Gas sensor and gas sensor control method

Abstract

The present invention provides a gas sensor capable of accurately measuring the oxygen concentration in a gas, even when oxygen and flammable gases such as hydrogen are present in the gas being measured. [Solution] In the gas sensor 100, the sensor element 101 includes a long plate-shaped base portion 102 containing an oxygen ion conductive solid electrolyte layer, a gas passage 15 formed from one end of the base portion 102 in the longitudinal direction, an oxygen pump cell 21 including an oxygen pump electrode 22 inside the passage and an oxygen pump electrode 23 outside the passage, and a heater 72 for heating the base portion 102. The heater control unit of the control device supplies power to the heater 72 to maintain the base portion 102 at a predetermined temperature, the pump control unit of the control device controls the operation of the oxygen pump cell 21, and the concentration calculation unit of the control device calculates the oxygen concentration in the gas to be measured based on the current flowing through the oxygen pump cell 21 and the power supplied to the heater 72.

Description

[Technical Field]

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

[0002] In various fields such as combustion control and exhaust gas control for internal combustion engines like automobiles, environmental control, medicine, biotechnology, and agriculture and industry, it is necessary to measure the concentration of target gas components (oxygen O2, water vapor H2O, carbon dioxide CO2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, etc.) in the gas being measured. Various measuring instruments are used to measure concentrations, but one example is a limiting current type gas sensor that uses an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) (for example, Japanese Patent Publication No. 3050781).

[0003] For example, in a limiting current type oxygen sensor, the sensor element has an internal cavity into which the gas to be measured is introduced under a predetermined diffusion resistance, and a pump cell containing a pair of electrodes arranged inside and outside the internal cavity. When a voltage is applied to the pump cell, the oxygen in the internal cavity is ionized, and a current flows through the pump cell due to oxygen ion conduction. Since the limiting current is proportional to the oxygen concentration in the gas to be measured within the internal cavity, it is known that the oxygen concentration can be measured from the value of the limiting current. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 3050781 [Overview of the project] [Problems that the invention aims to solve]

[0005] In the aforementioned limiting current type gas sensor, the sensor element is heated to a high temperature such that the solid electrolyte exhibits oxygen ion conductivity, and the target gas is measured in this state.

[0006] The gas being measured is typically a mixture containing multiple gas species. In some cases, oxygen and flammable gases such as hydrogen may be present in the measured gas. It is known that oxygen and flammable gases like hydrogen undergo combustion reactions under high-temperature conditions. It has been found that if such a combustion reaction occurs at the gas sensor's operating temperature, it becomes difficult for the gas sensor to accurately measure the oxygen concentration.

[0007] Therefore, the present invention aims to provide a gas sensor that can accurately measure the oxygen concentration in a gas to be measured, even when oxygen and a flammable gas such as hydrogen are present in the gas to be measured. [Means for solving the problem]

[0008] As a result of diligent research, the inventors have discovered that by calculating the oxygen concentration based on the current flowing through the oxygen pump cell and the power supplied to the heater, it is possible to accurately measure the oxygen concentration in a gas to be measured, even when oxygen and flammable gases such as hydrogen are present in the gas to be measured.

[0009] The present invention includes the following inventions.

[0010] (1) A gas sensor for detecting oxygen in a gas to be measured, comprising a sensor element and a control device for controlling the sensor element, The aforementioned sensor element is A long, plate-shaped substrate containing an oxygen ion conductive solid electrolyte layer, A gas passage for measurement formed from one end of the base portion in the longitudinal direction, An oxygen pump cell comprising an in-air oxygen pump electrode disposed within the gas flow space to be measured, and an out-of-air oxygen pump electrode located at a position different from the gas flow space to be measured on the base portion, which corresponds to the in-air oxygen pump electrode, A heater for heating the base portion, Includes, The control device is The heater control unit that controls the heater, A pump control unit that controls the operation of the oxygen pump cell, It includes a concentration calculation unit that calculates the oxygen concentration in the gas being measured, The heater control unit supplies power to the heater to maintain the base portion at a predetermined temperature. The pump control unit applies a predetermined pump voltage between the oxygen pump electrode inside the air and the oxygen pump electrode outside the air of the oxygen pump cell to pump oxygen from the gas flow air to be measured or to pump oxygen into the gas flow air to be measured. The concentration calculation unit is a gas sensor that calculates the oxygen concentration in the gas to be measured based on the current flowing through the oxygen pump cell and the power supplied to the heater.

[0011] (2) The gas sensor according to (1) above, wherein the concentration calculation unit further calculates the flammable gas concentration in the gas to be measured based on the current flowing through the oxygen pump cell and the power supplied to the heater.

[0012] (3) The concentration calculation unit stores in advance a first correspondence between the oxygen concentration in the gas to be measured, the flammable gas concentration, and the current flowing through the oxygen pump cell, and a second correspondence between the oxygen concentration in the gas to be measured, the flammable gas concentration, and the power supplied to the heater. The gas sensor according to (1) or (2) above, wherein the concentration calculation unit calculates the oxygen concentration in the gas to be measured based on the value of the current flowing through the oxygen pump cell, the value of the power supplied to the heater, the first correspondence, and the second correspondence.

[0013] (4) The gas sensor according to (2) or (3) above, wherein the flammable gas is selected from the group consisting of hydrogen, carbon monoxide, ammonia, methane, ethane, propane, butane, ethylene, propylene, butylene, and acetylene.

[0014] (5) The gas sensor according to any one of (1) to (4) above, wherein the heater control unit adjusts the electric power supplied to the heater based on a resistance value of the heater and / or a resistance value of the oxygen pump cell.

[0015] (6) The sensor element further comprises: a reference gas chamber formed inside the base portion, spaced apart from the measurement gas flow space; and a reference electrode disposed in the reference gas chamber, wherein the pump control unit applies the predetermined pump voltage between the in-space oxygen pump electrode and the out-space oxygen pump electrode of the oxygen pump cell based on a voltage between the in-space oxygen pump electrode and the reference electrode to pump oxygen out of the measurement gas flow space or pump oxygen into the measurement gas flow space, the gas sensor according to any one of (1) to (5) above.

[0016] (7) The sensor element further comprises: a measurement pump cell including an in-space measurement electrode disposed in the measurement gas flow space at a position farther from the one end portion in the longitudinal direction of the base portion than the in-space oxygen pump electrode, and an out-space measurement electrode disposed at a position different from the measurement gas flow space of the base portion and corresponding to the in-space measurement electrode, wherein the pump control unit further applies a predetermined pump voltage between the in-space measurement electrode and the out-space measurement electrode of the measurement pump cell to pump oxygen out of the measurement gas flow space, and the concentration calculation unit further calculates an oxide gas concentration in the measurement gas based on a current flowing through the measurement pump cell, the gas sensor according to any one of (1) to (6) above.

[0017] (8) The gas sensor according to (7) above, wherein the oxide gas is selected from the group consisting of nitrogen oxides, carbon dioxide, and water vapor.

[0018] (9) A control method for a gas sensor that detects oxygen in a gas to be measured, The gas sensor is the gas sensor described in any of (1) to (8) above. The control method described above is A heater control step of supplying power to the heater to maintain the base portion at a predetermined temperature, A pump control step of applying a predetermined pump voltage between the oxygen pump electrode inside the air and the oxygen pump electrode outside the air of the oxygen pump cell to pump oxygen from the gas flow air to be measured or to pump oxygen into the gas flow air to be measured, A control method comprising: a concentration calculation step of calculating the oxygen concentration in a gas to be measured based on the current flowing through the oxygen pump cell and the power supplied to the heater. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a gas sensor that can accurately measure the oxygen concentration in a gas to be measured, even when oxygen and a flammable gas such as hydrogen are present in the gas to be measured.

[0020] Furthermore, according to the present invention, it is possible to provide a gas sensor that can accurately measure both the oxygen concentration and the flammable gas concentration in the gas being measured. [Brief explanation of the drawing]

[0021] [Figure 1] This is a schematic diagram of a longitudinal vertical cross-section showing an example of the general configuration of the gas sensor 100. [Figure 2] This is a block diagram showing the electrical connection relationships between the control device 90 and the main pump cell 21, individual sensor cells 80, 83, and heater unit 70 of the sensor element 101 in the gas sensor 100. [Figure 3] This figure shows the relationship between the hydrogen concentration (H2 concentration) in the gas being measured and the O2 equivalent value of the gas sensor 100, assuming a constant oxygen concentration in the gas being measured. The horizontal axis represents H2 concentration [%], and the vertical axis represents the O2 equivalent value [%]. [Figure 4]This figure illustrates the correspondence between the O2 equivalent value and the oxygen concentration (O2 concentration) and hydrogen concentration (H2 concentration) in the gas being measured. Figure 4(1) shows an example of the correspondence between the O2 equivalent value and the O2 concentration and H2 concentration in the gas being measured. The horizontal axis represents the O2 concentration [%] and the vertical axis represents the H2 concentration [%]. Figure 4(2) is a schematic diagram showing the quantitative relationship between H2 and O2 for gas conditions A and B shown in Figure 4(1). [Figure 5] This figure illustrates the correspondence between the power supply (Ph) to the heater 72 and the oxygen concentration (O2 concentration) and hydrogen concentration (H2 concentration) in the gas being measured. Figure 5(1) shows an example of the correspondence between power supply (Ph) and the O2 and H2 concentrations in the gas being measured. The horizontal axis represents the O2 concentration [%], and the vertical axis represents the H2 concentration [%]. Figure 5(2) is a schematic diagram showing the quantitative relationship between H2 and O2 for gas conditions C and D shown in Figure 5(1). [Figure 6] These are schematic diagrams illustrating the procedure for calculating the oxygen and hydrogen concentrations in the gas being measured. Figure 6(1) is an explanatory diagram showing the relationship between the measured pump current Ip0 and the O2 equivalent value, O2 concentration, and H2 concentration. Figure 6(2) is an explanatory diagram showing the relationship between the measured heater power Ph and the Ph, O2 concentration, and H2 concentration. Figure 6(3) is an explanatory diagram showing how to obtain the oxygen and hydrogen concentrations. In all of Figures 6(1) to (3), the horizontal axis represents the O2 concentration [%] and the vertical axis represents the H2 concentration [%]. [Figure 7] This flowchart shows an example of operation when the gas sensor 100 is used for hydrogen engine control. [Figure 8] This is a schematic diagram of a vertical cross-section of the sensor element 201 in the longitudinal direction, showing an example of the general configuration of the gas sensor 200. [Figure 9] This block diagram shows the electrical connection relationships between the control device 290 and the pump cells 21, 50, 41, sensor cells 80, 81, 82, 83, and heater unit 70 of the gas sensor 200. [Modes for carrying out the invention]

[0022] The gas sensor of the present invention is a gas sensor that detects oxygen in a gas to be measured, and includes a sensor element and a control device that controls the sensor element.

[0023] The sensor element included in the gas sensor of the present invention is A long, plate-shaped substrate containing an oxygen ion conductive solid electrolyte layer, A gas passage for measurement formed from one end of the base portion in the longitudinal direction, An oxygen pump cell comprising an in-air oxygen pump electrode disposed within the gas flow space to be measured, and an out-of-air oxygen pump electrode located at a position different from the gas flow space to be measured on the base portion, which corresponds to the in-air oxygen pump electrode, A heater for heating the base portion, Includes.

[0024] The sensor element included in the gas sensor of the present invention is further, Inside the base portion, a reference gas chamber is formed at a distance from the gas flow space to be measured, It may include a reference electrode disposed in the reference gas chamber. Furthermore, The measuring pump cell may include an in-air measuring electrode disposed within the gas flow space to be measured, at a position further from one end of the base portion in the longitudinal direction than the in-air oxygen pump electrode, and an out-of-air measuring electrode disposed at a position different from the gas flow space to be measured on the base portion, corresponding to the in-air measuring electrode.

[0025] The control device included in the gas sensor of the present invention is The heater control unit that controls the heater, A pump control unit that controls the operation of the oxygen pump cell, It includes a concentration calculation unit that calculates the oxygen concentration in the gas being measured, The heater control unit supplies power to the heater to maintain the base portion at a predetermined temperature. The pump control unit applies a predetermined pump voltage between the oxygen pump electrode inside the air and the oxygen pump electrode outside the air of the oxygen pump cell to pump oxygen from the gas flow air to be measured or to pump oxygen into the gas flow air to be measured. The concentration calculation unit calculates the oxygen concentration in the gas to be measured based on the current flowing through the oxygen pump cell and the power supplied to the heater. The concentration calculation unit may further calculate the flammable gas concentration in the gas to be measured based on the current flowing through the oxygen pump cell and the power supplied to the heater.

[0026] An example of an embodiment of the gas sensor of the present invention will be described in detail below.

[0027] [Overall configuration of the gas sensor] The gas sensor of the present invention will be described below with reference to the drawings. Figure 1 is a schematic vertical cross-sectional view of the sensor element 101 in the longitudinal direction, showing an example of the general configuration of the gas sensor 100 including the sensor element 101. In the following, using Figure 1 as a reference, the top and bottom refer to the upper side of Figure 1 as the top and the lower side as the bottom, the left side of Figure 1 as the front end and the right side as the rear end.

[0028] In the embodiment shown in Figure 1, the gas sensor 100 is an example of a gas sensor that detects oxygen and hydrogen in the gas to be measured using a sensor element 101 and measures their respective concentrations.

[0029] Furthermore, the gas sensor 100 includes a control device 90 that controls the sensor element 101. Figure 2 is a block diagram showing the electrical connection relationship between the control device 90 and the sensor element 101.

[0030] (Sensor element) The sensor element 101 is a long, plate-shaped element that includes a base portion 102 having a structure in which multiple oxygen ion-conducting solid electrolyte layers are stacked. A long, plate-shaped element is also called a long plate or strip-shaped element. The base portion 102 has a structure in which six layers are stacked in this order from the bottom in the drawing view: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each made of an oxygen ion-conducting solid electrolyte layer such as zirconia (ZrO2). The solid electrolytes forming these six layers are dense and airtight. The six layers may all have the same thickness, or each layer may have a different thickness. The layers are bonded together via an adhesive layer made of solid electrolyte, and the base portion 102 includes this adhesive layer. Figure 1 illustrates the layer configuration consisting of the six layers, but the layer configuration in the present invention is not limited to this, and any number of layers and layer configuration may be used.

[0031] The gas flow space 15 to be measured is formed from one end of the base portion 102 and has a gas inlet 10 that opens to the surface of the base portion 102 and a first internal space 20 that communicates with the gas inlet 10 via a first diffusion rate-limiting passage 11 (first diffusion rate-limiting section).

[0032] A gas inlet 10 is formed at one end of the sensor element 101 in the longitudinal direction (hereinafter referred to as the tip), between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The gas passage space 15 to be measured, that is, the gas passage section to be measured, is formed adjacent to the gas inlet 10 in the longitudinal direction, with the first diffusion-limiting passage 11, the buffer space 12, the secondary diffusion-limiting passage 13 (secondary diffusion-limiting section), and the first internal space 20 communicating in this order.

[0033] The gas inlet 10, the buffer space 12, and the first internal cavity 20 are spaces inside the sensor element 101, provided in a manner in which the spacer layer 5 is hollowed out, with the upper part partitioned by the lower surface of the second solid electrolyte layer 6, the lower part partitioned by the upper surface of the first solid electrolyte layer 4, and the sides partitioned by the side surface of the spacer layer 5.

[0034] The first diffusion-limiting passage 11 and the secondary diffusion-limiting passage 13 are both provided as two horizontally elongated slits (with their openings perpendicular to the drawing in Figure 1). The first diffusion-limiting passage 11 and the secondary diffusion-limiting passage 13 can both be configured to provide the desired diffusion resistance, and their configuration is not limited to the slits described above.

[0035] Furthermore, a reference gas introduction space 43 is provided at a position further from the tip side than the gas flow space 15 to be measured, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, with its side portion demarcated by the side surface 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). For example, air is introduced into the reference gas introduction space 43 as the reference gas when performing concentration measurement.

[0036] The atmospheric introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the atmospheric introduction layer 48 through the reference gas introduction space 43. The atmospheric introduction layer 48 is also formed to cover the reference electrode 42. In this embodiment, the atmospheric introduction layer 48 and the reference gas introduction space 43 correspond to the reference gas chamber of the present invention.

[0037] The reference electrode 42 is an electrode disposed within the reference gas chamber. The reference electrode 42 is formed in such a manner that it is sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, an air introduction layer 48 connected to the reference gas introduction space 43 is provided around it. That is, the reference electrode 42 is disposed so as to be in contact with the reference gas via the porous air introduction layer 48 and the reference gas introduction space 43. Furthermore, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 using the reference electrode 42. The reference electrode 42 may be formed as a porous cermet electrode with a rectangular shape in plan view (for example, a cermet electrode made of Pt and ZrO2).

[0038] In the gas flow space 15 to be measured, the gas inlet 10 is open to the outside space, and the gas to be measured is drawn into the sensor element 101 from the outside space through the gas inlet 10.

[0039] In this embodiment, the gas to be measured is introduced into the gas flow space 15 through a gas inlet 10 opening on the tip surface of the sensor element 101. However, the present invention is not limited to this embodiment. For example, the gas flow space 15 does not need to have a recess for the gas inlet 10. In this case, the first diffusion-limiting passage 11 serves as both the gas inlet and the first diffusion-limiting passage.

[0040] Furthermore, for example, the gas flow space 15 to be measured may have an opening on a side surface along the longitudinal direction of the base portion 102 that communicates with the buffer space 12 or a position close to the buffer space 12 of the first internal space 20. In this case, the gas to be measured is introduced through the opening from the side surface along the longitudinal direction of the base portion 102.

[0041] Furthermore, for example, the gas flow space 15 to be measured may be configured such that the gas to be measured is introduced through a porous material.

[0042] The first diffusion rate-limiting passage 11 is a section that imparts a predetermined diffusion resistance to the gas to be measured, which is taken in from the gas inlet 10.

[0043] The buffer space 12 is a space provided to mitigate the effect of pressure fluctuations on the detected value when the pressure of the gas being measured fluctuates. The sensor element 101 may have a structure that does not include the buffer space 12.

[0044] The secondary diffusion-limiting passage 13 is a section that imparts a predetermined diffusion resistance to the gas to be measured, which is introduced from the buffer space 12 into the first internal cavity 20. The secondary diffusion-limiting passage 13 is provided in conjunction with the provision of the buffer space 12.

[0045] If the buffer space 12 and the secondary diffusion-limiting passage 13 are not provided, the first diffusion-limiting passage 11 communicates directly with the first internal cavity 20.

[0046] The first internal cavity 20 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured, which is introduced through the secondary diffusion rate-limiting passage 13. This partial pressure of oxygen is adjusted by the operation of the main pump cell 21. In other words, the main pump cell 21 functions as the oxygen pump cell of the present invention.

[0047] The main pump cell 21 is an electrochemical pump cell that includes an inner main pump electrode 22 of an in-air oxygen pump electrode disposed within the gas flow space 15 to be measured (in this embodiment, within the first internal space 20 of the gas flow space 15 to be measured), and an outer pump electrode 23 of an out-air oxygen pump electrode that corresponds to the inner main pump electrode 22 and is disposed at a different position on the base portion 102 from the gas flow space 15 to be measured (on the outer surface of the base portion 102 in Figure 1). "Corresponding to the inner main pump electrode 22" means that the outer pump electrode 23 is provided with respect to the inner main pump electrode 22 via a second solid electrolyte layer 6.

[0048] In other words, the main pump cell 21 is an electrochemical pump cell composed of an inner main pump electrode 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a manner exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0049] The inner main pump electrode 22 is formed spanning the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that partition the first internal cavity 20, and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side walls of the first internal cavity 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are arranged in a tunnel-shaped structure at the location where the side electrode portion is installed.

[0050] The inner main pump electrode 22 and the outer pump electrode 23 are porous cermet electrodes with a rectangular shape in plan view (electrodes in which metal and ceramic components are mixed). The metal component may contain a noble metal with catalytic activity (for example, at least one of Pt, Rh, Ir, Ru, and Pd). The ceramic component is not particularly limited, but it is preferable to use an oxygen ion conductive solid electrolyte, similar to the substrate 102. The inner main pump electrode 22 and the outer pump electrode 23 may be, for example, porous cermet electrodes made of Pt and ZrO2.

[0051] In the main pump cell 21, a pump voltage Vp0 is applied between the inner main pump electrode 22 and the outer pump electrode 23 by a variable power supply 24, and a pump current Ip0 flows between the inner main pump electrode 22 and the outer pump electrode 23 in the positive or negative direction, thereby enabling the pumping of oxygen from the first internal cavity 20 to the external space, or the pumping of oxygen from the external space to the first internal cavity 20.

[0052] Furthermore, the inner main pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 constitute an electrochemical sensor cell, namely, an oxygen partial pressure detection sensor cell 80 for main pump control. In the oxygen partial pressure detection sensor cell 80 for main pump control, an electromotive force (voltage V0) is generated between the inner main pump electrode 22 and the reference electrode 42 due to the difference in oxygen concentration between the atmosphere in the first internal cavity 20 and the reference gas in the reference gas introduction space 43.

[0053] The oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be determined by measuring the voltage V0 in the oxygen partial pressure detection sensor cell 80 for main pump control. Furthermore, the pump current Ip0 is controlled by feedback control of the pump voltage Vp0 of the variable power supply 24 so that the voltage V0 remains constant. This makes it possible to maintain the oxygen partial pressure in the first internal cavity 20 at a predetermined value. The current value of the pump current Ip0 that flows at this time corresponds to the current value corresponding to the oxygen concentration in the gas being measured that reaches the inner main pump electrode 22.

[0054] Furthermore, an electrochemical sensor cell 83 is constructed from a second solid electrolyte layer 6, a spacer layer 5, a first solid electrolyte layer 4, a third substrate layer 3, an outer pump electrode 23, and a reference electrode 42. The electromotive force (voltage Vref) obtained by this sensor cell 83 makes it possible to detect the partial pressure of oxygen in the gas being measured outside the sensor. In this way, the gas sensor 100 may also be equipped with the function of a so-called oxygen concentration cell.

[0055] Furthermore, the sensor element 101 is equipped with a heater section 70 that plays a role in temperature control by heating and maintaining the sensor element 101 to enhance the oxygen ion conductivity of the solid electrolyte. The heater section 70 comprises a heater electrode 71, a heater 72, a heater lead 76, a through hole 73, a heater insulating layer 74, and a pressure relief hole 75.

[0056] The heater electrode 71 is an electrode formed in such a manner that it is in contact with the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to an external power source, the heater unit 70 can be powered from the outside.

[0057] The heater 72 is an electrical resistor formed in such a manner that it is sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to a heater electrode 71 via a heater lead 76 that is connected to the heater 72 and extends to the longitudinal rear end of the sensor element 101, and a through hole 73. The heater electrode 71 generates heat when power is supplied from the outside, and heats and maintains the temperature of the solid electrolyte (i.e., the base portion 102) that forms the sensor element 101.

[0058] Furthermore, the heater 72 is embedded throughout the entire area of ​​the first internal cavity 20, and it is possible to adjust the temperature of the sensor element 101 to the temperature at which the solid electrolyte is activated. It is sufficient that the temperature is adjusted so that the main pump cell 21 can operate. The entire area does not need to be adjusted to the same temperature, and the sensor element 101 may have a temperature distribution. The sensor element 101 (base portion 102) may have a temperature distribution in the longitudinal direction, and may also have a temperature distribution in the thickness direction and / or width direction. For example, the sensor element 101 may be heated so that the temperature of the solid electrolyte and each electrode around the gas flow cavity 15 to be measured is about 700°C to 900°C.

[0059] In this embodiment, the heater 72 is embedded in the base portion 102 of the sensor element 101, but the embodiment is not limited to this configuration. The heater 72 only needs to be arranged to heat the base portion 102. That is, the heater 72 only needs to be able to heat the sensor element 101 to the extent that it exhibits oxygen ion conductivity that allows the main pump cell 21 described above to operate. For example, it may be embedded in the base portion 102 as in this embodiment. Alternatively, for example, the heater portion 70 may be formed as a separate heater substrate from the base portion 102 and arranged adjacent to the base portion 102.

[0060] The heater lead 76 consists of a pair of heater leads connected to both ends of the heater 72. Each heater lead is provided to have substantially the same shape, that is, to have the same resistance value. Each pair of heater leads is connected to a different heater electrode 71 via a corresponding through-hole 73.

[0061] The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 and heater lead 76 using an insulator such as alumina. The heater insulating layer 74 is formed to provide electrical insulation between the second substrate layer 2 and the heater 72 and heater lead 76, and between the third substrate layer 3 and the heater 72 and heater lead 76.

[0062] The pressure relief holes 75 penetrate the third substrate layer 3 and are formed to communicate with the heater insulating layer 74 and the reference gas introduction space 43. The pressure relief holes 75 can mitigate the rise in internal pressure caused by the temperature rise in the heater insulating layer 74. A configuration without pressure relief holes 75 is also possible.

[0063] Furthermore, a predetermined length of porous protective layer (not shown) may be used to cover the surface of the sensor element 101 from the leading edge in the longitudinal direction. The porous protective layer is formed to protect the areas of the sensor element 101 where internal cavities and electrodes exist from thermal shock caused by water splashing or the like. The porous protective layer is made of ceramics such as alumina, and is preferably about 10 μm to 2000 μm thick. It is also preferable that it is formed to withstand forces up to about 50 N.

[0064] The aforementioned sensor element 101 is incorporated into the gas sensor 100 in such a manner that the tip of the sensor element 101 is in contact with the gas to be measured, and the rear end of the sensor element 101 is in contact with the reference gas.

[0065] (Control device) The gas sensor 100 of this embodiment includes the sensor element 101 described above and a control device 90 that controls the sensor element 101. In the gas sensor 100, each electrode 22, 23, and 42 of the sensor element 101 is electrically connected to the control device 90 via lead wires (not shown). Figure 2 is a block diagram showing the electrical connection relationship between the control device 90 and the main pump cell 21, each sensor cell 80, 83, and heater unit 70 of the sensor element 101. The control device 90 includes the variable power supply 24 and heater power supply 77 described above and a control unit 91. The control unit 91 includes a heater control unit 92, a pump control unit 93, and a concentration calculation unit 94.

[0066] The control unit 91 is implemented by a general-purpose or dedicated computer, and the functions of the heater control unit 92, pump control unit 93, and concentration calculation unit 94 are realized by the CPU and memory installed in the computer. In the case where the gas sensor 100 measures oxygen O2 and hydrogen H2 contained in the exhaust gas from the engine of an automobile, and 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) may be realized by an ECU (Electronic Control Unit) installed in the automobile.

[0067] The control unit 91 is configured to acquire the electromotive force (voltage V0, Vref) in each sensor cell 80, 83 of the sensor element 101, the pump current Ip0 in the main pump cell 21, and the heater voltage Vh and heater current Ih in the heater unit 70. The control unit 91 is also configured to output control signals to the variable power supply 24 and the heater power supply 77.

[0068] The heater control unit 92 is configured to control the heater unit 70 (particularly the heater 72). The heater control unit 92 supplies power to the heater 72 and maintains the sensor element 101 (base unit 102) at a predetermined temperature (referred to as the drive temperature). The drive temperature should be a temperature at which the gas sensor 100 can detect oxygen, that is, a temperature at which the solid electrolyte of the sensor element 101 is activated to the extent that the main pump cell 21 can operate. The drive temperature may be, for example, around 700°C to 900°C.

[0069] Various known control methods can be used to heat the heater 72. For example, the heater control unit 92 may adjust the output of the heater power supply 77, i.e., the power Ph supplied to the heater 72, based on the resistance value of the heater 72 and / or the resistance value of the oxygen pump cell (main pump cell 21). Alternatively, the output of the heater power supply 77, i.e., the power Ph supplied to the heater 72, may be adjusted based on at least one selected from the group consisting of the resistance value of the heater 72, the resistance value of the oxygen pump cell (main pump cell 21), the resistance value of the oxygen partial pressure detection sensor cell 80 for main pump control, and the resistance value of the sensor cell 83.

[0070] In this embodiment, the heater control unit 92 feedback-controls the control signal output to the heater power supply 77 based on the heater resistance value Rh (=Vh / Ih) calculated from the heater voltage Vh and heater current Ih in the heater 72. The heater control unit 92 adjusts the power Ph supplied from the heater power supply 77 to the heater 72 so that the heater resistance value Rh (=Vh / Ih) calculated from the heater voltage Vh and heater current Ih in the heater 72 becomes a predetermined resistance value. The power Ph actually supplied to the heater 72 can be calculated by Vh × Ih.

[0071] Furthermore, the heater section 70 may include a heater resistance detection lead (not shown). The heater resistance detection lead is drawn out from the connection between the heater 72 and one of the heater leads 76 and extends to the longitudinal rear end of the sensor element 101. The resistance value of the heater 72 may be calculated using the voltage between the heater resistance detection lead and the aforementioned one of the heater leads 76, the voltage between the heater resistance detection lead and the other of the heater leads 76, and the heater current Ih.

[0072] The pump control unit 93 is configured to control the oxygen pump cell (main pump cell 21 in this embodiment) so that it can measure the amount of oxygen gas in the gas to be measured.

[0073] The pump control unit 93 is A predetermined pump voltage (pump voltage Vp0) is applied between the oxygen pump electrode inside the air space (inner main pump electrode 22) and the oxygen pump electrode outside the air space (outer pump electrode 23) of the oxygen pump cell (main pump cell 21) to pump oxygen out of the gas flow space 15 (in this embodiment, inside the first internal air space 20) and adjust the oxygen partial pressure in the first internal air space 20 of the gas flow space 15 so that substantially all of the oxygen in the gas to be measured introduced into the first internal air space 20 of the gas flow space 15 is pumped out. Alternatively, oxygen is pumped into the gas flow space 15 (in this embodiment, inside the first internal air space 20) and the oxygen partial pressure in the first internal air space 20 of the gas flow space 15 is adjusted so that substantially all of the combustible gas in the gas to be measured introduced into the first internal air space 20 of the gas flow space 15 is burned.

[0074] The pump control unit 93 may adjust the partial oxygen pressure in the internal cavities 20 by adjusting the pump voltage Vp0 applied to the oxygen pump cell (main pump cell 21). Alternatively, based on the electromotive force (voltage V0) generated in the main pump control oxygen partial pressure detection sensor cell 80, the unit may adjust the partial oxygen pressure in each internal cavity 20 by performing feedback control, for example, as follows. Specifically, in this embodiment, the control is performed as follows.

[0075] The pump control unit 93 checks when the electromotive force (voltage V0) generated between the air-filled oxygen pump electrode (inner main pump electrode 22) and the reference electrode 42 is constant (set value V0) SET The pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 is feedback controlled so that it becomes (referred to as Vp0). SET The oxygen partial pressure in the atmosphere within the first internal cavity 20 should be set to a value such that all or substantially all of the oxygen in the gas being measured is pumped out within the first internal cavity 20. In other words, the set value V0 SET It is best to set the pump current Ip0 flowing through the main pump cell 21 to a value that is the so-called limiting current. Since the voltage V0 indicates the partial pressure of oxygen near the inner main pump electrode 22, keeping the voltage V0 constant means keeping the partial pressure of oxygen near the inner main pump electrode 22 constant. As a result, the pump current Ip0 in the main pump cell 21 changes according to the oxygen concentration in the gas being measured that reaches the inner main pump electrode 22.

[0076] The partial pressure of oxygen in the gas being measured is set to V0 SET If the oxygen partial pressure is higher than the set value V0, the main pump cell 21 will discharge oxygen from the first internal cavity 20. On the other hand, if the oxygen partial pressure in the gas being measured is higher than the set value V0 SET If the oxygen partial pressure is lower than the corresponding value (for example, if there is an excess of flammable gas), the main pump cell 21 pumps oxygen into the first internal cavity 20 from the space outside the sensor element 101. Therefore, the pump current Ip0 can take on either positive or negative values. Note that the set value V0 SET If the partial pressure of oxygen in the atmosphere within the first internal cavity 20 is set to a value such that all or substantially all of the oxygen in the gas being measured is pumped out in the first internal cavity 20, then if the gas being measured contains an excess of flammable gas, all or substantially all of the flammable gas will burn.

[0077] The concentration calculation unit 94 is configured to calculate the oxygen concentration in the gas being measured. In this embodiment, the concentration calculation unit 94 may also be configured to calculate the flammable gas concentration in the gas being measured. In this embodiment, the concentration calculation unit 94 is configured to measure the oxygen concentration and the hydrogen concentration in the gas being measured.

[0078] The concentration calculation unit 94 calculates the oxygen concentration in the gas to be measured based on the current (pump current Ip0) flowing through the oxygen pump cell (main pump cell 21) and the power (Ph) supplied to the heater 72. Furthermore, the concentration calculation unit 94 calculates the flammable gas concentration (hydrogen concentration in this embodiment) in the gas to be measured based on the current (pump current Ip0) flowing through the oxygen pump cell (main pump cell 21) and the power (Ph) supplied to the heater 72.

[0079] For example, the concentration calculation unit 94 may pre-store a first correspondence between the oxygen concentration, the flammable gas concentration (hydrogen concentration in this embodiment), and the current (pump current Ip0) flowing through the oxygen pump cell (main pump cell 21) and the power Ph supplied to the heater 72. The concentration calculation unit 94 may calculate the oxygen concentration and flammable gas concentration in the gas to be measured based on the value of the current (pump current Ip0) flowing through the oxygen pump cell (main pump cell 21), the value of the power Ph supplied to the heater 72, the first correspondence, and the second correspondence.

[0080] In this embodiment, the concentration calculation unit 94 calculates the oxygen concentration in the gas to be measured based on the O2 equivalent value obtained by converting the current (pump current Ip0) flowing through the oxygen pump cell (main pump cell 21) to an oxygen concentration, and the power supply Ph supplied to the heater 72. The hydrogen concentration in the gas being measured is calculated based on the O2 equivalent value obtained by converting the current (pump current Ip0) flowing through the oxygen pump cell (main pump cell 21) to an oxygen concentration, and the power supply Ph supplied to the heater 72.

[0081] The concentration calculation unit 94 is, The relationship between the current flowing through the oxygen pump cell (main pump cell 21) (pump current Ip0) and the corresponding oxygen concentration (referred to as the Ip0-O2 conversion value relationship), The correspondence between the oxygen concentration, hydrogen concentration in the gas being measured, and the O2 equivalent value obtained by converting the current flowing through the oxygen pump cell (main pump cell 21) (pump current Ip0) to oxygen concentration (referred to as the O2 equivalent value - O2 concentration - H2 concentration correspondence), and, The system stores in advance the correspondence between the oxygen concentration, hydrogen concentration, and power supply (Ph) in the gas being measured and the power supplied to the heater 72 (referred to as the Ph-O2 concentration-H2 concentration correspondence). The O2 equivalent value-O2 concentration-H2 concentration correspondence corresponds to the first correspondence of the present invention, and the Ph-O2 concentration-H2 concentration correspondence corresponds to the second correspondence of the present invention.

[0082] The concentration calculation unit 94 acquires the pump current Ip0 in the main pump cell 21 and calculates the O2 equivalent value by converting the pump current Ip0 to O2 concentration based on the pre-stored correspondence between the pump current Ip0 and the corresponding oxygen concentration (Ip0-O2 equivalent value correspondence). The Ip0-O2 equivalent value correspondence is pre-stored in the memory of the control unit 91, which functions as the concentration calculation unit 94. The Ip0-O2 equivalent value correspondence can be appropriately determined in advance by a person skilled in the art through experiments or other means for the gas sensor 100. The Ip0-O2 equivalent value correspondence corresponds to the correspondence between the pump current Ip0 and the oxygen concentration in the gas being measured when no flammable gas such as hydrogen is present in the gas being measured. The Ip0-O2 equivalent value correspondence may be, for example, the coefficient of an approximate formula (such as a linear function) obtained experimentally, or it may be a map showing the correspondence between the pump current Ip0 and the oxygen concentration. The Ip0-O2 conversion value correspondence may be a parameter specific to each gas sensor 100, or it may be a parameter used in common for multiple gas sensors.

[0083] The concentration calculation unit 94 acquires the heater voltage Vh and heater current Ih to determine the power Ph (=Vh × Ih) supplied to the heater 72. Alternatively, power Ph may be obtained from the output signal of the heater power supply 77. Based on the O2 equivalent value calculated above, the obtained power Ph, and the pre-stored first correspondence relationship (O2 equivalent value - O2 concentration - H2 concentration correspondence relationship) and second correspondence relationship (Ph - O2 concentration - H2 concentration correspondence relationship), the O2 concentration and H2 concentration in the gas to be measured are calculated, respectively. The first correspondence relationship (O2 equivalent value - O2 concentration - H2 concentration correspondence relationship) and the second correspondence relationship (Ph - O2 concentration - H2 concentration correspondence relationship) are pre-stored in the memory of the control unit 91, which functions as the concentration calculation unit 94. The first correspondence relationship (O2 equivalent value - O2 concentration - H2 concentration correspondence relationship) and the second correspondence relationship (Ph - O2 concentration - H2 concentration correspondence relationship) can be appropriately determined in advance by a person skilled in the art through experiments or other means for the gas sensor 100. The first correspondence relationship (O2 equivalent value - O2 concentration - H2 concentration correspondence relationship) and the second correspondence relationship (Ph - O2 concentration - H2 concentration correspondence relationship) may be parameters specific to each gas sensor 100, or they may be parameters used in common for multiple gas sensors. Details of calculating the O2 concentration and H2 concentration in the gas to be measured will be described later.

[0084] [Measurement of oxygen concentration and water vapor concentration] Next, a method for measuring the concentrations of oxygen (O2) and hydrogen (H2) in a gas to be measured using the gas sensor 100 having the configuration described above will be explained.

[0085] The gas to be measured is introduced from the gas inlet 10, passes through the first diffusion-limiting passage 11, the buffer space 12, and the secondary diffusion-limiting passage 13 in that order, is given a predetermined diffusion resistance, and reaches the first internal cavity 20.

[0086] In the first internal cavity 20, the pump control unit 93 operates the main pump cell 21 as described above, so that oxygen is pumped out of the first internal cavity 20 so that the partial pressure of oxygen in the gas to be measured introduced into the first internal cavity 20 becomes low enough that all or substantially all of the oxygen contained in the gas to be measured is pumped out. Alternatively, oxygen is pumped into the first internal cavity 20 so that all or substantially all of the flammable gas is burned.

[0087] The current value of the pump current Ip0 flowing at this time corresponds to the oxygen concentration in the gas being measured that has been introduced into the first internal cavity 20 and reached the inner main pump electrode 22. Consider the case where oxygen and hydrogen coexist in the gas being measured. Figure 3 shows the relationship between the hydrogen concentration (H2 concentration) in the gas being measured and the O2 equivalent value of the gas sensor 100, when the oxygen concentration in the gas being measured is kept constant. The horizontal axis is H2 concentration [%], and the vertical axis is O2 equivalent value [%]. Here, the O2 equivalent value is obtained by converting the pump current Ip0 to the O2 concentration corresponding to that current value. The O2 equivalent value is the O2 concentration corresponding to the amount of oxygen pumped out as pump current Ip0. The O2 equivalent value corresponds to the O2 concentration in the gas being measured when hydrogen is not present in the gas being measured.

[0088] As shown in Figure 3, ideally, the O2 equivalent value is constant regardless of the hydrogen concentration, provided the oxygen concentration is constant. However, in reality, the O2 equivalent value decreases as the hydrogen concentration increases.

[0089] Hydrogen gas has an ignition temperature of 500°C to 571°C (see the Fire and Disaster Management Agency's Hazardous Materials Disaster Information Support System). Therefore, if the gas temperature is around 500°C or higher, hydrogen gas will react with oxygen and burn (2H2 + O2 → 2H2O). On the other hand, as mentioned above, the operating temperature of the gas sensor 100 must be such that the solid electrolyte is activated to the extent that it exhibits oxygen ion conductivity, allowing the main pump cell 21 to operate. For example, yttria-stabilized zirconia, which is commonly used as an oxygen ion conductive solid electrolyte, does not exhibit oxygen ion conductivity at temperatures below the ignition temperature of hydrogen gas, and therefore needs to be used at a temperature higher than the ignition temperature (for example, around 700°C or higher). Therefore, the operating temperature of the gas sensor 100 is usually around 700°C or higher.

[0090] Thus, the operating temperature of the gas sensor 100 is higher than the ignition temperature of hydrogen gas. The gas to be measured introduced into the first internal cavity 20 is heated to a temperature above the ignition temperature of hydrogen gas, so the hydrogen in the gas to be measured reacts with oxygen and burns. This combustion reaction consumes the amount of oxygen originally present in the gas to be measured that is necessary for the combustion of hydrogen, and the amount of oxygen in the gas to be measured within the first internal cavity 20 decreases. As a result, the amount of oxygen pumped out by the main pump cell 21 is less than the amount of oxygen originally present in the gas to be measured, so the pump current Ip0 becomes smaller. Consequently, the oxygen concentration (O2 equivalent value) calculated from the pump current Ip0 becomes smaller than the actual oxygen concentration in the gas to be measured (ideal value in Figure 3).

[0091] Such phenomena are an unavoidable challenge in gas sensors using oxygen ion conductive solid electrolytes. As a result of diligent research, the inventors have found that since the combustion reaction of hydrogen is an exothermic reaction, the power Ph supplied to the heater 72 of the sensor element 101 fluctuates depending on the amount of hydrogen burned. The inventors have found that by correcting the oxygen concentration calculated from the pump current Ip0 based on the power Ph supplied to the heater, it is possible to measure the oxygen concentration in the gas to be measured with high accuracy. Furthermore, they have found that it is possible to simultaneously measure the oxygen concentration and the combustible gas concentration (hydrogen concentration in this embodiment) based on the oxygen concentration calculated from the pump current Ip0 and the power Ph supplied to the heater.

[0092] As described above, the heater control unit 92 supplies power Ph to the heater 72 and maintains the sensor element 101 (base part 102) at the driving temperature. The gas to be measured introduced into the first internal cavity 20 is heated to a temperature above the ignition temperature of hydrogen, so a combustion reaction of hydrogen gas occurs within the first internal cavity 20. Since the combustion reaction is caused by the gas temperature, it can occur regardless of location, such as within the space of the first internal cavity 20, on the surface of the inner main pump electrode 22, or on the surface of the solid electrolyte. Furthermore, it can occur not only within the first internal cavity 20 but also near the surface of the sensor element 101.

[0093] The heat generated by the combustion reaction causes the temperature of the sensor element 101 (base portion 102) to rise. Therefore, the power supply Ph to the heater 72 decreases in order to maintain the sensor element 101 (base portion 102) at the operating temperature. The amount of heat generated by the combustion reaction is proportional to the amount of hydrogen burned. Therefore, the decrease in power supply Ph to the heater 72 is considered to be proportional to the amount of hydrogen burned. Using this relationship, it is considered possible to estimate the amount of hydrogen burned and the amount of oxygen consumed by the combustion of hydrogen from the power supply Ph to the heater 72.

[0094] Refer to Figures 4 and 5. The first correspondence between the oxygen concentration, hydrogen concentration in the gas being measured, and the O2 equivalent value obtained by converting the current flowing through the oxygen pump cell (main pump cell 21) (pump current Ip0) to oxygen concentration (O2 equivalent value - O2 concentration - H2 concentration correspondence), and, This section explains the second correspondence between the oxygen concentration, hydrogen concentration, and power (Ph) supplied to heater 72 in the gas being measured (Ph-O2 concentration-H2 concentration correspondence).

[0095] Figure 4 illustrates the correspondence between O2 equivalent values ​​and the oxygen concentration (O2 concentration) and hydrogen concentration (H2 concentration) in the gas being measured. Figure 4(1) shows an example of the correspondence between O2 equivalent values ​​and the O2 and H2 concentrations in the gas being measured. The horizontal axis represents O2 concentration [%], and the vertical axis represents H2 concentration [%]. Figure 4(2) is a schematic diagram showing the quantitative relationship between H2 and O2 for gas conditions A and B shown in Figure 4(1).

[0096] The O2 equivalent value-O2 concentration-H2 concentration correspondence can be represented, for example, as a set of correspondences between O2 concentration and H2 concentration for each of several O2 equivalent values. Figure 4(1) shows an example of the O2 equivalent value-O2 concentration-H2 concentration correspondence, illustrating the correspondence between O2 concentration and H2 concentration when the O2 equivalent values ​​are 0%, 3%, and 6%. In practice, the O2 equivalent value-O2 concentration-H2 concentration correspondence may include the correspondence between O2 concentration and H2 concentration for each O2 equivalent value across the entire measurement range of the O2 concentration required by the gas sensor 100.

[0097] An O2 equivalent value of 0% means that if all the H2 in the gas being measured were to burn (2H2 + O2 → 2H2O), all the O2 would be consumed. In other words, it means that the O2 concentration in the gas being measured is half the H2 concentration (O2 concentration = H2 concentration / 2). An O2 equivalent value of N% (for example, 3% or 6%) means that if all the H2 in the gas being measured were to burn, N% of O2 would remain as a concentration (the concentration of excess O2 is N%). In other words, it means that the O2 concentration in the gas being measured is N% higher than half the H2 concentration (O2 concentration = N + H2 concentration / 2).

[0098] Therefore, for each O2 equivalent value, the O2 concentration-H2 concentration correspondence is a straight line with the same slope (slope of 2). The O2 concentration-H2 concentration correspondence for each O2 equivalent value is usually uniquely determined from the gas composition, regardless of the configuration of the gas sensor 100. For example, gas conditions A and B on the line for the 6% O2 equivalent value in Figure 4(1) have the same amount of O2 remaining after all the H2 in the measured gas has burned (amount of excess O2), as shown in Figure 4(2).

[0099] Figure 5 illustrates the correspondence between the power supply Ph to the heater 72 and the oxygen concentration (O2 concentration) and hydrogen concentration (H2 concentration) in the gas being measured. Figure 5(1) shows an example of the correspondence between power Ph (labeled as heater power Ph in the figure) and the O2 and H2 concentrations in the gas being measured. The horizontal axis represents O2 concentration [%] and the vertical axis represents H2 concentration [%]. Figure 5(2) is a schematic diagram showing the quantitative relationship between H2 and O2 for gas conditions C and D shown in Figure 5(1).

[0100] The Ph-O2 concentration-H2 concentration correspondence can be represented, for example, as a set of correspondences between O2 concentration and H2 concentration for each of several power levels (Ph). Figure 5(1) shows an example of the Ph-O2 concentration-H2 concentration correspondence, illustrating the correspondence between O2 concentration and H2 concentration when the power levels (Ph) are 7.6W, 7.8W, and 8.0W. In practice, the Ph-O2 concentration-H2 concentration correspondence may include the correspondence between O2 concentration and H2 concentration for each power level (Ph) across the entire range of power levels (Ph) that can be supplied to the heater 72 in the gas sensor 100.

[0101] The dashed line in Figure 5(1) indicates that the gas being measured contains H2 and O2 in stoichiometric ratios (H2 concentration:O2 concentration = 2:1). At points on this dashed line, if all the H2 in the gas being measured is burned (2H2 + O2 → 2H2O), all the O2 will be consumed. Each solid line in Figure 5(1) shows the O2 concentration-H2 concentration correspondence when the amount of H2 burned is equal and the resulting power Ph supplied to the heater 72 is equal. The more H2 is burned, that is, the further to the upper right the intersection with the dashed line of the O2 concentration-H2 concentration correspondence is, the lower the power Ph supplied to the heater 72 becomes.

[0102] For each power level Ph, using the intersection with the dashed line in Figure 5(1) as a reference, the amount of H2 burned is equal regardless of the H2 concentration if the O2 concentration is the same, and the amount of H2 burned is equal regardless of the O2 concentration if the H2 concentration is the same. Therefore, the O2 concentration-H2 concentration correspondence for each power level Ph is an L-shape that passes perpendicularly through the intersection with the dashed line. For example, gas conditions C and D on the line for power level Ph 8.0W in Figure 5(1) have the same amount of H2 burned, as shown in Figure 5(2).

[0103] Next, using Figure 6, we will explain in detail how to calculate the oxygen and hydrogen concentrations in the gas being measured using the O2-converted value-O2 concentration-H2 concentration correspondence and the Ph-O2 concentration-H2 concentration correspondence, as shown in Figures 4 and 5. Figure 6 is a schematic diagram illustrating the procedure for calculating the oxygen and hydrogen concentrations in the gas being measured. Figure 6(1) is an explanatory diagram of the measured pump current Ip0 and the O2-converted value-O2 concentration-H2 concentration correspondence. Figure 6(2) is an explanatory diagram of the measured power Ph and the Ph-O2 concentration-H2 concentration correspondence. Figure 6(3) is an explanatory diagram of obtaining the oxygen and hydrogen concentrations. In all of Figures 6(1) to (3), the horizontal axis is O2 concentration [%] and the vertical axis is H2 concentration [%].

[0104] First, the concentration calculation unit 94 acquires the pump current Ip0 in the main pump cell 21 as described above, and calculates the O2 equivalent value based on the Ip0-O2 equivalent value correspondence relationship. Here, the acquired (measured) pump current Ip0 is referred to as Ip0(Measured). From the O2 equivalent value-O2 concentration-H2 concentration correspondence relationships, the O2 concentration-H2 concentration correspondence relationship (referred to as the O2-H2 correspondence relationship in Ip0(Measured)) corresponding to the O2 equivalent value calculated from Ip0(Measured) is identified (Figure 6(1)).

[0105] Next, the concentration calculation unit 94 acquires the heater voltage Vh and heater current Ih as described above, and determines the power Ph (=Vh × Ih) supplied to the heater 72. Here, the obtained (measured) power Ph is referred to as Ph(Measured). From the Ph-O2 concentration-H2 concentration correspondence relationships, the O2 concentration-H2 concentration correspondence relationship corresponding to Ph(Measured) (referred to as the O2-H2 correspondence relationship in Ph(Measured)) is identified (Figure 6(2)).

[0106] The O2-H2 correspondence at Ip0 (Measured) identified in Figure 6(1) and the O2-H2 correspondence at Ph (Measured) identified in Figure 6(2) are superimposed (Figure 6(3)). The O2 concentration at the intersection of these is the O2 concentration in the gas to be measured [O2 concentration (Measured)]. Similarly, the H2 concentration at the intersection is the H2 concentration in the gas to be measured [H2 concentration (Measured)].

[0107] The identification of the O2-H2 correspondence in Ip0 (Measured) (Figure 6(1)) and the identification of the O2-H2 correspondence in Ph (Measured) (Figure 6(2)) may be performed simultaneously or sequentially. If performed sequentially, the order in which they are performed does not matter.

[0108] In this way, by calculating the O2 concentration from the O2 equivalent value from the pump current Ip0 and the power supply Ph to the heater 72, the concentration of O2 originally contained in the gas being measured can be accurately measured regardless of the H2 concentration in the gas being measured. Furthermore, the O2 concentration and H2 concentration in the gas being measured can be measured simultaneously with high accuracy.

[0109] [Examples of gas sensor applications] Such a gas sensor 100 can be used for various applications. It is particularly useful when the gas to be measured is a mixture of oxygen and hydrogen.

[0110] The gas sensor 100 can be used, for example, as a gas sensor for hydrogen engine control. The gas sensor 100 may be attached to the exhaust pipe of a hydrogen engine vehicle and used to measure exhaust gas from the hydrogen engine. Figure 7 is a flowchart showing an example of operation when the gas sensor 100 is used for hydrogen engine control.

[0111] The gas sensor 100 installed in the hydrogen engine vehicle is normally driven when the hydrogen engine is started. When the gas sensor 100 is driven, the concentration calculation unit 94 of the gas sensor 100 measures the O2 equivalent value from the pump current Ip0 and the power supply Ph to the heater 72, as described above (step S10). From the measured O2 equivalent value and power supply Ph, the O2 concentration and H2 concentration are measured (calculated) as shown in Figures 6(1) to (3) (step S11). The measured O2 concentration is output to the hydrogen engine control device and fed back into engine control (step S12). Steps S10 to S12 are repeated to control the hydrogen engine.

[0112] In exhaust gas from a hydrogen engine, the H2 concentration is generally less than 1%, but it can temporarily rise to around 5%. When the H2 concentration rises, the O2 concentration is thought to be, for example, around 10% to 15%. In such an exhaust gas composition, the amount of H2 combustion is large enough that it cannot be ignored, raising concerns that the O2 conversion value from the pump current Ip0 will decrease. However, as mentioned above, the gas sensor 100 can accurately measure the O2 concentration regardless of the H2 concentration in the gas being measured. Therefore, by using the gas sensor 100, the accurate O2 concentration in the exhaust gas from the hydrogen engine can be fed back into the engine control, which is thought to allow for more optimization of the hydrogen engine control.

[0113] Furthermore, the gas sensor 100 can also be used, for example, to detect hydrogen leaks in fuel cells. The explosive concentration range of hydrogen is said to be 4.0% to 75% in air (i.e., in the presence of oxygen). By using the gas sensor 100 described above, it is possible to measure the H2 concentration and thus detect hydrogen leaks. In addition, since it is possible to measure the O2 concentration and H2 concentration simultaneously and in parallel, it can also be used to determine the explosion limit (for example, H2 concentration exceeding 4.0% in the presence of O2).

[0114] [Differentiation] In the above-described embodiment, the gas sensor 100 had one oxygen pump cell (main pump cell 21) as the pump cell, but the gas sensor of the present invention is not limited to this. The gas sensor of the present invention may have a plurality of pump cells.

[0115] For example, in the gas sensor of the present invention, The sensor element further includes a measuring pump cell comprising: an in-air measuring electrode disposed within the gas flow space to be measured, at a position further from one end of the base portion in the longitudinal direction than the in-air oxygen pump electrode; and an out-of-air measuring electrode disposed at a position different from the gas flow space to be measured on the base portion, corresponding to the in-air measuring electrode. The pump control unit further, A predetermined pump voltage is applied between the in-air measuring electrode and the out-of-air measuring electrode of the measuring pump cell to pump oxygen from the gas flowing air to be measured. The concentration calculation unit may further calculate the oxide gas concentration in the gas to be measured based on the current flowing through the measuring pump cell.

[0116] Figure 8 is a schematic diagram of a vertical cross-section of the sensor element 201 in the longitudinal direction, showing an example of the general configuration of the gas sensor 200 including a modified sensor element 201. Components identical to those in Figure 1 are denoted by the same reference numerals. Figure 9 is a block diagram showing the electrical connection relationship between the control device 290 and the sensor element 201 in the gas sensor 200.

[0117] The gas sensor 200 is an example of a gas sensor that detects oxygen, hydrogen, and NOx in the gas being measured using a sensor element 201 and measures their respective concentrations.

[0118] (Sensor element) In the sensor element 201, the gas flow space 215 to be measured is formed from one end in the longitudinal direction of the base portion 202 and has a gas inlet 10 opening to the surface of the base portion 202, a first internal space 20 communicating with the gas inlet 10 via a first diffusion-limited passage 11 (first diffusion-limited section), a second internal space 40 communicating with the gas inlet 10 via a second diffusion-limited passage 30 (second diffusion-limited section), and a third internal space 61 communicating with the second internal space 40 via a third diffusion-limited passage 60 (third diffusion-limited section). In other words, the sensor element 201 shows an example of a configuration having three internal spaces. It is preferable that the device has at least a gas inlet 10 opening on the surface of the base portion 202, a first internal cavity 20 communicating with the gas inlet 10 via a first diffusion-limited passage 11, and an oxide gas measurement cavity (third internal cavity 61 in the sensor element 201) communicating with the first internal cavity 20 via a diffusion-limited passage for measurement cavity (third diffusion-limited passage 60 in the sensor element 201).

[0119] The gas flow space 215 to be measured is formed adjacent to a first diffusion-limiting passage 11, a buffer space 12, a secondary diffusion-limiting passage 13, a first internal space 20, a second diffusion-limiting passage 30, a second internal space 40, a third diffusion-limiting passage 60, and a third internal space 61, all of which are connected in this order in the longitudinal direction from the gas inlet 10.

[0120] The second internal cavity 40 and the third internal cavity 61 are spaces inside the sensor element 101 that are provided in the same manner as the gas inlet 10, the buffer space 12, and the first internal cavity 20, by hollowing out the spacer layer 5, with the upper part being the lower surface of the second solid electrolyte layer 6, the lower part being the upper surface of the first solid electrolyte layer 4, and the sides being the sides of the spacer layer 5.

[0121] The second diffusion-limiting passage 30 is provided as two horizontally elongated slits (with their longitudinal openings perpendicular to the drawing in Figure 8), similar to the first diffusion-limiting passage 11 and the secondary diffusion-limiting passage 13. The second diffusion-limiting passage 30 can be in any form that provides the desired diffusion resistance, and its form is not limited to the slits described above.

[0122] The third diffusion-limiting passage 60 is provided between the spacer layer 5 and the second solid electrolyte layer 6 as a single horizontally elongated slit (with its longitudinal opening perpendicular to the drawing in Figure 8). The third diffusion-limiting passage 60 can be any form that provides the desired diffusion resistance, and its form is not limited to the slit described above.

[0123] In the sensor element 201, the inner main pump electrode 22 that comes into contact with the gas to be measured is formed using a material with weakened reducing ability to the NOx component in the gas to be measured. The inner main pump electrode 22 may contain a catalytically active noble metal (for example, at least one of Pt, Rh, Ir, Ru, and Pd) and a noble metal that reduces the catalytic activity of the catalytically active noble metal with respect to the gas to be measured (NOx in this embodiment) (for example, Au, Ag, etc.). In this embodiment, the inner main pump electrode 22 may be, for example, a porous cermet electrode of Pt and ZrO2 containing 1% Au.

[0124] The second diffusion rate-limiting passage 30 is a section that provides a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (partial oxygen pressure) is controlled by the operation of the main pump cell 21 in the first internal cavity 20, and guides the gas to be measured to the second internal cavity 40.

[0125] The second internal cavity 40 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured, which is introduced through the second diffusion rate-limiting passage 30, with greater precision. This oxygen partial pressure is adjusted by the operation of the auxiliary pump cell 50. The system can also be configured without the second internal cavity 40 and the auxiliary pump cell 50.

[0126] In the second internal cavity 40, after the oxygen concentration (partial pressure of oxygen) has been adjusted in advance in the first internal cavity 20, the partial pressure of oxygen is further adjusted by the auxiliary pump cell 50 for the gas to be measured introduced through the second diffusion rate-limiting passage 30. As a result, the oxygen concentration in the second internal cavity 40 can be kept constant with high precision, enabling highly accurate NOx concentration measurement in the gas sensor 200.

[0127] The auxiliary pump cell 50 is an electrochemical pump cell that includes an auxiliary pump electrode 51 disposed in the gas flow space 215, at a position further from the longitudinal tip of the base portion 202 than the inner main pump electrode 22 (in the second internal space 40 of the gas flow space 215), and an outer pump electrode 23 disposed at a different position on the base portion 202 from the gas flow space 215 (on the outer surface of the base portion 202 in Figure 8), which corresponds to the auxiliary pump electrode 51. "Corresponding to the auxiliary pump electrode 51" means that the outer pump electrode 23 is provided with respect to the auxiliary pump electrode 51 via the second solid electrolyte layer 6.

[0128] In other words, the auxiliary pump cell 50 is an auxiliary electrochemical pump cell composed of an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided over substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, any suitable electrode outside the sensor element 201 at a different location from the gas flow cavity 215 to be measured is sufficient), and the second solid electrolyte layer 6.

[0129] The auxiliary pump electrode 51 is disposed within the second internal cavity 40 in a tunnel-shaped structure similar to that of the inner main pump electrode 22 provided within the first internal cavity 20. Specifically, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 which forms the ceiling surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 which forms the bottom surface of the second internal cavity 40. Side electrode portions (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both walls of the spacer layer 5 which forms the side walls of the second internal cavity 40, creating a tunnel-shaped structure.

[0130] Furthermore, the auxiliary pump electrode 51 is formed using a material with weakened reducing ability to the NOx component in the gas being measured, similar to the inner main pump electrode 22. The auxiliary pump electrode 51 may contain, similar to the inner main pump electrode 22, a catalytically active noble metal (for example, at least one of Pt, Rh, Ir, Ru, and Pd) and a noble metal that reduces the catalytic activity of the catalytically active noble metal towards the gas being measured (NOx in this embodiment) (for example, Au, Ag, etc.). The auxiliary pump electrode 51 may be, similar to the inner main pump electrode 22, for example, a porous cermet electrode of Pt and ZrO2 containing 1% Au.

[0131] In the auxiliary pump cell 50, by applying a desired pump voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 using a variable power supply 52, it is possible to pump oxygen from the atmosphere in the second internal cavity 40 to the outside space, or pump oxygen from the outside space into the second internal cavity 40.

[0132] Furthermore, the first measuring electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3 constitute an electrochemical sensor cell, namely, an oxygen partial pressure detection sensor cell 81 for auxiliary pump control. In the oxygen partial pressure detection sensor cell 81 for auxiliary pump control, an electromotive force (voltage V1) is generated between the auxiliary pump electrode 51 and the reference electrode 42 due to the difference in oxygen concentration between the atmosphere in the second internal cavity 40 and the reference gas in the reference gas introduction space 43.

[0133] Furthermore, the auxiliary pump cell 50 is pumped by a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the oxygen partial pressure detection sensor cell 81 for auxiliary pump control. As a result, the oxygen partial pressure in the atmosphere inside the second internal cavity 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0134] Furthermore, the pump current Ip1 is used to control the voltage V0 of the oxygen partial pressure detection sensor cell 80 for main pump control. Specifically, the pump current Ip1 is input to the oxygen partial pressure detection sensor cell 80 for main pump control as a control signal, and by controlling its voltage V0, the gradient of the oxygen partial pressure in the gas to be measured, introduced from the second diffusion rate-limiting passage 30 into the second internal cavity 40, is kept 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 cell 21 and the auxiliary pump cell 50.

[0135] The third diffusion rate-limiting passage 60 is a section that provides a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (partial pressure of oxygen) has been further controlled to a lower level by the operation of the auxiliary pump cell 50 in the second internal cavity 40, and guides the gas to be measured to the third internal cavity 61.

[0136] The third internal cavity 61 is provided as a space for measuring the nitrogen oxide (NOx) concentration in the gas to be measured, which is introduced through the third diffusion rate-limiting passage 60. The NOx concentration is measured by the operation of the measuring pump cell 41.

[0137] The measuring pump cell 41 is an electrochemical pump cell that includes a measuring electrode 44 of an in-air measuring electrode, which is located in the gas flow space 215, at a position further from the longitudinal tip of the base portion 202 than the in-air oxygen pump electrode (in this embodiment, the inner main pump electrode 22) (in the third internal space 61 of the gas flow space 215), and an out-of-air measuring electrode, which is located at a different position on the base portion 202 from the gas flow space 215 and corresponds to the measuring electrode 44. In this embodiment, the outer pump electrode 23, which is located on the outer surface of the base portion 202, also functions as an out-of-air measuring electrode. "Corresponding to the measuring electrode 44" means that the outer pump electrode 23 is provided with respect to the measuring electrode 44 via the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4. In this embodiment, the measuring electrode 44 is positioned further from the longitudinal tip of the base portion 102 than the auxiliary pump electrode 51. The measuring electrode 44 is located within the oxide gas measurement cavity (third internal cavity 61).

[0138] The measuring pump cell 41 is an electrochemical pump cell composed of a measuring electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal cavity 61, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode outside the sensor element 201 at a different location from the gas flow cavity 215 to be measured), a second solid electrolyte layer 6, a spacer layer 5, and the first solid electrolyte layer 4. The measuring pump cell 41 measures the NOx concentration in the gas to be measured within the third internal cavity 61.

[0139] The measuring electrode 44 is a porous cermet electrode. The measuring electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere within the third internal cavity 61. The measuring electrode 44 is an electrode containing a catalytically active noble metal (e.g., at least one of Pt, Rh, Ir, Ru, and Pd). It is preferable that it does not contain noble metals that reduce the catalytic activity of the catalytically active noble metal towards the target gas (NOx in this embodiment) (e.g., Au, Ag, etc.). The measuring electrode 44 may be, for example, a porous cermet electrode of Pt and Rh and ZrO2.

[0140] Furthermore, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44, and the reference electrode 42 constitute an electrochemical sensor cell, namely, an oxygen partial pressure detection sensor cell 82 for controlling the measuring pump. In the oxygen partial pressure detection sensor cell 82 for controlling the measuring pump, an electromotive force (voltage V2) is generated between the measuring electrode 44 and the reference electrode 42 due to the difference in oxygen concentration between the atmosphere in the third internal cavity 61 and the reference gas in the reference gas introduction space 43. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected in the oxygen partial pressure detection sensor cell 82 for controlling the measuring pump.

[0141] The gas to be measured, introduced into the second internal cavity 40, reaches the measuring electrode 44 in the third internal cavity 61 via the third diffusion rate-limiting passage 60 under conditions where the oxygen partial pressure is controlled. Nitrogen oxides in the gas to be measured surrounding the measuring electrode 44 are reduced (2NO → N2 + O2) to generate oxygen. This generated oxygen is then pumped by the measuring pump cell 41, and at this time, the pump voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for controlling the measuring pump remains constant. Since the amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured, the nitrogen oxide concentration in the gas to be measured is calculated using the pump current Ip2 in the measuring pump cell 41.

[0142] In a gas sensor 200 having such a configuration, the gas to be measured, whose oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx) by operating the main pump cell 21 and the auxiliary pump cell 50, is supplied to the measuring pump cell 41. Therefore, the NOx concentration in the gas to be measured can be determined based on the pump current Ip2 that flows as oxygen generated by the reduction of NOx is pumped out from the measuring pump cell 41, which is approximately proportional to the NOx concentration in the gas to be measured.

[0143] (Control device) The gas sensor 200 of this embodiment includes the sensor element 201 described above and a control device 290 that controls the sensor element 201. In the gas sensor 200, each electrode 22, 23, 51, 44, 42 of the sensor element 201 is electrically connected to the control device 290 via lead wires (not shown). Figure 9 is a block diagram showing the electrical connection relationships between the control device 290 and each pump cell 21, 50, 41, each sensor cell 80, 81, 82, 83, and heater unit 70 of the sensor element 201. Components identical to those in Figure 2 are denoted by the same reference numerals. The control device 290 includes the variable power supplies 24, 52, 46 and heater power supply 77 described above, and a control unit 291. The control unit 291 includes a heater control unit 92, a pump control unit 293, and a concentration calculation unit 294.

[0144] The control unit 291 is configured to acquire the electromotive force (voltages V0, V1, V2, Vref) in each sensor cell 80, 81, 82, 83 of the sensor element 201, the pump current (Ip0, Ip1, Ip2) in each pump cell 21, 50, 41, and the heater voltage Vh and heater current Ih in the heater unit 70. The control unit 91 is also configured to output control signals to the variable power supplies 24, 52, 46 and the heater power supply 77.

[0145] In the gas sensor 200, the heater control unit 92 may adjust the output of the heater power supply 77, i.e., the power Ph supplied to the heater 72, based on at least one selected from the group consisting of the resistance value of the heater 72, the resistance value of the oxygen pump cell (main pump cell 21), the resistance value of the auxiliary pump cell 50, the resistance value of the measuring pump cell 41, the resistance value of the oxygen partial pressure detection sensor cell 80 for main pump control, the resistance value of the oxygen partial pressure detection sensor cell 81 for auxiliary pump control, the resistance value of the oxygen partial pressure detection sensor cell 82 for measuring pump control, and the resistance value of the sensor cell 83.

[0146] The pump control unit 293 is configured to control the oxygen pump cell (main pump cell 21 in the gas sensor 200) and the measuring pump cell 41 so that oxygen gas and oxide gas (NOx in the gas sensor 200) in the gas to be measured can be measured. In the gas sensor 200, the pump control unit 293 is also configured to control the auxiliary pump cell 50.

[0147] The pump control unit 293 is similar to the pump control unit 93 described above. A predetermined pump voltage (pump voltage Vp0) is applied between the oxygen pump electrode inside the air cavity (inner main pump electrode 22) and the oxygen pump electrode outside the air cavity (outer pump electrode 23) of the oxygen pump cell (main pump cell 21) to draw oxygen from within the gas flow cavity 215 (in the case of the gas sensor 200, within the first internal cavity 20) and adjust the oxygen partial pressure in the first internal cavity 20 of the gas flow cavity 215 so that substantially all of the oxygen in the gas to be measured introduced into the first internal cavity 20 of the gas flow cavity 215 is drawn out. Alternatively, oxygen is drawn into the gas flow cavity 215 (in the case of the gas sensor 200, within the first internal cavity 20) and adjust the oxygen partial pressure in the first internal cavity 20 of the gas flow cavity 215 so that substantially all of the combustible gas in the gas to be measured introduced into the first internal cavity 20 of the gas flow cavity 215 is burned.

[0148] The pump control unit 293 further, A predetermined pump voltage (pump voltage Vp2) is applied between the in-cavity measurement electrode (measurement electrode 44) of the measurement pump cell 41 and the out-of-cavity measurement electrode (outer pump electrode 23) to pump oxygen out of the measured gas flow cavity 215, and adjust the oxygen partial pressure in the third internal cavity 61 of the measured gas flow cavity 215 such that substantially all NOx in the measured gas introduced into the third internal cavity 61 of the measured gas flow cavity 215 is decomposed.

[0149] The pump control unit 293 may adjust the pump voltages (Vp0, Vp1, Vp2) applied to each of the pump cells 21, 50, 41 to adjust the oxygen partial pressure in each of the internal cavities 20, 40, 61. Further, based on the electromotive forces (voltages V0, V1, V2) generated in each of the sensor cells 80, 81, 82, for example, the following feedback control may be performed to adjust the oxygen partial pressure in each of the internal cavities 20, 40, 61. Specifically, control is performed in the gas sensor 200 as follows.

[0150] Similar to the aforementioned pump control unit 93, the pump control unit 293 controls the electromotive force (voltage V0) generated between the in-cavity oxygen pump electrode (inner main pump electrode 22) and the reference electrode 42 to be a constant value (set value V0 SET ), the pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 is feedback-controlled.

[0151] The pump control unit 293 controls the voltage V1 in the auxiliary pump control oxygen partial pressure detection sensor cell 81 to be a constant value (set value V1 SET ), the pump voltage Vp1 of the variable power supply 52 in the auxiliary pump cell 50 is feedback-controlled. Since the voltage V1 indicates the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51, keeping the voltage V1 constant means keeping the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51 constant. Thereby, the oxygen partial pressure in the atmosphere inside the second internal cavity 40 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0152] At the same time, the pump current Ip1 in the auxiliary pump cell 50 is set to a constant value (set value Ip1 SETThe set value of voltage V0 is determined based on the pump current Ip1, so that it becomes (referred to as) SET Feedback control is performed to set the value V0. Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell 80 for main pump control, and its voltage V0 is set to the set value V0 based on the pump current Ip1. SET By controlling this mechanism, the gradient of the oxygen partial pressure in the gas being measured, introduced from the second diffusion rate-limiting passage 30 into the second internal cavity 40, is always kept 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 cell 21 and the auxiliary pump cell 50. In other words, the oxygen concentration in the gas being measured, introduced from the third diffusion rate-limiting passage 60 into the third internal cavity 61, is maintained at a constant value of approximately 0.001 ppm.

[0153] The pump control unit 293 checks when the voltage V2 detected by the oxygen partial pressure detection sensor cell 82 for measuring the pump is at a constant value (set value V2). SET The pump voltage Vp2 of the variable power supply 46 in the measuring pump cell 41 is feedback controlled so that the voltage V2 becomes (referred to as Vp2). At the measuring electrode 44, nitrogen oxides in the gas to be measured are reduced (2NO → N2 + O2) to generate oxygen. The generated oxygen is then used when the voltage V2 reaches the set value V2. SET The measurement pump cell 41 pumps it out in this manner. Set value V2 SET The oxygen partial pressure in the atmosphere within the third internal cavity 61 should be set to a value such that all or substantially all of the NOx in the gas being measured is decomposed within the third internal cavity 61. In other words, the set value V2 SET The pump current Ip2 flowing through the measuring pump cell 41 should be set to a value that is the so-called limit current. SET By setting this parameter, virtually all NOx in the gas being measured is detected as pump current Ip2. Therefore, the current value of pump current Ip2 corresponds to the current value corresponding to the NOx concentration in the gas being measured.

[0154] The concentration calculation unit 294 calculates the oxygen and hydrogen concentrations in the gas to be measured, similar to the concentration calculation unit 94, and further calculates the oxide gas concentration (NOx concentration in the gas sensor 200) in the gas to be measured based on the current (pump current Ip2) flowing through the measuring pump cell 41.

[0155] The concentration calculation unit 294 acquires the pump current Ip2 in the measuring pump cell 41 and calculates the NOx concentration in the gas to be measured based on the pre-stored correspondence between the pump current Ip2 and the NOx concentration in the gas to be measured (Ip2-NOx concentration correspondence). The calculated NOx concentration is output as the detected value of the gas sensor 200. The Ip2-NOx concentration correspondence is pre-stored in the memory of the control unit 291, which functions as a concentration detection unit 293. The Ip2-NOx concentration correspondence can be appropriately determined in advance by a person skilled in the art through experiments or other means for the gas sensor 200. The Ip2-NOx concentration correspondence may be, for example, the coefficient of an approximate formula (such as a linear function) obtained experimentally, or it may be a map showing the correspondence between the pump current Ip2 and the NOx concentration in the gas to be measured. The Ip2-NOx concentration correspondence may be a parameter specific to each gas sensor 200, or it may be a parameter used in common for multiple gas sensors.

[0156] Thus, in addition to oxygen and hydrogen concentrations, the gas sensor 200 can also measure NOx concentration.

[0157] Although gas sensors 100 and 200 have been shown above as examples of embodiments of the present invention, the present invention is not limited to these forms. The present invention may include various forms of gas sensors, as long as they achieve the objective of the present invention, which is to accurately measure the oxygen concentration in a gas to be measured, even when oxygen and flammable gases such as hydrogen are present in the gas to be measured.

[0158] In the gas sensors 100 and 200 described above, hydrogen was detected as a flammable gas, but the present invention is not limited to this. The flammable gas to be measured may be selected from the group consisting of, for example, hydrogen, carbon monoxide, ammonia, methane, ethane, propane, butane, ethylene, propylene, butylene, and acetylene. When detecting oxygen and flammable gas, the gas sensor can also be applied, for example, to monitoring oxygen concentration at underground work sites such as tunnel construction sites.

[0159] In the gas sensor 200 described above, nitrogen oxide NOx was detected as the oxide gas, but the present invention is not limited to this. The oxide gas to be measured may be selected from the group consisting of, for example, nitrogen oxides, carbon dioxide, and water vapor. For example, when water vapor is detected as the oxide gas, a correction based on the hydrogen concentration in the gas being measured may be made when calculating the water vapor concentration based on the pump current Ip2. Alternatively, ammonia NH3 may be used as the gas to be measured instead of the oxide gas. In this case, the ammonia concentration may be measured by converting NH3 to NO in at least one of the inner main pump electrode 22 and the auxiliary pump electrode 51, and detecting the converted NO using the pump current Ip2.

[0160] In the gas sensor 100 described above, the concentration calculation unit 94 was configured to output oxygen concentration and hydrogen concentration as measured values ​​of the gas sensor 100, but the present invention is not limited thereto. The concentration calculation unit 94 may be configured to output only oxygen concentration as measured value of the gas sensor 100. In this case, the concentration calculation unit 94 does not need to calculate hydrogen concentration. Furthermore, in the gas sensor 200 described above, the concentration calculation unit 294 was configured to output oxygen concentration, hydrogen concentration, and NOx concentration as measured values ​​of the gas sensor 200, but the present invention is not limited thereto. The concentration calculation unit 294 may be configured to output oxygen concentration and NOx concentration as measured values ​​of the gas sensor 200. In this case, the concentration calculation unit 294 does not need to calculate hydrogen concentration.

[0161] In the gas sensors 100 and 200 described above, an O2 equivalent value was calculated from the pump current Ip0 and used to calculate the O2 concentration and H2 concentration. However, the current value of the pump current Ip0 may be used directly to calculate the O2 concentration and H2 concentration without calculating the O2 equivalent value. In this case, the concentration calculation units 94 and 294 may store in advance the Ip0-O2 concentration-H2 concentration correspondence relationship instead of the O2 equivalent value-O2 concentration-H2 concentration correspondence relationship as the correspondence relationship between the oxygen concentration, hydrogen concentration in the gas to be measured, and the pump current Ip0 flowing through the main pump cell 21.

[0162] In the gas sensors 100 and 200 described above, the inner main pump electrode 22 was composed of a ceiling electrode portion 22a formed on the ceiling surface of the first internal cavity 20, a bottom electrode portion 22b formed on the bottom surface of the first internal cavity 20, and a side electrode portion formed on the side surface of the first internal cavity 20 to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, but is not limited to this. The inner main pump electrode 22 may be formed only on the ceiling surface of the first internal cavity 20, or only on the bottom surface of the first internal cavity 20. Also, for example, when the inner main pump electrode 22 has a ceiling electrode portion 22a and a bottom electrode portion 22b, the ceiling electrode portion 22a and the bottom electrode portion 22b may be the same size or may be different sizes. The same applies to the auxiliary pump electrode 51 of the gas sensor 200. Furthermore, although the measuring electrode 44 of the gas sensor 200 was formed on the bottom surface of the third internal cavity 61, it is not limited to this. It may also be formed on the ceiling surface of the third internal cavity 61, or it may be in the form of a tunnel, like the inner main pump electrode 22.

[0163] In the gas sensor 200 described above, the sensor element 201 has a structure in which, as shown in Figure 8, it comprises three internal cavities: a first internal cavity 20, a second internal cavity 40, and a third internal cavity 61, with an inner main pump electrode 22, an auxiliary pump electrode 51, and a measuring electrode 44 arranged in each internal cavity, respectively. However, it is not limited to this structure. For example, it may have a structure in which it comprises two internal cavities: a first internal cavity 20 and a second internal cavity 40, with the inner main pump electrode 22 arranged in the first internal cavity 20 and the auxiliary pump electrode 51 and measuring electrode 44 arranged in the second internal cavity 40. In this case, for example, a porous protective layer covering the measuring electrode 44 may be formed as a diffusion rate-limiting part between the auxiliary pump electrode 51 and the measuring electrode 44.

[0164] In the sensor element 201 of the gas sensor 200 described above, the outer pump electrode 23 served the functions of three electrodes: the out-of-air oxygen pump electrode in the oxygen pump cell (main pump cell 21), the out-of-air auxiliary pump electrode in the auxiliary pump cell 50, and the out-of-air measurement electrode in the measurement pump cell 41. However, it is not limited to this. For example, the out-of-air oxygen pump electrode, the out-of-air auxiliary pump electrode, and the out-of-air measurement electrode may each be formed as separate electrodes. For example, one or more of the out-of-air oxygen pump electrode, the out-of-air auxiliary pump electrode, and the out-of-air measurement electrode may be provided separately from the outer pump electrode 23 on the outer surface of the base portion 102 so as to be in contact with the gas to be measured. Alternatively, the reference electrode 42 may serve the functions of one or more of the out-of-air oxygen pump electrode, the out-of-air auxiliary pump electrode, and the out-of-air measurement electrode.

[0165] In the sensor elements 101 and 201 described above, the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6 were all layers made of solid electrolyte, but are not limited to this. Layers not used to form each pump cell and each sensor cell do not have to be solid electrolytes, and may be made of an insulator such as alumina, for example.

[0166] In the gas sensors 100 and 200 described above, the variable power supplies 24, 52, and 46 were all composed of voltage sources, but the variable power supplies are not limited to these. One or more of the variable power supplies 24, 52, and 46 may be composed of current sources. Even if they are composed of current sources, a voltage will ultimately be applied to each pump cell.

[0167] In addition to the above configuration, the components of the element body 102, such as the gas flow space 15 to be measured and each electrode, can take on various forms depending on the type of gas to be measured, the purpose of use of the gas sensor, and the operating environment.

[0168] [Method of manufacturing a gas sensor] Next, an example of a method for manufacturing the gas sensor described above will be explained. After performing predetermined processing and printing circuit patterns on multiple unfired sheet-like molded products (so-called green sheets) containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component, the multiple sheets are laminated, the laminate is cut, and then fired to produce a sensor element. The fabricated sensor element can then be assembled to create a gas sensor.

[0169] In the following explanation, we will use the example of fabricating a sensor element 101 consisting of the six layers shown in Figure 1.

[0170] First, six green sheets containing an oxygen ion conductive solid electrolyte such as zirconia (ZrO2) as a ceramic component are prepared. Known molding methods can be used to produce the green sheets. All six green sheets may be the same thickness, or their thickness may differ depending on the layer to be formed. Sheet holes, etc., used for positioning during printing and lamination are pre-formed on each of the six green sheets by known methods such as punching with a punching device to create blank sheets. For the blank sheet used for the spacer layer 5, through-holes such as internal cavities are formed in the same way. Necessary through-holes are also pre-formed on the other layers.

[0171] The blank sheets used for the six layers—the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6—are subjected to printing and drying processes for various patterns required for each layer. Known screen printing techniques can be used for printing the patterns. Known drying methods can also be used for the drying process.

[0172] This process is repeated until various patterns have been printed and dried on each of the six blank sheets. Then, the six printed blank sheets are stacked in a predetermined order, positioned using sheet holes, etc., and pressed together under predetermined temperature and pressure conditions to form a laminate. The pressing process is carried out by heating and pressurizing using a laminating machine such as a known hydraulic press. The temperature, pressure, and time for heating and pressurizing depend on the laminating machine used, but can be appropriately determined to achieve good lamination.

[0173] The resulting laminate contains multiple sensor elements 101. The laminate is cut to separate it into units of sensor elements 101. The separated laminate is fired at a predetermined firing temperature to obtain sensor elements 101. That is, the sensor element 101 is obtained by integral firing of the solid electrolyte layer and the electrodes. The firing temperature should be such that the solid electrolyte constituting the base portion 102 of the sensor element 101 is sintered to become a dense body, and the electrodes and other components maintain the desired porosity. For example, firing is performed at a firing temperature of approximately 1200°C to 1500°C.

[0174] The obtained sensor element 101 is incorporated into the gas sensor 100 in such a manner that the tip of the sensor element 101 is in contact with the gas to be measured, and the rear end of the sensor element 101 is in contact with the reference gas.

[0175] As described above, the gas sensor of the present invention can accurately measure the oxygen concentration in a gas to be measured, even when oxygen and flammable gases such as hydrogen are present in the gas to be measured. Furthermore, the gas sensor of the present invention can accurately measure both the oxygen concentration and the flammable gas concentration in the gas to be measured. [Explanation of symbols]

[0176] 1. First substrate layer 2. Second substrate layer 3. Third substrate layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11. First diffusion-limited pathway 12 Buffer space 13. Sub-diffusion rate-limiting pathway 15,215 Gas flow voids under measurement 20 1st internal void 21 Main pump cell 22 Inner main pump electrode 22a Ceiling electrode section (of the inner main pump electrode) 22b Bottom electrode portion (of the inner main pump electrode) 23. External pump electrode 24. Variable power supply (for the main pump cell) 30 Second diffusion-limiting pathway 40 Second internal void 41 Measuring pump cell 42 Reference electrode 44 Measuring electrode 46. ​​Variable power supply (for measuring pump cells) 48 Reference gas introduction layer 43 Reference gas introduction space 50 auxiliary pump cells 51 Auxiliary pump electrode 51a Ceiling electrode section (of the auxiliary pump electrode) 51b Bottom electrode portion (of the auxiliary pump electrode) 52 Variable power supply (for auxiliary pump cells) 60 Third diffusion-limiting pathway 61 3rd internal void 70 Heater section 71 Heater electrodes 72 Heater 73 Through Holes 74 Heater Insulation Layer 75 Pressure relief holes 76 Heater Lead 80 Oxygen partial pressure detection sensor cell for main pump control 81. Oxygen partial pressure detection sensor cell for auxiliary pump control 82 Oxygen partial pressure detection sensor cell for measuring pump control 83 Sensor Cells 90, 290 control devices 91, 291 Control Unit 92 Heater control unit 93, 293 Pump Control Unit 94, 294 Concentration calculation section 100, 200 gas sensors 101, 201 Sensor elements 102, 202 Base part

Claims

1. A gas sensor for detecting oxygen in a gas to be measured, comprising a sensor element and a control device for controlling the sensor element, The aforementioned sensor element is A long, plate-shaped substrate containing an oxygen ion conductive solid electrolyte layer, A gas passage for measurement formed from one end of the base portion in the longitudinal direction, An oxygen pump cell comprising an in-air oxygen pump electrode disposed within the gas flow space to be measured, and an out-of-air oxygen pump electrode located at a position different from the gas flow space to be measured on the base portion, which corresponds to the in-air oxygen pump electrode, A heater for heating the base portion, Includes, The control device is The heater control unit that controls the heater, A pump control unit that controls the operation of the oxygen pump cell, It includes a concentration calculation unit that calculates the oxygen concentration in the gas being measured, The heater control unit supplies power to the heater to maintain the base portion at a predetermined temperature. The pump control unit applies a predetermined pump voltage between the oxygen pump electrode inside the air and the oxygen pump electrode outside the air of the oxygen pump cell to pump oxygen from the gas flow air to be measured or to pump oxygen into the gas flow air to be measured. The concentration calculation unit is a gas sensor that calculates the oxygen concentration in the gas to be measured based on the current flowing through the oxygen pump cell and the power supplied to the heater.

2. The gas sensor according to claim 1, wherein the concentration calculation unit further calculates the flammable gas concentration in the gas to be measured based on the current flowing through the oxygen pump cell and the power supplied to the heater.

3. The concentration calculation unit pre-stores a first correspondence between the oxygen concentration in the gas to be measured, the flammable gas concentration, and the current flowing through the oxygen pump cell, and a second correspondence between the oxygen concentration in the gas to be measured, the flammable gas concentration, and the power supplied to the heater. The gas sensor according to claim 2, wherein the concentration calculation unit calculates the oxygen concentration in the gas to be measured based on the value of the current flowing through the oxygen pump cell, the value of the power supplied to the heater, the first correspondence, and the second correspondence.

4. The gas sensor according to claim 2, wherein the flammable gas is selected from the group consisting of hydrogen, carbon monoxide, ammonia, methane, ethane, propane, butane, ethylene, propylene, butylene, and acetylene.

5. The gas sensor according to claim 1, wherein the heater control unit adjusts the power supplied to the heater based on the resistance value of the heater and / or the resistance value of the oxygen pump cell.

6. The aforementioned sensor element further, Inside the base portion, a reference gas chamber is formed at a distance from the gas flow space to be measured, A reference electrode is installed in the aforementioned reference gas chamber, Includes, The gas sensor according to claim 1, wherein the pump control unit applies a predetermined pump voltage between the oxygen pump electrode inside the air and the oxygen pump electrode outside the air of the oxygen pump cell based on the voltage between the oxygen pump electrode inside the air and the reference electrode, thereby pumping oxygen from the gas flow air to be measured or pumping oxygen into the gas flow air to be measured.

7. The aforementioned sensor element further, The measuring pump cell includes an in-air measuring electrode disposed within the gas flow space to be measured, at a position further from one end of the base portion in the longitudinal direction than the in-air oxygen pump electrode, and an out-of-air measuring electrode disposed at a position different from the gas flow space to be measured on the base portion, corresponding to the in-air measuring electrode. The pump control unit further, A predetermined pump voltage is applied between the in-air measuring electrode and the out-of-air measuring electrode of the measuring pump cell to pump oxygen from the gas flowing air to be measured. The gas sensor according to claim 1, wherein the concentration calculation unit further calculates the oxide gas concentration in the gas to be measured based on the current flowing through the measuring pump cell.

8. The gas sensor according to claim 7, wherein the oxide gas is selected from the group consisting of nitrogen oxides, carbon dioxide, and water vapor.

9. A control method for a gas sensor that detects oxygen in a gas to be measured, The gas sensor is the gas sensor described in claim 1, The control method described above is A heater control step of supplying power to the heater to maintain the base portion at a predetermined temperature, A pump control step of applying a predetermined pump voltage between the oxygen pump electrode inside the air and the oxygen pump electrode outside the air of the oxygen pump cell to pump oxygen from the gas flow air to be measured or to pump oxygen into the gas flow air to be measured, A control method comprising: a concentration calculation step of calculating the oxygen concentration in a gas to be measured based on the current flowing through the oxygen pump cell and the power supplied to the heater.

Images