NOx Sensor

Abstract

To provide a NOx sensor that can improve the detection accuracy of NOx concentration by suppressing an influence of ambient environmental temperature and more accurately performing correction based on moisture content contained in gas to be measured.SOLUTION: A 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 based on the second signal; a H2O concentration information detection section 102 that detects H2O concentration information based on the first signal or external information; and a NOx concentration correction section 103 that performs offset correction of the NOx concentration reference value. Offset correction information is based on a relation between the second signal and the H2O concentration information acquired at a predetermined reference environmental temperature. The reference environmental temperature is set to a steady temperature region of exhaust gas G in the environment in which the sensor element 1 is mounted.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 the offset based on the amount of moisture is not necessarily constant depending on the sensor surrounding environment, which differs depending on the mounting position and load conditions, and the sensor control state. This is presumably because the relationship between the amount of moisture and the offset is affected by the surrounding environment temperature, and the more the amount of moisture increases, the larger the offset error becomes, which may reduce the correction accuracy of the NOx gas concentration.

[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 influence of the surrounding environmental temperature 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) disposed upstream of a gas flow introduced in the measurement gas chamber, the first cell outputting a first signal (Ip) correlated with an HO concentration in the measurement gas; and a second cell (1s) disposed downstream of the first cell, the second cell 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 the H2O concentration based on the first signal or external information; a NOx concentration correction unit (103) that performs offset correction on the NOx concentration reference value based on the H2O concentration information; a detection control unit (200) that controls the voltage applied to the first cell and the second cell; having The NOx concentration correction unit stores offset correction information of the second signal corresponding to the H2O concentration information based on an output characteristic of the sensor element acquired at a predetermined reference environmental temperature (T0), and the reference environmental temperature is set to a steady temperature range of the measurement target gas in an installation environment of the sensor element. And, The detection control unit includes a voltage application circuit (201) that generates a voltage corresponding to an instruction voltage (V0) input from a voltage application instruction unit (202), and an instruction voltage correction unit (203). The instruction voltage correction unit includes a voltage detection unit (203a) that detects an actual voltage (V1) generated by the voltage application circuit, and a correction value calculation unit (203b) that corrects the instruction voltage based on a difference value between the instruction voltage and the actual voltage. This is located in the NOx sensor. 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 the first signal input from the first cell or H2O concentration information based on external information of the sensor element. The NOx concentration correction unit calculates an offset value corresponding to the detected H2O concentration information by referring to previously stored offset correction information, and corrects the NOx concentration reference value.

[0009] Here, the surrounding environment of the sensor element varies depending on the discharge environment of the measured gas, and the amount of moisture contained in the measured gas also varies. In this case, the effect on the sensor output is small in areas with low moisture content or low temperature. However, for example, when the temperature at the mounting position of the sensor element changes significantly due to load conditions, the sensor output increases significantly in areas with high moisture content or high temperature, and the effect on the sensor output cannot be ignored. This is presumably because, even if the temperature control of the sensor element is performed by a heater, when the temperature of the second cell, which outputs the second signal, rises due to the effect of the high-temperature measured gas, the decomposition of moisture is likely to proceed.

[0010] Even in such a case, the NOx concentration correction unit can perform correction taking into account not only the H2O concentration information but also temperature characteristics by using offset correction information previously acquired at a reference environmental temperature. The reference environmental temperature at which the offset correction information is acquired is set to a steady temperature range of the measured gas that reflects the actual mounting environment of the sensor element, so that the relationship between the H2O concentration information and the second signal in a state where the temperature of the measured gas is more stable can be obtained. By performing offset correction based on this relationship, correction variations due to changes in the moisture content and the surrounding environmental temperature are mitigated, and under- or over-correction is suppressed, allowing the NOx concentration to be calculated more accurately.

[0011] As described above, according to the above aspect, it is possible to provide a NOx sensor that can suppress the influence of the ambient temperature, perform correction based on the amount of moisture contained in the measured gas with higher accuracy, and improve 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]

[0012] [Figure 1] 1 is an overall schematic diagram showing a configuration of a NOx sensor in 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] FIG. 11 is a diagram for explaining offset correction information in the first embodiment, showing the relationship between NOx output and H2O concentration in a steady temperature range. [Figure 6] FIG. 4 is a graph showing temperature characteristics of NOx output of a sensor element in the first embodiment. [Figure 7] 5 is a diagram for explaining an offset correction method based on an offset correction value calculation map in the first embodiment. [Figure 8] 4 is a diagram for explaining a NOx concentration calculation procedure of a NOx concentration calculation unit of a sensor control unit in the first embodiment. FIG. [Figure 9] FIG. 3 is an engine map showing the relationship between engine speed and torque obtained through a running test in the first embodiment. [Figure 10] FIG. 4 is a diagram showing a comparison of NOx outputs before and after offset correction based on offset correction information in the first embodiment. [Figure 11] FIG. 4 is a diagram showing the relationship between NOx output error and H2O concentration obtained by a plurality of sensor elements in the first embodiment. [Figure 12] FIG. 11 is a schematic configuration diagram of a main part of a NOx sensor in a second embodiment. [Figure 13] FIG. 11 is a graph showing the relationship between pump cell voltage variation and NOx output error in the second embodiment. [Figure 14] FIG. 11 is a schematic configuration diagram of a detection control unit of a sensor control unit including an instruction voltage correction unit in a second embodiment. [Figure 15] FIG. 11 is a schematic configuration diagram of a case where a detection control unit of a sensor control unit does not include an instruction voltage correction unit in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] (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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] The NOx sensor S is used, for example, in a selective catalytic reduction (hereinafter, 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 this 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 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.

[0019] 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.

[0020] The NOx concentration calculation unit 100 includes a NOx concentration reference value calculation unit 101, an H2O concentration information detection unit 102, and a NOx concentration correction unit 103. The NOx concentration reference value calculation unit 101 receives the 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 the 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 NOx concentration correction unit 103 performs offset correction on the NOx concentration reference value C1 calculated by the NOx concentration reference value calculation unit 101 using the H2O concentration information Ci detected by the H2O concentration information detection unit 102, and outputs the offset-corrected value as a calculated value of the NOx concentration.

[0021] Specifically, the NOx concentration correction unit 103 stores offset correction information based on the relationship between the H2O concentration information Ci acquired at a predetermined reference environmental temperature and the second signal. This reference environmental temperature is set to be a steady temperature range of the exhaust gas G in the mounting environment of the sensor element 1. The steady temperature range is a temperature range that the exhaust gas G that reaches the mounting position of the sensor element 1 can take when the engine is in a steady operating state, and differs depending on the expected operating state of the engine, the distance between the engine and the sensor element 1 in the exhaust gas passage EX, etc.

[0022] At this time, the offset correction information used for the NOx concentration correction corresponds to the reference environmental temperature which is the steady temperature range of the exhaust gas G, and reflects the ambient environmental temperature in the environment in which the actual NOx sensor S is installed. This allows the offset correction based on the detected H2O concentration information Ci to be performed more appropriately, and the NOx output error in NOx detection can be reduced. The temperature characteristics of the NOx output and the offset correction will be explained next.

[0023] FIG. 5 shows the relationship between the NOx output and the H2O concentration information Ci in the steady temperature range as an example. As shown on the horizontal axis of the figure, the H2O concentration and the O2 concentration, which are the H2O concentration information Ci, are correlated with each other, and for example, the lower the O2 concentration (e.g., 0% to about 21%) known from the pump cell current Ip, the higher the H2O concentration (e.g., 0% to about 13%). Also, as shown on the vertical axis of the figure, for example, the NOx output known from the sensor cell current Is corresponds to the NOx concentration (unit: ppm). At this time, the NOx output includes an offset corresponding to the H2O concentration or the O2 concentration, and specifically, the higher the H2O concentration or the lower the O2 concentration, the larger the offset becomes, and the larger the rate of increase becomes. Therefore, even when there is substantially no NOx, the NOx output varies according to the H2O concentration or the O2 concentration.

[0024] Furthermore, the relationship between the NOx output and the H2O concentration information Ci has a temperature characteristic and varies depending on the environmental temperature at the mounting position of the sensor element 1. Here, the temperature range that the temperature of the exhaust gas G at the mounting position of the sensor element 1 can assume when the vehicle is in a stable running state after the engine is started is defined as a steady temperature range, and the NOx output characteristics within this range are compared. At this time, the NOx output characteristics change significantly between the lower limit temperature T1 (e.g., 300°C) and the upper limit temperature T2 (e.g., 700°C) of the steady temperature range, and the offset increases as the temperature increases. The difference in offset due to this temperature characteristic is relatively small in the region where the H2O concentration is low, but becomes large when the H2O concentration is high or the O2 concentration is low.

[0025] As shown in FIG. 6, the NOx output gradually increases with respect to changes in the gas temperature of the exhaust gas G (for example, from 0°C to 800°C), and the higher the temperature, the greater the increase rate of the offset. Therefore, when offset correction is performed using a fixed value according to the H2O concentration as in the past, based on characteristics obtained by a general test that does not take temperature characteristics into consideration, there is a risk that the NOx output error cannot be sufficiently reduced. Therefore, the NOx concentration correction unit 103 selects optimal offset correction information from, for example, the offset characteristics of the NOx output shown in FIG. 5, and performs correction based on the offset correction information in the NOx concentration correction unit 103.

[0026] 5, the NOx concentration correction unit 103 acquires offset correction information for correcting the NOx concentration reference value C1 in advance at a reference environmental temperature T0 in the steady temperature range of exhaust gas G. The steady temperature range is a temperature range that the exhaust gas G that reaches the periphery of the sensor element 1 when the engine is in a steady operating state, and indicates the range of the ambient temperature of the sensor element 1. The steady temperature range is determined by the type of engine, the mounting position of the gas sensor S in the exhaust gas pipe, and other conditions, and an appropriate reference environmental temperature T0 can be determined in accordance with the state of each sensor element 1 or multiple sensor elements 1 of the same type used under similar conditions.

[0027] Preferably, the reference environmental temperature T0 is set to a temperature range higher than the lower limit temperature T1 in the steady temperature range. For example, when the temperature of the exhaust gas G is in the range of 300°C to 700°C inclusive, the reference environmental temperature T0 is set to a temperature higher than the lower limit temperature T1 of 300°C. Also, preferably, the reference environmental temperature T0 is set to a temperature range lower than the upper limit temperature T2 of 700°C in the steady temperature range. More preferably, the reference environmental temperature T0 is set to an intermediate temperature that exhibits intermediate offset characteristics between the offset characteristics in the low temperature range including the lower limit temperature T1 and the offset characteristics in the high temperature range including the upper limit temperature T2.

[0028] Specifically, a predetermined intermediate temperature range including an intermediate temperature (e.g., 500°C) between the lower limit temperature T1 and the upper limit temperature T2 of the steady temperature range is set as the reference temperature range, and the reference environmental temperature T0 can be set within that range. The reference environmental temperature T0 may be 500°C, which is an intermediate temperature, or may be a temperature that shows a more intermediate offset characteristic, and can be set arbitrarily. Preferably, an intermediate temperature range centered on 500°C, for example, a range of 400°C to 600°C, can be set as the reference temperature range, and the reference environmental temperature T0 can be set appropriately according to the offset characteristic so that the error due to the mounting environment is reduced. In that case, the low temperature range is, for example, a range of 300°C to less than 400°C, and the high temperature range is, for example, a range of 600°C to less than 700°C.

[0029] The steady-state temperature range is not limited to a temperature range of 300° C. or more and 700° C. or less, but can be set appropriately depending on the mounting position of the sensor element 1, and may include a temperature range lower than 300° C. or a temperature range higher than 700° C. In that case, the ranges of the low temperature range, the reference temperature range, and the high temperature range are also changed appropriately depending on the set steady-state temperature range.

[0030] 3, the NOx concentration correction unit 103 has an offset correction value calculation unit 103A, and can store a correction map or a correction formula based on the output characteristics at the reference ambient temperature T0 as offset correction information. The offset correction value calculation unit 103A calculates an offset correction value C2 using H2O concentration information Ci detected by the H2O concentration information detection unit 102. The NOx concentration correction unit 103 corrects the NOx concentration reference value C1 using this offset correction value C2, and outputs it as the NOx concentration.

[0031] As shown in FIG. 7, by using the offset correction information acquired in this manner, the influence of the ambient temperature can be minimized. The center diagram in FIG. 7 shows the result of correcting the NOx output using a correction map based on the offset correction information acquired in a reference temperature range (for example, reference ambient temperature: 500°C), and when the temperature of the exhaust gas G is in the reference temperature range, almost no output error occurs. Also, even in the low temperature range or high temperature range, when the H2O concentration increases, the NOx output shifts to the positive or negative side, but the magnitude (absolute value) of the overall output error becomes relatively small. Therefore, if the H2O concentration is within the normal range, a stable NOx output can be obtained in a steady operating state even if the temperature approaches the lower limit temperature T1 or the upper limit temperature T2 of the steady temperature range.

[0032] In contrast, as shown in the upper diagram of Fig. 7, when a correction map based on offset correction information acquired in a low temperature range (e.g., lower limit temperature T1: 300°C) is used for the NOx output, the NOx output significantly deviates to the positive side with an increase in H2O concentration on the higher temperature side than the low temperature range. On the other hand, as shown in the lower diagram of Fig. 7, when a correction map based on offset correction information acquired in a high temperature range (e.g., upper limit temperature T2: 700°C) is used, the NOx output significantly deviates to the negative side with an increase in H2O concentration on the lower temperature side than the high temperature range. Therefore, when a correction map for the low temperature range is used, under-correction occurs, and when a correction map for the high temperature range is used, over-correction occurs.

[0033] In this way, by setting an appropriate reference environmental temperature T0 and acquiring the offset correction information, a more appropriate correction can be performed in the NOx concentration correction unit 103. That is, in the offset correction value calculation unit 103A, an offset correction value C2 reflecting the temperature characteristics is obtained based on the offset correction information and the H2O concentration information Ci from the H2O concentration information detection unit 102, and by using this to offset correct the NOx concentration reference value C1, the influence of the ambient environmental temperature on the detection of the NO concentration can be reduced.

[0034] Here, the H2O concentration information Ci detected by the H2O concentration information detection unit 102 may include H2O concentration or concentration information correlated with H2O concentration, such as O2 concentration information. The O2 concentration information can be acquired based on the pump cell current Ip input from the sensor element 1, and can be converted to O2 concentration based on a known relationship between the pump cell current Ip and the O2 concentration. The O2 concentration information may be the pump cell current Ip input from the sensor element 1, or 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.

[0035] Furthermore, the H2O concentration information Ci is not limited to information based on internal information of the sensor element 1, such as the pump cell current Ip, 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 this external information, or an estimated value of the H2O concentration or O2 concentration estimated based on this external information may be acquired as the external information.

[0036] In the H2O concentration information detection unit 102, external information is input to the sensor control unit 10 as information from, for example, a vehicle engine control unit (not shown) (i.e., Engine Control Unit; hereinafter referred to as ECU). 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.

[0037] 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. 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.

[0038] (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.

[0039] 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.

[0040] 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. 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, and NOx in the exhaust gas G is detected using the air A taken into the reference gas chamber 4 as a reference gas. A current Im flowing through the monitor cell 1m (hereinafter referred to as a monitor cell current) is output to the sensor control unit 10 as a third signal.

[0041] (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.

[0042] 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.

[0043] 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.

[0044] 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 1m are formed, and is capable of heating the entire tip side of the element that serves as the gas detection section.

[0045] 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.

[0046] 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.

[0047] The sensor electrode 22 and the monitor electrode 23 are arranged in parallel at the same position in the gas flow direction X, and are formed in the same shape with a smaller area than the pump electrode 21. As a result, the sensor electrode 22 and the monitor electrode 23 are in the same condition with respect to the flow of the 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. Kuko This can be done.

[0048] 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.

[0049] 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, appropriately referred to as a Pt-Rh electrode) may be used as such an electrode.

[0050] The pump electrode 21 of the pump cell 1p is made of an electrode material that has oxygen decomposition activity but does not have NOx decomposition activity. For example, an electrode containing platinum and gold (hereinafter, appropriately 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, appropriately referred to as a Pt electrode).

[0051] 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.

[0052] 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.

[0053] (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.

[0054] 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.

[0055] 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.

[0056] The detection control unit 200 controls the detection operations of the pump cell 1p, the monitor cell 1m, and the sensor cell 1s. 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 where 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.

[0057] 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 the switch circuit is turned on and off based on a control command, thereby making it possible to control the supply of electricity to the heater H via the heater terminals H+ and H-. As a result, each cell of the sensor element 1 heated by the heater H is maintained at a temperature suitable for detecting NOx.

[0058] 3, the NOx concentration calculation unit 100 calculates the NOx concentration reference value C1 (unit: ppm) 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, as shown in the upper diagram of FIG. 8, the relationship between the difference value [Is-Im] (unit: nA) between the sensor cell current Is and the monitor cell current Im and the corresponding NOx concentration reference value C1 is mapped and stored in advance, and the NOx concentration reference value C1 can be calculated based on the outputs from the sensor cell 1s and the monitor cell 1m. At this time, the NOx concentration reference value C1 increases in response to the output of the sensor cell 1s, and includes the influence of H2O if it is present.

[0059] The H2O concentration information detection unit 102 detects the O2 concentration, which becomes the H2O concentration information Ci, based on the input pump cell current Ip (unit: mA). Specifically, as shown in the middle diagram of FIG. 8, the relationship between the pump cell current Ip and the O2 concentration is mapped and stored in advance, and the O2 concentration (unit: %) that is correlated with the H2O concentration can be calculated based on the detected value of the pump cell current Ip. As described above, since the relationship between the H2O concentration and the O2 concentration in the exhaust gas G based on combustion or the like is known, the calculated O2 concentration can be used as the H2O concentration information Ci. At this time, as the pump cell current Ip increases, the O2 concentration increases, while the H2O concentration decreases.

[0060] The NOx concentration correction unit 103 corrects the NOx concentration reference value C1 input from the NOx concentration reference value calculation unit 101 using the offset correction value C2 calculated by the offset correction value calculation unit 103A. Specifically, as shown in the lower diagram of Fig. 8, the relationship between the O2 concentration as the H2O concentration information Ci and the offset correction value C2 can be acquired as offset correction information at a predetermined reference environmental temperature T0 and stored as an offset correction map. Based on this map, the offset correction value C2 corresponding to the detected O2 concentration is calculated, and the offset correction value C2 can be subtracted from the NOx concentration reference value C1 to calculate the offset-corrected NOx concentration.

[0061] A test for obtaining offset correction information can be performed, for example, using a model gas bench, and evaluation can be performed by changing the H2O concentration in the model gas within a predetermined range (for example, 0% to about 13%). Specifically, a model gas simulating exhaust gas is supplied at a predetermined flow rate from a gas supply unit to a gas passage in which the gas sensor S is installed, and the offset characteristic is evaluated based on the NOx output of the sensor element 1 when the H2O concentration is changed at a predetermined reference environmental temperature T0. The reference environmental temperature T0 is set from the range of gas temperatures that reach the mounting position of the sensor element 1 during steady operation in an actual vehicle environment, and the gas temperature at the mounting position of the sensor element 1 is adjusted to the reference environmental temperature T0 by a heater provided in the gas passage.

[0062] 9 is an engine map showing the results of a driving test in an actual vehicle environment, and shows the relationship between the engine speed and torque in the legally specified certified driving pattern (WLTC mode, RDE mode) and the ambient temperature at the mounting position of the sensor element 1. The range of gas temperatures obtained from this result (e.g., 300°C to 700°C) is set as the steady temperature range, and the center value (e.g., 500°C) is set as the reference ambient temperature T0.

[0063] FIG. 10 shows the offset characteristic (before correction) obtained in this way, and the NOx output after correction based on the offset correction information obtained from this characteristic line. The offset correction information may be a correction map, but for example, it can also be expressed by a correction formula that approximates the relationship between the NOx output before correction and the O2 concentration with a linear equation (y=Ax+b), and the corrected NOx concentration can be calculated by calculating an offset correction value C2 according to the O2 concentration and subtracting it from the NOx concentration reference value C1. In such a corrected NOx output, the NO output error due to changes in the O2 concentration or H2O concentration is reduced, making it possible to detect NOx with high accuracy.

[0064] The offset correction information may be information based on output characteristics acquired individually for the sensor element 1, or may be information obtained by averaging a plurality of output characteristics acquired for a plurality of sensor elements 1. Specifically, the offset correction information can be acquired for each NOx sensor S, for example, during product production, taking into consideration the installation environment of each sensor, vehicle information to which the sensor is applied, and the like. In this way, by acquiring characteristics individually for each product, correction based on appropriate offset correction information can be performed.

[0065] Alternatively, common offset correction information may be obtained for a plurality of the same type of NOx sensors S at the time of design or the like, and applied to all of the same type of NOx sensors S. In that case, as shown in Fig. 11, for example, characteristic data of NOx output error versus H2O concentration is obtained for several tens to several hundreds of NOx sensors S in a temperature range (e.g., 200°C, 500°C) according to the ambient temperature. By calculating an average value from the data, offset correction based on more average characteristics can be performed even when the offset correction information is common.

[0066] Thus, according to this embodiment, in the sensor control unit 10 of the NOx sensor S, the NOx concentration calculation unit 100 is equipped with a NOx concentration correction unit 103, and by using offset correction information that takes into account the temperature characteristics, the NOx output error caused by the moisture in the exhaust gas G can be appropriately corrected to calculate the NOx concentration.

[0067] (Embodiment 2) A second embodiment of a NOx sensor will be described with reference to the drawings. In this embodiment, an example of a specific procedure for detection control performed by the detection control unit 200 of the sensor control unit 10 described above will be shown. As shown in FIG. 12, the detection control unit 200 has a voltage application circuit 201 and a voltage application instruction unit 202, and controls the pump cell voltage Vp applied to the pump cell 1p for oxygen pumping. 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.

[0068] 12, the sensor control unit 10 applies a predetermined pump cell voltage Vp between the pump electrode 21 and the reference electrode 41 of the pump cell 1p by the detection control unit 200, and controls the power supply to the heater H by the heater control unit 300 (see, for example, FIG. 1) so that the pump cell 1p reaches a predetermined temperature. The detection control unit 200 generates a voltage signal corresponding to the command voltage V0 in the voltage application circuit 201 based on the command voltage V0 from the voltage application command unit 202, and applies the voltage signal to the reference electrode 41 of the pump cell 1p. The same voltage is also applied to the sensor cell 1s and the monitor cell 1m. As a result, the exhaust gas G introduced into the measurement gas chamber 2 generates oxide ions by the reduction and decomposition of oxygen at the pump electrode 21, which move toward the reference electrode 41, and a pump cell current Ip flows. NOx and H2O in the exhaust gas G move on the pump electrode 21 toward the downstream side.

[0069] For example, when the voltage actually applied to the pump cell 1p is higher than the command voltage V0 of the voltage application circuit 201, the catalytic reaction at the pump electrode 21 becomes more active, and a part of the H2O in the exhaust gas G is decomposed. At this time, it is considered that the higher the applied voltage, the less H2O reaches the sensor cell 1s. These reactions cause variations in the sensor cell current Is and the monitor cell current Im, which may cause variations in the NOx output based on the sensor cell current Is and the monitor cell current Im. In addition, when the H2O concentration information Ci is detected based on the pump cell current Ip, this also affects the calculation of the offset correction value C2.

[0070] On the other hand, even if the detection control unit 200 controls the voltage application circuit 201 to output a predetermined command voltage V0, the actual applied voltage may vary from the command voltage V0 because the product itself has a certain variation (for example, about ±20 mV at 20° C.). Therefore, as shown in Fig. 13, when comparing multiple NOx sensors S, if the variation in the applied voltage by the voltage application circuit 201 differs, the variation in the NOx output error caused by the decomposition of H2O also differs, and it is difficult to ensure correction accuracy when a fixed offset correction is standardized.

[0071] 14, a command voltage correction unit 203 is provided in the voltage application command unit 202 of the detection control unit 200, and a corrected command voltage V2 obtained by correcting the command voltage V0 is output to the voltage application command unit 202. Specifically, the command voltage correction unit 203 includes a voltage detection unit 203a that detects an actual voltage V1 actually applied to each cell from the voltage application command unit 202, and a correction value calculation unit 203b that calculates a difference between the actual voltage V1 and the command voltage V0 and outputs the difference as a correction value V1-V0. The command voltage V0 from the voltage application command unit 202 is output to the voltage application circuit 201 as a corrected command voltage V2 by subtracting the correction value V1-V0 in the command voltage correction unit 203.

[0072] FIG. 15 shows a configuration that does not include the command voltage correction unit 203, and the voltage application circuit 201 of the detection control unit 200 is configured to be able to generate the voltage applied to each cell of the sensor element 1 according to the command voltage V0 from the voltage application command unit 202. For example, the voltage application command unit 202 outputs a pulse signal corresponding to a preset command voltage V0 so that the oxygen pumping operation in the pump cell 1p is appropriate, and the voltage application circuit 201 outputs a pulse voltage corresponding to the pulse signal. As a result, a predetermined voltage is applied to each cell via the common terminal COM+ of the sensor control unit 10 shown in FIG. 1 described above. In this configuration, if the actual voltage V1 output from the voltage application circuit 201 varies with respect to the command voltage V0, as described above, the NOx output error may increase due to the high voltage.

[0073] In contrast, when the command voltage correction unit 203 is provided, the corrected command voltage V2 expressed by the following formula is applied. do. Corrected command voltage V2 = command voltage V0 - correction value [actual voltage V1 - command voltage V0] The corrected command voltage V2 thus obtained is output to the voltage application circuit 201, so that the actual applied voltage becomes the original command voltage V0. This suppresses the decomposition reaction of H2O in the pump cell 1p, and suppresses the variations in the sensor cell current Is and the monitor cell current Im, thereby reducing the NOx output error.

[0074] The command voltage V0 to the pump cell 1p can be arbitrarily set so that the oxygen concentration can be adjusted to a desired low level by oxygen pumping and the decomposition reaction of H2O in the pump cell 1p can be suppressed. Preferably, the command voltage V0 is set so as to eliminate variation between individual voltage application circuits 201, thereby effectively suppressing variation in the decomposition reaction of H2O and suppressing output errors due to variation in applied voltage, etc.

[0075] 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. For example, the NOx sensor S can be applied to detection of NOx not only in exhaust gas G from a vehicle engine, but also in gas emitted from various internal combustion engines, etc., as a measurement gas. In addition, the structure of each part of the sensor element 1 of the NOx sensor S and the control procedure of each part in the sensor control unit 10 can be appropriately changed. [Explanation of symbols]

[0076] 1 Sensor element 1p Pump cell (first cell) 1s sensor cell (second cell) 10 Sensor control unit 101 NOx concentration standard value calculation unit 102 H2O concentration information detector 103 NOx concentration correction section 2. Measurement gas chamber S NO sensor 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; In the measurement gas chamber, a gas flow detector is disposed upstream of the gas flow to detect H in the measurement gas. 2 a first cell (1p) for outputting a first signal (Ip) having a correlation with the concentration of NOx and H in the measurement gas; 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); The NOx concentration reference value is 2 a NOx concentration correction unit (103) that performs offset correction based on O concentration information; A detection control unit (200) that controls the voltage applied to the first cell and the second cell, The NOx concentration correction unit corrects the H based on an output characteristic of the sensor element acquired at a predetermined reference environmental temperature (T0). 2 offset correction information for the second signal corresponding to O concentration information is stored, and the reference environmental temperature is set to a steady-state temperature range of the measurement target gas in an environment in which the sensor element is mounted, The detection control unit includes a voltage application circuit (201) that generates a voltage corresponding to a command voltage (V0) input from a voltage application command unit (202), and a command voltage correction unit (203). The command voltage correction unit includes a voltage detection unit (203a) that detects an actual voltage (V1) generated by the voltage application circuit, and a correction value calculation unit (203b) that corrects the command voltage based on a difference value between the command voltage and the actual voltage.

2. 2. The NOx sensor according to claim 1, wherein the sensor element is mounted in an exhaust gas passage (EX) of an internal combustion engine from which the measured gas is discharged, and the steady-state temperature range is a temperature range that the measured gas that reaches a mounting position of the sensor element can assume when the internal combustion engine is in a steady operating state.

3. 3. The NOx sensor according to claim 1, wherein the reference environmental temperature is set to a temperature range higher than a lower limit temperature of the steady temperature range.

4. The NOx sensor according to any one of claims 1 to 3, wherein the reference environmental temperature is set to a reference temperature range in which the sensor element exhibits output characteristics intermediate between output characteristics acquired in a low temperature range including a lower limit temperature of the steady temperature range and output characteristics acquired in a high temperature range including an upper limit temperature of the steady temperature range.

5. 5. The NOx sensor according to claim 1, wherein the reference environmental temperature is set in a range of 300° C. or higher and 700° C. or lower.

6. 5. The NOx sensor according to claim 1, wherein the offset correction information is information based on output characteristics obtained individually for the sensor element.

7. 5. The NOx sensor according to claim 1, wherein the offset correction information is information obtained by averaging a plurality of output characteristics obtained for a plurality of the sensor elements.

8. A NOx sensor as described in any one of claims 1 to 7, wherein in the voltage application instruction section, the instruction voltage is set within a range in which the decomposition reaction of H 2 O in the first cell can be suppressed.

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