Gas Sensor

Abstract

To provide a gas sensor capable of downsizing and detecting an inflammable gas in a wide concentration.SOLUTION: A gas sensor 1 of the present invention includes a first detection element 2, a second detection element 3 and a control unit 4. The control unit 4 is configured to switch between a first detection mode in which a first voltage is applied to the first detection element 2 so that the first detection element 2 functions as a detection element and a second detection mode in which a second voltage lower than the first voltage is applied to the first detection element 2 so that the first detection element 2 functions as a compensation element and a third voltage is applied to the second detection element 3 so that the second detection element 3 functions as a detection element. The control unit 4 is configured to switch the first detection mode into the second detection mode when a gas concentration calculated in the first detection mode exceeds a predetermined threshold value, and / or is configured to switch the second detection mode into the first detection mode when a gas concentration calculated in the second detection mode is the predetermined threshold value or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas sensor. [Background technology]

[0002] Conventionally, catalytic combustion gas sensors and gas thermal conduction gas sensors have been used to detect combustible gases contained in a gas to be measured. Catalytic combustion gas sensors detect combustible gases by utilizing the phenomenon that when the sensing element is heated, the heat of combustion of the combustible gas burns, causing the temperature of the sensing element to rise, thereby increasing the resistance of the sensing element. Gas thermal conduction gas sensors detect combustible gases by utilizing the phenomenon that the resistance of the sensing element changes due to the heat balance between the sensing element and the gas to be measured, which contains combustible gases.

[0003] Catalytic combustion gas sensors are capable of detecting combustible gases with high accuracy, making them suitable for detecting concentrations within the combustible gas combustion range, but because they detect combustible gases by burning them, when combustible gas is present in high concentrations, there is a lack of oxygen and the gas cannot be completely burned by the sensor catalyst, making it impossible to accurately detect the gas concentration.Gas thermal conduction gas sensors do not burn combustible gases, so they can detect a wide range of combustible gas concentrations, but their low resolution makes it difficult to accurately detect low concentrations of combustible gases.

[0004] Because the range of combustible gas concentrations that each gas sensor can detect is limited, a gas sensor capable of detecting combustible gases over a wider concentration range is desired. Patent Document 1 proposes such a gas sensor, combining a catalytic combustion gas detection function and a gas thermal conduction gas detection function. The gas sensor of Patent Document 1 includes a low-concentration bridge circuit with a catalytic combustion gas detection function and a high-concentration bridge circuit with a gas thermal conduction gas detection function. The low-concentration bridge circuit incorporates a gas concentration detection element that functions as a detection element and a temperature compensation element that functions as a compensation element. The high-concentration bridge circuit incorporates a temperature compensation element that functions as a detection element and a reference resistor that functions as a compensation element, both of which are shared with the low-concentration bridge circuit. In the gas sensor of Patent Document 1, a selector switch switches between a low-concentration bridge circuit and a high-concentration bridge circuit depending on the concentration of hydrogen gas, a combustible gas. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-103605 Summary of the Invention [Problem to be solved by the invention]

[0006] In the gas sensor of Patent Document 1, two bridge circuits are switched between for use, and when one bridge circuit is in use, the other bridge circuit is not in use. Furthermore, when either bridge circuit is in use, at least one element is not in use. Thus, the gas sensor of Patent Document 1 requires extra space to accommodate components that are not always in use, resulting in a larger gas sensor. Furthermore, space is also required to accommodate a switch for switching between the two bridge circuits, which also results in a larger gas sensor.

[0007] SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a gas sensor that can be miniaturized and is capable of detecting combustible gases over a wide concentration range. [Means for solving the problem]

[0008] The gas sensor of the present invention is a gas sensor for detecting a combustible gas contained in a measurement target gas, and includes: a first detection element that detects the combustible gas based on a change in resistance caused by heating due to combustion heat of the combustible gas; a second detection element that detects the combustible gas based on a change in resistance caused by a heat balance with the measurement target gas containing the combustible gas; and a control unit configured to heat the first detection element and / or the second detection element by applying a voltage, and to calculate the gas concentration of the combustible gas based on an output of the first detection element and / or the second detection element, and the control unit controls the first detection element so that the first detection element functions as a detection element. and a second detection mode in which a second voltage lower than the first voltage is applied to the first detection element so that the first detection element functions as a compensating element, and a third voltage is applied to the second detection element so that the second detection element functions as a detection element, and the control unit is configured to switch from the first detection mode to the second detection mode when the gas concentration calculated in the first detection mode exceeds a predetermined threshold, and / or to switch from the second detection mode to the first detection mode when the gas concentration calculated in the second detection mode is equal to or less than the predetermined threshold.

[0009] Furthermore, it is preferable that the control unit is configured to apply the third voltage to the second detection element in the first detection mode so that the second detection element functions as a detection element, and to calculate the gas concentration based on a combined result of outputs of the first detection element and the second detection element.

[0010] Furthermore, it is preferable that the control unit is configured to apply a fourth voltage lower than the third voltage to the second sensing element in the first sensing mode so that the second sensing element functions as a compensation element, and to correct the output of the first sensing element using the output of the second sensing element obtained in that state.

[0011] Furthermore, it is preferable that the control unit is configured to apply the third voltage to the second detection element so that the second detection element functions as a detection element when the combustible gas is detected by the first detection element in the first detection mode while applying a fourth voltage lower than the third voltage to the second detection element so that the second detection element functions as a compensation element.

[0012] It is also preferable that the first sensing element and the second sensing element are formed as a MEMS type. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a gas sensor that can be miniaturized and that can detect combustible gases over a wide concentration range. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a configuration diagram of a gas sensor according to an embodiment of the present invention; [Figure 2] 1A and 1B are top views of a first sensing element and a second sensing element of a gas sensor according to an embodiment of the present invention. [Figure 3] FIG. 4 is a diagram showing changes over time in voltages applied to a first sensing element and a second sensing element. [Figure 4] FIG. 4 is a diagram showing changes over time in voltages applied to a first sensing element and a second sensing element. [Figure 5] FIG. 4 is a diagram showing changes over time in voltages applied to a first sensing element and a second sensing element. [Figure 6] FIG. 4 is a diagram showing changes over time in voltages applied to a first sensing element and a second sensing element. [Figure 7]10 is a graph showing the relationship (sensitivity characteristics) of the output of the first detection element with respect to the gas concentration (low concentration range) of a combustible gas. [Figure 8] 10 is a graph showing the relationship (sensitivity characteristics) of the output of the second detection element with respect to the gas concentration (low concentration range) of a combustible gas. [Figure 9] 10 is a graph showing the relationship (sensitivity characteristics) of the output of the second detection element with respect to the gas concentration (high concentration range) of a combustible gas. [Figure 10] 10 is a graph showing the relationship (sensitivity characteristics) of the combined output of the first sensing element and the second sensing element with respect to the gas concentration (low concentration range) of a combustible gas. [Figure 11] 10 is a graph showing the relationship (sensitivity characteristics) of the output of the first sensing element versus the hydrogen gas concentration (low concentration range) when the first sensing element functions as a sensing element or a compensating element. [Figure 12] 10 is a graph showing the relationship (sensitivity characteristics) of the output of the second sensing element relative to the hydrogen gas concentration (high concentration range) when the second sensing element functions as a sensing element or a compensating element. [Figure 13] 10 is a graph showing the relationship (temperature dependency) of the output of the first sensing element relative to the temperature of the gas to be measured when the first sensing element functions as a compensating element. [Figure 14] 10 is a graph showing the relationship (temperature dependency) of the output of the second sensing element relative to the temperature of the gas to be measured when the second sensing element functions as a compensating element. [Figure 15] 1 is a graph showing the change over time in sensitivity (methane reading) of a first sensing element (a) and a second sensing element (b) when exposed to 10 ppm hexamethyldisiloxane (HMDS). DETAILED DESCRIPTION OF THE INVENTION

[0015] A gas sensor according to an embodiment of the present invention will be described below with reference to the drawings. However, the embodiment described below is merely an example, and the gas sensor of the present invention is not limited to the following example.

[0016] The gas sensor 1 of this embodiment is used to detect combustible gases contained in a measurement target gas that constitutes an environmental atmosphere, such as atmospheric gas. The combustible gases that the gas sensor 1 detects are gases that have the property of combusting themselves, and examples of such gases include hydrogen, methane, ethane, butane, isobutane, propane, acetylene, and carbon monoxide.

[0017] 1, the gas sensor 1 includes a first sensing element 2, a second sensing element 3, and a control unit 4. In the gas sensor 1, the control unit 4 controls the first sensing element 2 and the second sensing element 3 to detect a combustible gas based on the output of the first sensing element 2 and / or the second sensing element 3, as will be described in detail below.

[0018] The first sensing element 2 functions as a sensing element that detects combustible gas based on a change in resistance due to heating by the heat of combustion of the combustible gas. The first sensing element 2 functions as a sensing element when heated above a predetermined temperature. When heated above the predetermined temperature, the first sensing element 2 burns the combustible gas present around the first sensing element 2. The first sensing element 2 is heated by the combustion heat of the burned combustible gas, thereby increasing its resistance. The first sensing element 2 detects the presence of combustible gas based on this increase in resistance. The increase in resistance of the first sensing element 2 varies depending on the concentration of the combustible gas in the measurement target gas, so the first sensing element 2 detects the concentration of the combustible gas in the measurement target gas based on this increase in resistance. In this embodiment, as shown in FIG. 7, the first sensing element 2 is configured so that its resistance increases approximately linearly with an increase in the concentration of the combustible gas in the measurement target gas (in FIG. 7, a potential difference is shown as a value corresponding to the resistance). By using such sensitivity characteristics of the first sensing element 2, the concentration of the combustible gas can be determined from the measured resistance value (potential difference) of the first sensing element 2. However, as long as the relationship between the resistance value and the combustible gas concentration is known in advance, the concentration of the combustible gas can be detected from changes in the resistance value. Therefore, the first sensing element 2 may be configured so that its resistance value changes nonlinearly with increases in the concentration of the combustible gas in the gas to be measured. The predetermined temperature at which the first sensing element 2 functions as a sensing element can be set appropriately within the range at which the combustible gas can be combusted, and is set, for example, to 400°C to 550°C, preferably 450°C to 550°C, more preferably 500°C to 550°C, and most preferably 500°C.

[0019] When the first sensing element 2 is maintained at a predetermined temperature or below, it also functions as a compensating element for temperature compensation of the second sensing element 3. When maintained at a predetermined temperature or below, the first sensing element 2 does not or barely combusts combustible gas. Therefore, even if combustible gas is present in the measurement target gas, the resistance value of the first sensing element 2 changes little or no. Referring to FIG. 11, which shows the change in output of the first sensing element 2 versus the gas concentration of combustible gas (hydrogen) in the measurement target gas (in FIG. 11, the change in potential difference corresponding to the resistance value is shown), when the first sensing element 2 is maintained at 500°C and functions as a sensing element, the output increases as the gas concentration of the combustible gas increases. However, when the first sensing element 2 is maintained at 100°C and functions as a compensating element, the output remains approximately constant even when the gas concentration of the combustible gas increases. On the other hand, the resistance value of first sensing element 2 changes in accordance with changes in temperature in response to changes in the temperature of the sample gas (see FIG. 13, which shows the change in potential difference corresponding to the resistance value, with the sample gas at 20°C as the reference temperature). Therefore, the temperature of the sample gas can be detected based on this change in resistance. First sensing element 2 can perform temperature compensation for second sensing element 3, which functions as a sensing element, based on the detected temperature of the sample gas. The predetermined temperature at which first sensing element 2 functions as a compensating element can be set appropriately within a range at which flammable gas does not burn or barely burns, and is set, for example, to 80°C to 200°C, preferably 80°C to 150°C, more preferably 80°C to 100°C, and most preferably 100°C.

[0020] The first sensing element 2 is not particularly limited in structure, as long as it functions as a sensing element when heated above a predetermined temperature and as a compensating element when below the predetermined temperature. For example, a known catalytic combustion gas sensing element can be used as the first sensing element 2. In this embodiment, the first sensing element 2 is formed as a MEMS (Micro Electro Mechanical System) type, as shown in FIG. 2(a). The MEMS type refers to a device structure in which at least some of the element components are integrated on a substrate such as a silicon substrate using microfabrication technology. Forming the first sensing element 2 as a MEMS type allows for smaller size and lower power consumption compared to a coil type. However, the first sensing element 2 may also be formed as a coil type or a substrate type that does not employ a MEMS structure.

[0021] 2(a), in this embodiment, the first sensing element 2 is provided on an insulating support film S12, such as a silicon oxide film, formed in a cavity S11 formed in a substrate S1, such as a silicon substrate, with a gap between the substrate and the cavity S11. The first sensing element 2 includes a first resistor 21 provided on the insulating support film S12, a first insulating oxide film 22 provided so as to cover the first resistor 21, and a catalyst 23 formed on the first insulating oxide film 22. The first sensing element 2 is incorporated into a circuit (for example, a first sensing element circuit C1, described later) that applies a voltage to the first sensing element 2 and measures the resistance value (or a physical quantity corresponding to the resistance value) of the first sensing element 2 via a lead wire L1 connected to the first resistor 21.

[0022] The first resistor 21 is heated by the application of a voltage and is heated by the heat of combustion of the combustible gas, causing a change in its resistance value. The first resistor 21 is not particularly limited and may be formed of platinum or the like using a film-forming technique such as sputtering. The first insulating oxide film 22 prevents the combustible gas from coming into contact with the first resistor 21. The catalyst 23 is not particularly limited, but preferably has a porous structure to increase the contact area with the combustible gas and promote combustion of the combustible gas. The first insulating oxide film 22 can be formed, for example, by applying a paste of fine powder of an insulating oxide such as alumina onto the first resistor 21. The catalyst 23 promotes combustion of the combustible gas. The catalyst 23 is not particularly limited and may be formed on the first insulating oxide film 22 by mixing a paste containing fine powder of an insulating oxide with fine powder of a catalytic metal such as platinum.

[0023] The first sensing element 2 may have any circuit arrangement, as long as it can apply a voltage to heat the first sensing element 2 and can measure the resistance (or a physical quantity corresponding to the resistance) of the first sensing element 2, which changes in response to the combustion heat of the combustible gas. In this embodiment, the first sensing element 2 is incorporated into a first sensing element circuit C1, as shown in FIG. 1 . The first sensing element circuit C1 applies a voltage to the first sensing element 2 to heat the first sensing element 2 and measures a potential difference corresponding to the resistance of the first sensing element 2, which changes in response to the combustion heat of the combustible gas. The first sensing element circuit C1 includes the first sensing element 2, a fixed resistor R1 electrically connected in series with the first sensing element 2, a first voltage supply unit VS1 that applies a voltage to the first sensing element 2 and the fixed resistor R1, and a potentiometer V that measures the potential difference between both terminals of the first sensing element 2. The first sensing element circuit C1 receives a control signal from the communicatively connected control unit 4, applies a voltage to the first sensing element 2 via the first voltage supply unit VS1 to heat the first sensing element 2, and measures the potential difference between both terminals of the first sensing element 2 via the potentiometer V. The potential difference measured by the potentiometer V is proportional to the resistance value of the first sensing element 2, and can therefore be treated as a value corresponding to the resistance value of the first sensing element 2. The first sensing element circuit C1 transmits the potential difference measured by the potentiometer V to the control unit 4 as an output of the first sensing element 2.

[0024] As shown in FIG. 1 , the gas sensor 1 may include at least two first sensing elements 2. The at least two first sensing elements 2 are incorporated in the first sensing element circuit C1 so that they can be switched between by a switch SW. By including at least two first sensing elements 2 in the gas sensor 1, if one of the at least two first sensing elements 2 deteriorates, it can be replaced with another of the at least two first sensing elements 2 that is not deteriorated. This can extend the life of the gas sensor 1. However, the gas sensor 1 does not necessarily have to include two or more first sensing elements 2; it is sufficient that at least one first sensing element 2 is included.

[0025] The second detection element 3 functions as a detection element that detects combustible gas based on a change in resistance due to a heat balance with the measurement gas containing combustible gas. The second detection element 3 functions as a detection element when heated above a predetermined temperature. When the second detection element 3 is heated above the predetermined temperature, a heat balance occurs between the measurement gas containing combustible gas (e.g., heat is absorbed by the measurement gas), causing a change in resistance (e.g., a decrease). The amount of heat absorbed by the second detection element 3 changes depending on the presence or absence of combustible gas in the measurement gas and the gas concentration of the contained combustible gas, causing a change in the resistance of the second detection element 3. The second detection element 3 detects combustible gas and the gas concentration of the combustible gas in the measurement gas based on the change in resistance. In this embodiment, as shown in Figs. 8 and 9, the second detection element 3 is configured so that its resistance value decreases approximately linearly with an increase in the gas concentration of the combustible gas in the measurement target gas (in Figs. 8 and 9, potential differences are shown as values ​​corresponding to resistance values). By using such sensitivity characteristics of the second detection element 3, the gas concentration of the combustible gas can be determined from the measured resistance value (potential difference) of the second detection element 3. However, as long as the relationship between the resistance value and the gas concentration of the combustible gas is known in advance, the gas concentration of the combustible gas can be detected from changes in the resistance value. Therefore, the second detection element 3 may be configured so that its resistance value changes nonlinearly with an increase in the gas concentration of the combustible gas in the measurement target gas. The predetermined temperature at which the second sensing element 3 functions as a sensing element can be set appropriately within a range in which there is a heat balance between the second sensing element 3 and the gas to be measured, including combustible gas, and in which the resistance value can change due to the presence of combustible gas; for example, it can be set to 200°C to 350°C, preferably 250°C to 350°C, more preferably 300°C to 350°C, and most preferably 300°C.

[0026] By maintaining the second sensing element 3 within a predetermined temperature range, it also functions as a compensating element for temperature compensation of the first sensing element 2. When the second sensing element 3 is within the predetermined temperature range, there is almost no heat balance between the second sensing element 3 and the sample gas, which would cause a change in resistance depending on the presence or absence of a combustible gas. Therefore, even if a combustible gas is present in the sample gas, the resistance value does not change or is minimal. The predetermined temperature range can be set appropriately within a range in which the resistance value does not change or is minimally affected by the presence or absence of a combustible gas, and is set, for example, to 30°C to 100°C, preferably 30°C to 75°C, more preferably 30°C to 50°C, and most preferably 50°C. FIG. 12 shows the change in output of the second detection element 3 with respect to the concentration of combustible gas (hydrogen) in the sample gas (the change in potential difference corresponding to the resistance value). When the second detection element 3 is maintained at 300°C and functions as a detection element, the output decreases as the concentration of the combustible gas increases. However, when the second detection element 3 is maintained at 50°C and functions as a compensation element, the output remains nearly constant even as the concentration of the combustible gas increases. Meanwhile, the resistance of the second detection element 3 changes with temperature changes in response to the temperature of the sample gas (see FIG. 14, which shows the change in potential difference corresponding to the resistance value, with the sample gas temperature at 20°C as the reference). Therefore, the temperature of the sample gas can be detected based on this change in resistance. The second detection element 3 can perform temperature compensation for the first detection element 2, which functions as a detection element, based on the detected temperature of the sample gas.

[0027] The second detection element 3 is not particularly limited in structure as long as it functions as a detection element when heated above a predetermined temperature and as a compensating element within a predetermined temperature range. For example, a known gas thermal conduction gas detection element can be used as the second detection element 3. In this embodiment, the second detection element 3 is formed as a MEMS type, as shown in FIG. 2(b). By forming the second detection element 3 as a MEMS type, it is possible to reduce the size and operate with lower power consumption compared to a coil type. However, the second detection element 3 may also be formed as a coil type or a substrate type that does not employ a MEMS structure.

[0028] 2(b), in this embodiment, the second sensing element 3 is provided on an insulating support film S22, such as a silicon oxide film, which is formed in a cavity S21 formed in a substrate S2, such as a silicon substrate, and spaced apart from the substrate body via the cavity S21. The second sensing element 3 includes a second resistor 31 provided on the insulating support film S22, and a second insulating oxide film 32 provided to cover the second resistor 31. The second sensing element 3 is incorporated into a circuit (for example, a second sensing element circuit C2, described later) that applies a voltage to the second sensing element 3 and measures the resistance value (or a physical quantity corresponding to the resistance value) of the second sensing element 3 via a lead wire L2 connected to the second resistor 31.

[0029] The second resistor 31 is heated by applying a voltage, and a heat balance occurs between the measurement gas containing a combustible gas and the second insulating oxide film 32 (for example, heat is absorbed from the measurement gas), causing a change (for example, a decrease) in its resistance value. The second resistor 31 is not particularly limited and may be formed of platinum or the like using a film formation technique such as sputtering. The second insulating oxide film 32 prevents the combustible gas from coming into contact with the second resistor 31. The second insulating oxide film 32 is not particularly limited as long as it prevents the combustible gas from coming into contact with the second resistor 31, but preferably has a dense structure sufficient to prevent the passage of the combustible gas. The second insulating oxide film 32 may be formed, for example, of a silicon oxide film that can be formed using a film formation technique such as sputtering.

[0030] The second detection element 3 may be configured in any suitable circuit arrangement, provided that a voltage can be applied to heat it and the resistance (or a physical quantity corresponding to the resistance) of the second detection element 3, which changes due to the heat balance between the second detection element 3 and the sample gas, can be measured. In this embodiment, the second detection element 3 is incorporated into a second detection element circuit C2, as shown in FIG. 1 . The second detection element circuit C2 applies a voltage to the second detection element 3 to heat it and measures a potential difference corresponding to the resistance of the second detection element 3, which changes due to the heat balance between the second detection element 3 and the sample gas. The second detection element circuit C2 includes the second detection element 3, a fixed resistor R2 electrically connected in series with the second detection element 3, a second voltage supply unit VS2 that applies a voltage to the second detection element 3 and the fixed resistor R2, and a potentiometer V that measures the potential difference between both terminals of the second detection element 3. The second sensing element circuit C2 receives a control signal from the communicatively connected control unit 4, applies a voltage to the second sensing element 3 via the second voltage supply unit VS2 to heat the second sensing element 3, and measures the potential difference between both terminals of the second sensing element 3 via the potentiometer V. The potential difference measured by the potentiometer V is proportional to the resistance value of the second sensing element 3, and can therefore be treated as a value corresponding to the resistance value of the second sensing element 3. The second sensing element circuit C2 transmits the potential difference measured by the potentiometer V to the control unit 4 as the output of the second sensing element 3.

[0031] The control unit 4 is configured to apply a voltage to heat the first detection element 2 and / or the second detection element 3, and to calculate the gas concentration of the combustible gas based on the output of the first detection element 2 and / or the second detection element 3 obtained by an interaction between the first detection element 2 and / or the second detection element 3 and the combustible gas. For this purpose, in this embodiment, as shown in FIG. 1 , the control unit 4 includes a voltage adjustment unit 41 that adjusts the voltage applied to the first detection element 2 and / or the second detection element 3, and a gas concentration calculation unit 42 that calculates the gas concentration of the combustible gas based on the output of the first detection element 2 and / or the second detection element 3. The control unit 4 is configured to implement either or both of a first detection mode in which the first detection element 2 functions as a detection element and a second detection mode in which the second detection element 3 functions as a detection element by adjusting the voltage applied to the first detection element 2 and the second detection element 3 by the voltage adjustment unit 41. The control unit 4 is configured to use the gas concentration calculation unit 42 to calculate the concentration of the combustible gas from the output of the first detection element 2 and / or the second detection element 3 obtained in each of the first and second detection modes, using sensitivity characteristics (calibration curves) that indicate the relationship between the output of the first detection element 2 and / or the second detection element 3 and the gas concentration of the combustible gas. The sensitivity characteristics to be used may be acquired in advance depending on the type of combustible gas, and may be stored in a storage unit (not shown) that the gas sensor 1 may include. The control unit 4 is not particularly limited and may be formed, for example, by a known CPU.

[0032] In the first detection mode, the control unit 4 is configured to apply a voltage to the first detection element 2 to heat the first detection element 2 so that the first detection element 2 functions as a detection element, and to calculate the gas concentration of the combustible gas based on at least the output of the first detection element 2. In the second detection mode, the control unit 4 is configured to apply a voltage to the second detection element 3 to heat the second detection element 3 so that the second detection element 3 functions as a detection element, and to calculate the gas concentration of the combustible gas based on at least the output of the second detection element 3. The first detection mode is preferably performed when a relatively low concentration (e.g., 100% LEL or less) of combustible gas is present in the measurement target gas, because the first detection element 2 functions as a catalytic combustion type detection element. The second detection mode is preferably performed when a relatively high concentration (e.g., above 100% LEL) of combustible gas is present in the measurement target gas, because the second detection element 3 functions as a gas thermal conduction type detection element.

[0033] The gas sensor 1 is configured to be able to perform both the first detection mode and the second detection mode, thereby enabling accurate detection of combustible gases over a wide concentration range. Furthermore, in both the first detection mode and the second detection mode, as will be described in detail below, the gas sensor 1 uses both the first detection element 2 and the second detection element 3, and there are no unused elements other than the first detection element 2 and the second detection element 3, eliminating the need to reserve space for the unused elements, thereby enabling the gas sensor 1 to be made smaller.

[0034] The control unit 4 may be configured to switch between the first detection mode and the second detection mode by adjusting the voltage applied to the first detection element 2 and the second detection element 3 using the voltage adjustment unit 41 depending on the gas concentration of the combustible gas in the measurement target gas. For example, the control unit 4 may be configured to switch from the first detection mode to the second detection mode when the gas concentration calculated in the first detection mode exceeds a predetermined threshold. The gas sensor 1 can accurately detect combustible gases with relatively high concentrations by switching to the second detection mode when the gas concentration exceeds the predetermined threshold while the first detection mode is being performed. The control unit 4 may also be configured to switch from the second detection mode to the first detection mode when the gas concentration calculated in the second detection mode is equal to or less than the predetermined threshold. The gas sensor 1 can accurately detect combustible gases with relatively low concentrations by switching to the first detection mode when the gas concentration becomes equal to or less than the predetermined threshold while the second detection mode is being performed. However, the first detection mode and the second detection mode may be switched by a user's instruction.

[0035] Here, the predetermined threshold serving as a criterion for switching between the first detection mode and the second detection mode is not particularly limited and can be set appropriately as the boundary between the gas concentration range suitable for detection in the first detection mode and the gas concentration range suitable for detection in the second detection mode. For example, by setting the predetermined threshold to the lower explosive limit concentration (100% LEL) of the flammable gas, the flammable gas can be accurately detected in the first detection mode in a concentration range where the flammable gas is unlikely to explode, and can be accurately detected in the second detection mode in a concentration range where the flammable gas is likely to explode.

[0036] The predetermined threshold value can be set based on factors other than the above-mentioned considerations of detection accuracy and safety. For example, if the combustible gas is hydrogen, when the first detection element 2 functions as a detection element in the first detection mode, water is generated during the catalytic reaction of hydrogen combustion. In this case, if the first detection element 2 and the second detection element 3 are formed as MEMS elements as in this embodiment, the generated water may adhere to the first detection element 2 (or the second detection element 3 in some cases), resulting in a phenomenon in which the resistance value fluctuates significantly (base drift). This phenomenon is likely to occur when the hydrogen concentration exceeds approximately 50% LEL. Therefore, by setting the predetermined threshold value to the lowest hydrogen gas concentration (approximately 50% LEL) at which base drift can occur, the second detection mode, which can suppress the occurrence of base drift, is implemented within the gas concentration range in which base drift can occur, thereby enabling stable gas concentration detection.

[0037] Next, the detection operation in the first detection mode and the second detection mode will be described with reference to FIGS. 3 to 6. FIGS. 3 to 6 show the change over time in the voltage applied to the first detection element 2 and the second detection element 3 after the power is turned on. For example, FIGS. 3 and 4 show a state in which the first detection mode is performed after the power is turned on, the mode switches to the second detection mode when the gas concentration of the combustible gas exceeds a predetermined threshold, and the mode switches back to the first detection mode when the gas concentration of the combustible gas falls below the predetermined threshold. Also, FIGS. 5 and 6 show a state in which the first detection mode switches between a detection mode in which the second detection element 3 functions as a compensating element and a detection mode in which the second detection element 3 functions as a detection element.

[0038] <First detection mode> In the first detection mode, as shown in FIGS. 3 to 6 , the control unit 4 is configured to apply a first voltage to the first detection element 2 so that the first detection element 2 functions as a detection element. The first voltage, when applied to the first detection element 2, is a voltage that can heat the first detection element 2 to a temperature at which the first detection element 2 can function as a detection element, i.e., a temperature at which combustible gas around the first detection element 2 can be combusted. The first voltage can be set, for example, to a voltage that can heat the first detection element 2 to at least 400°C to 550°C, preferably 450°C to 550°C, more preferably 500°C to 550°C, and most preferably 500°C. When the first voltage is applied, the first detection element 2 functions as a detection element and can detect combustible gas by catalytic combustion.

[0039] In the first detection mode, the second detection element 3 may function as a detection element or as a compensation element. For example, as shown in FIG. 3, the control unit 4 may be configured to apply a third voltage to the second detection element 3 in the first detection mode so that the second detection element 3 functions as a detection element. The third voltage, when applied to the second detection element 3, is a voltage that can heat the second detection element 3 to a temperature at which the second detection element 3 can function as a detection element, i.e., a temperature at which a change in resistance value associated with a change in the heat balance of the second detection element 3, which corresponds to a change in the thermal conductivity of the sample gas due to the inclusion of a combustible gas, can be detected. The third voltage can be set, for example, to a voltage that can heat the second detection element 3 to at least 200°C to 350°C, preferably 250°C to 350°C, more preferably 300°C to 350°C, and most preferably 300°C. When the third voltage is applied, the second detection element 3 functions as a detection element and can detect combustible gases by gas thermal conduction.

[0040] In this case, the control unit 4 is configured to calculate the gas concentration based on the combined output of the first detection element 2 and the second detection element 3. Referring to FIG. 10 , when both the first detection element 2 and the second detection element 3 function as detection elements, the combined output of the first detection element 2 and the second detection element 3 (the sum of the absolute values ​​of the outputs of the first detection element 2 and the second detection element 3) increases approximately linearly with an increase in the concentration of the combustible gas. By using such sensitivity characteristics of the first detection element 2 and the second detection element 3, the control unit 4 can calculate the gas concentration of the combustible gas from the measured outputs of the first detection element 2 and the second detection element 3. Calculating the gas concentration based on the combined output of the first detection element 2 and the second detection element 3 enables combustible gas detection with higher sensitivity and accuracy. However, the control unit 4 may also be configured to calculate the gas concentration of the combustible gas based only on the output of the first detection element 2.

[0041] In the first detection mode, the gas sensor 1 can achieve further advantages by having both the first detection element 2 and the second detection element 3 function as detection elements. When functioning as a detection element, the first detection element 2 burns combustible gases through a catalytic reaction. However, if a toxic gas such as a siloxane compound is present in the gas being measured, the first detection element 2 may be poisoned and deteriorated during the catalytic reaction. Deterioration of the first detection element 2 reduces the detection sensitivity of combustible gases, making it impossible to accurately detect combustible gases. In contrast, when functioning as a detection element, the second detection element 3 is hardly or never poisoned or deteriorated by the toxic gas. Therefore, the degree of deterioration (deterioration rate) of the first detection element 2 can be evaluated by using the output of the second detection element 3 as a reference. Therefore, the deterioration of the first detection element 2 can be compensated for (deterioration compensation) based on the evaluated deterioration rate, and the replacement time of the first detection element 2 can be determined. Details of this will be described later.

[0042] As shown in FIG. 4, the control unit 4 may be configured to apply a fourth voltage lower than the third voltage to the second detection element 3 in the first detection mode so that the second detection element 3 functions as a compensating element. The fourth voltage is lower than the third voltage and is a voltage that can maintain the second detection element 3 at a temperature at which the second detection element 3 can function as a compensating element. The fourth voltage, when applied to the second detection element 3, can maintain the second detection element 3 at a temperature at which the resistance value remains almost unchanged and the amount of heat exchanged between the second detection element 3 and the measurement target gas remains almost unchanged, even if the thermal conductivity of the measurement target gas changes due to the inclusion of a combustible gas. The fourth voltage is set to a voltage that can maintain the second detection element 3 at a temperature of, for example, 30°C to 100°C, preferably 30°C to 75°C, more preferably 30°C to 50°C, and most preferably 50°C. When the fourth voltage is applied to the second detection element 3, the second detection element 3 functions as a compensating element, with its resistance value remaining almost constant with the concentration of the combustible gas (see FIG. 12). However, its resistance value changes with the temperature of the target gas (see FIG. 14). The second detection element 3 detects the temperature of the target gas based on this change in resistance and can correct the output of the first detection element 2, which functions as a detection element, based on the detected temperature of the target gas. To this end, the control unit 4 is configured to correct the output of the first detection element 2 using the output of the second detection element 3 obtained when the fourth voltage is applied to the second detection element 3. The control unit 4 is then configured to calculate the concentration of the combustible gas based on the corrected output of the first detection element 2. By performing temperature compensation on the first detection element 2, the combustible gas can be accurately detected even if the target gas to be actually measured is at a temperature different from the temperature of the target gas when the sensitivity characteristic (calibration curve) of the first detection element 2 to the combustible gas concentration was obtained. If the temperature of the gas to be measured does not fluctuate significantly during the detection period, the voltage applied to the second detection element 3 may be changed from the fourth voltage to the third voltage, and the output of the first detection element 2 after the change may be corrected based on the output of the second detection element 3 before the change, as shown in Figures 5 and 6.

[0043] As shown in FIG. 5 , in the first detection mode, the control unit 4 may be configured to periodically and alternately apply a third voltage and a fourth voltage to the second detection element 3 so that the second detection element 3 alternately functions as a detection element and a compensation element. In the gas sensor 1, the second detection element 3 alternates between functioning as a compensation element and functioning as a detection element. This allows the output of the first detection element 2 to be temperature-compensated and degradation-compensated in response to temperature fluctuations of the measurement target gas or deterioration rate fluctuations of the first detection element 2 during the detection period. This allows the gas sensor 1 to detect flammable gases with greater accuracy. In particular, when the second detection element 3 is formed as a MEMS type, as in this embodiment, its temperature changes rapidly when the applied voltage changes. Therefore, even if the applied voltage is changed every 100 msec, the temperature of the second detection element 3 changes in response to the change in applied voltage. Therefore, even if the temperature of the gas to be measured or the deterioration rate of the first detection element 2 changes in units of, for example, one second, the temperature fluctuation and deterioration rate fluctuation can be detected, and the combustible gas can be detected with higher accuracy.

[0044] 6 , the control unit 4 may be configured to apply the third voltage to the second detection element 3 so that the second detection element 3 functions as a detection element when a combustible gas is detected by the first detection element 2 while applying a fourth voltage lower than the third voltage to the second detection element 3 so that the second detection element 3 functions as a compensating element in the first detection mode. In this way, in a situation where no or little combustible gas is present, the gas sensor 1 can accurately monitor the combustible gas by having the second detection element 3 function as a compensating element and perform temperature compensation for the first detection element 2. Furthermore, when a combustible gas is detected, the gas sensor 1 switches to having the second detection element 3 function as a detection element, enabling compensation for deterioration of the first detection element 2, thereby enabling more accurate detection of the combustible gas.

[0045] <Second detection mode> In the second detection mode, as shown in FIGS. 3 and 4 , the controller 4 is configured to apply a third voltage to the second detection element 3 so that the second detection element 3 functions as a detection element, and a second voltage lower than the first voltage to the first detection element 2 so that the first detection element 2 functions as a compensating element. The second voltage is lower than the first voltage and, when applied to the first detection element 2, maintains the first detection element 2 at a temperature at which the first detection element 2 can function as a compensating element, i.e., a temperature at which the first detection element 2 does not or hardly does not combust any combustible gas around the first detection element 2. The second voltage can be set to a voltage that can heat the first detection element 2 to at least 80°C to 200°C, preferably 80°C to 150°C, more preferably 80°C to 100°C, and most preferably 100°C. In the second detection mode, the first detection element 2 functions as a compensating element to detect the temperature of the measurement gas. The control unit 4 is configured to correct the output of the second detection element 3 based on the output of the first detection element 2 corresponding to the temperature of the measurement target gas. The control unit 4 is then configured to calculate the gas concentration of the combustible gas in the measurement target gas based on the corrected output of the second detection element 3. By performing temperature compensation for the second detection element 3, the combustible gas can be detected with high accuracy even if the measurement target gas actually measured is at a temperature different from the temperature of the measurement target gas when the sensitivity characteristic of the second detection element 3 to the gas concentration of the combustible gas was obtained.

[0046] The detection operation in the first detection mode and the second detection mode has been described above. However, the above description is merely an example, and the detection operation in the first detection mode and the second detection mode is not limited to the above example. Furthermore, the gas sensor 1 can also perform detection modes other than the first detection mode and the second detection mode.

[0047] Next, we will discuss the deterioration compensation for the first sensing element 2 in detail. As described above, when the first sensing element 2 functions as a sensing element, it may be poisoned and deteriorated if a toxic gas such as a siloxane compound is present in the gas to be measured. In contrast, when the second sensing element 3 functions as a sensing element, it is not poisoned by the toxic gas and deteriorates little or no. Figures 15(a) and 15(b) show the time-dependent changes in sensitivity (methane readings) of the first sensing element 2 and the second sensing element 3, respectively, when exposed to 10 ppm hexamethyldisiloxane (HMDS). As shown in Figure 15(a), the sensitivity of the first sensing element 2 decreases upon exposure to HMDS, indicating that it has been poisoned and deteriorated by HMDS. In contrast, as shown in Figure 15(b), the sensitivity of the second sensing element 3 remains unchanged even when exposed to HMDS, indicating that it is not poisoned or deteriorated by HMDS. Therefore, in the first detection mode, when both the first detection element 2 and the second detection element 3 are functioning as detection elements, the degree of deterioration (deterioration rate) of the first detection element 2 can be evaluated by using the output of the second detection element 3 as a reference, and the deterioration of the first detection element 2 can be compensated for (deterioration compensation) based on the evaluated deterioration rate.

[0048] For the purpose of evaluating the deterioration rate of the first detection element 2 and compensating for the deterioration of the first detection element 2, in this embodiment, the gas sensor 1 (controller 4 thereof) is equipped with a deterioration rate calculation unit 43 that calculates the deterioration rate of the first detection element 2, and a gas concentration calculation unit 42 that calculates the gas concentration of the combustible gas based on the output and deterioration rate of the first detection element 2, as shown in FIG.

[0049] The deterioration rate calculation unit 43 calculates the ratio between the output of the first detection element 2 and the output of the second detection element 3, and calculates the deterioration rate of the first detection element 2 from the calculated ratio. In this embodiment, as described above, both the first detection element 2 and the second detection element 3 are configured so that the outputs of the first detection element 2 and the second detection element 3 change approximately linearly with an increase in the gas concentration of the flammable gas (see FIGS. 7 and 8). Therefore, unless the first detection element 2 has deteriorated and its detection sensitivity has not decreased, the ratio (A=a1 / a2) between the output (a1) of the first detection element 2 and the output (a2) of the second detection element 3 remains constant regardless of the gas concentration of the flammable gas. If the first detection element 2 deteriorates and its detection sensitivity decreases, the ratio (B=b1 / b2) between the output (b1) of the first detection element 2 and the output (b2) of the second detection element 3 changes. The deterioration rate calculation unit 43 can calculate the deterioration rate of the first detection element 2 (for example, deterioration rate C=B / A×100(%)) using the ratio (A) before deterioration and the ratio (B) after deterioration.

[0050] The gas concentration calculation unit 42 calculates the gas concentration of the combustible gas based on the output of the first detection element 2 and the deterioration rate calculated by the deterioration rate calculation unit 43. In the gas sensor 1, the gas concentration calculation unit 42 calculates the gas concentration of the combustible gas based on the output and deterioration rate of the first detection element 2, so that even if the first detection element 2 deteriorates, the deterioration of the first detection element 2 is compensated for, and the combustible gas can be detected with high accuracy.

[0051] When the first detection element 2 is deteriorated, the method for calculating the gas concentration of the combustible gas by the gas concentration calculation unit 42 is not particularly limited as long as it is based on the output and deterioration rate of the first detection element 2. For example, the gas concentration calculation unit 42 can correct the output of the first detection element 2 based on the deterioration rate of the first detection element 2 and calculate the gas concentration of the combustible gas from the corrected output of the first detection element 2. In correcting the output of the first detection element 2, for example, by multiplying the output of the first detection element 2 by the reciprocal of the deterioration rate, it is possible to calculate the output of the first detection element 2 that would have been obtained if the first detection element 2 had not deteriorated. The gas concentration calculation unit 42 can calculate the gas concentration of the combustible gas from the calculated (corrected) output of the first detection element 2 using the sensitivity characteristics (calibration curve) of the first detection element 2 versus the gas concentration of the combustible gas that are stored in advance in the storage unit.

[0052] The gas concentration calculation unit 42 can also calculate the concentration of the combustible gas from the output of the first detection element 2 obtained in the actual gas to be measured and the deterioration rate calculated by the deterioration rate calculation unit 43, using a relationship (calibration curve) between the output of the first detection element 2 and the deterioration rate relative to the combustible gas concentration, which is stored in advance in the storage unit. The relationship between the output of the first detection element 2 and the deterioration rate relative to the combustible gas concentration can be obtained, for example, by measuring the sensitivity characteristics (calibration curve) of the first detection element 2 relative to the combustible gas concentration for multiple first detection elements 2 with known deterioration rates and different deterioration rates. The relationship between the output of the first detection element 2 and the deterioration rate relative to the combustible gas concentration is measured for each different type of combustible gas and is stored in the storage unit according to the type of combustible gas.

[0053] The gas sensor 1 may be configured to switch between at least two first detection elements 2 provided in the gas sensor 1 in accordance with the deterioration rate of the first detection element 2 calculated by the deterioration rate calculation unit 43. For this purpose, in this embodiment, the gas sensor 1 (control unit 4) further includes a detection element switching unit 44 that switches between the first detection elements 2, as shown in FIG. 1 . When the deterioration rate of one of the at least two first detection elements 2 exceeds a predetermined threshold, the detection element switching unit 44 switches from one first detection element 2 to another first detection element 2, so that another first detection element 2 of the at least two first detection elements 2 whose deterioration rate is equal to or less than the predetermined threshold is used. The predetermined threshold can be set appropriately within a range in which the above-described deterioration compensation is no longer possible. For example, the predetermined threshold is set to a range in which the first detection element 2 deteriorates and no output can be obtained, or a range in which the first detection element 2 deteriorates and the relationship between the combustible gas concentration and the output of the first detection element 2 becomes non-linear. When the deterioration rate of the first detection element 2 exceeds a predetermined threshold, the detection element switching unit 44 switches the first detection element 2 to another first detection element 2 whose deterioration rate is equal to or lower than the predetermined threshold by switching the switch SW in the first detection element circuit C1. This enables the life of the gas sensor 1 to be extended.

[0054] As described above, in order to detect the concentration of a combustible gas in the gas sensor 1, the following steps are executed: calculating the ratio between the output of the first detection element 2 and the output of the second detection element 3 and calculating the deterioration rate of the first detection element 2 from the calculated ratio; and calculating the gas concentration based on the output and deterioration rate of the first detection element 2. In this embodiment, these steps are executed by the control unit 4 of the gas sensor 1, but they may also be executed by a control device separate from the gas sensor 1. By executing these steps in the gas sensor 1, the combustible gas can be detected with high accuracy even if the first detection element 2 has deteriorated. [Explanation of symbols]

[0055] 1 Gas sensor 2. First sensing element 21 First Resistor 22 First insulating oxide film 23 Catalyst 3 Second sensing element 31 Second Resistor 32 Second insulating oxide film 4. Control section 41 Voltage adjustment unit 42 Gas concentration calculation section 43 Deterioration rate calculation section 44 Detector element switching unit C1 First detector circuit C2 Second sensing element circuit L1, L2 lead wires R1, R2 fixed resistor S1 and S2 boards S11, S21 cavity S12, S22 insulating support film SW switch V potentiometer VS1 First voltage supply VS2 Second voltage supply

Claims

1. A gas sensor for detecting a combustible gas contained in a measurement target gas, a first detection element configured to detect the combustible gas based on a change in resistance value caused by heating due to combustion heat of the combustible gas; a second detection element configured to detect the combustible gas based on a change in resistance due to a heat balance between the second detection element and the measurement target gas containing the combustible gas; a control unit configured to apply a voltage to heat the first detection element and / or the second detection element, and to calculate the gas concentration of the combustible gas based on an output of the first detection element and / or the second detection element; Equipped with The control unit a first sensing mode in which a first voltage is applied to the first sensing element so that the first sensing element functions as a sensing element; a second sensing mode in which a second voltage lower than the first voltage is applied to the first sensing element so that the first sensing element functions as a compensating element, and a third voltage is applied to the second sensing element so that the second sensing element functions as a sensing element; configured to switch between The control unit and / or configured to switch from the first detection mode to the second detection mode when the gas concentration calculated in the first detection mode exceeds a predetermined threshold. and switching the second detection mode to the first detection mode when the gas concentration calculated in the second detection mode is equal to or less than the predetermined threshold value. Gas sensor.

2. the control unit is configured to, in the first detection mode, apply the third voltage to the second detection element so that the second detection element functions as a detection element, and calculate the gas concentration based on a combined result of outputs of the first detection element and the second detection element.

2. The gas sensor according to claim 1.

3. the control unit is configured to, in the first detection mode, apply a fourth voltage lower than the third voltage to the second detection element so that the second detection element functions as a compensation element, and to correct the output of the first detection element using an output of the second detection element obtained in that state.

3. The gas sensor according to claim 1.

4. the control unit is configured, in the first detection mode, to apply the third voltage to the second detection element so that the second detection element functions as a detection element when the combustible gas is detected by the first detection element in a state in which a fourth voltage lower than the third voltage is applied to the second detection element so that the second detection element functions as a compensating element. The gas sensor according to any one of claims 1 to 3.

5. the first sensing element and the second sensing element are formed as a MEMS type; The gas sensor according to any one of claims 1 to 4.

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