Gas sensor and concentration measurement method using the same

Abstract

The present application provides a gas sensor with a simplified structure compared to the prior art. The sensor element has first to third cavities connected in order from a gas introduction port, oxygen is pumped between the measured gas and a reference pump electrode in contact with the reference gas by intermittently applying a voltage between the inner electrode facing each cavity and the reference pump electrode, in the first cavity, H2O and CO2 in the measured gas are reduced to H2 and CO, in the second cavity, oxygen is taken in, H2 is oxidized to H2O, the H2O concentration is determined based on the intake current when the electromotive force between the inner electrode and the reference pump electrode when no voltage is applied is maintained at a target value, in the third cavity, oxygen is taken in, CO is oxidized to CO2, the CO2 concentration is determined based on the intake current when the electromotive force between the inner electrode and the pump electrode when no voltage is applied is maintained at a target value.

Description

Technical Field

[0001] This invention relates to a multi-gas sensor capable of monitoring and determining the concentration of various gas components. Background Technology

[0002] Regarding the measurement of emissions from automobile exhaust, techniques for measuring the concentration of carbon dioxide (CO2) are known (see, for example, Patent Document 1 and Patent Document 2). The gas sensors disclosed in Patent Document 1 and Patent Document 2 are capable of measuring not only the carbon dioxide (CO2) component but also the water vapor (H2O) component.

[0003] Existing technical documents

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent No. 5918177

[0006] Patent Document 2: Japanese Patent No. 6469464 Summary of the Invention

[0007] Gas sensors with structures disclosed in Patent Documents 1 and 2 meet the general need to maintain accuracy while simplifying the structure. A simpler structure results in less manufacturing time, increased sensor reliability, and lower costs.

[0008] For example, in the case of the sensor element of the gas sensor disclosed in Patent Documents 1 and 2, electrodes containing precious metals are provided at 5 locations. It is desirable to reduce the number of these electrodes from the perspective of simplifying the structure and achieving cost reduction.

[0009] The present invention was made in view of the above-mentioned problems, and its object is to provide a gas sensor with a simplified structure compared with the past.

[0010] To address the aforementioned issues, a first aspect of the present invention is a gas sensor capable of measuring the concentrations of multiple target gas components contained in a gas to be measured, which includes at least water vapor and carbon dioxide. The sensor is characterized by comprising: a sensor element having a structure made of a solid electrolyte with oxygen ion conductivity; and a controller that controls the operation of the gas sensor. The sensor element includes: a gas inlet for introducing the gas to be measured; a first cavity, a second cavity, and a third cavity, which are sequentially connected from the gas inlet via different diffusion rate control units; a reference pump electrode in contact with a reference gas; and a first pump unit. A pump unit comprises a first inner electrode formed facing the first cavity, a reference pump electrode, and a solid electrolyte existing between the first inner electrode and the reference pump electrode; a second pump unit comprises a second inner electrode formed facing the second cavity, the reference pump electrode, and a solid electrolyte existing between the second inner electrode and the reference pump electrode; and a third pump unit comprises a third inner electrode formed facing the third cavity, the reference pump electrode, and a solid electrolyte existing between the third inner electrode and the reference pump electrode. The first pump unit is configured to allow gas to be introduced into the first cavity from the gas inlet. The gas being measured contains water vapor and carbon dioxide, which are substantially completely decomposed. Oxygen is drawn out from the first cavity in such a way that the second pump unit draws oxygen into the second cavity, thereby selectively oxidizing the hydrogen generated by the decomposition of water vapor in the gas being measured from the first cavity to the second cavity in the second cavity. The third pump unit draws oxygen into the third cavity, thereby oxidizing the carbon monoxide generated by the decomposition of carbon dioxide in the gas being measured from the second cavity to the third cavity in the third cavity. The controller includes: a first pump unit control mechanism, which controls the flow of oxygen from the first pump unit to the first inner electrode and the... The system includes: a second pump unit control mechanism that controls the voltage and application time applied between the reference pump electrodes; a third pump unit control mechanism that controls the voltage and application time applied between the third inner electrode and the reference pump electrode in the second pump unit; and a water vapor concentration determination mechanism that determines the concentration of water vapor in the gas being measured based on the magnitude of the current flowing between the second inner electrode and the reference pump electrode when oxygen is drawn into the second cavity through the second pump unit.And a carbon dioxide concentration determining mechanism, which determines the concentration of carbon dioxide in the gas being measured based on the magnitude of the current flowing between the third inner electrode and the reference pump electrode when oxygen is drawn into the third cavity through the third pump unit. The first pump unit control mechanism operates the first pump unit intermittently at intervals of less than 10 ms in such a way that the unapplied electromotive force V1 generated between the first inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 1000 mV to 1500 mV. The second pump unit control mechanism... The second pump unit is configured to operate intermittently at intervals of less than 10 ms, such that the unapplied electromotive force V2 generated between the second inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 250 mV to 450 mV. The third pump unit control mechanism is configured to operate intermittently at intervals of less than 10 ms, such that the unapplied electromotive force V3 generated between the third inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 100 mV to 300 mV.

[0011] The second aspect of the present invention is based on the gas sensor involved in the first aspect, characterized in that the first inner electrode is formed only on one of the surfaces that define the first cavity in the structure, the second inner electrode is formed only on one of the surfaces that define the second cavity in the structure, and the third inner electrode is formed only on one of the surfaces that define the third cavity in the structure.

[0012] The third aspect of the present invention is: a method for measuring the concentration of multiple target gas components contained in a gas to be measured, which contains at least water vapor and carbon dioxide, using a gas sensor, characterized in that the gas sensor comprises a sensor element having a strip-shaped structure made of a solid electrolyte with oxygen ion conductivity, the sensor element comprising: a gas inlet for introducing the gas to be measured; a first cavity, a second cavity, and a third cavity, the three cavities being sequentially connected from the gas inlet via different diffusion rate control units; a reference pump electrode in contact with a reference gas; a first pump unit comprising a first inner electrode formed facing the first cavity, the reference pump electrode, and the solid electrolyte present between the first inner electrode and the reference pump electrode; a second pump unit comprising a second inner electrode formed facing the second cavity, the reference pump electrode, and the solid electrolyte present between the second inner electrode and the reference pump electrode; and a third pump unit comprising a second inner electrode formed facing the second cavity, the reference pump electrode, and the solid electrolyte present between the second inner electrode and the reference pump electrode; and a third pump unit comprising a first cavity, a second inner cavity formed facing the second cavity, the second inner cavity, and the solid electrolyte present between the second inner electrode and the reference pump electrode; and a third pump unit comprising a second inner electrode formed facing the second cavity, the second inner cavity, and the third inner cavity; the second inner cavity formed facing the second cavity; the third inner cavity formed facing the second inner cavity; the second inner cavity formed facing the second inner ... The method comprises a third inner electrode formed in the third cavity, a reference pump electrode, and a solid electrolyte existing between the third inner electrode and the reference pump electrode. The method includes the following steps: a) drawing oxygen out of the first cavity through the first pump unit in such a way that water vapor and carbon dioxide contained in the gas to be measured introduced from the gas inlet into the first cavity are substantially completely decomposed; b) drawing oxygen into the second cavity through the second pump unit, thereby selectively oxidizing hydrogen generated by the decomposition of water vapor in the gas to be measured introduced from the first cavity into the second cavity in the second cavity; c) drawing oxygen into the third cavity through the third pump unit, thereby oxidizing carbon monoxide generated by the decomposition of carbon dioxide in the gas to be measured introduced from the second cavity into the third cavity in the third cavity; d) determining the concentration of water vapor contained in the gas to be measured based on the magnitude of the current flowing between the second inner electrode and the reference pump electrode when oxygen is drawn into the second cavity through the second pump unit.And e) Based on the magnitude of the current flowing between the third inner electrode and the reference pump electrode when oxygen is drawn into the third cavity through the third pump unit, the concentration of carbon dioxide contained in the gas being measured is determined. In step a), the first pump unit is operated intermittently at intervals of less than 10 ms in such a way that the unapplied electromotive force V1 generated between the first inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 1000 mV to 1500 mV. In step b), the second inner electrode is operated intermittently at intervals of less than 10 ms in such a way that no voltage is applied between the second inner electrode and the reference pump electrode. In step c), the second pump unit operates intermittently at intervals of less than 10 ms, such that the unapplied electromotive force V2 generated between the second inner electrode and the reference pump electrode when a voltage is applied between the side electrode and the reference pump electrode is maintained at a predetermined target value within the range of 250 mV to 450 mV. In step c), the third pump unit operates intermittently at intervals of less than 10 ms, such that the unapplied electromotive force V3 generated between the third inner electrode and the reference pump electrode when no voltage is applied between the third inner electrode and the reference pump electrode is maintained at a predetermined target value within the range of 100 mV to 300 mV.

[0013] The fourth aspect of the present invention is based on the concentration measurement method using a gas sensor involved in the third aspect, characterized in that the first inner electrode is formed only on one of the surfaces of the structure that delineate the first cavity, the second inner electrode is formed only on one of the surfaces of the structure that delineate the second cavity, and the third inner electrode is formed only on one of the surfaces of the structure that delineate the third cavity.

[0014] Invention Effects

[0015] According to the first and fourth aspects of the present invention, a multi-gas sensor is realized that, compared with conventional gas sensors, the number and area of ​​electrodes are reduced, the structure is simplified, the cost is reduced, and it is able to measure the concentration of a specified gas component in the gas to be measured in the same way as conventional gas sensors. Attached Figure Description

[0016] Figure 1 This is a diagram that schematically illustrates an example of the configuration of a gas sensor 100.

[0017] Figure 2 This is a block diagram representing the constituent elements of the functions implemented in controller 110.

[0018] Figure 3 This is a graph illustrating the time variation of the electromotive force V1 in the first cavity sensor unit 80 during feedback control.

[0019] Symbol Explanation

[0020] 10…Gas inlet, 11…First diffusion rate control unit, 13…Second diffusion rate control unit, 14…Structure, 20…First cavity, 21…First pump unit, 22…First inner pump electrode, 24, 52, 46…Variable power supply, 30…Third diffusion rate control unit, 40…Second cavity, 41…Third pump unit, 42…Reference pump electrode, 43…Reference gas inlet space, 44…Third inner pump electrode, 50…Second pump unit, 51…Second inner pump electrode, 60…Fourth diffusion rate control unit, 61…Third cavity, 72…Heater, 80…Sensor unit for first cavity, 81…Sensor unit for second cavity, 82…Sensor unit for third cavity, 100…Gas sensor, 101…Sensor element. Detailed Implementation

[0021] <Composition of Gas Sensors>

[0022] Figure 1 This diagram schematically illustrates an example of the configuration of the gas sensor 100 according to this embodiment. The gas sensor 100 is a multi-gas sensor that monitors various gas components and measures their concentrations using sensor elements 101. In this embodiment, at least water vapor (H2O) and carbon dioxide (CO2) are the main target gas components monitored in the gas sensor 100. The gas sensor 100 is used, for example, by being installed in the exhaust path of an internal combustion engine such as an automobile engine, and by using the exhaust gas flowing through that exhaust path as the gas to be measured. Figure 1 A vertical cross-sectional view along the length direction including sensor element 101.

[0023] The sensor element 101 includes: a strip-shaped structure (substrate) 14 formed of an oxygen-ion-conducting solid electrolyte; a gas inlet 10 formed at one end of the structure 14 (the left end in the figure) for introducing the gas to be measured; and a buffer space 12, a first cavity 20, a second cavity 40, and a third cavity 61 formed within the structure 14 and connected sequentially starting from the gas inlet 10. The buffer space 12 is connected to the gas inlet 10 via a first diffusion rate control unit 11. The first cavity 20 is connected to the buffer space 12 via a second diffusion rate control unit 13. The second cavity 40 is connected to the first cavity 20 via a third diffusion rate control unit 30. The third cavity 61 is connected to the second cavity 40 via a fourth diffusion rate control unit 60.

[0024] The structure 14 is constructed by stacking multiple substrates made of, for example, ceramic. Specifically, the structure 14 has a configuration in which six layers—a first substrate 1, a second substrate 2, a third substrate 3, a first solid electrolyte layer 4, an insulating layer 5, and a second solid electrolyte layer 6—are stacked in this order starting from the bottom. Each layer is made of a solid electrolyte with oxygen ion conductivity, such as zirconium oxide (ZrO2).

[0025] The gas inlet 10, the first diffusion rate control unit 11, the buffer space 12, the second diffusion rate control unit 13, the first cavity 20, the third diffusion rate control unit 30, the second cavity 40, the fourth diffusion rate control unit 60, and the third cavity 61 are formed sequentially on one end side of the structure 14, and are formed between the lower surface 6b of the second solid electrolyte layer 6 and the upper surface 4a of the first solid electrolyte layer 4. The portion from the gas inlet 10 to the third cavity 61 is also referred to as the gas flow section.

[0026] The gas inlet 10, buffer space 12, first cavity 20, second cavity 40, and third cavity 61 are formed such that they penetrate the isolation layer 5 along the thickness direction. In the above-mentioned cavities, the lower surface 6b of the second solid electrolyte layer 6 is exposed in the upper part of the attached drawing, and the upper surface 4a of the first solid electrolyte layer 4 is exposed in the lower part of the attached drawing. The sides of the above-mentioned cavities are demarcated by the isolation layer 5 or any diffusion rate control unit.

[0027] The first diffusion rate control unit 11, the second diffusion rate control unit 13, the third diffusion rate control unit 30, and the fourth diffusion rate control unit 60 each have two horizontally elongated slits. That is, they have openings that extend relatively long along a direction perpendicular to the drawing in the upper and lower parts.

[0028] Additionally, a reference gas inlet space 43 is provided at the other end of the sensor element 101 opposite to the end where the gas inlet 10 is provided (the right end in the figure). The reference gas inlet space 43 is formed between the upper surface 3a of the third substrate 3 and the lower surface 5b of the insulating layer 5. Furthermore, the side of the reference gas inlet space 43 is divided by the side of the first solid electrolyte layer 4. For example, oxygen (O2) and atmosphere are introduced into the reference gas inlet space 43 as reference gases.

[0029] The gas inlet 10 is an open portion relative to the external space, through which the gas to be measured is introduced from the external space into the sensor element 101.

[0030] The first diffusion rate control unit 11 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the gas inlet 10 into the buffer space 12.

[0031] The purpose of providing the buffer space 12 is to eliminate the concentration fluctuation of the gas being measured caused by pressure changes in the gas being measured in the external space. Examples of such pressure fluctuations include, for instance, the pulsation of exhaust pressure in automobile exhaust.

[0032] The second diffusion rate control unit 13 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first cavity 20.

[0033] The first cavity 20 is configured such that oxygen is drawn from the gas to be measured, which is introduced through the second diffusion rate control unit 13, and then the H2O and CO2 components contained in the gas to be measured, which are the gas components to be monitored, are reduced (decomposed) to generate hydrogen (H2) and carbon monoxide (CO), so that the gas to be measured not only does not contain oxygen, but also substantially does not contain H2O and CO2. This reduction (decomposition) of H2O and CO2 is achieved by the operation of the first pump unit 21.

[0034] The first pump unit 21 is an electrochemical pump unit consisting of a first inner pump electrode 22, a reference pump electrode 42, and a solid electrolyte present in the portion sandwiched between these two electrodes in the structure 14.

[0035] In the first pump unit 21, a voltage Vp1 is applied between the first inner pump electrode 22 and the reference pump electrode 42 using a variable power supply 24 external to the sensor element 101, thereby generating an oxygen pump current (oxygen ion current) Ip1. Accordingly, oxygen in the first cavity 20 can be drawn out into the reference gas introduction space 43.

[0036] The first inner pump electrode 22 is only disposed on the upper surface 4a of the first solid electrolyte layer 4, which delineates approximately the entire surface of the first cavity 20.

[0037] The first inner pump electrode 22 is configured as a porous metal-ceramic electrode with a top-view rectangular shape, for example, containing platinum and zirconium oxide, with platinum as the metal component.

[0038] The reference pump electrode 42 is an electrode formed between the first solid electrolyte layer 4 and the third substrate 3, for example, configured as a porous metal-ceramic electrode with a top-view rectangle comprising platinum and zirconium oxide.

[0039] A reference gas introduction layer 48, formed of porous alumina and connected to the reference gas introduction space 43, is disposed around the reference pump electrode 42. The reference gas in the reference gas introduction space 43 is introduced to the surface of the reference pump electrode 42 via the reference gas introduction layer 48. That is, the reference pump electrode 42 is in a state of constant contact with the reference gas.

[0040] On the other hand, the first inner pump electrode 22, the reference pump electrode 42, and the solid electrolyte present in the portion sandwiched between these two electrodes in the structure 14 constitute the first cavity sensor unit 80. The first cavity sensor unit 80 is an electrochemical sensor unit for measuring the oxygen partial pressure in the atmosphere within the first cavity 20.

[0041] In the first cavity sensor unit 80, an electromotive force (Nernst electromotive force) V1 is generated between the first inner pump electrode 22 and the reference pump electrode 42. As described later, the use of the first pump unit 21 is intermittent. The electromotive force V1 when the first pump unit 21 is not used and when the voltage Vp1 is not applied using the variable power supply 24 (also referred to as the electromotive force V1 when not applied) is the value corresponding to the difference between the oxygen concentration (oxygen partial pressure) in the first cavity 20 and the oxygen concentration (oxygen partial pressure) of the reference gas. Since the oxygen concentration (oxygen partial pressure) of the reference gas is substantially constant, the electromotive force V1 when not applied is the value corresponding to the oxygen concentration (oxygen partial pressure) in the first cavity 20 at that time.

[0042] The third diffusion rate control unit 30 is a part that imparts a predetermined diffusion resistance to the gas to be measured, which contains H2 and CO but does not substantially contain H2O, CO2 and oxygen, introduced from the first cavity 20 into the second cavity 40.

[0043] The second cavity 40 is configured such that only the H2 contained in the gas to be measured, and the H2 in CO introduced through the third diffusion rate control unit 30, are selectively and completely oxidized, causing them to regenerate H2O. This oxidation of H2 to generate H2O is achieved by the operation of the second pump unit 50.

[0044] The second pump unit 50 is an electrochemical pump unit consisting of a second inner pump electrode 51, a reference pump electrode 42, and a solid electrolyte present in the portion sandwiched between these two electrodes in the structure 14.

[0045] In the second pump unit 50, a voltage Vp2 is applied between the second inner pump electrode 51 and the reference pump electrode 42 using a variable power supply 52 external to the sensor element 101, thereby generating an oxygen pump current (oxygen ion current) Ip2. Accordingly, oxygen can be drawn from the reference gas introduction space 43 into the second cavity 40.

[0046] The second inner pump electrode 51 is only disposed on approximately the entire surface of the second cavity 40, which is located on the upper surface 4a of the first solid electrolyte layer 4.

[0047] The second inner pump electrode 51 is configured as a top-view rectangular porous metal-ceramic electrode with Pt-Au alloy as the metal component, for example, containing the Pt-Au alloy and zirconium oxide.

[0048] On the other hand, the second inner pump electrode 51, the reference pump electrode 42, and the solid electrolyte present in the portion sandwiched between these two electrodes in the structure 14 constitute the second cavity sensor unit 81. The second cavity sensor unit 81 is an electrochemical sensor unit used to measure the oxygen partial pressure in the atmosphere within the second cavity 40.

[0049] In the second cavity sensor unit 81, an electromotive force (Nernst electromotive force) V2 is generated between the second inner pump electrode 51 and the reference pump electrode 42. As described later, the use of the second pump unit 50 is intermittent. The electromotive force V2 when the second pump unit 50 is not used and when the voltage Vp2 is not applied using the variable power supply 52 (also referred to as the electromotive force V2 when not applied) is the value corresponding to the difference between the oxygen concentration (oxygen partial pressure) in the second cavity 40 and the oxygen concentration (oxygen partial pressure) of the reference gas. Since the oxygen concentration (oxygen partial pressure) of the reference gas is substantially constant, the electromotive force V2 when not applied is the value corresponding to the oxygen concentration (oxygen partial pressure) in the second cavity 40 at that time.

[0050] The fourth diffusion rate control unit 60 is a part that imparts a predetermined diffusion resistance to the gas to be measured, which contains H2O and CO but does not substantially contain CO2 and oxygen, introduced from the second cavity 40 into the third cavity 61.

[0051] The third cavity 61 is configured such that all CO contained in the gas to be measured, introduced through the fourth diffusion rate control unit 60, is oxidized, causing it to regenerate CO2. This oxidation of CO to generate CO2 is achieved by the operation of the third pump unit 41.

[0052] The third pump unit 41 is an electrochemical pump unit consisting of a third inner pump electrode 44, a reference pump electrode 42, and a solid electrolyte present in the portion sandwiched between these two electrodes in the structure 14.

[0053] In the third pump unit 41, a voltage Vp3 is applied between the third inner pump electrode 44 and the reference pump electrode 42 using a variable power supply 46 external to the sensor element 101, thereby generating an oxygen pump current (oxygen ion current) Ip3. Accordingly, oxygen can be drawn from the reference gas introduction space 43 into the third cavity 61.

[0054] The third inner pump electrode 44 is disposed on approximately the entire surface of the third cavity 61, which is defined by the upper surface 4a of the first solid electrolyte layer 4.

[0055] The third inner pump electrode 44 is configured as a porous metal-ceramic electrode with a top-view rectangular shape, for example, containing platinum and zirconium oxide, with platinum as the metal component.

[0056] On the other hand, the third inner pump electrode 44, the reference pump electrode 42, and the solid electrolyte present in the portion sandwiched between these two electrodes in the structure 14 constitute the third cavity sensor unit 82. The third cavity sensor unit 82 is an electrochemical sensor unit used to measure the oxygen partial pressure in the atmosphere within the third cavity 61.

[0057] In the third cavity sensor unit 82, an electromotive force (Nernst electromotive force) V3 is generated between the third inner pump electrode 44 and the reference pump electrode 42. As described later, the use of the third pump unit 41 is intermittent. The electromotive force V3 when the third pump unit 41 is not used and when the voltage Vp3 is not applied using the variable power supply 46 (also referred to as the electromotive force V3 when not applied) is the value corresponding to the difference between the oxygen concentration (oxygen partial pressure) in the third cavity 61 and the oxygen concentration (oxygen partial pressure) of the reference gas. Since the oxygen concentration (oxygen partial pressure) of the reference gas is substantially constant, the electromotive force V3 when not applied is the value corresponding to the oxygen concentration (oxygen partial pressure) in the third cavity 61 at that time.

[0058] In addition to the components mentioned above, the sensor element 101 includes a heater 72 sandwiched between the second substrate 2 and the third substrate 3. The heater 72 is heated by external power supplied via heater electrodes 71 disposed on the lower surface 1b of the first substrate 1. The heater 72 is embedded throughout the buffer space 12 to the third cavity 61, enabling it to heat the sensor element 101 to a predetermined temperature and maintain that temperature. Heating by the heater 72 improves the oxygen ion conductivity of the solid electrolyte constituting the sensor element 101.

[0059] Above and below the heater 72, for the purpose of obtaining electrical insulation with the second substrate 2 and the third substrate 3, a heater insulation layer 74 made of alumina or the like is formed. Hereinafter, the heater 72, the heater electrode, and the heater insulation layer 74 will be collectively referred to as the heater section. In addition, the heater section is provided with a pressure relief hole 75. The pressure relief hole 75 is provided to penetrate the third substrate 3 and communicate with the reference gas introduction space 43. The purpose of providing the pressure relief hole 75 is to mitigate the increase in internal pressure that accompanies the temperature rise within the heater insulation layer 74.

[0060] The sensor element 101 with the above configuration is different from the sensor elements of conventional gas sensors, such as those disclosed in Patent Documents 1 and 2, especially in terms of various electrode configurations.

[0061] Specifically, in the sensor element 101, the first inner pump electrode 22 of the first cavity 20 and the second inner pump electrode 51 of the second cavity 40 are both provided on approximately the entire surface of the upper surface 4a of the first solid electrolyte layer 4. However, in the case of conventional sensor elements, the electrodes corresponding to these electrodes are at least provided on the lower surface 6b of the second solid electrolyte layer 6, which delineates the position of each cavity.

[0062] Furthermore, in sensor element 101, a pump unit is constituted by each of the first to third inner pump electrodes, the reference pump electrode 42, and the solid electrolyte present between these electrodes. Oxygen is pumped (absorbed or drawn in) between the first to third cavities and the reference gas introduction space 43 through these pump units. However, in conventional sensor elements, an outer pump electrode is separately provided on the outer surface of the element. A pump unit is constituted by electrodes corresponding to the first to third inner pump electrodes, the outer pump electrode, and the solid electrolyte present between these electrodes. Oxygen is pumped between the first to third cavities and the external space through these pump units. Additionally, the electrode corresponding to the reference pump electrode 42 in sensor element 101 (the reference electrode) does not perform oxygen pumping; it is only used as a reference for the potential in each sensor unit.

[0063] That is, compared with the sensor elements of conventional gas sensors, the gas sensor 100 according to this embodiment has fewer pump electrodes and a smaller area, and achieves the integration of the functions of the reference electrode. Since the pump electrodes use precious metals such as platinum, the gas sensor 100 according to this embodiment can achieve low cost compared with conventional gas sensors.

[0064] In addition, the gas sensor 100 also includes a controller 110, which controls the operation of the sensor element 101 and performs the following processing: determining the concentration of the gas component to be monitored based on the current flowing through the sensor element 101.

[0065] Figure 2 This is a block diagram illustrating the constituent elements of the functions implemented in the controller 110. The controller 110 is composed of one or more electronic circuits having, for example, one or more CPUs (Central Processing Units) and storage devices. The electronic circuits are also software functional units that implement predetermined functions by the CPU executing, for example, a predetermined program stored in the storage device. Alternatively, it may be composed of integrated circuits such as an FPGA (Field-Programmable Gate Array) obtained by connecting multiple electronic circuits according to their functions.

[0066] It should be noted that when the gas sensor 100 is installed in the exhaust path of the automobile engine and the exhaust gas flowing through the exhaust path is used as the measured gas, part or all of the function of the controller 110 can be implemented by the automobile's ECU (electronic control unit).

[0067] The controller 110 includes, as a component of the functions implemented by executing a predetermined program in the CPU, an element operation control unit 111 that controls the operation of each part of the sensor element 101; and a concentration determination unit 112 that performs the process of determining the concentration of the target gas component contained in the gas being measured.

[0068] The component operation control unit 111 mainly includes: a heater control unit 111A, which controls the operation of the heater 72; a first pump unit control unit 111B, which controls the operation of the first pump unit 21; a second pump unit control unit 111C, which controls the operation of the second pump unit 50; and a third pump unit control unit 111D, which controls the operation of the third pump unit 41.

[0069] On the other hand, the concentration determination unit 112 mainly includes a water vapor concentration determination unit 112C and a carbon dioxide concentration determination unit 112D, which determine the concentrations of the main monitored gas components, namely H2O and CO2, in the gas sensor 100.

[0070] <Multi-gas monitoring and concentration determination>

[0071] Next, the monitoring of multiple gas types (multi-gas monitoring) and the method for determining the concentration of the monitored gas, implemented in the gas sensor 100 having the configuration described above, will be explained. In the following text, the gas to be measured is exhaust gas containing oxygen, H2O, and CO2.

[0072] In the gas sensor 100 of this embodiment, the gas to be measured introduced into the element from the gas inlet 10 is introduced into the first cavity 20 through the buffer space 12.

[0073] In the first cavity 20, oxygen is drawn from the introduced gas to be measured by the operation of the first pump unit 21. Accordingly, the H2O and CO2 contained in the gas to be measured undergo reduction (decomposition) reactions (2H2O→2H2+O2, 2CO2→2CO+O2), and H2O and CO2 are essentially completely decomposed into hydrogen (H2), carbon monoxide (CO), and oxygen, and the resulting oxygen is also drawn out. It should be noted that "essentially completely decomposed H2O and CO2" means that H2O and CO2 are not introduced into the second cavity 40.

[0074] The oxygen extraction described above is performed as follows: the first pump unit control unit 111B operates the first pump unit 21 intermittently, and sets a target value (control voltage) (preferably set to 1000mV) for the unapplied electromotive force V1 in the first cavity sensor unit 80 within the range of 1000mV to 1500mV, based on the desired oxygen partial pressure (oxygen concentration). Then, based on the difference between the actual electromotive force V1 value and the target value, feedback control is performed on the voltage Vp1 applied to the first pump unit 21 by the variable power supply 24 and the operating time of the first pump unit 21 to achieve the aforementioned target value. In this case, the voltage Vp1 is the sum of the IR overvoltage and the reaction overvoltage.

[0075] When a gas to be measured, for example, containing a large amount of oxygen, arrives at the first cavity 20, the value of the electromotive force V1 when no force is applied will deviate significantly from the target value. Therefore, the first pump unit control unit 111B controls the pump voltage Vp1 applied by the variable power supply 24 to the first pump unit 21 to reduce this deviation.

[0076] Figure 3 This is a graph illustrating the time variation of the electromotive force V1 in the first cavity sensor unit 80 when feedback control is performed using the first pump unit control unit 111B. Figure 3 During the time intervals Δta(1), Δta(2)..., voltages Vp1(1), Vp1(2)... are applied to the first pump unit 21 respectively. During the time intervals Δta(1), Δta(2)..., i.e., time intervals Δtb(1), Δtb(2)..., no voltage is applied to the first pump unit 21.

[0077] exist Figure 3 In the case shown, a voltage is intermittently applied to the first pump unit 21 at times Δta(1), Δta(2), ..., thereby intermittently drawing oxygen from the first cavity 20. When no voltage is applied, the electromotive force V1 gradually increases to V1(1) at time Δtb(1) and gradually increases to V1(2) at Δtb(2), ... While appropriately adjusting the applied voltage and application time based on the measured value of the electromotive force V1 when no voltage is applied, the above-described intermittent voltage application to the first pump unit 21 is repeated until the electromotive force V1 when no voltage is applied approaches the target value. Shortly afterward, the oxygen partial pressure in the sensor unit 80 of the first cavity remains at the desired value.

[0078] It should be noted that since the gas to be measured is continuously introduced from the gas inlet 10, while the operation of the first pump unit 21 is intermittent, the oxygen partial pressure (oxygen concentration) in the first cavity 20 may change due to gas diffusion during the period when the first pump unit 21 is not operating. If the oxygen partial pressure (oxygen concentration) changes, the electromotive force V1 will also change when no force is applied, so it may be difficult to reach the target value.

[0079] In this embodiment, in view of this, the interval during which the first pump unit 21 does not operate is ( Figure 3 Δtb(1), Δtb(2), ..., are each set to less than 10 ms, and the sum of this interval and the application time of voltage Vp1 applied by the first pump unit 21 is set to approximately 10 ms. Accordingly, the change in oxygen partial pressure (oxygen concentration) caused by gas diffusion is sufficiently small, and consequently, the change in electromotive force V1 when no voltage is applied is also suppressed. It should be noted that the time constant for gas diffusion, confirmed in advance by using the Cole plot of the electrodes obtained experimentally, is 100 ms; based on this, the upper limit of the interval time is set to 10 ms.

[0080] As a result, the target value (control voltage) of the electromotive force V1 when no voltage is applied is set to a value in the range of 1000mV to 1500mV, so that the oxygen partial pressure in the first cavity 20 is sufficiently reduced. When V1 = 1000mV, the oxygen partial pressure is 10. -20 The gas to be measured in the first cavity 20 contains H2 and CO, but does not actually contain H2O, CO2, or oxygen. This gas is then introduced into the second cavity 40.

[0081] In the second cavity 40, oxygen is drawn in by the second pump unit 50, which oxidizes only the H2 contained in the introduced gas to be measured.

[0082] The oxygen inhalation described above is performed as follows: the second pump unit control unit 111C operates the second pump unit 50 intermittently, and sets a target value (control voltage) (preferably set to 350mV) for the unapplied electromotive force V2 in the second cavity sensor unit 81 within the range of 250mV to 450mV, based on the desired oxygen partial pressure (oxygen concentration). Then, based on the difference between the actual electromotive force V2 value and the target value, feedback control is performed on the voltage Vp2 applied to the second pump unit 50 by the variable power supply 52 and the operating time of the second pump unit 50 to achieve the aforementioned target value. Although there is a difference between inhalation and exhalation, the specific method of feedback control is similar to... Figure 3 The same applies to the first pump unit 21. That is, for the second pump unit 50, the interval time is set to less than 10 ms, and the sum of the working time and the interval time is set to about 10 ms, and it is made to work intermittently.

[0083] The second pump unit 50 operates in the manner described above. Within the second cavity 40, the oxidation (combustion) reaction 2H2 + O2 → 2H2O is promoted, and an amount of H2O is regenerated that is correlated with the amount of H2O introduced from the gas inlet 10. It should be noted that in this embodiment, the correlation between the amount of H2O or CO2 and the amount of H2O or CO2 introduced from the gas inlet 10 and the amount of H2 produced by their decomposition and the amount of H2O or CO2 regenerated by the oxidation of CO are equal or within a certain acceptable error range for the sake of measurement accuracy.

[0084] By setting the target value of the unapplied electromotive force V2 within the range of 250mV to 450mV, the oxygen partial pressure in the second cavity 40 is maintained at a value within which H2 is almost entirely oxidized but CO is not oxidized. For example, when V2 = 350mV, the oxygen partial pressure is 10. -7 Around ATM.

[0085] Furthermore, when the electromotive force V2 is maintained at the target value without application, the oxygen pump current Ip2 flowing through the second pump unit 50 (hereinafter also referred to as the water vapor detection current Ip2) is approximately proportional to the concentration of H2O generated by the combustion of H2 in the second cavity 40 (there is a linear relationship between the water vapor detection current Ip2 and the concentration of generated H2O). The amount of H2O generated by this combustion is correlated with the amount of H2O in the measured gas that is temporarily decomposed in the first cavity 20 after being introduced from the gas inlet 10. Accordingly, by detecting the water vapor detection current Ip2 in the second pump unit control unit 111C, the H2O in the measured gas can be monitored.

[0086] Furthermore, a linear relationship exists between the water vapor detection current Ip2 and the water vapor concentration in the gas being measured. Data representing this linear relationship (water vapor characteristic data) is determined in advance using a sample gas with a known water vapor concentration and stored in the water vapor concentration determination unit 112C. In the gas sensor 100 according to this embodiment, the water vapor concentration determination unit 112C acquires the value of the water vapor detection current Ip2 detected in the second pump unit control unit 111C. The water vapor concentration determination unit 112C refers to the water vapor characteristic data to determine the value of the water vapor concentration corresponding to the acquired water vapor detection current Ip2. Based on this, the water vapor concentration in the gas being measured can be determined.

[0087] It should be noted that if the gas to be measured introduced from the gas inlet 10 does not contain H2O, then of course H2O will not decompose in the first cavity 20. Therefore, no H2 will be introduced into the second cavity 40, and thus the water vapor detection current Ip2 will be approximately zero.

[0088] H2 is oxidized to H2O, resulting in a gas containing H2O and CO but substantially free of CO2 and oxygen. This gas is then introduced into the third cavity 61. In the third cavity 61, oxygen is drawn in by the third pump unit 41, and the CO contained in the introduced gas is oxidized.

[0089] The oxygen inhalation is performed as follows: the third pump unit control unit 111D operates the third pump unit 41 intermittently. Based on the desired oxygen partial pressure (oxygen concentration), a target value (control voltage) (preferably set to 200mV) is set in the third cavity sensor unit 82 within the range of 100mV to 300mV for the unapplied electromotive force V3. Based on the difference between the actual electromotive force V3 value and the target value, feedback control is performed on the voltage Vp3 applied to the third pump unit 41 by the variable power supply 46 and the operating time of the third pump unit 41 to achieve the aforementioned target value. Although there is a difference between inhalation and exhalation, the specific method of feedback control is similar to... Figure 3 The same applies to the first pump unit 21. That is, for the third pump unit 41, the interval time is set to less than 10ms, and the sum of the working time and the interval time is set to about 10ms, and it is made to work intermittently.

[0090] The third pump unit 41 operates in the manner described above, and within the third cavity 61, the oxidation (combustion) reaction 2CO + O2 → 2CO2 is promoted, generating an amount of CO2 that is correlated with the amount of CO2 introduced from the gas inlet 10.

[0091] By setting the target value of the electromotive force V3 when no electromotive force is applied to a value within the range of 100mV to 300mV, the oxygen partial pressure in the third cavity 61 is maintained at a value within the range where CO is approximately completely oxidized. For example, when V3 = 200mV, the oxygen partial pressure is 10. -4 Around ATM.

[0092] Furthermore, when the electromotive force V3 remains at the target value without application, the oxygen pump current Ip3 flowing through the third pump unit 41 (hereinafter also referred to as the carbon dioxide detection current Ip3) is approximately proportional to the concentration of CO2 generated by the combustion of CO in the third cavity 61 (the carbon dioxide detection current Ip3 and the concentration of generated CO2 have a linear relationship). The amount of CO2 generated by this combustion is correlated with the amount of CO2 in the measured gas that is temporarily decomposed in the first cavity 20 after being introduced from the gas inlet 10. Accordingly, by detecting the carbon dioxide detection current Ip3 in the third pump unit control unit 111D, CO2 in the measured gas can be monitored.

[0093] Furthermore, a linear relationship exists between the carbon dioxide detection current Ip3 and the carbon dioxide concentration in the gas being measured. Data representing this linear relationship (carbon dioxide characteristic data) is determined in advance using a sample gas with a known carbon dioxide concentration and stored in the carbon dioxide concentration determination unit 112D. In the gas sensor 100 according to this embodiment, the carbon dioxide concentration determination unit 112D acquires the value of the carbon dioxide detection current Ip3 detected in the third pump unit control unit 111D. The carbon dioxide concentration determination unit 112D determines the value of the carbon dioxide concentration corresponding to the acquired carbon dioxide detection current Ip3 by referring to the carbon dioxide characteristic data. Based on this, the carbon dioxide concentration in the gas being measured can be determined.

[0094] It should be noted that if there is no CO2 in the gas to be measured introduced from the gas inlet 10, CO2 will not decompose in the first cavity 20. Therefore, no CO will be introduced into the third cavity 61, and the carbon dioxide detection current Ip3 will be approximately zero.

[0095] As described above, the gas sensor 100 according to this embodiment can accurately determine the concentration of water vapor and the concentration of carbon dioxide.

[0096] Furthermore, although indirect, the gas sensor 100 can also determine the oxygen concentration in the gas being measured. In summary, the difference C between the oxygen concentration drawn out from the first cavity 20 (denoted as C1) and the oxygen concentration drawn into the second cavity 40 and the third cavity 61 (denoted as C2 and C3, respectively) corresponds to the oxygen concentration in the gas being measured introduced through the gas inlet 10.

[0097] C = C1 - C2 - C3 ... (1)

[0098] C1, C2, and C3 are values ​​that are approximately proportional to the oxygen pump currents Ip1, Ip2, and Ip3, respectively. Therefore, if the relationships between C1 and Ip1, C2 and Ip2, and C3 and Ip3 (proportional constants) are determined in advance, the oxygen concentration in the gas being measured can be calculated based on the detected values ​​of the oxygen pump currents Ip1, Ip2, and Ip3.

[0099] As explained above, according to this embodiment, a multi-gas sensor is realized that reduces the number and area of ​​electrodes, simplifies the structure, reduces costs, and is capable of measuring the concentration of specified gas components (at least water vapor and carbon dioxide) in the gas being measured in the same way as conventional gas sensors.

Claims

1. A gas sensor capable of measuring the concentrations of multiple target gas components contained in a measured gas, which includes at least water vapor and carbon dioxide. Its features are, The gas sensor includes: A sensor element having a structure made of a solid electrolyte with oxygen ion conductivity; and A controller that controls the operation of the gas sensor. The sensor element includes: A gas inlet for introducing the gas to be measured; The first cavity, the second cavity, and the third cavity are connected sequentially from the gas inlet via different diffusion rate control units; A reference pump electrode that is in contact with a reference gas; The first pump unit comprises a first inner electrode formed facing the first cavity, the reference pump electrode, and the solid electrolyte existing between the first inner electrode and the reference pump electrode. The second pump unit comprises a second inner electrode formed facing the second cavity, the reference pump electrode, and the solid electrolyte existing between the second inner electrode and the reference pump electrode. as well as The third pump unit comprises a third inner electrode formed facing the third cavity, the reference pump electrode, and the solid electrolyte existing between the third inner electrode and the reference pump electrode. The first pump unit draws oxygen from the first cavity in such a manner that the water vapor and carbon dioxide contained in the gas to be measured, introduced from the gas inlet into the first cavity, are substantially completely decomposed. The second pump unit draws oxygen into the second cavity, thereby selectively oxidizing the hydrogen produced by the decomposition of water vapor in the gas being measured, which is introduced from the first cavity into the second cavity, within the second cavity. The third pump unit draws oxygen into the third cavity, thereby oxidizing the carbon monoxide, generated by the decomposition of carbon dioxide, contained in the gas being measured and introduced from the second cavity into the third cavity, within the third cavity. The controller has: A first pump unit control mechanism controls the voltage and application time applied between the first inner electrode and the reference pump electrode in the first pump unit. The second pump unit control mechanism controls the voltage and application time applied between the second inner electrode and the reference pump electrode in the second pump unit. The third pump unit control mechanism controls the voltage and application time applied between the third inner electrode and the reference pump electrode in the third pump unit. A water vapor concentration determining mechanism determines the concentration of water vapor in the gas being measured based on the magnitude of the current flowing between the second inner electrode and the reference pump electrode when oxygen is drawn into the second cavity through the second pump unit; and A carbon dioxide concentration determining mechanism determines the concentration of carbon dioxide in the gas being measured based on the magnitude of the current flowing between the third inner electrode and the reference pump electrode when oxygen is drawn into the third cavity through the third pump unit. The first pump unit control mechanism operates intermittently at intervals of less than 10 ms, such that the unapplied electromotive force V1 generated between the first inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 1000 mV to 1500 mV. The second pump unit control mechanism operates intermittently at intervals of less than 10 ms, such that the unapplied electromotive force V2 generated between the second inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 250 mV to 450 mV. The third pump unit control mechanism enables the third pump unit to operate intermittently at intervals of less than 10 ms, such that the unapplied electromotive force V3 generated between the third inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 100 mV to 300 mV.

2. The gas sensor according to claim 1, characterized in that, The first inner electrode is formed only on one of the surfaces that define the first cavity in the structure. The second inner electrode is formed only on one of the surfaces that define the second cavity in the structure. The third inner electrode is formed only on one of the surfaces that define the third cavity in the structure.

3. A concentration determination method using a gas sensor, which uses a gas sensor to determine the concentration of multiple target gas components contained in a gas to be measured, which includes at least water vapor and carbon dioxide. Its features are, The gas sensor includes a sensor element having a strip-shaped structure made of a solid electrolyte with oxygen ion conductivity. The sensor element includes: A gas inlet for introducing the gas to be measured; The first cavity, the second cavity, and the third cavity are connected sequentially from the gas inlet via different diffusion rate control units; A reference pump electrode that is in contact with a reference gas; The first pump unit comprises a first inner electrode formed facing the first cavity, the reference pump electrode, and the solid electrolyte existing between the first inner electrode and the reference pump electrode. The second pump unit comprises a second inner electrode formed facing the second cavity, the reference pump electrode, and the solid electrolyte existing between the second inner electrode and the reference pump electrode. as well as The third pump unit comprises a third inner electrode formed facing the third cavity, the reference pump electrode, and the solid electrolyte existing between the third inner electrode and the reference pump electrode. The method includes the following steps: a) Oxygen is drawn out from the first cavity by the first pump unit in such a way that the water vapor and carbon dioxide contained in the gas to be measured introduced from the gas inlet into the first cavity are substantially completely decomposed. b) Oxygen is drawn into the second cavity through the second pump unit, thereby selectively oxidizing the hydrogen contained in the measured gas introduced from the first cavity into the second cavity, which is generated by the decomposition of water vapor; c) Oxygen is drawn into the third cavity through the third pump unit, thereby oxidizing carbon monoxide, which is generated by the decomposition of carbon dioxide, in the gas to be measured and introduced from the second cavity into the third cavity, in the third cavity. d) Determine the concentration of water vapor in the gas being measured based on the magnitude of the current flowing between the second inner electrode and the reference pump electrode when oxygen is drawn into the second cavity through the second pump unit; and e) The concentration of carbon dioxide in the gas being measured is determined based on the magnitude of the current flowing between the third inner electrode and the reference pump electrode when oxygen is drawn into the third cavity through the third pump unit. In step a), the first pump unit is operated intermittently at intervals of less than 10 ms in such a way that the unapplied electromotive force V1 generated between the first inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 1000 mV to 1500 mV. In step b), the second pump unit is operated intermittently at intervals of less than 10 ms in a manner that maintains the unapplied electromotive force V2 generated between the second inner electrode and the reference pump electrode within a specified target value of 250 mV to 450 mV when no voltage is applied between the second inner electrode and the reference pump electrode. In step c), the third pump unit is operated intermittently at intervals of less than 10 ms in such a way that the unapplied electromotive force V3 generated between the third inner electrode and the reference pump electrode when no voltage is applied is maintained at a predetermined target value within the range of 100 mV to 300 mV.

4. The concentration determination method using a gas sensor according to claim 3, characterized in that, The first inner electrode is formed only on one of the surfaces that define the first cavity in the structure. The second inner electrode is formed only on one of the surfaces that define the second cavity in the structure. The third inner electrode is formed only on one of the surfaces that define the third cavity in the structure.

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