1, the pump cell current Ip of the pump cell 1p is detected as H2O concentration information Ci. When a predetermined DC voltage is applied by the detection control unit 200, oxygen is pumped in the pump cell 1p to discharge oxygen from the measurement gas chamber 2 and adjust the oxygen concentration to a predetermined low level. As shown in FIG. 5, the pump cell current Ip flowing at this time has a certain relationship with the O2 concentration and H2O concentration in the exhaust gas G. Therefore, the detected pump cell current Ip can be used as H2O concentration information Ci to calculate the offset correction value without being converted to the O2 concentration or H2O concentration.

[0076] As shown in the upper part of Fig. 18, the detection control unit 200 applies a predetermined DC voltage for oxygen pumping between a pair of electrodes of each cell, and also applies a predetermined AC voltage for detecting the cell impedance Zac of the monitor cell 1m at a constant cycle. In this way, by periodically switching the applied voltage and detecting the corresponding output current at predetermined detection timings t1 and t2 as shown in the lower part of Fig. 18, it is possible to obtain H2O concentration information Ci and sensor cell temperature information Ti for offset correction without significantly changing the sensor configuration.

[0077] t1 is the Zac detection timing for detecting the cell impedance Zac, and the cell impedance Zac (=Δv / Δi) can be calculated from the ratio of the change Δv (unit: V) in the applied AC voltage to the corresponding change Δi (unit: A) in the output current. t2 is the concentration detection timing for detecting a current corresponding to the gas concentration, and the applied DC voltage can detect a current generated as oxygen is discharged to the reference electrode 41. At this time, in the monitor cell 1m, a current is generated due to the decomposition of the remaining oxygen at the monitor electrode 23. In the sensor cell 1s, NOx and the remaining oxygen are decomposed at the sensor electrode 22, and a current is generated due to the decomposition of H2O.

[0078] Here, as shown in Fig. 19, each cell of the sensor element 1 is represented by a model diagram in which the resistance (R) component and capacitance (C) component of the solid electrolyte body 11 made of a zirconia-based material are electrically connected in parallel between a pair of electrodes. Of these, the resistance component (R) of the solid electrolyte body 11 is known to have a correlation with its temperature, and as shown in Fig. 18 above, it is possible to measure the AC resistance (R) inside the element by forcibly applying a voltage (Δv) and detecting the generated current (Δi).

[0079] In step S2, the cell impedance Zac of the monitor cell 1m is detected as the sensor cell temperature information Ti. This detection is performed by the detection control unit 200 at Zac detection timing t1 when a predetermined AC voltage for impedance detection is applied. As shown in Fig. 20, the cell impedance Zac of the monitor cell 1m is almost equal to the cell impedance Zac of the sensor cell 1s, and as the sensor cell temperature Ts (e.g., 500°C to 700°C) increases, the cell impedance Zac (e.g., 200Ω to 2500Ω) decreases and the rate of decrease becomes more gradual.

[0080] Therefore, the cell impedance Zac of the monitor cell 1m can be suitably used as the sensor cell temperature information Ti corresponding to the sensor cell temperature Ts. Note that, from the relationship shown in Fig. 20, the cell impedance Zac of the sensor cell 1s can be used instead of the cell impedance Zac of the monitor cell 1m, and in that case, there is an advantage that the temperature of the sensor cell 1s corresponding to the NOx output can be directly detected. However, as shown in Fig. 18, the current detection range when detecting the gas concentration is on the order of nA, whereas the current detection range required for impedance detection is on the order of μA to mA. Therefore, if both are supported, the current detection range required for detecting the output current becomes large, and the detection accuracy of the current on the low current side may decrease.

[0081] Regarding the current detection range, the relationship between the maximum current detection width and the resolution is as follows, and depends on the number of bits of the AD converter in the detection circuit. Maximum current detection width = Minimum current detection value (resolution) × AD (number of bits) In other words, if you try to make the resolution finer, the maximum current detection width becomes smaller. For example, with a 12-bit AD converter with a resolution of 1 nA, the maximum current detection width is 4096 nA (= 1 nA × 2 12 On the other hand, cell impedance Zac detection is μA to mA (10 -3 ~10 -6 A) Because the output is on the order of magnitude, if you try to detect an output current of nA with a sensor cell of 1s, it will not be possible to do so while maintaining the resolution.

[0082] Therefore, preferably, by using the cell impedance Zac of the monitor cell 1m, the sensor cell 1s can be configured to be suitable for detecting currents on the order of nA, and since there is no need to widen the current detection range, the impact on NOx detection can be suppressed.

[0083] Next, in step S3, an offset correction value C2 is calculated based on the H2O concentration information Ci and the sensor cell temperature information Ti. The pump cell current Ip detected in step S1 is used as the H2O concentration information Ci, and the cell impedance Zac of the monitor cell 1m detected in step S2 is used as the sensor cell temperature information Ti. The relationship between these pieces of information and the offset correction value C2 can be stored in advance as a calculation map or a calculation formula, etc.

[0084] In step S4, the sensor cell current Is of the sensor cell 1s and the monitor cell current Im of the monitor cell 1m are detected as sensor cell temperature information Ti. This detection is performed by the detection control unit 200 at the concentration detection timing t2 when a predetermined DC voltage is applied. In the following step S5, the NOx concentration reference value C1 corresponding to the difference value [Is-Im] between the sensor cell current Is and the monitor cell current Im is calculated based on the relationship shown in FIG.

[0085] Then, in step S6, the NOx concentration is calculated by subtracting the offset correction value C2 calculated in step S3 from the NOx concentration reference value C1 calculated in step S5, and this process is temporarily terminated.

[0086] With this procedure, the NOx concentration correction unit 104 can calculate the offset correction value C2 using the H2O concentration information Ci and the sensor cell temperature information Ti based on the input signal from the sensor element 1, and can correct the output error included in the NOx concentration reference value C1. Furthermore, when the sensor element 1 has a three-cell structure, the output current from the monitor cell 1m is used to detect the cell impedance Zac, thereby enabling efficient current detection without interfering with NOx detection in the sensor cell 1s, optimizing the circuit configuration, and enabling accurate detection of the NOx concentration.

[0087] 18, the detection control unit 200 applies a predetermined DC voltage to the monitor cell 1m to monitor the remaining oxygen, while periodically applying a predetermined AC voltage to output the monitor cell current Im for impedance detection. The NOx concentration calculation unit 100 acquires the monitor cell current Im generated in response to the applied DC voltage at a predetermined detection timing t1, and the sensor cell temperature information detection unit 103 acquires the monitor cell current Im generated in response to the applied AC voltage at a predetermined detection timing t2.

[0088] The NOx concentration correction unit 104 corrects the NOx concentration standard value C1 input from the NOx concentration standard value calculation unit 101 using the O2 concentration input from the H2O concentration information detection unit 102 and the cell impedance Zac of the monitor cell 1m input from the sensor cell temperature information detection unit 103, and calculates the NOx concentration.

[0089] (Embodiment 3) A third embodiment of a NOx sensor will be described with reference to the drawings. As shown in a block diagram in Fig. 21, this embodiment is another example of the offset correction performed in the NOx concentration calculation unit 100 of the sensor control unit 10 described above, and uses the monitor cell current Im output from the monitor cell 1m as the sensor cell temperature information Ti. Other than that, the basic configuration of the sensor element 1 and the basic control of the sensor control unit 10 are the same as those of the second embodiment, and the following description will focus on the differences.

[0090] 21, a NOx concentration calculation unit 100 detects the pump cell current Ip input to an H2O concentration information detection unit 102 as H2O concentration information Ci. As shown in Fig. 5 above, the pump cell current Ip corresponds to the O2 concentration in the exhaust gas G introduced into the measurement target gas chamber 2 and has a correlation with the H2O concentration. Therefore, the pump cell current Ip is used as the H2O concentration information Ci and is output to an offset correction value calculation unit 104A of a NOx concentration correction unit 104.

[0091] Furthermore, the NOx concentration calculation unit 100 detects the monitor cell current Im input to the sensor cell temperature information detection unit 103 as sensor cell temperature information Ti, and outputs it to the offset correction value calculation unit 104A. As shown in FIG. 4, the monitor cell 1m and the sensor cell 1s are located at the same position with respect to the gas flow in the measurement gas chamber 2, and are heated to the same temperature by the heater H. Therefore, the temperature of the monitor cell 1m substantially corresponds to the sensor cell temperature Ts. In the above embodiment, the cell impedance Zac is used as a parameter corresponding to temperature, but the monitor cell current Im can be used instead.

[0092] As shown in Fig. 22, in the sensor element 1, when a predetermined pump voltage (e.g., about 0.4 V) for pumping oxygen is applied between the pair of electrodes 21, 41 of the pump cell 1p, an equivalent voltage is also applied between the pair of electrodes of the sensor cell 1s and the monitor cell 1m. As a result, in the sensor cell 1s and the monitor cell 1m, free electrons (i.e., e in the figure) flow from the sensor electrode 22 and the monitor electrode 23 to the reference electrode 41. -) moves, and an associated current flow (indicated by the dotted arrow in the figure) occurs in the opposite direction. This current flow is considered to have temperature dependency, and can therefore be used as sensor cell temperature information Ts.

[0093] As shown in Fig. 23, the monitor cell current Im is correlated with the temperature of the sensor cell 1s, and the current increases in proportion to the rise in temperature due to the promotion of the movement of free electrons. When the H2O concentration is high, the current increases and the slope of the characteristic line changes. For example, in the temperature range from about 600°C to about 700°C, the monitor cell current Im increases from about 30 nA (H2O concentration 0%) to about 50 nA (H2O concentration 13%). Therefore, it is presumed that the monitor cell current Im is also correlated with the H2O concentration, and the current increases as the decomposition of H2O occurs together with the movement of free electrons.

[0094] 8, the Pt-Au electrode used for the monitor electrode 23 of the monitor cell 1m is configured as an electrode with low H2O decomposition activity, and is hardly affected by the decomposition of H2O under conditions of low H2O concentration. However, as the cell temperature or H2O concentration increases, the output due to the decomposition of H2O increases. Therefore, it is preferable to perform a correction taking the H2O concentration into consideration, so that the sensor cell temperature Ts can be detected with high accuracy. In this case, the H2O concentration information Ci (e.g., pump cell current Ip) detected by the H2O concentration information detection unit 102 can be used for the correction.

[0095] 21, the NOx concentration correction unit 104 uses the pump cell current Ip and the monitor cell current Im input to an offset correction value calculation unit 104A to calculate an offset correction value C2 based on a calculation map or the like indicating the relationship between previously acquired H2O concentration information Ci and sensor cell temperature information Ti. The monitor cell current Im, which is the sensor cell temperature information Ti, may be corrected in advance according to the H2O concentration, or a calculation map or the like prepared for each H2O concentration may be used.

[0096] In this way, the sensor cell temperature information Ti may be used as the monitor cell current Im, and the NOx concentration reference value C1 is offset-corrected using the acquired offset correction value C2 to accurately calculate the NOx concentration. Like the monitor cell current Im, the sensor cell current Is is also considered to have temperature dependency, but since the sensor electrode 22 contains Rh, it has a higher decomposition activity for H2O than the monitor electrode 23, making it difficult to separate the current increase due to H2O from the current increase due to temperature. In contrast, the monitor electrode 23 has little decomposition of H2O, so it can be used as the sensor cell temperature information Ti.

[0097] Here, as shown in FIG. 24, when the voltage (Vs; unit: V) applied between the pair of electrodes 22, 41 of the sensor cell 1s in the sensor element 1 is varied, the NOx output increases according to the H2O concentration. For example, when the H2O concentration is 13%, the NOx output increases sharply when the applied voltage is 0.35 V or higher. When the H2O concentration is 0%, the NOx output hardly changes depending on the applied voltage. This suggests that when H2O is present, the decomposition of H2O is promoted by increasing the applied voltage. Therefore, when the sensor electrode 22 is a Pt-Rh electrode, the applied voltage must be set to a voltage range in which H2O is not decomposed, or the electrode material or alloy ratio must be changed.

[0098] Even in such a case, as described above, by performing offset correction using the H2O concentration information Ci and the sensor cell temperature information Ti in the NOx concentration correction unit 104, suitable correction can be performed regardless of the H2O concentration and even if the sensor cell temperature Ts varies. In this case, the sensor electrode 22 is an electrode containing Rh, and the applied voltage is preferably set to 0.35 V or more, so that the offset correction can be performed effectively. Furthermore, by applying a voltage of 0.35 V or more to the sensor electrode 22 containing Rh, it is expected that the electrode life will be improved. The applied voltage can be appropriately set within a range of about 0.45 V or less, and a NOx sensor S with high accuracy and long life can be realized.

[0099] Also, as shown in FIG. 25, as the sensor cell temperature information Ti, the heater resistance of the heater H that heats the sensor cell 1s can also be used. In the sensor element 1, the heater H is temperature-controlled by energizing its heating element 62, and is in a certain relationship with the temperature of the sensor cell 1s heated by heat transfer through the heater insulation layer 61 and the solid electrolyte body 11 (for example, see FIG. 1). Specifically, as shown in the upper diagram of FIG. 26, when the heater temperature is controlled in the range from room temperature (20°C) to the maximum temperature Ta (unit: °C), the range of the heater resistance is, for example, about 2Ω to 8Ω. As shown in the lower diagram of FIG. 26, with respect to the range of the heater temperature (room temperature to Ta), the sensor cell temperature Ts increases proportionally to the heater temperature in the range from room temperature to the maximum temperature Tb (unit: °C; Tb < Ta).

[0100] From this relationship, the relationship between the heater resistance shown in FIG. 25 and the sensor cell temperature Ts is obtained, and the sensor cell temperature Ts in the range of, for example, room temperature to about 700°C can be detected according to the heater resistance. Also, since the temperature information is constantly acquired for heater control, there is no need to change the circuit configuration for acquiring the sensor cell temperature information Ti, and offset correction considering the sensor cell temperature information Ti can be easily performed using the conventional configuration. Thus, the sensor cell temperature information Ti is not limited to the monitor cell current Im or the cell impedance Zac, and may be information that has a correlation with the sensor cell temperature Ts and can estimate the sensor cell temperature Ts.

[0101] (Embodiment 4) Embodiment 4 related to the NOx sensor will be described with reference to the drawings. In the above embodiment, the NOx concentration calculation unit 100 acquires the H2O concentration information Ci using internal information such as the detection signal from the sensor element 1, but as shown in the block diagram of FIG. 27, the H2O concentration in the exhaust gas G can be estimated (hereinafter referred to as the H2O concentration estimated value) using external information of the sensor element 1 to obtain the H2O concentration information Ci. Specifically, the external information can be information that affects the H2O concentration contained in the exhaust gas G, such as driving information based on detection signals input from each part of the engine to the ECU, fuel information such as the fuel properties used for engine combustion, and environmental information based on the external environment of the gas sensor S. In the NOx sensor S of this embodiment, the basic configuration of the sensor element 1 and the basic control of the sensor control unit 10 are the same as those of the above embodiment 1, and the following description will focus on the differences.

[0102] The block diagram of Fig. 27 shows steps [A] to [C] performed in the H2O concentration information detection unit 102 of the NOx concentration calculation unit 100 to acquire H2O concentration information Ci. In step [A], the O2 concentration of the unburned mixture in the combustion chamber of the engine is calculated based on engine operation information, in step [B] the H2O concentration caused by the combustion of the unburned mixture is calculated, and further in step [C] the H2O concentration in the exhaust gas G is calculated. Step [A] may be modified to calculate the O2 concentration by step [a]. Each step will be described below.

[0103] In step [A], the H2O concentration information detection unit 102 uses the torque and rotation speed acquired by the ECU as engine operation information, and calculates the O2 concentration using an O2 concentration conversion map 111. As shown in FIG. 28, the O2 concentration conversion map 111 is a mapping of a relationship acquired in advance by performing tests or the like. In the figure, regions where the O2 concentration is 5%, 10%, and 15% are shown as examples, and there is a relationship in which the O2 concentration decreases as the torque increases or the rotation speed decreases. The acquired O2 concentration is used in step [B].

[0104] When using step [a] instead of step [A], the intake air volume and fuel injection volume are used as engine operation information, and when the engine employs an exhaust gas recirculation (EGR) system, the intake air volume is added to the EGR gas volume. Using these pieces of information, a calculation unit 112 performs a calculation based on a known relationship to calculate the A / F value. Next, the A / F value is converted to an O2 concentration using an O2 concentration conversion map 113. The obtained O2 concentration is used in step [B].

[0105] In step [B], the H2O concentration estimated to have been generated by fuel combustion is calculated from the O2 concentration obtained in step [A] or step [a] and the H / C ratio of the fuel used in the engine, using the H2O concentration conversion map 114. The calculated H2O concentration is added to the H2O concentration in the atmosphere in a calculation unit 116 in step [C] to calculate the H2O concentration in the exhaust gas G, and is output as an estimated H2O concentration value. The H2O concentration in the atmosphere is obtained by an atmospheric H2O calculation unit 115.

[0106] As shown in the upper diagram of FIG. 29, in the H2O concentration conversion map 114, the relationship between the O2 concentration and the H2O concentration varies depending on the fuel used, and the following two types of fuel are exemplified here. HVO (hydrogen biodiesel fuel) C / H=2.151 Light oil (quality standard EN590) C / H=1.886 As described above, when the H / C ratio of the fuel differs depending on the fuel properties, the amount of water vapor generated by combustion also differs (see, for example, FIG. 16). In that case, based on the formula shown in FIG. 13, for example, when diesel with C / H=1.886 is completely combusted, the H2O concentration is 12.5%, and when HVO with C / H=2.151 is completely combusted, the H2O concentration is 13.6%. Since the correlation between the O2 concentration and the H2O concentration changes slightly depending on the H / C ratio, it is preferable to calculate the H2O concentration taking into account the fuel properties using a map based on a relationship previously obtained according to the fuel used.

[0107] In addition, the atmospheric H2O calculation unit 115 calculates the H2O concentration in the atmosphere using temperature, air pressure, and humidity from environmental information of the NOx sensor S acquired by the ECU. As shown in the lower diagram of FIG. 29, the maximum water vapor concentration in the atmosphere is determined by temperature and air pressure, and has the illustrated relationship when the pressure is atmospheric pressure, for example. At this time, the maximum water vapor concentration has a relationship in which the maximum water vapor concentration increases as the temperature increases, and further, the H2O concentration in the atmosphere can be calculated by using humidity information. For example, as shown in the diagram, when the atmospheric pressure is 28°C and the humidity is 50%, the H2O concentration in the atmosphere is about 2%, which corresponds to 50% of the maximum water vapor concentration.

[0108] In this manner, the H2O concentration estimated value is calculated by steps [A] to [C] and output as H2O concentration information Ci to the NOx concentration correction unit 104. In the NOx concentration correction unit 104, as in the above embodiment, the NOx concentration reference value C1 is offset corrected using the H2O concentration information Ci and the sensor cell temperature information Ti. In this manner, when the H2O concentration that becomes the H2O concentration information Ci is calculated based on external information, the H2O concentration in the exhaust gas G can be estimated with high accuracy by taking into account not only engine operation information but also fuel information, environmental information, and the like.

[0109] (Embodiment 5) A fifth embodiment of the NOx sensor will be described with reference to the drawings. In the above embodiment, the NOx concentration calculation unit 100 acquires the sensor cell temperature information Ti using internal information such as a detection signal from the sensor element 1, but it is also possible to estimate the sensor cell temperature Ts using external information from the sensor element 1 (hereinafter referred to as a sensor cell temperature estimated value) and use this as the sensor cell temperature information Ti. As the external information, various information such as engine operation information and environmental information can be used. In the NOx sensor S of this embodiment, the basic configuration of the sensor element 1 and the basic control of the sensor control unit 10 are the same as those in the above embodiment 1, and the following description will focus on the differences.

[0110] In this embodiment, specifically, the sensor cell temperature Ts can be estimated based on exhaust gas information, which is information on the gas to be measured outside the sensor element 1. The exhaust gas information includes information on the operation of the engine, such as the gas temperature and gas flow rate of the exhaust gas G near the mounting position of the sensor element 1. The gas temperature and gas flow rate are input to the sensor cell temperature information detection unit 103 of the sensor control unit 10 as information from the ECU, for example.

[0111] As shown in FIG. 30, the sensor body 1A of the NOx sensor S has a housing S1 fixed to the passage wall EX1 of the exhaust gas passage EX, and the sensor element 1 accommodated in the housing S1 is located in the exhaust gas passage EX. As described above, the temperatures of the various parts of the sensor element 1 are not necessarily the same, and the reason for this is (1) the heat transfer from the housing S1 to the passage wall EX1, and (2) the heat transfer with the exhaust gas G. At this time, the heat transfer with the exhaust gas G changes depending on the gas temperature and gas flow rate of the exhaust gas G, and for example, the gas temperature and gas contact of the exhaust gas G introduced from the element cover S2 and reaching the tip of the sensor element 1 change. As a result, if the amount of power input to the heater H by the heater control unit 300 changes in order to maintain the control temperature of the sensor element 1, the balance between the heat transfer and the heat transfer is easily lost.

[0112] In this case, as described above in 14, the temperature of the sensor cell 1s located downstream of the pump cell 1p inside the sensor element 1 is likely to deviate from the control temperature of the heater H due to the influence of the gas temperature and flow rate of the exhaust gas G. Therefore, as shown in Fig. 31, by conducting tests or the like in advance to obtain and map the relationship between the exhaust gas temperature and flow rate of the exhaust gas at the mounting position of the sensor element 1 and the sensor cell temperature Ts, it is possible to obtain an estimated sensor cell temperature value based on the exhaust gas information.

[0113] The sensor cell temperature estimate thus obtained is output as sensor cell temperature information Ti to the NOx concentration correction unit 104. In the same manner as in the above embodiment, the NOx concentration correction unit 104 performs offset correction on the NOx concentration reference value C1 using the H2O concentration information Ci and the sensor cell temperature information Ti. In this manner, the sensor cell temperature information Ti can also be obtained based on external information, and the temperature of the sensor cell 1s located downstream of the pump cell 1p in the NOx sensor S that is heater-controlled based on the temperature of the pump cell 1p can be estimated with high accuracy.

[0114] Here, as shown in Figure 32, the relationship between the internal structure of the sensor element 1 and the NOx output offset was compared by changing the engine operating conditions. The single chamber in the figure is the sensor element 1 in which the pump cell 1p, the sensor cell 1s, and the monitor cell 1m are arranged in the measurement gas chamber 2 consisting of a single space, as shown in Figure 4 above, and the NOx output offset changed significantly depending on the load conditions. The multiple chambers in the figure is the sensor element 1 in which the measurement gas chamber 2 is divided into multiple chambers, and the pump cell 1p, the sensor cell 1s, and the monitor cell 1m are arranged in separate chambers, and the change in the NOx output offset is smaller.

[0115] The sensor output was measured under low load conditions equivalent to idling and high load conditions equivalent to driving at 130km / h in a WLTC mode driving test. In addition, the O2 concentration in the exhaust gas G was measured with an analyzer under each condition, and the results were as follows: Low load conditions: O2 concentration 10%, exhaust gas temperature 130℃ High load conditions: O2 concentration 9%, exhaust gas temperature 350℃

[0116] Since the measured O2 concentrations are almost the same under each condition, it is considered that the change in the NOx output offset indicates that the decomposition reaction of H2O in the sensor cell 1s has a temperature characteristic. Although the reason is not necessarily clear, it is presumed that in the one-chamber sensor element 1, the temperature of the sensor cell 1s is likely to rise due to the above-mentioned heat transfer with the exhaust gas G and the influence of the control of the heater H, and that the NOx output offset increases, especially on the high load side. Even in such a case, as in each of the above embodiments, the NOx concentration can be accurately calculated from low load conditions to high load conditions by performing offset correction in the NOx concentration calculation unit 100 of the sensor control unit 10.

[0117] The present invention is not limited to the above-mentioned embodiments, and can be applied to various embodiments without departing from the gist of the present invention. The configurations of the embodiments may be combined or partially modified. For example, the H2O concentration information Ci of the above-mentioned embodiment 4 may be combined with the sensor cell temperature information Ti of the above-mentioned embodiment 5. The NOx sensor S can be applied to detection of NOx using gas emitted from various internal combustion engines as the measurement gas, not limited to exhaust gas G from a vehicle engine. [Explanation of symbols]

[0118] 1 Sensor element 1p Pump cell (first cell) 1s sensor cell (second cell) 101 NOx concentration standard value calculation unit 102 H2O concentration information detector 103 Sensor cell temperature information detection unit 104 NOx concentration correction section 2. Measurement gas chamber Ip Pump cell current (first signal) Is Sensor cell current (second signal)

Claims

1. A NOx sensor (S) comprising: a sensor element (1) for detecting NOx contained in a measurement gas (G); a heater (H) for heating the sensor element; and a sensor control unit (10) for controlling operations of the sensor element and the heater and for calculating a NOx concentration based on a signal from the sensor element, The sensor element is a measurement gas chamber (2) into which the measurement gas is introduced via a gas inlet (3) and which has a solid electrolyte body (11) as a chamber wall; The measurement gas chamber has a first cell electrode (21) disposed upstream of the gas flow introduced therein, and H in the measurement gas is detected by the first cell electrode (21). 2 The measuring device includes a first cell (1p) for outputting a first signal (Ip) having a correlation with the concentration of NOx and H in the measurement gas, and a second cell electrode (22) disposed downstream of the first cell. 2 and a second cell (1s) that outputs a second signal (Is) according to the O concentration, The sensor control unit is a NOx concentration reference value calculation unit (101) that calculates a NOx concentration reference value (C1) based on the second signal; Based on the first signal or external information, H 2 O concentration or H 2 H containing concentration information correlated with O concentration 2 H to detect O concentration information (Ci) 2 An O concentration information detection unit (102); a second cell temperature information detection unit (103) for detecting second cell temperature information (Ti) having a correlation with the temperature (Ts) of the second cell; The NOx concentration reference value is 2 and a NOx concentration correction unit (104) that performs correction using an offset correction value (C2) based on the O concentration information and the second cell temperature information.

2. In the measurement gas chamber, the first cell outputs the first signal as oxygen contained in the measurement gas is discharged through the solid electrolyte body, 2. The NOx sensor according to claim 1, further comprising: a third cell (1m) for outputting a third signal (Im) corresponding to a residual oxygen concentration remaining in the measurement gas, the third cell (1m) being disposed in parallel with the second cell downstream of the first cell with respect to the gas flow introduced into the measurement gas chamber.

3. The NOx sensor according to claim 2 , wherein the second cell temperature information is a cell impedance (Zac) of the second cell or the third cell.

4. 3. The NOx sensor according to claim 2, wherein the second cell temperature information is the third signal output from the third cell.

5. 3. The NOx sensor according to claim 1, wherein the second cell temperature information is a heater resistance of the heater.

6. 3. The NOx sensor according to claim 1, wherein the second cell temperature information is acquired based on a flow rate and a temperature of the measurement target gas outside the sensor element.

7. The NOx sensor according to any one of claims 1 to 6, wherein the sensor control unit further comprises a detection control unit (200) that controls a voltage applied to the first cell and the second cell, and a heater control unit (300) that controls a temperature (Tp) of the first cell by applying current to the heater.

8. the second cell electrode is made of an electrode material containing Rh, 8. The NOx sensor according to claim 7, wherein the detection control unit controls the voltage applied to the second cell to be 0.35 V or higher.

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