gas sensor

Abstract

To more accurately cancel effects exerted on measurement results by the gas being measured.SOLUTION: A gas sensor 1 comprises: a gas sensor unit 11 that includes thermistors Rd1, Rd2 that are connected in series; a temperature sensor unit 12 that generates a temperature detection signal Vtemp_1; a variable resistor VR1 that is connected in parallel to the thermistor Rd1; and a control unit 25 that generates an output signal OUT that indicates the concentration of a CO2 gas, on the basis of a gas detection signal Vco2_1 that appears at the connecting point of the thermistor Rd1 and the thermistor Rd2. The control circuit 25 causes the resistance value of the variable resistor VR1 to change, on the basis of the temperature detection signal Vtemp_1. Thus, it is made possible to more accurately cancel effects exerted on measurement results by water vapor.SELECTED DRAWING: Figure 1

Description

【Technical Field】 【0001】 This disclosure relates to a gas sensor. 【Background Art】 【0002】 Patent Document 1 discloses a gas sensor that calculates the concentration of a gas to be measured based on the level of a signal that appears at the connection point of two series-connected thermistors. In the gas sensor described in Patent Document 1, a correction resistor is connected in parallel to one of the thermistors to cancel the measurement error caused by non-measured gases. 【Prior Art Documents】 【Patent Documents】 【0003】 【Patent Document 1】 Japanese Unexamined Patent Application Publication No. 2019-060848 【Summary of the Invention】 【Problems to be Solved by the Invention】 【0004】 However, the inventors have found that, depending on the type of non-measured gas, the influence of the non-measured gas on the measurement result changes with the ambient temperature. 【0005】 In this disclosure, a gas sensor that can cancel the influence of non-measured gases on the measurement result more accurately is described. 【Means for Solving the Problems】 【0006】 The gas sensor according to this disclosure includes a gas sensor unit including first and second thermistors connected in series, a temperature sensor unit that generates a temperature detection signal, a variable resistor connected in parallel to the first thermistor, and a control circuit that generates an output signal indicating the concentration of the gas to be measured based on the gas detection signal that appears at the connection point of the first thermistor and the second thermistor. The control circuit changes the resistance value of the variable resistor based on the temperature detection signal. [Effects of the Invention] 【0007】 According to this disclosure, a gas sensor is provided that can more accurately cancel out the influence of non-target gases on the measurement results. [Brief explanation of the drawing] 【0008】 [Figure 1] Figure 1 is a circuit diagram showing the configuration of a gas sensor 1 according to one embodiment of the technology described herein. [Figure 2] Figure 2 is a flowchart illustrating the operation of the control circuit 25. [Figure 3] Figure 3 shows an example of data table 26. [Figure 4] Figure 4 is a graph showing the changes in the resistance value of the variable resistor VR1 and the reference value Vref_co2 according to the ambient temperature. [Figure 5] Figure 5 is a graph showing measured values ​​to explain the effect of gas sensor 1. [Figure 6] Figure 6 is a flowchart illustrating the operation of a modified version of the control circuit 25. [Figure 7] Figure 7 is a waveform diagram illustrating the transition between Table No. 1 and Table No. 2. [Modes for carrying out the invention] 【0009】 The embodiments of the technology described herein will be described in detail below with reference to the attached drawings. 【0010】 Figure 1 is a circuit diagram showing the configuration of a gas sensor 1 according to one embodiment of the technology described herein. 【0011】 As shown in Figure 1, the gas sensor 1 according to this embodiment comprises a gas sensor unit 11, a temperature sensor unit 12, and a signal processing circuit 20. Although not particularly limited, the gas sensor 1 according to this embodiment detects the concentration of CO2 gas in the atmosphere. 【0012】 The gas sensor unit 11 is a thermal conduction type gas sensor for measuring the concentration of CO2 gas, which is the gas to be measured, and includes thermistors Rd1 and Rd2 connected in series, and heater resistors MH1 and MH2 for heating thermistors Rd1 and Rd2, respectively. The temperature sensor unit 12 includes a thermistor Rd3 connected in series and a fixed resistor R1. Thermistors Rd1 to Rd3 are detection elements made of materials with a negative temperature coefficient of resistance, such as composite metal oxides, amorphous silicon, polysilicon, and germanium. Both thermistors Rd1 and Rd2 detect the concentration of CO2 gas, but their operating temperatures are different, as will be described later. Thermistor Rd3 also functions as a temperature sensor to detect the ambient temperature. 【0013】 As shown in Figure 1, thermistors Rd1 and Rd2 are connected in series between the wiring supplied with the power supply potential Vcc and the wiring supplied with the ground potential GND. Thermistor Rd1 is heated to, for example, 300°C by heater resistor MH1, and thermistor Rd2 is heated to, for example, 150°C by heater resistor MH2. Thermistor Rd1 is designed to have a predetermined resistance value when heated to 300°C, while thermistor Rd2 is designed to have a predetermined resistance value when heated to 150°C. A gas detection signal Vco2_1 appears at the connection point of thermistors Rd1 and Rd2. 【0014】 When the sensing element, thermistor Rd2, is heated to 150°C and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of thermistor Rd2 change according to its concentration. This change manifests as a change in the resistance value of thermistor Rd2. On the other hand, even when the reference element, thermistor Rd1, is heated to 300°C and CO2 gas is present in the measurement atmosphere, the heat dissipation characteristics of thermistor Rd1 hardly change according to its concentration. Therefore, the change in the resistance value of thermistor Rd1 heated to 300°C due to the CO2 gas concentration is sufficiently smaller than the change in the resistance value of thermistor Rd2 heated to 150°C due to the CO2 gas concentration. The change in the resistance value of thermistor Rd1 heated to 300°C due to the CO2 gas concentration can be negligible. The gas detection signal Vco2_1 that appears at the connection point of thermistor Rd1 and thermistor Rd2 is supplied to the signal processing circuit 20. 【0015】 Here, when thermistors Rd1 and Rd2 are heated, if water vapor is present in the measurement atmosphere, the heat dissipation characteristics of thermistors Rd1 and Rd2 change depending on the concentration of water vapor. The change in resistance value of thermistor Rd1 heated to 300°C due to humidity is greater than the change in resistance value of thermistor Rd2 heated to 150°C due to humidity. The difference in sensitivity between thermistors Rd1 and Rd2 to humidity also changes with ambient temperature, and the higher the ambient temperature, the greater the difference in sensitivity between thermistors Rd1 and Rd2 to humidity. This difference in sensitivity between thermistors Rd1 and Rd2 to humidity is canceled out by the variable resistor VR1 connected in parallel to thermistor Rd1. 【0016】 Thermistor Rd3 and fixed resistor R1 are connected in series between the wiring to which the power supply potential Vcc is supplied and the wiring to which the ground potential GND is supplied. A temperature detection signal Vtemp_1 appears at the connection point between fixed resistor R1 and thermistor Rd3. The temperature detection signal Vtemp_1 is input to the signal processing circuit 20. 【0017】 The signal processing circuit 20 includes amplifiers 21, 22, an analog-to-digital converter (ADC) 23, a digital-to-analog converter (DAC) 24, and a control circuit 25. The amplifier 21 generates a gas detection signal Vco2_2 by amplifying the gas detection signal Vco2_1. The amplifier 22 generates a temperature detection signal Vtemp_2 by amplifying the temperature detection signal Vtemp_1. The gas detection signal Vco2_2 and the temperature detection signal Vtemp_2 are input to the ADC 23. The ADC 23 generates a gas detection value Vco2_adc and a temperature detection value Vtemp_adc, which are digital values, by performing analog-to-digital conversion on the gas detection signal Vco2_2 and the temperature detection signal Vtemp_2. The digital gas detection value Vco2_adc and the temperature detection value Vtemp_adc are supplied to the control circuit 25. 【0018】 The control circuit 25 has a data table 26. As will be described later, the control circuit 25 calculates the ambient temperature by performing an operation based on the temperature detection value Vtemp_adc, and changes the resistance value of the variable resistor VR1 by selecting a predetermined table in the data table 26 according to the ambient temperature. Furthermore, the control circuit 25 generates an output signal OUT indicating the concentration of CO2 gas by performing an operation based on the gas detection value Vco2_adc. On the other hand, the DAC 24 generates heater voltages Vmh1, Vmh2 by performing digital-to-analog conversion on the digital values supplied from the control circuit 25. The heater voltages Vmh1, Vmh2 are applied to the heater resistors MH1, MH2, respectively, thereby heating the thermistors Rd1, Rd2. 【0019】 The variable resistor VR1 may be a part of the signal processing circuit 20 or may be provided outside the signal processing circuit 20. Also, the variable resistor VR1 may be a part of the gas sensor unit 11. 【0020】 Next, the operation of the control circuit 25 will be described. 【0021】 Figure 2 is a flowchart for explaining the operation of the control circuit 25. 【0022】 First, when the control circuit 25 starts operating, it performs various initial settings such as provisional selection of the data table 26 (step 31), then acquires the temperature detection value Vtemp_adc (step 32), and calculates the ambient temperature based on the temperature detection value Vtemp_adc (step 33). The ambient temperature Temp can be calculated by performing the following equation (1). In equation (1), "B" is the B constant of the thermistor Rd3, and "Vtemp_adc@25" is the value of the temperature detection value Vtemp_adc obtained when the ambient temperature is 25°C. 【0023】 【number】 【0024】 Next, the control circuit 25 determines which temperature range the calculated ambient temperature Temp falls into by referring to the data table 26 (step 34). An example of the data table 26 is shown in Figure 3. The data table 26 shown in Figure 3 includes tables No. 1 to No. 5, each with a different temperature range. If the current ambient temperature falls within the temperature range corresponding to the table provisionally selected in the initial setup, the current table selection is continued (step 35). Conversely, if the current ambient temperature falls outside the temperature range corresponding to the table provisionally selected in the initial setup, the table selection is changed. In this case, if the current ambient temperature is lower than the temperature range corresponding to the table provisionally set in the initial setup, the table number is increased by one (step 36), and if the current ambient temperature is higher than the temperature range corresponding to the table provisionally set in the initial setup, the table number is decreased by one (step 37). Alternatively, the table corresponding to the current ambient temperature may be directly selected without provisionally selecting a data table 26 during the initial setup. 【0025】 As shown in Figure 3, each table is assigned a corresponding resistance value for the variable resistor VR1. The variable resistor VR1 is set so that the resistance value decreases as the table number increases, that is, as the ambient temperature increases. Therefore, as shown in Figure 4, the resistance value of the variable resistor VR1 decreases in steps as the ambient temperature increases. 【0026】 Next, heater voltages Vmh1 and Vmh2 are generated to heat the heater resistors MH1 and MH2 (step 38). The levels of the heater voltages Vmh1 and Vmh2 may be finely adjusted according to the calculated ambient temperature Temp so that thermistor Rd1 is heated to 300°C and thermistor Rd2 is heated to 150°C. Then, in this state, the gas detection value Vco2_adc is acquired (step 39), and the reference value Vref_co2 is calculated (step 40). The reference value Vref_co2 corresponds to the level obtained by AD conversion of the gas detection signal Vco2_2 when the concentration of the gas to be detected is at the normal concentration. In other words, when the concentration of the gas to be detected is at the normal concentration, the value of the gas detection value Vco2_adc that should be input to the control circuit 25 is the reference value Vref_co2. In this embodiment, the gas to be detected is CO2 gas, and the concentration of CO2 gas in the atmosphere under normal conditions is approximately 400 ppm. Since the concentration of CO2 gas in the atmosphere varies depending on the observation point, the "normal concentration" refers to the concentration of CO2 gas in the atmosphere at the observation point in question. Furthermore, indoors, the concentration of CO2 gas may be higher than that outdoors even under normal conditions; however, even in this case, the concentration of CO2 gas under adequately ventilated conditions is defined as the "normal concentration." 【0027】 The reference value Vref_co2 can be calculated by performing the following equation (2). Here, "Vref_co2_a", "Vref_co2_b", "Vref_co2_c", and "Vref_co2_d" are coefficients assigned to each of the tables shown in Figure 3. 【0028】 【number】 【0029】 As a result, in this embodiment, not only does the resistance value of the variable resistor VR1 change according to the ambient temperature, but the constants in the calculation formula used to calculate the reference value Vref_co2 also change. Consequently, as shown in Figure 4, the calculated reference value Vref_co2 decreases in steps in conjunction with the decrease in the resistance value of the variable resistor VR1 as the ambient temperature increases. The reason for gradually decreasing the reference value Vref_co2 in conjunction with the decrease in the resistance value of the variable resistor VR1 is to compensate for the decrease in the midpoint potential appearing at the connection point of thermistors Rd1 and Rd2 when the resistance value of the variable resistor VR1 decreases. Furthermore, since the ambient temperature Temp is included in equation (2) above, the correct reference value Vref_co2 can be obtained according to the ambient temperature Temp. 【0030】 Next, the control circuit 25 calculates the gas concentration value CO2_Data based on the difference between the gas detection value Vco2_adc and the reference value Vref_co2, as well as the gain Av and sensitivity coefficient CO2_sen of the amplifier 21 (step 41), and outputs this as the output signal OUT (step 42). 【0031】 As described above, in the gas sensor 1 according to this embodiment, the resistance value of the variable resistor VR1 is changed according to the ambient temperature, and the calculation formula used to calculate the reference value Vref_co2 is also changed according to the ambient temperature. Therefore, even if the sensitivity difference between the thermistors Rd1 and Rd2 to humidity changes according to the ambient temperature, it is possible to correctly measure the CO2 gas concentration value. Moreover, since the resistance value of the variable resistor VR1 and the calculation formula used to calculate the reference value Vref_co2 are changed in stages using the data table 26, control by the control circuit 25 is also made easier. 【0032】 Figure 5 is a graph showing measured values ​​to illustrate the effect of the gas sensor 1 according to this embodiment. In Figure 5, the symbol OUT indicates the output signal of the gas sensor 1 according to this embodiment, and the symbol OUT' indicates the output signal of a conventional gas sensor. The symbol T indicates ambient temperature, and the symbol H indicates absolute humidity. The horizontal axis of Figure 5 represents time, and the vertical axis represents CO2 gas concentration, temperature, or humidity. 【0033】 As shown in Figure 5, even if the concentration of CO2 gas in the atmosphere remains constant at 400 ppm, conventional gas sensors suffer from increased measurement errors due to humidity as the ambient temperature rises. In contrast, the gas sensor 1 according to this embodiment can output the correct CO2 gas concentration value regardless of ambient temperature and humidity. 【0034】 Figure 6 is a flowchart illustrating the operation of a modified version of the control circuit 25. 【0035】 The modified operation shown in Figure 6 differs from the operation shown in Figure 2 in that, in step 34, the upper and lower limits of the temperature range assigned to each table are extended by 0.5°C. Specifically, the lower limit is set to a temperature 0.5°C lower than the lower limit of the temperature range assigned to each table, and if the temperature falls below this, the process proceeds to step 36. Similarly, the upper limit is set to a temperature 0.5°C higher than the upper limit of the temperature range assigned to each table, and if the temperature exceeds this, the process proceeds to step 37. 【0036】 As a result, as shown in Figure 7, for example, when transitioning from table No. 1 to table No. 2 due to an increase in ambient temperature, the condition is that the ambient temperature exceeds 35.5°C, and when transitioning from table No. 2 to table No. 1 due to a decrease in ambient temperature, the condition is that the ambient temperature falls below 34.5°C. By introducing hysteresis into the table transitions in this way, so-called chattering becomes less likely to occur, making it possible to perform more stable control. 【0037】 While embodiments of this disclosure have been described above, it goes without saying that this disclosure is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of this disclosure, and such modifications are also included within the scope of this disclosure. 【0038】 For example, in the above embodiment, the case in which the target gas is CO2 gas and the non-target gas that causes noise is water vapor was described as an example, but the present invention is not limited to this. Furthermore, it is not essential that the sensor unit used in the present invention be a thermal conduction type sensor; other types of sensors, such as catalytic combustion type sensors, may also be used. For example, when the target gas is CO gas, a catalytic combustion type sensor unit can be used. 【0039】 Furthermore, in the above embodiment, the resistance value of the variable resistor VR1 is decreased as the ambient temperature increases. However, if the difference in sensitivity between the thermistors Rd1 and Rd2 to the non-measured gas increases as the ambient temperature decreases, the resistance value of the variable resistor VR1 may be decreased as the ambient temperature decreases. In addition, in the above embodiment, the variable resistor VR1 is connected in parallel with thermistor Rd1. However, if the change in the resistance value of thermistor Rd2 due to the concentration of the non-measured gas is greater than the change in the resistance value of thermistor Rd1 due to the concentration of the non-measured gas, the variable resistor VR1 may be connected in parallel with thermistor Rd2. 【0040】 The technology relating to this disclosure includes, but is not limited to, the following configuration examples. 【0041】 The gas sensor according to this disclosure comprises a gas sensor unit including first and second thermistors connected in series, a temperature sensor unit that generates a temperature detection signal, a variable resistor connected in parallel to the first thermistor, and a control circuit that generates an output signal indicating the concentration of the target gas based on the gas detection signal appearing at the connection point of the first and second thermistors, wherein the control circuit changes the resistance value of the variable resistor based on the temperature detection signal. This makes it possible to more accurately cancel out the influence of non-target gases on the measurement result. 【0042】 In the gas sensor described above, the first thermistor is heated to a first temperature by a first heater, and the second thermistor is heated to a second temperature different from the first temperature by a second heater. The sensitivity of the first thermistor heated to the first temperature to the target gas is lower than the sensitivity of the second thermistor heated to the second temperature to the target gas, and the sensitivity of the first thermistor heated to the first temperature to a non-target gas may be higher than the sensitivity of the second thermistor heated to the second temperature to a non-target gas. With this configuration, measurement errors caused by the difference in sensitivity between the first and second thermistors to the non-target gas can be canceled out by a variable resistor. 【0043】 In the gas sensor described above, the control circuit may lower the resistance value of the variable resistor as the temperature indicated by the temperature detection signal increases. This allows the variable resistor to cancel out measurement errors caused by the difference in sensitivity between the first and second thermistors for non-measured gases, even if this difference increases as the ambient temperature rises. 【0044】 In the gas sensor described above, the control circuit may change the resistance value of the variable resistor in steps based on the temperature detection signal. This allows measurement errors to be canceled out by simple control. 【0045】 In the gas sensor described above, the control circuit may set the level of the temperature detection signal that serves as the reference for changing the resistance value of the variable resistor from a first resistance value to a second resistance value when the temperature detection signal indicates a rise in temperature, and the level of the temperature detection signal that serves as the reference for changing the resistance value of the variable resistor from a second resistance value to a first resistance value when the temperature detection signal indicates a decrease in temperature, to different levels. This makes so-called chattering less likely to occur. 【0046】 In the gas sensor described above, the control circuit generates an output signal by comparing the gas detection value based on the gas detection signal with a reference value corresponding to the level of the gas detection signal when the concentration of the gas to be measured is at its normal concentration. The control circuit may also vary the reference value in steps in conjunction with the stepwise change in the resistance value of the variable resistor. This makes it possible to make the reference value follow the change in the midpoint potential of the first and second thermistors due to the change in the resistance value of the variable resistor. [Explanation of Symbols] 【0047】 1. Gas sensor 11 Gas sensor section 12 Temperature sensor section 20 Signal Processing Circuits 21,22 23 AD Converters 24 DA converters 25 Control circuits 26 Data Tables MH1, MH2 Heater Resistors R1 Fixed resistor Rd1~Rd3 Thermistor VR1 Variable resistor

Claims

[Claim 1] A gas sensor unit including first and second thermistors connected in series, A temperature sensor unit that generates a temperature detection signal, A variable resistor connected in parallel to the first thermistor, The system includes a control circuit that generates an output signal indicating the concentration of the gas to be measured based on a gas detection signal appearing at the connection point of the first thermistor and the second thermistor, The control circuit is a gas sensor that changes the resistance value of the variable resistor based on the temperature detection signal. [Claim 2] The first thermistor is heated to a first temperature by the first heater, The second thermistor is heated by the second heater to a second temperature different from the first temperature. The sensitivity of the first thermistor heated to the first temperature to the target gas is lower than the sensitivity of the second thermistor heated to the second temperature to the target gas. The gas sensor according to claim 1, wherein the sensitivity of the first thermistor heated to the first temperature to the non-measurement gas is higher than the sensitivity of the second thermistor heated to the second temperature to the non-measurement gas. [Claim 3] The control circuit lowers the resistance value of the variable resistor as the temperature indicated by the temperature detection signal increases, according to claim 1 or 2, for the gas sensor. [Claim 4] The gas sensor according to claim 1, wherein the control circuit changes the resistance value of the variable resistor in steps based on the temperature detection signal. [Claim 5] The gas sensor according to claim 4, wherein the control circuit sets the level of the temperature detection signal that serves as a reference for changing the resistance value of the variable resistor from a first resistance value to a second resistance value when the temperature detection signal indicates a rise in temperature, and the level of the temperature detection signal that serves as a reference for changing the resistance value of the variable resistor from the second resistance value to the first resistance value when the temperature detection signal indicates a decrease in temperature, to different levels. [Claim 6] The control circuit generates the output signal by comparing the gas detection value based on the gas detection signal with a reference value corresponding to the level of the gas detection signal when the concentration of the gas to be measured is at its normal concentration. The gas sensor according to claim 4 or 5, wherein the control circuit changes the reference value in steps in conjunction with the stepwise change in the resistance value of the variable resistor.

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