Gas sensor and control method for a gas sensor

Abstract

The present application is to accurately measure a target gas even at a low concentration. The present application relates to a gas sensor (100) that detects a target gas in a measured gas and includes a sensor element (101) and a control device (90), the sensor element (101) including: a base portion (101); a measured gas flow portion (15); a pump unit (21) for adjusting oxygen in the measured gas to a desired concentration, the pump unit (21) including an inner pump electrode (22) and an outer pump electrode (23); a pump unit (41) for measuring current, the pump unit (41) including an inner measurement electrode (44) and an outer measurement electrode (23), the pump unit (41) detecting the target gas in the measured gas as a current value; a reference electrode (42) disposed to contact a reference gas; and an electromotive force detection sensor unit (82) including the inner measurement electrode (44) and the reference electrode (42), the electromotive force detection sensor unit (82) detecting an electromotive force value between the inner measurement electrode (44) and the reference electrode (42), the control device (90) including a switching unit (95) that switches the current to flow or not to flow through the pump unit (41) for measuring current.

Description

Technical Field

[0001] This invention relates to gas sensors and control methods for gas sensors. Background Technology

[0002] Gas sensors are used to detect and determine the concentration of target gas components (oxygen O2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, carbon dioxide CO2, etc.) in gases such as automobile exhaust. For example, the concentration of the target gas component in automobile exhaust can be measured, and the exhaust gas purification system installed in the vehicle can be optimized based on the measured value.

[0003] As such gas sensors, gas sensors utilizing solid electrolytes with oxygen ion conductivity, such as zirconium dioxide (ZrO2), are 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 means of a limiting current.

[0004] Furthermore, Japanese Patent No. 5323752 discloses a NOx sensor comprising: a main pump unit and an auxiliary pump unit for adjusting oxygen concentration, and a measuring pump unit including a measuring electrode for detecting NOx. Regarding this NOx sensor, firstly, the oxygen partial pressure in the gas to be measured is controlled to a low partial pressure that substantially has no effect on the NOx measurement using the main pump unit and the auxiliary pump unit. NOx in the gas to be measured, with its oxygen partial pressure controlled, is reduced at the measuring electrode, resulting in the production of oxygen. This oxygen is then drawn out using the measuring pump unit and detected as a current value.

[0005] Japanese Patent Application Publication Nos. 2021-162580 and 2021-162581 disclose a gas sensor having two measuring pump units. The disclosure describes a method for detecting specific gas concentrations over a wide concentration range (e.g., 500 ppm to 10,000 ppm) by switching between using one of the two measuring pump units based on the concentration of a specific gas in the gas being measured.

[0006] Patent Document 1: Japanese Patent No. 5323752

[0007] Patent Document 2: Japanese Patent Application Publication No. 2021-162580

[0008] Patent Document 3: Japanese Patent Application Publication No. 2021-162581 Summary of the Invention

[0009] With stricter regulations on automobile exhaust emissions, there is a growing demand for gas sensors capable of accurately measuring even low concentrations of the target gas. Here, "low concentration" means, for example, concentrations of less than 500 ppm, less than 400 ppm, less than 300 ppm, less than 200 ppm, or less than 100 ppm.

[0010] Regarding existing gas sensors using the limiting current method, such as those described in Japanese Patent No. 5323752, the oxygen produced by reducing the target gas (e.g., NOx) is detected as a current value in the measuring pump unit. Therefore, this current value is detected as a value corresponding to the concentration of the target gas.

[0011] However, even when the target gas is absent from the gas being measured, the current value will not become zero; a tiny current will flow in the measuring pump unit. This tiny current is called the bias current. The bias current is not generated based on the concentration of the target gas. Therefore, when the bias current changes due to some factor, the current value detected in the measuring pump unit changes accordingly to the amount of the bias current change, not based on the concentration of the target gas. When measuring low concentrations of the target gas, the current value detected in the measuring pump unit based on the target gas concentration is relatively small. Therefore, the change in current value caused by the change in bias current is relatively large, showing a trend of increasing impact on measurement accuracy.

[0012] Therefore, the object of the present invention is to enable high-precision measurement of target gases even at low concentrations. That is, the object is to enable high-precision measurement of target gases covering a wide concentration range, including low-concentration target gases.

[0013] The inventors of this invention conducted in-depth research and discovered that: because the gas sensor has a switching unit that can be switched to allow current to flow through or not flow through the current measuring pump unit, the influence of bias current can be eliminated and the low concentration of the target gas can be measured with high accuracy.

[0014] This invention includes the following inventions.

[0015] (1) A gas sensor for detecting a target gas in a gas to be measured, comprising: a sensor element and a control device for controlling the sensor element.

[0016] The sensor element includes:

[0017] The elongated, plate-shaped base portion includes a solid electrolyte layer that is oxygen ion conductive;

[0018] The gas flow section to be measured is formed from one end of the base portion along its length.

[0019] The adjustment pump unit includes an inner pump electrode and an outer pump electrode, and adjusts the oxygen in the gas to be measured to a desired concentration. The inner pump electrode is disposed on the inner surface of the gas flow section, and the outer pump electrode is disposed at a different position on the base from the gas flow section and corresponds to the inner pump electrode.

[0020] A pump unit for current measurement includes an inner measuring electrode and an outer measuring electrode, and detects the target gas in the gas to be measured as the current value. The inner measuring electrode is disposed at one end of the inner surface of the gas flow section, which is further away from the base in the length direction than the inner pump electrode. The outer measuring electrode is disposed at a different position in the base from the gas flow section and corresponds to the inner measuring electrode.

[0021] A reference electrode, which is arranged inside the substrate to contact a reference gas; and

[0022] An electromotive force detection sensor unit includes an inner measuring electrode and a reference electrode, and detects the electromotive force value between the inner measuring electrode and the reference electrode.

[0023] The control device includes a switching unit that can switch between allowing current to flow through or not flow through the current measuring pump unit.

[0024] Regarding the electromotive force detection sensor unit, the reference electrode corresponds to the inner measuring electrode.

[0025] (2) Based on the gas sensor described in (1) above, the control device includes a measurement mode switching unit that switches between an electromotive force measurement mode and a current measurement mode. In the electromotive force measurement mode, the concentration of the target gas in the measured gas is detected based on the electromotive force value of the electromotive force detection sensor unit. In the current measurement mode, the concentration of the target gas in the measured gas is detected based on the current value of the current measurement pump unit.

[0026] During the switching process to the electromotive force measurement mode, the measurement mode switching unit switches the switching unit to a state where current does not flow in the current measurement pump unit, and during the switching process to the current measurement mode, the switching unit switches the switching unit to a state where current flows in the current measurement pump unit.

[0027] (3) Based on the gas sensor described in (1) or (2) above, the switching unit includes a switch for switching whether to cut off the conduction of the current measuring pump unit.

[0028] (4) Based on the gas sensor described in (3) above, the measurement mode switching unit disconnects the switch and cuts off the conduction of the current measuring pump unit during the process of switching to the electromotive force measurement mode, thereby switching to a state where the current does not flow. During the process of switching to the current measuring mode, the switch is turned on and the current measuring pump unit is turned on, thereby switching to a state where the current flows.

[0029] (5) Based on the gas sensor described in (1) or (2) above, the switching unit includes a variable power supply that changes the voltage applied to the current measuring pump unit.

[0030] (6) Based on the gas sensor described in (5) above, the measurement mode switching unit makes the voltage of the variable power supply zero and does not apply voltage to the current measuring pump unit during the process of switching to the electromotive force measurement mode, thereby switching to a state where current does not flow. During the process of switching to the current measuring mode, the voltage of the variable power supply is set to a predetermined value and a predetermined voltage is applied to the current measuring pump unit, thereby switching to a state where current flows.

[0031] (7) Based on the gas sensor described in any of (2) to (6) above, if it is determined that the concentration of the gas to be measured detected in the current measurement mode is lower than a predetermined first concentration threshold C1, the measurement mode switching unit performs: switching to the electromotive force measurement mode.

[0032] If it is determined that the concentration of the gas to be measured in the electromotive force measurement mode is higher than the predetermined second concentration threshold C2, the measurement mode switching unit switches to the current measurement mode.

[0033] That is, in the low concentration region where the concentration of the target gas in the gas being measured is lower than the first concentration threshold C1, the concentration of the target gas is detected using the electromotive force measurement mode, which has higher measurement accuracy in the low concentration region; and in the high concentration region where the concentration of the target gas in the gas being measured is higher than the second concentration threshold C2, the concentration of the target gas is detected using the current measurement mode, which has higher measurement accuracy in the high concentration region.

[0034] (8) Based on the gas sensor described in (7) above, the first concentration threshold C1 is: a concentration lower than the second concentration threshold C2.

[0035] (9) Based on the gas sensor described in any of (2) to (8) above, in the current measurement mode, the current value of the current measurement pump unit is controlled so that the electromotive force between the measuring electrode and the reference electrode of the electromotive force detection sensor unit reaches a specified value.

[0036] (10) Based on the gas sensor described in any of (1) to (8) above, the reference electrode functions as the outer measuring electrode.

[0037] (11) A control method for a gas sensor, the gas sensor being used to detect the target gas in a gas to be measured.

[0038] The gas sensor includes: a sensor element, and a control device for controlling the sensor element.

[0039] The sensor element includes:

[0040] The elongated, plate-shaped base portion includes a solid electrolyte layer that is oxygen ion conductive;

[0041] The gas flow section to be measured is formed from one end of the base portion along its length.

[0042] The adjustment pump unit includes an inner pump electrode and an outer pump electrode, and adjusts the oxygen in the gas to be measured to a desired concentration. The inner pump electrode is disposed on the inner surface of the gas flow section, and the outer pump electrode is disposed at a different position on the base from the gas flow section and corresponds to the inner pump electrode.

[0043] A pump unit for current measurement includes an inner measuring electrode and an outer measuring electrode, and detects the target gas in the gas to be measured as the current value. The inner measuring electrode is disposed at one end of the inner surface of the gas flow section, which is further away from the base in the length direction than the inner pump electrode. The outer measuring electrode is disposed at a different position in the base from the gas flow section and corresponds to the inner measuring electrode.

[0044] A reference electrode, which is arranged inside the substrate to contact a reference gas; and

[0045] An electromotive force detection sensor unit includes an inner measuring electrode and a reference electrode, and detects the electromotive force value between the inner measuring electrode and the reference electrode.

[0046] The control device includes a switching unit capable of switching between allowing current to flow through or not flow through the current-measuring pump unit.

[0047] The control method includes a concentration detection step of switching between an electromotive force measurement mode and a current measurement mode using the switching unit and performing concentration detection. In the electromotive force measurement mode, the adjustment pump unit is activated while the current measurement pump unit is not activated, and the concentration of the target gas in the measured gas is detected based on the electromotive force value of the electromotive force detection sensor unit. In the current measurement mode, both the adjustment pump unit and the current measurement pump unit are activated, and the concentration of the target gas in the measured gas is detected based on the current value of the current measurement pump unit.

[0048] (12) Based on the control method described in (11) above, in the concentration detection step, if it is determined that the concentration of the detected target gas is lower than a predetermined first concentration threshold C1, the switching unit is switched to the electromotive force measurement mode such that the current does not flow in the current measurement pump unit.

[0049] If it is determined that the concentration of the gas to be measured is higher than the predetermined second concentration threshold C2, the switching unit is switched to switch to the current measurement mode by allowing current to flow through the current measurement pump unit.

[0050] (13) Based on the control method described in (12) above, the first concentration threshold C1 is: a concentration lower than the second concentration threshold C2.

[0051] Invention Effects

[0052] According to the present invention, even low concentrations of the target gas can be measured with high precision. That is, according to the present invention, a wide range of concentrations of the target gas, including low concentrations, can be measured with high precision. Attached Figure Description

[0053] Figure 1 This is a schematic diagram of a vertical cross-section along the length of an example of the general structure of a gas sensor 100.

[0054] Figure 2 This is a block diagram showing the electrical connections between the control device 90 and the pump units 21, 50, 41, the sensor units 80, 81, 82, 83, and the heater section 70 of the sensor element 101.

[0055] Figure 3This is a schematic diagram illustrating an example of the relationship between NOx concentration and pump current Ip2 of gas sensor 100. The horizontal axis represents NOx concentration (ppm), and the vertical axis represents the value of pump current Ip2 (μA).

[0056] Figure 4 This is a schematic diagram illustrating an example of the relationship between NOx concentration and open-circuit electromotive force V2open for a gas sensor 100. The horizontal axis represents the NOx concentration (ppm), and the vertical axis represents the value of the electromotive force V2 (mV).

[0057] Figure 5 This is a schematic diagram illustrating an example of the time variation 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.

[0058] Figure 6 This is a flowchart illustrating an example of the NOx concentration detection process of the gas sensor 100.

[0059] Explanation of reference numerals in the attached figures

[0060] 1…First substrate layer; 2…Second substrate layer; 3…Third substrate layer; 4…First solid electrolyte layer; 5…Isolation layer; 6…Second solid electrolyte layer; 10…Gas inlet; 11…First diffusion rate control unit; 12…Buffer space; 13…Second diffusion rate control unit; 15…Measured gas flow section; 20…First internal cavity; 21…Main pump unit; 22…Inner main pump electrode; 22a…Top electrode section (of the inner main pump electrode); 22b…Bottom electrode section (of the inner main pump electrode); 23…Outer pump electrode; 24…Variable power supply (of the main pump unit); 30…Third diffusion rate control unit; 40…Second internal cavity; 41…Pump unit for current measurement; 42…Reference electrode; 43…Reference gas inlet space; 44…Measurement electrode; 46…Variable power supply (of the measurement pump unit); 47…Switching unit; 48…Atmosphere inlet layer; 50…Auxiliary pump unit; 51…Auxiliary pump electrode; 51a…(Top electrode part of auxiliary pump electrode); 51b…(Bottom electrode part of auxiliary pump electrode); 52…(Variable power supply of auxiliary pump unit); 60…Fourth diffusion rate control unit; 61…Third internal cavity; 70…Heater part; 71…Heater electrode; 72…Heater; 73…Through hole; 74…Heater insulation layer; 75…Pressure relief hole; 76…Heater conductor; 77…Heater power supply; 80…Oxygen partial pressure detection sensor unit for main pump control; 81…Oxygen partial pressure detection sensor unit for auxiliary pump control; 82…Electromotive force detection sensor unit; 83…Sensor unit; 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. Detailed Implementation

[0061] The gas sensor of the present invention includes: a sensor element and a control device for controlling the sensor element.

[0062] The gas sensor of the present invention includes a sensor element having:

[0063] The elongated, plate-shaped base portion includes a solid electrolyte layer that is oxygen ion conductive;

[0064] The gas flow section to be measured is formed from one end of the base portion along its length.

[0065] The adjustment pump unit includes an inner pump electrode and an outer pump electrode, and adjusts the oxygen in the gas to be measured to a desired concentration. The inner pump electrode is disposed on the inner surface of the gas flow section, and the outer pump electrode is disposed at a different position on the base from the gas flow section and corresponds to the inner pump electrode.

[0066] A pump unit for current measurement includes an inner measuring electrode and an outer measuring electrode, and detects the target gas in the gas to be measured as the current value. The inner measuring electrode is disposed at one end of the inner surface of the gas flow section, which is further away from the base in the length direction than the inner pump electrode. The outer measuring electrode is disposed at a different position in the base from the gas flow section and corresponds to the inner measuring electrode.

[0067] A reference electrode, which is arranged inside the substrate to contact a reference gas; and

[0068] An electromotive force detection sensor unit includes an inner measuring electrode and a reference electrode, and detects the electromotive force value between the inner measuring electrode and the reference electrode.

[0069] Here, in the electromotive force detection sensor unit, the reference electrode corresponds to the inner measuring electrode.

[0070] The control device included in the gas sensor of the present invention includes a switching unit that switches to allow current to flow through or not flow through the current measuring pump unit.

[0071] Hereinafter, an example of an embodiment of the gas sensor of the present invention will be described in detail.

[0072] [Overall Structure of a Gas Sensor]

[0073] The gas sensor of the present invention will now be described with reference to the accompanying drawings. Figure 1 This is a schematic vertical cross-sectional view along the length of an example of the general structure of a gas sensor 100 including sensor element 101. Hereinafter, [the following will be described using...] Figure 1 Based on the benchmark, regarding the upper and lower, Figure 1 The top side is designated as "top", and the bottom side is designated as "bottom". Figure 1 The left side is designated as the front-end side, and the right side is designated as the back-end side.

[0074] exist Figure 1 In the section on gas sensor 100, an example of a NOx sensor is shown that uses sensor element 101 to detect NOx in a gas to be measured and to measure its concentration.

[0075] Additionally, the gas sensor 100 includes a control device 90 for controlling the sensor element 101. The control device 90 includes a switching unit 47 that switches between allowing current to flow through or not flow through the current-measuring pump unit. Figure 2 This is a block diagram showing the electrical connection between the control device 90 and the sensor element 101.

[0076] In the gas sensor 100, the switching unit 47 only needs to have a mechanism to switch the flow of current in or out of the current measuring pump unit 41. For example, the switching unit 47 could be... Figure 1 The example shown is a switch 47, or a mechanism including a switch. The switch can be a contact switch that mechanically opens and closes contacts provided on a circuit, or a switch that uses a switching element to turn current flowing through the circuit on / off. Switching elements include diodes, thyristors, transistors, MOSFETs, etc. Those skilled in the art can appropriately define the structure of the switch. By opening the switch, the conduction of the current-measuring pump unit 41 can be cut off, thereby switching to a state where current does not flow. By closing the switch, the current-measuring pump unit 41 can be turned on, thereby switching to a state where current flows.

[0077] Alternatively, for example, the switching unit 47 may be a variable power supply 46, or a mechanism including a variable power supply 46. By setting the voltage of the variable power supply 46 to zero, voltage can be not applied to the current measuring pump unit, thereby switching to a state where current does not flow. By setting the voltage of the variable power supply 46 to a predetermined value, voltage can be applied to the current measuring pump unit, thereby switching to a state where current flows.

[0078] (Sensor element)

[0079] The sensor element 101 is a strip-shaped element, comprising a substrate 102 having a structure formed by stacking multiple oxygen ion-conducting solid electrolyte layers. "Strip-shaped" refers to a long plate or strip. The substrate 102 has a structure in which six layers—a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, an insulating layer 5, and a second solid electrolyte layer 6—are stacked sequentially from the bottom side in the figures, each composed of an oxygen ion-conducting solid electrolyte layer such as zirconium dioxide (ZrO2). The solid electrolyte forming these six layers is a dense and gas-tight solid electrolyte. The six layers can be all of the same thickness or have different thicknesses. The layers are bonded together by an adhesive layer made of solid electrolyte, which is included in the substrate 102. Figure 1 The example shown is a multi-layer structure consisting of the six layers, but the multi-layer structure of the present invention is not limited to this and can be set to any number of layers and multi-layer structure.

[0080] For example, after performing prescribed processing on the ceramic green sheets corresponding to each layer and printing circuit patterns, they are stacked and then fired to integrate them, thereby manufacturing the aforementioned sensor element 101.

[0081] A gas inlet 10 is formed at one end (hereinafter referred to as the front end) along the length of the sensor element 101, 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 section 15 is configured such that: a first diffusion rate control section 11, a buffer space 12, a second diffusion rate control section 13, a first internal cavity 20, a third diffusion rate control section 30, a second internal cavity 40, a fourth diffusion rate control section 60, and a third internal cavity 61 are formed adjacent to each other in the length direction starting from the gas inlet 10.

[0082] The gas inlet 10, buffer space 12, first internal cavity 20, second internal cavity 40 and third internal cavity 61 are: the internal space of the sensor element 101 provided by hollowing out the isolation layer 5, wherein the upper part of the internal space is divided by the lower surface of the second solid electrolyte layer 6, the lower part is divided by the upper surface of the first solid electrolyte layer 4, and the side part is divided by the side of the isolation layer 5.

[0083] The first diffusion velocity control unit 11, the second diffusion velocity control unit 13, and the third diffusion velocity control unit 30 are all configured with two horizontally long ( Figure 1 The slit (the length direction of the opening) is perpendicular to the direction shown in the attached figure. Both the first diffusion velocity control unit 11 and the second diffusion velocity control unit 13 can be configured to apply the desired diffusion resistance, and the configuration is not limited to the slit.

[0084] The fourth diffusion velocity control unit 60 is a horizontally long ( Figure 1 A slit (forming an opening along its length, perpendicular to the direction shown in the attached figure) is disposed between the isolation layer 5 and the second solid electrolyte layer 6. The fourth diffusion rate control unit 60 can be configured to apply the desired diffusion resistance, and is not limited to the slit.

[0085] Furthermore, a reference gas introduction space 43 is provided at a location further away from the front end than the gas flow section 15, between the upper surface of the third substrate layer 3 and the lower surface of the isolation layer 5, and further defined by the side of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end of the sensor element 101 (hereinafter referred to as the rear end). As a reference gas for measuring NOx concentration, for example, atmospheric air is introduced into the reference gas introduction space 43.

[0086] The atmosphere introduction layer 48 is a layer composed of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. In addition, the atmosphere introduction layer 48 is formed to cover the reference electrode 42.

[0087] The reference electrode 42 is an electrode formed by being sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, an atmospheric inlet layer 48 communicating with the reference gas inlet space 43 is provided around it. That is, the reference electrode 42 is configured to be in contact with the reference gas by means of the porous atmospheric inlet layer 48 and the reference gas inlet space 43. In addition, as described later, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61 can be measured using the reference electrode 42. The reference electrode 42 is formed as a porous metal-ceramic electrode (e.g., a metal-ceramic electrode of Pt and ZrO2).

[0088] In the gas flow section 15, the gas inlet 10 is open relative to the external space, and the gas to be measured is taken from the external space into the sensor element 101 through the gas inlet 10.

[0089] In this embodiment, although the measured gas flow section 15 is configured such that the measured gas is introduced through a gas inlet 10 that is open at the front end face of the sensor element 101, the present invention is not limited to this configuration. For example, the measured gas flow section 15 may not have a recess with a gas inlet 10. In this case, the first diffusion rate control section 11 substantially constitutes a gas inlet.

[0090] Alternatively, for example, the gas flow section 15 can be configured such that it has an opening communicating with the buffer space 12 on a side along the length of the base portion 102, or an opening communicating with the buffer space 12 near the first internal cavity 20. In this case, the gas to be measured is introduced from the side along the length of the base portion 102 through the opening.

[0091] Alternatively, for example, the gas flow section 15 to be measured can be formed as a structure in which the gas to be measured is introduced through a porous body.

[0092] The first diffusion rate control unit 11 is a part that applies a predetermined diffusion resistance to the gas to be measured taken from the gas inlet 10.

[0093] The buffer space 12 is a space provided for guiding the gas to be measured, which is introduced by the first diffusion rate control unit 11, to the second diffusion rate control unit 13.

[0094] The second diffusion rate control unit 13 is a part that applies a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal cavity 20.

[0095] The amount of gas to be measured introduced into the first internal cavity 20 is sufficient as long as it is within a specified range. That is, a specified diffusion resistance is applied to the entire area from the front end of the sensor element 101 to the second diffusion rate control unit 13. For example, it can be configured such that the first diffusion rate control unit 11 is directly connected to the first internal cavity 20, i.e., there is no buffer space 12 and no second diffusion rate control unit 13.

[0096] The buffer space 12 is a space provided to mitigate the impact of pressure fluctuations on the measured value in the event of pressure changes in the gas being measured.

[0097] When the gas to be measured is introduced from outside the sensor element 101 into the first internal cavity 20, the gas being measured, which is rapidly drawn into the sensor element 101 from the gas inlet 10 due to pressure fluctuations in the external space (pulsations of exhaust pressure in the case of automobile exhaust), is not directly introduced into the first internal cavity 20. Instead, it is introduced into the first internal cavity 20 only after the pressure fluctuations of the gas being measured are eliminated by the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13. As a result, the pressure fluctuations of the gas being measured introduced into the first internal space are almost negligible.

[0098] The first internal cavity 20 is configured as a space for adjusting the oxygen partial pressure in the gas to be measured, which is introduced through the second diffusion rate control unit 13. The main pump unit 21 operates to adjust the oxygen partial pressure.

[0099] The sensor element 101 includes an adjustment pump unit, which comprises: an inner pump electrode disposed on the inner surface of the gas flow section 15 to be measured; and an outer pump electrode disposed at a position on the base portion 102 different from that of the gas flow section 15 to be measured, and corresponding to the inner pump electrode. In this embodiment, the main pump unit 21 and the auxiliary pump unit 50 function as adjustment pump units. Furthermore, 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.

[0100] The main pump unit 21 is an electrochemical pump unit comprising: an inner main pump electrode 22 disposed on the inner surface of the gas flow section 15; and an outer pump electrode 23 disposed on the base portion 102 at a different position from the gas flow section 15. Figure 1 The outer pump electrode 23 is located on the outer surface of the substrate 102 and corresponds to the inner main pump electrode 22. "Corresponds to the inner main pump electrode 22" means that the outer pump electrode 23 is configured to be separated from the inner main pump electrode 22 by a second solid electrolyte layer 6.

[0101] That is, the main pump unit 21 is an electrochemical pump unit configured to include an inner main pump electrode 22, an outer pump electrode 23, and a second solid electrolyte layer 6 sandwiched between the inner main pump electrode 22 and the outer pump electrode 23. The inner main pump electrode 22 has a top electrode portion 22a disposed on the lower surface of the second solid electrolyte layer 6, facing approximately the entire area of ​​the first internal cavity 20. The outer pump electrode 23 is disposed on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the top electrode portion 22a, in a manner that exposes it to the external space.

[0102] The inner main pump electrode 22 is formed by spanning the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that divide the first internal cavity 20, and the isolation layer 5 that forms the sidewall. Specifically, a top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that forms the top surface of the first internal cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that forms the bottom surface. Furthermore, side electrode portions (not shown) are formed on the sidewall surfaces (inner surfaces) of the isolation layer 5 that forms the two side walls of the first internal cavity 20 in such a way that the top electrode portion 22a and the bottom electrode portion 22b are connected. The side electrode portions are arranged in a tunnel-shaped structure.

[0103] The inner main pump electrode 22 and the outer pump electrode 23 are porous metal-ceramic electrodes (electrodes in which metal and ceramic components are mixed together). The ceramic component is not particularly limited, but it is preferable to use a solid electrolyte with oxygen ion conductivity, similar to the substrate 102. For example, ZrO2 can be used as the ceramic component.

[0104] The inner main pump electrode 22, which is in contact with the gas being measured, is formed using a material that can reduce the reducing power of NOx components in the gas being measured. The inner main pump electrode 22 may contain: a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd), and a noble metal that is catalytically active and reduces the catalytic activity of the target gas (NOx in this embodiment) (e.g., Au, Ag, etc.). In this embodiment, the inner main pump electrode 22 is a porous metal-ceramic electrode containing 1% Au, Pt, and ZrO2.

[0105] The outer pump electrode 23 only needs to contain the aforementioned catalytically active noble metal. The same applies to the reference electrode 42, which also only needs to contain the aforementioned catalytically active noble metal. In this embodiment, the outer pump electrode 23 is a porous metal-ceramic electrode made of Pt and ZrO2.

[0106] In the main pump unit 21, a desired pump voltage Vp0 is applied between the inner main pump electrode 22 and the outer pump electrode 23 using a variable power supply 24, so that the pump current Ip0 flows between the inner main pump electrode 22 and the outer pump electrode 23 in either a positive or negative direction. This allows oxygen in the first internal cavity 20 to be drawn out to the external space, or oxygen in the external space to be drawn into the first internal cavity 20.

[0107] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, the electrochemical sensor unit, namely the oxygen partial pressure detection sensor unit 80 for main pump control, is configured to include an inner main pump electrode 22, a second solid electrolyte layer 6, an isolation layer 5, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42.

[0108] The oxygen concentration (oxygen partial pressure) within the first internal cavity 20 can be determined by measuring the electromotive force V0 of the oxygen partial pressure detection sensor unit 80 for main pump control. Furthermore, the pump voltage Vp0 of the variable power supply 24 is controlled by feedback to keep the electromotive force V0 constant, thereby controlling the pump current Ip0. Accordingly, the oxygen concentration within the first internal cavity 20 can be maintained at a predetermined constant value.

[0109] The third diffusion rate control unit 30 is a part that applies a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump unit 21 in the first internal cavity 20, thereby guiding the gas to be measured into the second internal cavity 40.

[0110] The second internal cavity 40 is configured as a space for adjusting the oxygen partial pressure in the gas to be measured, which is introduced through the third diffusion rate control unit 30, with higher precision. The auxiliary pump unit 50 operates to adjust the oxygen partial pressure. Alternatively, it can be configured without the second internal cavity 40 and the auxiliary pump unit 50. From the viewpoint of adjusting the oxygen partial pressure with precision, it is more preferable to have the second internal cavity 40 and the auxiliary pump unit 50.

[0111] The oxygen concentration (oxygen partial pressure) is pre-adjusted in the first internal cavity 20, and then the oxygen partial pressure of the gas to be measured, which is introduced through the third diffusion rate control unit 30, is further adjusted in the second internal cavity 40 using the auxiliary pump unit 50. This allows the oxygen concentration in the second internal cavity 40 to be maintained at a constant level with high precision, thus enabling high-precision NOx concentration measurement in the gas sensor 100.

[0112] The auxiliary pump unit 50 is an electrochemical pump unit comprising: an auxiliary pump electrode 51 disposed on the inner surface of the gas flow section 15, at a position further away from the base portion 102 in the longitudinal direction than the inner main pump electrode 22; and an outer pump electrode 23 disposed on the base portion 102 at a position different from that of the gas flow section 15. Figure 1 The outer surface of the substrate 102 is in the middle, and it corresponds to the auxiliary pump electrode 51. "Corresponds to the auxiliary pump electrode 51" means that the outer pump electrode 23 is configured to be separated from the auxiliary pump electrode 51 by a second solid electrolyte layer 6.

[0113] That is, the auxiliary pump unit 50 is an auxiliary electrochemical pump unit comprising an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, as long as it is a suitable electrode on the outside of the sensor element 101), and a second solid electrolyte layer 6, wherein the auxiliary pump electrode 51 has a top electrode portion 51a disposed on the lower surface of the second solid electrolyte layer 6 facing the entire area of ​​the second internal cavity 40.

[0114] The aforementioned auxiliary pump electrode 51 is disposed in the second internal cavity 40 with the same tunnel-shaped structure as the inner main pump electrode 22 previously disposed in the first internal cavity 20. That is, it is constructed with the following tunnel shape: a top electrode portion 51a is formed relative to the second solid electrolyte layer 6 constituting the top surface of the second internal cavity 40; a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 constituting the bottom surface of the second internal cavity 40; and side electrode portions (not shown) connecting the top electrode portion 51a and the bottom electrode portion 51b are formed on the two walls of the isolation layer 5 constituting the side wall of the second internal cavity 40.

[0115] Furthermore, the auxiliary pump electrode 51, like the inner main pump electrode 22, is formed using a material that weakens the reducing ability against NOx in the gas being measured. Similar to the inner main pump electrode 22, the auxiliary pump electrode 51 may contain: a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd), and a noble metal that has catalytic activity but reduces the catalytic activity against the target gas (NOx in this embodiment) (e.g., Au, Ag, etc.). In this embodiment, like the inner main pump electrode 22, the auxiliary pump electrode 51 is a porous metal-ceramic electrode containing 1% Au, Pt, and ZrO2.

[0116] In the auxiliary pump unit 50, a desired voltage Vp1 is applied between the auxiliary pump electrode 51 and the outer pump electrode 23 using a variable power supply 52, thereby enabling oxygen in the atmosphere inside the second internal cavity 40 to be drawn out to the external space, or oxygen to be drawn in from the external space into the second internal cavity 40.

[0117] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, the electrochemical sensor unit, namely the oxygen partial pressure detection sensor unit 81 for auxiliary pump control, is configured to include an auxiliary pump electrode 51, a reference electrode 42, a second solid electrolyte layer 6, an isolation layer 5, a first solid electrolyte layer 4, and a third substrate layer 3.

[0118] Furthermore, the auxiliary pump unit 50 utilizes a variable power supply 52 with a controlled voltage based on the electromotive force V1 detected by the oxygen partial pressure detection sensor unit 81 for auxiliary pump control. This controls the oxygen partial pressure in the atmosphere within the second internal cavity 40 to a low partial pressure that has virtually no impact on NOx measurement.

[0119] In addition, the pump current Ip1 is simultaneously used to control the electromotive force V0 of the oxygen partial pressure detection sensor unit 80 for main pump control. Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor unit 80 for main pump control, and its electromotive force V0 is controlled, thereby keeping the gradient of oxygen partial pressure in the gas to be measured, which is introduced from the third diffusion rate control unit 30 into the second internal cavity 40, constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of about 0.001 ppm by the action of the main pump unit 21 and the auxiliary pump unit 50.

[0120] The fourth diffusion rate control unit 60 is a part that applies a predetermined diffusion resistance to the gas to be measured in the second internal cavity 40, where the oxygen concentration (oxygen partial pressure) is controlled to be lower by the operation of the auxiliary pump unit 50, thereby guiding the gas to be measured into the third internal cavity 61.

[0121] The third internal cavity 61 is configured as a space for measuring the concentration of nitrogen oxides (NOx) in the gas to be measured, which is introduced through the fourth diffusion rate control unit 60. The NOx concentration is measured by the operation of the electromotive force detection sensor unit 82 or the current measuring pump unit 41.

[0122] The current measurement pump unit 41 is an electrochemical pump unit comprising: an inner measuring electrode (measuring electrode 44 in this embodiment), disposed on the inner surface of the gas flow section 15, at a position further away from the front end of the substrate 102 in the longitudinal direction than the inner pump electrode (in this embodiment, the inner main pump electrode 22 and the auxiliary pump electrode 51); and an outer measuring electrode, disposed on the substrate 102 at a position different from the gas flow section 15, and corresponding to the inner measuring electrode. In this embodiment, the outer pump electrode 23 disposed on the outer surface of the substrate 102 also functions as an outer measuring electrode. "Corresponding to the inner measuring electrode" means that the outer pump electrode 23 is configured to be separated from the measuring electrode 44 by a second solid electrolyte layer 6, an isolation layer 5, and a first solid electrolyte layer 4.

[0123] That is, the current measuring pump unit 41 measures the NOx concentration in the gas to be measured within the third internal cavity 61. The measuring pump unit 41 is an electrochemical pump unit configured to include a measuring electrode 44 disposed 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, an isolation layer 5, and the first solid electrolyte layer 4.

[0124] The measuring electrode 44 is a porous metal-ceramic electrode. The measuring electrode 44 also functions as a NOx reduction catalyst to reduce NOx present in the atmosphere within the third internal cavity 61. The measuring electrode 44 is an electrode containing at least one catalytically active noble metal (e.g., Pt, Rh, Ir, Ru, Pd). Preferably, it does not contain any noble metal that has catalytic activity and would reduce the catalytic activity against the target gas (NOx in this embodiment) (e.g., Au, Ag, etc.). In this embodiment, the measuring electrode 44 is a porous metal-ceramic electrode of Pt, Rh, and ZrO2.

[0125] Furthermore, the electromotive force detection sensor unit 82 is configured to include an inner measuring electrode (measuring electrode 44 in this embodiment) and the reference electrode 42, and to detect the electromotive force value between the inner measuring electrode and the reference electrode 42. Here, the reference electrode 42 corresponds to the measuring electrode 44 of the inner measuring electrode. "Corresponds to the measuring electrode 44" means that the reference electrode 42 is positioned such that a first solid electrolyte layer 4 and a third substrate layer 3 are spaced apart from the measuring electrode 44.

[0126] That is, the electromotive force detection sensor unit 82 is an electrochemical sensor unit configured to include a measuring electrode 44, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42. The electromotive force detection sensor unit 82 detects the oxygen partial pressure around the measuring electrode 44.

[0127] The current measuring pump unit 41 is configured such that it is switched using a switching unit 47, allowing current to flow or not flow within the current measuring pump unit 41. Hereinafter, refer to... Figure 1 The following explanation will be based on the case where the switching unit 47 is a switch 47. Figure 1 The circuit symbol for a contact switch is used to schematically illustrate the switching unit 47, but the switching unit 47 can also be a switch employing a switching element. Furthermore, the location of the switching unit 47 is not limited to... Figure 1 The position shown can be set at any position as long as it is on the circuit of the current measuring pump unit 41.

[0128] With the switching unit 47 disconnected, the electrical connection of the current measuring pump unit 41 is cut off, and therefore, no current flows in the current measuring pump unit 41. No current flow means that the current value of the current measuring pump unit 41 is zero or substantially zero.

[0129] The gas to be measured, guided into the second internal cavity 40 under controlled oxygen partial pressure, reaches the measuring electrode 44 in the third internal cavity 61 via the fourth diffusion rate control unit 60. Nitrogen oxides in the gas to be measured around the measuring electrode 44 are reduced (2NO→N2+O2) to produce oxygen. The produced oxygen remains around the measuring electrode 44. As a result, an electromotive force (EMF) is generated in the electromotive force detection sensor unit 82, corresponding to the difference between the amount of oxygen produced by the reduction of NOx components in the atmosphere surrounding the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere. This generated EMF is called the open-circuit EMF V2open. Since the amount of oxygen contained in the reference atmosphere is constant, the value of the open-circuit EMF V2open becomes a value corresponding 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-circuit EMF V2open detected by the EMF detection sensor unit 82.

[0130] When the switching unit 47 is turned on, the current measuring pump unit 41 is electrically connected, and thus, current flows through the current measuring pump unit 41.

[0131] Even under these conditions, the gas to be measured, guided into the second internal cavity 40, reaches the measuring electrode 44 in the third internal cavity 61 through the fourth diffusion rate control unit 60 while the oxygen partial pressure is controlled. Nitrogen oxides in the gas to be measured around the measuring electrode 44 are reduced (2NO→N2+O2) to produce oxygen. Furthermore, this generated oxygen is pumped using the current measuring pump unit 41. At this time, the electromotive force detected by the electromotive force detection sensor unit 82 is used as the control voltage V2 to perform feedback control on the voltage Vp2 of the variable power supply 46, so that the control voltage V2 remains constant. The amount of oxygen generated around the measuring electrode 44 is proportional 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 pump current Ip2 of the current measuring pump unit 41.

[0132] In addition, the electrochemical sensor unit 83 is configured to include a second solid electrolyte layer 6, an isolation layer 5, a first solid electrolyte layer 4, a third substrate layer 3, an outer pump electrode 23, and a reference electrode 42. The partial pressure of oxygen in the gas to be measured outside the sensor can be detected based on the electromotive force Vref obtained by the sensor unit 83.

[0133] Furthermore, in order to improve the oxygen ion conductivity of the solid electrolyte, the sensor element 101 includes a heater section 70 that performs temperature adjustment functions for heating and maintaining the temperature of the sensor element 101. The heater section 70 includes a heater electrode 71, a heater 72, a heater conductor 76, a through hole 73, a heater insulating layer 74, and a pressure relief hole 75.

[0134] The heater electrode 71 is an electrode formed in contact with the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power supply 77, which serves as an external power source, power can be supplied to the heater section 70 from the outside.

[0135] The heater 72 is a resistive element formed by being sandwiched between the second substrate layer 2 and the third substrate layer 3 from the top and bottom. The heater 72 is connected to the heater electrode 71 via the heater conductor 76 and the through hole 73. The heater electrode 71 is powered from the outside and generates heat, thereby heating and keeping the solid electrolyte forming the sensor element 101 warm. The heater conductor 76 is connected to the heater 72 and extends toward the rear end of the sensor element 101 in the longitudinal direction.

[0136] Furthermore, the heater 72 is embedded throughout the entire area from the first internal cavity 20 to the third internal cavity 61, thereby enabling the entire sensor element 101 to be adjusted to the temperature at which the solid electrolyte is activated. It is sufficient to adjust the temperature to allow the main pump unit 21, the auxiliary pump unit 50, and the measuring pump unit 41 to operate. It is not necessary to adjust the entire area to the same temperature; a temperature distribution can exist within the sensor element 101.

[0137] Regarding the sensor element 101 in this embodiment, although it is formed such that the heater 72 is embedded in the substrate 102, it is not limited to this method. The heater 72 can be configured to heat the substrate 102. That is, the heater 72 only needs to heat the sensor element 101 to a degree that exhibits oxygen ion conductivity sufficient to enable the main pump unit 21, the auxiliary pump unit 50, and the measurement pump unit 41 to operate. For example, it can be embedded in the substrate 102 as in this embodiment. Alternatively, for example, the heater 70 can be formed as a heater substrate different from the substrate 102 and disposed adjacent to the substrate 102.

[0138] The heater insulation layer 74 is an insulation layer formed of an insulator such as aluminum oxide on the upper and lower surfaces of the heater 72 and the heater conductor 76. The heater insulation layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and the heater conductor 76, and electrical insulation between the third substrate layer 3 and the heater 72 and the heater conductor 76.

[0139] The pressure relief hole 75 is formed such that it penetrates the third substrate layer 3 and connects the heater insulation layer 74 to the reference gas introduction space 43. The pressure relief hole 75 can mitigate the increase in internal pressure that accompanies the temperature rise within the heater insulation layer 74. Alternatively, a structure without the pressure relief hole 75 may also be used.

[0140] The aforementioned sensor element 101 is assembled in the gas sensor 100 such that the front end 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.

[0141] (Control device)

[0142] The gas sensor 100 of this embodiment includes: the sensor element 101 described above; and a control device 90, which controls the sensor element 101. In the gas sensor 100, each electrode 22, 23, 51, 44, 42 of the sensor element 101 is electrically connected to the control device 90 via wires (not shown). Figure 2This is a block diagram showing the electrical connections between the control device 90 and the pump units 21, 50, 41, sensor units 80, 81, 82, 83, and heater unit 70 of the sensor element 101. The control device 90 includes: the aforementioned variable power supplies 24, 46, 52; a switching unit 47 that switches to allow current to flow or not flow in the current measuring pump unit 41; 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 component that receives a control signal from the measurement mode switching unit 94 and switches to allow current to flow or not flow in the current measuring pump unit 41.

[0143] The control unit 91 is implemented by a general-purpose or special-purpose computer, utilizing the CPU, memory, etc., mounted on the computer to perform the functions of the drive control unit 92, the concentration calculation unit 93, and the measurement mode switching unit 94. Furthermore, the gas sensor 100 measures NOx contained in the exhaust gas from the automobile engine. When the sensor element 101 is installed in the exhaust path, some or all of the functions of the control device 90 (especially the control unit 91) can be implemented by the ECU (Electronic Control Unit) mounted on the automobile.

[0144] The control unit 91 is configured to acquire the electromotive forces (V0, V1, V2, Vref) of each sensor unit 80, 81, 82, 83 of the sensor element 101, the pump currents (Ip0, Ip1, Ip2) of each pump unit 21, 50, 41, and the heater voltage Vh and heater current Ih of the heater unit 70. Furthermore, 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.

[0145] The drive control unit 92 is configured to control the heater unit 70, the main pump unit 21, the auxiliary pump unit 50, and the measuring pump unit 41 so that the gas sensor 100 can measure the concentration of the target gas (NOx in this embodiment).

[0146] The drive control unit 92 heats the heater 72 and maintains the temperature of the heater 72 at the desired temperature.

[0147] Various known control methods can be used to heat the heater 72. For example, a constant voltage can be applied to the heater 72 to heat it. 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 ​​of the main pump unit 21, the auxiliary pump unit 50, and the measuring pump unit 41.

[0148] For example, the drive control unit 92 performs feedback control on the control signal output to the heater power supply 77 based on the heater resistance value Rh (=Vh / Ih) calculated according to the heater voltage Vh and heater current Ih of the heater 72, so that the heater 72 reaches the target temperature.

[0149] The drive control unit 92 performs feedback control on the pump voltage Vp0 of the variable power supply 24 of the main pump unit 21, so that the electromotive force V0 of the oxygen partial pressure detection sensor unit 80 for main pump control reaches a constant value (referred to as the set value V0). SET The electromotive force V0 represents the oxygen partial pressure near the inner main pump electrode 22. Therefore, keeping the electromotive force V0 constant means keeping the oxygen partial pressure near the inner main pump electrode 22 constant. As a result, the pump current Ip0 of the main pump unit 21 varies according to the oxygen concentration in the gas being measured.

[0150] The oxygen partial pressure in the gas being measured is higher than the set value V0. SET Under the condition of oxygen partial pressure, oxygen is discharged from the first internal cavity 20 in the main pump unit 21. On the other hand, when the oxygen partial pressure in the gas being measured is lower than the set value V0... SET Under certain oxygen partial pressure conditions (e.g., in the presence of hydrocarbons such as HC), oxygen is drawn into the first internal cavity 20 from the space outside the sensor element 101 in the main pump unit 21. Therefore, it is possible to achieve either a positive or negative value for the pump current Ip0.

[0151] The drive control unit 92 performs feedback control on the pump voltage Vp1 of the variable power supply 52 of the auxiliary pump unit 50, so that the electromotive force V1 of the oxygen partial pressure detection sensor unit 81 for auxiliary pump control reaches a constant value (referred to as the set value V1). SET The electromotive force V1 represents the oxygen partial pressure near the auxiliary pump electrode 51. Therefore, keeping the electromotive force V1 constant means keeping the oxygen partial pressure near the auxiliary pump electrode 51 constant. This controls the oxygen partial pressure in the atmosphere within the second internal cavity 40 to a low partial pressure that has no substantial effect on the determination of NOx.

[0152] In addition, at the same time, the set value V0 of the electromotive force V0 is set based on the pump current Ip1. SET Feedback control is used to ensure that the pump current Ip1 of the auxiliary pump unit 50 reaches a constant value (referred to as the setpoint Ip1). SET Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor unit 80 for main pump control, controlling its electromotive force V0 to a set value V0 based on the pump current Ip1. SETThis ensures that the gradient of oxygen partial pressure in the gas to be measured, introduced from the third diffusion rate control unit 30 into the second internal cavity 40, remains constant. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump unit 21 and the auxiliary pump unit 50. That is, it can be understood that the oxygen concentration in the gas to be measured, introduced from the fourth diffusion rate control unit 60 into the third internal cavity 61, is maintained at a constant value of approximately 0.001 ppm.

[0153] The drive control unit 92 has the following measurement modes:

[0154] In the electromotive force measurement mode, the main pump unit 21 and auxiliary pump unit 50, which function as adjustment pump units, operate as described above, while the current measurement pump unit 41 is not activated. The concentration of the target gas in the measured gas is detected based on the electromotive force value (open-circuit electromotive force V2open) of the electromotive force detection sensor unit 82; and

[0155] In the current measurement mode, the main pump unit 21, the auxiliary pump unit 50, and the current measurement pump unit 41 are activated to detect the concentration of the target gas in the gas being measured based on the current value (pump current Ip2) of the current measurement pump unit 41.

[0156] In electromotive force (EMF) measurement mode, the current is prevented from flowing through the current measuring pump unit 41 by the switching unit 47. The gas to be measured, after the oxygen concentration has been adjusted to a predetermined concentration by the main pump unit 21 and the auxiliary pump unit 50, is introduced into the third internal cavity 61. Oxygen is generated by reducing nitrogen oxides in the gas to be measured at the measuring electrode 44. In EMF measurement mode, the generated oxygen is not drawn out by the current measuring pump unit 41 but is instead retained around the measuring electrode 44. An open-circuit EMF V2open is generated in the EMF detection sensor unit 82, corresponding to the difference between the amount of oxygen generated by the reduction of NOx components in the atmosphere surrounding the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere. The drive control unit 92 detects the open-circuit EMF V2open generated in the EMF detection sensor unit 82. In other words, in EMF measurement mode, the current is prevented from flowing through the current measuring pump unit 41, meaning the EMF generated in the EMF detection sensor unit 82 becomes an open-circuit EMF V2open. In this specification, the electromotive force measurement mode is also referred to as the open-circuit electromotive force measurement mode.

[0157] In current measurement mode, the current is allowed to flow through the current measurement pump unit 41 using the switching unit 47. In current measurement mode, the drive control unit 92 also continuously acquires the electromotive force V2 detected by the electromotive force detection sensor unit 82. In current measurement mode, the drive control unit 92 performs feedback control on the pump voltage Vp2 of the variable power supply 46 of the current measurement pump unit 41, so that the electromotive force V2 detected by the electromotive force detection sensor unit 82 reaches a constant value (referred to as the set value V2). SET Preset value V2 SET The specification is as follows: the oxygen concentration around the measuring electrode 44 within the third internal cavity 61 is set to a predetermined low concentration. The gas to be measured, after its oxygen concentration has been adjusted to the predetermined concentration by the main pump unit 21 and the auxiliary pump unit 50, is introduced into the third internal cavity 61. At the measuring electrode 44, nitrogen oxides in the gas are reduced to produce oxygen. The drive control unit 92 causes the pump current Ip2 to flow through the current measuring pump unit 41, thereby substantially drawing out all the generated oxygen. (Set value V2) SET It can be set to: the value at which all NOx is essentially decomposed at measuring electrode 44. The setpoint V2 is set in this way. SET In essence, the measuring electrode 44 detects all NOx in the gas being measured as the pump current Ip2. In current measurement mode, the electromotive force V2 of the electromotive force detection sensor unit 82 is used as the control voltage for feedback control of the pump voltage Vp2 of the variable power supply 46 of the current measurement pump unit 41.

[0158] Here, the pump current Ip2 detected in the current measurement mode is explained in detail. Figure 3 This is a schematic diagram illustrating an example of the relationship between the NOx concentration in the gas being measured and the pump current Ip2 of the gas sensor 100. The horizontal axis represents the NOx concentration (ppm), and the vertical axis represents the value of the pump current Ip2 (μA).

[0159] As described above, in current measurement mode, the drive control unit 92 causes the pump current Ip2 to flow through the current measurement pump unit 41, thereby substantially drawing out all the oxygen generated by the reduction of nitrogen oxides. Therefore, the pump current Ip2 is 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 nitrogen oxides reduced. If all nitrogen oxides in the gas to be measured are substantially decomposed at the measuring electrode 44, then the amount of oxygen generated by the reduction of nitrogen oxides is proportional to the concentration of nitrogen oxides (NOx concentration) in the gas to be measured. That is, as... Figure 3As shown, a linear relationship exists between NOx concentration and pump current Ip2 over a wide range of NOx concentrations. Gas sensor 100 can measure NOx concentration over a large range based on this linear relationship between NOx concentration and pump current Ip2.

[0160] like Figure 3 As shown, the pump current Ip2 includes: a bias current Ip2offset that flows independently of the NOx concentration, and a concentration-dependent current Ip2conc that flows in accordance with the NOx concentration. The concentration-dependent current Ip2conc is the current that flows in accordance with the amount of oxygen produced by the reduction of nitrogen oxides, and its value is approximately proportional to the NOx concentration.

[0161] The bias current Ip2offset is the current flowing independently of NOx concentration. The bias current Ip2offset is the current generated by factors other than oxygen produced by the decomposition of NOx in the target gas. It can be understood that the bias current Ip2offset includes: residual oxygen in the gas being measured after the oxygen concentration has been adjusted to a specified concentration in the main pump unit 21 and auxiliary pump unit 50, or oxygen produced by the decomposition of a portion of water (H2O) in the gas being measured at the measuring electrode 44, etc. Furthermore, it can be understood that the bias current Ip2offset also includes: leakage current from the heater 72 energized to heat the gas sensor 100, and current generated by factors such as the measuring electrode 44 constituting the current measuring pump unit 41, the outer pump electrode 23, and the solid electrolyte layer (…). Figure 1 The current is caused by the movement of electrons or impurities contained in the second solid electrolyte layer 6, the isolation layer 5 and the first solid electrolyte layer 4).

[0162] When the gas sensor 100 detects the NOx concentration in the gas being measured, if the bias current Ip2offset changes due to some factor, the pump current Ip2 detected by the current measuring pump unit 41 changes with the amount of the bias current Ip2offset change ΔIp2offset, not based on the NOx concentration in the gas being measured. It is conceivable that the bias current Ip2offset might change due to, for example, changes in electrode temperature caused by changes in the temperature of the gas being measured, or due to changes in the output of the heater power supply 77. Furthermore, it is conceivable that it might change due to changes in the H2O concentration in the gas being measured.

[0163] When measuring a gas containing a high concentration of NOx, the pump current Ip2 detected by the current measuring pump unit 41 is relatively large. Therefore, the change in pump current Ip2 caused by the variation in bias current Ip2offset ΔIp2offset is relatively small. Thus, even with variations in bias current Ip2offset, the NOx concentration can be measured with high accuracy. For example, if the variation in bias current Ip2offset ΔIp2offset, converted to NOx concentration, is equivalent to 5 ppm, the measurement error is 10% when measuring a gas with a NOx concentration of 50 ppm, and 1% when measuring a gas with a NOx concentration of 500 ppm.

[0164] On the other hand, in order to measure the gas containing low concentrations of NOx with better accuracy, it is preferable to reduce the influence of the bias current Ip2offset variation.

[0165] When the pump voltage Vp2 of the variable power supply 46 is applied to the current measuring pump unit 41, causing the pump current Ip2 to flow, a bias current Ip2offset is generated. Therefore, if the pump voltage Vp2 is not applied to the current measuring pump unit 41, no pump current Ip2 flows, and thus no bias current Ip2offset is generated. In the gas sensor 100, the switching unit 47 is disconnected to cut off the electrical connection of the current measuring pump unit 41, forming a state in which no current flows in the current measuring pump unit 41, thereby achieving a state in which no bias current Ip2offset is generated. In this case, the drive control unit 92 controls the open-circuit electromotive force measurement mode and detects the open-circuit electromotive force V2open of the electromotive force detection sensor unit 82.

[0166] The open-circuit electromotive force V2open detected by the electromotive force detection sensor unit 82 is described in detail. Figure 4 This is a schematic diagram illustrating an example of the relationship between NOx concentration and open-circuit electromotive force V2open for a gas sensor 100. The horizontal axis represents NOx concentration (ppm), and the vertical axis represents the value of open-circuit electromotive force V2open (mV).

[0167] In electromotive force measurement mode, as described above, the electrical connection of the current measuring pump unit 41 is disconnected, creating a state where the pump current Ip2 is not flowing. In this case, the electromotive force detection sensor unit 82 generates an open-circuit electromotive force V2open corresponding to the difference between the amount of oxygen produced by the reduction of NOx components in the atmosphere surrounding the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere. There exists a relationship between the open-circuit electromotive force V2open and the NO concentration. Figure 4 The relationship shown.

[0168] In electromotive force measurement mode, the gas sensor 100 is in a state where the pump current Ip2 is not flowing, and therefore no bias current Ip2offset is generated. Therefore, it can be based on the principle of being unaffected by the bias current Ip2offset. Figure 4 The relationship between NOx concentration and open-circuit electromotive force V2open shown allows for high-precision measurement of NOx concentration.

[0169] like Figure 4 As shown, the lower the NO concentration in the measured gas, the greater the change in open-circuit electromotive force (EMF) V2open due to the change in NO concentration. That is, the lower the NO concentration in the measured gas, the larger the change in open-circuit EMF V2open becomes relative to a small change in NO concentration, thus exhibiting a trend towards higher measurement resolution. Therefore, when measuring a gas containing low concentrations of NOx, the NOx concentration can be measured with particularly high accuracy.

[0170] Thus, no bias current Ip2offset is generated in the electromotive force measurement mode. Therefore, NOx concentration can be measured with high accuracy over a large concentration range, unaffected by variations in the bias current Ip2offset. In particular, when measuring a gas containing low concentrations of NOx, the change in open-circuit electromotive force V2open relative to small changes in NO concentration is relatively large, resulting in higher measurement resolution. Therefore, when measuring a gas containing low concentrations of NOx, NOx concentration can be measured with exceptionally high accuracy.

[0171] Furthermore, in the current measurement mode, there exists a range between the pump current Ip2 and the NOx concentration, and particularly within a large range of NOx concentrations. Figure 3 The linear relationship shown allows for high-precision measurement of NOx concentration over a wide concentration range. In particular, when measuring a gas containing high concentrations of NOx, the change in pump current Ip2 due to the bias current variation ΔIp2offset is relatively small. Therefore, NOx concentration can be measured with high accuracy.

[0172] The concentration calculation unit 93 is configured to calculate the NOx concentration in the gas being measured and output it.

[0173] In electromotive force measurement mode, the concentration calculation unit 93 acquires the electromotive force V2open of the electromotive force detection sensor unit 82, calculates the NOx concentration in the gas being measured based on the pre-stored conversion parameter (open-circuit electromotive force-concentration conversion parameter) between the open-circuit electromotive force V2open and the NOx concentration in the gas being measured, and outputs it as the measured value of the gas sensor 100. The open-circuit electromotive force-concentration conversion parameter is used to represent... Figure 4 The data illustrating the relationship shown in the example are pre-stored in the memory of the control unit 91, which functions as the concentration calculation unit 93. Regarding the electromotive force-concentration conversion parameters, those skilled in the art can appropriately define them in advance for the gas sensor 100 through experiments, etc. The electromotive force-concentration conversion parameters can be, for example, coefficients of an approximation formula (logarithmic function, etc.) obtained experimentally, or a mapping table representing the correspondence between the electromotive force V2open and the NOx concentration in the measured gas. The electromotive force-concentration conversion parameters can be inherent parameters for each gas sensor 100, or parameters common to multiple gas sensors.

[0174] In current measurement mode, the concentration calculation unit 93 acquires the pump current Ip2 of the current measurement pump unit 41, calculates the NOx concentration in the gas being measured based on a pre-stored conversion parameter (current-concentration conversion parameter) between the pump current Ip2 and the NOx concentration in the gas being measured, and outputs it as the measured value of the gas sensor 100. The current-concentration conversion parameter is used as a representation... Figure 3 The linear relationship data illustrated in the example is pre-stored in the memory of the control unit 91, which functions as the concentration calculation unit 93. Regarding the current-concentration conversion parameters, those skilled in the art can appropriately define them in advance for the gas sensor 100 through experiments, etc. The current-concentration conversion parameters can be, for example, coefficients of an approximate formula (such as a linear function) obtained experimentally, or a mapping table representing the correspondence between the pump current Ip2 and the NOx concentration in the measured gas. The current-concentration conversion parameters can be inherent parameters for each gas sensor 100, or parameters common to multiple gas sensors.

[0175] The measurement mode switching unit 94 is configured to switch between the electromotive force measurement mode and the current measurement mode as described above.

[0176] When switching from current measurement mode to electromotive force (EMF) measurement mode, the measurement mode switching unit 94 switches the switching unit to a state where current does not flow or substantially does not flow in the current measurement pump unit 41. In this embodiment, a control signal is output to the switching unit 47 to be disconnected. Furthermore, the measurement mode switching unit 94 issues an instruction to the drive control unit 92 to control the operation in EMF 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 current measurement pump unit 41, and instead detects the open-circuit EMF V2open of the EMF detection sensor unit 82. The measurement mode switching unit 94 issues an instruction to the concentration calculation unit 93 to acquire the open-circuit EMF V2open of the EMF detection sensor unit 82 and calculate the NOx concentration based on the EMF-concentration conversion parameter.

[0177] When switching from electromotive force measurement mode to current measurement mode, the measurement mode switching unit 94 switches the switching unit to allow current to flow in the current measurement pump unit 41. In this embodiment, a control signal indicating "on" is output to the switching unit 47. Furthermore, the measurement mode switching unit 94 issues an instruction to the drive control unit 92 to control the pump in 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 unit 41 and detects the pump current Ip2 of the current measurement pump unit 41. The measurement mode switching unit 94 issues an instruction to the concentration calculation unit 93 to acquire the pump current Ip2 of the current measurement pump unit 41 and calculate the NOx concentration based on the current-concentration conversion parameter.

[0178] The electromotive force measurement mode and the current measurement mode can be switched based on the NOx concentration output by the concentration calculation unit 93. The measurement mode switching unit 94 (more specifically, the memory in the control unit 91 that functions as the measurement mode switching unit 94) pre-records a first concentration threshold C1, which is the threshold for switching from the current measurement mode to the electromotive force measurement mode, and a second concentration threshold C2, which is the threshold for switching from the electromotive force measurement mode to the current measurement mode. The measurement mode switching unit 94 can continuously acquire the NOx concentration output by the concentration calculation unit 93, or it can acquire the NOx concentration output by the concentration calculation unit 93 at predetermined intervals.

[0179] In the current measurement mode, the measurement mode switching unit 94 acquires the NOx concentration output by the concentration calculation unit 93. If it determines that the NOx concentration is in a low concentration region below the predetermined first concentration threshold C1, it switches to the electromotive force measurement mode. If it determines that the NOx concentration is above the predetermined first concentration threshold C1, it maintains the current measurement mode.

[0180] In addition, even in the electromotive force measurement mode, the NOx concentration output by the concentration calculation unit 93 is acquired. If it is determined that the NOx concentration is in a high concentration region above the predetermined second concentration threshold C2, the switch to the current measurement mode is performed. If it is determined that the NOx concentration is below the predetermined second concentration threshold C2, the electromotive force measurement mode is maintained.

[0181] Those skilled in the art can appropriately set a first concentration threshold C1 for switching the current measurement mode to the electromotive force measurement mode. The first concentration threshold C1 can be a range of NOx concentrations in the gas being measured, or a value varying depending on the required measurement accuracy for the gas sensor 100. The first concentration threshold C1 can be set as a lower limit of the NOx concentration at which the desired measurement accuracy can be obtained when measuring in current measurement mode. For example, the first concentration threshold C1 can be set as a lower limit of the NOx concentration at which the bias current Ip2offset is within the range permissible for the measurement accuracy. Alternatively, for example, it can be set as a lower limit of the NOx concentration at which the expected variation in bias current ΔIp2offset is within the range permissible for the measurement accuracy. The first concentration threshold C1 can be set, for example, in the range of 50 ppm to 500 ppm. For example, it can be set to 100 ppm.

[0182] Those skilled in the art can appropriately set a second concentration threshold C2 for switching the electromotive force measurement mode to the current measurement mode. The second concentration threshold C2 can be a range of NOx concentrations in the gas being measured, or a value varying depending on the required measurement accuracy for the gas sensor 100. The second concentration threshold C2 can be set to the upper limit of the NOx concentration at which the desired measurement accuracy can be obtained when measured in electromotive force measurement mode. For example, the second concentration threshold C2 can be set to the upper limit of the NOx concentration at which the change in open-circuit electromotive force V2open relative to the change in NO concentration is within the range permissible as the measurement resolution. The second concentration threshold C2 can be set, for example, in the range of 50 ppm to 500 ppm. For example, it can be set to 300 ppm.

[0183] The first concentration threshold C1 and the second concentration threshold C2 can be the same or different values. When the first concentration threshold C1 and the second concentration threshold C2 are set to the same value, the electromotive force measurement mode is executed in the low concentration region below the first concentration threshold C1 (= the second concentration threshold C2), and the current measurement mode is executed in the high concentration region above the first concentration threshold C1 (= the second concentration threshold C2).

[0184] The first concentration threshold C1 is more preferably set to a concentration lower than the second concentration threshold C2. That is, it is more preferable to use two thresholds with concentration ranges.

[0185] Figure 5 This is a graph illustrating an example of the time variation 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.

[0186] like Figure 5 As shown, if the NOx concentration in the measured gas (i.e., the NOx concentration output by gas sensor 100) is lower than the first concentration threshold C1, the system switches to electromotive force (EMF) measurement mode. After switching to EMF measurement mode, even if the NOx concentration in the measured gas fluctuates near the first concentration threshold C1 and exceeds it, the system continues to execute EMF measurement mode until the NOx concentration in the measured gas exceeds the second concentration threshold C2. Then, if the NOx concentration in the measured gas exceeds the second concentration threshold C2, the system switches to current measurement mode. After switching to current measurement mode, even if the NOx concentration in the measured gas fluctuates near the second concentration threshold C2 and falls below it, the system continues to execute current measurement mode until the NOx concentration in the measured gas falls below the first concentration threshold C1.

[0187] Thus, when the first concentration threshold C1 is set below the second concentration threshold C2, the concentration range (intermediate concentration range) between the first and second concentration thresholds C1 and C2 acts as a buffer zone to maintain the previous measurement mode without switching measurement modes. That is, when the NOx concentration in the measured gas fluctuates frequently around the first and second concentration thresholds C1 and C2, the switching of measurement modes can be adjusted to avoid excessively frequent changes. When switching measurement modes, the control of the current measurement pump unit 41 is changed, resulting in a state where the gas sensor 100 is temporarily unable to measure the NOx concentration. By setting the buffer zone between the first and second concentration thresholds C1 and C2, the switching of measurement modes can be adjusted to avoid excessively frequent changes. Therefore, the gas sensor 100 can measure the NOx concentration more continuously and with higher accuracy.

[0188] For example, the first concentration threshold C1 can be set to 50–200 ppm, and the second concentration threshold C2 can be set to 200–500 ppm. For example, the first concentration threshold C1 can be set to 100 ppm, and the second concentration threshold C2 can be set to 300 ppm.

[0189] The measurement mode can be switched within a buffer zone between the first concentration threshold C1 and the second concentration threshold C2. For example, it can be based on the time change of the NOx concentration detection value ( Figure 5 Switching can be based on the slope of the curve (predicting NOx concentration detection values). For example, switching can be based on the time the NOx concentration detection value is within the buffer zone.

[0190] [Concentration detection of the target gas]

[0191] Next, the method for measuring the concentration of the target gas in the measured gas using the gas sensor 100 will be described.

[0192] The control method for the gas sensor in this embodiment includes the following concentration detection steps:

[0193] The switching unit is used to switch between an electromotive force (EMF) measurement mode and a current measurement mode. In EMF measurement mode, the adjustment pump unit is activated while the current measurement pump unit is not activated, and the concentration of the target gas in the measured gas is detected based on the EMF value of the EMF detection sensor unit. In current measurement mode, both the adjustment pump unit and the current measurement pump unit are activated, and the concentration of the target gas in the measured gas is detected based on the current value of the current measurement pump unit. In the concentration detection step, either measurement mode is always executed, enabling continuous concentration detection. Furthermore, in this embodiment, the main pump unit 21 and the auxiliary pump unit 50 function as adjustment pump units.

[0194] The NOx concentration detection and processing of the gas sensor 100 of this embodiment will be described in detail below. Figure 6 This is a flowchart illustrating an example of the NOx concentration detection process of the gas sensor 100.

[0195] For example, if the gas sensor 100 receives a start signal (Dew point), it begins NOx concentration detection processing. When the gas sensor 100 is mounted in a vehicle or similar device, the start signal (Dew point) is, for example, a signal sent to the gas sensor 100 from the vehicle's ECU, exhaust gas treatment system, or similar source. For example, the processing can be initiated by manually turning on the power to the control device 90.

[0196] If the NOx concentration detection process is started, the drive control unit 92 of the control unit 91 first powers on the heater 72 to start heating the heater 72 (step S10), and maintains the sensor element 101 at a drive temperature (e.g., about 800°C) that enables the solid electrolyte to be activated and the NOx concentration to be measured with high accuracy.

[0197] Next, the drive control unit 92 begins controlling the main pump unit 21 (step S11) and begins controlling the auxiliary pump unit 50 (step S12). That is, for the main pump unit 21, control is performed based on the set value Ip1. SET and the set value V0 SET Feedback control is performed for the auxiliary pump unit 50: based on the set value V1 SET Feedback control. Regarding steps S11 and S12, either one can be executed first, or they can be executed simultaneously. In addition, regarding steps S11 and S12, they can be executed after the sensor element 101 reaches the driving temperature, or they can be executed at a temperature lower than the driving temperature.

[0198] The gas to be measured passes sequentially through the gas inlet 10, the first diffusion rate control unit 11, the buffer space 12, and the second diffusion rate control unit 13 to reach the first internal cavity 20, where the oxygen concentration is adjusted by the main pump unit 21. Then, the gas passes through the third diffusion rate control unit 30 to reach the second internal cavity 40, where the oxygen concentration is further adjusted by the auxiliary pump unit 50. After the oxygen concentration is adjusted to a constant low concentration by the main pump unit 21 and the auxiliary pump unit 50, the gas to be measured passes through the fourth diffusion rate control unit 60 to reach the third internal cavity 61.

[0199] Next, the measurement mode switching unit 94 of the control unit 91 switches to the current measurement mode (step S13). Specifically, the measurement mode switching unit 94 outputs a control signal to the switching unit 47, which is an example of a switching unit, indicating that the mode is turned on. Additionally, the measurement mode switching unit 94 issues 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 unit 41 and detects the pump current Ip2 of the current measurement pump unit 41. The measurement mode switching unit 94 issues an instruction to the concentration calculation unit 93 to acquire the pump current Ip2 of the current measurement pump unit 41 and calculate the NOx concentration based on the current-concentration conversion parameter. In the current measurement mode, as described above, the pump current Ip2 corresponding to the NOx concentration flows in the current measurement pump unit 41. Step S13 can be performed simultaneously with either or both of the above steps S11 and S12.

[0200] Next, the concentration calculation unit 93 acquires the pump current Ip2 of the current measuring pump unit 41 and calculates the NOx concentration in the gas being measured based on the pre-stored conversion parameter (current-concentration conversion parameter) between the pump current Ip and the NOx concentration in the gas being 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 the acquired NOx concentration is lower than the first concentration threshold C1 (step S15). The first concentration threshold C1 is, for example, a lower limit value of the NOx concentration at which the desired measurement accuracy can be obtained when measuring in the current measuring mode is preset.

[0201] In step S15, when the NOx concentration obtained from the concentration calculation unit 93 is above the first concentration threshold C1, the processing after step S14 is executed. That is, when the NOx concentration is above 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 to execute the current measurement mode.

[0202] In step S15, when the NOx concentration obtained from the concentration calculation unit 93 is lower than the first concentration threshold C1, the measurement mode switching unit 94 switches to the open-circuit electromotive force (EMF) measurement mode (step S23). Specifically, the measurement mode switching unit 94 outputs a disconnect control signal to the switching unit 47. Additionally, the measurement mode switching unit 94 instructs the drive control unit 92 to perform control in the open-circuit EMF 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 current measurement pump unit 41, and instead detects the open-circuit EMF V2open of the EMF detection sensor unit 82. The measurement mode switching unit 94 instructs the concentration calculation unit 93 to acquire the open-circuit EMF V2open of the EMF detection sensor unit 82 and calculate the NOx concentration based on the open-circuit EMF-concentration conversion parameter. In the electromotive force measurement mode, no pump current Ip2 flows in the current measurement pump unit 41. As described above, the electromotive force detection sensor unit 82 generates an open-circuit electromotive force V2open corresponding to the NOx concentration in the gas being measured.

[0203] Next, the concentration calculation unit 93 acquires the open-circuit electromotive force V2open of the electromotive force detection sensor unit 82, and calculates the NOx concentration in the gas to be measured based on the pre-stored conversion parameter (open-circuit electromotive force-concentration conversion parameter) between the open-circuit electromotive force V2open and 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). The second concentration threshold C2 is, for example, a preset upper limit value of the NOx concentration at which the desired measurement accuracy can be obtained when measuring in the open-circuit electromotive force measurement mode.

[0204] In step S25, if the NOx concentration obtained from the concentration calculation unit 93 is below the second concentration threshold C2, the processing after step S24 is performed. That is, if the NOx concentration is below the second concentration threshold C2, 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 to execute the open-circuit electromotive force measurement mode.

[0205] In step S25, when the NOx concentration obtained from the concentration calculation unit 93 is higher than the second concentration threshold C2, the measurement mode switching unit 94 switches to the current measurement mode (step S13) and performs the processing after step S14.

[0206] Thus, the control unit 91, in the measurement mode switching unit 94, determines, based on the NOx concentration obtained from the concentration calculation unit 93, whether to use a current measurement mode or an electromotive force measurement mode. As a result, NOx concentration is detected using either measurement mode. By distinguishing between the electromotive force measurement mode, which can measure the gas containing low concentrations of NOx with good accuracy, and the current measurement mode, which can measure the gas containing high concentrations of NOx with good accuracy, NOx concentration can be measured with good accuracy over a wide concentration range, including low concentrations.

[0207] In step S13, if the measurement mode switching unit 94 switches from the open-circuit electromotive force measurement mode to the current measurement mode, the pump current Ip2 flows in the current measurement pump unit 41, so that the electromotive force V2 detected by the electromotive force detection sensor unit 82 reaches the set value V2. SETThe atmosphere near the measuring electrode 44 is controlled to a state in which all oxygen originating from NOx is drawn out. In this state, after measuring the NOx concentration at least once in the current measurement mode (step S14), the system switches to the electromotive force measurement mode (step S23). Furthermore, in step S23, if the measurement mode switching unit 94 switches from the current measurement mode to the open-circuit electromotive force measurement mode, no current flows through the current measurement pump unit 41, thereby controlling the atmosphere near the measuring electrode 44 to a state in which oxygen originating from NOx is present. In this state, after measuring the NOx concentration at least once in the electromotive force measurement mode (step S24), the system switches to the current measurement mode (step S13).

[0208] Thus, when switching measurement modes, the determination of the next measurement mode switch is made after controlling the atmosphere near the measuring electrode 44 to form a state in which the NOx concentration can be measured. Typically, measurement mode switches are performed at intervals of more than one second. It is not intended to perform so-called pulse current-based on-off control, which is based on the switching of the pump current Ip2 based on a minute time interval and cannot achieve the aforementioned switching of the atmosphere near the measuring electrode 44.

[0209] In step S13, immediately after the measurement mode switching unit 94 switches from the open-circuit electromotive force measurement mode to the current measurement mode, the control of the current measurement pump unit 41 is changed, causing the pump current Ip2 to become unstable. Therefore, the concentration calculation unit 93 can execute step S14 after a predetermined waiting time. Furthermore, in step S23, even immediately after the measurement mode switching unit 94 switches from the current measurement mode to the open-circuit electromotive force measurement mode, the control of the current measurement pump unit 41 is sometimes changed, causing the open-circuit electromotive force V2open to become unstable. Therefore, the concentration calculation unit 93 can execute step S24 after a predetermined waiting time.

[0210] As described above, a gas sensor 100 for detecting the NOx concentration in a gas to be measured is shown as an example of an embodiment of the present invention, but the present invention is not limited to this method. The present invention may also include gas sensors with various sensor elements or control devices, as long as they fall within the scope of the present invention's objective of achieving high-precision measurement of a target gas across a concentration range, including low concentrations.

[0211] In the above embodiment, a switch is provided as an example of the switching unit 47, but the present invention is not limited to this method. For example, the variable power supply 46 can also be used as the switching unit 47. The measurement mode switching unit 94 can switch to a state where the pump voltage Vp2 of the variable power supply 46 is zero and no voltage is applied to the current measuring pump unit 41 during the switching process to the electromotive force measurement mode, thereby preventing current from flowing; and it can switch to a state where the pump voltage Vp2 of the variable power supply 46 is a predetermined value and a predetermined voltage is applied to the current measuring pump unit 41 during the switching process to the current measuring mode, thereby allowing current to flow. Furthermore, in the current measuring mode, as in the above embodiment, the pump voltage Vp2 of the variable power supply 46 of the current measuring pump unit 41 can be feedback controlled so that the electromotive force V2, which is the control voltage detected by the electromotive force detection sensor unit 82, reaches the set value V2. SET .

[0212] In the above embodiment, after step S12, the measurement mode switching unit 94 first switches to the current measurement mode in step S13, but it may also switch to the electromotive force measurement mode first in step S23 after step S12.

[0213] Alternatively, when the gas sensor 100 is activated, the control device 90 can be preset to the current measurement mode or the electromotive force measurement mode.

[0214] When the gas sensor 100 is activated, the interior of the third internal cavity 61 is filled with the gas to be measured. Compared to the state of stable drive control, the oxygen concentration in the gas to be measured inside the third internal cavity 61 is generally higher. In the current measurement mode at startup, the current measurement pump unit 41 is activated in addition to the main pump unit 21 and the auxiliary pump unit 50. As a result, oxygen in the gas to be measured inside the third internal cavity 61 can be actively drawn out, achieving drive control earlier and more stably. That is, the time from when the gas sensor 100 is activated until the NOx concentration can be measured (startup time) can be shortened.

[0215] 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 in the concentration calculation unit 93, but is not limited to this.

[0216] As a threshold for switching from current measurement mode to electromotive force measurement mode, those skilled in the art can appropriately set a lower limit value of the pump current Ip2 that allows for the desired measurement accuracy when measuring in current measurement mode, instead of the first concentration threshold C1. As a threshold for switching from electromotive force measurement mode to current measurement mode, for example, those skilled in the art can appropriately set an upper limit value of the open-circuit electromotive force V2open that allows for the desired measurement accuracy when measuring in electromotive force measurement mode, instead of the second concentration threshold C2.

[0217] For example, switching between electromotive force measurement mode and current measurement mode can be based on signals from other devices such as the vehicle's ECU and exhaust gas treatment system.

[0218] In the gas sensor 100 of the above embodiment, the current measuring pump unit 41 and the electromotive force detection sensor unit 82 are configured as different electrochemical units, but are not limited thereto. For example, the current measuring pump unit can be configured as a pump unit between the measuring electrode 44 and the reference electrode 42. That is, the reference electrode 42 can function as an outer measuring electrode of the current measuring pump unit. (Refer to...) Figure 1 The sensor element 101 and the current measuring pump unit can be configured to include a measuring electrode 44, a first solid electrolyte layer 4, a third substrate layer 3, and a reference electrode 42. Although the reference electrode 42 is formed inside the substrate portion 102, it can also be disposed at a different position than the measuring electrode 44 disposed on the inner surface of the gas flow section 15 and used as an outer measuring electrode, if the measuring electrode 44 is used as a reference. Furthermore, even in this case, the control device includes a switching unit that switches the flow of current in or out of the current measuring pump unit.

[0219] In this case, in electromotive force measurement mode, the switching unit is switched to a state where current does not flow in the current measurement pump unit, and the open-circuit electromotive force V2open generated between the measuring electrode 44 and the reference electrode 42 is detected. Furthermore, the NOx concentration is detected based on this open-circuit electromotive force V2open. In current measurement mode, the switching unit is switched to a state where current flows in the current measurement pump unit, and a constant pump voltage is applied between the measuring electrode 44 and the reference electrode 42, allowing the pump current to flow. The NOx concentration is detected based on this pump current.

[0220] In the above 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 can be other oxide gases besides NOx (e.g., carbon dioxide CO2, water H2O, etc.). When the gas to be measured is an oxide gas, in the current measurement mode, the gas to be measured, which contains the oxide gas itself, is introduced into the third internal cavity 61 in the same way as in the embodiment for detecting NOx concentration described above. The oxide gas in the gas to be measured is reduced at the measuring electrode 44 to generate oxygen. The generated oxygen is then detected as the pump current Ip2 of the current measuring pump unit 41.

[0221] If carbon dioxide (CO2) and water (H2O) are reduced, reducing gases such as carbon monoxide (CO), hydrogen (H2), and oxygen (O2) are produced, respectively. In electromotive force measurement mode, the setpoint V0 used for controlling the main pump unit 21 is... SET and the setpoint V1 for controlling the auxiliary pump unit 50. SET The oxygen concentration in the gas to be measured is controlled to a lower concentration by setting the value to be greater than NOx, and carbon dioxide (CO2) and water (H2O) are reduced (decomposed) at least one of the inner main pump electrode 22 and auxiliary pump electrode 51. As a result, the reducing gas produced by the reduction of carbon dioxide (CO2) and water (H2O), and the oxygen (residual oxygen) after its concentration is adjusted by the main pump unit 21 and auxiliary pump unit 50, reach the measuring electrode 44. At this time, by controlling the residual oxygen concentration in the gas to be measured to such that the ratio of reducing gas (concentrated gas) to residual oxygen is close to the stoichiometric air-fuel ratio (chemical air-fuel ratio) point, the concentration of reducing gas (concentrated gas) caused by carbon dioxide (CO2) and water (H2O) can be detected as the electromotive force V2open of the electromotive force detection sensor unit 82. In the region near the stoichiometric air-fuel ratio (SFR) point, compared to other regions, there is a trend of a large change in the open-circuit electromotive force V2open relative to a small change in the concentration of the rich gas. Therefore, it is possible to detect the low concentration of reducing gas (rich gas) caused by low concentrations of carbon dioxide (CO2) and water (H2O).

[0222] In the above-described embodiment for detecting NOx concentration, for example, the setpoint V0 used for controlling the main pump unit 21 can be... SET The voltage is set to approximately 150mV to 450mV, and the set value V1 used for controlling the auxiliary pump unit 50 can be adjusted. SET The setting is approximately 150mV to 450mV. On the other hand, when measuring carbon dioxide (CO2) and water (H2O), for example, the setpoint V0 used for controlling the main pump unit 21 can be... SETThe setting is approximately 450mV to 1000mV, and the setpoint V1 used for controlling the auxiliary pump unit 50 can be adjusted. SET The setting is approximately 450mV to 1000mV. This allows for appropriate setting based on the type of gas being measured: the setpoint V0 used for controlling the main pump unit 21. SET and the setpoint V1 for controlling the auxiliary pump unit 50 SET Regarding the setting value Ip1 used for controlling the auxiliary pump unit 50. SET And the set value V2 for controlling the pump unit 41 for current measurement in current measurement mode. SET It can also be set appropriately according to the type of gas being measured.

[0223] Alternatively, the target gas can be a non-oxide gas such as ammonia (NH3). When the target gas is a non-oxide gas, it is converted into an oxide gas (e.g., NO in the case of ammonia (NH3)). The gas to be measured, containing the converted oxide gas, is introduced into the third internal cavity 61. The measured electrode 44 reduces the converted oxide gas in the gas to produce oxygen. The generated oxygen is detected as the electromotive force V2open of the electromotive force detection sensor unit 82 in electromotive force measurement mode, and as the pump current Ip2 of the current measurement pump unit 41 in current measurement mode. At least one of the inner main pump electrode 22 and the auxiliary pump electrode 51 functions as a catalyst to convert the non-oxide gas into an oxide gas.

[0224] In the above embodiment, the drive control unit 92 of the control unit 91 performs the following: setting the set value V0 of the electromotive force V0 of the oxygen partial pressure detection sensor unit 80 for main pump control based on the pump current Ip1. SET Feedback control is used to ensure that the pump current Ip1 of the auxiliary pump unit 50 reaches the set value Ip1. SET The pump voltage Vp0 of the variable power supply 24 of the main pump unit 21 is controlled by feedback to ensure that the electromotive force V0 reaches the set value V0. SET However, the control method is not limited to this. For example, the drive control unit 92 can perform feedback control on the pump voltage Vp0 of the variable power supply 24 of the main pump unit 21 so that the pump current Ip1 of the auxiliary pump unit 50 reaches the set value Ip1. SET That is, the drive control unit 92 may not need to acquire the electromotive force V0 of the oxygen partial pressure detection sensor unit 80 for main pump control, or set the value V0. SET Instead of setting it up, it directly controls the pump voltage Vp0 based on the pump current Ip1.

[0225] Regarding the gas sensor 100 of the above-described embodiment, as follows: Figure 1 As shown, the sensor element 101 is configured to have three internal cavities: a first internal cavity 20, a second internal cavity 40, and a third internal cavity 61. Each internal cavity is equipped with an inner main pump electrode 22, an auxiliary pump electrode 51, and a measuring electrode 44, but this configuration is not limited to this. For example, it can be configured to have two internal cavities: a first internal cavity 20 and a second internal cavity 40. The first internal cavity 20 contains the inner main pump electrode 22, and the second internal cavity 40 contains the auxiliary pump electrode 51 and the measuring electrode 44. In this case, for example, a porous protective layer covering the measuring electrode 44 can be formed as a diffusion rate control unit between the auxiliary pump electrode 51 and the measuring electrode 44.

[0226] In the gas sensor 100 of the above embodiment, the outer pump electrode 23 serves as three electrodes: the outer main pump electrode of the main pump unit 21, the outer auxiliary pump electrode of the auxiliary pump unit 50, and the outer measuring electrode of the current measuring pump unit 41, but is not limited thereto. For example, the outer main pump electrode, the outer auxiliary pump electrode, and the outer measuring electrode can be formed as different electrodes. For example, any one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measuring electrode can be configured to contact the gas to be measured on the outer surface of the substrate 102, different from the outer pump electrode 23. Alternatively, the reference electrode 42 can also serve as any one or more of the outer main pump electrode, the outer auxiliary pump electrode, and the outer measuring electrode.

[0227] As described above, according to the present invention, it is possible to switch 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. Therefore, it is possible to measure a wide concentration range (e.g., 10 to 5000 ppm) of the target gas, including low concentrations. Here, in the present invention, it is not intended that the upper limit of the low concentration be less than 500 ppm. The target gas includes nitrogen-containing gases such as nitrogen oxides (NOx) and ammonia (NH3), as well as reducing gases such as carbon dioxide (CO2) and water (H2O) that produce reducing gases upon decomposition.

Claims

1. A gas sensor for detecting a target gas in a measured gas, comprising: A sensor element and a control device for controlling the sensor element, wherein, The sensor element includes: The elongated, plate-shaped base portion includes a solid electrolyte layer that is oxygen ion conductive; The gas flow section to be measured is formed from one end of the base portion along its length. The adjustment pump unit includes an inner pump electrode and an outer pump electrode, and adjusts the oxygen in the gas to be measured to the desired concentration. The inner pump electrode is disposed on the inner surface of the gas flow section, and the outer pump electrode is disposed at a different position on the base from the gas flow section, and corresponds to the inner pump electrode. A pump unit for current measurement includes an inner measuring electrode and an outer measuring electrode, and the target gas in the gas to be measured is used as the current value for detection. The inner measuring electrode is disposed at one end of the inner surface of the gas flow section, which is further away from the base in the length direction than the inner pump electrode. The outer measuring electrode is disposed at a different position in the base from the gas flow section and corresponds to the inner measuring electrode. A reference electrode, which is arranged inside the substrate to contact a reference gas; and An electromotive force detection sensor unit includes an inner measuring electrode and a reference electrode, and detects the electromotive force value between the inner measuring electrode and the reference electrode. The control device includes a switching unit capable of switching between allowing current to flow through or not flow through the current-measuring pump unit. The control device includes a measurement mode switching unit that switches between an electromotive force measurement mode and a current measurement mode. In the electromotive force measurement mode, the concentration of the target gas in the measured gas is detected based on the electromotive force value of the electromotive force detection sensor unit. In the current measurement mode, the concentration of the target gas in the measured gas is detected based on the current value of the current measurement pump unit. During the switching process to the electromotive force measurement mode, the measurement mode switching unit switches the switching unit to a state where current does not flow in the current measurement pump unit, and during the switching process to the current measurement mode, the switching unit switches the switching unit to a state where current flows in the current measurement pump unit.

2. The gas sensor according to claim 1, wherein, The switching unit includes a switch for switching whether to cut off the conduction of the current measuring pump unit.

3. The gas sensor according to claim 2, wherein, During the switching process to the electromotive force measurement mode, the measurement mode switching unit disconnects the switch to cut off the conduction of the current measurement pump unit, thereby switching to a state where current does not flow. During the switching process to the current measurement mode, the switch is turned on to turn on the current measurement pump unit, thereby switching to a state where current flows.

4. The gas sensor according to claim 1, wherein, The switching unit includes a variable power supply that changes the voltage applied to the current measuring pump unit.

5. The gas sensor according to claim 4, wherein, During the switching process to the electromotive force measurement mode, the measurement mode switching unit sets the voltage of the variable power supply to zero and does not apply voltage to the current measuring pump unit, thereby switching to a state where current does not flow. During the switching process to the current measurement mode, the unit sets the voltage of the variable power supply to a predetermined value and applies a predetermined voltage to the current measuring pump unit, thereby switching to a state where current flows.

6. The gas sensor according to claim 1, wherein, If it is determined that the concentration of the gas to be measured in the current measurement mode is lower than a predetermined first concentration threshold C1, the measurement mode switching unit switches to the electromotive force measurement mode. If it is determined that the concentration of the gas to be measured in the electromotive force measurement mode is higher than the predetermined second concentration threshold C2, the measurement mode switching unit switches to the current measurement mode.

7. The gas sensor according to claim 6, wherein, The first concentration threshold C1 is: the concentration that is lower than the second concentration threshold C2.

8. The gas sensor according to claim 1, wherein, In the current measurement mode, the current value of the current measurement pump unit is controlled so that the electromotive force between the measuring electrode and the reference electrode of the electromotive force detection sensor unit reaches a specified value.

9. The gas sensor according to claim 1, wherein, The reference electrode functions as the outer measuring electrode.

10. A control method for a gas sensor, the gas sensor being used to detect a target gas in a gas to be measured, wherein, The gas sensor includes: a sensor element, and a control device for controlling the sensor element. The sensor element includes: The elongated, plate-shaped base portion includes a solid electrolyte layer that is oxygen ion conductive; The gas flow section to be measured is formed from one end of the base portion along its length. The adjustment pump unit includes an inner pump electrode and an outer pump electrode, and adjusts the oxygen in the gas to be measured to the desired concentration. The inner pump electrode is disposed on the inner surface of the gas flow section, and the outer pump electrode is disposed at a different position on the base from the gas flow section, and corresponds to the inner pump electrode. A pump unit for current measurement includes an inner measuring electrode and an outer measuring electrode, and the target gas in the gas to be measured is used as the current value for detection. The inner measuring electrode is disposed at one end of the inner surface of the gas flow section, which is further away from the base in the length direction than the inner pump electrode. The outer measuring electrode is disposed at a different position in the base from the gas flow section and corresponds to the inner measuring electrode. A reference electrode, which is arranged inside the substrate to contact a reference gas; and An electromotive force detection sensor unit includes an inner measuring electrode and a reference electrode, and detects the electromotive force value between the inner measuring electrode and the reference electrode. The control device includes a switching unit capable of switching between allowing current to flow through or not flow through the current-measuring pump unit. The control method includes: a concentration detection step of switching between an electromotive force measurement mode and a current measurement mode using the switching unit to perform concentration detection; in the electromotive force measurement mode, activating the adjustment pump unit while preventing the current measurement pump unit from activating, and detecting the concentration of the target gas in the measured gas based on the electromotive force detection sensor unit; in the current measurement mode, activating both the adjustment pump unit and the current measurement pump unit, and detecting the concentration of the target gas in the measured gas based on the current value of the current measurement pump unit.

11. The control method according to claim 10, wherein, In the concentration detection step, if it is determined that the concentration of the detected target gas is lower than a predetermined first concentration threshold C1, the switching unit is switched to switch to the electromotive force measurement mode by causing the current to stop flowing in the current measuring pump unit. If it is determined that the concentration of the gas to be measured is higher than the predetermined second concentration threshold C2, the switching unit is switched to switch to the current measurement mode by allowing current to flow through the current measurement pump unit.

12. The control method according to claim 11, wherein, The first concentration threshold C1 is: the concentration that is lower than the second concentration threshold C2.

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