Sensor module

Abstract

To enable accurate measurement of concentration of a measurement target gas in a mixed gas atmosphere.SOLUTION: A heat conduction sensor module (1) for measuring concentration of a measurement target gas in a reference gas is provided, the sensor module comprising a microheater (10) configured to generate heat according to supplied power, and a control unit (100) configured to control power to the microheater (10) so as to bring heater temperature closer to target temperature at which a thermal conductivity of an interference gas matches a thermal conductivity of the reference gas.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a sensor module for measuring the concentration of at least one gas in a gas mixture. [Background technology]

[0002] Conventionally, sensors for measuring the concentration of at least one type of gas contained in a mixed gas are known. For example, Japanese Patent Application Laid-Open No. 2017-173126 (Patent Document 1) discloses a thermal conduction type gas sensor that measures the concentration of a gas by utilizing the thermal conductivity of the gas. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-173126 A [Summary] According to the gas sensor disclosed in JP 2017-173126 A, the temperature of the gas detection unit changes depending on the thermal conductivity of the gas to be measured that exists in the atmosphere, and the concentration of the gas can be measured by detecting the temperature of the gas detection unit. However, in the atmosphere, a mixed gas containing not only the gas to be measured but also other gases may exist. When the thermal conductivities of the multiple types of gases contained in the mixed gas are different from each other, the temperature of the gas detection unit is affected not only by the thermal conductivity of the gas to be measured but also by the thermal conductivity of the other gases contained in the mixed gas. For this reason, the gas sensor disclosed in JP 2017-173126 A may not be able to accurately measure the concentration of the gas to be measured in an atmosphere containing a mixed gas.

[0004] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to provide a technology for measuring the concentration of a gas to be measured with high accuracy in an atmosphere containing a mixed gas.

[0005] The present disclosure relates to a thermal conduction sensor module for measuring the concentration of a target gas in a reference gas. The sensor module includes a heater that generates heat in response to supplied power, and a control device that controls the power of the heater so that the temperature of the heater approaches a target temperature at which the thermal conductivity of an interference gas and the thermal conductivity of the reference gas match. [Brief description of the drawings]

[0006] [Figure 1] 1 is a diagram for explaining a configuration of a sensor module 1 according to a first embodiment. [Diagram 2] FIG. 1 is a diagram for explaining a configuration of a gas sensor according to a first embodiment. [Diagram 3] FIG. 11 is a diagram showing an example of the thermal conductivity of each of a plurality of types of gases. [Figure 4] FIG. 1 is a diagram showing an example of thermal conductivity versus temperature for each of a plurality of types of gases. [Diagram 5] 5 is a flowchart for explaining the power control of the microheater executed by the control device in the hydrogen sensor module of the present embodiment. [Figure 6] 1 is a diagram showing a comparison of the detected hydrogen concentration and the influence of the carbon dioxide concentration when the ambient temperature is 100° C. and when the ambient temperature is 450° C.

[0007] [Detailed Description] The embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals and the description thereof will not be repeated.

[0008] [Sensor module configuration] The configuration of a sensor module 1 according to embodiment 1 will be described with reference to Fig. 1 to Fig. 4. Fig. 1 is a diagram for explaining the configuration of a sensor module 1 according to embodiment 1. As shown in Fig. 1, the sensor module 1 according to embodiment 1 includes a microheater 10, a sensor 20, and a control device 100.

[0009] The microheater 10 and the sensor 20 are a chip-type gas sensor that uses MEMS (Micro Electro Mechanical Systems) technology to measure a target gas such as hydrogen.

[0010] The control device 100 is configured to measure the concentration of gas present in the atmosphere based on a detection value (resistance value of platinum of the sensor 20) obtained from the microheater 10. Specifically, the control device 100 measures at least the concentration of hydrogen present in the atmosphere in which the microheater 10 is disposed. The control device 100 is, for example, an information processing device such as a microcomputer, a notebook or desktop PC, a smartphone, a tablet terminal, an in-vehicle ECU (Engine Control Unit), or a PLC (Programmable Logic Controller) of a production facility.

[0011] The control device 100 mainly comprises an arithmetic unit 101, a storage device 102, a peripheral device interface 103, and a microheater interface 104.

[0012] The arithmetic device 101 is a computing entity (computer) that executes various processes by executing various programs. The arithmetic device 101 is configured with a processor such as a central processing unit (CPU) or a micro-processing unit (MPU). The processor, which is an example of the arithmetic device 101, has a function of executing various processes by executing a program, but some or all of these functions may be implemented using a dedicated hardware circuit such as an application specific integrated circuit (ASIC) or a field-programmable gate array (FPGA). The "processor" is not limited to a processor in the narrow sense that executes processes in a stored program manner such as a CPU or an MPU, but may include a hardwired circuit such as an ASIC or an FPGA. Therefore, the "processor" which is an example of the arithmetic device 101 can also be read as a processing circuitry in which processing is defined in advance by computer-readable code and / or a hardwired circuit. The arithmetic device 101 may be configured with one chip or multiple chips. Furthermore, some functions of the arithmetic device 101 may be provided in a server device (for example, a cloud-type server device) not shown.

[0013] The storage device 102 includes a volatile storage area (for example, a working area) that temporarily stores program codes, work memory, and the like when the arithmetic device 101 executes various programs. Examples of the storage device 102 include volatile memories such as dynamic random access memory (DRAM) and static random access memory (SRAM), and non-volatile memories such as read only memory (ROM) and flash memory. The storage device 102 may also include a hard disk drive (HDD) and a solid state drive (SSD). In the control device 100 according to the embodiment, the storage device 102 stores a control program 120 and thermal conductivity data 130.

[0014] The control program 120 causes the arithmetic device 101 to execute at least a process for measuring the concentration of hydrogen present in the atmosphere (hereinafter also referred to as a "concentration measurement process") based on the detection value acquired by the arithmetic device 101 from the microheater 10. The thermal conductivity data 130 is data on the thermal conductivity of each of multiple types of gases contained in the mixed gas, and includes data showing the thermal conductivity of each gas with respect to temperature as shown in Fig. 3 described later.

[0015] The peripheral device interface 103 is an interface for connecting peripheral devices such as an input device, a storage medium, etc. The peripheral device interface 103 realizes input and output of data between the control device 100 and the peripheral devices.

[0016] The microheater interface 104 is an interface for connecting the microheater 10. The microheater interface 104 detects a detection value corresponding to the heater temperature and acquires the detection value. The microheater interface 104 also supplies heater power to the microheater 10 under the control of the arithmetic device 101. Note that the microheater interface 104 may be divided into an interface for acquiring the detection value from the microheater 10 and an interface for supplying heater power to the microheater 10, which have different configurations.

[0017] 2 is a diagram for explaining the configuration of the gas sensor according to embodiment 1. As shown in FIG.

[0018] The thin film layer 12 is provided on the silicon substrate 11 and has a thickness of, for example, about 2.4 μm. The thin film layer 12 includes an insulating layer 321, an intermediate layer 322 provided on the insulating layer 321, a microheater 10 provided in the intermediate layer 322, and an insulating layer 324 provided on the intermediate layer 322. The microheater 10 includes at least one heater portion 323 made of a metal oxide. The insulating layer 321 and the insulating layer 324 are made of, for example, silicon nitride (SiN). The intermediate layer 322 is made of silicon dioxide (SiO2). The heater portion 323 includes, for example, a platinum (Pt) layer 330 and metal oxides 331 and 332 formed to cover the lower and upper surfaces of the platinum layer 330. The microheater 10 includes a first layer (metal oxide 331) and a second layer (metal oxide 332) formed on the first layer (metal oxide 331). At least one of the first layer and the second layer is an oxide barrier film.

[0019] The sensor 20 includes a metal oxide 201 disposed on the insulating layer 324 of the thin film layer 12 and a platinum (Pt) layer 202 disposed on the metal oxide 201 .

[0020] The metal oxide 331 functions as a barrier film between the platinum layer 330 and the intermediate layer 322. The metal oxide 201 functions as a barrier film between the insulating layer 324 and the platinum layer 202. The metal oxide 331 and the metal oxide 201 contain at least one of titanium oxide (TiO2), chromium oxide (Cr2O3), tantalum pentoxide (Ta2O5), and an oxygen-deficient metal oxide. The oxygen-deficient metal oxide is a material that has been attracting attention in recent years as a highly functional material.

[0021] The microheater 10 configured as described above generates heat in response to the power (hereinafter also referred to as "heater power") supplied from the control device 100. At this time, the temperature of the thin film layer 12 changes in response to the thermal conductivity of the gas present in the atmosphere in which the microheater 10 is placed.

[0022] Here, the thermal conductivity of each of the multiple types of gases will be described with reference to Figs. 3 and 4. Hydrogen, oxygen, air, nitrogen, water vapor, and carbon dioxide have different thermal conductivities. Fig. 3 is a diagram showing an example of the thermal conductivity of each of the multiple types of gases. As shown in Fig. 3, when the temperature of the atmosphere in which each gas exists is 0°C, hydrogen has the highest thermal conductivity, followed by oxygen, air, and nitrogen in descending order of thermal conductivity, and water vapor and carbon dioxide have the lowest thermal conductivity. Note that water vapor and carbon dioxide have approximately the same thermal conductivity. The air is standard air, for example, the air in the atmosphere at the time of shipment of the microheater 10.

[0023] Furthermore, the thermal conductivity of each of the gases, hydrogen, oxygen, air, nitrogen, water vapor, and carbon dioxide, changes according to the temperature change of the atmosphere in which the gas exists. Figure 4 is a diagram showing the change in thermal conductivity of each of a plurality of types of gases with respect to temperature. In the graph shown in Figure 4, the thermal conductivity of hydrogen is converted to 1 / 10.

[0024] As shown in Fig. 4, the thermal conductivity of each gas changes as the temperature changes between 200 K and 800 K. Specifically, the higher the temperature, the higher the thermal conductivity of each gas.

[0025] In this way, the thermal conductivity of the atmosphere differs depending on the type of gas present in the atmosphere in which the microheater 10 is placed. For example, the thermal conductivity of hydrogen is greater than the thermal conductivity of each of air, water vapor, and carbon dioxide. Therefore, when the gas present in the atmosphere is hydrogen, the heat of the thin film layer 12 is more likely to escape into the atmosphere than when the gas present in the atmosphere is air, water vapor, or carbon dioxide, and if the heat generation amount (or power supply) of the microheater 10 is the same, the temperature of the thin film layer 12 (hereinafter also referred to as the "heater temperature") is lower. In addition, the thermal conductivity of each of water vapor and carbon dioxide is smaller than the thermal conductivity of each of hydrogen and air. Therefore, when the gas present in the atmosphere is water vapor or carbon dioxide, the heat of the thin film layer 12 is more difficult to escape into the atmosphere than when the gas present in the atmosphere is hydrogen or air, and the heater temperature is higher.

[0026] That is, the heater temperature changes depending on the type (thermal conductivity) of the gas present in the atmosphere in which the microheater 10 is placed, and the resistance value of the platinum layer 202 of the sensor 20 changes depending on the heater temperature. By utilizing this phenomenon, the control device 100 can obtain the resistance value of the platinum layer 202 of the sensor 20 as a detection value, and measure the concentration of the gas present in the atmosphere based on the obtained detection value. Note that the control device 100 may directly detect the heater temperature of the thin film layer 12 as the detection value.

[0027] As described above, the thermal conductivity of hydrogen is nearly one order of magnitude higher than that of other gases, so the microheater 10, which produces a difference in the amount of heat dissipation, is used as a hydrogen sensor.

[0028] However, the proportion of carbon dioxide and water vapor in the atmosphere can be on the order of percent, and their impact on the accuracy of hydrogen sensors is not small. This poses the problem that the detection accuracy of hydrogen concentration becomes poor due to the effects of carbon dioxide and water vapor concentrations. Therefore, it is usually difficult to measure hydrogen concentrations of 1% or less with good accuracy. On the other hand, hydrogen is flammable, and there is a need for hydrogen sensors to detect the presence of hydrogen at an early stage, even at low concentrations, in order to detect and warn of leaks.

[0029] Looking at Figure 4, we can see that at approximately 723K (450℃), the thermal conductivity of air, nitrogen, and water vapor are all equal, at approximately 0.053 (W / mK). If the solid line is extended, it is estimated that the thermal conductivity of carbon dioxide will also be the same at approximately 723K (450℃).

[0030] As can be seen from FIG. 4, the thermal conductivity of air, water vapor, and carbon dioxide is close to each other in the temperature range of 673K ​​(400°C) to 773K (500°C). In other words, in this temperature range, even if the water vapor concentration and carbon dioxide concentration in the atmosphere change, there is almost no difference in thermal conductivity compared to that of air. Since the calibration curve for hydrogen concentration is created by changing the ratio of hydrogen in the reference gas (air) atmosphere, if a gas sensor is used in this temperature range, it is unlikely to be affected by the water vapor concentration and carbon dioxide concentration in the atmosphere, and the hydrogen concentration can be detected with high accuracy. Therefore, in this embodiment, the heater is controlled to keep the temperature constant, and the hydrogen concentration is detected based on the heater power at that time.

[0031] FIG. 5 is a flow chart for explaining the power control of the microheater executed by the control device in the hydrogen sensor module of this embodiment.

[0032] First, in step S1, the control device 100 feeds back the power P of the microheater 10 so as to maintain the temperature T0. Here, the temperature T0 may be within the temperature range of 400°C to 500°C shown in Fig. 4, and more preferably, the temperature T0 is 450°C, which is in the middle of this range. The control device detects the temperature from the resistance value of the platinum in the sensor 20, and increases or decreases the power P so that the detected temperature approaches the target temperature T0.

[0033] In step S2, the control device 100 derives the hydrogen concentration from the power P after feedback by referring to a data set prepared in advance.

[0034] The amount of heat dissipated into the atmosphere changes depending on the hydrogen concentration of the atmosphere even at the same temperature. The amount of heat dissipated has a one-to-one relationship with the power P applied to the heater. Therefore, if the relationship between the power P required to maintain the temperature of the microheater 10 at the target temperature T0 and the hydrogen concentration is prepared in advance as a calibration curve or map, the hydrogen concentration can be detected based on the power P.

[0035] Fig. 6 is a diagram showing a comparison of the detected hydrogen concentration value and the influence of carbon dioxide concentration when the ambient temperature is 100°C and 450°C. Fig. 6 shows the change in the measured value detected by the hydrogen sensor module of this embodiment when the carbon dioxide concentration in a gas to be measured, which has a hydrogen concentration of 0.1%, is changed from 0 to 2%. In this case, air is the reference gas, carbon dioxide is the interfering gas, and hydrogen is the gas to be measured.

[0036] When the carbon dioxide concentration is 0, the hydrogen concentration measurement correctly shows 0.10% whether the temperature is 100°C or 450°C.

[0037] When the temperature is 100°C and the carbon dioxide concentration is 1.8%, the measured hydrogen concentration will be 0.00%, which is an error of 0.1% compared to the true value of 0.10%, or 1000 ppm.

[0038] In contrast, when the temperature is 450°C, even if the carbon dioxide concentration increases to 2%, the measured value is between 0.09% and 0.10%, and the error is 0.01%, that is, smaller than 100 ppm.

[0039] Therefore, by operating the hydrogen sensor module with a target temperature of 450° C., the influence of the carbon dioxide (interfering gas) concentration on the measured value of the hydrogen (measurement target gas) concentration can be significantly suppressed.

[0040] Although FIG. 6 shows a case where the interfering gas is carbon dioxide, almost the same characteristics are shown when the interfering gas is water vapor.

[0041] In addition, although the target temperature is set to 450°C, the target temperature may deviate from 450°C. For example, since a difference may occur between the value detected by the sensor and the temperature of the heater surface depending on the heater structure, the target temperature may be experimentally determined to be the temperature at which the error is minimized as shown in FIG. 6. Specifically, it is preferable to compare the heater power in air and in an atmosphere containing carbon dioxide or water vapor, and to determine the heater temperature at which the difference in heater power is minimal by evaluating in advance using an actual element. For example, a gas with a carbon dioxide concentration of 2% and a hydrogen concentration of 0.1% is measured using a hydrogen sensor module, and the temperature at which the error between the measured value and the true value (0.1%) is within a preset value may be set as the target temperature.

[0042] As described above, the sensor module disclosed herein makes it possible to accurately measure the concentration of a gas to be measured in an atmosphere of a mixture of an interfering gas (such as carbon dioxide or water vapor) and a gas to be measured (such as hydrogen).

[0043] <Additional Notes> (Item 1) The present disclosure relates to a thermal conduction type sensor module for measuring the concentration of a target gas in a reference gas. The sensor module 1 shown in Fig. 1 includes a microheater 10 that generates heat in response to supplied power, and a control device 100 that controls the power of the microheater 10 so that the heater temperature approaches a target temperature at which the thermal conductivity of the interference gas and the thermal conductivity of the reference gas match.

[0044] With this configuration, the control device 100 can measure the concentration of at least one type of gas contained in the mixed gas with high accuracy based on the detection value from the microheater 10 even in an atmosphere in which the mixed gas exists.

[0045] (2) The sensor module according to the 1st paragraph further includes a temperature sensor 20 arranged to receive heat from the microheater 10 by thermal conduction. The controller is configured to control power to the heater 10 based on an output of the temperature sensor 20.

[0046] (Item 3) In the sensor module according to item 1 or 2, the heater 10 and the temperature sensor 20 are formed on a common substrate, as shown in FIG.

[0047] With this configuration, a compact and easy-to-use sensor module can be realized.

[0048] (4) In the sensor module described in any one of paragraphs 1 to 3, as shown in FIG. 5, the control device 100 is configured to detect the concentration of the gas to be measured based on the power of the heater 10 when the difference between the temperature detected by the temperature sensor 20 and the target temperature T0 becomes smaller than the threshold temperature.

[0049] According to this configuration, the control device 100 can select a target temperature that is less susceptible to the influence of interference gases in an atmosphere in which a mixed gas exists, and can measure the concentration of the gas to be detected with high accuracy.

[0050] (Item 5) In the sensor module described in item 4, the target temperature is T0=450°C, and the threshold temperature is 50°C.

[0051] (Item 6) In the sensor module described in item 4, the control device 100 controls the electric power of the heater so that the temperature detected by the temperature sensor 20 becomes 450° C. or higher.

[0052] (Item 7) In the sensor module according to item 1, the measurement target gas is hydrogen gas, the reference gas is air, and the interfering gas is carbon dioxide gas or water vapor.

[0053] According to this configuration, the control device 100 can measure the concentration of hydrogen in a mixed gas containing carbon dioxide gas or water vapor as an interfering gas with high accuracy.

[0054] (Item 8) In the sensor module according to any one of items 1 to 7, the heater 10 includes wiring made of a metal including a platinum layer 330, as shown in FIG.

[0055] (Item 9) In the sensor module described in any one of items 1 to 8, as shown in Fig. 2, the heater 10 includes a first layer (331) and a second layer (332) formed on the first layer (331). At least one of the first layer (331) and the second layer (332) is an oxide barrier film (oxide layer).

[0056] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the claims, not by the description of the embodiments described above, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0057] 1 sensor module, 10 micro heater, 11 silicon substrate, 12 thin film layer, 20 temperature sensor, 100 control device, 101 arithmetic device, 102 storage device, 103 peripheral device interface, 104 micro heater interface, 120 control program, 130 thermal conductivity data, 201, 331, 332 metal oxide, 202, 330 platinum layer, 321, 324 insulating layer, 322 intermediate layer, 323 heater portion.

Claims

1. A thermal conduction type sensor module for measuring the concentration of a target gas in a reference gas, a heater that generates heat in response to the supplied power; a control device that controls power to the heater so that the temperature of the heater approaches a target temperature at which the thermal conductivity of an interfering gas and the thermal conductivity of the reference gas match.

2. a temperature sensor disposed to receive heat from the heater by thermal conduction; The sensor module of claim 1 , wherein the controller is configured to control power to the heater based on an output of the temperature sensor.

3. The sensor module of claim 2 , wherein the heater and the temperature sensor are formed on a common substrate.

4. The sensor module according to claim 2, wherein the control device is configured to detect the concentration of the measurement target gas based on the power of the heater when the difference between the temperature detected by the temperature sensor and the target temperature becomes smaller than a threshold temperature.

5. The target temperature is 450° C. The sensor module of claim 4 , wherein the threshold temperature is 50° C.

6. The sensor module according to claim 4 , wherein the control device controls the electric power of the heater so that the temperature detected by the temperature sensor is 450° C. or higher.

7. The measurement target gas is hydrogen gas, The reference gas is air; The sensor module according to claim 1 , wherein the interfering gas is carbon dioxide gas or water vapor.

8. The sensor module according to claim 1 , wherein the heater includes a wiring formed of a metal including platinum.

9. The heater is A first layer; a second layer formed overlying the first layer, The sensor module according to claim 1 , wherein at least one of the first layer and the second layer is an oxide barrier film.

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