Gas sensor and method for controlling gas sensor

Abstract

To accurately measure low concentration measuring target gas .SOLUTION: A gas sensor 100 and a control method of the gas sensor 100 detect measuring target gas in gas to be measured. The gas sensor comprises a sensor element 101 and a controller 90. The sensor element 101 includes: a base portion 102; a gas-to-be-measured flow portion 15; an adjustment pump cell 21 that includes an inner pump electrode 22 and an outer pump electrode 23, and adjusts oxygen concentration in the gas to be measured to desired concentration; a current measurement pump cell 41 that includes an inner measurement electrode 44 and an outer measurement electrode 23 and detects measuring target gas in the gas to be measured as a current value; a reference electrode 42 disposed to be in contact with reference gas; and an electromotive force detection sensor cell 82 that includes the inner measurement electrode 44 and a reference electrode 42, and detects an electromotive force value between the inner measurement electrode 44 and the reference electrode 42. The controller 90 includes a switching unit 95 for switching whether or not a current flows through the current measurement pump cell.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a gas sensor and a method for controlling the gas sensor.

Background Art

[0002] Gas sensors are used to detect and measure the concentration of target gas components (such as oxygen O2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, carbon dioxide CO2, etc.) in a gas to be measured, such as the exhaust gas of an automobile. For example, the concentration of a target gas component in the exhaust gas of an automobile is measured, and based on the measured value, the exhaust gas purification system mounted on the automobile is optimally controlled.

[0003] As such a gas sensor, a gas sensor using an oxygen ion-conductive solid electrolyte such as zirconia (ZrO2) is known. For example, Japanese Patent No. 5323752 discloses a gas sensor that measures the concentration of a gas component in a gas to be measured by the limiting current method.

[0004] Further, Japanese Patent No. 5323752 discloses a NOx sensor having a main pump cell and an auxiliary pump cell for adjusting the oxygen concentration, and a measurement pump cell including a measurement electrode for detecting NOx. In the NOx sensor, first, the main pump cell and the auxiliary pump cell control the oxygen partial pressure in the gas to be measured to a low partial pressure that has substantially no influence on the measurement of NOx. NOx in the gas to be measured with the controlled oxygen partial pressure is reduced at the measurement electrode, and the oxygen generated as a result is pumped out by the measurement pump cell and detected as a current value.

[0005] Japanese Unexamined Patent Application Publication Nos. 2021-162580 and 2021-162581 disclose a gas sensor having two measurement pump cells. It is disclosed that switching is performed as to which of the two measurement pump cells is used depending on the concentration of a specific gas in the gas to be measured, and the concentration of the specific gas in a wide concentration range (for example, 500 ppm or more and 10,000 ppm or less) is detected.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] With the strengthening of exhaust gas regulations for automobiles and the like, gas sensors are required to accurately measure even lower concentrations of the gas to be measured. Here, the low concentration means, for example, a concentration of less than 500 ppm, less than 400 ppm, less than 300 ppm, less than 200 ppm, or less than 100 ppm.

[0008] In a conventional limiting current type gas sensor, for example, as described in the above-mentioned Japanese Patent No. 5323752, in the measurement pump cell, oxygen generated by reducing the gas to be measured (for example, NOx) is detected as a current value. Therefore, the current value is detected as a value corresponding to the concentration of the gas to be measured.

[0009] By the way, even when the gas to be measured does not contain the gas to be measured, the current value does not become zero, and a minute current flows through the measurement pump cell. This minute current is referred to as an offset current. The offset current is generated regardless of the concentration of the gas to be measured. Therefore, when the current value of the offset current fluctuates due to some factor, the current value detected in the measurement pump cell shifts by the amount of fluctuation of the offset current value regardless of the concentration of the gas to be measured. When measuring a low-concentration gas to be measured, since the current value detected in the measurement pump cell is relatively small according to the concentration of the gas to be measured, the change in the current value due to the fluctuation of the offset current value becomes relatively large, and the influence on the measurement accuracy tends to increase.

[0010] Therefore, an object of the present invention is to accurately measure a measurement target gas even at a low concentration. That is, an object is to accurately measure a measurement target gas in a wide concentration range including a measurement target gas at a low concentration.

Means for Solving the Problems

[0011] As a result of intensive studies, the present inventor has found that by providing a gas sensor with a switching unit that switches whether to pass current through the current measurement pump cell or not, the influence of the offset current can be eliminated and a measurement target gas at a low concentration can be accurately measured.

[0012] The present invention includes the following inventions. (1) A gas sensor including a sensor element and a control device for controlling the sensor element, The sensor element includes An elongated plate-shaped substrate portion including an oxygen ion-conductive solid electrolyte layer, A measurement target gas flow-through portion formed from one end in the longitudinal direction of the substrate portion, An inner pump electrode disposed on the inner surface of the measurement target gas flow-through portion, and an outer pump electrode disposed at a position different from the measurement target gas flow-through portion of the substrate portion and corresponding to the inner pump electrode, and an adjustment pump cell for adjusting oxygen in the measurement target gas to a desired concentration, An inner measurement electrode disposed at a position farther from the one end in the longitudinal direction of the substrate portion than the inner pump electrode on the inner surface of the measurement target gas flow-through portion, and an outer measurement electrode disposed at a position different from the measurement target gas flow-through portion of the substrate portion and corresponding to the inner measurement electrode, and a current measurement pump cell for detecting a measurement target gas in the measurement target gas as a current value, A reference electrode disposed inside the substrate portion so as to be in contact with a reference gas, An electromotive force detection sensor cell including the inner measurement electrode and the reference electrode, and detecting an electromotive force value between the inner measurement electrode and the reference electrode, Including The control device A gas sensor for detecting a gas to be measured in a gas to be measured, including a switching unit for switching whether to pass a current through the current measurement pump cell or not.

[0013] In the electromotive force detection sensor cell, the reference electrode corresponds to the inner measurement electrode.

[0014] (2) The control device includes a measurement mode switching unit that switches between an electromotive force measurement mode for detecting the concentration of the gas to be measured in the gas to be measured based on the electromotive force value in the electromotive force detection sensor cell and a current measurement mode for detecting the concentration of the gas to be measured in the gas to be measured based on the current value in the current measurement pump cell. The measurement mode switching unit switches the switching unit so that no current flows through the current measurement pump cell when switching to the electromotive force measurement mode, and switches the switching unit so that current flows through the current measurement pump cell when switching to the current measurement mode. The gas sensor according to (1) above.

[0015] (3) The switching unit includes a switch for switching whether to cut off the conduction of the current measurement pump cell. The gas sensor according to (2) above.

[0016] (4) The measurement mode switching unit switches the switch to OFF to cut off the conduction of the current measurement pump cell so that no current flows when switching to the electromotive force measurement mode, and switches the switch to ON to conduct the current measurement pump cell so that current flows when switching to the current measurement mode. The gas sensor according to (3) above.

[0017] (5) The switching unit includes a variable power source for changing the voltage applied to the current measurement pump cell. The gas sensor according to (2) above.

[0018] (6) The gas sensor according to (5) above, wherein, when switching to the electromotive force measurement mode, the measurement mode switching unit sets the voltage of the variable power supply to zero and applies no voltage to the current measurement pump cell to prevent current from flowing, and when switching to the current measurement mode, sets the voltage of the variable power supply to a predetermined value and applies the predetermined voltage to the current measurement pump cell to allow current to flow.

[0019] (7) the measurement mode switching unit switches to the electromotive force measurement mode when it determines that the concentration of the measurement target gas detected in the current measurement mode is lower than a predetermined first concentration threshold C1; The gas sensor according to any one of (2) to (6) above, which switches to the current measurement mode when it is determined that the concentration of the measurement target gas detected in the electromotive force measurement mode is higher than a predetermined second concentration threshold C2.

[0020] That is, in a low concentration region where the concentration of the target gas in the measured gas is lower than the first concentration threshold C1, the concentration of the target gas is detected in the electromotive force measurement mode, which has higher measurement accuracy in the low concentration region, and in a high concentration region where the concentration of the target gas in the measured gas is higher than the second concentration threshold C2, the concentration of the target gas is detected in the current measurement mode, which has higher measurement accuracy in the high concentration region.

[0021] (8) The gas sensor according to (7), wherein the first concentration threshold C1 is a lower concentration than the second concentration threshold C2.

[0022] (9) In the current measurement mode, the electromotive force detection sensor cell Inside The current value in the current measuring pump cell is controlled so that the electromotive force value between the measurement electrode and the reference electrode becomes a predetermined value. 2 13. The gas sensor according to claim 12, wherein the first and second electrodes are arranged parallel to each other.

[0023] (10) The reference electrode functions as the outer measurement electrode.Any one of the above (1) to (8) Either The gas sensor described in

[0024] (11) A method for controlling a gas sensor for detecting a gas to be measured in a gas to be measured, comprising: The gas sensor includes: a sensor element and a control device for controlling the sensor element; The sensor element includes: a long plate-shaped substrate portion including an oxygen ion conductive solid electrolyte layer; a gas to be measured flow portion formed from one end in the longitudinal direction of the substrate portion; an inner pump electrode disposed on the inner surface of the gas to be measured flow portion, and an outer pump electrode corresponding to the inner pump electrode and disposed at a position different from the gas to be measured flow portion of the substrate portion, and adjusting the oxygen in the gas to be measured to a desired concentration. An adjustment pump cell; an inner measurement electrode disposed at a position farther from the one end in the longitudinal direction of the substrate portion than the inner pump electrode on the inner surface of the gas to be measured flow portion, and a position different from the gas to be measured flow portion of the substrate portion. An outer measurement electrode corresponding to the inner measurement electrode, and a current measurement pump cell for detecting the gas to be measured in the gas to be measured as a current value; a reference electrode disposed inside the substrate portion so as to be in contact with a reference gas; an electromotive force detection sensor cell including the inner measurement electrode and the reference electrode, and detecting an electromotive force value between the inner measurement electrode and the reference electrode; and includes The control device includes: a switching unit for switching whether to pass current through the current measurement pump cell or not; The control method includes: An electromotive force measurement mode for detecting the concentration of the gas to be measured in the gas to be measured based on the electromotive force value in the electromotive force detection sensor cell by operating the adjustment pump cell and not operating the current measurement pump cell, and an adjustment pump cell and the current measurement pump cell are operated, and a concentration detection step of switching using the switching unit while detecting the concentration of the measurement target gas in the gas to be measured based on the current value in the current measurement pump cell is included. A control method for a gas sensor.

[0025] (12) In the concentration detection step, When it is determined that the concentration of the measurement target gas detected is lower than a predetermined first concentration threshold C1, the switching unit is switched so that no current flows through the current measurement pump cell, and the switching to the electromotive force measurement mode is performed. When it is determined that the concentration of the measurement target gas detected is higher than a predetermined second concentration threshold C2, the switching unit is switched so that current flows through the current measurement pump cell, and the switching to the current measurement mode is performed. The control method according to (11) above.

[0026] (13) The control method according to (12) above, wherein the first concentration threshold C1 is a concentration lower than the second concentration threshold C2.

Effect of the Invention

[0027] According to the present invention, it is possible to accurately measure a measurement target gas having a low concentration. That is, according to the present invention, it is possible to accurately measure a measurement target gas in a wide concentration range including a measurement target gas having a low concentration.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0029] The gas sensor of the present invention includes a sensor element and a control device for controlling the sensor element.

[0030] The sensor element included in the gas sensor of the present invention A long plate-shaped substrate portion including an oxygen ion conductive solid electrolyte layer, A measured gas flow portion formed from one end in the longitudinal direction of the substrate portion, An inner pump electrode disposed on the inner surface of the measured gas flow portion, and an outer pump electrode disposed at a position different from the measured gas flow portion of the substrate portion and corresponding to the inner pump electrode, and an adjustment pump cell for adjusting oxygen in the measured gas to a desired concentration. An inner measurement electrode disposed at a position on the inner surface of the measured gas flow portion, which is farther from one end of the substrate portion in the longitudinal direction than the inner pump electrode, and an outer measurement electrode disposed at a position different from the measured gas flow portion of the substrate portion and corresponding to the inner measurement electrode, and a current measurement pump cell that detects a measurement target gas in the measured gas as a current value; A reference electrode disposed inside the substrate portion so as to be in contact with a reference gas; An electromotive force detection sensor cell including the inner measurement electrode and the reference electrode, and detecting an electromotive force value between the inner measurement electrode and the reference electrode; Including. Here, in the electromotive force detection sensor cell, the reference electrode corresponds to the inner measurement electrode.

[0031] The control device included in the gas sensor of the present invention includes a switching unit that switches whether to pass current through the current measurement pump cell or not.

[0032] Hereinafter, an example of an embodiment of the gas sensor of the present invention will be described in detail.

[0033] [Schematic Configuration of Gas Sensor] The gas sensor of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic longitudinal vertical cross-sectional view showing an example of the schematic configuration of a gas sensor 100 including a sensor element 101. Hereinafter, with reference to FIG. 1, up and down refer to the upper side of FIG. 1 as up and the lower side as down, the left side of FIG. 1 as the front end side, and the right side as the rear end side.

[0034] In FIG. 1, the gas sensor 100 shows an example of a NOx sensor that detects NOx in the measured gas by the sensor element 101 and measures its concentration.

[0035] The gas sensor 100 also includes a control device 90 that controls the sensor element 101. The control device 90 includes a switching unit 47 that switches whether to pass current through the current measurement pump cell or not. FIG. 2 is a block diagram showing the electrical connection relationship between the control device 90 and the sensor element 101.

[0036] In the gas sensor 100, the switching unit 47 may have a mechanism for switching whether to pass current through the current measurement pump cell 41 or not. For example, the switching unit 47 may be a switch 47 as illustrated in FIG. 1, or a mechanism including a switch. The switch may be a contact switch that mechanically opens and closes contacts provided on an electric circuit, or a switch using a switching element that turns on and off the current flowing on the electric circuit. The switching element includes a diode, a thyristor, a transistor, a MOSFET, etc. The configuration of the switch may be appropriately determined by those skilled in the art. By turning off the switch, the conduction of the current measurement pump cell 41 can be interrupted and switched so that no current flows. By turning on the switch, the current measurement pump cell 41 can be made conductive and switched so that current flows.

[0037] Also, for example, the switching unit 47 may be a variable power supply 46, or a mechanism including the variable power supply 46. By setting the voltage of the variable power supply 46 to zero, it is possible to switch so that no voltage is applied to the current measurement pump cell and no current flows. By setting the voltage of the variable power supply 46 to a predetermined value, it is possible to switch so that a voltage is applied to the current measurement pump cell and current flows.

[0038] (Sensor element) The sensor element 101 is an elongated plate-shaped element including a substrate portion 102 having a structure in which a plurality of oxygen ion-conductive solid electrolyte layers are laminated. The elongated plate shape also means a long plate shape or a strip shape. The substrate portion 102 has a structure in which six layers, namely, a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each made of an oxygen ion-conductive solid electrolyte such as zirconia (ZrO2), are laminated in this order from the lower side in a plan view. The solid electrolytes forming these six layers are dense and airtight. The six layers may all have the same thickness, or may have different thicknesses for each layer. The spaces between the layers are adhered via an adhesive layer made of a solid electrolyte, and the substrate portion 102 includes the adhesive layer. In FIG. 1, the layer configuration consisting of the six layers is illustrated, but the layer configuration in the present invention is not limited to this, and any number of layers and layer configurations may be used.

[0039] Such a sensor element 101 is manufactured, for example, by performing predetermined processing and printing a circuit pattern on ceramic green sheets corresponding to each layer, then laminating them, and further firing and integrating them.

[0040] At one end in the longitudinal direction of the sensor element 101 (hereinafter referred to as the tip end), a gas inlet 10 is formed between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The gas flow path 15 to be measured is formed such that the first diffusion rate-limiting portion 11, the buffer space 12, the second diffusion rate-limiting portion 13, the first internal cavity 20, the third diffusion rate-limiting portion 30, the second internal cavity 40, the fourth diffusion rate-limiting portion 60, and the third internal cavity 61 are adjacent and communicate in this order in the longitudinal direction from the gas inlet 10.

[0041] The gas inlet 10, the buffer space 12, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 are spaces inside the sensor element 101 that are partitioned by the lower surface of the second solid electrolyte layer 6 at the upper part, the upper surface of the first solid electrolyte layer 4 at the lower part, and the side surface of the spacer layer 5 at the side part in a mode where the spacer layer 5 is hollowed out.

[0042] The first diffusion rate-determining section 11, the second diffusion rate-determining section 13, and the third diffusion rate-determining section 30 are all provided as two horizontally long slits (the openings have the longitudinal direction in the direction perpendicular to the drawing in FIG. 1). The first diffusion rate-determining section 11 and the second diffusion rate-determining section 13 may be in any form that imparts a desired diffusion resistance, and the form is not limited to the slit.

[0043] The fourth diffusion rate-determining section 60 is provided between the spacer layer 5 and the second solid electrolyte layer 6 as a single horizontally long slit (the opening has the longitudinal direction in the direction perpendicular to the drawing in FIG. 1). The fourth diffusion rate-determining section 60 may be in any form that imparts a desired diffusion resistance, and the form is not limited to the slit.

[0044] Also, at a position farther from the tip side than the measured gas flow section 15, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5 and at a position where the side portion is partitioned by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. For example, air is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0045] The air introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. Also, the air introduction layer 48 is formed so as to cover the reference electrode 42.

[0046] The reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, an air introduction layer 48 connected to the reference gas introduction space 43 is provided around it. That is, the reference electrode 42 is disposed so as to be in contact with the reference gas through the porous air introduction layer 48 and the reference gas introduction space 43. Also, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 using the reference electrode 42. The reference electrode 42 is formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO2).

[0047] In the measured gas flow portion 15, the gas inlet 10 is open to the external space, and the measured gas is taken into the sensor element 101 from the external space through the gas inlet 10.

[0048] In the present embodiment, the measured gas flow portion 15 is in a form in which the measured gas is introduced from the gas inlet 10 opened on the front end surface of the sensor element 101, but the present invention is not limited to this form. For example, the measured gas flow portion 15 may not have a recess of the gas inlet 10. In this case, the first diffusion rate-limiting portion 11 substantially serves as the gas inlet. Also, for example, the measured gas flow portion 15 may be in a form having an opening communicating with a buffer space 12 or a position close to the buffer space 12 of the first internal cavity 20 on a side surface along the longitudinal direction of the base portion 102. In this case, the measured gas is introduced from the side surface along the longitudinal direction of the base portion 102 through the opening. Also, for example, the measured gas flow portion 15 may be configured such that the measured gas is introduced through a porous body.

[0049] The first diffusion rate-limiting portion 11 is a portion that imparts a predetermined diffusion resistance to the measured gas taken in from the gas inlet 10.

[0050] The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate-determining section 11 to the second diffusion rate-determining section 13.

[0051] The second diffusion rate-determining section 13 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal cavity 20.

[0052] It is only necessary that the amount of the gas to be measured introduced into the first internal cavity 20 is within a predetermined range. That is, it is only necessary that a predetermined diffusion resistance is imparted from the tip of the sensor element 101 to the entire second diffusion rate-determining section 13. For example, a mode in which the first diffusion rate-determining section 11 communicates directly with the first internal cavity 20, that is, a mode in which the buffer space 12 and the second diffusion rate-determining section 13 do not exist may also be adopted.

[0053] The buffer space 12 is a space provided to mitigate the influence of pressure fluctuations of the gas to be measured on the detection value when the pressure of the gas to be measured fluctuates.

[0054] When the gas to be measured is introduced from outside the sensor element 101 into the first internal cavity 20, the gas to be measured suddenly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the gas to be measured in the external space (pulsation of the exhaust pressure if the gas to be measured is the exhaust gas of an automobile) is not directly introduced into the first internal cavity 20, but after the pressure fluctuations of the gas to be measured are canceled through the first diffusion rate-determining section 11, the buffer space 12, and the second diffusion rate-determining section 13, it is introduced into the first internal cavity 20. As a result, the pressure fluctuations of the gas to be measured introduced into the first internal space become negligible.

[0055] The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced through the second diffusion rate-determining section 13. Such oxygen partial pressure is adjusted by operating the main pump cell 21.

[0056] The sensor element 101 includes an adjustment pump cell including an inner pump electrode disposed on the inner surface of the measured gas flow portion 15 and an outer pump electrode disposed at a position different from the measured gas flow portion 15 of the base portion 102 and corresponding to the inner pump electrode. In the present embodiment, the main pump cell 21 and the auxiliary pump cell 50 function as adjustment pump cells. Further, the inner main pump electrode 22 and the auxiliary pump electrode 51 function as inner pump electrodes, and the outer pump electrode 23 functions as an outer pump electrode.

[0057] The main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 disposed on the inner surface of the measured gas flow portion 15 and an outer pump electrode 23 disposed at a position different from the measured gas flow portion 15 of the base portion 102 (in FIG. 1, the outer surface of the base portion 102) and corresponding to the inner main pump electrode 22. "Corresponding to the inner main pump electrode 22" means that the outer pump electrode 23 is provided via the second solid electrolyte layer 6 with the inner main pump electrode 22.

[0058] That is, the main pump cell 21 is an electrochemical pump cell constituted by an inner main pump electrode 22 having a ceiling electrode portion 22a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided in a manner exposed to the external space in a region corresponding to the ceiling electrode portion 22a on the upper surface of the second solid electrolyte layer 6, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0059] The inner main pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that partition the first internal cavity 20, and the spacer layer 5 that provides side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal cavity 20, a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface, and a side electrode portion (not shown) is formed on the side wall surface (inner surface) of the spacer layer 5 that constitutes both side wall portions of the first internal cavity 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and is disposed in a structure having a tunnel form at the disposed position of the side electrode portion.

[0060] The inner main pump electrode 22 and the outer pump electrode 23 are porous cermet electrodes (electrodes in a state where a metal component and a ceramic component are mixed). The ceramic component is not particularly limited, but it is preferable to use an oxygen ion-conductive solid electrolyte as in the base portion 102. For example, ZrO2 can be used as the ceramic component.

[0061] The inner main pump electrode 22 that contacts the gas to be measured is formed using a material that weakens the reduction ability with respect to the NOx component in the gas to be measured. The inner main pump electrode 22 preferably contains a noble metal having catalytic activity (for example, at least one of Pt, Rh, Ir, Ru, Pd) and a noble metal that reduces the catalytic activity of the noble metal having catalytic activity with respect to the gas to be measured (NOx in this embodiment). In this embodiment, the inner main pump electrode 22 is a porous cermet electrode of Pt containing 1% Au and ZrO2.

[0062] The outer pump electrode 23 only needs to contain the above-described noble metal having catalytic activity. Similarly, the above-described reference electrode 42 only needs to contain the above-described noble metal having catalytic activity. In this embodiment, the outer pump electrode 23 is a porous cermet electrode of Pt and ZrO2.

[0063] In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner main pump electrode 22 and the outer pump electrode 23 by the variable power supply 24, and a pump current Ip0 is caused to flow in the positive or negative direction between the inner main pump electrode 22 and the outer pump electrode 23, thereby pumping out the oxygen in the first internal cavity 20 to the external space, or pumping in the oxygen in the external space into the first internal cavity 20.

[0064] Also, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal cavity 20, an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 80 for main pump control is constituted by the inner main pump electrode 22, the second solid electrolyte Layer 6, the spacer layer 5, the first solid electrolyte Layer 4, the third substrate layer 3, and the reference electrode 42.

[0065] By measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for main pump control, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be known. Further, the pump current Ip0 is controlled by feedback-controlling the pump voltage Vp0 of the variable power supply 24 so that the electromotive force V0 becomes constant. As a result, the oxygen concentration in the first internal cavity Location 2 0 can be maintained at a predetermined constant value.

[0066] The third diffusion rate-limiting section 30 is a part that imparts a predetermined diffusion resistance to the measurement gas whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal cavity 20 and guides the measurement gas to the second internal cavity 40.

[0067] The second internal cavity 40 is provided as a space for more precisely adjusting the oxygen partial pressure in the measurement gas introduced through the third diffusion rate-limiting section 30. Such an oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50. It is also possible to adopt a configuration without the second internal cavity 40 and the auxiliary pump cell 50. From the viewpoint of the accuracy of adjusting the oxygen partial pressure, it is more preferable to have the second internal cavity 40 and the auxiliary pump cell 50.

[0068] In the second internal space 40, after the oxygen concentration (oxygen partial pressure) has been adjusted in the first internal space 20 in advance, the oxygen partial pressure of the measurement target gas introduced through the third diffusion rate-limiting section 30 is further adjusted by the auxiliary pump cell 50. As a result, the oxygen concentration in the second internal space 40 can be kept constant with high precision, so that highly accurate NOx concentration measurement is possible in such a gas sensor 100.

[0069] The auxiliary pump cell 50 includes an auxiliary pump electrode 51 disposed at a position on the inner surface of the measurement target gas flow section 15 that is farther from the longitudinal end of the base section 102 than the inner main pump electrode 22, and an outer pump electrode 23 disposed at a position on the base section 102 different from the measurement target gas flow section 15 (in FIG. 1, the outer surface of the base section 102), which corresponds to the auxiliary pump electrode 51. "Corresponding to the auxiliary pump electrode 51" means that the outer pump electrode 23 is provided via the second solid electrolyte layer 6 with the auxiliary pump electrode 51.

[0070] That is, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell composed of an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided substantially over the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, and any appropriate electrode outside the sensor element 101 is sufficient), and the second solid electrolyte layer 6.

[0071] The auxiliary pump electrode 51 is arranged in the second internal cavity 40 in a tunnel structure similar to the inner main pump electrode 22 provided in the previous first internal cavity 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal cavity 40. Side electrode portions (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are in a tunnel structure formed on both wall surfaces of the spacer layer 5 that provides the side walls of the second internal cavity 40, respectively.

[0072] Note that the auxiliary pump electrode 51 is also formed using a material with a reduced reduction ability for NOx components in the gas to be measured, similar to the inner main pump electrode 22. The auxiliary pump electrode 51 preferably contains, similar to the inner main pump electrode 22, a noble metal having catalytic activity (for example, at least one of Pt, Rh, Ir, Ru, Pd) and a noble metal that reduces the catalytic activity of the noble metal having catalytic activity with respect to the measurement target gas (NOx in this embodiment) (for example, Au, Ag, etc.). In this embodiment, the auxiliary pump electrode 51 is a porous cermet electrode of Pt containing 1% Au and ZrO2, similar to the inner main pump electrode 22.

[0073] In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 with a variable power supply 52, oxygen in the atmosphere in the second internal cavity 40 can be pumped out to the external space, or pumped into the second internal cavity 40 from the external space.

[0074] In addition, in order to control the oxygen partial pressure in the atmosphere in the second internal cavity 40, an electrochemical sensor cell, that is, an oxygen partial pressure detection sensor cell 81 for auxiliary pump control, is constituted by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3.

[0075] The auxiliary pump cell 50 performs pumping by means of a variable power source 52 that is voltage-controlled based on the electromotive force V1 detected by the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump. As a result, the oxygen partial pressure in the atmosphere within the second internal cavity 40 is controlled to a low partial pressure that has substantially no influence on the measurement of NOx.

[0076] At the same time, the pump current Ip1 is used to control the electromotive force V0 of the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell 80 for controlling the main pump, and by controlling the electromotive force V0 thereof, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-determining section 30 into the second internal cavity 40 is constantly controlled to be constant. When used as a NOx sensor, the oxygen concentration within the second internal cavity 40 is maintained at a constant value of approximately 0.001 ppm by the functions of the main pump cell 21 and the auxiliary pump cell 50.

[0077] The fourth diffusion rate-determining section 60 is a site that imparts a predetermined diffusion resistance to the measurement gas in which the oxygen concentration (oxygen partial pressure) is further controlled to be lower by the operation of the auxiliary pump cell 50 in the second internal cavity 40, and guides the measurement gas to the third internal cavity 61.

[0078] The third internal cavity 61 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the measurement gas introduced through the fourth diffusion rate-determining section 60. The measurement of the NOx concentration is performed by the operation of the electromotive force detection sensor cell 82 or the current measurement pump cell 41.

[0079] The pump cell 41 for current measurement is an electrochemical pump cell including an inner measurement electrode (in this embodiment, the measurement electrode 44) disposed at a position on the inner surface of the gas flow path 15 to be measured, which is farther from the tip of the substrate portion 102 in the longitudinal direction than the inner pump electrodes (in this embodiment, the inner main pump electrode 22 and the auxiliary pump electrode 51), and an outer measurement electrode disposed at a position different from the gas flow path 15 to be measured of the substrate portion 102 and corresponding to the inner measurement electrode. In this embodiment, the outer pump electrode 23 disposed on the outer surface of the substrate portion 102 also functions as an outer measurement electrode. "Corresponding to the inner measurement electrode" means that the outer pump electrode 23 is provided via the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4 with the measurement electrode 44.

[0080] That is, the pump cell 41 for current measurement measures the NOx concentration in the gas to be measured in the third internal cavity 61. The measurement pump cell 41 is an electrochemical pump cell composed of a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, and a first solid electrolyte layer 4.

[0081] The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the third internal cavity 61. The measurement electrode 44 is an electrode containing a noble metal having catalytic activity (for example, at least one of Pt, Rh, Ir, Ru, Pd). It is preferably free from a noble metal (for example, Au, Ag, etc.) that reduces the catalytic activity of the noble metal having catalytic activity with respect to the measurement target gas (NOx in this embodiment). In this embodiment, the measurement electrode 44 is a porous cermet electrode of Pt and Rh and ZrO2.

[0082] Further, the electromotive force detection sensor cell 82 includes an inner measurement electrode (in this embodiment, the measurement electrode 44) and the reference electrode 42, and is configured to detect the electromotive force value between the inner measurement electrode and the reference electrode 42. Here, the reference electrode 42 corresponds to the measurement electrode 44 of the inner measurement electrode. "Corresponding to the measurement electrode 44" means that the reference electrode 42 is provided with the measurement electrode 44 via the first solid electrolyte layer 4 and the third substrate layer 3.

[0083] That is, the electromotive force detection sensor cell 82 is an electrochemical sensor cell composed of the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42. The electromotive force detection sensor cell 82 detects the oxygen partial pressure around the measurement electrode 44.

[0084] The current measurement pump cell 41 is configured to be switched by the switching unit 47 to flow or not flow current through the current measurement pump cell 41. In the following, with reference to FIG. 1, the case where the switching unit 47 is the switch 47 will be described as an example. The switching unit 47 is schematically shown using the circuit symbol of a contact switch in FIG. 1, but the switching unit 47 may be a switch using a switching element. Further, the position where the switching unit 47 is provided is not limited to the position shown in FIG. 1, and it may be provided anywhere on the circuit of the current measurement pump cell 41.

[0085] When the switching unit 47 is turned OFF, the electrical connection of the current measurement pump cell 41 is interrupted, so no current flows through the current measurement pump cell 41. That no current flows means that the current value in the current measurement pump cell 41 is zero or substantially zero.

[0086] The gas to be measured guided into the second internal space 40 reaches the measurement electrode 44 in the third internal space 61 through the fourth diffusion rate-determining section 60 under the condition where the oxygen partial pressure is controlled. Nitrogen oxides in the gas to be measured around the measurement electrode 44 are reduced (2NO→N2+O2) to generate oxygen. The generated oxygen stays around the measurement electrode 44. As a result, in the electromotive force detection sensor cell 82, an electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere is generated. The generated electromotive force is referred to as the open electromotive force V2open. Since the amount of oxygen contained in the reference atmosphere is constant, the value of the open electromotive force V2open corresponds to the concentration of nitrogen oxides in the gas to be measured. Therefore, the concentration of nitrogen oxides in the gas to be measured can be calculated using the open electromotive force V2open detected by the electromotive force detection sensor cell 82.

[0087] When the switching unit 47 is turned ON, the current measurement pump cell 41 is electrically connected, so a current flows through the current measurement pump cell 41.

[0088] Also in this case, the gas to be measured guided into the second internal space 40 reaches the measurement electrode 44 in the third internal space 61 through the fourth diffusion rate-determining section 60 under the condition where the oxygen partial pressure is controlled. Nitrogen oxides in the gas to be measured around the measurement electrode 44 are reduced (2NO→N2+O2) to generate oxygen. And this generated oxygen is pumped by the current measurement pump cell 41. At this time, using the electromotive force detected by the electromotive force detection sensor cell 82 as the control voltage V2, the voltage Vp2 of the variable power supply 46 is feedback-controlled so that this control voltage V2 becomes constant. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the concentration of nitrogen oxides in the gas to be measured can be calculated using the pump current Ip2 in the current measurement pump cell 41.

[0089] Further, an electrochemical sensor cell 83 is composed of a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, a third substrate layer 3, an outer pump electrode 23, and a reference electrode 42. The electromotive force Vref obtained by this sensor cell 83 enables detection of the oxygen partial pressure in the gas to be measured outside the sensor.

[0090] Furthermore, the sensor element 101 includes a heater unit 70 that serves to adjust the temperature to heat and keep warm the sensor element 101 in order to enhance the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater electrode 71, a heater 72, a heater lead 76, a through hole 73, a heater insulating layer 74, and a pressure dissipation hole 75.

[0091] The heater electrode 71 is an electrode formed in a manner of contacting the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power source 77 which is an external power source, power can be supplied to the heater unit 70 from the outside.

[0092] The heater 72 is an electrical resistor formed in a manner of being sandwiched from above and below by the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via a heater lead 76 that is connected to the heater 72 and extends to the rear end side in the longitudinal direction of the sensor element 101 and a through hole 73, and generates heat when supplied with power from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte forming the sensor element 101.

[0093] Also, the heater 72 is embedded across the entire area of the first internal cavity 20 to the third internal cavity 61 so that the entire sensor element 101 can be adjusted to the temperature at which the above solid electrolyte is activated. It is only necessary that the temperature be adjusted so that the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 can operate. It is not necessary for these entire areas to be adjusted to the same temperature, and there may be a temperature distribution in the sensor element 101.

[0094] In the sensor element 101 of the present embodiment, the heater 72 is embedded in the base portion 102, but the present invention is not limited to this embodiment. The heater 72 may be arranged to heat the base portion 102. That is, the heater 72 may be any heater that can heat the sensor element 101 to such an extent that the above-described main pump cell 21, auxiliary pump cell 50, and measurement pump cell 41 can exhibit oxygen ion conductivity to operate. For example, it may be embedded in the base portion 102 as in the present embodiment. Alternatively, for example, the heater portion 70 may be formed as a heater substrate separate from the base portion 102 and arranged at an adjacent position to the base portion 102.

[0095] The heater insulating layer 74 is an insulating layer formed of an insulator such as alumina on the upper and lower surfaces of the heater 72 and the heater lead 76. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and the heater lead 76, and electrical insulation between the third substrate layer 3 and the heater 72 and the heater lead 76.

[0096] The pressure relief hole 75 penetrates the third substrate layer 3 and is formed so that the heater insulating layer 74 communicates with the reference gas introduction space 43. The pressure relief hole 75 can relieve the increase in internal pressure accompanying the temperature rise in the heater insulating layer 74. Note that a configuration without the pressure relief hole 75 may also be adopted.

[0097] The above-described sensor element 101 is incorporated into the gas sensor 100 in such a manner that the tip of the sensor element 101 is in contact with the gas to be measured and the rear end of the sensor element 101 is in contact with the reference gas.

[0098] (Control device) The gas sensor 100 of this embodiment includes the above-described sensor element 101 and a control device 90 that controls the sensor element 101. In the gas sensor 100, each of the electrodes 22, 23, 51, 44, 42 of the sensor element 101 is electrically connected to the control device 90 via lead wires (not shown). FIG. 2 is a block diagram showing the electrical connection relationship between the control device 90 and each pump cell 21, 50, 41, each sensor cell 80, 81, 82, 83 of the sensor element 101, and the heater unit 70. The control device 90 includes the variable power supplies 24, 46, 52 described above, a switching unit 47 that switches whether to pass current through the current measurement pump cell 41 or not, and a control unit 91. The control unit 91 includes a drive control unit 92, a concentration calculation unit 93, and a measurement mode switching unit 94. The switching unit 47 is a member that receives a control signal from the measurement mode switching unit 94 and switches whether to pass current through the current measurement pump cell 41 or not.

[0099] The control unit 91 is realized by a general-purpose or dedicated computer, and the functions as the drive control unit 92, the concentration calculation unit 93, and the measurement mode switching unit 94 are realized by a CPU, a memory, etc. mounted on the computer. When the gas sensor 100 measures NOx contained in the exhaust gas from an automobile engine as the measurement target gas and the sensor element 101 is attached to the exhaust path, some or all of the functions of the control device 90 (particularly the control unit 91) may be realized by an ECU (Electronic Control Unit) mounted on the automobile.

[0100] The control unit 91 is configured to acquire the electromotive forces (V0, V1, V2, Vref) in each of the sensor cells 80, 81, 82, 83 of the sensor element 101, the pump currents (Ip0, Ip1, Ip2) in each of the pump cells 21, 50, 41, and the heater voltage Vh and heater current Ih in the heater unit 70. Further, the control unit 91 is configured to output control signals to the variable power supplies 24, 52, 46, the switching unit 47, and the heater power supply 77.

[0101] The drive control unit 92 is configured to control the heater unit 70, the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 so that the gas sensor 100 can measure the concentration of the gas to be measured (NOx in this embodiment).

[0102] The drive control unit 92 heats the heater 72 and maintains the temperature of the heater 72 at a desired temperature.

[0103] To heat the heater 72, various known control methods can be used. For example, a constant voltage can be applied to the heater 72 for heating. The output of the heater power supply 77 can also be controlled based on the resistance value of the heater 72. Alternatively, the output of the heater power supply 77 can be controlled based on at least one of the resistance values in the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41.

[0104] For example, the drive control unit 92 performs feedback control on the control signal output to the heater power supply 77 so that the heater 72 reaches the target temperature based on the heater resistance value Rh (= Vh / Ih) calculated from the heater voltage Vh and the heater current Ih in the heater 72.

[0105] The drive control unit 92 performs feedback control on the pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 so that the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80 becomes a constant value (referred to as the set value V0 SET ). Since the electromotive force V0 indicates the oxygen partial pressure near the inner main pump electrode 22, making the electromotive force V0 constant means making the oxygen partial pressure near the inner main pump electrode 22 constant. As a result, the pump current Ip0 in the main pump cell 21 changes according to the oxygen concentration in the gas to be measured.

[0106] When the oxygen partial pressure in the gas to be measured is higher than the oxygen partial pressure corresponding to the set value V0 SET In the main pump cell 21, oxygen is discharged from the first internal cavity 20. On the other hand, when the oxygen partial pressure in the gas to be measured is the set value V0 SET(for example, when hydrocarbons HC or the like are contained), the main pump cell 21 pumps oxygen from the space outside the sensor element 101 into the first internal space 20. Therefore, the pump current Ip0 can be either positive or negative.

[0107] The drive control unit 92 controls the electromotive force V1 in the oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump to be a constant value (set value V1 SET The pump voltage Vp1 of the variable power supply 52 in the auxiliary pump cell 50 is feedback-controlled so that the electromotive force V1 indicates the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51. Therefore, keeping the electromotive force V1 constant means keeping the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51 constant. As a result, the oxygen partial pressure in the atmosphere in the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0108] At the same time, the pump current Ip1 in the auxiliary pump cell 50 is set to a constant value (set value Ip1 SET The set value V0 of the electromotive force V0 is set based on the pump current Ip1 so that SET Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell 80 for controlling the main pump, and the electromotive force V0 is set to a set value V0 based on the pump current Ip1. SET By controlling the diffusion rate of the fourth diffusion-controlling part 60, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-controlling part 30 into the second internal space 40 is always controlled to be constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is kept at a constant value of about 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50. In other words, it is considered that the oxygen concentration in the measurement gas introduced from the fourth diffusion-controlling part 60 into the third internal space 61 is kept at a constant value of about 0.001 ppm.

[0109] The drive control unit 92 is The main pump cell 21 and the auxiliary pump cell 50 that function as adjustment pump cells are operated as described above, and the electromotive force detection sensor cell is operated without operating the current measurement pump cell 41. 82 An electromotive force measurement mode for detecting the concentration of the gas to be measured in the gas to be measured based on the electromotive force value (open electromotive force V2open) in the 82 , and A current measurement mode in which the main pump cell 21, the auxiliary pump cell 50, and the current measurement pump cell 41 are operated, and the concentration of the gas to be measured in the gas to be measured is detected based on the current value (pump current Ip2) in the current measurement pump cell 41.

[0110] In the electromotive force measurement mode, the switching unit 47 causes no current to flow through the current measurement pump cell 41. In the third internal cavity 61, the gas to be measured after the oxygen concentration in the main pump cell 21 and the auxiliary pump cell 50 is adjusted to a predetermined concentration is introduced, and in the measurement electrode 44, nitrogen oxides in the gas to be measured are reduced to generate oxygen. In the electromotive force measurement mode, the generated oxygen stays around the measurement electrode 44 without being pumped out by the current measurement pump cell 41. In the electromotive force detection sensor cell 82, an open electromotive force V2open is generated according to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere. The drive control unit 92 detects the open electromotive force V2open generated in the electromotive force detection sensor cell 82. That is, in the electromotive force measurement mode, no current flows through the current measurement pump cell 41, that is, the electromotive force generated in the electromotive force detection sensor cell 82 becomes the open electromotive force V2open. In this specification, the electromotive force measurement mode is also referred to as the open electromotive force measurement mode.

[0111] In the current measurement mode, the switching unit 47 causes current to flow through the current measurement pump cell 41. Also in the current measurement mode, the drive control unit 92 continues to acquire the electromotive force V2 detected by the electromotive force detection sensor cell 82. In the current measurement mode, when the electromotive force V2 detected by the drive control unit 92 in the electromotive force detection sensor cell 82 reaches a constant value (set value V2SET The pump voltage Vp2 of the variable power supply 46 in the current measurement pump cell 41 is feedback-controlled so as to become a value (referred to as ) of the set value V2. SET The set value V2 is predetermined as a value such that the oxygen concentration around the measurement electrode 44 in the third internal cavity 61 becomes a predetermined low concentration. In the third internal cavity 61, the gas to be measured after the oxygen concentration is adjusted to a predetermined concentration in the main pump cell 21 and the auxiliary pump cell 50 is introduced. At the measurement electrode 44, nitrogen oxides in the gas to be measured are reduced to generate oxygen. The generated oxygen is substantially all pumped out by the drive control unit 92 flowing the pump current Ip2 through the current measurement pump cell 41. The set value V2 SET can be set as a value that substantially decomposes all NOx at the measurement electrode 44. By setting the set value V2 in this way, SET substantially all NOx in the gas to be measured is detected as the pump current Ip2 at the measurement electrode 44. In the current measurement mode, the electromotive force V2 in the electromotive force detection sensor cell 82 is used as a control voltage for feedback control of the pump voltage Vp2 of the variable power supply 46 in the current measurement pump cell 41.

[0112] Here, the pump current Ip2 detected in the current measurement mode will be described in detail. FIG. 3 is a schematic diagram showing an example of the relationship between the NOx concentration in the gas to be measured and the pump current Ip2 in the gas sensor 100. The horizontal axis represents the NOx concentration (ppm), and the vertical axis represents the value (μA) of the pump current Ip2.

[0113] As described above, in the current measurement mode, the oxygen generated by the reduction of nitrogen oxides is substantially all pumped out by the drive control unit 92 passing the pump current Ip2 through the current measurement pump cell 41. Therefore, the pump current Ip2 becomes a current value corresponding to the amount of oxygen generated by the reduction of nitrogen oxides. The amount of oxygen generated by the reduction of nitrogen oxides is proportional to the amount of reduced nitrogen oxides. Assuming that substantially all of the nitrogen oxides in the gas to be measured are decomposed at the measurement electrode 44, the amount of oxygen generated by the reduction of nitrogen oxides is proportional to the nitrogen oxide concentration (NOx concentration) in the gas to be measured. That is, as shown in FIG. 3, there is a linear relationship in a wide range of NOx concentrations between the NOx concentration and the pump current Ip2. The gas sensor 100 can measure the NOx concentration in a wide concentration range based on such a linear relationship between the NOx concentration and the pump current Ip2.

[0114] As shown in FIG. 3, the pump current Ip2 includes an offset current Ip2offset that flows regardless of the NOx concentration and a concentration-dependent current Ip2conc that flows according to the NOx concentration. The concentration-dependent current Ip2conc is a current that flows according to the amount of oxygen generated by the reduction of nitrogen oxides, and its current value is approximately proportional to the NOx concentration.

[0115] The offset current Ip2offset is a current that flows regardless of the NOx concentration. The offset current Ip2offset is a current generated by factors other than oxygen generated by the decomposition of NOx in the gas to be measured. The offset current Ip2offset is considered to include currents caused by residual oxygen in the gas to be measured after the oxygen concentration is adjusted to a predetermined concentration in the main pump cell 21 and the auxiliary pump cell 50, and oxygen generated by the decomposition of a part of water (H2O) in the gas to be measured at the measurement electrode 44. Further, the offset current Ip2offset is considered to include a leakage current from the heater 72 that is energized to heat the gas sensor 100, and currents due to the movement of charges such as electrons and impurities contained in the measurement electrode 44, the outer pump electrode 23, and the solid electrolyte layer (in FIG. 1, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4) that constitute the current measurement pump cell 41.

[0116] When the gas sensor 100 is detecting the NOx concentration in the gas to be measured, if the current value of the offset current Ip2offset fluctuates due to some factor, then regardless of the NOx concentration in the gas to be measured, the pump current Ip2 detected in the current measurement pump cell 41 shifts by the fluctuation amount ΔIp2offset of the offset current Ip2offset. The offset current Ip2offset is considered to fluctuate, for example, due to fluctuations in the electrode temperature caused by fluctuations in the temperature of the gas to be measured and accompanying fluctuations in the output of the heater power supply 77. It is also considered to fluctuate due to fluctuations in the H2O concentration in the gas to be measured.

[0117] containing a high concentration of NOx To be measuredWhen measuring gas, since the pump current Ip2 detected in the pump cell 41 for current measurement is relatively large, the change in the pump current Ip2 due to the variation ΔIp2offset of the offset current Ip2offset is relatively small. Therefore, even if the offset current Ip2offset varies, the NOx concentration can be measured with higher measurement accuracy. For example, when the variation ΔIp2offset of the offset current Ip2offset is equivalent to 5 ppm in terms of the NOx concentration, when measuring a gas to be measured with a NOx concentration of 50 ppm, the measurement error is 10%, but when measuring a gas to be measured with a NOx concentration of 500 ppm, the measurement error is 1%.

[0118] On the other hand, in order to measure a gas to be measured containing low-concentration NOx more accurately, it is preferable to reduce the influence of the variation of the offset current Ip2offset.

[0119] The offset current Ip2offset occurs when the pump voltage Vp2 of the variable power supply 46 is applied to the pump cell 41 for current measurement and the pump current Ip2 flows. Therefore, if the pump voltage Vp2 is not applied to the pump cell 41 for current measurement, the pump current Ip2 does not flow and the offset current Ip2offset does not occur either. In the gas sensor 100, by turning off the switching unit 47 to cut off the electrical connection of the pump cell 41 for current measurement and making the state where no current flows through the pump cell 41 for current measurement, a state where the offset current Ip2offset does not occur can be realized. In this case, the drive control unit 92 performs control in the open-circuit electromotive force measurement mode and detects the open-circuit electromotive force V2open in the electromotive force detection sensor cell 82.

[0120] The open-circuit electromotive force V2open detected in the electromotive force detection sensor cell 82 will be described in detail. FIG. 4 is a schematic diagram showing an example of the relationship between the NOx concentration and the open-circuit electromotive force V2open in the gas sensor 100. The horizontal axis represents the NOx concentration (ppm), and the vertical axis represents the value (mV) of the open-circuit electromotive force V2open.

[0121] In the electromotive force measurement mode, as described above, the electrical connection of the current measurement pump cell 41 is interrupted, and the pump current Ip2 does not flow. In this case, in the electromotive force detection sensor cell 82, an open electromotive force V2open is generated according to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measurement electrode 44 and the amount of oxygen contained in the reference atmosphere. There is a relationship as shown in FIG. 4 between the open electromotive force V2open and the NO concentration.

[0122] In the gas sensor 100, in the electromotive force measurement mode, since the pump current Ip2 does not flow, the offset current Ip2offset does not occur. Therefore, based on the relationship between the NOx concentration and the open electromotive force V2open as shown in FIG. 4, the NOx concentration can be accurately measured without being affected by the offset current Ip2offset.

[0123] As shown in FIG. 4, the lower the NO concentration in the gas to be measured, the greater the variation in the open electromotive force V2open due to the change in the NO concentration. That is, the lower the NO concentration in the gas to be measured, the greater the change in the open electromotive force V2open for a minute change in the NO concentration, so the measurement resolution tends to be higher. Therefore, when measuring a gas to be measured containing low-concentration NOx, the NOx concentration can be measured with particularly high measurement accuracy.

[0124] Thus, in the electromotive force measurement mode, since the offset current Ip2offset does not occur, the NOx concentration can be accurately measured over a wide concentration range without being affected by the variation of the offset current Ip2offset. In particular, when measuring a gas to be measured containing low-concentration NOx, since the change in the open electromotive force V2open with respect to a minute change in the NO concentration is large, the measurement resolution tends to be higher. Therefore, when measuring a gas to be measured containing low-concentration NOx, the NOx concentration can be measured with particularly high measurement accuracy.

[0125] In the current measurement mode, there is a linear relationship between the pump current Ip2 and the NOx concentration as shown in FIG. 3 over a wide range of NOx concentrations. Therefore, the NOx concentration can be accurately measured over a wide concentration range. In particular, when measuring a gas to be measured containing a high concentration of NOx, the change in the pump current Ip2 due to the fluctuation ΔIp2offset of the offset current is relatively small. Thus, the NOx concentration can be measured with higher measurement accuracy.

[0126] The concentration calculation unit 93 is configured to calculate and output the NOx concentration in the gas to be measured.

[0127] In the electromotive force measurement mode, the concentration calculation unit 93 acquires the electromotive force V2open in the electromotive force detection sensor cell 82, and based on the conversion parameter (open electromotive force-concentration conversion parameter) between the pre-stored open electromotive force V2open and the NOx concentration in the gas to be measured, calculates the NOx concentration in the gas to be measured and outputs it as the measurement value of the gas sensor 100. The open electromotive force-concentration conversion parameter is pre-stored in the memory of the control unit 91 that functions as the concentration calculation unit 93 as data representing a relationship as exemplified in FIG. 4. The electromotive force-concentration conversion parameter can be appropriately determined by those skilled in the art in advance through experiments or the like for the gas sensor 100. Electromotive force The -concentration conversion parameter may be, for example, the coefficient of an approximate formula (such as a logarithmic function) obtained by experiments, or a map showing the correspondence between the electromotive force V2open and the NOx concentration in the gas to be measured. The electromotive force-concentration conversion parameter may be a parameter unique to each gas sensor 100, or a parameter commonly used for a plurality of gas sensors.

[0128] In the current measurement mode, the concentration calculation unit 93 acquires the pump current Ip2 in the pump cell 41 for current measurement, and calculates the NOx concentration in the gas to be measured based on the conversion parameter (current-concentration conversion parameter) between the pump current Ip2 stored in advance and the NOx concentration in the gas to be measured, and outputs it as the measurement value of the gas sensor 100. The current-concentration conversion parameter is stored in advance in the memory of the control unit 91 that functions as the concentration calculation unit 93 as data representing a linear relationship as illustrated in FIG. 3. The current-concentration conversion parameter can be appropriately determined by those skilled in the art in advance through experiments or the like for the gas sensor 100. The current-concentration conversion parameter may be, for example, the coefficient of an approximate formula (linear function or the like) obtained through experiments, or a map showing the correspondence between the pump current Ip2 and the NOx concentration in the gas to be measured. The current-concentration conversion parameter may be a parameter unique to each gas sensor 100, or a parameter commonly used for a plurality of gas sensors.

[0129] The measurement mode switching unit 94 is configured to switch between the above-described electromotive force measurement mode and the current measurement mode.

[0130] When the measurement mode switching unit 94 switches from the current measurement mode to the electromotive force measurement mode, it switches the switching unit so that no current or substantially no current flows through the pump cell 41 for current measurement. In the present embodiment, a control signal for turning off is output to the switching unit 47. Further, the measurement mode switching unit 94 gives an instruction to the drive control unit 92 to perform control in the electromotive force measurement mode. In this case, the drive control unit 92 does not perform feedback control on the pump voltage Vp2 of the variable power supply 46 of the pump cell 41 for current measurement described above, but detects the open-circuit electromotive force V2open in the electromotive force detection sensor cell 82. The measurement mode switching unit 94 gives an instruction to the concentration calculation unit 93 to acquire the open-circuit electromotive force V2open in the electromotive force detection sensor cell 82 and calculate the NOx concentration based on the electromotive force-concentration conversion parameter.

[0131] When the measurement mode switching unit 94 switches from the electromotive force measurement mode to the current measurement mode, it switches the switching unit so that a current flows through the current measurement pump cell 41. In the present embodiment, a control signal for turning on is output to the switching unit 47. Further, the measurement mode switching unit 94 gives an instruction to the drive control unit 92 to perform control in the current measurement mode. In this case, the drive control unit 92 performs feedback control on the pump voltage Vp2 of the variable power supply 46 of the current measurement pump cell 41 described above, and detects the pump current Ip2 in the current measurement pump cell 41. The measurement mode switching unit 94 acquires the pump current Ip2 in the current measurement pump cell 41 from the concentration calculation unit 93, and gives an instruction to calculate the NOx concentration based on the current-concentration conversion parameter.

[0132] The switching between the electromotive force measurement mode and the current measurement mode may be performed based on the NOx concentration output by the concentration calculation unit 93. In the measurement mode switching unit 94 (more specifically, the memory that functions as the measurement mode switching unit 94 in the control unit 91), a first concentration threshold C1 that is a threshold for switching the current measurement mode to the electromotive force measurement mode and the electromotive force measurement mode Into current measurement mode A second concentration threshold C2 that is a threshold for switching to are pre-recorded. The measurement mode switching unit 94 may continuously acquire the NOx concentration output by the concentration calculation unit 93, or may acquire it at a predetermined interval.

[0133] The measurement mode switching unit 94 acquires the NOx concentration output by the concentration calculation unit 93 in the current measurement mode. When it is determined that the NOx concentration is in a low concentration region lower than a predetermined first concentration threshold C1, the measurement mode is switched to the electromotive force measurement mode. When it is determined that the NOx concentration is equal to or higher than the predetermined first concentration threshold C1, the current measurement mode is maintained.

[0134] Also, even in the electromotive force measurement mode, the NOx concentration output by the concentration calculation unit 93 is acquired. When it is determined that the NOx concentration is in a high concentration region higher than a predetermined second concentration threshold C2, the measurement mode is switched to the current measurement mode. When it is determined that the NOx concentration is equal to or lower than the predetermined second concentration threshold C2, the electromotive force measurement mode is maintained.

[0135] The first concentration threshold C1 for switching the current measurement mode to the electromotive force measurement mode can be appropriately set by those skilled in the art. The first concentration threshold C1 may be a different value depending on the concentration range of NOx in the gas to be measured that is assumed and the measurement accuracy required for the gas sensor 100. The first concentration threshold C1 may be the lower limit value of the NOx concentration at which the desired measurement accuracy can be obtained when measured in the current measurement mode. The first concentration threshold C1 may be, for example, the lower limit value of the NOx concentration within the range where the value of the offset current Ip2offset is within an acceptable range with respect to the measurement accuracy. Alternatively, for example, it may be the lower limit value of the NOx concentration within the range where the value of the assumed offset current variation ΔIp2offset is within an acceptable range with respect to the measurement accuracy. The first concentration threshold C1 may be in the range of, for example, 50 ppm to 500 ppm. It may be, for example, 100 ppm.

[0136] Electromotive force measurement mode Into current measurement mode The second concentration threshold C2 for switching to the threshold is one that can be appropriately set by those skilled in the art. The second concentration threshold C2 may be a different value depending on the concentration range of NOx in the gas to be measured that is assumed and the measurement accuracy required for the gas sensor 100. The second concentration threshold C2 may be the upper limit value of the NOx concentration at which the desired measurement accuracy can be obtained when measured in the electromotive force measurement mode. The second concentration threshold C2 may be, for example, the upper limit value of the NOx concentration within the range where the change amount of the open electromotive force V2open with respect to the change amount of the NO concentration is within an acceptable range as the measurement resolution. The second concentration threshold C2 may be in the range of, for example, 50 ppm to 500 ppm. It may be, for example, 300 ppm.

[0137] The first concentration threshold C1 and the second concentration threshold C2 may be the same value or different values. When the first concentration threshold C1 and the second concentration threshold C2 are set to the same value, in the low concentration region lower than the first concentration threshold C1 (= the second concentration threshold C2), the electromotive force measurement mode is executed, and in the high concentration region equal to or higher than the first concentration threshold C1 (= the second concentration threshold C2), the current measurement mode is executed.

[0138] It is more preferable that the first concentration threshold C1 is set to a concentration lower than the second concentration threshold C2. That is, it is more preferable to use two thresholds with a concentration range.

[0139] FIG. 5 is a schematic diagram showing an example of the time change of the NOx concentration detection value output by the gas sensor 100 and the switching of the measurement mode. The horizontal axis represents time (seconds), and the vertical axis represents the NOx concentration detection value (ppm) output by the gas sensor 100.

[0140] As shown in FIG. 5, when the NOx concentration in the gas to be measured (that is, the NOx concentration output by the gas sensor 100) falls below the first concentration threshold C1, the measurement mode switches to the electromotive force measurement mode. After entering the electromotive force measurement mode, even if the NOx concentration in the gas to be measured fluctuates around the first concentration threshold C1 and exceeds the first concentration threshold C1, the electromotive force measurement mode is continued without switching to the current measurement mode until the NOx concentration in the gas to be measured exceeds the second concentration threshold C2. Thereafter, when the NOx concentration in the gas to be measured exceeds the second concentration threshold C2, the measurement mode switches to the current measurement mode. After entering the current measurement mode, even if the NOx concentration in the gas to be measured fluctuates around the second concentration threshold C2 and falls below the second concentration threshold C2, the current measurement mode is continued without switching to the electromotive force measurement mode until the NOx concentration in the gas to be measured falls below the first concentration threshold C1.

[0141] Thus, when the first concentration threshold C1 is set to a concentration lower than the second concentration threshold C2, the concentration range (intermediate concentration range) between the first concentration threshold C1 and the second concentration threshold C2 serves as a buffer region that maintains the immediately preceding measurement mode without switching the measurement mode. That is, when the NOx concentration in the gas to be measured frequently fluctuates near the first concentration threshold C1 or the second concentration threshold C2, it can be adjusted so that the switching of the measurement mode does not occur too frequently. When switching the measurement mode, since the control of the current measurement pump cell 41 is changed, the gas sensor 100 may temporarily enter a state where it cannot measure the NOx concentration. By providing a buffer region between the first concentration threshold C1 and the second concentration threshold C2, it can be adjusted so that the switching of the measurement mode does not occur too frequently. As a result, the gas sensor 100 can measure the NOx concentration more continuously and accurately.

[0142] For example, the first concentration threshold C1 may be set to 50 - 200 ppm, and the second concentration threshold C2 may be set to 200 - 500 ppm. For example, the first concentration threshold C1 may be set to 100 ppm, and the second concentration threshold C2 may be set to 300 ppm.

[0143] In the buffer region between the first concentration threshold C1 and the second concentration threshold C2, the measurement mode may be switched. For example, it may be switched based on the time change of the NOx concentration detection value (the slope of the graph in FIG. 5) by predicting the NOx concentration detection value. For example, it may be switched based on the time that the NOx concentration detection value is within the buffer region.

[0144] [Detection of Concentration of Gas to be Measured]

[0145] Next, a method for measuring the concentration of the gas to be measured in the gas to be measured using the gas sensor 100 will be described.

[0146] The control method of the gas sensor of the present embodiment is An electromotive force measurement mode for detecting the concentration of the gas to be measured in the gas to be measured based on the electromotive force value in the electromotive force detection sensor cell by operating the adjustment pump cell and not operating the current measurement pump cell, and an adjustment pump cell and the current measurement pump cell are operated to detect the concentration of the gas to be measured in the gas to be measured based on the current value in the current measurement pump cell. A concentration detection step is performed while switching using the switching unit. In the concentration detection step, one of the measurement modes is always executed, and the concentration can be continuously detected. In the present embodiment, the main pump cell 21 and the auxiliary pump cell 50 function as the adjustment pump cell.

[0147] The NOx concentration detection process of the gas sensor 100 of the present embodiment will be described in detail below. FIG. 6 is a flowchart showing an example of the NOx concentration detection process in the gas sensor 100.

[0148] The NOx concentration detection process is started, for example, when the gas sensor 100 receives a start signal (Dew point). The start signal (Dew point) is, for example, a signal sent from the ECU of the vehicle or an exhaust gas treatment system to the gas sensor 100 when the gas sensor 100 is mounted on a vehicle or the like. It may also be started by manually turning on the power of the control device 90.

[0149] When the NOx concentration detection process is started, the drive control unit 92 of the control unit 91 first energizes the heater 72 to start heating the heater 72 (step S10), and maintains the sensor element 101 at a drive temperature (for example, about 800° C.) at which the solid electrolyte is activated and the NOx concentration can be accurately measured.

[0150] Next, the drive control unit 92 starts controlling the main pump cell 21 (step S11) and starts controlling the auxiliary pump cell 50 (step S12). That is, for the main pump cell 21, the set value Ip1 SET and the set value V0 SETPerform feedback control based on the set value V1 for the auxiliary pump cell 50 SET Perform feedback control based on SET . Steps S11 and S12 may be performed either first or simultaneously. Also, steps S11 and S12 may be performed after the sensor element 101 reaches the driving temperature, or may be performed at a temperature lower than the driving temperature.

[0151] The gas to be measured passes through the gas inlet 10, the first diffusion rate-limiting section 11, the buffer space 12, and the second diffusion rate-limiting section 13 in this order, reaches the first internal cavity 20, and the oxygen concentration is adjusted by the action of the main pump cell 21. Then, it passes through the third diffusion rate-limiting section 30, reaches the second internal cavity 40, and the oxygen concentration is further adjusted by the action of the auxiliary pump cell 50. The gas to be measured whose oxygen concentration has been adjusted to a certain low concentration by the main pump cell 21 and the auxiliary pump cell 50 passes through the fourth diffusion rate-limiting section 60 and reaches the third internal cavity 61.

[0152] Next, the measurement mode switching section 94 of the control unit 91 switches to the current measurement mode (step S13). Specifically, the measurement mode switching section 94 outputs a control signal to turn on the switching unit 47 as an example of the switching unit. Also, the measurement mode switching section 94 gives an instruction to the drive control section 92 to perform control in the current measurement mode. In this case, the drive control section 92 performs feedback control on the pump voltage Vp2 of the variable power supply 46 of the current measurement pump cell 41, and detects the pump current Ip2 in the current measurement pump cell 41. The measurement mode switching section 94 acquires the pump current Ip2 in the current measurement pump cell 41 and gives an instruction to the concentration calculation section 93 to calculate the NOx concentration based on the current-concentration conversion parameter. In the current measurement mode, a pump current Ip2 corresponding to the NOx concentration as described above flows through the current measurement pump cell 41. Step S13 may be performed simultaneously with either or both of steps S11 and S12 described above.

[0153] Next, the concentration calculation unit 93 acquires the pump current Ip2 in the pump cell 41 for current measurement, and calculates the NOx concentration in the gas to be measured based on the conversion parameter (current-concentration conversion parameter) between the pump current Ip2 stored in advance and the NOx concentration in the gas to be measured (step S14). The calculated NOx concentration is output as the detection value of the gas sensor 100. After step S14, the measurement mode switching unit 94 acquires the NOx concentration calculated by the concentration calculation unit 93, and determines whether or not the acquired NOx concentration is lower than the first concentration threshold C1 (step S15). As the first concentration threshold C1, for example, the lower limit value of the NOx concentration at which a desired measurement accuracy can be obtained when measuring in the current measurement mode is set in advance.

[0154] In step S15, when the NOx concentration acquired from the concentration calculation unit 93 is equal to or higher than the first concentration threshold C1, the steps S After 14 are performed. That is, when the NOx concentration is equal to or higher than the first concentration threshold C1, the measurement mode switching unit 94 does not switch the measurement mode, and the drive control unit 92 and the concentration calculation unit 93 continue the current measurement mode.

[0155] In step S15, when the NOx concentration acquired from the concentration calculation unit 93 is lower than the first concentration threshold C1, the measurement mode switching unit 94 switches to the open electromotive force measurement mode (step S23). Specifically, the measurement mode switching unit 94 outputs a control signal to turn off the switching unit 47. Further, the measurement mode switching unit 94 sends to the drive control unit 92 AndAn instruction to perform control in the open-circuit voltage measurement mode is given. In this case, the drive control unit 92 does not perform feedback control on the pump voltage Vp2 of the variable power supply 46 of the current measurement pump cell 41 described above, but detects the open-circuit voltage V2open in the open-circuit voltage detection sensor cell 82. The measurement mode switching unit 94 gives an instruction to the concentration calculation unit 93 to acquire the open-circuit voltage V2open in the open-circuit voltage detection sensor cell 82 and calculate the NOx concentration based on the open-circuit voltage-concentration conversion parameter. In the open-circuit voltage measurement mode, no pump current Ip2 flows through the current measurement pump cell 41, and as described above, an open-circuit voltage V2open corresponding to the NOx concentration in the gas to be measured is generated in the open-circuit voltage detection sensor cell 82.

[0156] Next, the concentration calculation unit 93 acquires the open-circuit voltage V2open in the open-circuit voltage detection sensor cell 82, and based on the conversion parameter (open-circuit voltage-concentration conversion parameter) between the open-circuit voltage V2open stored in advance and the NOx concentration in the gas to be measured, calculates the NOx concentration in the gas to be measured (step S24). The calculated NOx concentration is output as the detection value of the gas sensor 100. After step S24, the measurement mode switching unit 94 acquires the NOx concentration calculated by the concentration calculation unit 93 and determines whether the acquired NOx concentration is higher than the second concentration threshold C2 (step S25). As the second concentration threshold C2, for example, the upper limit value of the NOx concentration at which a desired measurement accuracy can be obtained when measured in the open-circuit voltage measurement mode is set in advance.

[0157] In step S25, if the NOx concentration acquired from the concentration calculation unit 93 is the second Concentration threshold C2 or less, the steps S after step 24 are performed. That is, when the NOx concentration is the second Concentration threshold C2 or less, the measurement mode switching unit 94 does not switch the measurement mode, and the drive control unit 92 and the concentration calculation unit 93 continue the open-circuit voltage measurement mode.

[0158] In step S25, if the NOx concentration acquired from the concentration calculation unit 93 is the second ConcentrationWhen the value is higher than the threshold C2, the measurement mode switching unit 94 switches to the current measurement mode (step S13), and the processes after step S14 are performed.

[0159] In this way, the control unit 91 determines, in the measurement mode switching unit 94, which of the current measurement mode and the electromotive force measurement mode to use based on the NOx concentration acquired from the concentration calculation unit 93, and as a result, detects the NOx concentration using one of the measurement modes. By properly using the electromotive force measurement mode capable of more accurately measuring the measured gas containing low-concentration NOx and the current measurement mode capable of more accurately measuring the measured gas containing high-concentration NOx, the NOx concentration can be measured more accurately in a wide concentration range including low concentrations.

[0160] In step S13, when the measurement mode switching unit 94 switches from the open electromotive force measurement mode to the current measurement mode, the pump current Ip2 is passed through the current measurement pump cell 41 so that the electromotive force V2 detected by the electromotive force detection sensor cell 82 becomes the set value V2 SET and the atmosphere near the measurement electrode 44 is controlled to a state where all the oxygen derived from NOx is pumped out. After the measurement of the NOx concentration in the current measurement mode (step S14) is performed at least once in that state, the switching to the electromotive force measurement mode (step S 23 ) is performed. Also, in step S23, when the measurement mode switching unit 94 switches from the current measurement mode to the open electromotive force measurement mode, no current is passed through the current measurement pump cell 41, and the atmosphere near the measurement electrode 44 is controlled to a state where oxygen derived from NOx is present. After the measurement of the NOx concentration in the electromotive force measurement mode (step S24) is performed at least once in that state, the switching to the current measurement mode (step S13) is performed.

[0161] As described above, when the measurement mode is switched, after the atmosphere near the measurement electrode 44 is controlled so that the NOx concentration can be measured, the determination of the next measurement mode switch is made. The measurement mode is usually switched at intervals of 1 second or more. Switching on and off the pump current Ip2 in a very short time so that the switching of the atmosphere near the measurement electrode 44 described above cannot be realized, that is, on-off control by a so-called pulse current is not intended.

[0162] In step S13, immediately after the measurement mode switching unit 94 switches from the open-circuit voltage measurement mode to the current measurement mode, since the control of the current measurement pump cell 41 is changed, the pump current Ip2 may not stabilize. Therefore, the concentration calculation unit 93 may perform step S14 after a predetermined standby time has elapsed. Also, in step S23, immediately after the measurement mode switching unit 94 switches from the current measurement mode to the open-circuit voltage measurement mode, since the control of the current measurement pump cell 41 is changed, the open-circuit voltage V2open may not stabilize. Therefore, the concentration calculation unit 93 may perform step S24 after a predetermined standby time has elapsed.

[0163] In the above, as an example of the embodiment of the present invention, the gas sensor 100 for detecting the NOx concentration in the gas to be measured has been shown, but the present invention is not limited to this form. The present invention can include gas sensors including various forms of sensor elements and configurations of control devices as long as the object of the present invention, which is to accurately measure a measurement target gas in a wide concentration range including a low-concentration measurement target gas, is achieved.

[0164] In the above-described embodiment, as an example of the switching unit 47, a switch is provided, but the present invention is not limited to this form. For example, the variable power supply 46 can also be used as the switching unit 47. The measurement mode switching unit 94 switches so as not to allow current to flow by setting the pump voltage Vp2 of the variable power supply 46 to zero and not applying a voltage to the current measurement pump cell 41 when switching to the electromotive force measurement mode, and switches so as to allow current to flow by setting the pump voltage Vp2 of the variable power supply 46 to a predetermined value and applying a predetermined voltage to the current measurement pump cell 41 when switching to the current measurement mode. In the current measurement mode, similar to the case of the above-described embodiment, the pump voltage Vp2 of the variable power supply 46 in the current measurement pump cell 41 may be feedback-controlled so that the electromotive force V2 as the control voltage detected by the electromotive force detection sensor cell 82 becomes the set value V2. SET

[0165] In the above-described embodiment, after step S12, the measurement mode switching unit 94 first switches to the current measurement mode in step S13, but after step S12, the switching to the electromotive force measurement mode may be first performed in S23.

[0166] Alternatively, at the start of the gas sensor 100, the control device 90 may be preset to be in the current measurement mode state, or may be preset to be in the electromotive force measurement mode state.

[0167] At the start of the gas sensor 100, the inside of the third internal cavity 61 is filled with the gas to be measured, and the oxygen concentration in the gas to be measured inside the third internal cavity 61 is often higher than that in the state where the drive control is constantly performed. When in the current measurement mode at startup, in addition to the main pump cell 21 and the auxiliary pump cell 50, the current measurement pump cell 41 is also operated. As a result, the oxygen in the gas to be measured inside the third internal cavity 61 can be actively pumped out, and the state of the drive control can be reached stably earlier. That is, the time (startup time) from when the gas sensor 100 starts up until the NOx concentration can be measured can be shortened.

[0168] In the above embodiment, the measurement mode switching unit 94 switches between the electromotive force measurement mode and the current measurement mode based on the NOx concentration calculated by the concentration calculation unit 93, but it is not limited thereto.

[0169] As a threshold for switching from the current measurement mode to the electromotive force measurement mode, instead of the first concentration threshold C1, those skilled in the art may appropriately set the lower limit value of the pump current Ip2 at which a desired measurement accuracy can be obtained when measured in the current measurement mode. Electromotive force measurement mode Into current measurement mode As a threshold for switching to, instead of the second concentration threshold C2, for example, the open electromotive force Force V The upper limit value of 2open at which a desired measurement accuracy can be obtained when measured in the electromotive force measurement mode may be appropriately set by those skilled in the art.

[0170] The switching between the electromotive force measurement mode and the current measurement mode may be performed, for example, based on a signal from another device such as an automobile ECU or an exhaust gas treatment device.

[0171] In the gas sensor 100 of the above embodiment, the current measurement pump cell 41 and the electromotive force detection sensor cell 82 are configured as separate electrochemical cells, but it is not limited thereto. For example, the current measurement pump cell may be configured as a pump cell between the measurement electrode 44 and the reference electrode 42. That is, the reference electrode 42 may function as an outer measurement electrode in the current measurement pump cell. Referring to the sensor element 101 of FIG. 1, the current measurement pump cell may be composed of the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42. The reference electrode 42 is formed inside the substrate portion 102 However, if the measurement electrode 44 disposed on the inner surface of the measured gas flow portion 15 is used as a reference, the reference electrode 42 is disposed at a position different from the measured gas flow portion 15 and can be used as an outer measurement electrode. In this case as well, the control device is provided with a switching unit that switches whether to pass current through the current measurement pump cell or not.

[0172] In this case, in the electromotive force measurement mode, the switching unit is switched so that no current flows through the current measurement pump cell, and the open electromotive force V2open generated between the measurement electrode 44 and the reference electrode 42 is detected. Then, the NOx concentration is detected based on this open electromotive force V2open. In the current measurement mode, the switching unit is switched so that current flows through the current measurement pump cell, and a constant pump current is applied between the measurement electrode 44 and the reference electrode 42. Based on this pump current, the NOx concentration is detected. Pressure By applying, the pump current Flow flows. This pump current Into flow Based on it, the NOx concentration is detected.

[0173] In the above-described embodiment, the gas sensor 100 detects the NOx concentration in the gas to be measured, but the gas to be measured is not limited to NOx. For example, the gas to be measured may be another oxide gas other than NOx (for example, carbon dioxide CO2, water H2O, etc.). When the gas to be measured is an oxide gas, in the current measurement mode, similar to the embodiment of detecting the NOx concentration described above, the gas to be measured including the oxide gas itself is introduced into the third internal cavity 61, and the oxide gas in the gas to be measured is reduced at the measurement electrode 44 to generate oxygen. The generated oxygen is detected as the pump current Ip2 in the current measurement pump cell 41.

[0174] When carbon dioxide CO2 or water H2O is reduced, a reducing gas such as carbon monoxide CO or hydrogen H2 and oxygen O2 are generated respectively. In the electromotive force measurement mode, the set value V0 SET used for controlling the main pump cell 21, and the set value V1 SETBy setting it to a value larger than that in the case of NOx, while controlling the oxygen concentration in the gas to be measured to a lower concentration, carbon dioxide (CO2) and water (H2O) are reduced (decomposed) at at least one of the inner main pump electrode 22 and the auxiliary pump electrode 51. As a result, the reducing gas generated by the reduction of carbon dioxide (CO2) and water (H2O), and the oxygen (residual oxygen) after the concentration is adjusted by the main pump cell 21 and the auxiliary pump cell 50 reach the measurement electrode 44. At this time, by controlling the residual oxygen concentration in the gas to be measured so that the ratio of the reducing gas (rich gas) to the residual oxygen is near the stoichiometric point, the concentration of the reducing gas (rich gas) caused by carbon dioxide (CO2) and water (H2O) can be detected as the open-circuit voltage V2open in the electromotive force detection sensor cell 82. In the region near the stoichiometric point, compared with other regions, the open-circuit voltage V2open tends to change more greatly with respect to a minute change in the rich gas concentration. Therefore, it is possible to detect the concentration of the low-concentration reducing gas (rich gas) caused by low-concentration carbon dioxide (CO2) and water (H2O).

[0175] In the embodiment for detecting the above-described NOx concentration, for example, the set value V0 used for controlling the main pump cell 21 SET may be set to about 150 mV to 450 mV, and the set value V1 used for controlling the auxiliary pump cell 50 SET may be set to about 150 mV to 450 mV. On the other hand, when measuring carbon dioxide (CO2) and water (H2O), for example, the set value V0 used for controlling the main pump cell 21 SET may be set to about 450 mV to 1000 mV, and the set value V1 used for controlling the auxiliary pump cell 50 SET may be set to about 450 mV to 1000 mV. Thus, according to the type of the gas to be measured, the set value V0 used for controlling the main pump cell 21 SET and the set value V1 used for controlling the auxiliary pump cell 50 SET may be appropriately set. The set value Ip1 used for controlling the auxiliary pump cell 50 SET , and the set value V2 used for controlling the current measurement pump cell 41 in the current measurement mode SETIt may be appropriately set according to the type of the gas to be measured as well.

[0176] Also, for example, the gas to be measured may be a non-oxide gas such as ammonia NH3. When the gas to be measured is a non-oxide gas, the non-oxide gas is converted into an oxide gas (for example, in the case of ammonia NH3, it is converted into NO), and the gas to be measured containing the converted oxide gas is introduced into the third internal cavity 61. In the measurement electrode 44, the converted oxide gas in the gas to be measured is reduced to generate oxygen. The generated oxygen is detected as the open-circuit voltage V2open in the open-circuit voltage detection sensor cell 82 in the open-circuit voltage measurement mode, and is detected as the pump current Ip2 in the pump cell 41 for current measurement in the current measurement mode. The conversion of the non-oxide gas into the oxide gas can be performed by at least one of the inner main pump electrode 22 and the auxiliary pump electrode 51 functioning as a catalyst.

[0177] In the above-described embodiment, the drive control unit 92 of the control unit 91 performs feedback control to set the set value V0 of the open-circuit voltage V0 in the main pump control oxygen partial pressure detection sensor cell 80 based on the pump current Ip1 so that the pump current Ip1 in the auxiliary pump cell 50 becomes the set value Ip1 SET and performs feedback control on the pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 so that the open-circuit voltage V0 becomes the set value V0 SET , but the control method is not limited to this. For example, the drive control unit 92 may perform feedback control on the pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 so that the pump current Ip1 in the auxiliary pump cell 50 becomes the set value Ip1 SET . That is, the drive control unit 92 may directly perform feedback control on the pump voltage Vp0 based on the pump current Ip1 without obtaining the open-circuit voltage V0 in the main pump control oxygen partial pressure detection sensor cell 80 and setting the set value V0 SET . SET

[0178] In the gas sensor 100 of the above-described embodiment, as shown in FIG. 1, the sensor element 101 includes three internal cavities, i.e., a first internal cavity 20, a second internal cavity 40, and a third internal cavity 61, and an inner main pump electrode 22, an auxiliary pump electrode 51, and a measurement electrode 44 are respectively disposed in each internal cavity. However, the present invention is not limited to this structure. For example, the gas sensor 100 may include two internal cavities, i.e., a first internal cavity 20 and a second internal cavity 40, and the inner main pump electrode 22 may be disposed in the first internal cavity 20, and the auxiliary pump electrode 51 and the measurement electrode 44 may be respectively disposed in the second internal cavity 40. In this case, for example, a porous protective layer covering the measurement electrode 44 may be formed as a diffusion rate-limiting portion between the auxiliary pump electrode 51 and the measurement electrode 44.

[0179] In the gas sensor 100 of the above-described embodiment, the outer pump electrode 23 has the functions of three electrodes, i.e., an outer main pump electrode in the main pump cell 21, an outer auxiliary pump electrode in the auxiliary pump cell 50, and an outer measurement electrode in the current measurement pump cell 41. However, the present invention is not limited to this structure. For example, the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be formed as separate electrodes. For example, one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may be provided on the outer surface of the substrate portion 102 so as to be in contact with the gas to be measured, separately from the outer pump electrode 23. Alternatively, one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measurement electrode may also serve as a reference electrode. 42 may also serve as.

[0180] As described above, according to the present invention, since measurement can be performed while switching between an electromotive force measurement mode with higher measurement accuracy at low concentrations and a current measurement mode with higher measurement accuracy at high concentrations, it is possible to accurately measure a gas to be measured in a wide concentration range (for example, 10 to 5000 ppm) including a gas to be measured at low concentrations. Here, in the present invention, the upper limit value of the low concentration is intended to be less than 500 ppm. The gas to be measured includes nitrogen atom-containing gases such as nitrogen oxides NOx and ammonia NH3, and carbon dioxide CO2, water H2O, etc. that generate reducing gases by decomposition.

Description of Reference Numerals

[0181] 1 First substrate layer 2 Second substrate layer 3 Third substrate layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion rate-limiting part 12 Buffer space 13 Second diffusion rate-limiting part 15 Measured gas flow part 20 First internal cavity 21 Main pump cell 22 Inner main pump electrode 22a Ceiling electrode part (of the inner main pump electrode) 22b Bottom electrode part (of the inner main pump electrode) 23 Outer pump electrode 24 Variable power supply (of the main pump cell) 30 Third diffusion rate-limiting part 40 Second internal cavity 41 Pump cell for current measurement 42 Reference electrode 43 Reference gas introduction space 44 Measurement electrode 46 Variable power supply (of the measurement pump cell) 47 Switching unit 48 Atmosphere introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a Ceiling electrode part (of the auxiliary pump electrode) 51b Bottom electrode part (of the auxiliary pump electrode) 52 Variable power supply (of the auxiliary pump cell) 60 Fourth diffusion rate-limiting part 61 Third internal cavity 70 Heater part 71 Heater electrode 72 Heater 73 Through hole 74 Heater insulator 75 Pressure release hole 76 Heater lead 77 Heater power supply 80 Oxygen partial pressure detection sensor cell for main pump control 81 Oxygen partial pressure detection sensor cell for auxiliary pump control 82 Electromotive force detection sensor cell 83 Sensor cell 90 Control device 91 Control unit 92 Drive control unit 93 Concentration calculation unit 94 Measurement mode switching unit 100 Gas sensor 101 Sensor element 102 Substrate part

Claims

1. A gas sensor including a sensor element and a control device for controlling the sensor element, The sensor element includes: A long plate-shaped substrate portion including an oxygen ion-conductive solid electrolyte layer; A gas to be measured flow portion formed from one end in the longitudinal direction of the substrate portion; An inner pump electrode disposed on the inner surface of the gas to be measured flow portion, and an outer pump electrode disposed at a position different from the gas to be measured flow portion of the substrate portion and corresponding to the inner pump electrode, and an adjustment pump cell for adjusting oxygen in the gas to be measured to a desired concentration; An inner measurement electrode disposed at a position farther from the one end in the longitudinal direction of the substrate portion than the inner pump electrode on the inner surface of the gas to be measured flow portion, and an outer measurement electrode disposed at a position different from the gas to be measured flow portion of the substrate portion and corresponding to the inner measurement electrode, and a current measurement pump cell for detecting a gas to be measured in the gas to be measured as a current value; A reference electrode disposed inside the substrate portion so as to be in contact with a reference gas; An electromotive force detection sensor cell including the inner measurement electrode and the reference electrode, and detecting an electromotive force value between the inner measurement electrode and the reference electrode; and includes The control device is a gas sensor for detecting a gas to be measured in a gas to be measured, including a switching unit for switching whether to pass a current through the current measurement pump cell or not.

2. The control device includes a measurement mode switching unit that switches between an electromotive force measurement mode for detecting the concentration of the gas to be measured in the gas to be measured based on the electromotive force value in the electromotive force detection sensor cell and a current measurement mode for detecting the concentration of the gas to be measured in the gas to be measured based on the current value in the current measurement pump cell. The measurement mode switching unit switches the switching unit so that no current flows through the current measurement pump cell when switching to the electromotive force measurement mode, and switches the switching unit so that current flows through the current measurement pump cell when switching to the current measurement mode. The gas sensor according to claim 1.

3. The switching unit includes a switch that switches whether to cut off the conduction of the current measurement pump cell. The gas sensor according to claim 2.

4. The measurement mode switching unit switches the switch to OFF to cut off the conduction of the current measurement pump cell so that no current flows when switching to the electromotive force measurement mode, and switches the switch to ON to conduct the current measurement pump cell so that current flows when switching to the current measurement mode. The gas sensor according to claim 3.

5. The switching unit includes a variable power source that changes the voltage applied to the current measurement pump cell. The gas sensor according to claim 2.

6. The measurement mode switching unit switches to not apply voltage to the current measurement pump cell by setting the voltage in the variable power source to zero so that no current flows when switching to the electromotive force measurement mode, and switches to apply a predetermined voltage to the current measurement pump cell by setting the voltage in the variable power source to a predetermined value so that current flows when switching to the current measurement mode. The gas sensor according to claim 5.

7. When the measurement mode switching unit determines that the concentration of the measurement target gas detected in the current measurement mode is lower than a predetermined first concentration threshold C1, it switches to the electromotive force measurement mode. When it is determined that the concentration of the measurement target gas detected in the electromotive force measurement mode is higher than a predetermined second concentration threshold C2, the gas sensor according to any one of claims 2 to 6 switches to the current measurement mode.

8. The gas sensor according to claim 7, wherein the first concentration threshold value C1 is a concentration lower than the second concentration threshold value C2.

9. The gas sensor according to any one of claims 2 to 8, wherein in the current measurement mode, the current value in the current measurement pump cell is controlled such that the electromotive force value between the inner measurement electrode and the reference electrode in the electromotive force detection sensor cell becomes a predetermined value.

10. The gas sensor according to any one of claims 1 to 8, wherein the reference electrode functions as the outer measurement electrode.

11. A method for controlling a gas sensor for detecting a measurement target gas in a gas to be measured, The gas sensor includes a sensor element and a control device for controlling the sensor element, The sensor element includes a long plate-shaped substrate portion including an oxygen ion-conductive solid electrolyte layer, a gas flow portion to be measured formed from one end in the longitudinal direction of the substrate portion, an inner pump electrode disposed on the inner surface of the gas flow portion to be measured, and an outer pump electrode corresponding to the inner pump electrode and disposed at a position different from the gas flow portion to be measured of the substrate portion, the adjustment pump cell for adjusting oxygen in the gas to be measured to a desired concentration, an inner measurement electrode disposed at a position farther from the one end in the longitudinal direction of the substrate portion than the inner pump electrode on the inner surface of the gas flow portion to be measured, and an outer measurement electrode corresponding to the inner measurement electrode and disposed at a position different from the gas flow portion to be measured of the substrate portion, the current measurement pump cell for detecting the measurement target gas in the gas to be measured as a current value, a reference electrode disposed inside the substrate portion so as to be in contact with a reference gas, an electromotive force detection sensor cell including the inner measurement electrode and the reference electrode and detecting an electromotive force value between the inner measurement electrode and the reference electrode, and includes The control device It includes a switching unit for switching whether to pass current through the current measurement pump cell or not. The control method is as follows. An electromotive force measurement mode in which the adjustment pump cell is operated and the current measurement pump cell is not operated, and the concentration of the gas to be measured in the measured gas is detected based on the electromotive force value in the electromotive force detection sensor cell, and an adjustment pump cell and the current measurement pump cell are operated, and the concentration of the gas to be measured in the measured gas is detected based on the current value in the current measurement pump cell. A concentration detection step of switching between the current measurement modes using the switching unit is included. A control method for a gas sensor.

12. In the concentration detection step, When it is determined that the detected concentration of the gas to be measured is lower than a predetermined first concentration threshold C1, the switching unit is switched so as not to pass current through the current measurement pump cell, and the switching to the electromotive force measurement mode is performed. When it is determined that the detected concentration of the gas to be measured is higher than a predetermined second concentration threshold C2, the switching unit is switched so as to pass current through the current measurement pump cell, and the switching to the current measurement mode is performed. The control method according to claim 11.

13. The control method according to claim 12, wherein the first concentration threshold C1 is a concentration lower than the second concentration threshold C2.

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